Impression materials Market Share by Volume tooth undercuts

Impression materials

Market Share by Volume %

tooth undercuts

stone

Impression materials - types • Elastic • Nonelastic

Impression materials - types Nonelastic • plastic • waxes (reversible) • impression compound (plastic, reversible)

Impression materials - types Nonelastic • rigid • plaster (irreversible) • zinc oxide - eugenol impression paste (irreversible)

Impression materials - types Elastic hydrocolloids (aqueous) • agar (reversible) • alginate (irreversible) elastomers (nonaqueous) • polysulfide • polyether • silicone (condensation, addition)

Hydrocolloids Aqueous Impression Materials

Elastic impression materials Hydrocolloids Colloid - a two phase mixture in which one of the phases (the dispersed phase) is very small (1 - 400 nm). 1000 nm = 1 μm

liquid - liquid milk emulsion gas – liquid soap foam liquid – gas fog liquid aerosol solid – gas smoke sol aerosol

Elastic impression materials Hydrocolloids solution - solute and solvent are not physically separate. sol - dispersed phase and dispersion medium are physically separate.

Elastic impression materials Hydrocolloids sol - a colloid in which small solid particles are suspended in a liquid. typical particles - 1 nm to 500 nm 1000 nm = 1 μm

Elastic impression materials Hydrocolloids hydrocolloid - a sol in which the dispersion medium is water

Agar Hydrocolloid Reversible Hydrocolloid

Agar - chemistry • a polysaccharide • sulfuric ester of a polymer a gallactose • molecular wt. ~ 150, 000 • insoluble in water

Agar – a polysaccharide (sulfuric ester of a polymer of gallactose) O H CH 2 OH HO OH O CH OH 2 HO H H H H O HO O H H H CH 2 OH. HSO 4 O H OH 8 ~ 90 N

Agar – structure of the gel • agar fibrils arranged in interpenetrating tangles – “brush heaps” • water is held in the interstices of the brush heap

Agar Hydrocolloid - composition Agar 12 - 15 % Water 80 – 85 % Potassium* 1 – 2 % Borax** 0. 2 % Fillers * hardens gypsum ** strengthens the gel

Agar Hydrocolloid – reversible reaction Forms sol 71 – 100 o. C reversible Forms gel 30 – 50 o. C

Agar Hydrocolloid – Conditioning Baths Bath Temp o C Time Boiling 100 10 - 15 Storage 66 – 69 Tempering 43 - 46 < 15

Agar Hydrocolloids – 3 bath system storage boiling tempering

Agar Hydrocolloid – tempering bath Removing tubes of agar hydrocolloid from the tempering bath.

Agar hydrocolloid: water cooled trays A water-cooled tray is used. The initial investment in agar (baths and water-cooled trays) is relatively high.

Alginate Irreversible Hydrocolloid

Alginate hydrocolloid:

Alginate Hydrocolloid - composition • brown seaweed – algae • linear polymer with numerous carboxyl groups • anhydro - β - d – manuronic acid • called alginic acid

Alginate Hydrocolloid - reaction H 2 O + Na-alginate + Ca. SO 4 (paste) soluble in water Ca-alginate + Na 2 SO 4 (gel) insoluble in water Reaction is irreversible

Alginate Hydrocolloid - mixing • gelation time dependent on waterpowder ratio – faster with less water • do not change water-powder ratio – may affect strength • warm water decreases gelation time – each 10 o. C will halve the gelation time

Alginate Hydrocolloid – storage in air Alginate impression of metal die simulating three-unit FPD preparation. Initial. 4 hours later.

Storage at 100% humidity minimizes wt. loss

Alginate Hydrocolloid – dimensional stability • set alginate exudes fluid – syneresis • rinse impression – exudate could inhibit setting of the surface of dental stone • dimensions will change at a rate that depends on the humidity at which the impression is stored • best advice – pour stone immediately

Both Hydrocolloids – elastic recovery • neither agar nor alginate is perfectly elastic • permanent deformation: alginate: 1. 5 % agar: 1. 0%

Both Hydrocolloids – reducing elastic recovery To reduce permanent deformation: • use the hydrocolloid in a thicker layer – smaller percentage deformation • remove the impression faster – permanent deformation is reduced at higher strain rates

Combined agar – alginate technique • alginate is tray material • agar is syringe material Basic idea: take advantage of the ability of agar to do a better job of replicating fine detail.

Combined agar-alginate technique: A single water heater is used to boil and store the agar cartridges. No need to temper. The agar will cool as it is being ejected from the syringe. agar is the syringe material

Combined agar-alginate technique: Injecting the agar wash around the prepared tooth. The impression is completed using the alginate in non-water-cooled tray. Note that the agar wash defined the margins of the impression.

Combined agar – alginate technique - method • boil agar – 5 minutes • 5 minutes at 70 o. C • Syringe agar over prepared teeth (syringe cools the agar) • Place tray filled with alginate material over the agar

Combined agar – alginate technique - advantages • can use custom tray • rapid set (3 minutes) • reduced equipment costs: syringe & one heater • low cost per impression

Hydrocolloids - disadvantages • low tear strength • large dimensional changes during storage • poor transfer of detail to stone • electroplated dies not possible