Cold polymerization

The original SBR process is carried out at. 50° C and is referred to as hot polymerization. It accounts for only about 5% of aU the mbber produced today. The dominant cold polymerization technology today employs more active initiators to effect polymerization at about 5°C. It accounts for about 85% of the products manufactured. Typical emulsion polymerization processes incorporate about 75% butadiene. The initiators are based on persulfate in conjunction with mercaptans (197), or organic hydroperoxide in conjunction with ferrous ion (198). The rest of SBR is produced by anionic solution polymerization. The density of unvulcanized SBR is 0.933 (199). The T ranges from —59" C to —64 C (199). 

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

Mercaptans are also used as chain transfer agents to provide a mechanism for molecular weight control. Commercially these types of polymerization are carried out at 5°C and are referred to as cold polymerizations to differentiate them from the previously discussed hot systems. A typical formula is listed below.30... 

The improved tire wear of cold polymerization SBR led to the very rapid replacement of hot SBR for most applications. This change was relatively easy to make, as all the equipment could be used with the only modification required being the addition of reactor cooling, which is achieved with either the reactor jacket, internal coils, or both. 

The polymerization process parallels the emulsion process used for styrene-butadiene rubber. Either a hot or a cold process can be used, with the cold polymerization providing the same improved processing and vulcanizate properties as seen in SBR. Polymerizations are carried to 70-80 percent conversion and terminated to avoid gel formation. The latex must be stripped to remove unreacted butadiene and acrylonitrile. 

Carruthers and Norrish [13] found that no polymerization occurred if the whole system was held above 100°C, even at 30 torr formic acid pressure. If one portion of the apparatus was cold, polymerization occurred in this portion. If the cold portion was cooled throughout the polymerization, this went to completion if the cooling was insufficient and the surface was exposed to the bombardment of hot gases depolymerization occurred and an equilibrium between polymer and monomer was established. 

In cold polymerization, the most widely used initiator system is the redox reaction between chelated iron and organic peroxide using sodium formaldehyde sulfoxide (SFS) as a reducing agent [see Eqs. (1) and 2]. In hot polymerization, potassium persulfate is used as an initiator. 

During polymerization, parameters such as temperature, flow rate, and agitation speed must be controlled carefully to get the right conversion. Polymerization is normally allowed to proceed to about 60% conversion in cold polymerization and 70% in hot polymerization before it is stopped with a terminal agent that reacts rapidly with the free radicals. Common terminal agents include sodium dimethyldithiocarbamate and diethyl hydroxylamine. 

In addition to the polymer viscosity, polymerization temperature also plays an important role in shaping the processability. Emulsion-polymerized SBR grades produced at low polymerization temperatures have less chain branching than those produced at higher temperatures. At an equivalent viscosity, cold polymerized E-SBR is normally easier to process than hot polymerized E-SBR, and this applies particularly... 

Kreft A, Busche G, Bernhards J, et al. Immunophenotype of hairy-cell leukaemia after cold polymerization of methylmethacrylate embeddings from 50 diagnostic bone marrow biopsies. Histopathology. 1997 30(2) 145-151. 

The molar mass is additionally fixed by these regulators so that mastication is no longer necessary. The cold polymerization is more favorable than the warm polymerization, since more /rnns-rich structures are produced. Ci5-rich polymers, of course, tend more to cyclization, which produces "stringiness,"" that is, an undesirable increase in viscosity, during subsequent processing. Buna S can be mixed directly with natural rubber. It is primarily used for the running surfaces of car tires. 

SBR is emulsion- and solution-polymerized from styrene and butadiene, plus small volumes of emulsifiers, catalysts and initiators, endcapping agents, and other chemicals. It can be sulfur-cured. SBR types are illustrated with Plioflex emulsion SBR (emulsion polymerized) and Solflex t solution SBR. Emulsion SBR is produced by hot polymerization for adhesives and by cold polymerization for tires and other molded automotive and industrial products. Solution SBR is used for tires. 

SBR/PVC blends with nitrile rubber (NBR) as a compatibilizer show improved mechanical properties at lower cost than NBR/PVC. i This was the conclusion of studies using a divinylbenzene cross-linked, hot-polymerized emulsion polymer with 30% bound styrene and a cold-polymerized emulsion polymer with 23% boimd styrene PVC with inherent viscosity from 0.86 to 1.4 NBR with Mooney viscosity from 30 to 86 acrylonitrile content of 23.5, 32.6, and 39.7% and ZnO, stabilizers, sulfur, and accelerators. ... 

Hot-polymerized nitrile rubbers are often preferred. However, this is not always the case e.g., cold-polymerized Hycar VT455 and Hycar VT480 are finding significant application in the adhesive area. 

Cold-polymerized, high-solids latex, often called high solids or cold latex. 

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