chemistry of synthetic resins

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DENTURE BASE MATERIALS VULCANITE (1855)– Rubber with 32 % Sulphur & Metallic oxides. Advantages – It is nono-toxic, non-irritant, has excellent Mechanical properties material is sufficient hard to polish. Limitation – Absorbs saliva – becomes unhygiene, leads to bacterial growth & unpleasant odour Poor esthetics Dimensional changes Thermal expansion Contraction of 2-4% by volume during addition of sulphur to the rubber


DENTURE BASE MATERIALS NITROCELLULOSE– Dimensionally unstable Contains unpleasant tasting plasticizers Excessive warpage High water absorption Poor colour stability


DENTURE BASE MATERIALS PHENOL FORMALDEHYDE Becomes discoloured & unesthetic It is thermoset type – so it is difficult to repair PORCELAIN – This material is tolerated well, But it is difficult to fabricate Can be easily broken


CLASSIFICATION OF DENTURE BASE MATERIALS METALLIC Cobalt – Chromium Gold Alloys Aluminium Stainless Steel Titanium TEMPORARY Self-cure Acrylic resin Shellac Base Plate Hard Base Plate Wax NON-METALLIC Acrylic Resin Vinyl Resin PERMANENT Heat-cure Acrylic resin (1937) METALLIC




CLASSIFICATION Synthetic Resins are often called as PLASTICS A substance that although dimensionally stable in normal use was plastic at some stage of manufacture Thermoplastic – they soften again when reheated (above GTT) Thermosetting – they are resistant to change after further application of heat

Third Group – ELASTOMERS Founded on Naturally occurring LATEX isolated from Hevea brasiliensis tree : 

Third Group – ELASTOMERS Founded on Naturally occurring LATEX isolated from Hevea brasiliensis tree Thermoplastic Resins Thermosetting Resins Are fusible, soluble in organic solvents Better flexural & impact properties Most plastics in Dentistry belong to this group PMMA, Polyvinyl, Polystyrene These become permanently hard when heated above critical temp. & they do not soften again on heating Usually cross-linked in state These are insoluble, infusible Crosslinked PMMA, Silicones. Superior abrasion resistance Superior Dimensional Stability


IDEAL REQUIREMENTS Tasteless, odourless, non-toxic, non-irrtant Esthetically satisfactory – transparent, translucent, easily pigmented, colour should be permanent Dimensionally stable, should not expand during Processing & subsequent use by the patient Should have adequate Strength, Resilience, Abrasion Resistance


IDEAL REQUIREMENTS Insoluble & Impermeable to oral fluids Low specific gravity Softening temp. should be well above temp. of any hot food Easy to fabricate Good thermal conductivity Radioopaque


USES Preparation of Dentures Artificial teeth Tooth restorations Orthodontic space maintainance Crown & Bridge facings, Temporary Crowns Maxillofacial prosthesis, Athletic Mouth Protector Inlay patterns Implants Dies, Impression trays Endodontic filling materials

Basic Nature of Polymers : 

Basic Nature of Polymers 1. Polymer – Molecule that is made up of may parts Chemical possessing a molecular weight of more than 5000 Monomer – Molecule from which polymer is constructed Molecular Wt. of various mers X no. of mers (determines its physical properties) Degree of Polymerization --- total no. of mers in polymers . Strength increases with increase in Deg. Of Poly.


STUCTURE OF POLYMERS (SPATIAL STRUCTURE) 1 LINEAR – linear homopolymer, random copolymer, block 2 BRANCHED – branched homopolymer, random copolymer, graft copolymer 3 CROSS-LINKED- homopolymer crosslinked with single crosslinking agent

Polymerization – series of chain reaction by which a macromolecule or polymer is formed from a single molecule : 

Polymerization – series of chain reaction by which a macromolecule or polymer is formed from a single molecule Condensation Addition Slow method Repeated Elimination of small molecules By- products – NH3, H2O, halogen acids Functional groups are repeated (Amide, Urethane, Ester or Sulfide) Here by-product formation is not necessary In Dental procedures No change in chemical composition & no by-products Giant molecules (unlimited size) Simple, but not easy to control

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CHEMICAL STAGES OF POLYMERIZATION INDUCTION (INITIATION) Is the time during which the molecules of the initiator becomes energized or activated & start to transfer the energy to the monomer. Impurity --- increases length of this period Increase temp. --- shorter is length of Induction period Initiation energy is 16000 to 29000 cal/mol.


3 INDUCTION SYSTEMS HEAT ACTIVATION –free radicals are liberated by heating Benzoyl peroxide CHEMICAL ACTIVATION – atleast 2 reactants --- chem. Reaction--- liberate free radicals Benzoyl peroxide + Aromatic Amine(dimetyl-p-toluidine) LIGHT ACTIVATION – photons of light energy activate the initiator – free radicals . Under visible light Camphoroquinone & an amine --- free radical CHEMICAL STAGES OF POLYMERIZATION

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CHEMICAL STAGES OF POLYMERIZATION PROPOGATION – After growth has started 5000 to 8000 cal/mol. Is required & the Process continues

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CHEMICAL STAGES OF POLYMERIZATION TERMINATION- direct coupling or exchange of H2 atom

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CHEMICAL STAGES OF POLYMERIZATION CHAIN TRANSFER . Chain termination can also result from chain transfer. Active state is transferred from activated radical to inactive molecule And new nucleus of growth is created


INHIBITION OF POLYMERIZATION Occurs when there is Complete exhaustion of monomer Or Formation of High Molecular Weight polymer Inhibited by : IMPURITIES (react with Activated Initiator / Nucleus) Hydroquinone (0.006%) is in Monomer for storage OXYGEN retards polymerization Influence the length of Initiation period & degree of polymerization


COPOLYMERIZATION Is required to improve physical properties Two or more chemically different monomers polymerize to form COPOLYMER TYPES Random Graft Block

Applications of copolymerization : 

Applications of copolymerization ETHYL ACRYLATE+ PMMA = FLEXIBILITY BLOCK & GRAFT Polymers = Improves IMPACT STRENGTH (good adhesive properties + surface characteristics) CROSS-LINKING (chemical bond between linear polymers) Applications – Improves strength, reduces solubility & water sorption Highly Cross-linked Material provides - increased resistances ------ to solvents, crazing & surface stresses

Plasticizers : 

Plasticizers Increases solubility of polymers in monomer Reduces brittleness But it also reduces Strength & Hardness & Softening point EXTERNAL – penetrates macromolecules & neutralizes secondary bond. It Evaporates / Leaches out INTERNAL - Copolymer

Types of resins : 

Types of resins Acrylic resin Vinyl resin Polystyrene Epoxy resins OTHER RESIN SYSTEMS Polycarbonates Polyurethanes Cyanoacrylates


ACRYLIC RESINS Are Derivatives of Ethylene & contain a vinyl group in their structural formula Acrylic resins used in dentistry are esters of 1 Acrylic acid 2 Methacrylic acid Available as Methyl methacrylate [liquid] & Poly (Methyl methacrylate) [powder]

Poly (Methyl Methacrylate) Resins : 

Poly (Methyl Methacrylate) Resins Widely used --- easy to process It is Thermoplastic resin Liquid [monomer] Methyl Methacrylate is mixed with Powder [polymer ] Monomer plasticizes the polymer to dough-like consistency which can be easily moulded Types ---- based on method used for its activation Heat activated resins Chemically activated resins Light activated resins


HEAT – ACTIVATED DENTURE BASE RESINS AVAILABLE AS Powder+ Liquid & Gels – Sheets &cakes COMPOSITION Liquid Methyl Methacrylate Dibutyl pthalate --- plasticizer Gylcol dimethylacrylate [1-2%] ---- cross-linking agent Hydroquinone ---- inhibitor Stored in tightly sealed Amber coloured bottle – to prevent evaporation , premature poymerization [by light or U.V radiation]


COMPOSITION Powder Poly (Methyl Methacrylate) Other copolymers (5%) Benzoyl Peroxide ---- Initiator Compounds of Mercuric sulphide, Cadmium sulphide ---- Dyes Zinc / Titanium oxides --- Opacifiers Dibutyl pthalate --- plasticizer Dyed organic filler Inorganic particles like glass fibers / beads

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High mol. Wt. polymers dissolves slowly in monomer So, to increase in solubility Additive – (Ethyl acrylate copolymer) Plasticizer – Dibutyl phthalate Adding low mol. Wt. PMMA POLYMERIZATION REACTION Powder (Poly)+ Liquid (mono) +heat = polymer + heat

Technical consideration : 

Technical consideration COMPRESSION MOULDING TECHNIQUE Prep of wax pattern [waxed dentures] Prep of Split mould [Investing & Dewaxing] Applictn of Separating Media Mixing of powder & liquid Packing Curing Cooling Deflasking Finishing & polishing

COMPRESSION MOULDING TECHNIQUEPrep of wax pattern [waxed dentures] : 

COMPRESSION MOULDING TECHNIQUEPrep of wax pattern [waxed dentures] Prosthetic teeth are selected & arranged – esthetic & functional requirements Impression making, cast generation, record bases Articulator mounting, teeth arrangement, wax contouring Waxed dentures are sealed to master casts – removed from articulator

Prep of Split mould [Investing ] : 

Prep of Split mould [Investing ] Master cast is coated with thin layer of separator Base flasking Counter flasking – dental stone in intimate contact with all external surfaces, Incisal & Occlusal surfaces are slightly exposed – to facilitate deflasking Third Pour – to fill remaining flask Lid is gently placed & stone is allowed to set

Prep of Split mould [Dewaxing] : 

Prep of Split mould [Dewaxing] On complete setting – record base & wax has to be removed Flask is immersed in boiling water for 4 mins Base flask & counter flask segments are separated Residual wax is removed by wax solvents Mold cavity is cleaned with mild detergent solution & rinsed with boiling water

Application of Separating Media : 

Application of Separating Media To prevent water from the mould to enter into Acrylic resin [affects rate of polymerization & colour of resin] To prevent Monomer penetrating into the mould [plaster to adhere to the acrylic resin & produce rough surface] Can lead to compromises in Physical & Esthetic properties TYPES Tinfoil Tinfoil substitutes - Cellulose lacquers, Solution of Alginate compounds, Evaporated milk, Soap, Sodium silicate, Starches

Mixing of powder & liquidPolymer:Monomer ratio : 

Mixing of powder & liquidPolymer:Monomer ratio Accepted ratio – 3:1 by volume or 2:1 by weight If more Monomer [lower polymer/monomer ratio] Greater poly. Shrinkage Additional time is reqd. to reach the packing consistency Tendency for porosity If less Monomer [lower polymer/monomer ratio] Less wetting – Granular acrylic Dough will be difficult to manage – not fuse into continous unit of plastic

Physical stages of Polymerization : 

Physical stages of Polymerization Stage 1 – Sandy / Wet sand stage – polymer gradually settles in monomer, forms a fluid, incoherent mass. Also described as ‘coarse or grainy’ Stage 2 – Stringy / Sticky stage – monomer enters into polymer, if the mixture is touched --- it forms cobweb like structure Stage 3 – Dough-like / Gel stage : mass becomes more saturated, smooth & dough like. It does not adhere to container or spatula. Mass is plastic & homogenous at this stage. Time reqd – 10 mins

Physical stages of Polymerization : 

Physical stages of Polymerization Stage 3 – Rubbery / Elastic Stage: Monomer disappears by penetration into the polymer & evaporation. Mass is cohesive, rubber-like, non-plastic & cannot be moulded as it rebounds when compressed or stretched, it does not flow freely Stage 5 – Stiff – due to evaporation of free monomer. Mix appears very dry & is resistant to mechanical deformation

Dough-forming Time : 

Dough-forming Time Time reqd for the resin mixture to reach a dough-like stage ADA specification no. 12 – in less than 40 mins Clinically – most resins reach a doughlike consistency in less than 10 mins Depends on Controlled by manufacturer 1 Deg. Of polyn. – higher the polyn, lower the Dough-forming Time 2 Particle size – Smaller the particle size, shorter the Dough-forming Time Controlled by operator 3 Polymer:Monomer ratio : If this is high (less monomer), there is shorter dough forming time 4 Temperature – Higher the temp., shorter dough forming time 5 Plasticizer – reduces the dough forming time

Working time : 

Working time May be defined as the time that a denture base material remains in dough like stage At least 5 mins Affected by temp., extended via refigeration (moisture can degrade properties) Can be avoided by storing in air tight container


PACKING Introduction of denture base resin into mould cavity Overpacking: excessive thickness, malpositioning of teeth Underpacking : noticeable denture base porosity Rope-like form ----- packed into flask Polyethylene sheet is placed ---- flask is assembled Application of pressure --- resin dough flows evenly into mold space Flask portions are separated – sheet is removed with a rapid, continuous tug Excess resin – flash Second trial closure Final closure – no polyethylene sheet

Polymerization Procedure / Curing : 

Polymerization Procedure / Curing DB Resins – Benzoyl Peroxide{Initiator} – when heated above 60*C – decompose to form Free Radicals - reacts with Monomer to initiate chain-growth polymerization Heat is termed as Activator After Final closure – flasks are kept at Room temp. for 30 to 60 mins – Bench Curing Longer flow period – equalization of pressure in Mold Allows time for more uniform dispersion of monomer Longer exposer of resin teeth to monomer- better bond

Curing cycle / Heating process : 

Curing cycle / Heating process 1. Processing denture base resin in Constant temp water bath at 74*C (165*F) for 8 hrs or longer, with no terminal boiling 2. Processing the resin at 74*C for approx. 2 hrs & then increasing the temp. of water bath to 100*C & processing for 1 hour more Other Methods of supplying heat for activation Steam, Dry air Oven, Dry heat (electrical), Infrared heating, Induction/Dielectric heating, Microwave radiation [Specially formulated resin & Non-metallic Flask: Speedy process]

Internal Porosity : 

Internal Porosity Resin & Dental stone – Poor thermal conductors, heat of reaction cannot be dissipated, so temp. of resin rises above that of stone & surrounding water Temp. exceeds the boiling pt. of Monomer (100.8*C) Porosity – not seen on surface, as heat is dissipated Centrally, heat generated in thick portions cannot be dissipated --- boiling of unreacted monomer ---- porosity

External Porosity : 

External Porosity 1. Lack of Homogenity – Portions containing more monomer will shrink more than the adjacent areas, results in voids & resin appears white.(proper powder:liquid, homogenous mix – pack in dough stage) 2. Lack of adequate pressure – Lack of dough during final closure (Flash indicates adequate material) OTHER PROBLEMS : Crazing[Cracks] & production of Internal Stresses

Cooling : 

Cooling After Curing – Denture flasks should be cooled slowly to room temp. Rapid Cooling – warpage of denture base because of differences in thermal contraction of resin & stone Slow Cooling – Minimizes potential difficulties So, Bench-Cooling for 30 mins, then flask should be immersed in cool tap water for 15 mins Cooling overnight is ideal

Deflasking, Finishing & Polishing : 

Deflasking, Finishing & Polishing Deflasking – has to be done with care to avoid flexing & breaking of Acrylic denture Finishing – Metal Trimmer, Acrylic/Alpine Stone, Dry & Wet Sand paper Polishing – suspension of finely ground pumice in water

Injection molding Technique : 

Injection molding Technique Mold space can be filled by injecting resin under pressure in specially designed flasks Sprue hole / Vent hole are formed in stone mold Soft resin (dough stage) is contained in injector & is forced into mold Resin under pressure until it has hardened Polystyrene resin – polymer is first softened under heat & injected while hot, then it solidifies in mold upon cooling No trial closures are required

Injection molding Technique : 

Injection molding Technique Advantages Disadvantages Dimensional accuracy Low free monomer content Good impact strength High capital costs Difficult mold design problems Less craze resistance Less creep resistance Special flask is required

Polymerization by Microwave energy : 

Polymerization by Microwave energy It is cleaner & faster than polymerization with conventional technique Fit of denture is comparable or superior Acrylic resins are less prone to porosity Advantages : good appearance, high glass transition temp, ease of fabrication, low capital cost & good surface finish Disadvantages : Radiolucency, Free monomer content/formaldehyde may cause sensitization, fatigue life too short & low impact strength

Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure) : 

Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure) Composition Liquid Methyl Methacrylate Dimethyl-p-toluidine ---- Activator Dibutyl phthalate----------- Plasticizer Glycol dimethacrylate ----- Cross linking agent Hydroquinone ------------- Inhibitor

Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure) : 

Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure) Composition Powder Poly (Methyl Methacrylate) Other copolymers - 5% Benzoyl Peroxide ---------------- Initiator Compounds of mercuric sulphide, Cadmium sulfide -------------- Dyes Zinc / Titanium Oxide ------------- Opacifiers Dibutyl phthalate ---------- Plasticizer Dyed organic fillers Inorganic particles like glass fibers / beads

Uses of Autopolymerizing Resin : 

Uses of Autopolymerizing Resin With fillers (pumice), for construction of custom trays For denture repair, relining & rebasing For making removable orthodontic appliances For adding a post-dam to adjust upper denture

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Advantages Disadvantages Better initial fit Less thermal contraction For repairing dentures, as it avoids warpage due to re-curing Colour stability is inferior, due to subsequent oxidation of the tertiary amine Lesser degree of polymerization, so these have slightly inferior physical properties

Manipulation : 

Manipulation 1. Sprinkle – On technique 2. Adapting technique 3. Fluid resin technique 4. Compression moulding technique 5. Injection moulding technique

Fluid resin technique (pour-type acrylic resin) : 

Fluid resin technique (pour-type acrylic resin) These have high molecular wt powder that are smaller in size & when they are mixed with monomer, the mix is very fluid They are used with lower powder-liquid ratio – 2 : 1 -2.5 : 1 This aids to prevent undue increase in viscosity during mixing & pouring stages This technique commonly involves use of Agar Hydrocolloid for the mould preparation Fluid mix is poured in the mould quickly & allowed to polymerize under pressure at 0.14 Mpa (20 psi).

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Advantages Disadvantages Better tissue fit Fewer open bites. Less fracture of porcelain teeth during deflasking procedure Reduced material cost Simplification of lab procedure for flasking (no trial closure),deflasking & finishing of denture. Air occlusion(bubbles) Shifting of teeth during processing Infraocclusion (closed bites) Occlusal imbalance due to shifting of teeth Incomplete flow of denture base material over neck of anterior teeth Formation of films of denture base material over cervical portions of plastic teeth that had not been previously covered with wax Poor bonding to plastic teeth.. Technique sensitivity.

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Autopolymerizing Heat cured Heat is not necessary for polymerisation Porosity is greater. Have lower average molecular weight. Higher residual monomer content. Material is not strong.(coz of their lower molecular weight mols.) Poor color stability. Easy to deflask. Rheological properties: A) Show greater distortion. B) More initial deformation. C) Increased creep & slow recovery. Heat is necessary for polymerisation Porosity is less Higher average molecular weight (5 lakhs-10 lakhs) Lower residual monomer content Material is strong Good color stability Difficult to deflask A) Show lesser distortion B) Less initial deformation C) Less creep & quicker recovery

Light activated denture base resin : 

Light activated denture base resin Composition Urethane dimethacrylate matrix Acrylic copolymer Microfine silica fillers Photoinitiator system – Camphoroquinone amine Supplied in pre-mixed sheets having clay-like consistency Provided in opaque light packages – to avoid premature polymerization Adapted to cast when in plastic form Polymerized in light chamber with light of 400-500 nm from high intensity quartz halogen bulbs

Properties of Denture Base Resin : 

Properties of Denture Base Resin Methyl Methacrylate Monomer: Clear, transparent, volatile, has sweetish odour. Melting pt: -48*C Boiling pt: 100.8*C Heat of polymerization: 12.9Kcal/mol Volume shrinkage during polymerization: 21% Poly (Methyl Methacrylate ) Tasteless, odourless, clear transparent, has adequate compressive & tensile strength, has low hardness-can be easily scratched & abraded Shrinkage ---- thermal shrinkage on cooling & polymerization shrinkage Volume shrinkage is 8% & Linear Shrinkage is 0.53%

Resin Teeth – PMMA copolymerized with a cross linking agent : 

Resin Teeth – PMMA copolymerized with a cross linking agent Resin teeth High fracture toughness Crazing, if not crosslinked Clinically significant wear Easily ground & polished Silent on contact Dimensional change (water sorpn) Cold flow under stress Loss of Vertical dimension Self adjusting Chemical bond to denture Minimal abrasion of opposing Porcelain teeth Brittle Crazing by thermal shock Insignificant wear Grinding is difficult (glaze) Sharp impact sound Dimensionally stable No permanent deformation Stable VDO Difficult to fit in diminished interarch space Mechanical retention necessary Abrades opposing teeth

Recent Advances : 

Recent Advances High Impact strength materials: butadenestyrene rubber-reinforced PMMA Rapid heat polymerized resins: hybrid acrylics that are polymerized in boiling water immediately after packing (place in boiling water & then full boil for 20 mins)

Denture Reliners : 

Denture Reliners Heat cure --- compression molding & low curing temp. There is a tendency for previously cured material to warp Self cure --- directly in mouth, but fades, smells SOFT/RESILIENT LINERS Purpose is to Absorb some of the energy produced by masticatory impact Used – irritation in mucosa, area of severe undercut, congenital/acquired defects of palate Plasticized acrylic resin- PEMA/PMMA/Copolymer + aromatic ester-ethanol liquid containing 60-80% plasticizer (dibutyl phthalate) Vinyl resin, Silicone rubbers – RTV, heat cured silicones, Polyurethanes Problems: inadequate bonding, loose stiffness as plasticizer is leached out, loss of denture base strength, trimming, polishing is difficult, disagreeable taste & odour, cannot be cleaned easily (fungal growth)

Materials in Maxillofacial prosthesis : 

Materials in Maxillofacial prosthesis To correct facial defects resulting from cancer surgery, accidents / congenital deformities Ideally – easy, inexpensive to fabricate, biocompatible, strong, stable, skinlike in appearance, soft & must be colour stable, easy to clean PMMA Latexes – but they are weak, degenerates rapidly with age Plasticized polyvinylchloride – has got plasticizers, crosslinking agent & UV stabilizers Silicone rubber --- RTV & heat-vulcanized Polyurethane polymers

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