BACKGROUND OF THE INVENTION

The invention relates to biscitraconimide (co)poly ers, a process for curing biscitraconimides with an anionic catalyst, and to articles of manufacture comprising the biscitraconimide (co)polymers.

Biscitraconimides are known compounds and can be prepared by the methods disclosed in, "The Synthesis of Biscitraconimides and Polybiscitraconimides," Galanti, A.V. and Scola, D.A., Journ. of Poly. Sc . : Polymer Chemistry Edition, Vol. 19, pp. 451-475, (1981), the disclosure of which is hereby incorporated by reference. These biscitraconimides are polymerized to tough amber-colored films that exhibit good thermal stability. In addition, the article points out that NMR analysis shows that the observed ratio of methyl protons at 2.1 ppm. to the methylene protons at 1.6 ppm. in the biscitraconimides is lower than the theoretical ratios for the i ide monomers. The difference is explained as being due to a small degree of polymerization that could occur when the acid is dehydrated thermally.

"The Synthesis of Bisitaconamic Acids and Isomeric Bisimide Monomers," Galanti, A.V. et al . , Journ. Poly. Sci.: Polymer Chemistry Edition, Vol. 20, pp. 233-239 (1982) also discloses a method for the preparation of biscitraconimides in the form of an isomeric mixture of the citraconic and itaconic imides.

In "The Development of Tough Bismaleimide Resins," Stenzenberger, H.D., et al., 31st International SAMPE Symposium, Vol. 31, pp. 920-932 (1986) it is disclosed that bismalei ides are prime candidates for

carbon fiber reinforced composites because of their properties. However, the article also points out that these materials tend to be brittle. Thus, several attempts have been made to improve the fracture toughness of the bismaleimides. First, the bismaleimides have been cocured with reactive elastomers such as carboxy terminated acrylonitrile-butadiene rubbers. Also, the bismaleimide polymers have been modified with comonomers which copoly erize via a linear chain extension reaction and include both diene type copolymerization reactions and "ene"-type copolymerization reactions. Thirdly, the bismaleimides have been modified with thermoplastic materials. Finally, the bismaleimides have been cured in the presence of ionic curing catalysts such as imidazoles and tertiary amines including diazobicyclo-octane (DABCO) .

In "Bismaleimide Resins the Properties and Processing of 'Compimide' BMI Resins," Segal, C.L., et al . , 17th Nat. SAMPE Conference 17, pp. 147-160 (1985) formulated bismaleimides are cured with the ionic catalysts DABCO and 2-methylimidazole. It was concluded that, in general, the curing of formulated bismaleimides improved the fracture toughness of the materials due to lower built-in cure stresses. All formulations cured in the presence of a the curing catalyst had been modified previously with a reactive elastomer and were cured in the presence of an allyl-type reactive diluent.

Other curing catalysts for bismaleimides are known as for example in Japanese Laid-Open Application no. 262823/1985 where it is disclosed that an N,N'-bisimide can be cured in the presence of a diamine and a tertiary a ine having a pH of at least 10.85. No appreciable difference in the properties of the resins cured with the catalyst could be discerned when compared with the same resins cured in the absence of the catalyst.

European Patent Application 0 108 461 published on May 16, 1984, discloses, in example 4, the curing of bismaleimide resins having therein styrene, diallyl phthalate and acrylic acid, in the presence

of DABCO. The products of this example exhibited improved fracture toughness and withstood a maximum strain of 3.9%.

Dutch Patent Application 6,514,767 published in 1966 describes the co-and homopoly erization of bismaleimides and biscitraconimides at high temperatures and under pressure. No copolymers of bismaleimides with biscitraconimides are disclosed. The polymers show good electrical properties and exhibited acceptable thermal stability.

U.S. Patent Number 4,568,733 issued on February 4, 1986, relates to mixed aromatic bismaleimide/aromatic biscitraconimide resins which produce materials which have better handling, processing and thermal properties than materials with individual resins. These resins are thermally cured without a curing catalyst. The incorporation of the biscitraconimide into the bismaleimide generally produced a significant reduction in the elongation percent, however.

Generally, the bismaleimide resins require difficult processing conditions, exhibit solvent retention in the prepregs, have a high melting point and high curing temperatures are required for the monomer. In addition, the maleimide polymers are often brittle due to the high cross-link density obtained in the network polymers. The foregoing body of prior art reflects the need for bisimide resin systems which are easily processable and exhibit improved mechanical and physical properties.

SUMMARY OF THE INVENTION

The present invention has for its object to eliminate the foregoing drawbacks of the prior art bisimide resins and to substantially improve the fracture toughness of molded compositions made from bisimide resins. For this purpose the present invention provides curable bisimide compositions containing at least one biscitraconimide unit having the general formula

wherein D is a divalent group, R is CH -R1, and R\ is independently selected from hydrogen and alkyl groups having 1-18 carbon atoms, characterized in that the composition comprises a sufficient amount of an anionic curing catalyst to convert at least 10% of the R groups on the biscitraconimide units into alkylene bridges in the cured composition.

The present invention further provides a process for the preparation biscitraconimide unit-containing (co)poly ers having at least 10% of the R groups on the biscitraconimide units converted to alkylene bridges comprising curing the curable compositions containing biscitraconimide units of the formula (I).

These polymeric compositions and the articles of manufacture produced therefrom offer several advantages over prior art bisimide formul tions. For example, these aliphatic biscitraconimide-containing materials can be processed at lower temperatures than bismaleimides and the resultant polymers show improved properties including a high Tg, good thermostability and the mechanical properties are significantly improved. Most noticeable among the improved mechanical properties is a substantial increase in the tensile strength of the polymers according to the present invention as a result of the formation of alkylene bridges therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Biscitraconimides are known compounds and can be prepared by any of the methods disclosed in Dutch Patent Application No. 6,514,767; "The Synthesis of Biscitraconimides and Polybiscitraconimides", Galanti, A.V., and Scola, D.A., Journ, of Polym. Sci.: Polymer Chemistry Edition, Vol. 19, pp. 451-475 (1981); and "The Synthesis of Bisitaconamic Acids and Isomeric Bisimide Monomers", Galanti, A.V., et al . , Journ. of Polym. Sci.: Polymer Chemistry Edition, Vol. 20, pp. 233-239 (1982), the disclosures of which are hereby incorporated by reference.

The aliphatic biscitraconimides employed in the present invention comprise compounds having the formula I:

wherein D is a divalent group, R is CH2- 1 , and R is independently selected from hydrogen and alkyl groups having 1-18 carbon atoms.

D may be selected from divalent groups including alkylene, cyclicalkylene groups, oligomers of biscitraconimides, and residues of one or more cocurable materials and biscitraconimide oligomers. D is preferably selected from a substituted or unsubstituted aliphatic divalent group. Most preferably D is a divalent group selected from tetramethylene; pentamethylene; hexamethylene; 2-methyl pentamethylene; neopentylene; (2,2,4-trimethyl)hexamethylene;

1,3-bis (methyl ene) cyclohexane; 4,4'-methylene bis-2-methyl cyclohexane; 2,2-dicyclohexylpropylene; m-xylylene; and tricyclodocecylene. In addition to biscitraconimide units wherein D is a divalent group, some citraconimide units wherein D is a onovalent

or trivalent group may also be present. For example, some citraconimide units wherein D is a monovalent or trivalent amine may be present along with biscitraconimide units wherein D is a divalent group.

Suitable aliphatic biscitraconimides are in particular N, N ' -ethyl ene-bi sci traconi c imi de;

-hexamethylene-biscitraconic i ide;

-tetramethyl erre-bi sci traconi c i mi de ;

-2-methyl -penta ethyl ene-bi sci traconi c imi de;

-propylene-biscitraconic imide;

-4,4' -di cycl ohexylmethane-bi sci traconi c imi de ;

-dicyclohexyl-bi sci traconi c imide; and

-α, '-4,4'-diraethylene cycl ohexane-bi sci traconi c imide.

The anionic catalysts employed in the present invention comprise generally known compounds which can be obtained commercially or can be prepared by known synthetic methods. In general, the anionic catalyst must exhibit catalytic activity on the polymerization of aliphatic biscitraconimide-containing compositions at suitable polymerization temperatures. Anionic catalysts within the scope of the present invention comprise diazo-bicyclo alkanes, diazo-bicyclo alkenes, imidazoles, substituted imidazoles, the alkali salts of organic alcohols, triphenyl phosphine and substituted or unsubstituted aliphatic and aromatic secondary and tertiary amines. The most preferred catalysts for both economic and performance reasons are the diazo-bicyclo octane, triphenyl phosphine and the imidazoles including 2-methyl imidazole. Other, less preferred catalysts include the alkali metal salts of t-butanol , ethanol , ethanol or di oi s .

The anionic catalyst is present in an amount sufficient to convert at least 10% of the R groups of the formula I, on the aliphatic biscitraconimide units into alkylene bridges. Typically, 0.01 to 3.0 weight percent of the anionic catalyst is employed. More preferably, 0.1 to 3.0 weight percent is used.

In parti cul ar, bi sci traconimi de compounds of the formul a ( I ) are polymeri zed i nto polymers such as the fol l owi ng :

( ID

0-C C=0 0=C C=0 0=C 0

\ / \ /

N N

D

wherein D, R and Rι_ have the same meaning as in the formula I. In these polymers there is included, in addition to the expected cross- linking, additional alkylene bridges between the polymer units such as the alkylene bridges shown in the formula II as indicated by the CH2-R1 groups. These alkylene bridges may link a carbon atom in the citraconi ide unit having an abstractable hydrogen atom attached. The alkylene bridges will always be formed by a linking methylene group which may optionally be substituted by an R group as shown in the formula II.

As is shown in formula II, there are essentially four possibilities with the two leftmost being the linking of the original biscitraconimide units and accounting for the majority of the polymeric units formed by 1,2 or 1,3 addition, and the two rightmost having a spiro form, resulting from iso erization of the biscitraconimide units into itaconi ide units under the reaction conditions. The itaconic isomerization can account for up to 30% or even slightly more of the units resulting from the present anionic curing process.

At least 10% of the original R groups on the biscitraconimide units are converted to these alkylene bridges in the cured polymer in order

to produce novel biscitraconimide polymers having superior properties. More preferably, at least 25%, and most preferably at least 40% of the original R groups are converted to alkylene bridges and nearly 100% of the original biscitraconimide R groups can be converted to alkylene bridges if desirable.

The curable composition may also comprise one or more cocurable materials. Suitable cocurable materials include bismaleimides, citraconic maleimides, itaconic maleimides, citraconic/itaconic maleimides, bis-(allyl trimellitate imides), bisitaconimides, and aromatic or aliphatic amines which may be present in an amount of up to 40% of the composition, and triallyl cyanurate, triallyl isocyanurate, and olefinically unsaturated monomers such as diallyl phenol, styrene and styrene derivatives such as α-methylstyrene, indene and diisopropenyl benzene which may be present in an amount of up to 25% of the composition.

The present invention also embodies cured polymeric materials comprising units derived from an aliphatic biscitraconimide of the formula (I) wherein at least 10% of the R groups on the biscitraconimide units are converted to alkylene bridges as a result of curing in the presence of an anionic catalyst. Again, these cured polymeric materials may include units derived from one or more of the cocurable materials specified herein. More preferably, the cured polymeric compound of the present invention has at least 25%, and most preferably at least 40% of the R groups on the biscitraconimide units converted to alkylene bridges.

The curable composition of the present invention must be cured in the presence of an anionic catalyst. The curing is carried out by simply heating a composition containing at least one aliphatic biscitraconimide of the formula (I), an anionic catalyst and, optionally, a cocurable material, to a temperature above the melting point of the biscitraconimide and maintaining the temperature at that level for a sufficient time to cure the material into a cured polymeric product. Curing can be accomplished at 150°C to 250°C.

Generally, the curing will be accomplished at a temperature in excess of 180°C. The curing time will vary depending upon the amount of catalyst present and type of material being cured.

The cured polymeric product of the present invention is particularly useful in fiber-reinforced composites because of its excellent properties. More particularly, the cured compositions of the present invention exhibit a significantly improved flexural strength and tensile strength when compared with known maleimide polymeric compositions. In addition, this improved flexural and tensile strength is obtained without detrimental effects to the other important properties of the material. As a result, a novel composition is obtained which is easily processable due to the low melting points of the aliphatic biscitraconimides. In addition, the aliphatic biscitraconimides have a large melt cure window which allows them to be more readily cocured with a large group of materials which would not be cocurable with bismaleimides because a curing temperature suitable for both the bismaleimide and the cocurable material could not be found. Additionally, the aliphatic biscitraconimide melt itself has a low viscosity which renders it easier to handle than bismaleimide-based melts. Further, the polymerization can be accomplished without the use of solvents without the formation of volatiles thus allowing the fabrication of void-free polymers.

For applications in the laminate field, it is necessary to make prepregs from the biscitraconimides in order to obtain the desired properties for the laminate. The impregnated fibre cloth must be tack-free, flexible and have the proper melt viscosity. The biscitraconimide monomers themselves are not suitable for these applications since they are either oils or are too crystalline in nature.

It is possible to make prepolymers having the desired properties which can be employed to make prepregs with the distinct advantage that these polymers do not require a solvent in the prepreg manufacturing process. In the present commercial prepreg manufacturing methods,

solvents must be employed which leads to costly solvent removal steps and some voids in the final product.

The invention will be further described with reference to the following examples which are not to be construed as limiting the scope of the invention.

Example 1

50 gram of l,6-N,N'-hexamethylene biscitraconimide is melted at 140°C. The low viscosity liquid is degassed for 5 minutes with vacuum. After the degassing 1 weight percentage of a catalyst (DABCO, diazo bicyclo octane) is added and stirred in very well. The mixture is poured in a preheated mould (130°C) to prepare unreinforced sheets of 10 by 10 by 0.3 cm. The temperature of the mould is slowly increased up to 220°C and held for 5 hours at that temperature. The flexural properties and Tg value measured by DMA of this sheet material are mentioned in Table 1. Further solid state 13C NMR analysis were carried out. A considerable decrease of the methyl group signal of the imide ring and a new methylene signal was found, indicating the occurence of the -CH2- bridge in this polymer.

Comparative Example la

50 grams of l,6-N,N'-hexamethylene biscitraconimide is treated by the same procedure as example 1. The catalyst DABCO- as substituted by 1 weight percentage of dicumyl peroxide. The result of the measurements are mentioned in Table 1. The solid state 13C NMR spectra for this polymer shows a regular pattern not having a methylene signal.

Comparative Example lb

50 grams of l,6-N,N'-hexamethylene biscitraconimide is treated by the same procedure as example 1. However, no catalyst is added. The molten imide is cured only thermally. The results are mentioned in Table 1. The solid state 13C NMR resemble those of the polymer formed by peroxide initiated polymerization (Example la).

Table 1: Properties of cured l,6-N,N'-hexamethylene biscitraconimides, cured 5 hours at 220°C.

Example Curing method Flexural properties Tg TGA

E-mod strain (DMA) % loss

GPa % °C at 400°C

0

Preparation of glass reinforced sheet material by resin transfer moulding. 5

Example 2

To prepare a 1.6 mm thick glass reinforced sheet material 10 plies of glass fabric with imide-compatible sizing are placed in a plain mould. The mould is closed and heated up to 130°C. A molten mixture of 0 l,6-N,N'-hexamethylene biscitraconimide and 1% DABCO is injected with a pressure of 3 bar. The mould is filled within 3 minutes. The reinforced sheet is cured at the standard temperature program of example 1. The composite material contained 35% of glass and showed, in the bending test, an elongation at break of 3.1% which corresponded ₅ with the expected value for the glass fibre.

Copolymerisations Example 3

A mixture of 40 grams of l,6-N,N'-hexamethylene biscitraconimide and _D 10 grams of N,N'-methylene dianiline bismaleimide is melted at 140°C. After degassing 1 weight percentage of DABCO is added. As described in example 1 the mixture is poured in a preheated mould and cured at 220°C. The flexural properties are determined and mentioned in Table 2. The 13C solid state NMR analysis show a considerable decrease of the intensity of the signal of the methyl group of the imide ring to

about 30% of the original value and a new methylene signal was observed.

Comparison Example 3a

As described in example 3 the mixture of l,6-N,N'-hexamethylene biscitraconimide and N.N ¹ -methylene dianiline bismaleimide is melted. The mixture is cured without added catalyst DABCO. The flexural strain at failure lies at a much lower level (Table 2). No methylene signal is observed.

Table 2: Properties of cured mixtures of 80% of l,6-N,N'-hexamethylene biscitraconomide and 20% of N,N'-methylene dianiline bismaleimide.

Cured unreinforced sheet material is made of 40 grams of n-hexyl biscitraconimide as described in example 1. Instead of DABCO, 1% of 2-methyl imidazole has been added. The flexural properties are on the same level. The 13C solid state NMR analysis shows a spectra comparable to that of example 1.

Example 5

Cured unreinforced sheet material is made of 40 grams of n-hexyl biscitraconimide as described in example 1. Instead of DABCO, 2 weight percent of trioctyl amine has been added.

The flexural properties and the 13C solid state NMR spectra are comparable with those of example 1.

Example 6

Cured unreinforced material has been made of 40 grams n-hexyl biscitraconimide by melting and curing at 230°C. As a catalyst, triphenyl phosphine has been added. The intensity of the signal of the C-atom of the methyl group, measured by solid state 13C NMR, has been decreased and a new methylene signal is observed.

Example 7

Preparation of Biscitraconimide Oligomers and Prepregs Thereof

A mixture of 25 grams of n-hexyl and 75 grams of 2-methyl-pentyl biscitraconimide and 16 grams of diamino diphenyl sulphone are melted and kept for 4 hours at 180°C. After cooling to 120°C 1% DABCO is added. Impregnation of a glass fabric (200 grams/square meter) at this temperature results in a prepreg containing about 50% resin. The prepregs, non-tacky and flexible, can be used for laminating. The cure cycle used to make the prepreg was 2 hours at 200°C with a postcure of 4 hours at 230°C.

Example 8 Preparation of Biscitraconimide Oligomers by Radical Polymerization

A mixture of 100 grams of n-hexyl biscitraconimide, 5 grams of styrene and 5 grams of dicumyl peroxide in 500 ml. of xylene was heated at 140°C for 16 hours. After the reaction was complete, the xylene was evaporated and a, clear product was obtained. The material is non- tacky and rubber-like at room temperature and has a viscosity at 120-140°C suitable for melt impregnation of fabrics. A mixture of this material and 1 weight percent of DABCO is used for the preparation of prepregs.

The foregoing description and examples of the invention have been presented for the purpose of illustration and description only and are not to be construed as limiting the invention to the precise forms disclosed. The scope of the invention is to be determined from the claims appended hereto.