Impression materials (requirement and classification)

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

Definition:

*The function of an impression material is to accurately record the dimensions of the oral tissues and their related structures.

*The impression gives a negative reproduction of these tissues.

*The positive reproduction is called a model or cast .

Requirements:

1 )Good dimensional accuracy.2 )Adequate flow properties.

3 )Sufficient mechanical strength not to tear or permanently deform during removal.

4 )Suitable setting time5 )Ease of manipulation and reasonable cost.

6 )Acceptability to the patient.7 )Safety “non toxic or irritating”

8 )Compatibility with die and cast materials

Classification:

*According to uses:I- Single tooth impression :

1 )Rubber 2) copper band + compoundII- Partial denture :

1 )Primary Alginate2 )Secondary : a) Alginate b) Rubber c)2 pieces impression

III- complete denture :1 )Primary : a) compound b) Alginate

2 )Secondary : a) Rubber b) Zinc oxide Eugenol c) plaster of parise

*According to behavior of material after setting:

I- Non – Elastic Impression:

1 )Plaster of Paris 2) compound 3) Zinc oxide eugenol 4) waxes

2 )Elastic Impression:

1 )Alginat 2) Agar, Agar 3) Rubber

Hydrocolloid impression materials

A colloid must be distinguished from a solution and suspension. A solution is a homogenous mixture. For example, in an aqueous solution the solute exists as small molecules or ions in the solvent. In contrast to this, a suspension is heterogeneous, consisting of particles of at least sufficient size to be seen microscopically, dispersed in a medium. Thus a suspension is two-phase system.

Colloids fall between these two extremes. They are heterogeneous, ( 2 phase systems), like suspensions, but the particle size of the dispersed phase is smaller, usually in the range 1-200 nm. However, it is not always possible to distinguish between a colloid and a solution on the one hand and a colloid and suspension on the other.

When the dispersion medium of a colloid is water, it is termed a hydrocolloid.

Colloid may exist in the sol and gel state. In the sol state, the material is a viscous liquid. A sol can be converted into a gel- a material of gelatin-like consistency, due to agglomeration of the molecules of the dispersed phase, to form fibrils, or chains of molecules, in a network pattern. Theses fibrils enclose the dispersion medium, for example water.

A sol may be converted into a gel in one of two ways:

1 )By a reduction in temperature: such processes are reversible, since on heating, a sol is formed again; an example of this is agar. In such a gel the fibrils are held together by Van Der Waals forces.

2 )Other materials can form a gel by a chemical reaction, which is irreversible (for example, alginates)

The strength or toughness of gel depends on:

1 )The concentration of fibrils – the greater the concentration, the stronger the material

2 )The concentration of fillers – inert powders can be added to a gel to render it less flexible.

A gel can lose or take up water or other fluids. Loss of water can occur by evaporation. Syneresis can also occur; this happened when the gel molecules are drawn closer together, for example by continuation of an setting reaction. As a result a fluid exudate appears on the surface of the gel.

Uptake of water is called imbibition .

In the use of hydrocolloids for dental impression, the material is inserted in the mouth in sol state, when it is sufficiency fluid to record detail. No gelation should have occurred at this stage. It is removed from the tissues after the gel is formed, when it exhibits elastic properties.

Clearly evaporation form, and imbibition by the gel should be avoided, as the former is associated with shrinkage and the latter results in expansion.

Agar ( Composition )

Constituent Approximate percentage Function

Agar 14 colloid

Borax 0.2 strengthens the gel, but retards the setting of dental stone model materials

Potassium sulphate 2 To accelerate the setting of stone

Water 83.8 Dispersion medium

Manipulation

a) The material is supplied in sealed containers to prevent evaporation of water.

b) It is brought to a fluid state by heating the tube in boiling water in a processing unit for 10-45 minutes.

c) After boiling, the material can be stored at 65ºC for up to 8 hoursd) It is important that the entire tube of material is softened e) The material is extruded onto metal trays with mechanical interlocking.

f) The filled tray is placed in a tempering bath at 45ºC for 2 minutes, before insertion into patient’s mouth.g) With water-cooled trays, the agar material set quickly.

A higher temperature is required for the conversion of gel to sol than for reverse reaction .

Properties

1 )Accuracy: *The material can be suffiently fluid to record fine

details if it has been correctly manipulated. *The first material to set is that which is in contact

with the tray (contrast with alginates) since this is cooler than the tissues. Thus the material in contact with the tissues stays liquid for the longest time, and can flow to compensate for any inaccuracy due to dimensional changes, or to inadvertent movement of

the tray .

*The set material can be withdrawn over undercuts. The adhesion of agar to metal is poor, so perforated trays are used.

*Models should be cast up immediately from agar impressions, to avoid the possibility of the evaporation or imbibition. If this is not possible, it has been claimed that stability is achieved by storage in 2% K2SO4 solution or humidor.

*The compatibility with cast materials depends on the chemicals in the impression material. Without an accelerator for the setting of stone (e.g. K2SO4) a soft surface may be obtained.

2 )These materials are non-toxic and non-irritant.3 )Their setting time is rather slow, unless efficient cooling is

achieved.4 )Tear resistance is poor.

5 )Their shelf-life is adequate. The material can be reused and can be sterilized. Loss of water, with an increase in viscosity of the sol, may occur. Water can be added if required.

6 )Can be sterilized by immersion for 10 minutes in dilute aqueous solutions of sodium hypochlorite or glutaraldehyde.

Applications

These materials may be used to some extend for prosthetic impressions, and in crown and bridge work. Agar can be employed in the laboratory, for model duplication, since they can be re-used many times, due to the reversible nature of the reaction.

Alginates(composition and setting)

•Constituent approx.% function•Sodium potassium 12 react with ca2+ to give calcium

gel •Slowly soluble calcium salt caso4 12 releases ca2+ to react with

alginate •Trisodium phosphaste 2 reacts with ca2+ to give

ca3(po4)2 to delayed gel formation

•Filler (diatomaceous earth) 70 increases cohesion of mix and strengths gel

•Silicon fluorides small quantity improves surface of stone model•Flavouring agents small quantity makes materials more acceptable

to patient •Chemical indicator small quantity changes color with PH change, to

indicate different stages in manipulation, e.g. violet color during spatulation, pink when ready to load the tray, white when

ready for insertion into the mouth .

On mixing the powder with water a sol is formed, and the alginate, the calcium salt and the phosphate begin to dissolve. The following reaction occurs to form an elastic gel of calcium alginate:

NaAlg +CaSO4 Na2So4 + CaAlg………..1Only the outer layer of each particle of sodium alginate dissolves and reacts.

However, the mixing and tray loading procedures. This is obviously undesirable, since the material should deform plastically, not elastically, on insertion into the mouth. Gel formation is delayed by trisodium phosphate, which reacts with calcium sulphate to give a precipitate of calcium phosphate, as following:

2Na3Po4 + 3CaSo4 Ca3(Po4)2 + 3Na2So4…………2This latter reaction does not contribute any elastic properties to the material.

Reaction 2 occurs in preference to 1, no substantial quantity of calcium alginate gel is formed until the trisodium phosphate is used up. The manufacture can therefore control the setting time of this product by

adjusting the quantity of this constituent .

Manipulation

The following points should be observed in order to obtain the best results:

1 )The container of powder should be shaken before use to get an even distribution of constituents.

2 )The powder and water should be measured, as directed by the manufacture. On brand of powder has been supplied in water soluble sachets, which help to ensure a uniform consistency of mix.

3 )Room temperature water is usually used, slower or faster setting times can be achieved, if required, by using cooler or warmer water respectively.

4 )Retention to the tray is achieved by one or both of two means:

a) Perforated tray

b) An adhesive such as molten sticky wax or methyl cellulose.

5 )There should be vigorous mixing by spreading the material against the side of the bowel (one minute)

6 )An alginate impression should be displaced sharply from the tissue- this sudden displacement ensure the best elastic behavior. The impression is removed about 2 minutes after set.

7 )On removal from the mouth, the impression should be:

a) wash with cold water to remove saliva.b) Converted with a damp napkin to prevent syneresis.

c) Cast up as soon as possible, preferably not more than 15 minutes after taking the impression.

Properties

1 )Alginate are sufficiently fluid to record fine detail in the mouth.

2 )During setting of the material it is important that the impression should not be removed. The reaction is faster at higher temperatures and so the material in contact with the tissues set first any pressure on the gel due to movement of the tray will set up stresses within the material, which will distort the alginate after its removal from the mouth.

3 )The material is sufficiently elastic to be withdrawn over undercuts, tearing of the impression material may occur with sever undercuts.

4 )Alginate are not dimensionally stable on storage, because of evaporation.

5 )Compatibility with plaster and stone can be good.

6)The materials are non toxic and non irritant, their taste and odor are usually acceptable

7 )The setting time depends on the composition and temperature of mixing

8 )The alginate powder is not stable on storage in the presence of moisture.

9 )Difficult to sterilize, spray disinfection diminish the sharpness of surface detail, while immersion in solutions adversely

affects dimensional accuracy .

Application

These materials are not generally used for impression for inlay, crown and bridge work, but are applied with great success for prosthetic and orthodontic purposes. Alginates are dimensionally less stable than the elastomer.

Elastomeric Impression Materials

Chemically there are 4 kinds:

1 )polysulphid

2 )Condensation polymerizing silicone

3 )Additional polymerizing silicone

4 )Polyether

1 -Polysulphide rubber impression materials:

1 )The first rubber impression material.2 )Didn’t have major changes during storage

that agar and alginate have.3)Impression was much stronger and more

resistant to tearing than agar and alginate.4 )Rubber could be electroplated therefore

metal die as well as gypsum models could be prepaired.

Chemistry of the polysulphides

Alternative names: rubber-base, mercaptan, Thiokol.

Composition:

The materials supplied as two past:

1 )The base past

2 )The reactor past

1 )past base contains:

*80% low molecular weight organic polymer containing reactive mercaptan gp (-SH)

*20% reinforcing agents (titanium oxides- zinc sulphate, copper carbonate, or silica.

2 )The reactor past ( accelerator or catalyst past)

*Lead dioxide (PbO2) this causes (polymerization and cross-linking), dioxide using it result in paste being dark brown to dark gray.

*Sulphur

*An inert oil (dibutyle or dicotyle phthalate)

To make past.

*Other catalyst system use copper hydroxide, organic peroxides (not used since they are volatile).

N.B. the viscosity of the material is controlled by the molecular weight of the mercaptan and

by the selection of the reinforcing agents .

Setting

The –SH groups can be oxidised by PbO2, giving S-S linkages, as follows:

R-SH+PbO2+HS-R R-S-S-R+PbO+H2OThis linear polymer contains approximately 1mol% of branches to provide enough pendant mercaptan groups as chain cross-linking sites. This polymer is usually cross-linked with an oxidizing agent such as lead dioxide. It is the lead dioxide that gives polysulfide its characteristic brown color. During the condensation reaction of the lead dioxide with the SH groups of the polysulphide polymer.

Two phenomena occur :1 )Chain- lengthening polymerization from the

reaction with the terminal-SH groups2 )Cross-linking from the reaction with the pendant-

SH groups.Because the pendant groups compose only a small percentage of the available –SH groups, initially, the polymerization reactions result in chain lengthening, which causes the viscosity to increase. The subsequent cross-linking reactions tie the chains together, forming a three-dimensional network that confers elastic properties to the material.

The polymerization reaction of polysulfide polymer is exothermic, the amount of heat generated depends on the amount of total material and the concentration of initiators. Moisture and temperature exert a significant effect setting of polysulfide impression material. The condensation reaction by-product is water. Loss of even this small molecule from the set material has a significant effect on the dimensional stability of the impression.

Silicon rubber impression materials

Were developed to over come the disadvantages of polysulphides which are:

1 )Objectionable odor.

2 )Staining clothes by the lead dioxides.

3 )Long setting times.

4 )Fairly high permanent deformation.

Composition

1 )Base:

-Past contains low molecular weight silicon liquid\ dimethyl siloxane which has terminal reactive “ OH” gp.

-Reinforcing agents “ silica”

1 )Proper consistency to the mix.

2 )Stiffness to the set rubber.

2 )Accelerator:

Usually supplied as a liquid but sometimes as a past by the use of thickening agent tin octate and an alkyl silicate such as artho -ethyl silicate.

3 )Silicone pastes supplied:

Consistency controlled by a) concentration of reinforcing agents.

b) Molecular weight of dimethyle siloxan

*light…………………….. 35% reinforcing agent

*regular.…………………

*heavy-bodies………mol.w

*putty………………mol.w……..75%

Setting reaction

1 )Dimethyl siloxan + ortho-ethyl silicate + tin octacle silicone rubber + ethylac.

2 )The multifunctional ethyl silicate produces a cross-linked structure that partly accounts for the low value of permanent deformation and flow of silicon rubber.

3 )Ethylac (by product) evaporates gradually reasonable for shrinkage during 24 hrs after setting.

4 )Setting reaction is more sensitive than polysulphid to moisture and heat-increase either of them the setting and working time( normally the silicon setting and working times are shorter than polysulphides)

Addition type

1 )Silane – containing siloxane + vinyl-terminal siloxane + chloroplatinic acid silicone rubber (by volatile byproducts) minimal dimensional changes during polymerize action

2 )In Temp the rate of reaction and shorten the setting time.3 )If hydroxyl gp. Present in the addition silicon

Side reaction occurs with the result of H2 gradually released from the set. Impression bubbles in the gypsum models prepared

after less than 1hr often impression taken. Overcome by: 1 )Control the presence of hydroxyl gp.

2 )Put H2 absorber such as palladium

Polyether rubber impression materials

It offers mechanical properties than polysulphides and dimensional changes the condensation silicone. However it possesses limitations such short working time and high stiffness.

Composition and setting reaction

1 )Base :1 )Lower molecular weight polyether contain ethyleneneamin terminal

gp.2 )Plasticizers

3 )Filler2 )Catalyst:

Aromatic sulphonate acid ester these terminal gps reacted together by the action of catalyst to form cross-linked mol.w.rubber.

2 )Plasticizer3)Filler

Polyether + sulphonic ester cross linked rubber

Visible light-cure impression material

1 )Polyether urethanedimethacrylate resin which visible light-cure photo initiator and photoaccelerater added.

2 )The silicon dioxide filler has a refractive index close to that of the resin in order to provide the translucency for maximum depth of cure.