Acid-base cements formation

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 setting reaction for the great majority of acid-base cements takes place in water. (The exceptions based on o-phenols are described in Chapter 9.) This reaction does not usually proceed with formation of a precipitate but rather yields a substance which entrains all of the water used to prepare the original cement paste. Water thus acts as both solvent and component in the formation of these cements. It is also one of the reaction products, being formed in the acid-base reaction as the cements set. 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

In nature, dissolution caused by carbonic and sihcic acids is a slow process. Because formation of carbonate and silicate complexes is very slow and mostly occurs on a geological time scale, it is difficult to reproduce the necessary reactions in the laboratory. On the other hand, acid-base cements may be produced within hours, and controlling the rate of reaction in these materials is easier than accelerating reactions in carbonate and sihcate minerals. Acid-base cements have considerable potential for commercial applications by exploiting the solubility of cation donors of oxides in acidic solutions. For this reason, we next explore the dissolution steps involved in formation of these cements in more detail. 

To form a CBC, control over the dissolution of the bases is crucial. The bases that form acid-base cements are sparsely soluble, i.e., they dissolve slowly in a small fraction. On the other hand, acids are inherently soluble species. Typically, a solution of the acid is formed first, in which the bases dissolve slowly. The dissolved species then react to form the gel. When the gel crystallizes, it forms a solid in the form of a ceramic or a cement. Crystallization of these gels is inherently slow. Therefore, bases that dissolve too fast will rapidly saturate the solution with reaction products. Rapid formation of the reaction products will result in precipitates and will not form well ordered or partially ordered coherent structures. If, on the other hand, the bases dissolve too slowly, formation of the reaction products will be too slow and, hence, formation of the gel and its saturation in the solution will take a long time. Such a solution needs to be kept undismrbed for long periods to allow uninterrupted crystal growth. For this reason, the dissolution rate of the base is the controlling factor for formation of a coherent structure and a solid product. Bases should neither be highly soluble nor almost insoluble. Sparsely soluble bases appear to be ideal for forming the acid-base cements. 

Overall, this review of minerals and acid-base cements provides insight into formation of CBCs in which control over dissolution of at least one of the participating components is crucial to formation of the ceramic. For this reason, a considerable part of this book is invested in describing the dissolution chemistry of oxides that form CBPCs. 

Calcium Chelates (Salicylates). Several successhil dental cements which use the formation of a calcium chelate system (96) were developed based on the reaction of calcium hydroxide [1305-62-0] and various phenohc esters of sahcyhc acid [69-72-7]. The calcium sahcylate [824-35-1] system offers certain advantages over the more widely used zinc oxide—eugenol system. These products are completely bland, antibacterial (97), facihtate the formation of reparative dentin, and do not retard the free-radical polymerization reaction of acryhc monomer systems. The principal deficiencies of this type of cement are its relatively high solubihty, relatively low strength, and low modulus. Less soluble and higher strength calcium-based cements based on dimer and trimer acid have been reported (82). 

Acid-base (AB) cements have been known since the mid 19th century. They are formed by the interaction of an acid and a base, a reaction which yields a cementitious salt hydrogel (Wilson, 1978) and offers an alternative route to that of polymerization for the formation of macro-molecular materials. They are quick-setting materials, some of which have unusual properties for cements, such as adhesion and translucency. They find diverse applications, ranging from the biomedical to the industrial. 

A final point needs to be made. Theory has indicated that AB cements should be amorphous. However, a degree of crystallization does sometimes occur, its extent varying from cement to cement, and this often misled early workers in the field who used X-ray diffraction as a principal method of study. Although this technique readily identifies crystalline phases, it cannot by its nature detect amorphous material, which may form the bulk of the matrix. Thus, in early work too much emphasis was given to crystalline structures and too little to amorphous ones. As we shall see, the formation of crystalUtes, far from being evidence of cement formation, is often the reverse, complete crystallinity being associated with a non-cementitious product of an acid-base reaction. 

This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated... 

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. 

They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from pore fluids. Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, AIPO4, which almost certainly contained magnesiiun. They ruled out a pure AlP04.nH20, for they considered that the reaction could not be represented by the equation... 

In 1968 Wilson published an account of his early search for alternatives to orthophosphoric acid as a cement-former with aluminosilicate glasses. Aluminosilicate glasses of the type used in dental silicate cements were used in the study and were reacted with concentrated solutions of various organic and inorganic adds. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses. 

Ellis Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation the phosphonic add band at 990 cm diminished as the phosphonate band at 1060 cm" developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an add-base readion, but of complex formation. Infrared spectroscopy showed a shift in the P=0 band from 1160 cm" to 1130 cm", indicative of an interaction of the type... 

Cement formation is the result of an acid-base reaction between zinc oxide and eugenol, leading to the formation of a zinc eugenolate chelate. Water plays a vital role in the reaction. 

Cowan Teeter (1944) reported a new class of resinous substances based on the zinc salts of dimerized unsaturated fatty acids such as linoleic and oleic acid. The latter is referred to as dimer acid. Later, Pellico (1974) described a dental composition based on the reaction between zinc oxide and either dimer or trimer acid. In an attempt to formulate calcium hydroxide cements which would be hydrolytically stable, Wilson et al. (1981) examined cement formation between calciimi hydroxide and dimer acid. They found it necessary to incorporate an accelerator, alimiiniiun acetate hydrate, Al2(OH)2(CHgCOO)4.3H2O, into the cement powder. 

By definition, heat of hydration is the heat generated during setting of the cement due to hydration. In the case of CBS, however, heat of hydration may not be an appropriate term, because CBS sets by acid-base reaction and not hydration. The most appropriate term would be heat of formation, which is the net change in the enthalpy during the reaction that... 

As Table 15.3 indicates, the heat offormation of Ceramicrete-based permafrost cement is typically 50-60% of the heat of formation of conventional cement. Even though the acid-base reaction is highly exothermic, i.e., it releases a significant amount of heat during setting. In CBS compositions, the binder that produces heat is only a part of the entire CBS formulation, and the remaining components are extenders. Thus, the net amount of heat generated is about half that in the equivalent amount of conventional cement. 

The surface area of cement catalysts, which carries aluminum- and calcium-containing oxide fragments, exhibits pronounced acid—base properties. These properties can manifest itself as a catalytic activity to the reactions of dehydrochlorination, which proceed via the formation of donor—acceptor complexes between the substrate and acid or base sites at the catalyst surface. 

There is confusion in the literature concerning the nature of the polymers used in glass-ionomers. This stems from the early research of Wilson et al., who studied a range of mono-, di- and tri-carboxylic acid monomers in polymers for cement formation, including itaconic and tricarballic acid [20] and this has led to the assumption that these must be used in practical cements. In fact proprietary materials are all based on the two polymers previously mentioned. 

Formation of at least part of the polysalt matrix from the acid-base components has been shown to reduce the efficiency of the polymerization process [19]. hi a study in which the application of the dental cure lamp was delayed for 20 min following mixing of the cement, the degree of conversion was found to be substantially reduced and the overall rate of polymerization to be lower than in systems that were irradiated immediately after mixing [ 19]. It is worth noting that conversion of HEMA to polymer is never particularly efficient in these systems, and that the maximum reported conversion in this study was only about 60%, and this was for a specific material, namely Fuji II EC (Fig. 7.2). 

The bonding of resin-modified glass-ionomer cements is associated with the formation of a gel phase at the interface between the material and the tooth surface [82,88]. This phase seems to originate from the acid-base part of the formulation, as it consists substantially of calcium polyacrylate, a substance that forms as the cement sets. However, the gel phase is more substantial in these materials than in conventional glass-ionomers, so that its occurrence owes something to the overall composition of resin-modified glass-ionomers [89]. 

These cements are based on the chemical reaction of solid basic magnesia powder and an aqueous acidic ammonium phosphate solution.The acid-base reactions that occur on mixing lead to the formation of insoluble magnesium ammonium phosphate hydrates, the primary binding material in the hardened product. Commercial products generally contain inert fillers and a set retarder to facilitate control of the exothermic setting process. 

Two reactive principles can be used in phosphocalcic cements. The first consists of achieving a reaction between one or more calcium phosphates, basic in nature (rich in calciirm) and one or more calcium phosphates, acidic in nature, (rich in phosphate). In aqueoirs medium, the reaction between these phases leads to less soluble new phases, whose crystallization brings about the setting. Variorrs combinations have been proposed. The second principle of cement formation consists in irsing only one phase, whose hydrolysis leads to a crystallized phase and the setting. The alpha tricalcium phosphate has been proposed more recently, an injectable cement with an amorphous phosphate base has been developed and marketed [KNA 98]. 

GI materials, the second component is a powder produced from an ion-leachable aluminosilicate glass (9), whereas in ZP cements, the powder is essentially pulverized zinc oxide, containing, in some cases, small amounts of magnesium oxide (10). Both powders are chemically basic, and thus react with the aqueous solution of the pol3mieric acid. The acid/base reaction that takes place when powder and liquid components are mixed, transforms the paste to a rigid mass within ten to twenty minutes. The mechanistic details of this reaction, as well as the structure/property relations obeyed by the solid product obtained are not well known at this time. Supposedly, the reaction involves the formation of ionic crosslinks between... 

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