Dicumyl peroxide, decomposition

This situation is substantially unchanged in the case of butyl rubber, where a few percent (0.6—3 mol %) of trans-1,4-isoprene units, i. e. C-CH2 -CH=C—CH2 -CH2 —C, are present and each methylene group is adjacent to at least one methyl substituted carbon atom. In fact, Loan has shown that the unsaturations of butyl rubber react more easily with the fragments arising from the dicumyl peroxide decomposition than the H atoms of isobutene methyl groups. The ratio of the two rate constants is... 

Ethylene-propylene and silicone rubbers are crosslinked by compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture. Peroxide cross-linking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the decomposition of the peroxide. Crosslinks are formed by coupling of the polymer radicals... 

The upper temperature limit for safe operation depends on the onset decomposition temperatures of peroxides and blowing agents employed. Preferred chemical crosslinking agents are organic peroxides, such as dicumyl peroxide (8). 

Aliphatic or aromatic peroxide curing agents can also be used, by reactions with vinyl side chains or even saturated alkyl groups. Specihc peroxides are chosen on the basis of their decomposition temperatures, and the reaction products they leave behind after the curing process is complete. Some peroxides used are Mv(2,4-dichloroben-zoybperoxide, benzoyl peroxide, dicumyl peroxide, and di-t-butyl peroxide.83-86... 

The decomposition of di-/-butyl and dicumyl peroxide in benzene, a poor hydrogen atom donor solvent, were studied in the presence of 2,2,6,6-tetramethyl piperidine JV-oxyl (II) over the temperature range of 95-125 The termination kinetics. 

Elastomers must be crosslinked to hold their final form. The crosslinking reaction takes place through generation of free radicals that promote bonding at sites of unsaturation. The most common crosslinking agents for this include reactive peroxides, such as dicumyl peroxide, diacetyl peroxide, di-tert butyl peroxide, and others. Since each has a different temperature at which thermal decomposition initiates, curing conditions vary with the peroxide type. 

The use of a different peroxide, Vulcup R, provides results comparable to dicumyl peroxide, a fact which can be interpreted as an indication of beneficial effects from its decomposition products also, in suppressing tree growth. 

Evidence suggesting a change in the mode of peroxide decomposition under conditions of rapid thermolysis is provided by the decomposition of dicumyl peroxide in mineral oil ). Thus, although the yield of cumyl alcohol is the same whether the decomposition occurs at 135 or 180 C, the yield of acetophenone and methane, the products of ketonic scission of the cumyloxy radical, essentially doubles at the higher temperature. 

The temperature of an initiator depends on the rate of decomposition as reflected in its half-life. A good rule-of-thumb in this regard is a ti/2 of about 10 h at the particular reaction temperature (Table 6.2). The practical use temperature ranges of some common initiators are diacetyl peroxide 70-90°C, dibenzoyl peroxide 75-95°C, dicumyl peroxide 120-140°C, and AIBN 50-70°C (Odian, 1991). 

For the decomposition of dicumyl peroxide in LDPE (2% w.) at 160 °C, the efficiency of peroxide in crosslinking was about 87% [103]. Such a hi efficiency may be ascribed to the contribution of physical entanglements to the overall number of chemical crosslinks and to the presence of unsaturated C=C bonds in a polymer. We may recall that during dimerization of low molecular alkanes initiated by rert.butyl... 

The choice of peroxide used is determined by the temperature of its decomposition. Peroxide should be effectively dispersed in the polymer melt brfore a substantial homolysis of 0—0 bonds can occur. For such a purpose, dicumyl peroxide which may be dissolved in vinyl trimethoxy silane (b.p. 120 °C) is suitable. The required degree of crosslinking was attained if 2 % w. of silane with 5-10% w. of peroxide were added to polyethylene. At the silylation, grafting should not commence before both compounds (peroxide, ane) are well dispersed in the polymer melt [141]. NonhcMno-geneous dispersion of additives reduces efficiency of grafting and of subsequent crosslinking. 

Table 20-1. Half-Lives and Activation Energies of Decomposition of Some Free Radical Initiators AI BN, Azobisisobutyronitrile BPO, Dibenzoyl Peroxide MEKP, Methyl Ethyl Ketone Peroxide IPP, Diisopropyl Peroxide Dicarbonate Dicup, Dicumyl Peroxide CuHP, Cumyl Hydroperoxide...

Acrylic esters and unsaturated polyesters are commercially cured with peroxides or peresters. The choice of per compound is determined on the basis of price, the achievable polymerization rate, and the side products formed. The polymerization rate is determined by the decomposition rate of the initiator, when mixed with the material to be cured, as well as on the free radical yield. In addition, attention should be paid to the fact that many per compounds decompose slowly during storage, thus reducing the polymerization activity per unit initiator mass. For this reason, crystalline per compounds are more stable because of the lower diffusion than amorphous or dissolved per compounds. Side products of initiator compounds can have an unfavorable effect on the long-term thermoset properties dibenzoyl peroxide, for example, forms acids dicumyl peroxide forms ketones. Acids can hydrolyze the ester bonds of polyester chains, causing scission, and ketones can... 

In one experiment, fibers were treated with a solution of benzoyl peroxide and dicumyl peroxide in acetone for about half hour after alkali treatment [44, 55-57]. High temperature favors decomposition of peroxides [74]. Studies on sisal fiber treatment were performed and composites were developed using benzoyl peroxide and dicumyl peroxide and toluene solvent with polyethylene at the time of mixing with fiber. The peroxide-treated matrix showed higher viscosity than untreated composites because of the grafting of polyethylene onto sisal fiber in the presence of peroxide [34]. Benzoyl peroxide-treated sisal fiber showed a tensile strength of 40.90 MPa [60, 61]. 

Many organic peroxides can be employed, one of the more widely used ones being dicumyl peroxide. Dicumyl peroxide decomposes either thermally to yield free radicals or in acid media by an ionic cleavage mechanism without the production of free radicals. Since the free-radical mechanism is required for the polymer vulcanization reaction, the ionic cleavage decomposition has to be suppressed by the use of a non-acidic medium. The factor determines what type of filler can be used. 

The thermal decomposition of dicumyl peroxide in rubbers has been the subject of numerous studies these are beyond the scope of this review, since they have not been carried out in solution. A few references are included here to provide the reader access to some of the excellent kinetic data available in this area [64 Loa 1,68 Lai 1]. 

Peroxide treatment The functional group of peroxide can be represented as -ROOR-. Most commonly used peroxides for this treatment are benzoyl peroxide and dicumyl peroxide. The main advantage of peroxide treatment is the quick decomposition of a peroxide yielding free radical that can react with the hydrogen group of the matrix and fiber. Like some of the other treatment methods, fibers are pretreated with alkali before treating with peroxides [2], The reactions that take place during peroxide treatment are represented in the Equations 9.7a, 9.7b, 9.7c, and 9.7d. The matrix used in these equations is polyethylene (PE). 

From an analytical perspective, the erosslinking materials and process should be well understood. A reactive peroxide, for example, will generally not be evident in the analysis of a polymer crosslinked with this material. This will be evident when not fully decomposed during a cure process, for example, and the presence of non-reaeted peroxide may be a valuable indieation of an under-cure condition. Typically, the deeomposition products of dicumyl peroxide are what will be found analytically. These include aeetie aeid, eumyl aleohol, acetophenone, and others. Heating of most thermoplastics to a level that initiates deeomposition of the curing agent would also oxidize the base polymer. It is therefore neeessary to proteet the polymer with an antioxidant. GC/MS analysis of a typical peroxide-crosslinked polyethylene will reveal the peroxide decomposition products, an antioxidant, and no trace of the original peroxide. 

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