In cold‑bond shoe‑making, poor bonding and delamination failures are frequently blamed on adhesives. Nevertheless, surface primers are often the decisive factor for stable bonding performance. Diverse shoe‑making substrates vary greatly in surface chemistry. Shoe‑material primers modify substrate surface conditions, remove surface contaminants and build transitional bonding layers, which effectively improve wettability and bonding durability. This article sorts out mainstream types of shoe‑material primers and explains their practical values in shoe‑sole production.
In actual shoe‑sole assembly production, manufacturers often face challenges in the application and formulation of adhesives and primers. Cold‑bond shoe delamination is commonly attributed to defective adhesives. In fact, most bonding failures stem from primers, though poor‑quality adhesives can also cause defects. To achieve satisfactory bonding results for cold‑bonded footwear, properly matched adhesives, curing agents and primers, together with qualified bonding process parameters are indispensable. Manufacturers generally attach great importance to adhesives, curing agents and processing conditions, yet the function and influence of primers are frequently overlooked by many shoe factories.
A wide variety of materials are adopted in shoe production, including rubber, leather, PVC artificial leather, PU synthetic leather, thermoplastic elastomer (TPR), EVA foam, nylon and other high‑molecular materials. These materials differ significantly in surface polarity and chemical status. Ideal bonding results cannot be achieved merely by applying adhesives without adjusting the surface chemical status of bonding interfaces. No single adhesive works universally for all substrates. Adhesives deliver favorable wetting and affinity only for certain materials, while showing poor compatibility with others.
When shoe‑material surfaces are wiped with primers prior to glue application, physical contaminants on surfaces can be removed. More importantly, a new transitional surface layer will form on the substrate. This newly‑formed layer features excellent wettability and compatibility with adhesives. It acts as a bridge between base materials and adhesives, enhancing substrate bondability and boosting bonding strength and service durability of shoe adhesives. For scenarios requiring anti‑yellowing performance, anti‑yellowing agent BETTERSOL 1513 delivers outstanding performance in primer formulations.
Main types of shoe‑material primers are listed as follows:
- Solvent‑based cleaning primer
It is mainly formulated with organic solvents such as toluene, dichloromethane and methanol. Solvents are blended at specific ratios according to material properties and solvent evaporation rates.
- Cyclizing primer
Synthetic rubber is immersed in concentrated sulfuric acid for 5‑15 minutes, then washed and dried. Micro‑fine cracks are generated on material surfaces, which contribute to mechanical interlocking during bonding.
- Halogenating primer
For polyolefin‑based shoe materials hard to treat by conventional methods, halogenating agents are applied for surface halogenation. Surface morphology and structure are modified with improved surface polarity. Accordingly, adhesives can fully wet the substrate interface and form stable chemical bonds.
- Diluted adhesive primer
Mixed solvents consisting of toluene, methyl ethyl ketone, acetone and other solvents are prepared. Adhesives are diluted to 3%‑5% concentration to serve as base‑coat primers for shoe‑making substrates.
- Grafted‑polymer primer
Monomers such as methyl methacrylate and acrylic acid are grafted onto SBS / SBR polymers to produce grafted polymer solutions, which are further diluted by mixed solvents into base‑coat primers. This type is widely used for thermoplastic elastomer (TPR) and rubber‑plastic shoe soles.
- Mixed‑formula primer
Prepared by mixing multiple adhesive solutions, or blending dissolved substrate solutions with adhesive liquids for base‑coating purposes.
- Plasma surface treatment
It is an emerging surface modification technology for rubber and plastic materials. Rubber or plastic workpieces are placed in low‑pressure gas glow‑discharge space. Activated chemical species generated by ionized plasma react with material surfaces and form ultra‑thin functional layers totally different from the original substrate.
The CASIG (Crosslinking by Activated Species of Inert Gases) treatment can generate polar groups including carbonyl, hydroxyl and carboxyl groups on hydrocarbon polymer surfaces. Research indicates that bonding improvement comes from combined effects of surface oxidation layers, polar functional groups and internal cross‑linking.
- Halogenation‑based primer
Based on addition reaction between halogenating agents and polybutadiene double bonds inside TPR materials, surface composition changes and surface polarity increases. Common halogenating agents include trichloroisocyanuric acid (TCCA), dichloramine T and N‑bromosuccinimide.
Halogenation accelerates crystallization of polyurethane adhesive films coated on surfaces. It raises initial tack strength, greatly cuts down adhesive consumption and improves final bonding strength. Halogenation technology has gained wide recognition among global shoe‑making industries. However, chlorine released from halogenation solutions will cause environmental pollution.
- Grafted‑polymer base‑coat primer
Also known as base primer, it is dominantly applied for TPR shoe soles. TPR materials cover SBS, SIS, EPDM and other grades. Combining high elasticity of rubber and high strength of plastic, TPR is cost‑effective and widely adopted for shoe soles. TPR features low surface polarity, while polyurethane adhesives are highly polar, which brings bonding difficulties. The graft‑modified SBS polymer with vinyl monomers is developed as TPR surface primer to solve bonding challenges of TPR shoe soles.
