US20180044858A1

US20180044858A1 - Coating composition comprising hydrolysed wheat proteins

Info

Publication number: US20180044858A1
Application number: US15/553,132
Authority: US
Inventors: Magalie PERALBA; Vittorio RIVOLTA; Sofie FREDERIX
Original assignee: Tereos Starch and Sweeteners Belgium
Current assignee: Syral Belgium NV; Tereos Starch and Sweeteners Belgium
Priority date: 2015-02-23
Filing date: 2016-02-23
Publication date: 2018-02-15
Also published as: BE1023282B1; FR3032979B1; FR3032979A1; BE1023282A1; CN107532390A; BR112017015729A2; JP2018511712A; CA2975499A1; EP3262232A1; EP3262232B1; WO2016135625A1

Abstract

A paper or cardboard coating composition having a dry matter content of between 45% and 80%, comprising i) binders which include at least one modified starch and one adhesive, ii) a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and iii) a mineral filler, and also the method for obtaining such a composition. A method for coating or brightening paper or cardboard, comprising the steps consisting of i) the provision of a composition as described, and ii) the depositing of said composition on a paper or cardboard substrate, and to the coated paper or cardboard thus obtained. Finally, the use of a hydrolyzed wheat protein in the replacement of latex in a paper or cardboard coating composition.

Description

This application claims the benefit of French patent application No. 15/00337, filed Feb. 23, 2015, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present application relates to a paper or cardboard coating composition comprising a modified starch and a hydrolyzed wheat protein, and also to the method for obtaining such a composition. The application also relates to a method for coating or brightening paper or cardboard using such a composition, and to the coated paper or cardboard thus obtained. Finally, the application relates to the use of a hydrolyzed wheat protein in the replacement of latex in a paper or cardboard coating composition.

TECHNICAL BACKGROUND

Coating is a paper or cardboard finishing step which makes it possible to confer on a sheet of paper or on a cardboard a certain number of properties, such as opacity, gloss or whiteness or else to improve the printability for printing processes such as offset printing or photogravure. During this coating step, a composition referred to as coating color is applied to the surface of the paper or of the cardboard. This composition generally comprises at least one mineral or pigment filler, at least one binder and other additives such as, in particular, dispersants, rheology modifiers, lubricants, optical brighteners or antifoams.
The paper industry uses many chemical products, such as surfactants, optical brighteners, water-resistance agents (ketone resin, anionic latex, etc.), to give paper various properties or to simplify the process for obtaining same. Latex, typically synthetic latex of styrene-butadiene type, is the binder most widely used. Its function is to enable cohesion between the various elements of the composition and to bind them to the fibers. Synthetic latex is produced from petroleum resources which are by definition non-renewable. In order to reduce the number of chemical products used in this industry, and to reduce the consumption of petroleum-derived products and also the costs, the replacement of latex in coating colors represents a potential market but also a sizeable technical challenge. This is because it is very difficult to maintain the performance levels of a coating color while reducing the proportion of synthetic latex used.
Numerous approaches have been tested, including the use of soy proteins (US 2006/174801). However, these products resulted in the obtaining of extremely viscous coating colors or coating colors that have to be used at very low dry matter contents (about 38% to 44% DM) in order to compensate for this viscosity or to ensure dissolving of these products.
The prior art also describes the use of cold-soluble starch in such compositions; mention may for example be made of international patent application WO 08/104574. However, these starches have a tendency to form aggregates during the dissolving thereof; they also require the use of a high-shear mixer with which most paper manufacturers are not equipped.
Moreover, it should be noted that coating colors are intended to be applied to the surface of the paper or of the cardboard in very thin layers and at very high speed. Their application is carried out by means of a blade or a threaded rod which exerts very high shear forces at the surface of the paper. Thus, in the case of non-uniformity or of excess viscosity, these shear forces lead not only, at the level of the application zone, to turbulences responsible for defects in the deposit, called “filaments” or “beads”, but also to an increase in the pressure exerted on the paper, increasing by the same token the risks of breakage and thus potentially hours of production interruption.
In order to solve this technical problem, the prior art proposes coating compositions which have a low dry matter content. However, the reduction of the dry matter of the composition (and thus the increase in the water content) in order to reduce the viscosity thereof is not an advantageous solution in the present application. This is because the coating color has a natural tendency to transfer into the sheet of paper all or some of the water and of the water-soluble parts that it contains. This has several consequences, the first of which is the weakening of the paper or of the cardboard which, due to the excess water, can lose its integrity and can lead to breaking of the paper produced and thus hours of production interruption. The second is the loss of gloss of the paper observed following the migration of the water-soluble parts of the coating color into the paper. This migration leads to a third consequence which is the weakening of the cohesion of the layer of the paper, leading to problems during printing. Mention may, for example, be made of deposits of fibers or of mineral fillers originating from the coat on the blankets during offset printing. The final consequence of this excess water of the coating composition is the increase in energy and/or in time required to dry the paper or the cardboard obtained.
In addition, the advantage of a coating color which has both a high dry matter content and a low viscosity, beyond the solving of the abovementioned problems, is the small amount of coating color that needs to be laid. In addition, a coating color of low viscosity would also have the advantage of allowing very-high-speed coating, which constitutes a very clear industrial advantage.
Moreover, among the approaches envisioned in the replacement of latex, none makes it possible to improve or at the very least maintain the pick resistance capacities of the paper or cardboard obtained. In point of fact, a strong pick resistance guarantees preservation of the integrity of the paper or cardboard when a force is exerted at its surface and thus a wider use of the latter. This property is essential in printing, in particular offset printing where the paper is subjected to high stresses at the output of the inking rollers. This is because, in this step, the film of ink separates into 2 parts, one remaining on the paper and the other on the blanket. During this separation, a normal force is exerted on the paper leading to picking of the particles (mineral fillers or fibers) poorly bonded to one another or to the paper and the depositing thereof on the blankets. This phenomenon is responsible for the fouling of the blankets and can make it necessary for the printer to interrupt production in order to clean them.
There is therefore currently no coating composition which has both a reduced latex content beyond what the prior art describes, a very high dry matter content and a low viscosity while at the same time conferring a high pick resistance on the paper or cardboard obtained.

DETAILED DESCRIPTION OF THE INVENTION

A new paper or cardboard coating composition has a dry matter content of between 45% and 80%, preferentially 50% and 78%, more preferentially 55% and 75%, comprising:

binders which include at least one modified starch and one preferentially synthetic adhesive,
a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and
a mineral filler.

The term “paper or cardboard coating composition” is intended to mean a composition that is particularly suitable for coating paper or cardboard. It is an aqueous formulation conventionally containing water, at least one mineral filler, one or more binders and also various additives.
The composition may comprise, in parts per 100 parts by weight of mineral filler:

1 to 99 parts of binders, preferentially 1 to 50 parts, more preferentially 1 to 15 parts, and
1 to 50 parts by weight of wheat protein, preferentially 1 to 20 parts, more preferentially 1 to 8 parts.

The term “wheat protein” denotes a water-insoluble protein fraction extracted from wheat flour via the wet process and subsequently dried, also known as wheat gluten. Typically, wheat proteins having an average molecular weight of between 7 and 1000 kDa are obtained by hydrolysis according to methods known well to those skilled in the art [Anfinsen, C. B. Jr. (1965), Advances in Protein Chemistry: v. 20, New York and London, Academic Press]. Typically, the hydrolysis may be thermal, acid or enzymatic. Enzymatic hydrolysis is preferred.
A wheat gluten that is particularly suitable is Solpro® 508 sold by Tereos Syral.
In some embodiments, the hydrolyzed wheat protein has a weight-average molecular weight of between 7 and 800 kDa, 5 and 500 kDa or 8 and 100 kDa, preferentially between 9 and 80 kDa, more preferentially between 10 and 70 kDa, even more preferentially between 12 and 50 kDa, even more preferentially between 13 and 40 kDa.
The term “binder” is intended to mean a compound having the function of sticking the particles of mineral filler (or pigments) to one another and of maintaining the coat at the surface of the paper.
In some embodiments, the composition comprises a binders/wheat protein ratio of from 1:5 to 5:1, preferentially from 1:3 to 3:1, more preferentially 1:2 to 2:1.
In some embodiments, the binders are at least one modified starch and one adhesive such as a synthetic adhesive; typically in a modified starch/synthetic adhesive ratio of from 1:5 to 5:1, preferentially from 1:3 to 3:1, more preferentially 1:2 to 2:1.
For the purposes of the present invention, the term “modified starch” is intended to mean any starch that has been chemically or physically treated.
The molecules of modified starches can originate from a plant source such as cereals, tubers, roots, vegetables and fruits. Thus, the starch(es) can originate from a plant source chosen from corn, peas, potato, sweet potato, banana, barley, wheat, rice, oats, sago, tapioca and sorghum.
More particularly, the modification reactions can be carried out for example:

by pregelatinization by causing the starch granules to burst (for example drying and cooking in a drying drum);
by acid hydrolysis using for example strong acids or by enzymatic hydrolysis;
by oxidation using for example strong oxidizing agents resulting in the introduction of carboxyl groups into the starch molecule and in the depolymerization of the starch molecule (for example by treating an aqueous starch solution with sodium hypochlorite);
by crosslinking with functional agents capable of reacting with the hydroxyl groups of the starch molecules which will thus be bonded to one another (for example with glyceryl and/or phosphate groups); phosphorus-containing compounds of monostarch phosphates (of the St-O—PO—(OX)₂type), of distarch phosphates (of the St-O—PO—(OX)—O-St type) or even of tristarch phosphates (of the St-O—PO—(O-St)₂type) or mixtures thereof are for example obtained. X denotes for example alkali or alkaline earth metals;
by esterification in an alkaline medium for the grafting of functional groups, in particular C₁-C₆acyl (acetyl), C₁-C₆hydroxyalkyl (hydroxyethyl, hydroxypropyl), carboxymethyl or octenylsuccinic groups. Mention may in particular be made of sodium carboxymethyl-modified starches;
by dextrinization, such as for example by a dry-phase treatment of a native starch, by the action of heat, in an optionally dry medium, optionally in the presence of a chemical agent, by the action of heat or a combination of these various means. Most of these methods, whether they are batchwise or continuous, call for transformation temperatures higher than 80° C. and for the optional presence of an acid, of an alkaline agent and/or of an oxidizing agent.

Suitable modified starches comprise, but are not limited to, pregelatinized starches, low-viscosity starches (for example, dextrins, hydrolyzed starches, oxidized starches), stabilized starches (for example, starch esters, starch ethers), crosslinked starches and starches which have received a combination of treatments (for example crosslinking and gelatinization treatments) and mixtures thereof
Dextrins are the preferred modified starches. For the purposes of the present invention, the term “dextrin” is intended to mean a modified starch obtained from native starch by dextrinization; typically, the dextrins according to some embodiments of the invention are not subjected to any other modification, in particular chemical modification. The dextrins that are suitable for some embodiments of the present invention are, for example, white dextrins, generally obtained by transformation of starch at temperatures often of between 100 and 170° C., in the presence of a chemical agent or chemical agents, in particular of acid, in relatively high amounts; yellow dextrins, often obtained by transformation of starch at higher temperatures, generally of between 170 and 230° C., in the presence of a chemical agent or chemical agents, in particular of acid; finally, dextrins termed “British Gum” obtained solely by the action of heat, at high temperature, often above 230° C. A dextrin that is particularly suitable for some embodiments of the present invention is a wheat-based dextrin, typically the Mylofilm® 214 or Mylofilm® 218 dextrin sold by the company Tereos Syral.
In some embodiments of the present invention, the particularly suitable modified starch has a weight-average molecular weight of between 20 and 300 kDa, preferentially 30 and 250 kDa, more preferentially between 35 and 233 kDa, even more preferentially between 40 and 200 kDa, even more preferentially between 42 and 150 kDa, and/or a viscosity of between 50 and 400 mPa·s (Brookfield, 70° C., 31% DM). The measurement, with a Brookfield viscometer, of the modified starch, such as, for example, of a dextrin, is performed in solution and is carried out on an RVDV-E model, the measurement is carried out at a speed of 20 rpm with spindle 3. The measurements are carried out at 70° C. The module is dipped in a composition of modified starch in suspension at 31% of dry matter up to the indicator line of the spindle; the value is recorded after 10 s of rotation.
The term “average molecular weight” is intended to mean the weight-average molecular weight.
In the context of the hydrolyzed protein, this average molecular weight is measured by size exclusion chromatography (SE-HPLC) coupled with a UV detector regulated at the wavelength of 214 nm. The size exclusion chromatograph is equipped with a pump which circulates an eluent composed of a PBS phosphate buffer (0.1 M Na₂HPO₄—NaH₂PO₄with 0.1% of SDS) at a flow rate of 0.7 ml/min in a TSKG4000SWx1 column. This measurement is expressed in daltons. The preparation of a sample can be carried out by dissolving the product studied in a phosphate extraction buffer with 1% of SDS, followed by centrifugation to recover the supernatant.
In the context of the modified starch and more particularly of dextrin, the average molecular weight is expressed in daltons and can be determined by those skilled in the art by size exclusion chromatography coupled with a detector of MALLS (Multi Angle Laser Light Scattering) type. The preparation of a sample can be carried out by dissolving 50 mg by dry weight of a modified starch and in particular of dextrin in a solvent consisting of a mixture of 90% (v/v) of DMSO (dimethyl sulfoxide) in deionized water containing 0.1% (w/v) of sodium nitrate. After stirring overnight, the mixture is preheated for 1 hour at 105° C. and then centrifuged for 15 min at 5300 g. A volume of 100 ml of the supernatant is injected into a size exclusion chromatography apparatus, the mobile phase of which is for example composed of a mixture of 90% (v/v) of DMSO (dimethyl sulfoxide) in deionized water containing 0.1% (w/v) of sodium nitrate, at a flow rate of 0.5 ml/min and a temperature of 70° C., the columns used preferentially being a combination in series of Gram columns. The detector is for example an angle laser such as the New Generation 3-angle miniDawn Treos. The calibration is carried out with Viscotec P82 Shodex standards. In the context of the present invention, the adhesive is preferentially synthetic. An example of synthetic adhesive suitable for the present invention is a latex, a vinyl acetate, polyvinyl alcohol, sodium carboxymethylcellulose and hydroxyethylcellulose.
The term “latex” refers to an aqueous dispersion of polymer which corresponds to a colloidal dispersion of synthetic polymers in an aqueous phase, i.e. a dispersion of polymer microparticles in suspension in an aqueous phase, sometimes also referred to as polymer suspension or polymer emulsion. Examples of latex suitable for the present invention are chosen from the group made up of styrene-butadiene latexes, polyvinyl alcohol latexes and acrylic copolymer latexes, preferentially latex of styrene-butadiene type.
In general in the coating color, the mineral filler introduced is carried in the form of an aqueous suspension. Conventionally, this filler is a calcium carbonate suspended in water by means of a dispersing agent. Typically, a mineral filler that is particularly suitable for a coating composition comprises a sufficient degree of whiteness (greater than 80% of the whiteness of barium sulfate at 457 nm), a particle size distribution of from 0 to 10 μm at most (the average particle size being between 0.2 and 2 μm) and a minimum degree of agglomeration of the particles. For example, the mineral filler can be chosen from the group made up of calcium carbonates, coating clay, calcined fine clay, alumina trihydrate, talc and titanium dioxide.
The term “calcium carbonate” comprises ground calcium carbonate (GCC), that is to say a calcium carbonate obtained from natural sources, such as limestone, marble, calcite or lime. The term “calcium carbonate” also comprises precipitated calcium carbonate (PCC), that is to say a synthesized substance, generally obtained by precipitation following a reaction of carbon dioxide and calcium hydroxide (hydrated lime) in an aqueous environment or by precipitation of a source of calcium and of carbonate in water.
The composition according to some embodiments of the invention may also comprise other agents such as one or more dispersing agents. The term “dispersing agent” is intended to mean an agent having the function of maintaining the particles of mineral filler in a state of electrostatic dispersion. By way of example, the dispersing agent is chosen from the group made up of sodium polyacrylate, tetrasodium polyphosphate, tetrasodium pyrophosphate, pentasodium tripolyphosphate, sodium tetraphosphate and sodium silicate.
The composition may also comprise at least one lubricant, typically chosen from the group made up of sodium stearate, calcium stearate, sulfonated oils, sulfated tall oil fatty acid and polyethylene emulsions.
The composition may also comprise at least one insolubilizing agent chosen from the group made up of urea resins, melamine resins, glyoxal, zinc compounds, formaldehyde and dimethylolurea.
A new method for producing the composition described may comprise the following steps:

mixing, with stirring, a mineral filler and binders which include at least one modified starch, preferentially a dextrin, and one preferentially synthetic adhesive, said modified starch preferentially having previously undergone a cooking step,
adding, with stirring, to the mixture obtained, a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, said hydrolyzed wheat protein preferentially being in a pulverulent form,
adding water, with stirring, so as to obtain a composition having a dry matter content of between 45% and 80%; optionally, the adding of water is carried out with the mineral filler and/or the modified starch and/or the hydrolyzed protein. Typically, during the method for producing the composition according to the invention, said modified starch such as in particular a dextrin can be dissolved in the water, and can preferentially undergo a cooking step, prior to it being mixed with the adhesive. Independently, the mineral filler can be dissolved beforehand in the water before being mixed with the binders.

A new method for coating or brightening paper or cardboard may comprise the steps of

- (a) the provision of a composition as described,
- (b) the depositing of said composition on a paper or cardboard substrate.

The step of depositing said composition on a paper or cardboard substrate can be carried out by means of blade coating, pencil coating, a threaded rod, size press curtain coating or a film press or any other technique known to those skilled in the art. Typically, the depositing step is carried out at a temperature of between 25 and 60° C.
Typically, said composition is applied to at least one face of said paper or cardboard substrate in an amount of between 3 g/m²and 15 g/m², preferably between 5 g/m²and 10 g/m².
A new paper or cardboard is coated with a composition as described.
A new use of a hydrolyzed wheat protein involves the replacement of latex in a paper or cardboard coating composition, said hydrolyzed wheat protein preferentially having an average molecular weight of between 7 and 1000 kDa.
A combination of a hydrolyzed wheat protein and of a modified starch, and more particularly a dextrin, may be used in the replacement of latex, preferentially in a modified starch/wheat protein ratio of 1:5 to 5:1, preferentially of 1:3 to 3:1, more preferentially 1:2 to 2:1.
Typically, said wheat protein or said combination of wheat protein with a modified starch, preferentially a dextrin, is used in the replacement of from 1% to 40% of the latex of said composition, preferentially from 10% to 35%, more preferentially from 15% to 30%.
Although they have distinct meanings, the terms “comprising”, “containing” and “consisting of” have been used interchangeably in the description of the invention, and can be replaced by one another.
The invention will be understood more clearly on reading the following examples given only by way of illustration.

EXAMPLES

Example 1

Replacement of Latex by a Modified Starch

Production of Coating Colors:

Preliminary tests showed that the best results were obtained with a dry matter content greater than 45% (machinability and energy yield). A clearer improvement is observed from 60%. Indeed, an increase in the breaking of the paper when the coating color is diluted (less than 45% of DM) was noted. Furthermore, an increase was noted in the drying time below 60% and even more below 45%, which appears to be the lowest acceptable dry matter content. Since the paper undergoes a drying step after coating, any excess water in the coating color results in an increase in the drying times and thus in the production cost. Consequently, the tests were continued with coating colors having a dry matter content of 70%.
A coating color was produced according to formulae R1 to R4 of table 1 below.

TABLE 1

Coating compositions
comprising dextrin as partial
replacement for latex

Calcium	Synthetic
carbonate	binder	Dextrin

R1	100	3.5	3.5
R2	100	3	4
R3	100	2.5	4.5
R4	100	2	5

The recipes are given in number of parts (as is standard practice in papermaking).
The dextrin (Mylofilm® 214) is a wheat-based dextrin (Mw=47 kDa, Wt=11.6) sold by Tereos Syral. It is first of all cooked at a concentration of 35% dry matter on a jet cooker (Temp.=130° C., residence time: 3 min), then diluted to 31%.
The coating color is produced using a stirrer (IKA type) first of all by suspending the calcium carbonate in water at 79.7% (Hydrocarb® 90 supplied by the company Omya). The synthetic binder (styrene-butadiene DL930 latex from the company Styron) and also the dextrin dissolved as previously specified are then added to the calcium carbonate. The concentration is adjusted with water in order to obtain a dry matter content of 70%. The stirring speed is adjusted to 1500 rpm; the pH is then adjusted to 9. The coating color is thus stirred for 10 min.
The coating colors of table 1 were tested in coating tests.
The viscosity of the coating colors is evaluated before coating of the paper.

Brookfield Viscosity

The measurement of the coating color with a Brookfield viscometer is carried out on an RVDV-E model; the measurement is carried out at a speed of 20 rpm with spindle 3. The measurements are carried out at 40° C. The module is dipped in the coating color up to the indicator line of the spindle, and the value is recorded after 10 s of rotation.

Paper Coating

The coating color is deposited on the paper in an amount of 6 g/m²on a single face using a DT blade coater coating pilot plant allowing drying combining infrared radiation and hot air. The coating speed is 20 m/min.
The paper used is an 80 g/m²thin paper supplied by the company Fedrigoni.
The paper thus coated is then stored in a humidity- and temperature-conditioned room (50% humidity, 23° C.) for 24 h before any test.

Dry IGT Pick Resistance

The dry pick measurement is carried out according to the IGT W31 method (ISO 3783:2006). This measurement makes it possible to evaluate the strength of the layer. Indeed, the binders (synthetic or natural such as starch) are used to maintain the mineral fillers required for the paper printing properties. If the binding capacity is too low, the mineral fillers are picked from the paper during printing and are deposited on the inking roll, leading to frequent interruptions. The higher the dry IGT measurement, the higher the pick resistance of the layer.

Results

The analysis of the characteristics of the paper obtained made it possible to demonstrate that replacing the latex with dextrin in solution leads to a loss of the pick properties (table 2).

TABLE 2

Brookfield viscosity of
the coating compositions
R1 to R4 and pick properties
of the papers obtained

	Brookfield	Dry
	viscosity	pick
	(mPa · s)	(m/s)

R1	125	0.45
R2	140	0.40
R3	145	0.37
R4	155	0.19

In fact, replacing the latex with dextrin alone does not make it possible to maintain the characteristics of the coating and thus leads to a clear decrease in the pick properties of the layer. Thus, dextrin alone does not make it possible to compensate for the reduction in latex in the coating color.

Example 2

Replacement of Latex with Hydrolyzed Soy or Wheat Proteins

The use of plant proteins as a replacement for latex was evaluated. After several tests, it was noted that the addition of proteins in addition to the dextrin makes it possible to observe better results in the replacement of latex than those observed for the dextrin alone, more particularly regarding the dry pick properties of the composition obtained. This effect was observed only for hydrolyzed proteins, not for native proteins. Thus, nonhydrolyzed wheat gluten, because of its low solubility, does not make it possible to obtain uniform coating colors of acceptable viscosity, and even less so the replacement of latex. In order to evaluate the effect of replacing the latex with hydrolyzed proteins of various botanical origins, the R1 mixture was chosen as reference recipe for a coating color.
The coating colors were produced as in example 1 according to the formulae of table 3 in which the synthetic latex is replaced with proteins in an amount of 14% (R5), 30% (R6) and 43% (R7).

TABLE 3

Coating compositions
comprising dextrin and proteins
as partial replacement for latex

Calcium	Synthetic
carbonate	binder	Dextrin	Protein

R1	100	3.5	3.5	—
R5	100	3	3.5	0.5
R6	100	2.5	3.5	1
R7	100	2	3.5	1.5

In order to reduce the amounts of latex in the coating compositions, hydrolyzed wheat and soy proteins were tested on the basis of the proportions provided in table 3.
The proteins tested are the following:

hydrolyzed wheat proteins having a low molecular weight (Mw=5.7 kDa),
hydrolyzed wheat proteins having a medium molecular weight of 15.5 kDa (Solpro® 508 sold by Tereos Syral) and
hydrolyzed soy protein (Sobind LVL from DuPont).

The proteins are added to the coating color without prior dilution.
The coating color is produced as in example 1, using a stirrer (IKA type), by suspending the calcium carbonate in water at 79.7% (Hydrocarb® 90 supplied by the company Omya). The synthetic binder (styrene-butadiene DL930 latex from the company Styron) and also the dextrin having been dissolved are then added to the calcium carbonate as previously specified. In this step, the protein is incorporated into the color in solution form or as powder, as appropriate. The concentration is adjusted with water in order to obtain a dry matter content of 70%. The stirring speed is adjusted to 1500 rpm; the pH is then adjusted to 9. The coating color is thus stirred for 10 min.

TABLE 4

Brookfield viscosity of the coating compositions in which 14%,
30% and 43% of latex are replaced with a mixture of dextrins and
hydrolyzed proteins and pick property of the papers obtained

Hydrolyzed wheat proteins

	Hydrolyzed	Low molecular	Medium molecular
	soy proteins	weight	weight

Brookfield

(Mw = 5.7 kDa)

(Mw = 15.5 kDa)

viscosity	Dry pick	Brookfield	Dry	Brookfield	Dry
(mPa · s)	(m/s)	viscosity	IGT	viscosity	IGT

R1	125	0.45	125	0.45	125	0.45
R5	254	0.49	128	0.50	147	0.60
R6	636	0.33	218	0.32	168	0.42
R7	853	0.25	230	0.24	182	0.25

On reading the results (table 4), it is noted that replacing the latex at more than 40% with any one of the hydrolyzed proteins tested does not make it possible to maintain the pick resistance characteristics provided by the latex. Only the partially hydrolyzed soy and wheat protein hydrolyzates enable a replacement that can reach 35% (R6).
However, replacing the latex with the hydrolyzed soy protein leads to a clear increase in the viscosity starting from 14% (R1). This increase is accentuated at 30% (R6) and 42% (R7), making the coating composition difficult to use. Such a viscosity does not allow application at the industrial level since it would result in a considerable increase in pressure during the depositing, leading to machinability and paper quality problems.
Conversely, the low-molecular-weight wheat protein hydrolyzates have only a limited effect on the viscosity of the composition obtained but do not make it possible to compensate for the reduction in latex starting from 30%. Indeed, at 30% replacement of the latex, a loss of pick resistance is observed (0.45 of IGT at 0% replacement; 0.5 at 14% replacement for 0.30 at 30% replacement).
Among the various hydrolyzed proteins tested, only the partially hydrolyzed wheat proteins allow a significant increase in pick resistance at 14% latex replacement (composition R5).
Finally, only the partially hydrolyzed wheat proteins confer a maintaining both of the pick characteristics and also of the viscosity characteristics. Indeed, 30% replacement of the latex (R6) occurs with the pick resistance properties and also the viscosity characteristics of the composition obtained being maintained (0.45 m/s for 0 part replaced (R1) compared with 0.42 m/s for 30% replaced (R6)).
In addition, the advantage of the hydrolyzed wheat proteins is that, contrary to the soy proteins, they are soluble enough to be directly added to the coating color and do not require a prior dilution adding a not insignificant amount of water and thus reducing the dry matter content of the coating color, thereby making it possible to vary quite freely the dry matter content of the composition.

Claims

1. A paper or cardboard coating composition having a dry matter content of between 45% and 80% comprising:

binders including at least one modified starch and one synthetic adhesive,

a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and

a mineral filler.

2. The composition as claimed in claim 1, characterized in that the hydrolyzed wheat protein has an average molecular weight of between 8 and 100 kDa.

3. The composition as claimed in claim 1, characterized in that it comprises, in parts per 100 parts by weight of mineral filler:

1 to 99 parts of binders, and

1 to 50 parts by weight of wheat protein.

4. The composition as claimed in claim 1, characterized in that it comprises a modified starch/synthetic adhesive ratio of from 1:5 to 5:1.

5. The composition as claimed in claim 1, characterized in that it comprises a binders/wheat protein ratio of from 1:5 to 5:1.

6. The composition as claimed in claim 1, characterized in that the modified starch is chosen from a pregelatinized starch, a dextrin, a hydrolyzed starch, an oxidized starch, a starch ester, a starch ether, a crosslinked starch and mixtures thereof.

7. The composition as claimed in claim 6, characterized in that the dextrin modified starch comprises a dextrin and the dextrin has a molecular weight of between 20 and 300 kDa, and/or a viscosity of between 50 and 400 mPa·s.

8. The composition as claimed in claim 1, characterized in that the synthetic adhesive is chosen from the group made up of latexes, vinyl acetates, polyvinyl alcohol, sodium carboxymethylcellulose and hydroxyethylcellulose.

9. The composition as claimed in claim 1, characterized in that the synthetic adhesive is a latex preferentially chosen from the group made up of styrene-butadiene latexes, polyvinyl alcohol latexes and acrylic copolymer latexes.

10. A method for producing a paper or cardboard coating composition, comprising the following steps:

mixing, with stirring, a mineral filler and binders which include at least one modified starch and one synthetic adhesive,

adding, with stirring, to the mixture obtained, a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and

adding water, with stirring, so as to obtain a composition having a dry matter content of between 45% and 80%.

11. A method for coating or brightening paper or cardboard, said method comprising the steps consisting of

(a) the provision of a composition having a dry matter content of between 45% and 80% and comprising binders, including at least one modified starch and one synthetic adhesive, a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and a mineral filler, and

(b) the depositing of said composition on a paper or cardboard substrate.

12. A paper or cardboard coated with a composition having a dry matter content of between 45% and 80% and comprising binders, including at least one modified starch and one synthetic adhesive, a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and a mineral filler, or a composition obtained by mixing, with stirring, a mineral filler and binders which include at least one modified starch and one synthetic adhesive, adding, with stirring, to the mixture obtained, a hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa, and adding water, with stirring, so as to obtain a composition having a dry matter content of between 45% and 80%.

13. A method of using a hydrolyzed wheat protein, comprising substituting the hydrolyzed wheat protein for latex in a process for making a paper or cardboard coating composition, said hydrolyzed wheat protein having an average molecular weight of between 7 and 1000 kDa.

14. The method of use as claimed in claim 13, characterized in that said composition comprises:

binders which include at least one modified starch and one preferentially synthetic adhesive,

a mineral filler.

15. The method of use as claimed in claim 13, characterized in that the binders/wheat protein ratio is from 1:5 to 5:1.

16. The composition as claimed in claim 1, wherein the composition has a dry matter content of between 50% and 78%.

17. The composition as claimed in claim 1, characterized in that the hydrolyzed wheat protein has an average molecular weight of between 9 and 80 kDa

18. The method of claim 10, wherein said modified starch has previously undergone a cooking step.

19. The method of claim 10, wherein said hydrolyzed wheat protein is in a pulverulent form.

20. The method of claim 10, wherein the adding of water is carried out with the mineral filler and/or the modified starch and/or the hydrolyzed protein.