Chemical and Engineering Properties of Polyurethane Isolator Pads Morris A. Mendelsohn' and George E. Rudd Research and Development Center, Westinghouse Nectric Corporation, Pittsburgh, Pennsylvania 15235
Glrard B. Rosenblatt Marine Division, Westinghouse Nectric Corporation, Sunnyvale, California 94088
Chemical and engineering properties of certain polyurethanes are described. Isolator pads with specific mechanical properties are cast from these materials for use as liners in missile launch tubes. The liner materials, consisting of pads employing prebuckled struts, support the missile, provide shock mitigation, and damp vibrations. Configurational studies include determination of the effects of variations in shape, thickness, and length of the elastomeric struts on the liner's compression-deflection characteristics. Relationships between the chemical composition of the polyurethanes and compressive stress-strain properties, rate sensitivity, and vibrational damping are presented.
1. Introduction The overall objective of this study was to enhance design knowledge for developing new missile launch systems. This involved the development and investigation of engineering properties of cast elastomeric polyurethane launch tube liner pads (Figure 1). The isolator pads provide an interface between the missile and launcher tube. While supporting the missile in an aligned position, they also provide shock mitigation, guide the missile out of the launch tube during launch, and provide vibration isolation properties compatible with missile response characteristics. In order to meet operational requirements, the isolator must possess proper relationships between mechanical configuration and polymer composition. These requirements include the static and dynamic compressive stiffness, rate sensitivity of compression-deflection characteristics (ratio of dynamic-to-static stress a t a given deflection), high excursion to bottoming, vibrational damping, drainability, good bondability, fatigue resistance, fungus resistance, resistance to degradation in the presence of sea water, and flame resistance. Effects of pad geometry and chemical composition of the polyurethane polymers on the compression-deflection (C-D) characteristics at low and high strain rates, rate sensitivity, and vibrational damping are discussed in this paper. 2. Experimental Section 2.1 Mechanical Tests. The mechanical tests performed in this study consisted of measuring the static and dynamic compression-deflection (C-D) characteristics and vibrational damping on specimens prepared in the laboratory. Usually two specimens, about 6 in. wide and 4 in. long (Figure 1) were cut from castings approximately 12 in. long. After completion of the oven cure, samples were permitted to equilibrate to ambient for about seven days a t room temperature and 50% R.H. and then tested. The mechanical tests performed in this program are similar to those used on previous studies of missile liners (Mendelsohn, et al., 1970, 1971) and will be described briefly. a. Static Compression-Deflection Tests. The static C-D tests are performed in an Instron Universal Test Machine. A pad is loaded to the bottomed deflection (Figure 2), normally at about 65 to 70% compression, and unloaded
without pause for three successive cycles. Three cycles are employed since almost no discernible difference in the C-D data results from additional loading cycles (Figure 3). Cross-head rates of 2.0, 2.0, and 5.0 in./min. were employed for pads having a thickness of 1.78, 3.00, and 4.30 in., respectively. b. Dynamic Compression-Deflection Tests. Dynamic C-D tests were run in a sand-drop, shock-test machine shown in Figure 4. A weight guided by a ball bushing bears against the test specimen. A deflectometer connected to the weight permits the liner deflection to be determined, while an accelerometer mounted on the weight is used to measure the dynamic compressive stress. The deflection and acceleration traces are then recorded on an oscillograph. An adjustable drop-table height, which permits test velocities up to 120 in./sec, provides the proper initial strain rate. Dynamic tests are performed on specimens that had been previously subjected to static C-D testing. Typical dynamic C-D curves are included in Figure 5 . The data reported throughout this study were taken from single cycle dynamic tests; however, the effect of a second dynamic cycle 24 hr later is also shown in Figure 5 . c. Damping Characteristics. The damping quality factor, 8 Q = -
1
2C,/C,
(where C/C, = damping ratio)
is measured in a servo-hydraulic test stand (Figure 6). The 4.30- and 3.00-in. pads (sizes B-E, Figure 1)were preloaded to 0.30 in. compression and subjected to a 10 Hz f 0.02 in. sinusoidal deflection. Figure 7 shows the resulting loadunload hysteresis loop from which Q is calculated. The same test was used for the 1.78-in. (size A) pads except that the precompression was 0.15 in. 2.2 Polymer Processing
a. Chemical Compositions. The polyurethane polymers (Adiprene L100, L167, and L315 are isocyanate terminated prepolymers, manufactured by DuPont, containing polyoxytetramethylene chain segments and having equivalent weights of approximately 1020, 660, and 450, respectively. Conap DP4736, manufactured by Conap, Inc., is a polyoxyisopropylene-based prepolymer having an isocyanate equivalent weight of approximately 800) were prepared by treating isocyanate terminated prepolymers and combiInd. Eng.
Chem., Prod. Res. Dev., Vol. 14, No. 3, 1975
181
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Bail Bushingto Guide Drop wt.
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Clrreer 01 pao geamerry on moae or eornpressmn.
nations of prepolymers with the chain extender, MOCA [4,4'-methylenehis(2-chloroaniline)], a t elevated temperatures. The quantity of MOCA employed for single prepolymers and for their combinations was adjusted to give a NCO: NH2 ratio of 1.00.95. This ratio was selected since lower concentrations of MOCA gave materials which exhibit excessive drift in their properties during aging due to the high NC0:NHz ratios, whereas greater MOCA concentrations resulted in properties which were too rate sensitive. b. Preparation of Cast Pad. The procedure helow describes a typical preparation of a polyurethane casting a t a nominal processing temperature of 8O0C.A solution consisting of 70 parts Adiprene L167 and 30 parts Adiprene L315 is heated to 78-80'C while 21.0 parts MOCA are brought to 124-126OC. Degassing of the Adiprene solution and MOCA a t a pressure of 1-2 Torr for about 5 and 1min, respectively, follows. When the temperature of the prepolymer blend has dropped to 70 f Z0C, it is stirred together with the M0CA;which is a t 122-124OC. After agitating for about 2 min, the resultant solution, a t a temperature of 79-8loC, is poured into the mold which was preheated to 182
Ind. Eng. Chem.. Prod. Res. Dev.. Vol. 14, No. 3. 1975
Figure tics.
the same temperature and then cured a t that temperature for a prescribed length of time.
3. Metchanical Properties 3.1 Effect of S t r u t Angle on Compression-Deflection ntnr:&nl w.,.ii0~ ..,A, a inxnnanionhn -+ .I i i a m Chara-~oLsm~su-. ,I.lrllu~loV1ll. 1971) and Meier e t al. (1972) describes properties obtained from pads having strut configurations that are hent and notched. Due to our current interest in materials having C-D curves that display relatively high compressive spring rate a t low deflections followed by a moderately high stress plateau, it was decided to investigate pads having strut configurations that are hent hut not notched. Simpler
--
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I
n
Owiliowope
n I5
c
‘Linear variable Differential Translormer
Figure 6. Damping measurement apparatus.
I
I
0
0.5
I
I
1.0 1.5 Deflection, inches
I
I
2.0
2. 5
Figure 8. Effect of strut angle on first cycle C-D characteristics.
t
010
-
-
ma
I
--
Structure wlth Straight Columns
,020
Frequency = 10 cps
___ X
--
__
4
Figure 7. Typical hysteresis loop for determining damping. molds are an added advantage with the unnotched strut. The study of the dependence of the shape of the C-D curve on strut configuration began with the preparation of a series of pads having different strut angles (cf. Figure 1 for definition of strut angle and Figures 8 and 9 for effect of strut angle on C-D characteristics). The structure of Figure 9, composed of straight columns (strut angle a = O ” ) , is a limiting condition which has been discussed in previous work (Mendelsohn et al., 1971, 1972). The shape of the static C-D curve is undesirable due to the negative slope a t low deflections. In this region, the dynamic C-D curve is unacceptable due to its sharply negative slope and pronounced rate sensitivity. The spike a t low dynamic deflection is understandable when one observes that as the static buckling load is approached, the column develops a considerable bow. Hence, when the static buckling load is reached, the column has a large bending moment acting in addition to straight compression load. If under dynamic loading the strut does not have time to “fly out,” as is the case shown in Figure 9, a larger compression load can develop before buckling occurs. Once buckling has occurred, the column abruptly loses its high compressive strength exhibiting a rapid decay of the stress. The negative slope of the static C-D curve a t low deflections is the consequence of the reduction in load bearing capability that results when straight columns are bent. That the stress values level out or “plateau” results from the contributions of the various bending forces involved in the deformation of the face sheets and struts. Viscoelastic properties of the polymer also affect the shape of the dynamic C-D curve. Polymers exhibiting greater rate sensitivity and vibrational damping properties give concomitantly higher dynamic spikes. Due to the slow rate of compressing the pads during measurement of static C-D characteristics, the shape of the static compressive
10
20
30 40 Percent Deflection
yl
W
Figure 9. Compression deflection curves for structure using straight columns.
stress-strain curves is less dependent on the viscoelastic properties of the polyurethanes. As expected, the compressive spring rate a t low deflections and the overall load bearing capability decrease with increasing bending of the strut. Figure 8 shows that with a 5 and 10” strut angle, the static C-D curve has a negative slope. The 15’ curve has a good static shape but displays an excessively high dynamic spike. An angle of 20” was then selected since it represents a compromise having desirable static C-D characteristics (Figure 8) and a relatively small dynamic spike (Figure 5 ) . The dynamic spike would be reduced by employing a greater strut angle or a notched design, as was used for previously designed liner pads (Mendelsohn et al., 1970, 1971); however, both of these alternatives would decrease the load bearing properties of the strut or require a compensating increase in strut thickness. Unfortunately, the increased strut thickness would result in an undesirable diminution of the excursion to bottoming. Use of a stiffer polymer with a greater strut angle might improve the compromise; however, this is by no means a certainty since stiffer polymers tend to exhibit greater rate sensitivity. 3.2 Reproducibility Studies. With the strut angle fixed a t 20’, a brief study was made of the reproducibility of the data. On 11 different days, 4.3-in. (Figure 1, size D) castings were poured from a formulation consisting of 65 parts Adiprene L100, 35 parts Conap DP4736, and 13.2 parts Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3, 1975
183
9, i
Prepolymer Compor Nion Adiprene Ll67/Adiprene 315 Stress at 1.0' Deflection
9
-s
a
"i7
zP
6
5 M i n i m u m . M e a n andMaximum of 21 Runs
4
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pads S t r u t Height - 3.6.'' S t r u t Thickness - 0.261 " S t r u t Angle 20° W e r a l l Pad Height - 4.30"
2
-
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P2
a4 (16
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,
8
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1.2 1.4 1.6 1.8
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Composition 65 Adiprene L-IM 35 -Conap DP4736 13.2 - MOCA Cure - 5 h d l W ° C
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l
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l
,
,
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2.6
2 8
0.10
3.0
miation
, 11
.I2
, 13
.14
, 15 .16 .17 .18 Strut Thickness. in
.19
.20 .21
.?2
Figure 12. Effect of strut thickness on compression-deflection Characteristics.
Figure 10. Reproducibility of compression-deflection characteristics.
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,17 . l S .19 Strut Thickness. In
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.Z
Figure 11. Effect of strut thickness on compression-deflection characteristics. MOCA. The detailed results are shown in Figure 10. Even though there are no data from which to conclude that the reproducibility of C-D characteristics will be the same for other formulations and processing conditions, this study nevertheless provides "intuitive" confidence in the results and conclusions made on other pads. (A detailed statistical analysis which would satisfy the multitude of combinations of formulations, curing conditions, and pad geometries would have been such a vast undertaking that it would have precluded many of the studies reported here.) 3.3 Effect of Strut Thickness on Compression-Deflection Characteristics. An investigation on the effect of strut thickness on C-D properties was performed using 3-in. pads (Figure 1, sizes B and C). The results show that the load bearing properties of pads having the 2.69-in. strut height are more sensitive to variations in strut thickness than pads having 2.80 in. high struts (Figures 11-14). This difference in sensitivity is explanatory when considering that the bending of the face sheet affords an important contribution to the C-D characteristics of the pads (Figure 184
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3. 1975
0.M
0.10
0.U Log I l O0.M hl
0.25
0.N
0.3
Figure 13. Relation between strut thickness and compressive stress. 2) and that pads having a strut height of 2.69 in. (face sheet thickness = 0.234 in.) have a face sheet thickness almost twice that of 2.80-in. strut pads (face sheet thickness = 0.125 in.). As the thickness of the strut increases, the thickness of the face sheet becomes more important since at the very low strut thicknesses the ratio of face-to-strut thickness approaches a value where the struts buckle easily while the face does not significantly distort. At low strut thickness such as 0.125 in., the effect of strut height, 2.69 in. vs. 2.80 in., is smaller on an absolute and percentage basis than for the heavier struts (Figures 11-15). Thus, pads having longer struts with a thinner face sheet exhibit less sensitivity of C-D characteristics to strut thickness due to the relatively greater contribution that the distortion of the face sheet makes to the deformation of the pad. The pad configurations of this study utilize opposed struts in order to avoid the problem of lateral shifting of the face sheets relative to one another during compressive loading. The unidirectional alignment of struts in a fullscale pad would be expected to alter the mode of the rippling of the face sheets and thus change their quantitative contribution to the C-D characteristics of the pads. This
Strut Hslghl = 2 . 8 l l n . Upper Face Shod Thickness
= 0.121 In.
PrPpolymer Composlllon
Adlprene LI67IAalprene 1315 o 70130 w20 qQ110
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o
ionlo
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0.9
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1.47
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0 0
In.
,Z .4
,8
.6
1.0
1.4 1.b 1.8 Mlcctlm, in.
1.2
LO L2 24 L b 28 3.0
Figure 16. Effect of strut height to thickness ratio on compression-deflectioncharacteristics.
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1
1
1
9t
17
I I Prepolymer System -Adiprenes t 167 and L315
I
1
I
1
Prepolymer System Adiprene t l W - C o ~ pDP 47%
Strut Dimensions
Heig hi- 3.45" Thickness-0.281'
40
1
I I 50 60,. 70 Prepolymer Composition, % L1W
80
Figure 17. Effect of prepolymer composition on compression-deflection characteristics.
Strut
2
1W
1
90
80 70 60 Prepolymer Composition, 5 L167
I
M
where PO = reference line load and ho = reference strut thickness. The value of ni had been determined by measuring the slope of a dimensionless plot of a reduced line load function, log (PiIPo),against a reduced strut thickness function, log (hilho).
-
1 1%
Figure 15. Effect of polymer composition and pad geometry on load bearing.
difference is minimized with increasing thickness of the face sheet. In earlier work (Kim and Rudd, 1973) on the effects of geometrical parameters, the following relations were determined.
Pi =
ci
[t] "i
where hi = strut thickness, lo = original strut height, Pi = line load a t an arbitrary axial displacement, Ci = variable coefficient, and ni = variable exponent. If strut thickness and resultant loads are the variables, the above equation can be rewritten to give log Pi - log P , ni = log hi - log ho
(2)
[3
=
IZi
log
[$I
(3)
This relationship was employed to determine values of ni and Ci for three different pad geometries. One of the shapes studied, designated as CD-3, has a bent, but unnotched, strut and therefore is similar to the 3-in. pads in this program. The CD-3 design having a strut height (I) of 1.165 in. and a face sheet thickness (f) of 0.150 in. has an Z:f ratio of 7.8,whereas shapes C and D (Figure 1) have 1:f ratios of 11.5 and 22, respectively. Thus, we would expect the C pads to conform more closely to the results obtained in the CD-3 design. Since there is no necessity in this work to apply a dimensionless analysis, a more direct and simpler relationship is employed in which the line load is converted to compressive stress and the various constants are grouped together. log ( S ) = n log ( k )
+
C
(41
where S = compressive stress a t 1.0 in. deflection and h = strut thickness. By plotting log ( S )against log (10h), values of n in the range of 2.40 to 2.52 were obtained for the C size Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3, 1975
185
pads (Figures 13 and 14). This is in excellent agreement with the n values of 2.46 to 2.52 obtained previously (Kim and Rudd, 1973) for a 24-32% deflection (bracketing the 1-in. deflection of the 3-in. pads) of a CD-3 pad having a 20° strut angle. When the face sheet becomes considerably thinner as in the case of shape D pads, the values of n drop to about 2.1, indicating that the stress becomes less dependent on strut thickness. Since the slopes of the log ( S ) vs. log ( h )plots are nearly the same over a series of different polymer formulations (Figures 13 and 14), it appears that over the range of compositions studied, the exponent, n, is essentially independent of prepolymer composition. However, the value of the constant, C, reflects the effect of polymer stiffness. From the plot of stress vs. prepolymer composition (Figure 151, one also observes that the effect of strut height and concomitantly the face sheet thickness diminishes as the strut thickness decreases. From eq 1,we may write the proportion
s
a
[f]
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3, 1975
I
I
80
90
Percent Adiprene L-100
Figure 18. Effect of prepolymer composition on vibrational damping.
Composition
-
8W2U 80 Parts LlW20 Part Conap DP 4736 10
F\
7
Theref ore
186
I
70
50
9
4. Polymer Composition and Properties Earlier studies of polymer compositions consisted of treating isocyanate-terminated prepolymers with a variety of chain extenders consisting of aromatic diamines and polyols. It was found that the polyoxytetramethylenebased prepolymers provide high stiffness and excellent recovery properties. As the equivalent weight of the prepolymers decreases, the stiffness increases, the resilience decreases, vibrational damping improves, and rate sensitivity increases. The polyoxyisopropylene-based prepolymers display extremely high rate sensitivities and cannot be used as a major component; however, their excellent vibrational
I
W
I
40
ni
Thus, if the face sheet were not a factor [f(h)
