Keeping Bacteria at Bay

I once had the privilege of spending half a year working with a scanning electron microscope. That’s a story in itself, but one unexciting day I scraped my fingertip with a scalpel and put the flake into the machine. After the chamber pumped down, and the usual fiddling to get contrast into the image, did I get a surprise! There’s more to our skin than just skin. Bacilli, bacteria, whatever you call them, are an invisible part of life everywhere, but that doesn’t mean we have to like them.

On plastic parts, bacteria can be anything from an objectionable stain or odor-causing nuisance to a serious health and safety issue. Why do they attack resin parts? Bacteria are like everything else that’s alive; they go where it’s comfortable and where there’s food. The food is carbon, which is largely locked into the resin chemistry. Synthetic polymers (skin is a natural polymer) are actually pretty microbe resistant, mainly because of their stability. Additives, however, like plasticizers, can provide enough free carbon to fuel bacterial growth, which can then degrade the resin part’s bulk properties and deplete the additive’s beneficial effects.

Bugs also need water and a comfortable temperature, ideally at or a little above room temperature. A perfect environment? Your kitchen or bathroom, where the battle to keep bacteria down is intense. Like everybody else, I don’t want to pay big for resin products that hold my toothbrush, or keep my bottom off the cold porcelain every morning. If a molder is processing for a warm, damp environment, he’ll have to consider the possibility of bacterial growth.

Anti-bacterial additives (officially “biocides” or “biostabilizers”) are one answer. There are many choices with exotic acronyms like OBPA, OIT, DCOIT, butyl-BIT, Triclosan as well as silver and organo-tin biocides.

So you have a sensitive commodity application and the customer wants some anti-bacterial properties? Your additive supplier suggests an appropriate product and you’re good to go, right? Not quite. One problem in sensitive quality concerns like bio-contamination is the test methodology used to prove the effectiveness of your solution. Naturally, the folks at the ASTM and ISO have a stable of lab tests that use resin samples to test the extent of growth inhibition. They’re far from perfect. Take ISO 846 Method B for example. It’s a classic Hollywood-style lab test, with a small wafer of resin placed in a Petri dish loaded with fungus. Incubate, then check to see if there’s a dead zone or “zone of inhibition” around the part. Unfortunately, a successful test, meaning a big zone of inhibition, means that the active inhibitor additive has migrated out of the sample. Leaching or bloom is never a good thing, but in this case you really need to keep the biocide in the part. This test could give misleading results, since a part with an additive that doesn’t migrate would look ineffective. Lab tests are good, but you need real world testing too.

Then consider this: are you sure that the bacteria are growing on your part? A common problem is bacterial growth on a thin film of dirt adhering to the part. It’s unlikely that you can expect your biocide to reliably migrate across that barrier even if you wanted it to. Naturally your end-user will clean the part with a detergent. But detergents are surfactants adding to the wettability of the resin surface, making it, you guessed it, more susceptible to bacterial growth. A scrubbable surface can be its own reason for an anti-bacterial additive. There’s so much science behind keeping bacteria off your plastic parts that you’ll need a close working relationship with both your customer and your compounder. But that’s what the business is all about, right?