Use of cold plasmas for lignin modification and improvement of lignin-polypropylene composites

Lignin and polypropylene (PP) were subjected to plasma treatments with the goal of improving composites made from these materials. Lignin was treated with SiCl4, argon, oxygen, styrene, and acryloylchloride, in three different plasma reactors. It was shown that efficient surface modification could be achieved by plasma treatment, avoiding long reaction times and large volumes of reactants for modification by conventional wet chemistry. Plasma-induced copolymerization of acryloylchloride on both lignin and PP, and plasma state polymerization under pulsing conditions for PP, resulted in thin film depositions that imparted the best mechanical properties to the composites.Lignin polypropylene composites were prepared in the range of 10--60% by weight lignin content. Blending of lignin with polypropylene resulted in materials with reduced tensile and flexural strength and a dramatic decrease in unnotched Izod impact strength compared to the properties of virgin PP, although moduli were improved. Using maleic-anhydride-grafted-polypropylene (MAPP) as a coupling agent improved strength properties.Surface modification of both lignin and polypropylene in several plasma environments was performed to enhance composite properties. SiCl4 Plasma modification of the materials resulted in a 35% improvement in tensile and 10% in impact strengths related to the untreated materials at 30% lignin content. Helium-plasma induced graft copolymerization of acryloyl chloride on lignin and PP resulted in an improvement of properties, especially when only lignin was treated with a 50% and 100% increase in tensile and impact strengths, respectively. Plasma-state polymerization of acryloyl chloride on PP under pulsing conditions resulted in composites with tensile and flexural properties similar to those obtained with MAPP. Scanning electron microscopy of the composites made evident the improvement in blending morphology when using coupling agents and pulsed-plasmas with acryloyl chloride or SiCl 4. DMTA of the composites indicated the anti-plastizicing effect of adding lignin to PP the composite storage modulus was increased with lignin addition. Composites made with coupling agents and pulsed plasmas were found to have lower polymer mobility at temperatures near to the melt, indicating better compatibility between the components in the composite.