Dolly preparation for adhesion testing with DeFelsko PosiTest AT adhesion testers

This article discusses the process of dolly preparation necessary for optimal bonding at the dolly-adhesive interface.

In addition, the article covers the importance of proper dolly preparation for maximizing the repeatability of trigger adhesion tests.

Products montioned:

PosiTest AT-A, PosiTest AT-M

Functional principle of the pull-off adhesion test

Adhesion testers such as the PosiTest AT measure the force required to peel an area of coating of a specific diameter from the substrate. This measured peel force provides a direct indication of the strength of the bond between the coating and the substrate. By eliminating sources of variation in peel, such as unintentional loss of adhesion between the adhesive and poorly prepared dollies, adhesion test results become even more meaningful and predictable.

The main components of an adhesion tester are the pressure pump, the pressure measuring device (manometer) and the actuator. During operation, the flat side of a test specimen (dolly) is glued to the coating under test. After the adhesive has cured, a coupling piece of the actuator is attached to the dolly. Activating the pressure pump slowly increases the pressure on the actuator in the system. When the pressure on the actuator becomes greater than the adhesion strength between the coating and the substrate, separation (tear-off) occurs and the actuator-dolly unit lifts the coating off the substrate (see Figure 1). The maximum pressure reading on the device's manometer directly indicates the pressure at which the pull-off occurred.

Figure 1: Cross-section of the PosiTest AT actuator

Figures 2 and 3 - mtv messtechnik supplies high-quality DeFelskos devices for adhesion testing, above the PosiTest AT-A Automatic, below the PosiTest AT-M Manual

Preparing the dolly and the surface for the adhesion test

Dollies for pull-off adhesion testers are made from a variety of metals, including aluminum, carbon steel, and stainless steel. While the tests performed in this study focused on the disposable aluminum dollies used with the DeFelsko PosiTest AT Adhesion Tester listed in proposed Appendix A5 of ASTM D 4541 (see Figure 2), the principles discussed apply to all dolly types. Dolly preparation typically consists of three key steps: degreasing, sanding, and cleaning.

Degreasing is the removal of traces of oil and grease from the surface to be bonded. This may include oils from the skin of the person handling the dolly. Sanding is an aggressive alteration of the profile of the dolly surface. Sanding serves two primary purposes: to increase the surface area available for bonding and to remove oxidation or rust. Cleaning is simply the removal of any loose particles from the surface to be bonded, especially those caused by abrasion.

Some dolly types are machined before shipping, eliminating the need for the customer to degrease the dollies as the machining process and subsequent careful handling and packaging eliminates any contamination.

Typically, the manufacturer will provide the customer with appropriate equipment, materials and instructions to facilitate preparation prior to bonding the dollies. The reconditioning methods recommended by the manufacturer should be based on extensive laboratory testing of abrasion and cleaning methods for its dollies. The reconditioning methods should also be clear, simple and detailed enough to ensure repeatable results for different users and applications.

Experimental basis for recommending a dolly preparation method

The following dolly preparation recommendation has been optimized for the tested adhesion tester according to Annex A5 based on the results of a study of generally accepted methods and subsequent experimental results. A special experiment was designed to verify and summarize the results of tests and comparisons previously performed during the product validation phase.

The purpose of the experiment was to directly measure the effects of oxidation and dolly preparation (degreasing, sanding and cleaning) on adhesion. The test method involved randomly bonding 48 aluminum “dollies” to a coated carbon steel plate using an appropriate adhesive that had already been used in other tests. Since the goal of the study was to evaluate the factors affecting the bond strength between the adhesive and the dolly, an attempt was made to develop a substrate and coating combination with an extremely strong adhesive and cohesive bond that would also bond well with the previously selected Araldite 2011 adhesive. To accomplish this, a 4 mil thick layer of the same Araldite 2011 was baked onto a ¼” thick carbon steel plate from a ship’s hull. The steel plate was carefully prepared by sanding away a few millimeters of potential surface corrosion and contamination and then cleaning the plate with alcohol and a dry cloth. The result was a coated substrate with sufficient adhesion strength to prevent undesirable coating failure during the 48 peel adhesion tests.

The test dollies were divided evenly by abrasion method, oxidation time, cleaning method and adhesive cure time. The four abrasion methods included machine flattening, end milling, sanding with fine sandpaper and scrubbing with a Scotch-Brite™ pad. Each group of sanded dollies was exposed to air for three different time intervals (7 days, 24 hours and a few minutes) before coating was applied. Before coating was applied, the dollies were either wiped with a dry cloth or cleaned with a cotton swab dipped in alcohol and then wiped with a dry cloth. This resulted in two samples prepared using every possible combination of methods. One sample from each of these pairs was then allowed to cure for either 24 hours or 5 days before the pull-off test was performed.

Results of the different abrasion methods

The results were tabulated for each factor and the average results by abrasion method for each set of 12 dollies are shown in Table 1. As expected, the critical factor for preparation was the abrasion method used. When comparing the results to the abrasion method for the dolly, a predictable pattern emerges. The weakest adhesive bonds were on the machined-only and the finish-milled dollies. These dollies experienced almost 100% adhesive-to-dolly bond failure. This was expected as both preparation methods resulted in relatively smooth surfaces, although the milling process creates large visible grooves in the dolly surface. These grooves increase the surface area of the dollie and are believed to be the reason for the slightly higher bond strength demonstrated prior to failure.

Table 1 - Failure test results related to different abrasion methods

As noted in previous tests, the Scotch-Brite pad achieved the highest overall bond strength, while the sandpaper came in second. Although the visual abrasion results are similar with a Scotch-Brite pad and sandpaper, it is likely that the microstructure (surface profile) is more favorable for adhesion when using a three-dimensional abrasive pad such as the Scotch-Brite. Adhesion requires the adhesive to be soaked into the microstructure through some sort of capillary reaction, so slight changes due to grit or abrasion method can have a significant impact.

Another interesting result was the significantly higher variance between the sandpaper and Scotch-Brite strippings. This result may be related to the rapid buildup of aluminum observed on the surface of the sandpaper, which is difficult to remove between applications. As a result, it is likely that not every dolly will receive the same microstructure. The Scotch-Brite pad allows the aluminum dust to trickle through the fabric, which appears to result in a more uniform microstructure. The Scotch-Brite pad also wears more slowly and needs to be replaced less often.

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Results of the oxidation test

The average bond failure based on oxidation time after abrasion is shown in Table 2. This relatively small difference in bond strength is likely due to a limited effect of oxidation on the aluminum dollies. Since aluminum typically forms a thin aluminum oxide layer immediately when exposed to air, it is plausible that uncontaminated dollies would not experience long-term effects of exposure. The effects of oxidation would potentially be significantly greater on other dolly materials, particularly carbon steel.

Table 2 - Results of failure test after a certain oxidation time

Results of cleaning tests

The average bond failure based on cleaning method before bonding is shown in Table 3. This negligible difference supports the theory that by processing the dollies before shipping, degreasing the dollies before use is no longer necessary as long as they are handled carefully by the customer. Bond Failure Based on Cleaning Method

Table 3 – PosiTest AT Dolly Bond Failure Test Results by Cleaning Method

Previous experiments



It is important to point out that the development of hypotheses and the selection and elimination of factors is based on several previous tests, comparisons and experiments. Some of the excluded factors with the corresponding test results are briefly described below.

The effect of shot peened dollies was compared to that of end milling. Three epoxies from different manufacturers were used. A total of 24 dollies were made using the two abrasion methods and then bonded to a highly adhesive, white epoxy coated blasted steel plate. The average fracture strength of the dolly-to-adhesive bond was 2686 psi for the blasted and 2786 psi for the milled dollies. A subsequent literature review helped to explain this result by determining that the expected benefits of shot peening only occur when the dollies are bonded within hours of preparation for coating. 2

It is generally accepted that aluminum that has been anodized and sealed with chromic or sulfuric acid can be bonded after degreasing and light abrasion. Phosphoric acid anodized aluminum has the optimal surface properties for direct bonding without pretreatment, but even the treated dollies must be bonded within a few hours of anodizing to achieve the desired effect.

It is generally accepted that aluminum that has been anodized and sealed with chromic or sulfuric acid can be bonded after degreasing and light sanding. Phosphoric acid anodized aluminum has the optimal surface properties for direct bonding without pretreatment, but the treated dollies must also be bonded within a few hours of anodizing to achieve the desired effect. This theory was tested with a small sample of dollies bonded about a week after anodizing. The delay in bonding the dollies was largely due to the inevitable shipping and handling times. These bond test dollies were also tested against end-milled dollies. The anodized dollies had 20 to 30% lower bond values.

An adhesion test was conducted using an aluminum surface primer to maximize the bond strength of the dollies. The surface primer tested was Henkel Alodine 1132, which was highly recommended as a military approved conversion coating. An equal number of dollies were used for the adhesion test with various abrasion methods including machining, end milling, sandpapering, and shot blasting. In addition, 6 different two-part epoxies were used. The end result was an average bond strength of 1776 psi with Henkel versus 2277 psi without.

This result is likely due to a combination of factors, most notably that the Henkel product, while a highly accepted pressure sensitive adhesive, does not necessarily have the tensile strength required for use in peel testing. It is worth noting that the Henkel product improved peel strength for some combinations of adhesives and abrasion methods, but resulted in lower overall bond strength values.

A final notable comparison involved the degreasing and cleaning of dollies. In summary, dollies that had been sanded and carefully handled did not require any special degreasing and cleaning methods. Whether the dollies were blotted with methyl ethyl keytone, acetone, or alcohol, or simply wiped with a dry cloth, the differences in bond strength were not statistically significant. Dollies that were sanded and then directly bonded without cleaning or degreasing had lower average bond strengths at the dolly-adhesive interface. When examined under a microscope, the adhesive left on the coating from these defects tended to have a higher visible concentration of contaminants on the surface (particularly from abrasion methods such as sanding) when lower bond strength values were observed.

These preliminary results, combined with the prohibitive costs of additional processes such as shot peening, anodizing, conversion coating and degreasing, led to the development of a simple but sophisticated process that is more easily implemented on-site at the customer.

Why is it not recommended to reuse aluminum dollies?

A common customer request that relates directly to dolly reconditioning is the reuse of disposable dollies. This request typically comes from customers who are used to adhesion testers that use the more expensive steel dollies. For such applications, custom-built equipment is offered that allows the customer to remove coating and adhesive from the dolly by heating and time-consuming scraping. The number of times the dolly can be reused is usually only limited by the wear and tear of the dolly surface before each reuse.

From speaking to numerous inspectors, it is clear that they often choose not to reuse the dollies because they need to retain the dollies as important evidence of the inspection results. Other customers choose to retain the dollies as permanent quality records that demonstrate the success of the stripping while also providing related details such as the thickness of the coating stripped. One approach to avoid dolly reuse is to provide a less expensive, disposable dolly that the customer can either retain or discard after the inspection.

If the dolly can be returned to its original condition, it is likely to be safe to use again. However, please note that if the dolly is significantly damaged or worn, reuse is strongly discouraged. This is more common and immediate with aluminum dollies, which can be damaged by the quick disconnect during high pressure pulls. All dolly surfaces may have imperfections caused by repeated sanding or machining when cleaning previous test coatings and adhesives.

conclusions

The experiments, tests and comparisons documented in this article demonstrate that manufacturers must provide the necessary equipment and methods to properly prepare dollies for repeatable bond strength tests. While it is recognized that a variety of chemical and mechanical solutions exist for preparing dollies, it is suggested that simpler and less expensive methods for sanding dollies should exist. In the case of the aluminum dollies studied, sanding with a Scotch-Brite pad followed by cleaning with a dry cloth was more than sufficient to eliminate poor peel adhesion tests due to adhesive failure.

The experimental results briefly mentioned in the article also show that different material and adhesive combinations can have their own optimal preparation methods. The experiments showed that changes in the abrasion method increased the bond strength for one adhesive type, while decreasing it for another. Not all of these alternatives were fully explored, as the preliminary failure strengths for a given adhesive and associated dolly preparation method were still significantly lower.

Because factors such as coating compatibility and cure time may limit the use of a particular adhesive for a particular application, it is recommended that the factors discussed in this article be used as a framework for selecting and verifying customer modifications to the manufacturer's recommended dolly preparation method.

For more information, see our PosiTest AT, Adhesion Testing resources, and dollies.

Please read the PosiTest AT series literature below, this article on measuring adhesion strength with DeFelsko PosiTest AT adhesion tensile tester

 and the product descriptions of the respective dollies ( pull stubs ) for PosiTest AT devices with 10 mm, 14 mm and 20 mm diameter.

What is an adhesion test? How does adhesion testing work?

In accordance with ASTM D4541/D7234, ISO 4624 and others, adhesion testers evaluate the adhesion (peel strength) of a coating by determining the greatest tensile force it can withstand before peeling. Failures, evidenced by broken surfaces, occur along the weakest plane within the system consisting of the dolly (loading device, peel nozzle), adhesive, coating(s) and substrate.

The PosiTest AT series of adhesion testers measures the force required to peel a specific test diameter of coating from its substrate using hydraulic pressure.

Why is it necessary to test the adhesion strength?

The objective of adhesion testing is to produce a coating failure. Coating adhesion is an indicator of how well the surface has been prepared and how well the coating has bonded to the surface and/or additional coating layers. Adhesion testing provides a quantifiable method to determine whether a paint or coating system is fit for purpose and can meet the quality requirements of the job specifications.

Choose between Automatic or Manual PosiTest AT Pull-Off Adhesion Testers:

Automatic Pull-Off Adhesion Tester

PosiTest AT - A

• The electronically controlled hydraulic pump automatically ensures a uniform and continuous trigger pressure. Significantly reduces the effort required by the user and the risk of influencing the pulling process.
• Shock- and scratch-resistant color touchscreen with keyboard for operation with or without gloves
• User-adjustable draw speed, draw limit and hold time
• Built-in battery performs over 200 tests per battery charge
• Internal memory stores maximum trigger pressure, trigger speed, test duration, dolly size, pass/fail, type of break, and user notes for 100,000 pull-pull tests in up to 1,000 blocks.

Manual Pull-Off Adhesion Tester

PosiTest AT - M

• Manual hydraulic pump that generates uniform and continuous pressure with a single stroke
• High-contrast, easy-to-read color display
• Traction force indicator for manual monitoring and adjustment of the traction force
• The internal memory stores the maximum trigger pressure, trigger speed, test duration and dolly size for up to 200 triggers

For coatings to perform effectively, they must adhere to their substrates. Several recognised methods, including knife tests and pull-off adhesion testing, measure how well coatings bond. After testing, it’s essential to note whether bond failure was adhesive (at the coating/substrate interface), cohesive (within the coating or substrate), or glue failure (separation within the glue). Many standards, both national and international, require documentation of the fracture type or nature of the fracture.

Determination of the type of coating failure (addition to step 5):

Detailed analysis of the fracture - is it a cohesive fracture, an adhesive fracture or a failure of the adhesive?

Also watch these videos on adhesion testing with the PosiTest AT adhesion tester:

PosiTest AT Automatic adhesion tester from mtv messtechnik

The automatic adhesive tensile tester PosiTest AT offers absolutely uniform tensile force! - No more rejects due to one-sided peeling!

Works without any effort and always delivers exact results. PosiTest AT Automatic Adhesion Tester is used for #tear-off testing