Ask the Expert: Adhesion

by Scott Sabreen, The Sabreen Group, Inc.

Question: Fluoropolymers
Could you explain fluoropolymers and their benefits?

A fluoropolymer is a polymer that contains fluorine atoms. Fluoropolymers inherently possess high resistance to solvents, acids, and bases. One of the most well-known fluoropolymers is Teflon (PTFE – polytetrafluoroethylene), which was invented by DuPont. Equally well-known polymers within this industry are PVDF, FEVE, PFA and ETFE. Fluoropolymers (typically clear) can be added to coatings to offer weatherability protection from UV, wind, rain, and corrosion. PVDF paints have extremely good gloss and color retention. Chemically, these polymers’ molecular structures are based on their C-F bond energy which is larger than the energy of UV rays in sunlight. Structurally, there are variations – including homopolymers, copolymers, and terpolymers – based upon the application requirements. Since fluoropolymers are extremely inert, typical end-use applications include outdoor products such as architecture, automotive, aerospace, and marine requiring long-term stability from the forces of nature.

Question: Standard Test Methods for Measuring Adhesion
Can you recommend test methods for measuring adhesion?

ASTM 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. Early versions of this test referenced the use of 3M 710 and Permacel tape.

Today, the most commonly used pressure-sensitive tapes include those listed in the table to the right. The following are the most basic procedures for conducting the tape test:

1) Agreement on the selection of tape.
2) Coatings and inks must be completely cured before testing.
3) Make X-cuts in the film per Method A or in a lattice pattern with either six or eleven cuts in each direction per Method B of the test.
4) Remove and discard two laps of tape from the roll dispenser.
5) Smooth the tape into place by finger, then rub firmly with an eraser on the end of a pencil, or similar. The color under the tape is a good indicator of complete contact.
6) Within 60-120 seconds of application, remove the tape by pulling the free end rapidly (do not jerk) back upon itself as close to an angle of 180 degrees as possible.
7) Inspect the cut area for removal of coating from the substrate and rate the adhesion relative to predetermined descriptions and illustrations classifications (0B, 1B, 2B, 3B, 4B, 5B).

Alternatively, ASTM D 4541 Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers also is a well-known standard, but unlike ASTM D 3359, it requires the use of instrument apparatus referred to as pull-off adhesion testers. Several different instrument apparatus models include Fixed Alignment, Hydraulic, and Pneumatic and may yield different results depending upon the instrument parameters and usage. The tensile adhesion test determines either 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). Based upon extensive industry testing from the three models on identical surfaces, tests varied considerably, which makes direct comparison impractical. Regardless of which pull-off adhesion tester is used, the method is performed by securing a loading fixture (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 fixture and aligned to apply tension normal to the test surface. The force applied to the loading fixture is gradually increased until either material detachment “pull-off” occurs or the specified value is achieved.