Protected Agriculture: A Global Review, Volumes 23-253
World Bank Technical Paper 253. Presents a broad overview of protected agriculture and its benefits for farm incomes, nutrition, and foreign exchange earnings from exports. In pursuing strategies to promote agricultural development and crop dive
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Page 119 - Carbon Black— A black pigment produced by the incomplete burning of natural gas or oil. It is widely used as a filler, particularly in the rubber industry. Because it possesses useful ultraviolet protective properties, it is also much used in polyethylene compounds intended for such applications as cold water piping and black agricultural sheet.
Page 118 - Blown Tubing— A thermoplastic film which is produced by extruding a tube, applying a slight internal pressure to the tube to expand it while still molten and subsequent cooling to set the tube. The tube is then flattened through guides and wound up flat on rolls. The size of blown tubing is determined by the flat width in inches as wound rather than by the diameter as in the case of rigid types of tubing.
Page 124 - A technique of packaging in which the strains in a plastics film are released by raising the temperature of the film thus causing it to shrink over the package. These shrink characteristics are built into the film during its manufacture by stretching it under controlled temperatures to produce orientation, qv, of the molecules.
Page 124 - The so-called major load (60 to 150 kilograms) is next applied and a second reading obtained. The hardness is then calculated as the difference between the two loads and expressed with nine different prefix letters to denote the type of penetrator used and the weight applied as the major load.
Page 119 - See rosin, condensation, n • a chemical reaction in which two or more molecules combine with the separation of water or some other simple substance. If a polymer is formed, the process is called polycondensation. (See...
Page 125 - ... an external or internal crack in a plastic caused by tensile stresses less than its short-time mechanical strength. NOTE — The development of such cracks is frequently accelerated by the environment to which the plastic is exposed. The stresses which cause cracking may be present internally or externally or may be combinations of these stresses.
Page 122 - Modified Containing ingredients such as fillers, pigments or other additives, that help to vary the physical properties of a plastics material. An example is oil modified resin. Modulus of Elasticity The ratio of stress to strain in a material that is elastically deformed. Moisture Vapor Transmission The rate at which water vapor permeates through a plastic film or wall at a specified temperature and relative humidity. Mold...
Page 125 - A material that will repeatedly soften when heated and harden when cooled. Typical of the thermoplastics family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, vinyls, nylons, and the various fluorocarbon materials.
Page 123 - When two or more monomers are involved, the product is called a copolymer. polymerization, n — a chemical reaction in which the molecules of a monomer are linked together to form large molecules whose molecular weight is a multiple of that of the original substance. When two or more monomers are involved, the process is called copolymerization or heteropolymerization.