Swelling of Nitrile Rubber Vulcanizates-Part 3: Factors Affecting Maximum Swelling

Abstract

As found from previous studies, a Gaussian curve describes the swelling, by various liquids, of a series of nitrile rubbers containing from 0 to 50% bound acrylonitrile. If only dispersion forces are involved, it is found that the value of maximum swelling is inversely proportional to the molar volume of the liquid causing the swelling. As a result, knowing the molar volume and the polarity index of a liquid, it is possible to draw curves relating the volume increase with the bound acrylonitrile level of the rubber. This relation also ap plies to associated liquids, such as acetone, methyl acetate, and alcohols, if the degree of association is known. For those liquids involved in acid-base interac tions with the rubbers, the swelling is the sum of two Gaussian equations. One equation relates to the swelling due to dispersion forces while the other gives the swelling due to acid-base interactions. Only the former equation can be pre dicted with any accuracy. Nitrile rubbers are involved not only in acid-base in teractions with acidic liquids, such as certain chlorinated hydrocarbons but also with basic liquids such as toluene, aniline, tetrahydrofuran, and cyclohex anone. Because nitrile rubbers interact with hard acids and soft bases, they are considered to be hard bases and soft acids.