Leach-resistant chemicals

Most FR formulations are not resistant to leaching by water. Therefore, there have been increased efforts to develop leach-resistant chemicals that can be impregnated into wood products for use in exterior or high-humidity applications. Some of the proposed leach-resistant systems include chemical combinations that form insoluble complexes, amino-resin systems, and monomers that polymerize in the wood. A common amino-resin system for exterior use is dicyandiamide phosphoric acid formaldehyde. Guanylurea phosphate-boric acid also is a commonly used organic phosphate salt for modern commercial FR wood. 

Leach-Resistant Chemicals. Insoluble Complexes. Leach-resistant fire retardants can be formed by reacting soluble salts with metal salts to form insoluble, metallic salt complexes. Sodium silicate reacted with calcium chloride formed an insoluble, hydrated calcium silicate (95). Application of a 20% diammonium phosphate solution, followed by a 20% magnesium sulfate solution, has been proposed as a ready-to-use treatment for wood roofs (96). This combination forms an insoluble magnesium ammonium phosphate and is recommended for roofs that are 5 years old or older. Test results indicate that this treatment provides increased flame-spread protection. 

Future Research. Improvements in leach-resistant chemicals have been a primary concern over the past decade. Advances have been made in leach-resistant systems such as the amino-resin systems however improvements still need to be made in leach-resistant compounds without increasing the cost. Other areas where research on fire retardants needs to be conducted are in coating systems, especially those that are durable to weathering and UV degradation reduction of smoke and toxic products, improvements in fire-retardant treatments for panel products and fundamental work on the mechanisms of particular formulations. 

Carbon supported powdered palladium catalysts have been widely used in the chemical industry. In addition to activity and selectivity of those catalysts, the recovery rate of the incorporated precious metal has a major impact on the economic performance of the catalyst. In this study, the effects of catalyst age, oxidation state of the incorporated metal and temperature treatment on the palladium leaching resistance as well as on activity and dispersion of carbon supported palladium catalysts were investigated. 

The first barrier is the form of the waste, which will immobilize the radioactive materials. The waste form should not be damaged by heat or radiation nor be attacked by groundwater. The waste is placed in a steel canister, which is resistant to leaching. The canister is surrounded by packing materials that prevent radioactivity from escaping, and the entire repository is backfilled with a material that absorbs or resists chemical intrusion. The final barrier is the host medium that separates the repository from the surrounding area. 

Fly ash increases the density, decreases the permeability, and increases the leaching resistance of Ordinary Portland Cement (OPC). It is a truism that The leach resistance of solidified cement-waste systems can be improved by any process which accelerates curing, limits porosity, or chemically bonds fission product or actinide elements. (Jantzen et al., 1984). Supercritical C02 treatment of a modified Portland cement is expected to further increase the density over the untreated material, so that a reduced porosity and improved leachability should result. In addition, the high silica content of fly ash, with its well-known sorbent properties toward actinides and certain other radionuclides, enhances the immobilization characteristics. 

Fire-retardant chemicals used by the commercial wood-treating industry are limited almost exclusively to mono- and diammonium phosphate, ammonium sulfate, borax, boric acid, and zinc chloride (4,8). It is believed that some use is also made of the liquid ammonium polyphosphates (9). Some additives such as sodium dichromate as a corrosion inhibitor are also used. Aqueous fire-retardant treatment solutions are usually formulated from two or more of these chemicals to obtain the desired properties and cost advantages For leach-resistant type treatments, the literature shows that some or all of the following are used urea, melamine, dicyandiamide, phosphoric acid, and formaldehyde (10-12) ... 

Fire-retardant-treated wood is durable and stable under normal exposure conditions. Treatments using inorganic water-soluble salts, however, are not recommended for exterior exposures to rain and weathering unless the treatment can be adequately protected by water-repellent coating. Exterior-type treatments in which the chemicals are "fixed" in the wood in some manner are leach resistant and nonhygroscopic. 

The success of this treatment system indicated a breakthrough in the development of a commercially successful system whereby fire-retardant chemicals are pressure impregnated into the wood and fixed or converted to a leach-resistant state without serious impairment of the desirable natural wood properties. This development has stimulated research with leach-resistant type treatments. Chemicals employed usually involve organic phosphates and compounds that can react with phosphorous-containing chemicals or with the wood cellulose structure to give permanence of treatment. 

Leach-Resistant Compounds. Progress for improving the leachability of fire retardants has been made in the past decades. Several commercial treatments are available for exterior use. However, even these demonstrate some leaching of chemicals. Further work needs to be done to increase the leach resistance of these treatments without excessively increasing the cost. Improved leach resistance will be necessary to expand wood products into commercial and institutional buildings. 

In 1975, Chem Systems developed the liquid phase methanol (LPMeOH) process which is based on the low pressure synthesis technology except that the new process is carried out in an inert oil phase [79], The catalytic system used is Cu/Zn0/Al203, that is modified for slurry operation (i.e., attrition resistant, finely powdered, and leaching resistant). The S3.85 and S3.86 catalysts of BASF and EPJ-19 and EPJ-25 catalysts of United Catalysts Inc. were developed for this process [14,19]. The process has been tested for commercial feasibility at a demonstration scale by Air Products and Chemicals, Inc [79]. 

Wood used in construction of decks, docks and buildings, utility poles, railroad ties, and bridge ties is typically treated with a chemical preservative to make it resistant against microbial degradation. Among those chemical preservatives, the most widely used were chromated copper arsenate (CCA), ammoniacal copper quat (ACQ), pen-tachlorophenol, and creosote. CCA was the most prevalent preservative due to its low cost, and because it provided a dry and paintable surface after its application and left the wood relatively leach-resistant [3]. 

Calcines are well known to be readily leachable in water. Hot pressed supercalcine shows a leach resistance similar to that of the other chemically stable products. 

One aspect of stabilizers effeetiveness is their resistance to leaching and chemical attack. Modem additives are usually large moleeular speeies, often polymerie, diffieult to remove from the polymer [5]. 

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