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 consistingPolymers Are long chain molecules consisting
of many (poly) repeating units (mers).of many (poly) repeating units (mers).
A monomer: Is the smallest repeating unit inA 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 monomerswhich polymers are formed from monomers
chain e.g. ethylene monomer.chain e.g. ethylene monomer.

CLASSIFICATIONS OFCLASSIFICATIONS OF
POLYMERSPOLYMERS
Classifications
according to
Classifications
according to
Polymerization
reactionSpatialThermalOrigin

The originThe origin
Natural polymersNatural polymers
Agar, celluloseAgar, cellulose
DNA, proteinsDNA, proteins
Natural rubberNatural rubber
Collagen and silkCollagen and silk

A) Synthetic PolymersA) Synthetic Polymers
ِِProduced in the laboratory by chemicalProduced in the laboratory by chemical
reactions. e.g. Acrylic resin ,nylon andreactions. e.g. Acrylic resin ,nylon and
polystyrene.polystyrene.
Originally the synthetic polymers regarded asOriginally the synthetic polymers regarded as
substitutes for existing natural polymers suchsubstitutes for existing natural polymers such
as rubber and silk .Nowadays a polymers canas rubber and silk .Nowadays a polymers can
be produced and have medical use such asbe produced and have medical use such as
dialysis and oxygenator membranes anddialysis and oxygenator membranes and
dental application such as filling materialsdental application such as filling materials ..

B) Natural PolymersB) Natural Polymers
Proteins polyamides of polypeptidesProteins polyamides of polypeptides
polyisoprenespolyisoprenes
e.g. rubber and gutta-perchae.g. rubber and gutta-percha
polysaccharidespolysaccharides
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 BehaviorAccording To Thermal Behavior
Thermoplastic resinThermoplastic resin
They are polymers which can beThey are polymers which can be
shaped by heat and after coolingshaped by heat and after cooling
they maintain their shape thethey maintain their shape the
reaction is reversible and can bereaction 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 shapeThey 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 butThey 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 primaryThe polymers form network with cross –links between them primary
covalent bonds. E.g. PMMA.covalent bonds. E.g. PMMA.

ThermalThermal
a) Thermosetting b) Thermoplastica) Thermosetting b) Thermoplastic
(polyvinyl chloride
polyethylene
Nylon
Polystyrene

b) Thermoset polymersb) Thermoset polymers
Harden during fabrication (irreversible or chemicalHarden during fabrication (irreversible or chemical
reaction).reaction).
ChemicalChemical
Soft hardSoft hard
reactionreaction
Examples are cross-linkedExamples are cross-linked
polymethylmethacrylate, silicons ,and bisphenolpolymethylmethacrylate, silicons ,and bisphenol
A-diacrylates.A-diacrylates.

According to the basis of structural or spatialAccording to the basis of structural or spatial
configurationconfiguration
linea
r
copolymer
Simple polymer
Has one type
monomer
homopolymer
Branched
Random
Block
OR
Homo
Copolymer
OR
graftRandom
3 Basic Types
Cross-linked
polymer

))a) Linear polymers
(simple polymer)
This may be homopolymerThis may be homopolymer it has one type ofit has one type of
monomer ,monomer ,or copolymer, this means thator copolymer, this means that it hasit has
more than one type of monomer,more than one type of monomer,
Homopolymer copolymer, randomHomopolymer copolymer, random
blockblock

b) Branched polymersb) Branched polymers
This type of polymers show higher strength hardness andThis 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 TheBranched 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, randomHomopolymer copolymer, random
graftgraft
2

C) Cross-LinkingC) Cross-Linking
Polymer chains are cross - linked or attached byPolymer chains are cross - linked or attached by
a cross - linking agent such as glycola cross - linking agent such as glycol
dimethaerylate.dimethaerylate.
Through primary and small degree of crossThrough primary and small degree of cross
linking will limit the amount of movements oflinking 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 deformationWhen 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 mayHigher 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 ofIt 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 fromThe chemical reaction by which polymers are prepared from
monomersmonomers..
Condensation polymerization
Occurs when two molecules react to form a large molecules
with the elimination of smaller molecule
)Often but not always water(
Addition polymerization
Occurring 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 condensationMost 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 largerThe 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 productPolymer + by product
MonomerMonomer polymerizationpolymerization
ExExample is the polysulphide rubber impression material.ample is the polysulphide rubber impression material.

22((Addition polymerizationAddition polymerization
The reaction between two molecules to giveThe reaction between two molecules to give
a larger molecule without the eliminationa 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, theNo change in composition takes place, the
structure of the monomer is repeated manystructure 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 ADDITIONOF 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 initiationA) Activation and initiation
A free radical is a compound with an unpairedA free radical is a compound with an unpaired
(unshared )electron (very reactive )(unshared )electron (very reactive )
The symbol (c= c) represents four electrons theThe symbol (c= c) represents four electrons the
monomer itself become free radical Benzoyelmonomer itself become free radical Benzoyel
peroxide is a substance most commonly used toperoxide 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 chemicalThe activation is done by light, heat or by a chemical
compound such as dimethyl paratoluidine. In thiscompound such as dimethyl paratoluidine. In this
way the peroxide decomposes giving up freeway the peroxide decomposes giving up free
radicals to initiate the polymerization reaction .radicals to initiate the polymerization reaction .

Three stages of addition polymerization HeatThree stages of addition polymerization Heat
1) Initiation1) Initiation
AminesAmines
C6H5 COOCC6H5C6H5 COOCC6H5 2 (C6H5COO) + CO22 (C6H5COO) + CO2
Benzoyl peroxide free radicals (R) + carbon dioxideBenzoyl peroxide free radicals (R) + carbon dioxide
CH3 CH3CH3 CH3
R+ CH2 = C R-CH2 – CR+ CH2 = C R-CH2 – C
COOCH3 COOCH3COOCH3 COOCH3
Free radical + monomer Free radical activated monomerFree radical + monomer Free radical activated monomer
Heat
Amines

B) PropagationB) Propagation
The chain reactions continues with the evolutionThe chain reactions continues with the evolution
of heat until all the monomer has beenof heat until all the monomer has been
changed to a polymerchanged 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 + monomerpolymer free radical + monomer growing chaingrowing chain

C) TerminationC) Termination
The reaction terminated either byThe reaction terminated either by
a) Direct couplinga) Direct coupling
This occurs when two free radicals reactThis occurs when two free radicals react
to form a stable moleculeto form a stable molecule
1- R1MN + R2MM1- R1MN + R2MM R1MNR1MN R2MMR2MM
b) Exchange of a hydrogen atomsb) Exchange of a hydrogen atoms
The chain reactions can be terminated eitherThe chain reactions can be terminated either
by direct coupling or by the exchange ofby 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 withFactors associated with
polymerizationpolymerization
A) Evolution of heat as theA) Evolution of heat as the
reactions is strongly exothermicreactions is strongly exothermic
due to breaking of bonds.due to breaking of bonds.
B) Reduction in volume.B) Reduction in volume.

Inhibition of polymerizationInhibition of polymerization
Any impurity in the monomer which can reactAny impurity in the monomer which can react
with free Radicals will inhibit or retard thewith 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 activatedor any activated nucleus or with an activated
growing chain to prevent further growthgrowing chain to prevent further growth

The presence of such inhibitors influences theThe presence of such inhibitors influences the
length of the initiation period, as well aslength of the initiation period, as well as
the degree of polymerization.the degree of polymerization.
The addition of small amount of inhibitorThe addition of small amount of inhibitor
(Hydroquinone) to the monomer(Hydroquinone) to the monomer
(Methyl methacrylate)(Methyl methacrylate)
will inhibit polymerization if no chemicalwill inhibit polymerization if no chemical
initiator is present and it will retard theinitiator is present and it will retard the
polymerization in the presencepolymerization in the presence
of an initiator.of an initiator.

Factors affecting the properties ofFactors affecting the properties of
polymerspolymers
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 andThe 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 polymersLonger chains and higher molecular weight polymers
characterized bycharacterized 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- linkedFor example composite resins highly cross- linked
matrix in which large number of strong covalentmatrix in which large number of strong covalent
linkages between chains transforms the moleculeslinkages between chains transforms the molecules
into a rigid very high molecular weight materialinto a rigid very high molecular weight material
resulting strength, stiffness and withstand occlusalresulting strength, stiffness and withstand occlusal
stresses.stresses.

In contrastIn contrast
Elastomeric impression materials composed ofElastomeric impression materials composed of
individual coiled chains with just few cross- linksindividual coiled chains with just few cross- links
this type of molecular structure permits the largethis type of molecular structure permits the large
scale uncoiling and recoiling of chains that givesscale uncoiling and recoiling of chains that gives
these materials high flexibility.these materials high flexibility.
Two basic properties which characterize polymersTwo 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 = MwT g = Mw
Molecular weight is another factor which affectsMolecular 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 orTo modify the properties of the polymer , use two or
more chemically different monomers such asmore chemically different monomers such as
copolymer process enabled chemists tocopolymer process enabled chemists to
““ tailor – make “ molecules of predicted properties fortailor – make “ molecules of predicted properties for
special applications e.g.special applications e.g.
Addition of sufficient amounts of octyl methacrylateAddition of sufficient amounts of octyl methacrylate
results in a copolymer that is soft and flexible atresults in a copolymer that is soft and flexible at
mouth temp.mouth temp.
And that has been used as a soft linerAnd that has been used as a soft liner
1.1. Hydroxyethyl methacrylate increase water sorptionHydroxyethyl 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 inThe 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 perA chemical compound with two double bonds per
molecules can act as cross-linking agent since eachmolecules 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 isThis product has superior properties an example is
ethylene glycol dimethacrylate .ethylene glycol dimethacrylate .
Small degree of cross-linking limits the amount of theSmall 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 moreThe polymer is harder more brittle and more
resistance to the action of solventsresistance to the action of solvents

PlasticizersPlasticizers
Liquids are able to penetrate between the chains ofLiquids are able to penetrate between the chains of
polymer .The molecules become further apart and thepolymer .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 plasticizers
CharacteristicsCharacteristics
(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 Ittemp . if sufficient plasticizers is added to a polymer It
becomes flexible , rubbery in the mouth below mouthbecomes flexible , rubbery in the mouth below mouth
temperature. So it is used in producing soft liningtemperature. So it is used in producing soft lining
materialsmaterials

Types of copolymersTypes of copolymers
Random copolymer
In 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-M1
Alternating copolymer
Contains 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 – M2
Graft 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 inApplication of polymers in
dentistrydentistry
Denture base materials.Denture base materials.
Artificial teeth.Artificial teeth.
Tooth restorativeTooth restorative
materials.materials.
Cements.Cements.
Crown and bridgeCrown 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

Non – metallic Denture base materialsNon – metallic Denture base materials
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 resinAcrylic resin
Is made by addition processIs made by addition process
free radical additionfree 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 denture baseThe ideal requirement of denture base
materialmaterial
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 acrylicComposition of acrylic
resinresin
An acrylic resin denture is made by theAn acrylic resin denture is made by the
process of free radical additionprocess of free radical addition
polymerization to formpolymerization to form
polymethylmethacrylate.polymethylmethacrylate.
The monomer is methylmethacrylate.The monomer is methylmethacrylate.
The resins are available in either heat curedThe resins are available in either heat cured
or self-cured.or self-cured.

Composition Of Heat Cured Resin
Powder
1.1. Beads or granules of polymethyl methacrylate.Beads or granules of polymethyl methacrylate.
The granules dissolve more rapidly and henceThe 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 lessPlasticizers - dibutyl phthalate. to produce a less
brittle.brittle.
4.4. Opacifiers- Titanium / zinc oxides .to be seen inOpacifiers- 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 likePigments / dyes to give the natural tissue like
appearance.appearance.
6.6. Synthetic fibers - nylon / Acrylic. to simulateSynthetic 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 reactInhibitor - Hydroquinone to react
with any free radical that may formwith any free radical that may form
in the liquid.in the liquid.
3.3. Cross - linking agent - ethyleneCross - linking agent - ethylene
glycol dimethacrylate cross linkingglycol dimethacrylate cross linking
agent ,to improve the mechanicalagent ,to improve the mechanical
propertiesproperties..

ManipulationManipulation
1-proportioning:1-proportioning: The polymer /powder ratio is 3:1 byThe 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 polymerizationUsing 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 byCould 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 awill 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 toSticky or fibrous stage: The polymer begins to
dissolve in the monomer .dissolve in the monomer .
3.3. Dough stage :The material becomes smooth and doesDough stage :The material becomes smooth and does
not stick to the mixing gar or hands and it is easilynot stick to the mixing gar or hands and it is easily
manipulatedmanipulated ,,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 becomesIf the material is left longer it becomes
stiff.stiff.

PackingPacking
When the material reaches the dough stage it is readyWhen the material reaches the dough stage it is ready
for packing in the flaskfor packing in the flask
If packed in sandy or sticky stages the material will beIf packed in sandy or sticky stages the material will be
of high fluidity and will flow out of the flask resultingof high fluidity and will flow out of the flask resulting
in denture porosity.in denture porosity.
If packed in rubbery o stiff stage, the material will notIf packed in rubbery o stiff stage, the material will not
flow under pressure, this result denture withoutflow under pressure, this result denture without
details and with fractured teeth.details and with fractured teeth.
The acrylic should be packed with excess to decreaseThe 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 toSufficient pressure should be applied on the mold to
avoid shrinkage porosity.avoid shrinkage porosity.

CuringCuring
After packing the flask underAfter packing the flask under pressure is placed inpressure is placed in
water path at 72water path at 72 00C using eitherC using either
long or short curing cycleslong or short curing cycles
Under curing will result in a denture with highUnder curing will result in a denture with high
residual monomer lead to tissue irritation andresidual monomer lead to tissue irritation and
inferior properties.inferior properties.
Over curing will result in boiling of the monomerOver curing will result in boiling of the monomer
(100.3(100.3 00C )C )due to added of heat of polymerizationdue to added of heat of polymerization
and as a result the monomer will vaporize leavingand as a result the monomer will vaporize leaving
pores inside the denture (gaseous porosity).pores inside the denture (gaseous porosity).

Cooling- Deflasking- Finishing andCooling- Deflasking- Finishing and
polishingpolishing
The flask should be cooled slowly to release the internal stressesThe flask should be cooled slowly to release the internal stresses
DeflaskingDeflasking
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 polishing
PropertiesProperties
Residual monomerResidual monomer
Even in a properly cured acrylic denture,0.2- 0.5 %of theEven in a properly cured acrylic denture,0.2- 0.5 %of the
monomer remains curing at too low temperature or formonomer remains curing at too low temperature or for
too short time results in higher residual monomer thistoo short time results in higher residual monomer this
should be avoided because it is more irritant to the oralshould be avoided because it is more irritant to the oral
tissues, it act as plasticizer and make the denture moretissues, it act as plasticizer and make the denture more
flexible and weakerflexible and weaker

Types of PorosityTypes of Porosity
It is undesirable effects on both strength and opticalIt 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, eachAcrylic dentures absorb water by 2% of its weight, each
1% increase in weight causes a liner expansion of 0.221% increase in weight causes a liner expansion of 0.22
%.%.
Also drying out the acrylic dentures is associated withAlso drying out the acrylic dentures is associated with
shrinkage.shrinkage.
Water sorption is an advantage to compensate theWater sorption is an advantage to compensate the
processing shrinkageprocessing shrinkage
which is 0.44 % (the polymerization and the thermalwhich is 0.44 % (the polymerization and the thermal
shrinkage occurring when cooling from (72shrinkage occurring when cooling from (72 00C toC to
2020 00C )So dentures should be kept wet all the time whenC )So dentures should be kept wet all the time when
not in service.not in service.

CrazingCrazing
Is an actual mechanical separation of the polymer chainsIs 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 wettinga- Continuous stresses due to repeated drying and wetting
of the denture.of the denture.
b-b- Difference in coefficiency of thermal expansionDifference 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 causemonomer when a denture is repaired and may cause
crazing.crazing.

Mechanical propertiesMechanical properties
Thermal conductivity:Thermal conductivity:
Is very low thermal conductivity, and thisIs 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 baseexcept 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 Color
The coloring pigment is usually incorporate in theThe coloring pigment is usually incorporate in the
polymer powder, but in some cases it may simply bepolymer powder, but in some cases it may simply be
on the surface of the polymer beads and may beon the surface of the polymer beads and may be
washed off by too rapid contact of the monomer inwashed off by too rapid contact of the monomer in
this case.this case.
Produce too light a shadProduce too light a shad
Should be added to the monomer slowly too littleShould be added to the monomer slowly too little
powder will mould -liningpowder will mould -lining
Separating medium must be employed to preventSeparating medium must be employed to prevent
adhering to the mould nowadays.adhering to the mould nowadays.
The separating medium is usually a solution of sodiumThe separating medium is usually a solution of sodium
alginate, although some still recommend the use ofalginate, although some still recommend the use of
tinfoil.tinfoil.

ProcessingProcessing
Two problems in the processing of acrylics for theTwo 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 porosityCauses of porosity
a)a) Polymerization shrinkagePolymerization shrinkage
(Contraction porosity )(Contraction porosity )
1.1. Volatilization of the monomerVolatilization of the monomer
( gaseous porosity( gaseous porosity

Contraction PorosityContraction Porosity
A monomer contracts by some 20% during processing byA monomer contracts by some 20% during processing by
using powder / liquid system the contraction isusing powder / liquid system the contraction is
minimized 5-8% (high linear shrinkage ) volumetricminimized 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 temperatureat the curing temperature
The resin is able to flow into the spaces created by theThe 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 asCold cure resins should give a better fit for the denture as
the processing temp . is considerably lower ( aroundthe 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 theDue to poor mixing of the components or to packing the
mould before the doughy stage is reached gaseousmould before the doughy stage is reached gaseous
porosity .porosity .
due to exothermic reaction the temp, of the resindue to exothermic reaction the temp, of the resin
increase above the boiling temp., which is just aboveincrease above the boiling temp., which is just above
100 co .100 co .
Cracks
The relief of internal strain can produce tiny surfaceThe relief of internal strain can produce tiny surface
crakes in the resin these are known as crazes and oncrakes in the resin these are known as crazes and on
be identified by a hazy or foggy appearance the crakesbe identified by a hazy or foggy appearance the crakes
may be visible .may be visible .

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