Processing, polymer

A variety of polymers are extrusion-foamed, including polyolefins, polystyrene, and polyesters if the polymer is subjected to high-pressure gas, and the pressure is suddenly decreased or the temperature rapidly increased, the gas will try to escape from the polymer, causing antiplasticization. This rapid escape of gas can cause the nucleation and growth of bubbles within the polymer. Once a significant amount of gas escapes, the of the polymer drops, and thus the structure formed is frozen . 

During polymer processing non-isothermal crystallization conditions, mechanical deformation, and shear forces may alter the morphology and orientation of polymers both at the surface and in the bulk. In addition, orientation effects of semicrystalline polymers that crystallize in contact with solids are considered. 

One example is the toughening of HDPE by rubber or calcite particles. To address this issue, the morphology and orientation of polyethylene in thin films 

A preferentially and a sheaf-like aggregation with random in-plane orientation are observed for the thinner films (thicknesses of 0.1, 0.2 and 0.4 pm in panels a-c). By contrast thick films (0.6 pm and thicker, panel d) show a morphology that resembles the well known (bulk) spherulitic form with a banded structure, characteristic of linear polyethylene crystallized from the melt at moderately high undercooling. 

Opdahl and Somorjai studied the surface deformation and surface elastic properties of stretched polyethylene in a device depicted in Fig. 3.73. It was found that the surface textures of both HDPE and LDPE changed and that the nodular structures present at the surface lengthened in the direction of the stretch and contracted perpendicular to the stretch at various elongations. This resulted in a roughening of the surface. 

In this chapter we have examined the reactions and reactors used in polymerizing monomers. In many situations the production of monomers from feedstocks, the polymerization, and the forming of the polymer into products are done in the same chemical plant. In some cases the monomer is too reactive to be shipped to a polymerization plant (isocyanates, for example), and in some cases the producer does not want to be dependent on monomer suppliers. 

There is a compelling reason to integrate PMMA and phenol-formaldehyde because the monomers phenol and acetone are both made from cumene oxidation (previous chapter). Therefore, one makes one mole of phenol for every mole of acetone, and a producer would have to sell one of these monomers if he did not have an integrated process to produce both polymers or some other products. 

Crystallization is a polymerization process that has comparable kinetics to the reactions making conventional polymers but whose products are quite different In crystallization one allows supersaturated monomer to nucleate and grow into crystals. We usually do not regard crystallization as a chemical reaction at aU because the process is 

Crystallization processes are very important in chemical processes whenever there are solid products in a reactor. We saw in Chapter 9 that crystallization and dissolution particle sizes could be handled with the same equations as chemical vapor deposition and reactive etching. We note here that crystallization reactions can be handled with the same equations as polymerization. 

Basically, crystallization occurs either by monomer addition to a growing crystal or by coagulation of smaller crystals unto larger crystals. Monomer addition produces more uniform and regular crystals and a narrower crystal size distribution. Coagulation produces irregularly shaped crystals with a wide range in crystal sizes. These processes are obviously addition crystallization and condensation crystallization, respectively. We will not consider these kinetics in any more detail here, but save them for a homework problem. 

In this chapter, we discuss just three of the most common polymer processing operations-extrusion, injection molding, and fiber spinning. Because 

Similarly, virtually all polymers in commercial use contain additives. Their purpose is two-fold to alter the polymer properties and to enhance ease of processing. Plasticizers are used to modify mechanical properties. Modifiers can be lubricants, cross-linking agents, emulsifiers, and thickening agents, just to name a few. Pigments and odorants are common additives used for aesthetic reasons. 

It is not feasible here to go in any detail into the history of processing methods let it suffice to point out that that history goes back to the Victorian beginnings of polymer technology. Thus, as Mossman and Morris (1993) report, the introduction of camphor into the manufacture of parkesine in 1865 was asserted to make it possible to manufacture more uniform sheets than before. Processing has always been an intimate part of the gradual development of modern polymers. 

rotating drum with inserted pins E, covered opening G, handle. The rubber is sheared between the pins 

PVC behaves quite differently during processing. The initial step is the same, the formation of macroalkyl radicals, but the latter can undergo a rapid unzipping of hydrogen chloride from the initial 

Hydroperoxides formed during processing of polymers have a profoundly deleterious effect on the long-term performance of products made from them. This is a consequence of the very great sensitivity of hydroperoxides to light which leads to the rapid deterioration of the polyolefins, PVC and rubber-modified polymers in the outdoor environment. Their formation is of crucial importance to the weathering of industrial polymers since these highly reactive free radicals produced are initiators for the photooxidation of polymers. 

1990-1997. (Data by courtesy of The International Rubber Study Group, London.) 

Further information about methods for producing oriented polymers is given in chapter 10. Further information on all the topics dealt with here can be obtained from the books cited as (7) in sections 1.6.1 and 1.6.2. 

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In this chapter the general equations of laminar, non-Newtonian, non-isothermal, incompressible flow, commonly used to model polymer processing operations, are presented. Throughout this chapter, for the simplicity of presentation, vector notations are used and all of the equations are given in a fixed (stationary or Eulerian) coordinate system. 

Middleman, S., 1977. Fundamentals of Polymer Processing, McGraw-Hill, New York. 

Pittman, J. F.T. and Nakazawa, S., 1984. Finite element analysis of polymer processing operations. In Pittman, J.F. T., Zienkiewicz, O.C., Wood, R.D. and Alexander, J. M. (eds), Num,erical Analysis of Forming Processes, Wiley, Chichester. 

Tadmor, Z. and Gogos, C. G., 1979. Principles of Polymer Processing, Wiley, New York. 

Two-dimensional models can be used to provide effective approximations in the modelling of polymer processes if the flow field variations in the remaining (third) direction are small. In particular, in axisymraetric domains it may be possible to ignore the circumferential variations of the field unlaiowns and analytically integrate the flow equations in that direction to reduce the numerical model to a two-dimensional form. 

Pittman, J.F.T. and Nakazawa, S., 1984. Finite element analysis of polymer processing... 

The thermal conductivity of polymeric fluids is very low and hence the main heat transport mechanism in polymer processing flows is convection (i.e. corresponds to very high Peclet numbers the Peclet number is defined as pcUUk which represents the ratio of convective to conductive energy transport). As emphasized before, numerical simulation of convection-dominated transport phenomena by the standard Galerkin method in a fixed (i.e. Eulerian) framework gives unstable and oscillatory results and cannot be used. 

IMPOSITION OF BOUNDARY CONDITIONS IN POLYMER PROCESSING MODELS... 

Pittman, J. F. T., 1989. Finite elements for field problems. In Tucker, C. L. Ill (ed.), Computer Modeling for Polymer Processing, Chapter 6, Hanser Publishers, Munich, pp. 237- 331. 

Nassehi, V. and Ghoreishy, M, H. R., 1998. Finite element analysis of mixing in partially filled twin blade internal mixers. Int. Polym. Process. XIII, 231 -238. 

Pearson, J.R. A., 1985. Mechanics of Polymer Processing, Applied Science Publishers, Barkings, Essex, UK. 

Families of finite elements and their corresponding shape functions, schemes for derivation of the elemental stiffness equations (i.e. the working equations) and updating of non-linear physical parameters in polymer processing flow simulations have been discussed in previous chapters. However, except for a brief explanation in the worked examples in Chapter 2, any detailed discussion of the numerical solution of the global set of algebraic equations has, so far, been avoided. We now turn our attention to this important topic. 

As mentioned in Chapter 2, the numerical solution of the systems of algebraic equations is based on the general categories of direct or iterative procedures. In the finite element modelling of polymer processing problems the most frequently used methods are the direet methods. 

Iterative solution methods are more effective for problems arising in solid mechanics and are not a common feature of the finite element modelling of polymer processes. However, under certain conditions they may provide better computer economy than direct methods. In particular, these methods have an inherent compatibility with algorithms used for parallel processing and hence are potentially more suitable for three-dimensional flow modelling. In this chapter we focus on the direct methods commonly used in flow simulation models. 

PRACTICAL ASPECTS OF FINITE ELEMENT MODELLING OF POLYMER PROCESSING... 

The utilization of commercially available finite element packages in the simulation of routine operations in industrial polymer processing is well established. However, these packages cannot be usually used as general research tools. Thus flexible in-house -created programs are needed to carry out the analysis required in the investigation, design and development of novel equipment and operations. 

Figure 2.5 Shearing force per unit area versus shear rate. The experimental points are measured for polyethylene, and the labeled lines are drawn according to the relationship indicated. (Data from J. M. McKelvey, Polymer Processing, Wiley, New York, 1962.)...

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