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TECHNOLOGY FEATURE                                                                                            By Scott R. ...
TECHNOLOGY FEATURE   continued from page 40                                               Thus, when a flexible adhesive ta...
Regardless of which pull-off adhesion tester is used, the pro-                                 as purchased, have smoothce...
TECHNOLOGY FEATURE                              continued from page 43disposable aluminum dollies. Documented procedures w...
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Techniques For Measuring Adhesion Abrasion Durability Of Coatings Inks By Scott R. Sabreen


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Techniques for Measuring Adhesion & Abrasion Durability of Coatings & Inks

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Techniques For Measuring Adhesion Abrasion Durability Of Coatings Inks By Scott R. Sabreen

  1. 1. TECHNOLOGY FEATURE By Scott R. Sabreen, The Sabreen Group, Inc.This is the first of a two-part series on techniques for measuring pulling the free end rapidly (not jerked) back upon itself as closeadhesion and abrasion durability of coatings and inks. This first to an angle of 180 degrees as possible.issue will examine adhesion. Part two will address abrasion dura- • Inspecting the cut area for removal of coating from thebility including new equipment and measurement techniques. substrate and rating the adhesion relative to pre-determined descriptions and illustrations classifications (0B, 1B, 2B, 3B,Adhesion and abrasion durability of cured coatings and inks are 4B, 5B).two critical performance properties, and their measurement andcontrol are essential for robust manufacturing. Adhesion and Early versions of ASTM D 3359 referenced the use of 3M 710 andabrasion durability are discrete physical properties and commonly Permacel tape. 3M discontinued its product and, over the years,misdiagnosed during failure mode analysis. Poor adhesion and Permacel changed the properties of its tape. The ASTM subcom-abrasion durability are leading causes of field failures, scrap and mittee advised users to check whether current Permacel givesrework, lower profits, and dissatisfied customers. Techniques exist comparable results to the original tape. This proved to be impracti-to measure these properties. This article addresses several different cal and opened the floodgates for companies to use virtually anytest methods and instruments. Among these is a new linear abraser type of pressure-sensitive tape. Many companies today use abbrevi-made by Taber Industries that is designed to evaluate actual product ated versions of ASTM D 3359, in which specific product nichesshapes involving contoured surfaces as well as flat specimens (part are tested as similarly as possible, e.g., cellular phones, computertwo). Advanced measurement techniques such as the ones cited keyboards, automotive components, etc. Several commonly usedin this paper are helping manufacturers achieve superior product techniques and pressure-sensitive tapes are shown:performance and six-sigma process excellence.Adhesion versus AbrasionOptimal adhesion and abrasion durability (resistance) of curedcoatings and inks onto plastic substrates must be achieved duringmanufacturing to produce high quality products that will endurethroughout their service life. These properties can be interdepen-dent but often are mutually exclusive. That is, cured coatings andinks can demonstrate good adhesion and poor abrasion, or poor There are important considerations to keep in mind when conduct-adhesion and good abrasion. Proper testing and understanding ing the tape test so that repeatable and reproducible results can beof adhesion and abrasion ensures conformance to specifications achieved to determine acceptable or unacceptable adhesion withoutand is invaluable in solving coating and printing problems. The bias. Batches (rolls) of tape as manufactured have inherent adhe-identification of the failure mechanism, adhesion (cohesion), sion variability, especially over time. For example, independentabrasion or both, will determine the engineering process solution. laboratories have noted differences of up to 50 percent. The tape testHistorical test methods and new instrumentation exist to measure is qualitative and cannot precisely assess quantitatively the actualand analyze these properties. strength of the adhesive bond (only a minimum value was met). However, it does provide relative adhesion performance. The testAdhesion Test Methods method was originally developed for ductile coatings applied to metalASTM D 3359 Standard Test Method for Measuring Adhesion by substrates, and not for coatings or inks applied to plastic products.Tape Test is historically perhaps the most well-known and widelyused method for testing adhesion of coatings and inks because of itssimplicity and low cost. The most basic procedure for conductingthe 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 film 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 finger and then rubbing firmly 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 by40 continued on page 42
  2. 2. TECHNOLOGY FEATURE continued from page 40 Thus, when a flexible adhesive tape is applied to a flexible 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 (fixed 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
  3. 3. Regardless of which pull-off adhesion tester is used, the pro- as purchased, have smoothcedure begins with securing a loading fixture (termed “Dolly”) machined and/or end milledperpendicular to the surface of the coating with an adhesive. surfaces, however they typi-After the adhesive is cured, a testing apparatus is attached to the cally yield weaker bonds.loading fixture and aligned to apply tension normal to the test Some users of pull-off ad-surface. The force applied to the loading fixture is gradually in- hesion testers may modifycreased until either material detachment “pull-off” occurs or the the dolly surface using either Scotch-Brite® padspecified value is achieved. When multiple coatings are applied, or sandpaper to provide a slightly roughened sur-the nature of the failure mechanism, adhesive or cohesive failure, face, creating microstructures for maximum bondwill be derived. Two examples are shown below of the many strength. After “dressing” the dolly, it must bedevices commercially available. The top two pictures show the cleaned (usually alcohol and dry cloth) to removeElcometer 106 Fixed Alignment apparatus, the bottom two pic- excess grit, oils, and oxidation. A common adhe-tures show the Defelsko sive used is Araldite 2011, a two-component epoxyHydraulic Self-Aligning and may require up to 24 hours cure before theapparatus. pull test can be conducted. Other adhesives can be selected, but it is paramount that the adhesive notFor any of the pull- effect or interact with the coating or the test resultsoff adhesion testers, will be of minimal value. Regardless of the pull-offsurface preparation of adhesion tester selected for use, the most criticalthe dolly and applica- aspect for conducting this test is the consistency oftion of adhesive are how the test is conducted. Even slight variationsimportant to achieving in procedure will cause statistically significantaccurate reproducible data results. Many users of this test method useresults. Many dollies, continued on next page 43
  4. 4. TECHNOLOGY FEATURE continued from page 43disposable aluminum dollies. Documented procedures will helpminimize variation as a result of inconsistent setup.ASTM D 4541 was also developed for the specific purpose of test-ing coatings on rigid structures such as metal, concrete, or wood.Unlike ASTM D 3351 that can be conducted on relatively smallparts, ASTM D 4541 typically requires product sizes of three inchesor larger to accommodate standard dolly sizes (although custom products using the methods and techniques presented and Six Sigmadollies can be purchased). ASTM D 4541 procedure should be techniques will ensure robust manufacturing. conducted in at least 2-3 locations on a part, which can make itimpractical for small products. The decision to use ASTM D 3351, AcknowledgementsASTM D 4541, or other adhesion tests is up to each company 3M Corporation Industrial Tape & Specialties Divisionand may be influenced by historical testing conducted within its American Society of Testing and Materials Internationalmarket niche. BYK GardnerTest Standards and Process Excellence Defelsko CorporationMany companies use some of the test methods discussed in this pa- Elcometer Inc.per out of necessity, i.e., to acquire business by demonstrating their Gardner Companymanufacturing processes conforms to specific industry standards. Permacel (Nitto Denko Company)Unfortunately, manufacturers can often be unaware of field failureproblems until their customers return nonconforming products. By Scott R. Sabreen is founder & president of The Sabreen Group,default, “poor adhesion” is the blame. Through proper understanding Inc. (TSG). TSG is a global engineering company specializing inof the many possible causes for adhesion versus abrasion failures, secondary plastics manufacturing processes – surface pretreat-engineers can adroitly identify the critical root causes and implement ments, bonding, decorating & finishing, laser marking and productcorrective actions. Testing should not be reserved only for product security. For more information, call toll-free (888) SABREEN ordevelopment and troubleshooting. Continuous in-line testing of visit: 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 75093 • www.sabreen.com44