Denture Base Materials

Dental Polymers

Three Basic Classes Of Materials In Dentistry:

MetalsPolymersCeramics

Metals

At the atomic level , the atoms are bonded together by metallic bond.-metallic luster.-excellent electrical and thermal conductivity.-ability to be shaped (malleability).-strength, hardness, and high density.-Opaque

Metals

What is alloy? Mixture of 2 or more metals

What is metal?? Metals versus alloys in dentistry.

Types of metals in dentistry: Noble (gold, pt, Pd, ….. Semi-noble ( old term).gold is replaced

with silver Non-noble.

Metals

Preformed metals: Cohesive gold Amalgam Wrought wires

Metals

Casting metalsThe restoration is fabricated outside the mouth utilizing the lost wax technique.

POLYMERS

They are widely used in dentistry.Polymers versus plastics???Plastic is any material cabable to be

shaped .Ductile metals considered as plastic

Polymers:this term means many parts

Polymerisation: process by which the monomer is converted into polymer.

Polymers

They are bonded by covalent bonding along the backbone, and ionic bonding .

Polymers are prepared in form of dough then shaped into desired shapes.

They harden by:-physical reaction,(cooling, or evaporation). Waxes, -chemical reaction,

Types Impression materials Cavity Filling Materials Cements Equipments Sealants Temporary Crown & Bridge Restorative

Material. Denture bases

Ceramics

They are made of metallic oxides. Porcelain. Produced by sintering Glass, produced by melting and

fusion of the oxides.

ceramics

They are bonded by ionic bonds. No free electron to conduct heat or

electricity. Poor thermal conductor Stable ,biocompatible Transparent Translucency, due to air bubbles Three dimensional crystal structure. Brittle

Denture Base Resins

Prosthetic Uses

Denture bases Denture teeth Relining Materials Repair of dentures Provisional acrylic

partial dentures Custom impression

trays Mouth guards

Fluoride and bleaching trays

Facing on esthetical crowns

Provisional restorations

Removable tooth movement devices

Orthodontic retainers

Denture base function

Distributes pressure over a wider

area

So reducing bone resorption

Retains artificial teeth

Replaces missing tissue

Forms a seal for retention

Denture Base materials• Carved ivory • Carved Wood• Vulcanite; dark, opaque

(Vulcanised rubber)

• Highly cross-linked Acrylic resin

• Other Resin and plastic alternatives

Plastic acrylic teeth Bind chemically to the denture Can be adjusted Not cause wear of opposing tooth Good colour match Minor resiliency Wear under high force occlusion May stain with time

Porcelain (high fusing ceramic)

composite;

X-linked acrylic)

Experimental fiber-reinforced

teeth

Other Teeth Materials

Maxillofacial prosthesis

Needed After trauma Surgical defects Birth defects

Other materials Silicone rubber Vinyls with

plasticizers

Acrylic repair

Materials Chemically cured acrylic Light cured acrylic

Relining MaterialsRelining Materials

Provisional partial dentures

Impression trays

Record bases

Materials Cold-curing acrylic resin Other material

Shellac Vacuum formed vinyl or

polystyrene Baseplate wax.

Occlusal Splint Night guards Bleaching and fluoride application

trays

Mouth guards

Orthodontic applications

Removable tooth movement devices Orthodontic retainers

Facing of crowns Provisional restorations

Other materials:

•Polycarbonate

•Aluminum

•Stainless steel

•celluloid

Requisites of Dental Resins Can be processed easily using simple

techniques Aesthetics Economical

Charactristics Biological compatibility Physical properties Ease of manipulation Low cost Chemical stabilty in mouth aesthetics

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Terminology

Monomer + monomer = polymer Monomer1 + monomer2 = copolymer Oligomonomer= 2-4 monomers

Poly= manyMono = singleMer = unitOligo =several

Morphology of spatial arrangements

Linear or chain polymerisation Easily manipulated, stretched, bent,

thermoplastic, Hard e.g. fitting surface of acrylic teeth- better

binding to denture baseBranched polymerisation

Easily manipulated, stretched, bent, thermoplastic, More hard

Morphology of spatial arrangements

Cross-linked polymerisation Strong, stiff, thermoset, wear

resistant E.g. Denture base materials,

Occlusal surfaces of actylic teeth

Coiled chains Flexible e.g. impression materials

Morphology of spatial arrangements

Crystalline polymers Very regular arrangement in space:

strong, stiff, absorb less water.

Amorphous or glassy polymers Irregular arrangement Behaves as a brittle solid

Plasticizers effects

Added to stiff, glassy uncross-linked polymersLowers glass transition temperature (Tg)Become

rubber-like, Flexible less brittle Eg. pipe

Dimensional and thermal changes Expansion on polymerization,

exothermic

Contraction on polymerization 21vol.% If unfilled acrylic resin

6% denture resin

1-3% composites

Expansion on swelling in water

Expansion or warpage on thermal change and reheating

Types and molecular weight

Addition polymerisation: No by products Polymer mol. wt = Σ mol. wt monomers

Step Growth or Condensation polymerisation:

By products are produced and lost in the final product

Polymer mol. wt ≠ Σ mol. wt monomers

Stages in Addition Polymerization

4 distinct stages Induction – Activation and initiation Propagation Chain Transfer Termination

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Polymethyl methacrylate (PMMA)

Activation Initiation

Chain Propagation Termination

MMALiquid

PMMAPowder

Old and newpolymer chainsIntertwined at themolecular level.

PMMAPowder

Particle swells

MMALiquid

Acrylic dough (Cohesive gel)

Heat and pressure

(entanglements)

Induction

To begin : a source of free radicals R R generated by activation of radical

producing molecules using a second chemical ,heat or UV light

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Propagation

The resulting free radical monomer complex then acts as anew free radical center when it approaches another monomer to form a di-mer which also becomes a free radical

This reaction continues……. The growth of polymer chain ceases

when the reactive center is destroyed by number of termination reactions

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Chain Transfer

The active free radical of a growing chain is transferred to another molecule and a new free radical for further growth is created.

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Termination

Most often terminated by direct coupling of two free radical chain ends or by exchange of hydrogen atom from one growing chain to another

Ideal properties

Natural appearance Easy processing Easy to clean Easy to repair Inexpensive Biocompatible Resistant to

bacterial contamination

High strength, stiffness, hardness, toughness, fatigue resistance

•Low density•Radiopaque •High thermal conductivity•High modulus of elasticity,

impact strength•Abrasive resistance•Dimensionally stable•Accurate reproduction of

surface detail

Curing methods Chemically cured

Tertiary amine ( dimethyl-p-toludine or sulfinic acid) (accelerator)

Benzoyl peroxide (initiator) Hydroquinone (inhibitor)

Heat cured Heat and pressure control

Avoids porosity Maximizes conversion of monomer to polymer

Light cured Photo-initiators (camphorquinone), Blue light,

Used for: record bases, custom tray, denture repair

Heat cured acrylic resin Powder ( can have limitless life)

Beads or granules of polymethyl methacrylate Initiator (benzoyl peroxide) Pigments/dyes (colour vitality as cadmium, iron, organic

dyes) Optical opacifiers (tio2/ Zno) Plasticizers (ethyl acrylate (internal), dibutylphthalate

(external) to make dough easier) Synthetic fibres (nylon) Coloured fibres (blood vessels)

Liquid ( in dark bottle, avoid contamination by powder)

Methyl methacrylate monomer Inhibitor (hydroquinone) Crosslinking agent

(diethylene glycol dimethacrylate, (1,4 butylene glycol dimethacrylate)

Bead Polymer

Chemical cured resin Cure is initiated by a tertiarv amine (e.g.

Dimethyl-p-toluidine or sulfinic acid)

Absence of heat: Lower molecular weight material Lower strength properties Higher residual monomer in the resin Color stability is not as good- yellowing Less contraction on cooling to room temp

Polymer beads are smaller Faster dissolution in the monomer to produce a dough Doughy stage is reached before the addition curing reaction – mix viscosity is high and prevents the adaptation of the mix to

the mould walls or cast -keep readapting

Lowering of the glass transition temperature Less build-up of internal strain Highly susceptible to creep- distortion when in use.

Light activated materials Components:

Urethane dimethacrylate { matrix} Acrylic copolymer Silica (filler) to control rheology

Forms Sheets Ropes

Curing Light chamber- 400-500 nm Photo-initiators (camphorquinone), Teeth added in a second exposure over

the base

Used for Record bases Custom tray Denture repair

Hardness and impact strength ≈ heat cured resin

Elastic modulus < heat cured resin; deform under mastication

Less shrinkage (3%) better fit Less residual monomer

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Auto-polymerizing/cold cure acrylic Reducing agent (tertiary aromatic amine or

barbituric acid derivative, NN’-dimethyl-p-toluidine) reacts with peroxide at room temp.

Excellent detail reproduction To be able to pour in mold, balanced size mol. wt,

plasticizers and x-link agents Reversible hydrocolloid (agar) mold can’t resist

teeth movement during pouring Hydro pressure flask reduces air bubbles and

monomer porosities Difficult to dewax, less monomer binding to teeth Shortcomings:

residual monomer ↓ Cross link densities Creep Variety of products

High-impact acrylic A rubber phase is added (phase inversion)

Uniformly distributed

Rubber cored polymer

Types Butadiene + styrene = polystyrene butadiene rubber

Butadiene + MMA

PMMA + polystyrene butadiene rubber + poly(2,3-dibromopropyl methacrylate) for opacity

PMMA = lucitone 199

Lightly xlinked or no cross linking agent is added

Rubber has a craze inhibitory effect

Experimental types of acrylic Added Bis-GMA and fiber

Flexural strength ≈ ceramics Can be used as lingual bars and connectors

Experimental (mwt polyethylene fiber-reinforced) Neutral color

Low density

Biocompatibility

Surface treated to enhance fabrication

Time consuming

Types of acrylic

Other (polystyrene,epoxy, SS)

PMMA Adhesion to Metal- use adhesive primers untreated porcelain teeth with

organo silane compounds

Setting reactionMixing of powder and liquid cause monomer

diffusion and softening of the surface of the powder producing the following gelling stages:

Ratio P/L (2/1 wt %, 1.6 -1 vol %) Sandy - initial melting of beads (not used) Stringy or sticky - entanglements with swollen

beads and thickened interstitial monomer(not used)

Dough - gelation (used) Rubbery - monomer penetrates to the core of

beads, plasticizing them, ↓Tg (not used)

Manipulation issues

P/L Inadequate filling by monomerWeak material properties Porosity

↓P/LExcessive polymerisation shrinkagePoor fit Light color as powder holds the pigments

Manipulation issues Curing before monomer diffuse to bead (before

dough stage)↓ flexural strengthcracks between linear polymerised interstitial gel and cross linked beadsMore shrinkage contraction by the loss of pressure produced by the dough to compensate for it

Curing in dough stage monomer penetrate the beads dissolves beads allows cross-linking agent to penetrate interpenetrating polymer network IPN.

Packing in the rubber stage Less extrusion of excess acrylic from flask Extra pressure in the mould

Fracture the cast less flow around teeth Dislodgment of teeth into mould

Manipulation issues Control of color

Pigments position Inside beads surface of beads

polymer should be added to the monomer slowly so it will not washed off by too rapidly

Blood vessel resembling Fibers aggregate in the bottom of bottle

Shake powder well before use

Mould Lining resin may penetrate rough plaster and adhere a separating medium must be employed

solution of sodium alginate tin foil.

Manipulation issues

Control of Processing strains Shrinkage in restricted mould cause internal strain On release of stress (flask opening) it may give

Crazing Warpage Distortion

These are reduced by the slightly extra packed material that flow into shrinkage spaces when temperature is higher than Tg (heated flask)

Manipulation further reduces strains by Using acrylic teeth Cooling the flask slowly

Flasking steps

Flasking Dewaxing Putting a separating medium Placing acrylic dough Packing Heat curing

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Flasking for heat cured resin

Flasking options with acrylic dough: Trial-packing, trimming, repacking Packing-only Poured resin (e.G., Lucitone fas-por) Injection moulding

Heat and pressure control Aim to produce radicals and initiate polymerization Reaction is thermally activated and generates

heat as well Reaction conversion is about 98 to 99.5% MMA: tbp = 100c (p= 1 atm); 140c (p= 2 atm)

Heat curing cycles Fast cycle

Cure at 71-72°C for 30-90 min 100°C for 30 min.

Slow cycle = cure at 71-72°c for 10 hrs [A slow cycle is better with larger

amounts of material.] [Generally, slow cures result in

better dimensional accuracy.] Other cycles are done as

recommended by manufacturers

Heat curing cycles

Rapid heating: Excess radical release Extra xlinking and branching of

interstitial polymer More residual monomer Reduced toughness Heat builds up from exothermic rxn Porosity

Loss of strength Bad esthetics (opaque and cloudy color) Possible fouling

Heat curing cycles

Slow : Sufficient radical release Adequate x-linking and branching between

high mol.wt polymer chains Increased toughness Sufficient radical ends increase monomer

incorporation in growing chains X-linking agents polymerized, reducing

their plasticizing effect (in their non bound state) and reduce creep

Produce an annealing effect easing stresses produced from shrinkage, reducing crazing and distortion

Heat curing cycles

Pressure control Places compressive force Compensates for polymerization shrinkage Increase flow of dough around teeth, more

monomer wetting and surface dissolution, stronger bond

Oozes out excess dough Some hybrid systems begin polymerization

from one side to allow dough to cover for shrinkage

Heat curing cycles

Microwave curing Uses a microwave Flasks are non metalic Reduced time

Denture Radiolucency

Problems when accidents displace fractured segments Lungs Skull stomach

Salts and fillers reduce esthetics, strength Organo-metalics are toxic

Bromine containing organics lack heat stability, must be added in quantities that plasticize the denture, causing creep and water sorption

Phase separating bromo-polymer in beads reduce the previous effects

Mechanical properties

Failure to Moderate strengths: impact resistant denture is low Low elastic and flexural modulus lack of fracture toughness 30% of denture repairs involve midline fractureswhich are most prevalent among upper dentures. dropped denture does not necessarily break

instantly a crack continue to grow and failure due to flexural

fatigue. Failure due to poor quality processing

Lack of bonding between the resin and the acrylic teeth and weak interface

Crazes due to processing faults or exposure to solvents is another possibility.

Creep Reduced by cross linking Heat cured < cold cured

Internal denture porosity

InherentInherent porosityporosity: : Not seen by vision 1-2% of residual

monomer Leaks Replaced by fluids

Minimized byMinimized by Use heat cured

resin Pack denture under

correct pressure Use correct P/L Use the glaze after

polishing

Internal denture porosity

Irregular porosityIrregular porosity: : Seen by vision Not regular on

denture surface P/L heterogeneity Air incorporation

(spherical pores)

Minimized byMinimized by Use correct P/L Add liquid first Mix well Cover the mix before

dough stage Can use the vibrator

External denture porosity

Irregular surface Irregular surface deficienciesdeficiencies: :

Seen by vision Insufficient pressure Dough was not

molded correctly by hand leaving surface blisters and pores

Insufficient dough

Minimized byMinimized by Mold dough by

hand into small areas

Place sufficient material in flask

Pack under correct pressure

External denture porosity

Irregular porosityIrregular porosity: : Shrinkage by

polymerisation (5-8% vol or 0.2 -0.5% linear)

Further shrinkage by cooling to room temperature

Can compensated for by the post dam technique

Minimized to byMinimized to by Pack under pressure Slight extra denture

material can overcome shrinkage and maintain pressure (single packing)

Pack in dough stage

Internal denture porosityGaseous porosity

Seen by visionVolatisation of monomer by Localized MMA boiling

Common in thicker portions

Minimized byMinimized by Avoid high

processing temperatures

Avoid extra monomer than recommended for P/L

Raise heat slowly and evenly around the flask

Gaseous porosity

Avoid high processing temperatures

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Correct cycles

Incorrect cycle

Crazing Area of localised region of high plastic

deformation which may fill by voids Crazed region can still support stress As the voids in the crazed region grow, they become

separated only by thin fibrils of polymer Fibrils fail and a crack is formed Crack will grow under an externally applied load Cause denture failure by brittle fracture.

Caused by Internal strains in flask Heat (due to polishing) Differential contraction around porcelain teeth Attack by solvents such as alcohol

Crazing Avoid internal strain during polymerisation

Slow cooling of the flask Use single trial packing Use cross linked polymer types

Avoid extra stress during function Use acrylic rather than porcelain teeth Do not overheat on polishing Keep denture away from solvents Avoid denture drying Polish after each adjustment Use glazes for surface

Dimensional changes on processing

Expansion on heating flask; heat evenly Expansion on polymerization,

exothermic Contraction on polymerization (21vol.%); Contraction on cooling to room

temperature; Expansion on swelling in water; Expansion on thermal change to 32c.

Net result– should be near zero

Warpage on drying

Contraction on evaporation of absorbed water

Don’t leave denture outside the mouth dry

Adverse reactions to PMMA

Most common in dental laboratories

Associated with regular contact with monomer when handling the dough

Must avoid direct contact

Rubber gloves may not provide sufficient protection

Barrier creams can help

Irritant contact dermatitis

Adverse reactions to PMMA

Allergic contact dermatitis Usually associated with

release of residual monomer Benzoic acid

Types Immediate Delayed hypersensitivity

(type IV) Heat cured resin <

chemical cured

Must ensure full cure of

denture

Avoid relining procedures

May use an extra cycle of

polymerisation – but

denture may warp

May need to consider

alternative material such as

polycarbonate if Delayed

hypersensitivity

Adverse reactions to PMMA

Thermal properties

Low Thermal conductivity

during denture processing heat cannot escape – prone to gaseous porosity

isolates from any sensation of temperature – throat burns

High Coefficient of Thermal Expansion

Porcelain teeth may be lost due the differential expansion and action

Warpage if denture is cleaned with hot water

Water Sorption

PMMA will absorb water by polar nature (1.0-2.0% wt)

May compensate for processing shrinkage Weeks of continuous immersion in water to

reach a stable weight

Solubility Solvents (e.G. Chloroform, alcohol) Xlinked are insoluble in most of fluid intakes Weight loss will occur, due to leaching of the

Monomer Pigments and dyes.

Ideal properties achieved?

Natural appearance Easy processing Easy to clean Easy to repair Inexpensive Biocompatible Resistant to bacterial contamination

x High strength, stiffness, hardness, toughness X Low density Radiopaque X High thermal conductivity X Dimensionally stable X Accurate reproduction of surface detail

Injection molded plastic Types Polycarbonates Nylon

Advantage: Consistent mwt Substitute acrylics in sensitive patients

Disadvantage Must use dry mold, slow heating and cooling Under filled molds by inadequate spruing or underheating Low melt temp cause high injection forces, moving teeth in

mold Cost of equipment Difficult to attach to teeth Small market segment Can explode if high heat and wet molds Overheating cause depolymerization, oxidation,

porosties Loss of strength Bad esthetics (opaque and cloudy color) Possible fouling

Polycarbonates

Tough plastic Injected in dry molds A high melt viscosity

Problems in binding to teeth

May de-polymerize explosively in the presence of heat and water

No cross linking – Poor solvent resistance Poor craze resistance

Nylons and polyamides

Polyamide = diacid + diamine

Conventional nylon failed Excessive water sorption

Poor creep resistance

Biodegradation

Glass (beads or fibers) reinforced nylon Less water sorption

Fibers better in stiffness(≈ acrylic) than beads

Fibers may irritate patient if denture fitting surface was abraded

Cellulose product

Camphor used as plasticiser Warpage in mouth Camphor leached out

Loss of color Taste Blistering Staining

Phenol Formaldehyde (Bakelite)

Difficult to process Lost its color with function

Vinyl resins

Low resistance to fracture Fatigue failure

Denture base reprocessing:

Hard and soft tissue changes every 5-8

years

Require modifying denture base:

Relining resurfacing of the tissue

surface

Rebasing replacement of entire

denture base

Soft denture lining material

Uses: After surgery Immediate dentures Sores Undercuts which are not removed by surgery Ill fitting denture

can be done In lab Chair side

Ideal lining material properties Durability: but hardens in short time(1-4w, 1-3 y) Dimensional stability Resistance to fouling Water absorption Osmotic presence of soluble material Resistance of Biodegradation Could it bond old acrylic Inhibit candida growth

Lining materials–acrylic based

Glassy MMA + high conc. of plasticizers Plasticizers:

Free: diffuse out reducing the resiliency Bound in cured matrix – failed clinically

Has lower rate of polymerization Phase separation Water accumulate in plasticizer rich phase Soluble impurities cause more osmotic pressure Swells and distorts Discoloration Bad taste Exothermic rxn Bad taste

Lining materials–acrylic based

Soft acrylics that have ↓Tg EMA (ethylmethacrylates)

Beads coploymer Ethyl methacrylate + isobutyl methacrylate Ethyl methacrylate + ethoxyethyl methacrylate

Have unpleasant odour

Monomer MMA Tg > room temp Less irritant to patients Isobutyl methacrylate Tg < room temp (polished after

placing in iced water), Dimensional instability

Plasticizer in monomer trapped in beads (25-50%) Phthalate ester – leach out by time Avoid heat, strong bleaching agents that reduce

resilience

Lining materials–acrylic based

Soft acrylics that have ↓TgHydroxy EMA

Water is the plasticizer Swelling of liner may make it distort Ions enter and may crystallize inside matrices thus

hardening the liner

Polymerisable plasticisers Beads ploymer

Ethyl methacrylate + isobutyl methacrylate or Monomer

Alkyl maleate orAlkyl itaconate + Tridecyl methacrylate + 2-diethylhexyl maleate, ethylene glycol dimethacrylate

Tissue conditionersDiffer from soft lining material by the following

Different viscoelastic properties Flowable on insertion responding to

Masticatory forces Lingual forces Border moulding forces

Increase viscosity on setting Flows slowly responding to persistent heavy

masticatory forces after setting Useful to fill space after tissue swellings resolve Can be used as a functional impression

Reaction Gel formation not polymerization Alcohol swells beads and ↓ their Tg Beads become tacky by entanglements and cohesive

strength

Tissue conditioners

Differ from soft lining material by the following Composition

Old- plasticine Old- chewing gum Ethyl methacrylate copolymers Or small mwt polymers

Plasticisers:

ethyl alcohol or

aromatic esters (butylphthalyl butylglycolate)

hemical cleaning damages the liner

Use plain soap and water

Tissue conditioners

Differ from soft lining material by the following Alcohol problems:

Leak and replaced by water- so harden days up to 14

days

High conc. Can give a sting sensation

Can give a false positive on breathalyser test

Reduce leach of plasticisers by glazing or semiset MMA

Very susceptible to infection

Incorporate antimicrobials as

silver zeolite

itraconazole

Chemical cleaning damages the liner

Use plain soap and water

Silicon - RTV

Room temperature vulcanizing silicones (RTV) Polymethyl siloxane polymer It sets by crosslinking of existing polymers

Heat Tetraethyl silicate

Condensation minimal xlinking Poor tear resistance Poor abrasion resistance Poor adhesion to denture

Use adhesive or coupling agent Osmotic pressure effects

Buckling and swelling with water Poor resistance to cleansers Biocompatible Dimensional stability May foul by Candida

Silicon – Heat cured

More xlinking Poor tear resistance Adequate adhesion to denture

Can use siloxane methacrylate as a binder to heat cured additional silicon

Resistant to aqueous environment and Osmotic pressure effects

better resistance to cleansers Poor tear resistance Poor abrasion resistance

Denture base hygiene

1. Clean with toothbrush and warm soap-and-water2. Use low abrasive cleaners

2. Avoid oxidizing or Cl-containing materials• Bleaching the color• Reduces strengths of denture• Reduces fatigue resistance

3. Diligently clean both the top and tissue-borne surfaces 4. Clean with benzalkonioum