Guanidine accelerators

AnUine, however, is too toxic for use in mbber products. Its less toxic reaction product with carbondisulfide, thiocarbanihde, was introduced as an accelerator in 1907. Further developments led to guanidine accelerator [4]. Reaction products formed between carbon disulfide and aliphatic amines (dithiocarbamates) were first used as accelerators in 1919 [5]. These were and still are the most active accelerators in respect to both cross-finking rates and extent of cross-link formation. However, most dithiocarbamates accelerators give little or no scorch resistance and therefore cannot be used in aU applications. 

This reference documents the chromatographic properties of over 100 rubber-related amine and phenolic antioxidants, antiozonants, guanidines, accelerators, and amine hydrochlorides. This reference examined the chromatographic characteristics of the cited additives and, thus, did not characterize acmal polymers. 

Sometimes curing agents, antioxidants and accelerators can interact, creating new toxic chemicals e.g., when guanidine accelerators are used during the vulcanisation of rubber with sulfur with phenylene diamine-based antioxidants, aromatic amines and isothiocyanates can be produced, both of which are suspected carcinogenic agents [11]. 

Other guanidine accelerators may substitute for DPG, but they also have toxicity issues. They will also impart different cured physical properties that must be evaluated. The strong toxicity concerns with the use of DPG are driving an effort to find new substitutes for it. 

Vamac terpolymers are usually cured with a combination of primary diamines and guanidine accelerators... 

Guanidines. Guanidines (10) were one of the first aniline derivatives used as accelerators. They are formed by reaction of two moles of an aromatic amine with one mole of cyanogen chloride. Diphenylguanidine (DPG) has enjoyed a resurgence ia demand as an activator for sulfenamides and a co-accelerator ia tire tread compounds which employ siUca fillers for low rolling resistance. Guanidines alone show too Htde activity to be extensively used as primary accelerators. There were no U.S. producers as of mid-1996. 

It is common practice in the mbber industry for a compounder to use combinations of several accelerators in developing a cure system. Typically these cure systems are comprised of a primary accelerator and one or more secondary types. Primary accelerators are generally the thiazole and sulfenamide classes the secondary types (kickers) are the thiurams, dithiocarbamates, guanidines, and to a much lesser extent, certain amines and the dialkylphosphorodithioates (20). 

Fig. 5. Cure characteristics of accelerators A, thiuram B, dithiocarbamate C, sulfenamide D, thiazole and E, guanidine. The induction period represents...

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

Sulfenamide accelerators generally requite less fatty acid because they release an amine during the vulcanization process which acts to solubilize the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

New efficient vulcanization systems have been introduced in the market based on quaternary ammonium salts initially developed in Italy (29—33) and later adopted in Japan (34) to vulcanize epoxy/carboxyl cure sites. They have been found effective in chlorine containing ACM dual cure site with carboxyl monomer (43). This accelerator system together with a retarder (or scorch inhibitor) based on stearic acid (43) and/or guanidine (29—33) can eliminate post-curing. More recently (47,48), in the United States a proprietary vulcanization package based on zinc diethyldithiocarbamate [14324-55-1]... 

Curing. Carboxyl cure sites are incorporated in the ethylene—acryhc terpolymer to permit cross-linking with primary diamines (1,7). Guanidines are added to accelerate the cure. Peroxides may also be used as curing agents in the terpolymer, but generally give inferior properties to vulcanizates based on diamine systems (8). Dipolymers are cured only with peroxides. 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

Several of the commonly used rubber chemicals tike accelerators, retarders, antidegradants, etc. are classified as hazardous on account of values of LD50 (lethal dose 50%) [6]. Many of the guanidine... 

For the purpose of polymer/additive analysis most applications refer to vulcanisate analysis. Weber [370] has determined various vulcanisation accelerators (Vulkazit Thiuram/Pextra N/Merkapto/AZ/DM) in rubbers using PC. Similarly, Zijp [371] has described application of PC for identification of various vulcanisation accelerator classes (guanidines, dithiocarbaminates, thiuramsulfides, mercapto-substituted heterocyclic compounds, thioureas, etc.). The same author has also... 

FD-MS is also an effective analytical method for direct analysis of many rubber and plastic additives. Lattimer and Welch [113,114] showed that FD-MS gives excellent molecular ion spectra for a variety of polymer additives, including rubber accelerators (dithiocar-bamates, guanidines, benzothiazyl, and thiuram derivatives), antioxidants (hindered phenols, aromatic amines), p-phcnylenediamine-based antiozonants, processing oils and phthalate plasticisers. Alkylphenol ethoxylate surfactants have been characterised by FD-MS [115]. Jack-son et al. [116] analysed some plastic additives (hindered phenol AOs and benzotriazole UVA) by FD-MS. Reaction products of a p-phenylenediaminc antiozonant and d.v-9-lricoscnc (a model olefin) were assessed by FD-MS [117],... 

Guanidine itself is imido-urea, but the term guanidines usually refers to the accelerators of vulcanisation, diphenylguanidine and diort/wtolylguamdme. 

The W types require additional acceleration and ethylene thiourea (ETU), gives the best balance of all properties. However, the use of this accelerator is increasingly being restricted due to fears of its effects on pregnant women, and more recently men. DETU, thiurams and guanidines can also be used. Sulphur is sometimes used to increase the degree of cure in the W types, but this detracts from the ageing performance of the vulcanisate. 

Reduction of the adhesion level will occur if certain compounding ingredients are not avoided. The acceleration system has a direct effect on the adhesion level dibenzothiazole disulphide (MBTS) gives the highest adhesion levels. If a second accelerator is used, e g., tetramethyl thiuram disulphide (TMTD) or diphenyl guanidine (DPG), then the adhesion is significantly affected. Other ingredients which cause problems are plasticisers and process oils. 

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