Scorch resistance

An important consideration of any vulcanization system is scorch resistance. Scorch time is that period after heat has been applied until a significant change in compound stiffness occurs indicating that cross-linking of the polymer has initiated. As the rubber compound is hot when the accelerators and sulfur are added, it appears that the scorch period starts immediately. As the compoimd is processed through the factory to form tire components, it is heated and is subject to premature vulcanization (search) if heat is excessive (scorch). Scorched compoimds are more difficult to process to their finished component. 

Pentabromodiphenyl Oxide. Pentabromodiphenyl oxide [32534-81-9] (PBDPO) is prepared from diphenyl oxide by bromiaation (36). It is primarily used as a flame retardant for flexible polyurethane foams. For this appHcation PBDPO is sold as a blend with a triaryl phosphate. Its primary benefit ia flexible polyurethanes is superior thermal stabiUty, ie, scorch resistance, compared to chloroalkyl phosphates (see Phosphate esters). 

Extruded Articles. In extmded article compounding, the most important parameters are scorch safety and flow characteristics (53). The bisphenol cure system again offers the best scorch resistance of the available fluorocarbon elastomer cure systems. Good flow characteristics can be achieved through proper selection of gum viscosities. Also, the addition of process aids to the formulation can enhance the flow characteristics. Typical formulations for extmsion grade fluorocarbon elastomers are given iu Table 7. 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

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. 

Although ultra accelerators or sulfur donors can be used together with primary accelerator (such as sulfenamide, TBBS) to improve cure rate as well as the heat resistance [16-18], their use is restricted because of the associated nitrosamine issue [19]. Accelerators derived from secondary amines, for example, MBS, TMTD, TETD, TMTM, and OTOS fall into this category. The combination of sulfenamide, such as CBS or TBBS, and a thiuram, such as TMTD or TETD, shows high-cure rates but suffers from the adverse effects on scorch resistance and vulcanizate dynamic property [20]. Additionally as previously mentioned, the use of TMTD or Tetraethylthiuram disulhde (TETD) or A-oxidiethylene dithiocarbamyl-A -oxidiethylene sulfenamide (OTOS) or 4,4 -Dithiodimorpholine (DTDM) is undesirable [21] due to concerns over carcinogenic nature of the A-nitrosamines formed from the parent amines. The solution to this originated by introduction of nitrosamine safe ultra accelerator such as TBzTD [22,23]. 

Cure characteristics are shown in Figure 14.15 the antireversion agent has no effect on scorch resistance and time to optimum cure. The beneficial effect becomes apparent when reversion occurs, as observed in the control compound. The compound containing the antireversion agent on the other hand maintains a torque level close to the maximum. 

Diamine, e.g., hexamethylene diamine Magnesium oxide or Calcium oxide General purpose cure system, fairly resistant to scorch. Best heat resistance, but not particularly resistant to acids. 

Red Lead Best acid and steam resistance, but exhibits strong tendency to scorch. 

Zinc oxide/dibasic lead phosphite Least tendency to scorch, intermediate acid resistance. 

In terrestrial plants, paraquat s action is at the point of local absorption (Anonymous 1963). Characteristic damage signs to susceptible species include wilting and general collapse in herbaceous plants. Regrowth may occur in some perennial plants, but in resistant species temporary scorch may be the most marked effect (Anonymous 1963). In sugarcane (Saccharum officinarum), paraquat... 

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 care of microfiber products is similar to that of the normal fiber materials made from the same polymer. One caution is heat sensitivity. Because the fibers are so fine, heat penetrates easily causing them to scorch or glaze more quickly than normal fibers if too much heat is applied or heat is applied over too long a period. Typically, microfibers are wrinkle-resistant, but if ironing is done, it should be accomplished using lower temperatures and only as directed. 

Processibility is dependent on the viscosity or plastic flow of the rubber compound,i.e., resistance to flow. Plasticity or viscosity determines the energy requirement of the rubber during milling, calendering or extrusion while the time to the onset of curing, i.e., scorch time, indicates the amount of heat history which can be tolerated before the rubber is converted from the plastic to the elastic state at which time processing becomes virtually impossible. 

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. 

The scorched area is shown as a hatched range in Fig.2.4, corresponding to a size of about 5m 2. The Hachinohe Police Office reported that property damaged by the fire included the inside wall (made of fire-resistant board) on the side of the room facing the road, the ceiling (made of fire-resistant board), the floor (about lm 2), and pillars and the roof of the entrance. 

FIBRAMOLL CN Liquid is an excellent textile softener recommended for finishing natural and synthetic fabrics and their blends. A soft desirable hand can be accomplished without obtaining a greasy, slick finish. Treated fabrics show good scorch resistance and good non-yellowing properties. 

Extremely high resistance to scorch, elevated temperature yellowing and discoloration due to oxidation. 

CNC SOFT C-2 provides excellent fiber softness and lubricity and is resistant to scorching and chlorine retention. 

CNC SOFT SR is a stable compound of a polyglycol mono-oleate base. It is non-ionic with excellent stability and compatibility and has high resistance to scorch yellowing. 

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