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Investigation of High Rate Delamination Between Label and Release Liner

Tim Mitchell1, Zhenbin (Ben) Niu1, Daria Monaenkova2, and Ellen Keene3

1Dow Chemical Company, 2200 West Salzburg Road, Auburn, MI 48611 2Dow Chemical Company, 633 Washington Street, Midland, MI 48667 3DuPont, 1501 Larkin Center Drive, Midland, MI 48674

INTRODUCTION

Release coatings are materials applied on substrates to prevent a sticky surface from prematurely adhering. These coatings have unique low release force properties which are used to achieve a weak interaction between the coating and a pressure sensitive adhesive (PSA) layer. This aids to protect the laminate construction prior to use but allows easy delamination of the label when ready for application. There are several types of release coatings based on the types of the chemical materials used including polyacrylates, carbamates, polyolefins, fluorocarbons, chromium stearate complexes, and silicones. Among them, silicone based release coatings play a dominant role

due to their unique advantages, such as low migration, lower release forces, and relative low cost.1 Silicone based release coatings are extensively used in tape, labeling, hygiene, medical, and food industries. There has been a strong market need for higher production efficiency over the past couple of decades. As a result, the high-speed delamination process has become more important. Predicting the release performance of a label construction during high-speed operations is critical for the pressure sensitive industry and a significant factor that impacts the release force is the delamination speed. In high-speed machine operations, the orientation of the laminate construction can change and result in differences observed with release forces. There are two major ways that convertors delaminate the release liner/label during application processing: a) peeling liner from label during processes such as bottle labeling or b) peeling label from liner during die cutting and matrix removal steps (Figure 1). Generally, the former affords noticeably lower release force than the latter.

Figure 1: Delamination methods: (left) bottle labeling – release liner from label; (right) die cutting process with matrix stripping – label from release liner.

LabelLabel Matrix

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The manufacturers of bulk label constructions use high rate peel/release testing equipment to predict the processing performance of the laminate. Four peeling speeds (0.3, 10, 100, and 300 meters/minute) are commonly employed to construct a representative release force profile of a laminate system. There are numerous factors that can impact the final laminate performance including release coating composition, adhesive composition, material coat weights, type of substrates, and changes in the processing conditions during production. The impact of several of these factors are demonstrated in Figure 2.

Figure 2: Release force profiles of a silicone release coating with added release modifier and processed with two different emulsion acrylic pressure sensitive adhesives.

In a system that utilizes a modified silicone release coating, the release profiles exhibit an inclining behavior for both adhesives. Release forces are generally lowest when the sample orientation is pulled “liner from label”. When the sample is pulled “label from liner”, the release forces become significantly higher for both adhesives at delamination rates higher than 0.3 m/min. The magnitude of change is greatest for Adhesive B at the highest testing rate. Depending on sample orientation during testing, Adhesive B has both the lowest and highest release forces at a delamination rate of 300 m/min. These types of results can make it difficult to design the ideal laminate system when processing factors play a role. When large differences are seen in the release force profiles of the same laminate construction with only a simple change in sample orientation, it is necessary to understand the fundamental reason behind this behavior. Due to the fast nature of high rate delamination, it is hard to visually monitor changes in the test with just the naked eye. Therefore, the detailed mechanism behind high rate delamination can remain elusive and alternative techniques must be considered when investigating the problem.

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F3A - Liner from Label (I) F3B - Liner from Label (I)

F3A - Label from Liner (A) F3B - Label from Liner (A)

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EXPERIMENTAL PROCEDURES

Laminate Preparation

All laminate constructions were prepared by coating on silicone and adhesive pilot coater lines. The same paper is used for both the liner and facestock to eliminate that as a factor in the testing. The naming terminology for each sample was determined by the specific silicone formulation and adhesive used in each laminate construction during pilot trials. For example, F3B means the combination of silicone formulation “F3” and adhesive “B”. Samples F13-16L, F13-16M, F13-16H, F13-15H, and F13-17H were produced in a trial in which the silicone and adhesive coat weights were varied. The silicone coating (non-modified) and adhesive (Adhesive C) were different in this coater trial. F13-16L means the combination of silicone formulation “F13” coated in Run 16 and finished with a low coat weight of adhesive (L). Each coated laminate construction was cut into 1-inch wide strips using a Chemsultants sample cutter. All samples from both trials were stored in a controlled temperature and humidity room (23°C / 50% RH) under 40-pound weights using standard release industry protocols.

Coat Weight Measurements

The coat weights of the silicone liner were tested immediately after exiting the coating oven. Three sample disks were punched from the coated liner using a die cutter (1-3/8'' diameter). The silicone coat weight on each sample disc was determined using an Oxford Instruments Lab-X 3500 Benchtop XRF analyzer. The XRF was calibrated to detect the amount of silicon present on the surface of the coated liner using an x-ray fluorescence technique. The average of three disks was reported as the final silicone coat weight. The coat weight of the adhesive was determined by obtaining the actual physical weight of a 1” x 6” sample strip on a Mettler AE100 analytical balance. The tare weight of an uncoated paper sample was subtracted from the weight of the adhesive/paper sample to obtain the final adhesive coat weight.

Release / Peel Testing

High rate release testing was completed on a ZPE-1000 High Rate Peel Tester supplied by Instrumentors, Inc. (Strongsville, OH). The release testing was performed at a peeling angle of 180 degrees with a delamination rate of 300 meters/minute employing the standard FINAT test protocol – FTM 3. The testing equipment set-up and resulting testing output are shown in Figure 3.

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Figure 3: (Top) ZPE-1000 High Rate Peel Tester - Upper Configuration; (Middle) ZPE-1000 High Rate Peel Tester - Lower Configuration; (Bottom) Representative release force curve at 300 m/min peel speed.

Testing laminates in the “label from liner” orientation must be performed from the upper deck to avoid having the adhesive label wrap around the wheel and motor of the test equipment. A “testing tail” made from multiple layers of a rubber-based tape is first attached to the laminate. Then the test tail is wrapped around the half-moon fixture prior to being positioned between the wheel and motor. Once the test button in the software is

Upper Deck

Lower Deck

Wheels / MotorHalf-moon

Test Laminate

Load Cell

Testing “Tail”

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Wheels / Motor

Test Laminate

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pushed, the top wheel clamps the test tail onto the motor. The motor immediately engages and pulls the test tail through the wheels at the desired testing speed. When testing from the lower deck, the half-moon fixture is removed and the load cell is repositioned in front of the lower deck. The test tail is attached to the laminate and directly positioned between the wheel and motor prior to testing. Testing laminates in the “liner from label” orientation can be performed from either the upper or lower deck.

Imaging Setup

Peel test experiments were illuminated using 2 high intensity LED spots (Advanced Illumination, SL185-WHI-I3-M12) and imaged using a Photron APX RS high speed camera with a Micro-Nikkor 105mm f/2.8 lens. The camera was mounted on a tripod and positioned approximately 15 ½” from the peel tester to the front of the camera lens. Lights were held by 2 ring stands and were positioned at a height of approximately 9” off of the lab bench and approximately 5 ½” from the sample to the lights. Additionally, the surfaces of the peel tester instrument were covered with matte black leneta paper and black gaffer tape. This instrument “resurfacing” was done to obtain an optimized, high contrast image and help minimize imaging artifacts from reflections off the instrument. All image acquisition settings were controlled through the camera software (Photron FastCam Viewer; PVF 3). The camera was set to acquire images at 5,000 fps and 1/10,000 s shutter speed. Images were acquired at 512 x 1024 pixel resolution. Images were analyzed in ImageJ to measure the rate of peel, as well as the peel curvature, angle, and sample lift off the instrument deck.

RESULTS AND DISCUSSION

Proof-of-Concept Imaging Experiments

In the first round of proof-of-concept experiments, a variety of laminate configurations were tested on one sample (F3B). These included two different strip orientations such that the strip was peeled “label from liner” or “liner from label”. Additionally, two experiments were conducted in which the sample was freely suspended above the instrument deck or “fixed” with a double-sided Scotch tape to the instrument desk (Figure 4).

Figure 4: Simplified experimental schematics: (A) Free label configuration: the pulling process is controlled by sample lift and the radius of sample curvature. (B) Fixed label configuration: the pulling process is controlled only by the radius of sample curvature.

Release force measurements for the freely suspended samples are summarized in Table 1. Both samples orientations were of interest because different measured forces result depending on the sample orientation, and an understanding of the source of this difference was the main driver for this work. The data shows that pulling “label from liner” requires more force. Release force measurements cannot be obtained when the

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sample is fixed to the deck. Taping the sample to the instrument isolates the ability for the load cell to record a response to the delamination. Table 1: Coat weight and release data for top deck experiment using sample F3B.

Run Release coating coat

weight (g/m2) Adhesive coat weight

(g/m2) Force measure (g/in)*

Label from liner – R1 0.89 +/- 0.005 18.2

90.77

Liner from label 44.98

* For general release testing at least three laminates are tested for each sample. However, for this high-speed imaging proof-of-concept study, only one laminate was tested to guarantee good correlation between the release force and high-speed imaging data. Table 2 summarizes experimental results extracted from the image analysis (Figures 5 and 6) of both the free and fixed sample configurations.

Figure 5: Light micrographs of free sample (F3B) peel testing in two sample orientations. Lift and curvature varies with orientation of mounted sample in the instrument.

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Figure 6: Light micrographs of fixed sample (F3B) peel testing in two orientations. Curvature varies with orientation of mounted sample in the instrument.

Table 2: Measured parameters from first round of peel test imaging experiments of F3B.

*All measurements are in pixels unless otherwise noted **Lift measurement from the “End” was not possible, because the instrument ledge was not visible in the image ***Rates in [m/min] are approximate based on calibration from ~100 um film thickness ****Image field of view is ~ 3 x 1.6 cm

Comparing data from the imaging experiments, it can be noted that the average sample peeling rate is faster in the “liner from label” configuration. That configuration also has a higher radius of curvature when compared to the “label from liner” configuration. In the initial set of experiments with the freely suspended laminate, the sample was observed to lift off the instrument deck to different heights depending on the sample orientation. To understand differences in mechanical behavior of liner and label, simplified experiments with “fixed” samples were conducted. The sample was fixed to the instrument deck with double-sided Scotch tape. This is contrary to a typical release testing experiment because the peel forces cannot be measured. It was thought that isolating the lift could potentially provide insight into the instrument operation. What was especially interesting, fixed “liner from label” configurations still had higher velocities, higher curvatures, and the difference in tape curvature was more prominent. Experiments with fixed samples clearly demonstrate differences in mechanical properties of the laminate when tested in different orientations. Since the peel rates and radius of curvature trends of the fixed samples were similar to those observed with the free samples, it was decided to continue experimentation using normal release testing protocols. This would enable the ability to capture release force data to supplement the imaging analysis.

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Second Round of Imaging Experiments

From the first round of imaging experiments, a lot of previously unknown information was learned about samples as they were peeled at high rates. However, many outstanding questions remained including:

1) Since all samples start from rest, was the sample imaged at a location where it had reached the steady-state velocity? Could this account for the lower than predicted measured rates?

2) Was the instrument testing motor calibrated properly before this testing? 3) Is there sample slippage at the wheel/motor, where the instrument velocity is set, which results in the

lower observed (by imaging) velocity than the instrument reports? 4) Does the sample follow a similar peel behavior at different testing locations on the instrument (upper

versus lower deck)? Are there significant friction forces between the testing tail and half-moon fixture which impacts testing in the upper deck configuration?

5) What is the impact of coat weights of coating and adhesive on the observed differences in peel velocity/measured forces?

Therefore, a set of follow-up experiments were conducted. This included repeats of the initial set of samples (F3B), imaging of the wheel/motor position during testing, and evaluation of samples with varying coat weights of release coating and adhesive.

Instrument Calibration and Field of View Adjustment

A full maintenance calibration on the test motor and load cell was scheduled for the instrument prior to starting any additional imaging experiments. It was found that both the test motor and load cell were in acceptable working order. Therefore, this could be eliminated as a cause for the deviation in measured velocity of samples during testing. The initial proof-of-concept experiments found that all peeling speeds measured were slower than anticipated based on the instrument settings. It was thought that the region of interest imaged may have been in the peel acceleration zone such that the sample had not yet reached steady-state velocities. When normal release testing is completed on a day-to-day basis, the operator will adjust cursors in the software to bracket the plateau region of the release curve (Figure 7) based on the knowledge that the sample must move a certain distance before reaching a steady-state velocity. Therefore, the camera was re-positioned further down the instrument deck from the load cell clamp to better capture steady-state peel velocities in all subsequent experiments.

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Figure 7: Release force curve showing the location of the initial and adjusted field of view for imaging experiments. The white lines indicate a typical plateau location where the release force value is averaged.

Imaging of Instrument Wheel and Testing Tail Tape

The instrument wheel is where the speed of the peel test is set by the instrument. It was of interest to determine if there was slippage of the testing tail (or film strip) observed here and if further understanding by motion tracking would help gain insight into the resulting instrument measurements. Figure 8 shows the instrument wheel that was marked with a permanent marker to aid in motion tracking. Two separate imaging experiments of the wheel were tested, with (“film”) and without (“free”) a film strip (Figure 8, right column).

Initial Field of View Adjusted Field of View

Release Force Measurement Range

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Figure 8: Instrument wheel with markings to track motion (left). Two experiments conditions tested, with film clamped (“film”, top right) or without a film (“free”, bottom right).

Figure 9 summarizes the motion tracking of the “free” instrument wheel setup. In this case, only the small wheel rotated.

Figure 9: Velocity determined by marker tracking on small wheel of “free” experiment.

The imaging wheel experiment was repeated with the film strip being pulled by the large and small wheels. In this case, both large and small wheels rotate periodically (Figure 10).

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Figure 10: Two replicates of velocity determined by marker tracking on large and small wheels of film clamped experiment.

The rate of peel was determined by counting the number of frames the film took to travel the length of the image, as well as knowing that each frame represents 0.2 ms of elapsed time. This calculation represented an average rate of film travel for each experiment. It was, however, of interest to determine the instantaneous speed of the instrument. Therefore, dots were marked at ½ inch intervals on the black film strip and it was pulled through the instrument. This experiment was imaged and analyzed by particle tracking of the marked dots (Figure 11).

Figure 11: Dots, with ½” spacing, on black tape (top). Particle tracking by image analysis of 3 dots (red, yellow, green) from frame to frame (bottom).

Figure 12 summarizes the instantaneous velocity as measured by the particle tracking. This particle tracking experiment was completed three times. The first time was on the original equipment without any modification. The second time was on the original equipment after cleaning the wheels to remove any old sample debris. The third time was on a new instrument (ZPE-1100W) that had not been put into commission. A polynomial curve was fit to each plot and the data showed similar behavior among instruments including the time for film acceleration to reach steady state. The measured instantaneous velocity was quite noisy. Therefore, the

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instantaneous velocity versus distance was also measured for a real peeled sample and found to be linear (Figure 13).

Figure 12: Velocity determined by particle tracking on the original instrument (A), original instrument after the wheels had been cleaned of built-up debris (B), and the new instrument (C).

Figure 13: Delamination region travel distance plotted versus time of experiment showing that motion is linear and that in “liner from label” configuration the peeling is faster.

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Upper versus Lower Deck Testing – Sample F3B

Testing of sample F3B was repeated on both the upper and lower deck after adjustments were made in the camera positioning. For upper deck peel test experiments, samples were peeled in both the “label from liner” and “liner from label” configurations. Coat weight and release force measurements are summarized in Table 3. Table 4 summarizes the imaging results and the measured parameters of the F3B samples. Table 3: Coat weight and release data for the top deck experiment using sample F3B.

Run Release coating

coat weight (g/m2)

Adhesive coat weight (g/m2)

Force measure for each laminate (g/in)

Average force measure +/- STD

(g/in)

Label from liner

0.89 +/- 0.005 18.2

66.84

71.79 +/- 3.72* 72.71

75.82

Liner from label

34.84

31.85 +/- 2.12 30.10

30.63 * The release force measured during the second round of experiments is lower than the release force measured during the first round of experiments. This can be attributed to the laminate aging during the extended time between experiments.

Table 4: Measured parameters for peel testing of sample F3B on instrument top deck.

All trends remained the same when compared to the proof-of-concept experiments. The force required to pull the sample is higher, the radius of curvature is smaller, and the velocity is slower when tested in the “label from liner” configuration. The overall velocities for both sample configurations were higher than the original testing but still much lower than the anticipated target of 300 m/min. For lower deck peel test experiments, samples could only be peeled in the “liner from label” configuration. Coat weight and release force measurements are summarized in Table 5. Table 6 summarizes the imaging results and the measured parameters of the F3B samples.

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Table 5: Coat weight and release data for the bottom deck experiment using sample F3B.

Run Release coating

coat weight (g/m2) Adhesive coat weight (g/m2)

Force measure for each laminate (g/in)

Average force measure +/- STD

(g/in)

Liner from label 0.89 +/- 0.005 18.2

39.70

30.79 +/- 5.75 31.89

24.89

26.68

Table 6: Measured parameters for peel testing of sample F3B on instrument bottom deck.

The comparison between the upper and lower deck testing results indicate that peeling “liner from label” using either instrument position will result in similar testing values. The release force, peel angle, curvature, and lift were nearly the same. The only difference was found between the velocities of the samples. Testing from the upper deck resulted in a higher velocity than the lower deck. The trend was opposite of what was anticipated. It was expected that the testing tail moving over the half-moon fixture would generate some friction forces and result in a lower velocity using the upper deck.

Evaluating the Impact of Coat Weight

A series of samples that vary by the coat weight levels of silicone and adhesive were evaluated in both the “label from liner” and “liner from label” configurations. Samples F13-15H, F13-16H, and F13-17H contained increasing levels of silicone release coating. Samples F13-16L, F13-16M, and F13-16H contained increasing levels of adhesive. The coat weights of the release coating and adhesive are summarized in Table 7. Figures 14 and 15 summarize the imaging results and the measured parameters of each sample. Table 7: Coat weights for release coating and acrylic adhesive layers.

Run Release coating coat weight

(g/m2) Adhesive coat weight

(g/m2)

F13-15H 0.67 +/- 0.02 64.82 +/- 1.64

F13-16H 1.15 +/- 0.01 66.41 +/- 1.38

F13-17H 1.84 +/- 0.08 65.67 +/- 1.05

F13-16L 1.15 +/- 0.01 20.27 +/- 0.51

F13-16M 1.15 +/- 0.01 43.92 +/- 0.56

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Figure 14: Summary of measurements from increasing levels of silicone release coat weight experiments.

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F13-16H - Label from Liner (A) F13-16H - Liner from Label (I)

F13-17H - Label from Liner (A) F13-17H - Liner from Label (I)

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F13-16H - Label from Liner (A) F13-16H - Liner from Label (I)

F13-17H - Label from Liner (A) F13-17H - Liner from Label (I)

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Figure 15: Summary of measurements from increasing levels of adhesive coat weight experiments.

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F13-16M - Label from Liner (A) F13-16M - Liner from Label (I)

F13-16H - Label from Liner (A) F13-16H - Liner from Label (I)

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F13-16M - Label from Liner (A) F13-16M - Liner from Label (I)

F13-16H - Label from Liner (A) F13-16H - Liner from Label (I)

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The release forces decreased as the coat weight of silicone increased from 0.67 to 1.84 g/m2. The release forces were higher when tested “label from liner” but the decreasing trend remained. There was no statistical difference between the velocity, peel angle, or lift for any of the samples regardless of orientation. The curvature of the samples tested “label from liner” were lower than “liner from label”, as seen in previous experiments. Silicone thickness did not appear to influence the magnitude of curvature within samples tested in same orientation. Changing the coat weight of the adhesive from 20.27 to 66.41 g/m2 resulted in greater changes in laminate performance. As the adhesive coat weight increased, the release forces increased while the curvature and velocity decreased. The trends of lower curvature, lower velocity, and higher release followed when testing “label from liner”. There was no statistical difference seen with the peel angle or lift for any sample.

CONCLUSIONS

High-speed imaging experiments provided the unique opportunity to monitor the physical behavior of labels during the delamination process. It provided new insights into the fundamentals of peel release mechanisms at high speeds which could not previously be observed with the naked eye. Samples tested “liner from label” had typically larger curvatures in comparison to samples peeled “label from liner” regardless if the experiments were conducted with fixed or freely suspended samples. Although it was discovered that the sample lifts off the instrument deck during testing, no consistent dependence of the sample lift height on the configuration of the experiment was deduced. It did confirm that the samples are not truly being tested at 180° and the resulting force measurements may not truly reflect those forces obtained during converting processes. “Liner from label” experiments were also associated with higher peeling rates and lower peeling forces. Experiments conducted on the upper and lower deck of the instrument produced similar peeling forces, peel angles, lift, and sample curvatures. The only difference was found between the velocities of the samples. Testing from the upper deck resulted in a higher velocity than the lower deck. The trend was opposite of what was anticipated. It was expected that the testing tail moving over the half-moon fixture would generate some friction forces and result in a lower velocity using the upper deck. High-speed imaging has also confirmed suspected stick-slip behavior between the testing tail and instrument wheels. This behavior improved somewhat when the instrument was cleaned. Release forces decreased as the coat weight of silicone increased and were higher when tested “label from liner”. The curvature of the samples tested “label from liner” were lower than “liner from label”, as seen in previous experiments. Silicone thickness did not appear to influence the magnitude of curvature within samples tested in same orientation. Changing the coat weight of the adhesive resulted in greater changes in laminate performance. As the adhesive coat weight increased, the release forces increased significantly while the curvature and velocity decreased. The trends of lower curvature, lower velocity, and higher release followed when testing “label from liner”.

ACKNOWLEDGMENTS

The authors would like to thank Lacey Brissette for testing release on the initial laminates which indicated differences in release force depending on orientation of the test strip and Dave Rich for allowing us to use sample laminates from his pilot coater study to complete our preliminary imaging work. We would also like to thank PSTC for the opportunity to present the work.

REFERENCES

1. Randall, G. S.; Loretta, A. J.; In Technology of Pressure-Sensitive Adhesives and Products; CRC Press: 2008, p 9.