![]() Some of this strain is non-recoverable, since the elastic properties of these materials are far from ideal. They can, typically, withstand about 700% tensile strain before tearing. ![]() The polysulphide elastomers have very good tear resistance. Setting times of 10 minutes or more are not uncommon, particularly for light-bodied materials. Setting commences immediately on mixing of the two pastes and is characterised by a gradual increase in viscosity and a rather slow development of elasticity. Properties: The setting characteristics of the polysulphides differ considerably from those of the alginates (Section 18.3). When the degree of cross-linking reaches a certain level the material develops elastic properties. As chain extension proceeds, the viscosity increases. The reaction is of the condensation polymerization type since one molecule of water is produced as a byproduct of each reaction stage. Some of these reactions result in chain extension and cross-linking as shown in Fig. Setting reaction: On mixing the two pastes, terminal and pendant thiol groups of the prepolymer chains undergo a reaction with lead dioxide. Table 19.1 Classification of elastomeric impression materials according to consistency. An adhesive is used to promote adhesion between the impression material and tray. The colour contrast between the two pastes is an aid to efficient mixing, which is continued until a homogeneous colour, with no streaks, is achieved. The colour of the catalyst paste is governed by the nature of the oxidizing agent used materials containing lead dioxide are normally dark brown. The base paste is normally white, due to the filler, and has an unpleasant odour caused by the high concentration of thiol groups. Three grades of paste are normally available to the practitioner – ‘light-bodied’, ‘regular-bodied’ and ‘heavy-bodied’, having increasing filler contents and viscosity values. The viscosity of the paste is governed by the quantity of filler incorporated by the manufacturer. For mechanistic purposes the structure of the pre-polymer is conveniently represented by the simplified structure given in Fig. The liquid polysulphide prepolymer in the base paste can be represented by the formula:Īlthough the actual structure is known to be slightly more complex than this and contains pendant thiol groups (-SH) in addition to the terminal groups shown in the structural formula. The composition of a typical material is given in Table 19.2. One paste is normally labelled base paste whilst the other is labelled catalyst paste. 19.2 PolysulphidesĬomposition: These materials are generally supplied as two pastes which are dispensed from tubes (see Fig. The techniques for using the combined high and low viscosity materials are described later in this chapter. Manufacturers often package their materials in a manner which reflects their preferred combination of high viscosity (type 0 or 1) and low viscosity (type 3) product. Types 0 and 1 are normally used in combination with a type 3 material whilst type 2 materials are normally used alone and are sometimes referred to as monophase elastomeric materials. The situation is clarified somewhat by consideration of the manner in which the materials are used. The situation is further complicated by the use of terms such as ‘soft putty’, which generally indicates a type 0 material, with a relatively low viscosity (high consistency disc diameter). It is the manufacturer who makes the choice in this situation. Hence, a material producing a disc of 31–34 mm diameter could be classified as type 0, 1 or 2. The specification limits defining consistency overlap significantly between types. Materials are categorized as types 0–3, as shown in Table 19.1. ![]() The consistency, defined by the average diameter of the resulting disc of the material, is related to the viscosity of the material at the time when the force is applied. This is measured by pressing 0.5 ml of mixed material between two flat plates using a force of 1.5 N. Apart from the chemical nature of the material, the primary method of classification is according to consistency. Silicone rubbers (addition curing type) Ī classification and some properties of these materials is given in the ISO Standard for Dental Elastomeric Impression Materials (ISO 4823).Silicone rubbers (condensation curing type).They were quick to gain acceptance in dentistry because they offered potential solutions to the two main problems associated with hydrocolloids – poor tear resistance and poor dimensional stability.įour types of elastomers are in general use: Synthetic elastomers were developed mainly for industrial applications but their potential in medicine and dentistry was quickly realized and they are now widely used as impression materials. Chapter 19 Elastic Impression Materials: Synthetic Elastomers 19.1 Introduction
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