Polyurethane, abbreviated as PUR, is produced by the polyaddition of polyfunctional polyisocyanates, dihydrac and higher alcohols. Characteristic for polyurethanes is the urethane group having the chemical structure:

Materials of this class usually also contain other types of bonds, such as urea, amide, biuret, allophanate, ester and/or other ether compounds. Therefore polyurethane is a collective term that is used for quite differently structured polymers.
Depending on the selection and the stoichiometric ratio of the starting materials, one arrives at very different properties, that are used as components of adhesives and varnishes (polyurethane resins), as thermoplastic material for bearing parts, wheels, tires, rollers and as a more or less hard elastomers in fibrous form or as polyether and/or polyester urethane rubber, as thermoset casting resins and, above all, as moulded foam with a variety of applications.

Source: BASF

Polyurethane foams result from polyaddition, when water and/or foaming agents are present. Water reacts with the isocyanates, eliminating buoyant and foaming carbon dioxide.
Depending on the choice of reactants, their stoichiometric ratios and chemical reaction conditions, one arrives at flexible polyurethane foams, rigid polyurethane foams and structural or integral foams.

Integral foams (structural foams) are foams that are chemically identical over the entire cross-section, but the density of which decreases continuously from the outside inwards. They are characterised by a porous core and an almost massive rim. The temperature gradient setting for the foaming process from the mould interior to the mould wall causes the formation of integral skin foams, a different expansion of the vaporised foaming agent that condenses in the cold ridge zone, over which cross-sections form and thereby lead to the density differences in the foam described.
Additional auxiliary materials or additives required for polyaddition can be catalysts, emulsifiers, foam stabilizers, pigments, aging agents and fire retardants, etc. These are included, for example, in an A-component (polyol blend). The polyaddition is a step-like running coupling reaction of bi-, tri- or more functional basic molecules (monomers) into large chain molecules (polymers). In contrast to polycondensations no molecules are split during polyaddition. A familiar example of polyaddition is the reaction of diisocyanates with diols (polyfunctional alcohols) to form polyurethanes. In this case the isocyanate group (-N = C =O) reacts with a hydroxy group (-OH) to a bridge urethane (-NH-CO-O-).

baur-hauptformel-enIn general the chemistry of all organic compounds can be characterised as alcohols whose characteristic functional group is the hydroxal group (OH). In the polyurethanes, polyether polyols and/or polyester polyols with relatively high molecular weight are used and are e.g. contained in a two-component system in the A-component and are also referred to in short as polyols.
Polyisocyanates (B components) can be divided into aliphatic and/or cycloaliphatic and aromatic isocyanates. They can be summarized by the following general structural formula:
The aromatic polyisocyanates have a greater economic impact. They are used in more than 90% of polyurethane applications. The two materials mainly used are the 4,4′-methylenedi (phenyl isocyanate), referred to in short as MDI, and toluene diisocyanates, referred to in short as TDI. Aliphatic and cycloaliphatic iIsocyanates are used mainly in coatings due to their resistance to light.

The preparation of the polyurethane foams is carried out by so-called foam moulding, wherein the reaction mixture is introduced into tempered moulds that it completely fills after completion of the foaming reaction. In order to produce complex moulded objects out of polyurethane components, in the 70s the so-called RIM (Reaction Injection Moulding) was invented. The RIM process is based on the rapid dosing and mixing of components (e.g. 2 components: A and B components) in a mixing chamber and injection of the reactive mixture e.g. into a mould (additionally for foams: the foaming of the reaction mixture and the filling in of the mould, as well as rapid setting). The cycle time is a few minutes..