By Cynthia A. Gosselin, Ph.D., The ChemQuest Group

One of the more discussed paint adhesion and hardness tests involves “scratching” paint off a substrate. Not only does this type of testing depict a qualitative level of adhesion, but in many instances, it also provides a relative idea of degree of cure and hardness of the organic coating.

The genesis of scratch testing is found in the very old (pre-1960) National Coil Coating Testing Manual, Appendix A–Test Method NMCTP–17-92. Scratching could be preceded by environmental testing or conducted immediately after painting. The paint surface was scratched through the paint film with the edge of a nickel—hence also known as the “Nickel Adhesion Test.”

Every paint line superintendent had their favorite nickel in their pocket that, according to their experience, produced consistent results. A photographic standard was used to determine the quality of adhesion as depicted by the thickness of the paint removal line. (Figure 1) The thicker the substrate exposed within the scratch line, the poorer the adhesion. Acceptable adhesion levels were determined between the paint vendor, coated product producer, and the final customer to ensure success in the field.

FIGURE 1 Nickel adhesion photographic standard.

While the Nickel Adhesion Test provided surprisingly consistent relative results, more sophisticated and reliable quick adhesion and hardness testing was needed. In 1974, ASTM published ASTM D3363-74 Standard Test Method for Film Hardness by Pencil Test. This test method was a quick, inexpensive determination of film hardness. Relative adhesion and degree of cure could be inferred during testing. In 1980, the National Coil Coating Association issued an update of the ASTM specification as Technical Bulletin 4.2.5 (NCCA No. 11-12). The next iteration of this specification took place in June 1996 when a revised version was published in a compilation of testing for the coil coating industry. This revision was heavily influenced by the methodology utilized by ASTM.

Currently, ASTM D3363-74 (22) is an updated staple within myriad testing protocols included in ASTM Volume 06.01, routinely undergoing the rigorous five-year review process.

Pencil hardness testing involves holding a variety of flattened pencil leads of differing hardness at a 45° angle. The leads are pushed into the paint, away from the operator, using a 6.5mm (0.25″) stroke.

The process starts with the hardest pencil and continues down the hardness scale to the point where the lead will not cut into the film (hardness) or will not mar the film (scratch hardness). Any defacement of the film other than a gouge (cut) is considered a scratch. Both endpoints are reported to provide a full view of the paint film.

Calibrated pencil drawing leads or equivalent calibrated wood pencils are used. These leads meet the following hardness scale, with the difference between two adjacent leads considered as one unit of hardness.

Sometimes the point at which gouging occurs points to the relative degree of cure, which can affect hardness. High-reflectance white paint is one such product. The system required an H-pencil standard for the paint to adhere without incurring damage during the stamping process. Anything less results in paint peeling in the die, even if volume solids, paint film, color, reflectance, and crosshatch adhesion are perfect. Using the strict H-pencil criteria with mutually calibrated leads allows for the verification of enough cure to withstand the stamping process and a significant decrease in rejected material.

However, critics of this type of human/manual test are always looking for a combination adhesion/hardness test that can be perfectly calibrated and demonstrate excellent precision, repeatability, and reliability. There have been many attempts, but nothing comes close. Often alternative testing has proven too complicated and time-consuming, or it requires expensive, hard-to-calibrate equipment. That may be fine for a research laboratory, but for production labs and field evaluation, it is not practical.

To determine just how good (or bad) this test is, results of precision testing were added to the ASTM standard. A detailed interlaboratory study was conducted, analyzing data both within and between different laboratories. The study found that an interlaboratory standard deviation was 0.52 and 0.61 between laboratories. Based on this study, criteria were developed to judge the acceptability of results at a 95% confidence level. If two operators using the same panels and pencils produce results that differ by more than one pencil unit, the repeatability and reproducibility should be considered suspect.

This surprisingly good result makes pencil hardness one of the quickest and most relevant adhesion/hardness tests within the industry. Yes, it is qualitative and relative (not mathematically absolute), but good enough to provide confidence in the ultimate success of the product in the field.

Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group, www.chemquest.com. Email: cgosselin@chemquest.com.