CASE (Coatings, Adhesives, Sealants, and Elastomers), foam and elastic fiber applications all utilize a diverse family of polyols that includes:
- Polytetramethylene ether glycols (PTMEG)
- Polypropylene glycols (PPG)
- Adipate- and phthalate-based polyesters
- Polycaprolactone polyols
- Polycarbonate polyols
Selecting the best polyol for specific formula can be the difference between making a high quality product or a low performance one. The key for making a proper material selection is a good understanding of the inherent characteristics of each polyol chemistry.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.
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. This makes PTMEG 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, 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. Polyester segments can be crystalline or amorphous. The 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.
Following is a summary of the attributes of the main classes of polyols to the corresponding urethane elastomers:
Polyether-based polyurethanes exhibit excellent resistance to hydrolysis, even at higher temperatures. They are the preferred material for applications involving immersion in water or applications that require good property retention in warm and humid environments.
Polyesters offer higher initial tensile and tear resistance, but 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:
- Hydrocarbon solvents
If resistance to moisture and mild acids and bases is critical, polyether-based polyurethanes are a very good choice.
Low Temperature and Thermal Performance
Polyethers have lower glass transition temperature (Tg) and better retain flexibility and impact resistance at low temperatures.
Polyesters exhibit better thermo-oxidative stability and property retention at elevated temperatures.
Polyether-based polyurethanes generally exhibit higher rebound (resilience) compared to polyester-based polyurethanes.
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 build-up, 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 be challenging.
Polyether-based polyurethanes, due to their higher resilience, offer better performance in applications where impingement abrasion is the dominant form of wear. This is especially true for PTMEG based elastomers.
Although very generalized, it can be said that 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 will negatively impact their long-term abrasion resistance.
PTMEG polyols are exact difunctional primary diols exhibiting:
- A very low degree of acidity.
- Melting points below room temperature for low molecular weight grades, such as PTMEG 650.
- Higher MW grades melt slightly above room temperature.
- Lower viscosity because the molecular weight distribution is narrower.
- 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.
Polyester polyols can have high melting points and higher degrees of acidity, which affect catalyst reactivity. They exhibit broad molecular weight distributions and viscosities.
To understand which polyol is suited for your company’s unique needs, partner with Gantrade. Our wealth of technical knowledge and expertise can guide you to the best polyurethane solution for your products