Introduction
Nonwoven fabric substrates, due to their unique fiber structure and high specific surface area, are often used as carrier materials in experimental testing, such as biochemical test strips or filter media. As a semi-solid formulation, the coating process of gel agents must meet multiple requirements for uniformity, thickness controllability, and substrate wettability. Traditional manual coating often fails to avoid thickness fluctuations and edge effects, whereas adjustable scraper coaters provide an engineered solution for uniform spreading of gel agents on nonwoven fabrics through precise gap control. This coating method draws on mature experience in paper coatings and film preparation, but adjusts for the porosity and flexibility of nonwoven fabrics.
Equipment principle
The core of the adjustable scraper coating machine is a height-adjustable scraper mechanism, usually used in conjunction with conveyor belts or platforms. The gap between the squeegee and the substrate determines the thickness of the wet film, which can be set using micron-level threads or linear motors. When the nonwoven fabric substrate passes under the scraper at a constant rate, the pre-applied gel agent is smoothed by the scraper and filled into the fiber gaps. Coating uniformity depends on the straightness of the squeegee edge, the flatness of the substrate, and the rheological properties of the gel. Compared to roller coating or spraying, the scraper coating method allows for more precise control of coating thickness, especially when handling high-viscosity gels.
Material matching
There are many types of nonwoven fabric substrates, including polyester, polypropylene, or viscose fibers. Different fibers have varying affinity for gel agents, which directly affects the integrity of the coating. For example, hydrophobic polypropylene nonwoven fabric has poor wettability for water-based gels and is prone to missed spots. Therefore, in practical operation, plasma treatment or pre-coating agents are usually needed to improve interfacial bonding on nonwoven fabrics. The viscosity, thixotropy, and solid content of the gel agent itself also need to match the squeegee process parameters. Referring to relevant technical literature, when the gel viscosity is between 1000 and 5000 mPa·s, a scraper gap of 0.1 to 0.5 millimeters can be used to obtain a stable coating layer.
Parameter regulation
Coating speed is a key variable; if too fast, the gel agent will accumulate or cause stringing before the squeegee, while if too slow, the gel may over-penetrate the bottom layer of the non-woven fabric. Based on experimental experience, when applying gel to nonwoven fabric, the linear speed is usually controlled within the range of 0.5 to 3 meters per minute. The setting of the squeegee gap must consider the thickness shrinkage rate after gel drying or curing. For example, if the target dry film thickness is 0.1 mm and the gel solid content is 30%, the wet film gap should be adjusted to about 0.33 mm. Additionally, the squeegee angle (usually 30 to 60 degrees) affects shear stress transfer, and acute squeegee blades are more effective for high-viscosity gels.
Process Case
In one laboratory test, a hydrogel containing activated carbon particles was coated using spunlace polyester nonwoven fabric weighing 50 grams per square meter as the base. In the initial trial, the squeegee gap was set to 0.4 millimeters, and the coating rate was 1.2 meters per minute. The results showed uneven gel penetration and carbon particle aggregation in some areas. After adjustments, the gap was increased to 0.6 millimeters and the speed reduced to 0.8 meters per minute, while another pre-wetting step was added, ultimately reducing the surface roughness of the coating to within ±5 microns. Another case involves applying oil gel to polypropylene nonwoven fabric, effectively eliminating tiny bubble defects in the coating layer by introducing ultrasound-assisted defoaming.
Effect evaluation
Coating quality must be evaluated through multiple indicators, including thickness uniformity, penetration depth, and surface roughness. When measuring the total field thickness with a micrometer or laser profilometer, the coefficient of variation should be kept within 15%. Penetration depth can be observed through a sectional microscope; too deep wastes gel material, too shallow causes poor adhesion. Compared with manual scraping, the adjustable squeegee method excels in reducing batch-to-batch variation, with intra-batch thickness standard deviation reduced by more than two-thirds. The repeatability of this process provides stable starting conditions for subsequent curing or crosslinking reactions, as the saying goes, 'To do a good job, one must first sharpen one's tools.'
Notes:
In practice, be alert to drying and peeling of the gel agent, especially around the edges of the spatula. The blade edge should be cleaned regularly and its wear checked. It is recommended to replace the blade or regrind it every 500 meters of coating. Tension control in nonwoven fabric cannot be ignored; excessive tension causes fiber stretching and deformation, while too little causes wrinkles. Environmental temperature and humidity have a significant impact on gel viscosity; for example, for every 10-degree increase in temperature, some hydrogels lose viscosity by 20% to 30%. Therefore, coating laboratories should be equipped with constant temperature and humidity control, especially during batch coating, where stable conditions must be maintained.