Abstract

UV curable coatings are becoming an increasingly popular alternative to conventional coatings.  Compared with thermal coatings, UV coatings provide a number of benefits to plastic part manufacturers including enhanced appearance, improved performance, and various process worker safety, and environmental advantages. But, same high speed curing and highly-crosslinked chemistry that underlie these benefits can also make adhesion failures more likely.  This paper examines the problem of adhesion common to UV curable liquid and powder coatings, and describes some tradeoffs associated with popular methods to reduce adhesion problems. Atmospheric plasma provides an especially attractive method of enhancing adhesion of UV cure coatings to a wide range of plastic materials.

Background

Since the early 1970s, UV curable coatings have gained slow, but steady acceptance as an alternative finishing technique for a wide range of substrates from wood flooring, to glass optical components, and from pipe and tube to plastic cosmetics containers. Today for example, nearly all polycarbonate headlight lenses, most plastic commercial eyewear, and a large percentage of plastic consumer electronic devices are UV coated. 

A number of factors are believed to be responsible for the success of UV coatings growth (Cohen, 2012).  First, UV curing is an extremely rapid process compared with conventional thermal baking and curing. UV formulations cure almost instantaneously when exposed to ultraviolet light (Walton, 2012). By contrast, conventional waterborne and solvent borne systems require substantial oven dwell times and relatively higher temperatures. This makes UV curing attractive for coating applications such as graphic arts and printing, optical fiber coating, wood molding and panel finishing, and similar high speed applications.

This speed advantage is even more impressive when comparing traditional thermoset powder coatings to UV curable powders.  Once applied, traditional thermoset powder coatings require a substantial amount of time in order for the powder to melt and flow to produce a smooth, continuous film. This melt/flow process is important to achieve both aesthetic and performance qualities.  After flowing, thermoset powder coatings also require substantial dwell time at high temperatures to achieve the complete polymer cross-linking needed for full performance. Taken together, this two-stage process routinely requires between 20 and 60 minutes (Walton, 2012).  By contrast, UV curable powder coatings also use thermal energy to melt and flow the powder coating, but rely on ultraviolet light instead of heat to achieve crosslinking. With UV cure powders, total process times of less than 10 minutes have been reported (Schwarb and Knoblauch, 2011) .

Another feature of UV coatings is their tough surface properties; particularly high scratch and mar resistance. It is these properties that make UV coatings especially well-suited for applications such as hardwood flooring, optical coatings, and CD/DVD coatings where surfaces often must take a good deal of abuse. The surface durability of UV coatings stems from the high cross-link density common to UV formulations particularly those using popular (meth)acrylate chemistry (Schwalm, 2006).  But while high crosslink density provides tough surfaces, it has some drawbacks as well. For example, physical shrinkage is closely associated with the high crosslink density found in UV films. The acrylate monomers and oligomers shrink considerably as longer-distance Van der Waals forces are replaced by strong but shorter covalent bonds. Schwalm (2006) suggest that shrinkage as high as 35% can occur in UV formulations. Shrinkage causes internal stress that can result in defects and dimensional changes leading to decreased adhesion (Jian et al. 2013).

A third feature of UV cure liquid coatings is that instead of conventional organic solvents, UV formulas frequently employ low molecular weight additives such as monomers and other reactive diluents.  These are fully consumed in the curing process, leading to the notion that some UV coatings are “100% solids” materials.  This feature has attracted the attention of government regulatory agencies and environmentally conscious manufacturers (Loof, 2001). But again, while eliminating solvents results in fewer hazardous air pollutants and lower VOCs, removing these solvents presents a challenge to attaining adhesions, since solvents help wet-out the surface of the part. Powder coatings emit virtually no VOCs or hazardous air pollutants, and contain no solvents (Whitfield, 1995). However, most powder applicators must invest heavily in chemical pretreatment prior to powder coating to attain sufficient adhesion.

To make matters worse, many popular plastics are tough to begin with.  Their surfaces are more chemically inert. Table 1 illustrates this by comparing the surface energy of common plastics with the surface energy required to attain adequate adhesion for various coating technologies.  UV curable coatings require higher surface energy to achieve adequate performance than their conventional counterparts.

Taken together, UV coatings provide attractive benefits but also present formidable obstacles to achieving adhesion for coatings, inks and adhesives.  The inherent high cross-link density of UV formulas results in mechanical stresses that, combined with the absence of conventional solvents, and their higher surface energy requirements, make it more difficult to ensure proper adhesion. 

Improving Adhesion to Plastic

A number of possible routes for improving the adhesion of coatings to plastic substrates are available. The most popular alternatives include reformulating the coating, adding adhesion promoting agents to the process, modifying the composition of the substrate, applying an additional layer of primer coating, or raising the surface energy level of the substrate using corona, flame or plasma surface treatment (Ryntz, 1994).

Frequently the presence of contaminants is also a factor in adhesion failures. Contaminants include soils, mold release agents, or oily fingerprints or can stem from chemicals within the plastic as materials migrate to the surface. Wiping parts manually with solvent creates a concern for worker safety since exposure to caustic cleaners and harmful solvents, as well as hazardous VOCs. Manual wiping is also time consuming, so automated methods like plasma removal are better suited to high speed processing, and thin deposits of contamination.

Reformulating a coating is another potential path, but it is often means sacrificing other coating properties (Burak, 2003). Suppliers are often unwilling to modify coatings unless the user is willing to pay for additional formulation and tolerate long delays as new versions of the coating are tested. Also, improvements in adhesion can sometimes come only at the expense of other properties such as changes in gloss, less surface hardness, or an increase in the coating’s cost. In some industries, reformulation may also require requalification or recertification of the material or process, incurring additional testing time and cost.

Another alternative is to modify the composition of the substrate itself. But since designers often choose plastics for a range of other mechanical properties such as machinability, weight, mold time or dimensional stability, replacing a plastic may be difficult if there are few substitutes that provide these desired properties, or can meet the target cost (Ryntz, 1998).

Still another route to attaining adhesion is to incorporate a thin ‘tie-coat’ of chlorinated polyolefin to assist in promoting adhesion of the coating to polyolefins. The thickness of the tie coat critical to obtaining good adhesion. If the coat is too thick, cohesive failure within the tie-coat can occur, while too thin a tie coat will not provide adhesion (Ryntz, 1994).
The remainder of this article discusses attaining adhesion by treating the substrate surface in order to remove contaminants and to increase the surface energy so enable strong adhesion. This method is safe, economical, does not require reformulation of either the coating or the substrate, or applying an additional coating.

Adequate adhesion requires the presence of strong forces where the coating and surface meet.  Plasma can significantly increase the surface energy at this interface by replacing less active saturated hydrocarbons with more reactive hydrophilic and hydrophobic species. Using oxygen to create greater chemical functionality improves the wettability of the surface. Figure 1 illustrates this effect of plasma on increasing the surface energy of a typical polypropylene plastic.

Open-air plasma (plasma that can be used on a benchtop with no special environment) produces a stream of electrons, radicals and ions that strike the plastic surface with sufficient energy to cleave molecular bonds of most plastic substrates. This cleavage produces free radicals that react quickly in the presence of oxygen to form more chemically active groups such as hydroperoxide (HOO-), hydroxyl (HO-), carbonyl (C=O), and carboxyl (HOOC), groups. Even a relatively small number of these functional groups can be highly effective at improving adhesion to the plastic surface.

Plasma Treatment and UV Coating Adhesion

Atmospheric plasma treatment has been proven to be especially effective at improving adhesion of UV cure liquid coatings. Successes include using plasma to improve adhesion to polyamide fascia used in automotive interiors (Melamies, 2012). shows the beneficial effects of plasma treatment for coating biomedical devices (Oehr, 2003) and surface treatment for coating PC and PMMA plastics (Gururaj et al. 2011). 

Contact angle measurements provide a highly accurate, and quantitative means to assess surface energy since a smaller contact angle is directly associated with greater wettability. Figure 2 shows the effect of plasm treatment on PC and PMMA plastics. Plasma increased the contact angle on the PC substrate from 80o before treatment, to 43o after plasma treatment and on PMMA from 65o before treatment to 55o after plasma treatment.

The successful use of plasma for UV liquid coatings can also be extended to UV cure powder coatings.  UV powder was commercialized in 1998, and has expanded the applications for powder coating beyond metal goods to markets that require more heat-sensitive substrates such as plastics and wood.  The allure of UV powder coatings is that they combine the durability, cost efficiency, and environmentally friendly characteristics of conventional powder coatings with the low temperatures and fast speed afforded by UV crosslinking (Mills, 1998).  However, difficulties in achieving adhesion of UV powder coatings has also been reported (Skinner, 2003).

Recently, some of these adhesion problems have been overcome using atmospheric plasma surface treatment. Plasma treatment is a safe, inexpensive and environmentally desirable alternative to traditional cleaning methods such as solvent wipe, reformulation or flame treatment. Active species in the oxygen combine with UV energy to drive a chemical reaction that removes surface contaminants, eliminating the need to clean the plastic surface manually. Plasma treatment is an effective process for both cleaning and activating difficult plastic surfaces prior to powder coating. 

This article illustrates this solution with examples of standard test plaques molded from of various blends of polypropylene, ABS, polycarbonate, ABS/Polycarbonate, and Nylon. Plasma surface treatment was performed identically on each test panel at a line speed of 20 FPM using a Plasmatreat RD1004 rotating nozzle laboratory system.

In order to promote electrostatic attraction of the powder coating to the non-conductive plastics, a thin (10-12 micron) conductive coating (Chemical Technology Inc. CTI-4386 or CTI-1693 or similar product) was spray applied and air dried. Next, an acrylated polyester UV curable powder coating was electrostatically applied at a film thickness of 55-60 microns. 

The test panels were heated in a 230F electric convection oven for 10 minutes to allow the powder coating to melt and flow smoothly over the surface of the panel surface. The test plaques were cured using a (Fusion, 300W/in) gallium additive UV lamp.

Full cure was confirmed by using 50 double rubs of methyl ethyl ketone, with no measurable loss using a 60o gloss meter.  Adhesion on each panel was evaluated using standard crosshatch adhesion test method ASTM D 3359 (see Table 2).

This work provides several interesting insights.  First, plasma treatment had a pronounced improvement on coating adhesion for a number of substrates. Polypropylene, polycarbonate, ABS and ABS/PC panels which had no coating adhesion without surface treatment, showed excellent adhesion after atmospheric plasma treatment (see Figure 3). Second, there is still work to be done. For example, the polypropylene tested according to this method did not produce the same level of adhesion as we obtained on ABS or ABS/polycarbonate blends and we could not obtain adequate adhesion to Nylon 6 with plasma surface treatment conditions test so far. Further improvements may be achievable with refinement of the surface treatment process. 

Another important observation is that while Some coating and substrate combinations (for example the ABS/polycarbonate blend used here) sufficiently might work well without surface treatment, this does not mean that plasma provides no added benefit. Since many manufacturers report turning to using regrind or recycled materials to reduce cost, plasma provides added insurance against failures by controlling the process even when the resin composition might change from batch to batch.

We should also point out that our testing intentionally used a “stock” UV powder coating and standard plasma lab system. Since it is common practice to tweak the powder chemistry or fine-tune the plasma settings to optimize adhesion, we consider these results even more illuminating. The fact that the majority of powder/substrate combinations showed robust evidence that good results are possible without much fuss.  In three of the four cases plasma treatment made the difference between an acceptable and unacceptable process. 

Certainly follow up work can focus on testing with variants of the powder coating, and different surface treatment parameters to improve the adhesion of UV powder coating both to these substrates, and to other popular plastics such as SMC, BMC,  and Nylon.

This article shows how atmospheric plasma treatment can overcome adhesion failures commonly associated with UV liquid and powder coatings to plastic. Our testing also suggests that plasma surface treatment may offer a robust solution to applying a wide selection UV powder to a number of common plastics. 

Our initial results provide evidence that plasma treatment yields acceptable results on otherwise un-coatable surfaces. Plasma offers a safe, cost-efficient, and environmentally friendly means for improving the performance of both liquid and powder UV coatings for a growing range of plastic applications. 

This paper was originally presented at the 2nd Biennial Eastern Coatings Show, June 1-4 in Atlantic City, New Jersey.

Acknowledgements

The authors wish to thank Kevin Biller, and the research staff at the Powder Coating Research Group, Columbus, Ohio for their expertise, help and dedication to this project. 

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