Coatings World - October 2015

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.