OPTIMIZATION OF THE PROCESS OF OBTAINING EPOXIDIZED NATURAL RUBBER FOR THE DEVELOPMENT OF NEW COMPOSITE MATERIALS ON ITS BASIS

The object of research is the process of epoxidation of natural rubber scrap. Epoxidized natural rubber (ENR) has a wide range of applications, for example, in treadmill coatings, special tires, belt drives, hoses, shoes, adhesives, sealants, floor coverings and other areas where only special synthetic rubbers are used. Natural rubber (NR) is modified by the epoxidation reaction to achieve higher oil resistance, increased adhesion, weather resistance and damping characteristics of materials with its use. Promising is the processing of secondary, non-standard, natural rubber (scrap) as a raw material for the ENR production. Thus, the task of scrap disposal and its return to the production cycle is solved. To accomplish the task of epoxidation of secondary rubber, the possibility of conducting combined physicochemical processes in a two-phase water-xylene medium in one reaction space was studied to reduce the total energy costs. The use of a combined reaction-separation process for the epoxidation of scrap of natural rubber allows to solve the problem of accumulation and disposal of rubber waste in the most efficient way. It is possible to obtain a product with a regulated functionalization degree without a significant amount of by-products. To find the optimal regime for conducting the combined reaction-separation process of epoxidation, the method of the planned experiment was used to obtain the regression equation with its subsequent analysis. The obtained regression equation makes it possible to optimize the conditions for conducting the process of epoxidation of nanocrystals with obtaining products with desired properties. As a result of the implementation of the planned experiment, it is found that epoxidation at a temperature of 93 °C of a diluted (10 % wt.) solution of natural rubber with peracetic acid formed "in situ" provides a higher epoxidation degree. The conditions and ratios of the components are selected under which NR retains aggregative stability during epoxidation in a water-xylene medium.


Introduction
Obtaining new composite materials based on epoxidized natural rubber (ENR) is a promising area of research [1,2]. In industry, ENRs are synthesized by carrying out an epoxidation reaction of natural rubber in a suspension containing a significant amount of gel particles with pera cetic acid formed in situ. The epoxidation reaction is a random process and, therefore, the addition of oxygen to double bonds occurs randomly, distributed along the polymer molecule. The rate of epoxidation increases with increasing rubber concentration [3]. ENR latexes are obtained by epoxidation of natural rubber (NR) at the latex stage in a suspension containing a significant amount of gel particles with peracetic acid [4] or using glacial acetic acid and hydrogen peroxide [5].
It is found that, using the technology of processing latex of nanocrystals, openring components increase with the reaction temperature and reaction time. It has been established that as the degree of epoxidation increases, the number of tetrahydrofuran rings increases [6].
The possibility of conducting combined physicochemi cal processes in a twophase waterxylene medium in one reaction space with a decrease in the total energy costs was studied. A scheme of the process of epoxidation of scrap of NR with peracids in a waterxylene petroleum medium in a thermally insulated reactor was proposed [7].
Epoxidized natural rubber in the form of latex has good performance properties and has a wide range of applica tions. ENRbased latexes are potentially useful materials that have unique properties, such as: high resistance to oils and aging; have a high glass transition temperature. At a temperature of 20 °C, the relative permeability of air varies from NR, ENR 25, ENR 50, ENR 70, to the most permeable synthetic rubber [8].
Two degrees of epoxidation of 25 mol % (ENR25) and 50 mol % (ENR50) were studied for their potential use as commercial rubbers, and both are proposed as materials ISSN 2664-9969 for the development of composite materials. Excellent enhancement of ENR is achieved using siliceous fillers, which allows to obtain a given level of strength even in the absence of binding agents [9].
An industrial methodology and hardware design for the scrap processing (waste products of natural rubber produc tion) are developed and proposed by the authors [10].
Therefore, it is relevant to obtain new epoxidation products based on natural rubber scrap with improved physicochemical and technological properties.
Thus, the object of research is the process of epoxida tion of natural rubber scrap.
The aim of research is obtaining products with a control led epoxidation degree.

Methods of research
To implement the planned experiment, it is necessary to consider in detail the reaction process that occurs dur ing the epoxidation of natural rubber and select the in fluencing factors.
The epoxidation reaction proceeds in two stages: at the first stage, the formation of a strong oxidizing agent -peracetic acid, at the second stage, the peroxy acid reacts with double bonds of 1,4cispolyisoprene with the formation of an oxirane ring [11]: Epoxidized natural rubber in the solution is unstable, reactions can occur over time: -hydration of the oxirane ring in the presence of water and acid: . (3) -opening of the epoxy ring with the formation of carboxyl groups or crosslinking [12]: . (4) Taking into account the complex mechanism of or ganic reactions and the presence of a number of adverse reactions, the yield of the target product is a function of many variables (temperature, pressure, component concen trations, time, hydrodynamic conditions, physicochemical properties of solutions, etc.). To solve the problem, the experiment planning method was selected [13].
This involves setting the planned experiment accord ing to plan 23 into three factors and 8 experiments. The experiments are performed without randomization.
The epoxidation process is carried out in a batch reac tor -a thermally insulated threenecked flask equipped with a thermometer, reflux condenser and heater. Let's use a 10 % by weight solution of natural rubber scrap in petroleum xylene, as well as aqueous solutions of 5 % by weight acetic acid and 35 % by weight hydrogen peroxide.
The study of the kinetics of the epoxidation process [11] allows the following limitations to be formed for factors: -temperature -89 < X 1 < 93 °С; -reaction time without taking into account the heat ing time -0.5 < X 2 < 1.5 hours; -the amount of hydrogen peroxide -20 < X 3 < 40 mol. %. The temperature variation range is adopted on the basis of data on the maximum reaction rate with the mini mum degree of occurrence of adverse reactions [11]. Based on certain restrictions, an experiment planning matrix is formed (Table 1). After isolation from the reaction mass, the technical product of epoxidation of nanocrystals is analyzed for the residual content of double bonds (iodine number) according to the Ganus method [15]. The percentage of epoxy oxygen in the epoxidized product (epoxy number) is determined by reverse titration of excess HCl [16].

Research results and discussion
Based on the data obtained, it is calculated: -conversion (C) EA: ratio of the iodine number of the epoxidation product to the iodine number of the starting rubber (Table 2); -epoxidation degree (ED): the ratio of the proportion of EA that went into the formation of epoxy groups to the initial amount of EA (Table 3); -conversion degree (CD) :the ratio of the EA propor tion that went into the practical formation of epoxy groups to its theoretical amount ( Table 3). As a result of the implementation of the planning ma trix (Table 1), the linear regression equation is obtained: As follows from equation (5), the third factor, the con centration of hydrogen peroxide, has the greatest influence on the yield of the finished product. The following factors for the degree of influence are: temperature and time of the epoxidation process.
To create composite materials and industrial applica tions, the products of experiments 1 and 2 (ENK25) are recommended, for the preparation of which the degree of conversion of the epoxy agent exceeds 60 %.
Qualitative analysis of the products was carried out using infrared spectroscopy (IR spectroscopy) on a Specord 75 IR spectrophotometer (Germany) of films with a thickness of 40-50 μm of the original scrap of natural rubber and the obtained epoxidized rubbers on silica glass in a wide wave length range. Particular attention is paid to the absorption bands of epoxy, hydroxyl, and carboxyl groups (Fig. 1).   The presence of epoxy groups at the maximum degree of epoxidation is confirmed by the presence of absorption bands in the range of 1260-1240 cm -1 in the IR spectra of the products (Fig. 1, b) corresponding to stretching vibrations of the epoxy ring [17,18]. This mode of produc tion of ENR is characteristic at a temperature of 93 °C and a maximum concentration of EA -40.0 mol. % The presence of absorption bands of hydroxyl groups at 3750 cm -1 and intense at 1650 cm -1 confirms the pas sage of the reaction of the opening of the epoxy cycle (4). Therefore, with an insignificant epoxidation degree (less than 11.0 % mol), the concentration ratio of the epoxy group/acetic acid increases, which leads to its opening by reaction (4).

Conclusions
The regression equation obtained in the work allows optimizing the conditions for conducting the process of epoxidation of nanocrystals and obtaining products with desired properties. Synthesized epoxidized rubber can be used in the development of new composite materials, in cluding as a protective coating of metals [19,20].
Thus, it is found that epoxidation at a temperature of 93 °C of a diluted (10 % wt. %) solution of natu ral rubber with peracetic acid formed «in situ» provides a higher epoxidation degree. The conditions and ratios of the components are selected under which NR retains aggregative stability during epoxidation in a waterxylene medium.