The term "prepolymer" can be misleading to someone not fully conversant in the patois of polyurethane terminology. A polyurethane prepolymer is one in which all of the polyol hydroxyl end groups have been reacted with isocyanate groups leaving isocyanate functionality at the termini instead of hydroxyls. In principle, a prepolymer molecular weight should be the molecular weight of the polyol plus two times the molecular weight of the diisocyanate capping the polyol. In practice, things are rarely that neat . Figure 2.45 shows a simplified view of the reactants to products of prepolymer production.
In principle, a prepolymer could be formed by reacting diisocyanates with an excess of polyol to produce a polyol functional prepolymer with urethane functionality at its center, but this is not practiced commercially and the term "prepolymer" is always used to refer to polyols terminated with isocyanate functionality. The excess isocyanate is required to prevent molecular weight growth caused by opportunistic chain extension of unreacted polyol reacting with the isocyanate-terminated prepolymer. In practice, chain extension always occurs but can be limited by increasing the ratio of isocyanate to hydroxyl functionality. A true "prepolymer" in this regard would then distill the excess isocyanate from the product. In the case where the excess isocyanate is left in the product, this is referred to as a "quasi-prepolymer" . In practice, quasi-prepolymer refers to systems with relatively high levels of free isocyanate (>12%), while prepolymer refers to systems with low levels of free isocyanates (<12%). Overall, over 400 million pounds a year of prepolymer is used commercially for many applications where the advantages of prepolymers can be realized.
Prepolymers find application for several reasons. Among the advantages are that there is improved handling since prepolymers are liquids, they are storage stable (when kept dry), and they provide a source of isocyanate functionality with lower vapor pressure than free isocyanates. Additionally, a prepolymer will usually provide better compatibility between additional polyurethane formulation components since the reacted systems provide lower surface tension to other chemicals within the solubility parameter range. Lastly, it is believed that the prepolymer allows one to "prebuild in" the final polymer properties and obtain a more uniform final polyurethane structure [107, 108]. This is particularly true in cast urethane systems where mixing and physiochemical polymer kinetics are more limited and make obtaining equilibrium-phase structures more difficult (see Chapter 9).
FIGURE 2.45 General preparation of a TDI—polypropylene oxide prepolymer.
As a practical consideration, prepolymers are usually used and defined on the basis of their percent isocyanate or more commonly as "percent NCO." On reflection, the calculation of this parameter is not as intuitively defined as desirable and so merits exposition. The definition of %NCO is by equation
where Y= the equivalent weight of the isocyanate, X= equivalent weight of the polyol, the weight of NCO = 42, and X+ Z= the number of grams of isocyanate that must be reacted with Y grams of polyol to make a M100% NCO prepolymer or
As an example, if it is desired to prepare a 12.5% NCO prepolymer with methylene bisphenyl diisocyanate (MDI) (equivalent weight = 125 g/NCO eq) with a polyol having equivalent weight= 1633 g/eq; using the equation for Z, one would need
Thus, N+Z= 125 +1041 = 1166g of isocyanate per 1633 g of polyol (total mass of 2799 g) to make the desired 12.5% NCO. Smaller quantities would be prepared by a simple ratio adjustment.
Prepolymers offer an attractive approach allowing one to prepare cast systems, especially elastomers, from entirely liquid components. Using pure MDI, for instance, would require that all ingredients be heated to a temperature greater than the MDI melting point (ca. 55 °C), which could shorten the window for convenient processing considerably. As a liquid, it is additionally convenient to begin with a system that is as low viscosity as possible to avoid problems such as entrainment of air during processing. Minimization of viscosity is usually a function of pH control, which under basic conditions can result in trimerization of the isocyanate. Other viscosity-building cross-linking reactions occur in making prepolymers such as formation of allophanates and biurets that are more prevalent in NCO-rich environments.