Gypsum products and investment materials · Types of investment materials 2. Inorganic binder 1....
Transcript of Gypsum products and investment materials · Types of investment materials 2. Inorganic binder 1....
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Pavel Bradna
1st. Faculty of Medicine, Charles University, Prague, Czech Republic
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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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