“Why Is My Plastic Cracking?”

Posted: February 19, 2015 in Education
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We wanted to bring up something amateur plastic project enthusiasts and “Do-It-Yourselfers” often don’t consider until something goes wrong: the weathering of plastic. This was inspired by the a recent visit from a “diy’er” Jason, a hunter. Jason wanted to show us a table he built for processing his game. It was a homemade outdoor table with a white plastic surface likely natural or virgin-white HDPE or UHMW polyethylene. The surface was discolored and had numerous small cracks where Jason (rightfully) was concerned about bacteria growing in the cracks but even more so, he was curious on what caused the plastic to degrade.

The answer for Jason and the culprit with many plastics is UV (ultra violet light or sun) exposure. The chemistry would take to long to explain but suffice to say the sun has damaging effects on plastic. Many plastics become more brittle and crack while others discolor, usually by becoming yellow. If you’ve ever been to an aircraft museum and seen the yellow tinted plastic (polycarbonate) canopies on aircraft? That’s from weathering and UV-exposure. If you look, very closely, at those canopies you’ll see subtle cracks that create a type of haze obscuring vision, as in the picture below.

In most plastic applications for home machinists and project enthusiasts the UV exposure is more of an annoyance than anything. We recently had a sailboat owner frustrated with the discoloration of his polycarbonate hatches he made: the same issue with those aircraft canopies. The most important thing for your project is to figure out of there would be a safety issue caused by a part weathering. In the case of Jason the hunter the cracks on his table might harbor bacteria but since the meats would eventually be cooked (and raw meat has lots of bacteria anyways) this probably isn’t a critical issue. However, in another application – such as the increasingly common homemade roller coasters – the failure of a part, such as a wheel, could be very dangerous.

If you want UV protection there is some good news. Most plastics can be procured in UV-stabilized versions. The problem is these versions are more expensive and sometimes prohibitively more expensive if the material needs to be shipped in on a special order. In the competitive world of plastic distribution there simply isn’t margin or warehouse space to stock the UV-stabilized version of every plastic. The other thing you need to realize is that UV-stabilization in plastics isn’t permanent, it just buys you time – usually no visible degradation over 10 years – but the plastic will eventually lose its UV-stabilizing properties and degrade.

The point of this article is to get you thinking and remember to consider on your next project if weathering is a concern. And for those of you with weathered plastic in your applications? Well now you know why!

For more information, contact Redwood Plastics.

weathered_Canopy

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