Rubber materials have special physical and chemical properties such as low surface energy, smooth surface, no active functional groups, and weak interfacial layers on the surface. Without activation treatment, it is difficult for adhesives to extend and penetrate on the surface of rubber materials, which significantly affects the bonding performance and thus affects the reliability of bonding.

Surface treatment of rubber materials is usually divided into two types: one is physical methods, such as solvent method and mechanical polishing method; Another method is chemical, including isocyanate coating method, concentrated sulfuric acid cyclization method, concentrated hydrochloric acid chlorination method, etc. The promotion degree of physical methods on the adhesive performance of rubber materials is limited, while acid treatment method has the advantages of good treatment effect, no need for special equipment, and easy operation. It has been used in the rubber material processing process for a long time, but its operation process is dangerous, the process is complicated, and time-consuming, usually requiring 2-4 people to work for 6-10 hours. Moreover, the cost of treating waste acid generated in the production process is high, and if not handled properly, it can cause harm to the environment. The low-temperature plasma technology has excellent effects on surface modification of various polymer materials. Compared with acid treatment, it has significant advantages such as low cost, no waste, and no pollution. It can not only effectively clean the surface oil stains, oxides, etc. of rubber sheets, but also form active groups on the surface to improve surface activity, effectively promote the adhesive performance of rubber materials, and achieve excellent treatment results.

Surface modification principle of low-temperature plasma treatment technology for rubber

Plasma is a collection of charged positive and negative particles, whose energy can act on the surface of materials through radiation, collisions of neutral particle flows, and ion flows, generating free radicals or undergoing chemical reactions with the material surface to improve its surface properties. The high-energy particle groups generated by plasma interact strongly with the surface of rubber materials, usually through free radical chemical reactions. After plasma treatment, free radicals are formed on the surface of polymer materials. When exposed to an oxygen-containing atmosphere, peroxides or hydroxyl peroxides are generated on the surface, which are triggered by ultraviolet light, heat or other methods to generate oxygen-containing free radicals. At the same time, the vinyl bonds of monomers are opened for graft polymerization, resulting in the introduction of more polar groups such as - COOH, - NH2, epoxy groups on the substrate surface. These functional groups are reactive groups that can react chemically with the substance being bonded. At the same time, rubber materials undergo various physical and chemical reactions such as etching, cross-linking, and polymerization, which work together to promote the improved adhesive properties of the treated rubber materials. Plasma can promote the modification of rubber materials, but it only modifies the surface of the material (usually from a few nanometers to a few hundred nanometers) and does not affect the fundamental properties of the material itself.

Changes in contact angle of rubber surface after plasma treatment

Contact angle, also known as droplet angle, is an important parameter for measuring the liquid infiltration performance, surface roughness, chemical diversity, material heterogeneity, and other physicochemical properties of a material surface. The smaller the contact angle, close to 0, the better the surface wettability; On the contrary, the poorer the wettability. Plasma graft polymerization can introduce grafting groups on the surface of materials, thereby enhancing the wettability of the material surface, changing the surface roughness of the material, and possibly promoting the diffusion and infiltration of adhesives on the rubber surface. This article measured the contact angle changes of rubber after plasma treatment and placed for different times, and further characterized the surface physicochemical properties of rubber sheets. The experimental results are shown in Figure 1.

Figure 1 Schematic diagram of changes in contact angle of rubber surface after plasma treatment with different parameters

As shown in Figure 1, plasma treatment can reduce the surface contact angle of rubber sheets from around 110 ° to about 30 °; When placed for 6-12 hours, the contact angle value does not change significantly; After being left for 24-36 hours, there is a slight decline in the effect; After being placed for 48 hours, there is a significant decline. This indicates that plasma treatment can significantly improve the liquid infiltration on the surface of rubber sheets, but the treatment is time-dependent.

Shear strength measurement and analysis

After plasma treatment (direct sample preparation after 0 hours, the average shear strength was 5.85 MPa, and most of the sample failure types were interface failure. Sample preparation after 12 hours, the average shear strength was 9.27 MPa, and all sample failure types were rubber failure. Sample preparation after 24 hours, the average shear strength was 11.0 MPa, and all sample failure types were rubber failure. Sample preparation after 48 hours, the average shear strength was 13.7 MPa, and all sample failure types were rubber failure. Sample preparation after 72 hours, the average shear strength was 10.88 MPa, and all sample failure types were rubber failure. Sample preparation after 96 hours, the average shear strength was 7.26 MPa, and interface failure began to occur. Sample preparation after 120 hours, the average shear strength was 10.88 MPa. The sample was prepared with an average shear strength of 3.95MPa, and the failure type of the sample was almost entirely interface failure, as shown in Figure 2.

Figure 2 Rubber shear strength under different placement times of plasma treatment

From Figure 2, it can be seen that the adhesive performance of the rubber sheet is significantly enhanced after plasma surface treatment. With the increase of storage time after treatment, the degree of increase in adhesive strength also further increases, and the performance is optimal after 48 hours of storage.