How to repair brittle plastic

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How to repair brittle plastic

Post by WingAdmin » Fri Jun 22, 2012 7:39 am

I saw this article in this month's Rock Auto newsletter, and thought it was good enough to republish:

Tired of breaking the old, brittle plastic clips that hold on exterior and interior trim? Worried that the plastic thermostat housing will either leak or crack after being over tightened? Hoping for a way to rejuvenate the yellowed plastic gears, slides and rollers in that window regulator? Having trouble sliding the plastic handles onto the metal rods that came with that new foosball table? The answer to these and other plastics conundrums is an elixir available at the nearest kitchen sink.

My dad was a plastics engineer for 30+ years. One of the best tips he has given me thus far is to rejuvenate and limber up old and new thermoplastic polymers by soaking them for a minute or so in boiling water. The results with nylon can be especially dramatic. Yellowish nylon goes into the water brittle and comes out supple and milky white. New composite thermostat housings, foosball table handles, and other plastic pieces that must be mounted to metal become slightly more flexible and are an easier and better fit.

The boiling water does not melt the plastic. Soaking nylon in room temperature water for a long time has the same impact as a short bath in boiling water. The chemistry is too complex for just the son of a plastics engineer to explain. Basically the moisture releases tension between polymer molecules that was created when the plastic was first made (molded, extruded, etc.) or that built up over time as the plastic was exposed to sunlight, heat, chemicals or otherwise aged.

Over the years I have only seen good results from putting my plastic parts in boiling water. At worst the plastic seems unaffected, probably a thermoset plastic (rigid body parts, distributor caps, Bakelite, etc.). However, there are myriad plastic resin recipes and plastic products out there and I must include a disclaimer and encourage common sense and caution. Do not soak plastic pieces that include electronics, gaskets, lubricants, paint, adhesives, decals, etc. that are not supposed to be exposed to water. Do not bring a plastic part out of a freezing garage and immediately dunk it into a boiling pot of water. Thin, molded plastic pieces like interior trim or milk jugs might lose their shape if exposed to heat. Heat and moisture from boiling water might not be uniformly transferred through very thick plastic pieces. If you are at all concerned about the temperature of boiling water, then maybe instead try soaking the plastic piece in unheated water for a day or two. Don’t boil a greasy composite valve cover in your spouse’s favorite spaghetti kettle…

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Re: How to repair brittle plastic

Post by Ghostman » Fri Jun 22, 2012 10:38 am

After working in plastics for 25 years I have to agree with you. Some of the plastic parts on wings this will work great on. With nylon and some of the others being hygroscopic, this will help extend their life. Some of the pieces because of the types of plastic you wil just have to live with the ageing and if you do have to replace them ,look for NOS parts.

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Re: How to repair brittle plastic

Post by thrasherg » Fri Jun 22, 2012 11:50 am

Thanks for posting that, I had never heard of this, but you have very little to loose by trying it and a lot to gain if it works!! :D


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Re: How to repair brittle plastic

Post by B8ssist » Sat Jan 03, 2015 9:08 am

I remember in High School Chemistry class watching a plastic butter tub reshaping into a square piece of plastic when a hair dryer on high was aimed at it. It showed us how plastics can be molded into any type of shape with heat. That was over 30 years ago. :o

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Re: How to repair brittle plastic

Post by ka9nyn » Thu Mar 19, 2015 8:28 am

The same holds true for RC Airplane nylon propellors. Those that have been stress-relieved in boiling water stand ground impacts much better and last longer than untreated ones. Food for thought...
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Re: How to repair brittle plastic

Post by Ol' Man » Thu Mar 19, 2015 5:46 pm

In the FWIW dept., I remember, as a kid, soaking 78 rpm phono records in boiling water and reforming into candy bowls, etc. :)
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Re: How to repair brittle plastic

Post by surveywaters » Fri Feb 03, 2017 9:43 am

Just tried this on my rear master cylinder reservoir and it cracked in several places....

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Re: How to repair brittle plastic

Post by Alesco » Sun May 28, 2017 4:27 am

Hmm, was the garage cold where yo had the bike? Did you dump the whole plastic thing into boiling water?
Was said the water should be about 78 deg?
Just wondering.

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Re: How to repair brittle plastic

Post by FM-USA » Sun May 28, 2017 6:19 am

I'm using common sence on this softening old plastics.
Those plastics that have harden with age are losing their elasticity due to the loss of hydrocarbons. As they age, the hydrocarbons at the surface evaporate. Placing plastic in boiling water the hydrocarbons within move to where they aren't. It's much like water in a sponge moving from wet to dry only much slower in plastics due it being less porusness, we know it as capulary action.

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Re: How to repair brittle plastic

Post by Corkster52 » Sun May 28, 2017 10:12 am

FM-USA » Sun May 28, 2017 7:19 am

Placing plastic in boiling water the hydrocarbons within move to where they aren't. It's much like water in a sponge moving from wet to dry only much slower in plastics due it being less porusness, we know it as capulary action.
Very interesting! As I continued working on my old bike, I noticed the side panel (where the CB would be installed on an SE version) was loose at the bottom as I removed the screw. Found out the piece was cracked and the part that it mates to was broken. So I am back to more creative Dremel work gouging out the cracked portion and ABS gluing (plus adding a reinforcing support) on the broken part. Not sure if I will ever get to the end of the damage I am finding on this bike until absolutely every part has been removed and inspected. :twisted:

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