Process for curing layers of radiation curable adhesives

The invention relates to a method for the metered crosslinking of pressure-sensitive adhesive layers by radiation, in particular UV light.

With conventional pressure sensitive adhesives PSA (= pressure sensitive adhesives) based on acrylate, SBC or NR [SBC = styrene block copolymers such as SIS (= styrene / isoprene / styrene) or SBS (= styrene / butadiene / styrene), SBR ( = styrene / butadiene rubber); NR = nitrile rubber = butadiene / acrylonitrile rubber] in hotmelt, dispersion or solvent application, the PSA properties can be influenced by the addition of tackifying substances (tackifiers). This includes both tackifying resins and plasticizers, such as oils.

The latest generation of pressure sensitive adhesives is based on solvent-free hotmelt pressure sensitive adhesives which, after being applied as a layer to a base or substrate, can be crosslinked by irradiation, especially with UV light. Such adhesives are described, for example, by Auchter, Barwich, Rehmer and Jäger in the article "UV-crosslinkable acrylate hotmelt pressure-sensitive adhesives" in "tack & seal" 37 (1993), pages 14 to 20 and in EP-A 377 199 and 448 741 . The base polymer is made so that it is close to the so-called gel point. With the help of photoinitiators, which are either admixed or chemically bound, the polymer is sensitized to high-energy radiation, in particular UV-A radiation. The degree of crosslinking of the polymer can be set by targeted exposure to a specific UV dose. This degree of crosslinking largely determines the final properties of the pressure sensitive adhesive. For example, with one and the same polymer, only by irradiation with different UV doses, a very sticky adhesive label can be produced, which even adheres to moist substrates, or an adhesive label that is cross-linked to such an extent that it can be removed from any surface without residue .

Such final properties can basically be set as described above. It is only irradiated with so much UV light until the desired properties are set. An "overexposure" would otherwise change the product properties drastically.

Optimal pressure sensitive adhesive properties, which are described by the "tack", the peel resistance and the shear strength (holding power and shear adhesion failure temperature = SAFT), are difficult to reconcile in a homogeneous PSA. High tack and high peeling resistance are associated with poor shear strength and vice versa.

The object of the invention was to influence the properties of a radiation-crosslinkable pressure-sensitive adhesive layer in such a way that an overall optimal combination of the properties described above is achieved.

According to the invention, a method for crosslinking radiation-crosslinkable pressure-sensitive adhesive layers by metered irradiation with actinic radiation is proposed, which consists in exposing the layer surface to a regular or random radiation pattern from areas of different intensities.

According to one embodiment of the method, the radiation pattern consists of areas of finite intensity in addition to non-radiation areas.

In the context of the present invention, actinic radiation is to be understood to mean any radiation which causes irreversible crosslinking in the crosslinkable layer to be irradiated. UV light, short-wave visible light and electron radiation are particularly suitable.

The method according to the invention offers an expanded possibility of controlling the final properties of radiation-crosslinkable, in particular UV-crosslinkable, pressure-sensitive adhesives: localized radiation in the adhesive surface makes it possible to add adjacent, more or less strongly crosslinked or non-crosslinked areas in a layer composed of a radiation-crosslinkable PSA produce.

This can be done by selective irradiation of the areas to be cross-linked, for example by UV or electron radiation focused with optical or magnetic lenses, or by grid-like irradiation, for example with suitable, temporally and locally controlled laser light.

However, this can also be done by targeted shading of the areas of the pressure-sensitive adhesive layer that are less cross-linked by means of masks, stencils or by means of dark or shadow areas generated by interference phenomena. In a continuous processing process, the devices required for this can either run along with the web-shaped supports to be irradiated or can be permanently mounted and generate a locally movable shadow or intensity pattern.

The above-mentioned possibilities for locally inhomogeneous irradiation bring about a locally inhomogeneous crosslinking which produces a crosslinking pattern in the layer of PSA. This networking pattern can randomly have statistically distributed more and less strongly networked areas or can be a repeating regular pattern. The more cross-linked areas can be connected and enclose island-shaped areas of less strongly cross-linked PSA, but the less cross-linked areas can also be connected and enclose the more cross-linked areas.

The size of the island-shaped areas can range from only a few nanometers to a few centimeters, but is preferably in the order of 0.001 to 10 mm.

The proportions of the more or less cross-linked areas in the total area of ​​the PSA can vary between 2 and 98%. They are preferably between 10 and 90%.

Through the application of the method according to the invention, it becomes possible, in addition to the composition of the adhesive layer and the radiation intensity, a further parameter, namely the area structure (pattern) and the area portion (distribution) of more and less strongly cross-linked areas, for the targeted influencing of the adhesion properties To make available.

In particular, less cross-linked areas can contribute to an improvement in the tack of the irradiated material. Although these less strongly networked areas have only a low shear strength, they can be supported by a surrounding pattern of more networked areas and secured against shearing or flowing under loads.

This enables the production of novel products with self-adhesive PSAs that can be crosslinked by UV or electron radiation and which are superior to the products available today in terms of the combination of tack (tack) and shear strength.

On the other hand, the proportions and properties of the more and less crosslinked areas can be selected by a suitable choice of the radiation parameters in such a way that an adhesive strip which can be removed easily and almost without residue is obtained. This is achieved in particular when the entire adhesive surface is exposed to cross-linking radiation and individual areas are cross-linked so strongly that they are virtually deactivated with regard to stickiness.

In the production of packing tapes, the use of the locally selective crosslinking of the adhesive application led to a reduction in the noise development when the packing tape rolls off the roll. This can reduce the annoying noise at packaging workplaces to an acceptable level.

A further application arises from the possibility that uncrosslinked areas of the adhesive application become cohesive during removal (i.e. with stringing and separation in the adhesive mass even with adhesive residues left behind both on the carrier of the adhesive strip and on the bonded surface) and more crosslinked areas of the Separate adhesive application adhesive (ie almost residue-free with respect to the living surface) when removing. If the non-crosslinked areas of the adhesive form a pattern or a lettering, the attempt to remove the adhesive tape produced in this way leaves clear traces in the surface of the adhesive tape and the stuck-on surface which confirm the attempt at removal, which is why an adhesive tape produced in this way is used as a seal-like seal can be, or can be used in the field of security labeling.

The degree of crosslinking of the pressure-sensitive adhesive layers used in the process according to the invention is generally determined by measuring the absorption on the irradiated layer. This preferably contains polymerizable or crosslinkable compounds together with crosslinking or polymerization initiators which are activated by actinic radiation and changed by participation in the reaction. Their absorption usually changes, so that the absorption measurement can be used to determine the degree of conversion.

The absorption measurement is preferably carried out by the method described in the older, not previously published German patent application 197 07 967.9 by confocal Raman spectroscopy or confocal fluorescence spectroscopy.

The photoinitiators are preferably chemically bound to the crosslinkable compounds in the pressure-sensitive adhesive layer, for example by covalence. All compounds which can change into an excited state by irradiation with UV light, in which they are able to react with the crosslinkable compounds, are suitable as photoinitiators. Aromatic carbonyl compounds, in particular benzophenone, its derivatives and substitution products - which are referred to collectively as benzophenone compounds - have proven to be particularly suitable for this purpose. According to the current state of knowledge, carbonyl groups activated by radiation react during the crosslinking to form a benzhydrol crosslinking structure, as described in the publication "UV-crosslinkable acrylate hotmelt PSAs" mentioned at the beginning.

The benzophenone residues react during the reaction, i.e. characteristic bands in the spectrum disappear.

Suitable crosslinkable compounds are in particular polymers and oligomers of (meth) acrylic esters. Suitable compounds are described, inter alia, in EP-A 448 741. These are copolymers of (meth) acrylic acid derivatives which are substituted by phenone residues, in particular benzophenone residues, with unsubstituted (meth) acrylic acid esters.

The crosslinkable compositions can generally be processed as melts, solutions or dispersions. They are particularly suitable for the production of pressure sensitive adhesives, pressure sensitive adhesive films and pressure sensitive adhesive labels as well as embossing forceps films. The masses can be applied in the usual manner, if appropriate at elevated temperature, for example at 20 to 150 ° C., to customary substrates such as paper, cardboard, wood, metals and plastic films, for example made of plasticized PVC, polyethylene, polyamides, polyesters or polypropylene.

The optionally dried or pre-dried applications are then crosslinked by irradiation with UV light, giving good-adhesive coatings which have high cohesion and good peel strength with excellent resistance to aging. It does not need to be irradiated under an inert gas atmosphere, but one can work in the air. The usual lamps, for example low-pressure, medium-pressure and high-pressure mercury vapor lamps, can be used as UV lamps. In some cases, crosslinking radiation also removed residual solvent or water through the IR portion of the lamps.

The degree of crosslinking of the crosslinked PSA layers is determined by Raman or fluorescence spectroscopy, the degree of conversion of the photoinitiator being determined on the basis of the intensity of characteristic spectral bands.

Raman spectroscopy allows chemical identification via the analysis of molecular vibrations, similar to the widespread infrared spectroscopy.

Confocal Raman microscopy combines the chemical structural information of Raman spectroscopy with the high spatial resolution of confocal optical microscopy of approx. 1 µm³ (see Figure 1). The setup links a confocal microscope with a Raman spectrometer in such a way that a laser is focused on a very small measurement volume via the imaging optics of the microscope. The resulting Raman light is first spectrally separated from the laser light in the detection beam path (eg via a holographic notch filter). The depth plane is selected via a confocal aperture in the detection beam path and thus the measurement volume three-dimensionally to a few µm³ narrowed down. A simple bright grating spectrograph can take over the spectral decomposition of the Raman light. The detection is carried out, for example, with a two-dimensional CCD chip.

In addition to the extensive mapping of chemical structures, depth profiles with µm resolution are also possible. Elaborate sample preparations are usually not necessary. Doping with labeling substances is usually unnecessary.

This measurement method enables the degree of crosslinking of a pressure-sensitive adhesive, which is responsible for the final properties, to be recorded exactly.

This takes advantage of the fact that the photoinitiator changes chemically during the UV exposure. This reaction can be followed very precisely via the decrease in characteristic Raman or fluorescence bands and provides information about the crosslinking. In particular, it can also be determined whether the photoinitiator has been completely used up, that is to say there is no fear of UV post-curing, which can change the adhesive effect.

In addition to fulfilling the tasks listed above, confocal Raman or fluorescence spectroscopy can be used for quality control of pressure sensitive adhesive products. Here, on the one hand, quality control in adhesive production comes into consideration and, on the other hand, quality control in cross-linked adhesive film.

A device for carrying out the measuring method described above is described below with reference to FIG. 1. This shows the schematic structure of a device for using confocal Raman spectroscopy to measure the degree of crosslinking.

The device according to FIG. 1 is essentially based on a combination of a confocal microscope with a laser light source and a Raman spectrometer. The excitation light of a laser light source 1 is redirected here by a notch filter 2, so that it reaches the sample 10 to be examined via the objective 3 of the microscope. A notch filter is particularly suitable for redirection because it reflects the specific laser wavelength almost completely. However, conventional mirror arrangements can also be used. The objective 3 defines the cross section of the observation volume in the sample 10, since it focuses the excitation light of the laser 1 and the resulting light cone limits the propagation of the laser light in the sample perpendicular to the direction of incidence. The laser light scattered inelastically in the sample 10 is received from the sample via the objective 3 and passed on to the confocal diaphragm 4 via the notch filter 2, which filters away excitation light reflected from the sample 10. This aperture 4 selects the depth of field of the observation volume in the sample 10 by its masking effect, that is, its position in the sample in the direction of incidence of the excitation light. After the aperture 4, the Raman scattered light reaches a grating spectrograph 6 via a lens 5, which spectrally decomposes the scattered light. The individual spectral lines are then passed to a two-dimensional CCD chip 7 as a photodetector, with the aid of which the intensity of the band is measured. Different depths can be set by changing the distance between the sample and the objective.

With an analog design of the measuring device, the depth-resolved degree of crosslinking of PSA layers can also be determined by confocal fluorescence spectroscopy. For this purpose, it is useful to use a UV light source in the device according to FIG. 1 and advantageously an edge filter instead of the notch filter 2. The fluorescence emission spectrum of the sample can be evaluated in a manner known per se analogously to the Raman spectroscopy described above.

The adhesive properties of flat substrates which have a pressure-sensitive adhesive layer can be determined by measuring the shear strength as a measure of the cohesion and the peel strength as a summary measure of cohesion and adhesion. For the test, films made of polyethylene terephthalate or aluminum are coated with a pressure-sensitive adhesive layer in such a way that a dry layer thickness of 25 to 80 μm results.

To test the solvent-free hotmelt PSAs, polyethylene terephthalate films are coated with the PSAs at 85 to 120 ° C. on a heated spreading table in such a way that a layer thickness of approximately 25 μm is obtained.

If dissolved copolymers are used for the test, the solvents are evaporated at 70 ° C. and 1 bar for 1 minute. The coated and dried foils are irradiated with the light from medium pressure mercury lamps.

Irradiation is carried out with one or more medium pressure mercury lamps arranged one behind the other, each with a power of 80 to 160 watts per cm of beam length. The coated and dried films are placed on a running endless belt, so that the coated films run under the lamps at a distance of 5 to 50 cm at a speed of 6 to 20 m / min. The radiation takes place in the air.

The foils produced in this way are cut into 2.5 cm wide strips and these strips are rolled onto an aluminum sheet with a rubberized steel roller. The plate and the strips are stored for 24 hours at 23 ° C. and 65% relative humidity.

To measure the peel strength, the test strips are peeled backwards parallel to the adhesive layer at a speed of 100 mm per minute. The force required for this is measured.

To measure the immediate tack (tack), a 2.5 cm wide adhesive tape with a length of 150 mm is formed into a loop and the two ends of the adhesive tape are clamped into the jaws of a tensile testing machine. The adhesive tape loop is brought into contact with a test plate made of stainless steel or aluminum and the adhesive tape is immediately removed at a speed of 300 mm per minute. The force required to pull the strip off the steel plate is measured. The mean value is formed from 6 measurements.

When measuring the shear strength, a 25 mm wide substrate strip coated with adhesive and irradiated as described is glued to a length of 25 mm with a glass or stainless steel test plate inclined at 3 ° to the vertical. The inclination of the plate guarantees that no peeling forces act on the adhesive. The overhanging part of the adhesive tape is loaded with a weight of 1 kg. The contaminated sample is then kept at room temperature for 5 hours and then heated at a heating rate of 2 ° C. per hour. The time and temperature are determined until the bond comes off. The temperature of the sample in ° C and the deformation of the sample (creepage distance) in µm are measured throughout the test.

The parts and percentages given in the following examples relate to the weight. The K values ​​are determined in accordance with DIN53726 in a 1% solution in tetrahydrofuran at 25 ° C. The melt viscosities are measured with a cone-plate rheometer, for example Rotovisko® RV 20 with measuring device PK 100 (Haake, Karlsruhe) (D = shear rate in s^-1).

The Staudinger index (viscosity number) is determined in tetrahydrofuran at 25 ° C using known methods (e.g. GV Schuh, H.-J. Cantow in Houben - Weyl, Methods of Organic Chemistry, G. Thieme Verlag, 1955, Vol. 3/1, Pages 431 to 445 and B. Vollmert: floor plan of macromolecular chemistry, Volume III, page 55 ff).

EXAMPLE 1

Copolymer solution P1

To a mixture of<dl id="dl0001" compact="compact"><dt>160 G</dt><dd>Ethyl acetate,</dd><dt>50 G</dt><dd>Tetrahydrofuran and</dd><dt>9 G</dt><dd>tert-butyl peroxy-2-ethyl hexanoate</dd></dl> were 150 g of a monomer mixture<dl id="dl0002" compact="compact"><dt>500 G</dt><dd>Isoamylacrylate,</dd><dt>300 G</dt><dd>2-ethylhexyl acrylate,</dd><dt>170 G</dt><dd>Methyl acrylate,</dd><dt>30th G</dt><dd>Acrylic acid and</dd><dt>7.5 g</dt><dd>a benzophenone derivative of the formula<chemistry id="chem0001" num="0001"><img file="EP0904853A2_D0001.tif" /></chemistry></dd></dl> given. The mixture was polymerized at 85 ° C for 15 minutes. The remainder of the monomer mixture was added to the reaction mixture over the course of 2 hours, and at the same time, but distributed over 3 hours, a solution of 5 g of tert-butylperoxy-2-ethylhexanoate in 40 g of ethyl acetate was added. After the addition had ended, the polymerization was continued for 5 hours.

A copolymer with a K value of 38.5 and a Staudinger index (viscosity number) of 0.39 was obtained. The copolymer P1, freed from solvent and volatile components, has a melt viscosity of 15 Pa · s (D = 100 s.) At 120 ° C^-1).

EXAMPLE 2

Copolymer solution P2

To a mixture of<dl id="dl0003" compact="compact"><dt>160 G</dt><dd>Ethyl acetate,</dd><dt>50 G</dt><dd>Tetrahydrofuran and</dd><dt>10th G</dt><dd>tert-butyl peroxy-2-ethyl hexanoate</dd></dl> were 150 g of a monomer mixture<dl id="dl0004" compact="compact"><dt>500 G</dt><dd>n-butyl acrylate,</dd><dt>330 G</dt><dd>2-ethylhexyl acrylate,</dd><dt>150 G</dt><dd>Methyl methacrylate,</dd><dt>20 G</dt><dd>Acrylic acid and</dd><dt>6.5 g</dt><dd>a benzophenone derivative of the formula<chemistry id="chem0002" num="0002"><img file="EP0904853A2_D0002.tif" /></chemistry></dd></dl> given. The mixture was polymerized at 80 ° C for 15 minutes. The remainder of the monomer mixture and, at the same time, but distributed over 3 hours, a solution of 5 g of tert-butylperoxy-2-ethylhexanoate in 40 g of ethyl acetate were added to the reaction mixture within 2 hours. After the addition had ended, the polymerization was continued for 5 hours.

A copolymer with a K value of 48 and a Staudinger index of 0.41 was obtained. The copolymer P2, freed from solvents and volatile components, has a melt viscosity of 40 Pa · s (D = 200 s.) At 120 ° C^-1).

EXAMPLE 3

Copolymer solution P3

270 g of a monomer mixture were obtained from a mixture of 280 g of a n-hexane-poor gasoline with a boiling range of 60 to 95 ° C. and 70 mg of 2,2'-azobis-isobutyronitrile<dl id="dl0005" compact="compact"><dt>870 G</dt><dd>2-ethylhexyl acrylate,</dd><dt>100 G</dt><dd>Methyl methacrylate,</dd><dt>30th G</dt><dd>Acrylic acid and</dd><dt>2.0 g</dt><dd>of the benzophenone derivative of the formula<chemistry id="chem0003" num="0003"><img file="EP0904853A2_D0003.tif" /></chemistry></dd></dl> given. The reaction mixture was polymerized at reflux temperature for 15 minutes. The remainder of the monomer mixture was then added over the course of two hours and the reaction mixture was kept under gentle reflux for a further two hours after the addition was complete. 10% by weight of a solution of 10 g of tert-butyl peroxypivalate in 50 g of the above-mentioned low n-hexane gasoline were then added to the mixture in the course of 5 minutes. After a further hour, the rest of this solution and at the same time 670 g of the same gasoline were added.

The solution of a copolymer P3 with a K value of 68 was obtained.

EXAMPLE 4

Copolymer P4

The procedure was as for copolymer P2, but a solution was used for the polymerization<dl id="dl0006" compact="compact"><dt>800 G</dt><dd>Isoamyl acrylate</dd><dt>180 G</dt><dd>Vinyl acetate,</dd><dt>20 G</dt><dd>Methacrylic acid and</dd><dt>7.0 g</dt><dd>of the benzophenone derivative of the formula<chemistry id="chem0004" num="0004"><img file="EP0904853A2_D0004.tif" /></chemistry></dd></dl> used. A copolymer with a K value of 36 was obtained.

EXAMPLE 5

Copolymer P5

The procedure was as for copolymer P2, but a solution was used for the polymerization<dl id="dl0007" compact="compact"><dt>900 G</dt><dd>Isoamylacrylate,</dd><dt>100 G</dt><dd>Macromer® 13 K-RC (styrene oligomer with a methacrylic acid end group; manufacturer Sartomer Inc.) and</dd><dt>6.5 g</dt><dd>of the benzophenone derivative of the formula<chemistry id="chem0005" num="0005"><img file="EP0904853A2_D0005.tif" /></chemistry></dd></dl> used. A copolymer with a K value of 45 was obtained.

Examples of suitable patterns

The following table shows the typical properties of some patterns with circular individual surfaces, which can be created on the adhesive strip by mask irradiation or one of the other methods mentioned. Polymer solution P2 was used to prepare the samples. Similar results could also be obtained with the other polymer solutions.<tables id="tabl0001" num="0001"><img file="EP0904853A2_D0006.tif" /></tables>

Results of the test of the pressure sensitive adhesive properties

The test of the peel strength is shown in Fig. 2.

The abscissa (x-axis) shows the number of passes under the irradiation unit with a lamp power of 160 W / cm, a distance of 10 cm and a speed of 10 cm / s. The ordinate (y-axis) indicates the peeling force in N / 25 mm sample width.

The non-hatched columns (for the multiple passes on the left in the picture) indicate the peeling forces after irradiation without a mask; the columns shown with a regular grid (on the right in the picture) indicate the peeling forces after irradiation under a shadow mask with pattern 5 at a distance of 3 mm from the adhesive surface.

When not cross-linked, the homogeneously irradiated adhesive strips show low peel values ​​with cohesive failure. The peel values ​​go through a maximum after 2 to 4 runs in the event of mixed failure (partly cohesive, partly adhesive) and after 8 irradiations the adhesive properties are reduced to such an extent that there is a sharp drop in the peel values ​​under purely adhesive failure. When using the perforated mask, the adhesive strips were previously passed through the installation without a mask in order to produce a homogeneous basic crosslinking.

The drop in the peel values ​​when passing through the UV system several times is significantly reduced, as shown in FIG. 2.

The immediate tack is shown in Fig. 3.

The gray grid fields show the immediate liability after irradiation without a mask; the dark fields after mask 5 irradiation. This shows that after two homogeneous irradiations (without a mask) with the same settings as for the peel strength test, the immediate adhesion is significantly reduced and almost disappears after 8 passes.

In the case of grid-type radiation, the immediate adhesion is retained even after 8 runs. A homogeneous pre-irradiation was carried out as described above by a pass under the UV source.

The shear strength is shown in Fig. 4.

The abscissa is the time in hours, the ordinate shows the shear path in µm for curves A to D on the left and the temperature in ° C for curve T on the right. Curve A shows the behavior after two UV irradiations without a mask, curve B after four times and curve C after eight times without a mask. Curve D shows the behavior after a single homogeneous UV irradiation and seven times UV irradiation under mask No. 5. The curve T indicates the temperature in ° C that was reached when the samples were heated at the point in time shown in the abscissa.

The measurements show that the twice homogeneously irradiated adhesive strips (curve A) have only a low shear strength and fail after only about 12 hours at 37 ° C. Irradiation four times (curve B) or eight times (curve C) increases the service life to 16 or 34 hours and increases the failure temperature to 44 or 78 ° C.

The comparison of a homogeneously cross-linked adhesive strip (curve C) with a sample which was produced by single homogeneous UV irradiation and subsequent locally selective mask irradiation (curve D) with a mask spacing of 3 mm through 7 passes shows only a slight difference in shear strength . The properties of immediate adhesion and peel strength, however, are not lost, as shown above, in contrast to the eight times homogeneously irradiated adhesive strips. This is the key advantage of the process.

5 shows the computed tomographic representation of a surface area of ​​a pressure-sensitive adhesive layer crosslinked by UV radiation under a shadow mask with circular holes of 0.35 mm in diameter. For this purpose, the surface was scanned with laser light and the intensity of the Raman scattered light produced was measured for each point. The absorption values ​​obtained by confocal Raman spectroscopy are shown as a measure of the degree of cross-linking in the image by means of different gray levels. The cross-linking pattern obtained can be seen.