高分子材料与工程专业英语课文.doc

UNIT 12 Bulk PolymerizationBulk polymerization traditionally has been defined as the formation of polymer from pure, undiluted monomers. Incidental amounts of solvents and small amounts of catalysts, promoters, and chain-transfer agents may also be present according to the classical definition. This definition, however, serves little practical purpose. It includes a wide variety of polymers and polymerization schemes that have little in common, particularly from the viewpoint of reactor design. The modern gas-phase process for polyethylene satisfies the classical definition, yet is a far cry from the methyl methacrylate and styrene polymerization which remain single-phase throughout the polymerization and are more typically thought of as being bulk. A common feature of most bulk polymerization and other processes not traditionally classified as such is the need to process fluids of very high viscosity. The high viscosity results from the presence of, dissolved polymer in a continuous liquid phase. Significant concentrations of a high molecular-weight polymer typically increase fluid viscosities by 104 or more compared to the unreacted monomers. This suggests classifying a polymerization as bulk whenever a substantial concentration of polymer occurs in the continuous phase. Although this definition encompasses a wide variety of polymerization mechanisms, it leads to unifying concepts in reactor design. The design engineer must confront the polymer in its most intractable form, i. e. , as a high viscosity solution or polymer melt.The revised definition makes no sharp distinction between bulk and solution polymerizations and thus reflects industrial practice. Several so-called bulk processes for polystyrene and ABS use 5%15% solvent as a processing aid and chain-transfer agent. Few successful processes have used the very large amounts of solvent needed to avoid high viscosities in the continuous phase, although this approach is sometimes used for laboratory preparations.Bulk polymerizations often exhibit a second, discontinuous phase. They frequently exhibit high exothermicity, but this is more characteristic of the reaction mechanism than of bulk polymerization as such. Bulk polymerizations of the free-radical variety are most common, although several commercially important condensation processes satisfy the revised definition of a bulk polymerization.In all bulk polymerizations, highly viscous polymer solutions and melts are handled. This fact tends to govern the process design and to a lesser extent, the process economics. Suitably robust equipment has been developed for the various processing steps, including stirred-tank and tubular reactors, flash devolatilizers, extruder reactors, and extruder devolatilizers. Equipment costs are high based on working volume, but the volumetric efficiency of bulk polymerizations is also high. If a polymer can be made in bulk, manufacturing economics will most likely favor this approach. It is tempting to suggest that polymer processes will gradually evolve toward bulk. Recently, the suspension process for impact polystyrene has been supplanted by the bulk process, and the emulsion process for ABS may similarly be replaced. However, the modern gas-phase process for polyethylene appears to represent an opposite trend. It seems that polymerization technology tends to eliminate solvents and suspending fluids other than the monomers themselves. When the monomer is a solvent for the polymer, bulk processes as described in this article are chosen. When the monomer is not a solvent, suspension and slurry processes like those for polyethylene and polypropylene are employed. Hence, it is worthwhile avoiding a highly viscous continuous phase, but not at the price of introducing extraneous material. Reading MaterialsPolymerization ViscostitiesViscosity in itself is a nebulous term when describing the polymer or polymer solution in most polymerizations. Some polymerizations are carried out in water with small beads being formed and suspended in the water. The viscosity of such a system could actually mean the viscos.ty of the water, the viscosity of the slurry present with the beads in the water, the impeller viscosity, the process viscosity, the bulk viscosity, or the viscosity at the heat transfer surface.In the bulk polymerization method, knowledge of viscosity is of vital importance. Bulk polymerizations typically operate between 100 000 and 500 000 cP (100 and 500Pa s) bulk viscosity. An accurate determination of the bulk viscosity is extremely important in addition to the rheology associated with the particular polymer. Because bulk polymerizations are generally high viscosity in nature, the corresponding mixing Reynolds number is very low, normally less than 100. This is in the laminar region. Power is proportional to N2D3u in the laminar range; so the actual horsepower which the mixer will draw is proportional to viscosity. Because of this, it is a requirement that viscosity vs. shear rate data be known. For example, assume two separate companies manufacturing bulk polystyrene have presented viscosity data to mixer vendors. Both companies have stated that the bulk viscosity of this material is 300 000 cP (300Pa s). Company A furnished only this information to the mixer suppliers. Company B furnished the bulk viscosity information in addition to the viscosity vs. shear rate data. Because the mixer manufacturer could determine the proper viscosity to load the impeller from the information that customer B furnished, the mixer recommended was a 25-hp design (18. 5 kW). Company A received a quote for a 50-hp mixer (37kW). Both mixers were for tanks of the same size and shape and operating at the same speed. Naturally the quotation for customer A will be at a higher price and a higher operating cost than for customer B. However, both mixers will accomplish the required results.About 85% of all high-viscosity materials are pseudoplastic and viscoelastic in nature. Bulk polystyrene, polyesters, and polyelectrolytes are pseudoplastic in nature. Most materials have a slope of 0. 2 to 0. 6 when viscosity is plotted vs. shear rate. By reviewing these data and comparing the viscosity-vs. -shear rate information with the known shear rate constant of close-clearance impellers, the impeller viscosity can be determined. The shear rate constant for anchors and helical impellers is 30.As indicated earlier, helical impellers and anchors are typically used in bulk polymerizations. However, neither of these two devices can operate effectively without viscous drag at the wall of the vessel. Without some drag the material in the tank will turn as one entire mass, and almost no mixing will occur. Therefore, in bulk polymerizations it is important to be sure that inlet pipes of low-viscosity material and reflux lines are directed toward a point at the liquid level one-half of the distance from the mixer shaft to the tank wall. This will allow incorporation of the low-viscosity material and prevent its migration to the tank walls where it could act as a lubricating layer, thereby reducing the agitation. It is also important to optimize the temperature differential AT between the bulk fluid and the heat transfer surface. Normally bulk viscosity applications only require tank jackets to obtain temperature control. A very high jacket temperature could reduce the viscosity of the material at the tank wall to a point where it acts as a lubricating boundary layer. Too cold a temperature at the tank wall could increase the viscosity dramatically to a point where the mixer is not designed to handle it. In this case a totally stagnant boundary layer at the wall could occur and product quality could be affected. Further more, damage could result to the mixer, drive motor, and vessel.Viscosities for solution polymerizations are normally 25 000 and 500 000 cP (25 and 500 Pa s) bulk viscosity. The same problems exist with the term viscosity in this type of polymerization as in bulk polymerizations. An exact knowledge of the bulk viscosity and viscosity at the impeller are important. In lower viscosity materials, where open impellers are used , the importance of the viscosity determination is slightly reduced because the mixing Reynolds numbers are normally in the transition region where horsepower is not proportional to viscosity. Therefore, a minor change in the viscosity will have little effect on the horsepower drawn by the mixer.Most solution polymerizations use tank jackets as heat transfer media; however, some solution polymerizations require additional surface area. Again, the AT optimization is important in solution polymerizations.UNIT 14 Styrene-Butadiene CopolymerThe synthetic rubber industry, based on the free-radical emulsion process, was created almost overnight during World War I . Styrene-butadiene (GR-S) rubber created at that time gives such good tire treads that natural rubber has never regained this market. The GR-S Standard recipe isThis mixture is heated with stirring and at 50C gives conversions of 5% 6% per hour. Polymerization is terminated at 70%75% conversion by addition of a short-stop, such as hydroquinone (approximately 0. 1 part), to quench radicals and prevent excessive branching and microgel formation. Unreacted butadiene is removed by flash distillation, and styrene by steam-stripping in a column. After addition of an antioxidant, such as iV-phenyl-naphthylamine (PBNA) (1.25 parts), the latex is coagulated by the addition of brine, followed by dilute sulfuric acid or aluminum sulfate. The coagulated crumb is washed, dried, and baled for shipment.This procedure is still the basis for emulsion polymerization today. An important improvement is continuous processing illustrated in Fig. 14.1; computer modeling has also been described.In the continuous process, styrene, butadiene, soap, initiator, and activator (an auxiliary initiating agent) are pumped continuously from storage tanks through a series of agitated reactors at such a rate that the desired degree of conversion is reached at the last reactor. Shortstop is added, the latex warmed with steam, and the unreacted butadiene flashed off. Excess styrene is steam-stripped, and the latex finished as shown in Fig. 14.1.SBR prepared from the original GR-S recipe is often called hot rubber, cold rubber is made at 5C by using a more active initiator system. Typical recipes are given in Table 14.1. At 5*C , 60% conversion to polymer occurs in 1215h.Cold SBR tire treads are superior to those of hot SBR. Polymers with abnormally high molecular weight (and consequently too tough to process by ordinary factory equipment) can be processed after the addition of up to 50 parts of petroleum-base oils per hundred parts of rubber (phr) . These oil extenders make the rubbers more processible at lower cost and with little sacrifice in properties; they are usually emulsified and blended with the latex before coagulation. Recent trends have been toward products designed for specific uses. The color of SBR, which is important in many nontire uses, has been improved by the use of lighter-colored soaps, shortstops, antioxidants, and extending oils. For example, dithiocarbamates are substituted for hydroquinone as shortstop ; the latter is used on hot SBR where dark color is not objectionable. A shortstop such as sodium dimethyldithiocarbamate is more effective in terminating radicals and destroying peroxides at the lower temperatures employed for the cold rubbers.Free-radical dissociative initiators that function by dissociation of a molecule or ion into two radical species are normally limited to inorganic persulfates in the case of butadiene polymerization. The other important class of free-radical initiators, redox systems, contain two or more components that react to produce free radicals. Dodecyl mercaptan added to control molecular weight also appears to aid free-radical formation by reaction with persulfate. The commercial importance of such chain-transfer agents or modifiers cannot be overemphasized. Without molecular weight control the rubbers would be too tough to process.Reading MaterialsSteady-State Multilicity in Continuous Emulsion PolymerizationThe phenomenon of multiple steady states is seen in emulsion polymerization. Fig. 14. 2 is a plot of steady-state monomer conversion as a function of reactor residence time for methyl methacrylate emulsion polymerization in a CSTR. A region of multiplicity is indicated by the fact that the upper and lower branches of the curve overlap between residence times of 30 and 50 minutes. The dotted line is an estimate of the shape of the unstable middle branch which is experimentally unobservable. The dashed lines indicate experimental instances of ignition and extinction. At 50 minutes residence time the system has been observed to move from the lower steady state of 54% conversion to the upper steady state at approximately 80% with no discernible change in operating conditions (ignition). Extinction has been observed when the residence time is changed from 30 minutes to 20 minutes on the upper branch, resulting in a drop in conversion from the upper to the lower steady-state values. The phenomenon of multiple steady states arises in emulsion polymerization for much the same reason as it appears in solution polymerization: the autocatalytic nature of the polymerization (due to the gel effect), combined with the mass balance, results in the possibility of steady-state multiplicity.Steady-state multiplicity can be an operational problem for a number of reasons. If one wishes to operate at an intermediate level of monomer conversion (perhaps to minimize viscosity or prevent excessive chain branching), one may be forced to operate in the unstable region, relying on closed-loop control to stabilize the operating point. This is tricky at best. Additionally, the steady state (upper or lower) to which the system goes on start-up will depend on how the start-up is effected. A careful start-up policy may be needed to assure that the system arrives at the desired steady state. In general, a conservative start-up, with the temperature and initiator concentration brought to steady-state values slowly will result in operation on the lower branch, while aggressive start-up (high temperature and/or high initiator concentration during start-up) will result in steady-state operation on the upper branch. Finally, large upsets in the process may cause ignition or extinction. This may lead to loss of temperature control in the case of ignition, or loss of reactor productivity in the case of extinction. A system designed to operate at the upper steady state will be operating way below design product yield at the lower steady state. Additionally, the product quality (MWD, CCD, etc. ) will be different for the two operating points. The polymerisation reactor designer should be aware of the potential for multiplicity, and, if possible, design the system to operate outside this region.CSTR polymerization reactors can also be subject to oscillatory behavior. A nonisothermal CSTR free radical solution polymerisation can exhibit damped oscillatory approach to a steady state, unstable (growing) oscillations upon disturbance, and stable (limit cycle) oscillations in which the system never reaches steady state,and never goes unstable, but continues to oscillate with a fixed period and amplitude. However, these Phenomena are more commonly observed in emulsion polymemation.High-volume products such as styrene-butadiene rubber (SBR) often are produced by continuous emulsion polymerization. This is most often done in a train of 515 CSTRs in series. Sustained oscillations (limit cycles) in conversion, particle number, and free emulsifier concentration gave been reported, under isothermal conditions in continuous emulsion polymerization systems. This limit cycle behavior leaves its mark on the product in the form of disturbances in the molecular weight distribution and particle size distribution which cannot be blended away. Fig. 14. 3 shows evidence of a sustained oscillation (limit cycle) during emulsion polymerization of methyl methacrylate in a single CSTR. Com parison of the monomer conversion and surface tension data graphically illustrates the mechanism of oscillation. It will be noted that the surface tension oscillates with the same period as the conversion (67 residence times). This can be explained with the classical micellar initiation mechanism (or with homogeneous nucleation) . Beginning at a time of about 300 minutes, the conversion rises rapidly as new particles form and old particles grow. As the particle surface area increases , additional surfactant is adsorbed on the particles. Meanwhile micelles dissociate to keep the aqueous phase saturated. Once all of the micelles have dissociated, it is no longer possible to maintain the aqueous phase at saturation, and the surface tension begins to rise. This is observed at about 320 minutes. At the point at which micelles are no longer present, micellar initiation stops and the rate of polymerization slows. Eventually, since particles are washing out while no new particles are being formed, the conversion begins to fall. Since the total particle surface area is decreasing at this point, and since surfactant is continually being introduced with the feed, the surface tension falls as the aqueous phase reapproaches saturation. As the aqueous phase becomes saturted initiation begins again. Saturation of aqueous phase may be observed by noting the point at which the surface tension reaches its CMC value. As new micelles are formed they adsorb free radicals, become polym