polymers

Dental Bio-materialsDental Bio-materials

Ass. Prof. Dr. Mohamed Abed el-Aziz SouhailAss. Prof. Dr. Mohamed Abed el-Aziz Souhail Head of Dental Bio-material Department,Head of Dental Bio-material Department,

Faculty of Dental Medicine,Faculty of Dental Medicine, Al- Azhar University.Al- Azhar University.

POLYMERSPOLYMERS

Polymers are Plastic and rubbersPolymers are Plastic and rubbers

Polymers Are long chain molecules consisting Polymers Are long chain molecules consisting of many (poly) repeating units (mers). of many (poly) repeating units (mers).

A monomer: Is the smallest repeating unit in A monomer: Is the smallest repeating unit in the polymerthe polymer

PolymerizationPolymerization : Is the chemical reaction by : Is the chemical reaction by which polymers are formed from monomers which polymers are formed from monomers chain e.g. ethylene monomer.chain e.g. ethylene monomer.

CLASSIFICATIONS OF CLASSIFICATIONS OF POLYMERSPOLYMERS

Classifications

according to

Classifications

according to

Polymerization reaction Spati

al

ThermalOrigin

The originThe origin

Natural polymersNatural polymers

Agar, celluloseAgar, celluloseDNA, proteinsDNA, proteinsNatural rubberNatural rubber

Collagen and silkCollagen and silk

A) Synthetic PolymersA) Synthetic Polymers

--Produced in the laboratory by chemical Produced in the laboratory by chemical reactions. e.g. Acrylic resin ,nylon and reactions. e.g. Acrylic resin ,nylon and polystyrene. polystyrene.

Originally the synthetic polymers regarded as Originally the synthetic polymers regarded as substitutes for existing natural polymers such substitutes for existing natural polymers such as rubber and silk .Nowadays a polymers can as rubber and silk .Nowadays a polymers can be produced and have medical use such as be produced and have medical use such as dialysis and oxygenator membranes and dialysis and oxygenator membranes and dental application such as filling materialsdental application such as filling materials . .

B) Natural PolymersB) Natural Polymers

Proteins polyamides of polypeptides Proteins polyamides of polypeptides polyisoprenes polyisoprenes

e.g. rubber and gutta-percha e.g. rubber and gutta-percha polysaccharides polysaccharides

e.g. starch , cellulose, agar and alginate .e.g. starch , cellulose, agar and alginate .

poly nucleic acidspoly nucleic acids

such as DNA and RNA.such as DNA and RNA.

According To Thermal According To Thermal BehaviorBehavior

Thermoplastic Thermoplastic resinresin

They are polymers which can be They are polymers which can be shaped by heat and after cooling shaped by heat and after cooling they maintain their shape the they maintain their shape the reaction is reversible and can be reaction is reversible and can be repeatedrepeated

characteristicscharacteristics

The polymer chains are bonded to each other by secondary bond.The polymer chains are bonded to each other by secondary bond.They are relatively soft and their mechanical propertiesThey are relatively soft and their mechanical properties

are sensitive to heat.are sensitive to heat.They dissolve in organic solvent thermosetting polymersThey dissolve in organic solvent thermosetting polymers

They are formed into a permanent shape They are formed into a permanent shape and set by chemical reaction.and set by chemical reaction.

They can not be remelted and reformed into another shape but They can not be remelted and reformed into another shape but degrade or decompose up on being heated to high temperature..degrade or decompose up on being heated to high temperature..

Thermoset cannot be fused.Thermoset cannot be fused.The polymers form network with cross –links between them primary The polymers form network with cross –links between them primary

covalent bonds. E.g. PMMA.covalent bonds. E.g. PMMA.

ThermalThermala) Thermosetting b) Thermoplastica) Thermosetting b) Thermoplastic

(polyvinyl chloridepolyethyleneNylonPolystyrene

b) Thermoset polymersb) Thermoset polymers

Harden during fabrication (irreversible or chemical Harden during fabrication (irreversible or chemical reaction).reaction).

ChemicalChemical

Soft hardSoft hard

reactionreaction

Examples are cross-linked Examples are cross-linked polymethylmethacrylate, silicons ,and bisphenol polymethylmethacrylate, silicons ,and bisphenol A-diacrylates.A-diacrylates.

According to the basis of structural According to the basis of structural or spatial configurationor spatial configuration

linear

copolymer

Simple polymerHas one type monomer homopolymer

Branched

RandomBlock

OR

HomoCopolymer

OR

graftRandom

3 Basic Types

Cross-linked polymer

((a )Linear polymers

(simple polymer)

This may be homopolymerThis may be homopolymer it has one type of it has one type of monomer ,monomer ,or copolymer, this means that or copolymer, this means that it has it has more than one type of monomer,more than one type of monomer,

Homopolymer copolymer, random Homopolymer copolymer, random

blockblock

b) Branched polymersb) Branched polymersThis type of polymers show higher strength hardness and This type of polymers show higher strength hardness and

less water sorption than linear polymer .less water sorption than linear polymer .

Branched polymers are either homo or copolymer The Branched polymers are either homo or copolymer The branched copolymer may be random or graft types.branched copolymer may be random or graft types.

Homopolymer copolymer, random Homopolymer copolymer, random

graftgraft

2

C) Cross-LinkingC) Cross-LinkingPolymer chains are cross - linked or attached byPolymer chains are cross - linked or attached by

a cross - linking agent such as glycol a cross - linking agent such as glycol dimethaerylate. dimethaerylate.

Through primary and small degree of cross Through primary and small degree of cross linking will limit the amount of movements of linking will limit the amount of movements of the polymer chains relative to each other.the polymer chains relative to each other.

When the material is stressed the deformation When the material is stressed the deformation

is elastic rather than plastic.is elastic rather than plastic.

CharacteristicCharacteristic

1.1. Higher strength and hardness.Higher strength and hardness.

2.2. Higher resistance to crazing which may Higher resistance to crazing which may be due to Alcohol drinking or solventsbe due to Alcohol drinking or solvents

3.3. It is the best type used for construction of It is the best type used for construction of denture base and artificial teethdenture base and artificial teeth

4.4. Low water sorptionLow water sorption

PolymerizationPolymerization::

The chemical reaction by which polymers are prepared from The chemical reaction by which polymers are prepared from monomersmonomers..

Condensation polymerizationOccurs when two molecules react to form a large molecules with the elimination of smaller molecule

( Often but not always water)Addition polymerizationOccurring when a reaction between two molecules

(either the same to form a homopolymer or dissimilar to form heteropolymer )

produces a large molecule without the elimination of a smaller molecule such as water

Types of polymerizationTypes of polymerization

Most polymerization reactions are of two types condensation Most polymerization reactions are of two types condensation and addition polymerization.and addition polymerization.

1)Condensation polymerization:1)Condensation polymerization: The reaction between two molecules to form a larger The reaction between two molecules to form a larger

molecule with the elimination of a smaller such as watermolecule with the elimination of a smaller such as water as a by product.as a by product. condensationcondensation Polymer + by product Polymer + by product

MonomerMonomer polymerizationpolymerizationExExample is the polysulphide rubber impression material.ample is the polysulphide rubber impression material.

22))Addition polymerizationAddition polymerization

The reaction between two molecules to give The reaction between two molecules to give a larger molecule without the elimination a larger molecule without the elimination of a smaller molecule (no by-product).of a smaller molecule (no by-product).

No change in composition takes place, the No change in composition takes place, the structure of the monomer is repeated many structure of the monomer is repeated many times in the polymer e.g. times in the polymer e.g.

Poly (ethylene), poly (acrylic acid), Poly (ethylene), poly (acrylic acid), polymethacrylic acid) poly (methacrylate).polymethacrylic acid) poly (methacrylate).

STAGESSTAGES OF ADDITION OF ADDITION POLYMERIZATIONPOLYMERIZATION

Initiation

Activation and initiation (by free radical mechanism)

STAGES OF ADDITION POLYMERIZATIONSTAGES OF ADDITION POLYMERIZATION

TerminationPropagation

CBA

Direct coupling

Exchange of a hydrogen atom

A) Activation and A) Activation and initiationinitiation

A free radical is a compound with an unpaired A free radical is a compound with an unpaired (unshared )electron (very reactive ) (unshared )electron (very reactive )

The symbol (c= c) represents four electrons the The symbol (c= c) represents four electrons the monomer itself become free radical Benzoyel monomer itself become free radical Benzoyel peroxide is a substance most commonly used to peroxide is a substance most commonly used to generate free radicals before initiationgenerate free radicals before initiation

The activation is done by light, heat or by a chemical The activation is done by light, heat or by a chemical compound such as dimethyl paratoluidine. In this compound such as dimethyl paratoluidine. In this way the peroxide decomposes giving up free way the peroxide decomposes giving up free radicals to initiate the polymerization reaction .radicals to initiate the polymerization reaction .

Three stages of addition Three stages of addition polymerization Heatpolymerization Heat

1) Initiation 1) Initiation

AminesAmines

C6H5 COOCC6H5 C6H5 COOCC6H5 2 (C6H5COO) + CO2 2 (C6H5COO) + CO2

Benzoyl peroxide free radicals (R) + carbon dioxideBenzoyl peroxide free radicals (R) + carbon dioxide

CH3 CH3CH3 CH3

R+ CH2 = C R-CH2 – C R+ CH2 = C R-CH2 – C

COOCH3 COOCH3COOCH3 COOCH3Free radical + monomer Free radical activated monomerFree radical + monomer Free radical activated monomer

Heat

Amines

B) PropagationB) Propagation

The chain reactions continues with the evolution The chain reactions continues with the evolution of heat until all the monomer has been of heat until all the monomer has been changed to a polymer changed to a polymer And so onAnd so on

R1M +M + R1MM + M +R1MMMR1M +M + R1MM + M +R1MMM

R1MN where n is any integral number.R1MN where n is any integral number.polymer free radical + monomer polymer free radical + monomer growing chain growing chain

C) TerminationC) TerminationThe reaction terminated either by The reaction terminated either by

a) Direct coupling a) Direct coupling

This occurs when two free radicals reactThis occurs when two free radicals react

to form a stable molecule to form a stable molecule

1- R1MN + R2MM 1- R1MN + R2MM R1MN R1MN R2MM R2MM

b) Exchange of a hydrogen atoms b) Exchange of a hydrogen atoms

The chain reactions can be terminated either The chain reactions can be terminated either by direct coupling or by the exchange of by direct coupling or by the exchange of

a hydrogen atom.a hydrogen atom.

Chain transferChain transfer

H H H H by transfer of

R1Mn – C – C + R2Mm – C – C

H H H H a hydrogen atom

H H H H

R1Mn – C = C + R2Mm – C – C - H H H H

Factors associated with Factors associated with polymerizationpolymerization

A) Evolution of heat as the A) Evolution of heat as the reactions is strongly exothermic reactions is strongly exothermic due to breaking of bonds.due to breaking of bonds.

B) Reduction in volume.B) Reduction in volume.

Inhibition of Inhibition of polymerizationpolymerization

Any impurity in the monomer which can react Any impurity in the monomer which can react with free Radicals will inhibit or retard the with free Radicals will inhibit or retard the

polymerization reactionpolymerization reaction

It can react either with the activated initiatorIt can react either with the activated initiator

or any activated nucleus or with an activated or any activated nucleus or with an activated growing chain to prevent further growthgrowing chain to prevent further growth

The presence of such inhibitors influences the The presence of such inhibitors influences the length of the initiation period, as well as length of the initiation period, as well as the degree of polymerization.the degree of polymerization.

The addition of small amount of inhibitor The addition of small amount of inhibitor (Hydroquinone) to the monomer(Hydroquinone) to the monomer

(Methyl methacrylate)(Methyl methacrylate)

will inhibit polymerization if no chemical will inhibit polymerization if no chemical initiator is present and it will retard the initiator is present and it will retard the polymerization in the presence polymerization in the presence

of an initiator.of an initiator.

Factors affecting the Factors affecting the properties of polymersproperties of polymers

Many factors affect the properties of polymersMany factors affect the properties of polymers

1.1. Chemical composition of the chain. Chemical composition of the chain.

2.2. Degree of polymerization.Degree of polymerization.

3.3. The number of branches and The number of branches and

4.4. The cross – links between polymer chainsThe cross – links between polymer chains..

In generalIn general

Longer chains and higher molecular weight polymers Longer chains and higher molecular weight polymers characterized by characterized by

(a) Increased strength, hardness Brittleness and (a) Increased strength, hardness Brittleness and stiffnessstiffness

(b) Increased resistance to creep.(b) Increased resistance to creep.

For example composite resins highly cross- linked For example composite resins highly cross- linked matrix in which large number of strong covalent matrix in which large number of strong covalent linkages between chains transforms the molecules linkages between chains transforms the molecules into a rigid very high molecular weight material into a rigid very high molecular weight material resulting strength, stiffness and withstand occlusal resulting strength, stiffness and withstand occlusal stresses. stresses.

In contrastIn contrast

Elastomeric impression materials composed of Elastomeric impression materials composed of individual coiled chains with just few cross- links individual coiled chains with just few cross- links this type of molecular structure permits the large this type of molecular structure permits the large scale uncoiling and recoiling of chains that gives scale uncoiling and recoiling of chains that gives these materials high flexibility.these materials high flexibility.

Two basic properties which characterize polymers Two basic properties which characterize polymers are glass transition temperature (Tare glass transition temperature (Tgg))

and melting temp.and melting temp.

(T(Tgg ) Is the temperature at which the polymer ceases ) Is the temperature at which the polymer ceases to be glassy , brittle and become rubber – like.to be glassy , brittle and become rubber – like.

T g = Mw T g = Mw

Molecular weight is another factor which affects Molecular weight is another factor which affects (T (T gg))

The two properties is related by an equation:The two properties is related by an equation:

Co- polymerizationCo- polymerization To modify the properties of the polymer , use two or To modify the properties of the polymer , use two or

more chemically different monomers such as more chemically different monomers such as copolymer process enabled chemists tocopolymer process enabled chemists to

“ “ tailor – make “ molecules of predicted properties for tailor – make “ molecules of predicted properties for special applications e.g. special applications e.g.

Addition of sufficient amounts of octyl methacrylate Addition of sufficient amounts of octyl methacrylate results in a copolymer that is soft and flexible at results in a copolymer that is soft and flexible at mouth temp.mouth temp.

And that has been used as a soft liner And that has been used as a soft liner 1.1. Hydroxyethyl methacrylate increase water sorption Hydroxyethyl methacrylate increase water sorption and wetability of the copolymer by saliva .and wetability of the copolymer by saliva . The material become soft and flexible if placed in The material become soft and flexible if placed in

water or saliva It is used as soft contact lenseswater or saliva It is used as soft contact lenses and as soft liner for denturesand as soft liner for dentures

CROSS-LINKING

Is a chemical bond formed between polymer chains. Is a chemical bond formed between polymer chains. A chemical compound with two double bonds per A chemical compound with two double bonds per

molecules can act as cross-linking agent since each molecules can act as cross-linking agent since each C =C bond can react with different chain.C =C bond can react with different chain. This product has superior properties an example is This product has superior properties an example is

ethylene glycol dimethacrylate .ethylene glycol dimethacrylate .Small degree of cross-linking limits the amount of the Small degree of cross-linking limits the amount of the

polymer chains when the material is stressed.polymer chains when the material is stressed. The deformation is elastic rather than plastic. The deformation is elastic rather than plastic. The polymer have high glass transition temperature. The polymer have high glass transition temperature. The polymer is harder more brittle and more The polymer is harder more brittle and more

resistance to the action of solvents resistance to the action of solvents

PlasticizersPlasticizers

Liquids are able to penetrate between the chains of Liquids are able to penetrate between the chains of polymer .The molecules become further apart and the polymer .The molecules become further apart and the forces between them become less. forces between them become less.

such liquids is called plasticizerssuch liquids is called plasticizersCharacteristicsCharacteristics (a ) Soften the material (a ) Soften the material (b) Make it more flexible by lowering its glass transition (b) Make it more flexible by lowering its glass transition

temp . if sufficient plasticizers is added to a polymer It temp . if sufficient plasticizers is added to a polymer It becomes flexible , rubbery in the mouth below mouth becomes flexible , rubbery in the mouth below mouth temperature. So it is used in producing soft lining temperature. So it is used in producing soft lining materials materials

Types of copolymersTypes of copolymers

Random copolymerIn which the two monomers enter into the copolymer in a relatively random manner along the co polymer

chain: M1-M2 – M2 - M2 – M1 – M2-M2-M1Alternating copolymerContains the two monomer units in equimolar amount in regular alternating distribution . M1-M2 – M1 - M2 Block copolymer Is a linear copolymer with one or more long uninterrupted sequences of each polymeric species M1-M1 – M1- M1 – M2 – M2 –M2 – M2Graft copolymer Is a branched copolymer with a back bone of one monomer to which are Attached one or more side chains of another monomer . M1-M1 – M1- M1 –M1 M1 M1 M1

Application of polymers in Application of polymers in dentistrydentistry

Denture base materials.Denture base materials.

Artificial teeth.Artificial teeth.

Tooth restorative Tooth restorative materials. materials.

Cements.Cements.

Crown and bridge Crown and bridge facings.facings.

Impression materials.Impression materials.

DiesDies..Endodontic fillingsEndodontic fillings..Mouth protectorsMouth protectors . .Maxillofacial prosthesisMaxillofacial prosthesis . .

VarnishesVarnishes,, pit and fissure sealantpit and fissure sealant

THANK YOUTHANK YOU

Dental Bio-materialsDental Bio-materials

Ass. Prof. Dr. Mohamed Abed el-Aziz SouhailAss. Prof. Dr. Mohamed Abed el-Aziz Souhail Head of Dental Bio-material Department,Head of Dental Bio-material Department,

Faculty of Dental Medicine,Faculty of Dental Medicine, Al- Azhar University.Al- Azhar University.

Non – metallic Denture base Non – metallic Denture base materialsmaterials

Heat cured PMMA

Auto polymerized PMMA

Injection Molded

Conventional

High impact

PMMA Polycarbonate

Nylon

Unfilled

Reinforced

Glass fiberCarbon

Soft liner

Acrylic

Silicone

RTV

Heat cured

Acrylic resin Acrylic resin Is made by addition process Is made by addition process

free radical addition free radical addition polymerizationpolymerization

a denture consists of denture base and artificial teeth,

different materials have been made to construct a denture

bases

The ideal requirement of The ideal requirement of denture base materialdenture base material

1-Acrylic resin more aesthetics.1-Acrylic resin more aesthetics.2-High impact strength , fatigue strength, stiffness and hardness.2-High impact strength , fatigue strength, stiffness and hardness.3-Good thermal conductivity to maintain healthy tissues.3-Good thermal conductivity to maintain healthy tissues.4- Low density to help denture retention.4- Low density to help denture retention.5-Easy to repair.5-Easy to repair.6-Easy to manipulate.6-Easy to manipulate.7-Goog shelf life.7-Goog shelf life.8-Inexpensive.8-Inexpensive.9-Dimensionally stable and maintain surface details.9-Dimensionally stable and maintain surface details.10-Good retention to base metals ,porcelain teeth and acrylic teeth.10-Good retention to base metals ,porcelain teeth and acrylic teeth.11-Radio-opaque to be seen in x-ray if swallowed.11-Radio-opaque to be seen in x-ray if swallowed.21-Not toxic or irritant to the oral tissues. 21-Not toxic or irritant to the oral tissues.

Composition of Composition of acrylic resinacrylic resin

An acrylic resin denture is made by the An acrylic resin denture is made by the process of free radical addition process of free radical addition polymerization to form polymerization to form polymethylmethacrylate.polymethylmethacrylate.

The monomer is methylmethacrylate.The monomer is methylmethacrylate.

The resins are available in either heat cured The resins are available in either heat cured or self-cured.or self-cured.

Composition Of Heat Cured Resin

Powder1.1. Beads or granules of polymethyl methacrylate. Beads or granules of polymethyl methacrylate.

The granules dissolve more rapidly and hence The granules dissolve more rapidly and hence shorten the dough time.shorten the dough time.

2.2. Benzyl peroxide- Initiator.Benzyl peroxide- Initiator.3.3. Plasticizers - dibutyl phthalate. to produce a less Plasticizers - dibutyl phthalate. to produce a less

brittle.brittle.4.4. Opacifiers- Titanium / zinc oxides .to be seen in Opacifiers- Titanium / zinc oxides .to be seen in

the x-ray if swallowed.the x-ray if swallowed.5.5. Pigments / dyes to give the natural tissue like Pigments / dyes to give the natural tissue like

appearance.appearance.6.6. Synthetic fibers - nylon / Acrylic. to simulate Synthetic fibers - nylon / Acrylic. to simulate

blood vessels of the oral tissues.blood vessels of the oral tissues.

LiquidLiquid

1.1. Methylmethacrylate monomerMethylmethacrylate monomer

2.2. Inhibitor - Hydroquinone to react Inhibitor - Hydroquinone to react with any free radical that may form with any free radical that may form in the liquid.in the liquid.

3.3. Cross - linking agent - ethylene Cross - linking agent - ethylene glycol dimethacrylate cross linking glycol dimethacrylate cross linking agent ,to improve the mechanical agent ,to improve the mechanical propertiesproperties. .

ManipulationManipulation

1-proportioning:1-proportioning: The polymer /powder ratio is 3:1 by The polymer /powder ratio is 3:1 by volume or 2.5:1 by weight,volume or 2.5:1 by weight,

Using this ratio will decrease the polymerization Using this ratio will decrease the polymerization shrinkage from 21 % to 6-7 %shrinkage from 21 % to 6-7 %

Too Much PowderToo Much Powder Could result in under - wetting of the polymer beads by Could result in under - wetting of the polymer beads by

the monomer resulting in a weak material .the monomer resulting in a weak material .Too Much MonomerToo Much Monomer will produce excessive polymerization shrinkage and a will produce excessive polymerization shrinkage and a

loss of quality of fit to the denture bearing area surfaceloss of quality of fit to the denture bearing area surface

Polymerization stagesPolymerization stages

1.1. Sandy stageSandy stage :It is incoherent mass is formed like sand:It is incoherent mass is formed like sand

2.2. Sticky or fibrous stage: The polymer begins to Sticky or fibrous stage: The polymer begins to dissolve in the monomer .dissolve in the monomer .

3.3. Dough stage :The material becomes smooth and does Dough stage :The material becomes smooth and does not stick to the mixing gar or hands and it is easily not stick to the mixing gar or hands and it is easily manipulated manipulated ,,this is the ideal stage for packing.this is the ideal stage for packing.

4.4. Rubbery stage: The material can not be shaped.Rubbery stage: The material can not be shaped.

5.5. Stiff stage:Stiff stage: If the material is left longer it becomes If the material is left longer it becomes stiff.stiff.

PackingPackingWhen the material reaches the dough stage it is ready When the material reaches the dough stage it is ready

for packing in the flaskfor packing in the flaskIf packed in sandy or sticky stages the material will be If packed in sandy or sticky stages the material will be

of high fluidity and will flow out of the flask of high fluidity and will flow out of the flask resulting in denture porosity.resulting in denture porosity.

If packed in rubbery o stiff stage, the material will not If packed in rubbery o stiff stage, the material will not flow under pressure, this result denture without flow under pressure, this result denture without details and with fractured teeth.details and with fractured teeth.

The acrylic should be packed with excess to decrease The acrylic should be packed with excess to decrease polymerization shrinkage from 7 %to0.2 %polymerization shrinkage from 7 %to0.2 %

Sufficient pressure should be applied on the mold to Sufficient pressure should be applied on the mold to avoid shrinkage porosity.avoid shrinkage porosity.

CuringCuringAfter packing the flask underAfter packing the flask under pressure is placed in pressure is placed in

water path at 72 water path at 72 00C using eitherC using either

long or short curing cycleslong or short curing cyclesUnder curing will result in a denture with high Under curing will result in a denture with high

residual monomer lead to tissue irritation and residual monomer lead to tissue irritation and inferior properties.inferior properties.

Over curing will result in boiling of the monomer Over curing will result in boiling of the monomer (100.3 (100.3 00C )C )due to added of heat of polymerization due to added of heat of polymerization and as a result the monomer will vaporize leaving and as a result the monomer will vaporize leaving pores inside the denture (gaseous porosity).pores inside the denture (gaseous porosity).

Cooling- Deflasking- Finishing Cooling- Deflasking- Finishing and polishingand polishing

The flask should be cooled slowly to release the internal stressesThe flask should be cooled slowly to release the internal stressesDeflaskingDeflasking Should be done with care to avoid breaking of the denture.Should be done with care to avoid breaking of the denture.Finishing and polishingFinishing and polishingPropertiesPropertiesResidual monomerResidual monomer

Even in a properly cured acrylic denture,0.2- 0.5 %of the Even in a properly cured acrylic denture,0.2- 0.5 %of the monomer remains curing at too low temperature or for monomer remains curing at too low temperature or for too short time results in higher residual monomer this too short time results in higher residual monomer this should be avoided because it is more irritant to the oral should be avoided because it is more irritant to the oral tissues, it act as plasticizer and make the denture more tissues, it act as plasticizer and make the denture more flexible and weakerflexible and weaker

Types of PorosityTypes of PorosityIt is undesirable effects on both strength and optical It is undesirable effects on both strength and optical

properties of the dentureproperties of the denture

Water sorptionWater sorption

Acrylic dentures absorb water by 2% of its weight, each Acrylic dentures absorb water by 2% of its weight, each 1% increase in weight causes a liner expansion of 0.22 1% increase in weight causes a liner expansion of 0.22 %.%.

Also drying out the acrylic dentures is associated with Also drying out the acrylic dentures is associated with shrinkage.shrinkage.

Water sorption is an advantage to compensate the Water sorption is an advantage to compensate the processing shrinkageprocessing shrinkage

which is 0.44 % (the polymerization and the thermal which is 0.44 % (the polymerization and the thermal shrinkage occurring when cooling from (72 shrinkage occurring when cooling from (72 00C to C to

20 20 00C )So dentures should be kept wet all the time when C )So dentures should be kept wet all the time when not in service. not in service.

CrazingCrazingIs an actual mechanical separation of the polymer chains Is an actual mechanical separation of the polymer chains

or group of chains under tensile stresses.or group of chains under tensile stresses.

And it has weakening effect on the denture.And it has weakening effect on the denture.

Causes:Causes:

a- Continuous stresses due to repeated drying and wetting a- Continuous stresses due to repeated drying and wetting of the denture.of the denture.

b- b- Difference in coefficiency of thermal expansion Difference in coefficiency of thermal expansion between acrylic denture base and porcelain teeth.between acrylic denture base and porcelain teeth.

c- Action of solvents e.g. alcohol ,acetone chloroform , c- Action of solvents e.g. alcohol ,acetone chloroform , monomer when a denture is repaired and may cause monomer when a denture is repaired and may cause crazing.crazing.

Mechanical propertiesMechanical properties

Thermal conductivity:Thermal conductivity: Is very low thermal conductivity, and this Is very low thermal conductivity, and this

is disadvantages.is disadvantages.Coefficient of thermal expansion:Coefficient of thermal expansion: It is quite high and this does not problem, It is quite high and this does not problem,

except that porcelain teeth in denture base except that porcelain teeth in denture base may gradually loosen and lost.may gradually loosen and lost.

UsesUses: Denture bases and artificial teeth.: Denture bases and artificial teeth.

Control Of ColorControl Of ColorThe coloring pigment is usually incorporate in the The coloring pigment is usually incorporate in the

polymer powder, but in some cases it may simply be polymer powder, but in some cases it may simply be on the surface of the polymer beads and may be on the surface of the polymer beads and may be washed off by too rapid contact of the monomer in washed off by too rapid contact of the monomer in this case.this case.

Produce too light a shad Produce too light a shad Should be added to the monomer slowly too little Should be added to the monomer slowly too little

powder will mould -liningpowder will mould -liningSeparating medium must be employed to prevent Separating medium must be employed to prevent

adhering to the mould nowadays.adhering to the mould nowadays.The separating medium is usually a solution of sodium The separating medium is usually a solution of sodium

alginate, although some still recommend the use of alginate, although some still recommend the use of tinfoil.tinfoil.

ProcessingProcessing

Two problems in the processing of acrylics for the Two problems in the processing of acrylics for the dentures .dentures .

1) Porosity: Is the presence of processing strains . 1) Porosity: Is the presence of processing strains .

During Processing Stage

Causes of porosity Causes of porosity

a)a) Polymerization shrinkage Polymerization shrinkage

(Contraction porosity )(Contraction porosity )

1.1. Volatilization of the monomer Volatilization of the monomer

( gaseous porosity( gaseous porosity

Contraction PorosityContraction Porosity

A monomer contracts by some 20% during processing by A monomer contracts by some 20% during processing by using powder / liquid system the contraction is using powder / liquid system the contraction is minimized 5-8% (high linear shrinkage ) volumetric minimized 5-8% (high linear shrinkage ) volumetric contraction (1.2-2%) thermal contraction (0.2-0.5%) contraction (1.2-2%) thermal contraction (0.2-0.5%) from curing contraction .from curing contraction .

at the curing temperature at the curing temperature The resin is able to flow into the spaces created by the The resin is able to flow into the spaces created by the

curing contraction.curing contraction.Cold cure resins should give a better fit for the denture as Cold cure resins should give a better fit for the denture as

the processing temp . is considerably lower ( around the processing temp . is considerably lower ( around 60co compared to 100co for the heat cured resin ).60co compared to 100co for the heat cured resin ).

Localized PorosityLocalized Porosity

Due to poor mixing of the components or to packing the Due to poor mixing of the components or to packing the mould before the doughy stage is reached gaseous mould before the doughy stage is reached gaseous porosity .porosity .

due to exothermic reaction the temp, of the resin due to exothermic reaction the temp, of the resin increase above the boiling temp., which is just above increase above the boiling temp., which is just above 100 co .100 co .

CracksThe relief of internal strain can produce tiny surface The relief of internal strain can produce tiny surface

crakes in the resin these are known as crazes and on crakes in the resin these are known as crazes and on be identified by a hazy or foggy appearance the crakes be identified by a hazy or foggy appearance the crakes may be visible .may be visible .