Synthesis of a graft copolymer of chloroprene rubber with acrylic acid as a shoe adhesive

. Synthesized graft copolymers of chloroprene rubber with acrylic acid. A mixture of benzene and dimethylformamide was used as a solvent, and potassium persulfate was used as a polymerization initiator. Based on the dependence of the copolymerization rate on the concentrations of components and temperature, the order of the reaction rate in terms of the concentration of acrylic acid (2.0), potassium persulfate (1.2) and the activation energy of the process (24.4 kJ/mole) were determined. The degree of grafting of polyacrylic acid increases from 2% to 140% with increasing process time, monomer and initiator concentrations. Grafting efficiency is high (70-90%) in a wide range of time and component concentrations. The graft copolymer has high adhesion to the surface of the skin and tissue.


Introduction
Currently, a solution of chloroprene rubber is used to glue the main parts of shoes. The introduction of units with polar groups into the rubber macromolecule significantly improves the adhesion of the adhesive to natural leather and textile material. The grafting of acrylic acid to polychloroprene is an effective method for the formation of adhesive macromolecules with polar groups that tend to interfacial interaction with the surface of various materials. On the other hand, grafting a thermoplastic polymer to rubber makes it possible to synthesize thermoplastic elastomers (TPE) that combine the properties of elastomers during operation and thermoplastics during processing [1]. The applicability of TPE is almost universal, so the latest achievements of science and technology related to their production and processing are becoming increasingly relevant [2,3]. TPE is widely used in the shoe industry as a shoe sole and adhesive. The development of technologies determines the development and research of new types of TPE products based on polyolefin [4], polybutadiene [5], acrylic acid [6], compositions with hydroxide groups [7], polymer nanocomposites [8].
Graft copolymerization of methyl methacrylate on natural rubber was carried out, initiated by the redox system hydrogen peroxide -sodium thiosulfate. The reaction was studied in an aqueous medium by varying the concentrations of the monomer, hydrogen peroxide, and sodium thiosulfate [9]. In order to improve the thermal and mechanical properties of natural rubber, its graft copolymerization with acrylic monomers was carried out [10]. Graft copolymerization of methyl methacrylate on chloroprene rubber in the presence of hydrogen peroxide as an initiator is carried out in order to improve the adhesive ability of adhesives [11]. The graft copolymerization of ethyl acrylate and methyl methacrylate onto polychloroprene was carried out in toluene using benzoyl peroxide as an initiator, also in order to improve the adhesive ability of adhesives [12].
The kinetics of graft copolymerization of styrene with natural rubber was studied using cumene hydroperoxide -tetraethylenepentamine as a redox initiator [13,14]. The values of the kinetic parameters of graft copolymerization differ significantly from the parameters of free radical polymerization of monomers. Thus, in the case of graft copolymerization of sodium styrenesulfonate on films of polyvinylidene fluoride irradiated with an electron beam, the order of the reaction with respect to monomer concentration and absorbed dose of grafting turned out to be 2.84 and 1.20, respectively. The calculated total activation energy of the graft copolymerization reaction was 11.36 kJ/mole [15]. High values of reaction orders (2.0 and 0.75) and low activation energy (12.5-36.9 kJ/mole) were obtained for the process of graft copolymerization of styrene on polyethylene-tetrafluoroethylene films [16].
The composition, structure, and properties of graft copolymers are inextricably linked with the method and patterns of their preparation [17]. The molecular characteristics and properties of graft copolymers are determined by grafting parameters such as the extent and efficiency of grafting. Establishing the kinetic parameters and parameters of the grafting of acrylic monomers to synthetic rubber macromolecules is an urgent problem in the chemistry of macromolecular compounds. The aim of this study is to synthesize a graft copolymer of chloroprene rubber with acrylic acid in the presence of potassium persulfate as an initiator, to determine the kinetics and parameters of grafting, and to increase the adhesive strength of the adhesive layer.

Materials and methods
Chloroprene rubber (CPR), produced in the Russian Federation, was provided by "Tashkent-Rezina" LLC (Republic of Uzbekistan).
Potassium persulfate (PP) was purified from impurities by recrystallization from water. Synthesis of graft copolymers was carried out in a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. The flask was charged with 20 ml of dimethylformamide (DMF), 30 ml of benzene and 5 g of CPR. The rubber first swells, and then completely dissolves with vigorous stirring. AA and PP were added to the rubber solution, which also dissolve in the reaction mixture. The flask was immersed in a thermostat; the synthesis of copolymers was carried out with constant stirring of the reaction mass in solution for 8 hours. Then the reaction mixture was taken out of the thermostat, cooled to room temperature, poured into a beaker with ethanol. The unreacted monomer dissolves in ethanol, the copolymer and homopolymer precipitate. Non-grafted polymers were separated from the grafted copolymer by repeated extraction with distilled water. The extraction is continued until the weight reduction of the grafted copolymer stops. The copolymer purified from unreacted monomer and homopolymer was dried at a temperature of 60°C to constant weight.
The copolymerization kinetics was studied gravimetrically. To do this, the required amounts of CPR, AA, and PP solutions were loaded into the ampoule with a thin section and purged with nitrogen. Then the ampoule was stoppered and placed in a thermostat. The ampoules were opened every 20 minutes, the contents were poured into ethanol, the precipitate was separated, dried, and the mass was weighed. The copolymer was separated from the homopolymer by extraction, dried to constant weight, and weighed again.
The degree of grafting is defined as the ratio of the mass of grafted chains (polyacrylic acid (PAA)) to the mass of the original CPR.
The grafting efficiency was determined as the ratio of the mass of grafted chains to the mass of AA that entered into polymerization.
The average molecular mass of grafted chains is determined by the formula: where: is the average molecular weight of grafted chains, is the degree of conversion (conversion), [ ] is the concentration of AA, [ ] is the concentration of PP, is the grafting efficiency, ( ) is the molar mass of AA.
Determination of the adhesive property of the copolymer. Using a 20% solution of the synthesized copolymer in benzene, and a 20% solution of polychloroprene for comparison, the skin was glued to the skin and fabric. The adhesion strength between the layers of materials was determined according to the interstate standard "GOST 17922 Textile fabrics and piece goods. Breaking load determination method" on the AG-1 tensile testing machine. A sample of 300x50 mm in size was cut out of the finished material. After cutting the adhesive layer with a sharp object at a distance of 50 mm, delamination into two layers was performed from the narrow edge of the sample. The layers were loaded individually into clamps. When the START button is pressed, the upper active clamp starts to rise at a travel speed of 100 mm/min. The maximum load at break of the layers is taken as the adhesion strength between the layers of the material [18].

Results and discussion
According to the results of gravimetric studies at a constant concentration of the initiator and at a constant temperature, at a variable concentration of the monomer, graphs of the dependences of the degree of conversion over time were plotted (Fig. 1a). According to the logarithmic dependence of the grafting rate and the AA concentration, the order of the reaction according to the AA concentration was determined (Fig. 1b). According to the results of gravimetric studies at a constant concentration of the monomer and at a constant temperature, at a variable concentration of the initiator, graphs of the dependences of the degree of conversion over time were plotted (Fig. 2a). From the logarithmic dependence of the grafting rate and the PP concentration, the reaction order with respect to the PP concentration was determined (Fig. 2b).
The following kinetic indices were obtained: the order of the reaction in terms of AA concentration = 2.0, in terms of PP concentration = 1.2. The values obtained are approximately two times higher than in the case of radical polymerization of AA in the presence of PP. Such kinetic orders of graft copolymerization reactions are known in the literature [15,16] and indicate the interaction of AA and PP with the CPR macromolecule and their participation in the acts of initiation and growth of grafted polymer chains as a result of this interaction. According to the results of gravimetric studies at a constant concentration of the monomer and initiator, at a variable temperature of the process, graphs of the dependences of the degree of conversion over time were plotted (Fig. 3a). The total activation energy of the graft copolymerization of AA with CPR was determined from the dependence of the logarithm of the grafting rate and the reciprocal of the temperature (Fig. 3b). The total activation energy of the graft copolymerization reaction turned out to be = 24.4 kJ/mole, which corresponds to the literature data, and is much less than the energy of free decomposition of the initiator. The obtained value of the activation energy indicates that the formation of active grafting sites occurs under energetically favorable conditions as a result of the interaction of CPR macromolecules with PP.
The calculated grafting parameters and the average molecular weight of the grafted chains at different concentrations of the monomer and initiator, temperatures, process times are presented in tables 1-3. According to the tables, the degree of grafting changes significantly with changes in the concentrations of the monomer and initiator, temperature, and process time. The grafting efficiency changes insignificantly with changing synthesis conditions. An increase in the degree of grafting with an increase in the concentration of the monomer and initiator is in full agreement with the general laws of polymerization. An increase in the initiator concentration leads to an increase in the active centers of initiation of graft copolymerization, and an increase in the monomer concentration leads to an increase in the length of grafted polymer chains and, accordingly, the degree of grafting.  In the obtained values, two facts deserve attention: an increase in the average molecular weight of the grafted chains with increasing temperature, contrary to the general laws of polymerization, and an increase in the grafting efficiency with an increase in the duration of the process. The first fact is directly related to the degree of conversion; with an increase in temperature, the rate of the process increases, the degree of conversion of the monomer into a polymer, which leads to a natural increase in the length of the grafted chains.
The second fact seems to be more interesting; at the beginning of the process two processes compete: the growth of grafted and non-grafted polymer chains, and over time, the growth of grafted polymer chains begins to predominate. Hence the rather high value of the effectiveness of grafting and its increase with increasing duration of the process. To some extent, this explains the assumption about the detachment of the proton of the = − group of the rubber macromolecule during the formation of the active center. The interaction of the CPR macromolecule with PP with the formation of free radicals can be represented by the following scheme: Initiation and growth of grafted PAA chains: In the deep stages of the process, persulfate free radicals are exhausted, and chain growth occurs predominantly along grafted chains.
The synthesized graft copolymer has excellent adhesion to the surface of the skin and tissue. The adhesive strength of gluing leather to leather and leather to fabric with CPR glue averages 38 and 5.2 N, respectively. The bonding strength of these materials with the graft copolymer of CPR and AA is 118 and 64 N. Apparently, the carboxyl groups of AA are strongly bound both to the surface of the skin and to the surface of the textile material.

Conclusion
The low value of the total activation energy of the process, the possibility of changing the degree of grafting and the length of grafted chains of polyacrylic acid to chloroprene rubber macromolecules makes it possible to synthesize graft copolymers with a given structure and properties. Relatively high molecular weights of grafted chains (25000-38000) were obtained as a result of high grafting efficiency. The bonding strength of leather to leather and fabric with polychloroprene-acrylic acid graft copolymer is 3-12 times greater than the bonding strength of pure polychloroprene.