PPG based polyether polyols are made by reacting propylene oxide and/or ethylene oxide in the presence of a catalyst with an initiator which can be a diol, water, glycerin, TMP, sucrose or sorbitol. Catalysts often are a strong base like potassium hydroxide or double metal cyanide (DMC) complexes prepared by reacting ZnCl2 and K3[Co(CN)6]2 in the presence of complexing agents or multi-metal cyanide (MMC) catalyst prepared by reacting ZnCl2, K3[Co(CN)6]2 and K4Fe(CN)6) in the presence of complexing agents. When producing high molecular weight PPG-polyols, the DMC catalyzed products are specified since they result in much lower contents of unsaturated chain-ends (i.e. mono-ols). The unsaturated end-functionality with KOH catalyzed PPG-polyols results from the base catalyzed isomerization of propylene oxide to allyl alcohol.
PTMEG is produced from tetrahydrofuran (THF) through Ring-Opening Polymerizations using water as a proton donor and a terminator and heteropolyacids as a catalyst. Strong acids catalysts like fluorosulfonic acid and polymeric sulfonic acid catalysts are also used with water as the chain terminator.
What distinguishes the PPG based polyether polyols from PTMEG polyether polyols?
The two major families of polyether polyols are polypropylene glycols (PPG) and polytetramethylene ether glycols (PTMEG).
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 temperatures, high resiliency, good processing characteristics, and excellent mechanical and dynamic properties. Strain-induced crystallization of the PTMEG soft segments, exact bifunctionality, 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. They are lower in price, a key factor in cost driven polyurethane foam applications.
How are these differences between the two primary classes on polyether polyol explained?
The superior performance with PTMEG based polyurethane elastomers in CASE, TPU and fiber applications is attributed to strain-induced crystallization of the amorphous PTMEG soft-segment, which is not present with amorphous PPG-based polyurethanes. The linear polytetramethylene ether segments vs. the "branched" polypropylene ether segments increase the molecular alignment and intermolecular forces within the PTMEG soft segments. This provides a reinforcing effect in PTMEG based polyurethanes, but also facilitates microphase separation of the hard block.
In addition, PTMEG polyols are exactly bifunctional, whereas standard PPG polyols contain some unsaturated non-function end-group (i.e. less than 2.0 functionality). As a result, the PTMEG polyols achieve higher molecular weight in polyurethane. These features of the PTMEG polyols contribute to superior tensile and dynamic properties, hysteresis and resiliency, better cut, chip and abrasion resistance and elevated temperature compression set properties. With the use of DMC catalysts, significant advances have been made in improving the functionality of the polyol end-groups.