Plastics As Corrosion-Resistant Materials

228 Pages·1966·5.925 MB·English

by  V. Evans, A. Kennaway and G. Tolley (Auth.)

Plastics as Corrosion-
Resistant Materials
BY
V. EVANS, Msc, F.R.I.C, A.P.I.
Special Director and Chief Chemist,
Prodorite Limited
PERGAMON PRESS
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Copyright © 1966 Pergamon Press Ltd.
First edition 1966
Library of Congress Catalog Card No. 66-23846
Printed in Great Britain by A. Wheat on & Co. Ltd., Exeter
This book is sold subject to the condition
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Foreword
CLASSICAL trades associated with the working of wood, metal,
ceramics, etc., may be learned by apprentices in the traditional
manner. In contrast, information on modern arts such as plastics
fabrication are too new to be passed on in the usual way.
The dynamic plastics industry consists of many branches.
Books should be provided for background information on seg
ments of the plastics art. It is also essential that written informa
tion be provided describing new improvements in plastics
fabrication.
The use of plastics in corrosive environment is necessary for
industrial progress. Unfortunately, there are few sources of the
information required for this type application. This book does
provide the "know how" on many different applications of plastics
as materials of construction.
Both the corrosion engineer already engaged in the use of
plastics and the technician who plans to make a contribution in
this field will find this book to be of considerable value. Plastics as
Corrosion-Resistant Materials is a welcome and worth-while con
tribution to the plastics art literature.
RAYMOND B. SEYMOUR
Associate Chairman,
University of Houston
Chemistry Department
Houston, Texas
vn
Preface
CONSIDERABLE information has been made available during the
past 20 years on the many applications of plastics in the field of
corrosion protection. Much of this information has been in the
form of articles and papers scattered through a very wide range of
technical journals and other publications. In this short book an
attempt has been made to collect together the more important of
these applications. Emphasis has been placed on the requirements
of the potential user who may be looking to plastics for help in the
solution of some of his corrosion problems. Consequently, con
siderable attention has been given to properties and applications
in the hope that these will be of practical help.
Obviously, in a short book it has been impossible to cover
comprehensively the whole field of corrosion-resistant plastics.
Attention has been concentrated, therefore, on materials and
methods which have given good results in practice.
Some bibliographical references are given as suggestions for
further study.
IX
Acknowledgements
THE AUTHOR is indebted to a great many firms and individuals for
assistance in the preparation of this book and for permission to
reproduce certain drawings, photographs and previously published
information. He would like especially to thank Dr. G. Tolley and
Dr. R. B. Seymour for reading through the draft manuscript and
for their helpful suggestions, and also Dr. Seymour for so kindly
writing a foreword.
Thanks are also due to Prodorite Ltd., for information, per
mission to use a number of drawings and photographs and for
helpful discussions with several colleagues, especially Mr. R. R.
Guest; to the late Mr. J. G. Harding and to Mr. Mitchell for line
drawings; to the Temple Press Ltd., for permission to reproduce
Fig. 26 which first appeared in British Plastics, April 1959 (with
modified scale); to the National Association of Corrosion
Engineers of America for permission to reproduce part of data
previously published in Corrosion, Vol. 15, No. 12, 635t-641t;
to the Victaulic Co. Ltd., for Figs. 32 and 33; to B.X. Plastics Ltd.,
for Figs. 11 and 12; to Yorkshire Imperial Metals Ltd., for Fig. 29;
and to Durapipe and Fittings Ltd., for Fig. 27.
Finally, the author wishes to thank the following firms for
information and, in many cases, permission to reproduce photo
graphs: Acalor (1948) Ltd.; F. W. Berk & Co. Ltd.; Boyd & Co.
Ltd.; British Visqueen Ltd.; Bristol Aeroplane Plastics Ltd.;
Calvinac Sakaphen Ltd.; Ciba (A.R.L.) Ltd.; Commercial Plastics
Ltd.; Dunlop Chemline Services Ltd.; Durapipe & Fittings Ltd.;
Ensecote Ltd.; Fothergill & Harvey Ltd.; F. Haworth (A.R.C.)
Ltd.; Fibreglass Ltd.; Imperial Chemical Industries Ltd.; Kestner
Evaporator & Engineering Co. Ltd.; Parglas Ltd.; Permali Ltd.;
xi
xii Acknowledgements
Plastic Coatings Ltd.; Plastic Constructions Ltd.; Rediweld Ltd.;
John Summers & Sons Ltd.; Tufnol Ltd.; The Victaulic Co. Ltd.;
Whessoe Ltd.; and Yorkshire Imperial Metals Ltd.
CHAPTER 1
Introduction
What are Plastics?
Plastics is today a household word, but a simple, concise and
comprehensive definition is not readily available. One of the best is
that given in Post-war Building Studies No. 3, Plastics (H.M.S.O.
1944, London), namely, Materials which, although stable at am
bient temperatures are plastic at some stage in their manufacture
and in this condition can be shaped by the application of heat and
pressure. This definition is wide and comprehensive, including
substances such as bitumens, pitches, gums, rosin, natural rubber
and even sulphur, all of which have been known for many years.
Nevertheless, most people associate plastics with the man-made
or synthetic products which have become available in the last
40 years or so. It is these latter materials with which this book is
mainly concerned, although several of the substances mentioned
above do have useful corrosion-resistant properties.
Plastics are commonly divided into two classes, thermoplastics
and thermosetting; useful corrosion-resistant properties are found
in both groups. Thermoplastics are materials which under suitable
temperature conditions are permanently plastic, that is, they can
be softened by heat over and over again without any hardening
taking place. On the other hand, thermosetting resins are converted
by heat or by heat and pressure into permanently infusible materi
als. Broadly speaking, thermoplastics consist of long, chain-like
molecules, whereas thermosetting plastics comprise large, cross-
linked, three-dimensional molecules.
The distinctive properties of plastics as compared with many
other materials is due essentially to their structure; basically to
1
2 Plastics as Corrosion-Resistant Materials
their molecular structure. Plastics have large, often very large,
molecules, so that popularly they are often referred to as "giant
molecules". This molecular structure is built up of many, small
repeating units which can be identical or dissimilar and the com
bination of these repeating units is called a polymer. With dis
similar units the term copolymer is used.
It is in this large size of the plastics molecules that their distinc
tive properties—compared with metals—lie. Consequently, for a
particular plastic material some differences in properties can be
expected according to the molecular size, larger molecules gener
ally giving tougher, harder products than smaller ones. For
thermoplastics the molecular size is dependent on the degree of
polymerisation or the number of basic units which are linked to
gether in the molecule. This is measured by the average molecular
weight which may be roughly explained as the weight of the
polymer molecule in terms of the weight of a hydrogen atom as the
unit. Of course, for one grade of any particular plastic it must not
be thought that if the average molecular weight is about 30,000,
then all the molecules are of this particular size. There will be an
appreciable scatter around this figure. Another feature is that
although thermoplastics comprise very largely long, chain-like
molecules, there is often a certain degree of "branching" with
smaller chains branching from the main chain. Branching affects
the properties of the plastic quite appreciably; for instance it
reduces the possibility of "cold drawing" and orientation which is
referred to later in this chapter.
General Properties of Plastics
As materials of construction plastics show several important
differences from metals and concrete, particularly the former;
true appreciation of these differences is very important for design
considerations. In the first place plastics have much lower tempera
ture resistance than most metals with one or two exceptions.
Nevertheless, in spite of this limitation, a material such as
unplasticised polyvinylchloride (PVC), has quite important
Introduction 3
applications even though its upper safe working temperature limit
is only around 60°C.
Again in contrast to metals most plastics do not have sharp,
well-defined melting points but slowly and almost imperceptibly
change from more or less rigid solids to highly viscous liquids.
In some ways many thermoplastics resemble the behaviour of glass
on heating but at much lower temperatures. But this behaviour
can be put to good use in fabrication techniques, as some thermo
plastics can be blown, pressed, drawn or extruded into desired
shapes under the appropriate temperature conditions.
This lower temperature resistance must be carefully born in
mind when design data are studied; for whereas with metals such
as mild steel creep effects are not important under temperatures
of 500°C, they have profound effects on some plastics even at
normal temperatures. Practically, this means that the long term
figures for many plastics are very much lower than the correspond
ing short term tests. In the case of unplasticised PVC, the short
term tensile strength at 20°C is around 525 kg/cm2 but the long
term figure at the same temperature is only 190 kg/cm2. This is
the consequence of working with materials relatively close to their
softening points. Many important features relating to design are
considered in Chapter 2.
It is interesting to compare stress-strain curves for, say, mild
steel and a typical plastic given in Figs. 1 and 2. With mild steel
the initial portion of the curve OA is practically a straight line, the
strains are very small and proportional to the applied stresses in
conformity with Hooke's law. In other words the ratio of stress to
strain is a constant, viz. Young's modulus. The point A is termed
the elastic limit, as up to this point the stressed specimen will
return to the original dimensions when the stress is released.
Beyond A the point B is quickly reached where there is a sudden
increase of strain with little or no change in stress and this is
termed the yield point; permanent set occurs here and if the stress
is released the specimen no longer returns to the original dimen
sions. Beyond the yield point the strain increases rapidly and the
curve BC is obtained. The point C marks the maximum stress the
4 Plastics as Corrosion-Resistant Materials
specimen will sustain and the stress at this point is termed the
ultimate tensile stress. After C the specimen thins down consider
ably or forms a "neck" and at D fractures. From the CD portion
of the curve it would appear that fracture occurs at a stress lower
than the ultimate tensile stress. But stresses are usually based on
the original cross-sectional area of the stressed specimen, so that
after C where an appreciable decrease in cross-section begins, the
calculated stress increases.
FIG. 1. Stress-strain curve for typical metal (mild steel).
Turning to the corresponding curve for a plastics material
(Fig. 2) it will be seen that although resemblances to the mild steel
stress-strain are found, important differences are also apparent.
The first portion of the curve OA corresponds to that of Fig. 1,
but the slope is steeper corresponding to larger strains for com
paratively small stresses. This section is also a straight line with
an approximate adherence to Hooke's law and the strain is
recoverable on releasing the stress. Just beyond A the elastic limit
is reached with rapid increase in strain with no change in stress,
and this also corresponds to the yield value B. The general form
of the curve beyond the yield point can vary considerably accord
ing to the type of plastic under test and for different plastic
materials the point of rupture may be anywhere along the BC