Coatings World - October 2015

Atmospheric Plasma Treatment
October 2015 www.coatingsworld.com Coatings World | 69

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, improve-
ments 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, reformu-
lation 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 sub-
strate 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 ei-
ther 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.