Alginate Setting Reaction — MFDS Part 1 MCQ
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Correct answer: E — Ionic cross-linking of soluble alginate chains by calcium ions to form a gel
The correct answer is E. Alginate is an irreversible hydrocolloid: calcium ions supplied by calcium sulphate ionically cross-link soluble alginate chains, converting the sol into an insoluble calcium-alginate gel. A phosphate salt acts as a retarder by reacting preferentially with available calcium; once the retarder is depleted, calcium-alginate cross-linking proceeds rapidly. Addition polymerisation describes addition-cured silicones, while condensation polymerisation applies to materials such as condensation silicones and produces a small-molecule by-product. Formation of a polysalt matrix by an acid-base reaction is characteristic of materials such as glass-ionomer cement. Alginate does not set through evaporation of a volatile solvent.
Reference: Cook W. Alginate dental impression materials: chemistry, structure, and properties. Journal of Biomedical Materials Research. 1986;20(1):1–24. https://pubmed.ncbi.nlm.nih.gov/3949820/