Synthesis and molecular weight characterization of rubber seed oil-modified alkyd resins

Alkyd resins of 40% (I), 50% (II), and 60% (III) oil length (OL) were prepared with rubber seed oil (RSO), phthalic anhydride (PA), and glycerol (GLY), employing the two-stage alcoholysis method. Changes in the physical characteristics of the reaction medium were monitored by determination of the acid value and the number-average molecular weight, Mn(overbar), of in-process samples withdrawn at different stages of the reaction. The mode of variation of these properties denotes that the preparation of RSO alkyds is complex. Molecular weight averages and the molecular weight distribution (MWD) of the finished alkyds were determined by GPC, cryoscopy, and end-group analysis. Molecular weight averages and the MWD vary with differences in the formulation, with sample II exhibiting the narrowest size distribution. Values of Mn(overbar) with the corresponding polydispersities in brackets are 3234 (1.91), 1379 (1.56), and 3304 (2.56) for samples I, II, and III respectively. Mn(overbar) values obtained by cryoscopy are comparable to those obtained by gel permeation chromatography (GPC), while end group analysis seems to grossly overestimate their molecular weights. Correlation of Mn(overbar) and the MWD with the quality of the finished alkyds shows that the narrower the size distribution the better the quality of the alkyd. Properties such as the rate of drying and resistance of the alkyds are optimum at 50% OL.