Polyurethanes and diisocyanates

Polyurethanes and diisocyanates
Polyurethanes are a type of extremely versatile plastic material that comes in many forms and are used in applications that most people come in contact with every day. Flexible polyurethane foam is used in cushioning for furniture, footwear, cars, mattresses and other applications. Rigid polyurethane foam is used to make several types of insulation found in buildings, appliances, cars and other equipment. Other forms of polyurethane, such as coatings, adhesives and sealants, are used in building and construction, industrial and commercial coating applications, as well as a variety of automotive applications. 
Diisocyanates are chemical building blocks used to make polyurethane products. The most widely used aromatic diisocyanates are toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). Less widely used, but still important, are the aliphatic diisocyanates, including hexamethylene diisocyanate (HDI), hydrogenated MDI (H12MDI), and isophorone diisocyanate (IPDI). TDI is mainly used to make flexible polyurethane foam that can be found in everyday products, including furniture, bedding, carpet, and packaging. TDI is used in the manufacture of some coatings, sealants, adhesives and elastomers. MDI is used primarily to make rigid polyurethane foams such as insulation for home or refrigerator. Vehicle parts like dashboards, steering wheels and bumpers are also made of MDI. HDI, H12MDI and IPDI are most often further reacted to form polyisocyanates, or pre-polymers, which act as building blocks to form color-stable polyurethane coatings and elastomers, that can significantly enhance a product’s appearance, lengthen its lifespan and offer high abrasion resistance.

Manufacturing of polyurethanes

Polyurethanes are made from the reaction between an isocyanate and a polyol.

TDI (toluene diisocyanate or methylbenzene diisocyanate) and MDI (methylene diphenyl diisocyanate or diphenylmethane diisocyanate) form the basis of about 95% of all the polyurethanes.

The starting material of TDI is methylbenzene (toluene). When it reacts with mixed acid (nitric and sulfuric), two isomers of nitromethylbenzene (NMB) are the main products. If this mixture is nitrated further, a mixture of dinitromethylbenzenes (dinitrotoluene or DNT) is produced. The mixture of dinitrobenzenesis then reduced to the corresponding amines. In turn, the amines, known commercially as Toluene Diamines or TDA, are heated with carbonyl chloride (phosgene) to produce the diisocyanates and this process can be carried out in the liquid phase with chlorobenzene as a solvent at about 25°C (80°F). Alternatively, these reactions are carried out in the gas phase by vaporizing the diamines at about 325°C (620°F) and mixing them with carbonyl chloride.
MDI is more complex and permits the polyurethane manufacturer more process and product versatility. The starting materials are phenylamine (aniline) and methanal (formaldehyde) which react together to form a mixture of amines, known as MDA (methylenedianiline).
This mixture reacts with carbonyl chloride (phosgene) to produce MDI in a similar way to the manufacture of TDI. The polyols used are either hydroxyl-terminated polyethers (in about 90% of total polyurethane manufacture) or hydroxyl-terminated polyesters.  They have been developed to have the necessary reactivity with the isocyanate that will be used and to produce polyurethanes with specific properties. The choice of polyol, especially the number of reactive hydroxyl groups per polyol molecule and the size and flexibility of its molecular structure, ultimately control the degree of cross-linking between molecules. This has an important effect on the mechanical properties of the polymer. If the polyol has two hydroxyl groups and is mixed with either TDI or MDI, a linear polymer is produced. If the polyol has more than two reactive hydroxyl groups, adjacent long-chain molecules become crosslinked at intermediate points thus creating a stiffer polymer.

GAB Neumann’s process equipment

The diisocyanates manufacturing process involves the recovery (absorption and further processing) of very large amounts of hydrochloric acid (HCl) therefore the need for corrosion resistant process equipment. GAB Neumann supplies annular groove graphite heat exchangers, graphite heaters, graphite coolers, graphite block heat exchangers, graphite absorbers, graphite columns, and HCl recovery units to the diisocyanates producers.


Associated products:

Impervious graphite annular groove interchangers

Impervious graphite annular groove heavy-duty condensers

Impervious graphite annular groove partial condensers

Impervious graphite block heat exchangers

Impervious graphite annular groove isothermal absorbers

Impervious graphite columns

Hydrochloric acid recovery units

GAB Neumann GmbH

Alemannenstrasse 29

79689 Maulburg


Tel: +49 (7622) 6751 0

Fax: +49 (7622) 6751 20


GAB Neumann GmbH | Alemannenstrasse 29 | D-79689 Maulburg | Phone +49 (7622) 6751 0 | Fax +49 (7622) 6751 20 | E-Mail info@gab-neumann.de | www.gab-neumann.com