Combined Surface Treatment And Adhesion Mechanism For Polyethylene Wood Plastic Composites

As the typical representative of wood-plastic composites(WPCs),polyethylene wood-plastic composites have the dual advantages of both wood and plastic,and have been widely used in many fields,but their application has been greatly limited due to the limitations of their connection method.Compared with the current widely used mechanical connection,adhesive bonding with low cost,easy operation,light weight and other characteristics,can effectively avoid stress concentration,but also make seamless connection of wood-plastic products,which is especially suitable for the design and application of large scale complex structural products of wood-plastic composite materials.However,due to the low surface energy polyethylene component of the polyethylene wood plastic composite material,it is difficult to be connected by a gluing method,and the bonding durability is poor.Surface treatment of the sticky material is often used in the bonding process,but there are not many methods for the surface treatment of the polyethylene wood-plastic composite materials,and there are some defects in each single method.In order to achieve the rapid bonding connection of wood-polyethylene composites as well as the good durability for the bonded joint,this paper is based on the synergistic surface treatment of coupling agent coating and plasma discharge to deal with the surface of the composites.Using adhesive performance measurement and the analysis of contact angle,Fourier transform infrared spectroscopy(FTIR),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS)and the water absorption test to study the synergistic effect of coupling agent coating treatment and plasma discharge on surface properties,bonding strength and bonding durability under different environment.And the action mechanism of synergistic effect for coupling agent coating treatment and plasma discharge on the bonding properties for the composites was also explored through the characterization of the destruction type and breaking mechanism for the bonded joint.The study for the paper would helpful to establish the technique of rapid surface treatment and bonding of the wood-polyethylene composites,and the above research would also provide the scientific theoretical basis and practical methods for achieving the seamless connection of wood-polyolefin composites and enlarge the application scope.After the combined surface treatment of sanding then coating with coupling agent followed by plasma discharge was implemented,the surface properties of PE-WPCs changed greatly.The surface contact angle decreased and the surface wettability of PE-WPCs improved significantly.The oxygen-containing polar groups and the O/C values on the surface of PE-WPCs all increased.A large amount of—OH,—C=O and—O—C=O and other oxygen-containing polar groups appeared,especially the strong chemical bonds connection formed between the coupling agent and the material surface.All of above improved the adhesion properties of the adhesive joints.Compared with the single surface treatment otherwise,not only the shear strength increased,but the durability in water environment,thermal environment and natural aging environment have also been greatly improved,which reflected the synergistic effect of two treatment methods.The adhesive joints of combined-treated PE-WPCs failed in the long-term water and thermal aging environment,and the temperature change can exacerbate the degree of failure.Through the study of the evolution of the surface properties on the combined-treated PE-WPCs in the water environment,find the surface properties of the materials in the water environment have changed greatly,and those surface properties changes would cause the failure of PE-WPCs adhesive joints.In the water environment,desorption between the coupling agent and the material surface caused by water molecules and internal stress caused by the expansion and degradation of the adhesive and material are the main reasons for the failure of the PE-WPCs adhesive joint.In the natural aging environment,the interaction between internal stress and external environment is the main reasons for the failure of the PE-WPCs adhesive joint.In the thermal aging environment,the change of adhesive joints chemical composition and structure caused by thermal oxidation degradation and interfacial stress are the main reasons for the failure of the PE-WPCs adhesive joint.In the water environment,the PE-WPCs surface treated by coupling agent coating and plasma discharge can maintain its original appearance for a long time,and can reduce the production and development of defects in the adhesive interface effectively.Compared with the single surface treatment otherwise,the combined-treated sample surface was significantly less damaged by the water molecules.Combined treatment of coupling agent coating and plasma discharge improved the adhesion between PE-WPCs surface and adhesive and the ability of adhesive interface to resist water molecular damage,and reduced the damage of the water molecules to the PE-WPCs surface by the sealing effect of coupling agent.This is the reason for the good water resistance of PE-WPCs with combined surface treatment.Combined treatment of sanding then coating with coupling agent,followed by plasma discharge are fully utilized the "molecular bridge" structure of coupling and the rapid,efficient,cleaning characteristics of plasma.It shows excellent effect of surface modification.The combined surface treatment can simultaneously affect the polyethylene and wood flour composition in the PE-WPCs,and the treatment effect is much greater than that of the single treatment method.It also effectively compensates for defects in the coupling agent coating or plasma treatment alone,showing a synergistic effect of the treatment method.On the premise of ensuring high strength and good durability of PE-WPCs adhesive joint,fast processing and adhesive bonding are realized.