Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Technical Field of the Invention
This invention pertains to manufacturing of natural fibre reinforced thermoset
composite
materials through a modified resin transfer molding technique. In this method,
moisture
sensitive reinforcement fibres can be used in a number of different
arrangements to
impart desired properties to finished product.
Background of the Invention
Natural and wood fibre composites are manufactured by combining wood or other
natural
fibres such as flax, hemp, jute or kenaf, with polymers including
polyethylene,
polypropylene, or polyvinyl chloride (PVC). Composites based on natural and
wood
fibres are one of the fastest growing markets in the plastics industry. They
can be used to
produce products for building, automotive, infrastructure and consumer
applications.
These types of composites present many advantages compared to synthetic fibre
reinforced plastics such as low tool wear, low density, cheap cost,
availability and
biodegradability. For high performance composites bast fibres, extracted from
the stems
of plants such as jute, kenaf, flax, and hemp, are widely accepted as the best
candidates
due to their very good mechanical properties. Hemp especially was shown to
have very
promising tensile properties for such applications 1-3.
Natural fibres consist mainly of cellulose fibres. These fibres are made of
microfibrils in
a matrix of lignin (or pectin) and hemicellulose. The strength and stiffness
of the fibres
are provided by hydrogen bonds and other linkages. The overall properties of
the fibres
depend on the individual properties of each of its components. Hemicellulose
is
responsible for the biodegradation, moisture absorption and thermal
degradation of the
fibre. On the other hand lignin (or pectin) is thermally stable but is
responsible for the
UV degradation of the fibre. On average natural fibres contain 60-80%
cellulose, 5-20%
lignin (or pectin) and up to 20% moisture'.
The thermal stability of the reinforcing fibres is a key parameter in
composite processing,
especially in the case of thermosetting resins and their exothermic curing
behaviour.
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Wielage et al:4 studied the thermal stability of flax and hemp fibres using
differential
scanning calorimetric (DSC) and thermo-gravimetric (TGA) methods. Their
results
suggest that hemp and flax fibres have the thermal stability to endure
thermoset cure
reactions encountered during composite manufacturing.
Another important aspect is the moisture content of natural fibres. These
fibres are
hydrophilic and absorb water. The moisture content can go as high as 20%, but
in most
cases it will be in the range 5 to 10%. This can affect the final properties
of the
composites. During processing the presence of water can create voids in the
matrix and
also lead to a poor adhesion of the fibres with the hydrophobic resini'3. The
hydrophilic
nature of natural fibres can be a problem in the finished composites as well.
Li et al.5 reviewed many papers concerning the mechanical properties of
natural fibres. It
was shown that the tensile properties of these fibres are not uniform along
their length.
In their extensive report on "Composites Reinforced with cellulose based
fibres" Bledzki
and Gassan2 gave some data for various natural fibres as well. As observed
previously the
characteristic values of natural fibres are comparable to those of glass
fibres. The strength
of natural fibres greatly depends on the process used to produce them. In
theory the
elastic moduli of cellulose chains can reach values of 250 GPa. However there
is no
existing procedure to separate these chains from the microfibrils and
therefore obtain
such values. Right now the pulp and paper industry is able to produce
cellulose fibres
with moduli around 70 GPa. Moreover some experimental data obtained from flax
and
pineapple fibres show that the tensile strength of these fibres is
significantly more
dependent on the length of the fibre than for the case of glass fibres.
Natural fibres seem
to be less homogeneous than synthetic fibres. From these papers it can be
concluded that
even if natural fibres are well suited to replace glass fibres in composite
materials many
improvements can still be done concerning their mechanical properties.
Experimental
data giving the tensile strength, flexural strength, modulus, impact force and
compressive
force are available in the literature for different types of natural-fibre
composites.
Research on hemp fibre composite is still in its early stage and only few
publications can
be found in the literature. Keller6 worked on a biodegradable system based on
thermoplastic resins. The mechanical properties of the resulting composites
were found to
be quite low compared to polypropylene. Pervaiz and Sain7 studied the strength
data for
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sheet molded polyolefin hemp fiber composites, and found that the tensile and
impact
strength of these materials were shown to be substantially lower than their
glass fiber
counterparts.
In this work a thermoset resin is the system of choice and hence the following
paragraphs
contain review of papers dealing with such polymers. Among the techniques
available for
the production of thermo-set composites, Resin transfer Molding is a very
popular
process in the automotive and aerospace industries to produce large and
complex parts.
Sebe et al.8 manufactured hemp fibre/polyester composites using RTM. They
obtained
good quality parts with high flexural properties, but the impact strength of
these materials
was found to be very low. Richardson and Zhang9 presented an experimental
study of the
mold filling process for a non woven hemp/phenolic resin system. Fiber washing
was
shown to be a problem at low fibre concentration due to poor clamping. Edge
flow was
observed during the mold filling as well. The use of performs larger than the
mold solved
this problem. The injection pressure and the fiber concentration were shown to
be the
critical parameters to achieve proper mold filling. A few other publications
presented
natural fibre composites manufactured by RTM'""
.
Recently hemp fibre/unsaturated polyester composites were manufactured in our
lab
using a Resin Transfer Molding (RTM) process12. These materials have promising
mechanical properties. Surface modifications of the fibres were proposed in
order to
improve these mechanical properties as well as the fibre/matrix interface
interaction".
The results did not provide substantial changes in the materials properties.
The strength
tests gave promising results. It is anticipated that moisture in fibres is a
key factor to
influence the curing mechanism of resin and hence the final properties of the
composites.
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Summary of the Invention
In this work natural fibre reinforced epoxy vinyl ester resin composites were
manufactured using a Resin Transfer Molding (RTM) process. RTM composites with
fibre contents, up to 40 % by volume, were manufactured. The wetting of the
fibres was
very good. The resin injection time was observed to increase dramatically at
high fibre
contents due to the low permeability of the mat. Surface treatment of fibres
provided
improved moisture resistant properties and also enhanced composite properties.
Loose
fibre, mats and woven as well as non-woven natural fibres were used with
different
design and construction. Examples of woven fabrics are jute, cotton and glass.
Examples
of nonwovens are hemp, flax, hemp-polyester, and flax-polyester. The typical
loose
fibres were hemp, flax, soy, wheat, cotton, corn. Natural fibres were used
either alone in
combination with a thermosetting resin or a hybrid system where glass fibre
and /or glass
mat was used in addition to the natural fibres substrates. Natural fibres and
their
substrates used were either untreated or surface treated with any functional
chemicals.
Typical examples of functional treatment chemicals were maleic coplymers,
maleimide coplymer, aryl- maleimide, their quaternary salts with variable
charge density
and aryl or maleic content. Surface chemicals were also used with variable
molecular
weight. Other surface chemicals used were alkyl imines, and their combinations
with
rosin ester and rosin derivatives. Keeping a constant mold temperature is the
key to
obtain fast and homogenous curing of the part. The experimental procedure
designed in
this research resulted in the production of parts with a good finish and very
promising
mechanical properties. The performance of these samples was evaluated by
measuring
tensile strength and flexural strength.
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Brief Description of the Drawings
Figure 1: Diagram of resin transfer molding equipment
Figure 2: Tensile strength of Silane treated composites (20 vol% fibres)
Figure 3: Tensile modulus of Silane treated composites (20 vol% fibres)
Figure 4: Flexural strength of Silane treated composites (20 vol% fibres)
Figure 5: Flexural modulus of Silane treated composites (20 vol% fibres)
Table 1: Summary of tensile, impact and flexural properties of composites
containing 20
to 30% hemp fibre.
Detailed Description of the Invention
Experiments
A number of polyester/natural fibre composites were manufactured in the lab.
The
final dimensions of the parts were 380mm by 380 mm by 3.4 mm thick. The mold
(30),
comprising two plates and made of aluminium, was opened and closed manually
with 16
screws distributed around the cavity which contained fibres of choice. The two
inlet ports
(20) were situated under the mold and a vent port (40) was located on the top.
It was kept
at constant temperature during the curing reaction by water flowing inside its
upper and
lower sections (50). The water, circulated in a closed loop through a tank,
was kept at
constant temperature with a temperature controller (60) connected to a
thermocouple and
an immersion heater of 2000 watts. To compensate for the heat produced during
the
exothermic crosslinking reaction cold water was kept running permanently in a
copper
coil placed in the tank. This system balanced itself around the preset
temperature during
the experiment. It should be noted that the thickness of the composite was
defined by a
frame placed between the upper and lower plates of the mold; it could
therefore be
modified in further experiments.
Testing
The tensile and flexural strength of the composites were determined using a
SATEC 110000 Materials Testing System. The tensile properties of the materials
were
measured following the ASTM standard method D638-99. The flexural properties
were
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obtained according to the ASTM standard method D790-99. The size and shape of
the
different samples were chosen depending of their thickness as mentioned in
these
methods.
Example 1:
Prior to a typical experiment the surfaces of the mold were cleaned with the
Frekote PM mold cleaner and then coated with the Frekote B-15 Sealer and the
FrekotelD 700-NC mold release agent. Once these coatings were cured layers of
natural
fibres' mats having the mold's size were placed in the cavity (30). The mold
was tightly
closed and a vacuum of 725 mm of mercury was created in the cavity through the
vent
port (40) connected to an aspirator placed on a tap. At this point the fibres
were dried for
2 hours by circulating water at 55 C. The mold was then cooled down with cold
water
(50). In the meantime the resin was mixed with the initiator and placed in the
injection
pot (10). From there the resin was injected (20) in the mold with compressed
air at a
constant gauge pressure of 2.00x105 Pa. This pressure was kept constant in the
pressure
pot by continuously adjusting manually the compressed air valve. The injection
time of
course varied with the amount of fibers present in the mold. Once the resin
was observed
at the outlet, the vent port (40) was closed. A small flask was placed between
the vent
port and the tap for safety, to prevent any resin from flowing to the tap
water. The resin
was left flowing at the inlet for 5 more minutes to make sure that the mold
was filled
completely. Then the inlet ports (20) were closed as well and hot water at
constant
temperature was circulated in the mold (50). The composite was cured under
these
conditions for an hour. Meanwhile the injection pot and all the tubes were
cleaned with
acetone to avoid any clogging due to cured resin.
Example 2:
In this study the polymer used was Derakane[TM] 8084, epoxy vinyl ester resin
obtained from the Dow Chemical Co14. and it contained 45 wt% dissolved
styrene. This
resin is manufactured for use in closed mold processes such as RTM. It is a
low viscosity
resin, which starts reacting by addition of an initiator. In this case the
chosen initiator was
MEKP DDM-9 from Ato-FINA. The resin manufacturer recommends using an initiator
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concentration between 1.5% and 3% by weight. Therefore three different
concentrations
were investigated during the pilot study: 1%, 1.5% and 2%. Following this
study it was
decided that a MEKP concentration of 1.5% should give the best results. To
allow for
curing to occur at 30 C, the resin was mixed with 0.3% by weight of 6% cobalt
naphthenate catalyst (Sigma Aldrich Co.). Additionally, 0.025% of 99% N, N
dimethyl
aniline (Sigma Aldrich Co.) was used as an accelerator, while 1.5% of methyl
ethyl
ketone peroxide (9% active oxygen) was the initiator.
The fibres used in this study were manufactured by Flax craft, Inc. The
Bastmat
100 was a 4mm mat made of 67% hemp fibres and 33% kenaf fibres. Hybrid fiber
mats
consisting of hemp fibres sandwiched between slim glass fibre mats (randomly
oriented)
were prepared. The mats were pre-press at a high temperature (above 80 C) and
at a
defined thickness to reduce their spring back behaviour and allow more fibres
to be
placed in the mold.
Example 3:
A 1% by weight aqueous solution of 3-aminopropyltriethoxysiane was prepared
using distilled water. The solution was then poured in a bottle and sprayed on
the hybrid
fibre mats until soaking them. The mats were left on the bench for 30 min to
allow the
hydrolysis of the silane. Then the fibres were dried first in an oven at 100 C
for one hour
followed by 12 hours at 80 C. Fibres were also separately treated with styrene
maleic
anhydride copolymer with different molecular weight and maleic anhydride
content.
Similar treatment was also carried out with rosin ester and polyethylene-imide
(PEI).
Finally, treatment of fibres was also carried out with maleated imide cationic
polymers
such as styrene maleimide and its quaternary salts with different charge
density and
molecular weights.
Example 4:
The composites with various fibre contents were prepared using the modified
RTM process. Once treated the fibre mats were first placed in the mold (30)
and dried
under vacuum (40). The water evaporating from the fibres could be observed in
the liquid
trap connected to the vent port (70). Once the fibres were dried for 2 hours,
resin mixed
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with initiator was injected (20) at a constant gauge pressure of 2.00 x105 Pa.
This
pressure was chosen by trial and error to provide the shortest injection time
possible
coupled with a proper wetting of the fibers (knowing that the maximum gauge
pressure in
the system should not exceed 2.5 x105 Pa for safety). Then the composite was
cured at
40 C for an hour. Finally each part obtained was post cured in an oven at 105
C to ensure
complete and homogeneous curing of the polyester matrix.
The mold being entirely made of aluminium the flow front could not be observed
during
the resin's injection. The injection time increased dramatically with
increasing fibre
content.
The natural fibber composites manufactured by the RTM process in this work
were found
to be of good quality. An excellent wetting of the fibres was obtained and the
drying of
the fibres prior to resin's injection permitted to avoid the formation of
small gas bubbles
in the part due to water evaporation.
Example 5: (Tensile strength)
The tensile strength tests were performed using samples made from parts that
reached
final degree of cure. Figure 2 presents the tensile strength of Silane treated
Stypol
composite and Silane treated Derakane composite samples. As expected the
tensile
strength of the samples increased with change in resin from Stypol to
Derakane. A
substantial increase was observed between the Stypol resin and the Derakane
sample with
20.6% fibers, from 54.86 MPa to 64.52 MPa. After break, very less fiber pull
out could
be observed on the specimens with Derakane, proving that the fiber-matrix
adhesion was
substantially improved. For information the tensile strength of a glass
fiber/unsaturated
polyester composite i.e. Stypol of similar volume fraction and prepared using
the same
process was added in Figure 2 as well. It can be seen that the natural fibre
composites
manufactured in this work have tensile strengths approximately 20% lower than
their
glass fiber/Stypol counterpart.
Example 6 (Tensile modulus)
The tensile modulus of the Silane treated Stypol composite and Silane treated
Derakane
composite samples are shown in Figure 3. The Stypol hemp fibre composites have
the
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same tensile modulus as the Derakane. Once again the glass fiber/Stypol
results were
added for information showing that the Derakane fibre composites had tensile
modulus
very close to the synthetic fibre.
Example 7 (Flexural strength)
The flexural strengths are reported in Figure 4. The Figure shows the same
trend as the
results of tensile strengths. The flexural strength of the samples increased
with change in
resin from Stypol to Derakane. An increase was observed between the Stypol
resin and
the Derakane sample with 20.6% fibers, from 129.96 MPa to 132.76 MPa. The
explanations for these results are that the flexural properties are influenced
by the
fiber/matrix interface interaction as well. Once again the strength value for
the glass fiber
sample is greater than that for the natural fiber composites.
Example 8 (Flexural modulus)
The modulus results can be seen in Figure 5. The flexural modulus for Derakane
composites exhibits similar modulus when compared to Stypol composite.
Example 9 (Impact properties)
Table 1 gives the summary of tensile, impact and flexural properties of
composites containing 20 to 30% hemp fibre. In this work the manufacturing of
natural
fiber composites using a modified Resin Transfer Molding was investigated. The
drying
process before the resin's injection permitted to obtain a good wetting of the
fibers as
well as to avoid any formation of gas bubbles during curing. In this work data
concerning
the curing behaviour of Derakane 8084, an epoxy-vinyl ester resin were
presented. In
order to achieve high fiber contents with hemp fibres in a process such as RTM
the need
of pre-pressing stage at 100 C was asserted. This additional step reduced
greatly the
spring back behaviour of the fibres, making the closure of the mold much
easier.
The natural fiber composites obtained by this process were found to be of high
quality.
No voids could be observed within the parts. The tensile, flexural properties
were found
to increase with change in resin from Stypol to Derakane. It was observed that
the
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optimum properties were not reached in this study and that the fiber content
higher than
20 vol% should yield better mechanical properties.
The highlights of this invention could be further summarized as:
- The technique has been designed for optimization of RTM
process to achieve the
composites with high mechanical performance from hemp/glass fibers and
synthetic thermoset resin. The resin injection pressure and temperature have
been
optimized.
- The curing system has been standardized by selecting
appropriate resin, catalyst,
accelerators, retarders, coupling agent with their concentrations
- It has been observed that the composite with 26% hemp fiber
and 7% glass fiber
exhibits the optimum tensile strength of 75MPa, Flexural strength of 187 MPa
and modulus of 8GPa and impact strength of 200 J/m.
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