Slow releasing

Slow-Release Fertilizers. Products containing urea—formaldehyde are used to manufacture slow-release fertilisers. These products can be either soHds, Hquid concentrates, orHquid solutions. This market consumes almost 6% of the formaldehyde produced (115) (see Controlled release TECHNOLOGY, AGRICULTURAL). 

Zinc, calcium, and magnesium peroxides prepared from hydrogen peroxide are used as specialized oxidants where a slow release of hydrogen peroxide is desired. These find use in various medicinal treatments. 

Vinyl Phosphates. Dichlorvos [62-73-7] 0,0-dimethyl 0-(2,2-dichloroviayl) phosphate, (CH20)2P(0)0CH=CCl2 (bp 140°C at 27 kPa, d 1.314, vp 1.6 Pa at 20°C), is soluble ia water to about 10 g/L. The half-hfe ia water is 8 h. The rat oral LD qS are 80, 56 mg/kg. Dichlorvos is used ia aerosols and sugar baits to control flies and mosquitoes. Slow release formulations have been used ia plastic strips and pet collars to control animal ectoparasites. 

Slow release formulations incorporate nonpersistent compounds, eg, methyl parathion, insect growth regulators, and sex pheromones, in a variety of granular, laminated, microencapsulated, and hoUow-ftber preparations. 

Applications. These materials are stiU in developmental infancy. Current production is limited to one commercial process in Europe and a demonstration-scale process in North America. The lignins produced in these processes have potential appHcation in wood adhesives, as flame retardants (qv), as slow-release agents for agricultural and pharmaceutical products, as surfactants (qv), as antioxidants (qv), as asphalt extenders, and as a raw material source for lignin-derived chemicals. 

Coatings, Paints, and Pigments. Various slightly soluble molybdates, such as those of zinc, calcium, and strontium, provide long-term corrosion control as undercoatings on ferrous metals (90—92). The mechanism of action presumably involves the slow release of molybdate ion, which forms an insoluble ferric molybdate protective layer. This layer is insoluble in neutral or basic solution. A primary impetus for the use of molybdenum, generally in place of chromium, is the lower toxicity of the molybdenum compound. 

Other. 2-Nitro-1-butanol is an excellent solvent for many polyamide resins, cellulose acetate butyrate, and ethylceUulose. It can be utilized in paint removers for epoxy-based coatings. 2-Hydroxymethyl-2-nitro-l,3-propanediol is usebil for control of odors in chemical toilets. Its slow release of formaldehyde ensures prolonged action to control odor, and there is no reodorant problem which sometimes is associated with the use of free formaldehyde. 2-Hydroxymethyl-2-nitro-l,3-propanediol solutions are effective preservative and embalming fluids. The slow Uberation of formaldehyde permits thorough penetration of the tissues before hardening. 

Erodings of Slow-Releasing Core Tablets. The sustained-dose portion of a dmg is granulated with hydrophobic materials such as waxes, fatty acids, or fatty alcohols and compressed into a core. The initial dose is added to the core by a modified sugar coating process or by compression coating. Thus, a tablet within a tablet is created. The core erodes slowly to release the active ingredient. 

Standard Mosquito Repellents. Since its initial report as a promising repellent in 1954, DEET has been considered the best all-around repellent having generally acceptable characteristics, despite a continuing search for a superior chemical. Improvements include many commercial products with added cosmetic agents that use slow release technology, such as the U.S. Armed Services slow release 35% DEET formulation (16). There were 35 EPA-registered repellent products in 1994 that contained only DEET under different trade names (2). DEET is present in 192 of the 212 products mentioned previously (2). 

Sodium selenite has also been incorporated into styrene—butadiene mbber and used in a pellet form which results in the slow release of selenium into water. These pellets have been placed in lakes in Sweden which have fish contaminated with mercury owing to high levels of that element in the water. The selenium released by the pellets reacts with mercury to form insoluble, heavy mercury selenide which setties to the lake bottom and removes mercury from the ecosystem (126). 

A mixture of dimethyl sulfate with SO is probably dimethyl pyrosulfate [10506-59-9] CH2OSO2OSO2OCH2, and, with chlorobenzene, it yields the 4,4 -dichlorodiphenylsulfone (153). Trivalent rare earths can be separated by a slow release of acid into a solution of rare earth chelated with an ethylenediaminetetraacetic acid agent and iodate anion. As dimethyl sulfate slowly hydrolyzes and pH decreases, each metal is released from the chelate in turn and precipitates as the iodate, resulting in improved separations (154). 

Ivermectin is widely used as an endectocide for catde as an injectable, oral, topical, or slow release bolus for sheep as an injectable or oral formulation for swine as an injectable for horses as a paste or drench and for goats as an injectable or oral formulation. Ivermectin has recently been introduced for heartworm prophylaxis in dogs and it is being studied for use with cats, many other mammals, birds, fish, and reptiles. 

Isobutyraldehyde reacts with aqueous ammonia at 0—10°C to give hexahydro-2,4,6-ttiisopropyl-r-ttiazine, (4) (18), whereas under refluxing conditions the enea2omethine [5339-41-3] (5), is formed (19). Isobutyraldehyde condenses with two mole equivalents of urea in the presence of an acid catalyst to give isobutyUdenediurea [2224-20-6] (IBDU), (6) a slow release fertiliser (20). 

Isobutyhdene diurea (IBDU), a slow release fertilizer, is formed from isobutyraldehyde and two moles of urea. 

Piaacidil has a short half-life and most human studies were carried out ia slow-release formulatioas. The reductioa ia blood pressure produced by piaacidil is accompanied by tachycardia and fluid retention. Plasma catecholamines and renin activity are iacreased. Other side effects are headache, di22iaess, and asthenia. 

Urea—Other Aldehyde Reaction Products. Urea can also react with other aldehydes to form slow release nitrogen fertilizers. However, cost constraints associated with higher aldehydes have either precluded or limited broad commercial development of these products. Two exceptions are isobutyhdene diurea (IBDU), registered trademark of Vigoro Industries, and crotonyHdene diurea (CDU), registered trademark of Chisso-Asahi Fertilizer Co. 

Composite Resins. Many composite restorative resins have incorporated fluoride into the filler particles. One commonly used material, yttrium trifluoride [13709-49-4] is incorporated as a radiopaque filler to aid in radiographic diagnosis, and is also responsible for slow release of fluoride from the composites (280). This same effect is achieved with a barium—alumina—fluoro-siUcate glass filler in composite filling and lining materials. Sodium fluoride [7681-49-4] has also been used in composites by incorporating it into the resin matrix material where it provides long-term low level release (281-283). 

Polycaprolactones (see also Section 25.11), although available since 1969, have only recently been marketed for biodegradable purposes. Applications include degradable film, tree planting containers and slow-release matrices for pharmaceuticals, pesticides, herbieides and fertilisers. Its rate of biodegradability is said to be less than that of the polylactides. 

Direct interaction of the substance, followed by slow release into the water may give long-term problems The substance may simply collect in hollows and be gradually dispersed back into the main flow of water to give medium-term problems... 

Substances which function as tissue adhesives have the unique ability to remain in contact with tissues in localized areas for prolonged periods of time. Not only does the tissue adhesive function as a glue but it has the capacity to serve as a slow release mechanism for drug delivery as it is biodegraded. 

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