Dental Materials

Polysulfide impression material: Uses, Setting Reaction, Composition

Updated on: July 11, 2017

Polysulfide impression material

– Polysulfide impression material was the first non aqueous elastomeric “rubber” impression material developed for dentistry.
– Often, polysulfide materials are called “rubber” or “rubber base” materials, even though
polyether and silicone materials are also rubber materials.
Polysulfide, polyether, and silicone materials are also called non aqueous
elastomeric impression materials.
– They all undergo cross-linking and chain lengthening polymerization reactions.
– They have similar, but not identical, mixing and handling properties.
– One very important difference between types of elastomeric impression materials is the
adhesive that is used to bond the impression material to a non perforated tray.
– Each impression material has its own adhesive, which will not work with other types of
material.

A. Condensation Polymerization

– Polysulfide materials set via a condensation polymerization reaction.
– This is the same chemical reaction that joins together the building blocks of biologic
polymers.
– Biologic polymers (proteins) are important components of the tissues of the body.
– In a typical condensation reaction, a hydrogen atom and a hydroxyl group (OH) are taken from monomers and are combined to form water (H2O).
– The functional groups of the monomers can be carboxylic acid and amine groups that
produce proteins (or nylon).
– The functional groups of polysulfide impression material are mercaptan groups (sulfur and hydrogen atoms) and oxygen from lead oxide.
– Reaction by-products other than water are produced by other condensation polymerization reactions, but water is the most common by-product.
– The name “condensation” polymerization is based on the production of water.

B. Composition of Polysulfide Impression Materials

– Polysulfide impression materials are supplied as two pastes in tubes.
– Typically, one paste is dark brown, and the other paste is white.
– The white “base” paste contains a low-molecular-weight polysulfide polymer mixed with an inorganic filler, such as titanium oxide.
– The brown “accelerator” paste contains lead oxide and an “oily” organic chemical that does not react.
– A small percentage of sulfur is also included in the brown paste because it promotes the polymerization reaction.

C. Mixing Polysulfide Impression Materials

– The two pastes are dispensed in equal lengths on a paper mixing pad.
– An impression material spatula is used to mix the two pastes.
– The impression material spatula has a long (~4”), straight-sided blade.
– The pastes are swirled and stropped together until a homogeneous paste is obtained.
– Mixing may take from 30 to 90 seconds depending on the amount and viscosity of the
material.
– The side of the spatula blade is used to scrape unmixed material off the paper pad and then to mix it into the rest of the material.
– The mixed material is loaded into the tray and placed in the mouth as a viscous paste.
– The same procedure is used to mix other non aqueous elastomeric impression materials, such as polyether and silicones.

Polysulfide impression

D. Polymerization Reaction of Polysulfide Impression Materials

– The polymerization reaction starts when mixing begins and proceeds slowly.
– The low-molecular weight polysulfide polymer has mercaptan groups (–SH) on the end of the short polymer chains and as pendant groups hanging off the middle of the chains.
– Two hydro gen atoms from two different short polymer chains react with oxygen (from the lead oxide) to form water.
– The sulfur acts as a catalyst and helps to link the two sulfur atoms together, joining the two polymer chains together.
– The same reaction lengthens the polymer chains and crosslinks the chains.

E. Properties of Polysulfide Impression Materials

– Polysulfide impression materials are much more accurate than alginate.
– With proper handling, polysulfide impression materials can be used for inlays, crowns, and bridges.
– However, they are not as accurate as other non aqueous elastomeric materials.
– A polysulfide impression should be poured within several hours after mixing.
– Custom trays are recommended for optimum results.
– Polysulfide impression materials have a disagreeable smell and taste.
– They stain clothing and are generally regarded as unpleasant materials.
– Polysulfide impression materials do have one advantage: they have the longest working time of any elastomeric material (4 to 6 minutes).
– As a result, they are useful for impressions of multiple preparations.
– Along with the long working time comes the longest setting time.
– Impressions need to be held in place in the mouth for as long as 15 minutes.
– The water by-product of the setting reaction can be lost through evaporation, resulting in
distortion.
– The working and setting times of polysulfide impression materials are significantly
accelerated by heat and humidity.
– The setting time on hot, humid summer days will differ from that on cool, dry days.
– One technique using polysulfide impression material for a full-denture impression is to mix a drop of water with the material to accelerate the setting.

F. Use of Polysulfide Impression Materials

– Polysulfide impression material is often used with custom trays to increase the accuracy of the impression.
– For impressions of crowns and bridges, light-body material is injected around the
preparation, and heavy-body material is used in the tray.
– Light and medium-body materials are used for full- denture impressions.
– Polysulfide impression materials are relatively inexpensive and easy to pour with gypsum
materials.
– They are somewhat hydrophilic; however, so only a single pour (one model)
is recommended.
– Subsequent pours using the same impression may not have the required accuracy.