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191
The bulk of flexible foam is produced in block (slab stock) form
using machines of the Henecke type or some simple modification of it. In
this machine polyester and isocyanate are fed to a mixing head which
oscillates in a horizontal plane. The other ingredients, known as the
“activator mixture”, are then injected or bled into the isocyanate-polyester
blend a n d t he w h ol e mixture i s vi go r o u sly stir re d and forc e d o ut of the bas e
of the mixing head. The emergent reacting mixture runs into a trough which
is moving backwards at right angles to the direction of traverse of the
reciprocating head. In this way the whole of the trough is evenly covered
with the reacting mass, which has frequently foamed within a minute or so
of issuing from the mixing head. The principle of the Henecke machine is
illustrated in Figure 25.
Figure 25 – Principle of the Henecke machine (Farbenfabrik Bayer)
Because of the drag effect of the side-walls of the trough on the
expanding and cross-linking foam the process as described above gives a
domed block. Hence when the block is sliced up into sheet and slab there is
an undesirable level of scrap. To some extent the fraction of scrap is
reduced by increasing the block size. Over the years block sizes have been
increased and widths of 2.20m and heights of 1.2m are produced although
this is more common with polyether rather than polyester foams.
Much effort has been expended to try and produce flat-top foams.
In one process polyethylene sheets placed along the side-walls of the trough
rise with the foam. In another technique the reactants are metered from the
192
mixing head into a fixed trough in which partial expansion takes place. The
foaming material is then drawn over a weir by a mo ving band of paper and
then drawn down a slope so that the top surface maintains a constant level
as the material expands.
In another variation of the process, the foaming mix is fed to the
bottom of a cylinder and the foaming mixture is pushed upwards (the
Vertifoam process). The mass of material above the reacting f oam can be
used to control density, whilst in addition volatiles and gases find it more
difficult to escape from the system. The solidified cylinder of foam may
then be sliced horizontally into large discs.
Both the Henecke process and the variations described above are
today widely used in conjunction with polyether polyols.
Foam may be made from such polycaprolactones by reaction with
poly-isocyanates in the presence of tin catalysts.
Polyether prepolymers
Flexible polyester foams are not altogether satisfactory for
upholstery applications and in the 1950s the attention of American chemists
turned to the use of polyethers. These materials could be obtained more
cheaply than the polyesters but the products were less reactive and with the
catalyst systems then available could not be directly converted into foams
by a one-shot process. As a result a prepolymer technique, reminiscent of
that used with Vulkollan and which had already been used with certain
polyesters, was developed.
In this process the polyether is reacted with an excess of isocyanate
to give an isocyanate-terminated prepolymer which is reasonably stable if
kept in sealed tins in dry conditions. If water, catalysts and other
ingredients are added to the product a foam will result. Where linear
polyethers are used it is found that this foam has rather poor load-bearing
and cushioning properties and where this is important a low molecular
weight triol, such as glycerol or trihydroxy-methylpropane, is added to the
polyether before reaction with isocyanate. This will then provide a site for
chain branching. Alternatively a small amount of water could be added to
the system. This would react with terminal isocyanate groups, which link
up to produce an urea link. This urea group is more reactive than an
urethane link and reacts with isocyanates to give a biuret link as a si te for
chain branching. It is important that carbon dioxide evolved in the
isocyanate-water reaction be allowed to escape and also that the reaction is
kept down so that premature foaming does not occur.
193
Although prepolymer processes have become less important with
the advent of the one-shot process they have certain advantages. Because
there is less exotherm large blocks of foam can often be produced, there is
often a greater flexibility in design of compounds, the reduced amount of
free isocyanate reduces handling hazards and there is some evidence that
two-stage foams have slightly better cushioning properties. On the other
hand prepolymers have limited stability, are often rather viscous to handle,
and do involve an extra process.
Quasi-prepolymer polyether foams
This process, which is intermediate between the prepolymer and
one-shot process, is useful where prepolymers are too vi scous, where the
resin does not easily adapt itself to one-shot processes and where the
equipment available is more suitable for two-part systems. In principle a
polyol is reacted with a large excess of isocyanate so that the prepolymer
formed is of low molecular weight and there are a large number of free
isocyanate groups. This product is then reacted at the time of foaming with
additional hydroxyl compound, water and catalyst to produce the foam. The
additional hydroxyl compound may be a polyol or a simple molecule such
as ethylene glycol or glycerol which has the additional function of a
viscosity depressant. The system has the advantage of flexibility and of
having low-viscosity components, but as with one-shot foams there are
problems with high exotherms and with a high free isocyanate content.
Quasi-prepolymer systems (also known as semi-prepolymer systems) are
based on both polyesters, and polyethers are of interest in shoe soling; the
former are most wear resistant and the latter the easiest to process.
Polyether one-shot foams
The one-shot polyethers now form the bulk of the flexible
polyurethane foam now being manufactured. This is a result of the
favourable economics of polyethers, particularly when reacted in a one-shot
process, and because the polyethers generally produce foams of better
cushioning characteristics. A typical formulation for producing a one-shot
polyether foam will comprise polyol, isocyanate, catalyst, surfactant and
blowing agent.
A variety of polyethers have been used and may be enumerated in
their order of developm ent as follows:
(1) Polymers of tetrahydrofuran introduced by Du Pont as Teracol
in 1955:
194
O
HO [(CH2)4O]nH
CH
3
CHOHCH
2
O
(CH
2
CH
CH
3
O)
n
CH
2
CH
OH
CH
3
These polyethers produced good foams but were rather expensive.
(2) Polymers of ethylene oxide, cheaper than the tetrahydrofuran
polymers, were found to be too hydrophilic for successful use.
(3) Propylene oxide polymers are less hydrophilic and also lower in
cost and may be prepared by polymerizing the oxide in the presence of
propylene glycol as an initiator and a caustic catalyst at about 160°C. They
have the general structure:
The secondary hydroxyl groups of these poly(oxypropylene) glycol
diols are less reactive than the primary hydroxyl groups of the earlier
polyesters. At the time of the introduction of these polyethers, the catalysts
then available were insufficiently powerful for one-shot processes to be
practical and so these polymers have been used primarily in prepolymer
processes.
(4) Block copolymers of ethylene oxide and propylene oxide, less
hydrophilic than poly(oxyethylene) glycol and more reactive than the
propylene oxide polymers, were introduced by Wyandotte Chemical (USA)
under the trade name Pluronic.
(5) Today most polyether polyols are based on propylene oxide,
usually in conjunction with 10-15% of ethylene oxide. Reaction is typically
carried out at about 100°C at 2-3 atm. pressure using KOH as a catalyst. It
is desirable that the polyether is branched and of a moderate molecular
weight so that there is a level of cross-link density more typical of an
elastomer rather than a rigid thermosetting plastic. The use of such
branched polymers also confers better load-bearing characteristics for the
foam as compared to foam made from unbranched polyol. Branching is
brought about by initiating the reaction with a trifunctional material such as
trihydroxymethylpropane, hexane-1,2,6-triol or, most commonly, glycerol.
Where only propylene oxide is used the r esultant polymers will be
of the following general type:
HO•(C3H6O)n•CH2•CH(OH)•CH2(C3H6O)n•OH
Molecular weights are usually in the range 3000-6000.
For the bulk of domestic upholstery applications the polyol used is
195
made by co-feeding propylene oxide with the minor component of ethylene
+
OCNRNCO
+
H2N
NH
2
NH
2
H2N
NH
NH C
O
NH
R NH C
O
NH NH
oxide. On statistical considerations, the bulk of the end groups will derive
from the propylene oxide and thus be secondary hydroxyls. Higher
reactivity may be achieved by the process known as tipping to gi ve tipped
polyols. In this process a propylene oxide homopolymer is grafted with a
short block of ethylene oxide units to form a block copolymer which will
have primary hydroxyl end groups. Such tipped polyols tend to be preferred
for higher quality applications such as automotive moulding trim.
(6) There is an increasing market for higher resilience foams using
the so-called polymer polyols. Amongst the earliest to become established
were suspensions of styrene-acrylonitrile copolymer in the polyol. A
variation involved some grafting of SAN, either instead of or in addition to
the use of a suspension.
In the 1990s this approach became more common in order to ensure
sufficient compressive strength with the trend to lower bulk densities.
Furthermore the proportion of SAN to polyol has been increased to about
40%. This may lead to serious stability problems and care must be taken to
control the size and distribution of the particles and prevent agglomeration.
Polymer polyols using polystyrene as the polymer component have recently
become available (Postech-Shell) and are claimed to exhibit good stability,
low viscosity and less discolouration as well as providing price advantages.
In a further variation developed by Bayer, hydrazine (NH2NH2) is
dissolved in the polyol and then allowed to react during the foaming stage
with some of the 80/20 TDI present. This leads to a polyurea of general
form:
This remains as a fine dispersion in the foam.
In a yet further variation of the process developed by Shell,
diethanol-amine (HOCH2CH2NHCH2CH2OH) is used instead of hydrazine
and this leads to what is referred to as a polyurethane/polyurea supension.
The second largest component of a foam formulation is the
isocyanate. 80:20 TDI is found to be the most suitable of the various
isocyanates available and was, for many years, used almost exclusively.
In recent years there has been some substitution of TDI by MDI
derivati ves. One-shot polyether processes became feasible with the advent
of sufficiently powerful catalysts. For many years tertiary amines had been
196
used with both polyesters and the newer polyethers. Examples included
alkyl morpholines and triethylamine. Catalysts such as triethylenediamine
(“Dabco”) and 4-dimethylaminopyridine were rather more powerful but not
satisfactory on their own. In the late 1950s organo-tin catalysts such as
dibutyl tin dilaurate and stannous octoate were found to be powerful
catalysts for the chain extension reactions. It was found that by use of
varying combinations of a tin catayst with a tertiary amine (which catalyse
both the gas evolution and chain extension reaction) it was possible to
produce highly active systems in which foaming and cross-linking
reactions could be properly balanced. Although stannous octoate is more
susceptible to hydrolysis and oxidation than dibutyl tin dilaurate it does not
cause such rapid aging of the foam, a problem with organometallic
catalysts, and thus it is somewhat more popular.
During the 1990s concern increased about the odour and volatility
of amino catalysts, particularly in enclosed spaces such as automobiles.
Odourless low volatility (and hence low-fogging) catalysts based on saltlike or ionic carboxylates containing active amine centres became
available. Another approach was to incorporate amine groups into the
polymer to provide a built-in rather than a free-standing catalyst.
Surface active agents are important components of foam
formulations. They decrease the surface tension of the system and facilitate
the dispersion of water in the hydrophobic resin. In addition they can aid
nucleation, stabilize the foam and control cell structure. A wide range of
such agents, both ionic and non-ionic, has been used at various times but
the success of the one-shot process has been due in no small measure to the
development of the water-soluble polyether siloxanes. These are either
block or graft copolymers of a polydimethylsiloxane with a polyalkylene
oxide (the latter usually an ethylene oxide-propylene oxide copolymer).
Since these materials are susceptible to hydrolysis they should be used
within a few days of mixing with water.
The water present reacts with isocyanate to produce carbon dioxide
and urea bridges. The more the water present (together with a
corresponding additional amount of isocyanate) the more the gas evolved
and the more the number of active urea points for cross-linking. Thus the
foams of lower density do not necessarily have inferior load-bearing
characteristics. When soft foams are required a volatile liquid such as
fluorotrichloromethane may be incorporated. This will volatilize during the
exothermic reaction and will increase the total gas present but not increase
the degree of cross-linking.
197
The use of CFCs such as fluorotrichloromethane became quite
widespread, particularly as for many years the material was believed to
cause few toxic and environmental problems. However, evidence that such
materials were damaging the ozone layer became substantial and the use of
such materials is to be discouraged and is illegal in many countries. To
some extent CFCs have been substituted by methylene chloride (also illegal
in some countries) and other fluoro compounds, but these too may prove to
be environmentally unacceptable. For this reason there has been increased
dependence on the use of the isocyanate-water reaction to generate
sufficient carbon dioxide to give products of the required density.
In some cases it may be desired to increase the cross-link density
and hence the rigidity independently of the isocyanate-water reaction.
Compounds such as glycerol, pentaerythritol and various amines have been
employed as additional cross-linking agents.
Formulations should be based on stoichiometric considerations.
Based on a knowledge of the hydroxyl value of the polyol the amount of
isocyanate necessary to cause chain growth should be calculated. The gas
evolved will depend on the water content and additional isocyanate must be
incorporated corresponding to the water present. When the isocyanate used
equals the theoretical amount the system is said to have a TDI index of 100.
In practice a slight excess of isocyanate is used (TDI index 105-110) to
ensure complete reaction and to make available some free isocyanate for
the biuret and allophanate reactions. A typical formulation would be
Polyether triol 100
80:20 TDI 40
Water 3
Triethylenediamine 0.5
Stannous octoate 0.3
Silicone block copolymer 1.0
Commercial formulations may also include other additives.
Prominent amongst these are anti-aging additives (including tetravalent tin
compounds, mercaptans and organic phosphites), fillers, colorants and cell
regulators. In the last class may be mentioned solvents such as
dimethylformamide which lead to reticulated foams with no cell
membranes and agents such as lecithin and water-soluble silicone oils
which can lead to cell structures resembling those of natural sponges.
The use of flame retardants has become increasingly important.
They were originally primarily of concern for institutional bedding, but the
increased number of domestic fatalities due to fires has led to mandatory
198
use of fire retardant in flexible foams in a number of countries. In this
connection it is to be noted that a substantial proportion of fatalities
involving fires with polyurethane foams was due to inhalation of toxic
substances arising from burning of the polymer rather than through
individuals being burnt to death. In the late 1980s melamine became the
preferred fire retardant, being used at levels of 10-30 pts per 100 pts polyol
for domestic applications but at levels up to 100 pphp polyol for
institutional applications such as hospitals, nursing homes and aircraft. It
may be used with 5-10 pphp polyol of a liquid fire retardant such as,
preferably, ammonium polyphosphate trichlorethyl phosphate or
trichlorpropyl phosphate. Organobromo compounds are also sometimes
used but these can introduce toxic hazards.
For the most rigorous specifications it may be necessary to use
expanded graphite as a flame-retarder but its use can pose other difficulties.
Most foam is produced on machines based on the Henecke process
but in many cases it is necessary t o have at least four streams to the mixing
head; e.g. polyol and fluorocarbon (if any); isocyanate; water, amine,
silicone; and tin catalyst. Reaction is carried out with slightly warmed
components and foaming is generally complete within a minute of the
mixture emerging from the head. Although slab stock flexible foam
remains the largest single outlet for polyurethane materials, directly
moulded foam now claims some 30% of the market. Such direct moul ding
may be carried out for the following reasons:
(1) Where it is required to use metal or other inserts for fastening
of upholstery elements or coverings.
(2) Where the shape of the product is complex and it is difficult to
cut this readily from slab stock.
(3) Where it is uneconomic, because of scrap, to cut from
slabstock.
Such conditions are particularly prevalent in the car industry where
moulded foam is used for chair backs, chair seats, head restraints and knee
strips. The furniture industry also widely uses moulded products.
There are also two variants of the direct foam moulding process:
the so called hot moulding process and the cold moulding process.
For hot moulding somewhat more reactive polyethers with a higher
proportion of primary hydroxyl groups are used than for slab stock foams.
These are then reacted with TDI and most of the foaming is brought about
by the isocyanate-water reaction (they are said to be “water blown”) rather
than by fluorocarbons although these may be used as supplementary
blowing agents.
199
Cold-curing foams use polyethers of somewhat higher molecular
weight (~4500-6000) and which have a higher proportion of primary
hydroxyls than are used for hot moulding. In addition the isocyanates used
have a functionality greater than 2, this being achieved by the use of
modified isocyanates.
Typical hot moulding requires mould residence times of about 12
minutes at 150°C, and cold moulding 5-8 minutes at 40-60°C. Whilst coldcure foams have greater flexibility leading to greater comfort when sitting
down onto the seat the hot-cure foams have greater load-bearing capacity
and this is often associated with better damping of vehicle vibrations by the
seat. In general, in comparison with coil-less spring constructions, all-foam
seats give more reliable support to the user over a wide variety of driving
situations, can, by good design, avoid high load concentrations which could
affect blood circulation in the skin and in addition considerably reduce the
transmission of vehicle vibrations.
Properties and applications of flexible foams
Flexible polyurethane foams are resilient open-cell structures.
Compared with foams from natural rubber and SBR latex they are less
inflammable and have better resistance to oxidation and aging. The major
interest of flexible polyurethane foams is for cushioning and other
upholstery materials and for this reason the load-compression
characteristics are of importance. People differ considerably in their
opinions as to what constitutes an ideal cushioning material and, as a result,
manufacturers have tended to try to reproduce the characteristics of natural
rubber latex foam which has become widely accepted as a cushioning
material. The early polyester foams unfortunately did not correspond well
in their load-deflection characteristics for, although they had an initially
high modulus, they tended to collapse or “bottom out” above a certain
loading. Thus in many applications the foam became essentially a solid
piece of rubber. In addition the foam showed a slow recovery from
compression and a pronounced hysteresis loop in the load-compression
curve. Later polyether foams tended to be much more in line with late foam
but with a slightly greater damping capacity which in many instances may
be considered a desirable feature.
In addition to freedom from “bottoming out”, most people prefer a
seat which effectively provides a soft surface with a firm interior. One
measure of the relationship between such surface softness and inner support
200
is the sag factor or support factor. In one commonly used test this is
obtained by dividing the force required to compress a foam by 65% of its
height by the force needed to obtain 25% sample compression. This
generally increases with density but is typically <2.5 for a conventional
slabstock foam but >2.5 for a high-resilience foam.
Today polyether foam with a density of less than half that of rubber
latex foam is widely used as a cushioning material. Polyester foams,
although tending to be more expensive, continue to have a number of
outlets, particularly where a high initial modulus is desirable. In addition to
miscellaneous upholstery applications polyester foams are useful as “foam
back”, that is a foam backing in order to stiffen or shape some softer fabric.
Examples include car door and roof trim, quilting, shoulder pads and coat
interlinings. Amongst the many miscellaneous uses for both types of foam
are paint rollers, sponges, draught excluders and packaging for delicate
equipment.
Polyurethane foams do, however, suffer from one serious
disadvantage. Unless modified they burn with copious evolution of smoke
and toxic by-products, which has led to a number of fatal fires, particularly
in domestic accommodation. To some ext ent the problem may be reduced
by suitable upholstery covering, but a number of countries have now made
mandatory the use of fire retardent additives. There is consid erable a ctivit y
in the development of new safer systems, particularly in the use of amino
materials such as melamine as additives. Further developments may also be
expected in the near future.
2.8.4 Rigid and Semi-rigid Foams
The flexible foams discussed in the previous section have polymer
structures with a low degree of cross-linking. If polyols of higher
functionality, i.e. more hydroxyl groups per molecule, are used, tougher
products may be obtained and in the case of material with a sufficiently
high functionality rigid foams will result.
As with the flexible foams the early products were invariably based
on polyesters, but more trifunctional alcohols such as glycerol or
trihydroxy-methylpropane was added to the initial polyester reaction
mixture. These materials could then be reacted with isocyanate, catalyst,
water and emulsifying agent in the presence of a flame retarder such as triβ-chloroethyl phosphate. Although TDI was used initially, the increasing
use of rigid foams for in situ applications l ed to the development of less
volatile and subsequently less unpleasant isocyanates such as the