chemistry and use in dentistry

2. Chemistry and its use in Dentistry

4.  Kholi Maheni Roll No. 2
 Muhibur Rahman Roll No. 7
 Gituparna Pradhan Roll No. 14
 Hage Yasse Roll No. 26
 Bindiya Thokchom Roll No. 34

5.  Introduction
 Keywords used
 Classification and W/P ratio
 Primary Application
 Manipulation
 Setting
 Strength
 Conclusion
 Bibliography

6.  Gypsum is a white powdery mineral
widely found in nature.
 Chemical name: Calcium sulphate
dihydrate (CaSO4.2H2O)
 In dentistry, gypsum is used in the
form of Calcium sulphate hemihydrate
(CaSO4.½H2O)
 Crystalline forms of gypsum existing
in nature:
Selenite
Satin spar
Alabaster

7.  Gypsum
 Calcium sulphate dihydrate (CaSO4.2H2O)
 Model
 A miniature representation of something.
 Cast
 A lifesized likeness of some desired form
 Impression
 A negative likeness or copy in reverse of the surface of an
object
 An imprint of the teeth and adjacent structures for use in
dentistry

8.  Die
 The positive reproduction of the form of a prepared tooth
in any suitable substances
 Dental plaster
 β form of calcium hemihydrate
 Dental stone
 α form of calcium hemihydrate
 Calcination
 Process by which different gypsum products are obtained
from gypsum by heating it to a certain temperature (110⁰C
- 130⁰C)

9.  Water Powder ratio:
 The proportion of water to powder used to make a workable mix
of a particular gypsum product is called the water/powder ratio.
 Mixing Time
 Time from addition of the powder to water till the mixing is
complete.
 Setting Time
 Time duration between start of mix & point where material
hardens.
 Working Time
 Time from start of mix to the point where the consistency
of the mix is no longer acceptable for the usage of intended
purpose.

10.  Based on manufacturing method
Dry calcination
Wet calcination
Synthetic
 Based on the crystal structure
Beta spongy irregular
Alpha prismatic regular
 Based on applications
Type I Impression Plaster
Type II Model Plaster
Type III Dental stone
Type IV Dental stone (High strength, Low expansion)
Type V Dental stone (High strength, Low expansion)
 Based on expansion
Minimum setting expansion (Type I, III and IV)
Maximum setting expansion(Type II and V)

11.  Composed of Plaster of Paris (β CaSO4. H2O)
 Rarely used any longer for dental impression making.
 Currently, it is more useful as a bite registration material.
 Advantages
 Used in mucostatic impression
 Accurate reproduction of fine details
 Negligible dimensional change
 Disadvantages
 Non elastic
 Bad taste
 Messy to work with
2
1

12.  Beta form of Calcium
hemihydrate (βCaSO4.½H2O).
 Also known as
 Laboratory Plaster
 Mounting Plaster
 White in color to provide
contrast with stones which are
generally colored.
 Uses
 Principally used to fill a flask used in
denture fabrication where setting
expansion is not critical and the
strength is adequate.

13.  Alpha form of Calcium
hemihydrate (αCaSO4.½H2O).
 Produced by wet calcination.
Uses
 For fabrication of models and cast of
both dentulous and edentulous cases.
 For making orthodontic study models.
Advantage
 Stronger than dental plaster.
 Can be removed easily after
processing.
Disadvantage
 More expensive than Dental Plaster or
Impression Plaster.

14.  Also known as
Class II stone
Die stone
Densite
Improved stone
 Produced by wet calcination
 Used as a common die material as it has
High strength
High resistance to abrasion

15. Uses
 Used for fabrication of cast
crowns
 Used in die for compensation of
solidification shrinkage
Advantage
 Higher compressive strength
than type IV dental stone
 Most recent gypsum product
 Manufactured by adding small amount of
surfactants (eg Lignin sulphonate)

16. Properties Type I Type II Type III Type IV Type V
Water Powder
Ratio 0.50 – 0.75 0.45 – 0.50 0.28 – 0.30 0.22 – 0.24 0.18 – 0.22
Setting Time
(min) 4±1 12±4 12±4 12±4 12±4
Setting
Expansion (2hr) 0.00-0.15 0.00-0.30 0.00-0.20 0.00-0.10 0.00-0.30
Compressive
Strength (2hr)
MPa
4.0 9.0 20.7 34.5 48.3

17.  Production of study models of oral and
maxillofacial structures and their use as
auxiliary materials for dental laboratory
operations involved in the production of dental
prosthesis.
 Making of models and casts on which wax
models of dental prostheses and restorations
are constructed.

18.  Care of Gypsum Products
 Hemihydrate of gypsum absorbs water from the air
readily.
 Best means of storage is to seal the product in a
moisture-proof metal container.
 Proportioning
 Recommended W/P ratio should be used
 Water and powder should be measured by using an
accurate graduated cylinder for the water volume and a
weighing balance for the weight of the powder

19.  If mixing is performed by hand, the bowl should be
parabolic in shape, smooth and resistant to
abrasion.
 Mixing should continue until a smooth mix is
obtained, usually within a minute. A longer
spatulation time drastically reduces the working
time which is of prime importance when pouring
models.
 Entrapment of air must be avoided since porosity
can lead to weak spots and surface inaccuracies.
 The mix is stirred vigorously.

20.  The inside of the bowl is periodically wiped with a
spatula to ensure wetting of the powder and
breaking up of lumps.
 Stirring is continued till a smooth, creamy mix is
obtained.
 Spatulation should be completed in 45-60
seconds.
 The mix is vibrated (using a mechanical vibrator or
by repeated tapping against a bench) and poured
in into the impression, taking care not to entrap
air.

21.  When calcium sulphate hemihydrate is mixed with
water, the hemihydrate is changed back to
dihydrate by the process of hydration.
 Heat is liberated, as shown by the following
reaction:
 Being less soluble than the hemihydrate, the
calcium sulphate dehydrate precipitates out of the
solution as interlocking crystals, which form a hard
mass.
heatO.2HCaSOOHO.½HCaSO 24224 

22. I. Colloidal theory
When mixed with water, plaster enters into a colloidal
state through a sol-gel mechanism. In the sol state,
hemihydrate combines with water to form dihydrate. As
the water is consumed, the mass turns to a solid gel.
II. Hydration theory
Rehydrated plaster particles join together through H-
bonding to the sulphate groups to form the set material.
III. Dissolution-precipitation theory
i. As hemihydrate is more soluble than dihydrate, it
dissolves until it forms a saturated solution.

23. ii. Some dihydrate is formed due to the reaction and
being less soluble, it precipitates out, leaving the
non reacted hemihydrate supersaturated.
iii. As hemihydrate is more soluble than dihydrate, it
dissolves until it forms a saturated solution.
iv. Some dihydrate is formed due to the reaction and
being less soluble, it precipitates out, leaving the
non reacted hemihydrate supersaturated.
v. As the dihydrate precipitates out, the solution is no
longer supersaturated and so it continues to
dissolve. The process continues until no further
dihydrate precipitates out of the solution.

24.  The time elapsed from the beginning of mixing
until the material hardens.
 Initial Setting time: As the reaction proceeds, more
hemihydrate crystals react to form dihydrate crystals. The
viscosity of the mass increases and it can no longer be
poured. The material becomes rigid (but not hard). It can be
carved but not moulded. This is known as initial setting
time.
 Final Setting Time: The time at which the material can be
separated from the impression without distortion or
fracture.

25.  Substances which are added to gypsum products to
modify setting time
 Accelerators
 Reduces the setting time
 Retarders
 Enhances the setting time

26.  All gypsum products show a linear expansion
during setting, due to the outward thrust of the
growing crystals during setting.
 In dentistry setting expansion may be both
desirable and undesirable depending on the use.
It is undesirable in impression plaster, dental plaster and
stone as it will result in an inaccurate cast or change in the
occlusal relation if used for mounting.
Increased setting expansion is desired in case of
investment materials as it helps to compensate the
shrinkage of the metal during casting.

27.  When a gypsum product is placed under water
before the initial set stage, a greater expansion is
seen.
 This method is used to expand some gypsum
bonded investment.

28.  The strength increases rapidly as the material
hardens after the initial setting.
Factors affecting strength
I. Free water content:
The greater the amount of free water in the set stone, the
less is the strength.
Wet strength: Strength of gypsum when excess free water
is present in the gypsum.
Dry strength: Strength of gypsum when the excess free
water evaporates.

29. II. Temperature
Gypsum is stable only below 40⁰C. Drying at higher
temperatures must be carefully controlled.
Other factors affecting strength
 W/P Ratio: The more the water, the greater the porosity
and less the strength.
 Spatulation: Within limits, strength increases with
increased spatulation.
 Addition of accelerators and retarders lowers strength.
 Tensile Strength: Gypsum is a brittle material, thus
weaker in tension than in compression.

30.  Desirable properties of Gypsum
Accuracy
Dimensional stability
Ability to reproduce fine details
Strength
Resistance to abrasion
Compatibility
Color
Biological safety
Ease of use

31.  “Basic Dental Materials”
:by John J Manappallil
 “Phillips Science of Dental Materials”
:by Arvind Shenoy &
K Chandrashekharan Nair
 Class Notes
 Internet

32. For your patience