The glass has a formulation based on the firing of a combination of chemicals (Table 9.1). Figure 9.2 shows some typical glass formulations for commonly available cements and highlights the substantial variation in the formulations of glass which may be used to produce a glass ionomer cement.

Table 9.1 Compounds contained in the glass of a typical glass ionomer cement

Compound	Percentage composition
Alumina (aluminium oxide)	14.2–28.6
Silica (silicon dioxide)	30.1–41.9
Calcium fluoride	12.8–34.5
Aluminium fluoride	1.6–11.0
Aluminium phosphate	3.8–24.2
Sodium fluoride	3.6–12.8

Fig. 9.2 Typical glass formulations of glass ionomer cements in commercial use.

Manufacturing process for the glass component

The glass mixture is heated to a temperature between 1150°C and 1450°C. The molten mass is then poured onto a metal plate and then into water, a process termed shock cooling. The glass is then broken up to form a glass frit (Figure 9.3). All the glasses for the currently available cements are then either wet or dry milled to produce small particles of glass. Various ranges of particle size are used depending on the requirements and usage of the cement. The larger glass particle sizes are used in those cements intended as restorative materials (up to 20 μm) while the smaller particle sizes (<5 μm) are used for luting cements.

Fig. 9.3 Glass frit produced after firing.

Frit: This is the product of pouring molten glass into water. It usually consists of small particles of glass which will be ground up to smaller size later in the glass preparation process. The size of the glass particles is determined by the use or application.

Wet milled: The glass particles are placed in a cylindrical ceramic vessel with a volume of water and a number of ceramic balls. The whole assembly is then rotated. The ceramic balls tumble around the vessel grinding the glass frit down in size. The longer the tumbling process the finer the particles of glass.

Dry milled: as for wet milling but with the exclusion of the water.

If the glass is mixed with a polyacid at this point, it would set very rapidly with too short a working time for clinical use. The manufacturers all now adopt a passivation treatment to reduce the reactivity of the glass. The glass is washed in acetic acid for up to 24 hours and then dried. This process leads to ion depletion of the glass surface, resulting in fewer ions being immediately available to form the salt matrix when mixing starts. Passivation of the glass is of greater significance when the particle size of the cement is reduced, as the smaller the particle the more rapid the set.

Acid

The acid used is part of a series of polyacids, including polyacrylic acid, polymaleic acid and a number of copolymers of polyacrylic acid. The combination of polyacids that are suitable for copolymerization can be varied widely so conveying different properties to the final product. The acid in the glass ionomer cement is a variant of either the homopolymer of acrylic acid, its co-polymer or maleic acid, depending on the manufacturer. The functional group which all these acids contain and which plays a part in the chemical reaction is a carboxylate group – which in water ionizes to carboxyl and hydrogen ions. A small number of cements also include a polyphosphonic acid. This is thought to provide a short sharp set but does not appear to enhance the physical properties to any great extent.

Homopolymer: a polymer that contains one single monomer type.

Ion: An ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.

Copolymerization: the method of chemically synthesizing a copolymer.

The performance of the cement is related to the molecular weight of the acid used and this explains why two cements with apparently similar compositions can behave differently. It is generally considered that the higher the molecular weight of the acid used, the better the mechanical properties. However, this benefit is offset by the increase in viscosity of the aqueous acid solution with increasing molecular weight. To overcome this, some manufacturers vacuum dry the acid solution and then mix the anhydrous powder so formed with the glass. Activation of the cement is achieved by adding water. This addition from a dropper bottle starts the chemical reaction as the vacuum-dried acid starts to go into solution, which then is followed by the initial dissolution of the glass surface.

Anhydrous: a material which does not contain water. The term anhydrous refers to the presentation of some glass ionomer cements in which the powder has been dried.

Maleic acid, one of the alternatives to polyacrylic acid, is a less viscous aqueous solution and this has permitted one manufacturer (3M ESPE) to use the acid in aqueous form in the encapsulated presentation more easily.

Changes in molecular weight will also influence the working and setting time of the cement. The higher the molecular weight (and so the viscosity), the shorter the working and setting times. A small amount of tartaric acid is added to all materials to accelerate the setting phase of the reaction while maintaining the working time (Figure 9.4).

Fig. 9.4 Bar graph illustrating the effect of the addition of tartaric acid on working times (WT) and setting times (ST) of a glass ionomer cement. There is no change in working time but there is a marked reduction in setting time on addition of 2% tartaric acid.

(Data from Wilson AD, Nicolson J Acid-Base Cements: Their Biomedical and Industrial Applications. Cambridge University Press, Cambridge, 1993 (Chapter 5).)