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

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

3. 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

4. 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?

5. 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

6. 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 Parts 1 and 2

7. The Cross-Cut Method ISO16276 Part 2
Summarised by ISO 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

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

9. 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

10. The Cross-Cut Method

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

12. Pull-Off Adhesion Testing
ISO 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

13. 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!

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

15. 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

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

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

18. 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

19. Inspection Areas - How often should we test?
ISO 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

20. 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

21. 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

22. Interpretation & Expression of Results

23. Interpretation & Expression of Results
Adhesive and Cohesive failure modes

24. 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

25. 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

26. 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

27. 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

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

29. 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

30. 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

31. 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 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria.

32. 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 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria.
Choice of adhesive is important as the adhesive must damage the coating

33. 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 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

34. 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 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

35. 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 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

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