GB2045792A - Process and installation for the continuous manufacture of starch adhesives - Google Patents

Abstract

A process for producing starch pastes, in particular for manufacturing corrugated paper or cardboard, of the type comprising the stage of preparing a primary paste cold from a gelatinized starch in the presence of an alkaline agent, is characterised by the fact that the primary paste is made from a first portion of an aqueous starch suspension which is subjected to the action of an alkaline agent in excess, that the primary paste is mixed with a second portion of the said suspension, and that at least part of the alkaline agent present in excess is neutralised with boric acid. A plant for operating the process is also described.

Description

SPECIFICATION Process and installation for the continuous manufacture of starch adhesives The present invention relates to a process for producing starch pastes intended in particular for the manufacture of corrugated papar and cardboard.

The invention also includes a plant for the automatic and continuous implementation of the above process.

At the present time starch-based pastes, in particularthose intended for the production of corrugated paper, are generally prepared by the following processes: 1) Stein-Hall process, the oldest and most common process, according to which some of the starch in aqueous suspension is gelatinised at a high temperature in the presence of an alkaline agent, generally caustic soda, and constitutes a primary paste acting as a support or "carrier" for further starch added to the mixture as a powder or in the form of an aqueous suspension. The caustic soda is added only to the carrier portion or together with the carrier portion and the crude starch portion, which should then be in the form of a "milk", but in both cases the total amount of caustic soda should be such that it does not produce any swelling of the crude starch in the final paste, which would indicate instability.After the carrier portion has been boiled it is at a fairly high temperature, generally above 60"C, and it is necessary to reduce this temperature to a maximum value of 45"C before adding the crude starch if one is to avoid the risk of swelling the latter.

This objective is achieved in practice by adding cold water to the primary paste.

It is thus necessary in the whole preparation procedure to keep a sufficient amount of cold water for this operation, which means that the primary paste of the carrier is relatively highly concentrated before dilution, that is to say contains of the order of 10% of starch, and is consequently very viscous. This high viscosity is a serious drawback as regards the design of an automatic and continuous boiling apparatus.

2) The so-called "no carrier" process, according to which all the starch is partially swelled under the action of the alkaline conditions and temperature.

The degree of gelatinisation of the starch is monitored by measuring the viscosity, and the reaction is stopped at the desired point by adding an acidic compound, for example aluminium sulphate. A process of this type is difficult to adapt to continuous production.

3) Cold processes using a complete mixture containing crude starch, pre-gelatinised starch and chemical additives. The paste is obtained by simply dispersing powder in water the pre-gelatinised starch forming the carrier. The process can be adapted to automatic and continuous production, but there is the disadvantage of the high cost of the pre-gelatinised starch and mixtures, and the lack of flexibility in the choice of paste formulations.

4) The so-called "Cold Burst" process, according to which a modified or unmodified starch in granular form is gelatinised in an aqueous medium by an alkaline agent, generally caustic soda, and not by raising the temperature. This process has a certain number of advantages, including the fact that boiling at high temperatures or the need for costly pre -gelatinised starch are avoided, and the length of preparation of the paste is reduced and the temperature monitoring procedures are simplified, and consequently the labour costs are reduced.

This last method of preparation appears to be most capable of implementation by means of an automatic and continuous arrangement, since the primary paste or carrier does not have to be cooled and may be prepared in a relatively low concentration, namely of the order of 5% of starch, and thus forms a not very viscous solution that can easily be poured.

However, the cold process when carried out using natural starches has serious disadvantages, which include a lack of stability of the pastes under the action of shear forces that inevitably arise in a production unit for corrugated paper, when the paste circulates between the pasting machine and the paste reservoir under the action of circulation pumps; the modification of the paste is further accentuated by the rise in temperature, to 35"-38"C, when the paste passes over the pasting rollers. The result is a fall in viscosity of the paste and consequently a change in the thickness of the paste film deposited on the cardboard, or an imperfect pasting of the latter.

In order to obviate this defect it is not sufficient to subject the paste or even the carrier portion, the most sensitive part of the paste, after gelatinisation in a conventional alkaline medium, to a mechanical action, even to a very intensive mechanical action, such as can be obtained with a powerful stirrer in an enclosure having the smallest volume compatible with the desired flow rate, and instead it is necessary to employ a starch whose resistance has been strengthened by a structural modification, namely either a fluidification or a cross-linking produced a polyfunctional compound such as epichlorhydrin, phosphorus oxychloride, sodium trimetaphosphate, etc., this cross-linking being able to be produced in situ by employing the alkaline medium which catalyses the reaction. The use of a modified starch however results in a large increase in the cost of the paste.

It is known, moreover, that starch pastes should contain a certain proportion of borax, in particular when used to manufacture corrugated paper.

The present invention is thus directed to a process for producing starch paste which obviates the aforementioned disadvantage while providing a product having the desired properties and which can be produced economically and continuously.

The process according to the invention, which is of the type comprising the stage of producing cold a primary paste from a gelatinised starch in the presence of an alkaline agent, is essentially characterised by the fact that the primary paste is produced from a first portion of an aqueous starch suspension which is subjected to the action of an alkaline agent in excess, that the primary paste is mixed with a second portion of the said suspension, and that at least part of the alkaline agent present in excess is neutralised by means of boric acid.

According to other characteristic features, the contact time between the gelatinised starch and the second portion of starch suspension is prolonged, and the final paste obtained may be diluted and completely neutralised as desired.

Advantageously the process starts from water heated to a temperature between about 38 and 42"C and a concentrated solution of alkaline agent is prepared, the hot water being used both for diluting the solution of alkaline agent to the desired concentration, for preparing the starch suspension, and, if necessary, for diluting the final paste.

In order to implement the above process, the plant according to the invention comprises a reservoir of water heated to a constant regulable temperature, an assembly of three metering pumps for producing the primary paste from a first portion of aqueous starch suspension, a first pump connected to the outlet of a reservoir of concentrated alkaline agent, a second pump connected to the water reservoir, a third pump connected to the outlet of a tank for preparing the aqueous starch suspension situated opposite a feed hopper, the outlet of the three metering pumps being connected to a means for bringing the constituents of the primary paste in contact with one another, a fourth pump connected to the said tank and whose outlet is connected to a mixer for the primary paste and the second portion of aqueous suspension, the outlet of the said mixer being itself connected to a storage tank provided with a boric acid metering device and whose outlet is connected to at least one final paste vat provided with level probes connected to the control means for the outlet of the four metering pumps.

According to other characteristic features, the assemblies of metering pumps are provided with three-way valves for returning the flow to the respective reservoirs; the mixers are preferably of the static type; the reservoirs and tanks are provided with constant level control means; the assembly of the liquid circulation means are controlled automatically by means of level probes placed in the final paste vats.

Other characteristic features and advantages of the present invention will be more readily understood on reading the following description, in conjunction with the attached drawing which diagrammatically represents the plant and illustrates the process according to the invention.

In accordance with the invention, the primary paste or carrier is made from a natural starch, gelatinised in the presence of an amount of caustic soda or other alkaline agent in excess with respect to the amount contained in a normal paste for corrugated paper or cardboard. The excess caustic soda is then neutralised and this operation is profitably employed to reform borax, which is always present in this type of paste, from the boric acid added during the mixing or immediately after the mixing of the gelatinised starch and crude starch. The amount of borax may be modified as desired as a function of the excess caustic soda.Under these conditions, during the course of the gelatinisation of the carrier portion in a static mixer and in the storage enclosure connected thereafter, the starch is sufficiently degraded to reach the resistance point, which enables it to retain a stable viscosity during the various actions to which the paste is subjected.

The water reservoir is kept at a constant temperature of the order of 38"C to 42 C, and is used: - on the one hand to prepare, in a feed vat, an aqueous starch suspension of constant concentration fixed at a value of 30 to 43% of dry matter which can be regulated as desired within this range; - on the other hand to dilute caustic soda at a concentration of about 400 litre to a concentration of about 10 g to 20 g/litre.

The assemblies of meteringpumps consist of a four-head metering pump which ensures a perfect synchronisation and enables a constant relationship between the flow rates of the various constituents of the paste to be maintained, or, by way of variation, consists of an assembly of four variable flow metering pumps operating continuously, the liquids being recycled automatically outside the production.The following simultaneous operations may accordingly be realised: 1) Metering of the solution of concentrated caustic soda; 2) Metering of the water of dil ution for the caustic soda, the mixing being effected in a pipe arrangement by means of a T or, better, by means of a static mixer; 3) Metering oftheamountofstarch suspension necessary to produce the carrier part or primary paste; 4) Metering of the amount of starch suspension necessary to form the crude starch part of the paste.

This secondary starch suspension is mixed with the carrier paste in a second static mixer; homogenisation is completed by passing the suspension through a pump connected in series and operating in counter-current, as will be explained in detail hereinafter.

The flow rate of the secondary starch suspension to the static mixer is controlled by a probe located in the outlet of the storage vessel in which the gelatinisation of the primary starch takes place; the supply of crude starch may also be effected by means of a timing system.

The final mixture of primary starch and secondary starch is then passed to a storage vat where it remains for a specific duration determined by means of an overflow level device.

The storage tank located at the end of the circuit results in a good homogenisation of the paste and brings itto its final equilibrium state. All the desired additives, for example resins intended to make the paste resistance to moisture, can easily be added to this tank.

With reference to the accompanying drawing, the starch is distributed from a hopper or silo 1 and, via a volumetric metering device 2, to the starch milk preparation vat 3. This vat is provided with a high speed stirrer 4 and is connected via a line 5 to the water reservoir 6 maintained at the desired temperature by means of, for example, a steam coil 7. The vat 3 constitutes the device for preparing the starch milk, and is able to deliver continuously an aqueous suspension within a wide range of constant and regulable concentration, of the order of 30 to 43% by weight of dry matter.

The water reservoir 6 provides depending on requirements, and via automatic stopcocks (not shown), water at a fixed chosen temperature, generally of the order of 38 to 42"C, thanks to the thermostatic regulation of the heating by means of the steam coil 7. The temperature of the water is adjusted so as to obtain a final mixture which arrives at a temperature of about 38"C at the storage tank 8.

A reservoir 9 for caustic soda having a concentration of about 400 g/l is connected via a line 10 to a metering pump P1, forming part of an assembly P1, P2, P3, P4, described hereinafter.

This assembly may consist of a single four-head metering pump or of four metering pumps each driven by a variable transmission motor, having the general reference numeral 11, and it enables the flow rate for each liquid to be chosen within a defined range, the flow rate being read on a flow meter in a manner known per se. The pumps operate continuously, and the liquids are recycled, outside production, to their respective vats or reservoirs by a set of automatic three-way valves, namely the valve 12 connected to the concentrated caustic soda reservoir 9 via the line 13, the valve 14 connected to the water reservoir 6 via the line 15, and the valves 16 and 17 connected to the vat 3 via the lines 18 and 19 respectively.

The inlet of the pump P2 is connected to the outlet 20 of the water reservoir 6 via the line 21, and the inlets of the pumps P3 and P4 are connected to the outlet of the vat 3 via the line 22.

The pumps P1 and P2 inject their liquid flows through the lines 23 and 24 respectively to a first point A, and then the pump P3 injects a first portion of starch suspension through the line 25 to a second point B upstream of a first static mixer 26 intended to homogenise the various constituents and gelatinise the starch. This gelatinisation is prolonged and completed by contact with a coil 27 connected to the outlet of the static mixer 26 via a line 28 whose dimensions are such that the storage time is at least one minute.

The pump P4 injects the second portion of starch suspension into the primary paste at a third point C, via the line 29, just upstream of a second static mixer 30, and at the outlet of the coil 27. The simultaneous arrival of the two flows at the point C is regulated by a probe (not shown) located at the outlet of the coil 27 or, more simply, by a suitable timing device. The static mixer 30 homogenises the primary paste/ crude starch mixture, this homogenisation being completed for example by an in-line pump 31 operating in a counter-current manner.

The final mixture flows via the line 32 from the outlet of the pump 31 to the storage tank or vat 8 provided with an efficient stirrer 33. This vat 8 is furthermore provided with an automatic volumetric metering device 34 for metering the boric acid in the desired amount, as well as with an overflow level device 35 intended to allow the paste to remain in the vat 8 for 30 to 40 minutes, while being stirred, before the paste is distributed.

To this end the outlet of the vat 8 is connected via a line 36 containing a suitabie circulation pump 37 to at least two storage vats 38 and 39. These vats are fed with fresh paste under instructions from a probe S situated at a low level and which initiates the production procedure, a three-way valve 40 arranged in the line 36 directing the distribution by a rocking or tilting action to the required position.

Advantageously, and particularly in the case where the starch paste is produced for corrugated paper, the vat 38 receives directly the final starch, which has the characteristics of so-called "double face" paste. The vat 39 may be allocated for the so-called "simple face" paste and, in this case, its feed inlet 41 is connected to the valve 40 via a line 42 comprising an inlet 43 for water of dilution, which may be connected to the reservoir 6 and, if necessary, an inlet 44 for the addition of an acid such as hydrochloric acid, these additives being mixed by means of a third static mixer 45 whose outlet forms the feed 41 for the vat 39, the hydrochloric acid being contained in a vat 46.

Suitabie probes 47 and 48 placed at a high level in the vats 38 and 39 respectively are intended to stop the distribution and interrupt the whole production cycle, the assembly of pumps P1 to P4 then being in the recycling position via the valves 12, 14, 16 and 17, until production is restarted by one of the low level probes S.

The following examples, which are given by way of illustration and are in no way meant to be limiting, will provide a better understanding of the scope and value of the invention. In these examples all the percentages are given by weight unless otherwise indicated.

EXAMPLE 1 Preparation of a paste for corrugated paper, having a starch concentration of2O'/0 This test is carried out using a natural maize starch containing 12.5% moisture. The temperature of the water in the reservoir is maintained at 40"C and the starch milk is prepared in a concentration of 40% of commercial starch, i.e. 35% of dry matter.

The afore-described installation is used, in conjunction with thefolowing regulation of the pumps: The pump P1 delivers a solution of caustic soda in a concentration of 420 g/litre at a rate of 36 litres/hour.

The pump P2 delivers water from the reservoir at a rate of 1200 litres/hour.

The pump P3 feeds the circuit with starch milk for the formation of the carrier, at a rate of 140 litres/hour.

The pump P4 provides the crude starch or secondary starch at a rate of 940 litres/hour.

The boric acid is fed to the paste arriving at the storage tank at a rate of 5 kg/hour.

The characteristics of the components employed are as follows: Concentration of dry matter in the starch of the carrier paste = 4.05%; Ratio of carrier starch to total starch = 13%; Concentration of the caustic soda after dilution = 12.2 grams/litre.

The flow rate of final, undiluted paste is 2320 litres/hour and the characteristics of this paste on arrival at the storage vat for the double face paste are as follows: Concentration of starch = 20.1% Temperature = 38"C Stein-Hall viscosity at 35"C = 50 s Brookfield viscosity at35 C (20 revs./min.) = 3000 mPas Alkalinity in % NaOH/starch = 2.70 Borax as % Na2B407,1 10 H2O/starch = 1.5 Stability of the paste: no notable change in viscosity at 35"C after shear forces produced by a turbine blade rotating at 600 revs./minute for 4 hours.On settling at ambient temperature for 96 hours, the Stein-Hall viscosity normally increases to a value of the order of 70 s, but re-adopts its initial value after stirring and adjusting the temperature to 35"C.

Gelatinisation temperature of the paste = 58"C.

The simple face paste is obtained by dilution with cold water at a flow rate of 280 litres of water per hour, and has the following characteristics: Flow rate = 2600 litres/hour Concentration of starch = 18% Temperature = 35"C Stein-Hall viscosity at 350C = 36s % alkalinity/starch = 2.7 Borax in % Na2B407,10 H2O/starch = 1.5 Gelatinisation temperature = 600C Stability of the paste: same remarks as for the double face paste.

EXAMPLE 2 Preparation of a paste for corrugated paper, having a starch concentration of 25% The test is carried out as previously, using natural maize starch containing 12.5% moisture.

The temperature of the water in the reservoir is maintained at 39"C and the starch milk is prepared having a concentration of 40% of commercial starch, i.e 35% of dry matter.

The pump P1 delivers a solution of caustic soda in a concentration of 415 g/litre, at a rate of 43 litres/hour.

The pump P2 delivers water at a rate of 940 litres/hour.

The pump P3 feeds starch milk for the carrier at a rate of 156 litres/hour.

The pump P4 feeds crude starch milk at a rate of 1260 litres/hour.

The boric acid is added to the storage tank at a rate of 6.55kg/hour.

Concentration of dry starch matter in the carrier paste = 5.4% Ratio of carrier starch/total starch = 11% Concentration of caustic soda after dilution = 18.16gel The flow rate of undiluted final paste is 2400 I/h Characteristics of the double face paste: Starch concentration = 25% Temperature = 38"C Stein-Hall viscosity at 35"C = 52s Alkalinity in % NaOH/starch = 2.4 Borax in % Na2B407,10 H2O/starch = 1.5 Gelatinisation temperature of the paste = 58"C Stability of the paste: no notable change in the viscosity at 35"C after the action of shear forces produced for 4 hours or after settling at ambient temperature for 2 to 4 days.

Simple face paste: obtained by dilution with cold water at a rate of 290 litres/hour and by adding hydrochloric acid, density 1.16 (content of pure acid equal to 370 g/l) at a flow rate of 6.46 litres/hour.

The characteristics of this simple face paste are as follows: Flow rate = 2690 litres/hour Starch concentration = 22.5% Temperature = 34"C Stein-Hall viscosity at 35"C = 33s % alkalinity/starch = 2.0% Borax in % Na2B407,10 H2O/starch = 1.5 Gelatinisation temperature = 64"C Stability of the paste: identical to that of the double face paste during exposure to shear forces at 35"C, and is even improved as regards the viscosity behaviour when the paste is allowed to settle at ambient temperature.

The process and plant according to the invention provide a really continuous and automatic production of starch pastes, and permit a considerable reduction in the storage capacity.

The plant is simple and reliable, and the constant relationship between the flow rates of the various constituents of the paste ensures a constancy of the characteristics of the pastes obtained from a given starch. It is not necessary to subject the operation of the plant to control by a device comprising a viscosimeter.

The various operating parameters can be regulated precisely and rapidly.

The gelatinisation of the carrier in the cold constitutes an economy in the use of steam and requires only expanded steam to maintain the water at a temperature of about 40"C. It also enables a carrier with a low starch concentration and which is thus not very viscous and can flow easily, to be obtained.

The plant according to the invention is highly flexible in operation, and is capable of producing pastes having regulable starch concentrations of between 18 and 30% by varying the alkalinity as desired. The hydrochloric acid neutralisation device enables the alkalinity of the simple face paste to be adjusted independently of that of the double face paste.

The replacement of borax by boric acid also has several advantages, and represents a device for increasing the degree of degradation of the carrier starch in order to reach its resistance point, by using an amount of additional caustic soda equivalent to the caustic soda necessary to reform the borax. It is easier to meter dry than borax, since the flow behaviour of the powder is better and, above all, it dissolves more rapidly in the starch paste.

The storage tank located at the end of the circuit enables the paste to be well homogenised and brings the latter to its final equilibrium state. All the desirable additives, for example resins intended to make the paste resistant to moisture, can easily be added to this tank.

As regards the properties of the paste, the process according to the invention enables the following features in particular to be obtained: - a constant viscosity at the desired level; - a good stability with regard to the temperature and shear force; - a good stability over time, without any noticeable change between 35"C and ambient temperature, for a storage period as long as 96 hours; no irreversible retrodegradation; - a good water retention, and no separation on storage or settling.

It should of course be understood that the present invention has been described and illustrated only by way of example, which is in no way limiting, and any modifications within the field of technical equivalence may be made to the invention without going beyond its scope.

Claims (10)

1. A process for producing starch pastes, in particular for manufacturing corrugated paper or cardboard, of the type comprising the stage of preparing a primary paste cold from a gelatinised starch in the presence of an alkaline agent, characterised bythe fact that the primary paste is made from a first portion of an aqueous starch suspension which is subjected to the action of an alkaline agent in excess, that the primary paste is mixed with a second portion of the said suspension, and that at least part of the alkaline agent present in excess is neutralised with boric acid.

2. A process according to claim 1, characterised by the fact that the aqueous starch suspension is made at a temperature between about 38 and 42"C, at a starch concentration of between about 30 and 43% by weight of dry matter.

3. A process according to claim 2, characterised by the fact that the primary paste is made by contacting the aqueous starch suspension with a solution of an alkaline agent such as caustic soda having a concentration of between about 10 and 20 g/l.

4. A process according to any one of claims 1 to 3, characterised by the fact that the boric acid is added in an amount of about 1.5% by weight of Na2B407,10 H2O, with respect to the concentration of starch.

5. A plant for producing starch paste continuously and automatically, for implementing the process according to any one of claims 1 to 4, characterised by the fact that it comprises a reservoir of water heated to a constant regulable temperature, an assembly of three metering pumps for producing the primary paste from a first portion of aqueous starch suspension, a first pump connected to the outlet of a reservoir of concentrated alkaline agent, a second pump connected to the water reservoir, a third pump connected to the outlet of a tank for preparing the aqueous starch suspension, situated opposite a feed hopper, the outlet of the three metering pumps being connected to a means for bringing the constituents of the primary paste in contact with one another, a fourth pump connected to the said tank and whose outlet is connected to a mixer for the primary paste and the second portion of aqueous suspension, the outlet of the said mixer being itself connected to a storage tank provided with a boric acid metering device and whose outlet is connected to at least one final paste vat provided with level probes connected to the control means for the outlet of four the metering pumps.

6. A plant according to claim 5, characterised by the fact that the outlets of the four metering pumps contain three-way valves, for recycling outside production to the respective reservoirs, and controlled by the level probes.

7. A plant according to one of claims 5 and 6, characterised by the fact that the means for bringing the constituents of the primary paste into contact consists of a static mixer followed by a coil designed to ensure a contact time of at least one minute.

8. A plant according to any one of claims 5 to 7, characterised by the fact that the outlet of the storage tank for the final mixture is connected via a three-way valve to a storage vat for the final paste and to a storage vat whose feed line comprises an inlet for neutralising agent such as hydrochloric acid, and if necessary an inlet for water of dilution, and a stirrer.

9. A plant according to any one of claims 5 to 8, characterised by the fact that the storage tank for the final mixture includes a timing device for ensuring a contact time of about 40 minutes.

10. A starch paste obtained by means of the process according to any one of claims 1 to 4.