Gypsum product Flashcards
What is gypsum ?
Gypsum is a mineral that is mined in various parts of the world.
The word gypsum is derived from Greek word meaning ‘to cook’ referred to burnt or calcined mineral.
Chemically it is CaSO4.2H2O i.e calcium sulphate dihydrate
Three types of gypsum available in dentistry
- model plaster
- dental stone
- high strength dental stone or die stone
Other forms of gypsum?
- Selenite
- Alabaster
- Satin spar
- Plaster of Paris
Classification of gypsum product?
Type I : impression plaster
Type II : dental plaster
Type III : dental stone
Alpha hemihydrate, hydrocol
Type IV : die stone( high strength , low expansion)
Modified alpha hemihydrate,densite
Type V : die stone ( high stone, high expansion)
Type I gypsum product; manufacture; use
IMPRESSION PLASTER
Earlier plaster was modified for use as impression material by addition of chemicals and flavouring agents.
Preliminary impression of edentulous ridge in fabrication of dentures [ Mucostatic Impression ]
Name of Type II gypsum product, manufacture , property, use
Plaster of Paris
Model plaster
Dental plaster
Beta hemihydrate
Heating the gypsum powder in an open container. This direct and rapid heating results in powder that consists of porous and irregular particles.
It is the weakest and least expensive of all gypsum product
- Preliminary cast for CD
- To secure cast in articulator
- To fill a flask in denture construction
Name Type III gypsum product; manufacture; use
Dental stone
Alpha hemihydrate
Hydrocol
Class I stone
Carefully controlled calcination under steam pressure in a closed container. This method releases the water slowly and resultant powder particles are more uniform in shape and less porous.
Stone is more stronger and expensive than plaster.
- Cast for diagnostic purpose
- Cast for RPD and CD construction
Type IV gypsum product name, manufacture, use
Dental stone high strength Die stone low expansion Class II stone Densite Improved stone
Made by calcining in calcium chloride solution. This calcining results in a powder particle that is very dense and cuboidal in shape with reduced surface area.
It is the strongest and most expensive. Densite is the most densest of all material.
Use mainly for cast and dies for inlay and crown fabrication.
Type V gypsum product
Dental stone high strength high expansion
Made by calcining in calcium chloride solution. This calcining results in a powder particle that is very dense and cuboidal in shape with reduced surface area.
Added with accelerators and retarders and surface tension reducing agent like lignon sulfate
For making very strong dies ( has highest compressive strength) this is achieved by decreasing the w/p ratio and increasing setting expansion to compensate for alloy solidification shrinkage.
ADA specification no
25
Classification of gypsum product based on crystal structure?
- Beta hemihydrate
- fibrous aggregate of fine crystal with capillary pores
- fluffy porous and least dense - Alpha hemihydrate
- cleavage fragments and crystal in form of rods and prism
- higher density and more crystalline
Manufacture
Commercially, the gypsum is ground and subjected to 110-130 Celsius to drive off the water of crystalline and produce calcium sulphate hemihydrate.
110-130 CaSO4.2H2O ———>CaSO4.1/2H2O Gypsum Plaster
or
CaSO4
Stone
130-200
————> CaSO4
Hexagonal anhydrite
200-1000
————> CaSO4
Orthorhombic anhydrite
Calcination
CaSO4.2H2O
Open kettle
——————> type II
High pressure steam
————-—————> Type III
Boil, CaCl2
—————————> type IV
Boil, cacl2
—————————> type V
the differences of alpha and beta hemihydrate due to?
The difference between alpha and beta hemihydrate are the result of differences in crystal size, surface area and degree of lattice imperfection.
Differences of alpha and beta hemihydrate?
ALPHA HEMIHYDRATE
- when dihydrate is heated under steam pressure
- dense and prismatic crystal
-stronger and harder products on mixing with water
(Type III, IV, V)
- smaller w/p ratio
BETA HEMIHYDRATE
-When dehydrate is heated in an open kettle or kiln
-spongy and irregularly shaped crystal
-less strong product on mixing with water
( type I, II)
-larger w/p ratio
Reaction takes place in the setting of gypsum product?
(CaSO4)2.H2O + 3H2O->
2CaSO4.2H2O + heat(3900cal/g mol)
Product of reaction?
Gypsum
Solubility of hemihydrate and dihydrate?
Hemihydrate is 4 times more soluble in water than dihydrate near room temperature
The understanding of the setting process?
- hemihydrate is mixed with water , a suspension is formed that is fluid and workable.
- water become saturated with hemihydrate
- this saturated solution is supersaturated with dihydrate so the latter precipitates out
- solution is no longer saturated with hemihydrate
- so it continues to dissolves until no further dihydrate precipitate out of solution
- the process continues until most of the hemihydrate is converted to dihydrate
- the crystal of dihydrate are spherulitic in nature and grow from specific growth sites called nuclei of crystallisation.
Theories of setting of gypsum?
- Colloidal theory
- Hydration theory
- Dissolution precipitation theory or crystalline theory
What is colloidal theory?
When mixed with water, plaster enters into the colloidal state through sol-gel mechanism.
In the sol state , hemihydrate particles are hydrated to form the dihydrate , thereby entering into an active state.
As the measured amount of water is consumed, the mass converts into a solid gel.
What is hydration theory?
This theory suggests that rehydrated plaster particles join together through hydrogen bonding to the sulphate group to form the set material.
Dissolution precipitation theory? Who invent? When?
- by Louis Chatelier in 1885
- this theory is based on dissolution of plaster and instant recrystilization of gypsum, followed by interlocking of the crystal to form the set product\
Mechanism of dissolution precipitation theory?
Nuclei of crystallisation
|
Spherulitic growth
|
Outward thrust as spherulites make contactWhat is water powder ratio? What does it determines?
- the proportion of water to powder used to make a workable mix of a particular gypsum product is water / powder ratio
- it is the quotient obtained when the weight or volume of the water is divided by the weight of powder
- the w/p ratio is an important factor in determining the physical and chemical properties of the final gypsum product.
Chief factors that determine the amount of gauging water required?3
- Particle size
- Total surface area
- Particle size distribution
What is excess amount of water ?
An excess amount of measured water is always necessary above the theoretically correct amount required for hydration which is 0.186%
The excess is needed to make a workable mix that can be poured and shaped
The excess water is distributed as free water in the set mass without taking part in chemical reaction and it contributes to the subsequent porosities or voids in the set product weakening it.
The proper w/p ratio of products depends on?
Physical characteristics of powder particles
Which require more water ? Plaster / stone ?
Plaster requires more measured water to float the irregular porous particles than the dense particles of stone
What reduces the amount of water necessary to mix gypsum products?
A mixture of calcium oxide (0.1%) and gum Arabic (1%) reduces the amount of water necessary to mix gypsum product resulting in improved properties.
Mixing water (mL/100g of powder) for model plaster? What is the required water and excess water?
Mixing water : 37-50 mL
Required water : 18.6 mL
Excess water : 18-31 mL
Mixing water (mL/100g of powder) for dental stone? What is the required water and excess water?
Mixing water : 28-32 mL
Required water : 18.6 mL
Excess water : 9-13 mL
Mixing water (mL/100g of powder) for high strength dental stone? What is the required water and excess water?
Mixing water : 19-24 mL
Required water : 18.6 mL
Excess water : 0-5 mL
Recommended ranges of w/p ratio for type I
0.50-0.75
Recommended ranges of w/p ratio for type V
0.18-0.22
Recommended ranges of w/p ratio for type II
0.45-0.50
Recommended ranges of w/p ratio for type IV
0.22-0.24
Recommended ranges of w/p ratio for type III
0.28-0.30
Relate w/p ratio to setting time
Directly proportional
Relate mixing time to setting time?
Inversely proportional
What is mixing time?
- time from addition of powder to water until mixing is completed
- mechanical mixing : 20-30 sec
- hand spatulation : 1 min
What is working time?
The time available to use a workable mix, one that maintains an even consistency that may be manipulated to perform one or more tasks
- generally 3 min working time is adequate
What is setting time?
When the powder is mixed with water, the time that elapse from the beginning of mixing until the material is hardens is known as setting time.
How to control the setting time of gypsum product by manufacture?
- The solubility of hemihydrate can be increased or decreased.
- Number of nuclei of crystallisation can be increased or decreased.
- Rate of crystal growth is increased or decreased.
How to control the setting time of gypsum product by operator?
To decrease the setting time
- more mixing time
- low w/p ratio
- addition of certain chemicals called accelerators
Vice versa
How retarder acts?
Retarder act by forming an adsorbed layer on the hemihydrate to reduce its solubility and on the gypsum crystal to present to inhibit growth.
How NaCl acts as retarder and accelerator ?
NaCl when added in small quantities will act as accelerators , but in large quantity will act as retarder
What is setting expansion?
This thrust results in an external expansion with resulting internal porosity in set mass.
How to control setting expansion?
- less w/p ratio more the setting expansion
- more the mixing time higher the setting expansion
- accelerator and retarders have an effect of reducing setting expansion and are referred as Anti expansion agents
What is accelerators? How does it work? Examples?
It increases the solubility of hemihydrate without increasing the solubility of dihydrate. Thus accelerates the process.
Eg- NaCl upto 2 %
NaSO4 upto 3.4 %
K2SO4 >2%
How pH works in accelerating or retarding the process?
Liquids of high pH accelerate the setting reaction while the liquid of low pH like saliva retard the setting reaction.
What is retarders? Give example?
Certain chemicals form a coating on the hemihydrate particles and thus prevent the hemihydrate from going in the normal manner.
Eg- citrates, acetates, borates
What is hygroscopic setting expansion?
If the setting process is allowed to occur under water, the setting expansion is more than double in magnitude.
Factors influencing hygroscopic setting expansion?
- less w/p higher HSE
- more mixing time higher HSE
