Polymerization Chemistry of glycol, and trimethylolpropane (TMP, 2,2-
bis(hydroxymethyl)-1-butanol).
Polyurethanes To obtain short chain linkages between isocyanate-
terminated prepolymers, diols of variable chain length
1. Raw Materials from ethylene glycol to dodecane diol (sometimes
1.1 Isocyanates termed ‘‘cross-linkers’’) are used. This applies es-
pecially to polyurethane elastomers.
Four diisocyanates account for more than 95% of Amino groups react faster with isocyanates than
annual industrial production. These are tolylene di- hydroxyl groups, whereas primary hydroxyl groups
isocyanate (TDI, a mixture of 2,4- and 2,6-isomer), usually react faster than secondary ones.
methylenediphenyl diisocyanate (MDI, in the form
of various isomer mixtures), hexamethylene-1,6-
diisocyanate (HDI), and isophorone diisocyanate, 5- 1.3 Catalysts
isocyanato-3-(isocyanatomethyl)-1,1,3-trimethylcy- Catalysts are crucial to achieve economic material
clohexane (IPDI). processing and optimized end product properties.
Others such as bis(4-isocyanatocyclohexyl)me- Both the chemical nature and the amount of catalyst
thane, 1,5-naphthalenediisocyanate, and 1,4-phenyl- used are important, and thus it requires some ex-
enediisocyanate are produced in small amounts for perience to optimize polyurethane systems effectively.
special applications. The possible partners in reactions of isocyanates
The reactivity of the isocyanate moiety depends include:
strongly on its substitution pattern and the steric $ isocyanates themselves to yield dimer (uretdione)
environment. These differences can be useful, e.g., in or trimer (isocyanurate) structures,
isophorone diisocyanate where the two isocyanate $ lihydroxyl groups yielding urethanes,
groups are so different in reactivity that two-step $ amine groups yielding ureas,
reactions take place yielding rather well-defined inter- $ urea groups yielding biuret, and
mediates and end products. The reactivity of aromatic $ urethanes yielding allophanates,
isocyanates is higher than the reactivity of aliphatic Common catalysts are tertiary amines and tin
isocyanates. compounds. Depending on the exact chemical nature
and the amount of catalyst employed, the processing
and the end properties of polyurethane materials can
1.2 Polyols and Polyamines
be fine tuned to a wide extent without changing the
Because polymers based exclusively upon polyure- reacting raw materials.
thane linkages are too brittle to be used as materials,
additional linkage types are used, the most common
1.4 Addities
ones being polyether, polyester, and polycarbonate.
There are a few applications where other linkages such Most importantly, additives include water, foam
as simple hydrocarbon chains are employed for their stabilizers (mainly of the silicon type), flameproofing
hydrophobicity, but they do not offer the advantage of agents, solid fillers, color pastes, etc.
being able to form hydrogen bonds with urethane and Water is used in nearly every polyurethane foam
urea groups. These hydrogen bonds provide strong formulation to provide carbon dioxide as a blowing
interchain interactions in the polymer that give the agent from the reaction with the isocyanate and heat.
combination of high elasticity and strength typical for The amine thus formed quickly reacts further with
polyurethane materials. more isocyanate-yielding urea groups in the polymer
The dominating chemistry used for the manufac- network.
turing of polyols and polyamines is the addition of Foam stabilizers (surfactants) determine the mor-
epoxides, especially oxirane and methyloxirane, to phology of polyurethane foams. Usually, they consist
polyfunctional starter compounds like alcohols or of a silicon backbone and polyether moieties attached
amines or mixtures thereof. The products thus ob- via Si–C or Si–O–C bonds. Depending upon the length
tained possess hydroxyl end groups, functionalities and structure of both submoieties, very different foam
usually between 2 and 8, and molecular masses in the morphologies can be obtained.
range 200 and 8000. For special applications, poly- Flameproofing agents usually contain phosphorus
(tetrahydrofuran)diols are used. and\or halogen to make polyurethanes compliant
To obtain amino end group functionalities, cata- with specific burning standard procedures as defined,
lytic processes have been developed to react ammonia for example, by ASTM or DIN depending on the
with polyether polyols. application. Melamine is also used.
In much smaller quantities than polyether polyols, Solid fillers such as glass fibers, silicates, etc. can be
polyester polyols are being produced by condensation used to improve mechanical properties, but their
of difunctional carboxylic acids with alcohols of application is confined to special uses such as reaction
functionality 2 to 3 such as ethandiol, diethylene injection molding (RIM).
1
Polymerization Chemistry of Polyurethanes
segregated morphology consisting of crystalline ‘‘hard
1.5 Auxiliary Blowing Agents
segments’’ (urethane, urea) surrounded by a larger,
Since the implementation of the Montreal Protocol, continuous, amorphous ‘‘soft segment’’ (polyether,
the use of ozone-layer damaging blowing agents such polyester, etc.) phase. Because both the amount and
as chlorotrifluoromethane (CFC 11, trichlorofluoro- the distribution of crystallinity are strongly influenced
methane) has been reduced and will soon be elimin- by kinetics, it is obvious that the chemical structure of
ated. For insulating applications, hydrocarbons such the raw materials determines physical properties only
as pentane or cyclopentane are being used in Europe, in part, while factors governing the kinetics of the
whereas in the NAFTA region, partially halogenated different polyaddition reactions, especially catalysis,
products such as trifluorochloroethane (HCFC 141 b, have their roles as well.
1-fluoro-1,1-dichloroethane) will be phased out by A very important factor is the calculated stoichi-
2002 and then be replaced either by hydrocarbons or ometry for the polymer. It is usually expressed as the
by ozone layer friendly fluorinated hydrocarbons like ratio of isocyanate to hydroxy\amino groups times
HFC 134 a (1,1,1,2-tetrafluoroethane), HFC 365 mfc 100 and is called the NCO\OH index. Most poly-
(1,1,1,3,3-pentafluorobutane), or 245fa (1,1,1,3,3- urethanes are produced with an index between 100 and
pentafluoropropane). 110 because the completion of the reaction is reached
For flexible foams, special dispensing technologies earlier compared with indices below 100. The index
have been developed to use liquid carbon dioxide as a also strongly influences all final properties so that
naturally occuring compound and efficient blowing special care has to be taken to provide accurate dosage
agent, but for reasons of technical simplicity, dichloro- of the raw materials.
methane is still being used, especially in developing
countries.
3.2 Thermoplastic Polyurethanes and Cast
Elastomers
2. Processing In terms of physical properties, thermoplastic poly-
2.1 Processing of Raw Materials urethane (TPUs) and cast elastomers do not differ
materially, so both can be dealt with simultaneously.
The dominating processing method in the poly- The reason for their widespread use lies in their
urethane industry is the use of liquid–liquid mixheads unique combination of:
where the raw materials are put in contact with each $ high elasticity within a wide hardness and tem-
other for a time ranging up to several seconds. perature range,
Depending on the miscibilities and the viscosities of $ good weatherability and solvent resistance,
the raw materials involved, different pump types and $ durability,
mixhead designs are used. The fact that the initial mix $ abrasion resistance, and
is still liquid is exploited as the expanding mixture is $ higher Young’s modulus compared to rubber of
able to flow and fill complicated mold geometries similar hardness.
using the heat and the gaseous carbon dioxide released Typical molecular masses produced are in the range
from the chemical reaction between the components. of 15 000 up to several hundred thousand. In order to
Main differences arise from the difference between secure thermoplasticity, functionality of raw materials
the needs of continuous (e.g., slabstock flexible foam) has to be kept strictly at, or slightly below, two.
vs. discontinuous (e.g., molded foam for seating) Shore hardness can be varied between approxi-
production of polyurethane parts. A wide range of mately 75 Sh A and 60 Sh D, bridging the gap between
specialized solutions is available on the market that rubber and technical thermoplastics such as poly-
can be found in detail in Oertel (1997). amides, ABS, and polycarbonates.
Highly elastic fibers for textiles can be made from a
special variety of polyurethane elastomers (see also
2.2 Processing of Thermoplastic Polyurethanes Block Copolymers : Segmented).
Processing equipment used for thermoplastic poly-
urethanes is identical to that used for standard
thermoplastics and includes injection molding, ex- 3.3 Flexible Foams
trusion, and calander machinery. Flexible foams are by far the largest application by
volume of raw materials processed. Their properties
can be summarized as follows:
3. Physical Properties $ high air permeability,
$ low compression set, and
3.1 General
$ high elasticity.
The dominating feature needed to understand the The density range obtainable on modern machines
physical properties of polyurethanes is the phase- reaches down to approximately 8 kg m−$. The usual
2
Polymerization Chemistry of Polyurethanes
range is 15–60 kg m−$, with a whole range of hardnesses
3.6 Coatings, Sealants, and Adhesies
(measured as force needed to compress the foam to
60% of its original height) available. These applications are minor with regard to annual
Main applications are furniture upholstery and consumption and the amounts of polyurethanes in-
bedding. volved are small compared to the parts on which they
are used. Thus, the polyurethane raw-material cost per
kg is not as decisive as in higher-volume polyurethane
3.4 Rigid Foams applications, and a large proportion of the more
expensive aliphatic isocyanates is used here, for
Rigid polyurethane foams are mainly used for effective example, for high-performance coatings. The same
thermal insulation. They are characterized by their applies to the polyol side, where the range of raw
closed-cell structure which is needed to contain the materials is far too wide to be described here.
insulating gaseous compositions within the foam cells Further details concerning applications and raw
and which necessitate a minimum strength of the materials involved can be found in Oertel (1997),
polymer matrix to withstand external pressure without Saunders and Frisch 1963, and Thermoset Adhesies).
shrinkage. The lowest densities are around 30 kg m−$.
The thermal conductivity is the lowest obtainable with
polymeric materials and can be as low as 17 mW m−"
K−" at room temperature. Bibliography
Main application areas include construction and The academic interest in polyurethanes has been considerably
appliances for cooling. weaker than for most other polymers, probably because the
fundamentals were developed in chemical industry laboratories
and because of the difficulty in formulating precise chemical
structures even for many raw materials. Developments in
3.5 Integral Foams polyurethane chemistry have always been best followed via the
By taking advantage of the temperature gradient in patent literature. As a consequence, the selection of literature
molds, foams with an outside skin and a foam contains just two monographs as references where references to
structure on the inside can be produced. These so- noteworthy patents can be found.
called integral foams have the advantage of low density Oertel G (ed.) 1997 Polyurethane Handbook. Hanser, Munich
combined with a smooth surface. They can be pro- Saunders J H, Frisch K C 1963 Polyurethanes: Chemistry and
duced as flexible foams (e.g., automobile seats) or rigid Technology. Interscience, New York
foams (e.g., ski cores). The ability to fill complicated
molds without defects is crucial. B. Klesczewski
Copyright ' 2001 Elsevier Science Ltd.
All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted
in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or
otherwise, without permission in writing from the publishers.
Encyclopedia of Materials : Science and Technology
ISBN: 0-08-0431526
pp. 7632–7635
