Switchable adhesives

Abstract

The present invention provides switchable adhesives comprising a mixture, in proportions by weight, of 20% to 98% of an adhesive, 2% to 80% of curable molecules and 0.05% to 10% of photoinitiator in which the weight proportion of the adhesive is calculated on the basis of its dry weight and wherein the adhesive includes an internal cross-linker for cross-linking the adhesive during drying to provide a cohesive strength of between 5 and 100 N/12.7×12.7 mm measured according to FINAT test method No. 18. Preferably, the adhesive and curable molecules are mutually soluble when dry, or the curable molecules and adhesive may be uniformly dispersed in each other. Preferably the amount of adhesive in the mixture is in the range 40% to 98% by weight, more preferably 60% to 95% by weight, even more preferably 70% to 85% by weight. Preferably the proportion of curable molecules in the mixture ranges from 2% to 60% by weight, more preferably 5% to 40% by weight, even more preferably 15% to 30% by weight. Preferably, the photoinitiator is present in the mixture in the proportions 0.5% to 5% by weight, more preferably 1% to 3% by weight. Such switchable adhesives are useful in medical dressings and other removable sheet products, and may be simply prepared by stirring the adhesive, the curable molecules and the photoinitiator together at room temperature.

Claims

1. A switchable pressure sensitive medical adhesive composition comprising a mixture, in proportions by weight based on the weight of the composition, of: 20% to 98% of a base adhesive polymer constituent which has no bound-in curable groups that are curable by free radical polymerization; 2% to 80% of curable molecules that are curable by free radical polymerization and have a minimum weight average molecular weight of 500 dalton, and 0.05% to 10% of a photoinitiator, wherein the weight proportion of the base adhesive is calculated on the basis of its dry weight and wherein said composition includes an internal cross-linker that is curable by a mechanism other than free radical polymerization for cross-linking the adhesive during drying to provide a cohesive strength of between 5 and 100 N/12.7×12.7 mm measured according to FINAT test method No. 18.

2. A switchable pressure sensitive medical adhesive composition according to claim 1 comprising a mixture, in proportions by weight based on the weight of the composition, of: 40% to 98% of the base adhesive polymer constituent; 2% to 60% of the curable molecules, and 0.5% to 5% of the photoinitiator.

3. A switchable pressure sensitive medical adhesive composition according to claim 1 comprising a mixture, in proportions by weight based on the weight of the composition, of: 60% to 95% of the base adhesive polymer constituent; 5% to 40% of the curable molecules, and 0.5% to 5% of the photoinitiator.

4. A switchable pressure sensitive medical adhesive composition according to claim 1 comprising a mixture, in proportions by weight based on the weight of the composition, of: 70% to 85% of the adhesive polymer constituent; 15% to 30% of the curable molecules, and 1% to 3% of the photoinitiator.

5. A switchable pressure sensitive medical adhesive composition according to claim 1 wherein the proportion of internal cross-linker is from 0.1 to 6% in proportion by weight based on the wet base adhesive polymer constituent.

6. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the base adhesive polymer constituent is selected from the group consisting of polyacrylates, polyurethanes and polysilicones.

7. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the base adhesive polymer constituent is a mixture of at least two adhesives selected from a group consisting of polyacrylates, polyurethanes and polysilicones.

8. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the base adhesive polymer constituent is a polyacrylate.

9. A switchable pressure sensitive medical adhesive composition according to claim 1 wherein the curable molecules are unsaturated compounds.

10. A switchable pressure sensitive medical adhesive composition according to claim 9 wherein the curable molecules have more than one unsaturated site.

11. A switchable pressure sensitive medical adhesive composition according to claim 10 wherein the curable molecules have multiple functionalities of 3 or greater.

12. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the curable molecules are selected from the group consisting of acrylic acid esters and methacrylic acid esters of alcohols, glycols, pentaerythritol, trimethylpropane, glycerol, aliphatic epoxides, aromatic epoxides, aliphatic urethanes, silicones, polyesters, polyethers, ethoxylated species thereof, propoxylated species thereof, or mixtures thereof.

13. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the photoinitiator is selected from the group consisting of titanocene photoinitiators; dye/co-initiator systems; dye/borate salt systems; dye/peroxide systems and 1,2-diketone/co-initiator systems.

14. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the photoinitiator is reactive to visible light.

15. A switchable pressure sensitive medical adhesive composition as claimed in claim 1 wherein the reduction in peel force of the pressure sensitive adhesive after switching is 30 to 98%.

16. A switchable pressure sensitive medical adhesive composition as claimed in claim 15 wherein the reduction in peel force of the pressure sensitive adhesive after switching is 50 to 95%.

17. A method for manufacturing a switchable pressure sensitive medical adhesive composition according to claim 1, the method comprising stirring the adhesive polymer constituent, the curable molecules, the photoinitiator and the internal cross-linker together in darkness or under red light conditions for 30 to 60 minutes at a temperature from 10° C. to 40° C.

18. A method for manufacturing a switchable pressure sensitive medical adhesive composition according to claim 17 wherein the stirring temperature is room temperature.

19. A method for manufacturing a switchable pressure sensitive medical adhesive composition as claimed in claim 17 including a step of drying at 80° C. to 150° C.

20. A method for manufacturing a switchable pressure sensitive medical adhesive composition as claimed in claim 17 further comprising spreading the composition onto a release means and drying the composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be illustrated by way of example only with reference to the drawings, in which:

(2) FIG. 1 is a cross-section of an adhesive tape using a switchable adhesive in accordance with the invention;

(3) FIG. 2 is a cross-section of a dressing of the invention, and

(4) FIG. 3 is a schematic cross-sectional view of a simple adhesive label using the switchable adhesive of the invention.

(5) FIG. 4 is a graph showing the change in peel force with different amounts of cross-linker for various switchable PSAs formulated in accordance with Example 6.

(6) FIG. 5 is a graph showing the change in dynamic shear force with different amounts of cross-linker for various switchable PSAs formulated in accordance with Example 6.

(7) FIG. 6 is a graph showing the change in peel force with different amounts of cross-linker for various switchable PSAs formulated in accordance with Example 7.

(8) FIG. 7 is a graph showing the change in dynamic shear force with different amounts of cross-linker for various switchable PSAs formulated in accordance with Example 7.

(9) FIG. 8 is a graph showing the change in peel force with different amounts of cross-linker for various switchable PSAs formulated in accordance with Example 8.

(10) FIG. 9 is a graph showing the change in dynamic shear force with different amounts of cross-linker for various switchable PSAs formulated in accordance with Example 8.

(11) Referring to FIG. 1, an adhesive tape is designated by reference numeral 1. The tape comprises a backing layer 2 and an adhesive layer 3. The backing layer 2 comprises an occlusive layer 4 and a transparent layer 5 intermediate the occlusive layer 4 and the adhesive layer 3. The tape may optionally be provided with appropriate carrier layers and protector layers.

(12) Such an adhesive tape could be in the form of a surgical tape for medical applications. In use, the tape 1 is adhered to the skin of a patient when the adhesive layer 3 is in a tacky state. When it is desired to remove the tape 1 from the patient's skin, the occlusive layer 4 is removed to reveal the transparent layer 5 and thereby expose the adhesive layer 3 to visible light. The visible light causes the photoinitiator to initiate free-radical cross-linking of the pressure sensitive adhesive through the curable molecules incorporated in the adhesive mix. This results in the adhesive losing its tackiness and peel strength. The time required for complete switching of the adhesive from the tacky state to the non-tacky state may vary, e.g., from a few seconds to several minutes. The tape may then be removed from the patient's skin with reduced trauma to the patient.

(13) Referring now to FIG. 2, this shows a schematic cross-section of a medical dressing according to the present invention when in use on a patient.

(14) Medical dressing 10 is shown attached to a patient's skin 20. The dressing 10 comprises a wound facing absorbent layer 30 disposed beneath a protective backing layer 40. At opposed edges 50, 60, the backing layer 40 is provided with adhesive 70 which comprises groups that can be cross-linked under the influence of visible and/or UV light.

(15) The backing layer 40 is provided with a cover 80 which is releasably secured to the backing layer 40 by a weak adhesive 90. In an alternative arrangement, not shown here, the cover 80 may be laminated to the backing layer 40. For ease of removal, the cover 80 overlaps the backing layer 40 at its edges 100, 110.

(16) When it is desired to remove the dressing from the skin of the patient, the cover 80 can be gripped at its edges 100, 110 and peeled from the backing layer 40 to expose the adhesive 70 to UV or visible light irradiation. This irradiation acts so as to generate free radicals that cause the curable molecules to undergo a curing reaction which, after a certain time (depending upon the adhesive used), causes the adhesive 70 to lose its tackiness to such an extent that the dressing can be removed without causing trauma to the patient.

(17) In order that the removal of the cover 80 does not itself cause trauma to the patient, the peel strength of the adhesive 90 adhering the cover 80 to the backing layer 40 should be less than the peel strength of the adhesive 70 adhering the dressing 10 to the patient's skin.

(18) Since the adhesive 70 loses tackiness on exposure to visible and/or UV light, it is desirable that the adhesive 70 is not exposed to the light for a substantial period when the dressing 10 is applied to a patient. Thus, the adhesive 70 may be initially provided on the surface with release paper (not shown) which is preferably opaque to UV and visible light and which can be readily removed from the adhesive so that the dressing is ready for use when required.

(19) It is another requirement of the constituents of the adhesive mixture used in medical applications that they should be capable of undergoing sterilisation without causing switching of the adhesive from its tacky state to a non-tacky state. High-temperature sterilisation by autoclaving is not appropriate for medical products and/or utensils that are sensitive to heat; ethylene oxide sterilisation can be used instead.

(20) In many applications, a release film or release paper layer is applied over the switchable PSA layer and is removed just before the switchable PSA is applied to its working site. The release layer needs to be light occlusive to prevent switch of the switchable PSA during storage. As examples, the release layer may be a black siliconized PET film, or a PET film with an aluminium foil laminated to it.

(21) Turning now to FIG. 3, this shows in a schematic cross-sectional view how the invention can be applied to a simple two-layer device such as a removable product label.

(22) The label is generally denoted by the reference 210 and comprises an adhesive layer 213 composed of switchable adhesive in accordance with the present invention, and an occlusive layer 214 (which may be in the form of a film) overlying the adhesive layer and preventing access of light to the adhesive layer. In this embodiment, the occlusive layer is also provided with a peel tab 215 that facilitates removal of the label from the product surface to which it is applied.

(23) It is not essential in the case of a removable product label for the peel force required to remove the occlusive layer to be less than the peel force required to remove the adhesive in its tacky form from the product. In practice, it may be preferred if some of the adhesive layer is removed with the occlusive layer. The residue of the adhesive which remains on the product is then able to switch to its non-tacky state through exposure to light. The non-tacky adhesive residue can be easily removed from the product surface by rubbing or washing.

(24) The occlusive layer 214 may have a design/indicia on its surface. In some embodiments, the design/indicia may provide the occlusive effect and may therefore be disposed at the interface between the occlusive layer 214 and the adhesive layer 213 so that it is protected from scuffing damage.

(25) As mentioned above, one problem associated with switchable adhesives having bound-in curable groups is that they require painstaking synthesis. Also, they often require switching times of more than 1 minute. In particular, the polymerisation of acrylate internally functionalised switchable adhesives requires the use of multiple solvents, firstly in order to produce a polymer having a sufficiently high molecular weight and low monomer concentration for suitability as a medical adhesive, and secondly to carry out the reaction of the functionalising moiety with the main polymer chain.

(26) An adhesive manufacturing process requiring multiple solvents, and hence solvent exchange steps, is complicated, time consuming and expensive.

(27) By contrast, the switchable adhesives of the present invention can be made from a mixture of readily available adhesives and curable monomers or oligomers containing acrylate functions which, upon irradiation, form chemical bonds between the oligomers or monomers and hence create polymeric cross-linking. The effect of such cross-linking is to build a three-dimensional polymeric network interlocking with the adhesive polymer chains, thereby reducing their mobility and free volume. This change, which takes place very quickly, causes the PSA to become low tack. The force required to peel the adhesive has been found to reduce considerably, by at least 60% to 90%, after illumination.

(28) Examples of suitable curable monomers and oligomers, used alone or in mixtures, are acrylic acid esters or methacrylic acid esters of alcohols, glycols, pentaerythritol, trimethylpropane, glycerol, bisphenol A epoxides, aliphatic epoxides, aromatic epoxides, aliphatic urethanes, aromatic urethanes, silicones, polyesters and polyethers as well as ethoxylated or propoxylated species thereof.

(29) The invention will now be further illustrated with reference to Examples. In the Examples below, the constituents are listed in the order:

(30) 1 base adhesive(s)

(31) 2 curable molecules

(32) 3 photoinitiator and, where present,

(33) 4 stabilizer for preventing premature switch during storage

(34) 5 Stabilizer of internal cross-linker in solution.

(35) 6 Internal cross-linker.

EXAMPLE 1

(36) TABLE-US-00002 Component Amount (g)  1a Aroset 1450 Z 40 100.60  1b Aroset 1450 Z 40* 0.00 2 CN 925 33.10 3 Irgacure 784 0.52 4 Irganox 1010 0.10 *without internal cross-linker

EXAMPLE 2

(37) TABLE-US-00003 Component Amount (g)  1a Aroset 1450 Z 40 26.24  1b Aroset 1450 Z 40* 15.82 2 CN 925 10.43 3 Irgacure 784 0.30 4 Irganox 1010 0.03 *without internal cross-linker

EXAMPLE 3

(38) TABLE-US-00004 Component Amount (g)  1a Aroset 1450 Z 40 23.14  1b Aroset 1450 Z 40* 21.87 2 Omnilane P9200Z 14.61 3 Irgacure 784 0.32 4 Irganox 0.05 *without internal cross-linker

COMPARATIVE EXAMPLE 4

(39) TABLE-US-00005 Component Amount (g)  1a Aroset 1450 Z 40 20.00  1b Aroset 1450 Z 40* 20.00 2 CN 925 12.10 3 Irgacure 784 0.22 4 Irganox 0.03 *without internal cross-linker

COMPARATIVE EXAMPLE 5

(40) TABLE-US-00006 Component Amount (g)  1a Aroset 1450 Z 40 0.00  1b Aroset 1450 Z 40* 37.60 2 CN925 5.40 3 Irgacure 784 0.51 4 Irganox 0.03 *without internal cross-linker

EXAMPLE 6

(41) To a master batch consisting of:

(42) TABLE-US-00007 Component Amount (w/w %) 1 GMS 1753u 73.6 2 CN925 20.8 3 Irgacure 784 0.41 4 Irganox 1010 0.05 5 Methanol 5.1
a poly(melamine-co-formaldehyde), methylated solution (6) was added to a final concentration of 0, 0.31, 0.48, 0.62 and 0.77 weight percent, in total five samples, in preparation for peel and shear tests. The results are tabulated in Tables 3 and 4 and illustrated in FIGS. 4 and 5.

EXAMPLE 7

(43) To a master batch consisting of:

(44) TABLE-US-00008 Component Amount (w/w %) 1 Aroset 1910-TH-52 62.4 2 Ebecryl 870 22.8 3 Irgacure 784 0.4 4 Irganox 1010 0.1 5 Isopropanol 14.2
Aluminium acetylacetonate (6) was added to a final concentration of 0, 0.035, 0.061, 0.11 0.21 and 0.77 weight percent, in total six samples, in preparation for peel and shear tests. The results are tabulated in Tables 5 and 6 and illustrated in FIGS. 6 and 7.

EXAMPLE 8

(45) To a master batch consisting of:

(46) TABLE-US-00009 Component Amount (w/w %) 1 Polytex SP8002 75.2 2 CN925 23.5 3 Irgacure 784 1.2 4 Irganox 1010 0.1 5 none 0
Tolylene 2,4-diisocyanate (6) was added to a final concentration of 0, 0.016, 0.025, 0.039, 0.054 and 0.097 weight percent, in total six samples, in preparation for peel and shear tests. The results are tabulated in Tables 7 and 8 and illustrated in FIGS. 8 and 9.

(47) TABLE-US-00010 TABLE 1 Table of Suppliers Component Description Company Aroset 1450 Thermosetting acrylic solution Ashland Inc. Z 40 polymer, dry content 40% CN 925 Aliphatic urethane tetraacrylate Sartomer Co., Inc. (Cray Valley SA) Omnilane Tri functional polyester acrylate IGM resins P9200Z B.V. Netherlands Irgacure 784 Bis.(.eta.5-cyclo-pentadienyl)-bis Ciba Specialty (2,6-difluoro-3-[pyrrol-l-yl]-phenyl) Chemicals titanium Irganox 1010 Pentaerythritol Tetrakis(3-(3,5-di- Ciba Specialty tert-butyl-4-hydroxyphenyl) Chemicals propionate Hostaphane 23 my Polyester film Mitsubishi RNK 2600 Polyester Film SP 8002 Acrylic adhesive dry content 45% Avery Dennison Aroset Acrylic adhesive with 0.1-0.5%, Ashland Inc 1910-TH-52 according to MSDS, aluminium pentadionate as a cross-linker dry content 52% GMS 1753u Acrylic adhesive dry content 42% Poly(melamin-co- Cross-linker Sigma-Aldrich formaldehyde), methylated [or polyhexamethoxy methyl melamine] 84 wt. % solution in 1-butanol Aluminium Cross-linker Sigma-Aldrich acetylacetonate, ReagentPlus, 99% Tolylene 2,4- Cross-linker Sigma-Aldrich diisocyanate, 95% Ebecryl 870 Polyester acrylate Cytec Hostaphane 50 my Polyester film Mitsubishi RNK 2600 Polyester Film

(48) Preparative Details

(49) All components in the respective examples were loaded into a sealable glass jar and mixed to a homogenous solution over a period of approximately 60 minutes under red light conditions using a magnetic stirrer. The resulting adhesive solution was then spread onto a release liner using a spreader to a coating thickness of about 60 μm and left to dry at room temperature for 10 minutes.

(50) The adhesive coating was then further dried in a ventilated fan assisted oven at 110° C. for an additional 10 minutes. After drying, the thickness of the adhesive coating was about 30 μm.

(51) Finally, for peeling studies, a 23 μm Hostaphane RNK 2600 (polyester) film was transferred to the exposed side of the adhesive in preparation for peeling studies. For the dynamic shear tests, a 50 μm RNK 2600 (polyester) film was used; a thicker film was necessary in order to prevent film expansion during the tests. All procedures using Irgacure 784 were carried out under red light conditions.

(52) Peel Force Measurements

(53) Peel strengths were determined after a dwell time of 20 minutes using a LLOYD testing rig (L2000R) according to FINAT test method FTM1, with the exception that high density polyethylene (HDPE) plates were used as the substrate and that a peeling rate of 100 mm/min was used in order to collect all of the necessary data within the time frame of one peel force measurement.

(54) Dynamic Shear Strength Measurements

(55) Dynamic shear strength was obtained according to Finat test method (Finat technical handbook 6.sup.th edition 2001) FTM18 utilizing the same instrument as above.

(56) Adhesive switching was achieved by exposing the adhesive film (adhered to the HDPE plate) to light through the PET carrier film backing with a light intensity of approximately 12000 lux from a 500 W halogen lamp having a broad spectrum. It should be noted that switching times achievable with a light source intended for consumers (LEDs with spectra adjusted to the photoinitiator) will be less than the values obtained with the broad spectrum lamp mentioned above. However, using a light source in these tests that is adjusted to the photoinitiator would make it difficult to get an accurate measurement of the switching time for purposes of comparison, since the switching times would all have been very short. Switching times for the different coatings were measured as the time between the starting time of irradiation and the time when the substantially instantaneous loss of tack occurred, during a continuous peel strength test of about 1 minute (i.e., the adhesive was peeled for a period of time whilst being irradiated). Peel strengths and switching times were measured in quadruple and the average values of switch time and peel strength (before and after switch) were calculated.

(57) Adhesive Switching

(58) Results quoted below in Table 2 as %-switch refer to the percentage reduction in peel strength after exposure to light, calculated as follows:
(1−P1/P2)*100=%-switch
where P1 is the peel strength after exposure to light and P2 is the initial peel strength.

(59) On the following pages, tables of peel strength test results and dynamic shear strength test results are provided.

(60) Table 2 is a table of results of peel strength tests for Examples 1 to 3 and comparative examples 4 and 5.

(61) Table 3 is a table of results of peel strength tests for various switchable PSAs formulated in accordance with Example 6 with different proportions of cross-linker, the peel force being measured before and after switch. The results are also illustrated in FIG. 4.

(62) Table 4 is a table of results of dynamic shear force tests for the switchable PSAs formulated in accordance with Example 6 with their different proportions of cross-linker. The results are also illustrated in FIG. 5.

(63) Table 5 is a table of results of peel strength tests for various switchable PSAs formulated in accordance with Example 7 with different proportions of cross-linker, the peel force being measured before and after switch. The results are also illustrated in FIG. 6.

(64) Table 6 is a table of results of dynamic shear force tests for the switchable PSAs formulated in accordance with Example 7 with their different proportions of cross-linker. The results are also illustrated in FIG. 7.

(65) Table 7 is a table of results of peel strength tests for various switchable PSAs formulated in accordance with Example 8 with different proportions of cross-linker, the peel force being measured before and after switch. The results are also illustrated in FIG. 8.

(66) Table 8 is a table of results of dynamic shear force tests for the switchable PSAs formulated in accordance with Example 8 with their different proportions of cross-linker. The results are also illustrated in FIG. 9.

(67) Table 9 is a summary table of the average values obtained from the shear and peel tests for the PSAs of Examples 6 to 8 with their different proportions of cross-linker.

(68) TABLE-US-00011 TABLE 2 Results of Peel Strength Tests Peel force Shear value Peel force Switch Example before switch before switch after switch time Number (N/25 mm) (minutes) (N/25 mm) (Seconds) %-switch Failure mode 1 2.04 >2000 0.11 3.75 94.60 CP 2 3.36 275 0.17 2.50 94.93 CP 3 3.7 125 0.45 5.70 87.84 CP 4 5.0 75 0.2 3.05 96.00 PCF 5 15.5 16 0.45 2.15 97.10 CF Key to failure modes CP = Clean Panel, i.e., no residuals left on the test plate PCF = Partial Cohesive Failure, i.e., some adhesive left on the test plate CF = Cohesive Failure

(69) TABLE-US-00012 TABLE 3 Results of peel force tests on Examples 6 Peel force (N/25 mm) X linker before switch Peel force (N/25 mm) after switch Switch time (s) %- Coat weight (wet w/w %) S1 S2 S3 S4 Average Failure mode S1 S2 S3 S4 Average S1 S2 S3 S4 Average Switch (g/m.sup.2) 0.00 6.1 7.6 7.0 7.4 7.02 PCF 0.11 0.11 0.11 0.12 0.11 3.0 3.0 3.0 3.0 3.0 98.4 39 0.14 5.9 5.3 3.6 3.4 4.52 CP 0.23 0.13 0.17 0.16 0.17 3.3 3.3 3.3 3.0 3.2 96.2 36 0.31 3.3 3.5 4.2 n/a 3.69 CP 0.18 0.15 0.12 0.37 0.15 3.0 3.0 3.0 3.0 3.0 95.9 37 0.47 2.8 2.9 3.3 2.9 2.97 CP 0.19 0.16 0.17 0.20 0.18 3.0 3.0 3.0 2.8 2.9 93.9 36 0.62 1.9 2.1 2.3 2.5 2.19 CP 0.12 0.15 0.13 0.10 0.13 3.3 3.0 3.0 3.0 3.1 94.3 34 0.77 1.3 1.3 1.8 1.7 1.54 CP 0.15 0.13 0.12 0.16 0.14 3.0 3.0 3.0 3.0 3.0 90.9 37

(70) TABLE-US-00013 TABLE 5 Results of peel force tests on Examples 7 Peel force (N/25 mm) X linker before switch Peel force (N/25 mm) after switch Switch time (s) %- Coat weight (wet w/w %) S1 S2 S3 S4 Average Failure mode S1 S2 S3 S4 Average S1 S2 S3 S4 Average Switch (g/m.sup.2) 0.00 7.3 6.1 7.5 6.7 6.97 CP 0.15 0.10 0.18 0.21 0.16 4.2 4.0 3.9 4.0 4.0 97.7 29 0.035 6.8 6.0 6.5 5.9 6.43 CP 0.17 0.20 0.25 0.20 0.21 4.0 4.0 4.0 4.5 4.1 96.8 31 0.061 6.5 7.1 5.9 5.0 6.50 CP 0.25 0.39 0.30 0.25 0.30 4.0 4.3 4.3 4.3 4.2 95.4 34 0.114 3.7 3.0 2.9 3.3 3.17 CP 0.21 0.20 0.19 0.18 0.20 4.5 4.0 3.8 4.0 4.1 93.8 31 0.212 1.9 2.1 2.3 2.3 2.11 CP 0.15 0.20 0.14 0.19 0.17 4.5 4.0 4.5 4.0 4.3 91.9 29 0.387 0.9 1.2 1.2 1.10 CP 0.21 0.17 0.16 0.18 5.5 5.0 4.8 5.1 83.6 33

(71) TABLE-US-00014 TABLE 7 Results of peel force tests on Examples 8 Peel force (N/25 mm) X linker Peel force (N/25 mm) before switch after switch Switch time (s) Coat (wet Aver- Failure Aver- Aver- %- weight w/w %) S1 S2 S3 S4 age mode S1 S2 S3 S4 age S1 S2 S3 S4 age Switch (g/m.sup.2) 0.00 7.9 5.3 6.2 6.2 6.65 CF 0.60 0.95 0.33 0.43 0.58 15.2 14.0 10.5 12.4 13.0 91.3 34 0.015 12.0 12.9 (4.4 11.5 12.13 CF 0.88 0.63 0.41 0.67 0.65 12.0 11.3 12.5 11.5 11.8 88.1 29 CP) (6.5 CP) 0.025 3.0 3.4 6.0 5.3 4.43 CP 0.50 0.46 0.65 0.39 0.50 13.0 14.0 13.0 12.5 13.1 88.7 35 0.039 2.9 4.1 3.7 3.5 3.55 CP 0.53 0.41 0.60 0.60 0.54 13.0 13.0 12.3 13.0 12.8 84.9 36 0.054 1.6 1.7 1.3 1.4 1.51 CP 0.50 0.37 0.40 0.38 0.41 14.5 15.0 15.0 15.5 15.0 72.7 34 0.097 1.0 0.9 1.0 1.0 0.96 CP 0.50 0.39 0.36 0.39 0.41 15.8 15.0 15.0 15.0 15.2 57.4 35

(72) TABLE-US-00015 TABLE 4 Results of dynamic shear force tests on Examples 6 Coat x-linker dynamic shear force (N/161 mm.sup.2) weight (wet w/w %) S1 S2 S3 S4 Average failure (g/m.sup.2) 0.000 4.91 4.82 4.82 4.96 4.88 CF 39 0.137 21.2 22.1 21.7 21.5 21.6 CF 36 0.311 31.2 34.3 33.7 32.9 33.0 CF 37 0.474 53.5 56.3 55.4 57.6 55.7 PCF 36 0.623 61.1 63.5 64.7 60.3 62.4 PCF 34 0.770 60.5 58.9 60.4 60.0 60.0 PCF 37

(73) TABLE-US-00016 TABLE 6 Results of dynamic shear force tests on Examples 7 Coat x-linker dynamic shear force (N/161 mm.sup.2) weight (wet w/w %) S1 S2 S3 S4 Average failure (g/m.sup.2) 0.000 38.8 39.5 39.6 35.5 38.3 CF 29 0.035 47.8 45.1 47.8 44.7 46.4 CF 31 0.061 53.0 52.7 50.4 45.9 50.5 PCF 34 0.114 49.2 55.9 55.0 55.0 53.8 CP 31 0.212 45.6 44.6 43.5 44.9 44.6 CP 29 0.387 45.3 38.8 37.7 37.6 39.8 CP 33

(74) TABLE-US-00017 TABLE 8 Results of dynamic shear force tests on Examples 8 Coat x-linker dynamic shear force (N/161 mm.sup.2) weight (wet w/w %) S1 S2 S3 S4 Average failure (g/m.sup.2) 0.000 7.2 2.2 2.2 2.1 2.2 CF 34 0.016 15.8 15.8 16.5 15.1 15.8 CF 29 0.025 18.9 19.3 19.8 18.5 19.2 CF 35 0.039 32.4 35.5 35.6 35.7 35.6 CP 36 0.054 30.0 22.5 29.5 31.1 27.7 CP 34 0.097 34.0 31.4 38.5 35.2 35.1 CP 35

(75) TABLE-US-00018 TABLE 9 Summary table of average values obtained from sheer and peel tests for Examples 6 to 8 Dynamic Dynamic Peel force Peel force Example shear force shear failure before after switch Switch time Peel failure number x linker (%) (N/161 mm.sup.2) mode switch (N/25 mm) (N/25 mm) (seconds) %-switch mode 6 0.00 4.88 CF 7.0 0.11 3.0 98.4 PCF 0.14 21.6 CF 4.5 0.17 3.2 96.2 CP 0.31 33.0 CF 3.7 0.15 3.0 95.9 CP 0.47 55.7 PCF 3.0 0.18 2.9 93.9 CP 0.62 62.4 PCF 2.2 0.13 3.1 94.3 CP 0.77 60.0 PCF 1.5 0.14 3.0 90.9 CP 7 0.00 38.3 CF 7.0 0.16 4.0 97.7 CP 0.04 46.4 CF 6.4 0.21 4.1 96.8 CP 0.06 50.5 PCF 6.5 0.30 4.2 95.4 CP 0.11 53.8 CP 3.2 0.20 4.1 93.8 CP 0.21 44.6 CP 2.1 0.17 4.3 91.9 CP 0.39 39.8 CP 1.1 0.18 5.1 83.6 CP 8 0.000 2.16 CF 6.7 0.58 13.0 91.3 CF 0.016 15.8 CF 5.5 0.65 11.8 88.1 PCF 0.025 19.1 CF 4.4 0.50 13.1 88.7 CP 0.039 34.8 CP 3.6 0.54 12.8 84.9 CP 0.054 28.3 CP 1.5 0.41 15.0 72.7 CP 0.097 34.8 CP 1.0 0.42 15.0 56.1 CP

EXAMPLE 6

Adhesives

(76) Referring to FIG. 4, the peel force decreases with increasing amount of cross linker, which is because cross linking makes the adhesive more stiff and less capable of flow and of wetting the surface to it has been attached. At the same time the switched peel force values stay practically independent of the concentration of internal cross-linker. This is due to the inter-molecular cross linking that takes place during the switch totally overwhelming the contribution of the intra-molecular cross linking used for controlling the tack-shear balance.

(77) In opposition to this, as seen in FIG. 5, the dynamic shear force increases with increasing concentrations of internal cross linker up to a certain point, after which it declines. The increase at the beginning is explained by the cohesive strength being improved through cross linking of the polymer chains. As a result, it requires an increasing amount of force to shear the adhesive. However this increase in shear force reaches a maximum. Where the weakest point in the adherent chain ceases to be the cohesive strength and instead changes to become the adhesive strength, the failure mode changes from cohesive failure (CF) to so-called “clean panel” (CP) where no residual adhesive is left on the test plate. The decline after this point occurs because higher cross linking makes the adhesive less capable of flow and less able to wet the test surface, which thereby lowers the adhesive force. It is possible that some of the decrease observed here can can be attributed to stretching the film above the adhesive. A tendency to stretching of the film also was the reason for using a thicker film during the dynamic shear tests compared to the film used for the peel tests. The reasoning for not including the switched values in the dynamic shear force graph is that, at least for low concentration of cross linker, in this case the PSA will benefit from good wetting and flow properties when first applied to the surface and from a very high shear strength after switching it, which often will result in breaking the carrier film. This is a phenomenon that can find a number of applications where the product is not exposed to any significant peel force but where high shear strength and/or residual-free removal is of great importance.

EXAMPLE 7 Adhesives

(78) Referring to FIGS. 6 and 7, the graphs for peel force and shear force, respectively, for Example 7 basically follow the same pattern as in Example 6 with the exception that the maximum value in shear force appears in the middle of the explored concentration range of cross linker.

EXAMPLE 8 Adhesives

(79) Referring now to FIGS. 8 and 9, the graphs for peel and shear force for the various Example 8 adhesives, mimic the earlier described examples except for a maximum in peel force in the unswitched state present somewhere around 0.016% concentration of cross linker. In fact, the balance between CF and CP is so delicate at this concentration, as can be seen from the line of peel force values in Table 7 at 0.016 wet w/w % cross-linker, that while some samples show CF others show CP or even changes between CF and CP during the peel. The reason for the maximum in the peel force curve mirrors the one present in the dynamic shear force values mentioned earlier. When the failure changes from CF to CP the force needed to detach the adhesive from the test surface decreases. That the decline is more abrupt here than in the shear force curves is most probably due to the energy loss from forcing the adhesive to flow and split during a peel test under CF is much higher than during a shear test because they take place at different rates, namely 100 and 5 mm/min, respectively. Also, the peel force value is an average number while the shear force value is the maximal value of the shear force curve.

(80) Note that, in some applications, a partial cohesive failure leaving only a small residue of adhesive behind after switching will be acceptable.

(81) An adhesive composition which might find application in a medical dressing for use in treating a chronic wound or a permanent stoma that needs dressing repeatedly and where, if a non-switchable adhesive were used, skin trauma would be significant is an adhesive that exhibits strong adhesion in the unswitched state. After switching, the adhesive should have a significantly reduced peel force. A medical dressing using this adhesive could be easily removed after switching without causing discomfort to the patient or traumatising fragile skin.

(82) An adhesive suitable for very sensitive materials e.g. delicate production line work is one which has an initial tackiness that is sufficient to position components reliably on a production line for certain steps in a production process. An occlusive layer may be removed at this low tackiness without perturbing the components. After switching, the peel force should be reduced to an almost negligible value, allowing the processed components to be removed easily from the production line.

(83) The present invention is not limited to use in adhesive dressings. Examples of other technical applications include: removable labels; for shipping and handling of fragile or sensitive parts; production line applications where one or several pieces attached to a switchable PSA tape can be mounted into a structure—the tape can then easily be removed after irradiation. Other examples are vehicle labels, shop floor markers, wallpaper and adhesive fixings for posters and/or notices.