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201
diphenylmethane di-isocyanates. These foams can be produced without
difficulty using one-shot techniques either on large factory-installed
machines of the Henecke type or alternatively on small portable equipment.
In most systems the reaction is rather slower than with the flexible foam
and conditions of manufacture rather less critical. In the United States
prepolymer and quasi-prepolymer systems corresponding to those
discussed under flexible foam were developed, largely to reduce the
hazards involved in handling TDI on portable equipment in places where
there were severe ventilation problems.
As with the flexible foams there has been a shift to the use of
polyethers. These are largely adducts based either on trifunctional hydroxy
compounds, on tetrafunctional materials such as pentaerythritol or a
hexafunctional material such as sorbitol. Ethylene diamine and, it is
understood, domestic sugar are also employed. Where trifunctional
materials are used these are of lower molecular wei ght (~500) than with the
polyethers for flexible foams in order to reduce the distance between
hydroxyl groups and hence increase the degree of cross-linking.
In the 1990s novel polyols included polyether-esters, which
provided good prerequisites for flame retardancy in rigid foams and
polyether carbonates with improved hydrolysis stability.
Formulations for one-shot polyether systems are similar to those
used for flexible foams and contain polyether, isocyanate, catalyst,
surfactant and water. Trichloroethyl phosphate is also often used as a flame
retardant. As with polyesters, diphenylmethane di-isocyanate is usually
preferred to TDI because of its lower volatility. Tertiary amines and
organo-tin catalysts are used as with the flexible foams but not necessarily
in combination. Silicone oil surfactants are again found to be good foam
stabilizers. Volatile liquids such as trichlorofluoro-methane have been
widely used as supplementary blowing agents and give products of low
density and of very low thermal conductivity.
Halocarbons have the further advantage of reducing the viscosity of
the reaction mixture and, where used as the main blowing agent instead of
the carbon dioxide produced by the isocyanate-water reaction, cheaper
foams are obtained since less isocyanate is used.
While melamine is widely used in flexible foams as a fire-retardant,
trichlorphenyl phosphate has been the preferred agent for use in rigid
foams. However, the introduction of specifications stipulating halogen-free
additives has led to a search for alternatives such as halogen-free
phosphorus esters, red phosphorus and ammonium polyphosphate.
202
In addition to one-shot processes, quasi-prepolymer systems are
used commercially with rigid polyether foams. The quasi-prepolymer is
commonly produced using excess TDI rather than diphenylmethane diisocyanate. Since the former isocyanate is light in colour and the latter
dark, quasi-prepolymer foams are usually lighter in colour. The quasiprepolymer systems are also more tolerant to variations in processing
conditions and often less careful control of the process can be tolerated.
Products intermediate to the flexible and rigid foams may be
obtained from castor oil (a trihydroxyl molecule), synthetic triols of
moderate molecular weight and polyesters with a moderate amount of
trifunctional hydroxyl compound in the structure. Current practice,
however, is to use tipped polyols of the type used for flexible foams with
MDI. Semi-rigid foams are used for such purposes as crash pads, car
steering wheels and packaging equipment.
Although some rigid foams are used in sandwich constructions for
aircraft and building structures the major interest of rigid foams has been in
the field of thermal insulation. In such application the foams encounter
competition from polystyrene and U-F foams. With both the polystyrene
and the polyurethane foams there has been intensive development in recent
years leading to improved products of lower cost. The polystyrene foams
have the economic advantage of being made from cheaper starting
materials, can be produced successfully at lower densities (0.016 g/cm3
instead of 0.021 g/cm for polyurethane foam) and are generally less friable.
One particular advantage of polyurethanes is that they may be formed in
situ and themselves act as an adhesive to most cavity surrounds or skins. At
the present time where it is necessary only to lay a piece of foam in
position, expanded polystyrene is cheaper. Where, however, it is necessary
to bond the foam on to the skin material, such as in a sandwich
construction, the cost of the adhesives necessary with polystyrene makes a
substantial addition to the overall cost. The relative economics of the two
materials will therefore depend very much on the end use in question.
For materials of equivalent density water-bl own polyurethanes and
the hydrocarbon-blown polystyrene foams have similar thermal
conductivities. This is because the controlling factor determining the
conductivity is the nature of the gas present in the cavities. In both of the
above cases air, to all intents and purposes, normally replaces any residual
blowing gas either during manufacture or soon after. Polyurethane foams
produced using fluorocarbons have a lower thermal conductivity (0.017-
0.022 W/mK) because of the lower conductivity of the gas. (For example,
203
the thermal conductivity for air is 0.024 W/mK, for carbon dioxide (CO2)
Density range, g/cm
3
Typical application
0.016-0.027
1.2
1. In situ packaging; 2. Flower arrangements
ers, cold stores, ships, chemical plant,
properties, e.g. refrigerators; 2. Decorative
1. Decorative applications: imitation wood,
parts, TV, radio and
speaker cabinets, brush handles and gun
1. As above
0.015 W/mK and for monofluorotrichloromethane (CCl3F) 0.008 W/mK).
Except where the foam is surrounded by a skin of relatively
impermeable material, it would be expected that the blowing gas would
diffuse out and be replaced by air and that the thermal conductivities of the
foams would increase until they approached that of expanded polystyrene
of similar density. Whilst this is true of foams which generate carbon
dioxide it is found that this does not happen when fluorocarbons are used.
In this case diffusion of the fluorocarbon proceeds very slowly and it
appears that an equilibrium is eventually reached when the ratio of air to
fluorocarbon in the cell is about 1:1. For this reason fluorocarbon-blown
foams have ultimate thermal conductivities significantly lower then those
of CO2-blown foams or expanded polystyrene of similar densities.
Foam density is largely a function of the concentration of blowing
agents. There has been a strong development towards the use of less
expanded, i.e. higher density rigid cellular polyurethanes. This includes not
only the so-called structural foams for “simulated wood” but also
unexpanded solid materials used for brush handles and gun stocks. This
range is clearly indicated in Table 12.
Table 12 – Typical applications of cellular rigid polyurethanes and
polyisocyanurates
0.027-0.096
0.098-0.16
0.16-0.48
0.48-0.96
1. In situ insulation of refrigerators, deep
freez
houses, building panels; 2. Buoyancy goods; 3.
Chair shells
1. Moulded insulation using self-skinning
mouldings, e.g. wood beams
picture frames
1. Structural plastics mouldings: furniture of all
kinds, car body
loud
stocks
204
Self-skinning foams and the RIM process
For many applications it is desirable that the surface of a foam
moulding be non-porous and have a good finish. It is particularly desirable
that in these cases both the cellular core and the skin be produced in one
moulding step. This is best achieved by using a system employing a
volatile blowing agent such as chlorotrifluoromethane or methylene
dichloride rather than a “water blown” system involving the evolution of
carbon dioxide on reaction of isocyanate and water.
A critical factor is the boiling temperature of the blowing agent and
its relationship to the temperature of the walls of the mould and of the
reacting mixture. There should be sufficient exotherm to vaporise the
blowing agent in the centre of the reacting material but the mould walls
should be sufficiently cool to condense the blowing agent in the reaction
mixture close to the walls. In addition porosity near the wall can largely be
suppressed under the correct moulding conditions by the pressure exerted
internally on the skin by the vapour pressure developed in the core. Success
in operating the process clearly requires close control over the metering of
the raw materials and of mould temperatures. In respect of the latter, metal
moulds with their good conductivity are preferred to moulds from such
materials as epoxide resins.
The successful development of self-skinning foam technology is
largely due to the process originally known as reaction casting but which
has more commonly become known as reaction injection moulding (RIM)
(or the German equivalent RSG). In this process the reaction compone nts
are metered into a reaction chamber adjacent to the mould cavity, and the
reacting mixture then flows into the cavity. Mixing in the reaction chamber,
which may have a capacity in the range 0.3-4 cm3, is brought about by
injecting the components towards each other at high speed from opposite
sides of chamber so that impingement or counter-current mixing takes
place. At this stage turbulence is encouraged. Such a static impingement
system also allows precise temperature control by the ability to continually
recycle material except when it is being mixed. It is also self-cleaning and
there are few moving parts. Typical mould temperatures are in the range
40-60°C.
Advantages of the RIM process over conventional injection
moulding include:
(1) Low plant investment.
(2) Low process energy.
(3) Low clamping pressures required–thus allowing production of
very large mouldings.
205
(4) Variations in thickness without sink marks due to presence of
an internal pressure caused by the entrapped gases.
(5) Low product densities.
Disadvantages include the facts that painting of the moulding is
often necessary to obtain a good finish, and the difficulty in using any
cross-linked waste.
The RIM process was originally developed for the car industry for
the production of bumpers, front ends, rear ends, fascia panels and
instrument housings. At least one mass-produced American car has RIM
body panels. For many of these products, however, a number of injection
moulding products are competitive, including such diverse materials as
polycarbonate/PBT blends and polypropylene/EPDM blends. In the shoe
industry the RIM process has been used to make soling materials from
semi-flexible polyurethane foams.
Interest in the RIM process appears to have abated somewhat in the
1990s. Nevertheless, nearly 100000 tonnes of polyol and polyisocyanate
were consumed for this application in the USA alone in 1993.
The reinforced reaction injection moulding (RRIM) process is a
development of RIM in which reinforcing fillers such as glass fibres are
incorporated into the polymer. One advantage of such a system is to reduce
the coefficient of thermal expansion, and with a 40-50% glass fibre content
the coefficient is brought into line with those of metals.
One specific wish of the RIM technologist is the extension of the
system to produce fast-running vehicle tyres and some progress has been
made in this direction. One approach to overcome current problems, such
as a low heat distortion tempe rature and too soft a compound, has been the
development of glass, fibre-filled materials produced by the “reinforced
RIM” technique.
2.8.5 Coatings and Adhesives
A wide range of polyurethane-type products has become available
in recent years for coating applications. These include simple solutions of
linear polyurethanes, two-pot alkyd-isocyanate and polyether-isocyanate
systems and a variety of prepolymer and adduct systems. The coatings can
vary considerably in hardness and flexibility and find use mainly because
of their toughness, abrasion resistance and flexibility. Uses include metal
finishes in chemical plant, wood finishes for boats and sports equipment,
finishes for rubber goods and rain-erosion-resistant coatings for aircraft.
206
One type of coating is potentially competitive with PVC leathercloth. Both
RNCO
N
C
N
C
N
C
O
R
R
R
O
O
alkyd-di-isocyanate and adduct-di-isocyanate compositions may be coated
on to fabrics from solutions of controlled visc o sit y a nd sol i ds c ont en t. S uc h
coated fabrics are soft, flexible and, unlike PVC leathercloth, free from
plasticizers.
Many isocyanates have good adhesive properties and one of them,
triphenyl-methane-pp’p’’-triyl tri-isocyanate, has been successfully used f or
bonding of rubber. Isocyanates are, however, rather brittle and somewhat
limited in application. Somewhat tougher products are obtained from
adhesives involving both polyols and isocyanates, i.e. polyurethane-type
materials. The major application of these materials to date is in the boot and
shoe industry.
2.8.6 Polyisocyanurates
Whilst rigid closed-cell polyurethanes are excellent thermal
insulators they do suffer from a limited and often unsatisfactory level of
fire resistance, even in the presence of phosphorus-containing and halogencontaining fire retardants. Considerable promise is now being shown by the
polyisocyanurates, which are also based on isocyanate chemistry.
These materials not only have a good resistance to burning and
flame spread but are also able to withstand service temperatures of up to
150°C. At the same time polyisocyanurate foams have the very good
hydrolytic stability and low thermal conductivity associated with rigid
polyurethane foams.
The underlying reaction for polyisocyanurate formation is the
trimerization of an isocyanate under the influence of specific catalysts:
The most commonly used isocyanate is a modified form of MDI.
Such polymeric forms may be prepared, for example, by reacting phosgene
with formaldehyde-aniline condensates which have average functionalities
207
of between 2 and 7 and may be represented by the formula given in Figure
OCN
CH
2
OCN
CH
2
NCO
n
C
N
C
N
C
N
O
O
O
N
X
N
H2C
[
]
]
[
CH
2
N
X
N
H
2
C
N
X
N
[
]
26.
Figure 26
Polymeric MDIs, which are also used in polyurethane foams,
usually have a lower reactivity than the monomeric material but are also
less volatile. The polyisocyanurate produced from this material will be of
the type shown in Figure 27.
Amongst the catalysts used or the polymerization-trimerization
reactions are alkali metal phenolates, alcoholates and carboxylates and
Figure 27
compounds containing o-(dimethylaminomethyl)phenol subgroups.
Fluorocarbons such as trichloro-fluoromethanes are used as the sole
blowing agents in the absence of any isocyanate-water reaction.
208
Because of the high cross-link density of polyisocyanurates as
OCNRNCO
+
CH
2
CH
O
R
1
CH CH
2
O
RN
CO
O
CH
CH
2
R
1
CH
C
CO
CH
2
N
prepared above, the resultant foams are brittle, so that there has been a
move towards polyisocyanurate-polyurethane combinations. For example,
isocyanurate-containing polyurethane foams have been prepared by
trimerization isocyanate-tipped TDI-based prepolymers. The isocyanurate
trimerizing reaction has also been carried out in the presence of polyols of
molecular weight less than 300 to give foams by both one-shot and
prepolymer methods.
An alternative route involves the reaction of 1,2-epoxides with
isocyanates to yield poly-2-oxazolidones.
Whilst reaction can take place in the absence of catalysts it is more
common to use such materials as tetra-alkylammonium halides and tertiary
amines such as triethylenediamine. A major side reaction leads to the
production of isocyanurate rings, particularly in the presence of tertiary
amines.
The conventional polyisocyanurate may be prepared with a two-
component system using standard polyurethane foaming equipment. It is
usual to blend isocyanate and fluorocarbon to form one component whilst
the activator or activator mixture form the second component.
Typical properties of isocyanura te foam :
Density, kg/m3 38-48
Compression strength, MPa 0.14-0.28
Shear strength, MPa 0.10-0.24
Resistance to elevated
temperature distortion (DIN 53424) 200ºC
2.8.7 Polycarbodi-imide Resins
Besides trimerization, leading to the production of
polyisocyanurates, isocyanates can react with each other to form
polycarbodi-imides with the simultaneous evolution of carbon dioxide:
OCN–R–NCO + NCO–R–NCO → –(=C=N–R–N=C=C–R–N=C=)– +CO
2
When this is carried out in suitable solvents at temperatures in the
range 75-120°C, soluble products will be obtained. Polymeric MDI is
usually used as the isocyanate component and this results in a stiff chain
molecule. One such product is reported to have a Tg of 200-220°C.
209
In the absence of solvents and with suitable catalysts the evolution
of carbon dioxide simultaneously with the polycarbodi-imide formation
gives rise to a foamed product. These foams are cross-linked because of
reactions between carbodi-imide groups and free isocyanate groups. Raw
materials for such foams are now available from Bayer (Baymid).
The polymers combine a high level of flame retardancy with good
thermal insulation and sound absorption characteristics. Densities are
somewhat high (16-20 kg/m3).
Amongst applications reported are underfloor footfall sound
insulation, thermal insulation between cavity walls and pipe insulation.
2.8.8 Polyurethane-Acrylic Blends
Over the years many blends of polyurethanes with other polymers
have been prepared. One recent example is the blending of polyurethane
intermediates with methyl methacrylate monomer and some unsaturated
polyester resin. With a suitable balance of catalysts and initiators, addition
and rearrangement reactions occur simultaneously but independently to
give interpenetrating polymer networks. The use of the acrylic monomer
lowers cost and viscosity whilst blends with 20% (MMA + polyester) have
a superior impact strength.
2.8.9 Miscellaneous Isocyanate-based Materials
Because of their great versatility there continues to be a steady
stream of developments of polymers made by reaction of isocyanates. In
addition to the materials discussed in this chapter there are the polyureas,
the polyoxazolidinones and polybenzoxazinediones.
There is also growing interest in multi-phase systems in which hard
phase materials are dispersed in softer polyether diols. Such hard phase
materials include polyureas, rigid polyurethanes and urea melamine
formaldehyde condensates. Some of these materials yield high-resilience
foams with load deflection characteristics claimed to be more satisfactory
for cushioning as well as in some cases improving heat resistance and
flame retardancy.
Aqueous dispersions of polyurethanes have also become available
which may be used instead of solutions in organic solvents for such
applications as leather treatment, adhesives and surface coatings.
The polycarbamylsulphonates are water-soluble reactive bisulphite
adducts of polyisocyanates and are being investigated as possi ble materials
to render woollen fabrics crease-resistant.
210
This tutorial was written using the follo wing l iterat u re:
1. J.A.Brydson. Plastics Materials. – Butterworth Heinemann. –
Oxford. – 1999. – 954 p.
2. Manas Chanda, Salil K.Roy. Plastics Technology Handbook. –
Taylor&Francis Group. – USA. – 2007. – 816 p.
3. Kroschwitz J., Howe-Grant M. Encyclopedia of Chemical
Technology (vol.1-27). – 1998. – 4-th edition.