WATER-SOLUBLE POLYMERS POLYMERIZED BY GEL FORM

Abstract

The present invention relates to a water-soluble polymer P1 with an average molecular weight of less than 2 million daltons in powder form prepared by a gel polymerization process comprising the following successive steps: a) Forming a solution S1 by adding and mixing, in a polymerization reactor, between 25 and 150 wt% of water-soluble monomers with a solution S2 containing between 1 and 25 wt% of a polymer P2, by weight relative to the weight of solution S2, polymer P2 containing at least 1 wt% of hydrophobic monomers; b) Adding a pH regulator to solution S1; c) Degassing solution S1; d) Adding at least one polymerization initiator to solution S1, with solution S1 at a temperature between -5°C and 30°C; e) Forming a gel of polymer P1 by polymerizing the solution S1 from step d) and allowing the polymer P1 gel to age at a final polymerization temperature between 80 and 150°C,for at least 60 minutes without heating, f) Discharge the resulting P1 polymer gel into a granulator, g) Dry the P1 polymer gel, h) Grind and sieve the P1 polymer into a powder.

Description

Title of the invention: WATER-SOLUBLE POLYMERS POLYMERIZED BY GEL PROCESS FIELD OF THE INVENTION

[0001] The present invention relates to water-soluble synthetic polymers with a weight-average molecular weight of between 500,000 daltons and 2 million daltons in powder form used as coagulants, flocculants or thickeners in multiple applications. More specifically, the invention relates to water-soluble synthetic polymers and a gel polymerization process for obtaining water-soluble synthetic polymers of low molecular weight. PRIOR STATE OF THE ART

[0002] High molecular weight water-soluble synthetic polymers are commonly used in many applications due to their flocculating or thickening properties. Indeed, these polymers are used in the oil and gas industry, hydraulic fracturing, papermaking processes, sludge dewatering, water treatment, construction, mining, cosmetics, agriculture, textiles and detergents.

[0003] For example, the flocculent nature of these high molecular weight water-soluble synthetic polymers is used in the field of water treatment / sludge dewatering. Indeed, after an optional coagulation step where the colloidal particles (similar to spheres of a size less than 1 micrometer) of a given water are destabilized, flocculation represents the step where the particles are gathered into high molecular weight aggregates to generate rapid sedimentation. The water-soluble polymers used for water treatment are mainly in the form of powder or water-oil inverse emulsion. Depending on the water to be treated, the physical properties of the flocculant are modulated. Thus, the ionic character (non-ionic, anionic, cationic, amphoteric, zwitterionic), the molecular weight or the structure (linear or structured, or even crosslinked) of the water-soluble polymer can be adapted.

[0004] The thickening character of these polymers can be exploited in the field of enhanced oil recovery (EOR). The efficiency of water injection sweeping is generally improved by the addition of high molecular weight synthetic (co)polymers soluble in water. The expected and proven advantages of using these (co)polymers, by the "viscosification" of the injected water, are the improvement of sweeping and the reduction of the viscosity contrast between the fluids to control their mobility ratio in the field, in order to recover the oil quickly and efficiently. efficiently. These (co)polymers increase the viscosity of water.

[0005] The weight average molecular weight of these water-soluble polymers is generally between 500,000 daltons and 30 million daltons. To obtain these polymers in powder form, gel polymerization can be used. The main steps of this polymerization method are the polymerization of the water-soluble monomers in the aqueous phase, the discharge of the polymer gel thus obtained from the reaction vessel, the granulation of the polymer gel in a granulator, the drying of the polymer gel to obtain a water-soluble polymer in powder form and finally the grinding and sieving of the powder.

[0006] For the discharge of the polymerization reactor, the granulation and the drying step, the gel must be sufficiently viscous to "self-support". In order to measure this property, a cylinder of gel 10 cm in diameter and 10 cm thick is cut from the polymer mass, placed on a flat surface and left to stand for 1 hour at 25°C. A form factor F is then defined on the volume obtained as the ratio of the horizontal section to the vertical section: F = L / H (see [Fig.l]).

[0007] If the ratio is less than 3, the gel is "self-supporting". Otherwise, it is not self-supporting.

[0008] In order to be self-supporting, the gel must be viscoelastic. And when the weight average molecular weight of the water-soluble polymers is between 500,000 daltons and 2 million daltons, the gel might not be viscoelastic enough to be processable (unloaded, granulated and dried).

[0009] An increase in the concentration of monomers in the polymerization reactor may be a means of obtaining the required consistency of the gel but, as a consequence, in the case of an exothermic polymerization reaction, the final polymerization temperature and the final pressure inside the reaction vessel will increase, which will require adaptation of the reaction vessel. Disclosure of the invention

[0010] Unexpectedly, the Applicant has discovered that a water-soluble polymer with a weight-average molecular weight of less than 2 million daltons in powder form can be prepared by a gel polymerization process with a polymer gel consistency sufficient for the reactor discharge, granulation, grinding and drying steps, by carrying out the polymerization in a solution containing, among other things, a water-soluble polymer containing at least 1% by weight of hydrophobic monomers.

[0011] More specifically, the invention relates to a water-soluble polymer PI with a weight-average molecular weight of less than 2 million daltons in powder form. This water-soluble polymer PI is prepared by a gel polymerization process comprising the following successive steps: a) Forming a solution SI by adding, and mixing, in a polymerization reactor between 25 and 150% by weight of water-soluble monomers, to a solution S2 containing between 1 and 25% by weight of a polymer P2, by weight relative to the weight of the solution S2, polymer P2 containing at least 1% by weight of hydrophobic monomers, b) Add a pH regulator to the SI solution, c) Degas the SI solution, d) Adding polymerization initiators to the SI solution, the SI solution being at a temperature between - 5°C and 30°C, e) Forming a PI polymer gel by polymerizing the SI solution (polymer P2 + water-soluble monomers + pH regulator + polymerization initiators) from step d) and allowing the PI polymer gel to age at a final polymerization temperature between 80°C and 150°C for at least 60 minutes without heating, f) Discharging the PI polymer gel thus obtained into a granulator, g) Dry the PI polymer gel to obtain the PI polymer in powder form, h) Grind and sieve the PI polymer in powder form.

[0012] The invention also relates to a process for preparing the water-soluble polymer PI according to steps a) to h). Advantageously, the gel polymerization process does not comprise an intermediate step between the different steps a) to h).

[0013] The value ranges include the lower and upper bounds. Thus, the value ranges “between 0.1 and 1.0” and “from 0.1 to 1” include the values ​​0.1 and 1.0.

[0014] The term “polymer” designates both homopolymers and copolymers of at least two distinct monomers.

[0015] As used herein, the term "water-soluble polymer" or "water-soluble monomer" means a polymer or monomer that gives an aqueous solution without insoluble particles when dissolved under stirring for 4 hours at 25°C and with a concentration of 20 gL 1 in deionized water.

[0016] According to the present invention, a solution, for example S1 or S2, denotes a liquid composition of at least one compound (polymer, monomers, etc.). The solution may optionally comprise insolubles. This may in particular be the case when the solution S2 comprises water and a polymer P2 of hydrophobic monomer(s).

[0017] The polymer PI may be non-ionic, anionic, cationic or amphoteric. An amphoteric polymer is a polymer comprising cationic charges and anionic charges, preferably as many anionic charges as cationic charges.

[0018] Thus for step a) of the gel polymerization process of polymer P1, the monomers added to the solution S2 of polymer P2 may be non-ionic and / or anionic and / or cationic and / or zwitterionic. Step a) consists of (1) preparing a solution S2 containing 1 to 25% by weight of a polymer P2, relative to the weight of the solution S2, then (2) adding 25 to 150% by weight of water-soluble monomers, relative to the weight of the solution S2. S2 is advantageously a mixture of water and polymer P2.

[0019] In other words and by way of example, step a) may correspond to the preparation of a solution SI, by mixing in a polymerization reactor (1) 25 to 150 g of water-soluble monomers, and (2) 100 g of a solution S2 containing between 1 and 25 g by weight of a polymer P2.

[0020] The non-ionic monomers are preferably chosen from the group containing acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkyl acrylamides, N,N-dialkylmethacrylamides, alkoxylated esters of acrylic acid, alkoxylated esters of methacrylic acid, N-vinylpyridine, N-vinylpyrrolidone, hydroxyalkylacrylates, hydroxyalkyl methacrylates, and mixtures thereof. Among these non-ionic monomers, the alkyl groups are advantageously C1-C5, more advantageously C1-C3. They are preferably linear alkyls.

[0021] The anionic monomers are preferentially chosen from the group comprising monomers having a carboxylic acid function and their salts, including acrylic acid, methacrylic acid, itaconic acid, maleic acid; monomers having a sulfonic acid function and their salts, including acrylamido tert-butyl sulfonic acid (ATBS), allyl sulfonic acid and methallyl sulfonic acid, and their salts; and monomers having a phosphonic acid function and their salts.

[0022] Generally, the salts of anionic monomers of the polymer PI are salts of an alkali metal (preferably sodium), of an alkaline earth metal (preferably calcium or magnesium) or of an ammonium (preferably a quaternary ammonium).

[0023] The cationic monomers are preferably chosen from the group comprising quaternized or salified dimethylaminoethyl acrylate (ADAME), quaternized or salified dimethylaminoethyl methacrylate (MADAME), diallyldimethylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC).

[0024] Advantageously, the cationic monomers of the polymer PI have as counterion a halide, preferably a chloride ion.

[0025] The zwitterionic monomers are preferably chosen from the group comprising sulfobetaine monomers such as sulfopropyl dimethylammonium ethylmethacrylate, sulfopropyl dimethylammonium propylmethacrylamide, or sulfopropyl 2-vinylpyridinium; phosphobetaine monomers, such as phosphato ethyltrimethylammonium ethylmethacrylate; and carboxybetaine monomers.

[0026] The polymer PI can be linear, structured or crosslinked. The crosslinking agents allowing the structuring can in particular be chosen from sodium allyl sulfonate, sodium methallyl sulfonate, sodium methallyl disulfonate, methylenebisacrylamide, triallylamine, triallylammonium chloride, tetraallylammonium chloride.

[0027] According to the present invention, the weight average molecular weight of the synthetic water-soluble polymer PI is determined by measuring the intrinsic viscosity. The intrinsic viscosity can be measured by methods known to those skilled in the art and can in particular be calculated from the reduced viscosity values ​​for different concentrations by a graphical method consisting of plotting the reduced viscosity values ​​(on the y-axis) as a function of the concentrations (on the x-axis) and extrapolating the curve to zero concentration. The intrinsic viscosity value is read on the y-axis or using the least squares method. Then, the weight average molecular weight can be determined by the well-known Mark-Houwink equation: [q] = KM" [q] represents the intrinsic viscosity of the polymer determined by the solution viscosity measurement method, K represents an empirical constant, M represents the molecular weight of the polymer, a represents the Mark-Houwink coefficient, a and K depend on the particular polymer-solvent system. Tables known to those skilled in the art give the values ​​of a and K depending on the polymer-solvent system.

[0028] The water-soluble polymer PI of the invention has a weight-average molecular weight of less than 2 million daltons, preferably between 500,000 daltons and less than 2 million daltons.

[0029] For step a) of the process, the polymer P2 contains at least 1% by weight of hydrophobic monomers. Thus, the polymer P2 can be non-ionic, anionic, cationic or amphoteric. Beyond the hydrophobic monomers, it can be made up of non-ionic and / or anionic and / or cationic and / or zwitterionic monomers, preferably chosen from the same lists previously described for PI.

[0030] The hydrophobic monomers of the polymer P2 are preferably chosen from the following list: - (meth)acrylic acid esters having an alkyl, arylalkyl and / or ethoxylated and / or propoxylated chain; (meth)acrylamide derivatives having an alkyl, arylalkyl or dialkyl and / or ethoxylated and / or propoxylated chain; cationic allyl derivatives having an alkyl, arylalkyl or dialkyl chain and / or an ethoxylated and / or propoxylated chain; anionic or cationic hydrophobic derivatives of (meth)acryloyl; and anionic or cationic monomeric derivatives of (meth)acrylamide bearing a hydrophobic chain; - n-hexyl (meth)acrylate, n-octyl (meth)acrylate, octyl (meth)acrylamide, n-tert-butyl (meth)acrylamide, diethyl (meth)acrylamide, lauryl (meth)acrylate, lauryl (meth)acrylamide, myristyl (meth)acrylate, myristyl (meth)acrylamide, pentadecyl (meth)acrylate, pentadecyl (meth)acrylamide, cetyl (meth)acrylate, cetyl (meth)acrylamide, oleyl (meth)acrylate, oleyl (meth)acrylamide, erucyl (meth)acrylate, erucyl (meth)acrylamide, and combinations thereof; - hydrophobic monomers corresponding to the general formula CH2=CR1-COO-(EO)n-(PO)m-R2 in which R1 is hydrogen or methyl, n is an integer of at least two, preferably from 6 to 100 or from 10 to 40, m is an integer from zero to 50, preferably from zero to 20, EO is an ethylene oxide group (-CH2-CH2-O-), PO is a propylene oxide group (-CH2-CH(CH3)-O-) and R2 is a C8-C30 alkyl group or a C8-C30 arylalkyl group, and n+m is preferably from 6 to 100 or from 10 to 40. These are preferably linear alkyls.

[0031] Among (meth)acrylic acid esters having an alkyl chain, the alkyl groups are advantageously C1-C5, more advantageously C1-C3. These are preferably linear alkyls.

[0032] More preferably, the hydrophobic monomers of P2 are chosen from the following list: haloalkylated (preferably bromoalkylated) derivatives of methacrylamidodimethylaminopropyl with a C8-C16 alkyl chain, ethoxylated behenyl methacrylate, diethylacrylamide, n-tert-butylacrylamide. These are preferably linear alkyls.

[0033] Even more preferably, the polymer P2 is a terpolymer of diethylacrylamide, n-tert-butylacrylamide and sodium 2-acrylamido-2-methylpropanesulfonate.

[0034] The polymer P2 contains between 1 and 100% by weight of hydrophobic monomers, preferably between 2 and 100% by weight, even more preferably between 5 and 100% by weight, even more preferably between 10 and 100% by weight, and even more preferably between 20 and 100% by weight of hydrophobic monomers.

[0035] In a preferred embodiment, the polymer P2 is functionalized with the end of the polymer chain by a hydroxyl, cyano, amine, phosphate, phosphonate, sulfate, sulfonate, xanthate, trithiocarbonate, dithiocarbamate, or dithioester moiety.

[0036] According to another preferred embodiment, the polymer P2 contains at least one carbon-carbon double bond.

[0037] For step a) (formation of solution SI) of the process for obtaining polymer PI, between 25 and 150% by weight of monomers are added to solution S2 of polymer P2, advantageously between 25 and 120% by weight and even more advantageously between 33 and 100% by weight, relative to the weight of solution S2.

[0038] The solution S2 of polymer P2 contains between 1 and 25% by weight of polymer P2, advantageously between 2 and 20% by weight, and even more advantageously between 3 and 15% by weight.

[0039] For step b) of the process for obtaining the polymer PI, the pH regulator is advantageously composed of one or more of the following elements: hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, citric acid, formic acid, acetic acid, adipic acid, propionic acid, oxalic acid, benzoic acid, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate. A person skilled in the art will be able to define the pH to be reached at the end of step b) as well as the quantity and choice of pH regulators depending on the chemistry of the polymer to be synthesized, in particular depending on the nature of the monomers (cationic, anionic, etc.).

[0040] For step c), in order to remove residual oxygen from the solution obtained at the end of step b), an inert gas is introduced so as to degas the SI solution. The inert gas is usually passed through the solution. Suitable inert gases for this purpose are, for example, nitrogen, carbon dioxide or noble gases such as neon or helium. Argon can also be used.

[0041] When adding the polymerization initiator (step d)), the SI solution is at a temperature between -5°C and 30°C, preferably between 0 and 10°C.

[0042] The gel polymerization of the process of the invention is carried out by radical means. It includes free radical polymerization using UV, azo, redox or thermal initiators as well as controlled radical polymerization (CRP) techniques or, more particularly, RAFT (Reversible Addition Fragmentation Chain Transfer) type techniques.

[0043] The usual polymerization regulators can be used. These may in particular be sulfur compounds such as thioglycolic acid, mercapto alcohols, dodecyl mercaptan, amines such as ethanolamine, diethanolamine, morpholine and phosphites such as sodium hypophosphites. In the case of RAFT type polymerization, specific polymerization regulators such as those comprising a transfer group comprising the -S-CS- function, can be used. In particular, we can cite these compounds from the family of xanthates (-S-CS-O-), dithioesters (-S-CS-Carbon), trithiocarbonates (-S-CS-S-), or dithio-carbamates (-S-CS-Nitrogen). Among the compounds of the xanthate family, O-ethyl-S-(l-methoxy carbonyl ethyl) xanthate can be advantageously used due to its compatibility with acrylic monomers.

[0044] The polymerization initiators used in step d) of the process for obtaining the polymer PI can be any compounds which dissociate into radicals under the polymerization conditions, for example: organic peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and redox catalysts. The use of water-soluble initiators is preferred. In some cases, it is advantageous to use mixtures of various polymerization initiators, for example mixtures of redox catalysts and azo compounds.

[0045] Suitable organic peroxides and hydroperoxides are, for example, sodium or potassium peroxodisulfate, acetylacetone peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butyl pemeohexanoate, tert-butyl perbuto-butylate, -ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, or tert-butyl per-3,5,5-trimethylhexanoate and tert-amyl pemeodecanoate.

[0046] Suitable persulfates may be chosen from alkali metal persulfates such as sodium persulfate.

[0047] Suitable azo initiators are advantageously water-soluble and selected from the following list: 2,2'-azobis-(2-amidinopropane) dihydrochloride, 2,2'-azobis(N,N'-dimethylene)isobutyramidine dihydrochloride, 2-(azo(l-cyano-l-methylethyl))-2-methylpropane nitrile, 2,2'-azobis[2-(2'-dimidazolin-2-yl)propane] dihydrochloride and 4,4'-azobis(4-cyanovaleric acid).

[0048] Said polymerization initiators are used in usual quantities, for example in quantities which can vary from 0.001 to 2%, preferably from 0.01 to 1% by weight, relative to the monomers to be polymerized (water-soluble monomers added during step a)).

[0049] As oxidizing component, the redox catalysts advantageously contain at least one of the above-mentioned compounds and, as reducing component, for example ascorbic acid, glucose, sorbose, hydrogen sulfite, sulfite, thiosulfate, hyposulfite, pyrosulfite or an alkali metal, metal salts, such as in the form of iron (II) ions or silver ions or sodium hydroxymethyl sulfoxylate. The reducing component of the redox catalyst preferably used is Mohr's salt (NH4)2Fe(SO4)2, 6 H20.

[0050] Based on the amount of monomers used in the polymerization, from 5 x 106 to 1 mole % of the reducing component of the redox catalyst system and from 5 x 105 to 2 mole % of the oxidizing component of the redox catalyst may, for example, be used. Instead of the oxidizing component of the redox catalyst, one or more water-soluble azo initiators may also be used.

[0051] The polymerization is carried out in the absence of oxygen (step c) of degassing), by introducing the initiators in the appropriate order, known to those skilled in the art, into the solution to be polymerized. The initiators are introduced either in soluble form in an aqueous medium, or in the form of a solution in an organic solvent.

[0052] During step e) (advantageously also during step d)), all the constituents of the solution SI (polymer P2 + water-soluble monomers + pH regulator + polymerization initiators) are advantageously solubilized, more advantageously in water. Indeed, the different constituents can allow the solubilization of constituents which would not be soluble in the main solvent, for example water. For example, the water-soluble monomers can allow the solubilization of the polymer P2 in water, even when it mainly comprises hydrophobic monomers. In this case, the water-soluble monomers play the role of co-solvent of the polymer P2.

[0053] Thus, at the end of step d), the solution SI is advantageously free of insolubles.

[0054] According to a preferred embodiment, the solution SI is an aqueous solution.

[0055] According to a preferred embodiment, the solution S2 is an aqueous solution.

[0056] Generally speaking, polymer PI is water-soluble while polymer P2 is not necessarily so.

[0057] As soon as the polymerization begins, for step e) of the process for obtaining the polymer PI, the reaction mixture is heated or heats up (exothermic reaction) depending on the starting conditions chosen. Advantageously, due to the released heat of polymerization, the temperature of the reaction mixture rises from 80 to 150°C, preferably from 80°C to 100°C. The polymerization is advantageously carried out at atmospheric pressure.

[0058] The reactor used from step a) of the process is in most cases jacketed so that the reaction mixture can be cooled or heated as required. Once the polymerization reaction is complete, the resulting polymer gel can be rapidly cooled, for example by cooling the reactor wall.

[0059] At the end of the polymerization reaction in step f) (end of step e)), after having allowed the polymer gel PI to age for at least 60 minutes, the product resulting from the polymerization is a gel so viscous that it “self-supports”.

[0060] As already indicated, a self-supporting gel has a form factor F less than 3. In order to measure this form factor F, a cylinder of gel 10 cm in diameter and 10 cm thick is cut from the polymer mass, placed on a flat surface and left to rest for 1 hour at 25°C. The form factor F corresponds to the ratio between the horizontal section and the vertical section of the cylinder left to rest for 1 hour: F = L / H (see [Fig.l]).

[0061] In order to facilitate the discharge of the gel from the reactor at the end of the reaction (step f)), the reactor is advantageously in the form of an inverted conical tubular (cone downwards) in order to discharge the gel downwards by applying pressure, for example an inert gas or air, to the surface of the gel or in the form of a rocker in order to discharge the mass of gel by rocking the reactor.

[0062] Preferably, the reactor is in the form of an inverted conical tubular shape (cone downwards).

[0063] Step f) of the process of the invention consists of discharging the PI polymer gel obtained in step e) into a granulator. Granulation consists of cutting the gel into small pieces. Advantageously, the average size of these gel pieces is less than 1 cm, more advantageously it is between 4 and 8 mm. Those skilled in the art will know how to choose the appropriate means for optimal granulation. Granulation is also described in the section relating to the presentation of the state of the art.

[0064] Step g) of the process consists of drying the polymer PI. The drying means and its conditions (duration + temperature) are routine choices for those skilled in the art. Industrially, the drying is advantageously carried out by a fluidized bed or rotor dryer, advantageously using air heated to a temperature between 70°C and 200°C, the air temperature being a function of the nature of the product as well as the drying time applied. At the end of the drying (end of step g)), the water-soluble polymer PI is physically in powder form.

[0065] For step h) of the process, the polymer powder PI is ground and sieved. The grinding step consists of breaking the large polymer particles into smaller particles. This can be done by shearing or by mechanically crushing the particle between two hard surfaces. Different types of equipment known to those skilled in the art can be used for this purpose. Examples include rotor mills, where the rotating part crushes the particle on a compression blade, or roller mills, where the particle is crushed between two rotating rollers. The purpose of sieving is then to eliminate, depending on the specifications, the medium-sized particles that are too small or too large.

[0066] The following examples illustrate the invention without, however, limiting its scope. DESCRIPTION OF FIGURES

[0067] [Fig.l] [Fig.l] illustrates the determination of the factor F = L / H (upper face of the polymer gel cylinder). EXAMPLES OF CARRYING OUT THE INVENTION

[0068] Example 1: Gel synthesis of an acrylamide / sodium acrylate copolymer Pla, adding to the polymerization charge 3% by weight of polymer P2a containing 42% by weight of hydrophobic monomer.

[0069] In a first step, the polymer P2a of composition by weight: 3% n-tert-butyl acrylamide, 39% diethylacrylamide, 8% sodium 2-acrylamido-2-methylpropanesulfonate, 50% acrylamide is synthesized in aqueous solution by radical polymerization.

[0070] In a second step, the polymer Pla is synthesized by a radical gel polymerization process from an aqueous charge comprising 3% by weight of polymer P2a according to the following protocol: 30 g of polymer P2a (361 g of aqueous solution S2a at 8.3% by weight of P2a), 79 g of acrylic acid, 403 g of acrylamide at 50% by weight in water and 70 g of sodium chloride are introduced into a 1.5 L beaker. The neutralization of the charge is carried out using 87 g of sodium hydroxide at 50% by weight in water in order to reach a pH of the Sla solution between 6.5-7.5. The dry matter of the aqueous solution Sla is 40.6% by weight. This aqueous Sla solution is placed in a 2 L beaker and then cooled to 0°C before being placed in a Dewar.1.5 g of azobisisobutyronitrile are introduced into the aqueous Sla solution which is then homogenized using a hand mixer at a speed of 500 rpm for 20 seconds before being degassed under nitrogen bubbling for 20 minutes.

[0071] 1.2 x 10 1 moles% of sodium hypophosphite, 2.4 x 10 4 moles% of diethylene triamine penta acetic acid (DTPA) are then added to the aqueous solution Sla, expressed relative to the total quantity of monomers involved, then the reaction is initiated by successive additions of 1.3 x 10 3 moles% of sodium persulfate then 5.2 x 10 4 moles% of Mohr's salt. The reaction time is 60 minutes, for a final temperature of 94°C. The Pla polymer obtained is in the form of a gel having a factor F = 2.1. Consequently, it is possible to granulate it, then to dry it in a flow of air at 70°C for 60 minutes. The dry grains of Pla polymer are then ground in order to obtain a particle size of less than 1.7 mm. The resulting Pla polymer is 100% water-soluble and has a molar mass of 966,000 Da.

[0072] Example 2 (counter-example): Gel synthesis of an acrylamide / sodium acrylate copolymer Pib under the same conditions as described in Example 1, except for the addition of polymer P2a.

[0073] In this example, the polymer Pib is synthesized as described in Example 1, by replacing the 361 g of the 8.3% solution of the polymer P2a with 30 g of urea and 331 g of deionized water. The quantity of dry matter of the charge to be polymerized is therefore identical to that of example 1, namely 40.6% by weight.

[0074] The polymerization conditions and the catalytic system are identical to those described in Example 1.

[0075] The reaction time is 80 min, for a final temperature of 90°C. The Pib polymer gel obtained has a factor F = 5.2 and is not self-supporting. After drying in an oven, a 100% water-soluble Pib polymer is obtained with a molar mass of 867,000 Da.

[0076] Here we clearly observe differences in gel texture between two polymers (Pla and Pib) which nevertheless have a similar molar mass and which come from two polymeric charges having the same quantity of dry matter.

[0077] Example 3: Gel synthesis of an acrylamide / sodium acrylate copolymer Pic, by adding to the polymerization charge 2% by weight of polymer P2b containing 83% by weight of hydrophobic monomer and carrying a carbon-carbon double bond.

[0078] In a first step, the polymer P2b of composition by weight: 5% n-tert-butyl acrylamide, 78% diethylacrylamide, 17% sodium 2-acrylamido-2-methylpropanesulfonate is synthesized in aqueous solution by radical polymerization in the presence of 2-aminoethanethiol as a limiting agent. The functionalization of the polymer thus formed with the double bond is carried out by adding acryloyl chloride at basic pH.

[0079] In a second step, the Pic polymer is synthesized by a bulk polymerization process and by radical gel polymerization from an aqueous Sic solution comprising 2% by weight of polymer P2b according to the following protocol: 20g of polymer P2c (361g of aqueous solution S2b at 5.5% by weight of P2c), 79g of acrylic acid, 403g of acrylamide at 50% by weight in water and 70g of sodium chloride are introduced into a 1.5L beaker. The neutralization of the charge is carried out using 87g of sodium hydroxide at 50% by weight in water in order to reach the pH of the Sic solution between 6.5-7.5. The dry matter of the aqueous Sic solution is 39.6% by weight. This Sic solution is placed in a 2 L beaker and then cooled to 0°C before being placed in a Dewar.1.5g of azobisisobutyronitrile are introduced into the Sic solution which is then homogenized using a hand mixer at a speed of 500 rpm for 20 seconds before being degassed under nitrogen bubbling for 20 minutes.

[0080] 1.2 x 10 1 moles% of sodium hypophosphite, 2.4 x 10 4 moles% of diethylene triamine penta acetic acid (DTPA) are then added to the Sic solution, expressed relative to the total quantity of monomers involved, then the reaction is initiated by successive additions of 1.3 x 10 3 moles% of sodium persulfate then 5.2 x 10 4 moles% of Mohr's salt. The reaction time is 60 minutes, for a temperature final temperature of 76°C. The resulting Pic polymer is in the form of a gel with a factor F = 1.8. It can be granulated and then dried in an air flow at 70°C for 60 minutes. The dry grains of Pic polymer are then ground to obtain a particle size of less than 1.7mm. The resulting Pic polymer is 100% water-soluble and has a molar mass of 964,000 Da.

[0081] Example 4 (counter-example): Gel synthesis of an acrylamide / sodium acrylate copolymer Pld under the same conditions as described in example 3, except for the addition of the polymer P2b.

[0082] In this example, the polymer Pld is synthesized as described in Example 3, replacing the 361 g of 5.5% solution of polymer P2b with 20 g of urea and 341 g of deionized water. The quantity of dry matter of the charge to be polymerized is therefore identical to that of Example 3, namely 39.6% by weight.

[0083] The polymerization conditions and the catalytic system are identical to those described in Example 3.

[0084] The reaction time is 60 minutes, for a final temperature of 80°C. The resulting Pld polymer gel has a factor F = 4.3. The gel is soft and does not self-support. It requires oven drying to obtain the 100% water-soluble Pld polymer with a molar mass of 994,000 Da.

[0085] Here again we observe a strong difference in gel structure between two polymers (Pic and Pld) whose molar mass is close and which are derived from two polymeric fillers having the same quantity of dry matter.

Claims

Claims

1. Water-soluble polymer PI with an average molecular weight of less than 2 million daltons in powder form prepared by a gel polymerization process comprising the following successive steps: a) Forming a solution SI by adding, and mixing, in a polymerization reactor between 25 and 150% by weight of water-soluble monomers, to a solution S2 containing between 1 and 25% by weight of a polymer P2, by weight relative to the weight of the solution S2;the polymer P2 containing at least 1% by weight of hydrophobic monomers, b) Adding a pH regulator into the SI solution, c) Degassing the SI solution, d) Adding at least one polymerization initiator into the SI solution, the SI solution being at a temperature between -5°C and 30°C, e) Forming a polymer gel PI by polymerizing the SI solution from step d) and allowing the polymer gel PI to age at a final polymerization temperature between 80 and 150°C, for at least 60 minutes without heating, f) Discharging the polymer gel PI thus obtained into a granulator, g) Drying the polymer gel PI, h) Grinding and sieving the polymer PI in powder form.;

2. Water-soluble polymer PI according to claim 1, characterized in that the polymer PI has an average molecular weight between 500,000 daltons and less than 2 million daltons.

3. Water-soluble polymer PI according to one of claims 1 to 2, characterized in that the solution S2 of step a) contains between 2 and 20% by weight of polymer P2.

4. Water-soluble polymer PI according to one of claims 1 to 3, characterized in that the solution S2 of step a) contains between 3 and 15% by weight of polymer P2.

5. Water-soluble polymer PI according to one of claims 1 to 4, characterized in that the polymer P2 contains between 2% and 100% by weight of hydrophobic monomers.

6. Water-soluble polymer PI prepared according to one of claims 1 to 5, characterized in that the polymer P2 contains between 5% and 100% by weight of hydrophobic monomers.

7. Water-soluble polymer PI prepared according to one of claims 1 to 6, characterized in that the polymer P2 contains between 10% and 100% by weight of hydrophobic monomers.

8. Water-soluble polymer PI prepared according to one of claims 1 to 7, characterized in that the polymer P2 contains between 20% and 100% by weight of hydrophobic monomers.

9. Water-soluble polymer PI prepared according to one of claims 1 to 8, characterized in that the hydrophobic monomers of the polymer P2 are chosen from: - (meth)acrylic acid esters having an alkyl, arylalkyl and / or ethoxylated and / or propoxylated chain; (meth)acrylamide derivatives having an alkyl, arylalkyl or dialkyl and / or ethoxylated and / or propoxylated chain; cationic allyl derivatives having an alkyl, arylalkyl or dialkyl chain and / or an ethoxylated and / or propoxylated chain; anionic or cationic hydrophobic derivatives of (meth)acryloyl; and anionic or cationic monomeric derivatives of (meth)acrylamide carrying a hydrophobic chain;- n-hexyl (meth)acrylate, n-octyl (meth)acrylate, octyl (meth)acrylamide, n-tert-butyl (meth)acrylamide, diethyl (meth)acrylamide, lauryl (meth)acrylate, lauryl (meth)acrylamide, myristyl (meth)acrylate, myristyl (meth)acrylamide, pentadecyl (meth)acrylate, pentadecyl (meth)acrylamide, cetyl (meth)acrylate, cetyl (meth)acrylamide, oleyl (meth)acrylate, oleyl (meth)acrylamide, erucyl (meth)acrylate, erucyl (meth)acrylamide, and combinations thereof;- hydrophobic monomers corresponding to the general formula CH2=CR1-COO-(EO)n-(PO)m-R2 in which R1 is hydrogen or methyl, n is an integer of at least two, preferably from 6 to 100 or from 10 to 40, m is an integer from zero to 50, preferably from zero to 20, EO is an ethylene oxide group (-CH2-CH2-O-), PO is a propylene oxide group (-CH2-CH(CH3)-O-) and R2 is a C8-C30 alkyl group or a C8-C30 arylalkyl group, n+m being preferably from 6 to 100 or from 10 to 40.;

10. Water-soluble polymer PI prepared according to one of claims 1 to 9, characterized in that the hydrophobic monomers of the polymer P2 are chosen from: haloalkylated derivatives of methacrylamido-dimethylaminopropyl with a C8-C16 alkyl chain, the me- ethoxylated behenyl acrylate, diethylacrylamide, and n-tert-butylacrylamide.

11. Water-soluble polymer PI prepared according to one of claims 1 to 10, characterized in that the polymer P2 is a terpolymer of diethylacrylamide, n-tert-butylacrylamide and sodium 2-acrylamido-2-methylpropanesulfonate.

12. Water-soluble polymer PI prepared according to one of claims 1 to 11, characterized in that, prior to step a), the polymer P2 is functionalized at the end of the polymer chain by a hydroxyl, cyano, amine, phosphate, phosphonate, sulfate, sulfonate, xanthate, trithiocarbonate, dithiocarbamate, or dithioester fragment.

13. Water-soluble polymer PI prepared according to one of claims 1 to 12, characterized in that the polymer P2 contains at least one carbon-carbon double bond.

14. Water-soluble polymer PI prepared according to one of claims 1 to 7, characterized in that the polymerization reactor is an inverted conical tubular reactor.

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