Vulcanization scorch

It should not be prone to causing premature vulcanization (scorching) during mixing, extrusion, calendering or other prevulcanization operations. 

Sulfur and accelerators are also present. If these are not well dispersed in the compound, they can lead to early vulcanization (scorch) during processing. 

Step 1. May be to examine the scorch time of the compound for seasonal or special cause trends. In rubber molding, we are trying to flow the material and vulcanize the material in one process. The onset of vulcanization, scorch, may be providing some of the viscosity and pressime increases we need in the product cavity to eliminate blisters and minimize flash. We may need to establish a range of acceptable scorch rates and adjust the accelerator level in the formula to keep the scorch rate in the good molding window. This adjustment may need to be a one-time affair, or it may need to be seasonal as shown in Figure 16C.12. 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

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]... 

When two polymers interact or react with each other, they are likely to provide a compatible, even a miscible, blend. Epoxidized natural rubber (ENR) interacts with chloro-sulfonated polyethylene (Hypalon) and polyvinyl chloride (PVC) forming partially miscible and miscible blends, respectively, due to the reaction between chlorosulfonic acid group and chlorine with epoxy group of ENR. Chiu et al. have studied the blends of chlorinated polyethylene (CR) with ENR at blend ratios of 75 25, 50 50, and 25 75, as well as pure rubbers using sulfur (Sg), 2-mercapto-benzothiazole, and 2-benzothiazole disulfide as vulcanizing agents [32]. They have studied Mooney viscosity, scorch... 

Mooney viscometer is also used to measure the time it takes, from initial exposure of the compound to a particular temperature, to the time of onset of cure at that temperature [2]. This is known as the scorch time. Scorch time is an important parameter to the rubber processor, as a short time may lead to problems of premature vulcanization. As the test is taken past the onset of cure, the rotor tears the cured rubber, and therefore this device cannot be used to investigate rheological properties after the scorch time. 

Scorch resistance, in vulcanization, 22 811 Scorch time, in vulcanization, 22 803 SCORE cleaning system, 24 22 Scotch-Marine boiler, 22 319 Scouring, 9 171, 183, 189, 192, 197 of fibers, 22 180 in wool processing, 26 384-385 Scoville Heat test, 23 159 Scrap... 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

A. B. Sullivan, C. J. Hann, and G. H. Kuhls, "Vulcanization Chemistry—Fate of Elemental Sulfur and Accelerator during Scorch Delay as Studied by Modem HPLC", Paper No. 9, presented at th eMCS Rubber Division Meeting, Toronto, Canada, May 21 —24, 1991, American Chemical Society, Washington, D.C., 1991. 

Another cure system consideration is the compound scorch behavior. Prior to vulcanization, rubber is plastic-like and can be processed into desired shapes such as tires, hoses, belts, or other articles. The time available to accomplish this processing depends laigely on the cure system and is referred to as the scorch time. If a compound cures prematurely during the processing step, it usually becomes useless scrap. Therefore, a key requirement of the vulcanization step is to minimize premature vulcanization or scorch (Fig. 4). 

Fig. 4. Effect of heat history on processing (scorch) safety where A shows the time after processing and storage and B shows vulcanization without...

Sulfur sample Scorch time Optimal vulcanization Min. torque Max. torque... 

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