Cement powder (proton acceptor)
Cement liquid (proton donor)Type of cement
Related materials
Zinc oxide
Eugenol
Zinc oxide-eugenol(ZOE)
EBA cement
Zinc oxide
Aqueous solution of phosphoric acid
Zinc phosphate
Copper and silver cement
Zinc oxide
Aqueous solution of poly acrylic acid
Zinc polycarboxylate or polyacrylate
Poly acrylic acid may be in solid form as a powder components
Fluorine containing aluminosilicate glass
Aqueous solution of phosphoric acid
Silicate cement
Silicaphosphat cement
Fluorine containing aluminosilicate
Aqueous solution of poly acrylic acid or polymer
Glass-ionomer
Table: chief constituents of acid –base reaction cements
Zinc phosphate cement The compositions of powder and liquid in a typical cement are given in table: Powder ZnO 90% Other metallic oxide 10% Liquid aqueous solution of phosphoric acid 50-60% Al3(PO4)2 up to 10% as buffers Zn3(PO4)2
Setting reactionOne or both of two things may occur on mixing the powder and liquid: Chemical reaction, to form a compound called zinc eugenolate.The reaction between zinc oxide and eugenol begins gives the structural formula of eugenol. The basis of the reaction is that the phenolic –OH of the eugenol act as a weak acid and undergoes an acid –base reaction with zinc oxide to form a salt, zinc eugenolate, as follows:2C10H12O2 + ZnO -------------------- Zn(C10H11O2)2 +H2O Two molecules of eugenol reacts with zinc oxide to form the salt. The structural formula of zinc eugenolate.The ionic salt bonds are formed between zinc and the phenolic oxygen of each molecule of eugenol. Two further co-ordinate bonds are formed by donation of pairs of electrons from the methoxy oxygens to zinc. These bonds are indicated by the arrows in figure
H2O
Effect on the pulp is small, thus the material has been recommended for use in deep cavities near the pulp. Chemical properties: the solubility of the set cement in water is high- by the far the highest off all dental cements-mainly due to the elution of eugenol. Mechanical properties: (ZOE cement)these are the weakest of all the cement(except calcium hydroxide). Protection of the pulp. Optical properties. Adhesion : ZOE cement do not adhere to enamel and dentine. This is one reason why they are not frequently used for the final cementation of dental restoration ZOE cement are bacteriostatic and abundant. Thermal insulation: a cement is used under a large metallic restoration (e.g. amalgam) to protect the pulp from temperature changes. Chemical protection : a cement should be able to prevent penetration into the pulp of harmful chemicals from the restorative material.
Composition of conventional amalgam alloy
MetalsWt%
Silver
65
Tin
29
Zinc
6 max
copper
2max
Mercury
3 max
The quantities of silver and tin specified ensure a preponderance of the silver/tin inter metallic compound Ag3Sn, this compound known as the δ (gamma) phase of the silver-tin system, is formed over only a small composition range and is particularly advantageous since it readily undergoes an amalgamation reaction with mercury. The role of zinc is a “scavenger” during the production of the alloy. The alloys is formed by melting all the constituents metals together. At the elevated temperatures required for this purpose there is a tendency for oxidation to occur.
Oxidation of tin, copper or silver would seriously affect the properties of the alloy and amalgam. Zinc reacts rapidly and preferentially with the available oxygen, forming a slag of zinc oxide which is easily removed. many alloys and oxidation during melting is preventing by carrying out majority in an inert atmosphere. The majority of alloys powders contain no mercury . those product containing up to 3%. Mercury are called pre-amalgamated alloys. They are said to react more rapidly when mixed with mercury.
The reactions which take place when alloys powder and mercury are mixed is complex. Mercury diffuses into the alloys particles, very small particles may become totally dissolved in mercury. The alloys structure of the surface layer is broken down and the constituent metals undergo amalgamation with mercury. The reaction products crystallize to give new phase in the set amalgam. A large quantity of the initial alloys remains unreacted at the completion of setting. The structure of the set materials is such that the unreacted cores of alloy particles remain embedded in a matrix of reaction products.In simplified terms, the reaction for conventional amalgam alloys may be given by the following unbalanced equation Ag3Sn +Hg Ag2Hg3 +SnxHg +Ag3SnOr δ Hg δ1 + δ2 + δ
The primary reaction products are a silver-mercury phase (the δ1 phase) and a tin –mercury phase(the δ2 phase) . the δ2 phase has a rather imprecise structure and the value of X in the formula SnxHg may vary from 7 to 8. The equation emphasizes the fact that considerable quantities of unreacted alloy (δ phase) remain unconsumed. For copper –enriched alloys the reaction may be presented by Ag3 Sn + Cu +Hg Ag2Hg3 + Cu6Sn5 +Ag3SnOr δ + Cu + Hg δ1 + Cu6Sn5 + δ
the essential difference between this and the reaction for conventional alloys is the replacement of the tin-mercury, δ2 phase in the reaction product with a copper-tin phase. In the case of the dispersion-modified, copper-enriched material. It is believed that the particles of conventional lathe-cut alloy initially reacts to form δ1 and δ2 phases. The δ2 phase than reacts with copper from the silver-copper eutectic spheres to form the copper- tin phase. Thus in these materials, the δ2 phase exists as an intermediate reaction product for a short time during setting.