Preparation of Seed Oil-Derived Polyols
Epoxidation and Ring Opening There are numerous ways to create functionality on the seed oil internal olefin bonds for use in polyurethane chemistry. One of the simplest methods is to ring open epoxidized seed oil with ring-opening reagents such as methanol and HC1 to produce hydroxy functional esters. Epoxidized soybean oil and linseed oil have been converted this way and tested in rigid foam applications, but performance issues have generally limited their application . A related but more complicated method involves alkoxylation of ring-opened epoxidized seed oil. The epoxides are ring opened with a diol such as ethylene glycol and subsequently alkoxylated to the desired structure (Fig. 2.41). These structures have found use by virtue of the reactivity provided by the primary hydroxyl product .
Ozonolysis is a seed oil modification technique that opens avenues to numerous polymerization building blocks. Ozonolysis of the fatty acid double bonds creates ozonide functionalities that can be subsequently reacted to diacids, acid aldehydes, terminal acids, and terminal aldehydes . The difunctional materials can then be modified for convenient polymerization for polyurethanes. While attractive for its versatility and seeming simplicity, the number of steps necessary to get to useable polymerization feedstocks can be a roadblock to economic feasibility. Figure 2.42 shows the pathways available to polymerization building blocks from ozonolysis of oleic acid .
FIGURE 2.41 Oxidation of a triglyceride to the epoxidized form followed by alcoholysis to the ester alcohol of the triglyceride. The alcohols can react subsequently with isocyanates to form urethanes.
Hydroformylation and Reduction
Hydroformylation of seed oil unsaturation forms an aldehyde at each olefin group, which can then be reduced to form a primary hydroxy methyl group in place of each aldehyde. This technology has the benefits of quantitatively producing a single primary hydroxyl group for every olefin resulting in polyester polyol building blocks with uniform reactivity . The hydroformylation of the olefin to the aldehyde is accomplished with very expensive homogeneous rhodium catalysts that must be recovered by quantitative extraction, or the product can quickly become very expensive to produce. Reduction of the aldehyde can be accomplished by any number of well-known industrial reduction procedures. When this procedure is applied to the triglyceride, the equivalent weight of the polyol is on the order of 300g/OH group. This equivalent weight is useable in rigid foam formulations but might require formulation with
FIGURE 2.42 Simplified ozonolysis of oleic acid. Other products are also possible .
even lower equivalent weight polyols to provide the necessary cross-link density to result in rapid gelation, high tensile strength, and low gas transmission necessary for most rigid foam applications.
It is also possible to separate the fatty acids from the triglyceride to produce the fatty acid or the methyl ester. Hydroformylation and reduction then result in an A-B monomer having ester and hydroxyl functionality on a single monomer (Fig. 2.43) .
Using a low-molecular-weight polyol as a polymerization starter or "initiator," it is thus possible to obtain a polyester polyol of nearly any molecular weight or functionality typical for flexible applications. Separation of the different hydroformylation and reduction products resulting in diol and triol esters, for instance, can allow for each product to find its highest value application.
Along with the process requirements associated with catalyst recovery, several issues can pose as obstacles to this elegant procedure. One problem is that, even with substantial optimization, the overall cost can be prohibitive due to the number of steps necessary to get from seed oil to final building blocks including separations and recycle. Another potential problem is related to the saturates content found in most seed oils, the presence of which reduces overall asset utilization by occupying volume in the chemical process but on which no modification occurs . The stearate and palmitate saturates furthermore serve no useful role in polyurethane chemistry and therefore will either degrade performance or require finding an alternative commercial outlet for their production .
FIGURE 2.43 Procedure for formation of polymerizable ester alcohols from seed oil triglycerides by a hydroformylation and reduction procedure. From Ref. . © Elsevier.
FIGURE 2.44 Illustrative procedure of olefin metathesis reactions and metathesis applied to the etheneolysis of oleic acid.
Metathesis is an organic reaction that enables the redistribution of olefin bonds through a scission and recombination mechanism. In the context of seed oil modification for the purpose of creating urethane capable feedstocks, this modification creates useable products for urethane and polyolefin applications [102,103]. A pictorial image of the action of cross-metathesis exchange envisioned useful in this context is shown in Figure 2.44.
The product ester olefin can be then be readily functionalized to a terminal ester alcohol A-B monomer for polymerization by hydroformylation and reduction as described in Section "Hydroformylation and Reduction". The alpha olefin shown can be directly incorporated into polyolefin polymerizations.
New catalysts for metathesis have improved their compatibility with modern industrial processes. The commercialization of these catalysts has resulted in products from metathesis becoming available commercially . Obstacles to fully realizing a large-scale application of this technology are similar to those of the hydroformylation and reduction technology described in Section "Hydroformylation and Reduction" associated with separations and the cost of chemical production requiring large numbers of unit operations.