Acid anhydrides are commonly used as curing agents for epoxy resins. Whereas many liquid anhydrides are easily mixed with liquid epoxy resins and are thereby cured in thermoset applications, certain solid anhydrides, such as, for example, trimellitic anhydride (TMA) are more difficult to use as curing agents. Because TMA has a low solubility in most liquid epoxy resins and has a relatively high (165.degree. C.) melting point, its efficiency as a curing agent is dependent on its uniform distribution in the liquid resin formulation, generally followed by heating to effect dissolution and subsequent reaction. Finely divided TMA is especially useful in heat-curable compositions disclosed and claimed in U.S. Pat. No. 4,284,753, and known methods for obtaining a preferred particle size include ball milling and jet milling. The resulting finely divided material is however very difficult to handle because of the extreme dusting tendencies combined with the toxic effects of inhalation.
It was demonstrated that the dusting tendency can be greatly reduced if the preground TMA is post-wetted with up to about 10% by weight of liquid methyl hexahydrophthalic anhydride (MHHPA). The resulting curing agent still maintains good but slightly lower high temperature performance properties in a cured resin system like the composition disclosed in U.S. Pat. No. 4,284,753. In fact, when laboratory prepared samples were subjected to a quantitative dusting test as hereinafter more clearly described, the dusting level of the post-wetted sample was diminished by a factor of 100 compared with the non-wetted material. When this post-wetting technique was sealed up using various types of equipment such as Littleford mixer, Schugi agglomerator and ribbon blender, it was found that it was possible to produce a low dust level product at &lt;10% by weight loading of MHHPA on TMA while achieving improved high temperature cured resin properties. In some instances, however, there were occasional soft agglomerates indicating uneven wetting in the products prepared by this post-wetting route. If insufficient blending is achieved, the resulting agglomerates have, in some cases, resulted in curing agent settling during the resin system formulation operation resulting in an undesirable cured product.
It has now been further found that if the raw or neat TMA is first prewet, usually in flake form, with liquid MHHPA, and then this premixture is processed through a size reducing means such as a jet mill, the resulting epoxy curing agent is a non-agglomerated product exhibiting exceptionally low dusting properties.
Thus, a low dusting acid anhydride epoxy curing agent is prepared by (a) post-wetting a finely divided TMA powder with liquid MHHPA or (b) pre-wetting the TMA flakes with liquid MHHPA prior to reducing the particle size of the premixture. The most preferred process is the (b) process, i.e., the pre-wetting process.