DENTAL POLYMERS

PRESENTED BY,

SMIJAL

CONTENTS

Resins used in Dentistry Classification of Resins Polymerization Classification of Polymers Stages of Polymerization Inhibition of Polymerization Physical properties of polymers

RESINS USED IN DENTISTRY

Resins are hard, brittle glassy polymers. Principal polymers- Vulcanized rubber for denture bases. Later- Acrylic resins. Polymers in dentistry are: Vinyl acrylics- relining material. Epoxy resins- die material. Polyether- impression material. Polysulphide - “ “ Silicone “ “ Polycarbonates- temporary crown material Polyacrylic acid- denture base material Polystyrene, Polyethylene, Poly vinyl acetate

DENTAL POLYMER

THERMOPLASTIC Resins are softened &

molded under heat & pressure.

No chemical change. Soluble in organic solvents

& fusible. Eg: Acrylic resin

Impression comp.

Plastics in dentistry

THERMOSET Final product cannot

softened by reheating. Set by chemical reaction. Insoluble in organic solvents

& infusible. Superior abration

resistence.

CLASSIFICATION OF RESINS

Based upon their origin. Natural resin - e.g : Gum Synthetic resin Thermoplastic Thermoset Synthetic elastomers Based on Thermal Behaviour. Thermoplastic Thermoset

Based on structural units. Vinyl Resins Polystyrene Acrylic Resins Epoxy “ Multifuctional acrylate resins

APPLICATIONS IN DENTISTRY

Dentures (bases, liners, artificial teeth). Cavity filling materials ( “Composites” ). Sealants. Impression materials. Cements (resin based). Orthodontics (elastics). Equipments (mixing bowl)

POLYMERIZATION

Chemical reaction in which monomers of a low molecular weight are converted into chains of polymers with a high molecular weight.

Monomer molecules are bonded by covalent bonds. It is an ongoing reaction, never fully complete. Average degree of polymerization= no. of structural

units/ total no. of units.

DENTAL POLYMERIZATION

CLASSIFICATION OF POLYMERS

Based on Nature of monomer.

1) Homopolymer :Same chemical

composition.

2) Copolymer : Different chemical

composition. Based on Spatial Arrangement :

1) Linear

2) Branched

Based on Type of Poymerization.

Addition Polymerization Condensation “

Based on Type of Copolymerization.

Random copolymerization Graft type “ Block type “

Polymerization unit

ADDITION POLYMERIZATION

Reaction between two molecules to form a larger molecule by addition reaction without the formation of byproducts.

Formed polymers have better dimensional stability. 3 types : Free- radical polymerization. Ring- opening “ Ionic “

Free- radical polymerization

Initiator releases free- radicals which bring about the Polymerization reaction.

Eg : Benzoyl peroxide releases free- radicals to bring polymerization in acrylic resins.

Ring- opening poymerization

Ring structure in the polymer chain is opened & crossing- linking occurs.

Eg : Epoxy resin, Polyether impression material.

Ionic polymerization : Catalyst bring about exchange of ions

resulting in cross- linked polymer. Eg : Addition silicones

CONDENSATION POLYMERIZATION[ Step- growth ]

Reaction b/w 2 molecules to form a larger molecule with elimination of a small molecule

As the rtns proceed, longer chains including trimers, tetramers form through other estrification rtns until the rctn contain a mixture of polymer chains of large m.wt.

Thus, formation of polymers by this process is slow due to step – growth.

2 types

a) Condensation rctn where it gives a byproduct

Eg : Condensation silicone impression material.Gives ethyl alcohol as byproduct.

Byproduct may evaporate & affect the dimensional stability.

b) Condensation rctn which occurs by the additon of a reactive part & the elimination of a byproduct.

Eg ; Polyurethane

COPOLYMERIZATION

Polymer chain contains 2 or more chemically different types of monomer units.

3 types 1) Random copolymerization 2) Graft type “ 3) Block type “

CO POLYMER LIQUED CRYSSTALINE COPOLYMER

Random copolymerization

Different monomer units are randomly distributed.

They could be either Linear or Branched.

Block Copolymerization When specific number of monomer units are

repeated together in a particular pattern.

Graft copolymerization

If the same occurs in a branched polymer it is called Graft copolymer.

Requirements of Polymerization 1) Momomer (reactive gp) with functional gp C=C. 2)Side gp on polymer determines the physical

properties of resulting polymer. Side group is usually pendent group.

STAGES OF POLYMERIZATION

1) Initiation 2) Propagation 3) Termination 4) Chain transfer

1) Initiation

First step involves formation of free- radical. An initiator is converted to a free- radical by

the action of heat / chemical rctns. This free- radical initiates polymerization rctn Eg : Conversion of benzoyl peroxide into

free- radical initiates polymerization in acrylic resins.

Activation by heat or chemical rctns.

Second step is rctn of free-radical & monomer.

Free- radical converts monomer into polymer The electrons from lower energy levels pair

with electrons of higher energy levels resulting in atomic bonding.

Results in repeated linking of monomer units to form polymer

Egs :

In Heat cure acrylic resins, Initiator : Benzoyl peroxide Activator : Heat In chemical activated resins, Initiator : Benzoyl peroxide Activator : Tertiary amine In light activated resins, Initiator : Camphoroquinone Activator : Visible light

2) Propagation

Initiation followed by propagation & linking more monomer units to form polymer units.

Increase in polymer chain requires energy. The process continues with considerable velocity, is

accompanied with evolution of heat. If ideal conditions is there,propagation continue untill

all monomer get changed to polymer. Usually this never happens.

3) Termination

Polymerization rctn can be controlled by termination.

This can be either by direct coupling or by exchage of H2 atom from the growing chain to another.

4) Chain transfer

Termination can occur due to chain transfer. The activated radical is converted into an

inactive molecule. A new nucleus growth is created.

INHIBITION OF POLYMERIZATION

Inhibited by impurities. These react with activated free- radical or

any activated nucleus or with an activated growing chain to prevent further growth.

Eg: Hydroquinone Presence of O2

PHYSICAL PROPERTIES OF POLYMERS.

1) Deformation & Recovery Applied forces within polymers produces’ a) Plastic deformation- Permanent change b) Elastic “ - Reversible “ c) Viscoelastic “ - Combination of both In viscoelastic, recovery occurs over time.

2)Rheometric properties

The rheometry, or flow behaviour, of solid polymers involves a combination of elastic & plastic deformation (viscous flow) & elastic recovery when stresses are eliminated.

3)Solvation properties

Polymers are usually slow to dissolve. Solvation charecteristics are sensitive to

Mw,Mw/Mn (piolydispersty), cross-linking, crystallinity & chain branching.

Longer chain- polymer dissolve slowly. Highly cross-linked – not dissolve. Polymers absorb a solvent, swell, &soften,

rather than dissolve.

4) Thermal properties

Higher the temperature, softer & weaker the polymer .

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