Chemical compatibility charts can tell you whether a plastic is resistant to a certain chemical or not, but just looking at the legend gives an incomplete picture of how a plastic might react under specific conditions. When choosing a chemically resistant plastic for your application, you'll need to find out how the material reacts under the following circumstances.
How concentrated a chemical is affects how it will interact with plastic. Some chemical compatibility charts will list certain chemicals by concentration, which can range anywhere from "aqueous solution" to 40% and beyond. However, not all chemicals are listed this way, so it can be difficult to tell whether a plastic will interact positively with them or not. For example, acrylic is more susceptible to concentrated butyric acid than an aqueous solution of butyric acid.
Even acid resistant plastic can begin to show signs of stress and wear when exposed to harsh chemicals for extended periods of time. For example, plastic filler parts at dairy factories sometimes become "chalky" after repeated washdown and sterilization cycles, even though they were considered appropriate for the job. When choosing chemically resistant plastics, it's a good idea to "over-engineer," or choose a plastic that's overqualified for the application — this will ensure that plastic parts will last as long as possible and offer the best performance.
At room temperature, chemically resistant plastic generally has no trouble doing its job. However, when temperatures get to extremes, plastics become more susceptible to chemical attack, which occurs when the chemicals react with the molecular makeup of a polymer and begin to break it down. It can also increase how quickly chemicals are absorbed into the plastic, which will also change the plastic's properties and make it softer than it should be.
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