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The feasibility of using red mud in coatings based on glyptal resins

The possibility of industrial waste – alkaline red mud to be used as the filler of glyptal resins and coatings thereof is considered. It was determined that the most effective way of compounding of said components was in situ combinations during the synthesis of the polymer, providing systems with filling factor up to 36 wt.% with decreased moisture absorption ability in comparison to the initial resin. It was demonstrated that the most probable cause of this performance increase is the formation of chemical bonds between the hydroxyl groups of glyptal and weak van der Waals bonds between the carbonyl group of the resin and surface groups on red mud.


Introduction
The so-called "red mud" is the industrial waste that forms during the alkaline processing of bauxite ores at the alumina production. It is a fact, that at least 35-40 % of the processed ore converts into the red mud. [1][2][3][4][5].
The negative environmental influence is caused by the strong alkalinity of the red mud (its pH can be in range [11][12][13]. Considering the fact, that typically the volumes of such waste are significantly large, this leads to noticeable contamination of soil. [6] This problem is known for a long time [7], prompting the proposal and development of numerous technical solutions, mainly through the use of this waste as the raw material for other industries: for example, the production of alkaline activation cements, composite concrete and clinkers [8,9].
In the research presented in reference [7], it was shown that the red mud can be successfully used as the reinforcing extender of plastic composites, for example, of polyvinylchloride base. Besides the comparatively low cost of such raw material, the outstanding advantage of it is the content of the bonded water, which varies in the range 4-8 wt. % [10], providing a certain level of fire retardant properties.
Despite the large body of work on the possible uses of red mud -plastic compounds, the application of this waste material in the formulation of protective and organic coatings has to date not been developed and is limited to its consideration as a pigment [11] or fire-retardant additive [12]. At the same time, taking into account the high alkalinity of such waste, there is unanswered question about interphase compatibility in the thin film coating.
In particular, an conceivable interaction between the polymer matrix and the red mud at a molecular level would be of interest. In the work presented in reference [13], it was shown that a certain level of such interactions may be

Materials and methods
The red mud of PSC "Zaporizhsky aluminium industrial complex" (GPS 47.860921, 35.129808) was employed as the raw filler material in this work.
Its chemical composition was studied by oxide analysis and is characterized by the high content of Fe2O3, TiO2, the mixture of alkali and alkali-earth materials with common formula RO + R2O = 8.62 wt. % (Table 1).  The basic components for glyptal synthesis used were glycerol and phthalic anhydride. The synthesis was carried out in accordance to a known procedure [15], and the initial weight ratio of components was 1:1,65 respectively. The water absorption parameter was determined as the change of the sample weight in wt. %.

Results and discussion
Infrared spectra of the synthesized polymer ( Fig. 1. (1) verified to be the targeted glyptal resin with the following structure ( Fig.1), accordingly to reference [19]. the filler concentration level 56 wt. % (Fig. 3.). It should be noted that the spectrum of glyphthalic resin almost completely masks the spectrum of the filler. The wide absorption band at 3450 сm-1, which is corresponding to the vibrations of the surface ОН-groups of the filler (Fig. 3, 6) is almost completely obscured in the spectra of the compositions.
In the spectra of compositions, obtained from the in situ formulations (Fig. 4), the shift of carbonyl peak to 1729 cm-1 at the 56 wt. % loading also can be noted.  In the case of compositions, obtained by in situ formulations, these trends are more evident (Fig. 6). There appear to be van der Waals, hydrogen bond and chemical interactions between the red mud and the polymer matrix through carbonyl and hydroxyl groups of the resin and red mud, respectively.