Which High-Performance Plastics Offer Chemical Resistance?

High-performance plastics are integral to numerous engineering applications due to their unique properties, including high strength, thermal stability, and chemical resistance. Understanding the chemical compatibility of these materials is crucial for selecting the appropriate plastic for specific applications, especially in environments where exposure to various chemicals is inevitable. In this blog post, we evaluate the compatibility of several high-performance plastics with common chemicals, such as solvents, hydrocarbons, acids, bases, and oxidizers.

Chemical Resistance Varies

The plastic molecular structure and the chemical compound’s composition determine the chemical resistance of the plastic. Polymers are large molecules comprised of repeating monomers chemically bonded to form long chains. The chains may be linear, branched, or cross-linked, with the branches connected to different polymer chains. Thermoplastics, those that can be remelted, are categorized by their morphology as either crystalline, semi-crystalline, or amorphous. Crystalline polymers have highly structured and closely arranged molecules as opposed to amorphous polymers, which have randomly arranged and intertwined molecules.

A chemical will have a chemical or physical reaction when exposed to the plastic. It may adhere to the surface – called adsorption, or enter the plastic – called absorption. If there is a chemical reaction, a breakdown of the chemical chains can occur, resulting in plastic that is brittle, discolored, and has weakened mechanical properties. With physical reactions, the chemical penetrates the plastic and causes swelling, which can result in the plasticizing or softening of the polymer.

What Impacts the Chemical Resistance of a Polymer?

Some plastics can withstand certain chemicals while others, under the same conditions, break down or even dissolve. For critical applications,  there is no room for error, as a chemical incompatibility could be catastrophic.

Some factors influencing chemical compatibility include molecular structure (cross-linking and crystallinity), functional groups, polymer backbone, and polarity.

• Molecular structure – Polymers with a high degree of cross-linking tend to be more resistant to chemicals because the cross-links create a network that can resist solvent penetration and degradation. However, this is true for purposely cross-linked polymers. Some chemicals may cause unintentional cross-links in the polymer chains by forming covalent or hydrogen bonds or physical entanglement of polymer chains, resulting in a harder, more brittle, or less flexible plastic. Polymers with higher crystallinity have closely packed molecular chains, making it harder for chemicals to penetrate and react with the polymer compared to amorphous polymers.
• Functional groups – Specific functional groups can react with certain chemicals. Some groups are more reactive, making the polymer more susceptible to chemical attack. For example, a plastic with ester groups in its formula might be vulnerable to hydrolysis (breakdown by water), while ether bonds (like in polyethylene oxide) can offer better resistance.
• Polymer backbone – The polymer’s backbone, such as those with a benzene ring or saturated polymers (carbon atoms are bound to other atoms), can make a polymer more chemical resistant. However, even with a strong backbone, functional groups attached to it can be vulnerable points, reacting with a specific chemical and causing chain scission (breaking) or other degradation.
• Polarity – Polar polymers generally have better resistance to other polar chemicals because they can form favorable interactions through hydrogen bonding, dipole-dipole interactions, and other intermolecular forces. Conversely, nonpolar polymers will tend to resist nonpolar chemicals for the same reason.

Other Considerations for Chemical Resistance

Several other factors beyond the polymer’s inherent chemical makeup significantly impact its resistance. The product’s use and environment must be considered when choosing a polymer. Some factors to consider include:

• Contact time – The longer a plastic is exposed to a chemical, the more likely it is to experience some level of degradation or swelling. Even resistant plastics can be compromised over extended periods.
• Concentration of the chemical – The strength or concentration of the attacking chemical plays a significant role. A more concentrated solution will generally be harsher on plastic than a dilute one.
• Temperature – Chemical reactions typically accelerate with increasing temperature. So, a plastic that might resist a chemical at room temperature could degrade much faster at elevated temperatures.
• Mechanical stress – When a plastic is under stress (e.g., holding weight or being stretched), it becomes more susceptible to chemical attack. The combined stresses can weaken the polymer chains and make them more prone to breaking under chemical exposure.
• Nature of contact – The form of contact between the plastic and the chemical can also influence resistance. For example, continuous immersion is generally more detrimental than occasional splashes. Additionally, agitation can increase the rate of attack by promoting better contact between the plastic and the chemical.

Specific High-Performance Plastics and Their Chemical Resistance

The chart below shows a variety of common chemicals (solvents, hydrocarbons, acid, base, oxidizers) and their compatibility with several common high-performance plastics. The tests were performed at 20°C. A more extensive chemical list is available here.

+ Good
* Fair
– Poor

While a chart like the above is a great starting point, speaking with a plastics expert is always wise to ensure a suitable polymer is chosen. You may also need to check the plastic’s behavior and possible unexpected interactions under realistic application conditions.

Partner with Ensinger

When your application requires chemical resistance, we are here to help. With engineers who have a depth of plastics knowledge and the ability to tap into resources of the global Ensinger Group, from which we are a subsidiary,  we are confident we can solve your toughest plastic challenges, whether it’s chemical compatibility or something else.  Contact us to get started.