dm 6 & 7 - polymers & composites

POLYMERS & COMPOSITES IN DENTISTRY

DR. DASMAWATI MOHAMADB.Eng., MSc., PhD (UK)

School of Dental Sciences, USM

Lectures Outline

• What is Polymer?• Chemistry of Polymerization• Structure & Properties of Polymers• What is Composite?• Components of Composites• Curing of Composites• Classification of Composites

What is Polymer?• High molecular weight, chain like

molecules• Distinct repeating group of atoms, derived

from small group of molecules or monomers.

• The process of monomers are converted into polymers is called polymerization. This process can either be addition or condensation reaction.

• Monomers are generally liquids or gases and during polymerization the become converted to crystalline or amorphous solids.

Polymers• Chain branching and cross-linking

– Ideally polymerization should yield linear macromolecules however in practice, structural units of the polymers are often connected together to form nonlinear, branched or cross-linked polymer.

– Cross-linking forms bridges between chains and increases molecular weight.

– Eg. During curing of polysulfide impression material, or during the formation of calcium alginate from sodium alginate.

PolymersExamples of some specific monomers

McCabe-Applied Dental Materials

Chemistry of Polymerization

• Addition Polymerization• Condensation Polymerization

Addition Polymerization Involves the joining together of two molecules to form a third, larger molecule without the elimination of a smaller molecule.Involves four main stages:

1. Activation2. Initiation3. Propagation4. Termination

ActivationA process of producing free radicals (•), very reactive chemical species that have an odd (unpaired) electron.For instance, the decomposition of a peroxide.

Addition PolymerizationInitiation

The free radicals can react with a monomer involving vinyl (CH2=CHX) such as ethylene and can be represented schematically as

R-R' R· + R· ' The excited photochemical initiator attacks the π-bond of a molecule of monomer to produce free radicals (R·), thus created an active center (R·').Propagation

These radicals attack the double bonds of methacrylate groups during the propagation reaction, thus creating new radicals.

R· + M R-M·R-M· + M R-M-M·

Addition Polymerization

TerminationDirect coupling of two free radical chain ends is often terminated for addition polymerization.Both molecules combine and become deactivated by formation of a covalent bond.

R-M·x + ·yM-R R-M(x+y)-R

Condensation Polymerization

• Sometimes refer as step-growth polymerization, often with the formation of by-products such as water, alcohols, halogen acids and ammonia.

• Is a slow process from monomer to dimer to trimer and so forth until large polymer molecules containing many molecules are formed.

• Eg: the synthesis of polysulphide rubber reactionHO---(Silicone)----OH + nHO---(Silicone)---OHHO---(Silicone)---(O—Silicone)n---OH + n H2O

Structure & Properties of Polymers

• Factors affected;– The molecular structure of the repeating units

including the use of copolymers– The molecular weight or chain length– The degree of chain branching– The presence of cross-linking and the cross-

link density– The presence of plasticizers of fillers


• Characterization of polymers by two basic properties which are glass transition temperature (Tg) and melting temperature (Tm). Polymers

Crystalline Amorphous

Tg & Tm Tg


• Amorphous polymers < Tg, behave as rigid solids. > Tg behave as viscous liquids, flexible solids or rubbers

• Q: If a denture constructed from a polymer which had a Tg value of 60°C, how does the denture would behave at normal mouth temperature and when taking a hot drink at 70°C?

Points to Ponder

• Anyone who stops learning is old, whether at twenty or eighty.

• Anyone who keeps learning stays young.• The greatest thing in life is to keep your

mind young.

Henry Ford

What is Composite?

• A combination of materials in which the individual components retain their physical properties.

• Consists of a mixture of two or more materials

• The product material has a better combination of properties than could be realized by any of the component phase.

Composites as a restorative material

Tetric Ceram (Ivoclar Vivadent)

Filtek Supreme (3MESPE)

Compositions of dental composites

• Resin• Filler• Coupling Agent• Initiator

Compositions of dental composites• Resin:

Bis-glycidyl Dimethacrylate (BISGMA)

C29H36O8 RMM 512 (a) lower volatility; (b) lower polymerisation shrinkage; (c)more rapid hardening, and (d) production of stronger and stiffer resin.

C14H22O6 RMM 286Triethyleneglycol dimethacrylate (TEGDMA) –as a diluent

Compositions of dental composites: Resin

Abbreviation Resin RMM Molecular Formula

Bis-GMA Bis-glycidylDimethacrylate

512 C29H36O8

TEGDMA Triethyleneglycoldimethacrylate

286 C14H22O6

UDMA Urethane dimethacrylate

470 C23H38N2O8

Bis-EMA (6) Bisphenol A polyethylene

glycol dietherdimethacryl

ate Bisphenol A

628 C35H48O10


• Filleri.e, Silicate glass, barium, strontiumimprove mechanical properties, reduction in shrinkage, provide radiopacity, controlling aesthetic features; colour, translucency, fluorescence

• Coupling Agenteg. SilaneDifunctional to binds both:1.-OH group on silica 2.monomers in the bis-GMA matrix


• Initiator- Camphorquinone (CQ)

absorbs wavelength 450-460nm (visible light)- Benzoyl peroxide

Note: Inhibitor is also added to minimize or prevent spontaneous polymerization of monomers.eg. Butylated hydroxytoluene(BHT).

Curing of Composites: Method of Curing

• UV Light Cure Polymerisation initiated from light cure unit. A single paste technique.

• Visible Light Cure

Curing of Composites: Method of Curing

• Chemical CurePolymerisation reacts when two paste of activator and initiator mix together.

Curing of Composites: Light Cure Unit (LCU)

Light Emitting Diode (LED) Quartz Tungsten Halogen (QTH)

Curing of Composites: Light Cure Unit (LCU)

• QTH– Consists of a halogen bulb with a filament. As a current

passes through the filament, the wire heats up and as a result electromagnetic radiation is emitted from the filament.

– Wavelength produced very broad, hence has to be filtered.• LED

– semi-conductor devices of an n–p type, constructed from two layers of semi-conducting materials, one doped with electrons (n-doped), the other doped with ‘holes’ (p-type). When a small voltage is applied to the terminals, electrons are injected from the n-region into the p-region.

– Emits light within a narrow wavelength ~ 460-480 nm

Curing of Composites• Degree of Conversion (DC)

A measure of the percentage of carbon-carbon double bonds that have been converted to single bonds to form a polymeric resin.

• DC depends upon the:a) light irradiance reaching the composite surface b) time of exposure c) thermal energy within the system.

• C-Factor– C-factor is the ratio of the bonded to unbonded surface area

of the specimen.– C = total bonded area / total unbonded area

Curing of Composites

• Oxygen inhibition– A sticky surface will be occurred if the resin is

not fully cured.– This is due to the contact of oxygen in the air

during curing.– Avoidable if using incremental placement

procedure rather than bulk procedure and also possible to use a matrix strip to exclude the oxygen.

Classifications of dental composites

Filler Size(mean particle size)

Composite Category

<20 microns Macrofilled

<0.05 microns Microfilled

<10 microns Hybrid

<100nm Nanomer


Courtesy from 3MESPE

Microfilled Nanocomposite

Classifications of dental composites• Macrofilled

– High filler loading, the composites featured a high rigidity andlow fatigue. However, these composites had the disadvantage of being difficult to polish due to the relatively large size of the filler particles, hence resulting in very poor surface finish.

• Microfilled– Due to the extremely small size of the inorganic filler particles,

the surface of these materials can be polished and are aesthetic.• Hybrid

– The concept of hybrid composite has been developed in an attempt to combine the polishability and superior aesthetics of the microfills with the improved mechanical properties of the macrofills to yield a compromise situation.

• Nano– High translucency, high polish and polish retention similar to

those of microfills while maintaining physical properties and wear resistance equivalent to several commercial hybrid composites.


SEM photograph of the roughened surface of a conventional composite material showing protruding filler particles

Smooth outer surface contributes to high strength (Z250)

Problems with Dental Composite

• Polymerisation shrinkage• Lower strength compared with amalgam• Wear

Shrinkage

Shrinkage

0

0.5

1

1.5

2

2.5

3

3.5

TB THB1 THB2 THB3 TC1 TC2 TC3

Shrinkage at 23CShrinkage at 37C

Shrinkage

Several strategies have been done by the researchers to reduce the shrinkage;

1. to increase the volume of the inorganic filler incorporated in the composite

2. to use different co-monomers such as multi-methacrylate, highly branched methacrylates

3. to develop new monomers with low volumetric shrinkage

4. to improve the photo-initiator system

Improvement of Composites

DEVELOPMENTS IN COMPOSITE

Resin Technology Filler Technology

Application of CompositesClass III RestorationClass III Restoration

Application of Composites

Class IV RestorationClass IV Restoration

Application of Composites

Class II RestorationClass II Restoration

dm 6 & 7 - polymers & composites

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