Rigid impression material

Dr. Raied Al-Attar 2017-2018 7th lecture

RIGID IMPRESSION MATERIALS
Impression plaster Impression compound Zinc oxide pastes

1. IMPRESSION PLASTER

It is a rigid impression material, used in shellac special trays to make 2ry impressions for edentulous patient has no or shallow undercuts. It is one of the gypsum products that based on - Ca2 So4. Ѕ H2O (-calcium sulfate hemihydrate)

Presentation

The material is presented as powder to be mixed with water to form a slurry (soft creamy mix)

Composition of the powder

-calcium sulfate hemihydrate….basic constitutePotassium sulfate….anti expansion and accelerator Borax……retarderColoring agent…… Alizarine redFlavoring agent….MentStarch….Facilitate the removal of stone cast -calcium sulfate hemihydrate

Setting reaction

The material sets via chemical reaction named crystallization The reaction is characterized by heat generation and setting expansion The reaction could be influenced by; P/L ratio, Mixing rate (time and speed), Particle size, Temperature, Additives (Acc. & Ret.)



The calcium sulfate hemihydrate gets some water and changes into inter-meashed calcium sulfate dihydrate crystals

Properties

1. Biological properties Non toxic and non-irritant Cause dehydration to oral soft tissues 2. Interfacial properties Requires separating medium before pouring of gypsum cast and die materials 3. Mechanical properties Rigid after setting and should not be used in patients with sever undercuts Brittle after setting and could be fractured in clean cut lines during impression removal from shallow undercuts (i.e. fractured parts could be assembled).

Properties

4. Chemical propertiesLittle degree of dimensional changes due to the presence of anti-expansion additives.Material’s powder should be stored in dry, tightly closed containers to elongate the shelf life5. PracticabilityEasy manipulationResenable setting timeSomewhate short working time

2. IMPRESSION COMPOUND

It is a rigid thermoplastic impression material, used in stock trays to make 1ry impressions for edentulous patient has no or shallow undercuts.TypesLow fusing (True impression material, ..…softening temp. 45-60oC)High fusing (Tray compound, ….. softening temp. 70-130oC)

Presentation

L.F. Cakes for full impression H.F. Cakes for making special trays L.F. Green sticks for border tracing L.F. Cones for copper ring impressions

Composition

Thermoplastic resins… gives thermoplastic behaviorWaxes… Flow, stickiness, and give thermoplastic behaviorFiller (Talc, chalk)…consistency,  stickinessPlasticizer (Stearic acid) … moldability (plasticity) and facilitate the flowColoring agent


Setting Reaction
Thermoplastic material changes its Physical state by temperature changeOn Heating ……Hard  SoftOn Cooling ……Soft  HardThe material is bad thermal conductor, so sufficient heating is required for homogenous softening. Sufficient cooling is also required for proper hardening

Properties

1. Biological properties Non toxic and non-irritant Overheated material could burn the soft tissue 2. Interfacial properties Does not require separating medium before pouring the gypsum cast 3. Mechanical properties Rigid after cooling and should not be used in patients with sever undercuts

Properties

4. Chemical propertiesThermoplastic materialLittle degree of dimensional changes due to thermal change and release of stressesOverheating evaporates the plasticizer   Moldability5. PracticabilityEasy manipulation, Reasonable W.&S. TimesThe material could be reusedImpression should be disinfected before pouring in gypsum and sterilized before reusing

3. ZINC OXIDE PASTES

It is a rigid impression material, used in acrylic special trays with border tracing to make 2ry impressions for edentulous patient has no or shallow undercuts. Uses of Zinc oxide paste 1. Impression material 2. Bite registration 3. Surgical pack


Types 1. ZOE paste.. contains eugenol in its formulation 2. Eugenol-free paste.. contains carboxylic acid derivative instead of eugenol The eugenol-free material overcomes the drawbacks of the eugenol; 1. Eugenol taste could be unaccepted by some patients 2. Eugenol could irritate the soft tissue 3. Leaching of the eugenol out of the material could cause gastric troubles

Both types are presented as two-paste systems (Base and Catalyst)

Base Catalyst ZOE white paste red paste E-free white paste blue paste
Presentation



Composition
Catalyst paste
Base paste
Eugenol- containing materials Eugenol (Oil of clove) Fillers (Kaoline, talc) Accelerator (Zinc acetate) Moisture
Zinc oxide Oils (olive oils) Resins Accelerator (Zinc acetate )
Eugenol-free materials Carboxylic acid drevative Ethyl alcohol as vehicle

Setting Reaction

ZOE sets through chemical reaction named chelation(1)….ZnO + H2O  Zn(OH)2 (2)…. Zn(OH)2 + Eugenol (2HE)  ZnE2 + H2OThe reaction could also be accelerated in presence of higher temperatures, little amount of moisture, 1ry alcohol Eugenol-free material sets through chemical reaction named saponificationZnO + 2RCOOH  (RCOO)2 Zn + H2O

Properties

1. Biological properties Non toxic and non-irritant Eugenol could irritate the soft tissue 2. Interfacial properties Does not require separating medium because of its oil content 3. Mechanical properties Rigid after setting and should not be used in patients with sever undercuts

Properties

4. Chemical properties Impression inaccuracy could result from the warpage of acrylic trays Dimensionally stable on storage 5. Practicability Easy manipulation, Reasonable W.&S. Times Impression should be disinfected before pouring in gypsum, however long term immersion could cause erosion of impression surface

Make comparison between elastomeric impression material according to their biological property What are the uses of agar impression material

Make comparsim between elastomeric impression material according to their chemical property Mention the disadvantage of alginate impression material