Impression material

Elastomer Impression Materials. Dentistry requires impression materials that are easily handled and accurately register or reproduce the dimensions, surface details, and interrelationship of hard and soft oral tissues. Elexible, elastomeric materials are especially needed to register intraoral tooth stmctures that have undercuts. The flexibility of these elastomers allows their facile removal from undercut areas while their elasticity restores them to their original shape and size. 

An elastic impression material must be easily and quickly prepared set quickly to an elastic mass in the mouth not be harmful or cause discomfort to the oral tissues and flow to all areas without the need of excessive force. It also must copy detail accurately possess sufficient strength, toughness, and elasticity to resist permanent deformation when removed from the mouth not adversely affect the set properties of the cast material be capable of being... 

Reversible HydrocoUoids (Agar). The agar-based impression materials are thermally reversible, aqueous gels (230,231), that become viscous fluids in boiling water and set to an elastic gel when cooled below 35°C. The popularity of agar-based impression materials has diminished with the introduction of elastic impression materials such as alginate-based, polysulfide, silicone, and polyether impression materials, but agar [9002-18-0] materials are still used in substantial quantities. 

Impressions of inlay and crown preparations, and all gingival areas, are best obtained by filling the preparation or gingival area with impression material injected from a hypodermic syringe. This eliminates trapping air in the corners and recesses and gives a more faithful reproduction without nodules or other imperfections. 

The agar-based impression materials are used extensively for dupHcating casts. Frequendy, it is desired to retain the original model for reference and do the actual work on a dupHcate cast. Partial-denture fabrication requires that the original stone cast be dupHcated in an investment. For dupHcating, the agar-based impression material is usually diluted with water, boiled, cooled to the desired temperature, and carefully poured over the model to be dupHcated. 

Agar-based impression materials must have a compressive strength of at least 0.2 MPa (29 psi). They should have a strain in compression of 4—20% in stresses of 9.8-98 kPa (1.4—14.2 psi) per specification test method, and should not have a permanent deformation exceeding 3% after 12% strain is appHed for 30 seconds. 

Alginate impression materials are chemically reactive mixtures. AH factors that influence reaction rates are, therefore, important in the use of these materials, ie, correct proportioning temperature of the water, powder, and mixing equipment and spatulation rate and duration. 

Polysulfide Impression Materials. In 1953 the first nonaqueous, elastic dental impression material based on the room-temperature conversion of a Hquid polymer, a polyfunctional mercaptan (polysulfide), to a strong, tough, dimensionally accurate elastomer, was introduced. The conversion of the Hquid polymer to an elastic soHd has been achieved in most products by lead peroxide [1309-60-0]. Significant improvements in strength, toughness, and especiaHy dimensional stabiHty of the set polysulfide elastomers over the aqueous elastic impression materials made these materials popular. 

The polysulfide impression materials can be formulated to have a wide range of physical and chemical characteristics by modifying the base (polysulfide portion), and/or the initiator system. Further changes may be obtained by varying the proportion of the base to the catalyst in the final mix. Characteristics varied by these mechanisms include viscosity control from thin fluid mixes to heavy thixotropic mixes, setting-time control, and control of the set-mbber hardness from a Shore A Durometer scale of 20 to 60. Variations in strength, toughness, and elasticity can also be achieved. 

Only two types of systems have found appHcation ia dentistry. Lead peroxide is the curing agent most frequently used for the polysulfide polymers that serve as dental impression materials. Lead peroxide converts the Hquid polymer to an elastic soHd within a time short enough for oral appHcations. 

A.ddition Silicones. Perhaps the most important development ia the area of elastic impression materials has been the addition siloxane (or siHcone) system. Several reviews have been pubHshed on the materials (238,239). 

The siHcone impression materials are very compatible with gypsum products, give casts having exceUent hard surfaces, and can be electroplated with either copper or silver. However, the acidic copper sulfate bath gives more acceptable results. 

To improve the rheological properties and extend the very short working time, a simple polyester is kicluded as thinner. Mixing is easy, and dimensional change ki ak is less than 0.1% over several hours. Elastic recovery and reproduction of detail are exceUent. The elastomeric cycHc imine impression materials have a higher modulus of elasticity than the condensation siHcone or polysulfide mbbers, and are more difficult to remove from the mouth. The materials have relatively low tear strength and an equUibrium water sorption of 14% thus, polyether impression materials tear readily. Because of thek poor dimensional stabUity ki water, they should be stored ki a dry environment. 

A new type of impression material that has a polyether backbone with urethane—acryHc end groups and that cures free-radicaUy by visible light kradiation, has been developed (244). 

The ZOE impression paste is used in taking impressions of the mouth prior to constructing a denture. It is used as a corrective impression material after a preliminary impression has been taken (Phillips, 1982a). A preliminary impression lacks detail so it is necessary to take a secondary or corrective impression which is placed on a tray that has the contours of the preliminary impression. 

Elastomer impression materials, 8 327 Elastomer-modified epoxy resins, 10 436 Elastomer-modified epoxies, 10 375-376 Elastomer production, tetrahydrofurfuryl alcohol in, 12 279... 

Polyetherimide-polysiloxane multiblock copolymers, 24 716 Polyetherimides (PEI), 10 217—218 Polyether impression materials, 8 332-333 Poly(ether ketones) (PEK), 10 197-199 Polyether polyols, 25 455-456,464,468t, 470 propylene oxide polymerization to, 20 793-794, 812 Poly ethers, 12 663... 

In dentistry, silicones are primarily used as dental-impression materials where chemical- and bioinertness are critical, and, thus, thoroughly evaluated.546 The development of a method for the detection of antibodies to silicones has been reviewed,547 as the search for novel silicone biomaterials continues. Thus, aromatic polyamide-silicone resins have been reviewed as a new class of biomaterials.548 In a short review, the comparison of silicones with their major competitor in biomaterials, polyurethanes, has been conducted.549 But silicones are also used in the modification of polyurethanes and other polymers via co-polymerization, formation of IPNs, blending, or functionalization by grafting, affecting both bulk and surface characteristics of the materials, as discussed in the recent reviews.550-552 A number of papers deal specifically with surface modification of silicones for medical applications, as described in a recent reference.555 The role of silicones in biodegradable polyurethane co-polymers,554 and in other hydrolytically degradable co-polymers,555 was recently studied. 

Polyelectrolyte-based dental cements or restorative materials include zinc polycarboxylates, glass ionomers, a variety of organic polyelectrolyte adhesives as well as alginate-based impression materials. Dental cements are primarily used as luting (cementing) agents for restorations or orthodontic bands, as thermal insulators under metallic restorations, and as sealents for root canals, pits and fissures. They are also sometimes used as temporary or permanent (anterior) restorations. For further introduction to dental materials the reader is referred to standard texts [122,123]. 

A variety of organic adhesives which are capable of forming strong bonds between a polymeric (acrylate) restoration and the hydrophilic tooth material have recently been developed. A number of these monomers, which possess a pendent ionizable group, are polymerized in the mouth to form an adhesive layer. Alginates, which are used as impression materials, are formed by the reaction of the sodium salt of anhydro-beta-d-mannuronic acid with calcium sulfate. Calcium ions crosslink the linear polymer to form a gel. This reaction is carried out inside the mouth, and the gel formed retains the shape of the oral cavity. 

Impressive materials properties can also be achieved by the solvent-specific aggregation properties of appropriately designed amphiphilic materials. A brief account on the organogel or hydrogel formation by amphiphilic molecules is now presented, citing a few examples from the current literature. Both... 

Metathesis-curable compositions of polyethylene glycol monomers, (V), using ruthenium derivatives, (VI), were prepared by Angeletakis [5] and used as dental impression materials and orthodontic appliances. 

The taking of tooth impressions to assess wear in vivo has become popular in recent years, as silicone impression materials are available with excellent detail reproduction and good dimensional stability. Several authors [35-38] have reported the use of these materials to capture the dimensions of selected teeth (or teeth surfaces) over a period of time. The silicone impressions are scanned using a profilometry system, typically a laser profilometer, to build an electronic image of the tooth surface. Several scans of the same tooth or surface, taken over a period of time, can then be electronically overlaid and subtracted, to calculate the change in surface contour, or the volume of material removed. This technique can only work if the two scans are precisely overlaid. To avoid... 

---