Selecting Plastic Materials for Use in Chemical Environments
The question of whether or not a plastic material is appropriate for use in a chemical environment is rarely straightforward. Chemicals can affect the strength, flexibility, color, surface appearance, dimensions, and weight of plastics. Depending on the application, these changes might disqualify a material for use.
Chemical compatibility charts typically rate plastics as either “resistant” or “not resistant” to a long list of substances. These charts are a good reference, but relying on this information alone could lead a specifier to mistakenly believe that a plastic could be used for an application in which the part might not perform as required.
Factors that affect the chemical resistance of a plastic:
- Length of exposure
- Presence of internal and external stresses
- Concentration of the chemical
This article will examine how these factors affect the chemical resistance of plastics and to explore how to best select plastic materials for chemical environments.
Three basic modes of interaction between a chemical and a plastic are:
- Chemical attack
- Physical absorption
- Environmental stress cracking
In chemical attack, a reaction takes place on the molecular chain of the polymer. Oftentimes that reaction causes the chain to break or “unzip”. When a plastic part has experienced chemical attack, the smaller segments of the molecular chain will have reduced entanglement. This results in the material having lower tensile strength, tensile elongation, and impact resistance, which can result in part failure.
Physical absorption does not involve a chemical reaction with the polymer chain; instead the chemical is absorbed into the plastic in a manner reminiscent of how water is absorbed into a sponge. Once inside, the chemical can cause changes in the weight, hardness, and dimensions of a plastic part. In some cases the chemical acts as a plasticizer, making the plastic softer and more flexible. In other cases, the chemical draws out a plasticizer that was part of the original formulation, causing the plastic to become brittle.
Environmental Stress Cracking
The presence of internal and external stresses makes plastic parts more vulnerable to chemical-related failures. Internal stresses are introduced into a part during processing and fabrication. External stresses are the result of the mechanical loads applied to a plastic part. Environmental stress-cracking (ESC) occurs when a chemical, referred to as a “stress-cracking agent”, weakens the plastic sufficiently to allow internal and external stresses to initiate localized cracking.
A chemical attack is considered ESC when the chemical would not have reacted with the plastic in an unstressed state. It is common to see ESC manifest as thin spiderweb-like cracks called crazing around areas where the stresses are concentrated. Relieving internal stresses through a controlled heating and cooling cycle reduces the likelihood of ESC. Minimizing stress concentrations due to part geometry such as sharp internal corners and threads also helps to mitigate ESC.
The operating temperature range is one of the most important considerations when designing a plastic part that will be exposed to chemicals. Higher temperatures promote chemical attack and increase the rate of physical absorption into plastic materials. It is important to note that temperature and pressure affect the equilibrium between the liquid and gaseous states of a chemical, which can have an impact on the performance of a plastic part. For example, the fumes above a heated chemical bath may attack a plastic guard despite there being no liquid spatter.
Concentration, Exposure Time, and Surface Area
Knowing the manner of exposure to a chemical is essential to assessing the expected longevity of a plastic part. Chemical reactions sometimes require considerable time before appreciable damage takes place. Wiping away incidental spatter before it can absorb into the plastic can go a long way toward preserving the longevity of a part. Having more surface area exposed to a chemical will generally increase the likelihood of chemical attack. Creating designs that limit the surface area exposed to the chemical can also help to extend part life.
Some chemicals are not used in their pure form and instead are kept in solution with water. The higher the concentration of a chemical, the more likely it is to damage a plastic part. Because of this, chemical compatibility tables often have multiple entries showing the resistance of a polymer to various concentrations of a chemical.
Poor compatibility between a plastic and a chemical may be due to one or more modes of interaction as well as environmental factors. For example, at room temperature a plastic may be susceptible to absorbing a particular chemical, but the material does not experience any dramatic losses in physical properties. At elevated temperatures however this can change and at the same concentration and length of exposure, the plastic exhibits serious degradation.
Similarly, a non-stressed plastic part may absorb a particular chemical and show no signs of damage. However, when the same part is placed under load, cracks may begin forming despite the load being well below the yield strength of the material.
When reviewing chemical compatibility charts it is important to recognize that the results represent the performance of plastic test specimens exposed to chemicals under controlled conditions. More extensive laboratory testing can provide valuable insights about the unique relationship between a chemical and plastic.
For example, conducting tests at varying temperatures and under different mechanical loads can be an excellent method of determining whether a plastic is strongly resistant to a chemical or only marginally so. This is an important distinction to make given how difficult it is to quantify the effects of other factors such as internal stresses.
It is highly recommended to “over-engineer” when selecting a plastic material for use in a chemical environment. Choosing a plastic that exhibits minimal property changes in test conditions more extreme than the actual application environment will often prevent plastic part failures and provide significant long term cost savings.
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