MY-720/DDS epoxy samples were treated with three selected chemical compounds to render the active H-sites inactive for moisture absorption. Treating the epoxy castings with acetyl chloride and dichlorodimethyl silane leads only to surface changes indicating that these molecules are too large to penetrate the epoxy castings. Boron trifluoride, on the other hand, does penetrate the epoxy chain as is indicated by the formation of green domains in the interior of the castings. However, the process of saturating the specimens with moisture appears to leach out the chemical additives--thereby nullifying their possible ameliorative effects

Singh, J. J.

St.clair, T. L.

Stoakley, D. M.

English

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N 8 4 - 2 7 8 3 4
NASA Technical Memorandum 85826
AN INVESTIGATION OF CHEMICALLY-INDUCED IMPROVEMENT IN
SATURATION MOISTURE CHARACTERISTICS OF EPOXIES
JAG J, SINGH, TERRY L, ST, CLAIR, AND DIANE M, STOAKLEY
JUNE 1984
IWVSA
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23665
https://ntrs.nasa.gov/search.jsp?R=19840019765 2020-03-20T23:11:42+00:00Z
ABSTRACT
MY-720/DDS epoxy samples were treated with three selected chemical compounds
to render the active H-sites inactive for moisture absorption.- Treating.the epoxy
castings with acetyl chloride and dichlorodimethyl silane leads only to surface
changes indicating that these molecules are too large to penetrate the epoxy castings.
Boron trifluoride, on the other hand, does penetrate the epoxy chain as is indicated
by the formation of green domains in the interior of the castings. However, the
process of saturating the specimens with moisture appears to leach out the chemical
additives—thereby nullifying their possible ameliorative effects.
INTRODUCTION
'Most epoxies are susceptible to moisture pickup because of the tendency of
H on N or 0 atoms to interact with H in the HgO molecule. It can thus be argued
that it may be possible to reduce their moisture susceptibility if we can render
active H sites in the epoxy chain inactive. With this goal in mind, tetraglycidyl
methylene dianiline (TGMDA or MY-720) cured with U, l^'-diamino diphenyl sulfone
(DDS) was selected as the candidate epoxy. It is widely used in aerospace and
electronic industries. The chemical additives selected for passivating the test
epoxy were: (l) Acetyl chloride (CH^COCl), (2) dimethyldichloro silane [ SiCl2
(0113)2] , and (3) Boron Trifluoride (BF3). The saturation moisture characteristics
of treated and untreated MY-720/DDS epoxy samples were measured. It was also planned
to measure positron annihilation characteristics in the most promising Moisture-
proofed varieties in order to elucidate the mechanisms responsible for their hydro-
phobicity. The results of these studies are discussed in the following sections.
EXPERIMENTAL PROCEDURE AND RESULTS
The purpose of this study was to develop a technique that could render fully
cured epoxy specimens less susceptible to moisture pickup. As indicated above,
a highly cross-linked thermosetting resin (TGMDA/DDS) was selected for this study.
Epoxy moldings in the form of 2" diameter x 0.1" discs were prepared. Some of
these discs were chemically treated and their saturation moisture contents compared
with those of the control (untreated) specimensv*). The results are discussed in
the following sections.__
The saturation moisture contents were determined by comparing the weights of
the specimens desiccated at 100°C with the weights of the specimens saturated with
water at 90°C. (Notice that the specimens were actually immersed in water at 90°C
for saturating them with water.)
Epoxy Preparation
Sixteen discs -of MY-T20/DDS epoxy vere prepared in two "batches . The first
"batch contained eight discs. The second batch, prepared h weeks later, also
contained eight discs. All. discs were prepared from an epoxy solution containing
1*9$ by weight of tetraglycidyl methylene dianiline (TGMDA or MY-720), lk% by weight
of I*, -U'-diaminodiphenyl sulfone (DDS) and 31% by weight of methyl ethyl ketone ''(NEK)
solvent. The epoxy solution was poured into aluminum pans and degassed under
vacuum at 120°C for 1 hour to remove the volatiles. Vacuum was then released, and
the discs were cured by gradually heating from 120 ->• 150°C over a 3-hour period.
The temperature was held at 150°C for 1 hour and then increased to 177°C for a
2 -hour final cure. Figure 1 shows a structural diagram of the MY-720/DDS epoxy
system' -*-' .i
The sample discs were treated with three different types of chemicals — acetyl
chloride, dichlorodimethyl silane and boron trifluoride. These chemicals are
expected to react with active hydrogens in the MY-720/DDS epoxy.
1. Acetylation Process : Four discs from the first batch were treated in the
following manner:
i. The discs were left in contact with acetyl chloride vapors at room
temperature for ^  days.
ii. They were then dried in a vacuum oven at 60-80°C for 2U hours.
iii. The discs were then soaked in liquid acetyl chloride at room tempera-
ture 'for 6 hours.
iv. They were next placed in boiling acetyl chloride at ^ 0°C for 30 minutes.
v. Finally, the discs were washed in distilled water and wiped dry.
The discs changed from a yellow to a green color after initial treatment with
acetyl chloride vapors. No further physical change was noted during the acetylation
process. Figure 2 shows a structural diagram for acetylation of MY-720/DDS epoxy.
The remaining four discs were left untreated. The saturation moisture contents
of the two sets of discs were then -measured using the standard procedure (see
page 1 footnote). It was noted that the treated discs had regained their original
color/appearance after the saturation-desiccation cycle. The results are summarized
in Table I. It is apparent from the data that acetylation makes no difference in
the saturation moisture content of MY-720/DDS samples.
i
2.. Silanization Process. The four untreated discs from the previous batch
were silanized as follows:
j i. The discs were immersed in a closed container of SiClgCCI^Jg at room
temperature for 5 days.
ii. The samples were then washed in heptane.
iii. The samples were finally washed in distilled water and wiped dry.
It was noted that the silanized samples had acquired a dark green color. Figure 3
shows a structural diagram of silanized MY^ 720/DDS samples.
Since the acetylated samples from the previous test had regained their original
state after saturation-desiccation cycle, they were treated as control samples for
comparison with the silanized samples.
The saturation moisture contents of the silanized samples are summarized in.
Table II. During the saturation process, it was observed that the treated samples
had recovered their original color after they had .been in water for 65 hours.
.' It is apparent from the data shown in Table II that the silanized samples do
pick up slightly less moisture than the untreated samples. It may be due to the
fact that H in the original amine function of the epoxy has been replaced by a
dimethyl chlorosilyl group which can react with water to form a slightly more
hydrophobic dimethyl silanol group. However, the difference is rather small.
3- Boron Fluoridation Process. Four new discs from the second batch were
subjected to boron trifluoride by placing them in a vessel containing a mixture
ofvhexanes. Boron trifluoride gas was bubbled into the liquid and then the vessel
was closed. This procedure was repeated.twice daily for h days. At the end of
this time, the samples were removed and examined. There was no appreciable weight
gain. However, the amber-colored sample discs had developed green spots in their
interiors. The samples were photographed and placed back into the BFg/hexane bath
over the weekend. Figure k shows a structural diagram of BF^-treated MY-720/DDS
samples.
The saturation moisture contents of BFg-treated samples are summarized in
Table III. During the saturation process, it was again noted that the BF-^-treated
samples had recovered their original color after they had been in the hot water
bath for about 3 days.
,. It is apparent from the data shown in Table III that the BF3~treated samples
pick up the same amount of water as the reference samples. However, the BF3~treated
samples took longer to lose their moisture content than the reference samples.
1
 As seen in photographs 5 and 6, there were two different green spot patterns
in ..the BF3~treated discs before they were treated with water. In one case, the
spots were larger in size but fewer in numbers. In the other case, the spots were
finer but more numerous. Positron lifetime measurements were made in these BF^-
treated.samples in order to explain the reasons for different spot size/distribu-
tions^- . Results are summarized in Table IV.
DISCUSSION
' The three chemical additives selected for passivating the.epoxy samples are
expected to react with active hydrogen atoms on N or 0 sites in the cross-linked
TGMDA/DDS chains. (See figures .1-U.) During these reactions, they will generate
aci'ds of the general form HX which can subsequently react with amines present
in the cured epoxy samples to form hydrochloride salts (see figure 7).
In the case of the acetyl chloride-treated epoxy moldings, a green color
formed on the surface. The same phenomenon occurred in dichlorodimethyl silane
treatment. This color change may be related to the formation o'f hydrochloride
salts on the surface.
When boron trifluoride was used, green areas formed in the center of epoxy
moldings. This again may be related to the formation of hydrochloride salts.
However, in this case, the HF molecule was evidently small enough and polar enough
to pentrate the molding. The green specks started out as small spots and grew
with time.
In all cases, the effects of the chemical additives were reversed after the
specimens had been in the hot-water bath for some time. This explains why the
saturation moisture contents of the treated and untreated samples are essentially
equal in all cases.
As indicated earlier, the distribution of the green spots in the interior
of the samples subjected to BF3 treatment followed two distinct patterns: Half
of the samples had smaller but more numerous spots whereas the remaining samples
had larger but fewer numbers of spots. Positron annihilation characteristics in
these samples are different, as seen from the data in Table IV. The pickoff ortho-
positronium annihilation rate in samples with smaller spots is larger, accompanied
by a larger probability of Ps-atom formation. On the other hand, the pickoff
annihilation rate in samples with larger spots is slower, accompanied by a lower
probability of Ps-atom formation. These results indicate the presence of uncured
regions in the samples where BF-$ appears to concentrate, thereby affecting the
local free electron density and distribution.
CONCLUDING REMARKS
MY-720/DDS epoxy samples were treated with three selected chemical compounds
to render active H sites inactive. Treating the epoxy castings with acetyl
chloride and dichlorodimethyl silane leads only to surface changes indicating that
these molecules—as well as the hydrogen chloride that forms—are too large to
penetrate the epoxy castings. Boron trifluoride, on the other hand, does penetrate
the epoxy chain as is indicated by the formation of green domains in the interior
of the castings. However, the process of saturating the specimens with moisture
appears to leach out the additives, thereby nullifying their possible ameliorative
effects. Positron annihilation characteristics in the test epoxy samples indicate
rather large free volume, thus attesting the appropriateness of MY-720/DDS epoxy
as the test medium.
REFERENCES
.1. Lynn S. Penn and T. T. Chiao: Epoxy Resins (Handbook of Composites, edited
by George Lubin, Van Nostrand Reinhold Publishing Company, New York, 1982,
PP. 57-88).
2. William H. Holt and Willis Mock, Jr.: Positron Lifetime Technique for Non-
destructive Evaluation of Materials. NSWC-DL-TR-3573, U. S. Navy,
December 1976. (Available from DTIC as AD A038305).
3. Jac J. Singh, Terry L. St. Clair, William H. Holt and Willis Mock, Jr.:
Moisture Dependence of Positron Annihilation in Nylon-6. Nucl. Instr. and
Methods, Vol. 221, 1*27, 19814.
Table I. Piunmnry of Saturation Moisture Contents of Untreated and Acetyluted
MY-7.-0/DDS Epoxy Samples
Untreated Samples
oanp] c No .
.1
^
^\
(Average )
Saturation' Moisture Content
5.3>i w/o
5.37 w/o
5-32 w/o
5.38 w/o
(5.35±0.03) w/o
Treated Samples
Sample No .
1
2
3
h
(Average)
Saturation Moisture Content
5.29 w/o
5.35 w/o
5.32 w/o
5.32 w/o
(5.32+0.03) w/o
II. Surrnary of Saturation Moisture Contents of Silanized MY-720/DDS
Epoxy Samples
Untreated Samples
Sample No.
1
£
3
'Average)
Saturation Moisture Content
5.16 w/o
5.28 w/o
5.09 w/o
5.17 w/o
(5.18+0.08 w/o)
Treated Samples
Sample No.
1
2
3
U
( Average )
Saturation Moisture Content
U.9U w/o
5.01 w/o
U.90 w/o
5.07 w/o
(H. 98+0.08 w/o)
Table III. Summary of Saturation Moisture Contents of Untreated and BFo-Treated
MY-720/DDS Samples.
Sample No.
1
2
e
(.Average )
Saturation Moisture Content
k.79 w/o
It. 90 w/o
(It. 85+0. 05) v/o
Sample No.
1 (larger
spots)
2 (smaller)
spots)
(Average )
Saturation Moisture Content
U.83 w/o
U. 87 w/o
(I*. 85+0. 02) w/o
(*) .Table IV. Summary of Positron Lifetimes in BF-^-Treated MY-720/DDS Samples (Dry)
Parameter Sample With Smaller Green Spots Sample With Larger Green Spots
366+17 ps
1521+68 ps-
32.0+1.1%
370+10 ps
1596+U3 ps
(*)
TI - Short Lifetime Component Lifetime
Tp - Long Lifetime Component Lifetime
IP - Long Lifetime Component Intensity
(.Pick-off orthopositron lifetime (19) is very sensitive to the atomic environment
at the end of the range of the positrons injected into the target epoxies^).)
DOS
NH,
A
« — CH~ ^x.
N CH, N
/
A
« — GH1— CHx
MY-720
Figure-L Structure of MY-720/DDS Epoxy.
R-OH
H
I
R-N
I
0
II
C-C£
0
II
CM. c-a
0
R-O-C-CH
0 = C-CH,
I
R-N
I
HGr
Figure-2. Structure of Acetylated MY-720/DDS; Epoxy,
R -OH
H
1
a
i
+ CH--SJ-CH- —
1
a
a
i
-
*- R-O-
H
1
CK,
1 <
Si-Cf +
1
CH3
CH7
. 3
na
R-N + CH--SI-CH- —-*- R-N-Si-CF + HCf
I I I
a CH3
Figure-3. Structure of Silanized MY-720/DDS Epoxy.
F
I
R-OH + BF3 —^ R-O-B-F + HF
H F
I I
R-N + BF. —*- R-N-B-F +. HF ,
I I • *
Figure-4. Structure of BF.-treated MY-720/DDS Epoxy.
-II
co
-5
o.
oo
0>
§
O.
C\J
I
>-
CT3
O>
CO
*O
Q.
CO
O
Q_
ir\
10
*"> CT>
11
H
I
-N + HX
I
R-N
I
-H
Effect of HX
720/DDS Epoxy.
on
12
1. Report No.
NASA TM-85826
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
An Investigation of Chemically-Induced 'Improvement
in Saturation Moisture Characteristics- of Epoxies-
5. Report Date
June 1984
6. Performing Organization Code
505-33-90-01
7. Author(s) Jag J. Singh, Terry L. St. Clair, and
Diane M. Stoakley
8. Performing Organization Report No.
10. Work Unit No.
9. Performing Organization Name and Address
NASA Langley Research Center
Hampton, VA 23665
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, DC 205^6
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
16. Abstract
MY-T20/DDS epoxy samples were treated with three selected chemical compounds
to render the active H-sites inactive for moisture absorption. Treating the
epoxy castings with acetyl chloride and dichlorodimethyl silane leads only to
surface changes indicating that these molecules are too large to penetrate the
epoxy castings. Boron tri fluoride., on the other hand, does penetrate the epoxy
chain as is indicated by the formation of green domains in the interior of the
castings. However, the process of saturating the specimens with moisture appears
to leach out the chemical additives—thereby nullifying their possible ameliorative
effects.
17. Key Words (Suggested by Author(s)) Moisture-Resistant
Epoxy, MY-T20/DDS Epoxy, Acetyl..Chloride,
Dichlorodimethyl Silane, Boron Tri-
fluoride, Surface Effects, Bulk/Interior
Effects, Positron Lifetime
18. Distribution Statement
Unclassified - Unlimited
Subject Category - 2h
19. Security Qassif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No. of Pages
13
22. Price
A02
For sale by the National Technical Information Service, Springfield. Virginia 22161


An investigation of chemically-induced improvement in saturation moisture characteristics of epoxies

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