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Lesson 3-3
Impression Materials

An impression is a negative reproduction of a given area of the oral cavity. The area reproduced may be composed of either hard or soft tissues or both. The material must be inserted into the mouth while it is too soft to retain its shape. A rigid base is needed to carry it to the mouth and hold it against the tissues until it hardens. For this purpose, a variety of trays, called stock trays, are available. These are shaped to fit over the average maxillary and mandibular arches. Some can be trimmed and bent to the requirements of the individual patient. Trays may also be fabricated for each individual patient.

REQUIREMENTS FOR IMPRESSION MATERIAL

a.  An impression material must meet a wide range of requirements in order to provide an accurate impression of the different tissues.

b. List of Requirements. The following are some of the more important requirements.

(1) The material should flow or be pliable at a temperature that will not injure the oral tissue.

(2) It should set quickly, preferably within 2 to 4 minutes, at body temperature.

(3) It should unite into a solid mass without adhering to the oral tissues or to the material used for the cast.

(4) It should fall into all irregularities and fine lines in the area to reproduce without displacing soft tissue.

(5) It must retain an accurate reproduction of surface detail when it solidifies and is withdrawn from the mouth.

(6) It must have dimensional stability. It must not expand, contract, or become deformed in any way because of temperature changes, atmospheric conditions, or the pouring of the cast.

(7) It must not be too unpleasant to the patient.

(8) It must not flake (after solidifying) when trimmed with a sharp knife at room temperature.

TYPES OF IMPRESSION MATERIALS

Impression materials are of three types: the rigid type, the thermo- plastic type, and the elastic type. Thermoplastic materials soften when warmed and harden when cooled with no change in chemical makeup. Elastic materials expand and contract with no change in structure or shape. All three types have advantages and disadvantages. The dental officer determines which material best meets the requirements of each particular case. He frequently will use two or more materials to make a single impression.

AGAR-TYPE HYDROCOLLOID--CHARACTERISTICS

a.  The agar-type hydrocolloids are thermoplastic, elastic materials. They are called reversible hydrocolloids because they are softened by heating, hardened by cooling, and used repeatedly. In the hardened state, they are flexible and elastic.

b. Composition. The basic component of these hydrocolloids is agar-agar, a product extracted from certain types of seaweed. The exact composition of the material varies with different manufacturers. Most preparations contain about 80 percent water, 15 percent agar-agar, and 5 percent chemicals and inert substances.

c. Inert Substances. Inert substances in the material are fillers that increase its strength and stiffness. Small amounts of borax and potassium sulfate are usually included. These chemicals do not affect agar-type hydrocolloids the same way they affect gypsum products. Borax is used to increase the strength of the hydrocolloid. However, both borax and hydrocolloid retard the setting time of gypsum products and may prevent casts from hardening. Consequently, potassium sulfate is added to the impression material to partly counteract the action of the borax.

d. Instruments and Materials. Figure 3-1 shows typical instruments and materials for agar-type hydrocolloid impressions. Manufacturers furnish agar-type hydrocolloids either in tubes for making impressions or in bulk form for duplicating casts. Both forms must be stored in 100 percent relative humidity in airtight containers in a cool place. A relative humidity of 100 percent indicates that the air contains as much water vapor as it can take up at a given temperature, usually room temperature. Some of the properties of the agar-type hydrocolloid impression materials are discussed below.

MIXING AND SETTING TIME

Agar-type hydrocolloids become fluid at temperatures much higher than the temperature at which they gel. The temperature range varies slightly from one manufacturer to another. The tube containing the material used in making impressions is placed in boiling water to soften the material. The tube then is stored in a water bath at a temperature of 145o to 155o F (63o to 68o C) until it is needed. Before using the tube, it is "tempered" by cooling it to about 115o F (46o C) so that it will be of a consistency to remain in the impression tray, set quickly, and not be uncomfortable to the patient. Since most brands gel at 97o F (36o C) (slightly below mouth temperature), a water-cooled tray is used. Water at about 70o F (21o C) is circulated through the tray for about 5 minutes to gel the impression.

DIMENSIONAL STABILITY

a. Water Content. The water content of agar-type hydrocolloid impression materials is most important for dimensional stability.

(1) Syneresis. When an impression made of this material is removed from the mouth into the air at room temperature, the surface contracts by giving off water to the air. This process is called syneresis and causes the outer layer of the impression to shrink and become distorted.

Figure 3-1. Instruments and materials for agar hydrocolloid impressions.

(2) Imbibition. If the impression is placed in water, it will expand (take up water). This process is called imbibition. Unfortunately, the expansion caused by imbibition will not restore an impression to its original dimensions.

(3) Expansion after shrinkage. The expansion does not equal the shrinkage either in volume or direction. Therefore, any attempt to restore an impression after syneresis has occurred will result in a distorted cast or die.

b. Brief and Abrupt Stress Recommended. The structure of the hydrocolloid gel is such that it can withstand an abrupt, brief strain (change of form or size) without fracture or permanent distortion more easily than it can withstand a gentle, prolonged strain. Therefore, the impression should be removed from undercut areas quickly rather than be "teased" over these areas. The impression always undergoes stress during removal. Stress is the internal resistance of a material to forces that disarrange its normal molecular structure. After the internal force is removed, the gel "relaxes," but the impression does not quite resume its original shape because the material is not perfectly elastic. The material is also under some slight pressure while the impression is being made. The stresses induced either relax or set when the pressure is removed. This set distorts the impression after a short time. Therefore, it is important that the casts made from agar-type hydrocolloid impression materials be poured immediately after the impressions are made.

EFFECT ON GYPSUM PRODUCTS

Some agar-type hydrocolloid impression materials retard the setting of gypsum products. The surface of a cast in contact with the gel may harden very slowly or not at all. The cast will also absorb water from the gel. As a result, the surface will be soft and rough. The hardness of the cast can be improved by "fixing" the impression with a hardening solution. For this purpose, a 2 percent solution of potassium sulfate is recommended. The impression is immersed in the solution for 5 to 10 minutes. Immersion for longer time may affect the dimensional stability of the hydrocolloid gel. The stone cast or die should be left in the impression for at least 30 minutes--preferably for 60 minutes--before the impression is separated from the cast. This is recommended because the setting time of the gypsum product in contact with the impression material will be lengthened even though a proper hardening solution is used.

ALGINATE-TYPE HYDROCOLLOID--CHARACTERISTICS

a.  The alginate-type hydrocolloids are an elastic type impression material. An alginate is a salt of alginic acid (an extract from seaweed). Alginate-type hydrocolloids gel by chemical action. Once the gelation process begins, it is irreversible.

b. Composition. The composition of the alginate-type hydrocolloids varies with different manufacturers. The basic components are a soluble alginate (either potassium alginate or sodium alginate) and a reactor (calcium sulfate), which causes the alginate to gel. The product also contains a retarder (sodium or potassium sulfate, oxalate, or carbonate) to prevent gelation from occurring too rapidly. A fluoride is usually added to prevent retardation of the setting time of the casts. The remainder of the material is composed of fillers that increase the strength and stiffness of the gel.

c. Usage. Alginate-type hydrocolloids are supplied in powder form, either in bulk or in measured portions packaged in foil envelopes. The powder must be stored in a cool place. The bulk form must be kept in a tightly closed container to protect it from contamination and to prevent it from absorbing moisture from the air. The containers are agitated to loosen the bulk powders before they are measured, thus preventing use of an excessive proportion of the powder. The powder is mixed with a measured amount of water. Further discussion follows below.

DISPENSING

Water and powder measuring cups are provided by the manufacturer in each can of alginate-type material. Lines on the water measurer correspond to the number of scoops of powder used. The ratio of use is 1 to 1 or, for example, three scoops of powder to three units of water. The actual amount will vary, depending on the size of the impression tray. Prior to mixing, it is necessary to tumble the container in order to fluff the powder.

MIXING AND SETTING TIME

The best method of controlling the gelation time of alginate-type hydrocolloid materials is to alter the temperature of the water used in the mix. The higher the temperature of the water, the faster gelation will occur. Higher temperatures accelerate the chemical reaction. The temperature of the water must be regulated carefully within a few degrees of that recommended by the manufacturer to obtain a constant and reliable gelation time. The average recommended temperature of water is 70o F (21o C). Changing the water-powder ratio and the mixing time will alter the gelation time, but these methods of control also impair certain properties of the material. The amount of the retarder in the material can be regulated only by the manufacturer since the action of the retarder changes the nature of the material.

STRENGTH

The water-powder ratio recommended by the manufacturer must be used. Too little or too much water will weaken the gel. Mixing must be timed. Undermixing may prevent the chemical reaction from occurring evenly, and overmixing may break up the gel. Either can decrease the strength of the material by as much as 50 percent. The strength of alginate-type hydrocolloids increases for several minutes after the initial gelation. Consequently, the impression must not be removed from the mouth for at least 2 or 3 minutes after gelation has occurred.

DIMENSIONAL STABILITY

Alginate-type hydrocolloid impression materials are influenced by syneresis, imbibition, strain, and stress in the same way as the agar-type materials. Hence, for the most accurate results, the impression should be fixed and the cast poured soon after the impression is removed from the mouth. If the impression must be stored for a short period of time, it should be placed in a humidor in which the relative humidity is 100 percent.

EFFECT ON GYPSUM PRODUCTS

Alginate-type hydrocolloid impression materials affect gypsum products in the same manner as the agar-type materials affect them. Some alginates do not require the use of hardening solutions because the manufacturer has incorporated these materials in the powder. However, the hardness of the surfaces of the cast can always be improved by "fixing" the impression with a hardening solution.

SYNTHETIC RUBBER BASE IMPRESSION MATERIALS--CHARACTERISTICS

a.  Synthetic rubber base impression materials are flexible, rubber-like, and sufficiently elastic to return to their original shape after slight distortion. They are used for making impressions of areas containing undercuts, especially for crowns, inlays, and removable and fixed partial dentures. Figure 3-2 shows typical instruments and materials setup for rubber base impression materials.

b. Two Types. There are two types of rubber base impression materials. Type one has a synthetic rubber base of silicone. Type two has a synthetic rubber base of polysulfide. Both types are polymeric compounds. These compounds are composed of molecules of the same elements in the same proportions but differing in size. The compound containing small, simple molecules is called monomer. The compound containing large complex molecules in which the atoms are joined in chains or rings is called polymer. Because of their different molecular structure, the compounds have different physical properties. The chemical process by which the molecules of monomer are combined to form polymer is called polymerization.

c. Two Liquid Bases. Both the silicone and polysulfide bases are liquids. They are mixed with liquid chemical reactors which polymerize them. The manufacturers add inert substances or plasticizers to the bases and sometimes to the chemical reactors to make paste of a consistency that will remain in an impression tray until polymerization has taken place. The silicone type must be stored in a cool place and will deteriorate after about 6 months. The polysulfide type does not require special storage and can be stored indefinitely.

d. Usage. A thin layer of this material, uniform in thickness, is required to obtain the most accurate impression with the material. For this reason, it is used in individually designed (custom) acrylic resin trays. Synthetic rubber base materials are not adhesive; therefore, a tray adhesive is needed to prevent the impression from pulling away from the tray. If the impression material pulls away from the tray, distortion will result when the tray is removed from the patient's mouth.

Figure 3-2. Setup for rubber base impression material.

DISPENSING

Equal lengths (usually 6 inches each) of both the rubber base impression material and the chemical reactor (a catalyst) should be laid out side by side onto a polymer paper or parchment pad. The base material has a tendency to spread once it is dispensed. For that reason, it is better to put the rubber base impression material on the pad first in order to give it time to spread. The chemical reactor should not touch the base material until everything is ready to mix.

MIXING AND SETTING TIME

The polysulfide base and its chemical reactor are both furnished as pastes that are packaged in separate tubes. The silicone base is also a paste that is packaged in tubes. Its chemical reactor may be either a paste or a bottled liquid. With both types, the proportions and the method of spatulation recommended by the manufacturer must be followed exactly. The bases and the chemical reactors are of different colors. They must be spatulated together until no streaks remain. Undermixing will prevent material from polymerizing evenly and overmixing will increase both the set and the strain pattern, but especially the set. Increased relative humidity and temperature tend to shorten both setting and mixing time, particularly for the polysulfide type. The chemical reactor is necessary to polymerize the material. It is often called an accelerator because the setting time may be shortened by increasing the amount used. It does not change the structure of the materials as does the accelerator used with gypsum products. Varying the amount of chemical reactor is the only method of changing the setting time of the silicone type.

DIMENSIONAL STABILITY

During polymerization, synthetic rubber base impression materials undergo some shrinkage that continues for some time after the impression is made, particularly with the silicone type. However, these materials are not subject to syneresis and imbibition because they are hydrophobic (water-hating). They have more dimensional stability than do the hydrocolloids. They react to strain and stress in the same way the hydrocolloids do but do not change in volume. The cast must be poured within 30 minutes after an impression is made. This is necessary because the material continues to polymerize, and bubbles trapped in the material are apt to collapse causing the cast to be faulty. No separating material is required between the rubber base impression materials and the cast or die material.

EFFECT ON GYPSUM PRODUCTS

Polysulfide-type synthetic rubber base impression materials do not interfere with the hardening of gypsum products. A cast poured into an impression made of this material will have a smooth, hard surface. Polymerization of some brands of the silicone type, however, produces gases that mar the surface of the cast.

MODELING PLASTIC--CHARACTERISTICS

a.  Modeling plastic (modeling composition) is an impression material (thermoplastic type) which can be softened by heat into a soft plastic mass and then hardened by cooling with either a stream of cold water or a blast of air. Modeling plastic is used primarily to make impressions of the edentulous arches (the tooth ridges without teeth).

b. Properties and Materials. Modeling plastic is composed of shellac, talc, and glycerides derived from certain tallow oils. The temperature range at which softening occurs depends upon the proportions of the ingredients contained in the material. Modeling plastic is supplied in cakes, wafers, or sticks, and in various colors to aid in distinguishing between products of different softening (fusing) temperature ranges.

c. Recommended Procedures. Any of the gypsum products can be poured against a modeling plastic impression without the use of a separator. Also, modeling plastic is one of the impression materials against which an amalgam die can be packed. (An amalgam die is a model of a tooth in silver amalgam, used for making an inlay or crown.) Although no separating medium is required in either of these procedures, extreme care must be exercised in drawing the impression material from the cast or die, since both materials are hard and relatively unyielding. Therefore, the modeling plastic is softened in heated water, at 120 F (49 C), and removed gently so that the cast is not damaged.

IMPRESSION PASTE--CHARACTERISTICS

a.  Impression paste is a thermoplastic-type impression material. It is usually supplied as two separate units, a base and a hardener. The principal ingredients are zinc oxide and eugenol. When the base and the hardener are mixed together in specific proportions, they form a paste. No separator is required when pouring the cast in an impression taken with this material.

b. Usage. Impression paste is used primarily as a corrective material inside an individual impression tray. It is also widely used for rebase impressions for both complete and partial dentures. (A rebase impression replaces the base material of a denture without changing the occlusal relations of the teeth.) Occasionally, it is used in immediate denture fabrication as a lining for a sectional compound impression. Impression paste can be used to provide a lining for a complete denture baseplate to make it fit both the cast and the mouth accurately.

 

 

 

David L. Heiserman, Editor

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All Rights Reserved

Revised: June 06, 2015