HEATHARDENABLE COMPOSITIONS, CONTAINING A BENZOXAZINKOMPONENTE AND A ACRYLONITRILE BUTADIENE COPOLYMER WITH SECONDARY AMINE FINAL'S GROUPS AS THE IMPACT STRENGTH-BETTER

Curable compositions, such as benzoxazine-based ones, are useful in applications within the aerospace industry, such as for example as a heat curable composition for use as a matrix resin or an adhesive, and form the basis of the present invention.

Epoxy resins with various hardeners have been used extensively in the aerospace and electronics industries both as adhesives and as matrix resins for use in prepreg assembly with a variety of substrates.

Benzoxazines themselves have been reported in the literature as generally having a high glass transition temperature, good electrical properties (e.g., dielectric constant), and low flammability.

Blends of epoxy resins and benzoxazines are known.Seee.g.U.S. Patent Nos. 4,607,091 (Schreiber), 5,021,484 (Schreiber), 5,200,452 (Schreiber), and 5,445,911 (Schreiber). These blends appear to be potentially useful in the electronics industry, as the epoxy resins can reduce the melt viscosity of benzoxazines allowing for the use of higher filler loading while maintaining a processable viscosity. However, epoxy resins oftentimes undesirably increase the temperature at which benzoxazines polymerize.

Ternary blends of epoxy resins, benzoxazine and phenolic resins are also known. SeeU.S. Patent No. 6,207,786 (Ishida), and S. Rimdusit and H. Ishida, "Development of new class of electronic packaging materials based on ternary system of benzoxazine, epoxy, and phenolic resin," Polymer, 41, 7941-49 (2000).

Other resins, such as reactive monomers, polymers and oxazolines, polyimide/siloxane co-polymers and cyanate esters, are too known. Seee.g.U.S. Patent No. 4,806,267 (Culbertson), in connection with oxazolines and J. McGrath et al., "Synthesis and Characterization of Segmented Polyimide-Polyorganosiloxane Copolymers", Advances in Polym. Sci., Vol. 140, Springer-Verlag, Berlin 61-105 (1999) in connection with polybenzoxazine polyimide-polyorganosiloxane co-polymers.

Moreover, in J. Jang et al., "Performance Improvement of Rubber Modified Polybenzoxazine", J. Appl. Polym. Sci., 67, 1-10 (1998), the authors report the use of polybenzoxazine modified with amine-terminated, butadiene acrylonitrile rubber and with carboxyl-terminated, butadiene acrylonitrile rubber to improve mechanical properties. The polybenzoxazine chosen was synthesized from bisphenol A, formaldehyde and the aromatic amine, aniline.

Notwithstanding the state of the technology, there has been no disclosure, teaching or suggestion to prepare a heat curable composition based on the combination of a benzoxazine and amine-terminated, butadiene acrylonitrile rubber, let alone one with improved performance properties.

The inventive compositions include a heat curable composition comprising the combination of a benzoxazine component and a toughener component.

In one aspect, the invention provides a benzoxazine component comprising where o is 1-4, X is a direct bond (when o is 2), alkyl (when o is 1), alkylene (when o is 2-4), carbonyl (when o is 2), thiol (when o is 1), thioether (when o is 2), sulfoxide (when o is 2), and sulfone (when o is 2), and R₁ is alkyl, such as methyl, ethyl, propyls and butyls, and 5 weight percent or more of a toughener component comprising acrylonitrile-butadiene co-polymer having secondary amine terminal groups ("ATBN").

In a more specific embodiment of that aspect of the invention, the benzoxazine component is embraced by one or more of where X is selected from a direct bond, CH₂, C(CH₃)₂, C=O, S, S=O and O=S=O, and R₁, R₂, and R₃ are the same or different and alkyl, such as methyl, ethyl, propyls and butyls and R₄ is selected from hydrogen, alkyl, alkenyl and aryl.

In a more particular embodiment, the benzoxazine is embraced by where X is selected from a direct bond, CH₂, C(CH₃)₂, C=O, S, S=O and O=S=O, and R₁ and R₂ are the same or different and are selected from methyl, ethyl, propyls and butyls.

In yet a more specific embodiment of that aspect of the invention, the benzoxazine component is embraced by where R₁ and R₂ are the same or different and are selected from methyl, ethyl, propyls and butyls, though in a particularly desirable embodiment R₁ and R₂ are each methyl.

Cured reaction products of the inventive compositions are capable of demonstrating at least one of a wet Tg of at least 176.7 °C (350 °F), such as in the range of 176.7 to 232.2 °C (350 to 450 °F), a toughness measured by G_Ic of at least 332.5 J/m² (1.9 in-lb./in²), such as in the range of 332.5 to 700.0 J/m² (1.9 to 4.0 in-lb./in²), a percent decrease in ΔH of at least 15% compared with a benzoxazine prepared from bisphenol F and aniline, and a percent decrease in wet Tg compared with dry Tg with increased toughener concentration of less than 6%.

Cured reaction products of the inventive compositions are also capable of demonstrating a Tg and toughness measured by GI_c increase as the amount of toughener in the composition increases.

Moreover, cured reaction products of the inventive compositions should also have a cured density of less than 1.2 g/cc.

The invention is further directed to prepregs of the inventive compositions and cured reaction products thereof, prepregs of the inventive compositions and cured reaction products thereof, towpregs of the inventive compositions and cured reaction products thereof, and processes for producing the prepregs and towpregs, adhesive compositions of the inventive compositions and films thereof, methods of preparing and using the inventive compositions, articles of manufacture assembled with the inventive compositions and the like.

Figure 1 depicts a plot of glass transition temperature (left y axis) vs. toughness vs. G_IC (right y axis) for benzoxazine with increasing ATBN concentration.

Figure 2 is a representation of Figure 3 from J. Jang et al., depicting a plot of G_IC vs. ATBN and acrylonitrilebutadiene co-polymer having carboxyl terminal groups ("CTBN") concentration.

Figure 3 is a representation of Figure 6 from J. Jang, et al., depicting a plot of Tg vs. ATBN and CTBN concentration.

As noted above, the present invention provides generally a heat curable composition comprising the combination of a benzoxazine component and a toughener component.

In one aspect, the invention provides a benzoxazine component comprising where o is 1-4, X is a direct bond (when o is 2), alkyl (when o is 1), alkylene (when o is 2-4), carbonyl (when o is 2), thiol (when o is 1), thioether (when o is 2), sulfoxide (when o is 2), and sulfone (when o is 2), and R₁ is alkyl, such as methyl, ethyl, propyls and butyls, or where p is 2, Y is selected from biphenyl (when p is 2), diphenyl methane (when p is 2), diphenyl isopropane (when p is 2), diphenyl sulfide (when p is 2), diphenyl sulfoxide (when p is 2), diphenyl sulfone (when p is 2), and diphenyl ketone (when p is 2), and R₄ is selected from hydrogen, halogen, alkyl and alkenyl; and a toughener component comprising ATBN.

In a more specific embodiment of that aspect of the invention, the benzoxazine component is embraced by one or more of where X is selected from a direct bond, CH₂, C(CH₃)₂, C=O, S, S=O and O=S=O, and R₁, R₂, and R₃ are the same or different and alkyl, such as methyl, ethyl, propyls and butyls and R₄ is selected from hydrogen, alkyl, alkenyl and aryl.

In a more particular embodiment, the benzoxazine is embraced by where X is selected from the group consisting of a direct bond, CH₂, C(CH₃)₂, C=O, S=O and O=S=O, S, and R₁ and R₂ are the same or different and are selected from methyl, ethyl, propyls and butyls.

In yet a more specific embodiment of that aspect of the invention, the benzoxazine component is embraced by where R₁ and R₂ are the same or different and are selected from methyl, ethyl, propyls and butyls, though in a particularly desirable embodiment R₁ and R₂ are each methyl.

Cured reaction products of the inventive compositions are capable of demonstrating at least one of a wet Tg of at least 176.7 °C (350°F), such as in the range of 176.7 to 232.2 °C (350 to 450°F), a toughness measured by G_IC of at least 332.5 J/m² (1.9 in-lb./in²), such as in the range of 332.5 to 700.0 J/m² (1.9 to 4.0 in-lb./in²), a percent decrease in ΔH of at least 15% compared with a benzoxazine prepared from bisphenol F and aniline, and a percent decrease in wet Tg compared with dry Tg with increased toughener concentration of less than 6%.

Cured reaction products of the inventive compositions are also capable of demonstrating improved Tg and toughness measured by GI_c increase as the amount of toughener in the composition increases.

Moreover, cured reaction products of the inventive compositions should also have a cured density of less than 1.2 g/cc.

The benzoxazine component should be present in an amount in the range of about 10 to about 99 percent by weight, such as about 25 to about 75 percent by weight, desirably about 35 to about 65 percent by weight, based on the total weight of the composition.

The toughener component, ATBN, should be present in an amount in the range of about 1 to about 90 percent by weight, such as about 10 to about 70 percent by weight, desirably about 15 to about 30 percent by weight, based on the total weight of the composition.

The benzoxazine component of the present invention may typically be prepared by reacting a phenolic compound, such as a bisphenol A, bisphenol F, bisphenol S or thiodiphenol, with an aldehyde and an alkyl amine. U.S. Patent No. 5,543,516, hereby expressly incorporated herein by reference, describes a method of forming benzoxazines, where the reaction time can vary from a few minutes to a few hours, depending on reactant concentration, reactivity and temperature. SeealsoBurke et al., J. Org. Chem., 30(10), 3423 (1965); seegenerallyU.S. Patent Nos. 4,607,091 (Schreiber), 5,021,484 (Schreiber), 5,200,452 (Schreiber) and 5,443,911 (Schreiber).

Benzoxazines are presently available from several sources commercially, including Vantico, Inc., Brewster, New York, Georgia-Pacific Resins, Inc. and Shikoku Chemicals Corporation, Chiba, Japan, the last of which offers among others B-a, B-m, F-a, C-a and F-a benzoxazine resins. Of these, the benzoxazine component of the present invention is oftentimes desirably within the B-m benzoxazine resin family.

Benzoxazine polymerization can also be initiated by cationic initiators, such as Lewis acids, and other known cationic initiators, such as metal halides; organometallic derivatives; metallophorphyrin compounds such as aluminum phthalocyanine chloride; methyl tosylate, methyl triflate, and triflic acid; and oxyhalides.

The inventive compositions may also include coreactants, curatives and/or catalysts for the benzoxazines component. Examples include Lewis acids, such as phenols and derivatives thereof, strong acids, such as alkylenic acids and cationic catalysts.

The inventive compositions may also include inorganic fillers, such as silicas. For instance, the inventive composition may also include an additional silica nanoparticle component. The silica nanoparticles can be pre-dispersed in epoxy resins, and may be selected from those commercially available under the tradename NANOPOX, such as NANOPOX XP 0314, XP 0516, XP 0525, from Hanse Chemie, Germany. These NANOPOX products are silica nanoparticle dispersions in epoxy resins, at a level of up to about 50% by weight. These NANOPOX products are believed to have a particle size of about 5 nm to about 80 nm. NANOPOX XP 0314 is reported by the manufacturer to contain 40 weight percent of silica particles having a particle size of less than 50 nm diameter in a cycloaliphatic epoxy resin. The invention relates also to prepregs formed from a layer of fibers infused with the inventive heat curable composition.

In this regard, the invention relates also to a processes for producing a prepreg. One such process includes the steps of (a) providing a layer of fibers; (b) providing the inventive heat curable composition; and (c) joining the heat curable composition and the layer of fibers to form a prepreg assembly, and exposing the resulting prepreg assembly to elevated temperature and pressure conditions sufficient to infuse the layer of fibers with the heat curable composition to form a prepreg.

Another such process for producing a prepreg, includes the steps of (a) providing a layer of fibers; (b) providing the inventive heat curable composition in liquid form; (c) passing the layer of fibers through the liquid heat curable composition to infuse the layer of fibers with the heat curable composition; and (d) removing excess heat curable composition from the prepreg assembly.

The fiber layer may be constructed from unidirectional fibers, woven fibers, chopped fibers, non-woven fibers or long, discontinuous fibers.

The fiber chosen may be selected from carbon, glass, aramid, boron, polyalkylene, quartz, polybenzimidazole, polyetheretherketone, polyphenylene sulfide, poly p-phenylene benzobisoaxazole, silicon carbide, phenolformaldehyde, phthalate and napthenoate.

The carbon is selected from polyacrylonitrile, pitch and acrylic, and the glass is selected from S glass, S2 glass, E glass, R glass, A glass, AR glass, C glass, D glass, ECR glass, glass filament, staple glass, T glass and zirconium oxide glass.

The invention further contemplates a towpreg formed from a bundle of such fibers infused with the inventive heat curable composition.

In this regard, the invention relates also to processes for producing a towpreg. In one such process, the steps include (a) providing a bundle of fibers; (b) providing the heat curable composition and (c) joining the heat curable composition and the bundle of fibers to form a towpreg assembly, and exposing the resulting towpreg assembly to elevated temperature and pressure conditions sufficient to impregnate the bundle of fibers with the heat curable composition to form a towpreg.

In another such process, the steps include (a) providing a bundle of fibers; (b) providing the inventive heat curable composition in liquid form; (c) passing the bundle of fibers through the liquid heat curable composition to impregnate the bundle of fibers with the heat curable composition; and (d) removing excess heat curable composition from the towpreg assembly, thereby forming the towpreg.

The inventive composition may also include an additional toughener component, examples of which include poly(propylene) oxide; amine-terminated polyethylene sulfide, such as PES 5003P, available commercially from Sumitomo Chemical Company, Japan; core shell polymers, such as PS 1700, available commercially from Union Carbide Corpoartion, Danbury, Connecticut; and BLENDEX 338, SILTEM STM 1500 and ULTEM 2000, which are available commercially from General Electric Company. ULTEM 2000 (CAS Reg. No. 61128-46-9) is a polyetherimide having a molecular weight ("Mw") of about 30,000 ± 10,000.

The inventive composition may be in the form of an adhesive, in which case one or more of an adhesion promoter, a flame retardant, a filler (such as the inorganic filler noted above, or a different one), a thermoplastic additive, a reactive or non-reactive diluent, and a thixotrope may be included. In addition, the inventive adhesive may be placed in film form, in which case a support constructed from nylon, glass, carbon, polyester, polyalkylene, quartz, polybenzimidazole, polyetheretherketone, polyphenylene sulfide, poly p-phenylene benzobisoaxazole, silicon carbide, phenolformaldehyde, phthalate and napthenoate may be included.

Compositions of the present invention may ordinarily be cured by heating to a temperature in the range of about 120 to about 180°C for a period of time of about 30 minutes to 4 hours. Thus, the inventive compositions can be used at relatively moderate temperatures to achieve very good productivity.

The inventive compositions (and pregregs and towpregs prepared therefrom) are particularly useful in the manufacture and assembly of composite parts for aerospace and industrial end uses, bonding of composite and metal parts, core and core-fill for sandwich structures and composite surfacing.

The invention also provides a process for producing a heat curable composition. The steps of this process includes:

1. (a) providing a benzoxazine comprising where o is 1-4, X is a direct bond (when o is 2), alkyl (when o is 1), alkylene (when o is 2-4), carbonyl (when o is 2), thiol (when O is 1), thioether (when o is 2), sulfoxide (when O is 2), sulfone (when O is 2) and R₁ is alkyl.
2. (b) providing with mixing 5 weight precent or more of a toughener component comprising acrylonitrile-butadiene co-polymer having secondary amine terminal groups; and
3. (c) mixing the benzoxazine and the toughener component under conditions appropriate to produce the heat curable composition.

This invention is further illustrated by the following representative examples.

In a first comparative example, the following components were used to prepare Sample Nos. 1-2 as noted in Table 1.

Each of the samples was prepared as follows:

The benzoxazine was warmed at a temperature in the range of 82.2-93.3 °C (180-200°F) to render it flowable, without initiating curing.

Where an epoxy component was present, the flowable benzoxazine was mixed with the epoxy at a temperature of 82.2 °C (180°F) until a homogeneous mixture was formed.

Where ATBN was present, the ATBN was mixed into the mixture at a temperature in the range of 71.1-82.2 °C (160-180°F).

The composition so formed was mixed under a vacuum, at a temperature in the range of 71.1-82.2 °C (160-180 °F) for a period of time of 15-30 minutes. The so formed composition was stored in a closed container at room temperature.

The samples noted in Table 1 may be cured in an open face mold placed in an autoclave using the following cure profile;

The samples were exposed to 620528 N/m² (90 psi) pressure within the autoclave and the temperature therein was increased at a 2.8 °C/min (5°F/min) ramp rate to a temperature of 176.7 °C (350 °F), for a period of time of about 3 hours. The cured samples were then cooled to a temperature of about 32.2 °C (90 °F) in the mold at a 2.8 °C/min (5 °F/min) ramp rate for a period of time of about 1 hour before use or evaluation.

The cured samples were evaluated using the following property tests:

Dynamic mechanical thermal analysis ("DMTA") was performed on cured samples using a double cantilever fixture. The cured samples were isothermally equilibrated at a temperature of 40°C in an oven and the temperature was increased at a 5°C/min ramp rate to a temperature of 250°C. Tg values were obtained from this DMTA evaluation from onset G', storage shear modulus.

The cured samples were also subjected to boiling water for 3 days, and weight gain was recorded. Tg values were obtained from these samples using a DMTA evaluation.

Density of uncured and cured samples was measured according to ASTM D 792, and cure shrinkage was then calculated.

Flexure strength and modulus was determined in accordance with ASTM D 790, using cured samples with the following specimen dimensions: 0.003 x 0.013 x 0.102 m (0.125 x 0.5 x 4 in.), span 0.051 m (2 in.), test speed: 0.001 m/min (0.05 in./min).

K_IC and G_IC was determined in accordance with ASTM D5045-96, using single edge notch bending ("SENB") specimens of the cured samples.

The results of these evaluations on Sample Nos. 1-2 are set forth in Table 2.

In comparing these results from Sample Nos. 1 and 2, it is clear that Tg did not change appreciably by the addition of ATBN in this benzoxazine-epoxy system. Also, fracture toughness in terms of G_IC increased over two fold. However, the comparison between Tg and Hot/Wet Tg of Sample Nos. 1-2 each had a large decrease. This is undesirable because under service conditions, a finished part made with such a composition would experience a low service temperature. Next, Sample Nos. 3-11, which are within the scope of the invention, were prepared using Bm-type benzoxazine and, where noted, ATBN in the amounts noted in Tables 3a, 3b and 3c. Sample No. 3, without ATBN, was used for comparative purposes. And Sample Nos. 21-22 were used for comparative purposes as regards an inorganic filler.

The preparation process for each of Sample Nos. 3-11 and 21-24 (Sample Nos. 21-22 being the same as Sample Nos. 6 and 8 but presented for comparison with silica nanoparticle containg samples) was as described above with respect to Sample Nos. 1-2 (except for Sample Nos. 23-24, where silica nanoparticles were added and ATBN was added in the former) as was the cure profile.

Sample Nos. 3-11 and 21-24 were cured and evaluated for property performance, results of which are shown below in Tables 4a, 4b and 4c.

In comparing these results from Sample Nos. 3-11 and 21-24, it is clear that Tg increased with the addition of ATBN into the Bm-type benzoxazine. Also, fracture toughness in terms of G_IC increased with increased amounts of ATBN. And, the comparison between Tg and Hot/Wet Tg of Sample Nos. 3-11 and 21-24 showed that there was not an appreciable difference in these values, quite unlike the values observed with Sample Nos. 1-2.

The advantages of using silica nanoparticles, for example, NANOPOX XP 0314 include increasing the modulus and improving toughness of the cured composition, without compromising Tg and Hot/Wet Tg. In addition, the use of silica nanoparticles (contrasted with larger silica particles) allows for the inclusion of such silica nanoparticles in the composition even after filtration processes (used in prepreg or RTM), which would otherwise trap such larger particles.

For compositions made from B-m type benzoxazine with ATBN, good toughness and high Tg (dry and hot/wet) were observed. With the amount of ATBN increased, the modulus was observed to decrease (seee.g. Sample Nos. 6-8). By adding silica nanoparticle (in an epoxy resin dispersion), such as NANOPOX XP 0314, the modulus was observed to improve without affecting Tg. Therefore, by adjusting the amount of ATBN and silica nanoparticles, the present invention provides heat curable compositions with high toughness, high Tg and a variable modulus from low to high. A low modulus composition is suitable for adhesive applications while a high molulus composition oftentimes can satisify high performance prepreg requiremnts.

For further comparative purposes, Sample Nos. 12-20 were prepared using either a bisphenol F-type, bisphenol A-type, Bm-type benzoxazine (as indicated) and, where noted, an epoxy resin, together with the tougheners incidated, ATBN in the amounts noted in Tables 5a and 5b.

The preparation process for each of Sample Nos. 12-20 was as described above with respect to Sample Nos. 1 and 2, as was the cure profile.

Performance property evaluation of Sample Nos. 12-20 was performed as follows, with the results recorded in Tables 6a-6b.

Uncured samples were evaluated by Differential Scanning Calorimetry ("DSC"), within a temperature range of 40-350°C, where the temperature range increased at a 20°C/min ramp rate, and from which ΔH was recorded.

Cured samples were evaluated as above.

Reference to Figures 1-3 shows that with the inventive composition (illustrated in Figure 1), an increase of ATBN concentration improves G_IC and dry Tg and Hot/Wet Tg, while with benzoxazines outside of the claimed structure (illustrated in Figures 2-3), an increase of ATBN concentration improves G_IC but decreases Tg. Figure 1 shows that in the inventive composition Tg and toughness measured by G_IC increase as the amount of toughener in the composition increases, whereas Figures 2 and 3 show that with either an aryl substituted benzoxazine (that is, where R₁ and/or R₂) and/or CTBN instead of ATBN the opposite result is illustrated.

An increased Tg such as 200 (dry), together with increased toughness such as 3, are not ordinarily found in the same thermoset system. One known exception to this is Cytec Industries 977-2, which provides an epoxy prepeg thermoset. However, this Cytec product is known to have a moisture uptake of about 3 to 5%, which contributes to low Hot/Wet Tg (such as about 300), and a high ΔH, such as 500-700 J/g, compared to the inventive compositions, which are demonstrated to be lower than 500 J/g with one as low as 163 J/g. A lower ΔH is desirable so that stresses within the assembled structure are not caused during cooling. And the thermosets from the inventive compositions have the potential to reach an increased Tg ordinarily not found, unless insoluble toughener materials are added, or have an unacceptable viscosity for many commercial applications.