Reducing Hardness for Polyester Resin

Q: Is there any way to reduce the hardness for powder coating? We are using a polyester resin for a powder coating with a pencil hardness of 3H. However, our customer requires 1H hardness.

A: Yes, you can reduce the hardness of a polyester powder coating by adjusting the formulation. Basically, you can do this by reducing the crosslink density (distance between chemical bonds in the cured powder).

This can be accomplished by switching to a carboxyl polyester, which has a lower acid value. Traditional polyesters designed for tri glycidyl isocyanurate (TGIC) or hydroxy alkyl amide (HAA) have acid values around 30-35. Switching to a polyester with an acid value of 20-25 will require less crosslinker (TGIC or HAA), which will reduce the pencil hardness. Be aware that this will also reduce the chemical resistance and probably the corrosion resistance, so be careful when making this formulation adjustment.

Another way to reduce the crosslink density and hence the pencil hardness is to simply reduce the crosslinker level (TGIC or HAA). Traditional polyester TGIC powders are formulated at a 93:7 ratio (polyethylene [PE] to TGIC) and HAA polyesters at a 95:5 ratio (PE to HAA). Reducing the crosslinkers to 94:6 (TGIC) or 96:4 (HAA) will reduce the pencil hardness. Again, the coating will not be as robust for chemical resistance and corrosion resistance.

Furthermore, “superdurable” polyesters are inherently harder than standard durable polyesters. As a result, if you are using a superdurable-grade polyester, replacing it with a standard grade will lower the pencil hardness (probably one pencil).

Choice of mineral filler also affects pencil hardness. If your formulation contains a mineral filler, check its Mohs hardness. If it is over 5.0, you may want to change it to calcium carbonate (3.0) or barium sulfate (3.5). This will reduce the pencil hardness a little.