Polyurethanes are a special class of segmented block copolymers, consisting of alternating sequences of soft and hard segments. The soft segment is generally based on polyether or polyester polyols with glass transitions (Tg) well below room temperature, while the hard segment is composed of a diisocyanate and chain extender. The hard segment is often crystalline.
All polyol classes used in polyurethanes have a set of unique attributes that make them useful as the soft-segment in specific polyurethane elastomer applications. Selecting the best polyol for a specific formula can be the difference between making a high-quality product or one that’s low-performing. The key for proper material selection is a good understanding of the inherent characteristics of each polyol chemistry.
Polyurethanes based on polyester polyols, in general, exhibit higher tensile and tear properties, abrasion resistance, thermal stability, chemical resistance and weathering performance compared to polyether-based polyurethanes. They are preferred in the most demanding environments and applications. However, there is one weak point of polyester based polyurethanes. They are much more susceptible to hydrolysis compare with polyethers in polyurethanes. A part made of polyester based polyurethanes can gradually lose properties in a humid environment or water emersion.
2-Methyl-1,3-propanediol (MPO) based polyester polyols largely overcomes this deficiency due to the steric shielding of the ester linkage by the pendant methyl group on MPO, and greater hydrophobicity. MPOis an ideal diol intermediate for polyester polyols used in polyurethane elastomers (PURs). This is because MPOs unique structure enhances hydrolytic stability, affords liquid polyester polyols that are easier to handle, and demonstrates excellent compatibility in formulations.
The two major families of polyether polyols are polytetramethylene ether glycols (PTMEG) and polypropylene glycols (PPG). PTMEG is the premier polyol used in high-performance polyurethane elastomers. PTMEG-based polyurethanes exhibit superior resistance to hydrolytic cleavage, good mechanical property retention at low temperature, high resiliency, good processing characteristics, and excellent mechanical and dynamic properties. Strain-induced crystallization of the PTMEG soft segments, exact di-functionality, and low acid values are all contributing factors to the good mechanical properties of the associated polyurethane elastomers. PPG polyols have excellent hydrolysis resistance and low temperature properties as well. However, when compared to PTMEG polyols, the PPG polyols have lower mechanical properties and are more prone to thermo-oxidative degradation.
The rising demand for “green,” bio-sourced raw materials and intermediates originates from the concept of sustainability. In our industry, sustainability is about employing materials that are safe, efficient to use and derived from renewable resources.
Cast polyurethane elastomers based on 1,4-butanediol (BDO)-extended MDI systems using PTMEG as the soft segment are among the most useful polyurethane materials, offering cost and performance advantages. We see these elastomers used in a myriad of diverse cast urethane applications including wheels, heavy duty casters and tires for lift trucks, mowers; and all-terrain vehicles; industrial belts, wiper blades, and chute liners; pipe, hose, pump components, and tank liners used in abrasive service; mining, power, and oil-field equipment such as gaskets, valves, hydro cyclones, screens, and flotation parts; athletic shoes, boots and recreational equipment; and medical devices.
Using the broad range of unique polycaprolactone polyols available from Gantrade and Daicel Corporation, formulators can custom-tailor polyurethanes to achieve greater levels of performance. Polycaprolactone polyols in polyurethanes can enhance the capabilities of existing polyurethanes or enter new applications by displacing other material systems, such as rubber and epoxies.
We have written in this space about different performance polyols used to achieve the desired performance of elastomers in various applications. In addition to the disocianate type, polyols offer formulators an ability to achieve a variety of processing and performance advantages.
Caprolactone monomer is a reactive cyclic-lactone ester which is easily ring-opening polymerized to produce a wide variety of performance intermediates with advanced polymer architectures based on the polycaprolactone unit.
All polyol classes used in polyurethanes have a set of unique attributes that make them useful as the soft-segment in specific polyurethane elastomer applications.
