FR2780422A1 - Heat thickening composition for coating baths - Google Patents

Abstract

Heat thickening composition for coating baths comprises at least a heat-sensitive copolymer with a comb structure, consisting of a polymeric 'skeleton' segment, on which are grafted at least two lateral polymeric segments, identical or different, with either the skeleton or lateral segments having a low critical solubility temperature (LCST) = 30-80 deg C. An Independent claim is also included for use of said composition in coating bath for paper or cardboard.

Description

i The present invention relates to a new composition

  thermo thickener for bedding baths, can be substituted or combined

  conventional thickening additives used in these baths.

  The present invention relates more particularly to a new thickening composition which improves the coverage of the surface layer of

  cellulosic supports and reduces the mottling of printing.

  In conventional coating operations, the heterogeneity of air, inherent in any cellulosic or cardboard support, as well as its porous and absorbent nature induce defects in coverage of the support by the layer.

of surface.

  In fact, these defects essentially stem from two distinct phenomena. Although the coating operation aims to specifically deposit this bath on the surface and to avoid its penetration into the support, there is also a significant absorption of the coating bath by the support. This results in a loss of coating efficiency since, to achieve the same level of property, a higher layer weight must be deposited to compensate for the layer absorbed by the support. The losses of quality induced are losses of opacity, smoothness and / or gloss. This absorption of the bath in the support is a defect all the more penalizing as the layer weights are low and it

  therefore becomes critical below the threshold of a layer weight of approximately 6 g / m2.

  The second type of defect corresponds to heterogeneities in the structure and composition of the layer coated on the support. Its most common and penalizing manifestation is the mottling or swarm of printing. Mottling is a phenomenon which manifests itself at the level of the printing of the layer by fluctuations in the printing results. This type of defect is notably caused by phenomena of migration of the bath in the support. For a bath which does not correspond to the present invention, these migration phenomena and the defects which they generate are in particular caused and amplified by the drying of the layer. Thus, the heating applied at the level of the layer to dehydrate it has the effect of reducing its viscosity, then amplifying as much

plus migration phenomena.

  Conventionally, the coating bath compositions comprise, in addition to fillers, at least one thickening agent which has the main function of precisely controlling the viscosity and the water retention of the coating bath. These are generally cellulose derivatives such as

  carboxymethylcellulose or highly carboxylated synthetic polymers alkali-

  thickeners. Unfortunately, these compounds have the drawback of having a rheological profile which increases significantly at a temperature above 45 C. They therefore promote migration phenomena during the dehydration operation and do not prevent the manifestation of

faults mentioned above.

  The present invention therefore aims to propose a new thickening composition for bedding baths, useful in particular in that it allows

  avoid migration phenomena on drying and the resulting defects.

  More specifically, it offers a new thickener which has the advantage of causing an increase in the viscosity of the applied layer when it is dried, an increase which therefore blocks migration phenomena. This results in improved coverage of the surface layer of the

  cellulosic support and a significant reduction in printing mottling.

  Consequently, the present invention firstly relates to a heat-thickening composition based on at least one heat-sensitive copolymer.

  which overcomes the problems mentioned above.

  The term “heat-sensitive copolymer” is understood to mean a polymer whose viscosity in an aqueous medium increases with temperature above a threshold temperature, unlike conventional polymers whose viscosity decreases continuously with

the rise in temperature.

  The present invention takes advantage in particular of the ability of certain water-soluble polymer chains to associate locally above a certain threshold temperature. It follows the formation of a physical network of a high molar mass and therefore an increase in viscosity thus creating the

thermo-thickening property.

  Copolymers of this type are in particular already known in the petroleum industry and more particularly in the field of drilling fluids. Particular advantage is taken of the ability of the polyoxyalkylene chains, which are water-soluble at ambient temperature, to become hydrophobic at a temperature known as temperature.

critical, greater than 100 C.

  Other polymers derived from cellulose such as methylcellulose or ethylhydroxyethylcellulose have the desired heat-thickening property but are generally used as ordinary rheological agents. However, the solutions of these polymers develop high turbidity, beyond their critical temperature, which is characteristic of the formation of concentration heterogeneities. Now, in applications relating to the coating of paper, in order to improve the homogeneity of the quality of the layer, it is necessary that the increase in viscosity occurs in a perfectly homogeneous medium, this

  which makes such polymers unsuitable for these applications.

  Unexpectedly, the inventors have shown that it is possible to reproduce the water-soluble / hydrophobic transition phenomenon mentioned above at lower concentrations and this for a temperature range well below 100 C and therefore more suitable for a use in

the paper coating industry.

  In addition, the copolymers selected according to the invention have the characteristic of having, in solution, an almost zero variation in turbidity beyond the critical temperature. This property means that the solutions of polymers according to the invention retain, in the temperature range of their use, a degree of homogeneity which makes it possible to obtain a more regular structure and

  more uniform of the paper layers.

  Consequently, the present invention firstly relates to a thermo-thickening composition for coating baths comprising at least one thermosensitive copolymer characterized in that said copolymer has a comb structure and consists of a polymeric segment called backbone to which are grafted at least two segments polymeric sidewalls, identical or different, with either the skeleton segment or the side segments having a temperature

  lower solubility critical, LCST, between 30 and 80 C.

  According to a first variant of the invention, it is the so-called skeleton polymer segment which has a lower critical solubility temperature, LCST,

between 30 and 80 C.

  According to a second variant of the invention which is preferred, it is the polymeric lateral segments which have a lower critical temperature

  solubility, LCST, between 30 and 80 C.

  Finally, according to another variant of the invention, the thermo-thickening composition claimed comprises several copolymers as defined above. These copolymers are arranged therebetween so as to form a crosslinked structure in which their polymeric segments having the lower critical solubility temperature, LCST, appear the crosslinking nodes and at least part of their segments having no lower critical temperature of solubility between 30 and 80 C establish

connections between said nodes.

  According to these variants, the segment which does not have the required LCST, namely between 30 and 80 C, is for its part water-soluble at least in this

temperature range.

  It can also be envisaged in the context of the present invention that the segment not exhibiting the LCST of between 30 and 80 ° C. is nevertheless provided with an LCST. However, it is then necessary that in the temperature range considered (30-80 C), this segment is water-soluble. This implies

  its LCST is greater than 80 C.

  More specifically, the polymer segments having a lower critical solubility temperature exhibit a water-soluble character at a temperature below their LCST. As regards the polymer segments which do not have a lower critical solubility temperature in the temperature range concerned, they are water-soluble at least in the temperature range of use of the heat-thickening composition, preferably

between + 10 C to + 100 C.

  As regards the polymer segment having no critical temperature, it is more preferably a polymer of the type

water-soluble ethylene.

  These water-soluble polymers can be derived from the (co) polymerization of water-soluble ethylenic monomers. These monomers can be in particular of vinyl, acrylic, styrenic, diene type or also of type

vinyl ester.

  By way of example of vinyl monomers, mention may be made of vinyl acid

  sulfonic, methallyl sulfonic acid or their salts.

  As examples of acrylic monomers, mention may be made of (meth) acrylic acid, diacids such as fumaric acid, itaconic acid or their salts, maleic anhydride, acrylamide and its derivatives such as that acrylamido acid

  methyl propane sulfonic acid or their salts.

  Examples of styrenic monomers that may be mentioned include styrene acid

  sulfonic, vinyl benzoic acid or their salts.

  With the water-soluble monomers mentioned above, it is also possible to combine hydrophobic monomers, the units of which, once incorporated in the polymer chain, can be transformed, in particular by chemical treatment such as hydrolysis, into water-soluble units. These are, for example, methyl (meth) acrylate, ter-butyl (meth) acrylate, glycidyl (meth) acrylate and

vinyl acetate.

  Finally, any organosoluble monomers can also be used and incorporated into the polymer chain in the form of hydrophobic units. Present in small quantities, in the polymer segment, they

  allow to control the solubility in water of the corresponding copolymer.

  Of course, the different monomers are selected so that the corresponding polymer segment has a solubility in aqueous medium according to the invention. This adjustment of the relative quantities in

  corresponding monomers fall within the competence of a person skilled in the art.

  Are particularly preferred according to the invention, the monomers such as acrylic or methacrylic acid, acrylamides and their derivatives, fumaric and maleic acids and sulfonated monomers such as 2acrylamide-methyl-propane acid

  sulfonic (AMPS) and its alkaline salts and vinylsulfonate.

  More preferably, this type of polymer segment has a molecular weight at least greater than 1000 and preferably at least greater

at 20,000.

  Advantageously, these polymer segments result from the polymerization of acrylic acid (AA) and / or 2-acrylamidemethylpropane acid

sulfonic (AMPS).

  With regard to the polymer segments having a lower critical temperature for LCST solubility of between 30 and 80 ° C., they

  are derived from polyoxyalkylene polymers.

  According to a preferred embodiment of the invention, the different oxyalkylene unit (s) present in the polyoxyalkylene polymer have at most 6 atoms

of carbon.

  Preferably, the segments having a critical temperature (LCST) consist of oxyethylene (OE) units and / or of units

oxypropylene (OP).

  The OE and OP motifs can be arranged in the thermosensitive polymer segment in statistical, block or sequenced form. It turns out to be possible to adjust the critical solubility temperature in particular through the length and the composition of these polymer segments. Preferably, the segments having a critical temperature in accordance with the invention are

  made up of at least 5 oxyalkylene units.

  More preferably, they are corresponding macromonomers.

  Within the meaning of the present invention, a macromonomer designates a macromolecule carrying one or more (co) polymerizable ethylenic functions

by radical way.

  The grafting of the lateral polymer segments onto a so-called skeleton polymer segment can be carried out according to conventional techniques familiar to those skilled in the art (European Polymer Journal 4, 343 (1968); US

3,719,647).

  Among these conventional techniques, mention may be made in particular of those known as

  direct grafting and copolymerization.

  Direct grafting consists in (co) polymerizing the monomer (s) chosen (s) by the radical route, in the presence of the polymer selected to form the skeleton of the final product. If the monomer / skeleton couple as well as the operating conditions are judiciously chosen, then there may be a transfer reaction between the growing macroradical and the skeleton. This reaction generates a radical on the skeleton and it is from this radical that the graft believes. The primary radical

  from the initiator can also contribute to transfer reactions.

  For its part, the copolymerization firstly implements the grafting at the end of the heat-sensitive segment of a (co) polymerizable function by the radical route. This grafting can be carried out by usual methods of organic chemistry. Then, in a second step, the macromonomer thus obtained is copolymerized with the monomer chosen to form the skeleton and a

so-called "comb" copolymer.

  It is obvious to a person skilled in the art that when a macromonomer and a monomer chosen are copolymerized in such a way that these two species associate strongly by hydrogen bonds, then there is simultaneously direct grafting on the polymer segment of the macromonomer. and incorporation of this macromonomer into the copolymer chain by simple copolymerization of its (co) polymerizable end. In this case, the structure obtained is substantially

  more connected or even cross-linked than in the two previous cases.

  Preferably, the copolymer comprises 0.1% to 50 mol% and preferably 0.1 to 5.0 mol% of polymer segments having a

  lower critical solubility temperature (LCST) between 30 and 80 C.

  In solution, the copolymers of the invention advantageously exhibit a slight variation in turbidity as a function of the temperature, which distinguishes them.

  other polymers having the thermo-thickening character.

  Preferably, in aqueous solution, the polymers according to the invention exhibit an insensible variation in turbidity such that at a concentration equal to 2%, the variation in absorbance of the solution measured with a turbidimeter (662 Photometer of Metrohm brand) is less than 0.1 in the range of

  temperature between 20 and 60 C.

  Particularly suitable for the invention, the heat-thickening compositions comprising at least:

  - A copolymer prepared from POE-POP tri-block macromonomer-

  POE and acrylic acid (respective molar percentages: 2.3%, 97.7%), preferably by direct grafting,

  - A copolymer prepared from macromonomer of tri-POE blocks-

  POP-POE and acrylic acid (respective mol%: 1.6%, 98.4%), preferably by copolymerization and / or

  - A copolymer prepared from a macromonomer of POE diblocks-

  POP and acrylic acid (respective mol%: 3%, 97%), preferably by

copolymerization.

  The compositions according to the invention are particularly useful as a thickening agent in the paper coating industry and more particularly

  as an agent improving the homogeneity and the quality of the diaper cover.

  Preferably, they are introduced into the coating bath at rates

  of the order of 0.1 to 3 parts by weight expressed per 100 parts of fillers.

  This composition can be mixed with the other components

  conventionally used in sleeping baths.

  These components include fillers of the pigment type. These are mineral pigments such as kaolin, satin white, calcium carbonate, talc, titanium oxide or so-called plastic pigments. It can also be binders

  latex type, at a rate of 5 to 25 parts by dry weight per 100 parts of pigments.

  An optical brightening agent in an amount of 0.1 to 1 parts by weight and a brightening medium such as polyvinyl alcohol in an amount of 0.2 to 2 parts by weight

  are also commonly found in conventional coating baths.

  Of course, this coating bath composition can comprise S other ancillary agents of the slip agent type such as calcium stearate, and / or

insolubilizing agent.

  Generally, the coating bath composition comprises 60 to 75%

  by weight of dry extract and has a pH between 8 and 10.

  The layers are coated on the surface with a cellulosic support according to

  conventional coating techniques.

  Advantageously, the presence of at least one copolymer as defined above in the coating bath makes it possible to improve the homogeneity of layer coverage. The copolymer contributes to the optimization of the immobilization of the paper layer and therefore makes it possible to effectively oppose migrations

  generally occurring during the drying of the paper layer.

  It is observed in particular that the printing properties of the paper layers obtained by using a heat-thickening composition according to the invention are better with regard to the "mottling" criterion than those obtained with methylcellulose which has the property increase in viscosity with temperature but not that of constant turbidity with temperature. The present invention also relates to a coating bath for coating papers and / or cardboards comprising at least one composition

  thermo thickener according to the invention.

  It also relates to the use of a thermo-thickening composition as defined above in a coating bath or a coating bath in accordance with the invention to obtain better layer coverage, to improve layer coverage for the coating of paper and cardboard with low layer weight and / or improving the smoothness, opacity and / or gloss of

  coated paper and cardboard with low layer weight.

  For the purposes of the invention, coated papers or cardboards with a low layer weight mean papers or cardboards coated with a lower coating or

equal to 6 g / m2.

  The present invention also relates to the use of a thermo-thickening composition in accordance with the invention in a coating bath or a coating bath according to the invention to reduce and / or avoid flaking of the print. The examples and figures submitted below are presented for illustration and

  non-limiting of the present invention.

FIGURES

  Figure 1: Evolution of the turbidity of a copolymer according to the invention and of a control

  (methylcellulose) as a function of temperature.

  FIG. 2: Evolution of the viscosity of a coating bath comprising a heat-thickening composition according to the invention (copolymer 1) as a function of the temperature

  and for a constant imposed constraint.

  FIG. 3: Evolution of the viscosity of a coating bath comprising a heat-thickening composition according to the invention (copolymer 2a) as a function of the

  temperature and for a constant imposed stress.

  FIG. 4: Evolution of the viscosity of a coating bath comprising a heat-thickening composition according to the invention (copolymer 2b) as a function of the

  temperature and for a constant imposed stress.

  FIG. 5: Evolution of the viscosity of a coating bath comprising a heat-thickening composition according to the invention (copolymer 3) as a function of the temperature

  and for a constant imposed constraint.

EXAMPLE 1

  Preparation of a “BRANCHED” STRUCTURE copolymer by direct grafting of PAA onto a tri-block POE-POP-POE copolymer: Copolymer 1: AIBN azobisisobutyronitrile (0.030 g, 0.2.10 -3 moles) is successively introduced, I 'acrylic acid (6 g, 8.33.10-2 moles) and Pluronic PE6400 (number-average molar mass: approximately 3,000; mass percentage of OE units:%; LCST: approximately 69 C) marketed by BASF (6 g, 0.002 moles) in one

  tube which is then placed in an oven whose temperature is maintained at 70 C.

  After 12 h, the polymer is gradually dissolved in basic water

(soda: 3.4 g; water: 276 g).

EXAMPLE 2

  Preparation of macromonomers: - Macromonomer 1: The synthesis is carried out under nitrogen. Pluronic PE6400 (100 g, 0.033 moles) and maleic anhydride (3.596 g, 0.0366 moles) are introduced into a 100 ml three-necked flask topped with a condenser and immersed in a heating oil bath. The temperature is raised from 25 C to 60 C in 20 min, maintained at 60 C for 15 min, then increased to 140 C and maintained for 12 h. The macromonomer is

then used as is.

  - Macromonomer 2: The synthesis is carried out under nitrogen. The Antarox E-400 (marketed by RHODIA) is introduced into a 100 ml three-necked flask surmounted by a condenser and immersed in a heating oil bath (number-average molar mass: approximately 3,200; mass percentage of OE units: 39 %; LCST: about 55 C) (100 g, 0.031 moles) and maleic anhydride (3.37 g, 0.0343 moles). The temperature is raised from 25 C to 60 C in 20 min, maintained at 60 C for 15 min, then increased to 140 C and maintained for 12 h. The macromonomer is then used as it is.

EXAMPLE 3

  Synthesis of Deiane-structure copolvmers: - Copolymer 2 (from macromonomer 1: 1.6 mol%):

  The synthesis is carried out under nitrogen.

  In a glass reactor of 11, surmounted by a condenser and provided with magnetic or mechanical stirring as well as a temperature probe, are successively introduced: - 11.8 g of macromonomer 1 (3.8.10-3 moles) - 480 g of water

  - 0.0415 g of ammonium peroxodisulfate.

  The temperature is brought from 25 C to 40 C in 20 min, then to 45 C in 10 min. It is then maintained at 45 C. The acrylic acid (17.346 g, 0.241 moles) and the ascorbic acid (0.0166 g) are introduced separately continuously over 5 h, while the

  temperature is maintained at 45 C for a total of 14 h.

  The complex formed during the polymerization is then separated from the aqueous phase, redissolved in basic water (pH: about 8.4; concentration

mass: approximately 8.0%).

  - Copolymer 3 (from macromonomer 2: 3 mol%):

  The synthesis is carried out under nitrogen.

  In a glass reactor of 11 surmounted by a condenser and provided with a magnetic or mechanical stirring as well as a temperature probe, are successively introduced: - 30 g of macromonomer 2 (9.1.10-3 moles) - 381 g of water

  - 0.0384 g of ammonium peroxodisulfate.

  The temperature is brought from 25 C to 40 C in 20 min, then to 45 C in 10 min. It is then kept at 45 C. The acrylic acid (21.164 g, 0.294 moles) and the ascorbic acid (0.0153 g) are introduced separately continuously over 5 h, while the

  temperature is maintained at 45 C for a total of 14 h.

  The complex formed during the polymerization is then separated from the aqueous phase, redissolved in basic water (pH: about 8; mass concentration:

about 15.5%).

EXAMPLE 4

  Use of a composition according to the invention for thickening a bath of

sleeping.

  In this example, sleeping baths are prepared according to the same

  formulation in which the nature of the thickener is changed, from one test to another.

  The coating formulation used is a typical top layer formulation of woodless paper comprising: parts * calcium carbonate 75 * kaolin 25 * binding latex 11 * APV (Rhodoviol 4/20) 0.5 (polyvinyl alcohol marketed by Rhône-Poulenc; mass mass average molar: approximately 25,000; hydrolysis rate: approximately 99%) * variable thickener (see tables) The various thickeners tested are - a carboxymethylcellulose (Finnfix FF30, marketed by METSASERLA), which is a binder currently used in ceuvre by the papermakers to minimize the phenomenon of "mottling". This thickener is taken as

witness: witness 1.

  - a methylcellulose (MC2000 marketed by Aqualon / Hercules): control 2. This modified cellulose, which enters the

  composition of some paints, has thermo-thickening properties.

  Unlike the copolymers according to the invention, it manifests a strong whitening

in solution when passing the LCST.

  - the copolymer 1 produced according to the synthesis process described in

Example 1.

  a first copolymer 2, said 2a, manufactured according to the process of

synthesis described in Example 3.

  a second copolymer 2, said 2b, produced according to the process of

synthesis described in Example 3.

  - the copolymer 3 manufactured according to the synthesis process described in

Example 3.

  In solution, the copolymers of the invention exhibit a slight variation in turbidity as a function of temperature, which distinguishes them from other polymers having the thermo-thickening character but manifesting a high turbidity when passing the LCST. This property is illustrated in FIG. 1, where the evolution of the turbidity of the copolymer 3 and of the control 2 (methylcellulose) is plotted as a function of the temperature. The measurements are carried out with a turbidimeter (662 Photometer from Metrohm brand) in "absorbance" mode on 2% aqueous solutions whose temperature is regulated between 20 and 60 C using a thermostatically controlled bath. The viscosity of the baths is measured on a rheometer (Carri-med CSL100 manufactured by TA Instruments) in flow mode. The viscosity is recorded as a function of the temperature, keeping the imposed stress constant. The results are presented in FIGS. 2 to 5, where the measurements relating to the baths are given, the characteristics of which are given in Tables 1 to 4. For FIGS. 4 and 5, the stress imposed varies from one sample to another. It is adjusted for each of them so that the flow at 20 C takes place at a gradient of 10 s-1, to allow a direct comparison of the viscosity of the

  S sleeping baths at this temperature.

  Figures 2 to 5 clearly show the heat-thickening character of the copolymers 1, 2 and 3. This property is preserved in the case of a mixture of the copolymer with carboxymethylcellulose. Control 2 is also heat-thickening, only its variant of turbidity with temperature distinguishes it,

  in terms of properties, copolymers according to the invention.

  The coating baths are deposited on a woodfree paper pre-

  coated (LEICAM PM9, 105 g / m2) with a laboratory blade machine

EUCLID TOOL brand.

  The quantity of bath deposited corresponds to a specified layer weight

in the tables.

  For the coating tests to be more representative of the conditions found on an industrial scale, and for the drying to be carried out at high temperature in order to reveal the heat-thickening properties of the copolymers according to the invention, instant drying of the layer is produced directly after removal. For this purpose, a heat gun is installed immediately at the exit of the blade, ensuring a convective drying of the layer

  by a flow of air heated to around T = 150 C.

  The quality of the homogeneity of the layer cover is judged by a "mottling" test. The prints are made on a Prufbau laboratory press. The printing conditions are those, recognized by the men of

  art to judge the heterogeneity of the print rendering or "mottling".

  A strip of paper is printed with a metal wheel covered with an ink film. After a time of one second, during which the paper fixes a certain amount of ink, the unbound ink is removed by performing a counter-printing: the printed surface is wiped with a metal wheel coated with rubber. This wiping operation is repeated 3 times so as to eliminate the loose ink. Printing and counterprints are performed

  at a speed of 0.5 m / s under a pressure of 800 kN / cm.

  Ideally, printing should result in a solid color distributed evenly across the paper. If this is not the case, there are heterogeneities of color, the "mottling", which is attributed to an irregularity in the structure of the layer (in the broad sense). The polymers according to the invention have an action on the homogeneity of the layer. Therefore, their effect is therefore judged according to the criterion

of "mottling".

  The assessment is made according to the practices of the profession. It consists in making a judgment on the homogeneity of the printing coloration determined by visual observation. A score is assigned on a scale of 0 to 5, 0 being assigned to a completely homogeneous impression, and 5 to an impression exhibiting very significant color fluctuations. A panel of 5 people performs the rating; the values given in Tables 1 to 4 are the arithmetic mean of the results. In this context, the results must be compared within the same series (each series is the subject of a separate table)

  taking as reference the value of the control bath (control 1).

  The mottling notes, depending on the nature of the thickener, are

  reported in Tables 1 to 4 below.

  Control thickener 1 copolymer 1 copolymer 1 + Nature thickener CMC PAA-g- (POE / POP / POE) PAA-g- (POE / POP / POE) + CMC Molar fraction of grafts 2.3% 2.3% + 0% Quantity of thickener 0.7 parts 2 parts 1 + 0.7 Dry extract of the bath 64.8% 54.7% 57.3% Weight of the layer 5.5 g / m2 4.5 g / m2 6.5 g / m2 Mottling score (ls) 2 I 0.2

Table 1

  Control thickener 1 control 2 copolymer 2a Nature CMC MC PAA-g- (POE / POP / POE) Molar fraction of grafts 1.8% Quantity of thickener 0.7 parts 0.4 parts 1 part Dry extract of the bath 63.3 % 63.3% 63.3% Layer weight 7.0 g / m2 10 g / m2 8.5 g / m2 "Mottling" rating (ls) 2.6 1.9 0.8

Table 2

  Pli Oj o CD rli Control thickener 1 copolymer 2b copolymer 2b Nature CMC PAA-g- (POE / POP / POE) PAA-g- (POE / POP / POE) Molar fraction of grafts 1.8% 1.8% Quantity of thickener 0.7 parts 0.9 parts 0.7 parts Dry extract of the bath 64% 64% 64% Layer weight 6.5 g / m2 4.5 g / m2 5.0 g / m2 "Mottling" rating "(1s) 4 2.4 2.6

Table 3

  Control thickener 1 copolymer 3 copolymer 3 Nature CMC PAA-g- (POE / POP) PAA-g- (POE / POP) Molar fraction of grafts 3% 3% o Quantity of thickener 0.7 parts 0.7 parts 1 part Dry extract of the bath 66% 66% 66% Weight of the layer 5.5 g / m2 7.4 g / m2 6.7 g / m2 "Mottling" score (1s) 1.2 0.4 0.6

Table 4

  OD o CD - The copolymers 1, 2 and 3, introduced into coating baths the formula of which is given above, lead to paper layers whose printing properties are better, with respect to the criterion of " mottling ", than those

  obtained using carboxymethylcellulose.

  - The copolymer 1 can be used in combination with carboxymethylcellulose and lead to an improvement in the printing properties,

  vis-à-vis the criterion of "mottling".

  - Copolymers 2 and 3 lead to an improvement in printing properties, vis-à-vis the "mottling" criterion, for mass rates less than or equal to 1 part (value compared to 100 parts of pigments) The results obtained with the copolymers of the invention are better with respect to the "mottling" criterion than with methylcellulose, which has the property of increasing viscosity with temperature but not that of a

  constant turbidity with temperature.

Claims (23)

  1. Thermo-thickening composition for coating baths comprising at least one thermosensitive copolymer characterized in that said copolymer has a comb structure and consists of a polymer segment called backbone to which are grafted at least two polymeric lateral segments, identical or different, with either the skeleton segment or the lateral segments having a lower critical solubility temperature, LCST, between and 80 C.   2. Thermo-thickening composition according to claim 1 characterized in that the polymer backbone has the critical temperature   lower solubility, LCST, between 30 and 80 C.   3. Thermo-thickening composition according to claim 1 characterized in that the polymeric lateral segments have the   lower critical solubility temperature, LCST, between 30 and 80 C.   4. Thermo-thickening composition according to one of claims 1 to 3   characterized in that it comprises several copolymers arranged together so as to form a crosslinked structure in which their polymeric segments having the lower critical solubility temperature, LCST, include the crosslinking nodes and at least a portion of their segments not having of lower critical solubility temperature between 300C and 80 C establish the connections between said nodes.   5. Thermo-thickening composition according to one of claims 1, 2,   3 or 4 characterized in that the segments not having the lower critical solubility temperature of between 30 C and 80 C are water-soluble   in at least this temperature range.   6. Thermo-thickening composition according to one of claims   previous characterized in that the polymer segments having no lower critical solubility temperature are water soluble in the range of   temperature of use of said thermo-thickening composition.   7. Thermo thickening composition according to one of claims   previous characterized in that the polymer segment having no   critical temperature is a water-soluble ethylenic type polymer.   8. Thermo-thickening composition according to claim 7 characterized in that said polymer derives from the (co) polymerization of water-soluble ethylene monomers of vinyl, acrylic, styrenic type,   diene and / or vinyl ester type.   9. Thermo thickening composition according to claim 7 or 8 characterized in that the monomers are chosen from (meth) acrylic acid, diacids such as fumaric acid, itaconic acid or their salts, maleic anhydride, l acrylamide and its derivatives such as acrylamido methyl propane sulfonic acid or their salts, styrene sulfonic acid, vinyl benzoic acid or their   salts, vinylsulfonic acid, methallylsulfonic acid or their salts.   10. Thermo thickening composition according to claim 9 characterized in that the monomers are acrylic or methacrylic acid, acrylamides and their derivatives, fumaric and maleic acids and sulfonated monomers such as 2-acrylamide-methyl-propane sulfonic acid (AMPS) and its alkaline salts and vinylsulfonate.   11. Thermo-thickening composition according to one of   Claims 7 to 10 characterized in that the polymer segments have   a molecular weight at least greater than 1,000 and preferably at least more than 20,000.   12. Thermo-thickening composition according to one of   Claims 7 to 11, characterized in that the polymer segments are   from the polymerization of acrylic acid (AA) and / or 2acrylamide acid- sulfonic methyl-propane (AMPS).   13. Thermo-thickening composition according to one of   previous claims characterized in that the polymer segments   having a lower critical LCST solubility temperature between 30   and 80 C are derived from polyoxyalkylene polymers.   14. Thermo thickening composition according to claim 13 characterized in that the one or the various oxyalkylene units present in the   polyoxyalkylene have at most 6 carbon atoms.   15. Thermo thickening composition according to claim 13 or 14 characterized in that the polymer segments having a lower critical solubility temperature consist of oxyethylene (OE) and / or oxypropylene (OP).   16. Thermo-thickening composition according to one of   Claims 13 to 15 characterized in that the segments having a   lower critical solubility temperature consist of at least 5 oxyalkylene units. 17. Thermo-thickening composition according to one of   previous claims characterized in that the copolymer comprises 0.1% to   molar% of polymer segments having a critical temperature   lower solubility (LCST) between 30 and 80 C.   18. Thermo-thickening composition according to one of   previous claims characterized in that the copolymer comprises 0.1% to   molar% of polymer segments having a critical temperature   lower solubility (LCST) between 30 and 80 C.   19. Thermo-thickening composition according to one of   previous claims characterized in that the copolymer is chosen from:   - A copolymer prepared from POE tri-block macromonomers -   POP-POE and acrylic acid (respective molar percentages: 2.3%, 97.7%).   - A copolymer prepared from macromonomer of tri-POE blocks-   POP-POE and acrylic acid (respective mol%: 1.6%, 98.4%) and   - A copolymer prepared from a macromonomer of POE diblocks-   POP and acrylic acid (respective mol%: 3%, 97%).   20. Coating bath for coating paper and / or cardboard comprising at least one thermo-thickening composition according to one of claims 1 to 18.   21. Use of a thermo-thickening composition or according to one   of claims 1 to 19 or of a coating bath according to claim 20 for   obtain better layer coverage for the coating of paper and cardboard.   22. Use of a thermo-thickening composition according to one   of claims 1 to 19 or of a coating bath according to claim 20 for   improve the layer cover for the coating of weighted paper and cardboard weak layer.   23. Use of a thermo-thickening composition according to one   of claims 1 to 19 or of a coating bath according to claim 20 for   improve the smoothness, opacity and / or shine of weight coated paper and cardboard weak layer.