Revisiting Dynamic Lap Shear Method for Evaluating Tapes Marc I. Johnson, Texture Technologies Corp.

Transcription

1 Revisiting Dynamic Lap Shear Method for Evaluating Tapes Marc I. Johnson, Texture Technologies Corp. Abstract For many decades the Static Holding Power test has been the accepted method of evaluating the shear resistance of a pressure sensitive adhesive. This method tends not only to give variable results, but they are then reported in mixed units, either as time to fall, or the amount of movement in a given time, making comparisons difficult. What is proposed in the conversion of this test to a dynamic one, by determining the force required to disentangle the molecular structure at a very slow rate. The proposed method is fully explained and examples given. Marc I. Johnson, Texture Technologies Corp. 6 Patton Drive, Hamilton, MA tel marcj@texturetechnologies.com Marc Johnson was graduated from Hobart College with a BA in Economics and from The Wharton School, University of Pennsylvania with an MBA. Mr. Johnson is Executive Vice President of Texture Technologies Corp, the North American distributor of SMS s TA.XT2Plus Texture Analyzer. Mr. Johnson designs fixtures and test protocols to solve industrial and academic client testing problems in the adhesive, pharmaceutical, cosmetic and food industries. He is a frequent lecturer on issues related to industrial texture and adhesive measurement. Prior to Texture Technologies Mr. Johnson worked in commercial real estate acquisitions for domestic and international pension funds.

2 Revisiting Dynamic Lap Shear Method for Evaluating Tapes Marc I. Johnson, Texture Technologies Corp. Introduction For many decades the Static Holding Power test has been the accepted method of evaluating the shear resistance of a pressure sensitive adhesive tape. This method tends not only to give variable results, but they are then reported either as time to fall or the amount of movement in a given time, making comparisons difficult. What is proposed is the conversion of this test to a dynamic one, by determining the force required to disentangle the molecular structure at a very slow shear rate. This method has been desired for many years, but it became possible only recently with the advancement of instrumentation capable of very slow shear rates. The method s importance is to allow companies to quantify the shear behaviors of their PSAs more quickly than the often days-long conventional tests. The time savings alone would help companies ship products sooner after manufacture and the information that can be derived from the plots may allow people to better understand the results of their formulations and allow them to have faster development velocities as they develop PSAs for specific applications. Test Principal The test premise is that by lap shearing PSA tapes at extremely slow shear rates the polymer chains that help form their bond will disentangle rather than break. The shear rate used for the research in this paper was conducted at a shear rate of 0.01 mm/second ( inches/minute). The test principle is illustrated in Diagram 1. As the shear progressed from step 1 to step 4, the polymer chains are stretched and then either disentangle, break, or hold. An A D optimal test occurs when the polymer 1 C B chains disentangle and the practical results is a shear behavior rather than 2 a peel behavior. Types & Sources of Tape We used two sources of tapes. The A D primary PSAs were donated by Intertape Polymer Corp. Those tapes C B Clamped here had adhesives based on a Solvent This side moves Acrylic, a Water-based Acrylic, a Hot Diagram 1 Test Principle Melt Block Copolymer, and a Natural Rubber. The adhesives were all mounted and applied to similar bi-axially-oriented polypropylene backings. Steps 3 4 Clamped here This side stationary In addition to the tapes supplied by Intertape Polymer Corp. we also tested three commercially recognized PSAs so people can compare them relative to their own product behaviors. These store-bought tapes were a 3M Magic 810 Office Tape, a 3M Transparent Duct Tape, and a Manco Crystal Clear packaging tape. The chemistry of their adhesive layers and their backing material for these additional tapes is unknown. Test Settings The instrument that was used for the research was the TA.XTPlus Texture Analyzer which was programmed to conduct a test in tension using a Return to Start option. The upper clamp traveled at 0.5 mm/second until it took up any slack in how the tapes were mounted to a consistent standard of 5 grams of tension. From that

3 position the clamp then pulled upward at a speed of 0.01 mm/second ( inches/minute) for a total distance of 40 mm (1.575 ). When the adhesive faces released in shear failure from each other and the forces fell to below 10 grams, then a break was determined to have occurred and the clamps returned to their starting position. During the test forces were captured at 50 points per second. All tests were conducted between 71 o F and 73.5 o F. Sample Preparation One of the vagaries of this test dynamic shear method is that it is very sensitive to not only subtle changes in product behaviors but also differences in sample preparation. We originally tested PSAs that were mounted and aligned with our alignment rig (Picture 1), however, the resulting tests were too variable. Even when PSAs were overlapped perfectly we observed that variability was introduced as the tape segments were stored and subsequently mounted directly onto the grips. Some of the variability was due to different techniques by the operators as they clamped the PSAs. In order to have statistically valid Picture 1 Original Alignment Rig differentiation and strong repeatability we had to establish exacting new protocols and fixturing for controlling the sample handling. To that end we established a mounting frame technique to assist the alignment of the tapes, to control the history of the tapes, and to manage how the tapes are aligned and mounted into the grips. Cutting & Mounting Tapes For each type of tape we removed and discarded three revolutions of tape to prevent any edge of roll effects. We then took lengths of tape from each roll, and we elevated the rolls across an 18 span so that 10 mm wide strips of tape could be cut from the tape strips without touching the adhesive face of each segment. We used a custom fixture mounted with parallel razor blades so that the strips of PSAs were exactly parallel at 10 mm wide. (Picture 2) Use of Frames to Handle Tape Strips and Assist Alignment The tape strips were then applied onto frames of card stock which had been prepared in advance. The card stock strips (Picture 3) were C shape frames. Picture 2 Parallel Cutting of Sample The opening in the frame was 0.75 tall and 0.75 wide. The entire frame was 1.25 wide and 1.5 tall. The frames allowed the tapes to be mounted and stored in preparation for the pending tests without risk of any over- manipulation. Without the frames the tape strips could be accidentally stressed, torqued, twisted or even accidentally overly-bonded/squeezed. The frames also assisted in ensuring that a tight grip was held by the miniature clamps and the each tape sample could be clamped in a perfectly vertical orientation. We used four strips of tape for each test replicate. Two segments were designed to be face to face with each other (Diagram 1; segments A & B) so that they could be sheared apart. The other two segments were to normalize the sample thickness to prevent distortion as it stretched and to ensure that the shear action was perfectly parallel. The overlapped segments of the PSAs were exactly 10 mm wide by 10 mm long. A B Picture 3 Frame to Hold Tape The initial strip of tape was then placed adhered across the frame (Picture 3, Frame A) and then the tape was cut perpendicular across the strip s width (Picture 3, Frame B). This cut served as the bottom edge of the overlapped shear section.

4 We placed the frame with the adhesive side of the PSA facing up on a light table in order to perfectly align the two face-to-face adhesive tapes (Picture 4). We used an Exacto knife to guide the second adhesive down onto the facing up tape, since otherwise static electricity would bring the two tape faces prematurely into contact. We placed colored adhesive tapes on the light table exactly 10 mm apart to serve as a guide for the exact amount of overlap (10 mm wide x 10 mm long). Once the two overlapped shear tapes were mounted, we then mounted the segments C and D (as per Diagram 1) to complete the frame. We did not apply any bonding force between the tapes, since the adhesive layers appeared to adequately wet out the complete contact A B Picture 4 Overlapping the Tape Segments area on their own. With a little practice we were able to create a set of six mounted frames in less than five minutes. We rested the tapes in their frames for 24 hours before testing. Once each test was ready we placed the frame holding the tape into the TA-96B Miniature Grips. The grips close from each side and have brass faces that are.75 wide x.375 tall. The grips are open at the bottom and top to allow the mounting frame to extend beyond the grip face. The grips faces were calibrated to a starting position exactly.75 apart so that they would perfectly grip the frame, leaving only the exposed tape. The frame helped us align the PSAs perfectly parallel and unstressed through the mounting sequence. Once the frame was mounted and tightened we cut the top and bottom of the frame so that only the overlapped shear faces of the tapes were exposed. (Picture 5). A B C D Picture 5 Mounting Tapes and Cutting Frame Conducting the Tests Once the test was initiated the load cell zeroed any pre-stress, typically very low because of the use of the mounting frame, and then initiated the search for the 5 grams of trigger force. As the tension built up the complete tape (backing and adhesive) was pulled in shear tension. In Picture 6, frame A is the sample immediately prior to the initiation of the test. Frame B shows the tape as the tension builds up. Frame C is where the shear starts to be sufficient to disentangle the polymer chains and the shear begins. Frame D is ¾ of the way through the shear and Frame E is right before the tape faces completely debond in shear mode. These pictures are matched frame by frame in Diagram 2.

5 A B C D E Picture 6 Tapes During Typical Test Sequence After each test replicate the sheared tapes were observed to determine whether any adhesive failures from the backing occurred. No such adhesive failures were observed with any of the primary or store-bought tapes. A B C D E Diagram 2 Tapes During Typical Test Sequence Annotated Graph The test generates plots like that shown in Graph 1, which is of one of the Solvent Acrylic PSA replicates. The force originally climbs as the tension builds up in the PSA and the backing and the tape system is stretched. Once the nominal tensile forces are completed then the focus of the force profile becomes the shear plane between the two tape segments A & B (as per Diagram 1). In most, but not all, cases the shear plane stretches at a lesser modulus until the shear failure begins. After the shear failure starts forces typically drop as the contact area between the two tape segments becomes less than the initial 10 mm by 10 mm dimension. Calculations & Measurements The behaviors of the tapes were all very different, and could be well differentiated using a variety of the measurements. One of the purposes of this method is to examine the shear behavior, so we programmed This inflection point is where the tensile forces have built up to their limit and the polymers start to be the focus of the stretch. Distance to onset of shear This early section is where the tension on the tape builds up before the shear initiates Solvent Acrylic This peak force is where the adhesive layers start to shear Graph 1 - Annotated Plot As the tape face shear against each other the amount of face-to-face contact decreases and the shear forces drop. our analytical macro to determine the distance to which the tape was stretched before the onset of shear began. This point was captured in two ways. The first was with the first significant descending inflection point, and the second was with the intercept position of the tapes initial modulus (from beginning to 20% of the maximum force) with the later modulus immediately before the peak force. The intercept concept is

6 illustrated in Graph 2. We used the second method since the force and distance measurements at the intercept time were relatively repeatable and extremely discriminating (Appendix 1). The other measurements that were captured with the analytical macro were the absolute peak force, the distance at absolute peak force, distance from the end of the nominal stretch to the onset of shear at the peak force, and the complete area of work (Graph 3). Each test was replicated six times. Graph 2 Illustrated Graph 3 - Other Measurements Comparative Results The test results for all graphs are summarized in presented in Table 1, depicted in Graph 4, and the detailed results are in Appendix 1. Appendix 2 has typical individual tests and Appendix 3 has the complete set of replicates for each PSA. 3M Magic 810 Office Tape Solvent Acrylic Hot Melt Block Copolymer Natural Rubber The method worked very well Manco Crystal Clear Packaging for the water-based acrylic tape, the solvent acrylic, the 3M office tape, 3M duct tape and Water Based Acrylic the Manco packaging tape. All of these products clearly experienced cohesive face to 3M Transparent Duct Tape face lap shear failures in the course of the test. The natural rubber PSAs never failed in Graph 4 Typical Graphs shear mode during the tests, while the hot melt block copolymer PSAs failed in three out of six instances. The hot melt failures, however, were still after relatively long times of shear stress, indicating that the technique may not be suitable. For the two products that the technique did not work on and also the 3M office tape, we then experimented with conducting the shear tests immediately after the PSAs were mounted on the tapes. The hot melt and the natural rubber products each sheared at approximately ¾ of the forces shown in Graph 4 when the tests were conducted immediately after mounting. The freshly mounted office tape sheared at approximately ½ the forces shown in the longer rest time. The data is not presented here since we did not conduct the same tests immediately after mounting for all seven products. Further research should be conducted at different set times

7 after mounting to determine whether the method using a different resting period is suitable for a broad range of those two classes of products, and whether the shorter rest time might be appropriate for all product classes. The water based acrylic PSA initiated sheared at a very low peak force (~1,200 grams) and exhibited only ~260,000 gs as its complete energy to shear. The solvent acrylic PSA, by comparison, initiated shear at a peak force of ~1,800 grams and took ~800,000 gs to completely shear. The 3M Transparent Duct Tape initiated shear at the lowest peak force (~185 grams) and exhibited only ~80,000 gs of energy to completely shear. The 3M Magic 810 office tape, initiated shear at a peak force of ~2,050 grams and took ~550,000 gs to completely shear. Interestingly, the 3M office tape initiate shear at the shortest distance from after the tape was taut (.26 mm), while the same value for the 3M duct tape was.45 mm, perhaps because it was a thicker and softer adhesive layer. The Manco Crystal Clear High Performance packaging tape exhibited the oddest behavior, whereby the shear forces stayed extremely high and level for between 3 to 4 mm of travel once the shear initiated. All of the other products experienced sharp decreases in shear forces once the shear initiated. Please note that we have not made any judgments as to which values from this Dynamic Lap Shear method are good or bad. We have focused solely on developing a method which is reproduciable, repeatable and differentiating. Time Table 1 Summary of Test Results Force at Distance at Peak Force Distance at Peak Force Distance to Initiate Shear Complete Area of Work1:6 seconds g mm g mm mm g.sec Solvent Acrylic Avg: , , ,668 Std Dev ,200 % CV 5% 7% 5% 10% 14% 19% Water Acrylic Avg: , ,807 Std Dev ,961 % CV 21% 7% 21% 11% 27% 51% Hot Melt Avg: , , ,916,201 Std Dev ,680 % CV 28% 16% 28% 5% 11% 15% Natural Rubber Avg: , ,513,583 Std Dev ,110 % CV 61% 22% 61% 7% 3% 7% 3M Office Avg: , , ,915 Std Dev ,315 % CV 11% 4% 11% 3% 6% 15% Manco Packaging Avg: , , ,761 Std Dev ,029 % CV 16% 4% 16% 7% 19% 28% 3M Duct Avg: ,460 STD DEV ,861 % CV 31% 10% 31% 8% 19% 11% Repeatability The repeatability of the test varies according to the parameter measured, but by and large the repeatability is relatively tight with % cv values for peak forces ranging from 3% to 11%. With very few exceptions each

8 product using all of the captured measures are statistically differentiated from each other with a greater than 95% confidence level. The variability of the tests can be observed graphically in Appendix 3 where we present a complete set of the test replicates for each tape tested. Effect of Tape Backing For every product tested we also conducted several replicates of tension tests on two segments of tapes adhered face to face without being configured to fail in shear mode. This experiment was to determine the tensile characteristics of each product and to ensure that the forces and distances measured were due to the shear failures and not due to tensile failures of the tape backing. The tensile characteristic of the backings were always such that the forces built up to a relatively large load, say around 3 ½ kilos as shown in Graph 5, and then the doubled up Face to face adhered two segments of 3M Magic 810 Office Tape All replicates of 3M Magic 810 Office Tape Graph 5 - Tensile Test on Two Segments of Tape Adhered Face to Face. tape backing weakened and started to stretch out and elongate at a lesser modulus. In every case, even for the hot melt block copolymer and the natural rubber, the force it took to weaken and begin to elongate the tapes were less than the maximum peak forces of the shear behaviors of the tapes. Thus the dynamic shear experiment remains valid since the shear will occur before the tapes might otherwise stretch. Conclusion This proposed Dynamic Lap Shear Method appears to be a very differentiating and repeatable method for evaluating the shear performance of some varieties of PSAs. For those products, the results can be obtained in 2 ½ to 16 minutes, rather than perhaps hours or days with more traditional methods. Preliminary work suggests that all PSAs, even those PSAs that do not work with this method (hot melt block copolymer and natural rubber), might be able to take advantage of the method with shorter resting times after mounting. This would allow people to move inventory faster through production based on sooner understandings of the PSAs lap shear behaviors. The technique should also allow manufacturers to have faster development velocities as they develop and evaluate PSAs for specific applications. The method is extremely easy to conduct once the PSAs are properly mounted into frames. We recommend the use mounting frames to reduce operator related test variability. Acknowledgments I would like to thank Ms. Beverly Zimmerman of Intertape Polymer Corp for providing the tapes and for patiently tolerating my lack of understanding of chemistry, and Mr. John Johnston for the strong downhill push to get this project initiated and completed.

9 Gradient F-T 1:2 Time Appendix 1 Test Results Force at Distance at Peak Force Distance at Peak Force Distance to Initiate Shear Complete Area of Work1:6 g/sec seconds g mm g mm Mm g.sec SOLVENT ACRYLIC , , ,634 SOLVENT ACRYLIC , , ,765 SOLVENT ACRYLIC , , ,024 SOLVENT ACRYLIC , , ,417 SOLVENT ACRYLIC , , ,586 SOLVENT ACRYLIC , , ,037,583 Average: , , ,668 STD DEV ,200 % CV 8% 5% 7% 5% 10% 14% 44% 19% ACRYLIC WATER , ,081 ACRYLIC WATER , ,007 ACRYLIC WATER , ,610 ACRYLIC WATER , ,240 ACRYLIC WATER , ,362 ACRYLIC WATER , ,541 Average: , ,807 STD DEV ,961 % CV 30% 21% 7% 21% 11% 27% 61% 51% Hot Melt Block Copolymer , , ,957,776 HMBC , , ,251,627 HMBC , ,518,784 HMBC , , ,607,663 HMBC , , ,058,410 HMBC , , ,102,948 Average: , , ,916,201 STD DEV ,680 % CV 21% 28% 16% 28% 5% 11% 17% 15% Natural Rubber , ,603,901 Natural Rubber , , ,389,655 Natural Rubber , ,542,570 Natural Rubber , ,418,454 Natural Rubber , ,613,335 Average: , ,513,583 STD DEV ,110 % CV 33% 61% 22% 61% 7% 3% 8% 7% 3M MAGIC 810 TAPE , , ,591 3M MAGIC 810 TAPE , , ,519 3M MAGIC 810 TAPE , , ,323 3M MAGIC 810 TAPE , , ,303 3M MAGIC 810 TAPE , , ,838 Average: , , ,915 STD DEV ,315 % CV 11% 11% 4% 11% 3% 6% 67% 15% MANCO CRYSTAL , , ,213 MANCO CRYSTAL , , ,022 MANCO CRYSTAL , , ,344 MANCO CRYSTAL , , ,172,228 MANCO CRYSTAL , , ,486 MANCO CRYSTAL , , ,273 Average: , , ,761 STD DEV ,029 % CV 20% 16% 4% 16% 7% 19% 27% 28% 3M DUCT ,872 3M DUCT ,199 3M DUCT ,737 3M DUCT ,396 3M DUCT ,094 Average: ,460 STD DEV ,861 % CV 20% 31% 10% 31% 8% 19% 24% 11%

10 Appendix 2 Test Examples Solvent Acrylic Water Based Acrylic Hot Melt Block Copolymer Natural Rubber 3M Magic 810 Office Tape 3M Transparent Duct Tape Manco Crystal Clear

11 Appendix 3 Complete Overlay of Test Replicates per PSA Solvent Acrylic Water Based Acrylic Hot Melt Block Copolymer Natural Rubber 3M Magic 810 Office Tape 3M Transparent Duct Tape Manco Crystal Clear