Roobol on Painting
Usually start with a header?I am painting brass pen caps and barrels with a polyurethane black and clear coating. Cured parts are tested in a package to mimic shipping conditions. The coating behaves as if it were a thermoplastic resin. Parts held at 140°F with a relative humidity of 50 percent become marked by a string which holds the parts in the box. Information about both the coating application process and the paint chemistry is below.
• Parts are cleaned using ultrasonics in an aqueous solution, then chromated to provide good adhesion.
• We use a chain-on-edge conveyor on which parts rotate in front of HVLP guns so that the coating covers 360° on these cylindrical parts.
• We start with a black base coat at about 0.35-0.60 mil thickness.
• After 30 min flash-off, we apply a clear coat. On average, 5 coats of clear (about 0.2-0.3 mils per coat) are applied with 4 min flash-off between each coat.
• Then parts are baked at 290°F for one hour. Our supplier recommends baking at only 160°F for 30 min, which we far exceed.
(Two different suppliers not named here): Formulation is based on volume %.
Black basecoat: modified polyester resin 38%; a modified aliphatic polyisocyanate resin catalyst 38%; thinner 23%; and an additive 1.0%. Coating viscosity is around 16 to 17 sec in a Zahn #2 cup.
Clear : two-component linear aliphatic polyester polyurethane 44.5%; catalyst 44.5% (same catalyst as the basecoat); thinner 11% and the same additive as for basecoat 1%. Coating viscosity is around 15.50 to 16.50 sec with a Zahn #2 cup.
Parts coated as described pass all our other tests for adhesion, wear resistivity, corrosion, engraving (proper hardness vs. softness) and pencil hardness of 2H. The only test they failed is the marking of the parts in the box (in which we ship pen products to our customers).
We have also tried another chemistry for the paints but still get the same concern marking defects. The process is same as above. The paint was acrylic urethane:
• Black base 2 parts; hardener 1 part diluted by 25%.
• Clear was a high-solids urethane 2 parts and hardener 1 part.
We have asked both suppliers for help but neither was able to. I have done some testing in order to identify sources of the problem with some interesting results.
• First, I varied ratios of resin and catalyst to see if decreasing the amount of catalyst would reduce marking. As we know, isocyanates love moisture which the test condition (140°F and 50% RH was abundant. There was no visible difference in terms of marking.
• I added glass beads based on a recommendation from a friend in order to improve mar resistance, but the glass beads did not help either.
• I tested same parts in an oven where there was no added humidity. With just a regular dry oven the marking problem did not repeat. Thus, moisture in the chamber is creating the problem.
• After parts were coated for more than 60 days, the problem still persisted. So, aging of the coating is not a solution.
So, the questions I have are:
1. "Why is moisture creating a problem?"
2. "If there is excess isocyanate, why has the problem still persisted when I reduced the amount of catalyst by 75%?"
The frustration you are feeling is very apparent in these questions. Yes, I can provide answers and at the same time simplify the situation greatly for you and your company. To answer your first question, the moisture is definitely swelling the coating so that the marking becomes visible in a humid oven but not a dry one. This water adsorption is not surprising given the number of polar sites in the cured paint resin macromolecules. I'd like to have you try placing marked parts in a dry 140°F oven to see if the marks disappear by driving out some or all of the moisture. I'm betting that it will.
As to question number 2, I'm convinced that if the paint is mixed in the proportions advised by your paint suppliers, no excess isocyanate will be present in the cured paint film. In fact, your tests with reduced isocyanate proved exactly that.
So now we have to deal with what you should do in the future, and that boils down to two choices. Let me ask if the "string hold" test at 140°F and 50% RH is a proper one to use to simulate shipping conditions. If the answer is yes, then can you package the product differently to avoid any marking? If the answer is no, then the appropriate action is to use a harder paint that resists marking. Either changing packaging or changing paint (or combination of both?) will solve the problem.
Is there an additive that can make coating more hydrophobic so that it does not adsorb moisture in the chamber? I was told glass beads, "Prizmalite," would help to make the coating to behave more like it was hydrophobic. The packaging is our standard box so it cannot be changed. Do you know any chemistry (supplier/product name) that would give pencil hardness of 2-3H? We need this hardness to make sure parts do not mar during our processing and at the same time remain flexible enough so that it does not chip when we engrave indicia into it.
Glass beads will not significantly alter the hydrophilic character of a given resin; actually, no additive is able to accomplish that. Don't pay anyone to develop a new coating. There are already suppliers that can provide suitable coatings with the appropriate hardness/flexibility ratio. But unfortunately, the suitability of any paint to your requirements cannot be assured without actually testing that formulation. You need to contact potential paint suppliers and obtain coating samples for mar and chip resistance testing. This testing can be readily done in-house on painted parts. Use the heat/humidity test for determining marring, and engrave the parts to see if they chip. The suppliers may also be willing to test their products on your pen parts. Only by physical testing of the paints can you find the best material for your purposes.