When designing a polyurethane elastomer, selecting the best polyol for a specific application can be the difference between making a high-quality product or a low performance one. A good understanding of the inherent characteristics of each polyol chemistry and the key properties and environmental exposures for a given application are keys for making a proper material selection.
The selection criterial includes economics and the performance attributes required. In this article, we are bringing it together by addressing Frequently Asked Questions (FAQs) regarding selection criteria for each polyether polyol family. A companion FAQs does the same for the families of polyester-based polyol.
Polyether polyols account for about 80 % of the polyol types used in polyurethane, polyurea and polyisocyanurate applications. Polyurethane foam products, Spandex fibers and CASE (Coatings, Adhesives, Sealants, and Elastomers) applications all utilize a diverse family of polyether polyols. The largest polyether polyol market is in flexible foams, where polypropylene glycol based polyols are dominant. PTMEG is the main soft segments of Spandex polyurea elastic fiber, cast thermoset polyurethane elastomers, thermoplastic polyurethane elastomers (TPUs), thermoplastic polyester and polyamide elastomers (TPEEs & TPEAs).
The polyether polyol market segment was valued at US$ 2.3 billion globally in 2019 with a CAGR of about 6%. Classifications of polyether polyols are as follows:
What are key attributes of polyurethanes based on polyether polyols?
What are the end-use market shares for polyether polyols by application area?
The major market for polyether polyols is in foam products, with flexible foam representing the dominant area. Automotive cushioning and furniture are the major drivers of PPG based polyether polyol volumes. Automotive coatings are another significant market area. For PTMEG polyols, spandex elastic fibers represent about 85% of total demand for this family of polyols.
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.
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.
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.
PPG-repeat unit PTMEG-repeat unit
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.
The reactivity of PTMEG towards MDI is faster by a factor of about 10-fold vs. the reactivity of PPG-type polyether polyols. This is due to the more reactive primary hydroxyl chain-ends in PTMEG vs. a preponderance of secondary hydroxyl chain ends in a PPG-polyols. In PPG polyols, both secondary and primary hydroxy chain-ends are present, in a ratio of about 90:10. Using DMC (Double Metal Catalysts) catalysis for PPG-polyol production increases the content of primary hydroxy chain end-groups. In applications where fast curing times are desirable, PTMEG (and also ethylene oxide end-capped PPG polyols) are preferred. The secondary hydroxyl moieties do increase hydrophobicity, so balancing hydrophobicity and reactivity may be important.
PPG-polyols are available in higher molecular weights that PTMEG and can be higher functional using a triol and tetraol initiators.
The PPG-polyols contain one tertiary hydrogen atom adjacent to an ether or alcohol linkage on each repeat unit. These tertiary hydrogen atoms are very prone to oxidative degradation and hence limit the thermo-oxidative stability of PPG-based polyurethane elastomers. The PTMEG polyol backbone only possess the more stable secondary hydrogen atoms, with some adjacent to an ether or hydroxyl linkage.
The PTMEG polyol grade slate consists of diol with molecular weight ranges of 210, 250, 650, 1000, 1400, 1800, 2000 and 3000. The PTMEG grades 210, 250 and 650 are liquids at room temperature. If crosslinking is desired, triols like TMP or low molecular weight PPG-triols are recommended.
The PPG-based polyols have a rich diversity of product grades. Molecular weights range from, 400 or lower to above 5000. Functionalities range from diol, triols and tetraols to above seven. Sucrose initiated polyols can have a functionality of greater than seven. The increase crosslinking of the higher functionality polyols provides for enhanced stiffness and hardness, and improved chemical resistance and thermal resistance. When the PPG based polyols are end-capped with ethylene oxide, the resulting end-functionalities are the more reactive primary hydroxyls.
For more information on polyether polyols and critical selection criteria, please contact Gantrade today .