As expected, the procedures for cleaning and preparing plastic surfaces are almost the same as those for polymer types. Some programs are low cost and easy to complete; others are not. Most plastic types have some highly recommended procedures to achieve a better coating finish, but users are often faced with many options that are equally valid, or worse, conflicting recommendations on strength levels.
Plastics are available in many forms: mouldings, sheets, formed parts, foams and films. Many polymers can be easily blended together to achieve specific properties. Fillers and plasticizers are also included in the resin matrix to impart certain properties. Obviously, the same finishing procedure can be inappropriate in every case, although the main ingredients remain the same. Manufacturers of plastics and coatings should be prepared to advise on the appropriate surface preparation techniques to use with the product. However, because user needs vary so widely, even manufacturers may not have a clear answer for individual cases. Extensive experimentation may be required to determine the appropriate technique for a given operation.
The procedures outlined in the remainder of this chapter focus on technology, not plastics. However, where appropriate, certain techniques listed are suitable for them.
Finally, many processes use hazardous, corrosive, toxic, flammable or poisonous chemicals and safe handling methods, worker training and proper control procedures are essential to minimize risks in the work environment.
solvent cleaning
Solvent action removes surface contamination by dissolving unwanted substances. This is a simple and commonly used procedure, often used as the first step in a more complex procedure. Organic solvents and water are recommended. Organic solvents can be flammable or non -flammable.
Commonly used plastics are acetone, methyl ethyl ketone, isopropanol, methanol, toluene, 1,1,1-trichloroethane, naphtha, and sometimes Freon (alone or mixed with another solvent). Water is cheap and readily available, but often contains traces of impurities, which can also stain surfaces. Distilled or deionized water is usually recommended. Drink water frequently to use as a rinse for other surface preparation procedures.
These solvents can be used for simple procedures such as wiping plastic with a damp cloth. Immersion is a technique usually done by immersing the part in a heated rotating bath to accelerate solvent action. The advantage of spray cleaning is that contaminants can be washed away with the force of the spray. Vapor degreasing involves suspending plastic parts in boiling solvent tanks. As steam condenses on the part, a constant flow over the surface washes it clean. Ultrasonic degreasing uses the high-frequency vibrations of sound waves to remove contaminants in a solvent bath.
In all cases, it is recommended that the solvent be changed or filtered frequently to prevent residue buildup and recontamination. Compatibility of solvents with plastics should be checked thoroughly. Care should be taken since some plastics absorb water. Heat is often used to dry parts after cleaning, but heat can easily deform thermoplastics.
detergent cleaning
Soaps and detergents do the same to plastic parts as they do in dishwashers and washing machines at home.
Emulsions of oils, greases and some mold release agents are easy solutions in hot or cold water . Detergent cleaning is often used as a preliminary step in mechanical treatment. Cleaners have familiar names because they are a recurring item in the home. Ivory soap, Ajax, borax, and trisodium phosphate are all recommended for various cleaning operations and work well.
Dip washing works great unless the plastic is water sensitive. Scrubbing with a medium-bristle brush is great for removing many contaminants.
The cleanliness of the solution needs to be monitored. Clean water rinsing is required because soap can act if not wash off contaminants. Thorough drying on high heat is recommended.
mechanical treatment
Physical scrubbing of plastic surfaces removes oxide and contamination layers. Solvent or detergent cleaning is required prior to mechanical treatment to avoid scrubbing surface contaminants into rough surfaces. This process usually occurs prior to chemical treatment.
Sanding, either wet or dry, is a common procedure, using 40 to 400 grit, depending on the amount of surface to be removed and the desired surface finish. Softer plastics are obviously more susceptible to damage. For parts with complex configurations, sandblasting (wet or dry) or wire brushing are appropriate. Grain size and type can be varied to obtain the proper finish. Machining can be used to expose new layers of plastic for coating.
In all cases, rough surfaces should be vacuumed or air blown to remove residual dust or grit. It is also recommended to use a solvent wipe or water rinse, followed by drying on high heat.
chemical treatment
Typically, an effective surface treatment is chemical etching of the plastic to be coated. Both can alter physical and chemical properties to improve wetting and film adhesion. Almost always, chemical treatment is preceded by one or more cleaning operations to remove surface contamination. These prior manipulations reduce solution contamination and ensure better interaction between solution and substrate.
In most cases, surfaces are cleaned or immersed in acids, bases, oxidizing agents, chlorinating agents or other highly reactive chemicals. Each procedure requires control of part active ingredient, solution temperature, and soak time by weight. Some programs have broad ingredient ratios, while others are very specific. Solution temperature is inversely proportional to immersion time; that is, the higher the temperature, the shorter the exposure time.
Almost all chemical etching procedures require a water rinse (once or twice) and high temperature drying is recommended. For active ingredient treatment, it is imperative that the solution strength be monitored and refreshed at appropriate intervals.
Other processing
Various other cleaning and preparation techniques can be used. Many of these procedures are unique to plastics processing and were developed to overcome the low surface activity exhibited by many plastics. To be most effective in most cases, these treatments require prior decontamination of the surface with solvents or cleaning with detergents.
1) Primers
A primer is an adhesion-promoting coating used to make the final coating adhere better to the plastic substrate. Primers can be of any number of chemical types including silane, polyurethane polymer, isocyanate, nitrile phenolic or vinyl. To protect the substrate from recontamination, a primer is typically applied to the surface shortly after other surface preparation procedures have been completed.
2) flame treatment
Flame impact on many plastics such as polyolefins, acetals, fluoropolymers and polycarbonates oxidizes the surface to provide higher levels of surface energy and better film adhesion. The process is especially effective on complex shapes and molded parts. Superheated air (1000°F) performs roughly the same function as a flame.
3) Exposure to UV radiation
High intensity radiation from UV sources provides ionized or highly polar surfaces.
4) dry
Since many plastics readily absorb moisture from the atmosphere, simple oven drying may be effective.
5) Plasma treatment
Most plastics benefit from plasma treatment. Parts are exposed to gases that are discharged by radio ionization frequencies or microwaves. Valid gases include neon, helium, oxygen, and water vapor. Although very effective at improving surface conditions to promote better film adhesion, this process is often equipment limited and limited to smaller components and parts.
6) Corona discharge
For films and other thin gauge plastics, the surface tension can be increased by passing between two electrodes. This processing is suitable for high-speed operation.
