Note: Descriptions are shown in the official language in which they were submitted.
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DEGUMMING AND SCOURING OF BAST MATERIAL FOR
PRODUCTION OF TEXTILE AND PULP-QUALITY FIBER
Background of the Invention
Cellulosic bast fibers harvested from plants like hemp, flax and sisal are
composed of
large proportions of undesired material other than cellulose, including
lignin, pectin and
hemicellulose, known collectively as gum. Gum serves as a natural glue
adhering cellulose fibers
together to provide the plant with structural rigidity. For cellulose and
fiber manufacturing
purposes, gum is undesired. Its presence is detrimental to the manufacturing
apparatus and
product quality, and undermines the efficient conversion of raw bast into
marketable cellulose
fiber.
Processing bast-derived cellulose fiber requires the removal of gum through
chemical and
mechanical means, also known as degumming. Chemical treatments seek to target
non-cellulosic
molecules with minimal damage to cellulose polymers composing the fibers
themselves.
Mechanical treatment is meant to enhance chemical transport as well as
physically separate gum
from its cellulose structure. Historically and generally, bast fiber degumming
utilizes natural
processes and ambient conditions to render the raw cellulose fiber and
separate it from the rest of
the plant - referred to as retting. Retting takes various forms but generally
utilizes natural
microbiological and chemical activity for degumming. For field retting, cut
stalks are left
exposed in the field for some time. In water retting, stalks are placed in
ponds or streams.
Neither method guarantees consistent production of marketable textile fiber,
creating
=
uncertainty for markets. As recently as April 4, 2019, in the Wall Street
Journal, page D3,
Jeffrey Silberman, a professor and chairperson of textile development at the
Fashion Institute of
Technology in New York, stated that it takes a specific type of processing
equipment to turn
hemp into a fiber, and that type of machinery is still scarce. He is quoted as
saying "New York
state doesn't have the processing equipment for it, at least as far as I can
tell. I haven't found a
spinning mill that can handle hemp."
The fibers obtained by conventional field and water retting yield a silver-
grey colored
fiber due to the byproducts and staining of biological processing. Enzymatic
methods and other
chemical methods result in a slightly yellow or blonde to white fiber.
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Industrial scale quality-controlled degumming involves the formulation of
chemical
reagents and catalysts to specifically target gum and can generate problematic
waste products, as
well as fibers that are difficult to process.
The art is in need of a process whose effluents are readily reusable, or
reducible to an
environmentally inert form with minimal impact on water quality. A successful
process
execution should yield suitable cellulose fibers of consistent quality within
a wide range of
governing parameters.
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Summary of the Invention
The present invention fulfills one or more of these needs in the art by
providing a method
for degumming (removal of structural non-cellulosic material) bast fibers
comprising the soaking
a source of bast fiber in a saline or ionic solution. The source of bast
fibers may be hemp.
In one embodiment, the source of bast fiber is soaked in a saline solution
having a
concentration ranging between about 1 part per thousand up to about 200 parts
per thousand. The
ion concentration of the saline solution may be varied while the source of
bast fiber is soaking in
the saline solution. This step may be conducted without electrolysis of the
solution.
In one embodiment, the source of bast fiber is soaked in seawater. In another
embodiment, the source of water is soaked in brackish water. The saline
concentration may be
varied over time; for example, either natural (tides) or man-made (alternating
salt baths) may be
used to fluctuate ion concentration. Because the ionic or saline concentration
varies in water in
estuaries as the tide ebbs and flows, immersing the source of bast fiber in an
estuary for a series
of tidal cycles can provide the salinity variations.
The saline solution may be amended with a gum-targeting reagent.
The method may include soaking the bast fiber in a solution containing a base
to degrade
lignin into water-soluble forms for removal by water. A basic saline solution
of a base at a pH
from about 7 to about 14 may be used.
The method may include soaking the source of bast fiber in a solution
including sulfur (S
oxidation number < VI) to sulfonate lignin for removal from the source of bast
fiber by water.
The source of bast fiber may be soaked in an oxidizing solution to scour
lignin, pectin
and hemicellulose from the source of bast fiber for removal by water.
The method may include rinsing the source of bast fiber with detergent and
surfactants to
further remove lignin, pectin and hemicellulose from the source of bast fiber.
The source of bast
fiber may be mechanically agitated. For example, the source of bast fiber may
tumbled.
The present invention may also be considered a method for degumming hemp
fibers from
a hemp plant comprising soaking a hemp plant in a saline solution, soaking the
hemp plant in a
sulfur solution to sulfonate lignin for removal from hemp fiber in the hemp
plant, and rinsing the
hemp fiber with detergent to further remove lignin, pectin and hemicellulose
from the hemp
fiber.
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The method may include soaking the hemp fiber in an oxidizing solution to
remove
lignin, pectin and hemicellulose and their partially oxidized intermediates
from the hemp fiber.
For instance, the hemp fiber may be soaked in a hydrogen peroxide solution.
The hydrogen
peroxide and other oxidant sources may be generated in situ e.g. via
electrolysis. The hemp fiber
may be soaked in the oxidizing solution at a pH between about 6 to about 14
and at a
temperature below a boiling point of the oxidizing solution. This step may be
conducted without
electrolysis of the solution.
In one embodiment, the hemp fiber may be soaked in the sulfur solution at a pH
between
about 6 to about 14 and at a temperature between ambient (20-30 C) to about 95
C.
In one embodiment, soaking the hemp plant in a saline solution comprises
soaking the
hemp plant directly into or within a vessel connected to natural, tidally
flushed coastal waters so
that the ionic composition and associated alkalinity, acidity, and density of
the solution
proximate to the hemp plant varies with tidal fluctuations.
The resulting fiber is suitable for spinning into textile-quality yam using
conventional
textile methods and machinery, wherein the processing is selected from the
group consisting of
making a yam by spinning, braiding, felting, making nonwovens such as by
needle-punching,
and more than one of them. Bleaching or oxidation need not be used,
eliminating the inherent
production of potentially toxic by-products by this method and allows the bast
material to retain
elements of its original natural green color. The fiber (fiber containing
chlorophyll), thus
degummed, maintains at least some of its green color and is sufficiently free
of lignin, pectin,
and hemicellulose, and the fiber can be processed in conventional textile
machinery, wherein the
processing is selected from the group consisting of making a yam by spinning,
braiding, felting,
making nonwovens such as by needle-punching, and more than one of them.. The
green color of
the bast fiber is typically imparted to the bast fiber by chlorophyll. The
bast fiber may be hemp.
The source of green color of the bast fiber may, upon extraction into a
solvent, show a
photometric absorption spectrum indicative of general chlorophyll pigments
(Chi a, b & c). As
described in Jeffrey and Humphrey, "New Spectrophotometric Equations for
Determining
Chlorophylls a, b, ci, c2 in Higher Plants, Algae and Natural Phytoplankton,"
Biochem. Physiol.
Pflanzen (BPP) Bd 167, S. 191-194 (1975), data from 750, 664, 647 and 630 nm
wavelengths
can be used to calculate chlorophyll concentration directly from a suitable
absorption spectrum.
For example, a 500 mg sample of green-colored, degummed, hemp fiber sample
produced in
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accordance with an embodiment of the method was soaked in 100 mL of deionized
water for 12
hours and filtered through a 0.45 'um Supor membrane. Data from an absorption
spectrum per
Jeffrey and Humphrey (1975) showed 1.6 mg/L total chlorophyll in the extract,
equivalent to at
least 0.03% chl in the fiber sample by mass.
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Brief Description of the Drawings
The invention will be better understood by a reading of the Detailed
Description of the
Examples of the Invention along with a review of the drawings, in which:
Figure 1 is a black and white photograph of four samples.
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Detailed Description of Examples of the Invention
The present invention is directed to a method for processing plants that
include bast fibers
to remove lignin, pectin and hemicellulose (collectively referred to as "gum")
and other
undesirable materials. The result is a cellulose fiber partially, mostly, or
wholly free from gum so
that it may be used in manufacturing for conversion into marketable materials
such as textile
fibers. The bast fibers may be hemp, flax or sisal. Jute and other fibers are
also known as bast
fibers.
The method comprises soaking a plant that is a source of bast fibers in a
saline solution.
The source of bast fiber may be soaked and agitated in a brine solution. The
source of bast fiber
may be soaked and agitated in an alkaline solution. The source of bast fiber
may be soaked and
agitated in solution with a pH > 6, and this is typically performed without
electrolysis of the
solution. In one embodiment, the solution is comprised of water and inorganic
salt with a total
salinity ranging from less than 1 to about 200 parts per thousand (ppt).
Salinity sources can
include those found naturally such as in tidally flushed estuaries, creeks,
coastal and sea water.
Natural freshwater can be used, though consideration for ionic discharge into
freshwater
watersheds should be made. Generally, freshwater has a salinity ranging from
about 0 to about
0.5 ppt, brackish waters has a salinity ranging from about 0.5 ppt to about 30
ppt, and seawater
has a salinity ranging from about 30 ppt to about 50 ppt.
The submersion of the source of bast fibers into an aqueous solution leads to
uptake of
water and aqueous ions into interstices between cellulosic fibers and gum and
into the cellulose
matrix due to cellulose wetting and capillary action. This imparts a pressure
on the cellulose
structure causing it to swell. The presence of ions in solution within the
cellulose matrix alters
the wetting behavior of cellulose, alters the capillary and pore pressures,
and enhances interstitial
transport and swelling. Swelling imparts mechanical stress on the bast
structure, helps expose the
fiber body to reagent, increases substrate surface area, delivers reagent for
subsequent steps, and
enhances fiber separation. Conversely, reduction of ion concentration reduces
the swelling
behavior, enhances diffusive export of dissolved material from the cellulose
matrix (effectively
flushing it), and imparts stress on the fiber matrix, further enhancing
separation. The process
may be enhanced through additional mechanical treatment. Interstitial pressure
gradients may be
sustained by periodic increase and decrease in the ion concentration of the
solution, through salt
or brine addition or dilution, and will enhance fiber separation as well as
exposure of gum to
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reagent. The use of tidally-flushed estuarine water sources permits the
naturally available and
non-interfering use of sea salt as a source of interstitial pressure. The use
of tidally flushed water
in an estuary also removes gum and/or its partially oxidized intermediates
from the bast fibers
safely through flushing.
Exposing the source of bast fiber to the variations in ion concentration can
be carried out
under increased temperature and pressure to enhance diffusion and reaction
rates, and increase
interstitial pressures.
The method may also include exposing lignin present in the bast fiber to a
degumming
reagent. This may include a base to depolymerize lignin into smaller, lighter
weight, soluble
lignin products. A basic saline solution of a base at a pH from about 7 to
about 14 may be used.
The method may also include sulfonating the lignin present on the bast fiber
to create
water-soluble products that can be removed by flushing the bast fiber with
water. A neutral to
alkaline solution of strong base and a sulfur anion at a pH from about 7 to
about 14 may be used.
Sulfur in neutral to alkaline solution will react with ring structures in
lignin to yield a sulfonated
soluble product readily removed from the plant material through flushing.
Sufficient sulfur is
provided to account for the range of lignin content of the source of bast
fiber (generally 5-15%
lignin by mass for raw fiber, and less for retted or otherwise pre-processed
fiber). The source of
bast fiber may be soaked in a sulfonating solution for 10 minutes to several
hours at pressure
ranging from ambient to 10 atmospheres. Temperatures may vary with pressure
per the Clausius-
Clapyeron relation.
The bast fibers may also be soaked in an oxidizing solution to bleach the
fibers and scour
remaining gum. The oxidant may be added to the solution directly or generated
in situ (e.g. by
electrolysis). The addition of oxidant stabilizers may be included in some
embodiments. The
needed water temperature and soak time are dependent on the oxidant used. A
water solution
with a pH ranging from about 8 to about 12 is made with a combination of a
strong base and a
buffer. As a non-limiting example of the proportions, 50 mL of 3% H202 is
added to a solution
of 2 g/L NaOH and 1 g/L NaHCO3. The hemp fiber is soaked from about 0.1 to
about 1 hour at
a temperature below the boiling point of the solution. Since some naturally
occurring heavy
metals (Fe, Mg) interfere with H202 oxidation of gum (e.g. see Fenton's
Reagent), the addition
of chelators may be used to prohibit or minimize their interference.
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Additional steps may also be added to further degum the bast fibers. For
example
detergent may be used to rinse and remove any remaining soluble, partially
soluble, or insoluble
(but mobile) gum in the bast fiber. Pectinase or other natural enzymes (either
directly added or
through in situ biological production) may be used as a non-essential
component to assist in the
removal of gum from the bast fibers. The steps may include mechanical
agitation. Mechanical
agitation may be tumbling, stirring or other agitation.
The steps for degumming the bast fibers may vary depending upon the chemical
and
physical conditions of the water source. Potential sources of water for
soaking the bast fibers
include most natural waters as well as municipal and purified water. Since the
method utilizes
saline solutions, sources of naturally saline water may be used to advantage.
Raw water (non-municipal or otherwise untreated, saline, brackish or fresh)
may be used
in one or more steps. Some natural sources of water contain components that
may be utilized as
reagents for one or more of the steps described above. For instance, waters
available from
wetlands contain naturally abundant sulfur (i.e., having an oxidation number
less than or equal to
VI) that can be used for sulfonating lignin. The presence of naturally
available reactants or
reagent components within the water source may be accounted for when
formulating the desired
reagent composition. For example, naturally available chelators may be used in
place of
manually added ones. The natural salinity of the water source may be accounted
for in all steps,
and its natural salinity variability (due to tides or other factors) may be
used as a source of
enthalpy for physical and chemical processing. Naturally available alkaline
water sources may
also be useful for certain processes wherein the pH of the solution is within
a range from about 8
to about 12.
Moreover, certain steps of the method may vary for several reasons. For
example, the
bast fibers may be pretreated or decorticated, thus requiring a less
aggressive treatment. Certain
steps may be repeated (with or without a more aggressive reagent) if it is
determined by an
operator that the results of one or more steps are deemed insufficient. In
some examples, the
plant stalks may still be intact (i.e. the bast is not separated from the
whole plant), requiring
additional steps/repetitions to remove additional gum. On the other hand, some
sources of bast
fibers may be already be partially processed ("retted") and require a less
aggressive treatment,
permitting omission of one or more steps. Material of a specified quality due
to variable control
available to the method may also be desired.
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The effluent from the degumming process may be a solution and mixture of gum,
its
partially oxidized intermediates, and its oxidation byproducts, residual
cellulose fiber lost due to
agitation, as well as inorganic salts, residual reagent, detergent, and
chelators. Post-treatment of
effluent includes additional oxidation (through addition of oxidants and/or in
situ generation) or
biological activity, per standard municipal water treatment protocols.
Residual fiber, lignin and
sulfonated lignin may be recovered from the effluent as marketable material.
Suitable,
detoxified effluent may be used as a nutrient additive to soils for
agriculture. Inorganic ions may
be added to the effluent to adjust ion ratios and salinity to match that of
surface waters into
which effluent will be discharged. Effluent alkalinity or acidity may be
neutralized through
titration with a suitable acid or base.
The fiber obtained by the process outlined above is a high-quality, separated,
soft,
spinnable fiber. The bast fiber is sufficiently free of lignin, pectin and
hemicellulose that the
fiber can be processed in conventional textile machinery. Such processing can
include making a
yarn by spinning, braiding, felting, and making nonwovens such as by needle-
punching.
The bast fiber may retain a green color. The color can be ascertained by
extraction of the
bast fiber into a solvent, showing a photometric absorption spectrum
indicative of general
chlorophyll pigments (CM a, b & c). As described in Jeffrey and Humphrey, "New
Spectrophotometric Equations for Determining Chlorophylls a, b, cl, c2 in
Higher Plants, Algae
and Natural Phytoplankton," Biochem. Physiol. Pflanzen (BPP) Bd 167, S. 191-
194 (1975), data
from 750, 664, 647 and 630 nm wavelengths can be used to calculate chlorophyll
concentration
directly from a suitable absorption spectrum. For example, a 500 mg sample of
green-colored,
degummed, spinnable hemp fiber sample produced in accordance with an
embodiment of the
method was soaked in 100 mL of deionized water for 12 hours and filtered
through a 0.45 Jim
Supor membrane. Data from an absorption spectrum per Jeffrey and Humphrey
(1975) showed
1.6 mg/L total chlorophyll in the extract, equivalent to at least 0.03% chl in
the fiber sample by
mass. Evaluations using the methodology of Jeffrey and Humphrey (1975) yields
this result:
Wavelength (nm) 750 664 647 630
Absorption 0.015 0.047 0.059 0.058
When the processing of the source of the bast fiber is performed without
bleaching or
oxidation, production of potentially toxic by-products is virtually eliminated
and the green color
may be retained.
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Figure 1 is a black and white photograph of four samples. From left to right
these
samples are:
= raw green hemp (not degummed)
= degummed hemp fiber that shows the remaining slightly green color (the
actual sample used
to extract chlorophyll for the UV-VIS scans)
= degummed fiber (different variety - original fiber was green) slightly
darker green
= degummed fiber - no green (the original fiber was not green)
Certain modifications and improvements will occur to those skilled in the art
upon
reading the foregoing description. By way of example, the method disclosed
herein is not limited
to the order described above. In certain embodiments, the above steps may be
omitted, repeated,
combined, or carried out in a different order. For example, limited resources
may require
combining steps, or reusing effluent from one step in another. The step order,
parameters and
iterations may be modified in some embodiments to enhance or diminish specific
fiber properties
or composition, or to unify the properties of the end product relative to
those of the unprocessed
bast. It should be understood that all such modifications and improvements
have been omitted
for the sake of conciseness and readability, but are properly within the scope
of the following
claims.
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