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Dental Materials

CLEANING AND PICKLING ALLOYS

The surface oxidation or other contamination of dental alloys is a troublesome occurrence. The oxidation of base metals in most alloys can be kept to a minimum or avoided by using a properly adjusted method of heating the alloy and a suitable amount of flux when melting the alloy . Despite these precautions, as the hot metal enters the mold, certain alloys tend to become contaminated on the surface by combining with the hot mold gases, reacting with investment ingredients, or physically including mold particles in the metal surface. The surface of most cast, soldered, or otherwise heated metal dental appliances is cleaned by warming the structure in suitable solutions, mechanical polishing, or other treatment of the alloy to restore the normal surface condition.

Surface tarnish or oxidation can be removed by the process of pickling. Castings of noble or high-noble metal may be cleaned in this manner by warming them in a 50% sulfuric acid and water solution . . After casting, the alloy (with sprue attached) is placed into the warmed pickling solution for a few seconds. The pickling solution will reduce oxides that have formed during casting. However, pickling will not eliminate a dark color caused by carbon deposition 

The effect of the solution can be seen by comparing the submerged surfaces to those that have still not contacted the solution. the ordinary inorganic acid solutions and do not release poisonous gases on boiling (as sulfuric acid does). In either case, the casting to be cleaned is placed in a suitable porcelain beaker with the pickling solution and warmed gently, but short of the boiling point. After a few moments of heating, the alloy surface normally becomes bright as the oxides are reduced. When the heating is completed, the acid may be poured from the beaker into the original storage container and the casting is thoroughly rinsed with water. Periodically, the pickling solution should be replaced with fresh solution to avoid excessive contamination.

Precautions to be taken while pickling

With the diversity of compositions of casting alloys available today, it is prudent to follow the manufacturer's instructions for pickling precisely, as all pickling solutions may not be compatible with all alloys. Furthermore, the practice of dropping a red-hot casting into the pickling solution should beavoided. This practice may alter the phase structure of the alloy or warp thin castings, and splashing acid may be dangerous to the operator. Finally, steel or stainless steel tweezers should not be used to remove castings from the pickling solutions. The pickling solution may dissolve the tweezers and plate the component metals onto the casting. Rubber-coated or Teflon tweezers are recommended for this purpose.

Temporary Filling Materials

Applications / Use

While waiting for lab fabrication of cast restoration
While observing reaction of pulp tissues


Objectives

Provide pulpal protection
Provide medication to reduce pulpal inflammation
Maintain the tooth position with an aesthetic restoration

Classification

Temporary filling cements
Temporary filling resins


Components

Temporary filling cements

1. Zinc oxide-eugenol cement with cotton fibers added
2. Polyme r powder-reinforced zinc oxide eugenol cement

Temporary filling resins

•    MMA / PMMA filling materials
•    Polyamide filling materials
•    BIS-GMA filling materials
 

Root canal sealers

Applications

Cementation of silver cone gutta-percha point
Paste filling material

Types

Zinc oxide-eugenol cement types
Noneugenol cement types
Therapeutic cement types

properties

Physical-radiopacity
Chemical-insolubility
Mechanical-flow; tensile strength
Biologic-inertness

Gingival tissue packs

Application-provide temporary displacement of gingival tissues
Composition-slow setting zinc oxide-eugenol cement mixed with cotton twills for texture and strength


Surgical dressings
1.Application-gingival covering after periodontal surgery
2. Composition-modified zinc oxide-eugenol cement (containing tannic, acid. rosin, and various oils)

Orthodontic cements

Application-cementation of orthodontic bands
Composition-zinc phosphate cement 

Manipulation

Zinc phosphate types are routinely mixed with cold or frozen mixing slab to extend the working time
Enamel bonding agent types use acid etching for improved bonding
Band, bracket, or cement removal requires special care
 

COMPOSITE RESINS

Reaction

  • Free radical polymerization

Monomers + initiator. + accelerators-+ polymer molecules

  • Initiators-start polymerization by decomposing and reacting with monomer
  • Accelerators-speed up initiator decomposition
  • Amines used  for accelerating self –curing  systems
  •  Light  used for accelerating light-curing systems

Retarders or inhibitors-prevent premature polymerization

Physical reaction-cooling causes reversible hardening

Chemical reaction-irreversible reaction during setting

Dental Solders

Applications-bridges and orthodontic appliances

Terms

Soldering -joining operation using filler metal that melts below 500° C

Brazing -joining operation using filler metal that melts above 500°C

Welding-melting and alloying of pieces to be joined

Fluxing
 -Oxidative cleaning of area to be soldered
 - Oxygen scavenging to prevent oxidation of alloy being soldered
16- 650 -- 650 fineness solder to be used with 16-karat alloys; fineness refers to the gold content

Classification

a. Gold solders-bridges
b. Silver solders-gold-substitute bridges and orthodontic alloys

Structure of gold solders

Composition-lower gold content than of alloys being soldered

Manipulation-solder must melt below melting temperature of alloy

Properties

1. Physical-similar to alloys being joined
2. Chemical-more prone to chemical and electrochemical corrosion
3. Mechanical-similar to alloy  being joined
4. Biologic-similar to alloys being joined
 

Cement Bases

Applications

•    Thermal insulation below a restoration
•    Mechanical protection where there is inadequate dentin to support amalgam condensation pressures

Types

•    Zinc phosphate cement bases
•    Polycarboxylate cement bases
•    Glass ionomer cement bases (self-curing and light-curing)

Components

o    Reactive powder (chemically basic)
o    Reactive liquid (chemically acidic)

Reaction

o    Acid-base reaction that forms salts or cross linked matrix
o    Reaction may be exothermic

Manipulation-consistency for basing includes more powders, which improves all of the cement properties

Properties

Physical-excellent thermal and electrical insulation

Chemical-much more resistant to dissolution than cement liners

    Polycarboxylate and glass ionomer cements are mechanically and chemically adhesive to tooth structure

    Solubility of all cement bases is lower than cement liners if they are mixed at higher powder- to-liquid ratios

Mechanical- much higher compressive strengths (12,000 to 30,000 psi)
  
 Light-cured hybrid glass ionomer cements are the strongest
    Zinc oxide-eugenol cements are the weakest

Biologic (see section on luting cements for details)

    Zinc oxide-eugenol cements are obtundent to the pulp
    Polycarboxylate and glass ionomer cements are kind to the pulp
 

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