Dr. F. Parodi - web paper #2 (high-performance isocyanate-epoxy FPR resins)

Among polymeric materials, cross-linked glasses resulting from the in situ polymerization of reactive liquids or low-melting solids are recognized to play from decades a key role for hundreds of industrial, civil and military applications. As well known, the broad definition of thermosetting resins adopted worldwide for such reactive products embraces a multitude of chemically multi-functional systems of oligomers and/or low-molecular weight organic compounds + reaction initiators, catalysts, and additives and ancillary components of many types [1,2].

Ecological and environmental issues over the last 10-15 years are pushing for an increasing adoption of thermoplastic polymers (whenever viable, and particularly for high-volume consumer articles and components) by virtue of the (at least potentially) easy recyclability of such materials with respect to thermosets. Despite that, the infusibility and insolubility of thermosets, representing per se a severe obstacle to recycling, are precisely premium attributes making cross-linked resins still nowadays and for a reasonably long future materials not replaceable for a broad variety of qualified purposes:

as matrices of light-weight, structural and semi-structural composite materials (containing high volume fractions of high-modulus reinforcing fibers) [1];
as encapsulation/embedding materials for electrical/electronic components (primarily for high service-temperature devices);
as electrically insulating and dielectric materials for high-power/heavy-duty electromechanical, as well as medium/high-voltage electrical, devices;
as composite materials matrices for printed circuit boards;
as protective (and primarily corrosion-resistant) coatings for metal surfaces.

A broad spectrum of physicochemical, thermal and mechanical characteristics qualify the numerous types of thermosetting resins developed and made commercially available according to the highly diversified technical requirements of their applications. Despite the complexity of the technological scenario, the fulfillment of requisites according to a rather narrow range of parameters may be considered as the discriminant between the two major classes of conventional thermosetting resins (Table 1a) and high-performance resins (Table 1b): glass transition temperature (Tg), heat distortion temperature (HDT) and permanent service temperature; hydrolytic and chemical resistance; impact resistance and adhesion to metals and mineral materials; fire reaction (flame retardancy and smoke emission characteristics).

Accordingly, a sort of performance borderline, beyond which we can place high-performance resins (complying with the most restrictive requisites of the aforementioned applications) may be depicted as in the following:

Tg and HDT: > 180 - 200 °C;
Permanent service temperature: > 160 - 180 °C;
Hydrolytic resistance: virtually unlimited;
Chemical resistance: almost complete chemical and physicochemical inertness, except, and limitedly to a slow and modest degradation, under the most aggressive chemicals (hot, strong acids and bases);
Impact strength and adhesion to metals, inorganic glasses and ceramics: equivalent to, or better than, those of the best epoxy + amine hardener resin systems;
Fire resistance: significant, inherent flame retardancy (V1-V0 according to UL 94).

The above requisites are not only by far beyond the performance characteristics of the entire category of the unsaturated polyester resins most commonly used (orthophthalic, isophthalic and bisphenolic ones), but also above those of the better-qualified and more expensive standard vinyl-ester (epoxy-acrylate) resins, and even the specialty (or multifunctional) vinyl-ester ones (i.e., multifunctional acrylates from epoxy-novolacs). Yet, the overall performance level outlined above cannot be reached by all conventional and semi-conventional epoxy resin systems: bisphenol A and F-derived epoxy resins homopolymerized by tertiary amine or boron halide-based catalysts; bisphenol A and F-derived epoxy resins and epoxy-novolacs + standard liquid amine or anhydride hardeners, or dicyandiamide. Other important thermosetting matrices are below the same high-performance standards: amino-resins (such as urea- and melamine-formaldehyde resins, etc.) and phenolics (though possessing an almost unparalleled fire resistance) own to their inherent brittleness and poor adhesion to metals and mineral materials in general, as well as many other common resins (such as alkyd resins, furane and indene-cumarone resins, etc.) being confined to coating applications due to lacks in their thermo-mechanical and/or chemical properties [1].

The severe performance requirements as per the above scheme restrict the field of thermosetting resins complying with a combination of high thermal, thermo-mechanical and chemical properties to a rather narrow range of high-priced products, including: a) epoxy systems based on standard bisphenol A-derived or various specialty tri- or tetra-functional epoxy resins + specialty solid amine or anhydride hardeners [such as the 4,4'-diaminodiphenylsulfone (DDS) or its 3,3'-isomer, and the benzophenone-3,3',4,4'-tetracarboxylic dianhydride, respectively]; b) condensation and PMR polyimide resins; c) standard and modified bismaleimide resins; d) polystyryl-pyridine resins, acetylene (or ethynyl)-functional resins, benzocyclobutene-, cyanato-, cyanamido- and N-cyanoureido-functional resins, etc. [1,2].

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Table 1a - *Conventional Thermosetting Resins:* Tg and price index values
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	ortophthalic	Tg = 90 ÷ 100°C	price index = 1.0
unsaturated polyesters	isophthalic	Tg = 115 ÷ 125°C	price index = 1.1 ÷ 1.2
	bisphenolic	Tg = 110 ÷ 130°C	price index = 1.2 ÷ 1.4

vinyl-esters	standard	Tg = 120 ÷ 130°C	price index = 2.7 ÷ 3.2
vinyl-esters	multifunctional	Tg = 160 ÷ 185°C	price index = 3.5 ÷ 4

epoxy resins + standard hardeners	standard	Tg = 120 ÷ 165°C	price index = 2.8 ÷ 3.5
epoxy resins + standard hardeners	epoxy-novolacs	Tg = 120 ÷ 165°C	price index = 4.8 ÷ 5.5

phenolics
amino-resins (urea-formaldehyde, melamine-formaldehyde, etc.)
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Table 1b - *High-Performance* *Thermosetting Resins:* Tg and price index values
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conventional epoxy resins & epoxy-novolacs + specialty hardeners	Tg = 180 ÷ 280 °C	price index = 4.5 ÷ 6.5

specialty multifunctional epoxy resins + specialty hardeners	Tg = 260 ÷ 340 °C	price index = 8 ÷ 15

condensation polyimide resins	Tg > 450 °C
PMR polyimide resins	Tg = 400 ÷ 450 °C	price index = > 60
bismaleimide resins (std.)	Tg = 350 ÷ 400 °C

polystyryl-pyridine resins
acetylen- (or ethynyl-) functional
benzocyclobutene resins		price index = 20 ÷ 50
cyanato-functional resins
N-cyanoureido-functional resins

ISOCYANATE-EPOXY resins FPR S (standard grades)	Tg = 270 ÷ 300 °C	price index = 3.6 ÷ 4.5
ISOCYANATE-EPOXY resins FPR H (specialty grades)	Tg = 300 ÷ 320 °C	price index = 4.3 ÷ 5.0
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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).Because font>

2. Poly(isocyanurate)s and Poly(2-oxazolidone)s

The high thermal and thermo-mechanical properties, and the good fire resistance, proper to thermoset materials deriving from high-performance resins are strictly linked to their own chemical structure, based on, or including considerable fractions of, sulfonyl groups –SO2– and/or (hetero)cyclic and (hetero)polycyclic structures (a variety of which is given in Scheme 1). Such chemical groupings are characterized by an outstanding structural stiffness, thermal stability and oxidative inertness, as well as by remarkable attitudes to generate (or indirectly promote, as in the case of –SO2– groupings) carbon-rich/graphitic by-products by pyrolysis (by-products universally "acclaimed" as self-generating and efficient barriers to flame propagation in polymeric materials) [3,4].

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).

Table 2 - Processabiliy of ISOCYANATE-EPOXY FPR Resins: Typical Curing and Post-Curing Temperatures and Times, and Comparison with Current Liquid Epoxy Resin Formulations.
processing stage	FPR resins	epoxy resins + hardeners of different type
processing stage	FPR resins	anhydrides (catalyzed)	amines	DDS
curing	80 ÷ 100 °C 1 ÷ 10 min	85 °C 2 h	80 ÷ 120 °C 1 ÷ 2 h	180 °C 3 h
post-curing	180 ÷ 240 °C 1.5 ÷ 2 h	150 ÷ 230 °C 4 h	150 ÷ 230 °C 4 h	250 °C 2 h

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.


(a) complete TTT diagram for a generic thermosetting resin		(b) complete CHT diagram for a generic termosetting resin

Figure 1 - Transformation diagrams for thermosetting resins: a) for isothermal curing treatments [Time–Temperature–Transformation diagrams (TTT diagrams)]; b) for curing treatments under continuous heating, at constant heating rate [Continuous Heating Transformation diagrams (CHT diagrams)].

Figure 2 - TTT diagram of the std. Isocyanate–Epoxy FPR S-1 resin (medium-slow catalysis).

Figure 3 - Transformation diagram under continuous heating, at constant heating rate [Continuous Heating Transformation diagram (CHT diagram)] of the specialty Isocyanate–Epoxy FPR H-0 resin (slow catalysis).

Figure 4 - Process stages of the overall dynamic curing of the specialty Isocyanate–Epoxy FPR H-1 resin (with slow catalysis), as evidenced through the evolution of its dynamic-mechanical properties (shear moduli G' and G") under a linear heating ramp at 2°C/min up to 360°C.

3.2b Microwave Processability

Thanks to the peculiar physico-chemical properties and chemical mechanisms of action of their specialty catalysts, these isocyanate-epoxy resins are exceptionally-well suited to be cured and/or post-cured by microwave heating. By means of such processing method, the curing and/or post-curing times can typically be minimized to 1/4 ÷ 1/10 of those required under conventional thermal conditions at the same temperature [10-12]. For instance, the 2 hour-long post-curing cycle at 180-220°C of (S-RIM-molded) glass fiber-reinforced FPR S-1 plates can be accomplished in just 15 minutes under microwave heating at an average temperature of 225°C of the resin plates.

Novel proprietary catalysts, specifically developed for the microwave processing of FPR resins ( FPC W1 e FPC W2 ), allow for the preparation of FPR resin compositions endowed with the following, extremely interesting combination of features: a prolonged pot-life at room temperature (up to 4-6 hours), coupled with particularly short vitrification times under microwave irradiation minimized to 1/8 - 1/10 of those under conventional thermal treatments, at the same resin temperature.

3.2c Rheological Properties

Initial viscosity: 100 ÷ 600 cps at 23°C, depending on the resin formulation, i.e. (without any resin diluents) viscosity values 5-10 times lower than those of known epoxy resin formulations, and 2-4 times lower than those of unsaturated polyester, and conventional & multifunctional vinyl-ester resins. Table 3 shows a comparison of the viscosity ranges for FPR resins at 25 and 50°C with those of different, liquid epoxy resin formulations currently in use.
Chemorheology: typical "snap-curing" profile, as required, e.g., by RTM and RIM technologies (as exemplified in Figure 5).
Thixotropy: FPR resins can easily be endowed with thixotropic properties, by addition of conventional thixotropic agents (e.g. 0.5 ÷ 1 % by weight of colloidal silica).

Figure 5 - Apparent Viscosity–vs–Time curves for the std. FPR S-1 resin (with slow catalysis) at different, constant temperatures.

Table 3 - Processabiliy of ISOCYANATE-EPOXY FPR Resins: Resin Viscosities as Compared to Current Liquid Epoxy Resin Formulations. Viscosity Values in cPs [mPa · s].
Temperature (°C)	FPR resins	DGEBA & DGEBF epoxy resins	epoxy-novolacs
25	100 ÷ 600	1000 ÷ 6000	1200 ÷ 9000
50	25 ÷ 200	150 ÷ 1000	250 ÷ 2000

3.3 Properties of Cured Resins

3.3a Heat Distortion Temperature

Glass Transition Temperature ( Tg ~ HDT ). Depending on the resin formulation: Tg of standard ISOCYANATE-EPOXY FPR S resins = 250 ÷ 300°C (typically: 270 ÷ 300 °C); Tg of specialty ISOCYANATE-EPOXY FPR H resins = 300 ÷ 320 °C. Plasticized formulations possess a glass transition temperature lowered to180 ÷ 240°C; for partially plasticized grades, the Tg spans the 230 ÷ 270 °C range. As an example of this, the dynamic-mechanical spectra of Figure 6 display a Tg ~ 300°C for the standard FPR S-1 resin (in excellent agreement with the value of ~ 290°C resulting from DSC analysis), and a Tg = 265-275 °C for the partially plasticized, low-viscosity FPR S-1 LV resin.



(a) std. *FPR S-1* resin (HDT > 250°C)		(b) partially plasticized *FPR S-1 LV* resin (HDT = 230-240 °C)

Figure 6 - Dynamic-thermal-mechanical (DMTA) spectra, under 2°C/min heating scans, for two neat, fully cured, Isocyanate–Epoxy *FPR* resins.

3.3b Thermo-Oxidative Resistance

Fully cured FPR resins possess an excellent thermal stability up to 280°C, being currently able to sustain continuous use temperatures > 150°C (typically, of 160 ÷ 180 °C, and up to 200°C), and peak temperatures up to 350-360 °C. Their high-temperature aging, in both inert atmosphere and air, implies a smooth and slow weight loss, without any bulk and surface micro-structural damages (with the resin surface remaining in fact smooth and brilliant). For example, the weight loss of neat resin specimens is of 5 - 6 % after 200 hours at 250°C in air; and that of glass fiber-reinforced or mineral powder-filled resins (i.e., quartz-, calcined clay-, silica-filled, etc.) [60 wt. % of fibers or mineral powder] spans the 2.5 - 2.8 % interval after 2000 hours of continuous exposure at 200°C in air. To give a better picture of this, Table 4 compares the thermo-oxidative resistance of FPR H resins with that of qualified, current epoxy resin thermosets. The data of Table 4 show a remarkable superiority of FPR H resins, even with respect to anhydride-hardened epoxy resins, whose thermo-oxidative resistance is currently acclaimed as optimal among thermoset polymeric materials, and weaker only than the much more expensive imide, cyanato- and ethynyl-functional resins.

Table 4 - Long-Term Thermo-oxidative Resistance of ISOCYANATE-EPOXY FPR Resins as Compared to Qualified, Current Epoxy Resin Thermosets: % weight loss under continuous exposure to hot air (specimens of neat, fully crosslinked resins).
% weight loss after	FPR H resins	DGEBA, DGEBF & epoxy-novolacs		epoxy-novolacs + DDS
% weight loss after	FPR H resins	+ std. anhydrides	+ std. amines	epoxy-novolacs + DDS
200 h at 210°C		0.3 ÷ 1.3 %	2.5 ÷ 5.8 %
100 h at 260°C		4.8 ÷ 6.1 &	> 6 % *
200 h at 250°C 200 h at 260°C	5 ÷ 6 %	9.0 ÷ 10.5 %	> 10 % *	> 10 % *
Tg of the various materials	300 ÷ 320 °C	140 ÷ 220 °C	130 ÷ 220 °C	210 ÷ 255 °C
* associated with significant to strong oxidative degradations

3.3c Fire Resistance

Fully-cured, neat FPR resins are inherently flame retardant (with respect to neat unsaturated polyester, vinyl-ester and conventional epoxy resins), and display a fire behavior not so much different from that of phenolic and imide resins. Classification according to the UL 94 test method (Underwriters Laboratories), for 3.2 mm-thick specimens:

neat resins	=	V1
resins filled with common, inert mineral fillers (talcs, micas, clays)	=	V0
resins filled with commercial, mineral flame retardants (10-20% by weight)	=	better than V0
partially brominated resins	=	much better than V0

3.3d Water Uptake and Chemical Resistance

In boiling water, as well as in saturated moist air, the fully-cured resins display water uptake levels remarkably below those exhibited by the best epoxy thermosets (maximum, equilibrium water uptake capacities of FPR thermosets = 0.9 ÷ 1.0 % by weight). The comparative data of Table 5 show the superiority of FPR resins over the entire category of epoxy materials, and particularly with respect to the DDS-cured ones (with the highest Tg levels) for structural, aerospace-grade applications (whose hydrophilicity problems, hydration attitudes in moist air, and resulting remarkable rigidity losses, dimensional instability issues, etc. are well recognized and critically evaluated in the field of structural composite materials).

Table 5 - Saturation Water Uptake of ISOCYANATE-EPOXY FPR Resins as Compared to Qualified, Current Epoxy Resin Thermosets.
	FPR resins	DGEBA, DGEBF & epoxy-novolacs		epoxy-novolacs + DDS
	FPR resins	+ std. anhydrides	+ std. amines	epoxy-novolacs + DDS
H2O uptake (wt. %)	0.9 ÷ 1.0 %	1.4 ÷ 2.4 %	1.8 ÷ 2.5 %	3.4 ÷ 4.1 %
Tg	270 ÷ 320 °C	140 ÷ 220 °C	130 ÷ 220 °C	210 ÷ 255 °C

Thanks to the absence of hydrolytically- and chemically-weak groupings (such as esters, amides and urethanes), as well as to the perfect chemical neutrality of their specialty FPC catalysts, the glass transition temperature and mechanical properties of FPR resins are unaffected (or just minimally affected) by aging in both moist environments and boiling water (even in the presence of surfactants). By virtue of their chemical structure, the resistance to very aggressive chemicals is excellent as well. Only concentrated, strong acids and bases can attack, and slowly degrade, the surface of the fully-cured resins, which sustain indeed, e.g., 12 hours in aqua regia at room temperature, or 24 hours in boiling aqueous (20 wt. %) caustic soda. Table 6 displays a comparison of the resistance of ISOCYANATE-EPOXY FPR H resins and qualified epoxy thermosets to hydrolytic attack and to a variety of chemicals. Such data enlighten a substantial and overall (even astonishing) "chemical superiority" of FPR resins over all the qualified epoxy systems of well established use; such advantages become "overwhelming" with respect to unsaturated polyester and vinyl-ester resins when hydrolytic resistance is being considered, and particularly the chemical resistance to bases, as well known, literally destroying all ester-containing polymeric materials in general.

Table 6 - Long-Term Chemical Resistance of ISOCYANATE-EPOXY FPR Resins as Compared to Qualified, Current Epoxy Resin Thermosets.
resistance	FPR H resins	DGEBA, DGEBF & epoxy-novolacs		epoxy-novolacs + DDS
resistance	FPR H resins	+ std. anhydrides	+ std. amines	epoxy-novolacs + DDS
hydrolytic	"unlimited"	very critical	good	good
to mineral oils up to 200°C	excellent	good	very good	very good
to common organic solvents	very good	good	very good	very good
to strong bases (diluted) to strong bases (concentrated)	excellent very good	poor very poor	very good good	excellent very good
to strong acids (diluted) to strong acids (concentrated)	excellent very good	good fair	poor very poor	critical poor
to nitric acid & nitric mixtures	fair	poor	very poor	poor
Tg of the various materials	300 ÷ 320 °C	140 ÷ 220 °C	130 ÷ 220 °C	210 ÷ 255 °C

3.3e Mechanical and Thermo-Mechanical Properties of Neat, Fully-Cured Resins

The overall spectrum of their mechanical and thermal-mechanical properties makes FPR resins excellent matrices for structural composites, especially for high-temperature applications.
From the standpoint of flexural, tensile and impact properties of dry-conditioned materials (at a relative humidity of 50-55 %) at room temperature, FPR resins are equivalent to the best unsaturated polyester and vinyl-ester resins, as well as to most conventional epoxy thermosets:
flexural strength at 23°C (ASTM D790) = 90 ÷ 110 MPa
flexural modulus at 23°C (ASTM D790) = 3 ÷ 4 GPa

A comparison of these values of flexural properties with the variability intervals for different types of high-quality epoxy thermosets (given in Table 7) shows a comparable rigidity, and a realistically ~ 20%-lower strength of FPR resins [for dry-conditioned specimens at 23°C (at a relative humidity of 50-55 %)]. However, because of the higher-to-much higher water uptake attitudes of epoxy thermosets with rispect to FPR resins (as said in paragraph 3.3d and shown in Table 5), comparative characterizations performed on samples conditioned for long times at 23°C at a relative humidity of 95-100 % evidenced a general equivalence of flexural and tensile strength between epoxy thermosets and FPR resins, and significantly higher values of elastic moduli of cured FPR resins with respect to the same reference epoxy materials.

As exemplified in Figure 6a, fully cured FPR resins maintain their mechanical properties almost unchanged over a very broad temperature range, at least up to 50°C below their glass transition temperature: elastic modulus decays of just 20 ÷ 25% over the entire -50 ÷ 200-220 °C temperature range for the standard grade resins (FPR S resins), and over the -50 ÷ 260 °C interval for the high-Tg ones (FPR H resins). As expected, proportionally smaller stiffness decreases are being displayed by glass or carbon fiber-reinforced materials.

Table 7 - Mechanical Properties of ISOCYANATE-EPOXY FPR Resins as Compared to Qualified, Current Epoxy Resin Thermosets: flexural characteristics at 23°C according to ASTM D790 (materials dry-conditioned at 23°C)
property	FPR S & H resins	DGEBA, DGEBF & epoxy-novolacs		epoxy-novolacs + DDS
property	FPR S & H resins	+ std. anhydrides	+ std. amines	epoxy-novolacs + DDS
flexural strength (MPa)	90 ÷ 110	125 ÷ 145	90 ÷ 130	120 ÷ 140
flexural modulus (GPa)	3 ÷ 4	3.2 ÷ 3.5	2.6 ÷ 3.4	3.2 ÷ 3.4
Tg of the various materials	270 ÷ 320 °C	140 ÷ 220 °C	130 ÷ 220 °C	210 ÷ 255 °C

3.3f Manufacturing and Properties of Structural Composite Materials

Wettability of reinforcing fibers and permeation of fiber structures by FPR resins. By virtue of both their particularly low viscosity and the fiber-wetting effects exerted by FPC catalysts, FPR resins possess the best attitudes to wet and permeate with incredible rapidity commercial glass and carbon fibers, and their structures (such as clothes, woven rovings, mats, various combinations of them, etc.), even those the most densely packed in manufacturing molds. Figures 7 and 8 display very clearly this through the macro-photographic and micrographic monitoring of the typical vacuum infusion of a glass fiber structure by the standard ISOCYANATE-EPOXY FPR S-1 resin, at room temperature. More specifically, Figures 7 and 8b show the strong physicochemical affinity between the resin and the reinforcing fibrous material, and the fast capillary advancement of the liquid resin along and within the fiber strands. Figures 7 and 8c-8e put in evidence the prompt and complete permeation of the fiber structure, leading as desirable to a translucent composite material just behind the advancing front of the liquid resin. The excellent translucence of composites resulting from impregnation of commercial glass fiber reinforcements is perfectly maintained upon curing and cross-linking completion of the resins.

*resin feeding*	►

Figure 7 - Optical macro-photograph depicting the evolution of the vacuum infusion of the glass-fiber filling (4 plies of woven-roving, 345 g/sqm each) of a Bag Molding envelope by the *FPR S-1* resin (with slow catalysis, at room temperature). Back + front lighting.

*resin feeding*	►

		(a) front lighting	(b) front lighting



(c) back visible lighting + front UV lighting (Wood lamp)		(d) back lighting	(e) back lighting

Figure 8 - Optical stereo-micrographs showing the evolution of the vacuum infusion of the glass-fiber filling (4 plies of woven-roving, 345 g/sqm each ) of a Bag Molding envelope by the *FPR S-1* resin (with slow catalysis) at room temperature: virgin and fully impregnated fibers, and advancing front of the liquid resin.

Short-term mechanical properties: the mechanical properties of structural laminates fabricated by fast Liquid Injection Molding with FPR resins are equivalent to those of the best epoxy laminates obtained by bag molding from commercial pre-pregs. Typical mechanical properties of structural composites from FPR resins (reinforcement: glass fiber woven roving; overall fiber content: 70-75 % by weight, 55-59 % by volume) are the following:

quasi-isotropic 8-ply glass fiber-reinforced laminates (0, 90, ±45°, symmetrical); properties at 23°C

flexural strength	ASTM D790	500 - 550 MPa
flexural modulus	"	19 - 21 GPa
tensile strength	ASTM D638	300 - 350 MPa
compression strength	ASTM D695	300 - 350 MPa

orthotropic 8-ply glass fiber-reinforced laminates (0, 90°); properties at 23°C

flexural strength	ASTM D790	600 - 650 MPa
flexural modulus	"	21 - 24 GPa

The excellent mechanical properties of fiber-reinforced FPR thermosets are linked primarily to the high adhesion of such resins to reinforcing fibers, as evidenced by the SEM micrographs of Figure 9, showing the fracture morphology of glass fiber-reinforced FPR resins. These micrographs exemplify, with both standard and partially plasticized FPR matrices, the post-impact permanence of significant portions of the glassy polymeric matrix firmly linked to the fiber surfaces, despite the micro-morphology proper to a brittle fracturing of the composites. Such strong FPR resin-fiber adhesion is further demonstrated by the values of the interlaminar shear strength (short-beam shear strength, according to ASTM D2344) of glass and carbon fiber-reinforced FPR resin orthotropic laminates (at 23°C):

laminate type	short-beam shear strength
glass fiber-reinforced orthotropic laminates	55 MPa
carbon fiber-reinforced orthotropic laminates	65 - 70 MPa

Post-impact mechanical properties: as compared to commercial, aerospace-grade carbon fiber/epoxy composites, evaluations of the residual mechanical strength (tensile, flexural, and impact strength) of structural, carbon fiber-reinforced FPR laminates after non-destructive ball-drop impacts allowed for rating them as aerospace-grade materials, with the advantage of a 60-80°C-higher distortion temperature.



(a) polymer matrix: standard *FPR S-1* resin



(b) polymer matrix: partially plasticized *FPR S-1* resin

Figure 9 - SEM micrograph pictures of the fracture morphology of glass fiber-reinforced Isocyanate–Epoxy *FPR* resins after a destructive ball-drop impact at room temperature. RTM composites; fiber reinforcement: woven roving.

3.3g Electrical Properties

The FPR resins are per se characterized by a spectrum of electrical properties (i.e., dielectric strength, dielectric constant and loss factor, surface and volume resistivity, as well as thermal endurance) similar to that of the best epoxy materials qualified on the market for heavy-duty, low and medium-voltage electrical applications (high-power electrical transformers, big capacitors and insulators, etc.).

Cast thermosets from quartz powder-filled (60 wt. %) FPR H-0 and H1 resins exhibit the following, typical properties:

dielectric constant [25°C, 50 Hz]	IEC 60250	2.9 ÷ 3.3
dielectric loss factor (tan d) [25°C, 50 Hz]	"	0.010 ÷ 0.015
dielectric strength (specimen thickness = 1 mm)	IEC 60243	28 ÷ 32 kV/mm
dielectric strength (specimen thickness = 2 mm)	"	18 ÷ 22 kV/mm
Permanent Service Temperature	IEC 60216	175 ÷ 185°C
Thermal Class rating	IEC 60085	> F class

4. Main Grades of Isocyanate-Epoxy FPR Resin Systems Developed

4.1 FPR Resin Systems

Resin System FPR S-1 : Standard resin, with medium viscosity and medium thermal-mechanical properties (HDT > 250°C), flame resistance = V1 according to UL 94, transparent, pale amber color. General purpose, high-temperature resin, and for manufacturing of composite materials with glass, carbon and/or Kevlar® fiber reinforcements of standard type.
Resin System FPR S-1 FG [food-grade resin] (experimental resin) : Novel, food-grade, fast-curing and standard-performance resin, with medium viscosity and medium thermal-mechanical properties (HDT > 250°C), flame resistance = V1 according to UL 94, transparent, pale amber color. High-temperature and strongly chemically-resistant resin for items in contact with foods and beverages; this resin system implies the use of the special, food-grade Catalyst FPC FG-2.
Resin System FPR S-2 (experimental grade) : Standard resin, with medium viscosity and medium thermal-mechanical properties (HDT > 250°C), halogen-free flame-retardant (V0 according to UL 94), translucent, pale amber color. Resin for standard high-temperature composite materials; high-temperature resin for general purposes requiring halogen-free flame retardancy.
Resin System FPR S-3 : Standard resin, with medium viscosity and medium thermal-mechanical properties (HDT = 240-250°C), partially brominated flame-retardant (V0 according to UL 94), transparent, amber color. High-temperature resin for standard composite materials, and general purposes, implying high flame retardancy requirements.
Resin System FPR S-1 LV : Very low-viscosity resin, with medium thermal-mechanical properties (HDT = 220°C), flame resistance = V1 according to UL 94, transparent, pale amber color. High-temperature resin ideal fo manufacturing of composite materials containing particularly high volume fractions of reinforcing fibers, and especially for structural composites with very high mechanical properties made by infiltration of densely packed preforms of glass, carbon and/or Kevlar® fibers.
Resin System FPR S-3 LV : Resin similar to FPR S-1 LV, partially brominated flame-retardant (V0 according to UL 94).
Resin System FPR H-0 : Very high-performance, specialty resin, of dark brown color, with medium/high viscosity and outstanding thermal-mechanical and chemical properties (HDT = 300° C), flame resistance = V1-V0 according to UL 94. Resin for highly-demanding, heavy-duty applications (high-temperature, structural composites complying with aerospace standards; high-voltage electrical applications; parts, components and protective coatings for chemically and biologically strongly aggressive environments).
Resin System FPR H-1 : High-performance, specialty resin, similar to FPR H-0, of amber color, with medium viscosity and high thermal-mechanical and chemical properties (HDT = 280° C), flame resistance = V1-V0 according to UL 94.
Resin System FPR H-2 (experimental grade) : High-performance, specialty resin, similar to FPR H-1, with relatively-high viscosity, halogen-free flame-retardant (V0 according to UL 94).
Resin System FPR H-3 : High-performance, specialty resin, similar to FPR H-1, with medium viscosity, partially brominated flame-retardant (V0 according to UL 94).

4.2 Specialty and Proprietary FPC Curing Catalysts

All the FPC Catalysts are moisture-insensitive, non-toxic, non-noxious, non-corrosive, and nonflammable compounds, with a minimum shelf life of one year at room temperature when properly stored in closed containers, and kept protected from prolonged exposure to sunlight or artificial actinic light (at best, in metal drums or cans, or also in brown glass vessels). "Pure catalyst" grades are available together with complete FP Resin Systems only. "Catalyst concentrate" grades can be supplied separately from isocyanate and epoxy resin components of FP Resin Systems, liquid isocyanates and epoxy resins being directly purchased on the local market by the user according to our specifications for the various FP Resin Systems.

Catalyst FPC 1A : "Pure catalyst". Standard, medium/fast-curing catalyst, for the standard, Resin Systems FPR S. Clear, white to pale-straw oily liquid; promptly soluble in resins at room temperature. Low-cost, dual-function catalyst: fast-hardening catalyst + fast wetting and impregnation promoter for reinforcing fibers in composites. Best suited for RTM and LIM manufacturing of highly qualified commodity and structural composites for automotive, appliances and general construction uses.
Catalyst FPC 1B : "Pure catalyst". Standard, fast-curing catalyst, for the standard, Resin Systems FPR S. Clear, pale-straw oily liquid; promptly soluble in resins at room temperature. Convenient, dual-function catalyst: rapid-hardening catalyst + fast wetting and impregnation promoter for reinforcing fibers in composites. Ideal for R-RIM, S-RIM, RTM & HS-RTM, LIM, as well as pultrusion, manufacturing of structural composites for automotive, appliances and general construction uses.
Catalyst FPC 2A : "Pure catalyst". Very fast-curing catalyst, designed for the high-performance, Resin Systems FPR H. Very fast-curing or low-dosage catalyst for the standard, FPR S Resin Systems. Clear, refractive, honey-like, yellowish liquid; readily soluble in resins at room temperature. For applications requiring thermosets with a superior thermal and chemical performance level: recommended for high continuous-service temperatures, uses in strongly oxidizing and/or chemically aggressive environments, implying stressing wet/dry and/or warm/cold cycles, etc. Well suitable for heavy-duty electrical insulation purposes.
Catalyst FPC CP-2A : "Catalyst concentrate". Masterbatch solution of the fast-curing catalyst FPC 2A in a liquid mixture of epoxy resins. Clear, pale-yellow, honey-like liquid; easily miscible with resins at room temperature. Especially designed for a precise dosage of FPC 2A catalyst in small-scale or occasional manufacturing operations.
Catalyst FPC FG-2 (experimental catalyst) : "Pure catalyst". Special, fast-curing, food-grade catalyst, especially designed for the food-grade Resin System FPR S-1 FG. Clear, yellow, honey-like liquid.
Catalyst FPC 2B : "Pure catalyst". Ultrafast-curing catalyst, designed for the high-performance, Resin Systems FPR H. Ultrafast-curing or very low-dosage catalyst for the standard, Resin Systems FPR S. Clear, refractive, yellow, highly viscous liquid; easily soluble in resins at room temperature. For applications requiring thermosets with a premium thermal, chemical and dielectric performance spectrum: ideal for high-temperature structural composites for heavy-duty automotive parts, industrial constructions and appliances, aerospace-grade composites, high dielectric strength electric insulators for high-voltage/high-temperature uses, etc.
Catalyst FPC CP-2B : "Catalyst concentrate". Masterbatch solution of ultrafast-curing catalyst FPC 2B in a liquid mixture of epoxy resins. Clear, yellowish, honey-like liquid; easily miscible with resins at room temperature. Especially designed for a precise dosage of FPC 2B catalyst in small-scale or occasional manufacturing operations.
Catalyst FPC XF : NEW! "Pure catalyst". Ultrafast-curing catalyst, for all the FPR H and FPR S Resin Systems . Pale yellow, low-melting crystalline solid. Designed to minimize (i.e. further decrease the already low) concentration of the FPC 2B catalyst.
Catalyst FPC W1 : "Pure catalyst". Specialty, fast microwave-curing catalyst for all the Resin Systems FPR S & H. It enables a long pot-life and short curing and post-curing times under microwave irradiation. Honey-like, light amber, refractive liquid; promptly soluble in gently warmed resins. It imparts to isocyanate-epoxy thermosets the same high-performance spectrum provided by Catalyst FPC 2A.
Catalyst FPC W2 : "Pure catalyst". Specialty, superfast microwave-curing catalyst for all the Resin Systems FPR S & H. It ensures a long pot-life and particularly short curing and post-curing times under microwave irradiation. High-viscosity, amber-colored, refractive liquid; easily miscible in gently warmed resins. It provides isocyanate-epoxy thermosets with the same premium performance spectrum enabled by Catalyst FPC 2B.

5. General References

F. Parodi, "Thermoset Matrix Composite Materials", Proceedings of the 11th AIM (Italian Association of Macromolecular Science & Technology) Meeting-School "Structural Polymeric Materials" (Gargnano, Garda Lake, Italy, June 11-16, 1989), Pacini Publ., Pisa, Italy, 1989, pp. 165-227 (in Italian).
F.W. Harris and H.J. Spinelli (eds.), Reactive Oligomers, ACS Symp. Ser., 282, Am. Chem. Soc., Washington DC, 1985.
P.E. Cassidy, Thermally Stable Polymers, Dekker, New York, 1980; J.P. Critchley, C.J. Knight and W.W. Wright, Heat Resistant Polymers, Plenum Press, London, 1983.
F. Parodi, "Step-Growth Polymerization", in The Encyclopedia of Advanced Materials, eds. D. Bloor, R.J. Brook, M.C. Flemings and S. Mahajan, Pergamon (Elsevier Sci. Publ.), Oxford, England, 1994, vol. 4, pp. 2665-2679.
F. Parodi, "Isocyanate-Derived Polymers", in Comprehensive Polymer Science, vol. 5, eds. G. Eastmond, A. Ledwith, S. Russo and P. Sigwalt, Pergamon Press, Oxford, England, 1989, chapter 23 (pp. 387-412).
M. Uribe and K.A. Hodd, "The Catalysed Reaction of Isocyanate and Epoxide Groups: A Study using Differential Scanning Calorimetry", Thermochimica Acta, 77, 367-373 (1984); T.I. Kadurina, V.A. Prokopenko and S.I. Omelchenko, "Curing of Epoxy Oligomers by Isocyanates", Polymer, 33, 3858-3864 (1992).
J.B. Enns and J.K. Gillham, "Time-Temperature-Transformation (TTT) Cure Diagram: Modeling the Cure Behavior of Thermosets", J. Appl. Polym. Sci., 28, 2567-2591 (1983); M.T. Aronhime and J.K. Gillham, "Time-Temperature-Transformation (TTT) Cure Diagram of Thermosetting Polymeric Systems", Adv. Polym. Sci., 78, 83-113 (1986); J.K. Gillham and J.B. Enns, "On the Cure and Properties of Thermosetting Polymers using Torsional Braid Analysis", Trends Polym. Sci., 2, 15-25 (1994).
M.T. DeMeuse, J.K. Gillham and F. Parodi, "Evolution of Properties of an Isocyanate/Epoxy Thermosetting System During Cure: Continuous Heating (CHT) and Isothermal Time-Temperature-Transformation (TTT) Cure Diagrams", J. Appl. Polym. Sci., 64, 15-25 (1997).
M.T. DeMeuse, J.K. Gillham and F. Parodi, "Evolution of Properties of a Thermosetting Isocyanate /Epoxy/Glass Fiber Composite Model System with Increasing Conversion", J. Appl. Polym. Sci., 64, 27-38 (1997).
M.T. DeMeuse, F. Parodi, R. Gerbelli and A.C. Johnson, "Microwave Processing of Isocyanate/Epoxy Composites", 39th International SAMPE Symp. (Anaheim, California, USA, April 11-14, 1994), conference proceedings, vol. 1, pp. 13-23.
F. Parodi, "Microwave Heating: Polymerization Processes and Organic Syntheses", Chim. Ind. (Milan), 80(1), 55-61 (1998).
F. Parodi, "Microwave Heating and the Acceleration of Polymerization Processes", in Polymers and Liquid Crystals, ed. A. Włochowicz, Proceedings of SPIE - The International Society for Optics and Photonics, 4017, 2-17 (1999).

6. FAQs and Answers

Do FPR Resins contain any plasticizers, diluents or volatile organic compounds ? FPR Resin System were specifically designed to withstand very high temperatures (250 - 300°C), similarly to multifunctional epoxy resin systems with the highest glass transition temperature values for heavy-duty, structural, electrical, electronic and electromechanical applications, with the advantage over them of being both much less viscous and outstandingly more rapidly curable. According to their fundamental destination as high-temperature materials, FPR Resin Systems are absolutely free from any plasticizers, diluents, solvents and volatile organic compounds.
Can these thermosets be toughened with conventional toughening polymeric additives ? Chemically plasticized and toughened FPR Resin Systems can be developed on demand according to specific customer's requirements, though cured thermosets of this family with glass transition temperature values below 220°C are not recommended. This in order to minimize the always detrimental effects of plasticizers and toughening agents on the excellent thermal, hydrolytic, chemical, and fire resistance characteristics proper to these thermosetting materials. Several oligomeric/polymeric toughening agents of established adoption in epoxy resin technology may also be employed with epoxy-isocyanate systems, with similar, overall effects.
What are the tolerances for the processing/curing conditions, i.e. % moisture, temperature, etc. ? FPR Resin Systems are much more moisture-tolerant than commonly expected based on the high amounts of free aromatic isocyanates they contain. For manufacturing of structural composites and qualified casts, preliminary vacuum degassing of resins is advisable or even strongly recommended, as it is with all liquid epoxy resin systems used for the same purposes. Requirements for manufacturing of high-quality and structural composites by processing in closed molds (R-RIM, S-RIM, RTM, Vacuum Infusion Molding, and casting): clean and dry molds; reinforcing fibers and mineral fillers having, or pre-dried by hot air or infrared treatment to have, a moisture content below 0.05 % by weight, and preferably below 0.02 % (microwave or radio frequency drying being a nice option for glass fibers); resin transfers performed by clean and dry pipings; dry-air or dry-nitrogen purging and blanketing of resin vessels is recommended, though usually being not necessary.
What are the storage conditions of FPR Resin components, e.g. % moisture, maximum temperature, and shelf-life ? Epoxy component (moisture-insensitive): shelf-life of one year at room temperature (up to 35 °C) in closed containers. Isocyanate component (water and moisture-reactive): shelf-life typical for liquid MDI isocyanates, i.e., 6 months at maximum 20°C in the original, sealed drums, or in partially empty containers tightly closed after dry-air or dry-nitrogen blanketing. FPC Catalysts (moisture-insensitive, or just slightly hygroscopic): shelf-life of one year at room temperature (up to 35°C) in the original, sealed containers; 6 months in partially empty, tightly closed cans or drums.
Are uncatalyzed isocyanate-epoxy blends stable over time ? Isocyanate-epoxy mixtures must be prepared prior to their use, and can only be stored for limited periods of time without significant, spontaneous viscosity increases. Suggested storage time at room temperature for un-catalyzed isocyanate-epoxy blends: 48 hours maximum (typically, max. 36 hours at 20-25 °C; max. 12 hours at 30-35 °C).

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