Coating Adhesion Testing Methods and Equipment

Coating Adhesion Testing Methods and Equipment
Bill Corbett KTA-Tator, Inc. This one-hour webinar is on Coating Adhesion Testing Methods and Equipment, and will be conducted by Bill Corbett of KTA-Tator, Inc.

Webinar Content: Why Measure Adhesion Properties? What is Being Measured? Selecting an Adhesion Testing Method Measuring Adhesion by the Tape Test Measuring Adhesion by the Knife Test Measuring Pull-off Adhesion Fixed Alignment – Mechanical Self-Aligning – Hydraulic (3 devices) Self Aligning – Pneumatic Special Requirements for Testing Concrete Coating Adhesion Common Pitfalls When Testing Coating Adhesion Reporting The webinar will discuss a variety topics relating to adhesion testing of coatings on metallic surfaces, including: Why the adhesion of coatings and lining systems is assessed What property of a coating is being measured when it is evaluated for adhesion How to select an adhesion testing method How to measure the adhesion of coatings using the tape and knife tests How to measure the pull-off or tensile strength of a coating system using five different testing instruments Common pitfalls associated with performing adhesion evaluations; and The specific items to be reported when conducting an adhesion test. In addition, we will briefly describe the special requirements associated with conducting adhesion testing of coatings applied to concrete substrates. Note that this webinar is not meant to replace or be a substitute for the information and procedures described in the respective ASTM test methods, or the instrument manufacturer’s instructions. It provides an overview of basic adhesion testing methodology. Prior to performing adhesion testing, the user should carefully read the instrument manufacturer’s instructions and the corresponding ASTM test method to ensure a complete understanding of the procedure. Also, calibration of the pull-off adhesion testing devices is not described in this webinar. The user should follow the manufacturer’s instructions for frequency of calibration. This is typically performed by the equipment manufacturer or an authorized calibration laboratory.

Learning Outcomes Completion of this webinar will enable the participant to: Explain the purpose of testing the adhesion properties of coatings Describe the two types of adhesion testing mechanisms List four standard test methods used to test the adhesion of coating systems Describe the difference between shear adhesion and tensile adhesion Measure the adhesion of coating systems using the tape method Measure the adhesion of coating systems using the knife method There are twelve learning outcomes for this webinar. Completing this webinar will enable you to: Explain why coatings may be assessed for adhesion properties, Describe two adhesion testing mechanisms, List the four ASTM standard test methods for evaluating adhesion, Describe the basic differences between shear or “peel-back” adhesion and tensile or “pull-off” adhesion, Measure adhesion using the Tape Method, Measure adhesion using the Knife Method,

Learning Outcomes, continued
Describe the difference between fixed alignment and self aligning adhesion testers Explain the differences between testing the pull-off strength of coatings applied to steel and concrete surfaces Measure the tensile (pull-off) strength of coating systems using mechanical, hydraulic and pneumatically-operated testing devices List common pitfalls associated with adhesion testing of coatings Evaluate potential locations of break, including adhesion, cohesion and glue Describe the reporting requirements for adhesion testing 7. Describe the basic difference between adhesion testers that are fixed alignment verses those that are self-aligning, 8. Explain the basic differences in procedures when evaluating coating adhesion on concrete, verses coatings on steel, 9. Measure tensile adhesion of coatings using five different adhesion testers, including one fixed alignment tester and four self aligning testers, 10. List common pitfalls associated with performing adhesion testing on coatings, 11. Evaluate and document planes of separation or locations of break, and 12. Describe documentation requirements for adhesion testing. While hands-on learning is not a component to this webinar, it is highly recommended that you to practice with the instrumentation prior to actual shop or field use.

Why Measure Adhesion Properties?
Determine specification compliance Common in-process inspection of TSC Determine maintenance strategy for existing coating systems Adhesion data is not typically a predictor of coating performance Why are we interested in assessing the adhesion of a coating or lining system? First and foremost, the painting specification may invoke a requirement for a minimum adhesion value or rating prior to placing the structure into service. In this case, as long as the adhesion of the coating exceeds the minimum specified value or rating, the actual adhesion value (point of adhesion loss) is not critical. Section 6.4 of the SSPC/AWS/NACE Tri-Society Coating Specification for thermal spray coatings invokes a requirement for tensile bond measurements using a self-aligning adhesion tester, and provides minimum tensile bond requirements which vary based on the type of feedstock or wire. Adhesion testing is a valuable tool for determining whether an existing coating system can withstand the stresses of an overcoat system, or whether total removal and replacement of the existing system is the better maintenance strategy. In fact, deciding to overcoat an aged coating without performing some type of adhesion assessment of the existing system is risky and can lead to catastrophic failure. Despite all of the benefits of adhesion assessments, there is no direct correlation between adhesion ratings or values and performance. That is, a coating system with a rating of 5 is not necessarily going to last twice as long as a coating system with a rating of 2 to 3. And a coating with a pull-off strength of 1500 psi is not necessarily going to perform three time longer than a coating system with 500 psi tensile strength. In fact, both systems may perform equally, even if the adhesion values are dissimilar. While the coating system must remain attached to the substrate in order to protect it, the degree of attachment can vary.

What is Being Measured? Measuring Coating Strength at Different Planes
Adhesion Strength Bond strength between the substrate and the coating and/or the coating layers to one another Cohesion Strength Inner-strength of a coating layer Adhesion strength is highly variable No industry-wide adhesion acceptance value Minimum adhesion values may be on a PDS or in a specification When you perform an adhesion test, you are measuring the strength of a coating at several different “planes.” First, you are testing the adhesion of the coating system to the substrate. Secondly, you are testing the adhesion of the coating layers to each other, if there is more than one layer of coating on the surface. In both of these cases, you are testing the adhesion strength of a coating, or the bond of the layers to one another and to the substrate. Lastly, when you perform an adhesion test, you are also testing the inner-strength of each coating layer. This is known as the cohesion strength of a coating, or the ability of each layer to hold itself together. We will illustrate each of these in more detail later. The adhesion of a coating system to a surface is highly variable, and can be influenced by a multitude of factors too numerous to itemize during this webinar. That is why there is no industry-wide standard that says that a certain generic type of coating must have a minimum adhesion or cohesion strength. However, project specifications may require a minimum adhesion value as a contract requirement, and coating manufacturers will often report an adhesion value on their product data sheets. Sometimes this value is adopted into a project specification.

Selecting a Test Method
ASTM Standard Method D Adhesion By Tape Test D Knife Adhesion D Pull-off Strength of Coatings Using Portable Adhesion Testers D Pull-off Strength of Coatings on Concrete Using Portable Pull-off Adhesion Testers Prior to performing an adhesion test, you must select a test method. The project specification should indicate the required method, as the various methods can generate very different results. There are three primary ways to test the adhesion of a coating system, including tape, knife and pull-off. The three methods are described in four ASTM standard test methods. The chart on the slide lists the methods that are commonly used to test the adhesion of industrial coatings.

Selecting a Test Method
Tape and Knife Adhesion require minimal equipment and can be conducted quickly Evaluate “shear” or “peel” strength (peel-back) Pull-off adhesion requires specialized equipment and the testing takes comparatively longer to perform Evaluates pull-off strength (resistance to perpendicular pull) Cannot compare data, since testing mechanisms are different Adhesion testing is destructive to the coating film Generally the tape and knife methods are considered “field” test methods, since they do not require any special equipment and can be conducted rather quickly. Tensile or pull-off adhesion requires instrumentation and takes comparatively longer to perform. However, the instruments used to test the pull-off adhesion strength of coatings are portable and testing can and is routinely performed in the field. The tests described in this webinar evaluate two different adhesion properties, and they use different testing mechanisms. The tape and knife adhesion tests are used to evaluate the “shear” or “peel” strength of a coating, while the pull-off test is used to evaluate a coating’s tensile strength, or its resistance to a perpendicular pull. Since the testing mechanisms are different, the results obtained from each test should not be compared. Also, any type of adhesion test is “destructive.” That is, the coating in the area in which you perform the test will be damaged, and oftentimes must be repaired. Nondestructive pull-off adhesion testing can be performed by applying the load up to, but not exceeding the contract-specified tensile force. The load can then be decreased and the fixture left on the coated surface - provided the contract permits this practice.

Measuring Adhesion By Tape Test
Equipment Steel ruler or guide Utility knife Brush Adhesive tape Magnifier ASTM Test Method Cross-cut Tester (alternative to knife and guide) If the project specification or other document requires you to perform an adhesion test according to ASTM D3359, which is the “Adhesion by Tape Test,” you will need the testing equipment listed on the slide. This includes a steel ruler or cross-cut template; a utility knife equipped with a new blade; a small bristle brush; adhesive tape, a magnifier; and a copy of the ASTM test method, which is customarily provided with the test kit. A cross-cut tester equipped with cutter blades of various spacings can be used in lieu of a template or guide and knife blade.

Measuring Adhesion By Tape Test
Step 1: Select test area(s) and conduct triplicate tests per area Step 2: Determine the thickness of the coating system Step 3: Select the test method based on the coating thickness Method A (X-cut): Coating thickness > 5 mils Method B (Cross-cut): Coating thickness < 5 mils Up to 2 mils: Use 1mm spacing between incisions (11 cuts) From 2-5 mils: Use 2mm spacing between incisions (6 cuts) Selecting a test area is perhaps the most difficult part of testing adhesion, particularly if you are evaluating an older coating system that has been maintained by spot or area touch-up painting over many years. It is beyond the scope of this webinar to provide direction on where to test and how many tests to perform. However, you should test several areas and perform triplicate tests in each area to ensure you are obtaining representative adhesion data. If you are testing the adhesion of a new coating system, you may want to minimize the number of locations that you test to reduce the amount of touch-up required. Before you test the adhesion of the coating system using this procedure, you will need to determine how thick the coating system is. This will enable you to select a testing “method.” Method A is called an “X-Cut” and is used to test the adhesion of a coating system that is greater than 5 mils thick. Method B is called a “Cross-cut” and is used to test a coating system that is less than, or equal to 5 mils thick. If method B is selected, the spacing between the incisions varies, depending on the coating thickness. If the coating is less than 2 mils thick, the cuts are spaced 1mm apart and 11 incisions are made. If the coating is less between 2 and 5 mils thick, the cuts are spaced 2mm apart and 6 incisions are made. This procedure will be illustrated later.

Measuring Adhesion By Tape Test Method A (X-cut)
Make a single 1.5” cut through the coating system. Avoid multiple passes. Make a second cut to form an “X” 30-45° angle at intersection of the “X” To perform method A, mount a new razor blade in a utility knife holder and extend the blade outward. Use a straight edge and make a 1.5” long cut through the coating system down to the substrate then make a second 1.5” long cut across the first cut to form an “X”. The legs at the top and bottom of the “X” should be approximately 1” apart. The intersection of the “X” should be close to a 30-45o angle.

Remove cutting debris Remove 2 laps of tape (discard) Apply 3” piece of tape to X-cut Cover all of X-cut Assure good tape contact Remove tape within 90 +/- 30 seconds Remove tape 180° back across X-cut (do not pull upwards) Remove any debris from the “X” area using a soft brush. Remove two complete wraps of adhesive tape and discard it. Carefully remove a 3” piece of the adhesive tape and apply it to the “X” area. Because the tape and the “X” cut area are both 1” wide, the tape should cover the entire X-Cut. Use a soft pencil eraser to rub the tape over the X-Cut. Sixty to 120 seconds after applying the tape, remove it from the X-cut smoothly and rapidly, 180o back across the X-Cut by peeling it back. Do not pull the tape upwards.

Rating Description 5A No peeling or removal 4A Trace peeling or removal along the incisions 3A Jagged removal along the incisions up to 1/16” on either side 2A Jagged removal along the incisions up to 1/8” on either side 1A Removal of most of the coating from the area of the “X” under the tape 0A Removal of coating beyond the area of the “X” Examine the X-Cut area for coating delamination. Since there will be shavings of coating down in the grooves of the cuts, it is not a good idea to rate the adhesion based on what appears on the tape since the tape will almost always have coating debris on it. Instead, look at the X-Cut area on the coated surface and rate the condition of it according to the rating scale in the ASTM standard, which is shown on the slide. A rating of 5A indicates excellent adhesion, while a rating of 0A indicates very poor adhesion. If possible, the location of break should also be recorded as adhesion, which is a split between two layers or between the substrate and first layer; or cohesion, which is a split within a single coating layer.

Measuring Adhesion By Tape Test Method B (Cross-cut)
Make series of 6 or 11 parallel incisions through the coating system. Avoid multiple passes. Make a second series of incisions perpendicular to first series To perform method B, mount a new razor blade in a utility knife holder and extend the blade outward. Use a guide or template to make a series of 6 or 11 parallel knife blade cuts through the coating system down to the substrate. The number of cuts and the amount of space between the cuts is based on the total thickness of the coating system, as we described earlier. Alternatively, a cutter blade with pre-determined spacing can be used. Turn the guide or template 90o and make a second series of parallel knife cuts over top of the first set of cuts, but make them perpendicular to the first set to form a grid, cross-hatch or “Cross-Cut” pattern.

Remove cutting debris Remove 2 laps of tape (discard) Apply 3” piece of tape to grid Cover all of grid Assure good tape contact Remove tape within 90 +/- 30 seconds Remove tape 180° back across cross-cut (do not pull upwards) Remove any debris from the “Cross-cut” area using a soft brush. Remove two complete wraps of adhesive tape and discard it. Carefully remove a 3” piece of adhesive tape and apply it to the “Cross-cut” area. If the total width of the cross-cut area is more than 1”, you will have to apply a second piece of tape next to the first piece to cover the entire grid area. Make sure that you overlap the two pieces so that you can pull them from the grid simultaneously. Use a soft pencil eraser to rub the tape over the cross-cut area. This will help ensure good contact between the tape and the coating. Within 60 to 120 seconds, grasp one end of the tape and remove it from the cross-cut area smoothly and rapidly, 180o back across the grid. Peel the tape back, do not pull upwards.

Rating % Description 5B 0% Edges of cuts completely smooth 4B <5% Small flakes of coating detached at intersections 3B 6-15% Small flakes of coating detached along edges & at intersections 2B 16-35% Coating flaked along edges and parts of squares 1B 36-65% Coating flaked along edges in large ribbons and whole squares detached 0B >65% Flaking & detaching worse than 1B Examine the cross-cut area for coating delamination. Since there will be shavings of coating down in the grooves of the cuts, it is not a good idea to rate the adhesion based on what appears on the tape since the tape will almost always have coating debris on it. Instead, look at the cross-cut area on the coated surface and rate the condition according to the scale in the ASTM standard. The standard also provides a corresponding chart that illustrates the various percentages of delamination. A rating of 5B indicates excellent adhesion, while a rating of 0B indicates very poor adhesion. If possible, the location of break should also be recorded as adhesion or cohesion.

Measuring Adhesion By Knife Test
Equipment Steel ruler or guide Utility knife ASTM Test Method Adhesion can also be evaluated using ASTM D6677, which is Evaluating Adhesion by Knife Test. If the project specification or other document requires you to perform this test, you will need the testing equipment listed on the slide. This includes a steel ruler guide; a utility knife equipped with a new blade and a copy of the ASTM test method. You do not need adhesive tape for this method.

Select test area Make a single 1.5” cut through the coating system. Avoid multiple passes. Make a second cut to form an “X” 30-45° angle at intersection of the “X” Attempt to lift coating with tip of knife blade To perform the knife adhesion test, select a test area, then mount a new razor blade in a utility knife holder and extend the blade outward. Use a straight edge make a 1.5” long cut through the coating system down to the substrate followed by a second 1.5” long cut across the first cut to form an “X”. The legs at the top and bottom of the “X” should be approximately 1” apart. The intersection of the “X” should be close to a 30-45o angle. Starting at the intersection of the “X,” use the tip of the knife blade and attempt to lift the coating from the substrate or the underling coating layer(s).

Rating Description 10 Coating is extremely difficult to remove. Fragments no larger than 1/32” x 1/32” removed with great difficulty 8 Coating is difficult to remove. Coating chips from 1/16” x 1/16” to 1/8” x 1/8” removed with difficulty 6 Coating is somewhat difficult to remove. Coating chips from 1/8” x 1/8” to 1/4” x 1/4” removed with slight difficulty 4 Coating is somewhat difficult to remove. Coating chips in excess of 1/4” x 1/4” removed using light pressure with knife blade 2 Coating is easy to remove. Once started with the knife blade, the coating can be grasped with fingers and easily peeled to a length of at least 1/4” Coating is easy to remove. Once started with the knife blade, the coating can be grasped with fingers and easily peeled to a length greater than 1/4” Rate the adhesion according to the table in the ASTM standard, which is shown on the slide. A rating of 10 indicates excellent adhesion, while a rating of 0 indicates very poor adhesion. If possible, the location of break should also be recorded as adhesion or cohesion.

Pitfalls to Avoid ASTM D3359 is for coatings on metallic surfaces
Dull knife blades can produce erroneous results Maintain proper spacing between incisions when employing the cross-cut tape adhesion method When employing the X-cut methods (tape or knife adhesion procedures), maintain proper angle at intersection of “X.” The legs of the “X” should be ~ 1” apart (width of adhesive tape) Report tape used. Store tape properly Tape may not be effective in cold temperatures Here are six pitfalls to avoid: The scope of ASTM D 3359 standard applies to testing coatings on metallic surfaces. While similar testing can be performed on wood and plastic, the standard was not designed for this purpose. Incisions created with dull knife blades can produce “chatter” along the cut lines and can result in lower adhesion ratings. When conducting method B, the spacing between the incisions is critical. If the cuts are too close, the coating may “ribbon” off of the surface. When conducting method A, make sure the distance between the top two legs and the bottom two legs of the “X” is 1”, which will create the degree angle required by the standard. Since different tapes and different lots of tape can produce different results, it is important to report the manufacturer and type of tape employed, and all testing should be performed using tape from the same lot. Many adhesive tapes are adversely affected by extreme temperatures or relative humidity. The temperature of the air and surface, as well as the relative humidity at the time of testing should be recorded.

What Tape Should be Used?
Note 5 in the current standard lists Permacel 99 as suitable Permacel 99 discontinued in 2009 No immediate replacement Intertape Polymer Group LA26 will be investigated (backing, adhesive, thickness & adhesion to steel similar) Section 5.3 indicates 1” (25 mm) width and adhesion strength as agreed is acceptable Exercise caution when comparing data using different tapes/batches of tape Which tape is the correct tape to use? Good question! Note 5 in the 2009 version of the tape adhesion standard states that Permacel 99 is reported to be suitable for testing. This tape was discontinued in 2009 and there is no immediate replacement. Intertape Polymer Group LA26 is 1” wide and uses the same backing and adhesive. The tape thickness and adhesion to steel are similar to Permacel 99, so it is a reasonable substitute. Section 5.3 in the standard indicates that 1” wide tape with an adhesion strength agreed upon by both parties is acceptable. However, it is important to exercise caution when comparing data obtained using the two different tapes.

Reporting Substrate type and coating system; method of cure
Environmental conditions (temperature and RH) No. of tests performed Manufacturer and type of tape employed (when used) Mean and range of ratings Location of Break (between coats, within a coat, between substrate surface and primer, etc.) Recording of the data acquired during tape and knife adhesion testing includes: The substrate type The coating or coating systems and the method of cure The air and surface temperatures, and the relative humidity during testing The number of tests performed The manufacturer and type of tape used. This applies to tape adhesion testing only The average and range of adhesion ratings The location of break, if discernable

Measuring Pull-off Adhesion
ASTM D4541, “Pull-off Strength of Coatings Using Portable Adhesion Testers” Annex A1: Fixed Alignment Type II (mechanical) Annex A2: Self Aligning Type III (hydraulic) Annex A3: Self Aligning Type IV (pneumatic) Annex A4: Self Aligning Type V( hydraulic) Annex A5: Self Aligning Type VI (hydraulic) Note: Different testers may yield different pull-off values ASTM D4541 describes the procedures associated with evaluating tensile or pull-off adhesion of coatings applied to metal substrates. There are five annexes in the standard describing the apparatus and procedures for one fixed alignment tester and four self-aligning test devices, including three hydraulic devices and one pneumatic device. The standard also provides the results of an interlaboratory study performed in 2008 that demonstrates the variability in test data generated by different test devices. It is important to determine which devices were used to perform testing prior to comparing pull-off adhesion values.

Preparing the Test Surfaces
Remove dirt, chalking, oil or other interfering materials Lightly roughen smooth or glossy surfaces using fine grit paper or abrasive pad Remove any residual dust using a compatible solvent Once a test area is selected, you may need to prepare the surface prior to attaching the loading fixtures. Ensure the area selected is free of grease, oil, dirt or chalking, water, protrusions, or other conditions that will interfere with the attachment of the loading fixture. The area selected must also be flat and rigid, and be large enough to accommodate the test apparatus as well as the attachment of triplicate loading fixtures. According to the standard, a substrate thickness of less than 1/8” usually causes a reduction in adhesion, compared to a ¼” substrate thickness, so rigidity of the substrate is an important consideration. If the test surface is very smooth or glossy, you may elect to lightly roughen it using a fine grade of sandpaper. If you abrade the surface, be sure to wipe away any fine dust that was created using a solvent that will not affect the coating.

Loading Fixture Design
Fixed Alignment Type II Self-Aligning Type III Self-Aligning Type V There is no standard design for loading fixtures or “dollies” as they are sometimes referred to. The loading fixtures are designed to be compatible with the test device. The loading fixtures for the Type 2, Type 4 and Type 5 are aluminum and are generally used once then discarded. The Type 3 loading fixture is stainless steel and the Type 6 fixture is carbon steel. These fixtures can be cleaned and reused. While the aluminum fixtures can be cleaned and reused, it may be more economical to simply purchase new fixtures. Self-Aligning Type IV Self-Aligning Type VI

Preparing the Loading Fixtures
Loading fixtures for Type II and Type IV testers are available pre-abraded Loading fixtures for other testers may need to be abraded to enhance bond to surface (recommended) Clean fixtures before use Because the contact surface of the loading fixtures is smooth, you may need to roughen it using a coarse sand paper or abrasive pad. Type 2 loading fixtures are available pre-roughened by abrasive blast cleaning, and Type 4 loading fixtures are supplied pre-roughened by abrasive blast cleaning. Independent of whether you use pre-roughened loading fixtures or roughen them just prior to use, it is important to remove any dirt or debris that may interfere with the attachment of a loading fixture to the surface.

Selecting an Adhesive High solids epoxy adhesives
Cyanoacrylate adhesive Ensure compatibility with coating UV curable adhesive available with Type IV tester (requires special loading fixtures with transparent base) There are a variety of adhesives that can be used to attach the loading fixture to the surface. A 100% solids epoxy adhesive is often used, but many require an extended cure time and minimum temperature to achieve complete cure. While other “quick-set” adhesives are available and can be used, they may not be able to withstand the pulling forces of the test device, and may break before the coating does at a relatively low force. Cyanoacrylate adhesives or Super Glues may soften or penetrate the coating film, so their use may be restricted on certain coatings. It is recommended that you test the coating in an inconspicuous area to assess compatibility between the adhesive and the coating if you are unsure whether the adhesive will affect the coating. The Type 4 tester is available with specially designed loading fixtures which use an adhesive that cures using an ultraviolet light source. Because the adhesive cures more rapidly, adhesion testing can be initiated more quickly.

Attaching the Loading Fixtures
Apply adhesive to the loading fixture, surface, or both Addition of No. 5 glass beads (1%) assists with fixture alignment Attach the loading fixture to the coated surfaces Push downward; do not twist Remove excess adhesive To attach the loading fixtures, mix the adhesive you selected according to the manufacturer’s directions. If the adhesive is two-component it will have a maximum working time, so mix only as much as you anticipate using during the working time. Using a wooden stick, apply an even layer of adhesive to the entire contact surface of the loading fixture. The addition of about 1% of No. 5 glass beads to the adhesive assists with even alignment of the fixture to the surface, however this is not mandatory. Attach the loading fixture to the coated surface and gently push downward to displace any excess adhesive. Do not twist or slide the fixture during attachment, because you may generate air bubbles in the adhesive. Carefully remove the excess adhesive from the perimeter of the fixture using a cotton swab or other device. The Type 4 tester comes with clear masks containing a center hole that are placed on the surface prior to attaching the fixture. Any excess glue is deposited onto the mask instead of the coated surface. Allow the adhesive to fully cure before performing the adhesion test.

Attaching the Loading Fixtures (Type III Tester)
Insert Teflon® plug Apply adhesive to the loading fixture, surface, or both Attach the loading fixture to the coated surfaces Push downward; do not twist Remove Teflon® plug Remove excess adhesive To attach the Type 3 loading fixtures, insert the small diameter Teflon plug through the center hole of the stainless steel fixture until the small tip of the plug protrudes from the contact surface of the fixture. Using the adhesive dispenser bottle or a wooden stick, apply an even layer of adhesive to the entire contact surface of the fixture. Attach the fixture to the coated surface and gently push downward to displace any excess adhesive. Remove the Teflon plug from the center of the fixture and remove any adhesive that has accumulated on the end of the plug.

Attaching the Loading Fixtures
Secure loading fixture until adhesive cures Use magnetic or mechanical fixture holder or tape If using tape, ensure it doesn’t “relax” during adhesive cure The ASTM standard recommends applying constant pressure on the loading fixture until the adhesive sets. This is particularly important when the fixtures are attached to vertical surfaces, as they will likely slide down the surface before the adhesive has an opportunity to set. A magnetic or mechanical device may be used, however adhesive tape is more common. If tape is used, it should be routinely inspected to ensure it does not relax over time and allow air to enter the fixture-to-surface interface.

Scoring Around Loading Fixture
Light scoring can be performed to remove excess adhesive Scoring of the coating is not recommended If scoring is performed, must be done carefully Scoring (if done) must be reported with test results While light scoring can be used to remove excess adhesive from the perimeter of the loading fixture, the ASTM standard does not recommend scoring the coating down to the substrate prior to testing, as the standard requires the testing of an unaltered coating. Scoring may produce tiny fractures in the coating or may generate friction heat, which can change the properties of the coating and result in lower adhesion values. If the project specification requires you to score the coating around the perimeter of the pull stub, it must be done very carefully to avoid contact with the sides of the fixture. Scoring is recommended for elastomeric coatings, reinforced coatings and coatings greater than 20 mils thick. If you score around the perimeter, place the scoring device over the fixture and rotate the cutter using a back and forth motion. Make sure you score down through all coating layers to the substrate. For thick coatings you may need to use a hole saw mounted in a portable drill to penetrate down to the underlying substrate. This requires considerable skill to avoid contacting the sides of the fixture. A wood template with a pre-dilled hole may help prevent sideways movement of the hole saw. Alternatively, the coating can be scored prior to placement of the loading fixtures. If the coating is scored, it should be reported with the test results.

Measuring Pull-off Adhesion Using the Type II Tester
Model No. Range 106/1 0-500 psi 106/2 (shown) psi 106/3 psi (uses wrench) 106/4 psi (uses wrench) 106/4A Use w/ 2” diameter pull stubs (uses wrench) 106/5 0-30 psi Now that the fixtures are attached, we’ll describe the use of five pull-off testers starting with the Type 2 device. The Type 2 tester is a fixed alignment device and is available in various ranges. Three of the six models employ a spanner wrench to tighten a nut and apply the force, as opposed to a hand wheel, and one model is specifically designed for use with 2” diameter fixtures for testing coatings on concrete.

Measuring Pull-off Adhesion Using the Type II (fixed alignment) Tester
Lower the jaw by turning the hand wheel (nut) counterclockwise Slide the jaw opening over the head of the loading fixture Verify levelness Set the drag pin to zero (push downward) 1 2 The Type 2 adhesion tester consists of three basic components: A hand wheel or a hex nut at the top which is used to apply the force, a black column containing a dragging pin and scale in the middle, and a base containing three legs and a pulling “jaw” which is designed to fit the head of the loading fixture. The adhesion tester is attached to the loading fixture by rotating the hand wheel or hex nut counterclockwise to lower the “jaw” of the device. The jaw is positioned completely under the head of the fixture. The three legs are positioned so that they are sitting flat on the coated surface. The drag pin indicator is positioned to “zero” on the black column by pushing it downward. 3 4

Measuring Pull-off Adhesion Using the Type II (fixed alignment) Tester
Rotate hand wheel (nut) clockwise to increase load (<150 psi/second) Break (or maximum force) should occur in 100 seconds or less Record the value corresponding to the bottom of the drag pin, E.g. 6 is 600 psi Firmly hold the base of the instrument in one hand and rotate the hand wheel clockwise to raise the “jaw” of the device that is attached to the head of the fixture. If the adhesion tester is equipped with a hex nut at the top, use the spanner wrench supplied with the tester. The tension on the spring will place a perpendicular, upward force on the fixture. The dragging pin indicator will move upward on the black column as the force is increased, and will hold the reading once the force is released. Increase the tension smoothly and evenly using a moderate speed. The rate of force should not exceed 150 psi per second. Continue to increase the tension on the head of the fixture until the test is complete, ideally in 100 seconds or less. The black column near the middle of the adhesion tester contains two scales. The right scale is pounds per square inch (psi) and the left scale is MegaPascals. When the endpoint of the test is reached, the scale across from the bottom of the dragging pin indicator is read. When using the psi scale, multiply the scale value by 100. For example if the bottom of the dragging pin is at 6 on the psi scale, the value is recorded as 600 psi.

Measuring Pull-off Adhesion using the Type III (hydraulic) Tester
Insert pin through center hole of stainless steel loading fixture (may need to loosen black threaded hand lever) Pull upwards on the quick connect and allow the locking ring to engage the outside flange of the loading fixture. Pull upwards to verify locking ring is engaged The Type 3 adhesion tester is self-aligning and consists of five basic components: A black, threaded hand lever, a pressure indicator dial containing a black needle and a red “sweep” needle, a flexible hydraulic hose, a stainless steel cylinder, and a “quick connect” locking ring with a stainless steel pin protruding from the center. A digital pressure indicator display is also available. Attach the adhesion tester to the fixture by rotating the black threaded hand lever counterclockwise to retract the stainless steel pin. Next, insert the center stainless steel pin through the center of the pull stub, then attach the quick-connect locking ring to the fixture by pulling back on the snap-on ring and locking the ring onto the flange of the fixture. Pull slightly upward on the adhesion tester to ensure it is engaged.

Measuring Pull-off Adhesion using the Type III (hydraulic) Tester
Return red “sweep needle” to zero (or zero set the digital display) Rotate the black threaded handle clockwise until break occurs or maximum pressure is achieved Read pressure from analog or digital display Loading fixtures (stainless steel) can be cleaned with heat clamp Rotate the red sweep needle dial to “zero” or depress the zero key on the digital display The black hydraulic hose must remain straight during testing. Hold the knurled top of the tester in one hand and rotate the black, threaded hand lever clockwise. This will cause the center pin to push against the coated substrate and the snap-on locking ring to simultaneously pull upward on the fixture. The black and the red indicator needles will rotate on the scale, or the digitally displayed value will increase as the force is increased. The black needle will return to “zero” when the pressure is released, but the red “sweep” needle will hold the reading. The digital version will display the maximum force until the display is reset with the zero button. Increase the tension smoothly and evenly using a moderate speed (no faster than 150 psi per second). Continue to increase the tension until the test is complete, ideally in 100 seconds or less. After the loading fixtures are evaluated, they can be cleaned and reused.

Measuring Pull-off Adhesion using the Type IV (pneumatic) Tester
Device uses air pressure inside a piston to provide a constant, perpendicular pull on a threaded loading fixture Piston pressure is converted to psi (or Kpa) using conversion charts based on piston size The Type 4 adhesion tester is also self-aligning and consists of three basic components: An adhesion tester base component, an air hose and a piston. Compressed air is deployed at a constant rate to a piston. The top plate of the piston is treaded onto the shaft of the loading fixture, creating a perpendicular pull. The piston burst pressure is converted to psi or kilopascals using conversion charts, which are based on the fixture diameter and the piston size used to apply the force.

Piston Range F-1 0-500 psi F-2 0-1,000 psi F-4 0-2,000 psi F-8 0-4,000 psi F-16 0-8,000 psi F-8/12 0-4,000/0-10,000 psi The piston itself contains 3 parts: A round base with a short “pigtail” hose and threaded connector, a silicone rubber gasket (inside the piston base) and a round black threaded top plate. There are six pistons to choose from. The F2, F4 and F8 are the more common piston sizes for testing industrial coating systems Round base with pigtail hose Silicone rubber gasket (lifted upward) Round threaded top plate

Connect piston hose to the main air hose (brass coupling) Push air hose connection into the base unit Insert CO2 cartridge into sleeve Thread the CO2 sleeve onto the base unit Select a piston, then connect the “pigtail” hose from the piston to the longer blue hose using the brass coupling. Insert the opposite end of the blue air hose to the port on the base component. Insert the air cartridge into the black sleeve. Thread the sleeve containing the air cartridge onto the port on the base component. You will here a hissing noise as the tip of the cartridge is pierced and pressurized air enters the base component. Hand-tighten the sleeve until it is snug.

Rate Valve Place piston over the loading fixture Thread top plate onto the fixture until contact is made with the piston, then reverse the top plate ¼-1/2 turn to allow silicone gasket to raise Close the Rate Valve (don’t over-tighten) Place the piston base over the threaded end of the fixture until it is flush with the coated surface. Make sure the silicone rubber gasket is seated inside the piston base. Carefully thread the black plate onto the fixture until the top plate makes contact with the piston base, then reverse the black plate ¼-½ turn so that a small gap is created between the piston base and the top plate. Close the rate valve, but do not over tighten it. As shown in the diagram, compressed air traverses through the hose and enters the piston at a point underneath the silicone gasket, causing the gasket to elevate and impart force to the underside of the top plate. Since the top plate is threaded on to the loading fixture, upward force is applied at a constant rate of pull.

Turn the Power switch to the “on” position Push the peak reset button Push in and hold the “Run” button, while slowly opening the Rate Valve until pressure increases at ~2 psi/second Do not exceed 6 psi/ second Rate valve Run On/off Peak Reset To operate the tester, verify the unit is powered-up and depress the peak reset button. Push in and hold the run button, while simultaneously opening the rate valve until the pressure increases at a rate of approximately 2 psi per second, without exceeding 6 psi per second.

Read the Piston Pressure from the digital display Convert the Piston Pressure to psi (or Kpa) using the conversion chart corresponding to the piston used and diameter of the loading fixture Example: 46 psi piston pressure = psi adhesion strength (F4 piston and 0.5” diameter loading fixture) When you achieve the endpoint of the test, read and record the burst pressure from the display. Convert this value to psi or KPa using a piston pressure conversion chart. Each piston size has a unique chart. There are two charts for each piston range. One chart converts the burst pressure to pull-off strength in psi and the other chart converts the burst pressure to pull-off strength in KPa. The F4 piston chart is shown. The left hand column on each chart contains bold numbers The top row contains bold numbers 0-9. You can use these values on the chart to plot the burst pressure and convert it to pull-off strength. For example, if an F4 piston was used and the burst pressure was 46 psi, move down the left column and locate the bold value “40.” then move across the top row and locate the bold value “6.” Intersect the “40” and the “6” and record the psi value from the chart (937.3 psi).

Measuring Pull-off Adhesion using the Type V (hydraulic) Tester
Type V uses hydraulic pressure to apply perpendicular force to the loading fixture Pull-off strength is read directly from digital display Manual version is shown The Type 5 adhesion tester is self-aligning and consists of two basic components: An adhesion tester base component and an actuator. The base component contains a hydraulic cylinder with a pressure release valve, a lever and a pressure gauge. The design of the head on the loading fixture ensures a perpendicular pull even when the loading fixture is not attached flush to the coated surface.

Open the pressure relief valve and push actuator handle into assembly Power-up tester (green) Verify LCD reads zero Select pull-stub size (*) Select units (**) To operate the tester, verify that the pressure relief valve on the cylinder is completely open, then Ppush the black actuator handle completely down into the actuator assembly. Power-up the test device and verify that the display reads zero. If it does not, press the on-off button to zero the display. Select the fixture size and the units by pressing the corresponding buttons on the control panel. * **

Position actuator over loading fixture. Attach quick connect coupling to head of fixture Close the pressure relief valve Pump lever until digital display indicates pressure is applied Place the actuator assembly over the round head of the pull stub and attach the quick connect coupling to the loading fixture. Close the pressure relief valve on the pump tightly. Begin pumping the lever above the cylinder until the pressure increases.

Continue pumping uniformly (<150 psi/ second) Monitor pull rate using LCD display (psi/s) Display holds maximum pressure of pull-off Record value, or press memory pad (*) to store value Depress green button to zero display Continue pumping at a uniform rate of no more than 150 psi per second until the actuator detaches the fixture from the coating. The display contains a pull rate monitor bar directly above the pressure reading. The display will hold the maximum pressure reading for you until you clear it. Either record the value or press the memory pad on the control panel to store the value. Press the green on-off button to zero the display. *

Automatic version applies hydraulic pressure to actuator without pumping of a lever An automatic version of the Type 5 adhesion tester eliminates the need for pumping a lever to apply the hydraulic load to the fixture.

Measuring Pull-off Adhesion using the Type VI (hydraulic) Tester
Type VI tester uses hydraulic pressure to apply perpendicular force to the loading fixture Pull-off strength is read directly from the gage dial (up to ~ 2,470 psi) The Type 6 adhesion tester is self-aligning and uses hydraulic pressure to slowly and constantly increase the perpendicular pull on the attached loading fixture. This pull-off strength is read directly from the gage in pounds per square inch or MegaPascals. The testing head shown has a pressure capacity of 17 MPa which is approximately 2,470 psi.

Type VI tester consists of a turning crank/pressure gage & testing head Verify crank and tightening wheel are in start position Press 4 pistons against even surface to return oil to the pump The Type 6 adhesion tester consists of two basic components: A turning crank and a testing head. Verify that the crank on the right side and the tightening wheel are in the start position. Press the four testing head pistons against an even surface using hand pressure. This will return the hydraulic oil to the pump and prepare the equipment for testing.

Lift outer ring of the quick release coupling; mount the testing head to the loading fixture Release the quick connect and listen for “click,” indicating it is locked into the groove Return the red “sweep needle” to zero Lift the outer ring of the testing head’s quick release coupling and mount the testing head onto the loading fixture. Release the quick connect and listen for a “click” indicating that the quick connect has locked into the groove near the top of the fixture. Return the red sweep needle to zero.

Turn the tightening wheel at base of pump until piston legs contact surface and gage dial initiates movement Turn hand crank smoothly & evenly until a break occurs or the maximum pressure is reached Read pressure from gage dial (psi x 100 on red dial) Turn the tightening wheel at the base of the pump until all four of the piston legs on the testing head are fully in contact with the coated test surface and the pointer on the gage dial begins to move slightly. Turn the hand crank smoothly and evenly until the desired pressure is reached, or until the coating fractures. The pressure gauge dial of the adhesion tester contains two scales. The black outer scale is in MegaPascals and the inner red scale is pounds per square inch. The red sweep needle will hold the maximum until you return it to “zero.” Read and record the value from the pressure gauge. If the psi scale is used, multiply the value by 100; if the metric scale is used, there is no conversion required.

Special Requirements for Pull-off Adhesion of Coatings on Concrete
Standard Test Method ASTM D SSPC SP13/NACE No. 6, Appendix A1.6 describes procedures for testing concrete coating adhesion Testing procedures are similar; minor differences in equipment and test procedures This section is not comprehensive SSPC CCI (Concrete Coating Inspector) Training Program contains more detail Four considerations described The procedures used to test the pull-off adhesion strength of coatings on concrete are very similar to the procedures used to test the pull-off adhesion strength of coatings on steel. However, there is a different ASTM standard test method for testing coatings on concrete, and there are a few minor differences in the equipment and the procedures. This section highlights the major differences between testing coatings on steel and coatings on concrete, but does not provide the exact procedures you need to use for each instrument. You should review the ASTM standard prior to conducting testing coatings on concrete surfaces. In addition, an appendix to SSPC SP13 on Surface Preparation of Concrete describes procedures for testing adhesion of coatings to concrete, and SSPC’s concrete coatings inspector course provides training on adhesion testing procedures. We will be focusing on 4 basic considerations: Scoring; loading fixture size; loading rate; and compressive verses tensile strength.

Scoring the coating Thick film coatings (>20 mils) have lateral bond strength May produce misleading pull-off values (false, high) Scoring through coating (down to the concrete surface) is typically performed Scoring is done prior to attachment of loading fixture (except if loading fixture is square) Scoring coatings <20 mils may also be required When attaching loading fixture, verify adhesive does not flow into groove 1. Scoring the coating Thick film coatings greater than 20 mils may have lateral bond strength, which can produce misleading pull-off values. Therefore, you will likely need to score the coating down to the concrete prior to attaching the loading fixture. Scoring a coating on concrete that is less than 20 mils in thickness may also be required. Scoring can be done by hand if the coating is thin enough. For very thick coatings, a mechanical method of scoring the coating may be required. Independent of the scoring method you select, you will need to score the coating prior to attaching the fixture. Otherwise you may inadvertently shear it off. When attaching the fixture to the surface, make sure that any excess glue does not flow into the score groove.

Loading Fixture Size Unlike steel, concrete is non-uniform and contains cement and aggregate Round loading fixture diameter should be 2” (50 mm) to ensure test is performed over a representative area Square loading fixtures are used with some testers Loading Rate The pressure loading rate should be 30 psi/second; the test should be completed in 5-30 seconds 2. Loading Fixture Size Unlike steel, concrete is a heterogeneous or non-uniform substrate that contains cement paste and aggregate. While any diameter loading fixture can be used, a 2” or 50mm diameter fixture is recommended so that the adhesion strength of the coating is being evaluated over a representative surface. Square loading fixtures are used with some test devices. 3. Loading Rate The loading rate applied to the fixture by the adhesion tester should be approximately 30 psi per second, so that the test pull is completed in 5 to 30 seconds.

4. Compressive Vs. Tensile Strength Concrete has high compressive strength (i.e., 4000 psi), but low tensile strength (i.e., 400 psi) Low pull-off values common Plane of fracture commonly within the concrete When fracture occurs in the concrete, report the minimum coating tensile adhesion properties; actual coating tensile strength remains unknown 4. Compressive Vs. Tensile Strength Concrete has extremely high compressive strength, typically in excess of 4000 psi, but relatively low tensile strength, typically less than 400 psi. Therefore, it is common to get a relatively low value when you test coatings on concrete, and the plane of fracture is typically within the concrete substrate, since the coating tensile strength often exceeds the concrete tensile strength. In this case, the minimum coating adhesion pull-off strength can be reported, but the actual coating adhesion strength is unknown.

Recording the Type and Location of Break
Adhesion Break: A break between coating layers or between the substrate and first coating layer Cohesion Break: A break within a single coating layer Glue Break: Coating adhesion and/or cohesion strength exceeds bonding strength of the adhesive If multiple locations of break occur, estimate the percentage of each (e.g., 75% cohesion within primer; 25% adhesion between primer and intermediate coats) In addition to recording the pressure, the type and location of break should be described. An adhesion break is defined as a break between coating layers or between the substrate and first coating layer. A cohesion break is a break within a single coating layer; and a glue break is used to describe an occurrence where the coating adhesion or cohesion strength exceeds bonding strength of the adhesive used to attach the loading fixture to the coated surface. If multiple locations of break occur, estimate the percentage of each.

Illustration of Various Locations of Break
The right image in the top figure illustrates a 100% adhesion break between the topcoat and the primer. The left image in the bottom left figure illustrates a glue break, while the right image in the same figure illustrates a cohesion break within the primer layer. The left image in the lower right figure illustrates an adhesion break between the substrate and the primer, while the right image illustrates a cohesion break within the topcoat.

Recording the Type and Location of Break
Disregard any test where glue break represents more than 50% If glue break occurs and the pull-off value is greater than the acceptance value, re-attachment is not necessary (report “pass with pull-off strength > X psi”) According to the ASTM test method, any pull test that results in a glue failure over more than 50% of the loading fixture surface should be discarded. If an adhesion acceptance value is provided and a glue break occurs, re-attachment of new loading fixtures is only necessary if the pull-off value is below the acceptance value. If the glue break occurs at a pull-off rate higher than the acceptance value, then no re-testing is necessary, even though the actual pull-off strength of the coating remains unknown.

Pitfalls to Avoid Determine which tester to use. Different testers generate different pull-off values on the same coating Verify testing equipment has current calibration Perform testing in triplicate The rigidity of the substrate can affect the testing Clean the surface and the loading fixtures to reduce the possibility of glue failures. Use care when abrading the surface to prevent introduction of surface flaws Verify adhesive is compatible with the coating Allow the adhesive to cure. Conducting pull tests before adhesive is cured often results in glue breaks Verify perpendicular pull Support pull-off device on vertical surfaces Here are some common pitfalls to avoid: The ASTM standard discloses the variability in test data generated by different test devices. It is important to determine which devices were used to perform testing prior to comparing pull-off adhesion values. Calibration of test devices varies by manufacturer. Verify the currency of calibration before performing testing A minimum of three loading fixtures should be applied in each area. Substrate thickness less than 1/8” may cause a reduction in adhesion, since the substrate may be flexing while the load is being applied to the fixture. Oil or debris on the coating surface or on the contact surface of the loading fixture can result in glue breaks. Careful cleaning of these contact surfaces is important. Glossy surfaces may be lightly abraded to enhance the bond of the glue to the coated surface, but excess abrading may alter the coating. Verify that the adhesive will not dissolve the coating by testing it in an inconspicuous area. Follow the adhesive manufacturer’s directions regarding cure time and temperature to avoid glue breaks at low values. Verify that the loading fixture is attached flush to the surface to ensure a perpendicular pull. Self-aligning adhesion testers can compensate for some un-evenness. Attachment of the testing device to the loading fixture on a vertical surface can apply lateral force to the fixture, resulting in a shear affect. Carefully support the test device when it is used in this manner.

Reporting General Nature of Test
Substrate type and coating system (thickness, type, surface orientation) Field or laboratory testing Air temperature and relative humidity Testing apparatus employed; loading fixture type/dimensions Adhesive type employed No. of tests performed Load applied (pull-off values, in psi, KPa or MPa) Mean and range of pull-off values Location of break (between coats, within a coat, between substrate surface and primer, etc.) Note scoring, if employed (and any other deviations from procedure) Recording of the data acquired during pull-off adhesion testing includes: The substrate type The coating or coating systems and the thickness, type and test surface orientation Whether the testing was performed in the field or in a laboratory environment The air temperature and the relative humidity during testing The test apparatus type, along with the loading fixture type and size The adhesive type used The number of tests performed The load applied The average and range of pull-off values The location of break Finally, note if scoring was employed and any other deviations from the standard test method

THE END Thank you for attending his one-hour webinar on Coating Adhesion Testing Methods and Equipment

Coating Adhesion Testing Methods and Equipment

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