SE444575B - PRESSURE, ADHESIVE BANDS WITH MICROSPHERES DISPERSED IN THE ADHESIVE LAYER - Google Patents

Description

7805488-9 The present invention provides what can be considered as the “first truly economical tape with a pressure sensitive adhesive layer having a thickness of 0.2 - 1.0 mm. However, the new strip can be economically produced with thicknesses as small as 0.1 mm and thicknesses as large as 2.5 mm or more.

The pressure-sensitive, adhesive layer on the new belt offers good resistance to both peeling and shear forces and also exhibits an excellent combination of properties, e.g. that it is relatively elastic during short-term application stresses but shows a very low elasticity after stress has been exerted for a certain period of time. When the adhesive is pressed against a rough surface, it flows in and remains in intimate contact with extremely small contours after the pressure has been removed. For example, the final softness of the adhesive allows the new band to be used to mount a decorative metal plate to hide screws with round heads, since the adhesive can run around the screw heads and provide permanent contact with the underlying surface. In contrast, the elasticity of a conventional pressure-sensitive adhesive tape with a foam backing would tend to lift it from the underlying surface.

Typically, the adhesive layer of a pressure-sensitive adhesive tape of the present invention, when tested at a thickness of 3 mm, exhibits a Shore 00 hardness of at least 50 at 1 second and at most 30 at 30 minutes.

Briefly, the strip according to the invention comprises a pressure-sensitive adhesive layer consisting essentially of a polymer pressure-sensitive adhesive mass and glass microspheres with a density not exceeding 1 (measured as bulk density), distributed in the mass. Although the new strip exhibits the physical appearance of a pressure-sensitive adhesive tape with a foam backing, its polymer mass is substantially free of cavities in addition to the hollow cavities of the individual microspheres.

In tests on a number of tapes according to the invention, the pressure-sensitive adhesive layer did not show any water absorption. 7805488-9 The new strip can be prepared in essentially the same way as the strip according to Belgian patent specification 675 420 except that glass microspheres are dispersed in the polymerizable mixture before it is spread. Ideally, the polymerization is initiated by ultraviolet light, whereby both the polymerizable mixture and the microspheres must be relatively permeable to ultraviolet light. The ultraviolet permeability is increased if the walls of the microspheres are thin. Furthermore, glass microspheres with thinner walls tend to be cheaper, based on volume. Thus, their density is preferably less than 0.2, more preferably below 0.1.

Instead of using ultraviolet light, the pulp may contain a heat-activatable polymerization initiator and thus be polymerized by the action of heat. This allows the use of microspheres which are opaque to ultraviolet light, but the process may be slower and thus more costly than ultraviolet light polymerization.

The mass can be coated on and polymerized against a backing sheet, which has a low adhesion surface, from which the adhesive layer can be easily removed, or on a backing sheet, on which it remains permanently adhered, e.g. aluminum or steel foil, crepe paper or a plastic film, such as cellulose acetate film or biaxially oriented polyethylene terephthalate film.

The average diameter of the glass microspheres should be 10-200 microns. Microspheres with smaller average diameters would be too expensive, while it would be difficult to apply a polymerizable mixture containing microspheres with larger average diameters.

Preferably, the average diameter of the microspheres is in the range of 20-80 .mu.m. The glass microspheres should constitute 20-65% by volume of the pressure-sensitive adhesive layer. It would be unreasonably cumbersome to attempt to produce a coherent coating, free of cavities, at higher percentages, while the benefits of the invention would not be significantly met, at less than 20% by volume of the microspheres. Preferably, 45-55% by volume of the pressure-sensitive adhesive layer consists of glass microspheres.

The thickness of the pressure-sensitive adhesive layer should exceed three times the average diameter of the microspheres and twice the diameter of the microspheres of substantially each microsphere. This allows the microspheres to migrate within the adhesive under applied pressure instead of rupturing and the adhesive to flow in intimate contact with rough or uneven surfaces while maintaining its foam-like character. Optimal properties in this respect are achieved if the thickness of the pressure-sensitive adhesive layer exceeds seven times the average diameter of the microspheres.

Since the polymerizable mixture has a viscosity of less than 1000 centripoys before addition to the microspheres, it is convenient to use a thixotropic developer, such as sublimated silica, to keep the microspheres homogeneously distributed. Even in the presence of a thixotropic agent, it is advisable to stir the mixture immediately after storage before smearing it to ensure a homogeneous distribution of the microspheres.

Because the glass microspheres are homogeneously distributed throughout the polymerizable mixture prior to application, and are small relative to the thickness of the coating, the exposed surface of the resulting pressure sensitive adhesive tape tends to be smooth and smooth and can be expected to have an effective surface. coarseness (root mean square) not exceeding 8 .mu.m. When the exposed surface of the adhesive layer is covered with a temporary protective web with low adhesion, it assumes the contour of the protective web over time. If this contour is rough, the adhesive layer has a rough surface after removal of the protective web but adapts quickly under pressure to substrates on which it is applied and forms strong adhesive bonds.

When it is desired to attach the new tape to a surface to which the pressure-sensitive adhesive layer would not form a strong bond, it may be convenient to apply a layer of unfilled pressure-sensitive adhesive to one or both of the surfaces of the microsphere-filled adhesive layer. which is chosen especially for its adhesion to this surface. For example, strong bonding to certain car paint surfaces can be achieved only by means of pressure-sensitive adhesives, which cannot be polymerized by the action of ultraviolet light. 7sø54ss-9 Generally, the effective root mean square roughness of the exposed surface of the unfilled layer is less than 5 .mu.m.

The accompanying drawing shows a curve, on a semilogarithmic scale, which shows the hardness of the pressure-sensitive adhesive layers of two strips according to the invention as a function of time.

In the following example, all parts are by weight unless otherwise indicated.

Example 1 In a 1000 ml stainless steel beaker, 294 g of isooctyl acrylate and 6 g of acrylic acid were mixed with a three-bladed propeller stirrer at 500 rpm. To this mixture was added 0.75 g of benzoethyl methyl while stirring to dissolve. 9 g of sublimated silica was mixed in until a homogeneous dispersion was reached, about 10 minutes. The stirring speed was increased to 1500 rpm and 33.3 g of glass microspheres were added. The microspheres showed a density of 0.07 (measured as bulk density, and value 0.11) and had a diameter of 20 - 150 .mu.m (mean 55 .mu.m).

This material was applied with a knife to a thickness of 0.75 mm on a paper backing sheet with a low adhesion silicone surface.

The coating was placed under an ultraviolet lamp (Sylvania FR 40BL-235) at a distance of 15 cm in a nitrogen atmosphere containing not more than 150 parts per million of oxygen. After 5 minutes, the coating was completely polymerized without any noticeable shrinkage. Its surface was smooth and free of wrinkles. The resulting pressure-sensitive adhesive tape exhibited the appearance and feel of a pressure-sensitive adhesive tape with a foam backing.

Examples 2 - 3 Additional tapes of the present invention were prepared in the same manner as in Example 1 using the following materials to form the microsphere-filled, pressure-sensitive adhesive layer, except that the UV-polymerizable mixture of Example 3 was applied at a thickness of 1 .0 mm. -.:. w _ 7805488 ~ 9 6 E x e m p e l Parts by weight 2 3 isooctyl acrylate 67.09 80 acrylic acid 11.84 benzoethyl ethyl 0.19 0.25 polyvinyl ethyl ether 2.24 3 hydrocarbon type tackifiers. 7.45 - wetting agent 3.73> 15 sublimated silica 1.5 7 glass microspheres according to Example 1 5.96 On each surface of the microsphere-filled, pressure-sensitive adhesive layer according to Example 3, an unfilled, pressure-sensitive adhesive layer of acrylate copolymer was laminated of the type described in U.S. Pat. 24,906 with a thickness of 0.075 mm and is supported by a paper backing sheet having a low adhesion surface. An unfilled layer and the microsphere-filled layer of Example 3 were passed surface-to-surface between a rubber roller and a steel roller heated to 120 ° C.

The pressure applied by the rollers was 275 kPa. Thereafter, the backsheet of the microsphere-filled layer was removed so that its second surface was exposed for lamination to a second strip of the unfilled layer. The obtained laminated tape according to Example 3 was used in the test given below.

The microspheres occupied the following volume percentages in the pressure sensitive adhesive layers, close to the unfilled surface layers of the tape of Example 3: 'Adhesive layers of Example 1' 2 3% by volume of microspheres 47 35 27 Test Tape of Example 1 '2 3 T ¥ peeling ig / cm width (ASTM 19-1876-72) 3200 '4450 5320 3 =: f 4.,: r 7805488-9 7 Bands according to example V 1 2 3 shear strength at 21OC aluminum base for stainless steel, 2.54x1, 27 cm 500 g 1,000 g 1,000 g time to rupture 20 min> 10,000 min> 10,000 min Strips of the 2.5 cm wide strip were stretched 300% (12.5 cm to 50 cm) while placed in water and the tensile stress was removed after 24 hours. The following table indicates the degree of elongation 5 seconds after removal of the stress and 24 hours later.

Band according to Example 1 2 _ 3% elongation after seconds 260% 258% 285 24 hours 217% 230% 260 xo \ ° o \ ° Another set of strips with a width of 2.5 cm was stretched 100% (from 12.5 cm to 25 cm) in air and the tensile stress was removed after 10 seconds. The following table indicates the lengths 30 seconds after removal of the stress. b) Strips according to Example 1 2 Return to 14 cm 16 cm I 13 cm The laminated strip according to Example 3 was particularly suitable for attaching metal plates to embossed vinyl fabric of the type used for car roofs. Its pressure-sensitive adhesive flowed under pressure into the contours of the fabric and showed no tendency to come loose.

To obtain samples of sufficient thickness to test the hardness of the adhesive layer, two strips of the tape were laid face-to-face and laminated together using a manual pressure hard rubber roller. Then a backing sheet was peeled off so that a third strip could be laminated and its backing sheet was peeled off so that a fourth strip could be laminated on. The total thickness of 3 mm ensured that the hardness values were not too much affected by the underlying table on the hardness test device.

Using a Shore hardness tester on the 00 scale, readings were taken at different time periods after the probe first came in contact with the adhesive. The reading was: S hore 00 hardness Time in minutes example 1 1 example 2 0.017 A 56 ._ 0.05 49 _ 55 0.083 V 43 _ _ 0.1 - 50 0.17 _ V 38 - 0.2 - 46 0.25 35 - 0,5 28 * 42 1,0 '20 39 2,0 13 36 3,0 9 - 34 6,0 4 - 9,0 2 -, 0 _ - 30, 0 0 - 28,0 . - 26 45.0 - 25 90.0, - 23 The attached drawing shows these values on a semilogarithmic scale, curve 1 for example 1 and curve 2 for example 2Q The adhesives in both examples 1 and 2 were relatively elastic during short-term application of stresses but very softer under prolonged stress. The adhesive in Example 1 responded to prolonged stress as if it were substantially soft. If the probe was applied for only a few seconds and then removed, in all cases the mark of the probe quickly disappeared.

At the end of each test, the mark of the probe remained essentially unchanged after 24 hours, indicating that the adhesive had assumed a substantially permanent shape.

Example 4 The following ingredients were mixed and applied to a low adhesion substrate essentially by the procedure of Example 1: 100 g of 90/10 isooctyl acrylate / acrylic acid partially reacted to a viscosity of 1,000 - 1,200 centipoise and containing 0.25 g of benzoethyl ethyl and , 1 g of a crosslinking agent, g of an unreacted mixture of 90 parts of isooctyl acrylate and parts of acrylic acid, 7 g of glass microspheres according to Example 1 The coatings were transferred to a biaxially oriented polyethylene terephthalate substrate with a thickness of 0.075 mm and cut up to a width of 1.27 cm for testing, with the following results: \ _ Shear resistance at 1800 peeling soo g and at 650 c rupture from Lim- from stainless steel glass at 30 cm / min. thickness (minutes) and 210 C (g / cm width) 0.125 mm 910 128C 0.5 mm 1088 2624 In each test, a hard rubber roller with a mass of 6.8 kg was used to provide the bonds to the test surfaces. During the peeling test, the strip then remained in contact with the glass surface for 24 hours at ordinary room temperature before testing.

Examples 5 to 6 Two pressure sensitive adhesive tapes were prepared as in Example 1, except that the average diameter of the microspheres was 63 .mu.m, the ratio of isooctyl acrylate to acrylic acid was 87.5: 12.5 and the thickness of the polymerized adhesive layer was 1.0 mm. One of the bands (Example 5) was unmodified. To the exposed surface of the second (Example 6), an unfilled, pressure-sensitive adhesive layer of the copolymer of 90 parts of isooctyl acrylate and 10 parts of acrylic acid, Q i., __ l_l_4_ was laminated. ll ___._._ ~ __ __-, bl, .l i.,. - _. , i. _ .. -..., ... ~, .._.......... «_ V ..H _, _, _..., _. ._ ,, ..,.-._...._...

W ... 7805488 ”9“ 10 whose thickness of 50 / um.

Using a "proficorder", the effective root medium square roughness was determined for exposed surfaces on the strips: Example 5 (unmodified) 3.8 .mu.m Example 6 (laminated) 1.8 .mu.m Both strips showed essentially no moisture uptake after immersion in water for 24 hours at ordinary room temperature.

Claims (4)

7805488-9 Patent claims 1. A pressure-sensitive adhesive tape, the adhesive layer of which exceeds a thickness of 0.2 mm and is substantially free of cavities, characterized in that glass microspheres are dispersed throughout the adhesive layer, that the density of the microspheres does not exceed 1 that the average diameter of the microspheres is 10 - Q00 μm, that the microspheres constitute 20 - 60% by volume of the adhesive layer, and that the thickness of the adhesive layer exceeds 3 times the average diameter of the microspheres and 2 times the diameter of substantially each microsphere. The pressure gauge via the legs according to claim 1, characterized in that the thickness of the adhesive layer 1 exceeds 7 times the average diameter of the microspheres. Pressure-sensitive, viahäfrenae bands according to Claim 1 or 2, characterized in that the microspheres have a density of less than 0.2 and are ultraviolet-transparent. Pressure-sensitive adhesive tape according to one or more of the preceding claims, characterized in that the average diameter of the microspheres is 20-80 μm.