denture base material

7/27/2019 Denture Base Material

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Dr Purnendu Bhushan1st yr MDS

Dept. of Prosthodontics

SCB Dental College


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Denture base:thepart of a denture thatrests on thefoundation tissues

and to which teethare attached


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HISTORY The first dental prosthesis was believed

to have been constructed in Egypt about 2500 BC.

Skilfully designed dentures were made as early as

700 BC. During medieval times, dentures were seldom

considered as a treatment option. They were hand

carved and tied in place with silk threads and had

to be removed before eating.


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WOOD:

For years, dentures weredesigned from wood

because it was readily available,relatively inexpensive

and could be carved to desiredshape. However, it

warped and cracked in moisture,lacked aesthetics

and got degraded in the oralenvironment


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BONE:

Dentures made from bone became very popular due to

its availability, reasonable cost and carvability. It is

reported that Fauchard fabricated dentures by

measuring individual arches with a compass and

cutting bone to fit the arches. It had better dimensional

stability than wood, however, esthetic and hygienic

concerns remained.


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IVORY:

Ivory denture bases and prostheticteeth were

fashioned by carving this materialto desired shape.

These were relatively stable in theoral environment,

offered esthetic and hygienic

advantages compared to Wood or bone. However, ivory was

not readily available and wasexpensive.


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PORCELAIN:

Alexis Duchateau (1774) was the first to fabricate porcelain dentures.

In 1788 AD, a French dentist, Nicholas Dubois de Chemant, made a baked-porcelain complete denture in a single block. The advantages were that it could be shaped easily, ensured intimate contact with the underlying tissues, was stable, had minimal water sorption, smooth surfaces after glazing, less porosity, low solubility and could be tinted but its drawbacks were brittleness and difficulty in grinding and polishing. Loomis (1854), Charles H Land (1890) and Alexander Gutowski (1962) experimented with different types of porcelain dentures.


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GOLD: In 1794 AD, John

Greenwood began to swagegold

bases for dentures. He also

made dentures of George Washington.

Usually 18 to 20 carat goldwas alloyed

with silver and teeth were

riveted to it.


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VULCANITE (1855) Rubber with 32 % Sulphur & Metallicoxides.Advantages It is non-toxic, non-irritant,has excellent Mechanical propertiesmaterial is sufficient hard to polish.

Limitation Absorbs saliva becomes unhygiene, leads tobacterial growth & unpleasant odourPoor esthetics

Dimensional changes

Contraction of 2-4% by volume during addition of sulphur tothe rubber


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.

TORTOISE SHELL:CF Harrington (1850) introduced the firstthermoplastic denture material, the tortoise shell base.GUTTA PERCHA:

Edwin Truman (1851) used Gutta percha as a denturebase but it was unstable.CHEOPLASTIC:

Alfred A Blandy (1856) made dentures from a lowfusing alloy of silver, bismuth and antimony but it wasnever accepted.ALUMINIUM:Dr. Bean (1867) invented the casting machine and didthe first casting of a denture base in aluminium.


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CELLULOID:

J. Smith Hyatt (1869) introduced celluloid thatwaslater used as a denture base material because ofitstranslucency and pink colour. However, this

materialdid not gain much popularity because ofdistortion anddiscolouration.

BAKELITE:Dr. Leo Bakeland (1909) introduced this phenolformaldehyde resin which was easily availablebutlacked colour quality.


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STAINLESS STEEL and

BASE METAL ALLOYS: Ni-Cr and Co-Cr were

obtained byE. Haynes(1907)

but they gained popularityafter 1937 because of their low density, low material

cost, higher resistance to tarnish and corrosion and

high modulus of elasticity. Allergy to Nickel and

difficulty in adjustmentposed a practical problem

VINYL RESIN:

Mixtures of polymerizedvinyl chloride and vinyl

acetate were underexperimentation during1930 due

to their pleasing colourbut had difficult

processing methods


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POLYMETHYL METHACRYLATE:

Rohm and Hass (1936) introduced PMMA in sheet form and Nemours (1937) in powder form.

Dr. Walter

Wright (1937) introducedPolymethyl methacrylate as

a denture base material which became the major

polymer to be used in the next ten years.


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Classification of denture base

material METALLIC

Cobalt Chromium Gold Alloys

Aluminium Stainless Steel Titanium

TRY IN Self-cure Acrylic resin Shellac Base Plate Hard Base Plate Wax

NON-METALLIC

Acrylic ResinVinyl Resin

DEFINATIVE

Heat-cure Acrylic resin(1937)

METALLIC


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classification Synthetic Resins are often called as PLASTICS

A substance that although dimensionally stable innormal use was plastic at some stage of manufacture

Thermoplastic they soften again when reheated(above GTT)

Thermosetting they are resistant to change afterfurther application of heat


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Thermoplastic Resin Thermosetting Resin

Are fusible, soluble in

organic solvents Better flexural & impact

properties

Most plastics in

Dentistry belong to thisgroup

PMMA, Polyvinyl,Polystyrene

These become permanentlyhard when heated above

critical temp. & they do notsoften again on heating

Usually cross-linked in state

These are insoluble, infusible

Crosslinked PMMA,Silicones.

Superior abrasion resistance

Superior DimensionalStability

I ti t


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I ea properties o a enture ase

materialMechanical Physical Biocompatible Aesthetic Other

Adequate flexural&fatigueStrength

Same thermalexpansion co-efficientas denture toothmaterial

Non-toxic Pigmentable Radioopaque

High modulus of

elasticity

Conduct heat Non-irritant Translucent Easy

to manufactureHigh proportionallimit

Low density (lightWeight)

In-soluble in oralfluids

Highlypolishable

Low cost

High impactstrength

Melting point higherthan ingestedfood/drinks

Non-absorbent

Abrasion resistant Dimensionally stable(during processingand function)

Inert

Capable of

maintaining highpolish


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ALTERNATIVE USESArtificial teeth

Tooth restorations

Orthodontic spacemaintainance

Crown & Bridge facings,

provisional crown &bridges

Maxillofacial prosthesis,Athletic Mouth

Protector Inlay patterns

Dies, Impression trays Endodontic filling

materials


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Denture Base Resin
http://www.google.com.sa/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=9-rn8NXDwR4FxM&tbnid=RsnBbqx4UM96EM:&ved=0CAUQjRw&url=http://webbdental.blogspot.com/2010/09/confused-about-complete-denture.html&ei=NQZ-UZLZDIrdPaPdgZAF&psig=AFQjCNEbQYXsJczEn3dJNbEVZUPg_XHdnw&ust=1367299971852050


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Basic Nature Of Polymer . 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 polymers(determines its physical properties)

Degree of Polymerization --- total no. ofmonomers inpolymers .

Strength increases with increase in Deg. Of Poly.


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Polymerization series of chain reaction by which a

macromolecule or polymer is formed from a single

molecule

Condensation Addition

Slow method

Repeated Elimination ofsmall molecules

By- products NH3, H2O,halogen acids

Functional groups arerepeated (Amide, Urethane,Ester or Sulfide)

Here by-product formation isnot necessary.

In Dental procedures

No change in chemicalcomposition & no by-products

Giant molecules (unlimitedsize)

Simple, but not easy tocontrol


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CHEMICAL STAGES OF

POLYMERIZATIONINDUCTION (INITIATION)

PROPOGATION

TERMINATION

CHAIN TRANSFER


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INDUCTION (INITIATION)

Is the time during which the molecules of the initiatorbecomes energized or activated & start to transfer theenergy 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.


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3 INDUCTION SYSTEMS

HEAT ACTIVATION free radicals are liberated byheating Benzoyl peroxide

CHEMICAL ACTIVATION atleast 2 reactants ---chem. Reaction--- liberate free radicalsBenzoyl peroxide + Aromatic Amine(dimetyl-p-

toluidine) LIGHT ACTIVATION photons of light energy

activate the initiator free radicals . Under visible lightCamphoroquinone & an amine --- free radical


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(C6H5COO)2 2 C6H5-

A free radical is a compound with an unpairedelectron, usually a fragment of a larger

molecule which has been split by heating.

This unpaired electron makes the radical

very reactive.


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Propagation The chain reaction continues with

the evolution of heat (about 25 C)

until the monomer has been

changed into polymethyl

methacrylate.


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C6H5

C

H

H

C

CH3

COOCH3

C

H

H

C

CH3

COOCH3

C

H

H

C

CH3

COOCH3

C

H

H

C

CH3

COOCH3

Linear chain of polymethyl methacrylate


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Termination The chain reaction can be terminated

either by direct coupling or by the

exchange of hydrogen atom from one

growing chain to another.


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CHAIN TRANSFERThe active free radical of a growing chain istransferred to another molecule (eg monomer orinactivated polymer chain) and a new free radicalfor further growth is created, termination occurs inthe latter.


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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


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Copolymerization Is required to improve physical properties

Two or more chemically different monomers, each withdesirable propertypolymerize to form COPOLYMER eg.Butyl methacrylate & methyl methacrylate

TYPES

Random

Block

Graft


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Application Of Copolymerization BUTYL METHACRYLATEACRYLATE+ PMMA =

FLEXIBILITY BLOCK & GRAFT Polymers = Improves IMPACT

STRENGTH (good adhesive properties + surfacecharacteristics)

CROSS-LINKING (chemical bond between linearpolymers)

Applications Improves strength, reducessolubility & water sorption Highly Cross-linked Material provides - increased

resistances ------ to solvents, crazing & surface stresses


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Plasticizers These are often added to resin to reduce their

softening or fusion temperature

Reduces brittleness

But it also reduces Strength & Hardness

EXTERNAL penetrates macromolecules &neutralizes secondary bond. It Evaporates / Leaches

out eg. Dibutyl pthalate

INTERNAL - Copolymer


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Acrylic ResinAre 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)Resins Widely used --- easy to process

It is Thermoplastic resin

Liquid [monomer] Methyl Methacrylate is mixed withPowder [polymer ]

Monomer plasticizes the polymer to dough-like consistencywhich can be easily moulded

Types ---- based on method used for its activation Heat activated resins

Chemically activated resins

Light activated resins


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Heat-Activated Denture Base Resin AVAILABLE AS Powder+ Liquid & Gels Sheets &cakes

COMPOSITION

Liquid Methyl Methacrylate

Gylcol dimethylacrylate [1-2%] ---- cross-linking agent

Hydroquinone ---- inhibitor

Stored in tightly sealed Amber coloured bottle to preventevaporation , premature poymerization [by light or U.Vradiation]


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Composition Powder

Poly (Methyl Methacrylate)

Other copolymers (5%)

Benzoyl Peroxide ---- Initiator

Zinc / Titanium oxides --- Opacifiers Dibutyl pthalate --- plasticizer

Dyed organic filler

Inorganic particles like glass fibers / beads


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Technical Consideration COMPRESSION MOULDING TECHNIQUE

Prep of wax pattern [waxed dentures]

Prep of Split mould [Investing & Dewaxing]

Application of Separating Media

Mixing of powder & liquid

Packing

Curing Cooling

Deflasking

Finishing & polishing


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Prosthetic teeth are selected & arranged esthetic &functional requirements

Impression making, cast generation, record bases

Articulator mounting, teeth arrangement, waxcontouring

Waxed dentures are sealed to master casts removed from articulator


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Preparation of the mold Application of separating

mediumFailure to place an

separating medium1. Water from mold

surface may difuse in todenture resin, it may affectthe polymerization rate as

well as optical and physicalproperties

2. Free monomer may

soak into mold surfaceportions of investing mediummay become fused to thedenture base


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Tin foil

Paint on separating media like cellulose, lacquers,solution containing alginate compounds, soaps,starches were introduced..

Most popular -- water soluble alginate solution

Produce thin, relatively insoluble calcium alginatefilms..


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Mixing Of Powder & Liquid 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 packingconsistency 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


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Polymer-Monomer Interaction When mixed in proper proportions, the resultant masspasses through five distinct stages

1.Sandy

2. stringy 3. Dough like 4. rubbery 5. Stiff

1.Sandy

During sandy stage, little or no interaction occurs on amolecular level. Polymer beads remain unaltered.


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2. Stringy stage

Later, mixture enters stringy stage. Monomer attacks thesurfaces of individual polymer beads.

Stage charcterized by stringiness,

3. dough like stage

The mass enters a dough like stage. On molecular level increased number of polymer chains

are formed. Clinically the mass becomes as a pliabledough. It is no longer tacky ( sticky)

This stage is ideal for compression molding.


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4.Rubbery or elastic stage

Following dough like stage, the mixture enters rubberyor elastic stage. Monomer is dissipated by evaporationand by further penetration into remaining polymerbeads. In clinical use the mass rebounds when

compressed or stretched5. Stiff Stage

Upon standing for an extended period, the mixturebecomes stiff.

This may be due to the evaporation of free monomer.From clinical point, the mixture appears very dry andresistant to mechanical deformation


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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 isshorter dough forming time 4 Temperature Higher the temp., shorter dough forming time 5 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 stage

At least 5 mins

Affected by temp., extended via refigeration (moisturecan degrade properties)

Can be avoided by storing in air tight container


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PACKING Placement and adaptation of

denture base material within themold cavity is termed packing

Overpacking- leads to excessivethickness and malpositioing pfprosthetic teeth

Underpacking- leads to

noticeable denture base porosity Trial packing is done to ensure

proper packing of resin mass inthe mold.

After the final closure of theflasks, they should remain at

room temperature for 30- 60min. it is called bench curing


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Bench curing

It permits equalizationof pressure throughoutthe mold

Allows more time foruniform dispersion of

monomer throughoutthe mass of dough If resin teeth are used, it

provides a longerexposure of resin teeth to

the monomer producinga better bond of theteeth with the basematerial


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POLYMERIZATION PROCEDURE/ CURING

When heated above 60 c, molecules of benzoylperoxide decompose to yield free radicals.

Each free radicals, rapidly reacts with an availablemonomer molecule to initiate polymerization

Heat is required to cause decomposition ofbenzoyl peroxide. Therefore heat is termed asactivator.

Decomposition of benzyol peroxide moleculeyields free radicals that are responsible forinitiation of chain growth. Hence it is termed asinitiator


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Temperature rise

The polymerization of denture base resin is exothermicand the amount of the heat evolved may affect theproperties of the processed denture bases.

temperature of resin should not allowed to exceed theboiling point of the monomer (100.8oC) whichproduces significant effects on the physicalcharacteristics of the processed resin.


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Curing cycle. Processing denture base resin in Constant temp water bathat 74*C (165*F) for 8 hrs or longer, with no terminalboiling

2. Processing the resin at 74*C for approx. 2 hrs & thenincreasing the temp. of water bath to 100*C & processingfor 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]


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Polymerization by Microwave Microwave energy may also be used to polymerise poly methylmethacrylate resins.

For this technique, a non-metallic investment flask must be used along with resin specifically formulated for microwave processing.

The microwave is used to provide thermal energy which allows the polymerisation reaction within the resin denture base to take place.

The main advantage of microwave polymerisation techniques is the speed with which the denture base can be produced. Current evidence suggests that denture bases produced by this

method have a dimensional accuracy and physical properties similar tothose of conventional materials and resins (Keenan et al., 2003; Memon et al.,

2004)


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Internal Porosity Resin & Dental stone Poor thermal conductors, heat

of reaction cannot be dissipated, so temp. of resin risesabove

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 bedissipated --- boiling of unreacted monomer ----porosity


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External Porosity . 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 doughstage)

2. Lack of adequate pressure Lack of dough duringfinal closure (Flash indicates adequate material)

OTHER PROBLEMS : Crazing[Cracks] & production ofInternal Stresses


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CoolingAfter Curing Denture flasks should be cooled slowly

to room temp.

Rapid Cooling warpage of denture base because ofdifferences 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 & Polishing Deflasking has to be done with care to avoid f lexing

& breaking of Acrylic denture

Finishing Metal Trimmer, Acrylic/Alpine Stone, Dry& Wet Sand paper

Polishing suspension of finely ground pumice inwater


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Injection Molding Technique Mold space can be filled byinjecting resin under pressure inspecially designed flasks

Sprue hole / Vent hole areformed in stone mold

Soft resin (dough stage) iscontained in injector & is forcedinto mold

Resin under pressure until it hashardened

Polystyrene resin polymer isfirst softened under heat &injected while hot, then itsolidifies in mold upon cooling

No trial closures are required


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Injection Molding TechniqueAdvantages Disadvantages Dimensional accuracy

Low free monomer content

Good impact strength

High capital costs

Difficult mold designprblems

Less craze resistance

Less creep resistance

Special flask is required


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Chemically activated

denture base resins Chemical activators are used

to induce polymerization.

Does not require thermalenergy and therefore may becompleted at roomtemperature.

Hence often referred to ascold curing, self curing or

autopolymerizing resins


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Chemical activation is accomplished through the

addition of a tertiary amine such as dimethyl- para-toluidine to the liquid.

Upon mixing, the tertiary amine causesdecomposition of benzoyl peroxide. Consequently,

free radicals are produced and polymerization isinitiated


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Advantages Disadvantages

Better initial fit

Less thermal contraction

For repairing dentures, as

it avoids warpage due tore-curing

Lesser degree ofpolymerization, so thesehave slightly inferiorphysical properties

Colour stability is inferior,due to subsequent

oxidation of the tertiaryamine


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Manipulation . Adapting technique

1. Sprinkle On technique

3. Fluid resin technique

4. Compression moulding technique 5. Injection moulding technique


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Fluid Resin Technique These have high molecular wt powder that are smaller insize & when they are mixed with monomer, the mix is veryfluid

They are used with lower powder-liquid ratio 2 : 1 -2.5 : 1

This aids to prevent undue increase in viscosity duringmixing & pouring stages

This technique commonly involves use of AgarHydrocolloid for the mould preparation

Fluid mix is poured in the mould quickly & allowed topolymerize under pressure at 0.14 Mpa (20 psi).


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Advantages Disadvantages

Improved adaptation tounderline soft tissue

Decrease probablity todamage prosthetic teeth &

denture bases duringdeflasking Reduced material cost Simplification of lab

procedure for flasking (notrial closure),deflasking &finishing of denture

Shifting of teeth duringprocessing

Air entrapment within

denture base material Poor bonding betweenthe denture basematerial & acrylic resinteeth

Technique sensitive


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Autopolymerizing Heat-Cured Heat is not necessary for

polymerisation Porosityis 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


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Light activated Denture Base Resin Composition Urethane dimethacrylate matrixAcrylic 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 of400-500 nm from

high intensity quartz halogen bulbs


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Properties of Light-activated Resin 1. These are indicated for PMMA sensitive patient as it

does not contain methyl methacrylate monomer2.They exibit smaller polymerization shrinkage

3. Elastic modulus and flexural strength are lower thanconventional resin4. Inferior bond strength with resin denture teeth butcan be improve with use of bonding agent

5.Biocompability is concern with report ofhypersensitivity & cytotoxicity in epithelial cells inculture.


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Properties of Denture Base Resins Methyl Methacrylate Monomer: Clear, transparent, volatile, hassweetish 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 0.69%


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Compressive and tensile strengths:

These materials are typically low in strength. However,

they have adequate compressive and tensile strengthfor complete or partial denture

Compressive strength - 75 MP

Tensile strength - 52 MP Hardness:

Acrylic resins are materials having low hardness. Theycan

be easily scratched and abraded. Heat cured acrylic resin : 18.20 KHN

Self cured acrylic resin : 16 18 KHN


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Modulus of elasticity:

Acrylic resins have sufficient stiffness (modulus of

elasticity 2400 MPa) for use in complete and partial dentures.

However, when compared with metal denture bases it is low.

Self cured acrylic resins have slightly lower values.

Dimensional stability:

A well processed acrylic resin denture has good dimensional stability. The processing shrinkage is balanced by

the expansion due to water sorption. Shrinkage: Acrylic resinsshrink during processing due to

two reasons: 1. Thermal shrinkage on cooling

2. Polymerization shrinkage


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Polymerization shrinkage Clinically, the polymerization of Denture base Resinproduces volumetric & linear shrinkage.Molecular EventsEach molecule of methylmethacrylate an electric field that

repels nearby molecule.The distance between molecule significantly greater thanC-C bond.During polymerization C-C linkage is formed thatproduces net decrease in space occupied by the molecules.

When methylmethacrylate polymerizes to formpolymethylmethacrylate, the density of mass changes from0.94-1.19g/cm3.This change in density result in volumetric shrinkage of21%


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But clinically when suggested polymer-monomer ratio

of 3:1 is use than volumetric change is about 6-8% onlyand linear shrinkage is about 0.69%

Thermal shrinkage of resin is primarly responsible forlinear shrinkage. It depends on Tg & linear cofficientof thermal expansion.Range of linear shrinkage: 0.12-0.97% for various

comercial denture resins


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Water Absorption. Polymethacrylate absorbs relatively small amount ofwater when placed in aqueous environmentWater molecules penetrate the polymethacrylate mass

and occupy positions between polymer chain.Effects1.It causes slight expansion of polymerized mass.2.It act as plasticizers- interfer with entaglement ofpolymer chains.

It exhibit water absorption value of 0.69mg/cm2.It has been estimated that for 1% increase in weightproduce by water absorption, acrylic resin expands0.23% linearly.


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Crazing

Crazing is formation of surface cracks on the denture base resin. These cracks may be microscopic or macroscopic in

size .In some cased it has a hazy or foggy appearance raphethan cracks

Crazing has a weakening effect on the resin and reduces

the esthetic qualities. Cracks formed on crazing areindicative of

the beginning of a fracture.

Causes: Crazing is due to 1. Mechanical stresses or

2. Attack by a solvent

R i f f A li R i


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Reinforcement of Acrylic Resin

Denture Base Reinforcement of a

denture base is theprocess of strengthening

the denture base to makeit more resistant tofatigue and fracture


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Heat-polymerizing acrylic resin, since its introduction

several decades ago, has been the material of choicefor the construction of denture bases for numerousreasons:

Excellent appearance.

Ease in processing.

Repairability.

Economical.

Stable in oral enviroment


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However it is associated with two important clinicaldisadvantages:

1. low flexure fatigue and impact resistance.

2. Fractures in acrylic dentures result from impact orbending forces.

Impact forces typically are created during an accidentalfall into a washbasin or onto the floor.

Bending forces are developed mainly during masticationbecause of poor adaptation of the denture to the

underlying mucosa, improper occlusion, morphology ofthe palate, excessive masticatory forces, or denturedeformation during use. Those bending forces in long-term contribute to fatigue of the material.


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When to make reinforcement:

Patient dentures are fitting well but are still breakingA full denture combined with natural dentition in the

opposing jaw

Thin dentures where vertical space is limited

Patients with strong bites

Removable implant-supported dentures


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Strengthening the acrylic resin prosthesis can beapproached by modifying or reinforcing the resin.

Methods of reinforcement:

1. High impact resin:

One method is to incorporate a rubber phase(butadine

styrene) in the bead polymer which produces highimpact resin but unfortunately the high cost of thesematerials restricts their use.

2. Metal wires and strips:

One of the most common reinforcing technique is theuse of metal wires embedded in prosthesis. however,the primary problem of this technique is pooradhesion between resin and wires.


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3. Cast metal plates and mesh:

Cast metal plates also have been used to replace someparts of the denture. Although metal plates increase

the flexural and impact strength, they may beexpensive, and prone to corrosion; moreover, metal-reinforced dentures may be unesthetic as well


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4. Fiber reinforcement:

Another approach is the reinforcement of acrylic resin

dentures with fibers.Various types of fibers including carbon, Kevlar, glass, and

polyethylene have been tested.

Carbon and Kevlar fibers are useful in strengthening

PMMA; however, they produce clinical problems such as,difficulty in polishing and poor esthetics.

Woven polyethylene fibers normally develop anisotropicproperties to the composite. They are more esthetic butthe process of etching, preparing and positioning layers

of them may not be practical in the dental office


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Glass fibers are the most common form of all used fibers;

they improve mechanical properties of denture basepolymers, have easy manipulation, and they are esthetic.

Light cured glass fibers, new technique more esthetic,stronger and easy manipulation.

Li it ti f R i f d D t


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Limitation of Reinforced Denture

Base Resin 1. Tissue irritation can occure from protruding glass

fibers2.Poor esthetics is associated with dark carbon fibers

or straw colored kevlar fibers3. Require increased production time4.Difficulties in handling, orientation, placement orbonding of the fibers within the resin

5.Metal insert has been associated with failures due tostress concentration around embeded insert.


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Rapid Heat-Polymerized Resin These are hybrid acrylics that are polymerized in boiling

water immediately after being packed into a denture flask.

After being placed into the boiling water, the water isbrought back to a full boil for 20 minutes.

After the usual bench cooling to room temperature,

the denture is deflasked, trimmed, and polished in the

usual manner

The initiator is formulated to allow for rapid

polymerization without the porosity that one might expect.

R li i & R b i f D t


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Relining & Rebasing of Denture

BasesDEFINITIONS(Winkler)Relining is the process of adding some material to the tissueside of a denture to fill the space between the tissue and thedenturebase

Rebasing is a process of replacing all the base material of adenture.The purpose such a process is to fill the space between thetissue and the denture base without changing the position of

the teeth and the relation of the denture.


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Indications for Relining or Rebasing1. 1.Immediate dentures at 3-6 months after their

original constructions.2.When the residual ridges have resorbed andthe adaptation of the denture bases to the

ridge is poor.3. When patient can not afford the cost of newdenture.4. when seris of appointment for new denturemay cause physical and mental stress e.ggeriatric or chronically ill patients.


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Contraindication Excessive resorption of the alveolar ridge Highly inflamed/ abused soft tissues

Poor, unacceptable esthetics

TMJ problems

Unsatisfactory jaw relation Horizontal, vertical and orientation relations

Severe osseous undercuts which require surgicalcorrection

Severe speech problems


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Materials1. PMMA

Heat cured acrylic resin

Cold cured acrylic resin

2. Modifications of PMMA Butyl meth acrylate

3. Soft liners/ tissue conditioners Plasticized acrylic resin

Chemically activated. short term denture liners

Heat activated. long term denture liners

Vinyl resins

Silicone materials Chemically activated

Heat activated


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Problems with soft liners 1. Leachig out- plasticized acrylic resin

2.poor adherence3. It decreares the strength of denture4. It can not be cleaned effectivly

5. Disagreable taste & odors related to these materials6. most common fungal growth candida albicansfound on these liners

Growth of Biofilms on denture


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Growth of Biofilms on denture

Base The predominant oral fungus isolated from dentures is

C. albicans(75%).These fungi and other bacterialspecies collectively form biofilms. Adhesion of such

film cause adverse effect like denture inducedstomatitis.--- In absence of adequate oral hygiene, watersorption, cracks, surface imperfections and

microporosity are some of the contributing factor forbiofilm formation.


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Control of Biofilm 1.Frequent cleansing and soaking the dentures in cchemical cleansers.2. The temporary treatment of resin surface with

Nystatin and chlorhexidine3.Copolymerization with methacrylic acid,creates asurface-modified PMMA(mPMMA) that alters ionicinteraction between the resin base and candidahyphae and decreasing adhesion of micoorganism to

base4. Coating the resin surface with self bondingprotective polymer eg. Poly(dimethylsiloxane)

Recent Advancement Flexible


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Recent Advancement- Flexible

Denture Bases Unilateral or bilateral undercuts are frequently encountered and may complicate

successful fabrication of denture prosthesis.

Management of these situations conventionally includes alteration of the denture prosthesis

bearing area, adaptation of denture base ,careful planning of the path of insertion and the

use of resilient lining material.

An alternative denture prosthesis design

in which optimal flange height and thickness can

be achieved is by using flexible denture base


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It is nylon based

thermoplastic

material that does not sacrifice

aesthetics.Soft dentures are anexcellent

alternative to traditionalhard-fitted dentures

Some of the commerciallyavailable products are

Valplast, Duraflex, Flexite,

Proflex, Lucitone, Impak where as valplast

and lucitone are monomer free.reserves


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Advantages Translucency of the material picks

up underlying tissue tones, making it

almost impossible to detect in the mouth.

No clasping is visible on tooth surface.

The material is exceptionally strong and flexible.

Free movement is allowed by the overall flexibility.

Complete biocompatibilityis achieved because the material is

free of monomer and metal, these being

the principle causes of allergic reactions in

conventional denture materials.

Disadvantages

Flexible dentures generally notused for

long-term restorations and isintended only for

provisional or temporaryapplications.Flexible

dentures tend to absorb thewater content and

will discolor often.Procedure is technique sensitive.

Extreme caution is necessary

when processing


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References 1. Philips Science of Dental Material

2.Essential of Complete Denture Prosthodontics-SheldonWinkler3.Tandon R,Gupta S,Agrawal SK, Journal of Dental Sciences,

vol.2 Issue 2, 20104.Kumar MV,Bhagat S,Jei JB SRM University Journal of DentalSciences, Vol 1 Issue 1, June 20105.Sharma SN,Jagdeesh KN,Kalvathi SD,Kashinath KR Journal ofDental Sciences & Research Vol 1 Issue 1 Page 74-796.Alla RK,Sajjan S,Ramaraju V, Ginjppalli K,Upadhaya N Journalof Biometric and Nanobiotechnology Vol.4(2013)7.htt//theses.gla.ac.uk/2245/8.Prosthodontic Treatment for Edentulous Patients-Zarb.Hobkirk.Eckert.Jacob9.Craigs Restorative Dental Materials


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THANK YOU

denture base material

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