PREPARATION

The invention relates to a particulate polymer composition and a method for preparing a particulate polymer composition according to the preambles of the enclosed claims.

Polymers and polymer compositions comprising polyacrylamide are commonly prepared by using catalysts, which comprise copper. Minute amounts of copper, originating from the used catalysts, are still present in the dried particulate polymer composition.

Polymer compositions may also come into contact with transition metals when the polymer compositions are used. For example, in enhanced oil recovery applications polymer compositions in solution form are injected to oil reservoir in order to increase the viscosity of the aqueous phase, which may often contain high concentrations of transition metals, such as iron.

Transition metals, such as copper and iron, induce the degradation of the polyacrylamide in the polymer composition. Degradation is mainly due to formation of free radicals that react with polyacrylamide chains and leads to dramatic viscosity drop. Transition metals, such as iron, can oxidize and produce free radicals, and also catalyse other radical reactions degrading the polymer. Polyacrylamide polymers and polymer compositions comprising polyacrylamide may be stabilized by addition of external stabilizer. Possible stabilizers are, for example 2-mercaptobenzothiazole 2-MBT and diethylthiourea. However, these stabilizers have not provided fully satisfactory results. Both 2-MBT and diethylthiourea has problems with limited solubility. Diethylthiourea has also safety issues, as it is suspected to cause cancer. It is clear that there is a need for a stabilizer that is effective and safe to use, even in demanding applications, such as enhanced oil recovery. An object of this invention is to minimise or even totally eliminate the disadvantages existing in the prior art.

Another object of the present invention is to provide a polymer composition that shows good stability and reduced degradation even in environments comprising transition metal ions, such as copper and/or iron.

Another further object of the present invention is to provide a method for preparing a polymer composition that shows good stability and reduced degradation.

The invention is defined in the characterising parts of the enclosed independent claims. Some preferable embodiments of the invention are defined in the dependent claims. All described features apply both for the polymer composition, the use as well as the method of the invention, whenever applicable, even if it not necessarily always stated so.

A typical particulate polymer composition according to the present invention comprises an acrylamide polymer and at least one substituted and/or unsubstituted benzotriazole as stabilizer.

A typical method according to the present invention for preparing particulate polymer composition comprising an acrylamide polymer comprises

- polymerising a reaction mixture comprising ethylenically unsaturated monomers in presence of water and initiator(s) by radical polymerisation and obtaining a polymer composition,

- comminuting the polymer composition by chopping or shredding, and

- drying the polymer composition, whereby a particulate polymer composition is obtained,

wherein at least one substituted and/or unsubstituted benzotriazole is added as stabilizer.

Now it has been surprisingly found out that benzotriazole, either substituted or unsubstituted benzotriazole, effectively functions as stabilizer for polymer composition comprising an acrylamide polymer, especially in enhanced oil recovery applications. Benzotriazole is able, even at relatively low dosages, improve the stability of the acrylamide polymer and reduce its degradation in a solution in presence of transition metals, especially reducing degradation catalyzed by both copper and iron. Benzotriazole is also non-hazardous so it is safe to use, even at higher dosages.

Furthermore, the price of benzotriazole is advantageous, which provides economical benefits when benzotriazole is used as stabilizer in comparison to other stabilizers.

Benzotriazole C6H5N3 is a polar heterocyclic compound containing a five- membered ring with three nitrogen atoms directly bonded to one another as substituents on a benzene ring. In the present context, a general reference to benzotriazole encompasses both substituted and unsubstituted benzotriazoles, if not explicitly otherwise stated.

According to one preferable embodiment benzotriazole is a substituted benzotriazole having at least one substituent. Benzotriazole may also have two or more substituents. Each substituent is preferably attached to the carbon atoms of the benzene ring structure of benzotriazole. The substituent may be selected from alkyl, amine, halogen and/or hydroxyl groups. Preferably the substituent may be selected from alkyl, such as methyl, amine and/or hydroxyl groups, more preferably from alkyl groups. When benzotriazole contains two or more substituents, the all substituents may be identical, some of the substituents may be identical, or all the substituents may be different from each other. According to another preferable embodiment benzotriazole is disubstituted benzotriazole, i.e. substituted benzotriazole has two substituents. The substituted benzotriazole may be selected from a group consisting of chloro- benzotriazole, hydroxy-1 H-benzotriazole, amino-1 H-benzotriazole, methyl-1 H- benzotriazole, methylbenzotriazole, propylbenzotriazole, nitrobenzotriazole, 4,5,6,7-tetrahydrobenzotriazole, 5-methyl-4-hydroxybenzotriazole and 5,6- dimethyl-1 H-benzotriazole, preferably from a group consisting of chlorobenzo- triazole, hydroxy-1 H-benzotriazole, amino-1 H-benzotriazole and methyl-1 H-benzotriazole. According to one preferable embodiment of the present invention benzo- triazole is methyl-1 H-benzotriazole.

The particulate polymer composition may comprise one or more different benzotriazoles as stabilizers. For example, stabilizer may be a mixture of methyl- 1 H-benzotriazole and hydroxyl-1 H-benzotriazole. The polymer composition may comprise at least 0.001 weight-%, preferably at least 0.01 weight-%, more preferably at least 0.1 weight-%, of benzotriazole, calculated from the dry matter of composition. According to one embodiment the amount of benzotriazole in the polymer composition may be in the range of 0.01 - 10 weight-%, preferably 0.01 - 5 weight-%, more preferably 0.1 - 2 weight-%. Benzotriazole has low toxicity and shows low health hazards, especially to mammals, such as humans, which enables use of relatively high amounts of benzotriazole as stabilizer in polymer compositions.

According to one embodiment of the invention the polymer composition may further comprise a free radical scavenger. The free radical scavenger may be selected from thioureas and alkyl thioureas, mercaptobenzothiazole, mercaptobenzoimidazole and thiocyanates. According to one preferable embodiment the free radical scavenger may be selected from ammonium or alkali metal salts of thiocyanate, preferably sodium thiocyanate. The polymer composition may comprise at least 0.001 weight-%, preferably at least 0.01 weight-%, more preferably at least 0.1 weight-%, of alkali metal salt of thiocyanate, preferably sodium thiocyanate, calculated from the dry matter of composition. According to one embodiment the amount of alkali metal salt of thiocyanate, preferably sodium thiocyanate, in the polymer composition may be in the range of 0.01 - 10 weight-%, preferably 0.01 - 5 weight-%, more preferably 0.1 - 2 weight- %. It has been unexpectedly observed that when the polymer composition comprises both benzotriazole and a free radical scavenger, preferably an alkali metal salt of thiocyanate, as stabilizers they provide a synergetic effect in degradation reduction, which effect is higher than the sum of the effects obtained by using the individual components alone. The synergetic effect is especially observable in enhanced oil recovery applications, where both copper and iron ions are present in the environment.

The polymer composition according to the invention comprises preferably water- soluble acrylamide polymer. The polymer composition is also preferably water- soluble. Water-soluble in this context denotes that the polymer composition and its constituents are fully miscible with water and dissolve in water. When mixed with excess of water the composition is preferably fully dissolved and the obtained solution is preferably essentially free from any discrete particles or granules, preferably without any remaining solid particulate material or swelled particles of acrylamide polymer. Excess of water means that the obtained solution is not a saturated solution in regard of any of the constituents present.

The polymer composition may comprise an acrylamide polymer, which is a homopolymer of acrylamide or methacrylamide or a copolymer of acrylamide or methacrylamide and one or more ethylenically unsaturated monomers. Acrylamide polymer may be a copolymer of acrylamide and ethylenically unsaturated hydrophilic and/or ionic monomers, or a copolymer of methacrylamide and ethylenically unsaturated hydrophilic and/or ionic monomers.

Suitable ethylenically unsaturated hydrophilic monomers may be selected from group comprising, for example, N-alkyl(meth)acrylamides, such as N- methylacrylamide, N-ethylacrylamide, N-propylacrylamide, N-ethylmethacrylamide; N,N-dialkyl(meth)acrylamides, such as Ν,Ν-dimethylacrylamide, N,N- diethylacrylamide, Ν,Ν-dimethylmethacrylamide; hydroxyalkyl (meth)acrylates, such as hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate; diacetone acrylamide; N-acryloyl pyrrolidine; N- acryloylpiperidine; N-acryloylmorpholine; methoxypolyethylene glycol (meth)acrylates; N-vinyl-2-pyrrolidone and any of their mixtures. Preferably the acrylamide polymer is an anionic copolymer of acrylamide or methacrylamide. Suitable ethylenically unsaturated anionic monomers may be selected from group comprising acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, aconitic acid, mesaconic acid, citraconic acid, crotonic acid, isocrotonic acid, angelic acid, tiglic acid, vinylsulphonic acid, allylsulphonic acid, maethallylsulphonic acid, styrenesulphonic acid, 2-acrylamido-2- phenylpropanesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and their salts. Preferably the anionic monomer is selected from group comprising acrylic acid, methacrylic acid and itaconic acid.

The reaction mixture comprising acrylamide or methacrylamide monomers and optionally ethylenically unsaturated hydrophilic and/or ionic monomers is polymerised in presence of water and initiator(s) by free radical polymerisation. Total monomer concentration of the reaction mixture is typically 20 - 70 weight-%, preferably 30 - 55 weight-%, sometimes 30 - 40 weight-%. The polymerisation may be performed as a batch process or a continuous process.

In the continuous process, where reaction mixture is transferred during the polymerisation in a reaction apparatus, such as a belt conveyor, screw extruder or a tubular reactor. The reaction mixture comprising the above described monomers and initiator(s) are introduced to the reaction apparatus near or at the first end/inlet of the reaction apparatus and the reaction mixture proceeds, while the polymerisation reaction is taking place, towards the second end/outlet of the reaction apparatus. A polymer composition is removed from the reaction apparatus at the second end/outlet of the reaction apparatus. The reaction time may be adjusted by adjusting the forward feeding speed of the reactor apparatus. Preferably the residence time in the reactor apparatus is selected such that a 99 % conversion is obtained before the second end/outlet of the reaction apparatus. Generally, the polymerization reaction reaches required conversion degree within 30 - 200 minutes. The radical polymerisation may be performed in air or in an inert atmosphere. In a batch process, the reaction mixture polymerises in a reaction apparatus, such as a drum or a batch reactor. The reaction mixture comprising monomers and initiator(s) are introduced to the reaction apparatus in an inert atmosphere, and the polymerisation takes place. The residence time in the reaction apparatus may vary between 40 and 240 min. The residence time may be adjusted by adjusting the initiator dosage. At the end of the polymerisation a polymer composition is removed from the reaction apparatus.

Further, reaction mixture comprises normally at least one initiator, which initiates the free radical polymerisation process, such as UV-sensitive initiator, temperature sensitive (thermal) initiator or redox initiator. The initiator may be organic or inorganic. Suitable polymerisation initiators are known to those skilled in the art. Typical organic initiators include, for example, various azo-compounds, whereby water-soluble azo-compounds are being preferred. Examples of suitable organic initiators are such as 2,2'-azo-bis(isobutyronitrile), 4,4'-azo-bis(4-cyanopentanoic acid) 2,2'-azo-bis(N,N'-dimethylene iso-butyramidine) dihydrochloride; 2,2'-azobis (2-amidinopropane) dihydrochloride and 2,2'-azobis [2-methyl-N-(2-hydroxyethyl) propionamide]. Other possible initiators include potassium persulfate and peroxides, such as t-butyl hydroperoxide. It is also possible to use redox systems as an initiator, for example t-butylhydroperoxide and sodium bisulphite. A further example of possible redox initiators are ammonium persulphate and ferrous ammonium sulphate, which can be used as aqueous solutions. Combinations of azo-initiators and redox initiator systems are also possible. Other useful initiators are various heavy-metal activated catalyst systems. Typically initiator(s) is/are used in amount of 0.0001 - 0.5 weight-%, calculated from the total weight of the monomers.

It is also possible to add or use further reagents during the preparation process such as chain transfer agents, buffers, diluents, derivatizing agents and cross- linking agents.

The acrylamide polymer in the polymer composition may be non-crosslinked or only slightly crosslinked by using a conventional cross-linker in the polymerisation in amount of 0.25 - 100 mg/kg monomers, preferably 0.5 - 10 mg/kg monomers, preferably 0.75 - 5 mg/kg monomers. Suitable conventional cross-linkers are, for example, methylenebisacrylamide, ethylene glycol divinyl ether, di(ethylene glycol) divinyl ether, tri(ethylene glycol) divinyl ether, methylenebisacrylamide being preferred. According to one preferable embodiment of the invention the acrylamide polymer in the polymer composition is non-crossl inked. This means that the acrylamide polymer is obtained by free radical polymerisation of an aqueous reaction mixture comprising acrylamide or methacrylamide monomers and optionally ethylenically unsaturated hydrophilic and/or ionic monomers, whereby the reaction mixture is free of crosslinking agents.

The acrylamide polymer in the polymer composition is preferably a linear polymer.

The free radical polymerisation of monomers produces a polymer composition, which is in gel form or in form of a highly viscous liquid.

The polymer composition is comminuted by chopping or shredding. Depending on the used reaction apparatus, shredding or chopping may be performed in the same reaction apparatus where the polymerisation takes place. For example, polymerisation may be performed in a first zone of a screw mixer, and the shredding of the obtained polymer composition is performed in a second zone of the said screw mixer. It is also possible that the shredding, chopping or other particle size adjustment is performed in a treatment apparatus, which is separate from the reaction apparatus. For example, the obtained polymer composition may be transferred from the second end of a reaction apparatus, which is a belt conveyor, through a rotating hole screen or the like, where it is shredded or chopped into small particles.

After shredding or chopping the comminuted polymer composition is dried, milled to a desired particle size and packed for storage and/or transport. The polymer composition typically has a particle size of 0.1 - 2 mm. The process may further comprise a number of other post-treatment steps such as pelletizing and/or sieving. It is also possible to add or use further reagents, such as buffers and diluents. According to one embodiment benzotriazole may be added to the polymer composition after the polymerization step, but before the polymer composition is comminuted. Benzotriazole may be sprayed on the obtained polymer composition, for example, when the polymer composition is transferred from the second end of the reaction apparatus towards the shredding or comminuting step.

Alternatively benzotriazole may be added to the reaction mixture, so that it is embedded to the formed polymer matrix. When this manner of preparation is used, the stabilizer is incorporated into the structure of the polymer composition, and there is no risk for separation of stabilizer and other composition components during transport and/or storage.

According to one embodiment benzotriazole is added at or after the comminuting step. For example, benzotriazole may be sprayed on comminuted, optionally dried, polymer composition.

According to yet another alternative benzotriazole is added as dry powder to the comminuted and dried polymer composition by mixing. When this manner of preparation is used, the amount of stabilizer, i.e. benzotriazole, is to adjust according to the intended use of the polymer composition.

According to an aspect of the invention benzotriazole is used for stabilizing polymer composition comprising an acrylamide polymer against degradation. For example, according to one embodiment benzotriazole may be added in form of a solution in an industrial process flow comprising dissolved polymer composition, which comprises an acrylamide polymer. Benzotriazole interacts in the process flow with transition metals and other disturbing substances which might otherwise induce degradation of the polymer. EXPERIMENTAL

An embodiment of the invention are more closely described in the following, non- limiting example.

Example 1

Polymer solution is prepared at 2500 ppm concentration with carbonate buffer generating approximate pH of 7.3 oxygen free inside glovebox. Copper residue in the polymer solution was 1 .4 ppm. Radical generating chemistry simulating field conditions in enhanced oil recovery applications is added to polymer solution in order to obtain 2 ppm Fe(ll) and 5 ppm sulphite. Samples are split into vials (50 ml each) and total addition volume is 5 ml. Stabilizers are added from their respective mother solutions at calculated volumes to reach desired concentration, oxygen is added (3.5 ml O2 saturated water) and rest of addition volume is satisfied using oxygen free deionized water. All chemical mother solutions are made into deionized water, thus every sample receives roughly the same amount of deionized water and salt difference between samples will be negligible. Polymer solution will be diluted 10 %, but every sample is diluted exactly the same amount, thus preserving the comparability between samples.

After the samples have been prepared, they are closed with butyl-rubber cap which has PTFE-liner and aluminum jacket to provide tight seal. Closed sample vials are taken into oven to 62 °C and stored there overnight. Samples are measured inside glovebox with viscometer the following day. Results are given in Table 2. Table 1 : Solution simulating field conditions

Table 2: Results A= Diethylthiourea

B= Sodium Thiocyanate

C= Methyl-1 H-benzothazole

From Table 2 it can be seen that at same concentration sample C is more effective than samples A and B. Also a synergistic effect of combination B and C can be seen, as the combination sample (B 0.5 w-% + C 0.5 w-%) with 1 .0 weight-% total concentration was clearly more efficient than B or C alone at same 1 .0 weight-% concentration. Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.