SDL
Name: Justin Ling Wei Jie Roll number: 101323018 Topic: Dental Plaster Introduction Dental plaster, is also known as dental model, is the Type II gypsum products, is commonly used in the dental laboratories for large sized models and casts due to its low cost and adequate mechanical properties. Composition Dental plaster contains dry calcined β calcium sulphate hemihydrates, β-HH+ uncalcined impurities, Borax, K 2SO4, and other impurities. Manufacture of Dental Plaster Dental plaster is manufactured by the mean of dry calcination of CaSO 4.2H2O where gypsum ore is purified by washing, frying, powdering. Firstly, gypsum is heated on a pan or kettle, open to atmosphere for some time to a temperature of 110 C to 130 C so that most of the gypsum will be converted to its hemihydrates. Impurities will eventually form by the remaining small amount of uncalcined gypsum as well as calcium sulphate anhydrite formed by overheating. The impurities play an important role as the nuclei of crystallization which accelerates the reaction. °
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Crystal structure The crystal structure of dental plaster is monoclinic II-β hemihydrates which are spongy and irregular due to its method of calcinations. Β hemihydrates produced from dry calcinations are slightly modified by some additions to obtain suitable properties so that they can be used as model plaster. Generally, they have low surface hardness and abrasive resistance. r esistance.
Setting action & reaction 2CaSO4.1/2H2O + 3H2O 2CaSO4.2H2O + HEAT (3900 cals/gm mol) when hemihydrates is mixed with water, a fluid suspension is formed. The hemihydrates dissolve partly and reacts to form dehydrate which the solubility is ¼ of the solubility of hemihydrates. The solution become supersaturated and precipitates out like crystal. The calcium sulfate dehydrate remaining in the powder are nuclei for precipitation. Water/Powder Ratio Water/Powder Ratio is the amount of water added(ml) to 100gm of powder. The higher the water/powder ratio, the thinner is the mix, resulting in longer setting time. Β hemihydrates have a relatively larger Water/Powder Ratio which is 45% to 50% which implies that theoretically, 45 to 50ml of water should be added to 100gm of powder. However, this amount of water added will produce a dry and crumbly mix which is difficult to manipulate and pour into impression. Hence, excess water should be added so that the mass is workable and manageable for optimum results. Excess unreacted water will be trapped in the mass and vaporises to form porosities. Setting expansion Expansion is gypsum products occur during the change of hemihydrate to dihydrate. Minimal or no setting expansion is always favoured to obtain an accurate cast or die. The setting expansion of dental plaster is <0.3 . Strength
1.
wet strength
Wet strength is the strength of the set mass of which there is still excess water content, measured after 1 hour. The wet strength of dental plaster is low, 9MPa. a.
2. Dry strength refers to the strength of the set mass when there is absence of excess water, measured after 48 hours. The dry strength of dental plaster is 20 -30MPa.
Setting time For a theoretical Water/Powder Ratio, the mass of dental plaster will set in 12±4 minutes. Uses The uses of dental plaster in the dental laboratories is mainly for making positive replicas of oral structures including dentulous cases, edentulous cast bases, orthodontic study models. Other uses of dental plaster include mounting plaster in articulations, impression and part of investment.
