Properties and handling characteristics

The properties and handling characteristics of various contemporary impression materials are discussed in this section. The hydrophilic versus hydrophobic nature of materials is discussed as it relates to flow characteristics, which result in more bubble-free impressions. In recent years, dentists have turned toward using polyvinyl siloxanes and polyethers because of their improved physical and mechanical properties . These properties include improved dimensional accuracy, stability, wettability, excellent elastic recovery, flexibility, ease of handling, tear strength, ability to produce multiple casts from one impression, and superior ability to reproduce detail.

Dimensional accuracy

With elastomeric impression materials such as polyvinyl siloxane, polyether, and polysulfide, the dimensional accuracy is usually time dependent, with greater dimensional accuracy occurring immediately after polymerization is complete but declining as the impression is stored for extended periods of time . Polyvinyl siloxane and polyether impression materials remain dimensionally accurate for 1 to 2 weeks . Polysulfide impression material is dimensionally accurate if poured within 1 to 2 hours of making the impression . Practitioners should take this characteristic into consideration when selecting impression materials given the time available to the practitioner to pour casts during office hours.

Hydrophilic versus hydrophobic nature of impression materials

There are definite differences in the hydrophilic properties of elastomeric impression materials. Limitations of the polyvinyl siloxanes involve their hydrophobic nature . Polyvinyl siloxanes are hydrophobic because of their chemical structure. They contain hydrophobic aliphatic hydrocarbon groups around the siloxane bond . Polysulfides and polyethers are more hydrophilic. They contain functional groups that chemically attract and interact with water molecules via hydrogen bonding . The hydrophilic nature of polyether impression material is manifested in carbonyl (C=O) and ether (C-O-C) groups, whereas polysulfide material has hydrophilic disulfide (-S-S-) and mercaptan (-S-H) groups . The hydrophobic aspect of polyvinyl siloxane impression materials has an adverse effect on surface quality of the polymerized impression material .

Presence of moisture results in impressions with voids or pitted surfaces, and the detail reproduced is inferior. This result has been reported even with the new “hydrophilic” polyvinyl siloxane impression materials. These hydrophilic polyvinyl siloxanes have improved wettability , and they are only clinically acceptable under dry conditions . The hydrophilization of polyvinyl siloxanes is enhanced with the incorporation of nonionic surfactants. They have a hydrophilic part and a silicone-compatible hydrophobic part. These surfactants act through a diffusion transfer of surfactant molecules from the polyvinyl siloxane into the aqueous phase. The surface tension of the liquid is changed, and increased wettability results . When using polyvinyl siloxanes, moisture control is critical to ensure success for predictable clinical impression making. Because of their hydrophilic nature, using polyether and polysulfide impression materials is more compatible with the inherent moisture present in mucosal tissues .

If a comparison of the various categories of impression materials is made based on hydrophilic versus hydrophobic nature, wettability, the amount of detail reproduced, their dimensional stability, the rigidity of the material, the tear strength of the material, and the contact angle of the material, the selection of the right material is made easier. The hydrophilic nature of an impression material relates to its ability to work in a wet environment and still provide accuracy in impression making. If a material can tolerate some moisture, it is considered to be hydrophilic. Hydrocolloids would be considered the most hydrophilic. The hydrophobic nature of an impression material relates to its inability to work in a wet environment and still provide accuracy in an impression. Claims are made with respect to polyvinyl siloxane materials being hydrophilic, but in reality they are somewhat hydrophobic .

Dimensional stability

The dimensional stability of an impression material reflects its ability to maintain the accuracy of the impression over time . These materials should have low shrinkage upon polymerizing and remain stable, which allows them to be poured days after making the impression. High impression dimensional stability materials usually can be poured within 1 to 2 weeks after the impression is made and still produce an accurate cast . Materials with high dimensional stability are the polyethers and polyvinyl siloxanes, in contrast to alginate, which has a low dimensional stability. The polysulfides distort over time . Because many dentists send their impressions to a laboratory to be poured, this characteristic should be considered when choosing an impression material .

Rigid impression materials require less support from trays. They distort less on pouring and make good bite registration materials . They work well for implant impressions, in which posts must be transferred accurately . They would be detrimental in making full arch impressions of periodontally compromised or mobile teeth. Polyethers and some polyvinyl siloxanes fall into this category.

Wettability (or flow characteristics)

Wettability of an impression material relates to the ability of the material to flow into small areas . Impressions that wet the teeth well displace moisture and result in fewer voids. Materials with a high wetting angle do not flow easily into small crevices and are poor candidates for use in fixed prosthodontics. Materials with a low wetting angle flow extensively. Water is the ideal example of a material with a low wetting angle. Wettability results in fewer voids and less entrapment of oral fluids, providing more accurate impressions .

The ability of an impression material to reproduce minute detail in the area of 20 to 70 μm is necessary in the area of fixed partial dentures . Impression materials with the ability to produce detail in the range of 100 to 150 μm work well and are acceptable in the areas of removable prosthodontics .

Elastic recovery

A set impression must be sufficiently elastic so that it will return to its original dimensions without significant distortion upon removal from the mouth . Polyvinyl siloxane has the best elastic recovery, followed by polyether and polysulfide .

Flexibility

Flexible impressions are easier to remove from the mouth when set. Polyethers tend to be the most rigid impression materials . Polyvinyl siloxanes are fairly stiff, and depending on the viscosity of the material, they flow readily to capture areas of detail . 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 . Accuracy of the impression is also affected when the percentage of deformation and the time involved in removing the impression are increased. In these instances, permanent deformation occurs relative to the type of elastomeric impression material used . Alginate would be considered the most flexible of the impression materials, whereas polyethers would be considered the least flexible.

Ease of handling

Working times can be varied with respect to standard-set versus quick-set impression materials as prepared by various manufacturers . Various viscosities and flow characteristics are also made available per individual manufacturer formulations.

Tear strength

The tear strength of an impression material relates to how resistant a particular material is to tearing after setting . Where subgingival margins are concerned, this can be an important criterion. Polyethers are considered to have the highest tear strengths, whereas hydrocolloids have relatively low tear strengths . Polysulfide impression materials have a high resistance to tearing but stretch and do not recover completely elastically .

Contact angle (and ability to reproduce detail)

Impression materials with low contact angle enable dental stone to flow easily, and relatively bubble-free casts are produced. Materials with high contact angle require more careful pour technique and attention to produce accurate casts . Polyvinyl siloxane materials may require surfactants to lower the contact angle before pouring casts. Hydrocolloids, polyethers, and polysulfides have relatively low contact angles.

Miscellaneous

In addition to these criteria, the following criteria should be considered: how well a material is tolerated by patients, obtaining the best results for the least amount of expense, and occurrence of minimal changes when in contact with disinfection chemicals. Materials such as hydrocolloids, polyethers, and methacrylates may require specific disinfection protocols to prevent distortion of the material after setting .

Disinfection of impression materials

Impressions should be rinsed with water and then disinfected . Diluted sodium hypochlorite (bleach 5.25%, 1:10 dilution, 10 minutes at 20°C) provides American Dental Association–accepted disinfection but not sterilization for all materials, except zinc-oxide eugenol paste. Glutaraldehydes are the disinfectant of choice for zinc oxide eugenol impression pastes .

Types and characteristics of specific impression materials

Irreversible hydrocolloids

When alginic acid (prepared from a marine plant) reacts with a calcium salt (calcium sulfate), it produces an insoluble elastic gel called calcium alginate. When mixed with water, the alginate material first forms a sol. The following chemical reaction forms a gel to create the set impression material. In an alginate impression compound, calcium sulfate dehydrate, soluble alginate, and sodium phosphate are in the powder. When water is added, calcium ions from the calcium sulfate dehydrate react preferentially with phosphate ions from the sodium phosphate and pyrophosphate to form insoluble calcium phosphate. Calcium phosphate is formed because it has a lower solubility; thus the sodium phosphate is called a retarder and provides working time for the mixed alginate. After the phosphate ions are depleted, the calcium ions react with the soluble alginate to form insoluble calcium alginate, which with water forms the irreversible calcium alginate gel. It is insoluble in water and its formation causes the material to gel .

After reviewing the types and characteristics of the most common impression materials, it becomes apparent that hydrocolloids have a high hydrophilic nature that allows this material to capture accurate impressions in the presence of some saliva or blood . It has a low wetting angle so it easily captures full arch impressions. It has moderate ability to reproduce detail and costs relatively little compared with other impression materials. It is not accurate enough for fixed partial dentures but is used for partial framework impressions . It has poor dimensional stability (imbibition or dessication is a problem), must be poured within 10 to 12 minutes of impression making or distortion becomes a major issue, and is good for only one pour per impression . Impressions made in hydrocolloid are easier to remove than other materials and require rigid trays to prevent distortion in impression making and pouring of dental casts. Because the tear strength of hydrocolloids is low, it may capture subgingival contours and anatomy but may tear upon removal . It is not as strong as polyethers or polyvinyl siloxane impression materials. It is relatively low cost and comes in flavors that are more patient friendly. Distortion can be a problem if disinfection guidelines are not strictly adhered to. Because hydrocolloids are hydrophilic, they swell if immersed in water or disinfectant . It is recommended that a disinfectant spray be used while the impression is placed in a plastic bag for 10 minutes, at which time the impression is rinsed with water immediately and the cast poured . If immersion disinfection (1% sodium hypochlorite or 2% potentiated glutaraldehyde) is performed (10–30 minutes), statistically significant dimensional changes are observed; these changes are on the order of 0.1%, and the quality of the surface is not impaired. (Such changes would be insignificant for clinical applications, such as study models and working casts.) Immersion disinfection also may differ between different brands of alginate with respect to different immersion systems, such as iodophor and glyoxal glutaraldehydes .

The setting reaction of hydrocolloids is not affected by latex proteins from gloves. Some water supplies contain large amounts of minerals that can adversely affect the accuracy and the setting time of alginate impression materials, however. If concerned about mineral content of local water supplies, distilled or demineralized water can be substituted . Once set, hydrocolloid does not adhere to itself and cannot be used to border mold. A potential problem when using irreversible hydrocolloid is the tendency for this material to stick to teeth, which occurs when alginate radicals in the impression material form chemical bonds with hydroxylapatite crystals of the enamel. On removal of the impression, the alginate tears. Factors that may cause sticking of the alginate include polishing of teeth, which removes a thin film overlying the teeth and actually prevents the hydrophilic nature of this material from wetting the teeth and reproducing detail . There is also a greater tendency for alginate to stick to teeth if they are dry. Dryness minimizes the moisture content of tooth surfaces and contributes to sticking of the alginate; ultimately, it leads to inaccurate cast pours. Finally, if repetitive impressions are made, the film over the teeth is lost and prevents satisfactory impression. Either placing a small amount of silicone lubricant over the teeth in a prophylactic paste or rehydrating through a rinse is necessary to produce a new film over the teeth for accurate impressions. Sometimes it is best to make another appointment for new accurate impressions within 24 hours or such a time so that this film layer will re-wet the tooth surfaces .

Polyethers

Polyethers consist of a base paste that is composed of a long-chain polyether copolymer with alternating oxygen atoms and methylene groups (O-[CH ₂ ] _n ) and reactive terminal groups. Also present are fillers, plasticizers, and triglycerides. The catalyst paste has a cross-linking agent (aliphatic cationic starter) and filler and plasticizers. Polyethers involve the reaction of the polyether-containing imine ringed side chains with a reactant that opens the rings and causes chain lengthening and cross-linking to form a polyether rubber .

Polyether impression materials are moderately hydrophilic and capture accurate impressions in the presence of some saliva or blood. Because their wetting angle is low, they capture a full arch impression easier than with polyvinyl siloxanes . Their ability to reproduce detail is excellent and they are dimensionally stable and allow multiple pours of accurate casts for 1 to 2 weeks after impressions are made, provided there is no tearing of the impression. They are rigid materials and may be more difficult to remove than polyvinyl siloxanes . They do not tear easily (high tear strength), which enables the dentist to get good subgingival detail without tearing the impression on removal.

This material adheres to itself and can be used to border mold or make correctable impression techniques. Improved polyether formulations such as the “soft” polyethers are easier to remove, maintain proper rigidity for a wide range of applications, and capture fine detail even in moist conditions . The snap-set behavior of the soft polyether materials allows the material to not start setting before the working time ends. When it does set, it does so immediately . These characteristics make it highly desirable for clinical and laboratory use. Polyether has properties such that it can flow into critical areas with low pressure exerted, which results in accurate impressions and makes for fewer adjustments and remakes for the practice of dentistry. They are a superior material to hydrocolloids and somewhat better than polyvinyl siloxanes . Because these materials are moderately hydrophilic, strict attention to disinfection guidelines is necessary to prevent swelling of the material. Spray with disinfectant for 10 minutes and rinse and dry immediately before pouring casts . This material does taste bitter, although it is currently flavored to offset the taste. The setting times are relatively short (4–5 minutes), and the set is not altered or contaminated by latex gloves.

Polyvinyl siloxanes

Addition silicones (which are the most popular because no reaction byproducts are formed) involve the linking of a vinyl siloxane in the base material with a hydrogen siloxane via a platinum catalyst . The reaction produces hydrogen, which is scavenged by the platinum. Viscosity is altered by changing the amount of silica filler, which produces either a putty or less viscous wash material. Vinyl polysiloxane silicones (also called addition silicones, polyvinyls, vinyls, and polyvinyl siloxane) are considered state-of-the-art for fixed partial denture impressions. They constitute the most widespread use of impression materials for fixed prosthetics . They are virtually inert after set, and they can be trimmed and poured in any die material.

Before they set, however, they are susceptible to contamination. Because the addition silicones require a small amount of catalyst (platinum compound) to initiate the setting reaction, anything that interferes with the catalyst (preventing cross-linking of the material) causes the surface of the impression to remain tacky . Polyvinyl siloxane contamination is usually a result of sulfur or sulfur compounds . This is usually seen in the dental office in the form of latex gloves or rubber dams. Small amounts of sulfur interfere with setting of the critical surface next to the tooth and produce major distortion . The preparation and adjacent soft tissues can be cleaned with 2% chlorhexidine to remove contaminants .

If wearing latex gloves, one should avoid touching the unset impression material, the teeth and adjacent gingiva, the interior of the tray, the mixing spatula or mixing pad, the end of a mixing tip, and the retraction cord. The way to avoid latex contamination is to wear polyethylene gloves over the latex gloves or not wear latex gloves during the impression procedures. Some vinyl gloves also may have the same effect because of the sulfur-containing stabilizer used in the manufacturing process . Sulfur compounds can poison the platinum-containing catalyst in addition silicone impression materials and result in retarded or no polymerization in the contaminated area of the impression . It has been reported that vapor given off by polysulfide impression material may cause contamination. It is a good idea not to store polyvinylsiloxane impression material close to polysulfide impression materials. Another source of contamination is the oxygen-inhibited layer on the surface of resin materials that appears immediately after curing. This thin layer causes impressions to remain tacky around new composite placed restorations .

Polyether and polysulfide impression materials also leave the mouth coated with a chemical film that inhibits polyvinyl siloxanes. If you make an impression with either of these two types of materials and then decide to make an impression with polyvinyl siloxane, it inhibits the set . Polyvinyl siloxane materials are also thermally sensitive . The warmer they are, the faster they set. If this material is overheated it may not recover to its normal setting time even after cooling, and it is recommended that this material be stored in a cool place and not in the sun (refrigerator or cool space). If cooled, the material sets slower. If kept in a refrigerator, it is advisable to let the material come to room temperature before use, otherwise it takes a longer time than normal to set. The material is thicker when it is cold and more difficult to express and mix .

Most impression materials require a 1:1 ratio of base to catalyst. More catalyst added also speeds the setting time. When using automix cartridges, it is recommended to extrude 0.25 inches of material and discard before placing the mixing tip to remove any contaminated material or material that has been exposed for long periods of time to the environment. Because some polyvinyl siloxane materials exhibit a phenomenon known as hydrogen out-gassing, if you pour casts too soon the stone captures these bubbles and produces a cast with pitted areas . The newer materials are said to contain a proprietary component that eliminates hydrogen bubbles, but it is best to read the guidelines for pouring specific brands of polyvinyl siloxanes before pouring stone.

The newer materials are supposedly able to be poured in 5 minutes after the impression material is removed from the mouth. It is recommended that one wait at least 30 minutes for the setting reaction to be completed before the gypsum casts and dies are poured . Epoxy dies should not be poured until the impression has stood overnight . The difference in the delay with gypsum and epoxy is that gypsum products have a much shorter setting time than epoxy die materials. Some products contain a hydrogen absorber, such as palladium, and gypsum and epoxy die materials can be poured against them as soon as is practical .

Bubbles in the impression can occur when you spatulate and entrap air into the mix. Automixing cartridges tend to create fewer bubbles than hand spatulation. This is probably true with respect to any impression material when comparing hand spatulation to automixing. Polyvinyl siloxanes are generally hydrophobic. Aquasil (Caulk/Dentsply) is slightly hydrophilic . Moisture from saliva or blood can interfere with accurate impressions. Loss of detail at impression margins is caused by moisture presence . It has a moderately high wetting angle, which makes it a little more difficult to create an accurate full arch impression than with hydrocolloid, polyether, or polysulfide. It has an excellent ability to reproduce detail and is dimensionally stable, which allows multiple pours of accurate casts for several weeks after impressions are made if no tears are present in the material. The material is moderately rigid and can be more easily removed than polyether materials. Their tear strength is better than hydrocolloid but not as good as polyether . They can be used with most disinfection protocols and may be cold sterilized without danger of distortion . Note that addition silicones release hydrogen on setting and many require a 30-minute to 1-hour de-gassing period before pouring a master cast or the cast develops surface porosity.

There is a greater tendency to trap air bubbles when pouring stone because of its moderately high contact angle, so greater care is required when pouring stone . This material comes in flavors and is not much of a problem from the standpoint of taste. The setting time is also relative short (4–5 minutes). However, contamination from the latex proteins in gloves may interfere with setting of this material. Most materials in this category do not adhere to themselves after they have set and would not be able to be used for border molding or correctable impression technique. Aquasil is an exception because it does adhere to itself after setting. Once set, polyvinyls are fairly inert, and there have been no reports of any disinfectants that damage them. High ambient room temperature does not distort them, and they can be trimmed and poured with any die material for casts.

Polysulfides

Polysulfide impression materials are supplied as two paste systems. The base consists of a polysulfide polymer (terminal/side chain −SH groups), titanium dioxide, zinc sulfate, copper carbonate, or silica. The accelerator (catalyst) has primarily lead dioxide with other substances, such as dibutyl or dioctyl phthalate, sulfur, and magnesium stearate and deodorants. The viscosity is altered by adding different amounts of titanium dioxide powder to the base. It sets by oxidation of the −SH groups, which results in chain lengthening and cross-linking and gives it elastomeric properties .

Polysulfide impression materials are generally low to moderately hydrophilic and make an accurate impression in the presence of some saliva or blood. Because the material has a low wetting angle it makes a full arch impression easier than with polyvinyl siloxanes or polyethers. It reproduces detail with excellent results but its dimensional stability is only fair . It may allow for more than one pour if it is not too thin in areas. It is not a rigid material, and impressions are easier to remove than with polyethers and polyvinyl siloxanes. It generally captures a subgingival margin upon impression without tearing on removal, which is much better than hydrocolloids or polyvinyls. It distorts from disinfection if not performed correctly because of its hydrophilic nature and may swell if placed in water or disinfectant for a period of time. Researchers recommend that it be sprayed with disinfectant for 10 minutes, rinsed, and dried immediately before pouring in dental stone . It has a terribly bitter taste and is relatively inexpensive. It is not affected by latex gloves. Unfortunately, it does not adhere to itself, which makes it unavailable for border molding or correctable impression techniques.

Tissue conditioners (polyethyl or methyl methacrylates)

Tissue conditioners are composed of a powder that contains poly (ethyl methacrylate) and a liquid that contains an aromatic ester-ethyl alcohol (up to 30%) mixture. Tissue conditioners are soft elastomers. They show a weight loss of 4.9% to 9.3% after 24 hours as a result of the loss of alcohol. Within a few days, tissue conditioners become stiffer as a result of the loss of alcohol. Tissue conditioners are formulated to have specific viscoelastic properties. The viscoelastic properties are influenced by the molecular weight of the polymer powders and the power/liquid ratio . Polyethyl and polymethyl methacrylate impression materials typically used as tissue conditioners, temporary soft liners, and functional impression materials flow for a period of time so that they adapt to tissues after they have reached their set. Because they have an extended flow period, they serve well as functional impression materials. They are all polyethyl or polymethyl methacrylate materials combined with an alcohol-based plasticizer .

The plasticizer makes each material unique and offers a different period of flow after the set. Plasticizers are moderately hydrophilic and make an accurate impression in the presence of some saliva or blood. Because of their low wetting angle, they easily capture full arch impressions. They are suitable for complete and partial dentures because they reproduce detail moderately. Their dimensional stability is fair and usually provides only one pour per impression. They have low rigidity and require rigid trays to support borders; otherwise they tend to distort. They are fairly easy to remove. These materials have low tear strength and usually tear on removal if not careful . These materials are excellent for reline or rebase procedures in removable prosthetics . As with all materials, there are learning curves with respect to working with these different tissue conditioners and other types of impression materials. They adhere to themselves and are excellent for border molding and correctable impression technique. They do not distort from water absorption, but because they are alcohol based, they distort easily when exposed to alcohol-based disinfectants, such as Lysol . For the most part they have a neutral taste.