Curing epoxy resins with novalac phenolic resins is a feasible approach for increasing intact char of the resin system. Char yields above 40% at 700 C were achieved with epoxy novalac (DEN 438)/novalac phenolic (BRWE 5833) resin systems with or without catalyst such as ethyl tri-phenyl phosphonium iodide. These char yields are comparable to commercially used epoxy resin systems like MY-720/DDS/BF3. Stable prepregs are easily made from a solvent solution of the epoxy/phenolic system and this provides a feasible process for fabrication of same into commercial laminates

Keck, F. L.

English

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.. 
NASA CONTRACTOR REPORT 166294 
NASA-CR-166294 
19820018515 
Thermochemical Tests on Resins - Char Resistance 
of Selected Phenolic Cured Epoxides 
Fred L. Keck 
CONTRACT NAS2-10 2 0 7 
February 1982 
NI\SI\ 
111111111111111111111111111111111111111111111 
NF02344 
llBRARV GOPY 
FeB "161982 
LANGLEY RESEARl.H CENTER 
LIBRARY, NASA 
HAMPTON, VIRGINIA 
https://ntrs.nasa.gov/search.jsp?R=19820018515 2020-03-21T07:31:30+00:00Z
NASA CONTRACTOR REPORT 166294 
Thermochemical Tests on Resins - Char Resistance 
of Selected Phenolic Cured Epoxides 
Fred L. Keck 
Lockheed Missiles & Space Company, Inc. 
P. O. Box 504 
Sunnyvale, California 94086 
Prepared for 
Ames Research Center 
under Contract NAS2-10207 
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Ames Research Center 
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TITLE 
FORWARD 
SUMMARY 
INTRODUCTION 
TABLE OF CONTENTS 
PAGE 
ii 
I 
z 
EPOXIDE-PHENOLIC EVALUATION 3 
TABLE 1 - Compilation of Results 5 
TABLE Z - Effect of Catalyst on DEN 438/BRWE 5833 7 
System 
, . 
FIGURE I - Laminate Photomicrograph 8 
CONCLUSIONS 10 
i 
-' 
FORWARD 
This report summ~rizes work conducted by the Advanced Manufacturing 
Technology organization, Missile Systems Division (MSD), of Lockheed 
Missiles & Space Company, Inc., Sunnyvale, California, 94086, under 
NAS 2-10207 Contract. Previous work had been done by the Manufacturing 
Research organization which was' partially absorbed into Advanced Manu-
facturing Technology. The administration of the work is under the direc-
tion of NASI Ames Research Center with D. A. Kourtides the Technical 
Monitor. Technical assistance on novalac phenolic cured epoxides was 
provided by Dr. H. A. Newey and C. J. Busso of Newey & Busso 
Associates. 
ii 
" 
SUMMARY 
Curing epoxy resins with novalac phenolic resins is a feasible approach 
for increasing intact char of the resin system. Char yields above 400/0 
at 7000 C were achieved with epoxy novalac (DEN 438)/novalac phenolic 
(BRWE 5833) resin systems with or without catalyst such as ethyl tri-
phenyl phosphonium iodide. These char yields are comparable to 
commercially used epoxy resin systems like MY -720/DDS/BF 3' Stable 
prepregs are easily made from a solvent solution of the epoxy/phenolic 
system and this provides a feasible process for fabrication of same into 
commercial laminates. 
1 
INTRODUCTION 
A basic problem associated with graphite composites is the release of 
free carbon fibers when burned. These carbon fibers can provide con-
ductive paths which can lead to serious electrical problems. To mini-
mize this possibility, the resin matrix used in the composite should 
have the property of producing a large quantity of intact char when burned 
that clings to the graphite fibers. Other properties desired of the resin 
matrix include good handling properties, good mechanical properties 
. 
when cured, good moisture resistance~~and a glass transition temperature 
high enough for the intended application. A class of resin systems 'that 
offer promise of meeting these requirements is phenolic cured epoxides. 
Accordingly the objective of NAS 2-10207 is to study the char resistance 
of this type of resin system using readily available commercial .resins. 
If feasible, this approach would be cost effective. 
2 
EPOXIDE-PHENOLIC EVALUATION 
It is known that highly crosslinked phenolic resins give higher char yields 
.lp 
than epoxy resins cured with amines or anhydrides. It is also known that 
phenolic resins such as novalacs, i. e. polyphenols made by condensing 
phenol and formaldehyde under acidic conditions, will cure epoxide resins. 
Practically all vaulting poles and many of the best skis are made with a 
liquid bisphenol A epoxy resin, novalac phenolic and latent catalyst. Al-
though this resin matrix has limited aircraft applications because of its 
low heat distortion temperature (900 C), it is included in the char resist-
ance evaluation. 
The initial screening evaluation for obtaining maximum char yield on 
epoxy/phenolic resin systems included the following epoxy resins cured 
with Celanese Epicure 8451. 
Epoxy 
MY-720 
Epon 828 
SU-8 
XD-7855 
Epon 1031 
DEN 438 
Supplier 
Ciba-Geigy 
Shell 
Celanese 
Dow 
Shell 
Dow 
In all cases stoichimetric amounts of the epoxy and phenolic resins pro-
duced a black char prior to satisfactory fUSing regardless of how slow the 
application of heat. Further investigation concluded that Celanese 8451 
phenolic is too reactive to form stable resin systems, probably because it 
has a "built-in" catalyst. Replacing the Celanese 8451 with Carbide 
BRWE 5833 novalac phenolic resulted in satisfactory systems; however, 
3 
/ 
sample preparation through fusing was not suitable for practical applica-
tiona. Consequently the solvent system technique was used for sample 
preparation. This consists of dissolving the epoxy resin and novalac 
phenolic (Carbide BRWE 5833) in methyl ethyl ketone. After solution, add 
catalyst if applicable and stir .to complete solution. 
The following cure with slight differences in time was used for the 
, 
various systems. 
1. Air dry overnight to remove majority of solvent. 
2. Vacuum dry at room temperature increasing vacuum gradually to 500 
microns exercising care to prevent boil over of the solvent system. 
3. After removal of solvent return to atmospheric pressure, heat for 
2 hours at 1200 C followed by 6 hours at 160oC, then cool. 
A compilation of epoxy/phenolic resin systems investigated plus "reference" 
epoxy resin systems and their respective char resistance (TGA) values at 
700°C and 7500 C is shown in Table 1. A near stoichiometric amount of 
novalac phenolic was used to cure the epoxy. The various types of epoxy 
resins as well as polyphenol evaluated along with their respective chemical 
formulations include: 
. a) Epoxy novalac - DEN 438 
o 
/\ 
O-C-C-C 
~--c-~ 
o 
/\ O-C-C-C 
:"l---C-~ I 
4 
o-c-cQ 
VI 
RES IN SYSTEHS 
REFERENCE EPOXY/PHENOLIC 
Resin System A(l) B~ll CPl D~21 E~2l F~2} CPl H~21 1~3l J(3} K(3l L~31 
Narmeo 
!l!.!!.!I Supplier 5208 
MY-720 Ciba-Geigy 100 90 100 100 
Epon 828 Shell 100 100 
SU-8 Celanese 10 100 100 
Epon 1031 Shell 100 100 
DEN 438 Dow '\ 100 100 
Phenolic 
BRW! 5833 Carbide SO SO 79.4 79.4 SO 60 SO 
TPE Sc:hneetady 45 45 
Catalyst 
ETPPI. 0.1% 0.15 0.18 0.16 0.37 0.36 0.36 
BF3 .400 1.5 
BDHA 0.6 
Coreaetant 
DDS 32 30 
HIlA 27 
TeA Values, % 
700°C 43.2 29.1 38.4 37.3 31.1 32.1 32.1 18.0 42.8 39.8 40.2 38.9 
7S00C 39.8 26.3 3S.2 34.0 33.2 31.1 31.1 17.S 39.3 37.3 37.S 3S.1 
1. Cured 16 brs at 93°C (2000y). 2 brs at 121°C (2S00P), 2 hrs at 149°C (3000,). 2 hrs at 177°C (3S00,). 2 hrs at 
20SoC (400°F). 
2. Cured 1 hr at 120°C (248°F). 4 brsat 160°C (320°F) 
3. Cured 2 hra at 120°C (248°F). 6 hrs at 160°C (320Op) 
TABLE 1 
b) Epoxy bisphenol novalac - Celanese SU-8 
c) Glycidyl ether of polyphenol - Shell Epon 1031 
d) Clycidyl amine - Ciba-Geigy MY -720 
6 
/0, -
/
CH -CH-CH-2· 2 N, 
CH -CH-CH 
2 \ / 2 
o 
e) Tetraphenylol ethane - TPE (precurser of Epon 1031) 
From the data in Table 1 the char resistance of most resin systems is 
lower than the best reference system, MY -720/DDS/BF 3. The only epoxy 
phenolic resin system having comparable char resistance is DEN 438/ 
BRWE 5833. The use of triphenylol ethane in place of BRWE 5833 with 
Epon 1031 or DEN 438 did not increase char resistance. Higher char 
resistance had been expected because of higher aromatic ring content and 
,cros s link density. 
Since DEN 438/BRWE 5833 had the highest char resistance of any epoxy/ 
phenolic system evaluated, it was selected to study the effect of catalyst 
concentration. The data shown in Table 2 indicates that the amount of ethyl 
triphenyl phosphonium iodide catalyst has no significant effect on char, 
resis tance. 
SYSTEM 
Material 
DEN 438 
BRWE 5833 
ETPPI, % 
1 
100 
60 
o 
TABLE 2 
2 3 4 
0.1 0.25 0.50 
7 
~./ 
TABLE 2 (Cont'd) 
TGA Values, % 1 2 3 4 
700°C 43.7 42.8 46.1 41. 8 
750°C 39.4 39.3 42.7 38.9 
Since the system containing 0.25% ETPPI had slightly better char resist-
ance, it was chosen as the epoxy/phenolic candidate resin system for 
fabricating a graphite laminate under NAS 2-10130 contract. The physical 
properties.of a 9-ply laminate cured at 350°F and 100 psi in an autoclave 
(detailed procedure in appendix of NAS 2-10130 contract) follow: 
Thick.ness 
Fiber volume 
Void volume 
Short Beam Shein 
Photomicrograph (42X) 
Tg 
118 mils 
62.4% ) 
0.2% ) 
8430 psi 
Figure 1 
105°C 
FIGURE 1 
8 
Based upon 
Resin sp. g. = 1.223 
Graphite sp. g. = 1·.782 
No indication of voids 
"--"- -
It should be noted that the char resistance was determined at7000 C and 
7500 C using a DuPont thermal analyzer. The test condition determined 
under NAS 2-10130 contract to produce maximum TGA values used 99.999% 
pure nitrogen at a flow rate of 50 ml per minute. A sample size of 8-9 
mg obtained by filing the cast resin was used each time for comparison 
purposes. 
9 
CONCLUSIONS 
As a result of the epoxy/phenolic investigation conducted under the 
NAS 2-10207 contract, the following conclusions are made. 
1) The DEN 438/BRWE" 5833 systems with or without ETPPI catalyst 
have char yields comparable to commercially used resin systems 
like MY -720/DDS/BF 3~ 
2) The DEN 438/BRWE 5833 resin system is cost effective . 
• uses readily available commercial materials 
• stable prepregs from a solvent system provide a feasible 
proces s for fabricating laminates for commercial applications. 
3) Curing epoxy resins with phenolic resins is a feasible approach for 
increasing intact char. 
4) The optimum cure cycle including post cure was not established under 
this contract as time and funds were not available. 
10 
1. R~ No. 2. GCMmrnent ~on No. 3. Recipient's t.talog No. 
NASA CR-166294 
4. Tltl. end Subtitle 5. Report Dlte 
THERMOCHEMICAL TESTS ON RESINS 
- CHAR RESISTANCE OF February 1982 
SELECTED PHENOLIC CURED EPOXIDES . 'e. Pwfannin9 Orvinlzatlon Code 
7. Au~I'1 8. Performing Organization Report No. 
Fred L. Keck 
10. Work Unit No. 
9. Performing Organization Heme' end Address T4319 Lockheed Missi1es& Space Company, Inc. 
11. Contract or Grant No. P. O. Box 504 
Sunnyvale, California 94086 NAS2-10207 
12. Sponsoring ~y Heme end Add,.. 
13. Type of Report and Period Covered 
Contract Report-Fina 
National Aeronautics and Space Administration 
Washington, DC 20546 14. Sponsoring AfJllflr::v Code 
RTOP No. 505-44-21 
15. SuppllllWftUfY Notes 
Ames Technical monitor: D. A. Kourtides, MS 223-6- Ames Research Center, 
Moffett Field, CA 94035, (415) 965-5226, FTS:' 448-5226 
16. Abstract 
Curing epoxy resins with novalac phenolic resins is a feasible approach 
for increasing intact char of the resin system. Char yields above 40% 
at 700°C were achieved with epoxy novalac (DEN 438)/novalac phenolic 
(BRWE 5833) resin systems with.or without catalyst such as ethyl tri-
phenyl phosphonium' iodide. These ch~r yie~ds are comparable to 
commercially used epoxy resin systems like ~~-720/DDS/BF. Stable 
prepregs are easily made from a solvent solution of the ~poxy/phenolic 
system and this provides a feasible process for fabrication of same into 
commercial laminates. 
17. Key Words (Suggested by Authorls)) 18. Distribution Statement 
Resins, Epoxies ~nclassified~Unlimited 
19. Security OlSsif. lot this rapo"1 20. Security Classif. lof this pagel 21. No. of Pages 22. Price' 
Unclassified Unclassified 15 
·For .. II by the Nltional Tec:hnic:ellnformation Sarvic:e, Springfield, Virginia 22161 
End of Document 


Thermochemical tests on resins: Char resistance of selected phenolic cured epoxides

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