The effect of particle size distribution on an experimental glass-ionomer cement
Introduction
Glass-ionomer cements (GICs), more properly referred to as glass polyalkenoate cements, are formed from powdered glass and aqueous polyacids. The glass is generally calcium or strontium fluoroaluminosilicate; strontium provides the radiopacity and fluorine, in the form of fluoride ions, enters the matrix phase, from where it can be released to promote remineralisation of caries-affected tooth structures, and confer antimicrobial properties to the final cement [1], [2], [3], [4]. The polyacids react with the glass in an acid–base reaction, leaching calcium/strontium and aluminium ions to form the set restorative cement, which is a mixture of unreacted glass in a calcium/strontium and aluminium polycarboxylate matrix.
Extensive work has been undertaken examining the interfacial reaction between the glass and the polyacid solution [5], [6], [7], [8], [9]. However, little of the conventional understanding of solid or liquid slurries and composite materials, such as the role of filler size in dental resin composites, or the flow characteristics of slurries in mineral processing, has been applied to GICs, even though variation in particle size distribution has been identified as a major route to improved mechanical properties [10]. There has been minimal investigation into the effects of particle size, and in particular particle size distribution, of the glass in conventional GICs.
The early work of Kent and Wilson [11] has been continued recently by Brune and Smith [12], who found mean particle sizing (based on sieve techniques) had little effect on compressive strength. A comparison with composites was used to explain an increase in abrasion resistance in association with a decrease in glass particle sizes [13]. It is commonly known that GICs have larger mean particle sizes than other restorative materials [14], which is recognised as a contributing factor to the relative weakness of the material [10], [14]. The lower limits of particle size in GICs are set by the viscosity of the resultant cement and by the necessity of significant glass content, to ensure that strength, ease of use and setting time remain suitable for a restorative material.
It is hypothesised that variation in particle size distribution has no significant effect on the handling, working time, setting time, and compressive strength of an experimental GIC (null hypothesis); this investigation aims to explore this hypothesis.
Section snippets
Preparation of glass-ionomer powder and liquid
A strontium fluoroaluminosilicate glass was milled using a laboratory ball mill, wet-sieved through a 25-μm mesh, and the suspension allowed to settle for 10 min. The supernatant was removed, and the settled glass filtered, dried, and re-sieved through a 150-μm mesh to remove agglomerates, resulting in a uniform glass powder (powder ‘A’). Powder A had a mean particle diameter of 9.60 μm (Table 1), measured with a laser-diffraction particle size analyser (Mastersizer S, Malvern Instruments,
Working time, setting time, and clinical handling
Fig. 1 shows the variation in working and setting time for pastes formed at the powder:liquid ratio of 2:1. The initial setting time decreased as the proportion of small particles (powder B) increased, while the working time showed a peak around 10% powder B. At this point also the paste achieved its highest score for handling (Table 3). The decrease in initial setting time followed an s-shaped curve, dropping more steeply after approximately 10% powder B before flattening out.
The graph of the
Discussion
A theoretical modelling of the flow and viscosity of glass-ionomer pastes is extremely difficult, given the complexity of the particle–particle and particle–liquid interactions, polymeric characteristics, and gelation reaction. Even two-phase dense solid–liquid systems do not compare well with theoretical calculations [16]. Although, conventional glass-ionomer cements fall into the two broad categories of ‘conventional Type II’ cements and ‘highly-viscous’ cements [17], these are not defined
Conclusion
Modern GICs provide for the clinician both challenges and opportunities. The requirements of a strong material, combined with optimal cohesion and good working and set times, means that experimental investigation into the fundamentals of particle interaction remains a necessary research area. GICs composed of large particles (around 10 μm) formed a clay-like, non-cohesive paste, while those composed of finer particles (around 3.4 μm) were strong but too fast-setting and viscous for clinical
Acknowledgements
The authors wish to acknowledge the support of SDI Ltd (Melbourne, Australia) in the supply of materials and use of experimental equipment.
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