Modified bismide composition

Abstract: A thermosetting composition comprising a bisimide monomer and a reactive triene which contains a conjugated double bond capable of Diels-Alder reaction with the bisimide and an isolated double bond. The copolymerization of a bisimide with a reactive triene such as myrcene provides a modified bisimide having good fracture toughness.

Description: This invention relates to thermosetting bisimide compositions having good processability and enhanced fracture toughness in the cured state. Advanced composites are high-performance materials made up of a fiber- reinforced thermoplastic or thermosettable material. The thermosettable materials useful in advanced composites must meet a set of demanding property requirements. For example, such materials optimally have good high-temperature properties such as high (above 200 °C) cured glass transition temperature and relatively low (less than 4%) water absorbance at elevated temperature. Such materials must also exhibit high mechanical strength, as measured by fracture toughness and compression after impact. For ease of processing in preparing prepregs for composite parts, the uncured material will ideally have a low (below 120 °C) melting temperature and a wide temperature range of processable viscosity (a wide "processing window"). Bismaleimide resins are thermosettable materials with much promise for use in advanced composites. Disadvantages of bismaleimides, however, include brittleness and high melting points, the latter of which requires bismaleimides to be used with solvents for acceptable processability. Standard modifiers for bismaleimides include allyl-substituted phenols (see USP 4288583, 4100140 and 4526925); bisbenzocyclobutenes (see USP 4730030, and 4927907); dihydrazides (see USP 4211860) and olefins (see USP 4220741 and 4159754). However, those modifiers have a number of disadvantages. Some, such as styrene, divinylbenzene, diisopropenylbenzene and certain derivatives thereof, are extremely reactive with maleimide groups by both Diels-Alder and radical mechanisms. This leads to a very short pot life for bismaleimide mixtures with these reactive diluents. These materials, while providing more easily processable bismaleimide resins, tend to be ineffective as tougheners for the resins. Diamines and dithiols generally share this disadvantage of high reactivity, and also the adducts of maleimide groups with amines or thiols are of somewhat low thermal stability. Other diluent modifiers, such as bisallyl, bis(allyloxy) or bispropenyl aromatic compounds, including diallyl bisphenol-A and adducts of allylphenols with epoxy resins, are somewhat less reactive than the vinyl aromatics but still have a pot life in the melt which is relatively short for many types of liquid processing applications. It is therefore an object of the invention to provide bisimide resin compositions having enhanced physical properties and good processability for composites applications. According to the invention, a curable composition is provided comprising: (a) a bisimide of an unsaturated dicarboxylic acid of the formula I: in which Y is a substituted or unsubstituted divalent group containing at least 2 carbon atoms and a carbon-carbon double bond, and Z is a divalent linking group; and (b) a triene represented by one of formulas IV and V: in which each R is selected independently from hydrogen and C₁₋₃ alkyl, and R' is a divalent linking group. R' can be, for example, alkylene, preferably C₂₋₁₂ alkylene; -(CH₂) n R''(CH₂) n -; and -O-R''-O-, in which n is an integer from 1 to 6 and R'' is C₁₋₁₂ alkylene, carbonyl and phenylene. The preferred R' linking group is C₂₋₁₂ alkylene, as in myrcene and trans-1,3,7-octatriene. The conjugated diene group must be capable of Diels-Alder reaction with the Y moiety of the bisimide and must not contain substituents in positions which would block the approach of a Diels-Alder dienophile when the diene group is in the cisoid confirmation, as in cis-1,3,7-octatriene, for example. The isolated double bond should be adjoined by at least one hydrogen-bearing carbon atom. Because of the fracture toughness and high-temperature properties achievable in the resulting bisimide copolymer, myrcene is the preferred triene comonomer. The invention composition includes a bisimide. Preferred bisimides of formula I are those in which Y is a substituted or unsubstituted divalent group containing 2 to 6 carbon atoms and a carbon-carbon double bond, and Z is a divalent radical comprising at least 1, generally of from 1 to 40 carbon atoms. Z can be aliphatic, cycloaliphatic, aromatic or heterocyclic. A particularly preferred class of bisimides are difunctional bismaleimides derived from an aromatic diamine which can be represented by the formula II: in which each R₁ is selected independently from H, C₁₋₂ alkyl or halide; R₂ is selected from divalent hydrocarbon radicals containing from 1 to 10 carbon atoms, -O-, -SO₂-, -COO-, -CONH-, -CO- and -S-; and each R₃ is selected independently from H, C₁₋₃ alkyl and halide. Specific examples of bisimides include: 1,2-bismaleimidoethane 1,6-bismaleimidohexane 1,3-bismaleimidobenzene 1,4-bismaleimidobenzene 2,4-bismaleimidotoluene 4,4'-bismaleimidodiphenylmethane 4,4'-bismaleimidodiphenylether 3,3'-bismaleimidodiphenylsulfone 4,4'-bismaleimidodiphenylsulfone 4,4'-bismaleimidodicyclohexylmethane 3,5-bis(4-maleimidophenyl)pyridine 2,6-bismaleimidopyridine 1,3-bis(maleimidomethyl)cyclohexane 1,3-bis(maleimidomethyl)benzene 1,1-bis(4-maleimidophenyl)cyclohexane 1,3-bis(dichloromaleimido)benzene 4,4'-biscitraconimidodiphenylmethane 2,2-bis(4-maleimidophenyl)propane 1-phenyl-1,1-bis(4-maleimidophenyl)ethane α,α-bis(4-maleimidophenyl)toluene 3,5-bismaleimido-1,2,4-triazole and various bismaleimides disclosed in U.S. 3,562,223, 4,211,860 and 4,211,861. Bismaleimides can be prepared by methods known in the art, as described in U.S. 3,018,290, for example. The bismaleimide resin can contain imide oligomer according to the formula III: in which x is a number within the range of from 0 to 0.5. Such oligomers may be present as an impurity in difunctional bisimides. The most preferred bisimide resin is N,N'-4,4'-diphenylmethane bismaleimide. The bisimide can contain various additives and modifiers as processing aids. The bisimide resin component can be a reaction product or prepolymer of a bisimide and an effective chain-extending agent such as an amine group-containing compound. Suitable amine group-containing compounds include diamines and polyamines represented by the general formula (H₂N) n Q or (RNH) n Q and aminophenols represented by the general formula (NH₂) n Q(OH) n or (RNH) n Q(OH) n , in which Q is a divalent aromatic or alicyclic group and n is a number such that the average number of amine hydrogens per molecule in the amine mixture falls between 1.95 and 2.5. Examples include bis(4-(N-methylamino)phenyl)methane, and N,N'-dimethyl-1,3-diaminobenzene. Such reaction products can be prepared by methods known in the art, such as contacting 0.1 to 0.8 mole of the chain-extending agent with each mole of the bisimide in an organic solvent at a temperature of from 40° to 200 °C for a time of from 5 minutes to 5 hours. The bisimide can be, for example, a hydrazide-modified bismaleimide as described in U.S. 4,211,860 and 4,211,861. Suitable N,N'-unsaturated bismaleimide resins are commercially available from Technochemie GmbH as Compimide resins (Compimide is a trade mark), for example. The bisimide can be a mixture of bisimides described above tailored to meet specific processing requirements. The bismaleimide and the triene are combined in a molar ratio of triene:bisimide within the range of from 0.2 to 1.0, preferably of from 0.3 to 0.9, most preferably of from 0.4 to 0.8, in order to achieve the desired physical properties in the cured composition. The bisimide and triene may be combined in any manner desired, such as melt, solution or powder blending. The preferred technique involves melting a mixture of the solid reactants at a temperature above the respective melting points but below the polymerization temperature of either monomer, and stirring the melt until a homogeneous mixture is achieved. The melt may also be held at temperatures above about 150 °C for desired periods of time in a process of prepolymerization to increase the crystallization resistance of the melt and/or to increase its viscosity to desired levels. The mixture can then be poured directly into a mould for polymerization, or it can be directly cooled for later polymerization. The composition may contain an optional free radical inhibitor to inhibit free radical polymerization of the bisimide monomer. Generally, the free radical inhibitor will be present in the composition in an amount within the range of from 0.0002 to 0.02 moles per mole of the bisimide, preferably of from 0.001 to 0.01 moles. The free radical inhibitor can be added to the monomers in any manner effective for intimate blending of the monomers and free radical inhibitor. Free radical inhibitors include phenols such as t-butylcatechol, hydroquinone and p-methoxyphenol; quinones such as 1,4-benzoquinone and 1,4-naphthoquinone; polynitro aromatics such as picric acid and 2,4,6-trinitrotoluene; hydroxylamines such as diethylhydroxylamine; stable radicals such as di-t-butylnitroxide or diphenylpicrylhydrazyl; and certain polycyclic heterocycles such as phenothiazine. The preferred free radical inhibitor is phenothiazine. Polymerization is effected by heating the mixture to a temperature effective to initiate reaction between the functional groups of the bisimide and the double bonds of the triene. The temperature is generally at least 150 °C, preferably within the range of from 170 to 350 °C, held for a time of from 2 hours or more, with the required reaction time dependent on the temperature-staging program used. In order to achieve optimum properties in the copolymers, a mixture of the monomers and free radical inhibitor is heated at a temperature near or above the ultimate (fully cured) glass transition temperature of the copolymer composition for a time sufficient to produce essentially complete reaction of the monomers. "Essentially complete" reaction of the monomers has been reached when no further reaction exotherm is observed by differential scanning calorimetry (DSC) upon heating the copolymer. The time of the heat treatment, or "post-cure," will vary depending upon the monomers, the degree of pressure applied and any precuring of the monomer mixture. The copolymers are useful as resin matrices for composites in aerospace and electronics applications, including large structural parts and circuit boards. Based on their long shelf life and relatively low melting point, some of the uncured mixtures are useful for making tacky prepregs which can then be moulded into composites. They are also suitable for liquid resin processing methods such as filament winding, resin transfer moulding, resin infusion moulding, and pultrusion if the mixtures are heated to provide sufficiently low viscosity for fiber impregnation. The low dielectric constants of the cured copolymers make them suitable for electrical applications such as the manufacture of circuit boards. For preparation of reinforced laminate materials, a fibrous substrate of glass, carbon, quartz, poly(p-phenyleneterephthalamide), polyester, polytetrafluoroethylene, poly(p-phenylenebenzobisthiazole), boron, paper or like material, in chopped, mat or woven form, is impregnated with a bisimide/triene composition in molten or solution form. A prepreg is formed by heating the impregnated substrate in an oven at a temperature sufficient to remove the solvent and optionally to partially cure without gelation or "B-stage" the resin system, generally at a temperature of from 180 °C to 230 °C, preferably of from 200 to 220 °C, for a time of up to 2 hours, preferably of from 10 to 40 minutes. A laminate is fabricated by subjecting a set of layered prepregs to conditions effective to cure the resins and to integrate the prepregs into a laminated structure. The laminate can optionally include one or more layers of a conductive material such as copper. Laminating generally involves subjecting the prepregs to a temperature above 200 °C, preferably from 210 to 350 °C, for a time of at least 1 hour, at a pressure within the range of from 3.5 to 35 bar. For some laminating applications, especially if the bisimide/triene mixture will be dissolved in a solvent before impregnation of the substrate, it may be advantageous to heat treat, or upstage, the bisimide/ triene mixture prior to application to a laminating substrate, particularly if the mixture will be stored prior to use. Suitable heat treatment involves subjecting the bisimide/triene to an elevated temperature for a time sufficient to cause sufficient reaction and viscosity increase to inhibit crystallization of either or both monomers from the mixture upon storage, but not sufficient to gel the composition. Example 1 Myrcene (SCM Glidco P&F grade) and bis(4-maleimidophenyl)methane (MDAB) were weighed into glass beakers in the proportions shown in Table 1 below (mixture sizes were from 50 to 80 grams). To all mixtures was added phenothiazine at a level of 0.53-0.54 mol % (based on MDAB). The beakers were then heated in an oil bath at approximately 150 °C, with stirring, until the contents were homogeneous. The mixtures were then poured into a rectangular stainless steel mould with a 2.8 (or 3.2) mm thick cavity, with the mould parts separated by an airtight rubber gasket such that the mould could be pressurized during cure. The mould was then placed into an oven (under 4.8-6.9 bar nitrogen pressure) and the systems were cured as shown in Table 1. (Small portions of mixtures 4,6 and 8 from Table 1 were cooled to room temperature and saved for use in the cure rheological studies described in Example 2.) Two control castings were also prepared. The first (control casting 1) was from a mixture of 70 mol % MDAB, 15 mol % of 2,4-bismaleimidotoluene, and 15 mol % of 1,3-bismaleimidobenzene (containing 0.53-0.54 mol % phenothiazine, based on total amount bismaleimides). This mixture was used instead of unmodified MDAB because of the difficulty in pouring molten MDAB into casting moulds due to its tendency to crystallize on slight cooling. The initial cure temperature used for the unmodified bismaleimide (210 °C) was higher than for the other systems (150 °C) because of the extremely slow cure rate of the unmodified BMI blend at the lower temperature. Control casting 2 was made from a standard commercial toughened bismaleimide system containing 63.4 mol % MDAB, 34.5 mol % of 2,4-bismaleimidotoluene, and 2.1 mol % of 1,5-bismaleimido-2,2,4-trimethylpentane. This BMI mixture was chain-extended and toughened with the 2:1 adduct of o-allylphenol with resorcinol diglycidyl ether (31.5 mole% with respect to total bismaleimides) and with m-aminobenzhydrazide (8.6 mole% with respect to total bismaleimides). The control castings were similarly cured as shown in Table 1. The moulds were then allowed to cool and the cured resin castings were removed from the moulds. Physical properties of the castings were then determined as shown in Table 1. One can see that the myrcene/MDAB blends provide a number of advantages over the control castings of the unmodified BMI mixture and the commercial toughened system. The unmodified BMI mixture formed a casting (control 1) which was so brittle that samples for mechanical testing could not be machined successfully without cracking. All of the myrcene blends were considerably lower in water absorption than the commercial toughened BMI system (control 2). Similarly, all of the MDAB/myrcene blends tested (except one containing a relatively low amount of myrcene) were superior to the commercial toughened BMI system (control 2) in wet flexural strength and modulus at 93 °C. The myrcene/MDAB blends were also somewhat lower in dielectric constant than the commercial material (desirable in many electrical applications). Example 2 This example shows the wide "processing window" during cure of myrcene blends in comparison with a standard commercial BMI mixture. The retained uncured mixtures 4,6 and 8 from Table 1 were placed between the plates of a parallel plate viscometer and heated at a rate of 10 °C/min. Viscosity (under 11 Hz oscillatory shear) was determined for each mixture as a function of temperature at this heating rate. The same viscosity test under heating was also performed on a control sample of the commercial toughened BMI system (control 2) from Example 1. The two systems showed similar viscosity behavior at lower temperatures, dropping in viscosity from semisolids at room temperature to 1 Pa.s in viscosity at approximately 100 °C. (On further heating of a curable resin system, viscosity will pass through a minimum followed by an upturn as the system reaches a temperature where cure reactions progress rapidly.) The commercially toughened BMI system rose again to 1 Pa.s in viscosity at 219 °C, while the myrcene blends did not reach this viscosity until 270 °C. (The exact position of the viscosity minimum was unknown because the viscometer used could not accurately measure viscosities below 0.5 Pa.s.) The more delayed viscosity increase in the myrcene blends, however, illustrates their wider "processing window" for liquid processing applications. Example 3 For comparison, a series of experiments were done to co-cure a bismaleimide mixture with various unsaturated C₁₀ compounds. Into a glass beaker were weighed 231 g of 4,4'-bismaleimidodiphenylmethane, 38.9 g of 2,4-bismaleimidotoluene, 36.9 g of 1,3-bismaleimidobenzene, and 0.929 g of phenothiazine. The mixture was heated in an oil bath to 205 °C, stirred until homogeneous, and allowed to cool to room temperature, solidifying to an amorphous mass. The mass was then broken up into small pieces and cocured with various terpenes as follows. (This mixture was used instead of a single bismaleimide resin for ease of processing, in order to hinder the bismaleimide from crystallizing from its molten mixture with the terpene.) Approximately 2.4 g of the above bismaleimide mixture was weighed into each of five 30 ml glass beakers. Limonene, (1S)-β-pinene, α-terpinene, γ-terpinene, and allo-ocimene, respectively, were added to the beakers. The molar ratio of terpene to total bismaleimides in each mixture ranged from 0.99 to 1.01. The beakers were then heated in an oil bath at approximately 150 °C, with stirring, until the contents were homogeneous. The mixtures were then poured into moulds formed from two sheets of glass separated by a 1.6 mm polytetrafluoroethylene spacer and held together by clamps. The moulds were then placed into an oven and the mixtures were cured for 1 hour at 150 °C, 1 hour at 180 °C, 1 hour at 210 °C, 30 minutes at 230 °C, 30 minutes at 250 °C, 30 minutes at 270 °C, and 1 hour at 290 °C. The moulds were then allowed to cool and the cured resin castings were removed from the moulds. Toughness of the resulting solids was assessed with a knife-peel screening test. Results are shown in Table 2.

Claims: A thermosetting composition comprising: (a) a bisimide of an unsaturated dicarboxylic acid of formula I: in which Y is a substituted or unsubstituted divalent group containing at least 2 carbon atoms and a carbon-carbon double bond, and Z is a divalent linking group; and (b) a triene represented by one of formulas IV and V: in which each R is selected independently from hydrogen and C₁₋₃ alkyl, R' is a divalent linking group, and the isolated double bond adjoins at least one hydrogen-bearing carbon atom. A composition as claimed in claim 1 in which R' is selected from the group consisting of alkylene, -(CH₂) n --R''-(CH₂) n - and -O-R''-O- in which n is an integer of from 1 to 6 and R'' is C₁₋₁₂ alkylene, carbonyl and phenylene. A composition as claimed in claim 2 in which R' is C₂₋₁₂ alkylene. A composition as claimed in any one of the claims 1 to 3 in which each R is selected from hydrogen and methyl. A composition as claimed in any one of the claims 1 to 4 in which the triene is myrcene. A composition as claimed in any one of the claims 1 to 5 in which the molar ratio of the triene to the bisimide is within the range of from 0.2-1.0:1. A composition as claimed in any one of the claims 1 to 6 in which the bisimide comprises N,N'-4,4'-diphenylmethane bismaleimide. A composition as claimed in any one of the claims 1 to 7 which in addition comprises from 0.0002 to 0.2 mol, per mol of the bisimide, of a free radical polymerization inhibitor. A composition as claimed in claim 8 in which the free radical polymerization inhibitor is phenothiazine.

Inventors: CORLEY LARRY STEVEN, CORLEY, LARRY STEVEN
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1991
Main classification: C08G73
IPC codes: C08F36, C08F222, C08F22, C08G61, H05K1, C08G73, C07C11, C08F236