Rigid Impression Materials

Four main types of products form the group of impression materials

classified as non-elastic or rigid materials:

1. Impression plaster

2. Impression compound

3. Zinc oxide/eugenol impression paste

4. Impression waxes.

1- Impression plaster

Impression plaster is similar in composition to the dental plaster used to

construct models and dies.

Composition

It consists of β-calcium sulphate hemihydrate which when mixed with

water reacts to form calcium sulphate dehydrate.

Properties

The material is used at a higher water/powder ratio (approximately 0.60)

than is normally used for modeling plasters.

The fluid mix is required to enable fine detail to be recorded in the

impression.

The setting expansion of dental plaster is reduced to minimal proportions by

using anti-expansion agents. Potassium sulphate is the most common of

these and has the secondary effect of accelerating the setting reaction.

Freshly mixed plaster is too fluid to be used in a stock impression tray is

normally used in a special tray, constructed using a 1-1.5 mm spacer.

The tray may be constructed from acrylic resin or shellac.

The mixed impression material is initially very fluid and is capable of

recording soft tissues in the uncompressed state.

In addition, the hemihydrate particles are capable of absorbing moisture

from the surface of the oral soft tissues, allowing very intimate contact

between the impression material and the tissues. The fluidity of the

material, combined with the ability to remove moisture from tissues and a

minimal dimensional change on setting, results in a very accurate

impression which may be difficult to remove.

The water-absorbing nature of these materials often causes patients to

complain about a very dry sensation after having impressions recorded.

Disinfection of a plaster impression can be achieved with a 10 minute

soak in sodium hypochlorite solution.

The main differences between impression plaster and model plaster are:

- More rapid setting in order to avoid inconvenience/discomfort to

both the patient and dentist,

- Smaller setting expansion for greater accuracy,

- And much lower strength so that fracture can occur easily if the

material engages an undercut.

2- Impression compound

Impression compound is described as a rigid, reversible impression

material which sets by physical change.

Impression compound demonstrates thermoplastic behavior. Thermoplastic

solids can be transformed from a hard, solid material into a softened, moldable

material simply by raising their temperature to an appropriate level.

The process can be reversed by cooling to room temperature.

Classification

Two types of impression compound are available:

1. Type I (lower fusing) materials are impression materials.

2. Type II (higher fusing) materials are used for constructing

impression trays.

The difference in fusing temperature between type I and type II materials

naturally reflects a difference in composition of the thermoplastic

components of each.

The lower fusing, type I impression materials may be supplied in either

sheet or stick for many various colors.

Composition

In general impression compound is a mixture of waxes, thermoplastic

resins, organic acids, fillers and coloring agents.

Component Example Function

Thermoplastic

material (47%)

resins and waxes Characterizes the softening temperature

Filler (50%) Talc Gives "body" by

increasing viscosity of the

softened material; reduces

thermal contraction

Lubricant (3%) Stearic acid Improves flow properties

Applications

1. The sheet material (type I) is used for making a preliminary

impressions in edentulous ridges (mouth without teeth), normally

using stock trays.

2. The stick material (type I) is used for:

- border extensions on impression trays

- border molding,

- For recording individual tooth impressions of single crowns using

the copper ring technique.

- For check undercuts in inlay preparation.

3. Type II is used for making special tray.

Properties of impression compound

Thermal conductivity

Impression compound has very low thermal conductivity, i.e. they are

poor conductors of heat.

During softening of the material, the outside will soften first and

the inside last. So to ensure uniform softening the material should

be kept immersed for a sufficiently long time in a water bath.

Kneading of the material ensures further uniform softening.

Due to poor thermal conductivity, the layer adjacent to the mouth

tissues will remain soft. Removal of the impression at this stage

will cause serious distortion. Thus it is important to cool the

compound thoroughly before removing if from the mouth.

Coefficient of thermal expansion (CTE)

The CTE of a compound is comparatively high because of the presence of

resins and waxes. The contraction from mouth temperature to room

temperature is 0.3% and undergoes considerable shrinkage on removal

from the mouth.

Three factors combine to produce significant internal stresses within the

compound impression.

1. The high value of coefficient of thermal expansion.

2. The poor thermal conductivity.

3. The relatively large temperature drop from the softening

temperature to room temperature.

Dimensional stability

The gradual relief of internal stresses may cause distortion of the

impression. For the most accurate results, the model should be poured as

soon as possible after recording the impression.

Flow

The type I material should flow readily at just above mouth temperature,

whilst type II material should ideally not distort at mouth temperature.

The very high viscosity of impression compound is significant in

the degree of fine detail which can be recorded in an impression.

Compound is fairly rigid after setting and has poor elastic properties. A

large stress would be required to remove an impression from undercut

areas.

Manipulation

Sticks: Small amounts of compound are softened over a flame. When a

direct flame is used, the compound should not be allowed to boil or

ignite, otherwise, the plasticizers are volatilized.

Cakes: When a large amount of compound is to be softened, it is difficult

to heat the compound uniformly over a flame. The compound is softened

in warm water in a thermostatically controlled water bath (usually in

range of 60 to 70o C). After the compound is removed from the water

bath, it is usually kneaded with the fingers in order to obtain uniform

plasticity throughout the mass.

Precautions

Prolonged immersion in a water bath is not indicated; the

compound may become brittle and grainy because some of the low

molecular weight ingredients may be leached out.

Overheating in water makes the compound sticky and difficult to

handle.

Avoid incorporating water while kneading.

.

Advantages

1. The material can be reused a number of times (for the same

patient only) in case of errors.

2. Inaccurate portions can be remade without having to remake the

entire impression.

3. Accuracy can be improved by flaming the surface material

4. The material has sufficient body to support itself to an extent

especially in the peripheral portions.

Disadvantages

1. Difficult to record details because of its high viscosity.

2. Compresses soft tissues while making impression.

3. Distortion due to its poor dimensional stability.

4. Difficult to remove if there are severe undercuts.

3- Zinc Oxide/Eugenol impression pastes

These materials are normally supplied as two pastes which are mixed

together on a paper pad or glass slab.

Base paste.

Accelerator or catalyst paste.

There is normally a good color contrast between the two pastes, the zinc

oxide paste, typically, being white and the eugenol paste, a reddish brown

color. The enables thorough mixing to be achieved as indicated by a

homogeneous color, free of streaks, in the mixed material.

Composition of a Zinc Oxide–Eugenol Impression Paste

Components Percentage

Tube No. 1 (base)

Zinc oxide 87

vegetable or mineral oil 13

Tube No. 2 (accelerator)

Oil of cloves or eugenol 12

Gum or polymerized rosin 50

Filler (silica type) 20

Lanolin 3

Canada balsam 10

Accelerator solution (CaCl2) and color 5

Setting reaction

Some formulations do not contain water in the paste and the setting of the

material is retarded until the mixed paste encounters moisture in the

patient’s mouth. In other case, water provides additional acceleration.

When mixing the two pastes, a reaction between zinc oxide and eugenol

begins. The basis of the reaction is that the phenolic – OH of the eugenol

acts as a weak acid and undergoes an acid – base reaction with zinc oxide

to form a salt, zinc eugenolate.

Two molecules of eugenol react with zinc oxide to form the salt. It can

be seen that the ionic salt bonds are formed between zinc and the phenolic

oxygens of each molecule of eugenol.

- Setting time can be decreased by adding zinc acetate or a drop of

water or acetic acid (acetic acid is a more effective than water. It

increases speed of formation of the zinc hydroxide.

Properties

These materials are normally used to record the major impressions of

edentulous arches. The impression is recorded in special (custom) tray,

constructed on the model obtained from the primary impression, or inside

the patient’s existing denture. The periphery of the special tray or denture

needs to be modified with impression compound to ensure proper

contour of the impression and to give support to the paste in these

critical areas.

The thickness of paste used is normally around 1 mm.

For the majority of patients the zinc oxide eugenol impression pastes may

be considered nonirritant. Occasionally, eugenol may promote an

allergic response in some patients. For this type of patient, eugenol-free

zinc oxide impression pastes are available (to overcome the burning

sensation and irritation). The eugenol is replaced by an alternative

organic acid.

Dimensional stability

The dimensional stability is quite satisfactory. A negligible shrinkage

(less than 0.1%) may occur during hardening.

The thickness of paste used in normally around 1mm. this thin section of

material results in an insignificant dimensional change on setting and

subsequent storage of the impression.

The relatively low initial viscosity of the mixed paste, coupled with its

pseudoplastic nature, allows fine detail to be recorded in the impression.

Defects sometimes arise on the surface of the impression but these can be

corrected using an impression wax.

Biological considerations

Some patients experience a burning sensation in the mouth due to

eugenol. It can also cause tissue irritation. This may be overcome by

using a non-eugenol paste. Eugenol-free zinc oxide impression pastes are

available. The eugenol is replaced by an alternative organic acid.

Advantages

1. It has enough working time.

2. It can be checked in the mouth repeatedly without deforming.

4. It registers accurate surface details.

5. It is dimensionally stable.

6. Minor defects can be corrected locally without discarding a

good impression.

Disadvantages

1. It requires a special tray for impression making.

2. It is sticky in nature and adheres to tissues.

3. Eugenol can cause burning sensation and tissue irritation.

4. It cannot be used for making impression of teeth and undercut

areas as it is inelastic in nature.

4- Impression waxes

Impression waxes are rarely used to record complete impressions but are

normally used to correct small imperfections in other impressions,

particularly those of the zinc oxide/eugenol type.

They are thermoplastic materials which flow readily at mouth

temperature and are relatively soft even at room temperature.

Waxes can also be used to produce a mucocompressive impression of the

edentulous saddles.

These materials consist, typically, of a mixture of a low melting paraffin

wax and beeswax in a ratio of about 3: 1.

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