Tensile, peel, and shear testing was performed on combinations of primers, adhesives, tycements and rubber compounds cured at various times and temperatures. The materials used in the fabrication of the solid rocket motor flex bearing as well as in other systems were evaluated. A compatibility study between adhesives and tycements was initiated. The flex bearing mold design was reviewed by our tooling experts

Wood, T. E.

English

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INASA TECHNICAL
MEMORANDUM
NASA TM -78266
EVALUATION OF SRM FLEX BEARING
MATERIALS AND PROCESSES
By Thomas E. Wood
Materials and Processes Laboratory
February 1980
(NASA-TM-7826 6) EVALUA'11ON OF SRM FLEX
BEARING MATERIALS AND P,RCCESSLS (NASA) 20 P
11C A02/MF A01	 CSCL 11A
K
s`
N80-19264
Unclas
G3/27 47491
NASA
George C. Marshall Space Flight Center
Marshall Space Flight Center, Alabama,
MSFC - Form 3190 (R.ev June 1971)
	 t.'
TABLE OF COPITEPITS
Page
T. INTRODUCTION	 .................. . .......... . ........... I
II. HARDWARE DESCRIPTION ............................... 1.
III. APPROACH ............... .. . . .... . . .. .. .... . . . . . . ....... 2
v	 IV. TEST PROCEDURES AND EXPERIMENTAL DATA ......... 2
A.	 Tensile
	 Specimens	 .....................
	 ............. 2
B.	 Rubber	 Shrinkage .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
C.	 Peel	 Specimens	 .....	 ........	 ..	 ................. 7
D.	 Primer and Adhesive Thickness Specimens ........... 9
V. RESULTS AND DISCUSSION .............. 9
VI. CONCLUSIONS	 ......
	 ...	 ................	 ..	 .. 15
PRECEDING PAGE SLAW NOT FILMED
iii
I
^w	 xbN:	 ._,_._..
LIST OF TABLES
Table Title Page
1. TYPICAL DISC SPECIMENS ........................... 2
2. EFFECT OF TYCEMENT THICKNESS .................. 3
3. EFFECT OF TIME AT TEMPERATURE ................. 3
4. EFFECT OF TEMPERATURE ON TYCEMENT-CHEMLOK
ADHESION............................. 9 .............. 4
5. INTENTIONAL CONTAMINATION ...................... 4
6. SHRINKAGE TEST OLD SAMPLE ...................... 6
7. IN-HOUSE VERSUS THIOKOL TYCEMENTS ............ 9
8. THICKNESS MEASUREMENTS 	 ......................... 10
9. TYPICAL	 PEEL ..... ,. ................................. 11
iv
Ap
4e
THMINICAL MEMORANDUM 78-266
EVALUATION OF SRM FLEX BEARING
MATERIALS AND PROCESSES
I. INTRODUCTION
As a result of the lark; number of failures at Thiokol in .fabricating
a full scale flex bearing, an in-house effort encompassi;kg a materials pro-
gram, a tooling evaluation, and a process analysis was established. A
list of nine problem areas of concern was prepared, and Materials and
Processes Laboratory personnel with expertise applicable to the SRM flex
bearing established the priority of each problem area. 'Three of the nine
areas of concern were given a high priority. A materials, mold design,
and tempert4ure controls study was undertakcii in an effort to resolve
the ftulu: , e modes occurring in the fabrication of the flex bearing.
II, NARD14ARE DESCRIPTION
The SIM flex bearing is a large (n. 7000 lb) complicated structure.
It consists of 12 metal pieces and 11 rubber pads, cured and bonded
together atone time. The metal shims and end rings tire primed and then
coated with two more adhesive materials. Presently, Chemlok 205 is the
primer, and Chemlok 220 and 2658 tycetnent are the two adhesives.
The purpose of this bearing is to :[unction as a universal joint which
allows the motor nozzle to swivel in tiny direction to achieve thrust control.
The movement is atttutled by stretching the 11 rubber pads between the
metal shims and the two end ring structures.
The complexity of' this bearing; is further indicated by a unique series
of functional requirements which require a very soft natural rubber with
special requirements of modulus, torque load, bondability in a large assem-
bly and stability with time.
The special rubber physical properties were obtained by using a
relatively incompatible liquid diluent to acquire the necessary softness and
low modulus.
III. APPROACH
It was desirable to simulate the various types of loadin( seen in
fabricating and curing the 81151 flex bearing. To assess those conditions,
specimens were prepared to obtain tensile, shear, pool, elongation, mod-
ulus, and shrinkage characteristics. Initial materials including primers,
adhesives, tycements, and rubber stock were obtained from Thiokol to
begin this investigation.Additionally similar materials were obtained from
other sources, along with in-house compounded tycements and rubber
stocks.
Test specimens were prepared using several combinations of primer,
adhesive, tycement and rubber stock cured under various time and tem-
perature conditions.
IV, TEST PROCEDURES AND EXPERIMENTAL DATA
A. TENSILE SPECIMENS
The tensile specimens consisted of a 0.300 in. thick rubber disc
vulcanized between two 0.75 by 2 in. steel discs. All tensile tests were
made on an Instron test machine with certified load cells. The specimens
were installed and pulled at a constant speed of 5 in. /min.
1. Typical Tensile Specimens. Eleven specimens prepared side by
side using Chemlok 205, Chemlok 220, tycement 2658 and TR 3005 rubber
stock were cured at 300 OF for 1 lir. These data presented in Table 1
show the typical strength spread obtained using these bonding materials.
TABLE 1. TYPICAL DISC SPECIMENS
Specimen No. I	 Strength (psi)
1,. 146
2. 338
3. 1.29
4. 280
5. 304
G. 221.
7. 121
8. 229
9. 202
10. 360
11. 231
Average 234
2
^.
	
	
2, Effect of Tycement Thickness. A series of tensile specimens was
prepared Wilicreas ng tile tyeenient thiclu.ess. Two tycements 2658
(Hyear-incorporated diluent) and 3827 tycement (No Ilyear) were evaluated
in this thickness study. The Oata presented in Table 2 intricate that tyce-
went thickness is critical, with six colas (3 , 4 mils) being near the optimum.
TABLE', 2. EWFECT OF TYCl:j5ll,NT `1'IIICKNI?SS
Tycemeln With llyc,-w Tycement Without Ilycar
Strength Coliesive Strength Cohesive
(psi) Failure (°,) (p SO Failure ( `„ )
1 Coat 244 273
2 Coats 220 78 272 98
9 Coats 311 95 482
6 Coats 1167 98 600 96
8 Coats 393 99 r17.? 97
a. Average of 3 specimens.
3. Effect of Time at 'Temperature. A set of seven specimens was
press cured tit300°l :inci"iliclivictual Ssunples were withdrawn for testing
at 30-min intervals. The data collected for the time study are presented
in Table 3. No discernable trend is evident over this time span.
`.l'11131,13 3. EWF1,C'T OF 'TIME,
AT 'I"HAIPERATURI:
Time
(Mill)
Strength
(I)SO
30 506
60 152
JO 187
120 210
150 321.
180 221.
210 1115
3
4, Effect of Temperature on Cured Tycement-Chemlok Adhesion.
Seven tensese ins wardep^red us ►n; C,henilok205	 iok''20,
tyeement 2827 plus a 0.20 in. thick disc of tycement gum and TR 3005
rubber stock. The data obtrdned in this study are presented in Table 4
and show strength decreased as the temperature Increased.
TABLE 4. EFFBCT OF TEMPERATURE ON TYCRMI NT-
CHEMLOK ADHESION
Temperature (°r) Strength (psi)
RT 422
200 241.
22" 5 234
250 208
275 1.71.,
300 1.55
5, Intentional Contaminati on. 'Tensile test specimens were prepared
with various ]finds of coat ii inants in tin attempt to create voids and/or
debonds in the process of bonding rubber to metal. The t ype of contrim-
inants do not drastically affect the bond strength in this particular case.
TABL E
 5, INTENTIONALN`.l`IONAL CONTAMINATION
Adhesive - Chemlok 205/233
Tycemont - 2827
Sample Strength (psi)
(1) Control 191
(2) Dirty Lands 125
(3) Dust 236
(4) Uncured Silicone 181
Adhesive - Chemlok 205/220
Tycement - 2658
Sample Strength (psi)
(1) Control 210
(2) Dirty Hands 135
(3) Dust 232
(4) Uncured Silicone 281
4
r
TABU' 5. (C oncludoo )
Adlivsive
	 Cho11loh 205/223
No `1'yconuint
C"o hosive
Strength hov"Ition l'`11i11u'e
Saillple SurfllCCa (psi) la(? 111 ati t)11 or Failure
1A Clean 1 90 1.5" A 95
113 Cleatl
'A Clean 1.:.1 1.37 A 00
213 Dirty
	
11111v^1s
3A C'Ioan 235 1.68 A 07
313 Shop 1)list
4A C1r,111 181 1.55 13 95
413 Uncured
Silivoue
Adhosivo
	
Clionflok 205/2:311
5`„ Siliconc, Added to 1ZUbbor
Samplo Strength (psi)
Gonlrol 229
Silic^ono 189
G. Loads Duo to Shrinkage. A 2 in. disc specimen was cured for
60 min at-:ffO'0F* . A fillo tilis ' Snnlplu was ,till hot and inside the mold cavity,
it was connected to all Instroll Lwid Coll. `rime, temperature, and load
were recorded ovary 5 min from 300 00 to 75 0
 for it hr. A "delta" load of
81 lb rosultod during coolciown of this samplo, as shown in Tnblo G.
In tho first tour tompor.lture roadings, the stninloss stool hookup
rods were absorbing hunt and wore explu1di111; • caushig 11 load loss whiell
would lalave resulted In higher fond rond^ing 11' this problem were minimized
or eliminated,
Two larger stimplos of the configuration orlp;i illy designed to simu-
late a sltinl 11116 rubber bond wore tested to get a larger test piece. `1`heso
are identified as "Old" told 	 `"Now" sanlplos.	 he old sample was cured
about 3 years ago. It was installed in a hot nir oven preheated to 15000,
stabilized 30 min at temperature, and connectod to all Instron test machine.
5
F
ate	 ^
TABLE 6. SHRINKAGE TEST OLD SAMPLE
Time (min) Temperature ( 0c) Load (lb)
0 133 2
30 88 26
60 63 51
90 49 63
120 40 72
150 35 76.5
240 28 81
New Sample
0 0 150
60 170 98
120 171 82
180 174 70
240 170 65
300 165 60
360 165 56
2 In. Disc
30 88 26
60 63 51
90 49 62.5
120 40 71.5
150 35 76.5
Over a 5 hr period, a total load of only 5 lb resulted during cooldown.
It was apparent from this very small load that these results were not
consistent with the previous 2 in. disc sample. We cleaned this sample,
cured a new rubber sample 60 min at 300 0F, and hooked it up to the load
cell as fast as possible. The hot air oven was already at 300 0r, and the
sample was allowed to return to this temperature. The data collected for
these shrinkage specimens are presented in Table 6. It is evident that
cooling in the mold adds stress to the rubber bond line.
6
B, RuBBER SHRINKAGE
To determine if the rubber was separating due to shrinkage duriaal;
mold cooldown, rive samples of SHM flox bearing rubber were molded and
cured, and measured by the Reliability and Quality Assurance Office per-
sonnel. The meld dimensions were 0.755 by 0.059 by 8.566 tai. The
shrinkage was not uniform in tall directions. The sample shrtualc nix aver-
age of 2.32 percent in length, 2.89 percent in width and 2.62 percent In
height, The shrinkage values for etch sample are as follows,
Rubber "amble
No,	 lvidth (u)
	
height ( 0 )
	
Length (u)
1	 2.33 1.52 2, 50
2	 3,34 3.13 2.22
3	 3.20 2.81 2.18
4	 3.20 2.81 2.40
5	 2.40 2.81 2.29
These samples were made in a mold coated with mold release so that the
samples were free to change dimensions ill any direction. In n second
series c,f tests, an 11 by 0.309 in. molded disc was heat cured and cooled
under a simulated time/temperature Ilex bearing; cure cycle. The 11 in.
specimen cure cycle is illustrated in Figure, 1. The resultant volume
shrinkage was 5.3 percent. A second 11 by 0,309 in., disc: was molded
and bonded to all surfaces of the mold. After cooling, the assembly was
subjected to nondestructive evaluation. Using x-ray and acoustical tech-
niques, no debond or non-bond could be detected. The mold was then
cut into eight pie shaped segments. hxcopt for one very small area ( ,k,
2 cm 2 ) , the rubber was tonaciously bonded to all surfaces of the mold in
spite of the 5.37 percent volume shrinhaf,-e. This failuro mode was lack
of bond to one surface. The two plate.,, when torn apart, had part of
the rubber bonded to each hall'. The visual appearance of the very sn1a111
debond zone was very sintilaar to the type of debond failure observed on
the shim of flex bearing No. 9, rrhe debond apparently occurred between
the tycement and the Cltemlok 220 adhesive. As a result of these tests,
it can be stated that 1ua adequate rubber to stool bond htas enough internal
volumetric "stretch" to easily withstand the 5.37 percent shrinkage expo®
r1unced during the cool-down cycle.
C. PEEL SPECIMENS
The peel specimens are machinod 2 by 4 by 1/4 in. stool plates with
8 by 1-1/2 in, rubber strip vulcanized to one Side. All pool testing; wns
made on an lnstron test machine with certified load cells. The specimens
were installed at a 30 dogroo angle and pulled at a constant speed of 0.2
in. /min.
E
Z
J
U)
N
TEMPERATURE °F
Figure 1. Eleven inch specimen cure cycle,
8
u
Typical test specimens with various combinations of lirimors, adhe-
sives, tyre ►mints, mid rubber stocks tale p'rosented in Table 7. lour
sets of pool ^speelmo s were prepared using two tycemonts prepared
In-house and two furnished by Tblolcol to doicrradne any differences in
piel strongth frog
 tlae two sources of materials,
T L?3711} 7. IN 11011 ',4H. 1*1111SVS TH.IOK'01, TYCHWHiNTS
W-11011se Thiokol
2658 e8.}.7 2658 3827
(PSI) (psi))434 35 qp38 'f3u
3.1 36 +14 36
99 31 39 35
96 32 36 35
28 37 39 1113
Av g.	 38 34 1115 39
D. PR IMER AND ADHESIVE THIC KNESS SPECIMENS
The specimen sul)straat s ware 2 by 9 by 1/16 in. 1010 steel plates,
Coating thickness meostaroments wero made using a (leneral Electric, Type
D and Gardo Dry VIlm thickness gages. The gages were adjusted with
plastic and brass shims to give readings of 1, 2, 3, and 5 mils oat a 1010
steel blank, then a 2 mil brass shish was placed over the tacky tycemeait
coating and the reading ,.  are aas shown in Table 8. A third type thickness
gage, a Pormascope by 'Twin City Corporatlon, wags also used in measuring
the adhesive system thickness.
V. RESULTS AND DISCUSSION
The tonsils tosting resulted in a raant),c at moony temperature of values
from 125 to 900 psi (Table 9) , The failures typically observed were occur-
ring tit the adhosion, tycement and rubber stock interfaces. A 2 in, disc
tensile specimens was cured and immediately installed )lot in the Ins+ran
testes* while still mountod in the mold. The slirinkage-generated load dur-
ing cooldown was measured at K lb. No evidence of damage was observed,
and the specimen pulled to failure in excess of 302 psi.
The intentionally "contaminated" tensile specimens yield sitaailar
strength data to that ohtahiod from clean, "uncontaminated" specimens.
k
9
TABLE 8. THICKNEW MEASUREMENTS
Speeinion No. 205 220 Tycement C.I . Gardo Perninncope
1 1 0.2 0.25
2 1 1 0.3 015
3 1 2 0.85 0185
9 1 2 2 1.5 1.75
5 1 2 2 115 1.75
6 1 0.5 0.50
7 2 1..1,0
8 2 1.27
0 1 0.00
10 1 0.60
11 1 0.97
12 2 0,32
13 1 1 1 1.18
19 1 1 1 1.02
15 2 1 1 1.92
16 1 2 1 1.70
17 1 1 1 1.00
18 1 1 2 1.32
19 1 2 2 1.5 1.75
20 2 0.75 0.80
21 3 1.60 1.50
22 4 2.50 1.20
23 5 3.00 3.00
24 6 3.50 4.30
25 8 6.50 7.50
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Peel testing was considered more representive of the conditions seen
in the fabrication of the SlIal flax bearing, reel testing resulted in
strengths between 20 to 40 psi at 30 degree peel. One set of pool sNeci-
teens was cured by introducing licat from one side only. Half the specs-
mens were cured from the metal side and half with the heat from the
rubber side. There were no observed differences between these specimens
and specimens cured by normal procedures
Using an in-house technique for meal .^ ri^R , Poisson 's Ratio, n, a
measurement of the n of TR 3005 stock was 0. ,WS. In tine same test, the
tensile modulus in the 0 to 6 percent elongation ringe was 390 psi. Using;
these values, the stress at the bond line of a totally enclosed and fully
bonded specimen was calculated to be 26.7 psi.
The shrinkage specimens showed that shrinkage was not uniform.
The average values found were 2.32 percent in length, 2.89 percent in
width, and 2.62 percent in height. Loads obtained due to shrinkage on
the 11 in. specimen were measured at up to 175 lb.
In handling; the cured 11 in. diameter specimens the exposed part of
the cemented mold surface was observed to be tacky and appeared to be
reverted. Subsequent testing of compatibility of the Cliemlok 220 Fund
tycement combinations showed that
1) Tycement in contact with Chemlok 220 and exposed to 300 OF in
air reverts after several hours exposure.
2) Tycement not in contact with Chemlok 220 and given tine saute
exposure cures without rever sion .
3) Tycement in contact with Chemlok 220 and exposed to 300 OF for
6 lnr in the absence of air does not revert.
TIAckness measurements were made using throe types of thickness
gages. Coated specimens (2 by 9 by 1/16-In. steel plates) were averaged
at top, middle, and bottom. lsach specimen was measured three times and
shuffled after each reading; to determine if a reproducible reading, was
obtained. Measurements on the G. E. and Gardo pages gave similar and
reproducible results.
Fillers were .removed from samples of Chcmlok 205 and 220 with a
high speed centrifuge. Using Gel Permeation Chromatographic methods,
the materials were found to contain two high and one low molecular weight
components. These three components were isolated and submitted for
chemical analysis. Infrared analysis revealed mixtui es rather than pure
compounds.
14'
f
VI, CONCLUSIONS
The tensile and heel test specimens consisting; of Chemlok 205, 220
and tycement cured for 1 hr at 300 OF gave a wide range of data; but all
were well above the strength required for bonding a usable SRM flux
bearing. There was no significant difference round in using three tyce-
meats prepared in-house and two furnished by 'Thiokol.
From data collected, there are indications of incompatibility between
Chemlok 220 and tycoments. Varying the time at temperature during cure
shows no adverse trends from data obtained from the normal cure cycle.
There is no adhesion obtained between rubber stock and Chomlolc 220
without a tycement coat. At least 1. mil of tycement is essential in obtain-
ing a good bond line.
The G.L. and Gardo thickness gages give accurate measurements of
the uncured adhesive system thickness and either should be adequate Lis
an inprocess control tool.
Rubber shrinkage during mold cooling adds stress to the rubber
bond line; but not enough to cause concern of debonding in the flex
bearing,
An indepth mold design study by the WK"C tooling experts revealed
no tool redesign necessary at this time. However, several recommendations
have been made to create a more uniform temperature distribution through-
out the mold.
Data collected from the Thixon adhesive systems study indica, ys that
satisfactory bonds are obtained, but no clear advantagc over the Chemlok
205/220 adhesive system is apparent.
15
APPROVAL
EVALUATION OF SRP1 FLEX BEARING
MATERIALS AND PROCESSES
By Thomas iii. Wood
The information in this report has been reviewed for security class-
ifieation. Review of rainy information concerning Department of Defense
or nuclear enemy programs or activities has been made by the MSFC
Security Classification Officer. This report, in its entirety, has been
determined to be unclassified.
C hi
vd H N 'i` . S C: EI IJLL
H
 
Polymers and Composites
Branch
J MES E. CURRY
hief, Non-Aletallic Materit
Division
2	 "
O ERT J .I	 xII MER
Director, Materials anti
Processes Laboratory
._	 1G
{^ U.S. GOVERNMENT PRINTING OFFICE 1980-640
. 247/405 REGION NO. 4


Evaluation of SRM flex bearing materials and processes

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