ASTM D4541-95e1
(Test Method)Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
SCOPE
1.1 This test method covers a procedure for evaluating the pull-off strength (commonly referred to as adhesion) of a coating on rigid substrates such as metal, concrete or wood. The test determines either the greatest perpendicular force (in tension) that a surface area can bear before a plug of material is detached, or whether the surface remains intact at a prescribed force (pass/fail). Failure will occur along the weakest plane within the system comprised of the test fixture, adhesive, coating system, and substrate, and will be exposed by the fracture surface. This test method maximizes tensile stress as compared to the shear stress applied by other methods, such as scratch or knife adhesion, and results may not be comparable.
1.2 Pull-off strength measurements depend upon both material and instrumental parameters. Results obtained by each test method may give different results. Results should only be assessed for each test method and not be compared with other instruments. There are five instrument types, identified as Test Methods A-E. It is imperative to identify the test method used when reporting results.
1.3 This test method uses a class of apparatus known as portable pull-off adhesion testers. They are capable of applying a concentric load and counter load to a single surface so that coatings can be tested even though only one side is accessible. Measurements are limited by the strength of adhesion bonds between the loading fixture and the specimen surface or the cohesive strengths of the adhesive, coating layers, and substrate.
1.4 This test can be destructive and spot repairs may be necessary.
1.5 The values stated in MPa (inch-pound) units are to be regarded as the standard. The values given in parentheses are for information only.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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e1
Designation: D 4541 – 95
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Pull-Off Strength of Coatings Using Portable Adhesion
Testers
This standard is issued under the fixed designation D 4541; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Research report added editorially in August 1995.
1. Scope 2. Referenced Documents
1.1 This test method covers a procedure for evaluating the 2.1 ASTM Standards:
pull-off strength (commonly referred to as adhesion) of a D 2651 Guide for Preparation of Metal Surfaces for Adhe-
coating by determining either the greatest perpendicular force sive Bonding
(in tension) that a surface area can bear before a plug of D 3933 Guide for Preparation of Aluminum Surfaces for
material is detached, or whether the surface remains intact at a Structural Adhesives Bonding (Phosphoric Acid Anodiz-
prescribed force (pass/fail). Failure will occur along the weak- ing)
est plane within the system comprised of the test fixture, D 3980 Practice for Interlaboratory Testing of Paint and
adhesive, coating system, and substrate, and will be exposed by Related Materials
the fracture surface. This test method maximizes tensile stress 2.2 ANSI Standard:
as compared to the shear stress applied by other methods, such N512 Protective Coatings (Paints) for the Nuclear Industry
as scratch or knife adhesion, and results may not be compa- 2.3 ISO Standard:
rable. Further, pull-off strength measurements depend upon 4624 Paints and Varnish—Pull-Off Test for Adhesion
both material and instrumental parameters. Results obtained
3. Summary of Test Method
using different devices or results for the same coatings on
substrates having different stiffness may not be comparable. 3.1 The general pull-off test is performed by securing a
loading fixture (dolly, stud) normal (perpendicular) to the
1.2 This test method uses a class of apparatus known as
portable pull-off adhesion testers. They are capable of apply- surface of the coating with an adhesive. After the adhesive is
cured, a testing apparatus is attached to the loading fixture and
ing a concentric load and counter load to a single surface so
that coatings can be tested even though only one side is aligned to apply tension normal to the test surface. The force
applied to the loading fixture is then gradually increased and
accessible. Measurements are limited by the strength of adhe-
sion bonds between the loading fixture and the specimen monitored until either a plug of material is detached, or a
specified value is reached. When a plug of material is detached,
surface or the cohesive strengths of the adhesive, coating
layers, and substrate. the exposed surface represents the plane of limiting strength
within the system. The nature of the failure is qualified in
1.3 This test can be destructive and spot repairs may be
necessary. accordance with the percent of adhesive and cohesive failures,
1.4 The values stated in inch-pound units are to be regarded and the actual interfaces and layers involved. The pull-off
strength is computed based on the maximum indicated load,
as the standard. The values given in parentheses are for
information only. the instrument calibration data, and the original surface area
stressed. Pull-off strength results obtained using different
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the devices may be different because the results depend on
instrumental parameters (see Appendix X1).
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Significance and Use
bility of regulatory limitations prior to use.
4.1 The pull-off strength of a coating is an important
performance property that has been used in specifications. This
test method serves as a means for uniformly preparing and
This test method is under the jurisdiction of ASTM Committee D-1 on Paint
testing coated surfaces, and evaluating and reporting the
and Related Coatings, Materials, and Applications and is the direct responsibility of
Subcommittee D01.46 on Industrial Protective Coating.
Current edition approved Feb. 15, 1995. Published April 1995. Originally Annual Book of ASTM Standards, Vol 15.06.
published as D 4541 – 93. Last previous edition D 4541 – 93. Annual Book of ASTM Standards, Vol 06.01.
2 5
The term adhesion tester may be somewhat of a misnomer, but its adoption by Available from American National Standards Institute, 11 W. 42nd St., 13th
two manufacturers and at least two patents indicates continued usage. Floor, New York, NY 10036.
D 4541
results. This test method is applicable to any portable apparatus 5.6 Cotton Swabs, or other means for removing excess
meeting the basic requirements for determining the pull-off adhesive and defining the adhered area. Any method for
strength of a coating. removing excess adhesive that damages the surface, such as
4.2 Variations in results obtained using different devices or scoring, must generally be avoided since induced surface flaws
different substrates with the same coating are possible (see may cause premature failure of the coating.
Appendix X1). Therefore, it is recommended that the type of 5.7 Circular Hole Cutter (optional), to score through to the
apparatus and the substrate be mutually agreed upon between substrate around the loading fixture.
the interested parties.
6. Test Preparation
5. Apparatus 6.1 The method for selecting the coating sites to be prepared
for testing depends upon the objectives of the test and
5.1 Adhesion Tester, commercially available, or comparable
agreements between the contracting parties. There are, how-
apparatus specific examples of which are listed in Annex
ever, a few physical restrictions imposed by the general method
A1-Annex A4.
and apparatus. The following requirements apply to all sites:
5.1.1 Loading Fixtures, having a flat surface on one end that
6.1.1 The selected test area must be a flat surface large
can be adhered to the coating and a means of attachment to the
enough to accommodate the specified number of replicate tests.
tester on the other end.
The surface may have any orientation with reference to
5.1.2 Detaching Assembly (adhesion tester), having a cen-
gravitational pull. Each test site must be separated by at least
tral grip for engaging the fixture.
the distance needed to accommodate the detaching apparatus.
5.1.3 Base, on the detaching assembly, or an annular bearing
The size of a test site is essentially that of the secured loading
ring if needed for uniformly pressing against the coating
fixture. At least three replications are usually required in order
surface around the fixture either directly, or by way of an
to statistically characterize the test area.
intermediate bearing ring. A means of aligning the base is
6.1.2 The selected test areas must also have enough perpen-
needed so that the resultant force is normal to the surface.
dicular and radial clearance to accommodate the apparatus, be
5.1.4 Means of moving the grip away from the base in as
flat enough to permit alignment, and be rigid enough to support
smooth and continuous a manner as possible so that a torsion
the counter force. It should be noted that measurements close
free, co-axial (opposing pull of the grip and push of the base
to an edge may not be representative of the coating as a whole.
along the same axis) force results between them.
6.2 Since the rigidity of the substrate affects pull-off
5.1.5 Timer, or means of limiting the rate of stress to less
strength results and is not a controllable test variable in field
than 150 psi/s (1 MPa/s) so that the maximum stress is obtained
measurements, some knowledge of the substrate thickness and
in less than about 100 s. A timer is the minimum equipment
composition should be reported for subsequent analysis or
when used by the operator along with the force indicator in
laboratory comparisons. For example, steel substrate of less
5.1.6.
than ⁄8-in. (3.2-mm) thickness usually reduce pull-off strength
NOTE 1—Obtaining the maximum stress in 100 s or less by keeping the
results compared to ⁄4-in. (6.4-mm) thick steel substrates.
maximum rate of shear to less than 150 psi/s (1 MPa/s) is valid for the
6.3 Subject to the requirements of 6.1, select representative
levels of pull-off strength measured with these types of apparatuses.
test areas and clean the surfaces in a manner that will not affect
5.1.6 Force Indicator and Calibration Information, for
integrity of the coating or leave a residue. Surface abrasion
determining the actual force delivered to the loading fixture.
may introduce flaws and should generally be avoided. A fine
5.2 Solvent, or other means for cleaning the loading fixture
abrasive (see 5.3) should only be used if needed to remove
surface. Finger prints, moisture, and oxides tend to be the
loose or weakly adhered surface contaminants.
primary contaminants.
6.4 Clean the loading fixture surface as indicated by the
5.3 Fine Sandpaper, or other means of cleaning the coating
apparatus manufacturer. Failures at the fixture-adhesive inter-
that will not alter its integrity by chemical or solvent attack. If
face can often be avoided by treating the fixture surfaces in
any light sanding is anticipated, choose only a very fine grade
accordance with an appropriate ASTM standard practice for
abrasive (400 grit or finer) that will not introduce flaws or leave
preparing metal surfaces for adhesive bonding.
a residue.
NOTE 2—Guides D 2651 and D 3933 are typical of well-proven meth-
5.4 Adhesive, for securing the fixture to the coating that does
ods for improving adhesive bond strengths to metal surfaces.
not affect the coating properties. Two component epoxies and
6.5 Prepare the adhesive in accordance with the adhesive
acrylics have been found to be the most versatile.
manufacturer’s recommendations. Apply the adhesive to the
5.5 Magnetic or Mechanical Clamps, if needed, for holding
fixture or the surface to be tested, or both, using a method
the fixture in place while the adhesive cures.
recommended by the adhesive manufacturer. Be certain to
apply the adhesive across the entire surface. Position fixture on
Araldite Adhesive, available from Ciba-Geigy Plastics, Duxford, Cambridge, the surface to be tested. Carefully remove the excess adhesive
CB2 4QA, England, Hysol Epoxy Patch Kit 907, available from Hysol Div., The
from around the fixture.
Dexter Corp., Willow Pass Rd., Pittsburg, CA 94565, and Scotch Weld Adhesive
1838B/A, available from 3M, Adhesives, Coatings amd Sealers Div., 3M Center, St.
NOTE 3—Caution: Movement, especially twisting, can cause tiny
Paul, MN 55144, have been found satisfactory for this purpose.
bubbles to coalesce into large holidays that constitute stress discontinuities
Versiloc 201 and 204 with accelerator, available from Lord Corp., Industrial
during testing.
Adhesive Div., 2000 W. Grandview Blvd., P.O. Box 10038, Erie, PA 16514, have
been found satisfactory for this purpose. 6.6 Based on the adhesive manufacturer’s recommendations
D 4541
and the anticipated environmental conditions, allow enough into the actual force applied in units of pounds-force (1
time for the adhesive to set up and reach the recommended N 5 0.1 kgf).
cure. During the adhesive set and early cure stage, a constant
8.2 Either use the calibration chart supplied by the manu-
contact pressure should be maintained on the fixture. Magnetic
facturer or compute the relative stress applied to each coating
or mechanical clamping systems work well, but systems
sample as follows:
relying on tack, such as masking tape, should be used with care
X 5 4F/pd (1)
to ensure that they do not relax with time and allow air to
intrude between the fixture and the test area.
where:
6.7 Scoring around the fixture violates the fundamental
X 5 greatest mean pull-off stress applied during a pass/fail
in-situ test criterion that an unaltered coating be tested. If
test, or the pull-off strength achieved at failure. Both
scoring around the test surface is employed, extreme care is
have units of psi (MPa 5 1 N/mm ).
required to prevent micro-cracking in the coating, since such F 5 actual force applied to the test surface as determined
cracks may cause failures at diminished strengths. Scored
in 8.1, and
d 5 equivalent diameter of the original surface area
samples constitute a different test, and this procedure should be
clearly reported with the results. stressed having units of inches (or millimetres). This
is usually equal to the diameter of the loading fixture.
6.8 Note the approximate temperature and relative humidity
during the time of test.
8.3 For all tests to failure, estimate the percent of adhesive
7. Test Procedure
and cohesive failures in accordance to their respective areas
7.1 The general procedure for conducting pull-off tests is
and location within the test system comprised of coating and
described in this section. More specific procedures are given in
adhesive layers. A convenient scheme that describes the total
Annex A1-Annex A4 for the types of testers used in the
test system is outlined in 8.3.1 through 8.3.3. (See ISO 4624.)
round-robin.
NOTE 5—A laboratory tensile testing machine is used in ISO 4624.
7.2 Select an adhesion-tester with a detaching assembly
having a force calibration spanning the range of expected
8.3.1 Describe the specimen as substrate A, upon which
values along with its compatible loading fixture. Mid-range
successive coating layers B, C, D, etc., have been applied,
measurements are usually the best, but read the manufacturer’s
including the adhesive, Y, that secures the fixture, Z, to the top
operating instructions before proceeding.
coat.
7.3 If a bearing ring or comparable device (5.1.3) is to be
8.3.2 Designate cohesive failures by the layers within which
used, place it concentrically around the loading fixture on the
they occur as A, B, C, etc., and the percent of each.
coating surface. If shims are required when a bearing ring is
8.3.3 Designate adhesive failures by the interfaces at which
employed, place them between the tester base and bearing ring
they occur as A/B, B/C, C/D, etc., and the percent of each.
rather than on the coating surface.
8.4 A result that is very different from most of the results
7.4 Carefully connect the central grip of th
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