Plasticizer migration-resistant, UV-curable hotmelt adhesive for graphics films and labels made of plasticized PVC

Abstract

Plasticizer migration-resistant, UV-curable hotmelt adhesive for graphics films and labels made of plasticized PVC Described here is a UV-curable hotmelt adhesive largely resistant to plasticizer migration and comprising a UV-crosslinkable poly(meth)acrylate formed from methyl acrylate, C4-18 alkyl (meth)acrylate, monomer with acid groups, copolymerized photoinitiator and optionally further monomers. The hotmelt adhesive further comprises an aliphatic polyester polymer. The use of the hotmelt adhesive on graphics films and self-adhesive labels made of plasticized PVC is also described.

Claims

1. A UV-curable hotmelt adhesive, comprising: (a) a UV-crosslinkable poly(meth)acrylate formed from (i) 25% to 60% by weight of methyl acrylate, (ii) 35% to 60% by weight of an alkyl (meth)acrylate comprising 4 to 8 carbon atoms in the alkyl group, (iii) 1% to 10% by weight of an ethylenically unsaturated monomer comprising an acid group, (iv) 0.2% to 5% by weight of an ethylenically unsaturated copolymerizable photoinitiator monomer and (v) 0% to 25% by weight of a further monomer distinct from (i) to (iv) and (b) an aliphatic polyester polymer which at 20? C. has a dynamic viscosity of 500 to 20,000 mPa s in an amount of 1% to 20% by weight, wherein the aliphatic polyester polymer is formed from an aliphatic dicarboxylic acid comprising 3 to 10 carbon atoms and an alkanediol comprising 2 to 10 carbon atoms, and wherein a plasticized polyvinyl chloride (PVC) film coated with the UV-curable hotmelt adhesive exhibits a 180 degree peel strength of >10 N/25 mm after storage for 3 days at 70? C. and 50% relative humidity; and wherein the plasticized polyvinyl chloride (PVC) film coated with the UV-curable hotmelt adhesive exhibits a shear strength value of >15 h after storage for 3 days at 70? C. and 50% relative humidity.

2. The UV-curable hotmelt adhesive of claim 1, wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a glass transition temperature of not more than 10? C.

3. The UV-curable hotmelt adhesive of claim 1, wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a K value of at least 20.

4. The UV-curable hotmelt adhesive of claim 1, wherein the aliphatic dicarboxylic acid is an adipic acid and the alkanediol comprises 4 to 8 carbon atoms.

5. The UV-curable hotmelt adhesive of claim 1, wherein the UV-crosslinkable poly(meth)acrylate comprises from 0.5% to 15% by weight of further monomer (v) distinct from (i) to (iv).

6. The UV-curable hotmelt adhesive of claim 1, wherein the further monomer (v) does not contain N-containing monomers and aromatic monomers.

7. The UV-curable hotmelt adhesive of claim 1, wherein (ii) the alkyl (meth)acrylate comprising 4 to 8 carbon atoms in the alkyl group is at least one selected from the group consisting of n-butyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate and the aliphatic dicarboxylic acid comprising 4 to 8 carbon atoms and alkanediol comprising 4 to 8 carbon atoms.

8. The UV-curable hotmelt adhesive of claim 1, further comprising a tackifier.

Description

EXAMPLES

(1) Input materials:

(2) nBA n-butyl acrylate

(3) 2-EHA 2-ethylhexyl acrylate

(4) MA methyl acrylate

(5) AA acrylic acid

(6) F1 photoinitiator monomer: polymerizable photoinitiator (35% solution in MEK) of formula F-1.

(7) MEK methyl ethyl ketone

(8) t-BPPiv tert-butyl perpivalate (75% solution in mineral oil)

(9) Palamoll? 632: polyester made of adipic acid and 1,2-propanediol, polymeric adipate dynamic viscosity 2000-3500 mPas

(10) Plastomoll? DOA: di-i-octyl adipate; monomeric adipate; dynamic viscosity 13-15 mPas (mixture of di-n-octyl adipate and diethylhexyl adipate)

(11) Lutonal? M 40: polyvinyl methyl ether

(12) Foral? 85-E: glyceryl ester of hydrogenated colophony resin

(13) acResin? A 260 UV: MA-free polyacrylate, UV-crosslinkable

(14) acResin? A 204 UV: polyacrylate, UV-crosslinkable having MA content of more than 15% and less than 20% by weight

(15) Polymer 1: UV-crosslinkable poly(meth)acrylate made of 44% by weight of 2-EHA, 40% by weight of MA, 5% by weight of AA, 10% by weight of MMA, 1% by weight of F1

(16) Production of polymer 1 (V7)

(17) In a polymerization apparatus consisting of a glass reactor, a reflux condenser, a stirrer and a nitrogen inlet 210 g of MEK are initially charged under a light nitrogen stream and heated to 80? C. 25 g of altogether 920 g of a monomer mixture composed of 2-EHA, MA, AA, MMA and F1 and having the abovementioned composition are added. Upon reattaining 80? C. 1.9 g of a starter solution of 3.6 g of tert-butyl perpivalate and 42.3 g of MEK are added and the mixture is polymerized for 3 min. Then the remaining 895 g of monomer mixture and 44 g of starter solution are added over 3 h. The temperature is then increased to 90? C. and a solution of 3 g of tert-butyl perpivalate in 28 g of MEK is added over 30 min. A vacuum is then applied and the solvent is distilled off at not more than 135? C. and less than 50 mbar. The mixture is then degassed with slow stirring for 1 h at 135? C. and the maximum achievable vacuum. The melt is drained into a PP cup.

(18) K value (1% in THF): 42

(19) Zero-shear viscosity at 130? C.: 51 Pas

(20) Production of the mixtures:

(21) V2: 50 g of acResin? A 260 UV are heated to 80? C. and 3.5 g Palamoll? 632 are added with stirring, then the mixture is cooled.

(22) K value of acResin? A 260 UV (1% in THF): 48-52

(23) Zero-shear viscosity of acResin A 260 UV at 130? C.: 30-70 Pas

(24) V4: 50 g of acResin? A 204 UV are heated to 80? C. and 2 g of Palamoll? 632 are added with stirring, then the mixture is cooled.

(25) K value (1% in THF): 49

(26) Zero-shear viscosity at 130? C.: 37 Pas

(27) V5: 50 g of acResin? A 204 UV are heated to 80? C. and 10 g of Lutonal? M40 are added with stirring, then the mixture is cooled.

(28) K value of acResin? A 204 UV (1% in THF): 48-52

(29) Zero-shear viscosity of acResin? A 204 UV at 130? C.: 20-55 Pas

(30) V6: The polymer is synthesized as described for polymer 1 (V7) but before application of the vacuum the solution is cooled in MEK and 64.5 g of Palamoll? 632 are added. The mixture is subsequently worked up as for polymer 1.

(31) K value (1% in THF): 42

(32) Zero-shear viscosity at 130? C.: 40 Pas

(33) B1: The polymer is synthesized and worked up as described in V6. In contrast to V6 the coated film is crosslinked through UV curing.

(34) B2: The polymer is synthesized as described for polymer 1 but before application of the vacuum the solution is cooled in MEK and 64.5 g of Palamoll? 632 and 69 g of Foral? 85-E are added. The mixture is subsequently worked up as for polymer 1.

(35) K value (1% in THF): 42

(36) Zero-shear viscosity at 130? C.: 45 Pas

(37) V8: 50 g of polymer 1 (V7) are heated to 80? C. and 3.5 g of Plastomoll? DOA are added with stirring, then the mixture is cooled.

(38) K value (1% in THF): 40

(39) Zero-shear viscosity at 130? C.: 24 Pas

(40) Measurement of Zero-Shear Viscosity:

(41) The zero-shear viscosity is the threshold value of the viscosity function at infinitely low shear rates. It is measured with an Anton Paar MCR 100 rheometer (US 200 evaluation software) in plate/plate geometry. The samples are measured under oscillatory shear at a small shear amplitude of 10%. Temperature 130? C. (or as stated), angular frequency ramp log 100-0.1 1/s, measuing gap 0.5 mm, evaluation according to Carreau-Gahleitner I, piston diameter 25 mm.

(42) Performance Tests:

(43) The measurements are carried out at room temperature (20? C.) unless explicitly stated otherwise.

(44) The pressure-sensitive adhesives were heated to 120? C. and doctor coated from the melt onto siliconized PET film at an application rate of 25 g/m.sup.2 or 15 g/m.sup.2 (only V7) and irradiated with UVC light or not irradiated (V6). The film is then transferred to a commercially available 100 ?m plasticized PVC film (SK-S-P Transparent from Renolit?) as a carrier. The carrier coated with pressure-sensitive adhesive was cut into 25 mm wide test strips. The tests were either continued immediately or the test strips were stored for 3 days at 70? C. and 50% rel. humidity before testing to determine plasticizer resistance.

(45) a) peel strength

(46) To determine peel strength the 25 mm wide test strips were bonded to the test surface made of steel (AFERA steel) and rolled on once with a 1 kg roller. One end of the test strips is then clamped in the upper jaws of a tensile strain tester. The adhesive strip was removed from the test surface at 300 mm/min and an angle of 180? , i.e. the adhesive strip was bent and removed parallel to the steel test sheet and the force required therefor was measured. The measure for peel strength is the force in N/25 mm obtained as the average value from five measurements. The peel strength was determined 24 hours after bonding. The adhesive strength has fully developed after this time.

(47) b) shear strength

(48) To determine shear strength the test strips were bonded to steel sheet (AFERA steel) with a bonded area of 25?25 mm, rolled on once with a 1 kg roller and then loaded with a 1 kg hanging weight. The shear strength (cohesion) was determined under standard climatic conditions (23? C.; 50% relative atmospheric humidity). The measure for shear strength is the time in hours until the weight drops off. An average was formed from five measurements in each case.

(49) TABLE-US-00001 TABLE 1 UV-curable hotmelt adhesives UV-crosslinkable Example poly(meth)acrylate Additive V1 acResin? A 260 UV V2 acResin? A 260 UV 7% by weight Palamoll 632 V3 acResin? A 204 UV V4 acResin? A 204 UV 4% by weight Palamoll 632 V5 acResin? A 204 UV 20% by weight Lutonal M 40 V6 polymer 1 7% by weight Palamoll 632 V7 polymer 1 B1 polymer 1 7% by weight Palamoll 632 B2 polymer 1 7% by weight Palamoll 632 7.5% by weight Foral 85E V8 polymer 1 7% by weight Plastomoll DOA

(50) TABLE-US-00002 TABLE 2 Test results 24 h Peel steel Shear 1 h steel; [N/25 mm] 25 ? 25 mm, 1 kg [h] Irradiation, Before After Before After Example UVC dose storage 3 d/70? C. storage 3 d/70? C. V1 20 mJ/cm.sup.2 16 10 28 5 V2 20 mJ/cm.sup.2 14 10 13 4 V3 20 mJ/cm.sup.2 12 6 >50 11 V4 20 mJ/cm.sup.2 12 6 35 10 V5 10 mJ/cm.sup.2 24 9 38 4 V6 27 24 1 0.2 V7 10 mJ/cm.sup.2, 23 13 >50 23 (15 g/m.sup.2) B1 20 mJ/cm.sup.2 20 18 >50 >50 B2 20 mJ/cm.sup.2 25 18 >50 38 V8 20 mJ/cm.sup.2 15 9 >50 31 V1 to V8: Comparative tests; B1 to B2: inventive

(51) The following adhesion values are preferred:

(52) TABLE-US-00003 24 h Peel after 3 d/70? C. less than 50% drop and >10 N/25 mm 1 h Shear after 3 d/70? C. less than 50% drop and >15 h

(53) The results show:

(54) TABLE-US-00004 V1 cohesion value too low, cohesion drop (shear) > 50% V2 cohesion value too low, cohesion drop (shear) > 50% V3 peel value falls to too low a value, cohesion drop (shear) > 50% V4 peel value falls to too low a value, cohesion drop (shear) > 50% V5 peel value drop (peel) and cohesion drop (shear) > 50% V6 no cohesion V7 cohesion drop (shear) > 50% B1 inventive B2 inventive V8 peel values too low

(55) The results show that an MA-free polymer (V1 and V2), both with and without the addition of plasticizer, has too little plasticizer resistance as reflected in a more than 50% drop in cohesion values. A polymer having an MA content of less than 20% (V3) shows excessively low peel values after storage. The addition of plasticizer (V4) does not increase the peel values as desired. Furthermore both samples show too pronounced a drop in cohesion after storage (too little plasticizer resistance). The addition of polyvinyl ethers (V5) does not improve plasticizer resistance either. A composition according to the invention but without crosslinking by means of UV curing (V6) results in very low cohesion values even before storage. However, with crosslinking it is possible to achieve a pressure-sensitive adhesive system that meets the required demands on adhesion values and plasticizer resistance both with additional tackifier (B2) and without additional tackifier (B1). The pure, plasticizer-free polymer (V7) only just fails to meet the demands on plasticizer resistance in respect of shear values after storage. Replacing the polymeric plasticizer with a monomeric plasticizer (V8) results in excessively low peel values.