IMPRESSION MATERIALS

Guided by

Dr.A. Chandrasekar

Prof. and Head

Presented by

G. Mahalakshmi

1st year PG

CONTENTS Introduction Definition Ideal impression material Review of literature Inelastic impression materials - impression compound - zinc oxide eugenol paste - impression waxes Elastic impression materials - Reversible hydrocolloids - Irreversible hydrocolloids

CONTENTS- Polysulfide polymers - Condensation silicones - Polyether - Addition silicones - Polyether urethane dimethylacrylate Problems with impression materials - to the patient - to the dentist - to the environment

Disinfection Conclusion References

INTRODUCTION An impression is a negative

reproduction of hard and soft tissues. It can serve as a mould to obtain a cast, on which a planned restoration is fabricated

Impression materials are used to register or reproduce the form and relationship of the teeth and supporting oral tissues.

MODEL, CAST AND DIE From the impression, an exact replica of

the dental structures of interest is obtained using a cast or die materials.. The replica thus, is a positive reproduction. The positive reproduction is called a model or cast when large areas of oral tissues are involved or a die when a single preparation is recorded

Ideal Properties of Impression Materials

(1) Non toxic and non irritant

(2) Acceptability to the patient: (a)Setting time, (b)Taste, (c)Consistency

(3) Accuracy : both (a) Surface reproducibility

(b) Dimensional stability

(4) Use of material : (a) Ease of mixing (b) Working time (c) Setting time (d) Handling of the material

(5) Compatible with model materials

(6) Economics of material (a) Cheap (b) Long shelf life (c) Accuracy (save redoing impression)

7)readily disinfected without loss of accuracy

8)No release of gases during setting9)Adequate strength so it will not break or

tear on removal from mouth10)Hydrophilic

Imp

ressio

n

Mate

rials

Non-elastic

Elastic

Aqueous Hydrocolloids

Non-aqueous Elastomers

PolysulfideSilicones

Polyether

Agar (reversible)

Alginate (irreversible)

Plaster

Compound

ZnO - Eugenol

Waxes

CLASSIFICATION OF IMPRESSION MATERIALS based on nature of the material

CLASSIFICATION

ACCORDING TO THE MANNER IN WHICH THEY HARDEN

 

(a) set by chemical reaction(irreversible) 

-plaster of Paris -zinc oxide eugenol -alginate -non aqueous elastomers 

(b)set by physical change(reversible)

-impression compound -agar 

ACCORDING TO THE VISCOSITY/TISSUE DISPLACEMENT

(A) Mucostatic – materials which are initially fluid- less likely to compress soft tissues.

(b) Mucocompressive – materials which are initially viscous

High viscosity Medium viscosity Low viscosity

1)Imp com 1)regular elastomers 1)imp paste

2)Putty elas 2)hydrocolloid

3)light body elas

4)imp plaster

BASED ON THEIR USE:A) Suitable for edentulous jaws: Plaster Impression compound Zinc oxide eugenol paste Irreverisible hydrocolloidB) Suitable for dentulous jaws; Irreversible hydrocolliod Reversible hydrocolloid Elastomersc) Suitable for crown and bridge work Green stick compound Elastomers Reversible hydrocolloids

IMPRESSION MATERIAL SYSTEMS Powder-liquid Paste-liquid Paste-paste Materials softened by heat

IMPRESSION TRAYSUsed to carry the impression material into

the mouth stock trays custom trays special use trays (bite registration trays)

IN ELASTIC IMPRESSION MATERIALS

   

         IMPRESSION COMPOUND

Also called as modelling plastic. It is one of the oldest impression material to be used TYPES:

1. TYPE-1: IMPRESSION PURPOSE 2. TYPE-2: TRAY FABRICATION

It is a thermoplastic compound i.e. it softens when heated and hardens when cooled. This process does not involve a chemical reaction.

Type-1 - used to take impressions of the mouth. Softening temp is 55-600C.

Type-2 - used for modifying impression trays. Softening temperature is 700C

INGREDIENTS Made up of1. Resins (e.g. wax) - principal ingredient2. Filler (e.g. talc or soapstone) – improve their

physical properties like strength flow and viscosity)

3. Lubricants (stearic acid or stearin, shellac, gutta-percha) - plasticizer-reduce brittleness

4. Colouring agent ( appropriate amt )

Impression Compound - MANIPULATION

1. The compound is completely immersed in a water bath at 55-60 degree Celsius for about 4-5 minutes to ensure complete softening.

2. Gauze is placed at the bottom of the water bath to prevent adherence. 3. If left too long some of the constituents may be leached out into the

water bath, altering the properties of the material (it is often the plasticiser stearic acid that is leached out).

4. The compound is loaded on to the tray and firm pressure is used to seat the tray in the mouth.                                                                                

An impression compound impression taken in an upper edentulous stock tray

The temperature at which it is placed inside the mouth is 43.5-45 degree celcius

Conductivity of heat is low, hence material should be allow to cool completely before removal to avoid distortion and also should be uniformly softened before making the impression.

SOFTENING(A)OVEN(B)FLAME(C)HOT WATER

Care should be taken not to over heat the impression to avoid boiling/burning/leaching out of low molecular weight ingredients. This will make compound brittle and grainy.

FLOW According to ADA specification ,

impression compound should have maximum flow not less than 85% at safe temperature 45 degree celcius. And should have minimum flow at 37 degree celcius (mouth temperature) not more than 6%.

PROPERTIES

1.Poor surface detail

2.High coefficient of thermal expansion (contraction of up to 0.3% when removed from mouth to room temperature)

3. Distorts when removed over undercut areas

4. Mucocodisplacive

5. Poor dimensional stability

6. Can be modified by re-heating

7. Non toxic and non irritant

8. Good shelf life

9. Can be sterilized in autoclave for 10 minutes

INDICATIONS FOR USE:

• To make an impression of completely edentulous ridge

• In operative dentistry – impression of single tooth -tube impression(with the help of cylindrical copper band or matrix band)

• construction of custum trays – tray compound

• border moulding of acrylic custum tray

ADVANTAGES:

1. Non irritant and non toxic2. Reusable 3. Can be reheated and readapted4. Can support other materials for wash impressions5. Mucocompressive6. Beading and boxing can be done

DISADVANTAGES:

1. Poor dimensional stability- relaxation of stresses

2. Poor surface detail

3. Expansion coefficient

4. Will distort if removed from undercuts

5. Not suitable for dentulous cases

ZINC OXIDE EUGENOL IMPRESSION PASTE Zinc oxide and eugenol react and yield a

relatively hard mass which possess certain medicinal advantages as well as mechanical benefits in certain dental operations.

COMPOSITIONDispensed as two separate pastes (base

and catalyst pastes)TUBE NO 1: Zinc oxide - main ingredient –

87% Fixed vegetable or mineral oil - plasticizer -

13% (offsets the action of eugenol)

TUBE NO 2: contains Oil of cloves or Eugenol 12% Gum or polymerised rosin 50%(facilitates reaction speed and helps in

yielding a smoother and homogenous mix) Filler(silica) 20% Resinous balsam 10%(improve mixing properties and flow) Accelerator(calcium chloride,zinc acetate,

glacial acetic acid) 5% Lanolin 3%

CHEMISTRY ZnO +H2o Zn(oH)2 +eugenol ZnE2 +2H2o AUTOCATALYTIC REACTIONSo the reaction proceeds faster in

humid environment

Zn(oH)2

MANIPULATION Oil- impervious paper or glass slab is

used Squeeze 2 strips of paste in equal

length Flexible stain less spatula is used to mix Two strips are combined with first sweep

of spatula and mixing continued for 1 minute.

Uniform color should be obtained.

SETTING TIME Initial setting time – 3-6 mins - includes

time of mixing, filling into the tray and seating the impression in mouth

Final setting time: is the time at which the material is hard enough to resist penetration under a load

Type 1 paste(hard) - 10 mins Type 2 paste(soft) - 15 minsWhen the final set occurs the impression

can be withdrawn from the mouth

CONTROL OF SETTING TIME Adding accelerator or a drop of water to

the eugenol paste Cooling the spatula and mixing slab

prolong setting time Adding olive oil, mineral oil, and

petrolatum prolong setting time – this reduces rigidity and gives inhomogenous mix

Altering ratio of two pastes Longer the mixing time , shorter the

setting time

Accelerating action of body temperature and saliva on the surface of tissues cause the adjacent surface of the impression to harden first

Impression should be removed after complete hardening of the material

PHYSICAL AND MECHANICAL PROPERTIES Flow: less flow – shorter working time Rigidity and strength: same or superior

that of compound . Dimensional stability good

NON- EUGENOL PASTEDisadvantage with eugenol is stingy and produce burning sensation ZOE reaction is never completed so free

eugenol may leach out Disagreeable taste

So, carboxylic acid can be used in place of eugenol.

Orthoethoxybenzoic acid (EBA) is usedBactericides and medicaments can also

be incorporated

USES Impression material for edentulous

ridges - wash impression(paste is

spreaded over compound) Surgical dressing- soft pastes are used Bite registration paste offers less

resistance to closure of the mandible and also gives a stable record

Temporary relining material for dentures

IMPRESSION WAXESWAXES: 1)pattern waxes 2) impression waxes – corrective

wax biteplate

wax 3) processing waxesBITE REGISTRATION WAX: used for

occlusal registration. formulated from bees wax or

hydrocarbon waxes and contain few metal particles

CORRECTIVE IMPRESSION WAX: for edentulous impressions. used as wax veneer over an

original impression to contact and register the detail of the soft tissue

formulated from hydrocarbon waxes (such as paraffin, ceresin) and beeswax and may few metal particles

flow at 37 degrees is 100%.

ELASTIC IMPRESSION MATERIALS

Material susceptible to being stretched compressed or distorted and then tending to resume the original shape.

Elastic impression materials are capable of accurately reproducing both the hard and the soft structures of the mouth including the undercut areas and interproximal spaces

ELASTIC

ADVANTAGE OF ELASTIC IMPRESSION MATERIAL OVER RIGID IMPRESSION MATERIAL

can be used in both dentulous and edentulous

cases with undercuts. The distortion when removed from an undercut is

minimal.

• Colloids are classified as the fourth state of

matter , they lie between suspension and solutions. A solid liquid or gaseous substance made up of large molecules or masses of smaller molecules that remain in suspension in a surrounding continuous medium of different matter

HYDROCOLLOIDS

.TYPES OF COLLOIDS

Aerosols----– liquids or solids in airLysosols ------gas or liquid or solid in

liquid.Foams---------gases in solid

Solid emulsion – liquids in solid Solid suspension -- solids in solid

• The colloidal materials that are dissolved in water are termed

hydrocolloids

• If the change of sol to gel is thermal and reversible –

reversible hydrocolloid (agar)

• If the change of sol to gel is chemical and irreversible -

irreversible hydrocolloid (alginates)

HYDROCOLLOID

A colloid that contains water as

the dispersion phase

GEL

A network of fibrils that form a weak slightly elastic brush heap

structure of hydrocolloid

SOL GEL TRANSFORMATIONIf a hydrocolloid contains an adequate

concentration of dispersed phase, a sol may change to semisolid material known as gel. In the gel state the dispersed phase agglomerates to form chain of fibrils also called as micelles. The fibrils may branch and intermesh to form a brush heap structure. The dispersed medium is held in the interstices between the fibrils by capillary attraction or adhesion .

For agar secondary bonds hold the fibrils together, these bonds break at slightly elevated temperature and become re-established as the hydrocolloid cools to room temperature. This process is reversible. In case of alginate the fibrils are formed by chemical action and the transformation is not reversible.

REVERSIBLE HYDROCOLLOID – AGAR Introduced by “Alphous poller of Vienna” in 1925

It was the first successful elastic impression material

Agar is an organic hydrophilic colloid extracted from certain

“Seaweed”

It is a sulphuric ester of a linear polymer of galactose

Though highly accurate, it has been largely replaced by

alginates and elastomers due to its cumbersome manipulation

Component Function Composition (%)

Agar Brush Heap structure 13 – 17

Borate Strength 0.2 – 0.5

Pot. Sulfate Gypsum hardener 1.0 – 2.0

Wax, ZO, Silica, , etc. Filler 0.5 – 1.0

Thixotropic materials Thickener 0.3 – 0.5

Water Reaction medium Balance 84%

Alkylbenzoates Perservative 0.1

Composition:

GELATION

Setting of reversible hydrocolloid

THERE IS A PHASE CHANGE FROM

SOL GEL

GELATION TEMPERATURE:

Gelation temperature - approximately 37°C. If the gelation temperature is too high it is possible that injury may result to the oral tissues involved , a severe surface stress may also develop . If the gelation temperature is too low it will be difficult or even impossible to chill the material to a temperature sufficiently low to obtain a firm gel adjacent to the oral tissues .

According to ADA specification no. 11 gelation temperature must not be less then 37°C or more then 45°C.

LIQUIFACTION TEMPERATURETemperature at which gel change to sol. (70 -100°c)

GELATION TEMPERATURETemperature at which sol changes to gel.(37-50°c)

HYSTERESISThe temperature lag between the liquefaction temperature and the gelation temperature.IMBIBITIONThe process of water sorption i.e. the gel swells when placed in water.SYNERESISExpression of fluid on to the surface of gel structure.

Based on viscosity type 1 - high consistency type 2 - medium consistency type 3 - low consistency

The Material is supplied as:

Gel in collapsible tubes (for impressions)

A number of cylinders in a glass jar (syringe material) In bulk containers (for duplication) Available forms: Syringe material Tray materialThe only difference between the syringe and the tray material is colour and the greater fluidity of the syringe material

Manipulation:

Agar hydrocolloid requires special equipments

- Hydro colloid conditioner

- Water cooled rim lock trays

PREPARATION OF THE MATERIAL :• First step is to reverse the hydrocolloid gel to

the sol form.• Usually done at 100C for 10min• 3 min should be added to this time whenever

the material is being reused. As it becomes difficult to break down the agar brush heap structure.

• After liquefaction ,material may be stored in sol condition.

• Storage temperature: 65C to 68C.

Conditioner Consists of:

a. Boiling or liquefaction section: 10 mins in boiling water (1000 C).

b. Storage section: 65-680 C is ideal it can be stored till needed

c. Tempering section: 460 C for about 2mins

CONDITIONING OF THE MATERIAL:

This refers to the cooling of the material (also called as ‘tempering’ of the material)

Tempering is usually done at 43C for 7min.

Uses of tempering:It increases the viscosity of the material so

that the hydrocolloid does not flow out.It reduces the temperature of the material

so that it is not uncomfortable for the patient.

WATER COOLED RIM LOCK TRAYS

IMPRESSION MAKING

• The tray material is seated over the syringe material

• Excess water from the surface of the tray material should be removed to facilitate proper union of the tray and the syringe material.

• Gelation is accomplished by circulating cool water at 18C to 21C through the tray for at least 5min. Care should be taken to prevent the movement of the tray during gelation.

After complete gelation the impression is removed from the mouth with a single jerk to prevent the tearing of the impression.

Laminate technique or agar-alginate combination technique

• Injecting syringe agar material onto the prepared tooth

• Placing an impression tray with chilled cooled alginate over the injected syringe agar .

• Alginate sets by chemical reaction . Agar sets by coming in contact with chilled alginate.

•

LAMINATE TECHNIQUE OR AGAR-ALGINATE COMBINATION TECHNIQUE Advantages:• Less agar used so cost effective• Accurate and quick• No need for water circulation

Disadvantages:• Bond between agar and alginate may

break• Suitable for single tooth preparation only• Thick consistency alginate may displace

thin consistency agar material

•Area to be recorded is flooded with warm water

• Syringe agar material – quickly & liberally over prepared tooth only at the occlusal or incisal areas

• Immediately, tray agar placed over the syringe agar

•Logic here is thick consistency tray material will push down the thin consistency syringe material into all areas of prepared tooth through hydraulic pressure

Wet field technique

FINAL IMPRESSION WITH AGAR

ADVANTAGES:

1. Hydrophilic Impression material

2. Good elastic properties, Good recovery from distortion

3. Can be re-used as a duplicating material

4. Long working time and low material cost

5. No mixing technique

6.High accuracy and fine detail recording

7.It gives good model surface as it is not hydrophobic and it

takes up the moisture

8.It is palatable and well tolerated by patients.

9)Cheaper as compared to non-aqueous elastomeric

impression materials

DISADVANTAGES:1. Extensive and expensive equipment required 2. It can not be electroplated 3. Impossible to sterilize for reuse

4. Low dimensional stability & tear resistance

5. Multiple models cannot be poured

Uses of Agar:

1. Widely used at present for cast duplication

2. Final impressions in dentulous patients for fixed and

removable prosthesis fabrication

3. Was used for crown & bridge before the advent of

elastomers

Disinfection of the impression:

Disinfection of agar is very important to prevent cross infection since the material can be re-used.

Disinfectant choice varies with the product according to the specifications of the manufacturer. Glutaraldehyde 2% or 0.5% sodium hypochlorite(submerge the impression for 10 mins).Recent protocol include synthetic phenols, iodophor.

STORAGE Storage of the impression in 100% relative

humidity is suggested to prevent dimensional changes.

May be wrapped in water soaked towel 2%pottassium sulfate solution

REMOVAL OF THE CAST Contact of the stone with impression

should be for 60 mins before the cast is removed.

PROPERTIES

Dimensional Stability of the impression:

Gels are invariably subject to changes in dimension by syneresis and imbibition.Dimensional changes begin as soon as the impression is removed from the oral cavity.

According to the ADA specification number:- 11 the compressive strength should not be less the 0.245 MPa

1) Tear strength ----------- 800 to 900 gms/cm22) Flow- sufficient to record the fine details 3) Flexibility sufficient4) Working time is 7-15 min5) Setting time is approximately 5 min6) Elasticity and elastic recovery : recovery occurs

upto 98.8%7) Reproduction detail:upto 25microns is achievable

with agar

FAILURES REASONS Grainy material Inadequate

boiling Storage

temperature too low

Storing for too long

Not poured immediately

FAILURES REASONS Distortion

Separation of syringe and tray material

Movement of tray during gelation

Improper removal Premature removal

Water soaked layer on tray material not removed

Premature gelation of either tray or syringe material

FAILURES REASONS Tearing

Air bubbles

Inadequate bulk Premature

removal

Gelation of syringe material preventing flow

FAILURES REASONS Rough or chalky

stone model Excess water left

in the medium delayed removal

of die Inadequate

cleaning of impression before pouring of cast

IRREVERSIBLE HYDROCOLLOID-ALGINATE

At the end of the 19th century, a chemist from Scotland noticed that certain brown sea weed (algae) yielded a peculiar mucous extraction. He named it "algin" . This natural substance was later identified as anhydrous -B-d- mannuronic acid (alginic acid)

In England 40 years later another chemist S.Willing received a basic patent for the use of algin as a dental impression material.

Substitute for agar during world war II.

Currently, alginate is more popular than

agar because

It is easy to manipulate

It is comfortable for the Patient

It is relatively inexpensive and does

not require elaborate equipment

TYPES 1. Type I - fast setting _ 1-2 mins

2. Type II - Normal setting _ 2-4.5mins

MODE OF SUPPLY It is supplied as a powder that is

packed In bulk or in tins or in sachets. In pre-weighed individual containers A plastic scoop is supplied for dispensing

the bulk powder and a plastic cylinder is supplied for measuring the water required for the bulk or the pre-weighed alginate powder.

Latest advances - Modified Alginates:

1.In the form of a sol, containing the water. A calcium reactor of ( plaster of Paris) is supplied separately.

2. 2 paste system, one containing the alginate sol, the second the calcium reactor..

3. Chromatic alginates: contain acid/base indicator that change colour at different critical points, indicating mixing time, loading into mouths & setting.

4. Dustless Alginate:

To avoid dust inhalation - coating the material with a glycol.

5. Siliconised Alginates:

Alginates modified by the incorporation of silicone polymers which improve the physical properties.

Component Function Wt%

Potassium alginate Soluble alginate 15

Calcium sulfate Reactor 16

Zinc oxide Filler particles 4

Potassium Titanium fluoride Accelerator 3

Diatomaceous earth Filler Particles 60

Sodium phosphate Retarder 2

COMPOSITION OF ALGINATE

GELATION PROCESS

The typical sol - gel reaction can be described simply as a reaction of soluble alginate with calcium sulfate to form an insoluble calcium alginate gel. The production of calcium alginate is so rapid that it does not allow sufficient working time, thus a third water soluble salt, such as tri sodium phosphate, is added to prolong the working time. The strategy is that the calcium sulfate will react with the trisodium phosphate in preference to the sodium alginate and delay the formation of the insoluble calcium alginate. The reaction occurs as a follows

2 Na3 Po4 + 3 Ca S04 Ca(P04)2+ 3Na2 S04

When the tri sodium phosphate is exhausted , the calcium ions begins to react with the potassium alginate to produce calcium alginate as follows

K2Alg + n CaSo4 -> n K2 S04 + Can Alg

GEL STRUCTURE The gel structure can be envisioned as a

brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water , filler particles and reaction byproducts, such as sodium sulfate and calcium phosphate. Calcium is responsible for cross linking.

CONTROL OF GELATION TIME

The gelation time measured from the

beginning of mixing until gelation, must allow

sufficient time for the dentist to mix the

material, load the tray and place it in the

patient's mouth. Once gelation starts the

impression material must not be disturbed

because the growing fibrils will fracture and

the impression would be significantly

weakened.

Probably the optimal gelation time is between 3-4 minutes at room temperature. Gelation time is best regulated by the amount of retarders added during manufacturing.

The dentist can best control the gelation time by altering the temperature of water used for mixing the alginate material. The higher the temperature the shorter is the gelation time and vice versa.

The mixing bowl and spatula can be cooled to prolong the gelation time.

Altering the gelation time by altering the W/ P ratio and mixing time can have marked effects on the properties of the gel, impairing the tear strength and elasticity.

The practical method of determining gelation time is to observe the time from the start of mixing until the material is no longer tacky or sticky when touched with clean dry, gloved finger tip.

Types Mixing time

Working time

Setting time

I- Fast set 45 sec 1.25 mins 1-2 mins

II- Normal set

60 sec 2 mins 2 - 4.5 mins

Manipulation: - 15 gm. Powder : 40 ml of water

-

MANIPULATIONPreparing the mix: The measured powder is sifted into pre measured

water that as been placed in a clean rubber bowl . The powder incorporated in to the water by careful mixing with a curved ,clean, metal spatula. Avoid whipping of air into the mix. A vigorous figure eight motion is best, with the mix being swiped or stropped against the sides of the rubber bowl with intermittent rotations ( 180 degrees) of the spatula to press out air bubbles. It is important to get all the powder dissolved, if residual powder remains, a good gel cannot form and the properties are compromised.

A mixing time of 45 seconds to 1 minute is sufficient, depending on the brand and type of alginate. Strength of the gel is reduced if the mixing is not complete the result should be a smooth, creamy mixture that does not drip off the spatula when it is raised from the bowl . A variety of mechanical devices are also available for spatulating the alginate materials. Their principal benefit is convenience, speed and elimination of the human variable.

Making the impression: The mixture is placed in a suitable tray,

which is carried into place in the mouth. It is imperative that the impression adhere to the tray, so that the impression can be withdrawn from around the teeth. Therefore a perforated tray is generally used.

If a plastic or a metal rim lock tray is used, a thin layer of tray adhesive should be applied and allowed to dry completely before loading the tray.

Thin layers of alginate are weak; therefore the tray must fit the patient's arch. So, sufficient bulk of the material (at least 3 mm) between the tray and the tissues be present.

The impression should not be removed for at least 2-3 minutes after the gelation has occurred . If the impression is left for too long significant distortion results.

STRENGTHMaximum gel strength is required to 1)prevent fracture and 2) to ensure elastic recovery of the impression on its removal form the mouth. All manipulative factors that are under the control of the clinician affect the gel strength. Like 1)altering water powder ratio . 2)Insufficient spatulation results in failure of the ingredients to dissolve sufficiently . 3)Over mixing breaks up the gel network as it is forming and reduces its strength. The directions supplied with the product should be followed in all respects.

PROPERTIES Taste and odour: pleasant taste and

smell. available in different flavours Elasticity and elastic recovery: highly

present Reproduction of tissue details: less when

compared to agar. Reproduce a line that is 75 microns in width.

Shelf life and storage: stored at temp not above 37 degrees. Lid should be always closed, to minimize moisture contamination.

Viscoelasticity:

Hydrocolloids are strain-rate dependent. Thus, the tear strength is increased when the impression is removed with a snap. The speed of removal must be a compromise between a rapid movements and the comfort of the patient. Usually, an alginate impression does not adhere to the oral tissues as strongly as some of the nonaqueous elastomers, so it is easier to remove the alginate impression rapidly. However, it is always best avoid torquing or twisting the impression in an effort to remove it quickly.

Dimensional stability:

Gels are invariably subject to changes in dimension by syneresis , evaporation and inhibition. Therefore the impression should be exposed to air for as short a time as possible and a cast should be constructed immediately.

Disinfection:

Hydrocolloid impression must be poured within a short time after removal from the mouth, so the disinfection procedure should be relatively rapid to prevent dimensional change.

The irreversible hydrocolloids may be disinfected by 10- minutes immersion in, or spraying with, an antimicrobial agent such as sodium hypochlorite and glutaraldehyde without significant dimensional changes.

Compatibility with gypsum:

Gypsum casts are not compatible with hydrocolloid impressions. They may be too soft for waxing procedures. This disadvantage can be over come in two ways1) By immersing the impression in a solution containing an accelerator for the setting of the gypsum cast.2) By incorporating a plaster hardener or accelerator in material by the manufacturer.

Alginate impression material is available in the market by some of the following trade namesTRADE NAMES:1) Jeltrate2) Zelgan3) Algitex4) Vericol aroma etc.

ADVANTAGES1) It is easy to mix and manipulate2) Minimum requirement of equipment3) Flexibility of the set impression4) Accuracy if properly handled.5) Low cost6) Comfortable to the patient.7) It is hygienic, as fresh material must be used

for each impression8) It is gives a good surface detail even in very

wet mouths.

Disadvantages:

1. Cannot be electroplated.2. Distortion occurs easily3. Poor dimensional stability (poured within 15 min.)4. Poor tear strength 5. Not accurate enough in crown and bridge preparations

Uses:

1. Complete denture prosthesis & orthodontics

2. Mouth protectors3. Study models and working casts 4. Duplicating models

TYPE CAUSE1. Grainy material a. Improper mixing

b. Prolonged mixic. Water ; powder ratio

too low2. Tearing a. Inadequate bulk

b. Moisture contaminationc. Premature removal

from mouthd. Prolonged mixing

3. Bubbles a. Undue gelation, preventing flow

b. Air incorporated during mixing.

4. Irregularly shaped voids a. Moisture or debris on tissues.

TYPES OF FAILURES IN ALGINATE

5. Rough or chalky stone cast

a. Adequate cleaning of impressions.

b. Excess water left in impressions.

c. Premature removal of cast

d. Leaving cast in impression too long

e. Improper manipulation of stone.

6. Distortion a. Impressions not poured immediately.

b. Movement of tray during gelation.

c. Premature removal from mouth.

d. Improper removal from mouth.

e. Tray held in mouth for too long.

COMPARISON BETWEEN HYDROCOLLOIDS

Properties Agar Alginate

Flexibility 20 % 14 %

Elasticity & elastic recovery

98.8 % 97.3 %

Reprodn. of details

25 um < agar

Tear strength 715 gm/cm2 350-700 gm/cm2

Comp. strength

8000 gm/cm2 500-8000 gm/cm2

Diml. Stability Better Poor

Reuse Possible Not possible

Manipulation Conditioner & rim lock trays

Normal trays

Linke B.A., Nicholls J.I., Faucher R.R. (1985)- conducted a study on the distortion analysis of stone casts made from impression materials. Six different impression materials were tested. They concluded:- 1. The impression materials all produced casts with an arch perimeter larger than the standard reference model. 2. Reversible hydrocolloid impression material produced significantly less interabutment distortion than the other impression materials. 3. The silicone-irreversible hydrocolloid produced less interabutment distortion than the irreversible hydrocolloid.

REVIEW OF LITERATURE

4.Significant differences exist among the impression materials, however, relative to the impression materials that are currently being used successfully, the combinations of irreversible hydrocolloid with silicone & modified reversible hydrocolloid with irreversible hydrocolloid were clinically acceptable

HISTORY OF ELASTOMERS• 15th Century – Christopher Columbus on his trip to

South America–found the natives playing with a strange looking solid mass ,which had the property of bouncing back on throwing onto the ground.Their natives called it weeping wood.

• 1770-Joseph Priestly – English scientist found that this material could `Rub out` the pencil mark so called is as Rubber. Later called as latex (milky fluid) –exudate of Rubber producing tree – Hevea Brasiliansis.

• 1839 –Charles Good Year (America) discovered Vulcanization process-polymer chain of rubber are cross linked through Sulfur atoms at high temperature to increase rubber hardness & durability- Rubber Tyres came

• 1927- A German Scientist ,invented the first synthetic rubber from a petrochemical called Butadiene. Polymerization done with Sodium Metal – called as “Bu Na”

• 1950s –Mercaptan rubber polymers were introduced followed by Silicone materials

GENERAL PROPERTIES OF ELASTOMERS All natural & synthetic rubbers are polymers

These molecules are tied together to form a 3 dimensional network. On stretching ,the chain uncoils ,and on removal of the stress ,they snap back to their relaxed entangled state. This coiling & uncoiling gives the Elastic property.

Synthetic resins (plastics) are also polymers but less elastic .So rubbers are called ELASTOMERS

Elastomers show excellent reproduction of surface details Generally hydrophobic except polyether Repeated pouring of impression is possible Tear strength excellent Tray adhesives used to retain the material in tray Can be electroplated with copper & silver Shelf life -2 yrs. under cool conditions-increased

USES OF ELASTOMERS

In FPD for impressions of prepared teeth Impressions of Dentulous mouth for RPD Impressions of edentulous mouth for CD Polyether –border molding of special trays For bite registration Silicone duplicating material used for making refractory

casts during cast partial denture construction

CLASSIFICATION OF ELASTOMERSI. According to chemistry 1.Polysulfide 2.Condensation polymerizing silicones 3.Addition polymerizing silicones 4.Polyether

II. ADA classified nonaqueous elastomers based on 24 Hr dimensional change following setting & certain elastic properties (max.permanent deformation& max.flow in compression)as

Type I Type II Type III

III. According to viscosity1.Low viscosity- light body or syringe consistency

2.Medium viscosity- Medium body or regular body

3.High viscosity – Heavy body or tray consistency

4.Putty consistency

5.Monophase consistency

IV. Elastomers also classified as

Hydrophobic

Hydrophilic

V.Recent advances – Visible light cure polyether impression material

POLYSULFIDE IMPRESSION MATERIAL First dental elastomers by Thiokol Corpn-Germany Called as Mercapton Impression material, vulcanising

impression material. Indications

complete denture removable fixed partial denture crown and bridge

Examples Permlastic (Kerr) Omni-Flex (GC America)

Mode of suply• 2 paste system in collapsible tubes as base

& acceleratorComposition:Base - white

– Liq.Polysulfide polymers - 80- 85 %– Inert fillers -Titanium dioxide, zinc Sulfate, Copper

carbonate or Silica - 16- 18%– Plasticizer – dibutyl phthalate- confers appropriate

viscosity– Sulfur – to enhance the reaction

Reactor paste - Brown– Lead dioxide 60-68% - Reactant– Dibutylpthalate 30- 35% - plasticizer

– Castor oil 5-8%– Magnesium Stearate - (retarder)

and deodorants -2%

• Recently developed PS replaces lead oxide by organic hydroperoxide accelerator system –cleaner to handle. Copper is also used as cross linkng system but not become popular.

• Tray adhesive - Butyl rubber or styrene /acrylonitrile dissolved in a weak solvent like chloroform or a ketone

--SH HS---------------------SH HS--

S H

O

=

Pb

=

O

O

=

Pb

=

O

O = Pb = OH

S

-S-S---------------S-S-

S

S

+3PbO+H2O

Mercaptan + Lead dioxide Polysulfide rubber + Lead oxide +

water

POLYSULFIDE REACTION

Reaction is exothermic with rise of 3-4 ˚c

Heat & moisture accelerates the reaction.

SETTING REACTION A condensation reaction (oxidation )of pendant and terminal

Thiol(mercapton- SH) groups with lead dioxide and sulfur to produce disulfide linkages & water.

Chain lengthening predominates first by oxidation of terminal –SH groups causing increase in viscosity

Cross linking occurs secondarily by oxidation of pendant SH groups which produce elastic properties

AVAILABLE VISCOSITIES

Light bodied –syringe material for dentulous impressions & for some mucostatic edentulous impressions

Medium- regular bodied –routine edentulous impressions Heavy bodied –mandiblar dentulous impressions & tissue

displacive edentulous impressions The viscosities of 3 types determined by the amount of

fillers in the base component

MANIPULATION

Adhesive to tray Uniform layer

custom tray Equal lengths of pastes Mix thoroughly

within one minute Pour within 1 hour

HANDLING TECHNIQUE Syringe and tray material used- multiple mix technique. Two materials should cure together, otherwise they may

not cohere. Under no circumstances should impression material be

allowed to develop elastic properties before the tray is seated - impression will be too narrow and too short.

PROPERTIESWorking & Setting Time

Type Working time Setting Time

Light Body 4-7 min 7-10 min

Medium Heavy&

3-6 min 6-8 min

Accelerators &(water) & retarders (Oleic acid ) can be used to adjust the working time & setting timeWarm ambient conditions also shortens Setting & Working timesBase catalyst ratio not altered above 20%-affects physical properties& dimensional changes

Poly sulfides are extremely viscous and sticky.Hence Mixing is difficult

They exhibit pseudoplasticity ie,Material will seem easier to handle if sufficient speed & force is used for spatulation.

Unpleasant odor & color .Stains linen & messy to work

Excellent reproduction of surface detail

Dimensional stability: lesser than other elastomers

Exhibit more permanent deformation than other elastomers (3-5 %).Impression removed with a single swift pull to allow elastic recovery

Curing shrinkage is high (0.45% -24 Hrs) and continues even after setting

Impression should be poured within 30 min as they undergo 50% shrinkage in the first half hour itself. (Evaporation of water (by product ) during polymerization)

Higher tear strength up to 7000gm/cm

Good flexibility - 7% & low hardness.

2mm spacing in the tray sufficient for making an impression.

Hydrophobic - mouth dried well before impression Electroplated More with silver than copper Shelf life -2 yrs-refrigerated to prolong shelf life Biocompatibility: good. Problem arises when a piece

of material is left in the sulcus.

ADVANTAGES Lower cost

compared to silicones and polyethers Long working time High tear strength High flexibility Good detail reproduction Long shelf life Good biocompatibility Only impression material that can be electroplated

DISADVANTAGES Poor dimensional stability

water by-product pour within one hour

Messy paste-paste mix bad odor may stain clothing

Long setting time Requires custom tray Hydrophobic Second pour less accurate

CONDENSATION POLYMERIZING SILICONES Silicone Rubber impression materials were developed to

overcome the disadvantages of polysulfide materials 2 types of silicones are available as elastomeric

impression materials based on their chemical reaction A)condensation silicone B)Addition silicone

CONDENSATION SILICONE

Condensation Polymerizing silicone is also known as conventional silicone. This was the earlier of the 2 silicone impression materials.

Also called as RTV (room temperature vulcanization) silicones as the setting occurs in room temperature.

Examples Speedex (Coltene/Whaledent)

-Primasil (TISS Dental)

Mode of supply: Available in 2viscosities - Light Bodied. - Putty.

Supplied aspaste - pastepaste - liquid

• COMPOSTION:Base Paste: 1. Polydimethyl siloxane 2. Colloidal silica or micro sized metal oxide filler – 35 to

75% depending on viscosity. 3. color pigments.Accelerator Paste: Alkyl Silicate such as Ortho Ethyl Silicate – cross linking agent. Stannous Octoate – catalyst. Inert fillers - colloidal silica. Coloring agents

In one paste liquid system • Base paste is same• Catalyst liquid – contains tin octoate and coloring

pigments.

Tray Adhesive: Contains poly Dimethyl siloxane and ethyl

silicate Hydrated silica, the ethyl silicate to create a physical bond with the tray and poly Dimethyl siloxane bonds with the rubber.

SETTING REACTION

It is a condensation reaction. Polymerization occurs as a result of cross linkage

between the ortho ethyl silicate and the terminal hydroxyl group of the dimethyl siloxane, to form a three dimensional network.

Stannous octoate acts as the catalyst . The reaction is exothermic with 1 ˚c rise

The ethyl alcohol formed as a by-product evaporates gradually from the set rubber leading to shrinkage

Dimethyl siloxane+Orthoethylsilicate Silconerubber + Ethylalcohol

Stannous Octoate

HO – Si – O – Si - O - H

CH3

CH3 CH3

CH3 n

HO – Si – O – Si - O - H

CH3

CH3 CH3

CH3 n

C2H5O OC2H5

Si

C2H5O OC2H5

HO – Si – O – Si - O -

CH3

CH3 CH3

CH3 n

HO – Si – O – Si - O -

CH3

CH3 CH3

CH3 n

OC2H5

Si

OC2H5

+ 2C2H5OH

Condensation Silicone Reaction

metal organic ester

ethanol

PROPERTIES Setting time 8 – 9 mins . Mixing time 45 seconds Pleasant color & odor. Excellent reproduction of surface details.

Dimensional stability

Comparatively less - 0.4 to 0.6% of high curing shrinkage due to evaporation of the ethyl alcohol by-product. To avoid this cast must be poured immediately. Permanent Deformation is also high 1 – 3% .

Tear strength -3000gm/cm is lower than poly sulfides.

Stiffer & harder than polysulfide. Hardness increases with time.

Spacing in the tray increased to 3mm to compensate for the stiffness.

It is Hydrophobic – Impression field dried well before making an impression.

Shelf life– less than 2 years due to unstable nature of the Orthoethyl silicates.

Because of poor tear strength of condensation silicone it, may tear away and remain in the sulcus.

Putty wash technique is used for making impression with condensation silicones.

ADVANTAGES Better elastic properties on removal Clean, pleasant odor Stock tray

putty-wash Good working and setting time

DISADVANTAGES Poor dimensional stability

high shrinkage evaporation of ethanol

pour immediately within 30 minutes

Hydrophobic poor wettability

Shelf life not adequate Low tear strength Avoid latex gloves

ADDITION SILICONES Most popular and widely used Elastomers at present. It is

also known as poly vinyl siloxane. Better properties than condensation silicones.

Indications crown and bridge denture bite registration

Available in 5 viscosities.

Light bodied. Medium bodied. Monophase Heavy bodied. Putty.

Supplied as 2 Paste system – Base and catalyst paste

equal sized tubes. Different viscosities comes in different colors like

orange, blue, green etc. Putty silicones (jars) -2 equal sized plastic jars one with

base & other with catalyst. Gun and cartridge system.

Examples Extrude (Kerr) Express (3M/ESPE) Aquasil (Dentsply

Caulk) Genie (Sultan Chemists) Virtual (Ivoclar Vivadent

COMPOSITIONBase paste

Polymethyl hydrogen siloxane other siloxane Prepolymers Main ingredients Hybrid silicone - Colloidal silica – filler Surfactant – eg: soap to make hydrophilic

Catalyst Paste Divinyl polydimethyl siloxane Other siloxane prepolymers. Platinum salts – catalyst (chloroplatinic acid) Palladium or Hydrogen absorber. Fillers – colloidal silica.

SETTING REACTION This is an addition Reaction

Vinyl siloxane + SilaneSiloxane silicone

Rubber

Pt salt

CH3 CH3 CH3 CH3

Pt salt

Si-H + CH2=CH-Si Si-CH2-CH2-Si activator

CH3 CH3 CH3 CH3

Vinyl + Silane Siliconesiloxane siloxane rubber

SETTING REACTION:

• No by product, but imbalance hydrogen gas air bubbles in

the stone models

• To avoid this palladium is added.

There are no by products as long as there is proper balance between the vinyl siloxane and the silane siloxane. If not balanced H2 gas is evolved which causes air bubbles in stone models . Palladium is added to absort the hydrogen or pouring of cast delayed by 1hour.

Sulfur compounds also retard the setting of silicones .Residual sulfur in the latex gloves inhibits the action of platinum salt catalyst and thereby retards setting time. Vinyl gloves can be used as an alternative.

Sulfur containing retraction cord additives such as ferric sulfate and aluminum Sulfate may also produce this effect.

PROPERTIES Pleasant color & odor Excellent reproduction of surface details. Setting time is 5 to 9 min. Mixing time 45

seconds ,Working time 2 to 4min. Changes in Base to catalyst ratio can produce inconsistent

change in working and setting times and may increase hydrogen gas production.

Has the best dimensional stability among Elastomers It has low curing shrinkage (0.17%) and lowest permanent

deformation (0.05 to 0.3%).

Got good tear strength 3000gm/cm. Extremely Hydrophobic – so surfactant added to

reduce contact angle & improve castability of gypsum. Electroplated with silver or copper – Hydrophilic

silicones are difficult to electroplate because of the added surfactant.

Low flexibility and harder than polysulfides. shelf life 1 to 2 yrs

ADVANTAGES

Highly accurate High dimensional stability

pour up to one week Stock or custom trays Multiple casts Easy to mix /automix dispense Pleasant odor

DISADVANTAGES Expensive Sulfur inhibits set

latex gloves ferric and Al sulfate

retraction solution Putty too stiff & displaces wash Short working time Lower tear strength Possible hydrogen gas release

bubbles on die palladium added to absorb

POLYETHER IMPRESSION MATERIAL

Introduced in Germany in late 60’s. First elastomeric material exclusively for dental

impression It has good mechanical properties & dimensional

stability Indications

crown and bridge bite registration

Available viscosities – Light ,Medium,& Heavy

Examples Impregum F (3M/ESPE) Permadyne (3M/ESPE) Pentamix (3M/ESPE) P2 (Heraeus Kulzer) Polygel (Dentsply Caulk)

Supplied as 2 paste system –base & accelerator in collapsible tubes

A third paste containing a thinner paste is provided to reduce the viscosity of the mixed material

COMPOSITION OF POLYETHER Base paste

Polyether polymer (Imine terminated) – cross links to form the set material

Colloidal Silica - Filler Glycolether or Pthalate – Plasticizer gives bulk & controls

viscosity Accelerator paste

Aromatic sulfonate Ester- cross linking agent Colloidal silica – filler Glycolether or Pthalate – Plasticizer

SETTING REACTION Material sets by the reaction between aziridine rings at

the end of branched polyether molecule Copolymer of Ethylenedioxide & Tetrahydrofuran forms the

main chain Crosslinking brought about by the aromatic Sulfonate

ester via the Imine end groups Exothermic with 4 to 5 ˚c

The set material is hydrophilic. It can absorb water and swell resulting in dimensional change

Polyether +Sulfonic ester Cross linked rubber

CHEMISTRY & SETTING REACTION H O O H

CH3 – C - CH2 – C – O – R – O – C - CH2 – C – CH2

N N

CH2-CH2 CH2-CH2

Polyether + Sulfonic ester Crosslinked rubber

Exothermic reaction 4-50 C

PROPERTIES Working & setting time – Mixing should be done quickly -30

sec. Setting time 8.3 min . Heat decreases setting time Dimensional stability – very good .Curing shrinkage low

0.24% Permanent deformation is low – 1to 2 % Extremely stiff –Flexibility 3% Hardness , higher than polysulfide and increase with time.

removing from undercuts is difficult ,4mm spacing given

Tear strength good 3000 g/cm

Hydrophilic –best compatibility with stone

Electroplated with silver or copper

Sulfonic ester may cause skin reaction

Shelf life more than 2 yrs

TRAY ADHESIVE

Chemical Composition

Acetone -N-Hexane -Hexane Isomers -

Toluene -Adhesive+Resin -

15 – 40% 5 - 15%15 – 40% 5 – 15%

Remainder

ADVANTAGES

Highly accurate Good dimensional stability Stock or dual-arch trays Good surface detail Pour within one week -kept dry Multiple casts Good wettability

DISADVANTAGES

Expensive Short working time Rigid

difficult to remove from undercuts Bitter taste Low tear strength Leaches components Absorbs water

changes dimension

VISIBLE LIGHT – CURE IMPRESSION MATERIAL

These materials are based on Polyether Urethane dimethacrylate resin with visible light –cure photo initiator & photo accelerators.

Available viscosities – Light Bodied - Heavy Bodied

Composition: 1.Polyether urethane dimethacrylate 2.Diketone- photo initiator

3.Transparent silica – Filler(40-60%)

Polymerization by Light 480 nm wave length by using conventional Light curing units

MANIPULATION

Transparent stock trays are used Light bodied is syringed & heavy bodied placed above

it Light exposed from posterior to anterior region with 30

sec exposure After polymerization, the surface of the material was

tacky because of air inhibition

Advantages – Long working time - Low flow - High tear strength 6000to7500gm/ cm - Low dimensional change - Impression can be corrected

Disadvantages dificulty in curing a full arch tray - Post polymerization tackiness - Costly - Special equipment needed

COMPARATIVE PROPERTIES OF ELASTOMERS

Setting Properties:

• 1. There is slight rise in Temperature for all elastomers during setting –No clinical concern

• 2. Viscosity:- Viscosity increases for the same material from a low to high consistency

Rapid increase of viscosity with time occurs with silicones & polyether

• All materials show a decrease in viscosity with increasing shear rates- more in polyether, condensation silicones & polysulfide.

This effect is called Shear thinning useful in single viscosity materials- polyether & polysulfide

• 3. Working Time & Setting Time: Polysulfide have the longest time, followed by silicones

& polyethers

• 4. Dimensional change on setting: (after 24 hrs) Largest Dimensional change- 0.6%in Condensation silicone Smallest Dimensional change – 0.15% in addition silicones

From best to worst AS > PE > PS > CS

MATER IAL

Temp.Rise˚C

Working timemin

Setting timemin

Dimensional change24 hr %

PS 3.4 4-7 7-10 -0.40

CSAS

1.1 2.5-42-4

6-84-6.5

-0.60-0.15

PE 4.2 3 6 -0.23

SETTING PROPERTIES OF ELASTOMERS

MECHANICAL PROPERTIES Properties like permanent deformation ,flow and

dimensional changes according to ADA spn.no 19 classify elastomers as Type I, II, III

TYPE MAX.PERMANENT DEFORMATION

MAX.FLOW IN COMPRESSION

MAX.DIMENSIONAL CHANGE IN 24 Hrs

I 2.5 0.5 -0.5%

II 2.5 0.5 -1.00%

III 5.5 2.0 -0.5%

PERMANENT DEFORMATION

Permanent deformation increases in the order of AS>CS>PE>PS

• Polyethers are the stiffest,followed by Addition silicones,condensation and polysulfide in decreasing order

PE > AS > CS > PS

● FLOW• Highest flow exhibited by PS & comparitively, Silicones and

Polyethers have the lowest value.• Tear strength• Highest tear resistance by

PS > AS>PE > CS• Hardness:• PE are Hardest of all.Low flexibility and high hardness of PE

allows the impression material to be taken even below 2 cm

• Detail reproduction: According to ADA spn.0.020mm wide line should be accurately reproduced by an impression material & should be transformed into gypsum die material. Elastomers meet this criteria except for some putty materials

• Wettability: Wettability is directly is correlated to the ease of pouring the cast. Hydrophilic Addition Silicones & PE can be wetted best, CS &Hydrophobic AS the least

• Wettability assessed by measuring the advancing contact angle of the set impression material or by using tensiometer

POLYETHER POSSESSES HIGH DEGREE OF WETTABILITY BECAUSE THEIR MOLECULAR STRUCTURES CONTAINS POLAR OXYGEN ATOMS WHICH HAVE AFFINITY FOR WATER.

SURFACTANTS HAVE BEEN ADDED TO ADDITION SILICONE– DECREASES SURFACE TENSION AND IMPROVES THE WETTABILITY.

PLASMA TREATMENT AND ARGON GLOW

DISCHARGE ALSO IMPROVE THE WETTABILITY.

COMPARITIVE PROPERTIES OF ELASTOMERS

TYPE EASE OFREMOVAL

POURINGTIME

TEARSTRENGTH

MOISTURE TOLERANCE

PS Moderate 60 min Good Acceptable

CS Easy 60 min Adequate Poor

AS ModerateToDifficult

7 days Adequate Poor-hydrophobic&Adequate-hydrophilic

PE Difficult 7 days Adequate Good

MANIPULATION MIXING SYSTEMS 1. Hand mixing 2.Static Automixing 3. Dynamic Mechanical Mixing

1. HAND MIXING Equal lengths of Catalyst & Base paste are displaced

on a paper pad. Initial mixing accomplished with a circular motion Final mixing with broad strokes of the spatula to

produce a mix free from streaks Mixing time of 45 secs.

When catalyst is supplied as a liquid – a specified no. Of drops per unit length of base paste is used and mixing done in same manner

With 2 putty systems as in condensation & AS putties are most often kneaded with fingers until free from streaks

Liquid catalyst mixed with spatula initially before kneading with fingers. Latex gloves is not used.

2.STATIC AUTOMIXING

Base & catalyst are supplied in separate cylinders of plastic cartridges

The cartridge is placed in a mixing gun containing 2 plungers that are advanced by a ratchet mechanism to extrude equal amounts of base & catalyst

The base & catalyst are forced through the static mixing tip containing a stationary plastic internal spiral. The two components are folded over each other many times as they are pushed through the spiral resulting in uniform mix at the tip end .

The mixed material can be extruded directly into an injection syringe or onto the impression tray.

Auto mixing systems results in mixes with fewer voids than hand mixed.

Average loss of material at tip is only 1-2 ml Initially Auto mixing done for only low consistencies.

Now all consistencies except putty can be mixed. AS, CS & PE are available with the mixing

3. DYNAMIC MECHANICAL MIXING Catalyst & base are supplied in large plastic bags

housed in a cartridge which is inserted into the top of the machine. A new plastic mixing tip placed on the front of the mixing

When the button is pressed the parallel plungers push against the collapsible plastic bag forcing the matl. into dynamic mixing tip

In the mixing tip, the internal spiral is motor driven Thorough mixing can be ensured by this.

Advantage is ease of use ,speed & thoroughness in mixing

Disadvantage is high cost

PE & AS are mixed using this system

IMPRESSION TECHNIQUES 3 Methods• 1. Dual viscosity technique• 2.Single viscosity or monophase technique• 3. Putty wash technique

1. DUAL VISCOSITY TECHNIQUE Low consistency material is injected into critical areas with

a syringe & high consistency material is mixed and placed in an impression tray

After injecting the low viscosity matl. The tray containing higher viscosity matl.is placed in the mouth which forces the lower viscosity material, to flow into fine aspects .As both are mixed at nearly the same time ,the materials join, bond & set together

Dual viscosity technique

SINGLE VISCOSITY OR MONOPHASE TECHNIQUE

Medium bodied AS &PE are mixed Pseudoplastic matls.demonstrate a decreased viscosity

when subjected to high shear stress as occurring with syringing.

In this technique medium bodied matl. is used in both the syringe & in the custom tray.

When the medium viscosity matl.is forced through an impression syringe ,the viscosity is reduced , whereas the viscosity of the same matl residing in the tray is unaffected

In this manner the same matl. is used for both syringe & trays and the dual viscosity technique is applied as stated previously

Fill the tray with Monophase Material.

Syringe monophase impression materialaround the clean, dry tooth preparation(s).

Slowly seat the tray in the mouth. The tray must be seated within 1 minute of the start of oral syringing and within 2minutes of the start of tray filling.

Immobilize the tray using passive pressure. Oral Setting Time: 4:00minutes.

Remove the impression, rinse, dry and disinfect.

3. PUTTY WASH TECHNIQUE

Developed mainly for CS to minimize the effects of dimensional change during polymerization

2 step impression procedure Preliminary impression taken with high or putty

consistency before tooth preparation with application of a separator to provide space for low consistency matl.

After tooth preparation ,low consistency matl. Is syringed into the area and the preliminary impression reinserted into the mouth

The low & high viscosity materials bonds and after low consistency matl. sets , the impression is removed.

This procedure is also called as wash Technique This technique is also now extended to Addition Silicones

DISINFECTION OF NONAQUEOUS ELASTOMERS ADA guidelines recommend disinfecting by spray or

immersion technique

Polysulfide &silicones –Gluteraldehydes, chlorine compounds, Iodophors ,phenolics(10 min Immersion)

Polyether –chlorine compounds or Iodophors

( sodium hypochlorite 5.25%) and sealed in plastic bag for ten mins

OPTICAL DENSITY

Polyether – low optical density Polysulphide- high optical density

Composition changes are recommended to allow radiographic detection and improve safety during clinical use of impression material

WASTE DISPOSAL METHOD OF IMPRESSION MATERIAL Sanitary landfill Incineration – costly Handover it to an agency who collects the

biomedical waste

COMMON FAILURE OCCURRING IN ELASTOMERS

FAILURES CAUSES

1 Rough or uneven surface

Incomplete polymerization by premature removal from mouthToo rapid polymerization from high humidity or temp.High accelerator /base ratio with CS

2 Bubbles Too rapid polymerization preventing flowAir incorporated during mixing

3 Irregularly shaped voids

Moisture or debris on surface of teeth

FAILURES CAUSES

4 Rough or chalky stone cast

1. In adequate cleaning of impression2. Excess water left on impression 3. Premature removal of cast4. Improper manipulation of stone5. Failure to delay pour of AS atleast

20 min

5 Distortion1. Lack of adhesive or wrong

adhesive2. Excessive bulk of matl3. Premature removal from the

mouth4. Movement of tray during

polymerization5. Insufficient relief for the reline

matl6. Continued pressure against

impression matl that has developed elastic properties

CONCLUSION Thanks to advances in polymer technology various

Elastomeric impression materials have been developed with marked accuracy and elastic properties.

The choice is based on personal preference ease of manipulation and to some extent economics. But ,the final success of the impression material depends on the skill of the operator during its manipulation.

REFERENCES Phillips science of dental materials-Anusavice

Restorative Dental Materials- Robert G. Craig

Essentials of Dental materials- S.H. Soratur

Clinical applications of dental materials- John F.Mcabe

Textbook of complete dentures- heartwell

Prosthodontic treatment for edentulous patients-Zarb-Bolender

Dental material science – B.K.Basu

Introduction to dental materials – Micheal bagby

Clinical aspects of dental materials - vanmoort

A review of properties and techniques. JPD 1992;68:728-732.

Cook WD, Thomas F. Rubber gloves and addition silicone impression materials. Aust Dent J 1986;3:140-144. Kahn RL, Donovan TE, Chee WW. Interaction of gloves and rubber dam with a poly (vinyl siloxane) impression

material: ascreening test. Int J Prosthodont 1989;2:342-346.

Pratten DH, Craig RG. Wettability of a hydrophilic addition silicone impression material [Abstract]. J Dent Res 1987;66:331.

Marcinak and Draughn. Linear dimensional changes in addition curing silicone impression materials. J Prosthet

Dent1982;47:411-413. ADA Reports. Council on Dental Materials, Instruments, Equipment: Disinfection of impressions. J Am Dent Assoc

1991;122:110.

Thank you