Spreading Out

Not everything we do in this industry involves turning pellets of resin into polymer parts. In many cases, the processor has to do something post-mold that is often decorative.

While the ability to mold or extrude parts with the colour “built in” has been an important advantage of thermoplastics technology for about 70 years, I think that coatings (“paint”) don’t get the credit they’re due in our industry. I was around when the future in automotive bodies was predicted to be plastic, with exotic technologies like “sheet molding compound” and vehicles like the Pontiac Fiero — few survive today, technologically speaking.

Why? The main reason, at least as OEM types tell me, is the inability to obtain a Class A finish on resin body panels in a cost effective way. Of course, the other reason is that the steel industry counterattacked with low-cost, easy to paint solutions like high strength low alloy sheet, making car bodies cheaper and lighter.

Plastics will be back in automotive bodies, but in the meantime, a lot of research into coating plastics has led to “paint” becoming a viable option for many parts, especially consumer durables and novelties. Any-one who’s ever paid to have a resin bumper repainted, however, knows it’s surprisingly expensive — and when pressed, the body shop service writer inevitably explains that it’s because of the high level of preparation required to get the expensive paint to stick. Why? The answer, like so much of plastics technology, isn’t simple, but here’s an “executive summary” of what’s in play when you’re looking at top coating a resin part. If we ignore the coating method at this point, the paint problem distills down to two related problems: wetting and adhesion. Put simply, how do you get the coating to flow over the surface evenly and completely, and then how do you get it to stick?

The first problem, wetting, is intuitively simple, but there’s a little physics involved. Materials, all materials, from a PE pipe to a cheese sandwich, have an intrinsic energy associated with them. The energy comes from the individual energies of the atoms and molecules that are bonded together to form the material. At the surface of a material, the attractive forces between the atoms and molecules are greater than the attraction between the molecules and the surrounding air, which is why it’s a solid and not a gas or vapour.

That’s fine for the solids vs. air comparison, but what about liquids? Water has intermolecular bonds strong enough to keep it from instantly flashing into vapour at normal temperatures and pressures. In space or in free fall (rain) it forms spherical droplets. This is because the lowest energy state for a drop of water is the one with the smallest exposed surface area for a given volume, and that’s always a sphere.

Throw that droplet against a waxed fender and it also tries to form a sphere, or bead as the droplet moves to its lowest energy configuration.

We commonly call the phenomenon “surface tension,” and the point is that getting the droplet to turn into a thin covering film means making the energy available on the part surface large enough to overcome the surface tension, so the thinly coated surface is the lowest energy configuration. Car wax makers want to go one way, plastics processors the other.

Unfortunately for us, neat commodity resins have few “polar groups” (molecules with energy available as electrostatic charge “built in”) compared to highly polar liquids like water, so the tendency is to bead and resist wetting. If the coating you or your customer needs has relatively high surface tension, you need to provide it with a part surface with enough surface energy to encourage that coating to wet spontaneously.

Of course Murphy’s Law rears its head: commodity resins like polyethylene and polypropylene are two of the lowest surface energy materials around, so some surface modification is a certainty to get most coatings to wet the part. Getting that surface energy up usually involves the same technique that water uses, namely adding an atom to the molecule that makes it polar. Oxygen is good choice, since it’s really cheap and doesn’t have to be shipped, mainly because it’s in the air we breathe. Hoe do you get that oxygen to bond to the resin’s weakly polar hydrocarbon molecules? Of course, energy. Plasma, UV radiation, corona discharge, and flame treatment are good, fast techniques. It’s possible to use reactive chemicals like acids where it’s difficult to get at the surface with flame or radiation.

No matter how you do it though, wetting is only half the battle. Getting the coating to stick is can be a lot more difficult. I’ll look at adhesion next issue. CPL