Solid Phase Processing of Polymers

The last forty years have seen considerable advances in polymer processing, one key area of progress being the production of highly oriented polymers. Processing is receiving increasing emphasis for several reasons. First, it has been recognised that the properties of polymers do not depend only on their chemical composition, but also on the structure which is imparted during processing. Secondly, there is the challenge that by optimising processing operations, polymers can be used for significant load bearing applications, with properties equivalent to or in some cases better than competitive materials. Finally, the combination of improved modelling and increasing computer power should make possible higher degrees of process control.

Polymeric materials are especially valuable for their exceptional toughness and plasticity, properties which are, nevertheless, continually sought to be combined with enhanced stiffness and strength appropriate to the molecular chain. To this end it is important to have a framework of understanding within which to address such matters. One can note at the outset that their toughness derives from molecular length and the capability this bestows of being able both to blunt incipient cracks by spreading and so reducing the local tensile stress and of having to break the strong covalent bonds which constitute the backbone of organic polymers. Falling temperature reduces the former accordingly, leading to the onset of brittleness, while crystallization tends to stiffen and harden polymers. In a crystalline polymer yield is linked to that of the crystal lamellae but, if crystallinity is too high, it can bring easier failure in the form of cleavage and fibrillation. To approach these matters it is necessary to decide whether it is adequate to treat the material as a continuum, homogeneous at the molecular or lamellar levels, or whether larger features of the microstructure must be taken into account on the principle that it is failure of the weakest internal link which causes the whole specimen to founder and macroscopic properties to fall well below those ofa defect-free material or the crystal lattice.

Orientation of polymers enhances many of their properties, in particular mechanical, impact, barrier, and optical. The products of orientation processes can generally be classified into three categories: fibres, films, and parts (sheets, bottles, rods, etc.). The fibre spinning proq:ss is the most simple and induces uniaxial orientation in the polymer. Films, on the other hand, constitute a major application and most often involve biaxial orientation, which is an added advantage allowing enhancement of properties in two directions, avoiding any weakness in the transverse direction. The most widely used biaxial orientation processes for films are tubular film blowing and cast film biaxial orientation (or tentering). Other processes such as blow molding, compression and injection molding, and thermoforming also produce mostly biaxial orientation. Finally, the solid state deformation process may involve either uniaxial or biaxial orientation in the parts and shapes formed. In pipes, for example, the orientation is usually biaxial whereas in wire production it is uniaxial and in roll-drawing planar.

Fibers are intrinsically one-dimensional articles; i.e. for all practical purposes, they have infinite aspect ratios. The uniqueness of fibers lies in this anisotropy: anisotropy of geometry, anisotropy of properties and anisotropy ofmicrostructure. Fiber processing is the art and science of producing and controlling the anisotropic geometry and molecular microstructure. It is the control of the microstructure that allows the polymer processor to design fiber properties and performance.

The starting point for the high modulus melt spun polyolefine and polyacetal fibres was the cold drawing studies of Ward and co-workers [1,2] on polyethylene. Two key results which underpinned the science and technology were:

Polyethylene terephthalate (PET) films find wide ranges of applications especially as substrates for magnetic storage and in the packaging market. It is clearly established that mechanical as well as end-use properties of the final films are mainly controlled by the structure and orientation at a molecular level, which originate from the various thermal and deformation conditions experienced by the film throughout its process. Therefore, numerous studies have been devoted to the characterisation of molecular orientation, mechanical properties and end-use properties such as gas permeability or hydrolysis, showing the importance of being able to afford an accurate characterisation of molecular orientation and structure in oriented samples.

This chapter discusses the rolling and roll-drawing processes, in particular for semi crystalline thermoplastics, with special emphasis on polyethyleneterephthalate (PET). This process has the advantages of being continuous, capable of high production rates, and able to achieve high deformation ratios with some degree of biaxial orientation (double orientation). The roll-drawing process allows the extent of biaxial orientation to be controlled by adjusting the tension applied by drawing.

For over fifty years scientists have been keenly making and studying the property development resulting from oriented morphologies of thermoplastics. A particularly interesting morphological state is that for polymers oriented in a planar direction. The mechanical properties of tensile modulus, strength and impact particularly of semicrystalline thermoplastics are markedly increased by planar orientation. For the former the measurement and use properties are generally in tension, and for the last, impact in the transverse direction. The impact enhancement through thickness is most prominent even at a low draw. Optical clarity is also commonly enhanced by planar draw processes, while permeation through thickness can be changed dramatically.

A very simple method of producing an oriented rod of a thermoplastic such as polyethylene and polypropylene is to apply pressure to a piston which pushes an isotropic plug of polymer through a die of reducing cross-section, most simply a conical cross-section. The first reports of the application of this technique to polymers came from Takayanagi and co workers [I] in Japan, but it has also been used extensively by Porter and co-workers [2] in the USA, in some cases with spectacular results in terms of the high stiffness and strength of the oriented extrudates. As in the case of tensile drawing the deformation is most effective at comparatively high temperatures (∼100°C for polyethylene) and there is a similar relationship between modulus and deformation ratio to that observed for tensile drawing.

An adequate constitutive equation to describe the solid phase processing behaviour of polymers requires the following ingredients: