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Vydehi Institute Of Dental Sciences And Research Center # 82 EPIP Area, Narullahalli Road, Whitefield, Bangalore Seminar on “Acrylic Materials In Orthodontics” submitted in partial fulfilment to Masters Of Dental Surgery Degree requirement by rajiv Gandhi University of Health Sciences

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Page 1: Acrylic material in Orthodontics

Vydehi Institute Of Dental Sciences And Research Center

# 82 EPIP Area, Narullahalli Road,

Whitefield, Bangalore

Seminar on “Acrylic Materials In Orthodontics” submitted in partial fulfilment to Masters Of Dental Surgery Degree requirement by rajiv Gandhi University of Health Sciences

Page 2: Acrylic material in Orthodontics

Vydehi Institute of Dental Sciences

& Research Center

CERTIFICATE

This is to certify that the Seminar entitled “Acrylic Materials in Orthodontics” was presented by Dr. Nilofer Vevai Post Graduate

Student of the Department of Orthodontics under my guidance.

Dr. Chandralekha B

Professor and Head,

Department of Orthodontics

Vydehi Institute of Dental Sciences

& Research Center

Page 3: Acrylic material in Orthodontics
Page 4: Acrylic material in Orthodontics

Contents : What Are Acrylic Resins ?

History & Evolution .

Polymeric Science – The Basic !!!

Chemistry & Chemical Nature .

Classification Of Acrylic Resins.

Morphology –Spatial Arrangement.

Stages Of Polymerization .

Ideal Physical Properties .

Factors Affecting Physical Properties.

Types in detail

Biologic Considerations

Uses a) In General

b) In Orthodontics

Page 5: Acrylic material in Orthodontics

Introduction

Ever since Orthodontics has been established as a specialty it has developed with leaps and bounds. Acrylic material as such is a very integral part in our specialty whose importance cannot be diminished or nullified in any way ...it is as important a compound to us in our speciality as it is to a Prosthodontist making it a mandatory to know as well as learn more about it .

Acrylics in general have a vast variety of properties that are useful to us in orthodontics, prosthodontics specifically and also other allied fields of dentistry.

In this seminar we shall explore various aspects of Acrylic materials right from a brief historical perspective ... to its physical nature, its chemistry, different types, components, uses as in Orthodontic and non orthodontic etc. Hence giving us a brief

overview of this topic.

Page 6: Acrylic material in Orthodontics

What Are Acrylic Resins ? Synthetic Resins are non metallic compounds molded into various forms and then hardened for commercial use.

Acrylic resins are derivatives of ethylene and contain a Vinyl group.

Acrylics are basically polymeric compounds and follow the general principles of Polymeric science which shall be outlined in detail in the following pages.

These resins are generally tasteless, colourless, odourless, non- irritating and non harmful to the oral tissues .They have adequate strength and resiliency and are extensively used for these reasons in the oral cavity.

The most significant features of polymers are that they consist of very large molecules and that their molecular structure is capable of virtually limitless configurations and conformations.

Their molecular weight could often exceed 5000 and when this happens such molecules are called Macromolecules hence most polymer molecules can be called macromolecules.

These come in the popular variety of clear and coloured acrylic materials . Designer appliances can also be made with these as mentioned in Clarke’s Twin block Functional Therapy Applications in Orthodontics second Ed. By these the author means appliances with various designs made into the acrylic.

Page 7: Acrylic material in Orthodontics

H istory & Evolution Prior to 1871 Orthodontic metal bands were used given by W.E Magill … self curing resin (methylmethacrylate) first thought of in an attempt to cement tooth attachments to tooth surface ..which later gave rise to Bonding & composites.

(SOURCE: Orthodontics State Of The Art Essence Of The Science by Lee W Graber )

There were disadvantages of these metal bands namely:

1. Time and skill requirement was high.2. Consequently Increased chairside time 3. Resultant tooth decalcification was high.4. Discoloration5. Periodontal tissue irritation

And finally banding of teeth causes banding space to be created .... there is hence necessity for band space closure.

Self –curing resin first thought of in an attempt to cement attachments to tooth surface

A small history of how polymers came into existence :

Approx 1840 Charles Goodyear discovered Vulcanized Rubber , it was used as a denture base material for a lot many years… even though this had poor esthetics , tasted bad and became foul smelling this was used a denture base material for around 75 years.

Page 8: Acrylic material in Orthodontics

In about 1868 a printer by name John Hyatt discovered the first plastic molding material - Celluloid

He discovered this by dissolving nitrocellulose a cotton derivative under pressure (his goal was to make billiard balls ). This celluloid was tough but somewhat flexible better esthetics but tasted and smelled bad. Not dimensionally stable and difficult.

In 1909 Dr. Leo Bakeland discovered Phenol Formaldehyde .

This material was known as Bakelite was not dimensionally stable as it was a condensation polymer. He discovered this while trying to make artificial shellac .

Finally 1930’s Dr. Walter Wright & the Vernon Brothers working at the Rohm & Haas company in Philadelphia developed Poly methyl methacrylate – a hard plastic .

Today this material has a vast field of application in an orthodontists and prosthodontists life.

Polymeric Science:Definitions..

Polymer Definition :

A Polymer Is A Long Chain Organic Molecule Produced By The Reaction Of Many Smaller Molecules Called Monomer /Mer.

Monomer + monomer = polymer

C o – Polymer :

If The Reaction Occurs between two different but compatible monomers the polymeric material is called copolymer

Monomer1 + monomer2 = copolymer

Page 9: Acrylic material in Orthodontics

Poly = many

Mono = single

Mer = unit

Oligo =several

Oligomonomer= 2-4 monomers

Examples of Polymers :

Acrylic . Composites . Silicones . Vinyl Rubbers . Relining Materials . Polystyrene . Celluloid . Polycarbonate

The longer the polymer chain greater is the entanglements ( temporay connections ) Therefore the longer the chain the more difficult it is to distort the polymeric material and thus such properties as rigidity , strength and the melting temperature increases with increasing chain length.

Page 10: Acrylic material in Orthodontics

Biologically , it is important to realize that polymerization seldom is entirely complete & that residual monomer molecules can be leached an cause adverse reactions . The residual monomer also has a pronounced effect on the molecular weight of the polymer .

Page 11: Acrylic material in Orthodontics

Definition Of The Components

Acrylics :

1. Resilient Plastics formed by joining multiple methyl methacrylate molecules or mers

2. A family of organic molecules (containing a carbon –carbon double bond ) that forms polymers useful in many applications.

Activator :

A component ( eg if chemical ,usually an amine compound) , used to cause an initiator molecule to become active and begin a polymerization reaction.It could be a chemical , heat ,light , wave energy etc.

Initiator :

A molecule that cause or Initiates a polymerization reaction .

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

Theoretically, R* can be almost any free radical.

Page 12: Acrylic material in Orthodontics

A free radical is an atom / group of atoms possessing unpaired electron (*). The unpaired electron confers electron withdrawal ability to the free radical

When the free radical and its unpaired electron approach a monomer with its high electron density double bond ,an electron is extracted , and it pairs with the R* electron to form a bond between the radical and the mnonomer molecule. Leaving the other electron of the double bond unpaired. Thus the original free radical bonds to one side of the monomer molecule and forms a new radical site at the other end . The reaction is now initiated .

The Free radical is not a catalyst … it is a component that causes initiation of the reaction and is more appropriately called the INITIATOR.

Inhibitor :

Chemical component that prevents / inhibits undesirable polymerization of the monomeric liquid during storage.

Classification Of Acrylics :

I . Based On Thermal Behavior :

1. Thermoset .

2. Thermoplastic.

II . Based On Type Of Chemical Reaction

1. Condensation Polymerization

2. Addition Polymerization

III . Based On Type Of Acrylic:

Page 13: Acrylic material in Orthodontics

1. Conventional Heat Cure .

2. Cold / Chemical Cure .

3. Light Cure .

4. Modifications .

THERMOPLASTIC :

Thermoplastic refers to resins that are softened & molded under heat / heat & pressure without any chemical change occurring . Usually in an organic solvent. They are made of linear/ branched chains and harden irreversibly when heated beyond a certain temperature.

e.g.. PMMA , Polyvinyl Acrylics Polystyrene

THERMOSET :

Thermoset refers to resins in which a chemical reaction takes place during molding the final product is chemically different from the original substance if it is a heat cure it begins to harden on its application and and these do not soften again on reheating to same temperature.

e.g.. Cross linked PMMA ,Silicones

CONDENSATION POLYMERIZATION :

Condensation polymerization is a type of polymerization in which small molecules are periodically given out as by products.

Page 14: Acrylic material in Orthodontics

In this form of reaction a small by product is released such as water / alcohol as a result of reaction between 2 ‘mer’ units . The final polymer wt is not equal to monomer wt.

Disadvantages : Slow reaction time.

The polymerization within a stipulated time length not adequate.

Generally high amounts of shrinkage is seen.

Inadequate chain length can affect properties of the acrylics such as its tear resistance, dimensional stability .

Condensation Reaction

(X)n –OH + (Y)m -H Xn –Ym + H2O

(n , m > 1)

ADDITION POLYMERIZATION :

Addition Polymerization is a type of reaction in which by products are not given out but in which the

reaction proceeds by simple conversion of double/ triple bonds into single bonds. Addition reaction is usually in monomers with C=C which is of high energy & is relatively unstable .

Though unstable they need a combination of initiator activator & inhibitors to function .

Initiators can be

a) Heat

b) Light

c) Chemicals

Page 15: Acrylic material in Orthodontics

Addition Reaction

( X ) + (Y ) X + Y

Steps of Addition Reaction

INDUCTION

This consists of two parts Initiation and activation . For an addition polymerization process to begin a source of free radicals is required. Free radicals can be generated by source of free radical molecules which is provided by activation of radical producing molecules using a second chemical, heat , visible light , ultraviolet light or energy transfer from some other compound that acts as a free radical.

Of these chemical agents heat and light are most commonly used in dentistry

Post activator action

I * + M I-M*

Propagation

I – M * + M I – M – M*

The resulting free radical – monomer complex then acts as a new free radical center when it approaches another monomer to form dimer which also becomes a free – radical .

Chain transfer :

The active free radical of a growing chain is transferred to another molecule { eg a monomer / inactivated polymer chain } and a new free radical for further growth is created.

Termination

I -(M)n – M*+ I–(M)m –M* I- (M)n – M –M- (M)m-I

Page 16: Acrylic material in Orthodontics

This can occur during chain transfer ... in addition reactions it occurs mostly due to direct coupling of 2 free radical ends or from exchange of Hydrogen atom from one growing atom to another.

The reactions are carried forward due to free radicals / free unpaired electrons which is why the addition reactions are also known as ‘ Free Radical Polymerization ‘

Most of the reactions in dentistry are by Addition reactions .

Numerous sites of polymerization occurs therefore numerous polymer chains form simultaneously this is Propagation .

2 main events during addition polymerization are that

a) Exothermic reaction

b) Shrinkage

Exothermic Reaction :

since the breaking of C=C bonds occur .

Shrinkage :

Volumetric shrinkage occurs because in liquid stage the molecules are further apart with just secondary bonds but when they react they found covalent bonds so the forces of attraction increase resulting in overall shrinkage .

E.g. Acrylics, Polyether , Epoxy resin dies

Ring Opening Polymerization :

A molecular of atoms ….opening of the ring causes elongation ….Covalent bonding causes shrinkage .;.

Net result balances out !!

Some Epoxy resin dies & Polyether Impression Materials are based on this principle ..

CONVENTIONAL HEAT CURE :

Conventional heat cure resin is a type of acrylic requiring heat as activator for the initiator to work .

Page 17: Acrylic material in Orthodontics

COLD CURE/CHEMICAL CURE :

Cold cure/chemical cure

This is a type of acrylic in which the reaction proceeds to its fullest extent under the environmental circumstances without application of heat or any special/ specific form/ wavelength of light.

LIGHT CURE :

Light cure acrylicis acrylic material in which a specific wavelength of light is required to activate the initiator .

MODIFICATIONS :

High Impact Resistant Acrylic

Injection Molded Polymers

Light Activated Polymer

Rapid Heat Polymerized Polymer

Microwave Polymerized Polymers

Fiber Reinforced Polymer

MORPHOLOGY - Spatial Arrangement

Space Lattice

1. For Polymers…

Linear Polymer [linear homopolymer, random, block ]

The molecules are arranged in straight line linear chains.

Page 18: Acrylic material in Orthodontics

Linear homopolymer

Straight chains of similar basic unit :

Branched Polymer

The linear chain has side links forming branches.

[Types Branched homopolymer, random copolymer of branched type, graft copolymer of branched type ]

Cross – Linked Polymer.

Co- Polymers…

Random Type .

Graft Type .

Block Type.

3. Coiled chains

Linear Type Polymer

a) Linear Homopolymer : It has mer units of the same type .

… M- M - M- M- M- M….

b) Random Copolymer of Linear Type :

It has 2 mer units randomly distributed along the chain

M - M - M - Y – M – M - Y – M

Page 19: Acrylic material in Orthodontics

c) Block Copolymer :

It has 2 types of mers in segments or blocks .

M-M-M-Y-Y –Y -M –M-M-Y-Y-Y

Branched

a ) Branched Homopolymer :

The mer units are of the same type .

… M – M –M –M –M –M –M –M –M – M –M …

| | |

M M M

| | |

M M M

. . .

. . .

Random Copolymer Of Branched Type

It has 2 types of mer units distributed randomly .

M –M –M –M –Y –Y –M –M –Y –M –Y –M

| | |

Y Y M

| | |

M M Y

. . .

. . .

Page 20: Acrylic material in Orthodontics

Graft Copolymer Of Branched Type :

One type of mer unit on the main chain & other on the branches ..

….- M –M –M –M –M –M –M –M –M –M ….

| | |

Y Y Y

| | |

Y Y Y

| | |

Y Y Y

. . .

. . .

Cross – Linked Polymer :

It is made up of a homopolymer across – linked with a single cross linking agent .

…M –M –M – –M –M –M –M –M –Y –M –M –Y –M …

|

Y

|

Y

|

… M –M –M – –M –M –M –M –M – –M –M ––M …

| |

Y Y

Page 21: Acrylic material in Orthodontics

Block Type :

In a block copolymer identical monomer units occur in relatively long sequences along the main polymer chain.

...M –M –M –Y –M –M –M –Y –M –M –M –Y –M –M –M..

Type Of Copolymers :

Random Type :

The different mers are randomly distributed along the chain .

-M –M –M –Y –M –M –Y –M –M –Y

Graft Type :

Sequences of one of the monomers are grafted on to a backbone of the second monomer .

…M –M –M –M –M –M –M –M –M –M –M –M –M –M …

| |

Y Y

| |

Y Y

Linear or chain polymerisation

Easily manipulated, stretched, bent, thermoplastic, Hard

e.g. fitting surface of acrylic teeth- better binding to denture base

Page 22: Acrylic material in Orthodontics

Branched polymerisation

Easily manipulated, stretched, bent, thermoplastic, More hard

Cross-linked polymerisation

Strong, stiff, thermoset, wear resistant

E.g. Denture base materials, Occlusal surfaces of acrylic teeth

Coiled chains

Flexible

e.g. impression materials.

Stages Of Polymerization

There are 6 Stages of Polymerization :

1. Sandy stage .

2. Stringy stage .

3. Dough stage.

4. Rubbery stage.

5. Stiff .

Sandy Stage …

1. Monomer diffuses into polymer beads .2. Causes the polymer beads to swell . 3. Low molecular weight polymer beads surface begins to dissolve .4. The texture is coarse or grainy .5. Also Known as Wet Sand Stage

Page 23: Acrylic material in Orthodontics

Stringy Stage …

1. The bead surface dissolves .2. The mass turns sticky and fibrous .3. If pulled apart fine threads of the material are seen , observed as “stringiness” or

“stickiness” .4. This stage lasts for one minute .

Dough Stage …

1. The material loses glossy surface appearance .2. Can be kneaded like dough .3. At the Molecular level …Continued diffusion of monomer into the beads and polymer

from beads to monomer increasing viscosity of the mix .4. This stage lasts for 2 minutes .5. Packing is done in late dough stage .

Rubbery Stage …

1. At the Molecular level - The monomer completely penetrates into the center of the polymer beads .

2. No longer retains shape i.e. No longer moldable .3. Springs back …if shape is tried to be changed .4. Second last stage

Stiff

In Cold cure the reactionary mechanisms continue to take it upto this level automatically …but in heat cure it has to be heated to take it to this level else it stays at rubbery level .

Last physical stage where in it is stiff and brittle .

Page 24: Acrylic material in Orthodontics

Shrinkage occurs during heating and cooling leading to a build up of stresses . Higher temp has more internal stresses & distortion cooling is done slowly so that residual stresses are allowed to relax easily .

Slow rate of heating is preferred since internal porosities may result .

Reducing the time or temperature of heat cure is not an option since it may cause residual monomers to remain .

According to ANSI/ADA Specification no. 12 the dough forming time should be within 40 min

This is however mostly achieved within 10 min .

Minimum working time is 5 min.

The working time can be increased by refrigeration .

Risk is of moisture contamination so the containers are made airtight before refrigeration .

And when removed the container temperature is made to come to room temperature .

Plasticizers effects

Plasticizers are added to stiff, glassy uncross-linked polymers to lower glass transition temperature (Tg) which makes it more malleable at lower heating temperatures.To make it rubber-like, at the same time increases flexibility making it less brittle.

Page 25: Acrylic material in Orthodontics

Ideal Physical Properties ..

1. Natural appearance2. Easy processing3. Easy to clean4. Easy to repair5. Inexpensive6. Biocompatible 7. Resistant to bacterial contamination8. High strength, stiffness, hardness, toughness, fatigue resistance9. Low density10. Radiopaque 11. High thermal conductivity12. High modulus of elasticity, impact strength13. Abrasive resistance14. Dimensionally stable15. Accurate reproduction of surface detail

Currently Available Acrylics

(source McCabe)

Are mostly Thermoplastic with a Glass Transition Temperature.

Polymerization Shrinkage a) Volumetric.

b) Linear .

Volumetric MMA shrinkage 21% density change from 0.94 g/cm3 1.19 g/cm 3

If Prepolymerized powder is used 7% volumetric shrinkage approx occurs .

For Linear shrinkage for acrylic resin undergoing 7% volumetric shrinkage is 2% mainly

occuring due to thermal shrinkage .Linear shrinkage affects the fit .

Page 26: Acrylic material in Orthodontics

According To A Study By Stafford GD ,Bates JF , Hugget R , & Handley R W “A Review Of The Properties Of Some Denture Base Polymers “ J Dent 1960

Material Linear shrinkage

a)High Impact Acrylic Resin 0.12%

b)Vinyl Acrylic Resin . 0.33%

c)Conventional Acrylic Resin 0.43%

d)Pour Type Acrylic Resin 0.48%

e)Rapid Heat Cured Resin 0.97%

The Tg for PMMA is 105 deg C approx.

Linear coefficient of thermal expansion is 81 ppm/deg C

a formula for linear shrinkage has been devised as

Linear Shrinkage = 81 x Change in Temperature .

A chemically activated resin shows less shrinkage compared to heat activated resin ( 0.26% , 0.53% ).

Porosities …

These can weaken the denture .

Can be a) Localized /generalized

b) Surface / subsurface voids .

c) Spherical /irregular .

Regions of increased thickness will have increased voids

Page 27: Acrylic material in Orthodontics

Regions with very thick investment around it and away from metal flask increased chances of porosities.

Cause

a) Improper MIXING of monomer polymer .

b) Improper RATIO of monomer polymer .

c) Packing in the WRONG stage .

d) Inadequate KNEADING of material.

e) RAPID temperature rise.

f) Inadequate PRESSURE .

g) AIR INCLUSION Porosity seen mainly in Fluid Resin Technique.

Page 28: Acrylic material in Orthodontics

Water Absorption :

Relatively small amounts of water Absorption occurs.

Diffusion mechanism is held responsible for the ingress of water.

Water absorbed then acts as plasticizer.

PMMA exhibits water sorption value of 0.69 mg/cm2

For 1% increase in wt by water absorption linear expansion of 0.23 % occurs.

However it is compensated by thermal shrinkage during polymerization.

The diffusion co-efficient of water in representative heat – cured acrylic is about 0.011 x 10-6 cm2/s.

Chemically cured acrylic is 0.023 x 10-6cm2/s .

A typical denture base may take 17 days to become saturated with water .

ANSI /ADA Specification no. 12 states the value of weight gain must not exceed 0.8gm/cm2.

Solubility Of Poly Methyl Methacrylate:

PMMA is virtually insoluble in oral fluids .

Page 29: Acrylic material in Orthodontics

Cross linked acrylics are insoluble in most of fluid that is taken.

Crazing :

Crazing is the production of flaws or microcracks produced by stress relaxation adversely affecting the aesthetics and physical properties of acrylic.

These appear as small linear cracks .

These Microcracks are directed at right angles to tensile forces.

In transparent dentures it gives a hazy / foggy appearance.

This property of acrylic predispose them to fracture.

Causes Of Crazing :

a. Stress application b. Attack from a solvent.

Those microcracks formed from the solvent action in acrylics are more random .

To Avoid internal strain during polymerisationthe following simple steps can be followed :

CRAZE CRACK

Page 30: Acrylic material in Orthodontics

Slow cooling of the flask

Use cross linked polymer types

Avoid extra stress during function

Do not overheat on polishing

Keep denture away from solvents

Avoid denture drying

Polish after each adjustment

The Dimensional changes that can be seen in acrylic on processing...

Expansion on heating flask; heat evenly

Expansion on polymerization, exothermic

Contraction on polymerization (21vol.%);

Contraction on cooling to room temperature;

Expansion on swelling in water;

Expansion on thermal change to 32°c.

Strength Of The Acrylic Resin

There are various factors affecting the strength of the acrylic resin these factors include

a) Composition of the resin.

b) Processing technique.

c) Conditions presented by the oral environment .

d) Strength depends upon the degree of polymerization

Composition of the acrylic resin basically depends on the type if it is fibre reinforced it is naturally stronger.

Page 31: Acrylic material in Orthodontics

The degree of polymerization is perhaps the most important factor which affects the strength of the acrylic resin more the polymerization the more is the strength since the amount of bonds primary bonding that occurs is more also the induction forces i.e. the Van der Waals and the London forces will also be consequently more.

The processing technique is important since it affects the amount of primary bonds occurring the more the curing the more are the bonds hence the strength is more.

e.g Between Heat cure acrylic and cold cure acrylic heat cure is stronger .

Creep

Creep is generally defined as a time dependent plastic deformation on application of sustained load.

Resins display viscoelastic behavior.

Creep does occur in resins on application of sustained load.

Creep rate may be elevated by increase in temperature in applied load ,high residual monomer being present, and the presence of plasticizers.

Creep rates are low at low stresses but rapidly increase at high stresses.

The Charpy impact strength for heat activated resin is 0.98to 1.78 J. Chemically activated resin is 0.78 J

The Knoop Hardness No. of heat activated resins is 20. And the KHN of chemically activated resins are 16-18.

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Rheometric Properties : These are 2 namely

Plastic Flow :

Irreversible strain behaviour that occurs when polymer chains slide over one another and become relocated within material resulting in permanent deformation.

Elastic Recovery :

Reversible strain behaviour that occurs in the amorphous regions of polymers when the randomly coiled chains straighten and then recoil , like springs that return to their original location without sliding past one another when an applied force is removed.

Thus deformation is of 3 types Plastic, Elastic Viscoelastic.

Abrasion Resistance of acrylics is generally low compared to metals /ceramics.

Acrylics give good esthetics.

Acrylics are good Thermal insulator other (2 properties thermoplastic/ thermosetting explained earlier).

Good electrical insulator.

Physical Properties of Methyl Methacrylate :

Methyl methacrylate is a transparent liquid at room temperature.

Molecular weight 100.

Highly Flammable

B.P. 100.8 deg C

Density 0.945g/ml at 20 deg C

Melting point -48 deg C

21% Volume shrinkage on evaporation

Page 33: Acrylic material in Orthodontics

Heat of polymerization 12.9 kcal/mol

Physical irritation to the skin & is a known allergen.

It is an excellent organic solvent.

According to Vermilyea SG , Powers JM , Koran A who conducted a study on the rheological properties of fluid denture base resin published in Journal of Dentistry…

“ The rheological properties of six fluid resins were determined . Materials displayed initial non- newtonian behavior and increased viscosity over time”

It Is Advisable To Soak Any Acrylic Product { Especially Dentures } For 24 Hours In Water To Offset Some Degree Of Shrinkage Also Most Of Residual Monomers Removed In This Period.

Page 34: Acrylic material in Orthodontics

Factors Affecting Properties Of Acrylic Resins…

Size of the individual polymer chains .

The longer the polymer chains the higher the strength , stiffness , stability .This is because increased secondary bonding is seen .

High frictional forces do not allow molecules to slip over one another therefore making it difficult to pull apart or change .

This is called “entanglements”.

2. The length of the molecules/chains in a polymer depend upon the degree of polymerization .

With increase of polymerization there is

a) increase of Strength

b) increase of Stiffness

c) increased dimensional stability

d) increased resistance to solubility .

These indicate the amount of polymerization roughly.

Another method of seeing the the degree of conversion is to see the % of double bonds that disappear during reaction with infrared spectroscopy providing the degree of cure of the polymer system.

It is important for expressing the extent of the polymerization reaction in cross linked network polymers such as dental composites ,adhesives , Pit and fissure sealants.

The Quality of monomers ..

Certain monomers produce a rigid glass like polymer because it is difficult to bend the main polymer chain or backbone .Others have flexible linkages

Page 35: Acrylic material in Orthodontics

Large bulky side groups tend to push the polymer chains apart reducing entanglements.

4. Polymer structure..

Linear structure amorphous no identifiable order .

Cross linked network shows increased stability and resistance to solvent.

Types Acrylic Material In Detail

Conventional Heat Cured Acrylic

Supply : Powder in bulk

Liquid in brown glass jar

Contents

Powder

Polymethylmethacrylate Main prepolymerized beads

Benzoyl Peroxide initiator

Coloring Pigments

Opacifiers Zinc /Ttanium oxide

Glass fibers or beads

Dibutyl thalate Plastcizer

Liquid

Methylmethacrylate monomer

Ethylene glycoldimethacrylate crosslinking agent

Page 36: Acrylic material in Orthodontics

Hydroquinone inhibitor

Activation

Initiation

Propagation

Termination

Residual monomer levels are lowest in heat cure acrylic resin around - 0.2 % - 0.3%

Thorough polymerization occurs due to high temperature changes though greater dimensional change of around 0.4 % occurs across molar region .

PMMAPowder

Heat and pressure

PMMAPowder

Particle swellsMMALiquid

Old and newpolymer chainsIntertwined at themolecular level.

Page 37: Acrylic material in Orthodontics

Ratio P:L generally used is about 2:1 by wt% and 1.6 :1 by vol%.

CURING IF DONE AT DIFFERENT STAGES AND THE EFFECTS :

1. Curing Before Monomer Diffusion(Before Dough Stage)

Decreased flexural strength

Cracks between linear polymerised interstitial gel and cross linked beads

More shrinkage contraction by the loss of pressure produced by the dough to compensate for it

2. Curing In Dough Stage

Monomer penetrate the beads

Dissolves Beads Allows Cross-Linking Agent To Penetrate

Interpenetrating Polymer Network Ipn Formed.

3. Packing In The Rubber Stage

Less extrusion of excess acrylic from flask

Extra pressure in the mould

Fracture of the cast and less flow around teeth along with dislodgement of teeth in the mould

Steps Of Flasking.

Flasking.

Dewaxing.

Putting a separating medium.

Placing acrylic dough.

Packing.

Heat curing.

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The wax up of cast is invested in brass flask .

Application of separating media

Closing the flask

After 45 min placed in hot boiling water.

Dewaxing

Separating media

Packing

Piece of cellophane placed over the acrylic and gently pressed , reopened excess removed

Sufficient pressure to avoid voids .

Page 39: Acrylic material in Orthodontics

Finally the flask is closed under pressure left for 1-2 hrs .

Denture placed in water bath with one of the following cycles being used .

a) 75 deg for 8 hr

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b) 75 deg for 1 ½ - 2 hrs

c) 74deg 2 hrs followed by 100 deg 1 hr

Heating and cooling done slowly.

Separating Media:

Separating media used may be thin sheets of tin foil (old concept ).

Now we have “tin foil substitutes” which are namely

Cellulose lacquers , Solutions of alginate , soaps starches

Purpose :

i) prevents water penetration from mould surface to denture base .

ii) Fusing of monomer to investing materials.

Chemical Cure Acrylic…

This is also known as Cold cure resin, Autopolymerizing, Self cure or Pour resin .

The beads are of lower molecular weights.

The same chemical composition holds true except for the activator which is a tertiary amine …dimethyl paratoluidine .

Processing is at room temperature

Residual monomer is at 1-4 %

Lower dimensional change o.2%

Processing is by a) Dough method.

b ) Salt n Pepper method.

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Takamata T , setcos JC ,Phillips RW & Boone ME

Conducted a study on the Adaptation of acrylic resin dentures influenced by the Activation mode of polymerization and published it in the Journal of American Dent Assoc their results were as follows ..

“ Five resins and four processing techniques were evaluated. The two best fitting groups were prepared from an autopolymerizing resin and the microwave –activated resin.”

Another study conducted by Bates J F, Stanford GD, Huggett R & Handley RW Current study of pour type denture resins published in Journal of Dentistry stated that

“ The mechanical properties of pour type denture resins were somewhat lower than those of conventional heat –cured resins and they were more sensitive to laboratory variables”

High Impact Resistant Acrylic…

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Similar processing to Heat cured resin. Much higher impact strength than conventional resins generally double e.g . Lucitone 199

Much less chances of fracture on dropping .

PMM A substituted with a copolymer (rubbery monomer )

This makes it behave as if it has an internal shock absorber .

No Cross linker

Adv : High impact strength . Decreased crazing

Comparable fit is seen.

A rubber phase is added (phase inversion)

Uniformly distributed

Rubber cored polymer

Types

Butadiene + styrene = polystyrene butadiene rubber

Butadiene + MMA

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

PMMA = lucitone 199

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Lightly cross-linked may or may not be added.

Injection Molded Polymers ..

Made of nylon / polycarbonate .

Supplied in the form of a gel as a cake .

This is heated and injected into a mold .

Requires expensive equipment .

Craze resistance not equivalent to conventional acrylic

High Impact resistance

Good alternative for those with MMA Allergy .

Slightly improved clinical accuracy of fit .

Light Activated Polymer ..

This is a composite of UDMA with fillers .

Used as a custom tray material as a denture base resin or repair resin

Properties in general considered to be lower than those of conventional resins .

Slightly rougher surface .

ONLY ONE STUDY showed better fit than heat cured microwave processed .

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COMPONENTS:

Urethane dimethacrylate matrix

Acrylic copolymer

Silica filler to control rheology

Photo-initiators (camphorquinone

Forms

Sheets

Ropes

CURING

Light chamber- 400-500 nm

According to a study done byTan HK ,Brudvik JS , Nichols JI & Smith DE on the Adaptation of a visible light cured denture base material published in Journal Prosthetic Dentistry Various methods were used to adapt light polymerizing resin sheets to casts vacuum forming produced the best adaptation.

Rapid Heat Polymerized Polymer…

Have an altered initiator system .

Else similar to conventional resins .

They are processed in boiling water 20 min .

Base thicker than 6mm have high level of porosity

(Convention says that base up to 20 mm should have no porosity )

Short duration of heating causes higher level of residual monomers . Which in this case is at a level of 3-7 times higher .

Microwave Polymerized Polymers…

Almost same composition as conventional material .

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The Optimal properties come up when special liquid is used .

Has a special polycarbonate flask .

Processing time shorter .

Properties as good as or better than heat cured material .

According to a study done by Bafile M, Grazer GN, Myers ML Li EKH published in Journal Of Prosthetic Dentistry 1991..it was found that…

“The Porosity Is Measured In Heat Cured & Microwave Cured Dentures And Is Found To Be The Same As Long As Special Liquid Is Used For The Microwave Material.”

Fiber Reinforced Polymer ...

Glass, Carbon ,Graphite ,Aramid and ultra high molecular weight polymer polyethylene used .

Metal wire may be used .

Graphite and metal wire reduces esthetics .

Fibers stronger than the matrix polymer thus their inclusions strengthens the composite structure .

Reinforcing agent can be in the form of multidirectional weaves.

Reduced fracture and accidental breakage is its main advantage.

Chemical & Heat Cure Comparison

Chemical Cure

Degree of Polymerization lower .

Increased unreacted monomer ,

Decreased Shrinkage .

Higher dimensional stability

Color stability low .

(due to tertiary amine which is susceptible to oxidation )

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Working time shorter .

Heat Cure

Degree of Polymerization higher.

Decreased unreacted monomer .

Higher shrinkage

Lower dimensional stability .

Color stability higher

Working time higher

Biologic Considerations…

Methyl methacrylate can cause allergic reactions .

Burning /Itching upon contact with skin .

Usually associated with release of residual monomer/Benzoic acid

The dental personnel in constant contact wit MMA

Is at risk for toxic exposure and the main symptom is mild headache .

On animals high quantities have shown varied affects therefore adequate ventilation is required .

Since denture resins are sites for retention for microorganism can cause mucosal irritations .

In a study done by Smith DC Baines MED on Residual methylmethacrylate in the denture base & its relation to denture sore mouth it was found that

“ Free monomer in a properly polymerized denture leaches out within 17 hrs suggesting that this component is not a likely cause for tissue irritation.”

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Uses…

In General

- Complete Dentures

- Partial Dentures

- Artificial Teeth

- Crown & Bridge Facings

- Provisional restorations

-Dies

- Impression Trays .

- Tooth Restoration

- Inlay Patterns

Orthodontic Uses

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Myofunctional Appliances - Twin Block Appliances

- Frankel’s FR

- Activator

- Bionator etc

Obturators

Acrylic adhesives (old)

Space Maintainers

Surgical Splint

Bruxism Splint

Retainers

Mouth Guards

And last but not the least Fluoride and bleaching trays.

Conclusion …

Dentistry in itself is a vast science with new materials turning up practically everyday and the more conventional materials undergoing a high amount of modifications.We saw the importance of Acrylics in the field of dentistry especially in the field of our speciality through this short seminar highlighting its properties chemical and physical…its molecular structure/spatial relationships and its modifications etc.

References:

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Phillip’s Materials in Dental Sciences .

Anusavice Dental Materials

Materials In Dentistry II Ed Jack L Ferracane .

McCabe Prosthodontic Materials

Journal Of Prosthodontics

Journal Of Orthodontics