Elastomers material

Introduction:

Accuracy and dimensional stabil i ty of impression materials have been the

tradit ional 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 t ime.

This need for a more stable, accurate and elastic impression material sponsored the

introduction of elastomers in dentistry. When l iquid polymers are mixed with a

suitable catalyst , they are converted to elastomers.

USES:

1) For crown and bridge work.

2) For part ial denture prosthetic procedures.

3) Where there are severe undercuts.

4) In patients exhibit ing xerostomia.

5) In patients with lesions of the mucosa, such as l ichen planus or pemphigus.

6) For master impression in rigid individual trays.

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%

1

Or

Base Weight (%)

Polysulfide polymer 80-85

Titanium dioxide, 16-18

Zinc-sulfate, si l ica

Or copper carbonate

Accelerator

Lead dioxide 60-68

Dibutyl phthalate 30-35

Sulfur

Other substances such as

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 fi l ler .

Fil lers l ike l i thopone, t i tanium 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 quanti ty of sulfur is also added. The

particle size of the fi l lers is about 0.3 microns. In general , the weight percent of

the fi l ler in the base paste increases fro low to medium to high consistencies. The

base paste is normally white, due to the fi l ler 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 consti tutes the l iquid vehicle, as well as a quanti ty of the same

fi l ler . Oleic or stearic acids are retarders added to control the rate of set . Lead

dioxide is the active catalyst .

Modifications:

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1) One materials avoids the use of lead dioxide and replaces i t by an organic

reactor, such as cumene hydroperoxide or t-butyl hydroperoxide or hydrated

copper oxide, (CuCoH)2. however, this consti tuent is volati le and i ts 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 Sil icone

Paste Liquid

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

prepolymer

Interfi l ler Tin compound such as dibutyl t in dilaurane

The base contains a moderately low molecular weight si l icone called a

dimethyl si loxane which has reactive terminal hydroxyl groups.

Liquid si l icone prepolymer undergoes cross-l inking to form rubber. Since

the si l icone polymer is a l iquid, f i l lers are added to form a paste. The selection and

pretreatment of the fi l ler are of extreme importance, since si l icones possess a low

cohesive energy density and therefore weaker intermolecular interaction. The

influence of the fi l ler on the strength of si l icone elastomer is much more cri t ical

than when i t is added to polysulfides. Fil lers give a proper consistency to the paste

and st iffness to the set rubber. The consistency of the si l icone paste is controlled

by the selection of the molecular weight of the dimethyl si loxane and the

concentration of the reinforcing agent. Higher molecular weights are used with the

heavier bodied materials. The concentration of the fi l ler increases from 35% for

l ight bodied consistency to 75% for the putty consistency. Colloidal si l ica or

microsized metal oxide, with an optimum particle size of 5 and 10mm; are added as

fi l lers. According to Craig the fi l lers may be copper carbonate or si l ica having

particle sizes from 2 to 8mm. The smaller part icled tend to aggregate, but larger

ones do not contribute to reinforcement. The particles are often surface-treated to

provide better compatibil i ty with, and reinforcement of the si l icone rubber. 3

Colorants l ike organic dyes and pigments are commonly used as an aid in obtaining

a homogenous mix. The accelerator may be a l iquid that consists of stannous

octolate suspension and alkyl si l icate ortho or tetra ethyl si l icate or i t may be

supplied as a paste by the addit ion of a thickening agent.

Tin compound act as reaction catalyst . The accelerator does not have

unlimited shelf l ife because the stannous octoate may oxidize and the ortho

ethylsi l icate is not entirely stable in the presence of the t in ester.

III] Addition Sil icone

One paste contains a poly dimethyl si loxane prepolymer. In which some of

the methyl groups are replaced by hydrogen. The other paste also contains a

prepolymer with a platinum salt l ike chlorplatinic acid activator. The polymer has

vinyl groups replacing some of the methyl groups. Vinyl si l icones are expensive

because of the high cost of platinum. Fil lers give a proper consistency to the paste

and st iffness to the set rubber. Both pastes contain fi l lers. Surfactants have been

added to addit ion si l icones by some manufacturers, which reduces the contact

angle, improves the abil i ty and simplifies the pouring of gypsum models. These

materials are said to be hydrophilic. The addit ion of surfactant makes the

preparation of electroformed dies more difficult because the metalizing powder

does not adhere as well to the surface of hydrophilic addit ion si l icone impression.

IV] Polyether

The base paste contains a moderately low molecular weight polyether,

containing ethylene imine terminal groups, si l ica fi l ler , and a plasticizer such as

glycoether pthalate. The accelerator paste contains 2, 5 dichloro benzene sulfonate

as a cross-l inking agent, along with a fi l ler 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.

Light-cured polyether urethane dimethacrylate has visible l ight-cure

photoinit iators, photo accelerators and si l icone dioxide fi l ler 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) Polysulfide4

The terminal and pendant mercapton groups (-SH) of adjacent molecules are

oxidized by the accelerator to produce chain extension and cross l inking

respectively. Because the pendant groups compose only a small eprcent of the

available –SH groups, chain lengthening will predominate at f irst . This will

principally increase viscosity. I t is the subsequent cross-l inking reaction that l inks

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-l inking

reaches a certain level, the material develops elastic properties. The reaction

results in a rapid 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 sl ightly 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 init iators.

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, l ike organic hydroperoxide have poor

dimensional stabil i ty 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 si l icone

Terminal hydroxyl groups of prepolymer chains react with the cross l inking

agent under the influence of the catalyst . The polymer consists of a hydroxyl

terminated poly (dimethyl si loxane). Cross l inking occurs through a reaction with

tr i-and tetrafunctional alkyl si l icates, commonly tetraethyl orthosil icate in the

presence of stannous octoate [Sn (C7 H15 Coo)2]. Each molecule of cross-l inking

agent may potential ly, react with upto 4 prepolymer chains causing extensive cross

l inking. Cross l inking produces an increase in viscosity and the rapid development

of elastic properties.

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These retractions are affected at ambient temperatures and the materials are

therefore called RTV (room temperature vulcanization) si l icones in technical

l i terature. Ethyl alcohol is a by-product of the sett ing reaction is exothermic with a

temperature rise of 1°C.

3) Additional si l icone

In this case the polymer is terminated with vinyl groups and is cross l inked

with hybride groups activated by a platinum salt catalyst , by an addit ion reaction.

There are no reaction by products as long as there is a good balance of vinyl

si l icone and hybrid si l icone. If proper balance is not maintained, hydrogen gas is

produced. Noblem salts l ike platinum or palladium is not maintained, hydrogen gas

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

l inked 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 l inking and thus sett ing is

brought about by an aromatic sulfonate ester. This produces cross l inking by

cationic polymerization via the imine end groups. The sett ing reaction is sl ightly

more exothermic than that of other elastomers, with a temperature rise of about

4°C.

Properties includes:

1. Rheological properties / viscosity.

2. Working and sett ing t ime.

3. Dimensional stabil i ty.

4. Permanent deformation / elastici ty.

5. Strain.

6. Flow.

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7. Hardness.

8. Tear strength.

9. Detain reproduction.

10. Creep.

11. Wettabil i ty.

12. Shelf l ife.

13. Biological properties.

1) Rheological Properties / Viscosity:

These play an important role in the successful application of elastomers.

Viscosity is a function of t ime after the start of mixing. The most rapid increase in

viscosity with t ime occurred with the si l icones and polyethers, with the lat ter

increasing sl ightly more rapidly than the former. Attention must be paide to proper

mixing t imes and t imes of insert ion of the impression material into the mouth if the

materials are to be used to their best advantage. Sil icones are more fluid and hence

easier to mix than polysulfides. But because of shorter sett ing t imes for the

si l icones, the flow is present for a shorter period of t ime. 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 si l icone 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 viscosit ies during injection with a syringe than when inserted in a tray

during mixing. I t 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 t imes, followed by si l icones and

polyethers. A reciprocating rheometer is a useful instrument to estimate practical

working and sett ing t imes. The working and sett ing t imes of elastomers are 7

shortened by increases in temperature and humidity. The sett ing t ime does not

correspond to the curing t ime. In condensation si l icone material the polymerization

may continue for 2 or more weeks after mixing. Working t ime is measured at room

temperature and sett ing t ime at mouth temperature. Working t ime may be prolonged

by a low room temperature or by mixing on a chil led, dry glass slab.

Alteration of the base-accelerator ratio is an effective method of changing

the curing rate of condensation si l icones. In contrast , the curing rate of addit ion

si l icones appears to be even more sensit ive to temperature changes than are

polysulfides. The curing rate of polyethers is less sensit ive to temperature change

than is that of addit ion si l icones. I t has the shortest working t ime among the

elastomers.

Condensation si l icones have the largest dimensional change (-0.6%). The

shrinkage is a result of the evaporation of volati le by products and the

rearrangement of the bonds result ing from polymerization. The addit ion si l icones

have 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 addit ion si l icones release hydrogen after sett ing 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 si l icones have the best recovery from deformation during removal

from the mouth, followed by condensation si l icones and the polyether and

polysulfides. Lower values of si l icones are related to the higher cross-l inking in

si l icones, al though the fi l ler 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, l i t t le permanent deformation takes place in the putty since i t is so st iff

that l i t t le deformation occurs during removal of the putty wash impression.

8

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

flexibil i ty of the material . In general , the l ight 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 si l icones are st iffer

than the polysulfides of comparable consistency and the addit ion si l icones are

sl ightly st iffer than the condensation si l icones.

5) Flow

This property is of part icular importance because i t 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 specimen 1 hour

after spatulation and the percent deformation is determined 15 minutes after a load

of 100gm is applied.

The si l icones and polyethers have the lowest values of f low and the

polysulfides have the highest values. Low flow of polyethers is caused by the

rubber being crosslinked and i ts high st iffness.

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 addit ion si l icones do not change hardness

significantly with t ime where as the hardness of condensation si l icones, the

addit ion si l icone putt ies and the polyethers does increase with t ime. The hardness

and the strain as well affect the force necessary for removal of the impression from

the mouth. Low flexibil i ty and high hardness can be compensated for cl inically

when more space for the impression material between the tray and the teeth is

provided. The high st iffness of polyether is indicated by the low flexibil i ty of 3%

compared with 5% and 7% for condensation si l icone and polysulfide regular bodies

types. The low flexibil i ty may cause problems in the removal of the impression

9

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 i t indicates the abil i ty of material to

withstand tearing in thin interproximal areas. Tear strength is a measure of the

force needed to init iate 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

st iffness. As a result , long tags of impression materials may tear during removal of

the impression more easily than occurs with the other 2 types. The resistance of

polysulfides to tearing is about 8 t imes the values reported for hydrocolloid

materials. I t 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 t ime in the mouth has a practical l imitation.

8) Detail Reproduction

In general si l icones and polyethers are capable of registering or reproducing

detail better than the polysulfides. Whereas the resolution capabil i ty of the lat ter 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 l ine 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 l i t t le difficulty in

meeting this requirement.

9) Creep Compliance

Elastomers are viscoelastic and their mechanical properties are t ime

dependent. For example, the higher the rate of deformation, the higher the tear 10

strength, and the longer the impressions are deformed, the higher the permanent

deformation. As a result the plots of the creep compliances t ime describe the

properties of these materials better than the stress-strain curves.

Polysulfide is the most f lexible and the polyether the least . The flatness or

parallel ism of the curves with respect to the t ime 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 si l icone and then the addit ion si l icone and polyether.

The recoverable viscoelastic quali ty of the materials is indicated by

difference between the init ial creep compliance and the creep compliance value

obtained by extrapolation of the l inear portion of the curve to zero t ime.

1) Wettabil i ty:

Wettabil i ty may be assessed by measuring the advancing contact angle of

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

si l icones and the polyethers were wetted the best , and the condensation si l icones

and hydrophobic addit ion si l icones the least . The wettabil i ty was directly

correlated to the case of pouring high strength stone models.

MaterialAdvancing contact

algne of water (°)

Castabil i ty of high-

strength dental stone

(%)

Polysulfide 82 44

Condensation si l icone 98 30

Addition si l icone

i) Hydrophobic 98 30

ii) Hydrophilic 53 72

Polyether 49 70

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10) Shelf Life

A properly compounded polysulfide or polyether impression material does

not deteriorate appreciably in the tubes when i t is stored under normal

environmental condit ions [10° to 27°C (65° to 80°F)] for 2 years. The shelf l ife for

si l icones is reasonable but is usually shorter than for polysulfides; thus large

quanti t ies should not be purchased or stored. Although the si tuation is greatly

improved over what i t was some years ago, occasionally the si l icone gum may

stiffen in the tube if stored for too long a t ime.

Continuous exposure of ei ther the si l icone paste or the reactor to the air

hastens deterioration. For this reason, the containers should be kept t ightly 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 st i l l 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. I t 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.

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b) Condensation si l icone:

The materials are non-toxic, al though direct contact of skin with the

accelerator is to be avoided since allergic reactions have been noted.

c) Addition si l icone

The culture tests on both the base and catalyst pastes have been negative and

indicate that addit ion si l icones caused less t issue reaction than the condensation

si l icones.

d) Polyether

The aromatic sulfonic acid ester can cause skin irri tat ion and direct contact

with the catalyst should be avoided. Thorough mixing of the catalyst with the base

should be accomplished to prevent any irri tat ion of the oral t issues.

Evaluation program:

American Dental Association Specification No. 19 applied to the properties

of elastomers.

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 si l icones have excellent dimensional stabil i ty, even in cold

steri l izing solutions.

7. Wide range of different viscosit ies available to match different cl inical

si tuations.

8. Low viscosity si l icones suitable for wash techniques.

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9. Putty si l icones are useful as space-fi l l ing 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.

12. More easily prepared for use.

13. More dimensionally stable over a period of t ime than hydrocolloids.

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

Disadvantages:

1. They are hydrophobic and so tend to sl ip on wet, mucus-covered mucosa.

2. Prolonged sett ing t ime, especially polysulfides.

3. Tear resistance of si l icones is low.

4. Condensation si l icones are dimensionally unstable.

5. Sil icone putty can easily distort peripheral t issues.

6. most extensive of al l 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 sett ing.

10. The ratio of the material is also cri t ical; 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.

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14. Polysulfides need custom made rather than stock tray due to greater chance

of distort ion.

Clinical presentation:

a) Polysulfides are supplied in 3 consistencies: low (syringe /wash),

medium (regular) and high (tray).

b) Addition si l icones are available in these three consistencies plus a

putty (very high) type. Addition si l icones are also supplied as a single

consistency product with sufficient shear thinning so that i t can be

used as both a low and a high consistency material .

c) Condensation si l icones are usually supplied in a low and putty l ike

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

si l icones supply the catalyst as a l iquid. They very high consistency is supplied as

a base putty and a catalyst putty or l iquid.

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

composit ion of the tube of the rubber base material is balanced with that of the

accelerator, the same matched tubes originally supplied by the manufacturer should

always be used for certain products some flexibil i ty in working and sett ing t imes

can be obtained by changing proportions.

The catalyst paste is f irst 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 unti l the mixed paste is of uniform color. If the

15

mixture is not homogenous curing will not be uniform and a distorted impression

will result .

An automatic dispensing and mixing device for addit ion si l icone is generally

used for l ight 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 addit ion, mixing

t ime is reduced. The possibil i t ies 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 t ip is in place.

In case of condensation si l icone, the reactor may be supplied in the form of

a colored oily l iquid. When the base paste is dispensed from the tube, a certain

length is extrude onto the mixing pad and the l iquid 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 l iquid 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 l iquid 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 unti l

free from streaks. The putty materials that have a l iquid catalyst are init ial ly mixed

with spatula unti l 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 that can be contoured closely to the oral t issues, 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 insert ion of the impression material .

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The adhesives furnished with the various types of rubber impression

materials are not interchangeable. Adhesives employed with polysulfides include

butyl rubber or styrene / acrylonitri le dissolved in a suitable volati le solvent such

as chloroform a ketone. The base for adhesive employed with the si l icone rubber

materials may contain poly(dimethyl si loxane) or a similar reactive si l icone and

ethyl si l icate. A slightly roughened surface on the tray will increase the adhesion.

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 f i l led with a uniform

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

sett ing t ime 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 fi l led

syringe into the prepared cavit ies. The fi l ler tray is then carried to place.

The procedure should be t imed 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 stabil i ty. The thin layer of the impression

adjacent to the oral t issues is recorded in l ight-bodied material assuring optimum

fine-detail reproduction.

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 si l icone rubber. Relief for the final or “wash” impression is

provided either by cutt ing away some of the “tray” si l icone or by using a thin

resin, rubber or wax sheet as a space between the si l icone and the prepared teeth.

This area is then fi l led with a thinner-consistency si l icone 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, i t can lead to a grossly in

accurate impression if a cri t ical portion of the primary impression is held under

pressure while the wash material is sett ing.

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c) Singe impressions:

The tray employed is usually a copper matrix band, approximately 30 gauge

in thickness. The band should be fi t ted to the tooth and the reinforced with

compound or self-curing resin. Otherwise the impression will be squeezed with the

fingers when i t is removed from the mouth and a distort ion will occur. The

adhesive is applied to the band and band fi l led 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 unti l the

curing has progressed sufficiently to provide adequate elastici ty so that distort ion

will not occur. The curing t imes 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 t ime of mixing allowing 6 to 8 minutes for the impression to

remain in the mouth. The rubber impression should be removed suddenly.

3) Disinfection of the impression:

The elastomers can generally be disinfected by various antimicrobial

solutions without adverse dimensional changes, provided that the disinfection t ime

is short . Prolonged immersion may produce measurable distort ion and certain

agents may reduce the surface hardness of poured gypsum casts. In part icular,

polyethers are susceptible to dimensional change if the immersion t ime is longer

than 10 minutes, because of their pronounced hydrophilic nature.

2% glutaraldehyde is a satisfactory solution for most elastomers. The

impression material i tself may contain disinfectant.

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Types of Failure

Type Cause

1) Rough / uneven a ) I n c o m p l e t e p o l y m e r i z a t i o n c a u s e d b y :

i . P r e m a t u r e r e m o v a l f r o m t h e m o u t h .

i i . I m p r o p e r r a t i o o r m i x i n g o f c o m p o n e n t s .

i i i . O i l o r o t h e r o r g a n i c m a t e r i a l o n t h e t e e t h .

b ) T o o r a p i d p o l y m e r i z a t i o n f r o m h i g h h u m i d i t y o r t e m p e r a t u r e .

c) E x c e s s i v e l y h i g h a c c e l e r a t o r b a s e r a t i o w i t h c o n d e n s a t i o n

s i l i c o n e .

19

Bubbles a ) T o o r a p i d p o l y m e r i z a t i o n , p r e v e n t i n g f l o w .

b ) A i r i n c o r p o r a t e d d u r i n g m i x i n g .

3) Irregularly

shaped

a ) M o i s t u r e , d e b r i s o n s u r f a c e o f t e e t h v o i d s .

b ) I n a d e q u a t e c l e a n i n g o f i m p r e s s i o n .

4) Roughly or

chalky store

cast

a ) I n a d e q u a t e c l e a n i n g o f i m p r e s s i o n .

b ) E x c e s s w a t e r l e f t o n t h e s u r f a c e o f i m p r e s s i o n .

c ) E x c e s s w e t t i n g a g e n t l e f t o n i m p r e s s i o n .

d ) P r e m a t u r e r e m o v a l o f c a s t .

e ) I m p r o p e r m a n i p u l a t i o n o f s t o n e .

f ) N o t d e l a y i n g f o r 2 0 m i n u t e s w h i l e p o u r i n g a d d i t i o n s i l i c o n e .

5) Distortion a ) R e s i n t r a y n o t a g e d s u f f i c i e n t l y a n d s t i l l u n d e r g o i n g p o l y m e r i z a t i o n

s h r i n k a g e .

b ) L a c k o f a d h e s i o n o f r u b b e r t o t r a y c a u s e d b y :

i . N o t e n o u g h c o a t s o f a d h e s i v e .

i i . F i l l i n g t r a y w i t h m a t e r i a l t o o s o o n a f t e r a p p l y i n g a d h e s i v e .

i i i . U s i n g w r o n g a d h e s i v e .

d ) L a c k o f m e c h a n i c a l r e t e n t i o n f o r t h o s e m a t e r i a l w h e r e a d h e s i v e i s

i n e f f e c t i v e .

e ) D e v e l o p m e n t o f e l a s t i c p r o p e r t i e s i n t h e m a t e r i a l b e f o r e t r a y i s s e a t e d .

f ) E x c e s s i v e b u l k o f m a t e r i a l .

g ) I n s u f f i c i e n t r e l i e f f o r t h e r e l i n e m a t e r i a l i f s u c h t e c h n i q u e i s u s e d .

h ) C o n t i n u e d p r e s s u r e a g a i n s t i m p r e s s i o n m a t e r i a l t h a t h a s d e v e l o p e d e l a s t i c

p r o p e r t i e s .

i ) M o v e m e n t o f t h e t r a y d u r i n g g e l a t i o n .

j ) P r e m a t u r e r e m o v a l f r o m m o u t h .

k ) I m p r o p e r r e m o v a l f r o m m o u t h .

l ) D e l a y e d p o u r i n g o f p o l y s u l f i d e i m p r e s s i o n .

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6) Faulty

electroplating

Recent advances in elastomers

1) A visible l ight-cure impression material was marked in 1988. As supplied, this

material contained a polyurethane dimethacrylate resin with SiO2 fi l ler and

consti tuents to enable the resin to polymerized in the presence of l ight of

around 480nm.

This material is available in 2 visocit ies: the l ight body material is packaged

in disposable syringes and the heavy-body material is packaged in tubes.

Properties: This material has excellent elastici ty and very low dimensional

shrinkage upon storage. I t may be poured immediately or upto 2 weeks later. The

material is r igid and i t 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 l ight 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 fi l l l ine with the heavy body material .

After the tray is seated in the mouth, both viscosit ies are cured simultaneously

using a visible l ight curing unit having an 8mm or larger diameter probe. The

curing t ime is approximately 3 minutes. The periphery of the impression which is

tacky from air-inhibit ion, will not cause clinical problems.

Advantages:

i . The dentist has complete control over working t ime.

i i . Curing t ime is relatively short (3 minutes).

i i i . The material has excellent physical , mechanical and clinical

properties.

Disadvantages:

21

i . The need for special trays that are transparent to the visible l ight

required to cure the material .

i i . If a delay occurs before placement, the material should be stored

in a dark place away from light.

i i i . Difficulty may be encountered when using the l ight source to cure

remote areas.

iv. The material should not be used with patients with a known allergy

or sensit ivity to urethanes, acrylics or methacrylates.

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