Effective measures to solve the problem of oil loss after oil spraying on low-hardness soft plastic parts
To address the problem of oil shedding after spraying on low-hardness soft plastic parts (such as TPU and silicone products with a Shore hardness of 30A-60A), the first step is to optimize the surface pretreatment process to remove surface release agents, oil stains, and low-molecular-weight precipitates, thereby enhancing coating adhesion. Soft plastic parts are typically molded with large amounts of release agent. Any residual release agent on the surface forms a barrier layer, preventing paint from adhering. Therefore, solvent cleaning or plasma treatment is necessary for removal. Solvent cleaning can use isopropyl alcohol or ethyl acetate. Use a dust-free cloth dipped in the solvent and repeatedly wipe the surface of the part until no stains remain. After cleaning, spraying should be completed within 30 minutes to avoid secondary contamination. Plasma treatment uses high-energy ions to bombard the surface, removing organic contaminants and introducing polar groups (such as hydroxyl and carboxyl groups). This increases the surface tension from 30mN/m to over 50mN/m. For example, plasma treatment (500W for 60 seconds) can improve coating adhesion by over 30% on TPU parts. For silicone plastic parts, special surface activation treatment is also required, such as soaking in γ-aminopropyltriethoxysilane (KH550) solution (concentration 5%, temperature 60°C, time 30 minutes) to form siloxane chemical bonds and provide anchor points for paint.
Choosing a paint system that matches soft plastic parts is key to preventing paint shedding. Ensure the paint’s flexibility matches the part’s elasticity to avoid cracking and shedding due to thermal cycling or deformation. For spraying soft plastic parts, it’s best to use an elastic paint with an elongation at break of at least 300%, matching the part’s elongation (typically 200%-500%). For example, polyurethane elastic paint (elongation 400%) is suitable for TPU parts, while silicone-modified acrylic paint (elongation 350%) is suitable for silicone parts. The paint’s curing system must be adapted to the soft material’s heat resistance. For TPU parts, the curing temperature for spraying should be controlled between 60-80°C (avoid temperatures exceeding 90°C, which can soften the material). For silicone parts, the curing temperature can be increased to 100-120°C. The curing time should be adjusted based on the thickness (typically 30-60 minutes). Adding an appropriate amount of adhesion promoter to the paint, such as a TPU-specific isocyanate curing agent (5%-8%) or a silicone coupling agent (2%-3%) for silicone, can enhance the chemical bond between the paint and the plastic surface. A TPU mobile phone case was experiencing paint loss after spraying. By replacing the paint with a polyurethane elastic paint and adding 6% isocyanate curing agent, the adhesion improved from level 1 (cross-cut test peeling area > 30%) to level 5 (no peeling).
Controlling spraying process parameters to ensure uniform coating and sufficient wetting of the plastic part surface is crucial for minimizing oil shedding. The spray viscosity should be adjusted based on the nozzle model and spray distance, typically between 15-25 seconds (using a 4-cup viscometer). Excessively high viscosity can lead to poor coating leveling and pinholes, while too low can easily cause sagging. For example, controlling the viscosity of TPU spraying within 20 seconds ensures uniform coating while preventing sagging. Maintain a spray distance of 150-200mm, a spray gun pressure of 0.3-0.4 MPa, and a cross-spray method (first horizontally, then vertically) to ensure uniform coating thickness (dry film thickness 15-30μm) with a thickness deviation of no more than ±3μm. For complex-shaped parts (such as those with grooves or ribs), adjust the spray gun angle to ensure even coating of recessed areas. For example, spray at a 45° angle in grooves to avoid localized oil shedding due to insufficient paint coverage. After spraying, the coating needs to be fully leveled (5-10 minutes) before entering the curing oven. The leveling temperature should be controlled at 25-35°C and the humidity at 50%-60% to prevent the coating surface from curing prematurely to form pinholes or bubbles, which will affect adhesion.
Strengthening post-spray curing and post-treatment to ensure complete crosslinking of the paint can further improve the coating’s weather resistance and adhesion. For two-component paints, the curing agent ratio and pot life must be strictly controlled. For example, for polyurethane paint, the curing agent ratio deviation should not exceed ±0.5%, and the pot life (at 25°C) should be controlled within 4 hours. Paint that has exceeded the pot life should be discarded to prevent oil loss due to incomplete curing. Cured plastic parts require post-curing treatment, such as room temperature for 24 hours or baking at 60°C for 2 hours, to promote the evaporation of residual solvents and complete the crosslinking reaction. For example, after spraying silicone plastic parts, curing at 60°C for 2 hours will increase the coating hardness from HB to H, and improve adhesion. For soft plastic parts that require bending (such as seals and buttons), a bend test (10mm bend radius, 180° bend, 100 cycles) is required. The coating will be considered acceptable only if there is no cracking or peeling after the test. A soft PVC toy lost oil at the bends after being sprayed with oil. By extending the post-curing time to 48 hours, the coating was intact after the bending test, thus solving the oil loss problem.
Optimizing protective measures for different usage environments can effectively prevent oil shedding on low-hardness soft plastic parts after spray painting. For plastic parts exposed to oils, fats, or chemicals (such as kitchen utensils), chemical-resistant paints, such as fluorocarbon-modified elastic paints, are recommended. These are resistant to alcohol, cooking oil, and other corrosion, and should show no wrinkling or peeling after immersion for 24 hours. For plastic parts used outdoors (such as automotive weather strips), UV-resistant paints should be selected with the addition of UV absorbers (such as benzotriazoles, at a dosage of 1%-2%). After QUV aging testing (1000 hours), the coating should demonstrate a color difference ΔE <3 and no loss of adhesion. A layer of clearcoat (such as polysiloxane clearcoat) with a thickness of 5-10μm can be applied to the coating to form a protective layer, enhancing wear and stain resistance. For example, a TPU sports bracelet coated with clearcoat after spray painting showed no visible underside after an abrasion test (1000 rubs under a 500g load). By comprehensively optimizing pretreatment, paint selection, process parameters and protective measures, the oil loss rate of low-hardness soft plastic parts after oil spraying can be reduced from more than 30% to less than 1%, meeting the requirements of use.
