Polymer composition, its use and a surface size

Abstract

The invention relates to a water-soluble polymer composition obtained by polymerizing in an aqueous polymerisation medium, which comprises degraded starch, at least following monomers: acrylamide and/or methacrylamide, and >2 mol-% of at least one unsaturated mono- or dicarboxylic acid(s) or salts thereof. The polymer composition has an anionic net charge at pH 7 and a dry solids content of >5 weight-%. The invention relates also to the use of the polymer composition for surface sizing of paper, board or the like as well as to a surface size composition comprising it.

Claims

1. A water-soluble polymer composition obtained by polymerizing in an aqueous polymerization medium, which comprises: degraded starch; at least following monomers: acrylamide and/or methacrylamide; and >2 mol-% of at least one unsaturated mono- or dicarboxylic acid(s) or salts thereof; wherein the polymer composition has an anionic net charge at pH 7 a dry solids content of >5 weight-%, and a viscosity in a range of 50-1500 mPas, measured at 10-with % solids concentration, at 25° C., pH 4, using Brookfield DV1 viscometer.

2. The polymer composition according to claim 1, wherein the polymer composition has an anionic net charge in the range of −0.5-−2.5 meq/g at pH 7.

3. The polymer composition according to claim 1, wherein the degraded starch is degraded cationic starch or degraded non-ionic starch.

4. The polymer composition according to claim 3, wherein the degraded starch is degraded cationic starch, which has a degree of substitution DS in the range of 0.015-0.2.

5. The polymer composition according to claim 1, wherein the degraded starch solution has a viscosity in the range of 3-100 mPas, measured at 10 weight-% solids concentration, at 60° C., with Brookfield DV1 viscometer.

6. The polymer composition according to claim 1, wherein the amount of the at least one unsaturated mono- or dicarboxylic acid or a salt thereof is 3-30 mol-%, calculated from total monomer content.

7. The polymer composition according to claim 1, wherein the polymer composition is obtained by polymerizing (meth)acrylamide and at least one unsaturated mono- or dicarboxylic acid or a salt thereof selected from the group of acrylic acid, maleic acid, fumaric acid, itaconic acid, aconitic acid, mesaconic acid, citraconic acid, crotonic acid, isocrotonic acid, angelic acid, tiglic acid, and any salt thereof.

8. The polymer composition according to claim 1, wherein the aqueous polymerization medium comprises one or more initiators, and the total amount of used initiator(s) is at most 0.9 weight-%, based on the amount of the monomers.

9. The polymer composition according to claim 1, wherein the polymer composition has a cationic charge density of 0.02-0.3 meq/g at pH 2.7.

10. The polymer composition according to claim 1, wherein the polymer composition is obtained by solution polymerization or gel polymerization.

11. The polymer composition according to claim 10, wherein the polymer composition is obtained by solution polymerization and has a dry solids content of 10-30 weight %.

12. The polymer composition according to claim 11, wherein the amount of degraded starch in the polymer composition is in a range of 1-50 weight-%, calculated from the total dry solids of the composition.

13. The polymer composition according to claim 11, wherein the polymer composition has a viscosity in a range of 60-400 mPas, measured at 10 weight-% solids concentration, at 25° C., pH 4, using Brookfield DV1 viscometer.

14. A method of surface sizing paper or board, the method comprising a step of adding the polymer composition according to claim 1 on a surface of the paper, or board.

15. The method according to claim 14, wherein the board is liner, fluting, folding boxboard (FBB), white lined chipboard (WLC), solid bleached sulphate (SBS) board or solid unbleached sulphate (SUS) board.

16. The method according to claim 14, wherein the board has a grammage from 60 to 500 g/m.sup.2.

17. A surface size composition for application on a surface of paper, or board comprising: 0.5-10 weight-%, of the polymer composition according to claim 1, and starch, wherein the surface size composition having a solids content of 0.5-25 weight-%.

18. The surface size composition according to claim 17, wherein the size composition has a viscosity in a range of 3-50 mPas measured at 60° C. by using Brookfield DV1 viscometer.

Description

EXPERIMENTAL

(1) Some embodiments of the invention are described in the following non-limiting examples.

(2) The following methods are used to characterise the polymer compositions obtained in the following examples.

(3) Dry content of a polymer composition is determined by placing a sample in an oven at 110° C., for 5 h.

(4) pH values are determined by pH meter, Knick Portamess, Van London-Phoenix company, Texas, USA

(5) Viscosity of a polymer composition in solution form is determined by using Brookfield DV1 viscometer, equipped with small sample adapter, at 25° C. Spindle SC4-18 is used for solution which has viscosity of 1000 mPas or less, and spindle SC4-31 is used for solution which has viscosity over 1000 mPas. Maximum rotation speed is used in the determination to get maximum torque value in each determination.

(6) Viscosity of cooked starch solution is determined at 10% concentration at 60° C. Starch is cooked at 10% concentration at 97-99° C. temperature for 60 min. Evaporated water is added to the starch solution to get the exact concentration, and the viscosity is then determined by using Brookfield DV1 viscometer, equipped with small sample adapter, at 60° C. Spindle SC4-18 and maximum rotation speed is used. Characteristics of starch products which are used in the following production examples are given in the Table 1.

(7) TABLE-US-00001 TABLE 1 Characteristics of starch products which are used for the production of polymer compositions. Charge Viscosity of Degree of density at cooked starch pH of cooked Starch cationic pH 2.7 at 10% at 60° C. starch at 10% product substitution [meq/g dry] [mPas] at 25° C. Starch 1 0.050 0.30 40.6 7.6 Starch 2 0.043 0.26 4.5 5.1 Starch 3 0.00 0.00 6.5 4.95

(8) Charge densities in the following examples are determined by using Mütek PDC 03 pH—particle charge detector (BTG Instruments GmbH, Herrsching, Germany) equipped with Mütek PCD Titrator Three-titrator unit (BTG Instruments GmbH, Herrsching, Germany), and by using a) 0.001 M PES-Na as titrant polymer for net cationic polymers, and b) 0.001 N poly-DADMAC as titrant polymer for net anionic polymers. Both titrant polymers are supplied by BTG Instruments GmbH, Herrsching, Germany.

Production of Polymer Compositions Obtained by Solution Polymerisation

Example 1: Production of Polymer Composition Which Comprises 20 Weight-% of Cationic Oxidized Starch, “Product 1”

(9) Starch used in the polymerisation is “Starch 1”, which is a cationic oxidized waxy potato starch. The charcteristics of “Starch 1” are given in Table 1.

(10) Monomer mixture is obtained by mixing 62 g of de-ionized water, 275 g of acrylamide (50%), 20 g of acrylic acid, 1.1 g of formic acid (50%), 0.7 g of EDTA aqueous solution (39%) in a monomer tank. The monomer mixture is purged with nitrogen gas for 15 min.

(11) 0.83 g of ammonium persulfate is dissolved in 34 g of de-ionized water in a catalyst tank.

(12) 540 g of de-ionized water is dosed into a polymerisation reactor, equipped with mechanical agitator, a condenser and a jacket for heating and cooling. 49 g of “Starch 1” (82%) is sludged into water under agitation. 1.0 g of citric acid is dosed into the polymerisation reactor, pH of the mixture is 2.8. The resulting mixture is heated to 100° C. and mixed for 30 min. Temperature is then decreased to 80° C.

(13) Feed of the monomer mixture from the monomer tank and feed of the ammonium persulfate solution from the catalyst tank are started at the same time. Constant feed of monomer mixture is performed under 60 min and constant feed of ammonium persulfate solution is performed under 90 min. During feeding the temperature is kept at 80° C. with the aid of the heating and cooling jacket. When the feed of the ammonium persulfate solution is ended, the reaction mixture is agitated for 30 min at 80° C.

(14) 558 g of de-ionized water is added and the mixture is cooled to 25° C. pH of the obtained polymer composition solution is 3.0 and the pH is adjusted to 3.7 with sodium hydroxide (20 weight-%). Dry content of the polymer composition solution is 13.0%, and the viscosity is 10 900 mPas, measured as defined above.

Example 2: Production of Polymer Composition Which Comprises 20 Weight-% of Cationic Oxidized Starch, “Product 2”

(15) “Starch 1”, as in Example 1, is employed in the polymerisation.

(16) 1.24 g of ammonium persulfate is dissolved in 34 g of de-ionized water in a catalyst tank.

(17) Monomer mixture is obtained by mixing 50 g of de-ionized water, 220 g of acrylamide (50%), 15.9 g of acrylic acid, 0.9 g of formic acid (50%), 0.7 g of EDTA aqueous solution (39%), and 0.068 g of thioglycolic acid in a monomer tank. The monomer mixture is purged with nitrogen gas for 15 min.

(18) 525 g of de-ionized water is dosed into a polymerisation reactor, equipped with mechanical agitator, a condenser and a jacket for heating and cooling. 41 g of “Starch 1” (82%) is sludged into water under agitation. 0.8 g of citric acid is dosed into the polymerisation reactor. The resulting mixture is heated to 100° C. and mixed for 30 min. Temperature is then decreased to 80° C.

(19) Feed of the monomer mixture from the monomer tank and feed of the ammonium persulfate solution from the catalyst tank are started at the same time. Constant feed of the monomer mixture is performed under 60 min and constant feed of the ammonium persulfate solution is performed under 90 min. During feeding the temperature is kept at 80° C. with the aid of the heating and cooling jacket. When the feed of the ammonium persulfate solution is ended, the reaction mixture is agitated for 30 min at 80° C.

(20) 109 g of de-ionized water is added and the mixture is cooled to 25° C. pH of the obtained polymer composition solution is 2.9 and the pH is adjusted to 3.7 with sodium hydroxide (20 weight-%). Dry content of the polymer composition is 16.6%, and the viscosity is 1 950 mPas, measured as defined above.

Example 3: Production of Polymer Composition Which Comprises 30 Weight-% of Cationic Oxidized Starch, “Product 3”

(21) “Starch 1”, as in Examples 1 and 2, is employed in the polymerisation.

(22) 1.24 g of ammonium persulfate 1.24 g is dissolved in 34 g of de-ionized water in a catalyst tank.

(23) Monomer mixture is obtained by mixing 43 g of de-ionized water, 192 g of acrylamide (50%), 13.9 g of acrylic acid, 0.8 g of formic acid (50%), 0.7 g of EDTA aqueous solution (39%), and 0.068 g of thioglycolic acid in a monomer tank. The monomer mixture is purged with nitrogen gas for 15 min.

(24) 525 g of de-ionized water is dosed into a polymerisation reactor, equipped with mechanical agitator, a condenser and a jacket for heating and cooling. 60 g of “Starch 1” (82%) is sludged into water under agitation. 0.8 g of citric acid is dosed into the polymerisation reactor. The resulting mixture is heated to 100° C. and mixed for 30 min. Temperature is then decreased to 80° C.

(25) Feed of the monomer mixture from the monomer tank and feed of the ammonium persulfate solution from the catalyst tank are started at the same time. Constant feed of the monomer mixture is performed under 60 min and constant feed of the ammonium persulfate solution is performed under 90 min. During feeding the temperature is kept at 80° C. with the aid of the heating and cooling jacket. When the feed of ammonium persulfate solution is ended, the reaction mixture is agitated for 30 min at 80° C.

(26) 128 g of de-ionized water is added and the mixture is cooled to 25° C. pH of the obtained polymer composition solution is 2.7 and the pH is adjusted to 3.7 with sodium hydroxide (20 weight-%). Dry content of the polymer composition is 16.3%, and the viscosity is 1 580 mPas, measured as defined above.

Example 4: Production of Polymer Composition Which Comprises 20 Weight-% of Cationic Oxidized Potato Starch, “Product 4”

(27) Starch used in the polymerisation is “Starch 2”, which is cationic oxidized potato starch. The characteristics of “Starch 2” are given in Table 1.

(28) Monomer mixture is obtained by mixing 343 g of acrylamide (37.5%), 18.6 g of acrylic acid, 0.0005 g of methylenebisacrylamide, 0.33 g of sulfuric acid (93%), 0.72 g of DTPA aqueous solution (40%), and 0.068 g of thioglycolic acid in a monomer tank. The monomer mixture is purged with nitrogen gas for 15 min.

(29) 1.24 g of ammonium persulfate is dissolved in 34 g of de-ionized water in a catalyst tank.

(30) 498 g of de-ionized water is dosed into a polymerisation reactor, equipped with mechanical agitator, a condenser and a jacket for heating and cooling. 46 g of “Starch 2” (82%) is sludged into water under agitation. 0.8 g of citric acid is dosed into the polymerisation reactor. The resulting mixture is heated to 100° C. and mixed for 30 min. Temperature is then decreased to 80° C.

(31) Feed of the monomer mixture from the monomer tank and feed of the ammonium persulfate solution from the catalyst tank are started at the same time. Constant feed of the monomer mixture is performed under 60 min and constant feed of the ammonium persulfate solution is performed under 90 min. During feeding the temperature is kept at 80° C. with the aid of the heating and cooling jacket. When the feed of the ammonium persulfate solution is ended, the reaction mixture is agitated for 30 min at 80° C.

(32) 55 g of de-ionized water is added and the mixture is cooled to 25° C. pH of the obtained polymer composition solution is 2.8 and the pH is adjusted to 3.7 with sodium hydroxide (20 weight-%). Dry content of the polymer composition is 19.0%, and the viscosity is 2670 mPas, measured as above described.

Example 5: Production of Polymer Composition Which Comprises 30 Weight-% Cationic Oxidized Potato Starch, “Product 5”

(33) “Starch 2”, as in Example 4, is employed in the polymerisation.

(34) 1.24 g of ammonium persulfate is dissolved in 34 g of de-ionized water in a catalyst tank.

(35) Monomer mixture is obtained by mixing 49 g of de-ionized water, 216 g of acrylamide (50%), 15.6 g of acrylic acid, 0.9 g of formic acid (50%), 0.7 g of EDTA aqueous solution (39%) in a monomer tank. The monomer mixture is purged with nitrogen gas for 15 min.

(36) 525 g of de-ionized water is dosed into a polymerisation reactor, equipped with mechanical agitator, a condenser and a jacket for heating and cooling. 64 g of “Starch 2” (82%) is sludged into water under agitation. 0.8 g of citric acid is dosed into the polymerisation reactor. The resulting mixture is heated to 100° C. and mixed for 30 min. Temperature is then decreased to 80° C.

(37) Feed of the monomer mixture from the monomer tank and feed of the ammonium persulfate solution from the catalyst tank are started at the same time. Constant feed of the monomer mixture is performed under 60 min and constant feed of the ammonium persulfate solution is performed under 90 min. During feeding the temperature is kept at 80° C. with the aid of the heating and cooling jacket. When the feed of the ammonium persulfate solution is ended, the reaction mixture is agitated for 30 min at 80° C.

(38) 91 g of de-ionized water is added and the mixture is cooled to 25° C. pH of the obtained polymer composition solution is 2.8 and the pH is adjusted to 3.7 with sodium hydroxide (20 weight-%). Dry content of the polymer composition is 18.6%, and the viscosity is 3730 mPas, measured as described above.

Properties of Polymer Compositions of Examples 1-5

(39) A summary of the properties of the obtained polymer compositions of Examples 1 to 5 is presented in Table 2.

(40) TABLE-US-00002 TABLE 2 Properties of the polymer compositions of Examples 1-5, obtained by solution polymerisation. Product Product Product Product Product Product Property 1 2 3 4 5 Dry starch of dry solids 20 20 30 20 30 of the composition, % Dry solids of the 13.0 16.6 16.3 19.0 18.6 composition, % pH 3.7 3.7 3.7 3.7 3.7 Viscosity at 25° C., 10 900 1 950 1 580 2 670 3 730 mPas Viscosity at 10% dry  1 300   171 159 116 175 solids at 25° C., mPas Charge density at pH 0.056 0.057 0.084 0.050 0.078 2.7, meq/g dry solids Charge density at pH −1.31 −1.29 −1.09 −1.34 −1.16 7.0, meq/g dry solids

Production of Polymer Compositions Obtained by Gel Polymerisation

Example 6: General Procedure for Production of Polymer Compositions

(41) Starch is dissolved in water by heating it to almost boiling point for 30 minutes, followed by cooling down at room temperature.

(42) A polymerisation reactor is charged with the cooked starch dissolved in water. The polymerisation of acrylamide and acrylic acid is done in the aqueous starch solution by adding aqueous acrylamide solution (50%) and acrylic acid in a molar ratio given for each composition in Table 3.

(43) The initial pH of the aqueous starch solution comprising the monomers is about 2-4. For the polymerisation reaction the pH of the monomer mixture is adjusted to neutral pH, about pH 6, by adding NaOH (50 weight-%). Other chemicals, such as chain transfer agents, chelating agents, and thermal initiators are added to the aqueous starch solution comprising the monomers. Then, the solution is degassed at low temperature by a continuous supply of nitrogen gas. Polymerisation is initiated by injecting a redox pair initiator system into the polymerisation reactor.

(44) An anionic polyacrylamide gel is obtained as a result of the polymerisation reaction. The gel is dried and finally powder or particles are obtained. The polymer compositions have dryness of 90-95 weight-%. Polymer properties are measured by using the obtained powder.

(45) Dried polymers have molecular weight of about 1 MDa. No difference in molecular weight is observed between polymer compositions prepared using cationic starch or non-ionic starch (dextrin).

(46) The amounts of starch, either cationic or non-ionic, in the polymer composition are also given in Table 3. The amounts are given based on 100 weight-% total monomers. Characteristics of the starches are given in the Table 1.

(47) Viscosities of the obtained polymer compositions are measured as follows:

(48) Polymer composition is dissolved in water at 0.5 weight-% concentration. NaCl was added in order to obtain 5 weight-% concentration. pH of the polymer composition is adjusted to pH 6.06. Brookfield DV1 viscometer is used for viscosity measurements as describe above.

(49) Measured viscosity values are for the polymer compositions are given in Table 3.

(50) Charge density values for polymer compositions are measured as described above by using Mütek PDC 03, pH 7.0, 0.001 N poly-DADMAC as titrant polymer.

(51) Insoluble particles are determined as follows:

(52) 0.5 weight-% of dry polymer composition is mixed with 1500 ml of deionized water mixed with a magnetic stirrer for 60 min. NaCl is added to the sample solution, to obtain salt concentration of 5 weight-%, and mixed for further 5 minutes. Insoluble particles are measured using a stainless steel sieve with aperture 500 microns. The sieve is filled with 1500 ml of the aqueous sample and allowed to drain. The sieve is washed with 1000 ml of cold water. Total drainage time does not exceed 5 minutes. Gels and/or particles remaining on the sieve are visually counted. The results are given in Table 3.

(53) TABLE-US-00003 TABLE 3 Properties of polymer composition obtained by gel polymerisation. Charge Molar ratio Amount of Number of density, Used Acrylamide:Acrylic starch Viscosity insoluble at pH 7 Starch acid [weight-%] [cP] particles [meq/g] Reference 1 — 92:8  0 5.1 0 −1.2921 Reference 2 — 7:1 0 5.1 2 −1.84 Product 5 Starch 2 7:1 26.46 8 NA −1.42 Product 6 Starch 3 7:1 29.37 4.55 4 −1.4 Product 7 Starch 2 7:1 11.06 5.5 0 −1.58 Product 8 Starch 2 7:1 6.36 5.5 4 −1.68

(54) It can be seen from Table 3 that the solubility of the obtained polymer composition in the water is good as the number of insoluble particles is generally low. The results of Table 3 indicate that relatively large amounts of non-ionic “Starch 3” can be incorporated into the polymer composition without negatively affecting the water-solubility of the polymer composition.

Surface Sizing Experiments

(55) Size press parameters were as follows:

(56) Size press manufacturer: Werner Mathis AG, CH 8155 Niederhasli/Zürich; Size press model: HF 47693 Type 350; Operation speed: 2 m/min; Operation pressure: 1 bar; Operation temperature: 60° C.; Sizing solution volume: 140 ml/test; Sizing times/sheet: 2.

(57) Sizing is performed in machine direction and the surface size composition is applied as 9-12 weight-% solution. Commercial starch products, which are thermally modified dextrins, tradenames C*film 07311 (Cerestar Cargill) or C*film 07312 (Cerestar Cargill), are used as surface size starch. The starch is selected to simulate enzymatically degraded native starch.

(58) Sizing composition is prepared as follows: A 15 weight-% solution of surface size starch, as defined above, is cooked for 30 min at 95° C. Surface size compositions are prepared by mixing of water, cooked starch solution and polymer composition, in this order. Thus polymer composition, calculated as dry, is added to the cooked surface size starch solution, and mixed at 70° C., for at least 2 min. Viscosity of the obtained composition is measured by using Brookfield DV1, at 60° C., at 9% concentration, otherwise as described above. The surface size compositions are stored at 70° C. until surface sizing experiments are carried out.

(59) Base paper was Schrenz paper, 100 g/m.sup.2, 100% recycled fibre based liner grade without size press. The base paper had an ash content of 16.4% (measured with standard ISO 1762, temperature 525° C.) and bulk value 1.57 cm.sup.3/g (measured with standard ISO 534).

(60) The test samples are sized twice, and the properties of the sized sheets are measured. The used measurements, testing devices and standards are given in Table 4.

(61) TABLE-US-00004 TABLE 4 Sheet testing devices and standards used. Measurement Device Standard Basis weight Mettler Toledo ISO 536 SCT GM Index Lorentzen & Wettre ISO 9895 (Short Span Compression Compression Strength tester test) Burst strength IDM Test EM-50/80 ISO 2758 CMT30 Index Sumet-Messtechnik SC-500 ISO 7263: 1994 Fluter: PTA Group AV-S Wet web tensile Lorentzen & Wettre Tensile SCAN-CM strength tester 69:09

(62) Drying of the sized sheets is made in one-cylinder felted steam heated dryer drum at 95° C. for 1 min. Shrinkage is restricted in dryer.

(63) Solution polymers, which are used in comparative experiments, are copolymers of acrylamide and acrylic acid. The properties of comparative polymers are given in Table 5. Dry solids content is determined by placing composition sample in an oven at 110° C., for 5 h. Viscosities are determined with Brookfield DV1 viscometer, equipped with small sample adapter, spindle SC4-31, at 25° C. Charge densities are determined by Mütek PDC 03 at pH 7.0 using 0.001 N poly-DADMAC as titrant polymer.

(64) TABLE-US-00005 TABLE 5 Properties of solution polymers used for comparative experiments. Dry solids Charge density, content Viscosity at pH 7.0 Polymer [%] [mPas] [meq/g dry] pH APAM 1 14.8 6 950 −1.1 5.1 APAM 2 20.4 9 100 −1.1 5.1 APAM 3 13.7 8 100 −1.8 5.2

(65) The results for the surface sizing experiments are given in Tables 6-8. The measured results for a surface size composition comprising starch C*film 07311 and a polymer component (as indicated in the column “Polymer”) after one or two passes are given in Table 6 (as indicated in the column “Passes”). The measured results for a surface size comprising starch C*film 07311 and a polymer component (as indicated in the column “Polymer”) after one pass are given in Table 7. The measured results for a surface size composition comprising starch C*film 07312 and a polymer component (as indicated in the column “Polymer”) after two passes in Table 8. The percentage values for pick-up in Tables 6, 7 and 8 are calculated from weight increase of an air-conditioned sheet, where the basis weight of the sheet is measured before and after sizing. The indexed values in Tables 6, 7 and 8 are given as the strength divided by the basis weight of the paper/board. The geometric (GM) value is the square root of (MD value)*(CD value). MD value is the measured strength value in machine direction and CD value is the measured strength value in machine cross direction.

(66) TABLE-US-00006 TABLE 6 The measured results for a surface size composition comprising starch C*film 07311 and a polymer component. Polymer Sizing Burst SCT GM Wet tensile dosage, % concentration, Viscosity, Dosage, Pick-up, index, index, index, Experiment Remark Polymer of size dry Passes % mPas kg/t % kPam2/g Nm/g Nm/g 1-1 ref. 0 1 9 1.7 4.09 1.97 23.71 1-2 ref. 0 2 9 1.7 6.36 1.85 24.32 0.38 1-3 comp. APAM 1 2.5 2 9 6 1.44 5.75 1.86 24.86 0.40 1-4 comp. APAM 1 5 2 9 9.6 2.79 5.59 1.99 25.44 0.42 1-5 Product 2 2.5 2 9 2.9 1.31 5.24 2.02 25.04 0.51 1-6 Product 2 5 2 9 2.9 2.49 4.98 2.06 25.33 0.48

(67) From Table 6 it is seen that Experiments 1-5 and 1-6 show better burst index and wet tensile index values than the reference experiments and comparative experiments. The SCT GM index obtained by using surface size composition comprising polymer composition according to the invention is similar to the values obtained in the comparative experiments but was achieved with lower pick-up level of the surface size composition, and thus with a lower surface size starch consumption.

(68) From Table 7 it is seen that experiments from 2-5 to 2-8 show better burst index values than the reference experiment and comparative experiments. Experiments 2-5 and 2-6 show better SCT GM index values than the reference experiment and the comparative experiments, even with lower pick-up level of the surface size composition. The surface starch consumption is thus lower. Experiment 2-7 achieved better SCT GM index value with lower dosage compared to comparative experiment 2-3. Experiments from 2-5 to 2-8 achieved higher dry content after sizing than the reference experiment and comparative experiments, leading into lower energy consumption at drying. The web has also higher strength at higher dry content, whereby the web is less susceptible to web break. Higher dry content and higher web strength may enable an increase in machine speed.

(69) TABLE-US-00007 TABLE 7 The measured results for a surface size composition comprising starch C*film 07311 and a polymer component after one pass. Polymer Burst SCT GM Dry content dosage % Dosage, Pick-up, index, index, after sizing, Experiment Remark Polymer of size dry kg/t dry % kPam.sup.2/g Nm/g % 2-1 ref. 0.0 0.0 7.7 1.90 22.8 53.8 2-2 comp. APAM 2 2.5 1.8 7.1 2.09 22.9 55.8 2-3 comp. APAM 2 5.0 3.4 6.9 2.04 23.6 56.7 2-4 comp. APAM 3 2.5 1.7 6.7 2.05 23.9 57.3 2-5 Product 2 2.5 1.6 6.3 2.12 24.1 58.7 2-6 Product 2 5.0 3.0 5.9 2.11 24.2 60.4 2-7 Product 5 2.5 1.5 5.9 2.11 23.9 60.5 2-8 Product 5 5.0 2.7 5.4 2.15 23.8 62.7

(70) TABLE-US-00008 TABLE 8 The measured results for a surface size composition comprising starch C*film 07312 and a polymer component and after two passes. Polymer Sizing Burst SCT dosage, % concentration, Pick-up, index, index GM, Experiment Remark Polymer of size dry % % kPam.sup.2/g Nm/g 3-1 Unsized 0.0 1.50 17.2 3-2 ref. 0 9 4.5 2.16 25.0 3-3 Product 2 2 9 4.1 2.22 25.6 3-4 Product 2 4 9 3.9 2.23 25.7 3-5 Product 3 2 9 3.9 2.22 25.2 3-6 Product 3 4 9 3.8 2.20 25.5 3-7 Product 5 2 9 3.9 2.25 25.4 3-8 Product 5 4 9 3.9 2.17 26.2 3-9 Product 4 2 9 3.8 2.24 25.7 3-10 Product 4 4 9 3.9 2.20 26.1

(71) From Table 8 it is seen that all the tested surface size compositions comprising polymer compositions according to the invention, achieve desired strength results and lower pick-up values and thus lower size consumption compared to the reference. The surface starch consumption is thus lower. Additionally, experiments 3-3, 3-4, and 3-9 achieve both high burst index and high SCT GM index.

Penetration Experiment

(72) Penetration measurement is performed as follows:

(73) The equipment used for penetration dynamic analysis (PDA) is Surface & Sizing Tester EST12.2. A sample of paper is immersed to a dish of starch solution, which is placed to water basin. Temperature in starch dish is 60° C. and the temperature in water basin varied between 25° C. and 35° C. The paper sample is attached to a sample holder with two-sided adhesive. The ultrasonic movement through the sample changes as the wetting advances. Penetration is calculated from as 100%−PDA signal %. The result is given as percentage of penetration at any given time.

Dynamic Penetration Results

(74) The penetration study is started by testing surface size compositions comprising a polymer composition and commercial starch C*film 07312. Used polymer compositions are “Product 2” and APAM3, as defined earlier. Experiments are performed by using three different solids concentration for the surface size composition. For each size concentration both polymer compositions are tested at two dosages, 2 and 4 weight-%. The reference experiment for each size concentration comprises pure surface size starch without addition of a polymer composition. Each experiment is conducted using 4 repeats. The objective is to find out the magnitude by which concentration of size and polymer composition affect the dynamic penetration. The results are given in Table 9.

(75) TABLE-US-00009 TABLE 9 Penetration and viscosity test result with C*film 07312 starch. Polymer Size Penetration at Dosage, % concentration, 2,4 s, % of Time at 30% Viscosity, Experiment Remark Polymer of size, dry % paper pentration, s mPas 4-1 ref. — 0 6 82.4 0.24 2.0 4-2 Product 2 2 6 81.8 0.24 3.0 4-3 Product 2 4 6 77.1 0.26 2.9 4-4 comp. APAM3 2 6 78.9 0.27 5.9 4-5 comp. APAM3 4 6 77.6 0.26 9.1 4-6 ref. — 0 9 62.5 0.39 4.3 4-7 Product 2 2 9 66.1 0.40 7.6 4-8 Product 2 4 9 63.0 na 7.6 4-9 comp. APAM3 2 9 58.2 0.58 11.9 4-10 comp. APAM3 4 9 61.5 na 21.7 4-11 ref. — 0 12 50.0 na 9.3 4-12 Product 2 2 12 57.1 na 20.8 4-13 Product 2 4 12 51.0 na 22.4 4-14 comp. APAM3 2 12 48.3 na 29.7 4-15 comp. APAM3 4 12 49.4 na 44.8

(76) It is seen from Table 9 that “Product 2” gives more or faster penetration than the comparative polymer composition. Also the lower viscosities, which are obtained when using “Product 2”, allow more even size distribution and metering at film size press. Penetration with “Product 2” is almost as fast as in reference experiments, using pure starch solution. Penetration speed is preferably fast enough in size press application to allow high enough pick-up in short dwell time. Good penetration is advantageous for obtaining SCT-strength and Z-directional tensile or Scott bond strength.

(77) Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.