WO2011081662A1

WO2011081662A1 - Polyurethane compositions for composite structures

Info

Publication number: WO2011081662A1
Application number: PCT/US2010/003197
Authority: WO
Inventors: Martine M. Rousse; Ulrich Herold; Alain M. Sagnard
Original assignee: Dow Golbal Technologies Inc.
Priority date: 2009-12-29
Filing date: 2010-12-17
Publication date: 2011-07-07
Also published as: TW201132664A; WO2011081662A8

Abstract

A polyurethane composition that includes a polyisocyanate; and a branched polyol that reacts with the polyisocyanate at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form a cured polyurethane having a glass transition temperature of at least 70 °C. Embodiments of the present disclosure further include a composite structure that includes the polyurethane composition and a reinforcement material, where the composite structure can be a blade for a wind generator.

Description

Polyurethane Compositions for Composite Structures

Technical Field

The present disclosure relates generally to polyurethane compositions and in particular to polyurethane compositions for composite structures.

Background

A wind generator transforms the kinetic energy of wind into electrical energy. Commercial wind generators that produce this electrical energy usually have three blades, which are attached to an electrical generator and are pointed into the wind by computer-controlled motors. The blades can have tip speeds of over 320 km/h (200 miles per hour), are designed to be of high efficiency and have low torque ripple, all of which contribute to good reliability. The blades for the commercial production of electrical power are also very large, ranging in length from 20 to 60 meters (66 to 197 ft) or more.

The blades for the commercial production of electrical power are also very expensive to produce. It is generally assumed that blades account for 15 to 20 percent of the total purchase price of wind turbines. In addition, the production of these blades is one of the largest single applications of engineered composites in the world. Components that go into producing the composites for the blades include glass fiber, carbon fiber, thermoset resins (primarily epoxy and vinyl ester), core materials such as balsa and polyvinyl chloride foam and metal for fittings and bolts. Finding cost savings in the selection and efficient use of these components is of great commercial interest.

Summary

Embodiments of the present disclosure include a polyurethane composition having a polyisocyanate and a branched polyol. For the various embodiments, the branched polyol reacts with the polyisocyanate at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form a cured polyurethane having a glass transition temperature of at least 70 °C.

Embodiments of the present disclosure also include a composite structure that includes the polyurethane composition. For the various embodiments, the composite structure can include the polyurethane composition and a reinforcement material, where the cured polyurethane formed from the polyurethane composition has a glass transition temperature of at least 70 °C after reacting at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes.

Embodiments of the present disclosure also include a process for preparing a composite structure. For the various embodiments, the process for preparing the composite structure includes impregnating a reinforcement material with the polyurethane composition and reacting the polyisocyanate and the branched polyol of the polyurethane composition at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form the cured polyurethane having a glass transition temperature of at least 70 °C. An example of a structure that can be formed with the polyurethane composition and/or the composite structures of the present disclosure includes, but is not limited to, a blade for a wind generator.

Brief Description of the Figures

Figure 1 illustrates a comparison of the gel-time of Examples 1 through 4 as compared to Comparative Example A.

Figure 2 illustrates a comparison of the Young's Modulus (Ε') and the Loss Modulus (E") for Example 1 as compared to Comparative Example A.

Detailed Description

Embodiments of the present disclosure include a polyurethane composition that includes a polyisocyanate and a branched polyol. For the various embodiments, the polyurethane composition is able to cure at temperatures as low as room temperature (approximately 20 °C to 25 °C) to provide a cured polyurethane having a glass transition temperature (Tg) of at least 70 °C without the need of a post-curing process.

For the various embodiments, the polyurethane composition of the present disclosure can be used in a composite structure. As discussed herein, such a composite structure can includes those in which reinforcement fibers are impregnated with the polyurethane composition, which is then cured at temperatures as low as room temperature to form the composite structure. The composite structure of the present disclosure is particularly well suited for the manufacture of, among other structures, blades for wind generators. For the various embodiments, the suitability of the polyurethane composition of the present disclosure for use in a composite structure is due at least in part to its viscosity at the time of achieving a mix of the branched polyols and polyisocyanates (i.e., a uniform dispersion). For the various embodiments, the viscosity of the polyurethane composition at the time of achieving the mix is sufficiently low enough to allow the polyurethane composition to be infused into a composite structure that is very large, such as a blade of a wind generator. In addition, the cure rate of the polyurethane composition of the present disclosure is sufficiently fast and has a cure temperature that is sufficiently low that the amount of heat generated by the curing polyurethane composition is kept to a level that is suitable for use with a wide variety of materials. This later aspect of the present disclosure may be particularly advantageous when it is desired to use a material in the composite structure that has a melting temperature that is 90 °C or less. These aspects of the present disclosure are discussed herein.

For the various embodiments, the polyurethane composition of the present disclosure includes a polyisocyanate and a branched polyol. For the various embodiments, the branched polyol reacts with the polyisocyanate at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form a cured polyurethane having a Tg of at least 70 °C. For the various embodiments, the cured polyurethane of the present disclosure can achieve the Tg of at least 70 °C without the need for a post-curing step. In other words, the Tg of at least 70 °C is achieved after the polyurethane composition of the present disclosure reacts at the cure temperature and reaction time provided herein.

As discussed herein, the cure temperature of the polyurethane composition of the present disclosure is 20 °C to 90 °C. Preferably, the cure temperature of the polyurethane composition of the present disclosure is 20 °C to 70 °C. More preferably, the cure temperature of the polyurethane composition of the present disclosure is 20 °C to 50 °C.

These ranges of cure temperature allows the polyurethane composition of the present disclosure to be used with a variety of materials that have melting temperatures that are both above 90 °C and those that are 90 °C or below. Examples of such materials include, but are not limited to, polystyrene (PS) foams such as COMPAXX™ brand foams, polyvinyl chloride (PVC) foam such as Divinylcell® structural foam from DIAB, Inc., Airex® brand foams from Alcan Airex, polyurethane (PU) foam, and balsa wood, among others. For these examples, the glass transition temperature (Tg) for the polymer foams recited herein are 90 °C to 95 °C for the PS foams, 95 °C to 105 °C for the COMPAXX™ brand foams, and 1 10 °C to 120 °C for crosslinked PVC foams.

PVC foams are in wide use as the core material in blades for wind generators. Epoxy systems used with the PVC foams in forming the blades tend to develop rather high exothermic temperature during the curing process. The maximum temperature reached can rise above 100 °C depending on the system reactivity. As such, PVC foam might be perceived as a good choice versus other foams such as PS foams, which have a Tg of 90 °C to 95 °C and would offer higher risk of foam melting. On the other hand, PVC foams appear to have an issue of undergoing out-gassing at temperatures as low as 90 °C. Out-gassing creates bubbles that weaken the resin matrix. As such, the polyurethane composition of the present disclosure may allow for curing temperatures that avoid operating at the limit of a foams performance.

For the various embodiments, the branched polyols of the present disclosure include at least one branch point intermediate between the boundary constitutional units of the polyol. As used herein, the branch point on the branched polyol is a point from which at least three chains emanate. For the various embodiments, the chains that emanate from the branch point (i.e., the branch point can be an ^functional branch point from which chains emanate) are polymeric offshoots of the constitutional units of the polyol.

For the various embodiments, each chain of the branched polyols can include a functional unit, making the branched polyol of the present disclosure a

multifunctional polyol. As used herein, a multifunctional polyol is a polyol having at least three functional groups (e.g, a triol, a tetrol, etc.), where a functional group for a polyol is defined as the number of hydroxy 1 (-OH) groups per molecule. For the various embodiments, each of the chains of the branched polyol can independently have a functionality of at least 1 . According to the present disclosure, the used of the multifunctional polyol allows for sufficient cross-linking of the polyurethane composition so as to achieve the cured polyurethane having a Tg of at least 70 °C.

For the various embodiments, blends of branched polyols, being

multifunctional polyols or not, are also possible. For example, such blends can include, but are not limited to, the multifunctional polyols and branched polyols and/or unbranched polyols that are monofunctional and/or bifunctional (i.e., a diol). The use of monofunctional and bifunctional branched polyols and/or unbranched polyols can allow for chain advancement during curing, which can allow for different rate of viscosity increases in the curing polyurethane composition.

So, for example, the branched polyol of the polyurethane composition can be a blend of at least a first branched polyol and a second branched polyol. For the various embodiments, each of the first branched polyol and the second branched polyol can have a functionality that is either the same (e.g., each having a functionality of three) or different (e.g., the first branched polyol having a functionality of three, and the second branched polyol having a functionality of two). As such, it is possible to have one or more branched polyols that are multifunctional polyols (a functionality of three or more) in the polyurethane composition. It is also possible to have a blend of one or more branched polyols that are multifunctional polyols and branched and/or unbranched polyols having a functionality of two or less in the polyurethane composition. For each of these possibilities, it is also possible that the branch structures have different chain lengths and/or different functionalities as discussed herein.

For the various embodiments, the chains of the branched polyols can have approximately the same number average molecular weight (Mn). In an additional embodiment, one or more of the chains can have approximately the same Mn, while others chains of the branched polyol can have different Mn. In another embodiment, each of the chains of the branched polyol can have a different Mn relative the other chains of the branched polyol. So, for example, the first branched polyol and the second branched polyol, as discussed above, can each have two or more chains having a Mn that is either the same or different, regardless of the functionality of each of the first and second branched polyols and/or their respective chains. As such, the first branched polyol can have a Mn that is different than a Mn of the second branched polyol.

For the various embodiments, each chain of the branched polyols can independently have a Mn of 70 to 250 grams/mole (-OH equivalent weight).

Preferably, each chain of the branched polyols can-independently have a Mn of 80 to 200 grams/mole (-OH equivalent weight). More preferably, each chain of the branched polyols can independently have a Mn of 80 to 160 grams/mole (-OH equivalent weight). For the various embodiments, the selection of the Mn for each chain of the branched polyols can be driven by the physical and thermal properties of both the polyurethane composition and the cured polyurethane that are trying to be achieved. For example, the Mn for each chain and the functionality of each chain can influence both the viscosity of the polyurethane composition and the Tg of the cured polyurethane. Selection of the Mn for each chain of the branched polyols can also influence the exotherm produced during the curing of the polyurethane composition.

For the various embodiments, examples of suitable branched polyols which are multifunctional polyols include, but are not limited to, polypropylene glycol glycerol ether, glycerol propoxylate, polyether polyols, sugar based polyols, and combinations thereof.

For the various embodiments, the polyols of the present disclosure can include glycerin based propylene oxide triols, glycerin based ethylene oxide triols, triols from glycerin and combinations of propylene oxide and ethylene oxide, and glycerin based butylenes oxide triols. Specific examples of suitable glycerin based branched polyols which are multifunctional polyols include, but are not limited to, Voranol™ CP 260, Voranol™ CP 300, Voranol™ CP 450, Voranol™ RH 360, Voranol™ RN 490, and combinations thereof, (all commercially available from The Dow Chemical

Company).

Additional examples include multifunctional polyols derived from polyesters. Examples of such multifunctional polyols include, but are not limited to Voranol™ RN 490 (The Dow Chemical Company).

For the various embodiments, the polyisocyanate of the polyurethane composition includes compounds, oligomers and polymers having at least at least two isocyanato (-N=C=0) functional groups, such as diisocyanates or triisocyanates, as well as dimers and trimers or biurets of the polyisocyanates discussed herein.

Polyisocyanates useful in the present d isclosure can be branched or unbranched. Monofunctional isocyanates can be used as chain terminators or to provide terminal groups during polymerization. Suitable polyisocyanates are capable of forming a covalent bond with a reactive group such as hydroxyl groups, but can also form covalent bonds with thiol or amine functional groups if compounds having such groups are included in the polyurethane composition.

For the various embodiments, the polyisocyanates of the present disclosure can also be "modified", "unmodified" and mixtures of "modified" and "unmodified" polyisocyanates. The term "modified" means that the aforementioned isocyanates are changed in a known manner to introduce biuret, urea, carbodiimide, urethane or isocyanurate groups.

The molecular weight of the polyisocyanate can vary widely. For the various embodiments, the Mn of the polyisocyanate can be 280 to 470 grams/mole.

Preferably, the Mn of the polyisocyanate can be 3 10 to 450 grams/mole. Most preferably, the Mn of the polyisocyanate can be 340 to 420 grams/mole.

For the various embodiments, examples of suitable polyisocyanates include, but are not limited to, aliphatic, cycloaliphatic, aromatic and heterocyclic

polyisocyanates, dimers and trimers thereof and mixtures thereof. For the various embodiments, the polyisocyanate of the present disclosure can have a functionality of at least 2, where the functional group for the polyisocyanate is defined as the number of isocyanato (-N=C=0) functional groups per molecule.

Useful cycloaliphatic polyisocyanates include those in which one or more of the isocyanato groups are attached directly to the cycloaliphatic ring and

cycloaliphatic polyisocyanates in which one or more of the isocyanato groups are not attached directly to the cycloaliphatic ring. Useful aromatic polyisocyanates include those in which one or more of the isocyanato groups are attached directly to the aromatic ring, and aromatic polyisocyanates in which one or more of the isocyanato groups are not attached directly to the aromatic ring. Useful heterocyclic

polyisocyanates include those in which one or more of the isocyanato groups are attached directly to the heterocyclic ring and heterocyclic polyisocyanates in which one or more of the isocyanato groups are not attached directly to the heterocyclic ring.

Specific examples of suitable polyisocyanates of the present disclosure include, but are not limited to, Voranate™ M 220 (a polymethylene

polyphenylisocyanate available from The Dow Chemical Company) and Isonate™ M 143 (available from The Dow Chemical Company), among other Voranate™, Isonate™, and Specflex™ polyisocyanate products.

For the various embodiments, achieving the desired properties for the cured polyurethane, such as a Tg of at least 70 °C and/or a modulus comparable to many epoxy resin systems, can be accomplished through the selection of the branched polyols and polyisocyanates. For the various embodiments, this selection can be based in part on the characteristics of the branched polyols and polyisocyanates, such as the branched polyols being multifunctional, as discussed herein. In addition, characteristics of the polyisocyanates can also be used in achieving the desired properties of the cured polyurethane. For example, the relative stiffness versus flexibility of the branched polyols and polyisocyanates can be used to achieve a desired Tg and/or modulus for the cured polyurethane.

The polyurethane compositions of the present disclosure can be formed into articles having a variety of shapes and dimensions. Examples of such articles include, but are not limited to, flat sheets or curved shapes. Non-limiting examples of useful methods for forming articles include heat treatment, coating, pressure casting, injection molding, and vacuum injection molding, among others, and curing the polyurethane composition to form a molded article.

For the various embodiments, the present disclosure may also include articles formed from multiple layers of the polyurethane compositions of the present disclosure. The thickness of each layer and overall thickness of the article can vary as desired. For the various embodiments, the present disclosure may provide multi!ayered articles or laminates that include at least one layer of the polyurethane compositions of the present disclosure. Laminates having the polyurethane compositions of the present disclosure can include at least one layer of a substrate selected from the group consisting of paper, glass, ceramic, wood, masonry, textile, metal or organic polymeric material and combinations thereof.

For the various embodiments, the polyurethane composition of the present disclosure can also be used as a matrix material in a composite structure. For the various embodiments, the composite structure can include, in addition to the matrix material, reinforcement material, which can be impregnated by the matrix material in forming the composite material. Examples of suitable reinforcement materials can be selected from the group of glass fibers, basalt, mineral wool, carbon fibers, silicon carbide, carbon, quartz, graphite, mullite, aluminum oxide, piezoelectric ceramic materials, polyamides such as aramid and/or nylon fibers, natural fibers such as cotton, cellulose, natural rubber, flax, ramie, hemp, sisal and wool, among others, thermoplastic polyesters (such as polyethylene terephthalate and polybutylene terephthalate), acrylics (such as polyacrylonitriles), polyolefins, polyurethanes and vinyl polymers (such as polyvinyl alcohol) and combinations thereof. For the various embodiments, reinforcing materials can be in the form of mats or fabrics comprised of the reinforcement materials provided herein. The composite structure of the present disclosure can also include one or more of an additional component that is used with the matrix and reinforcement materials. For example, a core can be used to help define a shape, provide weight reduction and use the matrix and reinforcement materials efficiently in producing a composite structure. One example where a core is used in forming a composite structure is in the production of a blade for a wind generator. The blade can include, among other things, a core of a rigid material that is surrounded by a reinforcement material impregnated with the polyurethane composition. The use of the composite structures as provided herein for producing a blade is but one example of many in the production of components for wind generators.

For the various embodiments, the process of preparing the composite structure, such as the blade, can include impregnating the reinforcement material with the polyurethane composition of the present disclosure. For example, a vacuum infusing process can be used to impregnate the polyurethane composition into the reinforcement material. In forming the blade for a wind generator, a mold can be used to hold a core that is at least partially surrounded by reinforcement material. The polyurethane composition can be vacuum infused into the mold to both impregnate the reinforcement material and to at least partially contact the core of the blade with the polyurethane composition.

For the various embodiments, the viscosity of the polyurethane composition is sufficiently low to allow for the polyurethane composition to be fully injected into the mold of the blade so as to fully coat the core while impregnating the reinforcement material of the composite structure. The polyurethane composition reacts at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form the cured polyurethane having a glass transition temperature of at least 70 °C to form the blade.

The cure time and the cure temperature of the polyurethane composition also allow for a wide variety of rigid materials to be used for the core. As discussed herein, rigid materials useful for the core material can include, but are not limited to, polystyrene (PS) foams such as COMPAXX™ brand foams, polyvinyl chloride (PVC) foam, metal foams from manufactures such as the Mitsubishi Companies, and balsa wood, among others. Because of the low cure temperatures of the polyurethane composition, it is now possible to used rigid materials having Tg values and/or melting point temperatures that are 90 °C or less in composite structures that use the polyurethane composition.

For the various embodiments, the polyurethane composition also has a viscosity at the time of mixing the branched polyols and polyisocyanates that is sufficiently low to make the polyurethane composition highly suitable for forming composite structures, such as the blade of a wind generator. For example, the polyurethane composition can have a viscosity upon achieving a mix of the branched polyols and polyisocyanates of 1500 megapascal-second (mPa s) or less at 25 °C. Preferably, the viscosity of the polyurethane composition upon achieving the mix of the branched polyols and polyisocyanates is less than 1000 mPa s at 25 °C. As used herein, achieving the mix indicates the time and the state at which the branched polyols and polyisocyanates of the polyurethane composition achieve a uniform dispersion through the act of mixing, such as by a mechanical agitation. Examples of mechanical agitation can include, but are not limited to, the use of impellers, agitators and/or revolving paddles.

For the various embodiments, the viscosity of the polyurethane composition can then have a first viscosity of 1000 mPa s measured at 40 °C after 10 minutes to 70 minutes of a reaction time, where the reaction time is measured from achieving the mix of the branched polyols and polyisocyanates. Preferably, the first viscosity of 1000 mPa s measured at 40 °C is after 30 minutes to 50 minutes of the reaction time.

The viscosity of the polyurethane composition can then have a second viscosity of at least 5000 mPa s measured at 40 °C after 20 to 120 minutes of the reaction time. Preferably, the second viscosity of at least 5000 mPa s measured at 40 °C is after 40 minutes to 70 minutes of the reaction time.

For the various embodiments, the polyurethane composition of the present disclosure can have a viscosity at the time of achieving the mix and during the first viscosity that is suitable for use in a vacuum infusion process, such as are used in forming the blade for a wind generator. Vacuum infusion processes can include the use of one or more injection points through which the polyurethane composition can be injected. As the polyurethane composition is injected it is driven under pressure through the entire length of the blade (e.g., 40 to 60 meters) where it impregnates the reinforcement material surrounding the core.

In addition to having a viscosity that is sufficiently low, as described herein, for use in forming the blade, the polyurethane composition of the present disclosure also have a rapid cure rate at a cure temperature that is low enough to minimize the exposure of the composite structure to temperatures that may damage the composite. For example, the cure temperature for the polyurethane composition of the present disclosure can be from 20 °C to 90 °C, where the cure rate at this temperature is sufficiently fast to allow the polyurethane composition to cure in a reaction time of 20 to 40 minutes to form a cured polyurethane having a glass transition temperature of at least 70 °C.

For the various embodiments, the polyurethane composition can further include at least one inorganic filler. For the various embodiments, the inorganic filler can be in the form of, for example as particles, including but not limited to nanoparticles, agglomerates, fibers, chopped fibers, and combinations thereof. The particles also can be hollow particles formed from materials selected from polymeric and nonpolymeric inorganic materials, polymeric and nonpolymeric organic materials, composite materials, and combinations thereof. Non-limiting examples of suitable materials from which the hollow particles can be formed are described above. In some embodiments, the hollow particles can be hollow glass spheres.

For the various embodiments, the polyurethane composition can further include at least one additive. Such additives can include, but are not limited to, light stabilizers, heat stabilizers, antioxidants, colorants, fire retardants, ultraviolet light absorbers, light stabilizers such as hindered amine light stabilizers, mold release agents, static (non-photochromic) dyes, fluorescent agents, pigments, surfactants, flexibilizing additives, and combinations thereof.

EXAMPLES

MATERIALS

Isonate™ M 143 (a polyisocyanate, available from The Dow Chemical Company).

Voranol™ CP 260 (a branched polyol, available from The Dow Chemical Company).

Voranol™ CP 450 (a branched polyol, available from The Dow Chemical Company).

Voranol™ RH 360 (a branched polyol, available from The Dow Chemical Company). Voranol™ RN 490 (a branched polyol, available from The Dow Chemical Company).

D.E.R.™ 330 (liquid Epoxy Resin, available from The Dow Chemical Company).

1,4-butanediol diglycidyl ether (BDDGE) available from Polystar®.

Jeffamine® D-230 (amine terminated polypropylene glycols, available from Huntsman Corporation).

Isphorone diamine (1PD), available from BASF.

Amino ethyl piperazine (AEP) available from The Dow Chemical Company. TEST METHODS

Viscosity and Gel Time were measured using a Paar Physica UDS 200. The shear viscosity was determined using a parallel-plate geometry (25 mm diameter plate, 0° angle). The experiments were performed under isothermal conditions at 40 °C and frequency of 1 Hz.

Young's Modulus (Ε') and Loss Modulus (E") were measured using Dynamic Mechanical Thermal Analysis (DMTA) with a Rheometric Solid Analyzer RSA II with Orchestrator software. Experiments were run on using one point bending, dynamic temperature step test (from 25 °C to 150 °C) and at 10 rad/s, under strain control.

Glass Transition Temperature (Tg) was measured using a Mettler-Toledo International Inc. DSC 821 Differential Scanning Calorimeter. Tg was measured on samples cured for 7 hours (h) at 70 °C.' The following temperature profile was used in measuring the Tg:

Step 1 : Heating step from 25 °C to 150 °C at 10 °C/min.

Step 2: 10 minutes Isothermal step at 150 °C.

Step 3: Cooling step to 25 °C at 30 °C/min.

Step 4: Heating step from 25 °C to 150 °C at 10 °C/min.

Step 5: Cooling step to 25 °C at 30 °C/min.

Tg 1 was determined after Step 1 , and Tg 2 (final Tg) was determined after Step 4. The reported Tg correspond to the mid-point on the curve.

Examples 1 -4 Examples 1 through 4 were prepared as follows. Fifty (50) grams (g) of each of Examples 1 through 4 was prepared according to the weight percentages given in Table 1 . Examples 1 through 4 were gently mixed manually until homogenization. Care was taken to avoid the incorporation of air that would favor the formation of bubble during cure. Weighting and mixing of the Examples 1 through 4 was conducted at room temperature (approximately 20 °C to 25 °C) under a fume hood.

A few grams of the mixture was poured in an aluminum cup that was placed into a 70°C oven for seven (7) hours in order to prepare sample for measuring Tg values. The remaining amount of Examples 1 through 4 were formed into 45 mm x 12 mm x 2 mm sheets which were placed into the 70°C oven for seven (7) hours in order to prepare samples for measuring Young's Modulus (Ε') and Loss Modulus (E") values.

Table 1

a - The mass of polymer which has one mole of reactive side-chain groups.

b- OH equivalent is the mass of the Polyol divided by the Equivalent Weight of the Polyol.

c- NCO equivalent is the mass of the Isocyanate divided by the Equivalent Weight of the Isocyanate.

As indicated in Table 1 , the OH/NCO ratio for each of Examples 1 through 4 is at least 1.0. For the various embodiments, this better ensures that most if not all of the cyanate (NCO) groups react with the hydroxy (OH) groups.

Comparative Example A

Fifty (50) grams (g) of Comparative Example A was prepared according to the following weight percentages: 86 weight percent D.E.R.™ 330 and 14 weight percent BDDGE for the epoxy resin and 73 weight percent of Jeffamine® D-230, 13.5 weight percent of 1PD and 13.5 weight percent of AEP for the amine hardener. The epoxy resin and the amine hardener of Comparative Example A were

gently mixed manually until homogenization. Care was taken to avoid the

incorporation of air that would favor the formation of bubble during cure. Weighting

and mixing of Comparative Example A was conducted at room temperature

(approximately 20 °C to 25 °C) under a fume hood.

A few grams of Comparative Example A was poured in an aluminum cup that

was placed into a 70°C oven for seven (7) hours in order to prepare sample for

measuring Tg values. The remaining amount of Comparative Example A was formed

into 45 mm x 12 mm x 2 mm sheets which were placed into the 70°C oven for seven

(7) hours in order to prepare samples for measuring Young's Modulus (Ε') and Loss

Modulus (E") values. Comparative Example A had a Tg 1 of 75 °C and Tg2 of 80 °C.

Figures 1 and 2

Figure 1 provides a comparison of the gel-time of Examples 1 through 4 as

compared to Comparative Example A. As illustrated, at 40 °C the increase in

viscosity for Examples 1 through 4 was much more rapid then the increase in

viscosity for Comparative Example A. In addition to having a faster gel-time than

Comparative Example A, Examples 1 through 4 also demonstrated mechanical

properties that were comparable to those of the epoxy system of Comparative

Example A. For example, the Tg values obtained with Examples 1 to 4 are

j comparable with or even higher then those of Comparative Example A. In addition,

Figure 2 illustrates that the both the Young's Modulus (Ε') and the Loss Modulus (E")

for Examples 1 to 4 were comparable to the epoxy system of Comparative Example

A,

Claims

We Claim:

1 . A polyurethane composition, comprising:

a polyisocyanate; and

a branched polyol that reacts with the polyisocyanate at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form a cured polyurethane having a glass transition temperature of at least 70 °C.

2. The polyurethane composition of claim 1 , wherein the branched polyol includes a branch point from which at least three chains emanate, each chain independently having a number average molecular weight of 100 to 750.

3. The polyurethane composition of claim 2, wherein each chain independently has a functionality of at least 1.

4. The polyurethane composition of any one of the preceding claims, wherein the branched polyol is a blend of a first branched polyol and a second branched polyol.

5. The polyurethane composition of any one of the preceding claims, wherein the branched polyol is glycerol propoxylate.

6. The polyurethane composition of any one of the preceding claims, wherein the branched polyol is a polyether polyol.

7. The polyurethane composition of any one of the preceding claims, wherein the polyisocyanate is an aromatic polyisocyanate having a functionality of at least 2.

8. The polyurethane composition of any one of the preceding claims, wherein the branched polyol has a functionality of at least 3.

9. The polyurethane composition of any one of the preceding claims, wherein the polyurethane composition has a first viscosity of 1000 mPa s measured at 40 °C after 10 minutes to 70 minutes of the reaction time and a second viscosity of at least 5000 mPa s measured at 40 °C after 20 to 120 minutes of the reaction time.

10. A composite structure, comprising:

a poiyurethane composition as claimed in any one of claims 1 through 9; and a reinforcement material, wherein the cured poiyurethane has a glass transition temperature of at least 70 °C after reacting at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes.

1 1. The composite structure of any one of the preceding claims, where the composite structure is a component of a wind generator.

12. The composite structure of claim 1 1, where the component of the wind generator is a blade having a core of a rigid material.

13. A process for preparing a composite structure, comprising:

impregnating a reinforcement material with the poiyurethane composition as claimed in any one of claims 1 through 9; and

reacting the polyisocyanate and the branched polyol of the poiyurethane composition at a cure temperature of 20 °C to 90 °C for a reaction time of 20 to 40 minutes to form the cured poiyurethane having a glass transition temperature of at least 70 °C.

14. The process of claim 13, including at least partially contacting a core of polystyrene with the poiyurethane composition.

15. A blade for a wind generator molded from a poiyurethane composition as claimed in any one of the claims 1 through 9.