SynopsisThe induction period for the autoxidation of polypropylene increases with increasing sample thickness (from 1 to 75 mils) and with decreasing oxygen pressure. The rate of oxidation shows the opposite dependences. In contrast, neither the rate nor the induction period in well stirred oxidations of squalane is oxygen pressuredependent. It is concluded that autoxidation of polypropylene is controlled by diffusion of oxygen into the sample. 1543

The rate constants for the autoxidation of polypropylene were determined by a combined ESR, volumetric, and chemical method. The values of ki, kp, and kt at 110°C. are 3 × 10−4 sec.−1, 1.9 l./mole‐sec., and 3 × 106 l./mole‐sec., respectively. The values of ki and its activation energy are the same as those for the decomposition of polypropylene hydroperoxide, thus identifying the latter as the principal initiation process. The values of the temperature‐independent kt suggest that secondary peroxy radicals are the terminating species. The rate constants are compared with rate constant ratios for initiated autoxidations of squalane and other related systems.

SynopsisThe autoxidation of polypropylene inhibited by 2,6-di-tertbutyl-pcresol (AH) and dilauryl thiodipropionate (S) was studied by the combined methods of electron spin resonance, oxygen absorption, and chemical analysis. With AH alone, there is a critical concentration of about 6 X 10-3 mole/l. below which there is no inhibition. This critical concentration agrees with that determined for inhibited squalane autoxidation and that calculated from known rate constants. Above the critical concentration there is a welldefined induction period duFing which the ROO. concentration is estimated to be lo-* mole/l. [ROOH] decreased rapidly as did [A.] and [AH]; the latter are kinetically related. The rate constant for the reaction between A. and ROO. is estimated to be 7 x lO71./molesec. at 130°C. At the end of the induction period, [ROOH], [ROO.], and --d[Oz] /dt increased rapidly until steady-state values were attained for all of them. With S alone, there are only retarded oxidation but no well-defined induction periods.[ROOH] is greatly reduced by S. I n all systems where the oxidation rates were appreciably suppressed there was formed a very stable paramagnetic species, S., which was inert toward AH and IZ but reactive toward triethyl phosphite. Because of its similarity with spin centers in carbon black, Sis postulated to be a delocalized polysulfide spin center. With both S and'AH p y e n t , the combined effect of stabilization is synergistic. The obsemed time-dependent variations of [ROOH], [ROO.], and [A.] follow familiar mechanisms. Mathematical relationships describing each of these three systems are included.

synopsisThree methods of measuring thermal oxidative stability of polypropylene have been compared. The oxygen absorption and the sealed tube tests give comparable results. The polymer lifetimes in circulating oven-aging tests ditrer significantly from those obtained by the first two methods. The oven-aging lifetime is apparently influenced to a large extent by the volatilization of the added stabilizer and of the oxidation products, the former decreases whereas the latter increasw the polymer lifetime. The optimum composition of a synergistic stabilizer system was found to vary with the test method.

No abstract