Method for the preparation of reticulate carbon structures

A method for preparing reticulate thermoset resin structures is described. Thermoset or thermosettable resin containing foams, prepared by a method wherein thin membranes dividing contiguous cells in a thin membraned, thick stranded thermoset or thermosettable resin foam with interconnected cells are produced, and are thermally reticulated. The foams are preferably thermally reticulated by providing a combustible gas mixture inside the cells of the foam and then igniting the mixture to destroy the foam membranes. The thermosettable or thermoset reticulate resin structures so produced are particularly useful for preparing carbon structures with the same geometry by heating at elevated temperatures under reducing, inert or vacuum conditions.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for preparing novel thermoset or 
thermosettable reticulate resin structures. It particularly relates to 
reticulate carbon structures derived from the resin structures. 
U.S. Pat. No. 3,927,186 to Vinton and Franklin shows a method for producing 
reticulate carbon structures by rapid firing of reticulate polyurethane 
structures which have been infused with a liquid furan resin or resin 
precursor. U.S. Pat. No. 3,922,334 to Marek shows phenolic resin 
containing structures prepared by infusion of a reticulate polyurethane 
structure with a thermosettable phenolic resin dissolved in 
tetrahydrofuran as a solvent and then carbonization for a longer period. 
These methods work very well but after the infusion step considerable care 
is required to remove unalloyed or non-infused resin from the surfaces of 
the polyurethene reticulate structure. The liquid thermosettable resins or 
resin precursors require an infusion step followed by an excess removal 
step, both entailing handling operations. It would be preferred if the 
thermoset resin structures could be produced directly by foaming and then 
reticulated so that the infusion and excess removal steps could be 
avoided. 
The use of a combustible gas mixture to reticulate polyurethane foams, 
particularly flexible foams as well as other cellular materials, is 
described in U.S. Pat. No. 3,175,025. The method is used commercially 
world-wide for reticulating flexible polyurethane foams by destroying the 
cell membranes; however, it has not been commercially used for other types 
of foams. The polyurethane foams have a specific foam structure with thin 
relatively uniform cross-sectioned cell membranes attached to much thicker 
strands defining the cells and also defining the intersection of membranes 
which are most susceptible to thermal reticulation. The cell membranes 
join the strands with a relatively very small radius of curvature between 
them. As a result the heat from the combustion, even though transient, 
destroys the membranes. On the other hand, thermoset resins usually do not 
form polyurethane-like foams, but rather the cells tend to be spherical 
with relatively irregularly cross-sectioned, usually closed cellular 
partitions between the cells. There is a relatively poor or non-existent 
definition between strands and membranes in the cells. The thermoset resin 
foams of U.S. Pat. No. 3,121,050 to Ford are generally believed to be of 
this type. 
The prior art thermoset resin foams have the advantage of being converted 
to carbon rather than being volatilized during heating. Polyurethane foams 
which are flexible volatilize virtually completely when heated above about 
400.degree. C. and do not carbonize. Rigid polyurethane foams which are 
more cross-linked will carbonize, as shown by U.S. Pat. No. 3,302,999 to 
Mitchell, and are essentially closed cell (unicellular) foams. 
U.S. Pat. Nos. 3,345,440 to Googin et al.; 3,574,548 to Sands et al.; 
3,635,676 to Sands et al.; 3,857,913 to Crow et al.; and 3,975,318 to 
Larsen et al. show unreticulated thermoset resin membrane-containing foams 
some of which are described as being carbonized over relatively long 
periods of time and wherein the carbon product contains essentially all of 
the membranes present in the uncarbonized foam. It would be desirable to 
be able to reduce the carbonization period and preferably at the same time 
produce reticulated carbon structures without having to infuse a 
polyurethane reticulate structure as in U.S. Pat. No. 3,972,186 to Vinton 
et al. 
OBJECTS 
It is therefore an object of the present invention to provide rapidly 
carbonizable reticulate thermoset or thermosettable resin structures. It 
is particularly an object of the present invention to provide a method for 
thermally reticulating thermosettable or thermoset resin foams which have 
a cell structure like a flexible polyurethane foam. Further, it is an 
object of the present invention to provide a preferred method which 
eliminates the need for infusion of a reticulate polyurethane structure 
with a liquid thermosettable resin or resin precursor. These and other 
objects will become increasingly apparent by reference to the following 
description and to the drawing.

DESCRIPTION OF THE INVENTION 
The present invention relates to the process for producing a reticulate 
structure of interconnected strands which comprises: providing a thermoset 
or thermosettable resin containing foam formed with interconnected cells 
with membranes attached to the strands dividing contiguous cells and which 
is carbonizable to a reticulate carbon structure; and thermally destroying 
the membranes in the resin foam to form a reticulate resin structure. 
Preferably the thermal destruction of the membranes is accomplished by 
igniting a combustible gas mixture provided in the cells of the foam so as 
to destroy the membranes and thus thermally reticulate the foam. 
The process of the present invention uses a thermoset or thermosettable 
resin foam (including membranes joined to strands forming cells) as a 
starting material and converts the foam into a non-foam product, herein 
referred to as a "reticulated structure." A foam is cellular since by 
definition the word "cell" means an enclosure having walls. In foams where 
the membranes, which are the walls forming the cell, are removed leaving 
only strands, there is no longer an enclosure and only the very open 
reticulate structure remains which therefore is not a foam or cellular 
structure. The distinction is used herein. 
An important feature of the present invention is that the cells of the 
thermoset or thermosettable resin foam must have a geometry which allows 
them to be thermally reticulated. The cells must be interconnected to 
allow the combustible gas for thermal reticulation to fill the cells and 
surround the membranes. The foam must have the usual flexible polyurethane 
type cell structure with reasonably uniformly thin cross-sectioned 
membranes attached to relatively very thick strands and with openings in 
at least some of the membranes to cause interconnections between the cells 
(so called "open cell" foam). Whether the thermosettable resin is foamed 
or it is infused into an unreticulated foam structure, the cell membranes 
must be substantially thinner in cross-section that the much thicker 
strands which define the intersections of the membranes. Because of this 
fact the process of forming the unreticulated thermosettable or thermoset 
foam is important to achieve satisfactory thermal reticulation and must 
produce a product with a geometry comparable to flexible polyurethane 
foam. 
Preferably the thermoset or thermosettable resin foam which can be 
reticulated is prepared by direct foaming, although infusing an 
unreticulated polyurethane foam in the manner of pending application Ser. 
No. 634,615, filed Nov. 24, 1975, now U.S. Pat. No. 4,022,875, using a 
liquid furan resin or furan resin precursor or in the manner of U.S. Pat. 
No. 3,922,334 to Marek using a phenolic resin in a solvent can be used. If 
infusion is used, care must be taken to remove excess thermosettable resin 
from the surfaces of the polyurethane foam to achieve rapid carbonization. 
The thermosettable resins are directly foamed with a polyisocyanate and 
water or in conjunction with a long chain polyester or polyurethane 
forming polyhydroxyl compound. The foams which eliminate the conventional 
polyester or polyester polyol and use a polyisocyanate reacted directly 
with a liquid thermosettable resin or its precursor are particularly 
unique in that they have the desired geometry for thermal reticulation and 
yet eliminate the conventional polyurethane component which in any event 
is usually vaporized and not carbonized during heating to carbonization 
temperatures. Considerable economy is achieved in producing such foams 
which can be thermally reticulated and are also directly carbonizable to 
unreticulated or reticulated carbon structures. 
As indicated previously, thermal reticulation with heated gases is 
described in U.S. Pat. No. 3,175,025 to Geen et al. Unexpectedly it has 
been found that the method can be applied to selected thermoset or 
thermosettable resin foams to produce reticulate structures which are 
rapidly carbonizable to reticulate carbon structures. As used herein, the 
phrase "rapidly carbonizable" means in less than about 5 hours from the 
time the carbonization of the sample begins (usually about 400.degree. C. 
to 500.degree. C.). This carbonization period is in contrast to the prior 
art methods, particularly with unreticulated thermoset resin foams, which 
require many hours of very slow heating to achieve carbonization without 
causing self-destruction. 
Generally the combustible gas is a mixture of an oxidizable gas and an 
oxidizer gas. Natural gas or hydrogen are the preferred oxidizable gases 
because of their availability; however, many other oxidizable gases are 
suitable. In particular, lower alkanes, containing 1 to 10 carbon atoms, 
individually or in mixture, can be used and these are the preferred 
oxidizable gases. Other oxidizable gases which can be used are for 
instance: ammonia, hydrazine, hydrogen sulfide and various hydrocarbons 
particularly lower alkenes and alkynes such as acetylene and ethylene. It 
will be appreciated that liquid oxidizable materials can be used simply by 
heating them to the gaseous state before introduction into a chamber or by 
heating after introduction into the chamber or by introducing them at 
pressures sufficiently low to cause them to volatilize at the ambient 
chamber temperatures. Suitable oxidizer gases are for instance, oxygen or 
ozone and halogens such as chlorine. It will be appreciated however, that 
oxygen is preferred. Further, it is preferred to use oxidizer materials 
which are gaseous at room temperatures particularly oxygen or oxygen 
enriched air or pressurized air. 
Various sealed chambers can be used for the gaseous combustion as described 
in U.S. Pat. No. 3,175,025 as is known to those skilled in the art. 
Preferably the chamber is a closed rigid chamber. The process of the 
present invention is used to reticulate such thermoset or thermosettable 
resin foamed materials even though they have widely varying cell sizes. It 
was found that the energy produced upon combustion was easily regulated by 
adjusting the pressure of the gaseous mixture in the chamber. It was 
further found that the energy produced upon combustion could be easily 
reduced by the use of non-reactive gaseous diluents, such as nitrogen. The 
process of the present invention can be used to reticulate the structures 
while the thermoset resin foam is uncured or when it is cured. 
Carbonization of the thermosettable or thermoset resin reticulate 
structures is achieved at temperatures in excess of about 400.degree. C. 
under neutral, reducing or vacuum conditions as described in detail by the 
prior art for carbonization in general. A vitreous reticulate carbon 
structure is preferred which starts to form at temperatures above about 
500.degree. C. 
SPECIFIC DESCRIPTION 
Examples I, II and VII show the thermal reticulation of thermosettable or 
thermoset resin infused foams. Examples III to VI show the reticulation of 
directly foamed thermosettable or thermoset resins. As will be seen from 
the Examples, there are many variations of the method of the present 
invention. 
EXAMPLE I 
A sample of an unreticulated polyester polyurethane foam having a bulk 
density of about 0.029 gms/cc and about 40 pores per cm was infused with 
furfuryl alcohol catalyzed with 1% of methyl paratoluene sulfonate by 
weight for 10 minutes and excess solution was then removed by squeezing 
against a screen. The furfuryl alcohol was cured to a furan resin in the 
foam by heating in an oven at 150.degree. C. for 5 hours. The infused and 
cured sample had polyurethane-like foam membranes as shown in FIG. 2. 
The sample was then placed in an enclosed sealed rigid chamber in the 
manner of U.S. Pat. No. 3,175,025. A vacuum pump was then used to evacuate 
the air from the chamber. The chamber was then pressurized to 15 psig 
(2.02 atmospheres) with a 3 to 1 by volume mixture of hydrogen and oxygen, 
which is a reducing atmosphere, and then ignited. The combustion of the 
gas removed the cell membranes, leaving the strands intact to form a 
reticulate structure as shown in FIG. 3. 
The sample was then placed in a retort at room temperature and heated to 
1000.degree. C. and carbonized over a period of 4 hours in a reducing 
atmosphere, after which it was held at 1000.degree. C. for 1 hour. The 
rate of heating was about 250.degree. C. per hour. Upon removal from the 
furnace, the reticulated carbon product was found to be about the same 
volume as the original non-infused (unreticulated) foam bulk volume, as 
shown in FIG. 4. 
EXAMPLE II 
The polyurethane foam of Example I was infused with furfuryl alcohol mixed 
with 1% by weight methyl paratoluene sulfonate as the catalyst and looked 
like the product of FIG. 2. The uncured, infused samples were reticulated 
as in Example I with a 20, 30, 40 psig (2.4; 3.0 and 3.7 atmospheres, 
respectively) mixture of 3:1 by volume hydrogen and oxygen. The 20 psig 
sample was not satisfactorily reticulated, but the other two samples were 
reticulated, as shown in FIG. 5. The reticulated samples were then cured 
and then placed in a furnace with a reducing atmosphere and heated, as in 
Example I. 
The carbon product, reticulated at 30 psig, had a bulk density of 0.063 
g/cc and a crushing strength of 26 psi (1.8 kilograms per square 
centimeter). The sample reticulated at 40 psig had a bulk density of 0.076 
gm/cc and a crushing strength of 31 psi (2.2 kilograms per square 
centimeter), as shown in FIG. 6. An unreticulated carbon foam comparative 
control had a bulk density of 0.088 gm/cc and a crushing strength of 23 
psi (1.6 kilograms per square centimeter). 
EXAMPLE III 
A block of foam was made by mixing in the following order: 
100 gms of Hypol.TM. FHP 3000 made by W. R. Grace Co. (a polyurethane 
prepolymer having the following characteristics): 
Equivalent weight/NCO group of 400 to 450 
Density at 25.degree. C. of 1.15 g/ml 
Viscosity in cps at 25.degree. C. of 15,000 to 20,000 
Nco content meq/g of 2.2 to 2.5 
100 gms of Durez.TM. 14383 furfuryl alcohol resin prepolymer (which had 
been on the shelf at room temperature for about 5 years and thus was very 
viscous) 
100 gms of water with 2% by weight paratoluene sulfonic acid catalyst; and 
4 drops of 1034 silicone surfactant made by General Electric Co. 
The resultant foam was then set aside for a day to partially cure and then 
totally cured at 150.degree. C. for 5 hours, as shown in FIG. 7. 
A sample was cut from the block of foam and reticulated at 15 psig (2.02 
atmospheres) with a 3:1 by volume mixture of hydrogen and oxygen, as shown 
in FIG. 8. 
The reticulated structure was then carbonized in a reducing atmosphere in a 
furnace as in Example I, as shown in FIG. 9. The final bulk density of the 
carbon structure was 0.131 gm/cc and the maximum crushing strength was 
about 160 psi (11.3 kg/sq cm.). For comparative purposes the parent 
(unreticulated) foam from the same block was fired at 1000.degree. C. to 
carbonize it. Its maximum crushing strength was 130 psi (9.2 kg/sq cm) and 
it had a bulk density of 0.129 gm/cc. 
EXAMPLE IV 
A foam sample was made by mixing: 40 gms of Durez.TM. 14383 resin 
prepolymer as used in Example III; 20 gms toluene diisocyanate; and 1 gm 
of a 50/50 by weight ethanol and paratoluene sulfonic acid mixture as the 
catalyst. No conventional polyurethane resin polyol was used. 
Using the procedure of Example III, the foam was reticulated at 1 
atmosphere with a 3:1 by volume hydrogen and oxygen mixture, then cured at 
150.degree. C. for 5 hours and then carbonized at 1000.degree. C. as in 
Example I. The final carbon product bulk density was 0.026 gm/cc and the 
crushing strength was about 2 psi (0.14 kg/sq cm). The corresponding 
unreticulated carbon foam had a bulk density of 0.025 gm/cc and a maximum 
crushing strength of about 1 psi (0.07 kg/sq cm). 
EXAMPLE V 
A sample was made by mixing: 
30 gms of Durez.TM. 14383 resin as used in Example III 
15 gm toluene diisocyanate; and 
1 gm of a 50/50 by weight ethanol and paratoluene sulfonic acid mixture. 
The sample was cured at 150.degree. C. for 5 hours to a rigid foam, as 
shown in FIG. 10, then reticulated at 15 psig (2.02 atmospheres) with a 
3:1 by volume mixture of hydrogen and oxygen, as shown in FIG. 11. It was 
then fired under reducing conditions at 1000.degree. C. in the same 
fashion as Example I. The final product bulk density was 0.053 gm/cc and 
had a maximum crushing strength of about 6 psi (0.42 kg/sq cm) and is 
shown in FIG. 12. 
EXAMPLE VI 
A foam was prepared using the Durez.TM. 14383 resin of Example III and 
toluene diisocyanate without a catalyst. Forty gms of the Durez.TM. resin 
was added to 20 gms of toluene diisocyanate. It took about 3 minutes for 
the two to be completely mixed and about 4 minutes for the mixture to 
start foaming. After about 10 minutes the foam had rigidified. It was 
brittle and it was difficult to reticulate. The foam was thermally treated 
at 1 atmosphere with a 3:1 by volume hydrogen to oxygen mixture, and was 
destroyed. Nitrogen was added as a diluent to the hydrogen and oxygen 
mixture to reduce its energy per unit volume. Another sample of foam was 
reticulated at 10 psig (1.7 atmospheres) with a mixture of 3 parts 
hydrogen, 1 part oxygen, and 4 parts nitrogen. This was found to be about 
the maximum energy level that could be used without destroying the sample. 
The 10 psig pressure of the combustible gas partially reticulated the 
foam. The sample was then carbonized under reducing conditions at 
1000.degree. C. in a retort. The final bulk density of the carbon product 
was 0.07 gm/cc and it had a maximum crushing strength of about 13 psi 
(0.91 kg/sq cm). 
Polyurethane foams containing furan polymers, such as described by Sands in 
U.S. Pat. No. 3,574,540, are suitable as starting foams for this invention 
and can be thermally reticulated in the usual fashion and subsequently 
carbonized to a reticulate carbon structure. Care must be taken not to 
damage the foams during reticulation if they are only partially cured. 
EXAMPLE VII 
Using the procedure of U.S. Pat. No. 3,922,334 to Marek, an unreticulated 
polyester polyurethane foam having about 4 pores per cm and having a bulk 
density of 0.029 gms/cc was infused with a phenolic resin in 
tetrahydrofuran (THF). Excess impregnant solution was removed by squeezing 
from the surfaces of the foam. The sample was dried at a temperature of 
about 50.degree. C. for 4 hours to volatilize and remove the THF. The 
sample was then reticulated with a combustible gas mixture. The sample was 
then carbonized in the fashion of Example I. 
The process of the present invention rapidly produces the reticulate carbon 
structures, usually in less than about 5 hours. The carbon products are 
comparable in form to those shown in U.S. Pat. No. 3,927,186.