Gypsum products and investment materials · Types of investment materials 2. Inorganic binder 1....

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2012 1 Pavel Bradna 1st. Faculty of Medicine, Charles University, Prague, Czech Republic [email protected] Gypsum products and investment materials Institute of Clinical and Experimental Dental Medicine

Transcript of Gypsum products and investment materials · Types of investment materials 2. Inorganic binder 1....

2012 1

Pavel Bradna

1st. Faculty of Medicine, Charles University, Prague, Czech Republic

[email protected]

Gypsum products and investmentmaterials

Institute of Clinical and Experimental Dental Medicine

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Gypsum products

110-130oC plaster (β-hemihydrate) (CaSO4.0.5H2O)130-200oC γγγγ-anhydrite (soluble CaSO4)

200-1000oC β-anhydrite (insoluble CaSO4)

Indications: models, casts, dies

Preparation:

Thermal decomposition (dehydratation, calcination) of natural or artificial CaSO4 . 2H2O (gypsum)

Calcium sulfate dihydratemonoclinic

Calcium sulfate β-hemihydrate(orthorhombic)+impurities, anhydrite

110-130oCCaSO4 . 2H2O → CaSO4 . 0.5H2O + 1.5H2O

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Properties and types of gypsum productsdepend on dehydration conditions

Decomposition in an open reactor: ββββ-hemihydrate

(Plaster of Paris, Type I a II1 gypsum products)

Decomposition in the presence of water steam and underpressure: αααα-hemihydrate (hydrocal gypsum, model plaster/dental stone /stone, Type III gypsum products)

Boiling in the presence of CaCl2: αααα-hemihydrate (die stone/ high-strength dental stone/Densite, Type IV a V gypsumproducts.)

The same chemical composition, crystallographicalmodification, but differences in shape, size and porosity

of crystals

1Dentistry - Types of gypsum products EN ISO 6873, five types

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CaSO4 . 2H2O

β-hemihydratePlaster of Paris Small, irregularcrystals, spongy, high porosity

mixing ratio 50-60 mL H20 /100 g powder

α-hemihydrateStone, hydrocal

Prismatic, more regu-lar, low porosity

crystals, mixing ratio30-35 mL/100 g

α-hemihydrateDie stone, high-strength dental

stone Large, dense crystals

of low porositymixing ratio 19-24 mL/100 g

Open reactor Pressure, steam H2O Boiling, CaCl2

CaSO4 . 0.5H2O

Phillips´ Science of Dental Materials, 11th. edition

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CaSO4.0.5H2O + 1.5H2O CaSO4.2H2O + heatβ,α-hemihydrate

Setting reaction:

dihydrate, gypsum

Sol

ubilit

y%

Dihydrate

Temperature oC

Hemihydrate

10020

0.1

0.5

1.0

3. As dihydrate precipitates more andmore hemihydrate disolves till it is consumed

1.Mixing hemihydrate with water – itsdissolution untill saturation with respectof Ca2+ and SO4

2- is reached.Composition of saturated solution cis characterized by

the blue curve

2. Saturated hemihydrate solu-tion issupersaturated with respect to dihyd-rate causing crystallization of CaSO4.H2OComposition of saturated solution cis characterized by

the blue curve

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A decrease in viscosity with shear rate e.g - mixing, vibrations, flow with the aim to decrease stress inducing the flow of fluids

Vis

cosi

ty[P

a.s-

1 ]

Flow rate γ

non-Newtonian liquidViscosity decreases with flow rate and its

time - PSEUDOPLASTIC and THIXOTROPIC behavior

Newtonian liquidviscosity is constant

Important properties of gypsum products

1. Pseudoplasticity/thixotropy (shear thinning)

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Theoretical water requirement: 19 mL/100 g CaSO4 . 0.5H2O

Excess of water (above theoretical value and necessary to prepare

workable mass which can be poured into an impression) evaporatesleaving voids and porosity which decrease strength and

abrasion resistance of gypsum

4020

103

07

0

mixing ratio W/P [mL water/100 g powder]50

Com

press

ion

stre

ngth

MPa

Type IV-V

Type III

Type II

Type I

2. Strength

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Theoretical setting shrinkage is app. 6.9 vol. % but in reality gypsumexpands during its setting reaction from 0.1 to 0.3 %

Needle-like crystals grow freely – crystals push each other from a nucleation centre and increase the volume occupied resulting in setting expansion. Their size is, however, constrained by surfacetension of water

Mahler DB, Ady AB. Explanation for the hygroscopic setting expansion of dental gypsum products. J Denr Res 1960, p. 578

initial mix crystalgrowth

expansionclosecontact ofcrystals

Terminationof crystal

growth

Normal setting conditions

3. Setting expansion

2012 9Mahler DB, Ady AB. Explanation for the hygroscopic setting expansion of dental gypsum products. J Denr Res 1960, p. 578

initial mix crystalgrowth

expansionclosecontact ofcrystals

termination

Hygroscopic setting conditions

An increase in the expansion when dihydrate crystalizes

under water - hygroscopic expansion – (e.g. from 0.15 %

to 0.30 vol. %) as a result of not constrained crystal

growth by the surface tension

4. Hygroscopic expansion

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How can be controlled physicaland mechanical properties of

gypsum produts?

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1. Control of setting rate using chemicaladditives in the produts:

Particles of dihydrate in mixing water act as nuclei of crystallization

Accelerators:Inorganic salts – such as potassium sulfate which increases the rate

of dihydrate crystallization

Alginates, agar - usually form adsorbed layers on hemihydrate par-ticles inhibiting their dissolution and on gypsum crystals inhibitingtheir growth

Retarders:Borax, NaCl (at higher concentrations), citric acid – cover hemihydrateparticles with calcium salts less soluble than the sulfate – reduce hemihydrate dissolution which prolongs setting time and lowers setting gypsum expansion

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2. Alginate impression debries in the mixing bowl – act as nucleation inhibitor – decreases rate ofcrystallization– slower setting

4. More water – less nuclei formed and less supersatu-ration – slower setting

3. Prolonged and rapid mixing – accelerate hemihydratedissolution – accelerate setting

Setting rate can be negatively affected alsoby:

1. Gypsum debries in the mixing bowl – act as nuclei of crystallization – accelerate setting

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2. By improvment of gypsum strength andabrasion resistance:

3. By decreasing the gypsum setting expansion

- decreased mixing ratio W/P (water/powder), more intensiveand longer mixing which increases number or nucleation centres andthus mutual interaction between crystalls and supports thesetting expansion negatively affecting model accuracy

- by reducing hygroscopic expansion - after initial setting

avoid contact of a model with water stimulating growth of needle-likedihydrate crystals and volume expansion

- Drying a gypsum model

- Using colloidal sols of SiO2 (contain up to 30 % SiO2 in water) to mix the gypsum powder

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Investment materials

Indications: casting mould preparation

1. Resistance to high temperatures of molten metals, mechanical strength to resist pressures during preheating and casting, permeability to gasses

Main requirements:

2. Compensation of metal solidification shrinkage by the mould expansion

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Types of investment materials

2. Inorganic binder

1. Gypsum-bonded investment materials – casting of Au alloys melting point <1000oC

2. Phosphate-bonded investment materials – casting of Co-Cr-Mo, Ag-Pd etc alloys, melting point app. 1500oC

Composition:1. Refractory component (filler)

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Types of refractories:Allotropic modifications of SiO2 (quartz, cristobalite, tridymite)

2. Quartz – β to α tranformation at 575oC

2.53573-1050hexagonalα-quartz

2.65<573trigonalβ-quartz

2.20270-1700cubicα-cristobalite

2.33220-270tetragonalβ-cristobalite

Density

[g/cm3]

Temperature of transformation

[oC]

Crystal systemModification of SiO2

1. Cristobalite – transformation from β to α modifi-

cation at 275oC

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Temperature oC400100

0,4

0,8

2,0

300200 500 600

1,2

1,6

cristobalite

Quartz

Thermal expansion of cristobalite and quartzE

xpa

nsio

nvo

l. %

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Gypsum-bonded investment materials

Composition:

• α-hemihydrate CaSO4.0.5H2O

• Quartz, cristobalite (broader range of expansion temperatures to reduce internal stresses during mold preheating)

• Setting regulators of gypsum

CaSO4.0.5H2O + 1.5H2O CaSO4.2H2O + heat

α-hemihydrate gypsum

Setting reaction (mixing with water):

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400100

0,2

0,4

1,0

300200 500 600

0,6

0,8

Typical volume changes of gypsum-bondedinvestment materials during heating

Shrinkage due to gypsum

decomposition on hemihydrate and

anhydriteExpansion β→α

cristobalite

275oC

575oC

Expansion β→αquartz

CaSO4

decompositionSO2 release

Expa

nsio

nvo

l. %

Temperature oC

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Phosphate-bonded investment materials:

Composition:

• MgO, NH4H2PO4

• Quartz, cristobalite

• Adittives – graphite (reduction of metal surface oxidation

Setting reaction after mixing with water:

MgO + NH4H2PO4 + 5H2O MgNH4PO4 .6H2O + heat

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400100

0,2

0,4

1,0

300200 500 600

0,6

0,8

Typical volume changes of phosphate-bondedinvestment materials during heating

Contraction due to water releaseMgNH4PO4 .6H2O

MgNH4PO4 .H2O+5 H2O

Contraction due to decomposition of

MgNH4PO4 .H2O Mg2P2O7+2NH3+H2O

Expansion βα-quartz

Expansion α-β cristobalite

Expa

nsio

nvo

l. %

Temperature oC

sol SiO2

water

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Expansion control:

• By using mixtures of cristobalite and quartz (also to decrease expansion stresses during molds heatingand risk of mold fractures)

• Mixing with SiO2 sols