Sheet Scorched

Preheat an oven to 250°F for ten minutes. Turn olf Place the tobacco on a cookie sheet and dry for about 10 minutes. Leave the oven door cracked and make sure you don t scorch it. 

Sometimes the surface of a rubber sheet might have been scorched or pre-vulcanized due to high processing temperatures and storage under improper storage conditions. To avoid this, modify the compound suitably. The preferred temperature for storing the in process stock of rubber can be 20°C to 24°C. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

Fortunately for our project and for the development of block polymers, we had an acutely observant team member, Hendricks, who prepared unvulcanized sheets of the three-block polymer for rheological measurements. He immediately reported that the "unvulcanized" sheets were unusually "snappy" and appeared to be "scorched." Subsequent stress-strain measurements showed high tensile strength and high ultimate elongation. However, solution data showed complete solubility, with no gel or microgel present. 

Perhaps the more important drawback of this method is the fact that for a 0.5-mm-thick sample, the temperature through its thickness should instantaneously reach that of the heated slabs of the press. This assumption is not exactly true, as shown in Figure 1.6 with the curve expressing the increase in the state of cure at the mid-plane of the rubber sheet as a function of time. Thus, the S shape for the curve representing the cure (or rather the mean cure) versus time (Figure 1.1) is logical. Moreover, the scorch time [2,3], which is the time necessary for the onset of vulcanization to take place, will perhaps need another explanation. 

Chem. Descrip. Zinc-dialkyidithiophosphate (67%) bound to silica (33%) Uses Accelerator for EPDM and other diene rubbers, tech, molded and extruded goods, e.g., profiles, hoses, sheeting, coatings Features Scorch-resist. nonstaining Properties Wh. crumbling powd. sp.gr. 1.3 Rheocin [Stid-Chemie Inc]... 

The cure time will vary according to the compound in use, the rubber sectional thickness, and the moulding conditions. The optimum value should be determined by the trials planned in step 6.3.4. For guidance see Table 6.2. The overall cure time will depend on the temperature of the injected rubber. The hotter that this can be sustained without incurring scorch penalties, the more even will be the crosslink density across the rubber section. The optimum value that is often quoted for crosslink density is 90%. This value was determined from a 2 mm thick sheet tested for tensile and related properties. 

Tensile properties of different EVA/ATH formulations are summarized in Table 4.3. The basic formulation contained 160phr of ATH, 1 phr stabilizer, and variable amounts of monomeric and oligomeric silanes and peroxide. A corotating twin-screw extruder was used to produce sheets for the tests. SUane content is based on filler, the silane was preblended with the EVA, dicumyl peroxide (DCP) and Irganox 1010 (phenolic stabilizer, Qba) were used as peroxide and stabilizer, respectively. A control without silane is not included since it leads to scorch. 

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. 

If you want to avoid chemicals, you can also strip paint with a heat gun. Hold the heat gun in one hand and a paint scraper in the other as you heat and then scrape away the paint, one small section at a time. Be sure to keep the heat gun moving constantly so it doesn t scorch the surface or create dangerous embers. On small jobs, you can even substitute a hair dryer for the heat gun just hold it very close to the paint and don t expect it to peel off in sheets. 

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