Stage Terms Links

A layer of tetrahedra can be considered as being built up by linking parallel chains. That this is not a mere formalism is shown by the existence of intermediate stages. Two linked silicate chains result in a ribbon of the composition [Si4Of ] it has two kinds of tetrahedra, one kind being joined via three and the other kind via two vertices, [SiO Oj SiOj/jOj ]3 Silicates of this type are termed amphiboles. They are fibrous and also used to be used as asbestos. 

The second-stage cross-linking (cure) reaction is initiated by organic peroxides MEK peroxide for room-temperature cure, and benzoyl peroxide or t-butyl perbenzoate or other stabler peroxides for higher-temperature cure processes. Peroxide action may be speeded by heat and/or activators such as cobalt soaps and tertiary amines. (Nonchemists are apt to use the terms catalyst and activator ratber loosely, which can he confusing or even dangerous in practice.)... 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

Whilst the term thermosetting plastics arose out of the fact that early products of this type were cross-linked by subjecting the intermediate-stage materials to elevated temperature, the term is also widely used where cross-linking takes place at normal ambient temperatures. 

The conditions under which the drug is used need to be estimated as do acceptable residues linked to the level of acceptable risk to the consumer. The acceptable level of risk, which is determined in theory at the risk management stage, has already been expressed in terms of residues by the ADI under hazard characterization. Moreover, the elements considered for hazard identification, hazard characterization, and exposure assessment make it possible, for a given form of utilization of a particular substance, to establish a profile of residues in animal tissues and to associate this with a profile of consumer exposure. Comparison of this consumer profile and ADI indicates whether the mode of utilization of the substance is acceptable or not. Analysis of the different results of residue content in animal products then provides an indication of level of residues in one or several animal tissues, making it possible to differentiate between veterinary drug applications that do or do not permit compliance with the ADI. 

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