" 113~8~
I'RESSURE-SENSITIVF TAPE HA~ING
A PLURALITY O~ ~DHESlVE LAYERS
Users of pressure-sensitive adhesive tape have frequently sought a product ~hich possesses the combined characteristics of (1) good adhesion to irregular substrates and (2) good resistance to failure when subject to shearing forces after application. These two desiderata are essentially mutually exclusive, since a soft adhesive is required for conformability but a hard, or firm, adhesive is required for good shear properties.
Thus, while adhesives possessing intermediate properties have been made (cf., e.g., U.S. Patent 2,956,904, which teaches the use of an electron beam to crosslink an adhesive, and U.S. Patent 3,658,740 which describes the blending of a tackifier with two types of rubbery copolymer), it has generally been considered impossible to obtain a tape in which the adhesive simultaneously possesses both good conformability and good shear properties.
One application for normally tacky and pressure-sensitive adhesive tape which imposes the conflicting demands just discussed is found in tape used for diaper closures. Such tape must bond quickly and firmly to the irregular surfaces of conventional cloth diapers or to the frequently powder-contaminated polyethylene outer face of disposable diapers. On the other hand, the stresses imposed on a tape product used as a diaper closure are such that almost all adhesives ~ ,,, ~-1, ' ~. .
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which are consi(lered "sof`t" (a characteristic which is required f`or conrormability and contamination tolerance) quickly fail in use because of the shearing action applied to the adhesive layer.
Normally tacky and pres3ure-sensitive adhesives of` certain water-resistant types have also been used in the manufacture Or identification labels for shirts and other launderable garments; see, e.g. U.~. Patent ~o.
4,074,004. Again while softness of the pressure-sensitive adhesive is essential to obtain conformabilityto the cloth, the stresses ir;,parted during washing-drying cycles tend to loosen the edges Or a fabric label having a soft adtlesive ancl hence to result in a shorter life for l;tie iderltificltiorl label than is desirable. A hard adhesive, on the other hand, lacks sufficient conforM~bility to develop adec~uate initial adhesion and ~lence peels of`f` readily. It has been extremely dif`Iicult to balarlce aclhesive properties to arrive at a label which not only adheres well but also resists the stiearing stress encountered in norlnal launderirlg.
l'he present invention provides a novel but simple pressure-sensitive adhesive tape construction which simultaneously possesses the normally mutually exclusive properties Or (1) excellent adhesion and conformability, and (2) excellent shear. Tapes made in accordance with the invention thus find utility in a wide variety of mounting laminating and other applications;
~37~3~
thes( tar~ls are ~)articularly suited, however, for use in the preparatiorl ol diaper closures and identific~tion labels ~or washable garments.
In accordance with the invention, a sheet backing material is provided with a layer of normally tacky and pressure-sensitive adhesive which comprises a plurality of pressure-sensitive adhesive strata, the strata for a given tape being part o~ the same general adhesive ~amily but the exposed adhesive straturn being significantly softer than the immediately underlying stratunl. Thus, for example, while both strata May be rubber-resin pressure-sensitive adhesives, both may be normally tacky and pressure-sensitive acrylate copolymer adhesives, etc., it is not generally wise to combine rubber-resin and acrylate adhesives.
The "softness" or "hardness" of the adhesives comprisin~ the pressure-sensitive adhesive layer is measured in terms of "shear creep compliance", using a test to be described in more detail. The exposed first adhesive stratum has a 10-second shear creep compliance (J1o) of at least about 3.5 x 10-6 (preferably at least about 7 x 10-6) cm2/dyne, while the underlying adhesive stratum has a J1o value of no more than about 2.5 x 10-6 cm2/dyne.
Applying this measure of sortness, the invention may be described as normally tacky and pressure-sensitive adhesive tape of the type wherein a `
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layer of` normally tacky and pressure-sensitive adhesive is coated over and adhered to a flexible sheet material, characterized in that the pressure-sensitive adhesive layer comprises a plurality of superposed and firmly united adhesive strata selected from the same general class of adhesives, the exposed adhesive stratum having a 10-second shear creep compliance of at least about 3.5 x 10 6 cm2/dyne (preferably at least about 5 x 10 6 dyne) and the immediate-ly underlying adhesive stratum having a 10-second shear creep compliance of not more than about 2.5 x 10 6 cm /dyne (preferably no more than about 2 x 10 6 dyne).
Others have previously made pressure-sensitive adhesive tape employing as a primer a resin-tackified natural rubber latex blended with chloroprene-acrylonitrile copolymer (see, e.g., U.S. Patent No. 3,340,088), but the primer is applied in an extremely thin layer and in any event it is not itself a pressure-sensitive adhesive composition.
For some purposes (e.g., bonding two fabrics together), it is desirable to have a 3-stratum adhesive layer in which a comparatively hard adhesive stratum is sandwiched between two comparatively soft adhesive strata.
Speaking in general terms, the thickness of each adhesive stratum can range between about 5 and 60 microns (preferably between about 15 and 45 microns). The thickness of the composite adhesive layer is about 60-120 microns for a 2-stratum product and 90-180 microns ~1 for a 3-stratulrl pro(iuct.
F[G[II~E 1 depicts, in greatly enlarged cross-section, a tape which is useful as a diaper closure; and FIGU~E 2 shows, likewise in greatly enlarged cross-section, a tape construction having particular utility as a garment label.
In Figure 1, film backing ll is provided with normally tacky and pressure-sensitive adhesive layer 12, comprising comparatively hard pressure-sensitive adhesive stratum 13 and comparatively soft adhesive stratum 14.
In Figure 2, tape 20 comprises cloth backing 21, on one side of which is coated normally tacky and pressure-sensitive adhesive layer 22, comprising relatively hard adhesive stratum 24 sandwiched between relatively soft adhesive strata 23 and 25.
Creep Compliance of Adhesive Layer. In Properties and Structure of Polymers, John Wiley ~ Sons, Inc., 1960, Chapter II, Section 6, Tobolsky discusses the five regions of viscoelastic behavior possessed by linear amorphous polymers, e.g., polystyrene. He characteri~es the elastic properties of a polymer by its tensile relaxation modulus, Er(t), which is obtained by measuring stress as a function of time in a sample maintained at constant extension and constant temperature. As a polymer is heated, it successively passes through regions which can be designated as glassy, transition, rubbery plateau, rut)bery rlow, and liquid flow. ~nly the latter two re~ions are of significance to the present invention.
~or a 10-second reference time, the dividing line between the rubbery plateau and the rubbery flow regions Er(lO), is approximately 2.5 x 106 dynes/cm2. In the rubbery flow region, the shear modulus, Gr(lo), is approximately one-third the tensile modulus or o.83 x 106 dynes/cm2;
shear creep compliance, Jr(lo), which is the reciprocal of the shear modulus, is thus 1.2 x 10-6 cm2/dyne at the boundary between the rubbery plateau and the rubbery flow regions.
In measuring shear creep compliance, the adhesive to be tested is spun-cast on a smooth film of polytetrafluoroethylene to a thickness of 500 micrometers. Two test pieces of equal area are die cut from the adhes:ive sheet and placed in a parallel plate shear creep plastometer*, one piece being on each side of the center plate, with an outer plate contacting the exposed surface of each. Screws connecting the two outer plates are then tightened so as to compress the adhesive layers 10 percent. T~le parallel plates are placed in horizontal arrangement in an oven and one end of the center plate connected to a linear displacement voltage transformer, which in turn is connected to a chart *See, e.g., ~erry, John D., Viscoelastic Properties of Polymers, John Wiley & Sons, Inc., New York, N.Y.~ 1961, Chapter 6.
recorder. A hook is attached to the opposite end of the center plate, with a flexible wire extending from the hook over a pulley. The outer plates are held in a fixed position. The oven is raised to the desired temperature and stabilized there at + C.5 C., after which a suitable weight (20 to 1,500 gms, whatever will both measurably deform the sample and remain within the linearity limit of the sample) is attached to the free end of the wire, and the chart recorder started. From the chart recorder the time and displacement can be read and the shear creep compliance, J, of the adhesive at a given temperature calculated from the equation J(t) = 2AXi/hF, where t is the time at which the measurement is taken, A
is the area of one face of one of the adhesive samples, h is the thickness of one of the adhesive saMples, Xi is the displacement at time t (where Xi is less than h), and F is the force due to gravitational acceleration of the mass attached to the wire connected to the middle plate.
When A is expressed in cm2, h in cm, Xi in cm, and F in dynes, J(t) is given in cm2/dyne. It has been found that an adhesive has satisfactory "softness" or shear compliance to function as an upper adhesive stratum, in accordance with the invention, if its lO-second shear creep compliance value, J(10), is at least about 3.5 x 10-6 cm2/dyne. Similarly an adhesive has adequate "hardness" to function as a lower adhesive stratum if its ~3'~~3 J(10) value is no more than about 2.5 x 10-6 cm2/dyne.
While the evaluation of a diaper closure tape i5 o~ten highly subjective, three specific tests can be employed to aetermine whether a given tape ~and more particularly the pressure-sensitive adhesive) possesses the properties which recommend it for use as a contaminant-tolerant diaper closure tape. These tests will now be described in more detail.
Quick Stick. This test is a modification of Pressure-Sensitive Tape Council test PSTC-5*, which measures the ability of a pressure-sensitive adhesive tape to adhere to a surface instantly, using no external pressure to secure more ~thorough contact. It is measured as a force resisting peeling of the tape at 90 angle from a standard surface upon which it has been applied.
In accordance with the modification, the test surface is a polyethylene film of the type commonly employed as the outer surface of a disposable diaper; this film is carefully and uniformly bonded to a steel test panel. The tape is placed in contact with the polyethylene Eilm and rolled down with a 100-gram roller at a rate of 30.5 cm per minute. The force required to remove a strip of tape *described in detail in "Test Methods for Pressure- Sensitive Tapes", 6th Edition, developed by the Specifications and Technical Committee of the Pressure-Sensitive Tape Council, 1201 ~aukegan Road, Glenview, ILL 60025.
1~37~31 9 _ at 90 from the exposed sur~ace of the polyethylene at a rate of 30.5 cm/minute should be at least about 100 grams per centimeter width, and preferably at least about 200 grams per centimeter widtil.
Contamination Tolerance. This test is substantially identical to the "Quick Stick" test just described, except that the surface of the polyethylene is first covered uniformly with talc and the excess shaken off, leaving approximately 2-2.5 g/m2. The force required to remove the tape from the talc-contaminated polyethylene film should be at least about 15 grams, and preferably at least about 40 grams per centimeter width.
Shear Adhesion. In this test, a 2.54-cm wide strip of tape is obtained and the adhesive surface partially masked with paper or the like so as to leave a
2.54-cm length of adhesive exposed. The 2.54 x 2.54 cm square of adhesive is then positioned against the polyethylene cover of a section cut from a disposable diaper, and, using a 500 gram roller, pressed into firm contact. One end of the diaper section is then gripped in a pair of jaws, the free end of the tape extending downward therefrom, and a l,000-gram weight connected to the free end of the tape. The time required for the tape to pull free from the polyethylene film is measured in minutes. A satisfactory shear adhesion value is considered to be at least 3 minutes, and preferably at least about 10 minutes.
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In the following illustrative but non-limita-tive examples, all parts are by weight unless otherwise noted. The following abbreviations have been employed to reier to the various components of the adhesive:
Rubbery Polymers ABBR~VIATION COMPONENT
K1101 Styrene:butadiene:styrene block copolymer, having a styrene:butadiene ratio of 30:70 and a solution viscosity (25% in toluene, 23C) of 4,000 cps, available under the registered trademark "Kraton" 1101.
K1107 Styrene:isoprene:styrene block copolymer having a styrene:isoprene ratio of 14:86, and a Brookfield viscosity of 1600 cps at 25C (25% solution in toluene), available under the registered trademark "Kraton" 1107.
S423P Styrene:isoprene radial block copolymer having a styrene:isoprene ratio of 16:84 and a Brookfield viscosity of 2300 cps at 25C (25% solution in toluene), available under the registered trademark "Solprene" 423P
S1205 Linear styrene:butadiene block copolymer having a styrene:isoprene ratio of 25:75, a Brookfield viscosity of 2345 cps at 25C (25~ solution in toluene~
and a molecular weight of 83,000, available under the registered trademark "Solprene" 1205 SMR Natural crepe rubber Tackifier Resins ABBREVIATIGN COMRONENT
~135 Polymerized alpha-pinene having a ring-and-ball softening temperature of about 135C., available under the registered trademark "Piccolyte" A 135 1~37~3~ -~115 ~olymerized beta-pinene having a ring-and-ball softening temperature of 115-120C., commercially available under the registered trademark "Zonarez" B-115 S115 Polymerized beta-pinene having a ring-and-ball softening temperature of 115-120C, available under the regis-tered trademark "Piccolyte" S-115 W10 Synthetic polyterpene resin having a molecular weight of 450, liquid at room temperature (ring-and-ball softening temperature of 10C.), available under the registered trademark "Wingtack" 10.
~115 Synthetic polyterpene resin having a molecular weight of 1400 and a ring-and-ball softening temperature of 115-1~0C, available under the registered trademark "Wingtack" 115 In each example, a first normally tacky and pressure-sensitive adhesive stratum was bonded to a polypropylene film backing using conventional techniques, either coating a solution of the adhesive on the backing and then evaporating the solvent, applying the adhesive as a hot melt, or forming the adhesive on a release liner and transferriny it to the polypropylene film. In each case, the thickness of the first adhesive stratum, when dry, was on the order of 15-45 micrometers. The second adhesive stratum was then bonded over the first in similar manner, applying a sufficient quantity to leave an overall adhesive layer thickness of approximately 60 micrometers.
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~7831 A transfer tape was prepared by coating the "hard" and "soft" adhesives of ~xarnple 7 on separate silicone-coated release liner sheets to form strata having respective thicknesses of 33 and 40 micrometers and thereafter laminating the two adhesives to each other. If desired, 3-stratum adhesives can be prepared by laminatirlg a second "soft" adhesive stratum to the opposite face of` the "hard" adhesive straturn. The 2- or
3-stratum adhesive layer can then be transferred to a substrate, the release liner removed, and another substrate adhered to the newly exposed adhesive surface in the manner well-known for transfer tapes having a one-stratum adhesive layer.
Multiple layer acrylate adhesives are particularly suited for use in identification label tape - for garments made of several different fabrics, where they are capable of remaining firmly bonded after multiple wash-dry cycles, as an aid to evaluating performance under wet conditions, the following test is utilized:
Wet Per ormance. Tape is prepared by coating a lightly creped neoprene beater-treated semi-bleached fiber paper backing weighing about 105 g/m2 with a solution of pressure-sensitive adhesive ànd evaporating the solvent to leave 58 g/m2 of adhesive solidsO To determine the ability of the tape to adhere to fabrics ~7~31 utlder ~et con(~it;oll;, a one-inch wide strip of the tape is applic~(~ to a 2-ply ~0 gracle cheese cloth which has beetl doubled over on itself, the tape overlapping the cheese cloth to provide 6.Ll5 cm2 cloth contact and extending beyond the folded edge of the cheese cloth. The tape is bonded firmly to the cheese cloth by twice rolling it with a 2-kg automated roller traveling at about 30 cm per minute. Af`ter rolling, the samples are allowed to remain for a period OI' one-half hour and then immersed for one hour in a jar containing distilled water. The taped sarnple is then removed and subjected to dynamic shear measured as the maximum value obtained when the folded cheese cloth is gripped by the upper jaw of a tensile testing machine and the extended strip of tape by the lower jaws, separating them at a rate of approximately 5 cm per minute. A value of about 140 g/cm2, and preferably about 200 g/cm2, is considered necessary for the adhesive to be deemed satisfactory for use as a label adhesive. The average of three samples is reported as the wet shear value.
Table II shows the performance of normally tacky and pressure-sensitive adhesive tapes which were made in substantially the same way as the tapes in Table I except that each of the two adhesives was an acrylate copolymer, the lower adhesive being prepared generally as taught in U.S. Patent Re. 24,906 and the upper adhesive being prepared generally as taught in U.S. Patent No.
4,074,004, the disclosures of both bein~ incorporated herein by rererence. T~e following abbreviations for the various monomers are employed:
ABBREVIATION CO~IONENT
AA acrylic acid IOA iso-octyl acrylate AAm acrylamide ' 1'137831 ~ -- 1 6 e ~o t~
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~ ' 7i~3 rl'he tern1 "tapc~" :in tne appended claims ref`ers oot!l to structures in which the adhesive is firmly bonded to a conventional backin~ and to structures in which the adi-lesive is removably bonded to the release-coatet3 surface of a backing. Tapes of the latter type, commonly known as "transfer tapes", can be applied to a desired substrate and the release liner backing removed to expose the other side of the adhesive layer.
A second substrate can then be adhered to the exposed surface.
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