Plastics for Engineers. An Introductory Course

177 Pages·1967·2.201 MB·English

by  G. R. Palin, A. Kennaway and G. Tolley (Auth.)

Plastics
for Engineers
AN INTRODUCTORY COURSE
by
G. R. PAL1N, B.Sc, Ph.D.
Head of Chemistry Department, Royal Air Force College
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Copyright © 1967 Pergamon Press Ltd.
First edition 1967
Library of Congress Catalog Card No. 66-28420
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Preface
PLASTICS find application in all fields of engineering, from pre-
fabricated kitchen-bathroom units to insulation in coaxial cables;
from gears and bearings to boat hulls and car bodies. Plastics are
no longer just useful materials for the fabrication of toys, kitchen
ware and similar articles. They are an extremely important and
versatile group of engineering materials, second only in usefulness
to the metals.
There are few engineers whose training does not include a fairly
detailed study of the nature, properties and applications of
metallic materials. Unfortunately, all too few receive a similar
grounding in plastics. The aim of this book is to present an
elementary account of the nature, engineering properties and
applications of this group of materials. It is intended primarily for
students in all fields of engineering, whether they are receiving
formal courses of instruction in the subject, or not. It should also
be useful to qualified engineers, who require a simple introduction
to the subject, either for its own sake, or as the foundation on
which to build a more detailed study of particular materials with
which they are concerned.
It is not possible to write a book about plastics without the
introduction of some chemistry. Nevertheless, every attempt has
been made to use as few chemical concepts as possible. The only
chemical knowledge assumed is an elementary appreciation of the
nature and structure of organic compounds, and of the inter-
molecular forces acting in solids. A number of suitable books
which cover these topics to the required level, are listed in the
Bibliography.
The book is based on courses given to engineering students
at the Royal Air Force College, and I am grateful to my
vii
viii PREFACE
colleague Fit. Lt. G. Scott for his help. Thanks are also due to
Dr. G. Tolley and Dr. B. Jennings for their many helpful sug-
gestions, and to Mrs. L. J. Stirling for her help in preparing the
manuscript.
Introduction
IN THE last thirty years the importance of plastics has grown
enormously. They are no longer just useful materials for the
fabrication of toys and kitchen ware, nor are they alternatives or
substitutes. They now represent a very important and versatile
group of engineering materials, second in rank only to the metals.
They are used in all fields of engineering from prefabricated
kitchen-bathroom units to cable insulation; from gears and
bearings to boat hulls and car bodies; from printed circuits to
aircraft cockpit canopies.
There is a wide variation in properties among the materials
which are classified as plastics. This makes it difficult to discuss
the advantages and disadvantages of plastics, as a group, com-
pared with other materials. Some generalizations can be made,
but they are not always applicable to all plastics. Most plastics can
be easily fabricated into items with quite intricate shapes, using
special techniques which are suitable for mass production. Many
plastics can also be worked, using normal workshop techniques.
On the other hand, plastic pieces are often difficult to repair if
broken. Nor are plastics cheap, although their cost will fall as
their use spreads. Plastics are not usually as strong as metals and
they are much more prone to dimensional changes, particularly
under stress. They are much more sensitive to high temperatures
than metals. Nevertheless, they are much less dense than metals,
so that comparison with metals, in terms of strength to weight
ratios, are not so unfavourable. Plastics are replacing light alloys
in many applications in which high temperatures are not en-
countered. Composite materials, such as glass-filled or glass-
reinforced plastics, can compare in strength with metals, but
unfortunately reinforcement often reduces the ease of fabrication.
ix
χ INTRODUCTION
The high-temperature characteristics of plastics can also be im-
proved by using them in composites. Reinforced plastics are being
widely used as structural materials, and this field is being de-
veloped rapidly. Plastics are good thermal insulators. They also
show high mechanical damping and many of their applications
make use of these properties. Plastics are much more resistant to
corrosion and chemical attack than metals. They can be produced
in a wide range of colours and are, in some cases, transparent.
Against this, a number of them deteriorate when exposed to
sunlight. Plastics fill the need for a group of electrical insulators
with adequate mechanical properties. They are superior to other
insulators in ease of fabrication, flexibility and mechanical
strength. Their only limitation in this field is the small temperature
range in which they can operate.
These generalizations indicate the advantages and disadvantages
of plastics. Many of their applications do not depend so much on
these general characteristics, as on some special and important
property of the individual plastic. Some of those which are made
use of are : very low coefficient of friction, complete chemical in-
ertness, retention of flexibility at very low temperatures and good
dielectric properties. There are many more and they indicate the
great versatility of this group of materials.
Design with plastics is not easy. There are not so many data
available as there are for the older materials. Nor is it so easy to
use available data for prediction, as variations in performance,
due to production and fabrication differences, are greater. There
have been many unsatisfactory applications of plastics. Some of
them because plastics were not the best type of material, but
others because the wrong plastic was chosen. As the use of
plastics grows, so will the available data and "know how". This
will enable plastics to be used to better and better advantage.
Plastics are so called because they are capable of being moulded.
A more general definition of plastics is that they are polymeric
materials. This defines them in terms of the arrangement of the
atoms within the material. A polymeric material consists of a
small unit of atoms combined in a given way. A large number of
INTRODUCTION xi
these units or "mers" are combined with one another to give a
very large molecule, known as a polymer. Apart from the silicones,
which will be dealt with separately, plastics are based on carbon.
Many naturally occurring substances are polymeric. Most of the
oils, resins and gums used in varnishes, paints and adhesives are
polymeric, as are natural rubber and cellulose. Materials of this
type are still widely used, but the growth of the plastics industry
is due to the discovery of methods of synthesizing polymers of
many types. This is done by starting with simple materials, which
correspond to the small structural units of atoms, and combining
them to produce the plastic.
The general classification of polymeric materials includes
rubbers, synthetic fibres and many substances used in coatings
and adhesives, as well as those commonly thought of as plastics.
Mention will be made in the following discussion of all types of
polymeric material, but it is mainly concerned with the last
group.
A wide variety of plastics, with very different characteristics,
can be made. The properties of the plastic depend upon the
nature of the structurally recurring unit, and the way in which
these units are combined. For this reason, the various types of
polymer structure and their effect on the properties will be dis-
cussed first. The methods of fabricating plastics are then outlined,
followed by a brief survey of the nature, properties and applica-
tions of some of the more important plastics.
CHAPTER 1
Polymers
Linear and Network Polymers
In polymers of the organic type, carbon is usually in its tetra-
hedral valency state. In this state the carbon atom forms four
single bonds with other atoms or groups, the bonds being directed
towards the corners of a regular tetrahedron.
The simplest type of polymer molecule is one in which each
carbon atom forms two single bonds with other carbon atoms, and
two with other atoms or simple groups. This is known as a
straight chain or linear polymer and can be represented as
The chain will be terminated by carbon atoms which form three
bonds with other atoms or simple groups. It must be remembered
that there is free rotation about a single carbon-carbon bond. In
a four-carbon system there are a number of different spatial
arrangements of the carbon atoms, four of which are shown in
Fig. 1. The system will be continuously changing from one of
these to another, as rotation about the bonds occurs. In a chain
with many carbon atoms the number of possible arrangements in
space will be enormous, so that the probability of the molecule
being in the form indicated by the above representation is very
small. The term straight applies only to the chemical form of the
chain and not to its physical form, which will be far from straight,
and will be changing all the time.
2 PLASTICS FOR ENGINEERS
-c c -c
c c c — c
\
I
j 180°
-c - c
\ 180° \
c ^ ^ C
c
\
c
FIG. 1. Changes in the spatiafarrangement^of a four-carbon system
as a result of rotation about the bonds.
The two bonds from each carbon atom, which are not involved
in the chain, may also be joined to carbon atoms which are part
of a simple organic group. Such a group will occur regularly
along the chain, and the molecule is still said to be linear. Two
such polymers are
CH, H CTL H C.HH CH. H
-c II— c—cI — 3cI and ι -S I I6 15
I I I I -c — c — c — α-
CH H CH H Ι I I I
3 3 H H H H
There are one or two polymers in which branch chains occur.
These are linear molecules in which the occasional carbon atom
is joined to another linear chain. These branches will be few in
number, and will occur irregularly along the main chain. The
branch will be joined to only one chain.
If a number of the carbon atoms in the polymer form three or
even four bonds with carbon atoms, other than those in simple
groups, a network polymer will result. This has a complicated
three-dimensional structure. A small section of a typical network
polymer is shown in Fig. 2. A chemically bonded system of this
POLYMERS 3
type may become so large that the term molecule ceases to have
any significance. These network polymers are often formed in two
steps. The first step is the production of fairly small molecules,
which are mainly linear in structure. These are then caused to
react in such a way that chemical bonds form between them, and
the network is built up. This second process is known as cross
/° V '
, / /CH—Ο ^CH XΟ
(CH) / )C0 /
2 ηn — CH, / CH,
CO > , 2
( C nH )
CO
FIG. 2. A small section of the network polymer formed by the
reaction of the acid HOOC(CH) COOH with glycerol.
2 n
linking. It is possible to produce network polymers whose
structure corresponds to a number of linear molecules with a few
chemical bonds connecting the chains together. These are said to
be lightly cross linked, whereas the complicated network polymer
with no linear nature, is said to be highly cross linked.
Addition and Condensation Polymers
Polymer molecules are often classified as being of the addition
or of the condensation type. This is somewhat misleading, as
there is little difference between the resultant materials. The
difference lies in the chemical reactions involved in the synthesis
of the material, and a more accurate classification would be
polymers formed by addition polymerization and polymers
formed by condensation polymerization.
Carbon can exist in other valency states than the tetrahedral
one already mentioned. In one of these the carbon atom forms a
double bond with another carbon atom and two single bonds