by Duncan Geddes
A polymer may be defined as a large molecule comprised of repeating structual units. The word POLYMER is derived from the Greek word of poly meaning many, and meros meaning part.
A large molecule is usually regarded as one having at least a molecular weight of 1000. The term MACROMOLECULE means a large molecule and comes from the Greek word macro meaning large. Macromolecule is often used synonymously with the term POLYMER.
A pre-polymer can be defined as a reacted but not completely polymerised product. In the field of polyurethanes the pre-polymer is a product formed by reacting polyol(s) with aliphatic or aromatic diisocyanates. The properties of the pre-polymer obtained are dependent on the types of polyols and isocyanates used. In the early days of polyurethane technology hydroxil tipped pre polymers were used but today most pre-polymers tend to be isocyanate (NCO) tipped. If the pre-polymer has an excess of isocyanate then the product is called a quasi pre-polymer.
Early polyether polyurethane slabstock foams were based on a two stage pre-polymer or quasi pre-polymer process. The polyol used then was based on a poly (propylene oxide) diol which was first reacted with TDI isocyanate to produce a pre-polymer which was then reacted with water and optionally further polyether polyol to produce a foam. However with the introduction of organotin catalysts and silicone surfactants, such systems were rapidly superseeded by one shot formulations. In one shot formulations all the formulations all the ingredients polyol, isocyanate, water, catalysts, stabilisers etc, are mixed together simultaneously to produce foam.
On continuous slabstock foam making machines the manufacure of foam is carried out by metering all of the ingredients mentioned to a mixing head and then dispensing the mixed material onto a moving converyor provided with sidewalls. If the number of seperate streams for polyol and isocyanate are limited then this is one reason why a foam manufacturer may make use of the pre polymer.
Other advantages for using pre polymers for foam manufacture are :
- Manufacture of speciality foams
- Reduced isocyanate vapour hazard
- The increased viscosity can lead to more efficient mixing
Polyurethane pre-polymers are used to manufacure PU foams for various industries :-
Construction : Isocyanurate foams, integral skin foams
Transportation : Seting, Headliners
Medical & Hygiene : Hydrophilic/absorbent foams wound management
Bedding : Slow recovery/memory foams for mattresses