Oxidation experiments were conducted with some experimental and commercial mechanical carbons at 650 C in dry air flowing at 28 cc/sec (STP). In general, purification of these carbon-graphites with chlorine at 2800 C improved oxidation resistance. Additional improvements in oxidation resistance were obtained from purification followed by an antioxidant (zinc phosphate) treatment. For the commercial materials, purification alone gave greater oxidation resistance than the antioxidant treatment alone. The reverse, however, was the case for the experimental materials

Allen, G. P.

Wisander, D. W.

English

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NASA TECHNICAL NOTE NASA TN D-7688
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PUIF 7688) CAFFC' OF CHLORINE
SOLCI C I OIDATION RESISAuCE OF J74-2 69 8 {51~ '~ ECHANICAL CABISS (NASA) C-26987
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H1/15 41968
EFFECT OF CHLORINE PURIFICATION
ON OXIDATION RESISTANCE OF
SOME MECHANICAL CARBONS
by Donald W. Wisander and Gordon P. Allen
Lewis Research Center o
Cleveland, Ohio 44135
> 6 - 1 9 1 7
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION * WASHINGTON, D. C. * JUNE 1974
https://ntrs.nasa.gov/search.jsp?R=19740018874 2020-03-23T08:44:48+00:00Z
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TN D-7688
4. Title and Subtitle 5. Report Date
EFFECT OF CHLORINE PURIFICATION ON OXIDATION, June 1974
RESISTANCE OF SOME MECHANICAL CARBONS 6. Performing Organization Code
7. Author(s) 8. Performing Organization Report No.
Donald W. Wisander and Gordon P. Allen E-7809
10. Work Unit No.
9. Performing Organization Name and Address 501-24
Lewis Research Center 11. Contract or Grant No.
National Aeronautics and Space Administration
Cleveland, Ohio 44135 13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address Technical Note
National Aeronautics and Space Administration 14. Sponsoring Agency Code
Washington, D.C. 20546
15. Supplementary Notes
16. Abstract
Oxidation experiments were conducted with some experimental and commercial mechanical
carbons at 6500 C in dry air flowing at 28 cc/sec (STP). In general, purification of these
carbon-graphites with chlorine at 28000 C improved oxidation resistance. Additional
improvements in oxidation resistance were obtained from purification followed by an anti-
oxidant (zinc phosphate) treatment. For the commercial materials, purification alone gave
greater oxidation resistance than the antioxidant treatment alone. The reverse, however,
was the case for the experimental materials.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Oxidation resistance; carbon; graphite; Unclassified - unlimited
purification; inhibitors; seal materials; Category 15
additives; antioxidants; boron carbide;
zinc phosphate; mechanical carbons
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price*
Unclassified Unclassified /9 $3. 00
* For sale by the National Technical Information Service, Springfield, Virginia 22151
EFFECT OF CHLORINE PURIFICATION ON OXIDATION
RESISTANCE OF SOME MECHANICAL CARBONS
by Donald W. Wisander and Gordon P. Allen
Lewis Research Center
SUMMARY
Experiments were conducted on some commercial and experimental carbon-
graphites to determine the effect of purification (with chlorine at 28000 C) on oxidation
resistance. Results are presented for specimens at 6500 C in dry (100 ppm H20) air
flowing at 28 cubic centimeters per second STP.
The materials used in this investigation were selected from a previous oxidation
study. Two of the three experimental materials were boronated (B4C containing) carbon-
graphites. All materials were studied without and with antioxidant (zinc phosphate)
treatment ("phosphated") after purification.
The commercial materials showed an improvement in oxidation resistance due to
purification. Also the nonboronated experimental material showed an increase in oxida-
tion resistance, but the boronated materials showed no significant increase. (However,
even without an increase, the boronated material had the longest life.)
In general, impregnation with zinc phosphate ("phosphating") improved oxidation
resistance of the purified and unpurified materials.
In general, purification before phosphating provided an improvement in oxidation
resistance.
INTRODUCTION
Advanced aircraft turbine engines require dynamic shaft seals to operate in air at
temperatures up to 6500 C. Mechanical carbons have advantages over other seal mate-
rials at high temperatures; these are high strength to mass ratios, very high sublimation
temperature (>35000 C) and low elastic moduli (conformability to mating surfaces). The
only major disadvantage is relatively poor oxidation resistance.
One factor that affects the oxidation resistance (of carbon-graphites) is the catalytic
effect of impurities. In general, commercial materials contain appreciable amounts of
1
impurities as indicated by ash contents of over 1 percent in extreme cases (ref. 1).
Reference 2 indicates that the oxidation rate of carbon is increased by most elements
found as impurities.
The purpose of this investigation was to determine the effect of purification on the
oxidation life (time to lose weight equivalent to 0. 05 cc from 6.45 cm of surface) of
some commercial and experimental mechanical carbons. Most of the nMterials investi-
gated were selected from a previous study (ref. 3) in which oxidation resistance had been
shown to be improved by inclusion of B4 C and by subsequent zinc phosphate treatment.
Results are presented for materials oxidized at 6500 C in air flowing at 28 cubic
centimeters per second STP (standard temperature and pressure).
APPARATUS AND PROCEDURE
The specimens used were hemispherically tipped cylinders (friction specimens)
(fig. 1). The nominal hemisphere and cylinder radii were the same, 0. 475 centimeter,
and the nominal length was 1.9 centimeters. A hole was drilled near the base of the
hemisphere to admit a Chromel wire hook.
The volume and apparent (or geometrical) surface area was determined. After pur-
ification and/or phosphating, the specimens were submerged in n-heptane (ASTM refer-
ence fuel) and subjected to ultrasonic cleaning to remove loose material and contami-
nants. They were then evacuated (20 to 30 torr) for at least 36 hours. High-purity ni-
trogen was used to repressurize the vacuum chamber. The specimens were weighed
just before use.
A schematic of the oxidation apparatus is shown in figure 1. Specimen weight was
continuously measured with a recording microbalance. The specimen was heated by an
induction heated susceptor located in a 5-centimeter-diameter pyrex tube. Specimen
temperature was monitored with an infrared pyrometer. The pyrometer emissivity
setting was 0.85, established by thermocouple measurement on a carbon-graphite speci-
men. This emissivity is in the range expected for a rough graphite surface (ref. 4).
The error due to incorrect estimate of emissivity is probably low since varying the
emissivity setting by 0.05 changes the indicated temperature by 70 at 6500 C. The py-
rometer output was electrically coupled to a temperature controller which maintained
the specimen temperature within an indicated maximum deviation of 100 C.
Specimens were suspended from one arm of the microbalance by a preoxidized
Chromel wire. The size and shape of the holes appeared to be unchanged indicating no
catalytic effect from the wire. The test envelope was purged with high purity nitrogen
gas until the test temperature was reached. The nitrogen flow was then replaced by
2
clean, dry (to about 100 ppm water) air flowing at 28 cubic centimeters per second
(STP). The dry (room temperature) air was directed to the induction heated specimen.
Previous observations indicated that the major oxidation loss occurs in the pores; the
air temperature has a negligible effect on oxidation. Those specimens for which the
mean cylinder radius was remeasured after testing showed no apparent loss from the
geometrical surface. Oxidation was continued until about 5 percent of the specimen
weight was lost. The time required to lose a weight equivalent to 0. 05 cubic centimeter
(p x 0. 05 cc) was determined for each specimen. This time was multiplied by the ratio
apparent surface area (sq cm)/6. 45 to give an oxidation life that is a normalized 5 per-
cent loss.
MATERIALS
The commercial carbon-graphite materials, designated A and B, were mechanical
base grades which, according to the manufacturers, contain no antioxidant or other
additives. Some properties are presented in table I.
The experimental materials (prefix E) are part of a series reported in reference 3.
Properties of these materials are also presented in table I. These materials contain
30 pph of particulates plus 70 pph of binder (no. 30 medium hard pitch). The particu-
lates consist of 80 pbw CHP graphite, 20 pbw thermax and 0, 5, or 7. 5 pbw B4 C (the 0,
5, or 7. 5 in the designation). The materials designated EP were impregnated with
additional pitch (see step 8 under Base Material Processing). The numbers 22 and 28
denote the graphitization temperatures (22000 and 28000 C) of the compacts.
Base Material Processing
The following outline describes the processing of the experimental materials (More
detailed information is available in ref. 5.):
(1) Blend particulates except B4 C.
(2) Preheat particulates in mixer, add pitch (binder), and mix (basic E-mix).
(3) Pelletize the mix, then cool.
(4) Micromill the mix.
(5a) For nonboronated materials, blend the milled mix.
(5b) For boronated materials, preblend half and half B4 C and E-mix, add the re-
maining required E-mix and blend, screen, and reblend.
(6) Mold blended material at 1500 C and 345 N/cm 2 (green compact).
(7) Pack green compacts with coke, cover with charcoal, and bake. Heat rapidly
to 500 C; heat at 22 C per hour to 500 C then at 50 C per hour to 10000 C, and hold
3
for 4 hours (carbonized compact).
(8) Impregnate with pitch if desired (EP materials) at 70 N/cm 2 . Heat at 5° C per
hour to 10000 C and hold 4 hours.
(9) Pack the carbonized compacts (from step 7 or 8) with graphite and heat to the
graphitization temperature indicated for each material. Heat at 4000 C per hour to
9000 C; 2000 C per hour to 16000 C; 3000 C per hour to final temperature and hold for
1 hour.
Purification Treatment
The purification was a commercial process. Specimens were heated to 28000 C in
a chlorine atmosphere and held at temperature for 1/2 hour. During this treatment, im-
purities are converted to chlorides which are vaporized and removed. Reference 1 indi-
cates that graphite does not react with chlorine.
Antioxidant Treatment
The antioxidant treatment done on some of the specimens can be considered to de-
posit zinc phosphate. Details are as follows:
(1) Carbon specimens are placed in a closed stainless-steel container, evacuated to
15 to 20 torr, then heated to 8000 C, and held at temperature for 60 minutes.
(2) The container, still evacuated, was allowed to cool to room temperature.
(3) The container was backfilled (by suction) with a 25 percent aqueous solution of
zinc phosphate. The solution included enough phosphoric acid to give a zinc oxide to
phosphorous pentoxide ratio of 1 to 3.
(4) The container was pressurized to 35 newtons per square centimeter with nitro-
gen and held for 60 minutes.
(5) After depressurization, the specimens were removed and oven dried for about
24 hours at 1750 C.
RESULTS AND DISCUSSION
The results presented are median values based on two or three specimens for a
given set of variables (material, purification, antioxidant). The specimens of each ma-
terial were fabricated from a single batch of material. This minimized the variations
which usually occur in the manufacture of carbon-base materials. Oxidation life is the
4
time required to lose 5 weight percent of a 1 cubic centimeter specimen with a 6. 45
square centimeter apparent surface area.
Effect of Purification
The effect of purification on oxidation resistance (or life) is presented in figure 2.
For the commercial materials (A and B), purification increases the relative life several
(5 and 10) times as compared to the unpurified materials. For experimental material
E-0-28 (nonboronated), the relative increase of three times was less than for the com-
mercial materials. It should be noted, however, that the oxidation life of unpurified
E-0-28 was greater than that of either unpurified commercial material. Also purified
E-0-28 falls between the purified commercial materials in oxidation life. These results
indicate that the commercial materials contain substantially more impurities than the
experimental material. Therefore the superiority of untreated E-0-28 can be attributed
to higher purity.
In an earlier study (ref. 3), boron carbide (B4C) as a constituent of the basic mix
was shown to increase the oxidation resistance of carbon-graphite. The effect of purifi-
cation on two B 4C containing materials is also shown in figure 2. Purification of the ex-
perimental boronated materials did not increase oxidation resistance. This lack of im-
provement is attributed to loss of B4C during purification. During the work reported in
reference 3, weight gains attributed to oxidation of B4 C (to B2 0 3 + CO 2 ) were observed.
In this study, however, no weight gains were observed for the purified materials.
Therefore, at least the B 4 C at and near the surfaces must have been removed during
purification.
Effect of "Phosphating" on Purified and Unpurified Materials
Loss of oxidation inhibitor can occur during purification (as with the boronated
materials previously mentioned). This is not a problem when the inhibitor (e.g., zinc
phosphate) is added after purification. Treatment with zinc phosphate ("phosphating")
increases oxidation life of unpurified carbon-graphites (ref. 3). It is desirable to
determine how purification followed by "phosphating" compares with purification alone
and with "phosphating" alone.
The effects of "phosphating" on purified and unpurified materials are shown in fig-
ure 3. In all cases except the unboronated experimental material (E-0-28), there are
significant increases in oxidation resistance. As can be seen, the commercial materials
(A and B) again show the greatest relative increases (x10 and x7). The boronated mate-
rials (EP-5-22 and EP-7. 5-22), despite loss of B4 C during purification, show an ulti-
5
mate benefit from purification when "phosphated. " And "phosphated" E-0-28 shows
very nearly the same life when purified as when unpurified.
Figure 4 gives an overall comparison of the results. For each material, there are
two variables - purification and phosphate treatment.
The maximum oxidation resistance was produced by a combination of purification
and "phosphating. " However, for E-0-28, the difference between "phosphating" alone
and the combination of treatments was slight.
For unpurified material A, "phosphating" increased life two times. However (for
material A), purification followed by "phosphating" increased life four times. Thus,
for material A, purification doubled the increase in life obtained from "phosphating."
Similar increases in the benefits of "phosphating" were also observed with EP-5-22
(x2 unpurified against x5 purified) and EP-7. 5-22 (<x2 unpurified against x4 purified).
For the commercial materials (A and B), it is plain that purification alone is su-
perior to "phosphating" alone. The specimens with purification alone averaged some-
what more than twice the life of the specimens with "phosphating" alone for both mate-
rials. For the experimental materials, however, it can be seen that "phosphating"
alone is superior to purification alone. The reason for this difference between com-
mercial and experimental materials is considered to be due to the difference in material
purity.
SUMMARY OF RESULTS
Studies of the oxidation lives of limited numbers of specimens of several commer-
cial and experimental mechanical carbons (carbon graphites) indicated the following re-
sults:
1. Purification by a chlorine process increased the oxidation resistance of non-
boronated experimental and commercial carbon-base materials. Purification did not
increase the oxidation resistance of the boronated materials.
2. Purification followed by phosphate treatment gave the highest oxidation life for
each material. But for the nonboronated experimental material phosphate treatment
alone gave very nearly the same life as the combination of treatments.
3. For the commercial materials studied, purification alone was superior to the
phosphate treatment alone. For the experimental materials, phosphate treatment alone
was superior to purification alone.
Lewis Research Center,
National Aeronautics and Space Administration,
Cleveland, Ohio, March 7, 1974,
501-24.
6
REFERENCES
1. Mantell, Charles L.: Carbon and Graphite Handbook. Interscience Publishers,
1968.
2. Maahs, Howard G.; and Schryer, David R.: Chemical Impurity Data on Selected
Artificial Graphites with Comments on the Catalytic Effect of Impurities on Oxida-
tion Rate. NASA TN D-4212, 1967.
3. Allen, Gordon P.; and Wisander, Donald W.: Oxidation Resistance of Selected Me-
chanical Carbons at 6500 C in Dry Flowing Air. NASA TN D-7381, 1973.
4. Walker, P. L., ed.: Chemistry and Physics of Carbon. Vol. 3. Marcel Dekker,
Inc., 1968.
5. Fechter, N. J.; and Petronich, P. S.: Development of Seal Ring Carbon-Graphite
Materials (Tasks VIII, IV, and X). Union Carbide Corp. (NASA CR-121092), Jan.
1973.
7
TABLE I. - PROPERTIES OF MATERIALS USED IN THIS INVESTIGATION
Material Average flex Average Young's Average Nominal Source
strength modulus hardness density,
2 psi 2 psi Rockwell, g/ccN/m psi N/m psi S
A 7.7x10 6 11 500 2.0x109  2.8x10 6  90 1.85 Manufacturer's
literature
B 6.1 8 800 2.6 3.7 --- 1.77 Manufacturer's
literature
E-0-28 2.63 3 810 1.20 1.74 106 1.83 Ref. 5
EP-5-22 3.83 5 560 1.68 2.44 112 1.86 Ref. 5
EP-7.5-22 4.13 5 990 1.77 2.5 109 1.85 Ref. 5
Recording microbalance
Air Counterweight ,
Glass
envelope
Nitrogen
Susceptor
Figure L - Schematic of oxidation apparatus.
coil P
\Specimen
viewhole
CD-11514-18
Gas exhaust
i   ti  f i ati  r t s.
10-
O Unpurified (ref. 3 except A)
EI Purified
A B E-0-28 EP-5-22 EP-7.5-22
Figure 2. - Effect of purification on oxidation resistance of carbon-
graphite materials. Time to lose weight equivalent to 0.05 cubic
centimeter from 6.45 square centimeter surface at 6500 C.
35-
30O Unpurified (ref. 3 except A)
T77 Purified
25-
20
15--
10
A B E-0-28 EP-5-22 EP-7.5-22
Figure 3. - Effect of purification on oxidation resistance of carbon-
graphite materials subsequently treated with antioxidant (zinc
phosphate). Time to lose weight equivalent to 0.05 cubic centi-
meter from 6.45 square centimeter surface at 6500 C.
10
35 - Zn
Zn
30 -
25 ~ Unpurified (ref. 3 except A)
ezza Purified
Zn Treated with zinc phosphate
20
15 Zn Zn Zn
Zn
10
Zn
5-
Zn
0 
-
A B E-0-28 EP-5-22 EP-7. 5-22
Figure 4. - Effect of purification on oxidation resistance of carbon-graphite mate-
rials without and with antioxidant (zinc phosphate). Time to lose weight equiva-
lent to 0. 05 cubic centimeter from 6.45 square centimeter surface at 6500 C.
NASA-Langley, 1974 E-7809


Effect of chlorine purification on oxidation resistance of some mechanical carbons

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