Plastic products are used in many applications where they encounter chemicals; in these cases, finding chemical resistant polymers is critical. Whether submerged or in contact with an occasional aerosol, the plastic must be resistant to the specific chemical or chemicals your product will encounter. Often using a material spec sheet isn’t enough to determine chemical resistance. Finding an issue with your plastic product once it is in the field is an expensive discovery. Putting the effort in to make a suitable choice will pay off in the long run.
Understanding Plastics and Polymers
Understanding the structure of polymers is essential for learning how some chemicals impact certain plastics. Polymers are large molecules comprised of repeating units called monomers that are chemically bonded together, creating long chains. The chains can be formed in many ways, including linear, branched, and cross-linked (branches are connected to different polymer chains). Plastics are polymers that can be formed or molded into solid or elastic parts.
Thermoplastics are plastics that soften when heated, harden when cooled, and can be remelted as opposed to thermosets that can’t be remelted. Thermoplastics are categorized by their morphology (arrangement of molecules) – crystalline or amorphous. A crystalline plastic will have molecules that are arranged closely and highly structured when it cools, while amorphous molecules are arranged randomly and intertwined. Semi-crystalline polymers have amorphous regions. Many thermoplastics are semi-crystalline. The processing (heating, cooling) of plastic can impact the crystallinity of a polymer. Some compatibility issues may be due to the molding process.
How Chemicals Interact with Plastics
When plastic is exposed to a chemical, a chemical or physical reaction within the plastic can occur. The chemical may be adsorbed, which means it adheres to the surface, or absorbed, which means it enters the structure. Physical interactions can result in plasticizing or softening of the polymer. The chemical penetrates the free regions through diffusion and causes swelling. Amorphous polymers have less structure and more free regions than semi-crystalline and, therefore, are typically more susceptible to diffusion and absorption. Environmental stress cracking (ESC), a leading cause of part failure, is accelerated by chemical exposure, and often it is a secondary chemical that wasn’t anticipated.
Chemical degradation occurs when a chemical reacts with the plastic at a molecular level, causing a breakdown of the polymer chains. This can result in the plastic becoming brittle, discolored, or losing its mechanical properties. Chemical interactions can lead to cross-linking or scission – in some cases, both can happen.
Cross-linking is a process by which polymer chains are linked together. In some cases, cross-linking of a polymer is done on purpose to enhance the properties of the plastic. However, when a chemical unintentionally reacts with plastic to cross-link polymer chains by forming covalent or hydrogen bonds or physical entanglement of polymer chains, the plastic can become harder, more brittle, or less flexible.
Scission also occurs at a molecular level when the chemical causes bonds to break in the polymer backbone. Depending on the polymer, it may have breaks at the chain ends or at random positions in the chain, forming shorter chains. This process changes the physical and chemical properties of the plastic.
It is important to note that semi-crystalline is not always more chemically resistant than amorphous plastics. So, it becomes important to ensure the plastic you choose is resistant to the primary and secondary chemicals it encounters.
Also, keep in mind that multiple chemical exposures can have a synergistic effect. While exposure to the chemicals independently may not cause a problem, together, the chemicals may react and negatively impact the plastic.
Other Elements to Consider For Chemical Resistant Polymers
There is more to consider than just whether plastic is compatible with a specific chemical. Therefore, when you work with a polymer supplier to find a suitable chemical resistant polymer, they will ask a series of questions. While knowing the chemical is critical, it is also important to understand how the product is in contact with it (e.g., immersion, splashed, aerosol), how frequently (e.g., constant, cyclic, random), how long (e.g., hours, minutes) and the chemical concentration. Other important considerations that directly impact chemical resistance are the environment it is exposed to (temperature and humidity) and applied stress on the part.
In some cases, a dilute concentration of a specific chemical may not cause a reaction but higher concentrations may. As temperature increases, the chemical resistance generally decreases because the rate of reactivity or permeation rate of the chemical increases. Likewise, adding stress reduces chemical resistance. For example, the presence of certain chemicals may cause environmental stress cracking where an unstressed part would have good resistance to that chemical. In combination, the effects of temperature and stress can be more significant. The plastic should be tested to ensure compatibility.
Testing Chemical Resistant Polymers
Many engineers use plastic technical data, such as UL prospector, to determine if materials are chemical resistant polymers. But as we previously stated, there could be circumstances where the datasheet doesn’t provide the whole picture or data sheets are unavailable for the plastic formulation. In these instances, testing can help to determine chemical compatibility. Two common tests are Environmental Stress Crack Resistance (ESCR) and Immersion Test.
The ESCR test evaluates the effect of a chemical or chemical product (cleaner, food, lubricants, etc.) on plastic with structural, load-bearing requirements in its finished part. In a special fixture, plastic test bars are bent to specific strain levels, and the chemical is applied to the most highly strained area of the test bar. The bar is kept under strain with chemical exposure for a set period of time. After exposure, the test bars are evaluated for a change in specific material properties.
There are several test methods for chemical immersion tests, but generally speaking, the plastic samples are exposed to the chemical through immersion, wiping, spraying, or saturated gauze, depending on the level of exposure the finished product will encounter. The sample may also be tested under stress. Depending on the test method, the sample may be placed in an oven for a specified time. When testing has been completed, the properties of the sample, such as tensile strength, are evaluated for change.
Partnering With Plastic Experts
The best way to ensure you have chemical resistant polymers is to consult with a plastic expert at EnCom Plastics. With decades of experience, our polymer experts can provide polymer chemistry solutions to your toughest challenges, including those involving chemical compatibility. We have an in-house quality lab to ensure your specifications are met. Not only will our engineering team match the polymer chemistry to the end application and processes, but they will provide a solution in a timely manner, keeping your production on schedule.
For More Information
Please call our main office at (866) 481-7700 and ask to speak to a technical specialist.