Athletic tracks are commonly surfaced with a mixture of a moisture-curing polyurethane binder and recycled rubber crumb. The selection of the components of the binder is influenced by several factors including the cost of the components, the physical properties of the binder, and the quality of the resulting product. Additionally, a polyurethane binder having volatile irritating components is objectionable to individuals who must use the material regularly.
Two formulations of a polyurethane binder are in common use: one containing methylene diphenyl diisocyanate (MDI), and another containing a mixture of toluene diisocyanate (TDI) and MDI. The moisture-curing one component adhesive binder that employs only MDI as the polyisocyanate component is inexpensive and safer to use than the TDI-containing binder. However, the commonly used MDI-only binder does not have a long pot life and typically gels in 1–1.2 hours. An adhesive's pot life is the length of time during which an adhesive remains suitable for use. A long pot life is important in applications such as athletic track surfacing because several hours may be required to apply the track material. Additionally, the material that is applied initially can be required to form a joint with the material that is applied last. Ideally, the joint between the material that is applied first and the material that is applied several hours later should be as seamless and aesthetically pleasing as possible. To prevent a seamed joint from forming that is undesirably susceptible to wear and potentially dangerous to the users of the track, the polyurethane material must not have completely cured before installation is complete. Adhesives with a long pot life, that are still workable when the final material is applied, form joints that are nearly, if not entirely, seamless. A long pot life provides a long working time to construct a resilient surface and creates a more seamless, aesthetically pleasing result.
The second commonly used moisture-curing one component polyurethane binder formulation, that employs TDI, is a popular choice in the athletic surfacing industry because the binder has a long pot life, typically 7 hours, and the resulting product demonstrates good physical properties. Unfortunately, because of the high vapor pressure of TDI, formulations containing TDI are unpleasant to work with. Health risks of an adhesive formulation containing TDI can include irritation of the nose and throat; choking and paroxysmal cough; chest pain and retrosternal soreness; nausea, vomiting, and abdominal pain; bronchial spasm; dyspnea, asthma, and pulmonary edema; and conjunctivitis and lacrimation (from the Merck Index, 12th edition, 9668).
Both the currently used formulations, the TDI-containing and MDI-only polyurethane binders, use prepolymer mixtures having an excess of isocyanate groups. The TDI binders are obtained by reacting TDI and MDI with a polyol, typically a polyether or polyester polyol. The MDI-only binders are obtained from the reaction of a polyol with an excess of MDI. The TDI and MDI-only binders harden through reaction with atmospheric moisture.