1. Colloidal theory 2. Hydration theory 3. Dissolution – precipitation theory

L2 Gypsum Products Flashcards by Irish Alicpala

mineral mined in various parts of the world with chemical name CaSO4.2H2O

Gypsum

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first mined in Montmartre in France
hence its name originated from there

Plaster of Paris

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ADA specification no. 25 classification types

type i - impression plaster
type ii - dental plaster
type iii - dental stone
type iv - dental stone (⬆️strength, ⬇️expansion)
type v - dental stone (⬆️strength, ⬆️expansion)

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TYPES OF GYPSUM:
- one of the earliest impression material in dentistry
- very rigid; often has to be fractured to remove it from undercut areas in the mouth

TYPE I or IMPRESSION PLASTER

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TYPE OF GYPSUM:
- Much harder and stronger than Type II
- uses less water

TYPE III or DENTAL STONE, MODEL

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TYPE OF GYPSUM:
- Stronger than Type III
- Also known as Class II stone, die stone, densite, improved stone

TYPE IV or DENTAL STONE, HIGH STRENGTH, LOW EXPANSION

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synonyms of type iv dental stone

Class II stone
die stone
densite
improved stone

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Strongest and hardest variety of
gypsum product

die stone

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TYPE OF GYPSUM:
- Expands to compensate for the shrinkage of the metal

TYPE V or DENTAL STONE, DIE STONE, HIGH STRENGTH, HIGH EXPANSION

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Also called Task investment material

DENTAL STONE, DIE STONE, HIGH STRENGTH, HIGH EXPANSION

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The process of heating of gypsum for the
manufacture of plaster.

Calcination

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Depending on the method of calcination there are two forms of hemihydrate.

ALPHA and BETA HEMIHYDRATE

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MANUFACTURE OF DENTAL PLASTER
Gypsum is ground and heated in an open kettle on
kiln at a temperature of __-__C. This process is called ____.

110 to 130◦C; dry calcination

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MANUFACTURE OF DENTAL STONE
Gypsum is calcined under steam pressure in an autoclave at __ to __◦C at 17Ibs/sq. inch for 5 to 7 hrs.

120-130

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When plaster is mixed with water it takes up one and a half molecule of water.

Hemihydrate + water → Dihydrate + unreacted hemihydrate + heat

SETTING REACTION

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The product of the reaction is gypsum.

setting reaction

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The heat evolved in the exothermic reaction is equivalent to the heat used originally in calcination.

SETTING REACTION

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enumerate theories

Colloidal theory
Hydration theory
Dissolution – precipitation theory

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The time elapsing from the beginning of mixing until the
material is hard and can stand alone

SETTING TIME

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● Hemihydrate crystals react to form dihydrate crystals. The viscosity of the mass is increased and it can no longer be poured. The material becomes rigid. It can be carved but not moulded.
● Can stand alone but still soft

INITIAL SETTING TIME

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● The time at which the material can be separated from the impression without distortion or fracture.
● Measure scientifically using needle which penetrates the set plaster stone, if it still penetrates it is not yet set.

FINAL SETTING TIME

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3 MEASUREMENT OF SETTING TIME

[1] Loss of gloss method
[2] Exothermic reaction
[3] Penetration tests

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MEASUREMENT OF SETTING TIME
- Occur when excess water in the mix is taken up in forming the dihydrate
- This occurs approximately at 9 mins and the mass has no measurable compressive strength.

Loss of gloss method

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MEASUREMENT OF SETTING TIME
- The setting reaction produces or releases heat.
- If it is still hot, it is still not set; when heat disappears, it is set.

Exothermic reaction

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MEASUREMENT OF SETTING TIME - using PENETROMETER - The time elapsing from the start mixing till the needle does not penetrate to the button of the plaster is the setting time

Penetration tests

TYPES OF PENETROMETERS

1. Vicat needle 2. Gillmore needles (Initial Gillmore & Final Gillmore)

TYPES OF PENETROMETERS - it weighs 300 gm and the needle diameter is 1mm.

Vicat needle

TYPES OF PENETROMETERS - ___ - Smaller one used to measure initial setting time ■ Weight : 1/4 lb ■ Diameter : 1/12 in (2.12mm) - ___ - Bigger one used to measure final setting time ■ Weight : 1 lb ■ Diameter : 1/24 in (1.06mm)

- initial gillmore (small) - final gillmore (large)

5 FACTORS AFFECTING SETTING TIME

1. Manufacturing process 2. Mixing and spatulation 3. Water/Powder ratio 4. Temperature 5. Modifiers [MaMiWPTeMo]

The more water used, the ___ the setting time The less water used, the ___ the setting time

longer; shorter

The higher the temperature, the ___ the setting time The lower the temperature, the ___ the setting time

shorter; longer

Chemicals added in order to alter some of the properties and make it more acceptable to the dentist.

MODIFIERS

If the chemical added decreases the setting time, it is called an ___. it increases the setting time, it is called a ___.

accelerator; retarder

(⬆️ / ⬇️) - accelerator: __ setting time, __ rate of rxn - retarder: __ setting time, __ rate of rxn

- accelerator: ⬇️ setting time, ⬆️ rate of rxn - retarder: ⬆️ setting time, ⬇️ rate of rxn

4 PROPERTIES OF GYPSUM PRODUCTS

1. Setting expansion 2. Strength 3. Hardness and abrasion resistance 4. Reproduction of detail [SeStHaRe]

The dimensional increase that occurs concurrently with the hardening of various materials.

SETTING EXPANSION

2 TYPES OF SETTING EXPANSION

1. Normal setting expansion 2. Hygroscopic setting expansion

All gypsum products show linear expansion during setting, due to the outward thrust of the growing crystal during setting

Normal setting expansion

Factors affecting strength (wet and dry)

- W/P ratio (⬆️water, ⬇️strength) - Spatulation (⬆️spatulation, ⬆️ strength) - Addition of accelerator and retarder (⬇️strength)

The strength when excess free water is PRESENT in the set gypsum

Wet Strength

- The strength of gypsum when the excess free water is LOST due to EVAPORATION - It’s two or more times greater than wet strength

Dry Strength

Name the varieties of gypsum

Alabaster → pure white, fine grained and translucent Satin spar → fibrous needle like with silky lustre Selenite → colorless, crystalline and transparent

different uses of Plaster of Paris

- impression plaster - Making moulds, study casts, diagnostic cast, preliminary cast, master cast - To mount a cast on articulator - Bite registration

(DIE / CAST / MODEL) - ___ - positive, dimensionally accurate replica of oral soft and hard tissues of either the maxillary or mandibular jaw - ___- positive, dimensionally accurate replica of a prepared natural tooth - ___ - positive, dimensionally accurate replica of oral soft and hard tissues

cast; die ; model

(DIE / CAST / MODEL) USES - ___- construction of dental appliances which fit on the soft tissues of the oral cavity - ___- for the construction of inlay, crown or bridges - ___ - as a study model or for patient education

cast; die; model

- Removing ¾th of water of crystallization by burning gypsum - Process of heating gypsum for manufacturing plaster

calcination

process of manufacturing ____ - Gypsum is calcined by boiling it in 30% calcium chloride solution - Chlorides are then washed away or autoclaved in presence of sodium succinate 0.5% - Then it is grinded to obtain ____

Alpha Modified Stone or Improved stone

This reaction occurs in the dental laboratory (hemihydrate and water mixture), not in the manufacturing process

setting reaction of plaster

It acts by providing additional nuclei of crystallization [lalabas sa exam]

TERA-ALBA

Finely powdered gypsum /white earth-like substances used to control setting time of Plaster of Paris [lalabas sa exam]

tera-alba

solid-liquid separation and purification technique in which mass transfer occurs from the liquid solution to a pure solid crystalline phase

crystallization

Slow precipitation of crystals from a solution of a substance

CRYSTALLIZATION

The invisible water present in the gypsum

water crystallization

When impression plaster contains potato starch to make the impression soluble

soluble plaster

The actual amount of water necessary to mix the calcium sulphate hemihydrate is greater than the amount required for the chemical reaction, this is called

excess water

A stone with a setting time established by the addition of proper quantities of both accelerator and retarder is called ---

balanced stone

- mixing time: __ min - working time: __ min - setting time:

1; 3

Initial time during setting before release of exothermic heat

induction period

- strength when the set mass has no excess water and the specimen is dry to touch - It is 2-3 times more than wet strength.

dry strength

Gypsum is stable only below about __°C. Drying at higher temperature rapidly at __°C or higher, loss of water of crystallisation occurs which causes shrinkage and reduction in strength.

40; 100

it is a mixture of die stone and any one of the dental investment materials used to make die or master models.

divestments

As plaster is ___, it should be kept in air tight containers.

hygroscopic (absorb moisture from air)

Presence of more than 20% of NaCl , colloids like coagulated blood, etc. act as retarder to setting of plaster by wetting of the nuclei and their growth

nuclei poisoning

It is a linear expansion during setting due to outward thrust of the growing crystals during setting which intermesh and intercept each other during growth.

normal setting expansion