Effect of antioxidants on the natural and the accelerated aging of rubber

Abstract

Five different rubber compounds, in each of which were incorporated differe nt antioxidants, were subjected to the following five conditions : (1) Dark storage, (2) weather exposure, (3) air at 70° C, (4) a ir at 90° C, and (5) oxygen at 300 pounds press ure per sq uare inch at 60° C. T ensile-strength t es ts were made on these compounds at s uitable intervals. The p eriods of exposure ranged from a few days in the case o f high-temperature t ests to seven or eig ht years und er conditions of dark st orage. The results a re shown in graphs whi ch consist of 10 sets of tensile-strength-time curves. Two means a re suggest e d for express ing the effectiveness of antioxidants numericall y-a time index, which is a measure of the increase in time during which a rubber compo und will r emain ser v iceable , and a tensile index, which is a measure of the improvement in av e rage tensile strength when compared with samples containing no added antioxidant. The effectiveness of antioxidants varied between wide limits, de pe nding IIpon the type of compound, the particular ant ioxidant used, and the conditions of exposure. A comparison of the indexes obtained b y diffe rent accelerated test s with those obtained unde r some conditions o f natural ag ing affords a m eans for choosing an aging test best s uited to a particular rubber product. CONTENTS Page I. In trod uction. ____ _____ .. ___ .. __ _ _ ____ __ ___ _ __ ______ __ _ _ ___ ___ ______ _____ _______ __ ____ _ 667 II. Antioxidants and rubber com pounds employed__ ____ ___ ___ _________________ _ 668 I II. Conditions of t est _____ __ ___ ___ _ _____ __ ____ _________ ________ _____ ___ _____ ___ ____ ___ _ 671 IV. Interpretation of results____ ____ _____ _____ _____ _____________ ______ __ ________ ________ 6 71 1. Tensile index___ ____________ __ __ ___ __ _______________________ ____ ______ __ ____ _ 673 2. Time index_________________ _____ ______ __ __________ _____ __________ __________ _ 674 3. Computation of tensile and time indexes_ ______ __ _________________ 676 4. General effect of antioxidants_____________ ______ _____ __ ________ _______ _ 680 5. Behavior of antioxidants in different compo uJlo s _ _ _ _ ___ _______ 680 6. Behavior of antioxidants under different conditions of exposure .. _____ ___ ___ ________________ __ _________ _______ ________ ______ ________ ___ 68] 7. R elative value of antioxidants___________ _________ __ ______ __________ ___