The use of unsaturated polyester resins in the molding of reinforced products enjoys broad application in the manufacture of automotive, industrial, and home products. With the advent of gasoline shortages, the automotive industry has increased its efforts to replace metal parts with fiber-reinforced composites to reduce weight and gasoline consumption. In this respect, it has been an important objective to improve physical properties of the polyester composites.
To improve crack resistance of fiber-reinforced composites, polyester resins of the type used in SMC have been modified by addition of reactive liquid polymers, such as vinyl terminated butadiene-acrylonitrile. The fracture surface energy of the modified polyester resin was thus improved by better than 9 times at a loading of 10 parts of the liquid polymer per 100 parts of about 40/60 alkyd/styrene resin and better than 3 times for a 42/58 alkyd/styrene resin. Although the toughening effect of the liquid polymers was very attractive, the resulting modified polyester resins presented mixing problems due to the incompatiblity of the liquid polymers with the polyester resins.
Numerous attempts have been made to improve mechanical properties of filled polyester composites by admixing fillers, polyester resins, reactive liquid polymers, and other additives with the polyester resins, however, such attempts have been only marginally successful or totally unsuccessful due to inherent brittleness and incompatibility of the materials, or other problems.
U.S. Pat. No. 3,518,221 to Kenyon, et al., discloses a molding composition and a process for preparing same by coating reinforcing particles with a first thermosettable resin, and then embedding the coated particles in a second thermosettable matrix resin, the second matrix resin being more rigid than the first resin. The first and second resins can be same or different as long as the second resin is more rigid than the first resin. The resins can be selected from epoxy, polyester, phenolic, amide, imide, amine, or urethane resins. In a preferred embodiment, the resins are selected from epoxide resins such as the Epons and Araldites. A particularly suitable commercially available resin is Epon-815 having average molecular weight of about 330. Such compositions can be molded to form shaped articles having very good tensile strength and modulus.
U.S. Pat. No. 3,956,230 to Gaylord discloses preparation of a composition by coating hydroxyl-containing filler particles with a first thermoplastic polymer containing labile atoms in the presence of an ethylenically unsaturated carboxylic acid or anhydride coupling agent under conditions which will generate free radicals on the thermoplastic polymer. Free radicals are formed by using a catalyst, such as a peroxide. The acid is thereby coupled onto the thermoplastic polymer and is reacted with the hydroxyl groups on the filler particles by means of esterification and hydrogen bonding. The coated filler particles are then dispersed in a thermoplastic polymer matrix, which can be same or different as the first thermoplastic coating resin. Suitable thermoplastic resins, which are disclosed at bottom of col. 2 and in col. 3 of the Gaylord patent, include polyurethanes which may be obtained by reaction of a diisocyanate with a polyol such as polyethylene oxide, polypropylene oxide, polytetramethylene glycol, hydroxyl-terminated polyesters, hydroxyl-terminated polyisobutylene and hydroxyl-terminated polybutadiene.
As the Kenyon, et al. and Gaylord patents will verify, which patents are discussed above, the prior art is replete with disclosures of epoxy resins used as a coating on particulate filler particles which are thereafter mixed with a matrix resin to form molding and other compositions. Epoxy resins of lowest molecular weight, such as Epon.RTM. 815 and 828, are based on diglycidyl ether and bisphenol A and have number average molecular weight (Mn) of about 380 to 400 and heat distortion temperature (HDT) of about 65.degree. to 75.degree. C., see p. 66 of the text entitled "Epoxy Resins" by Lee and Neville, published by McGraw-Hill Book (1957). Higher molecular weight resins have higher HDTs. HDT depends to a degree on the curing agent used. For instance, diethylene tetramine curing agent will yield epoxy resins having low HDT whereas aromatic amines as curing agent will give epoxy resins having high HDT of about 150.degree. C. Generally, HDT and Tg of a material will be within 10.degree. C. of each other, with HDT being lower than Tg. Comparison of the hereinclaimed functionally terminated reactive liquid polymers with the lowest molecular weight epoxy reins shows that Tgs of the epoxy resins are at least 50.degree. C. higher than Tgs of the reactive liquid polymers. Of course, for higher molecular weight epoxy reins, the difference will be greater.