GYPSUM PRODUCTS

GYPSUM PRODUCTSAllah el-most3an1What is gypsum ? Name the varieties of gypsum.It is a natural mineral mined around the various parts of the world having a chemical formula, CaSO4s2H2O(calcium sulphate dihydrate).TYPES:Albaster:-pure white ,fine grained and translucent.

What are different uses of plaster of Paris?Impression of mouth and face(impression plaster)

For making moulds, study casts, diagnostic cast,prliminary cast, master cast, and dies over which dental prostheses and restorations are made.

To mount cast on articulator.

For bite registration

In dental investments.

- cast,die and model.

CAST:- It is positive ,dimensionally accurate replica of oral soft and hard tissues of either the maxillary or mandibular jaw and used for the construction of dental appliances which fit on to the soft tissues of the oral cavity.DIE:-It is a positive , dimensionally accurate replica of a prepared natural tooth and used for the construction of inlay , crown or bridges.MODEL:-It is a positive, dimensionally accurate replica of oral soft and hard tissues used as a study model or for patient education .The progress of treatment can be shown to the patient with these models.

Classify the gypsum products according to ADA specification no.

According to ADA specification number 25:

TYPE I:Dental plaster,impressionTYPE II:Dental plaster,modelTYPE III:Dental stone, modelTYPE IV:Dental stone,die,high strength,low expansionTYPE V:Dental stone,die,high strength,high expansionWhat do you mean by calcination?

The process of heating gypsum for manufacturing plaster is known as calcination. OR Removing 3/4th of water of crystallization by burning of gypsumWrite the process for manufacturing of Dental plaster( hemihydrate). ORWhat do you mean by dry calcination ? Give its reaction.

Gypsum is ground and heated in an open kettle on kiln at a temperature of to for manufacturing of beta type crystals or dental plaster. This process is known as dry calcination.

(beta hemihydrate)

Write the process for manufacturing of dental stone( hemihydrate).

17 lbs/sq. inch pressure (alpha hemihydrate) Write the process of manufacturing for alpha modified stone or improved stone.Gypsum is calcined by boiling it in 30% calcium chloride solution . The chlorides are then washed away or autoclaved in presence of sodium succinate 0.5%.Than grinded to obtain alpha() modified stone. Give the reaction for manufacturing of orthorhombic anhydrite.

Write the setting reaction of plaster.

Hemihydrate + water Dihydrate + unreacted hemihydrate + Heat

EXOTHERMIC REACTIONWhat are modifiers?They are the chemicals added in order to alter some of the properties like expansion , setting time etc. or to enhance the other properties like working time , dimensional stability etc.EX.:accelerator & retarder. What are accelerator and retarder?ACCELERATORS:-The substances which decreases the setting time of gypsum products by increasing the rate of reaction are called accelerator. E.g., Tera alba, sodium sulphate, potassium sulphate and sodium chloride(low concentration)RETARDERS:-They are the substances which increases the setting time of gypsum products by decreasing the rate of reaction. E.g., Borax, sodium chloride, sodium sulphate(high concentration) ,glue ,agar, coagulated blood.What is impression plaster? Give its uses.Impression plaster is one of earliest rigid impression materials in dentistry.

USES:For making impressions in complete denture and maxillofacial prosthesis.For bite registration material.

What is the chemical formula for dental plaster/ hemihydrate?

What do you mean by hydrocal?Give its uses.It is the type III gypsum product called DENTAL STONE used for model preparation.USES:For preparing master casts and to make moulds.

Stone & plaster

pressure (alpha hemihydratestoooooooone (beta hemihydrate) plaster What is the difference between the crystal structure of dental plaster and dental stone? DENTAL PLASTER DENTAL STONENeedle like crystalsRods and prism like crystalsIrregular in shape with capillary poresRegular in shapeLoosely packedClosely packed

What is excess water?

The actual amount of water necessary to mix the calcium sulphate hemihydrate is greater than the amount required for the chemical reaction, this is called excess water.Write the synonyms for TYPE IV gypsum products.Class II stoneDie stone Densite Improved stone

Write the uses of die stone.

It is used when high strength and surface hardness is required E.g., dies used for inlays , crown and bridge wax pattern.

How type V is better than type IV stone?It has higher compressive strength.Setting expansion is increased from a maximum of 0.10 to 0.30% to compensate for the shrinkage of base metal alloys during solidification.

What do you mean by setting time? Give its two types. It is the time elapsing from the beginning of mixing until the material hardens.TYPES:- 1. Initial setting time 2.Final setting timeDefine initial setting time.As the reaction proceeds,more hemihydrate crystals react to form dihydrate crystals.The viscosity of the mass is increased and it can no longer be poured.The material become rigid(but not hard).It can be carved but not moulded.This is known as initial setting timeWhat is final setting time?The time at which the material can be separated from the impression without distortion or fracture.What are the methods to measure the setting time?Loss of gloss method ----The set plaster when loses it surface shine and smoothness it is considered as the end of working time and initial set has taken place this method is known as loss of gloss method.

Exothermic reactionPenetration testsWhich are the types of penetrometers?Vicat needle.Gillmore needle.

(Vicat needle) (Gillmore needle)

Properities of gypsum 1- setting time Factors affecting setting time a- w/p ratio?It is defined as water required for 100gm of powder.Higher liquid ____less nuclei of crystallization ______prolonged time .What is the w/p ratio of the gypsum products?Impression plaster : 0.50 to 0.75 much Dental plaster : 0.45 to 0.50Dental stone : 0.28 to 0.30Die stone,Type 4 : 0.22 to 0.24Die stone,Type 5 : 0.18 to 0.22 little

What is the of low and high water powder ratio on gypsum products? LOW W/P RATIO(thick mix) HIGH W/P RATIO (thin mix)Quick settingSlow setting High expansionLow expansionMore strengthLess strengthMore surface hardnessPoor surface hardnessLess porosityMore porosityB- mixing time and Give its value for gypsum products.It is the time from the addition of the powder to the water until mixing is complete.1 minute Increase mixing time lead tooooooo accelerate setting time due to mixing can break up some of the formed dehydrate crystals thus forming more nuclei of crystalization c-Water temprature From 0 -50 accelerate setting 50 -80 retard setting At100 no setting D- Factors controlled by manufacturing Fineness of powders will increase surface area of hemihydrate exposed for water ,this accelerate setting Impurities in the powder ,accelerate , act as nuclei of crysrtalization Addition of accelerator or retarder

2- hygroscopic expansion and how much it is?It is increased expansion of setting plaster in presence of water .It can be double the amount of normal setting expansion.USE:-It is used to expand some gypsum bonded investments.

Write the factors controlling expansion. Water powder ratio :- setting expansionModifiers :- setting expansion :- 4% solution decreases the setting expansion from 0.5 to 0.06 %.NaCl and borax :- setting expansion mixing time ... Setting expantion

Write the factors affecting strength and give its two types.Water powder ratio :- water , porosity , strengthSpatulation :- strength (within limit)Addition of accelerators and retarders :- strength TYPES:- 1.Dry strength2.Wet strengthWhat is wet strength ? How much it is for model plaster, dental stone and die stone?It is strength when the set mass is still contains excess water and specimen is wet to touch.E.g., Model plaster 12.5 Mpa Dental stone - 31 Mpa Die stone 45 Mpa3- strengthDry strength It is strength when the set mass has no excess water and specimen is dry to touch. It is 2- 3 times more than wet strength.

Wet strenght

What care should be taken with temperature to avoid shrinkage and reduction in strength of gypsum products?Gypsum is stable only below about . Drying at higher temperature rapidly at or higher ,lose of water of crystallisation occurs which causes shrinkage and reduction in strength.

Manipulation Storage . Closed container to prevent reaction with moisture from the atmosphere which accelerate the setting Correct w\p ratio :wet \ dry strengthIncorporation of air in the mix should be avoidedVibration air bubbles Mechanical mixures under vacum Die Silcophosphate cementHigh setting shrinkage

Amalgam die Packing Long time for setting Need sepeating media High thermal conductivity >>> cool wax pattern rapidly

Acrylic resin dieShrinkage Epoxy die Toxic & shrinkage Ceramic die Expensive Electroplated dieSilver plated rubber base material Not used with compoud