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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 acids – 2%

– 44%

Plasticiser / sulfur – 5% Perfume

– 1%

Inert oil

– 37%

Oleic Filler 50%

Or Base

Weight (%)

Polysulfide polymer

80-85

Titanium dioxide,

16-18

Zinc-sulfate, silica Or copper carbonate Accelerator Lead dioxide

60-68

Dibutyl phthalate

30-35

Sulfur Other substances such as

2

and

stearic


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 silicone

Alkyl silicate such as tetraethyl silicate.

prepol ymer Interfiller

Tin compound such as dibut yl tin dilaurane

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 1째C. 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 4째C. 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 syringetray 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 crosslinking 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

15


practice, 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

containing

consistency

thinner

are

elastomers.

more

flexible

The than

polyethers 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

compensated

flexibility for

and

clinically

high

when

hardness

more

space

can

be

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 (째)

Castability of high-strength dental stone (%)

Polysulfide

82

44

Condensation silicone

98

30

i) Hydrophobic

98

30

ii) Hydrophilic

53

72

Polyether

49

70

Addition silicone

10)

Shelf Life A

properly

compounded

polysulfide

or

polyether

impression material does not deteriorate appreciably in the tubes

when

it

is

stored

under

21

normal

environmental


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

22


mouth for only a few minutes and is hydrophobic, reducing the chances of washing out of lead compounds by saliva.

23


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.

24


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

resistance.

25

of

greater

abrasion


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.

26


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

27

with

sufficient

shear


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

28


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.

29


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

30


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.

31


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

reproduction.

32

optimum

fine-detail


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 selfcuring resin. Otherwise the impression will be squeezed with the fingers when it is removed from the mouth and a

33


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 changes,

solutions

without

provided that the

adverse

disinfection

dimensional

time

is

short.

Prolonged immersion may produce measurable distortion and

34


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 1) Rough / uneven

Cause a)

Incomplete caused by:

polymerization

i.

Premature from the mouth.

removal

ii.

Improper ratio mixing of components.

or

iii. Oil or other organic material on the teeth. b)

Toorapid from high temperature.

c)

Excessively high accelerator base ratio with condensation silicone.

35

polymerization humidity or


2) Bubbles

3) Irregularly shaped

4) Roughly or chalky store cast

a)

Too rapid polymerization, preventing flow.

b)

Air incorporated mixing.

during

a) Moisture, debris on surface of teeth voids. b) Inadequate impression.

cleaning

of

a)

cleaning

of

Inadequate impression.

b) Excess water left on surface of impression. c)

the

Excess wetting agent left on impression.

d) Premature removal of cast.

5) Distortion

e)

Improper stone.

manipulation

of

f)

Not delaying for 20 minutes while pouring addition silicone.

a) Resin tray not aged sufficiently and still undergoing polymerization shrinkage. b) Lack of adhesion of rubber to tray caused by:

36

i.

Not enough coats of adhesive.

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 mouth.

removal

from

k)

Improper mouth.

removal

from

l)

Delayed pouring polysulfide impression.

6) Faulty electroplating

37

of


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 heavybody 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 line with the heavy body material. After the tray is seated in the mouth, both

38


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

urethanes, acrylics or methacrylates.

40

to


Elastomers / dental implant courses by Indian dental academy