Process for preparing aqueous polymer dispersions for adhesives

Abstract

The present invention relates to aqueous polymer dispersions having a polymer content of at least 55% by weight that are suitable for pressure sensitive adhesives. The invention also relates to a process for preparing these aqueous polymer dispersions and also to the use of the aqueous polymer dispersions in pressure sensitive adhesives and, respectively, pressure sensitive adhesive compositions, especially in pressure sensitive adhesives for transparent substrates, such as plastics labels. The aqueous polymer dispersions have a polymer content of at least 55% by weight, based on the total weight of the dispersion, the polymer being composed of ethylenically unsaturated monomers M and having a glass transition temperature below 10 C., in which the dispersed polymer particles have a polymodal particle size distribution, in which at least 70% by weight have a particle diameter of below 350 nm.

Claims

1. A process for preparing an aqueous polymer dispersion, the process comprising: performing free-radical aqueous emulsion polymerization of ethylenically unsaturated monomers M by a monomer feed process, wherein the monomer feed process comprises: adding at least 95% of the monomers to be polymerized under polymerization conditions to a first particulate seed polymer 1 located in the polymerization reactor, and adding at least one further seed polymer 2 in the form of an aqueous dispersion to the polymerization reactor in the course of the polymerization to obtain the aqueous polymer dispersion, wherein the aqueous polymer dispersion has a polymer content of at least 55% by weight, based on the total weight of the dispersion, in which the polymer comprises the ethylenically unsaturated monomer units M and has a glass transition temperature of below 10 C., and in which dispersed polymer particles have a polymodal particle size distribution, wherein 10% to 60% by weight of the polymer particles have a particle diameter in a range from 70 to 180 nm and 40 to 90% by weight of the polymer particles have a particle diameter in a range from 180 to 350 nm, and wherein the particle size distribution has a polydispersity index Q of from 0.5 to 1.5: $Q=\frac{d_{90}-d_{10}}{d_{50}}$ in which d.sub.10 is a particle diameter below which 10% by weight of the polymer particles fall, d.sub.50 is a particle diameter below which 50% by weight of the polymer particles fall, and which is in the range from 90 to 320 nm, d.sub.90 is a particle diameter below which 90% by weight of the polymer particles fall, which is not more than 350 nm, and all particle diameters are determined at 23 C. by centrifuging and discriminating according to mass fractions, and wherein the monomers M, based on a total amount of the monomers M, comprise at least 65% by weight of at least one C.sub.2-C.sub.12 alkyl acrylate.

2. The process according to claim 1, the seed polymer 2 being added when 10% to 60% by weight of the monomers to be polymerized are located in the polymerization reactor.

3. The process according to claim 1, the seed polymer 1 being used in an amount of 0.1% to 2% by weight, based on the monomers to be polymerized.

4. The process according to claim 1, the seed polymer 1 having a weight-average particle size in the range from 10 to 150 nm.

5. The process according to claim 1, the seed polymer 2 being used in an amount of 0.05% to 2% by weight, based on the monomers to be polymerized.

6. The process according to claim 1, wherein seed polymer 1 and seed polymer 2 comprise vinylaromatic monomer units.

7. The process according to claim 1, wherein seed polymer 1 and seed polymer 2 comprise styrene.

8. The process according to claim 1, wherein seed polymer 1 and seed polymer 2 comprise C.sub.1-C.sub.10 alkyl acrylates and/or C.sub.1-C.sub.10 alkyl methacrylates.

9. The process according to claim 1, wherein seed polymer 1 and seed polymer 2 comprise butyl acrylate and methyl methacrylate.

10. The process according to claim 1, wherein less than 10% by weight of the polymer particles have a particle diameter of below 70 nm.

11. The process according to claim 1, wherein the polymer comprises at least 99% by weight of monoethylenically unsaturated monomer units M comprising: a) 65% to 99.8% by weight of at least one monomer a), wherein said monomer a) is a C.sub.2-C.sub.12 alkyl acrylate; b) 0.1% to 30% by weight of at least one monomer b) selected from the group consisting of methyl acrylate, a C.sub.1-C.sub.12 alkyl methacrylate, and a vinylaromatic monomer; c) 0.1% to 10% by weight of at least one monomer c), wherein said monomer c) is a monoethylenically unsaturated monomer having at least one hydroxyalkyl group; and d) 0% to 5% by weight of at least one monomer d), wherein said monomer d) is a monoethylenically unsaturated monomer having at least one acid group.

12. The process according to claim 1, wherein the polymer dispersion has a Brookfield viscosity, determined in accordance with DIN EN ISO 3219, at 20 C., 100 sec.sup.1, of not more than 1.5 Pa.Math.s.

13. The process according to claim 12, wherein the Brookfield viscosity is from 0.05 to 1.5 Pa.Math.s, determined in accordance with DIN EN ISO 3219 at 23 C., 100 sec.sup.1.

14. The process according to claim 1, wherein 15% to 50% by weight of the polymer particles have a particle diameter in a range from 70 to 180 nm and 50 to 85% by weight of the polymer particles have a particle diameter in a range from 180 to 340 nm.

15. The process according to claim 1, wherein 15% to 50% by weight of the polymer particles have a particle diameter in a range from 80 to 150 nm and 50 to 85% by weight of the polymer particles have a particle diameter in a range from 200 to 330 nm.

16. The process according to claim 1, wherein the polydispersity index Q is from 0.6 to 1.2.

Description

A PREPARATION AND CHARACTERIZATION OF THE POLYMER DISPERSIONS

(1) Determination of particle sizes: the particle size was determined by the method described by H. Clfen, Analytical Ultracentrifugation of Nanoparticles in Encyclopedia of Nanoscience and Nanotechnology, (American Scientific Publishers, 2004), pp. 67-88, at 23 C., on a 0.1-0.5% by weight dilution (based on solids content, light transmittance about 10%) of the polymer dispersion by means of an ultracentrifuge (Beckmann model XL) in a sedimentation field ramp from 600 to 40 000 rpm, corresponding to an acceleration of 2250 to 150 000 g, using a turbidity-based optical system (see also W. Mchtle and L. Brger in Analytical Ultracentrifugation of Polymers and Nanoparticles, (Springer, Berlin, 2006); W. Mchtle in Analytical Ultracentrifugation in Biochemistry and Polymer Science, S. E. Hrting et al. (editors), Cambridge, Royal Society of Chemistry, 1992, pp. 147-175; and in W. Mchtle, Makromolekulare Chemie 185 (1984), pages 1025-1039). The diluent used was D.sub.2O with 0.5 g/l Emulgator K30 (emulsifier; sodium salt of an alkanesulfonate).

(2) Determination of viscosity: the Brookfield viscosity was determined in a method based on DIN EN ISO 3219 by means of a rotational viscometer (Physica MCR 301 rheometer with sample changer and CC27 measuring system, from Anton Paar) at 23 and a shear rate of 0 to 500 sec.sup.1). The figure reported is the value at 100 sec.sup.1.

(3) The glass transition temperature was determined by means of DSC in accordance with the DSC method described in ASTM 3418/82, with determination of the midpoint temperature.

(4) Light transmittance (LT): the LT value was determined photometrically using a photometer on a 0.01% by weight dilution of the dispersion at 23 C.

(5) Ingredients:

(6) Emulsifier solution 1: 58% strength by weight commercial solution of the sodium salt of a sulfuric monoester of a C.sub.12-C.sub.14 alkanol ethoxylate (30 EO).

(7) Emulsifier solution 2: Dowfax 2A1: 45% strength by weight solution of the sodium salt of a mixture of mono- and di-C.sub.12-alkyl-bishydroxysulfonyl diphenyl ether.

(8) Emulsifier solution 3: 58% strength by weight solution of the sodium salt of di-2-ethylhexylsulfosuccinate.

(9) Seed dispersion 1: aqueous polymer dispersion of a styrene polymer having a solids content of 33% by weight and an average particle size (weight average, d.sub.50) of 30 nm.

(10) Seed dispersion 2: aqueous polymer dispersion of an acrylate polymer (65% by weight butyl acrylate, 34% by weight methyl methacrylate, 1% by weight acrylic acid) having a solids content of 41% by weight and an average polymer particle size (weight average, d.sub.50) of 50 nm.

1. Comparative Example 1

(11) A polymerization reactor was charged with 0.75 g of ascorbic acid in 206 g of deionized water. The mixture was heated to 90 C. 9.62 g of a 7% strength by weight aqueous solution of sodium peroxodisulfate were added, and, after a further 5 minutes, at 90 C., the addition of feed streams 1 and 2 was commenced. Feed stream 1 was metered in as follows: 0.4% over 6 minutes 3.8% over 22 minutes 7.6% over 22 minutes 11.4% over 22 minutes 76.8% over 2 hours 18 minutes

(12) Feed stream 2 was metered in over 3 hours 30 minutes. During the addition, the temperature in the reaction mixture was maintained at 90 C. After the end of feed steam 1, a further 40 g of deionized water were added. This was followed by successive neutralization with 15 g of 10% strength by weight aqueous sodium hydroxide solution and 7.5 g of 10% strength aqueous ammonia solution. A further 10 g of deionized water were added and then over an hour at 90 C. feed streams 3 and 4 were metered in. Subsequently 12.9 g of aqueous emulsifier solution 3 and 30 g of deionized water were added, and the batch was stirred at 90 C. for 15 minutes more and then left to cool to room temperature.

(13) The polymer dispersion obtained had a solids content of 64.2% by weight, a pH of 5, and an LT (0.01% by weight) of 37%. The glass transition temperature was 42 C. The viscosity was 620 mPa.Math.s.

(14) The particle size distribution is shown in FIG. 1. The d.sub.10, d.sub.50, and d.sub.90 values are reported in table 1. Feed stream 1: 1920 g of an aqueous emulsion of 15 g of acrylic acid, 29.7 g of 2-hydroxypropyl acrylate, 29.4 g of styrene, 118.8 g of methyl acrylate, 120.3 g of methyl methacrylate, and 1186 g of 2-ethylhexyl acrylate in deionized water, containing 70.3 g of emulsifier solution 1, 6.7 g of emulsifier solution 2, and 10.3 g of emulsifier solution 3. Feed stream 2: 77.14 g of a 7% strength by weight aqueous sodium peroxodisulfate solution. Feed stream 3: 22.5 g of a 10% strength by weight aqueous solution of tert-butyl hydroperoxide. Feeds stream 4: 18 g of a 10% strength by weight aqueous solution of sodium hydroxymethylsulfinate.

2. Comparative Example 2

(15) A polymerization reactor was charged with 0.7 g of ascorbic acid in 192 g of deionized water. The mixture was heated to 90 C. 8.98 g of a 7% strength by weight aqueous solution of sodium peroxodisulfate were added, and, after a further 5 minutes, at 90 C., the addition of feed streams 1 and 2 was commenced, in the manner described for comparative example 1. During the addition, the temperature in the reaction mixture was maintained at 90 C. 84 minutes after the start of feed streams 1 and 2, respectively, 9.76 g of seed dispersion 1 were added in one portion. After the end of feed streams 1 and 2, a further 37.5 g of deionized water were added. This was followed by successive neutralization with 14 g of 10% strength by weight aqueous sodium hydroxide solution and 7 g of 10% strength aqueous ammonia solution. A further 9.5 g of deionized water were added and then over an hour at 90 C. feed streams 3 and 4 were metered in. Subsequently 12.1 g of aqueous emulsifier solution 3 and 28 g of deionized water were added, and the batch was stirred at 90 C. for 15 minutes more and then left to cool to room temperature.

(16) The d.sub.10, d.sub.50 and d.sub.90 values are reported in table 1. Feed stream 1: 1980 g of an aqueous emulsion of 14 g of acrylic acid, 27.7 g of 2-hydroxypropyl acrylate, 27.4 g of styrene, 110.9 g of methyl acrylate, 112.3 g of methyl methacrylate, and 1107.7 g of 2-ethylhexyl acrylate in deionized water, containing 65.6 g of emulsifier solution 1, 6.2 g of emulsifier solution 2, and 9.7 g of emulsifier solution 3. Feed stream 2: 72 g of a 7% strength by weight aqueous sodium peroxodisulfate solution. Feed stream 3: 21 g of a 10% strength by weight aqueous solution of tert-butyl hydroperoxide. Feed stream 4: 16.8 g of a 10% strength by weight aqueous solution of sodium hydroxymethylsulfinate.

(17) The polymer dispersion obtained had a solids content of 60.4% by weight, a pH of 5, and an LT (0.01% by weight) of 41%. The glass transition temperature was 42 C. The viscosity was 190 mPa.Math.s.

(18) The d.sub.10, d.sub.50, and d.sub.90 values are reported in table 1.

3. Inventive Example 1

(19) A polymerization reactor was charged with 15.2 g of seed dispersion 2 in 188.3 g of deionized water. The mixture was heated to 90 C. 8 g of a 7% strength by weight aqueous solution of sodium peroxodisulfate were added, and, after a further 4 minutes, at 90 C., the addition of feed streams 1 and 2, which were metered in at a constant feed rate over 4 h, was commenced. During the addition, the temperature in the reaction mixture was maintained at 90 C. 72 minutes after the start of feed streams 1 and 2, respectively, 8.71 g of seed dispersion 1 were added in one portion. After the end of feed streams 1 and 2, a further 40 g of deionized water were added. This was followed by neutralization with 16.3 g of 10% strength aqueous ammonia solution. A further 10 g of deionized water were added and then over an hour at 90 C. feed streams 3 and 4 were metered in. Subsequently 10.8 g of aqueous emulsifier solution 3 and 30 g of deionized water were added, and the batch was stirred at 90 C. for 15 minutes more and then left to cool to room temperature.

(20) The polymer dispersion obtained had a solids content of 59.1% by weight, a pH of 6, and an LT (0.01% by weight) of 52%. The glass transition temperature was 42 C. The viscosity was 450 mPa.Math.s.

(21) The particle size distribution is shown in FIG. 3. The d.sub.10, d.sub.50, and d.sub.90 values are reported in table 1. Feed stream 1: 1737.3 g of an aqueous emulsion of 12.5 g of acrylic acid, 24.7 g of 2-hydroxypropyl acrylate, 24.5 g of styrene, 99 g of methyl acrylate, 100.3 g of methyl methacrylate, and 989 g of 2-ethylhexyl acrylate in deionized water, containing 58.6 g of emulsifier solution 1, 5.6 g of emulsifier solution 2, and 8.6 g of emulsifier solution 3. Feed stream 2: 64.3 g of a 7% strength by weight aqueous sodium peroxodisulfate solution. Feed stream 3: 18.8 g of a 10% strength by weight aqueous solution of tert-butyl hydroperoxide. Feed stream 4: 15 g of a 10% strength by weight aqueous solution of sodium hydroxymethylsulfinate.

4. Inventive Example 2

(22) Preparation took place in the same way as for inventive example 1, with the difference that the initial charge additionally contained 1.5 g of ascorbic acid.

(23) The polymer dispersion obtained had a solids content of 58.7% by weight, a pH of 6, and an LT (0.01% by weight) of 46%. The glass transition temperature was 42 C. The viscosity was 350 mPa.Math.s.

(24) The d.sub.10, d.sub.50, and d.sub.90 figures are reported in table 1.

5. Inventive Example 3

(25) Preparation took place in the same way as for inventive example 1, with the difference that the seed dispersion was carried out over a period of 192 minutes, beginning 24 minutes after the start of feed stream 1. For neutralization, instead of 16.3 g of the 10% strength by weight aqueous ammonia solution, added successively were 12.5 g of 10% strength by weight aqueous sodium hydroxide solution and 7.25 g of 10% strength by weight aqueous ammonia solution.

(26) The polymer dispersion obtained had a solids content of 59.4% by weight, a pH of 5.5, and an LT (0.01% by weight) of 49%. The glass transition temperature was 42 C. The viscosity was 380 mPa.Math.s.

(27) The particle size distribution is shown in FIG. 4. The d.sub.10, d.sub.50, and d.sub.90 figures are reported in table 1.

6. Inventive Example 4

(28) A polymerization reactor was charged with 13.7 g of seed dispersion 2 in 188 g of deionized water. The mixture was heated to 85 C. 8.02 g of a 7% strength by weight aqueous solution of sodium peroxodisulfate were added, and, after a further 4 minutes, at 85 C., the addition of feed streams 1 and 2, which were metered in at a constant feed rate over 4 h, was commenced. During the addition, the temperature in the reaction mixture was maintained at 85 C. 90 minutes after the start of feed streams 1 and 2, respectively, 24.4 g of seed dispersion 1 were added in one portion. After the end of feed streams 1 and 2, a further 40 g of deionized water were added. This was followed by neutralization with 16.3 g of 10% strength by weight aqueous ammonia solution. A further 10 g of deionized water were added and then over an hour at 85 C. feed streams 3 and 4 were metered in. Subsequently 10.8 g of aqueous emulsifier solution 3 and 27.6 g of deionized water were added, and the batch was stirred at 85 C. for 15 minutes more and then left to cool to room temperature.

(29) The polymer dispersion obtained had a solids content of 55.9% by weight, a pH of 6.2, and an LT (0.01% by weight) of 59%. The glass transition temperature was 42 C. The viscosity was 180 mPa.Math.s.

(30) The d.sub.10, d.sub.50, and d.sub.90 figures are reported in table 1. Feed stream 1: 1894.9 g of an aqueous emulsion of 12.5 g of acrylic acid, 24.7 g of 2-hydroxypropyl acrylate, 24.5 g of styrene, 99 g of methyl acrylate, 100.3 g of methyl methacrylate, and 989 g of 2-ethylhexyl acrylate in deionized water, containing 58.6 g of emulsifier solution 1, 5.6 g of emulsifier solution 2, and 8.6 g of emulsifier solution 3. Feed stream 2: 64.3 g of a 7% strength by weight aqueous sodium peroxodisulfate solution. Feed stream 3: 18.8 g of a 10% strength by weight aqueous solution of tert-butyl hydroperoxide. Feed stream 4: 15 g of a 10% strength by weight aqueous solution of sodium hydroxymethylsulfinate.

7. Inventive Example 5

(31) Preparation took place in the same way as for inventive example 4, with the difference that 9.15 of seed dispersion 2 were included in the initial charge, and, instead of the addition of 24.4 g of seed dispersion 1, 40 minutes after the start of feed stream 1, 30.5 g of seed dispersion were added.

(32) The polymer dispersion obtained had a solids content of 55.6% by weight, a pH of 6.1, and an LT (0.01% by weight) of 69%. The glass transition temperature was 42 C. The viscosity was 170 mPa.Math.s.

(33) The d.sub.10, d.sub.50, and d.sub.90 figures are reported in table 1.

8. Comparative Example 3

(34) Commercial aqueous PSA dispersion of a polymer having the monomer composition stated for comparative example 1. The polymer dispersion had a solids content of 61.1% by weight, and a pH>5. The glass transition temperature was 42 C. The viscosity was 510 mPa.Math.s.

(35) The particle size distribution is shown in FIG. 2. The d.sub.10, d.sub.50, and d.sub.90 figures are reported in table 1.

9. Comparative Example 4

(36) Commercial aqueous PSA dispersion of a polymer having the monomer composition stated for comparative example 1. The polymer dispersion had a solids content of 60.9% by weight, and a pH>5. The glass transition temperature was 42 C. The viscosity was 550 mPa.Math.s.

(37) The d.sub.10, d.sub.50, and d.sub.90 figures are reported in table 1.

(38) TABLE-US-00001 TABLE 1 Particle size distribution Example d.sub.10 d.sub.50 d.sub.90 Q C1 111 317 723 1.92 C2 126 258 626 1.93 I1 101 299 318 0.72 I2 115 316 334 0.69 I3 102 312 333 0.73 I4 73 209 228 0.74 I5 87 95 208 1.28 C3 101 376 407 0.81 C4 49.3 342 363 0.91 C = comparative example, I = inventive example

(39) Performance Testing

(40) 1. Production of Test Strips

(41) The dispersion under test was applied using a doctor blade in a thin layer to a siliconized paper and is dried at 90 C. for 3 minutes. The slot height of the doctor blade is chosen so as to give an application rate of 19-21 g/m.sup.2 for the dried adhesive. A commercial PE film (100 m, Corona pretreated) was placed on the dried adhesive and rolled on firmly using a manual roller. The film laminate thus produced is cut into strips 2.5 cm wide. These strips are stored for at least 24 hours under standard conditions prior to testing.

(42) 2. Determination of Whitening Behavior

(43) Whitening behavior and moisture exposure is determined by the following method.

(44) Strips of the coated film with a width of 25 mm were immersed in a water bath and a record was made of the time for their visual appearance to change. The hazing of the strips was evaluated visually after 30 seconds, 60 seconds, 5 minutes, 10 minutes, 20 minutes, 30 minutes, and 60 minutes. The evaluation was made according to a ratings scale from 0 to 4, where:

(45) 0 means no perceptible hazing

(46) 1 means very slight hazing

(47) 2 means distinctly recognizable hazing

(48) 3 means severe hazing

(49) 4 means very severe hazing

(50) The results are summarized in the table below.

(51) TABLE-US-00002 TABLE 2 Whitening behavior Time Example [min] I1 I2 C1 I3 C2 I4 I5 C3 C4 0.5 0 0 0-1 0 0-1 0 0 0-1 0-1 1 0 0 1 0 1 0 0 1 1 5 0-1 1 2 0-1 1-2 0-1 0 2 2 10 1 1-2 2-3 1 2 0-1 0 3 2-3 20 1 2 3 1 2-3 0-1 0 3 3 30 1-2 2 4 1-2 3-4 1 0 4 4 60 2 2-3 4 2 4 1 0-1 4 4

(52) 3. Testing of Peel Strength as a Measure of Adhesion (Based on FINAT FTM1)

(53) After the siliconized paper has been peeled off, the 2.5 cm wide film test strip is adhered to the test substrate. Ambient conditions: 23 C., 50% relative humidity. 1 minute after bonding (polyethylene substrate) or 24 hours after bonding (polyethylene or glass substrate), the strip is peeled at an angle of 180 and a speed of 300 mm/min with the aid of a tensile testing machine. The peel strength reported is the average force needed to achieve this, in N/2.5 cm, as the mean value of the results from three tests.

(54) TABLE-US-00003 TABLE 3 Peel strength [N/2.5 cm] PE Glass Fraction Fraction Fraction Example Instantaneous mode 24 h mode 24 h mode I1 3.6 A 5.7 A 13.1 A/C I2 3.4 A 3.8 A 9.9 A C1 3.2 A 3.5 A 10.1 A I3 3.6 A 4.9 A 13.8 A & C C2 4.2 A 5.7 A 12.6 A I4 3.4 A 3.8 A 10.1 A I5 4.5 A 4.2 A 11.6 A C4 2.3 A 3.5 A 16.3 C A = Adhesive fracture C = Cohesive fracture

(55) 4. Test of Surface Tack (Loop Tack Based on FINAT FTM9)

(56) The investigation takes place under ambient conditions: 23 C., 50% relative humidity. The release paper is peeled from the test strip. The two ends of the test strip are folded round to a length of approximately 1 cm, with the adhesive side inward. A loop is then formed from the test strip, with the adhesive side outward, and the two ends are held together and clamped into the upper jaw of a tensile testing machine. The test substrate is clamped into the lower jaw. The adhesive strip loop is moved at a speed of 300 mm/minute and at an angle of 90 in the direction of the surface of the test substrate, so that the adhesive side of the test strip bonds to the substrate without additional pressure. The tensile testing machine is halted, and is moved upward again at the same speed as soon as the bottom edge of the upper jaw is located 40 mm above the substrate.

(57) The loop tack reported is the average force required to achieve this, in N/2.5 cm, as a mean value from the results of three tests.

(58) TABLE-US-00004 TABLE 4 Loop tack Glass PE Fraction Fraction Example [N/2.5 cm] mode [N/2.5 cm] mode I1 10.4 A 6.6 A I2 10.2 A 6.0 A C1 11.7 A 6.1 A I3 14.2 A 6.6 A C2 12.6 A 6.5 A I4 10.1 A 6.0 A I5 11.6 A 6.3 A C4 9.6 A 5.0 A