Сraig. Dental Materials

352 Chapter 12 IMPRESSION MATERIALS

material, the tray containing the higher-viscosity material is placed in the mouth. In this manner, the more-viscous tray impression material forces the lower-viscosity material to flow into fine aspects of the areas of interest. Because they are both mixed at nearly the same time, the materials join, bond, and set together. After the materials have set, the tray and the impression are removed. An example of an impression for a bridge using this procedure is shown in Fig. 12-16.

In the single-viscosity or monophase technique, impressions are often taken with a medium-viscosity impression material. Additionsilicone and polyether impression materials are well suited for this technique because both have a capacity for shear thinning. As described in Chapter 4, pseudoplastic materials demonstrate a decreased viscosity when subjected to high shear rates such as occurs during mixing and syringing. When the medium viscosity material is forced through an impression syringe, the viscosity is reduced, whereas the viscosity of the same material residing in the tray is unaffected. In this manner, such materials can be used for syringing and for trays, as previously described for the simultaneous, dual-viscosity technique. The mechanism for shear thinning is discussed in the later section on the viscosity of impression materials.

The putty-wash technique is a two-step impression procedure whereby a preliminary impression is taken in highor putty-consistency material before the cavity preparation is made. Space is provided for a low-consistency material by a variety of techniques, and after cavity preparation a low-consistency material is syringed into the area and the preliminary impression reinserted. The lowand high-consistency materials bond, and after the low-consistency material sets, the impression is removed. This procedure is sometimes called a wash technique. The puttyconsistency material and this technique were developed for condensation silicones to minimize the effects of dimensional change during polymerization. Most of the shrinkage during polymerization takes place in the putty material when the preliminary impression is made, confining final shrinkage to the thin wash portion of

Fig. 12-16A elastomeric impression of a maxillary anterior bridge case. Dark material is of a low or injection consistency, and light material of a high or tray consistency. Note that the palate is omitted from the tray to facilitate removal of the impression.

the impression. Care must be taken so the wash material can freely escape via vents in the putty material when the wash impression is made. If not, the wash material can compress the putty in the second-stage impression, inducing permanent distortion and inaccuracies to the impression. The putty-wash technique was extended to addition silicones after their introduction, even though their polymerization shrinkage is significantly lower.

Manufacturers add coloring agents to the accelerator and/or base as an aid in determining the thoroughness of the mix. Normally a different color is used for each consistency of a particular product line so one can distinguish the wash (low) consistency from the tray consistency in the set impression. Retarders may be added as well to control working and setting time of the products.

COMPOSITION AND REACTIONS

The next four sections describe the general composition and setting reactions of polysulfide, condensation silicone, addition silicone, and polyether impression materials. The following section describes their physical properties, rather than presenting the information material by material.

BASE

Polysulfide polymer

Titanium dioxide, zinc sulfate, copper carbonate, or silica

ACCELERATOR

Lead dioxide

Dibutyl or dioctyl phthalate Sulfur

Other substances such as magnesium stearate and deodorants

This approach permits a more direct comparison of the various types and their properties.

Polysulfide Polysulfide impression materials are supplied as two pastes in collapsible tubes, one labeled base and the other labeled accelerator or catalyst. A typical list of ingredients and their concentrations is given in Table 12-6. The polysulfide polymer has a molecular weight of 2000 to 4000 and terminal and pendant mercaptan groups (-SH). The terminal and pendant groups of adjacent molecules are oxidized by the accelerator to produce chain extension and crosslinking, respectively. This reaction can be represented diagrammatically as shown in the equation on p. 354. The reaction results in a rapid increase in molecular weight, and the mixed paste is converted to a polysulfide rubber. The reaction is only slightly exothermic, with a typical increase in temperature of 3" to 4" C. Although the mixes set to a rubber consistency in about 10 to 20 minutes, polymerization continues, and the properties change for a number of hours after the material sets. Cross-linking is used to reduce the permanent deformation (increase the elastic recovery) of the set material under compression or extension during removal from the mouth.

The ingredients and their weight percent may vary from one product to another. In general, the

weight percent of the filler in the base paste increases from low to medium to high consistencies. The particle size of the fillers is about 0.3 p m Although the most common active ingredient in the accelerator is lead dioxide, some magnesium oxide may also be present. Whitening agents cannot cover the dark color of the lead dioxide; thus these pastes range from dark brown to gray-brown. Other oxidizing agents such as hydrated copper oxide, Cu(OHj,, have been used as a substitute for lead dioxide, producing a green mix.

Condensation Silicone Condensation silicones are supplied as a base and an accelerator. The base contains a linear silicone called a polydinzethylsiloxane, which has reactive terminal hydroxyl groups. Fillers may be calcium carbonate or silica having particle sizes from 2 to 8 ym, and in concentrations from 35% for low consistencies to 75% for puttylike consistencies. The accelerator may be a liquid that consists of stannous octoate suspension and alkyl silicate, or it may be supplied as a paste by adding a thickening agent. The reaction proceeds as mentioned, producing a three-dimensional network with the liberation of ethyl alcohol and an exothermic temperature rise of about 1" C. The polymerization accompanied by the release of the byproduct causes a shrinkage that is greater in the low consistency than in the puttylike consistency. In the product shown in Fig. 12-17, the shrinkage has been reduced by having only two reactive groups on the cross-linking agent, thus only half the amount of byproduct is formed. The two-step putty-wash impression technique also reduces polymerization shrinkage. The accelerator does not have unlimited shelf life, because the stannous octoate may oxidize and the ortho-ethyl silicate is not entirely stable in the presence of the tin ester.

Addition Silicone The addition type is available in extra low, low, medium, heavy, and very heavy (putty) consistencies. A representative product line of addition silicones is shown in Fig. 12-18. The base paste of this class of impression materials contains a moderately low-

molecular weight polymer (polymethylhydrosiloxane) with more than three and up to ten pendant or terminal hydrosilane groups per molecule (see formulas at top of p. 356 and 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

356 Chapter 12 IMPRESSION MATERIALS

Pendant hydrosilane groups

CH3

I

-0-Si-O-

I

H

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 on p. 357 (AS2).

Polymethylhydrosiloxane

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 on p. 357 (AS3). Another possible source of hydrogen gas is a side reaction of the Si-H units of the polymethylhydrosiloxanewith each other, under the influence of the platinum catalyst,also shown on p. 357 (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

Terminal hydrosilane groups

CH3

I

-0-Si-H-

I

CH3

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 15minutes against addition silicone, with and without a hydrogen absorber, are shown in Fig. 12-19.

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 gloves do not have such an effect. The preparation and adjacent soft tissues can also be cleaned with 2% chlorhexidine to remove contaminants.

Polyether Polyethers are supplied in low, medium, and heavy-body consistency, and the three mixing systems previously described are available for polyethers. The base paste consists of a long-chain polyether copolymer with alternating oxygen atoms and methylene groups (0-[CH,],) and reactive terminal groups (PE1, p. 358). Also incorporated are a silica filler, compatible plasticizers of a non-phthalate 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.

I

The catalyst also includes a 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 Fig. 12-14.

The reaction mechanism is shown above (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 (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.

SETTING PROPERTIES

Typical values of the setting properties of elastomeric impression materials are presented in Table 12-7. The temperature rise in typical mixes of impression materials was pointed out in the previous section, but Table 12-7 illustrates that the temperature rise is small and of no clinical concern.

>Copolymer -

Copolymer -

Viscosity The viscosity of materials 45 seconds after mixing is listed in Table 12-7. As expected, the viscosity increases for the same type of material from low to high consistencies. Viscosity as a function of time after the start of mixing is shown in Fig. 12-20 for mixes stored at 25" C. The most rapid increase in viscosity with time occurred with silicones and polyether materials, with the latter increasing slightly more rapidly than the former.

Attention must be paid to proper mixing times and times of insertion of the impression material into the mouth if the materials are to be used

to their best advantage. For example, lowconsistency polysulfide injected or placed in the mouth at 5.5 minutes would have the same viscosity as medium-consistency polysulfide at 3 minutes. Similarly,a medium-consistency polysulfide at 4 minutes would have the same viscosity as a high-consistency material at 2 minutes.

A shearing force can affect the viscosity of polyether and 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 Fig. 12-21. 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

Fig. 12-20Viscosity of elastomeric impression materials after mixing at 25' C. Polysulfide light, 0; polysulfide regular, x; polysulfide heavy, +; condensation silicone, A; addition silicone, 0; polyether, 0.

(Adapted from Herfort TW, Gerberich W, Macosko CW et al:

J Prosthet Dent 38:396, 1977.)

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 lowand mediumconsistency 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 Fig. 12-22. 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 elevenfold 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

Rotational speed (rpm)

Fig. 12-22 Viscosity in centipoise as a function of shear rate (rotational speed of the viscometer) for five single-consistency addition-silicone impression materials. A rotational speed of 0.5 rpm would represent a shear rate comparable with that observed when placing the material in a tray, and a speed of 10 rpm would represent a shear rate comparable with that experienced when syringing the material.

(From Kim KN, Craig RG, Koran A, Ill: J Prosthet Dent

67:794, 1992.)

used as syringe material and another portion to be used as tray material in the syringe-tray technique.

Working and Setting Times The working and setting times of elastomeric impression materials are listed in Table 12-7. Polysulfides have the longest times, followed by silicones and polyethers. 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

by the structural triglycerides, whereas the polymerization of copolymer chains thereafter provides the quick increase in viscosity as the material sets.

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 addition silicones, which was shown in investigations of rheological properties of setting materials (Fig. 12-23).

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 Fig. 12-24, 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 non-maximum penetration readings. When the material has set, the elasticity