Chemical reactions in the course of curing of organosilicate composites and aging of organosilicate coatings

Abstract

INTRODUCTIONOrganosilicate coatings are represented by films with specific functional properties [1][2][3]. Commercial organosilicon oligomers have been used as film formers. In the case of high-temperature composites, the functional properties are provided by the use of mineral components (finely dispersed layered silicates), inorganic pigments, and special modifiers (for example, vitreous additives, finely dispersed metal powders, etc.).The curing of organosilicate composites (OSCs) and coatings in the course of heat treatment, as a rule, is associated with the condensation of silanol groups of polyorganosiloxanes and the removal of water thus formed from the coating. An increase in the temperature accelerates these processes and makes it possible to achieve a more complete curing (this manifests itself in a decrease in the content of the soluble fraction in the coating). The kinetics of curing and aging of organosilicate composites is affected by oxidants of the environment (for example, oxygen) and the oxidizing and reducing agents introduced into the composition of organosilicate composites.The purpose of this work was to investigate the role of chemical reactions in the formation and aging of heat-resistant organosilicate composites and coatings. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUEFive model organosilicate composites ( I -V ) in the "polydimethylphenylsiloxane (PDMPS)-mica-asbestos-zirconia" system were prepared for investigation. The composites differed in the presence or absence of a vitreous additive, vanadium oxide V 2 O 5 , and barium peroxide. The organosilicate composites were prepared through mechanochemical treatment of different components in a ball mill (Table 1).Composite I contains the polymer, mica, asbestos, glass, and zirconia and does not involve the vanadium oxide and barium peroxide. The glass is absent in composite II . The vanadium oxide is absent in composites III and IV ; in this case, the latter composite does not involve the glass. All the above components are contained in composite V .In this work, we used the organosilicon varnish, which was a 65-67% solution of polydimethylphenylsiloxane of the composition {[C 6 H 5 SiO 1.5 ][[CH 3 SiO 1.5 ] 0.42 [(CH 3 ) 2 SiO] 1.33 ]} n modified by an organic polyester in toluene. The polyester (the product of polycondensation of ethylene glycol with phthalic and maleic anhydrides) modified by the castor oil was introduced into the polydimethylphenylsiloxane at the final synthesis stage in the ratio polymer : polyester = 10 : 1.Muscovite mica [ GOST (State Standard) 855], chrysotile asbestos [ GOST (State Standard) 12 871], zirconia, vanadium oxide V 2 O 5 , and barium peroxide (reagent grade) were used as fillers. The aluminoborosilicate glass of the composition (wt %) 53.6SiO 2 · 9.6B 2 O 3 · 14.7Al 2 O 3 · 20.9CaO · 0.7MgO · 0.5Na 2 O (according to the chemical analysis) served as a vitreous additive.The content of nonvolatile compounds in organosilicate composites I -V was determined from the weight losses of weighed portions of t...

Composition of Organosilicate Coatings High-Temperature Breakdown Products and Their Distribution in the Weld

“…The data were obtained by the thermal cycles acquiring using K-type thermocouples on 10 × 200 × 300 mm samples (presented below). From the decomposition onset temperature of the polymer component and until the steel melting temperature is reached, a number of physicochemical processes occur in OSCs, which may be classified into groups [17]:…”

“…In this case, the formation of new phases common for mild heating is significantly slowed down or completely suppressed. [9,17,23]. Among the possible breakdown products of the OSCs organosilicon polymer base (regardless of its composition), hydrogen and water, dissociating in turn into atomic oxygen and hydrogen, have the main effect on the pore and nonmetallic inclusion formation [24].…”

“…Phyllosilicates' high-rate thermal breakdown products appear to be an X-ray amorphous phase and poorly crystallized oxides; their amount among other breakdown products is considerably high [9,17,23]. For example, in the case of muscovite, SiO 2 and Al 2 O 3 , in the case of talc, SiO 2 and MgO, and, in the presence of both silicates, magnesium-aluminum spinel MgO•Al 2 O 3 .…”