Nonvolatile inhibitors

Nonvolatile Inhibitors. Glycosides A number of toxic constituents are known to be released by the enzymatic degradation of various glycosides. Some of the volatile components have been mentioned previously—i.e., isothiocyanates from mustard oil glycosides and hydrogen cyanide from cyanogenic glycosides. 

Reagent grade ether and styrene monomer were rid of dissolved gases by vacuum freeze-thaw cycles. The ether was distilled, while the styrene was used as is. Thus, styrene still contained nonvolatile inhibitor. The reactor was initially charged with 1 ml styrene, 100 ml ether, and AIBN (equivalent to 0.3 wt% in the system). The setpoint pressure in the reactor was slowly increased and the reactor temperature was recorded. 

Copper and copper salts have also been suggested as nonvolatile inhibitors of these acids. Nitric oxide may be of value as a volatile inhibitor. 

The question as to whether a flame retardant operates mainly by a condensed-phase mechanism or mainly by a vapor-phase mechanism is especially comphcated in the case of the haloalkyl phosphoms esters. A number of these compounds can volatilize undecomposed or undergo some thermal degradation to release volatile halogenated hydrocarbons (37). The intact compounds or these halogenated hydrocarbons are plausible flame inhibitors. At the same time, thek phosphoms content may remain at least in part as relatively nonvolatile phosphoms acids which are plausible condensed-phase flame retardants (38). There is no evidence for the occasionally postulated formation of phosphoms haUdes. Some evidence has been presented that the endothermic vaporization and heat capacity of the intact chloroalkyl phosphates may be a main part of thek action (39,40). 

Polymers with much higher average molecular weights, from 90,000 to 4 x 10 , are formed by a process of coordinate anionic polymerization (43—45). The patent Hterature describes numerous organometaUic compounds, aLkaline-earth compounds, and mixtures as polymerization catalysts. Iron oxides that accumulate in ethylene oxide storage vessels also catalyze polymerization. This leads to the formation of nonvolatile residue (NVR) no inhibitor has been found (46). 

Some compounds tend to polymerize when distilled unless chemicals are added to inhibit polymerization. These polymerization inhibitors tend to be nonvolatile, ending up in the column bottoms. If this is the case, it normally prevents finished products being taken from column bottoms. 

Acrylonitrile of 99.5-99.7% purity is available commercially, with the following specifications (ppm by weight, maximum) acidity (as acetic acid), 10 acetone, 75 acetonitrile, 300 acrolein, 1 hydrogen cyanide, 5 total iron, 0.1 oxazole, 10 peroxides (as hydrogen peroxide), 0.2 water, 0.5% and nonvolatile matter, 100. Hydroquinone monomethyl ether (MEHQ) is added as an inhibitor at concentrations of 35-45 mg/kg (ppm) (Cytec Industries, 1994, 1997). Trade names for acrylonitrile include Acritet, Acrylon, Carbacryl, Fumigrain and Ventox. 

Butadiene is available commercially as a liquefied gas underpressure. The polymerization grade has a minimum purity of 99%, with acetylene as an impurity in the parts-per-million (ppm) range. Isobutene, 1-butene, butane and cis-l- and Zrc//7.s-2-butcnc have been detected in pure-grade butadiene (Miller, 1978). Typical specifications for butadiene are purity, > 99.5% inhibitor (/c/V-butylcatecliol). 50-150 ppm impurities (ppm max.) 1,2-butadiene, 20 propadiene, 10 total acetylenes, 20 dimers, 500 isoprene, 10 other C5 compounds, 500 sulfur, 5 peroxides (as H2O2), 5 ammonia, 5 water, 300 carbonyls, 10 nonvolatile residues, 0.05 wt% max. and oxygen in the gas phase, 0.10 vol% max. (Sun Wristers, 1992). Butadiene has been stabilized with hydroquinone, catechol and aliphatic mercaptans (lARC, 1986, 1992). 

Citrus peel oils may also be used for their antioxidative, antitumor, and radicalscavenging activities. The radical-scavenging ability of citrus peel oil may help prevent free radical-induced and various chronic diseases (48, 55, 56). Monoterpenes from volatile components and polymethoxylated flavones from nonvolatile residues have been reported to be effective inhibitors of tumor cell growth, implicating that citrus peel oils may be good cancer preventive food additives (57-59). Furthermore, citrus peel oils are useful to alleviate pain from burnt skin (60). Demonstrating anxiolytic and sedative effect, they could also be used in primary medical care against insomnia, anxiety, and epilepsy (61). 

Unlike saturated FAs, carbon-labeled polyunsaturated arachidonate and docosahexaenoate can be used with autoradiography or PET scanning without the inhibitor, because they are minimally oxidized (Osmundsen, Cervenka, Bremer, 1982) only 15% of their label is found in the brain nonvolatile aqueous compartment 20 min after an intravenous injection. [ H]Arachidonate or [ Hjdocosahexaenoate produce only 10% nonvolatile aqueous background activity. 

Nonvolatile, odorless liquid. pKa 7.9. LDW s.c. in rabbits 15.4, .g/kg. J. J. Gordon, L. Leadbeater, Toxicol Appi Pharmacoi 40, 109 (1977). Caution Cholinesterase inhibitor more potent than sarin, q.v. 

Limit distillations or evaporation of peroxide-forming chemicals. Solvents or chemicals that can form peroxides should not be distilled to dryness or allowed to evaporate to dryness—the resulting residue could contain enough peroxides to explode. There are reports of explosions from distillations of peroxidizable chemicals. At times distillations of peroxide formers may be deemed essential. In these cases, it is recommended that peroxide content be tested and if above 100 ppm, the chemical should not be distilled. When carrying out any distillation of a peroxidizable chemical, you should ensure that the process is halted when about 20% of the starting volume is left. There have been suggestions to add a nonvolatile oil, such as mineral oil, to the distillation to ensure that it does not go to dryness. Furthermore, remember that the product of any distillation will not contain any inhibitor and is more prone to peroxide formation in the future. 

Obviously a great deal of research effort has been directed toward developing stabilizers for cyanoacrylate adhesives. Two major types have evolved the volatile acid gases and the nonvolatile acids. These are used in varying concentrations, depending upon the type of stabilizer. Table III is a summary of the types of stabilizers used and typical concentrations. The end use of the adhesive also dictates the type and level of inhibitor used a fast-curing industrial ethyl cyanoacrylate adhesive will tolerate much... 

The last-mentioned inhibitor is of particular interest. In the acid, it obviously exists as a nonvolatile salt that can act as a still-pot inhibitor when small amounts of acrylic or methacrylic acid have to be distilled under reduced pressure to prepare an inhibitor-free sample of the acids. 

---