Dental Materials Lecture 6, 7, 8, 9

GYPSUM

Is a mineral made a from gypsum rock, which is found in various parts of the world.  

CaSO4\*2H2O

Chemical formula for gypsum

Gypsum products

are supplied as fine hemihydrate powders that are produced by heating ground gypsum particles.

Calcium Sulfate Dihydrate (CaSO42H2O)

Chemically, gypsum rock is

Gypsum products

are supplied as fine powders that are mixed with water to form a fluid mass or slurry that can be poured and shaped and that later hardens into a rigid, stable mass.  

Gypsum products

used mainly for positive reproductions or replicas of oral structures

Gypsum products

used in dentistry for the preparation of study models for oral and maxillofacial structures.

Various types of dental plaster

produced, modified for specific property requirements and used to form molds and cast on which dental prostheses and restorations are constructed.

Gypsum dental investment

Plaster mixed with fillers is known as

Plaster mixed with fillers

Such dental investment are used to form molds for the casting of dental restorations with molten metal.

dental investment

When plaster is mixed with silica, mainly used to form moulds into which molten metals are cast.

\-       Accuracy

\-       Dimensional stability

\-       Ability to reproduce fine detail

\-       Strength and resistance to abrasion

\-       Compatibility with the impression material

\-       Color

\-       Biological safety

Desirable Properties of Gypsum Products:

Dry Calcination

Wet Calcination

Dehydration by boiling with chemicals

Synthetic Method

Different Methods of Manufacturing Gypsum Products:

CALCINATION

\-       The process of heating gypsum for the manufacture of plaster is known as

__*DRY CALCINATION*__

\-       When gypsum is finely ground and subjected to high temperatures of 110-1300C to eliminate the water of crystallization

Dental stone and die stones   

Raising to a higher temperatures in DRY CALCINATION METHOD, we further get

WET CALCINATION

\-       When gypsum is ground and heated under steam pressure at a temperature of 110 to 1300C in a closed kettle or kiln, or an autoclave.

Alpha-calcium sulfate hemihydrate

formed in __**wet calcination**__ which consists of smaller, regularly shaped crystalline particles in form of rods and prisms

Type III (dental stone or hydrocal or class I stone)

formed by wet calcination method

rods and prisms

Microscopically, crystals type III (Dental stone) are in the form of

DEHYDRATION BY BOILING WITH CHEMICALS

\-       The gypsum is calcined by boiling it with __**30% CaCl2 solution**__

Improved stone ( a-modified hemihydrate )

is manufactured by DEHYDRATION BY BOILING WITH CHEMICALS method, these methods are denser among all three types

SYNTHETIC GYPSUM PRODUCTS

The possibility of making an alpha & beta Hemihydrate from the by-products or waste products

ADA specification number 25

The gypsum products are classified according to

Type I – Impression Plaster

Type II – Dental Plaster

Type III – Dental Stone

Type IV – Improved Dental Stone or Die stone or High Strength Stone

Type V – Dental Stone, High Strength, High Expansion

Types of Gypsum Products:

Type I (Impression Plaster)  

\-       Considered as an obsolete impression material and it is no longer used in taking impression.

Type I (Impression Plaster)  

Nowadays it is used in mounting cast on articulator, bite registration

Type I (Impression Plaster)  

\-       Making impression for maxillofacial prosthesis

Dental Plaster or laboratory Type II plaster

\-       the beta form of calcium sulfate hemihydrate (CaSO4.1/2H2O)

Type II (Dental Plaster) Plaster of Paris

\-       It is used to fill flask during complete denture constructions

Type II (Dental Plaster) Plaster of Paris

\-       Used for making preliminary cast for CD

Type II (Dental Plaster) Plaster of Paris

For mounting cast on articulator

Type II (Dental Plaster) Plaster of Paris

\-       Usually marketed in natural white color

o It should set fast but should have enough time for manipulation.

o It should set to a very hard and strong mass.

o It should neither expand nor contract appreciably.

o It should not lose its strength when subjected to moulding and curing procedures.

Requisites for a dental plaster:

Type II (Dental Plaster) Plaster of Paris

\-       It has an irregular orthorhombic crystals shape with capillary pores and requires **more water** for hydration to get converted in calcium sulfate dihydrate.

0\.45 – 0.50

water powder ratio of Type II (Dental Plaster) Plaster of Paris

9-12.5MPa

wet strength of dental plaster

Plaster of Paris

prepared by heating calcium sulfate dihydrate, or gypsum, to 120 – 1800C (248 - 3560F)

Calcinations

 Plaster is produced by a process known as

Colloidal Theory

proposes that, when mixed with water, hemihydrate enters into the colloidal state through a sol-gel mechanism. In the sol state, hemihydrate particles are hydrated to form dehydrate, thereby entering into an active state. As the measured amount of water is consumed, the mass converts to a solid gel.

Hydration Theory

suggests that rehydrated plaster particles unite through hydrogen bonding with sulfate groups to form the set material.

the __**Dissolution precipitation Theory**__ __or__ __**Crystalline**__ __**Theory**____.__

The most widely accepted mechanism

the __**Dissolution precipitation Theory**__ __or__ __**Crystalline**__ __**Theory**____.__

This theory is based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set product.

Water/Powder Ratio

The proportion of water to powder used to make a workable mix of a particular gypsum product

Water/Powder Ratio

It is the quotient obtained when the weight or volume of the water is divided by the weight of powder.

Water/Powder Ratio

It is usually abbreviated as W:P

The W:P ratio

an important factor in determining the physical and chemical properties of the final gypsum product.

the strength of the gypsum product decreases, and the setting expansion decreases.

As the W/P ratio increases, the setting time increases,

45 to 50mL/100g (0.45-0.50)

for the average mix of plaster

28 to 30mL/100g (0.28-0.30)

for the average mix of stone

19 to 24mL/100g (0.19-0.24)

for the average mix of improved stone

Setting Time

It is the time elapsing from the beginning of mixing until the material hardens.

1\. Working Time or Initial Setting Time:

2\. Final Setting Time

The clinicians should be aware of the **two types or two time intervals** in the setting process:

Working Time or Initial Setting Time:

\-       Is the length of time from the start of the mix until the setting mass reaches a semi-hard stage.  

Working Time or Initial Setting Time:

\-       It represents the available time for manipulating the product, and it indicates partial progress of the setting reaction.

3 minute

\-       Generally, a _____ working time is adequate.

Final Setting Time

\-       Represents the length of time from the start of the mix until the setting mass becomes rigid and can be separated from the impression without distortion or fracture.

Final Setting Time

\-       The final setting time indicates the major completion of the hydration reaction.

Loss of gloss test

\-       Occurs when excess water in the mix is taken up in forming the dihydrate, so the mix loses its gloss.

Loss of gloss test

\-       This occurs approximately at __**9mins**__ and the mass has no measurable compressive strength. Therefore, it could not be safely removed from the mold.

Initial Gillmore’s Test

Smaller needle (1/4 lb weight and dia of 1/12”) is used for initial set testing 

Initial Gillmore’s Test

Mixture is spread out and the needle is lowered onto the surface.

INITIAL SET- Initial Gillmore’s Test

The time at which no longer leaves an impression is called an

FINAL SETTING TIME- Final Gillmore’s Test

The time elapsed when it leaves only a barely perceptible mark on the surface is called the

Final Gillmore’s Test

Heavier gillmore needle (1lb, 1/24”) is used for final set testing

Final Gillmore’s Test

It is rarely used as an indication for the ready-for-use stage

Vicat test for setting time

it has an inbuilt gauge for judging the setting characteristics.

Vicat test for setting time

The setting material is indented by a needle of __**1mm dia.**__  

Vicat test for setting time

\-       Under a load of __**300gm,**__ setting time is achieved when needle can no longer penetrate to a depth of __**2mm**__ into the material.

Increased Setting Time

(A Slower-Setting Product)

Increased Setting Time (A Slower-Setting Product)

\-       Decreased mixing

Increased Setting Time (A Slower-Setting Product)

\-       High water/powder ratio **(creates a thinner mix)**

**Retarders**

Addition of certain chemicals in Increased Setting Time called

Borax

A commonly used retarder

Retarders

generally act by forming an adsorbed layer on the hemihydrate to reduce its solubility and on the gypsum crystals present to inhibit growth.

Decreased Setting Time

(A Faster-Setting Product)

Decreased Setting Time (A Faster-Setting Product)

\-       Increased mixing

the shorter the setting time

the longer the mixing time

Decreased Setting Time (A Faster-Setting Product)

\-       Lower water/powder ratio **(creates a thicker mix)**

Accelerators

Addition of certain chemicals in Decreased Setting Time (A Faster-Setting Product) is called

Potassium sulfate

A commonly used accelerator

Improper storage and use of gypsum products

 can also change the setting characteristics.

water

essential for the setting reaction, any moisture that inadvertently comes in contact with the product can change the setting time.

gypsum products, airtight containers

_____ should be stored in ________ to prevent any uptake of water resulting from high relative humidity.

Setting Expansion

due to the outward thrust of growing crystals

Setting Expansion

It is observed based on the mechanism of crystallization.

Setting Expansion

The process of outgrowth of crystals from the nuclei of crystallization.

0\.06%-0.5%

Setting expansion may range from

Normal Setting Expansion

refers to the setting expansion when a gypsum product is allowed to expand in air

Hygroscopic Setting Expansion

refers to the expansion of a gypsum product when it is allowed to set, immersed in water.

STRENGTH

The strength of gypsum products is usually measured in terms of crushing or compressive strength.

Wet Strength

is the strength that is measured when the sample contains some or all of the water in excess of the theoretical amount required for hydration. The material feels wet to touch for many hours.

Dry Strength

is the strength that is measured when the excess water is not present in the sample. It may be two or more times the wet strength.

water/powder ratio

The strength depends on the _________; thicker mixes will increase strength within limits, and thinner mixes will decrease the strength.

surface hardness

related to the compressive strength but reaches its maximum value more rapidly because the surface is the first to dry.

dry strength

The greatest surface hardness occurs when the product reaches its _______.

1 to 2 hours, or preferably overnight or longer.  

Cast and dies should be allowed to set for

Calcium Sulfate Dihydrate

The chemical composition of all gypsum products:

Beta-hemihydrate

Dental Plaster is also known as:

Final Setting Time

The time from the beginning of the mix until the model no longer emits heat. About **1 hour.**