T H E D Y N A M I C T E N S I L E T E S T

Rich St. Coeur, Manager of New Technology Platform Development, Intertape Polymer Group, Marysville, MI

Jennifer Feys, R&D Product Development Specialist Intertape Polymer Group, Marysville, MI

Abstract: This paper will examine the use of a new method to measure creep resistance of pressure-

sensitive adhesives. To prove validity, the new method is directly compared to the face-to-face shear test. In this comparison, the new method delivers test results in less time with less variability and higher reproducibility than the face-to-face test. An analysis of the data presented in the report yields an R 2 adjusted correlation of 77.2% between the two tests for all tape samples values measured and 91.0% for those samples with face-to-face shear values greater than 100 minutes.

Introduction For an adhesive to be pressure-sensitive, it must be able to adhere when contacted with a

substrate under light pressure. An adhesive must possess a rheology that will allow it to flow and conform to a variety of surfaces. This same rheology must allow the flexibility to allowdissipation of the mechanical stresses during removal. The product must have enough internal strength to allow it to be peeled from the surface without leaving residue while overcoming the forces of adhesion (1). The terms cohesive strength and creep resistance are used to describe internal strength.

Creep resistance is one of the physical properties that differentiates product performance in pressure sensitive tape applications. High stress applications, such as splicing, demand that creep resistance be high. The OEM automotive painting application creates a high-temperature environment that effectively lowers the cohesive strength of pressure-sensitive adhesives (PSA's). An adhesive used in this application must possess high creep resistance to offset the effect of these temperatures.

Creep is conventionally defined as the response of a polymer to a fixed load, measured as the change in strain over time with the application of a constant stress. The literature is replete with engineering analyses of creep, but these analyses are far beyond the scope of this paper. For purposes of simplicity and ease of knowledge transfer, this paper focuses specifically on a test designed to measure creep resistance in pressure-sensitive adhesives.

A time-tested and officially accepted method of determining creep resistance is the shear adhesion test, PSTC-107 (2), more commonly known as the static shear test. In this test, a tape sample is applied to a standard stainless steel test panel to leave an effective contact area of 1" X 1". A 1000g weight is attached to the other end of the tape and the panel is then hung vertically so that the stress is applied directly to the adhesive / substrate interface (fig 1). The test results are given in time to failure, where failure is a complete separation of tape from the panel.

Broad use and acceptance of the static shear test is testimony to the benefits of this method. The equipment used in PSTC-107 is simple and inexpensive. Test set-up is fast and does not require a high skill level. The results of shear tests are easy to record and report. These benefits and subsequent widespread acceptance, have led to the greatest benefit of the shear test, that of historical, inter, and intra-lab test result cross-referencing.

With the many positives of the static shear test, an inexperienced researcher might question the need for designing a method to replace it. However, the drawbacks of the static shear test are many. The test purports to measure a constant stress (force / area) over time and measures the amount of time

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to failure. This is deceiving. As a sample begins to slip from the test plate, the force per unit area increases. If the slippage over time is measured, the result yields a chart giving a logarithmic change in slippage over time. These measurements have been performed, analyzed and reported by Johnston (3). With this understanding one can conclude that the forces involved in a shear test are complex and that recording time to failure does not provide an accurate measure of these forces. Without an accurate measure of these forces there is an accompanying increase in variability and decrease in test reproducibility. Poor reproducibility and high variability are the two greatest drawbacks to the static shear test. According to Johnston (3) "Anyone who has performed static shear testing knows how variable two test results can be from the same tape sample. As such it is a rather unreliable test, and at best can give only rough approximations." A less critical drawback of the shear test is the extended test time associated with crosslinked adhesives. In some cases time to failure extends over 100 hours. This length of time does not allow for in-process testing and represents a significant delay in determining the results of designed experiments.

As designed, the change in force per unit area over time is the greatest contributor to-poor reproducibility and high variability in PSTC-107. There are two other lesser, yet significant, contributors: the explainable effects of misinterpreting adhesive failure as cohesive failure (popping) and of backing irregularities and elongation differences (backing effect). Researchers at IPG have made changes to the standard shear test to moderate or eliminate these effects. The use o f PET substrates can be used to reduce or eliminate backing effect. The face-to-face shear test (FTF), where a sample is applied adhesive-face-to-adhesive-face, is utilized to reduce or eliminate the interpretation of adhesive failure as cohesive failure (fig 2). Internal studies havedetermined that the FTF shear test configuration has the greatest contribution to lower variability in shear test results when test samples have high crosslink density.

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Although a formulator must measure cohesive strength in the finished product, the detriments of static shear testing have limited its use. Even when used, the poor reproducibility and high variance have not allowed the formulator to discern small changes in the shear performance.

Two years ago IPG purchased a new generation tensile tester. This instrument was designed with an extremely slow crosshead speed andhighly sensitive load cell. The instrument had the additional benefit of a computer interface for continuous data acquisition and sophisticated software to analyze the acquired data. This purchase initiated several independent method development efforts.

One of the methods targeted for development was a fast, reproducible, and reliable test to measure creep resistance that could be correlated with results from PSTC-107.

The static shear test method development effort focused on modifying the "old" PSTC-107 test method to the "new" tensile tester as the method development team believed that this marriage would yield the intended benefits. The correlation with PSTC-107 would be derived by the use of identical test panel configurations and measurements given in force over time. Continuous and accurate measures of force(g), slippage(mm), and time(s) coupled with the more sophisticated analysis of these data was expected to decrease variability and increase reproducibility. With a 0. l mm/min crosshead speed, test time was reduced to minutes.

An analysis of the data derived from the new test indicated that reproducibility was low and variability high. Correlation between the new method and PSTC-107 was low. With many other test method developments in the queue, this initial effort was suspended.

A fresh effort to replace the static shear test was initiated in 2000. The method development team was given the same challenge- design a fast, reproducible, and reliable test to measure creep resistance that could be correlated with results from PSTC-107. The difficulties encountered by the first team led to the decision to conduct a literature search for prior art. This search uncovered a study published by Johnston (3) that described his dynamic shear test. This test method is very similar to that which was used by the first IPG design team with the exception that it employs a 6mm sample length. Johnston indicated that this test was more reliable than PSTC-107 and the design team attempted to use this method in their study. Unfortunately, the team encountered a significant amount of variation in the test results and after several unsuccessful attempts to reduce this variation, abandoned the effort.

A further search of the literature led the team to a paper by A. Zosel (4) in which he reports on a study to compare static shear and dynamic shear to dynamic viscosity data obtained via DMA. In this study Zosel indicates that "The type of failure seems to change in the dynamic shear test too, for polymers without viscous flow. Uncrosslinked materials exhibit an approximately linear decrease of force as expected from the calculation, whilst crosslinked samples show a rather high force maximum and separation at small elongation." Zosel continues" "For polymers with comparatively low viscosities, the shear strain vs. time characteristics and accordingly the shear strength can be calculated from the master curves of the dynamic shear modulus. This exact calculation cannot be applied to highly viscous or slightly crosslinked polymers which, however, are the materials mainly used in the pressure-sensitive adhesive industry. That leads to the conclusion that it does not seem possible to replace the nowadays applied shear tests by any other mechanical methods, such as the determination of the dynamic shear modulus, and to calculate the shear strength from the results obtained with this method."

The method development team reviewed Zosel's work and although they agreed with the conclusion, they were not entirely convinced that the failure of the correlation was entirely due to the inability of DMA to predict shear properties at higher crosslink densities. Instead, the team believed that the ability of static and dynamic shear tests to measure creep resistance diminished with higher crosslink density.

With their failure to achieve a correlation with the Dynamic Shear test design, the team was forced to shift their paradigm. Although believed to be counterintuitive when first discussed, the use of a tensile test as an indirect measure of creep resistance was considered. This idea entailed a test design that employed a jig to apply a tensile force, a force perpendicular to the adhesive film. This idea further deviated from PSTC-107 by having a jig and fixture oriented so that two tapes made contact perpendicularly with adhesive contacting adhesive (fig 3). The test instrument brought the two samples together at a controlled force, contact area, and contact time. The two contacted adhesives were then separated at a controlled rate and tensile forces were measured over time.

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The proposed method employed a face-to-face contact between the test samples. As indicated earlier in the paper, internal IPG studies have determined that FTF shear tests yield lower variability in shear test results when test samples are highly crosslinked. This led the development team to change the test objective, with a new goal of designing a fast, reproducible, and reliable test to measure creep resistance that could be correlated with results from FTF tests.

Because the new test method did not fit the existing paradigm, the development group was surprised when the first samples were tested and the curves representing different shear values did not overlap (fig 4). Statistical analysis of 10 samples yielded an R 2 adjusted of correlation of 80.6% between this test and FTF. Given these favorable results, the development group went to the next phase" a controlled study to correlate the new method with FTF, utilizing 30 samples for statistical significance.

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Experimental A commercially available IPG tape structure was chosen for this study. This structure was

selected for the high degree of crosslink density designed into the product. Standard laboratory test conditions were maintained throughout the study; test rolls were conditioned for a minimum of 48 hours at 72°F / 50%RH, with all testing conducted under the same conditions

The standard static shear test was modified, utilizing a face to face construction (½" X ½" X 1000g, test to failure). Ten replicates were used for each sample, the results for each sample were calculated as an average of the samples remaining after the highest and lowest values were eliminated. Samples were chosen to span a range of crosslink densities. ,

The Dynamic Tensile test method utilized two test jigs: The base plate (fig 5) allows for a tape sample to be adhered to the bottom and when turned over, is adhesive side up and ready to be placed on the instrument test platform. The test jig (fig 6) accepts a second tape sample, held in place adhesive side out.

The Dynamic Tensile test method is programmed to have the test jig lower into the base plate at a rate of 0.5 mm/sec until it makes contact between the two samples and a force of 100 grams is reached. The tape samples are maintained in contact at 100 grams for 30 seconds. The test jig is then raised at a rate 0.1mm / sec for 135 seconds. The curve generated from this test is a plot of force vs time. Area under this curve is segmented and measured, yielding 1:2 and 2"3 area: force over time, abbreviated AFT.

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Results The test data are given in table A. The data were evaluated via the regression analysis tool in MiniTab© (5)

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Predictor Coefficient Standard Deviation t-Ratio p-value Constant - 19940 2373 -8.40 0.000 AFT 1:2 2.145 0.3513 6.10 0.000 AFT 2"3 -2.0619 0.8705 -2.37 0.025 S = 3399 R-squared = 78.8% R-squared adj. = 77.2% The regression equation is FTF = - 19940 + 2.14 AFT 1:2 - 2.06 AFT 2 3

The regression analysis of AFT 1:2 and AFT 2"3 to FTF minutes indicates that the R-squared adjusted correlation between the FTF shear test and the Dynamic Tensile test was equal to 77.2%. This indicates that there is a strong correlation between the FTF and the Dynamic Tensile test results.

Tests for Variability in data" FTF and Dynamic Tensile Test for Equal Variances (Bartlett's & Levenes test)

There are many methods for testing variance. Bartlett's and Levene's tests can be used to compare variance of two or more factors. Levene's test is used for continuous distribution and Bartlett's test is used for normal distribution of population. If the p value is close to 1.0 it means the variances are equal and if close to zero, the variance are significantly different. By the rule of thumb, the p value should be greater than 0.8 to claim the variances are same. The results of these two analyses indicate that the variance in FTF data is significantly different than that of the Dynamic Tensile.

Test for Variance -Standard Deviation percent of the mean Standard deviation percent of the mean provides an idea of variability in specific data sets. A

low percentage indicates a low variability. From the analysis it is clear that FTF data have higher variability compared to AFT 1"2 and AFT 2:3.

Discussion In his analysis of Creep, Dahlquist (1) states that "although the term creep has a connotation of

slow processes involving minutes, days, or months, one can, in principle, determine the short-term, or

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dynamic, stress-strain response of a viscoelastic solid as well as the long term response by means of creep measurements and time-temperature superposition." Later in the chapter Dahlquist indicates that "when sheafing forces are small and the rate of creep is very slow, the creep rate depends primarily on the steady flow viscosity of a PSA." He defines long term creep and uses the PSTC-107 Shear Adhesion test to study long term creep and steady state viscosity.

Johnston (6) reports on the use of a dynamic shear method as an improvement on the PSTC- 107 shear adhesion test. Johnston indicates that the slower speeds used in this test (0.25mm/min) allow for measurements of polymer dis-entanglement, implying that steady flow viscosity is maintained.

In his paper on the Avery adhesive test, Chuang (7) utilizes an Avery Adhesive Tester (AAT), test equipment similar to the instrument used in this study. In Chuang's study, the AAT is utilized to study tapes of varying adhesive composition. Although focused on method development for PSA tack measurements, Chuang indicates that a second peak in the AAT curve profile is a function of cohesive strength. Chuang further correlates the peak energy of this curve with dynamic shear test results

In a paper outlining alternative test methods, Johnston (8) describes the butt tensile test for purposes of determining degree of cure in pressure sensitive tapes. Johnston indicates that a portion of the curve derived from this test represents the degree of crosslink and with this information "shear failure can be quickly identified as inadequate cure." The test set-up for a butt tensile is very similar to that of the Dynamic Tensile test with the main difference being the contact between adhesive and a polished steel plate.

This short review of previous work is not complete but from this sampling it becomes obvious that the method described in this paper is not entirely novel. However, the more exhaustive search of the literature indicates that this study is the first known published record of a high reproducibility and low variability test method that is highly correlated to the FTF static shear test in crosslinked polymers.

In addition, the unique combination of the newest test equipment, test jig geometry, perpendicular contact between samples, and adhesive to adhesive contact is novel. The novelty is not casual but measured. The test equipment increases sensitivity, reproducibility, and reliability. Test jig geometry simplifies set-up and increases reproducibility. The adhesive to adhesive contact increases sensitivity and decreases variance because with this contact, the entire force/time curve now becomes a measure of the shear holding power. This decreases the variation introduced by other methods that segment the curve and increases the sensitivity with the increase of measurable area.

Although the static shear and Dynamic Tensile tests differ in substantial ways, there are three fundamental similarities" 1. Both methods entail stress-strain measurements. 2. Both methods measure the energy in force / time.

a. In the FTF test the constant is force and time to failure is measured. b. In the Dynamic Tensile method, time is constant al~d force varies.

3. In both methods, the force per unit area changes over time These similarities help explain a regression analysis R-squared adjusted correlation between the

FTF shear test and the Dynamic Tensile test of 77.2%. These similarities do not explain the remaining 22.8% of variation.

The analysis of variability leads to the conclusion that a major contributor to the remaining 22.8% of variation lies in the variation of static shear test data. If there was less variability in the FTF test, the R-squared adjusted correlation between it and the Dynamic tensile would increase.

During the process of analyzing data, the development team took a closer look at the test variance associated with low crosslink density samples. Returning to Zosel's study, it was noted that his procedure for the calculation of static shear and dynamic shear from DMA data agreed in the time range up to about 100min. The calculated and actual values began to diverge after this time. Using this

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value as a guideline, the team reevaluated their data, removing all values that had FTF failure times less than 100 minutes. If only these data are used, the R-squared adjusted correlation jumps to 91.0%.

Ln (FTF) Vs Dynamic Tensile Predicted Value

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Dynamic Tensile Predicted Value

Ln (FTF) Vs Dynamic Tensile Predicted Value For FTF Values above 100 mln.

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Dynamic Tensile Predicted Value

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A regression analysis of remaining values indicates there is no correlation between the Dynamic Tensile test and the FTF test below 100 minutes. Figures seven and eight provide visual representations of the relationship between FTF and values predicted by the Dynamic Tensile test. The most descriptive representation is achieved when the data is transformed via log function. Figure seven includes all 30 data points, figure eight includes only the values that predicted FTF failure times greater than 100.

Conclusions: The results of this study indicate that there is a high correlation between the Dynamic Tensile

test and the FTF shear test. Final analysis of the data concludes that the correlation between Dynamic Tensile and FTF increases with increased crosslink density.

The failure of the Method Development Team to correlate the Dynamic Shear test with FTF is likely due to the predominance of tape samples that had higher crosslink density. Further internal studies with the Dynamic Shear test will employ samples with lower crosslink densities.

The successful correlation of Dynamic Tensile with higher crosslink density samples indicates great utility. It is these samples that normally take days or weeks to complete with FTF static shears. The Dynamic Tensile method yields results in a significantly shorter time period that is measured in minutes. The shorter time period allows for use of this test as an in-process measure of cohesive strength and a shorter test feedback loop in designed experiments.

The other significant benefit derived from the Dynamic Tensile method is a much lower variability and higher reproducibility than the FTF static shear test.

The remaining benefits of the Dynamic Tensile method include: 1. Test set-up time is short 2. The test method requires minimal training 3. The test can be conducted in a heated cabinet for SAFT tests or to simulate high

temperature applications 4. The test can be conducted in a chilled cabinet to simulate low temperature applications.

Results Table A Each value is an average of ten tests. Roll ID identifies the different test samples FTF (minutes) is the face to face test results reported in time to failure.

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AFT 1:2 represents area under the curve from force = 0 until maximum force is achieved. AFT 1 "3 represents area under the curve from maximum force until time = 135 seconds

Roll ID F T F (minutes) AFT 1:2 AFT 2:3

1

2

3

5

6

7

8

9

10

11

12

13

14

15

16

17

19

20

21

22

23

24

25

26

27

28

29

30

L28

L29

M24

M28

M29

M31

M52

M34

L35

L36

L48

L37

L41

M342

L43

M362

M51

M35

28367.2

23478.9

15582.5

30249.5

27627.9

19456.3

10669.9 18541.6

4944.3 16646.7

1023.0 15324.2

739.3 15127.8

609.4 15088.4

530.5 19365.2

478.3 14592.0

461.0 16075.3

428.6

377.0

371.5

357.1

172.2

166.9

154,7

M32 139.3

M49 132.0

114.9

87.1

77.0

76.6

73.9

67.5

65.5

62.1

60.8

53.2

M50

M36

M46

M47

M42

M43

M38

M41

M44

M40

16841.2

15200.9

16610.4

18836.2

15756.8

14707.5

13597.6

14034.4

14786.5

14279.8

12364.7

10949.7

12835.5

10019.1

10861.1

12814.7 t

11427.0

10050.6

13367.4

10621.9

9496.5

5428.9

4752.9

5048.7

4547.0

4558.8

4517.1

7370.9

6484.0

4751.6

5386.1

4939.5

6025.0

8378.9

5893.6

4605.6

3994.9

3839.4

5302.5

4107.0

3422.5

3645.4

4018.6

3549.1

3407.3

4131.2

3837.2

3272.4

3827.4

The data in Table A was evaluated via the regression analysis tool in MiniTab©

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Predictor Coefficient Standard Deviation t-Ratio p-value Constant -19940 2373 -8.40 0.000 AFT 1:2 2.145 0.3513 6.10 0.000 AFT 2:3 -2.0619 0.8705 -2.37 0.025 S = 3399 R-squared = 78.8% R-squared adj. = 77.2% The regression equation is FTF = - 19940 + 2.14 AFT 1"2 - 2.06 AFT 2:3

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Test for variance - Standard Deviation percent of the mean Variable Mean Standard deviation FTF 2997 7122 Area 1:2 15621 4447 Area 2:3 5108 1795

Test for Equal Variances (Bartlett's & Levene's test) Test for Equal Variances: FTF v/s Area 1:2

F-Test (normal distribution)Bartlett's test Test Statistic" 0.063 P-Value • 0.000

Levene's Test (any continuous distribution) Test Statistic" 1.681 P-Value :0.200

Test for Equal Variances" FTF v/s Area 2 • 3 F-Test (normal distribution)Bartletts's test Test Statistic" 6.140 P-Value : 0.000

Levene's Test (any continuous distribution) Test Statistic: 6.003 P-Value • 0.017

Std dev % of mean 237% 28% 35%

References 1. Dahlquist, Carl, "Creep" in Satas, D. (editor), Handbook of Pressure Sensitive Adhesive Technology, Satas & Associates, Warwick, RI, 1999, pgs 121 - 138.

2. Shear adhesion of pressure sensitive tapes, PSTC-107, Test Methods for Pressure Sensitive Tapes, Pressure Sensitive Tape Council, Northbrook, IL. 2000.

3. Johnston, John. Notes and Observations on the Simple testing of Pressure Sensitive Adhesive Tapes, Proceedings of the 20th Annual Technical Seminar, 1997, pgs 79 - 101.

4. Zosel, Albrecht. Shear Strength of Pressure Sensitive .~dhesives and its Correlation to Mechanical Porperties, Technical Seminar Proceedings, PSTC First World Congress, PSTC, Skokie, IL, 1992, pgs 191 - 205

5. Copyright (C), Minitab, Inc. 6. Johnston, John, "Pressure Sensitive Adhesive Tapes", Pressure sensitive Tape Council, Northbrook IL, 2000, pg 155.

7. Chuang, H.K., "Avery Adhesive Test Yields More Performance Data Than Traditional Probe", Adhesives Age, September 1997, pgs 1 8 - 23

8. Johnston, John. "Using alternate test methods for pressure sensitive products", Adhesives Age, Dec. 19, 1990, pgs 30 - 35.

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