|by Lloyd Smith, Ph.D.
Theres more than one way to test an industrial coating, and Subcommittee D01.46 on Industrial Protective Coatings oversees several standards for this process. Lloyd Smith of Corrosion Control Consultants and Labs overviews some of the groups most popular standards.
ASTM Subcommittee D01.46 on Industrial Protective Coatings is responsible for standards related to paints applied to structures and buildings in corrosive environments. Active members on the subcommittee represent paint manufacturers, owners and end users, test equipment suppliers, and industrial coatings consultants. Some of the active members are laboratory-oriented, being most concerned with testing and evaluation of industrial coating formulations and coating systems. Other members are field-oriented. Their focus is on the field evaluation of applied or aged coatings. D01.46 develops tests, practices and guides related to the performance, evaluation, specification, application and proper use of industrial protective coatings.D01.46 currently has responsibility for 18 standards. These include guides for preparation of zinc-coated (galvanized) steel surfaces for painting and for assessing the condition of aged coatings on steel surfaces; practices on conducting patch tests for coatings compatibility, for field sampling of coating films for laboratory analysis, and solvent rub tests to determine the degree of cure of coatings; and test methods for analysis of ionic contaminants in abrasive, pull-off strength of coatings, and measuring anchor profile of blast cleaned steel.
The subcommittee has been very active lately, having published three new standards in the last two years and performing a major revision on an existing standard. In addition, at least four or five new standards are under development. This article will highlight the new and revised standards, and briefly discuss the new initiatives being undertaken.
D 4541, Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, is one of the more widely used standards under the jurisdiction of D01.46. The test method consists of securing a loading fixture to the coating surface with an adhesive. The testing apparatus is then affixed to the loading fixture and aligned to pull the loading fixture perpendicular to the coating surface. The force is gradually increased until the loading fixture detaches. The user then reports not only the pull-off strength, but also the location of the break in the coating system, i.e., adhesive between the primer and substrate, adhesive between other coats, cohesive within a certain coating layer, etc. The test method maximizes tensile stress as compared to shear stress in other ASTM test methods such as the tape test (ASTM D 3359).
D 4541 is used by coatings manufacturers, specifiers, inspectors and coatings specialists. Some coatings manufacturers report the pull-off strength on their Product Data Sheets. Some specifiers require a minimum pull-off strength for qualifying coatings and sometimes pull-off strength tests are required as part of the execution of the project. Coatings specialists may use the method in evaluating existing coatings during a coating condition survey or in a failure analysis.
D 4541 used to cover the use of four instruments. It was recently revised to add a fifth instrument to the standard. In doing so, it was also decided that clarifications had to be made so that users better understood the method. Specifically, it has always been recommended that the specifier or user identify the exact instrument as different instruments give different results. Yet specifiers have not always told which instrument to use and users have not always reported the instrument used. The value of the pull-off strength achieved is meaningless without this information.
Table 1 presents the summary of the round robin data performed before the new instrument was added to the standard. As can be seen from this Table, Type III and Type IV instruments give comparable results. Type II instruments give pull-off strengths of about half that of the Type III and IV instruments, while Type I instruments give pull-off strengths that are significantly lower. (While a new round robin is being planned to include the new, Type V instrument, users have indicated that the pull-off strengths obtained with it are similar to the Type III and IV instruments.) Therefore, to emphasize the importance of identifying the instrument, D 4541 has been changed to specify five different methods to correspond with the five different instruments. This will make it simpler to identify the instrument used, and further highlight the need to report it.
Another requirement in D 4541 that casual users of the method, especially, should be aware of is the requirement to perform at least three replications to statistically characterize the surface. For the Type I, II and III instruments, the intralaboratory maximum acceptable difference between two sets of tests consisting of three pulls was 41 percent, while the interlaboratory maximum acceptable difference was 58.7 percent (see Table 2). Examination of the raw data shows that differences between individual results were much higher. It is quite common to get individual results of 1.4 MPa (200 psi), 4.2 MPa (600 psi) and 5.5 MPa (800 psi) in one test area. Why such varying results? The answer lies in the difficulty of performing the test. Some of the reasons for the varying results include misalignment of the apparatus or loading fixture that was not perpendicular to the surface, poor definition of the area stressed due to improper application of the adhesive, holidays in the adhesive caused by voids or inclusions, improperly prepared surfaces, and sliding or twisting of the test fixture during the initial adhesive cure. Almost anything that can go wrong in performing the test will give low results, not higher results.
It must be remembered that the precision statement was developed from tests performed in a laboratory setting. The painted panels were set on a laboratory bench, the operator was comfortably positioned, and the test apparatus was in a vertical position supported by the laboratory bench. Wider variations in individual results would be expected when pull-off strengths are measured in a field setting. Now the loading fixtures are applied to both horizontal and vertical surfaces, the test apparatus may have to be supported and properly oriented by the operator, and footing may be less stable. Experienced field users find that five to 10 percent of pulls end up being invalid. The common causes are adhesive runs underneath the loading fixture on vertical surfaces and knocking the loading fixture off prior to or during the attachment of the test apparatus. Therefore, applying at least three test fixtures per location in the field is a must when performing this test.
The changes made to D 4541 are only a beginning. Those involved with pull-off testing of coatings from concrete and masonry have been perfecting the method for these substrates. The unique qualities with these substrates are their heterogeneous nature and their inherent tensile strength.
Portland cement concrete is a mixture of aggregate and cement paste. The inherent tensile strength of concrete is about 8 to 12 percent of the compressive strength (i.e., 2.1 MPa (300 psi)). If a loading fixture with a small contact area was used, the pull-off strength achieved could be significantly influenced by the composition of the surface of the concrete directly underneath the loading fixture. This may consist of a significant percentage of large aggregate. This will reduce the stress applied to the cement paste, the weaker constituent in concrete. A much greater variability in pull-off strength occurs, including results that exceed the inherent tensile strength of the concrete. Equipment manufacturers have developed loading fixtures with a larger surface area to accommodate cementitious substrates. This information needs to be added to the standard.
Cutting (scoring) around the loading fixture has always been a controversial issue. Scoring can cause micro-cracking, which results in diminished pull-off strengths. But what about thick-film coatings such as flooring systems applied to concrete? Pull-off strengths obtained in this situation exceed the tensile strength of the substrate if scoring was not performed. Now the cohesive strength of the coating can come into play. So scoring is necessary on thick-film systems applied to concrete when trying to determine if the coating is firmly bonded to the substrate.
But how thick is thick? What about thick-film coatings applied to metal substrates? Should scoring be performed before or after the loading fixture is attached? There are a number of questions that need to be answered. The literature contains no authoritative studies in this area. This presents an opportunity to D01.46 to develop the needed information through round-robin testing prior to the next revision of D 4541.
The need to revise D 4541 or develop a new standard for concrete and/or thick film systems is great. ASTM Committee C03 on Chemical-Resistant Nonmetallic Materials and Committee D33 on Protective Coating and Lining Work for Power Generation Facilities have identified a need for a pull-off strength standard test method using the identical instruments in D4541 for their particular uses. Subcommittee D01.46 will spearhead this activity with assistance from these other ASTM committees.
One of the most widely used tests for assessing adhesion is D 3359, Test Methods for Measuring Adhesion by Tape Test, which is under the jurisdiction of Subcommittee D01.23. This method consists of scribing through the coating, applying a piece of tape, and quickly removing it by pulling parallel to the surface. The adhesion rating is determined by the amount of coating removed. This method is commonly used by coating manufacturers in product development and evaluation, as well as engineers responsible for determining if an existing coating system is over coated or must be totally removed, and in failure analysis. This type of peel test is more useful than the pull-off strength test in making over coating decisions as most coating failures of industrial coatings used in atmospheric exposure occur in shear rather than tension. It is also a very quick test to perform compared to D 4541 as there is no need to wait a day for the adhesive to cure.
One of the controversies with D 3359 is the tape. The tape originally specified is no longer manufactured. Other commercially available tapes indicated by D 3359 have variations in bond strength to the surface of the coating of over 50 percent. Bond strength of the tape may also change over time with storage of the tape. In addition, some of the modern industrial coatings are not amenable to the tape test due to their high cohesive strength compared to the bond strength of the adhesive.
Many people involved with industrial coatings have modified the method for their own use by scribing through the coating in the shape of the letter X and probing the cross-section of the incision with the tip of a sharp blade. Practioners who use this method also find it more informative when intercoat delaminations occur. They can then probe the primer/substrate interface to determine adhesion, which is not part of the D 3359 method.
Therefore the subcommittee saw a need to develop a standard using just a knife. D 6677, Test Method for Evaluating Adhesion by Knife, passed society ballot in 2001. The procedure uses a utility knife to make the incisions and to attempt to lift the coating from the intersection of the incisions. The rating scale is dependent upon the size of the coating removed and the difficulty in removing it.
Graffiti on buildings and structures is an ongoing and increasing problem. A number of coatings have been developed or advertised as being resistant to the application of graffiti or the graffiti can be easily removed from the surface. Coating products have been promoted or advertised as graffiti resistant, but what does that really mean? Coatings manufacturers that make these types of coatings were in need of a method to show that their product worked, and users needed a method to specify or select products that would meet their needs.
D 6578, Practice for Determination of Graffiti Resistance, was developed by D01.46 to meet these needs. The practice identifies four commonly used graffiti marking materials and five cleaners. The graffiti material is applied and then removed either using a straight-line washability apparatus or manually with a lint-free cotton cloth soaked in solvent that is wrapped around a cellulose sponge. The cleaning agents are used in a particular order of increasing aggressiveness to determine which one can remove the graffiti. The order consists of a dry cloth, mild detergent, citrus cleaner, isopropanol, and methyl ethyl ketone (MEK). The cleanability levels are determined by the cleaning agent that first removes the graffiti. But to obtain that level of rating, no more than 20 percent loss in gloss (1) of the coating can occur, and color change (2) cannot exceed 1deltaE.
Graffiti resistance of a coating can change with time as the coating fully cures or deteriorates under exposure. The practice covers evaluating graffiti resistance on freshly-applied coating, coating that has been artificially weathered in the laboratory, and coating that has been naturally weathered outdoors. More weight should be given by the user to graffiti resistance after weathering, particularly natural weathering.
A word of caution is needed for users of this practice. D 6578 evaluates the inherent graffiti resistance of a product. The tests are performed on smooth surfaces. While the coating may have a certain cleanability level, the roughness of the surface would affect the ability to adequately rub the cleaning agent completely over the surface. An example would be cleaning a graffiti-resistant coating applied to concrete or brick. The graffiti in the texture created by the substrate may be difficult to remove unless the proper tools and procedures are used.
Another recent standard developed by D01.46 is D 6577, Guide for Testing Industrial Protective Coatings. This document provides guidance on selection of tests to perform on industrial protective coatings. There are over 100 ASTM standards that might apply to testing of industrial coatings. This guide describes the various test methods applicable to liquid coating properties, application and film formation, appearance of the dry film, properties of the dry film, and performance of the dry film. This guide is intended to assist both the specifier as well as the coatings manufacturer in selecting tests to perform. Specifiers and users can benefit by selecting or requiring tests that are most appropriate for the intended coating use.
D01.46 is actively working on other new standards. These include a practice for immersion testing of coatings for interior surfaces of storage and processing vessels, and a practice for determining resistance of coatings to freeze-thaw cycling.
Another new standard being developed is a test method for measurement of surface roughness of abrasive blast cleaned surfaces using a portable stylus recorder. Currently, one of the critical properties specified and measured when abrasive blasting metal surfaces is the anchor profile, which is the average distance between the top of the peaks and bottom of the valleys on the surface. But what about the number of peaks and valleys per unit surface area? This is an important factor in determining coating adhesion and coating performance. Some specifiers have used subjective terms, such as stating that the anchor profile shall be in a dense and uniform pattern. Portable stylus recorders exist that can measure peak counts. These recorders can be used in the laboratory, shop or field. This test method will describe how to use this type of instrument on blast-cleaned steel. Hopefully, this test method will promote studies on coating performance that will identify the relationship between actual peak count and corrosion protection by coatings. The ultimate goal is the ability for owners to objectively specify and measure the minimum peak counts that will optimize more consistent, and long-term performance of industrial protective coatings.
D01.46 has been very active lately updating widely used standards under its jurisdiction and developing new standards. The subcommittee will continue its efforts on developing standards for use by both laboratory personnel involved in developing and testing coatings formulations, as well as field personnel involved in evaluating industrial protective coating application and performance. //
(1) Loss in gloss as determined by D 523, Test Method for Specular Gloss
(2) Color change as determined by E 1347, Test Method for Color and Color-Difference Measurement by Tristimulus (Filter) Colorimetry
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