Introduction
One of the more frequently asked questions when it comes to testing the tensile strength of industrial protective coatings is, “What glue should I use to attach the loading fixture?” This brief article lists the standard test methods for evaluating tensile adhesion and describes some of the considerations when selecting an adhesive.
Standardized Test Methods
Tensile (pull-off) adhesion testing provides a quantitative method for assessing coating system adhesive/cohesive strength.
There are several standardized test methods for performing pull-off testing, including:
- ASTM D4541, “Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers,” developed for testing the adhesion of a coating to metal substrates, but may also be used for other rigid substrates such as wood and plastic;
- ASTM D7234, “Standard Test Method for Pull-Off Adhesion Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers,” developed out of ASTM D4541, Method A for testing coatings on concrete; and
- BS EN ISO 4624, “Paints and Varnishes – Pull-off Test for Adhesion,” which describes three methods for use on a wide range of substrates, including deformable substrates.
Pull-off tensile testing is typically done in the field, shop, or laboratory using portable adhesion testers. The testers are designed to apply a force to separate the bonded loading fixture from the coated surface. They also keep the force perpendicular to the surface, which is important so as not to introduce any cleaving effect during the test, which could cause excessive variation in the test results. If the test is performed correctly, either the coating (or one layer of a multi-layer coating system) is separated from the substrate or other layers (referred to as “adhesive break”), or the coating (or layer) is fractured internally (referred to as “cohesive break”).
The glue used to attach the loading fixture to the coated surfaces must be stronger than the adhesion strength of the coating to either the substrate or to other layers of the coating system (if multiple coats are applied), and the internal strength of the various coatings layers themselves. This will ensure that the strength of the coating or coating system under test is being assessed and not the strength of the glue itself.
The pull-off testing procedure typically involves the following:
- preparation of a section of the surface of a fully dried and cured coating or coating system that has been applied on the surface of a rigid substrate (typically either steel or concrete with relatively uniform surface texture), usually by lightly sanding a section of the coating and subsequently wiping it clean,
- bonding the head of a loading fixture directly onto the surface of the coating or coating system, usually using a two-component, 100 % solids, viscous liquid adhesive (glue), and
- once the glue has sufficiently cured, detaching the loading fixture perpendicular to the surface (thus applying a “tensile” stress) with an apparatus specifically designed for measuring the amount of force needed to do so. Occasionally “scoring” is performed, in which the coating is scored just outside of the test area to minimize the potential contribution of lateral bonding from the surrounding area to the test result.
Adhesion Theory
To understand how to best choose the correct adhesive to use for pull-off testing, it is beneficial to understand a bit about the various theories of adhesion. A material can adhere to another in one, several, or all the following ways:
- Mechanical – This describes penetration of the adhesive into the pores and cavities of the surface of the substrate. A good example of this is white or wood glue; when applied, the polymers of the glue wick into the substrate (paper, fabric, wood) to create a physical bond after the carrier (water) has evaporated.
- Diffusion – The polymers in the adhesive diffuse into the surface of the substrate on a microscopic level; e.g., when adhering plastics together, a good adhesive will slightly penetrate the surface of the plastic on a microscopic level to provide a bond.
- Chemical – The adhesive forms chemical bonds directly with the substrate, e.g., using a “super glue” to repair a broken item.
- Wetting – If the surface tension of the adhesive is lower than the surface energy of the substrate, the adhesive can sufficiently spread out over the surface. Think of how water beads-up on a freshly waxed car; if an adhesive behaves like the water, it will not properly wet and bond with the substrate.
- Acid-Base – A type of chemical adhesion specifically involving the reaction of acidic and basic chemical groups on both the adhesive and the substrate to form a bond. Imagine how Velcro® works, where the “hooks” might be the acidic groups and the “loops” might be the basic groups. Both are required to form the bond, and neither will bond with itself. Note that Velcro® works by mechanical interlocking of the hooks and loops but try to use your imagination on this one.
- Electrostatic – This is on a sub-atomic level and works by the electrostatic effects between the adhesive and the substrate. Think of rubbing an inflated balloon to generate static charge to then adhere it to a wall.
Additional Considerations
Keeping in mind the adhesion theories described earlier, there are several additional factors that may affect the quality of the adhesive bond created for pull-off testing:
- Degree of cure – the adhesive and coating used must be allowed to fully cure.
- Cure and test temperatures – the temperature that both the coating system and the adhesive used for the pull-off test has been cured can affect the quality and strength of the bond being tested.
- Environment and chemical exposure – if the surface of the coating is exposed to any condition or chemical that might degrade it prior to testing, it could either interfere with the loading fixture bond, or create a weak layer at the coating surface.
- Preparation and cleanliness of the surfaces – anything on the surface of the coating (e.g., chalking) that may interfere with any of the adhesion mechanisms described earlier can also lead to abnormally low tensile strength values by inhibiting the adhesive bond to the coating. Often both the surface of the coating and the base of the loading fixture are abraded and then cleaned to provide a quality surface for the adhesive to adhere to.
Adhesive Types
There are several different adhesive chemistries that may be used for pull-off adhesion testing; the key is to maintain a balance of adhesive strength to the substrate, internal cohesive strength, and a reasonable cure time. Since there are a variety of coatings systems that can be applied to a variety of substrates, only trial and error and/or previous experience can help to determine the correct adhesive to use. It is important to consult the adhesive manufacturer’s product information sheets and consider the following:
- the ultimate tensile strength of the cured adhesive (some coatings can reach tensile strengths of nearly 4,000 psi),
- what coatings the adhesive should be used with (or compatibility), and
- the cure time of the adhesive at the prevailing temperature and humidity where the loading fixtures will be attached (typically several hours up to 24 hours)
The following types of adhesives can be considered for use in pull-off testing:
- Epoxy (2-component solvent-less epoxide). Epoxy adhesives are the predominant type used for pull-off testing. They are compatible with most coatings and adhere well to properly roughened loading fixtures, have good tensile strength, and reasonable cure times. The components must be mixed together prior to application. Products include:
- Huntsman Araldite 2011 Slow Cure/Araldite 2010 Fast Cure
- J-B Weld
- 3M DP460/Scotch-Weld 1838
- Hardman Double Bubble Extra Fast Setting
- Lord Loctite Hysol 1C Slow Cure
- Acrylic (2-component acrylic). Acrylic adhesives have been used in the past but have mostly been replaced by epoxy adhesives. However, they may be useful in situations where epoxies do not work well. The components can be mixed together before application, or the accelerator can first be applied to the substrate and allowed to dry, followed by application of the adhesive. Products include:
- Lord Versilok 204
- Palm Labs Turbo Bond 300
- Cyanoacrylate (“super” glues) typically have very fast cure times and are much less viscous and less strong than epoxies. Even though they cure rapidly, allowing several additional hours of curing will improve their bond strength. Products include:
- Palm Labs Turbo Fuse 170
- Permabond 240
- Loctite 416
- Polyester (peroxide-catalyzed). Polyester adhesives have essentially been replaced with epoxy adhesives but are described in the ISO 4624 method for adhesion testing.
- UV Curable (requires special loading fixture). One-component acrylic adhesives can be specially designed to cure upon exposure to ultraviolet (UV) light. One supplier, M. E. Taylor Engineering, has designed a kit (P.I.P.S., or “PATTI Instant Pull Stubs”) where this type of adhesive is used along with a specially designed pull stub containing a translucent base) and UV light fixture that transmits UV light to the adhesive. Once the adhesive is applied and the pull stub is attached to the coated surface, a portable UV-emitting light fixture is placed over the pull stub, and the adhesive is cured within several minutes. The tensile adhesion is then determined using the pneumatic adhesion tester.
Evaluation of Bond Failure/Failure Modes
After a pull off test has been performed and the tensile strength has been determined, the resulting failure mode(s) needs to be assessed to properly characterize the adhesion properties of the coating system. ASTM D4541 describes such a process, which involves evaluating the surfaces of the coating and loading fixture after detachment and rating the break based on where and how much fracture occurred. One of the possible failure modes is a glue break, where the adhesive and/or cohesive strength of the coating system exceeds the bond of the loading fixture to the coated surface. When this occurs, it may or may not be necessary to reattach new loading fixtures and repeat the testing.
Managing Glue Failures
According to ASTM D4541, a test result is indeterminate when there is a visible amount of glue fracture (defined as glue failure of 5% or more of the loading area) that occurred during the test and the maximum load is less than the pass/fail criteria. An indeterminate test may be redone to determine a passing or failing result. If the test is redone, and glue fracture persists at a load below the test criteria, it is recommended that the operator review the test procedure and make adjustments to reduce glue fracture or indicate the test is indeterminate. If the glue fracture occurs at an applied load greater than the pass/fail criteria, there is no need to retest (report the result as “pass; > [criteria] psi”).
Summary
When performing tensile (pull-off) adhesion according to ASTM or ISO standards, the adhesive (glue) selected to attach the loading fixture to the coated surface is critical to the test. If the glue fractures below the acceptance criteria the actual adhesion characteristics of the coating system remains unknown. There are a variety of adhesives on the marketplace that can be used; however, they must be compatible with (cannot dissolve) the coating and must possess high tensile strength properties. It is important to follow the adhesive manufacturer’s instructions and allow the adhesive to cure thoroughly prior to testing.
