Production of pressure sensitive hot melt-adhesive coated tape

Abstract: In the production of a pressure sensitive adhesive tape comprising the steps of compounding and applying a hot melt adhesive composition to a tape backing, and allowing the hot melt adhesive to cool and thereby solidify to a pressure sensitive adhesive coating on said tape, the improvement which comprises incorporating into the hot melt adhesive composition a blowing agent which does not decompose under the conditions of compounding and application of the hot melt adhesive composition, and thereafter decomposing the blowing agent, thereby to impart to the hot melt coating a cellular structure.

Description: The present invention relates to the production of pressure sensitive adhesive tapes. Pressure sensitive adhesive tapes are used widely as maskings during painting, for sealing cartons, and myriad other uses. They were traditionally produced by dissolving an adhesive composition in a volatile solvent, applying the solution to a tape backing and then evaporating the solvent to leave a pressure sensitive adhesive coating on the backing. The products perform satisfactorily, but the process entailed the use of a solvent and the need for evaporation. This involved significant cost and raised the possibility of environmental pollution by the solvent. Accordingly, there were devised processes for applying the adhesive in the form of a hot melt rather than as a solution, see US-A-3 392 328. The molten adhesive upon cooling sets to a solid pressure sensitive adhesive. While such hot melt adhesive tapes are satisfactory for many purposes, they do not perform as well as their solution-produced counterparts in certain uses, exhibiting less desirable physical and performance attributes. In addition, a larger amount of adhesive is required for even this lesser performance. Efforts have been made to incorporate into the melt a blowing agent which decomposed and foamed during the acts of compounding and/or application to the tape backing, as disclosed in JP-A-60 76 583, upon which the preamble of claim 1 is based. However, the melt indeed foamed, but, by the time of wind up, the appearance of the hot melt adhesive was substantially the same as when produced from blowing agent-free hot melt adhesive compositions. It is accordingly an object of the present invention to produce a superior hot melt adhesive pressure sensitive tape which performs as well as or even better than solution applied adhesive tapes. These and other objects and advantages are realized in accordance with claim 1 and subclaims. A somewhat similar but less cellular structure is characteristic of solution-applied adhesive tapes but much less so of hot melt adhesive tapes without the blowing agent. The cellular structure exhibits itself in superior performance. It is surprising that the recited process produces a high performing product since other seemingly similar processes, in which the blowing agent decomposed and foamed during compounding and/or application, were nowhere comparably acceptable. The hot melt adhesive composition can be any of those heretofore employed, e.g., a styrene-isoprene-styrene block copolymer compounded with various known additives such as resins, oils, blending agents, and the like. In accordance with a preferred aspect of the present invention, there can be used a composition of a lower melting point that those generally employed heretofore. Heretofore, the melt temperature was generally about 171°C (340°F) to 188°C (370°F), but it has been found that compositions melting as low as about 155°C (310° F) perform quite satisfactorily. As employed herein, for example, "melting" and "hot melt" do not necessarily mean a fully liquid mass in the conventional sense. Instead they mean a mass of a viscosity low enough to be processible. To some extent that depends upon the processing equipment but as a general rule of thumb it means a material whose viscosity at the indicated "melt" temperature is at most about 1,000,000 cP, advantageously at most about 500,000 cP and preferably at most about 100,000 cP. Representative compositions into which blowing agents may be incorporated in accordance with the invention include, but are not limited to, those shown in U. S. Patents 3,239,478 and 3,932,328, the disclosures of which are incorporated herein by references. The concept of incorporating blowing agents in pressure-sensitive adhesives is not limited to hot melt adhesives, but is also applicable with similar advantage to pressure-sensitive adhesives which are coated from aqueous or solvent media. The blowing agents can be any of those known to decompose at the desired temperatures. However, it has been found that, when formulated into hot melts in accordance with the invention, the blowing agents decompose at somewhat lower temperature than their ratings, apparently determined in uncompounded state. Suitable blowing agents include azodicarbonamide sold by Uniroyal under the designation Celogen AZ, and preferably p-toluene sulfonyl semicarbazide sold under the designation Celogen RA, particularly mixed with other blowing agents effectively to drop the decomposition temperature of the Celogen RA. Other blowing agents decomposing at about 166°C (330°F) and higher can similarly be employed, e.g. 5-phenyl tetrazole itself decomposing above 204°C (400°F) but capable of lower decomposition by suitable compounding. The blowing agent can be mixed dry with the hot melt adhesive composition or can be dissolved or dispersed in a liquid to effect a better distribution through the adhesive with which it is mixed. Advantageously the ingredients making up the hot melt are compounded and then melted, the melt advancing to a die where it is applied to the backing. The blowing agent may be injected into the melt on its way to such die. The backing to which the hot melt adhesive composition is applied can be any sheet material which will withstand the oven conditions wherein decomposition of the blowing agent is effected, e.g. crepe paper, a flat sheet of paper, high melting plastic film, and the like. The products of the instant process are characterized by high performance, e.g. quick stick, good holding, and smooth unrolling, i.e. unwinding without jerking. The invention will be further described with reference to the accompanying drawing which is a flow sheet of a system for producing pressure sensitive hot melt-adhesive coated tapes in accordance with the present invention. The components making up a hot melt pressure sensitive adhesive composition are charged to a jacketed oil-heated sigma blade mixer 10, thereby forming a hot melt. The melt is fed into a holding tank 12 which discharges into a positive displacement pump 14. The outlet side of the pump discharges into a line 16 which feeds the melt into the cavity 18 of a die 20, the melt leaving the die upwardly. Blowing agent is advantageously slurried in a liquid non-solvent in tank 22 and by positive displacement pump 24 is positively discharged into line 16 which conveys the hot melt adhesive composition. If needed, mixing means (not shown) may be provided to ensure uniform distribution of the blowing agent through the melt. In addition the positive displacement pumps 14 and 24 may be interconnected to maintain a fixed discharge ratio, even if outputs are to be changed, i.e. slave-master. A crepe paper 26 is unwound from a roll 28 and pulled over rollers 30 and 32 and over the die 18 so as to pick up and be coated with the hot melt. The coated paper enters a heating chamber 34 wherein the temperature is raised to decompose the blowing agent. The paper upon leaving the oven passes over a chilled roll 36 to cool the paper after which it passes over a coated direction-changing release roll 38 and is then wound up on a roll 40. Alternatively, the hot melt pressure sensitive adhesive can be compounded in an extruder on a continuous basis and the blowing agent injected into the extruder which directly supplies the coating die with the necessary adhesive material. Thereafter the roll may be wound into shorter lengths and/or cut into narrow tapes of conventional width. The invention will be further described in the following illustrative examples wherein all parts are by weight unless otherwise stated. EXAMPLE 1 Employing an apparatus as shown in the drawing the blowing agent sold by Uniroyal under the trademark Celogen RA is injected into a pressure sensitive hot melt adhesive composition, the mixture having the following composition in parts by weight: SIS Polymer - (Kraton 1112) 100 Antioxidant- (Butyl Zimate) 2 Antioxidant- (Irganox 1010) 0.5 Pigment - (TiO 2 ) 6 Escorez 1580 Tackifying Resin 60 Liquid Resin Wingtack 10 20 Resin Kolon 90 30 Resin Pentalyn H 30 Cellogen RA 1.25 The adhesive composition is melted at about 155°C (310°F) and by the positive displacement pump is pumped to the applicator where it is applied to a crepe paper weighing approximately 18,1 kg (40 pounds) per 278 m 2 (3000 square feet) in an amount of 9,05 kg (20 pounds) per 278 m 2 (3000 square feet), the crepe paper moving at a linear speed of about 30 m (100 feet) per minute. The coated paper is passed through an oven wherein its temperature is raised to 171°C (340°F), directly thereafter passing over a chilled roll which brings its temperature down to about 43°C (110°F), and then wound up. The adhesive surface of the tape under a magnifying glass exhibits a foamed cellular structure characteristic of solution-applied adhesives, low density and superior performance. EXAMPLE 2 The process of Example 1 was carried out with some slight procedural modifications. Between the coating applicator 12 and the oven 16, the coated tape was chilled and rolled up. Thereafter it was unrolled and introduced into the oven. Various runs were made with different blowing agents, different conditions of heating (sometimes with no oven), different weights of adhesive, different speeds, all as specified, and the products were measured with regard to performance employing a variety of tests. The details and results are set forth in Table 1 hereinbelow. In such table, the various column headings and the text procedures and their results are as follows: The rolling ball tack test measures the "wet grab" or "quick stick" properties of a pressure sensitive tape, measured by seeing how far a 19 mm (3/4") stainless steel ball travels on an adhesive surface before it comes to a complete stop. A shorter distance indicates better performance; 46 is the maximum reading. The box load test measures the ability of a tape to hold closed the flaps of a carton. A longer time indicates better performance; 24 hours is the maximum reading. In the holding power tests the top one inch of a three inch length of the finished tape, 12.7 mm (0.5 inch) wide, is pressed tightly onto a stainless steel strip or to cardboard as indicated, and a 1 Kg weight is suspended from the other end. The time for the tape to pull away from the stainless steel or cardboard strip is determined. A longer time indicates better performance; test discontinued after 28 hours for cardboard, after 45 hours for steel. RUN TEMP °C(°F) IN HEATING CHAMBER 34 ROLLING BALL TACK, mm(INCHES) BOX LOAD HRS.(TWO MEASUREMENTS) HOLDING POWER TO CARDBOARD, HRS.(TWO MEASUREMENTS) HOLDING POWER TO STEEL, HRS. 1 24(75) 660(27) 0.15 20.8 1.8 0.83 3.4 2 171(340) 76(3) 6.5 21.6 >45 6.5 >28 3 24(75) 117(46) 6.5 13.5 42 0.9 >28 4 171(340) 38(15) 24 >28 >45 24 >28 This table shows that triggering of the blowing agent results in enhanced performance. In other runs wherein the blowing agent was triggered prematurely, the product performance was essentially the same as when no blowing agent was included. It will be appreciated that the instant specification and claims are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the scope of the present invention as laid out in the appended claims.

Claims: Process for producing a pressure sensitive adhesive tape comprising the steps of compounding a hot melt adhesive composition and incorporating a blowing agent into said composition, applying said composition to a tape backing, and allowing the hot melt adhesive to cool and thereby solidify to a pressure sensitive adhesive coating on said tape, characterised by subjecting the hot melt adhesive composition during compounding and application to conditions under which the blowing agent does not decompose, and decomposing the blowing agent after application, thereby to impart to the hot melt coating a cellular structure. The process according to claim 1, wherein the maximum temperature encountered by the hot melt adhesive composition during compounding and application is at least 13.9°C (25°F) lower than the temperature at which the blowing agent decomposes, the tape after hot melt coating being subjected to a temperature sufficient to decompose the blowing agent. The process according to claim 2, wherein during compounding and application the hot melt adhesive composition achieves a maximum temperature of about 155°C (310°F). The process according to claim 1, wherein promptly after decomposition of the blowing agent the tape is positively cooled to effect solidification of the hot melt adhesive composition. The process according to claim 1, wherein the blowing agent is present in the hot melt adhesive composition in about 0.1 to 5% by weight. The process according to claim 1, wherein the blowing agent is present in the hot melt adhesive composition in about 0.25 to 2% by weight. The process according to claim 1, wherein the backing is a crepe sheet. The process according to claim 1, wherein the backing is a flat sheet of paper. The process according to claim 1, wherein the backing is a polymer film. A pressure sensitive adhesive tape produced by the process of claim 1, comprising a backing carrying an adhesive which melts at a temperature of at most about 155°C (310°F), the adhesive having a foamed cellular surface.

Inventors: LULLA JACK, TANRIKULU NILUFER, LULLA, JACK, TANRIKULU, NILUFER
Applicants: BEIERSDORF AG, BEIERSDORF AKTIENGESELLSCHAFT
Application year: 1993
Main classification: B05D5
IPC codes: C09J7, C09J201, C09J11, C09J5