Thermally crosslinked acrylate hotmelts

The invention relates to crosslinked polyacrylates by thermal treatment, a process for their preparation and their use.

The technological process for the production of PSAs is constantly evolving. In the industry, hot melt processes (hot melt processes) with solvent-free coating technology for the production of PSAs are of increasing importance. This development is further accelerated by ever increasing environmental regulations and rising prices for solvents. Therefore, one wants to eliminate solvents as much as possible from the manufacturing process for PSA tapes. The introduction of hot melt technology places increasing demands on the adhesive compositions. Acrylic PSAs in particular are being examined very intensively for improvements. Polyacrylates are preferred for high-quality industrial applications because they are transparent and weather-resistant. In addition to these advantages, the acrylic PSAs also have to meet high requirements in the area of ​​shear strength. This is achieved through high molecular weight, high polarity polyacrylates and subsequent efficient crosslinking. In the simplest case, efficient crosslinking is achieved by metal chelates, which react with carboxylic acid functions at higher temperatures and thus crosslink the acrylic PSA. This method is state of the art for solvent-based PSAs.

Alternatively, multifunctional isocyanates can also be used. However, these methods are unsuitable for hotmelt processes since the PSAs are processed at high temperatures and therefore crosslink in the processing process and would therefore gel. After crosslinking, these PSAs can no longer be coated.

Electron beam hardening (ES hardening or ESH) is preferred for hotmelt processes because it can also be used to crosslink thicker layers. No thermal energy is required for electron beam curing, and crosslinking takes place in a relatively short time.

The first ES-curing polyacrylate hotmelts were described in DE 21 31 059 A1. Further ES-curing hotmelts were developed in JP 05017726. No. 5,194,55 describes the addition of N-tert-butylacrylamide as a monomer in order to force ES hardening.

A general disadvantage of ESH is damage to the wearer. In addition to the adhesive, the carrier material or the release paper is also penetrated by the electron beams. This leads to damage that is noticeable in discoloration or in high rolling forces for the adhesive tape. There is therefore a need for a crosslinking method for hot-melt pressure sensitive adhesive that is gentle on the carrier but also efficient.

The object of the invention is therefore to offer a polymer mixture which enables a corresponding gentle crosslinking, but the crosslinking process does not occur during the early processing stage when using the solvent-free production technology. The carrier should not be damaged and its use for an adhesive tape should not be restricted.

This object is achieved by a polyacrylate as described in the main claim. The subclaims and subsidiary claims relate to advantageous developments of the polyacrylate, processes for their preparation and the use of the polyacrylates.

The invention is eminently suitable for fulfilling the tasks presented, since such gentle crosslinking can be carried out in the polymer mixture shown. The invention accordingly relates to polyacrylates which can be obtained from the following components by thermal crosslinking of a polymer mixture:<ul id="ul0001" list-style="none" compact="compact"><li>a) A polyacrylate copolymer of the following monomers<ul id="ul0002" list-style="none" compact="compact"><li>a1) Acrylic acid esters and / or methacrylic acid esters of the following formula         CH₂ = CH (R¹) (COOR²), where R² = H or CH₃ and R² is an alkyl chain with 1 - 20 C atoms, 65 - 99% by weight, based on a),</li><li>a2) olefinically unsaturated monomers with functional groups, 0 - 15% by weight, based on a),</li><li>a3) acrylates and / or methacrylates, the alcohol component of which contains tert-butoxycarbonyl (BOC) and / or hydroxyl groups, 1 - 20% by weight, based on a),</li></ul> 85 - 99.8% by weight, based on the polymer mixture,</li><li>b) a photoinitiator regulating the polymerization 0.1-10% by weight, in particular 0.5-1% by weight, based on the polymer mixture,</li><li>c) bifunctional isocyanate and / or bifunctional epoxy 0.1 - 5% by weight, in particular 0.5 - 1% by weight, based on the polymer mixture.</li></ul>

The invention further relates to a process for the production of crosslinked polyacrylates, in which the polymers to be crosslinked are first protected by introducing tert-butoxycarbonyl groups and the crosslinking only takes place after deprotection by thermal treatment of the now deprotected polyacrylates.

The introduction of the protective groups serves to avoid the crosslinking reaction that is only sought afterwards, when high process temperatures already exist during earlier processing stages, as is the case, for example, in the hotmelt process. The protection applies in particular to the crosslinking reaction at this point in time, but also to all other competitive reactions which would attack the unprotected functional groups of the polymer to be processed, in particular its hydroxide groups. Competitive reactions in this sense are reactions initiated and / or promoted by thermal energy as well as reactions that are initiated and / or promoted by other forms of energy and which can be avoided by introducing the protective group.

It is particularly advantageous to add additional crosslinking substances to the polymer mixture to be crosslinked before the crosslinking reaction. Suitable crosslinking substances in this sense are bi- or multifunctional isocyanates or bi- or multifunctional epoxides. However, all other bi- or multifunctional compounds which are known to the person skilled in the art and are capable of crosslinking polyacrylates can also be used here.

The invention also relates to the use of the polyacrylate as a pressure-sensitive adhesive, in particular the use as a pressure-sensitive adhesive for an adhesive tape, the acrylic pressure-sensitive adhesive being present as a single-sided or double-sided film on a carrier film.

The carrier material, for example for adhesive tapes, can be the materials which are familiar and customary to the person skilled in the art, such as foils (polyester, PET, PE, PP, BOPP, PVC), nonwovens, fabrics and fabric foils and, if appropriate, release paper. This list is not intended to be exhaustive.

The general reaction principle for the crosslinking reaction controlled by the introduction of BOC protective groups is shown below: The polymer mixture is crosslinked by two successive reactions. In the first step, a BOC-functionalized polymer is produced. A relatively simple possible way of doing this is by a polymer-analogous reaction, that is to say by a reaction on the macromolecules which modifies the chemical composition but does not significantly change the degree of polymerization. Reactions occurring at this stage and caused by the production process do not attack the protected hydroxide groups.

The polymer now containing BOC groups is then deprotected again by an acid-catalyzed reaction, the BOC groups being converted to the free hydroxyl group:<chemistry id="chem0001" num="0001"><img file="EP1127907A2_D0001.tif" /></chemistry>

This reaction is also used in photoresist technology (photoresists: light-sensitive, film-forming materials (photoresists), the solubility behavior of which changes due to exposure or radiation; in the case of negative-working photoresists, this occurs through crosslinking or photopolymerization) for the production of chip structures. The acid required for this process is produced by the UV radiation of an added photocation initiator.

In the second step, the hydroxy functions are used for the reaction with a crosslinker. Generally one can distinguish between two reactions:<ul id="ul0003" list-style="none" compact="compact"><li>a) Reaction with isocyanates as crosslinkers</li><li>b) reaction with epoxides as crosslinkers</li></ul>

The reaction with isocyanates proceeds without the influence of acid at elevated temperatures.<chemistry id="chem0002" num="0002"><img file="EP1127907A2_D0002.tif" /></chemistry>

In contrast, the reaction with the epoxides only takes place under acid catalysis. The supply of thermal energy is also necessary here.<chemistry id="chem0003" num="0003"><img file="EP1127907A2_D0003.tif" /></chemistry>

If these reaction sequences are now transferred to PSAs, at least the base polymers must contain BOC functions. Polymers containing hydroxyl groups, which can be prepared, for example, conventionally via free radical polymerization, are then reacted with BOC anhydride in a polymer-analogous reaction, whereby the BOC-protected polymers are obtained. Both the base polymers and the polymers provided with the BOC groups are notable for their low viscosity. The flow viscosity of the uncrosslinked BOC-containing PSAs is relatively low, since there are hardly any polar comonomers which could form pronounced hydrogen bonds. At this stage, the polymer mass therefore has very good processability in the hotmelt process.

After processing and adding UV cation initiators as well as diisocyanates or diepoxides or multifunctional isocyanates or multifunctional epoxides as crosslinking agents, the component mixture is then applied as a hot melt pressure sensitive adhesive, e.g. via a coating nozzle, to a carrier (e.g. polyester, PVC, fleece, BOPP) or on release paper Formed a thin layer and irradiated with UV light (for example mercury low pressure lamp or emission lamp). This is where reaction (1) takes place; due to the photocation generator, an acid is formed which reacts with the BOC group to form the free hydroxy function. This reaction takes place at temperatures&gt; 100 ° C. A range between 120 and 140 ° C. is preferred as the optimum. With the thermal release of the hydroxy reaction, reactive groups are now available for the multifunctional crosslinkers, so that the reactions (2a) or (2b) run off: at these temperatures, the isocyanates react directly with the resulting hydroxyl functions and crosslink the PSA on the backing. The protons required for crosslinking with the multifunctional epoxides are present due to the acid generated by the photocation initiator. Therefore, the cross-linking with the multifunctional epoxides is in situ.

Crosslinking with isocyanates forms amide groups that link the polymer chains. Linking increases the cohesion of the adhesive and thus the shear strength. The same applies to crosslinking via epoxides for the ether groups formed.

The physical properties of the end product, in particular its viscosity, adhesive strength and tack, can be influenced by the degree of crosslinking, so that the end product can be optimized by a suitable choice of the reaction conditions. By selective irradiation of the acrylic PSA, i.e. irradiation only in selected areas, or The local variation in the radiation intensity makes it possible to remove the BOC protective groups only at selected points in the PSA. Accordingly, only the areas previously exposed to UV radiation are available for the later crosslinking reaction, for which the presence of polar groups is a prerequisite. Therefore, in the networking step, highly networked segments can be created alongside non-networked or poorly networked segments. The selection of areas which are exposed to high radiation intensity, in addition to those into which the radiation is not or only slightly intensified, can be done by irradiation through a mask, in particular a grid or a shadow mask, which is only locally permeable for the radiation, or alternatively an embossed film which has an inhomogeneous thickness and / or density and therefore zones of different transmission for ultraviolet radiation.

FIG. 1 shows the irradiation of the acrylate mass (2) through a shadow mask (1), the acrylate mass (2) being on the carrier (3) (a). The initially polar groups of the acrylate composition (1) have been converted into non-polar groups by introducing BOC protective groups; by UV light (4) and at high temperatures (&gt; 80 ° C) they are converted back into polar acrylic acid groups. The ultraviolet rays (4) can only penetrate the mask (1) in the area of ​​the holes (11), so that after the irradiation the situation shown in part (b) of the figure results:

Polar groups can only be found in certain areas (21) of the acrylate composition (2), in addition to these there are other areas (22) in which only non-polar functions protected by BOC groups are present.

If the acrylic PSA is now heated, a crosslinking reaction occurs only in the polar areas (23) due to hydrogen interactions between the polar groups, so that these areas (23) are characterized by a high hardness , while the non-polar areas (24) are one have lower viscosity (c).

Another effect is the release of isobutene gas and carbon dioxide at the cross-linking points. This reaction reduces the molecular weight of the crosslinked areas (23) of the PSA (2) and a volume contraction (231) is observed. The layer thickness of the acrylic PSA (2) is reduced at the irradiated points (23); this changes the surface topography of the PSA (2) and thus also the technical adhesive properties. The contracted areas (231) have a property comparable to suction cups, so that the adhesive force of the acrylic PSA (2) is improved due to this "suction cup" effect.

The soft segments (24) make it easier for the adhesive to flow onto the substrate and thus also increase the adhesive strength and the tack (tack). The percentage of the irradiated area and the size of the segments produced have a major influence on the adhesive properties.

Examples

The following exemplary experiments are intended to explain the invention in greater detail without the invention being restricted unnecessarily by the choice of the examples given.

Test methods:

The following test methods were used to examine the samples:

Shear strength (test A)

A 13 mm wide strip of the adhesive tape was applied to a smooth steel surface which was cleaned three times with acetone and once with isopropanol. The application area was 20 mm × 13 mm (length × width). The adhesive tape was then pressed onto the steel support four times with constant contact pressure. A 0.5 kg weight was attached to the adhesive tape at 70 ° C., a 1 kg weight was measured at room temperature, and the time until the weight dropped was measured in each case. The measured shear life is given in minutes and corresponds to the average of three measurements.

Dynamic mechanical analysis

<u style="dash">1</u>

DMA (Test B)

The measurements were carried out using the Rheometrics Dynamic Stress Rheometer. At 25 ° C the mechanical loss factor tan δ was tracked as a function of frequency in an interval from 0.1 to 100 rad / s. The temperature dependence of the loss factor was measured at 10 rad / s in a temperature range from -25 ° C to 130 ° C. All experiments were carried out with a parallel plate arrangement.

Sample preparation

The preparation of the starting polymers is described below.

The investigated polymers were conventionally produced via free radical polymerization; the average molecular weight was about 800,000 g / mol. Polymers containing hydroxyl groups were reacted with BOC anhydride in a polymer-analogous reaction. A UV cation generator was added as a second component to 0.1-15 parts by weight. The third component is 0.1 to 5 parts by weight of the multifunctional (containing at least two isocyanate or epoxy functions) isocyanates or epoxides. This component mixture is then applied as a hotmelt pressure-sensitive adhesive, for example via a coating nozzle, to a support (for example polyester, PVC, nonwoven, BOPP) or release paper in the form of a layer and irradiated with UV light (for example low-pressure mercury lamp or emission lamp).

Introduction of the protective function

For the introduction of the BOC groups into the base polymers, the following monomers were conventionally incorporated into the polymer by radical polymerization:<chemistry id="chem0004" num="0004"><img file="EP1127907A2_D0004.tif" /></chemistry><ul id="ul0004" list-style="none" compact="compact"><li>HEMA = hydroxyethyl methacrylate</li><li>HPA = hydroxypropyl acrylate</li><li>4-HS = 4-hydroxystyrene</li></ul>

example 1

A 2 L glass reactor conventional for radical polymerizations was charged with 40 g HEMA, 40 g methyl acrylate, 320 g 2-ethylhexyl acrylate, and 300 g acetone / isopropanol (97: 3). After nitrogen gas had been passed through for 45 minutes with stirring, the reactor was heated to 58 ° C. and 0.4 g of AIBN [2,2'-azobis (2-methylbutyronitrile)] was added. The outer heating bath was then heated to 75 ° C. and the reaction was carried out constantly at this outside temperature. After 4 and 6 hours, the mixture was diluted with 150 g of acetone / isopropanol mixture in each case. After a reaction time of 48 hours, the polymerization was stopped and the mixture was cooled to room temperature.

Example 2

The procedure was analogous to Example 1. 40 g of HPA, 80 g of methyl acrylate and 280 g of 2-ethylhexyl acrylate were used for the polymerization. The amount of solvent was maintained.

Example 3

The procedure was analogous to Example 1. 40 g of 4-hydroxystyrene, 300 g of 2-ethylhexyl acrylate and 60 g of methyl acrylate were used for the polymerization. The amount of solvent was maintained.

The resulting polymers are summarized in Table 1. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="6" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">example</entry><entry namest="col2" nameend="col2" align="center">HEMA [%]</entry><entry namest="col3" nameend="col3" align="center">2-EHA [%]</entry><entry namest="col4" nameend="col4" align="center">HPA [%]</entry><entry namest="col5" nameend="col5" align="center">MA [%]</entry><entry namest="col6" nameend="col6" align="center">4-HS [%]</entry></row></thead><tbody valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">1</entry><entry namest="col2" nameend="col2" align="center">10</entry><entry namest="col3" nameend="col3" align="center">80</entry><entry namest="col4" nameend="col4" align="center">0</entry><entry namest="col5" nameend="col5" align="center">10</entry><entry namest="col6" nameend="col6" align="center">0</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">2</entry><entry namest="col2" nameend="col2" align="center">0</entry><entry namest="col3" nameend="col3" align="center">70</entry><entry namest="col4" nameend="col4" align="center">10</entry><entry namest="col5" nameend="col5" align="center">20</entry><entry namest="col6" nameend="col6" align="center">0</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">3</entry><entry namest="col2" nameend="col2" align="center">0</entry><entry namest="col3" nameend="col3" align="center">75</entry><entry namest="col4" nameend="col4" align="center">0</entry><entry namest="col5" nameend="col5" align="center">15</entry><entry namest="col6" nameend="col6" align="center">10</entry></row><row><entry namest="col1" nameend="col6" align="justify">HEMA = hydroxyethyl methacrylate</entry></row><row><entry namest="col1" nameend="col6" align="justify">2-EHA = 2-ethylhexyl acrylate</entry></row><row><entry namest="col1" nameend="col6" align="justify">HPA = hydroxypropyl acrylate</entry></row><row><entry namest="col1" nameend="col6" align="justify">MA = methyl acrylate</entry></row><row rowsep="1"><entry namest="col1" nameend="col6" align="justify">4-HS = 4-hydroxystyrene</entry></row></tbody></tgroup></table></tables>

The polymers 1-3 were then protected in a polymer-analogous reaction. Di-tert-butyl dicarbonate with catalytic amounts of N, N-dimethylaminopyridine or the reaction with chloroformic acid tert-butyl ester with previous deprotonation by sodium hydride are suitable for the reaction.

a) Polymer-analogous reaction with di-tert-butyl dicarbonate

The polymers from Examples 1-3 (100 g each) were dissolved in an acetone / toluene mixture (500 ml) and 1.2 equivalents of di-tert-butyl dicarbonate (based on the number of hydroxyl groups) were dissolved with stirring in 150 ml of toluene. After 24 h the reaction was stopped and the solvents were removed in vacuo.

b) Polymer-analogous reaction with chloroformic acid tert-butyl ester

The polymers from Examples 1-3 (200 g each) were dried in vacuo and then dissolved in 500 ml of toluene. With stirring and under an argon atmosphere, 1 equivalent of NaH (based on the number of hydroxyl groups) was added. After a reaction time of 60 minutes, 1.5 equivalents of tert-butyl chloroformate were added dropwise and the reaction was heated to 80 ° C. for 24 h. After cooling to room temperature, aqueous ammonium chloride solution was added, the organic phase was extracted several times with aqueous solution and the polymer was dried at 60 ° C. for 24 hours.

c) reaction of the monomers with di-tert-butyl dicarbonate

The monomers HEMA, HPA or 4-HS were dissolved in THF and 1.2 equivalents of di-tert-butyl dicarbonate (based on the number of hydroxyl groups) were added with stirring. After 24 h the reaction was stopped and the solvent was removed in vacuo. The BOC-protected products were purified by distillation.

The comonomer composition did not change as a result of the reaction, only the hydroxyl groups were protected BOC. The resulting protected compounds are referred to below as 1-BOC (from Example 1 and BOC), 2-BOC (from Example 2 and BOC) and 3-BOC (from Example 3 and BOC).

Alternatively, Examples 1-BOC to 3-BOC can be prepared by polymerizing the BOC-protected monomers:<chemistry id="chem0005" num="0005"><img file="EP1127907A2_D0005.tif" /></chemistry>

In order to check the suitability for the hot melt technology, the PSAs were examined by DMA. Examples are given in FIG. 1 (flow viscosity at 130 ° C. of 1-BOC) and FIG. 2 (DMA measurement of 1-BOC. Temperature sweep at 10 rad / s).

The dynamic glass transition point (T_G), this is around -10 ° C for 1-BOC. The flow viscosity at 130 ° C. was determined from FIG. Processing temperatures of over 100 ° C are usual temperatures for the hotmelt process, therefore pressure sensitive adhesives are required which have a relatively low flow viscosity at these temperatures. The flow viscosity measured for 1-BOC (diagram 1) is in the range of 1000 - 5000 Pa^*s at 1 rad / s, this material can therefore be processed very well in the extruder.

Networking of the samples

Example 1va

100 g of the adhesive 1-BOC (50% solution) were mixed with 2 g of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]; 50% solution) and 1 g of aromatic polyisocyanate (Desmodur L75 ® [BAYER AG]). After homogenization (removal of the solvent), the adhesive was spread onto release paper (siliconized paper), irradiated with UV light for 60 seconds and heated at 120 ° C. for 10 minutes. The xenon chloride lamp (VIB 308 Bulb [FUSION]) was used for UV radiation. The mass was then tested using adhesive technology.

Example 2va

100 g of the adhesive 2-BOC (50% in solution) were mixed with 2 g of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]; 50% solution) and 1 g of aromatic polyisocyanate (Desmodur L75 ® [BAYER AG]). After homogenization (removal of the solvent), the adhesive was spread on release paper (siliconized paper), irradiated for 60 seconds with UV light of 308 nm and heated at 120 ° C. for 10 minutes. The xenon chloride lamp (VIB 308 Bulb [FUSION]) was used for UV radiation. The mass was then tested using adhesive technology.

Example 3va

100 g of the 3-BOC adhesive (50% in solution) were mixed with 2 g of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]; 50% solution) and 1 g of aromatic polyisocyanate (Desmodur L75 ® [BAYER AG]). After homogenization (removal of the solvent), the adhesive was spread on release paper (siliconized paper), irradiated for 60 seconds with UV light of 308 nm and heated at 120 ° C. for 10 minutes. The xenon chloride lamp (VIB 308 Bulb [FUSION]) was used for UV radiation. The mass was then tested using adhesive technology.

Example 1vb

100 g of the adhesive 1-BOC (50% in solution) were mixed with 2 g of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]; 50% solution) and 1 g of bisepoxidized bisphenol A (Rütapox 164 ® [BAKELITE AG]). After homogenization (removal of the solvent), the adhesive was spread on release paper (siliconized paper), irradiated for 60 seconds with UV light of 308 nm and heated at 120 ° C. for 10 minutes. The xenon chloride lamp (VIB 308 Bulb [FUSION]) was used for UV radiation. The mass was then tested using adhesive technology.

Example 2vb

100 g of the adhesive 2-BOC (50% in solution) were mixed with 2 g of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]; 50% solution) and 1 g of bisepoxidized bisphenol A (Rütapox 164 ® [BAKELITE AG]). After homogenization (removal of the solvent), the adhesives were spread on release paper (siliconized paper), irradiated for 60 seconds with UV light of 308 nm and heated at 120 ° C. for 10 minutes. The xenon chloride lamp (VIB 308 Bulb [FUSION]) was used for UV radiation. The mass was then tested using adhesive technology.

Example 3vb

100 g of the adhesive 3-BOC (50% in solution) were mixed with 2 g of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]; 50% solution) and 1 g of bisepoxidized bisphenol A (Rütapox 164 ® [BAKELITE AG]). After homogenization (removal of the solvent), the adhesives were spread on release paper (siliconized paper), irradiated for 60 seconds with UV light of 308 nm and heated at 120 ° C. for 10 minutes. The xenon chloride lamp (VIB 308 Bulb [FUSION]) was used for UV radiation. The mass was then tested using adhesive technology.

Due to the UV-initiated thermal crosslinking with the aromatic polyisocyanate (Desmodur L75 ® [BAYER AG]) the flow viscosity has increased significantly (Figure 3: Flow viscosity at 130 ° C. Comparison of 1-BOC with 1va). This is a clear indication of an increase in cohesion and thus also of successful networking on the carrier. Furthermore, the glass transition temperature of 1va was again measured using a temperature-dependent DMA measurement and compared with the uncrosslinked starting material 1-BOC (FIG. 4: temperature sweep at 10 rad / s. Comparison of 1-BOC with 1va).

The difference in the dynamic glass transition temperature is extremely small. In contrast, the mechanical loss factor tan δ changes at higher temperatures. For the cross-linked polyacrylate, the curve is significantly flatter, which in turn indicates efficient cross-linking.

In order to investigate the influence of crosslinking on the adhesive properties, the thermally crosslinked hotmelts were tested in comparison to the starting polymers. The results are listed in Table 2:<tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="3" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center">example</entry><entry namest="col2" nameend="col2" align="center">SSZ 10 N, RT [min]</entry><entry namest="col3" nameend="col3" align="center">SSZ 5 N, 70 ° C [min]</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">1-BOC</entry><entry namest="col2" nameend="col2" align="center">50</entry><entry namest="col3" nameend="col3" align="center">40</entry></row><row><entry namest="col1" nameend="col1" align="center">2-BOC</entry><entry namest="col2" nameend="col2" align="center">75</entry><entry namest="col3" nameend="col3" align="center">40</entry></row><row><entry namest="col1" nameend="col1" align="center">3-BOC</entry><entry namest="col2" nameend="col2" align="center">110</entry><entry namest="col3" nameend="col3" align="center">50</entry></row><row><entry namest="col1" nameend="col1" align="center">1va</entry><entry namest="col2" nameend="col2" align="center">1940</entry><entry namest="col3" nameend="col3" align="center">1205</entry></row><row><entry namest="col1" nameend="col1" align="center">2va</entry><entry namest="col2" nameend="col2" align="center">2150</entry><entry namest="col3" nameend="col3" align="center">1180</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">3va</entry><entry namest="col2" nameend="col2" align="center">3475</entry><entry namest="col3" nameend="col3" align="center">2030</entry></row></tbody></tgroup></table></tables> 1va, 2va, 3va cross-linked with aromatic polyisocyanate Desmodur L75 ® [BAYER AG] SSZ: Shear life

It can be seen from Table 2 that the crosslinking significantly improves the cohesion of the PSAs. Comparing Examples 1-BOC and 1va with one another, it is found that the shear life increases from 50 to 1940 minutes at room temperature and from 40 to 1205 minutes at 70 ° C. The same trend is also found for Examples 2 and 3 and 2va and 3va respectively.

Networking with bi- and multifunctional epoxies has also been experimentally investigated. The adhesives 1-BOC, 2-BOC and 3-BOC - analogous to the isocyanate crosslinkers - were mixed with 1 part by weight of epoxy crosslinker and 2 parts by weight of triacrylsulfonium hexafluorophosphate (Cyracure UVI-6990 ® [UNION CARBIDE]) in solution, the solvent evaporated and the adhesives are applied as a hot melt on a carrier coated with an adhesion promoter. It was then irradiated with UV light under the same conditions as for the isocyanate crosslinking, but then tempered for a period of 20 minutes.

Again, as the results in Table 3 show, the thermal crosslinking reaction increases the cohesion of the mass and thus also the shear strength. <tables id="tabl0003" num="0003"><table frame="all"><title>Table 3</title><tgroup cols="3" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center">example</entry><entry namest="col2" nameend="col2" align="center">SSZ 10 N, RT [min]</entry><entry namest="col3" nameend="col3" align="center">SSZ 5 N, 70 ° C [min]</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">1-BOC</entry><entry namest="col2" nameend="col2" align="center">50</entry><entry namest="col3" nameend="col3" align="center">40</entry></row><row><entry namest="col1" nameend="col1" align="center">2-BOC</entry><entry namest="col2" nameend="col2" align="center">75</entry><entry namest="col3" nameend="col3" align="center">40</entry></row><row><entry namest="col1" nameend="col1" align="center">3-BOC</entry><entry namest="col2" nameend="col2" align="center">110</entry><entry namest="col3" nameend="col3" align="center">50</entry></row><row><entry namest="col1" nameend="col1" align="center">1vb</entry><entry namest="col2" nameend="col2" align="center">1560</entry><entry namest="col3" nameend="col3" align="center">950</entry></row><row><entry namest="col1" nameend="col1" align="center">2vb</entry><entry namest="col2" nameend="col2" align="center">2340</entry><entry namest="col3" nameend="col3" align="center">1380</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">3vb</entry><entry namest="col2" nameend="col2" align="center">3125</entry><entry namest="col3" nameend="col3" align="center">2120</entry></row></tbody></tgroup></table></tables> 1vb, 2vb, 3vb cross-linked with bis-epoxidized bisphenol A (Rütapox 164 ® [BAKELITE AG]) SSZ: Shear life

For the crosslinked examples 1vb, 2vb and 3vb, an increased cohesion, based on the starting polymer, was also measured. The shear life increased significantly. For example, in Example 2vb the shear life increased from 75 to 2340 minutes at room temperature and from 40 to 1380 minutes at 70 ° C. Nevertheless, the patterns made with different crosslinkers diverge in shear strength. For example, Example 1-BOC, provided it was thermally crosslinked with the aromatic polyisocyanate (1va), 1940 minutes, and the product crosslinked with the diepoxide (1vb) only 1560 minutes. This can be explained by the different molecular weight of the crosslinkers and thus also the different network arc length of the crosslinking.

In general, no damage to the carrier occurred in all UV-initiated thermal crosslinking of the hotmelts. The pressure-sensitive adhesive tapes produced can thus be used very well, for example, as “transfer tapes” for double-sided bonding. The carriers or release papers are not changed or destroyed by ionizing radiation.