In the production of Kraft silicone paper, the uniformity of the silicone coating is a core indicator determining its release performance, surface quality, and stability in downstream applications. Controlling coating uniformity requires a coordinated effort across multiple stages, including raw material selection, process parameter optimization, equipment precision improvement, environmental control, and testing feedback, forming a closed-loop management system throughout the entire process.
The stability of raw materials is fundamental to coating uniformity. As a core component of the coating, the molecular weight distribution, viscosity, and curing characteristics of silicone oil directly affect the coating's leveling properties. If the molecular weight difference between batches of silicone oil is too large, localized hard lumps or soft spots can easily form after curing, leading to uneven coating thickness. Therefore, silicone oils with narrow molecular weight distribution and stable performance between batches must be selected, and its viscosity fluctuations must be strictly controlled within a reasonable range through rigorous testing. Simultaneously, the proportion of additives (such as catalysts and leveling agents) in the release agent formulation must be precisely controlled to avoid uneven additive distribution causing differences in coating surface tension, leading to pinholes or sagging problems.
Precise control of coating process parameters is crucial. Taking roller coating as an example, the linear speed difference between the coating roller and the back pressure roller must be strictly matched: if the back pressure roller speed is too fast, the silicone oil will be excessively squeezed, resulting in a thicker edge and thinner center; conversely, insufficient coating may lead to streaks. In actual operation, the roller speed needs to be monitored in real time using a laser tachometer and automatically adjusted by a PLC system to ensure that the linear speed difference remains stable within a reasonable range. Furthermore, coating temperature significantly affects the fluidity of the silicone oil; increased temperature reduces the viscosity of the silicone oil, easily causing excessive leveling; excessively low temperature may result in a rough coating surface. Therefore, an infrared thermometer needs to be installed at the coating head to control temperature fluctuations within a small range.
Equipment precision directly affects the physical uniformity of the coating. The surface roughness of the coating roller needs to be controlled at an extremely low level. If the roughness exceeds the standard, the silicone oil is prone to accumulate in the pits, forming an "orange peel" defect. Simultaneously, the cylindricity error of the roller must be less than a minimum value; otherwise, the coating thickness will exhibit periodic fluctuations. Furthermore, the uniformity of airflow in the drying system is crucial. Excessive airflow variation within the oven can lead to inconsistent curing speeds of the silicone oil, potentially causing localized blistering or cracking. Therefore, it is necessary to regularly monitor the airflow in each area of the oven using an anemometer and adjust the airflow using guide vanes to keep the deviation within a small range, ensuring synchronized coating curing.
Environmental cleanliness is an easily overlooked factor. Micro-dust (larger particle sizes) adhering to the kraft paper surface can disrupt the continuity of the silicone oil film, creating pinhole defects. The production workshop must meet high cleanliness standards and utilize equipment such as air showers and adhesive mats to reduce contamination brought in by personnel. Humidity control is equally critical: excessively high humidity causes the low-boiling-point components in the silicone oil to absorb moisture, leading to whitening of the coating; excessively low humidity may cause dust to be attracted due to electrostatic discharge. Therefore, a dehumidifier/humidifier unit should be installed to stabilize the humidity within a reasonable range, providing a stable environment for coating curing.
Online monitoring and feedback adjustment are the core of the quality closed-loop system. Real-time scanning of coating thickness using a laser profilometer, with a high sampling frequency, allows for timely detection of thickness deviations and triggers alarms. A machine vision system identifies surface defects (such as flow marks and pinholes) and locates problem areas using image analysis software. Inspection data is synchronously transmitted to the central control system, automatically adjusting coating parameters (such as roller speed and temperature) to form a rapid "detection-analysis-adjustment" mechanism. Furthermore, samples from each batch of production undergo release force testing; if the test value deviates from the standard range, the machine must be stopped immediately to investigate the cause.
The skill level and quality awareness of operators are equally important. Regular coating process training is necessary, focusing on parameter adjustment, equipment maintenance, and troubleshooting capabilities. For example, operators must master the "three looks and three touches" method: look at the uniformity of the coating surface gloss, the neatness of the edges, and the stability of the equipment operating parameters; touch the coating surface for smoothness, check for warping of the dried paper, and feel for overheating of key equipment components. Standardized operating procedures and accumulated experience can reduce coating unevenness caused by human factors.
Controlling the uniformity of the silicone coating on kraft silicone paper requires coordinated efforts from six aspects: raw materials, processes, equipment, environment, testing, and personnel. By establishing a comprehensive process control system, the coating thickness variation can be kept within a small range, and the release force fluctuation range can be significantly reduced, thereby meeting the stringent requirements of the high-end adhesive industry for product consistency.
