Usage of Paint Film Solvent Rub Tester in Rapid Detection of Curing Degree

How it works:

A paint film solvent resistant wipe is an instrument that evaluates the physicochemical stability of paint film by simulating the solvent wiping process. The core principle is that the fully cured paint film has a high cross-linking density, which can effectively resist the dissolution or swelling of specific solvents. The instrument usually drives the solvent-coated wipe head to rub back and forth against the paint surface at a fixed pressure, frequency, and stroke. The degree of curing of the paint film can be indirectly and quickly inferred by measuring the number of wipes required to be wiped through, or the change in the surface state of the paint film after a fixed number of wipes. This method converts complex chemical cross-linking states into quantifiable mechanical wipe cycles, enabling rapid, non-destructive, or minimally damaging field and laboratory testing.

Degree of curing and solvent resistance

The curing process of paint film is essentially a process of cross-linking between the molecular chains of film-forming substances to form a three-dimensional network structure. The degree of curing (or cross-linking density) directly determines the physical properties of the paint film. The relationship with solvent resistance can be expressed by the following simplified model: assuming that the penetration and swelling capacity of the solvent is constant, the cohesive energy density of the paint film against swelling is directly proportional to its crosslinking density. Thus, the wipe work (W) required to achieve a particular failure state can be approximated as a function of the cross-linking density (ρ): W ∝ ρ. In the actual test, the wiping work is reflected by the product of the number of wipes (N) and the single wipe force (F) and stroke (S), and under the condition that the instrument parameters are fixed, the wipe times N becomes the relative index to measure the degree of curing. The incompletely cured paint film has a loose network structure, and the solvent is easy to penetrate, leading to the swelling or even dissolution of the polymer chain, which can be worn through with fewer wipes. On the other hand, a fully cured paint film requires more wiping.

Instrument operation

To ensure repeatability and comparability of test results, operations must be standardized. The main steps include: preparing and curing the paint film sample of the specified thickness; Choose a solvent that corresponds to the paint film chemistry (such as ketones, esters, or mixed solvents); Evenly impregnate the solvent on the standard wipe head and install it on the instrument. Set the wiping stroke, frequency and constant pressure applied to the sample. The test usually lasts until the paint film is rubbed through to expose the substrate, or after a predetermined number of times it has been observed for changes in gloss, color, wrinkles, etc. The control of key parameters directly affects data accuracy, and the following table lists the core test conditions and their impacts:

Application:

This method is widely used in the coating industry for online quality control and process optimization. For example, in automotive painting, furniture spraying or coil coating lines, samples can be taken at different baking stages or in different batches of products for rapid solvent wipe resistance. By comparing the number of wipes tested with standard sample data or historical qualification data that is known to be fully cured, qualitative or semi-quantitative assessment of the curing status of the current paint film can be made in half a minute. Data interpretation should be noted: the number of wipes is high, indicating good solvent resistance, which indirectly reflects the high degree of curing; The number of wipes was significantly lower than that of the control group, indicating that there may be poor curing problems such as insufficient baking temperature, too short time, and improper ratio. This method can be used as an effective supplement to precise analysis methods such as infrared spectroscopy and differential scanning calorimetry for rapid screening at production sites.

Advantages and limitations

As a rapid detection method, the paint film solvent-resistant wiping method has the advantages of easy operation, fast testing speed, low cost, and low requirements for sample shape, which is very suitable for the on-site environment. It can intuitively reflect the practical properties of the paint film, and has a certain correlation with the hardness, adhesion and other properties of the paint film. However, this method also has limitations: the results are greatly affected by factors such as solvent selection, ambient temperature and humidity, paint film thickness, etc., which are indirect measurements and cannot provide an absolute value of cross-linking density. In addition, for some special resin systems, the correspondence between solvent resistance and curing degree may not be a simple linear relationship. Therefore, it is recommended to incorporate this method into the company's internal quality control specifications and establish a standard database of its own products, while regularly correlating and calibrating with the laboratory's absolute measurement methods to ensure the reliability of its guiding production.

Cited Literature

1. Determination of liquid resistance of coatings and varnishes, National Standard GB/T 23989-2009.

2. Determination of solvent resistance of colored paints and varnishes, international standard ISO 2812-5:2018.

3. Wang, Li. Review of coating curing degree evaluation methods. Modern Coatings & Coatings, 2020.