Note: Descriptions are shown in the official language in which they were submitted.
.~ 1
The invention relates to a process for producing starch glues,
in particular glues of the Stein-Hall type for corrugated boards,
by continuously gelatinizing optionally alkalized aqueous starch
slurries at temperatures ranging from about 95 to 16GC,
applying high shearing forces and mixing the resulting starch
paste ~carrier) with granular or ungelatinized starch (raw starch)
and, optionally, water and/or common additives, in particular
alkalis, boron compounds and/or resins for wet-resistant gluing.
While the possibility of using starch as an adhesive had been
known for centuries, it was only after the development of the
so-called "Stein-Hall glues" that glues on the basis of starch
gained any remarkable practical importance as industrial glues
which consisted, in principle, of a dispersion of granular or
ungelatinized starch (hereinafter briefly referred to as "raw
starch") in an aqueous starch paste (hereinafter ~riefly referred
to as "carrier") and as a rule solve the chief problem connected
with the use of starch glue which is essentially caused by the
fact tha,t, on the one hand, raw starch slurries (in water~ lack
segregation stability, raw starch has no adhesive power and starch
glues give sufficiently firm glue lines only if their solids content
is relatively high, whereas, on the other hand, starch pastes even
with a relatively low content of fully gelatinized starch show a
viscosity which is too high for use as industrial glues.
So while the development of the Stein-Hal1 glues did mean a
breakthrough, it did not, of course, solve all problems arising
in connection with the use of starch glues. Quite a few of these
problems in spite of a remarkable developments, have not or at
least not in a fully satisfactory manner been solved to this day.
It is true that in the course of time comparatively uncomplicated,
reliable continuous processes of the above-mentioned kind have
been de~eloped (USP 3,228,781), but all of them show a number of
deficiencies and are unsatisfactory especially in the following
respects:
'
,
...
lo The viscosity of the finished starch glue which is pre-
determined fairly accurately by the purpose for which
the product is to be used, in connect~on with a given
type of carrier starch depends very largely on the con-
tent of gelatinized starch (carrier starch) which for
that reason may be varied at most slightly.
2. Since for a specific use (with a given type of starch)
the total starch content, too, may be varied only
within a fairly narrow range, the freedom of choice
with regard to the ratio of carrier to raw starch fre-
quently is likewise impedingly restricted in practise,
the extremely limited possibilitites of lowering the
raw starch portion being particularly inconvenient.
3. Another unsatisfactory point is the low viscosity stabi-
lity of the classic Stein-Hall starch glues, especially
when adding agents, mostly resins, for wet-resistant
bonding.
There have of course been many attempts to remove these de-
ficiencies, a number of the prior art approaches being use-
ful (to some extent) in so far as they are in fact capableof eliminating or at least substantially mitigating one or
the other and in some cases even several of the deficiencies
inherent in the classic Stein-Hall starch glues, but the
advantages have to be paid for by disadvantages in other
respects.
Most of the deficiencies of the classic Stein-Hall starch
glues actually can be largely avoided by using so-called
"no-carrier" starch glues, which are advantageous especially
for the large freedom of choice they offer in terms of vis-
cosity with a given starch content (and vice versa) (DE-AS
25 12 810), but which in view of the large amount of
--2--
regulating work required in preparing them and the high risk
of process failures due to operating errors up to now have
not been accepted in practice to any major extent.
The attempts to remove the deficiencies of the known Stein-Hall
starch glues mentioned under 3) above main~aining the Stein-Hall
principle were concentrated chiefly on adjusting the conti-
nuous production of starch glue to consumption in such a
way that the stock of carrier and, notably in the case of
glues for wet-resistant bonding (hereinafter briefly called
"wet-resistant glues") the stock of finished starch glue,
and thus the time until it is consumed were kept as low as
possible (US-PS 3,228,781 and DE-AS 25 12 810). By this
method the problem of viscosity stability may in fact be
solved, but one has to accept in return that even briefly
reparable failures of the carrier preparation system (here-
inafter briefly referred to as l'converter") and/or of the
dosing, mixing and/or conveying equipment arranged down
- stream of the converter lead to an interruption of the glue
supply to the consumers, the result in practise being in
most cases the unsatisfactory compromise of arranging a
buffer or storage tank between the glue preparation syst~m
and the consumer(s).
The other known attempts at removing the above-mentioned
deficiencies basically amount to using for the carrier modi-
fied, mostly degraded starches which are fitted to the needs
of the individual case. It is possible in such instances
to use special ready-mixes of a modified, "cold-swelling"
carrier starch, raw starch and, optionally, "chemicals"
which may be made into a size by simple stirring into luke-
warm water, to charge the converter with adequately pre-
modified special starches, or to modify the carrier starch
in and/or after the converter mechanically, thermally,
chemically and/or enzymatically~
The inevitable disadvantages of the two alternatives men-
tioned first, i.e. comparatively high cost of material,
the need to keep large stocks and the danger of missing
the right starch type, are obvious.
Prior art approaches following the last-mentioned alterna-
tive have not proved satisfactory in practise either. While
the known processes for continuously producing Stein-Hall
starch glues with carrier starches modified du~ing and/or
after carrier preparation frequently do allow to (somehow)
vary the viscosity of the carrier and sometimes even to im-
prove other functional .....
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-- 4 --
properties, like viscosity stability, to a certain extent ("Die
Starke" (Starch), 14, 1962, pages 197 to 208; DE-AS 25 12 810~,
a selective variation of the viscosity allowing a reproduci-
bility which meets practical requirements and an improvement of
other Eunctionally important properties, in particular visco-
sity, can be achieved by the prior art processes operating
according to this principle, e.g. processes in which the car-
rier starch is subjected to enzymatic liquefaction, only with a
fairly large amount of process technology. Hence, these prior
art processes compare with the prior art processes for produ-
cing no-carrier starch glues.
The general object of the invention therefore was to pro-
vide a process of the above-mentioned type which overcomes the
above deficiencies of the state of the art in that it permits
in the continuous production of Stein-Hall starch glues and
with a given total starch concentration to selectively vary
the viscosity and/or the ratio of carrier starch to raw starch
in the finished glue over wide limits without requiring any
major additional effort compared to any of the prior art pro-
cesses in which the starch for purposes of preparing thecarrier is merely gelatinized, and thus to obtain starch glues
with at least equivalent or even better functional properties,
especially in terms of viscosity stability.
The present invention provides a process for producing
starch glues, for example glues of the Stein-Hall type for
corrugated boards, by continuously gelatinizing aqueous starch
slurries at temperatures ranging from about 95 to 160C,
applying high shearing Eorces and mixing the resulting starch
paste (carrier) with granular or ungelatinized starch (raw
starch) characterized in that the starch of the carrier
(carrier starch) is degraded in situ by adding a water-soluble
oxidant producing nascent oxygen under the prevailing reaction
conditions, with the proviso that degradation of the carrier
starch take place in a reaction system which in respect of the
residence time spectrum shows close to ideal pipe reactor char-
acteristics, that oxidant is added to the carrier starch dis-
persion and uniformly distributed therein before the viscosity
of the latter exceeds 15 000 cP and that the carrier through-
put is selected so as to ensure that the contact time between
f j~ ~
7()
-- 5 --
the oxidant and the at least partially gelatinized carrier
starch at a temperature of at least 95C (reaction time) is
in the range of 1 to 5 minutes.
It has proved to be advisable in the process of the in-
vention to carry out carrier preparation in such a way that
the carrier starch is degraded as uniformly as possible.
For this purpose it is preferable not only to use reaction
systems or converters featuring the characteristics of a
tube reactor, but to select the measures for heating the
carrier starch slurry to a temperature of at least about
95C at which, as a matter of experience, quick and largely
complete gelatinization is guaranteed in the manner known,
so that at least the temperature range in which the starch
is only partially gelatiniæed and/or reacted with -the oxi-
dant is passed as fast as possible, for which purpose inparticular direct heating with heating media in the form of
vapor and/or gas is applied which offer the additional ad-
vantage of the reaction medium being intimately mixed.
The working temperatures during carrier preparation
according to the process of the invention preferably are
kept within the range commonly applied in carrier prepara-
tion by simple gelatinization, it having been found advis-
able to work within a temperature range of 100 to 150, more
preferably 103 to 125 and most preferably 105 to 115C in
order to achieve a maximum degree of gelatinization on the
one hand and to avoid damage being caused to the starch by ...
j
subsequent and/or secondary thermal reactions on the other.
The oxidants preferred to be used according to the inven-
tion are inorganic per compounds, more preferably peroxides,
in particular hydrogen and/or an alkalisuperoxide, persul-
fates, most preferably alkali and/or ammonium persulfate
which up to now have proved to be best, and/or perborates
and/or percarbonates.
As the o~idants according to this invention are used in
relatively small amounts and, at least when added shortly
before and/or in the converter, should be uniformly dis-
tributed in the reaction mixture quickly, they are prefer-
ably added in the form of aqueous solutions.
The amount of oxidant employed in the process of the inven-
tion may be varied over wide limits. The optimum amount in
the individual case depends on a number of factors, in
particular ~he type of starch used as a starting material,
the desired extent of viscosity reduction, and the oxidant
used, and should and may t~erefore be determined by those
skilled in the art empirically. By experience it has been
found that amounts equivalent to between 0.02 and 2.5, more
preferably 0.08 and 2 and mos~ preferably 0.2 and 1.2 w/w %
of ammGnium persulfate, based on the weight of the carrier
starch, are best suited for the purpose as a rule.
The starch content of Stein-Hall starch glues generally
depends on the intended use. Prior art processes frequent-
ly did not permit to select the content as high as would
have been desirable, in consideration of the viscosity and
the content of raw starch. The process of the invention
offers greater freedom of choice in this respect and is
therefore particularly advantageous for the manufacture of
starch glues with a relatively high starch content.
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Accordingly the preferred practise of the invention is the
manufacture of starch glues with a total starch content of
between 15 and 35, more preferably 18 and 30 and most pre-
ferably 20 and 28 w/w %.
According to the state of the art (where no special starches
are used for carrier preparation and/or no complex and time-
consuming processes are applied for carrier preparation,
e.g. enzymatic liquefaction) the weight ratio of carrier
starch to raw starch, in particular in the case of starch
glues with a higher total starch content, may be varied but
slightly and the freedom of choice is unsatisfactorily
limited especially regarding increased ratios. The inven-
; tion, by contrast, offers those skilled in the art a very
large freedom of choice in this respect, and according to
a preferred embodiment of the invention enables them to
manufacture Stein-Hall starch glues with a weight ratio of
carrier starch to raw starch of between 1:1 and 1:7, more
preferably 1:1.5 to 1:5, and most preferably 1:2 to 1:4
A repeatedly mentioned advantage of the invention is the
unusually high viscosity stability of the carriers and
starch glues obtained by it, which makes it possible to
select the viscosity of the finished, freshly made glue
within a wide range, since subsequent changes in viscosity
have to be taken into account to a minor degree only.
Another point is that in Stein-Hall starch glues produced
according to this invention the viscosity surprisingly may
be adjusted at a lower level than in Stein-Hall starch glues
produced according to prior art and that, as a result,
various functional properties may be remarkably improved.
30 Thus in a prefPrred embodiment of the invention the carrier
starch is degraded to a point where the Stein-Hall viscosity
of the finished starch glue within the temperature range of
--7--
20 to 45C is 22 to 70, more preferably 25 to 35 and most
preferably at most 32 seconds.
It is known that Stein-Hall starch glues as a rule are
added with alkalies in order to promote gelatinization of
the starch, the addition of alkalies being generally made
at one or several optional point(s) in the course of manu-
facture. However, to keep dosing and mixing times as
short as possible it is preferable to dose in the alkalies
only at one point of the process, and to promote gelatini-
zation in the converter it is preferable to pass at leastpart of the mixture through the converter if possible.
Since it has surprisingly been found that in the process
of the invention the presence of alkalies does not inter-
fere with the degradation of the carrier starch, or at
least not significantly, the commonly known and functional-
ly advantageous addition of at least part of the alkali
component as well as, by the way, other commonly known
"chemicals", if any, like boric acid or borax, to the car-
rier may be made before and/or during passage through the
converter.
The advantages of the invention are apparent particularly
in wet-resistant glues whose manufacture is therefore
preferred.
All common or per se known functionally advantageous mea-
sures of the continuous processes for the production of
Stein-Hall starch glues according to the state of the art
may likewise be used readily in the process of the invention.
The high viscosity stability mentioned several times before
of wet-resistant Stein~Hall starch glues made according to
this invention, enables those skilled in the art to select
the ratio of buffer (amount~ to current starch glue
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0
consumption or averaye residence time upto consumption,
which in terms of viscosity stability and for purposes of
changing the glue type quickly should be (as) low (as pos-
sible), but in the interest of an uninterrupted operation
should be (as) high (as possible), to safely take into
account the aspect of uninterrupted operation more than
they could in the prior art processes, for which reason a
preferred embodiment of the invention provides that the
speed at which finished starch glue is continuously pro-
duced and fed into the storage tank(s) arranged between theglue preparation system and the consumer(s) may be and is
preferably selected in a way ensuring that the stock of
starch glue in the storage tank is permanently kept at a
level high enough for the connected consumer to be supplied
from the glue stock in the case of an interruption of the
continuous starch production due to machine failure for
at least another 0.5, more preferably at least 1 and most
preferably at least 2 hour(s).
In this text it is particularly advantageous that simple
starch glues (i.e. Stein-Hall type starch glues without
wet-resistance improving additives) produced according to
the process of this invention retain their unusual vis-
cosity stability even when mixed with wet-resistant glues
made according to the invention, which means that a change
from wet-resistant glue to simple Stein-Hall starch glue
according to the process of the invention may be accomplished
simply by processing the latter directly on the wet-resistant
glue not yet used; in other words, it is not necessary to
interrupt the production of starch glue until the glue type
produced first has been used up completely.
Another advantage of the invention may be seen in the fact
that while it is generally possible to use any kind of
_9_
ungelatinized starch for the carrier, the comparably low-
price native starches under technological aspects are at
least equal, frequently even superior to modified and in
particular chemically modified granular starches. The use
of native starches therefore is preferred in the process
of the invention, whereas chemically modified granular
starches, in particular those modified with aggressive
oxidants, should not be used.
The below examples and comparative tests are cited to
illustrate the invention and its advantages over the state
of the art.
The equipment employed for the purpose was a common con-
tinuously operating glue preparation system comprising an
agitated mixing vessel for preparing a starch slurry, a
cooker consisting of a steam jet heater heatable with
direct steam, a subsequently arranged holding vessel, a
dosing device for (re-)diluting water and a mixing section,
dosing devices for the supply of raw starch slurry and,
optionally, chemicals, resins for wet-resistant gluing
and/or water to the diluted carrier, a high-power mixing
chamber and a subsequentlv arranged storage tank, which
system had been additionally equipped with a dosing device
for continuously adding the starch slurry fed to the steam
jet heater with oxidant.
; The functional properties were tested by using the respec-
tive product as glue in the indus~rial-scale production of
single- and multi-wall corrugated boards on a corrugator
of standard design.
Example 1
Production of simple Stein-Hall starch glue of the follow-
ing characteristics made from native corn starch according
--10--
o
to the process of the invention:
Concentration: 24.2% starch (D.S.)
Ratio carrier : ungelatinized
starch: 1 : 4.0
Temperature: 35C
Viscosity: 26 sec. Stein-Hall
Concentration NaOH,
based on starch (c.b.). 2.0%
Concentration boric acid,
based on starch (c.b.): 0.85%
A slurry of a concentration of 38% starch (c.b.) (master
slurry) is produced in the manner known by mixing native
corn starch with water under stirring. To prepare the
carrier, master slurry is continuously supplied to the
steam jet heater at a rate of 5.5 l/min (equivalent to 2.4
kg starch c.b.) and, shortly before entry into the steam
jet heater, oxidant is dosed thereto continuously in an
amount equivalent to 6.6 g/min or 0.275%, based on carrier
starch, of ammonium persulfate.
This mixture is heated to 105C by introducing steam and
then continuously mixed with aqueous sodium hydroxide
solution in an amount equivalent to 2% NaOH, based on the
total starch (c.b.) .....
-lOa-
--ll--
present in the finished starch glue, and then fed to the
holding vessel.
The stream of (carrier) starch paste leaving the holding vessel
is dosed with diluting water in an amount equivalent to 17.7 l/min
(including the heater steam condensate) and is intimately mixed
with the latter in the mixing section.
The diluted carrier thus obtained is then continuously added
in the high power mixing chamber with master slurry (22.2 1
equivalent to 9.6 kg starch (c.b.), per minu.e) and with boric
acid in an amount equivalent to 100 g/min, and intimately mixed,
whereupon the finished starch glue is supplied to the storage
tank.
,
Example 2
Example 1 is repeated, but by way of deviation from it for
purposes of producing wet-resistant starch glue (without any
other changes in recipe) a commercial wet~resistant agent on 3
j the basis of ketone/formaldehyde resin is additionally supplied
to the high power mixing chamber in an amount equivalent to
11 % based on total starch (c.b.).
The characteristics of the starch glue thus obtained, apart of
course from the content of wet-resistant agent, are the same
as those of the (simple) starch ~lue produced according to
Example 1.
Example 3
Example 1 is repeated, but by way of deviation from it only ¦
16.7 instead of 22.2 1 of master slurry, equivalent to 7.2 kg
starch (c.b.), are supplied to the high power mixing chamber
per minute.
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-12-
The (simple) Stein-Hall starch glue thus obtained shows the
following characteristics:
'
Concentration: 21 % starch (c.b.)
Ratio carrier : ungelatinized starch: 1 : 3
Temperature: 37.5C
Viscosity: 33 sec. Stein-Hall
Concentration NaO~/
based on starch (c.b.): 2.6 %
Concentration boric acid,
b,ased on starch (c.b.): 1.09
Example 4
Example 3 is repeated, but by way of deviation from it and
analogous to Example 2, a commercial wet-resistant agent is
additionally supplied to the high power mixing chamber.
The characteristics of the finished starch glue thus obtained,
apart of course from the content of wet-resistant agent, are
the same as those of the (simple) starch glue obtained according
- to Example 3.
Testing of Functional Properties
A test of the starch glues produced according to the above
examples showed the following properties advantageous for
processing:
a) Simple starch glues ~Examples 1 and 3):
Extremely stable viscosity. In spite of the notoriously
~ high shearing load in the glue circulating system, the
;- drop in viscosity within 5 hours was no more than max.
1 sec. Stein-Hall.
Even a~ high peripheral speeds o~ the glue applicator rolls
the glue did not "spatter" thanks to its favorable
rheological properties.
Due to the extremely low viscosity of the glue, appli-
cation was very economical with bonding (even of papers
otherwise difficult to glue together) being neverthe-
less very good in all speed ranges.
The machine performance was substantially higher than
in normal operation, the flatness of the carton blanks
was excellent. Immediate further processing was pos-
sible without interim storage.
b) Wet-resistant starch glues (Examples 2 and 4):
In spite of the addition of a wet-resistant agent
there was practically no change in viscosity (same as
in the case of the simple starch glues made according
to this invention) even during test runs conducted
over several hours.
The wet-resistant starch glues could be mixed at any
ratio with the corresponding simple Stein-Hall starch
glue produced according to this invention and processed
without any adverse consequences (gradual smooth change
from simple to wet-resistant starch glue and vice versa).
The wet-resistance of the bond fully met the specifi-
cations of the official FEFCO Test.
Comparative test
For comparison, Stein-Hall starch glues are produced from
native corn starch according to the conventional method,
i.e. without carrier starch being degraded in situ, their
composition resembling that of the glue obtained according
to Example 1 or 2 as far as this was possible considering
the applicability of the finished comparative glue as a
corrugating glue.
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7(~
The simple comparative starch glue shows the followiny
characteristics:
Concentration: 22.4% starch (D.S.)
Ratio carrier : ungelatinized
starch: l : 5.7
Temperature: 33C
Viscosity immediately after
production: 50 sec. Stein-Hall
Concentration NaOH, based
on starch*: 1.8%
Concentration borax (dekahydrate),
based on starch*: 1.15%
* Raising the addition of NaOH or borax i5 not possible
because it immediately causes disturbing gellings to
form at the guide plates of the single facers.
During processing of this starch glue made according to
the state of the art the following disadvantages are noted:
~; In the circulating system the viscosity drops constantly
to 32 sec. Stein-Hall.
During processing of papers more difficult to glue and/or
at low machine speeds the glue lines are very brittle thus
giving unsatisfactory bonding results.
When processing multi-wall boards the machine speed has to
20 be throttled from 160 m/min to 75 m/min.
At higher peripheral speeds of the applicator rolls the
starch glue starts to "spatter", whereby too much glue is
transferred to the board inevitably and uncontrollably.
The addition of wet-resistant agent leads to a steadily
rising viscosity, as is usual with this type of starch glues.
After two hours the glue is no longer fit for processing.
Utilizing the glue by mixing (even of relatively small por-
tions) with the .....
5~
-15-
corresponding simple starch glue is not possible either.
The wet-resistance of the bond not only is substantially
lower than with the wet-resistant starch glue made according
to the invention, but varies greatly depending on the
operating conditions.
