GB2061307A - Process for producing starch glues - Google Patents

Abstract

A process for producing starch glues of the Stein-Hall type by continuously gelatinising aqueous starch slurries at elevated temperatures and high shear and mixing the resulting starch paste with raw starch, is characterised by adding a water-soluble oxidant producing nascent oxygen under the prevailing reaction conditions, such as an inorganic per compound, to the carrier starch prior to or during gelatinisation so that the starch and oxidant are contacted at a temperature of at least 95 DEG C for from 1 to 5 minutes. The oxidant is added and uniformly dispersed in the carrier starch dispersion before the viscosity of the latter exceeds 15000 cp and 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 DEG C is in the range of 1-5 minutes.

Description

SPECIFICATION Process for producing starch glues 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 1 600 C, 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 briefly 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 that; 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-Hall 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 developed (USP 3,228,781), but all of them show a number of deficiencies and are unsatisfactory especially in the following respects: 1. The viscosity of the finished starch glue which is predetsrmined fairly accurately by the purpose for which the product is to be used, in connection with a given type of carrier starch depends very largely on the content of gelatinized starch (carrier starch) which for that reason may be varied at most slightiy.

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 frequently is likewise impedingly restricted in pactice, the extremely limited possibilities of lowering the raw starch portion being particularly inconvenient.

3. Another unsatisfactory point is the low viscosity stability 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 deficiencies, a number of the prior art approaches being useful (to some extent) in so far as they are in fact capable of 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 viscosity with a yiven 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 maintaining the Stein-Hall principle were concentrated chiefly on adjusting the continuous 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 (hereinafter briefly referred to as "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 practice being in most cases the unsatisfactory compromise of arranging a buffer or storage tank between the glue preparation system and the consumer(s).

The other known attempts at removing the above-mentioned deficiencies basically amount to using for the carrier modified, 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, "chemicais" which may be made into a size by simple stirring into lukewarm water, to charge the converter with adequately premodified 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 mentioned 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 alternative have not proved really satisfactory in practice either. While the known processes for continuously producing Stein-Hall starch glues with carrier starches modified during and/or after carrier preparation frequently do allow to (somehow) vary the viscosity of the carrier and sometimes even to improve other functional properties, like viscosity stability, to a certain extent ("Die Stärke" (Starch), 14, 1962, pages 197 to 208;; DE-AS 25 12 810), a selective variation of the viscosity allowing a reproducibility which meets practical requirements and an improvement of other functionally important properties, in particular viscosity, can be achieved by the prior art processes operating according to this principle, e.g. processes in which the carrier 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 producing no-carrier starch glues.

The general object of the invention therefore was to provide 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 processes in which the starch for purposes of preparing the carrier is merely gelatinized, and thus to obtain starch glues with at least equivalent or even better functional properties, especially in terms of viscosity stability.

This problem is solved by the invention in the manner described in the main claim based on the surprising finding that under the conditions prevailing in conventional converters for the continuous production of carrier from raw starch slurries, when adding oxidants commonly known to be used for modifying starches or in the presence of same there not only takes place a reaction which results in a viscosity reduction but a) this reaction, even when using relatively "mild" oxidants which supply nascent oxygen under normal reaction conditions and are desired for their comparatively low corrosiveness proceeds so quickly to the point where practically all the oxidants have been used up that it is completed even if relatively large amounts of oxidant are added during periods shorter than the short periods commonly required in these converters for heating the slurry, gelatinizing the starch and homogenizing the paste, b) the extent of the viscosity reduction caused by the oxidant under otherwise constant conditions is a function of the amount of oxidant used and so the viscosity reduction may be selectively adjusted and varied as desired over wide limits, and c) the reaction with the oxidant, apart from resulting in a viscosity reduction, causes a remarkable improvement of the viscosity stability of the carrier and the Stein-Hall starch glues (even so-called wetresistant glues) made from it.

It has proved to be advisable in the process of the invention 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 950C 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 gelatinized and/or reacted with the oxidant is passed as fast as possible, for which purpose in particular direct heating with heating media in the form of vapor and/or gas is applied which offer the additional advantage 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 preparation by simple gelatinization, it having been found advisable to work within a temperature range of 100 to-1 50, more preferably 103 to 125 and most preferably 105 to 1 50C in order to achieve a maximum degree of gelatinization on the one hand and to avoid damage being caused to the starch by subsequent and/or secondary thermal reactions on the other.

Preferably degradation and gelatinization of the carrier starch takes place in a reaction system which in respect of the residence time spectrum shows close to ideal pipe reactor characteristics. The oxidants preferred to be used according to the invention are inorganic per compounds, more preferably peroxides, in particular hydrogen and/or an alkalisuperoxide, persulfates, most preferably alkali and/or ammonium persulfate which up to now have proved to be best, and/or perborates and/or percarbonates.

As the oxidants 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 distributed in the reaction mixture quickly, they are preferably added in the form of aqueous solutions.

The amount of oxidant employed in the process of the invention may be varied over wide limits.

The optimum amount in the individual case depends on a number of factors, in particular the type of starch used as a starting material, the desired extent of viscosity reduction, and the oxidant used, and should and may therefore be determined by those skilled in the art empirically. By experience it has been found that amounts equivalent to between 0.02and 2.5, more preferably 0.08 and 2 and most preferably 0.2 and 1.2 w/w % of ammonium 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 frequently 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 practice of the invention is the manufacture of starch glues with a total starch content of between 1 5 and 35, more preferably 18 and 30 and most preferably 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 invention, by contrast, offers those skilled in that 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 preferablyl :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.Thus in a preferred 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 450C 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 pf manufacture. 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 gelatinization in the converter it is preferable to pass at least part 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 interfere with the degradation of the carrier starch, or at least not significantly, the commonly known and functionally 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 carrier 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 measures 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 average residence time upto consumption, which in terms of viscosity stability and for purposes of changing the glue type quickly should be (as) low (as possible), but in the interest of an uninterrupted operation should be (as) high (as possible), to safely take into account the aspect of uninterrrupted 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 produced and fed into the storage tank(s) arranged between the glue 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 viscosity 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 ungelatinized starch for the carrier, the comparably low-price native starches under technological aspect are at least equal, frequently even superior to modified and in particular chemically modified granular starches. The use of native starches therefor 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 advantage over the state of the art.

The equipment employed for the purpose was a common, continuously 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 subsequently 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 respective product as glue in the industrialscale 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 following characteristics made from native corn starch according to the process of the invention: Concentration: 24.2% starch (D.S.) Ratio carrier:ungelatinized starch: 1:4.0 Temperature: 350C Viscosity: 26 sec. Stein-Hall Concentration NaOH, 2.0% based on starch (c.b.): Concentration boric acid, 0.85% based on starch (c.b.): 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 I/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 1050C-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.) 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 I/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 minute) 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 the basis of ketone/formaldehyde resin is additionally supplied to the high power mixing chamber in an amount equivalent to 1 1% based on total starch (c.b.).

The characteristic of the starch glue thus obtained, apart of course from the content of wetresistant agent, are the same as those of the (simple) starch glue 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.

The (simple) Stein-Hall starch glue thus obtained shows the following characteristics: Concentration: 21% starch (c.b.) Ratio carrier:ungelatinized starch: 1:3 Temperature: 37.50C Viscosity: 33 sec. Stein-Hall Concentration NaOH, 2.6% based on starch (c.b.): Concentration boric acid, 1.09% based on starch (c.b.): 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 accordingly to the above examples showed the following properties advantageous for processing: a) Simple starch glues (Examples 1 and 3): Extemely 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 at high peripheral speeds of the glue applicator rolls the glue did not "spatter" thanks to its favorable rheological properties.

Due to the extremely low viscosity of the glue, application was very economical with bonding (even of papers otherwise difficult to glue together) being nevertheless 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 possible 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 specifications 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.

The simple comparative starch glue shows the following characteristics: Concentration: 22.4% starch (D.S.) Ratio carrier: ungelatinized starch: 1:5.7 Temperature: 330C Viscosity immediately after production 50 sec. Stein-Hall Concentration NaOH, 1.8% based on starch*: Concentration borax (dekahydrate), 1.15% based on starch*: * Raising the addition of NaOH or borax is 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 be throttled from 1 60 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 additional 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 portions) with the 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 (10)

1. 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 1 600C, applying high shearing forces and mixing the resulting starch paste (carrier) with granular or ungelatinized (raw starch) and, optionally, water and/or common additives, in particular alkalis boron compounds and/or resins for wet-resistant gluing, characterised in that the starch of the carrier (carrier starch) is degraded during the gelatinization step by adding a water-soluble oxidant producing nascent oxygen under the prevailing reaction conditions, the oxidant being added to the carrier starch dispersion and uniformly distributed therein before the viscosity of the latter exceeds 15000 cP and the carrier throughput being 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 950C (reaction time) is in the range of 1 to 5 minutes.

2. The process of claim 1, characterised in that the carrier starch slurry is heated quickly to at least 950C.

3. The process of claim 1 or 2, characterised in that gelatinization and degradation are carried out at a temperature of 103 to 1 25- C.

4. The process of one of claims 1 to 3, characterised in that the oxidant is an inorganic per compound.

5. The process of one of claims 1 to 4, characterised in that the oxidant is added in an amount which in terms of the oxygen amount provided by it is equivalent to from 0.02 to 2.5, w/w% of ammonium persulfate, based on the weight of the carrier starch.

6. The process of one of claims 1 to 5, characterised in that the carrier starch is degraded to a degree that the Stein-Hall viscosity of the finished starch glue within the temperature range of 20 to 450C is between 22 to 70 seconds.

7. The process of one of claims 1 to 6, characterised in that gelatinization and degradation of the carrier starch are carried out in the presence of alkalis.

8. The process of one of claims 1 to 10, characterised in that a wet-resistant starch glue is produced by the addition of a formaldehyde condensation resin.

9. The process of one of claims 1 to 8, characterised in that the finished starch glue is continuously fed to a storage tank at a speed sufficient to maintain there constantly a stock equivalent to the amount used by the consumers supplied from it in at least 0.5 hours.

10. The process of claim 9 characterised in that when changing from wet-resistant to simple starch glue the storage tank is continuously charged without interruption as required by current consumption.