TECHNOLOGY FEATURE

This is the fi rst of a two-part series on techniques for measuring adhesion and abrasion durability of coatings and inks. This fi rst issue will examine adhesion. Part two will address abrasion dura-bility including new equipment and measurement techniques.

Adhesion and abrasion durability of cured coatings and inks are two critical performance properties, and their measurement and control are essential for robust manufacturing. Adhesion and abrasion durability are discrete physical properties and commonly misdiagnosed during failure mode analysis. Poor adhesion and abrasion durability are leading causes of fi eld failures, scrap and rework, lower profi ts, and dissatisfi ed customers. Techniques exist to measure these properties. This article addresses several different test methods and instruments. Among these is a new linear abraser made by Taber Industries that is designed to evaluate actual product shapes involving contoured surfaces as well as fl at specimens (part two). Advanced measurement techniques such as the ones cited in this paper are helping manufacturers achieve superior product performance and six-sigma process excellence.

Adhesion versus AbrasionOptimal adhesion and abrasion durability (resistance) of cured coatings and inks onto plastic substrates must be achieved during manufacturing to produce high quality products that will endure throughout their service life. These properties can be interdepen-dent but often are mutually exclusive. That is, cured coatings and inks can demonstrate good adhesion and poor abrasion, or poor adhesion and good abrasion. Proper testing and understanding of adhesion and abrasion ensures conformance to specifi cations and is invaluable in solving coating and printing problems. The identifi cation of the failure mechanism, adhesion (cohesion), abrasion or both, will determine the engineering process solution. Historical test methods and new instrumentation exist to measure and analyze these properties.

Adhesion Test MethodsASTM D 3359 Standard Test Method for Measuring Adhesion by Tape Test is historically perhaps the most well-known and widely used method for testing adhesion of coatings and inks because of its simplicity and low cost. The most basic procedure for conducting the tape test includes the following:

• Agreement on the selection of tape.• Complete cure of the coatings and inks before testing.• Making “X”-cuts in the fi lm per Method A or lattice pattern with either six or eleven cuts in each direction per Method B. • Removing and discarding two laps of tape from the roll dispenser.• Smoothing the tape into place by fi nger and then rubbing fi rmly with an eraser on the end of a pencil, or similar (the color under the tape being a good indicator of complete contact). • Within 60-120 seconds of application, removing the tape by

pulling the free end rapidly (not jerked) back upon itself as close to an angle of 180 degrees as possible. • Inspecting the cut area for removal of coating from the substrate and rating the adhesion relative to pre-determined descriptions and illustrations classifi cations (0B, 1B, 2B, 3B, 4B, 5B).

Early versions of ASTM D 3359 referenced the use of 3M 710 and Permacel tape. 3M discontinued its product and, over the years, Permacel changed the properties of its tape. The ASTM subcom-mittee advised users to check whether current Permacel gives comparable results to the original tape. This proved to be impracti-cal and opened the fl oodgates for companies to use virtually any type of pressure-sensitive tape. Many companies today use abbrevi-ated versions of ASTM D 3359, in which specifi c product niches are tested as similarly as possible, e.g., cellular phones, computer keyboards, automotive components, etc. Several commonly used techniques and pressure-sensitive tapes are shown:

There are important considerations to keep in mind when conduct-ing the tape test so that repeatable and reproducible results can be achieved to determine acceptable or unacceptable adhesion without bias. Batches (rolls) of tape as manufactured have inherent adhe-sion variability, especially over time. For example, independent laboratories have noted differences of up to 50 percent. The tape test is qualitative and cannot precisely assess quantitatively the actual strength of the adhesive bond (only a minimum value was met). However, it does provide relative adhesion performance. The test method was originally developed for ductile coatings applied to metal substrates, and not for coatings or inks applied to plastic products.

By Scott R. Sabreen, The Sabreen Group, Inc.

continued on page 4240

TECHNOLOGY FEATURE continued from page 40

Thus, when a fl exible adhesive tape is applied to a fl exible or semi-rigid plastic, rather than a rigid metal, the peel phenomenon and failure mecha-nisms are different.

Differences in tapes, backing stiff-ness, and adhesive theology, can yield different results even on the same product tested. A given tape for-mulation inherently does not adhere equally to all coatings and plastics. Surface texture is also an important factor. ASTM D 3359, by design, was made to be simple and not require calibrated equipment. Thus, the test accuracy is operator dependent.

ASTM D 4541 Standard Test Method for Pull-Off Strength of Coatings Us-ing Portable Adhesion Testers is also a well-known standard, but unlike ASTM D 3359 requires the use of instrument apparatus referred to as pull-off adhesion testers. Several dif-ferent instrument apparatus models are cited (fi xed alignment, hydraulic, pneumatic) and may yield different results depending upon the instru-ment parameters and usage. The tensile adhesion test determines ei-ther the greatest perpendicular force (in tension) that a surface area can bear before any material is detached, or whether the surface remains intact at a predetermined force (pass/fail). Results from extensive industry test-ing with the three models on identical surfaces vary considerably, making direct comparison impractical.

42

Regardless of which pull-off adhesion tester is used, the pro-cedure begins with securing a loading fi xture (termed “Dolly”) perpendicular to the surface of the coating with an adhesive. After the adhesive is cured, a testing apparatus is attached to the loading fi xture and aligned to apply tension normal to the test surface. The force applied to the loading fi xture is gradually in-creased until either material detachment “pull-off” occurs or the specifi ed value is achieved. When multiple coatings are applied, the nature of the failure mechanism, adhesive or cohesive failure, will be derived. Two examples are shown below of the many devices commercially available. The top two pictures show the Elcometer 106 Fixed Alignment apparatus, the bottom two pic-tures show the Defelsko Hydraulic Self-Aligning apparatus.

For any of the pull-off adhesion testers, surface preparation of the dolly and applica-tion of adhesive are important to achieving accurate reproducible results. Many dollies,

as purchased, have smooth machined and/or end milled surfaces, however they typi-cally yield weaker bonds. Some users of pull-off ad-hesion testers may modify

the dolly surface using either Scotch-Brite® pad or sandpaper to provide a slightly roughened sur-face, creating microstructures for maximum bond strength. After “dressing” the dolly, it must be cleaned (usually alcohol and dry cloth) to remove excess grit, oils, and oxidation. A common adhe-sive used is Araldite 2011, a two-component epoxy and may require up to 24 hours cure before the pull test can be conducted. Other adhesives can be selected, but it is paramount that the adhesive not effect or interact with the coating or the test results will be of minimal value. Regardless of the pull-off adhesion tester selected for use, the most critical aspect for conducting this test is the consistency of how the test is conducted. Even slight variations in procedure will cause statistically signifi cant data results. Many users of this test method use

continued on next page

43

TECHNOLOGY FEATURE continued from page 43

products using the methods and techniques presented and Six Sigma products using the methods and techniques presented and Six Sigma techniques will ensure robust manufacturing.

Acknowledgements3M Corporation Industrial Tape & Specialties DivisionAmerican Society of Testing and Materials InternationalBYK GardnerDefelsko CorporationElcometer Inc.Gardner CompanyPermacel (Nitto Denko Company)

Scott R. Sabreen is founder & president of The Sabreen Group, Inc. (TSG). TSG is a global engineering company specializing in secondary plastics manufacturing processes – surface pretreat-ments, bonding, decorating & fi nishing, laser marking and product security. For more information, call toll-free (888) SABREEN or visit: www.sabreen.com

disposable aluminum dollies. Documented procedures will help disposable aluminum dollies. Documented procedures will help minimize variation as a result of inconsistent setup.minimize variation as a result of inconsistent setup.

ASTM D 4541 was also developed for the specifi c purpose of test-ASTM D 4541 was also developed for the specifi c purpose of test-ing coatings on rigid structures such as metal, concrete, or wood. ing coatings on rigid structures such as metal, concrete, or wood. Unlike ASTM D 3351 that can be conducted on relatively small Unlike ASTM D 3351 that can be conducted on relatively small parts, ASTM D 4541 typically requires product sizes of three inches parts, ASTM D 4541 typically requires product sizes of three inches or larger to accommodate standard dolly sizes (although custom dollies can be purchased). ASTM D 4541 procedure should be conducted in at least 2-3 locations on a part, which can make it impractical for small products. The decision to use ASTM D 3351, ASTM D 4541, or other adhesion tests is up to each company and may be infl uenced by historical testing conducted within its market niche.

Test Standards and Process ExcellenceMany companies use some of the test methods discussed in this pa-per out of necessity, i.e., to acquire business by demonstrating their manufacturing processes conforms to specifi c industry standards. Unfortunately, manufacturers can often be unaware of fi eld failure problems until their customers return nonconforming products. By default, “poor adhesion” is the blame. Through proper understanding of the many possible causes for adhesion versus abrasion failures, engineers can adroitly identify the critical root causes and implement corrective actions. Testing should not be reserved only for product development and troubleshooting. Continuous in-line testing of

44

TOTAL SOLUTIONS FOR HIGH CONTRAST & COLOR LASER MARKING

The Sabreen Group (TSG) is a global engineering company specializing in laser marking of plastics, metals and other substrates. We bring total solutions encompassing material science, laser technology and systems integration. The newest generation of laser additives utilized by TSG yields unprecedented marking quality and speed. Since 1992, our laser solutions continue to be used in applications including aerospace, animal tags, automotive, electronic, medical (FDA), packaging and product security. Contact us today to learn how our innovative technologies can provide your company critical competitive advantages.

THE SABREEN GROUP, INC.1-888-SABREEN • 972-250-4664

5837 Wavertree Lane • Plano, TX USA 75093engineering@sabreen.com • www.sabreen.com