Coating solvent resistance wipe testing is a critical test method to evaluate the ability of coatings to resist damage such as swelling, dissolution, loss of shine, or softening under the action of specific solvents. This test provides an important basis for the durability of the coating by simulating scenarios where the coating may come into contact with solvents during actual use or cleaning. The core of the test is the accurate determination of the endpoint, which is directly related to the reliability and comparability of the test results.
Influencing factors for endpoint determination
The endpoint is not determined by a single indicator, but is affected by multiple factors. The chemical composition of the coating determines the nature of its interaction with the solvent; The curing degree of the coating affects its cross-linking density and solvent penetration resistance. The chemical properties (such as polarity, solubility parameters) and physical properties (such as volatilization rate) of the solvent are the key variables. The material used for wiping (e.g., cotton cloth, sponge) and the force and frequency applied can significantly affect the mechanical and chemical strength of the coating surface. Therefore, the endpoint determination method needs to be performed under standardized test parameters.
Specific methods
Endpoint determination is usually based on observable or measurable changes in the coating surface. Here are some core determination methods and their operational essentials.
Visual and tactile observation methods
This is the most basic and straightforward approach. The operator needs to observe the coating surface at a specific angle under a standard light source and cooperate with tactile inspection. The basis for determining whether the coating surface has lost light, fog shadows, color changes, swelling, wrinkling, cracking or exposing the substrate. Tactile inspection focuses on whether the coating is sticky, softened, or whether the material is transferred to the wipe. The method is highly dependent on the operator's experience, so personnel comparison training is required to ensure consistency.
Wipe contamination determination method
This method is done by checking the wipe materialWhether it is contaminated with a coating substance to determine the endpoint. The specific operation is to check the white cotton cloth or similar material for visible coating color or substance residue after completing the specified number of wipes. If there is obvious contamination, it indicates that the surface of the coating has been damaged. To reduce subjectivity, standard pollution ratios can be usedColor charts for comparison.
Performance change measurement
A more objective judgment relies on the quantitative measurement of the physical properties of the coating before and after the test. Common metrics include:
Gloss change: Use a gloss meter to measure the difference in gloss between the tested area and the untested area. When the difference exceeds the standard value (such as a 50% reduction), it can be determined as the end point. The formula can be expressed as: ΔG = | G₁ - G₀| / G₀ × 100%, where G₀ is the initial gloss and G₁ is the gloss after the test.
Chromatic Aberration Change: Color change ΔE is measured using a colorimeter. When the ΔE value exceeds a predetermined threshold (e.g. 1.5 or 2.0 depending on the application area), it indicates an unacceptable change in the appearance of the coating.
Coating Thickness Loss: Use a thickness gauge to measure coating thickness before and after testing. A significant reduction in thickness indicates that the coating material has been dissolved and removed.
Judgment process and record
A stepped decision process is recommended to improve efficiency and accuracy. Initial visual and wipe inspection is performed first, and if no significant damage is found, the wipe cycle is continued to a predetermined maximum number of times or until signs of damage appear. For high-performance coatings, it is recommended to combine performance measurement methods for final determination. All observations, measurement data, solvent type, wiping conditions, ambient temperature and humidity, etc. should be recorded in detail.
Standard reference
Different industries and application standards have specific regulations on the determination of the endpoint. For example, standards involving furniture, automobiles, industrial protection, etc. may have different requirements for the number of wipes, solvent types, and conformity criteria. During the test, it is necessary to pay attention to the stability of the laboratory environmental conditions, the protection of operators, and the compliant storage and disposal of solvents. When making a decision, avoid pre-setting the scene and make a judgment strictly based on the phenomenon or data threshold specified in the standard.
References
ASTM D5402-19, Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs.
ISO 15184:2020, Paints and varnishes — Determination of film hardness by pencil test.
GB/T 23989-2009, Determination of solvent resistance to coatings.
Industry Technical Manual: Coating Performance Testing and Evaluation.