Designation:D 4541–02
Standard Test Method for
Pull-Off Strength of Coatings Using Portable Adhesion Testers 1
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.
1.Scope
1.1This 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 pre-scribed 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.1.2Pull-off strength measurements depend upon both ma-terial 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.
1.3This test method uses a class of apparatus known as portable pull-off adhesion testers.2They are capable of apply-ing 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 adhe-sion bonds between the loading fixture and the specimen surface or the cohesive strengths of the adhesive,coating layers,and substrate.
1.4This test can be destructive and spot repairs may be necessary.
1.5The values stated in MPa (inch-pound)units are to be regarded as the standard.The values given in parentheses are for information only.
1.6This 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 appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.
2.Referenced Documents 2.1ASTM Standards:
D 2651Guide for Preparation of Metal Surfaces for Adhe-sive Bonding 3
D 3933Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding (Phosphoric Acid Anodiz-ing)3
D 3980Practice for Interlaboratory Testing of Paint and Related Materials 42.2ANSI Standard:
N512Protective Coatings (Paints)for the Nuclear Industry 52.3ISO Standard:
4624Paints and Varnish—Pull-Off Test for Adhesion 53.Summary of Test Method
3.1The general pull-off test is performed by securing a loading fixture (dolly,stud)normal (perpendicular)to the surface of the coating with an adhesive.After the adhesive is cured,a testing apparatus is attached to the loading fixture and aligned to apply tension normal to the test surface.The force applied to the loading fixture is then gradually increased and monitored until either a plug of material is detached,or a specified value is reached.When a plug of material is detached,the exposed surface represents the plane of limiting strength within the system.The nature of the failure is qualified in accordance with the percent of adhesive and cohesive failures,and the actual interfaces and layers involved.The pull-off strength is computed based on the maximum indicated load,the instrument calibration data,and the original surface area stressed.Pull-off strength results obtained using different
1
This test method is under the jurisdiction of ASTM Committee D01on Paint and Related Coatings,Materials,and Applications and is the direct responsibility of Subcommittee D01.46on Industrial Protective Coatings.
Current edition approved Feb.10,2002.Published April 2002.Originally published as D 4541–93.Last previous edition D 4541–95{1.2
The term adhesion tester may be somewhat of a misnomer,but its adoption by two manufacturers and at least two patents indicates continued usage.
3
Annual Book of ASTM Standards ,V ol 15.06.4Annual Book of ASTM Standards ,V ol 06.01.5
Available from American National Standards Institute,11W.42nd St.,13th Floor,New York,NY 10036.
1
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devices may be different because the results depend on instrumental parameters(see Appendix X1).
4.Significance and Use
4.1The 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 testing coated surfaces,and evaluating and reporting the results.This test method is applicable to any portable apparatus meeting the basic requirements for determining the pull-off strength of a coating.
4.2Variations in results obtained using different devices or different substrates with the same coating are possible(see Appendix X1).Therefore,it is recommended that the type of apparatus and the substrate be mutually agreed upon between the interested parties.
4.3The purchaser or specifier shall designate a specific test method,that is,A,B,C,D or E,when calling out this standard.
5.Apparatus
5.1Adhesion Tester,commercially available,or comparable apparatus specific examples of which are listed in Annex A1-Annex A5.
5.1.1Loading Fixtures,having aflat surface on one end that can be adhered to the coating and a means of attachment to the tester on the other end.
5.1.2Detaching Assembly(adhesion tester),having a cen-tral grip for engaging thefixture.
5.1.3Base,on the detaching assembly,or an annular bearing ring if needed for uniformly pressing against the coating surface around thefixture either directly,or by way of an intermediate bearing ring.A means of aligning the base is needed so that the resultant force is normal to the surface. 5.1.4Means of moving the grip away from the base in as smooth and continuous a manner as possible so that a torsion free,co-axial(opposing pull of the grip and push of the base along the same axis)force results between them.
5.1.5Timer,or means of limiting the rate of stress to less than1MPa/s(150psi/s)so that the maximum stress is obtained in less than about100s.A timer is the minimum equipment when used by the operator along with the force indicator in 5.1.
6.
N OTE1—Obtaining the maximum stress in100s or less by keeping the maximum rate of shear to less than1MPa/s(150psi/s)is valid for the levels of pull-off strength measured with these types of apparatuses. 5.1.6Force Indicator and Calibration Information,for determining the actual force delivered to the loadingfixture.
5.2Solvent,or other means for cleaning the loadingfixture surface.Finger prints,moisture,and oxides tend to be the primary contaminants.
5.3Fine Sandpaper,or other means of cleaning the coating that will not alter its integrity by chemical or solvent attack.If any light sanding is anticipated,choose only a veryfine grade abrasive(400grit orfiner)that will not introduceflaws or leave a residue.
5.4Adhesive,for securing thefixture to the coating that does not affect the coating properties.Two component epoxies6and acrylics7have been found to be the most versatile.
5.5Magnetic or Mechanical Clamps,if needed,for holding thefixture in place while the adhesive cures.
5.6Cotton Swabs,or other means for removing excess adhesive and defining the adhered area.Any method for removing excess adhesive that damages the surface,such as scoring(see
6.7),must generally be avoided since induced surfaceflaws may cause premature failure of the coating. 5.7Circular Hole Cutter(optional),to score through to the substrate around the loadingfixture.
6.Test Preparation
6.1The method for selecting the coating sites to be prepared for testing depends upon the objectives of the test and agreements between the contracting parties.There are,how-ever,a few physical restrictions imposed by the general method and apparatus.The following requirements apply to all sites: 6.1.1The selected test area must be aflat surface large enough to accommodate the specified number of replicate tests. The surface may have any orientation with reference to gravitational pull.Each test site must be separated by at least the distance needed to accommodate the detaching apparatus. The size of a test site is essentially that of the secured loading fixture.At least three replications are usually required in order to statistically characterize the test area.
6.1.2The selected test areas must also have enough perpen-dicular and radial clearance to accommodate the apparatus,be flat enough to permit alignment,and be rigid enough to support the counter force.It should be noted that measurements close to an edge may not be representative of the coating as a whole.
6.2Since the rigidity of the substrate affects pull-off strength results and is not a controllable test variable infield measurements,some knowledge of the substrate thickness and composition should be reported for subsequent analysis or laboratory comparisons.For example,steel substrate of less than3.2mm(1⁄8-in.)thickness usually reduce pull-off strength results compared to6.4mm(1⁄4-in.)thick steel substrates. 6.3Subject to the requirements of6.1,select representative test areas and clean the surfaces in a manner that will not affect integrity of the coating or leave a residue.Surface abrasion may introduceflaws and should generally be avoided.Afine abrasive(see5.3)should only be used if needed to remove loose or weakly adhered surface contaminants.
6.4Clean the loadingfixture surface as indicated by the apparatus manufacturer.Failures at thefixture-adhesive inter-face can often be avoided by treating thefixture surfaces in accordance with an appropriate ASTM standard practice for preparing metal surfaces for adhesive bonding.
6Araldite Adhesive,available from Ciba-Geigy Plastics,Duxford,Cambridge, CB24QA,England,Hysol Epoxy Patch Kit907,available from Hysol Div.,The Dexter Corp.,Willow Pass Rd.,Pittsburg,CA94565,and Scotch Weld Adhesive 1838B/A,available from3M,Adhesives,Coatings and Sealers Div.,3M Center,St. Paul,MN55144,have been found satisfactory for this purpose.
7Versiloc201and204with accelerator,available from Lord Corp.,Industrial Adhesive Div.,2000W.Grandview Blvd.,P.O.Box10038,Erie,PA16514,have been found satisfactory for this
purpose.
N OTE2—Guides D2651and D3933are typical of well-proven meth-ods for improving adhesive bond strengths to metal surfaces.
6.5Prepare the adhesive in accordance with the adhesive manufacturer’s recommendations.Apply the adhesive to the fixture or the surface to be tested,or both,using a method recommended by the adhesive manufacturer.Be certain to apply the adhesive across the entire surface.Positionfixture on the surface to be tested.Carefully remove the excess adhesive from around thefixture.(Warning—Movement,especially twisting,can cause tiny bubbles to coalesce into large holidays that constitute stress discontinuities during testing.)
N OTE3—Adding about1percent of#5glass beads to the adhesive assists in even alignment of the testfixture to the surface.
6.6Based on the adhesive manufacturer’s recommendations and the anticipated environmental conditions,allow enough time for the adhesive to set up and reach the recommended cure.During the adhesive set and early cure stage,a constant contact pressure should be maintained on thefixture.Magnetic or mechanical clamping systems work well,but systems relying on tack,such as masking tape,should be used with care to ensure that they do not relax with time and allow air to intrude between thefixture and the test area.
6.7Scoring around thefixture violates the fundamental in-situ test criterion that an unaltered coating be tested.If scoring around the test surface is employed,extreme care is required to prevent micro-cracking in the coating,since such cracks may cause reduced adhesion values.Scored samples constitute a different test,and this procedure should be clearly reported with the results.
N OTE4—It is common to score around the testfixture when performing tests on cementitious substrates where the tensile strength of the substrate is significantly lower than either the pull-off or cohesive strength of the coating system.
6.8Note the approximate temperature and relative humidity during the time of test.
7.Test Procedure
7.1Test Methods:
7.1.1Test Method A—Fixed Alignment Adhesion Tester Type I:
7.1.1.1Operate the instrument in accordance with Annex A1.
7.1.2Test Method B—Fixed Alignment Adhesion Tester Type II:
7.1.2.1Operate the instrument in accordance with Annex A2.
7.1.3Test Method C—Self-Alignment Adhesion Tester Type III:
7.1.3.1Operate the instrument in accordance with Annex A3.
7.1.4Test Method D—Self-Alignment Adhesion Tester Type IV:
7.1.4.1Operate the instrument in accordance with Annex A4.
7.1.5Test Method E—Self-Alignment Adhesion Tester Type V:
7.1.5.1Operate the instrument in accordance with Annex A5.
7.2Select an adhesion-tester with a detaching assembly having a force calibration spanning the range of expected values along with its compatible loadingfixture.Mid-range measurements are usually the best,but read the manufacturer’s operating instructions before proceeding.
7.3If a bearing ring or comparable device(5.1.3)is to be used,place it concentrically around the loadingfixture on the coating surface.If shims are required when a bearing ring is employed,place them between the tester base and bearing ring rather than on the coating surface.
7.4Carefully connect the central grip of the detaching assembly to the loadingfixture without bumping,bending,or otherwise prestressing the sample and connect the detaching assembly to its control mechanism,if necessary.For nonhori-zontal surfaces,support the detaching assembly so that its weight does not contribute to the force exerted in the test. 7.5Align the device according to the manufacturer’s in-structions and set the force indicator to zero.
N OTE5—Proper alignment is critical,see Appendix X2.If alignment is required,use the procedure recommended by the manufacturer of the adhesion tester and report the procedure used.
editor evaluating revision7.6Increase the load to thefixture in as smooth and continuous a manner as possible,at a rate of less than1MPa/s (150psi/s)so that failure occurs or the maximum stress is reached in about100s or less(see Note1).
7.7Record the force attained at failure or the maximum force applied.
7.8If a plug of material is detached,label and store the fixture for qualification of the failed surface in accordance with 8.3.
7.9Report any departures from the procedure such as possible misalignment,hesitations in the force application,etc.
8.Calculation and Interpretation of Results
8.1If instructed by the manufacturer,use the instrument calibration factors to convert the indicated force for each test into the actual force applied.
8.2Either use the calibration chart supplied by the manu-facturer or compute the relative stress applied to each coating sample as follows:
X54F/p d2(1) where:
X=greatest mean pull-off stress applied during a pass/fail test,or the pull-off strength achieved at failure.Both
have units of MPa(psi).
F=actual force applied to the test surface as determined in8.1,and
d=equivalent diameter of the original surface area stressed having units of inches(or millimetres).This
is usually equal to the diameter of the loadingfixture.
8.3For all tests to failure,estimate the percent of adhesive and cohesive failures in accordance to their respective areas and location within the test system comprised of coating and adhesive layers.A convenient scheme that describes the total test system is outlined in8.3.1through8.3.3.(See ISO
4624.)
N OTE6—A laboratory tensile testing machine is used in ISO4624. 8.3.1Describe the specimen as substrate A,upon which successive coating layers B,C,D,etc.,have been applied, including the adhesive,Y,that secures thefixture,Z,to the top coat.
8.3.2Designate cohesive failures by the layers within which they occur as A,B,C,etc.,and the percent of each.
8.3.3Designate adhesive failures by the interfaces at which they occur as A/B,B/C,C/D,etc.,and the percent of each. 8.4A result that is very different from most of the results may be caused by a mistake in recording or calculating.If either of these is not the cause,then examine the experimental circumstances surrounding this run.If an irregular result can be attributed to an experimental cause,drop this result from the analysis.However,do not discard a result unless there are valid nonstatistical reasons for doing so or unless the result is a statistical outlier.Valid nonstatistical reasons for dropping results include alignment of the apparatus that is not normal to the surface,poor definition of the area stressed due to improper application of the adhesive,poorly defined glue lines and boundaries,holidays in the adhesive caused by voids or inclusions,improperly prepared surfaces,and sliding or twist-ing thefixture during the initial cure.Scratched or scored samples may contain stress concentrations leading to prema-ture fractures.Dixon’s test,as described in Practice D3980, may be us
ed to detect outliers.
8.5Disregard any test where glue failure represents more than50%of the area.If a pass/fail criterium is being used and a glue failure occurs at a pull-off strength greater than the criterium,report the result as“pass with a pull-off strength >{value obtained}...”
8.6Further information relative to the interpretation of the test results is given in Appendix X2.
9.Report
9.1Report the following information:
9.1.1Brief description of the general nature of the test,such as,field or laboratory testing,generic type of coating,etc. 9.1.2Temperature and relative humidity and any other pertinent environmental conditions during the test period. 9.1.3Description of the apparatus used,including:appara-tus manufacturer and model number,loadingfixture type and dimensions,and bearing ring type and dimensions.
9.1.4Description of the test system,if possible,by the indexing scheme outlined in8.3including:product identity and generic type for each coat and any other information supplied, the substrate identity(thickness,type,orientation,etc.),and the adhesive used.
9.1.5Test results.
9.1.5.1Date,test location,testing agent.
9.1.5.2For pass/fail tests,stress applied along with the result,for example,pass or fail and note the plane of any failure(see8.3and ANSI N512).
9.1.5.3For tests to failure,report all values computed in8.2 along with the nature and location of the failures as specified in 8.3,or,if only the average strength is required,report the average strength along with the statistics.
9.1.5.4If corrections of the results have been made,or if certain values have been omitted such as the lowest or highest values or others,reasons for the adjustments and criteria used.
9.1.5.5For any test where scoring was employed,indicate it by placing a footnote superscript beside each data point affected and a footnote to that effect at the bottom of each page on which such data appears.Note any other deviations from the procedure.
10.Precision and Bias8
10.1Precision—In an interlaboratory study of Test Methods A-D,operators made measurements,generally in triplicate but in a few cases in duplicate,on coated panels covering a moderate range at the intermediate adhesion level using four different types of instruments(see Annex A1-Annex A5and Appendix X1).The number of participating laboratories varied with each instrument and in the case of one instrument with the material.Only two laboratories had access to Type I instru-ments but two operators in each made the triplicate tests. During the statistical analysis of the results three individual results and one set of triplicates obtained with Type II instruments were rejected as outliers;one single test with Type III instruments and three single results with Type I instruments were rejected.The pooled intra-and inter-laboratory coeffi-cients of variation were found to be those shown in Table1. Based on these coefficients the following criteria should be used for judging,at the95%confidence level,the acceptability of results:
10.1.1Replicate Repeatability—Triplicate results obtained by the same operator using instruments from the same category should be considered suspect if they differ in percent relative by more than the values given in Table1.
N OTE7—Difference in percent relative to two results,x
1
and x
2
,is the absolute value of
~x12x2!
~x11x2!/23100.(2)
8Supporting data are available from ASTM International Headquarters.Request RR:D01-1094.
TABLE1Precision of Adhesion Pull-Off Measurements
Instrument
Coefficient of
Variation,v,%
Degrees of
Freedom
Maximum
Acceptable
Difference,% Intralaboratory Instrument:
Type IV8.54829.0 Type I J
Type II12.212941.0
Type III
Total177
Interlaboratory Instrument:
Type IV8.72025.5 Type I J
Type II20.65858.7
Type III
Total
78
10.1.2Reproducibility —Two results,each the mean of trip-licates,obtained by operators in different laboratories using instruments of the same category should be considered suspect if they differ in percent relative by more than the values given in Table 1.
10.2Bias —This test method has no bias statement since there is no acceptable reference material suitable for determin-ing the bias of this test method.
11.Keywords
11.1adhesion;coatings;field;paint;portable;pull-off strength;tensile test
ANNEXES
(Mandatory Information)
A1.FIXED-ALIGNMENT ADHESION TESTER,TYPE I
A1.1Apparatus:
A1.1.1A fixed-alignment portable tester as shown in Fig.A1.19
N OTE A1.1—Precision data for Type I instruments described in Table 1were obtained using the devices illustrated in Fig.A1.1.
A1.1.2The tester is comprised of detachable aluminum loading fixtures,50mm (1.97in.)in diameter,screws with spherical heads that are screwed into the center of a fixture,a socket in the testing assembly that holds the head of the screw,pressure gage,dynamometer,wheel and crank.
A1.1.3The testers are available in four models,with maxi-mum tensile forces of 5,15,25,and 50kN (1125,3375,5625,and 11250lb )respectively.For a fixture having a 50mm (1.97in.)diameter,a 5kN device corresponds to a range of 2.5MPa (0to 360psi).
A1.2Procedure:
A1.2.1Follow the general procedures described in Sections 6and 7.Procedures specific to this instrument are described in this section.
A1.2.2Set the pointer on the zero mark by first pressing the push-button located on the left of the indicator.While holding the push-button,turn the little knob located on the upper part of the indicator to set the pointer at zero.Set the zero after testing by pressing the push-button.
A1.2.3After fixing a loading fixture to a substrate,insert a screw with a spherical head into the center of the fixture.Position the testing equipment on the metal disc.Then by means of the notched wheel,fix the head of the spherical screw into the socket at the base of the equipment.For the first mechanical approach,stop screwing down the wheel when the pointer on the indicator shifts from the ZERO mark.Tests are done by turning the crank.After each test,turn the crank in the opposite direction until it stops.
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The Dyna Z5tester is available from PROCEQ SA,Riesbachstrasse 57,CH-8034,Zurich,
Switzerland.
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