Key definitions
Term | Definition |
|---|---|
Degradation | Any chemical or physicochemical process that changes a material's composition or structure |
Water sorption | Uptake of water into a polymer causing swelling and plasticisation |
Soluble fraction (solubility) | Mass of water-soluble components leached out of a material |
Scission | Covalent bond rupture in polymers that lowers molecular weight and mechanical properties |
Tarnish | Surface discolouration without bulk material loss; removable by polishing |
Corrosion | Chemical reaction of metal with its environment involving anodic oxidation (metal → ions + e⁻) and cathodic reduction; leads to lateral loss and weakening |
Static fatigue / stress corrosion cracking | Subcritical crack growth in a ceramic accelerated by a corrosive environment and stress |
Why dental materials degrade in the mouth
The oral cavity is chemically and mechanically aggressive: saliva (water, ions), dietary acids/alkalis, temperature swings, and biofilm components can dissolve, oxidise/corrode, swell, stain, or embrittle materials. Consequences include loss of strength, discoloration, surface roughening, interfacial failure, and biologic exposure to leached products. Broadly: polymers absorb water/lose solubles and can undergo bond scission; metals tarnish and corrode; ceramics undergo chemical dissolution and environmentally assisted crack growth.
Polymers: water sorption and soluble fraction
Mechanism: Water (and alcohols) diffuse into polymer matrices, push chains apart, and plasticise the network → softer material, lower glass transition (Tg), and often lower strength.
Examples:
Nylon (e.g., toothbrush bristles): notable water uptake limits service life.
Resin composites: water sorption contributes to discolouration and hydrolytic weakening at the resin–filler interface; porosity or glass dissolution can raise apparent sorption values.
Soft denture liners: loss of water-soluble plasticisers → less flexibility, more creep, possible fracture(osmotic stresses).
Glass ionomer cements: too much water → loss of translucency; too little (desiccation) → surface crazing.
Polymers: bond rupture (scission) and its triggers
Covalent bond cleavage reduces average molecular weight and undermines mechanical properties such as strength and toughness. Ultraviolet radiation can ionise susceptible bonds, causing chain scission and embrittlement; in some systems, high-energy radiation instead promotes useful cross-linking, as with γ-modified polyethylene that resists softening and flow at elevated temperatures. Heat can also drive scission, sometimes simply from localised overheating during aggressive polishing, with thermal stability governed by the relative bond energies within the polymer. Chemical exposure, particularly to alcohols, swells polar matrices, weakens secondary interactions, softens the material, and increases wear. Not all degradation is detrimental: bioresorbable devices such as soluble sutures are designed to break down into small, biologically manageable products (carbon dioxide, water, salts) that cells can clear from the implantation site.
Metals: tarnish and corrosion
Tarnish is a superficial discoloration, typically from sulphide or chloride films, that does not entail bulk material loss and is readily removed by polishing. Corrosion, by contrast, is an electrochemical reaction between a metal and its environment that removes metal from service and can release reactive products. In aqueous media the anodic reaction oxidises the metal to ions while the cathodic reaction consumes electrons, commonly via dissolved oxygen reduction. Because these coupled half-reactions reduce free energy, virtually all structural metals are susceptible in sufficiently aggressive conditions, with the consequences ranging from aesthetic compromise to loss of strength, fracture, and adverse tissue responses.
Dry oxidation (air)
Outside of aqueous environments, most metals (apart from noble elements such as gold and platinum) spontaneously form surface oxides. If the oxide is adherent and stable, growth slows as diffusion through the film limits further reaction, producing a parabolic weight-gain profile and a protective “passive” layer. If the oxide is non-adherent or prone to cracking, newly exposed metal repeatedly oxidises and mass increases more linearly. At high temperatures some oxides volatilise as they form, leading to net mass loss and continued exposure of fresh metal. These dry processes explain why metals last longer in hot, arid conditions than in humid ones.