INVESTIGATION OF THE PROCESS OF OBTAINING ADDITIVE FOR GYPSUMBOARD BASED ON POLYMETHYLENENAPHTHALINE CARBOXYLIC ACIDS

ABSTRACT

In the technology for the synthesis of additives for drywall based on polymethylenenaphthalene carboxylic acid, it is proposed to carry out an additional stage, which allows reducing the mass fraction of residual formaldehyde. Studies have been carried out to study the effect of the synthesized additive on the plasticizing properties of the gypsum mixture.

АННОТАЦИЯ

В технологии синтеза добавки для гипсокартона на основе смеси полиметиленнафталин карбоновых кислот предложено проведение дополнительной стадии, которая позволяет снизить массовую долю остаточного формальдегида. Проведены исследования по изучению влияния синтезированной добавки на пластифицирующие свойства гипсовой смеси.

Keywords: residual formaldehyde; plasticizer properties; polymethylenenaphthalene carboxylic acid; Cannicaro reaction.

Ключевые слова: остаточный формальдегид; пластицифицирующие свойства; полиметиленнафталин карбоновая кислота; реакция Канницаро.

Introduction

At the same time, the material that provides moisture stability in the building, has air permeability, and forms the basis of plasterboard is gypsum. Disadvantages of gypsum-based binding materials include: a large amount of water is required for the preparation of gypsum mixtures, low water resistance, low cold resistance. During the production of plasterboard, gypsum is mixed with water and additives that improve its properties. Various organic dispersants, in order to ensure even distribution of the gypsum mixture between the cardboard sheets, and to reduce the amount of water required to ensure the fluidity of the gypsum mixture at the required level, diluents and superplasticizers (modified starch, lignosulfonates, sulphite-alcohol bars, sodium polymethylenesulfonate, etc.) are used. In addition, such additives reduce the time and energy resources required for gypsum drying[1]. Plasticizers are structured film quality substances that are adsorbed at the boundary of solid and liquid phases as a dispersant-stabilizer. Immobilization of bound water in gypsum flocs, reduction of the coefficient of internal friction of gypsum water suspension, smoothing of the microrelief of hydrated gypsum particles and, in some cases, a significant change in the electrostatic repulsion of particles due to their electrokinetic potential are the main factors of the plasticizing effect of surfactants on the gypsum-water surface, which reduces their water demand and binder consumption[2].

Literature review

The additive polymethylenenaphthalenesulfonic acid, which is the subject of research, is a highly effective diluent with a high molecular weight fraction, sodium and calcium salts. Additives used today contain free formaldehyde, which has a negative effect on the ecological characteristics of the finished product. The technological process of production of polymethylenenaphthalene sulfonates (addition base) consists of the following stages:

• β- sulfonation of naphthalene with sulfuric acid in order to obtain β-naphthalene sulfo acid;
• process of polycondensation of β-naphthalene sulfo acid with formaldehyde to obtain polymethylene naphthalene sulfo acid; - neutralization of the obtained reaction product with sodium hydroxide or calcium hydroxide;
• filter the additive solution to remove sodium/calcium deposits.

Depending on the conditions of the process, substances of different properties are formed. We analyze all stages of the technological process. At the stage of sulfonation of naphthalene with sulfuric acid, the production of β-naphthalene sulfo acid is the main process.

Depending on the temperature of the sulfonation process, a mixture of α- and β-naphthalene sulfo acids in different proportions is formed:

• 14% β- and 86% α-naphthalene sulfonic acid at a temperature of 110°C;
• 90% β- and 10% α-naphthalene sulfonic acid is formed at a temperature of 160°С.

Therefore, the sulfonation process is carried out at a temperature higher than 160°С. When the temperature rises above 170°С, the level of side processes increases and the amount of dinaphthylsulfonic acid (substances insoluble in water) increases, decreasing the process temperature below 160°С leads to an increase in the amount of naphthalene sulfonic acid α-isomer. The polycondensation reaction of naphthalene sulfonic acids with formalin proceeds through the following scheme:

The polycondensation process is carried out at a temperature of 100ºC for several days. The longer the process continues, the higher the content of the polymerized substance in the product, and the higher the content of the active substance in the product. The completion of the process is monitored by sampling. When cooled, the obtained polycondensate turns into a viscous mass, when stretched it stretches into thin fibers and dissolves well in water (a clear, non-turbid solution) [3]. Research methodology. Scientific research is mainly aimed at studying the process of polycondensation of naphthalene carboxylic acids with formalin, the product of oxidation of naphthalene homologues in nitric acid, and using the obtained product as a superplasticizer in plaster mixtures, in which oxidation, neutralization, mixing, filtering, polycondensation processes were used. Hydrometers, IK-spectroscopic, gas chromatographic, mass spectrometric methods were used to determine the physical and chemical properties and chemical composition of the selected raw materials and the obtained products.

Purpose of work

Reducing the amount of residual formaldehyde by adding a new process to the additional production technology for gypsum board based on studying the synthesis process of polymethylenenaphthalene carboxylic acid mixture, and carrying out research on the effect of the synthesized additive on the plasticizing properties of the gypsum mixture.

Methods and results

Polymethylenenaphthalene carboxylic acid (PMNK), which is the subject of research, is a highly effective diluent with high molecular weight fractions, sodium and calcium salts. Synthesized polycondensate contains free formaldehyde, which has a negative effect on the ecological characteristics of the finished product.

The technological process of production of polymethylenenaphthalene carboxylic acids (additional base) consists of the following stages:

• oxidation of 2-methylnaphthalene with concentrated acid to obtain β-naphthalene carboxylic acid;
• process of polycondensation of β-naphthalene carboxylic acid with formaldehyde to obtain polymethylenenaphthalene carboxylic acid;
• neutralization of the obtained reaction product with sodium hydroxide or calcium hydroxide;
• filter the additive solution to remove sodium/calcium deposits. Depending on the conditions of the process, substances of different properties are formed.

We analyze all stages of the technological process. At the stage of oxidation of 2-methylnaphthalene with concentrated nitric acid, the production of β-naphthalene carboxylic acid is the main process.

Depending on the temperature of the oxidation process, a mixture of different naphthalenesulfo acids is formed. Therefore, the sulfonation process is carried out at a temperature higher than 120°С. When the temperature rises above 150°C, the level of side processes increases, aromatic compounds are released, and the amount of various acids increases, the process temperature increases. A decrease from 120°С leads to an increase in the amount of other oxygenated organic compounds without oxidation to 2-methylnaphthalene acid.

The polycondensation reaction of naphthalene carboxylic acids with formalin proceeds through the following scheme:

The polycondensation process is carried out at a temperature of 110 ºC for several days. The longer the process continues, the higher the content of the polymerized substance in the product, and the higher the content of the active substance in the product. The completion of the process is monitored by sampling. The obtained polycondensate turns into a viscous mass when it is cooled, when it is stretched it becomes a thin fiber and it dissolves well in water [4].

In order to reduce the time of the polycondensation process and the consumption of energy resources, the process was carried out at a high temperature in a high pressure setting, where the reaction is completed in a few hours. Formalin is introduced into the reaction mixture from several points and below the reaction mass, the goal is to ensure uniform distribution of formalin throughout the reaction mass. If formalin is given from one point, it can increase the viscosity of the reaction mass, and as a result, it can lead to unpleasant consequences, such as the failure of the mixer and other parts.

At the stage of neutralization of polycondensation process products with sodium hydroxide, sodium salts of polymethylenenaphthalene carboxylic acid are formed. The condensed mass is mixed with a certain amount of water and diluted, cooled and an alkali solution is added. The sodium hydroxide solution is stirred until the mixture is neutral. The reaction equation for the neutralization process is as follows:

During the synthesis of the additive, the residual formaldehyde mass fraction is 0.001% more than usual, which cannot be used in the composition of building materials used for the interior decoration of buildings with a lot of people [3].

In order to reduce the mass fraction of residual formaldehyde in the production process of the additive, they proposed to use the Cannitzsaro reaction. In this case, formaldehyde molecules interact and turn into various harmless substances. In such a process, a disproportionation reaction occurs, one molecule of formaldehyde is reduced and the second molecule is oxidized, alkalis play the main role as a catalyst of the process [5]:

2СН₂О + Н₂О = СН₃ОН + НСООН

Aldehydes without a hydrogen atom in the alpha position undergo disproportionation under the influence of concentrated alkali solutions to form carbonic acid and alcohol. The mechanism of the Kannitzer reaction combines a two-stage nucleophilic coupling reaction: in the first stage, the hydroxyl anion is attached to the carbonyl group of the formaldehyde molecule, then hydrogen is released from this adduct compound in the form of a hydride-anion and combines with the second molecule of formaldehyde. For example, formaldehyde is converted into methyl alcohol with potassium formate (since there is potassium hydroxide in the medium).

The Cannitzsaro reaction was carried out at a high temperature of 100ºС for several hours. After the process was completed, the product was neutralized with a low concentration sulfuric acid solution. As a result, the mass fraction of formaldehyde in the liquid product of the process did not exceed 0.001%.

In order to evaluate the transportability of the synthesized additive solution, the relationship between its concentration and density was determined. The results of the experiments are presented graphically in Figure 1. One of the most important characteristics of the diluent-plasticizer, which affects the gypsum mixture, determined by the Suttard method, is the plasticity index [5-7].

Figure 1. A graph of the dependence of the additive density on the concentration of the solution

Table 1 presents the results of the experiments conducted to determine the spreadability of the gypsum mixture prepared using an additive based on polymethylenenaphthalene carboxylic acid.

Table 1.

Dependence of the spread of gypsum mixture on the amount of plasticizer

Conclusion

The additive synthesized with a similar additive added to the mixtures was analyzed comparatively and the maximum dispersion value was determined. The value of the obtained additive was 0.4% compared to the gypsum mass. An excess amount of the additive reduces the plasticity of the gypsum mixture and, accordingly, the consumption property. Vic's apparatus was used to determine the setting time of gypsum mixture of standard consistency. The essence of the analysis method is to determine the time of beginning and end of hardening of the gypsum mixture from the time of the beginning of the effect of the gypsum binder with water. It was found that the time of hardening of gypsum mixture with 0.4% addition is not less than 28 minutes.

Thus, in order to reduce the mass fraction of formaldehyde in the dispersant-plasticizer additive used for the production of plasterboard based on polymethylenenaphthalene carboxylic acid, it was proposed to add the Kannitzsaro reaction to the technological process.

References

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3. European standard EN 13986:2004. Wood-based panels for use in construction - Characteristics, evaluation of conformity and marking. Text: electronic. – URL: https://docplayer.net/30412682-Wood-based-panels-for-use-in-constructioncharacteristics-evaluation-of-conformity-and-marking.html.
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