Plastics

436 Pages·1991·46.295 MB·English

by  G. Pál and H. Macskásy (Eds.)

Studies in Polymer Science
Other titles in the series
1. Elastomers and Rubber Compounding Materials edited by I. Franta
2. Molecular Conformation and Dynamics of Macromolecules in
Condensed Systems edited by M. Nagasawa
3. Design of Plastic Moulds and Dies by L. Sors and I. Balázs
4. Polymer Thermodynamics by Gas Chromatography by R. Vîlcu and
M. Leca
5. Optical Techniques to Characterize Polymer Systems edited by H.
Bässler
6. Plastics: Their Behaviour in Fires by G. Pal and H. Macskásy
Studies in Polymer Science 6
Steoir h(Bhm\\úw
GD
by
G. Pal and H. Macskásy
Research Institute for the Plastics Industry
Budapest, Hungary
ELSEVIER
Amsterdam—Oxford—New York—Tokyo 1991
This is a revised version of the Hungarian A müanyagok éghetosége, published by
Muszaki Könyvkiado, Budapest
Translation by A. Grobler
The distribution of this book is being handled by the following publishers:
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Library of Congress Cataloging-in-Publication Data
Pal, Károlyné.
[Müanyagok éghetosége. English]
Plastics: their behaviour in fires/by G. Pal and H. Macskásy.
p. cm. — (Studies in polymer science; 6)
Translation of: Müanyagok éghetosége.
ISBN 0-444-98766-5
1. Plastics—Fire—testing. I. Title. II. Series.
TH9446. 5. P45P3513 1990
628.9'222-dc20
89-77550
CIP
ISBN 0-444-98766-5 (Vol. 6)
ISBN 0-444-42994-8 (Series)
© G. Pal and H. Macskásy 1991
© English translation A. Grobler 1991
Joint edition published by Elsevier Science Publishers, Amsterdam, The Netherlands
and Akadémiai Kiadó, Budapest, Hungary
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system
or transmitted in any form or by any means, electronic, mechanical, photocopying, recording
or otherwise without the prior written permission of the copyright owner
Printed in Hungary
by Akadémiai Kiadó és Nyomda Vállalat, Budapest
Preface
It is a characteristic feature of the present time that plastics are gaining more and
more ground in industry, in agriculture, in many other sectors of the economy, in
fact in almost all walks of life.
The world production of plastics was 1.5 million tons in 1950. It increased twenty-
fold by 1970, and doubled during the following decade, reaching 75 million tons by
1985. The consumption of plastics has increased at a much higher rate than any
other engineering material, such as steel, other metals, wood, and cement. More and
more kinds of plastics replace conventional structural materials. Technical nov-
elties that could not exist without plastics appear on the market continuously.
Industrially-used plastics are combustible. They are very often used in exposed
situations where fire safety is crucially important, such as in buildings and vehicles,
together with their interior design, electronic equipment and household appliances.
For this reason, plastic materials and products must be selected and used in such a
manner that the fire risk is not thereby increased. This requires a thorough knowledge
of the flammability characteristics of plastics and how such characteristics can be
modified.
With this aim in view, research was started at the Hungarian Research Institute for
the Plastics Industry in the early sixties, leading to the first Hungarian book on the
flammability of plastics in 1967. With the rapid increase in the use of plastics, a
comprehensive book on this subject was issued by Muszaki Könyvkiado (the Hun-
garian Publishing House of Technics), Budapest, in 1980. It covered all the Hun-
garian and foreign results up to that time, based on the international literature and
on the experimental work at the Hungarian Research Institute for the Plastics In-
dustry, systematizing all the necessary information about flammability problems of
plastics both in the plastics industry and in sectors where plastics products are
used.
This present English edition is not a direct translation of the Hungarian version.
It has been thoroughly revised, principally because of the continuing rapid devel-
opment of plastics. A couple of years is enough for the manufacture of a range of
novel resins and products with markedly improved flammability characteristics.
Brand-new concepts and products have appeared in the interval since the Hungarian
edition was published, so that some chapters had to be completely re-written. The
other chapters have also been carefully revised and supplemented as necessary.
V
In view of the anticipated wider readership of this English edition, the international
(ISO, IEC) and leading national (ASTM, BS, DIN, AFNOR, etc.) standards are
discussed in greater detail and other changes have been made to cater for an interna-
tional readership.
Finally, the authors are indebted to the translator, Dr. András Gröbler, who has
used his extensive expertise in close co-operation with the authors for the clear and
unequivocal drafting of the English version.
Gizella Pal
Hugo Macskásy
VI
1. A preliminary survey of plastics
1.1 Terms and definitions
According to the international standard ISO 472, plastics are materials
— which contain a high polymer as an essential ingredient and
— which, at some stage in their processing into finished products, can be shaped
by flow.
A similar definition is given by the American standard ASTM D 883-85 in a
more detailed form. Plastics are materials that
— contain as an essential ingredient one or more organic polymeric substances
of large molecular mass,
— are solid in their finished state, and
— at some stage in their manufacture or processing into finished articles, can be
shaped by flow.
Polymers are organic compounds. For this reason, these extensively manu-
factured, processed and used plastics are usually combustible. Even the silicon
plastics are not exceptions since their inorganic polymeric chains of repeated -Si-O-
units comprise organic side groups, designated R and R :
x 2
Ri
I
-Si-O-
I
R
2
Referring to the standard ASTM D 883 again, a polymer is a substance consisting
of molecules characterized by the repetition (neglecting ends, branch junctions and
other minor irregularities) of one or more types of monomeric units.
A monomer is a relatively simple compound which can react by joining to other
monomers to form a polymer.
Polymerization is a chemical reaction in which the molecules of monomers are
linked together to form polymers. The two principle kinds of polymerization are
addition and condensation polymerization. Addition polymerization has two sub-
groups : chain propagation polymerization and stepwise chain growth. (It should be
noted that much of the literature, especially in Europe, uses "polymerization" only
for the chain propagation process. Stepwise growing is referred to as polyaddition,
condensation polymerization is named polycondensation.)
1
Industrial monomers are low-molecular organic compounds such as ethylene, vinyl
chloride and styrene which can be polymerized into polyethylene, poly(vinyl chlo-
ride) and polystyrene, respectively.
In the polymerization reaction, a monomer molecule links to a second, then a
third,. . . hundredth, . . . thousandth,. .. n-th monomer molecule by primary
chemical bonds forming a polymeric molecule consisting of n consecutive monomeric
units. Both ends of the polymeric molecule are terminated by diverse groups, accord-
ing to the following schematic pattern (where X and Y are the terminal groups) :
polymerization
ΛΜ >- X-M-M- . . . -M-Y
monomer 1st 2nd n-th
molecule monomer unit
or more simply :
polymerization
n M ^ X- [M]„-Y
In the case of ethylene:
polymerization
n CH =CH ^ X-CH ~CH -CH -CH - . . . -CH -CH - Y
2 2 2 2 2 2 2 2
ethylene terminal 1st 2nd /i-th terminal
group monomer unit group
or more simply:
polymerization
«CH =CH ^ X-tCIV-CH^-Y
2 2
ethylene polyethylene
The terminal groups X and Y are each hydrogen in this example.
The value n is the number of monomeric units in one polymeric molecule, named
degree of polymerization.
The polymerization of ethylene is an example of chain propagation polymeriza-
tions. The polymerization of propylene, vinyl chloride, styrene, aery lates, methacry la-
tes, and many other monomers proceeds in the same way.
Stepwise polymerization is characteristic, for example, of the polyurethane for-
mation from diisocyanates and polyols. Nylon 66 is formed by condensation poly-
merization from hexamethylenediamine and adipic acid.
The essential ingredients of technical plastics, are mainly high polymers i. e. their
degree of polymerization exceeds 100. In practice, it is generally some thousands, so
that their molecular mass is greater than 104, commonly by one or two orders of
magnitude.
High polymers belong to the family of macromolecular materials which are distin-
guished from the low-molecular substances (such as water, benzene, ethylene, etc.)
consisting of few (less than 100) atoms in each molecule.
Macromolecular materials are frequently encountered in natural substances such
as cellulose, natural rubber, and proteins. Cellulose is a polymer of glucose con-
taining several thousand repeated glucose units in its macromolecule. Natural rubber
is a polymer of isoprene. The natural and plant proteins are also macromolecular ma-
2
terials but they are not polymers in the strict sense because their molecules are not
constructed by repetition of one or a few kinds of monomeric units; instead, about
20 different amino acids are linked together by primary chemical bonds in definite
sequences.
Historically, the first plastics were prepared by chemical modification of the natural
macromolecular materials. Cellulosic plastics such as cellulose acetate, cellulose
acetobutyrate, cellulose propionate, ethyl cellulose, viscose fibres and films are still
used in the industry.
Macroscopic amounts of natural and synthetic polymers comprise a huge number
of macromolecules (unless they consist of a completely cross-linked network). The
individual polymeric molecules are different in the degree of polymerization, i. e. in
molecular mass, as another principal difference from the low-molecular materials
having a definite molecular mass common to all molecules. A polymer can be charac-
terized by an average degree of polymerization or average molecular mass on a num-
ber or mass basis.
Atoms in both the low-molecular and macromolecular compounds are linked to-
gether by intramolecular primary chemical forces. In polymers, covalent bonds are
the most commonly occurring ones but other bond types may also occur. Intermole-
cular secondary forces such as dispersion forces, polarization effects, and hydrogen
bonds may also act in a plastic material.
The formation or decomposition of a primary chemical bond requires or releases
a definite energy which is nearly independent on the size of the molecule. In
consequence, the heats of combustion per gram of a polymer and a low-molecular
compound of identical composition are very similar though not necessarily equal.
The secondary forces, on the other hand, are additive according to the size of the
molecule, their effects increasing with the molecular mass. This is the reason for
many of the important differences between the low-molecular and the polymeric
materials.
For example molecules of methane, propane, butane and other «-alkanes as well as
polyethylene can be symbolized as
CH -[CH ] -H
3 2 m
where m = 0 for methane. Methane is a gas under ambient conditions. It can be liq-
uefied at — 162°C due to some weak intermolecular secondary forces. Ethane,
propane, and butane (where m = 1 to 3) are also gases with successively increasing
boiling points; the latter two compounds can be liquefied even at room temperature
when sufficient pressure is applied. The further members of this series from pentane
(m = 4) to hexadecane (m = 15) are liquids at room temperature while the higher
alkanes are solids. The industrial product "paraffin wax" is a solid mixture of such
higher alkanes (m > 16) with low mechanical strength (scratched by finger-nail)
at room temperature; it becomes plastic at a little higher temperature then melts
completely soon.
The much higher strength of polyethylene (m > 1000) can be attributed to the
summation of the secondary forces among the macromolecules. For the same reason,
it becomes plastic at a higher temperature than the paraffin. At more elevated temper-
3
atures, polyethylene is liquefied but its melt viscosity is much higher than that of the
paraffin. Thus, molten polyethylene can only be processed by special machinery
(extruders, injection moulding machines, etc.) at a high pressure.
A polymer consisting of a single kind of monomer is called a homopolymer. A
polymer manufactured from more than one kind of monomers is a copolymer. The
most frequent copolymerization processes use two monomers (e. g. copolymerization
of vinyl chloride with vinyl acetate, styrene with butadiene, etc.). If a polymer con-
tains three different monomers, it may be called a terpolymer. The most common
terpolymer is ABS consisting of acrylonitrile, butadiene, and styrene.
Molecules of high polymers can consist of chain structures (linear or branched) or
are cross-linked. In linear chains, the monomer units are linked consecutively by
primary chemical forces. In branched chains, shorter or longer side chains are bonded
also by primary chemical connections. Both linear and branched-chain polymers can
be molten and frozen repeatedly by alternating heating and cooling processes. They
are soluble in some organic solvents.
Cross-linked plastics have the essential ingredient in chain-molecular form (which
is not necessarily a high-polymer, it may be an oligomeric or even a monomeric
compound) before final processing. In the process of manufacturing the end-products,
primary chemical bonds are formed in all spatial directions. These cross-linked poly-
mers cannot be molten, they remain solid up to the decomposition temperature.
They may be swollen but not dissolved by solvents. Since the atoms or units are linked
together continuously in all directions by primary chemical forces, such notions as
molecule, molecular mass, degree of polymerization can no longer be strictly defined.
Most of the plastics contain as an essential ingredient a single homopolymer or
copolymer. There are, however, industrial plastics comprising two or more kinds of
macromolecular compounds. For instance, the impact strength of rigid PVC is gener-
ally enhanced by incorporation of another polymeric material.
Oligomers are intermediates between the low-molecular materials and the high-
polymers. Their degree of polymerization may be 2 (dimer), 3 (trimer), or more up to
100. Some of the plasticizers for PVC are also oligomers even though they are called
polymeric plasticizers.
The industrial plastics may contain other materials besides the high-polymers.
Even the polymer itself may comprise some traces of monomer and oligomer, some
residues of the polymerization catalysts and other auxiliary materials of the polymer-
ization. In order to modify the processability of the plastics and the properties of the
end-products, various additives, plasticizers, reinforcing materials, and fillers may be
added to the polymer. Additives such as antioxidants, stabilizers, lubricants, pigments,
flame retardants, etc. are incorporated at the level of a few per cent of the polymer,
while the proportion of plasticizers, reinforcements and fillers may be much higher.
For example, the most important application of plasticizers is in the manufacture of
flexible PVC, where the mass ratio of plasticizer to PVC may reach 40 : 60. In most
cases additives and plasticizers are organic materials. Fillers and reinforcing mate-
rials are either inorganics (chalk, talc, glass fibre, etc.) or organics (wood-flour, cel-
lulose, man-made fibres, etc.).
The original definition of plastics includes several kinds of industrial products such
as rubbers, man-made fibres, synthetic resins used in paints, lacquers, and adhesives,
4
etc. In some technological respects, all these materials are considered as plastics. In
statistical data, however, plastics and synthetic resins are treated together while
rubbers and man-made fibres are classified into two separate groups.
1.2 Classification of plastics
Plastics are divided initially into two classes in terms of their production method :
synthetic and natural plastics.
— Synthetic plastics containing one or more high-polymers as essential ingredients
are produced by
— chain-propagation polymerization,
— condensation polymerization, or
— stepwise polymerization.
— Natural plastics contain as an essential ingredient a high-polymer prepared by
modification of a natural substance (e. g. cellulose).
Another classification is based on the mechanical behaviour of the material as a
function of temperature. This feature is associated with the molecular structure of the
polymeric ingredient.
— Thermoplastics can be repeatedly softened by heating and then hardened by
cooling. In the softened state, they can be shaped by flow into articles by injection
moulding, extrusion, calandering, or in other ways. In the heating and cooling proces-
ses, mainly physical changes take place. The essential ingredients of thermoplastics
are chain-molecular high-polymers. Examples are polyethylenes, PVC, and poly-
styrene.
— Thermosets, when cured by heat, transform into substantially infusible and in-
soluble products through reactions with chemical additives induced by curing agents,
irradiation, or in other ways. Molecules of the essential ingredients of thermosets are
linked by primary chemical bonds forming a dense cross-linked structure in the curing
process. The cross-linked end-products are manufactured from the uncured material
by compression moulding, transfer moulding, injection moulding, laminating, etc.
Some examples of thermosets are phenolic plastics, amino plastics, unsaturated poly-
esters, epoxy plastics, and most of the polyurethanes.
— Elastomers are macromolecular materials which, within a certain temperature
range including the ambient temperature, return rapidly to their initial size and
shape after a substantial deformation by application and release of a relatively weak
stress. A characteristic representative of elastomers is the natural rubber. Its essential
ingredient is polyisoprene, a chain-molecular high-polymer. In the original state, it
is a thermoplastic which is soluble in some solvents such as in benzene and is capable
of repeated softening and hardening. If, however, a few per cent of sulphur is incor-
porated along with some chemical additives, the sulphur forms cross-links between
the macromolecules during a subsequent heat treatment, producing a sparse cross-
linked structure. This vulcanized rubber holds the original elasticity but it becomes
insoluble in organic solvents and it loses its thermoplasticity.
Many kinds of synthetic rubbers, such as styrene/butadiene rubber or chloroprene
rubber, are also vulcanized for manufacturing the end-products.
5