Introduction:

Accuracy and dimensional stability of impression

materials have been the traditional goals of researchers and

clinicians. Due to a host of contingencies, many dentists do

not pour their own impressions immediately. Thus

impressions must be stable enough to produce accurate casts

over extended periods of time. This need for a more stable,

accurate and elastic impression material sponsored the

introduction of elastomers in dentistry. When liquid

polymers are mixed with a suitable catalyst, they are

converted to elastomers.

USES:

1) For crown and bridge work.

2) For partial denture prosthetic procedures.

3) Where there are severe undercuts.

4) In patients exhibiting xerostomia.

5) In patients with lesions of the mucosa, such as lichen

planus or pemphigus.

6) For master impression in rigid individual trays.

1

Composition

I] Polysulfide:

These were the first synthetic rubbers to be used as

impression materials:

Typical base paste Catalyst paste

Liquid polysulfide – 55% Lead dioxide – 10%

Filler – 44% Oleic and stearic acids – 2%

Plasticiser / sulfur – 5% Filler 50%

Perfume – 1%

Inert oil – 37%

Or

Base Weight (%)

Polysulfide polymer 80-85

Titanium dioxide, 16-18

Zinc-sulfate, si lica

Or copper carbonate

Accelerator

Lead dioxide 60-68

Dibutyl phthalate 30-35

Sulfur

Other substances such as

2

magnesium stearate and deodorants

The polysulfide polymer has a molecular weight of

2000 to 4000 with terminal and pendant mercaptan groups

(-SH). The polysulfide compounded with a suitable filler.

Fillers like lithopone, titanium oxide or zinc sulphide

are added to provide required strength. Plasticizer such as

dibutyl or dioctyl pthalate confer the appropriate viscosity to

the paste. A small quantity of sulfur is also added. The

particle size of the fillers is about 0.3 microns. In general,

the weight percent of the filler in the base paste increases fro

low to medium to high consistencies. The base paste is

normally white, due to the filler and has an unpleasant odour

caused by the high concentration of thiol groups. Some

magnesium oxide may also be present. Whitening agents

cannot cover the dark color of the lead dionide and thus base

pastes are dark brown to gray-brown in colour. The same

plasticizer as is used in the base paste constitutes the liquid

vehicle, as well as a quantity of the same filler. Oleic or

stearic acids are retarders added to control the rate of set.

Lead dioxide is the active catalyst.

3

Modifications:

1) One materials avoids the use of lead dioxide and

replaces it by an organic reactor, such as cumene

hydroperoxide or t-butyl hydroperoxide or hydrated

copper oxide, (CuCoH)2. however, this constituent is

volatile and its loss by evaporation leads to shrinkage

of the set mass. Hydrated copper oxide produces a

green mix while the others can be any color desired by

the manufacturer.

2) A recently developed polysulfide replaces the lead

dioxide by a zinc carbonate / organic accelerator

system. It is claimed that this is much cleaner to

handle than a conventional polysulfide.

II] Condensation Silicone

Paste Liquid

Liquid si l icone Alkyl sil icate such as tetraethyl si l icate.

prepolymer

Interfi l ler Tin compound such as dibutyl t in di laurane

4

The base contains a moderately low molecular weight

silicone called a dimethyl siloxane which has reactive

terminal hydroxyl groups.

Liquid silicone prepolymer undergoes cross-linking to

form rubber. Since the silicone polymer is a liquid, fillers

are added to form a paste. The selection and pretreatment of

the filler are of extreme importance, since silicones possess

a low cohesive energy density and therefore weaker

intermolecular interaction. The influence of the filler on the

strength of silicone elastomer is much more critical than

when it is added to polysulfides. Fillers give a proper

consistency to the paste and stiffness to the set rubber. The

consistency of the silicone paste is controlled by the

selection of the molecular weight of the dimethyl siloxane

and the concentration of the reinforcing agent. Higher

molecular weights are used with the heavier bodied

materials. The concentration of the filler increases from 35%

for light bodied consistency to 75% for the putty

consistency. Colloidal silica or microsized metal oxide, with

an optimum particle size of 5 and 10mm; are added as fillers.

According to Craig the fillers may be copper carbonate or

5

silica having particle sizes from 2 to 8mm. The smaller

particled tend to aggregate, but larger ones do not contribute

to reinforcement. The particles are often surface-treated to

provide better compatibility with, and reinforcement of the

silicone rubber. Colorants like organic dyes and pigments are

commonly used as an aid in obtaining a homogenous mix.

The accelerator may be a liquid that consists of stannous

octolate suspension and alkyl silicate ortho or tetra ethyl

silicate or it may be supplied as a paste by the addition of a

thickening agent.

Tin compound act as reaction catalyst. The accelerator

does not have unlimited shelf life because the stannous

octoate may oxidize and the ortho ethylsilicate is not entirely

stable in the presence of the tin ester.

III] Addition Silicone

One paste contains a poly dimethyl siloxane

prepolymer. In which some of the methyl groups are replaced

by hydrogen. The other paste also contains a prepolymer

with a platinum salt like chlorplatinic acid activator. The

polymer has vinyl groups replacing some of the methyl

groups. Vinyl silicones are expensive because of the high

6

cost of platinum. Fillers give a proper consistency to the

paste and stiffness to the set rubber. Both pastes contain

fillers. Surfactants have been added to addition silicones by

some manufacturers, which reduces the contact angle,

improves the ability and simplifies the pouring of gypsum

models. These materials are said to be hydrophilic. The

addition of surfactant makes the preparation of

electroformed dies more difficult because the metalizing

powder does not adhere as well to the surface of hydrophilic

addition silicone impression.

IV] Polyether

The base paste contains a moderately low molecular

weight polyether, containing ethylene imine terminal groups,

silica filler, and a plasticizer such as glycoether pthalate.

The accelerator paste contains 2, 5 dichloro benzene

sulfonate as a cross-linking agent, along with a filler and

plasticizer. Coloring agents may be added to base and

accelerator as desired. A separate tube contains a thinner

that includes octyl pthalate and about 5% methyl cellulose as

a thickening agent.

7

Light-cured polyether urethane dimethacrylate has

visible light-cure photoinitiators, photo accelerators and

silicone dioxide filler which has a refractive index close to

that of the resin in order to provide the translucency

necessary for maximum depth of cure.

Chemistry

1) Polysulfide

The terminal and pendant mercapton groups (-SH) of

adjacent molecules are oxidized by the accelerator to

produce chain extension and cross linking respectively.

Because the pendant groups compose only a small eprcent of

the available –SH groups, chain lengthening will

predominate at first. This will principally increase viscosity.

It is the subsequent cross-linking reaction that links all the

chains together in a three dimensional network that confers

elastic properties to the material. The reaction is of the

condensation polymeriation type since one molecule of water

is produced as a byproduct of each reaction stage. As chain

extension proceeds, the viscosity increases. When the degree

of cross-linking reaches a certain level, the material

develops elastic properties. The reaction results in a rapid

8

increase in molecular weight and the mixed paste is

converted to a rubber. The molecular weight of the

mercaptan is 2000 to 4000; thus each reaction with two –SH

groups increases the molecular weight by about this amount.

The reaction is only slightly exothermic, with a typical

increase in temperature of 3°C to 4°C. The amount of heat

generated depends on the amount of total material and the

concentration of initiators.

Although the mixes set to a rubber in about 10-20

minutes, polymerization continues and properties change for

a number of hours after the material sets.

Alternatives to led dioxide, like organic hydroperoxide

have poor dimensional stability while inorganic hydroxides

have obscure chemical mechanisms.

The chemical reaction is much more effective if a

small amount of sulfur is present. Moisture and temperature

exert a significant effect on the course of the reaction.

2) Condensation silicone

Terminal hydroxyl groups of prepolymer chains react

with the cross linking agent under the influence of the

9

catalyst. The polymer consists of a hydroxyl terminated poly

(dimethyl siloxane). Cross linking occurs through a reaction

with tri-and tetrafunctional alkyl silicates, commonly

tetraethyl orthosilicate in the presence of stannous octoate

[Sn (C7 H15 Coo)2]. Each molecule of cross-linking agent

may potentially, react with upto 4 prepolymer chains causing

extensive cross linking. Cross linking produces an increase

in viscosity and the rapid development of elastic properties.

These retractions are affected at ambient temperatures

and the materials are therefore called RTV (room

temperature vulcanization) silicones in technical literature.

Ethyl alcohol is a by-product of the setting reaction is

exothermic with a temperature rise of 1°C.

3) Additional silicone

In this case the polymer is terminated with vinyl

groups and is cross linked with hybride groups activated by a

platinum salt catalyst, by an addition reaction. There are no

reaction by products as long as there is a good balance of

vinyl silicone and hybrid silicone. If proper balance is not

maintained, hydrogen gas is produced. Noblem salts like

platinum or palladium is not maintained, hydrogen gas

10

scavenger for the hydrogen. Hydrogen gas could also be

forced if moisture on residual sianol groups are present to

react with the hybrids of the base polymer. As the reaction

proceeds, the viscosity increases and eventually a relatively

rigid cross linked rubber is produced.

4) Polyether

Polyether base polymer is cured by the reaction

between aziridine rings, which are at the end of branched

polyether molecules. The main chain is probably a

copolymer of ethylene oxide and tetrahydrofuran. Cross

linking and thus setting is brought about by an aromatic

sulfonate ester. This produces cross linking by cationic

polymerization via the imine end groups. The setting

reaction is slightly more exothermic than that of other

elastomers, with a temperature rise of about 4°C.

Properties includes:

1. Rheological properties / viscosity.

2. Working and setting time.

3. Dimensional stability.

11

4. Permanent deformation / elasticity.

5. Strain.

6. Flow.

7. Hardness.

8. Tear strength.

9. Detain reproduction.

10. Creep.

11. Wettability.

12. Shelf life.

13. Biological properties.

1) Rheological Properties / Viscosity:

These play an important role in the successful

application of elastomers. Viscosity is a function of time

after the start of mixing. The most rapid increase in viscosity

with time occurred with the silicones and polyethers, with

the latter increasing slightly more rapidly than the former.

Attention must be paide to proper mixing times and times of

12

insertion of the impression material into the mouth if the

materials are to be used to their best advantage. Silicones are

more fluid and hence easier to mix than polysulfides. But

because of shorter setting times for the silicones, the flow is

present for a shorter period of time. The viscosity of

polyether mixes can be reduced by using a thinner.

All elastomers show a decrease in viscosity with increasing

shear rate. The effect was more pronounced with polyether,

condensation silicone and polysulfide with a Cu(OH)2

accelerator than with polysulfide with PbO2 accelerator. The

effect is sometimes called shear thinning and is important

with single viscosity materials such as polyether and

polysulfide with Cu(OH)2 accelerator. These materials have

lower viscosities during injection with a syringe than when

inserted in a tray during mixing. It has been estimated that

the shear rate is about 10 seconds for mixing and 1000

seconds for syringing. A single mix can be used in a syringe-

tray technic as a result of the shear thinning effect.

2) Working And Setting Time

In general, polysulfides have the logest times, followed

by silicones and polyethers. A reciprocating rheometer is a

13

useful instrument to estimate practical working and setting

times. The working and setting times of elastomers are

shortened by increases in temperature and humidity. The

setting time does not correspond to the curing time. In

condensation silicone material the polymerization may

continue for 2 or more weeks after mixing. Working time is

measured at room temperature and setting time at mouth

temperature. Working time may be prolonged by a low room

temperature or by mixing on a chilled, dry glass slab.

Alteration of the base-accelerator ratio is an effective

method of changing the curing rate of condensation

silicones. In contrast, the curing rate of addition silicones

appears to be even more sensitive to temperature changes

than are polysulfides. The curing rate of polyethers is less

sensitive to temperature change than is that of addition

silicones. It has the shortest working time among the

elastomers.

Condensation silicones have the largest dimensional

change (-0.6%). The shrinkage is a result of the evaporation

of volatile by products and the rearrangement of the bonds

resulting from polymerization. The addition silicones have

14

the smallest change (-0.05% to 0.15%) followed by

polyethers (-0.2%) and the PbO2 and Cu(OH)2 accelerated

polysulfides (-0.04%).

The shrinkage rate of elastomers is not uniform during

the 24 hours after removal from the mouth. In general, about

half of the shrinkage observed at 24 hours occurs during the

first hour after removal and for greatest accuracy casts

should be poured immediately.

Some addition silicones release hydrogen after setting

and to avoid bubbles, casts should be poured after 1-2 hours.

Polyether impressions should not be stored in water, since

they will slowly absorb water and change dimensions.

3) Permanent deformation

Addition silicones have the best recovery from

deformation during removal from the mouth, followed by

condensation silicones and the polyether and polysulfides.

Lower values of silicones are related to the higher cross-

linking in silicones, although the filler content obviously has

an effect, as seen by the value of 22% for the putty class

compared with less than 1% for the other classes. In practice,

15

little permanent deformation takes place in the putty since it

is so stiff that little deformation occurs during removal of

the putty wash impression.

Since polysulfide is not perfectly elastic, compression

during removal of the impression material should be kept to

a minimum.

4) Strain

The strain in compression under a stress of 100gm/cm2

is a measure of the flexibility of the material. In general, the

light consistency materials of each type are more flexible

than heavy consistency elastomers. The polyethers

containing thinner are more flexible than the regular

material. Also the silicones are stiffer than the polysulfides

of comparable consistency and the addition silicones are

slightly stiffer than the condensation silicones.

5) Flow

This property is of particular importance because it

relates to the amount of deformation a polymerized

impression material undergoes after being poured up with a

gypsum product. The flow is measured on a cylindrical

16

specimen 1 hour after spatulation and the percent

deformation is determined 15 minutes after a load of 100gm

is applied.

The silicones and polyethers have the lowest values of

flow and the polysulfides have the highest values. Low flow

of polyethers is caused by the rubber being crosslinked and

its high stiffness.

6) Hardness

The shore A hardness increases from low to high

consistency. Where two numbers are given, the first

represents the hardness 1.5 minutes after removal from the

mouth, and the second number is the hardness after 2 hours.

The polysulfides and the low, medium and high viscosity

addition silicones do not change hardness significantly with

time where as the hardness of condensation silicones, the

addition silicone putties and the polyethers does increase

with time. The hardness and the strain as well affect the

force necessary for removal of the impression from the

mouth. Low flexibility and high hardness can be

compensated for clinically when more space for the

impression material between the tray and the teeth is

17

provided. The high stiffness of polyether is indicated by the

low flexibility of 3% compared with 5% and 7% for

condensation silicone and polysulfide regular bodies types.

The low flexibility may cause problems in the removal of the

impression from the mouth and a 4mm rather than 2mm

thickness of rubber between the tray and teeth is

recommended.

7) Tear Strength

The tear strength is important because it indicates the

ability of material to withstand tearing in thin interproximal

areas. Tear strength is a measure of the force needed to

initiate and continue tearing specimen of unit thickness. A

few polysulfides have high tear strengths of 7000gm/cm but

the majority have lower values in the 2500-3000gm/cm

range. There is a small increase in tear strength as the

consistency of the impression type increases, but most of the

values are between 2000 and 4000gm/cm.

It would be desirable to have higher tear strengths for

elastomers. One of the problems associated with polyethers

is their lower tear strength but higher stiffness. As a result,

long tags of impression materials may tear during removal of

18

the impression more easily than occurs with the other 2

types. The resistance of polysulfides to tearing is about 8

times the values reported for hydrocolloid materials. It

should be emphasized that the strength and permanent

deformation properties of the polysulfides continues to

improve for a number of hours after they are set. Several

minutes extra in the mouth result in noticeable improvement;

however the time in the mouth has a practical limitation.

8) Detail Reproduction

In general silicones and polyethers are capable of

registering or reproducing detail better than the polysulfides.

Whereas the resolution capability of the latter is

approximately 8 to 10mm, the resolution of the other types

may be a great as 1 to 2mm.

Except for the very high viscosity products they all

should reproduce a v-shaped groove, a 0.020mm wide line in

the rubber and the rubber should be compatible with gypsum

products so that the 0.020mm line is transferred to gypsum

die materials. Low medium and high viscosity elastomers

have little difficulty in meeting this requirement.

19

9) Creep Compliance

Elastomers are viscoelastic and their mechanical

properties are time dependent. For example, the higher the

rate of deformation, the higher the tear strength, and the

longer the impressions are deformed, the higher the

permanent deformation. As a result the plots of the creep

compliances time describe the properties of these materials

better than the stress-strain curves.

Polysulfide is the most flexible and the polyether the

least. The flatness or parallelism of the curves with respect

to the time axis indicates low permanent deformation and

excellent recovery from deformation during the removal of

an impression material; polysulfides have the poorest

recovery from deformation followed by the condensation

silicone and then the addition silicone and polyether.

The recoverable viscoelastic quality of the materials is

indicated by difference between the initial creep compliance

and the creep compliance value obtained by extrapolation of

the linear portion of the curve to zero time.

1) Wettability:

20

Wettability may be assessed by measuring the advancing contact angle

of water on the surface of the set impression material. The hydrophilic

addition silicones and the polyethers were wetted the best, and the

condensation silicones and hydrophobic addition silicones the least. The

wettability was directly correlated to the case of pouring high strength stone

models.

Material Advancing contact algne of water (°)

Castability of high-strength

dental stone (%)

Polysulfide 82 44

Condensation silicone 98 30

Addition silicone

i) Hydrophobic 98 30

ii) Hydrophilic 53 72

Polyether 49 70

10) Shelf Life

A properly compounded polysulfide or polyether

impression material does not deteriorate appreciably in the

tubes when it is stored under normal environmental

conditions [10° to 27°C (65° to 80°F)] for 2 years. The shelf

21

life for silicones is reasonable but is usually shorter than for

polysulfides; thus large quantities should not be purchased

or stored. Although the situation is greatly improved over

what it was some years ago, occasionally the silicone gum

may stiffen in the tube if stored for too long a time.

Continuous exposure of either the silicone paste or the

reactor to the air hastens deterioration. For this reason, the

containers should be kept tightly closed when they are not in

use. Also storage in a cool environment is advisable. ADA

specification No. 19 requires that after storage of the base

and accelerator for 7 days at 60±2°C (140±3.6°F), the

material still meet the test for permanent deformation.

11) Biological Properties:

a) Polysulfide:

The use of lead compounds in polysulfide material has

been questioned because of the known toxic effects of lead.

It is unlikely that the lead contained in these products is able

to exert a harmful effect as the material in the patient’s

mouth for only a few minutes and is hydrophobic, reducing

the chances of washing out of lead compounds by saliva.

22

b) Condensation silicone:

The materials are non-toxic, although direct contact of

skin with the accelerator is to be avoided since allergic

reactions have been noted.

c) Addition silicone

The culture tests on both the base and catalyst pastes

have been negative and indicate that addition silicones

caused less tissue reaction than the condensation silicones.

d) Polyether

The aromatic sulfonic acid ester can cause skin

irritation and direct contact with the catalyst should be

avoided. Thorough mixing of the catalyst with the base

should be accomplished to prevent any irritation of the oral

tissues.

Evaluation program:

American Dental Association Specification No. 19

applied to the properties of elastomers.

23

Advantages:

1. Excellent surface detail.

2. Dimensional accuracy.

3. No separator required before pouring casts.

4. Record undercuts but polysulfides may suffer from

permanent deformation on removal.

5. Polysulfides have good tear resistance.

6. Additon silicones have excellent dimensional stability,

even in cold sterilizing solutions.

7. Wide range of different viscosities available to match

different clinical situations.

8. Low viscosity silicones suitable for wash techniques.

9. Putty silicones are useful as space-filling materials.

10. Pleasant appearance and feel in the mouth.

11. Can be electroformed to give metal die, an advantage

over stone dies because of greater abrasion

resistance.

24

12. More easily prepared for use.

13. More dimensionally stable over a period of time than

hydrocolloids.

14. Do not affect hardness of the surface of stone.

Disadvantages:

1. They are hydrophobic and so tend to slip on wet,

mucus-covered mucosa.

2. Prolonged setting time, especially polysulfides.

3. Tear resistance of silicones is low.

4. Condensation silicones are dimensionally unstable.

5. Silicone putty can easily distort peripheral tissues.

6. most extensive of all impression materials.

7. After set, the boders cannot be adjusted.

8. Polysulfides have strong odour of rubber and untidy to

handle.

9. Tray must be held rigidly for accuracy for 8-12

minutes for setting.

25

10. The ratio of the material is also critical; if the ratio is

not accurate, the mechanical properties may be

changed.

11. The impression material must be poured within 1 hour

after removal from the mouth.

12. Complete adhesion to a prefabricated tray is

essential.

13. Polysulfides tend to run down patient’s throat

because of lower viscosity.

14. Polysulfides need custom made rather than stock tray

due to greater chance of distortion.

Clinical presentation:

a) Polysulfides are supplied in 3 consistencies: low

(syringe /wash), medium (regular) and high

(tray).

b) Addition silicones are available in these three

consistencies plus a putty (very high) type.

Addition silicones are also supplied as a single

consistency product with sufficient shear

26

thinning so that it can be used as both a low and

a high consistency material.

c) Condensation silicones are usually supplied in a

low and putty like consistency.

d) Polyethers are supplied as a medium consistency

type plus a thinner or as a low and a high

consistency.

The low, medium and high consistencies are supplied

as two pastes labeled bases and accelerator (catalyst) in

collapsible tubes. A few manufacturers of silicones supply

the catalyst as a liquid. They very high consistency is

supplied as a base putty and a catalyst putty or liquid.

Manipulation

1) Spatulation:

Elastomers are mixed as described for the impression

pastes (ZOE). The proper length of the two pastes are

squeeze onto a mixing pad. Since the composition of the tube

of the rubber base material is balanced with that of the

accelerator, the same matched tubes originally supplied by

27

the manufacturer should always be used for certain products

some flexibility in working and setting times can be obtained

by changing proportions.

The catalyst paste is first collected on a stainless steel

spatula and then distributed over the base and the mixture is

spread out over the mixing pad. The natural contrasting

colours of the 2 pastes enables the progress of mixing to be

monitored. Mixing is continued until the mixed paste is of

uniform color. If the mixture is not homogenous curing will

not be uniform and a distorted impression will result.

An automatic dispensing and mixing device for

addition silicone is generally used for light and medium

viscosity materials and has certain advantages in comparison

with hand dispensing and spatulation. There is greater

uniformity in proportioning and in mixing and fewer

bubbles in the mix. In addition, mixing time is reduced. The

possibilities for contamination of the material are much less.

The mixed impression material is ejected directly onto the

adhesive-coated tray and onto the prepared teeth if the

syringe tip is in place.

28

In case of condensation silicone, the reactor may be

supplied in the form of a colored oily liquid. When the base

paste is dispensed from the tube, a certain length is extrude

onto the mixing pad and the liquid is placed beside the rope

of paste with a stated number of drops per unit length of

paste. If the mixing pad absorbs the oily liquid accelerator, a

less permeable pad or a glass slab should be used. The

absorption of the accelerator by the pad can also be reduced

by placing the drops of liquid on the spatula rather than the

pad.

The two-putty systems use scoops supplied by the

manufacturer for dispensing and may be mixed with a heavy

spatula or kneaded in the hands until free from streaks. The

putty materials that have a liquid catalyst are initially mixed

with spatula until the catalyst is reasonably incorporated and

completion of mixing is accomplished by hand (using vinyl

gloves).

2) Preparation of the tray:

The bulk of the impression material should be less;

optimal thickness is 2 to 4mm and the bulk should be evenly

distributed. Although stock impression trays are available

29

that can be contoured closely to the oral tissues, a better

method is to construct a tray with a plastic material.

Adhesion to the tray:

Complete adhesion to the tray is imperative when the

impression is removed from the mouth. Otherwise, a

distorted impression will result. Adhesion can be obtained by

the use of perforated trays or by the application of adhesive

to the plastic tray previous to the insertion of the impression

material.

The adhesives furnished with the various types of

rubber impression materials are not interchangeable.

Adhesives employed with polysulfides include butyl rubber

or styrene / acrylonitrile dissolved in a suitable volatile

solvent such as chloroform a ketone. The base for adhesive

employed with the silicone rubber materials may contain

poly(dimethyl siloxane) or a similar reactive silicone and

ethyl silicate. A slightly roughened surface on the tray will

increase the adhesion.

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3) Impression Techniques:

a) Multiple mix technique:

The method of using both the syringe and tray types of

elastomers is often referred to as the multiple mix technique

because two separate mixtures are required. When the tray

material is mixed first, the tray is filled with a uniform

thickness of material and set a side, or the manufacturer may

have adjusted the setting time of the two materials so that

the syringe material should be mixed first or at the same

time as the tray material. The materials is injected from the

filled syringe into the prepared cavities. The filler tray is

then carried to place.

The procedure should be timed so that neither the tray

no the syringe material cures to a point at which they will

not cohere when they are brought together. The bulk of the

impression is recorded in heavy-bodies material assuring

optimum accuracy and dimensional stability. The thin layer

of the impression adjacent to the oral tissues is recorded in

light-bodied material assuring optimum fine-detail

reproduction.

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b) Reline technique:

The rapid curing putty materials placed in a stock tray

and a preliminary impression is taken. This results in what is

essentially an intraoral custom-made tray formed by the

silicone rubber. Relief for the final or “wash” impression is

provided either by cutting away some of the “tray” silicone

or by using a thin resin, rubber or wax sheet as a space

between the silicone and the prepared teeth. This area is then

filled with a thinner-consistency silicone and the tray is

reseated into the mouth. The tray should be held under

pressure only during seating of the tray and not while the

wash material is curing. If not, it can lead to a grossly in

accurate impression if a critical portion of the primary

impression is held under pressure while the wash material is

setting.

c) Singe impressions:

The tray employed is usually a copper matrix band,

approximately 30 gauge in thickness. The band should be

fitted to the tooth and the reinforced with compound or self-

curing resin. Otherwise the impression will be squeezed with

the fingers when it is removed from the mouth and a

32

distortion will occur. The adhesive is applied to the band and

band filled with the previously mixed elastomer. Either a

syringe or a tray-type material can be used, but usually only

one type is employed.

Removal of the impression:

Under no circumstances should be the impression be

removed until the curing has progressed sufficiently to

provide adequate elasticity so that distortion will not occur.

The curing times may vary for the two different

consistencies, hence both the tray and syringe material

should be tested for curing. With a satisfactory elastomer,

the impression should be ready to be removed within atleast

10 minutes from the time of mixing allowing 6 to 8 minutes

for the impression to remain in the mouth. The rubber

impression should be removed suddenly.

4) Disinfection of the impression:

The elastomers can generally be disinfected by various

antimicrobial solutions without adverse dimensional

changes, provided that the disinfection time is short.

Prolonged immersion may produce measurable distortion and

33

certain agents may reduce the surface hardness of poured

gypsum casts. In particular, polyethers are susceptible to

dimensional change if the immersion time is longer than 10

minutes, because of their pronounced hydrophilic nature.

2% glutaraldehyde is a satisfactory solution for most

elastomers. The impression material itself may contain

disinfectant.

Types of Failure

Type Cause

1) Rough / uneven a) Incomplete polymerization caused by:

i. Premature removal from the mouth.

ii. Improper ratio or mixing of components.

iii. Oil or other organic material on the teeth.

b) Toorapid polymerization from high humidity or temperature.

c) Excessively high accelerator base ratio with condensation silicone.

34

2) Bubbles a) Too rapid polymerization, preventing flow.

b) Air incorporated during mixing.

3) Irregularly shaped a) Moisture, debris on surface of teeth voids.

b) Inadequate cleaning of impression.

4) Roughly or chalky store cast

a) Inadequate cleaning of impression.

b) Excess water left on the surface of impression.

c) Excess wetting agent left on impression.

d) Premature removal of cast.

e) Improper manipulation of stone.

f) Not delaying for 20 minutes while pouring addition silicone.

5) Distortion a) Resin tray not aged sufficiently and still undergoing polymerization shrinkage.

b) Lack of adhesion of rubber to tray caused by:

i. Not enough coats of adhesive.

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ii. Filling tray with material too soon after applying adhesive.

iii. Using wrong adhesive.

d) Lack of mechanical retention for those material where adhesive is ineffective.

e) Development of elastic properties in the material before tray is seated.

f) Excessive bulk of material.

g) Insufficient relief for the reline material if such technique is used.

h) Continued pressure against impression material that has developed elastic properties.

i) Movement of the tray during gelation.

j) Premature removal from mouth.

k) Improper removal from mouth.

l) Delayed pouring of polysulfide impression.

6) Faulty electroplating

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Recent advances in elastomers

1) A visible light-cure impression material was

marked in 1988. As supplied, this material contained a

polyurethane dimethacrylate resin with SiO2 filler and

constituents to enable the resin to polymerized in the

presence of light of around 480nm.

This material is available in 2 visocities: the light body

material is packaged in disposable syringes and the heavy-

body material is packaged in tubes.

Properties: This material has excellent elasticity and very

low dimensional shrinkage upon storage. It may be poured

immediately or upto 2 weeks later. The material is rigid and

it is recommended that severe undercuts should be blocked

out to ease removal of the impression. This material has the

highest resistance to tearing – 6,000 to 7,500 g/cm.

Manipulation : No mixing or syringe loading is necessary.

The light body material is syringed into the sulcus around

and over the preparations and portions of the adjacent teeth.

A clear tray is loaded to the fill l ine with the heavy body

material. After the tray is seated in the mouth, both

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viscosities are cured simultaneously using a visible light

curing unit having an 8mm or larger diameter probe. The

curing time is approximately 3 minutes. The periphery of the

impression which is tacky from air-inhibition, will not cause

clinical problems.

Advantages:

i. The dentist has complete control over working

time.

ii. Curing time is relatively short (3 minutes).

iii. The material has excellent physical,

mechanical and clinical properties.

Disadvantages:

i. The need for special trays that are transparent

to the visible light required to cure the

material.

ii. If a delay occurs before placement, the

material should be stored in a dark place away

from light.

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iii. Difficulty may be encountered when using the

light source to cure remote areas.

iv. The material should not be used with patients

with a known allergy or sensitivity to

urethanes, acrylics or methacrylates.

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