Wood-plastic composites are relatively new materials as they started to emerge at the beginning of the 90’s. The most widespread use of WCPs is in outdoor decking but it is also used in railings and siding as well as park benches and indoor furniture. Wood-plastic composites (WPCs) are composed of three main ingredients: recycled thermoplastics (such as polyethylene, polypropylene and polyvinyl chloride), wood filler and additives.
For some uses, the ability to coat or bond the WPC is needed. For example, overlaying the WPC panels with veneer sheets improves their appearance and properties for furniture manufacturing. Bonding of WPC elements is required to fabricate laminated beams for structural applications. Adhesion is extremely important in both these processes. The adhesion mechanism can be divided into three categories:
1. mechanical interlocking 2. physical bonding 3. chemical bonding
Mechanical interlocking strength depends on the size and the geometry of the locking sites; thus surface roughness. The surface roughness will increase the surface area and thereby increase the binding sites for other adhesion mechanisms. Available physical and chemical bonds are sometimes collectively described as thermodynamic adhesion. Thermodynamic adhesion depends on the surface chemistry of the substrate.
Most surface modification methods used to increase adhesion will influence both the roughness of the surface and the surface chemistry. Therefore the separation of these two parameters is important for comprehensive understanding of the adhesion process. Surface roughness can be analyzed by 3D topography measurements and surface chemistry can be evaluated by contact angle measurements. Performing these measurements simultaneously on the same sample spot enables the separation of the impact from each parameter.