E-BEAM DRUM TRANSFER: PROCESS TO ELECTRON IRRADIATE A PSA IN THE ABSENCE OF A BACKING.

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Juniper Freeman
6 years ago
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1 E-BEAM DRUM TRANSFER: PROCESS TO ELECTRON IRRADIATE A IN THE ABSENCE OF A BACKING. James DiZio, Senior Research Specialist, 3M, St. Paul, MN Douglas Weiss, Senior Research Specialist, 3M, St. Paul, MN Extruded pressure sensitive adhesives (s) are sometimes post-crosslinked via electron radiation. Crosslinking the by electron radiation increases polymer molecular weight and thereby enhances shear strength. Unfortunately, a problem is encountered when electron irradiating a on a backing; the electrons not only interact with the adhesive but also penetrate into the backing, often degrading the backing. Ionizing (electron) radiation degrades most organic tape backings such as cellulose, PVC, silicone, Teflon, or polypropylene. In the quest to develop a method that would completely eliminate degradation to the backing, we created a process entitled electron beam drum transfer. Using the beam drum transfer method, the in a tape construction can be crosslinked while completely avoiding radiation exposure and damage to the tape s backing. An added benefit to this method is that any electron beam voltage and dose can be chosen to elicit a desired level of crosslinking and gradient through the, without harming the backing. We define electron beam drum transfer as a sequence of events that includes applying a hot melt to a rotating beam drum, subjecting the adhesive to electron radiation, and then stripping the adhesive off the drum by preferential attachment to an incoming, non irradiated backing. To start the process, the can be directly applied to the beam drum via an extrusion die, or it can be transferred to the drum from a previously coated, incoming web. Figures 1A and 1B depict two possible scenarios for the general process. 1A. 1B. Release Belt Structured Die Finished tape Figure 1A. Extruded is coated directly on the rotating electron beam drum. Figure 1B. Scenario where the is first coated onto a moving belt and then transferred to the drum. The key to this continuous process is the method by which the is manipulated within the electron beam compartment. The drum surface must be made of metal since any organic release coatings will not withstand the high radiation environment within the electron beam. We have accomplished critical transfer steps via the "structured drum method" and the hot drum method." In the structured drum method, we control the surface area contact of the on a structured metal drum. This allows for adequate, sustained manipulation of a continuous web, ultimately making usable tape constructions in the process. For the hot drum method, temperature is used to create a differential adhesion

2 between the hot drum and a cooler backing. Controlling both the contact area and the temperature of the drum gives the maximum flexibility. Hot Method The force needed to peel a from a substrate is dependent on the temperature of the /substrate interface. This peel force/temperature relationship can be used to transfer a from a hot surface to a cooler surface. Studies were performed to determine the viability of a process employing this transfer method. Bench-top transfer experiments, performed to determine process temperature windows, included applying to a metal surface, heating the metal to a specified temperature, electron irradiating the adhesive, and then (while keeping the metal hot) transferring the adhesive to an incoming cool backing attached to a roller. The transfer step is depicted in Figure 2. Adhesive Stainless Steel Hot Plate Wrapped with backing Figure 2. Small scale transfer process used to develop hot drum method. This procedure afforded consistent and complete transfer of a. Both polyester and paper backings were tested, each showing positive results. Table 1 shows some of the temperature levels at which a rubber based adhesive was completely transferred to a backing. Table 1. Temperature profiles needed to afford transfer. Metal temp. ( F) required to afford transfer Temp. F Paper backing Polyester backing Irradiation or 7 MRad or 7 MRad or 7 MRad Unirradiated High metal temperatures promoted better transfer because lower tack is apparent at the hot metal/adhesive interface. It was never necessary to cool the backing at high metal temperatures because the difference in tack between the metal/adhesive and adhesive/backing interfaces was already sufficient. An important aspect of the process was to transfer quickly ( 50ft/min). If the backing was in contact with the adhesive/metal plate assembly for too long, the backing temperature rose and tack differential was lost. This is probably the most important factor to keep in mind for this type of transfer process.

3 We also performed larger scale experiments on the electron beam. For patch transfer, a 4"x6" section of adhesive was coated onto the drum, irradiated, and then transferred to a paper or polyester backing. A depiction of the patch transfer process is shown in Figure F Cooling chamber single screw extruder Heated hose Heated Die Sequence of Events 1. Coat adhesive onto drum. 2. Irradiate 3. Engage nip roll 4. Adhesive is transferred. Figure 3. Larger scale, patch transfer process In the Energy Sciences Electrocurtain apparatus, a vertical die position was chosen. The drum was Ni and had a smooth finish. Patches of were successfully transferred at drum and backing temperatures indicated in the small scale experiments. Structured drum method Limiting the surface area contact between an adhesive and substrate can also lower the force needed to peel a from a substrate. This peel force/surface area relationship can be used to transfer a from one surface to another. Studies showed excellent viability for this type of transfer method. Bench-top transfer experiments were performed to determine optimum structure dimensions. A film was applied to a structured plate by first obtaining the adhesive on release liner, pressing the coated adhesive onto a structured plate (adhesive side toward plate), and then peeling the release liner off the adhesive. At this point the assembly can be irradiated. A roller, wrapped with the backing of choice, is then rolled over the adhesive, transferring the adhesive to the backing. The transfer step is similar to that depicted previously in Figure 2, other than no heat is involved and the metal surface is structured. The structured surfaces ranged from virtually smooth to macroscopic patterns. Some of the best patterns were those comprising a multiplicity of structured recesses. The geometry of these recessed patterns was such that the film lay on top of the structure (on the land area between recesses) without

4 substantial sagging. Many of the patterns were 3 or 4 sided at the base. A section of a 4 sided pattern is depicted in Figure 4. Figure 4. Section of an inverted square base pyramid structure. For application of the adhesive to the structure, the adhesive was laminated onto the structure using a roller weight of 10 or 40 Lbs. The surface area contact of a 1 wide adhesive, pressed onto the structured metal, was measured along with the average contact width at any one surface. Table 2 details the contact areas, contact widths, and dimensions of a few adhesive/structured substrate combinations. Table 2. Percent adhesive contact vs. pattern style Roller Force Adhesive Contact Width Pattern Dimensions (mil) Pattern (Lb.) % Contact (mil) Base Depth A. Trigonal , , 67.7, , 3.9 B. Square , C. Square , D. Trigonal , , 16.6, , 1.5 E. Hexagonal >50 No pattern (flat) Flat surface A successful process requires an adhesive/structure contact area such that transfer to and from the structure is possible. Table 3 shows data for one particular combination of, release liner, and backing. The table details the ease by which a rubber based was applied and then removed from a structured substrate. percent contact area is also reported.

5 Table 3. Transfer-ability vs. % contact Transferability Pattern % Contact Releasto-Structure Structurto- A. Trigonal B. Square C. Square D. Trigonal E. Hexagonal No pattern (flat) = Completely and cleanly transferred 2= Some incomplete or flawed transfer but acceptable 3= Failure (worse at speeds >50 fpm) The data show that there is an optimum range of percent contact area for which adhesive transfer, both to and from the structured surface, is successful. High contact areas facilitate transfer to the structure, whereas low contact areas facilitate transfer from the structure. In this case, the optimum percent contact range is from 17% to 23%, although contact areas outside this range might also be acceptable. Contact line width and spacing is also important in determining a successful pattern. Notice that the hexagon pattern is not optimal for either transfer to and from the structured pattern. Even though the percent contact area is relatively low (13%), the contact line width is high (9.5 mils). Transfer from the structure is inhibited because the unsupported area (130 mils spacing between contacts) sags and therefore does not mate with the backing effectively. Also, the supported contact areas are large (9.5 mil line width). All of these patterns perform better than a flat surface from which the adhesive completely fails to transfer to the backing at room temperature. It should be noted that faster speeds allow less time for the adhesive to wet out onto the affected surfaces and therefore there will be a speed (residence contact time) at which transfer is not possible for any adhesive [This last sentence confuses me. WHY will it not transfer? Going too fast to stick to the surface? Are we talking about flat surfaces or patterned surfaces? Any idea how fast that might be we say we did it at 200 fpm and could have gone higher]. To demonstrate a larger scale continuous process, the set-up shown in Figure 5 was designed. Modifications to the beam included adding extra rollers to the base cabinet, along with fitting a structured metal sleeve over the drum. This process afforded reliable, continuous production of tape at 200 feet/min, the maximum speed of the web drive. All indications are that higher speeds would certainly have been possible if allowed by the drive.

6 Release belt Structured Finished Tape Figure 5. Larger scale, continuous transfer process. To demonstrate the effect of dose and voltage on backing degradation, a series of adhesive tape examples were prepared via this continuous transfer process. Although all the samples were subjected to the transfer process, some of these samples were left non-irradiated (for comparison purposes). The non-irradiated samples were later irradiated in a conventional manner where the adhesive constructions were passed through the electron beam (electrons passing from adhesive to backing) in a common web line fashion. This allowed for a direct comparison of tape that is irradiated via the drum transfer method versus tape irradiated by a conventional method. The rubber based adhesive was measured to be 2.3 mil thick on the backing. Degradation of the paper backing was measured by the reduction in its flexibility as indicated by the number of cycles to failure in the MIT Folding Endurance Test (on a 0.5 inch wide strip of tape). Table 4 shows the comparison of conventionally irradiated tape versus drum transfer processed tape. Table 4. MIT flex test, cycles to failure for tape strips. Voltage Dose Irradiation process (KeV) (MRads) Cycles to failure Conventional Transfer

7 The more degraded a backing, the lower the number of flex-cycles required before failure occurs. The conventionally irradiated samples show backing degradation due to increasing voltage and dose conditions. Conversely, samples irradiated during the drum transfer process show no sign of backing degradation, even at the highest voltage and dose setting for this test. This is shown by comparing the cycles-to-failure for the drum transfer processed samples to the control samples, which received no radiation. Since the backing receives no radiation in the drum transfer process and therefore degradation is not an issue, it is possible to vary the penetration voltage to obtain interesting dose gradients in the adhesive. Modeling the energy deposition through the use of radiation simulation codes correctly detail the gradients. To experimentally demonstrate the dose gradients that can be obtained using this invention, a series of adhesive tape examples were prepared via the continuous transfer process. The adhesive in these examples was measured to be 1.8 mil thick on the backing. Dosimeters were placed on each side of the adhesive to be irradiated in a staggered arrangement. Voltages were chosen that gave varying dose gradients (and therefore crosslink gradients) though the adhesive. The data is shown in Table 5. Table 5. Dose "gradients" through adhesive vs. accelerating voltage. Accelerating Adhesive Side Voltage (KeV) Normalized Dose (MRads) Top Bottom Top Bottom Top Bottom The top side of the adhesive designates the front surface and the bottom side designates interface with the backing. The dosimeters themselves have nearly the same thickness as the adhesive (1.7 mil) and therefore it was not possible to define the gradient but a general trend can be observed. At an accelerating voltage of 125 KeV, the dosimeters indicate a dose gradient through the adhesive such that less energy is deposited on the surface than at the interface. At 300 KeV, the opposite effect is observed. At 200 KeV, the dose is uniform. In all cases, the tape backing did not receive any dose. Concluding Remarks We have shown that transfer using the structured drum method and hot drum method can be an effective way to crosslink an adhesive by electron beam without damaging the backing. This method allows all possible variations in surface dose and dose gradients without affecting the tape backing. We have shown continuous transfer using rubber based, Kraton, and acrylic adhesives. Our maximum transfer speed was 200 fpm, the maximum speed of the web drive. Indications point to the fact that the process would be effective at higher speeds.

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