6.4 Dental Materials Chemistry, Impression Media & Restorative Auxiliaries

Dental Materials Chemistry, Impression Media & Restorative Auxiliaries

Gypsum Products & Chemistry

Gypsum products are widely utilized in dentistry for fabricating diagnostic study casts, working models, and high-precision dies for fixed prosthodontic restorations. All dental gypsum products originate from the natural mineral calcium sulfate dihydrate ($CaSO_4 \cdot 2H_2O$).

Calcination Process and Chemical Reaction

Through industrial heating (calcination), water of crystallization is driven off from the dihydrate mineral to form calcium sulfate hemihydrate ($CaSO_4 \cdot \frac{1}{2}H_2O$):

$2(CaSO_4 \cdot 2H_2O) + \text{Heat} \longrightarrow 2(CaSO_4 \cdot \frac{1}{2}H_2O) + 3H_2O$

When calcium sulfate hemihydrate powder is mixed with water in the dental clinic, a reverse exothermic rehydration reaction occurs, releasing heat as the material recrystallizes back into calcium sulfate dihydrate:

$CaSO_4 \cdot \frac{1}{2}H_2O + 1.5 H_2O \longrightarrow CaSO_4 \cdot 2H_2O + \text{Heat}$

ADA Classification of Gypsum Products (Types I - V)

Gypsum Types (Density & Strength Increase -> W/P Ratio Decreases)
├── Type II Model Plaster (Beta-hemihydrate, Porous, W/P 0.45-0.50)
├── Type III Dental Stone (Alpha-hemihydrate, Prismatic, W/P 0.28-0.30)
├── Type IV Die Stone (Modified Alpha, Cuboidal/Dense, W/P 0.22-0.24)
└── Type V High-Expansion Stone (Densite, Base Metal Castings, W/P 0.18-0.22)

1. Type I: Impression Plaster: Obsolete for impression taking due to high rigidity.
2. Type II: Model Plaster:
   • Produced by calcination in an open kettle at 110-120°C.
   • Consists of $eta$-hemihydrate crystals that are irregular, porous, and spongy.
   • Requires the highest Water-to-Powder (W/P) ratio: 0.45 to 0.50 (45-50 mL water per 100 g powder).
   • Possesses the lowest compressive strength and highest setting expansion; used for diagnostic study models and mounting casts.
3. Type III: Dental Stone:
   • Produced by calcination under steam pressure in an autoclave at 125°C.
   • Consists of $\alpha$-hemihydrate crystals that are uniform, prismatic, and less porous.
   • W/P Ratio: 0.28 to 0.30.
   • Higher compressive strength; used for full or partial denture working casts.
4. Type IV: High-Strength / Low-Expansion Dental Stone (Die Stone / Densite):
   • Produced by boiling gypsum in a 30% calcium chloride ($CaCl_2$) solution.
   • Consists of modified $\alpha$-hemihydrate crystals that are dense, smooth, and cuboidal.
   • Requires the lowest W/P ratio: 0.22 to 0.24.
   • Highest hardness, maximum abrasion resistance, and minimal setting expansion; used for crown, bridge, and inlay dies.
5. Type V: High-Strength / High-Expansion Dental Stone:
   • W/P Ratio: 0.18 to 0.22.
   • Designed with high setting expansion to compensate for the high thermal shrinkage of base metal alloys used in crown fabrication.

Setting Time & Expansion Modifiers

• Accelerators: Potassium sulfate ($K_2SO_4$) and terra alba (set gypsum fine crystals) act as chemical accelerators, shortening setting time. Increasing spatulation speed/time also accelerates set.
• Retarders: Borax (sodium tetraborate) and trisodium phosphate slow crystal growth, lengthening setting time.
• Temperature Effects: Water temperature above 37°C retards the set; water approaching 100°C stops the setting reaction entirely because hemihydrate and dihydrate solubilities become equal.

Impression Materials & Media

Impression materials capture an accurate negative reproduction of oral tissues. They are divided into Hydrocolloids and Elastomeric Impression Materials.

Impression Materials
├── Hydrocolloids
│   ├── Reversible (Agar): Thermal Gelation (Gel -> Sol -> Gel)
│   └── Irreversible (Alginate): Chemical Gelation (Subject to Syneresis & Imbibition)
└── Elastomeric Materials
    ├── Polysulfide: Condensation (Water byproduct, high tear strength)
    ├── Condensation Silicone: Condensation (Alcohol byproduct, high shrinkage)
    ├── Polyether (Impregum): Addition (No byproduct, HydroPHILIC, stiff)
    └── Addition Silicone (PVS): Addition (No byproduct, Highest stability & detail)

Hydrocolloids

Hydrocolloids use water as the dispersion medium.

1. Reversible Hydrocolloid (Agar):
   • Undergoes a physical change dependent on temperature (thermal gelation).
   • Transition: Solid $\text{Gel} \xrightarrow{\text{Heat } 100^\circ\text{C}} \text{Liquid Sol} \xrightarrow{\text{Cool } 43^\circ\text{C}} \text{Solid Gel}$.
   • Requires specialized 3-bath conditioning units (liquefaction, storage, and tempering baths).
2. Irreversible Hydrocolloid (Alginate):
   • Undergoes a chemical reaction that cannot be reversed.
   • Main ingredients: Potassium/sodium alginate + Calcium sulfate dihydrate $\longrightarrow$ insoluble calcium alginate gel.
   • Trisodium phosphate is added as a chemical retarder to control working time.
   • Ideal for study models and mouthguards, but lacks detail for crown/bridge dies.

Dimensional Stability Problems in Hydrocolloids

• Syneresis: The exudation of water from the hydrocolloid gel onto its surface, causing shrinkage and distortion.
• Imbibition: The absorption of water when exposed to liquids, causing swelling and distortion.
• Clinical Rule: Alginate impressions must be disinfected and poured immediately (within 15-30 minutes) or stored in 100% relative humidity.

Elastomeric Impression Materials

Elastomers are synthetic rubber materials that undergo chemical polymerization to set.

• Polyvinyl Siloxane (PVS / Addition Silicone): The gold standard for crown and bridge impressions. PVS produces no volatile byproduct. (Secondary side-reactions may release hydrogen gas; dies should be poured after 1 hour unless palladium hydrogen absorbers are formulated into the material).

Composite Resin Restorative Materials

Dental composite resins consist of three primary components:

1. Organic Polymer Matrix: Bis-GMA (bisphenol A-glycidyl methacrylate) or UDMA (urethane dimethacrylate), blended with low-viscosity diluent monomers such as TEGDMA.
2. Inorganic Filler Particles: Silica, quartz, barium glass, or strontium glass (barium and strontium render composites radiopaque on radiographs).
3. Coupling Agent: Organosilane (silane). Silane chemically bonds the inorganic filler particles to the organic resin matrix, ensuring stress transfer and preventing water sorption degradation.
4. Initiator System: Camphorquinone (CQ) photoinitiator absorbs blue light at 460 to 470 nm, reacting with a tertiary amine accelerator to initiate free-radical polymerization.

Composite Classification by Filler Size

• Macrofilled: 75-80% filler, particle size 8-12 $\mu m$. High strength, but rough surface that stains easily (obsolete).
• Microfilled: 35-50% filler, particle size 0.04 $\mu m$ (submicron silica). Exceptional polishability and smoothness, but lower mechanical strength; indicated for non-stress Class III and V restorations.
• Hybrid / Microhybrid / Nanohybrid: 75-85% filler, mix of 0.4-1.0 $\mu m$ and submicron particles. Combines strength of macrofills with finish of microfills; universal for Class I-V.
• Nanofilled: 1-100 nm nanomers and nanoclusters. Superior polish retention, high strength, excellent physical properties.

Polymerization Shrinkage & C-Factor

Composites undergo 2% to 5% volumetric shrinkage during polymerization. This shrinkage creates tensile stress at the tooth-restoration interface.

$\text{C-Factor} = \frac{\text{Number of Bonded Cavity Surfaces}}{\text{Number of Unbonded Cavity Surfaces}}$

• Class I Occlusal Cavity: 5 bonded surfaces / 1 unbonded surface = C-Factor of 5 (highest stress).
• Class IV Cavity: 2 bonded surfaces / 4 unbonded surfaces = C-Factor of 0.5 (lowest stress).
• Clinical Management: Placing composite in incremental layers of $\le 2\text{ mm}$ minimizes C-factor stress, microleakage, and post-operative tooth sensitivity.

Dental Amalgam Chemistry & Metallurgy

Dental amalgam is an alloy formed by mixing liquid mercury with a silver-based alloy powder (trituration).

Amalgam Metallurgy
├── Composition: Ag (40-70%), Sn (15-30%), Cu (>12% High-Cu), Zn (0-2% Deoxidizer)
├── Low-Copper Reaction: Gamma (Ag3Sn) + Hg -> Gamma-1 (Ag2Hg3) + Gamma-2 (Sn8Hg)
└── High-Copper Reaction: Eliminates weak Gamma-2 (Sn8Hg) -> Forms Eta-prime (Cu6Sn5)

Alloy Components

• Silver (Ag): 40-70%; increases strength and setting expansion.
• Tin (Sn): 15-30%; controls expansion and reduces strength.
• Copper (Cu): High-copper alloys contain >12% to 30% Cu; low-copper alloys contain $<6%\text{ Cu}$.
• Zinc (Zn): 0-2%; acts as a deoxidizer. If zinc-containing amalgam is contaminated with moisture/saliva during condensation, water reacts with zinc to produce hydrogen gas, causing delayed expansion, severe pain, and marginal breakdown.

High-Copper vs Low-Copper Metallurgy

• Low-Copper Setting Reaction: $\gamma (Ag_3Sn) + Hg \longrightarrow \gamma_1 (Ag_2Hg_3) + \mathbf{\gamma_2 (Sn_8Hg)}$ The Gamma-2 ($ ext{Sn}_8 ext{Hg}$) phase is soft, weak, and highly susceptible to rapid chemical corrosion and marginal breakdown.
• High-Copper Setting Reaction: $Ag_3Sn + Cu_3Sn + Hg \longrightarrow \gamma_1 (Ag_2Hg_3) + \mathbf{Cu_6Sn_5 (\eta')}$ High copper reacts with tin to form the $ ext{Cu}_6 ext{Sn}_5$ ($\eta'$) phase, completely eliminating the corrosion-prone Gamma-2 phase. High-copper amalgams exhibit higher early compressive strength, lower creep, superior marginal integrity, and resistance to corrosion.

Tarnish vs Corrosion

• Tarnish: A superficial, non-destructive surface discoloration caused by reaction with sulfide ions. Can be polished off.
• Corrosion: Chemical or electrochemical breakdown penetrating into the body of the alloy. High-copper amalgams resist corrosion.