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Five different rubber compounds, in each of which were incorporated different antioxidants, were subjected to the following five conditions : (1) Dark storage, (2) weather exposure, (3) air at 70° C, (4) air at 90° C, and (5) oxygen at 300 pounds pressure per sq uare inch at 60° C . T ensile-strength t es ts were made on these compounds at suitable intervals. The p eriods of exposure ranged from a few days in the case of high-temperature t ests to seven or eight years under conditions of dark st orage. The results a re shown in graphs which consist of 10 sets of tensile-strength-time curves. Two means a re suggested for express ing the effectiveness of antioxidants numerically-a time index, which is a measure of the increase in time during which a rubber compound will r emain ser viceable , and a tensile index, which is a measure of the improvement in average tensile strength when compared with samples containing no added antioxidant. The effectiveness of antioxidants varied between wide limits, depe nding IIpon the type of compound, the particular ant ioxidant used, and the conditions of exposure . A comparison of the indexes obtained by diffe rent accelerated test s with those obtained under some conditions of 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 compounds employed__ ____ ___ ___  _ 668 I II. Conditions of t est ___ __ __ ___ ___ _  __ ____    ___  ___ ____ ___ _ 671 IV. Interpretation of results____ ____      __   671 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 compouJlo s _ _ _ _ ___  680 6. Behavior of antioxidants under different conditions of exposure ..  ___ ___  __   __ ____ ____   ___ 68] 7. Relative value of antioxidants___   __ ____ __  ___ 683 V. Summary .... _ .... _ .... ____ ..  .. _  __ ....  .... __ .. ____ ___ .... __ 684

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