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 OXFORD • LONDON • EDINBURGH • NEW YORK TORONTO • SYDNEY • PARIS • BRAUNSCHWEIG Pergamon Press Ltd., Headington Hill Hall, Oxford 4 & 5 Fitzroy Square, London W. 1 Pergamon Press (Scotland) Ltd., 2 & 3 Teviot Place, Edinburgh 1 Pergamon Press Inc., 44-01 21st Street, Long Island City, New York 11101 Pergamon of Canada, Ltd., 6 Adelaide Street East, Toronto, Ontario Pergamon Press (Aust.) Pty. Ltd., 20-22 Margaret Street, Sydney, New South Wales Pergamon Press S.A.R.L., 24 rue des Ecoles, Paris 5e Vieweg & Sohn GmbH, Burgplatz 1, Braunschweig 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 that it shall not, by way of trade, be lent, resold, hired out, or otherwise disposed of without the publisher's consent, in any form of binding or cover other than that in which it is published. (2954/66) 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