Polyimide resin laminates

A thermosetting resin is comprised of (a) a bisimide, particularly a bismaleimide, (b) a polyphenol, particularly a bisphenol, and (c) dicyandiamide. The polyphenol may be styrene-terminated (i.e. a vinyl benzyl ether of a bisphenol). Flame retardance is improved by the addition of octabromodiphenyl oxide.

FIELD OF THE INVENTION 
The invention relates generally to cross-linked, heat resistant, 
thermosetting polyimide resins, used in the manufacture of laminates for 
the electronics industry. More particularly, it relates to a novel 
polyimide resin which avoids the aromatic diamines now in common 
commercial use. 
PRIOR ART 
Thermosetting resins prepared from bisimides and aromatic diamines are in 
commercial use. In some instances, such compositions are prepared as 
prepolymers and combined with epoxy compounds and crosslinked with 
dicyandiamide or other agents. 
While resins derived from bisimides and aromatic diamines perform well in 
electronic laminates, they have several disadvantages. If substitute 
materials could be found that had improved performance, contained no free 
aromatic diamines, were less brittle and met UL flammability rating V-O, 
they should find ready acceptance in the marketplace. 
In commonly assigned application Ser. No. 850,660 now abandoned, 
thermosetting resins are produced by reacting bismaleimides (including a 
prepolymer chain-extended with an aromatic diamine) with 
styrene-terminated bisphenol (and/or the tetrabromo substituted compound). 
It is characteristic of these compositions to be blended together, 
combined with reinforcing materials, and cured to prepare laminates. Such 
compositions are not chemically reacted until the laminates are formed. 
Such resins are shown to have a desirably low dielectric constant, but 
they exhibit low peel strength and are too brittle and improved resins 
have been sought. 
In the parent of this application, Ser. No. 187,158, an improved 
thermosetting resins was disclosed which had many desirable properties. It 
has been found that better flame retardant properties could be obtained 
for laminates using such resins, but the resin-flame retardant 
combinations have been found to have unexpected synergistic properties, as 
well as seen in the discussion which follows. 
SUMMARY OF THE INVENTION 
An improved thermosetting resin suitable for use in laminates for the 
electronics industry is prepared from (a) a bisimide, preferably a 
bismaleimide, (b) a polyphenol, preferably a bisphenol, which may be 
styrene-terminated (i.e. a vinyl benzyl ether of a bisphenol), and (c) 
dicyandiamide. Additional flame retardant properties are achieved by 
including octabromodiphenyl oxide in effective amounts while retaining the 
desirable properties of the resin. 
The resin is principally comprised of a bisimide with minor amounts of 
components (b) and (c). The weights of (a), (b), and (c) preferably will 
be between 80 to 98%/1 to 10%/0.1 to 10%, respectively, most preferably 94 
to 98%/1 to 2%/1.5 to 3%, respectively, based on the total of (a), (b), 
and (c). The octabromodiphenyl oxide will be added in effective amounts up 
to about 5 weight percent based on the resin.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The thermosetting resins of the invention can replace those made by 
reacting a bisimide with an aromatic diamine. Such resins are polymerized 
and then crosslinked by use of dicyandiamide or related compounds. The 
present resins, however, introduce dicyandiamide as an intrinsic element 
of the composition, and thereby producing a superior laminate, as will be 
seen. 
Bisimides 
Bisimides are compounds having the general formula D(CO).sub.2 
N--A--N(CO).sub.2 D where D is a divalent radical containing a 
carbon-carbon double bond and A is a linking group which may be a divalent 
radical having at least 2 carbon atoms. Particularly preferred are 
bismaleimide (BMI) compounds where A comprises an isopropyl radical or an 
ether linkage between two or more phenyl radicals whiCh are attached to 
the nitrogen atoms. 
For purposes of the present invention, the bisimide compound will be 
selected to provide resins having improved toughness, low water 
absorption, low dielectric constant, and excellent thermal resistance. 
Resins of the invention are usually comprised mainly of the bisimide, 
preferably from about 80 to 98 weight %, most preferably 94 to 98 weight 
percent. Typically, the prepolymers of bisimides and aromatic diamines of 
the prior art contain smaller amounts of the bisimides than are employed 
in the present resins. The bisimide can react with the diamine through the 
unsaturated carbon-carbon bond of the imide ring, but it may also react 
with itself in the same manner. In the resins of the invention, a further 
component of the composition is dicyandiamide, which is more commonly used 
in other resin systems as a crosslinking agent once the bisimide and 
diamine have formed a prepolymer. The dicyandiamide here is believed to 
take an essential, although not fully understood, role in the polymer 
formation and not to be merely a cross-linking agent. 
Dicyandiamide 
This compound has the formula 
##STR1## 
It is capable of reacting with the carbon-carbon double bond of the 
bismaleimide to extend the polymer chain. 
Only relatively small amounts of dicyandiamide are required, broadly about 
0.1 to 10 wt. % of the resin may be used. Preferably, about 1.5 to 3 wt. % 
would be reacted to form the resin. 
Polyphenols 
The polyphenols used in the resins of the invention preferably are 
bisphenols, particularly those within the following formula: 
##STR2## 
where: Z=H, CH.sub.2 =CH--C.sub.6 H.sub.4 --CH.sub.2 -- 
##STR3## 
B=H, Br, CH.sub.3 
In the above formula and those which follow, --C.sub.6 H.sub.4 --represents 
a phenylene radical. 
Of particular interest are the group consisting of bisphenol A, 
tetrabromobisphenol A, vinyl benzyl ether of bisphenol A, and vinyl benzyl 
ether of tetrabromobisphenol A. 
The polyphenols are present in the resin in minor amounts, preferably about 
1 to 10 wt. %, most preferably 1 to 2 wt. %. 
Composition of the Resin 
A thermosetting resin composition according to the invention may comprise 
(a) a bisimide having the formula: 
##STR4## 
where: A comprises at least one of alkylene radicals having 1 to 20 carbon 
atoms, cycloalkylene radicals having 4 to 40 carbon atoms, heterocyclic 
radicals containing at least one of O, S, and N, and phenylene or 
polycyclic aromatic radicals 
D is a divalent radical containing a carbon-carbon double bond 
(b) a bisphenol, and 
(c) dicyandiamide. 
In a preferred embodiment the bisimide is a bismaleimide and the 
thermosetting resin composition may comprise (a) a bismaleimide having the 
formula: 
##STR5## 
where: 
##STR6## 
and the bisphenol has the formula 
##STR7## 
where: Z=H, CH.sub.2 =CH--C.sub.6 H.sub.4 --CH.sub.2 -- 
##STR8## 
B=H, Br, CH.sub.3 and (c) dicyandiamide. The term "sigma bond" refers to 
a covalent bond between the aromatic rings and corresponds to a bisphenol 
in which the aromatic rings are directly joined. 
The resin composition may comprise 1-10 mols of (a), greater than zero to 1 
mols of (b), and greater than zero to 2 mols of (c). 
Particularly preferred embodiments include those which have bismaleimides 
where A is 
##STR9## 
Flame Retardants 
Use of bisphenols containing bromine improves flame retardant properties of 
laminates prepared from the above described resins. For some compositions 
and for some end uses it may be necessary to add additional flame 
retardants in order to meet required tests. The inventors have discovered 
that, contrary to experience with epoxy-based laminates, that 
bismaleimide-based laminates respond differently to the addition of 
bromine containing compounds. In particular, certain compounds are not 
sufficiently effective and/or may adversely affect other properties of the 
laminates. Generally, the amount of bromine present in the finished 
laminate does not predict the results of the usual flammability tests. It 
has been found that octabromodiphenyl oxide is capable of providing 
laminates which meet the UL-94 test for flammability while retaining the 
other physical properties characteristic of the 
bismaleimide-bisphenol-dicyandiamide resins described above. Sufficient 
amounts of the octabromodiphenyl oxide are added to the resin formulation 
to meet the UL-94 test for flammability, up to a maximum of about 5 weight 
percent, after which the other physical properties are adversely affected. 
Octabromodiphenyl oxide is commercially available from Great Lakes 
Chemical Corporation and is diphenyl oxide which has been brominated to 
have predominately eight atoms of bromine per molecule. 
Resin Synthesis 
The improved bismaleimide resins may be prepared by reacting the three 
components in a solvent at an elevated temperature to form a prepolymer. 
One convenient procedure is to warm a suitable solvent, such as dimethyl 
formamide (DMF), N-methylpyrrolidone, dimethyl acetamide, acetone, 
benzene, toluene, and the like to a temperature at which the chosen 
bismaleimide will be dissolved, say about 90.degree. to 100.degree. C. The 
bismaleimide is added to the solvent and mixed until dissolved. Then the 
temperature is increased to the desired reaction temperature, about 
120.degree. to 140.degree. C., at which time the second and third 
components are added and mixed. The three-component mixture in the solvent 
will be maintained at the reaction temperature for a sufficient period of 
time to partially complete the reaction. Polymerization is completed 
during the manufacture of laminates. The octabromodiphenyl oxide is added 
before impregnating the reinforcing fibers. 
Use of the Resins 
The resins of the invention may be used to prepare laminates for the 
electronics industry by techniques generally in use in the field. 
Generally, the resins are diluted with a solvent and then used to 
impregnate a fabric with fibers of glass, high strength organic polymers, 
and the like familiar to those skilled in the art, and then dried at an 
elevated temperature. The resulting composite may then be laminated with 
other layers, such as copper foil, and then baked to fully cure the 
finished laminate. Such laminates may be used in fabrication of printed 
circuit boards having improved properties. 
In the following examples, unless otherwise specified, the resins were 
prepared by this procedure. Dimethylformamide (DMF) was added to a glass 
flask and heated to 100.degree. C. The bismaleimide resin (Skybond 3000 
supplied by Monsanto) was added to the DMF and the temperature returned to 
100.degree. C., when dicyandiamide and the bisphenol ("STTBBPA", a styrene 
terminated tetrabromo bisphenol A, i.e., a vinyl benzyl ether of a 
brominated bisphenol A) were added to the mixture The flask was heated to 
140.degree. C. and maintained at that temperature until the desired degree 
of reaction is reached. This was determined by sampling the mixture and 
determining the gel time by stroke cure method on a cure plate. When the 
resin had a gel time of about 6-7 minutes at 171.degree. C., the reaction 
was stopped by cooling the flask. The flame retardent compounds were added 
and the resin was then ready for use. 
EXAMPLE I 
A series of laminates were prepared with varying amounts of two bromine 
flame-retardant compounds, octabromo diphenyl oxide according to the 
invention (samples 1-3) and for comparison tetrabromobisphenol A (samples 
4-6). The composition of each resin, its processing into laminate, and the 
results of tests on the laminates are shown in the following tables. 
TABLE A 
______________________________________ 
1 2 3 
______________________________________ 
Composition 
Bismaleimide, grs. 
562 645 562 
STTBBPA, grs 25 29 25 
Dicyandiamide, grs. 
11 13 11 
Octobromodiphenyl Oxide, grs. 
18 35 42 
Dimethylformamide, grs. 
546 640 568 
Processing 
Resin Solids 53% 53% 53% 
Bromine % per Solids 
4.02% 5.52% 6.83% 
Resin Gel Time at 171 deg. C. 
355 sec. 357 sec. 367 sec. 
Resin Content 108 glass style 
60.6% 62.3% 61.6% 
Resin Content 7628 style glass 
39.5% 40.8% 38.8% 
Cure Time at 176 deg. C. 
4 min. 4 min. 4 min. 
Press Cycle at 176 deg. C. 
2-1/2 hrs. 
2-1/2 hrs. 
2-1/2 hrs. 
Postbake at 218 deg. C. 
16 hrs. 16 hrs. 16 hrs. 
Test Results 
Bromine % 4.02 5.52 6.83 
Resin % (1) 59.1 62.2 60.6 
Flow % (1) 26.3 29.6 26.4 
Volatile % (1) 2.80 3.58 4.57 
Flammability-A (2) 
average 3.2 1.0 0.7 
single high 8.4 3.9 2.0 
Solder Blister-550 F (3) 
1169 1044 1165 
______________________________________ 
(1) Style 108 glass bonding sheet, target 62% .+-. 5% resin; 30% .+-. 5% 
flow; 4% maximum for volatiles 
(2) 8 ply style 7628 glass Target flammability values are 2.5 seconds 
average and 5.0 seconds maximum for any one test. 
(3) 5 mil thickness, 2 ply style 108 glass, 1 oz copper both sides, targe 
600 seconds 
TABLE B 
______________________________________ 
1 2 3 
______________________________________ 
Composition 
Bismaleimide, grs. 
562 595 666 
STTBBPA, grs 25 27 30 
Dicyandiamide, grs. 
11 12 13 
Tetrabromobiphenol A, grs. 
26 43 68 
Dimethylformamide, grs. 
552 600 688 
Processing 
Resin Solids 53% 53% 53% 
Bromine % per Solids 
4.05% 5.42% 6.79% 
Resin Gel Time at 171 deg. C. 
309 sec. 300 sec. 275 sec. 
Resin Content 108 glass style 
60.9% 60.7% 61.0% 
Resin Content 7628 style glass 
41.7% 40.3% 42.6% 
Cure Time at 176 deg. C. 
3-1/2 min. 
3-1/2 min. 
3-1/2 min. 
Press Cycle at 176 deg. C. 
2-1/2 hrs. 
2-1/2 hrs. 
2-1/2 hrs. 
Postbake at 218 deg. C. 
16 hrs. 16 hrs. 16 hrs. 
Test Results 
Bromine % 4.05 5.42 6.79 
Resin % (1) 60.0 59.5 59.3 
Flow % (1) 26.0 26.7 28.3 
Volatile % (1) 3.36 4.48 5.87 
Flammability-A (2) 
average 3.0 3.3 2.3 
single high 9.2 11.0 8.1 
Solder Blister-550 F (3) 
498 215 7 
______________________________________ 
(1) Style 108 glass bonding sheets, target 62% .+-. 5% resin; 30% .+-. 5% 
flow; 4% maximum for volatiles 
(2) 8 ply style 7628 Glass, Target flammability values are 2.5 seconds 
average and 5.0 seconds maximum for any one test. 
(3) 5 mil thickness, 2 ply style 108 glass, 1 oz. copper both sides, 
target 600 seconds 
It can be seen from Table A that increasing the amount of octabromodiphenyl 
oxide reduces the flammability test results to within the target range, 
while the solder blister test apparently is not significantly affected. 
However, tetrabromobisphenol, even at about the same total bromine content 
does not reduce the flammability results effectively but at the same time 
reduces the solder blister test and percent volatiles results to 
unacceptable values. It is concluded that the octabromo-diphenol oxide is 
uniquely effective as a flame retardant additive for the 
bismaleimide-based resins of the invention.