PROTECTIVE ADHESIVE FILM

Abstract

Disclosed herein is a pressure-sensitively adhesive protective film formed from a polymer film coated with a crosslinked pressure-sensitive adhesive composition. The pressure-sensitive adhesive composition contains a pressure-sensitive adhesive polymer which is formed by emulsion polymerization in the presence of saccharified starch from soft monomers which when polymerized as a homopolymer have a glass transition temperature of less than 0 C., from ethylenically unsaturated acids or acid anhydrides, and optionally further monomers different from (i) and (ii). The pressure-sensitive adhesive composition contains defined crosslinkers, with the crosslinking reaction brought about by the crosslinker being at an end before the pressure-sensitively adhesive protective film is adhered to a substrate.

Claims

1. A pressure-sensitively adhesive protective film formed from a polymer film coated with a crosslinked pressure-sensitive adhesive composition, the pressure-sensitive adhesive composition comprising at least one pressure-sensitive adhesive polymer formed by emulsion polymerization from (i) at least 60 wt %, based on the sum of the monomers, of at least one soft monomer which when polymerized as a homopolymer has a glass transition temperature of less than 0 C., (ii) 0.1-5 wt %, based on the sum of the monomers, of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride, (iii) optionally further monomers different from (i) and (ii), wherein: the emulsion polymerization of the monomers occurs in the presence of at least one saccharified starch, the pressure-sensitive adhesive composition comprises at least one crosslinker selected from the group consisting of polyisocyanates and polyaziridines, and the crosslinking reaction brought about by the crosslinker is at an end without the pressure-sensitively adhesive protective film being adhered to a substrate.

2. The protective film according to claim 1, wherein the pressure-sensitive adhesive composition is an aqueous dispersion with pressure-sensitive adhesive polymer dispersed therein, the composition comprising from 0.05 to 10 parts by weight of crosslinker, based on the sum of the weight fractions of the at least one saccharified starch and of the at least one pressure-sensitive adhesive polymer.

3. The protective film according to claim 1, wherein the at least one soft monomer (i) is selected from the group consisting of n-butyl acrylate, ethyl acrylate, 2-ethylhexyl acrylate, and mixtures of these monomers.

4. The protective film according to claim 1, wherein the at least one ethylenically unsaturated acid (ii) is selected from the group consisting of acrylic acid, methacrylic acid and mixtures of these monomers.

5. The protective film according to claim 1, wherein the further monomers are present in an amount of 0 to 30 wt % and are selected from the group consisting of C1 to C20 alkyl acrylates, C1 to C20 alkyl methacrylates, vinyl esters of carboxylic acids comprising up to 20 C atoms, vinylaromatics having up to 20 C atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 C atoms, aliphatic hydrocarbons having 2 to 8 C atoms and one or two double bonds, monomers comprising hydroxyl groups, (meth)acrylamide, phenyloxyethyl glycol mono(meth)acrylate, glycidyl (meth)acrylate, aminoalkyl (meth)acrylates, diacetoneacrylamide, acetoacetoxyethyl methacrylate, and mixtures of these monomers.

6. The protective film according to claim 1, wherein: the saccharified starch is present in amounts of 1 to 50 parts by weight, based on 100 parts by weight of monomers of the pressure-sensitive adhesive polymer, and the saccharified starch has a DE of 1 to 25.

7. The protective film according to claim 1, wherein the weight ratio of saccharified starch to crosslinker in the dispersion is from 1:2 to 100:1.

8. The protective film according to claim 1, wherein the crosslinkers are selected from the group consisting of aliphatic diisocyanates, cycloaliphatic diisocyanates, aromatic diisocyanates, polyisocyanates containing isocyanurate groups and derived from aromatic, aliphatic and/or cycloaliphatic diisocyanates, uretdione diisocyanates having aromatically, aliphatically and/or cycloaliphatically bonded isocyanate groups, polyisocyanates containing biuret groups and having aromatically or aliphatically bonded isocyanate groups, polyisocyanates containing urethane groups and/or allophanate groups and having aromatically, aliphatically, or cycloaliphatically bonded isocyanate groups, polyisocyanates comprising oxadiazinetrione groups, trimethylolpropane tris(beta-aziridino)propionate, neopentyl glycol di(betaaziridino)propionate, glycerol tris(beta-aziridino)propionate, pentaerythritol tetra(betaaziridino)propionate, 4,4-isopropylidenediphenol di(beta-aziridino)propionate, 4,4-methylenediphenol di(beta-aziridino)propionate, 1,6-hexamethylene di(N,N-ethyleneurea), 4,4-methylenebis(phenyl-N,N-ethyleneurea), 1,3,5-tris(omega-hexamethylene-N,N-ethyleneurea) biuret, and mixtures thereof.

9. The protective film according to claim 1, wherein the glass transition temperature of the noncrosslinked adhesive polymer is from 60 to 0 C.

10. The protective film according to claim 1, wherein the pressure-sensitive adhesive composition comprises at least one pressure-sensitive adhesive polymer which is formed by emulsion polymerization from (i) 60 to 99.9 wt %, based on the sum of the monomers, of at least one soft monomer selected from the group consisting of n-butyl acrylate, ethyl acrylate, 2-ethylhexyl acrylate, and mixtures of these monomers, (ii) 0.1-5 wt %, based on the sum of the monomers, of at least one monomer selected from the group consisting of acrylic acid, methacrylic acid and mixtures of these monomers, and (iii) 0 to 30 wt % of monomers different from (i) and (ii), selected from the group consisting of C1 to C20 alkyl (meth)acrylates, vinyl esters of carboxylic acids comprising up to 20 C atoms, vinylaromatics having up to 20 C atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 C atoms, aliphatic hydrocarbons having 2 to 8 C atoms and one or two double bonds, monomers comprising hydroxyl groups, (meth)acrylamide, phenyloxyethyl glycol mono(meth)acrylate, glycidyl (meth)acrylate, aminoalkyl (meth)acrylates, diacetoneacrylamide, acetoacetoxyethyl methacrylate, and mixtures of these monomers, wherein: the emulsion polymerization of the monomers occurs in the presence of at least one saccharified starch in an amount of 0.1 to 50 parts by weight, based on 100 parts by weight of monomers of the at least one pressure-sensitive adhesive polymer, and the pressure-sensitive adhesive composition comprises at least one crosslinker selected from polyisocyanates and polyaziridines, the weight ratio of saccharified starch to crosslinker in the dispersion being from 1:2 to 100:1.

11. A process for producing protective films, comprising forming a pressure-sensitive adhesive composition comprising at least one pressure-sensitive adhesive polymer formed by emulsion polymerization from (i) at least 60 wt %, based on the sum of the monomers, of at least one soft monomer which when polymerized as a homopolymer has a glass transition temperature of less than 0 C., (ii) 0.1-5 wt %, based on the sum of the monomers, of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride, and (iii) optionally further monomers different from (i) and (ii), wherein: the emulsion polymerization of the monomers occurs in the presence of at least one saccharified starch, the pressure-sensitive adhesive composition comprises at least one crosslinker selected from the group consisting of polyisocyanates and polyaziridines, and the protective film is a polymer film which is coated with the pressure-sensitive adhesive composition, and the crosslinking reaction brought about by the crosslinker is at an end without the pressure-sensitively adhesive protective film being adhered to a substrate.

12. A pressure-sensitive adhesive composition in the form of an aqueous polymer dispersion comprising at least one dispersed pressure-sensitive adhesive polymer formed by emulsion polymerization from (i) at least 60 wt %, based on the sum of the monomers, of at least one soft monomer which when polymerized as a homopolymer has a glass transition temperature of less than 0 C., (ii) 0.1-5 wt %, based on the sum of the monomers, of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride, and (iii) optionally further monomers different from (i) and (ii), wherein: the emulsion polymerization of the monomers occurs in the presence of at least one saccharified starch, the pressure-sensitive adhesive composition comprises at least one crosslinker selected from the group consisting of polyisocyanates and polyaziridines, and the weight ratio of starch to crosslinker in the dispersion being from 1:2 to 100:1.

Description

EXAMPLES

[0075] Abbreviations used are as follows: [0076] DE Dextrose equivalent [0077] C Plus Maltodextrin C Plus 10998, Cargill, Del. 16.5 [0078] C-Sweet C-Star-Sweet 01403, Cargill, degraded starch, glucose syrup, DE 26-32 [0079] Roquette Maltodextrin Roquette 1967, Roquette, Del. 18 to 20 [0080] Basonat Basonat HW 100, water-dispersible polyisocyanate based on isocyanurated hexamethylene diisocyanate, BASF SE [0081] Emuldur Emuldur 3643, BASF SE, water-dispersible polyaziridine [0082] Lutavit C L(+)-Ascorbic acid solution, BASF SE [0083] Dowfax 2A1 Alkyldiphenyl oxide disulfonate, emulsifier [0084] Disponil LDBS 20 Emulsifier [0085] pphm Parts by weight per 100 parts by weight of monomer (parts per hundred parts monomers) [0086] SC Solids content [0087] LT Light transmittance; parameter for determination of differences in particle size. The polymer dispersion here is diluted to 0.01% solids content and the light transmittance is measured in comparison to pure water [0088] Tg (calculated) Glass transition temperature calculated by the Fox equation from the glass transition temperature of the homopolymers of the monomers that are present in the copolymer and from their weight fraction:

1/Tg=xA/TgA+xB/TgB+xC/TgC+ . . . [0089] Tg: calculated glass transition temperature of the copolymer [0090] TgA: glass transition temperature of the homopolymer of monomer A [0091] TgB, TgC: Tg correspondingly for monomers B, C, etc. [0092] xA: Mass of monomer A/total mass of copolymer, [0093] xB, xC correspondingly for monomers B, C, etc.

Example Dispersion 1a

[0094] A mixture of 200 g of water, 100 g of a 50% strength maltodextrin solution (C Plus), and 6.06 g of a 30% fine polystyrene seed (in water) is heated to 85 C. and stirred for 5 minutes. Then 10 g of a 2.5% strength sodium peroxodisulfate solution are added, and stirring is repeated for 5 minutes. This is followed by the metering of the monomers over 2 hours and, in parallel, the metering of 40 g of sodium peroxodisulfate (2.5% strength solution in water).

Monomer Feed 1a:

[0095] 200 g water
22.22 g Dowfax 2A1 (45% strength in water)
5 g acrylic acid
495 g n-butyl acrylate

[0096] This is followed by postpolymerization of 45 minutes. Thereafter 10 g of a 5% strength hydrogen peroxide solution are added, and 10 g of a 10% strength Lutavit C solution are metered in over 30 minutes. 30 g of water and 3.6 g of a 12.5% strength ammonia solution are added.

Example Dispersions 1b, 1c and Comparative Examples 1d, 1e

[0097] In examples 1 b-d, with the same polymerization process and monomer composition, the amount of starch added is varied. Example 1 b adds 75 g of maltodextrin solution, example 1c adds 50 g, and example 1d adds no maltodextrin solution, with this example serving as a noninventive reference. In example 1e, 100 g of a 50% strength maltodextrin solution are added to the dispersion from example 1d, after the polymerization and after cooling, and the system is stirred.

Example Dispersions 2a and 2b

[0098] In examples 2a and b, the monomer composition is varied. The maltodextrin quantity and the course of the polymerization are as for example 1a.

Monomer feed 2a:
200 g water
22.22 g Dowfax 2A1 (45% strength in water)
5 g acrylic acid
250 g n-butyl acrylate
245 g 2-ethylhexyl acrylate

Monomer Feed 2b:

[0099] 200 g water
22.22 g Dowfax 2A1 (45% strength in water)
5 g acrylic acid
410 g n-butyl acrylate
85 g 2-tert-butyl acrylate

Example Dispersions 3a and 3b

[0100] In examples 3a and b, the nature of the starch added is varied. In 3a, instead of the maltodextrin, a much more degraded starch (C-Star-Sweet 01403, as a 50% strength solution) is used; example 3b uses the maltodextrin from Roquette (Roquette 1967, as a 50% strength solution). In both cases the monomer composition and the quantity of starch are analogous to those in example 1a.

Example Dispersion 4a

[0101] In example 4a, instead of Dowfax 2A1, the emulsifier Disponil LDBS 20 is used, in an equal quantity. The composition and polymerization conditions are analogous to those in example 1a.

[0102] The characteristic values of the example dispersions are described in the table below.

TABLE-US-00001 TABLE 1 Wet specimen values of the dispersions from examples 1-4. Example SC [%] pH LT Tg (calculated) 1a 49.3 6.5 72 42 C. 1b 51.8 5.1 73 42 C. 1c 49.6 5.1 53 42 C. 1d comparative 49.9 4.7 72 42 C. 2a 50.2 4.4 70 50 C. 2b 50.6 4.8 72 30 C. 3a 49.6 3.9 74 42 C. 3b 49.4 4.1 74 42 C. 4a 50.4 4.9 73 42 C.

Performance Testing

Production of the Protective Film

[0103] The polymer dispersions obtained were investigated for their suitability as PSAs for protective film applications. Both unformulated adhesives and adhesives formulated with crosslinkers are tested. For this purpose, 100 g of each polymer dispersion is admixed dropwise, with stirring, with the quantity of crosslinker indicated in the corresponding tables, and the system is homogenized by further stirring for 5 minutes. Within a maximum of 2 hours, the resulting adhesives are coated using a bar coater directly onto corona-pretreated polyethylene film (film thickness 50 m), and the coated films are dried in a forced air oven at 90 C. for 3 minutes, then lined with siliconized paper and stored under standard conditions (23 C., 50% relative humidity) for at least 16 hours, in the case of crosslinked adhesives for 7 days, prior to performance testing.

[0104] The test specimens each have an adhesive coatweight of (10+/1) g/m.sup.2. For investigations of the water absorption and of the blushing behavior, the adhesives are applied to PET film (Hostaphan BN50) with an adhesive coatweight of (15+/1) g/m.sup.2; other conditions of the sample preparation are the same.

Testing for Suitability as Protective Film Adhesive

Water Resistance

a) Water Absorption

[0105] Sections with an area of 100 cm.sup.2 are cut, using a circular cutter, from PET film specimens coated with adhesive as described above, and the silicone paper is removed, and the specimens are weighed on an analytical balance. With knowledge of the adhesive coatweight (15+/1) g/m.sup.2, the total amount of adhesive in the dry state is calculated. In a shallow tray, mains water is first conditioned at room temperature for 24 hours. The coated PET film sections are then stored in water, with the adhesive side upward, at room temperature for 24 hours; floating is prevented by weighting with small pieces of brass. The specimens are then removed, adhering water drops are dabbed off immediately with a cotton cloth, and the film disk is folded together by the adhesive side in order to prevent further drying. The specimens are again weighed on the analytical balance. The increase in mass determined is interpreted as water absorption by the adhesive, and is expressed as a percentage relative to the original mass of adhesive.

TABLE-US-00002 TABLE 2 Water absorption of the polymer films after water storage for 24 hours. Monomer Amount of Amount of Water composition starch crosslinker absorp- [parts by [parts by [parts by tion Example weight] weight], type weight], type [%] 1a 99 nBA, 1AA 10, C Plus 0 1.7 1a 99 nBA, 1AA 10, C Plus 1.5 (Basonat) 0.1 1b 99 nBA, 1AA 7.5, C Plus 0 17.6 1b 99 nBA, 1AA 7.5, C Plus 1.5 (Basonat) 1.1 1c 99 nBA, 1AA 5, C Plus 0 28.0 1c 99 nBA, 1AA 5, C Plus 1.5 (Basonat) 1.1 1d 99 nBA, 1AA 0 0 31.2 comparative 1d 99 nBA, 1AA 0 1.5 (Basonat) 19.6 comparative 1e 99 nBA, 1AA 10*, C Plus 0 3.1 comparative 1e 99 nBA, 1AA 10*, C Plus 1.5 (Basonat) 20.0 comparative 2a 49 EHA, 10, C Plus 0 19.2 50 nBA, 1 AA 2a 49 EHA, 10, C Plus 1.5 (Basonat) 0.5 50 nBA, 1 AA 2b 17 tBA, 10, C Plus 0 7.4 82 nBA, 1 AA 2b 17 tBA, 10, C Plus 1.5 (Basonat) 0.4 82 nBA, 1 AA 3a 99 nBA, 1AA 10, C Sweet 0 8.6 3a 99 nBA, 1AA 10, C Sweet 2.0 (Basonat) 0.5 3b 99 nBA, 1AA 10, Roquette 0 10.6 3b 99 nBA, 1AA 10, Roquette 2.0 (Basonat) 0.6

b) Blushing Behavior

[0106] PET film strips coated with (15+/1) g/m.sup.2 adhesive are stored in mains water at room temperature, and any blushing that occurs to the adhesive is assessed visually in qualitative terms after defined intervals as per table 3. Here, a rating of 0=no haze, 1=very slight haze, 2=more pronounced haze, 3=severe haze, and 4=very severe haze.

TABLE-US-00003 TABLE 3 Blushing of the polymer films after water storage at different times, with varying amounts of starch and of crosslinker Amount of Blushing, ratings Dispersion crosslinker Starch:crosslinker 0-4 after example [pphm], type ratio 10 min 1 h 24 h 1a 0 10:0 4 4 4 1a 1 (Basonat) 10:1 1 1 1 1a 2 (Basonat) 5:1 0 0 0 1a 3 (Basonat) 3.3:1 4 4 4 1a 0.3 (Emuldur) 33:1 3 3 3 1a 0.6 (Emuldur) 17:1 4 4 2 1a 0.9 (Emuldur) 11:1 0 0 0 1c 0 10:0 4 4 4 1c 2 (Basonat) 2.5:1 4 4 4 1d 0 10:0 4 4 4 comparative 1d 2 (Basonat) 5:1 4 4 4 comparative 1e 0 10:0 4 4 4 comparative 1e 1.5 (Basonat) 6.7:1 4 4 4 comparative 3a 0 10:0 4 4 4 3a 2 (Basonat) 5:1 1 0 0 3b 0 10:0 4 4 4 3b 2 (Basonat) 5:1 2 1 1
c) Anchorage after Water Storage

[0107] PE strips coated with (10+/1) g/m.sup.2 adhesive (specimens as for the determination of the adhesive bonding profile; see below) are stored in mains water at room temperature for 24 hours. If the adhesive subsequently can be rubbed away from the PE film by thumb only with very great difficulty or not at all, the anchorage is assessed as being very good or good. If the adhesive, on the other hand, can be rubbed off with just a little effort, the anchorage is assessed as poor.

TABLE-US-00004 TABLE 4 Anchorage of the films of adhesive after 24 h water storage. Amount of crosslinker Example [%], type Anchorage to PE.sup.b 1a 0 very poor 1a 1.5 (Basonat HW 100) very good 1d comparative 0 very poor 1d comparative 1.5 (Basonat HW 100) poor 3a 0 poor 3a 1.5 (Basonat HW 100) very good 3b 0 very poor 3b 1.5 (Basonat HW 100) very good

Adhesive Bonding Profile

a) Quickstick

[0108] In the determination of the quickstick (surface tack, also called loop tack), a determination is made of the force with which an adhesive applied to a carrier material by bonding without pressure onto a substrate opposes removal from the substrate at a defined removal speed. From the carrier coated with adhesive as described above, a test strip 25 mm in width and 250 mm in length is cut and is stored under standard conditions (23 C., 50% relative humidity) for at least 16 hours, in the case of crosslinked adhesives for 7 days. The two ends of the test strip are folded over for a length of approximately 1 cm with the adhesive side inward. A loop is formed from the adhesive strip, with the adhesive side outward, and the two ends are brought together and clamped into the upper jaw of a tensile testing machine. The test substrate mount is clamped into the lower jaw, and the test substrate is inserted. The loop of adhesive strip is run downward through the tensile testing machine at a speed of 300 mm/minute, causing the adhesive side of the test strip to bond to the substrate without additional pressure. The tensile testing machine is halted and is immediately moved upward again when the bottom edge of the upper jaw is 40 mm above the substrate. The test result is reported in N/25 mm width. The maximum value on the display (Fmax) is read off as the measure of the surface tack. An average is formed from at least two individual results.

b) Shear Strength (Cohesion)

[0109] For the determination of the shear strength, the test strips are bonded to sheet steel with a bonded area of 2525 mm, rolled down once with a roller weighing 1 kg, and, after 10 minutes, loaded in suspension with a 1 kg weight. The shear strength (cohesion) is determined under standard conditions (23 C.; 50% relative humidity). The measure of the shear strength is the time taken, in hours, for the weight to drop; the average is calculated in each case from at least 3 measurements.

c) Peel Strength (Adhesion)

[0110] For the determination of the peel strength (adhesion), a test strip 25 mm wide is adhered in each case to a test element comprising the material characterized in the respective tables, and is rolled down once with a roller weighing 1 kg. The strip is then clamped by one end into the upper jaws of a tensile strain testing apparatus. The adhesive strip is peeled from the test surface at an angle of 180 and at 300 mm/minthat is, the adhesive strip is bent around and pulled off parallel to the test element, and the expenditure of force required to achieve this is recorded. The measure of the peel strength is the force in N/25 mm which results as the average value from at least two measurements. The peel strength is determined 1 minute and 24 hours after bonding. The test methods correspond essentially to Finat test methods (FTM) 1, 8 and 9. Explanation of the fracture mode: A corresponds to adhesive fracture at the substrate; in the case of F, a slight filmy residue is visible on the substrate surface.

TABLE-US-00005 TABLE 5 Adhesive bonding profiles of the example dispersions with and without starch. Peel strength [N/25 mm]; Shear Quickstick, fracture mode strength, steel, 1 min 24 h 1 min 24 h steel Ex. [N/25 mm] steel steel HDPE HDPE [h] 1a 2.1 1.4; A 2.6; A 0.5; A 2.0; A >100 1a.sup.a 1.1 1.2; A 2.1; A 0.4; A 0.6; A >100 1d.sup.b 3.7 2.9; A 5.1; F 0.9; A 1.3; A >100 1d.sup.a,b 2.4 2.0; A 4.0; F 0.3; A 0.4; A >100 3a 1.2 1.6; A 3.8; F 0.5; A 1.2; A >100 3a.sup.a 0.8 1.0; A 2.2; F 0.3; A 0.4; A >100 3b 1.5 2.5; A 3.7; F 0.8; A 0.9; A >100 3b.sup.a 1.4 1.6; A 2.1; F 0.3; A 0.4; A >100 4a 0.7 1.0; A 3.5; A 0.3; A 0.4; A >100 4a.sup.a 1.7 3.2; A 4.7; A 0.9; A 1.2; A >100 .sup.ablended with 1.5% Basonat HW 100 .sup.b1d is a noninventive, comparative dispersion