Application of molecular weight regulators for the synthesis of sodium polyacrylate thinners of mineral suspensions

The synthesis of additives for thinning mineral suspensions based on sodium polyacrylate was carried out. The effect of molecular weight regulators on the molecular weight characteristics of the polymer and the effect of such polymers on the rheological properties of suspensions was studied. Sodium acrylate polymers are synthesized by free radical polymerization in aqueous solution using molecular weight regulators. The molecular weight characteristics of the polymeric samples were estimated by viscometry using Mark-Houwink-Kuhn-Sakurada (MHKS) equation. Synthesized polymers were used as thinners ceramic slurries, prepared according to the recipe of the enterprises producing ceramic products. The thinning ability of polymer samples with different molecular weights was estimated using an Engler viscometer from the time of the ceramic slurry flow. The influence of the type and amount molecular weight regulator on polyacryates was revealed. It was found that molecular weight synthesized samples was in the range of 21000 91000. It was determined that samples with a molecular weight of 28000 35000 synthesized using mercaptoethanol (at a dosage of 0.5-1.5% by weight of the monomer) provide optimal fluidity to the ceramic slurry.


Introduction
Slurries based on clay minerals are used for the production of sanitary ware and ceramic tiles. The technology for the production of such items is very sensitive to the physical characteristics of the ceramic suspension. First of all, the requirement is high fluidity, with a limited amount of water content in this mineral suspension. The increased water content makes the slurry more fluid, but leads to an increase in energy consumption for moisture removal from finished products.
Also, a high water content in the slurry leads to a deterioration in its properties [1,2]. In this regard, there is a problem of synthesis chemical additives for thinning mineral suspensions. Such additives can be: salts of polyphosphoric, silicic and humic acids, etc. Nevertheless, synthesis of new additives for thinning slurries remains relevant [1,5,6]. One of the promising directions is the use of polymers and, in particular, sodium polyacrylate in this role. The ability of water-soluble acrylic polymers to prevent the aggregation of the mineral part of the suspensiondeflocculation, leads to a decrease of the viscosity of the mineral suspension, which makes it possible to reduce the energy consumption for its preparation, or to achieve the required viscosity without increasing the water content [3][4][5][6]. Thinning ability of acrylic polymers in mineral slurries depends on the monomer composition and molecular weight distribution. To regulate the chain length in radical polymerization, molecular weight regulators are used. The most effective are sulfur containing compounds that regulate MM also by the chain transfer mechanism [8][9][10][11][12][13][14].
Aim of the work: synthesis of sodium polyacrylate samples with different molecular weights using mercaptoethanol, thioacetic acid and sodium sulfite as molecular weight regulators; comparative analysis of the effectiveness of molecular weight regulators; determination of the thinning ability of sodium polyacrylate in mineral suspensions; establishing the optimum ratio of components to obtain an effective thinning additive.

Materials and Methods
For the synthesis of polymer compounds, we used: distilled water, acrylic acid (AA) (Sigma-Aldrich), ammonium persulfate (PSA) (Aldrich), sodium carbonate (Sigma-Aldrich), sodium sulfite (SS) (Sigma-Aldrich), mercaptoethanol (ME) (Sigma-Aldrich), thioacetic acid (TA) (Sigma-Aldrich). The reagents were used without preliminary purification. were simultaneously introduced, the rate of introduction of reagents was selected so as to prevent boiling of the reaction medium due to the exothermic effects of polymerization. The synthesis was carried out for 4 hours, then cooled and neutralized with sodium carbonate. The amount of water used in the synthesis was calculated to obtain a 20% final sodium polyacrylate solution. Four series of polymer samples based on sodium acrylate were synthesized: without a molecular weight regulator, using mercaptoethanol, sodium sulfite, and thioacetic acid as MM regulators.
Intrinsic viscosity was determined according to the method described in [7], for this, a 0.1% polymer solution in 3% NaCl was prepared. The measurements were carried out at 25 ° C in a capillary viscometer with a hanging level (capillary For the slurry, the following components were used: sand, clay (Nizhne-

Results
The molecular mass of compounds obtained by the method of radical polymerization depends on many factors: the nature of the monomer, initiator, solvent, temperature, also on the ratio of components, etc. To obtain polymers with a certain MW value, the concentration of the polymerization initiator can be varied (the higher the initiator concentration, the higher the polymerization rate, but the lower the molecular weight of the resulting polymer). However, this method increases the economic costs of production, and also promotes branching and crosslinking reactions of the polymer chain, as a consequence of this, the molecular weight distribution of the polymer is very wide.As can be seen from      In all cases, with the addition of a regulator, the MM decreases. Despite the difference in the structure of substances, all three regulators can be used as components of the reaction mixture to control the molecular mass of the resulting polymer. The mechanism of action of these substances, is well studied [6][7][8], but the comparative efficiency of these compounds and the effect of their effect on the properties of the target product, which is used to thinning clay suspensions, is of scientific interest [11][12][13][14]. Characteristics of the slurry are presented in Table 5. It was shown the dependence slurry fluidity on the molecular weight of sodium polyacrylate samples, synthesized with different molecular weight regulators. As shown in Table 5 3. Complexation. This mechanism consists in the addition of agents that bind free divalent cations before they can contact the surface of the starting material particle and cause flocculation.
The number of carboxyl groups, determining the first and third mechanisms of deflocculating action, did not change upon the addition of equal amounts of polyacryates with different molecular weights in slurry. So it can be concluded that, at a certain molecular weight corresponding to MM 28000 -35000, the conformation and size of sodium polyacrylate chain tangles provide maximum steric repulsion. This was achieved using mercaptoethanol and, with less potency, with thioacetic acid as a regulator.

Conclusions
Polymers and copolymers of sodium acrylate were obtained at various ratios of components, and the molecular weight of all samples was determined. MW values were obtained in the range of 21000 -65000 using molecular weight regulators and 45000-91000 without them.
Analysis of the effectiveness of molecular weight regulators was carried out and it was found that organic substances mercaptoethanol and thioacetic acid is the more effective.
The thinning ability of sodium acrylate polymers in mineral suspensions for production of ceramic products has been studied. It was found that in the case of sodium acrylate polymers, mercaptoethanol can be used as a regulator, at a dosage of 0.5 -1.5% of the monomer weight. The macromolecules of polyacrylate in this case have MM 28000 -35000, the optimal size and conformation of chains, which provide optimal fluidity of the mineral suspension.