Melt strength enhancing additives for reactive extrusion of polylactide – a comparative study

Polylactide (PLA) has a low inherent melt strength which is an obstacle to its use in film and blow-molding applications. This can be addressed through the use of reactive chain extension, but commercially available additives for this process vary widely in reactivity and chemical composition. This work compares the melt strength of polylactide modified with 11 different commercial additives marketed as chain extenders or viscosity enhancers for polyesters. To compare the effectiveness of these additives in enhancing PLA melt strength, blends were produced via both batch mixing and continuous extrusion using temperatures of 170-230 °C, processing times of 3-15 min and additive concentrations of 2-10 %. The melt strength of produced blends was evaluated using Rheotens draw-down measurement on a continuously extruded melt strand and changes to melt strength from reactive extrusion were determined through comparison with unmodified PLA processed under identical conditions. The effectiveness of melt strength enhancing additives was found to be dependent on functional group type and concentration within the additive. Maleic anhydride functionalized additives were shown to be completely ineffective. Epoxy-modified acrylates were demonstrated to provide only moderate melt strength enhancement at low functional group content but achieved up to 300% melt strength and 400% zero-shear viscosity over unmodified PLA when using additives with a functional group content of 15-20%. Increasing additive concentration in the blend was found to only be an effective means of further viscosity enhancement for low-reactivity additives with a functional group content below 10%.