Intercoat adhesion

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By Jim Hingst
Intercoat adhesion refers to either the
bond of a primer co
at to a substrate, or to the adhesion of one coating to
another. In many cases, a primer or top coat bonds to the primer or another
coating mechanically.  Although you may
not see microscopic imperfections on a surface, a primer or paint will grip
onto these rough areas.
You can generally improve adhesion by
lightly sanding a surface or scuffing it with a ScotchBrite® pad, which puts tiny
scratches on it. The rule of thumb is that the deeper scratch profile, the
better the bond. One advantage of using ScotchBrite® pad instead of sandpaper,
it that it creating a deep scratch without removing much material.
For scuffing a surface, such as aluminum
composite material, a red ScotchBrite® pad is generally recommended. To ensure
good surface adhesion, scuff the surface until the entire surface is dull with
no shiny areas.
Another way that a coating can adhere to
another coating is chemical adhesion. Ingredients in the paint chemistry can
improve the flow out of a coating allowing it to better penetrate a porous
surface. In addition, solvents in a coating can partially solvate another coating
causing it to bite into its surface and improving the bond between coating layers.
Causes of Intercoat Adhesion Failure
The roughness of a surface and the
chemistries of the primer and top coats are just two of many aspects affecting
intercoat adhesion. Other factors include:
● Moisture retention in a porous
substrate which could potentially result in blistering of the coating;
● Substrate contamination;
● Failure to apply a coating within its
specified application temperature range;
Incompatibility
of primers and paints; and
● Applying a coating
before an earlier coating has fully cured.
Ensuring Good Intercoat Adhesion and Preventing Failure.

Read the product specification sheets and technical
bulletins for the coatings that you use. Make sure that you prime, paint and
clear coat according to the manufacturer’s recommendations. As a general rule, do
not mix and match your materials. Stick with the products produced within one
manufacturer’s paint system to ensure that components are matched for compatibility.

Intercoat Adhesion Tests.

Before using a new substrate, a new primer, paint or clear
coat or trying a new production procedure, my recommendation is to “Test, Don’t
Guess”.  Make sure that when you try
something new, accurately record what you did so you can duplicate your
results.
Some of the tests that you can use to evaluate materials
and procedures are the ones that I learned in the screen print industry for
evaluating good intercoat adhesion of many coatings on various media. These tests
include the scratch test, tape test and cross hatch test.
While some sophisticated lab equipment has been developed
to evaluate intercoat adhesion, a shop can easily evaluate a coating using the
following procedures:
Thumb
Nail or Scratch Test. 

You
can’t perform a more elemental test for ink adhesion than a scratch test, also
referred to as 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.
Screen printers will
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 or Tape 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

You can generally evaluate the performance of coatings a
few days after they have cured. Manufacturers will often specify in their product
literature when you should conduct product tests.

In most cases, as time goes on, adhesion to the substrate
and between coating layers increases. Be aware though, that in some cases,
intercoat adhesion can decrease with the passage of time.




About Jim Hingst: Sign business authority on vehicle wraps, vinyl graphics, screen printing, marketing, sales, gold leaf, woodcarving and painting. 

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 190 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 500 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. 

© 2019 Jim Hingst, All Rights Reserved.
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