Warning: This article may contain content that is not safe for work! Just recently my significant other was going about his business in the loo when suddenly he felt an ominous movement directly beneath him. Understandably perturbed, he investigated further and discovered a radial crack in the wood composite toilet seat he’d installed just the week before. At this point I should note that 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. Unlike the case with, say, lithium ion batteries, he hadn’t heard of any recalls or advisories on toilet seats of late. So why did the seat crack?
In basic terms, a toilet seat is a simply supported beam whose upper surface is subjected to compressive stresses and lower surface is subjected to tensile stresses under load. Exceed the maximum allowable stress and craaaaaaaaaaaaak! Hunt says 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. Hunt related a recent case in which the large wood composite panels mounted on the ceiling of an executive board room went crashing to the floor. Engineers failed to compensate for the panels shrinking, and they simply fell through the frames holding them in place. I personally would have liked to see the video. 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. Wood composites have broad application today and in fact are used in 50% of the wood products in the typical home and for nearly all toilet seats. Hunt’s group is looking to extend the material’s use, working on such projects as a tornado-proof structure (see the video) and a line of lightweight furniture with a performance that’s comparable to their beefier counterparts at a fraction of the weight. For anyone who has ever lugged a particle board bookcase up a narrow, steep staircase, this development will be a godsend. Just ask my husband!