Part 5, Polymer Engineering, Open University Press, ISBN 0 335 10669 2

S F Bush and G Weidmann

Introduction

Composite materials such as wood, bone and horn, which are based on the naturally-occurring polymers cellulose, collagen and keratin, have a long history of use by mankind. The earliest composite using a synthetic polymer was almost certainly Bakelite, a phenol-formaldehyde resin (see Unit 1) with cellulose fibres added (in the form of wood flour) to enhance its strength and toughness. The first commercial product moulded in Bakelite is reputed to have been the gear-lever knob for a Rolls Royce, made in 1916. Since then, and especially with the impetus given (as in so many other branches of technology) by the Second World War, the types and applications of polymer composites have increased enormously. There are three principal motives for wishing to introduce a second phase material into a polymeric matrix. The first, as exemplified by Bakelite, is to improve or modify the physical properties of the polymer whilst retaining to a large extent its easy mouldability. Secondly, there is the opportunity to reduce the cost of the material by incorporating low-cost fillers – a factor given increased importance following the large increase in the price of oil in 1973. Finally, and associated with the fact that the materials with the highest specific strengths and specific moduli are generally only availalble in fibrous form (see DATA Book), is the use of the polymer matrix to hold an assembly of such fibres together, transfer stresses to them, and protect their surfaces from damage or environmental attack.

The number of possible combinations of second phase materials with polymeric matrices is enormous, with the form of the second phase as well as its properties critically affecting the overall properties of the composite. Polymeric composites can be broadly classifield according to whether the matrix is thermoplastic or thermosetting, and whether the second phase is in the form of continuous fibres, discontinuous fibres or is particulate. Although this classification is of some use in indicating which of the many processing options are available for a given composite, it only provides a fairly general guide to composite properties. Thus a thermosetting matrix is more likely to withstand higher temperatures than a thermoplastic one, and a continuous fibre composite should have a higher modulus than one of the same composition containing short chopped fibres. However, the end properties of any composite depend not only on the properties of the individual constituents, but also on how they are arranged and on the interface between the matrix and the second phase (i.e. on how effectively they are bonded together).

Part 4, Polymer Engineering, Open University Press, ISBN 0 335 10668 4

S F Bush with G Weidmann

Introduction

Polymer engineering is about making things. This and subsequent units show how the properties and processing knowledge of polymers acquired in the previous three units can be applied to realistic design problems.

It will be readily appreciated that, in any practical situation, a design for a product or component will have to fulfil a multitude of conditions. Usually only some of these can be defined in precise, let along strictly mathematical, terms. Nonetheless there is a systematic way of designing which can embrace both quantitative and qualitative factors, and this is outlined in Section 3. The subsequent discussion is then carried out within the framework of this design procedure and with reference to it.

While the quantitative analysis needed to attain the required processing and product performance will usually be one or more of a few types, the variety of qualitative factors which might possibly be involved is almost endless, as those who have experience of engineering projects or design will be aware. That being so, the qualitative, circumstantial factors can only be illustrated by case studies and the text is based around two of these. One case study, in Section 2, examines the failure of a polypropylene vessel in a chemical plant, whilst the other, on the design of a gas pipe, forms a theme which runs through a large part of the unit.

The earlier units have covered the basic aspects of polymer behaviour in the fluid state and have indicated how this behaviour influences product performance. More evidence of this relationship will be seen as this unit develops. The important point is that the analysis of polymer behaviour will now be made more specific and applicable to real cases. To do this takes a little time, so that most of this unit will be taken up with two matters of central importance in this branch of polymer engineering:

1. stresses and failure conditions in extruded products
2. melt flow and heat exchange in extruders and within dies and channels.

Both these matters will be dealt with in the context of the overall design procedure as applied to practical cases.