Though crude oil is rightly the ultimate source of many chemicals the adhesive industry uses, renewable materials can offer an alternative to aid economic and supply sustainability. To this end, how to produce polyol monomers from plants that can be used to make polyurethane adhesives has been broadly explored. Today, such monomers are commercially available, but their differences from conventional monomers can be an important consideration during formulation.
Figure 1: Plant oils, triglyceride esters of saturated and unsaturated acids, are the most common and cheapest sources of renewable polyols |
The most common and cheapest sources of renewable polyols are plant oils, which are triglyceride esters of saturated and unsaturated fatty acids.1 Castor oil in particular is a triglyceride of ricinoleic acid, a fatty acid that contains secondary hydroxyl groups that can be used produce polyols. Other natural oils, such as soybean oil, can be made into polyols by epoxidation at their unsaturated sites, for example. The resulting epoxy group can then be opened to produce secondary alcohols.
Figure 2: Reactions used to convert natural oils into polyols |
Polyol producers also catalytically hydroformylate vegetable oils to produce aldehyde functions that can be hydrogenated to yield primary alcohols.2 One industrial process has exploited this after first transesterifying soybean oil with methanol to yield fatty acids methyl esters (FAMEs). Finally, the manufacturers react these hydroxylated FAMEs with a polyol like propan-1,2,3-triol or a polyamine.
Isomerizing straight chain unsaturated fatty acids into branched chain forms, or oligomerising them, also makes polyols. The reaction is conventionally carried out using a clay catalyst and is generally performed at very high temperatures or in high cost ionic liquids.3 These acids are known as C36 dimer acid, but also often contain high amounts of trimer (C54) and higher fatty acids.
Conventional polyols are made by reacting a "starter" polyol with alkylene oxide to produce polyethers, or dicarboxylic acids or anhydrides to produce polyesters. An alternative natural oil polyol comes by replacing the conventional petrochemical starter polyol with one from a biological source. Options include saccharides like sorbitol or sucrose or propane-1,3-diol derived from glycerine. These can also be combined with adipic or succinic dicarboxylic acids made from renewable feedstocks.
The most straightforward way to formulate with renewable polyols would be direct replacement of petrochemical equivalents. Indeed, researchers developing novel polyols for adhesives frequently use them as a straight replacement in two-component polyurethane adhesives, where a polyol is mixed together with an isocyanate like toluene 2,4-diisocyanate by the user.4
Possibly the most valuable formulations exploit polyester polyols with tailored hydroxyl groups to enable one-component moisture-curing reactive hot melt polyurethane adhesives (HMPURs).5 Formulators have made moisture-reactive hot-melt adhesives for some time that incorporate castor oil into a polyurethane prepolymer-forming reaction mixture containing polyol and polyisocyanate.6 Castor oil is thought to provide crosslinking to the composition, and can provide adhesives that achieve good properties. However, polyhydroxylated soybean oil and hydrogenated castor oil have also been used in order to adjust formulations to achieve a specific balance of properties.7
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