Because of the complex chemistry and/or the expensive organic chemicals involved, such high-performance resins are affected by price levels, in practice from 4-20 times higher then those of the best conventional resins.  This still confines their industrial uses to the narrow fields of composite materials for missile, military, aeronautical and aerospace constructions, as well as to specialized electrical & electronic devices and components, whose very critical service conditions had dictated the development of most high-performance resins during the 1970s and 1980s.  Besides their heavy economics, intrinsic and often remarkable processing issues are proper to these resins:  i) many of them are solids to be hot-melted and kept warm during all the processing stages;  ii) many of them are extremely viscous liquids, whose manipulation is feasible only under adequate heating to moderate their viscosity, or even preferably as solutions in organic solvents (to be stripped thoroughly away after, e.g., the fiber impregnation operations as in the manufacturing of pre-pregs for structural laminates or printed circuit boards);  iii) the existing high-performance resins are inherently characterized by slow curing kinetics, requiring prolonged processing times at high temperatures (hardening temperatures typically above 150°C, followed by long post-curing treatments at 200-300°C, and even higher temperatures).

Numerous polymeric products containing (chemically and thermally-stable) heterocyclic chemical structures (exemplified in Scheme 2) are attainable from organic isocyanates through a plurality of cycloaddition or cyclocondensation reactions [5].  Among such polymeric products, poly(isocyanurate)s have a renowned industrial importance:  typically glassy, densely cross-linked and brittle polymeric materials containing a plurality of isocyanurate structures (A), largely employed as rigid cellular materials for thermal and/or acoustic insulation.  Such products are attainable by the direct, and optionally very fast, cyclotrimerization, promoted by a variety of catalysts, of liquid polyisocyanates and/or isocyanato-functional oligomers [5]:  Equation (1) of Scheme 3].

In parallel to poly(isocyanurate)s, R&D efforts were devoted years ago to poly(2-oxazolidone)s, thermoplastic polymers with a chemical structure comprising the disubstituted heterocyclic (penta-atomic) oxazolidine-2-one (or simply 2-oxazolidone) structures (B.1) and/or (B.2).  These products can be conveniently synthesized through the cycloaddition reaction, activated by suitable catalysts, between isocyanates and epoxides shown as Equation (2) in Scheme 3.

With respect to thermoplastic poly(2-oxazolidone)s, a truly promising role in specialty industrial applications has been gained since 20 years ago by cross-linked polymeric materials containing 2-oxazolidone, or jointly 2-oxazolidone and isocyanurate, structures, deriving from the variously catalyzed polymerization of reactive systems of epoxy resins + isocyanates and/or isocyanato-functional oligomers.  This is fully explained by the nice thermal stability and chemical resistance, the high softening temperature (easily and even well above 200°C), as well as by the convenient performance-to-cost ratio, of these materials.  A bright confirmation of such interest comes from the numerous international patents issued in 1980s and 1990s, claiming the preparation and a variety of heavy-duty uses of these thermosets as adhesives, polymeric matrices for composite materials, cellular materials for thermal and/or acoustic insulation, protective coatings, electrical insulators, etc.

Unfortunately, the amplitude and complexity of the isocyanate and epoxide chemistry are widely recognized, and such that a plurality of concurrent and often competing reactions and side-reactions must be considered in the curing of mixed isocyanate-epoxy reactive systems.  This variety of chemical processes (summarized in Scheme 3) tends to make inherently difficult the fast and consistent generation of hybrid isocyanurate-oxazolidone thermosets with the desired molecular structure and physico-mechanical properties [5].

More specifically, the kinetics of each reaction of Scheme 3 is influenced at a profoundly different extent by a multitude of catalytic and co-catalytic substances (either intentionally added or present as impurities or by-products), by the concentration ratio between reactive species (isocyanates and epoxides), and, greatly, by temperature and thermal history.

Many different catalysts have been proposed and investigated for this peculiar type of reactive systems:  ethyl-methyl-imidazoles and other alkyl-imidazoles, quaternary ammonium and phosphonium salts, alkali and alkaline-earth metal halides in dipolar aprotic solvents, complexes of Lewis-acids (such as boron halides, aluminum chloride, etc.) with tertiary amines, amides, phosphines, phosphine oxides, etc., as well as many catalysts typically used for the manufacture of conventional polyisocyanurate foams, e.g. alkali metal carboxylates, carboxylates of alkaline-earth and various heavy and transition metals, amino-phenols, and so on [5,6].

In general, however, the available catalysts either do not allow at all, or do that at high concentrations (detrimental to the thermal, oxidative and chemical resistance of resulting thermosets), for hardening times of liquid isocyanate-epoxy resins short enough (< 20-30 minutes) at reasonably low temperatures (20-80 °C), i.e. under thermal conditions complying with the majority of current thermosetting resin processing technologies (especially for composites).

Many of the most active catalysts among those cited above promote the preferential formation of isocyanurates rather than 2-oxazolidones, thus leading to too much densely cross-linked, and unacceptably brittle, materials.  As an alternative to this (or besides this), such catalysts cause at least one of the unwanted side-reactions (3) and (4) of Scheme 3.  Reaction (3) [carbodiimide formation] implies undesirable isocyanate consumptions, associated with a CO2 generation within the target thermosets being manufactured (i.e. easily leaving micro-cavities acting as subtle micro-structural defects).  Reaction (4) irreversibly subtracts epoxide groupings, to be spent for the generation of 2-oxazolidone structures.

On the whole, such drawbacks have been preventing in practice until today isocyanate-epoxy systems from playing their potential and significant role in the ambit of high-performance thermoset materials.  Recently, a deep knowledge of the peculiar curing mechanisms and related chemical and physical control parameters has generated the skills for setting up and managing neat and well-targeted processing protocols for such reactive systems.  This, associated with the development of the very active and selective FPC catalysts, has created the availability of a class of reliable thermosetting resins (ISOCYANATE–EPOXY FPR Resins) capable of surprisingly high thermo-mechanical performance, possessing an excellent thermal, oxidative and chemical resistance, and simultaneously being attractively much cheaper than the high-performance resins nowadays existing on the market (see Tables 1a and 1b).

For such reasons, these resins may thus be ideal for the following range of structural, elecrtical/electromechanical, and chemical applications:

a.   for articles and devices requiring high-performance thermosets, though designed for "not extreme" service conditions, and thereby such that the strong extra-costs associated with the use of the current high-performance resins could not be repaid;

b.   for items and devices with superior performance characteristics, though not justifying their integral fabrication by very expensive high-performance resins, and thus made of conventional thermosets, either associated with labor-intensive and money-consuming artifices (such as thickness multiplications of structural components or parts, additional chemically- or fire-resistant over-coatings, heavy additions of flame-retardant additives, etc.), or leading to fabrication and marketing of finished or semi-finished items whose characteristics are critically close to the specification limits for their service conditions;

c.   for items and devices whose destination would require the use of specialty and expensive thermosets, but whose fabrication and marketing are economically viable only if manufacturing methods, machines, and conditions suitable for conventional resin processing may be employed.

3.    Fast-Curing and High-Performance Isocyanate-Epoxy FPR Resins

3.1  General Characteristics

 Fast-curing and high-performance ISOCYANATE-EPOXY FPR Resins are two-shot thermosetting systems based on liquid aromatic polyisocyanates of the diphenylmethane-diisocyanate (MDI) family [component A] + liquid, di- or multifunctional, glycidylether-type epoxy resins [component B] + specialty & proprietary polymerization FPC catalysts . Upon mixing of the two components A + B in a 70:30 to 60:40 weight ratio (+ the appropriate FPC catalyst), the resulting resins are nearly odorless, low-viscosity liquids, with a pot-life at room temperature adjustable from 10-15 minutes to 6 hours, complying with the most widely different processing needs.  The subsequent polymerization converts them into densely cross-linked, hard and high-softening materials, with a mixed isocyanurate-oxazolidone molecular structure.

Through their own specialty & proprietary polymerization catalysts FPC, the resin hardening time at temperatures from 25 to 100°C may easily be varied at will within a broad interval from hours to a few minutes, and optionally made as short as 20-30 seconds at 80-100 °C.  After appropriate post-curing, the solid products are turned into densely crosslinked and absolutely insoluble, amber-colored polymeric glasses, with a distortion temperature typically comprised in the 250 ÷ 320 °C range.

By varying the catalyst type and concentration, hardening times can be adjusted to fulfill the wide range of processing requirements including those of fast manufacturing technologies, such as R-RIM, S-RIM, RTM, HS-RTM, and pultrusion, to those of relatively slow Liquid Injection Molding, Vacuum Infusion Molding, and resin casting techniques.

These hard, moderate-cost thermosets are further characterized by:  1) a superior hydrolytic, solvent and chemical resistance;  2) an intrinsic flame resistance;  3) outstanding adhesion to mineral glasses, ceramics and metals.  Besides such characteristics, durability at peak temperatures of 350°C, and at continuous service temperatures of up to 200°C, make these resins materials of choice as matrices for structural composites, for fabrication of parts and components, embedding/encapsulation or coating of electrical/electronic/electromechanical devices, and a variety of applications whenever a critical combination of heavy-duty performance, fast processing and competitive prices is a critical issue.

3.2     Processability Characteristics

3.2a    Pot-Life and Polymerization Rate

• Pot-life at temperatures up to 50°C:  perfect latency (stability of the initial viscosity) adjustable from 10-15 minutes to 1-1.5 hours, depending on temperature, FPC catalyst type and concentration (see, for instance, the viscosimetric diagram of Figure 5).

• Gelation times at 60-100°C:  from 20 seconds to 2 hours, depending on the catalyst type & concentration.

• Vitrification times at 60-100°C:  from 40 seconds to 6 hours, depending on the catalyst type & concentration.

• The resin hardening must be completed by thermal after-treatments:  1 to 6 hours at temperatures from 150 to 240°C (typically, 1.5 ÷ 2 hours at 180 ÷ 240°C).  Plasticized FPR resins (with lower final Tg) require shorter post-curing treatments, and/or lower post-curing temperatures (150 ÷ 180°C).

Deep investigations on the complex curing processes of FPR resins have been performed by mapping the evolution of their dynamic-mechanical properties during cure under isothermal treatments (TTT transformation diagrams, exemplified in Figure 1a), and under linear heating ramps (CHT transformation diagrams, exemplified in Figure 1b) [7].   Such analyses have demonstrated how the entire polymerization of such resin systems consists of two distinct and separable steps, as evidenced in Figure 2 [8,9]:  (i) the first reaction step, occurring at temperatures of up to 120°C, yielding a yellowish, brittle glass, having a maximum glass transition temperature (Tg1°°) of 100-120°C, still heat-moldable and soluble in many ordinary polar organic solvents;  (ii) the second reaction step, taking place at temperatures above 140°C, leads to the final amber to dark amber-colored, completely insoluble and high-softening material (up to its maximum glass transition temperature Tg2°°).  With low-to-moderate catalyst concentrations, the curing process may thus be interrupted at the first stage by rapid cooling;  the pre-polymerized resin can be stored, optionally ground, molded at a later time, and subsequently submitted to the full-curing thermal treatment.  This makes these resins applicable for glass or carbon fiber pre-pregging technologies in general (for manufacturing of structural composites and Printed Circuit Boards).  By virtue of their inherently-low initial viscosity, these resin systems present the advantage over epoxy resins of requiring no solvents (with subsequent solvent removal needs) in the fiber impregnation process.  The only care needed is to protect the solid, partially cured isocyanate–epoxy materials from excessive moisture during storage.

   The CHT diagram of Figure 3 shows the time-temperature values for the different fundamental steps (devitrification and liquefaction of the fresh resin, gelation, vitrification, and, finally, devitrification of the fully cross-linked resin) of the overall curing process of an ISOCYANATE-EPOXY FPR resin under linear, continuous heating ramps (specialty FPR H-0 resin, with slow catalysis), starting from the virgin glassy  resin at -50°C.  Besides the resin liquefaction, gelation, vitrification and final devitrification steps, the diagram of Figure 4 displays and exemplifies in the best way (by monitoring the evolution of the dynamic-mechanical properties of the neat FPR H-1 resin, with slow catalysis) the two distinct, aforementioned curing steps (i) e (ii) of ISOCYANATE-EPOXY FPR resins.

3.2c      Rheological Properties

• Initial viscosity:  100 ÷ 600 cps