Formulators of coatings, adhesives, sealants, and elastomers (CASE), as well as makers of foams and elastic fiber applications, utilize a diverse family of polyols to meet specifications for product performance. These polyols include polytetramethylene ether glycols (PTMEG), polypropylene glycols (PPG), adipate- and phthalate-based polyesters, polycaprolactone polyols, and polycarbonate polyols.
Performance Attributes of Polyols for CASE Applications
Strain-induced crystallization of the PTMEG soft segments, exact difunctionality and low acid values are all contributing factors to the superior mechanical properties of the associated polyurethane elastomers. These factors make PTMEG the material of choice for processors specializing in wheels, belts, tires, tubing, abrasion-resistant surfaces, and many other products.
Compared with polyester-type polyurethanes, the PPG-polyether polyols also exhibit excellent hydrolysis resistance and low temperature properties. However, when compared with the PTMEG and polyester polyols, the PPG polyols have inferior mechanical properties and are more prone to thermo-oxidative degradation.
In contrast to PPG polyols, polyester polyols possess better mechanical properties, such as tensile and tear strength and flex fatigue resistance. Polyester polyols are reaction products of dicarboxylic acids and diols, and polyester segments can be crystalline or amorphous. These polyesters are more resistant to oil, grease, solvents, and oxidation.
The polycaprolactone polyols exhibit lower melt viscosities, a narrower molecular weight distribution, and low acid values which improve their hydrolytic stability. The polycarbonate polyols are characterized by superior heat and moisture resistance vs. polyester polyols.
Performance with Corresponding Urethane Elastomers
Polyether and polyester polyols also exhibit a variety of performance characteristics with specific regard to polyurethanes, such as hydrolytic stability, chemical resistance, and more.
Hydrolytic Stability
Polyether-based polyurethanes exhibit excellent resistance to hydrolysis, even at higher temperatures. These polyurethanes are the preferred material for applications involving immersion in water or applications that require good property retention in warm and humid environments.
While polyesters offer higher initial tensile and tear resistance, they are susceptible to hydrolytic cleavage. In addition, the presence of residual esterification catalysts can accelerate the hydrolysis.
Polycaprolactone polyols and polycarbonate polyols are more hydrolytically stable than standard adipate and phthalate polyesters, due to their lower acid levels and a low propensity to generate acid moieties during hydrolysis.
Chemical Resistance
Polyester-based polyurethanes, particularly the semi-crystalline polyol-based polyurethanes, are more resistant to certain types of chemicals. Polyester-based polyurethanes will help your products resist exposure to oils, fuels, and hydrocarbon solvents.
If resistance to moisture and mild acids and bases is critical, polyether-based polyurethanes are a very good choice for your application.