Flame retarded poly(lactic acid) using POSS-modified cellulose. 2. Effects of intumescing flame retardant formulations on polymer degradation and composite physical properties

Poly(lactic acid), PLA, was extruded with intumescing flame retardant formulations based on ammonium polyphosphate, APP. Nanofibrillated cellulose fibers (NFC), POSS-modified NFC (PNFC), and pentaerythritol (PER) were used as the additional carbon source. The effects of each additive and their intumescing combinations on polymer degradation, flammability, crystallization, melt rheology, and tensile properties were systematically examined. APP and PER catalyzed the degradation of PLA during extrusion, which increased the crystallinity of PLA, lowered the viscosity of the melt, reduced the moduli, and decreased the tensile strength of the composite. APP had the largest effect on physical properties, acting as a nucleating agent, decreasing tensile strength, and increasing elongation to break. The POSS moieties on PNFC acted as a slight plasticizer in melt rheology studies, but did not affect the glass transition temperature. PNFC formed a cross-linked network with APP when melt-blended with PLA, which reduced polymer degradation, decreased PLA crystallinity, reduced the melt viscosity, and improved composite stiffness relative to the neat extruded PLA. The PLA composites containing APP and PNFC had the best tensile properties of all the intumescing composites studied. The cross-linked network formed between cellulose, POSS, and PLA helps produce composites with superior flame retardant, rheological, and mechanical properties relative to other intumescing formulations.