Physical chemistry of a phosphite processing stabilizer in polypropylene. Part 1: Solubility

The solubility of a commercial aryl phosphite processing stabilizer for pol yolefins, {tris (2,4-di-t-butylphenyl)-phosphite) (Irgafos 168, Ciba Specia lty Chemicals), has been measured in polypropylene over a range of temperat ures. Measurements are complicated by hydrolysis of the phosphite, releasin g 2,4-di-t-butylphenol, which is much more soluble in the polymer. We have measured the solubility of this phenol, and of the phosphate oxidation prod uct of the stabilizer. The results are interpreted on the basis of a regula r solution model. They show that the phosphite has a very low solubility bu t that it is not particularly anomalous in comparison with other stabilizer s of similar molecular weight and polarity. The phosphate is significantly more soluble than the phosphite at 70 degreesC, but their solubilities are comparable at 25 degreesC. This is mainly due to the lower heat of fusion a nd melting point of the phosphate, whose compatibility is actually lower th an that of the phosphite. The phenol is much more soluble because of its lo w melting point.