Blends of linear and peroxide-modified branched polylactide for extrusion coating

The purpose of the present work was to improve melt rheological properties of linear poly(L-lactide) (PLLA) by melt blending with peroxide-modified branched PLLA for extrusion coating. Peroxide-modified PLLA, ie, PLLA melt extruded with 0.3wt% of tert-Butyl-peroxybenzoate (TBPB); 2,5-dimethyl-2,5-(tert-butylperoxy)-hexane (Lupersol 101 [LOL1]); and benzoyl peroxide was added to linear PLLA in ratios of 5, 15, and 30wt%. All blends showed increased zero shear viscosity, elastic feature (storage modulus, decreased tan delta), and shear sensitivity. The blends' properties were mostly dependent on the peroxide applied for the modification and, to some extent, on the amount of added peroxide-modified PLLA. Rheological models suggest that all blend compositions are mainly immiscible. Thermal properties were unchanged; all materials remained amorphous, though the enthalpy of cold crystallization was slightly increased. Extrusion coating on paperboard was conducted with PLLA and peroxide-modified PLLA blends (90:10). All blends were processable; however, only that made with 2,5-dimethyl-2,5-(tert-butylperoxy)-hexane (LOL1) afforded a smooth high-quality coating surface with improved line speed. Adhesion levels between fibre and plastic and heat seal performance were marginally reduced compared with pure PLLA (3051D). The water vapour transmission measurements with 2,5-dimethyl-2,5-(tert-butylperoxy)-hexane (LOL1) showed acceptable water transmission levels, being only slightly higher than for neat PLLA coating.