Wood plastic composites (WPCs) are durable building materials that require less maintenance than pressure treated lumber. They are manufactured using wood flour and often incorporate recycled thermoplastics, thereby turning would-be waste streams into high value products. In addition to wood and thermoplastics such as polyethylene (PE), additives are used to improve compatibility between the wood and PE and as processing aids to reduce melt fracture, melt temperature, and extruder torque during composite production. Maleic anhydride grafted polyethylene (MAPE) is often used in WPCs as a coupling agent for the wood and PE and metal stearates are often used as process aids. However, metal stearates are known to react with MAPE, which reduces the coupling efficiency of MAPE and decreases the modulus and strength of composite materials.<br/><br/>Silicones can be used as an alternative to metal stearates. Nuclear magnetic resonance (NMR) analysis shows no reaction between silicone and MAPE. X-ray fluorescence (XRF) also indicates no reaction between silicone and cellulose, chosen as a proxy for wood flour. Because silicone does not react with MAPE or with cellulose (wood flour), the mechanical properties of WPC boards containing both silicone and MAPE are improved due to the enhanced wood-plastic coupling.<br/><br/>This enhanced coupling was directly interrogated by using scanning electron microscopy (SEM) to examine composite fracture surfaces. SEM imaging was paired with energy dispersive X-ray spectroscopy (EDS) to map the distribution of silicone in WPC cross-sections. Stearate-containing WPCs were cross-sectioned and analyzed using secondary ion mass spectrometry (SIMS) to map the stearate distribution. Results showed that the two process aids distribute quite differently within the WPC microstructure, which may also contribute to their performance differences. This presentation will introduce the combination of silicone and MAPE as an additive for WPCs, demonstrate methods for analyzing additive effectiveness, and discuss aspects of composite characterization that led to an understanding of the mechanism by which silicone functions as a process aid in these materials.

Silicone additives enhance sustainability in wood plastic composites

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