A typical alginate impression is compressed about 10% in areas of undercuts during removal. The actual magnitude depends on the extent of the undercuts and the space between the tray and the teeth. The ANSI/ADA specification requires that the elastic recovery be more than 95% when the material is compressed 20% for 5 seconds at the time it would normally be removed from the mouth. As indicated in Table 12-2, a typical value for elastic recovery is 98.2%. The corresponding permanent deformation is 1.8%.

The permanent deformation, indicated as percent compression set, is a function of percent compression, time under compression, and time after removal of the compressive load, as illustrated in Figure 12-4. Note that permanent deformation is a time-dependent property. Lower permanent deformation (higher elastic recovery) occurs (1) when the percent compression is lower, (2) when the impression is under compression a shorter time, and (3) when the recovery time is longer, up to about 8 minutes after the release of the load. Clinically these factors translate into requirements for a reasonable bulk of alginate between the tray and the teeth, appropriate retention of the alginate in the tray, and a rapid removal of the impression from the mouth. The usual procedures followed to disinfect the impression and produce a gypsum model provide adequate time for any recovery that might occur.

The compressive and tear strengths of alginates are listed in Table 12-2. Both properties are time dependent, with higher values obtained at higher rates of loading. Compressive strengths range from approximately 0.5 to 0.9 MPa. The ANSI/ADA specification requires that certified products have a compressive strength of at least 0.35 MPa. Tear strengths vary from 0.4 to 0.7 kN/m, and this property is probably more important than the compressive strength. The tear strength is a measure of the force/thickness ratio needed to initiate and continue tearing and is often determined on a specimen of the shape shown in Figure 12-5. Tearing occurs in the thin sections of the impression, and the probability of tearing decreases with increasing rates of removal. The effect of loading rate on the tear strength of several alginates is shown in Figure 12-6. Values for tray materials range from 0.38 to 0.48 N/mm at 20 mm/min to 0.6 to 0.7 N/mm at 500 mm/min. The lower tear strength at corresponding rates for the syringe materials reflects the decreased alginate in the syringe material.

The selection of an alginate-gypsum combination that produces good surface quality and detail is highly important. The surface quality and ability of alginate-gypsum combinations to reproduce fine V-shaped grooves are shown in Figure 12-7, A and B. A type III model plaster was poured against an alginate in Figure 12-7, A, and type IV dental stone was poured against the same alginate in Figure 12-7, B. The finest groove was 0.025 mm wide in each instance. The combination in Figure 12-7, B, was not as compatible as the one in Figure 12-7, A, with respect to either surface quality or detail. For purposes of comparison, in Figure 12-7, C, the same type IV dental stone used in Figure 12-7, B, was poured against a polysulfide impression.

The impression must be rinsed well in cold water to remove saliva and any blood, and then disinfected. Next, all free surface water should be removed before preparing a gypsum model. Saliva and blood interfere with the setting of gypsum, and if free water accumulates, it tends to collect in the deeper parts of the impression and dilute the gypsum model material, yielding a soft, chalky surface. The excess surface water has been removed when the reflective surface becomes dull. If the alginate impression is stored for 30 minutes or more before preparing the model, it should be rinsed with cool water to remove any exudate on the surface caused by syneresis of the alginate gel; exudate will retard the setting of the gypsum. Thereafter, it should be wrapped loosely in a moist paper towel and sealed in a plastic bag to avoid moisture loss.

The set gypsum model should not remain in contact with the alginate impression for periods of several hours because contact of the slightly soluble calcium sulfate dihydrate with the alginate gel containing a great deal of water is detrimental to the surface quality of the model.

Alginate impressions lose water by evaporation and shrink when standing in air. Impressions left on the bench for as short a time as 30 minutes may become inaccurate enough to require remaking the impression. Even if the impression stored for more than 30 minutes in air were immersed in water, it would not be feasible to determine when the correct amount of water had been absorbed, and in any case the previous dimensions would not be reproduced. For maximum accuracy, the model material should be poured into the alginate impression as soon as possible. If for some reason the models cannot be prepared directly, the impressions should be stored in 100% relative humidity in a plastic bag or wrapped in a damp (but not wringing-wet) paper towel. There is a greater chance for distortion the longer the impression is stored (Figure 12-8). New alginates have improved long-term storage ranging from 48 to 120 hours when stored in a plastic bag. Now, orthodontists routinely send alginate impressions to companies that offer digital fabrication of appliances.

Addition silicone is available in extra low, low, medium, heavy, and very heavy (putty) consistencies. A representative product line of addition silicones is shown in Figure 12-11. The base paste of this class of impression materials contains a moderately low-molecular-weight polymer (polymethylhydrosiloxane) with more than 3 and up to 10 pendant or terminal hydrosilane groups per molecule (see formulas below and addition silicone formula 1 [AS1]). The base also contains filler.

The accelerator (catalyst) and the base paste contain a dimethylsiloxane polymer with vinyl terminal groups, plus filler. The accelerator also contains a platinum catalyst of the so-called Karstedt type, which is a complex compound consisting of platinum and 1,3-divinyltetramethyldisiloxane. Unlike the condensation type, the addition reaction does not normally produce a low-molecular-weight byproduct, as indicated in the reaction shown below (AS2).

A secondary reaction can occur, however, with the production of hydrogen gas if —OH groups are present. The most important source of —OH groups is water (H—OH), the reaction of which under consumption of Si—H-units is illustrated above (AS3). Another possible source of hydrogen gas is a side reaction of the Si—H units of the polymethylhydrosiloxane with each other, under the influence of the platinum catalyst, also shown above (AS3).

Not all addition silicone impression materials release hydrogen gas, and because it is not known which do, it is recommended that one wait at least 30 minutes for the setting reaction to be completed before the gypsum models and dies are poured. Epoxy dies should not be poured until the impression has stood overnight. The difference in the delay with gypsum and epoxy is that gypsum products have much shorter setting times than epoxy die materials. Some products contain a hydrogen absorber such as palladium, and gypsum and epoxy die materials can be poured against them as soon as practical. Examples of high-strength stone poured after 15 minutes against addition silicone, with and without a hydrogen absorber, are shown in Figure 12-12.

Latex gloves have been shown to adversely affect the setting of addition silicone impressions. Sulfur compounds that are used in the vulcanization of latex rubber gloves can migrate to the surface of stored gloves. These compounds can be transferred onto the prepared teeth and adjacent soft tissues during tooth preparation and when placing tissue retraction cord. They can also be incorporated directly into the impression material when mixing two putties by hand. These compounds can poison

the platinum-containing catalyst, which results in retarded or no polymerization in the contaminated area of the impression. Thorough washing of the gloves with detergent and water just before mixing sometimes minimizes this effect, and some brands of gloves interfere with the setting more than others. Vinyl and nitrile gloves do not have such an effect. Residual monomer in acrylic provisional restorations and resin composite cores has a similar inhibiting effect on the set of addition silicone materials. The preparation and adjacent soft tissues can also be cleaned with 2% chlorhexidine to remove contaminants.

Polyethers are supplied in low-, medium-, and heavy-body consistency. The base paste consists of a long-chain polyether copolymer with alternating oxygen atoms and methylene groups (O—[CH₂]_n) and reactive terminal groups (see polyether 1 [PE1]). Also incorporated are a silica filler, compatible plasticizers of a nonphthalate type, and triglycerides. In the catalyst paste, the former 2,5-dichlorobenzene sulfonate was replaced by an aliphatic cationic starter as a cross-linking agent. The catalyst also includes silica filler and plasticizers. Coloring agents are added to base and catalyst to aid in the recognition of different material types. Examples of polyether impression materials are shown in Figure 12-9.

The reaction mechanism is shown (PE2) in a simplified form. The elastomer is formed by cationic polymerization by opening of the reactive terminal rings. The backbone of the polymer is believed to be

a copolymer of ethylene oxide and tetramethylene oxide units. The reactive terminal rings open under the influence of the cationic initiator of the catalyst paste and can then, as a cation itself, attack and open additional rings. Whenever a ring is opened, the cation function remains attached, thus lengthening the chain (see PE3). Because of the identical chemical base, all polyether consistencies can be freely combined with each other. A chemical bond between all materials develops during curing.

The viscosity of materials 45 seconds after mixing is listed in Table 12-3. As expected, the viscosity increases for the same type of material from low to high consistencies. Viscosity is a function of time after the start of mixing.

A shearing force can affect the viscosity of polyether and addition silicone impression materials, as was mentioned in the section on impression techniques. This effect is called shear thinning or pseudoplasticity. For impression materials possessing this characteristic, the viscosity of the unset material diminishes with an increasing outside force or shearing speed. When the influence is discontinued, the viscosity immediately increases. This property is very important for the use of monophase impression materials, and is illustrated for polyether in Figure 12-13. In the case of polyether, shear-thinning properties are influenced by a weak network of triglyceride crystals. The crystals align when the impression material is sheared, as occurs when mixed or flowing through a syringe tip. The microcrystalline triglyceride network ensures that the polyether remains viscous in the tray or on the tooth but flows under pressure. This allows a single or monophase material to be used as a low- and medium-consistency material. Cooling of the pastes results in substantial viscosity increase. Before using, pastes have to be brought to room temperature.

The effect of shear rate (rotational speed of the viscometer) on the viscosity of single-consistency (monophase) addition silicones is shown in Figure 12-14. Although all products showed a decrease in viscosity with increasing shear rate, the effect was much more pronounced for two products, Ba and Hy, with about an eightfold to eleven-fold decrease from the lowest to the highest shear rate. The substantial decrease in viscosity at high shear stress, which is comparable with the decrease during syringing, permits the use of a single mix of material, with a portion to be used as syringe material and another portion to be used as tray material in the syringe-tray technique.

The working and setting times of addition silicone and polyether impression materials are listed in Table 12-3. In general, for a given class of elastomeric impression materials by a specific manufacturer, the working and setting times decrease as the viscosity increases from low to high. Polyethers show a clearly defined working time with a sharp transition into the setting phase. This behavior is often called snap-set. This transition from plastic condition into elastic properties is rather short compared with older addition silicones, which was shown in investigations of rheological properties of setting materials (Figure 12-15).

Note that the working and setting times of the elastomeric impression materials are shortened by increases in temperature and humidity; on hot, humid days this effect should be considered in the clinical application of these materials.

The initial (or working) and final setting times can be determined fairly accurately by using a penetrometer with a needle and weight selected to suit these materials. The Vicat penetrometer, as shown in Figure 12-16, with a 3-mm diameter needle and a total weight of 300 g, has been used by a number of investigators. A metal ring, 8 mm high and 16 mm in diameter, is filled with freshly mixed material and placed on the penetrometer base. The needle is applied to the surface of the impression material for 10 seconds, and a reading is taken. This is repeated every 30 seconds. The initial set is that time at which the needle no longer completely penetrates the specimen to the bottom of the ring. The final set is the time of the first of three identical nonmaximum penetration readings. When the material has set, the elasticity still allows penetration of the needle, but it is the same at each application.

Addition silicone and polyether impression materials undergo shrinkage due to polymerization. The linear dimensional change between a die and the impression after 24 hours is listed in Table 12-3. The addition silicones have the smallest change, about −0.15%, followed by the polyethers at about −0.2%. The contraction is low for these two products because there is no loss of byproducts.

The rate of shrinkage of elastomeric impression materials is not uniform during the 24 hours after removal from the mouth. In general, about half the shrinkage observed at 24 hours occurs during the first hour after removal; for greatest accuracy, therefore, the models and dies should be prepared promptly, although in air the elastomeric impression materials are much more stable than hydrocolloid products.

The order in which the permanent deformation of the elastomeric impression materials is listed in Table 12-4 demonstrates that addition silicones have the best elastic recovery during removal from the mouth, followed by polyethers. A material with a permanent deformation of 1% has an elastic recovery of 99%.

The strain in compression under a stress of 0.1 MPa is a measure of the flexibility of the material. Table 12-4 illustrates that, in general, the low-consistency materials of each type are more flexible than the high-consistency elastomeric impressions. For a given consistency, polyethers are generally the stiffest, followed by addition silicones.

Flow is measured on a cylindrical specimen 1 hour old, and the percent flow is determined 15 minutes after a load of 1 N is applied. As seen in Table 12-4, silicones and polyethers have low values of flow.

Typical elastomeric impression materials apparently have no difficulty meeting the mechanical property requirements of ANSI/ADA specification No. 19 (see Table 12-6). Although the flow, hardness, and the tear strengths of elastomeric impression materials are not mentioned in the specification, these are important properties; they are also listed in Table 12-4.

Tear strength is important because it indicates the ability of a material to withstand tearing in thin interproximal areas and margins of periodontally involved teeth. The tear strengths listed in Table 12-4 are a measure of the force needed to initiate and continue tearing a specimen of unit thickness. As the consistency of the impression type increases, tear strength undergoes a small increase, but most of the values are between 2.0 and 3.9 kN/m. Values for very high consistency types are not listed because this property is not important for these materials. Higher tear strengths for elastomeric impression materials are desirable, but compared with the values for hydrocolloid impression materials of 0.3 to 0.7 kN/m, they are a major improvement.

Elastomeric impression materials 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, plots of creep compliance versus time describe the properties of these materials better than stress-strain curves. Creep-compliance time curves for low-consistency polysulfide, condensation silicone, addition silicone, and medium-consistency polyether are shown in Figure 12-17. The initial creep compliance illustrates polysulfide is the most flexible and polyether is the least flexible. 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; addition silicones and polyethers have the best elastic recovery.

The recoverable viscoelastic quality of the materials is indicated by differences between the initial creep compliance and the creep compliance value obtained by extrapolation of the linear portion of the curve to zero time. As a result, addition silicones have the lowest viscoelastic quality and require less time to recover viscoelastic deformation, followed by the polyethers.

The requirements of elastomeric impression materials are listed in Table 12-6. Except for the very high-viscosity products, all should reproduce a V-shaped groove and a 0.02-mm wide line in the elastomeric. The impression should be compatible with gypsum products so the 0.02-mm line is transferred to gypsum die materials. Low-, medium-, and high-viscosity elastomeric impression materials have little difficulty meeting this requirement.