Premature scorch

In the presence of ZnO, the dump temperature is a parameter of paramount importance in mixing of a silica compound containing a couphng agent with a polysulfidic moiety The sulfur eliminated from the coupling agent causes premature scorch in the presence of ZnO. This problem is partially solved when ZnO is added in the productive mixing step on the two-roU mill. 

The higher the temperature of the mbber in the barrel before it is injected, the faster the mbber can be injected and the faster it will be heated up to vulcanization temperature in the hot mold. However, if the mbber stock temperature is too high before injection, the mbber might begin to cure prematurely (scorch). Care must be taken to avoid this. 

A more serious problem is any tendency for the compound to cross-link prematurely (scorch) during compounding or shaping. Ideally the vulcanization process should not occur at all during... 

Premature scorch, whether severe, rendering the compound unfit for further use, or as a very mild condition, which may be smoothed out with milling through a tight, cool mill, needs to be addressed carefully. Caution renuUing may be deceiving since the compound, when further processed into a finished part, may have poorer physical properties and compression set than expected. It is important to re-test the compound for Mooney scorch and Rheometer properties to ascertain if processing and physical properties meet established standards. 

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

Meta.1 Oxides. Halogen-containing elastomers such as polychloropreae and chlorosulfonated polyethylene are cross-linked by their reaction with metal oxides, typically ziac oxide. The metal oxide reacts with halogen groups ia the polymer to produce an active iatermediate which then reacts further to produce carbon—carbon cross-links. Ziac chloride is Hberated as a by-product and it serves as an autocatalyst for this reaction. Magnesium oxide is typically used with ZnCl to control the cure rate and minimize premature cross-linking (scorch). 

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

Induction period. The curatives react with themselves in preparation for the cross-linking reaction. This period allows the ingredients to be safely mixed avoiding premature curing ( 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. 

The standard approach to reducing scorch (e.g. discoloration resulting from oxidation) involves addition of one or more antioxidants. Scorch retardants prevent premature decomposition of peroxides and cross-linking of polymers (pre-vulcanisation). 

Precure - Premature vulcanization taking place during the process prior to vulcanization. Similar to scorch. 

Scorch - Premature vulcanization of a rubber compound, generally due to excessive heat history. 

Mooney viscosity—measure of the resistance of raw or unvulcanized rubber to deformation, as measured in a Mooney viscometer. A steel disc is embedded in a heated rubber specimen and slowly rotated. The resistance to the shearing action of the disc is measured and expressed as a Mooney viscosity value. Viscosity increases with continued rotation, and the time required to produce a specified rise in Mooney viscosity is known as the Mooney scorch value, which is an indication of the tendency of a rubber mixture to cure, or vulcanize, prematurely during processing. 

Scorch Premature incipient vulcanization of a rubber compound. 

The compound 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]undecane together with a peroxide is utilized in the cross-linking or curing of polyethylene for wire and cable applications. Addition of 2,4-diphenyl-4-methyl-l-pentene (AMSd) inhibits scorch or premature cross-linking of the polymer, presumably through interception of the radicals.490... 

The unique character of Neoprene vulcanisation is that rubber accelerators as a class are not effective. As a matter of fact several have measurable retarding effects and it has to be understood that mercaptobenzothiazole is an extremely potent retarder in Neoprene vulcanisation. Salicylic acid is a fast accelerator leading many a times to scorching or premature vulcanisation at higher dose levels [3]. 

In the presence of Mo deficiency, low nitrate reductase activity results in high accumulations of NO," in plants, with many physiological implications. Disorders such as scald disease of beans (Phaseolus vulgaris) (Wilson, 1949) and leaf burn/scorch of clementine (Citrus sinensis) trees (Triboi, 1978 Cassin et al., 1982) have been related to nitrate accumulation in the presence of a short supply of Mo. Premature sprouting of... 

Premature cure at processing tenqperature ("scorch at 121 0 is conveniently delayed or prevented by snail amounts of various phenolic and amine free-radical inhibitors and antioxidants. When the inhibitor concentration is properly chosen, the retarding effect disappears on the curing temperature (177 0. 

The problem of scorching or premature vulcanization is very acute with ultra or last accelerators. Rubber stocks are usually bad conductors of heat and therefore flow of heat to the interior of a vulcanizing stock from outside is very slow. As a result, in thick items the outer layers may reach a state of overcuring before the core or interior layers begin to cure. For such thick items, a slow accelerator (Table 2.5) is most suitable. 

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