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Flawless Coatings
Bill Lewis, California Hardcoating Company and Barbara Kanegsberg, BFK Solutions LLC
Let’s suppose you see visible particles on a hardcoated product. This is not a desirable situation. Coatings have to be flawless for good aesthetics and for optimal function. How do you find the source of the problem? How do you fix it? There are simple solutions that are applicable to a range of high-performance engineered coatings. Success with high-performance coatings involves keeping the part clean before and during the coating and curing process.
What are hardcoatings
Generically, hardcoatings are sol-gels of silica and resin nanoparticles that are applied in liquid form and then cured using either ultraviolet light or heat. They are relatively thin, typically approximately 5 microns, and provide a glass-like hard finish that is thin enough to be adherent and thin enough to be a bit flexible and resist cracking. Hardcoatings are applied to substrates such as polycarbonate, acrylic, fiber-reinforced plastic or polished metal in order to protect them from abrasion and scratching as well as from chemical damage. They are used in optical plastics for instruments, for lenses, and for windows. They could also be used for computer or phone cases. Any surface that needs protection against scratching and abrasion is a candidate for the use of a hardcoating. This helps manufactured products to retain their “like-new” appearance and functionality longer.
Because hardcoatings are ten times thinner than traditional paints, particles stick out like a sore thumb; even small particles are greatly magnified when you add the thickness of the coating covering them.
Products with high-quality hardcoatings that meet your customer needs involve a well-formulated coating, a defined and controlled process for applying the coating, and – you guessed it! – careful surface preparation. Sources of particles include the coating and curing process, the air, and the substrate (the part being coated).
Coating and curing
The coating itself should not be a problem as supplied. If the coating should prove to be the source of particles, contact your supplier immediately. The coating process is more likely to be the source of the problem.
Coating processes are general classed as dip, spin, spray, or flow. In dip coating, the hardcoat could be compromised by particles carried into the bath by previously-coated parts, so equipment design and coating bath filtration is essential. Spin is used for single-sided coating of such product as lenses. Spray coating is used in high-volume production; it can be used for single-sided coating and for complex parts. In flow coating, the part is flooded with coating in a vertical position, so the part is effectively being washed with coating. This practice tends to minimize adhesion of particles. In spin coating, a similar effect is obtained as excess coating is applied and spun off.
In contrast with dip or flow coating (and sometimes spinning) where the coating is reused after filtration, spraying is single-pass and thus avoids potential particle contamination from reuse. However, the substrate does not get the benefit of being washed by the coating, so surface cleanliness is even more important. In any of these processes, the coating equipment can contribute to particle contamination; part of the solution involves appropriate equipment design. For example, in dip processes, a tank with a four-way overflow is essential.
Air
Particles are found even in very clean air, so with any coating process, particles in the surrounding air have to be minimized. Contamination from the air can occur not only during the coating process but also during drying of the solvents and curing, whether by U.V. or heat.
HEPA (High Efficiency Particulate Air) filtration is essential. Fortunately this does not necessarily involve constructing an entire cleanroom to achieve the needed degree of cleanliness. Cleanrooms involve a substantial capital outlay as well as ongoing monitoring and employee education/training. Fortunately, what are termed mini-environments are more fiscally-accessible. The coating booth can be turned into a mini-environment, with the filtered air directed toward the part to be coated.
People are another major source of particle contamination. Automating the process is useful not only in minimizing contamination but also in achieving a consistent process. In addition, developing a workforce that is skilled and well-educated in hardcoating and in contamination control is a great investment.
The product
This involves coating a clean part and keeping it clean before, during, and after coating. While particles are a major challenge in hardcoating, thorough removal of oils and thin films improves adhesion and also reduces particle contamination. Thin film residue can make the particulate problem worse because particles stick to surfaces via both physical and chemical forces. This means that critical cleaning is essential for hardcoating and for other high-performance coatings.
If you have issues with hardcoating, review the cleaning process. Do you do the cleaning? Are the parts clean enough as received? If you clean the parts, do you store them in a particle-free environment? Has the cleaning process itself been optimized? Is the cleaning agent appropriate for the soils? Review the wash, rinse, and dry steps of the cleaning process. Do any of the steps inadvertently promote recontamination?
With careful attention to details, high-quality, particle-free results can be obtained.
William Lewis, PhD is President of California Hardcoating Company in Chula Vista, CA. cahardcoating.com/index.php
Barbara Kanegsberg, is President of BFK Solutions LLC in Pacific Palisades, CA
bfksolutions.com.
