Thereto-oxidative degradation of linear low density poly(ethylene) in the presence of carbon black: a kinetic approach

The mechanistim of carbon black (CB) effects on the thereto-oxidative degradation of linear low density polyethylene (LLDPE) was studied. Quantitative measurement, in both the kinetic and diffusion mode, of the kinetics of LLDPE's thereto-oxidative degradation was done in four ways as follows: (i) in the absence of both CB and a stabilizer; (ii) in the absence of a stabilizer but in the presence of CB (Black Pearl 3700) (2), (iii) without CB but in the presence of an amine stabilizer (AI) (iv) with both CB and AI. The stabilizer chosen for this study was polymerized 1,2-dihydro-2,2,4-trimethylquinoline (Trademark: Naugard Super Q). Measurements were done at 180 degreesC, the AI concentration being (1.1-9.8) x 10(-2) mol/kg, based upon a molecular mass of 0.874 kg/mol for the monomer unit. CB concentration was 5% by weight while oxygen pressure pO(2) = (50-300) mm Hg. Quantitative parameters for the thereto-oxidative degradation of LLDPE were established for kinetic and diffusion conditions. The kinetics of the inhibited thereto-oxidative degradation of LLDPE, in the presence of an amine antioxidant such as Naugard Super Q, was found to be essentially no different than the degradation kinetics in the presence of hindered phenolic antioxidants. It was shown that CB may act as an inhibitor albeit a rather weak one. However, during inhibition of the thereto-oxidative degradation of LLDPE with AI, CB shows itself to be an effective synergist, especially over the AI concentration range of from (2 to 6) x 10(-2) mol/kg (0.4-1.2% by weight). The most probable explanation for this synergy lies in the adsorption of stabilizer radical In-. onto the surface of the CB particles and a resultant decrease in the value of the rate constant k(10) of the chain transfer reaction between a polymer molecule RH and Inhibitor radical In-.. (C) 2001 Elsevier Science Ltd. All rights reserved.