Makers of a wide variety of outdoor building products have increasingly turned to plastics to give their products longer service life. Roofing, siding, awnings, decking and high-end outdoor furniture are just a few examples of the types of products that were once made exclusively of asphalt, aluminum, wood, and fabric and now have alternatives made of plastic. 

Plastics can provide a number of in-use performance advantages over traditional substrates for outdoor products. They don’t degrade as quickly, and therefore they stand up better to ultraviolet rays. They’re stronger, resulting in minimized abrasions, scratching and splintering. They’re available in a wide range of colors and can maintain their original intensity longer. Plus, they have improved resistance against mold and bacterial growth. In short, plastics give consumers an ease of maintenance throughout a product’s lifecycle and an aesthetic advantage they can’t get with other materials.

However, many of these advantages require an important component for extended service life: the right coating chemistry. Because coatings are applied to the surface only and can be concentrated on a product’s “impact zone,” they provide a cost-effective alternative to imparting performance characteristics to the entire plastic compound.

With product manufacturers seeking to offer warranties of 15-20 years on their plastic products, they need to be confident that the coatings they use have a strong bond to the surface, restrict color changes, do not chalk (which is the result of the chemistry degrading under UV exposure), inhibit bacterial growth, and are flexible enough to expand and contract under varying temperatures.

Some Plastics Are Difficult to Coat

Certain plastics, such as hydrocarbons like polyolefins as well as various engineered thermoplastics, inherently have very low surface energy and are non-polar or hydrophobic, resulting in a non-stick surface (also referred to as not “wettable” with respect to coating the surface). The lower the surface energy, the more difficult it is for the coating to bond to the substrate. The key to achieving acceptable adhesion with low surface energy plastics is to formulate the coating to have a lower surface tension than that of the substrate. 

As a general rule, acceptable adhesion is achieved when the surface energy of a substrate (which is measured in dynes/cm) is approximately 10 dynes/cm greater than the surface tension of the liquid. Moreover, the higher the surface energy of the solid substrate relative to the surface tension of a liquid, the better its “wettability” and the smaller the contact angle. Determining the surface wettability of a plastic is accomplished by measuring the contact angle using a Goniometer or by the use of dyne test pens. Figure 1 shows the difference in contact angle between poor and good surface wettability.  

Achieving adhesion is critical to the success of the coating chemistry and it can impact other performance aspects, such as abrasion resistance, UV blocking, stain inhibiting and water repellency. ASTM D3359 is the Standard Test Method for Measuring Adhesion by Tape Test, which is a good test to indicate the level of adhesion to the substrate. In accordance with this test, an angled lattice pattern (cross-hatch) or “X” is cut into the coating, penetrating through to the surface of the material. Tape is applied on top of this area, pulled off, inspected, and then rated. 

Matching the flexibility of the coating to the substrate is another key challenge when working with hard-to-coat plastics. Consider a piece of vinyl siding: In the winter months, vinyl can expand and contract from extreme temperature fluctuations. In the summer months, the vinyl can warp and stretch under high temperature and humidity. If the coating is too hard, it may check or crack, then lose adhesion after being exposed to these environmental conditions. One test that will help determine the proper hardness of a cured coating formulation is pencil hardness. Pencil hardness is explained under ASTM D3363 and gives directional results during the formulation process. Tg or glass transition temperature is another a key indicator that quantifies the hardness of a coating. Today, new technologies in resin systems allow formulators to blend ratios of higher Tg and lower Tg polymers to achieve the flexibility necessary for end use.

Process & Engineering Options

Plastic manufacturers can overcome some of these aforementioned coating limitations by engineering their products with materials that are more compatible with coating chemistries. 

An additional option for helping coating chemistries bond properly to plastics is through in-line surface pre-treatments, such as flame, plasma treatment and corona treatment. These treatments change the surface tension of the plastic by oxidizing it, thus making it more receptive to bonding with the coating chemistry. Each type of treatment is application-specific and offers pros and cons.

In flame treatment, a flame treater device applies a controlled open flame to functionalize the surface of the substrate just before the material reaches the stage of the coating application. Typically flame treaters are the most cost-effective treatment option for coating lines; however, the heat can affect the appearance of the surface.  

Corona treaters use a high-frequency electronic discharge to increase the polarity of the surface by breaking down oxygen molecules into an atomic form. This surface modification can be debased over a short period of time, so it is necessary to incorporate corona treaters directly in-line with the printing or coating station. 

Plasma treatments are required for more complex surfaces, where corona treaters cannot be used. Plasma treater mechanisms remove any foreign contaminants on a non-wettable surface by way of oxygen gas or atmospheric air. Depending on the substrate, a plasma treatment can remain effective for days or sometimes months.  

What to Look for in a Coating Chemistry

When determining the proper coating chemistry, performance and adhesion are the primary factors of focus. Once performance criteria have directed the formulator to a specific resin chemistry, there are a few methods for promoting adhesion. Adjusting the solvent and co-solvent composition in the coating, incorporating primers and tie-coats, or introducing additives for covalent bonding, are all avenues for bonding a particular chemistry to a hard-to-coat surface. 

When choosing the appropriate solvent or co-solvent for a formulation, the chemistry of the plastic substrate should be investigated. The goal is to partially solubilize the surface of the polymer to allow the coating to penetrate into it and establish a cohesive bond between the two polymers. Sometimes a coating chemistry favored for performance purposes isn’t compatible with the plastic substrate, even when the solvents have been considered. 

A primer or tie-coat is a good solution to act as a compatible intermediate coating layer to bond the plastic with the topcoat chemistry. Primers offer a rougher, higher-surface-energy layer to maximize adhesion, and often incorporate an adhesion-promoting agent dispersed in solvent or water. Adhesion promoters are additives that can be incorporated into the primer as mentioned, or sometimes into the coating composition itself. Adhesion promoters create one of three types of bonds: covalent, forces of attraction, or chemical similarity. Substrates such as polyolefins and other hydrocarbon thermoplastics, which are becoming more prevalent in upholstery and roofing membranes, are extremely difficult for coating adhesion; hence, a polyolefin-based adhesion promoter may be the best way to bind the coating chemistry to the surface because of their similarity. 

Once the adhesion issue is addressed, plastic makers need to be concerned with other performance criteria demanded by end users, including:

• UV degradation — Certain chemistries like Kynar®, a fluoropolymer resin technology, will be more resistant to UV degradation than other resins such as vinyl acrylics.
• Mold resistance — Coating developers can incorporate bactericides, such as nano silver, to help the plastic product restrict or even kill mold growth.
• Abrasion and scuff resistance — Harder, tougher resin technologies, such as polycarbonates, can resist harsh impacts better than other polymers such as acrylics. 

Environmental Regulations

Because solvents tend to help improve adhesion of a coating to a substrate, some plastic makers continue to rely on traditional oil-based and solvent-borne coating chemistries for rigid plastics, and employ oxidation units to trap related emissions. However, federal, state and local environmental regulations limiting emissions that produce ground-level ozone — as well as employee safety and health concerns — have started pushing the coatings industry toward water-borne, zero or low-VOC formulations. What this means for plastics manufacturers: sourcing a high-performance, low-emitting coating for a hard-to-coat polymer can be a challenge.  

As regulations continue to change, traditional coating chemistries can be adversely affected, causing compromises in performance. Finding a hands-on expert in engineering coating chemistries, current regulations on exempt solvents, and application equipment is highly critical for the successful commercialization of a coated plastic. The plastic maker, coating developer, and in some instances the printer/coating applicator, will need to work collaboratively. Optimizing the plastic polymer, incorporating advanced polymers into a compliant, but compatible, coating chemistry and modifying the application equipment are all variables that function to achieve the required performance criteria. 

Some plastics can be difficult to coat, but with proper attention to pre-treating the surface, coupled with the right custom-engineered coating chemistry, plastics manufacturers can obtain strong adhesion for the type of long-wearing aesthetic advantages consumers demand. 

ABOUT THE AUTHOR

Michael Couchie has served 26 years at APV Engineered Coatings, focusing the vast portion of his career on new product development and sales for emerging markets. He has a Bachelor of Business Administration degree in sales management from the University of Akron and has contributed to $25M in sales growth over the course of his career at APV.
His expertise lies in project management and commercialization of high-performance finish technologies for advanced textiles, flexible films, and composite building products. As vice president of sales, he has spent the past 12 years successfully implementing a variety of new factory-applied technologies, including Kynar®-based coating systems for automotive upholstery, siding, roofing, awning, decking, wallcovering, commercial signage and advanced films.