Reinforced Plastics Durability

378 Pages·1999·5.911 MB·English

by  G. Pritchard (Eds.)

Reinforced
plastics durability
Edited by
Geoffrey Pritchard
CRC Press
Boca Raton Boston New York Washington, DC
Y
Cambridge England
ITYPR(new) 1 11/26/98, 12:30 PM
Published by Woodhead Publishing Limited, Abington Hall, Abington
Cambridge CB1 6AH, England
Published in North and South America by CRC Press LLC, 2000 Corporate Blvd,
NW Boca Raton FL 33431, USA
First published 1999, Woodhead Publishing Ltd and CRC Press LLC
© 1999, Woodhead Publishing Ltd
The authors have asserted their moral rights.
This book contains information obtained from authentic and highly regarded
sources. Reprinted material is quoted with permission, and sources are indicated.
Reasonable efforts have been made to publish reliable data and information, but
the authors and the publishers cannot assume responsibility for the validity of all
materials. Neither the authors nor the publishers, nor anyone else associated with
this publication, shall be liable for any loss, damage or liability directly or
indirectly caused or alleged to be caused by this book.
Neither this book nor any part may be reproduced or transmitted in any form
or by any means, electronic or mechanical, including photocopying, microfilming
and recording, or by any information storage or retrieval system, without
permission in writing from the publishers.
The consent of Woodhead Publishing and CRC Press does not extend to
copying for general distribution, for promotion, for creating new works, or for
resale. Specific permission must be obtained in writing from Woodhead
Publishing or CRC Press for such copying.
Trademark notice: Product or corporate names may be trademarks or registered
trademarks, and are used only for identification and explanation, without intent
to infringe.
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library.
Library of Congress Cataloging in Publication Data
A catalog record for this book is available from the Library of Congress.
Woodhead Publishing ISBN 1 85573 320 X
CRC Press ISBN 0-8493-0547-0
CRC Press order number: WP0547
Cover design by The ColourStudio
Typset by Best-set Typesetter Ltd., Hong Kong
Printed by St Edmundsbury Press, Suffolk, England
ITYPR(new) 2 11/26/98, 12:30 PM
Preface
Primitive reinforced plastics products were known in the 1920s and 1930s,
but the more advanced fibre reinforced materials we know today only
became significant commercially as structural materials in the 1950s, and
even then, the more recent reinforcing fibres such as carbon/graphite,
aramid (e.g. Kevlar®, Twaron®) and polyethylene (e.g. Dyneema®) fibres
were all still completely unknown. The great majority of reinforced plastics
articles we use today have been manufactured since 1975.
Thus, we have a very limited number of case histories of structural
applications with which to prove the durability of fibrous composites
beyond dispute, although the evidence we have is very encouraging. This
promising start is not surprising because, except in a very few special
cases, they resist microbial organisms and are unaffected by electrolytic
corrosion.
As we reach the end of the present millenium, we notice a great expan-
sion of reinforced plastics into new fields such as load bearing parts of
buildings, bridges over highways and various infrastructure pipelines. A
mere 25 years of useful life will not suffice for these applications. In many
cases the target is 60, 75 or even 100 years. This is a longer period than the
entire previous history of reinforced plastics. The contribution that theory
can make to predicting the achievable lifetimes of reinforced plastics is
growing, but it is too early yet to base decisions entirely on conclusions that
are unsupported by any practical or experimental confirmation. Therefore
we have to use an amalgam of medium term laboratory research, case
histories and a knowledge of the theoretical principles of the main degrada-
tion processes.
Major studies are currently being undertaken into composites durability.
One of these programmes, the Composites Durability Study, is being car-
ried out in the USA by the Civil Engineering Research Foundation and the
Society of the Plastics Industry’s Composites Institute Market Develop-
ment Alliance, for the benefit of the design and construction industry and
Y
the civil engineering profession. It is said that particular concerns include
ix
ITYPR(new) 9 11/26/98, 12:30 PM
xx CProenftaecnets
performance degradation in severe operating environments, fatigue life,
creep, fire resistance, weatherability and maintenance.
The problem with many technical books is that although they contain
valuable information, it is not always made sufficiently accessible to those
who most need it. This is often because of the failure to explain basic
terminology to readers from different backgrounds and specialties. The
editor has therefore made a considerable effort to ensure that at least the
early chapters of this book are readily understandable by people from
discipline areas other than composites science. It has not been possible to
eliminate altogether the chemical and mathematical equations which the
outsider often finds alienating, but they have been reduced to a minimum.
Those who wish that the subject matter had been treated more rigorously –
and no doubt there will be many – will find numerous references at the
end of each chapter, indicating further literature with a more specialized
orientation.
This book is therefore targeted at those concerned for the first time with
using reinforced plastics in building, highway engineering, offshore engi-
neering, civil and chemical engineering, marine and electrical areas. It is an
entry level book for engineers, architects, entrepreneurs, managers, design-
ers, and graduate students who have already completed courses in engi-
neering or science and who now need to be able to converse better with
consultants and specialists in the reinforced plastics world. If research
chemists, resin technologists or plastics process engineers should also find
something of interest, that will be a bonus.
The editor takes the blame for the frequent use of the phrase ‘reinforced
plastics’ rather than the increasingly favoured, up-market term ‘advanced
composites’. He recognizes that glass fibre reinforced composites are cus-
tomarily excluded from the advanced composite category in aerospace
circles, chiefly because of their relatively low modulus. But when their
technical qualities as a whole are considered, including their durability in
the broad sense, and when their cost effectiveness is also taken into account,
reinforced plastics scarcely need rebadging. They are among the best mate-
rials in the world in the context of durability.
Y
ITYPR(new) 10 11/26/98, 12:30 PM
List of contributors
Chapter 1 An introduction to plastics for non-specialists
Geoffrey Pritchard, Emeritus Professor, Kingston University, Surrey, UK
Consultant, York House, Moseley Road, Hallow, Worcester WR2 6NH,
England
Chapter 2 Fabrication, inspection and durability
Geoffrey Pritchard, Emeritus Professor, Kingston University, Surrey, UK
Consultant, York House, Moseley Road, Hallow, Worcester WR2 6NH,
England
Chapter 3 Durability of reinforced plastics in liquid environments
Frank Jones, Department of Engineering Materials, Sir Robert Hadfield
Building, Sheffield University, Mappin Street, S1 3JD, England
Chapter 4 Temperature–its effects on the durability of
reinforced plastics
John J Liggatt* Geoffrey Pritchard** and Richard A Pethrick*,
*Department of Pure and Applied Chemistry, Strathclyde University, 295
Cathedral Street, Glasgow, G1 1XL, Scotland **See Chapter 1
Chapter 5 Cyclic mechanical loading
Karl Schulte, Technical University of Hamburg-Harburg, Polymer and
Composites Section, Denickestraße 15, D-21071 Hamburg, Germany
Chapter 6 Weathering
John Layton, The Ash, 23 Newtown Road, Raunds, Wellingborough,
Northants NN9 6LX, England
Chapter 7 Review of the durability of marine laminates
Y
Tim J Searle and John Summerscales, Advanced Composites
xi
ITYPR(new) 11 11/26/98, 12:30 PM
xxiiii CLiosnt toefn ctsontributors
Manufacturing Centre, School of Manufacturing, Materials and
Mechanical Engineering, Plymouth University, PL4 8AA, England
Chapter 8 Survey of long term durability of fiberglass-reinforced plastics
tanks and pipes
Ben Bogner, Amoco Chemicals, 150 West Warrenville Road D-7,
Naperville, Illinois 60563–8460, USA
Chapter 9 Epoxy vinyl ester and other resins in chemical
process equipment
Paul Kelly, Dow Deutschland Inc., Postfach 20, Industriestraße 1, D-77836
Rheinmünster, Germany
Chapter 10 Repairs using fibre reinforced plastics
Wing Kong Chiu and Rhys Jones, Department of Mechanical Engineering,
Monash University, Wellington Road, Clayton Campus, Victoria 3168,
Australia
Chapter 11 Fatigue performance: The role of the interphase
Nikhil E Verghese and John J Lesko, Materials Response Group,
Department of Engineering Science and Mechanics, Virginia Polytechnic
Institute and State University, Blacksburg, VA 24061-0219, USA
Chapter 12 Computer models for predicting durability
Samit Roy, Department of Mechanical Engineering, University of
Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65401, USA
Y
ITYPR(new) 12 11/26/98, 12:30 PM
1
An introduction to plastics for non-specialists
GEOFFREY PRITCHARD
1.1 Durability
Most materials have a finite life. Metals corrode; they can also suffer from
fatigue. Wood rots, and concrete cracks or suffers from various chemical
degradation processes. Natural rubber can perish as a result of ozone at-
tack. All these materials have been around for long enough for us to know
and make allowance for their weaknesses.
Our concern here is with identifying and assessing the weaknesses,
if any, of reinforced plastics. The experience of museums is that
plastics artifacts, even when kept on display in glass cases and preserved
from rough handling, nevertheless eventually show signs of deteriora-
tion. We cannot assume that the same will apply to reinforced plastics,
because the resins used in reinforced plastics are different. Of all the
resins commonly used in reinforced plastics, phenolics are the oldest,
having been known for over nine decades. They date commercially from
about 1909, being used first as wood lacquers rather than in composites,
whereas polyesters have been used structurally in reinforced plastics
since the 1940s. All these resins have evolved considerably since then,
because new improved varieties of resin come out every seven years or
thereabouts, so it is arguable that we cannot know by experience what
the lifetime of any of the modern equivalents on sale today is likely to
be. Reinforced plastics are now being specified for applications designed
to last for 30, 40 or even 60 years without loss of functional effectiveness.
The accelerating trend towards using reinforced plastics in bridges and
buildings means a further extension of the required lifetime, possibly to
almost a century – longer than the entire history of the reinforced plastics
industry.
This book is designed to bridge the gap between composite materials
specialists and end-users. It will discuss in broad terms the ways in which
Y
deterioration in reinforced plastics occurs, and the ways to delay it. We shall
1
ITY1 1 11/26/98, 12:32 PM
2 Reinforced plastics durability
concentrate mainly, but not exclusively, on time-dependent processes, such
as weathering, corrosion, wear, fatigue and heat aging, rather than on
sudden destruction by impact or fire, although the latter dangers are men-
tioned too.
Inevitably, by its nature, the book highlights potential problems, rather
than opportunities, for the user of reinforced plastics. The resulting impres-
sion could easily be too negative, as when a layman anxiously consults a
medical textbook about his health – there seem to be too many things to go
wrong! Such a response would be unduly pessimistic. We should recall
that reinforced plastics have surpassed early expectations by their durable
and reliable performance in outdoor use. Some applications have been
very demanding, notably in the marine sector, the offshore oil industry,
aerospace, and perhaps most obviously, in the chemical process equipment
field.
Moreover, many of the early successes were achieved in the early
days of the new material, in the 1940s, 1950s and 1960s, without the
benefit of any substantial theoretical basis or long term performance
database. The approach then was understandably one of cautious
overdesign. Later, designers moved towards more cost effective and
lightweight structures, without sacrificing too much durability. It was
not sufficient to build a boat that survived, it had to win races. Aircraft
had to lose as much weight as possible. Actual performance continued
to exceed the pessimistic expectations produced by an obsession with
the literature on failure mechanisms. Whether this happy state of af-
fairs will continue, with ever more demanding applications and ex-
pectations, remains to be seen, but it now at least seems possible to
identify the application areas where reinforced plastics can safely be
used.
1.2 Cost effectiveness and product lifetimes
A product is at the end of its useful life when it no longer fulfils its technical
function. But technical performance alone is not enough. To be useful, the
product must continue to do its job in a cost-effective way. The criteria for
cost effectiveness depend on the application and on the financial situation
of the organization concerned, including its available investment capital
and its perceptions of likely future return on capital. Such concepts are
clearly outside our present terms of reference, but it is worthwhile to
remind ourselves that cost effectiveness is inseparable from technical con-
siderations, especially as energy saving features and low maintenance costs,
rather than initial outlay, are so often mentioned as reasons for using
Y
reinforced plastics products.
ITY1 2 11/26/98, 12:32 PM
An introduction to plastics for non-specialists 3
1.3 When does a fibre reinforced plastics product
have to be replaced?
When can we say that a product’s life has finally ended? Bridges must not
fall down. They must remain safe to use, more specifically, despite the
stresses and the external weathering they experience over decades. They
must not become too expensive to maintain. Fibre reinforced plastics
(FRP) boats are exposed to several hostile forces – water, weather, repeti-
tive wave action and occasional impact. The chief concern is again struc-
tural integrity, but most small boats also have cosmetic appeal to their
owners, and the cost of maintaining pristine appearance, free from worrying
defects, must also be considered.
The FRP cladding panels of buildings do not have to support large
stresses, because a steel framework can do this, but in the same way as with
boat hulls, their aesthetic appearance and moisture barrier qualities must
not be allowed to deteriorate. Often they have been subject to colour
changes caused by the action of ultraviolet light on flame retardant addi-
tives in the resin.
Translucent roofing over an indoor swimming pool must remain translu-
cent and failure to do so cannot be compensated by structural virtues such
as excellent retention of flexural strength and modulus. In contrast, the
appearance of a chemical storage tank is of minor importance provided that
it does not leak. Pipes and tanks need to maintain both their load carrying
and chemical resistance qualities.
In most of these applications, the visible signs of deterioration appear
gradually and can include one or more of the following: cracks of various
kinds, surface pitting, blisters, swelling, delamination, softening and occa-
sionally, discoloration. The possibility of repair is an attractive feature of
reinforced plastics. They can have an extension of their useful life because
they are more easily repaired than some other materials and can even be
used to extend the life of structures originally made from something else,
such as concrete or metals.
1.4 Health and safety
Health and safety considerations need to be taken into account. Legislation,
even though not yet enacted, could require a product to be replaced while
it is still functionally effective and cost effective. One of the best fibrous
reinforcements in the FRP world used to be asbestos. Its strength, stiffness
and especially its heat resistance were outstanding, but since the mid-1970s,
removing asbestos-based materials from factories, schools and other build-
Y
ings has been an industry in itself. It is therefore prudent to consider the
ITY1 3 11/26/98, 12:32 PM
4 Reinforced plastics durability
long term health and safety status of all materials proposed for use in
durable structures.
Although this author claims no special expertise in the toxicology of
materials, it seems fair to say that the weight of opinion at present is
reassuring about fibres in existing use. Glass, aramid and polyethylene
fibres are all much safer than asbestos. In common with traditional materi-
als such as wood and cotton, they must always be handled with care,
especially if they are finely divided and therefore in respirable forms, i.e.
small ((cid:2)3μm) diameter short fibres or fine dust.
Typical reinforcing glass fibres have much larger diameters (8–16μm)
and unlike asbestos, the filaments do not readily split along their axes to
form smaller diameter fibres. Once incorporated in a resin matrix, they are
no longer respirable except when the composite is machined.
Health-related issues have been discussed in more detail by Braddock
[1].
Another safety consideration is the flammability of reinforced plastics,
together with their capacity for smoke evolution during a fire. This matter
is therefore discussed in Chapter 4. It should be safe to assume that the
standards required by the relevant authorities will become progressively
more demanding over the next few decades. The actual lifetimes of rein-
forced plastics structures being designed today could well be determined by
future fire legislation as much as by weathering performance or fatigue
resistance. This fact may account for the increasing popularity of phenolic
resin matrix composites, which have good records in this respect. There is a
widespread residual prejudice against synthetic materials and it should be
remembered that timber, wool and cotton are also flammable.
Among thermoplastics, PEEK (polyether ether ketone) is also very
promising but at the time of writing, much too expensive for most purposes
outside medicine and aerospace. The way to improve the fire and smoke
performance of an ordinary resin is conventionally by using grades contain-
ing appropriate additives [2].
1.5 What causes deterioration in fibre
reinforced plastics?
We should not assume automatically that there will be any deterioration!
But if a reinforced plastics product was competently designed and soundly
fabricated, material deterioration usually begins through one or more of the
following four influences:
1 mechanical stress, including static loading, fatigue, repeated minor im-
pact, erosion (including water erosion) and abrasion
2 chemicals (water, solvents, fuels, oils, acids, cleaning liquids, atmos-
Y
pheric oxygen, oxidizing agents, caustic alkalis, etc.)
ITY1 4 11/26/98, 12:32 PM