Chemical Resistance

Introduction

Chemical resistance is a fundamental consideration of engineering and materials science; the capability of a material determines its suitability, life expectancy, and integrity of a component in a service environment. Chemical resistance is the ability of a material to resist (or prevent) an attack from a chemical or solvent, although it will be altered in little to no mechanical properties, dimensions, weight, or appearance. There are few, if any, materials that are resistant to all chemicals; glass is often referenced as resistant to most acids, but to hydrofluoric acid and strong bases, an attack can occur. 

Mechanisms of Chemical Attack

The breakdown of a material due to chemical exposure can proceed via several distinct mechanisms. One mechanism, known as dissolution, is/occurs as a physical process and is defined as the chemical overall (often a solvent or something similar) entering into the material and dissolving polymer chains, etc., leading to swelling and softening, and, ultimately, loss of mass and strength. A second mechanism, chemical reaction, occurs when a direct reaction occurs between the chemical agent and the material in question; examples with this would include oxidation of metals (rusting), or hydrolysis of ester bonds in some polymers (like PET), which results in chain scission and embrittlement. 

Factors Influencing Chemical Resistance

The resistance of any material to chemicals is not a fixed number rather, it will depend on an entire suite of external and internal factors. First and foremost, the concentration of the chemical will greatly affect resistance; for example, the material may have some resistance to a dilute acid or base, but fail quickly under the corresponding concentrated version. Temperature of exposure is similarly critical; in general (dependent on the chemical involved), increasing the temperature by 10°C will typically mean that the rate of the chemical reaction or diffusion of molecules will nearly double, thus affecting resistance. 

Testing and Evaluation Methods

Standardized testing is key for measuring and comparing the chemical resistance of materials in quantitative metrics. Most standardized tests consist of placing a sample in a chemical under prescribed conditions of temperature and time. Standardized chemical resistance tests include ASTM D543, or the resistance of plastics to various reagents, and ASTM D1308, or the effects of household chemicals on clear coats painted on a surface. For qualitative, rapid screening, a simple spot test can be employed. This test consists of putting a chemical on the surface of the material and waiting a defined period to see the effect. 

Application in Material Selection and Design

The real-world application of chemical resistance data is the foundation of reliable engineering design. In the chemical process industry, materials have included Hastelloy, Teflon (PTFE), and fiber-reinforced plastic (FRP) for piping, valves, and tanks due to their relative inability to be corroded by a corrosive process stream. In the pharmaceutical and food packaging landscape, high-density polyethylene (HDPE) and polypropylene (PP) have been chosen for their relative inertness, regardless of the contents, to provide assurance for the safety of the product consumed, as well as prolonged shelf life.

Chemical Resistance in Protective Coatings

For protective coatings, the effect of chemicals is the key performance characteristic. The coating must act as a barrier coating to corrosive chemicals and impede any further movement to the substrate. The chemical resistance of a coating is a function of the chemistry of the coating. Highly cross-linked thermosets, such as epoxies, are highly resistant to solvents, acids, and alkalis. Polyurethanes provide good resistance to UV degradation and some chemicals. Coatings are typically tested by exposing the coating to prolonged immersion or harsh environments, and then the adequacy for adhesion, blistering, gloss loss, and under-film corrosion is evaluated. 

Conclusion

In conclusion, chemical resistance is often a complicated, multifaceted property and is central to the durability and performance of materials in harsh environments. It is not a property that has an absolute criterion, it is relative and based on a specific combination of material (i.e., polymer or resin) and possible chemical permutations under specific service conditions. A systematic approach of being aware of degradation mechanisms, assessing exposure conditions, and extensive testing with a standard testing program is important for making reasonable material selections.