- A PU is made by mixing together the ingredient chemicals (Isocyanate and Polyol) in predetermined proportions, which then react to form the polymer PU.
- Uniquely, PUs utilise simultaneous polymerisation and shaping of the part.
- The production of consistent end products depends on mixing, in precise ratio, the ingredient chemicals and maintenance of the appropriate processing temperatures. As the liquid Isocyanate and Polyol react to form the PU, the liquid mix becomes increasingly viscous eventually forming a solid mass. The reaction is exothermic and therefore heat is involved.
- Other ingredients will be included in the polyol blend, for example the catalyst which controls the rate at which the liquid mixture reacts to become solid.
Success is due to well chosen technology
There are no hard and fast rules for obtaining the optimum PU end product, success is due to good formulation selection with well chosen and appropriate processing parameters, technology and mould geometry. The process by which liquid polymers are converted to elastomeric or glassy solids is fundamental to the manufacture of PU products.
Selection of a Polyurethane
- Consider the requirements which the application will demand of the PU with respect to chemical and physical properties.
- Based upon an understanding of what controls these properties select a few candidate PU systems.
- The properties of a PU are largely controlled by the chemical nature of the system and how it is processed so it is prudent to consult specialist suppliers and processors at this stage.
Basic Polyurethane Chemistry
- The simplest PU is linear in which the hydroxyl compound and the nitrogen compound each have a functionality of two. This can be represented by the following:
Isocyanate + Polyol = Polyurethane
- The isocyanate can react with different chemical groups, so the final properties of the polymer will vary according to the reaction route taken.
- Therefore the formulation of a PU must take into account every possible reactive constituent. PUs may have a very widely varying structure depending on the type of isocyanate and the type of reactive hydrogen components present in the formulation.
- The presence or otherwise of the various groups along the urethane linkage will control the end properties of the polymer.
- The curing of a PU can be regarded as the formation of a network, also called cross-linking, the extent or degree of cure is often expressed as the crosslink density.
- The extent of cross linking may vary and will be reflected in the final properties of the PU, ranging from longer, linear chains of flexible elastomers and foams to the rigid, heavily cross linked polymers.
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