RESEARCH OF THE PROPERTIES OF POLYVINYL ALCOHOL COATINGS MODIFIED WITH ANTIPIRENS TO INCREASE THE FIRE-RESISTANT NATURE OF WOOD MATERIALS

ABSTRACT

Modified oligomers obtained as a result of phosphorylation of polyvinyl alcohol were obtained, and studies were conducted to improve the fire resistance of wood materials. The effect of temperature on the phosphorus content in polyvinyl alcohol during phosphorylation, the resistance to thermal oxidative degradation of polyvinyl alcohol coatings treated with a phosphorus-boron-containing oligomer solution, and the effect of the concentration of phosphorus-boron-containing oligomer solutions on the fire resistance and oxygen index of wood samples were studied.

АННОТАЦИЯ

Получены модифицированные олигомеры, полученные в результате фосфорилирования поливинилового спирта, и проведены исследования для улучшения огнестойкости древесных материалов. Изучено влияние температуры на содержание фосфора в поливиниловом спирте в процессе фосфорилирования, устойчивость к термоокислительной деструкции покрытий из поливинилового спирта, обработанных раствором фосфор-борсодержащего олигомера, и влияние концентрации растворов фосфор-борсодержащих олигомеров на огнестойкость и кислородный индекс образцов древесины.

Keywords: fire resistance, oligomer, flammability, flame retardant, oxygen index, thermal oxidative degradation, incombustibility, weight loss.

Ключевые слова: огнестойкость, олигомер, воспламеняемость, антипирен, кислородный индекс, стойкость к горению, потеря массы.

Introduction. The nature of wood materials and based on it is such that they cannot be made completely fireproof. For this purpose, additives are used that make it difficult to ignite and reduce the rate of flame propagation - flame retardants. In the problem of fire safety of wood materials and based on it, flame retardants and their components are of primary importance [1].

The problem of reducing the fire hazard of construction timber is not only an economic task, but also has a social and environmental focus. In modern construction, the search for new highly effective fire protection products for wood is becoming more intensive [2]. Being one of the most popular building materials, wood must comply with the degree of fire protection provided by fire safety standards and regulations for protected objects. But, at the same time, fire protection today must ensure not only a reduction in the flammability of wood, but must ensure the preservation of its operational and aesthetic parameters, as well as solve the problems of environmental safety, durability and reliability [3,4].

Along with the widespread use of wood materials in almost all sectors of the economy, their rapid flammability is considered one of the negative characteristics. In the process of using wood materials, flame retardant coatings were used to eliminate this negative property. At the same time, for comparison, global production of flame retardants in 2019 amounted to 2.39 million tons, in 2020 - 2.45 million tons, and in 2021 - 2.51 million tons. These figures indicate that the production of flame retardants has increased in recent years. In addition, the development of methods for wood modification by fire and its processing is currently considered one of the most important tasks [5, 6].

Methods. When treated with wood materials with oligomer antipirene obtained by modification of polyvinyl alcohol with phosphorus-boron, its flammability decreases. To determine the effectiveness of the fire-resistant nature of wood materials, studies have been conducted on phosphorus-boron-containing oligomers and post-modification sample mass variation, water absorption, resistance to oxygen index [7, 8].

Results and Discussion. Under normal conditions of polyvinyl alcohol, a phosphorus-boron modification of polyvinyl alcohol obtained using a phosphorus-boron containing oligomer obtained by the action of dimethyl phosphate on boric acid in a 1:2 ratio. This oligomer has optimal technology properties as a flame retardant modifier for the polymer materials under study.

In practical experiments, the modification of polyvinyl alcohol carried out with an oligomer with a concentrated phosphorus-boron content in the temperature range of 20-50^oC and its solutions of 5-20% in water. Depending on the conditions of the change in composition, up to 14.5% phosphorus was introduced into polyvinyl alcohol (fig. 1).

Figure 1. The effect of temperature on the phosphorus content in polyvinyl alcohol in the process of phosphorylation

Phosphorylation of polyvinyl alcohol with a phosphorus-boron-containing oligomer accompanied by the formation of cross-linked structures. This probably indicates the formation of crosslinks in modified polyvinyl alcohol due to the formation of complex compounds insoluble in the boron atom. The hydroxyl groups of polyvinyl alcohol form a donor-acceptor bond during the transfer of a lone pair of electrons from the oxygen atom to the p- empty orbit of the boron atom, causing the polymer to lose its solubility in water.

In the process of obtaining these fire retardants, aqueous solutions of boric and orthophosphoric acids were mixed for 2 hours and heated to 100-110°C for their dehydration. Carbamide was added to the resulting product and mixed at a temperature of 180-250°C, and the resulting fire retardant was modified with a 3% solution of polyvinyl alcohol, lowering the temperature to 60-80°C.

When polyvinyl alcohol (PVA) is modified with a phosphorus-boron-containing oligomer, partial structural changes occur. This is mainly evidence of the formation of weak complexes with the boron atom in the modified polyvinyl alcohol. In the reaction mechanism, the phosphorus-containing groups interact with the hydroxyl (-OH) groups of polyvinyl alcohol, possibly forming phosphate esters or similar bonds. It is assumed that partial interactions with boron may occur to form a complex. These processes can generally be considered as bonds formed as a result of donor-acceptor interactions. This improves the mechanical and thermal properties of the modified polyvinyl alcohol. The general formula of polyvinyl alcohol modified with phosphorus-containing oligomeric flame retardants is as follows:

During the study, the effect of the concentration of aqueous solutions of phosphorus- boron-containing modified oligomers of polyvinyl alcohol obtained on the basic properties of polyvinyl alcohol studied. In this case, the 5% concentration of an aqueous polymer solution is the highest in terms of water absorption (table 1).

Table 1.

Dynamics of water absorption change in aqueous solutions of a phosphorus-boron- containing oligomer

The concentration of water solutions of a phosphorus-boron-containing oligomer used for modification has been found to have a significant effect on the properties of polyvinyl alcohol. The results of studies of changes in the weight and composition of phosphorus in coatings depending on the concentration of the phosphorus-boron oligomer

solution suggest that the higher the concentration of the phosphorus-boron-containing oligomer, the greater the mass of the coating. After modification, the amount of phosphorus obtained for the sample also increases. With an increase in the concentration of a phosphorus-boron-containing oligomer’s solution in accordance with the table above (see table 1), the water absorption of modified coatings significantly reduced.

One of the most important indicators characterizing the fire resistance of polymers is their ability to form Coke in the process of Thermo-oxidative degradation. Increasing the concentration of aqueous solutions leads to an increase in coke residue (table 2) and a decrease in the flammability of modified samples during phosphorus-boron-containing oligomer thermal oxidative destruction, which confirmed by the determination of the oxygen index.

Table 2.

Tolerance to Thermal-oxidation degradation of polyvinyl alcohol coatings in a solution of a phosphorus-boron-containing oligomer

The analysis of the data of this table shows an increase in the concentration of the solution of the phosphorus-boron-containing oligomer in the oxygen index of polyvinyl alcohol coatings. In addition, the concentration of phosphorus-boron-containing oligomer solutions has a significant effect on other properties of modified polyvinyl alcohol coatings. With an increase in the concentration of the oligomer solution containing phosphorus-boron, the tensile strength of the coatings increases from 9.50 MPa to 39.86 MPa, and in this the relative elongation decreased by 4.0% (fig. 2).

Figure 2. The effect of the concentration of an aqueous solution of a phosphorus- boron-containing oligomer on the relative elongation of polyvinyl alcohol coatings

Studies have also been conduct on phosphorus-boron-containing oligomers and optimal conditions for changing Wood to determine the effectiveness of fire resistance of wood materials, changes in sample mass after modification, water absorption, resistance to Thermal oxidation degradation, and oxygen index. According to the results, it found that a significant increase in the mass of wood samples occurs at intervals of 2 to 24 hours (table 3).

Table 3

Effect of exposure time on changes in wood mass

High resistance to combustion to wood samples was provided in the process of processing their surface, which is confirmed by the determination of the fire resistance of wood by the method of fire pipe according to GOST 16363-98 (table 4).

Table 4.

Effect of the concentration of solutions of phosphorus-boron-containing oligomers on the fire resistance property of wood samples

In addition, when wood samples with a 5% solution of polyvinyl alcohol modified with phosphorus-boron content studied by the method of fire-resistant properties of the sample in the treated state, a mass loss of 9,5% found.

During practical experiments, with a change in the concentration of phosphorus- boron-containing oligomer solutions, it was found that the flammability of samples does not change the indicators of the oxygen index according to the norms established in GOST 16363-98 (table 5).

Table 5.

The effect of the concentration of phosphorus-boron-containing oligomer solutions on the oxygen index of wood samples

Based on the results of the analysis carried out, polyvinyl alcohol modified with a phosphorus-boron-containing oligomer caused the oxygen index indicator of the wooden material to change to 48 when treated with antipirene.

Conclusions.

From the above, it can be concluded that in order to achieve a positive effect with fire-resistant modification of polyvinyl alcohol, the use of highly concentrated aqueous solutions of phosphorus-boron-containing oligomer, long modification time, high temperatures and complex equipment is not required. With the use of modified polyvinyl alcohol coatings, the tensile strength of coatings increases from 9.50 MPa to 39.86 MPa, while the elongation decreases by 4.0%. Studies have shown that modification of wood with phosphorus-boron-containing oligomers has a significant effect on its physico- chemical properties. It was found that a significant increase in the weight of wood samples occurs in the range from 2 to 24 hours after treatment, which indicates effective impregnation and interaction of oligomers with the wood matrix. Changes in water absorption, resistance to thermal and oxidative degradation and oxygen index confirm the improvement of flame-retardant properties of modified wood. The weight loss of wood materials treated with phosphorus-boron-containing oligomeric compounds is 9.5%, and the oxygen index (OI) reaches 48%. These indicators indicate that the processed samples of the 1st degree of fire resistance of wood materials comply with current regulatory requirements. A high oxygen index indicates a significantly reduced combustibility of modified wood.

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