The present invention relates to a process for the preparation
of vegetable fibres having an improved moisture resistance, and
more in particular to the preparation of long multicelled vegetable
fibres having an improved moisture resistance; to the moisture
resistant vegetable fibres prepared by said process and to their
use as fibre reinforcement in polymer matrices.
Vegetable fibres form a class of very desirable products for a
wide range of applications, for reasons of cost and performance.
For example, the mechanical performance properties of flax, which
is a well-known example of a long multicelled vegetable fibre, are
comparable to those of glass and metal fibres, when taken on weight
basis. The vegetable fibres not only have a much lower density than
glass and metal, but are moreover also considerably cheaper. Due to
their lower rigidity or higher flexibility it can be expected that
the vegetable fibres will result in a more favourable
processability compared to glass, especially with regard to
abrasion of equipment and to fibre-breakage, e.g. during fibre
dispersion.
A field of application wherein the hereinbefore mentioned
characteristics of vegetable fibres may play an important part, is
their use as reinforcing fibres, for example in polymer composites.
Notwithstanding the hereinbefore described positive
characteristics of vegetable fibres, their use as reinforcing
fibres has one serious drawback when compared with that of e.g.
glass, i.e. their sensitivity to moisture. When vegetable fibres
are contacted with water or employed in a moist atmosphere, they
tend to absorp water and swell, which phenomenon may also occur
when they are employed as fibrous reinforcement in e.g. polymer
composites, thereby affecting the dimension stability of said composites.
The vegetable fibres belong to the class of lignocellulosic
materials, which materials have three major components in common,
i.e. cellulose, hemicellulose and lignin. Cellulose is a high
molecular weight linear polysaccharide, which as result of its high
molecular weight is insoluble at room temperature in water and
dilute acids and alkali. It is a substantially crystalline material
and the major structural component of the cell wall of said
lignocellulosic materials, such as the vegetable fibres, and is
primarily responsible for the strength of these fibrous materials.
Hemicellulose, on the other hand, is a poorly ordered,
non-crystalline, relatively low chain length non-cellulosic
polysaccharide, and occurs in close association with the cellulose
in the cell wall as well as with lignin between the cells. Lignin
is chemically speaking a totally different type of compound, being
a phenol-based aromatic polymer comprising phenyl-propane units. It
occurs mainly as an encrusting agent between the fibres and on the
outer wall of the cell.
It is generally accepted that hemicellulose is not only the
most hygroscopic of the three components mentioned hereinbefore but
moreover also the most accessible. Hence the hemicellulose
component present in the lignocellulosic materials, is generally
considered to be the cause of their dimension instability due to
moisture absorption.
Cellulose is also hygroscopic, but contrary to hemicellulose
it hardly swells, if at all, as a result of moisture absorption and
hence does not contribute to the dimension instability of vegetable
fibres.
Lignin does not play a part in the process of moisture
absorption.
Processes for treating lignocellulosic materials and
comprising an at least partial removal and decomposition of the
hemicellose component, are known from numerous publications. For
example a method for treating green vegetable fibres is known from
GB-A- 388 561, which describes a process
wherein vegetable fibres are submitted to a steam treatment at high
temperatures and pressure for a relatively short time, followed by
extrusion under pressure from the reaction chamber through one or
more orifices. The latter treatment causes the fibres to be rent
apart by expansion of the steam and ultimately results in loose
free fibres i.e. materials wherein the cellulose walls have been
ruptured. When the steam treatment is prolonged the fibres are
converted to a more or less pulpy condition.
In GB-A- 476 569, a similar process
is described wherein the steam/water treatment is conducted in
two-stages, the pressure in the second stage being lower than in
the first. With both processes the disintegrated fibrous materials
are purified by washing and used e.g. as packing material or for
board-making.
In WO-A-8704194 a process is
described for the preparation of modified fibrous flax, which
comprises subjecting a fibrous plant to the action of high pressure
steam followed by expansion to atmospheric pressure and finally
washing the thus obtained fibrous materials. Said fibrous
materials, before spinning and after carding, essentially consist
of elementary fibres which are free from encrusting materials; and
thus lack mechanical strength.
In EP-A- 0 161 766, a process is
described for the preparation of composite products from ligno-cellulosic
materials, which comprises treating the lignocellulosic
material in divided form with steam to heat the lignocellulosic
material to a temperature high enough to release hemicellulose and
for a time sufficient to decompose and hydrolyse hemicellulose. The
thus treated lignocellulosic material is subsequently formed into a
mat and compressed at a temperature not exceeding the temperature
at which the mat would char, and at a pressure and for a time
sufficient to transfer the hemicellulose decomposition products
into a polymeric substance, which adhesively bonds together the
lignocellulosic material. In the thus obtained boards the cellulose
cell walls of the vegetable fibres are irreversibly deformed, i.e.
flattened, and due to a combination of compression and reaction of
the hemicellulose decomposition products, said vegetable fibres
have permanently lost the characteristics of individual vegetable
fibres. It is mentioned that the treated materials are almost
completely free from hemicellulose and hence have a better
dimension stability. In one embodiment of said process, which
appears to be preferentially applied to selected agricultural
fibrous materials, the steam pressure is suddenly released upon
completion of the steam treatment, which treatment is known to
explode and shred the treated material into a fibrous lump.
In EP-A- 0 373 726 a process is
described for the preparation of a fibrous aggregate wherein a
section of cellulosic fibrous material, i.e. softwood, is exposed
to the action of an aqueous softening agent at a temperature in the
range of from 150 to 220°C at a pressure which is at least the
equilibrium pressure of the softening agent at the operating
temperature, thereby at least partially disproportionating and
hydrolyzing the hemicellulose and the lignin present in the
cellulosic material, and a curing stage comprising drying the
product of the softening stage at a temperature in the range of
from 100 to 200°C, preferably in a mould under pressure, to yield a
shaped cross-linked cellulosic matrix.
The vegetable fibres which have been treated by anyone of the
hereinbefore mentioned processes have in common that at least part
of their hemicellulose has been removed by means of hydrolysis and
hence it can be expected that this will result in an improvement of
their moisture resistance. Regarding the end products formed there
is a considerable difference. With some of the processes the end
products are loose fibrous materials of which the cellulosic cell
walls have been ruptured, while with other processes the end
products are composite products or aggregates wherein the
cellulosic cell walls have frequently been ruptured and/or the
characteristics of the individual fibres have disappeared
altogether.
As known from numerous textbooks on the subject of
lignocellulosic fibres, such as "Structural Biomaterials", Julian
F.V. Vincent, The Macmillan Press Ltd., London and Basingstoke
(1982), "Cellular Solids - Structure and Properties", Lorna J.
Gibson and Michael F. Ashby, Pergamon Press. Oxford 1988, and "Wood
Chemistry-Fundamentals and Applications", Eero Sjöström, Acadamic
Press, New York (1981), the high level of performance properties of
these natural fibres, such as the vegetable fibres, are directly
attributed to the presence of specific cellulosic cell structures
in these fibrous materials, moreover, any distortion of or damage
to said cell structures resulting from e.g. mechanical and/or
chemical action, will be reflected in the level of performance
properties, e.g. by an appreciable reduction in mechanical
strength.
Hence it can be concluded that the methods for treating the
vegetable fibres as described hereinbefore, and which result in the
production of aggregate and non-aggregate fibrous materials having
potentially improved moisture resistance, and wherein at least an
appreciable amount of the cellulosic structures have been damaged
or distorted, are not suitable for the production of vegetable
fibres which combine increased dimension stability with high
strength.
As there is considerable need to make such dimension-stable
fibres available, the problem underlying the present invention is
to develop a suitable process for the production of these fibres,
i.e. fibres having an improved dimension stability as a result of
reduced moisture absorption combined with good mechanical
performance properties, i.e. fibres wherein the original cellulose
cell wall has essentially been maintained.
As a result of extensive research and experimentation it has
surprisingly been found possible to prepare vegetable fibres having
the desirable characteristics as described hereinbefore by a
process wherein long, multicelled vegetable fibres are
consecutively submitted to a steam treatment, a decompression/
cooling step and a curing step.
Accordingly the invention provides a process for upgrading
long, multicelled vegetable fibres or assemblies thereof, which
comprises the steps of a submitting long multicelled vegetable
fibres or an assembly based thereon, to a water and/or steam
treatment at a temperature in the range of from 130 to 250°C and at
a pressure which is at least the equilibrium pressure of the
operating temperature and for a time sufficient to decompose at
least part of the hemicellulose present in said fibres, b) a
controlled decompression/cooling of the reactor contents, c)
isolating the fibres or assemblies thereof, and d) submitting the isolated fibres or assemblies thereof in a
heating chamber to a temperature in the range of from 140 to 200°C to cure said fibres or assemblies thereof without aggregating them.
The nature of the vegetable fibres which may be employed in
the process of the present invention is not critical and may
include long, multicelled vegetable fibres which originate from the
leaves, stem and bark of plants. They may be employed as harvested
or after having been submitted to a subsequent treatment which
leaves the cellulosic cell wall essentially unaffected, such as
drying, retting, hackling, stripping, scraping and carding.
The vegetable fibres may be employed in the form of individual
fibres, i.e. the fibres as harvested or as obtained by anyone of
the treatments as described hereinbefore, but also in the form of a
fibre assembly, i.e. individual fibres which have for example been
converted to woven and non-woven fabrics, yarns, cords and paper
sheet.
In the context of the present invention the term fibres or
individual fibres refers to bundles of single fibrous cells,
wherein each cell contains a variety of layers which together form
a basically cylindrical arrangement, hence the term multicelled
fibre.
Examples of suitable leaf fibres include abaca, bowstring
hemp, pineapple and sisal; examples of bast fibres include jute,
flax, hemp, kenaf and ramie, while suitable stem fibres include
common bamboo, banana stalk and coconut.
As mentioned hereinbefore, the fibres may be employed as
harvested but also in a dried form. Advantageously the vegetable
fibres are presoaked in water to obtain fibres which are saturated
with water, before being introduced into the reactor. In said
reactor the fibres are contacted with water in a water/fibre weight
ratio which generally is in the order of at least 1 when working
batchwise, whereas the water/fibre ratio will typically be >10 and
preferably >20 when operating continuously under slurry conditions.
Subsequently the reactor contents are treated at a temperature in
the range of from 130 to 250°C and a pressure which is at least
equal to the equilibrium vapour pressure at the temperature of
operation, and for a time sufficient to decompose at least part of
the hemicellulose present in said vegetable fibres. Simultaneously
with the decomposition of the hemicellulose, lignin can also be
partially decomposed, which results in the formation of fenol type
or fenol derivative type of decomposition products. A complete
decomposition of the lignin would require a temperature which is
considerably higher than the temperature at which the process of
the present invention is conducted. Preferably the temperature for
treating the fibres is in the range of 160-190°C. When the
vegetable fibres have a high lignin content, such as bamboo, cocos
and jute fibres said temperature will be in the range of from
180-190°C, while for low-lignin fibres such as abaca, flax and
linen, said temperature will be in the range of from 160-170°C. In
general very good results were obtained when the vegetable fibres
had been submitted to a heat treatment at a time/temperature in the
range of from 60 min/160°C to 15 min/180°C, although shorter and
longer exposure times at the indicated temperatures should not be
excluded.
One of the hemicellulose decomposition products which may be
formed in addition to sugars and aldehydes when exposing vegetable
fibres to water/steam as described hereinbefore, is acetic acid,
and will result in a drop in the pH of the reaction medium. The
presence of acetic acid may accelerate the hemicellulose
decomposition, simultaneously in a partial decomposition of the
cellulose.
Should, however, the formation of acetic acid and the related
phenomena be unacceptable, it can be counteracted by conducting the
water/steam treatment of the vegetable fibres in the presence of a
pH buffer. Suitable buffering agents have a pH in the range of from
4-7 and more preferably from 4.5 - 6.5.
The buffering agent is suitably a mixture of a base or an
acid, and a salt of an organic acid. The buffering agent is
preferably a mixture of acetic acid and an ammonium or alkali metal
salt thereof; alkaline earth metal salts such as magnesium and
calcium salts may also be used. Preferred alkali metal salts are
sodium and potassium salts. The concentration of the buffering
agent in water is suitably between 0.01 and 5 mol/litre and
preferably between 0.05 and 2 mol/litre, and wherein the
concentration of the buffer is considered to be the joint-concentration
of the salt and acid or base. Should an aqueous
buffering solution be used in the process of the present invention,
it may, when appropriate, already be present during the presoaking
of the vegetable fibres as described hereinbefore. As an
alternative to the addition of the complete pH buffer, e.g. the
acid and alkali metal salt thereof, it is also possible to add only
the alkali metal salt, e.g. sodium acetale, and use the in situ
generated acetic acid as the acid part of said pH buffer.It will be
appreciated that the use of a pH buffer in the process of the
present invention is especially advantageous when conducting said
process on a large scale.
As mentioned hereinbefore the water/steam treatment of the
vegetable fibres is followed by a controlled cooling/decompression
step. Depending on the type of reactor employed for the water/steam
treatment of the vegetable fibres, the cooling of the reactor
contents can be accomplished by means of external and/or internal
cooling. A further possibility to lower the temperature of the
reactor contents is via decompression of the reactor, which will
result in "adiabatic" evaporation of the liquid phase and hence in
a reduction of the temperature thereof. When the decompression mode
of cooling is applied, care should be taken that it is a very
gradual and well controlled decompression which allows evaporation
and diffusion of moisture within the fibres, but does not result in
rupture of the cell walls due to an explosive evaporation of the
liquid phase within the fibres. At any one moment the pressure
within the reactor as a result of the decompression should not be
very different to that of the equilibrium vapour pressure at the
prevailing temperature. Advantageously cooling and decompression
are applied simultaneously to reduce the temperature of the reactor
contents.
When the temperature in the reactor is below 100°C and the
pressure within the reactor equals the atmospheric pressure the
vegetable fibres, of which at least part of the hemicellulose has
been decomposed, may be isolated from the aqueous reaction medium
via known techniques such as filtration and decantation.
Subsequently the isolated fibres are heated at a temperature
in the range of from 140 - 200°C. During this treatment, which will
be referred to as the curing step, it is believed that a reaction
will occur between the various decomposition products, which will
increase the moisture resistance and the dimension stability of the
treated vegetable fibres. This curing step is
conducted in a heating chamber. In order to achieve the highest
possible degree of reaction during said curing step, care should be
taken to reduce the loss of reactive decomposition products. In
this context it is considered advantageous to dewater and dry the
isolated fibres, preferably at ambient temperature, with the aid of
a drying agent, prior to the curing step. Suitable drying agents
include silicagel, magnesium sulfate and calcium chloride.
The time during which the fibres are submitted to the curing
step is largely determined by the actual cure temperature.
Conveniently the cure time may vary from 0.25 - 10 hours at a
temperature in the range of from 200 - 145°C respectively. When
green fibres, i.e. fibres which have not undergone any treatment,
are used in the preparation of the vegetable fibres having improved
moisture resistance and dimension stability, it is possible that
the ultimate products are coloured and/or soiled. Should this be
unacceptable for certain applications, then the steam/water treated
fibres may be submitted to an aqueous washing procedure for which
generally water having a temperature in the range of from room
temperature to 65°C is used, prior to being cured. The resulting
cured fibres will generally have a considerably lighter colour.
However, it should be borne in mind that during the aqueous
wash certain reactive organic compounds, especially water-soluble
compounds, may be removed or extracted from the fibres. Hence it is
possible that these fibres will have a somewhat lower moisture
resistance and dimension stability after curing than the
corresponding non-washed fibres, although still being considerably
superior in this respect than untreated or green fibres.
The long, multicelled vegetable fibres as described in claim 9 and prepared according
to the process of the present invention were indeed found to have
improved moisture resistance and dimensionstability when compared
with the corresponding untreated fibres.
These improved fibre characteristics can be determined by
measuring the moisture take up of the fibres after having been
exposed to moisture under varying conditions or after having been
soaked in water, and subsequently measuring the corresponding
degree of swell of the exposed fibres. Said degree of swell being a
useful yardstick for the dimensionstability of the vegetable
fibres. Moisture take up can conveniently be determined by
measuring the weight increase of the exposed fibres.
A suitable method for measuring the degree swell of the
exposed fibres is comparing the diameter of the fibres before and
after exposure to moisture with the aid of a microscope having
sufficiently large magnification, e.g. 52 x. A further method for
measuring the degree of swell is with the aid of a so-called
dedicated image analyzer (Vidas, ex Kontron, Germany). With this
technique, which is considerably faster than the microscopic route,
the volume of the fibre is projected on a screen. By comparing the
size of the images of the fibres before and after exposure, it is
possible to calculate the dimension stability.
With the aid of Confocal Laser Scanning Microscopy it was
demonstrated that with the fibres prepared according to the process
of the present invention the original cellulose cell structure had
essentially been maintained, i.e. the cell walls had remained
intact during the upgrading of the moisture resistance, and had not
been ruptured. Occasionally some of the cross sections of the cells
deviated slightly from those of the untreated fibres.
Moreover with the same technique it was also demonstrated that
with fibres which had been submitted to a process which comprized a
steam treatment step followed by a sudden decompression, no trace
of any cellulose cell structure and/or cell wall could be observed.
Confocal Laser Scanning Microscopy is a novel form of optical
microscopy having an advantage over the conventional light- and
electronmicroscopy in that it possesses a large depth of focus, and
moreover rejects all out of focus information.
The images of the fibre cell structures and walls produced via
the hereinbefore described confocal laser technique, can be made
visible by projection on e.g. a screen or on photographic paper.
The latter mode of operation having the advantage in that it makes
comparing the different images a lot easier, and moreover also
allows said results to be saved.
The long, multicelled vegetable fibres having improved
moisture resistance and dimension stability and wherein moreover
the original cellulose cell structure has been essentially
maintained, as described hereinbefore, are novel products and form
another aspect of the present invention. In view of the presence of
the original cell structures it can be expected that said fibres
will also have maintained their mechanical performance properties,
thus making them valuable materials e.g. for use as fibre
reinforcement in polymer matrices.
The use of the vegetable fibres which can be made via the
process of the present invention and especially their use as fibre
reinforcement in polymer matrices as described in claim 10 is another aspect of the present
invention.
The method or process used for the preparation of the
fibre-reinforced polymer matrices or composites as they will be
referred to hereinafter, is not critical, but will of course be
governed by the nature of the polymer(s) used, i.e. be it a
thermoplastic or thermosetting polymer. In general any process used
for the preparation of fibre reinforced composites employing
conventional fibrous reinforcements, e.g. glass fibres or green
vegetable fibres, can also be used when the fibrous reinforcement
comprises the vegetable fibres having improved moisture resistance
and dimension stability as described hereinbefore. Well-known
examples of such processes include melt-mixing, laminating and
pultrusion.
Melt-mixing will generally be conducted by mixing the loose
vegetable fibres and polymer at a temperature at which the polymer
is in the molten form. Conveniently melt-mixing can be conducted in
an extruder wherein the polymer and fibrous reinforcement can be
fed separately or jointly. In the latter case the joint polymer and
fibre addition may be preceded by a dry blending step. The ultimate
fibre containing polymer melt or extrudate can be employed in e.g.
moulding operations for the preparation of shaped articles.
When the preparation of the fibre reinforced composites is
conducted via a laminating technique, which composites are
generally referred to as laminates, both loose fibres and fibre
assemblies can be used. The general procedure for the preparation
of such laminates comprises stacking alternating layers of polymer
and reinforcement, e.g. in a mould; heating the contents of the
mould to melt the polymer, evacuating the air from the mould
followed by cooling and compressing the mould contents to obtain
the laminate. The polymer may be used in powder form but
advantageously the polymer is employed in the form of a film or
sheet. Suitable fibre assemblies which may be employed include
woven and non-woven cloth based on vegetable fibres as described
hereinbefore.
With pultrusion the fibre reinforcement, is conveniently is
contacted with the molten polymer by drawing the fibre
reinforcement with the molten polymer through an orifice or die,
followed by cooling of the resulting product. The resulting coated
yarn can then be cut to the desired fibre length, and the resulting
granules, each containing the correct blend of fibre and polymer,
can be applied in further processing to provide the final fibre-reinforced
article.
Generally the fibre reinforcement used in the preparation of
the fibre-reinforced composites as described hereinbefore will be
based on a single type of vegetable fibre. It is of course also
possible or sometimes even advantageous to use blends of two or
more types of vegetable fibres or use fibre assemblies based on
more than a single type of vegetable fibre. In general the fibres
will comprise in the range of from 10 to 90%W of the total
reinforced composite, and preferably from 20 to 80%W.
As mentioned hereinbefore the polymer matrix of the fibre
reinforced composites may be based on both thermoplastic and on
thermosetting polymers. Examples of suitable thermoplastic polymers
include polyethylene, polypropylene, polybutylene, polystyrene,
polyamides, polyethyleneteraphthalate, polycarbonate, polyketones,
polyphenyleneoxides, polyesters such as polymethacrylates,
functionalized, polyolefins such as those which have been modified
to carry one or more polar groups via grafting or copolymerization;
preferred polar groups being acid groups and especially carboxylic
acid groups or derivatives thereof. The composites may be based on
a single thermoplastic polymer or blends of two or more
thermoplastic polymers. It is moreover feasible that when employing
a laminating technique that the composites may combine polymer
sheets or films from different polymers.
Examples of suitable thermosetting polymer systems include
polyepoxides in combination with a wide range of curing agents,
unsaturated polyesters, phenolic resins and isocyanate curable
systems.
The fibre reinforced composites based on the vegetable fibres
or assemblies thereof as described hereinbefore as well as in claim 11, are part of the invention.
The use of said vegetable fibres as fibre-reinforcement in the
polymer composites as described hereinbefore allows said composites
to be prepared via a fully integrated process, i.e. a process
wherein the heattreatment or cure step of the vegetable fibres
takes place during the moulding operation of the composites. Under
said circumstances it is required that the isolated non-cured
vegetable fibres should be dry, i.e. water free.
The invention will be illustrated with the following examples
for which the following information is provided.
Reactors
1. The small-scale water/steam treatment of the vegetable fibres
was conducted in 20 cm3 pipe reactors. The closed reactors were
placed in an oil bath of sufficient heat capacity to allow heating
of the reactor contents to a temperature in the range of from
165-180°C within approximately 2 minutes. Likewise cooling of the
reactors was accomplished by placing them in a cold bath.
2. Larger scale batch experiments were conducted in a 13 l
autoclave containing a 5 l stainless steel wire basket as sample
holder. Between basket and reactor wall a thermal shield was
positioned, comprising a highly efficient copper cooling spiral,
which could be water cooled, or when so required could be drained
with compressed air. The autoclave was furthermore equipped with a
facility to allow steam to be introduced and a possibility to drain
off a liquid phase.
Test procedures
Water absorption was determined by measuring the weight increase of
fibres after having been exposed to moisture.
Degree of swell was determined with the aid of a so-called
dedicated image analyzer (Vidas, ex Kontron, Germany) by projecting
the volume of a fibre on a screen and measuring the dimensions of
the projection before and after exposure to moisture.
Composite preparation via compression moulding (stacking method)
Reinforcement and polymer film were cut in the dimensions of
the cavity of a positive mould and 14 layers of polymer film and
13 layers fibre based mats were stacked in alternating layers. The
stacked layers were compression moulded to a sheet of 4 mm
thickness using the following moulding conditions: preheating for
1 minute at 200°C and 4 bar, subsequently evacuating the air from
the mould followed by heating for 2.5 min at 200°C and 80 bar and
cooling to room temperature at 80 bar.
Composite preparation via wet deposition
10.5 g of disintegrated fibres, 24.5 g of polymer powder and
2 g of Triton X-45 were mixed for about 10 minutes in 20 l of demi
water and the resulting slurry was drained to form a sheet. The
sheet was dried in a vacuum oven between filter paper for at least
24 h at 40°C under a nitrogen stream.
The dried sheet was further converted to composites via the
compression moulding technique described hereinbefore.
Example 1
3 g size samples of ovendry abaca thread (ex Langman, the
Netherlands) were placed in a pipe reactor as described
hereinbefore and allowed to soak in 7.5 g of demiwater for 10 min.
Subsequently the reactor was closed and heated to the temperature
and for the time as indicated. After cooling, the reactor was
opened, and the fibres were removed, dried, and cured at 140°C for
2 hours.
With some of the experiments the water contained 0.05 mol/l of
sodium acetate. A further variable in the process condition was
submitting the fibres to a water wash before cure, to which end the
fibres were stirred in an excess of demi water at approximately
50°C. The thus obtained fibres were tested for water absorption
after having been placed in an environment of 98% RH at room
temperature.
From the resulting data which have been collected in Table 1
and also includes the corresponding data of the non-treated fibres,
it can be observed that the fibres prepared according to the
present process, show only a marginal reduction in moisture
absorption. It should however be remembered, as mentioned
hereinbefore, that the cellulose also contributes to the moisture
absorption. As this fibre component is not affected by the process
of the present invention, its moisture absorption is likewise not
affected.
Moreover it is also apparent that the influence of the use of
a pH buffer and/or of washing the fibres before cure is not very
clear with these small scale experiments. Confocal Laser Scanning
Microscopy confirmed that the original cellulose cell structures
had been essentially maintained.
exp. steam treatment buffer washing pH after water absorption (%w) |
no. | conditions | | | reaction after |
| | | | | 7 days | 16 days |
1 | 15'180°C | + | + | 4.5 | 18.8 | 19.7 |
2 | " | + | - | 4.5 | 22.0 | 24.2 |
3 | " | - | + | 3.5-4 | 17.8 | 18.9 |
4 | " | - | - | 4 | 20.0 | 22.4 |
5 | 1h 165°C | + | + | 4.5 | 18.8 | 19.7 |
6 | " | + | - | 4.5 | 21.6 | 22.7 |
7 | " | - | + | 3.5-4 | 17.1 | 18.1 |
8 | " | - | - | 3.5-4 | 19.2 | 19.8 |
ref | - | - | - | - | 23.4 | 24.3 |
Example 2
The procedure of example 1 was repeated using green flax and
abaca but restricting the steam treatment to 165°C for 1 hour,
followed by a wash step at 50°C and cure at 140°C for 2 hours.
The resulting fibres were tested for moisture absorption and
swelling after having been exposed in air of 98% RH for 650 hours,
which provided the following data
green flax | moisture absorption 26% swelling 60% |
treated flax | moisture absorption 19% swelling 6% |
abaca | moisture absorption 26% swelling 40% |
treated abaca | moisture absorption 19% swelling 6% |
The results obtained in the experiments described in this
example clearly demonstrate the very large reduction in the degree
of swelling which can be achieved by the process of the present
invention. The moisture absorption improvement is considerably less
impressive and confirm the results of the previous example.
Furthermore the presence of the original cellulose cell
structures was again confirmed.
Example 3
In a number of consecutive experiments 2 kg size wet samples
of flax fibres, non-woven flax, abaca fibres, woven jute (burlap)
and woven linen (muslin), which samples corresponded with
approximately 1 kg of dry material, were placed in the sample
holder of the autoclave described hereinbefore. The reactor walls
had already been heated to the temperature required for treating
the fibrous material and full cooling had been put on the thermal
shield. The reactor was closed, the cooling switched off, water
drained from the thermal shield and simultaneously a sufficient
amount of saturated steam having the required temperature for
conducting the fibre treatment was supplied to the reactor. Upon
completion of the steam treatment the steam supply was closed and
cooling of the thermal shield switched on. When the reaction
contents had reached a temperature < 100°C the slight overpressure
in the reactor was neutralized by careful decompression of the
reactor. The treated fibres were dewatered and cured as indicated
in Table 2 hereinafter.
The thus treated flax and abaca fibres were used for the
preparation of polypropylene composites having a fibre content of
30% w (20% v) via the wet deposition procedure as described
hereinbefore.
Via the same procedure but omitting the polypropylene powder
and the compression moulding step, abaca "paper" sheets were
prepared.
The abaca "paper" sheet, the treated non-woven flax, the woven
jute and linen were each used in combination with polypropylene
film for the preparation of fibre-reinforced polypropylene
composites having a fibre content of 30% w (20% V), via the
stacking method described hereinbefore. Reference composites based
on the corresponding non-treated fibres were prepared likewise.
The thus prepared composites were submerged at room
temperature in water for 650 hours, which was followed by measuring
the weight increase and degree of swell. The resulting data have
been collected in Table 2. It can be observed that the composites
based on the vegetable fibres prepared following the process of the
present invention show a considerable reduction in swell when
exposed to moisture and also a reduced moisture uptake.
The flexural properties (Modulus and Strength) of the thus
prepared composites were determined on a fully computerized Instron
testing machine using standard test specimens having a thickness of
4 ± 0.2 mm and a width of 10 ± 0.5 mm. Testing conditions: span 64
mm, crosshead speed 20 mm/min and straining rate 0.01 min-1.
From the results collected in Table 3, it can be observed that
the use of the vegetable fibres having improved moisture resistance
results in composites of which the flexural properties are very
simular to those of corresponding composites based on the untreated
fibres. Moreover it can be observed that the flexural properties of
linen based composites are on a considerably lower level compared
to those based on the other types of vegetable fibres, this applies
to both the treated and untreated linen fibres.
Fibre | treated | conditions of treatment | cure | weight increase % | swelling % |
abaca | + | 60'-170°C | 2h-165°C | 7.9 | 1.6 |
abaca | - | | | 9.0 | 3.7 |
jute | + | 15'-185°C | 1h-190°C | 7.2 | 1.2 |
jute | - | | | 11.9 | 4.9 |
flax | + | 60'-170°C | 10h-145°C | 6.9 | 1.0 |
flax | - | | | 9.0 | 2.8 |
linen | + | 60'-165°C | 3h-160°C | 4.7 | 0.7 |
linen | - | | | 6.1 | 2.4 |
Composite preparation | Flexural properties |
| Modulus, (GPa) | Strength, (MPa) |
| Untreated | Treated | Untreated | Treated |
Wetdeposited |
abaca | 5.4 | 5.4 | 84 | 87 |
flax | 4.9 | 5.1 | 66 | 70 |
Stacked |
jute, woven | 5.3 | 5.2 | 70 |
linen, woven | 3.6 | 3.0 | 65 | 62 |
flax, non-woven | 4.9 | 4.7 | 61 | 66 |
abaca paper | 4.3 | 4.1 | 75 | 72 |
Non-reinforced Polypropylene | 1.7 | 48 |