TAKING PRESSURE SENSITIVE ADHESIVES TO A DIFFERENT LEVEL OF PERFORMANCE

Transcription

1 TAKING PRESSURE SENSITIVE ADHESIVES TO A DIFFERENT LEVEL OF PERFORMANCE Robert Wigdorski, Senior Research & Development Scientist, Adhesives Research Inc., Glen Rock, PA Kevin McKinney, Senior Chemist, Adhesives Research Inc., Glen Rock, PA Deepak Hariharan, Ph.D., Group Leader, Adhesives Research Inc., Glen Rock, PA Liquid adhesives are routinely used for product assembly and process applications due to the strength these adhesives provide in the finished assemblies; however, on many occasions there is a desire to use pressure sensitive adhesives (PSAs) for these end uses due to their ease of use. This desire is often lost because the strength of the PSA systems does not match the end use requirements. Therefore, pressure sensitive adhesive compositions that would easily convert from the pressure sensitive state to high strength, approaching structural adhesion for permanent bonding of parts are of interest. Using these materials one could take advantage of all the benefits of pressure sensitive technology and replace liquid adhesives for many applications. It would also be of benefit to have pressure sensitive adhesives which are strong enough in their pressure sensitive state to hold parts together then easily convert to a nontacky ultra removable state to separate the parts once processing of the parts is complete. The work being presented in this paper describes some of our efforts to produce adhesives of this nature and it is hoped that the information presented will serve to foster ideas to advance the development of additional convertible pressure sensitive adhesives in our industry. The work is centered on information covered by United States Patent # 6,686,425 as well as extensions of this effort. Basic Model for the Development Effort It was our desire to produce convertible pressure sensitive adhesives for several different end uses. This being the case, we felt the most prudent way to accomplish this was to treat these materials like we were modifying standard base polymers with tackifiers and plasticizers to modify the Tg and modulus of the base polymer to produce the initial pressure sensitive properties. The base polymer needed to be compatible with many modifiers which would be blended with the film former to develop pressure sensitivity. These systems would then be converted by some means, at a point in the future, to produce the desired end result. Further, it was felt that the base polymer itself should not only have the capability to react with the modifiers when necessary but also have the ability to react with itself. This would be beneficial should modifiers be used that did not have the functionality to chemically bond to the base polymer during the curing phase of the adhesive. Additionally, we wanted the base polymer to require very high levels of modification to produce pressure sensitivity, at least 200 parts of modifier to 100 parts of base polymer, to enable us to take advantage of the capabilities of the modifying ingredients during the curing stage. The methods we wanted to be able to use to trigger cure of the adhesives to convert them from their pressure sensitive state to a different performance state were thermal and/or photo in nature; therefore, the base polymer needed to be designed with this in mind. Our work centered on the development of High Tg solution Polymers with polarity such that they would be compatible with many standard reactive materials such as epoxies, acrylate monomers or oligomers, epoxy acrylates, unsaturated polyesters etc., which could be used as modifiers. Several solution 11

2 polymers were made and evaluated leading us to choose a glassy acrylate with reactive functionality built into the backbone of the base polymer as our film former which is characterized as follows. Polymer Properties Molecular Weight 200,000 Initial Tg First Pass* 54 o C Figure1 Self reaction onset temperature 123 o C Second Pass Tg * 80 o C (Same sample re-run after cure in the first pass) Figure 2 Film Properties Hard and Brittle * DSC range -50 o C to 200C Figure 1. First Pass DSC Figure 2. Second pass DSC (Same sample from Figure 1 re-run after cure in the first pass) 12

3 The data generated for the polymer indicated that we achieved our goal of producing a base film forming polymer with molecular weight and Tg sufficiently high to require substantial modification to produce pressure sensitive properties. The final chemical composition of the polymer was such that it had the polarity required to be compatible with many different modifiers. Further, the base polymer had the ability to react with itself to produce a reacted film with higher Tg then the starting material. Convertible Pressure Sensitive Adhesive Development (2 stage adhesives) Several convertible pressure sensitive adhesives were formulated using this base polymer through modification with all of the reactive materials we were targeting for use, some of which are presented in this paper. In all cases where modification of the base polymer with reactive materials resulted in the formation of pressure sensitive adhesives, the resultant DSC s showed the Tg of the formulated systems to be in the pressure sensitive range. Additionally, the systems display only one Tg indicating compatibility. Further, the level of modifiers needed to produce pressure sensitive properties was greater than or equal to 200 parts per 100 parts of base polymer on a dry basis in all cases, thus achieving our desire to need high levels of modification to produce pressure sensitive properties. The DSC depicted in Figure 3 is an example of one of the systems, designed to be thermally cured, which shows the Tg of the base polymer being reduced from 54 o C to negative (-)4.7 o C. Samples prepared for determination of Tg and potential cure were made in the following manner. The adhesives prepared were coated on silicone coated release liner to achieve a 75 micron thickness when dry. Drying was performed at room temperature to eliminate the possibility of premature cure. Dry state was established when stable weight of the coated adhesive was achieved over three measurements at 1 hour intervals. DSC was used to determine Tg and onset temperature for cure. Figure 3. Example showing Tg shift after base polymer is modified to form a pressure sensitive adhesive designed to be thermally cured 13

4 Specific Modifications and Performance Properties Example of a simple epoxy modified system with latent amine hardener Parts Based on Solids Base polymer 100 Epoxy modifier 250 Latent amine hardener 100% of theoretical amount needed to cure the epoxy functionality Cure Profile of Dried Adhesive The DSC for this system (shown in Figure 3 above) shows the adhesive should cure at temperatures of 100 o C and higher and might cure at temperatures below this. This is demonstrated in the cure profile depicted in Figure 4. Percent Cure vs. Time and Temperature was calculated using DSC to determine residual reactivity after exposure to temperature for different lengths of time. Cure speeds are accelerated at temperatures above 100 o C as one would expect and cure at 80 o C is possible providing one is afforded the time necessary to do so. % Cure vs Time at Temperature % C ure min 10 min 20 min 30 min 40 min 50 mil 60 min 90 min 120 min 150 min Time minutes Figure 4 % cure at 80C % cure at 100 C 14

5 The before and after cure performance properties of this system were characterized using standard test methods incorporating polyester and aluminum carriers and stainless steel panels. All testing performed was carried out using a 75 micron dry adhesive thickness. The polyester film was direct coated while the aluminum was transfer coated with adhesive from a silicone coated release liner. Drying was accomplished at temperatures below the established thermal activation temperature of the adhesive. The results obtained are found in Table 1. Cure was accomplished at 120C for 1 hour. Table 1 Test Method Before Cure Result Test Method After Cure Result Peel adhesion Polyester to 4 pli Peel adhesion Polyester to stainless steel at 12 inches / Polyester Rupture Stainless Steel at 12 inches / minute after 5 minute residence at RT 17.8 N/25.4 mm minute at RT Peel adhesion Aluminum to 5.5 pli Peel adhesion Aluminum to Aluminum at 12 inches per Aluminum Rupture Aluminum at 12 inches per minute 5 minute residence at RT 24.5 N/25.4 mm minute at RT Shear Adhesion ¼ sq inch test area 500 gram load polyester to stainless steel 60 minutes to failure cohesive split Dynamic shear ¼ square inch test area stainless steel to stainless steel at RT >2000 psi >1379 N/sq cm Exceeds capabilities of the load cell being used RT = 21 o C, 50% Relative Humidity The performance data generated suggests the convertible pressure sensitive adhesive has before cure peel and shear which should be sufficient to hold parts in place prior to cure and after cure performance comparable to what one would expect from a 2 part epoxy adhesive designed for high performance applications. The following experiment was conducted to demonstrate the shear properties exhibited by this adhesive were sufficient to hold parts in place during cure. The wet adhesive was coated on silicone release liner to achieve a 75 micron dry film thickness and dried at temperatures below the thermal activation temperature of the adhesive. A second silicone release liner with lower release properties was laminated to the adhesive surface to form a transfer adhesive structure. Specimens were cut from the transfer adhesive film to ½ inch x ½ inch dimensions. The adhesive was transferred to a 2 inch x 3 inch glass microscope slide after removal of the easy release liner. Placement was as close to center as possible. The tight release liner was then removed and a second glass slide was placed on top of the adhesive coated slide such that all edges were aligned then pressed with light finger pressure. To determine if the adhesive would distort under mild load or fail when handled in a manner simulating a light parts assembly operation, the sandwich was lifted by 15

6 grasping the top slide only. The sample was inverted, held vertical and shaken by hand. No bond line failure or adhesive distortion was observed. The perimeter of the adhesive was traced onto the top and bottom glass slides using a fine point permanent marker to identify position and dimension. The sample was then cured for 1 hour at 120 o C without any support to keep the slides aligned. Upon removal from the oven, no adhesive distortion or slide movement was observed. This test shows convertible pressure sensitive adhesives with the shear and peel properties demonstrated should be capable of parts assembly operations in die cut form without the need for supporting the parts during transit or curing operations. Several epoxy modified pressure sensitive adhesives were prepared to determine if sufficient formulation latitude was available to permit the design of after cure performance to meet various targets. This work showed that systems could be formulated to meet many targets we had in mind and followed the trends one would expect when formulating conventional curing epoxy adhesive systems. An example of this was the desire to produce an adhesive system for multilayer plastic sheet laminations requiring after cure impact resistance at -25C, stiffness such that the laminated panels would not flex or distort at temperatures up to 65C and the ability of the laminated blocks to be sawed, face sanded and polished resisting the fluids used for these operations. Reactive modifiers were chosen targeting the end properties we wanted to achieve and blended with the base polymer to produce a pressure sensitive adhesive system. The ratio of modifier to base polymer was once again 250 parts of modifier; 100 parts of dry base polymer. The adhesive was cast onto silicone coated release liner to achieve a dry film thickness of 75 microns and dried below the thermal activation temperature. After drying the adhesive was laminated to 250 micron Poly Methyl Methacrylate (PMMA) sheets. The individual coated sheets were then laminated face to back to produce a 50 layer laminated block and cured for 150 minutes at 80 o C. In this case the laminated stack-up was supported top and bottom with 1/16 inch thick steel panels and held in place with spring clamps to minimize distortion of the PMMA sheets during cure. After cure the blocks were sawed, sanded, and polished for testing surviving the rigors of these operations. The pre-cure and post cure performance properties of the adhesive system is as follows (Ref Table 2). Table 2 Test Performed Before Cure Result Peel adhesion to stainless steel at 12 inches/minute at 21C 50% Relative Humidity 4 pli 17.8 N/25.4mm Shear adhesion ¼ sq inch to stainless steel 500 gram load at 21C 50% Relative Humidity 220 minutes to failure cohesive split Test Performed Cold Shock at negative (-)25C (6 foot drop test at temperature, 50 layer lamination) Flex resistance at room temperature 50 layer lamination Flex resistance at 150F (65.6C) 50 layer lamination Resistance to sawing 50 layer lamination Resistance to cutting fluid 50 layer lamination Face sanding across the layers Resistance to polishing fluids during polishing operation After Cure Result Pass - No bond failure Pass- Rigid lamination with no distortion Pass- Rigid lamination with no distortion Pass No bond line failure during sawing Pass No bond line failure after 24 hour immersion in cutting fluid Pass No bond line failure or adhesive chipping. PMMA melting during sanding evident Pass No bond failure, no adhesive smearing or adhesive cracking during polishing operation Dynamic shear Stainless Steel to Stainless Steel 1400 psi 965 N/sq cm Cold shock and resistance to Flexing were performed on test specimens 0.5 inches wide x 6 inches long 16

7 The example above shows convertible adhesives can be designed to meet the end use requirements one is targeting by selecting the materials being used as modifiers to fit the desired end properties of the cured adhesive. This is equivalent to the approach one would take to modify a conventional base polymer to produce a standard pressure sensitive adhesive system. Using our target of being able to formulate a wide variety of convertible pressure sensitive adhesives through modification of the base polymer with high levels of reactive modifiers selected for specific applications many systems were designed. These incorporated acrylate functional oligomers and unsaturated polyesters in many cases. Modifier selection was based on the capabilities of these materials to function as if they were being used without the base polymer to carry them. To demonstrate this, three systems will be discussed all incorporating 250 parts of modifier per 100 parts of base polymer on a dry basis. Two of the adhesives, a low and medium tack PSA are made with acrylate oligomers as the main modifying ingredients, and a high tack system was made with unsaturated polyester as the main modifier. Modifiers for the low tack system were chosen to have good adhesion to metals as well as good resistance to heat and humidity. Modifiers for the medium tack system were chosen for low residuals after cure, the ability to produce flexible films with good adhesion to metals and some plastics and low temperature performance. The modifiers for the high tack system were chosen for excellent bonds to steel as well as heat resistance and water resistance. The adhesives were evaluated for performance using various time/temperature conditions to establish trends and determine the best conditions for curing each adhesive. The systems were then rated for functionality on different substrates and evaluated for environmental resistance to determine if our approach was sound. Dynamic shear, used to compare performance, was performed on 1 square inch bond areas at a separation rate of 0.2 inch per minute using tensile testing equipment equipped with a load cell with a maximum capacity of 1000 pounds. Results of the Studies The three adhesive systems were evaluated for dynamic shear Stainless Steel to Stainless Steel, after cure at 90 o C, 120 o C and 150 o C for 10, 30 and 60 minutes. The results of the study showed all of the adhesives cured effectively at 120 o C. Increasing time at temperature and temperature alone, in general, improved dynamic shear. However; excessive time at 150 o C was detrimental to performance. Optimum bonds to stainless steel were obtained using the following cure conditions. (Ref Table 3) Table 3 Adhesive Cure Condition Dynamic Shear Strength Stainless Steel to Stainless Steel psi Dynamic Shear Strength Stainless Steel to Stainless Steel n/sq.cm High Tack PSA 120 o C for 30 minutes 1000 psi 690 N/sq. cm Medium Tack PSA 120 o C for 60 minutes 850 psi 586 N/sq. cm Low Tack PSA 120 o C for 30 minutes 790 psi 545 N/sq. cm 17

8 Performance on Various Substrates Using the 120 o C cure temperature established for the adhesive systems a study was performed bonding aluminum (Al), polycarbonate (PC) and fiber reinforced plastic (FRP) varying time at temperature to determine adhesion to the various substrates. Cure times of 10, 30 and 60 minutes were used once again for this study and the results obtained were compared to the results obtained for optimum bond to stainless steel. The results obtained, depicted in Figure 5, modeled what we expected based on the properties of the modifiers we chose and are summarized as follows: Adhesion to metals in general was best with the acrylate oligomers chosen for the low tack system. The flexible acrylate oligomer modifiers used for the medium tack adhesive produced a good balance in performance for metals and some plastics; however, adhesion to polycarbonate was not as good as expected. Adhesion to stainless steel was best with the polyester system. Dynamic Shear On Various Substrates shear strength psi High Tack Medium Tack Low Tack Stainless Steel control 10 Al 30 Al 60 Al 10 PC 30 PC 60 PC 10 FRP 30 FRP 60 FRP Cure time at 120C minutes and substrate Figure 5 The study was continued exposing the cured adhesives to various environmental conditions to determine if the performance would once again follow what we expected based on the potential performance of the reactive modifiers. Five sets of bonded specimens were prepared using stainless steel as the substrate and the optimum cure conditions found for the three adhesives for purposes of this study. Dynamic shears were performed at negative (-)23 o C, 37 o C, and 100 o C after equilibrating at temperature, after 18

9 seven (7) day exposure to 37 o C / 90% Relative Humidity and after seven (7) days submersion in water at room temperature. The results obtained, depicted in Figure 6, modeled what we expected based on the properties of the modifiers we chose and are summarized as follows: The high tack and low tack adhesives produced the best heat resistance. The low tack system produced the best heat and humidity resistance. The flexible medium tack system produced the best low temperature performance. The high tack system produced the best water resistance. Environmental Resistance Dynamic Shear psi room temprature Negative (-) 23.3C 37.8C 100C 37C / 90% RH Water Submersion High Tack Medium Tack Low Tack Figure 6 To determine if a cured system with low residual monomers was obtained with the medium tack system, cured samples were analyzed via GC/Mass Spec using Dynamic Head Space Techniques with sample collection at 85C for 3 hours. The results obtained for total out-gassing of micro grams/cm 2 provided evidence that this adhesive met our objective by design. Summary Overall, the results obtained from the work presented demonstrates that the model we wished to follow; modification of a designed base polymer with high levels of a wide variety of reactive modifiers, selected for use based on the capabilities of the modifiers in their cured state, was an effective means of producing convertible pressure sensitive adhesives. Using this model many convertible adhesive systems 19

10 were made achieving our goals. The systems include materials like those presented, being one adhesive layer cured to form high strength bonds between two substrates as well as bi-layer convertible adhesive systems designed with specific adhesion to the different substrates to which they were designed to bond, that would knit together through reaction at the adhesive/adhesive interface during the cure cycle as well as systems that would convert from the pressure sensitive state to a non-tacky ultra removable state after cure. Thermal and photo-cure mechanisms were incorporated effectively completing the targets we had. Conclusions Pressure sensitive adhesives that convert to provide extremely strong bonds as potential replacements for liquid adhesives can be made. Further, adhesives that convert from an aggressive pressure sensitive state to non-tacky ultra removable adhesives can also be produced. Our approach is only one of many, based on literature searches, that can be used to formulate these materials. Acknowledgments The work presented would not have been possible without the efforts of the following Adhesives Research Inc. associates as various stages of the project: Allyson Daron Donna Tompkins Michael Koons Denise Blue Michael Passariello Lindsey Markey 20

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