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The coating on the surface works as the main protection from external strains and interactions or a harsh environment. However, coatings get exposed to chemicals and acids either during cleaning products, spills, or fumes. So, it becomes imperative that the outmost layer of the coating has the required ability to resist chemical attacks and protect 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 chemical exposed
- Time and degree of exposure to the chemical
- Type of substrate
It is advisable to consider all factors while selecting components and formulating chemical resistant coatings to achieve optimal performance owing to various reasons.
For example, a coating might be resistant to a concentrated chemical at low temperature; however, it may degrade when exposed to dilute concentration at high temperature.
Further, these chemical attacks can lead to swelling, discoloration, adhesion loss, gloss reduction, blistering and so on which can ultimately lead to the deterioration of performance and, eventually, failure under the given condition.
Let’s explore the types of chemical resistant coatings and the test methods to evaluate chemical resistance of various chemicals.
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 corrosion and abrasion 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.
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.
Solvent and Fuel Resistance of Traffic Paints
- 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.
Source: Solvent Rub Test by TRL
The time required to reach a specific level of solvent resistance can be influenced by temperature, film thickness, air movement and, for water-borne or water-reactive 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.
Chemical Resistance of Pipeline Coatings
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.
Acid and Mortar Resistance on 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.
Source: Auto Technology
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.
- 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.
Continue at: https://coatings.specialchem.com/tech-library/article/standards-to-evaluate-chemical-resistance-of-coatings
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