Accelerant residues

If accelerators which leave amine residues in the rubber compound are used, then the above mentioned deterioration of a polyester fabric will occur. Thiurams and dithiocarbamates can also damage fabrics. These accelerator residues cause deterioration of both fabric and adhesion levels which usually occur under service conditions after long periods of exposure to high temperatures. 

Tinkler J, et al Risk assessment of dithiocar-bamate accelerator residues in latex-based medical devices Genotoxicity considerations. Food Chem Toxicol 36 9, 1998... 

Through the use of the various methods of recovery discussed, it is often possible for the forensic chemist to obtain a satisfactory sample of accelerant residue for examination purposes. Through utilization of gas chromatography, the identification of the accelerant can often be effected and differences and similarities between recovered and known standard specimens can be shown. However, success in the recovery and identification of accelerant residues is highly dependent upon the type and quantity of material received for examination and the care that has been taken in the preservation of the items to be examined. 

The experimental ratios of 2 ( 0.2) agree with the expected value of 2.0 converted ENB units for one chemical crosslink (ENB-Sn-ENB), indicating that crosslinks are formed predominantly. Several explanations are proposed for deviations from the theoretical value. Both pendent sulfur as in the crosslink precursor (ENB-Sn-X, X = accelerator residue) and sulfur loops (sulfur bridge between two ENB units in the same EPDM chain) will give rise to a ratio larger than 2.0. Formation of cyclic sulfide (addition of S to unsaturation) will give rise to a ratio lower than 2.0. The fact that within experimental error a ratio of 2.0 is found suggests that pendent sulfur, loops and cyclic sulfur hardly occur (less than 10% of the total amount of reacted sulfur). 

BLANKS A blank is of prime importance. Many materials can produce Figure E26-1. An arson sample combustion products that look like accelerant residues. For example, glues collection container, from carpets, hydrocarbons from floor tiles, or essential oils from grasses. I once had a house fire to investigate that supposedly was set by pouring gasoline... 

Equation (9.6) gives a somewhat simplistic schematic of the vulcanization reaction. On completion of the reaction, the pendant accelerator will cleave off (i.e., Captax) after generation of a crosslink. This accelerator residue, Captax, is an accelerator in its own right and continues to participate in further crosslinking as vulcanization continues. 

Any item or debris suspected of containing accelerant residue should be collected in a tightly sealed, clean, and previously unused paint can, a special evidence bag, or a clean glass jar to prevent further evaporation. The container should be of size similar to the amount of debris but should be no more than three-fourths full, leaving sufficient headspace volume for subsequent laboratory analysis. Care should be taken to avoid contamination as this will compromise the value of the evidence. The container should be labeled with the investigator s name as well as the date, department, location, area from which the sample was taken, case numbei and description of the item or contents. The chain of custody must be maintained if the evidence is to be used in court. 

Fig. 12. Sulfur-based cure system designs where conventional systems are polysulfidic, EV systems are mono- to disulfidic, and semi-EV are di- to polysulfidic. A is pendent sulfide group terminated with accelerator residue, B monosulfidic cross-link, C disulfidic crosslink, D polysulfidic cross-link (x > 20), E cyclic monosulfidic cross-link, F cyclic polysulfidic cross-link, and Acc accelerator residue.

Important initiators and accelerators - residual monomer, copper stearate ... 

Scheme 16.37. Products and generally accepted mechanism for accelerated sulfur vulcanization of EPDM (X = accelerator residue sulfur substitution at C3(exo) and C9 of ENB is just given as an example).

Figure 1 Generalized structure of a sulfur vulcanizate of a diene rubber, a, 6, x, y=l-6 X = accelerator residue 2-benzothiazolyl, dialkylthiocarbamyl, etc. curved arrow signifies cis,trans-isomerization...

Scheme 1 Outline reaction scheme for the sulfur vulcanization of diene rubbers. R represents the rubber chain and H is normally an allylic hydrogen atom X = accelerator residue...

The term arson analysis implies the search of materials taken from a fire scene for accelerant residues to establish... 

The list of the most common accelerants, shown in Table I, may at first give the impression that these would rapidly gasify along with the burning solid materials and thus not become detectable, but it is not always so. Traces of the fuel often remain in some closed areas of the fire scene even after the temperature has reached perhaps 1000°C, and these accelerant residues may be detectable after proper sample collection. 

Results are presented of a study in which the sulphur vulcanisation of polyisoprene was carried out at 150C and gas chromatography/mass spectrometry and HPLC were used to determine the chemical nature of intermediates and accelerator residues formed during the vulcanisation reaction. 15 refs. 

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