Сraig. Dental Materials

Fig. 12-24Vicat penetrometer used to determine setting time of impression materials and other restorative materials.

still allows penetration of the needle, but it is the same at each application.

Dimensional Change on Setting The impression material undergoes a dimensional change on setting. The major factor for contraction during setting is cross-linking and rearrangement of bonds within and between polymer chains. Additional shrinkage can arise from the loss of volatile components such as water in polysulfides and ethanol in condensation silicones. Impressions can expand if water sorption takes place and an impression can be distorted if seated after the material has set to any degree. Finally, distortion or creep will occur if the material does not recover elastically when the set

impression is removed from undercuts. Imbibition is discussed in the section on disinfecting impressions, and creep-induced distortion is discussed under permanent deformation.

All types of elastomeric impression materials undergo shrinkage due to polymerization, and those with reaction byproducts undergo additional contraction. The linear dimensional change between a die and the impression after 24 hours is listed in Table 12-7. The polysulfides and condensation silicones have the largest dimensional change during setting, in the range of -0.4 to -0.6%. The shrinkage is a result of the evaporation of volatile byproducts and the rearrangement of the bonds with polymerization. The addition silicones have the smallest change, about -0.15%, followed by the polyethers at about -0.2%. The contraction is lower for these two products because there is not loss of byproducts.

The rate of shrinkage of elastomeric impression materials is not uniform during the 24 hours after removal from the mouth. In general, about half the shrinkage observed at 24 hours occurs during the first hour after removal; for greatest accuracy, therefore, the models and dies should be prepared promptly, although the elastomeric impression materials are much more stable in air than hydrocolloid products.

MECHANICAL PROPERTIES

Typical mechanical properties of elastomeric impression materials are listed in Table 12-8. The permanent deformation (in the current specification, elastic recovery, which is 100% minus the permanent deformation), strain in compression, and dimensional change are properties used in ANSVADA Specification No. 19 (IS0 4823) to classify elastomeric impression materials as low, medium, high, or very high viscosity types. The requirements for these properties are given in Table 12-9. Further requirements of the specification for rubber impression materials are indicated in Table 12-10.The consistency diameter is used to classify viscosity by measuring the diameter of the disk formed when 0.5 ml of mixed material is subjected to a 575-g weight at 1.5min-

utes after mixing for 12 minutes. Because the setting times of elastomeric impression materials vary, the consistency diameter is affected not only by the viscosity but also by the setting time. The classification of a material by the consistency diameter may be different from that by a true viscosity measurement.

Permanent Deformation The order in

best elastic recovery during removal from the mouth, followed by condensation silicones and polyethers, and then polysulfides.

The trend is to report the elastic recovery rather than the permanent deformation. Thus a material with a permanent deformation of 1% has an elastic recovery of 99%.

Strain The strain in compression under a stress of 1000 g/cm2 is a measure of the flexibility of the material. Table 12-8 illustrates that, in general, the low-consistency materials of each

type are more flexible than the high-consistency elastomeric impressions. For a given consistency, polyethers are generally the stiffest followed by addition silicones, condensation silicones, and polysulfides.

Flow Flow is measured on a cylindrical specimen 1 hour old, and the percent flow is determined 15 minutes after a load of 100 g is applied. As seen in Table 12-8, silicones and polyethers have the lowest values of flow, and polysulfides have the highest values.

Typical elastomeric impression materials apparently have no difficulty meeting the mechanical property requirements of ANSI/ADA Specification No. 19 (see Table 12-9). Although the flow, hardness and the tear strengths of elastomeric impression materials are not mentioned in the specification, these are important properties; they are also listed in Table 12-8.

Hardness The Shore A hardness increases from low to high consistency. When two numbers are given, the first represents the hardness 1.5 minutes after removal from the mouth, and the second number is the hardness after 2 hours. The polysulfides and the low-, medium-, and highviscosity addition silicones do not change hardness significantly with time, whereas the hardness of condensation silicones, addition-silicone putties, and polyethers does increase with time. In addition, the hardness and strain affect the force necessary to remove the impression from

the mouth. Low flexibility and high hardness can be compensated for clinically by producing more space for the impression material between the tray and the teeth. This can be accomplished with additional block-out for custom trays or by selecting a larger tray when using disposable trays.

A new variation in polyether provides less resistance to deformation during removal of the impression from the mouth and the gypsum cast from the impression. To achieve this, the filler content was reduced from 14 to 6 parts per unit, thereby reducing the Shore A hardness from 46 to 40 after 15 minutes, and from 61 to 50 after 24 hours. The ratio of high-viscous softener to low-viscous softener was changed to achieve a consistency similar to that of the conventional monophase polyether.

Tear Strength Tear strength is important because it indicates the ability of a material to withstand tearing in thin interproximal areas. The tear strengths listed in Table 12-8 are a measure of the force needed to initiate and continue tearing a specimen of unit thickness. A few polysulfides have high tear strengths of 7000 g/ cm, but the majority have lower values in the 2500 to 3000 g/cm range. As the consistency of the impression type increases, tear strength undergoes a small increase, but most of the values are between 2000 and 4000 g/cm. Values for very high consistency types are not listed because this property is not important for these materials. Higher tear strengths for elastomeric impression

Time (min)

Fig. 12-25Creep compliance of elastomeric impression materials at the time recommended for removal from the mouth. Curves from top to bottom: polysulfide, condensation silicone, addition silicone, and polyether.

(From Craig RG: Mich Dent Assoc J 59:259, 1977.)

materials are desirable, but compared with the values for hydrocolloid impression materials of 350 to 700 g/cm, they are a major improvement. Although polysulfides have high tear strengths, they also have high permanent deformation that may result in inaccurate impressions.

Creep Compliance Elastomeric impression materials are viscoelastic,and their mechanical properties are time dependent. For example, the higher the rate of deformation, the higher the tear strength; and the longer the impressions are deformed, the higher the permanent deformation. As a result, plots of creep compliance versus time describe the properties of these materials better than stress-strain curves. Creep-compli- ance time curves for low-consistencypolysulfide, condensation silicone, addition silicone, and medium-consistency polyether are shown in Fig.

12-25. The initial creep compliance illustrates polysulfide is the most flexible and polyether is the least flexible.The flatness or parallelism of the curves with respect to the time axis indicates low permanent deformation and excellent recovery from deformation during the removal of an impression material; polysulfides have the poorest elastic recovery, followed by condensation silicone and then addition silicone and polyether.

The recoverable viscoelastic quality of the materials is indicated by differences between the initial creep compliance and the creep compliance value obtained by extrapolation of the linear portion of the curve to zero time. As a result, polysulfides have the greatest viscoelastic quality and require more time to recover the viscoelastic deformation, followed by condensation silicone, polyether, and addition silicone.

Detail Reproduction The requirements of elastomeric impression materials are listed in Table 12-10. Except for the very high-viscosity products, all should reproduce a V-shaped groove and a 0.02-mm wide line in the elastomeric. The impression should be compatible with gypsum products so the 0.02-mm line is transferred to gypsum die materials. Low-, medium-, and highviscosity elastomeric impression materials have little difficulty meeting this requirement.

WETTABlLlTY AND HYDROPHILIZATION

OF ELASTOMERIC IMPRESSION MATERIALS

Wettability may be assessed by measuring the advancing contact angle of water on the surface of the set impression material or by using a tensiometer to measures forces as the material is immersed and removed (Wilhelmy technique). The advancing contact angles for elastomeric impression materials are listed in Table 12-11.Of all the impression materials discussed in this chapter, only hydrocolloids can be considered truly hydrophilic. All of the elastomeric impression materials possess advancing and receding contact angles greater than 45 degrees. There are, however, differences in wetting among and within types of elastomeric impression materials. Traditional addition silicone is not as wettable as

366 Chapter 12 IMPRESSION MATERIALS

polyether. When mixes of gypsum products are poured into addition silicone, high contact angles are formed, making the preparation of bubblefree models difficult.

Surfactants have been added to addition silicones by manufacturers to reduce the contact angle, improve wettability, and simplify the pouring of gypsum models. This class with improved wetting characteristics is most accurately called hydrophilized addition silicone. Most commonly, nonionic surfactants have gained importance in this area. These molecules consist of an oligoether or polyether substructure as the hydrophilic part and a silicone-compatible hydrophobic part (Fig. 12-26, A). The mode of action of these wetting agents is believed to be a diffusion-controlled transfer of surfactant molecules from the polyvinylsiloxane into the aqueous phase, as shown, thereby altering the surface tension of the surrounding liquid. As a result, a reduction in surface tension and therefore greater wettability of the polyvinylsiloxane is observed (Fig. 12-26, I?).This mechanism differs from polyethers, which possess a high degree of wettability because their molecular structure contains polar oxygen atoms, which have an affinity for water. Because of this affinity, polyether materials flow onto hydrated intraoral surfaces and

are therefore cast with gypsum more easily than are addition silicones. This affinity also allows polyether impressions to adhere quite strongly to soft and hard tissues.

By observing water droplets on in~pression surfaces, it has been shown that hydrophilized addition silicones and polyethers are wetted the best, and condensation silicones and conventional addition silicones the least. Wettability was directly correlated to the ease of pouring highstrength stone models of an extremely critical die, as shown in Table 12-11. Using a tensiometer to record forces of immersed impression specimens (Wilhelmy method), polyether was shown to wet significantly better than hydrophilized addition silicones for both advancing (74" versus 108" C) and receding contact angles (50" versus 81' C).

To evaluate the ability of impression materials to reproduce detail under wet and dry surface conditions, impressions were made of a standard wave pattern used to calibrate surface analyzers. The surfaces of impressions were scanned for average roughness (Ra) after setting to determine their ability to reproduce the detail of the standard, the value of which is shown with a double line in Fig. 12-27.From a clinical standpoint, most impression materials produced acceptable detail under wet and dry conditions. Polyethers produced slightly better detail than did addition silicones, and were generally unaffected by the presence of moisture, whereas detail decreased some for addition silicones under wet conditions, even if hydrophilized.

DISINFECTION OF ELASTOMERIC

IMPRESSIONS

All impressions should be disinfected upon removal from the mouth to prevent transmission of organisms to gypsum casts and to laboratory personnel. Several studies confirm that all types of elastomeric impression materials, polysulfide, condensation silicone, addition silicone, and polyether, can be disinfected by immersion in several different disinfectants for u p to 18 hours without a loss of surface quality and accuracy.

RELATIONSHIP OF PROPERTIES

AND CLINICAL APPLICATION

Accuracy, the ability to record detail, ease of handling, and setting characteristics are of prime importance in dental impressions.

Silicones generally have shorter working times than polysulfides but somewhat longer times than polyethers. Single-mix materials have some advantage in that, as a result of shear thinning, they have low viscosities when mixed or syringed but higher viscosities when inserted in a tray. The time of placement of a elastomeric impression material is critical, because viscosity increases rapidly with time as a result of the polymerization reaction. If the material is placed in the mouth after the consistency or viscosity has increased via polymerization, internal stresses induced in the impression are released after the impression is removed from the mouth, resulting in an inaccurate impression.

Thorough mixing is essential; otherwise portions of the mix could contain insufficient accelerator to polymerize thoroughly or may not set at the same rate as other portions of the impression. In this event, removal of the impression would cause high permanent deformation and result in an inaccurate impression. Automixing and mechanical mixing systems produce mixes with fewer bubbles than hand mixing, save time in mixing, and result in a more bubblefree impression.

Polymerization of elastomeric impression materials continues after the material has set, and the mechanical properties improve with time. Removal too early may result in high permanent deformation; however, excessively long times in the mouth are unacceptable to the patient. The manufacturer usually recommends a minimum time for leaving the impression in the mouth, and this minimum is used for testing the materials according to ANSI/ADA Specification No. 19.

Dimensional changes on setting were the highest for condensation silicones and polysulfides. The effect of this shrinkage can be compensated for by use of a double-impression or putty-wash technique. When using a doubleimpression technique, a preliminary impression is taken in the highor puttylike-consistency

material, providing some space for the final impression in low-consistency material. The preliminary impression is removed, the cavity prepared, and the final impression taken with the lowconsistency material, using the preliminary impression as a tray. In this way, the dimensional change in the high consistency or puttylike consistency is negligible, and although the percent dimensional change of the low-consistency rnaterial is still large, the thickness is so small that the actual dimensional change is small. The double-impression technique is suitable for use with a stock impression tray, because the preliminary impression serves as a custonl tray. With the monophase and simultaneous dual-viscosity technique, a slight improvement in accuracy results when a custom-made tray is used because it provides a uniform thickness of impression material. Several studies have shown, however, that relatively stiff stock plastic or metal trays yield nearly the same accuracy.

Clinical studies have shown that the viscosity of the impression material is the most important factor in producing impressions and dies with minimal bubbles and maximum detail. As a result, the syringe-tray technique produced superior clinical results in the reproduction of fine internal detail of proximal boxes or grooves.

The accuracy of the impression may be affected when the percentage of deformation and the time involved in removing the impression are increased. In both instances, permanent deformation increases, the amount depending on the type of elastomeric impression material.

Because elastomeric impressions recover from deformation for a period after their removal, some increase in accuracy can be expected during this time. This effect is more noticeable with polysulfides than with other impression materials. However, polymerization shrinkage is also occurring, and the overall accuracy is determined by a combination of these two effects. Insignificant recovery from deformation occurs after 20 to 30 minutes; therefore dies should be prepared promptly after that time for greatest accuracy. Addition silicones that release hydrogen are an exception to this guide.

Second pours of gypsum products into elas-

tomeric impressions produce dies that are not quite as accurate as the first, because the impression can be deformed during the removal of the first die; however, they are usually sufficiently accurate to be used as a working die. Materials such as polysulfide are more susceptible to permanent deformation with cast removal than other types.

BITE REGISTRATION M A f E a W S * 5':

EIASTOMERIC IMPRESSION MATERIALS

Addition silicones and polyethers have been formulated for use as bite registration materials. Most of the products are addition silicones and most are supplied in automix cartridges. Properties of these bite registration materials are listed in Table 12-12. These materials are characterized by short working times and the length of time left in the mouth compared with typical elastomeric impression materials. They are also noted for their high stiffness, indicated by the low percent strain in compression, and for their low flow and dimensional change even after 7 days. The property that distinguishes addition silicones from polyether is their lower dimensional change after removal; however, either is superior to the stability of waxes for taking bite registrations.

ZINC OXIDE-EUGENOL

AND IMPRESSION PIASTER

Several materials become rigid when used to record bite relationships and anatomical features. These materials can be used to capture occlusal

relationships providing the materials do not flow into undercuts, such as beyond the height of contour of the teeth being recorded. If they do, these brittle materials can distort permanently or fracture upon removal.

Zinc oxide-eugenol, commonly used as a temporary cement or temporary filling material, can also be used to record tooth and arch relationships (Fig. 12-28). In this case, it is often used to reline an area in which the bite was initially recorded, to improve the accuracy of the record. Type I gypsum, also called impression plaster, can be used to record relationships of crowns and pontics for soldering and for recording relationships between arches. Detailed descriptions of zinc oxide-eugenol and Type 1 gypsum are given in Chapters 20 and 13, respectively.

WAX REGISTRATIONS

Bite registrations used to articulate upper and lower models have often been taken in wax, as described in Chapter 14. However, the properties of waxes limit their accuracy, because wax registrations (1) can be distorted upon removal,

(2)may change dimensions by release of internal stresses, depending on the storage condition,

(3)have high flow properties, and ( 4 ) undergo large dimensional changes on cooling from mouth to room temperature.

Wax is also used as a corrective impression technique in partial and complete denture prostheses. Impression waxes are available with a variety of softening temperatures, further dis-

cussed in Chapter 14. Waxes with lower softening points are used to register functional impressions. In one case, the posterior palatal seal area is delineated intraorally using a colored pencil. This line is transferred to the final impression. Thereafter, a thin layer of impression wax is placed over the posterior palatal seal area to form a raised area in the final denture, thereby creating a better posterior seal. In other cases, a thin layer of impression wax can be applied to a denture base with occlusal bite rims and left in place in the mouth for a period of time. In this way, the wax flows and adapts itself to the oral tissues under the influence of functional occlusion. Impression waxes with a higher softening point are used in extending the border of the denture base when necessary. The clinical techniques for their use are adequately described in textbooks on prosthetic dentistry.

IMPRESSION COMPOUND

One of the oldest dental impression materials is impression compound. Tray compound has been largely replaced by acrylic and plastic tray materials, but impression compound is still used for border-molding complete denture impressions and is conveniently used to secure rubber dam retainers so they remain stable during clinical

procedures. Impression compounds are available in the form of sheets, sticks, cylinders, and cones, some examples of which are shown in Fig. 12-29. Impression compounds are thermoplastic materials and are softened to their working consistency by immersion in hot water or by warming over a flame. Some variation of the temperature at which softening takes place exists among different compounds, and they can be divided into high-fusing (tray) and low-fusing (impression) compounds.

Composition The impression compound shown in Table 12-13 is a mixture of thermoplastic resins and waxes, a filler, and a coloring agent. By varying the proportions of the various ingredients, compounds of differing physical properties can be made. Resins and waxes soften on heating and provide flow and cohesion, and the filler adds body and gives a suitable working consistency. Rouge produces a characteristic reddish brown color and it is the most common pigment.

Thermal Conductivity The thermal conductivity of impression compounds is low. When immersed in hot water or heated over a flame, they rapidly soften on the outside, but time is required before the mass is softened throughout.

When heating over a flame, care is needed to prevent the outside from being overheated and the more volatile components from being vaporized or ignited. Prolonged immersion in hot water also leaches out the more soluble components and adversely alters physical properties.

The low thermal conductivity influences the cooling rate of these materials, because the outside of a mass of compound hardens fairly rapidly, whereas the inner regions remain soft. Impressions must be given adequate time to cool completely before they are removed from the mouth.

Softening and Flow Impression compounds should soften at a point just above mouth temperature and exhibit adequate flow to adapt

closely to the tissues and register surface detail. They should harden at mouth temperature and exhibit a minimum of flow to reduce the danger of distortion on ren~oval.

ANSI/ADA Specification No. 3 (obsolete) for dental impression compound listed the following criteria for flow qualities of the impression type: the flow at 37" C shall not be more than 6%. and the flow at 45" C shall not be less than 85%.

Sticks or small cones of compound are used for securing rubber dam retainers or for recording occlusal relationships. The sticks or cones should be softened by holding them some distance above a flame to avoid overheating the outside portion and ensure softening the center of the stick. The softened compound may be placed directly around the rubber dam retainer or on the occlusal registration plate, but care must be taken that it is not too hot. For this reason the heated compound should be tempered in air or by a brief immersion in water at a temperature just above the softening point of the compound.

Cooling Impressions formed by placing the softened compound into a suitable copper band for indirect inlay and crown techniques are usually cooled in the mouth with water from a syringe. The water used for cooling should be within the range of 16" to 18' C, because cold water is uncomfortable to the patient and tends