Many types of cement utilised in dentistry may be characterized by setting typical reactionswhich involve the creationof chelate compounds amongdivalent metallic ions such as zinc ions and ortho-disubstituted aromatic compounds. Three kinds of aromatic compound are commonly used, delineating the three groups of materials.
· Zinc oxide/eugenol cements.
· Ortho-ethoxybenzoic acid (EBA) cements.
· Calcium hydroxide cements, during which the aromatic ligands are silicylates.
Zinc oxide/eugenol cements
Composition and setting: The products can be supplied being a powder and liquid or as two pastes. The little number of zinc acetate within a powder acts just as one accelerator by helping to generate an ionic medium in which the setting reaction can occur. Some commercial products contain hydrogenated rosin or polystyrene and are known as resin-reinforced zinc oxide/eugenol cements. The relative proportions of powder and liquid are not normally measured accurately, although some manufacturers provide a scoop which gives a known volume of powder to which a given number of drops of liquid are added. Thin mixes, creating a low powder/liquid ratio, must be avoided since they produce inferior properties - lower strength and higher solubility. These possess the good thing about easier proportioning and mixing.
Properties: The settingcharacteristics from the zinc oxide/eugenol cements are, to some extent, ideal. They offer a mixture of adequate working time, during which little increase in viscosity occurs, coupled with rapid setting after placing into the cavity. The latter is the result of residual moisture in the cavity and the higher temperature of the mouth compared with room temperature. The effect of cavity moisture is noteworthy, particularly since efforts are generally made to dry the cavity before placement of a lining. Only very small amounts of water are required to cause the accelerating effect.
Ortho-ethoxybenzoic acid (EBA) cements
Composition: These cements are generally supplied as a powder and liquid. The ratio of o-ethoxybenzoic acid to eugenol in the liquid may vary from one product to a different but is frequently about 2: 1. Shows the similarity between the structure of o-ethoxybenzoic acid and eugenol. Both compounds are able to form chelate compounds with zinc ions.
Properties and applications: The setting characteristics are similar to those of the zinc oxide/eugenol materials and are similarly affected by moisture. It is possible to achieve higher powder/liquid ratio with these products since much of the powder consists of inert, reinforcing filler.
Calcium hydroxide cements
Composition: Some calcium hydroxide preparations consist simply of a suspension of calcium hydroxide in water. This is applied to the base of the cavity and dries out to give a layer of calcium hydroxide. These materials are both difficult to manipulate and form a very friable cavity lining which is easily fractured. A solution of methyl cellulose in water or of a synthetic polymer in a volatile organic solvent can be used instead of water. These additives produce more cohesive cement but the compressive strength remains very low at about 8 MPa.
Properties: The mixed materials have very low viscosity and setting can be relatively slow for a few products. Moisture has a dramatic effect on the speed of setting however, and also the materials set within a few seconds of being placed within the cavity, even when the cavity can be ‘dried’. Setting of the light-activated materials is more under the control from the operator and residual moisture inside cavity does not have the same influence on setting time. An exposure to activating light for only some seconds is required to activate polymerization of the skinny layer of cement. One characteristic of those cements which continues to be largely ignored is the relatively high temperature rise produced on setting. This results from the heating effect of the light source and the exothermic setting reaction.
The set material is comparatively weak matched against other cements, developing a compressive strength of about 20 MPa. This strength is gained rapidly, however, and under the constrained conditions of a cavity the fabric is able to resist flow and fracture during amalgam condensation, providing correct technique is used. The light-activated materials by which the set material has a cross linked resin matrix are less brittle than both paste products.
California Smile Design || Metal And Alloys Dr Ihab Emil Hawary