Note: Descriptions are shown in the official language in which they were submitted.
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Flexible reinforcing fiber yarn pre-impregnated with resin
Description:
The invention relates to a pre-impregnated yarn consisting of a bundle of
reinforcing fiber filaments having a bundle interior and a bundle outer side,
wherein the reinforcing fiber filaments are impregnated with a first resin
composition infiltrated into the pre-impregnated yarn, which composition can
be
multiply melted and converted to a solid state by cooling to ambient
temperature.
The invention further relates to a textile structure which comprises a yarn of
this
type.
Components made from fiber composite materials are increasingly used,
especially in the aircraft and space industrial sectors, yet also e.g. in
machine
building, wind power, or the automotive industry. Fiber composite materials
often
offer the advantage of lower weight and/or higher strength over metals. An
essential aspect thereby is the inexpensive production of this type of
resilient and
yet lightweight composite material components at the same time. In view of the
resistance, i.e. the rigidity and strength, the volume percent of the
reinforcing
fibers and especially also the direction of the reinforcing fibers have a
decisive
effect on composite material components.
A commonly used manufacturing method is currently based on the so-called
prepreg technology. In this case, the reinforcing fibers, such as glass fibers
or
carbon fibers, are arranged for example parallel to one another, embedded in a
matrix resin, and processed into sheet-like semi-finished products. For
component
manufacture, these sheets are cut according to the component contour and
laminated layer-by-layer into a tool by machine or by hand while taking into
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account the orientation of the reinforcing fibers as required by the component
load.
Subsequently, the matrix is cured under pressure and at temperature in an
autoclave. Prepregs (abbreviation for pre-impregnated fibers) already have as
a
rule the two components (fiber and matrix resin) in the final mixture ratio
and are
therefore already bending resistant as a semi-finished product. In order to
prevent
premature, undesired reactions, this material must additionally be stored
under
cool conditions and at the same time has only a limited storage period. Due to
the
bending stiffness and the production as wide rolled goods, the applications
for
prepregs are limited to large-surface and virtually flat components. The
matrix
resin already present does not allow for textile processing or laying of the
prepregs
without folds, for example along narrow radii or on strongly contoured
geometries.
Examples for achieving improved textile processing with impregnated yarn
products are described e.g. in US-A-5 275 883 and US-A-4 614 678, which
disclose reinforcing fiber yarns provided with a coating. According to these
documents, the reinforcing fiber yarns are initially loaded with a mixture of
a
polymer powder and subsequently coated with a sheath preferably made from a
thermoplastic polymer in order to stabilize the polymer powder in the
interior.
These yarn materials do indeed have a certain flexibility; however, as a
result of
the continuous thermoplastic sheath they are still relatively rigid, and
therefore are
only of limited suitability for e.g. further textile processing methods.
Similar products are disclosed in EP-A 0 554 950 Al, which relates to a method
in
which initially an open yarn bundle of reinforcing fibers is impregnated with
a
thermoplastic polymer powder and subsequently the impregnated fiber bundle is
provided with a continuous sheath made of a thermoplastic polymer. The
resulting
sheathed bundle is calendered at a temperature above the softening temperature
of the thermoplastic, after which the bundle is finally cut into granular
form. The
granules serve for producing composite components via methods like extrusion
or
injection molding.
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In EP-A-0 814 916 B1, so-called yarn prepregs ("towpregs") are described,
which
are suitable for use in textile preform processes, wherein, among others,
weaving,
braiding, or knitting processes or winding methods ("filament winding") belong
to
textile preform processes of this type, or the yarn prepregs can be processed
into
short-cut material. The yarn prepregs from EP-A-0 814 916 B1 comprise a
plurality
of fibers as well as a coating made of matrix resin, wherein the fibers are
structured in an arrangement of inner fibers, which are substantially free of
matrix
resin, and outer fibers, wherein the outer fibers are at least partially
embedded in a
non-continuous sheath made of the matrix resin. The production of the yarn
prepreg takes place by applying powdery particles of the matrix resin to the
outer
fibers and subsequently partially melting the matrix resin particles. The
matrix
resin used can be a thermoset or a thermoplastic material.
US-A-6 228 474 also deals with the production of yarn prepregs made of
reinforcing fiber bundles impregnated with an epoxy resin composition, wherein
the resin proportion in the yarn prepregs lies in the range from 20 to 50
wt.%. The
epoxy resin composition in one embodiment can comprise two or more epoxy
resins and be a mixture of monofunctional or bifunctional epoxy resins with
trifunctional or polyfunctional epoxy resins. This epoxy resin composition
further
comprises fine particles of a rubber material that is insoluble in the epoxy
resins as
well as curing agent components for the epoxy resins.
Yarn prepregs must have a sufficiently high proportion of matrix resin,
typically
more than 15 vol.%, in order to allow for consolidation to a component
structure
that is substantially free of cavities or pores without requiring the addition
of further
matrix resin. Yarn prepregs of this type do indeed have a higher flexibility
over
sheet-like prepregs. However, they can only be further processed in a limited
way
in textile processes due primarily to the high matrix resin proportion. In
addition,
the presence of the matrix resin often leads to an increased tackiness of the
yarn
prepregs, which results in an increased complexity during handling of these
yarn
prepregs. In addition, as a rule, permanent cooling of the yarn prepregs is
required
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up until the time of processing in order to prevent an uncontrolled curing of
the
matrix resin. Finally, yarn prepregs have disadvantages in the production of
three-
dimensional structures and, for example, cannot be repeatedly reformed.
Increasingly, fiber composite components made from reinforcing fibers are
produced via so-called near-net-shape fiber preforms. Essentially, these fiber
preforms are textile semi-finished products in the form of two- or three-
dimensional
configurations made from reinforcing fibers, in which, in further steps for
producing
the fiber composite component, a suitable matrix material is introduced via
infusion
or injection, also by application of vacuum. Subsequently, the matrix material
is
cured at, as a rule, increased temperatures and pressures into the finished
component. Known methods for infusion or injection of the matrix material are
the
so-called liquid molding (LM) method, or methods related thereto such as resin
transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin
film infusion (RFD, liquid resin infusion (LRI), or resin infusion flexible
tooling
(RIFT). For these applications, reinforcing fibers are used that are not yet
provided
with matrix resin in the amount required for the later composite component
because the matrix material is, as previously stated, introduced into the
finished
fiber preform in a subsequent process step. On the other hand, it is
advantageous
if the fiber material used for the production of the fiber preforms already is
impregnated with e.g. small amounts of a plastic material, i.e. a binder
material,
that is e.g. curable or becomes rigid at reduced temperature, in order to
improve
the fixing of the reinforcing fibers in the fiber preform and to impart a
sufficient
stability to the fiber preform.
WO 98/50211 relates to reinforcing fibers coated with a binder material and
suitable for use in the production of fiber preforms, to which fibers the
binder
material is applied in the form of particles or discrete regions on the
surface of the
reinforcing fibers. The binder material consists of 40 to 90 wt.% of a
thermoset
resin and 10 to 60 wt.% of a thermoplastic, which is tailored to the matrix
material
used in the later fiber composite component produced from the fiber preform.
The
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binder material applied to the reinforcing fibers is rigid and non-tacky at
ambient
temperatures. According to WO 98/50211, the reinforcing fibers thus coated or,
e.g. the woven fabrics produced therefrom, have a good drapability. According
to
WO 98/50211, the individual yarn strands can initially be provided with the
binder
material and subsequently processed into woven fabrics. The yarns from WO
98/50211 are not suitable for the production of flat yarn strands with a fixed
yarn
width, which flat yarn strands would be amenable to an automated, direct
processing into fiber preforms. In addition, the reinforcing yarns coated with
binder
material from WO 98/50211 can have in part relatively high proportions of
binder
material of up to 20 wt.%, which can result in significantly impaired
impregnation
behavior in the fiber preforms produced therefrom.
Pre-impregnated yarns for the production of fiber preforms are also described
in
WO 2005/095080. Regarding the yarns of WO 2005/095080, the filaments of the
pre-impregnated yarns are at least partially connected via a resin
composition,
wherein the yarns have 2.5 to 25 mrt.% of the resin composition in relation to
the
total weight of the yarns, wherein the resin composition is composed of a
mixture
of at least two epoxy resins, and wherein the epoxy resins are different with
respect to their epoxy value and molecular weight. The weight ratio of the
epoxy
resins in the mixture is selected such that the resin composition has an epoxy
value between 550 and 2100 mmol/kg of resin. Alternatively, a mixture of three
bisphenol A epichlorohydrin resins is proposed with defined characteristics of
the
resins with respect to epoxy value, molecular weight, and melting point. The
resin
compositions are selected such that they can be multiply melted and can be
converted again to a solid state by cooling to ambient temperature, and that
the
yarns impregnated therewith are non-tacky at ambient temperature, yet are
tacky
at increased temperatures. However, it has been shown that yarns impregnated
with the resin compositions from WO 2005/095080 do not have a sufficient
tackiness for all applications, e.g. for applications in which yarns are laid
over each
other at an angle of for example 900
.
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There is therefore a need for improved pre-impregnated yarns for the
production of
fiber preforms. It is therefore the object of the present invention to provide
improved pre-impregnated reinforcing fiber yarns of this type, in particular
for use
in the production of fiber preforms.
The object is achieved by a pre-impregnated yarn consisting of a bundle of
reinforcing fiber filaments with a bundle interior and a bundle outer side,
- wherein the reinforcing fiber filaments are impregnated with a first
resin
composition infiltrated into the pre-impregnated yarn and the filaments of the
pre-impregnated yarn are at least partially connected via the first resin
composition, and
- wherein the first resin composition contains at least two bisphenol A
epichlorohydrin resins H1 and H2 in a weight ratio H1:H2 of 1.1 to 1.4,
- wherein H1 has an epoxy value of 1850 to 2400 mmol/kg and an average
molecular weight MN of 800 to 1000 g/mol and is solid at ambient
temperatures, and
- H2 has an epoxy value of 5000 to 5600 mmol/kg and an average
molecular weight MN of < 700 g/mol and is liquid at ambient temperatures,
wherein the pre-impregnated yarn is characterized in that
- it has 0.1 to 2% wt.% of the first resin composition in relation to the
total yarn
weight, and the first resin composition further contains an aromatic
polyhydroxy
ether P1, which has an acid value of 40 to 55 mg KOH/g and an average
molecular weight MN of 4000 to 5000 g/mol, and that
- the pre-impregnated yarn has a second resin composition on the bundle
outer side in the form of particles or drops adhering to the reinforcing fiber
filaments,
- wherein the second resin composition is solid at ambient temperatures,
has a melting temperature in the range from 80 to 150 C and is present on
the bundle outer side in a concentration of 0.5 to 10 wt.% in relation to the
total weight of the pre-impregnated yarn,
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- wherein at least 50% of the surface of the bundle outer side is free of
the
second resin composition, and
- wherein the bundle interior is free of the second resin composition.
It has been shown that the yarn pre-impregnated in such a way possesses
excellent dimensional stability and can be multiply melted and converted to a
solid
state or quasi-solid state by cooling to ambient temperature. In addition, the
resin
for the yarns according to the invention can be selected such that the yarn to
be
coated therewith is non-tacky at ambient temperature. In this case, a non-
tacky
state is understood as a state, such as is e.g. also present for commercially
available standard carbon fibers and which enables a problem-free unwinding up
e.g. from bobbins. Therefore, a yarn of this type then can be not only wound
up,
but also stored in the wound up state while retaining its textile
characteristics and
can even be unwound again after a long storage period at ambient temperature.
For example, the yarn according to the invention can be unwound without
problems after 12 months storage time and shows at most negligible changes for
the characteristics of strength, elastic modulus, and elongation at break
measured
according to DIN 65 382. In a preferred embodiment, the first and/or the
second
resin composition is free of curing agents.
The yarn according to the invention can be a yarn made of short fiber
filaments or
a yarn made of endless filaments. In the case that the yarn consists of
endless
filaments, the number of filaments can lie preferably in the range from 6000
to
48,000 filaments, and particularly preferably in the range from 12,000 to
24,000
filaments. Likewise, yarns having a linear density in the range from 400 to
32,000
tex are preferred, and particularly preferred yarns are those having a linear
density
in the range from 800 to 16,000 tex.
In a further preferred embodiment, the yarn according to the invention is
obtained
from pitch, polyacrylonitrile, lignin, or viscose pre-products, or the yarn is
an
aramid, glass, ceramic, or boron fiber yarn, a synthetic fiber yarn or a
natural fiber
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yarn, or a combination of one or more of these fibers. The yarn according to
the
invention is particularly preferably a carbon fiber yarn.
As previously stated, the yarn according to the invention has a high
dimensional
stability, wherein this is understood to mean that the yarn has a stable,
fixed yarn
width or a stable ratio of yarn width to yarn thickness that remains unchanged
even if the yarn according to the invention is held unsupported over large
distances under tension or is further processed in textile processes. Due to
this
excellent dimensional stability, automated processing, for example automated
laying to form fiber preforms, is enabled. In addition, the fixed and
consistent yarn
width of the yarns according to the invention leads to more stable adhesion of
superimposed yarns during the production of fiber preforms. It was found that
the
dimensional stability of the yarn according to the invention is essentially a
result of
the first resin composition, with which the pre-impregnated yarn is
infiltrated,
wherein the proportion of the aromatic polyhydroxy ether P1 plays a major
role. In
a preferred embodiment, the first resin composition thereby contains the
bisphenol
A epichlorohydrin resins H1 and H2 in a weight ratio to the aromatic
polyhydroxy
ether P1, (H1+H2):P1, of 0.05 to 0.8. It was observed in tests that weight
ratios
lower than 0.05 can lead to increased yarn abrasion. Weight ratios greater
than
0.8 in contrast lead to yarns with an excessively low dimensional stability.
In view
of the dimensional stability on the one hand and the drapability on the other,
it is
also advantageous if the first resin composition is present in a concentration
of 0.4
to 1.2 wt.% in relation to the total weight of the pre-impregnated yarn.
The yarn according to the invention can be brought in a simple way into the
shape
of a flat band already in the production process of the yarn, in that the
initially
impregnation-free, easily spreadable yarn is fed into and through a bath via
suitable spreading devices and impregnated with the first resin composition.
The
first resin composition thereby connects the filaments of the yarn at least
partially
and ensures a very good consolidation. In addition, due to its composition,
the first
resin composition imparts a high dimensional stability to the spread and now
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impregnated yarn, by which means the ribbon shape remains unchanged and the
yarn can be wound up, e.g. on spools, in this shape after the application of
the
second resin composition. Later, then, the inventive pre-impregnated yarn can
be
processed, without additional measures such as the transition via suitable
spreading devices, by means of routine laying methods to produce textile
structures, such as two- or three-dimensional fiber preforms or two-
dimensional
structures, for example in the form of unidirectional wovens or multiaxial
composites. The high dimensional stability enables an advantageous embodiment
of the pre-impregnated yarn, such that said yarn is available as a flat band,
which
has a ratio of yarn width to yarn thickness of at least 20. In a particularly
preferred
embodiment, the flat band has a ratio of yarn width to yarn thickness in the
range
of 25 to 60.
Due to the second resin composition applied to the bundle outer side, it is
achieved in the pre-impregnated yarns according to the invention, that these
are
non-tacky at ambient temperatures and can be e.g. wound up as described. At
increased temperature, however, a high tackiness is achieved due to the second
resin composition, which tackiness also leads to a high stability of the
structure of
the fiber preform after cooling, even in structures in which the yarns
according to
the invention are laid superimposed over one another at an angle. When using
the
yarn according to the invention, preforms can therefore be produced without
requiring the costly addition of binder material for fixing the yarns, wherein
a better
binding still results between the yarns than in a preform of the prior art.
At the same time, it was found that in the indicated concentration of the
second
resin composition, in particular the type of application of the second resin
composition in the form of particles or drops adhering to the reinforcing
fiber
filaments, wherein at least 50% of the surface of the bundle outer side is
free of
the second resin composition and wherein the bundle interior is free of the
second
resin composition, leads to pre-impregnated yarns with high flexibility and
good
drapability. It thereby is shown to be advantageous when the particles or
drops
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adhering to the reinforcing fiber filaments have a size less than 300 pm, and
particularly advantageous if they have an average size in the range from 20 to
150 pm.
To achieve in particular the characteristics of the present yarn with respect
to its
tackiness or its adhesive strength, the second resin composition contains in a
preferred embodiment at least 50 wt.% of a bisphenol A epichlorohydrin resin
H3
with an epoxy value of 480 to 645 mmol/kg and an average molecular weight MN
of 2700 to 4000 g/mol, an aromatic polyhydroxy ether P2, a polyamide, a
polyethylene, an ethylene copolymer such as an ethylene vinyl acetate (EVA)
copolymer or a thermoplastic polyurethane resin or mixtures of these
compounds,
wherein these compounds have a melting temperature in the range from 80 to
150 C. Thereby, embodiments are also comprised, in which e.g. the bisphenol A
epichlorohydrin resin H3 is a mixture of two or more bisphenol A
epichlorohydrin
resins, so long as the mixture has an epoxy value of 480 to 645 mmol/kg and an
average molecular weight MN of 2700 to 4000 g/mol, and a melting temperature
in
the range from 80 to 150 C.
Particularly preferably, the second resin composition contains the previously
mentioned compounds in a ratio of at least 80 wt.% and more particularly
preferably at least 90 wt.%. In a particularly suitable embodiment, the second
resin
composition consists of the indicated compounds or mixtures of the indicated
compounds.
The aromatic polyhydroxy ether P2 used in the second resin composition and the
aromatic polyhydroxy ether P1 contained in the first resin composition can be
the
same or different. However, the condition must be fulfilled for the aromatic
polyhydroxy ether P2 that it has a melting temperature in the range from 80 to
150 C.
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To achieve a sufficiently high adhesive strength of the pre-impregnated yarns
for
the production of the fiber preforms, the second resin composition has a good
adhesive force or adhesive strength above the melting temperature thereof. In
a
preferred embodiment, the second resin composition has an adhesive strength or
adhesive force of at least 5 N at a temperature 20 C above the melting
temperature, in relation to an adhesive surface with a diameter of 25 mm. The
determination of adhesive force or adhesive strength is carried out basing on
ASTM D 2979. In this case, the adhesive force is considered to be the force
that is
required to separate a sample of the second resin composition from an adhesive
surface shortly after bringing the second resin composition and the adhesive
surface into contact under a defined load and temperature and during a defined
time. The details of the determination will be given later.
In view of the total characteristics of the inventive pre-impregnated yarns
and
especially in view of achieving good impregnation characteristics for the
fiber
preforms produced from the yarns during later infusion or injection with
matrix
resin, it is advantageous when the concentration of the second resin
composition
is greater than that of the first resin composition. It is likewise
advantageous when
the total concentration of the first resin composition and the second resin
composition lies in the range from 2 to 7 wt.% in relation to the total weight
of the
pre-impregnated yarn.
In principle, any technology is suitable for the infiltration of the first
resin
composition into the yarn or the impregnation of the yarn with the first resin
composition, which technology supports a fast and complete wetting of the
reinforcing fiber filaments of the yarn with the first resin composition.
Methods of
this type are described for example in EP 1 281 498 A. For example, the yarn
can
be sprayed with a dispersion or an emulsion of the first resin composition. A
film of
the resin dispersion or resin emulsion can also be applied to a smooth roller
or into
the grooves of a roller and the yarn can be pulled over the smooth roller or
through
the grooves of the roller. Preferably, the yarn is fed through a bath which
contains
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a dispersion or emulsion of the first resin composition. It is likewise
possible that
the yarn is successively impregnated with the individual components of the
first
resin composition, for example, in that the yarn is successively fed through
different dispersion baths which contain the individual components of the
first resin
composition. In this case, the yarn provided for the impregnation step can be
initially spread by means of a suitable spreading device to the desired width
so
that the individual fibers or individual filaments are easily accessible for
impregnation. Preferably, the bundle of the yarn to be impregnated is brought
into
the shape of a flat band having the ratio of yarn width to yarn thickness
desired for
the final inventive pre-impregnated yarn.
In principle, any liquid mixture is suitable as the liquid phase for the
previously
indicated resin dispersion or resin emulsion, which liquid mixture forms a
stable
dispersion or emulsion with the inventive resins. Among these liquid mixtures,
in
particular those which are aqueous and have a low VOC (volatile organic
content)
are suitable for emission protection reasons. The components of the first
resin
composition are thereby present advantageously as particles in the micrometer
range, particularly preferably with a size less than 0.1 pm.
Naturally, the application amount of the first resin composition, in relation
to the
total weight of the yarn, can be adjusted via the speed with which the yarn is
e.g.
fed through a bath that contains the dispersion of the first resin
composition, via
the immersion length, and via the resin concentration in the bath. In this
case, the
speed with which the yarn is fed through the bath lies preferably in the range
of
120 to 550 m/h, particularly preferably in the range of 150 to 250 m/h. The
immersion length lies preferably in the range from 0.2 to 1 m. The resin
concentration in the dispersion in relation to the weight thereof lies
preferably in
the range of 2 to 35 wt.')/0 and particularly preferably in the range from 2
to 7 wt.%.
Subsequent to the impregnation of the yarn with the first resin composition,
the
yarn or the yarn bundle is loaded on the outer side thereof with the second
resin
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composition. In this case, after the impregnation, the second resin
composition is
applied in the form of a powder to the outer side of the preferably still
moist yarn
bundle. The application of the second resin composition can take place e.g.
via
powder scattering methods, via aqueous dispersion or via fluidized bed
processes,
as is e.g. described in US-A-5 275 883 or US-A-5 094 883, wherein the
particles
can be preferably electrostatically charged, as is the case in electrostatic
powder
scattering.
The second resin composition present in particle form has a particle size
distribution, wherein in a preferred embodiment the particle size
distribution, as
determined by laser diffractometry, has characterizing values for the particle
size
D50 for the average particle size in the range of approximately 20 to 120 pm
and
D90 in the range from 70 to 160 pm. Particularly preferable is a particle size
distribution with a D50 value in the range of 30 to 100 pm and a D90 value in
the
range from 85 to 155 pm.
A drying temperature in the range of 100 to 160 C has been shown to be
particularly suitable for drying the yarn provided with the first and the
second resin
compositions. By this means, the second resin composition is simultaneously
melted and forms island-shaped particles or drops adhering to the bundle outer
side.
The production of the inventive pre-impregnated yarn can be integrated into
the
production process of the initial yarn, i.e. the impregnation of the yarn with
the first
resin composition and the application of the second resin composition on the
yarn
can directly follow the production process for the yarn provided. However, an
initial
yarn which is e.g. wound up on a spool can also be provided in a separate
process
with the first resin composition and subsequently with the second. It is
likewise
possible that an initial yarn which is impregnated with the first resin
composition is
provided wound up on a spool and is then furnished with the second resin
composition in a separate process step.
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The pre-impregnated yarn according to the invention can be used advantageously
for the production of textile structures like fiber preforms.
A further, underlying object of the present invention is therefore achieved by
a
textile structure which comprises the previously-described yarns according to
the
invention, wherein the yarns preferably are connected to each other via the
second resin composition at points of mutual contact. In a preferred
embodiment,
the textile structure is a fiber preform.
Although wovens can also be produced from the yarns according to the
invention,
said wovens, following melting and re-solidification of the resin
compositions,
resulting in e.g. an exceedingly non-slip fiber preform, it is advantageous to
construct fiber preforms of this type directly from the yarns according to the
invention because the yarns can thereby be positioned in the direction in
which,
during the use of a composite component produced from the fiber preform
according to the invention, the highest mechanical loads are expected.
Thus, in a preferred embodiment of the fiber preform according to the
invention,
the yarns are arranged unidirectionally, by which means the preform can be
further
processed into a composite component, during the use of which the maximum
mechanical load is expected to be in this one direction of the yarns.
In a further preferred embodiment of the fiber preform according to the
invention,
the yarns are arranged bidirectionally, tridirectionally, or
multidirectionally, by
which means the preform can be further processed into a composite component,
during the use of which the maximum mechanical load is expected to be in these
two or more directions of the yarns.
In addition to the previously mentioned flat embodiments of the fiber preform
according to the invention, the uni-, bi-, tri-, or multidirectionally
arranged yarns
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can be wound around a body having, e.g. a cylindrical shape, such that a three-
dimensional fiber preform results.
Further, an embodiment of the fiber preform according to the invention is
preferred
in which the yarns according to the invention were chopped (short-cut) into
short
pieces, and the pieces can be oriented in all spatial directions. By this
means, this
fiber preform is particularly suited for the production of a composite
component,
during the use of which mechanical loads can arise in all spatial directions.
Preforms according to the invention preferably can be produced by a method
comprising the steps
a) Provision of at least one of the yarns according to the invention,
b) Arrangement of the at least one yarn in a configuration
which corresponds to the configuration of the desired fiber
preform,
c) Heating of the configuration resulting from step b) to a
temperature above the melting point of the second resin
composition, and
d) Cooling the configuration resulting from step c) to at
least below the melting point of the second resin composition.
In a preferred embodiment of this method, the configuration resulting from
step b)
is simultaneously compacted during the heating in step c).
The fiber preform according to the invention or the fiber preform produced
according to the inventive method detailed above shows a pronounced anti-slip
property because the yarns of the fiber preform according to the invention are
connected to one another at least via the second resin composition. Therefore,
the
fiber preform according to the invention is easily handled, which is
advantageous
in particular during the further processing thereof into a composite
component.
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When the fiber preform according to the invention or the fiber preform
produced
according to the inventive method should have openings, these openings can be
realized by appropriate arrangement of the yarns and thus without any cutting
losses. Thus, an expensive and labor-intensive cutting is avoided and no waste
is
generated. By this means, the production of composite components having
openings is simplified and reduced in price.
Further, by using the yarn according to the invention instead of a textile
fabric
during the production of the fiber preform according to the invention or the
fiber
preform produced according to the method detailed above, the yarn can be
positioned in the directions in which, during use of the subsequently produced
composite component, the highest mechanical loads are expected.
For example, in a preferred embodiment of the method for producing a fiber
preform, yarns according to the invention are arranged unidirectionally in
step b)
such that following step d) a fiber preform according to the invention results
in
which the yarns are unidirectionally arranged.
In a further preferred embodiment of the method for producing the inventive
preform, the yarns according to the invention can be laid either in bi-, tri-,
or
multidirectional layers in step b) in a configuration that corresponds to the
desired
fiber preform. Yarns according to the invention can be used exclusively
therein.
Likewise, within a layer of yarns, only a part can consist of yarns according
to the
invention and the rest can be yarns, the filaments of which have no resin
coating
or have common yarn preparations used to improve the processability of carbon
fibers. The yarns configured in the indicated way are heated in step c) of the
inventive method at a temperature that is above the melting point of the
second
resin composition, whereby the yarns are compacted if necessary. By this
means,
the yarns become tacky. After cooling to at least below the melting point of
the
second resin composition in step d), an inventive preform is generated in
which
the yarns are arranged bi-, tri-, or multidirectionally.
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17
In a further preferred embodiment of the method for producing the inventive
preform, the yarns according to the invention are cut into short pieces, which
have
e.g. a length of 1 to 1000 mm, preferably Ito 40 mm, and the short yarn pieces
are placed into a mold in step a). Afterwards, in step b) of the inventive
method,
the short yarn pieces are heated to a temperature above the melting point of
the
second resin composition, by which means the short yarn pieces become tacky,
and are thereby compacted if necessary. After cooling to at least below the
melting
point of the second resin composition in step d), a fiber preform according to
the
invention is generated in which the yarns according to the invention are
present as
short yarns having isotropic directionality.
The fiber preform according to the invention, or the fiber preform produced
according to the inventive method can be used advantageously, due to the
previously specified reasons, to produce a composite component that comprises
a
matrix, which is selected from one of the groups of polymers, metals,
ceramics,
hydraulically setting materials, and carbon, wherein thermoplastics like
polyamides, copolyamides, polyurethanes and the like, or duromers such as
epoxides are suitable as the polymer matrix, steel (alloys) or titanium are
suitable
for the metal matrix, silicon carbide and boron nitride are suitable as the
ceramic
matrix, cement or concrete is suitable as the hydraulically setting material,
and
graphite is suitable as the carbon matrix.
The yarns according to the invention are arranged in the resulting composite
components in the direction in which, during use of the composite component,
the
greatest mechanical loads are expected. Thus, use of the yarns according to
the
invention and of the fiber preform produced therefrom leads to composite
components in which the directionality of the yarns is custom adapted to the
expected mechanical loads.
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18
The invention will be described in more detail using the following examples
and
comparative examples. In so doing, the following methods of analysis will be
used:
The epoxy value of the epoxy resins used is determined according to DIN EN ISO
3001:1999.
The molecular weight is determined by means of GPC analysis according to DIN
55672 after calibration with polystyrene (with tetrahydrofuran as the eluent).
The acid value in mg KOH/g is determined by titration with potassium hydroxide
according to DIN 53240-2.
The particle size distribution is determined by means of laser diffractometry
according to ISO 13320. The D50 and D90 parameters for the particle size are
subsequently determined from the particle size distribution.
The melting temperature is determined by means of DSC according to DIN 65467.
The adhesive strength or adhesive force of the second resin composition is
determined at a temperature of 20 C above the melting temperature, based on
ASTM D2979. The adhesive strength or adhesive force is measured as the force
that is required to separate a sample of the second resin composition from an
adhesive surface shortly after bringing the second resin composition and the
adhesive surface into contact under a defined load and temperature and during
a
defined time. For this purpose, a measuring apparatus is used, such as the MCR
301 rheometer (Anton Paar GmbH), which is equipped with corresponding force
sensors and suitable for tensile tests. The determination of adhesive strength
or
adhesive force thereby takes place with a plate/plate measuring geometry using
plates made of aluminum (AlCuMgPb, Wst.-Nr. 3.1645, EN AW 2007) and with a
plate diameter of 25 mm.
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19
Approximately 5 g of the resin composition to be tested (preferably in powder
form) is applied at ambient temperature to the lower plate of the plate/plate
measuring system. Shortly before the contact of the sample material by the
upper
plate, the plates of the measuring system are brought together to a distance
of
approximately 2.025 mm. The sample is subsequently heated by means of a
suitable temperature control device (e.g. Peltier temperature control system)
to the
required measuring temperature of 20 C above the melting temperature of the
second resin composition to be tested. After reaching the measuring
temperature,
the plates of the measuring system are brought together until contact with the
sample material at 2 mm and the sample material is pressed together at a
constant force of 10 N for 5 s.
Subsequently, the upper plate is moved upward at a constant withdrawal speed
of
2 mm/s and a constant temperature, and the force required thereby is
constantly
measured. The maximum value of the force required to pull the plates apart is
used as the measurement for the adhesive strength or adhesive force of the
sample tested.
The determination of the adhesive strength of the pre-impregnated yarns is
done
based on DIN EN 1465:2009. For this purpose, five pieces of yarn are laid
superimposed above one another and placed alternating against one another at 0
orientation in a receiving mold such that they lie on top of one another with
one
end thereof in the middle of the mold with an overlap length of 2 cm. The
adhesive
surface A results from the overlap length and the width of the yarns used. The
stack of yarn pieces is treated for 5 minutes in an oven at an oven
temperature
lying 20 C above the melting temperature of the second resin composition,
whereby the stack is loaded in the middle region thereof with a weight having
a
mass of 2 kg. By this means, the second resin composition is activated, i.e.
it
starts to melt. After cooling, the test body thus produced is subjected to a
shear-
tensile test, in which the ends of the test body are pulled apart at a test
speed of
mm/min. The shear-tensile strength characterizing the adhesive strength of the
CA 02843488 2014-01-29
yarns is determined from the resulting maximum force Fm,, [N] and the adhesive
surface area A [mm2] according to the formula
Fmax [N]
Shear-tensile strength - ______________
A [mm2]
The concentration of the resin composition, in relation to the total weight of
the
yarn and resin composition, is determined via extraction by means of sulfuric
acid/hydrogen peroxide according to EN ISO 10548, Method B.
Example 1:
A carbon fiber filament yarn with a linear density of 800 tex and 12,000
filaments
was fed dry at a speed of approximately 100 m/h at a thread tension of 1800 cN
through a bath containing an aqueous dispersion of a first resin composition.
The
bath was conditioned to a temperature of 20 C. The aqueous dispersion
contained
a first epoxy resin H1 and a second epoxy resin H2 as the first resin
composition
in a concentration of 1.6 wt.%, wherein the weight ratio of resins H1 and H2
was
1.2. The first epoxy resin H1 had an epoxy value of approximately 2000 mmol/kg
and an average molecular weight MN of 900 g/mol, and was solid at ambient
temperature; the second epoxy resin H2 had an epoxy value of approximately
5400 mmol/kg and an average molecular weight MN of < 700 g/mol, and was liquid
at ambient temperature. The aqueous dispersion further contained a linear
aromatic polyhydroxy ether P1 in a concentration of 14.4 wt.% with an acid
value
of 50 mg KOH/g, and an average molecular weight MN of 4600 g/mol, which
polyhydroxy ether was solid at ambient temperature.
After traversing the bath containing the aqueous dispersion of the first resin
composition, the yarn infiltrated with the first resin composition was dried
at a
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21
temperature of 150 C. After drying, the carbon fiber filament yarn had the
first
resin composition, comprising the components H1, H2, and P1, in a
concentration
in the range of 0.6 to 0.8 wt.% in relation to the yarn impregnated with the
first
resin composition, and showed a good compactness of the yarn, i.e. the
filaments
of the carbon fiber filament yarn were at least partially connected to each
other via
the first resin composition.
Directly subsequent to the drying, the yarn impregnated with the first resin
composition was fed through a second bath containing a second aqueous
dispersion. The second aqueous dispersion likewise had the first resin
composition, but in a concentration of 0.5 wt.%. Further, the dispersion
contained
a second resin composition in a concentration of 6.75 wt.%, which comprised an
epoxy resin H3 according to this example. The epoxy resin H3 had an epoxy
value
of 500 to 645 mmol/kg and an average molecular weight MN of 2900 g/mol, and
was solid at ambient temperature. The adhesive strength or adhesive force of
the
second resin composition was determined to be 10 N. The epoxy resin H3 was
present in the dispersion in the form of a powder having an average particle
size
D50 of 70 pm and a D90 of 125 pm.
After leaving the second bath, the yarn now loaded with the first and the
second
resin compositions was dried in that it was fed through two horizontal driers
arranged in series and was dried there at a temperature of 200 C and 220 C,
respectively. The resulting pre-impregnated yarn had the first and the second
resin
compositions in a total concentration of 4.8 wt.% in relation to the total
weight of
the pre-impregnated yarn. The finished pre-impregnated yarn showed island- or
drop-shaped adhesions of the second resin composition on the outer side while
the yarn interior was free of the second resin composition. The pre-
impregnated
yarn had a stable form with a ratio of yarn width to yarn thickness of 38. The
adhesive strength of the impregnated yarn was good. A force of 553 N was
required to separate the unidirectionally adhered yarns, which resulted in a
shear-
tensile strength of 4.03 N/mm2.
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22
Example 2:
This proceeded as in Example 1. Unlike Example 1, the concentration of the
first
and second epoxy resins H1 and H2 was 1.65 wt.% and the concentration of the
linear aromatic polyhydroxy ether P1 was 14.85 wt.%.
The epoxy resin H3 from Example 1 was likewise used as the second resin
composition. Unlike Example 1, the carbon fiber filament yarn loaded with the
first
resin composition was fed, still wet, after exiting the bath containing the
dispersion
of the first resin composition, without drying, through a conventional powder
coating chamber, in which the second resin composition was applied to the yarn
infiltrated with the first resin composition via powder coating. By this
means, the
concentration of the second resin composition on the outer surface of the yarn
was
controlled via conventional measures such as volume flow of the particles of
the
second resin composition and exhaust airflow.
After exiting the powder coating chamber, the carbon fiber filament yarn
provided
with the first and second resin compositions was dried at a temperature of 120
C.
After drying, the pre-impregnated yarn obtained had a concentration of H1, H2,
and P1 (first resin composition) of 0.7 to 0.9 wt.% and of H3 (second resin
composition) of 2.4 to 2.6 wt.%, in each case in relation to the total weight
of the
pre-impregnated yarn. The finished pre-impregnated yarn showed island- or drop-
shaped adhesions of the second resin composition on the outer side while the
yarn interior was free of the second resin composition. The pre-impregnated
yarn
had a stable form with a ratio of yarn width to yarn thickness of 48. A force
of
429 N was required to separate yarns adhering to each other, resulting in a
shear-
tensile strength of 3.68 N/rnm2. The yarns of this example thus had a good
adhesive strength.
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23
Example 3:
This proceeded as in Example 2. Unlike Example 2, a mixture of the epoxy resin
H3 and a copolyamide was applied as the second resin composition via powder
coating to the yarn infiltrated with the first resin composition. The epoxy
resin H3
had an epoxy value of 500 to 645 mmol/kg and an average molecular weight MN of
2900 g/mol, and was solid at ambient temperature. The copolyamide was an
aliphatic copolymer based on caprolactam and laurolactam. The copolyamide had
a particle size distribution with an average particle size D50 of 50 pm, a D90
of
100 pm, and a molecular weight of 10,000 g/mol. It was solid at ambient
temperature and had a melting point of approximately 135 C. Both components of
the second resin composition together were present at a mixture ratio of 1:1
in an
average particle size D50 of 45 pm and a D90 of 125 pm and were applied in
this
composition in the powder coating chamber to the yarn infiltrated with the
first
resin composition. The adhesive strength or adhesive force of the second resin
composition was determined to be 16 N.
After exiting the powder coating chamber, the carbon fiber filament yarn
loaded
with the first and second resin compositions was dried at a temperature of 140
C.
After drying, the pre-impregnated yarn obtained had a concentration of H1, H2,
and P1 (first resin composition) of 0.7 to 0.9 wt.% and of H3 and the
copolyamide
(second resin composition) of 4.3 to 4.5 wt.%, in each case in relation to the
total
weight of the pre-impregnated yarn. The finished pre-impregnated yarn showed
island- or drop-shaped adhesions of the second resin composition on the outer
side while the yarn interior was free of the second resin composition. The pre-
impregnated yarn had a stable form with a ratio of yarn width to yarn
thickness of
27. The adhesive strength was determined to be 678 N and the shear-tensile
strength was 4.44 N/mm2.
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=
24
Comparative example 1:
A carbon fiber filament yarn with a yarn linear density of 400 tex and 6000
filaments was fed dry at a speed of approximately 240 m/h at a thread tension
of
340 cN through a bath with an aqueous dispersion, conditioned to a temperature
of 20 C, of a resin composition comprising two bisphenol A epichlorohydrin
resins
H1* and H2*. The aqueous dispersion contained the first epoxy resin H1* in a
concentration of 8.4 wt.% and the second epoxy resin H2* in a concentration of
6.9 wt.%; the weight ratio of resins H1* and H2* was 1.2. The first epoxy
resin H1*
had an epoxy value of approximately 2000 mmol/kg and an average molecular
weight MN of 900 g/mol, and was solid at ambient temperature; the second epoxy
resin H2* had an epoxy value of approximately 5400 mmol/kg and an average
molecular weight MN of < 700 g/mol, and is liquid at ambient temperature.
After traversing the bath containing the aqueous dispersion of the resin
composition made of H1* and H2* (residence time = 12 s), the yarn infiltrated
with
the epoxy resins H1* and H2* was dried at a temperature decreasing from 250 C
to 140 C. After drying, the carbon fiber filament yarn had the resin
composition,
comprising the components H1* and H2*, in a concentration in the range of 1.2
to
1.4 wt.% in relation to the yarn impregnated with the first resin composition.
Directly subsequent to the drying, the yarn impregnated with the resin
composition
comprising H1* and H2* was fed through a second bath containing an aqueous
dispersion of a third bisphenol A epichlorohydrin resin H3*. The epoxy resin
H3*
had an epoxy value of 515 mmol/kg and an average molecular weight MN of
2870 g/mol, and a melting point of 120-130 C. The second aqueous dispersion
has the epoxy resin H3* in a concentration of 3.8 wt.%, wherein the epoxy
resin
H3* in the dispersion had a particle size in the range from 0.35 to 0.8 pm.
The
dispersion medium consisted of a mixture of 76 wt.% water and 24 wt.% 2-
propoxy
ethanol.
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The residence time of the yarn in the second bath was 15 seconds. After
leaving
the second bath, the yarn now loaded with the resins H1*, H2*, and H3* was
dried,
in that it was initially dried in a vertically arranged drier at 300 C and
subsequently
in a horizontally arranged drier at 330 C. This resulted in yarn impregnated
with
the resins H1*, H2*, and H3* with a total concentration of the resins of 3.6
wt.% in
relation to the total weight of the pre-impregnated yarn. This showed thereby
that
the resin H3* was distributed equally across the entire yarn cross section. At
the
same time, the yarn of the comparative example had no particles or drops of
the
epoxy resin H3* adhering to the outer side thereof; instead, the epoxy resin
H3*
was also distributed uniformly across the surface in the form of a film.
The pre-impregnated yarn had a high rigidity and a comparatively round shape
with a ratio of yarn width to yarn thickness of 3.75. The adhesive strength of
the
yarns of the comparison example was insufficient. A force of 99 N was required
to
separate yarns adhering to each other, which results in a shear-tensile
strength of
2.32 N/mm2.
Example 4:
The pre-impregnated yarn obtained according to Example 1 was wound on a
metal plate, the two faces of which were each covered with a separating film,
by
means of a laboratory winding system at a thread speed of 23.1 mm/s and a
thread tension of 400 cN in each case up to the edge of the metal plate. The
faces
of the metal plate had dimensions of 280 x 300 mm2, and each had first and
second winding axes, respectively, in the middle between the opposing edges.
Initially, a first winding layer with a fiber mass per unit area of 267 g/m2
with a 900
orientation to the first winding axis was generated on the two sides of the
metal
plate. Afterwards, the metal plate was rotated by 90 such that the already
present
winding layer was oriented parallel to the second winding axis. In the next
step,
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26
under identical winding conditions, a further winding layer with a 90
orientation to
the first winding layer was applied to the already present winding layer. In
this way,
a layered structure with a 00 thread layer and a 90 thread layer resulted on
each
of the two sides of the metal plate. The previously described winding process
was
repeated until in each case four winding layers lay on top of each other on
the two
faces of the metal plate, which layers had alternating 0 and 90 thread
orientations.
Subsequently, the winding layers on the two faces of the metal plate were each
covered with a separating film. The metal plate was thereafter conditioned,
complete with both respective four-layer winding structures and the separating
films, in a press for 5 min at a surface pressure of 2 bar and a temperature
of
125 C. The resulting pressing was cooled to below the melting point of the
second
resin composition (epoxy resin H3). Afterwards, the two winding packets were
cut
apart at the end faces of the metal plate and the four separating films were
removed. In this way, two preforms resulted with a respective four-layer
structure
alternating between 0 and 90 , i.e. with a bidirectional arrangement of the
yarns.
The nominal thickness per thread layer was 0.25 mm.
The preforms were very dimensionally stable due to the high adhesive strength
of
the pre-impregnated yarns used and could be handled problem-free for further
processing. In addition, after inserting the preforms in a mold, a good
impregnation
capability of the preforms was determined during the injection of the matrix
resin.
Comparative example 2:
This proceeded as in Example 4. However, the yarns obtained according to
Comparative example 1 were used as the pre-impregnated yarns.
27
The preforms of the comparative example had low dimensional stability due to
the
low adhesive strength of the pre-impregnated yarns used. The handling during
further processing was revealed as problematic due to the instability. In
addition,
during injection of matrix resin, a worsened impregnation capability of these
preforms was determined.
Example 5:
A square piece having an edge length of 200 mm was cut from a preform
analogous to that produced in Example 4 and inserted into a mold having equal
edge lengths and a height of 2 mm, which square piece however had an 8-layer
structure with fiber layers only in the 00 orientation. An epoxy resin (type
RTM6,
Hexcel), previously heated to 80 C, was injected into the mold so that a
composite
with a fiber volume proportion of 60 vol.% results. The preform now
impregnated
with resin was cured at 180 C. A composite laminate resulted having an eight-
layer structure with an orientation of the fibers in the 0 direction.
Test bodies were taken from the composite laminate to determine the inter-
laminar shear strength (ILSS) according to DIN EN 2563 and the compressive
strength and the compressive modulus according to DIN EN 2850. It was shown
that the mechanical characteristics of the composite laminate produced with
the
pre-impregnated yarns of the present invention were at the same level as
corresponding characteristics of a laminate based on standard carbon fiber
yarns
(Tenax HTS40 F 13 12 K 800 tex, Toho Tenax Europe GmbH), even though the
concentration and the composition of the resin application of the yarns
according
to the invention differed significantly from the concentration and composition
of the
standard carbon fiber yarns.
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