Introduction

The chief problem in studying the chemical nature of AB cements is that many are essentially amorphous, so that the powerful tool of X-ray diffraction (XRD) analysis cannot be used. Some AB cements do exhibit a degree of crystallinity, but rarely in significant amounts; indeed, complete crystallinity is usually a sign that the reaction product is not cementitious. The literature contains numerous examples of workers being misled by the results of XRD analysis into neglecting the presence and significance of the amorphous phase.

A number of techniques have been employed that are capable of giving information about amorphous phases. These include infrared spectroscopy, especially the use of the attenuated total reflection (ATR) or Fourier transform (FT) techniques. They also include electron probe microanalysis, scanning electron microscopy, and nuclear magnetic resonance (NMR) spectroscopy. Nor are wet chemical methods to be neglected for they, too, form part of the armoury of methods that have been used to elucidate the chemistry and microstructure of these materials.

In addition to spectrosopic studies of the setting chemistry of AB cements, numerous mechanical tests have been used to measure properties of the set materials. This latter group has included determination of compressive and flexural strengths, translucency, electrical conductivity and permittivity. The present chapter describes each of these techniques in outline, and shows how they have been applied. Results obtained using these techniques are described in earlier chapters which deal more thoroughly with each individual type of AB cement.

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.

Over the years, from 1964 to date, a team at the LGC, with its expertise in Materials Chemistry, has studied many of the materials described in this book, elucidating structures, setting reactions and behaviour. This experience has formed a strong experimental background against which the book was written. In addition we have maintained contact with leaders in this field throughout the world.

General

This chapter is devoted to a miscellaneous group of aqueous acid–base cements that do not fit into other categories. There are numerous cements in this group. Although many are of little practical interest, some are of theoretical interest, while others have considerable potential as sustainedrelease devices and biomedical materials. Deserving of special mention as biomedical materials of the future are the recently invented polyelectrolyte cements based on poly(vinylphosphonic acids), which are related both to the orthophosphoric acid and poly(alkenoic acid) cements.

Miscellaneous aluminosilicate glass cements

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 acids. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses.

(1) Silica gel is formed in the reaction but is not associated with cement formation.

(2) Water is essential to the reaction and cements are not formed when the acid is present in an organic solvent rather than in aqueous solution. Water acts as an effective reaction medium and probably hydrates reaction products.

(3) Cements are formed only with acids that are capable of forming complexes with calcium and aluminium. Thus, neither hydrochloric nor acetic acid forms cements.

Polyelectrolytes

General

The setting of AB cements is an example of gelation, and gelation is related to ion binding. A theoretical examination of the various phenomena associated with ion binding and gelation finds its clearest exposition in the field of polyelectrolytes. Moreover, this field may be wider than it seems at first.

Polyelectrolytes form the basis of those modern cements which are distinguished by their ability to adhere to reactive surfaces. At present the main use of such cements lies in the medical field, principally in dental surgery. They adhere permanently to biological surfaces where they have to withstand adverse conditions of wetness, chemical attack, the stress of biological activity, and chemical and biological changes within the substrate. Nevertheless, adhesive bonds are maintained.

Polyelectrolytes are polymers having a multiplicity of ionizable groups. In solution, they dissociate into polyions (or macroions) and small ions of the opposite charge, known as counterions. The polyelectrolytes of interest in this book are those where the polyion is an anion and the counterions are cations. Some typical anionic polyelectrolytes are depicted in Figure 4.1. Of principal interest are the homopolymers of acrylic acid and its copolymers with e.g. itaconic and maleic acids. These are used in the zinc polycarboxylate cement of Smith (1968) and the glass–ionomer cement of Wilson & Kent (1971). More recently, Wilson & Ellis (1989) and Ellis & Wilson (1990) have described cements based on polyphosphonic acids.

General

The non-aqueous or chelate cements are an exceptionally diverse group of materials (Wilson, 1975a,b, 1978; Smith, 1982b). The term chelate cement is not strictly speaking correct, as a minority of them do not form chelates, and some aqueous AB cements do. However, the term is a convenient one. They are of interest in that the reaction media for the acid–base reaction are non-aqueous, although sometimes water may play a role in cement formation. In these cements water is replaced by an organic acid that is liquid at room temperature and generally has chelating ability.

The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO and Bi2O3 (Brauer, White & Moshonas, 1958; Nielsen, 1963). In practice these are confined to two: calcium hydroxide and special reactive forms of zinc oxide.

Examples of liquid organic acids suitable for cement formation are:

(1) Alkoxyphenols, for example the 2-methoxyphenols, which include eugenol, guaiacol and vanillates. Also, 2,5-dimethoxyphenol.

(2) β-diketones, e.g. acetylacetone.

(3) β-keto esters, e.g. β-keto-ethylate.

(4) Keto acids, e.g. acetyl acetic acid.

(5) Other difunctional aliphatic carboxylic acids, e.g. lactic acid, pyruvic acid, ethoxyacetic acid.

(6) Aldehydic aromatic acids, e.g. salicylaldehyde.

(7) Alkoxy aromatic acids, e.g. 2-ethoxybenzoic acid.

Introduction

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.

Water as a solvent

It is widely recognized that the solvent in which any chemical reaction takes place is not merely a passive medium in which relevant molecules perform: the solvent itself makes an essential contribution to the reaction. The character of the solvent will determine which chemical species are soluble enough to enter solution and hence to react, and which species are insoluble, and thus precipitate out of solution, thereby being prevented from undergoing further chemical change. In the case of water, as will be seen, polar and ionic species are the ones that most readily dissolve. But even so, mere polarity or ionic character is not sufficient to ensure solubility. Solubility depends on a number of subtle energetic factors, and the possible interactions between water and silver chloride, for example, do not fulfil the requirements despite the ionic nature of the silver salt. Hence silver chloride is almost completely insoluble in water.

General

The phosphate bonded cements described in this chapter are the products of the simple acid–base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates.

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 [PO4 tetrahedron.

Orthophosphoric acid solutions

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.

Introduction

Oxysalt bonded cements trace their origin to studies by Sorel in the third quarter of the nineteenth century. The first of these cements which he studied was zinc oxychloride (Sorel, 1855). Later he described a series of magnesia-based cements which included both the magnesium oxychloride and magnesium oxysulphate types (Sorel, 1867). Of this second group, the magnesium oxychlorides in their hydrated form have been shown to have larger values of modulus of elasticity, microhardness and compressive strength than does Portland cement for a wide range of porosites (Beaudoin & Ramachandran, 1975). The magnesium oxysulphate cements have properties that have led to their being considered for nuclear applications, since they have good fire resistance, low thermal conductivity and above all, in marked contrast to the related oxychloride cements, no potential to initiate corrosion of the reinforcing steel (Beaudoin & Ramachandran, 1978). These cements are also employed as binders in lightweight panels, in insulating materials and in architectural applications (Urwongse & Sorrell, 1980b).

Oxysalt bonded cements are formed by acid–base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homogeneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex.

Introduction

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.

The polyelectrolyte cements may be classified by the type of basic powder used to form the cement.

1. The metal oxide cements (Section 5.6)

2. The zinc polycarboxylate cement (Section 5.7)

3. The mineral ionomer cements (Section 5.8)

4. The glass–ionomer or glass polyalkenoate cement (Section 5.9)

Only two of these materials are of practical importance: the zinc polycarboxylate cement of Smith (1968) and the glass–ionomer cement of Wilson & Kent (1971). Both are used in dental applications and both have been used as bone cements.

Our general understanding of molecular collisions and energy transfer rests mainly upon classical mechanics. Except for particular quantum mechanical processes, such as transitions between different electronic states, and quantum mechanical features like resonances or interferences classical mechanics is a very useful tool for the study of molecular encounters (Porter and Raff 1976; Pattengill 1979; Truhlar and Muckerman 1979; Schatz 1983; Raff and Thompson 1985; Levine and Bernstein 1987:ch.4). This holds true for photodissociation as well (Goursaud, Sizun, and Fiquet-Fayard 1976; Heller 1978a; Brown and Heller 1981; Schinke 1986c; Goldfield, Houston, and Ezra 1986; Schinke 1988b; Guo and Murrell 1988a,b).

Classical mechanics is the limit of quantum mechanics as the de Broglie wavelength λB = 2πħ/(2mE)½ becomes small. The total energy released as translational and internal energy in UV photodissociation often exceeds 1 eV and therefore λB is of the order of 0.1 Å or shorter. On the other hand, the range of the potential is typically much larger so that the quantum mechanical wavefunction performs many oscillations over the entire interaction region (see Figures 2.3 and 3.2, for example). Furthermore, in many cases the fragments are produced with high internal excitation (Figure 3.3) which additionally favors a classical description.

The classical picture of photodissociation closely resembles the timedependent view. The electronic transition from the ground to the excited electronic state is assumed to take place instantaneously so that the internal coordinates and corresponding momenta of the parent molecule remain unchanged during the excitation step (vertical transition).

Rotational excitation of photofragments is a wide field with many subtleties. In the foregoing chapters we have considered exclusively the scalar properties of rotational excitation, i.e., the distributions of final rotational states of the products and the forces that control them. For this purpose, it was sufficient to study the case that the total angular momentum of the entire molecular system is zero, J = 0. This restriction drastically facilitated the theoretical formulation and allowed us to concentrate on the main effects without being intimidated by complicated angular momentum coupling. In Section 11.1 we will extend the theory of rotational excitation to general total angular momentum states J ≠ 0. Our aim is the investigation of final rotational product states following the photodissociation of single rotational states of the parent molecule (Section 11.3). Before doing so, however, we discuss in Section 11.2 the distribution of the various electronic fine-structure states (Λ-doublet states) if the fragment possesses a nonzero electronic angular momentum which couples with the angular momentum of the nuclear motion. Important examples are OH and NO.

The vector of the electromagnetic field defines a well specified direction in the laboratory frame relative to which all other vectors relevant in photodissociation can be measured. This includes the transition dipole moment, μ, the recoil velocity of the fragments, v, and the angular momentum vector of the products, j. Vector correlations in photodissociation contain a wealth of information about the symmetry of the excited electronic state as well as the dynamics of the fragmentation. Section 11.4 gives a short introduction.

Dissociation via a single excited electronic state is the exception rather than the rule. The remarkable success with which all experimental results for the dissociation of H2O, for example, have been reproduced by rigorous calculations without any adjustable parameter rests mainly upon the fact that only one electronic state is involved (Engel et al. 1992). Many other photodissociation processes, however, proceed via two or even more electronic states with the possibility of transitions from one state to another. Figure 15.1 illustrates a common situation: the photon excites the molecule from the electronic ground state (index 0) to a dipole-allowed upper state (index 1) which further out in the exit channel interacts with a second electronic state (index 2). The latter may be dipole-forbidden and therefore not directly accessible by the photon. The corresponding diabatic potentials V1 and V2 cross at some internuclear distance Rc. In the proximity of this point the coupling between the two electronic states, which was ignored throughout all of the preceding chapters, can be large with the consequence that a transition from state 1 to state 2 and/or vice versa becomes possible (radiationless transition, electronic quenching). Electronic transitions manifest the break-down of the Born-Oppenheimer approximation, i.e., the motion of the electrons and the heavy particles can no longer be adiabatically separated.

Let us imagine a wavepacket starting in the Franck-Condon region on potential V1 When it reaches the crossing region it splits under the influence of the coupling into two parts.