Date Published: July 31, 2018
Publisher: Taylor & Francis
Author(s): John W. Nicholson.
Glass-ionomer cements are used for a variety of tooth-repair functions in clinical dentistry. They are formed by reaction of a basic glass powder with a solution of polymeric water-soluble acid, usually polyacrylic acid. After the initial neutralization reaction, by which the cement hardens, various maturation reactions occur. Changes induced by these maturation reactions are identified as: increase in strength; reduction in plasticity; improvement in opacity; and increase in proportion of tightly bound water. In addition, in contact with the tooth, an ion-exchange interfacial layer is gradually formed. This is mechanically strong and chemically-resistant. These changes are described in the current paper, which reviews the extent to which they occur, and reports what is know about the chemistry that underlies them. Processes involving slow diffusion of various ions and of water through the set cement bring about these changes. They include a secondary setting reaction to form a phosphate-based phase, binding of water to co-ordination sites around metal cations and to a hydration sheath around the polymer molecules, and possibly reaction of water with glass particle surfaces to form silanol groups. Evidence from a wide range of literature sources is used to be build up a detailed picture of the chemistry of the maturation processes, and gaps in our understanding are highlighted. The article concludes that, given the importance of glass-ionomers in contemporary dentistry, it is important to know the extent to which such maturation processes occur in current cement formulations, and also to determine how rapidly they take place.
Glass-ionomer cements are acid-base materials that are formed by the reaction of weak polymeric acids with basic alumino-silicate glass powders [1–3]. Modern versions of these materials typically comprise powders that contain some of the polymeric acid in dried form, so that the acid solution is not too viscous while allowing the freshly mixed cement to contain the high amounts of acid necessary for the achievement of rapid setting and high strength. This type of formulation is characteristic of the so-called “high-viscosity” glass-ionomers, a term typically applied to materials with powder:liquid ratios of at least 3.6 to 1.
Glass-ionomer cements undergo a rapid initial hardening reaction, but continue to undergo changes for some time after this hardening is complete. These later processes are known jointly as maturation, and they are the subject of this review paper. In this paper, the key changes are identified, and what is known about their underlying chemistry is described.
That glass-ionomer cements consist of more than simply ionically cross-linked polyacid molecules reinforced by unreacted glass particles was first proposed more than 25 years ago . This arose from the finding that hard, insoluble cement materials could be made by reacting ionomer glasses with ethanoic (acetic) acid . Later it was shown that similar cement materials could be made using lactic acid . These materials were later called pseudo-cements, a helpful term because it distinguishes them from the clinically useful proper glass-ionomer cements made with polymers .
The standard technique for measuring the strength of conventional glass-ionomer cements is in compression . However, other types of strength have been used, namely flexural , biaxial flexure , diametral tensile  and shear punch . Most of the data on change in strength concerns compressive strength, and much of it is concerned with very early glass-ionomer formulations. There is relatively little published data on how the strength of modern glass-ionomers changes with maturation, though what there is suggests that these materials, too, become stronger with time over the initial few weeks after preparation.
By comparison with composite resins, glass-ionomers show poor translucency and inferior aesthetics . However, unlike all other dental cements, modern materials do have a degree of translucency, and this changes with time during the maturation phase, so that after 24 hours translucency is much improved . Early glass-ionomer materials were relatively opaque due to the high fluoride content of the glass powder used, but this is one of the properties that have been improved in contemporary glass-ionomers.
Water is an essential component of glass-ionomer cements, and has several functions in these materials . It is the solvent for the dissolution of the polymeric acid, and allows it to ionise and donate protons, thereby behaving as a Bronsted-Lowry acid . It is also the medium in which the setting reaction takes place and it is a component of the set cement. The last is an important but overlooked feature. All of the water incorporated initially as the solvent for the acidic polymer eventually becomes entrained in the set cement. There is no phase-separation on setting, and no expulsion of water as the cement hardens.
Glass-ionomer cements are naturally adhesive to teeth at all stages of their development. They owe their initial adhesion to the presence in them of polyacrylic acid or acrylic/maleic acid copolymer . The hydrophilic nature of the cement paste causes it to fully wet the freshly prepared tooth surface. Adhesion develops rapidly after initial placement as hydrogen bonds are formed between the free carboxyl groups in the cement and strongly bound water layers on the tooth surface . These hydrogen bonds are gradually replaced by ionic bonds involving cations such as calcium in the mineral phase of the surface of the tooth and carboxylate groups on the polymer .
This review has shown that there are various maturation processes in glass-ionomer cements. They take place over the first month to 6 weeks of a cement’s existence and they generally combine to improve the physical properties of the cement. Materials become stronger, less susceptible to water loss and surface crazing, and their appearance improves as translucency increases. These processes occur relatively slowly and appear to be controlled by the rate at which ions and water are able to diffuse through the cement. Movement of water has been demonstrated experimentally in water-loss experiments carried out under severely desiccating conditions, and movement of ions has been shown by the formation of the ion-exchange bonding layer at the interface with the tooth; it has also been shown in fluoride-release studies.