Zinc-phosphate cements: chemistry and structure
The setting process of these materials is accompanied by a pronounced exothermic reaction (active heat release). This process is based on the chemical interaction between the powder (zinc oxide) and the liquid (phosphoric acid), which results in the formation of zinc phosphate. The equation is as follows: 3ZnO + 2H3PO4 → Zn3(PO4)2 + 3H2O.
The finished hardened mass has a heterogeneous structure. It consists of peculiar grains, where the core is unreacted zinc oxide (and other oxide additives), and the binding matrix and shell are formed by zinc phosphate.
An important aspect is the change in acidity level during the crystallization process. Immediately after mixing the components, the mass has an extremely high acidity (pH from 1.0 to 2.0) due to the presence of free orthophosphoric acid. After an hour, the pH gradually rises to 4.0–5.0, and the material becomes completely neutral only after twenty-four hours. The neutralization rate is directly affected by the thickness of the mix: the more liquid added to the mass, the longer it retains an aggressive acidic reaction. A thick mix neutralizes faster — from five to thirty minutes. The optimal readiness of the mass is determined simply: when the spatula is pulled away, short teeth up to one millimeter are formed, and the mixture breaks off without stretching into long threads.
Application and limitations of zinc-phosphate cements
The scope of application of this class of materials is quite wide, but requires strict adherence to consistency for each task. A thick mix is used to create insulating bases, as well as for temporary filling of primary teeth if no more than a year remains before their physiological shedding (in this case, modifications with the addition of silver are often used, for example, the preparation Argil). A more fluid consistency is necessary for the reliable cementation of various fixed prosthetic appliances: artificial crowns, inlays, posts, and fixed bridges. In addition, the material is used as a core build-up under a crown when filling permanent teeth, as a temporary dressing, and for retrograde root canal filling (exclusively during root-end resection surgery).
The main and absolute contraindication is the use of zinc-phosphate compositions without protection in deep carious cavities. Free phosphoric acid combined with thermal burn from the exothermic reaction exerts a powerful irritating and toxic effect on the pulp. To minimize this harm, the dentin must be pre-coated with a special insulating varnish.
The market offers both basic compositions (e.g., "Unicem", Adhesor) and modified variants. To increase strength and provide a bactericidal effect, silver ("Bactericidal Phoscin"), copper phosphates (Harvard Kupferzement), or bismuth oxides ("Dioxyvisphate") may be added to the powder.
Silicate cements: compositional features
Silicate materials are also supplied as a powder-liquid system. The powder part is a finely dispersed glass based on aluminosilicates and fluoride salts. The liquid is an aqueous solution of phosphoric acid (at a concentration of 30–40%) enriched with aluminum and zinc hydrates.
Each component of the powder is responsible for certain physical and mechanical characteristics of the future filling. Silicon dioxide (SiO2), the proportion of which reaches 47%, guarantees high transparency of the material and a characteristic luster after polishing. Aluminum oxide (Al2O3) provides mechanical strength exceeding that of phosphate alternatives. A small inclusion of zinc oxide (ZnO) causes rather weak adhesion (bonding) of silicate masses to hard tooth tissues. A popular representative of this group in domestic practice is the Czech preparation Fritex.
Mixing silicate masses requires compliance with the temperature regime (18–20 °C) and precise proportions: 7–8 drops of liquid (about 0.33–0.35 ml) are taken per one gram of powder. The mass is plastic and easily amenable to shaping.
Pros, cons, and handling rules for silicate materials
Silicate cements have a number of significant advantages. They are visually similar to natural tooth enamel due to transparency and luster, possess high mechanical strength, and have a coefficient of thermal expansion close to tooth tissues. A high fluoride content provides a local caries-static (preventive) effect. In addition, these materials are affordable and easy to mix.
However, they have serious drawbacks that limit their application. Like phosphate alternatives, silicate cements are extremely toxic to the pulp. The prolonged presence of uncombined phosphoric acid leads to its diffusion through the dentinal tubules, which inevitably causes pulp necrosis. Therefore, the most important rule of work: a silicate filling is placed only over an insulating base of phosphate cement.
Among other disadvantages are low adhesion, brittleness, fragility, and insufficient compressive strength. Because of this, the material is categorically unsuitable for creating contour fillings (restoring crown angles) — under masticatory load and contact with antagonist teeth, such restorations quickly chip. Also, silicate fillings exhibit noticeable shrinkage that disrupts marginal adaptation, are not radiopaque, possess high abrasiveness, and are subject to disintegration (dissolution) under the action of saliva and organic acids of the oral cavity.
Briefly: answers to topical questions
What components are included in the powder of alginate impression materials?
The main component of alginate impression materials is sodium alginate (the sodium salt of alginic acid). Depending on the material group, its composition includes a multicomponent powder or a complex powder composition. When mixed, the powder is combined with water, paste, or a 5% aqueous solution of sodium alginate, forming a plastic mass. The exact chemical list of all powder components is not provided in the sources.
What chemical reaction ensures the setting of the alginate mass?
The setting of the alginate mass is ensured by a chemical reaction of macromolecule cross-linking. This process of transition of the material from a plastic to an elastic state does not stop even after removing the impression from the oral cavity. The consequence of this reaction is the release of free liquid — alginic acid — onto the surface of the impression, which can slow down the gypsum setting process when pouring the model.
For which clinical situations is the use of alginate impression materials indicated?
The use of alginate impression materials is indicated for obtaining accurate impressions from various hard and soft surfaces of the prosthetic field. The main clinical situations include:
- Fabrication of crowns — including acrylic crowns, for which an impression made of alginate materials is optimal.
- Reproduction of prosthetic bed tissues — to achieve high relief accuracy.
The material has high elasticity, which makes it possible to take impressions in the presence of undercuts.
What are the main disadvantages of alginate impression materials?
The main disadvantage of alginate impression materials is the release of free liquid onto the impression surface. In the process of the ongoing chemical reaction of macromolecule cross-linking, alginic acid is released. This has a negative impact on subsequent work stages, as the acid disrupts the surface purity of the plaster model and slows down the gypsum setting process.
What requirements are imposed on impression trays when taking impressions with alginate masses?
Impression trays anatomically adapted to the shape of the dentition are used to take impressions. The main types of trays are:
- Perforated trays — have holes for draining excess material and retention of the impression mass.
- Non-perforated trays — equipped with a bordering retention rim.
- Individual trays are necessary in many cases when manufacturing removable partial dentures; in case of significant alveolar ridge atrophy, it is advisable to combine them with the volumetric base modeling technique.
Trays are selected by size (for children, adults, narrow jaws) and shape (for edentulous jaws, distal extension defects).
What is the difference between alginate and silicone impression materials?
Alginate and silicone materials differ in composition, physical and mechanical properties, and handling characteristics.
| Characteristic | Alginate materials | Silicone materials |
|---|---|---|
| Main component | Sodium alginate | Rubber, filler, plasticizer, catalyst |
| Tensile strength | At least 3 kg/cm² | At least 10 kg/cm² |
| Setting time | 5–7 min (setting) | 4–6 min (vulcanization) |
| Surface detail accuracy | Error margin 10 µm | Reproduction of a 0.04 mm wide groove |
| Handling characteristics | Release alginic acid | Require thorough drying before casting |
Class and exam questions
Free phosphoric acid remains in the material for a long time. Penetrating the dentinal tubules, it causes a chemical burn and pulp necrosis.
Free phosphoric acid remains in the material for a long time. Penetrating the dentinal tubules, it causes a chemical burn and pulp necrosis.
How to determine the correct consistency of zinc-phosphate cement?
When tearing the spatula away, the mass should break off rather than stretch into threads, leaving short spikes up to 1 mm high on the surface.
Can tooth angles be restored with silicate cement?
No. Despite its hardness, the material is very fragile and brittle, resists compression poorly, and easily chips upon contact with antagonists.
What else to study on this topic
- Mechanism of the toxic action of phosphoric acid on dental tissues
- Comparative strength analysis of silicate and phosphate cements
- The role of fluorides in dental glass-ionomer and silicate compositions
- Techniques for placing insulating liners in deep caries
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