High damping epoxy resin composite

An epoxy resin based composite is described with improved damping properties along with good strength and modulus of elasticity. The composition comprises a stiff epoxy such as epichlorohydrin-bisphenol A diglycydyl ether epoxy mixed with a flexible epoxy such as linoleic dimer acid glycidyl ester epoxy and a flexible cross-linking agent such as a long chain amine-fatty acid amide. The composition in admixture with high modulus fibers such as graphite forms composites with high damping properties, good strength and high modulus of elasticity. Typical uses for the composites of the present invention are flexural beams for hingeless-bearingless rotors and acoustical barrier material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of art to which the invention pertains is mixed epoxy resin 
compositions and fiber containing composites made therefrom. 
2. Description of the Prior Art 
While the prior art has considered various resin mixtures for various 
purposes, a high damping resin composition with good strength and modulus 
of elasticity properties is not available. For example, while U.S. Pat. 
Nos. 3,518,221; 3,598,693; 3,658,748; and 3,923,571 all teach epoxy resin 
composition mixtures including cross-linking agents, none recognize or 
address the problem of high damping properties. Similarly, while these 
same references teach additions of various fibrous filler material 
(3,923,571 excluded), none contemplate high damping composites so 
constituted. 
BRIEF SUMMARY OF THE INVENTION 
The present invention is directed to an epoxy resin composite with high 
damping, high strength and high modulus of elasticity properties. The 
resin component comprises about 12 to about 35% by weight of a high 
stiffness epoxy resin in admixture with about 20 to about 43% by weight of 
a flexible epoxy resin and about 35 to about 61% by weight of a 
flexibilizing curing agent. A composite of such resin in admixture with 
about 20 to about 50% reinforcing fibers results in a high damping, high 
strength and high modulus composite.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The main components of the high damping composites of the present invention 
are the admixture of a flexible long chain aliphatic epoxy component with 
a standard bisphenol-A stiff epoxy resin, and a flexible long chain amine 
fatty acid amide curing agent, blended in specific percentage ranges. 
As the flexible epoxy an epichlorohydrin-bisphenol A diglycidyl ether epoxy 
is used such as Epon 828. Structurally, this is shown as follows: 
##STR1## 
where n=0, 1 or 2 (G.P.C. determined molecular weights of 340, 608 and 876 
respectively). 
The stiff epoxy component is a diglycidyl ester linoleic dimer acid such as 
Epon 871 with the structural formula as follows: 
##STR2## 
The curing agent is a flexible cross-linking agent which is a long chain 
amine fatty acid amide such as Versamid V-40 (General Mills). 
In order to obtain the improved damping, strength and modulus properties of 
the composites of the present invention, these materials must be used in a 
specific percentage range. The flexible epoxy (such as Epon 828) is used 
in a range of from about 20 to about 43% by weight and preferably about 
27% by weight, the stiff epoxy (such as Epon 871) is used in a range of 
about 12 to about 35% by weight and preferably about 27% by weight, and 
the flexible curing agent (such as Versamid V-40) is used in a range of 
about 35 to about 61% by weight and preferably about 49% by weight. 
Composites were formulated by dissolving the resin formulation in a solvent 
such as methyl ethyl ketone to make an approximately 50% by weight 
solution. Graphite fibers were preferably used in reinforcing the 
composites such as HMS (Hercules) and Thornel 75 (Union Carbide) of 
continuous tow. Fiber loading was about 20% to about 50% by weight based 
on weight of fiber plus resin composition, and preferably about 42% by 
weight. The graphite fibers were directed by a pulley through the resin 
bath and wound onto a 17 inch diameter, 6 inch wide drum to produce a 
resin impregnated tape approximately 4 inches wide for testing purposes. 
The particular length and thickness of the tape can be varied depending on 
the ultimate use. After application to the drum the solvent was evaporated 
from the tape at room temperature. The tape was removed and cut into four 
12 inch sections and cured to B stage in an oven at approximately 
80.degree. C. for approximately 15 minutes under vacuum. The tapes were 
removed and cut into approximately 4 by 6 inch sections and laid one over 
the other in a mold for production of a multi-layered composite. While 
fiber laying can be in any desired orientation in the composite, 
unidirectional laying is preferred for flexbeam uses, for example, and 
cross-ply laying (e.g., 0.degree., 45.degree., 90.degree.; 0.degree., 
30.degree., 60.degree.; 0.degree., 90.degree., etc.) for other uses such 
as acoustical or spar uses. The mold was placed in a preheated press at 
about 100.degree. C. and a constant pressure of approximately 200 psi for 
10 minutes was imposed followed by curing under this pressure at 
100.degree. C. for approximately one hour. The molded composite was then 
post-cured for one hour at approximately 125.degree. C. After removal from 
the mold, the composite was cut into appropriate sample size shapes for 
physical, mechanical, and damping tests. Sample formulations are 
demonstrated by Table I and physical property measurements by Table II. 
While the composites of the present invention can be formed in any desired 
shape, depending on their use, square and rectangular cross-section 
composites are preferred, especially for flexbeam use, because of their 
ease of fabrication and particular high damping properties. 
TABLE I 
______________________________________ 
Flexible Epoxy Resin Compositions 
Formulation (wt., gm) 
Material A B C D E F G H 
______________________________________ 
Epon 828 50 40 75 75 60 60 50 50 
Epon 871 50 60 25 40 40 40 45 40 
Versamide 
90 100 100 75 90 80 92 90 
V-40 
______________________________________ 
The composites were tested statically on a universal testing machine using 
a four-point loading technique to measure flexure properties. The results 
are shown in Table III. 
The specimens were instrumented between the inner two-load points and the 
strain recorded on a two-axes plot. The specimens were loaded to failure 
as indicated by complete physical separation or by large excursions and 
strain. Flexural tests were made for two samples of each composite and the 
results averaged. The moduli were determined from the initial slope of the 
stress-strain curves. In some cases, particularly for the higher damping 
samples such as 1, 4 and 9, the curve became non-linear significantly 
prior to failure and thus the values of modulus given in Table III for 
these specimens are somewhat misleading. Nominal values of approximately 
75% of those listed will be more realistic for preliminary design 
purposes. It should also be noted that the flexure modulus determined from 
four-point loading tests correlates well with axial tests. However, it is 
found to be considerably higher than that determined from three-point and 
cantilevered tests. This difference is caused by the influence the matrix 
has in carrying load. 
TABLE II 
__________________________________________________________________________ 
Some Physical Properties of High Damping 
Graphite/Epoxy Composite 
Graphite 
Sample 
Resin 
Fiber Vol. % Density Thickness 
No. Form 
Type Fiber 
Resin 
Void 
gm/cc 
Plies 
in. 
__________________________________________________________________________ 
1 A HMS 18.7 
68.4 
12.9 
1.21 7 0.200 
2 A Thornel 75 
-- -- -- -- 16 0.124 
3 C Thornel 75 
31.9 
63.4 
4.8 
1.36 16 0.142 
4 B Thornel 75 
29.7 
59.6 
10.7 
1.28 15 0.137 
5 D Thornel 75 
28.7 
62.2 
9.18 
1.29 16 0.138 
6 E Thornel 75 
31.9 
58.9 
9.2 
1.31 16 0.137 
7.sup.2 
H Thornel 75 
45 55 -- 1.70 22 0.137 
8.sup.2 
F Thornel 75 
40 60 -- 1.50 21 0.155 
9.sup.2 
G Thornel 75 
42 58 -- 1.55 22 0.155 
10.sup.1,2 
G Thornel 75 
42 58 -- 1.55 22 0.155 
__________________________________________________________________________ 
.sup.1 An additional cure cycle was used on a portion of composite No. 9 
to yield No. 10. The additional cure was 2 hrs at 150.degree. C. 
.sup.2 The physical properties for these composites are estimated values. 
TABLE III 
__________________________________________________________________________ 
Mechanical and Damping Properties of High Damping Graphite/Epoxy 
Composites 
Torsional Shear 
Properties 
Flexural Properties Stress @ 
Sample 
Strength.sup.2 
Modulus.sup.2 
Bending Frequency, cps 
Damping, % Critical 
Modulus Elastic 
No. 10.sup.3 psi 
10.sup.6 psi 
w/o tip wt 
w/tip wt 
w/o tip wt 
w/tip wt 
psi Limit.sup.1 
__________________________________________________________________________ 
psi 
1 -- -- 340 53 4.40 5.45 &lt;5.8 .times. 10.sup.3 
&lt;70 
2 26.77 
23.3 313 -- 1.77 -- 1.0 .times. 10.sup.5 
.about.400 
3 42.89 
22.9 378 53 1.21 1.17 1.31 .times. 10.sup.5 
.about.615 
4 6.45 12.5 336 -- 5.12 -- &lt;1.03 .times. 10.sup.4 
&lt;135 
5 49.6 21.8 385 55 0.87 0.90 2.11 .times. 10.sup.5 
&gt;1500 
6 33.47 
21.1 373 52 1.15 1.19 1.01 .times. 10.sup.5 
.about.403 
7 24.12 
24.8 -- -- -- -- 1.82 .times. 10.sup.5 
660 
8 39.1 27.6 -- -- -- -- 2.49 .times. 10.sup.5 
&gt;1210 
9 15.6 14.3 244 -- 2.64 -- &lt;3.88 .times. 10.sup.4 
&lt;95 
10 18.14 
16.4 238 -- 1.3 -- 1.10 .times. 10.sup.5 
300 
__________________________________________________________________________ 
.sup.1 Determined as the stress at which creep occurred while loading, or 
the stress at which nonlinear behavior was observed. 
.sup.2 4point, S/D = 20/1. Average of two measurements. 
Axial and four-point loading does not subject the specimen to shear 
deformation and therefore the fibers have the major influence on modulus. 
Conversely, cantilever and three-point loading placed the specimens under 
shear deformation and thus, would be influenced to a greater extent by the 
properties (e.g., modulus) of the matrix. Previous results have shown that 
four-point tests have produced modulus values as much as 50% higher than 
those from cantilevered tests. This difference, of course, is less for 
composites with high modulus resins. This accounts in part for the 
apparent inconsistencies in comparing flexure modulus of the various 
specimens and then comparing the corresponding torsion modulus. The 
percent differences in the torsional properties are generally much larger 
due to the above reason. 
The shear properties were determined using a simple torsional loading 
fixture and manually loading cantilever specimens in torsion measuring the 
tip angular deflection and calculating the modulus from the formula 
G=(Q/.theta.'J) where Q is the applied torque, .theta.' the twist rate, 
and J the polar moment of inertia for the high damping specimens. The 
loading fixture was not sufficiently sensitive to accurately measure 
torsion since the weight pans themselves often cause the specimen to 
creep. For these cases estimates were made as noted in Table III. 
Damping measurements were made using six-inch samples supported at one end. 
The samples were excited at their natural frequencies by striking them at 
the free end. The strain was measured at the root end and recorded on a 
Techtronics Dual Beam Storage Oscilliscope. The response curves are shown 
in FIGS. 2-7. The frequencies and damping levels were measured directly 
from these curves. A tip weight of 76 gms was added in some cases as 
indicated on the figures. The tip weight clamp was securely affixed to 
avoid looseness but not so tight as to prevent shear deformation at the 
free end. Some reduction in the free end shear probably occurred which 
would tend to add stiffness and reduce damping; however, no estimate was 
made. The specimen length of six inches was not maintained precisely which 
would account for some minor differences in frequency. Variations in this 
dimension were less than .+-.0.1 inch for all specimens except 9 and 10. 
For these cases the specimen length was approximately 7.5 inches. Damping 
levels were calculated by comparing response amplitudes at two adjacent 
peaks and substituting in the equation for damping ratio: C/C.sub.c 
=.delta./.sqroot..delta..sup.2 +4.pi..sup.2 where C=actual damping, 
C.sub.c =critical damping, .delta.=log X.sub.n /X.sbsb.n+1, X.sub.n and 
X.sub.n+1 represent the dimensions of the two adjacent peaks. 
Damping levels were checked at various points to determine the effect of 
amplitude on damping and although the results showed some difference, no 
general trend was indicated. Also, the effect of tip weight did not 
indicate a consistent trend in its effect on damping. The results of the 
damping and modulus measurements are summarized in FIG. 8. The results on 
this figure should be viewed as qualitative since the moduli were 
difficult to define for the higher damping specimens. 
Attention should also be directed to FIGS. 1 and 9 for a comparison of 
conventional graphite/epoxy material (e.g., Canadian Patent 951,301) and 
the high damping material of the present invention and FIG. 9 when used in 
a matched stiffness composite bearingless rotor utilizing identical 
conditions and frequencies to compare stability characteristics under 
resonant conditions. 
FIG. 9 compares the system stability of a helicopter wind tunnel model 
using a high damping rotor according to the present invention (curve A) 
with that of a conventional or low damping rotor (curve B). As can be seen 
from the tests, the blade system according to the present invention never 
really went unstable. 
As evidenced by Table III composites of the present invention have the 
following properties: flexural strength greater than 5.times.10.sup.3 psi 
and preferably greater than 15.times.10.sup.3 psi; flexural modulus 
greater than 10.times.10.sup.6 psi and preferably greater than 
14.times.10.sup.6 psi; damping up to about 3% critical and preferably up 
to about 6% critical; torsional shear modulus less than about 
3.times.10.sup.5 and preferably less than about 4.times.10.sup.4 ; and 
stress at elastic limit greater than 60 psi and preferably greater than 90 
psi. 
Accordingly, it has been demonstrated that epoxy composites of the 
composition and percents specified have high internal damping. These 
damping levels can be controlled in this range chemically and through 
curing cycles as recited producing damping up to about 6% critical. The 
material at the high end of the range is generally unacceptable from the 
standpoint of stiffness and strength but the mid-range damping composites 
(e.g., sample No. 9 with a 2.64% damping) shows promise as a composite 
bearingless rotor flexbeam material for low edgewise stiffness designs. 
The damping levels do not appear to be affected by the addition of a 
concentrated mass, response frequency, or response amplitude, however, 
note the test in a rotating environment discussed above. In addition to 
the damping properties, the low shear modulus of these materials would 
allow significant reduction in flexbeam length. 
Although this invention has been shown and described with respect to a 
preferred embodiment, it will be understood by those skilled in this art 
that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.