way results in a die which is ovoid in shape with narrowing bucco-lingually. This occurs with both soft and hard putties but does not occur when the heavy/light body technique is used.

There are two solutions to this problem. Firstly, rigid metal stock trays can be used which do not flex under seating loads. The disadvantage of metal trays is that they are not disposable and hence need to be cleaned and sterilized before reuse which is a laborious task. Alternatively an initial putty impression can be recorded with a polythene spacing sheet between the putty and the teeth. This is allowed to set fully and then the light-body impression is recorded as a second stage. The spacing sheet is required to avoid the large hydraulic pressures which would develop if a light wash was placed inside a well fitting putty impression. These pressures would cause distortion in their own right.

Medium viscosity materials can be used in a similar way to the monophase polyethers, but their dimensional accuracy in stock trays is not quite as good as when using a two-viscosity technique.

There are two problems with the addition cured silicone rubbers. First they are markedly hydrophobic. As a consequence tooth surfaces into which the light body material is being syringed have to be dry. Any moisture contamination will simply result in a failure to record accurately the surface of the prepared tooth. Furthermore, if the tooth is in the upper arch the light body will tend to drip off the preparation under the influence of gravity which is very frustrating. As previously stated the more recently developed materials incorporate wetting agents to lower the surface tension between the impression material and the tooth. One manufacturer has gone so far as to provide a separate aerosol containing the wetting agent to be sprayed on the tooth after drying prior to placement of the light-body material. These developments have improved matters somewhat, but the material remains hydrophobic.

Second, the platinum catalyst system in the addition cured materials is relatively easy to poison, inhibiting the set of the material. Problems can be encountered with the plasticizers in rubber, from gloves or rubber dam, glove powders, some haemostatic agents, particularly those based on ferric salts and freshly placed methacrylate-based materials, including composites, compomers and light-cured glass ionomer cements. These latter

products can be used to patch up a crown preparation if there is some caries present or an existing restoration falls out during preparation. However, if this is done then the impression should not be recorded in the same visit, as the surface of the impression material adjacent to the patch will not set. The other agents can usually be either avoided or washed/cleaned off the prepared tooth surface prior to recording the impression. Obviously rubber gloves should not be used to protect the hands of somebody mixing a putty impression; vinyl or nitrile gloves are a viable alternative.

There are no reports in the literature of patients suffering an allergic response to these materials although they can induce a contact dermatitis in dental nurses who mix impression putties without wearing suitable gloves.

The standard disinfection regime of a 10 minute immersion in sodium hypochlorite will have no effect on the dimensional stability of these materials. They are also sufficiently stable that the more rigorous processes associated with managing patients with an established cross-infection risk (i.e. prolonged immersion in glutaraldehyde solution) can be undertaken without a marked effect on their accuracy (see Appendix 1).

19.5 Polyethers

Composition: These materials are normally supplied as two pastes. (see Fig. 19.11) The ‘base’ paste, containing the prepolymer and inert filler is supplied in a large tube. The ‘catalyst’ paste, containing a reaction initiator together with pasteforming oils and fillers, is supplied in a second, but much smaller tube. The composition is summarized in Table 19.5. Simplified structural formulae for the imine-terminated, ether initiator are given in Fig. 19.12. The group X shown in the structural formula of the polyether prepolymer (Fig. 19.12) has the structure:

− CO2 − [CH− (CH2)n− O]m − CH(CH2)n − CO2−

where R represents a hydrogen atom or alkly group. The values of m and n are such that the molecular weight of the difunctional imino molecule is about 4000. The polyester unit (shown in the square brackets) is typically produced by copolymerisation of ethylene oxide and tetrahydrofuran. The materials are generally supplied in

Fig. 19.11 This shows a typical polyether impression material. The two pastes have been extruded on to the mixing pad ready for mixing using a metal bladed spatula. This type of material is also available in bulk auto-mixed format similar to that seen in Figure 19.9.

Fig. 19.12 (a) Simplified structural formula of the imineterminated ether prepolymer present in the base paste of a polyether impression material. X represents repeating units of ethers. (b) Structural formula of an aromatic sulphonic acid ester. This is the main active ingredient of the catalyst paste of a polyether impression material. Group R is an alkyl group, for example, butyl.

only one viscosity, equivalent to the regularbodied materials of other elastomers. The manufacturers do supply a diluent oil, however, which can be used to produce a paste with viscosity akin to that of a light-bodied material.

The two pastes are proportioned by volume. Equal lengths of paste are extruded onto a mixing pad giving a base paste/catalyst paste volume ratio of about 8 : 1. The good colour contrast between the pastes aids mixing.

The materials are also available in bulk automixed format similar to that shown in Figure 19.9.

Setting reaction: When the two pastes are mixed together a cationic, ring opening addition polymerisation occurs. The ionized form of the sulphonic acid ester provides the initial source of

cations and each stage of the reaction involves the opening of an epimine ring and the production of a fresh cation, as illustrated in Fig. 19.13.

Distinct activation, initiation and propagation stages may be identified in the reaction as shown. The reaction is of the addition type with no byproduct being produced. Since each prepolymer molecule has two reactive epimine groups, individual propagation reactions may produce simple chain lengthening or cross-linking. As the reaction proceeds, the viscosity increases and eventually a relatively rigid cross-linked rubber is produced.

Properties: The polyether materials have adequate tear resistance and elastic properties approaching those of the silicones. They are relatively rigid when set and considerable force may be required

Fig. 19.13 Schematic representation of the cationic, ring opening polymerisation involved in the setting of polyether impression materials.

to remove the impression after setting, particularly when the undercuts are severe.

The accuracy of polyether impressions compares favourably with other regular-bodied elastomers. The lack of heavy-bodied and putty pastes, however, precluded the use of techniques using combined viscous/fluid pastes which are commonly used with other elastomers to optimize accuracy.

The manufacturers of polyether materials have recently introduced products having a range of viscosities. These enable the use of combined

heavyand light-bodied techniques described for polysulphides and silicones. This enables precise impressions to be recorded using stock impression trays.

Under conditions of low relative humidity, the polyether materials have very good dimensional stability. This is related, primarily, to the fact that the material contains no volatile constituents and sets by an addition reaction which produces no volatile byproducts. The set material is relatively hydrophilic and absorbs water under conditions of high humidity. This causes the impression material to swell and distort. The use of polyether materials should therefore be avoided in climates where humidity is high and where efficient air conditioning is not available.

Clinical applications: Commonly these materials are used as a monophase where a single viscosity of material is used for both the bulk of the impression in the tray and to be syringed around the prepared teeth in the mouth. The polyethers are sufficiently dimensionally stable on setting to allow this approach when using a stock impression tray.

Polyether materials are hydrophilic, consequently they can record an accurate impression when it is difficult to achieve perfect moisture control.

The greatest disadvantage from a clinical standpoint is the rigidity of these materials when set. This can make removal of impressions very difficult in dentate patients when there are marked undercuts present around the teeth, for example when there has been gingival recession exposing root surfaces with open embrasure spaces between the teeth or where bridgework is present elsewhere in the mouth. It is sensible to block out such undercut areas with wax prior to recording the impression if the operator is not to risk being unable to remove the impression or, in extreme cases, removing existing bridgework or even extracting teeth when the impression is removed.

The rigidity of these materials is used to good effect when recording impressions for dental implants. Whilst clinicians attempt to place implants parallel to each other this is not always possible. The transmucosal elements of some implants have a keyway on their upper surface over which a precision machined collar should fit and then be screwed to the implant. The impression process for this style of implant involves

attaching brass dummies of the collar to the surface of the implant with long screws. The brass dummies have undercut areas within their superstructure which allows them to be engaged by an overlay impression. However, this impression must exhibit high levels of rigidity to ensure that the brass analogues do not move within the impression once removed from the mouth. The implants are rigid within the base of the jaws so there is no leeway for movement of the analogues in the impression. If the implants are parallel to each other then impression plaster can be used for this purpose. However, when the fixtures are not parallel to each other a polyether is the material of choice for these locating impressions.

Allergic reactions have also been associated with the use of polyether impression materials, usually to the sulphonic acid catalyst system.

A ‘standard’ disinfection routine of 10 minutes immersion in sodium hypochlorite is unlikely to have a deleterious effect on the accuracy of these materials, although longer periods of immersion in water will result in water uptake and associated dimensional change (see Appendix 1).

19.6 Comparison of the properties of elastomers

Many of the clinically important properties of elastomeric impression materials are included as requirements in the International Standard (ISO 4823). These requirements are reproduced in Table 19.6. The setting characteristics of materials are important in determining ease of manipulation and patient discomfort levels. Materials should ideally possess sufficient working time for mixing, loading the tray (or syringe) and placing in the mouth. There should be a distinctive transition

from plastic to elastic behaviour which enables the dentist to judge when the material is set and safe to remove from the mouth without distortion. The transition should occur rapidly to reduce the time required for the dentist to hold the impression tray still and reduce patient discomfort. During the working time phase the material should remain plastic and mouldable whilst at the time of removal from the mouth the material should ideally be perfectly elastic. Methods of determining working and setting times do not always address these facts. Measurement of changing viscosity, for example, may not be adequate for predicting developed elasticity. As a general rule polysulphides take longer to set than the other materials whilst the polyethers have a very distinctive transition from plastic to elastic behaviour which can be helpful in estimating the earliest safe time for removal from the mouth.

The initial accuracy of any material is related in part to its ability to reproduce fine lines on a test block. This is more closely related to the viscosity of the material than to chemical nature (assuming the area to be recorded is dry). It is seen that type 0 materials (putty type) must be able to reproduce a line 0.075 mm wide whereas a type 3 material (lightbodied) must be able to reproduce a line only 0.020 mm wide. In addition to the ability to reproduce fine detail accuracy also depends on dimensional changes which occur during or immediately after setting. The validity of such tests of accuracy and detail reproduction must be questioned as the recording of detail on a dry block of metal is unlikely to reflect the ease with which detail on a relatively moist oral structure can be captured. The silicones tend to be relatively hydrophobic although some materials now contain additives to improve hydrophilicity. On the other

hand polyether materials are more inherently hydrophilic. Recording detail of moist surfaces with hydrophobic impression materials requires attention being paid to rigorous drying regimes prior to recording the impression.

Polymerisation shrinkage varies within the range 0.4–1.0% for regular viscosity materials. These values are greater for light-bodied products, and lower for heavy-bodied or putty products. A further dimensional change occurs when the impression cools from mouth temperature to room temperature. Values of the coefficient of thermal expansion are quite large for elastomers (190–300 × 10−6 ºC−1 for regular-bodied materials) and this can result in a dimensional change of 0.1–0.3%. These values are greater for lightbodied materials and smaller for heavy-bodied and putty materials.

The value of strain in compression gives an indication of the stiffness of the set material. It is determined by measuring compression under a stress of 0.1 MPa. The higher viscosity materials (types 0 and 1) tend to be stiffer than fluid materials (types 2 and 3), although it is clear from Table 19.6 that there are significant differences in stiffness within the type 0–3 categories (i.e. 2–20% for type 2). This reflects significant differences for the different types of material. In general, the polyether materials tend to be much stiffer than the other products whilst the polysulphides tend to be less stiff than the other products. The values of the limits of strain in compression required by the ISO Standard illustrate the difficulty of standardizing different types of materials within one document, i.e. the lower values of strain in the acceptance range are there to accommodate the polyether materials whilst the higher values accommodate the polysulphide materials. The significance of the value of strain in compression is that it gives an indication of the ease with which a set impression can be removed (less stiff material is easier) and an indication of the possibility that teeth may be fractured from models (greater stiffness gives a greater danger of fracture). For stiff materials it is important to use them in great enough thickness to allow sufficient deformation to occur under load.

The value of recovery from deformation (Table 19.6) gives an indication of the elastic characteristics of the material. It is determined by compressing a cylindrical sample by 30% of its height for one second at one minute after the end of its

setting time (the time recommended by the manufacturer for removing the material from the mouth). The recovery is measured 40 seconds after removing the compressing load. All elastomeric impression materials are required to undergo at least 96.5% recovery in this test. In practice, most silicone and polyether materials give 99– 100% elastic recovery and the lower limit of 96.5% recovery is set in order to accommodate the polysulphide materials. The latter products are markedly viscoelastic in nature and their recovery from deformation depends significantly on the time for which the deforming load is applied (i.e. the time the material is under stress).

The maximum dimensional change permitted by the ISO Standard is 1.5% for all materials. This is measured by recording the distance between two lines on a metal test block and comparing the measurement with an equivalent measurement in an impression of the test block which has been stored for 24 hours. The change in dimensions is negligible for polyether and addition silicone products and more significant for the other materials – particularly for condensation curing silicones. The dimensional change of the latter materials can be correlated with a weight loss which occurs on standing. This is caused by loss of ethyl alcohol which is formed as a byproduct of the setting reaction. A small dimensional change (and weight loss) can be measured for polysulphides. This may be due to loss of water which is formed as a byproduct. The final major requirement of the ISO Standard is that the fine detail which is recorded in the impression can be retained in the gypsum cast.

Two approaches have been used to assess resistance to tearing. One involves measurement of tensile strength, the other involves measurement of elongation to break. Both approaches can be considered less than ideal and this probably explains why there is no test for tear resistance in the ISO Standard. Polysulphide materials invariably give the highest elongation at break (500% or more for regular-bodied material) but very little of this is ‘usable’ elongation as much of it will result in permanent deformation. On the other hand, the addition silicones give the highest values of tensile strength (2–5 MPa for regular-bodied materials). The strength of these products can be influenced by the presence of pre-existing notches in the material. Perhaps a more acceptable way to compare materials is, therefore, to use a specimen