Adhesion Testing

Testing the Bond of an Ink or Paint

 

Whether you are screen printing or painting, the coating must bond to the substrate for the service life of the product.  Depending on the physical properties of the coating that you have selected for a project and the substrate to which the coating will be applied, the paint or ink adheres in a few different manners.

Some paints and inks bond through absorption into the substrate. This is how a coating will typically adhere to a porous material, such as wood or paper. Other coatings adhere chemically. Solvent screen print inks will primarily bond to a plastic, such as a pressure sensitive film, by solvating or dissolving the surface and chemically uniting with the film. Finally, coating will adhere by mechanically interlocking with the surface of the substrate. The strength of a mechanical bond is greatly dependent on how the coating wets the surface.

Coatings often adhere to a substrate by employing more than one of the aforementioned adhesion mechanisms. Regardless of how a paint or ink forms its bond, its adhesion strength must be sufficient for the application for which it is being used or the coating will fail. Typical adhesion failures include peeling and blistering.

The importance of adhesion testing is to determine the strength of the bond before the job leaves your shop. That way you avoid the embarrassment when a job fails.

To test adhesion, people have devised a variety of tests. In performing some tests, coatings are scratched with either a thumb nail or knife. In other methods, tape is adhered to the coating and then pulled from the surface to determine if the bond of the paint or ink is adequate. More scientific procedures involve sophisticated equipment to more accurately record the performance of a coating.

Whatever tests that you select to determine how well a coating bonds to a substrate, what’s really important is that you perform some type of testing, irrespective of its complexity. The fact is that some of the simplest test procedures are often the best. These rudimentary tests generally provide all the information that a printer or painter needs in producing a quality product.

Thumb Nail Test. You can’t perform a more elemental test for ink adhesion than a thumb nail test. Although many technicians scoff at this procedure for its simplicity, it is nevertheless a quick and easy go/no go check. Here’s how it works.

After printing or painting some sample pieces of film or substrate, allow the coating to completely cure. In conducting these tests, you should cure the coating using the same method as will be used in production.

Then simply scratch the surface of the ink with your thumbnail.  A properly cured ink will resist scratching. Painters will do a variation of this test, scraping the coating  with the blade of a utility knife.

Thumb Twist Test.

Another simple test is the thumb twist test. In this test, just press your thumb against the ink or paint and give it a twist.  If the ink is not thoroughly cured, a hardened layer of ink can slip over an uncured layer of ink. A properly cured coating also should not feel tacky. Instead it should feel dry to the touch.

Although the Thumb Twist and the Thumbnail test methods are regarded by some as being very subjective in nature and dependent on the skill and experience of the individual, these tests can alert printers to potential adhesion problems.

Cross Hatch Test. 

Your testing should include a cross hatch test to check the adhesion of the ink to the substrate; intercoat adhesion (the bond of one layer of a coating to another) and compatibility of the coating with the substrate or another coating.

The cross hatch test consists of lightly scoring the printed ink sample or paint sample eleven times with an X-Acto® knife and then scoring the ink again the same number of times over the first set of lines at a 90° angle. The cross hatch pattern of parallel lines should be approximately 1/8” (.32 cm) apart.  For about $500 or $600 you can also buy a cross hatch tester. This tools has blades evenly spaced and at a uniform depth.

Using an X-Acto knife lightly cut cross hatch lines at a 90° angle forming a grid.

After cutting the lines, use a plastic squeegee to burnish an aggressive tape, such as 3M Brand #600  clear tape, over the scored cutlines.   The tape is then pulled off 180° against itself, in one quick motion. If any of the ink comes off, the adhesion of the ink to the substrate is insufficient.

Burnish an aggressive tape over the scored cutlines.   Then pull the tape off  in one quick motion at a 180° angle against itself .

A variation of the cross hatch test is the Cross Cut Test. Using an X-Acto® knife you lightly score the surface with two knife cuts forming an “X”. After applying an aggressive tape over the cut and quickly ripping it from the surface at a 180° angle against itself, you should check the substrate and the tape for any paint or ink, which may have been removed. This test checks whether the coating is secure to the surface. Coatings, which have not formed a strong bond, are prone to peeling.

Conclusion. 

The tests that I have described obviously are not the only way to test the adhesion of a coating. Other tests have been developed. Some involve expensive laboratory equipment. The popularity of the cross hatch test is its simplicity and its reliability. It is an easy test which can be performed either on the line or in the lab.

Once you find a combination of raw materials that work for you, that you stick with that winning formula.  You should document the details of every job, so that nothing is left to chance when duplicating a reorder. Repeatability of results depends largely on reproducing all of the variables involved in printing or painting. 

Vinyl Application Videos

Squeegee Technique. Nothing is more basic in vinyl application than the squeegee. But some squeegees work better than others. And there are right ways and wrong ways to use this simple tool. This video clip reviews squeegee selection, squeegee care and squeegee technique. Click to Link

Wet Applications. Dry applications are typically recommended for most vinyl applications. For those exceptions to the rule, this instructional video explains the right way to perform a wet application. Click to Link

Application of Window Graphics. Installing vinyl graphics on glass can be challenging, because the adhesive aggressively grabs onto this high energy surface. Repositioning graphics on window is generally difficult, if not impossible. This instructional video explains how to dry apply window graphics right the first time. It also describes the necessary steps required for surface preparation. Click to Link

Transferring Frosted Window Graphics Films.  Because frosted window films are highly textured, transferring cut vinyl graphics can be problematic. This video provides direction in selection of the right application tape and how to apply these films quickly, easily and without problems. Click to Link

3-Step Surface Prep. This instructional video describes how to properly clean the surface of a vehicle before applying pressure sensitive vinyl graphics to a trailer or doing a full wrap of a car or van. This three step surface prep involves detergent washing, solvent cleaning and a final wipe down with IPA. Click to Link

Vinyl Application Over Rivets. Applying vinyl graphics on vehicle surfaces with rivets is challenging for most sign makers. Vinyl failures to these surfaces are all too common. This video demonstrates some tips and procedures, used by professional decal installers, which will make these demanding applications easier and more trouble-free.  Click to Link

Vinyl Application to Corrugations. Learn how to apply vinyl graphics to corrugations in a relaxed state. Forcing films into the valleys of corrugations puts too much mechanical stress on the film. Sooner or later, the vinyl will lift or tent in valleys. Click to Link

Applying Air Egress Vinyls. Vinyl films with air egress release liners have almost eliminated bubbles and wrinkles from applied graphics. Air egress release liners with their highly textured surface structure are problematic. Not much will stick to these liners other than the vinyl film. At RTape our best premask solution for air egress liners is a special Conform® tape called 4761RLA. Click to Link

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.  Vinyl Sign Techniques is available at sign supply distributors and at Amazon. 

© 2016 Jim Hingst