ZINC POLYCARBOXYLATE CEMENTS:
Zinc Polycarboxylate cement was the first cement to bond to tooth structure. It is also known as zinc poly acrylate cement. This cement was developed by Dr. DC Smith, dentist in 1968.
Composition
Powder
ZnO
Tin or Magnesium oxide 1-5%
Al2O3 10-40%
Stannous Fluoride 4% (Modifies the setting time and enhances manipulative properties. It does not impart any anti Cariogenic property because fluoride released is only 10–15% of that released by the glass Ionomer cement. However, it acts as a strengthening agent.)
Liquid
Aqueous solution of poly acrylic acid Or a copolymer of acrylic acid with other carboxylic acids such as itaconic acid
Note:
Anhydrous water-mixable Polycarboxylate cements are also available commercially with freeze-dried acid incorporated into the powder for reliable mixing, setting and handling of the cement
Setting Reaction:
The setting reaction is similar to that for glass ionomers. Zinc oxide behaves as a basic oxide and undergoes an acid– base reaction with acid groups in the polyacid to form a reaction product which consists of cores of un reacted zinc oxide bound together by a salt matrix in which poly acrylic acid chains are cross-linked through divalent zinc ions. The setting reaction involves particle surface dissolution by the acid that releases zinc, magnesium and tin ions, which bind to the polymer chain
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MANIPULATION:
P/L ratio: 1.5 parts of powder to 1 part of liquid by weight.
The powder bottle is fluffed and the powder dispensed on a glass slab. The liquid is dispensed just before mixing. Otherwise, it loses water to the atmosphere very rapidly and results in marked increase in its viscosity. About 90% of powder is added immediately to the liquid and mixed with a stiff spatula for 30 seconds. Remainder of the powder is added to adjust the consistency. Viscosity decreases as the rate of spatulation increases and the correct consistency for luting is when the spatula pulls up the mix but it strings back by its own weight. The cement should be used before it loses its glossy appearance (or before the onset of cob webbing). The glossy appearance indicates a sufficient number of free carboxylic acid groups that are vital for bonding to tooth structure. A dull looking mixture on the other hand, means that the insufficient number of unreacted carboxyl groups is available to bond to the calcium in the tooth surface
Practical tips:
1. Poly acrylic acid solutions are more viscous than the liquids of other cements, which affects the ease of mixing of the material.
2. The cement should be applied to the tooth as soon as possible after mixing, otherwise poor adhesion may result. If a cement mix begins to ‘cobweb’ on manipulation, or loses gloss, it should be discarded.
3. Polycarboxylate cement will adhere to instruments, particularly those made of stainless steel therefore,
a. It is useful to use alcohol as a release agent for the mixing spatula
b. Instruments should be cleaned before the cement sets on them
c. If cement does inadvertently adhere to a spatula, it can be removed by boiling in sodium hydroxide solution
Working time:
2.5-3.5 Min [Much shorter than zinc phosphate cement (5min). lowering the temperature of the reaction can increase the working time. Unfortunately, cool glass slab can cause the poly acrylic acid to thicken and makes the mixing procedure difficult. Therefore, it is suggested that the powder should be refrigerated before mixing. The liquid should not be chilled, as this encourages gelation due to hydrogen bonding
Setting time:
6-9 min
PROPERTIES:
Mechanical Properties:
1. Compressive strength:
55-85 MPa (Inferior to Zinc Phosphate cement)
2. Tensile strength:
8-12 MPa (More than Zinc Phosphate cement)
3. Modulus of elasticity:
2.4 to 4.4 GPa (Significance: As the modulus of elasticity of Polycarboxylate cement is one-third that of zinc phosphate(13GPa) it exhibits significantly greater plastic deformation than zinc phosphate thus, it is not well suited for use in regions of high masticatory stress or in cementation of long-span prosthesis.
Film thickness:
25microm or less
Pseudo plasticity:
Freshly mixed cement has honey-like consistency with the property of being pseudo plastic and shows shear-thinning behavior. Therefore, though the mixed cement appears too thick, it flows adequately under pressure to a film thickness of 25–35 m. It shows significant plastic deformation under dynamic loading for a long time after cementation which may suggest its use be limited to single unit restorations or short span fixed partial dentures.(Significance: Correctly mixed Polycarboxylate cement has a consistency similar to that of honey and the appears to be too viscous to allow proper seating. However, this is normal and should not be of concern since the cement undergoes ‘shear thinning’ which reduces the apparent viscosity during the seating of the crown.)
Effect on Pulp:
The Polycarboxylate materials are acidic, but not as irritant as phosphate cements, for two reasons.
1. Polyacrylic acid is a weaker acid than phosphoric acid and the polyacid chains are too large and
2. Lack the mobility required to penetrate dentinal tubules.
Note:
Despite the more biocompatible nature of these materials, they are not widely used as linings in very deep cavities unless a sub lining of a calcium hydroxide or zinc oxide/eugenol material is used.
Solubility:
The solubility of this cement in water is low (0.1- 0.6 wt%) but, when it is exposed to organic acid of less than pH 4.5, the solubility markedly increases. (Clinical significance: As it has relatively low resistance to erosion in an acidic environment, it may not be the best choice as a luting agent for patients who have gastric reflux or frequently consume acidic beverages.
Adhesive Property:
1. Adhesion to tooth:
The adhesive bond is primarily to enamel although a weaker bond to dentin also forms as a result of chelation reaction between the carboxyl groups of the cement and calcium in the tooth structure; hence, the more mineralized the tooth structure, the stronger the bond. This cement is hydrophilic so is capable of wetting dentinal surfaces. (Clinical significance: Unlike zinc phosphate cement, as this cement bonds chemically to the tooth structure, varnish should not be applied below Zinc Polycarboxylate base)
2. Adhesion with non-precious alloys:
They will bond with the non-precious alloys probably because of the presence of oxide layer. The materials form a strong bond with stainless steel which makes them useful for attaching orthodontic bands. (Significance: Care must be taken when using steel instruments for mixing and placing the cement Excess material should be removed from such instruments before it sets, otherwise a tenacious bond will form. A thin layer of petroleum jelly should be coated on the outer surface of the prosthesis made up of non precious alloy excluding the margins to prevent excess cement from adhering to its surface.)
3. Adhesion with noble alloys:
It forms weak bond with gold due to highly inert nature of gold alloys usually resulting in an adhesive failure at the interface. In order to improve the bond strength, the intaglio surface of the noble metal prosthesis should be sand blasted. (Note: Long sandblasting should be avoided to minimize the risk for margin deformation.
4. Adhesion with porcelain:
They show no perceptible bond with porcelain. The set materials are opaque due to a high concentration of un reacted zinc oxide cores. This may detract from the appearance of porcelain crowns, particularly if the cement lute margin is visible.
Conditioning of teeth:
As this cement provides chemical bond to the tooth, the surface of the tooth should be clean to ensure intimate contact and interaction between the cement and tooth. It is recommended to apply a10% Poly acrylic or Maleic acid solution for 10-15 sec, followed by rinsing with water.
Removal of excess cement:
During setting, the zinc Polycarboxylate cement passes through rubbery stage. The excess cement should not be removed at this stage because there is a danger that some of the cement may be pulled out form beneath the margins, leaving a void. Excess cement should be removed when it becomes hard or it should be removed as soon as the seating is completed.
USES:
1) The materials are primarily used as luting cements for metal crowns, bridges and inlays
2) As cavity base materials.
3) For the attachment of orthodontic bands.
4) It is recommended for vital or sensitive teeth with preparations close to the pulp and for cementing single units or short span bridges in areas of low stress
5) Zinc Polycarboxylate cement can occasionally be used for luting provisional restorations where maximal retention is needed or in non retentive provisional restorations).
Advantages:
1. Chemical bonding
2. Biocompatibility with the dental pulp due to:
- Rapid rise in pH after mixing
- Polyacrylic acid being weaker than phosphoric acid
- Lack of tubular penetration from large and poorly dissociated poly acrylic acid molecules
3. Favorable tensile strength(8–12 MPa)
Disadvantages:
1. Low tensile strength
2. Not resistant to acid dissolution
3. Deforms under loading
4. Manipulation critical
5. Can be difficult to obtain low film thickness. Early rapid rise in film thickness that may interfere with proper seating of a casting
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