Polycarboxylate cements
- Zinc carboxylase cements are also called polycarboxylates to recall their organic constituents polycarboxylic acid, they are classified into two types:
- Type I: for sealing (in prosthesis)
- Type II: for filling (in conservative dentistry).
1. Composition:
*The powder: it is essentially zinc oxide with sometimes a certain quantity of magnesium oxide and aluminum oxide. We can also find stannous fluoride, the effect of which is to improve the mechanical properties of the cement.
*The liquid: It is an aqueous solution containing about 40% polyacrylic acid and other organic acid such as itaconic acid.
2. Setting phenomena:
2.1. Setting reaction
*Mixing powder and liquid for 15 to 30 seconds gives a plastic mass which hardens over time.
*Zinc oxide reacts with polyacrylic acid to form a zinc polyacrylate structure.
Zinc oxide + polyacrylic acid zinc polyacrylate.
2.2. Setting time:
It is 1 to 5 minutes.
This setting time is influenced by certain factors:
-Powder liquid ratio
-The presence of adjuvant
-Grain granulometry
– It decreases when the temperature increases
3. properties
3.1. Physical properties:
Viscosity: viscosity is decreased as spatulation speed increases.
Thermal conductivity : very low.
Resistance : its compressive strength varies from 6.2 to 8.3 MPa (satisfactory).
Tensile strength 6 to 7 MPa
Solubility : after 7 days in water the solubility of this cement is 0.08% lower than the solubility of zinc phosphate cement. In citric acid, the solubility is very important.
3.2. Chemical properties:
Adhesion: Adhesion to dentin and enamel by the chelating power of carboxyl groups with respect to calcium.
3.3. Biological properties :
does not penetrate the canaliculi, because the molecular weight is high
● Low acidity of these cements at the start of mixing, the pH quickly evolves towards 7.
4. Handling:
*On a cold, dry glass plate , mix quickly after carefully proportioning the powder and liquid.
-the tooth surfaces must be clean
5. Indications:
Inlay sealing of prosthetic crowns
– protective base, intermediate base.
-temporary sealing.
6. Advantages:
-Its physicochemical and biological properties comparable to those of zinc phosphate and zinc oxide.
– kneading is easy and pulp reaction is minimal.
– adhesion to enamel is good.
7. Disadvantages:
– moderate compressive strength.
-working time is short.
Polycarboxylate cements
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Invisible orthodontic treatments are gaining popularity.
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