• Colloids are called "hydro"colloids when the dispersion medium is water.
• Can exist in sol state (viscous liquid) or gel state (semi-solid).
• Inserted in the mouth in the sol state (fluid enough to record details).
• Removed after the gel forms (exhibits elastic properties).

Molecules of the dispersed phase agglomerate to form fibrils, which form a network structure. The dispersion medium fills the spaces in the network by capillary action.

Causes of agglomeration:

A visco-elastic impression material that changes from sol to gel via a chemical reaction and cannot be reversed back to sol.

• Widely used for primary impressions: diagnostic casts, opposing casts for crowns/bridges, night guards, fluoride applications.
• NOT used for indirect restorations — doesn't produce fine details.

• Na/K alginate: salt of alginic acid — forms sol with water, reacts with calcium to form gel.
• Reactor — CaSO₄·2H₂O: source of Ca²⁺ ions to give calcium alginate gel.
• Na₃PO₄ (retarder): reacts with Ca²⁺ first, delaying gel formation and controlling working time.
  • Regular set: 3–4 min (higher retarder)
  • Fast set: 1–2 min (lower retarder) — used for children/high gagging reflex
• K·Ti fluoride: gypsum accelerator and hardener.
• Fillers (diatomaceous earth/silicate powder): strengthen the gel.
• Flavoring, coloring, disinfecting agents (chlorhexidine).

Chemical reaction, irreversible. On mixing powder + water, a sol forms:

Equipment: alginate powder, mixing rubber bowl, metal spatula, perforated/rim-locked stock tray.

A. Dispensing and mixing:

• Shake container before dispensing to distribute constituents evenly.
• Water goes into a clean bowl first, then powder (to avoid air bubbles).
• Mix with metal spatula in ONE direction (avoid destroying gel) against bowl walls (prevent air bubbles).
• Mixing time: 45 sec to 1 min depending on fast/regular set.
• Resulting mix should be creamy and smooth.

B. Impression taking:

• Loaded into a perforated stock tray (perforations retain the alginate) and inserted in the mouth.
• Remaining alginate in the bowl indicates setting: if bowl material is set, mouth material is also set.

C. Impression removal: Removed with a snap movement to increase tear strength and elastic recovery (viscoelastic material).

D. Disinfection: to avoid cross-infection — e.g. household bleach (sodium hypochlorite) 1:10 dilution.

• Spraying technique: rinse → spray disinfectant → wrap in soaked paper towel in sealed bag for 10 min → unwrap, rinse, shake, pour.
• Immersion technique: immerse in disinfecting agent for 10 minutes.

E. Impression pouring — no separating medium needed.

1. Dimensional stability — dimensionally unstable on storage due to:

• Syneresis: gel contracts, extrudes fluid → shrinkage.
• Dehydration: water evaporates in dry conditions → shrinkage.
• Imbibition: absorbs water in wet/humid conditions → swelling/expansion.

2. Biocompatibility: no adverse/allergic reactions; inhaling fine powder particles can cause pulmonary hypersensitivity.

3. Dimensional accuracy: not capable of reproducing details as fine as other elastomeric materials.

4. Flexibility: flexible → easy removal from undercuts with minimum stress.

5. Tear strength: relatively low. Clinical importance: impression thickness should not be less than 4–6 mm; remove with snap movement.

6. Elasticity: viscoelastic, elastic recovery 97.3%. Clinical importance: remove suddenly to reduce permanent deformation.

7. Compatibility with gypsum: no separating medium needed, but affects gypsum setting/surface:

• Water content inhibits gypsum surface setting → soft surface.
• Hydrophilic alginate absorbs water from gypsum mix → chalky appearance.

• Dust-free (dustless) alginate: glycol coats particles → prevents dust release during mixing (dust can cause silicosis and pulmonary hypersensitivity if inhaled).
• Chromatic alginate: color changes with pH to indicate manipulation stage — e.g. violet during spatulation, pink when ready to load tray, white when ready for insertion.

• Supplied as gel in sealed tubes.
• Uses: cast duplication; previously used as secondary impression for crowns, inlays, and onlays.
• Sets by a physical (reversible) reaction — cools and gels, can be re-liquefied by heating.

• Flexible, with very low dimensional changes compared with hydrocolloids.
• Different model and die materials can be poured with rubber materials.
• Much stronger and more resistant to tear than hydrocolloids.
• Produce more surface detail.

1. Polysulfide
2. Silicone — a. Condensation silicone, b. Addition silicone
3. Polyether

Present in 4 consistencies due to different filler concentrations:

Each consistency comes in two pastes (base + catalyst) — except putty base, which comes in a jar.

• For light, medium, and heavy consistencies: two pastes added in equal lengths on a waxed paper pad or glass slab.
• Spatula mixes until a homogeneous paste forms.
• Mixing takes 30–90 seconds depending on the material's amount and viscosity.

Hand kneading is used for putty consistency: a scoop of base is mixed with catalyst by hand.

Consists of a dispensing gun, base and catalyst cartridges, and disposable mixing tips.

Advantages of auto-mixing / dynamic mixing:

• Time saving.
• Uniform dispensing.
• Uniform mixing.
• Less air bubble incorporation.

I. Multiple Mix Technique

1. One-step impression technique:

• Light and heavy mixed separately on two mixing pads/spatulas.
• Heavy body loaded into a special tray; light body loaded into a syringe.
• Light body injected around the tooth preparation, then heavy material inserted over it in the mouth — forcing light body to adapt to prepared tissue.

2. Two-step impression technique:

• A primary impression is made first with heavy material.
• After removal, light body mix is added, then reinserted to record fine details (secondary impression).
• The primary impression acts as a special tray carrying the light body.

II. Putty Wash Impression Technique

1. Two-stage technique:

• Putty mixed, inserted in a stock tray, primary impression made.
• Putty in the tooth preparation area is cut away for light (wash) body space.
• Light body mixed, added to the putty impression, reinserted in the mouth.

2. Single-stage technique: light body injected into place, then mixed putty seated over it.

III. Monophase Impression Technique

• A mix of medium body is divided into two parts.
• First part inserted into a syringe and injected around the tooth preparation.
• Second part inserted into a special tray, placed over the injected material.

• Formulated so that under high shear forces (as injected from the syringe) → low viscosity (pseudoplasticity).
• Under low shear forces (placed in a tray with a spatula) → high viscosity.

• The first rubber impression material used clinically.
• Classified by viscosity: a. Light-body, b. Medium-body (regular consistency), c. Heavy-body.
• No putty version of this material.

Presented in two tubes: base and catalyst.

Polysulfide polymer (mercaptan) + Lead dioxide → Polysulfide rubber + H₂O

• A condensation polymerization reaction.
• Occurs by oxidation of mercaptan groups of two molecules, joining the molecules together.

• Mixing time: a streak-free mix can be made in 45 seconds.
• Working time: 3–5 min, depending on consistency.
• Setting time: 8–12 min.
• Flexibility: good, varies with consistency.
• Fine detail reproduction: excellent.
• Strength: tear strength 8× greater than hydrocolloids.
• Staining: the catalyst (lead dioxide) causes cloth/skin staining and bad odor.

• Light body may be injected into the preparation, with heavy body taken over it.
• Regular body impression is taken using a special tray.
• No need for snap removal (unlike alginate).

• Condensation silicone.
• Addition (vinyl) silicone.

Supplied according to consistency as: Light bodied, Very heavy bodied (Putty). Supplied as base and catalyst.

Dimethylsiloxane + Ethyl silicate --(Tin octoate catalyst)--> Silicone rubber + Ethyl alcohol

• Cross-linking occurs between siloxane polymer and ethyl silicate in the presence of tin octoate.
• Ethanol evaporates, causing high shrinkage in the first 24 hours.

Consistencies: Light bodied, Medium (regular) bodied, Heavy bodied, Very heavy (Putty).

Siloxane (terminal silane) + Siloxane (terminal vinyl) --(platinum salts)--> Silicone rubber

• No by-products formed → minimal dimensional changes.
• If hydroxyl group is present, a side reaction produces hydrogen gas → can cause air bubbles in gypsum dies poured within one hour.
• Slight palladium may be added as a hydrogen absorber.

• Addition silicones are naturally hydrophobic, but adding a surfactant makes them hydrophilic.
• This allows better wetting of soft tissue and more effective pouring of gypsum products.

• Working & setting times: shorter than polysulfide.
• Viscosity: lower than polysulfide → easier mixing.
• Dimensional changes: greater for condensation type than polysulfide over 24 h; addition silicone shows the LOWEST dimensional change of all impression materials.
• Fine detail: silicones readily reproduce fine detail.
• Strength: lower than polysulfide but higher than hydrocolloids.
• Shelf life: shorter than polysulfide.
• Permanent deformation: lower for silicones than polysulfide (higher cross-linking); addition silicone shows lower values than condensation type.
• Flexibility: lower for silicones than polysulfide; addition silicone lower than condensation type.

• A primary impression is taken before preparation with putty; a wash impression is taken over it after preparation — or the light body may be injected into the preparation directly.
• Putty mixing is by hand; light body is mixed with a spatula or automixer.
• Avoid contact between latex gloves and addition silicone — latex can prevent/retard setting by poisoning the platinum catalyst.

• Hydrophilic — good wetting properties.
• Short working time and high stiffness.

• Supplied as light, regular, and heavy bodied.
• Base: low-molecular-weight polyether polymer containing an aziridine ring.
• Catalyst: sulfonic acid ester.

• Addition polymerization reaction.
• Called ring-opening polymerization — occurs by opening of the aziridine ring.

• Shortest working time (2–5 min) among all rubber materials.
• Hydrophilic, good wetting properties — but absorbs water, so must be dried after washing.
• High stiffness (low flexibility).
• Permanent deformation is less than polysulfide but higher than silicones.
• Low flow properties — viscosity can be reduced using a thinner (increases working time and flexibility).
• Very good shelf life.
• The catalyst is irritant to skin and oral tissues.

• Also known as 3-D intraoral scanning.
• The latest technology for capturing a replica of the mouth.
• Performed using a hand tool connected to a computer with advanced software.
• Dentist creates a virtual model of the hard and soft tissues of the mouth.

• High degree of accuracy.
• Impression can be emailed directly to the dental laboratory.
• Often used for same-day dental restorations, reducing patient visits.
• Less chair time needed.
• More pleasant for patients who struggle with conventional impressions.
• Eliminates conventional-material impression errors → fewer remakes.
• Can easily be stored electronically.

• Dental Model (Cast): a positive replica of the teeth and adjacent structures.
• Dental Die: a positive replica of the individual prepared tooth on which inlays, onlays, and crowns are made.

• Ability to reproduce all fine details in the impression.
• Dimensionally stable.
• High abrasion resistance — to resist scratching during wax pattern carving.
• Good strength with a smooth surface.
• Compatibility with all impression materials.
• Reasonable setting time.
• Color contrast with wax pattern, to easily distinguish between them.

• A mineral found in nature.
• Formed of calcium sulfate dihydrate (CaSO₄·2H₂O), known as Alabaster.

CaSO₄·2H₂O is ground then heated, causing water loss and forming calcium sulfate hemihydrate:

1. Impression plaster (Type I) — used as a secondary impression material.
2. Model plaster (Type II).
3. Dental stone / Hydrocal (Type III).
4. High-strength dental stone / Densite (Type IV).
5. High-strength, high-expansion dental stone (Type V).

All gypsum products are chemically the same (CaSO₄·½H₂O) but differ in: manufacturing method, particle size/shape, water/powder ratio, physical/mechanical properties, and uses.

When hemihydrate is mixed with water, the reaction reverses and the dihydrate re-forms:

The reaction is called crystallization, forming calcium sulfate dihydrate crystals.

• Powder is added into water in a mixing rubber bowl (ensures wetting, avoids air bubbles).
• Metal spatula for hand mixing against the bowl wall; mechanical mixing can be done under vacuum to reduce entrapped air.
• Vibration is done after mixing and during pouring to minimize air bubbles.
• The mix flows onto the impression surface.
• Allowed to set for 45 to 60 minutes.
• After separation, the model should be disinfected.

• Casts should set for 24 hours before disinfection.
• Spraying is better than immersion.
• Common disinfectants: sodium hypochlorite, iodophors, chlorine dioxide.

• Initial setting time: time from start of mix until loss of gloss (8–10 min).
• Final setting time: reached when the material can be safely handled, cool (exothermic reaction over).

• Setting expansion: results from crystal growth and outward thrusting of growing dihydrate crystals during setting.
• Hygroscopic expansion: if gypsum is soaked/immersed in water during setting, water fills pores → additional crystal growth. It is double the setting expansion.

1. W/P ratio: increasing water/powder ratio decreases nuclei of crystallization → retards setting.
2. Spatulation (mixing): rapid, prolonged spatulation accelerates setting and increases setting expansion (breaks up dihydrate → more nuclei).
3. Temperature: increasing water temperature to a certain level accelerates setting.
4. Impurities: accelerate setting by providing extra nuclei of crystallization.
5. Particle size: smaller, finer particles set faster (more nuclei of crystallization).
6. Accelerators and retarders:
   • Accelerators: potassium sulfate or set gypsum — act as crystallization sites.
   • Retarders: blood, saliva, alginate.