Process for curing elastomer-based pressure sensitive adhesive tapes with low voltage electron beam radiation

Abstract: A pressure sensitive tape comprising an electron beam radiation crosslinkable pressure sensitive adhesive on a radiation-degradable backing, the adhesive having been crosslinked by exposure to electron beam radiation in a fashion such that the dosage reduction through the adhesive layer is no greater than about 40%; the average dosage which is received by the backing is less than about 80% of that received by the adhesive, and a sufficient dose of beam energy has been provided to produce a gel content in the adhesive of at least about 20% by weight of the elastomer.

Description: Technical Field The present invention relates to a method for preparing a pressure sensitive adhesive tape which is manufactured by crosslinking elastomer-based adhesive compositions contained on a radiation degradable substrate using controlled low voltage electron beam radiation. Surprisingly, a balance of desired adhesive and backing properties are obtained, a characteristic hereto unavailable by previously taught electron beam irradiation techniques. Background Art The use of electron beam radiation, i.e., a beam of electrons accelerated through an electric potential, to cure or crosslink polymers used in pressure sensitive adhesives is known in the art. For example, U.S. Patent No. 2,956,904 describes the use of high energy electron beam radiation to crosslink or cure pressure sensitive adhesives to improve the shear properties thereof. The benefits claimed for radiation cure versus conventional sulfur cure include higher shear performance without loss of tack, as well as the absence of staining on lightly colored painted surfaces, typically caused by the presence of sulfur compounds in the adhesive. The radiation disclosed therein was the discharge of high energy electrons from a cathode ray tube powered by a resonant transformer operated at 1000 kV. There is no disclosure therein of using low voltage radiation. It is also known that electron beam radiation applied at doses necessary to crosslink pressure sensitive adhesives to their desired level of performance (0.5 to 10.0 MRad) can damage many of the backings commonly used to form pressure sensitive tapes, such as cellulose-based backings (e.g., paper and cotton cloth), polypropylene film, polyvinyl chloride film, and polytetrafluoroethylene film. U.S. Patent No. 4,246,297 discloses the use of a critically adjusted electron beam taught to be capable of curing coatings on a radiation-sensitive substrate. The patent describes the effective beam energy as being in the range of 50 to 300 keV for curing pressure sensitive adhesives on temperature sensitive webs such as paper, plastic and the like, with doses of from 0.5 to 5 MRad. The beam energy of the reference is taught to be controlled through a combination of the accelerating potential and the energy loss of the beam as it travels through the various layers of material prior to reaching the coating to be cured. Such layers include the foil window separating the high vacuum chamber of the electron gun and the atmosphere, a layer of nitrogen gas between the foil and the sample to be cured, and any covering liner over the coating to be cured. This reference discloses a process for assertedly achieving substantially uniform electron beam cure of a coating while ensuring minimal irradiation of the substrate, e.g., a tape backing. However, the examples described therein disclose a ratio of the irradiation dose at the front of the coating to be cured to that at the back of the coating, i.e., at the coating/backing interface, of 5:1. We have determined that such a degree of dose variation through the thickness of the adhesive is unacceptable for pressure sensitive adhesive coatings because upon removal of the tape from a substrate, cohesive failure of the adhesive results and adhesive residue is undesirably left on the substrate. Pressure sensitive adhesives require a fairly uniform cure throughout the thickness of the adhesive. Summary of the Invention In accordance with the invention there is provided a method for making a pressure sensitive adhesive tape comprising: (1) applying at least one electron beam radiation crosslinkable elastomer-based pressure sensitive adhesive onto a radiation degradable backing, (2) exposing said adhesive to a beam of electrons which has been accelerated through a potential voltage in the range of between 75 and 175 kilovolts, said voltage utilized being dependent upon, (a) the thickness of the adhesive layer, (b) the thickness of said backing and, (c) the degradation of the electron beam prior to impingement on said adhesive layer and selected such that the reduction in dose from the front of said adhesive layer to the back of the adhesive layer is no greater than 40% and the average dose received by said backing is less than 80% of the average dosage received by said adhesive, said pressure sensitive adhesive layer further having been crosslinked with a sufficient dose of beam energy to produce a gel content in said adhesive of at least 20% by weight of said elastomer, said dose to the pressure sensitive adhesive being less than about 12 megarads. In this manner, a balance of properties is found in the tape not heretofore available in electron beam radiation cured systems. The balance of properties relates to the adhesive characteristics balanced against the potential degradation of the backing by an exposure to electron beam radiation. We have found that a narrow window of potential is available which will achieve this balance and thus provide a product exhibiting the desired characteristics. Detailed Description The pressure sensitive adhesive tape made according to the method of the present invention comprises a radiation degradable backing coated with an elastomer-based pressure sensitive adhesive. A release coating may be optionally coated on the opposite side or a separate release liner can be used. The adhesive typically comprises one or more electron beam crosslinkable elastomers and one or more tackifying resins, optionally containing fillers and antioxidants. Examples of unsaturated elastomers which can be utilized include natural rubber, synthetic polyisoprene, polybutadiene, styrene/butadiene rubber (SBR), and block copolymers where one of the blocks consists of one of the above elastomers, such as styrene-isoprene-styrene triblock or starblock polymers, as well as acrylic, and silicone elastomers. Tackifying resins include those known in the art of pressure sensitive adhesive manufacture, examples including polyterpenes, synthetic hydrocarbon resins, rosins, and rosin derivatives such as rosin esters. Antioxidants, similarly known in the art, include, as examples, phenolic types, such as 2,4-di(tert-amyl)hydroquinone and dithiocarbamate types such as zinc dithiobuytlcarbamate. Many fillers can be used, examples including zinc oxide, silica, carbon black and calcium carbonate. The adhesive can be applied to the backing by any of the commonly known means, examples including solvent casting, latex casting, calendering the adhesive into a film followed by lamination to the backing, and extrusion or hot melt coating of the adhesive onto the backing. Thicknesses of adhesive useful for the tapes of the present invention comprise those commonly used in the industry and are from about 15 µm to about 75 µm. Backings for the tapes of the invention are those which are radiation degradable, including paper, saturated papers, vinyl films, cellulose-based woven and non-woven fabrics, polypropylene based non-woven fabrics, polypropylene films, and polytetrafluoroethylene films. The backside, i.e., the non-adhesive containing side, of the backing may be precoated with a release material to facilitate easy unwind of a completed roll of tape. Furthermore, the side of the backing to which the adhesive is to be applied may also be pretreated or coated with a primer material prior to application of the adhesive to enhance bonding of the adhesive to the backing. The coated tapes are exposed to electron beam radiation from the adhesive side to suitably crosslink the elastomer in the adhesive to provide adequate resistance to flow in the adhesive. The precise degree of crosslinking depends on the specific tape construction and application, but tapes made in accordance with the method of the invention would be crosslinked to the point where at least 20% by weight of the elastomer is insoluble by the gel content test described hereinafter. The unique balance of adhesive and backing properties of the tape made in accordance with the method of the present invention are achieved through control of the voltage, i.e., the accelerating potential of a beam of electrons, to surprisingly narrow ranges. We have found that the tape constructions can be made within narrow voltage ranges that fall within a broader range of 75 kV to 175 kV. The range of voltages found useful is dependent on the adhesive and backing thicknesses as well as the degradation of the beam energy prior to impingement on the adhesive sample. For the present invention, it has been found that the accelerating potential must be high enough to provide a dose at the backing side of the adhesive which is no less than 60%, and preferably no less than 70%, of the dose at the front of the adhesive. In other words, the reduction in dose through the adhesive layer is no greater than 40% and preferably, no greater than 30%. It has also been found that the accelerating potential must not be so high as to deliver an average dose in the backing greater than 80% of that received in the adhesive, and preferably not greater than 60% of that received in the adhesive, in order to substantially reduce backing degradation or damage, while allowing adequate cure of the adhesive. These two conditions heretofore unrecognized will be described in more detail in the examples. Within the voltage ranges described above, we have further found that doses of less than 12 MRad are required. Above 12 MRad, even with properly chosen voltage, the backing will generally be unacceptably degraded. To demonstrate the voltage ranges as described herein, the following Table 1 has been constructed. For each of these cases, we assumed the presence of a titanium foil window 12.5 µm thick. The titanium density is 4.51 g/cm 3 . The nitrogen gap between the foil window and the adhesive surface is assumed to be 19 mm with a density of 1.25 x 10 -3 g/cm 3 . The adhesive density was assumed to be 1.0 g/cm 3 and the backing density was chosen to be 0.78 g/cm 3 (characteristic of a conventional saturated paper masking tape backing). Adhesive Thickness (µm) Backing Thickness (µm) Absolute Minimum Potential (kV) Preferred Minimum Potential (kV) Preferred Maximum Potential (kV) Absolute Maximum Potential (kV) 12.7 50.8 85 90 100 114 76.2 152 128 134 153 172 Examples 1 - 24 To demonstrate the value of electron beam curing of pressure sensitive adhesive tapes by the conditions of this invention, a series of tape examples were prepared. For all of these, a pressure sensitive adhesive with the formulation described in Table 2 was used. The rubbers were milled together on a conventional two roll mill and the zinc oxide and antioxidant added and blended. The millbase and the tackifying resin were then dissolved in heptane and coated onto a paper masking tape backing made as described in U.S. Patent 2,410,078. The paper backing is coated with a release coating such as exemplified by U.S. Patent Nos. 2,607,711 and 3,502,497. The adhesive was coated at three thicknesses and irradiated with an ESI "Electrocurtain" machine at accelerating potentials from 100 to 170 kV in 10 kV increments with a dose of 5 MRad. The current necessary to deliver this dose was determined by dosimetry with a thin (8 µm) Nylon chip containing a radiachromic dye from Far West Technologies, Inc. These chips are essentially uniformly irradiated at all of the voltages of interest because they are so thin. Adhesive thicknesses were measured by forming a sandwich of two pieces of tape, measuring the total thickness, subtracting twice the backing thickness and dividing by two. The optimal voltage ranges by the aforementioned criteria are given in Table 3. 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. The tests were conducted on a 0.5 inch (1.25 cm) wide strip of tape on an MIT Flex Tester Model #1 made by Tinius Olsen Testing Machine Company of Willow Grove, PA. Crosslinking of the adhesive was measured by the adhesive gel content and the swell ratio. The gel content of the examples is determined by soaking a piece of tape in toluene for 24 hours to extract the portion of the adhesive that is not crosslinked, determining the amount of gelled rubber in the extracted sample, and dividing the amount of gelled rubber by the amount of rubber in the adhesive formulation since only the rubber is typically crosslinked. The swell ratio is determined by soaking a tape sample in toluene for 24 hours, blotting off the excess solvent from the sample, and determining the amount of swelling by calculating the ratio of the swollen gel weight to that of the dried gel. We also observed whether or not the gel remained bonded to the backing. A 5 MRad dose was illustrative. From Table 5, one can see that the gel content for these adhesives is about 60% of the rubber which is within the range of useful gel contents. One can see from Tables 6 and 7 that when the accelerating potential is less than the lower limit defined by the invention, the adhesive is poorly cured at the backing side, as indicated by the fact that the cured portion of the adhesive (the gel) floats off the backing (Table 7), because the layer near the backing is soluble and because the average swell ratio of the cured adhesive is considerably higher than the limiting value at high voltage (Table 6). The swell ratio is a measure of the average crosslink density. The higher the swell ratio, the lower the crosslink density. On the other hand, at voltages higher than the upper limits set by the invention, the backing properties are degraded to an unacceptable level. This is illustrated in Table 4. We have found that when the MIT Flex life is below about 300 cycles, paper backings are unacceptably brittle and tend to tear when the tape is removed rapidly from a surface to which it is tightly adhered. Examples 25 - 42 A similar study was done with the same tape constructions, choosing the voltage and varying the dose. The gel content and MIT flex for these examples, are given in Tables 8 and 9, respectively. It can be seen that for this specific adhesive system, to achieve the useful range of gel contents (greater than 20%), one would need to use doses greater than 2 MRad. However, a dose of 12 MRad produces unacceptable backing properties, even within the optimal voltage range of the invention. Thus, there is a maximum useful dose range for the invention as well as a limited voltage range. Examples 43 - 45 Tape samples were prepared having an adhesive thickness of 34 µm and exposed to voltages of 100 kV, 125 kV, and 175 kV, at a dose of 3 MRad. The tapes were then rolled and the force required to unwind the tape from the roll was measured by attaching the free end of the tape to a scale and pulling on the tape at a rate of 90 inches per minute (228.6 cm per minute). The unwind force is measured in Newtons per centimeter, and the data are shown in Table 10. An unwind force of less than about 3 N/cm, and preferably less than 2.5 N/cm, is desired. At 175 kV, the unwind force is unacceptable. The following examples illustrate that operating within these limits can produce tapes with unique, functionally important properties. Example 46 The adhesive described in Table 2 was coated on the aforementioned saturated paper backing at a coating thickness of 34 µm and irradiated with an ESI "Electrocurtain" processor operated at 125 kV and a dose of 10 MRad. According to the methods of this invention, the ratio of the radiation dose at the adhesive backing side to that at the front adhesive surface was 0.87 and the ratio of average backing dose to average adhesive dose was 0.55. The tape produced had an average flex life of 317 cycles by the MIT flex test, and removed cleanly with no adhesive transfer from a painted panel after exposure to 129°C (265°F) and peeling hot. The peel adhesion removal force from the backside of the tape was measured by adhering a strip of tape to a hard surface with a 2 kg hard rubber roller. A second strip of tape was adhered onto the backside of the first strip in a like manner. The adhesion value was measured by attaching a scale to a free end of the tape and pulling the scale away at a rate of 90 inches per minute (228.6 cm per minute) at an angle of 180°. The peel adhesion removal force was 1.2 N/cm. Generally, the desired removal force is less than about 2.0 N/cm. Comparative Example 1: The construction of Example 46 was irradiated with the processor operating at 175 kV and a dose of 10 MRad. The ratio of dose at the back to that at the front of the adhesive layer was 1.06. The backing dose to adhesive dose ratio was 0.99 under these conditions. The tape had an average flex life of only 53 cycles, but did remove cleanly from the painted panel at 129°C (265°F). The peel adhesion removal force from the backing was 2.5 N/cm which is unacceptably high for masking tape. Comparative Example 2: The construction of Example 46 was irradiated with the processor operating at 100 kV and a dose of 10 MRad. The ratio of dose at the back to that at the front of the adhesive layer was 0.53. The backing dose to adhesive dose ratio was 0.13. The tape had a flex life of 430 cycles and a peel adhesion removal force from the backing of 1.2 N/cm, but some of the adhesive transferred to the painted panel when the tape was removed at 129°C (265°F), because of the insufficient cure of the back side of the adhesive. Component: Parts by weight: Ribbed smoked sheet natural rubber 50 Ameripol "Synpol" 1011A (SBR) 50 "Escorez" 1304, a tackifier available from Exxon 50 zinc oxide 12.5 Wingstay" L, an antioxidant available from Goodyear 1.0 Thicknesses and Voltage Ranges of Example Constructions Adhesive Thickness (µm) Absolute Minimum Potential (kV) Preferred Minimum Potential (kV) Preferred Maximum Potential (kV) Absolute Maximum Potential (kV) 14.5 87 92 119 135 36.6 105 111 129 146 73.2 126 133 145 162 MIT Flex life (cycles) vs. Adhesive Thickness and Accelerating Potential Coating Thickness (µm) kV 14.5 36.6 73.2 100 678 635 602 110 359 585 614 120 355 365 531 130 243 334 402 140 233 277 425 150 150 171 346 160 124 188 275 170 76 190 231 Gel Content (% of elastomer) vs. Adhesive Thickness and Accelerating Potential Coating Thickness (µm) kV 14.5 36.6 73.2 100 35.4 21.9 14.9 110 44.0 45.0 28.4 120 53.3 59.6 52.9 130 54.8 51.9 55.8 140 56.8 51.7 68.8 150 61.0 56.6 59.3 160 61.4 45.1 57.7 170 58.6 58.4 57.8 Swell Ratio vs. Adhesive Thickness and Accelerating Potential Coating Thickness (µm) kV 14.5 36.6 73.2 100 83 108 100 110 67 109 108 120 44 78 85 130 37 51 90 140 40 46 78 150 35 40 61 160 33 57 51 170 35 37 49 Floating or Bonded (+) Gel vs. Adhesive Thickness and Accelerating Potential Coating Thickness (µm) kV 14.5 36.6 73.2 100 + - - 110 + - - 120 + + - 130 + + + 140 + + + 150 + + + 160 + + + 170 + + + Gel Content (% of elastomer) vs. Adhesive Thickness and Dose Coating Thickness (µm) Thickness: 14.5 36.6 73.2 kV used: 110 125 135 Dose (MRad): 0.5 6 3 1 2.0 22 31 33 4.0 34 44 48 6.0 33 53 55 8.0 56 57 63 12.0 66 69 71 MIT Flex Life (cycles) vs. Adhesive Thickness and Dose Coating Thickness (µm) Thickness: 14.5 36.6 73.2 kV used: 110 125 135 Dose (MRad): 0.5 600 580 520 2.0 591 641 584 4.0 394 557 490 6.0 538 361 -332 8.0 367 376 300 12.0 303 200 262 Unwind Force vs. Voltage at 3 MRad Dose Voltage (kV) % Unwind Force - N/cm 100 2.18 125 2.08 175 3.82

Claims: A method for making a pressure sensitive adhesive tape comprising: (1) applying at least one electron beam radiation crosslinkable elastomer-based pressure sensitive adhesive onto a radiation degradable backing, (2) exposing said adhesive to a beam of electrons which has been accelerated through a potential voltage in the range of between 75 and 175 kilovolts, said voltage utilized being dependent upon, (a) the thickness of the adhesive layer, (b) the thickness of said backing and, (c) the degradation of the electron beam prior to impingement on said adhesive layer and selected such that the reduction in dose from the front of said adhesive layer to the back of the adhesive layer is no greater than 40% and the average dose received by said backing is less than 80% of the average dosage received by said adhesive, said pressure sensitive adhesive layer further having been crosslinked with a sufficient dose of beam energy to produce a gel content in said adhesive of at least 20% by weight of said elastomer, said dose to the pressure sensitive adhesive being less than about 12 megarads. The method of claim 1 wherein the reduction in dose from the front of said adhesive layer to the back of said adhesive layer is no greater than 30%, and the average dose received by said backing is less than 60% of the average dose received by said adhesive.

Inventors: MUNSON DANIEL C, YARUSSO DAVID J, MUNSON, DANIEL C., YARUSSO, DAVID J., MUNSON, DANIEL C., C/O MINNESOTA MINING AND, YARUSSO, DAVID J., C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1991
Main classification: C08J7
IPC codes: C09J201, C09J7