EP2408966B1

EP2408966B1 - Paper coating composition

Info

Publication number: EP2408966B1
Application number: EP10713151.8A
Authority: EP
Inventors: Franco Federici; Gabriele Costa; Thierry Bossi; Stefano Fumagalli; Riccardo Vago; Giuseppe Li Bassi
Original assignee: Lamberti SpA
Current assignee: Lamberti SpA
Priority date: 2009-03-16
Filing date: 2010-03-15
Publication date: 2017-11-22
Anticipated expiration: 2030-03-15
Also published as: WO2010106023A1; ITVA20090020A1; CA2754179C; CA2754179A1; ES2660743T3; SI2408966T1; CN102356197A; IT1393466B1; EP2408966A1; CN102356197B

Description

FIELD OF THE INVENTION

The present invention relates to paper coating compositions containing comb water soluble (hydrophilic) polyurethanes containing polyoxyethylene side-chains as deflocculants and water retention agents.

BACKGROUND OF THE ART

It is well known that the surface of printing paper sheets is commonly coated with a paper coating formulation to improve the printability and to make it smooth and glossy.
Paper coating compositions generally comprise fillers or pigments dispersed in water, polymeric binders, rheology modifiers, water retention agents and dispersing agents.
Dispersing agents are indispensable to reduce the viscosity in the presence of the high solid contents which are typical of paper coating compositions and to maintain a constant desired processing viscosity; examples of conventional dispersing agents are complex phosphates, salts of polyphosphoric acid and salts of polycarboxylic acids.
Water retention agents prevent dewatering of the coating composition upon contact with the surface of the paper sheet; a typical water retention agent for paper coating composition is carboxymethyl cellulose.
Rheology modifiers are added to modulate the paper coating viscosity.
Binders are responsible for cohesion of the final coating and for its grafting to the paper sheet.
Some conventional paper coating additives are known to perform more than one function; by way of example, carboxymethyl cellulose acts both as rheology modifier and water retention agent, polyvinyl alcohol acts as water retention agent and optical brightness enhancer.
WO 01/96007 , WO 2004/044022 , WO 2004/041883 and WO 2007/069037 describe the use of polyacrylic anionic copolymers in the paper industry, for making or coating paper; the polyacrylic anionic copolymers contain at least one anionic ethylenically unsaturated monomer having monocarboxylic functionality and at least one non-ionic ethylenically unsaturated monomer having poly(C_2-4-alkylene oxide) functionalities. They are said to be useful as dispersing and/or grinding agents, as agents for improving the optical brightening activation, as water retention agents, as viscosity and gloss enhancers.
Unfortunately, when the known ionic substances are used in aqueous dispersions in accordance with the prior art, their effectiveness is dependent on the pH value of the dispersion.
It has now been found that specific hydrophilic comb polyurethanes are suitable as water retention agents and gloss enhancer for paper coating compositions; the paper coating compositions of the invention are stable over a wide viscosity range irrespective of their pH value and impart good printability and gloss to coated paper.
Hydrophilic comb polyurethanes containing polyoxyethylene side-chains are known and have been described in the patent literature.
EP 60,430 discloses a process for making a polyurethane having polyalkylene oxide side-chains, but it does not mention the use of the polyurethane in paper coating compositions.
WO 03/046038 describes a broad family of polyurethane dispersants comprising from 35 to 90% by weight of poly(C_2-4-alkylene oxide) based on the total weight of the polyurethane polymer, but it is silent about the possibility of using the polyurethanes in paper coating compositions.

SUMMARY OF THE INVENTION

The object of the invention is a paper coating composition according to claim 1.

DETAILED DESCRIPTION

The paper coating compositions of the invention comprise: a) from 30 to 80% by weight of inorganic pigment; b) from 0.05 to 3.0 parts by weight each 100 parts by weight of pigment of the above described comb polyurethane; c) at least 15% by weight of water, d) from 0.01 to 3% by weight of a dispersing agent; e) from 1 to 15% by weight of a binder, and have Brookfield® viscosity at 25°C and 100 rpm of less than 3,000 mPa∗s, preferably from 500 to 2,000 mPa∗s.
The paper coating compositions according to the present invention also comprise from 0.01 to 3% by weight of a dispersing agent, because the comb polyurethane does not act per se as pigment dispersant; the compositions may also contain a specific rheologly modifier.
Examples of useful dispersing agents are anionic un-crosslinked polyacrylate derivatives, such as sodium polyacrylate, having molecular weight from 5,000 to 40,000.
Typical useful rheology modifiers are carboxymethyl cellulose, hydroxypropyl guar, hydroxypropylmethyl cellulose, xanthan, ASA polymers (i.e. "Alkali Swellable Acrylic" polymers).
Dispersing agents, which are common ingredients of paper coating compositions, are not generally able to prevent the flocculation of the finest particles, especially when the paper coating composition is being applied on the paper sheet, i.e. under high stress conditions, and the particles flocculation is detrimental to smoothness and gloss of the resulting coated paper.
The comb polyurethanes containing polyoxyethylene side-chains having each molecular weight higher than 500, preferably from 2,000 to 20,000, and ethylene oxide content from 80 to 99.9% by weight are particularly effective as deflocculants, avoiding the formation of clusters of fine particles, which may tend to settle, and they characterise the paper coating compositions of the present invention.
The inorganic pigments of the paper coating compositions, preferably having from 40 to 90% of the particles finer than 2 microns, are those normally employed in the coating of paper, and are particularly kaolin, calcium carbonate, talc, titanium dioxide, barium sulfate, gypsum, and mixture thereof. The paper coating compositions of the invention normally also comprise from 1 to 15% by weight of a binder, preferably a polymeric acrylic binder.
Among the polymeric acrylic binder preferred for the realisation of the invention we cite the polymers of acrylic or methacrylic acid esters, the copolymers of acrylic ester monomers and vinyl acetate, styrene, butadiene or mixture thereof.
Other conventional additives, such as defoaming agents, biocides, optical brighteners, may be present in the paper coating compositions.
The water soluble comb polyurethane useful for the realisation of the invention comprises a main chain (backbone) containing urethane and urea linkages with multiple trifunctional branch points (branch points) from each of which a polyoxyethylene side-chains emanates.
In the present text, with the expression "polyoxyethylene side-chains" we mean side chains containing -(CH₂CH₂O)_n- units with n≥9.
The main chain containing urethane and urea linkages has uniformly spaced branch points, whose distribution can be predicted from the molar ratios and chemical nature of reactants and does not depend from the reaction conditions or catalysts used. Therefore, the fine structure of the comb polyurethane can be advantageously reproduced and possibly modulated as desired.
The polyurethane backbone shall be per se insoluble in water and preferably it does not contain any internal or terminal polyoxyethylene chain.
In the comb polyurethane the total ethylene oxide content, i.e. the sum of the -(CH₂CH₂O)- units, is preferably higher than 50% by weight, more preferably higher than 70% by weight.
The preferred comb polyurethanes have molecular weight higher than 100.000, more preferably from 300.000 to 3.000.000, and polyoxyethylene side-chains having molecular weight from 2,000 to 20,000 and ethylene oxide content from 90 to 99.9% by weight.
The polyoxyethylene side-chains of the comb polyurethane are distributed along the backbone at intervals of less than 100, preferably of less than 50, covalent bonds.
The high molecular weight hydrophilic comb polyurethanes may be prepared by any method known in the art.
The polyoxyethylene side-chains of the comb polyurethane are introduced by reacting in one of the preparation steps an organic isocyanate group with at least one compound having molecular weight higher than 500, preferably from 2,000 to 20,000, more preferably from 4000 to 6000, ethylene oxide content from 80 to 99.9% by weight, preferably from 90 to 99.9% by weight, and having one group which reacts with isocyanates.
Comb polyurethanes having polyoxyethylene side-chains with molecular weight from 2,000 to 20,000 and ethylene oxide content from 90 to 99.9% by weight are preferred for use in paper coating compositions, because they impart better water retention, with only slight loss on gloss values.
Any compound having molecular weight higher than 500, ethylene oxide content from 80 to 99.9% by weight and having one group which reacts with isocyanates may be used to introduce the polyoxyethylene side-chains in the comb polyurethane.
Convenient examples of such compounds are C₁-C₄ poly(ethylene oxide) monoalkyl ether, such as poly(ethylene oxide) monomethyl ether and poly(ethylene oxide) monobutyl ether, poly(ethylene oxide) monomethyl ether having molecular weight from 4,000 to 6,000 being the most preferred compound.
The term molecular weight used in this text means the number average molecular weight, when polymers are concerned.
The methods of preparation of the comb polyurethanes of the invention comprise, as key intermediate, a bifunctional isocyanate containing polyoxyethylene side-chains having each molecular weight higher than 500, preferably from 2,000 to 20,000, and ethylene oxide content from 80 to 99.9% by weight which is reacted with one or more compounds having molecular weight from 34 to 300 and two groups which react with isocyanates, in order to chain extend the backbone, to increase the molecular weight of the bifunctional isocyanate and to obtain a comb polyurethane having high molecular weight.
According to one preferred method of preparation (Method A), the comb polyurethane is obtained by reacting in a first step about one mole of a compound having molecular weight from 92 to 1,200 and having three groups which react with isocyanates with about 3 moles of a diisocyanate (step a1); in a second step, the thus obtained trifunctional isocyanate is reacted with equimolar amounts of one or more compounds having molecular weight higher than 500, preferably from 2,000 to 20,000, ethylene oxide content from 80 to 99.9% by weight and having one group which reacts with isocyanates (step a2); in a third step, the thus obtained bifunctional isocyanate is reacted with one or more compounds having molecular weight from 34 to 300 and having two groups which react with isocyanates (step a3).
According to another preferred method of preparation (Method B) the comb polyurethane is obtained by reacting in a first step a polyisocyanate with one or more compounds having molecular weight higher than 500, preferably from 2,000 to 20,000, ethylene oxide content from 80 to 99.9% by weight and having one group which reacts with isocyanates (step b1) in order to obtain, in the average, a bifunctional isocyanate; in a second step, the thus obtained isocyanate is reacted with one or more compounds having molecular weight from 34 to 300 and having two groups which react with isocyanates (step b2).
According to still another preferred method of preparation (Method C) the comb polyurethane is obtained by reacting in a first step a diisocyanate with equimolar amounts of one or more compounds having molecular weight higher than 500, preferably from 2,000 to 20,000, ethylene oxide content from 80 to 99.9% by weight and having one group which reacts with isocyanates (step c1); in a second step, the thus obtained monofunctional isocyanate is reacted with equimolar amounts of one or more compounds having molecular weight from 34 to 300 and having at least one -NH- group which reacts with isocyanates and at least two hydroxyl groups which react with isocyanates (step c2); in a third step, the thus obtained compound having two groups which react with isocyanates is reacted with diisocyanates in order to obtain an intermediate compound capped by two lateral isocyanate groups (step c3); in a fourth step, the thus obtained bifunctional isocyanate is reacted with one or more compounds having molecular weight from 34 to 300 and having two groups which react with isocyanates (step c4).
The compound of step a1 having molecular weight from 92 to 1,200 and having three groups which react with isocyanates is preferably a trifunctional alcohol. Examples of utilizable trifunctional alcohols are glycerin, polypropylene glycol triol, trimethylolpropane, trimethylolethane,
Any organic diisocyanate having molecular weight below 500 and average -NCO functionality from 2.0 to 2.1 may be used in step a1 and c1 as the diisocyanate.
Examples of useful diisocyanates are 1,6-hexamethylene diisocyanate (HDI), tetramethylene diisocyanate, 1-isocyanate-3-isocyanate-methyl-3,5,5-trimethyl-cyclohexane (or isophoronediisocyanate) (IPDI), 4,4'-dicyclohexyl-methanediisocyanate, 2,4- toluenediisocyanate either alone or in admixture with 2,6-toluenediisocyanate (TDI), 4,4'-diphenyl-methanediisocyanate (MDI), meta-tetramethylxilylenediisocyanate (TMXDI), 1,5-naphthalene diisocyanate, and mixtures thereof; cycloaliphatic and aliphatic diisocyanate are preferred, the most preferred being IPDI.
The compounds having molecular weight from 34 to 300 and two groups which react with isocyanates used in steps a3, b2 and c4 are preferably diamines. Examples of utilizable diamines are hydrazine, ethylenediamine, piperazine, 1,5-pentanediamine, 1,6-dihexanediamine, isophoronediamine, diethylenetriamine.
Steps a3, b2 and c4 are preferably carried out by dispersing the bifunctional isocyanate in water and adding to the dispersion the diamine, possibly dissolved in water.
The compound having molecular weight from 34 to 300 of step c2 is preferably diethanolamine.
Any organic polyisocyanates with average -NCO functionality from 2,4 to 3,8 (trifunctional isocyanate), and having molecular weight below 800 may be used in step b1.
Examples of trifunctional isocyanates are the compounds obtained from trimerization, biurethization, urethanization or allophanation of difunctional isocyanates, such as those mentioned above, and mixtures thereof.
Useful trifunctional isocyanates are HDI biuret, HDI isocyanurate, IPDI trimers and the combination of the above trifunctional isocyanates with diisocyanates.
The preferred trifunctional isocyanates are the isocyanurate and biuret obtained from hexamethylenediisocyanate, for example HDI isocyanurate in asymmetrical form, HDI biuret in low viscous form, and those obtained by the combinations of IPDI trimers and HDI trimers.
For purpose of the present invention, in order to obtain the desired high molecular weight comb polyurethane, it is preferred to minimize the quantity of diisocyanate that may be present together with the trifunctional isocyanates during the step b1, as it is well known to person skilled in the art.
For the purpose of the present invention, the ratio between the equivalents of isocyanate groups and the equivalent of groups which react with isocyanate during the step c3 is between 1.1 and 2.3.
The water soluble comb polyurethanes are preferably non-ionic compounds, devoid of acid groups, such as carboxylic and sulphonic acid groups.
Suitable solvents may be used in the preparation steps, but it is also possible to perform all the preparation steps with the neat reactants, without the use of water or organic solvents.
The water soluble comb polyurethanes according to the invention are useful as water retention agents, deflocculant and viscosity stabilizer for paper coating compositions and provide coated paper with excellent printability, brightness and gloss.
Another advantageous characteristic of the polyurethanes of the invention is the fact that they act as rheology and water retention buffers over different batches of industrial paper coating compositions having same recipe; this means that, in the industrial coating process, the usual deviations from the theoretical amounts of paper coating ingredients does not affect the rheology and water retention
characteristics.

EXAMPLES

In the examples the following materials were used:

TRIOL1: polypropylene glycol triol, molecular weight 1000 g/mol, Voranol CP 1055, from Dow Chemical Company
ETHOXY1: butanol polyethoxylated, molecular weight 3000 g/mol
ETHOXY2: methanol polyethoxylated, molecular weight 5000 g/mol
ETHOXY3: butanol polyethoxylated, molecular weight 5000 g/mol
ETHOXY4: methanol polyethoxylated, molecular weight 750 g/mol
DIISOCYANATE1: isophoronediisocyanate, molecular weight 222.3 g/mol,
Desmodur I from Bayer Material Science.
TRIISOCYANATE1: hexamethylene diisocyanate trimer, NCO content 24,0%, NCO functionality 3,1, Desmodur XP2410 from Bayer Material Science.
AMINE1: isophorone diamine, molecular weight 170.3 g/mol, from Sigma Aldrich
AMINE2: diethanolamine, molecular weight 105,14 g/mol, from Sigma Aldrich
AMINE3: ethylendiamine, molecular weight 60,10 g/mol, from Sigma Aldrich
AMINE4: 24% hydrazine hydrate solution, molecular weight 32.3 g/mol, from Sigma Aldrich.

EXAMPLE I

Preparation of a comb polyurethane with Method A

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 100.0 g of TRIOL1 and 300.0 g of ETHOXY1. The mixture was heated under stirring condition to 85°C and 0.15 g of 85% phosphoric acid were added. At 85°C 69.0 g of DIISOCYANATE1 were charged to the homogeneous mixture, under stirring; after 20 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free
-NCO groups still present gave a calculated value of 1,7% (value determined in this example as well as in the other examples according to the standard method ASTM D2572),
300 g of the obtained product were dispersed by vigorous stirring into 890 g of water cooled at 18°C. After 20 minutes 9.7 g of AMINE1 dissolved in 39.3 g of water were dropped in.
The obtained product had 25.55% solid content, viscosity 48 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 6.5.

EXAMPLE II

Preparation of a comb polyurethane with Method A

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 50.0 g of TRIOL1 and 250.0 g of ETHOXY2. The mixture was heated under stirring to 85°C and 0.11 g of 85% phosphoric acid were added. At 85°C, 34.5 g of DIISOCYANATE1 were charged to the homogeneous mixture under stirring condition; after 20 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free -NCO groups still present gave a calculated value of 1.26%.
300 g of the obtained product were dispersed by vigorous stirring into 893.1 g of water cooled at 18°C. After 20 minutes 6.9 g of AMINE1 dissolved in 27.4 g of water were dropped in.
The obtained product had solid content 26.15%, viscosity 62 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 6.4.

EXAMPLE III

Preparation of a comb polyurethane with Method B

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 300.0 g of ETHOXY1 and g of 85% phosphoric acid. The mixture was heated under stirring condition to 85°C and 56.6 g of TRIISOCYANATE1 were charged; after 20 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free -NCO groups still present gave a calculated value of 2.5%.
300 g of the obtained product were dispersed by vigorous stirring into 862.6 g of water cooled at 18°C. After 20 minutes 12.8 g of AMINE1 dissolved in 51.5 g of water were dropped in.
The obtained product had solid content 25.98%, viscosity 196 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 6.6.

EXAMPLE IV

Preparation of a comb polyurethane with Method B

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 274.5 g of ETHOXY 2 and g of 85% phosphoric acid. The mixture was heated under stirring to 85°C and 31.1 g of TRIISOCYANATE1 were charged; after 20 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free - NCO groups still present gave a calculated value of 1.6%.
275 g of the obtained product were dispersed by vigorous stirring into 795.3 g of water cooled at 18°C. After 20 minutes 7.6 g of AMINE1 dissolved in 30.2 g of water were dropped in.
The obtained product had solid content 25.51%, viscosity 246 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 5.5.

EXAMPLE V

Preparation of a comb polyurethane with Method B

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 300.0 g of ETHOXY3 and g of 85% phosphoric acid. The mixture was heated under stirring to 85°C and 22.0 g of TRIISOCYANATE1 and 10.1 g of DIISOCYANATE1 were charged; after 20 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free -NCO groups still present gave a calculated value of 1.8%.
280 g of the obtained product was dispersed by vigorous stirring into 826.4 g of water cooled at 18°C. After 20 minutes 9.4 g of AMINE4 dissolved in 9.8 g of water were dropped in.
The obtained product had solid content 26.07%, viscosity 412 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 6.8.

EXAMPLE VI

Preparation of a comb polyurethane with Method B

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 300.0 g of ETHOXY3 and g of 85% phosphoric acid. The mixture was heated under stirring to 85°C and 22.0 g of TRIISOCYANATE1 and 10.1 g of DIISOCYANATE1 were charged; after 20 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free -NCO groups still present gave a calculated value of 1.8%.
280 g of the obtained product were dispersed by vigorous stirring into 838.1 g of water cooled at 18°C. After 20 minutes 2.7 g of AMINE3 dissolved in 10.0 g of water were dropped in.
The obtained product had solid content 25.84%, viscosity 256 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 6.8.

EXAMPLE VII

Preparation of a comb polyurethane with Method C

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 300.0 g of ETHOXY2 and g of 85% phosphoric acid. The mixture was heated under stirring to 85°C and 15.3 g of DIISOCYANATE1 were charged; after 10 minutes 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was brought to 85°C until the titrimetric determination of the free -NCO groups still present gave a calculated value of 0.9%. The mixture was then cooled to 55°C and 6.6 of AMINE2 dissolved in 6.6 g of N-methylpyrrolidone were charged dropwise. After 20 minutes the titrimetric determination of the free -NCO groups gave a calculated value of 0.0%. The reaction temperature was then brought to 85°C and 28.5 g of DIISOCYANATE1 was charged and it was reacted until the titrimetric determination of the free -NCO groups gave a calculated value of 1.5%.
300 g of the obtained product were dispersed by vigorous stirring into 898.0 g of water cooled at 18°C. After 20 minutes 2.9 g of AMINE3 dissolved in 10.7 g of water were dropped.
The obtained product had solid content 24.99%, viscosity 194 mPa∗s (measured by Brookfield® viscometer at 20 rpm) and pH 6.3.

EXAMPLE VIII

Preparation of a comb polyurethane with Method B

A reaction vessel, equipped with internal thermometer, stirrer and cooler, was filled, under nitrogen atmosphere and at room temperature, with 300.0 g of ETHOXY4 and g of 85% phosphoric acid. The mixture was heated under stirring to 85°C and 226.2 g of TRIISOCYANATE1 were charged; after 10 minute 0.3 g of dibutiltindilaurate (DBTL) were added. The reaction temperature was kept at 85°C until the titrimetric determination of the free - NCO groups still present gave a calculated value of 5.9%. 470 g of the obtained product was dispersed by vigorous stirring into 1360 g of water cooled at 18°C. After 20 minutes 16.7 g of AMINE3 dissolved in 62.1 g of water were dropped in.
The obtained product had solid content 25.1%.

Application Examples

Paper coating compositions based on 100% carbonate (Hydrocarb 90, from Omya, CH) were prepared using the comb polyurethanes from Examples I-VIII and with a water retention agent of the prior art.
The compositions of the paper coating compositions are reported in Table 1; the amounts of the ingredients are parts by weights.
The paper coating compositions were characterized by performing the following measurements:

pH
Brookfield@ viscosity, 100 rpm
Dry matter
Water retention -Tappi Method T710

The data obtained are also reported in Table 1.

The paper coating compositions were applied (13 g/m²) on offset sheets (80 g/m²); the sheets were conditioned for 24h at 21°C and 50% r.h. and calendared (cylinders temperature 55°C, pressure 67.5 Kg/cm; 4 nips). Brightness and gloss were measured and are reported in Table 2.

Table 1

PAPER COATING COMPOSITIONS:	1	2	3	4	5	6	7	8	9⁶⁾
HYDROCARB 90	100	100	100	100	100	100	100	100	100
Dow LATEX 935¹⁾	10	10	10	10	10	10	10	10	10
REOTAN A²⁾	0.045	0.045	0.045	0.045	0.045	0.045	0.045	0.045	0.045
DEFOMEX 108³⁾	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
TINOPAL ABP-Z⁴⁾	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8
Ex. 1	0.45	/	/	/	/	/	/	/	/
Ex. 2	/	0.45	/	/	/	/	/	/	/
Ex. 3	/	/	0.45	/	/	/	/	/	/
Ex. 4	/	/	/	0.45	/	/	/	/	/
Ex. 5	/	/	/	/	0.45	/	/	/	/
Ex. 6	/	/	/	/	/	0.45	/	/	/
Ex. 7	/	/	/	/	/	/	0.45	/	/
Ex. 8	/	/	/	/	/	/	/	0.45	/
Viscolam GP37⁵⁾	/	/	/	/	/	/	/	/	0.15

CHARACTERISTIC OF THE PAPER COATING COMPOSITIONS
Dry matter (%)	70.32	70.40	70.18	70.16	69.95	69.95	69.97	70.21	70.18
pH	8.85	9.02	8.85	8.91	8.97	9.08	9.06	8.87	8.95
Viscosity (mPa*s)	550	655	570	650	600	770	650	450	960
Water retention (g/m²)	150	140	155	145	130	145	140	165	112

1) Binder, styrene butadiene latex (Dow Chemical Co. US)
2) Dispersant, sodium polyacrylate (Lamberti SpA, IT)
3) Defoaming agent, (Lamberti SpA, IT)
4) Optical brightener (CIBA, CH)
5) ASE Thickener from Lamberti SpA
6) comparative

Table 2

PAPER COATING COMPOSITION :	°Brightness¹⁾	Gloss @ 75°²⁾
1	100.8	72.5
2	100.6	72.1
3	101.1	72.8
4	100.5	72.0
5	100.3	72.4
6	100.4	72.7
7	100.9	72.6
8	100.6	75.2
9³⁾	99.2	65.8

1) Brightness, Tappi Method T452
2) Specular Gloss at 75°, Tappi Method T480
3) comparative

Claims

Paper coating compositions comprising a) from 30 to 80% by weight of inorganic pigment; b) from 0.05 to 3.0 parts by weight each 100 parts by weight of pigment of at least one water soluble nonionic comb polyurethane containing polyoxyethylene side-chains having each molecular weight higher than 500 and ethylene oxide content from 80 to 99.9% by weight; c) at least 15% by weight of water d) from 0.01 to 3% by weight of a dispersing agent; e) from 1 to 15% by weight of a binder, the paper coating compositions having Brookfield® viscosity at 25°C and 100 rpm of less than 3,000 mPa∗s.
Paper coating compositions according to claim 1 wherein the comb polyurethane has total ethylene oxide content higher than 50% by weight.
Paper coating compositions according to claim 2 wherein the comb polyurethane has total ethylene oxide content higher than 70% by weight.
Paper coating compositions according to claim 2 or 3 wherein the comb polyurethane has molecular weight higher that 100.000 and polyoxyethylene side-chains having molecular weight from 2,000 to 20,000 and ethylene oxide content from 90 to 99.9% by weight.
Paper coating compositions according to claim 1 wherein the comb polyurethane is obtained by reacting a bifunctional isocyanate containing polyoxyethylene side-chains having each molecular weight higher than 500 and ethylene oxide content from 80 to 99.9% by weight with one or more compounds having molecular weight from 34 to 300 and two groups which react with isocyanates.
Paper coating compositions according to claim 1. wherein the dispersing agent is anionic un-crosslinked polyacrylate derivatives having molecular weight from 5,000 to 40,000.
Paper coating compositions according to claim 1 wherein the inorganic pigment has from 40 to 90% by weight of the particles finer than 2 microns and is chosen among kaolins, calcium carbonate, talc, titanium dioxide, barium sulfate, gypsum and mixtures thereof.