Note: Descriptions are shown in the official language in which they were submitted.
CA 02025039 1999-09-16
EXTREMELY TOUGH THERMOSETTING BISMALEIMIDE RESIN SYSTEMS
Background of the Invention
1. Field of the Invention
The subject invention pertains to thermosetting
bismaleimide resin systems. More particularly, the subject
invention pertains to bismaleimide resin systems containing
both a particulate thermoplastic toughener and a minor
quantity of a low viscosity epoxy resin. Such resin systems
exhibit surprisingly high compression after impact (CAI)
strengths when used as the matrix resin in carbon fiber
reinforced composites.
2. Description of the Related Art
Although many thermoplastics are tough, ductile
materials, their use in structural materials has been minimal
for several reasons. First, many of the thermoplastics do not
have the required solvent resistance, thermal stability, and
high softening points required in demanding aerospace
applications. Second, the high temperature engineering
thermoplastics are difficult to process, often requiring both
high temperature and pressure to produce acceptable fiber
reinforced parts.
1
2~~~~
For these reasons, and despite the proliferation
and improvement of high temperature, high performance
thermoplastics, thermosetting systems currently remain the
important commercial resin systems. Of the thermosets
available, by far the most common are the epoxies, the
bismaleimides, and the cyanates. Each of these resin
systems has its own unique set of physical and chemical
attributes, but all are glassy, generally crosslinked
systems which tend to be brittle. Thus attempts at toughen-
ing such systems have become increasingly important.
By the term toughness is meant resistance to
impact induced damage. Toughness in cured neat resin
samples may be assessed by the critical stress intensity
factor, K1C, among others. Toughness in fiber reinforced
composites prepared by laying up and subsequently curing
numerous plies of prepregs is best assessed by measuring the
compression strength after an impact of suitable energy.
Generally, an impact of 1000 or 1500 in-lb/in (respectively,
4.45 and 6.68 kJ/m) is used, and compression after impact
(CAI) values measured in accordance with Boeing test BSS
7260 on a quasiisotropic [+45/0/-45/90]4s layup. Similar
teats may be specified by other aerospace manufacturers.
-2-
Elastomers have been used with good success in
toughening a number of thermosetting resins, particularly
epoxy resins. Examples of such systems are given in eauer,
Epoxy Resin Chemistry II, Chapters 1-5, ACS Symposium Series
221, American Chemical Society, Washington, D.C., 1983.
Both soluble and infusible elastomers have been utilized,
the former generally increasing flexibility at the expense
of physical properties such as tensile modulus, while the
latter generally increase toughness without substantially
affecting bulk properties. Both types of modification
generally lead to lower thermal properties, an effect which
can be minimized when polysiloxane elastomers are utilized.
Soluble thermoplastics have also been used, for
example in the article by Hucknall and Partridge, "Phase
Separation in Epoxy Resins Containing Polyethersulfone,"
Polymer 24 639-646 (1983). .In Hucknall's examples, dissolu-
tion of up to 17 percent by weight of a polyethersulfone
having a molecular weight slightly greater than 20,000
Daltons in an epoxy formulation increased toughness as
measured by K1C by up to 50 percent. At the highest levels,
phase separation was noted upon cure of the system, the
resulting cured neat resin consisting of the glassy poly-
ethersulfone discontinuous phase dispersed within a glassy
-3-
epoxy continuous phase. With epoxy resins having an average
functionality of four, no phase separation was observed.
although the cured system still displayed enhanced tough-
ness.
Dissolution of up to 80 weight percent of soluble
polyimide PI2080 into the bismaleimide of bis[4-amino-
phenyl)methane was disclosed by Yamamoto in ~Preparation and
Characterization of Thermo-Plastic/Thermo-Setting Polyimide
Blends." published in SAMPE Journal, July/August 1985.
However, resin systems containing high levels of dissolved
polyimide are difficult to process and generally have little
if any tack, an important consideration in the laying up of
prepregs into composites. Furthermore, high levels of
dissolved thermoplastic make fiber impregnation by the film
method difficult.
Toughened systems have also been proposed which
rely for toughness, on the use of oligomeric curing agents
or monomers. Such monomers and curing agents have less
crosslink density and thus are inherently more flexible,
tougher systems. In U.S. patent 4,608,404, for example,
epoxy resin systems containing an epoxy resin and an
oligomeric amine-terminated polyethersulfone is disclosed.
Such systems were capable of providing composites having CAI
-4-
_.
(compression after impact, see infra) values of greater than
30 Ksi, particularly when diaminodiphenylsulfone (DDS) was
used as a co-curative.
In U.S. patents 4,656,207 and 4,656,208, the
principles of Bucknall and Partridge and of the '404
patentees were logically combined to provide epoxy systems
employing DDS and greater than 25 percent by weight of a
reactive polyethersulfone oligomer having a molecular weight
of from 2000 to 10,000 Daltons. These epoxy systems cure
into two phase systems having a glassy discontinuous phase
dispersed within a glassy continuous phase as disclosed by
Bucknall but utilizing a lower molecular weight, and thus
more soluble and less viscous, polyethersulfone oligomer.
Carbon fiber reinforced composites employing the resin
systems of the '207 and '208 patents are able to achieve CAI
values in excess of 40 Ksi. Other researchers have utilized
analogous technologies with bismaleimide resins.
In U.S. patent 4,604,319, discrete films of
thermoplastic, optionally containing up to 40 percent by
weight thermosetting resin, are applied under heat and
pressure to epoxy or bismaleimide prepregs containing carbon
fibers as reinforcement. When such film faced prepregs are
laminated together to form a composite, CAI values greater
-5-
than 40 Ksi can be obtained. Unfortunately, such prepregs
have not been accepted by the industry due to the possi-
bility of a mistake during layup wherein two thermoplastic
films might abut each other, promoting catastrophic inter-
laminar separation. Furthermore, such prepregs have little
tack, and thus make composite layup difficult.
In European patent EP-A-0 252 725, elastomeric
interlayers are formed in situ by the filtering out of
discrete, infusible particles by the fiber reinforcement
because the particles are larger (10-75 Wn) than the fiber
interstices. Prepregs such as these and composites formed
therefrom have the capability of having CAI values in the
40-50 Ksi zange, but may suffer from lower properties at
elevated temperatures.
In European patent EP-A-0 274 899, the addition of
thermoplastics, preferably in the form of solid, spherical
particles, to thermosettable resin systems is said to cause
an increase in toughness in carbon fiber laminates.
Examples of thermoplastics are polyamideimides,
polybutyleneterephthalate, and nylon, with transparent
nylons being preferred. When particles greater than 2Wa in
diameter are utilized, the thermoplastic is concentrated in
situ onto the outside of the prepreg as in EP-A-O 252 725.
-6-
__
When particles having a size less than 2Nm are used, the
thermoplastic remains homogenously dispersed within the
prepreg.
United States patent 4,131,632 indicates that
bismaleimides and epoxy resins may be combined, but that the
content of bismaleimide must be limited to no more than 30
weight percent due to the incompatibility of the
bismaleimide and epoxy resins. United States patent
4,212,959 also discloses these drawbacks of combination
epoxy and bismaleimide resin systems, as well as the further
drawback that such systems exhibit high shrinkage during
cure. Such resin systems would be expected to result in
distorted composites and/or severe microcracking when
utilized as thermosetting matrix resins in structural
composites.
In U.S. patent 4,743,647, epoxy resins are one of
many suggested comonomers which may be added to bismaleimide
resins, particularly with the use of diamine epoxy resin
curing agents. However, no guidance as to the selection of
epoxy resins or curing agents and no examples of such use
are given. In U.S. patent 4,510,272, the use of
bismaleimides in epoxy resin systems is taught, but once
again, no direction as to selection of epoxy resin is
CA 02025039 1999-09-16
given. The epoxies are cited as improving the high
temperature water resistance. No mention of toughness as
reflected by resistance to impact is mentioned. Furthermore,
the systems exemplified all contain about 30 weight percent
or more of the epoxy resin, generally in conjunction with an
amine curing agent.
Summary of the Invention
In application EP 383174 in the name of the
applicant, the use of epoxy resins as but one of a variety of
potential comonomers in bismaleimide resin system was
suggested. It has now been found that bismaleimide resin
systems with exceptional levels of toughness may be prepared
by including in the resin system a particulate thermoplastic
polyimide and a low viscosity epoxy resin. The use of these
epoxies would be expected to lower overall cured system
performance, for example use temperature, while providing
certain benefits to the uncured system such as greater tack
and drape. However, many low viscosity epoxy resins have
surprisingly been found to synergistically increase the
toughness of resin systems employing both bismaleimide
monomers and particulate thermoplastics. Further, it is
believed that the use of such epoxy comonomers increases the
variety of useful particulate engineering thermoplastics
useful in such systems. Quasiisotropic panels of carbon fiber
reinforced prepregs prepared utilizing the subject invention
resin systems are capable of exhibiting compression strengths
greater than 50 Ksi while maintaining the high temperature
use characteristics of bismaleimide resin systems, and
avoiding distortion and microcraking.
More particularly, the present invention proposes a
thermosetting bismaleimide resin system, comprising:
8
CA 02025039 1999-09-16
a) 30 weight percent or greater of one or more
thermosetting bismaleimide monomers:
b) from 1 to about 30 weight percent of a low
viscosity epoxy resin toughener;
c) from 5 to about 30 weight percent, based on the
total resin system, of a particulate thermoplastic
which is soluble or swellable in the bismaleide
resin system upon cure of the system, and which is
effective in toughening the bismaleimide resin
system as reflected by an increase in the
compression strength after impact when tested
according to Boeing Support Specification BSS 7260;
d) from 5 to about 50 weight percent of a comonomer
selected from the group consisting of alkenyl and
alkynyl group containing compounds,
wherein a substantial amount of the thermoplastic remains in
the uncured
resin system
in particulate
form having
a mean
particle size of from 2 ~m to about 80 dun.
The present invention also proposes a thermosetting
bismaleimide resin system, comprising:
a) 30 weight percent or greater of one or more
thermosetting bismaleimide monomers;
b) from 1 to about 30 weight percent of a low
viscosity epoxy resin toughner; and
c) from 5 to about 30 weight percent of one or more
particulate thermoplastics which are swellable or
soluble in the bismaleide resin system upon cure of
the system, wherein a major portion of the
thermoplastic comprises a thermoplastic polyimide
whose repeating units may be considered as having
been derived from
i) a dianhydride comprising:
Y-X-Y
9
CA 02025039 1999-09-16
wherein Y is a phthalyl anhydride group and X
is a linking group selected from the group
consisting of a covalent bond, --CR2--,
--CO--, --O--CO--, --O-CP--O, --NH--CO--,
--S--, --SO--, and --S02--,wherein R is
phenyl, C6-C10 cycloalkyl, or Cl-C4 alkyl,
--C(CF3)2--; pyromellitic dianhydride, and
mixtures thereof; and
ii) a diamine, wherein at least 50 weight
percent of the diamine residues comprise
residues of toluenediamine and
methylenedianiline;
d) from 5 to about 50 weight percent of a comonomer
selected from the group consisting of alkenyl and
alkynyl group containing compounds,
wherein a substantial portion of the thermoplastic remains in
the uncured resin system in particulate form having a mean
particle size of from 2 dun to 80 E.~m.
It is further proposes a thermosetting bismaleimide
resin system,
comprising:
a) 30 weight percent or greater of one or more
thermosetting bismaleimide monomers;
b) from 1 to about 10 weight percent of one or more
difunctional epoxy resins which are the glycidyl
ethers of a phenol selected from the group
consisting of bisphenol A, bisphenol F, and
resorcinol;
c) from 5 to about 30 weight percent of one or more
particulate thermoplastics, which are soluble or
swellable in the bismaleimide resin system upon
cure of said system, wherein up to about one third
CA 02025039 1999-09-16
of the thermoplastic comprises a swellable or
soluble engineering thermoplastic having a Tg
greater than about 150 C and the remainder
comprises a soluble or swellable thermoplastic
polyimide the majority of whose repeating units may
be considered to have been derived from
benzophenone tetracarboxylic dianhydride,
methylenedianiline, and toluenediamine~
d) from 5 to about 50 weight percent of a comonomer
selected from the group consisting of alkenyl and
alkynyl group containing compounds,
wherein a substantial portion of the particulate
thermoplastic (c) remains in the uncured resin system in
particulate form having a mean particle size of from 2 Eun to
about 80 Eun.
Description of the Preferred Embodiments
The present invention concerns the addition of a
select group of thermoplastic polyimides in solid form in
conjunction with an epoxy resin having a viscosity of less
than about 25,000 cps, to bismaleimide resin systems in order
to provide increased toughness, and, in particular,
significant resistance to impact induced damage. The
thermoplastic polyimides are added to the uncured
bismaleimide resin preferably by means of a slurry mixing
process by means of which a substantial amount of polyimide
remains in a particulate form having a mean size between 2
and 80 ~m in the neat uncured matrix resin. During the
prepregging operation, a substantial amount of these
polyimide particles are filtered out by the reinforcing
11
CA 02025039 1999-09-16
fibers, forming a thermoplastic rich and/or thermoplastic
particle-rich zone substantially exterior to the fibers.
Following cure, the polyimide may remain as a largely
continuous film in the interlayer zone or as a thermoplastic
enriched, cured, bismaleimide layer.
The bismaleimide monomers useful in the subject
invention are well known to those skilled in the art and in
most cases are commercial products of ready availability. The
bismaleimides are generally prepared by the reaction of an
unsaturated anhydride with the primary amino groups of a di-
or polyamine, and as used herein, the term bismaleimide
includes minor amounts of maleimide-group-containing monomers
of higher functionality, i.e. tris- and tetra-maleimides.
However, the preferred bismaleimides are difunctional. The
term bismaleimide as used herein also includes the closely
related nadic imides and allylnadic imides which are prepared
in substantially the same manner as the bismaleimides but
using an unsaturated anhydride such as the norbornene
dicarboxylic acid anhydrides formed from the Diels-Alder
reaction of malefic anhydride or substituted malefic anhydrides
with cyclopentadiene or substituted cyclopentadienes,
particularly methylcyclopentadiene.
Suitable bismaleimides and methods for their
preparation are disclosed, for example, in U.S. patents
4,604,437; 4,100,140; 4,130,564; 4,138,406; 4,154,737;
4,229,351; and 4,689,378. Particularly preferred
bismaleimides include the bismaleimides of the toluene
diamines and 4,4'-methylenedianiline, and the commercially
available eutectic bismaleimide mixtures. These latter
mixtures, one of which is COMPIMIDE~ 353, a product of Shell
Chemical Co., formerly available from Boots-Technochemie, are
mixtures of two or more bismaleimides, the mixture of which
has a lower melting poit than the bismaleimides alone.
11a
CA 02025039 1999-09-16
COMPIDIME~ 353, for example, contains the bismaleimides of
4,4'-diaminodiphenylmethane, toluene diamine, and 1,6-
diamino-2,2,4-trimethylhexane. Other eutectic bismaleimide
mixtures may be prepared through suitable combinations of
various aromatic bismaleimides and/or aliphatic
bismaleimides. Suitable aliphatic bismaleimides which may be
utilized to form eutectic bismaleimide components include the
bismaleimides of 1,6-hexanediamine, isophoronediamine and the
diaminocyclohexanes. Eutectic bismaleimides are particularly
preferred in preparing resin systems with suitable tack, but
do not necessarily result in composites with maximal physical
properties.
The term "bismaleimide monomer" as used herein
encompasses both low molecular weight as well as higher
mCl°r"lar '.r°irtht hicmalPimi~P~_ ThP 1~trPr are QenerallV
11b
prepared by the reaction of malefic anhydride with a suitable
amine terminated oligomer such as the polyoxylene oxides,
polyarylene sulfides, polyarylene sulfones, polyarylene
ether sulfones, polyarylene ether ketones, and the like.
Other higher molecular weight bismaleimides include
maleimide terminated prepolymers prepared by reacting an
excess of a bismaleimide with a diamine. Such products are
available commercially. Generally, the higher molecular
weight bi;smaleimide monomers have molecular weights of less
then about 12,000, preferably less than 5000 Daltons.
The bismaleimide monomers described above are
seldom used alone, but are most often used as a total resin
system which may contain other polymerizable species in
addition to fillers, rheology control agents, catalysts,
fibrous and non-fibrous reinforcement, and the like.
Particularly important in bismaleimide resin systems are
various comonomers and reactive modifiers.
Among the comonomers useful with bismaleimides are
the alkynyl and alkenylphenols and phenoxyethers. Alkenyl
group-containing compounds, particularly alkenyl aromatic
compounds may also be suitable comonomers. Examples of
these compounds are styrene, 1,4-divinylbenzene,
terephthalic acid diacrylate, cyanuric acid triacrylate, and
-12-
CA 02025039 1999-09-16
glyceryl triacrylate. The corresponding allyl, methallyl,
methacrylo, and methylvinyl group-containing compounds are
also suitable.
Among the alkenylphenols and alkenylphenoxy ethers
useful are particularly the allyl, methallyl, and propenyl
phenols such as o,o'-diallylbisphenol A, eugenol, eugenol
methylether, and similar compounds as disclosed in U.S.
4,100,140. Also useful are oligomers which are terminated
with allyl- or propenylphenyl or allyl- or propenylphenoxy
groups such the appropriately terminated polysiloxanes,
polyetherketones, polyethersulfones, polyimides, polyether-
imides, polymerized or chain extended epoxy derived resins,
and the like. Suitably terminated oligomers, for example, may
be prepared by allylating phenolated dicyclopentadienes and
subsequently rearranging to the allylphenol as taught in U.S.
patent 4,546,129. Most preferably, the alkenylphenol is o,o'-
diallylbisphenol A or o,o'-dipropenylbisphenol A. The
alkenylphenols and alkenylphenoxy comonomers are utilized in
amounts of up to 70 weight percent based on the total system
weight, preferably from 10 to 50 percent, and most preferably
from about 20 to about 40 percent; or from 5 to about 150
percent, preferably from 30 to about 100 percent based on the
weight of the bismaleimide(s). The corresponding alkynyl
compounds are also useful.
Also useful as comonomers are the cyanate ester
resins and their reaction products with bismaleimides. The
cyanate ester resins contain the -OCN reactive moiety and are
generally prepared by the reaction of a cyanogen halide with
a di- or polyphenol. Suitable cyanate ester resins and
methods for their preparation are disclosed in U.S. Patent
4,546,131. Prepolymers prepared by the reaction of the
cyanate resins with epoxy resins or with bismaleimide resins
13
CA 02025039 1999-09-16
are also useful. The latter are available commercially from
the Mitschubishi Gas Chemical Co. as "BT Triazine Resins."
Low viscosity epoxy resins are a necessary
component of the subject invention. Among such resins are
those described in the treatise Handbook of Epoxy Resins, Lee
and Neville, McGraw-Hill, ~1967; and Epoxy Resins Chemistry
and Technology, May and Tanaka, Marvel Dekker, ~1973,
Particularly preferred are epoxy resins having viscosities of
less than about 25,000 cps at 50°C, such as the epoxies
derived from resorcinol, bisphenol A, and bisphenol F. Also
exemplary are Cardolite~ NC-514 and Cardolite ~ NC-551, epoxy
resins from the Cardolite Corpora-
14
~, ~a
tion, Newark, New Jersey,, which have viscosities of 25,000
cps and 600 cps, respectively. Generally, minor quantities
of epoxy resins are utilized, for example up to about 30
percent by weight. Generally, however, lesser amounts, for
example up to 25 percent" preferably less than 20 percent,
and most preferably less than 10 percent by weight are
useful. Lower levels are especially preferred in
applications where a high Tg is required.
Not every low viscosity epoxy resin is expected to
increase the toughness of: every bismaleimide resin system.
Due to the complex interactions and interreactions, a minor
amount of experimentation may be required to identify the
optimal epoxy comonomer and the optimal amounts of the
various system components.. These experiments are routinely
performed by those skilled in the art of formulating heat-
curable resin systems of any type. In general, it has been
found that the difunctional low viscosity epoxy resins
perform the best with they systems tested thus far.
Trisglycidyl p-aminophenol, for example, provides no
increase in toughness with these systems although it does
increase both tack and drape. 8owever, it is quite likely
that this same epoxy, when used in conjunction with the
soluble or swellable thermoplastics of the subject
-15-
invention, will increase the toughness of other bismaleimide
resin systems.
Thus, in the remainder of the specification and in
the claims, the term "low viscosity epoxy resin toughener"
is meant to include only those epoxy resins which are
capable of increasing the toughness of the base resin system
(the same system minus the epoxy) by at least ten percent
when tested according to BSS 7260 after an impact of 1500
in-lb/in. A suitable test to determine whether a given
epoxy or mixture of epoxies is within the scope of the
claims is to prepare and cure identical quasiisotropic
panels with and without the epoxy(ies) and measure the
compression strength after impact (CAI). If the CAI, after
an impact of 1500 in-lb/in is about ten percent or more
greater with the epoxy than for the same system without the
epoxy, then the epoxy and the system containing it are
within the scope contemplated by the claims.
Toughening modifiers are also useful in the
practice of the subject invention. Generally, these are
reactive oligomers having molecular weights of between 600
and 30,000 Daltons. These modifiers may be terminated with
reactive groups or have medial reactive groups. Examples
are the allyl or propenylphenols and phenoxy ethers
-16-
CA 02025039 1999-09-16
discussed previously, or polymers or oligomers having amino,
maleimide, cyanate, isocyanate, or other groups inter-
reactive with bismaleimides. The backbone of these oligomers
may be of diverse nature, for example polyarylenes such as
the polyetherketones, polyetheretherketones, polysulfones,
polyethersulfones and the like as prepared in U.S. 4,175,175
and in the article Toughening of Bis Maleimide Resins:
Synthesis and Characterization of Maleimide Terminated Poly
(Arylene Ether) Oligomers and Polymers, J. E. McGrath, et.
al., NASA report n187-27036, Final Report Task 1-17000. The
backbone may also be derived from polysiloxanes or, in
particular, poly(dicyclopentadienes) terminated with allyl or
propenyl phenol or phenoxy groups.
The particulate thermoplastics useful in the
practice of the subject invention are limited to those which
swell or dissolve during cure as discussed in EP 38 3174, and
which are effective as toughening agents. Preferred
thermoplastics are the so-called "engineering
thermoplastics", having a Tg greater than 150°C, preferably
greater than 200°C. Especially prefered thermoplastics are
the thermoplastic polyimides, particularly those having
aliphatic groups attached to at least some of the aromatic
nuclei in the polymer chain.
17
It is difficult if not impossible to predict which
thermoplastic particles will be effective based on structure
and/or physical properties alone. Generally, identification
of a particular effective thermoplastic may be based as a
first,approximation on whether the particulate thermoplastic
swells or dissolves in the resin system. If no swelling or
dissolving takes place, then the thermoplastic is not
suitable. If, after this screening test the thermoplastic
is a suitable candidate for toughening, then a resin system
containing the bismaleimide, the particulate thermoplastic,
and any remaining system components other than the epoxy
resin is prepared. The toughness of panels prepared from
this matrix resin system is then compared to that of an
otherwise similar system not containing the thermoplastic.
If a statistically significant increase in toughness
results, then the thermoplastic is suitable for the subject
application. Preferably. the thermoplastic provides at
least a ten percent increase in toughness as reflected in
CAI values. It is conceivable that a particulate
thermoplastic which is effective with one system of
bismaleimides and associated non-epoxy comonomers might not
be effective. in another system.
-18-
,..
The thermoplastic particles most extensively
tested in preparing the resin systems of the subject
invention are the polyimide thermoplastics derived from a
bis(anhydride) and two particular diamines, toluenediamine
(TDA) and 4,4'-diaminodiphenylmethane (MDA). These
polyimides may also contain minor quantities, i.e. up to
about 20 percent by weight of other diamines, so long as the
amount does not affect the ability of the thermoplastic to
toughen the bismaleimide. Preferably, the polyimide
contains TDA and trlDA in a weight ratio of $0/20.
The dianhydride selected to prepare the thermo-
plastic polyimide may be selected from numerous dianhy-
drides. These dianhydrides may be mononuclear or dinuclear,
but are preferably Binuclear, i.e. the anhydride groups are
located on different aromatic rings connected by a divalent
linking group, or by a covalent bond. Examples of such
dianhydrides are diphthalyl dianhydride,
bis(phthalyl)methane dianhydride, bis(phthalyl)ketone
dianhydride, bis(phthalyl)sulfide dianhydride,
bis(phthalyl)sulfone dianhydride, their ~aixtures, and the
like. Thus the preferred dianhydrides have the formula:
-19-
p r y ~~~
~r<re,~v'~J
' 'CO CO'
O_ X ~ O
\ CO CO
wherein X represents the organic linking group or a covalent
bond. Preferably not more than about 20 weight percent of
the total dianhydride component may comprise a mononuclear
dianhydride such as 1,2,4,5-benzenetetracarboxylic acid
dianhydride. Polyimides containing the residues of
mononuclear dianhydrides are expected to be useful
particularly when used in conjunction with at least a
portion of aliphatic or alicyclic diamines. Preferably, the
linking group is the carbonyl group. This dianhydride is
more commonly called benzophenone tetracarboxylic acid
dianhydride (HTDA).
In the foregoing description of the soluble
polyimide thermoplastics useful in the subject invention, it
has been assumed that the polyimide preparation will occur
by polymerizing approximately equimolar quantities of a
diamine and a dianhydride. However, there are also other
means of preparing such polyimides. F'or example, suitable
polyimides having substantially the same properties as those
formed by the condensation polymerisation of dianhydrides
and diamines may be made by reacting a dianhydride with the
-20-
2~~~~ ~~
diisocyanate corresponding to the diamine as taught by U.S.
patent 4,001,186. In the specification and claims, the
polyimide is identified as containing residues of HTDA and
both TDA and MDA. This terminology also includes similar
polyimides which formally contain such residues but which
are produced in other ways, for example by the reaction of
HTDA or benzophenone tetracarboxylic acid with mixtures of
toluenediisocyanate and methylenediphenylenediisocyanate.
Most preferred as the thermoplastic is a polyimide
available from Lenzing, A.G., A-4860 Lenzing, Austria as
High Performance Powder P84, believed to be made in
accordance with the teachings of U.S. patent 4,001,186, and
may be considered as being derived from the residues of
HTDA, TDA, and MDA, the latter two in a weight ratio of
80/20. The product may be further characterized by nominal
physical properties as follows: a specific gravity of about
1.33: a heat deflection temperature [DIN 53461(A)j of 288°C;
a tensile strength (DIN 53455) of 110 MPa: and an elongation
at break (DIN 53455) of 5 percent.
Other swellable or soluble polyimides and other
types of swellable or soluble thermoplastics may also be
useful, particularly when used in conjunction with the
preferred polyimide. In such cases, less of the preferred
-21-
2~~
polyimide may be used without sacrificing impact strength.
Examples of other polyimides are the Ulteme polyetherimides
and Matrimide 5218 polyimides. However, Kaptonm polyimides
are not expected to be useful as they neither swell nor
dissolve in usual bismaleimide resin system components.
Examples of other thermoplastics include the polyether
sulfones, polyetherketones, and the like. Mixtures of
thermoplastics may be physical mixtures of discrete polymer
particles, or may be particles derived by size reduction of
a homogenous polymer melt or film.
Whatever the composition of the thermoplastic, it
must first be reduced to the appropriate particle size. The
average particle size should be less than about BOWn,
preferably less than SOwn, and most preferably in the range
of 4-50Wn. Particle size is expressed as mean diameters as
measured on a Brinkman model 2010 particle size analyzer
utilizing volume distribution. Such particle sizes may
obtained by traditional means, for example cryogenic
grinding, ball or sand milling, etc., but is most advan-
tageously prepared by air jet milling. All such grinding
techniques are well known to those skilled in the art.
Other means of size reduction, for example spray drying or
solution precipitation are also commonly practiced. These
-22-
~r ~l~ fed ~.~ ~ e.: e~
latter techniques may be useful, in particular solution
precipitation, to produce thermoplastic particles of roughly
spherical shape. Such particles have a minimum surface to
volume ratio which may be helpful in reducing the viscosity
of the overall resin system, particularly those having high
thermoplastic loading.
The amount of thermoplastic useful in the subject
invention is generally in excess of 5 weight percent.
Amounts of thermoplastic up to about 60 weight percent may
be useful, but in general, from 10 to about 30 weight
percent, preferably from 10 to about 25 weight percent are
used. Lower amounts of thermoplastic, i.e. less than 10
percent by weight may prove successful when used in conjunc-
tion with another thermoplastic which is also present in the
form of soluble or swellable particles or which is dissolved
in the other resin system components. The dissolved or
particulate thermoplastic may be of a similar type, for
example a different, soluble polyimide (e.g. t~latrimidee
5218), or may be a different type of polymer, for example a
reactive or non-reactive polysulfone, polyethersulfone,
polyetherketone, or the like. The latter are particularly
useful when dissolved in the resin system. The oolecular
weight of these additional, soluble thermoplastics may be
-23-
from about 2000 to about 150,000 Daltons, but is preferably
from 20,000 to about 100,000 Daltons. The additional
soluble or particulate thermoplastic may be used in amounts
from 1 to about 15 percent, preferably 5 to about 15 percent
by weight .
In addition to the primary tougheners such as the
toughening modifiers cited earlier, and the secondary
tougheners such as the thermoplastic particle tougheners and
dissolved thermoplastics, elastomeric particle tougheners
may be useful as tertiary tougheners. Such elastomers are
well known to those skilled in the art and include, for
example, the various ATHN and CTBN elastomers available from
the H.F. Goodrich Company, as the HYCARe and PROTEUSm
rubbers, and various polysiloxane elastomers, particularly
the reactive polysiloxanes such as the aminopropyl
terminated polymethyl and polyphenyl polysiloxanes. Such
tertiary elastomeric particle tougheners may have particle
sizes from 0.01 to 100Nm, preferably from 1.0 to 75um, and
more preferably from 10 to 20um.
Catalysts may also be useful in the resin systems
of the subject invention. Such catalysts are well known to
those skilled in the art, for example tertiary amines; metal
carboxylates, e.g. tin(II) octoate; and particularly the
-2d-
CA 02025039 1999-09-16
organophosphines, organophosphine salts, complexes, and the
reaction products of maleimide group-containing compounds and
organophosphines such as those disclosed as useful for epoxy
resin systems in U.S. patent 3,843,605. The resin systems of
the subject invention are generally prepared by mixing
together the various system components employing standard
techniques well known to those skilled in the art, until the
resulting mixture is substantially homogeneous.
Following the preparation of the uniform mixture
previously described, the thermoplastic particles,
particulate elastomers, and catalysts) are added at as low a
temperature as possible. A substantial amount, i.e. more than
30 percent, preferably more than 70 percent of particulate
thermoplastic should remain in particulate form in the resin
system.
The resin systems, prepared as described, may be
cast as a thin film for use as an adhesive or, preferably,
for use as a prepregging matrix resin. when used as a
prepregging resin, the resulting prepreg should be as uniform
as possible to achieve optimal properties when cured into
composite parts. prepregging techniques are well known to
those skilled in the art, as disclosed, for example, in U.S.
patent 3,784,433. The resin content of the prepreg may be
adjusted by varying the thickness, and hence the areal weight
of the resin films. The resin content is generally between 20
and 60 percent by weight, more preferably between 20 and 40
percent by weight.
By whichever method the prepreg is prepared, the
net result is a prepreg having a thermoplastic rich or
thermoplastic particle rich layer which is contiguous with
but substantially exterior to the fiber reinforcement and its
surrounding resin. During cure, the thermoplastic particles
are believed to swell and/or dissolve, producing a cured
CA 02025039 1999-09-16
resin having a thermoplastic concentration gradient which is
greatest at or near what was the surface of the prepreg.
When such prepregs are laved up into composite
structures and cured, the thermoplastic enriched area lies in
the interply zone substantially medially between the fiber
reinforcement layers. The composites prepared by this method
may contain thermoplastic in the form of a continuous or
quasi-continuous layer between the plies, this layer
containing both thermoplastic and thermosetting resin
components.
26
In the examples. which follow, C-353A is Compimidem
353A, a eutectic mixture of bismaleimides available from the
Shell Chemical Company; ~AHA is diallylbisphenol A; P84 is
Lenzing P84 polyimide: NC'.-514 is Cardolitee NC-514, an epoxy
resin with a viscosity of 25,000 cps and an epoxy equivalent
weight of 350; NC-551 is Cardolitee NC-551, an epoxy resin
with a viscosity of 600 cps and an epoxy equivalent weight
of 225; Heloxy is Heloxy~' 69 which is a resorcinol
diglycidyl ether available from the Wilmington Chemical
Company; XH-3336 is bisph.enol F diglycidyl ether having an
epoxy equivalent weight o~f 174 and a viscosity of
approximately 6000 cps. MY-0510 is trisglycidyl
p-aminophenol having an epoxy equivalent weight of 95 to 107
and a viscosity of 550 to~ 850 cps. BMI-1 is the
bismaleimide of 4,4'-methylenedianiline; BMI-2 is the
bismaleimide of toluenediamine; DMAB is dimethoxyallyl-
benzene and TAIC is triallyl-1,3-triazine-2,4,6(1H,3H,SH)-
trione, both low viscosity nonepoxy modifiers for comparison
purposes. TPP is triphenylphosphine.
itesin mixing procedures are well known to those
skilled in the art. The components are generally ~ised at
the lowest temperatures possible ~o as not to lead to
advancement of the resin. The prepared resin systems were
-27-
2~~~
coated onto release paper and used to impregnate
unidirectional intermediate modulus carbon fibers at a resin
content of 33 ~3 percent and an areal fiber weight of 145 +5
g/m2. The resulting preF>regs were layed up in a quasiiso-
tropic layup and autoclave cured at 420°F for 3-4 hours.
The panels were tested for compression strength after impact
by Boeing test method BS:.-7260 after being impacted at 1500
in-lb/in. Compositions and test results are presented in
Tables I and II.
-28-
CA 02025039 2001-08-13
29
M
LC1Lfl ~ O O U1 f~ ~ N
CO M ~ r1 '~ ~
N N O
~, O
M
O O O
U '-IP-~~ N
M M di , H
O '-'
O 01 I~ ~ U
. . . H O N
L~ l0 01 ~, ~ rl M
N N N H
O 61 l
L~ l0 Q1 '~ M
N N N
O
CO ~ Wit'O '~ U1
O O1 O
N N O ~' O N M
N N M
,5-I r-i
O
N
1-~
H
w
L l0 N ~ O p CO O
M Lf1LI-1 ~'d' ~ ''~~'
H N N M
N ~S
r-I
Q
LIlw
N N N M N v-Id'
N lD N d' rl
M ri M
-r-i r~
O
chi N
LIl_ ~I ~ l~ r-i
N N M N ~
~ ~ Q..,~ O .~
l0 N N d' ~ O ~ i
rl M M O
U Q, -~
u-ro
rd -~ o
Ul U~ 4-a
U~ H N i v QJ
W '~ O
~ O ~, Pa ~
M U~ N
H N 1~ ~-I di N
C(~ 0.1~ -~ O W
W '~
O ~-~r-~ d~ -a
W 'J-I v O N -~ r-1U
CJs FC ~I ~ ~-I Ul
Pa U O ~ Pa 4J O
H ~ 1-~
C~~ 1-I P .J--1~ H
O a1 W- o rd
W O mo ~ O
~ N ~l CJ U di
L''r-I N M
CA 02025039 2001-08-13
o ', ~, ~,O o u1
H i i LI1O ~ N Cf'
i i d' M
x
c0 m ~1'
~ H O
N N O U N
N N (''1 rZ
c0 c0 ~' '-I
~ ~ O L(1
N N O ~ N d'
N N C'1 rz
1~
M
N N O ~ N d' U7
~
N N
Q
r
-i
l0 l0 ~ ~ yJ
~-Ir-I CO ~pO O ~ Q
N N N ~ N ~' l-1
x
-- o
W
m
LI1 N N O ~ O O 01
N N M '-IN Wit' '
U
x
(~ ~I
N
~'
di di i O .-I N .~
N ,5
N N c'1~ N d' -r-1S-I
O O
W . . (~i U1
L L~ l0 i O d' ~ J-~r-I
N N ('~l~ r-Ic'~l ~ ~-I
O ~
U ~
W ~ ~ co N ~ o
i i d' c~'1i N M
~-I -riO
l~j O
r1 U~ Lfl
N U1 N r
1
r~
U7 ~ N c~I~ ~, ~ O
W W
~ r-IN c~'1 ~ ~ W . ?C ~ N
~ rtS~ b, ~-I
W i i Ul ~ ~ -~ '~ 1. W
QJ Ul N -riI
W H H Cl ~ ~ ~O ~ ~-I Ul
~ O 1..J
h7 L~ ~ P-~ ~ c-~'~'~~ U
p ~1 ~ 1, f~
W O U
P-iLJ ~l
N r-I N l'1
h
~_
Other low viscosity modifiers which are
coreactants with bismaleimides were found not to be
effective in increasing CAI. Thus Examples A and 8 both
gave lower CAI than the control with no low viscosity
modifier (Example G). While these non-epoxy low viscosity
modifiers are not effective in and of themselves in
increasing toughness, it is expected that they may be used
in conjunction with a low viscosity epoxy and thermoplastic
if desired, for example, to further increase the tack or
drape of a resin system.
The examples in the tables also show that the
additive of a suitable low viscosity epoxy resin to
bismaleimide formulations containing suitable thermoplastic
particles causes a sharp increase in the compression
strength after impact. In particular, bismalei~ide resin
systems capable of preparing quasiisotropic composites
exhibiting values of greater than 45 Rsi on intermediate
modulus carbon fiber when tested according to HSS-7260 were
previously unknown.
The CAI of Example F, for instance, with
thermoplastic but no epoxy, increased from 34 ksi to 4B ksi
(Example 3) when the only change was the addition of a small
amount (4.5 weight percent) of a suitable low viscosity
-31-
.m. ~ ~' ~°, ~1 "
~sf.~~'~~~
epoxy resin, a 41 percent increase in impact resistance.
Similarly, the CAI of Example E which contains thermoplastic
but no epoxy increased from 37 ksi to 45 ksi, a 22 percent
increase, when the epoxy is added to the formulation
(Example 10). These increases are statistically
significant, being far beyond the range of scatter, and
result in performance levels previously unheard of in
bismaleimide resin systems.
The ability of the epoxy to toughen is not
confined to a particular level of thermoplastic. Example F
contained 10 weight percent thermoplastic while Examples E
and G contained 20 weight percent. All showed large
increases in CAI with epoxy addition (Examples 3, 10 and 5
respectively).
The epoxy itself is not a toughener. The
combination of epoxy and thermoplastic is an essential
requirement. That this is so is illustrated by Examples C
and D which contain no thermoplastic. Example D, which
contains the epoxy, has approximately the same CAI as
Example C containing no epoxy. Thus the epoxy acts as a
toughener only in conjunction with the thermoplastic.
The particular bismaleimiae monomer content does
not greatly influence the toughness when both ther~oplastic
-32-
t ' n, ,~~ r~
... ..
and epoxy are present. Examples 1, 2, 10, and 5 demonstrate
a variety of bismaleimide monomer ratios, and all have
excellent CAI values in the range of 45 to 50 ksi.
The type of epoxy which is effective covers a wide
range. Examples 5, 7, 8, and 9,~containing different
epoxies, all showed significant increass in CAI over the
control formulation containing no epoxy, Example G.
Likewise, the effective amount of epoxy spans a wide
range. For instance, 8 weight percent (Example 6) was as
effective as 4 weight percent (Example 5).
-33-
