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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