Dental materials - Cements (ZnOE)

They are cements of low strength discovered in early 1860s

CLASSIFICATION

• Type I ZOE: For temporary cementation
• Type II ZOE: Permanent Cementation
• Type III ZOE: Temporary filling and thermal base
• Type IV ZOE: Cavity Liners 

ZOE cement is available as 

• Powder and liquid System

• Two – paste system 

SETTING REACTION 
In the First Step Hydrolysis of Zinc Oxide to its hydroxide takes place

Water is essential for the reaction (dehydrated zinc oxide will not react with dehydrated eugenol )

                                                ZnO + H2O →Zn (OH)2

The reaction proceeds as a typical acid – base reaction

                                          Zn (OH)2 + 2HE → ZnE2 +2H2O

                                (Zinc hydroxide)( Eugenol)- (Zinc eugenolate )

The Chelate formed is an amorphous gel that tends to crystallize imparting strength to the set mass

Structure of set cement: 

 The set cement consists of particles of zinc oxide embedded in a matrix of zinc eugenolate.

Setting time is around 4-10min.

FACTORS AFFECTING SETTING TIME

• Particle size: Smaller zinc oxide particles set faster
• Powder to liquid ratio: Higher the ratio, faster the set
• Addition of accelerators, e.g. alcohol, glacial acetic acid and water makes the cement set faster.
• Cooling the glass slab: Slows the reaction
• The set can be retarded by addition of glycol and glycerine which act as retarders. 

MANIPULATION

Power – Liquid System

Powder/liquid ratio: 4:1-6:1 wt%

After shaking the bottles gently, measured quantity of powder and liquid are dispensed onto a cool glass slab.

The bulk of the powder is incorporated into the liquid and saturated thoroughly in a circular motion with a stiff bladed stainless steel spatula.

Smaller increments are then added until the mix is complete.

TWO PASTE SYSTEM

Equal lengths of each paste are dispersed and mixed until a uniform colour is observed

MODIFIED ZINC OXIDE EUGENOL CEMENTS

EBA- alumina modified cements

Polymer reinforce ZOE cement

These were introduce to improve the mechanical properties of zinc oxide eugenol cement.

PROPERTIES

Compressive strength – ranges from 3 to 4 MPa up to 50-55 MPa 

Tensile strength – ranges from 0.32 to 5.3MPa 

Modules of elasticity – 0.22 to 5.4GPa 

Thermal properties – their thermal insulating properties are excellent

Solubility and disintegration – The solubility of the set cement is highest among the cements (0.4% - wt) they disintegrate in oral fluids

Adhesion – they do not adhere well to the enamel and dentin.This is one reason why they are not often used for final cementation of crowns and bridges.

Biological properties. 

PH and effect on pulp (pH is 6.6 to 8) they are the least irritating of all cements. Pulpal responses are mild.They have soothing effect on the pulp

USES

For temporary and permanent cementation

For temporary fillings

Cavity lines and basE

Pulp capping agent 

Secondary use – root canal restorations