Modification of low density polyethylene (LDPE) by graft copolymerization with some acrylic monomers

Graft copolymerization of low density polyethylene (PE) with acrylic acid (AA), ethyl acrylate (EA) and butyl acrylate (BA) using dicumyl peroxide (DCP) as the initiator was studied using differential scanning calorimetry (DSC) under the dynamic (non-isothermal) condition and in the shear mixer of a Brabender plasticorder at 170 degrees C (443 degrees K). DSC studies allowed measurements of the heat of reaction and the kinetics and energetics of the overall reaction during graft copolymerization. For each monomer system, the reaction was first order with respect to monomer concentration and the observed activation energy (150-170 kJ mole(-1)) followed the order AA > EA > BA. The grafting efficiency (%) was highest when using AA. The higher volatility of the EA monomer makes it less suitable for grafting on PE under a shearing action at 170 degrees C (443 degrees K). A low DCP dose (0.75 mmole per 100 g of PE) produced optimum conversion and grafting effects. Studies of melt rheology at 190 degrees C (463 degrees K) showed that 10-15% grafting of acrylic polymers on polyethylene resulted in a three-to five-fold increment in the melt viscosity of the polymer and substantial lowering in its melt flow index (MFI). Some cross-linking of PE during DCP-induced grafting unavoidably occured as a side reaction, particularly for grafting using the AA monomer and generally when using a high DCP dose ( > 1.0 mmole per 100 g of PE) for all the monomers. The nature and extent of changes in the mechanical properties of PE on acrylic grafting depended on the chemical nature of the monomer used and on the extent of grafting achieved. (C) 1997 Published by Elsevier Science Ltd.