THERMOPLASTIC THERMOSET Resins are softened & Final product cannot molded under heat & softened by reheating. pressure. Set by chemical reaction. No chemical change. Insoluble in organic solvents Soluble in organic solvents & infusible. & fusible. Superior abration Eg: Acrylic resin resistence. Impression comp. Plastics in dentistry
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
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.
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 “
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 Initiatorreleases 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 “
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 Ifthe 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 Inhibitedby 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 OFPOLYMERS.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 .