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1999 | Buch

Introduction to Rheology Kapitel lesen Erstes Kapitel lesen

Melt Rheology and Its Role in Plastics Processing

Theory and Applications

verfasst von: John M. Dealy, Kurt F. Wissbrun

Verlag: Springer Netherlands

Enthalten in: Professional Book Archive

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Über dieses Buch

This book is designed to fulfill a dual role. On the one hand it provides a description of the rheological behavior of molten poly­ mers. On the other, it presents the role of rheology in melt processing operations. The account of rheology emphasises the underlying principles and presents results, but not detailed deriva­ tions of equations. The processing operations are described qualita­ tively, and wherever possible the role of rheology is discussed quantitatively. Little emphasis is given to non-rheological aspects of processes, for example, the design of machinery. The audience for which the book is intended is also dual in nature. It includes scientists and engineers whose work in the plastics industry requires some knowledge of aspects of rheology. Examples are the polymer synthetic chemist who is concerned with how a change in molecular weight will affect the melt viscosity and the extrusion engineer who needs to know the effects of a change in molecular weight distribution that might result from thermal degra­ dation. The audience also includes post-graduate students in polymer science and engineering who wish to acquire a more extensive background in rheology and perhaps become specialists in this area. Especially for the latter audience, references are given to more detailed accounts of specialized topics, such as constitutive relations and process simulations. Thus, the book could serve as a textbook for a graduate level course in polymer rheology, and it has been used for this purpose.

Inhaltsverzeichnis

Frontmatter
Chapter 1. Introduction to Rheology
Abstract
It is anticipated that many readers will have little previous knowledge about rheology but will wish to find out how it can be useful to them in solving practical problems involving the flow of molten plastics. For this reason, it is our intention to supply sufficient basic information about rheology to enable the reader to understand and make use of the methods described. With this in mind, we begin at the beginning, with a definition of rheology.
John M. Dealy, Kurt F. Wissbrun
Chapter 2. Linear Viscoelasticity
Abstract
The simplest type of viscoelastic behavior is linear viscoelasticity. This type of behavior is observed when the deformation is sufficiently mild that the molecules of a polymeric material are disturbed from their equilibrium configuration and entanglement state to a negligible extent. Obviously, very small deformations would be in this category. This might be a deformation in which the total strain was very small, or the early stages of a larger deformation. For melts, which have a fading memory and can flow, linear behavior is also observed when a deformation occurs very slowly, as in steady simple shear at very low shear rates. This is because relaxation processes due to Brownian motion are always acting to return the molecules to their equilibrium state, and if the deformation is tending to take them away from this state only very slowly, this relaxation mechanism has plenty of time to “keep up” with this process, with the net result that no significant deviation from equilibrium occurs. One manifestation of this is that at very low shear rates, the viscosity of a polymeric liquid becomes independent of shear rate.
John M. Dealy, Kurt F. Wissbrun
Chapter 3. Introduction to Nonlinear Viscoelasticity
Abstract
The measurement of linear viscoelastic properties of polymers is a very useful tool for polymer scientists and plastics engineers. These properties are readily measured, and they can be related to certain aspects of the molecular structure of a polymer. Moreover, the theory of linear viscoelasticity presented in Chapter 2, i.e., the Boltzmann superposition principle, is useful in providing relationships between the data obtained in different types of experiment.
John M. Dealy, Kurt F. Wissbrun
Chapter 4. Steady Simple Shear Flow and the Viscometric Functions
Abstract
Steady simple shear is of central importance in applied rheology for two reasons. First, it is the flow that is by far the easiest to generate in the laboratory. Therefore, the data most often reported are based on this flow. Secondly, a number of processes of industrial importance, particularly extrusion and flow in many types of die, approximate steady simple shear flow. For these reasons it seems appropriate to devote an entire chapter to this subject.
John M. Dealy, Kurt F. Wissbrun
Chapter 5. Transient Shear Flows Used to Study Nonlinear Viscoelasticity
Abstract
The flow behavior of molten polymers can be described by the theory of linear viscoelasticity only when either the total strain or the maximum strain rate is very small. But there are very few, if any, commercial melt processing operations in which the flow satisfies either of these criteria. Thus, while linear viscoelastic behavior has some application in material characterization, it cannot, in general, be correlated with melt performance in processing machinery.
John M. Dealy, Kurt F. Wissbrun
Chapter 6. Extensional Flow Properties and Their Measurement
Abstract
For deformations that are either very small or very slow, the theory of linear viscoelasticity is a unifying concept that provides relationships between the material functions that are determined using various types of deformations. For example, this theory tells us that in start-up flow, the shear stress growth coefficient, η +(t), is independent of shear rate. Furthermore, it provides a simple relationship between this material function and the tensile stress growth function, \(\eta _E^ + \left( t \right)\), that is measured at the start-up of steady simple extension:
$$\eta _E^ + \left( t \right) = 3{\eta ^ + }\left( t \right)$$
(6-1)
Thus, as long as the total strain or the maximum strain rate that occurs during a particular deformation is very small, no new information is obtained from the use of an extensional flow, once the linear viscoelastic behavior has been established by use of a shearing deformation. A corollary of this statement is that the response of a melt to any small or slow extensional flow can be calculated from a material function determined using a small or slow shearing experiment.
John M. Dealy, Kurt F. Wissbrun
Chapter 7. Rotational and Sliding Surface Rheometers
Abstract
There are two basic types of instrument for measuring shear properties: capillary and slit rheometers in which the flow is generated by a pressure drop; and drag flow rheometers in which one bounding wall moves relative to a second, stationary wall. Pressure-driven rheometers are described in Chapter 8, and the present chapter deals with drag flow instruments.
John M. Dealy, Kurt F. Wissbrun
Chapter 8. Flow in Capillaries, Slits and Dies
Abstract
Pressure driven flow through tubes, slits and other types of channels is of central importance in experimental rheology and in polymer processing. Not only is this flow used as the basis for the most popular type of melt rheometer, but it is also a flow that occurs often in melt processing, for example in an extrusion die or in the runner feeding an injection mold. We will derive the basic equations for flow in tubes and slits and show how these can be used to interpret rheometer data and to design flow systems. The irregular flows that can occur at the entrance and exit of a die are described, and methods for estimating the pressure drop in dies are reviewed.
John M. Dealy, Kurt F. Wissbrun
Chapter 9. Rheo-Optics and Molecular Orientation
Abstract
A beam of light may be thought of as an oscillating electric field propagating through space. Matter is composed of electrically charged electrons and nuclei, which are affected by electric fields. Therefore, when a light beam encounters a material, its electric field interacts with the charged components of the matter, and the light beam is altered by this interaction. The type and magnitude of the interaction with the light can be used to probe the state of the matter. Application of optical techniques to deforming systems (“rheo-optics”) allows the measurement of components of the velocity vector and of the stress tensor.
John M. Dealy, Kurt F. Wissbrun
Chapter 10. Effects of Molecular Structure
Abstract
At various places in previous chapters we have discussed molecular theories of polymer melt rheology and the relationship of rheological parameters to molecular structure characteristics such as molecular weight. This chapter presents, for convenience of reference, some useful relationships in summary form. While there is some duplication of material given earlier, it is felt that the reader will find this useful. Numerical values of parameters are presented in the text to illustrate principles, but extensive tables of data are not given. References are given to literature in which data compilations are available.
John M. Dealy, Kurt F. Wissbrun
Chapter 11. Rheology of Multiphase Systems
Abstract
This chapter gives a brief account of how melt rheology is affected by the presence of more than one discrete phase. Polymers filled with rigid reinforcing agents such as fibers or minerals are such systems, as are immiscible blends of polymers. Phase-separated block or graft copolymers can be considered as immiscible blends, with the added constraint that the blend components are joined chemically. Foams are filled polymers in which the filler is a gas.
John M. Dealy, Kurt F. Wissbrun
Chapter 12. Chemorheology of Reacting Systems
Abstract
The term chemorheology was first introduced by Tobolsky et al. [1, 2] to describe their research on the “chemical stress relaxation” of cross-linked rubbers. They found that the stress in a stretched specimen decayed to zero over a long period of time, a behavior incompatible with the concept of a cross-linked structure. They concluded that cross-links had been gradually lost in the strained sample so that the apparent relaxation was actually due to a chemical change and was not a viscoelastic effect. More recently, the term has come to be used to describe the study of rheological changes occurring during the course of any chemical reaction.
John M. Dealy, Kurt F. Wissbrun
Chapter 13. Rheology of Thermotropic Liquid Crystal Polymers
Abstract
Liquid crystallinity is a state of matter that is intermediate between the crystalline solid state and the liquid state. Hence, it is sometimes called a “mesomorphic” state or “mesophase, ” and the molecular structural elements responsible for its formation are called “mesogens.” Liquid crystals do not have the three dimensional order of position of the molecules of solid crystals and are therefore capable of flowing like fluids. Unlike ordinary fluids, however, they do possess some order over macroscopic distances. This order may be solely that of orientation of the molecules, or it may also involve some positional order. There are many combinations of types of order, resulting in a wide variety of types of liquid crystal, which are described in a number of texts [1–3].
John M. Dealy, Kurt F. Wissbrun
Chapter 14. Role of Rheology in Extrusion
Abstract
Extrusion is the most important single polymer processing operation. Virtually every pound of thermoplastic polymer is subjected to an extrusion process at some point in its conversion to a finished article. It is more amenable to theoretical analysis than some other processing operations for a number of reasons:
1.
It is a continuous, steady state process, not discontinuous like injection molding
 
2.
For the most common mode of operation there are no free surfaces within the extruder, so that boundary conditions can be prescribed on known surfaces, and
 
3.
Viscoelastic behavior plays only a minor role, and viscous fluid models have been found adequate for the analysis.
 
John M. Dealy, Kurt F. Wissbrun
Chapter 15. Role of Rheology in Injection Molding
Abstract
In the injection molding process the objective is to produce a product that is free of voids and sink marks, is not subject to warpage, and has sufficient strength and stiffness for its end use. This requires that the melt flow freely into the mold cavity, and that the final part be reasonably free of residual stresses. At the same time, the product must be produced at minimum cost, and this implies the shortest possible cycle time. The challenge, then, is to produce a good quality part at a minimum cost, and melt rheology plays a central role in meeting this challenge.
John M. Dealy, Kurt F. Wissbrun
Chapter 16. Role of Rheology in Blow Molding
Abstract
The two principal types of blow molding process are extrusion blow molding and injection blow molding. In the latter process a “preform,” often similar to a test tube with a threaded end, is injection molded and subsequently reheated and inflated inside a mold. This process affords excellent control of the thickness distribution in the preform and is used to make small containers with high quality finishes. It also lends itself well to the stretch blow molding process used to make carbonated beverage bottles.
John M. Dealy, Kurt F. Wissbrun
Chapter 17. Role of Rheology in Film Blowing and Sheet Extrusion
Abstract
The essential elements of the film blowing process are illustrated in Figure 17-1. An extruder melts the resin and forces it through a screen pack and an annular die. The extruded melt, in the form of a tube, flows upward under the influence of a vertical, “machine direction” force, applied by means of nip rolls some distance above the die. There is an overall stretching of the polymer in the machine direction, and the ratio of the linear speed of the film through the nip rolls, divided by the average melt velocity at the die lips, is called the “draw down ratio” (DDR).
John M. Dealy, Kurt F. Wissbrun
Chapter 18. On-Line Measurement of Rheological Properties
Abstract
There are three types of application of on-line rheometers: process monitoring, quality control, and automatic process control. The first type arises primarily in the development of a new material or process, when it is useful to be able to monitor the effects of changes in formulation or operating parameters on product characteristics. In a quality control application, a slow and labor-intensive laboratory test procedure is replaced by a direct indication of product quality. The most sophisticated type of application is in automatic process control, where the rheometer is used as a sensor providing an input signal to the controller. A general review of process rheometers and their applications has been published [1].
John M. Dealy, Kurt F. Wissbrun
Chapter 19. Industrial Use of Rheometers
Abstract
Even with unlimited resources of equipment and personnel it would be necessary to make choices as to which of the infinite number of conceivable rheological properties to measure and which instruments to use to measure them. In the real world limitations of resources, and especially of time, make these choices even more necessary.
John M. Dealy, Kurt F. Wissbrun
Backmatter
Metadaten
Titel
Melt Rheology and Its Role in Plastics Processing
verfasst von
John M. Dealy
Kurt F. Wissbrun
Copyright-Jahr
1999
Verlag
Springer Netherlands
Electronic ISBN
978-94-009-2163-4
Print ISBN
978-0-7923-5886-2
DOI
https://doi.org/10.1007/978-94-009-2163-4