Evaluation of coupling mechanisms in wood plastic composites

Erica Fay Rude

The major components of wood plastic composites (WPCs) are wood polymers, thermoplastics, coupling agents, and lubricants. Coupling agents are used to increase the adhesion between the thermoplastic matrix and the wood, thus increasing the mechanical properties, while lubricants increase the speed of production and surface properties. Maleic anhydride polypropylene (MAPP) is a commonly used coupling agent in WPCs, while ethylene bisstearamide (EBS), the ester based Optipak 100 (OP100) and zinc stearate (ZnSt) are widely used lubricants. WPCs containing both MAPP and EBS or OP100 show no significant decreases in mechanical properties, while those containing ZnSt have a marked decrease. Currently the chemical interactions between wood and MAPP, and lubricants and MAPP are not well understood. This study focused on the chemical interactions between MAPP and wood polymers, MAPP and lubricants, and ternary blends of MAPP, wood polymers, and lubricants. Solid state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C CP MAS NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy were employed to determine the mechanism behind the coupling of MAPP with wood and how these interactions may be hindered by ZnSt. Initially the interaction between MAPP and cellulose, lignin and maple was examined to determine how MAPP interacts with these to produce higher mechanical properties Evidence of esterification and hydrogen bonding between MAPP and cellulose, lignin, and maple was uncovered. Secondly the interaction between MAPP and EBS, OP100, and ZnSt was examined to determine what, if any interactions were present. EBS and OP100 had no covalent interactions, while strong evidence of hydrogen bonding between these lubricants and MAPP was uncovered. The ZnSt/MAPP blend had significant anhydride bonding between the MAPP and the stearate groups. Zinc disassociates from the stearate groups leaving the free stearic acid groups to bond with the MA moieties of the MAPP. This warranted the study of ternary blends to determine if the MAPP/wood polymer interaction or the MAPP/ZnSt interaction would be favored. These studies showed significant favoring of the MAPP/ZnSt interaction over that of the MAPP/wood polymer interactions. The ZnSt successfully inhibits the ability of MAPP to effectively couple with wood, thus decreasing the efficacy of MAPP as a coupling agent.

Evaluation of coupling mechanisms in wood plastic composites

Files and links (1)

Abstract

Metrics

Details