What are Polyurethanes?

Contrary to the generality of plastic materials, which have well-defined and known properties, the versatility of polyurethanes is so great and the field of applications so vast that allows polyurethanes to be present and used in countless applications in our every day life. Polyurethanes exist as soft materials in the foams of mattresses and sofas and also in rigid materials such as skateboard wheels or surfboards. They are marketed in the oily form (thermoplastics), in water-based lilies, solvent-based (paints, varnishes), without solvents (pre-polymer) or in gel. They are used both in construction applications (adhesives, sealants) and biometric applications. Polyurethanes are a huge market that goes from "commodities" to market niches.

Despite the disparate characteristics, there is something in common in all these products: a chemical group called urethane. The beginning of the success story of polyurethanes dates back to 1957, when Otto Bayer studied and developed the polyurethane synthesis line. In a more technical way it is said that the urethane group results from the reaction between a hydroxyl group (OH) and an isocyanate (NCO).

Chemically when a molecule contains a hydroxyl group (OH group) it is said that this molecule belongs to the family of alcohols. A molecule containing several OH groups is usually called a polyol. If a molecule possessing an OH (hydroxyl) group "finds" a molecule with an isocyanate group (NCO) then the 2 molecules react to form a single molecule with a urethane group.

As with most polymers, in order to achieve the desired properties the molecules will have to "grow" in size. For this purpose are used molecules that contain more than one reactive group: they are used with 2 or more NCO groups. in this way the molecule will have 2 or more reactive points where it will grow and increase in size. A polyurethane is a molecule formed of multiple urethane groups, each resulting from the reaction between a hydroxyl and an isocyanate.

The isocyanates commonly used in the production of prepolymers for the agglomeration of cork are TDI and MDI, whereas in the production of dispersions for the bonding of discs IPDI is usually used.

In the production of prepolymers, the extent of the reaction between the polyols and the isocyanates is limited so that the polyurethane molecules do not reach too large sizes and at the end there is still an unreacted isocyanate group. By limiting the molecular size of these polyurethanes it is thus possible to obtain liquid materials that are ideal for use as binders.

Pre-polymers when used as binders will be cured primarily by the action of water. Once the curing process is complete, the isocyanate groups disappear making the prepolymers perfectly harmless materials.