Technical Field
[0001] The field of art to which this invention pertains is methods for using composites.
Background Art
[0002] Weight saving and manufacturing cost benefits have led to the increase in use of
organic matrix fiber reinforced composite structures in the aircraft and aerospace
industries. In order to be a viable alternative to metal these composites should maintain
the strength typical of conventional structural systems. In many applications composites
are put under a variety of environmental and mechanical stresses. For example, frequently
these composites are exposed over long periods of time to elevated temperatures which
can result in a loss of composite properties. The loss of properties can result from
heat induced microcracks that allow oxygen to attack the fibers. As a result of this
deficiency, extensive research and development efforts have been undertaken to define
methods and identify materials which improve composite performance in elevated temperatures.
For example, it is known that surface preparation of fibers can be important in the
formation of composites. Thus, the fiber can be coated with an organic primer or sizing
agent to produce a surface which with improved handling (i.e., weaveability) and less
subject to contamination when combined with the matrix resin develops the strengths
which meet application requirements. A variety of sizing agents have been used to
produce improved bondability including epoxy, polyimide and polyvinylacetate polymers.
In particular, commonly assigned U.S. Patent No. 4,678,820 describes an amorphous
hydrated metal oxide primer as a fiber size that provides improved wet strength to
a fabricated composite. Although the above surface preparations have provided advantages,
there is a need for new technology to aid in the advancement of lightweight aerospace-type
composite structures.
[0003] Accordingly, there is a constant search in this field of art for new methods of providing
lightweight, structurally sound composites.
Disclosure of Invention
[0004] The invention is directed to a method of using a polymeric composite wherein the
composite exhibits greater oxidative stability. The fiber reinforcement has a layer
of amorphous hydrated metal alkoxide formed by deposition onto the fiber and subsequent
hydrolysis of metal alkoxides. The improvement comprises exposing the composite to
temperatures of about 100°C to about 975°C.
[0005] This amorphous hydrated metal oxide sizing for composite fibers provides improved
oxidation resistance at elevated temperatures. Thus, this invention makes a significant
advance to the aerospace industry by providing new technology relating to structural
composites.
[0006] The foregoing, and other features and advantages of the present invention will become
more apparent from the following description, claims and accompanying drawings.
Brief Description of the Drawings
[0007]
The Figure graphically depicts the oxidation resistance of unsized graphite fiber
and fibers having the sizing of this invention.
Best Mode for Carrying Out the Invention
[0008] Any metal alkoxide that hydrolyzes to give an amorphous hydrated metal oxide (i.e.
a monohydroxy metal oxide) may be used in the practice of this invention. Metal alkoxides
having the formula M
x(OR)
y where x is 1 and y is 3 or 4 are preferred in forming the metal oxide primer of this
disclosure, y being determined by the particular valence of the metal. Typically,
a valence of at least 3 is preferable to form a monohydroxy metal oxide. However,
alkoxides where y is 2 are preferably combined with alkoxides having higher y values.
M is any metal capable of forming a stable alkoxide, which can be purified by, for
example, distillation or crystallization without decomposition; as y is defined above
essentially all metals meet this requirement. It is preferred that the metal is selected
from the group consisting of titanium, zirconium, silicon, nickel, iron and aluminum.
It is especially preferred that M is aluminum. Typically, R can be any organic radical
that can be distilled at temperatures below about 300°C. Since the alkoxide (-OR moiety)
is not incorporated into the primer, the important criteria associated with it is
that the resultant alcohol can be volatilized at temperatures that are not high enough
to damage the primer or substrate. It is preferred that R is an alkane radical from
C₁ to C₁₀. It is especially preferred that R is methyl, ethyl, propyl or sec-butyl
as these radicals are volatilized as alcohols at relatively low temperatures. In addition,
the alkoxides can be modified by incorporation of varying amounts of additives such
as phosphate, silicate or magnesium oxide without affecting the sizing properties.
Mixtures of the above metal alkoxides may also be used in the practice of this invention.
In addition, partial hydrolyzates of the metal alkoxides can also be used as starting
material, i.e., dimers, trimers, etc. of the monomeric alkoxides.
[0009] The above metal alkoxides hydrolyze to amorphous hydrated metal oxides (sizing) when
exposed to moisture such as atmospheric moisture or moisture on the fiber substrate
surface and optionally heat as described below. An exemplary reaction believed to
occur is that of aluminum alkoxide to alumina. The initial hydrolyzation reaction
of aluminum alkoxides is empirically illustrated as
Al(OR)₃ + H₂O Al(OR)₂(OH) + R(OH) (1)
This reaction proceeds rapidly with further hydrolyzation-polymerization to
2Al(OR)₂(OH)+H₂O >RO-

-O-

-OR+2 ROH, etc. (2)
to incorporate n aluminum ions, i.e. Al
nO
n-1(OH)
(n+2)-x(OR)
x assuming linear polymerization for simplicity. As the reaction proceeds the number
of OR groups, i.e. x, relative to n decreases to a value depending on the hydrolysis
temperature and available moisture concentration. Under normal application conditions,
the ratio of residual OR groups as designated by x is less than 4 and n is 28 or greater.
Such low levels of -OR do not impede the performance of the primer. In contrast, zirconium
alkoxide is believed to hydrolyze to a hydrated oxide, i.e. ZrO₂ · 1.7 H₂O having
no residual -OR or -OH groups.
[0010] The thickness of this primer layer can vary effectively from about 1.5 nanometers
(nm) to about 1000 nm. Above about 1000 nm, the layer can be so thick as to create
stress risers and to form a weak boundary layer. Below about 1.5 nm, the layer does
not provide the properties such as crack propagation resistance and stress transfer
at the levels typically required. In addition, it is preferable to apply the primer
to the fiber surface with a plurality of layers of metal alkoxide as this facilitates
removal of volatiles and solvent which can be more difficult to achieve from a single
thick application.
[0011] Any fiber may be used in the practice of this invention that is useful for making
composite articles. Examples include amide, carbon, metal, boron, glass, silicon carbide,
alumina and KEVLAR
TM fibers (DuPont DeNemours, E.I., Co., Wilmington, Delaware). Preferably graphite or
glass fibers are used as these provide the properties most desired of composites such
as strength and light weight. It is especially preferred to use graphite available
from BASF as it resists environmental stresses and produces lightweight composites
having good strength.
[0012] Any conventional resin matrix can be used for the practice of this invention that
is useful for making composite articles. Preferably epoxides, bis-maleimides or polyimide
resins are used as these provide the properties most desired such as good strength.
It is especially preferred to use 3501-6
TM resin available from Hercules, Inc. (Wilmington, Delaware) 5208 resin available from
Narmco,Arimide N
TM resin (E.I. DuPont DeNemours) and LARC-TPI resin (Nasa) as they resist environmental
stresses, are strong and are commercially readily available.
[0013] Any method of making a composite may be used for the practice of this invention that
provides composites having reinforcing amorphous hydrated metal oxide coated fibers.
For example, a chopped fiber composite can be made by mixing chopped fibers coated
with amorphous hydrated metal oxide and resin in a mold under pressure and optional
heat. However, it is preferred to apply a coating of metal alkoxide to the carbon
fibers by drawing the fiber through a solution of metal alkoxide. The metal alkoxide
coated composite fibers are then maintained at a temperature of about 25°C to about
300°C as below 25°C the reaction kinetics are typically too slow and above 400°C loss
of desirable fiber properties or crystallization of the coating may occur with an
accompanying loss of mechanical strength. It is especially preferred to heat the aluminum
alkoxide coated fibers to a temperature of about 100°C to about 200°C as the lower
temperatures minimize the risk of mechanical property degradation of fibers and morphological
transformation of the coatings leading to shrinkage and embrittlement. Surprisingly,
an increase in temperature from room temperature (R.T.) to about 325°C does not affect
an increase in metal alkoxide conversion to amorphous hydrated metal oxide.
[0014] Once coated with amorphous hydrated metal oxide, the reinforcing carbon fibers (tows)
are dipped into the resin solution to form a tape prior to the molding step. Typically,
the resin impregnated fibers are allowed to dry so that any solvent will evaporate.
The graphite tape can then be cut into plies (layers of impregnated fibers) of the
desired dimensions of the article to be fabricated. The plies are then stacked to
create the desired thickness, typically in metal molds coated with mold release agent
such as Micro Spray♢ (Micro-Spray Products Inc.). The assembled ply layup is then
placed in a press and exposed to pressures and temperatures conventional for the resin
system used and application desired.
Example 1
[0015] Inorganic primer was applied to unsized, surface treated, Celion 6000 graphite fiber
available from BASF Corporation by drawing the fiber tow through a 1% toluene solution
of Stauffer Chemical Company aluminum alkoxide, E-8385, and drum winding the coated
tows. The resulting wound tape was heated at 200-325°C in an air oven to dry and thus
produce the amorphous alumina coating.
[0016] The coated fibers and uncoated fibers as controls were tested for high temperature
oxidation resistance using thermogravimetric analysis (TGA). The results are depicted
in the Figure. In the Figure, percent weight loss (Y) is graphed against temperature
in degrees centigrade (X). There is a significant difference in the thermal oxidative
stability as shown by curve (A) (treated graphite) in comparison to curve (B) (untreated
graphite).
[0017] This primer may be used to advantage in a wide range of composites. For example,
chopped fiber, filament wound and ordered fiber composites benefit from this primer.
While this invention has been described in terms of a metal alkoxide, a mixture of
various metal alkoxides can be used.
[0018] This sizing coating provides improved oxidation resistance at elevated temperatures.
At temperatures of about 100°C to about 975°C, this sizing results in more oxidatively
stable fibers and hence composites. The resulting properties such as tensile and flexural
strengths of composites made with the inorganic fiber sizing are believed at least
equivalent or better than composites made using conventional organic fiber sizing.
Yet the inorganic sizing agent can be utilized at thinner layers than the 1.0 wt.
% loading of fibers typical of organic sizing. Thicker layers tend to set up stress
risers and to form a weak boundary layer as the components segregate. Also, because
of its oxidation resistance the inorganic sizing can be used equally as well with
high temperature resins such as polyimides or with low temperature resins such as
epoxy systems unlike organic sizing agents which are typically temperature specific.
[0019] It is also believed that composites using the inorganic sized fibers will have improved
wet strength retention, be less subject to microcracking and exhibit improved fracture
toughness and impact strength properties. Also, the inorganic insulating sheath will
have little, if any, deleterious effect on electrical equipment (e.g., cause shorting)
should such a composite disintegrate.
[0020] This invention provides an amorphous hydrated metal oxide sizing for fiber reinforced
composites resulting in improved oxidation resistance. Thus, it makes a significant
advance in the aerospace industry by providing new technology relating to composites.
[0021] It should be understood that the invention is not limited to the particular embodiments
shown and described herein, but that various changes and modifications may be made
without departing from the spirit and scope of this novel concept as defined by the
following claims.
1. A method of using a fiber reinforced polymeric matrix composite material wherein
the fiber reinforcement has a layer of amorphous hydrated metal alkoxide formed by
deposition onto the fiber and subsequent hydrolysis of a layer of MxORy or combinations thereof where
a) x is 1;
b) y is 2, 3 or 4;
c) M has a valence of 2, 3 or 4; and
d) R is an organic radical;
wherein the improvement comprises: exposing the composite to temperatures of about
100°C to about 975°C wherein the composite has greater oxidative stability.
2. The method as recited in claim 1 wherein M is selected from the group consisting
of titanium, aluminum, and silicon and the fiber is selected from the group consisting
of graphite, glass, silicon carbide, alulmina, boron and polyaramid.
3. The method as recited in claim 1 wherein the fiber is graphite and M is aluminum.
4. The method as recited in claim 1 wherein the composite is exposed to temperatures
of about 500°C to about 975°C.
5. The method as recited in claim 1 wherein MxORy is partially hydrolyzed prior to application of the fiber.