FR2861706A1 - METHOD FOR CONTROLLING HEAT CLEARANCE IN A SELF-ADHESIVE SYSTEM BY WATER ADDITION - Google Patents

Abstract

The present invention relates to a method for controlling the release of heat in a self-heating system triggered by addition of water.This method for controlling the temperature in a self-heating system triggered by addition of water, is characterized in that a sufficient quantity of at least one water-soluble heat-thickening polymer is added to the self-heating system. Preferably this addition is carried out in the aqueous part of the self-heating system triggered by the addition of water.

Description

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 The present invention relates to a method for controlling the release of heat in a self-heating system triggered by the addition of water.

The field of application of the invention relates to self-heating systems. There are several types of self-heating systems that are triggered by the addition of water.

By way of example, mention may be made of systems based on the hydration of anhydrides, such as, for example, CaO as described in US Pat. No. 4,809,673, CaO / P205 as described in US Pat. No. 5,935,486, MgCl 2 / CaO, CaCO3, CaCl2.

There may also be mentioned self-heating systems based on the initiation of an oxidation-reduction reaction by addition of water, as described in US 5,984,953. By way of example, mention may be made of the iron + magnesium reaction or a self-heating system based on permanganate. In such systems, once water is added to the self-heating system, the temperature rises to the boiling point of water. This behavior is acceptable for applications where you want to heat a complete meal for example, but for applications where you want to heat a drink (coffee, tea) or soup, the final temperature of the liquid must be less than about 70 C for security reasons. If the temperature is above about 70 C, you risk burning your lips and mouth. So, for liquid applications (ie drinks, soup etc.) existing self-heating systems are not satisfactory.

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One of the aims of the present invention is to find a means of controlling the temperature in a self-heating system triggered by the addition of water.

In particular, one of the aims of the present invention is to find a means of controlling the temperature in a self-heating system triggered by addition of water so that the maximum temperature obtained does not exceed 70 C.

Another object of the present invention is to find a means of controlling the temperature in a self-heating system triggered by addition of water which does not degrade the total amount of heat released by the self-heating system, but which can for example prolong the self-heating effect over time.

Another object of the present invention is to find a means of controlling the temperature in a self-heating system triggered by addition of water which is not sensitive to variations in the pH in the reaction medium.

These and other objects are achieved in the present invention which relates to a method for controlling the temperature in a self-heating system triggered by the addition of water, characterized in that a sufficient quantity of water is added to the self-heating system. at least one thermo-thickening polymer soluble in water.

This addition may be carried out either in the aqueous phase, the self-heating system, or in the solid phase of the self-heating system. Preferably, the addition of the thermo-thickening polymer soluble in water is carried out in the aqueous phase.

Thus, the subject of the present invention is more particularly a method for controlling the temperature in a self-heating system triggered by the addition of water, characterized in that a sufficient quantity of at least one thermoplastic polymer is added to the aqueous phase. thickener soluble in water.

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By self-heating system triggered by addition of water is meant self-heating systems such as those described above.

By thermo-thickening polymer soluble in water is meant a polymer comprising segments or unitary units which remain hydrophilic and therefore water-soluble regardless of the temperature, and so-called thermosensitive unit segments or units having a lower critical solubility temperature (LCST ).

When the temperature exceeds the lower critical temperature of solubility (LCST) also called cloud point, the so-called thermosensitive unit segments or units become hydrophobic (expulsion of bound water molecules) and therefore associate with each other by hydrophobic-hydrophobic interactions. A physical gel is thus obtained at temperatures above the LCST.

This gel persists as long as the temperature is maintained beyond the LCST; if the temperature is decreased below, then it disappears and the solution returns to its initial viscosity level: the phenomenon is totally reversible.

In the context of the present invention, it is possible to use a thermo-thickening polymer comprising so-called thermosensitive unit segments or units having a lower critical solubility temperature (LCST) of between 20 ° C. and 70 ° C.

Preferably, use is made of a thermo-thickening polymer comprising so-called thermosensitive unit segments or units having a lower critical solubility temperature (LCST) of between 30 ° C. and 50 ° C. Even more preferably, a heat-thickening polymer comprising segments or patterns is used. so-called thermosensitive units having a lower critical solubility temperature (LCST) of between 30 C and 40 C.

By way of example of so-called thermosensitive segments or unitary units, mention may be made of: polyethers

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 - polyethylene oxide (POE) - random ethylene oxide / propylene oxide (EO / PO) copolymers - POP-POE diblocks - POE-POP-POE triblocks - alkyl-POE - poly (methyl vinyl ether) - hydroxylated polymers: - poly (hydroxypropyl acrylate) (polyHPA) and its copolymers - partially hydrolyzed vinyl polyacetate (PVAC) - methylcellulose - hydroxyethylcellulose - hydroxypropylcellulose - hydroxypropylmethylcellulose, polymers containing substituted amide units: poly (N-substituted acrylamides), for example poly N-isopropyl acrylamide (polyNIPAM) -poly (N-vinyl caprolactam), poly (N-acryloyl pyrrolidine), poly (N -acryloyl piperidine), poly (ethyl oxazoline), or a mixture thereof.

All these synthetic polymers have an LCST or cloud point, which depends on their molecular weight, their exact composition, their structure, and the presence of additives.

By way of example of segments or unitary units which remain hydrophilic and therefore water-soluble whatever the temperature, mention may be made of: unitary segments or units having carboxylic functions, such as acrylic acid or methacrylic acid units, segments or unitary units precursors of carboxylic functions, such as maleic anhydride units,

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 segments or unitary units having sulphonate functional groups, such as AMPS, styrene sulphonate, sulphoethyl methacrylate, sulphopropyl acrylate or sulphopropyl methacrylate units, hydroxyl unit segments or units such as polyvinyl alcohol or hydrophobic unmodified polysaccharides, that is to say non-grafted with alkyl groups such as ethyl, propyl, hydroxyethyl, hydroxypropyl, as well as derivatives of more hydrophilic polysaccharides such as cellulose, carboxymethyl cellulose, guars and in particular cationic guars, - segments or unitary units based on acrylamide or acrylamide derivatives such as dimethylacrylamide, phosphonate unit segments or units such as vinylphosphonic acid, phosphate unit segments or units such as phosphatoethylmethacrylate, or a mixture thereof.

Preferably, a thickening polymer comprising, as water-soluble unit segments or unitary units, carboxylic unit segments or units, precursors of carboxylic functions, sulphonates, phosphonates or phosphates, is used. the sulphonate, phosphonate or phosphate units or unitary units also have the advantage of being well soluble in water over a wide pH range.

It is also conceivable in the context of the present invention that the segment not having the LCST between 20 and 70 C is nevertheless equipped with an LCST. However, it is then necessary that in the temperature range considered (20-70 C), this segment is water soluble. This implies that its LCST is greater than 70 C.

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The polymer comprising segments or unitary units that remain hydrophilic and therefore water-soluble regardless of the temperature, and so-called heat-sensitive unitary segments or units is a copolymer that may be of linear structure, or comb or grafted or plugged. This copolymer can be random, alternating, sequenced or block.

When the thermosensitive copolymer has a comb structure, it consists of a so-called skeletal polymer segment to which at least two identical or different polymeric side segments are grafted with either the backbone segment or the side segments having a lower critical temperature of solubility, LCST, between 20 and 70 ° C.

According to a first variant of the invention it is the so-called skeletal polymer segment which has a lower critical solubility temperature, LCST, of between 20 and 70 ° C.

According to a second variant of the invention which is the preferred one, it is the polymeric side segments which have a lower critical solubility temperature, LCST, of between 20 and 70 C.

Finally, according to another variant of the invention, several copolymers as defined above are used. These copolymers are arranged therebetween so as to form a crosslinked structure in which their polymer segments having the lower critical temperature of solubility, LCST, are the crosslinking nodes and at least a portion of their segments having no critical temperature. lower solubility between 20 and 70 C establish connections between said nodes.

According to these variants, the segment which does not have the required LCST, namely between 20 and 70 ° C., is, for its part, water-soluble at least in this temperature range.

The grafting of the lateral polymeric segments on a so-called skeletal polymer segment may 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 of those called direct grafting and copolymerization.

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

For its part, the copolymerization implements in a first time the grafting at the end of the thermosensitive segment of a radical (co) polymerizable function. 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 is obtained.

It is obvious to those skilled in the art that when a macromonomer and a monomer are copolymerized so that these two species associate strongly with hydrogen bonds, then simultaneously there is direct grafting on the polymeric 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 resulting structure is substantially more connected or even crosslinked 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) of 20 to 70 ° C.

The following thermo-thickening copolymers are particularly suitable for the invention: a copolymer prepared from POE-POP-POE tri-block macromonomer and acrylic acid (respective molar percentages: 2.3%, 97.7% ), preferably by direct grafting, - A copolymer prepared from POE-POP-POE tri-block macromonomer and acrylic acid (respective mol%: 1.6%, 98.4%), preferably by copolymerization and or

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 A copolymer prepared from POE-POP di-block macromonomer and acrylic acid (respective mol%: 3%, 97%), preferably by copolymerization.

A copolymer prepared from POE-POP di-block macromonomer and acrylic acid (respective mol%:%, 98%), preferably by copolymerization.

The amount of thermo-thickening polymer added to the self-heating system must be sufficient. Sufficient amount means an amount sufficient to control the temperature of the self-heating system in a range chosen according to the intended application.

Thus, in the case of beverages, sufficient amount is understood to mean an amount sufficient to control the temperature of the self-heating system in a temperature range of less than 70 ° C.

The sufficient amount depends on the nature of the self-heating system and the nature of the thermo-thickening polymer used. This amount can be easily determined by those skilled in the art.

The water-soluble heat-thickening polymer may be used when added to the aqueous phase at a concentration of from 0.1% to 20% by weight based on the total weight of the aqueous mixture.

Preferably it is used at a concentration of between 0.5% and 10% by weight relative to the total weight of the aqueous mixture. Even more preferably it is used at a concentration of between 1% and 5% by weight relative to the total weight of the aqueous mixture.

Without wishing to be limited to a scientific theory or a reaction mechanism, it can however be explained that thermo-thickening polymers increase the viscosity of the aqueous medium when the temperature increases which limits the diffusion of the reagents. On the contrary, when the temperature decreases, the thermo-thickening polymers reduce the viscosity of the aqueous medium and

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 again facilitate the diffusion of the reagents which contributes to maintaining the temperature.

They therefore act as regulators of exothermic reactions, and therefore limit the amount of heat released as a function of time.

An additional advantage of the method of the invention is that this method makes it possible to secure the self-heating systems triggered by the addition of water.

Indeed, when the perforation between the water compartment and the self-heating system is not performed correctly, the temperature due to the very small amount of water introduced can increase very strongly, up to 800 C in certain cases. which is very dangerous. The self-heating aluminum package may even melt.

The presence of thermo-thickening polymer in the water can prevent or at least limit this phenomenon.

The examples and figures submitted below are presented as non-limiting illustrations of the present invention.

FIG. 1: Synthesis of the copolymer 4 of Example 3. FIG. 2: Evolution of the viscosity of a thermo-thickening polymer according to the invention (copolymer 4) as a function of the temperature.

Figure 3: Evolution of the temperature in a self-heating system CaO / P205 with or without polymer according to the invention.

 EXAMPLE 1 Preparation of a "BRANCHED" STRUCTURE copolymer by direct grafting of PAA on a POE-POP-POE triblock copolymer:

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 Copolymer 1: AIBN azobisisobutyronitrile (0.030 g, 0.2 × 10 -3 mol), acrylic acid (6 g, 8.33 × 10 -2 mol) and Pluronic PE6400 (molar mass number: about 3 EO%: LCST: approximately 69 ° C) marketed by BASF (6 g, 0.002 mol) in a tube which is then placed in an oven whose temperature is maintained at 70 ° C. After 12 hours, the solubilization is solubilized progressively the polymer in basic water (sodium hydroxide: 3.4 g, water: 276 g).

EXAMPLE 2 Preparation of Macromonomers: Macromonomer 1: The synthesis is carried out under nitrogen. In a 100 ml three-neck flask surmounted by a condenser and immersed in a bath of heating oil are introduced Pluronic PE6400 (100 g, 0.033 mol) and maleic anhydride (3.596 g, 0.0366 mol). The temperature is raised from 25 ° C. to 60 ° C. in 20 minutes, maintained at 60 ° C. for 15 minutes and then increased to 140 ° C. and maintained for 12 hours. The macromonomer is then used as is.

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

EXAMPLE 3 Synthesis of copolymers with a comb structure

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 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 mol) - 480 g of water - 0.0415 g of ammonium peroxodisulfate.

The temperature is brought from 25 ° C. to 40 ° C. in 20 minutes and then to 45 ° C. in 10 minutes. It is then maintained at 45 ° C. Acrylic acid (17.346 g, 0.241 mol) and ascorbic acid (0.0166 g) are introduced separately continuously over 5 h, while the temperature is maintained at 45 ° C. for 14 hours. h in total.

The complex formed during the polymerization is then separated from the aqueous phase, redissolved in basic water (pH: about 8.4, mass concentration: about 8.0%).

- Copolymer 3 (from the macromonomer 2: 3 mol%): The synthesis is carried out under nitrogen.

In a glass reactor 11 surmounted by a condenser and provided with stirring, magnetic or mechanical as well as a temperature probe, are successively introduced: - 30 g of macromonomer 2 (9.1 × 10 -3 mol) - 381 g of water - 0.0384 g of ammonium peroxodisulfate.

The temperature is brought from 25 ° C. to 40 ° C. in 20 minutes and then to 45 ° C. in 10 minutes. It is then maintained at 45 ° C. Acrylic acid (21.164 g, 0.294 mol) and ascorbic acid (0.0153 g) are introduced separately continuously over 5 h while the temperature is maintained at 45 ° C. for 14 hours. h in total.

The complex formed during the polymerization is then separated from the aqueous phase, redissolved in basic water (pH: about 8, classical concentration: about 15.5%).

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- Copolymer 4 (from the macromonomer 2: 2 mol%): The synthesis is carried out under nitrogen.

In a glass reactor 11 surmounted by a condenser and provided with stirring, magnetic or mechanical as well as a temperature probe, are successively introduced: - 19.41 g of macromonomer 2 (6.07 × 10 -3 mol) - 381 g of water - 0.0384 g of ammonium peroxodisulfate.

The temperature is brought from 25 ° C. to 40 ° C. in 20 minutes and then to 45 ° C. in 10 minutes. It is then maintained at 45 ° C. Acrylic acid (21.164 g, 0.294 mol) and ascorbic acid (0.0153 g) are introduced separately continuously over 5 h while the temperature is maintained at 45 ° C. for 14 hours. h in total.

The complex formed during the polymerization is then separated from the aqueous phase, redissolved in basic water (pH: about 8, classical concentration: about 15.5%).

The thermo-thickeners are characterized in the following way: - dry extract (ES), by drying 1 h - 2h at 115 C then weighed, - Brookfield viscosity at ambient T (50 rpm) - pH - relative molar masses (aqueous GPC, POE calibration their thermo-thickening power is evaluated by rheology in flow mode at pH = 8.5 and at 4% solids. The viscosity leaps were measured in forced-constrained flow mode by carrying out a temperature sweep between 20 ° C. and 80 ° C. (Carrimed cone rheometer)
CSL2100). The configuration used is the cone-plan geometry 4cm / 1 degree.

 The constraint introduced in the program was chosen (in manual mode) so that the gradient at 25 C is 10 s-1. The viscosity ratio is measured at 60 ° C. / viscosity at 25 ° C., and is expressed in decades.

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Table I

<Tb>
<tb><SEP>SEP> SE <SEP> Viscosity <SEP> Reference <SEP> Mn <SEP><SEP><SEP> Jump <SEP>
<tb> Polymer <SEP> (%) <SEP> Brookfiel <SEP> relative <SEP> relative <SEP> viscosity, <SEP> viscosity,
<tb> thermo-thickener <SEP> d <SEP> (mPa.s) <SEP> (g / mole) <SEP> (g / mole) <SEP> from <SEP> 25 <SEP> to <SEP> from <SEP > 25 <SEP> to
<tb> 60 C <SEP> 80 C
<tb> Copolymer <SEP> 4 <SEP> 14.9 <SEP> 128 <SEP> 320 <SEP> 000 <SEP> 95 <SEP> 000 <SEP> 1.3 <SEP> 2.5 <SEP>
<tb> decades <SEP> decades
<Tb>
EXAMPLE 4 Use of a thermo-thickening polymer to control the temperature in a self-heating system triggered by the addition of water Example 1 The application test is a Dewar where 6 g of P205 / CaO mixture granules are placed. In the solid mixture a thermocouple is placed to measure the temperature. The Dewar is covered with a lid equipped with a syringe containing 6 g of a solution of the polymer in water. When the system is stable in temperature, the solution is added in one go and the temperature is measured.

Primary Materials: Granules The granules used have a mass composition comprising:
43. 3% CaO
43. 3% P205
13. 4% of a mixture consisting of 74% mineral oil and 26% surfactant "Actrafos 216".

Their method of synthesis is described in US 5,935,486 page 23 lines 20 to 40.

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 The thermo-thickening polymer used is the copolymer 4 of Example 3 above.

It was used at two different concentrations: 4% and 10%.

Results
Table II

<Tb>
<tb> Polymer <SEP> Concentration <SEP> Temp. <SEP> Max <SEP> Time <SEP> to <SEP> plus <SEP> of <SEP> 60 C
<tb> White <SEP> 0 <SEP> 85 <SEP> C <SEP> 30 <SEP> minutes
<tb> Copolymer <SEP> 4% <SEP> 71 <SEP> 31 <SEP>
<tb> 4
<tb> Copolymer <SEP> 10% <SEP> 76 <SEP> 27
<tb> 4
<Tb>
It is seen in Table II and in Figure 3 the maximum temperature decrease by the addition of the polymer in water.

Claims (19)

 1. A method for controlling the temperature in a self-heating system triggered by addition of water, characterized in that there is added to the self-heating system a sufficient amount of at least one water-soluble thermo-thickening polymer. 2. Process according to claim 1, characterized in that the thermo-thickening polymer soluble in water is added to the aqueous phase. 3- Method according to any one of claims 1 or 2, characterized in that the self-heating system triggered by addition of water is selected from: - systems based on the hydration of anhydrous such as for example CaO, CaO / P205, MgCl2 / CaO, CaCO3, CaCl2, or - self-heating systems based on triggering an oxidation-reduction reaction by addition of water, such as, for example, the iron + magnesium reaction or a self-heating system based on permanganate. 4. Process according to any one of claims 1 to 3, characterized in that the thermo-thickening polymer soluble in water is a polymer comprising segments or unitary units which remain hydrophilic and therefore water-soluble irrespective of the temperature, and so-called thermosensitive unitary segments or units having a lower critical solubility temperature (LCST). 5. Process according to any one of claims 1 to 4, characterized in that the thermo-thickening polymer comprises segments or units called so-called heat-sensitive having a lower critical temperature of solubility (LCST) between 20 C and 70 C. 6. Process according to any one of claims 1 to 5, characterized in that the heat-thickening polymer comprises segments or unitary patterns. <Desc / Clms Page number 16>  said thermosensitive having a lower critical temperature of solubility (LCST) between 30 C and 50 C. 7- Process according to any one of claims 1 to 6, characterized in that the thermo-thickening polymer comprises so-called thermosensitive unit segments or units having a critical lower solubility temperature (LCST) between 30 C and 40 C. 8- Process according to any one of claims 1 to 7, characterized in that the segments or unitary units known as heat-sensitive are chosen from: - polyethers: - polyethylene oxide (POE) - random copolymers ethylene oxide propylene oxide (EO / OP) - POP-POE diblocks - POE-POP-POE triblocks - alkyl-POE - poly (methyl vinyl ether) - hydroxylated polymers: - poly (hydroxypropyl acrylate) ) (polyHPA) and its copolymers - partially hydrolysed vinyl polyacetate (PVAc) - methylcellulose - hydroxyethylcellulose - hydroxypropylcellulose - hydroxypropylmethylcellulose - polymers containing substituted amide units: - poly (N-substituted acrylamides) for example, poly N-isopropyl acrylamide (polyNIPAM) -poly (N-vinyl caprolactam), poly (N-acryloyl pyrrolidine), poly (N-acryloyl piperidine), poly (ethyl oxazoline), or a mixture thereof. <Desc / Clms Page number 17> 9- Process according to any one of claims 1 to 8, characterized in that the segments or unitary units which remain hydrophilic and therefore water-soluble regardless of the temperature are chosen from: - segments or unitary units having carboxylic functions, such as acrylic acid or methacrylic acid units, - unitary precursor segments or units of carboxylic functions, such as maleic anhydride units, - unitary segments or units having sulphonate functional groups, such as AMPS, styrene sulphonate, sulphoethyl methacrylate, sulphopropyl acrylate units or sulphopropylmethacrylate, hydroxylated unit segments or units such as polyvinyl alcohol or hydrophobic unmodified polysaccharides, that is to say ungrafted with alkyl groups such as ethyl, propyl, hydroxyethyl, hydroxypropyl, as well as the derivatives of polysaccharides more hydrophilic agents such as cellulose, carboxymethyl cellulose, guars and in particular cationic guars, - segments or unitary units based on acrylamide or on acrylamide derivatives such as dimethylacrylamide, - segments or unitary phosphonate units such as vinylphosphonic acid, - segments or units phosphate units such as phosphatoethylmethacrylate, or a mixture thereof. 10- Process according to claim 9, characterized in that the segments or unitary units which remain hydrophilic and therefore water-soluble regardless of the temperature are selected from carboxylic unit segments or units, precursors of carboxylic functions, sulfonates, phosphonates or phosphates. 11- Process according to any one of claims 1 to 10, characterized in that the polymer comprising segments or unitary units which remain <Desc / Clms Page number 18>  hydrophilic and therefore water-soluble whatever the temperature, and so-called thermosensitive unit segments or units is a copolymer that is linear, or comb or grafted, or plugged. 12- Process according to claim 11, characterized in that the copolymer comprises 0.1% to 50% molar and preferably 0.1 to 5.0% molar of polymer segments having a lower critical solubility temperature (LCST) between 20 and 70 C. 13- Method according to any one of claims 11 or 12, characterized in that the copolymer is prepared from tri-block macromonomer POE-POP-POE and acrylic acid (respective molar percentages: 2.3%, 97%). , 7%). 14- Process according to any one of claims 11 or 12, characterized in that the copolymer is prepared from POE-POPPOE tri-block macromonomer and acrylic acid (respective molar%: 1.6%, 98, 4%). 15- Process according to any one of claims 11 or 12, characterized in that the copolymer is prepared from POE-POP diblock macromonomer and acrylic acid (respective mol%: 3%, 97%). 16- Process according to any one of claims 11 or 12, characterized in that the copolymer is prepared from POE-POP diblock macromonomer and acrylic acid (respective mol%: 2%, 98%). 17- Method according to any one of claims 1 to 16, characterized in that water-soluble thermo-thickening polymer is used when it is added to the aqueous phase at a concentration of between 0.1% to 20% by weight relative to the total weight of the aqueous mixture. 18- Method according to any one of claims 1 to 17, characterized in that thermo-thickening polymer soluble in water is used when it is <Desc / Clms Page number 19>  added to the aqueous phase at a concentration of between 0.5% and 10% by weight relative to the total weight of the aqueous mixture. 19- Process according to any one of claims 1 to 18, characterized in that water-soluble thermo-thickening polymer is used when it is added to the aqueous phase at a concentration of between 1% to 5% by weight relative to the total weight of the aqueous mixture.