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How water and working environment affect polyurethane?

Views: 175 Update date: Jun 10,2019

water resistance polyurethaneHow water and working environment affect polyurethane?

Water has two effects on polyurethane elastomers. One is the plasticization of water, that is, water molecules enter into macromolecular chains and form hydrogen bonds with polar groups in polymer molecules, which weakens the interaction between polymer molecules, and decreases the tensile strength, tear strength and wear resistance. The process is reversible and the original performance can be restored by drying and dehydration. It is reported that when the relative humidity of air changes within 100%, the water absorption of polyurethane elastomer changes within 2%. When the relative humidity is 50%, the water absorption of polyester and polyether polyurethane elastomers is about 0.6%. When the relative humidity was 100%, the water absorption of polyester and polyether increased to 1.1% and 1.4% respectively. At this time, the corresponding reduction rate of tensile strength is about 10% for the former and 20% for the latter. It was found that the water absorption of TDI/MOCA polyurethane elastomer and MDI/diol polyurethane elastomer were almost the same. The second is the degradation of water, that is, the elastomer undergoes chemical degradation.

The influence of external factors on hydrolysis cannot be ignored. Both acids and bases are enhancers of hydrolysis. The hydrolysis of esters produces acids, which have catalytic hydrolysis. Metal organic compounds, such as tin salts, catalytic hydrolysis is not inferior to amines. At the same time, the combination of tin salts and amines has synergistic effect on hydrolysis. Therefore, residual acids and bases should be avoided as far as possible in raw materials, and the amount of catalyst should be reduced. The carbodiimide (-N=C=N) structure was introduced into polyester polyurethane system to react with residual carboxylic acid and improve hydrolysis resistance. The appropriate dosage of carbodiimide is 1%-5%, and its effect decreases with the increase of water temperature. It is reported that for polyester polyurethane elastomer, adding carbodiimide in boiling water can improve the hydrolysis performance about 2 times, in 70 degree C water can improve about 3 times, in 40 degree C water about 4 times. However, polyether polyurethane has no effect. In addition, the water resistance of polyurethane elastomer crosslinked with biuret and urea formate is similar to that of carbodiimide by adding 3%-5% titanium dioxide. However, the effect of titanium dioxide is obvious only in boiling water, but not in 80 degree C water, and even less in 70 degree C water.

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