Monitoring physical and chemical changes during reactive processing

Reactive processing involves the production of a novel polymer as a result of chemical reactions that occur during the processing operation. These changes may be deliberate, as in functionalization, or inadvertent, as in chain scission due to undesired thermo- or mechano-chemistry. Section 1.4 gave the chemical changes to be expected when a thermoplastic polymer is subjected to elevated temperature for a period of minutes in the presence or absence of an oxidative atmosphere (Scott, 1993, Zweifel, 1998). For example, there will be the appearance of higher oxidation states of carbon such as in ether, alcohol, ketone and acid groups. These often accompany chain scission, which occurs during the free-radical-initiated oxidation chain reaction. In addition, there may be other chemical changes, such as grafting and functionalization, in which new chemical species are added to the polymer backbone in the course of the processing operation. The initiation of the oxidation, grafting or functionalization reactions will be through added initiator such as an organic peroxide and the novel chemistry will involve a monomer or other grafting agent, neither of which will necessarily be totally consumed during the reaction sequence. The measurement of the concentration profiles of these reagents as a function of time and, if possible, position in the reaction zone is crucial to the development of an understanding of the link between the chemical and rheological changes of the novel polymer system during processing.

Plastics are the most diverse materials in use in our society and the way that they are processed controls their structure and properties. The increasing reliance on plastics for high-value and high-performance applications necessitates the investment in new ways of manufacturing polymers. One way of achieving this is through reactive processing. However, the dynamics of reactive processes places new demands on characterization, monitoring the systems and controlling the complete manufacturing process.

This book provides an in-depth examination of reactive polymers and processing, firstly by examining the necessary fundamentals of polymer chemistry and physics. Polymer characterization tools related to reactive polymer systems are then presented in detail with emphasis on techniques that can be adapted to real-time process monitoring. The core of the book then focuses on understanding and modelling of the flow behaviour of reactive polymers (chemorheology). Chemorheology is complex because it involves the changing chemistry, rheology and physical properties of reactive polymers and the complex interplay among these properties. The final chapter then examines a range of industrial reactive polymer processes, and gives an insight into current chemorheological models and tools used to describe and control each process.

This book differs from many other texts on reactive polymers due to its

The book has been aimed at chemists, chemical engineers and polymer process engineers at the advanced-undergraduate, post-graduate coursework and research levels as well as industrial practitioners wishing to move into reactive polymer systems.

Introduction

This chapter highlights the importance of chemorheology in determining cure properties of reactive systems. A brief introduction to experimental rheology has been provided in Chapter 3 to provide a baseline knowledge of experimental rheology. In this chapter we examine a description of chemorheology in terms of basic chemorheology, chemoviscosity, gelation and vitrification transitions and ultimate properties. Finally, examples of chemorheological analysis will be discussed. (We will briefly summarize chemorheological data and models in this chapter, but only for reference to chemorheological testing. A more extensive examination of chemorheology and modelling of systems will be presented in Chapter 5.)

Chemorheology

The definition of chemorheology (in this text) is the study of the deformation properties of reactive polymer systems. Figure 4.1 shows a schematic representation of the structural development during thermoset cure.

Step (a) shows unreacted monomers, and cure proceeds to step (b), at which there is the formation of some branched molecules. By step (c) the cure has progressed to the gel point, such that an infinite network is formed across the whole structure. Further cure can occur to point (d), at which the material becomes fully cured and vitrification is reached.

The essential elements of a chemorheological study are

• fundamental chemorheology

• chemoviscosity profiles

• gelation

• vitrification

• ultimate chemorheological properties

• modelling

We shall focus on modelling in Chapter 5.

The purpose of this chapter is to provide the background principles from polymer physics and chemistry which are essential to understanding the role which chemorheology plays in guiding the design and production of novel thermoplastic polymers as well as the complex changes which occur during processing. The focus is on high-molar-mass synthetic polymers and their modification through chemical reaction and blending, as well as degradation reactions. While some consideration is given to the chemistry of multifunctional systems, Chapter 2 focuses on the physical changes and time—temperature-transformation properties of network polymers and thermosets that are formed by reactions during processing.

The attention paid to the polymer solid state is minimized in favour of the melt and in this chapter the static properties of the polymer are considered, i.e. properties in the absence of an external stress as is required for a consideration of the rheological properties. This is addressed in detail in Chapter 3. The treatment of the melt as the basic system for processing introduces a simplification both in the physics and in the chemistry of the system. In the treatment of melts, the polymer chain experiences a mean field of other nearby chains. This is not the situation in dilute or semi-dilute solutions, where density fluctuations in expanded chains must be addressed. In a similar way the chemical reactions which occur on processing in the melt may be treated through a set of homogeneous reactions, unlike the highly heterogeneous and diffusion-controlled chemical reactions in the solid state.

Introduction

Chapters 3 and 4 presented chemical, physical and chemorheological techniques useful for characterizing various reactive polymer systems. This chapter will now focus on a review of chemorheological analyses for a variety of polymer systems, including detailed experimental findings and chemorheological modeling.

Chemoviscosity and chemorheological models

Chemorheology is defined as the study of the viscoelastic behaviour of reacting polymer systems. This involves examining the effects on chemoviscosity of chemical reactions (cure conversion, cure kinetics) and processing conditions (temperatures, shear rates), as well as gelation and vitrification. In Chapter 4 we briefly summarized chemoviscosity models that highlight effects of cure (ηc=ηc(Τ, α)), shear rate (ηsr=ηsr(γ, Τ)) and filler (ηf=ηf(F, Τ, t)) in Tables 4.4–4.6. This chapter will examine the development of chemorheology and chemorheological modelling in more detail by examining the chemorheology and chemoviscosity models of unfilled reactive systems, overviewing the effects of fillers on chemoviscosity and then presenting chemoviscosity data and models for filled systems. It is hoped that, by presenting the data and models in more depth, a better understanding of the chemorheology of systems will be obtained.

Neat systems

Chemorheological models for neat (unfilled) curing systems can be grouped into the following categories:

• simple empirical models

• Arrhenius models

• structural and free-volume models

• probability-based and molecular models

Simple empirical models

Malkin and Kulichikin (1991) initially reviewed the rheokinetics of cured polymers and highlighted the first empirical chemorheological models.

Chapter rationale

This chapter focusses on the physical properties and models of network and reactively modified polymers. Understanding changes in physical properties during curing, in tandem with changes in chemical properties (Chapter 1), and chemorheological properties (Chapter 4), is essential to fully characterizing network and reactively modified polymer systems. This chapter will first give a brief introduction to polymer physics and dynamics before focussing on redefining network and reactively modified polymer systems. Then it will focus on defining the key changes in physical properties during cure. Finally this chapter will focus on key experimental techniques for describing changes in physical properties during cure.

Polymer physics and dynamics

Chapter 1 has already introduced basic concepts of polymers relating to their physical nature, such as crystalline and amorphous regions, molar mass, glass transition and rubbery regions. This section will focus on developing further basic polymer-physics and polymer-dynamics concepts that will be pertinent to reactive polymer systems. Specifically we will be interested in examining the physics behind polymer dynamics — to understand how to characterize the dynamics and stress behaviour of polymers under deformation and flow. This will be essential background for the chemorheology of polymer systems.

Polymer physics and motion — early models

Polymer chains consist of large molecules (macromolecules), which are composed of multiple repetition of one or more species of atoms or groups of atoms that are interlinked.