Potential retardation

Sorption also is a potential retardation mechanism for radionuclide transport through fractures. The surfaces of fractures are often lined with minerals that may be capable of sorbing some of the radionuclides however, there is limited characterization of the extent and distribution of fracture-lining minerals along potential flow paths (Carlos et al., 1995). In the conceptual model used for Yucca Mountain, it is conservatively assumed that there is no sorption (i.e., = 0) in fractured tuff... 

D) LOW-ENERGY ELECTRON BEAM (RETARDING-POTENTIAL) METHODS... 

Figure Bl.26.23. Current-voltage curves observed in the retarding potential difference method of work-fimction Miboxfmeasurement] (Hudson J B 1992 Surface Science (Stoneham, MA Butterworth-Heinemaim)).

A low-energy electron beam can also be obtained using a field emission tip and used in the field emission retarding-potential method. This combination provides an absolute measure of the sample work fiinction and the resolution is excellent [52]. 

The reaction with sodium sulfite or bisulfite (5,11) to yield sodium-P-sulfopropionamide [19298-89-6] (C3H7N04S-Na) is very useful since it can be used as a scavenger for acrylamide monomer. The reaction proceeds very rapidly even at room temperature, and the product has low toxicity. Reactions with phosphines and phosphine oxides have been studied (12), and the products are potentially useful because of thek fire retardant properties. Reactions with sulfide and dithiocarbamates proceed readily but have no appHcations (5). However, the reaction with mercaptide ions has been used for analytical purposes (13)). Water reacts with the amide group (5) to form hydrolysis products, and other hydroxy compounds, such as alcohols and phenols, react readily to form ether compounds. Primary aUphatic alcohols are the most reactive and the reactions are compHcated by partial hydrolysis of the amide groups by any water present. 

Another factor potentially affecting the market for halogenated fire retardants is the waste disposal of plastics (see Wastes, industrial). As landfiU availabihty declines or becomes less popular, two alternatives are incineration and recycling (qv). The nature of the combustion products from halogenated products requires carefiil constmction and maintenance of incinerators (qv) to avoid damage to the incinerator itself and a pubHc health problem from the exhaust. The ease of recycling used products also has a potential effect on fire retardants. 

The use of flame retardants came about because of concern over the flammabiUty of synthetic polymers (plastics). A simple method of assessing the potential contribution of polymers to a fire is to examine the heats of combustion, which for common polymers vary by only about a factor of two (1). Heats of combustion correlate with the chemical nature of a polymer whether the polymer is synthetic or natural. Concern over flammabiUty should arise via a proper risk assessment which takes into account not only the flammabiUty of the material, but also the environment in which it is used. 

A critical review of the toxicity of the haloalkyl phosphates and the potential metaboHc products is available (141). The toxicity of flame retardants used in textiles has also been reviewed (142). 

J. W. HoUeman, Health Effects of Haloalky I Phosphate Flame Retardants and Potential Metabolic Products, Oak Ridge National Laboratory, Term., 1984, DOE/NBM-4006848 (DE84 006848),... 

The identification of Tris as a potential carcinogen dealt a resounding blow to the flame-retardant finishing industry. From 1977 to 1984, several principal supphers of flame-retardant chemicals either reduced the size of their operations or abandoned the market completely. However, Albright and Wilson Corp. (UK) continues to produce THPC—urea precondensate and market it worldwide, and Westex Corp. (Chicago) continues to apply precondensate—NH finish to millions of yards of goods for various end uses. American Cyanamid reentered the market with a precondensate-type flame retardant based on THPS. 

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. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

---