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Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder

*

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Smithers Rapra Technology, 2013

Mechanical Properties of the
Composite Material Based on
Modified Scrap Tires and Polymer
Binder

R. Plesuma*, A. Megne, I. Mateusa-Krukle and L. Malers

Institute of Polymer Materials, Riga Technical University, Latvia

Received: 10 February 2012, Accepted: 25 October 2012

SUMMARY

The present work focuses on the investigation of the mechanical properties of
the composite material composed of preliminary modified rubber crumb and
a polymer binder. The modification of rubber crumb was realized by means of
treatment with water or sulphuric acid, in order to change the degree of polymer
crosslinking or to intensify adhesion interaction between rubber particles and
polymer binder. The Shore C hardness, compressive stress at 10% deformation
and compression modulus of elasticity, ultimate tensile strength and elongation
at break of composite material, in correlation with its composition and adhesion
between rubber and polymer binder were investigated.

Obtained results show possibility of attaining significant changes in the mechanical
properties of composite material by selected rubber crumb treatment methods,
in correlation with the composite material composition. This gives additional
opportunity for meaningful improvement of composite material production
technology.

INTRODUCTION

Recycling of used automobile tyres must be considered as an important
activity from the point of view of not burdening the environment from the
non-degradable waste. Various ways of utilization and recycling methods

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R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers

exist [1];one of the most common approaches for recycling tyres is mixing of
rubber crumb with a polymer binder to produce composite materials [2, 3].

In our previous investigations, optimization of the composition and technology
of composite material production from mechanically ground scrap tyres and
a polyurethane type binder, were realized [4, 5, 6]. It was elucidated that the
properties of the studied composite material are highly dependent, not only
on the material composition and the type of polyurethane binder used, but
also on several technological parameters and production conditions such as
molding pressure and formation temperature [6].

The present work must be considered as a continuation of previous
investigations on composite material and focuses on clarifying the influence
of preliminary rubber crumb modification on selected mechanical properties
of the composite material, in what must be considered as insufficiently
investigated question. Two different methods of modification were selected
based on experience and published composite material research [7, 9, 11-
13]. Hence ,the methods of rubber crumb modification detailed in this study
could be useful in improving the mechanical properties of composite material
and production technology.

EXPERIMENTAL

Description and Preparation of Materials and Samples

Mechanically ground at ambient temperature, modified and non-modified
rubber crumb obtained from used tyres, with a particle size ranging from
0.2-7.0 mm (

Figure 1), and polyurethane type binder with a different reactivity

(isocyanate group content - 2.42%, 5.4% and 7.4%) were used in order to
produce composite material samples. Different compositions of rubber crumb
and polymer binder (from 8-23 wt%) were used in the composite material.

Uniform samples of composite material were prepared under constant and
defined conditions: formation temperature (18-22°C), pressure (0.004 MPa),
molding time (24 h) and relative air humidity (23-30%).

It is well known [7] that environmental moisture content directly affects the
hardening degree of polyurethane polymers, containing isocyanate groups,
due to the degree of change of polymer crosslinking.

Direct preliminary

rubber crumb modification using water was implemented in order to clarify
the possibility of intensifying the degree of crosslinking of the polymer binder
used directly in the composite material. The conditions employed for the

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Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder

rubber treatment with water in the present work were selected taking into
account our previous investigations [8]. The modification of rubber crumb
was realized by the direct treatment of previously dried rubber crumb with a
defined amount of water.

Chemical modification of rubber crumb with 96% sulphuric acid at different
reaction times was used in order to explore the possibility of intensifying the
adhesion between the polymer binder and rubber particles, thereby affecting
the mechanical properties of the investigated composite material [9, 11, 12].

The particle size distribution of scrap tyres before and after surface modification
with sulphuric acid is shown in

Figure 1.

Testing

In order to examine constituent materials used in the production of the
composite material and determine the essential characteristics which could
affect the final properties of the material, the following test methods were used:

• Determinationofmoisturecontentinrubbercrumbwasascertainedby

simple calculations with preliminary dried (different drying times were
selected) and modified rubber particles.

• In order to determine some wetting properties, the contact wetting

angle q° of a water droplet on a rubber surface of the non-modified and

Figure 1.

Particle size distribution of scrap tyres before and after surface modification

with sulphuric acid

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R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers

modified solid rubber samples were examined. The possible change of
hydrophilic properties, depending on the rubber surface energy, was
estimated (

Figure 2).

• Special samples were prepared in order to examine the strength of

adhesive bonding between the polymer binder and modified rubber
surface. For this purpose Peel test (90°, speed 50 mm/min) was used
according to LVS EN 28510-1.

• The apparent density AD (kg/m

3

) of the composite material was

determined according to LVS EN 1602.

• ShoreChardnesswasinvestigatedbyusingaShoretester(TypeC;ISO

7619, ISO 868).

• Mechanical properties of the composite material (compressive stress

at 10% deformation and compressive modulus E) were determined by
using the testing apparatus, Zwick/Roell7020, according to EN 826. In
the tensile mode of loading, the ultimate tensile strength and elongation
at break, for specially prepared samples with defined dimensions, were
determined according to LVS EN ISO 527-3.

Figure 2. Contact angle q° of solid rubber samples before and after modification with

sulphuric acid

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Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder

RESULTS AND DISCUSSION

In previous investigations it was clarified that the composite material
composition and its hardness is highly interdependent [5]. Close correlation
between mechanical properties and Shore C hardness was demonstrated,
hence proving that Shore hardness can be used as a tool to determine other
mechanical properties (for example, compressive stress at 10% deformation)
without direct material testing

[5, 6].

It was also elucidated that elevated air humidity, during formation of the
composite material, reduces the composite material hardening time of due
to intensification of polymer crosslinking reactions: thus making it possible to
achieve significantly higher values of Shore C hardness after 24 h hardening,
compared with the testing results of composite material samples hardened
at ambient air humidity [10]. Therefore, the effect of moisture content on
the properties of the composite material must be considered as potentially
significant, and for that reason additional investigations were carried out using
a rubber crumb special treatment with water spraying before production of
the composite material samples.

The results show that values of apparent density increase upon increasing
the sprayed water and binder content (

Figure 3a). The correlation between

characteristics and properties of the composite material with the binder
content, used in the production of the samples, has also been investigated
and demonstrated in several previous investigations [4-6].

Similar tendencies were observed in the determination of Shore C hardness for
composite material samples containing modified rubber particles (

Figure 4a).

This can possibly be explained by the specific nature of polymer binder in
improving the hardening process, as the rubber particle-polymer binder

interface is highly dependent on the environmental moisture [8].

Chemical modification of rubber crumb was carried out by treatment with
sulphuric acid under defined conditions prior to the production of the
composite material samples. The results show a significant decrease of
apparent density compared with the composite material samples prepared
under the same conditions using non-modified rubber crumb (

Figure 3b).

This can possibly be explained by the changes in the particle size distribution
of rubber crumb after chemical modification (

Figure 1). At the same time,

experiment showed that Shore C hardness increased by up to 15% when
modified rubber crumb was used; probably a result of the destruction of double
bonds on the rubber surface and consequent loss of elasticity (

Figure 4b).

In addition, a more similar shape of rubber particles was observed after
chemical modification.

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R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers

The chemical modification of the rubber surface with sulphuric acid was
carried out mainly in order to investigate subsequent changes of adhesive
interaction (bonding) between the rubber and polymer binder. Surface oxidant
treatment, in order to improve interfacial compatibility in composite materials,
has also been demonstrated in systems containing polyolefins [12, 13].
Peel test results show that the chemical modification of the rubber surface
promotes adhesive bonding for selected samples (

Figure 5). The results of

the Peel test correlate with rubber surface wettability measurements shown

Figure 3. Variation in composite material apparent density AD (kg/m

3

) with

composition of material and modification of rubber crumb: a) treated with water, b)
treated with sulphuric acid

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Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder

in

Figure 2. A visual evaluation of the failure character of adhesive joints

after the Peel test demonstrated mainly adhesive type of destruction. It was
shown that with the increase of concentration of reactive isocyanate groups
in the polymer binder, Peel strength increased due to a more active interaction
between components of the adhesive joint (

Figure 5).This fact establishes

the importance of the individual properties of constituent materials used in
the production of composite material.

Figure 4. Variation in composite material Shore C hardness with composition of
material and modification of rubber crumb: a) treated with water, b) treated with
sulphuric acid

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R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers

Figure 5. Variation in Peel strength with polymer binder activity before and after
chemical modification of rubber surface

In order to investigate subsequent changes of mechanical properties - ultimate
tensile strength (σ

t

) and elongation at break (ε

t

) in the tensile mode of loading

of the composite material (polymer binder with 2.42% isocyanate group
content) were determined. The samples of composite material were prepared
using modified and non-modified rubber crumb. Results are given in

Table 1.

The results presented in

Table 1 show that the rubber crumb treatment with

water demonstrated mainly higher ultimate elongation values and lower
values of tensile strength for the tested material samples. This can be possibly
explained due to some of the water not being involved in the crosslinking
reactions of the polyurethane binder and therefore acting as a plasticizer of
the composite material.

Chemical modification of the rubber crumb resulted in the ultimate tensile strength
exhibiting lower values compared with the non-modified samples. However, the
elongation at break in the tensile mode of deformation demonstrated an inverse
situation. The explanation for this must take into account the fact that rather
different rubber particle shape and size distributions (

Figure 1) were observed

for modified and non-modified rubber crumb, and therefore the compactness
and also properties of the composite material samples differ [11].

Similar samples of composite material, produced with the methods mentioned
above, were also tested in a compressive mode of loading; compressive
modulus of elasticity and compressive stress at 10% deformation were also
determined (

Table 1).

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Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder

In the case of using water spraying as a modification method, test results
show that the mechanical properties of the composite material are highly
dependent on moisture content; explained by the improvement of the particular
polymer binder hardening. The significance of water treatment prior to the
production of composite material samples was established and therefore the
role of moisture presence is emphasized once again.

Similar tendencies were observed when chemical modification of the rubber
crumb was used (

Table 1). In this case, mechanical properties such as

modulus of elasticity and compressive stress at 10% of deformation were
enhanced due to the improvement of adhesion between the rubber and
polymer binder. This is highly connected with the improvement of wetting
capability (hydrophilicity) of the rubber crumb due to the processes arising
from the influence of sulphuric acid on the chemical structure of the rubber
surface [9]. This was also demonstrated by the experiment investigating the

Table 1. Ultimate tensile strength (σ

t

),elongation at break (ε

t

),

compressive stress at 10% deformation (σ

10%

) and compressive

modulus of elasticity (E) of the composite material

Nr.

Description of rubber crumb Polymer

binder

content

(wt.%)

σ

t

(MPa)

ε

t

(%)

σ

10%

(MPa)

E

(MPa)

1.

Non-modified

13

0,068

66

0.21

0,25

18

0,152

57

0,28

0,36

23

0,175

51

0,39

0,42

2.

Modified with water

2.1. Preliminary dried ( 0.5 h; 80°C)

13

0,086

58

0,42

1,09

18

0,088

-

-

1,10

23

0.090

63

0,51

1,40

2.2. Treated with water -1 ( 4 wt% )

13

0,070

73

0,59

1,38

18

0.118

-

0,60

1,60

23

0.116

75

0,69

1,63

2.3. Treated with water -2 (6 wt% )

13

0,074

67

0,61

1,30

18

0.088

71

-

1,36

23

0,142

75

0,79

1,70

3.

Modified with sulphuric acid

13

0,064

67

0,80

1,50

18

0,104

-

1,00

6,80

23

0,106

62

1,38

7,00

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R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers

adhesion between the preliminary treated rubber substrate and the polymer
binder (

Figure 5).

CONCLUSIONS

The results show that the selected properties of the composite material are
highly dependent on both the rubber crumb treatment with water spraying
and surface modification with sulphuric acid.

The remarkable influence of rubber crumb treatment with sulphuric acid on
selected properties (Shore C hardness, compressive stress and modulus of
elasticity, peel strength) of the composite materials was observed; attributed
to the increase of rubber surface hydrophilicity and thereby improved adhesion
with the polymer binder.

It is postulated that the rubber treatment with water leads to the increase
of Shore C hardness and ultimate tensile strength of composite material,
due to the intensification of crosslinking and thus hardening process of the
polyurethane binder.

Therefore, the methods of the modification of rubber crumb, detailed in this
study, could lead to a meaningful improvement of mechanical properties and
production technology of composite material.

REFERENCES

1.

Mark J.E., Erman B., and Erich R., The Science and Technology of Rubber,3

d.

Ed., Elsvier Inc., USA, (2006) 763.

2.

Lund H.F., McGraw – Hill recycling handbook, R.R.Donelly & Sons Company,

USA, (1993) 18.2-18.35.

3.

Adhikari B. and Maiti S., Progress in Polymer Science,

25, (2000) 909-948.

4.

Malers L., Plesuma R., and Locmele. L., Mech. Comp. Mat.,

45, (2009) 1-6.

5.

Malers L., Plesuma R., and Locmele L., Scientific Journal of Riga Technical

University: Material Science and Applied Chemistry

,

21, (2010) 35-38.

6. Malers L., Plesuma R., Locmele. L., Megne A., and Mateusa-Krukle I.,

Polymer composites: Book of abstracts,

Zapodočeska Univerzita v Pilzen,

the Czech Republic, ( 2011) 135-136.

7.

Wicks Z.W., Jones F.N., and Pappas S.P., Organic Coatings: Science and

Technology, Volume I

, John Wiley & Sons Inc., USA, (1992) 189-191.

187

Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 3, 2013

Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder

8.

Malers L., Plesuma R., and Locmele L., Scientific Journal of Riga Technical

University: Material Science and Applied Chemistry

,

23, (2011) 103-106.

9. Cepeda-Jimenez C., Pastor-Blas M.M., Ferrandiz-Gomez T.P., Martin-

Martinez J.M., J. Adhes.,

73, (2000) 135-160.

10. Megne A., Mateusa-Krukle I., Plesuma R., and Malers L., Baltic Polymer

Symposium: Book of abstracts

, Tallin University of technology, Estonia,

(2011) 29.

11. Goncharuk G.P., Knunyants M.I., Kruchkov A.N., Obolonkova E.S., and

Vysokomolekularnye Soedineniya,

40, (1998) 877.

12. Colom X., Carrillo.F., and Canavate J., Composites,

38, (2007) 44-50.

13. Pat.US 6833396B2 (21.12.2004).

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