60 years of excellence inspection equipment Adhesion Testing for Protective Coatings Understanding...

60 years of excellencewww.elcometer.com

inspection equipment

Adhesion Testing for Protective Coatings

Understanding Cross Hatch and Pull-Off Adhesion Testing

© Elcometer Limited 2011



A presentation for MECC 2012

Author: John FletcherASTM D01 Committee Chairman

Technical Support Manager, Elcometer

Presenter: Craig WoolhouseSales Manager, Elcometer

Adhesion Testing for Protective Coatings


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?

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


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 ISO16276 Part 2


The Cross-Cut Method

Typical Cross-Cut Adhesion Test Kit

Cutter Blades

Multi-blade Cutter with brush, magnifier and tape



Using the specialised self adhesive tape

Pulling the tapeUse on hard substrates only.

Test method and tape

ISO 2409 BS 3900-E6

BS 6496BS 6497

ASTM D3359

Time to complete the test

Less than 5 minutes

Less than 5 minutes

90 30 seconds

Angle of pull

60° 90° 180°




X-Cut MethodUse a sharp blade to create a X-Cut in the coating



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



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



Self-Alignment Adhesion Tester – Type III

Operated by manually applied Hydraulic Pressure

Often called Push-Off Testers or HATE Testers

Curved Dolly Options



Self-Alignment Adhesion Tester – Type IV

Operated by pneumatically applied Pressure



Self-Alignment Adhesion Tester – Type V

Operated by manually or automatically applied hydraulic pressure



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

Examples3 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



Adhesive and Cohesive failure modes



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


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


Pull-Off Adhesion tests give numerical values in force per unit area


Conclusions


Pull-Off Adhesion tests give values in force per unit area

ASTM D4541 describes five different types of apparatus and five different methods


Conclusions




ISO 16276-1 references D4541 and defines inspection areas, sampling plans and acceptance rejection criteria.


Conclusions





Choice of adhesive is important as the adhesive must damage the coating


Conclusions





Choice of adhesive is important as the adhesive must not damage the coating

The gluing process should minimise glue failures


Conclusions







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







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?

Date post:	22-Dec-2015
Category:	Documents
Upload:	veronica-briggs
View:	226 times
Download:	0 times

60 years of excellence inspection equipment Adhesion Testing for Protective Coatings Understanding...

Documents