WO2007075123A2 - Synthetic resin and a method for the production thereof - Google Patents

Abstract

The invention relates to a synthetic resin for producing wood chipboards, plywood and glued joints and to a method for the production thereof. The resin production component ratio consists of 52.2-68.1 mass % carbamide, 29.0-37.9 mass % formaldehyde, the rest being the oxidised starch reagent up to 100 mass %. The molecular mass of the obtained resin is equal to or greater than 1000. The inventive method consists in preparing a reaction mixture which is based on carbamide and formaldehyde with a pH of 7.0-7.5, in heating a mixture to a temperature equal to less than a boiling point for a time required for forming hydroxymethyl groups, in condensing by reducing pH to 4.2-4.5 for forming oligomer whose polycondensation degree ranges from 6 to 10, in neutralising to the pH ranging from 6.7-7.0, in reducing the temperature by 20-27 °C, in introducing an oxidised starch reagent in a quantity of 1-25 mass parts with respect to a dry substance by counting on the base of 100 mass parts of a dry formaldehyde and in finishing the condensation by adding carbamide and by cooling to a room temperature. Said invention makes it possible to reduce a free formaldehyde content and to increase the resin degree of miscibility with water.

Description

 SYNTHETIC RESIN AND METHOD

Technical field

The group of inventions relates to the chemical industry and relates to a synthetic resin and a method for its production, which is used mainly in the production of chipboards and fiberboards, plywood and adhesive wood compounds.

State of the art

Known and most widely used in industry is urea-formaldehyde resin of the brand KF-MT-15 [TU 6-05-12-88, Handbook for the production of chipboards / Otlev I.A. et al. - 2nd ed. reslave. and add. - M .: Lesn. industry, 1990. - p. 41-42]. Urea, formaldehyde in the form of an aqueous solution (formalin) or in the form of a urea-formaldehyde concentrate (CPA) and additives to control the pH of the medium (for example, aqueous solutions of caustic soda and ammonium chloride) are used to synthesize the resin.

The physicochemical properties of urea-formaldehyde resins (CFS) mainly depend on the molar ratio of urea: formaldehyde and on the synthesis conditions of the resin. The initial composition of the reaction mixture of the resin brand KF-MT-15 has a molar ratio of urea to formaldehyde equal to 1: 2, the final molar ratio of urea: formaldehyde is 1: 1.21 - 1: 1.24 due to the fact that additional urea is introduced in an amount 65-61 mass parts (May hours) for every 100 May. including urea consumed for the preparation of the reaction mixture. The resin has an index of maximum miscibility with water 1: 2 - 1:10, a shelf life of 1.5 months, the content of free formaldehyde is not more than 0.15%. The average molecular weight of this resin does not exceed 700. The use of resin in the manufacture of plates provides products with a formaldehyde content of 12-18 mg / 100 g abs. dry plate, which classifies it as E2 formaldehyde emission class according to GOST 10632-89, which has limited use.

Another resin is known that is used for the manufacture of plates, in order to reduce the toxicity of which to the level of requirements of the class El formaldehyde emissions, then there is no more than 10 mg / 100 g abs. dry plate, the composition of the reaction mixture for the synthesis of resin has a molar ratio of urea: formaldehyde 1: 1.15 - 1: 1.20

[Kondratiev B.P., Sholokhova G.V. New carbamide resins for wood-based panels.

Synthesis, modification and use of resins for wood boards / Scientific. - prakt. Seminar, November 18, 2004, St. Petersburg: Publishing House of the Polytechnic University, University of 2004, p. 30-40]. The resin has an index of maximum miscibility with water 1: 1 - 1: 4, a shelf life of 1 month, the content of free formaldehyde is not more than OD 1%.

Known compositions of the reaction mixture having a molar ratio of urea: formaldehyde, varying in the range from 1: 0.85 to 1: 1.15 [Anokhin A.E. On the adhesive ability of low molar urea resins. Wood Boards: Theory and Practice: Second Scientific Pract. seminar, March 17-18, 1999, St. Petersburg: SPbLTA, 1999, p. 30-32]. Resins have a free formaldehyde content of not more than 0.10% [Hotilovich P.A., Shabanov Yu.V. Studying the influence of conditions. synthesis of CFS on the formation of the properties of low-toxicity reduced toxicity. Wood Boards: Theory and Practice: Fourth Scientific Pract. Seminar, March 21-22, 2001, St. Petersburg: SPbLTA, 2001, p. 24-26]. Experience in the development and application of CFS with a low (below 1: 1.20) molar ratio revealed a limited shelf life of resins that are practically not transportable [Kondratiev V.P., Sholokhova G. B., p. 32]. Resins have a guaranteed shelf life of 2-7 days and are capable of pasting [Doronin Yu.G., Kondratiev V.P., Savelyeva T.V. Ways to improve the synthesis of urea-formaldehyde resins in order to reduce the toxicity of finished products: Overview, inform. - M .: VNIPIEIlesprom, 1988, p. 19].

As a formaldehyde-containing component for the synthesis of resins, formalin or urea-formaldehyde concentrate (KFK) is used, which is an aqueous solution of formaldehyde and urea [Kondratyev VP, Kondrashchenko V. I. Synthetic adhesives for wood materials. - M .: Scientific World, 2004, p. 254-264]. The use of urea-formaldehyde concentrate allows to exclude a long stage of resin synthesis - evacuation, due to the high concentration of formaldehyde in KFK (50 - 60% of the total composition). Non-evacuated resins are known, for example, grades KF-A, KF-HB. In the production of such resins from formalin, the vacuum operation is excluded [Kondratiev V.P., Kondrashchenko V.I. from. 245 - 253]. A feature of non-evacuated resins is the mass fraction of solids reduced to 52-55%, however the production of such resins is widespread, since the amount of wastewater is reduced, which allows to improve the environmental situation and increase the efficiency of production.

The disadvantage of the composition of the reaction mixture with a molar ratio of urea: formaldehyde from 1: 1.21 to 1: 1.30 is the increased content of free formaldehyde in the resin, as a result of which the products based on these resins do not meet the toxicity requirements (GOST 10632-89). The disadvantage of the composition of the reaction mixture with a molar ratio of urea: formaldehyde from 1: 0.85 to 1: 1.20 is the low miscibility of the resin with water, which complicates the preparation of the resin solution of the required concentration, as well as the short shelf life of the resin.

To reduce the formaldehyde content in the plates and plywood, it is proposed to introduce starch into the resin in an amount of 3-6% by weight of the finished resin [V. Kondratiev,

Kondrashchenko V.I. from. 123-127, p. 302-304]. The disadvantage of the composition is that the starch is insoluble in water and after introduction into the resin is in suspension, which requires constant mixing of the prepared mixture. In addition, when starch is mixed with the finished resin, formaldehyde binding with starch can occur during the short storage time of the prepared composition and the short time of hot pressing of the boards or plywood.

To increase the miscibility index of resins with water and increase their stability during storage, alcohol additives are introduced into them: propylene glycol and (or) triethanolamine [Kondratiev V.P., Kondrashchenko V.I. from. 293, 302] in the amount of 8.0 and 0.35%, respectively [Tupitsyn Yu.C., Miroshnichenko C.H., Notkin M.M. Processes and equipment for finishing wood-based panel materials. - M .: Lesn. industry, 1983, p.108]. Alcohols are added during the synthesis of CFS or in the finished resin. The lack of composition is the toxicity of alcohols and their inability to bind free formaldehyde.

A synthetic urea-formaldehyde resin of the KF-MT brand [Doronin Yu.G., Miroshnichenko C.H., Svitkina MM

Synthetic resins in woodworking. - 2nd ed., Revised. and add. - M .: Forest industry, 1987. - p. 10, 31-39]. KF-MT urea-formaldehyde resin is a mixture of low molecular weight polycondensation products with the average molecular weight of not more than 700, that is, it is a mixture of oligomers with a degree of polycondensation of 6-10. The resin has a mass fraction of solids of 65-67%, the mass fraction of free formaldehyde is not more than 0.3%, the other physicochemical properties of the resin are shown in the Table. The resin is characterized by good maximum miscibility with water (1: 4 - 1:10), its shelf life is 2-3 months, but it has a high content of free formaldehyde, which prevents its use in the manufacture of boards and plywood that do not meet toxicity requirements.

Resin KF-MT is obtained as follows. The reaction mixture is prepared from neutralized with a solution of caustic soda to a pH of 7.0-7.5 formalin and urea with a molar ratio of urea to formaldehyde equal to 1: 2. The first stage of the process involves heating to 90-92 ⁰ C, i.e. to a temperature not exceeding the boiling point of the mixture, and holding at this temperature for 30 minutes, i.e. during the time necessary for the formation of hydroxymethyl groups. In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5 and a temperature of 90-92 ° C, for 20-30 minutes, i.e. maintaining the initial temperature for the time required for the formation of the oligomer with a degree of polycondensation of 6-10. Excerpted by the introduction of caustic soda upon receipt of a milky white sample for miscibility with water. The condensation solution is neutralized to a pH of 6.7-7.0. In the third stage, the solution is cooled to 70-72 ⁰ C, i.e. the temperature is lowered by 2O ⁰ C from the original, and kept it under vacuum at a temperature of 65-70 ⁰ C and a discharge of 0.09-0.08 MPa. In the process of vacuum drying, water is distilled off in an amount of 21-23% by weight of the starting components. The fourth stage of the condensation process occurs with the additional introduction of urea in the amount of 54 mass parts (May hours) for every 100 May. including urea consumed for the preparation of the reaction mixture. The molar ratio of urea: formaldehyde is reduced to 1: 1.3. Subcondensation is carried out at 60 ⁰ C for 30 minutes, after which the resin is cooled to 25-30 ° C, i.e. to room temperature. The disadvantage of this method is its high duration of 6 hours, which is associated with the need to evacuate the resin for 2 hours, and also that it does not provide a resin with a low content of free formaldehyde, since the content of free formaldehyde in this resin is up to 0.3%, as described above.

SUMMARY OF THE INVENTION An object of the present invention is to provide a new synthetic resin and a method for its preparation, which provides a reduction in the content of free formaldehyde while increasing the miscibility of the resin with water.

The resin is obtained by the interaction of urea and formaldehyde in the presence of an oxidized starch reagent in the following ratio of components,% mass: urea 58.2 - 68.1; formaldehyde 29.0 - 37.9; oxidized starch reagent - the rest is up to 100. The resulting resin has a molecular weight of at least 1000.

A method of obtaining the inventive synthetic resin includes the preparation of a reaction mixture based on urea and formaldehyde with a pH of 7.0-7.5 with a ratio of urea to formaldehyde equal to 1: 2 mol. The first stage of the process involves heating to 90-92 ° C, i.e. to a temperature not exceeding the boiling point of the mixture, and holding at this temperature for no more than 30 minutes, i.e. during the time necessary for the formation of hadroxymethyl groups. In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5, maintaining the initial temperature, for no more than 30 minutes, i.e. during the time required for the formation of an oligomer with a degree of polycondensation of 6-10. Exposure is stopped upon receipt of a milky white sample for miscibility with water by neutralizing the condensation solution to a pH of 6.7-7.0. In the third stage, the solution is cooled to 65-72 ° C, i.e. the temperature is lowered by 20-27 ° C from the original. The fourth stage of the condensation process is carried out with the additional introduction of urea. Subcondensation is carried out at a temperature of at least 6O ⁰ C for at least 20 minutes (i.e., the time for free formaldehyde to bind), after which the resin is cooled to 20-30 ° C, i.e. to room temperature. In the process of resin synthesis, before the beginning of the fourth stage, starch oxidized reagent (PKO) is introduced in the amount of May 1-25. hours on absolutely dry matter, counting on 100 mass parts of absolutely dry formaldehyde, while the molar ratio of urea to formaldehyde after the fourth stage is 1: (0.85-1.30).

The oxidized starch reagent is obtained from grain raw materials, for example, according to the patent of the Russian Federation N ° 2017750 "Method for producing a product having adhesive properties from grain raw materials." The starch oxidized reagent is an aqueous suspension containing low molecular fatty acids, amino acids and carboxyl-containing derivatives of carbohydrates in the form of salts as a soluble part. As an insoluble part, a slightly hydrolyzed protein, salts of carboxylated fiber and high molecular weight starch are present in the suspension. The molecular weight of the starch oxidized reagent is in the range from 5.1-10 ² to 2.3-10 ⁷ , the average molecular weight of the starch oxidized reagent is 6-10 ³ - 7-10 ⁵ depending on the depth of oxidation of the feedstock.

The advantage of using a starch oxidized reagent in the composition of the reaction mixture is that the starch oxidized reagent is capable of chemically bonding part of the free formaldehyde due to the active functional groups of carbohydrates and protein for a longer period and at a higher temperature compared to introducing it or native starch into the finished resin. The presence of carboxyl-containing derivatives of carbohydrates in the form of salts gives the starch oxidized reagent surface-active properties, which improves the miscibility of the resin with water and is achieved when the molecular weight of the obtained resin is not less than 1000. This molecular weight also affects the ability to bind free formaldehyde present in the finished product , reducing its amount to 0.02 - 0.10%. An additional advantage in the synthesis of the resin is the possibility of reducing or completely replacing the alkali solution to neutralize the reaction mixture due to the fact that the starch oxidized reagent solution is alkaline in nature.

Embodiments of the invention

In more detail, the invention is disclosed in the examples below.

The following substances are used in the examples: - carbamide (GOST 2081-75); formaldehyde in the form of an aqueous solution - formalin (GOST 1625-89) and in the form of a urea-formaldehyde concentrate - KFK (TU 2181-032-00203803-2003); - oxidized starch reagent in the form of an aqueous solution with a concentration

eighteen%.

Example 1. The reaction mixture is prepared from neutralized with a solution of sodium hydroxide to a pH of 7.0-7.5 formalin in an amount of 270 may. hours and 100 may. including urea, that is, with a ratio of urea to formaldehyde equal to 1: 2 mol. The first stage of the process involves heating to 90-92 ⁰ C and holding at this temperature for 30 minutes. In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5 and a temperature of 90-92 ° C for 20-30 minutes. Exposure is stopped upon receipt of a milky white sample for miscibility with water. The degree of polycondensation of the oligomer is -6. Then, a solution of starch oxidized reagent is introduced in an amount of May 10. hours by abs. dry matter. The condensation solution is neutralized to a pH of 6.7-7.0. In the third stage, the solution is cooled to 65-72 ° C. The resin is not evacuated. The fourth stage of the condensation process occurs with the additional introduction of urea in the amount of May 135. h for every 100 may. including urea consumed for the preparation of the reaction mixture. The molar ratio of urea: formaldehyde is reduced to 1: 0.85. Subcondensation is carried out at 60 ° C for 30 minutes, after which the resin is cooled to 25-30 ° C.

The ratio of absolutely dry resin components,% mass: urea - 68.1; formaldehyde - 29.0; oxidized starch reagent - 2.9. The synthetic resin has an average molecular weight of 1.8-10 ⁴ , a free formaldehyde content of 0.02%, and a maximum miscibility of 1: 2 with water. The remaining parameters of the resin are shown in the Table.

Example 2. The reaction mixture is prepared from formalin in an amount of 270 may. hours and a solution of reagent starch oxidized in the amount of May 1. hours by abs. dry matter, neutralized with sodium hydroxide solution to a pH of 7.0-7.5, add May 100. including urea, that is, with a ratio of urea to formaldehyde equal to 1: 2 the mole. The first stage of the process involves heating to 90-92 ° C and holding at this temperature for 30 minutes. In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5 and a temperature of 90-92 ° C for 20-30 minutes. Excerpted by the introduction of caustic soda upon receipt of a milky white sample for miscibility with water. The degree of polycondensation of the oligomer is -7.

The condensation solution is neutralized to a pH of 6.7-7.0. In the third stage, the solution is cooled to 65-72 ° C. The resin is not evacuated. The fourth stage of the condensation process occurs with the additional introduction of urea in the amount of May 100. h for every 100 may. including urea consumed for the preparation of the reaction mixture. The molar ratio of urea: formaldehyde is reduced to 1: 1. Subcondensation is carried out at 60 ° C for 30 minutes, after which the resin is cooled to 25-30 ° C.

The ratio of absolutely dry resin components,% mass: urea -

66.5; formaldehyde - 33.2; oxidized starch reagent - 0.3. The synthetic resin has an average molecular weight of 1.0-10 ³ , a free formaldehyde content of 0.05%, and a maximum miscibility of 1: 1 with water. The remaining parameters of the resin are shown in the Table.

Example 3. For the synthesis of resins using urea-formaldehyde concentrate (CPK) containing 60% formaldehyde, 25% urea and 15% water.

Take CPK in the amount of May 167. hours, which contain May 100. h. abs. dry formaldehyde, May 42. including urea and May 25. including water. If necessary, using a solution of caustic soda, the pH of the concentrate is set at 7.0-7.5.

Add May 59. hours of water and May 58. including urea, that is, the ratio of urea to formaldehyde is 1: 2 mol. The first stage of the process involves heating to 90-

92 ⁰ C and holding at this temperature for 30 minutes In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5 and a temperature of 90-92 ⁰ C for 20-30 minutes. Exposure is stopped upon receipt of a milky white sample for miscibility with water. The degree of polycondensation of the oligomer is 8. Then, a solution of oxidized starch reagent is introduced in an amount of May 5. hours by abs. dry matter. The condensation solution is neutralized to a pH of 6.7-7.0. In the third stage, the solution is cooled to 65-72 ⁰ C. The resin is not evacuated. The fourth stage of the condensation process occurs with the additional introduction of urea in amount of May 100. h for every 100 may. including urea consumed for the preparation of the reaction mixture. The molar ratio of urea: formaldehyde is reduced to 1: 1. Subcondensation is carried out at 60 ⁰ C for 30 minutes, after which the resin is cooled to 25-30 ⁰ C. The ratio of absolutely dry resin components,% mass: urea -

65.6; formaldehyde - 32.8; oxidized starch reagent - 1.6. The synthetic resin has an average molecular weight of 1.6-10, a free formaldehyde content of 0.04%, and a maximum miscibility of 1: 3 with water. The remaining parameters of the resin are shown in the Table.

Example 4. For the synthesis of resin using urea-formaldehyde concentrate (CPK) containing 60% formaldehyde, 25% urea and 15% water. Take CPK in the amount of May 167. hours, which contain May 100. h. abs. dry formaldehyde, May 42. including urea and May 25. including water, add a solution of starch oxidized reagent in an amount of May 10. hours by abs. dry matter. If necessary, set the pH of the mixture at the level of 7.0 - 7.5. Add on May 38th. hours of water and May 58. including urea, that is, the ratio of urea to formaldehyde is 1: 2 mol. The first stage of the process involves heating to 90-92 ° C and holding at this temperature for 30 minutes. In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5 and a temperature of 90-92 ⁰ C for 20-30 minutes. Excerpted by the introduction of caustic soda upon receipt of a milky white sample for miscibility with water. The degree of polycondensation of the oligomer is 9. The condensation solution is neutralized to a pH of 6.7-7.0. In the third stage, the solution is cooled to 65-72 ° C. The resin is not evacuated. The fourth stage of the condensation process occurs with the additional introduction of urea in the amount of May 100. h for every 100 may. including urea consumed for the preparation of the reaction mixture. The molar ratio of urea: formaldehyde is reduced to 1: 1. Subcondensation is carried out at 60 ° C for 30 minutes, after which the resin is cooled to 25-30 ° C. The ratio of absolutely dry resin components,% mass: urea -

64.5; formaldehyde - 32.3; oxidized starch reagent - 3.2. Synthetic resin has an average molecular weight of 7.1-10 ³ , the content of free formaldehyde 0.03%; maximum miscibility with water 1: 4. The remaining parameters of the resin are shown in the Table.

Example 5. The reaction mixture is prepared from neutralized with sodium hydroxide solution to a pH of 7.0-7.5 formalin in an amount of 270 may. hours and 100 may. including urea, that is, with a ratio of urea to formaldehyde equal to 1: 2 mol. The first stage of the process involves heating to 90-92 ° C and holding at this temperature for 30 minutes. In this case, the pH of the medium decreases to 6.0-6.5. The second stage is carried out at a pH of 4.2-4.5 and a temperature of 90-92 ° C for 20-30 minutes. Exposure is stopped upon receipt of a milky white sample for miscibility with water. The degree of polycondensation of the oligomer is -10. Then, a starch oxidized reagent solution is introduced in an amount of May 15. hours by abs. dry matter. The condensation solution is neutralized to a pH of 6.7-7.0. In the third stage, the solution is cooled to 65-72 ° C and kept under vacuum at a temperature of 65-70 ° C and a discharge of 0.09-0.08 MPa for 2 hours. The fourth stage of the condensation process occurs with the additional introduction of urea in the amount of May 100. h for every 100 may. including urea consumed for the preparation of the reaction mixture. The molar ratio of urea: formaldehyde is reduced to 1: 1. Subcondensation is carried out at 6O ⁰ C for 30 minutes, after which the resin is cooled to 25-3O ⁰ C.

The ratio of absolutely dry resin components,% mass: urea - 63.5; formaldehyde - 31.7; oxidized starch reagent - 4.8. The synthetic resin has an average molecular weight of 2.5-10, a free formaldehyde content of 0.03%, and a maximum miscibility of 1: 5 with water. The remaining parameters of the resin are shown in the Table.

The authors carried out a number of additional experiments on the synthesis of resins with different ratios of the starting components (see the Table of items 6-17). The table for comparison shows the results of experiments in which the amount of PKO is less than the minimum declared value and greater than the maximum, as well as examples in which PKO was not used in items 18 and 19. Line 20 of the table in the prototype information In all these experiments, temperature and temporary conditions, as well as the pH of the medium at the stages of the process were maintained at levels, as described above. Physico-chemical properties of the obtained synthetic resins are presented in the Table. An analysis of the obtained test results of resins synthesized with different compositions of the reaction mixture shows a real possibility of reducing the content of free formaldehyde in the resin and increasing its miscibility with water when a starch oxidized reagent with a molecular weight of at least 1000 is included in the reaction mixture for the synthesis of urea-formaldehyde resin. So, in comparison with the prototype, the resin from experience

JChz 15. manufactured at the same molar ratio of urea to formaldehyde equal to 1: 1.3, but since May 10. including PKO, has a 26.7% lower content of free formaldehyde and 1.5 times better miscibility with water. Similar dependences are observed for resins with molar ratios of urea to formaldehyde of 1: 1.0 and 1: 0.85. When starch oxidized reagent is introduced into the composition of their reaction mixture, the content of free formaldehyde in the resin decreases and their miscibility with water improves. New properties make it possible to apply the obtained resin to the production of plates and plywood used in furniture and construction details.

Table

Type of formaldehyde-containing raw materials and compositions of the reaction mixture for the synthesis of synthetic resins, physicochemical properties of synthetic resins

to

Claims

CLAIM 1. A synthetic resin obtained by the interaction of urea and formaldehyde in the presence of an oxidized starch reagent, characterized in that it has an average molecular weight of at least 1000. 2. The synthetic resin according to claim 1, characterized in that it is obtained in the following ratio of components,% mass: urea 58.2 - 68.1; formaldehyde 29.0 - 37.9; oxidized starch reagent - the rest is up to 100. 3. A method of obtaining a synthetic resin, including the preparation of a reaction mixture based on urea and formaldehyde with a pH of 7.0-7.5, the first step is heating the mixture to a temperature not exceeding the boiling point and holding at this temperature for the time required for the formation hydroxymethyl groups; the second stage - condensation when the pH is lowered to 4.2-4.5, maintaining the initial temperature for the time required for the formation of an oligomer with a degree of polycondensation of 6-10, neutralization of the condensation solution to a pH of 6.7-7.0; the third stage - reducing the temperature by 20-27 ° C in relation to the initial one; the fourth stage is the introduction of an additional amount of urea, post-condensation, followed by cooling to room temperature, characterized in that before the start of the fourth stage, oxidized starch reagent is introduced in the amount of 1-25 parts by weight on absolutely dry matter, counting on 100 parts by weight of absolutely dry formaldehyde, while the molar ratio of urea to formaldehyde after the fourth stage is 1: (0.85-1.30). 4. The method according to p. 3, characterized in that the first stage is carried out at a temperature of 90-92 ⁰ C for no more than 30 minutes. 5. The method according to p. 3, characterized in that the second stage is carried out at a temperature of 90-92 ° C for no more than 30 minutes. 6. The method according to p. 3, characterized in that the third stage is carried out at a temperature of 63-72 ° C. 7. The method according to p. 3, characterized in that the post-condensation is carried out at a temperature of at least 60 ⁰ C for at least 20 minutes. 8. The method according to p. 3, characterized in that after the third stage is evacuation.