Sunday, October 29, 2017

Catalyzation and Curing

Gelcoat is mixed with MEKP (Methyl Ethyl Ketone Peroxide) catalyst.  The MEKP bonds with the resin molecules and forms a hard polyester plastic.  The chemical reaction gives off some heat and in theory, all the MEKP is bonded and used up in the reaction.

The ratio of MEKP to resin is between 1 and 4% depending on the temperature.  The more MEKP, or the higher the temperature, the faster the resin will cure.
A fast cure is also not a good cure.  The reaction takes time and if the resin 'kicks' too fast, there will be unused catalyst in the polyester matrix. A 20-30 minute cure time is a good reasonable time to have.  As can be seen from this chart, 4% catalyzation at 5C will cure in 2 hours while at 20C it will cure in 5 minutes.

The daytime temperature here in Malaysia is around 32C (90F).  This does not give a lot of time for catalyzed resin to be applied.  If you mixed a batch, put it in a spray gun and started applying, it would probably turn into a hard chunk of plastic before you were done spraying.  This would ruin the gun and obviously this would make it very difficult to do a large area like an entire boat. 

Even if you were to apply early on the cool morning with 1% MEKP, you have to thoroughly clean the gun spray gun between each batch or the residue will still harden in the gun even though more material is added.

There are spray guns for small projects that use disposable cups. Even using a gun like this, the material sprayed in the beginning could have a cure time of 20 minutes while the material at the end may only have a few minutes, if that.  These guns are also more like spatter guns than spray guns.  There is not a fine misting orifice nor a controlled airflow pattern like you would find on a proper spray gun.

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