On the face of it, ‘flame-treating’ a flammable material like plastic may seem like a strange thing to do. However, this useful technique is vital for creating a better bond between epoxy and plastic. Technical expert Hamish Cook is on hand to guide you through the process.
Plastic and duck feathers have a lot in common, would you believe? If you’ve ever seen a duck dive into the water and come out again, you’ll see the water simply run off its back, as per the proverbial phrase. Well, in their basic state, certain plastics behave in exactly the same way. Whatever liquid you apply will typically bead up and roll off.
That liquid can include epoxy. This ‘hydrophobic’ tendency of plastic can affect adhesion of plastic surfaces with epoxy, which ultimately affects the strength of the bond. That’s clearly no good if you’re trying to mend your kayak/car bumper/roofbox.
So how do you deal with this? If your plastic is PVC, HDPE or ABS, the simple answer is to flame-treat it before you apply the epoxy.
By flame-treating, I don’t mean heating. In fact, this is really important: you’re not looking to change the surface in any visible way whatsoever. The idea is just to invisibly oxidise the upper layer of the plastic so that it becomes more ‘hydrophilic’ and therefore bonds better with the epoxy.
Here’s how to do it.
Step 1: clean the surface
Use an alcohol wipe to clean the surface that you’re going to flame-treat.
Step 2: flame-treat the surface
To flame-treat the plastic, hold a propane torch about 15cm above the surface so that the flame just touches the plastic. Move it across the surface very quickly, at a rate of about 30-40cm per second. Keep the torch moving and make sure you work your way up and down the plastic carefully in sections, each time slightly overlapping the last section that you flame-treated.
Step 3: apply the mixed epoxy
Remember you only have between 15 and 30 minutes to apply the epoxy before the temporary effects of the flame treatment will wear off.
Not sure it’s worked? Here’s a quick test
If you’re not sure whether your flame treatment has worked, you can perform a simple test.
Apply two water droplets – one to an untreated area of plastic and one to a treated area. On untreated plastic, the droplet should be very spherical. On well-oxidised plastic the water will ‘wet’ the surface better, resulting in a flatter droplet.
If it doesn’t look like the flame treatment has worked, don’t worry – leave the plastic for at least half an hour and then flame-treat again.
Thanks to Hamish Cook for his contribution.
Have you got a ‘burning’ question for Hamish? Email us at firstname.lastname@example.org and we may even feature it in a future edition of epoxycraft!