Operation Summary
Surface Preparation
For all coating applications, adhesion of the coating to substrate is one of the most important aspects. To ensure proper adhesion of the coating material, proper surface preparation is critical. Each substrate/surface has unique properties and it takes different preparation methods for the optimum result. Following is typical preparation methods for different materials.
- Metal Surface
- For Wood, Foam, Concrete, or other Porous Surface
- Plastic Surface
- Mechanical Adhesion
- Other Preparation Methods
Surface Preparation for Metal:
The basic pattern of the preparation methods for metal surface is: roughen the surface, clean, and prim. Each application requires different levels of adhesion properties. Some may need the all steps, but some may not need all steps to obtain the level of adhesion property it requires. A user needs to test the methods to establish a procedure for a good adhesion for his application.
Roughening-up the Surface
Typically, metal surfaces are smooth; roughening-up surface increases the surface area at microscopic level and helps adhesion property. This can be done by sand-blasting, sanding, wire brushing, or other methods.
Cleaning:
Typically you can use commonly used industrial solvents such as acetone, MEK, Xylene, and isopropanol. Solvent materials need to be handled carefully since it has high VOC rate and often flammable. Follow the recommendation from your solvent supplier for safety and handling. Residual solvent may affect the quality of coating or hinder the adhesion. Be sure to dry all solvent. Water or high water content cleaning agent should be avoided as possible residual also affects the quality of the coating.
Primer (Undercoat)
Primer or undercoat can be used to enhance adhesion property. Typical primer materials are epoxy dispersions (2-part epoxy in solvent), or solvent base polyurethanes. Silane base material can also be used as a primer. Each primer material has its advantages and disadvantages to the others. Please follow the recommendations from the primer manufacturer for the safety and usage. Some metal, such as some aluminum, has tendency to oxidize the surface very quickly in the atmospheric air. The oxidized metal may hinder the adhesion of the primer and top coat. If the problem is eminent, the application of primer needs to be done soon after roughening surface and solvent cleaning.
Some primer material needs to cure over a certain time. Be sure to give enough time for primer to cure. Sometimes, it is creates better bonding property if you spray top-coat while the primer is still half-way cured. You may spray the top coat soon after solvent is dried, but the primer is not fully cured. The optimum timing of top coat application varies depending on the primer. Consult your supplier of the primer for details. Implement some method to prevent contamination of the surface if the top coat is to be applied later and the items to be coated need to be stored or left in the field.
If silicone is used in the same facility, silicone migration could cause problem with adhesion. Silicone migrates very easily in a building; sometimes by people’s hands and shoes, sometimes it is airborne. A careful control of silicone materials is required to avoid the problem.
Preparing Porous Surfaces (Such as wood Concrete, or Foams)
If you are coating a material with small holes, the preparation emphasis is to sealing the surface. Direct application of the top coat material to these surfaces could cause “dimples” and “bubbles”. As the coating material hits the surface, some material starts to sink into the porous surface. By doing so, it pushes out the air in the small hole. This coating material has such a short gel time that the air coming out of the porous surface cannot escape quickly enough and trapped in the coating as a bubble. Or, even if the bubbles pop, the surrounding material is too thick at that point to fill the void after the bubble pops. This makes dimples. Also, in some cases, the moisture content in the porous surfaced material could cause side reaction with this material to cause gassing.
Sealing materials can be shellac, acrylic, or any other sealer appropriate to the surface. Follow the manufactures recommendation for sealing materials. In this case, the adhesion property of the top coat relies on that of sealing material. Be sure to choose the material with sufficient adhesion property.
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Preparation for plastic surface
In general, plastic surface is very slick and hard to adhere the coating property. Some primer products carry relatively strong solvent to activate the plastic surface and attach functional chemical to the plastic to promote the adhesion. Also plasma surface treatment and halogen gas surface treatment are known to give plastic material a good adhesion.
If the adhesion of the coating is not sufficient with surface preparation alone, giving mechanical structure (only when possible to do so) to create mechanical adhesion can improve adhesion of the coating significantly. Example of mechanical adhesion is to weld steel mesh on the metal surface. Nail in, or screw in the woven fiberglass, or drilling holes. With the additional mechanical structure, the coating material encapsulates the structure, which is mechanically attached to the surface. This method only can be used in certain application that physically possible to create the structure and add one step more to production if used in OEM environment. But in some extreme cases, it is necessary to create mechanical adhesion structure.
Some applicators apply two-side adhesion tape on the surface and spray the top coat directly on it. This is easy way to control the adhesion strength. This material adheres to regular acrylic based pressure-sensitive tape. For special cases, internal adhesion promoter chemical may be added to the top coat to enhance the adhesion property.
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Northstar Polymers, LLC
3444 Dight Avenue South
Minneapolis, MN 55406
Tel: 612.721.2911
E-Mail: info@northstarpolymers.com
Notice: All of the statements, recommendations, suggestions, and data concerning the subject material are based on our laboratory results, and although we believe the same to be reliable, we expressly do not represent, warrant, or guarantee the accuracy, completeness, or reliability of same, or the material or the results to be obtained from the use thereof, neither do we warrant that any such use, either alone or in combination with other materials, shall be free of the rightful claim of any third party by way of INFRINGEMENT or the like, and NORTHSTAR POLYMERS DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, OF MERCHANTABILITY and FITNESS FOR A PARTICULAR PURPOSE.