Phil Stevenson
Good adhesion of your coatings should always be a concern in any type of coating system. As coating formulations change and new methods of finishing are developed, our first reaction is to check to insure that the coating meets the expected adhesion performance. As water base coatings become more commonly available, many finishers are wary of the adhesion properties of these types of coating systems. When water base finishes were first developed, adhesion was very poor with many of the early formulations. As technology improved, resin polymers became much more durable and resistant to adhesion finish failures. As new two-component finishes became available in water base, the adhesion properties varied greatly. Some coatings were brittle; others were soft and flaked off the substrate prematurely.
In recent years, many of these issues have been resolved. There are many formulations available that are widely accepted to be equal or better than solvent based formulations. This is true across the board from single component to two-component to UV cured water base formulations. However, whether you are using water base finishes or solvent base finishes, it is still highly recommended that you always check the adhesion of your coating system on a regular basis. This is particularly true of multi-step finishes using glazes, shaders, and toners. These types of materials, especially glazes, will almost always reduce the adhesion properties of the finish.
Adhesion is a very broad topic with much attention placed on adhesion properties of the coating in formulators’ laboratories; however, from an applicator’s viewpoint, adhesion is often overlooked or misunderstood. From a finisher’s perspective, it is our intention to provide a working knowledge of coating adhesion and methods of testing coating adhesion integrity.
Adhesion is the ability of the coating to adhere to the substrate – termed as “substrate adhesion,” and the ability of the coating to adhere to multiple coats in the system, which is termed “inter-coat adhesion.” There are two factors that affect the ability of the coating to adhere to the surface: mechanical adhesion and chemical adhesion.
Mechanical adhesion occurs when the coating adheres to the sanding scratch profile of the surface by flowing around the fiber of the substrate to mechanically secure the film to the surface. Deeper and larger sanding profiles will increase coating adhesion, whereas shallower profiles and smoother surfaces impede the ability of the coating to sufficiently adhere. Chemical adhesion refers to the ability of the coating to adhere to the chemistry of the coating formulation. Various components of the coating chemistry affect adhesion. The coating must have sufficient wetting properties to penetrate into the pores of the surface and to wet around the fibers of the substrate. Solvent blends of the coating system allow the coating to burn into coating layers within the system. Resin properties of the coating vary, and they affect the ability of the coating to adhere to the substrate. Flexibility and wetting properties of the polymer and coating system package influence the ability of the coating to adhere to the substrate or to each layer of the coating system. Therefore, it is important to understand and select the coating system that meets your performance requirements.
As a general rule, moving up the scale in coating technologies from lacquers to conversion varnishes, to two part urethanes and two-part water base and UV cured coatings, the adhesive properties will be significantly increased. It must not be assumed, however, that all products within each category will be equal in performance. Every product will exhibit its own specific degree of adhesion characteristics. The finisher is ultimately responsible for verifying that the selected coating system will meet the necessary adhesion requirements for each specific finishing project. The same coating system will often exhibit different levels of chemical adhesion over various wood species and substrates. Wood resins and pitch and oils in the wood will significantly impact the chemical adhesion ability of the coating. Keeping in mind all the different factors that affect the performance of adhesion, the finisher must make sound decisions on the coating system to be used. (Please refer to adhesion check list provided.) Publisher please place check list in layout where reader can easily refer to.
A detailed finishing schedule and standard operating procedure should then be written and approved with a signature from the coatings supplier representative. To verify that the selected coating system is sound and secure, the finisher must always perform adequate testing of the film for adhesion prior to production.
There are three main procedures to assess the resistance of the coating to separate from the substrate and to provide different adhesion analysis of the film; the cross-hatch test, the scrape adhesion test, and a pull-off test. The scrape test method utilizes a weighted balance beam with a rounded stylus or loop attached to the beam. The panels with the dry finish film are pushed underneath the stylus and beam that is loaded with increasing amounts of weight until the finish is scraped loose from the substrate. The evaluator uses the weight in kilograms necessary to remove the film to assign a value to the adhesive properties of the coating. This test method follows American standard test method (ASTM) – D2197, D2248, D2454, 5178, depending on the coating and substrate to be tested. Complete test methods may be found on ASTM website (www.astm.org).
The second and most commonly used test method in the wood industry is the cross-hatch cutter test. This test is fast and simple to perform with an inexpensive cross-hatch cutter test kit. A lattice pattern is cut into the finish film down to the substrate using a cross-hatch cutter. The test area is then brushed diagonally five times in each direction to remove any loose film finish particles. A special tape for testing adhesion is then firmly applied over the crosshatch test area and removed quickly by pulling the tape back off of the test area to reveal the amount of coating lifted off by the test tape. The cross-hatched test area is then visually compared to ASTM standards D3002 and D3359. In the standard list, there are five charts with visual diagrams of varying degrees of film pull-off. ASTM class 5B illustrates no film pull-off, representing the highest level of adhesion. ASTM class 1B illustrates between 35% and 65% of the film removed, representing the poorest adhesion. Class 2B-4B represent varying degrees of adhesion properties that fall between class 1B and 5B. (See illustration provided.) By the nature of this visual comparison, the results are sometimes somewhat subjective and not always definitive with a numerical value associated with this test.
The third method utilizes a hydraulic pull-off tester to measure the tensile stress in hydraulic pounds to detach the coating in a direction perpendicular to the substrate. To perform this test, we used a DeFelsko positest adhesion tester and followed ASTM D4541. A metal loading dolly is secured perpendicular to the surface of the dry coating film by the means of a CA adhesive or the adhesive supplied by the manufacturer. When the adhesive is cured, the dolly is attached to the hydraulic pull-off equipment with hydraulic pressure applied and gradually increased until the dolly pulls the coating off of the substrate. A gauge on the tester shows the number of pounds required to detach the coating from the substrate. The result is a definitive number that may be assigned to the adhesive property of the coating. There may be differing readings obtained on various areas of the test panel due to varying wood substrate densities. The high and low range of pull-off pounds that are acceptable to meet expectations should be assigned to the adhesive value of the coating. According to our research, there is no standard found for expected for pull-off pounds for each category of coating applied to various substrates and must be determined by the finisher for the specific coating system that is being evaluated.
How much curing time should elapse prior to testing for adhesion? It is recommended that the coating be checked after the initial curing and after the long term curing occurs. The coating manufacturer’s recommendations should be followed in regard to when the adhesion testing is performed. Factors such as number of mils applied, ambient air drying or forced cured drying method, and type of coating need to be considered when deciding when the testing should be performed. Normally, after 30 days a second test should be performed. As coatings continue to cure, the adhesive properties will change. Some coatings will improve in adhesion as the coating continues to cross-link; other coating systems decrease in adhesion due to different rates of shrinkage between substrate or coating layers within the system.
Finishers must never let down their guard against substandard adhesion. One major product failure may result in bankruptcy and cost you, the finisher, your job.
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Checklist for adhesion failures:
- Excessive substrate moisture content
- Improper sanding procedures- polish sanding substrate
- Incompatible coatings within the finishing system
- Insufficient curing and dry times.
- Contamination of substrate
- Extreme temperatures
- Excessive dry film build
- Poor spray atomization spray application
- Excessive pigment load in stains
- Incorrect catalization of coating
- Omitting scuff sanding between coats
- Natural oils and resins in teak, pine, etc.
- High moisture content in 2K urethane coatings
- Unstable resin systems