STUDY OF THERMAL AND THERMO-OXIDATIVE DESTRUCTION OF COPOLYMERS BASED ON STYRENE, METHYLMETACRYLATE, AND ACRYLONITRILE

ABSTRACT

The article studied the thermal and thermal-oxidative stability of copolymers based on styrene. It was revealed that the insertion of a small amount (0.5-3.0 mass %) of heterocyclic methylmethacrylate units in the composition of polystyrene contributes to a significant increase in their resistance to thermal oxidative degradation.

АННОТАЦИЯ

В статье изучены термическая и термоокислительная стабильности сополимеров на основе стирола, метилметакрилата и акрилонитрила. Выявлено что, введение малого количества (0,5-3,0 масс.%) звеньев гетероциклического метилметакрилата в составе полистирола, полиметилметакрилата и полиакрилонитрила способствует существенному повышению стойкости их к термической и термоокислительной деструкции.

Keywords: polymer, copolymer, degradation, stabilization, methyl methacrylate, styrene, polystyrene, thermogravimetric analysis, chromatography, monometric method, thermogravimetric analysis, thermooxidative degradation.

Ключевые слова: полимер, сополимер, деструкция, стабилизация, метилметакрилат, стирол, полистирол, термогравиметрический анализа, хроматография, монометрический метод, термогравиметрический анализ, термоокислительной деструкции

The efficiency of the use of polymeric materials in the national economy largely depends on the preservation of the properties of polymers under conditions of processing and operation.  An increase in the time of reliable operation of polymers is equivalent to the production of many hundreds of thousands of tons of additional products. In this regard, the study of degradation processes, the establishment of the mechanism of polymer decomposition under the interaction of various factors, and the development of ways to increase their stability are relevant [1]. The stabilization of polymers thus becomes one of the most rational ways to save labour costs, natural resources and energy [2].

It is known from the literature that an increase in the thermal stability of polystyrene can be achieved by introducing into their macromolecule stabilizing units that play a different role depending on their structure [3].. Thus, the effectiveness of some stabilizer monomers is due to the implementation of the “foreign link” effect, which leads to inhibition of the degradation process [4].

Of particular interest is the possibility of increasing the thermal stability of polymers by introducing into their chains monomer units with a structure close to that of the stabilizing object [5]. This applies, in particular, to monomers containing active sulphur and nitrogen atoms in the heterocyclic, due to their participation in the destruction of hydroperoxide and peroxide groups formed in the course of degradation and causing the onset of the chain depolymerisation process [6]. The introduction of a small amount of a monomeric stabilizer into the main polymer macromolecule leads to an increase in thermal stability and at the same time prevents migration, volatilization, and washing out of the stabilizer [7].

The thermal and thermooxidative degradation of polymers and copolymers has been studied by thermogravimetric analysis, chromatographic and monomeric methods, as well as changes in their molecular weights during degradation [8].

The results of dynamic thermogravimetry of polystyrene, its copolymers with insignificant (0.5-3.0 wt.%) amounts of hetero-cyclic esters of methacrylic acids, show that the modified samples have higher heat resistance than polystyrene.  The onset of thermal decomposition shifts to higher temperatures. The stabilizing properties of the synthesized stabilizers are most effectively manifested when the content in the polymer structure is 0.5-1.0 wt.% heterocyclic methacrylates [9].

Table 1 shows the experimental results of thermogravimetric analysis (TGA) of samples of homopolymers and copolymers and their compositions.  As can be seen from the table, the introduction of nitrogen-, oxygen-, sulfur-, and halogen-containing heterocyclic fragments into the polymer chain contributes not only to an increase in the temperature of the onset of weight loss of the samples (10%), but also to the temperature of maximum decomposition.

The maximum decomposition rate also shifts to higher temperatures compared to unstabilized samples.  This, apparently, is explained by the blocking effect of the kinetic chain of the decomposition of benzoxazolthionyl methyl methacrylate units.

Monomer units of benzoxazolthionylmethyl methacrylate, as in the case of a copolymer with styrene, have the strongest stabilizing effect than known analogues.

Apparently, during thermooxidative degradation, the stabilizing effect of heterocyclic units that have throne groups is associated with the formation of low-active compounds upon termination of chain processes, with the destruction of copolymer macromolecules.  Apparent activation energies of thermal-oxidative destruction, according to dynamic thermogravimetry, were calculated by the Reich method of double logarithm.

Table 1.

Parameters of thermal-oxidative degradation of styrene homo and copolymers during non-isothermal oxidation in air at a rate Heating 5⁰С/ minute

The analysis of volatile products of thermal and thermal oxidative degradation of stabilized polystyrene samples by mass spectrometry and electron paramagnetic resonance showed that, in fact, the main monomer, as well as benzoxazolethione radical and CO2 and CO, are formed in the process of thermal decomposition.  The formation of benzoxazolthione radicals during thermal degradation was confirmed by EPR spectroscopic data.

Thus, the introduction of a small amount of heterocyclic methyl methacrylate units in the composition of polystyrene contributes to a significant increase in their resistance to thermal oxidative degradation.

And also, the influence on the process of thermal-oxidative degradation of polystyrene, polyacrylonitrile and polymethylmethacrylate of the monomer phthalimidomethyl methacrylate, which was added to the polymer in the form of a conventional mechanical mixture, was studied (Table 2).  As can be seen, a small addition of phthalimidomethyl methacrylate to polymers increases the temperature of the onset of decomposition of polyacrylonitrile by 22-570, polymethyl methacrylate - by 3-170, polystyrene - by 48-610.  Moreover, an increase in the content of phthalimido-methyl methacrylate in the composition leads to an increase in the temperature of the beginning of decomposition and the maximum rate of development of the process.

Of greatest interest was the introduction of phthalimidomethyl methacrylate units into the polymer chains of polystyrene, polyacrylonitrile, and polymethyl methacrylate.  For this purpose, copolymers of phthalimidomethyl methacrylate with the indicated monomers containing 0.5–3.0% FIMMA units were synthesized.  Since the process was carried out to high degrees of conversion (89–95%), the composition of the copolymers practically corresponded to the composition of the initial monomer mixture.  On the table  2 shows the dependence of weight loss on heating in air of copolymers with phthalimidomethyl methacrylate.  As can be seen, the addition of phthalimidomethyl methacrylate units to copolymers leads to a significant stabilizing effect.  The initial decomposition rate shifts to higher temperatures compared to unmodified samples, which allows us to conclude that intramolecular stabilization is highly efficient.

Table 2.

Parameters of thermal-oxidative degradation of the composition of polyacrylonitrile, polymethyl methacrylate and polystyrene with phthalimidomethyl methacrylate in air at a heating rate of 50 С/ min

The results show (Table 2) that not only the temperature of the beginning of decomposition, the maximum rate of development of the process, but also the activation energy of stabilized samples is higher than that of unstabilized polymers.  In principle, an increase in the activation energy of decomposition is observed with an increase in the content of phthalimidomethyl methacrylate in the copolymer.  Separate results falling out of this regularity can be explained by experimental errors.  It is interesting to compare the data of tables 2 and 3. As can be seen from the comparison of the results, the copolymers exhibit a greater stabilizing effect compared to the mechanical mixture, which indicates a higher efficiency of intramolecular stabilization.  The introduction of a stabilizer not only shifts the decomposition start temperature, but also contributes to the conservation of the molecular weight.

Table 3.

Parameters of thermal-oxidative degradation of copolymers of acrylonitrile, methyl methacrylate and styrene with methacrylic acid phthalimidomethylene ester in air at a heating rate of 5⁰ minutes

Studies have been carried out on changes in the viscosity of copolymers depending on the content of phthalimidomethylene ester methacrylic acid in the last units.  The results are shown in Table 4. As can be seen from the above results, samples with different content of phthalimido-methylene ether methacrylic acid in the copolymer have approximately the same intrinsic viscosity and, after degradation, retain this parameter in different ways.  The decrease in intrinsic viscosity for all copolymers depends on the content of the phthalimidomethyl methacrylate unit in them.  Moreover, the more of them in the copolymer, the smaller the difference between the values of intrinsic viscosity and after destruction.

Consequently, the presence of phthalimidomethyl methacrylate units in the copolymer contributes to the preservation of the molecular weight of the samples prevents the flow of products of thermal-oxidative decomposition of copolymers of methyl methacrylate with phthalimidomethyl methacrylate at a content of the latter of 0.5-5.0% show that the main degradation products are methyl methacrylate monomers, in addition, a small amount of carbon oxides was found  CO2 and CO, the formation of which occurs, apparently, during the destruction of methyl methacrylate.

Table 4.

Dependence of the Reduced Viscosity of Solutions of Copolymers Phthalimidomethylene Ether Methacrylic Acid with Methyl Methacrylate and with Styrene on the Content of FIMEMAC Units in Them

Table 4.  the dependence of the amount of released methyl methacrylate monomer on the content of phthalimidomethyl methacrylate in the copolymer is presented.  The amount of released methyl methacrylate significantly decreases with increasing content of phthalimidomethyl methacrylate in the copolymer, which is in accordance with the above studies and indicates effective stabilization.

The analysis of volatile products of thermal-oxidative degradation of stabilized polystyrene samples by mass spectrometry, IR spectroscopy, and chromatography showed that, indeed, a monomer is released during decomposition.  In addition, products such as benzaldehyde, benzoic acid, carbon oxides are formed, which indicates the oxidation of styrene molecules.

It is known that the thermal-oxidative degradation of polyacrylonitrile and its copolymers is accompanied by cyclization of units.  This process is accompanied by shrinkage of the polymer, the appearance of a large number of small pores in it. The sample then acquires a black color and completely loses solubility.  When studying copolymers of acrylonitrile with phthalimidomethyl methacrylate by DTA, it was found that the cyclization process is shifted to a higher temperature region.

In the IR spectrum of polyacrylonitrile after heating at 528 K for 1 hour, an absorption band at 1620 cm-1 is found, which is characteristic of the nitrile group and indicates intramolecular cyclization. In the IR spectrum of copolymers of acrylonitrile with phthalimidomethyl methacrylate subjected to heating under similar conditions, an absorption band at 1620 cm^–1 also appears, but its intensity is significantly lower than in the case of acrylonitrile homopolymer subjected to degradation.  Thus, the presence of phthalimidomethyl methacrylate not only causes the induction period of depolymerization, but also prevents intramolecular cyclization of acrylonitrile.

The mechanism of thermal-oxidative degradation of polyacrylonitrile, polymethyl methacrylate, and polystyrene has been studied in detail and proceeds according to a radical mechanism through the formation of peroxide compounds.  In this regard, the stabilizing effect of phthalimidomethyl methacrylate units is apparently associated with the generation by it of rather inactive radicals that participate in recombination reactions with the resulting radicals.  The generation of radicals can occur when the C-N bond of phthalimidomethyl methacrylate is broken. Phthalimide radicals are generated due to the relatively weak C-N bond in phthalimidomethyl methacrylate.

Thus, the introduction of a small amount of units of heterocyclic esters of methyl methacrylate in the composition of polyacrylonitrile, polymethyl methacrylate and polystyrene contributes to a significant increase in their resistance to thermal oxidative degradation.

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