basics on silicone

7/29/2019 Basics on Silicone

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Presented by

Dr Rinu Sharma

1st year PG resident

Dept of Prosthodontics &

Maxillofacial Prosthetics

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Contents

Introduction

Chemistry & manufacturing process

History of Maxillofacial prosthetic material

Silicone elastomers

Classification

Properties

Advantages/ disadvantages Common problems associated

Conclusion

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Introduction

Silicon Chemical element with the symbol Si

Atomic number 14

Rarely occurs as pure free element but in its combined formaccounts for about 25% of earths crust

Elemental silicon is dark grey in color with metallic shine

It is hard & brittle octahedric material.

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Silicon and oxygen have a strong chemical affinity.

Occurs naturally only in stable form of Si-Ocompounds such as calcium, magnesium and ironsilicates.

As well as silicon dioxide (silica) as sand and quartz.

The tetravalent structure is common to all compounds

in which silicon is surrounded by oxygen atoms.

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Silicone Not to be confused with metalloid element silicon.

Silicone includes silicon together with carbon, hydrogen,oxygen & sometimes other chemical elements.

Silicon to oxygen bond forms the

backbone & in addition bonded to

organic group typically methyl.

Thus , silicones are combination of

organic & inorganic compounds

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Chemistry

Chemically Silicone is :

Polydimethylsiloxane(PDMS)

The basic repeating unit is known as siloxane.

Polymerized siloxane or polysiloxane with chemical formula[R2SiO]n, where R is an organic group eg. methyl .

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Manufacturing Process

( Silicon to Silicone)1. Silicon from Quartz :

Thermal reduction of quartz(SiO2) with carbon.

SiO2 + C ------------> Si + CO2

Thus obtained silicon is a solid metal and needs to becrushed into powder for further reaction.

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Rochow Process

2. Chlorosilanes from Silicon

Most commercially important reaction .

Finely ground and well mixed Silicon is reacted with Methylchloride in gaseous form in presence ofcopper catalyst &certain promoters to form Methylchlorosilanes.

300 Cent

2 CH3-Cl + Si ---------------> Me2SiCl2Catalyst

(Mueller-Rochow synthesis)

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Produces a silane mixture

According to the number of chlorine atoms on the basicsilane molecule,

Mono-, di-, tri- or other silane units with Si-O bonds.

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The predominant material obtained is

dimethyldichlorosilane (approx. 80% by weight).

All silicone fluids, emulsions and rubbers arebased on dimethyldichlorosilane. This is thereforethe decisive base product.

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3. Silicone from Chlorosilanes

Hydrolysis

The reaction of the dimethyldichlorosilanes with water(hydrolosis) or methanol (methanolysis) producessiloxane.

Linear siloxanes with OH groups as well as Cyclicsiloxanes with normally between three and six chainunits are formed.

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Organochlorosilanes react violently with waterreleasing hydrochloric acid.

Using HCl catalysis, thus formed siloxane directlyleads to formation of further reacted oligomers orpolymer siloxanes.

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QUARTZ

SILICON

CHLORO-SILANES

DIMETHYL

DICHLORO-SILANES

SILOXANE

POLYDIMETH

YL -SILOXANE(PDMS)

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Polydimethylsiloxane (silicone)

Polydimethylsiloxane (PDMS) is the most commonlyavailable silicone.

By adjusting -Si-O- chain lengths, the functionality ofthe side groups and the cross-linking betweenmolecular chains, silicones can be synthesized intoalmost infinite varietyof materials.

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Polydimethylsiloxane

Wherein n is an integer ,

Higher n = more elastic

Lower n = more plastic

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Grades of silicone

Fluids Emulsions Resins Elastomers

Silicone elastomers, or rubbers, are made fromlinear polymers that bear hydroxyl, vinyl or otherreactive side chains.

They can be cross-linked in various ways to yieldhighly elastic, more or less open-pored structures.

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Apart from wide range of Uses in Prosthodontics,

Most importantly,

Silicone Elastomers are used as MaxillofacialProsthetic Material for Rehabilitation of patients

with Oral or Facial defects.

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History

Auricular, nasal, and even ocular prosthesis fabricatedwith various materials, have been found in EgyptianMummies.

According to Beder, the first obturator was describedin 1541 byAmbrose pare. It consisted of a simple discattached to sponge.

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Tycho Brache (1546-1601), who used an artificial nosemade from gold to replace his own nose.

1600 to 1800:- Pierre Fauchard (1678) fabricated a silvermask for a French soldier. It was painted with oil

paints & margins covered with facial hair.

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1800 to 1990:-William Morton was credited with

fabrication of a nasal prosthesis using enameledporcelain to match the complexion of a patient.

In 1880:- Kingsleydescribed a combination of a nasalpalatal prosthesis in which the obturator portion was anintegral part of the nasal prosthesis.

In 1900 to 1940:- In the nineteenth century,vulcaniterubberwas widely used in dental profession and wasalso adopted for use in facial prosthesis.

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Upham described the fabrication of nasal andauricular prosthesis made from vulcanite.

In 1905, Ottofy, Baird and Baker all reported usingblackvulcanized rubber.

In 1913Gelatin-glycerin compounds were introduced

for use in facial prosthesis in order to mimic thesoftness and flexibility.

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Kazanjian described the use ofcelluloid prints forcoloring vulcanized rubber facial prosthesis.

From 1940 to 1960:-Acrylic resin was introduced in thedental profession.

From 1960 to 1970:- The introduction of various kindsofelastomers resulted in major changes

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1960 to 1970 Barnhart was the first to use silicone rubber for

construction and coloring of facial prosthesis bycombining silicon rubber base material with acrylic

resin polymer.

Tashma used dry earth pigments dispersed in colorlessacrylic resin polymer powder for intrinsic coloring of a

silicon facial prosthesis.

Thus , silicon elastomers came into practice.

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Silicone elastomers

Probably the most widely used material for facialprosthesis now a days.

Introduced in mid 19th century, but has been used in thefabrication of maxillo-facial prosthesis only for the pastfew years. 24


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Silicone elastomer = Silicone polymer + Filler+ Crosslinker (vulcanizing agent)

Thus, Silicone elastomers are crosslinked linearsilicone (fluids or gums )with a three-dimensional

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Cross-linkingofSilicones

There are three different types of cross-linking reactions:

Peroxide (-initiated) curing --where polymer containsvinyl groups.

Addition curing --where polymer contains vinyl groupsand crosslinking agent contains Si-H groups

catalyst- platinum , palladium , rhodium etc

Condensation curing betweendihydroxypolydimethylsiloxanes and silicic acid esters

Catalyst Tin.

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ClassificationSilicones are classified into 4 groups according totheir applications:

Class I: - Implant grade, which requires the material to

undergo extensive testing and must meet FDArequirements.

Class II: - Medical grade, which is approved for external use.This material is used for fabrication of maxillofacial

prosthesis. Class III: - Clean grade

Class IV: - Industrial grade commonly used for industrial

applications.

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Composition Inorganic

silicon , carbon, oxygen ,hydrogen

Organicmethyl / phenyl / vinyl group

Fillers1. Reinforcing fillers :

- Pyrogenic (fumed) silicas- Modifies properties( Modulus, Tensile strength or

Elongation at break )

2. Non Reinforcing fillers:

-Needed for bulking up silicone rubberseg. Quartz, diatoms or metal oxide

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Crosslinking / Vulcanizing agent

Additives / Colorants: organic or inorganicpigments.eg. Oxidative degradation can be retarded by iron oxides

Catalysts : platinum, tin and titanium complexes etc.

Surfactants / Antioxidants

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Ideal properties of MFP material

Ideal Physical and Mechanical Properties:

- High edge strength.

- High elongation.

- High tear strength. - High tensile strength

- Softness, compatible to tissue.

- Translucent. - Low coefficient of friction

- Low glass transition temperature

- Low specific gravity

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Ideal Biological Properties:

- Non-allergenic.

- Cleansable with disinfectants. - Color stability.

- Dimensionally stable

- Inert to solvents and skin adhesives.

- Resistance to growth of microorganisms.

-Resistance to environmental discoloration

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Why a MFP material ?

Silicone elastomers have been widely used to fabricatefacial prostheses primarily because of their chemicalinertness, strength, durability, ease of manipulation,and biocompatibility

Still far from ideal

The chemical combination of the organic groupsattached to the polysiloxane backbone leads toproducts with outstanding chemical and physicalproperties.

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Properties of Silicones

Thermally stable :

The thermal stability of silicones stems from the Si-O andSi-CH3 bonds which are themselves thermally stable.

(standard grades of silicone elastomers can handle

temperatures ranging from -40 to 200C/-40 - to 392F)

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Electric insulation

Methyl groups regularly spaced throughout siliconemacromolecules are non-polar and do not allow electriccurrent to pass through them.

Silicones are therefore non-conductive and are excellent

electrical insulators.

They are chemically inert after processing.

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Tensile strength Measurement of resistance resulting from stretching

till it breaks.Measured as pounds per square inch(psi).

Depends on amount & type of filler used.

High elongation is desirable when peeling prosthesis

from tissue surface

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The polymer chains, silica fillers, and the interactionsbetween these two components affect the overallsilicone strength and bonding .

(Muhanad M. Hatamleh, David C. Watts . Mechanical properties andbonding of maxillofacial silicone elastomers. Dent Mater 2 6 ( 2 0 1 0 )185191.)

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Tear strength :Measurement of material by pulling until it tears.

Measured as (psi) Silicone elastomers posses high tear resistance due to

cross-linking of material.

This property helps to overcome adhesive resistance &

allows the fabrication of thin margins.

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Viscosity Measured as internal resistance of material to flow.

Measured in centipois(cps) or millipascals(mPas).

Lower the viscosity, higher the flow.

Can be increased with both cross linker & fillers.

AdhesionPosses excellent adhesion propertiesSilicon based adhesives & sealants can withstand

highly acidic / basic chemicals, water and organicsolvents.

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Hardness:Measure of flexibility

Measurement of hardness is done by instrumentDurometer.

Shore A Scale most common (0-100)

Higher the value , harder is the rubber.

Similar hardness to missing facial tissue desired(shore A 5-20).

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Surface tension PDMS have a low surface tension (20.4 mN/m) & are

capable of wetting most of the surfaces.

Methyl group points outward making it hydrophobic withgood release properties.

Glass Transition Temperature is low i.e. (146 K forPDMS )

Excellent weatherability

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Biocompatible The material has minimal adverse impact on host

Hydrophobicity, its chemical & high molecular weight

polymeric nature provides the theoretical basis forlack of toxicity.

Biodurable The Host has minimal side effects on the material

Related to exceptional thermal & chemical stabilityproperties.

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Silicone elastomers

According to the type of vulcanisation (crosslinkingagents, temperature) and base-polymer viscosity

employed , it can be classified as

1. High Temperature Vulcanizing silicones (HTV)

2. Room Temperature Vulcanizing silicones (RTV)

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(HTV)

Usually white ,opaque material

Basically made from reactive silicone gums, or straight

chains with high molecular weight.

Available in semisolid or putty like consistency in 1 or 2component system

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Catalyst/vulcanizing agent : dichlorobenzoyl peroxideor platinum salt depending on type of polymerisationused

Copolymerisation with methyl vinyl or methyl phenylsiloxy radical varies the relative softness & tear

strength

HTV requires milling, packing under pressure & a 30min heat application cycle at 180 degrees.

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Advantages

Pigments are milled into these material so intrinsiccolor can be achieved.

Better color stability than RTV.

It exhibits better mechanical strength and can be usedat temperatures ranging from -50C to +300C. hencethermally stable

It is chemically & biologically inert Better Ageing Resistance & photo-oxidative stability

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Disadvantages

Requirement of a milling machine & a press

Metal mold is normally used & fabrication of mold isa lengthy procedure (stone mold increases risk ofdamage to the material during de-flasking)

It lacks sufficient elasticity to function in movabletissue.

Low edge strength (nylon can be reinforced)Has opaque & lifeless appearance .Do not readily accept extrinsic coloration

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Room Temperature Vulcanizing

(RTV)Available as single paste system or clear solution

Filler : Stannous octoate

Cross linking agent: Orthoalkyl silicate

Polymerisation by condensation or addition reaction

They are used more often than any other maxillofacial

material due to good physical properties & favorableprocessing characteristics.

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Advantages:

Can be polymerised by artificial stone molds but moredurable molds can be made from epoxy resins ormetals

Original RTV silicone are biologically inert & retaintheir physical & chemical properties at widetemperature ranges.

Excellent resistance to weathering

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Disadvantages:

Not as strong as HTV & generally monochromatic Poor edge strength & difficult to color

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Common problems of

Maxillo-facial Prosthetic material Discoloration over time

1. Intrinsic color change of elastomers

2. Intrinsic color change of colorants(pigment flocking)

Discoloration due to loss of external coloration

1. loss of adhesion of extrinsic coloration to prosthesis

2. primers & adhesives3. Poor patient handling

4. Staining

5. Medical adhesives & cleansers

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The degree of the color change observed clinicallyin maxillofacial prostheses may result fromstaining rather than aging of the polymers orpigments.

(RG.C.raig, k Koraann d R.Yu. Elastomers for maxillofacialapplications, Review. Biomaterials 1980, Vol 1)

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Degradation of physical & mechanical properties

1. Tear at margins ( tear strength , fatigue)

2. Change in surface texture3. Elongation at margins

4. Compatibility with medical adhesives

5. Weakening of margins by colorants, adhesives,solvents, cleansers(colorants do not adhere chemically)

6. Deterioration of static & dynamic mechanicalproperties.

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Regardless of type of maxillofacial prosthetic materialused in fabrication,

The service life time of a facial prosthesis is usuallyfrom 6 months to 2 years

The average wearing time of a facial prosthesis is 10months.

( Chen M, Udagama A, Drane JB : Evaluation of facial prostheses for head& neck cancer patients. J Prosthet Dent .46: 538 ; 1981 )

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Conclusion

Polydimethylsiloxane, commonly known as Siliconeare used in many applications including Maxillofacialprosthesis.

The ultimate challenge of a maxillofacial prostheticmaterial lies in

1. Improving physical & mechanical properties so thatit will behave more like human tissue & increase

service life of prosthesis2. Finding color stable coloring agents & developing

sceintific method of color matching to human skin.

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References1. John Beumer III, Maxillofacial rehabilitation: prosthodontics and

surgical considerations. Ishiyaku Euro-American. Inc. Pulblishers.

2. Restorative Dental Materials, Robert G. Craig 11th edition

3. Silicon Biomaterials : History & chemistry , Reprinted fromBiomaterials Science 2nd edition

4. Udita S Maller , Karthik K S , Sudhakara V Maller . MaxillofacialProsthetic Materials - Past and Present Trends. JIADS VOL -1 Issue 2

April - June,2010 |25|

5. Muhanad M. Hatamleh, David C. Watts . Mechanical properties andbonding of maxillofacial silicone elastomers. Dent Mater 2 6 ( 2 0 1 0 )185191.

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THANK YOU

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basics on silicone

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