Tipped polyols

Products intermediate to the flexible and rigid foams may be obtained from castor oil (a trihydroxyl molecule), synthetic triols of moderate molecular weight and polyesters with a moderate amount of trifunctional hydroxyl compound in the strueture. Current practice, however, is to use tipped polyols of the type used for flexible foams with MDI. Semi-rigid foams are used for such purposes as crash pads, car steering wheels and packaging equipment. 

The end hydroxy groups of the polymer are secondary groups and are ordinarily rather unreactive in the urethane reaction. Initially, this limitation was overcome by the preparation of isocyanate-terminated prepolymer and by the use of block copolymers with ethylene oxide. The latter products are known as tipped polyols and are terminated with primary hydroxy groups of enhanced activity. 

Table 3.2 Application of reactivity method to standard ethylene oxide tipped polyols ...

Table 7.32 - Application of Reactivity Method to Standard Ethylene Oxide Tipped Polyols... 

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

Materials. 1,4-Butanediol was vacuum dried (60°C., 2mm Hg) for six hours and then stored over molecular sieves. The PPG polyols used were PPG 1025 and PPG 2025 from Union Carbide Corporation. Tipped PPG polyols were supplied through the courtesy of F. J. Preston of the Olin Corporation Research Center, New Haven, Connecticut. Polyols were vacuum dried (100°C., 2 mm. Hg) for one hour immediately prior to use. The MDI, Isonate 125 MF from Upjohn Company, was stored at 50°C. and decanted prior to use so that only water white, clear material was used. The BHEHQ was from Eastman Chemical Products. 

Effect of Oxyethylene Content on Stability. The processing instability of TPU elastomers based on PPG 2000 can be overcome by the use of ethylene oxide "tipped" PPG polyols, especially those containing high levels of oxyethylene groups, e.g., 30% to 45% by weight. The improved stability of the polymers based on such polyols is demonstrated in Table 2. 

Figure 6. Effect of NCO/OH ratio on hardness build-up for polymer made from tipped PPG polyol (45% EO) at NCO/OH ratios of 1.01 (0), 1.03 (%), 1.05...

Figure 8. DSC cooling curves of TPU elastomers (based on 2000 M.W. tipped PPG polyol (45% EO) varying in NCO/OH ratio).

Chem. Descrip. Dipropylene glycoMnitiated ethylene oxide-tipped diol Uses Polyol in flexible urethane applies, ind. caulks, sealants, coatings, encapsulants, elastomers, adhesives, and flexible foams Properties Colorless dear liq. m.w. 3740 vise. 700-850 cps hyd. no. 30 ... 

Since the TDI and MDI automotive polyols are highly ethylene oxide (EO) tipped the reactive hydroxyl gronp can be represented by a primary alcohol. The electronic effect of... 

Since the Voranol 3010 polyol is fully PO tipped, the reactive hydroxyl group can be represented by a secondary alcohol. The unreactive polyether backbone of the polyol can be represented by the dimethyl ethers of dipropylene glycol and diethylene glycol. The diglyme represents the wt% EO incorporation into the polyol. Note the sum of the masses of the model alcohol and the two dimethyl ethers equals 100 grams, the total polyol mass. This ensures that the concentration of reactive groups is comparable to that in the actual foam. Water and catalysts are the same for both formulation and model. 

These are an entirely new class of polyol intermediates produced by Montedison under the trade name Fomblin Z series. A typical hydroxy end-tipped perfluoropolyether would have the structure shown below ... 

When terminal primary hydroxyl groups are desired ethylene oxide may be used either as the sole alkylene oxide with the polyol initiator, or it may be employed to "cap" or "tip" the oxypropylated polyol with ethylene oxide as shown in the following scheme ... 

Reaction Injection Moulding.— By contrast with the epoxy resins, polyurethanes cure at much lower temperatures, 40 °C upwards. The reactants are usually polyether or polyester triols and diols (polyols) and isocyanates or isocyanate-tipped polyester piepolymers. The process problems are essentially those of mixing the reactants sufficiently rapidly to achieve composition uniformity and to control the temperature in the mould. Where foams are produced, control of bubble nucleation is an additional complicating factor e.g. Menges and Schwesig ). 

In the stable prepolymer systems the isocyanate used to tip the polyol is more stable on storage in dry cool conditions. In addition a trace of an acid or acid chloride may be added to neutralize trace catalytic impurities that might promote undesired cross-linking reactions on storage. Such systems are offered by several suppliers, with polyesters used in some formulations and polyethers in others. Toly-... 

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