A TEST METHOD TO EVALUATE THE VISCOELASTIC PROPERTIES OF PRESSURE SENSITIVE ADHESIVES USING A TEXTURE ANALYZER

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1 A TEST METHOD TO EVALUATE THE VISCOELASTIC PROPERTIES OF PRESSURE SENSITIVE ADHESIVES USING A TEXTURE ANALYZER Nattakarn Hongsriphan, Graduate Student University of Massachusetts Lowell, Lowell, MA Carol M. F. Barry, Associate Professor University of Massachusetts Lowell, Lowell, MA Joey L. Mead, Associate Professor, University of Massachusetts Lowell, Lowell, MA Abstract A test method employing the Texture Analyzer was developed to evaluate the viscoelastic properties of pressure-sensitive adhesives. Dynamic testing of coated substrates was performed using the cycle until count mode and measurement of compression force. The available frequencies were 1 to 7 Hz, and the strain and temperature were varied from 0 to 25% and -10 to 120 C, respectively. Adhesive-substrate interactions were examined for several substrates. Results were assessed using single point data, Lissajou figures (stress-strain curves), and derived dynamic mechanical properties. Time-temperature superposition curves were developed from the single point data. Lissajou figures facilitated examination of adhesive hysteresis at various test conditions. Since the cycle until count mode effectively imposed a sinusoidal strain on the adhesive sample, the storage and loss moduli generated with new method was compared with dynamic mechanical properties obtained from conventional oscillatory shear. Introduction The Texture Analyzer, introduced as way to quantify the adhesive and tack parameters of pressure sensitive adhesive tapes 1, has been used with a variety of probes and test methods. In a tack test with a flat-ended cylindrical probe, Johnson 1 found that the force-time profile depended on the type of adhesive and backing. The Avery Adhesive Test (AAT) employed a probe with 25-mm ball at the tip 2. Chuang et al. 2 related data from the AAT test to the adhesive performance and molecular structure of pressure sensitive adhesives (PSAs). Using the combination of a brass base plate with slits and a wide, radiused probe, St. Coeur and Feys 3 measured the creep resistance of pressure sensitive adhesives. The resultant creep data correlated well with results from standard static shear (shear creep) tests, but was less variable and more reproducible 3. In this study, a dynamic test method was developed to evaluate the viscoelastic properties of PSAs. Experimental Approach Two pressure sensitive adhesives were used in this study. PSA-1 was a UV-curable, SBS-based hotmelt PSA (Natural Starch and Chemical Co., grade: DURO-TAK A) and PSA-2 was a polyester-based adhesive (Eastman, grade: REGALREZ 1018). The former adhesive was coated onto release liner (siliconized paper) using a slot coater while the latter adhesive was manually drawn down on the same release paper. Thickness of both coatings was about 0.45 mm. The coated release paper was die cut to make rectangular-shaped samples. All samples were placed in a controlled environment prior to testing. The PSA-1 and PSA-2 adhesives also underwent standard dynamic mechanical analysis (DMA). Testing was preformed using a TA.XT2i Texture Analyzer (Stable Micro Systems) with a 5-kg capacity, 0.1-g force resolution, and 1-μm distance resolution. Adhesives samples were mounted to a brushedaluminum base with ten 9-mm diameter holes (TA-303 indexable rig). The stainless-steel puncture

2 probe had a 7-mm diameter and a radius curvature of 2. Between tests, heptane was used to remove any residual adhesive from the probe and base. The test protocol was set as Cycle until Count mode with the measurement of compressive force. Although the Cycle until Count test allows for constant force (stress) or constant distance (strain) modes, the latter mode was selected for this study because it mimicked dynamic mechanical analysis (i.e., varied oscillation frequencies while keeping a constant oscillation amplitude (strain) and temperature). As shown in Figure 1, contact with the adhesive surface initiated probe oscillation and data collection. Since the downward motion of the probe was controlled by the pre-test speed, keeping the same starting point (i.e., the height that the probe at rest was away from the adhesive surface) and pre-test speed ensured the same first contact pressure for all samples. Therefore, the pre-test speed and post-speed were held constant at 5.00 mm/s while the test speed was varied from 0.10 mm/s to mm/s. A trigger force, that initiated probe oscillation, also started data collection at the default rate of was 200 points per second (pps). As discussed later, selection of the trigger force was critical to the measurements. The effects of travel distance in the dynamic testing were evaluated using PSA-1 samples. Dynamic testing was conducted at room temperature (i.e., outside the controlled temperature chamber) with test speeds of 0.10, 0.20, 0.50, 1.00, 2.00, 5.00, and mm/s. The travel distances were set at 0.10, 0.20, 0.30, and 0.40 mm. Thirty cycles and five samples were run for each test speed. Temperature effects in dynamic testing were determined for PSA-1 and PSA-2. The temperature control chamber (Stable Micro Systems) was electrically heated and cooled with carbon dioxide. For these tests, the trigger force was set at 10.0 N and travel distance at 0.10 mm. The test speeds were 0.10, 0.20, 0.50, 1.00, 2.00, 5.00, and mm/s. At each temperature, five samples underwent 30 cycles. The temperature was varied from 0 to 120 C for PSA-1 and -10 to 23 C for PSA-2. Raw data collected from these tests was used to calculate compressive modulus, perform time-temperature superposition, and determine the dynamic mechanical properties. A software program (Curve Expert, v. 1.3) was employed to fit the raw data (from the temperature tests) to a sinusoidal curve. Raw data between distance and time were analyzed to find the strain equations that represent the sinusoidal movement of the testing mechanism. Then raw data between compressive force and time, which were response from the setting travel distance (strain), were analyzed with the same method to generate the stress equations similar to the strain equations. Both equations were compared and find their relationship in order to generate the expression for the dynamic mechanical properties. Results and Discussion Trigger Force Although the trigger force could be set as low as N, such forces could be obtained from 1) vibration of the workstation, 2) vibration of the moving probe, or 3) friction between the misaligned probe and the inside of the hole in the sample mounting plate. Careful alignment of the probe with the mounting plate holes initially allowed dynamic testing without premature triggering when the trigger force was N. Addition of the temperature controller chamber, however, required that the probe be extended from its original length of 165 mm to 285 mm using three threaded aluminum blocks. While the extension allowed the probe to pass through the hole at the top of the chamber to the adhesive mounting base, the clearance between the probe (D = 18 mm) and the hole (D = 22 mm) was only 2 mm.

3 As a result, the probe surface contacted the inside surface of the hole even if the probe was carefully aligned before each trial. Vibration due to movement of the extended probe and friction generated from the contact between the probe and the hole produced sufficient force to trigger data collection. Premature trigger caused the probe to oscillate 1) in the air above the adhesive surface or 2) at different initial penetration distances. Although the trigger force was increased until trials exhibited no premature oscillation, the optimum trigger force was N or about a third of the force measured in the tests. This trigger force had a significant effect on test results. As illustrated in Figure 2, raising the trigger force increased the measured compressive force. When the trigger force was set at 10.0 N, the compressive force was N, which was 3.7 times of the force measured with the trigger force of N. High trigger forces also increased penetration distance, resulting in higher applied strain. Since the actual position of the probe was not measured directly, but recorded as distance from the trigger point, underestimation of strain was possible with higher trigger forces. Nevertheless, the high trigger force was required to overcome friction forces and vibrations of the extended probe. Characteristics of Dynamic Curves from the Texture Analyzer Figure 3a presents a typical force-time curve obtained for 30 cycles of oscillation. The top of each peak represented the compressive force required to press the probe into the adhesive and the bottom of each peak was the compressive force after the probe returned to the trigger point. For all test temperatures and test frequencies, the compressive force decreased exponentially with each cycle, eventually reaching a plateau. The difference between the top and the bottom of each peak, however, was constant throughout the 30 cycles. Compressive force also increased with the test speed and decreased with higher temperatures. The decrease in compressive force had three possible causes: 1) the probe pulled the adhesive off the substrate, 2) the probe pushed the adhesive away from the contact area, and 3) cycling induced stress relaxation in the adhesive. The first peak measured only compressive force because the adhesive was at rest and no movement had disturbed the contact between the adhesive and release paper. If the adhesive adhered preferentially to the probe tip, adhesive would be pulled from the release paper (see Figure 4a). As a result, the compressive force in next cycle would be lower since less adhesive was left in the contact area. In contrast, compression of the adhesive could force adhesive away from the contact area, leaving less material to compress in the next cycle (see Figure 4b). Since the samples were compressed under constant strain and assuming the adhesive thickness remained constant, relaxation of the adhesive successive cycling could also have reduced the compressive force. Adhesive pull out could not occur if the probe did not move far from the adhesive surface. As the probe was pressed into the adhesive and oscillated within a small distance, the adhesive was alternately compressed and relieved, but never placed in tension. Given that the travel distance during oscillation was 0.10 mm, the probe probably did not break the adhesive surface and deformation of adhesive was always in compression. To clarify determine whether pull was occurring, dynamic tests were performed on PSA-1/release paper and PSA-1/stainless steel plate systems. Adhesion between PSA-1 and the steel plate was unquestionable better than adhesion between the adhesive and the release paper. Moreover, PSA-1 adhered preferentially to the steel plate rather than the probe tip because the probe contacted the adhesive for a very short time. As shown in Figure 3, first peak of the compressive force was slightly higher for PSA-1/steel than for PSA-1/release paper and compression force decreased with cycling. For adhesive/steel system, the compressive force decreased 6.63% from the first to thirtieth cycle, whereas the difference was 12.87%

4 for the adhesive/release paper system. The difference in compressive force between the two systems may have been due to variations in the sample thickness, but most likely reflected the interfacial (adhesive) strength between adhesive and substrate. Good adhesion between the adhesive and substrate restricted polymer movement, thereby requiring a higher compressive force to deform the adhesive. These results confirmed that the reduction of compressive force during the dynamic test did not involve adhesives being pulled out of the substrate. Although the adhesive-coated samples were placed under a brass plate, a 1.0-mm clearance between the diameter probe and diameter hole in the place might have provided a place for adhesive to flow during oscillation. At a test temperature of 120 C, PSA-1 is near its melting temperature of 160 C. As a result, PSA-1 was very fluid and could be easily forced away from the contact area. At 120 C, the compressive force also reach a plateau much faster than at 23 C. The difference in compressive force between the first and second cycles was negligible (see Figure 5). When the high-temperature tests were completed, a little dimple remained on the test surface. This dimple disappeared after the tested sample was at rest for more than five minutes. When the adhesive was more viscous, a mark showed the contact by the probe and little strings, caused by snapping-back of the adhesive, adhered to the probe. These dimples and marks suggest that the decreasing compressive force did not involve stress relaxation, but only reflected deformation of the adhesive. No evidence confirmed whether the decrease in compressive force was due to stress relaxation or just the result of oscillation work on adhesive. Study of the stress relaxation in the pressure sensitive adhesives would have required modification of test jigs for viscous and fluid samples. Despite of lack of clarification, compressive force showed a tendency to decrease and gradually plateau, and these were values of compressive force used for dynamic mechanical analysis. Compressive Strain in Dynamic Tests Table 1 summarizes the effect of probe travel distance, d, on the compressive force, F 1. The applied strain, ε, was determined using d ε = (1) h where h was the average adhesive thickness (~0.45 mm). Increasing the travel distance increased the applied strain, thereby increasing of the compressive force and the time to complete 30 cycles, t 30. Greater travel distance, however, reduced the frequency of the test. For PSA-1, the frequency increased 29% when the travel distance was changed from 0.10 to 0.40 mm. These effects were important when selecting a nominal strain. The test speed range of 0.10 to mm/s limited the frequency range of the dynamic tests. Thus, a smaller travel distance provided for higher frequencies. Moreover, greater travel distances increased the probability that the adhesive would climb the probe. When the travel distance was more than half the thickness of the adhesive layer, adhesive climbed the sides of the probe, making the measured force a combination of compressive and shear forces. The adhesive on the sides of the probe also increased of the contact area, and thus, increased measured compressive force. Consequently, the minimum travel distance of 0.10 mm was employed during this testing.

5 Using a travel distance of 0.10 mm and test speeds of 0.10 to mm/s, the calculated frequencies were 0.49 to 8.09 Hz. With test speeds of 0.10 to 2.00 mm/s, frequency increased linearly with test speed (see Figure 6). The frequency, however, changed very little when the test speed increased from 2.00 to mm/s. This phenomenon resulted from the small travel distance. At 0.10 mm, this distance may not allow for the rapid probe movement required for the high-speed oscillations. The acceleration and de-acceleration may also have influenced the differences between the test speeds of 2.00, 5.00, and mm/s. Compressive Modulus Compressive moduli, E c, of the adhesives were calculated from the measured compressive forces and the applied strain. The first cycle of the dynamic test was employed because the adhesives were at rest when the probe made contact with the adhesive and no possible shear or stress relaxation effects complicated the first compression cycle. Time-temperature superposition was performed in order to obtain modulus over a wider range of frequencies. As shown in Figure 7, the compressive modulus of PSA-1 increased slightly with frequency. Similar results were obtained for PSA-2, but the second PSA exhibited a constant modulus until the frequency exceeded 3 Hz. For rubbery systems, the tensile modulus is almost three times the shear modulus. Since the tensile and compressive moduli of the PSAs were similar, this assumption was used to calculate the shear moduli, G, for PSA-1 and PSA-2. The calculated shear moduli were compared to the complex shear moduli, G*, obtained from dynamic mechanical analysis of the adhesives (see Figure 8). While the shear moduli of PSA-1 were 10 times of the complex shear moduli, this difference was 100 times for PSA-2. Both sets of curves showed similar trends, but different magnitudes for the changes in modulus with frequency. Dynamic mechanical analysis was performed at 10% strain while Texture Analyzer testing had a nominal strain of 22%. This would produce some difference in the shear moduli, but not the high magnitudes found in this testing. A Poisson s ratio of 0.50 was acceptable for the rubbery SBS-based PSA-1, but may not have been appropriate for PSA-2. Other factor affecting these values may have been the substrate and thickness of the adhesive. Texture Analyzer samples had a 0.45-mm thick layer of adhesive on a silicone release paper while dynamic mechanical analysis was performed with steel plates and a gap of 1 mm. Since PSA-1 was a rubber-based PSA, its compressive modulus not only depended on the level of strain, but also on the geometry of the test fixtures. Although the brass fixture secured the samples, the fixture probably reduced the ability of the adhesive to deform under the applied strain. Thus, the compressive moduli was increased significantly from the shape factor and the material s compression coefficient 4. In contrast, PSA-2 was polyester-based PSA which was fluid at room temperature. The test fixture was not used because the fixture s weight caused PSA-2 to flow into the holes and changed sample thickness. Without the test fixture, however, the adhesive on the release paper was free to flow when compressed by the probe. This flow changed the thickness of the samples. Calculation of moduli was done with the presumption that the thickness was constant during the test. Change of thickness changed the value of applied strain, and thus the compressive moduli were probably higher than they supposed to be from the change of strain. Moreover, the trigger force was the important factor for force measurement. Underestimation of strain happened when the trigger force was set at the high value. The current temperature control chamber was not suitable for the dynamic test, since its interference between the probe and the hole was too small. The test results were interfered by friction force, which it caused the probe to oscillate at different

6 positions, not always at the surface of the adhesive. Low trigger force was favored but the premature trigger was likely to happen. High trigger force was selected and caused underestimation of applied strain. In order to gain the absolute values of moduli, the Texture Analyzer for the dynamic test required a new temperature control chamber suitable for the oscillation, and new sets of compression jig suitable to handle adhesive samples. i.e. compression plunger for liquid-like PSAs. Dynamic Analysis: Stress-Strain Curves As shown in Figure 9, the characteristics of force-distance profiles changed with frequency (or test speed), but temperatures. At the slowest speed, the force-distance profile was relatively linear with each successive cycle exhibiting reduced stress (see Figure 9a). The compressive force decreased with each cycle, and the highest compressive forces were not in phase with the applied strain. This pattern is characteristic of a viscoelastic material when it was subjected to dynamic stress. If the material was perfectly elastic, the maximum stress (or force) should present at the maximum strain. When measured force is not in phase with the applied strain, the material is viscoelastic. In contrast, the force-distance profile at the fastest test speed was similar to a hysteresis curve for rubber (see Figure 9b). The lag in response of stress (force) to the applied strain (distance) was more evident with the higher test speed. When the rate of applied strain was faster, viscoelastic material showed response toward elastic behavior. At faster test speeds, low and high distances had multiple force values. These force measurements corresponded to movement of the probe. As the probe changed directions, it de-acceleration to make the turn at the set travel distance and acceleration to resume the setting test speed. Since the resolution of the recorded distance was only 0.001, this was not sufficient to differentiate the small change of distance at the turning points. Increasing the data collection rate from 400 to 600 pps did not improve the resolution. This phenomenon is also illustrated in Figure 10. For one cycle of the dynamic test, the stress curve was a sinusoidal curve corresponding to the applied strain while the strain curve was truncated. The stress, however, clearly lagged behind the applied strain. Dynamic Mechanical Analysis Nevertheless, the characteristic of the curve is closed enough to assume it is sinusoidal in order to generate the mathematic equation for dynamic mechanical analysis. The time lags were determined directly from the raw data in order to generate a form of delta, Δ, in DMA data. Since the cut-off at the top in the distance curve gave five points with the same value, the middle point was assumed to be the top of the curve. When the time lag was determined, this assumption made a variation of time lag. The time lag between the force and distance ranged from to s. There was no pattern to relate with test speeds or temperatures. Limitation of raw data at the top prevented the direct analysis of time lag. At slow speeds the stress (force) and strain (distance) curves were triangular (see Figure 11). The resolution of the recorded distance was not enough to differentiate small change of distance when the probe moved with very slow test speed. Therefore, the distance data showed a series of point bundles, which they were visually presented as big spots in the graph. For the force curve, recording, 400 pps of data recording with a very slow test speed produced a large amount of points, which were presented as a continuous triangle line. These triangle shapes produced lower correlation of data with the sinusoidal model, which was going to use for the dynamic mechanical analysis. With the slowest test speed, truncation of the distance curve gave 10 to 12 points with the same value of distance, but the time lag was to s. Therefore, the time lag was not able to determine directly from the raw data

7 producing from the current data acquisitions in this instrument, which was designed to examine the typical adhesive tests such as tack or peel. Dynamic test results were analyzed by employing a software program (Curve Expert 1.3) to fit all raw data into the sinusoidal model. The general equation for sinusoidal fit by this program was: ( c x d ) y = a + b * cos * + (2) where a, b, c and d were empirical constants. When distance and time were analyzed, the high test rate data provided low standard errors and correlation coefficients, r, approaching 99.88%. In contrast, low frequency data exhibited slightly greater standard errors and correlation coefficients of about 99.35%. At speeds of 0.10 mm/s, the error was expressed as a truncation of triangular form of the raw data. The fitting analyses, however, provided correlation coefficients that were greater than 95%. For PSA-1, constant a was directly related to the travel distance. The constant c was basically related to the frequency of the sinusoidal wave, and therefore, depended directly to the frequency (or the speed) of the testing condition. Constants b and d indicated patterns, but changed slightly with speeds. After obtaining constants from the raw data, a new strain fitting analysis was performed by fixing constant a equal to for all test speeds and setting the constant c at a fixed value for each speed. The second analysis produced better patterns, with the constant b related to frequency and the constant d ranging from to (i.e., they were close enough to assume that they were the same value). The average correlation coefficient was 98.86%. Tables 2 and 3 present the strain equations for PSA-1 and PSA-2, respectively. The constant a was the constant related to the travel distance (or strain), which could be set to the same value for any system in the dynamic test. Its value would be changed when the value of travel distance (or strain) was changed. Since the constant c was related to frequency of the sinusoidal curve, certain values could be set to represent the test speeds that were available in the instrument. The constant b was the constant that was multiplied to the cosine term in the sinusoidal equation. From the fitting analysis, these values could be selected certain values to represent test speeds similarly to constant c. The constant d was different when the adhesives were changed from PSA-1 to PSA-2. Since the travel distance (or strain) and test speeds were set similarly for both PSA-1 and PSA-2, all four constants in the equation should have been the same. Constant d showed its dependency on adhesive system rather than testing conditions. This might have been due to the effects of material viscosity toward the acceleration and de-acceleration of the probe. For stress fitting analysis, raw data between force and time obtained from PSA-1 and PSA-2 were used to determine the constants a, b, c and d in the sinusoidal equation. Tables 4 and 5 summarize the second stress fitting analyses, in which constant b was fixed at and constant c was the average of the values calculated from the first fitting analysis. Constant b was set at for 0.10 mm/s in the PSA- 2 to obtain the correlation coefficient higher than The c constants represented the frequencies used in the dynamic test. Constants b were set with the certain value (4.0000) to produce the pattern for the stress equation. Constants a, however, could not be set because they were directly related to the value of force measured in the test. Unlike the set distance (or strain), the values of compressive force were the response to the combination of the set travel distance, test speeds, and temperature. No values were obtainable for constant d since they also changed with respect to changing of constants a. Thus, the stress fitting analysis was completed by selecting regions, in which the compressive force were

8 equivalent. This technique could be performed for PSA-1, but not PSA-2 because the latter exhibited a change of state within the range of test temperatures. Table 6 shows a comparison between constant c that were obtained from the strain and stress fitting analysis for PSA-1 and PSA-2. Differences in constant c between the strain and stress analysis, Δc, indicated the lag in response during the dynamic test. In PSA-1, Δc increased with respect to frequency (or test speed) as shown in Figure 13. This was expected since the time lag would increase with frequency. For PSA-2, however, Δc varied randomly. In order to determine constant a for the stress fitting analysis, a new approach was performed to determine constant a and compare them with those constants in the strain equations. For this approach, raw data of compressive force and time in the plateau region were extracted for a cycle. The maximum compressive force was identified and used to calculate a new constant, z: 0.1 constant z = (3) the maximum force at the plateau By multiplying constant z into all raw data in the plateau region, the maximum measured compressive force would be 0.10 similar to the set distance. Sinusoidal fitting analysis produced constants a, b, c, and d for PSA-1 and PSA-2. Constants a and b changed with respect to the value of the measured compressive forces. The combination of these two constants reflected the maximum compressive forces. Thus, the constants were greater at higher frequencies and lower for high test temperatures. Constants c and d were directly related to frequency. The higher frequencies increased the values of constant c. While constant d was also related to frequency, but the constants fine tuned constant c values for each frequency. With the stress (force) equations and the strain (distance) equations from the data fitting analysis, a new set of sinusoidal curves were generated to analyze their time lags. As shown in Figure 13, the stress (force) and strain (distance) curves were out of phase. This, the time lag could be determined by calculating the time that gave a maximum value of y equal to 0.1 for both curves. Figure 14 presents the change in tan delta with frequency using tan delta time (tan Δt) from the Texture Analyzer and tan delta from dynamic mechanical analysis. Tan Δt decreased with increasing frequency while the tan δ increased with frequency. This phenomenon probably resulted from the movement of the probe. With higher frequencies, the maxima in the stress and strain curves closer to another, thereby producing lower Δt values. Delta time was determined directly from the time lag between the strain curve and the closest stress curve, which resulted in the trend of decreasing of lag time with frequencies. Conclusions Dynamic testing could be performed using the Texture Analyzer and the Cycle Until Count test protocol. Compression measurements permitted selection of the starting position, but the dynamic test required a set value for travel distance or applied strain, test speed (or frequencies of oscillation), and the trigger force. The trigger force was the most important factor for the dynamic test because it determined the position at which oscillation began.

9 In the ideal set-up, the trigger force was equal to the force detected by the force transducer when the probe made the first contact to the adhesive surface. Since that force could not be set, the minimum trigger force was used to initiate the oscillation. Vibration of the extended probe and friction forces generating from the small clearance between the probe and the hole inside the temperature control chamber, however, produced pre-test forces higher than the trigger force. These caused premature triggering of oscillation. To prevent premature triggering, a high trigger force was required in this study. This force caused the measurement of compressive forces to start after the probe penetrated the adhesive surface. Therefore, the actual applied strain was higher than the 0.22 calculated from the sample and test dimensions. Underestimation of strain caused the measured compressive moduli to be higher than expected. The geometry of the test fixture also produced greater-than-expected compressive forces. Since the test fixture constrained the sample, PSA-1 was not able to flow in response to the applied strain, and thus, exhibited high compressive forces. In contrast, without the fixture, PSA-2 flowed easily when it was pressed with the probe. The compressive forces for PSA-2, however, were still higher than predicted. It was postulated that a shape factor could correct for fixture-related constraints. Underestimation of strain resulted from changing of sample thickness when the oscillation took place. This probably caused the massive difference between test results from the Texture Analyzer and the DMA. With the Texture Analyzer, the distance-force curves changed with frequency and showed the characteristic time lag between the distance and the force curves. While these time lags confirmed the viscoelastic response of the test samples, the viscoelastic properties could not be determined directly from the raw data. Acceleration and de-acceleration of the instrument s mechanics caused the distancetime data to plateau at the top of the curves. When the middle points were used to estimate the time lag, the patterns did produce accurate dynamic data. Therefore, dynamic data were generated using data fitting analysis; specifically, a sinusoidal model was used to generate the strain and stress curves representing the oscillation of the probe in the dynamic tests. Since instrument limitations caused the analysis to incorporate some truncated data, the strain equations were flawed by acceleration and deacceleration of the instrument. Time lags were determined from the strain and the stress equation, thereby allowing the loss moduli to be calculated from the tan delta and the storage moduli. The latter were determined directly from the equations for stress and strain. The storage and loss moduli showed trend similar to those observed when the same materials were characterized using dynamic mechanical analysis. Their values, however, were not in the same magnitude as the DMA values. Both the storage moduli and tan delta values were higher than exhibited by dynamic mechanical analysis. The former was probably due to constraints imposed by the test fixture and underestimation of the strain caused by the high trigger force. The latter may have been a result of the differences in strain between the Texture Analyzer and dynamic mechanical analysis. Literature Cited 1. Johnson, Marc (1996), Ways to Differentiate Tackiness of Pressure Sensitive Tapes, Adhesives & Sealants Industry, October/November, pp Chuang, H. K., Chiu, C., and Paniagua, R. (1997), Avery Adhesive Test A Novel Pressure Sensitive Test Method, PSTC Tech XX, Chicago, IL, pp St. Couer, Rich, and Feys, Jennifer (2001), The Dynamic Tensile Test, PSTC Tech XXIV, Orlando, FL, pp

10 4. Gent, Alan (1992), Engineering with Rubber, How to Design Rubber Components, Hanser Publishers, New York, NY, pp.37 Acknowledgements The authors would like to thank Dr. Anne-Marie Baker of Tyco Adhesives for her support and funding of this study. Table 1. Effect of Travel Distance on Compressive Force, Time, and Frequency d (mm) ε F 1 (N) t 30 (s) ω (Hz) Table 2. Strain equations for PSA-1, where ε is strain and x is is time in s Speed (mm/s) Strain equation ε = cos x ( ) 0.20 ε = cos ( x ) 0.50 ε = cos ( x ) 1.00 ε = cos ( x ) 2.00 ε = cos ( x ) 5.00 ε = cos ( x ) ε = cos ( x ) Table 3. Strain Equations for PSA-2, where ε is strain and x is is time in s Speed (mm/s) Strain equation ε = cos x ( ) 0.20 ε = cos ( x ) 0.50 ε = cos ( x ) 1.00 ε = cos ( x ) 2.00 ε = cos ( x ) 5.00 ε = cos ( x ) ε = cos ( x )

11 Table 4. Force Equations for PSA-1, where F c is force in N, and x is time in s Speed (mm/s) Force equation = a cos x d 0.10 F c ( ) 0.20 F c = a cos ( x d ) 0.50 F c = a cos ( x d ) 1.00 F c = a cos ( x d ) 2.00 F c = a cos ( x d ) 5.00 F c = a cos ( x d ) = a cos ( x d ) F c Table 5. Force Equations for PSA-2, where F c is force in N, and x is time in s Speed (mm/s) Force equation = a cos x d 0.10 F c ( ) 0.20 F c = a cos ( x d ) 0.50 F c = a cos ( x d ) 1.00 F c = a cos ( x d ) 2.00 F c = a cos ( x d ) 5.00 F c = a cos ( x d ) = a cos ( x d ) F c Table 6. Comparison between Constant c from the Strain and Stress Fitting Analysis Test speed PSA-1 PSA-2 (mm/s) Strain Stress Δc Strain Stress Δc

12 a b c d e Figure 1. Schematic diagram of the dynamic test with measurement in compression: a) the probe moves with pre-test speed, b) the probe touches sample surface and the trigger fore is detected, c) the cycle at test speed is started, d) the probe retracts until reaching the travel distance, and e) the probe moves to contact the surface, completing the cycle Compressive force, 1 st peak (N) N N Trigger force (N) Figure 2. The effect of trigger force on the measured compressive force of PSA Time (s) Time (s) Figure 3. Force-time curves for a) PSA-1/release paper and b) PSA-1/steel. The test speed and temperature were 1.00 mm/s and 23 C, respectively.

13 a) b) Figure 4. Possible mechanisms for the reduction in compressive force: a) adhesive was pulled out of the release paper and b) the adhesive was flowed away from the contact area Time (s) Figure 5. The effect of cycling on the compressive force of PSA-1. The travel speed was 0.10 mm/s and temperature was 120 C Label denotes the test speed in mm/s Test speed (mm/s) Figure 6. The effect of test speed on frequency.

14 10 0 C 10 C 23 C 60 C 80 C 100 C 120 C E c (MPa) 1 1.0E E E E E E E+03 Figure 7. Time-temperature superposition curve for PSA a) b) 22 % strain, 23 o C % strain, 23 o C G (MPa) 1.0 G* (MPa) E E E E E E E E E E E E+02 c) d) E % Strain, 23 o 10 % Strain, 23 o C C 1.0E E-02 G (MPa) 1.0 G* (MPa) 1.0E E E E E E E E E E E E E+02 Figure 8. Modulus-frequency curves for a) PSA-1 generated from Texture Analyzer, b) PSA-1 generated from DMA data, c) PSA-2 generated from Texture Analyzer, and d) PSA-2 generated from DMA data.

15 a) b) cycle # 1 cycle # cycle # 1 cycle # 2 cycle # 3 cycle # 4 cycle # 5 cycle # cycle # 3 cycle # 4 cycle # 5 Cycle # Distance (mm) Distance (mm) Figure 9. Hysteresis characteristics obtained from dynamic testing of PSA-1 at a) 0.10 mm/s and 0 C, and b) mm/s and 0 C Distance Compressive force Distance (mm) Time (s) Figure 10. Stress-strain curve showing the lag between stress and strain curve. The lag, Δ, was s. Testing was performed at 0 C and mm/s (8.09 Hz) Distance Compressive force Distance (mm) Time (s) Figure 11. Stress-strain curve showing lag between stress (force) and strain (distance) curve. The test was performed at 0 C and 0.10 mm/s (0.49 Hz).

16 Delta C Figure 12. The effect of frequency on the Δc values obtained from PSA-1. a) b) Distance Force Distance Force Distance (mm) Distance (mm) Time (s) Time (s) Figure 13. Comparison force and distance curves when the test speed was a) 0.10 and b) mm/s. 1.0E+00 a) b) E-01 Tan delta t 1.0E-02 Tan Delta E E E E E E+02 Figure 14. Comparison of tan delta values obtained from a) the Texture Analyzer and b) DMA.