Mass process

In this process uncrosslinked rubber is dissolved in a mixture of the monomers and solvent(s). This solution is pumped into the first reactor which is connected to a series of reactors. The polymerization is started by increasing the temperature, eventually in the presence of an initiator. Most of the rubber grafting and particle sizing happen early in the process. Chain transfer agent level, initiator (type/amount) and shear have a great influence in this stage. Crosslinking of the rubber particles occurs later in the process. The final step is the removal of residual monomer and solvent. 

The advantages of the mass process are the absence of contaminants (emulsifiers) and a lower waste generation (no white water). Possible disadvantages are related to heat removal, viscosity and limitations in rubber particle sizing. 

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M ass Process. In the mass (or bulk) (83) ABS process the polymerization is conducted in a monomer medium rather than in water. This process usually consists of a series of two or more continuous reactors. The mbber used in this process is most commonly a solution-polymerized linear polybutadiene (or copolymer containing sytrene), although some mass processes utilize emulsion-polymerized ABS with a high mbber content for the mbber component (84). If a linear mbber is used, a solution of the mbber in the monomers is prepared for feeding to the reactor system. If emulsion ABS is used as the source of mbber, a dispersion of the ABS in the monomers is usually prepared after the water has been removed from the ABS latex. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Fig. 2. Transmission electron micrograph of ABS produced by a mass process. The mbber domains are typically larger in size and contain higher...

Classification of Processes and Reactors. Most styrene polymers are produced by batch suspension or continuous mass processes. Some are produced by batch mass processes. Mass in this sense includes bulk polymerization of the polymer... 

Figure 1. Batch-mass process flow diagram (1)...

Access to Practice. Publications and patents on the batch mass process are limited. Bishop s book CD contains the most detailed description of the polymerization press and mass-suspension processes for PS and HIPS. Fong (16) presents an economic analysis of the press process based on Bishop s description. Patent references are few for the batch-mass process the 1939 Bakelite patent on transfer of prepoly syrup to chambers or containers is of historical interest (17). 

The advantage of suspension processes over mass processes is the excellent temperature control that can be obtained through the suspending medium, water. This allows for rapid heat removal and shorter polymerization times. It reduces or eliminates hot spots or heat-kicks characteristic of mass reactors. It also allows the polymerization to be driven very close to completion so that no devolatilization step is normally required. 

General Considerations. Continuous mass processes for polystyrene have been in commercial use since the 1930 s, and for rubber-modified polystyrene (HIPS) since the 1950 s. Much of the information on equipment design, process configuration and operating parameters connected with continuous processes is found in the patent literature. There are inherent limitations to such sources. Recognizing this, we will provide... 

Figure 15, Union Carbide patented continuous mass process for polystyrene (2A)...

Figure 16, Schematic of a Dow Chemical Co, patented continuous mass process...

The continuous mass process is divided into 4 steps rubber solution in styrene monomer, polymerization, devolatilization and compounding. In 1970 N. Platzer (40) drew up a survey of the state of the art. Polymerization is divided into prepolymerization and main polymerization for both steps reactor designs other than the tower reactors shown in Figure 2 have been proposed. Main polymerization is taken to a conversion of 75 to 85% residual monomer and any solvent are separated under vacuum. The copolymer then passes to granulating equipment, frequently through one or more intermediate extruders in which colorant and other auxiliaries are added. 

The second large-scale process was the batch mass suspension process. Monsanto did the pioneer work on this (41). In this process, prepolymerization is carried out in bulk and main polymerization in suspension the latter is taken to conversions of over 99%. In contrast to the continuous mass process, peroxide starters are used in order to achieve a high conversion at tolerable reaction times. Figure 3 shows a basic flow diagram of such a plant. A detailed discussion of advantages and disadvantages of the two processes can be found in R. Bishop s monograph published in 1971 (42), and it is continued in a paper by Simon and Chappelear in 1979 (43). It was a decisive factor for the economic success of impact polystyrene that these processes had been completely developed and mastered in theory and practice. 

Figure 2. Continuous mass process (40) for crystal and high impact polystyrene.

Continuous mass process for impact-resistant polystyrene from styrene and SBR (Dow) goes onstream. 

Depending on the formulation and on the properties of the components in the powder it may be necessary to mass the moistened powder for some time before screening. However, it has to be checked as to whether the massing process can be avoided if the granulating liquid is pumped into the powder bed at a reasonable rate. 

Fielden KE, Newton JM, Rowe RC. A comparison of the extrusion and spheronization behavior of wet powder masses processed by a ram extruder and a cylinder extruder. Int J Pharm 1992 81 225-233. 

Sample Size - Because of their mass processing nature, comnercial food systems machinery tend to utilize large sample sizes. When novel products are tested, the experimental samples are sometimes too small to be used in such a manner. Model Systems employing smaller batch sizes provide the answers. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

The application of Eqs. (2.4) and (2.5) is restricted to nonflow (constant mass) processes in which only internal-energy changes occur. Far more important industrially are processes which involve the steady-state flow of a fluid through equipment. For such processes the more general first-law expression [Eq. (2.3)] must be used. However, it may be put in more convenient form. The term steady state implies that conditions at all points in the apparatus are constant with time. For this to be the case, all rates must be constant, and there must be no accumulation of material or energy within the apparatus over the period of time considered. Moreover, the total mass flow rate must be the same at all points along the path of flow of the fluid. 

Standardization of formats for capturing physical data would dramatically accelerate progress in the natural sciences, not only because of new discoveries stemming from mass processing of available data, but also because of the time saved by highly qualified people, which could be more fruitfully invested elsewhere. 

Although the emulsion process is commercially the most important, the mass process cannot be neglected because it has a number of advantages that will become clear from the more detailed description of both processes. 

No. Product Annual production (10 kg) Average percent annual change Usual organic feedstock Energy consumed per product mass Process Feed (MJ/kg) (MJ/kg) Average market Annual Price value ( /kg) ( 10 ) ... 

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