Cement-forming acids

Denis Smith, and the development showed that satisfactory cements could be made by reacting heat-treated zinc oxide of the type used in zinc phosphate dental cements with concentrated solutions of poly (acrylic acid). This demonstrated that an alternative cement-forming acid was available, and one, which offered the prospect that cements formed from it, would adhere to the tooth [22,23],... 

It was not a straightforward matter to take the next step of making an acceptable cement from dental silicate glass and aqueous poly(acrylic acid) [18], When it was first tried, the result was a disappointing material that set very slowly and was extremely weak. It was so poor that the result was not reported at the time in a pioneering study of novel cement-forming acids [24], It was only some years later that Wilson mentioned this experiment and its unfortunate outcome [18],... 

Cement-forming liquids are strongly hydrogen-bonded and viscous. According to Wilson (1968), they must (1) have sufficient addity to decompose the basic powder and liberate cement-forming cations, (2) contain an acid anion which forms stable complexes with these cations and (3) act as a medium for the reaction and (4) solvate the reaction products. 

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. 

Acid-base cements. Cement formation involves both acid-base and hydration reactions (Wilson, Paddon Crisp, 1979). These cements form the subject of this book. 

The nature of the association between cement-forming cation and anion is important. As we shall see from theoretical considerations of the nature of acids and bases in section 2.3, these bonds are not completely ionic in character. Also while cement-forming cations are predominantly a-... 

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

Group (1) Cations and anions which are incapable of donor-acceptor interactions. These are the large univalent ions. Bonding is purely by Coulomb and Madelung electrostatic interactions. From the Lewis point of view these are not acids or bases. They have no cement-forming potential. 

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). 

Poly(acrylic acid) and its salts have been known to have useful binding properties for some thirty years they have been used for soil consolidation (Lambe Michaels, 1954 Hopkins, 1955 Wilson Crisp, 1977) and as a flocculant (Woodberry, 1961). The most interesting of these applications is the in situ polymerization of calcium acrylate added to soil (de Mello, Hauser Lambe, 1953). But here we are concerned with cements formed from these polyacids. 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

Cement-forming liquid 45 % poly(acrylic acid)... 

Crisp, Merson Wilson (1980) found that the addition of metal fluorides to formulations had the effect of accelerating cement formation and increasing the strength of set cements the effect was enhanced by the presence of (-I-)-tartaric acid (Table 5.13). Strength of cements formed from an SiOj-AljOg-Cag (P04)2 glass, G-247, can be almost doubled by this technique. 

Cement formation with fluoride glasses - - -)-tartaric acid The presence of (+)-tartaric acid in a cement formulation exerts a profound effect on the cement-forming reaction. The nature of the underlying chemical reaction is changed and this is reflected in time-dependent changes in viscosity. 

The cement-forming reaction will be similar to glass polyalkenoate cement. The cement matrix will consist of metal polyacrylates, but boric acid will be produced instead of silica gel. Since boric acid has a water solubility oil-1 % compared with the near insolubility of silica gel, it would... 

The most important of these are the refractory cements formed by the heat treatment of aluminium acid phosphate solutions. This subject has been well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977) and O Hara, Duga Sheets (1972). The chemistry of these binders is extremely complex as the action of heat on acid phosphates gives rise to polymeric phosphates, with P-O-P linkages, and these are very complex systems (Ray, 1979). 

Here we are concerned with the cement-forming reaction between orthophosphoric acid solutions and basic oxides and silicates where the reaction is much simpler. Polymeric phosphates are not involved, there are no P-O-P bonds, and the structural unit is the simple [POJ tetrahedron. 

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd Salmon, 1956 O Neill et al., 1982). But aluminium has a special effect in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. 

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. 

Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. 

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. 

Cement formation between MgO and various acid phosphates involves both acid-base and hydration reactions. The reaction products can be either crystalline or amorphous some crystalline species are shown in Table 6.5. The presence of ammonium or aluminium ions exerts a decisive influence on the course of the cement-forming reaction. 

The early history of the cement is obscure. Dreschfeld (1907) and Sanderson (1908) attributed its invention to Fletcher. Fletcher (1878,1879) certainly described cements formed from concentrated orthophosphoric acid solutions and sintered mixtures of oxides which included SiOj, AljOj, CaO and ZnO. One was reported by Fletcher (1879) as being slightly translucent. These cements were not successful in clinical use. 

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. 

Ellis and Wilson studied cements formed from concentrated solutions of poly(vinylphosphonic acid) (PVPA) and oxides and silicate glasses, which they termed metal oxide and glass polyphosphonate cements (Wilson ... 

Ellis, J. Wilson, A. D. (1991). A study of cements formed between metal oxides and polyvinylphosphonic acid. Polymer International, 24, 221-8. 

Skinner, Molnar Suarez (1964) studied the cement-forming potential of 28 liquid aromatic carboxylic acids with zinc oxide. Twelve yielded cohesive products of some merit. Of particular interest were cements formed with hydrocinnamic, cyclohexane carboxylic, p-tertiary butyl-benzoic, thiobenzoic and cyclohexane butyric acids. One of these cements is on the market as a non-eugenol cement. It is very weak with a compressive strength of 4 0 MPa, a tensile strength of 11 MPa and a modulus of 177 MPa, and is only suitable as a temporary material (Powers, Farah Craig, 1976). 

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