This invention relates to a process for the preparation of a polyether polyol that has a reduced unsaturation content.
Polyether polyols suitable for use in preparing polyurethanes are usually prepared by the reacting in the presence of a basic alkoxylation catalyst an initiator compound having active hydrogen atoms with an alkylene oxide such as, for example, ethylene oxide or especially propylene oxide. Typical catalysts include tertiary amines, or hydroxide or alkoxide of sodium and potassium. However, under the overall conditions of the reaction including catalyst concentration, elevated temperature and pressure, the propylene oxide may isomerize and rearrange to give an allyl alkoxide before it can react with the initiator. The ability to prepare high equivalent weight polyether polyols in the presence of the allyl alkoxide is reduced as the allyl alkoxide competes in reaction for the remaining alkylene oxide, thereby limiting molecular weight build up on the initiator. The allyl adduct is commonly described as the unsaturation content of a polyether polyol.
When a high equivalent weight polyether polyol having a high unsaturation content is used in the preparation of, for example, a flexible polyurethane foam, a product with undesirable properties can result. Problems such as foam discoloration, inferior compressive and tensile strengths, low reactivity, low flexural modulus and poor humid aging may be observed. It would therefore be desirable to provide a means for preparing a polyether polyol having a reduced unsaturation content, permitting the manufacture of improved polyurethane polymers.
It is known from the art that the unsaturation content of a polyether polyol may be reduced by a subsequent acid treatment such as discussed by Dege et al., Journal of the American Chemical Society, p. 3374, Vol. 81 (1959), or as disclosed in U.S. Pat. Nos. 2,996,550 and 3,271,462. However, such acid treatment of polyether polyols is not always practical, adding to production costs and generating waste products which must be subsequently removed from the polyols and discarded. Accordingly, it would be more desirable to provide a means of preparing a polyether polyol in which the accompanying buildup of unsaturation during its manufacture is avoided or minimized.
The art contains various disclosures which teach the selection of certain substances functioning as alkoxylation catalyst, when preparing polyether polyols, and which additionally minimize formation of unsaturated substances. U.S. Pat. Nos. 3,393,243 discloses the use of cesium hydroxide for the preparation of polyoxypropylene polyether polyols having an equivalent weight of from 1500 to 2500 and reduced levels of unsaturation. U.S. Pat. No. 5,010,187 discloses the use of barium- or strontium-containing substances for a similar purpose. In an improvement of this latter disclosure, U.S. Pat. No. 5,114,619 discloses the controlled concomitant addition of limited amounts of water during addition of the alkylene oxide to provide for a further reduction in the unsaturation content of polyether polyols prepared in the presence of such barium- or strontium-containing catalyst substances. It is also reported that substitution of water by low molecular weight diols or triols does not provide for a reduction in the unsaturation content of the resulting polyether polyol. While studying further this latter improved process the present inventors have confirmed the value of water when preparing a polyether triol having a low unsaturation content, but are unable to observe a similar reduction of unsaturation content when preparing a polyether diol. Further investigation of this observation has led to the present discovery.