You may not need to know what is going on inside that paint you use every day but it’s still a fascinating subject, says Refinish Marketing Director ANZ, Kevin Woolerton.
A bowl of spaghetti
We paint chemists talk about paint being made up of a polymer or resin which is the same thing. Drill down to the molecular level and it’s essentially made up of long strands of carbon atoms with other atoms and compounds attached, depending the type of paint it is. Imagine these long strands as a bowl of spaghetti – it’s not a liquid but it will flow and the strands can twist and coil around each other. If you evaporate all the water and dry it, you get a solid – this is the basic concept of a 1K or one-pack paint product.
For a two-pack product, we take that bowl of spaghetti and add another ingredient, the hardener, which causes cross-links to be formed between the polymer strands, like the rungs on a ladder. For the typical polyurethane refinish product, the hardener is an isocyanate and the chemistry is the same, regardless if it’s a primer, a colour coat or a clearcoat. The cross-linking reaction happens quite slowly at ambient temperatures, therefore giving enough time for the paint to be applied before baking accelerates the chemical reaction to fully cured. Paint chemists refer to ‘cross-link density’ and the higher it is, the more rigid and robust it is, although taking it to the extreme will make it brittle. PPG’s ground-breaking CeramiClear® mar and scratch resistant clearcoat is a good example of a product that is super-tough thanks to its high cross-link density.
OEM vs refinish
The difference between OEM vehicle finishes and their refinish counterparts, largely comes down to temperature. In the OEM world, the paint chemistry is designed to react and cross-link at high bake temperatures (typically, 120°C to 130°C) without using a hardener. This is fine on an OEM production line because they are working with a bare bodyshell with nothing attached. In contrast, taking a car off the street and baking it in a booth at 120°C to 130°C would see the plastics melted on the floor, the electronics fried, etc. This is where isocyanate hardener comes in. In a refinish situation, it allows you to get the same degree of cross-link and durability, etc. after being cured at typical spray booth bake temperatures which are around half of those used by OEMs.