Zinc phosphates

The most important phosphate-containing pigments are zinc phosphate [5.67]-[5.69], [5.71] basic zinc phosphate [5.70], [5.71] chromium phosphate [5.72] aluminum triphosphate [5.73] barium phosphate [5.53] aluminum zinc phosphate [5.68] and zinc/iron phosphate [5.71], Phosphate ions form protective coatings of basic iron(III) phosphate on an iron surface. Their composition is described in [5.64]. 

Toxicological data for phosphate pigments are described in Section 5.2.14. 

The most important phosphate-containing pigment is zinc phosphate [7779-90-0], Zn3(P04)2-4 H20, Mr 458.1. It can be used with a large number of binders (see Table 39) and has a very wide range of uses [5.67], [5.68], [5.70], [5.71], [5.82], Zinc phosphate is usually produced on an industrial scale from zinc oxide and phosphoric acid, or from zinc salts and phosphates [5.83], Composition and properties are given in Table 40. 

Color white-beige white-beige green 

Basic zinc phosphate Fleucophos ZPO (Dr. H. Heubach, Germany) His-pafos SP (Colores Hispania, Spain). 

In order to replace lead and chromate pigments, much attention has been focused upon zinc phosphate. Today, zinc phosphate is one of the most commonly used phosphate-containing anticorrosive pigments. The formulation versatility, due to the extremely lower solubility compared with chromate and other pigments, and therefore the lower reactivity, has led to its economic importance in the market Zinc phosphate can be used in a wide variety of resin systems [5.53, 5.55]. 

Zinc phosphates are normally produced on an industrial scale by a wet-chemical reaction involving zinc oxide (ZnO) and orthophosphoric acid (H3PO4), followed by filtration, washing, drying and milling processes. Numerous technical publications have gone into detail regarding the mode of action of zinc phosphate [5.62-5.66]. 

The performance properties of zinc phosphate pigments are attributed to chemical effectiveness, and the ability to form adhesion and inhibitor complexes on the surface of the substrate. In addition, in the case of zinc phosphate, electrochemical effectiveness, preferably in anodic areas, is also of note, since small amounts of the zinc phosphate will hydrolyze under moist conditions. The result of this reaction is argued to be the formation of zinc hydroxide and secondary phosphate ions, which are able to build protective layers on the metal surface in anodic areas [5.67, 5.68]. 

It is also discussed in the literature that, under moist conditions, basic complexes can be formed by reaction of zinc phosphate with inorganic ions or with carboxylic groups of the resin used, which lead, by reaction with metal ions, to so-called adhesion, cross-linking and inhibitor complexes [5.67-5.69]. Under the assumption, that the hydrolyzation process is the prerequisite for the effectiveness of zinc phosphates, this means that such pigments need a certain time before becoming active [5.69]. 

This leads to the conclusion that zinc phosphates do not have the well-recognized electrochemical effectiveness of chromate pigments [5.69]. 

---

Resorcinol Derivatives. Aminophenols (qv) are important intermediates for the syntheses of dyes or active molecules for agrochemistry and pharmacy. Syntheses have been described involving resorcinol reacting with amines (91). For these reactions, a number of catalysts have been used / -toluene sulfonic acid (92), zinc chloride (93), zeoHtes and clays (94), and oxides supported on siUca (95). In particular, catalysts performing the condensation of ammonia with resorcinol have been described gadolinium oxide on siUca (96), nickel, or zinc phosphates (97), and iron phosphate (98). 

A cmcial development for zinc phosphate coatings came in 1943 when it was found that more uniform and finer crystals would develop if the surface was first treated with a titanium-containing solution of disodium phosphate (6). This method of crystal modification is a prime reason for the excellent paint (qv) adhesion seen on painted metal articles. 

The Zinc Phosphating Process. The zinc phosphating reaction involves acid attack on the substrate metal at microanodes and deposition of phosphate crystals at microcathodes (8). Liberation of hydrogen and the formation of phosphate sludge also occur. The equation for the dissolution of iron together with precipitation of dissolved iron as sludge in a nitrite accelerated system is as foUows ... 

Modified Zinc Phosphates. Coatings on steel have been identified as hopeite, 4H2O, phosphophyUite, 4H2O, and... 

Product Utilization. Zinc phosphate coatings form the basis for paint adhesion in a variety of industries. These are used when long-term quahty is of concern in appHcations such as for automotive parts and vehicles, cod-coated products, and appHances. 

Zinc phosphate, Zn2(P0 2> forms the basis of a group of dental cements. Chromium and zinc phosphates are utilized in some metal-treating appHcations to provide corrosion protection and improved paint adhesion. Cobalt(II) phosphate octahydrate [10294-50-5] Co2(P0 2 8H20, is a lavender-colored substance used as a pigment in certain paints and ceramics. Copper phosphates exhibit bioactivity and are used as insecticides and fungicides. Zinc, lead, and silver phosphates are utilized in the production of specialty glasses. The phosphate salts of heavy metals such as Pb, Cr, and Cu, are extremely water insoluble. 

Benzal chloride is hydrolyzed to benzaldehyde under both acid and alkaline conditions. Typical conditions include reaction with steam in the presence of ferric chloride or a zinc phosphate catalyst (22) and reaction at 100°C with water containing an organic amine (23). Cinnamic acid in low yield is formed by heating benzal chloride and potassium acetate with an amine as catalyst (24). 

Zinc Phosphate Cements. Zinc phosphate cements are the oldest of the aqueous-based cements (see Table 1) and are stiU used in a wide range of appHcations eg, cavity bases, temporary restoratives, and for the fixation of inlays, crowns, fixed partial dentures (bridges), posts, facings, and orthodontic bands. 

This is a process to provide a fine coat of zinc phosphate or zinc calcium phosphate on ferrous and non-ferrous... 

Fig. 31. Electron micrographs that compare crystal size of (top) a grain-refined microcrystalline coating and (bottom) a conventional zinc phosphate conversion coating [54].

Fig. 32. Micrographs of microcyslalline zinc phosphate coatings on (top) A514 and (bottom) A606 steel substrates showing the very different morphologies produced by identical processes [54],...

Some pigments exert an inhibitive effect on the corrosion of metal. The mechanisms are complex and not always fully understood. Inhibitive pigments include red lead, zinc phosphate and zinc chromate. 

These materials are now widely used for coating both steel and concrete surfaces that are subject to a particularly aggressive environment (e.g. North Sea oil platforms). There is less validity for their use under normal atmospheric conditions since they are relatively expensive and tend to chalk on exposure to sunlight. However, their use as zinc phosphate, pretreatment or blast primers for blast-cleaned steel which is subsequently overcoated by any other paint system is an extremely valuable contribution to the painting of new steel work. 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

Cleaners containing silicate can cause problems. They should not be used prior to an alkaline process on aluminium, owing to the formation on the surface of alkali-insoluble aluminium silicate. Silicated cleaners can also cause problems before some surface-sensitive zinc phosphating solutions, especially the more modern low-zinc type. 

Etch primers are widely used. They are mostly based on polyvinyl butyral and contain chromates and phosphoric acid. They are said to act both as primers and as etching solutions because it is believed that the chromates and phosphoric acid form an inorganic him, which provides adhesion, while oxidised polyvinyl butyral provides an organic him. For direct application to new galvanised steel, the best known primers are based on calcium orthoplumbate pigment and metallic lead, but these are now less used for environmental reasons. Zinc-dust paints and zinc-phosphate pigmented paints are also used, but the trend is to use pretreatments to assure good adhesion. 

Zinc phosphate is now probably the most important pigment in anticorrosive paints. The selection of the correct binder for use with these pigments is very important and can dramatically affect their performance. Red lead is likely to accelerate the corrosion of non-ferrous metals, but calcium plumbate is unique in providing adhesion to newly galvanised surfaces in the absence of pretreatment, and is claimed to behave similarly on other metals in this group. 

Zinc phosphate was introduced as an inhibitive pigment by Barraclough and Harrisonand in the early tests vehicles based on drying oils were used. Later it was claimed that it was an elfective inhibitive pigment when used with all paint media in current use. 

A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

First comes the pretreatment stage. After rust removal and alkaline degreasing, a zinc phosphate formulated pretreatment (see Section 15.2) is applied by dip or spray-dip. Crystalline iron-rich zinc phosphate forms on the metal surface at a coating weight of 0.5-4.5 g/m. ... 

Good quality steel is used and electrozinc is preferred for washing machines. Steel is pretreated with iron phosphate for economy electrozinc with a fine crystal zinc phosphate. No primer is normally used 25-40/im of finish is applied direct to metal. The required properties are best obtained with a thermosetting acrylic or polyester/melamine-formaldehyde finish. Self-reactive acrylics are usually preferred these resins contain about 15 Vo 7V-butoxymethyl acrylamide (CH2=CH —CO —NH —CHj—O —C4H,) monomer and cure in a manner similar to butylated melamine-formaldehyde resins. Resistance or anti-corrosive properties may be upgraded by the inclusion of small amounts of epoxy resin. Application is usually by electrostatic spray application from disc or bell. Shapes are complex enough to require convected hot-air curing. Schedules of 20 min at 150-175°C are... 

It is usual to define primers in terms of the principal inhibiting pigment e.g. zinc phosphate, zinc dust or zinc chromate, and the topcoats in terms of the binder, e.g. alkyd, chlorinated rubber, etc. This practice can be confusing, however, and lead to the selection of incompatible coatings. 

Zinc phosphate primer in quick- Airless spray. 2 80-100... 

A zinc phosphate solution tends to produce coatings more quickly than iron or manganese phosphate solutions, and dissociation of primary zinc phosphate proceeds rapidly through reaction 15.2 to 15.3 or directly to tertiary zinc phosphate via reaction 15.4. Even so, a processing time of 30 min is usual with the solution near boiling. 

---