Shortstop

Short spin processes Shortstopping agents Shortstops Shot mills Shotpeemng... 

Since the principal hazard of contamination of acrolein is base-catalyzed polymerization, a "buffer" solution to shortstop such a polymerization is often employed for emergency addition to a reacting tank. A typical composition of this solution is 78% acetic acid, 15% water, and 7% hydroquinone. The acetic acid is the primary active ingredient. Water is added to depress the freezing point and to increase the solubiUty of hydroquinone. Hydroquinone (HQ) prevents free-radical polymerization. Such polymerization is not expected to be a safety hazard, but there is no reason to exclude HQ from the formulation. Sodium acetate may be included as well to stop polymerization by very strong acids. There is, however, a temperature rise when it is added to acrolein due to catalysis of the acetic acid-acrolein addition reaction. 

The basic constituents of all commercial emulsion polymerization recipes are monomers, emulsifiers, and polymerization initiators. Other common components are modifiers, inorganic salts and free alkaH, and shortstops. The function of these different components and the mechanism of emulsion polymerization have been described (43,44). 

Neutral or alkaline salts, eg, KCl, K SO, K CO, or Na PO, are often present in synthetic latices in quantities of - <1%, based on the weight of the mbber. During emulsion polymerization the salts help control viscosity of the latex and, in the case of alkaline salts, the pH of the system. Many polymerizations are carried out at high pH, requiring the use of fixed alkaH, eg, KOH or NaOH. Very small amounts of ferrous salts can be employed as a component of the initiator system, in which case a sequesteriag agent, eg, ethyldiaminotetraacetic acid (EDTA) may be iacluded to complex the iron. Water-soluble shortstops, eg, potassium dithiocarbamate, may also be iacluded ia very small amounts (ca 0.1 parts). 

Another difference between hot and cold elastomeric SBR latices is that hot types are carried to < 90% conversion and not normally shortstopped. The cold latices are usually shortstopped at ca 60—80% conversion. Again the desired physical properties of the contained copolymer are responsible for these differences. Cold latices are used in applications where the modulus, eg, in foam, or retention of physical properties at high filler loadings, eg, in fabric backing, are required. The cold latices are generally suppHed at a higher soHds concentration than the hot series because of these uses. 

Fig. 3. Solution SBR manufacture by continuous process where Bd = butadiene, HC = hydrocarbon, AO = antioxidant, and SS = shortstop.

There is an industry trend to supply SBR certifiably free of volatile nitrosamines or nitrosatable compounds. This has generally been accomplished by replacing shortstop systems based on carbamates and hydroxyl amines with products that are not based on secondary amines or are secondary amines of high molecular weight, such as dibenzyldithiocarbamate. A more recendy issued patent for ESBR shortstop is based on isopropyUiydroxylamine, a primary amine that does not form nitrosamine (46). 

The original recipe adopted by the U.S. Government Synthetic Rubber Program was known as the "Mutual Recipe" and is shown iu Table 4. As can be seen, the reaction temperature was set at 50°C, which resulted iu 75% conversion to polymer iu about 12 h. The reaction was then stopped by addition of a "shortstop," such as 0.1 parts hydroquinone, which destroyed any residual catalyst (persulfate), and generated quiuone, which helped inhibit any further polymerisation. 

Develop and install emergency system and procedures to shortstop runaway reaction. 

Synonym Clarosan, GS 14260, Igran, Prebane, Shortstop, Terbutrex, Terbutrin, Terbutryn... 

In the polymerization of BD by Ti-, Co- and Ni-based catalyst systems the polymerization has to be shortstopped at a specific monomer conversion in order to avoid the formation of gel. In contrast, polymerization catalysis by Nd catalysts does not need control of monomer conversion. As gel formation is particularly low with Nd catalysts full monomer conversion can be accomplished [427,428]. 

If the polymerization is shortstopped at a specific monomer conversion water, acids and alcohols are used. Water has to be properly mixed with the... 

The first three benefits are a direct consequence from the extremely low tendency of the Nd-catalyst to form branches and gel. Because of this remarkable feature, Nd-catalysts allow monomer conversions up to 100%. Therefore, the polymerization reaction does not have to be shortstopped below a critical monomer conversion in order to avoid gel. In addition, polymerization temperature does not have to be controlled within a well-defined temperature range. As the maximum polymerization temperature (at complete monomer conversion) can be as high as 120 °C the polymerization process can be performed in a fully adiabatic manner. In this case energy costs for cooling and for the removal of low molar mass residuals can be very low. Another benefit of the Nd-catalyst is the low tendency to catalyze the Diels-Alder dimerization of BD to vinyl cyclohexene. 

The ampoules were removed from the bath at the desired time and opened. The polymerizations were shortstopped by quickly pouring the polymer cement into 30% of its volume of a 0.15M solution of ammonia gas in THF. This cement was then evaporated in air and finally dried in a vacuum oven to constant weight. 

Butadiene-Styrene Rubber occurs as a synthetic liquid latex or solid rubber produced by the emulsion polymerization of butadiene and styrene, using fatty acid soaps as emulsifiers, and a suitable catalyst, molecular weight regulator (if required), and shortstop. It also occurs as a solid rubber produced by the solution copolymerization of butadiene and styrene in a hexane solution, using butyl lithium as a catalyst. Solvents and volatiles are removed by processing with hot water or by drum drying. 

Polymerizations were carried out in 8-ounce glass bottles with metal caps containing self-sealing butyl rubber gaskets. The capped bottles with their contents were rotated end-oyer-end at 45 rpm at 50°C. in a thermostatted water-bath. Samples for conyersion and for particle size measurements were withdrawn at regular time interyals using hypodermic needle and syringe. Hydroquinone was used as a shortstop. 

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