CA1141070A - Process for producing starch glues - Google Patents

Abstract

ABSTRACT Starch glue is produced by gelatinizing an aqueous starch slurry at temperatures from about 95 to 160°C and applying high shearing forces. Before the viscosity exceeds 15000cP a water-soluble oxidant producing nascent oxygen is added, and gelatinization takes place in a reaction system which in respect of the residence time spectrum exhibits close to ideal pipe reactor characteristics. The contact time between the oxidant and the at least partially gelatinized starch at a temperature of at least 95°C is 1 to 5 minutes. The resulting starch paste is mixed with raw starch. The process permits the viscosity of the glue and/or the ratio of carrier starch to raw starch to be selectively varied while providing a glue with functional properties at least equivalent to those obtained with prior processes.

Description

.~ 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 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 .....

-3a-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() 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.

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 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 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 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.

- ~ r~

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.

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~

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.

Claims (37)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 160°C, applying high shearing forces and mix-ing the resulting starch paste (carrier) with granular or un-gelatinized 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 de-gradation of the carrier starch take place in a reaction sys-tem which in respect of the residence time spectrum shows close to ideal pipe reactor characteristics, that oxidant is added to the carrier starch dispersion and uniformly distribu-ted therein before the viscosity of the latter exceeds 15 000 cP and that the carrier throughput is selected so as to ensure that the contact time between the oxidant and the at least partially gelatinized carrier starch at a temperature of at least 95°C (reaction time) is in the range of 1 to 5 minutes.

2. The process of claim 1 in which the aqueous starch slurry is an alkalized starch slurry.

3. The process of claim 1 in which said starch paste is addi-tionally mixed with one or more compounds selected from the group consisting of water, an alkali, a boron compound, and a resin for wet-resistant gluing.

4. The process of claim 1 in which gelatinization of the car-rier starch also takes place in a reaction system which in respect of the residence time spectrum shows close to ideal pipe reactor characteristics.

5. The process of claim 1, characterized in that the carrier starch slurry is heated as quickly as possible by means of a heating medium in the form of vapor and/or gas to at least 95°C.

6. The process of claim 5 in which the heating medium is steam.

7. The process of claim 5 or 6 in which the carrier starch slurry is heated by direct heating by the heating medium.

8. The process of claim 1, characterized in that gelatiniza-tion and degradation are carried out at a temperature of 100 to 150°C.

9. The process of claim 8 in which the temperature is 103 to 125°C

10. The process of claim 9 in which the temperature is 105 to 115°C.

11. The process of claim 1 in which the oxidant used is a water-soluble per compound.

12. The process of claim 1, characterized in that the oxi-dant used is an inorganic per compound.

13. The process of claim 12 in which the per compound is one or more compounds selected from the group consisting of per-oxides, persulfates, perborates and percarbonates.

14. The process of claim 13 in which the per compound is one or more compounds selected from the group consisting of hy-drogen and alkali metal superoxides, and ammonium and alkali persulfates.

15. The process of claim 1, 5 or 8, characterized in that the oxidant is added to the carrier starch in the form of an aqueous solution.

16. The process of claim 12, 13 or 14, characterized in that the oxidant is added in an amount which in terms of the oxy-gen amount provided by it is equivalent to from 0.02 to 2.5 w/w % of ammonium persulfate, based on the weight of the car-rier starch.

17. The process of claim 12, 13 or 14, characterized in that the oxidant is added in an amount which in terms of the oxy-gen amount provided by it is equivalent to from 0.08 to 2 w/w % of ammonium persulfate, based on the weight of the carrier starch.

18. The process of claim 12, 13 or 14 characterized in that the oxidant is added in an amount which in terms of the oxy-gen amount provided by it is equivalent to from 0.2 to 1.2 w/w % of ammonium persulfate, based on the weight of the carrier starch.

19. The process of claim 1, characterized in that the starch glue is produced with a total starch content of 15 to 35 w/w %.

20. The process of claim 19 in which said starch content is 18 to 30 w/w %.

21. The process of claim 20 in which said starch content is 20 to 28 w/w %.

22. The process of claim 1, characterized in that the weight ratio of carrier starch to raw starch is adjusted to between 1:1 and 1:7.

23. The process of claim 22 in which said ratio is between 1:1.5 and 1:5.

24. The process of claim 23 in which said ratio is between 1:2 and 1:4.

25. The process of claim 1, characterized in that the car-rier starch is degraded to a degree that the Stein-Hall vis-cosity of the finished starch glue within the temperature range of 20 to 45°C is between 22 and 70 seconds.

26. The process of claim 25 wherein said viscosity is bet-ween 25 to 35 seconds.

27. The process of claim 26 wherein said viscosity is at most 32 seconds.

28. The process of claim 1, 5 or 8, characterized in that gelatinization and degradation of the carrier starch are carried out in the presence of alkalis.

29. The process of claim 1, charcterized in that a wet-resis-tant starch glue is produced.

30. The process of claim 29 in which the starch glue contains a formaldehyde condensation resin.

31. The process of claim 1, characterized in that one single starch slurry of a starch concentration at least equivalent to that of the finished starch glue is produced, continuously or batchwise, partially gelatinized and degraded and then contin-uously mixed with untreated starch slurry.

32. The process of claim 31 in which the starch slurry is diluted before being partially gelatinized.

33. The process of claim 31 in which the degraded starch slurry is diluted before being mixed with the untreated starch.

34. The process of claim 31, characterized in that the fin-ished starch glue is continuously fed to a storage tank at a speed sufficient to maintain there constantly a stock equi-valent to the amount used by the consumers supplied from it in a period of at least 0.5 hours.

35. The process of claim 34 in which said period is at least 1 hour.

36. The process of claim 35 in which said period is at least 2 hours.

37. The process of claim 34, 35 or 36 characterized in that when changing from wet-resistant to simple starch glue the storage tank is continuously charged without interruption as required by current consumption.