denture base materials ppt
TRANSCRIPT
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DENTURE BASE MATERIALS
BOOKS TO BE REFFERED PHILLIPS
SUBBARAO
MANNAPALLI
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DENTURE BASE MATERIALSVULCANITE (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 estheticsDimensional changesThermal expansion Contraction of 2-4% by volume during addition of
sulphur to the rubber
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DENTURE BASE MATERIALS
NITROCELLULOSE– Dimensionally unstableContains unpleasant tasting plasticizersExcessive warpageHigh water absorption Poor colour stability
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DENTURE BASE MATERIALSPHENOL FORMALDEHYDEBecomes discoloured & unestheticIt is thermoset type – so it is difficult to repair
PORCELAIN – This material is tolerated well,But it is difficult to fabricateCan be easily broken
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CLASSIFICATION OF DENTURE BASE MATERIALS
METALLICCobalt – ChromiumGold AlloysAluminiumStainless SteelTitanium
TEMPORARYSelf-cure Acrylic resinShellac Base PlateHard Base Plate Wax
NON-METALLICAcrylic ResinVinyl Resin
PERMANENTHeat-cure Acrylic resin
(1937)METALLIC
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CHEMISTRY OF SYNTHETIC RESINS
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CLASSIFICATIONSynthetic Resins are often called as PLASTICSA 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
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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, infusibleCrosslinked PMMA, Silicones.Superior abrasion resistanceSuperior Dimensional
Stability
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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
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IDEAL REQUIREMENTSInsoluble & 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
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USESPreparation of
DenturesArtificial teethTooth restorationsOrthodontic space
maintainanceCrown & Bridge
facings, Temporary Crowns
Maxillofacial prosthesis, Athletic Mouth Protector
Inlay patternsImplantsDies, Impression
traysEndodontic filling
materials
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Basic Nature of Polymers1. Polymer – Molecule that is made up of may partsChemical 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.
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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
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Polymerization – series of chain reaction by which a macromolecule or polymer is formed from a single molecule
Condensation Addition Slow methodRepeated Elimination of
small moleculesBy- products – NH3, H2O,
halogen acids
Functional groups are repeated (Amide, Urethane, Ester or Sulfide)
Here by-product formation is not necessary
In Dental proceduresNo 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)
PROPOGATION
TERMINATION
CHAIN TRANSFER
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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 periodIncrease temp. --- shorter is length of
Induction periodInitiation energy is 16000 to 29000 cal/mol.
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3 INDUCTION SYSTEMSHEAT ACTIVATION –free radicals are
liberated by heating Benzoyl peroxideCHEMICAL 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
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INHIBITION OF POLYMERIZATIONOccurs when there isComplete exhaustion of monomer OrFormation of High Molecular Weight polymer
Inhibited by : IMPURITIES (react with Activated Initiator / Nucleus)Hydroquinone (0.006%) is in Monomer for storage
OXYGEN retards polymerizationInfluence the length of Initiation period & degree of
polymerization
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COPOLYMERIZATIONIs required to improve physical properties
Two or more chemically different monomers polymerize to form COPOLYMER
TYPESRandomGraftBlock
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Applications of copolymerizationETHYL ACRYLATE+ PMMA = FLEXIBILITYBLOCK & 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
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Plasticizers Increases solubility of polymers in monomerReduces brittlenessBut it also reduces Strength & Hardness &
Softening point
EXTERNAL – penetrates macromolecules & neutralizes secondary bond. It Evaporates / Leaches out
INTERNAL - Copolymer
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Types of resinsAcrylic resinVinyl resinPolystyrene Epoxy resins
OTHER RESIN SYSTEMSPolycarbonatesPolyurethanesCyanoacrylates
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ACRYLIC RESINSAre 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]
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Poly (Methyl Methacrylate) ResinsWidely used --- easy to processIt 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 activationHeat activated resinsChemically activated resinsLight activated resins
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HEAT – ACTIVATED DENTURE BASE RESINSAVAILABLE AS Powder+ Liquid & Gels – Sheets &cakes
COMPOSITIONLiquid Methyl MethacrylateDibutyl pthalate --- plasticizerGylcol dimethylacrylate [1-2%] ---- cross-linking agentHydroquinone ---- inhibitor
Stored in tightly sealed Amber coloured bottle – to prevent evaporation , premature poymerization [by light or U.V radiation]
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COMPOSITIONPowderPoly (Methyl Methacrylate)Other copolymers (5%)Benzoyl Peroxide ---- InitiatorCompounds of Mercuric sulphide, Cadmium sulphide ---- DyesZinc / Titanium oxides --- OpacifiersDibutyl pthalate --- plasticizerDyed organic fillerInorganic particles like glass fibers / beads
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High mol. Wt. polymers dissolves slowly in monomer
So, to increase in solubilityAdditive – (Ethyl acrylate copolymer)Plasticizer – Dibutyl phthalateAdding low mol. Wt. PMMAPOLYMERIZATION REACTION Powder (Poly)+ Liquid (mono) +heat =
polymer + heat
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Technical considerationCOMPRESSION MOULDING TECHNIQUEPrep of wax pattern [waxed dentures]Prep of Split mould [Investing & Dewaxing]Applictn of Separating MediaMixing of powder & liquidPackingCuringCoolingDeflaskingFinishing & polishing
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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
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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 flaskLid is gently placed & stone is allowed to set
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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 solventsMold cavity is cleaned with mild detergent
solution & rinsed with boiling water
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Application of Separating MediaTo 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
TYPESTinfoilTinfoil substitutes - Cellulose lacquers, Solution of
Alginate compounds, Evaporated milk, Soap, Sodium silicate, Starches
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Mixing of powder & liquidPolymer:Monomer ratio
Accepted ratio – 3:1 by volume or 2:1 by weightIf more Monomer [lower polymer/monomer ratio]Greater poly. ShrinkageAdditional time is reqd. to reach the packing consistencyTendency for porosity
If less Monomer [lower polymer/monomer ratio]Less wetting – Granular acrylicDough will be difficult to manage – not fuse into continous
unit of plastic
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Physical stages of PolymerizationStage 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
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Physical stages of PolymerizationStage 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
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Dough-forming TimeTime reqd for the resin mixture to reach a dough-like stage ADA specification no. 12 – in less than 40 minsClinically – most resins reach a doughlike consistency in less than 10 mins
Depends on Controlled by manufacturer1 Deg. Of polyn. – higher the polyn, lower the Dough-forming
Time2 Particle size – Smaller the particle size, shorter the Dough-
forming Time
Controlled by operator3 Polymer:Monomer ratio : If this is high (less monomer),
there is shorter dough forming time4 Temperature – Higher the temp., shorter dough forming
time5 Plasticizer – reduces the dough forming time
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Working time May be defined as the time that a denture
base material remains in dough like stageAt least 5 minsAffected by temp., extended via refigeration
(moisture can degrade properties)Can be avoided by storing in air tight
container
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PACKINGIntroduction of denture base resin into mould cavityOverpacking: excessive thickness, malpositioning of teethUnderpacking : noticeable denture base porosity
Rope-like form ----- packed into flaskPolyethylene sheet is placed ---- flask is assembledApplication of pressure --- resin dough flows evenly into
mold spaceFlask portions are separated – sheet is removed with a
rapid, continuous tugExcess resin – flashSecond trial closureFinal closure – no polyethylene sheet
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Polymerization Procedure / CuringDB 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 MoldAllows time for more uniform dispersion of monomerLonger exposer of resin teeth to monomer- better bond
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Curing cycle / Heating process1. 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 activationSteam, Dry air Oven, Dry heat (electrical), Infrared
heating, Induction/Dielectric heating, Microwave radiation [Specially formulated resin & Non-metallic Flask: Speedy process]
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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 dissipatedCentrally, heat generated in thick portions cannot
be dissipated --- boiling of unreacted monomer ---- porosity
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External Porosity1. 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
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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
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Deflasking, Finishing & PolishingDeflasking – 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
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Injection molding TechniqueMold space can be filled by injecting resin under
pressure in specially designed flasksSprue hole / Vent hole are formed in stone
moldSoft resin (dough stage) is contained in injector
& is forced into mold Resin under pressure until it has hardenedPolystyrene resin – polymer is first softened
under heat & injected while hot, then it solidifies in mold upon cooling
No trial closures are required
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Injection molding TechniqueAdvantages DisadvantagesDimensional
accuracyLow free monomer
contentGood impact
strength
High capital costsDifficult mold design
problemsLess craze
resistanceLess creep
resistanceSpecial flask is
required
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Polymerization by Microwave energyIt is cleaner & faster than polymerization
with conventional techniqueFit of denture is comparable or superiorAcrylic resins are less prone to porosityAdvantages : 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
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Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure)
CompositionLiquidMethyl MethacrylateDimethyl-p-toluidine ---- ActivatorDibutyl phthalate----------- PlasticizerGlycol dimethacrylate ----- Cross linking agentHydroquinone ------------- Inhibitor
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Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure)CompositionPowderPoly (Methyl Methacrylate)Other copolymers - 5%Benzoyl Peroxide ---------------- InitiatorCompounds of mercuric sulphide,
Cadmium sulfide -------------- DyesZinc / Titanium Oxide ------------- OpacifiersDibutyl phthalate ---------- PlasticizerDyed organic fillersInorganic particles like glass fibers / beads
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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
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Manipulation 1. Sprinkle – On technique2. Adapting technique
3. Fluid resin technique
4. Compression moulding technique5. Injection moulding technique
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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 DisadvantagesBetter tissue fitFewer open bites.Less fracture of porcelain
teeth during deflasking procedure
Reduced material costSimplification of lab
procedure for flasking (no trial closure),deflasking & finishing of denture.
Air occlusion(bubbles)Shifting of teeth during
processingInfraocclusion (closed bites)Occlusal imbalance due to
shifting of teethIncomplete 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 curedHeat is not necessary for
polymerisationPorosity 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 lessHigher average molecular
weight (5 lakhs-10 lakhs)Lower residual monomer
contentMaterial is strong
Good color stabilityDifficult to deflask
A) Show lesser distortionB) Less initial deformationC) Less creep & quicker
recovery
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Light activated denture base resinCompositionUrethane dimethacrylate matrixAcrylic copolymerMicrofine silica fillersPhotoinitiator system – Camphoroquinone amine
Supplied in pre-mixed sheets having clay-like consistencyProvided in opaque light packages – to avoid premature
polymerizationAdapted to cast when in plastic formPolymerized in light chamber with light of 400-500 nm
from high intensity quartz halogen bulbs
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Properties of Denture Base Resin Methyl Methacrylate Monomer: Clear, transparent,
volatile, has sweetish odour.Melting pt: -48*CBoiling pt: 100.8*CHeat of polymerization: 12.9Kcal/molVolume 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%
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Resin Teeth – PMMA copolymerized with a cross linking agent
Resin teethHigh fracture toughnessCrazing, if not crosslinkedClinically significant wearEasily ground & polishedSilent on contactDimensional change
(water sorpn)Cold flow under stressLoss of Vertical dimensionSelf adjustingChemical bond to dentureMinimal abrasion of
opposing
Porcelain teethBrittleCrazing by thermal shockInsignificant wearGrinding is difficult (glaze)Sharp impact soundDimensionally stableNo permanent deformationStable VDODifficult to fit in diminished
interarch spaceMechanical retention
necessaryAbrades opposing teeth
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Recent AdvancesHigh 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)
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Denture RelinersHeat cure --- compression molding & low curing temp.There is a tendency for previously cured material to warpSelf cure --- directly in mouth, but fades, smells
SOFT/RESILIENT LINERSPurpose is to Absorb some of the energy produced by masticatory
impactUsed – irritation in mucosa, area of severe undercut,
congenital/acquired defects of palatePlasticized 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)
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Materials in Maxillofacial prosthesisTo correct facial defects resulting from cancer
surgery, accidents / congenital deformitiesIdeally – easy, inexpensive to fabricate, biocompatible,
strong, stable, skinlike in appearance, soft & must be colour stable, easy to clean
PMMALatexes – but they are weak, degenerates rapidly
with agePlasticized polyvinylchloride – has got plasticizers,
crosslinking agent & UV stabilizersSilicone rubber --- RTV & heat-vulcanizedPolyurethane polymers
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