Cement-forming acids phosphoric acid

Polyphosphoric acid supported on diatomaceous earth (p. 342) is a petrochemicals catalyst for the polymerization, alkylation, dehydrogenation, and low-temperature isomerization of hydrocarbons. Phosphoric acid is also used in the production of activated carbon (p. 274). In addition to its massive use in the fertilizer industry (p. 524) free phosphoric acid can be used as a stabilizer for clay soils small additions of H3PO4 under moist conditions gradually leach out A1 and Fe from the clay and these form polymeric phosphates which bind the clay particles together. An allied though more refined use is in the setting of dental cements. 

Generally, cement-forming liquids are aqueous solutions of inorganic or organic adds. These adds include phosphoric add, multifunctional carboxylic adds, phenolic bodies and certain metal halides and sulphates (Table 2.1). There are also non-aqueous cement-forming liqtiids which are multidentate acids with the ability to form complexes. 

Although this account of gelation is made with reference to organic polyelectrolytes, it is of wider application and may be applied to phosphoric acid cements. Orthophosphoric acid solutions used in these cements contain aluminium, and soluble aluminophosphate complexes are formed. Some appear to be multinuclear and there is evidence for polymers based on the bridging Al-O-P unit. These could be termed polyelectrolytes (Akitt, Greenwood Lester, 1971 Wilson et al., 1972 O Neill et al., 1982). 

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the... 

Zinc phosphate cement, as its name implies, is composed principally of zinc and phosphate. It is formed by mixing a powder, which is mainly zinc oxide, with a solution based on phosphoric acid. However, it is not as simple chemically as it appears because satisfactory cements caimot be formed by simply mixing zinc oxide with phosphoric acid solution. 

The early history of the material is obscure. According to Palmer (1891) it goes back to 1832, but this statement has never been corroborated. Rostaing (1878) patented a series of pyrophosphate cements which could include Zn, Mg, Cd, Ba and Ca. Rollins (1879) described a cement formed from zinc oxide and syrupy phosphoric acid. In the same paper he mentions zinc phosphate cements recently introduced by Fletcher and Weston. Similar information is given in a discussion of the Pennsylvania... 

The liquid is an aqueous solution of phosphoric acid, always containing 1 to 3 % of aluminium, which is essential to the cement-forming reaction (Table 6.2). Zinc is often found in amounts that range from 0 to 10% to moderate the reaction. Whereas zinc is present as simple ions, aluminium forms a series of complexes with phosphoric acid (Section 6.1.1). This has important consequences, as we shall see, in the cement-forming reaction. 

Komrska Satava (1970) showed that these accounts apply only to the reaction between pure zinc oxide and phosphoric acid. They found that the setting reaction was profoundly modified by the presence of aluminium ions. Crystallite formation was inhibited and the cement set to an amorphous mass. Only later (7 to 14 days) did XRD analysis reveal that the mass had crystallized directly to hopeite. Servais Cartz (1971) and Cartz, Servais Rossi (1972) confirmed the importance of aluminium. In its absence they found that the reaction produced a mass of hopeite crystallites with little mechanical strength. In its presence an amorphous matrix was formed. The amorphous matrix was stable, it did not crystallize in the bulk and hopeite crystals only grew from its surface under moist conditions. Thus, the picture grew of a surface matrix with some tendency for surface crystallization. 

Vashkevitch Sychev (1982) have identified the main reaction product of the cement-forming reaction between copper(II) oxide and phosphoric acid as Cu3(P04)2. SHjO. The addition of polymers - poly(vinyl acetate) and latex - was found to inhibit the reaction and to reduce the compressive strength of these cements. However, impact strength and water resistance were improved. 

A deficiency of water in the cement liquid has the same effect and this occurs when the H3PO4 content exceeds 60%. Wilson Mesley (1968) noted that in a cement formed from a solution of 65 % H3PO4 there was evidence of incomplete reaction even after 6 hours. We have noted in Section 6.5.3 that there is a sharp decline in the rate of reaction when the orthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14). The avidity of cements to absorb water from humid surroundings also increases sharply when the phosphoric acid in the cement-forming liquid exceeds 60%. It is difficult to avoid the conclusion that these two phenomena are related and that a deficiency of water retards the cementforming reaction. 

An old bottle (1 year) containing glycolonitrile with phosphoric acid used as a stabiliser showed the appearance of tars. It detonated during handling. The detonation was probably due to the polymerisation of nitrile that was made possible by the fact that phosphoric acid was isolated by tars. Besides, the cap was cemented by tars that had already formed round the cap. A similar accident happened thirteen days after distilling the same nitrile. 

In parallel to the work on zinc phosphate cements, porcelain dental cements also were developed. Steenbock [23] was the first to produce silicophosphate dental cement using 50 wt% concentrated phosphoric acid solution and an aluminosilicate glass. Schoenbeck [24] introduced fluoride fluxes in these glasses and vastly improved the dental cements. Fluorides lower the temperature of fusion of the glasses used in forming these cements. The same fluorides impart better translucency to the cement, and have some therapeutic effects. As a result, fluorides have become a part of modern dental cements. 

Wilson et al. [25] analyzed various brands of commercial cements and specified their possible composition, properties, and microstructure. Wilson et al. report the most representative and comprehensive data on commercial porcelain dental cements. These cements consist of powdered alumina-lime-silica glass mixed with phosphoric acid that formed a hard and translucent product. The starter glass powder consists of 31.5-41.6 wt% silica, 27.2-29.1 wt% alumina, 7.7-9.0wt% calcium oxide, 7.7-11.2 wt% sodium oxide, 13.3-22 wt% fluorine and small amounts of phosphorous and zinc oxides. Often very small amounts of magnesium and strontium oxides are also present. 

These cements are formed by a similar process to the silicate minerals described in Chapter 1, the difference being the rate. Silicate minerals are formed at a rate lower by orders of magnitude compared with dental cements. In the case of dental cements, the phosphoric acid releases protons in the solution and lowers its pH. This decomposes the glass and releases silicon in the solution, and silicic acid forms as an intermediate product [26,27]. Simultaneously, cations such as Ap, Ca " ", and Na" " and the anion F are also released [28]. The cations and anions are attracted to each other, and neutral bonding phases form. Such a bonding network, especially that of aluminum, results in gelation and subsequent polymerization of a hard product. 

Similarly, the dissociation constants of phosphoric acid or its subsequent ions, given in Eqs. 4.28 and 4.29, are comparable to the highest value of pAiso given in Table 4.2 or even higher. Thus, use of phosphoric acid that furnishes phosphate ions is not useful in forming practical ceramics. For this reason, as noted in Chapter 2, researchers have resorted to some neutralization of the acid by dissolving oxides of A1 or Zn to produce dental cements. 

Magnesium titanates have also been used to form such cements. Mg2Ti04, MgTiOs, and Mg2Ti20s have been reacted with phosphoric acid by Sychev et al. [35] and Sudakas et al. [36]. Newberyite is formed when Mg3Ti04 is used, but in other cases, amorphous phases are formed that are difficult to identify. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

CHEMICAL PROPERTIES noncombustible solid gradually absorbs carbon dioxide upon exposure to air reacts with hydrochloric acid to produce zinc chloride reacts with sulfuric acid to produce zinc sulfate, and it reacts with carbon monoxide or hydrogen to produce elemental zinc reacts lowly with fatty acids in oils and fats to produce lumpy masses of zinc oleate, stearate, etc. forms cement-like products when mixed with a strong solution of zinc chloride or with phosphoric acid, owing to the formation of oxy-salts hydrogen peroxide is produced when ointments containing zinc oxide and water are melted and exposed to UV light FP (NA) LFL/UFL (NA) AT (NA) HC (NA) HF (-350.5 kJ/mol crystal at 25°C) pH (6.95 American process zinc oxide, 7.37 French process zinc oxide). 

For many years, the dental silicate cement was thought to set by the formation of a sihcate stracture [9]. However, studies by Wilson et al. in the 1960s showed that it was, in fact, a phosphate cement [10-12]. Reaction of the glass with aqueous phosphoric acid was shown to yield a complex matrix that consisted of a mixture of calcium and alnminium phosphates. There was also some siUca gel formed, though... 

Several possible calcium and aluminium phosphates exist and they differ in their solubility in aqueous media and also in their resistance to acid attack. Among the factors determining which of these products are formed are powderiliquid ratio of the cement and concentration of the phosphoric acid solution. As a result, this material was easy to prepare in a soluble or acid-sensitive state. Incorrect metering of the powder to liquid components increased the solubility of the set cement, and leaving the bottle of phosphoric acid solution open to the air led to uptake of moisture from the atmosphere, with a corresponding reduction in acid concentration. This resulted in an increase in the proportion of more soluble metal salts in the set cement. These factors combined to make the dental silicate difficult to use in the clinic and gave the material a reputation for unreliability [8]. 

Zinc phosphate cement consists of finely powdered zinc oxide suspended in phosphoric acid. The setting and hardening of this cement results from a chemical reaction between these two constiments, in which zinc phosphate tetrahydrate is formed as the product of reaction ... 

Combinations of zinc oxide with concentrated solutions of phosphoric acids also exhibit cementing properties. These zinc phosphate cements can be formulated to set within a few minutes, and develop strength rapidly. Zinc phosphate is formed as a product of the hardening reaction (see also section 12.4). 

The reaction of phosphoric acid (usually in the form of ammonium phosphate) with magnesium oxide in the presence of water gives magnesium phosphate cements. These are used for rapid repairs, for example of pot holes in busy roads. 

Phosphoric acid combines rapidly at room temperatures with the oxides of Be, Mg, Ca, Sr, Ba, Zn, Cu, Mn and Pb producing both acid and neutral salts. Some of the products form hard cohesive masses which can be utilised as cements. These include cements for dental, electrical, refractory and constructional purposes (Section 12.10). 

---