In the production of Kraft silicone paper, blistering of the silicone coating is a critical issue affecting product quality. Its causes involve multiple aspects, including raw materials, process parameters, equipment design, and operating procedures. A comprehensive approach is needed, addressing the issue from three aspects: source control, process optimization, and end-of-pipe treatment, to improve coating uniformity and adhesion.
The selection and pretreatment of raw materials are fundamental to preventing blistering. The purity of the silicone raw material directly affects coating quality. If the moisture content is too high or volatile impurities are present, water vapor or gas volatilization during high-temperature vulcanization will cause pores to form within the coating. Therefore, silicone substrates with low moisture content should be selected, and fillers should be dried before mixing to avoid using hygroscopic materials. For example, reinforcing agents such as carbon black and clay need to be pre-baked to remove adsorbed moisture and prevent gas release during vulcanization. Furthermore, the selection of plasticizers must be cautious. Some oil-based plasticizers contain low-temperature volatiles that easily generate bubbles during mixing or vulcanization; therefore, more stable alternatives should be prioritized.
Precise control of process parameters is the core element in reducing blistering. Vulcanization temperature and time are key factors affecting coating performance. Too low a temperature will result in incomplete curing of the silicone, preventing internal gases from escaping; too high a temperature will cause premature surface curing, forming a sealed layer that further hinders gas escape. Therefore, a reasonable vulcanization temperature range must be set according to the silicone type and coating thickness, and temperature deviations caused by prolonged periods without mold heating should be avoided during external mold operations. Vulcanization time also needs strict control; too short a time will lead to under-vulcanization, resulting in a soft coating prone to blistering; too long a time may cause over-vulcanization, leading to embrittlement. In practice, the optimal vulcanization curve must be determined through experimentation to ensure sufficient cross-linking of the silicone.
Ventilation design and operating procedures are crucial to preventing air bubble retention. A large amount of air is entrained in the silicone material during mold closing. If the mold lacks sufficient vents or has an inadequate venting structure, air will be trapped inside the coating, forming air bubbles. Therefore, mold design needs to optimize product arrangement, partitioning, and parting line position to ensure smooth air venting. For example, adding venting grooves or venting pins to key parts of the mold can improve venting efficiency. Meanwhile, operators must regulate the mold closing speed and pressure to avoid trapping air due to excessively rapid pressurization. For existing air bubbles, extending the settling time or using a vacuum degassing process can utilize gravity or negative pressure to allow the bubbles to rise and burst.
Optimizing the mixing and stirring process can reduce the introduction of air bubbles. During adhesive preparation, if the adhesive viscosity is too high or the stirring method is improper, air is easily incorporated and air bubbles form. Therefore, the stirring speed and time must be adjusted according to the adhesive characteristics to avoid over-stirring and air entrapment. For adhesives with high viscosity, a step-by-step stirring method can be used: first, mix the main material at a low speed, then disperse the additives at a high speed, and finally degas under vacuum. In addition, the cleanliness of the stirring equipment must be checked regularly to prevent residual impurities from contaminating the adhesive and triggering chemical reactions that produce gas.
Adjusting the curing process can reduce the risk of air bubble formation. Excessive curing speed, excessively high heat dissipation temperature, or excessive adhesive curing shrinkage can all lead to stress concentration within the coating, forming air bubbles or cracks. Therefore, the curing agent ratio and curing environment must be optimized, and a staged heating process should be used to allow the adhesive to cure slowly and release internal stress. Simultaneously, control the humidity and temperature of the curing environment to avoid fluctuations in adhesive performance due to environmental factors. For solvent-based silicone, strictly control the amount of solvent and plasticizer added to prevent evaporation and bubble formation during curing.
End-of-life treatment and quality inspection are the last line of defense to ensure product quality. For molded Kraft silicone paper, a combination of visual inspection and instrumental testing is required to screen out defective products containing bubbles. Minor bubbles can be treated with local repair or rework; severe bubbles require disposal to prevent them from entering the market. Furthermore, a comprehensive quality traceability system must be established to record the production parameters and test results of each batch of products, in order to trace the root cause of problems and continuously improve the process.
