Adhesion Testing for Protective Coatings Understanding Cross Hatch and Pull-Off Adhesion Testing © Elcometer Limited 2011

Adhesion Testing for Protective Coatings A presentation for MECC 2012 Author: John Fletcher ASTM D01 Committee Chairman Technical Support Manager, Elcometer Presenter: Craig Woolhouse Sales Manager, Elcometer

Contents Introduction Cross Cut Adhesion Testing Pull-Off Adhesion Testing Types of Pull-Off Adhesion Testers Inspection Areas and Test Frequency Interpretation and Expression of the ASTM Comparison Test Results Issues arising from the ASTM Tests Conclusions

Introduction Clarifying the current Adhesion Testing options available in the market Different Standards exist Many types of Adhesion Testers exist How do they work? How do we compare results? What do the ASTM and ISO Standards mean?

Introduction Why test for adhesion? What is an adhesion test? It is not always simply to meet a standard Also to ensure that the following processes have been done correctly Surface Profile Surface Cleanliness Application Conditions Dry Film Thickness What is an adhesion test? A test designed to assess whether the coating will adhere to the substrate during its service life

Introduction Two Field Methods For Adhesion Testing The Cross-Cut and X-Cut Methods – Comparison Method The Pull-Off Method – Value Method Two key Standards ASTM D4541 ISO 16276 Parts 1 and 2

The Cross-Cut Method ISO16276 Part 2 Summarised by ISO 16276 Part 2: Cutting the coating, applying specific adhesive tape to remove non-adhered coating followed by comparison tests against pictorial standards Cross-Cut (Cross Hatch) X-Cut (St Andrews Cross) References ISO 2409 and ASTM D3359

The Cross-Cut Method Typical Cross-Cut Adhesion Test Kit Cutter Blades Multi-blade Cutter with brush, magnifier and tape

Use on hard substrates only. The Cross-Cut Method Using the specialised self adhesive tape Pulling the tape Use on hard substrates only. Test method and tape ISO 2409 BS 3900-E6 BS 6496 BS 6497 ASTM D3359 Time to complete the test Less than 5 minutes 90  30 seconds Angle of pull  

The Cross-Cut Method

X-Cut Method Use a sharp blade to create a X-Cut in the coating

Pull-Off Adhesion Testing ISO 16276 Part 1 and ASTM D4541 Determination of coating adhesion or coating fracture strength by mechanically pulling adhered ‘dollies’ and the coating from the substrate For clarity this review concentrates on adherence to ASTM D4341

Pull-Off Adhesion Testing ASTM D4541 Five Types of Portable Adhesion Testers (Type II to Type VI) 20 mm diameter dollies Force Application Rate – 1 MPa/s or less to complete test over full range within 100 s or less Force applied in a continuous and smooth manner Cutting round the dolly – or not!

Types of Pull-Off Testers Fixed Alignment Adhesion Tester – Type II Mechanically load applied by Compressed Spring Mechanically load applied by Compressed Washers

Types of Pull-Off Testers Self-Alignment Adhesion Tester – Type III Operated by manually applied Hydraulic Pressure Often called Push-Off Testers or HATE Testers Curved Dolly Options

Types of Pull-Off Testers Self-Alignment Adhesion Tester – Type IV Operated by pneumatically applied Pressure

Types of Pull-Off Testers Self-Alignment Adhesion Tester – Type V Operated by manually or automatically applied hydraulic pressure

Types of Pull-Off Testers Self-Alignment Adhesion Tester – Type VI Operated by manually applied Hydraulic Pressure Feature ‘self- aligning’ legs Often called PAT Testers

Inspection Areas - How often should we test? ISO 16276 Part1 and ASTM D4541 Normally defined by project specification If not defined then whole area of structure is the inspection area – one failure fails the whole job! Random measurements according to size of inspection area

Inspection Areas - How often should we test? Inspection Area (m²) Number of valid measurements < 1,000 3 for each 250 m² area or part thereof > 1,000 12 plus 1 for each additional 1,000m² area or part thereof Subdivision into smaller inspection areas is recommended Examples 3 areas of 900 m² is equivalent to four areas of 250 m². 16 valid measurements for each area. Total = 48 measurements 1 area of 2,700 m² is equivalent to three areas of 1,000 m². 12 valid measurements plus 2. Total = 14 measurements

Interpretation & Expression of Results Type of failure Glue failure is not a valid test unless the force applied exceeds the specified value Inspect the face of the dolly after the coating has failed The Coating must cover a minimum of 80% of the area of the dolly face to be considered as a valid test

Interpretation & Expression of Results

Interpretation & Expression of Results Adhesive and Cohesive failure modes

Interpretation & Expression of Results Test Acceptance or Rejection Criteria For the acceptance of an inspection area, the following three criteria must be fulfilled: The fracture strength values for substrate/primer (A/B) adhesive failures shall be equal or greater than the specified value When 10 or more measurements are made in a single inspection area, not more than one-tenth of fracture strength values shall be between 20% and 60% of the specified value Two-thirds of the fracture strength values shall be equal or greater than the specified value, with the additional requirement that the remaining one-third of the values are equal or greater than 60% of the specified value

Testing Issues – Adhesive choice Two-pack Epoxy Adhesive Easy to mix, tolerant of surface contamination and mix. Long cure time Cyanoacrylate (super glue) Adhesive Single component, need clean surfaces, can damage some types of coating. Quicker cure time Be aware of cure time and ambient temperature issues

ASTM Comparison Test ASTM D4541 Tester Type ‘Round Robin’ Testing 2007 Six coatings to cover high and low adhesion values Seven operatives, four tests per coating 6 mm thick hot-rolled steel coated test plates Coating A and F – adhesion performance exceeded range of adhesion testers used and so were eliminated from analysis ASTM deemed that the failure methods for each test type are different hence the results are not comparable

Maximum Recommended Difference (%) Testing Issues D4541 Summary of Reproducibility (R & r Test Data) Adhesion Tester Type Test Method Maximum Recommended Difference (%) Type II B 64.7 Type III C 33.8 Type IV D 14.8 Type V E 27.8 Type VI F 17.5

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give numerical values in force per unit area

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give values in force per unit area ASTM D4541 describes five different types of apparatus and five different methods

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give values in force per unit area ASTM D4541 describes five different types of apparatus and five different methods ISO 16276-1 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria.

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give values in force per unit area ASTM D4541 describes five different types of apparatus and five different methods ISO 16276-1 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria. Choice of adhesive is important as the adhesive must damage the coating

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give values in force per unit area ASTM D4541 describes five different types of apparatus and five different methods ISO 16276-1 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria. Choice of adhesive is important as the adhesive must not damage the coating The gluing process should minimise glue failures

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give values in force per unit area ASTM D4541 describes five different types of apparatus and five different methods ISO 16276-1 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria. Choice of adhesive is important as the adhesive must not damage the coating The gluing process should minimise glue failures Cutting round the dolly depends on the type of coating and the risk of damage to the coating under the dolly – user to be consistent in choice

Conclusions Cross-Cut and X-Cut tests compare the adhesion of a coating Pull-Off Adhesion tests give values in force per unit area ASTM D4541 describes five different types of apparatus and five different methods ISO 16276-1 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria. Choice of adhesive is important as the adhesive must not damage the coating The gluing process should minimise glue failures Cutting round the dolly depends on the type of coating and the risk of damage to the coating under the dolly – user to be consistent in choice ASTM conclude load values using different test methods can’t be compared

Adhesion testing for Protective Coatings Standards for Cross Hatch and Pull-Off Testing Questions?