Peroxide stabilisation

Hydrogen peroxide as supplied is normally stabilised with phosphates and sometimes tin(IV) compounds, the latter being effective at the product s natural (weakly acid) pH by hydrocolloid formation, which occludes adventitious transition metal ions and reduces their catalytic activity. In many cases, extra stabilisation is not required when H2O2 or its derivatives are used in synthesis. However, elevated temperature, high alkalinity and increased metal impurities all tend to destabilise peroxy-gens, and where these conditions are unavoidable, additional stabilisers may be employed added either to the peroxide (subject to reactivity) or separately to the reaction mixture. Such stabilisers fall into two categories. 

A useful nitrogen-free sequestrant for H2O2 and peracids is HEDP (1-hydroxyethylidene diphosphonic acid). In general, solutions containing both peracids and free hydrogen peroxide are more prone to catalytic decomposition than either component alone [60], since the two peroxides themselves represent a redox couple. 

The sequestrant approach to stabilisation is usually better than the scavenger approach, since it is based on prevention rather than cure. It can. 

It will be recognised that in-process stabilisation of peroxygens is related to safety of operations, discussed in the following section. 

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Acrylics, unsaturated polyesters and other monomer-based adhesives containing ethy-lenic unsaturation, cure by formation of free peroxide radicals formed with transition metal ion donators such as cobalt, iron, copper and nickel. In engineering applications the presence of transition metals on surfaces sees them act as initiators by forming the free peroxide radicals from the added peroxide compound(s). The base monomers usually contain inhibitors such as phenols or other materials classed as peroxide stabilisers to... 

The goods are entered into a bath containing 2-3 volume H O, 1 g/1 sodium hydroxide flake, 0.2 g/1 peroxide stabiliser, 0.25 g/1 sequestering agent and 0.002 to 0.05 g/1 free radical suppressor at 40 C, the temperature is raised to 85 C and then the treatment continued for 1 h. The treated goods are then cooled and rinsed thoroughly. When appropriate, selected optical brighteners may be incorporated in the peroxide bleach bath. 

Wash with cone H2SO4, then Na2C03 soln, dry with anhydrous Na2C03, and finally pass through a 50cm column of activated alumina before distn. Alternatively, wash with 10% ferrous sulfate soln to remove peroxides, then H2O, dry with CaS04, and dist in vac. Add 0.2% of catechol to stabilise it. VERY TOXIC. 

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

Phosphites Tris-(p-nonylphenyl) phosphite (X) No Widely used in conjunction with conventional stabilisers (q.v.) in PVC. Some types appear to be useful heat and light stabilisers in polyolefins. Function primarily as peroxide decomposers rather than chain-breaking antioxidants. 

Lead-platinum The alloy is lead together with 0-1-2% silver usually in rod form with platinum microelectrodes inserted every 150 mm. The purpose of these microelectrodes, which take the form of pins, is to stabilise the formation of lead peroxide on the anode surface. 

The iodometric method has the advantage over the permanganate method (Section 10.95) that it is less affected by stabilisers which are sometimes added to commercial hydrogen peroxide solutions. These preservatives are often boric acid, salicylic acid, and glycerol, and render the results obtained by the permanganate procedure less accurate. 

Po and Sutin " have disputed both the extent of the catalytic effect of chloride ion reported by Wells and Salam" and the formation constant of 5.54 (25 °C, [Cl ] = 0.300 M, n = 1.00) for FeCl estimated thereby. Wells " has replied that the value of k2 of Po and Sutin at zero chloride concentration is artifically increased because of the presence of stabiliser in their peroxide, consequently masking the catalysis. 

Dilution or simple mixing with a stable compound is sufficient to stabilise an unstable substance. In the case of a simple mixture with a neutral substance, this stabilisation process is called desensitisation . Thus hardeners such as benzoyl peroxide are nomially in the form of suspensions in heavy esters or oils. This peroxide is mixed with 30% of water by weight. Dynamite is nitroglycerine stabilised with the help of a neutral material. In all these cases, heat that is produced by the potential beginning of decomposition is absorbed by the inert substance. 

Similar accidents have happened when barium perchlorate is added to to C3 alcohols as well as 1 -octanol. The latter alcohol demonstrates the nature of this danger and is a counter-example of the typical observation of stabilisation of unstable species, when the number of carbon atoms increases (whereas it is the case for peroxides and peracids). 

GC is extensively used to determine phenolic and amine antioxidants, UV light absorbers, stabilisers and organic peroxide residues, in particular in polyolefins, polystyrene and rubbers (cf. Table 61 of Crompton [158]). Ostromow [159] has described the quantitative determination of stabilisers and AOs in acetone or methanol extracts of rubbers and elastomers by means of GC. The method is restricted to analytes which volatilise between 160 °C and 300 °C without decomposition. A selection of 47 reports on GC analysis of AOs in elastomers (period 1959-1982) has been published... 

Applications Conventional TLC was the most successful separation technique in the 1960s and early 1970s for identification of components in plastics. Amos [409] has published a comprehensive review on the use of TLC for various additive types (antioxidants, stabilisers, plasticisers, curing agents, antistatic agents, peroxides) in polymers and rubber vulcanisates (1973 status). More recently, Freitag [429] has reviewed TLC applications in additive analysis. TLC has been extensively applied to the determination of additives in polymer extracts [444,445]. 

Crompton [21] has reviewed the use of electrochemical methods in the determination of phenolic and amine antioxidants, organic peroxides, organotin heat stabilisers, metallic stearates and some inorganic anions (such as bromide, iodide and thiocyanate) in the 1950s/1960s (Table 8.75). The electrochemical detector is generally operated in tandem with a universal, nonselective detector, so that a more general sample analysis can be obtained than is possible with the electrochemical detector alone. 

Other noteworthy developments are carrier materials, such as Stamypor (DSM) and Accurel (AKZO), for production of concentrates with liquid or low-melting additives and reactants (see Section 1.2.1). The biggest growth area for additive carriers is coming from liquid peroxides and silanes, due to related health and safety issues for shopfloor staff. The NOR HALS stabiliser Tinuvin 123-S (a non-interacting, low-MW liquid) for TPO, PP and some blends is delivered in a solid carrier (Accurel). 

Ethers contain additives to stabilise them against peroxide formation. For instance, tetrahydrofuran is commonly stabilised by the addition of small amounts of hydroquinone. This absorbs uv radiation strongly and so interferes with uv absorbance detection. It can be removed by distilling the solvent from KOH pellets. If you use inhibitor-free tetrahydrofuran, it should be stored in a dark bottle and flushed with nitrogen after each use. Any peroxides that form should be periodically removed by adsorption onto alumina. 

This stems from the weakness, i.e. ease of thermal fission, of the Pb—R bond, and radicals may be generated in solution in inert solvents, as well as in the vapour phase, through such thermolysis of weak enough bonds, e.g. those with a bond dissociation energy of < w 165 kJ (40kcal)mol 1. Such bonds very often involve elements other than carbon, and the major sources of radicals in solution are the thermolysis of suitable peroxides (O+O) and azo compounds (C+N). Relatively vigorous conditions may, however, be necessary if the substrate does not contain substituents capable of stabilising the product radical, or... 

As the alkene monomers can absorb oxygen from the air, forming peroxides (c/. p. 329) whose ready decomposition can effect autoinitiation of polymerisation, it is usual to add a small quantity of inhibitor, e.g. quinone, to stabilise the monomer during storage. When subsequent polymerisation is carried out, sufficient radical initiator must therefore be added to saturate the inhibitor before any polymerisation can be initiated an induction period is thus often observed. 

Polycarboxylates may also be added to help prevent incrustations. It should be borne in mind, however, that magnesium is an essential component in most cases of stabilisation in peroxide systems, so any mixture of sequestrants should have minimum binding effect on this metal ion. 

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