*Article* **Effects of Coupling Agent and Thermoplastic on the Interfacial Bond Strength and the Mechanical Properties of Oriented Wood Strand–Thermoplastic Composites**

**Ziling Shen, Zhi Ye, Kailin Li and Chusheng Qi \***

MOE Key Laboratory of Wood Material Science and Utilization, Beijing Forestry University, Beijing 100083, China; shenziling@bjfu.edu.cn (Z.S.); ye.zhi037@gmail.com (Z.Y.); likailin2019@gmail.com (K.L.) **\*** Correspondence: qichusheng@bjfu.edu.cn

**Abstract:** Wood–plastic composites (WPC) with good mechanical and physical properties are desirable products for manufacturers and customers, and interfacial bond strength is one of the most critical factors affecting WPC performance. To verify that a higher interfacial bond strength between wood and thermoplastics improves WPC performance, wood veneer–thermoplastic composites (VPC) and oriented strand–thermoplastic composites (OSPC) were fabricated using hot pressing. The effects of the coupling agent (KH550 or MDI) and the thermoplastic (LDPE, HDPE, PP, or PVC) on the interfacial bond strength of VPC, and the mechanical and physical properties of OSPC, were investigated. The results showed that coupling agents KH550 and MDI improved the interfacial bond strength between wood and thermoplastics under dry conditions. MDI was better than KH550 at improving the interfacial bond strength and the mechanical properties of OSPC. Better interfacial bonding between plastic and wood improved the OSPC performance. The OSPC fabricated using PVC film as the thermoplastic and MDI as the coupling agent displayed the highest mechanical properties, with a modulus of rupture of 91.9 MPa, a modulus of elasticity of 10.9 GPa, and a thickness swelling of 2.4%. PVC and MDI are recommended to fabricate WPCs with desirable performance for general applications.

**Keywords:** wood–plastic composites; coupling agent; interfacial bonding; oriented strand–thermoplastic composites; wood veneer–thermoplastic composites
