Zinc Polycarboxylate cement was the First cement that was developed with the property of an adhesive bond to tooth structure along with some metallic cast restorations.
Zinc Polycarbolylate cement is available as powder and liquid
Also as a Water settable cement in which the polyacrylic acid is freeze dried powder that is mixed with the cement powder. The liquid is water or a weak solution of NaH2PO4
- Zinc oxide
- Magnesium oxide
- Bismuth oxide
- Aluminum oxide
- Stannous oxide – Modifies the setitng time, increases the STRENGTH and acts as Anticariogenic agent.
- 30 to 40% aqueous solution of Polyacrylic acid and its Copolymers of molecular weight between 30,00 to 50,000
- Unsaturated Carboxylic Acid Ex: Itaconic acid, Maleic Acid, Tricarboxylic acid.
The Setting reaction of Zinc Polycarboxylate cements involves the reaction between the outer portion of the powder particles and the ionized Copolymer of Acrylic acid and Itaconic acid.
- Powder and Liquid are mixed.
- The Acid attacks the powder and causes a release of Zinc, magnesium and tim ions.
- These ions react with carboxy groups of adjacent polyacid chains so that a cross linked salt is formed as the cement sets in the same way as occurs for the glass ionomer cements.
- Zinc provides the cross links rather than Calcium and Aluminum
- The result of the reaction is a cored structure in which the unreacted powder particles are bound by a matrix of zinc polyacrylate.
- The setting reaction is very fast which must be retarded to allow for a resonable working time.
How to control Setting time of Zinc Polycarboxylate cement:
- The manufacturer – depending upon the method of manufacture of the powder and liquid the setting time can be altered which is in the hands of the manufacturer.
- Use a Cold Glass Slab: Cold Glass slab retards the setting reaction and prolongs the setting time of Zinc Poly Carboxylate Cement. But this also causes the polyacrylic acid to thicken in consistency. This increases the viscosity making the mixing procedure more difficult. So make sure that you refrigerate only the powder before mixing.
- The powder should be incorporated into the liquid in one large increment because the reaction is not delayed by incremental mixing.
- Adding water and increasing the temperature of the mixing environment accelerates the setting time.
Bonding to Tooth Structure:
Bond strength is greater to the enamel than dentin because polyacrylic acid reacts with Calcium ions, enamel, dentin via carboxyl group.
Properties of Zinc Polycarboxylate Cement:
- pH of Liquid in Zinc Polycarboxylate: 1.7 which is readily neutralized by the addition of powder which makes the pH higher than Zinc Phosphate cement.
- In-spite of the initial low pH of Zinc Polycarboxylate cement it is not an irritant to the Pulp. Which can be attributed to the large size of the Polyacrylic acid molecules which limit the diffusion through dentinal tubules.
- It is highly bio compatible to the Pulp which is similar to ZOE cements.
- It is Anticariogenic, Fluoride release from this cement is only 15 – 20% of the amount released from glass ionomer cement.
- Working time: 2.5 minutes
- Setting time is 6 to 9 minutes
- Compressive Strength: 55Mpa (inferior to Zinc phosphate cement)
- Tensile Strength: 6.2 Mpa (Slightly higher than Zinc phosphate cement )
- Film Thickness: 21 micrometers. It is pseudoplastic and more viscous than Zinc phosphate cement.
- Solubility: 0.6% solubility of the cement in water is low ut more in acids. It is comparable to that of zinc phosphate cement.
- Viscosity is slightly higher than Zinc Phosphate cement. The mixed cement appears thick but flows readily when applied to the tooth surface.
- Thermal insulation: Good. In thickness of 1.5mm. At less than 1.5 mm thickness it is not an effective thermal insulator when compared to the other cements.
- Opaque nature: It is Opaque because of the large quantity of un reacted zinc oxide.
Manipulation or mixing of Zinc Polycarboxylate Cement:
- Dispense one drop of liquid and powder on a mixing pad. Powder/liquid ratio is 1.5 parts of powder to 1 part of liquid by weight.
- Incorporate half of the powder in the liquid in patting and folding motions using a small area of the pad surface. Add small increments of powder to the mix, using the same mixing motions until acquiring the proper consistency.
- Mixing time is 30 to 60 seconds
- Cement should be used mmediately because the working time is short.
- Working time – 2.5 Minutes
- The cement is no longer usable when it looses its lustre or start to “cobweb”
- Setting time is 6 to 9 minutes
- polycarboxylate cement will adhere to instruments particularly those made of stainless steel. Thus it is useful to use alcohol i.e spirit or dry powder as a release agent for the mixing spatula.
- Instruments should be cleaned before the cement sets on them.
- Low irritancy
- Adhesion to tooth
- Easy manipulation
- Strength tensile (similar to Zinc Phosphate)
- Solubility (similar to Zinc Phosphate)
- Film thickness (similar to Zinc Phosphate)
Disadvantages of using Zinc Polycarboxylate Cement:
Needs accurate proportioning of the powder and salt
Lower compressive strength (similar to Zinc Phosphate cement)
Shows greater visco elasticity (similar to Zinc Phosphate cement)