1
For the purpose of this lesson, polyurethanes are
defined as polymers which contain urethane groups
(-NH-CO-O-)
in the main chain. However, it is to be noted that in
technologically useful polymers of this type the
urethane group is not usually the principal group
present; other groups such as ester, ether, amine and
urea groups are generally contained in the chain in
appreciable number.
The commercial development of polyurethanes dated
from 1937 when O. Bayer found that reaction of
diisocyanates and glycols gave polyurethanes with
properties which made them of interest as plastic
and fibres. Further intensive research into
polyurethanes soon indicated that the materials
showed promise as adhesives rigid foams and surface
coating. These various applications reached
moderate commercial importance in Germany
during the Second World War, but were not
recognized elsewhere at this time. After the war,
Allied intelligence teams reported on these activities
and interest in polyurethanes was established in the
2
U.K. and U.S.A. although commercial development
was slow. The German chemical industry quickly
recovered from the effects of war; Bayer again
become active in the polyurethane field and
developed elastomers (1950) and flexible foams
(1952). In the period 1952- 54 this company
developed its diisocyanate- polyester flexible foam
system to a degree for commercial use and also
introduced novel machines for continuous
production of the foams. By 1955 the mass –
production of polyester –based foams was
established in the most industrial countries.
The urethane group results from the interaction of
an isocyanate and a hydroxyl compound.
R-NCO + HO-R’
R-NH-CO-O-R’
It will be apparent that this reaction leads to
polyurethanes when multifunctional reactants are
used. When a diisocyanate and a diol react together
a linear polyurethane is obtained whilst a
diisocyanate and a polyhydric compound(polyol)
lead to a cross-linked polymer. A cross-linked
polyurethane could also be derived from a
compound containing three or more isocyanate
groups and a diol but this approach is of limited
commercial importance.
3
Thus diisocyanates and diols and polyols are the
principal raw materials used in the manufacture of
polyurethanes. The more importance of these
reactants are given below.
Several reactions are known by which isocyanetes
are formed. However, there is only one method of
preparation of commertial importance, namely
phosgenation of primary amines
R’-NH2 + COCl2
R – N = C = O + 2 HCl
Toluene is the starting material for the production of
tolylene diisocyanate (TDI) and production process
may varied to give products of different isomer
contents end of the nitration;
4
5
Diphenylmethane diisocyanate (MDI) is derived
from aniline, the principal reaction involved are as
follows :
Naphthylene 1,5-diisocyanate (NDI) is prepared
from naphthalene as follows :
Hexamethylene diisocyanate (HMI) is prepared by
the phosgenation of hexamethylenediamine
6
COCl2
H2N-(CH2)6-NH2
OCN-(CH2)6-NCO
The earliest polyurethanes were based on aliphatic
diols (glycols) and, of the various glycols
investigated, 1,4 butandiol was generally preferred
for commercial operations. Since this time, however
production of polyurethanes has mainly involved
polymeric hydroxyl compounds. The use of these
materials permits the manufacture of a much wider
range products at relatively low cost. The polymeric
hydroxyl compounds which have received most
attention are polyesters and polyethers.
1,4 butandiol is prepared from acetylene by
following route
At one time this glycol was used to prepare fibre
forming polyurethanes but its use in now limited
mainly to molding materials and cast elastomers.
7
The polyesters used in the preparation of
polyurethanes generally have molecular weight
in the range 1000 -2000 and are liquids or low –
melting solids ; they are usually saturated. The
essential feature of these polyesters is that they
are hydroxyl –terminated and thus hydroxyl
groups are available to participate in the
urethane reaction.
Hydroxyl–terminated polyethers have now
assumed a dominant role in the commertial
production of polyurethanes the most widely
used polyethers are derivatives of propylene
oxide.
A polyester : Poly(ethylene terephthalate)PET
HO-CH2-CH2-O-[(O-C-Ph-CO-O-CH2-CH2-O)-H
A polyether : Poly(ethylene oxide )PEO
HO-[-CH2-CH2-O-]-CH2-CH2-OH
8
Isocyanates are very reactive materials and undergo
a great many reactions. In this section, those
reactions which have technological significance are
discussed. The common reactions of isocyanates may
be divided into two classes, namely addition
reactions with compounds containing active
hydrogen and self addition.
The more important reactions involving active
hydrogen compounds are shown below
9
R-NCO + R’-OH
R-NH-CO-O-R’
a Urethane
R-NCO + R’-NH2
R-NH-CO-NH-R’
a urea
R-NCO + R’-COOH
Reaction(v)
Reaction (vi)
[R-NH-CO-O-CO-R’]
Anhydride
-CO2
R-NH-CO-R’
amine
R-NCO + H2O
Reaction (vii)
[R-NH-COOH]
a carbamic acid
-CO2
R-NH2
Reaction (ii)
R’
R-NCO + R’-NH-CO-O-R’’
R-NH-CO-N-CO-O-R’’
allophanate
10
R’
R-NCO + R’-NH-CO-NH-R’
R-NH-CO-N-CO-NH-R’’
biuret
R’
R-NCO + R’-NH-CO-R’’
R-NCO + HO-Ph
R-NH-CO-N-CO-R’’
acylurea
R-NH-CO-O-Ph
a urethane
11
Polyurethane foams are produced by forming a
Polyurethane polymer concurrently with a gas
evolution process. Provided these two processes
are balanced bubbles of gas are trapped in the
polymer matrix as it is formed and a cellular
product results.
The gas used in the production of flexible foams is
usually carbondioxide formed by the interaction of
isocyanate and water. In the system containing a
diisocyanate, a polyol and water, two principal
reaction proceed simultaneously, namely :
Diisocyanate + polyol
polyurethane
Diisocyanate + water
carbondioxide
As indicated above, a satisfactory foam is obtained
only if these two reaction are in step
Polyether foams
Flexible polyether foams are most commonly produced
by a “one-shot” process In this procedure,
diisocyanate, polyol, water, catalysts and surfactant
are all mixed simultaneously. After 1-2 minutes the
foam reaches its maximum height.
12
A typical formulation for flexible polyether foam
would be as follows;
Polyether triol
80:20 Tolylene diisocyanate
Water
Triethylenediamine
Stannous octoate
Silicone block copolymer
: 100 parts by weight
: 40
: 3.0
: 0.5
: 0.3
: 1.0
Polyester foams
Flexible polyester foams are nearly always prepared
by a one-shot process; most commonly they are
produced as slabstock by a continuous method exactly
comparable to that described previously for polyether
foam.
A typical formulation for flexible polyester foam would
be as follows;
Polyester(slightly branched)
65:35 Tolylene diisocyanate
Water
N-methylmorpholine
N,N Dimethylcetylamine
Ammonium oleate
: 100 parts by weight
: 33
: 4.0
: 2.0
: 2.0
: 2.0(surfactant)
13
Flexible polyurethane foams are open-cell structure
which are usally produced with densities in the range
1.5-3 lb/ft3 Typical load deflection curves for polyether
and polyester foams are shown below. Compared to
polyether foams, polyester foams have higher tensile
strength, elongation at break and hardness;
consequently polyester foams are preferred in such
applications as textile laminates and coat shoulder
pads
Polyurethane foams are resistant to a wide range of
solvents. In this respect polyester foams are generally
superior to polyether foams, particularly in resistance
to dry cleaning solvents; in this further reason for
preferring polyester foams in textile applications.
14
Polyurethane foams are subject to degradation by
aqueous acids and alkalis and steam. Ester, amine and
urethane groups represent sites for hydrolytic attack.
Since the ether groups not ready attacked polyether
foams are generally more resistant to hydrolysis than
polyester foams.
The principle underlying the production of rigid
polyurethane foams is fundamentally the same as that
used for flexible foams, namely a reacting
isocyanate/polyol mixture is simultaneously expanded
by gas generation.
The essential difference between the two products lies
in their degree of cross-linking; whereas flexible
foams are lightly cross-linked, rigid foams are highly
cross-linked. This high degree of cross-linking is
achieved by using relatively low molecular weight
polyols for coupling with the isocyanate.
A typical basic formulation for rigid polyether foam
might be as follows ;
Polyether polyol
Crude diphenylmethane
diisocyanate
Trichlorofloromethane
Triethyleneamine
Silicone block copolymer
Glycerol
: 100 parts by weight
: Stoichiometric +5%
: 50
: 0.5
: 1.0
:10(cross-linking)
15
Rigid polyurethane foams are closed-cell
structures which are usually produced with a
density of about 2 lb /ft3. Since the major interest
in rigid foams has been for thermal insulation,
the thermal conductivity of the foams is a
physical property of some importance.
The chemical properties of rigid polyurethane
foams are similar to those of flexible foams.
Solid polyurethane elastomers (as distinct from
flexible foams) may be divided into three
categories, namely
* cast,
* millable and
* thermoplastic elastomers.
Cast elastomers
In the casting technique a liquid reaction mixture
comprising low molecular weight material is
poured into a heated mould, where the material is
converted to a solid, high molecular weight
elastomeric product.
16
The reactions which occur when a glycol is used
in the above operation are not fully understood,
but it is thought that firstly the glycol react with
the isocyanate-terminated propolymer to give an
extended polymer containing glycol-urethane
links. The amount of glycol used is slightly less
than that required to react with all the isocyanate
in the pre-polymer and so the extended polymer is
isocyanete terminated; the situation may be
illustrated as follows :
17
The extended polymer is generally supposed to
cross-link through reaction of terminal
isocyanete groups with urethane groups in the
polymer chain with the formation of
ALLOPHANATE links
Cross-linking may also occur through trimerization of terminal isocyanate groups to
isocyanurate
The reaction involved when a diamine is used in
the preparation of a cast elastomer are more
apparent. The amine and pre-polymer react to
give an extended polymer containing urea groups
18
which then form BIURET cross-link by reaction
with terminal isocyanete groups.
Millable elastomers
A disadvantage of the cast elastomers described
in the previous section is that the isocyanateterminated pre-polymer have limited shelf-life
and must be stored in the absence of moisture.
Further the practical procedures involved are
very different from those generally associated
with synthetic rubbers. These factors led to the
development of polyurethane elastomers to which
the conventional techniques of mill compounding
and vulcanization can be applied.
19
In this approach, stable (hydroxyl- terminated)
polymers are prepared by the reaction of lineer
polyesters(commonly adipates) or polyesters
(commonly poly(oxytetramethylene) glycol) with
diisocyanete. These polymers are rubber like
gums which may be compounded on the two-roll
mills with other ingradients, sulphur systems and
peroxides are the most widely used.
Reaction of the urea and amine groups with a
diisocyanete leads to the formation of BIURET
and ACYLUREA cross-link respectively
Thermoplastic Elastomers
Thermoplastic Elastomers are materials which
process,
at
normal
temperatures
the
characteristics resilience and recovery from
extension of cross-linked elastomers but which
exhibit plastic flow at elevated temperatures and
can be fabricated by the usual techniques applied
to thermoplastics, e.g. injection moulding and
extrusion.
These effects are associated with linear polymers
containing segments which give rise to interchain secondary valency forces of polar
attractions. At normal temperature, these
interactions have the effect of conventional
covalent cross-links but a elevated temperatures
20
the secondary bonds dissociate and the polymer
exhibits thermoplastic behavior.
Surface coating
Several polyurethane-type products are utilized in
coating applications by
Air curing System
Moisture-curing system
Head Curing system
Adhesives
Several polyurethane-type products are used as
adhesives. These general categories may be
distinguished namely isocyanete-polyol systems,
soluble elastomers and polyisocyanetes
The adhesive properties of the foregoing
materials may be attributed to the polar nature of
the polymers involved. Further, the isocyanates
present in the compositions may react with any
active hydrogen present in the substrate or with
the films of water which are often present on the
surface of such materials as ceramics, glass and
metals.