BACKGROUND OF THE INVENTION

In recent years polyimides have had increasing use as thermosetting resins in high performance applications, as the matrix resin for reinforced com¬ posites in spacecraft and missiles and for syntactic foams, as well as for lami¬ nates in printed circuit boards and other electronic applications. When poly- imide resins are cured they generally afford a polymer with a high glass transi- tion temperature and excellent chemical (environmental) stability with particu¬ larly good resistance to moisture and to oxidative degradation at elevated temperatures. However, cured resins typically are extensively crossiinked leading to products which are very brittle, that is, having low fracture toughness. Many bismaleimides manifest the unfortunate property of beginning to polymerize at a temperature which is at or just above the melting point of the monomer, that is, the temperature differential between melting and onset of polymerization is small. As a result it is difficult to maintain the uncured resin in a fluid -state, and the accompanying difficulty in attaining a homogeneous melt leads to well documented processing difficulties. The patentee of U.S. 4,464,520 addressed this problem and provided a class of bismaleimides (BMIs) with increased pot life, therefore a "larger processing window". How¬ ever, the compositions taught there still afforded cured polymers which were brittle.

Because the brittleness of the cured product arises from extensive crossHnking during polymerization, many efforts have been made to reduce the crosslink density in the cured product to afford toughened BMIs without ad¬ versely impinging on other desirable properties. See H. O. Stenzenberger et al., 19th International SAMPE Technical Conference, October 13-15, 1987, pages 372-85. One general approach has been to react BMI monomers with certain reactive b'rfunctiona! reagents having active hydrogens to afford Michael addi¬ tion products. This reaction and the resulting Michael adduct may be exempli¬ fied, using a diamine as the reactive b ^' rfunctional reagent by the equation,

As the foregoing equation shows Michael addition reduces double bond density in the BMI monomer (or oligomer) resulting in a lower crosslink density in the cured product The diamine also can be viewed as a chain extender in addition to its function of reducing crossiinking density. Michael addition generally is a base-catalyzed reaction, and since amines as bases serve as their own catalysts this is one reason why amines usually are quite reactive in Michael addition. Where alcohols are used, the re¬ action with nitrogen-substituted maleimides requires a .base catalyst as an addi¬ tional component; A. Renner et al., Helv. Chim. Acta., 61, 1443 (1978). These workers also have given the sole instance of the reaction via Michael addition of a polyhydric phenol (bisphenol A) to a typical BMI monomer, with chain ex¬ tension requiring a discrete base catalyst However, the use of a third compo¬ nent as a catalyst along with a chain extender polyhydric phenol generally is undesirable since the resulting product retains the catalyst as a component which might significantiy degrade the performance of the final cured resin. The necessity of using basic catalysts for chain extension with polyols is particularly unfortunate, since a significant advantage of polyols is that they are non-car¬ cinogenic whereas aromatic diamines used as chain extenders often are car¬ cinogenic. The bismaleimides of U.S. 4,464,520, representative of which is the structure

could be expected to undergo Michael addition -sluggishly, if at all, because of the relatively hindered nature of the maleimide double bond. Quite unexpect-

edly it was found that not only did such materials undergo Michael addition, but in fact they reacted facilely with the less reactive polyhydric phenols. But not only did the polyhydric phenols readily react witii the aforementioned BMIs, they did so in an uncatalyzed reaction, that is, in the absence of a base catalyst. This totally unexpected behavior afforded cured resins containing no perfor¬ mance-degrading components and led us to examine some relevant perfor¬ mance characteristics of representative chain-extended cured resins. We have found that relative to the parent cured resin, chain extension generally has re¬ duced brittleness and improved the toughness of the cured resin, with the latter having a superior dielectric constant and loss factors and comparable coeffi¬ cients of thermal expansion and chemical resistance. Most surprisingly, the chain-extended BMIs have wider processing windows than either the parent or diamine chain-extended BMI resins.

SUMMARY OF THE INVENTION

The puφose of this invention is to prepare bismaleimide resins chain extended with polyhydric phenols in the absence of a catalyst. An embodiment comprises the reaction of certain di-ortho-substituted BMIs, as exemplified by 1,2-bis(2-maleimidophenyithio) ethane, with polyhydric phenols in the .absence of any third component as catalyst In a more specific embodiment the poly- hydric phenol is a dihydric phenol. In a still more specific embodiment the dihy- dric phenol is hexaϋuorobisphenol A. Another embodiment is a thermosetting resin which is the chain-extended reaction product of certain di-ortho-substi¬ tuted bismaleimides such as 1,2-bis(2-maleimidophenyfthio)ethane with poly¬ hydric phenols and which contains no third component Yet another embodi- merit is the cured resin resulting from thermal treatment of the preceding ther¬ mosetting resin. Other embodiments will become clear from the ensuing de¬ scription.

DESCRIPTION OF THE INVENTION

Our invention arises from the unprecedented observation that a class of di-ortho-substituted BMIs undergoes Michael addition with polyhydric phenols in a reaction uncatalyzed by any third component and in the .absence of any base catalyst to yield chain-extended bismaleimides as reaction products. The chain-extended BMIs are thermosetting resins having an extended pot life and therefore having an increased processing window relative to non-extended

BMIs. The polymers from the fully cured chain-extended resins have not only improved fracture toughness, but also have a more favorable dielectric constant and dielectric loss factor, neither of which are predictable.

The BMI monomers used in the practice of our invention are those taught in U.S.4,464,520 and which have the formula

where R _a and R& each independently is hydrogen, an alky! or alkoxy group containing up to 4 carbon atoms, chlorine, or bromine, or R _a and R^ together form a fused 6-membered .hydrocarbon aromatic ring, with the proviso that R _a and R. _Q are not t-butyi or t-butoxy, where X is O, S, or Se, i is 1-3, and the alkyl- ene bridging group is optionally substituted by 1-3 methyl groups or by fluorine. in a preferred embodiment R _a ■ R^ » H, especially where X = S, and par¬ ticularly where X * S and i - 2. In the most favored embodiment the bis- maleimide is 1,2-bis(2-maleimidophenytthio)ethane.

The bismaleimide monomers are reacted in the absence of a third com- ponent as a catalyst via Michael addition with a polyhydric phenol acting as a chain extender. By "polyhydric phenol" is meant a compound haying at least 1 aromatic ring and .having at least 2 hydroxyl groups attached to the aromatiύ ring(s) in the compound. The most important class of polyhydric phenols is that of dihydric phenols, and within this class the subset of broadest availability is that of the resorcinols, i.e., 1,3-dihydroxybenzenes optionally substituted with one or more alkyl .groups on the aromatic ring, particularly where the alky group contains from 1 through -about 6 carbon atoms. Examples includ 2-methyiresorcinol, 4-methyiresorcinol, 5-methyiresorcinol 6-methylresorcinol 4-ethyiresorcinol, 4-propyiresorcinol, 5-pentyiresorcinol, 5-hexyiresorcinol 2,4-dimβthyiresorciπol, 2,5-dimethyiresorcinol, 4,5-dimethylresorcinol, 4,6-di methylresorcinol, and so forth.

Although the 1,3-dihydroxybenzenes may be the most widely available class of dihydroxybenzenes, nonetheless the 1,2-dihydroxy benzenes (pyrocatechols) and 1 ,4-dihydroxybenzenes (hydroquinones) also are suitable dihydric phenols in the practice of this invention. Illustrative examples include pyrocatechol, 3-methylpyrocatechol, 4-methylpyrocatechol, the ethylpyrocate- chols, propylpyrocatechols, butyipyrocatechols, pentylpyrocatechols, hexyl- pyrocatechols, hydroquinone, the alkyl-substituted hydroquinones where the alkyl group contains from 1 through 6 carbon atoms, the dialkyl-substiluted hy¬ droquinones, and so on. Another class of dihydric phenols is that of the dihydroxynaphthalenes where the hydroxyl groups may be on the same or on different rings. Illustrative members of this class include 1,2-dihydroxynaphthalene, 1,3-dihydroxy-naph- thalene, 1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,6-di-hydroxy- naphthalene, 1,7-dihydroxynaphthalene, 1,8-dihydroxynaphthalene, 2,3-dihydroxynaphthalene, 2,5-dihydroxyπaphthalene, 2,6-dihydroxy-naphtha- lene, etc.

Another important class of dihydric phenols used in this invention is that given by the formula

where Y * (bond), CH ₂ , C=0, C(CH ₃ ) ₂ , C(CF ₃ )2, O, S, S0 ₂ , SO, and where RQ, Rςj are hydrogen or alkyl groups containing from 1 through 6 carbon atoms. An important subgroup is that where X = (bond), that is, where the 2 aromatic rings are directly joined. Members of this class are illustrated by 4,4'-dihydroxydiphenyl, S^-dihydroxydiphenyl, and similar dihydroxydiphenyls. Another important subgroup is that where X - C(CH ₃ ) ₂ or C(CF3) ₂ . In the simplest case, where R _c * R^ « H, such materials are commonly referred to as bisphenol A and hexafluorobisphenol A. Other members of the group en¬ compassed by the aforegoing formula include 4,4-thiodiphenol, bis- (hydroxyphenyl)ether, bis(hydroxyphenyl)sutfoxide, bis(hydroxyphenyl)surfone, and bis(hydroxypheny methane.

Among the phenols which are at least trihydric may be mentioned tetrapheπolethane (1,1,2,2-tetrakis(hydroxyphenyl)ethane), 1,1,1-tris(hydroxy- phenyl)ethane, tris(hydroxypheπyl)methane, tetrakis(hydroxyphenyl)methaπe,

1,3,5-tris(hydroxyphenyl)benzene, 1,3,5-trihydroxybenzene, and the phenol- formaldehyde condensation products commonly known in the trade as No- volacs and having the formula

The polyhydric phenol will be used in a molar proportion relative to the bismaleimide as little as about 0.05 and as great as about 2. That is, the molar ratio of BMI to polyhydric phenol used in the preparation of the chain-extended BMI may be as great as 20:1 or as little as 0.5:1. Quite often little change is seen in the resulting product when less than about 0.1 molar proportion of polyhydric phenol is employed, and the glass transition temperature often is ad¬ versely affected when more than 1 molar proportion is employed. Conse¬ quently a molar proportion of polyhydric phenol relative to BMI which is pre¬ ferred in the practice of our invention is from about 0.1 to about 1.0. The chain extension reaction is carried out rather simply. The bis¬ maleimide and polyhydric phenol are mixed in a molar proportion from about 20:1 to about 1:1 and are reacted generally in a fluid melt state to achieve homogeneity. A temperature between about 160-170°C ordinarily will suffice, although an even lower temperature may be adequate where a fluid melt state can be achieved. When the reaction is complete the mixture is allowed to cool and solidify to afford a near quantitative yield of chain-extended bismaleimides.

The resulting chain-extended BMIs begin to undergo thermal polymer ization in the range from about 170°C up to at least 250°C. However, poly merization peaks at a temperature between about 250°C up to at least 320°C Thermal curing is perhaps most preferably done in an inert atmosphere, suc as nitrogen.

As the data in the following examples will show, the dielectric constan and loss factors for our chain-extended BMIs are superior to those of either th non-extended BMI or to a typical diamine chain-extended BMI. The coefficien of thermal expansion of all of our cured chain-extended BMIs are comparable t the parent or diamine chain-extended BMIs, as are water and methylene chlo

ride absorption properties. Rexural data show that polyhydric phenol chain extension has reduced brittleness and improved toughness of the cured resin over that of the cured non-extended BMI. The chain-extended BMIs of our in¬ vention also have broader processing windows than their diamine chain-ex- tended counterparts, exhibiting a difference of at least 100°C between their melting point and the onset of thermal polymerization. In summary, we have demonstrated chain extension with polyhydric phenols produces BMI resins having properties which are comparable or superior to those of diamine chain- extended counterparts, but without the complications of aromatic diamine toxicity and cardnogenicity.

EXAMPLES

The following list is of abbreviations used throughout this section. BPA » bisphenol A

6FBPA * hexafluorobisphenoi A

TDP » 4,4-thiodiphenol

PG = phloroglucinol

TPE = 1,1,2,2,-tetrakis(hydroxyphenyl)ethane APO-BMI = 2,2-bis(2-maleimidophenylthio)ethane

Chain-Extension of APO-BMI with Diols. The following example illus¬ trates the general procedure employed for the chain-extension of APO-BMI with diols. A 2-liter resin kettle was fitted with a reflux condenser, mechanical stirrer, N ₂ inlet, drying tube, thermocouple, and a heating mantle. Under a slight posi¬ tive N flow, the empty kettle was preheated to 120°C, then a mixture of 435.1 g of APO-BMI and 64.9 g of BPA (mole ratio 3.5:1) was added over the course of 18 minutes, while maintaining mechanical stirring to facilitate melting. The fluid melt was maintained at 160-170°C for a period of 2 hours while being stirred under N ₂ - The dear homogeneous reaction melt was poured into an enameled steel tray, and allowed to cool and solidify. The yield of resin was >95%.

Using the same procedure, reagents and proportions were varied to produce the various APO-BMI/BPA, APO-BMI/TDP, and APO-BMI/6FBPA systems cited.

Curing Diol Chain-Extended APO-BMI Resins. Small (5-10 g) sam¬ ples of dioi chain-extended APO-BMI resins were weighed into 57 mm diameter aluminum weighing dishes. The samples were placed into ^-purged ovens, and heated to 175°C; fluid melts resulted. The samples were cured at 175°C under N ₂ for 24 hours, then the temperature was increased to 240°C, and the samples were cured at this temperature under N ₂ for an additional 24 hours. It is recognized that these curing times are excessive, and that shorter cure times may be employed.

Chain-Extension and Curing of APO-BMI with Polyols. Because PG and TPE are polyfunctionai, it was antidpated that they would quickly yield in¬ fusible crosslinked gels. Therefore, chain-extension and curing were combined in a single step. Observation of the melt behavior of these systems suggests that the two steps could have been conducted separately, however. The fol¬ lowing procedure, describing chain-extension with TPE, is general. APO-BMI (6.7 g) and TPE (1.0 g), mole ratio 3.1:1, were mixed in a

57 mm diameter aluminum weighing dish. The mixture was placed into a N ₂ - purged oven, and heating was begun. At a temperature of 130°C, the mixture began to melt, and at 165°C a fluid, dear melt was obtained. After 1 hour at 180°C, the resin was still a free-flowing melt. The resin was cured at 180-190°C for 18 hours. The temperature was raised to 240°C under N ₂ for 8-1/2 hours. ft is recognized that these cure times are excessive and that shorter cure times may be employed.

Thermal Analyses. Both DSC (differential scanning calorimetry) and TGA (thermogravimetric -analysis) were performed using a DuPont Model 9900 Thermal Analysis system. DSC analyses of uncured resins were conducted at ΔT«5°C/min under N ₂ , and cured resins were analyzed at ΔT=10°C/min under N ₂ . All TGA analyses were conducted at ΔT=10°C/min in air. Coeffi dents of thermal expansion (CTE) were determined using a Mettier TA-3000 Thermal Mechanical Analysis system. Electrical Analysis. Dielectric constants ( and loss factors (tans) were determined using a Digibridge system at 1MHz and 23°C. Samples were preequilibrated at 0% and 50% relative humidity prior to testing.

Mold Curing. Resin formulations were placed into beakers, which were then placed into vacuum ovens purged with N ₂ . The -samples were heated to 150-160°C to give fluid melts. The melts were degassed under vacuum at

160°C for 30-60 minutes. Vacuum was released and was replaced by a N ₂

purge, and the degassed melted resins were poured into silicone robber flexural modulus molds. The resin-filled molds were placed into an N ₂ -purged oven which was preheated to 175°C. The samples were cured at 175°C for 24 hours. The samples were removed from the molds, and were further cured free-standing at 240°C under N ₂ for 24 hours. Samples were allowed to cool to room temperature slowly to prevent cracking.

Flexural Properties. Rexural properties of cured APO-BMI and APO- BMI/BPA (mole ratio 3.5:1) were determined by the 4 point bend te»st at room temperature following ASTM-D790. A loading span of 1.016", and a support span of 2.032" were used.

Water Uptake. Samples of cured resins were weighed before and after being suspended in a large excess of refluxing distilled water for 24 hours.

Methylene Chloride Uptake. Samples of cured resins were weighed before and after being suspended in a large excess of CH CI ₂ maintained at room temperature for 72 hours.

Tables 1-4 summarize some salient characteristics of chain extended resins and the cured resins therefrom.

Table 1. DSC Characterization of Uncured Diol/Polyol Chain-Extended APO-BMI Resins

a. Melting point. b. Estimated from bulk curing experinents. c. Glass transition tenperature. d. Heat of polymerization.

Table 2. Thermal Characterization of Cured Diol/Polyol Chain-Extended APO-BMI Resins

Wt. ratio DSC TGA, in Air, ^• C

Resin System (Mole ratio) Tg. *C ⁸ 5% Wt. Loss 10% Wt. Loss

APO-BMI >300 385 390

APO-BMI/BPA 6.7:1 (3.5:1) >300

APO-BMI/TDP 6.7:1 (3.35:1) >300

APO-BMI/6FBPA 6.7:1 (5.2:1) >300

APO-BMI/PG 6.7:1 (1.3:1) >300

AP0-BMI/TPE 6.7:1 (3.1:1) >300

APO-BMI/TPE 60:40 (1:1.4) 265

APO-BMI/MDA 6.7:1 (3.0:1.0) 275 370 375

a. Glass transition temperature.

Table 3: Physical and Electrical Properties of Cured Dlol Chain-Extended APO-BMI Resins. ¹

Wt. ratio c Wt.X H ₂ 0 Wt.X CH Cl Resin System (mole ratio) OX RH 50% RH Uptake Uptake

APO-BMI 3.28 3.60 2.51 3.06

APO-BMI/BPA 6.7:1 (3.5:1) 3.25 3.33 2.51 1.17 AP0-6FBPA 6.7:1 (5.2:1) 3.28 3.36 2.37 4.84

APO-BMI/MOA 6.7:1 (3.0:1) 3.31 3.71

a. RH - relative humidity b. Coefficient of thermal expansion at 260°C ₍ I.e., o^gg

Table 4: Cured Neat Resin Mechanical Properties for BPA Chain-Extended APO-BMI, Molar Ratio 3.5:1.

APO-BMI/BPA APO-BMI

Flexural Stress, KSI 4.46 2.96

Flexural strain, % 1.40 0.99

Flexural Modulus, KSI 321 258