Pressure-sensitive adhesives for bonding flexible printing plates

Abstract

A pressure-sensitive adhesive comprising at least one adhesive component comprising at least one polymer component based on a monomer mixture comprising at least one of the following monomers: at least one acrylic ester at least one methacrylic ester acrylic acid methacrylic acid
is particularly suitable for the bonding of printing plates, particularly to printing cylinders and/or printing sleeves, if the polymer component further comprises as monomer 5 to 25 wt %, preferably 7.5 to 20 wt % and more preferably more than 10 to 15 wt %, of vinyl acetate, the quantity figures being based in each case on the monomer mixture.

Claims

1. A method of bonding printing plates, comprising applying a pressure-sensitive adhesive to a substrate, said pressure-sensitive adhesive comprising at least one adhesive component comprising at least one polymer component based on a monomer mixture comprising at least one of the following monomers: at least one acrylic ester at least one methacrylic ester-acrylic acid methacrylic acid wherein the polymer component further comprises as monomer 5 to 25 wt % of vinyl acetate, the quantity figure(s) in each case being based on the monomer mixture.

2. The method according to claim 1, wherein the monomer mixture comprises at least the following monomers: a) 25 to 94.5 wt % of at least one acrylic ester and/or methacrylic ester whose homopolymer has a static glass transition temperature of <0 C.; b) 0 to 40 wt % of at least one acrylic ester and/or methacrylic ester whose homopolymer has a static glass transition temperature of >0 C.; c) 0.5 to 20 wt % of acrylic acid and/or methacrylic acid; d) 5 to 20 wt % of vinyl acetate; e) 0 to 20 wt % of further copolymerizable monomers, the quantity figures in each case being based on the monomer mixture.

3. The method according to claim 1, wherein the at least one polymer component comprises 5 to 20 wt % of acrylic acid, the quantity figure(s) in each case being based on the monomer mixture.

4. The method according to claim 1, wherein the monomer mixture comprises at least the following monomers: i.a) 50-94.5 wt % of at least one acrylic ester and/or methacrylic ester having the following formula: CH.sub.2C(R.sub.1)(COOR.sub.2), where R.sub.1H and R.sub.2 is a linear alkyl radical having 2 to 10 carbon atoms or is a branched, non-cyclic alkyl radical having at least 4 carbon atoms, and/or R.sub.1CH.sub.3 and R.sub.2 is a linear alkyl radical having 8 to 10 carbon atoms or is a branched, non-cyclic alkyl radical having at least 10 carbon atoms; i.b) 0 to 40 wt % of at least one acrylic ester and/or methacrylic ester having the following formula: CH.sub.2C(R.sub.3)(COOR.sub.4), where R.sub.3H or CH.sub.3 and R.sub.4 is a linear alkyl radical having at least 12 carbon atoms; i.c) 0.5-20 wt % of acrylic acid and/or methacrylic acid; i.d) 5 to 20 wt % of vinyl acetate; i.e) 0-25 wt % of at least one N-alkyl-substituted acrylamide, where the quantity figures in each case are based on the monomer mixture.

5. The method according to claim 1, wherein the monomer mixture comprises at least the following monomers: ii.a) 49.5-84.5 wt % of at least one acrylic ester and/or methacrylic ester and/or their free acids, having the following formula: CH.sub.2C(R.sub.5)(COOR.sub.6), where R.sub.5H or CH.sub.3 and R.sub.6 is an alkyl radical having 1 to 10 carbon atoms or H and the homopolymer has a static glass transition temperature of <30 C.; ii.b) 10 to 40 wt % of at least one acrylic ester and/or methacrylic ester having the following formula: CH.sub.2C(R.sub.7)(COOR.sub.8), where R.sub.7H or CH.sub.3 and R.sub.8 is a cyclic alkyl radical having at least 8 carbon atoms or is a linear alkyl radical having at least 12 carbon atoms and the homopolymer has a static glass transition temperature T.sub.g of at least 30 C.; ii.c) 0.5 to 10 wt % of at least one acrylic ester and/or methacrylic ester and/or their free acids, having the following formula: CH.sub.2C(R.sub.9)(COOR.sub.10), where R.sub.9H or CH.sub.3 and R.sub.10H or an aliphatic radical having a functional group X, where X comprises COOH, OH, NH(R.sub.11), SH, SO.sub.3H, and the homopolymer has a static glass transition temperature T.sub.g of at least 30 C., where R.sub.11H or a linear or branched alkyl radical having up to 10 carbon atoms, ii.d) 5 to 20 wt % of vinyl acetate, where the quantity figures in each case are based on the monomer mixture.

6. The method according to claim 1, wherein the monomer mixture comprises at least the following monomers: iii.a) 25 to 77 wt % of linear acrylic esters having 2 to 10 carbon atoms in the alkyl radical; iii.b) 10 to 40 wt % of branched, non-cyclic acrylic esters having a static glass transition temperature T.sub.g of not more than 0 C.; iii.c) 8 to 15 wt % of acrylic acid, based on the total amount of monomers to be polymerized; iii.d) 5 to 20 wt % of vinyl acetate; iii.e) optionally up to 10 wt % of further copolymerizable monomers, based on the total amount of monomers to be polymerized, wherein the ratio of the linear acrylic esters to the branched acrylic esters is in the range from 1:6 to 10:1 mass fractions.

7. The method according to claim 3, wherein the polymer component or polymer components based on monomer mixtures comprising the monomers a) to e) or i.a) to i.e) or ii.a) to ii.d) or iii.a) to iii.e) make(s) up at least 90 wt % of the adhesive component or components.

8. The method according to claim 1, wherein the pressure-sensitive adhesive is in crosslinked form.

9. The method according to claim 1, wherein the pressure-sensitive adhesive is provided in the form of an adhesive tape.

10. The method of claim 1, wherein the polymer component further comprises 7.5 to 20 wt % of vinyl acetate.

11. The method of claim 2, wherein component a) has homopolymer having a static glass transition temperature of <10 C.

12. The method of claim 2, wherein component b) has homopolymer having a static glass transition temperature of <10 C.

13. The method of claim 3, wherein the at least one polymer component comprises 7.5 to 17 wt % of acrylic acid.

14. The method of claim 4, wherein R.sub.4 of component i.b) is a stearyl radical.

15. The method of claim 6, wherein component iii.b) has a static glass transition temperature T.sub.g of not more than 10 C.

16. The method of claim 7, wherein the polymer component or polymer components make(s) up at least 95% of the adhesive component or components.

Description

FIGURES

[0168] FIG. 1 shows the layered sequence of construction of an adhesive tape according to a preferred embodiment of the invention as described herein.

[0169] FIG. 2 shows an alternative adhesive tape construction consisting of the following sections:

[0170] 3 PSA for mounting the plate,

[0171] 7a assembly of foamed carrier and PE film fused onto the surface on both sides, and

[0172] 9 PSA for mounting on the printing cylinder.

[0173] FIG. 3 shows a common method of bonding printing plates to printing cylinders and printing sleeves. Here, the plate (11) is bonded to the printing sleeve (13) or printing cylinder (13) by means of an adhesive tape (12) which is larger than the plate (11) and therefore projects by exposed regions (20) beneath the plate (11).

[0174] FIG. 4 shows detachment of plate edges (shown here for only one edge, for simplification; 21=printing plate; 22=bonded assembly; 23=cylinder).

[0175] Experiments

[0176] The pressure-sensitive adhesives (PSAs) investigated (inventive examples and reference examples) were produced as follows unless otherwise stated:

[0177] Polymerization Process

[0178] A 2 L glass reactor conventional for radical polymerizations which is also set up for semi-batch operation was charged (in accordance with the composition as set out for the individual examples in Table 1) with 30 g of vinyl acetate, 7.2 g of acrylic acid, 53.1 g of 2-ethylhexyl acrylate and 20.7 g of n-butyl acrylate, 37.0 g of acetone and 37.0 g of special-boiling-point spirit 60/95, the initial charge being thoroughly mixed.

[0179] After nitrogen gas had been passed through the reactor for 45 minutes, with stirring, an external heating bath was used to heat the reactor to 58 C. (internal temperature) and 0.15 g of 2,2-azodi(2-methylbutyronitrile) (Vazo 67) in solution in 10 g of acetone was added. Thereafter the external heating bath was heated to 75 C. and the reaction was carried out constantly with evaporative cooling (external temperature 75 C.).

[0180] 15 minutes after the first addition of initiator, 16.8 g of acrylic acid, 123.9 g of 2-ethylhexyl acrylate, 48.3 g of n-butyl acrylate, 63.0 g of acetone and 63.0 g of special-boiling-point spirit 60/95 (all components were mixed thoroughly prior to metering) were added to the reaction mixture at a metering rate of 3.15 g/min.

[0181] 1 hour after addition of the first quantity of initiator, a further 0.15 g of 2,2-azodi(2-methyl-butyronitrile) in solution in 10 g of acetone was added.

[0182] 3.25 hours after addition of the first quantity of initiator, dilution took place with 45 g of acetone and 45 g of special-boiling-point spirit 60/95.

[0183] 5.5 hours after addition of the first quantity of initiator, 0.45 g of bis(4-tert-butylcyclo-hexanyl) peroxydicarbonate in solution in 10 g of acetone was added.

[0184] 7 hours after addition of the first quantity of initiator, a further 0.45 g of bis(4-tert-butylcyclohexanyl) peroxydicarbonate in solution in 10 g of acetone was added.

[0185] 10 hours after addition of the first quantity of initiator, dilution took place with 45 g of acetone and 45 g of special-boiling-point spirit 60/95.

[0186] After a total reaction time of 24 hours, reaction was discontinued and cooling took place to room temperature.

[0187] The polymer was subsequently diluted with a 1:1 mixture of acetone and special-boiling-point spirit 60/95 to a solids content of 30%, blended with the additive (amount as specified for the individual examples in Table 1where, for example, for examples B11c and C11c the 7.5 wt % of additive correspond to 23.32 g of stearyl alcohol) and crosslinked with an epoxide, specifically either N,N,N,N-Tetrakis(2,3-epoxypropyl)-m-xylene-,-diamine, also called Erisys GA240, or (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate, also called Uvacure 1500, specifically in accordance with the composition as reproduced for the individual examples in Table 1. The properties of the resultant PSA were evaluated using a double-sided pressure-sensitively adhesively bonded assembly.

[0188] For the investigations, the resulting polymer solution was coated onto a siliconized polyethylene terephthalate film. After drying for 20 minutes at 80 C. and conditioning for seven days at 23 C. and 50 5% relative humidity, the coat weight was 35 g/m.sup.2 (adhesive tape b).

TABLE-US-00001 TABLE 1 PSA for carrying out the measurement series, inventive examples and comparative examples Polymer composition (wt %)* Amount Additive Acrylic Stearyl N-tert-Butyl 2-Ethylhexyl n-Butyl Vinyl of additive or co- acid IBOA acetate acrylamide acrylate acrylate acetate (wt %)** monomer Crosslinker [g] V1 compar- 12 59 29 0.60 Uvacure 1500 ative V2 compar- 12 59 29 3.8 Etho- 0.60 Uvacure 1500 ative meen C25 V3 compar- 3 15 10 72 0.18 Erisys GA 240 ative 1 compar- 1 15 84 0.6 Aluminium ative chelate 2 compar- 1 20 15 64 0.6 Aluminium ative chelate 3 compar- 1 20 15 64 0.6 Aluminium ative chelate 4 compar- 1 25 15 59 0.6 Aluminium ative chelate 5 compar- 3 15 82 0.18 Erisys GA 240 ative 6 compar- 3 15 82 0.18 Erisys GA 240 ative 7 compar- 3 20 15 62 0.18 Erisys GA 240 ative 8 compar- 3 10 87 0.18 Erisys GA 240 ative 9 compar- 3 10 87 0.18 Erisys GA 240 ative 10 compar- 3 15 10 72 0.18 Erisys GA 240 ative 11 compar- 3 25 10 62 0.18 Erisys GA 240 ative 12 compar- 3 35 10 52 0.18 Erisys GA 240 ative 13 compar- 3 15 10 67 5 0.18 Erisys GA 240 ative 14 compar- 3 10 67 20 Stearyl 0.18 Erisys GA 240 ative meth- acrylate 15 compar- 3 48.5 48.5 0.18 Erisys GA 240 ative 16 inventive 3 15 10 62 10 0.18 Erisys GA 240 17 inventive 3 15 10 62 10 0.18 Erisys GA 240 18 inventive 3 15 10 62 10 0.18 Erisys GA 240 19 inventiv 8 82 10 0.18 Erisys GA 240 20 inventive 8 59 23 10 0.18 Erisys GA 240 21 inventive 12 59 24 5 0.6 Uvacure 1500 22 inventive 12 59 21.5 7.5 0.6 Uvacure 1500 23 inventive 12 59 19 10 0.6 Uvacure 1500 24 inventive 12 59 14 15 0.6 Uvacure 1500 25 inventive 12 59 9 20 0.6 Uvacure 1500 26 inventive 3 15 72 10 0.18 Erisys GA 240 27 compar- 12 59 19 10 Vinyl 0.6 Uvacure 1500 ative propio- nate 28 compar- 12 59 19 10 Ethylvinyl 0.6 Uvacure 1500 ative ether 29 compar- 12 59 19 10 N-Methyl- 0.6 Uvacure 1500 ative N-Vinyl- acet- amide .sup.+ Total amount did not produce 100% NTBAM = N-tert-butylacrylamide 2-EHA = 2-ethylhexyl acrylate *parts by weight per 100 parts by weight of polymer **parts by weight per 100 parts by weight of overall adhesive component blend

[0189] Producing a Bonded Assembly V

[0190] First of all the PSA from the examples was coated from solution on customary tesa liner material, siliconized on both sides and grained/structured, in order to obtain the best possible transfer of the liner structure into the acrylate PSA. After drying for 20 minutes at 80 C., the coat weight was 35 g/m.sup.2.

[0191] On the adhesive side, the coated liner material was laminated with a PET film 23 m thick which had been etched on either side with trichloroacetic acid. A transfer carrier was then used to laminate a commercial acrylate adhesive with a coat weight of 20 g/m.sup.2 onto the uncoated side of the etched PET film in the assembly, and a PE-EVA foam with a thickness of 500 m and a density of 270 kg/m.sup.3 was laminated on.

[0192] Atop this foam carrier, using a transfer carrier, a commercial acrylate PSA was then laminated with a coat weight of 40 g/m.sup.2 onto the uncoated side of the previous assembly (open acrylate PSA layer).

[0193] The multilayer adhesive tape produced in this way is referred to as bonded assembly V.

[0194] Assessments of Suitability for Application

[0195] The evaluation yardsticks in the following test methods were each selected such that a o represents a result considered satisfactory for use in flexographic printing, whereas values (and especially values) lead, based on experience, to considerable problems in operation that are no longer tolerable.

[0196] + and ++ values characterise adhesive tape specimens which in operation cause hardly any or virtually no problems in respect of the property being tested.

[0197] Standard Plate Bonding Assessment (Test 1)

[0198] Specimens measuring 250 mm160 mm were cut from the double-sided bonded assembly under investigation. These specimens were adhered using the commercial, open acrylate PSA layer to a steel cylinder having a diameter of 110 mm, in such a way that the shorter edges of the specimens were aligned in the lengthwise direction of the cylinder. The liner material was then removed, now exposing the layer of the PSA of the invention. Atop the bonded assembly specimens thus bonded, a whole-area-exposed printing plate from DuPont Cyrel HOS with dimensions of 210 mm length120 mm width1.7 mm thickness was adhered to the PSA of the invention in such a way that the underlying bonded assembly stuck out at each edge by 20 mm (centred application on the bonded assembly specimen).

[0199] The procedure of applying a printing plate involved one of the shorter edges of the plate (transverse edge) being applied at a distance of 20 mm from one of the shorter edges of the bonded assembly specimen, parallel to that edge, to the specimen. Then, starting from this edge, the plate was rolled on using a plastic roller (width 100 mm, diameter 30 mm, Shore hardness A 45). The rolling movement took place in the lengthwise direction of the printing cylinder and perpendicularly, and was performed continuously from a longitudinal edge of the plate in each case to the opposite longitudinal edge of the plate, and back again. The rolling speed was 10 m/min in the transverse direction. At the same time, the printing cylinder rotated with a superficial velocity of 0.6 m/min, and so the plastic roller relative to the printing plate described a zig-zag movement in the direction of the second transverse edge of the plate. The printing plate was mounted on the bonded assembly using the corresponding pressing force needed to fix the plate over the whole area and without edge lifting. The entire procedure was repeated twice. The printing cylinder was aligned in such a way that both short edges of the bonded plate were at a height above the axis of rotation of the cylinder (open region oriented upwards). The specimens bonded to the printing cylinder were subsequently stored for three days (72 hours) under different climatic conditions.

[0200] The recovery behaviour of the plate means that it has a tendency towards edge lifting. The acrylate PSA of the invention on the plate side (see inventive examples in Table 1 and 2) is selected such that there is no edge lifting of the plate from the bonded assembly. Depending on the stability of the bond between the PSA of the invention and the printing plate, there is detachment of the plate edges (cf. FIG. 4; shown here for only one edge, for simplification; 21=printing plate; 22=bonded assembly; 23=cylinder). To asses this behaviour, a determination is made of the length L of the lifted plate edge up to the first remaining point of contact with the substrate (average value in each case from the evaluation of both edges and three measurement runs).

[0201] The test was carried out on the one hand with fresh adhesive tape specimens (one week old) and on the other with adhesive tape specimens having been stored for a period of 4 weeks at 40 C., and was carried out under two different sets of conditions: firstly, at room temperature (23 C. and 50% relative humidity), and secondly at 35 C. and a relative humidity of 855%.

[0202] Edge lifting evaluation scheme:

TABLE-US-00002 Length of edge lifting [mm] Evaluation 2 mm ++ 5 mm + 8 mm 0 11 mm >11 mm

[0203] A further determination is made as to whether the bonded assembly remains adhering reliably to the respective substrate, without forming bubbles and/or without forming creases, when the printing plate is removed again from the bonded assembly. Both requirements are regularly imposed by the customers in the printing industry, who on the one hand expect reliable fixing of the plate to sleeve or cylinder, but on the other hand are frequently required to adjust the plate for precise fixing, in the course of which the adhesive tape used for bonding is not to suffer damage and must not become detached.

[0204] Assessment of Ink ResistanceCellulose Nitrate Application Test (Test 2)

[0205] In order to assess the resistance of the adhesive to influences of the binders in printing inks, a 0.1% strength solution of cellulose nitrate in ethanol was prepared. The test was carried out using the low-viscosity Walsroder Nitrocellulose A400, which has a nitrogen content of 10.7%-11.3% and a degree of substitution of 1.89-2.05. A specimen printing plate is cleaned with ethanol and left for half an hour to evaporate. Using absorbent cotton, the cellulose nitrate solution is applied to the plate with a wiping motion. The treated plate, which is 1.14 mm thick, must be left to evaporate for at least 5 minutes for complete devolatilization of the ethanol used for the solution. If the plate is bonded earlier, the test does not work, since the residual ethanol on the bonding surface ensures that the cellulose nitrate is dissolved into the adhesive and there is therefore no longer any interaction. The plate applied in accordance with standard plate bonding is stored at 40 C. for three days. An assessment is made of the force required for demounting. The assessment parameter in each case is the subjective force which has to be exerted. The evaluation scheme below was drawn up such that a force expenditure denoted with o was considered by experts to be acceptable for use. Adverse evaluations () were considered no longer acceptable for daily use.

[0206] Evaluation Scheme

TABLE-US-00003 Little force expended + Moderate force expended High force expended

[0207] Solvent Resistance (Test 3)

[0208] The test specimens were mounted as in test 1 (assessment of edge lifting). The printing cylinder was oriented so that both short edges of the bonded plate were at a height above the axis of rotation of the cylinder (open region of the cylinder oriented upwards).

[0209] Using a pipette, a solvent mixture (1/8 ethyl acetate/1/8 n-propanol/6/8 ethanol) was applied to both edges for 1 minute so that the edges were continuously under the influence of solvent. The amount of solvent applied was just enough to wet both edges permanently with solvent (test conditions: 25 C., 505% relative humidity).

[0210] After the end of the wetting time, the cylinder was stood on one of its edges to allow excess solvent to run off.

[0211] Observation was made to determine whether the short edges of the plate had lifted from the bonded assembly. A measure of the edge lifting in this case was the length L of the no longer bonded part of the printing plate (of the lifted printing plate part), measured 60 minutes after the end of the wetting time, and measured in the tangential direction and from the free end up to the first point of remaining bonding (cf. FIG. 1: 1=printing plate, 2=bonded assembly, 3=printing cylinder, L=length of the lifted printed plate part).

[0212] Evaluation Scheme:

TABLE-US-00004 Edge lifting after solvent exposure Evaluation <5 mm ++ 5 to <10 mm + 10 to <15 mm 0/+ 15 to <20 mm 0 20 to <25 mm 0/ 25 to <30 mm 30 mm or more

[0213] The results of the tests are shown in the following Table 2.

TABLE-US-00005 TABLE 2 Results of assessment of application suitability (ink resistance, mounting and solvent resistance) Edge lifting, 1.7 mm plate* Ink Fresh 4 Wks 40 C. Solvent resistance RT 35 C./85% RT 35 C./85% resistance* V1 ++ + ++ ++ + V2 ++ + ++ + O V3 ++ ++ ++ ++ 1 ++ ++ 2 ++ ++ 3 ++ ++ 4 ++ ++ 5 ++ ++ 6 ++ ++ 7 ++ ++ 8 ++ ++ 9 ++ ++ 10 ++ ++ 11 ++ ++ 12 ++ ++ 13 ++ O ++ 14 ++ ++ 15 ++ ++ 16 + ++ ++ ++ + O 17 + ++ ++ ++ + O 18 + ++ O ++ O 19 + ++ ++ ++ + O 20 + ++ ++ ++ + O 21 O ++ ++ ++ ++ + 22 + ++ ++ ++ + 23 ++ ++ ++ ++ ++ ++ 24 ++ ++ ++ ++ ++ ++ 25 ++ ++ ++ ++ ++ + 26 + ++ + 27 ++ 28 ++ 29 ++