Chemical Resistance of Polymers

Chemical resistance describes the ability of a material to withstand a chemical attack for a specific period. During this exposure, the material maintains its performance properties without significant deterioration. 

Materials with high chemical resistance are also likely to be resistant to material breakdown due to a chemical attack. In the context of plastics, it is the capability of a plastic to resist chemical attacks, such as corrosion, dissolution, or swelling.¹ 

Chemical resistance is a significant advantage of engineering plastics over metals. This property allows end products to withstand harsh environmental conditions. Insufficient chemical compatibility between a material and the application can be severe. It could lead to cracking, swelling, softening, and shortened service life. 
 

The chemical resistance of a material is influenced by a variety of factors. Understanding these factors is crucial for selecting the right plastic for a specific application. 
 

Intrinsic material properties
 

1. Polymer Structure: The molecular structure of the polymer, including the type of bonds, chain length, and degree of crystallinity, significantly impacts its chemical resistance. For example, cross-linked polymers generally exhibit better chemical resistance than linear polymers.
2. Functional Groups: The presence of specific functional groups in the polymer chain can influence its interaction with chemicals. For instance, polymers with polar functional groups, such as hydroxyl or carbonyl groups, may be more susceptible to attack by polar solvents.
3. Molecular Weight: The average molecular weight of a polymer affects its properties, including mechanical strength and chemical resistance. Higher molecular weight polymers tend to have better chemical resistance due to increased chain entanglement and reduced chain mobility.
4. Additives: Additives, such as antioxidants and stabilizers, can improve the chemical resistance of rubber. The type and amount of filler used in the resin can influence its chemical resistance.
5. Cross-linking: The formation of covalent bonds between polymer chains, improving mechanical properties and chemical resistance.

External factors
 

1. Temperature: Elevated temperatures can accelerate chemical reactions and degradation processes, reducing the chemical resistance of a plastic.
2. Chemical Concentration: The concentration of a chemical can significantly impact its ability to attack a plastic. Higher concentrations generally lead to more severe degradation.
3. Exposure Time: Prolonged exposure to a chemical can increase the extent of degradation.
4. Stress: Mechanical stress, such as bending or tension, can weaken the plastic and make it more susceptible to chemical attack. It can contribute to chemical degradation.
5. UV Radiation: Exposure to UV light can degrade the polymer chains, reducing the plastic's chemical resistance. Discover how to resist UV degradation.
6. Environmental Factors: Factors like humidity and oxygen can also affect the chemical resistance of plastics.
7. Curing Conditions: The curing process, including temperature and time, can affect the final properties of the resin.

In the plastics industry, many polymers exhibit chemical resistance, ranging from high to low. These materials are broadly categorized as thermoplastics, elastomers, rubbers, and thermosets. Let us brief you about the chemical resistance profile of these materials in detail.
 

Thermoplastics^3,4
 

Thermoplastics are a class of polymers that can be repeatedly softened and hardened by heating and cooling. Their chemical resistance properties vary widely depending on their molecular structure and specific chemical composition (Figure 1).
 

 

Find out the subcategories of thermoplastics which show excellent or limited resistance to various chemicals.
 

Rubbers^5,6

Rubbers are polymeric materials characterized by their unique chemical composition and structure. These properties give them remarkable elasticity. Rubbers are versatile and come in various types. Each type has its unique chemical resistance profile. Find out the subcategories of rubbers which show excellent or limited resistance to various chemicals.
 

Thermosets⁷

Thermosets have excellent chemical resistance. This is because they have a highly crosslinked molecular structure. The crosslinking makes it hard for chemicals to penetrate and damage the material. The crosslinking also provides high chemical stability. This makes thermosets resistant to a wide range of chemicals. Find out the subcategories of thermosets which show excellent or limited resistance to various chemicals.
 

Thermoplastic elastomers

The chemical resistance of thermoplastic elastomers (TPEs) mainly depends on various factors. Firstly, it relies on the base polymer and secondly, the degree and type of crosslinking. TPEs with more polar and saturated polymer backbones generally resist non-polar chemicals better. The temperature, concentration of chemicals, and exposure time also affect the chemical resistance of TPEs. Find out the subcategories of elastomers which show excellent or limited resistance to various chemicals.
 

Chemical resistance testing involves exposing plastic specimens to specific chemicals under controlled conditions. The effects on the plastic's physical properties, such as weight, dimensions, mechanical strength, and appearance, are then evaluated.
 

Common testing methods¹
 

1. Immersion testing

   
   Immersion testing involves submerging a material sample in a liquid for a specified period. The exposure typically happens under controlled conditions like temperature, pressure, and humidity. The goal is to evaluate the material's:
    

   • Chemical resistance: Ability to withstand exposure to chemicals without degrading or losing its properties.
   • Water absorption: Ability to absorb or resist water, which can affect its mechanical properties.
   • Corrosion resistance: Ability to withstand exposure to corrosive substances without degrading.
      

2. Soak testing

   
   Soak testing is a type of testing method used to evaluate the chemical resistance of materials, including plastics, elastomers, and composites. This test involves immersing a material sample in a chemical solution for a specified period, typically under controlled conditions of temperature, pressure, and humidity. The sample is then evaluated for changes in its physical and mechanical properties, such as:
    

   • Weight gain or loss
   • Dimensional changes
   • Color changes
   • Cracking or crazing
   • Loss of mechanical properties (e.g., tensile strength, elongation)
      

3. Stress cracking

   
   Stress cracking occurs when a plastic is exposed to a chemical environment that causes it to crack or craze under stress. In stress crack testing, specimens are stressed mechanically while immersed in the test chemical to assess stress-cracking resistance.
   
    

International standards for evaluating chemical resistance
 

1. ASTM D543*: Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents.
2. ASTM D471*: Standard Test Method for Rubber Property (Tensile strength & Elongation) - Effect of Liquids.
3. ISO 175*: Plastics - Determination of the Resistance of Plastics to Chemicals.
4. ISO 4599*⁺: Plastics - Determination of the Resistance of Plastics to Chemical Vapors.
5. SAE J1742*: Test Method for Evaluating the Chemical Resistance of Automotive Plastics.

* Standards to measure chemical resistance by immersion and soak testing methods
⁺ Standards to measure chemical resistance by stress crack testing method
 

Choosing the right chemically resistant polymer matters while developing critical applications. Understanding the chemical resistance profile ensures product quality, safety, and longevity. One should always get proper guidance from chemical suppliers before formulating a product. Specific conditions like temperature, concentration, and exposure time can also be cross-checked.