Note: Descriptions are shown in the official language in which they were submitted.
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
1
A method for preparation of absorbing substances
The objective of this invention is a method for preparing absorptive
substances from
lignocellulosic materials such as straw, peels or hulls of cereal crop, plant
leaves,
wood chops, bagasse or jute.
The most important applications of absorbents are absorbing and release of
water,
flocculation of colloids, filtering aids, removal and a possible recovery of
harmful
organic or inorganic compounds, based among others on the ion exchange
properties of absorbents, and controlled release of medicines and
agrochemicals.
The range of applications is wide from hygiene and hospital articles to
various
purposes in the industry, agriculture and environmental protection.
In addition to the traditional absorbers such as Fuller's earth, other silica
minerals,
silica gel and activated carbon, synthetic or semi-synthetic organic
absorbents or so-
called super absorbents have been developed during the past few decennia. At
the
first stage of this development, the starting material was starch, to which
hydrophilic and water-absorbing atomic groups were added by using grafting
techniques. These groups can be positively or negatively charged. The most
usual
grafting chemicals were acrylic acid, methacrylic acid and their derivatives
such as
salts, esters, amides and nitriles. The~quality property most often followed
has been
the water absorbing capacity. It was "determined initially using excess of
water
under atmospheric pressure, and separating the solid matter from the mixture
by
centrifugation, later by following the absorption under pressure and in salt
solutions, thus imitating properties which axe essential when used for hygiene
or
hospital articles.
The starch-based products developed initially could have water absorbing
capacity
of several hundred times that of the weight of the dry absorbent. However,
along
with the increase of the amount water absorbed, mechanical properties of the
gel
formed were weakened, and a substantial part of the water was released under
pressure. Since hygiene articles have been a major field of application of the
organic absorbents, the main part of the demand has been directed to
absorbents,
which can imbibe and hold water and dilute solutions such as blood and excreta
even under mild pressures. A similar quality requirement is valid also for
substances used for absorbing and release of water in agricultural and
horticultural
applications. This has directed the development and marketing towards fully
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
2
synthetic absorbents, where the quality required is often presented to be a 25
to 35-
fold absorption of a physiological saline solution under a pressure of 0.2
bars. Only
few published data and applications exist on absorbing other materials but
water in
these absorbents.
For maintaining the absorbing capacity under pressure, cross linkages have to
be
created in the polymer. They diminish the total absorbing capacity. Cross
linkages
also form steric hindrances for continuation of the water absorption, and
limit the
penetration of water deeper than in the surface layers. Also in the cross-
linked
materials, a high water content causes a weakening of the mechanical
properties of
the gel. Diminishing the particle size causes easily agglomeration problems
and
makes it difficult to maintain an even distribution of the absorbing material
to the
other components of the final absorbing product. For these reasons, also
fibrous or
foil-formed absorbers have been prepared. Fibres can be formed by polymerizing
the same monomer or from another synthetic polymer. Alternatively, an
absorbing
layer can be prepared on the surface of isolated natural polymers such as
cellulose
or wool fibres. A fibrous absorbing material can be bound to other fibrous
materials
by weaving or by using non-woven techniques known as such. Advantages of using
fibrous materials are an easy separation from the liquid phase, which enables
uses
similax to filtering materials, or when mixed in large amounts of liquids, a
separation merely by sedimentation.
A hydrogel forming polymer, 2-methyl hydroxyethyl methacrylate (HEMA) has
been grafted on the surface of cellulose fibres, polyethylene, or silicon
rubber.
When using cellulose fibres, initiators used for grafting have been light, y-
irradiation, or chemical initiation. Weaknesses of these methods are,
according to
I~arlsson and Gatenholm (Polymer 38, 4727-4731, 1997), high equipment costs, a
weak controllability of the polymerization, and difficulties to avoid
homopolymerization. These researchers have used for chemical initiation a
treatment with ozone on the surface of moistened cellulose fibres, this
treatment
forming hydroperoxides on the surface. Polymerization is subsequently formed
in
methanol solution under nitrogen atmosphere. The duration of the ozone
treatment
has been 90 minutes. This treatment, however, caused breaking of cellulose
molecules thus weakening the mechanical properties of the fibrous material.
Grafting started from the pores and crevices of the surface, and for a
complete
covering of the surface with the hydrophilic layer, an amount of grafting
material
higher than 100% of the weight of the cellulose treated was needed. Absorbing
properties of the material obtained have not been published.
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
3
Graft polymerization has also been applied on non-isolated natural fibre
materials.
Mohanty, Parija and Misra (Journal of Applied Polymer Science 60, 931-937,
1996)
graft polymerized acrylonitrile on the surface of pineapple leaves, which had
been
previously treated chemically. The pretreatment consisted of three extraction
stages,
followed by treatments with ethylene diamine and zinc chloride. Washing and
drying operations were performed between all these stages. As initiators for
polymerization, cerium (IVY sulfate and N-acetylglycine were used, and the
reaction
was performed under nitrogen atmosphere. The amount of grafted material varied
depending on experimental conditions from 59.8 to 114.3°/O of the
weight of the
fibrous material. Crrafting was reported to increase the thermal stability of
the
material, but data on other properties have not been published.
As by-products of cereal industries and agriculture, large amounts of
lignocellulosic
material such as straw, peels and hulls are formed. They have been used as
absorbents as such or after some simple chemical treatments. The water
absorbing
capacity of untreated material is weak, being maximally two parts by weight
per
one part of the dry absorbing material. For this reason, this material as such
is
economically feasible for absorbing purposes only for absorbing excreta of
cattle or
poultry. Its water absorbing capacity can be elevated by treatments with
alkali or by
a combined treatment with alkali and peroxides, as it is presented in the
United
States patent no. 4,806,475. Fibre preparations obtained by such treatments
are
marketed as water absorbing food additives. Their water absorbing capacity is
6 to 8
parts per one part of dry matter of the absorbing material. No data exist on
their
water absorbing capacity under pressure, and these materials are not marketed,
for
example, for preparation of hygiene articles.
In the present invention it has been surprisingly found, that such easily and
economically available lignocellulosic materials can be in a relatively simple
process converted to absorbers having a high absorbing capacity especially
under
pressure. Characteristic for the method developed for preparing such absorbers
is
that it comprises the following steps:
(a) lignocellulosic material is treated with alkali to remove a part of its
lignin and/or
hemicellulose content,
(b) after step (a), the material is treated to provide its cellulose content
with reactive
radicals capable of functioning as polymerization initiators,
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
4
(c) after step (b), at least one polymerizable monomer and at least one cross-
linking
agent are added to the material,
(b) the composition obtained at step (c) is polymerized.
According to the invention, preparation of an absorbent is performed
advantageously by initiating the treatments by water washing of the
lignocellulosic
material, whereby finely dispersed and water-soluble compounds are removed,
and
among others, the contents of starch and protein are reduced. The pretreated
material is now treated with an alkaline solution and a treatment with
hydrogen
peroxide, persulfate, or another strongly oxidizing treatment for enabling the
fixation of the polymer and for initiating the polymerization. Into a moist
material,
one or several monomers and cross-linking agents, separately or previously
mixed,
are added, and the polymerization is performed at a temperature below
75°C.
Straw, peels, hulls or another lignocellulosic material deriving from an
industrial
process can contain extraneous material such as soil, and starch and proteins
deriving from other materials such as cereal grains. Since these materials can
weaken the fixation of the polymer formed andlor inactivate radicals formed
for
initiating the reaction, it is advantageous to preclean the material for
removal of
extraneous compounds. A great part of the said impurities can be removed by
washing with water. When materials with a waxy surface are used, the wax has
to
be removed. This can be most efficiently performed by a solvent treatment.
The purpose of the alkali treatment is to remove from the lignocellulosic
material
hemicellulose, lignin and other phenolic compounds, which could at the
following
stage hamper or disturb the initiation by capturing radicals, and by weakening
the
fixing of the polymer formed onto the fibres. A substantial cost advantage is
however obtained thereby, that according to the invention these materials or
components are only partially removed, without an attempt to purify the
cellulose
completely from other components. By alkali treatment and the possibly
preceding
water washing 30 to 95%, advantageously 50 to 80% of the total amount of
lignin
and hemicellulose can be removed.
Preferential starting materials such as cereal straw, peels and hulls, are
fibrous or
foil-formed in the structure. Additionally, it can be advantageous to separate
the
fibres in order to increase the reactive surface in relation to the total
weight, and to
modify physical properties of the material according to the requirements of
the end
use. In case a chemical defibration would be performed, the costs would be
easily
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
high and the yield would remain at about 40% of the initial weight. The
material
obtained would not be competitive as compared to cellulose from the present
large-
scale industrial processes. An analogous defibrating effect can however be
obtained
by treating the material according to the invention with alkali, whereby the
main
5 part of hemicellulose and a substantial part of lignin is dissolved and
removed. An
effective defibrating is achieved especially by treating with a strong
alkaline
solution at a temperature under 40°C. Economical defibrating treatments
are also
mechanical wet-millings in water suspensions, and chemo-mechanical wet
milling,
both of which can be performed at temperatures from 0 to 100°C, or at
higher
temperatures under pressure using, for example, extrusion techniques. The
advantage of chemo-mechanical wet milling as compared to alkaline extraction
without milling is a lower consumption of chemicals and a more effective
defibrating, the disadvantage is some disruption of the fibres in the process.
The initiation treatment whereby reactive radicals are formed is in this
invention
IS performed advantageously by using an oxidative chemical, such as hydrogen
peroxide or sodium persulfate. This stage is followed by addition of one or
several
monomers and cross-linking agents to the moist material, preferentially
without any
washing or other intermediate stages. Polymerization can be accelerated by
heating
the reaction mixture, maintaining the temperature, however, below 75°C.
Monomers to be used in this invention can be one or several compounds
containing
a vinyl group, such as acrylic acid, methacrylic acid, styrene, N-vinyl
pyrrolidone,
or their derivatives. Choice of the monomers and cross-linking agents depends
on
the properties desired for the end product, such as ion exchange properties,
water
binding capacity and the effect of acidity, ionic strength, and pressure on
these
properties.
The properties of the product can also be influenced by down stream treatments
following the polymerization. Thus, for example, when acrylic acid is used as
a
monomer, weakly dissociating carboxyl groups which can act as ion exchangers
are
formed in the polymer layer, and the water absorbing capacity can be elevated
by
treatments with alkali, whereby a part of these groups are neutralized.
Strongly
dissociating cation exchanging atomic groups can be obtained by using as one
of
the monomers vinyl monomers which contain a strongly or intermediately
strongly
dissociating atomic group such as sulfonic acid group. Alternatively, the
product
after the polymerization can be subjected to a treatment whereby such groups
are
formed, for example by treating with chlorosulfonic acid. Correspondingly,
anion
exchange properties can be obtained in the product by using as the monomer or
as
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
6
one of the monomers a vinyl compound containing basic atomic groups, or by
performing after the polymerization a treatment whereby such groups are formed
or
introduced, according to methods known as such.
The experimental material used in the investigations on which this invention
is
based has been oat hulls. Its content of cell walls is as a mean more than
83%, its
content of lignin being below 10%, of cellulose 30 to 35%, and of
hemicellulose 30
to 35%, respectively (Welch, Journal of the Science of Food and Agriculture
34,
417-426, 1983).
Implementation of the method is described in the following examples.
Example 1
100 parts by weight of oat hulls obtained from an industrial dehulling process
were
extracted for 2.5 hours in water heated to the boiling point. The water phase
containing also the finely dispersed fraction was separated. The separated
fraction
contained 10.25 parts by weight of dry matter. The wet solid fraction was
extracted
during three hours in a solution containing one part by volume of ethanol and
two
parts by volume of toluene, at the boiling point of the mixture. The solids
were
separated. The drying residue of the solution was 1.45 parts by weight, and it
consisted mainly of lignin and of a small proportion of carbohydrates. The
latter
result indicates that the amount of waxy compounds in this material is
negligible,
and thus their removal by extraction is not needed.
Example 2
To two parts by weight of the extracted and dried material from Example 1, 50
parts
by weight of 23% potassium hydroxide were added, and the mixture was kept at
room temperature (23°C) for 18 hours. The solution was separated by
decanting.
Dry matter of the solids was 58% of the weight of the extracted and dried
material
taken for treatment in this example. Its water absorption capacity was
determined by
immersing in distilled water and by removing the non-absorbed water by
centrifugation (2000 x g for 0.5 hours). The water absorption capacity was
sixfold
as compared to the dry weight.
Example 3
The mixture of oat hulls and potassium hydroxide obtained in Example 2 was
diluted to a twofold volume with distilled water, and one part by weight of
30%
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
7
hydrogen peroxide was added. Mixing was continued at room temperature for
three
hours. The solution was removed by decanting. After this treatment, the dry
weight
of the solids was 84.3% of that taken for the treatment in this example. Its
water
absorbing capacity, determined as in the example 2, was 7-fold as compared to
the
dry weight.
Example 4
The solids after decanting in the Example 3 were transferred without any
preceding
washing into a reaction vessel. 2.38 parts by weight of redistilled acrylic
acid and
0.13 parts by weight of redistilled ethyleneglycol dimethacrylate (EDMA) were
added. Air was removed by leading argon gas through the reaction mixture for 5
minutes, and 0.04% by weight of sodium persulfate were added. The temperature
was elevated to 60°C, and polymerization was continued for 1.5 hours,
maintaining
the temperature of the mixture below 75°C. The polymer formed was
cooled,
washed with a 0.0125 mol/L sodium hydroxide solution, separated from the
solution by filtering under vacuum, and dried in vacuum. The water binding
capacity, as measured with a 0.9% sodium chloride solution under pressure, was
16.5 fold as compared to the dry weight.
Example 5
The experiment according to Example 4 was repeated by using fibrous material
obtained from a treatment according to Example 2 as starting material. The
product
obtained had 12.5 fold water binding capacity as compared to the dry matter,
when
tested under pressure.
Example 6
The experimental serie according to Examples 2 to 4 was repeated in a
modification
where under the alkali treatment the temperature was elevated to 40°C
for one hour,
after which the solution was removed by decanting, and the duration of the
hydrogen peroxide treatment was one hour. The product obtained had a 18-fold
water binding capacity as compared to the dry matter, when tested under
pressure.
The examples indicate the operation principles, but do not limit ingredients
or their
proportions in the implementation. They may be selected depending on the
physical
form and functional properties desired.
CA 02447520 2003-11-12
WO 02/092669 PCT/FI02/00415
g
In addition to straw, peels and hulls of cereal crops, the method can be used
to treat
other lignocellulosic materials, which either are in thin layers or can be
brought to
thin layers. Examples of other materials are wood clips, bagasse, jute and
leaves of
plants.
The dissolved material obtained at the stages described in Examples 1 and 2 is
a by-
product which can be recovered and marketed separately, based on its high
content
of hemicellulose, for industrial raw materials or for feeds.
