Chemical Resistance of Paints & Coatings

The coating on the surface works as the main protection from external strains, interactions, or a harsh environment. However, coatings get exposed to chemicals and acids during cleaning, spills, or fumes. So, the outmost layer of the coating must resist chemical attacks. This protects the substrate from contamination. The key factors by which a chemical medium can affect the coating layer include:

• Type of chemical (solvents, acids, alkalis, salts, and other substances)
• The concentration of chemicals exposed
• Time and degree of exposure
• Temperature
• Type of substrate

Consider all factors when selecting components and formulating chemical-resistant coatings. This achieves optimal performance. For example, a coating may resist concentrated chemicals at low temperatures. However, it may degrade when exposed to a dilute concentration at high temperatures.

Furthermore, chemical attacks can lead to swelling, discoloration, adhesion loss, gloss reduction, blistering, and more. This can lead to deterioration of performance and eventual failure under certain conditions.

Let's get an in-depth knowledge about:

1. What are the types of chemical resistant coatings?
2. Why chemical resistance matters across various markets?
3. What are the factors influencing chemical resistance?
4. How to test the chemical resistance of coatings and paints?

Types of Chemical Resistant Coatings

Several types of chemical resistant coatings are available and used across various applications. Some of the major classes include:

• Epoxy-based – These coatings protect against acids, alkalis, solvents, caustic fluids, fuels, fatty acids, salt or sugar solutions, chemical cleaning detergents and more. Upon thermal curing, they offer excellent impact resistance, plus abrasion and corrosion resistance.


• Polyurethane-based – Such coatings are resistant to organic caustic fluids, acids and alkalis, fuel and hydraulic oils, and solvents.


• Phenolic-based – These coatings are resistant to a concentrated form of sulfuric acid, hydrochloric acid, transmission fluid, gasoline, formaldehyde, phenol, and numerous other chemicals. As compared to PU and epoxy coatings, these are ideal for low pH and high temperatures.


• Fluoropolymer-based – Such coatings are well known for its non-sticking properties. Fluoropolymers are resistant to a wide range of chemicals, including concentrated sulfuric, nitric, hydrochloric and phosphoric acids. For example, PTFE can withstand high temperatures (300°C) and FEP offers the same benefits as PTFE with better abrasion resistance.

Importance of Chemical Resistance

Chemical resistance is a critical property in paints and coatings as it:

• Impacts the durability and functionality.
• Extends the service life of materials.
• Preserves aesthetics and ensures structural integrity.
• Contributes to compliance with industry standards.

Hence, chemical resistance is a key parameter when selecting coatings for various applications.

Key applications

1. Industrial Applications: Chemical resistance is crucial when equipment is subjected to harsh chemicals. Paints and coatings act as a protective barrier against corrosion. They prevent deterioration caused by chemical exposure.


2. Automotive Sector: Vehicles encounter a diverse range of chemicals such as road salts, fuels, and cleaning agents. Chemical-resistant coatings help maintain the aesthetic appeal and structural integrity of automotive surfaces.


3. Household and Commercial Use: Kitchen appliances and furniture items are exposed to various chemicals. Chemical-resistant inks and coatings ensure they remain durable and aesthetically pleasing over time.

Factors Influencing Chemical Resistance

Polymer base

The choice of polymer in the formulation influences chemical resistance. Epoxy, polyurethane, and fluoropolymers are known for their excellent chemical resistance properties.

Additives and crosslinking agents

Incorporating additives and crosslinking agents enhances the chemical resistance of coatings. These compounds reinforce the molecular structure. This makes the coating more resilient to chemical attacks.

Curing process

The curing process is crucial in determining the final coating's chemical resistance. Proper curing ensures the formation of a robust and impermeable film that can withstand chemical exposure.

Coating’s Ability to Resist Chemicals – Test Methods

Chemical resistance tests evaluate the resistance of coatings to various test liquids. The main test procedures include spot tests and immersion tests. The test methods determine the effect of various chemicals on coatings, such as discoloration, adhesion loss, or any other paint defect.

ASTM D1308 - 02(2013) – Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes

This test method covers the determination of the effect of household chemicals on clear and pigmented organic finishes, resulting in any objectionable alteration in the surface, such as discoloration, change in gloss, blistering, softening, swelling, loss of adhesion, or special phenomena. This standard can be used to test materials, such as:

• Distilled water (hot or cold)
• Ethyl alcohol, 50%, by volume
• Vinegar (3% acetic acid)
• Alkali solution
• Acid solution
• Soap and detergent solutions
• Light fluids
• Lubricating oils and greases

Another similar standard is ASTM D3023 - 98(2017) – Standard Practice to Determine Resistance of Factory-Applied Coatings on Wood Products to Stains and Reagents. This method covers the evaluation of clear factory-applied coating systems on wood substrates.

ASTM D2792 - 17 – Standard Practice for Solvent and Fuel Resistance of Traffic Paints

This method describes a laboratory procedure to determine the resistance of a dried film of traffic paint to the action of a specified hydrocarbon solvent or gasoline fuel test fluid that causes blistering, wrinkling, loss of adhesion, and loss of hardness.

Procedure:

• The coating is applied to tin panels and air-dried for 90 h.
• Half the panel is immersed in the test liquid, and the vessel is covered for a period of 4 to 18 h as may be specified by the customer.
• The panels are then removed and examined for defects.
• The panels are allowed to dry for another 24 hours and re-examined for film defects and softening as compared to the unimmersed portion of the control panel.

ASTM D5402 - 93(1999) – Standard Practice to Assess the Solvent Resistance of Organic Coatings Using Solvent Rubs

Coatings that change chemically during the curing process, such as epoxies, vinyl esters, polyesters, alkyds and urethanes, become more resistant to solvents as they cure. These coatings should reach specific levels of solvent resistance before being top coated and before placing in service; the levels of solvent resistance necessary vary with the type of coating and the intended service.

Rubbing with a cloth saturated with an appropriate solvent is one way to determine when a specific level of solvent resistance is reached. However, the level of solvent resistance by itself does not indicate full cure and some coatings become solvent resistant before they become sufficiently cured for service.


The time required to reach a specific level of solvent resistance can be influenced by temperature, film thickness, air movement and, for water-reactive or water-borne coatings and humidity.

ASTM D4752 - 10(2015)– Standard Practice to Measure MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rubs

This practice describes a solvent rub technique to assess the MEK resistance of ethyl silicate (inorganic) zinc-rich primers. The MEK resistance of some two-component ethyl silicate zinc-rich primers has been shown to correlate well with the cure of the primer as determined by diffuse reflectance infrared spectroscopy. The technique can be used in the laboratory, field, or the fabricating shop. Practice D5402 is the preferred method for organic coatings.

ASTM G20 - 10(2015) – Standard Test Method for Chemical Resistance of Pipeline Coatings

This test method is intended to evaluate the resistance of pipe coating materials when exposed to various concentrations of reagents or suspected soil contaminants. The test serves as a guide to investigators wishing to compare the relative merits of pipe-coating materials in specific environments. The choice of reagents, concentrations, duration of immersion, the temperature of test, and properties to be reported are necessarily arbitrary and should be chosen to reflect conditions known to exist along the pipeline right-of-way.

ASTM D3260 - 01(2017) – Standard Test Method for Acid and Mortar Resistance of Factory-Applied Clear Coatings on Extruded Aluminum Products

This test method covers the evaluation of the mortar and acid resistance of a clear protective coating factory applied to extruded aluminum substrates.

ASTM D870 - 15 – Standard Practice for Testing Water Resistance of Coatings Using Water Immersion

Immersion of surface in water can cause the degradation of coatings. The knowledge of how a coating resists water immersion helps predict its service life.

Water immersion tests are used for research and development of coatings and substrate treatments, specification acceptance, and quality control in manufacturing. These tests typically result in a pass or fail determination, but the degree of failure may also be measured. A coating system is considered to pass if there is no evidence of water-related failure after a specified time.

ASTM B117 - 18 – Standard Practice for Operating Salt Spray (Fog) Apparatus

Salt fog resistance is important for marine, automobile, and aircraft coatings and any other exterior coating exposed to salt spray by being near the ocean or exposed to salted road conditions.


Procedure:

• The test requires a salt fog cabinet and coated panels.
• The coating is scored to the bare substrate with an X shape.
• The edges are sealed with a weatherproof tape, and the panel is placed in the cabinet for a specified time.
• The metal panels are exposed to the settling fog of an atomized neutral (pH 6.5 to 7.2) sodium chloride solution consisting of five parts by weight sodium chloride and 95 parts distilled or deionized water.
• The sample is then periodically checked to see if the rusted exposed metal has propagated under the coating causing coating failure.

Related standards include ASTM B368, E70, G85.