WO2000056775A1

WO2000056775A1 - A process of applying starch to a substrate

Info

Publication number: WO2000056775A1
Application number: PCT/GB2000/001036
Authority: WO
Inventors: David Malcolm Owen
Original assignee: Chemisolv Limited
Priority date: 1999-03-19
Filing date: 2000-03-20
Publication date: 2000-09-28
Also published as: AU3308600A; GB9906263D0

Abstract

The present invention provides a method of preparing starch or a starch derivative in the form of granules for use as a substrate strengthener, the method including the step of heating the granules to the point just before, just at or just after they rupture.

Description

A PROCESS OF APPLYING STARCH TO A SUBSTRATE

The present invention relates to a process of applying starch to a substrate. In particular, the present invention relates to a process for improving the mechanical characteristics of a substrate by the application of starch thereto. More particularly, the present invention relates to a process for increasing the strength of paper by adding starch during the production thereof. Furthermore, the present invention relates to a process for preparing starch for use as a substrate strengthener.

To increase the strength of a substrate, for example, paper, it is known to insolubulise materials on to the substrate, that is, to reinforce same. For example, during the production of paper, cellulose is firstly pulped to a slurry such that the resultant slurry can be applied to a screen to orient the fibres and drain away the liquids. Typically, from the screen, the paper is then squeezed between rollers to further reduce the water content and dried so as to produce a sheet of paper.

As will be appreciated, the strength of the paper produced is dependent upon the nature of the pulp material utilised. For example, virgin pulp of a given quality will generally produce a much stronger paper than pulp from repulped paper of the same quality. Also, the strength achievable from the pulp decreases each time the pulp is recycled. Thus, there has always been an interest in additives that could increase the strength of the paper.

The strength characteristics of paper are evaluated in many different ways. There is strength to be measured in the machine direction, cross machine direction or thickness direction. Various types of strength measurements are burst, tear, tensile, stiffness, taber stiffness, ring crush, fold endurance, concorra medium test (CMT) and STIFI. These physical properties, amongst others, can be measured using standards of the Technical Association of Paper Processing Industry (TAPPI). Heretofore, the strength of paper has been increased either by the use of a size press operation or by wet end addition.

Typically, size press additions involve wetting the finished paper on flooded rollers with a starch solution so as to soak the starch into the paper. Once soaked, the treated paper is then dried thereby providing paper of increased strength. However, such procedure suffers several drawbacks, among them being that there is a limited amount of starch that can actually be added in this manner. Accordingly, the strength increase possible is limited since the sheet can only absorb a fixed amount of solution. Furthermore, size presses are a large capital item requiring large amounts of space when coupled with the extra dryers required to dry the sheet. Therefore, it is believed to be apparent to the skilled person that size press additions are extremely inefficient from both a production and energy content standpoint.

Paper consumption has increased worldwide and is expected to increase further. A large proportion of solid waste generated is paper and paper products. Many nations have undertaken efforts to reduce all types of waste products, including paper, in order to conserve landfill space. As a result thereof, there is an increasing interest and desire to recycle paper products. As outlined above, one disadvantage and limitation heretofore of the recycling of paper was the inability to achieve the desired strength of paper made in whole or in part from recycled pulp. Indeed in Europe, where recycling is more intensive than in the United States, it has become commonplace to make certain grades of paper entirely from recycled fibre especially in the boxboard grades. Such products are made from recycled boxes and mixed waste without the use of any virgin fibre and for certain grades, for example, fluting grade paper, it is impossible to make the quality standard required without a large increase in the stiffness parameter as measured by the concorra medium test (CMT) or latterly, the STFI test. To effect such improvements as is necessary, paper mills have been forced to add starches by means of a size press in amounts varying between 3% and 10% based on fibre. To date, it has been generally accepted that addition of material such as starches, carboxymethyl cellulose, polyvinyl alcohol etc., in various states of charge density or charge sign at the wet end of the paper making process have failed to produce the quality improvements equivalent to a size press addition of say 4-10% starch for several reasons, including:

1. The cost of the wet end additives are excessive when compared to the total cost of size press addition of raw starch.

2. Technically, the technology of wet end addition has been incapable of adding sufficient reinforcing agent to the pulp such that it is retained in the correct fashion and provides the type of strength required.

However, and where possible, the paper maker has tended to use wet end addition, i.e. chemicals that are applied in the pulp slurry, to achieve the relevant increase in strength of a particular grade when using a recycled substitute furnish for the original virgin grade. There are many technical limitations on the use of wet end additives, for example, cationic starch, polyvinyl alcohol, xanthan gum etc., to achieve strength. Generally, it is the presence of tramp materials usually referred to as "anionic trash" that inhibit the performance of these additives such that the strength improvements achieved, if any, are not economically viable. As an alternative, the paper maker has resorted to size press technology to gain large increases in strength, as the uptake is quantitative based upon saturation of the finished sheet with a known strength solution of reinforcing agent. The results obtained by this technology are recognised by the industry as superior to wet end addition, especially in the area of the promotion of stiffness, a much valued property in many grades, particularly boxboard.

However, and as outlined above, it is recognised that size press additions are dry end additions which suffer large economic penalties. Thus, there has been a continuing need to eliminate the size press operation to increase efficiency and decrease cost, if the same quality improvements are available by wet end addition.

The prior methods for production of paper sought to improve the strength of paper by the addition of polyhydroxylated polymers such as starch, polyvinyl alcohol, carboxymethyl cellulose, xantham gum, guar gum and other such natural or synthetic binding agents, which act by hydrogen bonding to the substrate. However, in previous processes, it has not been possible to fix from solution the charge neutral versions of these polymers into the wet formed sheet as they remain in solution and simply drain through the paper being produced. Several processes have been demonstrated as methods for the attachment of such polymers to the substrate, for example:

(a) Cationization of the polymer to allow increased substantivity to the anionic sites of the substrate, eg: cationic starch.

(b) Insolubilisation of certain anionic species with polyvalent metal ions such as aluminum, iron or zirconium, to allow attachment of the polymer to the substrate, eg: carboxymethyl cellulose.

These processes allow incorporation of the additive into the wet end of the paper making process thus avoiding the expensive extra process of size press addition, which, and as discussed above, involves the extra drying step. However, these processes are subject to several limitations based on the quality of the fibre, anionic trash, pH etc., and have limited ability to increase the strength of the paper, especially with respect to stiffness.

Examples of other processes utilised to fix starch to a substrate are as follows: 1. IONICALLY CHARGED SOLUBLE STARCHES

Typically under these regimes a cationic, anionic or amphoteric starch is fully solubilised by cooking either in an open vessel or by jet cooking to produce starch which is fully dissolved and dispersed in water. The ionic charge of the dissolved starch polymer is then used to mordant the polymer chain, usually as a monomolecular film, either by self fixation in the case of cationic moieties, or by fixation with a cationic fixative, metallic or polymeric, in the case of anionic starches.

Usually, each of these technologies becomes limited with respect to the amount of included functional starch because of the electrochemistry of the substrate and the starch fixation mechanism. Simplistically this means that the anionic charge density on the substrate controls the amount of starch that can be absorbed onto its surface.

Typically, the limits of these technologies is 2% of starch by weight on the substrate fibre and at such a concentration and above adverse effects begin to appear both in the retention of starch efficiency and with runnability of paper machine. Typically, drainage starts to become adversely affected and there is no increase in strength for the added starch above the limited value. Additionally, it is found that in those systems where heavy recycling is practised and the backwater becomes seriously contaminated with interfering anionic substances such as fines, unretained filler, organic extractables and dissolved solids, it is common for the efficiency of these programmes to drop even further and it is not uncommon to find little or no effect for added starch values such as 1-2%. In a virgin system this might be expected to give between 20-200% improvement, dependent on pulp and product. 2. UNCOOKED/PARTIALLY COOKED STARCH ADDITIONS

In these methods it is typical that starch, modified or unmodified, is slurried with water and is either sprayed onto a formed sheet or introduced with the pulp slurry to the machine headbox to be retained in the sheet to suffer "cook out" in the dryers on the paper machine. One of the inefficiencies associated with such a process is that the starch needs to be fully cooked by the drier section. This is not as energetically advantageous as it would first appear. Based on the knowledge of the present application the problem is that the starch granule which is partially swollen or slurried has a limited time in which to cook to a useful adhesive form. Therefore, it is commonplace to achieve very limited success with this type of approach. Additionally, the extra energy required to cook the starch will often result in a significant slow down of production rate.

Furthermore, in the case of wet end added retention aid at conventional values of 100-500 grams per tonne with raw starch, especially in dirty systems, the retention of the starch is normally regarded as low, thus increasing the inefficiency of the process. Indeed such are the inefficiencies of this process that it is rarely seen on operating production machines since it is regarded as being uneconomical.

3. INSOLUBILISATION OF STARCH FROM SOLUTION..

Our earlier UK Patent Applications Nos. 9416520.6 and 9419323.2 disclose a method whereby oppositely charged polymers, preferably starch and polyacrylamide, react in specific ratios which control the absolute amount of starch that may be precipitated for a given amount of polymer. Fig. 5 shows a reaction ratio of 5.3:1 of starch to acrylamide, and Fig. 4 shows the precipitation of potato starch with the same polymer yielding a reaction ratio of 2.3:8. Initially it was assumed that the same mechanism prevailed in both these cases. However, on further investigation, it became apparent that this was not the case and that there are limitations to the achievable concentrations of starch and/or other reinforcing agents that could be added to the substrate as a result of the electrochemistry of the reacting species.

Whilst the method of our aforementioned applications gave improvements in performance of the substrate it was found, under practical conditions, that the anticipated benefits were not fully realised under specific conditions on real machines. In this connection, we used unmodified potato starch as a model; however, it is to be understood that modified starches of any origin, which possess similar degrees of ionisable moieties, positive or negative, have been shown to behave in a similar fashion. In our investigations we found that when the starch was cooked in an industrial jet cooker at conventional processes temperatures, i.e. at 120°C and greater, no improvement in strength was observed. Further investigation of this phenomenon showed that the cooking history of the starch played a major role in achievement of the desired properties. The amount of precipitable starch available to the polymer is initially approximately 80% of the added starch. Subsequent continued heating decreases the amount available, until eventually, at greater than 90 minutes no reaction occurs and no starch is precipitated.

Originally, it was believed that this represented some form of saponification and that the anionic moieties were in some way hydrolysed off the starch or somehow deactivated. However, this was found not to be the case upon examination of microscopic evidence regarding the nature of the active starch. In this connection, a study of the gelatinisation behaviour of a variety of anionic starches, and their subsequent reaction with cationic flocculants, revealed the following: As a test study potato starch (Farina supplied by Avebe Starches), which is naturally anionic with a DS of 0.003-0.005, was heated slowly as a 5% slurry in demineralised water at the following rates:

30-70°C 4°C/min

70-90X 2°C/min

It was observed that swelling of the starch granules progressed with time and contact with the water. It became apparent that the starch granule absorbs some water at ambient temperature as it starts to hydrate, until at 50^CC, the central structure of the starch granule began to deteriorate and apparently liquify leaving the outer structure of the granule intact. This continued all the way up to approximately 100°C until the whole of the starch granule eventually hydrated and broke up.

In our study, it was as though the starch granules wet and expand from the centre. This being confirmed by independent evidence by Hermansson (reference Biopolymers Mixtures, Pg 227, published 1995). In such paper very slow heating of starch shows that it is the amylose that is labile before loss of amylopectin during the swelling of the granule. For the purposes of this invention this is an undesired characteristic. Essentially the author postulates that in the case of potato starch the amylose content is below a certain critical value such that during the gelling process potato starch becomes an amylopectin continuous phase with amylose/amylopectin granule residuum embedded. This explains the different Theological behaviour of potato starches and grain starches wherein the higher amylose content exceeds the critical value such that when it is expelled from the granule it will form a continuous phase such that the mixture becomes an amylose/amylopectin residuum in an amylose continuous phase. No direct complete understanding is claimed, as work in this field is yet incomplete and the authors of the article suggest further work is needed to completely characterise the process for each species type of starch granule. Indeed it shows that the addition of extra amylose to potato starch can induce behaviour similar to grain starch with respect to viscosity of produced solutions.

Additionally, if the swelling is limited, solubilised starch, mainly amylose, will leach out of the granules and the swollen starch granules will be dispersed in a continuous phase of amylose. On the other hand, if the swelling is faster than the leakage of solubilised starch, the granules come into contact with each other and a close packed situation is obtained. The product that arises depends on the swelling power of the granules, as well as the concentration. The preferred quality for this embodiment is that the starch should swell rapidly with as little leakage as possible to provide an essentially fully swollen but unruptured granule.

This is achieved by the cooking process having as low a shear as possible, the correct temperature of gelatinisation and optimal time of exposure to heat during the gelling process.

Conventionally, if starch is cooked in an open batch cooker, typically the procedure is to slurry the starch in cold water and heat at the appropriate concentration until the starch gelatinises and it is heated with stirring until such time that the starch is fully solubilised. To feed a paper machine of significant output with a starch reinforcing aid, both the size and manpower required to operate such a system of starch preparation becomes inappropriate. Typically the starch is now cooked in a jet cooker whereby high slurry concentrations of the subject starch are introduced into a high shear, steam. Heated jet at elevated pressure to achieve almost immediate cook out to usable starch polymer. In these systems there is usually a post jet dilution state to reduce the viscosity of a high concentration starch. Additionally, in the case of dry end starches, typically size press oxidants, are introduced with the starch to reduce the viscosity to achieve runnability on the press. Therefore, heretofore it has been assumed that if a starch is batch cooked to 100°C or thereabouts it is fully cooked and unretainable on a paper machine. There is much evidence to support this. It has not been fully appreciated that if a starch is effectively scalded without shear within, certain control parameters, such as pH, conductivity, alkalinity etc., then the starch produced is equivalent to a soft bag of starch in which all the polymer is hydrated but is not in contact with the body of water in which it has been prepared. Even in this condition the starch when added to pulp at the requisite concentrations shows little more than marginal improvements in strength properties. If however, a coagulant and/or flocculant, organic or inorganic, is added of sufficient molecular weight and any charge sign or none, such that it can effect coagulation and/or flocculation of these bags (and in some cases associated exudate) and thereby remove from bulk dispersion in the aqueous phase the majority of the starch, such a prepared starch will participate in the embodiment of this reaction and produce strength effects close to or equivalent to starch applied at an equivalent concentration, based on fibre, at a size press.

In a first aspect of the present invention there is provided a method of preparing starch or a starch derivative in the form of granules for use as a substrate strengthener, the method including the step of heating the granules to the point just before, just at or just after they rupture.

In a preferred embodiment, a native maize starch will exhibit the desired characteristics when heated at a temperature of approximately 95°C for an appropriate amount of time. It can be seen that the actual values required regarding temperature of cooking and time of cooking will vary depending upon each species of starch utilised. Such characteristic values can be determined experimentally. For example, we have observed that depending on the heat source the desired effect can be obtained within seconds or within minutes. In this connection this may be up to a minute or between 5-10 minutes.

Preferably, the starch can be suitably modified to optimise the reaction. Preferred modifications are:

(a) Attachment of ionisable moieties or non-ionisable moieties such as alkyl epoxides, for example, ethylene oxide, propylene oxide (cationic, anionic or amphoteric) at such a concentration (DS) that during cooking the integrity of the starch granule is essentially preserved as described in the embodiment. Such moieties may be phosphate, carboxymethyl, sulphate, half esters of bifunctional carboxylic acids, quartemised nitrogen, alkyl expoxised and other such moieties conventionally used in the art to achieve nominally functionalised ionisable starches.

This has the advantage in that even under conditions where some fission of the starch bag occurs and the contents are released, they are still active as "micells" of starch, which are precipitable with the polymer system.

(b) The use of cross-linking agents, such as, but not limited to, phosphate, epichlohydrin, bifunctional acid anhydrides and other such agents obvious to those skilled in the art, to effect a degree of cohesion in the starch granule during cooking to produce the desired features of the embodiment. Experimental evidence shows that distarch phosphates exhibit improved precipitation performance when cooked in the manner of the embodiment, i.e. the residual solution starch after precipitation is close to zero. Obviously there is a matter of degree in the enhancement of this feature, such that if too much cross-linking is introduced to the granule it will be difficult to achieve both a full swelling and subsequent bursting of the granule in the driers. There is evidence to show that the degree of cross-linking can seriously effect the performance of a starch, both positively and negatively, dependent on the absolute value of cross-linking. Additionally, the use of cross-linking agents may be used to achieve more elevated cooking temperatures and a more robust starch, such that conventional jet cooking may be undertaken.

Additionally, low doses of retention aid type products typically 100-500 grams/tonne, may be used to at least marginally improve the retention of these starches. It is preferred to add sufficient polymer to create a network around the bags of starch and to charge neutralise, partially or fully, the backwater and thereafter achieve a significant degree of coating of the fibre matrix with the polymer/starch combination. Such a produced polymer starch combination is capable of being fed through the high shear sections of the paper machine but not limited to achieve the results anticipated by the invention. Typically, the reaction is conducted in thick stock.

In contrast, a typical anionic starch with a high DS value such as Retamyl AP, a phosphate starch manufactured by Avebe, exhibits entirely different gelatinisation behaviour when cooked. The granules of this starch wet at much lower temperatures such that they are fully swollen by 40°C and it is characteristic of the starches that this dissolution behaviour is observed both in the cold water dispersable and the cook up varieties. Additionally, it has been shown from laboratory experiments that when solutions of these starches are boiled for extended periods of time there is no loss of activity indicating that their solution chemistry is unaffected.

It is fairly typical of high DS starches, both anionic and cationic, to demonstrate this type of behaviour.

Additionally, it has been shown that the quality of the water in which the starch is cooked has a considerable effect upon the quality of the finished starch. Indeed, in demineralised water it is possible to jet cook at 120°C and preserve the effectiveness of the produced starch. Various ions can have differing effects on the degree of precipitation with specifically added ionic impurities. The most significant effect is that of alkali, be it bicarbonate, carbonate or hydroxide ion. Under the conditions of high temperature cooking, that is greater than 100°C, all of the above moieties revert to hydroxide very quickly. It is speculated that the destruction of the swollen bag is catalysed or enhanced by basic conditions, probably by proton removal and subsequent restructuring of the starch chains.

This would allow rapid solubiiisation of the starch to such a degree that it is no longer effective in the reaction.

Different types of water (hard, soft, backwater) can have varying effects on modified starches when cooked in a jet cooker at various temperatures.

Dependent on the water type and temperature it is possible to evaluate a condition of cooking such that the starch is produced in a precipitable form.

Since the conditions of cooking require strict control over the temperature and contact time of the starch with the water, a starch cooker design is required to achieve optimum results. Adequate results can be obtained with a conventional jet cooker, but under industrial conditions, it is anticipated that the shear experienced in the jets of the cookers may be excessive and reduce the final yield of the required starch type.

Therefore, it is preferable that the cooker should use steam or any other heat source to raise the temperature of the cooking water to the desired value (including greater than 100°C) whereupon, a high concentration starch slurry in cold water will be introduced to the hot water stream to achieve the desired cooking at or about 90-100°C. This may be achieved under atmospheric pressure, thereby reducing or eliminating the high shear experienced in the jets of a conventional cooker. In this connection, we have observed that by heating the starch or starch derivatives under atmospheric pressure reduces shear which in turn reduces the amount of granules from rupturing which eliminates or reduces the high shear being experienced by the starch slurry thereby presenting excessive rupturing thereof.

The optimum concentration of starch in water for cooking to the described condition appears to be between 1-8% by weight, preferably 3-8%, by weight, although, laboratory samples are cooked at between 0.5-1 % by weight. The upper limit of consistency to achieve the required characteristics is as yet unknown and normally is practically limited by the viscosity and pumpability of the starch produced.

From a study of the viscosity behaviour of a starch under classical cooking conditions, it is anticipated that a maximum viscosity is achieved on the way to the full cook out of the starch. This is a well documented effect and is understood by those skilled in the art. It is believed that the produced starch lies on the rising side of the curve somewhere close to the maximum viscosity seen in a conventional cooking curve. If the cooking is carried out to theoretical optimum there will be no amylose or other polymer release and the "solution" will effectively be as though there are bags of swollen starch capable of rolling past each other rather than crystallising polymers which interact with the aqueous phase. It should therefore be appreciated that the viscometry and viscometric behaviour may be unusual and the perceived viscosity may be lower than would be anticipated for a conventionally cooked starch at that temperature. This serves to indicate the general methodology required to achieve the required condition and helps to define the required condition without limitation and variation which will be obvious to those skilled in the art.

Indeed, it may be thought to be obvious to attempt to retain partially cooked starch in the sheet of paper and allow the retained partially cooked granules to cook out in the dryer section of the paper machine. There are many cases where it has been custom and practice to attempt such a method, but usually this meets with only qualified success, for example, starch spraying onto the wire or formed sheet or the use of partially swollen (low temperature) starches in the stock with the use of conventional doses of retention aid polymers at doses typically of 100-500 gram active. In all these cases, the results achieved are usually marginal giving typical increases in properties such as burst or stiffness of the order of 5-15% over an untreated blank. There are special cases where on particularly slow machines cold water slurries of natural starch are used via a spray technology to enhance ply bonding in multi-formed sheets and significant large increases of inter-ply bonding can be observed because of the localised effect and the extended periods of cooking in the dryer sections of these machines. Often it is reported that very large quantities of starch are required to achieve a given effect, for example, 6-8% may be required when theoretically it would be anticipated that 2-3% would be sufficient.

In a further aspect of the present invention there is provided a method of increasing the strength of a substrate, the method including the steps of adding a starch or a starch derivative which has been prepared in accordance with the method of the present invention to a slurry containing the substrate and flocculating and/or coagulating the starch or starch derivative.

It is believed that the process of the present invention allows for a greatly improved performance especially in "dirty systems" operating at neutral pH's and allows for greatly improved strength as compared to the strength available by the known processes outlined above. The present invention in many applications, will permit the elimination of the size press resulting in considerable cost savings and process simplification. The advantages of the present invention include: (a) ability to increase strength substantially over the known methods outlined above, (b) the elimination of sizing equipment, (c) the reduction of refining, (d) the ability to select reactants, reaction conditions to achieve desired strength increases or other desirable properties in relation to characteristics of the pulp being utilised. In a preferred embodiment, a cationic polymer, which is capable of flocculating and coagulating modified starch particles which have been correctly prepared by the method of the present invention are added to a slurry containing the pulp in an amount equal or less than the amount needed to neutralise the anionic charge of the slurry components. The specially prepared modified starches are added to the slurry at the required dose rate, either at the same time, before or after the addition of the cationic polymer. A gross flocculation is observed which can be subsequently sheared prior to formation of the sheet. In extreme cases, the flocculation can be very substantial and need the input of a shear value equivalent to light refining, or alternatively, in the weakest cases, a shear as may be observed in the cleaners of the pre-headbox system.

In a preferred embodiment, the cationic polymer is added to the slurry containing the substrate and thoroughly admixed therewith prior to the addition of the starch component. Preferably, the cationic polymer is a water soluble polymer or an acrylamide polymer having a molecular weight above 150,000. Preferably, the cationic polymers have a molecular weight of one million or more. Other polymers that are useful are polymers known to be flocculating agents which have a molecular weight above 150,000, preferably having a molecular weight of one million or more.

In a preferred embodiment, the amount of cationic polymer added is sufficient to neutralise 10% or more of the charge of the slurry and less than or equal to the amount necessary to completely neutralise the charge of the slurry. Also added to the slurry bath is a predetermined amount of starch followed by suitable shear.

In an embodiment of the present invention, when a starch is cooked by the above method, the starch, when subjected to coagulation and flocculation by a charged or uncharged flocculant, will be retained in the formed sheet of paper. A preferred dose rate of flocculant is between 0.1 and 10kg/tonne.

It is also possible to subject a starch, cooked in accordance with the method of the present invention, to coagulation by an inorganic coagulant, such as alum or PAC, whereby the starch is retained in the formed sheet of paper. Alternatively, the starch may be subjected to coagulation by an organic coagulant, such as a polyamine, polydadmac, cationic wet strength resins or any other coagulant system including cationic starch and cationic gums and resins, whereby the starch is retained in the formed sheet of paper. It is, of course, possible to utilise a mixture of inorganic and organic coagulants and/or flocculants.

In a further embodiment of the invention, a cationic polymer is used in conjunction with bentonite as a retention aid for addition to the starch prepared by the method of the present invention. It is also possible to utilise a cationic polymer in conjunction with a synthetic anionic microparticle, preferably, an anionic microcrystalline silica.

The starch produced by the method of the present invention can be applied by spraying, or any other suitable form, such that the starch is entrapped in the pulp matrix in order to generate its strength properties upon drying. In a preferred embodiment, the starch prepared by the method of the present invention may be used in recovered fibre systems to produce strength parameters approximately equal to size press applied starches.

Experimental data relating to our investigations is provided hereinbelow.

Photomicrographs of a sample of an anionically modified maize starch having a DS 0.003 which has been swollen in a jet cooker at 95°C, show that there are swollen granules embedded in a continuous phase of what is to be believed amylose. The same starch cooked conventionally at 125°C in a jet cooker shows no remnant starch granules swollen or otherwise, but does contain a highly dispersed phase which has 2-5 micron particles evenly distributed throughout the matrix. When the same starches are reacted with the preferred cationic polymer it is seen that the 95°C preparation coagulates/flocculates strongly to remove the starch, but that the 125°C preparation produces little or no coagulated/flocculated material.

Thus, it has been shown that the temperature of preparation is critical to achieving the required result in the interaction between the starch and the polymer.

A sample of anionically modified maize starch having a DS of 0.07, would normally represent a starch which upon cooking at 125°C in a jet cooker should perform as a water soluble anionic starch and precipitate from solution by the mechanism described in our co-pending UK patent applications identified above. If this starch is cooked at 95°C it has similar features to the previously mentioned starch of DS 0.003. It again looks like there are discreet bags of starch in an amylose substrate. The same starch having a DS. of 0.07 cooked at 125°C shows little or no remaining evidence of an amylose substrate and residual bags. Reaction of the starch with a polymer shows the following.

Photomicrographs showing the reaction product between starch granules heated to 95°C and the polymer indicates the presence of a gel like coating of the starch particles.

Photomicrographs showing the reaction product between starch granules treated to 125°C and the polymer, indicate that the starch has now become strand-like and typical of the reaction product for these high DS starches with the polymer. Thus it has been shown there are two mechanisms at work which can take place either exclusively or simultaneously in part or whole dependent on the cooking history and temperature of the starch.

The samples prepared above were prepared in a laboratory jet cooker using live steam. It is a design feature of these cookers to use the steam to create shear during the cooking process. Therefore, under ideal conditions, shear would be removed at the cooking stage to reduce the attntive effect of the shear. Hence a new design of cooker is proposed for the preferred embodiment but is not essential to practice the art.

Given the above observations, it is predicted that there should be a condition wherein, if the starch is cooked by scalding and the produced slurry is aged over a short period of time, then the produced strength of a paper using the starch as a reinforcing agent should be scalding temperature dependent and should reach a maximum at the temperature wherein the starch has expanded to the limit of the bag integrity, ideally without breaking the bag.

Accordingly, an experiment was conducted to test this hypothesis wherein the two starches (1) DS 0.003 (2) DS 0.07 were "scalded" between 20°-95°C for 5 minutes and handsheets were produced using 3% starch on fibre. The handsheets were measured for CMT and burst values and all results normalised. CMT was normalised to 105gsm as this represents a specific grade of paper used for fluting. The burst value was normalised to 100gsm so that ready comparisons of burst ratios (strength value/basis weight) could be made.

The tests were conducted with a single batch of waste chip paper from a local paper mill and represents a furnish of 50% Kraft Liner Substitute (KLS) and 50% mixed waste. The polymer level was maintained at 3kg/t and the handsheets made in the prescribed manner.

Examination of the results showed that for both CMT and burst a similar curve is produced in so much that up to 50°C the increases are relatively small. For burst the values increase by less than 10% and for CMT the value has increased by 30% for commercial papers; neither of these results represents a major improvement. Indeed it may be thought that a 30% increase in CMT value is significant but for the removal of a size press which is used to impregnate a sheet at 105gsm it will take an increase of at least 50% over the base sheet to achieve the commercial objective, i.e. a value of at least 175 newtons, in a handsheet, where directionality is nil.

However, as the temperature of the scald increases the produced strength is significantly higher and apparently reaches a maximum between 60-80°C which is significantly higher than the "cold slurried" starch. This represents an improvement on the existing art showing substantial increases in both burst and CMT values.

Additionally, a further and unanticipated increase in the strength is observed as the temperature is raised close to the boiling point of the water. The increase in burst represents an increase in burst ratio from approximately 2.8:1 to approximately 3.75:1 which is a substantial improvement. The increase in concorra is now approximately 50%. Hence, the increases in strength are of the order required to remove a size press when manufacturing fluting medium in the low basis weight grades.

The dependence of the produced strength on polymer concentration was also investigated. The starch with DS 0.07 was reacted with the polymer after preparation at 95°C by scalding as described in the method above. Various additions between 0 and 3kg/t of polymer were used at a constant 3% added starch. The strength parameters were again measured as in the experiment above.

It has been found that increasing levels of strength production occur with increasing level of polymer.

CMT

The polymer alone is contributing to the base concorra value and represents an increase of approximately 20%. The starch alone produces a small increase in concorra of approximately 10%; however, with increasing concentration of polymer the concorra value rises. Therefore, it is believed that there is a distinct synergy between the polymer and the starch to produce high values of concorra. This value will be polymer related and can be enhanced or reduced dependent upon polymer type.

BURST

A similar situation exists with burst as with concorra. The base value is enhanced by polymer alone and there is again a small increase due to starch alone but, with increasing addition of polymer the burst rises linearly and proportionally to the added polymer level.

All the above tests were conducted in clean water from the laboratory supply which has a low hardness and low alkalinity. The tests demonstrate the method; however, further tests have been conducted using machine backwaters as the medium in which the sheet formation was completed. Similar values have been obtained, demonstrating that the method is unaffected by the presence of anionic trash present in the backwaters. This is an improvement over the present art wherein cationic starch suffers severe inhibition of its effectiveness in "dirty systems". Equally, conventional anionic starches are normally subject to a similar limitation insomuch as they are cooked to full solubility and require mordanting to the fibre by a cationic moiety. Therefore, their ability to fix is governed by where the mordant fixes. Generally, the mordant will fix to the anionic trash and thus a similar situation emerges as found in the case of cationic starch.

In the case of the present method, the cationic fixative is used to swamp and retain all anionic trash on the fibre and subsequently to coagulate and flocculate a fully swollen, but essentially insoluble starch. Because the granule is essentially intact it is not subject to the same ionic interference as the ionically charged, fully cooked, starches imparted by the very high colloidal, and ionic loading of backwaters. As demonstrated in the experiment it takes a minimum temperature and time of exposure of the starch to create the very high bonding ability. If raw starch is included in the furnish of a paper and retained by conventional mechanism it is now obvious that it is unlikely to be able to develop the full strength in the drying section of a paper machine because there is insufficient time for the granule to fully swell and "cook out". Typically, the residence time in the dryers will be less than 1 minute and the average temperature of the paper will be lower than 90°C for most of that period. It is therefore a feature of the present invention that to obtain the maximum strength from the added starch, it is necessary to fully cook the starch prior to its journey through the drying section. Conventionally it has been the experience that if a starch is "fully cooked" then the starch is entirely in solution, and thus when applied to the wet end of a paper machine, partitions with the water phase and is lost from the paper by a washing action. Some starch will be retained by adsorption. Therefore due to the peculiar feature of starch as a natural product that it has a skin around the granule, it is possible to utilise the abnormal swelling characteristics in an inventive fashion to effect large changes in strength of papers from the wet end of a machine in a similar fashion to ionically charged starches used at present. The results show increases in strength which are of a much higher order than would be anticipated for additions of conventionally cooked modified starches.

Claims

1. A method of preparing starch or a starch derivative in the form of granules for use as a substrate strengthener, the method including the step of heating the granules to the point just before, just at or just after they rupture.

2. The method of claim 1 , wherein the starch is a native maize starch and is heated at a temperature of approximately 95°C.

3. The method of claim 1 or claim 2, further including the step of modifying the starch or starch derivative by attaching an ionisable or non- ionisable moiety thereto.

4. The method of claim 3, wherein the ionisable or non-ionisable moiety which is used is selected from phosphate, carboxymethyl, sulphate, half esters of bifunctional carboxylic acids, quartemised nitrogen and alkyl epoxides.

5. The method of claim 4, wherein the ionisable or non-ionisable moiety which is used is an alkyl epoxide which is cationic, anionic or amphoteric and is selected from ethylene oxide or propylene oxide.

6. The method of claim 1 or 2, further including the step of modifying the starch or starch derivative by adding a cross-linking agent thereto.

7. The method of claim 6, wherein the cross-linking agent used is selected from phosphate, epichlorhydrin and bifunctional acid anhydrides

8. The method of any one of the preceding claims, wherein a retention aid is added to the starch or starch derivative.

9. The method of any one of the preceding claims, wherein the starch or starch derivative is added to water and heated by the action of steam.

10. The method of any one of the preceding claims, wherein the starch or starch derivative to be heated is added to demineralised water.

11. The method of claim 9 or 10, wherein the concentration of the starch or starch derivative in water is between 1-8% by weight.

12. A method of increasing the strength of a substrate, the method including the steps of adding a starch or a starch derivative which has been prepared in accordance with the method of any one of the preceding claims to a slurry containing the substrate and flocculating and/or coagulating the starch or starch derivative.

13. The method of claim 12, wherein the starch or starch derivative is flocculated and coagulated by a cationic polymer.

14. The method of claim 13, wherein the cationic polymer is added to the slurry in an amount equal or less than the amount needed to neutralise the anionic charge of the slum/.

15. The method of claim 13, wherein the cationic polymer is added to the slurry in an amount which is sufficient to neutralise 10% or more of the charge of the slurry and which is less than or equal to the amount necessary to completely neutralise the charge of the slurry.

16. The method of any one of claims 13 to 15, wherein the cationic polymer is added to the slurry and admixed therewith prior to the addition of the prepared starch or starch derivative.

17. The method of any one of claims 13 to 16, wherein the cationic polymer used is a water soluble or an acrylamide polymer having a molecular weight of 150,000 or greater.

18. The method of claim 17, wherein the cationic polymer used has a molecular weight of 1 ,000,000 or greater.

19. The method of any one of claims 13 to 18, wherein the cationic polymer is used in conjunction with a retention aid or a synthetic anionic microparticle.

20. The method of claim 19, wherein the retention aid used is bentonite.

21. The method of claim 19, wherein the synthetic anionic microparticle used is an anionic microcrystalline silica.

22. The method of claim 12, wherein a flocculating agent having a molecular weight of 150,000 and above is used to flocculate the prepared starch or starch derivative.

23. The method of claim 22, wherein the flocculating agent used has a molecular weight of 1 ,000,000 or more.

24. The method of claim 22 or 23, wherein the flocculating agent is added to the slurry at a dose rate of 0.1 to 10Kg/tonne.

25. The method of claim 12, wherein the prepared starch or starch derivative is coagulated by the use of an inorganic coagulant and/or organic coagulant.

26. The method of claim 25, wherein the inorganic coagulant used is selected from alum or PAC.

27. The method of claim 25 or 26, wherein the organic coagulant used is selected from polyamine, polydadmac, cationic wet strength resin or any coagulant system including cationic starch and cationic gums and resins.