Note: Descriptions are shown in the official language in which they were submitted.
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STRUCTURES REINFORCED BY A TRANSPARENT
COMPOSITE MATERIAL
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BACKGROUND OF THE INVENTION
This invention relates to reinforced structures for the
storage and transpor-tation of gases, especially hydrogen
containing gases, at high pressure. More particularly, the
invention relates to reinforced structures such as pressure
vessels and pipe.
Pressure vessels have been produced in a wide variety
of designs. For example, early designs were fabricated from
high tensile strength alloy steels, which resulted in a
substantial weight per unit of volume of vessel, and were
subject to hydrogen embrittlement. Of course, these types of
vessels tended to be unwieldy and, therefore, had a limited
application for portable use.
With the advent of impact extruded aluminum, pressure
vessels were improved to the extent that an approximate thirty
percent weight reduction was achieved over the conventional
steel pressure vessels, while providing an extremely high
resistance to industrial and marine environments as well as to
many corrosive gases, albeit with relatively limited size and
capacity. After the aluminum pressure vessel became well-
established, further improvements involved the over-winding of
the circumference of an inner liner with a composite material,
such as a high strength filament material in an epoxy resin or
the like.
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A disadvantage of vessels overwound with such a
composite material is that currently used resins are opaque, so
that any defects or corrosion which would have been visible in
an unwrapped vessel are not visible. Moreover, defects in the
filaments and in the resin itself below the surface are not
visible. Of course, such defects or corrosion would also not be
visible in a similarly wrapped pipe.
SUMMARY OF THE INVENTION
The present invention relates to a reinforced structure
for containing fluid under pressure, comprising an elongated
hollow member and a band of continuous, high tensile strength
fibers in a resin matrix engaging and encompassing the member,
wherein the fibers have a predetermined index of refraction and
the resin matrix has an index of refraction substantially equal
to the index of refraction of the fibers, whereby the elongated
hollow member is visible through the band.
The refractive index of the resin is adjusted to match
that of the filament material so that the resultan-t composite
reinforcing material is transparent. As a result, any physical
damage to the composite material or to the structure underlying
the composite material is visible. Any labels or any
information stamped in the structure, such as lot numbers and
serial numbers, are visible through the composite material.
BRIEF ~ESCRIPTION OF THE ~RAWINGS
The above description, as well as the objects, features
and advantages of the present invention will be more fully
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appreciated by reference to khe following detailed description
of a presently preferred but nonetheless illustrative embodiment
in accordance with the present invention, when taken in
conjunction with the accompanying figures in which:
Fig. 1 is a front elevational view of a reinforced
pressure vessel according to the present invention;
Fig. 2 is an enlarged partial cross-sectional view taken
along the line 2-2 of Fig. l;
Fig. 3 is a cross-section of a portion of the wall of a
0 curved reinforced pipe according to the present invention;
Fig. 4 is a greatly enlarged cross-section of a portion
of the composite material of Fig. 1 showing the refraction of
incident light; and
Fig. 5 is a front elevational view oE a reinforced
pressure vessel according to the present invention which has
been exposed to excessive heat.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring specifically to Fig. 1 of the drawings, the
reference numeral 10 refers in general to a pressure vessel
formed according to the present invention having a cylindrical
portion 12 and two hemispherical end portions, or heads, 14
formed integral with the side wall 12. A pair of necks 16 are
formed at the respective ends of the vessel 10 integral with the
heads 14. The necks 16 protrude outwardly from the heads 14 and
are adapted to accommodate fixtures, such as reducer bushings or
the like, for accommodating valves for pressurizing or
depressurizing the vessel. A composite material, shown in
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general by the reference numeral 18, extends over the
cylindrical portion 12 and slightly onto a portion of the heads
14 and will be described in detail later. A label 19 which may
contain information about the gas in the pressure vessel 10,
proper use of the vessel, etc., is positioned between the vessel
10 and the composite material.
The composite material 18 is applied by rotating the
vessel 10 and transversely winding onto the vessel, at no
perceptible prestress, filaments 20 of a high tensile strength
material, such as type "E" fiberglass (see enlarged cross-
section of Fig. 4). The filaments 20 are preferably formed in
rovings which are wound around the circumference of the
cylindrical portion 12 of the vessel 10. These rovings, in
turn, are formed by thousands of filaments 20, each filament
having a diameter of less than .001 of an inch. The winding
operation can be performed with a filament winder, or the like,
and since it is conventional, it will not be described in any
further detail. Where the composite material 1~ is used with
pipe, individual pipe sections can be rotated like the pressure
vessel 1, or the pipe can be stationary and wrapped by
conventional machines in which the filament material revolves
around the pipe.
As can best be seen from Fig. 4, the filament material
is coated with a flexibili~ed isophthalic polyester resin 22
having a ~ilene binder which chemically couples the resin 22 to
the filaments 20. The resin 22 is available under the tradename
NCF HiFlex-2 from NCF Industries of Long Beach, California and
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under the name Polylite 98-4640 from Reichhold Chemicals, Inc.
of White Plains, New York. A catalyst, such as 1 1/2% MEK
peroxide catalyst, is added to the resin 22 before the resin is
applied to the filaments 2~, so that the resin 22 gels soon
after the composite material is in place on the vessel. This
amount of catalyst is based on an ambient temperature of 70F.
For a substantially cooler ambient temperature, additional
catalyst may be required, for example, 1 3/4% MEK peroxide
catalyst. For warmer temperatures, an inhibitor is used to slow
the gelling of the resin 22 after the catalyst is added in order
to prevent the resin from gelling before the resin is applied to
the filaments 22 or before the composite material 18 is in
place on the vessel. For example, at 80F, 1 to 1 1/4% of a
.002% solution of hydroquinone in methanol can be added to the
resin 22 after the MEK peroxide catalyst is added in order to
suitably slow the gelling process. The flexibilized isophthalic
polyester resin 22 has over 25% elongation, an elongation in
excess of that of the metal. The coating is done in a
conventional manner, such as by drawing the filaments 20 through
a resin bath or by spraying before the winding operation, to
form the composite material 18 of the filaments 20 in a matrix
of the flexibilized isophthalic polyester resin 22. The
filament material comprises approximately 67%-75% of the
composite material 18.
The composite material is wound helically over the
entire cylindrical portion 12 of the vessel 10 and slightly onto
the area defining the hemispherical heads 14 in a direction
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substantially perpendicular to the longitudinal axis of the
vessel, the helix having a very small pitch. The filaments 20
are wrapped around the vessel under a limited tension, that is,
a tension which is less than a wrapping tension which would
cause the filaments to break. The composite material 1~ is
wound to a sufficient thickness so that the hoop (radial) burst
strength of the vessel 10 reaches from 95% to 100% of the
longitudinal burst strength of the vessel.
After the vessel 10 has been wrapped with the composite
material in the foregoing manner, it is wrapped with a non-
adhering tape (not shown) which holds the resin in place toprevent sagging until it chemically gels due to, Eor example,
the MEK peroxide catalyst, and is then removed. The resin 22 is
then cured in an air furnace or with heat lamps at 135F-
150F for one or two hours, by which it also becomes bonded to
the vessel 10. As alternatives, ultraviolet radiation, a
catalyst or the like can be used for curing.
Fig. 2 depicts the particular configuration of a
hemispherical head 14 formed on one end of the vessel 10 and,
more particularly, depicts the increasing thickness along the
wall of the head 14 from the cylindrical portion to the neck 16
as a result of a conventional rolling technique. As an example
of the distances shown, the cylindrical portion 12 has a
diameter of 13 inches and a wall thickness of .500 inches at the
line A-A. The thickness at section B-B is .625 inches, at
section C-C, .800 inches, and at D-D, .925 inches. As mentioned
abover this increasing thickness in the wall of each head 14 as
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it progresses from its corresponding cylindrical portion 12 to
its neck portion 16 results in an increased vessel strength in
the longitudinal direction. In the example shown, the composite
material 18 has a thickness of approximately .2 inches along a
majority of its length and a slightly increased thickness at its
end portions, as shown by the reference numeral 18a, to avoid
any hinge effect between the respective walls oE the heads 14
and the cylindrical portion 12. This increased thickness at the
end portions 18a of the composite material 18 can be achieved in
the aforementioned winding process by causing the feed eye on
the filament winder to pause, or dwell, for an appropriate time,
which thus increases the amount of filament 20 wound onto the
end portions 18a.
The addition of the composite material 18 to the
cylindrical portion (and a portion of the heads) increases the
burst strength in the radial direction to a value substantially
equaling the longitudinal burst strength, which itself is
relatively high. Thus, the resulting reinforced pressure vessel
enjoys an increased burst strength throughout and behaves as a
0 sphere, with no stress imbalances at any point along the vessel.
Although not clear from the drawing, it ls understood
that the length of the vessel 10 would be in the neighborhood of
4-30 times greater than the diameter of the cylindrical portion
12 to avoid the problem of "end effect" in which resistance to
cyclic fatigue is relatively low due to the fact that the head
and base stiffness is transEerred to the side wall of the
vessel. For example, the vessel 10 discussed above could have a
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length of approximately 240 inches. Of course, the particular
dimensions set forth above have been given by way of example
only and can be varied without departing from the scope of the
invention
The filaments 20 are made of a material, such as
fiberglass, which is very light yet has a high tensile strength.
The aluminum tube and the resin 22 are also light, and the
pressure vessel 10 can withstand a high number of cycles of
pressurization and depressurization. As a result, the pressure
vessel 10 is small enough and light enough to be used as a fuel
tank in a vehicle. In addition, the pressure vessel 10 can
withstand pressures sufficient to contain an amount of gaseous
fuel which will provide the vehicle with a fairly long range on
a single tank of fuel.
As can be seen from the cross-section of Fig. 3, a pipe
26 can also be reinforced by winding the composite material 18
so that the filaments are substantially perpendicular to the
longitudinal axis of the pipe, in the manner described in
connection with the pressure vessel 10, except that the
thickened end portions 18a are not necessary. Pipes wound with
the composite material 18 can be curved or bent along their
length, as shown by the pipe 26 in Fig. 3, to fit various
applications, such as pipelines which must conform to hllls and
valleys. The portion of a wall 27 of the pipe 26 on the outside
of the curve undergoes expansion, with the expansion increasing
for tighter curves. Since the flexibilized isophthalic
polyester resin 22 has an elongation of more than 25%, it can
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accommodate the expansion of the pipe wall 27 on the outside of
the curve without cracking or otherwise failingO The elongation
of pipe materials, such as approximately 10%-25% for aluminum,
is less than the elongation of the resin 22. Thus, the pipe
material would fail as the result of bending the pipe 26 before
the resin 22 would fail.
The high strength unidirectional filament material is
transparent fiberglass, and the flexibilized isophthalic
polyester resin 22 is also transparent. However, it should be
noted that a plurality of layers of such fiberglass ~ilaments by
themselves are not transparent. It is believed that this is due
to the fact that there is air between adjacent filaments, and
that the refractive index of air is substantially different from
the refractive index of the fiberglass~ As a result, there are
numerous refractions and reflections in incident light rays
trying to penetrate the filaments. Thus, the light rays are
prevented from getting -through. Similarly, where a resin matrix
is used with the fiberglass filaments, even if the resin matrix
is transparent by itself, the composite material is not
transparent because the index of refraction of the resin matrix
is not equal to the index of refraction of the fiberglass.
In contrast, as can be seen from Fig. 4, when the
filaments 20 are contained in a matrix of -the resin 22, and the
spaces between the filaments 20 are filled with the resin, and
the refractive index of the resin 22 substan-tially equals the
refractive index of the fiberglass filaments 20, then, the
incident light rays 24 do not bend each time they travel from
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the resin 22 to a filament 20 and vice versa. The light rays 24
are refracted in going from the air into the portion of the
resin 22 defining the outer surface of the composite material
18, but there is not significant refraction of the rays after
they enter the composite material. For example, the refractive
index of type 2079 fiberglass produced by PPG Industries, which
is suitable for the filaments 20, is 1.549. Therefore, the
index of refraction of the flexibilized isophthalic polyester
resin is adjusted to approximately equal the index of
refraction oE the fiberglass, in this case 1.549. Thus, for
example, the index of refraction of the NCF HiFlex-2 resin can
be adjusted by making a cast of the resin, taking a reading of
the index in a refractometer, and by adding or subtracting
styrene monomer to the next batch of resin to raise or lower
its index of refraction to 1.549. Therefore, when the adjusted
resin 22 is used as a matrix for the filaments 20, the resultant
composite material 18 is transparent, and the pressure vessel 10
and the label 19 are visible through the composite material.
Furthermore, the composition of the resin can be varied
slightly so that discoloration occurs at different temperatures.
The flexibilized isophthalic polyester resin 22 discolors when
it is exposed to heat in excess of 400F. For example, it turns
brown, black or charred. Fig. 5 represents a reinforced
pressure vessel 10, like that of Fig. 1~ which has been exposed
in one area 28 to temperatures above 400F. The resin 22 in the
exposed are 28 has discolored, thereby indicating that the
pressure vessel has been exposed to such temperatures and is,
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thus, weakened and not suitable for further use.
The temperature of 400F was chosen for the resin 22 to
discolor, because 400F is a temperature above which aluminum,
the material of the pressure vessel 10 in the present example,
weakens rapidly and substantially. The temperature at which the
resin 22 discolors is adjusted by adding aniline to the liquid
resin before impregnating the filaments 20. By adding aniline,
the discoloration temperature of the NCF HiFlex-2 resin of the
present example can be lowered to as far as 350E`. It is
understood that other suitable resins may turn other colors upon
exposure to temperatures above other selected predetermined
temperatures.
A latitude of modification, change and substitution is
intended in the foregoing disclosure. Accordingly, it is
appropriate that the appended claims be construed broadly and in
a manner consistent with the spirit and scope of the invention
therein.
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