Fine, carbon-graphite fibers do not combust during the combustion of a composite and are expelled into the air as fine conductive particles. Coating of the fibers with a salt of a metal having a work function below 4.2 eV such as an alkaline earth metal salt, e.g., calcium acetate, catalytically enhances combustion of the fibers at temperatures below 1000.degree. C. such that the fibers self-support combustion and burn to produce a non-conductive ash. Fire-polishing the fibers before application of the coating is desirable to remove sizing to expose the carbon surface to the catalyst

Dowler, Warren L.

Humphrey, Marshall F.

Ramohalli, Kumar N. R.

NASA Technical Reports Server

United States Patent [19] 
Humphrey et al. [45] Jan. 19, I982 
[54] GASIFIABLE CARBON-GRAPHITE FIBERS 
[75] Inventors: Marshall F. Humphrey, Duarte; 
Kumar N. R. RamohaIIi, Pasadena; 
Warren L. Dowler, Sierra Madre, all 
of Calif. 
[73] Assignee: California Institute of Technology, 
Pasadena, Calif. 
[2I] Appl. No.: 30,836 
1221 Filed: Apr. 17, 1979 
[51] Int. C l . 3  .............................................. C08L 61/06 
[52] U.S. Cl. ................................. 260/38; 260/37 EP; 
260/37 N; 260/37 SB; 260/40 R 
[58] Field of Search ................ 260/37 EP, 37 SB, 38, 
260/40 R, 37 N; 106/307; 427/215, 35, 37; 
428/367, DIG. 902, DIG. 903 
r561 References Cited 
U.S. PATENT DOCUMENTS 
3,767,774 10,4973 Hou ..................................... I06/307 
3,833,402 9/1974 Elban et al. ......................... 428/367 
3,876,444 4/1975 McKee ................................ 428/367 
4,146,668 3/1979 Dorey et al. ........................ 428/367 
Primary Examiner-Lewis T. Jacobs 
Attorney, Agent, or Firm-Marvin E. Jacobs 
c57l ABSTRACT 
Fine, carbon-graphite fibers do not combust during the 
combustion of a composite and are expelled into the air 
as fine conductive particIes. Coating of the fibers with a 
salt of a metal having a work function below 4.2 eV 
such as an alkaline earth metal salt, e.g., calcium acetate, 
catalytically enhances combustion of the fibers at tem- 
peratures below looo" C. such that the fibers self-sup- 
port combustion and bum to produce a non-conductive 
ash. Fire-polishing the fibers before application of the 
coating is desirable to remove sizing to expose the car- 
bon surface to the catalyst. 
14 Claims, 2 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=20080004231 2019-08-30T02:17:47+00:00Z
Jan. 19, 1982 3 
T-300 GRAPHITE FIBER 
TEMPERATURE "C 
I? 
NO TREATMENT 
EPOXY RES1 N 
5% CaAc IN 
EPOXY RESIN 
T 
cn 
I- z 
3 
LL 
s 
0 FIBER SOAKED 
(1: SAT CaAc + 10% 
W IN EPOXY RESIN m 
s 
I, 
2 
TIME, sec 
4,311,630 
2 
quently. Such an event could cause disastrous conse- 
quences at or near airport, industrial or residential areas. 
Since the surface temperature of combustion (fast 
oxidation) of graphite is in the vicinity of 1300" C., fast 
oxidation of graphite is hardly reached by the simple 
combustion of a composite panel which occurs at typi- 
cal surface temperature of 400"-500" c. Also, even if 
the requisite temperatures are reached, the rates of com- 
bustion (oxidation) are too low compared to the same 
rates for the resin. This has the practical implication 
that the resin burns away fast leaving behind the graph- 
ite fibers that do not combust in the absence of the 
supporting flame. The fiber diameter of 8p. presents a 
2500 cm2 surface area per gram of mass. This is very 
large and leads to rapid heat loss and is conducive to 
early extinction even if the combustion is initiated. 
An additional property of the carbon fiber is the "red 
heat" behavior. It should be emphasized that in a short- 
ing situation a single carbon fiber is most difficult to 
burn or consume. Rather the literature suggests that the 
carbon fiber becomes a glowing filament and does not 
pyrolyze or burn at least to about 2300" K. And even 
above that temperature adequate air circulation is re- 
quired to consume the fiber fully. A minimum of 16 
grams of oxygen are needed to consume 12 grams of 
carbon, and hence in a closed area such as in the chassis 
of an electronic system, lack of air circulation and suffi- 
ciently high voltages may cause the fibers to develop a 
"red heat" condition and ignite adjacent flammable 
plastics and the like. 
In order to permit such widespread use of graphite 
composites, the recognized electrical hazards must be 
overcome economically without sacrificing or compro- 
mising the proven good features (strength, weight and 
cost). This aim should preferably be achieved so that 
the fiber and composite are compatible with state of the 
art processing and equipment. Furthermore, modifica- 
tion of the fiber by coating or treatment must provide a 
good bond to the fiber and to the resin matrix. 
There have been many proposals to prevent release 
of electricdly conducting graphite fragments from 
composites in a fire situation. Some approaches have 
been to gasify the fibers by oxidation or hydrogenera- 
tion, clump or retain the fibers so that they do not be- 
come airborne, insulate the fibers so no hazards are 
caused even if the fibers are airborne, resin modifica- 
tions, alternate fibers, secondary fiber inclusions or 
larger fibers which would be too heavy for aerosol 
formation. 
Most of these approaches would require several years 
of investigation at considerable cost, would necessitate 
qualification of the composite estimated to require 
several years and would result in substantial modifica- 
tion of properties of the composite and processing tech- 
niques for fabricating composites. 
STATEMENT O F  INVENTION 
Coating of carbon-graphite fibers with a salt of a 
metal having a work function below 4.2 eV such as an 
alkaline earth metal salt catalytically enhances combus- 
tion of the fibers at temperatures below 1000" C. such 
that the fibers self-support combustion and burn to pro- 
duce a non-conductive ash even in the absence of an 
external flame. The mechanical properties of the fibers 
and of the composite are not affected by the presence of 
the coating. The coating is present in an amount of 0.1 
to 10% by weight which does not entail a significant 
weight penalty. The coating is compatible with binder 
1 
GASIFIABLE CARBON-GRAPHITE FIBERS 
DESCRIPTION 
Origin of the Invention 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 83-568 
(72 Stat. 435; 42 USC 2457). 
TECHNICAL FIELD 
The present invention relates to coated carbon- 
graphite fibers and, more particularly, to such fibers 
containing a catalytic coating capable of combustion of 
such fibers in a burning composite. 
BACKGROUND ART 
Considerable interest exists in the future use of car- 
bon-graphite fibers due to their light weight and high 
strength. Presently, about 30 pounds of graphite are 
being utilized per plane in manufacture of several exist- 
ing aircraft and future projected use is IO00 Ibdaircraft. 
Due to the need to reduce weight of automobiles to 
increase fuel efficiency, use in cars is also expected to 
increase dramatically over the next decade. The pro- 
jected annual use of graphite fibers by 1990 is as follows: 
TABLE 1 
industry Pounds 
Aerospace 106 
Automobile io9 
Consumer IO6 
Commercial carbon fiber is usually sold as a stranded 
material or as a woven cloth, having from 100 to 10,OOO 
discrete thin fibers per strand. These fibers are prepared 
by heating a precursor such as rayon, pitch or poly- 
acrylonitrile fiber to carbonize the fibers followed by a 
high temperature (2000"-3000" C.) graphitization treat- 
ment in absence of oxygen during which it is believed 
that the carbon atoms rearrange into a hexagonal struc- 
ture. The industry has developed fine strand multifila- 
ment products as the result of difficulties in manufactur- 
ing large diameter fiber of sufficiently high molulus. It 
will be noted that an extremely small fiber diameter is 
now the industry standard, and is not predicted to 
change very much in the immediate future: 
Carbon Fiber Diameter-6.5 to 13 microns 
Modulus-50 million psi 
Fall Rate-about 2 c d s e c .  
Resistivity- IO00 ohms/cm. 
Burnout-0.5 to 1.0 watt/cm. 
Contact Voltage Drop-2 to 5 volts 
Recently a significant hazard has been recognized 
that could prevent the widespread use of graphite fi- 
bers. The fine fibers are conductive and are not oxidized 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
nor vaporized at the temperatures experienced during a 60 
typical fire. During a fire the epoxy resin binder is con- 
sumed at 400"-600" C. Fine graphite fibers and frag- 
ments are expelled from the composite, are entrained in 
the air and form aerosols. The aerosols can travel signif- 
icant distances, invade or settle in unprotected electrical 65 
or electronic equipment and cause shorting, equipment 
failure, power faiiure and blackouts. Automobile fires 
are quite a common event and aircraft fires occur fre- 
3 
4,3 1 1,630 
resins and its presence does not require modification of 
state-of-the-art fiber and composite processing tech- 
niques nor fabrication equipment. This will facilitate 
quick transfer of the technology to industrial practice. 
These and many other features and attendant advan- 5 
tages of the invention will become readily apparent as 
the invention becomes better understood by reference 
to the following detailed description when considered 
in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a series of thermal gravimetric analysis 
FIG. 2 is a series of burn test curves. 
10 
curves; and 
15 
DETAILED DESCRIPTION OF T H E  
INVENTION 
The catalytic coatings in accordance with this inven- 
tion are salts of electronegative metals having a work 
function less than 4.2 eV. The metals can be selected 20 
from Groups I to VI11 of the Periodic Table. However, 
the most cost effective catalysts are the alkaline earth 
metals, particularly barium, calcium and strontium. The 
anions can be inorganic or organic except for the anions 
that are fire suppressants such as fluorine, bromine, 25 
iodine, chlorine, phosphate or borate. Typical anions 
are carbonate, nitrate, hydroxyl, sulfate or salts of or- 
ganic acids containing from 2 to 20 carbon atoms such 
as acetates, gluconates, acrylates, tartrates, acetoaceton- 
ates or mixtures thereof. 30 
The coating is present in an amount from at least 
0.1 % to provide significant catalytic combustion en- 
hancement and amounts above 10% add too much 
weight penalty by weight to the fiber. Generally, effec- 
tive gasification can be achieved with from 0.5 to 5% by 35 
weight of the catalyst. It has further been discovered 
that the rate of gasification is further enhanced by add- 
ing from 0.1 to 20% by weight of the catalyst to the 
binder resin of the composite. 
The coating should be applied directly to the surface 40 
of the fiber. Therefore any coating on the fiber such as 
polyvinyl alcohol sizing should be removed by pretreat- 
ing the fiber in an inert gas arc or plasma under vacuum. 
This improves the catalytic effect and the mechanical 
properties of the composite by insuring an even coating 
firmly adhered to the surface of the fiber. The fibers are 
carbon-graphite monofilaments containing from at least 
10% graphite up to 100% graphite. The graphite forms 
a conductive coating on the exterior surface of the fi- 
bers. The graphite fibers have a diameter of at least 1 
micron. Fibers having diameter above 30 microns are 
too heavy to entrain in air. The fibers are usually pro- 
vided as a multifilament yarn containing 100 to 3000 
filaments per strand and can be woven into cloth. 
The coating can be applied to the fiber by dipping, 
boiling, ion plating, vapor deposition, sputtering or 
since the fiber is conductive by electrodeposition. For 
reasons of economics and ease of application the coat- 
ing is preferably applied to the fiber from a solution of 
the catalytic metal salt followed by evaporation of the 
solvent to deposit a coating of the salt on the surface of 
the fiber. The fiber should be observed to assure that 
salt crystals have not deposited qn the surface. The 
solution can be a dilute solution containing as little as 
0.1 to 10 mole percent or concentrated solutions up to 
saturation can be utilized. The solution can be applied 
by dipping the fiber into a bath of the solution or by 
brushing or spraying the solution onto the fiber whether 
45 
50 
55 
60 
65 
4 
in monofilament, multifilament, strand or woven form. 
The solution should penetrate, wet and coat each indi- 
vidual monofilament. 
The composites usually contain from 30% up to 70% 
of graphite fiber reinforcement, typically from 40% to 
60% graphite fibers dispersed in the matrix resin. Vari- 
ous resins can be utilized such as epoxy, polyester, phe- 
nolic, silicone or polyimide resins. The most widely 
used and qualified resin are Bisphenol A epoxy resins 
cured with from 10 to 30% of diamine curing agent 
hardeners. 
The development of the present invention was 
greatly aided by the development of a conductive fiber 
test apparatus. Use of this apparatus made it possible to 
eliminate shorting effects due to carbon debris from the 
degradation products of the matrix resin and to directly 
monitor the extent of graphite fiber fragment released 
during combustion. The detector includes a two layer 
interlaced comb-grid which allows the combustion 
products of graphite fiber to pass through but is immune 
to spurious shorts caused by soot and moisture. The test 
apparatus provides real-time data on release of graphite 
fibers during combustion in a, propane flame. 
Experiments were conducted with Thornel graphite 
fiber T-300 in the form of a monofilament or multifila- 
ment twisted strand of 3000 monofilaments having a 
diameter of 8 microns. 
EXAMPLE 1 
T-300 strands were pretreated by heating the fiber in 
an Argon arc for about thirty (30) minutes at about 1 
torr. The fiber was then dipped into a bath of a concen- 
trated aqueous solution of calcium acetate (CaAc), re- 
moved and dried to deposit about 2.5% by weight of 
calcium acetate. 
The coated fibers and uncoated T-300 was subjected 
to thermogravimetric analysis at a heating rate of 20" 
C./minute. Referring now to FIG. 1, it is observed that 
Ca+ + treated fiber demonstrated greatly enhanced rate 
of degradation (as indicated by the slope of the curve). 
EXAMPLE 2 
Strands of 67% of calcium acetate coated T-300 fi- 
bers prepared according to Example 1 were mixed with 
33% of an epoxy resin containing 10% calcium acetate; 
a second sample was prepared using 67% of untreated 
and uncoated fibers mixed with 33% of an epoxy resin 
containing 5% calcium acetate; these prepared compos- 
ites were used for subsequent burning experiments. In 
all cases, the epoxy used was an 80/20 mixture of RF- 
3000/RF-61 (E. V. Roberts Associates, Culver City, 
Calif.). This resin was applied to the fiber bundle with a 
spatula, then worked into the fibers with the hand. All 
excess resin was then squeezed out. The epoxy was then 
cured for a minimum of 2$ hours at 140" F. Burning 
tests were also conducted on these untreated compos- 
ites: an epoxy resin composite and an Ames charing 
resin. 
Two burning tests involved the use of a butane torch 
(Bernzomatic). The flame was adjusted so that the light 
blue portion of the flame was approximately 25 mm 
long. The fibers were held so that the end of the fiber 
bundle was about one to two millimeters above the tip 
of the light blue portion of the flame. The escaping 
fibers and binder debris were drafted up a 75 millimeter 
diameter glass cylinder and across the bar grid of the 
electrical detector described earlier. The time interval 
from heating to the detection of electrical shorts was 
% 
4.3 1 1,630 
5 
measured as an indication of electrical hazard. The 
escaping fibers would short on the grid and caused an 
integrated reading on a chart recorder. In this text, the 
number of counts totaled in a given time was used as a 
quantitative indicator of escaping fibers. 
Referring now to FIG. 2. it is seen that the epoxy 
resin evolved the most fiber counts. The calcium ace- 
tate (CaAc) filled resin initially showed low counts 
which increased significantly after 160 seconds. The 
treated fiber-CaAc filled composite showed very low 
counts initially and after I60 seconds. 
The composites of Examples 1 and 2 were soaked in 
water at 180" F. and then subjected to short beam shear 
stress. The composite containing 10% CaAc and CaAc 
treated fibers showed smaller loss in shear values than 
the control (untreated) or the sample containing 5% 
CaAc in the resin (uncoated fibers) with water treat- 
ment. Differential scanning calorimeter (DSC) analysis 
of the material of Example 1 and of the untreated graph- 
ite showed no reaction for the untreated graphite while 
the coated graphite exhibits large exotherms due to 
various chemical reactions. Thermogravimetric analy- 
sis (TGA) and DSC for calcium acetate (CaAc) shows 
three stages of reaction which are believed to correlate 
to evolution of water, evolution of acetone forming 
CaC03, and evolution of C02 to form CaO ash. Autoig- 
nition occurs at 467" C. DSC comparison of the alkaline 
earth metal acetates show that CaAc exhibits the largest 
exotherm, followed by SrAc, BaAc and MgAc. 
The catalytic coated fibers of this invention demon- 
strate self-sustained fiber gasification to a non-conduc- 
tive ash. The catalytic coating at small cost and weight 
penalty provides significant improvement in the allevia- 
tion of short circuits in burned graphite composites. 
It is to be realized that only preferred embodiments of 
the invention have been described and that numerous 
substitutions, modification and alterations are uermissi- 
6 
!. A composition of matter comprising: graphite fi- 
bers having a diameter from I to 30 microns coated with 
1.0 to 10% by weight of a salt of metal having a work 
function below 4.2 eV and said fibers being embedded 
2. A composition according to claim 1 in which the 
fibers are in the form of a multifilament yarn. 
3. A composition according to claim 1 in which the 
coating is present in an amount from 1% to 5% by 
4. A composition according to claim 1 in which the 
binder resin is selected from epoxy, polyester, phenolic, 
silicone or  polyimide resins. 
5. A composition according to claim 4 in which the 
6. A composition according to claim 4 in which the 
7. A composition according to claim 6 in which the 
8. A composition according to claim 7 in which the 
9. A composition according to claim 7 in which the 
10. A composition according to claim 1 further in- 
25 cluding 0.1 to 10% by weight of the salt dispersed in the 
111. A method of forming a gasifiable fiber comprising 
applying a coating of a salt having a work function 
below 4.2 eV to the exterior surface of a fine graph- 
ite fiber; and dispersing the coated fibers in a binder 
resin and curing the resin to form a composite. 
12. A method according to claim 11 in which the salt 
is applied to the fiber from a solution and further includ- 
13. A method according to claim 11 further including 
the step of pretreating the fiber by subjecting the fiber 
to an arc or plasma before applying the solution thereto. 
14. A method according to claim 11 further including 
5 in a binder resin. 
10 weight of the fibers. 
15 resin is an epoxy. 
coating material is an alkaline earth metal salt. 
anion of the salt is an organic acid. 
anion is acetate. 
metal is selected from calcium, barium and strontium. 
20 
binder resin. 
the steps of: 
30 
35 ing the step of drying the solution to form a coating. 
ble without department from the spirit and scope of the 40 the step of dispersing 0.1 to 20% by weight of the salt in 
invention as defined in the following claims. the binder resin. 
We claim: * * * * *  
45 
50 
55 
60 
65 


Gasifiable carbon-graphite fibers

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