Chemistry and Technology of Polyurethanes

Walter Dias Vilar, D. Sc.

Book: Chemistry and Technology of Polyurethanes
Third updated edition (2002), 360 pages,
with the Catalog of Companies and PU Suppliers
Price: US$ 100,00 (one hundred dollars).

Click here and send your name and address to receive the book.

For information contact: : Vilar Consultoria Técnica Ltda.
Av. Epitácio Pessoa, 3930/803, Lagoa, Rio de Janeiro, RJ, Brazil, ZIP CODE 22471-000
Phone/fax: [phone] and 55 21 9632-3704 - e-mail: [e-mail].

http://www.poliuretanos.com.br/

1.3 Polyols

1.3.1 Polyoxypropylene glycols (PPG's)

1.3.1.1 Polyoxypropylene/ethylene glycols

1.3.1.2 PPG's with low monol contents

1.3.1.3 Polymer polyols

1.3.2 Polyester polyols

1.3.2.1 Aliphatic polyester polyols

1.3.2.1.1 Polycaprolactone polyols

1.3.2.2 Aromatic polyester polyols

1.3.3 Poly(oxytetramethylene) glicol

1.3.4 Castor oil based polyols

1.3.5 Hydroxyl terminated polybutadiene (HTPB)

1.3.6 Acrylic polyols

1.4 Polyamines

1.5 Chain extenders

1.5.1 Hydroxylated chain extenders

1.5.2 Diamines as chain extenders

1.6 Relationship between structure and properties

1.6.1 Segmented polyurethanes

1.6.2 Hard segment effects

1.6.3 Soft segments effects

1.6.4 Molecular structures

1.6.5 Crosslinked PUs

Chapter 2 - Fundamentals & Raw materials - II - Additives

2.1 Catalysts

2.1.1 Catalysis

2.1.2 Tertiary amines

2.1.2.1 Catalysis mechanism

2.1.2.2 Gel and blow catalysis

2.1.3 Organometallic catalysts

2.2 Inhibitors

2.3 Blowing agents

2.3.1 Flexible foams

2.3.2 Rigid foams

2.4 Surfactants

2.4.1 Selection of the silicone surfactant

2.4.2 Low density foams stabilization mechanisms

2.4.3 Microcellular systems

2.5 Fillers

2.6 Antiaging agents

2.7 Coloring materials

2.8 Flame retardants

2.9 Release agents

2.10 Special addictives

2.10.1 Crosslinkers

2.10.2 Adhesion promoters

2.10.3 Coupling agents

2.10.4 Moisture scavengers

2.10.5 Rheology modifiers

2.10.6 Plasticizers

2.10.7 Antistatic agents

2.10.8 Cell openers

2.10.9 Lubricating

2.10.10 Hydrolysis stabilizers

2.10.11 Bacteriostats

1.3 - Polyols

A great polyol variety is used in PU's. Among the most widely employed are polyether polyols, which are obtained by the polymerization of propylene, ethylene and butylene oxides. Most commonly used are poly (propylene oxide) glycol and copolymers of (propylene/ethylene oxides) glycols (PPG's) (Table 1.5). Other polyether polyols, such as poly (tetramethylene oxide) glycol, are used in PU fibers and high performance elastomers, while polymeric polyols are used in high resilience flexible foams. Besides, one can cite the polyester polyols (Table 1.6), used in high performance applications, castor oil, hydroxyl-terminated polybutadiene (HTPB), etc. Usually, polyols having molecular weight between 1,000 and 6,000 and functionality between 1.8 and 3.0 are used in flexible foams and elastomers. Short chain (250<1,000), high functionality (3<12) polyols yield high cross-linked rigid chains and are used in rigid foams and high performance coatings. The molecular weight distribution of polyether polyols follows the Poisson probability equation, being narrower than the polyester distribution (thermodynamically controlled in agreement with the Flory equation).

1.3.1 - Polypropylene glycols (PPG's)

PPG's are obtained through anionic polymerization of propylene oxide (PO) and propylene and ethylene oxides (EO) copolymerization. The first stage of the PPG polymerization process is the reaction of an alcohol with a strong base (usually potassium hydroxide), forming the corresponding alcoholate. Figure 1.22 shows the mechanism of secondary hydroxyl formation resulting from the nucleophilic attack to the less hindered PO carbon atom.

Figure 1.22 - PPG preparation

PPG's narrow molecular weight distribution is due to the anionic polymerization process. The polyether functionality corresponds to the functionality of hydroxyl or amine group-containing compounds used as initiators (Tables 1.5 and 1.6). The PPG's viscosity is between 100 and 1,000 cP @ 25°C. PPG's diols of MW between 400 and 4,000 and hydroxyl number from 265 to 28 mg of KOH/g are used in CASE products, and the trifunctional ones, of molecular weight between 3,000 and 6,000, hydroxyl number from 56 to 28 mg of KOH/g are used in flexible foams, those of higher MW being used in high resilience foams (HR). High functionality polyols, of MW lower than 1,000, hydroxyl number between 300 to 800 and high viscosity (in some cases until 17,000 cP @ 25°C) yield highly crosslinked polyurethanes and are used in rigid foams. A few examples of polyether polyols based on propylene and ethylene oxides, used in different applications, are shown in Table 1.5.

Table 1.5 - Typical properties of polyether polyols

Applications	CASE^1	Flexible Foams			Rigid Foams
				Conventional			Conventional	HR
Polyol composition	propylene glycol + propylene oxide	glicerine + propylene and ethylene oxides	amine + propylene oxide	trimethylolpropane + propylene and ethylene oxides	trimethylolpropane + propylene oxide	sucrose + propylene oxide
Average MW	2000  100	3000  200	3750  200	4800  300	440  35	860   60
OH number  (mg KOH/g)	56 ± 3	56 ± 3	60 ± 3	35 ± 2	380 ±  25	380 ± 25
OH content (meq/g)	1.0	1.0	1.1	0.6	6.8	6.9
Average functionality^2	2.0	3.0	4.0	3.0	3.0	5.8
Insaturation (meq/g)	< 0.04	0.04	< 0.04	< 0.05	< 0.005	< 0.005
Viscosity at 25^oC (mPa.s)	250 - 350	450 - 550	580 - 720	750 - 900	600 - 700	11000 - 15000
Pour point (^oC)	- 36	- 31	- 35	- 38	- 22	- 2
pH	6.5 - 8.0	6.5 - 8.0	8.6 - 9.6	6.5 - 8.0	6.0 - 7.5	6.5 - 8.0
Density, 25^oC (g/cm)	1.00	1.01	1.00	1.02	1.03	1.1

1 - coatings, adhesives, sealants e elastomers, 2 - Average functionality = PM x OH content (meq/g) / 1000

Usually, diols such as propylene glycol are used as initiators for the production of polyether diols; triols such as glycerin and trimethylol propane to obtain polyether triols, and products of higher functionality such as sorbitol and sucrose in the production of high functionality polyether polyols (Table 1.6).

Table 1.6 - Common initiators for polyether polyols

Initiators	Structure	Functionality
Water	HOH	2
Ethylene glycol	HOCH2CH2OH	2
1,2-Propylene glycol	HOCH2CH(CH3)OH	2
Glicerine		3
Trimethylolpropane		3
Triethanol amine	N-(-CH2-CH2OH)3	3
Pentaerythritol	C-(-CH2OH)4	4
Ethylene diamine	H2NCH2CH2NH2	4
2,4-toluene diamine or (2,6-toluene diamine)		4
4'.4'-diphenyl methane diamine		4
Diethylene triamine	H2NCH2CH2NHCH2CH2NH2	5
Sorbitol		6
Sucrose		8

Primary amines can also be used as initiators to obtain polyether polyols. Due to the stronger nucleophilic character of the amines as compared to hydroxylated compounds, the use of KOH as catalyst may be dispensed with. Ethylene and toluene diamines are initiators for obtaining tetra functional polyether polyols. The higher basicity of the resulting polyols having tertiary amine groups makes them more reactive towards isocyanates.

 

1.3.1.1 - Copolymers with ethylene oxide

Reactive PPG's are used in cold molded PU foam systems, and their reactivity depends on the degree of primary hydroxyl groups (Figure 1.23). PPG's produced only with PO possess less reactive secondary hydroxyl groups. For obtaining more reactive primary hydroxyls, reaction with EO is carried out in the final stage of the polymerization. Usually the EO block is less than 20% of the polymer chain and it improves the polyol solubility in water. PO/EO copolymers increase the water solubility and decrease the heterogeneous phases, where the reaction of water with isocyanate occurs and forms rigid polyurea macrophases, resulting in softer flexible foams (Chapter 3).

Figure 1.23 - PPG's reactivity

 

1.3.1.2 - Low monol degree PPG's

The conventional propylene oxide based polyether polyols (PPG) are produced commercially through the base catalyzed, using an alkali metal salt such potassium hydroxide (KOH), propoxylation of so-called starters, which have two or more hydroxyl groups. Typically, propylene glycol is used to produce diols and glycerin to produce triols. However, their functionality is slightly lower than that of the initiator compound, this sometimes being due to small amounts of remaining water in reagents. It is well known that base catalyzes not only the addition of propylene oxide to the growing polyol molecule, but also a side reaction in which propylene oxide isomerizes to allyl alcohol (Figure 1.24).

Figure 1.24 - PPG monol formation

Allyl alcohol acts as a monofunctional starter resulting in propoxylated allyl alcohol referred to as monol. These monofunctional products act as chain terminators during the PU formation, resulting in a decrease in mechanical properties. As PPG monol is double bond- terminated, the monol degree can be quantified by the insaturation (in meq/g). The PPG's insaturation level or monol content increases with the molecular weight (Table 1.7). For example, a 2000 MW conventional PPG diol has a monol content of about 0,03 meq/g, which correspond to a functionality of 1,92, whereas, a 4000 MW PPG diol has a monol content of about 0.085 meq/g and functionality of only 1.7. Syntheses of low insaturation degree (<0.02 meq/g) polyether polyols have been carried out by using double metal cyanide (DMC) catalysts based on Zn₃[Co(CN)₆]₂ (zinc hexacyanocobaltate). The new generation of ultra low monol content polyols has a typical monol (insaturation) content of 0.005 meq/g or less, which correspond to a functionality of 1.98 for a 4000 MW diol.

Table 1.7 - Characteristics of commercial PPG's

Molecular Weight		OH number (mg de KOH/g)	Viscosity at (25^oC)	Insaturation (meq/g)	Monol % (mol)	Average Functionality*
Conventional	Low monol
1000		111	145	0.01	1	1.99
	1000	111	145	0.005	0,5	1.995
2000		56	335	0.03	6	1.94
	2000	56	335	0.005	1	1.99
3000		37	570	0.05	14	1.86
	3000	37	580	0.004	1	1.99
4000		28	980	0.09	31	1.69
	4200	28	860	0.005	2	1.98
8000		14		-	-	-
	8200	14	3000	0.05	4	1.96

* calculated using Carothers equation.

 

1.3.1.3 - Copolymer Polyols

Polyether polyols can be "filled" with grafted organic polymers. The successful preparation of stable, colloidal dispersions requires that the particles have some type of steric stabilization to prevent flocculation of the suspension. These white and viscous modified polyol dispersions are useful in making high hardness foams. Polymeric polyols are usually formed trough a free radical or step addition process by in situ monomer grafting to PPG. There are two types of modified polyether polyols.

Copolymer Polyols - Copolymer polyols are obtained through free radical grafting of styrene and acrylonitrile (SAN copolymer) to PPG. The product contains a mixture of PPG, SAN copolymer and graft copolymer polyols that act as a stabilizer (Figure 1.25).

Figure 1.25 - Copolymer polyol reactions

The first commercial copolymer polyols were based on acrylonitrile as the sole monomer. This dispersion contained 20% poly(acrylonitrile) and had viscosities of 3,000-5,000 mPa.s. These products were used for the production of cold molded, high resilience flexible foams (HR), with improved hardness, strength, foam processing, and cell opening. As the 100% acrylonitrile copolymer polyols cause discoloration problems in slabstock flexible foams, styrene/acrylonitrile (SAN) copolymer polyols were developed. These copolymer polyols require the use of a more efficient stabilizer molecule to form the grafted portion of the polymer. This is because the styrene and acrylonitrile copolymerization is quite favorable, and the tendency for free radicals to be generated on the polyol is reduced. The so-called macromer stabilizers are PPG's which have been functionalized with a vinyl moiety (for the PPG's reaction with maleic anhydride, ethylisocyanate methacrylate, methacryloyl chloride, etc), which will undergo copolymerization with SAN monomers (Figure 1.26).

Figure 1.26 - Copolymer polyol particle synthesis
a) graft macromer; b) "comb polymer"; c) phase separation; d) 0.01-0.05 microns particles; e) inside particle polymerization; f) 0.3-0.5 microns final particle.

Polymerization starts with the addition of base polyol and macromer, followed by initiator and monomer feed. In the beginning the reaction mixture is homogeneous and the grafted polyol is formed. At some point (corresponding to 1-3% of total added monomer) the polymeric "comb" associates into a spherical structure, in which the insoluble copolymer is located in the center of the sphere, and the macromer chains are in the surface, where they can interact with the continuous polyol phase. Polymerization continues inside the particles that grow from 0.01-0.05 to 0.3-0.5 microns, yielding a narrow distribution and monodisperse size particles. In molded and slabstock HR flexible foams copolymer polyols are used with 25-40% solids and viscosity 2,500-7,000 mPa.s, and for conventional flexible foams, with 40-43% solids and 4,000-6,000 mPa.s.

PHD Polyol (urea/urethane polyol) - PHD polyols (polyurea modified) are dispersions of polyurea particles, formed by the reaction between TDI and diamines (hydrazine, ethylene diamine, etc) in a conventional polyol. In the reaction of diamines and isocyanates, the low molecular weight polyurea formed separates from the continuous phase. Since the reaction of a polyol and diisocyanate is slower than that of a diamine and diisocyanate, their tendency to form small particles is less probable than that of the SAN copolymer polyols. This leads to lower incorporation of polyol grafts and broader distribution of particle sizes. The diamine quickly reacts with an excess diisocyanate and the formed polyurea- modified isocyanate reacts with the polyol, forming poly(urea/urethane) that acts as stabilizer for the polyurea dispersion in the PHD polyol (Figure 1.27). These polyureas, present in 20-30%, react with isocyanate during foam manufacture, eventually increasing the cross-linking degree.

Figure 1.27 - Formation of PHD polyol

 

1.3.2 - Polyester polyols

1.3.2 - Polyester polyols

Polyester polyols were the first polyols used in the beginning of PU development, and are produced by polycondensation of a diacid with excess diol (Figure 1.28).

Figure 1.28 - Obtaining polyol polyester

Difunctional monomers are used to obtain a linear polymer; and monomers with functionality larger than two as trimethylol propane and glycerin create ramified chains. The most used acids are adipic and phthalics. Adipic acid based polyester polyols are used in applications where flexibility is wanted, as in flexible foams and elastomers. Phthalic acids (or phthalic anhydride) based polyols, have rigid chains and are used in rigid foams and in high performance coatings (Table 1.8).

Table 1.8 - Typical properties of polyesters polyols

Application	Flexible foam	Semi-rigid foam	Rigid foam	Shoe soles	Elastomers	Coatings
												soft	hard
Structure	adipic acid, diethylene glycol, trimethylol propane	adipic acid phthalic acid, 1,2-propylene glycol, glycerine	adipic acid, phthalic acid, oleic acid, trimethylol propane	adipic acid, ethylene glycol, diethylene glycol	adipic acid, ethylene glycol, 1,4-butane diol	adipic acid, diethylene glycol	phthalic acid, maleic acid, trimethylol propane
Average MW	2400	1000	930	2000	2000	2750	2450
OH number (mgKOH/g)	57 - 63	205 - 221	350 - 390	58-62	52 - 58	38 - 45	250 - 270
OH content (meq/g)	1.1	3.8	6.6	1.1	1.0	0.7	4.6
Average functionality*	2.7	3.8	6.2	2.1	2.0	2.0	11.3
Viscosity at 75°C (mPa.s)	950 - 1100	570 - 750	1300 - 1550	500 - 700	500 - 600	700 - 800	17000 a 150^oC
Pour point (°C)	-12	-12	7	17 to 56	49 to 52	-9	90 to 100
Acid number	1.2	2.8	1.0	0.4	1.0	1.0	4.0
Density, 75°C (g/cm)	1.15	1.15	1.1	1.15	1.17	1.12	1.24

*Average functionality = PM x OH content (meq/g) / 1000

In polyester polyols production process, the diol, triol, etc is first heated to a temperature of 60-90°C. Then the dicarboxilic acid is added and removal of the reaction water begins. For obtaining the targeted molecular weight the excess diol is calculated by means of Flory Equation. Diol can be lost during removal of the water form the condensation reaction and through side reactions (formation of ethers and aldehydes). The amount of diol lost is dependent upon the processing conditions and upon type of diol. The amount of diol lost must be empirically determined. Usually the reaction is completed at temperatures up to 200°C. Nitrogen, carbon dioxide, or vacuum is used to remove the water and to reach the wanted conversion of 99.9%, and the resulting polyester should have an acid number less than two. This conversion is necessary to minimize the presence of residual carboxylic end groups that can reduce the reactivity. The polyesters are composed of all possible oligomers raging from the monomers to high molecular weight species: the molecular weight distribution follows a Frory probability. The properties of the PU based polyester elastomers are governed mainly by the overall molecular weight of the polyester and only to a minor degree by the molecular weight distribution.

Acids, bases and compounds of the transition metals can catalyze the esterification reaction. The dicarboxylic acids also exert a limited catalytic effect. In practice catalysts are used reluctantly because they cannot be removed and can have an undesirable effect on the following PU reaction, since inorganic substances even in the smallest quantities favor or retard de PU processing reaction. The p-toluenesulphonic acid can be used as an accelerator and left in the polyester. In cases where small amounts of catalysts do not later cause problems, compounds of tin, antimony, titanium, lead and other metals, have proved especially effective. The amounts added lie in the ppm range. Solid impurities are removed by hot filtration of the finished polyester.

 

1.3.2.1- Aliphatic polyester polyols

Usually aliphatic polyester polyols used in flexible polyurethanes are based on polyadipates diols such as ethylene glycol, diethylene glycol, propylene glycol, 1,4-butane diol, 1,6-hexane diol, etc. The growth of the diol chain results in greater PU flexibility and hydrolytic stability and reduction of polarity and glass transition temperature. Polyester polyols used in PU elastomers (Chapter 6), based on acid adipic and a glycol like ethylene glycol, 1,4-butane diol, 1,6-hexane diol, neopentyl glycol or mixtures of them (Table 1.9), are crystalline products with melting point between 50 and 60oC. The crystallinity can be reduced using mixed diols (as 1,4-butane diol and ethylene glycol) or mixed polyesters.

Lightly branched poly(diethyleneglycol adipates), which are used mainly to make flexible foams, and a wide range of adipates made with more than one aliphatic diol. These are used to make solid and microcellular elastomers, flexible coatings and adhesives. Relatively low cost polyester polyols, based on recovery materials are also available. Mixed adipic, glutaric and succinic acid polyesters are made using purified nylon waste acids (AGS acids). AGS acids are also hydrogenated to make a mixture of 1,4-butanediol, 1,5-pentanediol and 1,6-hexane diol, which is used to make polyadipates having a low melting point. Mixed polyadipates from hydrogenated AGS acids are used to make microcellular elastomers with good hydrolytic stability.

Table 1.9 - Polyester polyols of MW = 2000

Structure	Solidification point (°C)	Viscosity at 75°C (mPa.s)
adipic acid + ethylene glycol	52	540
adipic acid + ethylene glycol + 1,4-butane diol	17	625
adipic acid + 1,4-butane diol	56	670
adipic acid + hexamethylene glycol + neopentyl glycol	27	640

In comparison with PU based polyether polyols, the PU based polyesters are more resistant to oil, grease, solvents and oxidation. They possess better properties related to: tension and tear strength, flex fatigue, abrasion, adhesion and dimensional stability. On the other hand PU based esters are more sensitive to hydrolysis and microbiological attack. The high mechanical properties of PU based polyester can be explained by the greater compatibility between polar polyester flexible segments and polar rigid segments, causing a slower phase separation resulting in better distributed small crystalline rigid blocks (Chapter 1.7).

The hydrolysis stability of the ester linkage is inferior to that of the ether linkage in the polyethers, and residual esterification catalysts accelerate the hydrolysis. The hydrolysis resistance of the polyol polyester based PU increases with long chain glycols (1,6-hexane diol) or long chain diacids (dodecanoic acid), as a result of the largest hydrophobic portion and small amounts of ester groups. The hydrolysis stability can be improved with additives that react with carboxylic and alcoholic groups, formed during hydrolysis. These additives may be: oxazolines, epoxy compounds, aromatic polycarbodiimides and aliphatic monocarbodiimides. TPU's based polyester polyols are stabilized by addition of 1 to 2% in weight of aromatic hindered carbodiimides, that react with the acid generated by ester hydrolysis, which would act as catalyst of hydrolysis reactions (Figure 1.29).

Figure 1.29 - Reaction of carbodiimides with carboxyl

Polymeric polyester polyols are dispersions of vinyl polymers in polyadipate based polyol polyester stabilized by a dispersant. Polymeric polyester polyols, containing 10 to 20% of vinyl polymers are used in shoe soles and flexible PU foams with greater hydrolysis stability, higher hardness for same densities, more uniform cellular structures, and better dimensional stability.

1.3.2.1.1- Polycaprolactone polyols

Another process for production of aliphatics polyester polyols includes the ring opening polymerization of e-caprolactone with glycols (Chapter 1). Polycaprolactone diols are produced with diethylene glycol, 1,4 butane diol, neopentyl glycol or 1,6-hexane diol. The polycaprolactone triols use trimetilol propane or glycerin /ethylene glicol, and the tetrols are done with pentaerythritol. The polycaprolactone glycols are produced with MW from 400 to 4000, hydroxyl number from 560 to 28 mg KOH/g, and they have greater hydrolysis resistance and lower viscosity than the polyadipate glycols of same MW. They are used in production of high resistance PU, modification of resins, coatings, adhesives, shoe soles and orthopedic goods. Polycaprolactone and polyadipate copolymers diols are usually liquids of low viscosity at room temperature.

1.3.2.2 - Aromatic polyester polyols

Aromatic polyester polyols based on terephthalic or isophthalic acids are used in high performance hard coatings and adhesives, and in polyurethane (PUR) or polysocyanurate (PIR) rigid foams resistant to fire. In combustibility tests, the PIR and PUR foams based on aromatic polyester polyols form a charred backbone, and in many formulations reduces or eliminates the use of fire retardantes.

The polyterephthalate glycols are usually obtained by polymerization of dimethyl terephthalate (DMT) with ethylene glycol. Polyols with average equivalent weight of 181, functionality 2.3, hydroxyl number between 295 and 335 mg KOH/g, viscosities from 8,000 to 22,000 cP at 25°C, can be used in rigid foams and foundry systems. The ones with average equivalent weight of 238, functionality 2.0, hydroxyl number between 230 and 242 mg KOH/g, viscosity from 2,700 to 5,500 cP at 25°C, are used in PIR foams with minimum shrinkage and high weight retention. The ones with average equivalent weight 167, functionality 2.0, hydroxyl number between 315 and 350 mg KOH/g, viscosity of 1,300 to 3,000 cP at 25°C, are used in appliance thermal insulation and other low viscosity applications. Another polyterephthalates glycols obtaining process uses high molecular weight poly(ethylene terephthalate) (PET) scraps of polyester fibers or soft drinks bottles. The low molecular weight polyols are obtained by transesterification of milled PET residues with propylene glycol or mixture of ethylene/propylene glycols at 216°C for about 6 hours.

The polyisophthalates glycols are obtained by anhydride phthalic polymerization with glycols as diethylene glycol. The poly(diethylene isophthalate) glycol with average equivalent weight of 178 and 234, OH number of 230 to 330 mg KOH/g, viscosities from 2,000 to 4,500 cP at 25°C are used in PUR and PIR foams. The ones with equivalent weight of 288, OH number of 195 mg KOH/g, viscosity of 25,000 cP at 25°C can be used in resins and prepolymers for coatings, adhesives, sealants and elastomers, and also as additive in polyol polyether flexible foams to improve fire resistance and adhesion characteristics. The poly(neopentyl isophthalate) glycols with average equivalent weight of 510, OH number of 110 mg KOH/g are used in adhesives, coatings and elastomers with excellent hydrolysis resistance.

 

1.3.3 - Polytetramethylene ether glycols (PTMEG or PTHF)

The poly(oxytetramethylene) glycol or polytetramethylene ether glycol (PTMEG) are manufactured by the cationic polymerization of tetrahydrofuran (THF) (Figure 1.30). PTMEG's are linear chain polyols with reactive primary hydroxyls and functionality of 2.0. PTMEG's of molecular weights of 650, 1000 and 2000 (Table 1.10), are used in high performance PU and TPU's elastomers, coatings and elastomeric fibers.

Figure 1.30 - Obtaining of PTMEG's

PTMEG's are solid, white, waxy at room temperature, soluble in alcohols, esters, ketones and aromatic and chlorinated hydrocarbons, and insoluble in aliphatic hydrocarbons and water. They have variable solubilities for glycols: poly(oxypropylene) glycols and 1,6 hexanediol are completely miscible whereas only 20% of 1,4 butanediol can be dissolved in PTMEG 1000 and less than 10% in PTMEG 2000. PTMEG polyols are hygroscopic and can absorb 2% moisture in an unprotected environment. Gross amounts of water are removed by azeotropic distillation with toluene, and further reduction can be achieved by heating for several hours at 120-150oC under reduced pressure (less than 20mm Hg). PTMEG's are stabilized with antioxidants to prevent degradation during storage and normal handling. However, prolonged heating in the presence of air at 50-60°C will result in partial oxidation and degradation, and thermal decomposition will occur, in absence of air conditions at 210-220°C.

Table 1.10 - Characteristics of commercial PTMEG's

Molecular weight	OH number (mg de KOH/g)	Solidification point (°C)	Viscosity (mPas) at 75°C
650	173	25	55
1000	112	26	79
2000	56	35	350

 

1.3.4 - Castor oil based polyols

Castor oil (ricinus oil) is a pale yellow and viscous liquid (gardner viscosity U-V to 25°C), derived from the bean of the castor plant (ricinus communis) that occurs in all tropical and subtropical regions. It is triglyceride of fatty acids that contains 87-90% of ricinoleic acid (cis-12-hydroxyoctadec-9-enoic acid), with a hydroxyl number of 163 mg KOH/g, and average functionality of about 2.7 (Figure 1.31). Castor oil and its derivatives are used as polyols for the PU preparation mainly and in coatings, adhesives, and casting compounds with excellent hydrolytic stability, shock absorbing and electrical insulation properties. They also have been found to be very useful in the preparation of flexible, semi-rigid and rigid PU foams, resistant to moisture, sock absorbing, and with low temperature flexibility. The products with high purity are the recommended for PU's applications.

Figure 1.31 - Castor oil polyol

Transesterification of castor oil with polyhydroxylated compounds like glycerin trimethylolpropane, or propylene glycol results in polyols with higher or lower functionality. Transesterification with glycerin forms a trifunctional polyol mixture of mono and diglycerides (Figure 1.32), with OH number of 300 mg KOH/g. Castor oil based polyols with OH number of 310 mg KOH/g are used to promote pentanes blowing agent solubility, in rigid foams systems, with good thermal dimensional stability.

Figure 1.32 - Transesterification of castor oil with glycerin

 

1.3.5 - Hydroxyl terminated polybutadienes (HTPB's)

Several polyols with hydrocarbon structure are found in the marketplace. The main advantage of the PU based hydrocarbon polyols is the high resistance to hydrolysis, acids and bases. PU's based saturated hydrocarbon polyols have high temperature stability and are used in automotive electronic encapsulation. An important HTPB is obtained by free radical polymerization of butadiene, initiated by hydrogen peroxide and an alcohol as diluents (Figure 1.32).

Figure 1.32 - HTPB obtaining reaction

Due to the free radicals process it has ramifications in polymeric chain, and functionality slightly higher than two (2.1<2.3). HTPB possesses reactive allylic primary hydroxyl end groups, molecular weight of 2,800 and hydroxyl number of 46 mg KOH/g. HTPB's hydrophobic polymeric chains form Pus with exceptional hydrolysis stability, and its low humidity degree (<300 ppm), minimize or eliminate the previous drying. Due to its very low glass transition temperature the PUs formed have excellent elastomeric properties at extremely low temperatures. They possess great capacity to accept fillers as asphalt, aromatic and paraffinic oils, pentanes, plasticizers, carbon black, etc. HTPB microstructure is 60% of 1,4-trans, 20% of 1,4-cis and 20% of 1,2-vinyl insaturations (Figure 1.33) that turn possible further vulcanization and chemical modifications.

Figure 1.33 - HTPB microstructure

Another type of HTPB is obtained by anionic polymerization of butadiene initiated by sodium naphthalene, and terminated by reaction with ethylene or propylene oxides, following by hydrolysis, resulting in the formation of groups OH primary or secondary, respectively. This commercial HTPB's have functionality 2.0, molecular weights between 2000 and 5000, and usually possess secondary hydroxyl groups. They present microstructure with high quantity of 1,2-vinyl double bonds, which turns them extremely viscous (waxy) at room temperature. Due to the 2.0 functionality, these polybutadiene diols can be used in the thermoplastic elastomers (TPUs) (Chapter 6.3), with excellent hydrolysis and chemical stability, and insulating properties.

 

1.3.6 - Acrylic polyols

 1.3.6 - Acrylic polyols

Traditionally, acrylic resins are used in paints and coatings. Acrylic polyols are employed in PU coatings (Chapter 7) for automotive finish with good chemical resistance and durability. In Latin America these polyols are consumed at an annual rate of 3,000 tons. They are obtained from the copolymerization of conventional acrylic monomers, such as ethyl acrylates (EA) or butyl acrylates (BA), acrylic acid (AA), methyl methacrylate (MMA), or styrene (ST) with hydroxylated acrylic monomers such as 2-hydroxyethyl acrylates (HEA) or 4-hydroxybutyl acrylates (HBA). The increase in styrene content makes the acrylic polyol more hydrophobic, while the use of HBA makes it more reactive due to lower steric hindrance in the hydroxyl group. Usually, acrylic polyols used in solvent-based 2K-PUR systems are acrylic resins with OH content (based on solid resin) of 0.5 to 3.5%, equivalent weights from 3,400 to 500, non-volatile content from 40 to 100%, in solvents as xylene (X), naphtha (N), or butyl acetate (BA), or 1-methoxypropyl acetate (MPA), and viscosities between 1,800 and 9,000 mPa.s. Aqueous 2K-PUR systems are AA copolymers neutralized with ammonia or dimethyl ethanol amine (DMEA). The OH content is from 2.0 to 4.8%, non-volatile content is from 40-45%; there is further 0 to 10% organic co-solvent as butyl glycol (BG), butyl diglycol (BDG) pure or admixed with naphtha (N). Viscosities are between 200 and 1,500 mPa.s.

 

1.4 - Polyamines

There are several methods for the conversion of hydroxyl end groups into amine end groups, of which among the more commonly used is the reductive amination of secondary hydroxyl groups of polypropylene glycols. These specialized polyamines (amine-terminated polyethers) (Figure 1.34) are aliphatic in nature (making them suitable for both aliphatic and aromatic polyurea elastomers) and possess hindered amine groups. Given their flexible nature, polyetheramines include the soft-block segments of a polyurea chain. In addition to the polyetheramines, other low-molecular weight polyamines are employed as chain extenders. They react with the isocyanate component very fast, forming polyurea used in extremely fast processes such as RIM (reaction injection moulding) (Chapter 4) and elastomeric RIM-spray coatings (Chapter 7). The polyurea spray elastomer technology was introduced into the marketplace in the late 1980's, and is finding wide acceptance in a variety of commercial applications such as coating of concrete and secondary containment floorings. Moisture or low ambient temperature does not affect the very reactive and non-catalyzed polyurea systems. They have excellent adhesion and can be formulated into 100% solid systems so as to attend environmental regulations. These products exhibit excellent mechanical properties and durability even under adverse ambient conditions.

Figure 1.34 - Polyether amines

 

1.5 - Chain extenders

Chain extenders, curing agents, and crosslinkers are low molecular weight polyols or polyamines used to improve PUs properties. Normally, chain extenders and curing agents are used in flexible PUs as flexible foams (Chapter 3), microcellular elastomers (Chapter 4.8), casting elastomers (Chapter 6.2), polyureas (Chapter 4.10), adhesives (Chapter 7.1) and coatings (Chapter 7.3). Alcohols react with diisocyanate to form polyurethane hard segments and with amines to form polyurea hard segments. Those hard segments segregate, resulting in modulus increase and higher glass transition temperature (Tg). The higher density of hydrogen bonds of polyurea hard segments is responsible for the improved mechanical properties of polyurea and polyurethane/urea products.

 

1.5.1 - Hydroxylated chain extenders

Usually, chain extenders are difunctional compounds, such as glycols used in PUs, and diamines or hydroxylamines used in polyureas and polyurethane/ureas. Cure agents or curative are used in two-step processes for cast elastomers (prepolymer processes), and normally are chemically similar to chain extenders. Crosslinkers or crosslinking agents are tri- or polyfunctional compounds used to increase reticulation in rigid (Chapter 5) and semi-rigid (Chapter 4.5) foams. Diols are used as chain extenders in one-step and in two-step (prepolymers) processes. Table 1.11a shows hydroxylated compounds used as chain extenders or crosslinkers.

Table 1.11a - Hydroxylated chain extenders and crosslinkers

Compound	Structure	Functionality	MW
ethylene glycol	HOCH2-CH2OH	2	62
di-ethylene glycol	HOCH2-CH2-O-CH2CHOH	2	106
propylene glycol		2	76
di-propylene glycol		2	134
1,4 butane-diol	HOCH2-CH2-CH2-CH2OH	2	90
2-methyl-1,3-propylene- diol	HOCH2-CH(CH3)-CH2OH	2	90
water	HOH	2	16
N-N'-bis-(2 hydroxy-propylaniline) (DHPA)		2	221
1,4-di-(2-hydroxyethyl) hydroquinone (HQEE)		2	198
diethanol amine	HOCH2CH2NHCH2CH2OH	3	105
triethanol amine	N-(CH2CH2OH)3	3	149
trimethylol propane		3	134
glycerine	HOCH-CH2OH-CH2OH	3	92

 

1.5.2 - Diamines used as chain extenders

Low molecular weight diamines (Table 1.11b) are used as chain extenders in polyurea and polyurethane/urea processes. They react much faster with isocyanates than with polyols (Table 1.3). Due to their longer pot life, the less reactive aromatic diamines are used in cast PU elastomers prepared in two-step processes (Chapter 6.2). Aliphatic and aromatic amines are used as chain extenders in polyurea RIM processes (Chapter 4.10) and spray coatings (Chapter 7.3.6), where their higher reactivity results in shorter demold times. The use of more reactive aliphatic, or less reactive secondary aromatic diamines makes possible to vary the profile of the system reactivity. Aliphatic diamines such as hydrazine or ethylene diamine are used as chain extenders in processes directed to preparing PU aqueous dispersions (Chapter 7.3.5). Cyclo-aliphatic diamines are applied with aliphatic isocyanates in spray coatings to prevent the yellowing occurring with aromatic PUs.

Table 1.11b - Diamines used as chain extenders

Compound	Structure	MW
Hydrazine	H2N-NH2	32
Ethylene diamine	H2N-CH2-CH2-NH2	60
1,4-ciclohexanediamine		114
Isophorone diamine (IPDA)		170
4,4'-bis-(secbutilamine) diciclohexylmethane (SBADCHM)		322
4,4'-bis-(secbutilamine) diphenylmethane (SBADFM)		310
diethyl-toluene diamine (DETDA) isomers 2,4 (80) e 2,6 (20)		178
4,4'-methylene-bis(2-chloroaniline) (MOCA)		267
4-chloro-3,5-diamino-benzoic acid isobutylester (CDABE)		242,5
3,5-dimethylthio-toluenediamine (DMTDA) - isomers 2,4 (80) e 2,6 (20)		214
trimethyleneglycol-di-p-aminobenzoate (TMGDAB)		314
4,4'-methylene-bis-(3-chloro-2,6-diethylaniline) (M-CDEA)		365

1.6 - Correlations between structure and properties

1.6 - Correlations between structure and properties

PUs molecular structure can vary from rigid polymeric (crosslinked), to linear, elastomeric flexible chains (Figure 1.35). Flexible foams and TPU elastomers have segmented structures, made up of long flexible chains (polyols) linked by polyurethane and polyurea aromatic hard segments. Their character depend largely on hydrogen bonds between polar groups of the polymeric chain, mainly among N-H groups (electron acceptors) and carbonyl groups (electron donors) of urea and urethane groups. Hydrogen bonds can also be formed among N-H groups and polyester carbonyl groups and, more difficultly, with polyether oxygen atoms (weak bonds). Hard segments of flexible PUs, especially of polyurea, form strong secondary chemical bonds (hydrogen bonds) with a tendency to form hard segment domains. On the other hand, as a result of the polyfunctional reagents used, rigid PU foams are highly crosslinked, and they don't show the segmented structures present in flexible PUs. Besides urethane bonds, the PUs macromolecular chain possesses a multiplicity of other groups that contribute to the cohesive macromolecular forces (Table 1.12).

Table 1.12 - Cohesive molar energy of organic groups

Organic group	Choesive molar enrgy (kcal/ml)
-CH2-  (hydrocarbon)	0,68
-O-  (ether)	1,00
-COO- (ester)	2,90
-C6H4-  (aromatic ring)	3,90
-CONH-  (amide)	8,50
-OCONH- (urethane)	8,74

a) Soft, high elongation elastomers

a - urethane linkage b - polyol chain

b) Polymers with a segregated domain structure (high modulus elastomers and flexible foams)

a - hard block domain b - soft block domain

c) Rigid, highly cross-linked polymers

- = diurethane linkage T= triurethane linkage + = tetraurethane linkage * = polyurethane linkage

Figure 1.35 - Polyurethane structures

 

1.6.1 - Segmented polyurethanes

Properties of PUs produced from 1,6-hexane diisocyanate and 1,4-butane diol are like those of polyamides of similar structures. Amorphous PUs (prepared with TDI and diethylene glycol) are rigid and transparent, but they show low dimensional stability at high temperatures. On the other hand, there are PUs formed exclusively by soft segments, obtained by the stoichiometric reaction of a difunctional polyol with diisocyanate, resulting in amorphous products with elastomeric properties. Here, intermolecular forces are essentially between the polyol soft segments so that properties such as hardness and mechanical resistance are poor. All these products have a single phase and they don't present segmented structures. The only non-segmented PUs of commercial importance are the highly cross-linked PUs such as rigid foams and non-textile coatings. Segmented PUs is formed by the reaction of a polyol, diisocyanate and a chain extender, which can be a glycol, diamine or water. These PUs represent a class of products, characterized by a segmented structure (polymeric blocks) made up of two or more different polymeric phases. These segmented structures are responsible for the excellent PUs properties.

Hard and soft segments

A PU prepared with one mole of long chain linear polyol [poly(1,4-butane diol adipate)], two moles diisocyanate (MDI) and one mole chain extender (1,4-butane diol) presents the structure shown in Figure 1.36. Soft segments, which are quite mobile and are normally present in a coiled shape, and hard segment units, alternate.

a) sof segments, b) hard segments

Figure 1.36 - Representation of a segmented PU chain

 

Morphology of the segregated domains

Usually, Pus' soft segments are incompatible with the hard and polar ones. As a consequence a phase separation (segregation) occurs and covalently linked microphases are formed. The coherent matrix, which consists of flexible soft segments, results in high deformability of the resulting material. In contrast, within the hard segment domains, molecules are fixed by physical interaction. Because of covalent coupling to the soft segments, they inhibit plastic flow of the chains, thus creating elastomeric resiliency. Hard segmented domains can be looked at as multifunctional spacious crosslinked areas. The larger the phase segregation, the lower the polarity of the flexible segments. Therefore, segregation is less pronounced in polyester urethanes compared to polyether urethanes and is most pronounced in polybutadiene urethanes (Figure 1.37).

a - soft block domain b - hard block domain

Figure 1.37 - TPU structures

 

Hard domain morphology

Depending on the nature and length of the hard segments and the degree of segregation, three-dimensional organized proximity areas are formed with structures of predominantly paracrystalline nature. In case of very low cooling and sufficient length of the hard segments, even microcrystallites can be formed (Figure 1.38). The secondary structure depends on the proximity zone interaction between hard segments. This structure is mainly characterized by hydrogen bonding between adjacent aromatic rings of symmetrical isocyanates. Another important interaction is that existing between p electrons of the isocyanate aromatic rings.

a) sof segments, b) hard segments

Figure 1.38 - Interchain interaction between hard segments

 

1.6.2 - Effect of hard segments

Interactions between chains, mainly hydrogen bonds between hard segments, contribute to the distinguishing properties of PUs. Thermo-mechanical properties of linear segmented PUs are substantially different from those of chemically crosslinked products. Whenever mechanical forces are applied, changes in the orientation and mobility of structures within hard segment domains, which depend on temperature, can occur. In this process initial hydrogen bridges are broken and other, energetically more favorable, are formed. A change in structure occurs, causing alignment in the direction of the applied tension (Figure 1.39). As a consequence, applied tension is better distributed and as a result, the resistance of the material against further stress is increased. This effect contributes to the high tensile strain, elongation, tear strength, and permanent set values.

The melting range of the hard segment domains determines the dimensional thermal stability of linear segmented PUs. Above the melting range the material is thermoplastic. With the increasing length of the hard segment the melting range also rises and with the use of different chain extenders and isocyanates the melting range can be intentionally modified (Chapter 6.3). Above the melting range of the hard segments, linear PU's form a viscous homogeneous liquid that can be processed as a thermoplastic material. Whenever the melting range is above 250oC (which means higher than the PU decomposition temperature), even linear PUs will not exhibit thermoplastic properties. Upon increasing the amount of hard segment, PU shows an increase in hardness and modulus, and above 60% by weight there is a change in behavior from elastomeric to a brittle, high-modulus plastic.

Schematic representations of PU linear segmented structures
I -Relaxed structure (without stress)	II - Structure with 200% elongation	III - Structure with 500% elongation
a) soft segment, b) hard segment, c) reorientation after stress

Figure 1.39 - Effect of applied stress on PU segmented structure

 

1.6.3 - Soft segments effect

Chain mobility largely depends on the chemical nature and size of soft segments. Soft segments control properties such as cold flexibility, as well as PU chemical behavior, such as resistance to solvents, water, acids, bases and weather. In order to obtain suitable elastomeric properties, especially impact resistance, the soft segment should be amorphous and possess a low enough glass transition temperature. To prepare PUs with typical rubber elasticity an average molecular weight of 1,000 to 4,000 is desirable, corresponding to a chain length of 120 to 300 A. Whereas the elastomers freezing temperature TE (lower end of the glass transition) is about 20 to 30oC above that of the polyol used, T* (the upper end of the glass transition) is dependent on the degree of phase separation between the hard and soft segments. In products with large amounts of hard segment (>50%), the mobility of the soft segment is considerably reduced. As a result, cold flexibility properties are impaired. Tensile strength, 300% modulus and tear strength are substantially affected by the melting point (TM) of the soft segment. Increasing chain length of the soft segment and decreasing amounts of hard segments, as well as high linearity of the PU favor crystallization.

 

1.6.4 - Molecular structures

The viscous-elastic behavior of segmented linear PU elastomers was investigated through modulus/temperature experiments. Similar properties to those observed for block elastomers (e.g., a butadiene/styrene block copolymer) such as an extensive plateau in the high modulus area were also found for segmented linear PU elastomers. The absence of hydrogen bonds in hydrocarbon elastomers leads to the conclusion that hydrogen bridges are not the sole responsible for the observed properties in PU elastomers. In the two systems, physical interactions reinforce the structure until the melting temperature of high modulus blocks is reached. Through selective solvation of aromatic PUs and segmented polyester with different solvents, it was possible to demonstrate that an association of hard segments in the solid state (hard segment domains) is a prerequisite for the existence of a high transition temperature.

As previously mentioned, in segmented PU elastomers hydrogen bridges are formed between active hydrogen atoms of -NH- urethane groups and -CO- urethane groups, with polyols polyester -CO- groups, and more difficultly with the oxygen atom of polyol polyethers. Infrared investigations of N-H stretching-related adsorptions indicate that more than 90% of the N-H urethane group hydrogen forms hydrogen bonds. On the other hand, a study related to urethane -CO- groups shows that only about 60% of these groups are linked or associated. This indicates that a substantial portion of hydrogen bonds occurs between -NH- urethane groups and -CO- groups of the polyester soft blocks.

Other studies show that, for polyester-based PU elastomers, the formation of hydrogen bonds depends on the size of the polyester soft segment. In polymers with high urethane concentration, hydrogen bridges between N-H groups and urethane -CO- groups are more frequent. Though, when urethane concentration is reduced, connections between N-H urethane groups and polyester -CO- groups become more important. Spectroscopic infrared analyses of polyester and polyether systems indicate that in polyester systems hydrogen bonds are formed mainly with polyester -CO- groups, while in polyether systems, such bonds are formed with urethane -CO- groups.

 

1.6.5. - Crosslinked PUs

Drastic modifications in PUs properties can be introduced by varying the crosslinking degree. Reticulations may be formed by reaction of the isocyanate excess with urea or urethane groups yielding biuret or allophanate cross-linking, or by using tri or poly-functional alcohols or amines as chain extenders. Whereas elongation and permanent set decrease with increasing crosslinking density, tension strength initially increases, but later on decreases. When a predominantly linear segmented PU is reticulated, physical and chemical crosslinking effects overlap. However, if the polyaddition reaction is carried out directly in presence of tri- or higher functional polyisocyanate or polyols, which lead to an early network of primary valences, the formation of physically crosslinked areas (domains) can be prevented. Hence at low chemical crosslinking, modulus of elasticity decreases. Under extreme conditions, due to the fact that chains are already fixed, segregation will not occur, and even temperature treatment (annealing) will not result in an improved property level.

The effect of crosslinking density on physical properties of PUs elastomers is shown by data (Table 1.13) obtained by the substitution of 1,4-butane-diol for trimethylol propane for, as chain extender of PU based-polyester produced with MDI and ethylene glycol polyadipate. These data show the molecular weight increase of crosslinking units (Mc) (or decrease in crosslinking density) with the triol amount in the reagents. The initial decrease in modulus with the increase in crosslinking degree is opposite to the results observed in conventional polyhydrocarbon elastomers, where an increase in the reticulation corresponds to a modulus increase. With PUs, it happens that a larger crosslinking number reduces the chain orientation and formation of hydrogen bonds or other intermolecular interactions. This phenomenon prevails until the crosslinking density is sufficiently strong to exert its own effect towards increasing PU modulus.

Table 1.13 - Effect of crosslinking density (Mc) on PU physical properties

Mc	Tensile strenght (MPa)	Elongation at break (%)	Stress at 100% strain (MPa)	Tear resistence (kN/m)	Hardness (Shore B)	Tensile set  (%)	Compression set  (%)
2100	12,4	170	3,9	5,4	57	0	1,5
3100	12,0	200	3,0	4,5	53	0	16
4300	10,0	280	2,1	5,4	49	0	10
5300	19,3	350	1,9	5,4	46	0	0
7100	31,0	410	2,3	7,1	51	0	25
10900	38,6	490	3,2	10,8	55	5	40
21000	38,0	510	3,5	15,0	56	10	45
infinite	46,5	640	4,3	54,0	61	15	55

Mc - average molecular weight between cross-linking

2.1 - Catalysts

 

Chapter 2 - Additives

Several reagents are used in PU production, such as: isocyanates (Chapter 1.2), polyols (Chapter 1.3), polyamines (Chapter 1.4), chain extenders and crosslinkers (Chapter 1.5). Besides these, a great variety of chemical products can be added to control or modify the PU's formation as well their final properties. These additives include: catalysts, blowing agents (Chapter 2.3); surfactants (Chapter 2.4); fillers (Chapter 2.5), antiaging agents (Chapter 2.6), colorants (Chapter 2.7), flame retardants (Chapter 2.8), mold release agents (Chapter 2.9), lubricants, adhesion promoters, solvents, plasticizers, rheology promoters, moisture scavengers, etc.

 

2.1 - Catalysts

In terms of industrial productivity, in the absence of catalysts the isocyanate group reacts rather slowly with alcohols, water and itself. The catalyst choice for PU's manufacture is usually directed for obtaining an appropriate profile among the several reactions that can occur during PU production processes. Catalysts are used for producing cellular PU's (flexible, semi-rigid and rigid foams, microcellular elastomers) as well as solid PU's (elastomers, coatings, sealants, adhesives, etc). Different catalyst types are used for the reaction of isocyanate with water and polyols, the catalysts usually being aliphatic or aromatic tertiary amines, or organometallic compounds. Normally, tertiary amines are used for catalyzing the polyaddition reaction of isocyanate with a polyol-forming PU, as well as in the blowing reaction of isocyanate with water, forming polyurea and carbonic gas as blowing agent. Organometallic catalysts are mainly used in the catalysis of the polymerization reaction of an isocyanate with a polyol. Carboxylic acid salts of alkaline metals, phenols and symmetrical triazines are used to catalyze the polymerization of isocyanate with isocyanurate formation.

 

2.1.1 - Catalysis

Catalysts (Table 2.1) are used to promote selectivity when different chemical reactions occur at the same time, as is the case with PU's. PU's final properties depend on the amount of urethane, urea, allophanate, biuret, and isocyanurate bonds, along the polymer chain. On the other hand, these bonds depend on the type and concentration of the catalyst or its mixtures. This means that catalysts exert a considerable influence on PU structures and its end properties.

Table 2.1 - Catalysts used in PU

Reaction	Catalysts
NCO/NCO - trimerization	strong bases (potassium octoate), quaternary ammonium, phosphines
NCO/NCO - dimerization	phosphorous compounds
NCO/NCO - polymerization	alkaline metal hydroxides
NCO/OH	tertiary amines, organometals, metallic soaps
NCO/H2O	tertiary amines
NCO/NHCOOR (urethane)	metallic soaps
NCO/NHCONHR (urea)	tin and zinc soaps

The growing catalyst basicity promotes improved crosslink formation (alophanate and biuret). Generally, with tertiary amines the higher basicity increases the catalytic effect, except in the occurrence of steric hindrance. Triethylene diamine (TEDA) or 1,4-diazo (2,2,2)-bicyclo-octane (DABCO) have a stronger catalytic effect due to the absence of steric hindrance. It is important to emphasize that the catalyst specificity can vary according to the system used, hence the danger of trying to extract correlations from studies done in different systems. Basically the catalyst should be sufficiently nucleophilic to stabilize the isocyanate group by resonance, or to activate the active hydrogen atom-containing compound. According to the basic catalysis mechanism show in Figure 2.1, initially occurs the formation of a complex between the base and the isocyanate group, activating the NCO group and making easier the reaction with the non-paired electrons of the alcohol oxygen atom. Afterwards the complex is decomposed, forming PU and regenerating the base.

Figure 2.1 - Basic catalysis mechanism

Different catalysts (tertiary amines, alkaline compounds and organo metals) have different catalytic effects on the isocyanate group reactivity with active hydrogen atoms (Table 2.2) of urethane, urea, water and alcohol groups.

Table 2.2 - NCO group reactivity with active hydrogen-containing compounds

Hydrogen-containing compounds	Non catalyzed reaction	Tertiary amines^1	Alkaline compounds^2	Organometals^3
Urethane	1	-	strong	-
Urea	100	-	strong	weak
Water	400	strong	strong	weak
Alcohol	400	strong	strong	very strong

1 - tertiary amine: triethylene diamine (TEDA) or 1,4-diazo-(2,2,2,)-bicyclooctane, 2 - Alkali: HO(-) or RO(-) ,3 - Organometal: dibutyl tin dilaurate, stannous octoate, cobalt naphthenate, etc.

In Table 2.2, it may be seen that organometals have a very strong effect on PU formation, while the effect is weak on the blowing reaction, between isocyanate and water. Tertiary amines show a strong catalytic effect on PU formation and also on the blowing reaction, and both have a weaker effect on allophanate and biuret formation. This is very important in PU foam technology where organometals are commonly used to catalyze PU formation while tertiary amines are employed to catalyze the blowing reaction.

2.1.2 - Tertiary amines

2.1.2 - Tertiary amines

Tertiary amines are the most widely used catalysts in the manufacture of cellular and solid PU's. Some of the more commonly used tertiary amines are shown in Table 2.3. In PU foams, tertiary amines control the blowing and polymerization reactions, and play a relevant role in PU end properties.

Table 2.3 - Tertiary amine catalysts

Catalyst	Application
1. N,N-dimethylethanolamine (DMEA) (CH3)2NCH2CH2OH	Inexpensive, low-odor, isocyanate reactive and blowing catalyst used in polyether flexible foams.
2.Diaminobicyclooctane (DABCO) or triethylene diamine (TEDA)	General-purpose gelling catalyst, solid crystal soluble in water, glycols and polyethers polyols.
3. bis(2-dimethylaminoethyl)ether (BDMAEE) (CH3)2NCH2CH2OCH2CH2N(CH3)2	Low odor, strong blowing catalyst used in flexible foams.
4. N-ethylmorpholine	Low odor, used in polyester foams, and additionally improve the skin formation.
5. N'N'-dimethylpiperazine	Based on the high vapor pressure is used to improve the skin formation in molded foam.
6. N,N,N',N',N''-pentamethyl-diethylene-triamine (PMDETA)	Highly active blowing catalyst for flexible, semi-rigid and rigid foams.
7.       N,N-dimethylcyclohexylamine (DMCHA)	Liquid with intense odor used in rigid foams, leads to a well-balanced proportion of gelling and blowing reactions.
8.       N,N-dimethylbenzylamine (DMBA)	Liquid with smell characteristic employed in polyester flexible foams for reduced scorching, semi-rigid and in rigid foams for small cell size and good adhesion.
9.      N,N-dimethylcethylamine  CH3(CH2)14CH2N(CH3)2	Viscous liquid with a low odor used in polyester based flexible foams and some potting compounds.
11. N,N,N',N”,N”-pentamethyl-diproylene-triamine (PMDPTA)	Strong gelling catalyst with good flowability, strong ammoniacal odor used in polyether based slabstock foams, semi-rigid, and rigid foams.
12.Tritehylamine N-(CH2CH3)3	Highly volatile amine catalyst. Acts as a surface cure catalyst and reduce defects associated with lower temperature molds. Balanced blow and gel for molded and slabstock foams.
13.1-(2-hydroxypropyl) imidazole	Isocyanate reactive and gelling catalyst for polyether based foams and low-density rigid foams.

 

In agreement with its chemical behavior, flowability, and PU end properties, tertiary amines may be classified as: Gel catalysts, Blow catalysts, Delayed action catalysts, Skin cure catalysts, and Reactive catalysts.

Gel catalysts - Gellation catalysts promote the polyaddition reaction of isocyanate with polyol. These catalysts are non-sterically hindered tertiary amines having the free electronic pair available on the nitrogen atom; they activate the C=N moiety of the isocyanate group. This activated complex quickly reacts with the active hydrogen atom of the polyol, forming the PU. DABCO or TEDA is a strong general-purpose gel catalyst, due to the absence of steric hindrance, otherwise, dimethylcyclohexylamine (DMCHA) is sterically hindered but it is a strong base, widely used in PU rigid foams.

Blow catalysts - These catalysts promote the blowing reaction between the isocyanate and water, forming polyurea and carbonic gas, which acts as a blowing agent. They are non-sterically hindered tertiary amines with ethylene groups between the two (nitrogen or oxygen) active centers. These two active centers are able to chelate water (Figure 2.6a), thus increasing its reactivity.

Delayed action catalysts - Foam flowability is very important to properly fill the mold, especially with large and complicated profiles. On the other hand, good curing is essential to perform short demolding times and to avoid fingerprinting when the pad is taken out of the mold. To combine excellent in-mold flowability and fast curing, delayed-action catalysts were developed. When starting reactivity is too high, delayed-action blow catalysts should be employed; on the other hand, delayed-action gellation catalysts are used to delay the viscosity build-up of the reacting mixture allowing to flow easily in the mould cavity. The process to manufacture a delayed-action catalyst involves reacting a specified tertiary amine with a carboxylic acid. The resulting compound is composed of a salt and an excess of the starting amine. The salt has limited or no catalytic activity and when this blend is used in a foam formulation, the free amine launches the reaction, but when the reactions have progressed, with heat generation, the salt dissociates to yield back the amine and the acid. At this time the catalyst is de-blocked and has recovered the original activity. The released organic acid reacts with isocyanate-forming carbon dioxide and carbon monoxide, supplying an auxiliary source of blowing agent, resulting in less dense with more open cell foams. Normally, carboxylic acids such as formic and 2-ethylhexanoic acids are used as blocking agents. The unblocking temperature depends on the acid used. Strong acids require higher temperatures than the weak ones. In commercial applications the unblock temperature is located between 30oC and 60oC. There is some corrosion linked to the use of formic acid and the foams produced tend to be very closed and are therefore difficult to open.

Skin cure catalysts - The skin cure catalysts provide additional isocyanate reaction at the surface, improving the overall appearance of the part and eliminating defects such as fingerprint. There are two categories of surface cure catalysts. The first includes tertiary amines that have high vapor pressure and volatilize from the developing foam to the foam mold surface where they provide additional reactivity. Typical examples of these catalysts include triethylamine (TEA), N-methylmorpholine (NMM), and N-ethylmorpholine (NOR). The second group includes tertiary amines that are incompatible with the developing foam and migrate to the mold surface and provide the same effect as above. Typical amines of this category are modified morpholines.

Reactive catalysts - Reactive amine catalysts, as DMEA, DMAEE and TMAEE that contain a hydroxyl group, are able to react with isocyanates becoming chemically a portion of the polymer matrix. The disadvantage of polymer-bonded catalysts may be the deterioration of foam physical properties, especially when the foam is exposed to hot and humid climates. They are used in special applications, such as dashboard production, in order not to migrate into the PVC skin, this causing its discoloration.

 

2.1.2.1 - Catalysis mechanism

Since Otto Bayer's pioneering work, the catalysis mechanism of isocyanates reactions with alcohols (gel reaction) or with water (blow reaction) was studied by several authors and especially by Baker and Farkas. Baker postulated the formation of a complex consisting of an isocyanate and a tertiary amine catalyst, followed by the attack by nucleophilic reagents (water or alcohols) (Figure 2.2).

Figure 2.2 - Tertiary amines catalysis Baker's mechanism

Farkas based his theory on the initial formation of a complex between the nucleophilic reagent (alcohol or water) and the tertiary amine (Figure 2.3). This complex then reacts with the isocyanate, and the gel reaction (with polyol), or blow reaction (with water) occurs, with regeneration of the tertiary amine. In this mechanism the amine basicity is the predominant factor.

Figure 2.3 - Tertiary amine catalysis Farkas' mechanism

 

2.1.2.2 - Gel and blow catalysis

Alcohol interacts with tertiary amines like TEDA (triethylenediamine) through a single hydrogen bond (Figure 2.4).

 

Figure 2.4 - Interaction of water (R=H) and alcohol with TEDA

Water is able to interact with tertiary amines through different modes. Water interacts with TEDA (a strong gelling catalyst) through a single hydrogen bond like an alcohol. On the other hand, tertiary amines of higher blowing activity such as BDMAEE [bis(2-dimethylaminoethyl)ether] or PMDETA (N,N,N',N',N''-pentamethyl-diethylenetriamine) (Figure 2.5) are able to chelate water (Figure 2.6). This indicates that a single nitrogen-water hydrogen bond does not result in effective catalysis of the blowing reaction. The water-chelated complex, which is not feasible with TEDA, may then be the selective high-activity catalyst precursor.

BDMAEE

PMDETA

Figure 2.5 - BDMAEE and PMDETA structures

PMDPTA (N,N,N',N',N"-pentamethyldipropylenetriamine) is also unable to chelate water (Figure 2.6). Water chelation by the central nitrogen and one of the terminal nitrogens is disfavored by close contact between water hydrogens and hydrogens form the three-carbon bridge. These different modes of water binding provide an explanation for the experimental observation that acyclic tertiary amines with hetero atoms separated by a two-carbon bridge favor the blowing reaction more than tertiary amines with hetero atoms separated by a three-carbon bridge.

BDMAEE

PMDPTA

Figure 2.6 - BDMAEE and PMDPTA water binding

Table 2.4 includes the catalytic activity of many tertiary amines. The gelling and blowing catalytic activities were measured by a titration method in a standardized reaction system containing TDI, diethylene glycol and water. TEDA is a strong gelling catalyst. BDMAEE and PMDETA are strong blowing catalysts. TMHMDA, TMEDA and DMAEMP exhibit moderate activity between the gelling and blowing reactions. ROCONHC6H6 and R'NHCONHC6H6 are reactive tertiary amines.

Table 2.4 - Reaction rate constant and activating energies of amine catalysts

Name	Abreviation	Gelling activity (k1)	Blowing activity (k2)	Ratio blowing/gelling
		(x 10)	(x 10)	(x 10^-1)
Triethylenediamine	TEDA	10,90	1,45	1,34
TEDA 33% in DPG		3,63	0,48	1,34
N,N,N',N'-tetramethyl hexamethylenediamine	TMHMDA	2,95	0,84	2,85
N,N-dimethyl cyclohexylamine	DMCHA	2,22	0,83	3,76
N-(2-dimethylaminoethyl)-N'-methylpiperanize	DMAEMP	1,71	0,78	2,72
N,N,N',N'-tetramethylethylene diamine	TMEDA	4,19	1,14	2,72
N,N,N',N',N”-pentamethyldiethylene triamine	PMDETA	4,26	15,90	37,30
Bis(2-dimethylaminoethyl) ether	BDMAEE	2,99	11,70	39,00
BDMAEE 70% in DPG		2,09	8,19	39,00
N,N-dimethyllaminoethanol	DMEA	2,91	0,36	1,23
N,N-dimethylaminoethoxyethanol	DMAEE	1,84	2,55	13,90
N,N,N'-trimethylaminoethyl ethanolamine	TMAEEA	2,89	4,33	15,00
Imidazol based catalyst		3,54	0,39	1,10
Imidazol based catalyst		2,69	0,20	0,74
ROCONHC6H6		2,36	0,27	1,14
R'NHCONHC6H6		2,46	1,00	4,01

 

2.1.3 - Organometallic catalysts

Organometallic catalysts (Table 2.5) promote the polymer forming or gellation reaction between the isocyanate and a polyol. Of the many metals available, tin compounds like stannous octoate (SnOct) and dibutyltin dilaurate (DBTL) are the most popular. Stannous octoate is used in most flexible foam systems, except pre-blended two component ones where its low hydrolytic stability is unacceptable. Low levels of DBTL often catalyze microcellular elastomers, RIM systems and cast elastomers.

Table 2.5 - Organometallic catalysts

CATALYST	MAIN APPLICATION
Stannous octoate	Polyether based slabstock and moulded flexible foams catalysis.
Dibutyltin dilaurate	Microcellular, RIM and cast elastomers catalysis.
Potassium acetate	General purpose catalyst.
Potassium octoate	Isocyanate trimerization catalyst.
Dibutyltin mercaptide	Hydrolysis resistant catalyst.
Dibutyltin thiocarboxylates	Delayed action (hindered) catalysts for RIM and HR foams.
Phenylmercuric propionate	In glycol solution for potting compounds, as a powder for delayed action catalysis.
Lead octoate	Chain extension catalysis.
Alkaline metal salts, (K2CO3, NaHCO3 and Na2CO3)	General catalysts for urethane reaction and for isocyanate polymerization.
Calcium carbonate	Filler with catalytic effect.
Ferric acetylacetonate	Catalyst for cast elastomer systems.

Numerous studies have shown that organo tin catalyzed urethane reactions do not follow first order kinetics, and that the organo tin catalysts promote a number of side reactions, acting synergistically with at least one tertiary amine (TEDA). The catalytical activity of tin compounds can be significantly increased by addition of amines (Table 2.6 and Figure 2.7).

Table 2.6 - DBTL / TEDA Synergy

DBTL	TEDA	Activity order
0,0	0,0	1
0,0	0,3	330
0,3	0,0	340
0,3	0,3	1780

DBTL and TEDA, when used as sole catalysts, exhibit lower catalytical activity than if used together. Compared to the activity of the sole catalysts, and when both catalysts are combined, the activity increases by a factor 5. Similar activity increases are observed with SnOct and TEDA.

Figure 2.7 - DBTL and TEDA synergism

Organo metallic catalysts act as Lewis acids, and are generally thought to function by interacting with basic sites in the isocyanate and polyol compounds. They form an intermediate complex with an isocyanate group and a hydroxyl group of the polyol. This complex formation is inhibited by steric hindrance of the metallic atom. This steric effect is used in one type of delayed action catalyst; i.e. one that is not very active at room temperature but becomes so when the reaction temperature rises. There are three complementary mechanisms for activated complex formations. One conceptual mechanism involves activation of isocyanate molecules. Polyol attacks this complex at the isocyanate carbon atom to again propagate the polymer and regenerate de catalyst (Figure 2.8). In other mechanisms, the polyol is activated by the formation of a complex with the organometallic catalyst. This adduct can react with isocyanate to give a carbamate, which further reacts to additional polyol propagating the polymer and regenerating the catalytic species. The final mechanism attempts the synergism (Figure 2.7) between organometallic compounds and amine catalysts.

Figure 2.8 - Activation of isocyanate molecules by organo-metal.

The principal tin catalyst used in the manufacture of flexible PU foam is stannous octoate [Sn(C₈H₁₅O₂)₂]. This is an inorganic tin catalyst produced from tin metal and 2-ethylhexanoic acid. When used in combination with tertiary amines, this material affords an excellent balance of cost performance to provide the reaction profiles required by today's high volume flexible slabstock production lines. Tin can take on two oxidation states Sn²⁺ (stannous), which are found in SnOct and Sn⁴⁺ (stannic) found in organo tin compounds. Catalysts, like DBTL, have more a pronounced catalytic effect in the polymerization reaction between the isocianato and the alcohol than in the expansion reaction between the isocianato and the water. On the other hand, catalysts, such as the stannous octoate, the base of which is Sn²⁺, also present catalytic effects in the reaction of expansion of the isocianato with the water. Grignard reactions are used to produce the organo tin intermediates di-n-butyl tin oxide and di-n-butyl tin chloride. A wide range of organic acids reacted with this basic structure to produce compounds like DBTL (Figure 2.9). In addition, di-butyl tin can be reacted with dibasic acids, as maleic acid, and with mixtures of monobasic acids to provide numerous homologues.

Catalysts, like DBTL, have more a pronounced catalytic effect in the polymerization reaction between the isocianato and the alcohol than in the expansion reaction between the isocianato and the water. On the other hand, catalysts, such as the stannous octoate, the base of which is Sn2+, also present catalytic effects in the reaction of expansion of the isocianato with the water. Grignard reactions are used to produce the organo tin intermediates di-n-butyl tin oxide and di-n-butyl tin chloride. A wide range of organic acids reacted with this basic structure to produce compounds like DBTL (Figure 2.9). In addition, di-butyl tin can be reacted with dibasic acids, as maleic acid, and with mixtures of monobasic acids to provide numerous homologues.

Figure 2.9 - Basic structure of di-n-butyl tin dilaurate (DBTL)

For CASE applications, DBTL can be considered the workhouse catalyst. It is efficient; i.e., a very low level of catalyst will greatly increase the NCO/OH reaction rate. However, as with any catalyst, certain problems may be encountered, which may include issues of stability-reactivity, hydrolysis of ester groups, catalysis of the water/isocyanate reaction and environmental concerns. The process of selecting a catalyst depends on several factors. Addition of metal catalysts in concentrations that can be measured in ppm have a profound effect on the reaction rate; yet, very low concentrations of impurities can also effect the reaction rate. Often resins, additives and pigments contain impurities, which can interact with catalysts and deactivate them, or the impurity itself can act as a catalyst. Catalyst deactivation can also be a function of water content or acid number of the resins. In addition, impurities have different effects on different metal catalysts. Furthermore, catalyst selection will also depend on the desired properties, and pot life requirements. DBTL and dibutyltin diacetate (DBTA) are very versatile catalysts for the NCO/OH reactions. Moreover, a range of organotin free catalysts includes bismuth, aluminum and zirconium, which are environmentally more acceptable and offer performance advantages.

 

2.2 - Inhibiting

Bronsted or Lewis acids retard the proton transfer to the isocyanate groups. Common inhibitors are HCl, benzoylchloride and p-toluene sulfonic acid and others added in the ppm region to the isocyanate. These materials play an important role in the preparation of prepolymers based on highly reactive polyols or amines and isocyanates.

 

2.3 - Blowing agents

Cellular polyurethanes are manufactured by using blowing agents to form gas bubbles in the polymerizing reaction mixture. The oldest and most common blowing agent is water, which reacts with the isocyanates liberating carbon gas and forming polyurea rigid structures. The auxiliary blowing agents (ABAs) are low boiling point compounds, also used in foam formulation. The auxiliary blowing agents function is to absorb the heat from exothermic reactions; vaporizing, and providing additional gas, useful in expanding foam to a lower density. With the evidences that the chlorofluorocarbons (CFCs) are responsible for ozone depletion, many other products have been studied as alternatives being taken into account the toxicity, inflammability, environmental impact, cost and physical properties. The definitive substitution of CFCs affects different segments of the industry differently.

 

2.3.1 - Flexible foams

For the different segments of the industry involved in the production of slabstock flexible foams (Chapter 3), moulded flexible foams (Chapter 4) and semi-rigid foams, the elimination of CFCs as auxiliary blowing agent represents an additional minimum cost. The most common option is water, which reacts with the isocyanates, forming carbon gas and polyurea rigid structures. To reduce the density and the hardness of the slabstock foams, the auxiliary blowing agents (ABAs) may be used to aid in attaining densities and softness not obtainable with conventional water-isocyanate blowing chemistry. In slabstock flexible foam production, methylene chloride is one of the most popular products, but it suffers restrictions in certain areas: European countries, for example. Acetone is also used successfully. However, some precautions should be taken, due to its inflammability. Another alternative is the use of liquid carbon dioxide as blowing agent in continuous and discontinuous slabstock flexible foam processes. In PU molded systems, mainly integral skin foams (Chapter 4), CFCs were initially substituted by HCFCs, like HCFC-141b (not totally innocuous to ozone depletion) and other systems based in pentanes, or even by water as the only blowing agent.

 

2.3.2 - Rigid foams

The rigid foams for thermal insulation need the use of ABAs (Table 2.7) like CFCs, HCFCs, pentanes, HFCs, etc, to minimize their thermal conductivity (Chapter 5.4.3). These gases are kept in the closed cells of the PU rigid foams, and are responsible for their excellent insulating properties. Due to environmental problems, CFCs were phased out, and nowadays, we no longer have any universal blowing agents. The most common alternatives for the rigid foam production are: the use of water as the only blowing agent (resulting in smaller insulating properties); and the use of auxiliary blowing agents (ABAs) like chlorofluorohydrocarbons (HCFCs), or hydrocarbons such as pentanes, and hydrofluorocarbons (HFCs). The substitution of CFCs for the others ABAs happened in different ways, in different regions of the world. The choice depended on: thermal conductivity, foam quality, inflammability, cost, and end use. In Europe and Japan, CFC-11 was substituted by HCFC-141b or pentanes. In the USA, the most common option was HCFC-141b, due to uncertainties regarding the properties, environmental consequences, and inflammability of the rigid foam produced with pentanes. In Latin America, Middle East, Africa and Asia, CFC-11 is still used. The tendency, however, is to eliminate its use gradually. According the Montreal Protocol, the CFCs phase out is scheduled to 2010.

Table 2.7a - Alternative blowing agents

Structure	CFC-11 CCl3F	HCFC-141b CCl2FCH3	HCFC-22 CHClF2	HCFC-142b CClF2CH3	CO2
Molecular weight (g/mol)	137,4	116,9	86,5	100,5	44
Boiling point (°C)	23,8	32,2	-40,6	-9,8	-78,3
Liquid density @ 20°C (g/cm^3)	1,49	1,24	1,21	1,10	-
Atmospheric lifetime (years)	50	9,4	12,1	18,4	120
Ozone depletion potential (ODP)	1,0	0,11	0,055	0,065	0
Global warming potential (GPW)	4000	630	1500	1800	1
VOC status	no	no	no	no	no
Vapor flame limits (% vol)	none	7,6-17,7	none	6,7-14,9	none
Vapor thermal conductivity 25°C (kcal/m.hr.°C)	0,0071	0,0086	0,0102	0,0101	0,0140
Flash point (°C)	none	none	none	none	none
Miscibilty with polyol polyether (g/100g)	> 100	> 100	na	na	-
With polyol polyester (g/100g)	16	34	na	na	-
With MDI (g/100g)	> 100	> 100	na	na	-

HCFC-141b, the leading and most versatile CFC-11 substitute was introduced around a decade ago, and its introduction was relatively easy. In 2001, the world consumption of HCFC-141b was close to 135 thousand tons, mainly due to its low thermal conductivity. However, regarding the ozone depletion and global warming, HCFCs are not totally safe, and according to the Montreal Protocol, they will phase out in 2003 for European Union and United States and 2040 for developing countries. Two other HCFCs, HCFC-22, HCFC-142b and their blends have also been commercially used as foam blowing agents. Their applications have been limited because of their low boiling points. Nevertheless, because they have lower ozone depletion potentials (ODP) than HCFC-141b, lately, they have gained in importance.

Table 2.7b - Alternative blowing agents

Structure	HFC-134a CF3CFH2	HFC-245fa CF3CH2CF2H	HFC-365mfc CF3CH2CF2CH3	ciclo-pentano	n-pentano	iso-pentano
Molecular weight (g/mol)	102	134	148	70,0	72,0	72,0
Boiling point (°C)	-26,5	15,3	40,0	49,3	36,0	27,8
Liquid density @ 20°C (g/cm^3)	1,22	1,32	1,23	0,75	0,63	0,62
Atmospheric lifetime (years)	14	8,4	10,8	days	days	dias
Ozone depletion potential (ODP)	0	0	0	0	0	0
Global warming potential (GPW)	1300	820	810	11	11	11
VOC status	no	no	no	yes	yes	yes
Vapor flame limits (% vol)	none	none	3,5-9,6	1,4-9,4	1,3-8,0	1,4-7,6
Vapor thermal conductivity 25°C (kcal/m.hr.°C)	0,0113	0,0106	0,0931	0,0106	0,0119	0,0126
Flash point (°C)	none	none	-25	-37	-37	-37
Miscibilty with polyol polyether (g/100g)	5	50	na	3-100	3-24	3-21
With polyol polyester (g/100g)	1,1	8	na	9-30	5-17	5-17
With MDI (g/100g)	4,5	55	na	30-35	7-8	10-11

Other zero ODP options include hydrocarbons like pentanes and HFCs (Table 2.7b). Pentanes, especially the cycle-pentane that possesses smaller thermal conductivity, are cheaper, and represent a very attractive alternative since there are established process conditions adapted to these inflammable products. Cyclopentane has become the accepted blowing agent for the appliance industry in Europe. Besides its attractive environmental data, cyclopentane offers competitive processing and performance properties. Its boiling point is somewhat higher than it is for CFC-11 or HCFC-141b, what causes a minimal effect on rise profile, but together with its relatively good solubility in the polyols reduces blowing agent losses during processing. Cyclopentane has vapor thermal conductivity lower than HFC-134a, and aging studies showed that it remains in the cells. The disadvantages of cyclopentane are its flammability and its plasticizing effect on the polymer matrix, which demand appropriate safety precautions and higher densities compared to the water/CFC-11 co-blown systems.

The two most promising HFCs are HFC-134a and HFC-365mfc (scheduled to be commercialized in 2003). HFC-134a is used as blowing agent and as gas for compressors and can become a substitute of HCFC-141b in the USA. Though it possesses low solubility in polyols that limits its amounts in a lot of formulations. HFCs are zero ODP, but they contribute to global warming and, for that reason, they were included in the Kioto Protocol. Besides these, other different inflammable and non-flammable mixtures of different types of AEAs, have also been tested in the PU rigid foams production.

2.5 - Fillers

Particulate and fibrous fillers (Table 2.9) may each be used in most kinds of PU. There are many reasons for adding fillers. Particulate fillers are used in flexible PU foams to increase the weight and resistance to compression and to reduce their flammability. PU foams, which contain fillers, represent a well-established foam technology. Fillers used in foam formulations can be inorganic in nature, but it is difficult to produce a stable suspension, and for this reason, organic fillers are usually preferred.

Flexible foams employing organic fillers like styrene-acrylonitrile copolymers or PHD polyols meet fire performance specifications for bedding and furniture applications and, with melamine as flame retardant material, also pass ignition source tests. For low-density foam, it is better to use melamine consisting of fine particles (10 mm). In this case, the flame retardant effect is enhanced by improved distribution of the melamine incorporated into the foam cell structure. In general, lower density foams require higher levels of melamine to pass the flame retardancy test. Inorganic solid materials used in filled foam formulations include: hydrated alumina, carbonates, silicates, silica, glass fibers, and barium sulfate.

Historically, inorganic fillers such as barium sulfate and calcium carbonate have been used in flexible slabstock foams to achieve increased density and/or load bearing, and to reduce cost. Normal used concentrations range from 20 to 150 parts per hundred parts polyols. The use of inorganic fillers has several disadvantages including: difficulty of preparing and maintaining the dispersion; problems with removal of entrained air; difficulty of mixing and pumping the filler/polyol slurry; loss of the foam physical properties; difficulty of processing on all types of foam machinery, and, due to their abrasive nature, increased wear on machinery components.

Table 2.9 - Fillers and their applications in PU

Filler	Typical applications
Calcium carbonate (ground chalk, ground limestone, whiting)	Flexible foams, semi rigid foams, binder compositions, rigid integral skin foams.
Barium sulphate (barytes)	Flexible foams, semi rigid foams, especially for sound absorption.
Clays (china clay, kaolins, etc)	Flexible systems.
Expanded silicas, colloidal silicas	Flexible foams, cast elastomers.
Clay balls, vermiculite	Rigid foams..
Glass micro spheres	Flexible, microcellular foams, RIM.
Glass flakes	Elastomeric RIM.
Silicates, cements	Rigid foams, sealants, grouting.
Short fibers, milled and chopped glass fiber	Elastomeric RIM, rigid foams.
Glass cloths and scrims, wire mesh, organic fibers	Encapsulation in rigid foams, reinforcement of low-density flexible foam moldings.

Mineral fillers are also used to reduce costs, and to increase the compressive strength of rigid foams used in composite building panels. Finely divided fillers with a particle size ranging from a few microns up to about 100 microns are usually added as dispersers in the polyol component of the PU systems. Some low cost mineral fillers such as china clay, kaolins, and other aluminium silicates, which contain both free and combined water, which would otherwise be satisfactory, may be difficult to dry reproducibly and economically. Care must be taken to dry the fillers or to known the precise water content available and factor that data into the foam calculations. Fibrous fillers are reinforcers: they give increased stiffness, and they increase the range of operating temperatures of rigid foams, integral foams and flexible RIM products. The degree of reinforcement obtainable depends on the strength of the fiber, the concentration, the modulus and extensibility of the polymer matrix, the interfacial adhesion and the shear strength at the fiber/polymer interface, and on the orientation of the fibers.

 

2.6 - Antiaging agents

Like most polymeric materials the PU's are also susceptible to aging and the physical properties are normally negatively influenced. For example: PU's are subject to degradation by free radical pathways induced by exposure to heat or ultraviolet light; Polyester based PU's are more susceptible to hydrolysis (Chapter 1). However, the surface yellowing of PU based on aromatic isocyanates due to its exposure to light is merely an esthetic aging effect, which produces no loss of its mechanical properties. Light protection agents, such as hydroxybenzotriazoles, zinc dibutil thiocarbamate, 2,6-ditertiary butylcatechol, hydroybenzophenones, hindered amines and phosphites have been used to improve the light stability of PU's.

Degradation of both the polyol and urethane components will cause changes in the physical or mechanical properties of the PU's. Urethanes are susceptible to degradation by free radical pathways induced by exposure to heat or ultraviolet light. Autoxidation (Figure 2.16) may be initiated by heat, high-energy radiation UV (UV light), mechanical stress, catalyst residues, or through reaction with other impurities. Free radicals are generated (step 1) which react rapidly with oxygen to form peroxy radicals (step 2). These peroxy radicals may further react with the polymer chains leading to the formation of hydroperoxides (step 3). On exposure to heat or light, hydroperoxides decompose to yield more radicals that can reinitiate the degradation process (stage 4).

Figure 2.16- Degradation process of PU's

Antioxidants interrupt the degradation process in different ways according to their structure. Primary antioxidants, mainly acting in step 1 as chain breaking antioxidants, are sterically hindered phenols. Primary antioxidants react rapidly with peroxy radicals (ROO·) to break the cycle. Secondary arylamines, another type of primary antioxidant, are more reactive toward oxygen-centered radicals than are hindered phenols. Synergism between secondary arylamines and hindered phenols leads to regeneration of the amine from the reaction with the phenol (Figure 2.17).

Figure 2.17- Oxygen-centered radical traps

Phospite stabilizers are secondary antioxidants (hydroperoxide decomposers). Acting in step 4, they react with hydroperoxide (ROOH) to yield non-radical, non-reactive products. Secondary antioxidants are particularly effective in synergistic combination with primary antioxidants.

Hindered amine stabilizers (HAS) can in some cases, provide radical trapping effectiveness similar to hindered phenols. Traditionally used as light stabilizers, hindered amine stabilizers can also contribute to long-term thermal stability.

PU's are subject to degradation when exposed to natural and/or artificial UV lights. Degradation results in discoloration and/or loss of physical properties. In the photo-degradation mechanism, initially the polymer absorbs UV radiation, which excites the absorbing species, and raises them to a higher energy level (R to R*). If the molecule cannot be brought to its ground state, homolytic bond cleavage and the formation of free radicals will occur (R* to R·). The free radicals formed during photolysis readily react with oxygen to form peroxy radicals (ROO·), and the subsequent steps are similar to those in Figure 2.15. There are two classes of light stabilizers: 1) UV absorbers protect against photo-degradation by competing with the polymer for the absorption of ultraviolet light; 2) The mechanism of stabilization of hindered amine light stabilizers (HALS) involves efficient trapping of free radicals with subsequent regeneration of active stabilizer moieties.

Depending on the type of antioxidant used, the addition of antioxidants may also result in several undesirable side effects. Phenolic antioxidants (especially BHT) have long been known to migrate into fabric coverings, and, under certain conditions, to cause staining. The yellowing occurs when basic fabric finishes or basic cleaners are used. Acid finishes and rinses together with sunlight can reduce or eliminate the color. Secondary amine antioxidants are thought to be significant contributors to discoloration. Thioesters can react with oxides of nitrogen (exhaust fumes) to form orange to red color bodies. Finally, certain phospite antioxidants suffer from hydrolytic instability and can decompose to deleterious phosphoric acid during extended storage in polyol solutions.

 

2.7 - Coloring materials

Most low-density flexible foams are color-coded during manufacture to identify the grade and the density of the foam. Specialized products, such as foams for textile laminating and for packaging, may be highly colored to meet the requirements of the application. On the other hand, rigid foams, being mostly made from brown-colored polymeric MDI and sold enclosed within opaque covering materials, are often made without added colorants. The usual method of coloring adds pastes, made with polyols and inorganic or organic pigments, to the foam reaction mixture. Typical inorganic coloring agents included titanium dioxide, iron oxides and chromium oxide, organic pigments originated from the azo/diazo dyes, phthalocyanines and dioxazines, as well carbon black. Typical problems encountered with these colorants include high viscosity, abrasive tendencies, foam instability, foam scorching, migrating color and a limited range of available colors. The most popular coloring material is carbon black, which at levels above 0.1 parts per 100 parts of the polyol used gives some protection against the surface discoloration of the foam caused by UV light.

Polyurethanes based on aromatic isocyanates tend to yellow on exposure to daylight. Thus, to screen uncovered items such as self-skinning foams, microcellular molding and RIM products from UV light, carbon black pigmentation, or UV light absorbing additives must be used as protectors (Chapter 2.6). Another method of coloring is the surface coating of the finished PU. This is performed by subsequent painting of PU parts with light stable lacquers, preferably one-component PU lacquer systems based on blocked isocyanates, or two-component systems (Chapter 7). Two approaches are commonly used: first, in order to reduce the number of operations and to simplify any difficulties associated with the demolding of the product and with paint adhesion, in-mould coating is widely used. According to this procedure the coating is sprayed onto the mold surface before the PU reaction is carried out; then, flexible or rigid topcoats of PU are applied after demolding, to provide a high degree of abrasion resistance.

 

2.8 - Flame retardants

Given the application of sufficient heat in the presence of oxygen, PU's, as all organic materials will burn. The physical state of the polymer is also extremely important. Low-density, open-celled flexible PU foams have a large surface area and high permeability to air, and, thus, will burn most easily. Flame retardants are often added to reduce this flammability, at least, when they are to be measured by various specific, often small-scale tests, conduced under controlled laboratory conditions. The choice of flame retardants for any specific PU foam often depends upon the intended service application of that foam, and the attendant flammability-testing scenario governing that application.

Aspects of flammability that may be influenced by additives include the initial ignitability, burning rate and smoke evolution. On flame retardants, manifold requirements are placed: They should be compatible with the mixture of raw materials and additives and should not be able to migrate out of the finished products. In addition, the mechanical properties of the finished products should be affected as little as possible and, in case of burning, they should form little smoke and no toxic fumes.

In PU flexible foams, the most widely used flame retardants are the chlorinated phosphate esters. Chlorinated paraffin and melamine powders have also been used (Table 2.10). However, the incorporation of flame retardants in foams can present problems such as: to increase the possibility of burns in certain formulations; to increase the amount of smoke or burns; and to cause processing problems, especially with the reactive flame retardants.

According to theory, halogen compounds work in the gas phase, interrupting the free radical nature of the combustion process. They work either as molecules or interfere as halogen atoms, formed through their burning, in chain reactions. The phosphorous compounds effect a catalytic splitting of the PU and lead through dehydrogenation and dehydration reactions to a carbonized, protective surface. Technical important synergistic working combinations are halogen compounds / antimony trioxide, phosphorous compounds / halogen compounds and phosphorous compounds / nitrogen compounds.

Table 2.10 - Some flame retardants for PU

Flame retardants	Main application
Non-reactive liquids
Tris(1,3-dichloroisopropyl) phosphate (TCPP)	Flexible foams
Tris(2-chloroisopropyl) phosphate	All PU foams
Tris(2-chloroethyl) phosphate (TCEP)	Flexible foams and coatings
Pentabromodiphenyl oxide	Flexible foams
Tetrakischloroethyl-2,2-bis(chloromethyl)propylene-diphosphate	Flexible, rigid and molded foams
Tris(2,3-dichloropropyl) phosphate (TDCP)	Strip or block flexible foams
Reactive flame retardants
Dibromopropanol
Diester/ether diol of tetrabromophthalic anhydride	Rigid foam, elastomers and coatings
Tetrabromophthalate diol	Rigid foams, RIM, elastomers, adhesives, coatings and fibers
Tetrabromophthalic anhydride	Rigid foams
Solid flame retardants
Antimony Trioxide	Synergistic effect
Ammonium salts, sulphate, polyphosphate, etc	Together with halogenated additives in rigid foams
Aluminum hydroxide	All polyurethanes
Melamine	Flexible foams
Calcium carbonate	Heat absorbing filler
Flaked or powdered PVC

 

Solid flame retardants

Liquid flame retardants have always been preferred because other components for PU foam are liquid. However, solids, which are especially cost effective, have been in continuous use. Solids have the disadvantage of needing to be slurred with another component. Pumping and metering equipment life is shortened due to the abrasive nature of solids.

Flaked or powdered PVC is a soft and inexpensive flame retardant when comparatively mild flammability tests are required. There has been concern about the evolution of HCl when foam containing PVC is flame laminated. Ammonium polyphosphate is used with liquid halogenated phosphate esters in ester polyol based foams.

A combination of liquid and solid flame retardants often has a less deleterious effect upon physical properties than the same quantity of liquid or solid additive alone. In combustion modified high resilience, foams (CMHR) are often incorporated to the solids antimony trioxide, alumina trihydrate, and decabromodipheyl oxide as well as to a liquid halogenated phosphate ester. The presence of both chlorine and phosphorous is necessary for the optimum effect upon flammability.

The addition of aluminium trihydrate gives further reduction in flammability, and minimizes the increase in smoke formation on burning, resulting from the addition of halogenated organic phosphates. Aluminum hydroxide, between 180 and 200oC, splits off water and changes to aluminum oxide. Its effectiveness depends on the fact that endothermic splitting of water withdraws heat from the system, the formed water vapor dilutes the gas formed by polymer cracking, and the aluminum oxide forms an insulating protective layer. Melamine is lower in cost, less dense, and effective when used together with phosphate flame retardants, such as ammonium polyphosphate, in flexible foams for furniture cushions.

Melamine melts away from the flame and forms both a nonflammable gaseous environment and a molten barrier that helps isolate the combustible PU foam from the flame.

 

Liquid flame retardants

Due to their efficiency, compounds containing aliphatically bound halogen or combinations of aliphatically bound halogen and phosphorous were among the first flame retardant compounds used. Many ignitability standards for furniture and vehicle seating can be met by the incorporation of 5% to 10% chlorinated phosphate esters. Many of the most efficient flame retardants (Table 2.11) are no longer in use commercially due to toxicity concerns or to their effect upon foam properties.

Table 2.11- Flame retardants efficiency

Flame retardant	Level required (php)
Tribromoethanol	5
Dibromopropanol	5
Tris(dibromopropyl)phosphate	6.5
Bischloroethylethylphosphonate	6.5
Tetrakischloroethyl-2-2-bis(chloromethyl)ethylene-diphosphate	7.5
Hexabromocyclododecane	7.5
Tetrakischloroethyl-2,2—bis(chloromethyl)propylene-diphosphate	10
Tris(dichloropropyl)phosphate	10
Tris(chloroethyl)phosphate	10

The most used flame retardants in both flexible and rigid foam systems are chlorinated phosphate esters. These have a significant effect upon the ignitability of foams by a small heat source, and they may show marked reductions in the rate of burning in small-scale tests without adverse effects upon the processability of the foam system and the properties of the product.

Liquid flame retardants represent the largest volume of flame retardants used in flexible PU foam, but despite their ease of handling and their compatibility with other liquids, they have inherent problems. One of these is volatility, Internal temperatures in large burns can reach 160oC or higher during cure. Volatile, low molecular weight liquids are capable of migrating during foam cure, affecting the flammability of the burn center. The introduction of aging criteria to many flammability test standards has prevented this problem.

An even greater problem related to the exothermic of curing PU foam is the discoloration caused by inadequate thermal stability of liquid flame retardants. This discoloration or scorch is most prevalent when aliphatic halogen containing flame retardants are used. It has been suggested that acidity due to dehydrohalogenation catalyses the oxidation of the polyol.

The role of aromatically bound bromine is scorch reduction, and aromatic bromine containing flame retardants have been used in PU flexible foams to prevent scorch. The hydrohalogen acids formed reduce IFD (indentation force deflection) spread in HR foams. IFD spread is observed when dibutyltin dilaurate (DBTL) is used as a catalyst. It is believed that the acid formed inactivates DBTL, which can promotes depolymerization of the PU foam.

Hydrolytic decomposition of flame retardants may also be a problem, particularly in polyester PU foam subjected to humid aging at elevated temperature. Another property affected by flame retardant hydrolytic stability is the final cure rate. Acidic species generated neutralize the tin and amine catalysts, slowing cure.

 

2.9 - Release agents

The outstanding adhesion of PU to other material has led to a broad application in the adhesive sector (Chapter 7). This property is detrimental in foam molding. A release agent is necessary in order to easily and quickly remove the foam from the mold. The effectiveness of the release agent depends less on the amount than on the uniformity of the coating. The force per area necessary to open the mold can be used as scale for the release effectiveness. In order to choose the best release agent, a basic knowledge of the PU system and of the kind of mold material, surface quality and form geometry are decisive. The adhesion to the mold surface decreases with increasing reactivity and increasing density of the reaction mixture, due to the shorter period of wetting by the isocyanate component.

The release agent is best applied by spray coating the open mold. Sufficient ventilation must be provided. In general, the removed parts must be subjected to an after treatment to remove adhering release agent residues. This measure is indispensable if the finished part is to be painted afterwards. Removal of the release agent residues from the mold must be done after every one or two production shifts. This is done with solvents, such N-methylpyrolidone (wiping with soaked rags), or with cleaners offered by the release agent manufacturers (in the form of sprays, liquids or pastes).

There is no universally applicable alternative, and the best release agent can only be found by testing under production conditions. Several techniques are available to avoid the application or removal of release agents in particular areas:

Internal release agents - From the point of view of the processor, the ideal process only uses internal release agents, which are used rather successfully in RIM formulation, added to polyol component. The use of external release agents, however, remains a necessity. Normally, it is possible to achieve 20-100 demoldings until the external mould release agent has to be applied again. Metallic soaps such zinc stearate, ester based oils, waxes, and siloxanes are examples of products in use. These substances are insoluble in the resins and, therefore, they have to be distributed homogeneously. During the PU formation, these emulsions or dispersions beak respectively, the active ingredients migrate to the surface and, there, they form a very fine thin film. This film acts like a barrier and prevents the formation of physical and chemical interactions between the foam and the mould, finally providing a releasing layer for demolding the part. Siloxanes, in use as internal mould release agents, are normally potent denucleating additives or even defoamers, as well. They reduce the nucleation considerably so that the possible air-load comes down do only 20%. Internal mould release agents based on pure organic substances do not have this drawback. On the other hand, they are less effective. Disadvantages may occur during possible after treatment of the finished part because the release agent may still migrate to the surface even after painting, and lead to flaking of the paint.

Mould coating - The use of permanent mould coatings, such as PTFE coatings, increase substantially the productivity by reduction of the release agent application time, but it has some disadvantage. This is due to the limited life and difficult renewal. A further disadvantage is the smaller possibility of controlling the surface gloss of the finished part. Semi-permanent release agents are also used in many sectors. Polysiloxanes may polymerize on the mould surface, and, thus, produce a release reserve for several removals from the mould. The disadvantage may lie in the different surface of the finished part. The first molding, removed immediately after application of the release agent, often has different characteristics from the last molding, which can be removed only with difficulty. Semi-permanent release agents often act successfully in conjunction with internal release agents. Quickly reacting RIM systems frequently operate as so-called easy release systems, i.e., with and internal release agent in conjunction with an external semi-permanent application. This may be a hard wax, which is polished on the mould surface. The technical standard in most cases is the use o a hard soap, which does not have as long a life as a polished wax, but which can be applied far more quickly with a spray gun.

External mould release agents - External release agents, which are applied on the moulds using different techniques have to be very incompatible with the processed materials. Furthermore, they should have a low surface tension, to allow the formation of closed, homogeneous and very thin films on the surfaces with low energies. These films should not have any reactive groups; they should be chemically inert. However, a few polar groups are necessary to achieve a sufficient substantivity of the release substances on the moulds. Siloxanes, as external release agents, are preferred for microcellular systems, particularly for shoe soles. They meet the aforementioned conditions in an ideal way. The portion of polar groups necessary for the appropriate substantivity can be achieved by modifying the siloxanes with organic side groups. Besides good release, silicone based release agent provide good compatibility with coloring materials used for shoe soles. As the films are liquid, the formation of a hard buildup in the molds is not possible. The viscosity of the siloxanes may be reduced to such an extent that they become volatile. The result is that the surface of the mold is nearly free of any release agent. This technique is especially successful for polyesterpolyurethanes.

The external mould release agents for processing RIM systems are based either on waxes or metallic soaps. Aqueous solutions of metallic soaps are preferred whenever metallic soaps are applied as internal release agents. It is recommended that wax based external release agents be used when the internal release agent is a siloxane type one. Combinations of waxes with siloxanes are useful for integral skin foams, to provide surfaces with a silky finish. The use of solvents, in particular the ozone depleting halogenated hydrocarbons and CFC's, has become severely restated by several clean air acts.

Water based release agents - In global terms, they are the most common as they comply with the environmental norms. They have as disadvantage higher cost, higher water boiling point, and their reactivity with isocyanates, that can cause skin formation. In the case of molded flexible foams, the skin formation (closed cells) increases the hardness. The use of water based release agents has been the state of the art for several decades in the field of hot foamed material production, thanks to the high mould temperatures and the resulting-free evaporation of the release agent. The disadvantageous physico-chemical properties of water compared to other carriers such aliphatic hydrocarbons, chlorinated hydrocarbons, or CFC's were an important obstacle to the use of these systems in other PU fields.

Solvent based release agents - They constitute one of oldest technologies in use, and they are still the most commonly used in cold molded flexible foams. In them, a small amount of wax is suspended in a low boiling point solvent, as petroleum spirit, methylene chloride, etc. When the release agent is sprayed into the hot mold, the low boiling point solvent evaporates, resulting in a film formation in the mold surface that prevents the adherence of PU. Chlorine containing products such as methylene chloride are rejected for workplace and environmental protection reasons, although methylene chloride, unlike 1,1,1-trichloroethane and CFC 11, has not been banned so far. Only the pure hydrocarbons in their fractions then remain as suitable carriers for release agents. The reason lies in the favorable prices compared to chlorinated solvents. Important disadvantages such as flammability and the slower evaporation rates have been tolerated. A dearomatised white spirit with a flash point of about 25oC has established itself as carrier material. The low-solids solvent-based release agents possess 97% of solvent, which is liberated in the atmosphere during the application, and have also high VOC (value of oxygen consumption).

High-solid release agents - The solvent-base high-solid release agents have smaller VOC than low-solid solvent-based release agents, due to the reduction of the solvent level to 85-92%. If we consider the application of an equal amount of release agent, we would have a theoretical reduction of 70-80% in VOC. Though, in practice, it is very difficult to control the application level and the excess used results in increase of VOC. In the case of the water-based high solids release agents, a part of the hydrocarbon-based carrier was replaced by water, but by maintaining the properties of normal high-solids. However, this is only recommendable if the solvent emissions have to be reduced once again, when compared to normal high-solids.

Pastes - The history of release agents in PU industry began with pastes, such as floor waxes, which are still widely used today. This applies in particular to the application of prime coatings to the moulds. However, pastes with the most diverse compositions and hardnesses are also used in many applications in which the evaporation time is not a problem. The organic solvents have been replaced by water in many sectors. Many water-based release agents are used only in conjunction with water-based pastes as mould primers. Solvent-based mould primers often cause defects on the finished part.

 

2.10 - Special addictives

Besides the previously described auxiliary agents are also added special addictives to promote properties in PU's. For example, coatings, adhesives, sealants and the variety of topcoats, which have to be glued, require a large number of these special addictives.

2.10.1 - Crosslinkers

Organo silanes are useful as crosslinkers for PU sealants and adhesives (Chapter 7). They react with the NCO terminated PU prepolymer to form an endcapped silane prepolymer, call silylated PU (SPUR). The SPUR's based sealants and adhesives react fast with moisture at room temperature, have good durability, excellent adhesion performance and they are free from any unreacted isocyanate residual monomer. They can be formulated with a wide range of addictives, as fillers, plasticizers, adhesion promoters, moisture scavengers, etc.

2.10.2 - Adhesion promoters

Adhesion promoters are used to provide a reaction in the surface of the substrate and by this way to promote the adhesion, providing a bond between the PU's and the substrate. They can be applied in the surface of other materials, such as metallic parts. Adhesives and sealants of PU frequently can be combined with organo silanes (aminosilanes, mercaptosilanes, epoxysilanes, etc) that form a chemical bond between the adhesive or sealant resin and the substrate. This chemical bond is resistant to moisture, chemical products, and heat.

2.10.3 - Coupling agents

Silanes chemically bond organic polymers to inorganic material such glass fibers, glass spheres, silica, titanium dioxide, clays, metal and metallic oxides. Beyond providing a means of bonding inorganic filler to a PU resin, improving filler and pigment dispersion.

2.10.4 - Moisture scavengers

Water bonding agents are added for increasing the storage stability of one and two PU component systems. In PU solid systems, besides the careful drying of all components the addition of a drying agent (2 to 4%), like zeolite or molecular sieve, to the polyol component is recommended to bond possible residual moisture. Zeolite pastes ensure a quick and excellent dispersion in the PU and are, in addition, less sensitive ho humidity than zeolite powder. Silanes, especially vinyl silanes, have found wide use as moisture scavengers. The electro-withdrawing nature of the vinyl group, furthers enhances the water reactivity of the silicon-methoxy bond. As moisture scavengers it reacts with moisture faster than other alkoxy silanes, enabling it to function as a moisture scavenger in the presence of the other silanes incorporated as adhesion promoters, crosslinkers or coupling agent. A nominal amount (3% pbw) of vinyl silane is required to enhance the shelf-stability of a moisture sensitive product.

2.10.5 - Rheology modifiers

Rheology modifiers are used for control of the rheology by the reduction of the fluidity, avoiding the flowing of adhesives, selantes, paints, etc, and they include: pyrogenic silica, betonites and carbon black. The addition finely divided silica can prevent the no wanted penetration of adhesives in porous materials as leather, textile and concrete. The silicas (silicon dioxide) of extremely small particle size are used to modify the rheology of liquid systems. It is know as highly disperses, colloidal, or still fumed silica due to it production process. They find application mainly, in the modification of the rheology of PU sealants, adhesives, paints, coatings, etc.

2.10.6 - Plasticizers

Non-reactive liquids have been used to soften a PU or to reduce viscosity for improved processing. Plasticizers as the phthalates, benzoates and chlorinated paraffins are used for reduction the viscosity and cost, however they reduce the PU mechanical properties like tensile strength and hardness, and also decrease the PU glass transition temperature (Tg). Using a polyol of lower equivalent weight can compensate the softening effect so that a higher cross-linked polymer structure is obtained.

2.10.7 - Antistatic agents

The PU's are used in some packing and clothing applications as safety shoes, packing electronic goods, etc, that's requires a strong decrease of electrical resistance. The reduction of electrostatic charge build-up is attained through antistatic agents, for example tetra alkyl-ammoniumalkylsulfates. They are mixed with the polyol component or added to the isocyanate reactant and diminish the surface resistance by approx. 108 Ohm. Some flexible PU foams are also used in packing, clothing and other applications where it is desired to minimize the electrical resistance of the PU foam so that buildup of static electrical charges is minimized. This has traditionally been accomplished through the addition of ionizable metal salts, carboxylic acid salts, phosphate esters and mixtures thereof. These agents function either by being inherently conductive or by absorbing moisture from the air. The desired net result in orders of magnitude reduction in foam surface resistivity.

2.10.8 - Cell openers

In some PU foams it is necessary to add cell openers to obtain foam that does not shrink upon cooling. Known additives for inducing cell opening include silicone based antifoamers, waxes, finely divided solids, liquid perfluocarbons, paraffin oils, long chain fatty acids and certain polyether polyols made using high concentration of ethylene oxide.

2.10.9 - Lubricating

Waxes, soaps and other products are added in small amounts as lubricants and oiling agents. They usually improve the flow characteristics of fluid reaction mixtures by lowering the viscosity and facilitating the extraction of PU molded part. Lubricants act as processing aids for injection molding and extrusion of TPU's (Chapter 6.3).

2.10.10 - Hydrolysis stabilizers

Polyester based PU's are subject to aging. The ester bonds can be hydrolyzed under the influence of humidity and higher temperatures. Polyester based PU's are stabilized against hydrolytic degradation by adding 1-4% by weight of sterically hindered aromatic carbodiimides. The carbodiimide group reacts with acid residues, generated by the hydrolysis of ester groups, which otherwise catalyze further hydrolysis.

2.10.11 - Bacteriostats

Under certain conditions of warmth and high humidity, PU foams are susceptible to attack by microorganisms. When that is a concern, additives against bacteria, yeast or fungi are added to the foam during manufacture.

 

3 - Flexible Foams

---