(Disambiguation- "alginate" is the term used to describe both a type of water based molding material and also one of the components used in the formulation of the molding material- see Technical Details below.)
Alginate Molding Material is a product used to make high-detail casts at room temperature. One of the components, also known as alginate, is a naturally occuring substance rendered from seaweed. Alginate molding powder is sold as a fine dry powder. When mixed with water, dependant on the formulation and water temperature, alginate gels and sets in 2-15 minutes.
Alginate is especially popular in lifecasting. Alginate is also referred to as dental alginate or prosthetics alginate. There are slight variations in properties between the two products.
Casts made with alginate are only temporary, as the alginate will lose water and change shape in a matter of hours. This effect can be reduced by keeping the casting covered in damp paper towels. However, this cannot be a long-term solution as the substance will grow mold. Depending on the shape of the object being cast, and the product used to make the positive mold.
The cast is only slightly elastic, and has a low tear tolerance. In most situations, uses of alginate must be reinforced by a mother mold, such as plaster bandages.
Alginates are linear unbranched polymers containing β-(14)-linked D-mannuronic acid (M) and α-(14)-linked L-guluronic acid (G) residues.
Gelling depends on the ion binding (Mg2+ << Ca2+ < Sr2+ < Ba2+) with the control of the dication addition being important for the production of homogeneous gels (e.g. by ionic diffusion or controlled acidification of CaCO3). High G content produces strong brittle gels with good heat stability (except if present in low molecular weight molecules) but prone to water weepage (syneresis) on freeze-thaw, whereas high M content produces weaker more-elastic gels with good freeze-thaw behavior and high MGMG content zips with Ca2+ ions to reduces shear.
Alginate's solubility and water-holding capacity depend on pH (precipitating below about pH 3.5), molecular weight (lower molecular weight calcium alginate chains with less than 500 residues showing increasing water binding with increasing size), ionic strength (low ionic strength increasing the extended nature of the chains) and the nature of the ions present. Generally alginates show high water absorption and may be used as low viscosity emulsifiers and shear-thinning thickeners.
Alginate is nontoxic. In fact, it is in the list of acceptable food additives by the US FDA and the European Union.
As with all fine particulate matter, care must be taken not to inhale airborn alginate powder. A proper dust mask should be worn. Alginate should not be mixed by hand.
- Chaplin, Martin; Alginate: An explanation of its origin, structure and properties April 19, 2006; http://www.lsbu.ac.uk/water/hyalg.html