Aqueous polymer dispersion for paper with a copolymer of vinyl acetate and an acrylate monomer

Abstract

The invention relates to a process for preparing an aqueous dispersion of a polymer P, which comprises the emulsion polymerization of vinyl acetate, an acrylate monomer, which is a C.sub.1-C.sub.10 alkyl acrylate or a C.sub.1-C.sub.10 methacrylate, an ,-ethylenically unsaturated C.sub.3-C.sub.8 carboxylic acid, glycidyl methacrylate or glycidyl acrylate, optionally an ethylenically unsaturated sulfonic acid and optionally an ethylenically unsaturated other monomer. Furthermore, the aqueous polymer dispersion obtainable from this process and a powder form thereof are disclosed. They are useful for a paper coating slip containing one of the aforementioned forms. A paper or a cardboard, which is coated with the paper coating slip, shows surface strength, which is expressed by a good dry pick resistance, a good wet pick resistance or good offset test results. The aqueous polymer dispersion or the powder form thereof is furthermore useful as a binder.

Claims

1. A process for preparing an aqueous dispersion of a polymer P, the process comprising: emulsifying monomers consisting of (a), (b), (c), (d), (e) and optionally (f) in an aqueous medium: (a) vinyl acetate, (b) at least one acrylate monomer, which is a C.sub.1-C.sub.10 alkyl acrylate or a C.sub.1-C.sub.10 alkyl methacrylate, (c) at least one ,-ethylenically unsaturated C.sub.3-C.sub.6 carboxylic acid, (d) glycidyl methacrylate or glycidyl acrylate, (e) at least one ethylenically unsaturated sulfonic acid, and (f) at least one ethylenically unsaturated monomer, which is different from the monomers (a), (b), (c), (d) or (e), and is selected from the group consisting of a cycloalkene, a conjugated aliphatic C.sub.4-C.sub.9 diene, an ester of vinyl alcohol and a C.sub.2-C.sub.15 monocarboxylic acid, a C.sub.5-C.sub.10 cycloalkyl acrylate, a C.sub.5-C.sub.10 cycloalkyl methacrylate, di(C.sub.1-C.sub.10 alkyl) maleinate, di(C.sub.1-C.sub.10 alkyl) fumarate, a vinylaromatic compound, an ethylenically unsaturated C.sub.3-C.sub.8-monocarbonitrile, an ethylenically unsaturated C.sub.4-C.sub.8-dicarbonitrile, an ethylenically unsaturated C.sub.3-C.sub.8 monocarboxamide, an ethylenically unsaturated C.sub.4-C.sub.8 dicarboxamide, ureido methacrylate, ureido acrylate, allyl methacrylate or allyl acrylate, and polymerizing the monomers in the presence of a free-radical initiator, wherein the content of vinyl acetate is from 70 to 94.7 parts by weight of the monomers, the content of the at least one acrylate monomer is from 5 to 25 parts by weight of the monomers, the content of the at least one a,b-ethylenically unsaturated C.sub.3-C.sub.6 carboxylic acid is from 0.1 to 10 parts by weight of the monomers, the content of glycidyl methacrylate or glycidyl acrylate is from 0.1 to 10 parts by weight of the monomers, the content of the at least one ethylenically unsaturated sulfonic acid is from 0.1 to 5 parts by weight of the monomers, the content of the optional at least one other ethylenically unsaturated monomer when present, is from 0.1 to 15 parts by weight of the monomers, and the sum total of the parts by weight of the monomers (a), (b), (c), (d), (e) and (f) is 100.

2. The process according to claim 1, wherein the content of vinyl acetate is from 70 to 85 parts by weight of the monomers.

3. The process according to claim 1, wherein (b) the at least one acrylate monomer is n-butyl acrylate or ethylhexyl acrylate, and (c) the at least one ,-ethylenically unsaturated C.sub.3-C.sub.6 carboxylic acid is acrylic acid, methacrylic acid, crotonic acid, fumaric acid, maleic acid, maleic acid anhydride, 2-methyl maleic acid, 2-methyl maleic acid anhydride or itaconic acid.

4. The process according to claim 1, wherein (e) the at least one ethylenically unsaturated sulfonic acid is vinyl sulfonic acid or 2-acrylamido-2-methylpropane sulfonic acid.

5. The process according to claim 1, wherein (b) the at least one acrylate monomer is n-butyl acrylate, (c) the at least one ,-ethylenically unsaturated C.sub.3-C.sub.6 carboxylic acid is acrylic acid or methacrylic acid, and (e) the at least one ethylenically unsaturated sulfonic acid is vinylsulfonic acid.

6. The process according to claim 1, wherein the content of vinyl acetate is from 75 to 85 parts by weight of the monomers, the content of the at least one acrylate monomer is from 7 to 19 parts by weight of the monomers, the content of the at least one ,-ethylenically unsaturated C.sub.3-C.sub.6 carboxylic acid is from 0.5 to 5 parts by weight of the monomers, the content of glycidyl methacrylate or glycidyl acrylate is from 0.5 to 8 parts by weight of the monomers, the content of the at least one ethylenically unsaturated sulfonic acid is from 0.1 to 2 parts by weight of the monomers, the content of the at least one other ethylenically unsaturated monomer when optionally present is from 0.1 to 10 parts by weight of the monomers, and the sum total of the parts by weight of the monomers (a), (b), (c), (d), (e) and (f) is 100.

7. The process according to claim 1, wherein the monomers (a), (b), (c), (d), (e) and optionally (f) are chosen in a way such that the polymer P, which is obtained after removal of water from the aqueous dispersion, possesses a glass transition temperature in the range from 20 C. to 40 C. as determined by the norm ISO 11357-2.

Description

EXAMPLES

(1) Unless the context suggests otherwise, percentages are always by weight. A reported content is based on the content in aqueous solution or dispersion if not stated otherwise.

(2) Solids contents are determined by drying a defined amount of the particular aqueous polymer dispersion (about 5 g) at 140 C. in a drying cabinet to constant weight. Two separate measurements are carried out in each case and averaged.

(3) The glass transition temperature Tg is determined in accordance with the norm ISO 11357-2 by differential scanning calorimetry using a TA8000 series DSC820 instrument from Mettler-Toledo Int. Inc.

(4) The average particle diameters of the polymer particles are determined by dynamic light scattering on a 0.005 to 0.01 wt % aqueous polymer dispersion at 23 C. by means of an Autosizer IIC from Malvern Instruments, England. The cumulant z-average diameter of the measured autocorrelation function (ISO standard 13321) is reported.

(5) Materials Used:

(6) TABLE-US-00001 VA vinyl acetate nBA n-butyl acrylate AA acrylic acid VS vinyl sulfonate GMA glycidyl methacrylate (=2,3-epoxypropyl methacrylate) Amol N-methylolacrylamide (=N-hydroxymethyl acrylamide) EGDMA ethylene glycol dimethacrylate DCPA dicyclopentadieneyl acrylate tDMK tert-dodecylmercaptan

(7) The materials are commercially available for example from Aldrich Inc. or BASF SE.

(8) Emulsion Polymerization Procedure for Polymer Dispersion No. D1

(9) A polymerization vessel equipped with a stirrer, reflux condenser and dosing devices is initially charged with 262.94 g of water, 43.75 g of 20 wt % aqueous emulsifier solution of Lutensol AT 18 (RTM BASF, a polyethoxylated fatty alcohol) and 12.25 mg iron(II) sulfate heptahydrate. The mixture is heated to 30 C. under agitation and nitrogen. Then nitrogen flux is stopped and the emulsion feed as well as the initiators feeds are simultaneously started and fed during 4 hours under a constant feed rated. During the first 20 minutes after beginning of the addition of the emulsion and initiator feeds, the temperature is increased from 30 C. to 70 C. The emulsion feed is composed of 252.61 g of water, 32.81 g of 20 wt % aqueous emulsifier solution of Lutensol AT 18, 20.51 g of 32 wt % aqueous emulsifier solution of Disponil FES 77 (RTM BASF, an alkyl polyglyol ether sulfate), 17.50 g of 25 wt % aqueous solution of VS, 17.50 g AA, 43.75 g of GMA, 95.38 g of nBA and 714.00 g of VA. One initiator feed is composed of 105.00 g of a 2.5 wt % solution of sodium peroxodisulfate and the other initiator feed is composed of 43.69 g of water, 1.75 g of sodium acetate and 1.49 g of Rongalit C (RTM BASF, sodium hydroxymethylsulfinate). After the end of the emulsion and initiator feeds, in 10 minutes, 68.25 g of water and 13.67 g of a 32 wt % aqueous solution of Disponil FES 77 are fed in the reactor. Another 20 minutes are waited. After that at 70 C., 21.00 g of a 10 wt % aqueous solution of tert.-butyl hydroperoxide and 29.39 g of a 13.1 wt % aqueous solution of sodium acetone bisulfite are fed during 1 hour. The reactor contents are then cooled down to room temperature and 5.25 g of a 7.5 wt % aqueous solution of Acticid MBS 2550 (RTM Thor, 1,2-benzisothiazol-3(2H)-one/2-methyl-4-isothiazolin-3-one (BIT/MIT)) and 1.58 g of a 1.5 wt % aqueous solution of Acticid MV (RTM Thor, 5-chloro-2-methyl-4-isothiazolin-3-one/2-methyl-4-isothiazolin-3-one (CIT/MIT)) are added, followed by 13.51 g of a 10 wt % aqueous solution of sodium hydroxide. The final dispersion is filtrated with a 500 m filter to remove the possible coagulum. The dispersion D1 thus produced has a solids content of 50.8%, a pH-value of 5, a particle diameter of 193 nm and a Tg of 34 C.

(10) The polymer dispersions No. D2 to D10 are prepared in analogy. In case of polymer dispersion No. D6, the tDMK is added in the emulsion feed.

(11) Tables 1-A and 1-B shows the employed amounts of monomers and tDMK for the polymer dispersions No. D1 to D10.

(12) TABLE-US-00002 TABLE 1-A dispersion No. monomer/tDMK D1 .sup.a) D2 .sup.a) D3 .sup.a) D4 .sup.b) D5 .sup.b) D6 .sup.a) VA 81.6 81.6 81.6 81.6 81.6 81.6 nBA 10.9 12.9 14.9 15.9 12.9 10.9 GMA 5.0 3.0 1.0 0.0 5.0 5.0 AA 2.0 2.0 2.0 2.0 0.0 2.0 VS 0.5 0.5 0.5 0.5 0.5 0.5 Amol 0.0 0.0 0.0 0.0 0.0 0.0 EGDMA 0.0 0.0 0.0 0.0 0.0 0.0 DCPA 0.0 0.0 0.0 0.0 0.0 0.0 tDMK 0.0 0.0 0.0 0.0 0.0 0.15

(13) TABLE-US-00003 TABLE 1-B monomer/ dispersion No. tDMK D7.sup.b) D8.sup.b) D9.sup.b) D10.sup.a) VA 81.6 81.6 81.6 81.6 nBA 13.9 13.9 12.9 11.4 GMA 0.0 0.0 0.0 5.0 AA 2.0 2.0 2.0 2.0 VS 0.5 0.5 0.5 0.0 AAmol 2.0 0.0 0.0 0.0 EGDMA 0.0 2.0 0.0 0.0 DCPA 0.0 0.0 3.0 0.0 tDMK 0.0 0.0 0.0 0.0

(14) a): according to the invention

(15) b): comparative

(16) Procedure for a Paper Coating Slip Containing a Polymer Dispersion

(17) In a stirred assembly, the individual components are fed in succession. The pigments are added in a pre-dispersed slurry form. The other components are added after the pigments, the order corresponding to the order in the reported coating slip formulation. Final solids content is set by adding water.

(18) Composition of the paper coating slip formulation based on solids content of the components: 100 parts of finely divided calcium carbonate (Hydrocarb 60, RTM Omya) 7 parts of one of the polymer dispersions No. D1 to D10 as binder 0.25 parts of a thickener (Sterocoll FS, RTM BASF)

(19) The solids content of the paper coating slip is 64 wt % and its pH is about 9.

(20) Paper Coated with a Coating Slip Containing a Polymer Dispersion and its Surface Strength

(21) The paper coating slip containing a polymer dispersion, i.e. one of the polymer dispersions No. D1 to D10, is applied to one side of uncoated base paper (woodfree, 70 g/m.sup.2) using a laboratory coating machine and dried via IR radiator. The weight of the coat applied is about 10 g/m.sup.2 as determined by gravimetric measurement. The paper is cut into sheets and calendered on a lab calendar to give even and flat sheets. Before conducting the surface strength tests, the paper sheets are conditioned for at least 15 hours at 23 C. and 50% relative humidity.

(22) The received coated paper is tested for surface strength using the test methods IGT dry pick resistance, IGT wet pick resistance and Offset test, which are known to a person skilled in the art.

(23) Measurement of Dry Pick Resistance with IGT Test Printer (IGT Dry):

(24) Strips are cut out of the in-test papers and printed with the IGT test printer. The printing inks used are specific test inks from Lorilleux, which transmit different tensile forces. The test strips are fed through the press at continuously increasing speed (maximum speed 200 cm/s). To evaluate the result, the point at which 10 picks have occurred on the paper surface after the start of printing is determined on the sample printing strip. The measure reported for dry pick resistance is the speed in cm/sec present at this point during printing and also the test ink used. The higher this printing speed at the tenth pick point, the better the quality rating of the paper surface.

(25) Measurement of Wet Pick Resistance with IGT Test Printer (IGT Wet):

(26) Strips are cut out of the in-test papers and printed with the IGT test printer. The printer is set up such that the test strips are moistened with water prior to the printing operation. The printing inks used are specific test inks from Lorilleux (No. 3807), which transmit different tensile forces. Printing is done at a constant speed of 0.6 cm/s. Picks out of the paper surface are visible as unprinted spots. To determine wet pick resistance, a color densitometer is used to determine color density in % compared with the full hue. The higher the reported color density, the better the wet pick resistance.

(27) Offset Test:

(28) Samples having a size of 24046 mm are cut out of the in-test papers in the longitudinal direction. An appropriate amount of printing ink is applied to the inking roll and left to run for 1 minute. A printing disk is then inserted and inked for 30 s. The printing speed is 1 m/s. A paper strip is brought back to the starting position on a printing test support with the printed paper strip. After a specified time interval, the printing process is started again without replacing the printing disk. This operation is repeated more than once. After each printing cycle, the pick on the printed side of the paper strip is assessed by visual inspection. The table reports the number of cycles before picking occurred for the first time. The higher the number of cycles up to the occurrence of picking is, the better is the suitability of the papers for offset printing.

(29) The surface strength results of the papers coated with one of the polymer dispersions No. D1 to D10 are summarized in table 2.

(30) TABLE-US-00004 TABLE 2 wet pick applied dry pick resistance resistance offset Paper-No. dispersion-No. [cm/s] [cm/s] cycles P-1.sup.a) D1.sup.a) 65 20.9 6.75 P-2.sup.a) D2.sup.a) 60 18.4 5.50 P-3.sup.a) D3.sup.a) 45 13.4 5.25 P-4.sup.b) D4.sup.b) 31 8.9 3.50 P-5.sup.b) D5.sup.b) 30 16.0 4.25 P-6.sup.a) D6.sup.a) 32 19.6 4.25 P-7.sup.b) D7.sup.b) 21 6.0 2.00 P-8.sup.b) D8.sup.b) 22 5.2 2.50 P-9.sup.b) D9.sup.b) 27 12.5 3.50 P-10.sup.a) D10.sup.a) 30 17.0 3.25 .sup.a)according to the invention .sup.b)comparative

(31) The measured results of the papers P-1 to P-10 produced with paper coating slips containing one of the polymer dispersion-No. D1 to D10 show that a content of glycidyl methacrylate leads to improved results by comparison of P-1 to P-3 versus P-4; that a content of acrylic acid leads to improved results by comparison of P-1 versus P-5; that the absence of tert-dodecylmercaptan leads to further improved results by comparison of P-1 versus P-6; that a content of glycidyl methacrylate leads to better results than a content of N-methylolacrylamide by comparison of P-2 or P-3 versus P-7; that a content of glycidyl methacrylate leads to better results than a content of ethylene glycol dimethacrylate by comparison of P-2 or P-3 versus P-8; that a content of glycidyl methacrylate leads to better results than a content of dicyclopentadieneyl acrylate by comparison of P-1 versus P-9; that a content of vinyl sulfonate leads to further improved results by comparison of P-1 versus P-10.