resins/ academy general dentistry

www.indiandentalacademy.comwww.indiandentalacademy.com

Chemistry Chemistry of of

The Synthetic ResinsThe Synthetic ResinsINDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com


INTRODUCTIONINTRODUCTION HISTORYHISTORY REQUISITES FOR DENTAL RESINS REQUISITES FOR DENTAL RESINS BASIC NATURE OF POLYMERS BASIC NATURE OF POLYMERS

PHYSICAL PROPERTIEPHYSICAL PROPERTIEss THERMAL PROPERTIESTHERMAL PROPERTIES CHEMISTRY OF POLYMERIZATIONCHEMISTRY OF POLYMERIZATION CONCLUSION CONCLUSION



http://www.indiandentalacademy.com/

Introduction:

Polymers have been of importance for 60 years or more

and find use in almost every sphere of modern life.

It continues to be the material of choice for fabricating

many denture prosthesis because of its ease of processing

and generally favorable physical and mechanical properties


History:The loss of teeth by accident or disease has plagued mankind

throughout the ages. In ancient times the materials available were simple in nature, few in number and with much disappointment often with the final results.

Among the more valuable developments in very recent years has been the synthetic resins that now are so important to prosthetic and restorative dentistry.

Materials between 1840 and 1940:It was about 1855 that vulcanized hard rubber was discovered

and introduced as a denture base material with such names applied as ebonite or vulcanite.

During the following 75 years the use of vulcanized rubber was to remain. From the time it was introduced until about 1930 and 1940 vulcanite was the best denture base material that science and industry could produce. The principal disadvantage was the poor overall esthetic quality.


Celluloid, Bakelite, and other resins: Celluloid, Bakelite, and other resins: In 1868 John Wesley Hyatt is credited with preparing the first organic In 1868 John Wesley Hyatt is credited with preparing the first organic

plastic molding compound, which was cellulose nitrate, then popularly known plastic molding compound, which was cellulose nitrate, then popularly known as as “celluloid“celluloid”.”.

Cellulose nitrate was produced in pink tissue colors that were more pleasing Cellulose nitrate was produced in pink tissue colors that were more pleasing

thanthan vulcanitevulcanite.The presence of camphor (as a plasticizer), produced an .The presence of camphor (as a plasticizer), produced an unpleasant taste and odor. unpleasant taste and odor.

During the years to about 1930, various modifications of cellulose were used. During the years to about 1930, various modifications of cellulose were used.

Not until 1909 was phenol formaldehyde resin was discovered by Not until 1909 was phenol formaldehyde resin was discovered by Dr. LeoDr. Leo BakelandBakeland and known as and known as Bakelite.Bakelite.

Products were available in the form of sheets, cakes, and powder. Products were available in the form of sheets, cakes, and powder.

The physical qualities of the final denture depended to a large degree on the The physical qualities of the final denture depended to a large degree on the

processing conditionsprocessing conditions.. www.indiandentalacademy.comwww.indiandentalacademy.com

This lack of uniformity and control of qualities were the principal This lack of uniformity and control of qualities were the principal disadvantage of this material. disadvantage of this material.

The period from 1930 to 1940 was one of the rapid expansion in the The period from 1930 to 1940 was one of the rapid expansion in the resin industry.resin industry.

The introduction of a more satisfactory plastic denture base material The introduction of a more satisfactory plastic denture base material occurred in 1937 when occurred in 1937 when Dr. Walter WrightDr. Walter Wright described the results of described the results of his clinical evaluation of his clinical evaluation of methyl methacrylate resin. methyl methacrylate resin.

Vulcanite, celluloid, bakelite were rapidly displaced from dental Vulcanite, celluloid, bakelite were rapidly displaced from dental practice. practice. An ADA specification number 12 has been developed for acrylic An ADA specification number 12 has been developed for acrylic denture base material.denture base material.

Dental resin since 1940Dental resin since 1940 During the past 65 years since acrylic resins were introduced, the During the past 65 years since acrylic resins were introduced, the quality of resins has been more refined and improved.quality of resins has been more refined and improved.The methylmethacrylate polymers and copolymers continue to be The methylmethacrylate polymers and copolymers continue to be the most popular resin. the most popular resin.


CLASSIFICATION;CLASSIFICATION;BASED ON THE THERMAL BEHAVIOURBASED ON THE THERMAL BEHAVIOUR

1 THERMOPLASTIC1 THERMOPLASTIC

2 THERMOSETTING2 THERMOSETTING

poly(methyl methacrylate)poly(methyl methacrylate)polyethylenepolyethylenepolystyrene.polystyrene.

e.g.: cross linked poly(methyl methacrylate)e.g.: cross linked poly(methyl methacrylate)SiliconSilicon

BASED ON THE SPATIAL STRUCTUREBASED ON THE SPATIAL STRUCTUREA) LINEARA) LINEARRandom copolymer of the linear type.Random copolymer of the linear type.Linear block copolymerLinear block copolymer

B) BRANCHEDB) BRANCHEDBranched homopolymerBranched homopolymerGraft branchedGraft branched

C) CROSS LINKEDC) CROSS LINKEDwww.indiandentalacademy.comwww.indiandentalacademy.com

““REQUISITES FOR DENTAL RESINSREQUISITES FOR DENTAL RESINS”.”. Biological compatibilityBiological compatibility the resin should be tasteless, odorless, the resin should be tasteless, odorless,

non toxic, non irritating, and otherwise not harmful to the oral non toxic, non irritating, and otherwise not harmful to the oral tissues. tissues.

It should be It should be completely insoluble in salivacompletely insoluble in saliva or in any fluids or in any fluids taken into the mouth. taken into the mouth.

It should be impermeable to oral fluids to the extent that the resin It should be impermeable to oral fluids to the extent that the resin does not become does not become unsanitary or disagreeableunsanitary or disagreeable in taste or odor. in taste or odor.

Physical propertiesPhysical properties Should possess adequate strength and Should possess adequate strength and resilience, as well as resistance to biting, chewing forces, impact resilience, as well as resistance to biting, chewing forces, impact forces, and excessive wear. forces, and excessive wear.

The material should be The material should be dimensionally stabledimensionally stable under all under all conditions. conditions.

The resin should have a The resin should have a low specific gravitylow specific gravity when used for when used for

maxillary denturesmaxillary dentures. . www.indiandentalacademy.comwww.indiandentalacademy.com

Manipulation Manipulation The resin should not produce The resin should not produce toxic fumes or dusttoxic fumes or dust

during handling and manipulation. during handling and manipulation.

It should be easy to mix, insert, shape, and cure. It should be easy to mix, insert, shape, and cure.

The final product should be easy to polish, and in case of unavoidable The final product should be easy to polish, and in case of unavoidable

breakage it should be possible to repair the resin easily and efficiently. breakage it should be possible to repair the resin easily and efficiently.

Aesthetic propertiesAesthetic properties The material The material should exhibit sufficient should exhibit sufficient

translucency or transparencytranslucency or transparency so that it can be made to match the so that it can be made to match the

appearance of the oral tissues it replaces. appearance of the oral tissues it replaces.

The resin should be capable of being The resin should be capable of being tinted or pigmentedtinted or pigmented, but there , but there

should be no change in color or appearance of the material subsequent to should be no change in color or appearance of the material subsequent to

its fabrication. its fabrication.


Economic considerationsEconomic considerations The cost of the resin and its The cost of the resin and its

processing method should be relatively low, without requiring any processing method should be relatively low, without requiring any

expensive equipment.expensive equipment.

Chemical stabilityChemical stability The conditions in the mouth are highly The conditions in the mouth are highly

demanding, and only the most chemically stable and inert materials demanding, and only the most chemically stable and inert materials

can withstand such conditions without deterioration. can withstand such conditions without deterioration.


BASIC NATURE OF POLYMERS:BASIC NATURE OF POLYMERS:

Chemical compositionChemical composition The The term polymer denotes a molecule that is made up of term polymer denotes a molecule that is made up of

manymany(Poly) parts (mer). (Poly) parts (mer). The mer ending represents the simplest repeating chemical The mer ending represents the simplest repeating chemical

structural unit from which the polymer is composed. structural unit from which the polymer is composed.

Thus Thus poly (methyl methacrylatepoly (methyl methacrylate..) ) is a polymer having chemical structural units is a polymer having chemical structural units

derived from methyl methacrylate. derived from methyl methacrylate.

Copolymer structures – polymers that have only one type of repeating unit (mer) are Copolymer structures – polymers that have only one type of repeating unit (mer) are

homopolymers. homopolymers.

Those with two or more types of mer units are known as copolymers. Those with two or more types of mer units are known as copolymers.

There are 3 different types of copolymers. There are 3 different types of copolymers.

- Random co-polymer – no sequential order exists between the two (or more) mer units - Random co-polymer – no sequential order exists between the two (or more) mer units

along the polymer chain. along the polymer chain.

……. ABBABABAAABAAAABABBBB….. ABBABABAAABAAAABABBBB….

-- www.indiandentalacademy.comwww.indiandentalacademy.com

Block copolymerBlock copolymer – identical monomer units occur in relatively long – identical monomer units occur in relatively long sequences along the main polymer. sequences along the main polymer.

… … AAAAABBBBBBAAAABBBBBBBAAABBB….AAAAABBBBBBAAAABBBBBBBAAABBB….

-- Graft or branched copolymerGraft or branched copolymer – Sequences of one type of mer unit are – Sequences of one type of mer unit are attached as a graft onto a backbone of a second type of mer unit. attached as a graft onto a backbone of a second type of mer unit.

BB

BB

BB

BB

BB

BB

AAAAAAAAAA……AAAAAAAAAA……


Molecular weightMolecular weight : : The molecular weight of the polymer molecule which The molecular weight of the polymer molecule which

equals the molecular weight of the various mers multiplied by the number of mers It equals the molecular weight of the various mers multiplied by the number of mers It

may range from thousands to millions of molecular weight units, depending on the may range from thousands to millions of molecular weight units, depending on the

preparation conditions. preparation conditions.

The higher the molecular weight of the polymer made from a single monomer, the The higher the molecular weight of the polymer made from a single monomer, the

higher the degree of polymerization. higher the degree of polymerization.

The The degree of polymerizationdegree of polymerization is defined as the total number of mers in a polymer is defined as the total number of mers in a polymer

molecule. molecule.

Two poly (methyl methacrylate) samples can have the same chemical composition Two poly (methyl methacrylate) samples can have the same chemical composition

but greatly differ in physical properties because one of the samples has a high but greatly differ in physical properties because one of the samples has a high

percentage of low-mol weight molecules, whereas the other has a high % of high-percentage of low-mol weight molecules, whereas the other has a high % of high-

molecular weight molecules. molecular weight molecules. The higher the mol weight, The higher the mol weight, the higher the softening the higher the softening

and melting points and the stiffer the plastic.and melting points and the stiffer the plastic.


The longer the polymer chainThe longer the polymer chain – the more difficult it is to distort the – the more difficult it is to distort the polymer material. polymer material.

The strength, rigidity, melting temperature increases with theThe strength, rigidity, melting temperature increases with the increasing chain length.increasing chain length.

Synthetic resins polymerize randomlySynthetic resins polymerize randomly from local sites that have from local sites that have been activated. Thus, depending on the ability of the chains to grow been activated. Thus, depending on the ability of the chains to grow from their local activation sites, the chain varies in length. from their local activation sites, the chain varies in length.

The average molecular weight for various commercial denture base The average molecular weight for various commercial denture base polymers varies from polymers varies from 8,000 to 39,000. 8,000 to 39,000.

Two types of averages are commonly used The number average Two types of averages are commonly used The number average based on the average number of mer repeating units in a chain, based on the average number of mer repeating units in a chain, and the weight average based on the molecular weight of the and the weight average based on the molecular weight of the average chain. The residual monomer also has a pronounced effect average chain. The residual monomer also has a pronounced effect on the molecular weight – for ex- 0.9% of residual monomer in a on the molecular weight – for ex- 0.9% of residual monomer in a polymer, with a theoretical number average molecular weight of 22, polymer, with a theoretical number average molecular weight of 22, 400, if completely cured will reduce the molecular weight of the 400, if completely cured will reduce the molecular weight of the polymer to 7,300. polymer to 7,300.


Spatial structure: Spatial structure: There are 3 basic types of structures – linear, branched, and cross-linked. There are 3 basic types of structures – linear, branched, and cross-linked.


The linear The linear homopolymer has mer units of the same type.homopolymer has mer units of the same type.

And the And the random polymerrandom polymer of the linear type has the two mer units randomly of the linear type has the two mer units randomly

distributed along the chain. distributed along the chain.

The The linear block copolymerlinear block copolymer has segments or blocks along the chain where the has segments or blocks along the chain where the

mer units are the same.mer units are the same.


The The branched homopolymerbranched homopolymer consists of the same mer units, consists of the same mer units,

whereas the whereas the graft branched copolymergraft branched copolymer consists of one type of mer unit on consists of one type of mer unit on

the main chain and another mer for the branches. the main chain and another mer for the branches.


PHYSICAL PROPERTIES OF POLYMERSPHYSICAL PROPERTIES OF POLYMERS : :

Deformation and RecoveryDeformation and Recovery: : Applied forces produce stresses within the Applied forces produce stresses within the

polymers that can cause polymers that can cause elastic strain, plastic strain, or a combination of elastic strain, plastic strain, or a combination of elastic plus plastic strain. elastic plus plastic strain.

- Plastic deformation is - Plastic deformation is irreversible irreversible and results in a new permanent shape. and results in a new permanent shape.

- Elastic deformation is - Elastic deformation is reversiblereversible and will be completely recovered when and will be completely recovered when the stress is eliminated. the stress is eliminated.

Viscoelastic deformationViscoelastic deformation results in a combination of elastic and plastic results in a combination of elastic and plastic strain, however once the stress is eliminated, the process of recovery strain, however once the stress is eliminated, the process of recovery

occurs over time. occurs over time.


Rheometric propertiesRheometric properties : : The rheometry, or flow behavior of solid polymers The rheometry, or flow behavior of solid polymers

involves a combination of elastic and plastic deformation and elastic recovery involves a combination of elastic and plastic deformation and elastic recovery

when stresses are eliminated.when stresses are eliminated. This combination of elastic and plastic changes is termed viscoelasticity.

The chain length, number of cross links, temperature, and rate of force

application determine with type of behavior dominates. - Plastic flow – Irreversible strain behavior that occurs when polymer chains slide

over one another and become relocated within the material resulting in permanent

deformation.


-- Elastic recoveryElastic recovery – reversible strain behavior that occurs – reversible strain behavior that occurs in the amorphous regions of polymers when the in the amorphous regions of polymers when the randomly coiled chains straighten and then recoil randomly coiled chains straighten and then recoil without sliding past one another when an applied force is without sliding past one another when an applied force is removed. removed.

- The actual dental polymers are deformed by a - The actual dental polymers are deformed by a combination of elastic plus plastic strain processes. combination of elastic plus plastic strain processes. Plastics exhibit a high level of plastic deformation, but Plastics exhibit a high level of plastic deformation, but they also have at least some small degree of elastic they also have at least some small degree of elastic recovery. This phenomenon is called Viscoelastic recovery. This phenomenon is called Viscoelastic recovery. recovery.


Solvation propertiesSolvation properties : : Polymers are usually slow to dissolve, are seldom Polymers are usually slow to dissolve, are seldom clearly either soluble or insoluble in any given liquid, and their solvation clearly either soluble or insoluble in any given liquid, and their solvation characteristics are very sensitive to molecular weight, cross-linking, characteristics are very sensitive to molecular weight, cross-linking, crystallinity, chain branching. crystallinity, chain branching.

- - The longer the chains the more slowly a polymer dissolves.The longer the chains the more slowly a polymer dissolves.

- Polymers tend to absorb a solvent, swell, and soften, rather than - Polymers tend to absorb a solvent, swell, and soften, rather than dissolve. dissolve.

- - Cross-linking preventsCross-linking prevents complete chain separation and retards complete chain separation and retards dissolution. dissolution.

- - Highly cross-linked polymers cannot be dissolved Highly cross-linked polymers cannot be dissolved. .

- Elastomers swell more than plastic. - Elastomers swell more than plastic.

- A small amount of swelling of dental polymeric devices can have - A small amount of swelling of dental polymeric devices can have undesirable results on the fit of prosthesis. undesirable results on the fit of prosthesis.


- - Absorbed molecules spread polymer chains apart, facilitate slip between Absorbed molecules spread polymer chains apart, facilitate slip between chains. This effect is called chains. This effect is called PLASTICIZATION.PLASTICIZATION. Cross linkage provides Cross linkage provides sufficient number of bridges between linear macromolecules to form a 3-sufficient number of bridges between linear macromolecules to form a 3-dimensional network, that decreases water sorption, solubility and increases dimensional network, that decreases water sorption, solubility and increases strength and rigidity of the resin.strength and rigidity of the resin.

- Plasticizers are often added to resins to reduce their - Plasticizers are often added to resins to reduce their softeningsoftening or or fusion fusion temperatures.temperatures. A plasticizer acts to partially neutralize secondary bonds or A plasticizer acts to partially neutralize secondary bonds or intermolecular forces that normally prevent the resin molecules from slipping intermolecular forces that normally prevent the resin molecules from slipping past one another when the material is stressed. In some cases the action is past one another when the material is stressed. In some cases the action is analogous to that of a solvent, with the plasticizing agentanalogous to that of a solvent, with the plasticizing agent

increases the increases the intermolecular spacingintermolecular spacing. This type of plasticizer is referred to as an . This type of plasticizer is referred to as an external plasticizer. external plasticizer.

Plasticizing can also be accomplished by copolymerization with a suitable Plasticizing can also be accomplished by copolymerization with a suitable comonomer. In this case the plasticizing agent becomes part of the polymer and comonomer. In this case the plasticizing agent becomes part of the polymer and thus acts as an thus acts as an Internal plasticizer.Internal plasticizer.

For ex. When butyl methacrylate is added to methyl methacrylate For ex. When butyl methacrylate is added to methyl methacrylate before polymerization, the action of the butyl methacrylate is to increase before polymerization, the action of the butyl methacrylate is to increase intermolecular spacing via pendant groups.intermolecular spacing via pendant groups.


Impact strength:Impact strength: Impact strength is a measure of the energy Impact strength is a measure of the energy

absorbed by a material when a sudden blow breaks it. absorbed by a material when a sudden blow breaks it. The impact strength of polyvinyl acrylics is about The impact strength of polyvinyl acrylics is about

twice that of poly (methyl methacrylate). twice that of poly (methyl methacrylate). Although the addition of Although the addition of plasticizer may increase plasticizer may increase

the impact strength, the increase is accompanied by a the impact strength, the increase is accompanied by a decrease in hardness, proportionate limit, elastic decrease in hardness, proportionate limit, elastic modulus, and compressive strengthmodulus, and compressive strength. .

When surface defects are present the impact When surface defects are present the impact strength is significantly reduced. strength is significantly reduced.


Hardness:Hardness:

The The low knoop hardness numberlow knoop hardness number of the denture base of the denture base

plastic indicates these materials may be scratched and plastic indicates these materials may be scratched and

abraded easilyabraded easily..

Cross-linked poly (methyl methacrylate) is only slightly Cross-linked poly (methyl methacrylate) is only slightly

harder than regular poly (methyl methacrylate). The incorporation of harder than regular poly (methyl methacrylate). The incorporation of

fillers in plastics may alter the resistance to abrasion, but the fillers in plastics may alter the resistance to abrasion, but the

hardness of the plastic matrix remains unchanged. Polishing, shell hardness of the plastic matrix remains unchanged. Polishing, shell

blasting, cleaning denture bases by brushes should be carried out blasting, cleaning denture bases by brushes should be carried out

with this in mind.with this in mind.


THERMAL PROPERTIES:THERMAL PROPERTIES:

The physical properties of a polymer are influenced by The physical properties of a polymer are influenced by

changes in temperature and environment and by the composition, changes in temperature and environment and by the composition,

structure, and molecular weight of the polymer. structure, and molecular weight of the polymer.

Thermoplastic polymersThermoplastic polymers – – Are made of linear and / or Are made of linear and / or

branched chains. They soften when heated above the glass branched chains. They soften when heated above the glass

transition temperature (Tg), at which molecular motion begins to transition temperature (Tg), at which molecular motion begins to

force the chains apart. The resin can then be shaped and molded force the chains apart. The resin can then be shaped and molded

and upon cooling will harden in this form. However on reheating and upon cooling will harden in this form. However on reheating

again they soften and can be reshaped if required before hardening. again they soften and can be reshaped if required before hardening.

This cycle can be carried out repeatedly. Thermoplastic resins are This cycle can be carried out repeatedly. Thermoplastic resins are

fusible and they are usually soluble in organic solventsfusible and they are usually soluble in organic solvents


Thermosetting polymersThermosetting polymers: : Undergo a chemical change and Undergo a chemical change and

become permanently hard when heated above the temperature at which become permanently hard when heated above the temperature at which

they begin to polymerize, and do not soften again an reheating to the same they begin to polymerize, and do not soften again an reheating to the same

temperature. They are usually cross linked at this state and, thus, are temperature. They are usually cross linked at this state and, thus, are

insoluble and infusible, decomposing instead. insoluble and infusible, decomposing instead.

Thermosetting plastics generally have Thermosetting plastics generally have superior abrasion superior abrasion

resistanceresistance and and dimensional stability compareddimensional stability compared with thermoplastic resins, with thermoplastic resins,

which have better flexural and impact properties. which have better flexural and impact properties.


Glass Transition temperature: (Glass Transition temperature: (Tg) :Tg) :

Along each single polymer chain, valence electrons continuously Along each single polymer chain, valence electrons continuously move back and forth. Because of these electron movements, varying move back and forth. Because of these electron movements, varying electron densities exist along the chain at different times and locations. electron densities exist along the chain at different times and locations. Because of these interactions, interatomic induction forces (Van der Walls Because of these interactions, interatomic induction forces (Van der Walls and London forces) are developed between the chains. and London forces) are developed between the chains.

These forces, as well as hydrogen bonding, form polar bonds These forces, as well as hydrogen bonding, form polar bonds between the polymer chains, bonds that are much weaker than the primary between the polymer chains, bonds that are much weaker than the primary bonds along the polymer chain. When a polymer is heated to its Tg or to a bonds along the polymer chain. When a polymer is heated to its Tg or to a higher temperature, the weak polar bonds are broken and the polymer higher temperature, the weak polar bonds are broken and the polymer molecular chains can move freely relative to each other. molecular chains can move freely relative to each other.

The increased mobility has a strong impact on many physical The increased mobility has a strong impact on many physical properties such as strength, modulus of elasticity thermal expansion. properties such as strength, modulus of elasticity thermal expansion.

Strength and elastic modulus decrease as the temperature Strength and elastic modulus decrease as the temperature approaches Tg, whereas thermal expansion increases. approaches Tg, whereas thermal expansion increases.


Polymer chains with the higher molecular weight will also have a higher Polymer chains with the higher molecular weight will also have a higher Tg. If the straight-polymer chain length is increased, the number of polar Tg. If the straight-polymer chain length is increased, the number of polar bond sites increases along that chain. bond sites increases along that chain.

The longer chain length increases the chance for chain The longer chain length increases the chance for chain

entanglements. entanglements.

From a mechanical point of view, chain slippage also decreases From a mechanical point of view, chain slippage also decreases

as the chain length increases. as the chain length increases.

As the temperature increases the rotation of polymer segments As the temperature increases the rotation of polymer segments

increases. These rotations, coupled with thermal expansion increase increases. These rotations, coupled with thermal expansion increase

chain separation, break polar bonds, facilitate chain disentanglement. chain separation, break polar bonds, facilitate chain disentanglement.

These factors in turn facilitate chain slippage and explain the These factors in turn facilitate chain slippage and explain the thermoplastic behavior of the resin when it reaches Tg. thermoplastic behavior of the resin when it reaches Tg.

If cross-linkages exist, slippage cannot occur and the material If cross-linkages exist, slippage cannot occur and the material becomes more difficult to soften.becomes more difficult to soften.


Thermal conductivityThermal conductivity: : Resins are poor thermal and Resins are poor thermal and

electrical conductors, compared with gold, cobalt alloys or even electrical conductors, compared with gold, cobalt alloys or even

human dentine. Low thermal conductivity allows plastic denture human dentine. Low thermal conductivity allows plastic denture

bases to serve as an insulator between the oral tissues and hot or bases to serve as an insulator between the oral tissues and hot or

cold materials placed in the mouth. cold materials placed in the mouth.


Polymerization shrinkage:Polymerization shrinkage: The density of methylmethacrylate monomer is only 0.945 g / cm3 at The density of methylmethacrylate monomer is only 0.945 g / cm3 at 200C compared with 1.16 to 1.18 g / cm3 for poly 200C compared with 1.16 to 1.18 g / cm3 for poly (methylmethacrylate). (methylmethacrylate). This decrease in density is mainly accounted for by an approximate This decrease in density is mainly accounted for by an approximate 21%21% decrease in vol of monomer during polymerization. decrease in vol of monomer during polymerization. Because the ratio of polymer to monomer is Because the ratio of polymer to monomer is 3:13:1, the free volumetric , the free volumetric shrinkage amounts to approximately 6%. The light activated denture shrinkage amounts to approximately 6%. The light activated denture base material has low polymerization shrinkage of 3% because of base material has low polymerization shrinkage of 3% because of higher molecular higher molecular weight oligomersweight oligomers used. used.

However it should be noted that the linear shrinkage values, reported in However it should be noted that the linear shrinkage values, reported in the literature are generally much less than would be expected on the the literature are generally much less than would be expected on the basis of free volumetric shrinkage, because a portion of polymerization basis of free volumetric shrinkage, because a portion of polymerization takes place after the plastic has attained a solid condition, resulting in takes place after the plastic has attained a solid condition, resulting in residual stresses in the plastic rather than addition shrinkage, but residual stresses in the plastic rather than addition shrinkage, but thermal dimensional change would still result from cooling the plastic thermal dimensional change would still result from cooling the plastic from molding temperature to room temperature, from molding temperature to room temperature,


Dimensional stabilityDimensional stability:: - - If the denture is properly processes, the If the denture is properly processes, the original fit and the dimensional stability of the various denture base original fit and the dimensional stability of the various denture base plastics are good. plastics are good. However excess heat generated during finishing of the denture can However excess heat generated during finishing of the denture can easily distort a denture base by releasing residual stresses. easily distort a denture base by releasing residual stresses. It has been shown that chemically activated denture bases It has been shown that chemically activated denture bases processed by dough molding with a dimensional accuracy of – 0.1% processed by dough molding with a dimensional accuracy of – 0.1% were more accurate than heat- activated denture based to 0.4%. were more accurate than heat- activated denture based to 0.4%.

Water sorption and solubility: - Water sorption and solubility: - The sorption of water alters the dimensions of acrylic dentures, poly The sorption of water alters the dimensions of acrylic dentures, poly (methyl methacrylate) have relatively high water sorption (0.69mg / (methyl methacrylate) have relatively high water sorption (0.69mg / m2m2) ) polyvinyl acrylicspolyvinyl acrylics 0.26 mg / cm2. 0.26 mg / cm2.

Adhesion propertiesAdhesion properties:: The adhesion to untreated porcelain or metals The adhesion to untreated porcelain or metals is generally poor. As a result, porcelain teeth or combined metal and is generally poor. As a result, porcelain teeth or combined metal and plastic bases should be designed so that the porcelain or metal is plastic bases should be designed so that the porcelain or metal is held by mechanical retention. held by mechanical retention. The silane coupling agent, The silane coupling agent, - methacryloxypropyltrime - methacryloxypropyltrime thoxysilane provides a bond between the porcelain and the plastic thoxysilane provides a bond between the porcelain and the plastic surface. surface.


Esthetics Esthetics :: esthetic qualities induce properties such as colorability, esthetic qualities induce properties such as colorability, color stability, taste, odor, the ability of the plastics to be colored and color stability, taste, odor, the ability of the plastics to be colored and their compatibility with dyed synthetic fibers for characterization are their compatibility with dyed synthetic fibers for characterization are both good. both good.

Tissue compatibilityTissue compatibility It can be said that the completely polymerized It can be said that the completely polymerized poly (methyl methacrylate) or polyvinyl acrylics rarely cause allergic poly (methyl methacrylate) or polyvinyl acrylics rarely cause allergic reactions but that methyl / methacrylate monomer or other trace reactions but that methyl / methacrylate monomer or other trace components in the monomer may produce an allergic reaction. components in the monomer may produce an allergic reaction. Residual monomer is considered the allergen, and strict adherence Residual monomer is considered the allergen, and strict adherence to processing instructions recommended by the manufacturer can to processing instructions recommended by the manufacturer can keep the residual monomer to minimum. keep the residual monomer to minimum. When patients are known to have suffered from an allergic reaction, When patients are known to have suffered from an allergic reaction, processing the denture for extended periods (such as 24 versus 8) processing the denture for extended periods (such as 24 versus 8) may be helpful. may be helpful. Residual monomer levels can also be reduced dramatically by Residual monomer levels can also be reduced dramatically by processing heat polymerized poly (methyl methacrylate) in a water processing heat polymerized poly (methyl methacrylate) in a water bath for 7 hrs at 700C followed by boiling for 1 hr .bath for 7 hrs at 700C followed by boiling for 1 hr .


Chemistry of polymerizationChemistry of polymerization : :Monomers may be joined via one of two types of reaction Monomers may be joined via one of two types of reaction

Addition polymerization Addition polymerization Step-growth or condensation polymerization Step-growth or condensation polymerization

Addition polymerizationAddition polymerization Addition Addition polymerization starts from an active center, adding one monomer at polymerization starts from an active center, adding one monomer at a time to rapidly form a chain. The process is simple but not easy to a time to rapidly form a chain. The process is simple but not easy to control.control.

StagesStages -activation-activation - - initiationinitiation


A source A source of free radicals, of free radicals, RRis required.is required. Free radicals can be generated Free radicals can be generated by activation of radicals producingby activation of radicals producing molecules using a second molecules using a second chemical, heat, visible light, ultraviolet light, or energy transfer from chemical, heat, visible light, ultraviolet light, or energy transfer from another compound that acts as free radicals. another compound that acts as free radicals.

Heat, chemical agents, visible lights are most often used in dentistry. Heat, chemical agents, visible lights are most often used in dentistry.

R – R+ external energy = 2RR – R+ external energy = 2R..

One of the requisites of on addition polymerizable compound is the One of the requisites of on addition polymerizable compound is the presence of an unsaturated group that is, a double bond as well as a presence of an unsaturated group that is, a double bond as well as a source of fee radicals. source of fee radicals.

Free radicalsFree radicals is an atom or groups of atoms possessing an unpaired is an atom or groups of atoms possessing an unpaired electron (electron (). When the free radical and its unpaired electron approach ). When the free radical and its unpaired electron approach a monomer with its high electron – an electron is extracted and it pairs a monomer with its high electron – an electron is extracted and it pairs with the Rwith the R electron to form a bond between the radical and the electron to form a bond between the radical and the monomer, leaving the other electron of the double bond unpaired. monomer, leaving the other electron of the double bond unpaired.


The free radical forming chemical used to start the polymerization is not a catalyst, The free radical forming chemical used to start the polymerization is not a catalyst,

because it enters into the chemical reaction and becomes part of the final because it enters into the chemical reaction and becomes part of the final

chemical compound. It is more accurately called on initiator, because it is used to chemical compound. It is more accurately called on initiator, because it is used to

start the reaction. start the reaction.

The most commonly used initiator is The most commonly used initiator is benzoly peroxidebenzoly peroxide, which is activated rapidly , which is activated rapidly

between 500 and 1000C to release two free radicals per benzoyl peroxide between 500 and 1000C to release two free radicals per benzoyl peroxide

molecule. molecule.

InductionInduction is the period during which initiator molecules become energized and is the period during which initiator molecules become energized and

breaks down into free radicals, followed by these radicals reacting with monomer breaks down into free radicals, followed by these radicals reacting with monomer

molecules to initiate chain grown. molecules to initiate chain grown.

This period is greatly influenced by This period is greatly influenced by the purity of the monomerthe purity of the monomer. Any impurities . Any impurities

present can decrease the length of this period by consuming the activated initiator. present can decrease the length of this period by consuming the activated initiator.


However, However, the higher the temperature, the more rapid the formation of free the higher the temperature, the more rapid the formation of free radicals and consequently the shorter the induction period. radicals and consequently the shorter the induction period. A second type of induction system is chemically activatedA second type of induction system is chemically activated at ambient at ambient oral temperature. Such a system consists of at least two reactions that, oral temperature. Such a system consists of at least two reactions that, when mixed together undergo a chemical reaction that generates free when mixed together undergo a chemical reaction that generates free radicals. radicals. Ex: - is the tertiary amine (the activator) and benzoyl peroxide (initiator) Ex: - is the tertiary amine (the activator) and benzoyl peroxide (initiator) which are mixed together to initiate the polymerization of so-called “Self-which are mixed together to initiate the polymerization of so-called “Self-cured: resins at room temperature .This in fact is a special case of heat cured: resins at room temperature .This in fact is a special case of heat activation, because of the presence of the amine, reduces the thermal activation, because of the presence of the amine, reduces the thermal energy required to break the initiator into free radicals at ambient energy required to break the initiator into free radicals at ambient temperature. The amine form a complex with benzoyl peroxide, which temperature. The amine form a complex with benzoyl peroxide, which reduces the thermal energy, needed to split it into two free radicals.reduces the thermal energy, needed to split it into two free radicals.A third type of reaction system ie light activated. In this photons from a A third type of reaction system ie light activated. In this photons from a light source activate the initiator to generate free radicals that in turn, can light source activate the initiator to generate free radicals that in turn, can initiate the polymerization process. initiate the polymerization process. In the visible light-cured dental restoratives, In the visible light-cured dental restoratives, camphorquinone and camphorquinone and organic amine generate free radicals when irradiated by light in the blue organic amine generate free radicals when irradiated by light in the blue to violet region. to violet region. Light with a wavelength of about Light with a wavelength of about 470 mm470 mm is needed to trigger this is needed to trigger this reaction. reaction. However, factors such as However, factors such as light intensity, angle of illuminationlight intensity, angle of illumination, , distance of distance of resin from the light sourceresin from the light source can significantly affect the number of free can significantly affect the number of free radicals that are formed, thereby making this technique sensitive.radicals that are formed, thereby making this technique sensitive.


PropagationPropagation:: The resulting free radical monomer complex The resulting free radical monomer complex

than acts as a new free radical center when it approaches another than acts as a new free radical center when it approaches another monomer to form a dim er, with also becomes a free radical. monomer to form a dim er, with also becomes a free radical.

Because little energy is required once chain growth begins, the Because little energy is required once chain growth begins, the process continues with evolution of heat and leads to large polymer process continues with evolution of heat and leads to large polymer molecules within seconds. molecules within seconds.

Theoretically, the chain reactions should continue until all of Theoretically, the chain reactions should continue until all of

the monomer has been converted to a polymer between the initial set the monomer has been converted to a polymer between the initial set and the final set. and the final set.

However, the However, the polymerization reaction is never quite completedpolymerization reaction is never quite completed. .

The growth of the polymer chain ceases when the reaction center is The growth of the polymer chain ceases when the reaction center is destroyed by one of a number of possible termination reactions. The destroyed by one of a number of possible termination reactions. The reaction is exothermic and considerable heat is evolved. reaction is exothermic and considerable heat is evolved.


Chain transferChain transfer: -: - In this process the active free radical of In this process the active free radical of

a growing chain is transferred to another molecule and a new free a growing chain is transferred to another molecule and a new free

radical for further growth is created for ex:- a monomer molecule radical for further growth is created for ex:- a monomer molecule

may be activated by a growing macromolecule in such a manner may be activated by a growing macromolecule in such a manner

that termination occurs in the latter. Thus a new nucleus of growth that termination occurs in the latter. Thus a new nucleus of growth

occurs. occurs.

Termination:Termination: - - Addition polymerization reaction are most Addition polymerization reaction are most

often terminated either by direct coupling of two free radical chain or often terminated either by direct coupling of two free radical chain or

by the exchange of a hydrogen atom from one growing chain to by the exchange of a hydrogen atom from one growing chain to

another another


Inhibition of addition polymerization:Inhibition of addition polymerization: - - Any Any

impurity in the monomer that can react with the activated initiator or with an impurity in the monomer that can react with the activated initiator or with an

activated growing chain to prevent further growth. activated growing chain to prevent further growth.

For ex: addition of a initiator such as hydroquinone inhibits spontaneous For ex: addition of a initiator such as hydroquinone inhibits spontaneous

polymerization if no initiator is present and retards the polymerizations in the polymerization if no initiator is present and retards the polymerizations in the

presence of an initiator. Thus commercial dental resins contain a small presence of an initiator. Thus commercial dental resins contain a small

amount of an inhibitor such as methyl ether of hydroquinone to aid in the amount of an inhibitor such as methyl ether of hydroquinone to aid in the

prevention of polymerization during storage and in case of self cure resins prevention of polymerization during storage and in case of self cure resins

to provide adequate time for mixing and placement. to provide adequate time for mixing and placement.

Oxygen reacts rapidly with the free radicals and its presence Oxygen reacts rapidly with the free radicals and its presence

retards the polymerization reaction. retards the polymerization reaction.


Step growth polymerization:Step growth polymerization: The primary compounds reacts, often with the formation of a by-product The primary compounds reacts, often with the formation of a by-product

such as water, alcohols, halogen acids, ammonia. The formation of these by such as water, alcohols, halogen acids, ammonia. The formation of these by products is the reason step-growth polymerization is often called as products is the reason step-growth polymerization is often called as condensation polymerization.condensation polymerization.

A linear chain of repeating mer units is obtained by the stepwise inter A linear chain of repeating mer units is obtained by the stepwise inter molecular condensation or addition of the reactive groups in which molecular condensation or addition of the reactive groups in which bifunctional or trifunctional monomers are all simultaneously activated, as bifunctional or trifunctional monomers are all simultaneously activated, as opposed to the activation of one monomer at a time in addition opposed to the activation of one monomer at a time in addition polymerization. polymerization.

Step – growth polymerization resins are those in which polymerization is Step – growth polymerization resins are those in which polymerization is accompanied by repeated elimination of small molecules. accompanied by repeated elimination of small molecules.

The formation of polymers is slow because the reaction proceeds in a The formation of polymers is slow because the reaction proceeds in a stepwise to fashion from monomer to dimmer to trimer until large polymer stepwise to fashion from monomer to dimmer to trimer until large polymer molecules containing many monomer molecules molecules containing many monomer molecules


Copolymerization :Copolymerization :Two or more chemically different monomer each with some desirable Two or more chemically different monomer each with some desirable property can be combined to yield specific physical properties of a polymer. property can be combined to yield specific physical properties of a polymer.

The polymer formed is a copolymer, the process of formation is known as co The polymer formed is a copolymer, the process of formation is known as co polymerization. polymerization.

Acrylic resins :Acrylic resins :The acrylic resins are derivatives of ethylene and contain vinyl (-C=C-) group. The acrylic resins are derivatives of ethylene and contain vinyl (-C=C-) group. There are at least two acrylic resins series. There are at least two acrylic resins series.

Acrylic acid Acrylic acid

Methacrylic acid Methacrylic acid

Methyl methacrylateMethyl methacrylate – the monomer is mixed with the polymer which is supplied – the monomer is mixed with the polymer which is supplied in the powdered form. The monomer partially dissolves the polymer to form a in the powdered form. The monomer partially dissolves the polymer to form a plastic dough. This dough is packed into the mold, and the monomer is plastic dough. This dough is packed into the mold, and the monomer is polymerized. Methyl methacrylate is a transparent liquid at room polymerized. Methyl methacrylate is a transparent liquid at room temperature. temperature.


Molecular weight - 100 Molecular weight - 100 Melting point – 480CMelting point – 480CBoiling point – 100.80C. Boiling point – 100.80C. Density – 0.945 g/ml at 200CDensity – 0.945 g/ml at 200CHeat of polymerization – 12.9 K cal / molHeat of polymerization – 12.9 K cal / molA volume shrinkage of 21% occurs during A volume shrinkage of 21% occurs during

the polymerization of the pure the polymerization of the pure methylmethacrylate monomer. methylmethacrylate monomer.


Poly methyl methacrylate:Poly methyl methacrylate: Transparent resin of remarkable clarity. Transparent resin of remarkable clarity. Density – 1.19 g / cm3. Density – 1.19 g / cm3. Tensile strength – 60 Mpa Tensile strength – 60 Mpa knoop hardness number of 18 to 20. knoop hardness number of 18 to 20. Modulus of elasticity – 2.4 Gpa.Modulus of elasticity – 2.4 Gpa. Chemically stable to heat Chemically stable to heat Softens at 1250C Softens at 1250C Between 1250 and 2000C depolymerization Between 1250 and 2000C depolymerization

takes place takes place At appear - 4500C, 90% of the polymer At appear - 4500C, 90% of the polymer

depolymerizes to from the monomer. depolymerizes to from the monomer. www.indiandentalacademy.comwww.indiandentalacademy.com

HEAT – ACTIVATED DENTURE BASE RESINSHEAT – ACTIVATED DENTURE BASE RESINS::

The thermal energy required for polymerization of such material The thermal energy required for polymerization of such material

may be provided using a water bath or microwave oven. may be provided using a water bath or microwave oven.

Composition : Composition :

POWDERPOWDERa) Poly (methyl methacrylate) b) Benzoyl peroxide – a) Poly (methyl methacrylate) b) Benzoyl peroxide –

initiator initiator

LIQUID LIQUID a) Methyl methacrylate b) Hydroquinone acts as inhibitor. a) Methyl methacrylate b) Hydroquinone acts as inhibitor.

c) a cross linking agent such as glycol dimethacrylate can also be c) a cross linking agent such as glycol dimethacrylate can also be

adder. They are incorporated at a concentration of 1% to 2% by adder. They are incorporated at a concentration of 1% to 2% by

volume volume


Polymer To Monomer Ratio :Polymer To Monomer Ratio : The polymerization of denture base resin results in volumetric and linear The polymerization of denture base resin results in volumetric and linear

shrinkage. shrinkage. If we imagine two methyl methacrylate molecules, each molecule possesses If we imagine two methyl methacrylate molecules, each molecule possesses

an electrical field that repels nearby molecules, consequently the distance an electrical field that repels nearby molecules, consequently the distance between molecules is significantly greater than the length of a carbon to between molecules is significantly greater than the length of a carbon to carbon bond. carbon bond.

When the methyl methacrylate molecules are chemical bonded, a new When the methyl methacrylate molecules are chemical bonded, a new carbon – to – carbon linkage is formed. This produces a net decrease in the carbon – to – carbon linkage is formed. This produces a net decrease in the space occupies. space occupies.

Research indicates that there is Research indicates that there is 21% decrease in the volume of material21% decrease in the volume of material. . To minimize dimensional changes manufactures prepolymerize significant To minimize dimensional changes manufactures prepolymerize significant

fraction of the denture base material that may be thought as “preshrinking”. fraction of the denture base material that may be thought as “preshrinking”. The accepted polymer to monomer ratio is 3:1 by volume. The accepted polymer to monomer ratio is 3:1 by volume. Using 3:1 ratio the volumetric shrinkage may be limited to approximately 6% Using 3:1 ratio the volumetric shrinkage may be limited to approximately 6%

(0.5% linear shrinkage.)(0.5% linear shrinkage.)


Chemically activated denture baseChemically activated denture base : : In most cases chemical activation is accomplished through the In most cases chemical activation is accomplished through the

addition of a tertiary amine such as dimethyl – para toludine. addition of a tertiary amine such as dimethyl – para toludine. Upon mixing the tertiary amine cause decomposition of benzoyl Upon mixing the tertiary amine cause decomposition of benzoyl

peroxide – consequently free radicals are produced and polymerization is peroxide – consequently free radicals are produced and polymerization is initiated. initiated.

As a general rule, the degree of polymerization achieved using As a general rule, the degree of polymerization achieved using chemically activated resins is not as complete as that achieved using heat-chemically activated resins is not as complete as that achieved using heat-activated resins – this indicates there is a greater amount of unreacted activated resins – this indicates there is a greater amount of unreacted monomer – monomer – this unreacted monomer causes two major difficulties- first – it this unreacted monomer causes two major difficulties- first – it as a plasticizer that results in decreased transverse strength of the denture as a plasticizer that results in decreased transverse strength of the denture resin second – serves as a potential tissue irritant. resin second – serves as a potential tissue irritant.

But from a physical stand point – chemically activated resin display But from a physical stand point – chemically activated resin display slightly less shrinkage hence imparts greater dimensional accuracy.slightly less shrinkage hence imparts greater dimensional accuracy.

The color stability of chemically activated resins is poorThe color stability of chemically activated resins is poor – this is – this is related to the presence related to the presence of tertiary amines of tertiary amines such amines are susceptible to such amines are susceptible to oxidation. Discoloration may be minimized via the addition of stabilizing oxidation. Discoloration may be minimized via the addition of stabilizing agents that prevent such oxidation. agents that prevent such oxidation.


Modified acrylic materialsModified acrylic materials : :

The impact strength of acrylic polymers can be improved The impact strength of acrylic polymers can be improved significantly by incorporation of elastomers. significantly by incorporation of elastomers.

The elastomer is able to absorb energy on impact and thus protect The elastomer is able to absorb energy on impact and thus protect the acrylic resin from fracture. the acrylic resin from fracture.

An alternative to the direct addition of elastomers is the use of An alternative to the direct addition of elastomers is the use of acrylic elastomer copolymer. These are typically, methylmethacrylate acrylic elastomer copolymer. These are typically, methylmethacrylate butadiene-styrene copolymers. butadiene-styrene copolymers.

Attempts to improve the fatigue resistance of acrylic denture Attempts to improve the fatigue resistance of acrylic denture polymers has involved the use of carbon fiber inserts. They stiffen the polymers has involved the use of carbon fiber inserts. They stiffen the denture, reducing the degree of flexing and the possibility of fatigue fracture. denture, reducing the degree of flexing and the possibility of fatigue fracture. However bonding between the fibers and the acrylic resin may be difficult to However bonding between the fibers and the acrylic resin may be difficult to achieve and it bonding is not achieved the fibers may weaken the denture. achieve and it bonding is not achieved the fibers may weaken the denture. Other reinforcing fibers, including Kevlar and ultra high modulus Other reinforcing fibers, including Kevlar and ultra high modulus polyethylene are currently under investigation. polyethylene are currently under investigation.


Alternative polymers :Alternative polymers :

The major alternatives to acrylic polymer or modified acrylic The major alternatives to acrylic polymer or modified acrylic

are the polycarbonates and certain vinyl polymers. These may be are the polycarbonates and certain vinyl polymers. These may be

considered when the patient has proven allergy to acrylic resin or considered when the patient has proven allergy to acrylic resin or

when greater impact strength is required. when greater impact strength is required.

The other alternative to acrylic resin is vulcanite. The other alternative to acrylic resin is vulcanite.

The equipment required to process vulcanite denture is now The equipment required to process vulcanite denture is now

a rarity however and the material can no longer be considered as a a rarity however and the material can no longer be considered as a

serious alternative. serious alternative.


Conclusion:Conclusion:In the development of any biomaterial one In the development of any biomaterial one must consider not only the strength, must consider not only the strength, esthetics or functional aspect of the esthetics or functional aspect of the material, but, its biocompatibility as well. material, but, its biocompatibility as well. The dentist should have a through The dentist should have a through knowledge of the material that he is using knowledge of the material that he is using to make a successful prosthesis. to make a successful prosthesis.



http://www.indiandentalacademy.com/

resins/ academy general dentistry

Education