Overview
The coating machine is a familiar face in the laboratory, especially when applying liquid silicone to the fabric, which actually requires some attention. Liquid silicone has large viscosity fluctuations and complex rheological behavior. If applied thinly, the bottom is easily exposed; if thick, it wastes material and affects subsequent curing. Laboratory-grade coating machines, simply put, help us small-batch experimenters control variables tightly—coating speed, squeegee gap, silicone feeding pressure, every parameter can be adjusted, unlike workshop production lines that can't stop once started. I've seen many cases, such as applying silicone to nylon fabric as a waterproof layer, using a scraper coater with a blade angle of 30 degrees and a gap of 0.2 millimeters, achieving a uniformity of up to ±5 microns, which was already sufficient during the R&D stage.
Equipment selection
There are several types of laboratory coating machines. For liquid silicone, the most common are the squeegee type, roller coating type, and slit type. The scraper type is the simplest and is suitable for silicone with viscosity between 1000 and 50,000 mPa·s. During operation, the fabric is leveled, the squeegee is fixed, and silicone drips at the front end of the fabric, using the blade to smooth it out. However, be careful: if the squeegee is too pressed, silicone will seep through the fabric; if it's too small, the application won't be even. The slit coater is more precise; silicone is squeezed out from the slit nozzle and falls directly onto the fabric. It is suitable for silicone with low viscosity (such as below 5000 mPa·s), and can control the coating thickness between 10 and 200 microns. I remember a technical paper mentioning that when applying silicone coating on polyester fabrics, the thickness deviation for the slit type is only 3%, while for the scraper type it is 7%. When selecting a model, consider the thixotropy of the silicone. If the thixotropy is high, the scraper type tends to cause scratches, so switch to a roller coating type. Use a pair of rollers to press the material, and the fabric passes between the two rollers, allowing the silicone to adhere naturally.
Process parameters
If the parameters aren't set well, no matter how expensive the coater is, it's useless. First is the coating speed. If the fabric moves too fast, the silicone can't smooth out in time, causing streaks; If it's too slow, the silicone may cure part of it first. Generally, laboratory practice sets the speed between 0.5 and 5 m/min, adjusted according to the opening time of the silicone. Silicone with a short opening time (for example, cross-linking starts within minutes) requires fast speed, while it can be slower. Next is the squeegee gap, which directly affects thickness. The relationship between gap d and final coating thickness t is roughly t = k × d, where k is the compression ratio of silicone gel, usually between 0.3 and 0.7. Just write the formula
t = k d
The k value is corrected by actual measurement. For example, if the gap is set to 0.5 mm and the coating thickness is 0.2 mm, then k is 0.4. This linear relationship is more accurate on low-viscosity silicone; at high viscosity, k decreases because the silica's elasticity recovers significantly. Also, don't use too much silicone material, or the edges will overflow and dirty the fabric. I have seen a case where silicone was applied to cotton fabric for a soft touch, with a feed rate controlled at 2.5 grams per meter, a coating speed of 2 m/min, resulting in a thickness of 0.15 mm, with a very uniform hand feel.
Fabric pretreatment
If the fabric surface isn't clean and the silicone doesn't stick well, it will come off easily as soon as you tear it off afterward. Plasma treatment or primers are commonly used in laboratories. Plasma treatment is suitable for synthetic fibers such as polyester and nylon. After bombarding with air plasma for 30 seconds to 1 minute, the surface tension can be increased to over 40 mN/m, the contact angle of silicone gel drops below 30 degrees, and adhesion is significantly enhanced. As for the primer, it's suitable for natural fibers like cotton and linen. Apply a layer of dilute aminopropylsilane solution, let it dry, and then apply silicone. Be careful not to use strong acids or alkalis, as this can damage the fabric. I randomly thought of a point: the treated fabric must be applied within 24 hours, otherwise surface activity will decrease. This isn't explicitly stated in the standard documents, but it's often encountered in practice.
FAQs
Problems are inevitable during the coating process. For example, silicone foaming may be caused by air being wrapped during feeding. The solution is to add a settling section to the silicone jar and soak for a few minutes to let the bubbles float up. Also, if the coating thickness is uneven, it's most likely due to the spatula or roller being off-balance. Use a feeler gauge to calibrate it and keep the gap deviation within ±0.02 mm. Once, when I applied silicone in the lab, the edges of the fabric were always thick. Later, I found that the fabric tension wasn't adjusted properly, causing the edges to loosen and causing silicone buildup. When adjusting tension, use a tension meter to keep the fabric lateral tension difference less than 5 N/m, and the problem is solved. Additionally, if silicone overflows onto the back of the fabric, it means the squeegee pressure is too low or the silicone viscosity is too low. Move the squeegee forward by 0.5 mm or switch to higher-viscosity silicone. Don't panic, these issues are normal. After a few adjustments, you'll feel the right touch.
Test and verify
After applying silicone to the fabric, several indicators need to be measured. Adhesion is tested by peeling: take a 25 mm wide sample and peel it off at a speed of 100 mm/min. A force value between 0.5 and 2 N/cm is considered qualified, depending on the application. Thickness is measured with a thickness gauge, randomly selecting 10 points and averaging them, with a deviation coefficient not exceeding 8%. It also has water resistance and washability. According to the standard washing method, after washing at 40°C five times, the silicone will not peel off. I once tried applying silicone to nylon fabric to make waterproof clothing, and after 10 washes, the adhesion only dropped by 12%, which means the craftsmanship is decent. One thing to note: the curing conditions for silicone should also be remembered, such as 120°C temperature and 5 minutes oven time. Don't test if the curing is insufficient, as the data can be misleading.
That's basically all there is to it. Experimenting with liquid silicone with a lab coating machine isn't as mysterious as you might think. Try the parameters a few times, record the data, and gradually you'll figure out the patterns. If there's a regret, it's that every time I mix silicone, I have to stir it manually, which is exhausting. If there were an automatic defoaming machine, it would be much easier, but that's another matter. Anyway, that's all for this article. I hope it was helpful to you.