AQUEOUS COLD SEAL ADHESIVE

Abstract

Described is an aqueous cold seal adhesive in the form of an aqueous polymer dispersion comprising a mixture of a first polymer and a second polymer, wherein the first polymer is an emulsion polymer prepared in the presence of a protective colloid and is composed to an extent of at least 60% by weight of principal monomers selected from the group consisting of C1 to C20 alkyl (meth)acrylates and has a glass transition temperature of less than 0 C.; and wherein the second polymer is an emulsion polymer prepared in the presence of a protective colloid and is composed to an extent of at least 60% by weight of principal monomers selected from the group consisting of vinylaromatics having up to 20 C atom, and has a glass transition temperature of at least 50 C. Also described are cold sealable coated polymer films coated with the aqueous cold seal adhesive and a method for producing a cold-sealed pack by applying the aqueous cold seal adhesive to a packaging substrate.

Claims

1. An aqueous cold seal adhesive in the form of an aqueous polymer dispersion comprising a mixture of at least one first polymer and at least one second polymer, wherein the first polymer is an emulsion polymer comprising, to an extent of at least 60% by weight, one or more principal monomers selected from the group consisting of C1 to C20 alkyl (meth)acrylates, the first polymer being prepared by emulsion polymerization of free-radically polymerizable monomers in the presence of at least one protective colloid, and the first polymer having a glass transition temperature of less than 0 C.; wherein the second polymer is an emulsion polymer comprising, to an extent of at least 60% by weight, one or more principal monomers selected from the group consisting of vinylaromatics having up to 20 C atoms, the second polymer being prepared by emulsion polymerization of free-radically polymerizable monomers in the presence of at least one protective colloid, and the second polymer having a glass transition temperature of at least 50 C.

2. The aqueous cold seal adhesive according to claim 1, wherein a weight ratio of the at least one first polymer to the at least one second polymer is in a range from 100:5 to 100:30.

3. The aqueous cold seal adhesive according to claim 1, wherein the first polymer comprises at least one secondary monomer selected from the group consisting of acid monomers, 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 with at least one hydroxy group and mixtures of these monomers; and wherein the second polymer comprises at least one secondary monomer selected from the group consisting of acid monomers, C1 to C20 alkyl (meth)acrylates, vinyl esters of carboxylic acids comprising 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, and monomers with at least one hydroxy group.

4. The aqueous cold seal adhesive according to claim 1, wherein the glass transition temperature of the first polymer is from 30 C. to 5 C.; and the glass transition temperature of the second polymer is from +50 C. to +150 C.

5. The aqueous cold seal adhesive according to claim 1, wherein the emulsion polymerization of the first polymer and the emulsion polymerization of the second polymer is performed without emulsifier or with at most 0.5 parts by weight of a nonionic and/or an anionic emulsifier per 100 parts by weight of total monomers.

6. The aqueous cold seal adhesive according to claim 1, wherein the first polymer comprises, to an extent of 90% to 99% by weight, at least one C.sub.2 to C.sub.8 alkyl (meth)acrylate, and, to an extent of 1 to 10% by weight, acrylic acid, methacrylic acid or mixtures thereof; and wherein the second polymer comprises, to an extent of 80% to 99% by weight, styrene, alpha-methyl styrene or mixtures thereof, and, to an extent of from 1 to 20% by weight, acrylic acid, methacrylic acid or mixtures thereof, and, to an extent of from 0 to 15% by weight, at least one C.sub.1 to C.sub.8 alkyl (meth)acrylate.

7. The aqueous cold seal adhesive according to claim 1, wherein the protective colloids (i) are each present in an amount of 7 to 30 parts by weight, based on 100 parts by weight of the monomers of the respective first or second polymer to be polymerized, (ii) comprise, to an extent of at least 40% by weight, based on the monomers of the protective colloid, one or more nonionic principal monomers 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, and mixtures of these monomers, (iii) comprise, to an extent of at least 15% by weight, based on the monomers of the protective colloid, ethylenically unsaturated acid monomers, and (iv) have a number-average molecular weight of 1000 to 10 000.

8. The aqueous cold seal adhesive according to claim 1, wherein: the protective colloid in the polymerization of the first polymer comprises, to an extent of at least 40% by weight, based on the monomers of the protective colloid, one or more principal monomers selected from the group consisting of C2 to C8 alkyl (meth)acrylates, and the protective colloid in the polymerization of the first polymer comprises, to an extent of at least 20% by weight, based on the monomers of the protective colloid, one or more ethylenically unsaturated acid monomers, and wherein the protective colloid in the polymerization of the second polymer comprises, to an extent of at least 40% by weight, based on the monomers of the protective colloid, one or more principal monomers selected from the group consisting of vinylaromatics having up to 20 C atoms, and 1 to 20% by weight, based on the monomers of the protective colloid, of one or more monomers selected from the group consisting of C1 to C20 alkyl (meth)acrylates, and at least 20% by weight, based on the monomers of the protective colloid, of one or more ethylenically unsaturated acid monomers.

9. The aqueous cold seal adhesive according to claim 1, wherein: the first polymer has a glass transition temperature of from 30 C. to 5 C.; and comprises, to an extent of 90% to 99% by weight, at least one C.sub.2 to C.sub.8 alkyl (meth)acrylate, and, to an extent of 1 to 10% by weight, acrylic acid, methacrylic acid or mixtures thereof, and the second polymer has a glass transition temperature of from +50 C. to +150 C., and comprises, to an extent of 80% to 99% by weight, styrene, alphamethyl styrene or mixtures thereof, and, to an extent of from 1 to 20% by weight, acrylic acid, methacrylic acid or mixtures thereof, and, to an extent of from 0 to 15% by weight, at least one C.sub.1 to C.sub.8 alkyl (meth)acrylate; and wherein the protective colloids have a number-average molecular weight of 1000 to 10 000 and are each present in an amount of 7 to 30 parts by weight, based on 100 parts by weight of the monomers of the respective first or second polymer to be polymerized, wherein the protective colloid in the polymerization of the first polymer comprises, to an extent of at least 40% by weight, based on the monomers of the protective colloid, one or more principal monomers selected from the group consisting of C2 to C8 alkyl (meth)acrylates, and the protective colloid in the polymerization of the first polymer comprises, to an extent of at least 20% by weight, based on the monomers of the protective colloid, one or more ethylenically unsaturated acid monomers, and wherein the protective colloid used in the polymerization of the second polymer comprises, to an extent of at least 40% by weight, based on the monomers of the protective colloid, one or more principal monomers selected from the group consisting of vinylaromatics having up to 20 C atoms, and 1 to 20% by weight, based on the monomers of the protective colloid, of one or more monomers selected from the group consisting of C1 to C20 alkyl (meth)acrylates, and at least 20% by weight, based on the monomers of the protective colloid, of one or more ethylenically unsaturated acid monomers, wherein the emulsion polymerization of the first polymer and the emulsion polymerization of the second polymer are performed without emulsifier or with at most 0.5 parts by weight of a nonionic and/or an anionic emulsifier per 100 parts by weight of total monomers, and wherein a weight ratio of the at least one first polymer to the at least one second polymer is in a range from 100:5 to 100:30.

10. A cold sealable coated polymer film, comprising a polymer carrier film at least partly coated with the aqueous cold seal adhesive according to claim 1.

11. The cold sealable coated polymer film according to claim 10, wherein the film has an initial cold seal strength of more than 2 N/15 mm.

12. The coated polymer film according to claim 10, wherein the polymer carrier film comprises polyethylene or oriented polypropylene.

13. (canceled)

14. A method for producing a cold-sealed pack, comprising applying the aqueous cold seal adhesive according to claim 1 to a packaging substrate, drying the cold seal adhesive, and cold-sealing the cold seal adhesive.

Description

EXAMPLES

[0073] Unless the context dictates otherwise, the figures in percent always denote percent by weight. A content datum relates to the content in aqueous solution or dispersion, unless otherwise specified.

[0074] The ingredients used were as follows:

[0075] nBa n-butyl acrylate

[0076] AA acrylic acid

[0077] MAA methacrylic acid

[0078] EA ethyl acrylate

[0079] EHA 2-ethylhexyl acrylate

[0080] MMA methyl methacrylate

[0081] VAc vinyl acetate

[0082] HPA hydroxypropyl acrylate

[0083] EHTG 2-ethylhexyl thioglycolate

[0084] oPP carrier film: oriented polypropylene film

[0085] Lumiten I-SC Solution of sodium sulphosuccinate/isotridecanol ethoxylate in water; stabilizer and wetting agent; anionic emulsifier

Coating of the oPP Film

[0086] With the bar applicator, the adhesive is applied to the pretreated side of the oPP film Oppalyte 33MW247 and dried at 70 C. for 1 minute. The coated film is lined with an oPP film coated with a polyamide-based release varnish.

Seal Strength

[0087] Strips 15 mm wide are cut from the coated film and sealed to one another (adhesive to adhesive) on the sealing device for 0.5 seconds with 200 N (1.4 bar). 30 seconds after sealing has taken place, the peel strengths in N/15 mm are ascertained at a peel speed of 50 mm/min.

Blocking Resistance

[0088] The blocking resistance test is done manually. A dry film is formed from the cold seal adhesive to be tested and then the tackiness of the film surface is evaluated by pressing a dry cleaned finger on it and remove it from the surface. If there is no stickiness to be felt, the film is blocking-free.

[0089] Significant blocking means stickiness is clearly felt.

[0090] Extensive blocking means that the surface blocks more than in the case of significant blocking.

[0091] All tests take place at room temperature (23 C.).

Polymerdispersion A (Soft Polymer)

Preparation of Protective Colloid for First Polymer

[0092] 35 parts by weight of acrylic acid and 65 parts by weight of n-butyl acrylate were polymerized in a semi-batch process in isopropanol as solvent. Tert-Butylperoctoate was used as initiator. After finishing the polymerization, the solvent is distilled off and the residue is dispersed in water with ammonia to be used as protective colloid in emulsion polymerization. The degree of neutralization is 60%. The protective colloid has a number-average molecular weight of 6400.

Preparation of First Polymer (Soft Polymer)

[0093]

TABLE-US-00001 parts by weight deionized water 22.9 Protective Colloid 21.1 Ammonium Hydroxide 0.45 Ethyl Acrylate 50.51 EHTG 0.15 sodium persulfate 7% in water 2.89 Sodium persulfate 2.5% in water 2.0

[0094] Ethyl acrylate was polymerized using a semi-batch process in the presence of the protective colloid being neutralized in deonized water with ammonium hydroxide to a degree of neutralization of 60%. Sodium persulfate was used as initiator. Final product has a glass transition temperature Tg: 15 C. and calculated overall composition (including protective colloid): 7.2 parts by weight nBA/3.9 parts by weight AA/88.9 parts by weight EA

Polymerdispersion B (Soft Polymer, Comparative, Without Protective Colloid)

Preparation of First Polymer (Soft Polymer)

[0095]

TABLE-US-00002 parts by weight deionized water 25.62 Ascorbic Acid 0.03 Sodium Persulfate 3.76 Anionic Emulsifier .sup.1) 2.35 Sodium hydroxide 0.52 Acrylic Acid 0.32 Styrene 1.31 Methyl Methacrylate 5.21 Hydroxy Propyl Acrylate 1.31 Vinyl Acetate 5.21 Ethyl Hexyl Acrylate 52.3 t-Butyl Hydroperoxide 0.66 Acetone Bisulfite 0.83 Lumiten I-SC 0.57 .sup.1) mixture of 3 anionic emulsifiers: Dowfax 2 A1 (sodium salt of o-dodecyl diphenylether-disulfonate), Disponil FES 77 (sodium salt of an alkyl ether sulfate) and Lumiten I-SC

[0096] The monomers listed above were polymerized using a semi-batch process in the presence of sodium persulfate as initiator and anionic emulsifier for stabilization. The combination of tert-butyl hydroperoxide and acetone bisulfite was used after the polymerization for deodorization. No protective colloid was used in the polymerization. The final product has a glass transition temperature Tg: 43 C. and a calculated overall composition: 79.5 parts by weight EHA/8 part by weight MMA/8 parts by weight VAc/2 parts by weight styrene/2 parts by weight HPA/0.5 parts by weight AA

Polymerdispersion C (Soft Polymer, Comparative, Without Protective Colloid)

Preparation of First Polymer (Soft Polymer)

[0097] Synthesized in an emulsion polymerization process in presence of 1.7 wt.-% of anionic emulsifiers (mixture of Emulgator K 30 (sodium salt of a parrafine sulfonate) and Emulan OG (non-ionic fatty alcohol ethoxylate)); without protective colloid.

[0098] Glass transition temperature Tg: 23 C.

[0099] Calculated overall composition:

[0100] 55 parts by weight nBA/43 parts by weight VAc/2 parts by weight AA

Polymerdispersion D (Hard Polymer)

Preparation of Protective Colloid for Second Polymer

[0101] Monomers, solvent (2-ethylhexanol) and initiator (di-tert-butyl peroxide) were mixed together to form a uniform mixture. The mixture was continuously charged to a continuous stirred tank reactor operating at uniform temperature (220 C.) to form a polymer product. The polymer product was continuously withdrawn from the reactor at a rate equal to the feed rate to the reactor so as to maintain an average residence time of 15 minutes. The polymer product from the reactor was then continuously fed to a wiped film evaporator operating under vacuum and temperature where the unreacted monomer and residual solvent were evaporated from the polymer product to form a distillate stream and stripped polymer sample. The stripped polymer sample was then dispersed in water with ammonia to be used as protective colloid in emulsion polymerization. The final product has a number-average molecular weight of 8700, and a calculated overall composition: 24 parts by weight styrene/36 parts by weight alpha-methyl styrene/34 parts by weight AA/6 parts by weight EHA.

Preparation of Second Polymer (Hard Polymer)

[0102]

TABLE-US-00003 parts by weight deionized water 51.12 Protective Colloid 13.28 Ammonium Hydroxide 3.43 Styrene 30 Ammonium persulfate 0.25 t-butyl hydroperoxide 0.18 Ascorbic Acid 0.64

[0103] Styrene was polymerized using a semi-batch process in the presence of the protective colloid neutralized in deionized water with ammonium hydroxide. Ammonium persulfate was used as initiator. Combination of t-butyl hydroperoxide and ascorbic acid was used after the polymerization for deodorization. Final product has a glass transition temperature Tg: +110 C. with calculated overall composition (including protective colloid): 73.7 parts by weight styrene/11 parts by weight alpha-methyl styrene/10.4 parts by weight AA/1.8 parts by weight EHA.

Polymerdispersion E (Hard Polymer)

Preparation of Protective Colloid for Second Polymer

[0104] Monomers, solvent (2-ethylhexanol) and initiator (di-tert-butyl peroxide) were mixed together to form a uniform mixture. The mixture was continuously charged to a continuous stirred tank reactor operating at uniform temperature (217 C.) to form a polymer product. The polymer product was continuously withdrawn from the reactor at a rate equal to the feed rate to the reactor so as to maintain an average residence time of 15 minutes. The polymer product from the reactor was then continuously fed to a wiped film evaporator operating under vacuum and temperature where the unreacted monomer and residual solvent were evaporated from the polymer product to form a distillate stream and stripped polymer sample. The stripped polymer sample was then dispersed in water with ammonia to be used as protective colloid in emulsion polymerization. The final product has a number-average molecular weight of 9800, and a calculated overall composition: 33 parts by weight styrene/32 parts by weight alpha-methyl styrene/35 parts by weight AA.

Preparation of Second Polymer (Hard Polymer)

[0105]

TABLE-US-00004 parts by weight deionized water 48.82 Protective Colloid 12.24 Ammonium Hydroxide 2.80 Styrene 34.16 Ammonium persulfate 0.35 Nonionic surfactant 1.75

[0106] Styrene was polymerized using a semi-batch process in the presence of the protective colloid neutralized in deionized water with ammonium hydroxide and nonionic emulsifier. Ammonium persulfate was used as initiator. Final product has a glass transition temperature Tg: +98 C. and calculated overall composition (including protective colloid): 82.5 parts by weight styrene/8.4 parts by weight alpha-methyl styrene/9.1 parts by weight AA.

Polymerdispersion F (Hard Polymer, Comparative Without Protective Colloid)

Second Polymer (Hard Polymer)

[0107] Polyacrylate, synthesized in an emulsion polymerization process in the presence of 1.5% of anionic emulsifier (Disponil FES 27, sodium salt of alkyl ether sulfate); without protective colloid

[0108] Glass transition temperature Tg: +70 C.

[0109] Calculated overall composition:

[0110] 60 parts by weight MMA/30 parts by weight nBA/10 parts by weight MAA.

Polymerdispersion G (Hard Polymer, Comparative Without Protective Colloid)

Preparation of Second Polymer (Hard Polymer)

[0111]

TABLE-US-00005 parts by weight deionized water 43.11 Polystyrene Seed 5.15 Styrene 24.61 Methyl methacrylate 12.31 Methacrylic Acid 4.11 Sodium persulfate 3.52 Anionic emulsifier.sup.1) 5.45 t-butyl hydroperoxide 0.82 Acetone bisulfite 0.92 .sup.1)sodium salt of alkyl ether sulfate

[0112] The monomers listed above were emulsion polymerized using a semi-batch process in the presence of sodium persulfate as initiator. The polymers were prepared using a polystyrene seed latex and in the presence of 2% of anionic emulsifier. Final product has a glass transition temperature Tg: +118 C. with calculated overall composition: 60 parts by weight styrene/30 parts by weight MMA/10 parts by weight MAA.

Examples 1 to 2, Comparative Examples 1 to 4

[0113] Cold seal adhesives were made by mixing 100 parts by weight of a soft polymer dispersion (first polymer) with 15 parts by weight of a hard polymer dispersion. oPP films were coated with the cold seal adhesives and tested for seal strength and blocking resistance.

[0114] The results of the cold-sealing tests are set out in Table 1.

TABLE-US-00006 TABLE 1 Results of the cold-sealing tests Soft polymer Hard polymer Seal dispersion dispersion strength Blocking Example 100 parts* 15 parts* [N/15 mm] resistance Example 1 A D 6 No blocking Example 2 A E 6 No blocking Compara- A 6 Extensive tive exam- blocking ple 1 Compara- A F 6 Extensive tive exam- blocking ple 2 Compara- A G 6 Extensive tive exam- blocking ple 3 Compara- B D 2 Significant tive exam- blocking ple 4 Compara- C D 2 Significant tive exam- blocking ple 5 *All parts are parts by weight based on wet dispersion, that means the emulsion polymer plus the water it is dispersed in.

[0115] The target is a seal strength of more than 2 N/15 mm, measured 30 seconds after sealing and no blocking. Examples 1 and 2 meet the target. Comparative examples 1 to 4 do not meet the target.