Manufacturing of MDI

Manufacturing of MDI

MDI (diphenylmethane di-isocyanate) is a di-isocyanate used in certain specific slabstock applications, such as some specialized high resilience foams. MDI is manufactured commercially from aniline. Aniline is reacted with formaldehyde to give a mixture of aromatic amines containing 2 or more aromatic rings. These amines are then phosgenated to give a mixture of MDI’s, sometimes called “crude” MDI. Crude MDI is then distilled to separate pure MDI (containing 2 aromatic rings, which themselves can exist as several isomers) from “polymeric” MDI (which contains 3 or more aromatic rings):



The chemistry of MDI manufacture

Aniline + Formaldehyde = Mixture of aromatic amines (Phosgenation) = Crude MDI = Polymeric MDI

Aniline + Formaldehyde = Mixture of aromatic amines (Phosgenation) = Crude MDI (Distillation) = Pure MDI

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Manufacturing of TDI

Manufacturing of TDI
The second main ingredient used in the manufacture of flexible foam slabstock is a di-isocyanate. In most cases this is a toluene di-isocyanate, or TDO. TDI occurs in two different forms (isomers), depending on the position of the reactive isocyanate groups. As these isomers have a different reactivity, it is essential that the isomer ratio of the TDI is carefully controlled. The most commonly used TDI contains a mixture of the 2,4 and 2,6-isomers in the ratio 80/20, sometimes abbreviated to TDO-80 or T80.

TDI-80 is manufactured commercially from toluene. Toluene is first nitrated to give 2,4- and 2,6-dinitrololuene( again in the isomer ratio 80/20) This is then reduced to toluenediamine which is finally phosgenated to give TDI-80.

The chemistry of TDI manufacture

Toluene (Nitration) =
2-4dinitrotoluene + 2-6-dinitrotoluene (reduction) = 2-4-toluenediamine + 2-6-toluenediamine (Phogenation) =
2-4-toluene di-isocyanate + 2-6-toluene di-isocyanate

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Polymer Polyol

Polymer Polyol

In recent years there has been a trend in the foam slabstock market towards foam having improved load bearing characteristics. Such extended load bearing foams can be produced with polymer polyols which are conventional slabstock polyols containing suspended solid particles. These microscopic particles, which give the polyol an opaque white appearance, act as a reinforcing filler increasing the foam hardness. Polyols are available with different amounts of suspended particles: the higher the amount of solids the harder the foam.

Polymer polyols used for the production of enhanced load bearing foams often contain particles of polystyrene or a copolymer of styrene and acrylonitrile. In both cases the polymerization takes place in the base polyol and makes use of a pre-polymer stabilizer which surrounds each particle as it is generated. This prevents particles sticking together and ensures a stable suspension of microscopic particles.

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Important features of a Polyol

Important features of a Polyol

Molecular Weight

The more PO and EO molecules added to the glycerol initiator, the greater the molecular weight of the resulting polyol. The molecular weight of most polyols used for the production of slabstock foam is between 3000 and 6000. Increasing the molecular weight reduces the reactivity of the polyol. Furthermore it results in foam which has lower hardness and a higher tensile strength.

Functionality

The number of hydroxyl groups per molecule of initiator is referred to as the functionality. Glycerol has three hydroxyl groups, therefore the functionality of the final polyol is about 3 .In practice, certain side reactions usually result in functionality slightly below three.

Ethylene oxide Content

Polyols used for the production of of standard slabstock foam qualities can contain up to 20% EO. Increasing the amount of EO increases the reactivity of the polyol. The EO is usually distributed at random throughout the polyol chain. However, in special cases, the EO is present at the end of the polyol chains. These EO tipped polyols are more reactive than conventional polyol sans are used in specific applications such as the manufacture of high resilience foams.

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How is Polyol Made?

How is Polyol Made?

The glycerol initiator is added to a reaction vessel along with PO and, in some cases, some EO. Under the influence of heat, pressure and catalyst the oxides are reacted with the initiator, producing the polyol

Glycerol +Propylene oxide + Ethylene oxide = Polyol


Small amounts of byproducts are removed from the polyol in subsequent treatment steps. Antioxidant stabilizer is added to the polyol to protect the polyol during the storage and transportation and also to reduce the risk of foam scorching.

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What is polyol?

What is Polyol?

A polyol is the main raw material in any flexible foam formulation.

Two types of feedstock are used in the manufacture of a polyol: an initiator and an alkylene oxide. In the case of flexible slabstock polyols, the initiator is usually glycerol. The main alkylene oxide is propylene oxide (PO), but usually some ethylene oxide (EO) is used as well .The hydroxyl groups at the end of each chain can react with di-isocyanates, such as TDI to give polyurethane polymer.

Manufacture of Propylene oxide

Propylene oxide is the major feedstock for polyols. There are two basic commercial routes for the manufacture of PO, per oxidation process and the chlorhydrin process. Per-oxidation process produces both styrene monomer (SM) and PO.

Peroxidation Process (1)

In this process, the first step is per oxidation of ethyl benzene.

Ethyl benzene + Oxygen = Ethyl benzene Hydro peroxide


Peroxidation Process (2)

The ethyl benzene hydro peroxide is then used to oxidize propylene to give PO.

Ethyl benzene hydro peroxide + Propylene = Propylene Oxide + Methyl Phenyl Carbinol

Peroxidation Process (3)

The methyl phenyl carbinol, produced at the same time as the PO, is dehydrated to styrene monomer, an important raw material for the polystyrene industry

Methyl Phenyl Carbinol = Styrene Monomer + Water

Chlorhydrin Process

In the Chlorhydrin process, propylene is reacted with hypochlorous acid to give propylene chlorhydrin. In the presence of calcium hydroxide the chlorhydrin is converted to PO. The main by-product of this process is calcium chloride, which represents a disposal challenge making this process less attractive than the direct per oxidation route.

Propylene + Hypochlorous acid = Propylene Chlorhydrin

Propylene Chlorhydrin + Calcium Hydroxide = Propylene oxide + Calcium Chloride

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Polyurethanes - Raw Materials

Flexible polyurethane foam is a versatile material with many applications .Its properties can be precisely tailored to our requirements by using a suitable formulation and by careful selection of the raw material.
Two main components are used in the manufacture of flexible polyurethane slabstock foam are a polyol and a di-isocyanate.

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THE FOAMING PROCESS

The following sequence of events is a simple picture of what happens when flexible foam is made by mixing together the materials

MIXING

In the first step the foam ingredients are mixed by mean of a stirrer. Good mixing is essential to produce homogeneous foam. The silicone surfactant assistS in achieving good mixing since it lowers the surface tension of the polyol.


NUCLEATION

During the mixing air bubbles are created in the liquied. These act as nucleation point for the expansion gases. When making a box foam with simple equipment. It is not always possible to regulate the size or the number of these bubbles on a continuous slabstock machine however there are a number of way in which to ensure that there are sufficient of these initiating point of foam formation and that they are fit the right size.

After about 10 second the blowing gases (CO2 and an ABA if used) diffuse into these small air bubbles and enlarge them giving the mixture of ingredients a ‘creamy’ appearance. The time from initial mixing to change in appearance is called the cream time.

EXPANSION

As more blowing gas is generated, the bubbles expand and the foam begins to rise. During the foam rise the number of bubbles remains constant. The silicone surfactant stabilizes the bubbles preventing them from coalescing; without surfactant the mixture appears to boil and the foam collapses. At the same time as the bubbles are expanding, polymerization reaction also takes place after mixing, the gas reaction stops. The foam now occupies 30-50 time the original liquid volume. The polymer part of the foam has begun to gel in the form of gas-filled cells with thin walls and somewhat thicker struts along the edges.

CELL OPENING

In the flexible foams the cell walls are unable to contain the full gas pressure: at about the full rise time, they break and the polymer contracts into the struts. At the same time the polymer is sufficiently gelled for the struts to strong enough to stand while the gas escapes through the pen cells.



GELANATION

Continuing polymerization increases the strength of the polymer which reaches the gel time about-3 minutes after mixing.

CURING

The foam is then left for at least 24 hours to cure, during which time various slow cross-linking reactions take place to give the foam its final physical strength.

NOTE
In the foam formation just described the two types of reaction one causing gas evolution and the other polymerisation of the cell wall and struts, must be finely balanced. If the balance is wrong then various faults can occur into foam. In the simplest terms, the two main types of fault are caused by the polymerisation reaction occurring too soon or too late.

When polymerisation occurs too soon polymer strength develops too early. Some of the cell walls will not burst under the gas pressure. The result of this is that the foam will have poor resilience and feel ‘dead’, often called a ‘tight’foam: if many of the cells remain closed, then as the foam cools and the internal gas pressure falls below atmospheric, the foam shrinks.

When the polymerisation occurs too late the struts will be weak at the time when the cell walls burst. If this happens the struts can break and since the struts of one cEel are shared by all the immediately surrounding cells, the result is a series of struts breaking, forming a split. The extent of this split can be increased by the expanding gases forcing the spilt further apart and breaking even more struts.
(Thus it can be seen that the weaker the foam, i.e. the later the polymerisation, the larger the spilt.) Splits due to late polymerization usually occur ate the top edges (shoulders) of the blcok where the foam is weakest (coldest and therefore slowest to polymerize) and in the center of the blcok where the exothermic is highest (highest gas pressure).

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FLEXIBLE SLABSTOCK FOAM RAW MATERIAL AND THEIR FUNCTIONS

MAKING OF FOAM BLOCKS


A large proportion of flexible polyurethane foam is making in blocks, either discontinuously in simple boxes or in a continuous process known as slabstock foaming. In this process the raw materials, polyol, di-isocyanine, water, auxiliary blowing agents, catalysts, surfactant and other additives are mixed continuously into an in-line mixer and the mixed reactants, still in liquid form, are poured into a continuous paper and/or polyethylene mould on moving conveyor. In the paper mould, the liquids foam and expand to form a continuous block of foam. This blcok is out into sections, stored for at least a day to cure and cool, and then cut in variety of ways into the shapes required. The main uses for this foam are mattresses, furniture’s, automotive seating and textile laminating.

THE RAW MATERIALS AND THEIR FUNCTIONS


The essential components of flexible foam formulation are:

1)Polyol
2)Di-isocyanate
3)Water
4)Auxiliary Blowing Agents (ABA’s)
5)Catalysts: tertiary amines and tin salts
6)Silicone surfactants.

To these may be added optionally colours, fire retardants, combustion modifiers, fillers and other materials.

Taking each of the essential components in turn:

Polyol

Most flexible slabstock foam is made from polyther polyols.

The are essentially propylene oxide and ethylene oxide copolymers with a tri-functional initiator and are therefore trials. The polyols for standard block foams have hydroxyl values in the range 46-56 mgKOH/g (nominal molecular weights of 3500 to 3000 respectively).

Di-isocyanate

The most common di-isocyanate for flexible foams is toylene di-iscocyanate, also called toluene di-siocyanate or simply TDI. Commercial grades of TDI are mixtures of the 2,4 and 2,6 isomers in controlled proportions.The most frequently grade used to manufacture flexible polyurethane slabstock foams, consists of 80 parts of the 2,4 isomer and 20 parts of the 2,6 isomers.

Blowing agents

The primary blowing agent, which causes the foam to expand, is carbon dioxide, generated by the reaction between water and di-isocyanate. An auxiliary blowing agent may be used in combination with water to produce foams below 21 kg/m3 density or to produce soft foams at all densities. These auxiliary blowing agents are low boiling point liquids

Catalysts

Tertiary amines

These catalysts accelerate and control the rate of the water/di-isocyanate reaction.
Examples are:

Di-methyl diamine, often referred to as TEDA or DABCO TM , the original Houdry trade name; now marketed by Air Products and now most commonly supplied as DABCO TM33LV – a 33% solution of TEDA in dipropylene glycol
Bis (dimethylaminoethyl) ether 70% in di-propylene glycol; most commonly known as NIAZ TM A1, the original trade name of Union Carbide Co; now marketed by Witco.

Tin salts

These catalysts are specific for the reaction between polyol and di-isocyanate. Almost universally used is stannous octoate (the tin II salt of 2-ethyl-hexoicacide), often referred to as tin catalyst, SnOct or t-9, the original trade name of M & T Chemicals Inc.; now marketed as DABCO TM T-9 by Air Products.
The control accurately the small quantities required, the stannous octoate as supplied is normally diluted with polyol before use or can be purchased in an already diluted form. An alternative catalyst for use especially where polyol pre-blend (containing water, amines and silicone) are made is di-butyl tin dilaurate commonly known as DBTDL, DBTL or DABCO TMT-12.

Silicone Surfactant

A surfactant is essential to control of the foaming process. In slabstock foams this is siloxane based and is commonly referred to as ‘silicone’. It has two functions:
 To assist the mixing of the components to form a homogeneous liquid.
 To stabilize the bubbles in the foam during the expansion so preventing collapse before the liquid phase polymerises.

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Introduction to Maxfoam process for manufacture of Flexible Polyurethane Foam

The Maxfoam process is the most widely used flexible polyurethane slabstock foam process in the world.

INTRODUCTION

This introduction is designed to give you technical information about the running of the Maxfoam process. It is not intended as an instruction manual for your machine.
This Manual does give information about the main features of the Machine, as they affect the efficient running of the Maxfoam Process. It also gives information on setting up the Machine, chemical formulations and running conditions. There is also a comprehensive section on common faults which you may encounter from time to time, and how they may be overcome.

Safety is an important factor in running an efficient foaming operation.


Basic features of the Maxfoam machine

The basic mechanical features of the Maxfoam machine are shown in Figure 1.01 below.




All chemical streams are metered by accurate pumps and directed to the mixing head port-ring or to the polyol manifold through three-way diverter valves. This arrangement enables metering pumps to run at normal output conditions in a recirculation mode. During foam production, the chemicals are mixed in the mixing head which consists of a variable speed multi-pin stirrer closely fitted inside a cylindrical barrel; the speed can be varied either by a system of changeable pulleys or by an infinitely variable speed controller.

Two fundamental mechanical items are the trough and the fallplate. The chemical mix is fed from the mixing head through two flexible hoses into the bottom of a deep trough. The foam mix is allowed to start reacting and partially expand in the trough before spilling out onto the fallplate section.

The fallplate consists of a number of hinged sections. The vertical position of the trough and each section of the fallplate is controlled by motorised screw-jacks. This arrangement enables the profile shape of the fallplate to be changed to suit different foam formulations.

Onto the fallplate, to contain the foam from the trough, is fed the bottom paper - a continuous sheet of paper from horizontal unwind rolls at the back of the machine. Side papers are also fed from vertical unwind rolls at the start of the sidewalls. The feed tension of the unwind rolls is controlled by adjustable brakes.


From the lower end of the fallplate, the bottom paper runs onto the main conveyor, which is of metal slat construction. The speed of the main conveyor is variable and a critical control for the foaming process.

At the end of the main conveyor, where the side papers are removed from the foam block, are two vertical capstan rolls which are driven at the same surface speed as the main conveyor. These ensure that the side papers travel at the same speed as the foam block. The side papers are wound onto driven rewind rolls. The bottom paper is also removed from the foam block and wound onto a driven rewind roll below the level of the conveyor.

The block cut-off knife or saw is situated after the paper take-off positions. As it cuts, it moves synchronously with the main conveyor so as to ensure an accurate vertical cut.

After the blocks have been cut, they are transported to the cure hall where they will stand for about 12 to 18 hours (depending on the grade of foam) to allow completion of the chemical reactions and cooling of the foam before further processing.


Basic principles of the Maxfoam process

The main difference between Maxfoam and a conventional inclined conveyor slabstock process is the trough. The purpose of the trough is to cause a delay between mixing the chemicals and depositing them on the conveyor. This delay, which is typically about 20 seconds, allows some pre-foaming to occur in the trough, so that a froth instead of a liquid is laid down on the conveyor.


The advantage of this configuration is that the transporting surface where the foam expansion occurs (the Maxfoam Fallplate) can now be much steeper than the conveyor of a conventional machine. Whilst the angle of inclination of a conventional foam conveyor is normally less than 4o, the average inclination of the fallplate is greater than 10o.
The steeper angle is possible because the froth coming out of the trough has a much greater viscosity, and is of lower density than the liquid mix on a conventional machine. These two factors reduce the possibility of under-run, the running of heavier liquid material underneath older, lower density foam.

This steeper angle allows a shorter machine to be used, at lower outputs and conveyor speed - for the same height of block. Density distribution through the block is also improved.

A further advantage of Maxfoam is the ability to control the shape of the top of the block by adjusting trough height and full-rise position.

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Filled Foam on Maxfoam

The demand of filled foam is increasing in some parts of the world. It was produced quite successfully on conventional machines to a very limited height.

The use of an organic fillers in foam are an important tool to produce cheap quality foam without to much damage to the foam properties provided it is well applied and produced.

Years ago producers were satisfied to reach a block height of 26 inches or 650 mm but today a Maxfoam technology can produce a foam of almost 42 inches high by a width of 84 inches with 100% filled polyol.

In general the production of the foam type is not so difficult provided certain points are well noticed.

1. Type of Filler

Fillers must meet the following requirements;
a) The particle size must be very small, approx 5 micron.

b) The water content should not exceed 0.2% and the filler itself should not be hygroscopic.

c) The filler should not contain free metal parts to destroy the catalyst activity.

d) The filler must be easily dispersed in the polyol. In a weight ratio one to one the slurry must have a low enough viscosity to be pumped, preferably by means of a Monyo screw pump. One of the organic filler that meets all these requirement is Barium Sulphate with a specific gravity of 4.2 grams/cm3 in its purest foam.

Barium sulphate is to be used in preference to calcium carbonate due to the 55% higher specific gravity.

For this reason this report will continue on this type of filler.

2. How much filler can be used?

Any percentage from 0-100% or beyond this limit.

More important is the water rate/freon level in combination with the filler percentage/block height, because these four factors will determine the limit in which the production becomes critical. Also in the machine in particular the fallplate setting determines if it is possible to produce fault free foam.

For the sake of reproducibility it was found out that a 100% Barium filler with 2.35 water, 8 Freon and a block height of 42 inches will come pretty close to this limit of producing high density medium soft foams.


3. If conventional polyether foam is compared WITH FILLED foam what properties can we expect by filler increase percentage?

Elongation at tear strength reduced from 205% to 170%.
Tear strength reduces from 0.82 to 0,78Kg/cm2
Compression set shows 11.8% instead of normal 2.8 %.

4. What are the difficulties?

a) Very narrow tin spread by increasing filler percentage. Expressed in pph a spread of 0.025 pure stannous octate is possible for a 100% filled foam.
b) Mechanical faults can be made very easily in this type of foam due to somewhat different rise pattern.

The catalyst level on a filled foam is higher than on a conventional foam and still a total rise time of 270 seconds can be expected in a 3 lbs/cft density.

The trained observer soon finds out that the foam will almost reach a height in less than 120 to 150 seconds, it is very delicate at this stage and it is therefore easily damaged. Therefore a good fall plate settings which follows downward expansion of the foam and a smooth lay down are very essential, despite the little lost in the profile shape.

c) The index should not be lower than 106

d) A homogeneous slurry together a narrow range in specific gravity is required.

e) The water content of the slurry should be established before the production.

f) Preferably a machine box test should be made before any foam is poured onto the conveyor.

Unifoam’s standard start procedure can be used to obtain optimum shape and quality.


5. Description of additional equipment.

To produce a filled foam the following items are required.

a) The batch or slurry tank

Depending on the duration of the run, the tank volume has to be determined, an average output on slurry in approx. 500-600 lbs/min with a specific gravity of 1.3~1.6.

It is recommended to use a closed tank with a removable lid on the top. The bags containing fillers have to be emptied very carefully through a sieve into a predetermined quantity of polyol and during the preparation the slurry has to be circulated through metering circuit and the agitator (two blade propellers) has to run on a maximum speed of 55 revs/minute.

As soon as the filler is mixed in, the agitator speed can be turned down to 15~20 revs/minute and the metering pump keeps the running while the tank is closed.

Keep preferably dry air supply onto the when agitating.

b) Agitator

Two speed motor 8~10 H.P
Low speed 15~20 revs/min.
High speed 55 revs/min.

c) Propellers.

They can be made of metal sheets which are firmly connected to the axle with a clearance to the tank wall of not more than 12 inches.

The lowest propeller has to be installed close to the bottom and the second one 3 feet from the bottom.

The outlet opening of the tank into the feed line of the pump has to be provided with a coarse grid or sieve.

The return line must hit the wall of the tank on the top and splashing must be prevented.

d) Metering pump for polyol filler

Accurate metering at an output of 250 kg/min is achieved with a Monyo pump CDQ which is delivered by Robins and Meyers Springfield Ohio USA.

If some parts of nonhyrdolysable silicone oil are mixed into the slurry, the life of the pump will be extended.




e) Metering lines and filters

4 inch suction line and 3 inch delivery lines are required.
An oversized 6 inch filter in the pressure line will act as a strainer.


f) Temperature Control

The working temperature should not exceed 73F, a higher temperature will cut back on catalyst level but the major disadvantage of using heat as a catalyst lies in the fact of a long curing time and subsequently thick top skin with damage to the side skin.

When the slurry is prepared, it is obvious that the agitator is running.

The friction heat which is developed will raise the temperature of the slurry which is checked with a pyrometer or probe. Automatic temperature controlling the equipment can be used on the outside of the tank wall. Cooling on the outside and heat development on the inside will result in condensation if the slurry tank is open. Therefore it is better to have a closed tank with a dry air supply on the top.

6. Procedure

The best way to establish a stable formulation is to do some trials on lab scale, as described in the Maxfoam instruction book. Premix polyol and the desired quantity of Barites a couple of hours before that trial, for air will be trapped and it must have the chance to escape from the slurry.

Virgin foam formulation can be used to determine the catalyst levels.

If the density is within specification and if the ILD value is rather on the high side then it becomes worthwhile to prepare the machine for box trial.

From this point one can continue with short machine runs during which exact catalyst levels can be established and optimum fall plate setting can be determined.

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The Basic Chemistry of Polyurethanes

The basic chemistry of the isocyanate group can be tracked back to the pioneering work of Wurtz (1849) and Hofsman (1850). Dozen of reactions are known, but the ones of most commercial significance are those leading to the formation of polyurethane polymers. Polyurethanes are formed by the reaction of a polyisocyanate with a polyhydroxy, or polyol compound. The reaction of polyisocyanate with water is a convenient way to produce a gas useful for blowing the polymer to produce a foam structure.

BASIC CHEMISTRY

Polyurethane chemistry is based on the reaction of isocyanates with active hydrogen containing compounds. Isocyanates are compounds having one or more of the highly reactive isocyanate group. This group will readily react with hydrogen atoms more electronegative than carbon.

Electron density is expected to be the greatest on the oxygen atom and least on the carbon item. This results in the oxygen atom having the largest net negative charge, carbon a net positive charge, and the nitrogen, an intermediate net negative charge.

The normal reactions essentially involve addition to the carbon-nitrogen double bond. A nucleophilic center from an active hydrogen-containing compound attacks the electrophilic carbon. The active hydrogen atom then adds to the nitrogen atom. Electron-withdrawing groups attached to the isocyanate molecule increase the reactivity of the NCO group toward the nucleophilic groups. Electron-donating groups reduce reactivity. Thus, in most reactions, aromatic isocyanates are more reactive than aliphatic isocyanates. Steric hindrance effects on either the isocyanate or the active hydrogen compound will effect the reaction.

Formation of a flexible polyurethane foam is a complex process involving many ingredients and at least two competing reactions.

THE POLYMERIZATION REACTION

The polyurethane polymer forming reaction occurs between an isocyanate and an alcohol as follows

Isocyanate + Aocohol = Urethane

This is addition process for which the heat of reaction has been reported to be approximately 24 kcal/mole of urethane. Depending on the choice of starting materials, the R and R’ groups may also contain isocyanate or isocyanate-reactive groups respectively. When extended to polyfunctional reactants, this reaction provides a direct route to cross linked polymers.

The hydrogen on the nitrogen atom of the urethane group is capable of reacting with additional isocyanate to form an allophanate group.
Urethane + Isocyanate = Allophanate

Note that the formation of allophanante is a high temperature, reversible reaction. If actually formed in normal flexible foams, the allophanate linkage would serve to cross-link the polymer further. The catalysts generally used in the foam formulation do not promote this reaction, and temperatures greater than 110 Centigrade are necessary for significant allophanate formation.

THE GAS PRODUCING REACTION

To make foam, the polyurethane polymer must be expanded or blown by the introduction of bubbles and a gas. A convenient source of gas is the carbon dioxide produced from the reaction of an isocyanate group with water.

Isocyanate + Water → Carbamic Acid

Carbamic Acid → Amine + Carbon dioxide + Heat

The intermediate product of this reaction is a thermally unstable carbamic acid. Which spontaneously decomposes to an amine and carbon dioxide.. Diffusion of the carbon dioxide into bubbles previouslt nucleatedin the reacting medium causes expansion of the medium to make foam. Further reaction of the amine with additional isocyanate gives a distributed urea.

Isocyanate + Amine → Distributed Urea

The approximate total heat release per mole of water is 47 kcal.

Again, if the isocyanate and the amine molecules are polyfunctional, a cross-linked polymer will result. Another conceptual method of cross-linking the polymer is by reaction of a hydrogen from the distributed urea with a free isocyanate group to form a biuret linkage.

Distributed Urea + Isocyanate → Biuret

Since the reaction is also reversible, there is debate about whether allophanates and biurets actually exist in the final polyurethane foam.

Blowing can also be achieved by the physical addition of a low-boiling nonreactive liquid to a foam formulation. Historically, the most commonly used blowing agents are chlorofluorocarbons, urethane grade methylene chloride and trichloroethane. Vaporization of these liquids by heat from the exothermic reactions produces gas molecules which diffuse into nucleated bubbles and contribute to foam expansion.

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