Gypsum Materials - Dental Materials PDF

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Lecture notes on gypsum...

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DENT261 (Biomaterials)

Lecture 14

28 July 2014

Gypsum Materials  

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Most widely used material in dentistry Models of dentition o Dentures o Bridges o Crowns o Inlays o Implants o Orthodontic etc. Investment materials for casting metal

Gypsum   

Widely available material Calcium sulphate dehydrate Basis of many familiar products o Plaster for building products o Plaster of Paris…

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…

Classification     

Type I impression plaster Type II plaster, low-grade – Type 3 stone – Type 4 Die stone low expansion – Type 5 Die stone high expansion -

flasking and articulating diagnostic casts, casts for resin restorations casts for fixed restorations casts for metallic removable restorations

Gypsum Plaster Reactions 

Gypsum…

Manufacture    

Raw gypsum is ground into fine powder Heating to dry off the powder … Final product depends on the processing method o …

Plaster v Dental stone  

… …

Density of Different Products

DENT261 (Biomaterials) 

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Lecture 14

28 July 2014

Apparent density (bulk density): the mass of a sample divided by the volume it occupies, measured as a specific degree of compaction o Equation… Plaster has a low density o Open fire……

Setting Reaction    

Difference in the solubilities of calcium sulphate… …. Calcium sulphate dehydrate precipitate (less soluble than hemihydrate) …

Stages in Setting    

Fluid Plastic Friable Carvable

crystal nucleation, flowable crystals grow, will not flow but can be molded solid with relatively low strength solid, reached maximum strength

Exothermic Reaction   

Reaction is a reverse o manufacturing stages The more crystal nuclei present, the greater the area on which further deposition can form o Therefore higher the temperature generated Nucleation o Supersaturated dehydrate will deposit…

Setting Expansion 

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Three stages of dimensional change during setting o Contraction as water gets taken up in reaction to form the dehydrate o Minimal at ‘loss of gloss’ when excess water no longer present o Setting expansion as dehydrate crystals grow and make contact pushing themselves apart o Gypsum or dehydrate crystal grow from nuclei and tehn radiate as elongated or acicular needle spherulites o When crystals of spherulite make contact then mechanical itneracition occurs which imparts strength… All gypsum products expand linearly during setting o Plaster 0.2-0.3% o Dental stone 0.08-0.1% o High-strength dental stone …

Hygroscopic Expansion  

When setting stone is placed in water, greater expansion can be achieved Uninhibited crystal growth occurs

DENT261 (Biomaterials)

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Lecture 14

o Reduces strength Useful in making oversize models for casting metal

Water:Powder Ratio  

Equation… Mixing water with powder o A volume of water at least equivalent to volume of voids must be added o Additional volume of water… o … o …

Dilatancy  

Additional volume is needed for irregular particles to move/rotate around Especially during stirring

Dilution 

Additional water is needed to make the final slurry more flowable

Examples    

Plaster Dental stone … …

0.5-0.6 0.3-0.33

Role of Water:Powder Ratio 

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With increasing water, spaces between nuclei increase o Reduces mechanical interaction o Less resistance to expansion Reduction in strength of the set body with greater water:powder ratio …

Setting Control           

Nucleation Crystal growth Setting time Setting expansion Strength Impurities – increases setting speed Additives – accelerator or retarder Temperature Water:powder ratio Manipulation technique Humidity

28 July 2014

DENT261 (Biomaterials)

Lecture 14

28 July 2014

Impurities    

Impurities or foreign particles act as nuclei for dehydrate nucleation o Accelerates the setting Most common impurities o Residual gypsum in the rubber bowl or spatula Clinical tips o Always use clean tools for gypsum mixing…. ….

Additives  

K2SO4 ….

very effective accelerator

Mechanisms  

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K2SO4 accelerates the dissolution of hemi-hydrate Na2SO4 and NaCl o Low concentration – increases the rate of solution of hemihydrate o High concentration – nuclei poisoning Borax o Nuclei poisoning – coats the hemi-hydrate in calcium tetraborate, slowing solubility o Coats nuclei of dehydrate inhibiting the growth of dihydrate crystals

Manipulation (Spatulation)  

An increase in the amount of spatulation shortens the setting time Increased amount of spatulation causes more nuclei centres to be formed o Conversion of calcium sulphate hemihydrate to dehydrate requires less time

Temperature   

Increasing the temperature from 20C to 37C increases rate of reaction slightly If raised over 37C, rate of reaction decreases… ….

Water:Powder Ratio    

Generally, the rate of hydration during setting is independent of W:P ratio More water means fewer nuclei and greater diffusion distances…. …. ….

Physical Properties 

…..

Role of Porosity

DENT261 (Biomaterials)  

Lecture 14

28 July 2014

Porosity of set stone or dehydrate controls strength Three types of porosity o Presence of residual unreacted water o Air spaces between spherulites o Air bubbles entrapped during mixing

Factors  

W:P ratio has a critical effect on strength of the set dehydrate …

Wet or Dry   

Strength of gypsum is depended on the intermeshing of clusters of needle-like crystals o Inhibits the relative movement past each other Unreacted water inside material acts as lubricants between crystals o Lowers the apparent strength Loss of water is reversible

Clinical Tips  

Always dry the model before working on it Do not allow alginate to dry around the model

Surface Hardness and Abrasion Resistance   

The surface hardness of gypsum materials is related to their compressive strength High compressive strength of hardened mass corresponds to high surface hardness Die hardener o …

Reproduction of Detail   

Surface of set gypsum is porous on a microscopic level Air bubbles frequently are formed at the interface of the impression and gypsum casts o Freshly mixed gypsum does not wet some rubber materials well Too much water left with impression, especially alginates o Will change the local W:P ration significantly and generate poor surfaces

Clinical  

Run a thin layer of gypsum over the impression and vibrate it out Removes excess water pooling on the impression

Manipulation  

Portion the W:P ratio Add powder to water o Reduces air entrapment in the unset gypsum

DENT261 (Biomaterials) 

Lecture 14

28 July 2014...