Pressure-sensitive adhesive strip

Abstract

The invention relates to a pressure-sensitive adhesive strip composed of at least three layers, comprising an inner layer F composed of a non-extensible film carrier, a layer SK1 composed of a self-adhesive composition arranged on one of the surfaces of the film carrier layer F and based on a vinylaromatic block copolymer composition foamed with microballoons, a layer SK2 composed of a self-adhesive composition arranged on the opposite surface of the film carrier layer F from the layer SK1 and based on a vinylaromatic block copolymer composition foamed with microballoons, where the mean diameter of the voids formed by the microballoons in the self-adhesive composition layers SK1 and SK2 is independently 20 to 60 m.

Claims

1. A pressure-sensitive adhesive strip comprising three layers as follows: an inner layer F composed of a non-extensible film carrier, a layer SK1 composed of a self-adhesive composition arranged on a surface of layer F and based on a vinylaromatic block copolymer composition foamed with microballoons, a layer SK2 composed of a self-adhesive composition arranged on an opposite surface of layer F from layer SK1 and based on a vinylaromatic block copolymer composition foamed with microballoons, where a mean diameter of each of voids formed by the microballoons in layers SK1 and SK2 is independently 20 to 60 m.

2. The pressure-sensitive adhesive strip as claimed in claim 1, having a symmetric construction with respect to layer composition, in that the foamed vinylaromatic block copolymer compositions of layers SK1 and SK2 are chemically identical.

3. The pressure-sensitive adhesive strip as claimed in claim 1, having a structurally symmetric construction, in that layers SK1 and SK2 are of the same thickness and/or have the same density.

4. The pressure-sensitive adhesive strip as claimed in claim 1, wherein the self-adhesive composition layers SK1 and/or SK2 are based on vinylaromatic block copolymers comprising polymer blocks (i) predominantly formed from vinylaromatics (A blocks), and (ii) predominantly formed by polymerization of 1,3-dienes (B blocks).

5. The pressure-sensitive adhesive strip as claimed in claim 1, wherein the vinylaromatic block copolymer is at least one synthetic rubber in the form of a block copolymer having an A-B, A-B-A, (A-B).sub.n, (A-B).sub.nX or (A-B-A).sub.nX structure, in which A blocks are independently a polymer formed by polymerization of at least one vinylaromatic, B blocks are independently a polymer formed by polymerization of conjugated dienes having 4 to 18 carbon atoms, or a partly hydrogenated derivative of such a polymer, X is the radical of a coupling reagent or initiator, and n is an integer 2.

6. The pressure-sensitive adhesive strip as claimed in claim 4, wherein the vinylaromatics for formation of the A block include styrene, a-methylstyrene and/or styrene derivatives.

7. The pressure-sensitive adhesive strip as claimed in claim 4, wherein monomer for forming the B block is selected from the group consisting of butadiene, isoprene, ethylbutadiene, phenylbutadiene, piperylene, pentadiene, hexadiene, ethylhexadiene, dimethylbutadiene, and a mixture thereof.

8. The pressure-sensitive adhesive strip as claimed in claim 1, wherein proportion of the vinylaromatic block copolymers, based on overall self-adhesive composition layer SK1 or SK2, totals at least 20% by weight, and at most 75% by weight.

9. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layers SK1 and/or SK2 are formed on basis of vinylaromatic block copolymer and tackifying resin.

10. The pressure-sensitive adhesive strip as claimed in claim 9, wherein layer SK1 and/or SK2 includes 20% to 60% by weight of tackifying resin, based on the total weight of the self-adhesive composition layer.

11. The pressure-sensitive adhesive strip as claimed in claim 9, wherein the tackifying resin, to an extent of at least 75% by weight, is hydrocarbon resin or terpene resin or a mixture thereof.

12. The pressure-sensitive adhesive strip as claimed in claim 1, wherein proportion of the microballoons in layer SK1 and/or layer SK2 is up to 12% by weight, based in each case on overall composition of layer SK1 or layer SK2.

13. The pressure-sensitive adhesive strip as claimed in claim 1, wherein mean diameter of the voids formed by the microballoons in layers SK1 and/or SK2, is 20 to 50 m.

14. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer SK1 and/or SK2 consists of the following composition: TABLE-US-00006 vinylaromatic block copolymers 20% to 75% by weight, tackifying resins 24.6% to 60% by weight, microballoons 0.2% to 10% by weight, and additives 0.2% to 10% by weight.

15. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer SK1 and/or SK2 consists of the following composition: TABLE-US-00007 vinylaromatic block copolymers 35% to 65% by weight, tackifying resins 34.6% to 45% by weight, microballoons 0.2% to 10% by weight, and additives 0.2% to 10% by weight.

16. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer SK1 and/or SK2 consists of the following composition: TABLE-US-00008 vinylaromatic block copolymers 30% to 75% by weight, tackifying resins 24.8% to 60% by weight, and microballoons 0.2% to 10% by weight.

17. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer SK1 and/or SK2 have an absolute density of 400 to 990 kg/m.sup.3, and/or a relative density of 0.35 to 0.99.

18. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer SK1 and/or SK2 have a thickness between 20 and 200 m.

19. The pressure-sensitive adhesive strip as claimed in claim 1, wherein one or both surfaces of layer F have been physically and/or chemically pretreated.

20. The pressure-sensitive adhesive strip as claimed in claim 19, wherein the pretreatment is an etching operation and/or a corona treatment and/or a primer treatment.

21. The pressure-sensitive adhesive strip as claimed in claim 1, wherein materials used for layer F are polyesters.

22. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer F has a thickness between 5 and 125 m.

23. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer F has an elongation at break of less than 300%.

24. The pressure-sensitive adhesive strip as claimed in claim 1, wherein layer F has a tensile strength in longitudinal direction of greater than 100 N/mm.sup.2, and/or a tensile strength in transverse direction of greater than 100 N/mm.sup.2.

25. The pressure-sensitive adhesive strip as claimed in claim 1, having a thickness of 45 m to 4000 m.

26. A method of bonding components of accumulators or electronic devices, comprising application of a pressure-sensitive adhesive strip according to claim 1 to a substrate.

Description

FIGURES

[0186] With reference to the figures described hereinafter, particularly advantageous embodiments of the invention will be elucidated in detail, without any intention to unnecessarily restrict the invention thereby.

[0187] FIG. 1 shows the schematic construction of a three-layer pressure-sensitive adhesive strip of the invention, composed of three layers 1, 2, 3 in cross section.

[0188] The strip comprises a non-extensible film carrier 1 (layer F) in the form of a PET film that has been etched on both sides. On the top side and on the bottom side of the PET film 1 there are two outer self-adhesive composition layers 2, 3 (layer SK1 and layer SK2). The self-adhesive composition layers 2, 3 (layers SK1 and SK2) are covered in turn by a liner 4, 5 on each side in the illustrative embodiment shown.

[0189] In addition, the invention encompasses a process for producing a pressure-sensitive adhesive strip of the invention (see FIG. 2), wherein [0190] the constituents for formation of an adhesive composition, such as polymers, resins or fillers and non-expanded microballoons, are mixed in a first mixing unit and heated to expansion temperature under elevated pressure, [0191] the microballoons are expanded on exit from the mixing unit, [0192] the adhesive composition mixture along with the expanded microballoons is formed to a layer in a roll applicator, [0193] the adhesive composition mixture along with the expanded microballoons is applied to a non-extensible carrier material in sheet form, [0194] the same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise usable in accordance with the invention along with the expanded microballoons, is applied on the other side of the carrier material in sheet form (not shown).

[0195] In addition, the invention encompasses a process for producing a pressure-sensitive adhesive strip of the invention (likewise see FIG. 2), wherein [0196] the constituents for formation of an adhesive composition, such as polymers, resins or fillers and non-expanded microballoons, are mixed in a first mixing unit and heated to expansion temperature, [0197] the microballoons are at least partly expanded during mixing, and are preferably expanded fully, [0198] the adhesive composition mixture along with the expanded microballoons is formed to a layer in a roll applicator, [0199] the adhesive composition mixture along with the expanded microballoons is applied to a non-extensible carrier material in sheet form, [0200] the same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise of the invention along with the expanded microballoons, is applied on the other side of the carrier material in sheet form (not shown).

[0201] The invention likewise encompasses a process for producing a pressure-sensitive adhesive strip of the invention (see FIG. 3), wherein [0202] the constituents for formation of an adhesive composition, such as polymers, resins or fillers, together with non-expanded microballoons, are mixed in a first mixing unit under elevated pressure and heated to a temperature below the expansion temperature of the microballoons, [0203] the mixed, especially homogeneous adhesive composition from the first mixing unit is transferred into a second unit and heated to expansion temperature under elevated pressure, [0204] the microballoons are expanded in the second unit or on exit from the second unit, [0205] the adhesive composition mixture along with the expanded microballoons is formed to a layer in a roll applicator, [0206] the adhesive composition mixture along with the expanded microballoons is applied to a non-extensible carrier material in sheet form, [0207] the same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise usable in accordance with the invention along with the expanded microballoons, is applied on the other side of the carrier material in sheet form (not shown).

[0208] The invention likewise relates to a process for producing a pressure-sensitive adhesive strip of the invention (see FIG. 4), wherein [0209] the constituents for formation of an adhesive composition, such as polymers, resins or fillers, are mixed in a first mixing unit, [0210] the mixed, especially homogeneous adhesive composition from the first mixing unit is transferred into a second mixing unit, into which the non-expanded microballoons are simultaneously introduced, [0211] the microballoons are expanded in the second mixing unit or on exit from the second mixing unit, [0212] the adhesive composition mixture along with the expanded microballoons is formed to a layer in a roll applicator, [0213] the adhesive composition mixture along with the expanded microballoons is applied to a non-extensible carrier material in sheet form, [0214] the same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise usable in accordance with the invention along with the expanded microballoons, is applied on the other side of the non-extensible carrier material in sheet form (not shown).

[0215] In a preferred embodiment of the invention, the adhesive composition is shaped in a film applicator and applied to the carrier material.

[0216] There is generally no need to degas compositions foamed with microballoons prior to coating in order to obtain a homogeneous, continuous coating. The expanding microballoons displace the air incorporated into the adhesive composition during compounding. In the case of high throughputs, it is nevertheless advisable to degas the compositions prior to coating in order to obtain a homogeneous feed of composition in the roll gap. The degassing is ideally effected directly upstream of the roll applicator at mixing temperature and with a pressure differential from ambient pressure of at least 200 mbar.

[0217] In addition, it is advantageous when [0218] the first mixing unit is a continuous unit, especially a planetary roll extruder, a twin-screw extruder or a pin extruder, [0219] the first mixing unit is a batchwise unit, especially a Z kneader or an internal mixer, [0220] the second mixing unit is a planetary roll extruder, a single-screw or twin-screw extruder or a pin extruder and/or [0221] the shaping unit in which the adhesive composition along with the expanded microballoons is shaped to give a carrier layer is a calender, a roll applicator or a gap formed by a roll and a fixed doctor.

[0222] With the processes of the invention, solvent-free processing of all previously known components of adhesive compositions and those described in the literature, especially self-adhesive components, is possible.

[0223] The above-described processes within the concept of the invention in variants of particularly excellent configuration are illustrated hereinafter, without any intention to impose unnecessary restriction through the choice of the figures depicted.

[0224] The figures show:

[0225] FIG. 2 the process with one mixing unit, wherein the microballoons are added directly in the first mixing unit,

[0226] FIG. 3 the process with two mixing units, wherein the microballoons are added in the first mixing unit, and

[0227] FIG. 4 the process having two mixing units, wherein the microballoons are added only in the second mixing unit.

[0228] FIG. 2 shows a particularly advantageously configured process for producing a foamed pressure-sensitive adhesive strip.

[0229] In a continuous mixing unit, for example a planetary roll extruder (PRE), the pressure-sensitive adhesive composition is produced.

[0230] For this purpose, the reactants E that are to form the adhesive composition are introduced into the planetary roll extruder PRE 1. At the same time, the unexpanded microballoons MB are incorporated homogeneously into the self-adhesive composition during the compounding process.

[0231] The temperatures required for homogeneous production of the self-adhesive composition and for expansion of the microballoons are adjusted with respect to one another such that the microballoons at least begin to expand during mixing and preferably foam completely in the self-adhesive composition M on exit from the PRE 1 as a result of the pressure drop on exit from the die, and in so doing break through the surface of the composition.

[0232] With a roll applicator 3 as shaping unit, this foam-like adhesive composition M is calendered and coated onto a non-extensible carrier material TP in sheet form; in some cases, further foaming can still take place in the roll gap. The roll applicator 3 consists of a doctor roll 31 and a coating roll 32. The carrier material TP is guided onto the latter via a pick-up roll 33, such that the carrier material TP takes up the adhesive composition K from the coating roll 32.

[0233] At the same time, the expanded microballoons MB are forced back into the polymer matrix of the adhesive composition K, and hence a smooth surface is generated. The drop in bonding force resulting from the microballoons can thus be distinctly reduced.

[0234] The same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise usable in accordance with the invention along with the expanded microballoons, is subsequently applied in an analogous manner on the other side of the carrier material in sheet form (not shown).

[0235] FIG. 3 shows a further particularly advantageously configured process for producing a foamed pressure-sensitive adhesive strip.

[0236] The planetary roll extruder PRE 1 has two successive mixing zones 11, 12 in which a central spindle rotates. In addition, there are six planetary spindles per heating zone. Further reactants are added to the injection ring 13, for example plasticizer or liquid resin.

[0237] An example of a suitable apparatus is the planetary roll extruder from Entex in Bochum.

[0238] Subsequently, the microballoons are incorporated homogeneously into the self-adhesive composition in a second mixing unit, for example a twin-screw extruder, heated above the expansion temperature and foamed.

[0239] For this purpose, the adhesive composition K formed from the reactants E is introduced here into the twin-screw extruder TSE 2; at the same time, the microballoons MB are introduced. The twin-screw extruder TSE has a total of four heating zones over its length 21.

[0240] An example of a suitable apparatus is a twin-screw extruder from Kiener.

[0241] During the expansion caused by the pressure drop at the nozzle exit of TSE 2, the foamed microballoons MB break through the surface of the composition.

[0242] With a roll applicator 3, this foam-like adhesive composition M is calendered and coated onto a non-extensible carrier material TP in sheet form; in some cases, further foaming can still take place in the roll gap. The roll applicator 3 consists of a doctor roll 31 and a coating roll 32. The carrier material TP is guided onto the latter via a pick-up roll 33, such that the carrier material TP takes up the adhesive composition K from the coating roll 32.

[0243] At the same time, the expanded microballoons MB are forced back into the polymer matrix of the adhesive composition K, and hence a smooth surface is generated. The drop in bonding force resulting from the microballoons can thus be distinctly reduced.

[0244] The same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise usable in accordance with the invention along with the expanded microballoons, is subsequently applied in an analogous manner on the other side of the carrier material in sheet form (not shown).

[0245] FIG. 4 shows a further particularly advantageously configured process for producing a foamed pressure-sensitive adhesive strip.

[0246] In a continuous mixing unit, for example a planetary roll extruder (PRE), the pressure-sensitive adhesive composition is produced.

[0247] Here, the reactants E that are to form the adhesive composition are introduced into the planetary roll extruder PRE 1. The planetary roll extruder PRE 1 has two successive mixing zones 11, 12 in which a central spindle rotates. In addition, there are 6 planetary spindles per heating zone.

[0248] Further reactants are added to the injection ring 13, for example plasticizer or liquid resin.

[0249] An example of a suitable apparatus is the planetary roll extruder from Entex in Bochum.

[0250] Subsequently, the microballoons are incorporated homogeneously under elevated pressure into the self-adhesive composition in a second mixing unit, for example a single-screw extruder, heated above the expansion temperature and foamed on exit.

[0251] For this purpose, the adhesive composition K formed from the reactants E is introduced here into the single-screw extruder SSE 2; at the same time, the microballoons MB are introduced. The single-screw extruder SSE has a total of four heating zones over its length 21.

[0252] An example of a suitable apparatus is a single-screw extruder from Kiener.

[0253] During the expansion caused by the pressure drop at the nozzle exit of SSE 2, the microballoons MB break through the surface of the composition.

[0254] With a roll applicator 3, this foam-like adhesive composition M is calendered and coated onto a non-extensible carrier material in sheet form; in some cases, further foaming can still take place in the roll gap. The roll applicator 3 consists of a doctor roll 31 and a coating roll 32. The carrier material TP is guided onto the latter via a pick-up roll 33, such that the carrier material TP takes up the adhesive composition K from the coating roll 32.

[0255] At the same time, the expanded microballoons MB are forced back into the polymer matrix of the adhesive composition K, and hence a smooth surface is generated. The drop in bonding force resulting from the microballoons can thus be distinctly reduced.

[0256] The same adhesive composition mixture along with the expanded microballoons, or another adhesive composition mixture likewise usable in accordance with the invention along with the expanded microballoons, is subsequently applied in an analogous manner on the other side of the carrier material in sheet form (not shown).

[0257] With falling gap pressure in the roll gap, there is a decrease in the bonding areas of the coated foamed self-adhesive compositions, since the microballoons are then forced back to a lesser degree, as can be inferred from FIG. 2. FIG. 2 shows the bonding areas as a function of the coating process or parameter. The gap pressure required is highly dependent on the composition system used; the higher the viscosity, the greater the gap pressure should be, depending on the layer thickness desired and the coating speed chosen. In practice, a gap pressure of greater than 4 N/mm has been found to be useful; with exceptionally high coating speeds greater than 50 m/min, with low applications of composition (basis weights less than 70 g/m.sup.2) and high-viscosity compositions (50 000 Pa*s at 0.1 rad and 110 C.), gap pressures greater than 50 N/mm may even be required.

[0258] It has been found to be useful to adjust the temperature of the rolls to the expansion temperature of the microballoons. Ideally, the roll temperature of the first rolls is above the expansion temperature of the microballoons in order to enable further foaming of the microballoons without destroying them. The last roll should have a temperature equal to or below the expansion temperature in order that the microballoon shell can solidify and the smooth surface of the invention forms.

[0259] Many units for continuous production and processing of solvent-free polymer systems are known. Usually, screw machines such as single-screw and twin-screw extruders of different processing length and with different equipment are used. Alternatively, continuous kneaders of a wide variety of different designs, for example including combinations of kneaders and screw machines, or else planetary roller extruders, are used for this task.

[0260] Planetary roll extruders have been known for some time and were first used in the processing of thermoplastics, for example PVC, where they were used mainly for charging of the downstream units, for example calenders or roll systems. Their advantage of high surface renewal for material and heat exchange, with which the energy introduced via friction can be removed rapidly and effectively, and of short residence time and narrow residence time spectrum, has allowed their field of use to be broadened recently, inter alia, to compounding processes that require a mode of operation with exceptional temperature control.

[0261] Planetary roll extruders exist in various designs and sizes according to the manufacturer. According to the desired throughput, the diameters of the roll cylinders are typically between 70 mm and 400 mm.

[0262] Planetary roll extruders generally have a filling section and a compounding section.

[0263] The filling section consists of a conveying screw, into which all solid components are metered continuously. The conveying screw then transfers the material to the compounding section. The region of the filling section with the screw is preferably cooled in order to avoid caking of material on the screw. But there are also embodiments without a screw portion, in which the material is applied directly between central and planetary spindles. However, this is of no significance for the efficacy of the process of the invention.

[0264] The compounding section consists of a driven central spindle and several planetary spindles that rotate around the central spindle within one or more roll cylinders having internal helical gearing. The speed of the central spindle and hence the peripheral velocity of the planetary spindles can be varied and is thus an important parameter for control of the compounding process.

[0265] The materials are circulated between the central and planetary spindles, i.e. between planetary spindles and the helical gearing of the roll section, such that the materials are dispersed under the influence of shear energy and external temperature control to give a homogeneous compound.

[0266] The number of planetary spindles that rotate in each roll cylinder can be varied and hence adapted to the demands of the process. The number of spindles affects the free volume within the planetary roll extruder and the residence time of the material in the process, and additionally determines the size of the area for heat and material exchange. The number of planetary spindles affects the compounding outcome via the shear energy introduced. Given a constant roll cylinder diameter, it is possible with a greater number of spindles to achieve better homogenization and dispersion performance, or a greater product throughput.

[0267] The maximum number of planetary spindles that can be installed between the central spindle and roll cylinder is dependent on the diameter of the roll cylinder and on the diameter of the planetary spindles used. In the case of use of greater roll diameters as necessary for achievement of throughputs on the production scale, or smaller diameters for the planetary spindles, the roll cylinders can be equipped with a greater number of planetary spindles. Typically, up to seven planetary spindles are used in the case of a roll diameter of D=70 mm, while ten planetary spindles, for example, can be used in the case of a roll diameter of D=200 mm, and 24, for example, in the case of a roll diameter of D=400 mm.

[0268] It is proposed in accordance with the invention that the coating of the foamed adhesive compositions be conducted in a solvent-free manner with a multiroll applicator system. These may be applicator systems consisting of at least two rolls with at least one roll gap up to five rolls with three roll gaps.

[0269] Also conceivable are coating systems such as calenders (I,F,L calenders), such that the foamed adhesive composition is shaped to the desired thickness as it passes through one or more roll gaps.

[0270] It has been found to be particularly advantageous to choose the temperature regime for the individual rolls such that controlled further foaming can take place if appropriate, in such a way that transferring rolls can have a temperature above or equal to the foaming temperature of the microballoon type chosen, whereas receiving rolls should have a temperature below or equal to the foaming temperature, in order to prevent uncontrolled foaming, and where all rolls can be set individually to temperatures of 30 to 220 C.

[0271] In order to improve the transfer characteristics of the shaped composition layer from one roll to another, it is also possible to use anti-adhesively finished rolls or patterned rolls. In order to produce a sufficiently precisely shaped adhesive film, the peripheral speeds of the rolls may have differences.

[0272] The preferred 4-roll applicator is formed by a metering roll, a doctor roll, which determines the thickness of the layer on the carrier material and is arranged parallel to the metering roll, and a transfer roll disposed beneath the metering roll. At the lay-on roll, which together with the transfer roll forms a second roll gap, the composition and the material in sheet form are brought together.

[0273] Depending on the nature of the carrier material in sheet form which is to be coated, coating can be effected in a co-rotational or counter-rotational process.

[0274] The shaping system may also be formed by a gap formed between a roll and a fixed doctor. The fixed doctor may be a knife-type doctor or else a stationary (half-)roll.

[0275] In an alternative production process, all constituents of the adhesive composition are dissolved in a solvent mixture (benzine/toluene/acetone). The microballoons are converted to a slurry in benzine and stirred into the dissolved adhesive composition. For this purpose, it is possible in principle to use the known compounding and stirring units, and it should be ensured that the microballoons do not expand in the course of mixing. As soon as the microballoons are distributed homogeneously in the solution, the adhesive composition can be coated, for which it is again possible to use prior art coating systems. For example, the coating can be accomplished by means of a doctor blade onto a conventional PET liner. In the next step, the adhesive composition layer thus produced is dried at 100 C. for 15 min. In none of the aforementioned steps is there any expansion of the microballoons.

[0276] The non-extensible film layer F is laminated onto the free surface of the adhesive composition layer thus produced and dried. Laminated on the second surface thereof is the free surface of a second, likewise dried adhesive composition layer produced in this way, so as to result in an unfoamed three-layer composite composed of the inner film layer and two adhesive composition layers provided with liners.

[0277] Alternatively, the film layer F can be directly coated simultaneously or subsequently with the unfoamed adhesive compositions that have been provided with microballoons, and then these still-exposed adhesive composition layers are dried at 100 C. for 15 min and then covered with liners, so as to result in the unfoamed three-layer composite.

[0278] After the drying, the adhesive layers are foamed in the oven within an appropriate temperature/time window, for instance at 150 C. for 5 min or at 170 C. for 1 min, specifically covered between the two liners, in order to produce a particularly smooth surface.

[0279] The surface thus produced has a roughness R.sub.a of less than 15 m, more preferably less than 10 m, most preferably less than 3 m.

[0280] The surface roughness R.sub.a is a unit for the industrial standard for the quality of the final surface processing and constitutes the average height of the roughness, especially the average absolute distance from the center line of the roughness profile within the range of evaluation. In other words, R.sub.a is the arithmetic mean roughness, i.e. the arithmetic mean of all profile values in the roughness profile. R.sub.a is measured by means of laser triangulation.

[0281] The expansion temperature chosen is especially higher than the drying temperature in order to avoid the expansion of the microballoons in the course of drying.

[0282] The invention is elucidated in detail hereinafter by a few examples. With reference to the examples described hereinafter, particularly advantageous embodiments of the invention will be elucidated in detail, without any intention to unnecessarily restrict the invention thereby.

EXAMPLES

[0283] There follows a description of the production of pressure-sensitive adhesive strips of the invention, comprising a film carrier composed of PET with different thickness and self-adhesive composition layers (SACL) with a different microballoon content of Expancel 920 DU20. For this purpose, first of all, a 40% by weight adhesive solution in benzine/toluene/acetone was produced from 50.0% by weight of Kraton D1102AS, 45.0% by weight of Dercolyte A115, 4.5% by weight of Wingtack 10 and 0.5% by weight of Irganox 1010 aging stabilizer. The proportions by weight of the dissolved constituents are each based on the dry weight of the resulting solution. Said constituents of the adhesive composition are characterized as follows: [0284] Kraton D1102AS: styrene-butadiene-styrene triblock copolymer from Kraton Polymers with 17% by weight of diblock, block polystyrene content: 30% by weight [0285] Dercolyte A 115: solid -pinene tackifying resin with a ring and ball softening temperature of 115 C. and a DACP of 35 C. [0286] Wingtack 10: liquid hydrocarbon resin from Cray Valley [0287] Irganox 1010: pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) from BASF SE

[0288] The solution was subsequently admixed with 1% by weight, 2% by weight or 3% by weight of unexpanded microballoons, using the microballoons in the form of a slurry in benzine. The proportions by weight of the microballoons are based here in each case on the dry weight of the solution used to which they have been added (i.e. the dry weight of the solution used is fixed at 100%). The microballoons in each case were Expancel 920 DU20. The mixture obtained was then coated with a coating bar onto a PET liner provided with a silicone release agent in the desired layer thickness, then the solvent was evaporated off at 100 C. for 15 min and so the composition layer was dried.

[0289] A non-extensible film layer of PET was laminated onto the free surface of the adhesive composition layer thus produced and dried, where the film had a thickness of 12 m, 36 m or 50 m (the PET films of said thicknesses are each sold under the Tenolan OCN brand by the manufacturer Fatra). The tensile strength of the 12 m-thick PET film was 210 N/mm.sup.2 in longitudinal direction and 210 N/mm.sup.2 in transverse direction. The elongation at break of the 12 m-thick film was 124% in longitudinal direction and 147% in transverse direction. The tensile strength of the 36 m-thick PET film was 152 N/mm.sup.2 in longitudinal direction and 161 N/mm.sup.2 in transverse direction. The elongation at break of the 36 m-thick film was 145% in longitudinal direction and 83% in transverse direction. The tensile strength of the 50 m-thick PET film was 151 N/mm.sup.2 in longitudinal direction and 160 N/mm.sup.2 in transverse direction. The elongation at break of the 50 m-thick film was 148% in longitudinal direction and 81% in transverse direction.

[0290] Laminated on the second surface thereof was the free surface of a second, likewise dried adhesive composition layer produced in an identical manner, so as to result in an unfoamed symmetric three-layer composite composed of the inner film layer and two adhesive composition layers provided with liners. The three-layer composite accordingly firstly has, in relation to the composition of the layers, a symmetric construction, in that the two adhesive composition layers adjoining the film layer are chemically identical. Secondly, the three-layer composite accordingly has a structurally symmetric construction, in that the two adhesive composition layers adjoining the film layer are of equal thickness and have the same density.

[0291] After drying, the adhesive layers between the two liners were foamed in an oven at 150 C. for 5 min, which resulted in pressure-sensitive adhesive strips of the invention each having a thickness of about 150 m (examples 1 to 9). The thickness of about 150 m was obtainable in each case through suitable selection of the coat thickness of the adhesive composition comprising the unexpanded microballoons in the production process. The thickness is based on the pressure-sensitive adhesive strips of the invention, i.e. without PET liner.

[0292] Through the foaming between two liners, products having particularly smooth surfaces are obtainable. All the examples adduced have an R.sub.a value of less than 15 m. By die-cutting, pressure-sensitive adhesive strips with the desired dimensions were obtained.

[0293] For comparison, in addition, corresponding pressure-sensitive adhesive stripsbut without a non-extensible film layerwere produced, i.e. pressure-sensitive adhesive strips consisting of a single self-adhesive composition layer of the invention with 1% by weight, 2% by weight or 3% by weight of Expancel 920 DU20. The proportions by weight of the microballoons are based here in each case on the dry weight of the adhesive solution to which they have been added in the production process (i.e. the dry weight of the solution used is fixed at 100%). The self-adhesive composition layers were thus chemically identical to those of examples 1 to 9. In these cases, rather than a non-extensible film layer, a second PET liner as described above was laminated in each case onto the free surface of the dried adhesive composition layer produced and the adhesive composition layer was then foamed between the two liners in an oven at 150 C. for 5 min. Through the foaming between two liners, here too, products having particularly smooth surfaces are obtainable (with R.sub.a values less than 15 m). By die-cutting, pressure-sensitive adhesive strips with the desired dimensions were obtained (comparative examples 1 to 3). The pressure-sensitive adhesive strips without a non-extensible film layer were likewise produced such that they had a thickness of about 150 m. Here too, the thickness is based on the pressure-sensitive adhesive strips without PET liner.

[0294] In addition, likewise for comparison, a pressure-sensitive adhesive strip was produced that differed from the pressure-sensitive adhesive strip of the invention from example 5 merely in that, rather than a non-extensible film layer of PET of thickness 36 m, an extensible polyurethane film carrier having a comparable thickness of 30 m was used (comparative example 4). The elongation at break of the polyurethane film carrier both in longitudinal direction and in transverse direction was more than 300%. Production was effected analogously to the pressure-sensitive adhesive strip of the invention from example 5. The resulting pressure-sensitive adhesive strip likewise had a thickness of about 150 m. Here too, the thickness is based on the pressure-sensitive adhesive strip without PET liner.

[0295] Table 1 below shows the shock resistances of the pressure-sensitive adhesive strips of the invention with a PET film carrier as examples 1 to 9, the pressure-sensitive adhesive strips without a film carrier as comparative examples 1 to 3, and the pressure-sensitive adhesive strip with a polyurethane film carrier as comparative example 4.

TABLE-US-00004 TABLE 1 Shock resistances of inventive pressure-sensitive adhesive strips and comparative examples. Micro- PET film balloon SACL Ball Impact Push- thickness content density.sup.1 drop resistance.sup.2 out Experiment (m) (% by wt.) (kg/m.sup.3) (cm) (J) (N) Example 1 12 1 830 45 0.81 182 Example 2 36 1 822 25 0.75 170 Example 3 50 1 815 25 0.72 133 Comparative none 1 820 85 1.04 182 example 1 Example 4 12 2 720 185 1.12 140 Example 5 36 2 720 225 1.18 134 Example 6 50 2 725 125 0.97 131 Comparative none 2 715 165 1.10 163 example 2 Example 7 12 3 603 165 0.96 127 Example 8 36 3 618 125 0.78 118 Example 9 50 3 611 145 0.69 93 Comparative none 3 610 185 0.97 136 example 3 Comparative 30 (PU 2 718 145 0.93 140 example 4 film) .sup.1SACL = self-adhesive composition layer; .sup.2in z direction

[0296] The table shows that the pressure-sensitive adhesive strips of the invention have very good shock resistances, especially high ball drop values, impact resistances in z direction and push-out resistances. What is noticeable is that microballoon contents in the self-adhesive composition layers of 2% by weight, which results in an absolute density of the self-adhesive composition layers of 720 kg/m.sup.3, lead to pressure-sensitive adhesive strips having particularly good shock resistances. Excellent shock resistances are possessed especially by the pressure-sensitive adhesive strips from examples 4 and 5, wherein the thickness of the PET film is 12 m and 36 m respectively.

[0297] The inventive pressure-sensitive adhesive strip from example 5 comprising a PET carrier also surprisingly has distinctly improved shock resistance over the pressure-sensitive adhesive strip from comparative example 4 comprising a polyurethane carrier with comparable thickness. This is manifested especially by an elevated ball drop value and an elevated impact resistance in z direction. It is thus obviously a feature of inventive pressure-sensitive adhesive strips having a non-extensible film carrier that they have improved shock resistances over noninventive pressure-sensitive adhesive strips having an extensible film carrier, i.e. a film carrier having an elongation at break of at least 300% both in longitudinal direction and in transverse direction.

[0298] In further experiments, the effect of the size of the voids formed by microballoons (MB) in self-adhesive composition layers on the shock resistance thereof was tested. Likewise tested was the effect of the content of microballoons in self-adhesive composition layers or of the density that can be established as a result in self-adhesive composition layers on the shock resistance thereof. The self-adhesive composition layers were produced analogously to those of comparative examples 1 to 3, with variation in the content and type of the microballoons. The types of microballoons used were, as well as Expancel 920 DU20, also Expancel 920 DU40, Expancel 920 DU80 and Expancel 920 DU120. The self-adhesive composition layers were likewise produced such that they had a thickness of about 150 m. Here too, the thickness is based on the self-adhesive composition layers without PET liner.

[0299] Table 2 below shows the shock resistances of the self-adhesive composition layers as comparative examples 1 to 3 and 5 to 15.

[0300] The experiments show that those self-adhesive composition layers that have been produced using the comparatively small microballoons Expancel 920 DU20 and Expancel 920 DU40 have significantly higher shock resistances than those self-adhesive composition layers that have been produced using the comparatively large microballoons Expancel 920 DU80 and Expancel 920 DU120.

[0301] The experiments also show that the highest shock resistances can be achieved in the self-adhesive composition layers with microballoon contents of about 1.5% to about 2.5% by weight.

TABLE-US-00005 TABLE 2 Shock resistances of self-adhesive composition layers usable in pressure-sensitive adhesive strips of the invention. Impact Impact MB.sup.1 SACL Ball resis- resis- MB.sup.1 content density.sup.2 drop tance.sup.3 tance.sup.4 Experiment type (% by wt.) (kg/m.sup.3) (cm) (J) (J) Comparative DU20 1.0 820 85 1.04 0.73 example 1 Comparative DU20 2.0 715 165 1.10 0.77 example 2 Comparative DU20 3.0 610 185 0.97 1.22 example 3 Comparative DU20 2.3 656 185 1.45 1.22 example 5 Comparative DU20 3.3 590 145 1.21 1.06 example 6 Comparative DU40 0.9 723 85 1.24 0.80 example 7 Comparative DU40 2.0 630 125 1.13 0.80 example 8 Comparative DU40 3.0 567 85 0.85 0.64 example 9 Comparative DU80 1.5 652 85 1.00 0.72 example 10 Comparative DU80 2.0 639 105 0.97 0.70 example 11 Comparative DU80 3.0 522 85 0.75 0.60 example 12 Comparative DU120 0.5 788 25 0.75 0.61 example 13 Comparative DU120 0.9 667 25 0.97 0.75 example 14 Comparative DU120 2.0 530 85 0.81 0.57 example 15 .sup.1MB = microballoons; .sup.2SACL = self-adhesive composition layer; .sup.3in z direction; .sup.4in the x, y plane

[0302] It is also noticeable that, in general, the highest shock resistances can be achieved in the self-adhesive composition layers when the absolute density of the self-adhesive composition layers is within the range from 600 to 750 kg/m.sup.3, for example 600 to 700 kg/m.sup.3.

[0303] On the basis of these results, the self-adhesive composition layers used in the pressure-sensitive adhesive strips of the invention are those that have been foamed using comparatively small microballoons, for example Expancel 920 DU20 or Expancel 920 DU40. Moreover, it is advisable to use, in the pressure-sensitive adhesive strips of the invention, self-adhesive composition layers having microballoon contents of about 1.5% to about 2.5% by weight and/or an absolute density of 600 to 750 kg/m.sup.3, for example 600 to 700 kg/m.sup.3.

Test Methods

[0304] Unless stated otherwise, all measurements were conducted at 23 C. and 50% rel. air humidity.

[0305] The mechanical and adhesive data were ascertained as follows:

Elongation at Break, Tensile Strength (Test Method R1)

[0306] Elongation at break and tensile strength were measured in accordance with DIN 53504 using dumbbell specimens of size S3 at a separation speed of 300 mm per min. The test conditions were 23 C. and 50% rel. air humidity.

Modulus of Elasticity

[0307] Modulus of elasticity indicates the mechanical resistance that the material offers to elastic deformation. It is determined as the ratio of the strain required to the elongation achieved, where is the quotient of the change in length L and the length L.sub.0 in Hooke's regime of deformation of the specimen. The definition of the modulus of elasticity is elucidated, for example, in the Taschenbuch der Physik [Physics Handbook] (H. Stcker (ed.), Taschenbuch der Physik, 2nd ed., 1994, Verlag Harri Deutsch, Frankfurt, p. 102-110).

[0308] To determine the modulus of elasticity of a film, the tensile strain characteristics were ascertained using a type 2 specimen (rectangular test film strip of length 150 mm and width 15 mm) according to DIN EN ISO 527-3/2/300 with a test speed of 300 mm/min, a clamping length of 100 mm and an initial force of 0.3 N/cm, the test strip for ascertainment of the data having been cut to size with sharp blades. A Zwick tensile tester (model Z010) was used. Tensile strain characteristics were measured in machine direction (MD). A 1000 N (Zwick Roell Kap-Z 066080.03.00) or 100 N (Zwick Roell Kap-Z 066110.03.00) load cell was used. Modulus of elasticity was ascertained by graphical means from the measurement curves by determining the slope of the starting region of the curve which is characteristic of the behavior in respect of Hooke's Law and was reported in GPa.

DACP

[0309] 5.0 g of test substance (the tackifying resin sample to be examined) are weighed into a dry test tube, and 5.0 g of xylene (isomer mixture, CAS [1330-20-7], 98.5%, Sigma-Aldrich #320579 or comparable) are added. The test substance is dissolved at 130 C. and then cooled down to 80 C. Any xylene that escapes is made up for with fresh xylene, such that 5.0 g of xylene are present again. Subsequently, 5.0 g of diacetone alcohol (4-hydroxy-4-methyl-2-pentanone, CAS [123-42-2], 99%, Aldrich #H41544 or comparable) are added. The test tube is shaken until the test substance has dissolved completely. For this purpose, the solution is heated to 100 C. The test tube containing the resin solution is then introduced into a Novomatics Chemotronic Cool cloud point measuring instrument and heated therein to 110 C. It is cooled down at a cooling rate of 1.0 K/min. The cloud point is detected optically. For this purpose, that temperature at which the turbidity of the solution is 70% is registered. The result is reported in C. The lower the DACP value, the higher the polarity of the test substance.

Tackifying Resin Softening Temperature

[0310] The tackifying resin softening temperature is conducted by the relevant methodology, known as ring & ball and standardized in ASTM E28.

Diameter

[0311] The mean diameter of the voids formed by the microballoons in a self-adhesive composition layer is determined using cryofracture edges of the pressure-sensitive adhesive strip in a scanning electron microscope (SEM) with 500-fold magnification. The diameter of the microballoons in the self-adhesive composition layer to be examined that are visible in scanning electron micrographs of 5 different cryofracture edges of the pressure-sensitive adhesive strip is determined in each case by graphical means, and the arithmetic mean of all the diameters ascertained in the 5 scanning electron micrographs constitutes the mean diameter of the voids formed by the microballoons in the self-adhesive composition layer in the context of the present application. The diameters of the microballoons visible in the micrographs are determined by graphical means in such a way that the maximum extent in any (two-dimensional) direction is inferred from the scanning electron micrographs for each individual microballoon in the self-adhesive composition layer to be examined and regarded as the diameter thereof.

Density

[0312] The density of the unfoamed and foamed adhesive composition layers is ascertained by forming the quotient of mass applied and thickness of the adhesive composition layer applied to a carrier or liner.

[0313] The mass applied can be determined by determining the mass of a section, defined in terms of its length and width, of such an adhesive composition layer applied to a carrier or liner, minus the (known or separately determinable) mass of a section of the same dimensions of the carrier or liner used.

[0314] The thickness of an adhesive composition layer can be determined by determining the thickness of a section, defined in terms of its length and width, of such an adhesive composition layer applied to a carrier or liner, minus the (known or separately determinable) thickness of a section of the same dimensions of the carrier or liner used. The thickness of the adhesive composition layer can be determined by means of commercial thickness measuring instruments (caliper test instruments) with accuracies of less than a 1 m deviation. If variations in thickness are found, the mean of measurements at at least three representative sites is reported, i.e. more particularly not measured at creases, folds, specks and the like.

Thickness

[0315] Like the thickness for an adhesive composition layer as above, it is also possible to ascertain the thickness of a pressure-sensitive adhesive strip or a film carrier layer by means of commercial thickness measuring instruments (caliper test instruments) with accuracies of less than a 1 m deviation. If variations in thickness are found, the mean of measurements at at least three representative sites is reported, i.e. more particularly not measured at creases, folds, specks and the like.

Static Glass Transition Temperature T.SUB.g

[0316] Glass transition pointsreferred to synonymously as glass transition temperaturesare reported as the result of measurements by means of differential scanning calorimetry (DSC) according to DIN 53 765, especially sections 7.1 and 8.1, but with uniform heating and cooling rates of 10 K/min in all heating and cooling steps (cf. DIN 53 765; section 7.1; note 1). The sample weight is 20 mg.

Ball Drop Test (Impact Resistance)

[0317] A square sample in the shape of a frame was cut out of the adhesive tape to be examined (external dimensions 33 mm33 mm; border width 2.0 mm; internal dimensions (window cut-out) 29 mm29 mm). This sample was stuck to an ABS frame (external dimensions 45 mm45 mm; border width 10 mm; internal dimensions (window cut-out) 25 mm25 mm; thickness 5 mm). A PMMA window of 35 mm35 mm was stuck to the other side of the double-sided adhesive tape. The bonding of ABS frame, adhesive tape frame and PMMA window was effected such that the geometric centers and the diagonals were each superimposed on one another (corner-to-corner). The bonding area was 248 mm.sup.2. The bond was subjected to a pressure of 248 N for 5 s and stored under conditions of 23 C./50% relative humidity for 24 hours.

[0318] Immediately after the storage, the adhesive composite composed of ABS frame, adhesive tape and PMMA sheet was placed by the protruding edges of the ABS frame onto a framework (sample holder) such that the composite was aligned horizontally and the PMMA sheet faced downward in a freely suspended manner. A steel ball (weight 32.6 g) was allowed to drop vertically from a height of 25 cm (through the window of the ABS frame) centered onto the PMMA sheet in the sample thus arranged (test conditions 23 C., 50% relative humidity). Three tests were conducted with each sample, if the PMMA sheet had not become detached beforehand. The ball drop test is considered to have been passed if the bond did not part in any of the three tests. The height from which the weight dropped, in each case using a new sample, was increased in 20 cm steps until the bond (in at least one of the three tests) was parted. The drop heights reported for the double-sided adhesive tapes tested in the application relate to the last height at which the test is still passed.

Push-Out Resistance (Z Plane)

[0319] By means of the push-out test, it is possible to obtain conclusions as to how high the stability of a bond of a component is in a frame-like body, for example a window in a housing.

[0320] A rectangular sample in the shape of a frame was cut out of the adhesive tape to be examined (external dimensions 43 mm33 mm; border width in each case 2.0 mm; internal dimensions (window cut-out) 39 mm29 mm, bond area on the top and bottom side 288 mm.sup.2 in each case). This sample was bonded to a rectangular ABS polymer frame (ABS=acrylonitrile-butadiene-styrene copolymers) (external dimensions 50 mm40 mm, border width of each of the long borders 8 mm; border width of each of the short borders 10 mm; internal dimensions (window cut-out) 30 mm24 mm; thickness 3 mm). A rectangular PMMA sheet (PMMA=polymethylmethacrylate) with dimensions of 45 mm35 mm was bonded to the other side of the sample of the double-sided adhesive tape. The full available bonding area of the adhesive tape was utilized. The bonding of ABS frame, adhesive tape sample and PMMA window was effected such that the geometric centers, the angle bisectors of the acute diagonal angles and the angle bisectors of the obtuse diagonal angles of the rectangles were each superimposed on one another (corner-to-corner, long sides on long sides, short sides on short sides). The bonding area was 288 mm.sup.2. The bond was subjected to a pressure of 10 bar for 5 s and stored under conditions of 23 C./50% relative humidity for 24 hours.

[0321] Immediately after the storage, the adhesive composite composed of ABS frame, adhesive tape and PMMA sheet was placed by the protruding edges of the ABS frame onto a framework (sample holder) such that the composite was aligned horizontally and the PMMA sheet faced downward in a freely suspended manner.

[0322] A pressure ram is then moved vertically upward through the window of the ABS frame at a constant speed of 10 mm/min, such that it presses onto the center of the PMMA sheet, and the respective force (determined from the respective pressure and contact area between the ram and sheet) is registered as a function of the time from the first contact of the ram with the PMMA sheet until just before it drops away (test conditions: 23 C., 50% relative humidity). The force acting immediately prior to the failure of the adhesive bond between PMMA sheet and ABS frame (maximum force F.sub.max in the force-time diagram in N) is registered as the response of the push-out test.

Impact Resistance; Z Direction

[0323] A square sample in the shape of a frame was cut out of the adhesive tape to be examined (external dimensions 33 mm33 mm; border width 2.0 mm; internal dimensions (window cut-out) 29 mm29 mm). This sample was stuck to a PC frame (external dimensions 45 mm45 mm; border width 10 mm; internal dimensions (window cut-out) 25 mm25 mm; thickness 3 mm). A PC window of 35 mm35 mm was stuck to the other side of the double-sided adhesive tape. The bonding of PC frame, adhesive tape frame and PC window was effected such that the geometric centers and the diagonals were each superimposed on one another (corner-to-corner). The bonding area was 248 mm.sup.2. The bond was subjected to a pressure of 248 N for 5 s and stored under conditions of 23 C./50% relative humidity for 24 hours.

[0324] Immediately after the storage, the adhesive composite composed of PC frame, adhesive tape and PC window was braced by the protruding edges of the PC frame in a sample holder such that the composite was aligned horizontally and the PC window was beneath the frame. The sample holder was then inserted centrally into the intended receptacle of the DuPont Impact Tester. The impact head of weight 190 g was used in such a way that the circular impact geometry with a diameter of 20 mm impacted centrally and flush on the window side of the PC window.

[0325] A weight having a mass of 150 g guided on two guide rods was allowed to drop vertically from a height of 5 cm onto the composite composed of sample holder, sample and impact head thus arranged (test conditions: 23 C., 50% relative humidity). The height from which the weight dropped was increased in 5 cm steps until the impact energy introduced destroyed the sample as a result of the impact stress and the PC window parted from the PC frame.

[0326] In order to be able to compare experiments with different samples, the energy was calculated as follows:

E [J]=height [m]*mass of weight [kg]*9.81 m/s.sup.2

[0327] Five samples per product were tested, and the mean energy was reported as index for impact resistance.

Transverse Impact Resistance; X,Y Plane

[0328] A square sample in the shape of a frame was cut out of the adhesive tape to be examined (external dimensions 33 mm33 mm; border width 2.0 mm; internal dimensions (window cut-out) 29 mm29 mm). This sample was stuck to a PC frame (external dimensions 45 mm45 mm; border width 10 mm; internal dimensions (window cut-out) 25 mm25 mm; thickness 3 mm). A PC window of 35 mm35 mm was stuck to the other side of the double-sided adhesive tape. The bonding of PC frame, adhesive tape frame and PC window was effected such that the geometric centers and the diagonals were each superimposed on one another (corner-to-corner). The bonding area was 248 mm.sup.2. The bond was subjected to a pressure of 248 N for 5 s and stored under conditions of 23 C./50% relative humidity for 24 hours.

[0329] Immediately after the storage, the adhesive composite composed of PC frame, adhesive tape and PC sheet was braced by the protruding edges of the PC frame in a sample holder such that the composite was aligned vertically. The sample holder was then inserted centrally into the intended receptacle of the DuPont Impact Tester. The impact head of weight 300 g was used in such a way that the rectangular impact geometry with dimensions of 20 mm3 mm impacted centrally and flush on the end face of the PC window facing upward.

[0330] A weight having a mass of 150 g guided on two guide rods was allowed to drop vertically from a height of 5 cm onto the composite composed of sample holder, sample and impact head thus arranged (test conditions: 23 C., 50% relative humidity). The height from which the weight dropped was increased in 5 cm steps until the impact energy introduced destroyed the sample as a result of the transverse impact stress and the PC window parted from the PC frame.

[0331] In order to be able to compare experiments with different samples, the energy was calculated as follows:

E [J]=height [m]*mass of weight [kg]*9.81 kg/m*s.sup.2

[0332] Five samples per product were tested, and the mean energy was reported as index for transverse impact resistance.