X-Polymers-G-Polyurethane-1
POLYURETHANE FLEXIBLE FOAMS
Polyurethane foam is the most widely used flexible foam plastic. It is used to produce a
wide variety of items including thermal insulation and packaging materials, comfort
cushions, bed mattresses, carpet backings and resilient floor coverings.
Tolylene diisocyanate (TDI) and polyalcohols are the basic ingredients for the production
of polyurethane foam. The basic reaction is as shown below:
HO R OHNCOOCN +
H
N
O
C
OH
OROCN
Blowing agent, such as methylene chloride and water, and various additives are also
required.
Step 1 - Mixing of the raw materials
During production, the raw materials (TDI, polyalcohol, blowing agents and additives)
are pumped from their own storage tank to a common mixing chamber. Adequate
dispersion can be achieved by the stirring of high speed impeller installed in the mixer.
Step 2 - Foam forming and settling
The foam gradually solidifies when travelling up the settling chamber by the action of
paper conveyor. It is then cut into 2.2 m long blocks by an electric cutter after the foam is
hardened.
Step 3 - Curing
The newly formed foam blocks are still very hot when transported to the storage area.
They must be cured at room temperature for at least 18 hours before further processing.
INTRODUCTION
Solid foam is formed when gas is blown through solidifying plastic. Depending on its ability
to retain original shape after compression, solid foam can be classified as either flexible or
rigid. Polyurethane foam is the most widely used flexible foamed plastic, being used for
thermal insulation and packaging materials, cushions, bed mattresses, carpet backings and
resilient floor coverings.
There are four major polyurethane foam manufacturers in New Zealand. This article is based
on the process used by Dunlop Flexible Foam in Auckland, although all manufacturers use a
similar process. Dunlop has been using a continuous process since 1985, and has a daily
capacity of more the 15 tonnes of polyurethane foam.
The chemistry of polyurethane foam formation
Flexible polyurethane foams are open cell materials that allow free movement of air between
the foam cavities. They are commonly available in density of 13 - 80 kg m-3.
TDI and polyalcohol are the basic ingredients for polyurethane foam production. A range of
additives, blowing agents and water are also added.
X-Polymers-G-Polyurethane-2
CH3
N C ONCO
CH3
N
N C O
C O
toluene-2,6-diisocyanate toluene-1,4-diisocyanate
TDI is the abbreviation for toluene di-isocyanate, a mixture of toluene-2,4-diisocyanate and
toluene-2,6-diisocyanate with the 2,4- isomer being the major component. Polyalcohol
(HO—R—OH) is the other important ingredient in foam production, and the structure of the
R group in the alcohol is largely responsible for the final properties of the final product.
Generally speaking, using a longer R group will improve the flexibility of the resulting foam.
The relative molecular mass of polyalcohol for flexible foam production is as large as 20000.
When TDI reacts with polyalcohol, polyurethane forms in a highly exothermic reaction:
HO R OHNCOOCN +
H
N
O
C
OH
OROCN
This is a nucleophilic addition of RO-H across C=N.
The polymeric chain crosslinks with others via the following reactions:
R N C R
O
C
N
O
HR
TDI
R N C R
O
C
N
O
CR
O
NH R
R = other parts of the polymeric chain
R N C R
H
O
+ O C N R
This is a nucleophilic addition of >N-H across C=N.
Hard polyurethane plastic (such as that which many electric switches are made of) would be
formed in the above condensation reaction if the reaction is carried out without blowing
agent.
Two different types of blowing agents are added to the polymer - carbon dioxide and
methylene chloride. Carbon dioxide (the primary blowing agent) is produced in the reaction
mixture as water reacts with the isocyanate group:
X-Polymers-G-Polyurethane-3
NCO + H2O + CO2NH2NH C
O
OH
Due to its low reactivity and low boiling point (b.p. 40.7 oC), methylene chloride (CH2Cl2) is
selected as a supplementary blowing agent. It vapourises and leaves cavities in the plastic
framework after the exothermic reaction begins.
In practice, a range of additives are blended with the raw materials. A tin based additive is
used to stabilise the foam while a few amine based additives are added to control the extent
of cross linkage and the rate of reaction. Dyes are also added to distinguish different grades
and batches of foams.
THE MANUFACTURING PROCESS
Dunlop Flexible Foam produces polyurethane from a continuous production line. Unlike
conventional machines that extend horizontally for tens of metres, the polyurethane forming
machine in Dunlop adopts a vertical profile to save space.
A schematic diagram of the machine is shown in Figure 1 below.
TDI
blowing
agent
additives cutter
mixer
settling
chamber
paper
conveyor
polyalcohol
storage pile
Figure 1 - Schematic diagram of the Dunlop polyurethane process
Step 1 - Mixing of the raw materials
TDI and polyalcohol are transported to the plant by road tankers. They are stored in
cylindrical tanks before use.
X-Polymers-G-Polyurethane-4
Since the melting point of TDI is about 17 oC1, it would solidify in uninsulated tank during
winter. Therefore the contents in the tank are kept at 25 oC to solve the material handling
problem.
During production, the raw materials (TDI, polyalcohol, blowing agents and additives) are
pumped from their own storage tank to a common mixing chamber. An impeller driven by a
fast rotating motor is installed in the mixer to aid dispersion. Despite its small size ( less than
1 L in volume) the mixer can process more than 50 kg of raw materials each minute.
Step 2 - Foam forming and settling
Due to the foaming effect of the blowing agents, the reacting mixture expands rapidly on
ejection from the mixer. Since the mixture is still in liquid form when pumped to the bottom
of the settling chamber, it picks up the shape of the chamber readily.
The foam gradually solidifies when travelling up the settling chamber by the action of a paper
conveyor. It takes approximately nine minutes for the foam to travel from the bottom to the
top of the machine. By this time, the foam is fully hardened. It is cut into 2.2 m long blocks
by an electric cutter mounted on top of the settling machine.
Step 3 - Curing
The newly formed foam blocks are still very hot when transported to the storage area.
Although the peripheral of the foam is cooled to room temperature, the centre of the block
can still be hotter than 100 oC. It must be cured at room temperature for at least 18 hours
before further processing.
Fully cured foam blocks are either directly delivered to the customers or cut into smaller
sizes according to orders. Due to its low density, foam products are bulky and expensive to
transport. To save space, vacuum packing is prevalent during transportation. Being flexible
the foams reduce in size and revert to their full size when returned to atmospheric pressure.
THE ROLE OF LABORATORY
To assure the product quality, the laboratory performs routine tests on selected batches. Most
of the tests, such as density measurement, tensile and compression strength tests are
mechanical in nature.
Since expansion of polymer into a cellular state greatly increase the surface area, reaction of
the foam with environmental agents, e.g. moisture and air, are correspondingly faster than
that in solid polymer. Ageing problem is thus serious for polyurethane foam. The life time
of foam can be estimated from accelerated tests by keeping foam samples in an oven.
1Since TDI is a mixture of the two isomeric forms of toluene diisocyanate, it does not have a sharp melting
point.
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ENVIRONMENTAL IMPLICATIONS
TDI is a confirmed carcinogen. Its emission is closely monitored by the local council. TDI
tape detectors are installed around the factory. To minimise the emission of TDI, most of the
process equipment is shielded.
Traditionally CFC (chlorinated fluorocarbons) based solvents were used as blowing agents.
In the late 80's, the ozone layer depletion effect of CFC based chemical was established. The
use of CFC was gradually banned. Methylene chloride, which is inert to ozone, is used as a
substitute for CFC.
Lots of "offcuts" are generated in the foam cutting process. They are collected and sent to
the associated recycle plant to produce a lower grade flexible foam. This type of foam is
widely used in the production of carpet backings and packing materials. By this way the net
amount of waste produced in the process can be minimised.
Re-written by David Yuen from the original article by D. McRobie (Union Carbide (N.Z.)
Ltd.) using information provided by Keith G. Town (Dunlop Flexible Foams) and with
reference to:
• Kirk-Othmer, Encyclopedia of Chemical Technology (3rd Ed.), Vol 11, 88-89.
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