Note: Descriptions are shown in the official language in which they were submitted.
21-09-2000 B~l9g$ CA 02352222 2001-05-24 ~~J~J72649
EPO - DG 1 Prrnn.~
1
- Process for oxidising starch 21. 09. 2000
54
The invention relates to a process for lowering the viscosity of carbohydrates
by
oxidative degradation.
Starch must be subjected to a treatment to lower the viscosity before it can
be uscd
industrially. To this end the starch is generally subjected to oxidative
treatrnent at elevated
temperature (40-60 °C) in an alkaline medium. For example, if
hypochlorite is used as
oxidising agent at 40 °C, the reaction takes 4 -15 hours. The
disadvantage of this method is
the amount of salt formed and the extensive degradation of starch, accompanied
by
incorporation of chlorine. According to DE 738909, virtually dry potato starch
can be
gelled by treatment with hydrogen peroxide at 45 °C for 6 hours. Such a
treatrnent is
suitable for the preparation of blancmange, but the viscosity is too high and
not stable for
other applications.
According to another method, hydrogen peroxide is used with a transition metal
as
catalyst. However, the catalyst, which is usually toxic, is difficult to
remove afterwards,
even with the use of compounds such as EDTA. A further disadvantage is the
discolouration of the product.
US-A 5,362,868 discloses a method according to which the viscosity of (hydroxy
ethyl)starch is lowered by oxidation with a per-acid. The per-acid used is,
for example,
peroxysulphuric acid (HzS05) or peracetic acid. The peroxysulphuric acid has
to be
prepared in situ from hydrogen peroxide and sulphuric acid. The reactivity of
peracetic acid
is lower and leads to longer reaction times. It is also described in US-A
5,362,868 that
reaction with hydrogen peroxide leads to unacceptable reaction times.
According to WO 97/31951 the viscosity of starch and other carbohydrates can
be
lowered by treatment with hydrogen peroxide in the presence of an acylated
carbohydrate.
CS Patent 143708 (CA 77:166487) discloses the treatment of starch with 0.1-0.3
aqueous hydrochloric acid, 0.2-0.5% hydrogen peroxide 40% and about 2% water
at 70-
90°C for 4-7 h, resulting in a depolymerised product for use as textile
sizes and finishes.
The aim of the invention is to provide a method for lowering the viscosity of
starch
and other carbohydrates using simple reagents, without the use of heavy metals
and salt-
forming reagents, and without organic by-products. In this context it is
desirable that a
stable viscosity is obtained, that is to say a stability which does not
increase further at
ambient temperature.
Said aim is achieved by means of a method wherein the carbohydrate is oxidised
with
hydrogen peroxide in the presence of a catalyst and, after it has been brought
into contact
with the hydrogen peroxide, the carbohydrate is, if necessary, dried at a
temperature below
60 °C and is then treated at a temperature of 80 - 140 °C.
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The process according to the invention is an inexpensive method with short
reaction
times and with low consumption of hydrogen peroxide, consumption being
approximately
times as low as with the method according to WO 97/31951. There is also no
salt burden
or other waste stream worthy of mention. The method leads in high yield and
with little loss
5 to a soluble or dispersible carbohydrate, the solution or dispersion of
which has a low and
stable viscosity.
Any polysaccharide that is completely or partially soluble in water can be
used as the
carbohydrate that can be treated in accordance with the process according to
the invention.
Such polysaccharides include, firstly, starch and fractions and derivatives
thereof, such as
amylose, ethoxylated starch and carboxymethylstarch. Furthermore, the
solubility of
cellulose and chitin and derivatives thereof can be improved, or the viscosity
thereof can be
lowered, by means of the method according to the invention.
The quantity of hydrogen peroxide to be used is entirely dependent on the
desired
degree of oxidation. By virtue of the higher efficiency of the method
according to the
invention, a limited quantity of hydrogen peroxide can suffice. In general a
solution of 0.1 -
% (m/m) hydrogen peroxide, preferably of 1 - 10 % (m/m), is used. Preferably
the
quantity and concentration of hydrogen peroxide taken is such that the
carbohydrate does
not dissolve to a significant extent and, in the case of starch, remains in
granular form. The
catalyst can be an organic acid, such as acetic acid, formic acid, propionic
acid, lactic acid,
20 oxalic acid, citric acid, succinic acid or an anhydride such as acetic
anhydride, and in
particular a cyclic anhydride of a dicarboxylic acid such as succinic
anhydride, malefic
anhydride or citraconic anhydride, or possibly a lactone or lactide.
Phosphoric acid can also
be used. The quantity of catalyst is, for example, 0.1 - 4 % (m/m), in
particular 0.5 - 2
(m/m), with respect to the carbohydrate.
After hydrogen peroxide and catalyst have been added and mixed, the
undissolved
carbohydrate is filtered off if necessary. The water content must not be too
high for the
following step. If the water content is more than 20 or 25 % (with respect to
the mass of the
carbohydrate and water), the carbohydrate is then dried to a water content of
25 or lower,
especially between 10 and 20%, with respect to the mass of the carbohydrate
and water.
Drying is carried out at a temperature of at most 60, and in particular below
55 °C, down to,
for example, ambient temperature, preferably 20°C or higher, preferably
to a water content
of at most 25 % (m/m), in particular of 15-20 % (m/m), with respect to the
carbohydrate,
corresponding to at most 20 % and 13-17 %, respectively, on the basis of the
total mass of
carbohydrate and water. In the case of starch, drying must be carried out
under conditions
such that the starch does not gelatinise or barely gelatinises. Drying drums
can optionally
be used for drying on a large scale.
The dried carbohydrate is then kept at a temperature above 80 °C, in
particular
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above 100 °C, up to about 140 °C, preferably between 100 and
130°C, for a period of at
least 10 minutes, in particular for a period of 1 -10 hours, during which
period the actual
oxidation takes place. At higher temperature, for example 120 °C, use
can be made of
fluidised beds, microwave ovens and the like. It is important that an open
reactor is used, so
that any residual water released can be removed easily. If the residual water
is not
adequately removed, the carbohydrate, in the case of starch, can then still
gelatinise.
The viscosity of the carbohydrate of reduced viscosity that is obtained using
the
method according to the invention can be determined using a Brabender
viscometer.
According to the measurement method, a 25 % (m/m) solution or suspension is
heated from
40 °C at a rate of 1.5 °C/min; during this heating the viscosity
initially rises to a maximum
(peak viscosity); on further heating the viscosity falls again; at 90
°C the temperature is not
raised further and the viscosity reaches a minimum (valley viscosity); after
30 min at 90 °C
cooling is carried out, again at a rate of 1.5 °C/min, to a final
temperature of 40 °C; the
viscosity at this end point is termed the final viscosity. The product
according to the
invention preferably has a peak, valley and final viscosity of 4,000, 200 and
2,000,
respectively, with 2,000, 100 and 500 Brabender units, respectively, being
most preferred,
with a minimal reduction in the molecular weight. The treated starch product
preferably has
a molecular weight between 0.2 and 1 Mda, and a final viscosity which rises by
less than
100 %, in particular by less than 50 % (in Brabender units) at 40°C
within one hour of
completion of the treatment. The product of reduced viscosity can serve as the
basis for
various applications. One important application is in paper making, where it
can be used to
increase the initial wet strength of the paper and to obtain improved
printability, better
adhesion of cationic agents and improved reception of adhesive.
Example 1
A quantity of potato starch (100 - 1000 g) is suspended in a solution of 5 %
or 2.5
hydrogen peroxide in water. Acetic acid or malefic anhydride is optionally
added. After
filtration, the material filtered off contains 30-40 % water; the hydrogen
peroxide content is
therefore about 1 % (m/m) at 2.5 % H20z. The filter cake is dried in air for
three days.
Instead of this it is also possible to dry for a few hours under a stream of
air at
approximately SO °C. The powder obtained is then kept at 120 °C
in an oven for a number
of hours (reaction time). The material obtained is white and has retained its
granular
structure. The number of carboxylic acid groups is approximately 1 per 57
anhydroglucose
units. Brabender curves were recorded for the materials (25 % in water) at pH
7Ø The
results are summarised in the following table.
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Table 1
Reaction Catalyst HzOz Viscosity
time
(h) Type' %z %z Peak Valley Final
3 AA 1 5 320 40 100
3 - - 2.5 5480 300 7080
3 AA I _ 3 3
3 AA 1 2.5 3760 148 3400
4 AA 1 2.5 3160 132 2680
3 AA+MA 1+1 2.5 32 32 32
3 MA 0.5 2.5 1010 40 984
35 MA 0.5 2.5 850 64 104
' AA = acetic acid; MA = malefic anhydride
S z percentage by weight in water
3 viscosity too high to be able to be measured
4 after 30 minutes at 40 °C: 108
wheat starch
Example 2
A quantity of potato starch ( 100 - 1000 g) is dried to a moisture content of
at most
10 %. 2.5 % HZOz (2.5 gram per 100 gram dry starch, 100 % solids), 1 % or 0.5
% or
0.25 % acetic acid (with respect to dry starch) and 1.0 % or 0.5 % malefic
anhydride (with
respect to dry starch) are then added to the starch. The peroxide, acetic acid
and malefic
anhydride are all dissolved in water. The total quantity of water (water
already present in
the starch plus water added via the chemicals) is, however, no greater than 20
% (solids
content of 80 %). The whole is mixed well in order then to react for 3 hours
at 120 °C.
The material obtained is white and has retained its granular structure.
Brabender curves
were recorded for the materials (25 % in water) at pH 7Ø The results are
given in Table
2 below.
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Table 2
Quantity Catalyst Viscosity
of
water (%)
HzOz(%) AA(%) MA{%) Peak Valley Final
20 2.5 1 0.5 367 9 500
6 I .0 0.5 0.5 501 i 6 480
16 I.0 0.5 I.0 610 6 250(S)
S = Stable
