Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02234632 1998-04-09
WO 97/13788 PCT/US96/16285
DRY THINNED STARCHES, PROCESS FOR
PRODUCING DRY THINNED STARCHES
AND PRODUCTS AND COMPOSITIONS THEREOF
s
This invention relates to novel dry thinned starches, continuous processes for
producing dry thinned starches and to compositions and products thereof. More
particularly, the invention relates to dry thinned starches produced in a
continuous
process under specified interrelated process conditions. It also relates to
compositions
io incorporating such starches such as paper sizing and coating compositions
and to paper
products produced from such compositions.
- The present invention produces novel dry thinned starch. The dry thinned
starches are produced by continuously feeding a mixture of a base starch and a
chemical
is which hydrolyses the glycosidic linkage of starch to a plug flow reactor,
passing the
mixture through the reactor, recovering the mixture and neutralizing the
mixture.
The invention also includes starches produced by the process and paper
products
produced from the starch.
2o
Figure 1 is a schematic flow diagram of a dry thinning process.
Starches are widely used in paper coating formulations to give the required -
rheology, water holding, and binding properties. Typically, low viscosity
starches are
is required to achieve high solids in the coating system. High solids are
needed to reduce the
drying cost and to reduce surface deformation due to excessive shrinkage of
the surface
layer during drying. Another requirement of such starches is that they exhibit
resistance to
retrograding, i.e., the formation of insoluble precipitates from the
association of poorly
soluble linear dextrins present in starch solution and low DE (dextrose
equivalent)
3o hydrolyzates.
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Hydroxyethylated dent corn starches are presently used in such coatings
because
they are low cost and their cooked pastes resist retrogradation substantially
better than
unmodified dent corn starch. Thinned unsubstituted waxy starches have the same
or
greater resistance to retrogradation than the hydroxyethylated dent starches
and are less
s expensive to produce.
Low viscosity starches are prepared by reducing the molecular weight of the
starch
polymers. Most common processes use either an acid, an enzyme, or an oxidizing
agent
for molecular weight reduction. These thinning reactions are typically carried
out in an
io aqueous slurry of the starch. Upon thinning the starches to the preferred
viscosity, it
becomes necessary to filter the starch from the slurry so that it can be dried
and recovered.
Some of the soluble starch and salts of neutralization generated by the
thinning reaction are
substantially lost in the effluent. This results in a significant economic
loss due to yield
IDSS, drying inefficiencies and an increased load on the effluent treatment
facilities. In the
case of hydroxyethylated starch, a portion of the hydroxyethyl substituent is
also lost which
increases the economic disadvantages of its use.
An alternate route for making starches with the above performance
characteristics, but without the economic loss resulting from present
processes, has been
zo found. The essential characteristics of this method are the injection of,a
glycosidic
hydrolyzing chemical into starch which is in a non-slurry state, heating at a
temperature
and for a time necessary to produce a starch with the required viscosity, and
then
neutralization. No washing of the final product is required. This process can
be used for
any type of starch to be thinned.
is
Dry thinned waxy starch products allow higher solids concentration
compositions ~
that give the same viscosity as lower concentration modified starch
compositions. In
paper manufacture, this is an advantage at the sizepress and coater which are
both
viscosity constrained. Waxy starch is unique in this feature compared to
modified or
3o unmodified dent corn starch.
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A process has been developed for the continuous dry reaction of starch. This
process is used for reduction of molecular weight (thinning) via chemical
hydrolysis of
the glycosidic linkages of starch. A preferred means of chemical hydrolysis is
by
gaseous acidification. A primary use for these starches is in the paper
industry. They
will be used at the size press and on paper coaters. Their purpose is to add
viscosity
(flow properties) and water holding to the coating colors and strength to the
sheet at the
size press.
Any starch source including common dent corn, waxy maize, potato, waxy milo,
i o arrowroot, wheat, rice, tapioca and sago starches can be thinned by this
process. The
preferred base starches for paper applications are waxy starches
(amylopectins) derived
from waxy maize due to their resistance to retrogradation. Chemically modified
dent
starches (at least 20% amylose) also exhibit resistance to retrogradation.
Chemical
rpodification contributes resistance to retrogradation via steric hindrance.
Linear starch
i s chains are prevented from aligning due to the addition of substituents.
Such starches
include starches modified with epoxides such as ethylene oxide, propylene
oxide and
epichlorohydrin. Of these, starches modified with ethylene oxide are preferred
in paper
applications. The source starch can be chemically modified by other means such
as 1 )
bleaching with hydrogen peroxide, ammonium persulfate, chlorine, chlorite and
ao permanganate; 2) oxidation with chlorine; 3) esterificationwith acetic
anhydride,
phosphates, octenyl succinic anhydride, phosphorous oxychloride, succinic
anyhydride and
vinyl acetate; 4) etherification with acrolein or 5) formation of cationic
derivatives with
reagents such as 2,3-epoxypropyltrimethylammoniumchloride or (4-chlorobutene-
2)
trimethylammoniumchloride prior to the dry thinning process. Such starches are
well
as known in the art and are commercially available. For example, ETHYLEX~ gums
are
available from A.E. Staley Manufacturing Co. of Decatur, Illinois, are
derivatives of corn
starch in which hydroxyethyl groups have been attached to the starch Although
not as
resistant to retrogradation, useful starch products can also be prepared by
the process of the
invention from common dent corn starch. The dry thinned modified starches
produced by
3o this process can be used in food and textile applications in addition to
the described paper
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applications. In such cases, the dry thinned modified starches are used to
replace modified
starches which have been thinned in traditional aqueous slurry processes.
DRY THINNING PROCESS
s A dry thinning process has been discovered wherein the molecular weight of
base
starches is reduced by the reaction of the starch with a glycosidic
hydrolyzing chemical
in a continuous reactor. Any chemical which hydrolyses the glycosidic linkage
of starch
in the reaction conditions described can be used. There are three primary
groups of
hydrolyzing chemicals that can be used. Group I includes acids such as
hydrogen
io chloride gas (gaseaus HCl), hydrochloric acid (HCl) and sulfuric acid;
Group II includes
oxidants such as ammonium persulfate, hydrogen peroxide and chlorine gas; and
Group
III includes acids such as sulfur dioxide gas, carbon dioxide gas, nitric
acid, phosphoric
acid, monochloroacetic acid, ammonium chloride (Lewis acid) and calcium
chloride
(Lewis acid). Group I chemicals are preferred for use in the dry thinning
reaction, with
is the most preferred hydrolysis chemical being gaseous HCl. Pressurized
reactors maybe
required when sulfur dioxide and carbon dioxide are used as the hydrolyzing
chemicals.
In some instances it may be beneficial to use two or more hydrolyzing
chemicals
together in the process.
ao The following variables are critical to control the thinning rate:
temperature,
hydrolyzing chemical addition level, moisture level of the incoming starch,
and residence
time in the reactor. From these variables the final molecular weight of the
starch and the
amount of dry substances starch required to give 1000 centipoise of viscosity
at 35°C in
an RVA viscometer (VscGm DS) can be predicted. Typically this will be from
about 1.5
as to about 13 grams starch, dry basis (GmDS starch). The RVA viscometer is a
computer
controlled, electrically heated, water cooled, mixing and cooking device
designed to
measure viscosity of materials cooked under different time/temperature
profiles. The
RVA unit utilizes a 28 gram sample compartment. When used to evaluate products
made by this invention, it has been found that a 1.5 to 13 gram starch sample
should be
3o placed in the sample compartment. Sufficient water is added to the sample
compartment
to reach 28.0 grams total weight. The RVA is an instrument for measuring
starch
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cooking properties that provides a practical means for identifying the end
point of a
thinning reaction. The Rapid Visco Analyzer is made in Australia and marketed
in the
United States by Foss Food Technology of Eden Prairie, Minnesota. This
instrument
' allows for rapid generation of viscosity prof les. Standard oils with known
viscosities
s are used to calibrate the RVA over a specified temperature range. This
calibration is
used to convert RVA units to centipoise.
Experimental design response surfaces can be generated which describe the
relationship between the dependent variable (viscosity) and the independent
variables
io (temperature, hydrolyzing chemical level, retention time, initial moisture
content). Such
a relationship can be defined by an equation of the form:
VscGmDS=a+bT+cC+dR-eM
is wherein a, b, c, d and a are empirically derived constants that will vary
with reactor
design and configuration and T is temperature, C is chemical concentration
(weight %),
R is residence time in the reactor and M is the initial moisture content of
the starch (loss
on drying).
zo The starch products of the invention are prepared in a continuous reactor
exhibiting plug flow. The advantages of plug flow reactors versus reactors
wherein
mixing occurs is that the variables, most notably retention time, can be held
substantially
identical for all of the starch being processed whereas in mixed reactors the
starch will be
subjected to variable processing times.
is Plug flow reactors are known in the art. Such reactors, also referred to as
mass
flow purge and conditioning vessels, and mass flow bins are designed to allow
material
in the reactor to flow uniformly from the reactor in a first in first out
fashion with little or
no mixing of the material. It is recognized that it is difficult, if not
impossible, to
achieve perfect plug flow in mass flow bins with dry flowable products such as
starch.
so For purposes of this invention, it is the intent to approach plug flow of
the starch and
hydrolyzing chemical in the mass flow bin/reactor. Plug flow is meant to
describe a
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process in which substantially all of the starch flows through the reactor in
a first in first
out fashion with little or no channeling of the material in the mass flow
bin/reactor.
To obtain substantially mass flow in such a vessel, the conical slope of the
vessel
should be about 65 to about 75 degrees. The vessel should have a height to
width ratio
of at least 2 to 1.
Attainment of plug flow conditions can be determined by measuring certain
molecular weight parameters of the thinned product. Two measurements are made
by
using gel permeation chromatography (GPC) to determine molecular weight
parameters
io of the finished product. The first value, Mw, is the average molecular
weight of the
thinned starch product, while the second value, Mn, is the number average
molecular
weight of the thinned starch product. In an ideal dry thinning process such as
a batch
process, the width of molecular weight distribution decreases as the thinning
reaction
proceeds. The width of the molecular weight distribution is defined as the
difference
is between Mw and Mn. Perfect plug flow conditions should mimic batch process
results.
When results from the plug flow continuous process deviate substantially from
the batch
results, it would be an indication that channeling or non-plug flow conditions
exist in the
continuous process. The width of the molecular weight distribution can be
expressed as
(Mw-Mn)/1000. Plug flow conditions exist in the continuous process of this
invention
zo when the width of the molecular weight distribution of the thinned product
when
expressed as (Mw-Mn)/1000 is within + or - 25% of the same value for a batch
thinned
product of the same viscosity.
The process of the instant invention is carried out on a base starch having a
moisture content between about 5 percent and about 20 percent weight basis.
Preferably
is the base starch will have a moisture content of about 5 percent to about 17
percent. The
most preferred range is from about 10 percent to about 13 percent.
The base starch should be heated to a temperature of about 70° F (21
° C) to about
170° F (77 ° C) before entering the reactor, and before chemical
addition. Preferably the
so base starch will be at a temperature of about 100° F (38° C)
to about 140° F (60 °C).
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Prior to entering into the reactor, the hydrolyzing chemical is injected into
the
starch as a gas or in a finely dispersed liquid. Group I chemicals are
injected in an
amount from about 0.04 percent to about 0.5 weight percent basis dry starch.
Preferably
the Group I hydrolyzing chemical will be present in the starch at a
concentration of about
s 0.1 to about 0.4 weight percent. When Group II and III hydrolyzing chemicals
are used
as the thinning agents, higher amounts of the chemicals are required to obtain
the desired
thinning. Group II and III hydrolyzing chemicals will be present in the starch
at a
concentration of about 0.1 to about 2.0 weight percent.
io Injection of the hydrolyzing chemical into the starch base requires special
handling to avoid degradation and coloring of the starch and potential process
shut
downs. When anhydrous HCl comes in contact with the base starch, hydrochloric
acid is
formed from the reaction of the HCl and moisture in the starch. If sufficient
quantities of
hydrochloric acid form on the starch blackening can occur.
is
To prevent these problems, steps must be taken to insure equal dispersion of
the
hydrolysis chemical on the starch. This is accomplished by injecting the
hydrolyzing
chemical into the air stream of the pneumatic conveying line transporting the
base starch
to the reactor before the starch is added to the line
Zo The starch-hydrolyzing chemical mixture is introduced into a reactor vessel
adapted to provide a mass flow regime, i.e., the starch-hydrolyzing chemical
mixture
flows through the reactor in a first in and first out manner and without
substantial mixing
of the starch. Such a flow regime ensures that substantially all of the starch-
hydrolyzing
chemical mixture experiences substantially the same residence time in the
reactor.
While in the reactor, the starch-hydrolyzing chemical mixture is maintained at
a
temperature of about 70° F to about 170° F. Preferably the
temperature will be from
about 100° F to about 140° F.
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The starch-hydrolyzing chemical mixture will be resident in the reactor for a
time
of about .5 to about 6 hours. Preferably the resident time will be from about
1 to about 4
hours.
. s Upon exiting the reactor the starch-hydrolyzing chemical mixture is
neutralized:
For example, when HCl or sulfuric acid is used as the hydrolyzing chdrucal;
the thinned
starch can be neutralized with anhydrous ammonia or with aqueous bases. :
Spraying an
aqueous solution of soda ash (sodium carbonate) on the starch is the preferred
.
neutralization technique as it provides better uniformity of neutralization
and provides a
io convenient method to rehydrate the starch to any desired moisture content.
The starch
will preferably be neutralized to a pH of about 6 to. 8 and rehydtatod to a
moisdue
content about 10 to about 13 percent.
DESCRIPTION OF TYPICAL DRY THINNING PROCESS
a The dry,thinning process of the invention will be further described in
reference to
a typical processing scheme such as shown in Figure 1. The Faction variables
used in
such a typical process will, be as described above:
Dried starch, typically provided directly from a flash dryer as is known is
the art
Zo is provided~.through line 100 to a surge bin 101. The use of a vessel such
as the surge bin
is desired to facilitate the feed of the starch to the mass flow bins in a
constant and
continuous manner. The content of the starch in the surge bin can be monitored
conveniently through standard techniques such as the load cells 103. A
constant
temperature should be maintained throughout the mass flow bin. It may be
necessary to
Zs jacket or insulate the bin to maintain the temperature. The starch from the
feed bin
passes through a valve 105 and through an in-line weight indicator 107' and
through .
another valve 105 into pneumatic feed line 111. Heated air is provided from
unit 109
into pneumatic feed line upstream from the starch entry point. Also, upstream
from the starch
entry point into the pneumatic feed line the hydrolyzing chemical, is fed into
the heated air
so stream from hydrolyzing chemical storage tank 113 through line 115.
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The starch hydrolyzing chemical mixture is fed from the pneumatic conveying
line 111 to two mass flow bins 123. This feed is controlled by valves 117 and
would
typically be passed through dust collectors 121 as is known in the art. Heated
sir is
provided to the head space of the mass flow bins from the space air heater 133
through
s the airline 135. As with the surge bin, the contents of the .mass flow bins
can be
. monitored through load cells 103. ~ v
The starch hydrolyzing chemical mixture then passes through the mass flow bins
under a plug flow regime as described above. The dry thinned starch passes
from the
io mass flow bins and into the pneumatic conveying line 125 wherein aqueous
base is
~s
added. through valve and line 127 to' neutralize the .starch hydrolyzing
chemical mixhire
and water is added through valve 129 to rehydrate the mixhn~e. From the line
having
valve 1 T9 therein, the dry thinned starch is then moved to conventional
storage and package
facilities. Any hydrolyzing chemical such as HCl present in the air stream and
in the head
space of the reactors is vested through scrubber 131. .
APPLICATIONS IN THE PAPER INDUSTRY
The starch products of the invention can be utilized for the sizing and
coating of
paper prepared from all types of both cellulosic and combinations of
cellulosic with non-
cellulosic fibers. In addition, synthetic cellulose fibers of the viscous
rayon or
2o regenerated cellulose type can also be used as well as recycled waste
papers from various .
sources.
All types of paper dyes and tints, pigments and fillers may be added to the
starch
compositions (in the usual manner). Such materials include clay, talc,
titanium dioxide,
is calcium carbonate, calcium sulfate, and diatomaceous earths. Other surface
sizing
compounds as well as pigments, dyes and lubricants can also be used in
conjunction with
the size and coating compositions.
Application of non-pigmented starch compositions to paper at the sin press is
3o referred to as surface sizing. Application of size can also be made at the
calendar stack.
Dry thinned starches of this invention can .be used in surface sizing
applications for
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uncoated paper products by applying the thinned starch to a paper sheet
surface. A
continuous film of the starch composition is applied to both sides of the
paper.
Application of pigmented starch composition is referred to as color coating.
s Surface sizing improves surface finish, produces a better printing surface
and
improves the strength characteristics of the paper as well as other
properties. By proper
control of viscosity, starch sizes can be deposited primarily on the surface
of the sheet or
permitted to penetrated more deeply into the sheet to produce products with
the desired
physical properties.
io
Paper coating refers to the application of a layer of pigment, adhesive and
other
supplementary materials to the surface of dry paper or paper board. Prior to
coating the
paper surface maybe sized with dry thinned starch. The composition, commonly
referred
to as a coating color, is applied to the paper surface in the form of an
aqueous
is suspension. Typical coating compositions contain pigment as the primary
coating
material and a starch adhesive to bond the pigment particles to each other and
to the
paper. The most commonly used pigments are clay, calcium carbonate, titanium
dioxide
and combinations thereof. Other materials such as calcium sulfoaluminate, zinc
sulfide,
barium sulfate, calcium sulfate, calcium sulfite and diatomaceous silica
pigments are also
Zo used. . Other additives, such as polymeric latexes may be incorporated.
Such coatings
may be applied to the paper during the paper making process. Typically,
however,
coatings are applied as a separate step. In either case, the process differs
only in the
auxiliary equipment needed to perform the coating operation.
2s Paper or paper board is coated to produce a surface adequate for printing
processes. Coatings provide whiteness, brightness, gloss and opacity to paper
along with ,
a smoother more uniform surface. Different printing processes require
different sheet
properties and surface properties. Formulations of coating compositions for
such '
printing processes are well known.
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The starch adhesive component of coating compositions must not only act as a
binder for the pigment but also must act as a carrier, impact desirable flow
characteristics
and leveling, regulate the degree of water retention of the coating
composition and
produce the desired strength, ink receptivity and ink holdout for optimum
printing
s characteristics. The starch adhesive should be easy to prepare, have a high
adhesive
strength and have a stable viscosity during storage. Starch products of the
instant
invention satisfy these requirements.
Examples of two starch base coating formulation are given below:
io
Example 1 - Low-Grade Paper Coating
50 parts # 1 clay
50 parts delaminated clay
is 10 parts styrene butadiene latex (SBR)
8 parts dry thinned waxy maize starch
0.5 parts insolubilizer
0.3 parts CMC (carboxy methyl cellulose)
1 part calcium stearate
zo ~ The coating can be applied to paper to achieve a coat weight of
8g/m2/side
Example 2 - Medium Grade Paper Coating
85 parts #2 clay
2s 15 parts calcium carbonate
parts SBR latex
6 parts dry thinned waxy maize starch
1 part calcium stearate
.3 parts alginate
.0 0_5 parts insolubilizer
This coating can be applied to paper to achieve a coat weight of 8g/m2/side
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The preceding description of specific embodiments for the present invention is
not intended to be a complete list of every embodiment of the invention.
Persons who
are skilled in this field will recognize that modifications can be made to the
specific
embodiments described here that would be within the scope of the present
invention.
