Debbie Chrzanowski, Manager Product Development & Improvement, Intertape Polymer Group, Marysville, MI

Transcription

1 ACCURATELY PREDICTING HIGH SPEED UNWIND Debbie Chrzanowski, Manager Product Development & Improvement, Intertape Polymer Group, Marysville, MI Introduction We are all familiar with customer complaints of tape with low adhesion. Upon closer examination, the customer assumption of low adhesion is sometimes a function of low roll unwind force. It is commonly known that most consumers assume that tape, which dispenses easily, or with low force, will not adhere properly. Those of us in the tape industry acknowledge the misunderstanding and, when given the chance, educate the consumer on the separate mechanics of tape adhesion and roll unwind force, or ease of dispensing. Most pressure-sensitive tapes are "self-wound", that is, wound on themselves, without the benefit of a release liner. Therefore, the adhesive is in intimate contact with the tape's backing. When the tape is unwound, the adhesive must remain on the side to which it was applied. Thus, unwind force should be low, so as not to disturb the adhesive/backing interface. However, if the unwind is too low, the tape may have an unacceptably low adhesion to its own backing which will result in flagging when overlapped or applied to itself. 1 It would be ideal to supply the consumer with the same product dispensing performance (unwind condition) at all times. However, if a relationship between common in-process measurables and unwind force could be developed, the opportunity for better consistency and control would then exist. Accurately predicting high speed unwind (HSUW) with in-process testing during the manufacturing process has always been a challenge. Only when the product has been converted into f'mished rolls do we know the actual unwind characteristics. A new test method has been designed that, when used in conjunction with predetermined converting controls, will provide the opportunity to produce PSA tape products with consistent roll-toroll HSUW values. There were 2 significant factors identified in this work, HSAB (High Speed Adhesion to Backing) and Roll Hardness, which had high correlation to HSUW. HSUW along with these two factors will be described in detail. High Speed Unwind (HSUW) HSUW or release force is a critical component in the functionality and usability of a roll of pressure sensitive tape. As described by Scholz, et al. in U.S. Patent # 6,074,7473, release force is generally defined as the force required to peel the pressure-sensitive adhesive at a specified rate and angle from a release-coated surface. While moderate release characteristics upon unwinding the roll are important, equally important is high adhesion to its own backing in the customer application, especially for tapes designed for paint masking applications. It must be relatively easy to remove the tape from the roll, but it must have enough strength to adhere to its own backing during painting and masking applications. If there is weak adhesionto its own backing, the tape could fail and in the case of OEM (Original Equipment Manufacturer) paint masking, i.e., the tape and masking drape could blow off in a paint drying oven. HSUW is typically measured by the standard test methods described in PSTC-8 and PSTC-132, authored by the Pressure Sensitive Tape Council. Unwind measurements are conducted at rates that vary from 12"/min, 50 ft/min, or 200 ft/min. The measurable is unwind adhesion in pounds per inch width. 161

2 Many factors have been identified as contributing to HSUW properties. They include properties defined by adhesive, release and carrier and finished roll. Adhesive properties include crosslink density, (modulus, Tg), film condition and coat weight. Finished roll properties include wind tension and age. Carrier properties include paper surface geometry, paper basis weight or film caliper. Release coating properties include type, coating placement and weight. Release coatings are necessary in the production of pressure sensitive tapes so that self-wound product can be dispensed. Release coatings are designed to provide a surface to which the pressure sensitive adhesives have low affinity. The level of affinity depends on both the release coating and the adhesive. The release coatings primary function is to control adhesive adhesion to backing and to provide dispensability. 1 Surface wetability is probably the most important factor in release coating mechanisms ~. The rheology of the pressure-sensitive adhesive will allow it to flow, and become intimate with the releasecoated surface, forming higher bond strength and higher unwind force at a given speed 1. Because a pressure sensitive adhesive will flow under pressure, force applied during and after converting into finished rolls can increase or slow down this wet out or adhesive flow 1. Therefore, this property will influence the resultant HSUW on the finished roll of tape. High Speed Adhesion to Backing (HSAB) The first significant factor identified in this work was HSAB (High Speed Adhesion to Backing). Adhesion to backing is defined as the bond produced by contact between a pressure-sensitive adhesive and the tape backing when one piece is applied to the back of another piece of the same tape 5. Adhesion to backing is usually measured using the 180 peel adhesion test method, PSTC-14. PSTC-1 describes applying a ½ inch wide strip of tape to the release coated surface of its own backing, applying a 4.5 pound roll down pressure at 12 inches per minute followed by immediate removal at a speed of 12 inches per minute at a removal angle of 180. Adhesion to backing, in our experience, has shown poor correlation to finished roll HSUW properties. One of the disadvantages of the 180 peel configuration is that it does not reflect the angle at which the tape is removed from the finished roll. However, it is widely used because it is a simple test to perform using existing test fixtures. If one chooses to conduct the 90 peel adhesion test, which more closely resembles dispensing tape, a special test rig must be used to maintain the 90 angle at the test interface 6. Satas stated that the 90 peel test is preferred for theoretical analysis because it provides the best understood geometry and eliminates the buckling of flexible carriers sometimes seen in the 180 geometry 6. Another disadvantage is that PSTC-1 calls for a removal rate 'of 12 inches per minute, which does not reflect the rate at which tapes are typically dispensed from a roll. According to Johnson, no one unwinds tape at 12 inches per minute, and unwind either by hand or from a dispenser is typically around 50 to 60 feet per minute 7. It is also noted by Satas that in tape dispensing, the release values at high peel rates are important 1. The high-speed adhesion tester!9 used in this series of experiments eliminates both of the current test method disadvantages. The test is conducted at an angle of 90 and a rate of 600 inches per minute and removes a ½ inch strip of adhesive coated tape, which has been applied to the release-coated surface of its own backing. Reed switches control the start and stop points for data acquisition that is accomplished via a calibrated load-cell. It is recommended that four measurements be taken with an average HSAB reported. 162

3 Figure 1" High Speed Adhesion Tester Figure 2:90 Peel Adhesion Removal Angle 163

4 Figure 3" Close up of 90 High Speed Adhesion Test Geometry Development of Sample Preparation Method for HSAB Adhesives are viscoelastic materials. Both liquid and solid components are needed for an adhesive to function 7. The liquid component is what allows it toact as a pressure sensitive adhesive. It allows the adhesive to wet out onto the applied surface quickly to form a bond 7. The liquid component also allows the adhesive to deform under pressure. If the adhesive had only liquid components, the adhesive would never stop flowing, would not resist removal and would not fimction properly. Temperature, time, and pressure can affect the rate of adhesive wet out. For this test method, temperature could not be utilized as a variable, without advancing the adhesive crosslink density and altering the resulting adhesive properties. Time could not be utilized, because the objective was to develop a quick in-process test method. Therefore, this work focused on pressure to simulate the amount of adhesive wet out incurred in a self-wound roll of tape. The samples were tested using the high-speed adhesion tester at a removal angle of 90 and a rate of 600 inches per minute. The first experiment examined two sets of factors: pressure and time. Because the objective was to minimize test time, increasing pressure and reducing time were evaluated. 4 The standard configuration for adhesion to backing samples, according to PSTC-1, was used. The samples were prepared by applying a ½ inch wide strip of tape applied to the release coated backing of a 1 inch strip of the same tape. Standard steel panels as specified in PSTC, 1, Appendage B 4 were utilized. Once the samples were prepared, the panels were placed in a press 2 where the pressure exerted on the sample surface was varied from 8,000 to 16,000 pounds per square inch with a dwell time of onds to 1 minute. The HSAB (90 angle at 600 inches per minute) was measured immediately following the application of pressure. The experimental results were evaluated for standard deviation as a percent of the mean. (Refer to Table 7). The experimental results showed that standard deviation was minimized with a setting of 8,000 psi for onds. The new sample preparation results in 8% standard deviation from the mean while the standard PSTC-1 method results in 24.3% standard deviation from the mean. (Refer to Table 8). The results of this experiment demonstrated the lower variability of the new sample preparation method. 164

5 Roll Hardness Test Method The second significant factor identified in this work was Roll Hardness. The roll hardness test method was initially developed as a tool that could be used by converting department operators to adjust winding tension imparted by converting equipment. According to Smith, roll density or hardness is probably the most important factor in determining the difference between a good and bad roll s. The reduction of converting tension aids in minimizing adhesive edge ooze, which improves appearance and helps prevent rolls from sticking to each other and to the packaging used in single-roll wrap tape products. While it is relatively simple to reduce the tension to a minimum, this is not a practical solution since very low roll tension results in gapping, fluting and overall poor roll conditions. Typically, 1 or 2 barloads of tape will be cut, roll hardness measured and then slitter adjustments made. The pressure exerted (induced force) on the tape during the converting process directly influences the unwind force necessary to remove the tape from the roll as shown in the following table and figure. These data were collected for roll hardness measurements during an experiment that was conducted to examine the impact of converting tensions on a series of duct tape samples. Table 1" Data Calculated from the Duct Tape Roll Hardness Output Graphs vs. HSUW Sample Peak Force Area 1:2 Area 2:3.Area 1:3 ItSUW Very high tension High tension High tension Medium tension g g$ g$ g$ Medium Tension Low tension

6 Fol:ce (g) { O0 high tension Tension Tension,um Tension ~um Tension Tension 5000 il.o (see.) Figure 4: Roll Hardness Output- Scans Show Result of Different Converting Tensions Often, employees in converting departments determine roll hardness via the "thumb" test (resistance to deformation by thumb pressure on outside face of the roll of tape). They rely on experience to determine how soft or hard the roll is by pressing their thumb into the outer diameter of the finished roll of tape. As one can imagine, the determination in this test is subjective and highly dependent on the person conducting the test. There are several other less subjective test methods that can be adopted to measure roll hardness. They include, but are not limited to, ASTM D2240, Test Method for Rubber Property-Durometer Hardness1 ; ASTM C661, Indentation Hardness of Elastomeric- Type Sealants by Means of a Durometerll; ASTM D531, Rubber Property-Pusey and Jones IndentationS2; ASTM C836, High Solids Content, Cold Liquid-Applied Elastomeric Waterproofing Membrane for use with Separate Wearing Course Specificationl3; ASTM D1415, Rubber Property- International Hardness]4; TAPPI T834, Determination of Paperboard Roll Hardness 15. T834 is used as an indicator of quality problems such a loose wound rolls in paperboard production. 166

7 The new, non-destructive roll hardness test method was designed to mimic a standard Durometer test, while providing much more detailed and accurate information. This test utilizes a tensile tester 2~ with a special probe, which is pressed at a controlled speed and distance into the outside layers of a roll of tape. In the roll hardness test method, the roll of tape is supported on a specially designed mandrel. The mandrel is designed to support the roll and hold it securely in position immediately below the test probe. It also allows the roll to easily index to the next test position. Once the roll of tape is in position, a 7-mm diameter flat steel probe is pressed into the roll to a depth of 2 mm at a rate of 2.5 mm/sec. Four measurements spaced evenly at roughly 4 inches apart are taken around the outside diameter of the roll. The load-cell measures the force in both the compression and extension modes. The data acquisition rate can be adjusted. For thistest method, 400 points per second were utilized. Each individual test takes approximately 5 seconds to conduct, all 4 replicates on one roll can be completed in less than 1 minute, and an average response is generated. ~...~.,~.~..:~,..~,,~:.:,~.~.~,~.~... ~~,.,~.~,.,.~ i ii!iiiii!iiiiiiiiii!iii!ii;!i!i:iiii:iiii!!!n ' ii!i!i!!i!!! Figure 5: Roll Hardness Test Geometry 167

8 Figure 6: Close up of Probe and Roll Hardness Test Geometry The roll hardness test output consists of a stress vs. strain curve and calculations of area under the curve. The software associated with this test instrument allows for the parameters of the curve and its area to be measured, calculated and reported. For this application, the key parameters are peak force, area 1:2, gradient 1:2, area 2:3 and total area under the curve. Peak force is described as the maximum force recorded during the test. Area 1:2 is described as the area under the curve from force 0 until the peak or maximum force is achieved. Gradient 1:2 represents the slope of the force from force 0 to maximum force. Area 2:3 represents the area under the curve from the peak force to the return to minimum or 0 force. Area 1 "3 represents the entire area under the curve from the initial force 0 to the final force 0. The average curve generated from multiple samples can be archived and used as a fingerprint to match future samples both numerically and visually. Table 2: Typical Data Calculated from Roll Hardness Output Graphs Peak Force Area 1:2 Area 2:3 Area 1:3 g g$ g$ g$

9 {g) Force I ' '! I o o 0' '.8 1.o Time (s e c. ) Figure 7: Typical Roll Hardness Output- 4 Scans from One Roll (g) Force 7000"! ooo" =" I I I I ' Time (sec.) Figure 8: Roll Hardness Scans- 12 Average Roll Scans from One Production Run 169

10 , i' Experimental Parameters and Results Identified by cross web location, in-process checks from across the entire width of the coating web were sampled. A corresponding, numbered barload of rolls of finished tape was also sampled. Each in-process web sample was matched with its corresponding finished roll of tape. These sets of matched samples were obtained from several different production runs of the same product. The first product tested was a solvent-cast, natural rubber adhesive, general-purpose, crepe-paper, masking tape. Using the in-process web samples from documented points across the coating web, HSAB and total weight of the tape in pounds per ream (3000 square feet) were measured. HSAB and total weight were measured 4 times, and the average values reported. Utilizing the matched roll of finished tape, Roll Hardness was measured first followed by HSUW. The roll hardness measurement was performed 4 times with an averaged scan reported. The HSUW measurement consisted of dispensing approximately 10 yards of tape utilizing the HSUW tester at 50 feet per min and recording the average removal force indicated on the chart recorder output. Thirty-one individual sets of in-process web checks and matched rolls were tested and are reported in Table 9. Statistical analysis using commercial software ~8 was conducted on the data generated (Refer to Table 3 below). The R-squared adjusted correlation between the factors was determined and a stepwise regression analysis was conducted., Table 3: R-Sc uared Ad usted Correlation Between Factors..~ Solvent-cast HSAB! Weight Roll Hardness HSAB & HSAB, natural rubber, Crepe-paper, (Peak force, area 2:3) Weight Weight, Roll Hardness masking Production Run NA 84.3 Production Run Production Run Production Run ,,, 63.5,, Combined results The final regression equation resulted in an R-squared adjusted correlation of 77.5%. Therefore, for the solvent-cast natural rubber, crepe-paper masking tape studied the regression equation was: HSUW = Peak force area 2' HSAB weight In order to verify the reliability of this equation, in-process check samples from across the coating web and finished rolls from across the web were obtained from an additional production run of this product. The eleven individual results can be seen in Table 10 in the appendix. Table 4 shows the actual HSUW and the calculated value utilizing the regression equation above. 170

11 k Table 4: Actual vs. Calculated HSUW values for solvent-cast, natural rubber, crepe-paper production run #5 Actual HSUW Calculated HSUW Further testing was performed to determine if a similar relationship existed for different types of tapes. Twelve individual samples from a production run of a synthetic polymer adhesive, hot melt cast on a crepe-paper can be seen in Table 11. The final statistical analysis can be seen in Table 5. Synthetic polymer, hot melt cast, crepepaper Table 5: R-Squared Adjusted Correlation HSAB Weight Roll Hardness HSAB & (Peak force, Weight area 2:3) HSAB, Weight, Roll Hardness Production Run The final regression equation with the R-squared adjusted correlation of 69.3% for the synthetic polymer, hot melt cast, paper masking tape studied was: HSUW = Peak force area 2: HSAB weight Twelve individual samples for a synthetic polymer, solvent-cast adhesive on a crepe-paper can be seen in Table 12. The final statistical analysis can be seen in Table 6. Synthetic polymer, solvent-cast, crepe-paper Production Run 1 Table 6: R-Squared Adjusted Correlation HSAB Weight Roll Hardness HSAB & (Peak force, Weight area 2:3) HSAB, Weight, Roll Hardness 82.1 The final regression equation with the R-squared adjusted correlation of 82.1% for the synthetic polymer, solvent-cast, crepe paper tape studied was" HSUW = Peak force area 2" HSAB weight 171

12 Conclusions & Recommendations The new in-process test method, High Speed Adhesion to Backing (HSAB) utilizes a high-speed adhesion test machine 19 in conjunction with a novel sample preparation method. The adhesion to backing sample preparation, with applied pressure and time carefully controlled, simulates the forces incurred at the adhesive/backing interface when the PSA product is converted into a finished roll of tape. A regression equation utilizing HSAB, total weight, Roll Hardness peak force and Roll Hardness area 2:3 can be used to predict with 77,5% confidence the HSUW of the solvent-cast, natural-rubber, crepe-paper masking tape studied. In reviewing the regression equations generated for the three product types, it would appear that adhesive type and amount and backing type and weight have an influence on performance, which would indicate that a different regression equation would be applicable for different structures. Initial testing shows an R-squared adjusted correlation of 69.3% and an R-squared adjusted correlation of 82.1% for different tape structures. Further testing is recommended to enhance the statistical validity of the 2 alternate product types/structures tested. Further investigation into the effect of different pressures and time in sample preparation for HSAB is recommended. Each facility will need to identify the unique relationships between its converting controls in slitting and winding and the resultant roll hardness measurements, in order to take full advantage of the regression equation. Acknowledgements Brian Allen, Product Development Specialist, Intertape Polymer Group, for his assistance in conducting the testing. Tim Rummel, Paper Chemist, Intertape Polymer Group, for his assistance in developing the HSAB test method. Babu Hosangadi, Quality Engineer, Intertape Polymer Group, for generating the statistics reported in this paper. Literature Citations 1. Satas, Donatas (1989), "Release Coatings", Handbook of Pressure Sensitive Adhesive Technology, Van Nostrand Reinhold, NY, pp Unwind Force of Pressure-Sensitive Tapes, PSTC-8 and PSTC' 13, Test Methods for Pressure Sensitive Tapes, Pressure Sensitive Tape Council, Northbrook, IL Scholz, William F. Et al., (2000), "Ink-imprintable release coatings, and pressure sensitive adhesive constructions", U.S. Patent 6,074, Peel Adhesion for Single Coated Pressure-Sensitive Tapes 180 Angle, PSTC-1, Test Methods for Pressure Sensitive Tapes, Pressure Sensitive Tape Council, Northbrook, IL Test Methods for Pressure Sensitive Tapes, Pressure Sensitive Tape Council, Glenview, IL 6. Satas, Donatas (1989), "Peel", Handbook of Pressure Sensitive Adhesive Technology, Van Nostrand Reinhold, NY, pp Johnston, John (2000), "Notes and Observations on Simple Testing of Pressure Sensitive Tapes", Pressure Sensitive Tape Council, Northbrook, IL 8. Smith, Duane (April 1991), "The Art of Winding Good Rolls", Paper, Film & Foil Converter, Primedia Business Magazines & Media, Overland Park, KS 9. Johnston, John (2000), "Pressure Sensitive Adhesive Tapes", Pressure Sensitive Tape Council, Northbrook IL, pp Test Method for Rubber Property-Durometer Hardness, ASTM D2240, American Society of Testing and Materials, Philadelphia, PA 172.

13 11. Indentation Hardness of Elastomeric-Type Sealants by Means of a Durometer, ASTM C661, 1998, American Society of Testing and Materials, Philadelphia, PA 12. Rubber Property-Pusey and Jones, ASTM D531, 1989, American Society of Testing and Materials, Philadelphia, PA 13. High Solids Content, Cold Liquid-Applied Elastomeric Waterproofing Membrane for use with Separate Wearing Course Specification, ASTM C836, 1995, American Society of Testing and Materials, Philadelphia, PA 14. Rubber Property-International Hardness Rubber Hardness, ASTM D 1415, 1988, American Society of Testing and Materials, Philadelphia, PA 15. Determination of Paperboard Roll Hardness, T834, 1994, TAPPI, Technical Association of the Pulp and Paper Industry, Atlanta, GA 16. Peel Adhesion Of Pressure-Sensitive Tape at 180 Angle, ASTM , 1990, American Society of Testing and Materials, Philadelphia, PA 17. Satas, Donatas (1989), "Slitting", Handbook of Pressure Sensitive Adhesive Technology, Van Nostrand Reinhold, NY, pp "Minitab Statistical Software", Release 13.1, Minitab, Inc., State College, PA?,.. Equipment References 19. AR-1500 High Speed Adhesion Tester available from Chemlnstruments, Fairfield Ohio. 20. Carver Standard Benchtop Press, Model No. 3850, Carver, Inc, Wabash, IN 21. Texture Analyzer, Texture Technologies, 173

14 Test Results Table 7: Initial Experimental Results Sample Pressure Time HSUW In-Process HSAB Mean (psi) 8000 (seconds) 15 (oz/in) 25 (oz/0.5 in) Sample HSAB Std Dev. Std. Dev. % of Mean ,, Table 8: Experimental Results to Determine Variability Pressure Time HSUW In-Process HSAB Std. Dev. HSAB Std Dev. % of Mean 4.5 lbs. 12"/in (oz/in) Mean (oz/0.5 in) i lbs. 1 12"/in P lbs. J 12"/in bs. 7 12"/in lbs. 12"/in lbs. 12"/in lbs. 12"/in lbs. 12"/in! lbs. 12"/in lbs. 12"/in ; lbs. 12"/in i lbs. 12"/in i

15 !- Table 9" Roll Hardness, HSUW, HSAB, and Weight Results for Solvent-Cast, Natural Rubber, Crepe-Paper Masking Product Finished Roll Properties In-Process Check Properties Peak Force area 1: grad 1: area 2: area 1: HSUW 19.0 HSAB 5.85 Weight i , Regression Analysis: HSUW versus Peak force, area 2:3, HSAB, Weight The regression equation is HSUW = Peak force area 2: HSAB Weight Predictor Coef SE Coef T P 175

16 Constant Peak force area 2 : 3 HSAB Weight S = R-Sq = 80.4% R-Sq(adj) = 77.5% Analysis of Variance [ Source DF Regression 4 Residual Error 27 Total 31 SS MS F P Source DF Seq SS Peak force area 2: HSAB Weight Table 10: Roll Hardness, HSUW, HSAB, and Weight Results for Solvent-Cast, Natural Rubber, Crepe-Paper Masking Product - Run #5 k, i, ~ Finished Roll Properties Peak Force area 1:2 grad 1:2 area 2:3 area 1:3 HSUW ' ~ In-Process Check Properties HSAB Weight

17 ,, Table 11" Roll Hardness, HSUW, HSAB, and Weight Results for Synthetic Polymer Adhesive, Hot Melt Cast, Crepe-Paper Masking Tape Finished Roll Properties Peak Force area 1:2 grad 1"2 area 2:3 area 1:3 HSUW In-Process Check Properties HSAB Weight Regression Analysis: HSUW versus HSAB, Weight, Peak force, area 2:3 The regression equation is HSUW = HSAB Weight Peak force area 2:3 Predictor Coef SE Coef T P Constant HSAB Weight Peak force area 2: S = R-Sq = 80.5% R-Sq(adj) = 69.3% Analysis of Variance Source DF SS MS Regression Residual Error Total II F 7.22 P Source DF Seq SS HSAB Weight Peak force area 2: Unusual Observations Obs HSAB HSUW Fit SE Fit Residual I St Resid 2.16R R denotes an observation with a large standardized residual 177

18 i' Table 12: Roll Hardness, HSUW, HSAB, and Weight Results for Solvent-Cast Synthetic Polymer Adhesive on Crepe-Paper. Peak Force Finished Roll Properties area 1" grad 1: area 2: area 1: ' HSUW In-Process Check Properties HSAB Weight ,6, , Regression Analysis: HSUW versus HSAB, Peak force, area 2:3, Weight The regression equation is HSUW = HSAB Peak force area 2: Weight Predictor Coef SE Coef T P Constant HSAB Peak force area 2: Weight ,i / :/ S = R-Sq = 88.6% R-Sq(adj) = 82.1% Analysis of Variance Source DF Regression 4 Residual Error 7 Total II SS MS F P Source DF HSAB 1 Peak force 1 area 2 : 3 1 Weight 1 Seq SS