RU2174523C1 - Method preparing carbamide-formaldehyde resin - Google Patents

Abstract

FIELD: chemical industry. SUBSTANCE: described is method of preparing carbamide-formaldehyde resin by condensation of carbamide-formaldehyde concentrate with first portion of carbamide in the presence of viscosity and heating regulator in aqueous solution having variable acidity followed by aftercondensation of the resulting product with additional amount of carbamide in the presence of buffer additive at initial and final carbamide to formaldehyde molar ratio of 1:(1.9-2.1) and 1:(1.02-1.7) respectively and cooling of final resin. Condensation of carbamide-formaldehyde concentrate and carbamide is carried out throughout entire process in the presence of buffer additive introduced in two steps: prior to introducing first carbamide portion and in after condensation stage, respectively. EFFECT: simplified process and improved quality of the desired product. 4 ex, 1 tbl

Description

 The invention relates to methods for producing urea-formaldehyde resins used as binders and adhesives for gluing wood, in the manufacture of chipboard, fiberboard and MDF and other purposes. Due to increased requirements for the emission of formaldehyde from wood products synthesized using urea-formaldehyde resins, so-called low-molar resins, characterized by a minimal ratio of formaldehyde to urea, are widely used in woodworking enterprises. When using formalin technology to obtain them, significant disadvantages of resins are reduced quality indicators, low storage stability and low reactivity during curing, a significant amount of toxic formaldehyde-containing tar resins, and high formaldehyde emission from resin compositions.

A known method of producing urea-formaldehyde resin [Doronin Yu.G., Svitkina MM, Miroshnichenko S.N. Synthetic resins in woodworking. - M.: Forest industry. 1979. p. 41-49], which includes four main stages: obtaining methylol derivatives of urea (precondensate) in a slightly alkaline or neutral medium at a temperature of 80-100 ^o C for a duration of 30-60 minutes and a molar ratio of urea and formaldehyde 1: (1.97-2 , 0), condensation of gummy products in an acidic environment at pH 4.0-5.5 for 20-60 minutes, concentration of the resin under vacuum at pH 6-7.5 and a temperature of 65-70 ^o C, post-condensation of the reaction mixture with additional urea portion at 60-65 ^o C, pH 6,5-7,5 for 30 minutes to a final molar ratio urea to form degidom 1: (1,3-1,66). In the process of cooling the finished resin, it is stabilized with ammonia water, diethylene glycol or sodium tetraborate.

 The disadvantage of this method is its multi-stage and duration, which reduces the productivity of technological equipment, as well as a low degree of formaldehyde conversion, since distilled water at the stage of resin concentration contains up to 5-8% of unreacted formaldehyde. In addition, the resulting resins are quite soluble in water, they have a high concentration of methanol, which prevents the production of environmentally friendly products based on them.

A known method of producing urea-formaldehyde resin from urea-formaldehyde concentrate containing 55% of the "total" formaldehyde and 21% of the "total" urea [RU N 2059663, cl. C 08 G 12/12, 1996]. It is based on adding the first portion of urea to the urea-formaldehyde concentrate in order to bring the molar ratio of formaldehyde: urea (Ф: К) = 2.0: 1.0, heating the reaction mixture at 40 ^o C for two hours, introducing a 10% solution acetic acid to reduce the pH of the mixture to 5.2 and conduct acid condensation at 90 ^o C to achieve a viscosity of 1000 MPa s. After that, the pH of the reaction mixture was adjusted to 7.5, a second portion of urea was introduced and the pre-condensation process was completed for one hour at 60 ^° C.

 The disadvantages of this method include the increased viscosity of the urea-formaldehyde resin at the stage of acid condensation due to the low temperature of the process, the reduced reactivity of the finished urea-formaldehyde resin, characterized by the parameter "gelation time", the difficulty of controlling the synthesis of the resin.

где R₁ и R₂ = H, CH₂OH,
и 1-5 мас.% триазинонов общей формулы

где R₁ и R₂ = H, CH₂OH, R₃ = H, CH₃
Синтез карбамидоформальдегидной смолы осуществляют путем конденсации карбамидоформальдегидного концентрата с первой порцией карбамида в присутствии регулятора вязкости - добавки деминерализованной воды или 5%-ного водного раствора метанола с pH 7,5-9,0, в течение 10 минут при 90^oC с pH 7,6-8,8 до достижения мольного отношения Ф:К = 2,0:1,0. По завершении щелочной стадии проводят быстрое охлаждение реакционной смеси до (80 ± 2)^oC, ввод кислого агента с целью снижения pH до уровня 5,0 и менее, проведение при (81 ± 2)^oC кислой конденсации до начала коагуляции смолы, подщелачивание и одновременный ввод второй порции карбамида и буферной добавки и завершение доконденсации при (66 ± 2)^oC в течение 30 минут.The closest in technical essence can be considered a method of producing a urea-formaldehyde resin by condensation of a urea-formaldehyde concentrate with urea in several stages in a solution with variable acidity using a buffer additive only at the last stage of the process [RU N 2136703, cl. C 08 G 12/12, 12/40 // C 08 L 61/24; 09/10/1999]. At the same time, a product containing 54.7% of the “total” formaldehyde, 23.6% of the “total” urea and 21.7% of water, modified 1-15 wt.% Damage of the general formula was taken as a urea-formaldehyde concentrate

where R ₁ and R ₂ = H, CH ₂ OH,
and 1-5 wt.% triazinones of the general formula

where R ₁ and R ₂ = H, CH ₂ OH, R ₃ = H, CH ₃
The synthesis of urea-formaldehyde resin is carried out by condensation of a urea-formaldehyde concentrate with the first portion of urea in the presence of a viscosity regulator - the addition of demineralized water or a 5% aqueous solution of methanol with a pH of 7.5-9.0, for 10 minutes at 90 ^o C with a pH of 7, 6-8.8 to achieve a molar ratio of F: K = 2.0: 1.0. Upon completion of the alkaline stage, the reaction mixture is rapidly cooled to (80 ± 2) ^o C, an acidic agent is introduced to lower the pH to 5.0 or less, acid condensation is carried out at (81 ± 2) ^o C before resin coagulation begins, alkalization and the simultaneous introduction of a second portion of urea and a buffer additive and the completion of the condensation at (66 ± 2) ^o C for 30 minutes.

 The disadvantage of this technical solution is that the urea-formaldehyde concentrate used, with a low content of impurities (methanol, sodium formate, etc.), has increased reactivity, especially at the stage of acid condensation, which makes it difficult to control the process of obtaining highly viscous resin at low pH values.

 The aim of the invention is to optimize the conditions for the synthesis of low molar urea-formaldehyde resins while improving their quality.

 This is achieved by the fact that in the known method, in order to reduce pH fluctuations, the synthesis of urea-formaldehyde resin during the whole process is carried out in the presence of a stabilizing buffer additive (sodium tetraborate, trisodium phosphate, etc.), introduced in two doses: in the urea-formaldehyde concentrate at the alkaline stage before loading the first portions of urea and during precondensation together with the second portion of urea, respectively.

The essence of the proposed solution is the synthesis of urea-formaldehyde resins by condensation of a urea-formaldehyde concentrate containing 0.02-0.2% buffer additives with the first portion of urea in the presence of a viscosity regulator — demineralized water or an aqueous solution of methanol with a pH of 7.5–9.0, in for 10 minutes at (88 ± 3) ^o C and pH 7.6-9.0 until the molar ratio F: K = 2: 1 is reached. Upon completion of the alkaline stage, a quick cooling of the reaction mass to (80 ± 2) ^o C, introduction of an acidic agent in order to lower the pH to 4.0-5.0, and acid condensation at (80 ± 3) ^o C for 20-50 minutes before the coagulation of the resin, simultaneous injection of a second portion of urea and 0.02 - 0.20 wt.% buffer additives per urea-formaldehyde concentrate and completion of the condensation at (65 ± 2) ^o C for 30 minutes.

 The proposed technical solution differs from the known ones in that the process of obtaining a urea-formaldehyde resin throughout, starting from the stage of alkaline condensation, is carried out in the presence of a buffer additive that regulates the reactivity of the system and avoids the undesirable transition of the reaction mass to a solid state when an acidic agent (ammonium chloride) is added , formic acid, etc.).

 A distinctive feature of the proposed technical solution is that the buffer additive is introduced in two portions at the stages of alkaline condensation and pre-condensation, respectively.

 The invention is illustrated by the following examples: urea-formaldehyde concentrate is used in the form of an aqueous composition containing 41.6 - 59.0% of the "total" formaldehyde and 21.0 - 26.0% of the "total" urea. The molar ratio of urea: formaldehyde is 1: 3.2-5.4.

 To neutralize and acidify the reaction mixture, aqueous solutions of sodium hydroxide, formic acid, ammonium chloride and other bases and acids are used.

 Comparative characteristics of resins made according to examples of the proposed method and the prototype are presented in the table.

 EXAMPLE 1

4600 kg of urea-formaldehyde concentrate containing 54.9% of "total" formaldehyde, 22.7% of "total" urea and 22.4% of water (molar formaldehyde: urea molar ratio is 4.84: 1) are charged into a 10 m ³ reactor. 1060 kg of demineralized water and 4.6 kg of borax are added to it. The contents of the reactor are heated to 30-40 ^o C, the pH of the reaction mixture should be 8.5 ± 0.3. If the pH does not match this value, it is brought to the required level by the addition of a small amount of a 10% sodium hydroxide solution.

After that, the first portion of urea is fed into the reactor until the molar ratio urea: formaldehyde = 1.0: 2.0 is reached. The condensation solution with vigorous stirring is heated to 90 ^o C and maintained at this temperature for 10 minutes. Upon completion of alkaline condensation, the temperature of the reaction mixture is reduced to 80 ± 2 ^° C. and 35 kg of a 10% solution of ammonium chloride are introduced into the reactor from the measuring device in order to lower the pH. The end of the polycondensation is determined by the technological sample: in a beaker with 50 cm ^{3 of} water, cooled to 12-14 ^o C, add 3 cm ^{3 of the} reaction box. Condensation is stopped after the appearance of stable turbidity or small coagulation. Its duration depends on temperature and steady-state pH.

 Upon completion of the acidic stage of polycondensation, the contents of the reactor are neutralized with a 10% sodium hydroxide solution to a pH of 7.3-7.8, adding it from a measuring device in an amount of 34 kg, and a second portion of urea is introduced to achieve a molar ratio Ф: К = 1.15 : 1, as well as 4.6 kg of borax or trisodium phosphate.

The temperature of the reaction mixture spontaneously decreases to 66 ± 2 ^o C and is maintained at this level for 50 minutes. The finished resin is cooled to 20-25 ^o C and pumped into a container for storage.

 EXAMPLE 2

 The condensation conditions of example 1.

 As a urea-formaldehyde concentrate, a product was used containing 26% of the "total" urea, 41.6% of the "total" formaldehyde and 32.4% of water (molar ratio Ф: К = 3.2: 1).

 At the stage of alkaline condensation, 120 kg of water and 4.6 kg of borax were introduced into the reactor.

 EXAMPLE 3

 The condensation conditions of example 1.

 Instead of borax, a similar amount of trisodium phosphate was added.

 EXAMPLE 4

 The condensation conditions of example 1.

 At the stage of alkaline condensation, instead of borax, 2.3 kg of trisodium phosphate was introduced, and during pre-condensation - 6.9 kg of borax. As an acidic agent that lowers the pH level, 10% formic acid was used.

 From the description of the invention and the table it can be seen that according to the claimed technical solution, it is possible to control the duration of the acidic condensation stage and thereby obtain high-quality urea-formaldehyde resins with the necessary set of working properties: viscosity, miscibility with water, gelatinization time, free formaldehyde content.

Claims (1)

 A method of producing a urea-formaldehyde resin by condensing a urea-formaldehyde concentrate with a first portion of urea in the presence of a viscosity regulator and heating in an aqueous solution with varying acidity, followed by condensation of the resulting product with an additional amount of urea in the presence of a buffer additive with an initial and final molar ratio of urea and formaldehyde of 1: (1 , 9 - 2.1) and 1: (1.02 - 1.7), respectively, and cooling the finished resin, characterized in that the condensation of urea-formaldehyde and urea-stand concentrate throughout the process is carried out in the presence of buffer additives introduced in two steps: before loading the first portion of urea and in step dokondensatsii respectively.

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