CA2009018A1 - Polyhydric phenols as chain extenders for certain bismaleimide resins - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A class of di-ortho-substituted bismaleimides undergo uncatalyzed Michael addition with polyhydric phenols to afford chain-extended bis-maleimides having a significantly wider processing window than the non-ex tended parent. The fully cured resins show improved fracture toughness, gen-erally have comparable or superior dielectric constant an dielectric loss, and show no degradation in other properties such as resistance to moisture, to methylene chloride, and coefficient of thermal expansion.

Description

Z(~ 8 POLYHYDRIC PHENObS AS CHAIN ExrENDER$
EO~ C~RTAIN RIS~ALEIMIDE RESINS

BA~<GROUND OF THE INVENT ON
In recent years po~imides have had increasing use as therrnosetting resins in high performance applications, as the rnatrix resin for reinforced com-posites in spacecraft and missiles and for syn~actic 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-o ffon 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 crosslinked leading to products which are very brittle, that is, having low fracture toughness.
Many bismaleimides manifest the unfortunate property of beginning to 15 polymerize at a temperature which is at or Just above the melting point of the monomer, that is, the temperature dfflerential between meltin~ and onset of polymerization is small. As a result it is difficult to maintain the uncured resin in a fluid state, and th0 accompanying difficulty in attain.ng a homogeneous melt leads to well documented processing difficulties. The patentee of U.S.
20 4,464,520 addressed this problem and providad a class of bismaleimides (BMls) with increased pot life, therefore a Rlarger processing window~. How-ever, ths compositions taught there still afforded cured polymers which were brittle.
Becauss the brittleness of the cured product arises from extensive 25 crosslinking during polymerizatlon, many efforts have been rnade to reduce the crosslink density in the cured product to afford toughened BMls without ad-versely impin~ing on other desirable properties. See H. D. Stenzenberger et al.,l9th International SAMPE Technical Conference, October 13-15, 1987, pages 372~5. One ~eneral approach has been to react BMI monomers with certain 30 reactive bifunctional 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 bifunctional reagent by the equation, .

. .~ . .

30~018 NH2~ NH2 J~ ~ o~o As the foregoing equation shows Michael add tion reduces double bond density in the BMI monomer (or oligomer) resui~ng in a lower crosslink densi~y in the cured product. The diarnine also can be viewed as a chain extender in addition to its function of reducing crosslinkin~ density.
o Michael addition aenerally is a bæe~talyzed r~action, and since amines as bases seNe 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., Heh~. Chlm. 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 ~enerally is undesirabls sinc~ the resulting product retains the catalyst as a component 2 0 which might sign-~icantly degrade the perfonnance 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 i5 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 ~ S--CH2CH2-S~

3 o 0~ N ~0 Oq, N ~o 35 could be expected to undergo Michael addition sluggishly, n at all, because of the relatively hindered naturs of the maleimids double bond. Quite unexpect-. .
, ": . :

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3 ~ 0~8 edly it was found that not only did sueh materials undergo Michael addition, butin faet they reacted facilely with the less r0active polyhydrie phenols. But notonly did ths polyhydric phsnols readily r~act with the aforementioned BMls, they did so in an uncatalyzed reaction, that is, in the absence of a base catalyst.
5 This totally unexpected behavior afford0d cured r~sins eontaining no perfor-mance-degrading eomponents and led us to examine some r~levant perfor-manee characteristics of representative ehain-~xt~nded eured resins. We have found that relative to the parent eured resin, ehain extension generally has re-duced brittleness and improved the toughnsss of the eured resin, with the latter10 having a superior dieleetrie eonstant and loss factors and eomparable coeffi-cients of thermal expansion and chemical resistance. Most surprisingly, the chain-extended BMls have wider processing windows than either the parent or diamine chain-extended BMI resins.
SUMMARY OF THE INVENTION
The purpose of this invention is to prepare bismaleimide resins chain extended with polyhydrie phenols in the absence of a eatalyst. An embodiment eomprises the reaetion of eertain di-ortho-substituted BMls, as exemplified by 1,2-bis(2-maleimidophenylthio)ethane, with polyhydric phenols in the absence of any third component as catalyst. In a more specific embodiment the poly-2 o hydrie phenol is a dihydrie phenol. In a still more specific embodiment the dihy-drie phenol is hsxafluorobisphenol A. Another embodiment is a thermosetting resin which is the chain-extended reaction product of eertain di-ortho-substi-tuted bismaleimides such as 1,2-bis(2-maleimidophenylthio)ethane with poly-hydrie phenols and whieh eontains no third eomponent. Yet another embodi-25 ment is the eured resin resulting from thermal treatment of the preeeding ther;mosetting resin. Other embodiments will become elear from the ensuing de-scription.
DESCptlPTlO~LOF THE INVENTION
Our invention arises from the unprecedented observation that a class of 30 di-ortho-substituted BMls undergoes Michael addition with polyhydric phenols in a reaction uncatalyzed by any third component and in the absence of any base cata~st to yield ehain-extended bismaleimides as reaction products. The chain-extended BMls are therrnosetting resins having an extended pot life and therefore having an increased processing window relative to non-extended ~ . ............. . - . . . :
. . . . . . .. . . . .

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4 ~ 018 BMls. The polym~rs from the fully cured chain~xtendcd resins have not only improved fracture toughness, but also have a more ~avorable dielectric constant and dielectric loss factor, neither of which are predic~able.
The BMI monomers used in the practice ~f our invention are thsse 5 taught in U.S. 4,464,520 and which have the formula .

~ Rb ~:

10Ra~X--(CH2); -X~R, 0~0 0~0 . :, where Ra and Rb each independently is hydrogen, an alkyl or alkoxy group containing up to 4 carbon atoms, chlorine, or bromine, or Ra and Rb together form a fused 6-membered hydrocarbon aromatic ring, wi~h ths proviso that Ra and Rb are not t-butyl or t-butoxy, where X is 0, S, or Sc, i is 1-3, and the alkyl-20 ene bridging group is optionally substituted by 1-3 methyl groups or by fluorine.
In a preferred embodiment Ra = Rb = H, especially where X = S, and par-ticularly where X = S and i = 2. In th~ most favored embodiment the bis-maleimideis 1,2-bis(2-maleimidophenylthio)ethane.
The bismaleirnide monomers are reacted in the absence of a third com-25 ponent as a catalyst via Michael addition with a polyhydric phenol acting as a chain extender. By "polyhydric phenol~ is meant a compound having at least 1 aromatic ring and having at laast 2 hydroxyl groups attached to the aromati~
ring(s) in the compound. The most important class of polyhydric phenols is that of dihydric phenols, and within this class tha subset of broadest availability is 30 that of th~ resorcinols, i.e., 1~3-dlhydroxybenzenfls optionally substituted with one or more alkyl groups on the aromatic ring, partlcularly where the alkyl group contains from 1 through about 6 carbon atoms. Examples include 2-methylresorcinol, 4-methylresorcinol, 5-methylresorcinol 6-methylresorcinol, 4-ethylresorcinol, 4-propylresorcinol, 5-pentylresorcinol, 5-hexylresorcinol, 3 5 2,4-dimethylresorcinol, 2,5-dimethylresorcinol, 4,5-dimethylresorcinol, 4,6-di-methylresorcinol, and so forth.

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`` 5 ;~ )18 Although the 1,3-dihydroxybenzenes may be the most widely available class of dihydroxybenzenes, nonetheless the 1 ,2~ihydroxybenzenes (pyrocatechols) and 1,4-dihydroxybsnzenes (hydroquinones) also are suitable dihydric phenols in the practice of this invention. Illustrative examples include 5 pyrocatechol, 3-methylpyrocatechol, ~methylpyrocatechol, the ethylpyrocate-chols, propylpyrocatechols, butylpyrocatechols, pentylpyrocatechols, hexyl-pyrocatechols, hydroquinone, the aikyl-substituted hydroquinones where the alkyl group contains from 1 through 6 carbon atoms, the dialicyl-substituted hy-droquinones, and so on.
lC Another class of dihydric phenols is that of the dihydroxynaphthalenes where the hydroxyl groups may be on the sama or on dfflerent rings. Illustrativemembers 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-dihydroxynaphthalene, 2,6-dihydroxy-naphtha-lene, etc.
Another important class of dihydric phenols used in this invention is that given by the formula ZO HO~_ ~OH

25 where Y = (bond), CH2, C=O, C(CH3)2, C(CF3)2, O, S, S02, SO, and wherë
Rc, Rd are hydrogen or alkyl groups containing from 1 through 6 carbon atoms.
An important subgroup is that where X 2 (bond), that is, where the 2 aromatl'c rings are directiy pined. Members of this class are illustrated by 4,4'-dihydroxydiphenyl, 3,3'-dihydroxydiphenyl, and similar dihydroxydiphenyls.
30 Another important subgroup is that where X = C(CH3)2 or C(CF3)2. In the simplest case, where Rc = Rb = 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)sulfoxide, bis(hydroxyphenyl)sulfone, 35 and bis(hydroxyphenyl)methane.

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2~G~0~8 Among the phenols which are st least trihydric may be mentioned tetraphenolethane (1~1~2~2 tetrakisQlydroxyph~ny~thane)~ 1,1,1-tris(hydroxy-phenyl)ethane, tris~ydroxyphenyl)methane~ tetrakis~ydroxyphenyl)methane, 1,3,5-tris(hydroxyphcnyl)benz~ne, 1,3,5-trihydroxybenz0ne. and the phenol-5 formaldehyde condensation products commonly known in the trada as No-volacs and having the formula l ~ -1C [~ ]t 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 molarratio 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. Quits often little change is 15 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 practics of our invention is from about 0.1 to about 1Ø
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 homogensty. A temperature between about 160-170C ordinarily will suffice, although an even lower temperature may be adequate where a fluid melt state 25 can be achieved. When the reaction is complete the mixture is allowed to cooland solidify to afford a near quantitative yield of chain~xtsnded bismaleimides.The resulting chain-extended BMls be~in to undergo thermal polymer;
kation in the range from abou~ 170C up to at least 250C. However, poly-merization peaks at a temperature between about 250C up to at least 320C.
30 Thermal curing is perhaps most preferably done in an inert atmosphere, such as nitrogen.
As the data in the following examples will show, the dielectric constant and loss factors for our chain-extended BMls are superior to those of either thenon-extended BMI or to a typical diamine chain-extended BMI. The coefficient 35 of thermal expansion of all of our cured chain-extended BMls are comparable to the parent or diamine chain-extended BMls, as are water and methylene chlo-. ., , .. ~ . . -- ,. , . .- ~

:. . .: : . . .

7 ~ 8 rids absorption properties. ~exural data show ~hat polyhydric phsnol chain extension has reduced brittleness and improved toughness of the cured resin over that of the cured non-extended BMI. The chain-~xtended BMls of our in-vention also haYe broader processing windows than ~eir diamine chain-ex-5 tended counterparts, exhibiting a dfflcrence of at least 100C between theirmeiting 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 diarr ine chain-~xtended counterparts, but without the complications of aromatic diamine o toxicity and carcinogenicity.

EXAMPLES
The following list is of abbreviations used throughout this section.
BPA = bisphenol A
6FBPA = hexafluorobisphenol A
TDP = 4,4'-thiodiphenol PG e phloroglucinol TPE = 1,1,2,2,-tetrakis(hydroxyphenyl)ethane 2 o APO-BMI = 2,2'-bis(2-maleimidophenylthio)ethane Chain-Extension of APO-BMI with Dlols. The following example illus-trates the general procedure employed for the chain-extension of APO-BMI with diols. A 2-liter resin kettle was fnted with a reflux condenser, mechanical stirrer, 25 N2 inlet, drying tube, thermocouple, and a heating mantle. Under a slight posi-tive N2 flow, the empty kettle was preheated to 120C, then a mixture of' 435.1 9 of APO-BMI and 64.9 9 of 8PA (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 160-170C for a period of 2 hours while being 30 stirred under N2. The clear 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 AP0-BMI/BPA, APO-BMI/TDP, and APO-BMI/6FBPA
3 5 systems cited.

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- -Curin~ Dlol Chain-Extended APO-BMI R~sins. Small (5-10 9) sam-ples of diol chain-extended APO-BMI resins were weighsd into 57 mm diameter aluminum weighing dishes. The samples wer~ placed into N2-purged ovens, and heated to 175C; fluid melts r0sult~d. Th~ sarnples wer~ cured at 175C
5 under N2 for 24 hours, then the temperature was incrcæcd to 240C, and th2 samples were cured at this t~mperature under N2 for an additional 24 hours. It is recognked that these curing Umes are excessive, and that shorter cure times may be employed.
Chain~ nslon and Curlng of APO-BMI with Polyols. Because PG
lO and Ti'E are poiyfunctional, it was anticipated that ~ey would quickly yield in-fusible uosslinked gels. Therefore, chain-~xtension and curing were combined in a single step. Observation of the melt behavior of thes~ 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 9) and TPE (1.0 9), mole ratio 3.1:1, were mixed in a B7 mm diameter aluminum weighing dish. The mixture was placed into a N2-purged oven, and heating was begun. At a temperature of 130C, the mixture began to melt, and at 165C a fluid, clear melt was obtained. After 1 hour at 180C, the resin was still a free-flowing melt. The resin was cured at 180-190C20 for 18 hours. The temperature was raised to 240C under N2 for 8-1/2 hours.
It is recognized that these cure times are excessive and that shorter cure Umes may be empioyed.
Thermal Analyses. Both DSC (differential scanning calorimetry) and TGA (thermogravimetric analysis) were performed using a DuPont Model 9900 2!!i Thermal Analysis system. DSC analyses of uncured resins were conducted at ~ T=5C/min under N2, and cured resins were analyzed at ~T=10C/min under N2. All TGA analyses were conducted at ~T=10C/min in air. CoeffP~
cients of therrnal expansion (CTE) were determined using a Mettler TA-3000 Thermal Mechanical Analysis system.
Electrlcal Analysls. Dielectric constants (~ and loss factors (tan~) were determined using a Digibridge system at 1MHz and 23C. Samples were preequilibrated at 0% and 50% relative humidity prior to testing.
Mold Curlng. Resin formulations were placed into beakers, which were then placed into vacuum ovens purged with N2. The samples were heated to 150-160C to give fluid melts. The melts were degassed under vacuum at 160C for 30-60 minutes. Vacuum was released and was replaced by a N2 : - - . ,. ~ . .~ , . . .

. .

9 ;~ ~D~O~L8 purge, and the degassed melted r~sins were pourad into silicone rubber flexural modulus molds. The resin-filled molds were placed into an N7-purged oven which was preheated to 175C. The sarnples were cured at 175C for 24 hours. The sarnples were removed from the molds, and were further cured 5 free-standing at 240C under N2 for 24 hours. Samples were allowed to cool to room temperature slowly to prevent uacking.
Flexural Propertles. ~exur~ propertiss of cured APO-BMI and AP0-BMI/BPA (mole ratio 3.5:1) were deterrnined by the 4 point bend test at room t~mperature following ASTM-D790. A loading span of 1.016~, and a support o span of 2.032~ wer~ used.
Water Uptake. Sarnples of cured resins were weighed before and after being suspended in a large excess of reflwdng distilled water for 24 hours.
Methylene Chlorlde Uptake. Samples of cured resins were weighed before and after being suspended in a large excess of CH2C12 maintained at 15 room temperature for 72 hours.
Tables 1~ summarize some salient characteristics of chain sxtended resins and the cured resins therefrom.

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Claims (9)

1. A method of uncatalyzed chain extension of bismaleimides with polyhydric phenols comprising reacting in the absence of a catalyst a bismaleimide selected from the group consisting of where Ra and Rb each independently is hydrogen, an alkyl or alkoxy group containing up to 4 carbon atoms, chlorine, or bromine, or Ra and Rb together form a fused 6-membered hydrocarbon aromatic ring, with the proviso that Ra and Rb are not t-butyl or t-butoxy, where X is O, S, or Se, i is 1-3, and the alkylene bridging group is optionally substituted by 1-3 methyl groups or by fluorine, with from about 0.05 to about 2.0 molar proportion of a polyhydric phenol and recovering the chain-extended product.

2. The method of Claim 1 where the bismaleimide is 1,2-bis(2-maleimidophenylthio)ethane.

3. The method of Claim 1 where the polyhydric phenol is a dihydric phenol.

4. The method of Claim 3 where the dihydric phenol is selected from the group consisting of resorcinols, pyrocatechols, hydroquinones, dihydroxynaphthalenes, and phenols of the formula where Y is selected from the group consisting of (bond), CH2, C=O, C(CH3)2, C(CF3)2, O, S, SO2, SO, and Rc, Rd are independently selected from the group consisting of hydrogen alkyl or alkoxy containing from 1 to about 10 carbon atoms.

5. The method of Claim 1 where the polyhydric phenol is at least a trihydric phenol.

6. The method of Claim 1 where the polyhydric phenol is selected from the group consisting of tris(hydroxyphenyl)methane, 1,1,1-tris(hydroxyphenyl)ethane, tetrakis(hydroxyphenyl)methane, 1,3,5-tris(hydroxyphenyl)benzene, 1,1,2,2-tetrakis(hydroxyphenyl)ethane, and Novolacs.

7. The method of Claim 1 where the bismaleimide is reacted with from about 0.1 to about 1.0 molar proporation of the polyhydric phenol.

8. A thermosettig resin which is the chain-extended reaction product of the method of any one of Claims 1-7.

9. A polymer resulting from thermally curing the thermosetting resin of Claim 8.