Good Plastics, Bad Plastics
F or us in the industry, it's easy to make some broad and incorrect assumptions about how plastics fit into society as a whole. We know that they're indispensable, but the ongoing debate about plastic...
May 1, 2008 by Jim Anderton, Technical Editor
For us in the industry, it’s easy to make some broad and incorrect assumptions about how plastics fit into society as a whole. We know that they’re indispensable, but the ongoing debate about plastics in food contact applications, and specifically bisphenol A (BPA) shows that when plastics do make the headlines, it’s rarely for positive reasons. This month, I’m going to digress a little and break down the fundamentals of the issue for those of us that didn’t sit through that lecture in undergrad. Most importantly, it’s crucial to understand that when unexpected things are found in plastic products, there can be more than one possible source.
Generally, there are three ways that unwanted compounds appear in resin products. The first is by decomposition of the resin itself. Plastics are generally carbon-based macromolecules, as are most of the compounds in living cells, including DNA. Cleavage of parts of the resin molecule can occur by multiple causes, such as UV light exposure, chemical reactions or by ionizing radiation.
Stabilizers are often added to resin products to inhibit this breakdown, which is the second way that toxins can surface in plastics: as additives. The chemistry of the various additives is generally considerably different from the base resin, and is also more reactive. Depending on how the additive is designed to function and how it reacts with the bulk resin, it may migrate to the surface of plastic products, or break down itself.
The third route is through processing aids such as mold release agents and post-mold coatings such as paints or lubricants. Washing the affected parts may solve the problem, but in a common example — BPA — the situation is more complicated. BPA is a building block molecule for common resins such as polycarbonate and many epoxies, and it’s also used as an additive, an antioxidant and a stabilizer. Breakdown of the bulk resin in consumer goods is a very slow process, so it’s reasonable to assume that the majority of detected BPA comes from additive use. Current testing shows that although migration levels vary from well below to above regulatory levels for BPA, the amount migrating to the surface of resin products decreases with time and each washing of reusable containers.
What’s not widely understood is that additives, like stabilizers, are commonly used to prevent breakdown of the bulk resin into often toxic reaction products themselves. It’s possible to have a highly toxic additive in tiny but measurable amounts on the surface of a resin product, but with greater overall product safety because of the protective effect of the additive in preventing the formation of even more toxic breakdown products.
If it sounds complex, it is, and as a heath and safety issue it’s not going to go away until government, industry and the media start looking at safety from a systems approach instead of at each substance in isolation.
Jim’s Buzzword of the Month
Durometer (doo-rom-eter) an instrument (usually a “Shore” durometer) that measures the hardness of a material’s surface. A handheld or bench-mounted instrument, it uses a calibrated indenter with a mechanical or digital display showing a reading. Scales are unique, dimensionless and are normally specified in a quality standard or by a test such as ASTM D2240. Colloquially used as a verb or a noun instead of the more accurate “Shore” letter scale, e. g. “Shore A”.