**2.1.1 Film-forming materials**

*Water-based nano lithium silicate of 7.5/1.0 silica/alkali molar ratio*. Previous experiences with these solutions on glass as substrate allowed infer that as silicon dioxide content in the composition increases the film curing velocity also increases and that in addition the dissolution rate decreases.

For this study, a commercial colloidal lithium silicate (3.5/1.0 silica/alkali molar ratio in solution at 25% w/w) was selected; with the aim of increasing the silica/alkali ratio, a 30% w/w colloidal alkaline solution of nanosilica was used (sodium oxide content, 0.32%). The aim was to develop a system consisting of an inorganic matrix (alkaline silicate) and a nanometer component (silica) evenly distributed in that matrix with the objective of determining its behaviour as binder for environment friendly, anticorrosive nano coatings.

*Solvent-based, partially hydrolyzed tetraethyl orthosilicate.* The tetraethyl orthosilicate is synthesized from silicon tetrachloride and anhydrous ethyl alcohol. This product commercializes as condensed ethyl silicates and usually contains approximately 28% w/w of SiO2 and at least 90% w/w monomer. The additional purification removes waste products of low boiling point (mainly ethanol) and the dimmers, trimmers, etc.; in some cases, this treatment allows obtaining pure tetraethyl silicate conformed by 99% w/w monomer.

Theoretically, the complete hydrolysis of ethyl silicate generates silica and ethyl alcohol. Nevertheless, the real hydrolysis never produces silica in form of SiO2 (diverse intermediate species of polysilicates are generated). Through a partial hydrolysis under controlled conditions, it is possible to obtain a stable mixture of polysilicate prepolymers. The stoichiometric equation allows calculating the hydrolysis degree X (Giudice et al., 2007 & Hoshyargar et al., 2009).

The pure or condensed ethyl silicate does not display good properties to form a polymeric material of inorganic nature. In this paper, ethyl silicate was prepared with 80% hydrolysis degree in an acid medium since catalysis carried out in advance in alkaline media led to a fast formation of a gel.

The empirical equation of ethyl silicate hydrolyzed with degree X was used to estimate the weight of the ethyl polysilicate and the hydrolysis degree, through the calculation of the necessary amount of water. The weight was obtained replacing the atomic weights in the mentioned empirical formula; the result indicates that it is equal to 208-148 X.

The percentual concentration of the silicon dioxide in the ethyl polysilicate is equal to the relation molecular weight of SiO2 x 100 / weight of the ethyl polysilicate; consequently, SiO2, % = 60 x 100 / (208-148 X). On the other hand, to calculate the water amount for a given weight of tetraethyl orthosilicate and with the purpose of preparing a solution of a predetermined hydrolysis degree, the equation weight of water = 36 (100 X) / 208 was used.

Finally, the amount of isopropyl alcohol necessary to reach the defined percentual level the silica content was calculated. It is possible to mention that after finishing the first hydrolysis

Reinforcement Fibers in Zinc-Rich Nano Lithiun Silicate Anticorrosive Coatings 161

**Fine Regular**

0 20 40 60 80 100 **Cumulative undersize particles, w/w**

Fig. 1. Commercial spherical zinc dusts. D 50/50 average particle diameters: 4 μm (fine) and

*Silicon nitride (Si3N4).* It displays three different crystal structures (α, β and γ). It is industrially obtained by direct reaction between silicon and nitrogen at temperatures between 1300 and 1400 ºC. Silicon nitride is a material frequently used in the manufacture of structural ceramics with high requests of mechanical stress and wear resistance; it displays a moderately high modulus of elasticity and an exceptionally high tensile strength, which makes it attractive for its use in the form of fiber as reinforcement material for coatings films. It behaves like a semiconductor and it has 3.443 g.cm-3 density at 25 °C. For this experience, hexagonal β phase in the form of fiber has been selected, with average values of

1205 μm and 102 μm for length and transverse dimension, respectively (Figure 2.B).

*Quartz.* It is rhombohedral crystalline silica reason why it is not susceptible of exfoliation; chemically it is silicon dioxide (SiO2). Usually it appears colorless (pure), but it can adopt numerous tonalities if it has impurities; its hardness is such that it can scratch the common steels. It is often used in coatings as extender after being crushed and classified by size (average diameter between 1.5 and 9.0 μm). It is an insulating material from the electrical point of view; it has 2.650 g.cm-3 density at 25 °C (Chen et al., 2010 & Lekka et al., 2009). In this experience, quartz fibers were used with average values of 1118 μm and 95 μm for

These reinforcements cannot be classified as nano materials since they no have at least one of the dimensions inferior to 100 nm (Aluru et al., 2003; Radhakrishnan et al., 2009; Behler et

0

length and transverse dimension, respectively (Figure 2.C).

al., 2009 & Li & Panigrahi 2006).

8 μm (regular).

10

20

**Particle diameter, µm**

30

40

50

stage of tetraethyl orthosilicate that leads to the silicic acid formation, the absence of alcohol would generate the polycondensation of mentioned acid with silica precipitation and the null capacity to conform a polymeric silicic acid (Wang et al., 2009 & Yang et al., 2008).

In a first stage, the pure tetraethyl silicate and the isopropyl alcohol were mixed under agitation. Later, the water and the hydrochloric acid solution selected as catalyst were added (the final pH of the solution was slightly acid, 0.01% w/v, expressed as hydrochloric acid); agitation continued until the end of the dissipation of heat (exothermic reaction).

The conclusions from the experiences indicate that: an excessive amount of water (higher than calculated) generates a rapid gelling in the package, a high pH leads to a fast silica precipitation that reduces the capacity of formation of an inorganic polymer of elevated molecular weight and, in addition, a large quantity of acid retards the condensing reaction due to the repulsion of protonated hydroxyl groups (Giudice et al., 2007).
