CCI2014050735

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Technical and Practical Aspects of Lyophilizatian

Only at its "triple point" can water exist simultaneously in its three physical states. For each temperaturę below 0°C, a corresponding vapor pressure at eąuilibrium exists over the ice. When at a given temperaturę of the object, the corresponding vapor pressure at eąuilibrium cannot be reached in the lyophilization chamber, one risks precipi-tation of ice on the object or melting of the solid phase. For lyophilization to be both efficient and economic, the pressure should be as high as possible, provinding as high as possible a temperaturę for fast lyophilization; but caution should be employed never to let the temperaturę reach parameters high enough to permit melting. The temperaturę of the object to be lyophilized should also be sufficiently Iow to prevent melting during transfer manipulations and the evacuation of the lyophilization chamber. A large dewar (Minnesota Valley Engineering, New Prague, Minnesota, USA; model TA90, 94 x 46cm) with liąuid air on the bottom provides an excellent prefreezing vessel wherein the temperaturę can be chosen by varying the location of the object above the liąuid air. Temperatures around -80°C can easily be reached and main-tained for morę than 12 hours. We used a bench top freeze dryer with 31 ice capacity and a cooling trap at -50°C provided with a stainless Steel chamber type 401 (The Virtis Canpany, Gardiner, New York, USA). The pump has a capacity of 1801/min (Alcatel, Montrouge, France); this high capacity considerably reduces the time to reach high vacuum.

Consolidation with Folyethyleneglyools, Followed by Lyophilization

A Literaturę review

Caniaris et al. (1 ) do not give the concentration of PEG400 used for impregnation. Ihe impregnated leather was frożen at -20°C and lyophilized under high yacuum. They report good results, although 12% shrinkage is mentioned. In their comparative investigation with a.o. glycerin impregnation, they noticed that under their prefreezing conditions, 30% glycerin in water was a liąuid. Rosenąuist (3) used 10-15% PEG400 in water, then froze the pieces for 20 hours (no temperaturę is specified) and "ąuickly" lyophilized them.

No dimensional changes are reported.

Morris and Seifert (2) used PBG1500 at a 10% concentration. They report that freezing occured at -18°C. Freeze-drying was performed over a long period, whereby the vacuum was stepwise altered from 2 to O.ITorr over three days, where-after the vacuum was left to reach 5mTorr. The freeze-dried leather was stiff but its flexibility was restored through application of the British Museum Leather Dressing.

Elmer (4) reports that, when properly administered, the lyophilization of PEG impregnated leather should not entail any contraction. He used 8% PEG400 and 5% Luviskol K30. Prefreezing was done at -30°C. In linę with his emphasis on keeping the impregnation medium frozen during lyophilization, he performed the process in a cooled vacuum chamber. Peacock (9) used glycerin/PEG400 in ethanol at 10% each. The impregnated samples were not lyophilized, but were removed to an ethanol enriched atmosphere. Shrinkage was about 5%; the moisture content 10% (50% relative humidity, 23°C) and the visual and mechanical aspects are reported to be very good.

Scme Chemical and Physical Aspects of Polyethyleneglyools

For a given polymerization grade, a typical spreading of 10% in molecular weight is always obtained (Huls Chemie: Polydiols, Polywaxes; information leaflet).

Due to the presence of hydroxylgroups, PEG polymers are very soluble in water. The polarized ether linkages may contribute to their hygroscopicity.

In the presence of concentrated minerał acids, the PEG polymer may be hydrolyzed at the ether linkages. This leads to loss of the polymer from the treated object. Precautions should be taken with respect to the environment in which the impregnated leather will be kept. A poiluted atmosphere can promote formation of sulfuric acid in leather. Solubilization of any PEG in water will cause the boiling point of the mixture to rise and the freezing point to be lowered. Since water is removed from the object during lyophilization, the PEG concentration increases. This im-plies a continuous lcwering of the solidification point of the mixture. A lcwer temperaturę must accompany the mixture so that it may keep its solid form. The mixture will final-ly reach its eutetic temperaturę (Et)-the lcwest melting tanperature that can be reached for a given solute.

To prevent melting, the temperaturę of the mixture should