Crack in Toilet Seat Caused by Faulty Wood Composite

Detail of the offending crack

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Some time back my significant other was going about his business in the loo when suddenly he felt an ominous movement directly beneath him. Earthquakes are rare here in the Eastern part of the United States so, understandably perturbed, he investigated further. Much to his surprise, he discovered a radial crack in the wood composite toilet seat he’d installed just the week before.

Pink is a ridiculous color for a toilet, I know, but it’s not relevant to this story.

Yes, my husband’s weight is appropriate for his height, which is the first question that Research Mechanical Engineer John Hunt delicately raised when I rang him up. A wood composite expert with the Forest Products Lab of the US Dept of Agriculture, he was surprised to hear about the failure. It turns out that the rather lowly toilet seat is actually a highly engineered structure designed to sustain high bending loads while in use. Otherwise we’d fall in.

In my original post over at Electronics Weekly, Hunt described that your typical wood composite toilet seat consists of a high-density composite top and bottom layer and a lower-density core and is designed to support even the heftiest among us. The trickiest part comes in the processing of the material - basically a mixture of epoxy and wood particles — which requires a certain degree of engineering know-how to control the resin content, density gradients, temperature, and moisture levels. All of which can impact performance.

These parameters must be carefully controlled, and specifications meticulously written to account for the fact that wood composites don’t behave like other materials, namely wood. And one unusual characteristic is that wood composites are hydroexpansive, meaning design engineers need to account for moisture expansion and contraction in normal use

As for our toilet seat, Hunt speculated that most likely one of the processing parameters got out of whack, leading to its early demise. “Maybe too much moisture got into the batch or the glue content was not controlled well enough - that would do it.”

Early composite toilet seats probably failed left and right. But they don’t now, Hunt says, because over the past 30 years wood composite processing techniques have been moving from an art to a science - meaning engineers are getting very good at controlling the parameters and even engineering the material to achieve desired performance characteristics.

And the best part of the story? We’ve now replaced our pink toilet with a very cool black unit.