Calcium-containing rigid materials

A number of water-soluble calcium salts provide convenient vehicles for the administration of therapeutic anions. Probably the most widely encountered is the acetylsalicylate of soluble aspirin (patented 1935) the urea complex of calcium acetylsalicylate (water-solubility 231 gdm-3 at 310K, pH 4.8 (717)) is also widely used. Other examples include calcium bromide and bromolactobionate (sedatives), calcium 2-hydroxy-3-mercapto-l-propanesulfonateaurate(III) (chrysanol, antiarthritic), and calcium Af-carbamoylaspartate (tranquillizer). Calcium iron(II) citrate has been used to remedy iron deficiency - it has the advantage of being stable to air oxidation of the iron(II). The cyclamate anion is certainly not therapeutic, but is relevant here. 

Its freely water-soluble calcium salt (patented 1957) was a much-used and very effective sweetener, as was the sodium salt, whose sweetness was first noted in 1937 cyclamic acid is ineffective as it is essentially insoluble in aqueous media. 

Calcium oxalate (723) occurs as the monohydrate (whewellite, the thermodynamically stable form under ambient conditions (724)), the dihydrate (weddellite) in plant calcium stores and in sap, or the trihydrate (725). Calcium oxalate also plays a structural role in plants. Oxalate, for example from excessive amounts of rhubarb or spinach, inhibits absorption of Ca2+ from the GIT precipitation of calcium oxalate is the reason for the toxicity of oxalates. Calcium oxalate may also occur in man, where it can appear as minute star-shaped crystals in the urine. It is the main constituent of the majority of urinary calculi in man (726,727). The relationships between dietary calcium 

Solubilities and stabilities of calcium phosphates in natural waters have been described (735), as have the nucleation and growth of calcium phosphate from solution (736). Several species have long been known to inhibit the precipitation of calcium phosphates, for example carbohydrates (646) and statherin, the tyrosine-rich peptide which occurs in saliva (737). The role of 

CCP in milk is mentioned in connection with casein above (Section VI.C). Fluorapatite is a major constituent of phosphate rocks, and a constituent, probably important, of human tooth enamel for those whose drinking water contains significant amounts of naturally occurring or added fluoride. Fluorapatite is significantly less soluble than hydroxyapatite - the relationship between the solubilities of fluorapatite and hydroxyapatite parallels (but is much less extreme than) that between calcium fluoride (Ksp — 3.9 x 10 11 mol3 dm-9) and calcium hydroxide (Ksp = 7.9 x 10 6 mol3 dm 9). Calcium diphosphate, Ca2P207, is believed to be the least soluble of the calcium phosphates. 

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Molecular sieve zeolites constitute a class of stationary phase that combines exclusion with specific adsorption properties. These materials, which are crystalline aluminum silicates (commonly sodium or calcium aluminum silicates), have rigid, highly uniform three-dimensional porous structures containing up to 0.5ml/g of free pore volume, resulting when water of crystallization is removed by heating. Although munerous natural zeolites are known, most practical work is done with... 

The additives in PVC bottles for cooking oil and other food products must have FDA clearance. For flexible packaging materials, the most common stabilizers are mixed metals such as barium-zinc and calcium-zinc, which replaced the older cadmium-zinc formulations. For rigid blow molded containers and calendered sheets, or-... 

Lead, barium and cadmium based stabilisers are widely used for rigid PVC [16]. However, calcium and zinc containing materials are preferable as they offer good heat and weather stability and are used in flexible PVC applications. Foamed PVC can be considered more... 

Lead-stabilized products, used in some parts of the world in rigid extrusions, often contain 0.4-0.8 phr of calcium stearate as internal lubricant, 0.2-0.5 phr of stearyl alcohol to improve the mobility of the stabilizer (normally dibasic lead phosphate or combinations with tribasic lead sulfate), and 0.4-0.75 phr of partly saponified mon-tanic ester as a multifunctional lubricant. For the ultimate in process stability, calcium stearate may be replaced by a barium/cadmium stearate blend. The inclusion of cadmium also reduces the severity of staining by airborne sulfur compounds. Although lead-stabilized rigid compounds offer advantages in material cost and process safety, their use should be abandoned because of the hazard involved and because such usage is prejudicial to acceptance of vinyl products. 

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