A process for the preparation of composite laminate structures of glass cloth preimpregnated with polybismaleimide resin and adhered to a polybismaleimide glass or aromatic polyamide paper honeycomb cell structure filled or partially filled with a syntactic foam consisting of a mixture of bismaleimide resin and carbon microballoons. The carbon microballoons are prepared by pyrolyzing phenolic microballons and subsequently bonded using a 2% bismaleimide solution. The laminate structures are cured for two hours at 477.degree. K and are ahered to the honeycomb bismaleimide adhesive using a pressure of 700kN/m.sup.2 pressure at 450.degree. K. The laminate composite is then post-cured for two hours at 527.degree. K to produce a composite laminate having a density in the range from about 95 kilograms per cubic meter to 130 kilograms per cubic meter.

BACKGROUND OF THE INVENTION: 
1. Field of the Invention 
The present invention relates to a process for the preparation of low 
density composite laminate structures consisting of a bismaleimide-glass 
laminate face sheets adhered to a bismaleimide honeycomb structure 
containing carbon microballoons bonded with bismaleimide resin. Panels and 
structures prepared in accordance with the present invention are useful as 
aircraft panels and panels for vessels which require high ignition 
temperature and minimum of gassing and evolution of noxious and toxic 
fumes upon ignition. 
1. Description of the Prior Art 
Panels and structures employed by manufacturers as the interior paneling of 
aircraft and vessels subject to confined or constricted environments have 
generally employed glass-epoxy resins combined with a polyamide core. The 
prior art structures due to the presence of epoxy exhibit large smoke 
evolutions and a low fire containment capacity when subjected to heat or 
flame. 
In general the panels and aircraft lining structures have been prepared by 
silk screening the required decorative surface on a 0.005 cm polyvinyl 
fluoride film (Tedlar) by a continuous web process. After drying, a 0.0025 
cm transparent polyvinyl fluoride film coated on one side with polymethyl 
methacrylate is bonded to the decorative film to provide protection for 
the printed surface. 
The laminate is then bonded to one ply of epoxy-preimpregnated 181E glass, 
and may have a surface texture imposed during the bonding operation. 
One core material presently employed for sandwich paneling is an aromatic 
polyamide (sold under the tradename Nomex HRH-10) hexagonal-cell honeycomb 
structure. The cell size may be 0.312 cm, 0.625 cm, or 0.937 cm, depending 
upon the properties desired in the finished panel. 
The prior art method of binding an outer panel to the core consists of 
using an epoxy resin-preimpregnated bond ply over which is applied the 
prepared glass/polyvinyl fluoride decorative laminate. The resin in the 
bond ply provides the adhesive to bond the outer panel to the honeycomb 
and the decorative laminate to the bond ply. Curing is accomplished at 
110.degree. C with 50 cm Hg minimum vacuum bag pressure. For a panel 
requiring a decorative laminate on one side only, a bond ply is used as 
the outer panel on the other or back side. 
Structures and panels prepared in accordance with the prior art have 
exhibited relatively lower fire containment capabilities because of the 
absence of insulative material in the honeycomb core. The prior art 
structures also evolve large quantities of smoke and toxic fumes when 
exposed to fire or heat due to the presence of the epoxy material and the 
glass-epoxy resinous substrate. 
DESCRIPTION OF THE INVENTION 
Summary of the Invention 
The disadvantages and limitations of the prior art are obviated by the 
process and material compositions of the present invention which employ a 
low density bismaleimide-carbon microballoon composite. The advantages of 
the invention are achieved by structures composed of a face sheet 
preimpregnated with bismaleimide resin bound with a polyimide film 
adhesive to a core containing carbon microballoons bound in a honeycomb 
core with bismaleimide resin. 
Structures prepared in accordance with the present invention possess 
excellent thermophysical properties. Their rather low thermal conductivity 
renders them attractive for use as insulation where high temperatures are 
involved. Also of interest in these situations are the lower oxygen index 
values that these structures possess which render them more difficult to 
ignite than the materials of the art. Furthermore, when exposed to heat 
and flame, the structures of this invention exhibit a marked reduction in 
the density of smoke produced and in the toxicity of pyrolytic products, 
two characteristics which greatly improve the chances of survival of 
people in confined spaces. Such improved properties, coupled with 
increased mechanical capacities, highly recommend the new structures for 
use in ground vehicle and aircraft interiors. 
DETAILED DESCRIPTION 
The resins that are employed in the construction of the structures of the 
invention are those prepared by the polymerization of various 
bismaleimides, alone or in the presence of diamino compounds. These 
monomers and prepolymers can be illustrated by the following formulas: 
##STR1## 
wherein n may be any integer that yields a prepolymer of suitable 
viscosity under the conditions of use, R.sub.1 is a divalent radical 
having at least two carbon atoms, and R.sub.2 represents a divalent 
radical not having more than 30 carbon atoms. When a diamine is used, the 
bismaleimide-diamine ratio should be between about 1.2:1 to 50:1. R.sub.1 
and R.sub.2 may be identical or different and may be alkylene 
radicals-linear or branched or cyclic, heterocyclic radicals, phenylene or 
polycyclic aromatic radicals. These various radicals, as well as the 
hydrogen-bearing carbon atoms in the maleimide ring, may carry 
substituents that do not give side reactions under use conditions. The 
preferred monomers for use in the present invention are those in which 
R.sub.1 and R.sub.2 are aromatic in nature. Usable species of the resins 
just described are listed in U.S. Pat. No. 3,652,223 and French Pat. No. 
1,455,514. 
The resins can be prepared by heating the monomer or monomers until 
maleimide-terminated prepolymers are obtained [equations (1) and (2)], 
said prepolymers then being capable of undergoing cure in situ simply by 
heating them at a temperature within the range of about 450.degree. to 
510.degree. K to form dense, insoluble, nonmelting polymer networks. 
FABRICATION OF COMPOSITE STRUCTURES 
The advantages of the present invention can be achieved by preparing panels 
in the manner that will now be illustrated, using the resins already 
described. For convenience, a panel having dimensions of 30 cm .times. 30 
cm .times. 2.5 cm will be constructed. It must be understood however that 
those skilled in the art can vary greatly the panel sizes and 
configurations, as well as the fabrication techniques, without departing 
from the spirit of the invention. 
Preimpregnation of Face Sheet Material 
A resin powder consisting essentially of 
N,N'-(4,4'-diphenylmethane)-bismaleimide is sifted into an equal weight of 
n-methyl-2-pyrrolidone under vigorous agitation to prevent lumping. The 
viscosity of the resulting 50% by weight solution is less than 10 poises 
at room temperature initially and increases rapidly on aging. For best 
results, this resin preimpregnation solution is prepared immediately prior 
to use since its maximum useful life is about 20 days. 
The process used to impregnate the resin solution into the glass cloth 
consists in passing a dried fiberglass cloth through a preferably 50% by 
weight solution of the bismaleimide resin. The wet cloth is then led 
between a steel roller and a wiper blade to assist impregnation and remove 
excess resin. Uniform impregnation of the glass fabric is achieved by 
pulling the cloth through the resin solution at a constant speed of 0.6 
meters per minute, with a constant wiper blade pressure exerted on the 
impregnated cloth. The prepregged cloth is then dried and partly cured by 
heating it in an oven for 15 minutes at 355.degree. K and then for 30 
minutes at 366.degree. K. Prepregged cloth prepared in this manner has an 
average resin content of 41.3% and an average volatile content of 5.2%. 
The prepregged sheet may then be cured by using either an autoclave vacuum 
bag technique or by the platen pressure method, both of which being well 
known to those skilled in the art. In the autoclave vacuum bag technique, 
a 33 cm .times. 33 cm prepregged single layer cloth is sandwiched between 
porous Teflon-coated glass fabric sheets and is placed on a 0.6 thick 
aluminum plate. A glass bleeder cloth is placed against the sandwiched 
prepreg. The assembly is then vacuum bagged and cured at an external 
pressure of about 345 kN/m.sup.2 at a temperature of about 450.degree. K 
for one hour. In the press cure method, a 33 cm .times. 33 cm prepregged 
sheet is sandwiched between porous Teflon-coated glass fabric sheets and 
cured between 0.6 cm thick aluminum plates treated with a mold release 
agent. The prepreg is cured at about 345 kN/m.sup.2 for one hour at about 
450.degree. K. 
After curing, the face sheets are evaluated for visual defects including 
flaws, voids, thickness uniformity, and for resin content. The sheets are 
generally about 0.025 cm in thickness and contain 30% to 34% of the resin 
dry basis. The resin face sheets may thereafter be bonded with a polyamide 
adhesive to a honeycomb core filled with carbon microballoons, to provide 
a sandwich structure panel assembly in the manner hereinafter described. 
Core Fabrication Process 
The core structure is fabricated by introducing carbon microballoons into a 
suitable fiberglass-bismaleimide honeycomb core and securing the 
microballoons by means of a bismaleimide adhesive. The core may consist of 
either an aromatic polyamide paper honeycomb, with 0.3 cm diameter cells 
(Nomex HRH-10), or preferably a bismaleimide-glass fabric honeycomb, with 
0.935 cm diameter cells (Kerimid 601). 
The carbon microballoons are prepared by pyrolizing phenolic microballoons 
in a large stainless steel container which has been purged with nitrogen 
to contain an oxygen-free atmosphere. Typical phenolic microballoons that 
can be used for this purpose are described in various patents, 
particularly in U.S. Pat. No. 3,030,215. The specific phenolic 
microballoons pyrolized in the present instance had the following 
specifications: 
Bulk density; 0.10 - 0.12 g/cc 
True density; 0.322 - 0.334 g/cc 
Particle size; 20 mesh, 90% 50-100 mesh 
Broken balloons; 3% 
Volatiles; 8.0% maximum 
The pyrolysis of the phenolic microballoons is preferably accomplished by 
placing the stainless steel container in a furnace and then cycling from 
room temperature to about 1089.degree. K in four hours, holding at that 
temperature for four hours, and thereafter cooling to room temperature 
over a period of two days. The resulting carbon microballoons are 
preferably cooled to at least 311.degree. K or lower, before removal of 
the nitrogen atmosphere, to prevent spontaneous ignition. The carbon 
microballoons, after pyrolysis, are generally agglomerated in the form of 
large cakes which require separation. Separation may be accomplished by 
placing the microballoon cakes in a container with isopropanol present in 
the ratio of one kilogram of carbon microballoons per seven liters of the 
liquid. The mixture is then vigorously shaken for 15 minutes, as in a 
paint shaker, with the resulting slurry being passed through a 20-mesh 
screen to remove any non-separated agglomerates. The screened 
isopropanol-carbon microballoon slurry can be used directly for core 
impregnation to provide a core to which the face sheet may be attached. 
Impregnation may be conveniently achieved through various processes known 
to those skilled in the art, including vacuum screen processes in which 
the carbon microballoon-isopropanol slurry is drawn by vacuum into the 
honeycomb core. In some applications, a partially filled honeycomb core 
may be advantageous to provide the space necessary for expansion of the 
carbon microballoons during rapid temperature changes. 
The filled honeycomb core is then sandwiched between two nylon fine mesh 
screens and two aluminum support honeycombs, and dried for about 16 hours 
in an air-circulating oven at about 366.degree. K. After drying, the 
microballoon-filled core is saturated with a 2% n-methyl-2-pyrrolidone 
solution of the bismaleimide resin used earlier for impregnation of the 
facing sheet. In the case of cores partially filled with carbon 
microballoons, the solution is sprayed into the cores at low air pressure. 
With cores that are completely filled with microballoons, on the other 
hand, the microballoons have a tendency to blow out during spraying and, 
therefore, the application of the 2% solution is most effectively achieved 
by brushing the bismaleimide resin solution over the cells. After 
saturation of the microballoon filled core with the 2% bismaleimide resin 
solution, the honeycomb is heated for about 2 hours at about 366.degree. K 
and then for about 1 hour at about 477.degree. K, to completely cure the 
bismaleimide binder. 
Prior to final assembly, the carbon microballoon-filled honeycomb core is 
examined for uniformity of microballoon fill and tested for the combined 
weight of the microballoons and the bismaleimide resin binder. The 
microballoon-resin combination should contain approximately 4 to 10% of 
the resin by weight of the combination, and for a half filled 2.4 cm thick 
core, the fill weight should approximate 145g/10.sup.3 cm.sup.2. 
Sandwich Structure Panel Assembly 
The assembly of a panel in accordance with the invention is achieved by 
bonding the face sheet or sheets to the microballoon filled fiberglass 
bismaleimide honeycomb panel with a bismaleimide hot melt adhesive which 
again consists of the monomer used in the face sheets and the core. Prior 
to bonding, the face sheets and the carbon microballoon filled core are 
cleaned to enhance adhesion. Loose microspheres are removed from the 
bonding faces with a methyl ethyl ketone-soaked cleaning cloth. Each face 
sheet is then bonded to the core with the bismaleimide adhesive. The 
assembly is then placed in a platen press at about 477.degree. K and cured 
for about two hours under a pressure of about 700 kN/m.sup.2. After cure, 
the panel is postcured for about 24 hours at about 527.degree. K, to 
remove volatile materials and achieve minimal smoke characteristics. 
Panels prepared in accordance with the invention exhibit excellent thermal, 
fire-resistant and mechanical properties, making them particularly suited 
for various high temperature applications and where fire safety is a 
primary consideration. In particular, the panels of the invention may be 
utilized as aircraft interior panels as well as for walls for aircraft 
compartments where the maximum in fire containment and the smallest 
production of smoke and toxic products is desired. 
Additional applications include light weight composite structural walls for 
lightweight ships and other transportation vehicles where light weight and 
fire resistance are needed. The invention may also be used as interior 
wall panels in space shuttle vehicles to provide maximum of fire 
protection and least smoke and toxic pyrolysis product generation.

The present invention will be further illustrated by way of the following 
specific examples which are not intended to limit the scope of 
applicability of the invention. In this application, parts and percentages 
are by weight unless expressly stated to be otherwise. 
EXAMPLE 1 -- Comparison with the Prior Art 
Low density carbon microballoon-composites made in accordance with the 
detailed description of the invention above, each having a density of 
132.593 kg/m.sup.3, were prepared, and are compared with a representative 
prior art composite structure in Table 1. 
EXAMPLE 2 -- Thermogravimetric Analytical Comparison with the Prior Art 
Two composites, identified in Table 2 below as Sample Number 1 and Sample 
Number 2 respectively, and similar to those described in column 2 of Table 
1, but having a density of 110 kg/m.sup.3, were also prepared, following 
generally the process described in the above detailed description of the 
invention. These composites were subjected to thermogravimetric anaylsis. 
The analyses were made on a DuPont 950 Thermogravimetric Analyzer using 
both nitrogen and air atmospheres with a sample size 10 mg. The 
thermogravimetric analysis data, at a 10.degree. C/min. heating rate in 
nitrogen and in air, are shown in Table 2. As the data demonstrates, the 
composite systems of the present invention are more generally stable 
thermally than those of the prior art. 
TABLE 1 
__________________________________________________________________________ 
COMPOSITE NUMBER 
Composite Components 
1 2 3 
(In order of lamination) 
(Present Invention) 
(Present Invention) 
(Prior Art) 
__________________________________________________________________________ 
Decorative Surface 
0.002 PVF 0.002 PVF 
Thickness (cm) 
Acrylic Ink None Acrylic Ink 
0.005 PVF 0.005 PVF 
Face Sheet Bismaleimide/181 
Bismaleimide/181 
Epoxy Resin/181 
Resin/Fabric 
E-Glass E-Glass E-Glass 
Bond Sheet Bismaleimide Hot 
Bismaleimide 
Epoxy/120 
Resin/Fabric 
Melt Adhesive 
Melt Adhesive 
Glass 
Core Type Aromatic Polyamide 
Bismaleimide 
Aromatic Polyamide- 
Thickness, (cm) 
Paper Honeycomb 
Glass Honeycomb 
Paper Honeycomb 
Cell size, (cm) 
2.54 2.54 2.54 
Density 
Core Filler Bismaleimide-Carbon 
Bismaleimide-Carbon 
% of fill Microballoon, Syntactic 
Microballoon, Syntactic 
None 
Foam, 50 Foam, 50 
Bond Sheet Bismaleimide Hot 
Bismaleimide Hot 
Epoxy Resin/120 
Resin/Fabric 
Melt Adhesive/ 
Melt Adhesive/ 
Glass Fabric 
Glass Fabric 
Glass Fabric 
Face Sheet Bismaleimide/181 
Bismaleimide/181 
Epoxy Resin/181 
Resin/Fabric 
E-Glass Fabric 
E-Glass Fabric 
E-Glass Fabric 
Decorative Surface 0.005 PVF 
Thickness (cm) 
None None Acrylic Ink 
Total Panel Weight 0.002 PVF 
in Kg/M.sup.3 for 2.54 cm 
132.593 132.593 95.000 
Thick Panels 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
THERMOGRAVIMETRIC ANALYSIS OF COMPOSITES IN NITROGEN 
(HEATING RATE 10.degree. K/min.) 
__________________________________________________________________________ 
Percent Weight Remaining at Various Temperatures .degree. K 
(Nitrogen) 
Composite Type 
373.degree. K 
473.degree. K 
573.degree. K 
673.degree. K 
773.degree. K 
873.degree. K 
973.degree. K 
1073.degree. K 
1173.degree. K 
State of the art 
98 97.8 
96.5 
89.5 
77.5 
64 51 41 34 
Sample Number 1 
98 97.5 
97 96 91.5 
82.2 
73.5 
68.5 65 
Same Conditions as above (AIR) 
State of the art 
98.5 
98 92.5 
88 80.3 
66.2 
45 32.4 
31.5 
Sample Number 2 
96.5 
96 95.7 
94 86 53 25 25 24.5 
__________________________________________________________________________ 
EXAMPLE 3 -- COMATIVE TEST DATA 
A bismaleimide-carbon microballoon composite was prepared in accordance 
with the procedure given in the detailed description of the invention 
above. The composite was similar to composite No. 2 in Table No. 1, above, 
and had a density of 113 kg/m.sup.3. The following series of tests were 
run to compare said composite structure of the present invention with 
those of the prior art. 
Heat Transmission Test 
In this test, a test fixture was employed that is known as the Ames 3-T 
facility. This fixture is essentially an oil burner burning JP-4 jet fuel 
in a fire brick lined combustion chamber at a rate of about 1 1/2 gallons 
per hour. The combustion chamber has various openings so that samples can 
be tested at a desired heating rate. The heating rate selected was 
10.4-10.9w/cm.sup.2, and the test consisted of placing a sample of the 
composite over an opening in the combustion chamber. A temperature sensing 
device was placed over the backing plate so that the time required for the 
back face to reach a selected temperature could be measured. 
The results of the experiment are set forth in Table 3 below. 
TABLE 3 
______________________________________ 
Heat Transmission Comparison 
Thick- 
ness Density 
Time to Reach: 
473.degree. K 
Sample 
Material (cm) Kg/m.sup.3 
373.degree. K (sec.) 
(sec.) 
______________________________________ 
3-1 Prior Art 
2.54 95 80 140 
3-2 Present 
Invention 
2.54 113 300 465 
______________________________________ 
It can be appreciated from Table 3 above that in the case of Sample 1, 
which is typical of the prior art composites, the sample reached 
373.degree. K in 80 seconds, whereas Sample 2, prepared in accordance 
with the present invention, required 300 seconds to reach the same 
temperature. 
Pyrolysis Test 
A low density bismaleimide-carbon microballoon composite, similar to 
Composite No. 2 in Table 1, was ground to a fine particle size. The 
particles (Sample No. 3-3) were subjected to pyrolysis in a vacuum at 
about 973.degree. K for about 5 minutes. The major volatile products 
obtained are shown in Table 4 below, as determined at 296.degree. K. 
TABLE 4 
______________________________________ 
Volatile Products from Pyrolysis at 973.degree. K 
Sample No. Compound Quantity 
______________________________________ 
3-3 Mg of compound per g 
initial sample 
(present 
Invention) CO.sub.2 155.3 
CO 14.3 
CH.sub.4 1.3 
HCN 5.9 
NH.sub.3 3.6 
______________________________________ 
This data, which accounts for about 20% of the combustion products released 
in the atmosphere, does not indicate any improvement of the present 
structure over those of the prior art. 
Thermal Conductivity Test 
A low density bismaleimide-carbon microballoon composite (Sample 3-4) 
similar to Composite No. 2 in Table 1, was tested to determine thermal 
conductivity (ASTM + C-177-45). The thermal conductivity of the composite, 
having a density of 108 kg/m.sup.3, was 4.932 .times. 10.sup.-4 W. 
cm/cm.sup.2 .degree. C. Due to the carbon microballoons-bismaleimide 
syntactic foam present in the honeycomb, the conductivity is lower than 
that of prior art composites, thus providing better insulation and better 
fire resistance than prior art structures. 
Smoke Evolution Test 
Another bismaleimide-carbon microballoon composite, similar to composite 
No. 2 in Table 1, with a density of 100 kg/m.sup.3 (Sample 3-5) was 
subjected to a series of tests run to compare smoke evolution from 
composites of the present invention with that of composites of the prior 
art (Sample 3-1). The NBS Smoke Chamber was utilized as described by the 
National Fire Protection Association, Bull. 258-T (1974), and Lee T. B., 
Interlaboratory Evolution of Smoke Density Chamber, National Bureau of 
Standards Technical Note 708 (Dec. 1971). 
Smoke measurements are expressed in terms of specific optical density, 
within a chamber of unit volume produced from a specimen of unit surface 
area. 
In the standard procedure for conducting a test with the NBS chamber, the 
percent light transmission, T, is determined as a function of time until 
the minimum value is attained. The data is then converted to the specific 
optical density, D.sub.s, where 
##EQU1## 
The chamber volume, V, is 0.509 m.sup.3 ; the light path length, L, is 
0.914 m; and the exposed material surface area, A, is 0.004236 m.sup.2. 
The maximum value of D.sub.s reached in the chamber is termed D.sub.m. The 
tests were conducted with a heat source which gave a heat flux of 2.5 w/cm 
.sup.2 under flaming conditions. The results obtained are reported in 
Table 5 below: 
TABLE 5 
______________________________________ 
Smoke Evolution as Measured by Light Transmission 
Sample Material D.sub.s in 1.5 min. 
D.sub.s in 4.0 min. 
D.sub.m 
______________________________________ 
3-1 Prior Art 53.0 58.1 58.7 
3-5 Present 1.0 4.9 18.2 
Invention 
______________________________________ 
It can be seen from Table 5 that Sample 3-1, representing the prior art, 
generated the higher specific optical density value, indicating much more 
smoke produced. This fact could obscure vision much faster in a fire 
situation than composites make in accordance with the invention (Sample 
3-5). Furthermore, the greater toxicity of the prior art composites, as 
determined by the next test, is most probably due to this phenomenon, 
rather than to the quantity and nature of toxic gases emitted. 
Toxicity Test 
A bismaleimide-carbon microballoon composite, similar to composite No. 2 of 
Table 1, having a density of 120 kg/m.sup.3 (Sample 3-6) was tested to 
provide a comparison of the relative toxicity of the pyrolysis or thermal 
degradation products of said composite, with those of specimens of the 
prior art. Samples of 1.0 g size of each were powdered, then pyrolyzed in 
a series of tests in a quartz tube, to an upper temperature limit of about 
700.degree. C. The effluents of each were conveyed by natural thermal flow 
into a 4.2 liter hemispherical chamber containing four Swiss albino male 
mice. The apparatus and procedure used was similar to the one described by 
C. J. Hilado, "Evaluation of the NASA Animal Exposure Chamber as a 
Potential Chamber for Fire Toxicity Screening Test, " J. Combustion 
Toxicology, Vol. 2, No. 4, 298-314 (November 1975). 
The test was conducted for 30 minutes, unless terminated earlier upon the 
death of all four animals. Table 6 below reports the relative toxicity to 
mice of the degradation products from the powdered composites upon 
heating. 
TABLE 6 
______________________________________ 
Relatively Toxicity of Pyrolysis Products 
Time to Incapacitation 
Time to Death of 
Sample 
Material of Mice (minutes) 
Mice (minutes) 
______________________________________ 
3-1 Prior Art 
18.5 27.5 .+-. 1.9 
3-6 Present No incapacitation 
No deaths 
Invention 
______________________________________ 
It is apparent that the composite structures of the present invention 
product less toxic fumes than prior art composite structures. 
Mechanical Properties Test 
Two types of bismaleimide-carbon microballoon composites were prepared 
having properties similar to composites No. 1 and No. 2 of Table 1, each 
having a density of 132 kg/m.sup.3. Tests were conducted to compare the 
mechanical properties of the novel composites with the composites of the 
prior art. The test results are reported in Table 7 below. 
As the results indicate, it is now possible to prepare composites, 
according to this invention, that have greater flexural strength both in 
the long beam and short beam configurations than those of the prior art. 
Similarly, improved flatwise tensile strength and compressive strength may 
be achieved. These improvements in mechanical properties are attributed to 
the bismaleimide resin adhesive. 
TABLE 7 
__________________________________________________________________________ 
MECHANICAL TEST DATA (as per MIL-STD-401 B) 
TYPE OF TEST 
Flatwise 
Long Beam Flexure 
Short Beam Flexure 
Flatwise Tension 
Compress- 
Core Shear 
Composite 
Test Load at 
Face Load at 
Face Load at 
Type of 
ion Load at 
Load 
Type of 
Number Number 
Failure 
Compressive 
Failure 
Compressive 
Failure 
Failure 
Failure 
Failure 
Failure 
__________________________________________________________________________ 
1 1 299 1712 199 1515 15.7 core 12.9 7.0 core 
present 
2 297 1699 177 1300 17.0 core 10.7 6.9 core 
invention 
3 295 1687 200 1522 14.7 core 11.7 7.5 core 
mean 297 1699 192 1446 15.8 11.8 7.1 
2 1 177 847 165 1262 5.5 skin to 
33.6 6.4 skin to 
core core 
present 
2 101 576 76 581 7.9 skin to 
32.7 5.5 skin to 
core core 
invention 
3 190 1083 130 993 6.7 skin to 
38.3 6.6 skin to 
core core 
mean 159 615 124 945 6.7 34.9 6.2 
3 1 180 942 123 942 7.0 skin to 
6.5 4.6 skin to 
prior art core core 
__________________________________________________________________________ 
Several properties of a representative prior art composite shall now be 
compared with those of a composite representative of the novel low density 
bismaleimide-carbon microballoon composites of the invention for 
summarization in Table 8, below. From this comparison, it becomes evident 
that while the composite of the invention is slightly poorer than that of 
the prior art in flatwise tensile strength and volatile decomposition 
product at high temperatures, it is as good in terms of several properties 
and more importantly, it excells in such crucial aspects as fire 
endurance, smoke production and relative toxicity of pyrolysis products. 
These latter properties are of course of vital concern in any confined 
space applications. 
TABLE 8 
__________________________________________________________________________ 
SUMMARY OF COMPOSITE PROPERTIES 
Prior Art Invention 
Property (Epoxy-aromatic polyamide) 
(Bismaleimide-Carbon)* 
__________________________________________________________________________ 
Bulk Density Range Kg/m.sup.3 
90-96 100-132 
(ASTDM 1622) 
Flatwise Tensile Strength, at 300.degree. K 
690 500-610 
kN/m.sup.2 (ASTM C-307 
Vertical Burn Test, FAA FAR 25.853 
Passes Passes 
Thermal Conductivity at 300.degree. K, 
1.296 .times. 10.sup.-3 -1.44 .times. 10.sup.-3 
4.932 .times. 10.sup.-4 
W. cm/cm.sup.2 .degree. C (ASTM C-177-45) 
Smoke Density (NBS), 
D.sub.s 1.5 min./53.0 
1.0 
Specific Optical Density 
D.sub.s 4.0 min./58.1 
4.9 
Flaming Condition D.sub.s Max./58.7 
18.2 
Limiting Oxygen Index 
Epoxy-glass:29 Bismaleimide-Glass 
O.sub.2 /(N.sub.2 + O.sub.2) 
Aromatic Polyamide:32 
Polyimide:62 
(ASTM D-2863) Composite:29.6 Carbon Microballoons/ 
Bismaleimide:85 
Bismaleimide-glass:58 
Composite:66.7 
Relative Toxicity of pyrolysis 
Time to Incapacitation: 
products from composite (1.0g) 
18.5 min. &gt;30 min. 
powdered at 40.degree. K/min. to 973.degree. K; 
Time to Death: 
4 Swiss albino mice in 4.2 liter 
27.50 .+-. 1.86 min. 
&gt;30 min. 
exposure chamber, 30 min. 
Fire Endurance, NASA Ames T-3 
2 min. 20 sec. 7 min. 45 sec. 
Thermal Test Facility, Time to 
Reach Back Face Temperature of 
477.degree. K at Front Face Heat Flux 
11 .times. 10.sup.4 W/m.sup.2 
Major volatile products at 296.degree. K 
CO.sub.2 : 85.5 155.3 
from the pyrolysis of composites 
CO : 6.2 14.3 
in vacuum at 973.degree. K for 5 min. Mg 
CH.sub.4 : 9.4 1.3 
of volatile compounds per g of 
HCN : 3.2 5.9 
initial sample C.sub.6 H.sub.6 : 4.1 
3.6 - NH.sub.3 : -- -- 
H.sub.2 : 1.2 
Thermal Stability at 973.degree. K (N.sub.2, 
51.0 53.5 
heating rate 10.degree. K/min.) percent 
weight remaining. 
__________________________________________________________________________ 
*Composition of Composites, Percent by Weight: 
Bismaleimide-glass laminate 14.1 
Bismaleimide Adhesive 5.1 
Carbon microballoons with 2% bismaleimide 50.5 
Bismaleimide-glass laminate 30.3 
The composites that were representative of the invention, and evaluated, 
were similar in structure to composite No. 2, Table 1. 
The invention may be modified for particular applications to include 
various honeycomb core configurations and by modifying the size of the 
carbon microballoons and the concentration of the bismaleimide solutions 
employed. Composite structures having densities of from about 95 
kg/m.sup.3 to about 132 kg/m.sup.3 or more, and with thicknesses in the 
range from 0.4 cm to 2.5 cm, are readily produced. While the preferred 
core structure is a honeycomb having hexoganal cells, any open-pored core 
structure, that can be filled with the carbon microballoons, can be 
employed in practicing the invention. It will also be appreciated that the 
bonding of the core to the face sheet may be accomplished by a variety of 
techniques. 
It therefore will be appreciated that the present invention can be 
implemented in a variety of ways by those skilled in the art to suit 
particular requirements which are within the scope of the invention. While 
the invention has been disclosed herein by reference to the details of 
preferred embodiments, it is to be understood that such disclosure is 
intended in an illustrative, rather than a limiting sense, as it is 
contemplated that various modifications in the construction and 
arrangement of the honeycomb, and in the process of making it, will 
readily occur to those skilled in the art, within the spirit of the 
invention and the scope of the appended claims.