19.3 Silicone rubbers (condensation curing)

Composition: These materials may be supplied as two pastes or as a paste and liquid. Whichever method of dispensation is used the principle of the setting reaction is similar and depends on the cross-linking of hydroxyl-terminated polydimethylsiloxane chains, brought about by an alkyl silicate cross-linking agent and a tin compound as catalyst. The ingredients required for this reaction to occur are reflected in the composition of a typical paste/liquid material, given in Table 19.4. These materials are very similar to the room temperature polymerizing silicones used as denture soft liners. Figure 19.4 gives the structural formula of the silicone prepolymer. The viscosity of the paste is controlled by the amount of inert filler, as in the case of the polysulphides. Light-bodied, regular-bodied, heavy-bodied and ‘putty’ materials are available. The latter is a paste of a very high viscosity and its availability denotes an important difference between the silicones and polysulphides.

Proportioning of the paste/liquid materials is by mixing a given volume of paste with a fixed number of drops of liquid. For paste/paste materials equal lengths of pastes are mixed together. A colour contrast between the pastes enables the operator to see when proper mixing has been achieved.

Setting reaction: On mixing the two components, either two pastes or paste and liquid, a reaction begins immediately in which the terminal hydroxyl groups of prepolymer chains react with the crosslinking agent under the influence of the catalyst (see Fig. 19.5). Each molecule of cross-linking agent may, potentially, react with up to four prepolymer chains causing extensive cross-linking. Each reaction stage also produces one molecule of ethyl alcohol as a byproduct. Cross-linking pro-

duces an increase in viscosity and the rapid development of elastic properties.

Properties: The setting characteristics of the silicone materials tend to be more favourable than those of the polysulphides. Setting times are generally shorter and elasticity is developed earlier.

The silicone impression materials are very hydrophobic and are readily repelled by water or saliva. As a result, it is necessary to dry areas of the mouth for which an accurate impression is required. If a dry field is not secured ‘blow holes’ are likely to occur in the impression as the material will fail to drive away the residual moisture.

The set material has adequate tear resistance for most purposes. A regular-bodied silicone material can undergo only about 300% extension before fracturing, (compared with 700% for polysulphides) but most of this strain is recoverable. The silicones have elastic properties which most closely approach the ideal of complete and instantaneous recovery following stretching or compression.

Many of the properties are related to the filler content of the pastes. The trends are identical to those given in Table 19.3 for polysulphides. In the case of the silicones an additional, very high viscosity or ‘putty’ paste exists which has even lower setting and thermal contraction values than the conventional heavy-bodied materials. It also has better dimensional stability.

Dimensional changes after setting, for condensation curing silicones, may be due to continued slow setting or due to loss of alcohol produced as a byproduct of the setting reaction. The latter effect produces a measurable weight loss which is accompanied by a shrinkage of the impression material. Dimensional changes of regular-bodied condensation silicones are slightly greater than those of regular-bodied polysulphides but are small compared to the changes which occur with

alginates. In order to obtain optimum accuracy, the models should be cast as soon as possible after recording the impression.

Silicone elastomers may be considered essentially non-toxic, despite the fact that they contain a heavy metal catalyst. The materials are extremely hydrophobic and are in the patient’s mouth for only a few minutes. The liquid component of the paste/liquid materials may be hazardous if not handled carefully. Accidental splashes may cause considerable irritation and blistering of the eyes.

Applications: For clinical applications of these materials see the end of Section 19.4.

The increased usage of additional curing materials (next section) has led to a gradual decline in the use of the more traditional condensation curing materials.

19.4 Silicone rubbers (addition curing)

Composition: These materials are supplied as two pastes. Each paste contains a liquid silicone prepolymer and filler and one of the pastes contains

a catalyst. One paste contains a polydimethylsiloxane prepolymer in which some of the methyl groups are replaced by hydrogen (Fig. 19.6a). The other paste contains a prepolymer in which some methyl groups are replaced by vinyl groups (Fig. 19.6b). One of the pastes contains a catalyst which is normally a platinum-containing compound such as chloroplatinic acid. Four viscosities are available depending on the amount of filler incorporated by the manufacturer.

Proportioning is carried out by extruding equal lengths of each paste onto the mixing pad. A good colour contrast between the pastes enables thorough mixing to be achieved.

A significant advance in recent years has been the availability of addition curing silicones in an auto mixed, cartridge format (Fig. 19.7). The two pastes are housed in separate compartments of the cartridge and are brought together and mixed in the nozzle during extrusion. The mixed material can be extruded into an impression tray or, for light-bodied material, into an impression syringe or directly into the mouth. Light-bodied and regular-bodied materials lend themselves readily to the cartridge extrusion system. Some manufacturers have also managed to package heavy-bodied and even ‘soft putty’ materials in this way. At least one manufacturer has taken automixing a stage further by producing an electrically driven mixing

Fig. 19.6 Structural formulae of the silicone prepolymers used in the two pastes of addition curing silicone impression materials. (a) One paste contains some Si—H groups. (b) The other paste contains some Si—CH=CH2 groups.

Fig. 19.7 This shows a typical example of a polyvinylsiloxane impression material. The two pastes to be mixed are in separate parts of a cartridge and the mixing takes place when the pastes are extruded through the nozzle. The mixed material can be extruded directly into an impression tray or directly around the teeth to be recorded.

device which is loaded with bulk quantities of material (sufficient for about 20 impressions). When required, mixed material is produced at the touch of a button.

Fig. 19.9 shows such a mixing device which, in principle, is a larger and more automated version of the hand-held cartridges (Fig. 19.7). Both

Fig. 19.8 Platinum-catalysed addition reaction which causes cross-linking of prepolymer chains.

systems suffer the drawback that the mixing nozzle is disposable and a new one is required for each mix and the discarded nozzle contains significant quantities of ‘wasted’ material.

Setting reaction: On mixing the two pastes a platinum- catalysed addition reaction occurs, causing cross-linking between the two types of siloxane prepolymer (Fig. 19.8). It is noteworthy that the reaction does not involve the production of byproducts although it has been reported that these materials occasionally evolve hydrogen. Some manufacturers recommend that pouring of the cast is delayed until the evolution of hydrogen is complete in order that the cast surface does not become pitted. The mechanism of hydrogen release is unclear but may involve reaction of the platinum catalyst with moisture. Cross-linking produces an increase in viscosity coupled with the development of elastic properties.

Properties: In most respects, the addition curing silicone rubbers have properties similar to those of the condensation type. They have adequate setting characteristics and tear resistance coupled with near ideal elasticity. The combined use of

Fig. 19.9 This shows the bulk packaging of an elastomeric impression material. The machine can provide numerous mixes of material from larger cartridges than those seen in Fig. 19.7. The pastes are extruded through the mixing nozzle using an electrically powered motor inside the device. The mixed material can be extruded directly into an impression tray which is held underneath the nozzle. The nozzle itself is disposable and is replaced with a fresh nozzle for each individual mix.

putty and light-bodied materials enables accurate impressions to be recorded. The most significant difference between the addition curing and the condensation curing materials is in their relative dimensional stability. The production of little or no byproduct in the cross-linking reaction of the addition curing material results in a very stable impression.

Silicone elastomers are inherently hydrophobic in nature – a characteristic which can cause imperfections in impressions if the area to be recorded cannot be thoroughly dried. The mixed silicone material is repelled by moisture and this can result in blow holes in the impression. Attempts have been made to overcome this problem by the incorporation of surface-active agents into the materials in order to make the materials more hydrophilic. The newer materials, which are described as hydrophilic addition silicones, should strictly be described as ‘less hydrophobic’. Nevertheless, the improvement in surface characteristics can result

in an impression with better fine detail reproduction and fewer blow holes. An alternative approach to solving this problem has been through the use of a surface-active spray which is used to coat the surface of the hard and/or soft tissues prior to recording an impression.

The use of hydrophilic materials or surfaceactive sprays helps not only to improve the affinity between the impression materials and the oral tissues during recording of the impression, but also in the compatibility with the water-based gypsum model material. This ensures that detail recorded in the impression is transferred through to the model.

Clinical applications: There is virtually no difference between the addition and condensation cured material in terms of their handling characteristics. The addition cured materials are preferred on the basis of their greater dimensional stability. Both are available in a range of viscosities from lightbodied materials used to record accurate surface detail on prepared tooth surfaces, through medium viscosities, commonly used as a monophase material during either crown and bridgework or denture manufacture, and heavy-bodied materials used to support light body in stock trays for crown and bridge impressions, to putties which are now available in both soft and hard format.

The light-bodied materials can record very accurately the surface detail of tooth preparations but have inadequate dimensional stability to maintain their shape during the production of working casts. They are syringed into place onto the prepared tooth to provide a surface wash giving high quality detail. A more heavily filled material is then used to support the wash, either a heavy body or a putty.

Fig. 19.10 shows a putty-wash type impression and a section through such an impression showing the relative thickness of the two components (putty and wash). Note the similarity of this technique with that described earlier for polysulphides (Fig. 19.4). A feature of these systems is the clear colour contrast between the high viscosity and low viscosity components.

Heavy body and wash or putty and wash impressions can be performed as a one-stage procedure during which both viscosities of material are mixed simultaneously. The clinician then syringes the light-bodied material around the preparation/teeth whilst an assistant places the

(a)

(b)

Fig. 19.10 (a) Diagram to show a two component elastomeric impression, with a low viscosity light body material adjacent to the preparation (yellow arrowed) and the bulk of the impression in a material with greater viscosity (green). (b) A cross section through an impression for a crown on a molar tooth, the thickness of the light body (arrowed) material (in orange in this image) varies considerably with its position on the preparation but is present over the whole surface of the impression. Note similarity with Fig. 19.4.

heavier material into a stock tray. The loaded tray is then inserted into the mouth and the two viscosities blend together and set. Obviously this approach is ergonomically attractive. There are, however, concerns about the accuracy of the onestage putty wash impressions recorded in some plastic stock trays. The viscosity of the putties is such that on seating the impression tray into the mouth the sides of the tray flex outward. When the impression is removed from the mouth they then rebound inwards, distorting the impression. Unfortunately the distortion is not uniform, as an impression of a circular object when taken in this