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.