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DENTAL MATERIALS

ASSIGNMENT

HIBA HAMID OKARVI

BDS 1ST YEAR

ROLL # 32

COMPARISON OF SYNTHETIC ELASTOMERS

Comparison

topic

Polysulphides Condensation silicones Addition silicones Polyethers

Composition Two paste system dispensed

from tubes – one paste normally

base paste and other catalyst

paste.

Base paste contains:

- polysulphide prepolymer with

terminal and pendant thiol

groups

- plasticizer

- inert filler

Catalyst paste contains:

- lead oxide

- sulphur

- inert oil normally paraffinic oil

Two paste system or paste and

liquid.

Paste contains:

- hydroxyl terminated

polydimethylsiloxane

- inert filler such as silica

Liquid contains:

- alkyl silicate such as

tetraethylsilicate

- tin compound such as dibutyl tin

dilaurate

Supplied as two pastes.

- each paste contains a liquid

silicone prepolymer and filler.

- one of the paste contains a

catalyst

- one paste contains a

polydimethylsiloxane prepolymer

in which some methyl groups are

replaced by hydrogen

- other paste contains a

prepolymer in which methyl

groups are replaced by vinyl

groups

- catalyst in the paste is normally

platinum containing compound

such as chloroplatinic acid

Supplied as two pastes

Base paste (large tube) contains:

- imine-terminated prepolymer

- inert filler – silica

Catalyst paste (small tube)

contains:

- ester derivative of aromatic

sulphonic acid

- inert filler – silica

- plasticizer – phthalate

Viscosity Available in three viscosities;

light-bodied, regular-bodied and

heavy-bodied; no putty

Available in all four viscosities:

light-bodied, regular-bodied,

heavy-bodied and putty material

Available in all four viscosities:

light-bodied, regular-bodied,

heavy-bodied and putty material

Available in single viscosity

(regular-bodied).

Setting reaction On mixing two pastes, terminal

and pendant thiol groups of

prepolymer undergo reaction

with lead oxide. Reaction results

in chain extension and cross

linking. This reaction is of

condensation polymerization

type as a water molecule is

produced as a byproduct of each

reaction stage

On mixing, reaction begins

immediately in which terminal

hydroxyl groups of prepolymer

chains react with the cross linking

agent under influence of catalyst.

Due to the each molecule of cross

linking agent reacting with four

prepolymer chain, extensive

cross-linking occurs. Each

reaction stage produces one

On mixing two pastes, platinum-

catalysed addition reaction

occurs, causing cross-linking

between two types of siloxane

prepolymer. Reaction does not

produce any byproducts although

it has been reported that these

materials occasionally evolve

hydrogen

When two pastes mixed

together; cationic, ring opening

addition reaction occurs. Ionized

form of sulphonic acid ester

provides initial source of cations

and each stage of reaction

involves opening of an epimine

ring and production of a fresh

cation. Reaction is of addition

type with no by-product being

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molecule of ethyl alcohol as a

byproduct

formed.

Tear resistance Very good tear resistance.

Typically can withstand about

700% tensile strain before

tearing

Adequate tear resistance.

Undergo only about 300%

extension before fracturing.

Adequate tear resistance Adequate tear resistance

Elasticity Viscoelastic materials. Recover

only slowly and not completely

after being compressed or

stretched.

Elastic properties which most

closely approach the ideal of

complete and instantaneous

recovery following stretching or

compression

Near ideal elasticity Elastic properties near to those

of silicones. Adequate elastic

properties

Accuracy Heavy-bodied materials are

potentially more accurate, since

they exhibit lest setting and

thermal contraction. Their high

viscosity means that they are

unable to record the fine details

as the more fluid, light-bodied

materials.

Very hydrophobic materials and

are readily repelled by water or

saliva. As a result, it is necessary

to dry areas of the mouth for

which an accurate impression is

required. If a dry field is not

secured “blow holes” are likely to

occur in the impression because

the material will not be able to

drive away the residual moisture.

To obtain maximum accuracy,

model should be cast as soon as

possible after recording the

impression

The combined use of putty and

light-bodied materials enables

accurate impressions to be

recorded. Inherently hydrophobic

in nature, which can cause

imperfections in impressions if

the area to be recorded is not

thoroughly dried. Repelled by

moisture which can result in the

formation of blow holes.

Accuracy of polyether

impressions compares

favourably with other light-

bodied elastomers. The lack of

heavy-bodied and putty pastes,

however, precluded the use of

techniques using combined

viscous/fluid pastes which are

commonly used with other

elastomers to optimize accuracy

Dimensional

stability

Dimensional changes occur after

apparent setting of polysulphides

due to two major factors:

- continued reaction occurs for

some time after the apparent

setting time, causing further

shrinkage of the impression

- water produced as the

byproduct of the setting reaction

may be lost by evaporation from

the surface. In this case

dimensional change associated

Dimensional changes after setting

may be continued due to loss of

alcohol produced as byproduct of

setting reaction. This effect

produces measurable weight loss

accompanied by shrinkage of

impression material.

Dimensional changes of regular-

bodied condensation silicones

slightly greater than those of

regular-bodied polysulphides.

Production of little or no

byproduct in the cross-linking

reaction of the addition curing

material results in a very stable

impression.

Under conditions of low

humidity, have very good

dimensional stability. Material

contains no volatile constituents

and sets by an addition reaction

which produces no volatile

byproducts. Set material is

relatively hydrophilic and

absorbs water under conditions

of high humidity. This causes

impression to swell and distort.

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with change in weight of the

material

Setting time Take longer to set Less setting time as compared to

polysulphides

Less setting time as compared to

polysulphides

Distinctive transition from

plastic to elastic behavior which

can be helpful in estimating the

earliest safe time for removal

from the mouth

Decontamination Soaking in hypochlorite or

glutaraldehyde to decontaminate

has little effect and does not

harm the impression due to the

hydrophobicity of the material.

Soaking in hypochlorite or

glutaraldehyde to decontaminate

has little effect and does not harm

the impression due to the

hydrophobicity of the material.

Soaking in hypochlorite or

glutaraldehyde to decontaminate

has little effect and does not harm

the impression due to the

hydrophobicity of the material.

As these materials are

hydrophilic, great care must be

taken when soaking impressions

in aqueous solutions – lest the

material absorbs water and

swells.

Clinical use Materials most commonly used

in two viscosities, high and low,

and must be used in a special

tray. Commonly used for crown

and bridge impressions.

Limited reports of allergic

responses to these materials,

from people with and allergy to

latex products

Available in range of viscosities

from light-bodied materials used

to record accurate surface detail

on prepared tooth surfaces,

through medium viscosities,

commonly used a monophase

material during either crown and

bridgework or denture

manufacture, and heavy-bodied

materials used to support light

body in stock trays for crown and

bridge impressions, to putties

which are now available in both

soft and hard format

Preferred due to their greater

dimensional stability.

Available in range of viscosities

from light-bodied materials used

to record accurate surface detail

on prepared tooth surfaces,

through medium viscosities,

commonly used a monophase

material during either crown and

bridgework or denture

manufacture, and heavy-bodied

materials used to support light

body in stock trays for crown and

bridge impressions, to putties

which are now available in both

soft and hard format

Commonly used as a monophase

where a single viscosity of

material is used for both the bulk

of the impression in the tray and

to be syringed around the

prepared teeth in the mouth.

Greatest disadvantage from a

clinical standpoint is the rigidity

of these materials when set. This

can make removal of the

impressions very difficult in

dentate patients when there are

marked undercuts present around

the teeth.