Tensile Adhesion Tips and Tricks:

NOTE: The following comments are the opinion of the author based on experience using the following equipment in both a laboratory and field environments. Any comments are welcome and can be forwarded to me at tswan@cci-inspection.com

There are two basic types of adhesion tests, Peel and Tensile. For additional information on Shear Adhesion Testing (Tape Test), click on the Shear Adhesion Testing Tips and Tricks on the menu.

NOT ALL TESTERS ARE EQUAL

The measured pull is highly dependent on the tester used. I.e. The psi reading taken with an Elcometer 106 will not equal the psi reading taken with an Elcometer 110. There is NO valid way to compare psi readings taken with different model testers.

Adhesion testing is addressed in ASTM D4541, “Standard Method for Pull-Off Strength of Coatings using Portable Adhesion Testers”. It identifies 5 types of portable adhesion testers.

When choosing an Adhesion Tester, in general, the Tensile Adhesion Strength between the different models,  in order is:

Unless otherwise stated, published PSI ratings for adhesion are almost always done using the Elcometer 110 Patti^® Meter because it gives the BEST adhesion results.

My Rule of Thumb for the Elcometer Models based on MY personnel experiences with the different testers:

Expected Pull Rate for 400 PSI pull with an Elcometer 106

NOTE: Many factors can affect the PSI obtained especially operator technique. The above chart is not a hard and fast rue but only intended to be a guideline. By understanding the different testers, you can optimize results.

MATERIALS

Coating must be on a firm substrate, such as on metal, on concrete or on stiff wood products. Flexible or soft substrates will distort when under load and this may interfere with the bond, reducing the adhesion. The tester needs to be supported by the sample to deliver the reactive force. This means the choice of tester is limited by the size of the sample it can stand upon.

Small diameter pipes can be a problem due to using flat dollies on a curved surface. The Elcometer 108 provides concave and convex dollies for pipe diameters about 8 inches to 75 inches. If using the 106 you can minimize the effects of curvature by using the support ring to make sure the legs are set firmly and squarely on the surface. On smaller diameter pipes, below about 16 inches, adhesion will be compromised by using a flat dolly on a curved surface. As the pipe diameter increases, the effect of the curvature becomes less.

If Adhesion tests are a specification requirement consider coating test panels in lieu of testing the equipment. This solves the problems of testing a curved surface, prevents repairs and is usually acceptable to the owner when it is explained to them.

Elcometer Adhesion Testers

Elcometer models:

Type 2, 4, and 5 Testers pull on the test stub (dolly) from the outside..

Type 3 Tester pulls the dolly and delivers the reaction through a hole in the center of the dolly along its axis.

There are three factors that come into play in pulling dollies”

• Manual or automatic
• Rate and Evenness of pull
• Squareness of pull

Method of pull (applying force) is applied in 3 ways.

Manual vs. Automatic

The rate and smoothness of the pull can affect the final tensile adhesion measured. Automatic testers ensure the increase of force is at a slow uniform rate. Operators sometimes increase the pull in quick steps. For manual testers, those that allow for a continuous pull rate will normally give the most consistent  and highest results.

Rate and Evenness of pull

An important aspect of pulling the test fixture is to provide a direct axial (perpendicular) force. There must be no attempt to twist the dolly off the surface, nor to lever it off. The straighter the pull, the closer the test is to the ‘tensile’ rather than to the easier ‘peel’ test.  Testers where the pull head is separated from the crank or knob, minimize twisting and levering during the pull

Squareness of Pull

To Be A “Tensile Pull”, the test fixture must be pulled perpendicular to the surface. While some testers use ball bearings to adjust for misalignment, this does not ensure a perpendicular pull. The only self aligning testers to date, are the Elcometer 1910, 1920, 1930 and the 1940.

The Elcometer 19XX PAT ‘legs’ adjust to be square to the dolly before doing any work. Other testers stay perpendicular to the surface so some leverage is possible. But even when the incline is less than 5 degrees (<1% reduction in force), some tests show a break that is off-center, as if a crack moved along the diameter of the dolly.

The Elcometer 108 HATE avoids this problem by concentrating the force and reaction within the area of the dolly.

RATING THE TEST

It is not enough to record the maximum stress (tensile adhesion). Standard methods require some analysis of the material remaining on the dolly.

There could be a break between layers of a coating system. One layer may be split, indicating the cohesion is less than the adhesion. The system may break from the substrate. The cause of this may be inadequate preparation or even dust.

It is quite possible for the glue to break between the dolly and the surface of the coating; this does not constitute an adhesion value.

Usually, most dollies have “multiple” mode failures. Record per cent adhesive, cohesive and glue failure

Example: Text fixture failed at 750 psi.

Definitions

Adhesion Failure – defined as a failure between layers of paint or between the substrate and the paint

Cohesive Failure – defined as a failure or break within one coat of paint.

Glue Failure – Break occurs between the glue and the pant or the glue and the dolly. This is not an adhesion failure.

FOR ADDITIONAL HELP IN GRADING PULLS --> CLICK HERE

TO SCORE OR NOT TO SCORE

The specification should say whether or not to score around the dolly. ASTM D-4541 leaves it open to the owner and the person doing the test. It cautions scoring can cause micro-cracking that can cause lower adhesion but also acknowledges that not scoring can lead to higher pull rates due to the influence of the adjacent coating.. I

The ISO 4624 and BS EN 24624 standards require a tensile test over the area of the dolly. This means the surface must be separated from that adjacent, so a circular cutter is used around the dolly before testing.

Generally I recommend not to score unless the coating is elastomeric, very thick or a sold surface glued to the substrate. If the specification do not tell you what to do, discuss with all parties involved with the test.

WHICH METER IS BEST FOR ME?

If you are working to specifications, the specification should give both the pull rate expected and the type of meter the test is to be done with

If cost and range are considerations, the following Table will help:

Cost Range (psi) Meter

NOTE: In the above table most meters come in multiple ranges. Range can be affected by dolly size.

By far, the most common meter in use today is the Elcometer 106, not because of it’s accuracy or its repeatability, but because of its low cost, portability and ease of use. The most common meter for testing coatings on steel is the 0-1000 psi meter for $995.00 and for Concrete the 0-3,200 psi meter for $1,199. (NOTE: with 2 inch dollies, meter range is 0-800 psi). It does require recalibration on a regular basis to ensure accuracy.

The Elcometer 110 generally gives the best pull but this meter is better suited to laboratory use, although I have used it successfully in the field. The pistons require a large footprint and a flat area. The cost is $4,600 and can be ordered with piston to 10,000 psi.

The Elcometer 108 is commonly specified by companies based in Europe. It is a good choice if you require better accuracy than given by the Elcometer 106 and is the least expensive of the other options costing $2,299 to $2,499 and measures up to 2,600 psi.

For field testing, my favorite is the Elcometer 1910, Pat Handy. While it is a little more expensive, at $4,524, it is light and easy to use in the field, gives repeatable readings and comes with a self aligning head that is separate from the crank. It works with a variety of dolly sizes and goes to over 14,000 psi.

WHAT GLUE SHOULD I USE?

Instant gratification or 24 hours is one of the decisions to be made when choosing glue. Generally, the most acceptable method is using a 2 part epoxy glue and allowing 24 hours of cure time of at least 50^oF for the to cure to a maximum adhesion. Ardilite Epoxy is usually used because it is compatible with most surfaces and generally, (when mixed properly) give pull strengths to in excess of 3,000 psi.

Sometimes it is necessary to get results the same day. It is possible to use “Superglue” (Cyanoacrylate) on many coating types. If available use a high viscosity glue such as 3m Pronto CA-100 (Elcometer part no. T99911135). Superglue should NOT be used on porous coatings because it will penetrate the coating and can glue the dolly to the substrate. Also, Cyanoacrylates are not compatible with some coatings such as Fluoropolymers and can dissolve, penetrate or soften coatings. For maximum pull, generally it is best to let Cyanoacrylate cure for a minimum of 3 hours.

If you need a pull of over 3,000 psi, your best option is a heat cured epoxy. These can provide pulls in excess of 10,000 psi without adhesive failure and are available through Elcometer. Lastly for high PSI pulls, you may want to use the coating being tested as the adhesive by putting the dolly in the coating prior to pulling.

Remember, there is no difference between putting on a top coat or gluing on a dolly. If you would expect low adhesion such as top-coating a polyurethane with an epoxy, then the adhesion of the epoxy glue will be effected by the same properties. If you have exceeded the overcoat time of the coating being tested, it may be necessary to slightly abrade the surface tested to improve adhesion. In addition, glue will not sick to oil, grease or dust any more than paint would stick to it, so make sure the surface is clean.

CAN I AVOID DAMAGING THE SURFACE IF ADHESION TESTING IS REQUIRED?

The answer is a definite maybe. If the specification requires the coating must reach a specific psi rating, there is no reason to pull to failure. At this point you can leave the dolly in place which may be acceptable for some applications. (The 106 dollies are aluminum and can easily be cut off at the neck to reduce their profile on the surface) If the dolly needs to be removed, often a hit on the side of the dolly with a hammer will remove the dolly without pulling off the coating. There is no guarantee this will work but the Peel force required to remove the dolly is usually less than the tensile force.

Heat can often be used to soften the glue to make the dolly easier to remove.

Always assume the test will be destructive, and if you can remove the dolly without destroying the surface you are one step ahead of the game.

This is a good question without any good answers. There are many factors that affect the longevity of coatings and Adhesion can be one of them. Adhesion tests work best to help determine a mode of failure rather than determining how long a coating will last. Adhesion is often more of a MARKETING TOOL than then a true indicator of how good a coating system is.

One manufactures product data sheet may state the Adhesion of their coating is minimum 1000 psi while another coating manufacturer states their coating system is a minimum of 500 psi. Will the 1000 psi coating outperform the 500 psi coating? Maybe.

If the 500 psi reading was taken with an Elcometer 106 and the 1000 psi reading was taken with an elcometer 110, then the readings may actually be about the same.

If they were both taken with the same type tester, there is still no guarantee that the 1000 psi system will out perform the 500 psi system. Adhesion is only one of many factors that may affect a coatings longevity and is probably not a good test to use as a predictor.

WHEN IS ADHESION TESTING APPROPRIATE?

There are several good reasons for testing adhesion.

• There are questions about surface preparation, intercoat cleanliness or overcoat window times.
• When you want to overcoat an existing coating
• When there is a premature coatings failure and you want to determine the cause.

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