Scorchy

The /V-alkyl-/V-aryl- -PDAs (where the aryl is phenyl and the alkyl may be cyclohexyl, 1,3-dimethylbutyl or 1-methylethyl) ate the most widely used /)-PDAs. These derivatives reduce the rate of crack growth and also the number of cracks. The alkyl-aryl- -PDAs are in general excellent antiozonants, particularly in dynamic environments. These derivatives are destroyed only slowly by oxygen and increase the scorchiness of the stock only slightly. These are intermediate in staining among the three classes of -PDAs. 

The best oxidation inhibitors are not usually the best antio2onants (qv). A disubstituted i ra-phenylenediamine such as AJ-isopropyl-AT-phenyl- -phenylenediamine is often selected for that purpose. -Phenylenediamine derivatives iaterfere with cure chemistry and scorchiness, and can stain objects ia contact with the vulcani2ate (114). On balance, /V-(1,3-dimethy1buty1)-/V-phenyl- -phenylenediamine and phenyl /to1y1- -pheny1enediamines have the best combination of properties. They are less scorchy and provide excellent o2one and heat resistance. Additional protection is gained ia blends with a small amount of EPDM mbber (126). 

Functionally, accelerators are classified as primary or secondary. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. Secondary accelerators, on the other hand, are usually scorchy and provide very fast cure. There are a wide variety of accelerators available to the compounder including accelerator blends this number well over 100. In order to rationalize the extensive range of materials it is useful to classify them in terms of their generic chemical structure listed below and shown in Figure 14.4. 

Symmetrical diphenylthiourea, one of the oldest accelerators. It is slow-curing but scorchy and is little used nowadays. It survived as the accelerator for pneumatic tyre curing bags until the late 1960s. 

An ultra accelerator too scorchy for use as the sole accelerator in dry rubber compounding, but popular in latex work. In dry rubber compounding it is a useful booster for thiazole type accelerators. In some low ammonia latices it is used as a preservative. 

Premature vulcanization of a rubber compound (called scorch) can occur if the reaction temperature of the vulcanizing ingredients is reached before the desired time. If this temperature is reached in the mixing process before the proper viscosity and level of dispersion is obtained, then the addition of accelerators and vulcanizing agents will cause scorchiness and poor processability. 

Due to its advanced technology and low cost the rubber industry often prefers the use of sulphur-vulcanized polyurethane elastomers even though some of their technical properties, e.g. resistance to thermal degradation, are inferior to the peroxide- and diisocyanate-cured grades which often have short processing times (i.e. they are scorchy) and whose cure may be adversely affected by the presence of moisture in the unvulcanized rubber mix water is present in rubber fillers, e.g. carbon blacks usually contain about 0-5-1% and some non-black fillers such as silicas and clays 2-10%. Also to maximize scorch time it is common practice to quench-cool the rubber after internal mixing by immersion in cold-water tanks or by cold-water spray application to the surface of the hot-milled sheet. 

The early forms of VITON processed poorly and were hard to cure. The Kellogg chemists had discovered that their CTPE/VF2 elastomer could be cured with hexamethylene diamine carbamate which also was effective with VITON , but the compounds were very scorchy. Thermal stability of the cured products was good but poorer than the raw polymer because the curing reactions introduced olefinic bonds and a relatively unstable crosslink. More Importantly for critical sealing applications high temperature compression set fell far short of demands. Further research delivered delayed action amine type curing agents which improved processability. 

Then bisphenols such as bisphenol A and also hydroquinone were found to be less scorchy and to give better cures. However, good compression set remained an elusive goal as uses of these FKM elastomers expanded rapidly in both military and civilian markets. 

MBT and its derivatives form the dominant group of accelerators used with dienes. MBT is powerful but tends to be scorchy... 

Vulcanizates prepared using p-quinone dioxime show better ageing and thermal stability than those based on sulphur cures. The vulcanizates are however coloured, show lower mechanical properties before ageing and are somewhat scorchy. This last problem is reduced by the use of the dibenzoate of p-quinone dioxime. 

Basic organic nitrogen compounds such as the amines react rapidly with sulphonyl chloride groups to form sulphonamide cross-links. As a group however these compounds are either too scorchy or they yield vulcanizates with poor physical properties. 

As the harder stocks are generally more scorchy , because they increase in temperature more rapidly when being mixed, lower speeds are used. To get increased torque at lower speeds very often a DC motor which is slant rated using field weakening is specified. What this means is that it is possible to specify a DC electric motor so that it develops its full horsepower at 80% full speed as well as at full speed, e.g. 100 hp at 1200 rpm and 100 hp at 1500 rpm. This means that the torque up to 1200 rpm is constant, and it is reduced between 1200 and 1500 rpm. The slant rated or field weakened specification is ideal for a mixer drive, so as to have high torque at low speeds and reduced torque at high speeds. 

Other sulfur donors such as tetramethylthiuram disulfide (TMTD) can also be used however, TMTD is much more scorchy (imparts less scorch safety time) compared to DTDM. 

Diamines give effective vulcanizates, but those leading to the best properties are very scorchy [19,21]. Derivatives of diamines designed to stiffen the amine fimction, so as to reduce the scorching tendency, are by far the most widely used curing agents [19,21]. The most common examples are the carbamate and bis-cinnamylidene derivatives of the hexamethylene diamine (HMDA) ... 

Levels of 0.5-1.0 pphr methyl mercapto benzimidazole have been shown to improve resistance to flex cracking of CR vulcanizates, but tend to be scorchy. The zinc salt of MMBI (ZMMBI, Vulkanox ZMB-2) is more effective in this respect. 

Trimethyl dihydroquinoline (TMQ) is not recommended because of its pronounced accelerator effect, which causes scorchiness. 

First, fluoroelastomers were cured with amines. However, the scorchy nature of those cure systems as well as a rather poor compression set resistance led to the development of bisphenol cure systems [13,14]. Peroxide curing is performed thanks to the addition of a cure site monomer [4,15-18] for compositions that cannot be cured with bisphenol, such as high fluorine compositions, low-temperature polymers where HFP has been substituted by PMVE, and non-VF2 containing polymers such as FEPMs (TFE/P and FTP polymers), unless a bisphenol cure site has been added to the polymer [19,20]. 

ACM polymers tend to be scorchy and slow curing, thus requiring a post-cure to obtain the optimum physical properties. While the NPC (No Post-Cure) system available from Zeon Chemicals is very fast curing, it is also very sensitive to process temperatures hence the compound must be kept under 90°C after the curative has been incorporated to prevent scorch. If the compound is handled properly, the bin stability and scorch resistance are excellent. 

Corrqrlete curing system (activated lead oxides plus curing agent) in predispersed form Provides good processing safety and bin stability, moderate vulcanizate properties Acts as a retarder when scorchiness is caused by excess moisture... 

Due to the thermoplastic nature of CSM, a loading factor of 70%-75% is most effective. Highly loaded or highly plasticized compounds need to be at 75% fill factor, whereas lower loaded, high viscosity or scorchy stocks require a lower load factor. The power capabilities and wear of the mixer also dictates some further adjustment in load factors. 

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