Dicumyl peroxide, determination

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

The 13C NMR crosslink density results were compared with the crosslink density obtained by the mechanical measurements. In the determination of the crosslink density by mechanical methods, the contributions of the topological constraints on the results were neglected and the density was expressed as G/2RT. The 13C and mechanical-crosslink densities were obtained for both sulfur and dicumyl peroxide (DCP)-cured samples to ensure the effect of wasted crosslinks (pendent or intramolecular type sulfurisations), which are expected in the typical sulfur-vulcanisation of NR. In the major range of crosslink densities, the crosslink densities for those two systems are described by the same linear function with a slope of 1.0. Based on these observations, it is shown that the crosslink density of the sulfur-vulcanised NR as determined by 13C is identical with the true crosslink density, and the influence of the wasted or ineffective crosslinks (pendent and cyclic crosslinks) and chain ends is negligible. However, this conclusion seems to be only valid if the effect of topological constraints or entrapped entanglements on the mechanical modulus is negligible which is rarely the case in real systems. 

Figure 26. Superposed spectra of cured els-polybutadiene minus its components. Spectrum A, polybutadiene and the products of the degrading dicumyl peroxide. Spectrum B, cured spectrum minus uncured polybutadiene spectrum C, B minus dicumyl peroxide spectrum D, C minus dicumyl alcohol and spectrum E, D minus acetophenone. The amount of each component determined by least squares analysis.

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. 

Three urethane-crosslinked polybutadiene elastomers (TB-1, TB-2, and TB-3) of varying crosslinking levels, along with a similarly crosslinked styrene-butadiene copolymer (HTSBR) and two polybutadiene polymers randomly crosslinked with dicumyl peroxide (PB-1 and PB-2), have been investigated to determine their viscoelastic behavior. Elsewhere, TB-1, TB-2, and TB-3 have been designated as HTPB-1, HTPB-2, and HTPB-3, respectively. 

In radical polymerization (see Scheme 2), radicals are formed in a first stage determining the rate. Examples used industrially for radical sources, apart from electron radiation, are aroyl peroxides such as bis(2,4-dichlorobenzoyl) peroxide or bis(2,4-methylbenzoyl) peroxide and alkyl peroxides such as dicumyl peroxide or 2,5-dimethyl-2,5-di-/er/-butyl peroxyhexane. In the second stage, the actual crosslinking reaction, a radical addition based on the usual pattern for a radical chain reaction, takes place via the double bonds of the vinyl groups in the polymer. Radical polymerization is today used virtually exclusively for crosslinking solid silicones. 

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... 

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. 

In order to form the plastic phase of the semi-IPNs of the first kind and the full IPNs, styrene monomer solutions were prepared containing 0.4% (w/v) dicumyl peroxide and the several quantities of divinylbenzene shown in Table 5.3. To swell in the monomer, a known weight of crosslinked rubber was immersed in the solution at ambient conditions. The ratio of styrene to divinylbenzene actually imbibed was not determined. The duration of imbibing was dependent upon the desired final IPN composition. The swollen polymer then was placed in an airtight container with a saturated styrene atmosphere for approximately 12 hr so that a uniform distribution of monomer could be achieved throughout the sample. Next, the styrene was polymerized thermally at 50°C for a period of 4 days and at lOO C for 1 hr. Finally, the IPN was subjected to a vacuum-drying operation to remove any unreacted monomer. Semi-IPNs of the second kind were prepared by... 

Because of the speed of data acquisition of NIR instrumentation with nonmoving parts, opportunities arise to study reaction rates and mechanisms. One example includes the crosslinking reaction of liquid carboxylated poly(acrylonitrile-Co-butadiene) or nitrile rubber (NBR) with dicumyl peroxide (DCPO). This reaction was studied by electron spin resonance (ESR) spectroscopy and the crosslinking reaction was followed in situ in dioxane by monitoring of the disappearance of the pendant vinyl double bond with Fourier transform NIR (FT-NIR). The overall activation energy and rate equation of the reaction were able to be determined and provided insight into the reaction mechanism [42]. 

Fedors and Landel [103] point out that stress-strain behavior of swollen elastomers can be determined experimentally more conveniently by measurements in uniaxial compression than uniaxial extension. In extension, strains of the order of a few hundred percent are required whereas, in compression, strains of the order of only a few percent provide sufficient data for analysis. SBR-glass bead composites cured by means of dicumyl peroxide were used for stress-strain measurements to estimate the concentration of the eftective network chains per unit volume of ttie whole rubber. It was found that with decreasing volume fraction of the composite, tire effective network density decreased linearly at first and then rather rapidly in an unexpected and inexplicable manner. 

Brammer et al have described a method for determining dicumyl peroxide in polystyrene, which is not subject to interference by other organic peroxides or additives that may be present in the polymer. The dicumyl peroxide is extracted from the polymer with acetone and then separated from any other additives present by thin-layer chromatography on silica gel. The gel in the area of the plate containing dicumyl... 

Epoxidized PBD and PBD were radically crosslinked by dicumyl peroxide to investigate the influence of epoxidation on crossUnking. The heat of crosslinking was measured by DSC and the crosslinking efficiency determined as the amount of heat of reaction in relation to one repeating unit of PBD and per molecule of dicumyl peroxide. The crosslinking efficiency was four times lower for... 

Figure 2 illustrates the length-temperature data for one of the elastomers, poly-cis-1, U-butadiene cured with 0.5 p.h.r. dicumyl peroxide, at a series of loads. From the slopes of the straight lines in this plot, the linear thermal expansion coefficients of the strained rubber can be readily obtained. In order to compute the relative energy contribution to rubber elasticity by eq. 6, we need the linear thermal expansion coefficients of the unstrained rubber and the temperature coefficient of the shear modulus. These can be determined by plotting a as a function of the quantity (X -l)/(X +2). If eq. 7 is indeed valid, then one would expect a linear relation. From the intercept and slope of such a plot, values of and dilnG/dT can be easily obtained. 

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