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.
Selecting the best polyol for a specific formula can be the difference between making a high-quality product or a low-performance offering. The key for proper material selection is a good understanding of the inherent characteristics of each polyol chemistry, a level of business intelligence that’s a hallmark of the Ganrade team. PTMEG is the premier polyol used in high-performance polyurethane elastomers. PTMEG-based polyurethanes are known for superior resistance to hydrolytic cleavage, good mechanical properties retention at low temperature, high resiliency, good processing characteristics and excellent mechanical and dynamic properties.
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.
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.
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.
Low Temperature and Thermal Performance
Polyethers have lower glass transition temperature (Tg) and better retain their flexibility and impact resistance at low temperatures. Polyesters, meanwhile, exhibit better thermo-oxidative stability and property retention at elevated temperatures.
Polyether-based polyurethanes generally exhibit higher rebound (resilience) compared to their polyester-based counterparts.
Dynamic and Mechanical Properties
In applications requiring products with higher tensile strength and cut and tear resistance, polyesters are the preferred polyols. Polyethers impart lower hysteresis or heat buildup, which makes them a preferred material for dynamic applications like wheels, casters and rollers.
Abrasion wear is most often a result of the combination of sliding and impingement abrasion. There are numerous abrasion tests designed to accurately predict a material’s service performance, as many different factors can impact abrasion performance of urethane elastomers. Selecting the right abrasion test, which will most closely correspond with the actual end-use application, can thus be quite challenging.
Polyether-based polyurethanes, due to their higher resilience, offer better performance in applications where impingement abrasion is the dominant form of wear. This performance dynamic is especially true for PTMEG based elastomers.
In a general sense, the higher tensile and tear resistance of the polyester-based polyurethane materials offer an advantage in applications where sliding is the dominant form of abrasion.
The environment in which the material is expected to perform must also be considered. A potential for hydrolysis on the surface of the ester-based polyurethanes, for example, will negatively impact their long-term abrasion resistance.
PTMEG polyols are exact difunctional primary diols exhibiting several key attributes for processing. PTMEG polyols exhibit a very low degree of acidity. Their melting points measure at below room temperature for low molecular weight grades, such as PTMEG 650. Higher molecular weight grades melt slightly above room temperature, and they exhibit lower viscosity because their molecular weight distribution is narrower. Furthermore, PTMEG polyols promote consistency in the production of polyurethanes.
PPG polyols are not exact difunctional and contain levels of mono-functionality. They also have secondary hydroxyl moieties which are lower in reactivity. Accordingly, their molecular weight distribution and viscosity is higher than PTMEG based polyurethanes, and the molecular weights achieved are generally lower.
Finally, polyester polyols can have high melting points and higher degrees of acidity, which affect catalyst reactivity. These polyols exhibit broad molecular weight distributions and viscosities.
Experience the Gantrade Difference
To understand which polyol is suited for your company’s unique needs, partner with Gantrade. Our team is committed to excellence in customer service, and with our wealth of technical knowledge and expertise, we can guide you to the best polyurethane solution for your products. Make your polyol supply more sustainable in availability and targeted to your exact specifications with Gantrade. Expect responsive people you can trust every step of the way. Contact Gantrade today to get started.