Definition: A water-based dental cement formed by an acid–base reaction between zinc oxide and polyacrylic acid (or related polycarboxylic acids). The set material is a zinc polycarboxylate matrix containing unreacted zinc oxide and filler particles. It is the first cement to offer true chemical adhesion to enamel and dentine.
Clinical Case:
A 52-year-old patient attends for cementation of a metal–ceramic crown on an upper second premolar. The preparation is somewhat short and slightly over-tapered, so you would like a luting cement that gives you some chemical adhesion to enamel and dentine, not just mechanical retention. The pulp is not especially close, and aesthetics at the margin are not critical. You decide to use a zinc polycarboxylate cement as your luting agent.
Uses
Zinc polycarboxylate cements are used mainly as:
Luting cements for:
Metal and metal–ceramic crowns and bridges
Inlays and onlays
Orthodontic bands
Bases under metallic restorations (e.g. amalgam), where adhesion and reduced pulpal irritation are desirable.
They are less commonly used now than in the past, having been partly superseded by glass-ionomer and resin-modified cements, but they remain important to understand conceptually.
How to use them in clinical practice?
You can think of using zinc polycarboxylate cement as a sequence:
Case selection
Indirect restoration or band where you would like:
A chemically adhesive water-based cement.
Lower pulpal irritancy than zinc phosphate.
Aesthetics at the margin are not critical (opaque cement).
Tooth and restoration preparation
Finish and clean the preparation; remove debris and plaque.
Do not desiccate dentine; a slightly moist surface favours adhesion.
Clean and, where appropriate, air-abrade the internal surface of the restoration or band.
Dispense
Depending on the product:
Powder–liquid system: polyacrylic acid in water as liquid, zinc oxide powder.
Freeze-dried acid system: polyacrylic acid incorporated into powder, water in the liquid.
Dispense recommended proportions onto a cool glass slab or mixing pad.
Mix
Incorporate powder into liquid fairly rapidly.
Mix to a smooth, glossy consistency.
Working time is shorter than for zinc phosphate; once the mix loses its gloss, it is no longer suitable for use and should be discarded.
Apply and seat
Apply cement to the internal surface of the restoration or band.
Seat the restoration promptly with firm, continuous pressure.
Maintain pressure until the cement has set sufficiently to resist displacement.
Clean-up
Remove excess cement as it reaches a rubbery consistency, using an instrument and floss.
After setting, check margins and occlusion.
For bases, a thicker mix can be placed on the cavity floor in much the same way, with care to avoid trapping air.
Constituents
Powder
Zinc oxide (primary reactive component)
May contain magnesium oxide or other oxides as modifiers
In some formulations, freeze-dried polyacrylic acid is incorporated into the powder
Liquid
Either an aqueous solution of polyacrylic acid (often around 40–50% concentration)
Or distilled water (when the polyacid is in the powder)
Some products also contain small amounts of tartaric acid or other additives to modify setting behaviour.
Chemistry
Zinc polycarboxylate cements set by a classic acid–base reaction:
Polyacrylic acid (a polycarboxylic acid) attacks the surface of zinc oxide particles.
Zinc ions are released and form cross-links between the polymer chains, creating a zinc polycarboxylate salt (a cross-linked polyacrylate matrix).
Unreacted zinc oxide particles remain embedded as filler.
Initially, the liquid is quite acidic, but the pH rises more quickly than with zinc phosphate because many acid groups are bound into the set salt. The reaction is exothermic but less so than with phosphoric acid.
Adhesion
This is where zinc polycarboxylate differs fundamentally from zinc phosphate: