Note: Descriptions are shown in the official language in which they were submitted.
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Process for Preparing Chitosan Particles
Technical Field
The present invention relates to a process for preparing particles of
chitosan.
Chitosan particles in this form which are prepared according to the process
herein are
"activated" in the sense that they are especially useful for providing plant
care benefits.
Background of the Invention
Chitosan is a modified carbohydrate polymer derived from the chitin component
of the shells of crustaceans such as crabs, shrimp and cuttlefish. Chitosan is
used for a
wide variety of purposes including plant care, cosmetics additives, food and
nutrition
supplements and medical care uses.
Both Polish Patent No. 125,995 and the periodical "Journal of Applied Polymer
Science" Vol. 33 p. 177, 1987 disclose a method for producing a form of
chitosan with a
developed internal surface. This is done via a batch process in which the
chitosan is
periodically precipitated from its solutions in aqueous organic or inorganic
acids or their
salts by means of neutralization with hydroxides of alkali metals. The
precipitation
mixture is vigorously stirred. The precipitated chitosan in gel or suspension
form is
washed several times with water. This known method permits production of
chitosan
having a developed internal surface with a yield of 70 - 90% of theoretical.
However, the
procedure is a batch process which requires at least 12 - 24 hours for a
production cycle.
The single batches of the product are lacking homogeneity. The product has a
distinct
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tendency toward degradation and its sorption capacity is rather poor resulting
from the
relatively low degree of development of the inner surface of the chitosan
particles.
Polish Patent No. 164,247 and Finnish Patent No. 83,426 both disclose a
continuous method for producing microcrystalline chitosan. In this method a
solution of
chitosan in aqueous acids and/or their salts is introduced to a reactor along
with an
aqueous solution of alkali metal hydroxides or salts. This results in
formation of
suspension of microcrystalline chitosan particles with a suspension pH > 7.
Simultaneously, this alkaline suspension of the formed microcrystalline
chitosan particles
is continuously removed from the reactor. The alkaline solution may also be
introduced .
directly to a recirculation system. This method has several drawbacks
including a yield
below 90%, and realization of chitosan agglomerates with an average particle
size above
1 ~.m and water retention value below 5000%. The water retention value is an
indication
of the development of the inner surface. Moreover, it is not possible to
control the
molecular, superinolecular and morphological structure of the chitosan
particles
produced. This continuous process also causes a substantial decrease of the
average
molecular weight of the generated microcrystalline chitosan as result of
intensive
degradation effects.
Summary of the Invention
The present invention provides a process for preparing particles of
"microcrystalline" chitosan in the form of an aqueous suspension of such
particles. As a
consequence of the specific steps used to carry out the process, the chitosan
particles in
the suspension are "activated" in the sense that the chitosan material in this
form is
especially useful for a number of purposes including in particular the
provision of a
variety of plant care benefits.
In the first step of the process herein, an aqueous solution is formed,
generally
containing at least 0.001% and preferably from 0.01% to 10.0% by weight of
chitosan
which can be provided by any conventional chitosan source. This solution
contains
organic or inorganic acids and/or salts of such acids in an amount sufficient
to maintain
the solution at a pH which is low enough to completely solubilize the
chitosan.
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In the next step of the process, the aqueous solution of the first step is
partially
neutralized by adding a neutralizing agent while subjecting the solution to
shear agitation.
The neutralizing agent addition and the agitation are sufficient to convert
the solution into
a continuous gel phase having a pH of from 5.0 to 6.9
In the next process step, the partially neutralized gel formed in the previous
step is
maintained under, shear agitation for at least 10 seconds after the gel phase
has been
formed in order to homogenize the gel phase. This agitated homogenized gel is
then
further neutralized under continuing agitation by addition of more
neutralizing agent
sufficient to raise the pH within the homogenized gel phase to above 6.9,
preferably
above 7.3. Agitation is preferably continued for at least another 10 seconds
after this
elevated pH is reached. This then forms a gel-like suspension of discrete
particles of
activated microcrystalline chitosan having desirable properties.
In preferred embodiments of the present process, the aqueous solution formed
in
the initial step can be filtered prior to neutralization to remove insoluble
matter such as
may have been introduced with the chitosan source. Also preferably the
chitosan
particles of the eventually formed gel-like suspension may be washed, e.g.,
with water, to
remove such materials such water soluble salts.
Preferably the chitosan particles produced by the process of this invention
are
characterized by having an average particle size of from 2 to 20 ~.m.
Preferably also such
particles further have a water solubility of at least 90% at pH 6 after 24
hours.
Detailed Description of the Invention
In the first step of the process described above, an aqueous solution of
chitosan is
formed. This can be accomplished by combining chitosan with an aqueous
solution
containing organic or inorganic acids or their salts. Such acids can include,
for example,
acetic acid, lactic acid, salicylic acid, hydrochloric acid, and the like.
Sufficient acid or
salt thereof must be utilized to maintain this solution at a pH wherein the
chitosan is
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essentially completely dissolved, i.e., solubilized. Agitation sufficient to
facilitate
chitosan dissolution may be utilized.
Any conventional source of chitosan may be utilized to form the chitosan
solution.
Such chitosan sources may be those derived from shellfish or may be fungally
derived.
Commercially available chitosan sources are marketed, for example, under the
tradenames "Chitosan" by Vanson, Inc. of Redmond, Washington , USA and
"Chitoclear" by Primex Ingredients SA of Avaldsnes, Norway.
The chitosan is utilized to an extent which is sufficient to form a
concentration of
chitosan in the aqueous solution of at least 0.001 % by weight. More
preferably, the
solution will contain from 0.01 % to 10.0% by weight, most preferably from
0.01 % to
1.5% by weight.
After the aqueous chitosan solution has been formed in the first essential
step of
the process herein, such a solution may optionally be filtered before further
processing in
order to remove therefrom any insoluble material which might remain therein.
Such
insoluble material, for example, may have been introduced into the solution
from the
chitosan source.
In a second essential step of the process herein, the chitosan solution formed
in the
first essential process step is partially neutralized. This is accomplished by
adding a
neutralizing agent to the solution while maintaining the solution under shear
agitation.
Neutralization agents which can be used in this step include hydroxides such
as sodium,
potassium and ammonium hydroxides and their salts. Preferably these
neutralization
agents are combined with the first step chitosan solution in the form of
aqueous solutions
having a neutralizing agent concentration of from 0.01 % to 20% by weight.
Partial neutralization in this second essential process step is effected while
the
combined chitosanlneutralizing agent solution is maintained under shear
agitation. Both
the addition of the neutralizing agent and the shear agitation should be
sufficiently carried
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out in order to convert the solution into a continuous gel phase having a pH
within the
range of from 5.0 to 6.9. Typically shear agitation to the extent of from 10
to 1000 sec''
(rpm) can be used.
After this continuous gel phase has been formed, the gel phase is next
subjected to
continued shear agitation in order to homogenize this gel phase. Generally
such
continued shear agitation is carried out for a period of at least 10 seconds
and preferably
for a period of from 1 to 60 minutes. Typically shear agitation utilized at
this point is
applied to the extent of from 100 to 5000 sec 1.
Next the homogenized gel phase from the previous step is further neutralized
while still being subjected to shear agitation. In this step, neutralizing
agent is added to
the extent needed to raise the pH within the gel to above 6.9, and preferably
to within the
range of 7.1 to 7.5, e.g., above 7.3. This generally occurs under shear
agitation of from
to 1000sec-'. After the requisite pH is reached, high shear agitation (100 to
5000 sec-')
can be used to homogenize the suspension. The same type and form of
neutralizing
agents as specified above for the partial neutralization step can be utilized
in this further
neutralization step. Alternatively, the neutralizing agent used in this
further
neutralization step does not have to be identical to the neutralizing agent
used in the
partial neutralization step.
This further neutralization step produces a gel-like suspension of discrete
particles
of chitosan. Preferably this suspension of particles at this point can be
washed, e.g., with
water, to remove therefrom any residual soluble salt impurities. The chitosan
particles in
the suspension may also be concentrated, recovered and/or dried in
conventional manner.
The process of this invention may be carried out in either a batch-wise or
continuous
format.
In the process according to this invention, during the initial partial
neutralization
step, the sol form of the chitosan salt is transformed into gel.
Simultaneously, nucleation
points appear for the agglomerates of chitosan. Without being bound by theory,
it is
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believed that the intensive agitation causes this. An agglomerate structure
emerges and a
physical-chemical modification of the initial gel form proceeds. Further on in
the process
during further neutralization, the final agglomerate structure of the chitosan
particles
forms. The stabilization of the agglomerates of the physical/chemical modified
chitosan
particles proceeds in the final step of the process. During this chitosan
manufacturing
procedure, a controlled structure of the product is ~ formed with respect to
the molecular,
supermolecular and morphological characteristics of the resulting material.
The chitosan particles produced by this invention are characterized herein as
"microcrystalline" even though their degree of crystallinity is very low, and,
in fact is
much lower than the degree of crystallinity of commercial chitosan products
which have
a significant crystalline content. Perhaps more accurately, the particles
herein can be
characterized as "activated" given their utility in a number of contexts as
described
hereinafter. The chitosan material of the particles which are produced has a
modified
specific molecular structure having specific molecular weight and degree of
polydispersity. It also has a supermolecular structure having certain
morphological
characteristics which provide relatively high porosity and relatively high
ability to retain
water in a capillary system.
The modified chitosan can be prepared according to this invention with a yield
of
over 90%, usually 95-99.5%. The material is highly homogenous and well
reproducible.
It has average molecular weight which is close to that of the initial chitosan
with lowered
molecular polydispersity. The material also has a Water Retention Value (WRV)
which
is generally much higher than that for the starting chitosan. WRV will, in
fact generally
exceed 1000% and can even exceed 5000%, a value not attainable via previously
known
methods.
The process of the instant invention permits realization of activated chitosan
with
a modified structure, in the form of a gel-like suspension, paste or powder.
The gel-like
form is stable with attainable agglomerate sizes below 1 pm. Preferably,
however, the
average particle size of the chitosan particles in suspension will range from
2 to 20 Vim.
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Preferably also these particles will have a water solubility of at least 90%,
preferably
95%, at pH 6 after 24 hours.
The modified chitosan prepared according to the present invention can be
applied
in agriculture, medicine, pharmacy, veterinary medicine, chemical industry,
health and
beauty care products and environment protection. As indicated hereinbefore,
the activated
chitosan produced by this invention is particularly useful in the area of
plant care.
The process of this invention, and the material produced thereby, are
illustrated by
following examples which do not limit the scope of the present invention.
Example I
800 parts by weight of a 1% aqueous solution of commercially available
chitosan
(Primex Chitoclear) in 2% acetic acid are introduced to a reactor equipped
with an
agitator. The initial polymer is characterized by: average molecular weight My
= 550
kD, deacetylation degree DD = 80.6%, polydispersity degree Pd = 4.85, ash
content
1.70%. At rotation speed 60 rpm of the agitator, a 1.5% aqueous solution of
sodium
hydroxide is introduced through a metering pump to achieve pH = 6.85 which is
the
turning point of the chitosan salt mixture from the sol to gel form. The gel
so obtained is
agitated for 5 minutes at 1000 pm. Thereafter the neutralization is continued
with
addition of the 1.5% aqueous sodium hydroxide at agitation speed 4000 rpm
during next
minutes to attain pH = 7.5. Next the mixture is homogenized over 10 minutes.
The
chitosan obtained in a gel-like suspension is continuously washed with water.
The
suspension is next concentrated. 840 wt parts of chitosan are obtained in form
of a white
gel-like suspension with 0.94% concentration and with following
characteristics: My =
488.9 kD, DD = 80.6%, WRV = 4500%, Pd = 4.45 and average size of the
agglomerates
0.5 to 3 Vim. Ash content is 0.32% wt.
Example II
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To a reactor, as in Example I, 1000 parts by wt are introduced of a 1 %
aqueous
chitosan solution (with properties as in Example I) along with a 1.5% solution
of lactic
acid. With continual agitation at 120 rpm, a 0.75% aqueous solution of sodium
hydroxide
is continuously introduced through a metering pump to achieve pH 6.0 - 6.8.
This results
in the transformation of the chitosan salt from the sol form into gel and a
partial
formation of the chitosan form. The intermediate product obtained is
homogenized for 1
minute at 1000 rpm followed further by 10 minutes of neutralization with the
0.75%
aqueous sodium hydroxide solution at 4000 rpm to achieve pH = 7.5. This is
followed by
minutes of homogenizing. The chitosan obtained in a gel-like suspension is
purified as
in Example I.
In this case 640 parts by wt of chitosan are obtained in the form of a white
gel-like
suspension with 1.5% concentration of the polymer and following
characteristics: My =
491.5 kD, DD = 80.6%, WRV = 2300%, Pd = 4.88, ash content 0.1% and average
agglomerate size = 0.5 - 5 ~,m
Example III
To a reactor as in Example I, 800 parts by weight are introduced of a 1%
aqueous
chitosan solution, with properties as in Example I, along with a 0.5% solution
of lactic
acid. With continual agitation at 90 rpm, a 0.25% aqueous solution of
potassium
hydroxide is continuously introduced through a metering pump to achieve pH =
6.9%.
This results in the transformation of the chitosan salt from the sol form into
gel. The
intermediate product obtained is homogenized for 10 minutes at 1000 rpm,
followed
further by 1 minute of neutralization with the 0.25% aqueous potassium
hydroxide
solution at 4000 rpm to achieve pH = 7.4. The mixture is then homogenized for
15
minutes at the same agitator speed. The product obtained is purified as in
Example I.
1020 parts by wt of chitosan are obtained in form of a white, gel-like
suspension
with 0.75% concentration of the polymer and the following characteristics: My
= 398.2
kD, DD = 80.6%, WRV = 5200%, Pd = 4.52, ash content = 0.5% and average
particle
size in the 0.05- 10 ~.m range.
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Example IV
To a reactor as in Example I, 900 parts by wt of a 1% aqueous solution of
chitosan
(with following properties My = 328.2 kD, DD = 82.5%, Pd = 3.185 and ash
content =
0.34%) are introduced along with a 0.65% aqueous salicylic acid solution. With
continual
agitation at 60 rpm, a 0.25% aqueous solution of sodium hydroxide is
introduced
continuously through a metering pump to achieve pH 5.0 to 6.9 at which pH the
chitosan
salt is transformed from sol to gel with partial forming of the
"microcrystalline" form.
The intermediate product obtained is homogenized for 2 minutes at 4000 rpm,
followed
by 90 seconds of neutralization with 0.25% aqueous sodium hydroxide at the
same
agitation speed to achieve pH = 7.3. The homogenization is continued at that
reaction pH
= 7.3 for 5 minutes. The product obtained is purified as in Example I.
445 parts by wt of chitosan are obtained in the form of a white, gel-like
suspension with 2.0% concentration of polymer with the following
characteristics: My =
327 kD, DD = 82.5%, WRV = 1380%, Pd = 3.05 ash content = 0.05% and average
agglomerate size = 1-10 ~,m.
Example V
To a reactor as in Example I, 1000 parts by wt of a 1 % aqueous solution of
chitosan (with properties as in Example I) are introduced along with a 1.5%
solution of
lactic acid. With continual agitation at 100 rpm, a 0.75% solution of aqueous
sodium
hydroxide is continuously introduced through a metering pump to achieve pH 5.0
which
allows the sol/gel transition to begin. The resulting gel is agitated for 5
minutes (Trial 1)
or 30 minutes (Trial 2) at 4000 rpm followed by 1 minute of neutralization
with 0.75%
aqueous sodium hydroxide and simultaneous homogenizing at 4000 rpm to achieve
pH =
7.2. The homogenization is continued for 10 minutes further at 4000 rpm. The
chitosan
obtained is purified as in Example I.
In both trials chitosan is obtained as a white gel-like suspension with
properties as
shown in Table 1 below:
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Table 1
Trial Out - put Concentration My DD WRV
No
wt. parts wt % kD
Triall 345.0 2.72 395.4 80.6 1005
Trial2 350.0 2.75 277.8 ~ 80.6 ~ _
1600
Example VI
To a reactor as in Example I, 1000 parts by wt of a 1% aqueous solution of
chitosan (properties as in Example I) are introduced along with a 1.S%
solution of lactic
acid. With continual agitation at 900 rpm, a 0.75% aqueous solution of sodium
hydroxide
is introduced through a metering pump to achieve pH 6.S which allows the
sol/gel
transition. The generated gel is agitated at 4000 rpm for S minutes (Trial 1)
and for 30
minutes (Trial 2), followed by 60 seconds of neutralization and simultaneous
homogenizing at 4000 rpm to achieve pH = 7.2. Under such conditions,
homogenizing is
continued for 10 minutes at 4000 rpm. The chitosan obtained is purified as in
Example I.
The chitosan produced is in the form of a white gel-like suspension with the
following properties for the two trials as shown in Table 2:
Table 2
Trial Out - put Concentration My DD WRV
No
wt. parts wt % kD
Triall 382.0 2.56 402.0 80.6 1240
Trial2 340.0 2.91 423.8 80.6 2600
Example VII
A set of equipment consisting of a reactor equipped with agitator, metering
pumps
and a recirculation assembly with pump and an outlet for the intermediate
product
connected to a second reactor is used in this Example. To the first reactor,
1000 parts by
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wt of an aqueous solution containing 1.5% aqueous lactic acid and 1% chitosan
with
properties as in Example IV are introduced. With continual agitation at 1000
rpm, a
0.75% aqueous solution of sodium hydroxide is continuously introduced to
achieve a pH
= 6.8 and to achieve the sol/gel transition. Next with the agitator running at
1000 rpm, a
1.5 % solution of chitosan in lactic acid is continuously introduced with the
rate of 1200
wt parts per hour. A 0.75% NaOH solution is also introduced at a rate of 585
wt parts per
hour which allows the pH to be kept at 6.7-6.9. At the same time with the
recirculation
assembly switched on, the gel which is produced is directed to the second
reactor. The gel
in the second reactor is continuously neutralized to pH = 7.5 to form the
activated
chitosan. The mixture in the second reactor is homogenized at 4000 rpm. The
average
retention time in the reactor is 30 minutes.
The product obtained from the second reactor is purified as in Example I. The
resulting chitosan in the form of a white gel-like suspension is obtained with
an output of
20 wt. parts per hour and has following characteristics: concentration =
1.94%, My =
315 kD, DD = 82.5%, Pd = 2.95, WRV = 5900%.
n
