GB1571336A - Starch-based adhesive composition - Google Patents

Description

(54) STARCH-BASED ADHESIVE COMPOSITION (71) We, CPC (UNITED KINGDOM) LIMITED, a British Company, of Claygate House, Esher, Surrey, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to starch-based adhesive compositions and methods for preparing them. More particularly, the invention is concerned with starch-based adhesive compositions suitable for use in the manufacture of paperboard, such as corrugated paperboard, which comprise a gelatinized starch in minor proportion and an ungelatinized starch in major proportion.

At the present time, corrugated paperboard is manufactured in a continuous operation involving several steps. A strip of paper, slightly moistened, is passed through heated flute rolls.

To one side of the heated fluted paper, adhesive is applied at the top edge of each flute.

Immediately thereafter, the fluted paper is contacted, under heat and pressure, with a strip of liner paper to form a strong adhesive bond between the two layers. This product is known in the art as "single facer" corrugated paperboard. Another type of corrugated paperboard is the rigid sort used, for instance; in the fabrication of paper boxes. In forming this type of paperboard, additional adhesive is applied to the exposed, fluted tips in a further operation, and a second strip of liner paper is brought into contact with the fluted interlayer and bonded thereto, under heat and pressure, employing a double backer machine.

The successful production of corrugated paperboards in the high-speed machines (200 metres or more per minute) currently used is very sensitive to the properties of the adhesive.

Thus, the viscosity of the adhesive, to give a satisfactory product, must be such that the adhesive flows readily in the equipment while being circulated to keep it uniform. Also the adhesive must be sufficiently viscous to remain in position on the tip of the flute when applied, and to fill any depressions inherently present in the paper. In addition, because of the high speed of present machines, it must be capable of "instant bonding" when heat and pressure are applied.

Starch-based corrugating adhesives are traditionally formulated around the Stein Hall process. This is a system in which the main part of the adhesive is unburst unmodified maize starch granules slurried in water, at about 20% solids, the granules being maintained in suspension with the aid of a carrier portion consisting of a cooked starch paste.

The adhesive is prepared in principle by cooking the carrier portion in an open tank with live steam, conventionally on a batch basis. However, recent developments have achieved the continuous production of adhesive by the use of a jet cooker under atmospheric pressure conditions to prepare the carrier portion. This is then added to a second tank containing uncooked starch slurry (noncarrier portion). A typical Stein Hall adhesive has the following properties: Gelatinization temperature 620C to 650C Stein Hall viscosity at 350C - Single facer, 30 to 35 seconds Double backer 45 to 55 seconds Solids content - 20% commercial basis (c/b) pH - 11.8 to 12 Ratio of carrier starch to non-carrier starch about 1:5.5.

Such adhesives suffer from various disadvantages: a) The amount of carrier' (gelatinized) starch determines the initial tack of the adhesive, and this and the solids content of the adhesive determine the speed at which a corrugating machine can be run. It is difficult to provide a Stein Hall adhesive which has acceptable initial tack and solids content without also having an unacceptably high viscosity. Efforts to overcome this disadvantage have in the past included the use of chemically tinned starches, which do give rise to gelatinized solutions of lower viscosity, but they are substantially more expensive than unmodified starch, and adhesives for paperboard are necessarily rather low-cost products. b) The viscosity stability of the adhesive is poor, due to the tendency of the gelatinized starch to retrograde or set-up on standing. c) When the paperboard product is to be water-resistant, it is usual to include crosslinking resins in the adhesive. The heat used to gelatinize the carrier portion also effects reaction between the starch and the cross-linking resin to achieve a water-resistant bond. But the reactivity of conventional Stein Hall adhesive with cross-linking resins is poor.

It is an object of the present invention to overcome the above-mentioned disadvantages of the conventional adhesive and to provide a continuous method by which improved adhesives may be made.

Accordingly, the present invention provides, in one aspect, a starch-based aqueous adhesive having a solids content from 10% to 40%, preferably 10% to 35% by weight, a Stein Hall viscosity at 350C of from 20 to 100 seconds, preferably 25 to 55 seconds, and comprising, in a weight ratio of from 1:2 to 1:8 respectively: a) a carrier portion comprising a gelatinized molecularly dispersed starch, and b) a non-carrier portion comprising ungelatinized starch having a gelatinization temperature, in said adhesive of from 55"C to 75 C.

Unless otherwise indicated, solids contents in this specification are expressed on a commercial basis (c/b), that-is to say the proportion of commercially available starch and other solids to water. Commercially available starch contains a small proportion, typically about 12%, of water, so that a 20% c/b solids adhesive is equivalent to approximately an 18% d/b (dry basis) solids adhesive.

Raw starch comprises bundles of molecules held together by hydrogen bonding. Conventional gelatinization, e.g. by treatment with steam at nor more than 100"C, loosens and partially, but not completely, breaks up these bundles. In molecularly dispersed starch such as is used in the carrier portion of adhesives of the present invention, the bundles are essentially completely broken up and the iridividual starch molecules essentially separated. Molecular dispersion of starch can be achieved by gelatinization under severe conditions, such as the use of excess steam and temperature above 100"C. as more fully described below. Molecularly dispersed starches are characterised by having a viscosity of not more than 1000 centipoises, measured at a solids content of 10% and a temperature of 50"C, typically of the order of 250 centipoises. Conventionally gelatinised starch has a viscosity under these conditions well above 1000 centipoises.

We prefer to use unmodified starches for making the adhesives of this invention. Modified starches, such as etherified or esterified starches can be used, but since they are normally more expensive than ordinary starch and little or no advantage results from their use, they are not preferred. Partly oxidised or acid or enzyme hydrolysed starches may be used and will result in a lower viscosity on gelatinization. But unless they are also molecularly dispersed, they do not have the necessary viscosity stability and are not suitable for use according to the present invention.

Although any cereal or vegetable starch can in principle be used in the adhesive and method of this invention, maize or wheat starch is normally used for corrugated paper board adhesives.

A number of water-dispersable high viscosity polyhydroxy polymeric materials can be used in replacement of up to half the starch used in the carrier portion of the adhesive. Examples are cellulose derivatives such as sodium carboxy methyl cellulose, polyvinyl alcohols and hydrolysed polyvinyl acetates, and polyvinyl pyrrolidone.

As noted above, the features of solids content and carrier: non-carrier ratio are related.

Preferred adhesives according to this invention are formulated to fall within an area of a linear graph of solids content against weight ratio of carrier to non-carrier portion bounded by the four points: Solids Content 10 20 35 35 Weight Ratio 1:2 1:2 1:6 1:8 This area is shown graphically in Figure 1 of the accompanying drawings.

By virtue of the lower viscosity of the molecularly dispersed carrier starch, adhesive compositions of the present invention of given solids content and viscosity have a lower carrier: non-carrier ratio (i.e. contain more carrier) than conventional Stein-Hall adhesives.

Advantage can be taken of this property in two ways. If the solids content is held the same, then the adhesive has a better initial tack than a Stein-Hall one. Under these circumstances the adhesive preferably has a carrier: non-carrier ratio of from 1:2.5 to 1:4.5, particularly 1:3 to 1:4, and a solids content of from 15% to 25% by weight. Alternatively, the solids content can be increased, without disastrous loss of initial tack, to provide a drier adhesive, and so enable a corrugating machine to work faster than when using a Stein Hall adhesive. Under these circumstances the adhesive may preferably have a carrier: non-carrier ratio of from 1:4 to 1:6 and a solids content of from 20% to 30% by weight.

The gelatinization temperature of the noncarrier portion of the starch in said adhesive is preferably in the range 600C to 700C., particularly 63 to 660C. If this temperature is too low, premature gelatinization of the starch and thickening of the adhesive may occur. If the temperature is too high, an inordinate amount of heat may be required to render the adhesive fully effective. The gelatinisation temperature is principally determined by the proportion of sodium hydroxide in the adhesive. The use .of up to 50/0, preferably from 2 to 3.5% by weight on the weight of the starch, of sodium hydroxide is conventional and is also appropriate in the present invention.

Borax is also conventionally used in Stein Hall adhesives to stabilise the viscosity and increase tack. We prefer to use up to 5% borax in our adhesives, suitably in an amount of 0.75 to 3.25% by weight on the weight of the starch. Where a waterproof bond is required, crosslinking agents or pre-formed resins may be incorporated in the adhesive in accordance with conventional practice. Other additives may also be included in accordance with conventional practice.

The present invention provides in another aspect, a method for the preparation of a starchbased aqueous adhesive as herein described which method comprises: a) preparing a carrier portion of gelatinized molecularly dispersed starch at a solids content of from 3% to 40% by weight by cooking a starch slurry using a jet cooker operated at a temperature of from 110"C to 1700C with at least 1.1 times the amount of steam necessary to heat the slurry to the required temperature, b) mixing the carrier portion with a slurry of ungelatinized starch at a solids content of from 10% to 40% by weight, in relative proportion to provide a total solids content of from 10% to 40% by weight and a weight ratio of gelatinized to ungelatinized starch of from 1:2 to 1:8, c) and providing alkali in the resulting mixture in an amount of up to 5%, by weight on the weight of the starch, such that the gelatinization temperature of the ungelatinized starch is from 55"C to 75 C.

An important feature of the method of this invention is the use of a jet cooker with excess steam at superatmospheric pressure to cook the carrier portion of starch. Such cooking is not novel, it has been described, for example, in U.S. Patent No. 3,133,836, but has not previously been used in the production of the carrier portion of starch-based adhesives. A jet cooker operates under conditions of very high shear, and these are made still more severe by the use of superatmospheric pressure, cooking temperatures of from 1 100C to 1 700C, pre- ferably from 1300C to 1550C, and an excess of steam. If the cooking temperature is predetermined, the amount of steam that will just be sufficient to heat the starch slurry to that temperature, and will be completely condensed in doing so, can be readily calculated. According to the present invention, excess. steam over that required to heat the slurry is used, the excess passing uncondensed through the cooker.

The amount of steam used should be at least 1.1 times that required to heat the slurry to the cooking temperature, preferably from 1.25 to 5, particularly from 1.5 to 3 times the required amount. The use of these severe conditions results in much more complete breakdown of the starch granules than is usual in gelatinisation processes. Starch which has been subjected to this treatment is in a molecularly dispersed state. The viscosity of the resulting starch paste (at a given solids content) depends on the time and temperature of cooking and on the amount of excess steam used, but is always substantially less than that of a starch solution which has been gelatinized under normal conditions.

Specifically the viscosity of starch gelatinized in ajet cooker using a temperature of at least 110 and at least 1.1 times the required amount of steam (measured at 10% solids content and a temperature of 500 C) is not more than 1,000 centipoises, preferably from 100 to 500 centipoises particularly from 150 to 200 centipoises. By contrast, a normal gelatinized starch solution would have a viscosity under the same conditions of 6-7,000 centipoises.

The solids content of the final adhesives is from 10 to 40% by weight c/b, preferably from 15 to 30 c/b, e.g. 20% c/b, and is selected with an eye to the viscosity, the initial tack strength, and the final adhesive strength of the adhesive. An adhesive having a desired solids content (say 20% c/b) can be obtained by mixing appropriate proportions of 20% solids c/b carrier portion and 20% solids c/b noncarrier portion; or by mixing a more dilute carrier portion with a more concentrated noncarrier portion; or by mixing a more concentrated carrier portion with a more dilute noncarrier portion. The solids content of the carrier portion may be from 3 to 40%, preferably 5 to 35%, particularly 10 to 20% by weight c/b. Above 20% solids, the gelatinized starch solution tends to gel on standing, which is a nuisance, though not an insoluble problem.

When operating the jet cooker at below 10% solids, we have had some difficulty in obtaining the required temperature in the final adhesive blend. As far as the non-carrier portion of the starch is concerned, slurries containing 10 to 40% solids by weight c/b can readily be handled.

The solids content of the non-carrier slurry should be chosen, in relation to the solids content of the carrier solution and the proportions in which the two are mixed together, to provide a final adhesive composition of desired solids content.

The non-carrier starch slurry is normally used at ambient temperature. The carrier portion has, however, just passed through ajet cooker at high temperature. If the carrier and non-carrier portions are simply mixed together, premature gelatinization of the non-carrier starch is likely to take place. It is therfore normal practice to ensure that the temperature of the mixture does not rise above 400C. To achieve this, the carrier portion of the starch would normally need to be cooled; the temperature to which it needs to be cooled will depend on the relative volumes of the two portions, and can readily be calculated in any particular instance.

The carrier and non-carrier portions are mixed together in proportions to provide an adhesive composition of the desired viscosity.

At conventional solids contents of around 20% by weight c/b, because the viscosity of the carrier portion is substantially less than that of gelatinized starches previously used as carriers, the carrier: non-carrier ratio will necessarily be less than the conventional 1:5.5 by weight, and is in fact found to be in the range of 1:2.5 to 1:4.5, particularly 1:3.0 to 1:4.0 by weight. At ratios above 1:4.0 we have found that the viscosity of the adhesive composition is rather too low for convenience. At ratios below 1:3.0, our adhesive compositions have an excessively high viscosity which could only be reduced to an acceptable level by excessive dilution with water. In our work, a 20% c/b solids adhesive composition having a carrier:non-carrier weight ratio of 1:3.5 has provided an adhesive composition having a desirable viscosity, or which can readily be diluted with a small amount of water to give a desirably viscosity. But the use of different cooking conditions, or of a different solids content, would be expected to give rise to a different preferred proportion of carrier to non-carrier. Adhesive compositions of the present invention have a preferred Stein Hall viscosity at 350C. of from 25 to 55 seconds, preferably from 28 to 38 seconds for single facer adhesive compositions and from 45 to 55 seconds for double backer compositions.

As previously stated, adhesive compositions of the present invention preferably contain sodium hydroxide to lower the gelatinization temperature of the non-carrier starch. This sodium hydroxide is most conveniently incorporated in the carrier portion of the starch, and may be included in the raw starch slurry passed to the jet cooker. Alternatively, the sodium hydroxide may be incorporated into the non-carrier portion or admixture of the carrier and non-carrier portions. The sodium hydroxide is preferably distributed between carrier and non-carrier supply slurries.

As previously stated, adhesive compositions of the present invention preferably contain borax. The borax may conveniently be included in the non-carrier portion of the starch.

Alternatively, it may be added during the final mixing of the carrier and non-carrier portions.

Inclusion of all of the borax in the starch slurry being passed to the pressurized jet cooker may give rise to viscosity problems.

It may be advantageous to provide a single raw starch slurry, possibly containing some or all of the sodium hydroxide and borax that is desired in the final adhesive composition, and to split this slurry into two streams, one of which passed through the jet cooker to gelatinize the starch, and then to re-mix the two streams, adding at this stage any further sodium hydroxide or borax that may be desired.

The composition and method of the present invention offer the following advantages over the conventional Stein Hall adhesive and method of making it: 1) A higher ratio of carrier to non-carrier is possible without having to resort to expensive low viscosity modified starches. Carrier to non-carrier ratios of 1:2.5 to 1:4.5 can be achieved without the usual viscosity increase associated with unmodified carrier starch gela tinized by conventional means.

2) The adhesive has a greater initial tack strength to assist during early formation of the bond.

3) Alternatively, a higher solids content is possible at the same carrier:non-carrier ratio without resulting in an impossibly high viscosity, as a result of which corrugating machines can be run faster.

4) The carrier portion, being fully dispersed on a mono-molecular level, has greater viscosity stability than conventionally prepared starch and the final adhesive is less susceptible to shear forces.

5) The final mix has greater adhesiveness due to the way in which the carrier is dispersed and a higher proportion of carrier present.

6) The higher carrier ratio gives an adhesive which reacts better with cross-linking resins to achieve a water-resistant bond.

7) The method allows corrugating adhesive to be made on a continuous basis.

In Examples 1 and 2 below, the following generalised procedure was adopted. Two separate aqueous slurries of maize starch were prepared, one to serve as the carrier portion and the other as the non-carrier portion. The sodium hydroxide was divided pro-rata between the carrier and non-carrier slurries; all the borax was added to the non-carrier slurry. The carrier slurry was cooked at superatmospheric pressure in a jet cooker under the following conditions: Steam Pressure 75-80 psig Cooking temperature 305-310 C (152-1 550C) Back pressure 70-76 psig Carrier flow rate 2 gal/min The resulting carrier portion had a viscosity (measured at 10% by weight solids and 50"C) of from 150 to 200 centipoises, and was mixed with the non-carrier portion to provide the desired adhesive composition.

EXAMPLE 1 In this experiment, the solids content of the adhesive composition was maintained constant at 20% c/b by weight, and the effect of varying the ratio (carrier:non-carrier) on the viscosity of the resulting adhesive was determined.

Results are set out in Table 1 on page 5.

Temperatures of the carrier and non-carrier portions, and of the final adhesive are included in this Table; it will be noted that in three cases, the temperature of the final adhesive was RESULTS TABLE 1 Effect of varying the carrier: uncooked starch ratio on the properties of the final adhesive

Ratio (carrier:uncooked starch) 1:6 1:5 1:4 1:3.5 1:3.0 Carrier solids (% c/b) 20 20 20 9.3 7.2 7.2 20 20 10 20 Uncooked starch solids (% c/b) 20 20 20 28 36 36 20 20 28 20 Temperature of carrier ( C) 70 92 92 79 77 50 80 50 88 90 Temmperature of uncooked starch ( C) 16 12 12 17 20 21 19 21 24 12 Temperature of final adhesive ( C) 23 26 29 41 50 36 30 27 51 33 Stein Hall viscosity at 35 C (secs) 21 22 30 28 32 32 42 44 31 > 100 Brookfield viscosity at 35 C (cps) - - 136 138 152 156 180 186 140 440 For the above mixes:- Gelatinisation temperature 64-65 C Solids contents 20% c/b, 17.5-18.5% d/b pH 11.9-12.3 Sodium hydroxide: 2.5% on c/b starch Borax: 2.5% on c/b starch TABLE 2 Effect of varying the NaOH and Borax concentrations

% NaOH on the starch 2.5 2.5 2.5 2.5 2.5 2.0 1.5 1.0 0.5 % Borax on the starch 0 1.25 2.5 3.75 5.0 2.5 2.5 2.5 2.5 Stein Hall viscosity at 35 C (secs) 25 32 42 stringy 51 100 > 100 82 Brookfield viscosity at 35 C (cps) 168 176 180 long-textured 188 250 420 208 pH 12.0 12.0 11.9 gel-like 11.6 11.4 10.4 9.3 Gelatinisation temperature ( C) 64/65 64/65 64/65 adhesives - 67/68 - 69/70 Solids content (% d/b) 17.6 - 17.6 17.3 - - 17.2 TABLE 3 Reproducibility trials and storage stability of the experimental adhesive

Adhesive type Experimental Conventional Stein Hall Stein Hall viscosity at 35 C(secs) 45 43 41 45 40 Brookfield viscosity at 35 C (cps) 216 208 210 224 200 Gelatinisation temperature ( C) 64/65 65/66 64/66 64/65 63/65 Solids content (%) 17.7 18.1 18.0 18.3 18.3 pH 11.9 12.0 11.9 12.0 11.8 Stability:24 hrs at 20 C, comments long-textured, gel-like gel structure formed Stein Hall viscosity (secs) > 100 > 100 - > 100 > 100 Brookfield viscosity (cps) 570 465 - 720 895 24 hrs at 35 C, comments slight separation, easily restirred slight separation Stein Hall viscosity (secs) 43 48 - 43 41 Brookfield viscosity (cps) 200 210 - 206 208 greater than 40"C; this does not appear to have done any harm under the small scale conditions employed in the experiment, but would be undesirably in commercial operation. The viscosity of the adhesive at a carrier:non-carrier ratio of 1:3 was rather high, but this could have been reduced either by modifying the carrier cooking conditions, or by diluting the final adhesive.

EXAMPLE 2 In this experiment, the solids content of the adhesive was maintained at 20% c/b, arid the carrier:non-carrier ratio was maintained at 1:3.5. The effect of varying the sodium hydrox- ide and borax contents of the adhesive were determined, and the results are set out in Table 2 on page 6. In this Table, the adhesives containing 3.755to and 5% of borax were not usable. The adhesives containing 1.5%, 1.0% and 0.5rye of sodium hydroxide were of rather high viscosity. but could have been improved by dilution.

EXAMPLE 3 Reference is directed to Figure 2 of the accompanying drawings, which is a schematic diagram of plant for operating the method of this invention. Percent of sodium hydroxide and borax are by weight on c/b starch.

Referring to Figure 2, numeral 10 denotes a slurry tank to which are fed: water via line 12; sodium hydroxide via line 14; borax via line 16; and granular maize starch via line 18. The slurry tank contains 20% solids by weight c/b, 2.5% of sodium hydroxide and 0.1% by weight of borax. The slurry is continuously withdrawn from the bottom of tank 10 via line 20, and divided into two separate streams, 22% in line 22 and 78% in line 24 (i.e. a ratio of 1.3.5).

The slurry in line 22 passes to a jet cooker 26, available commercially under the Trade Mark Nemo Junior, where it is cooked as in Examples 1 and 2 with excess steam supplied via line 28 and then passed to a holding tank 30. The stream of slurry in line 24 is passed to another holding tank 32, where it is admixed with a further 3% by weight of borax from line 33 (to provide 2.5% by weight in the final composition).

The carrier portion of the composition from tank 30, and the non-carrier portion from tank 32, are passed to an in-line mixer 34 where they are thoroughly mixed, and thence to a storage tank 36. It may be advantageous, at least during start-up of the system and possibly throughout operation, to recirculate a proportion of the adhesive composition from storage tank 36 via line 38 to the mixer 34. The resulting adhesive composition has a Stein Hall viscosity at 350C of 41 to 45 seconds. It is withdrawn as required via line 40 for use in a double backer machine, or via line 42 for dilution prior to use in a single facer machine if applicable.

The properties of adhesives produced in pilot plant trials using the above equipment are set out in Table 3 on page 7, in which conventional Stein Hall adhesive properties are also given for comparison. It will be seen that reproducibility is quite good, and that the properties given satisfactorily duplicate those of the conventional Stein Hall adhesive. It should be noted that Table 3 is designed to demonstrate that a corrugating paper adhesive with commercially acceptable properties can be produced on a continuous basis; the Table is not intended to demonstrate the superiority of the present adhesive compositions over the conventional ones.

Examples 4 and 5 are comparative examples designed to show the difference in carrier:noncarrier ratio required to make adhesives at two different solids contents using the approach of the invention and the conventional Stein Hall approach.

EXAMPLE 4 20% Solids 1. Standard Stein Hall Technique 18 gm maize starch was slurried into 80 gm water and heated to 700C. To this slurry was added 3.2 gm sodium hydroxide dissolved in 8 gm water. The mixture was agitated and maintained at 700C for 15 minutes, after which time 100 gm cold water was added.

This cooked starch paste (the carrier) was added to a slurry containing 88 gm maize starch, 260 gm water and 3.2 gm powdered borax, and agitated for 15 minutes.

Adhesive properties: Stein Hall Viscosity = 48 secs at -20"C</RT cold water was added.

This cooked starch paste (the carrier) was added to a slurry containing 112 gm maize starch, 220 gm water and 3.9 gm powdered borax, and agitated for 15 minutes.

Adhesive properties: Stein Hall Viscosity = 50 secs at 200C Solids Content = 25.2% commer cial basis Ratio Carrier Starch: Carried = 1:6.2 2. Technique of Invention A slurry containing 83.5 parts water, 13.2 parts maize starch and 3.3 parts of 10% caustic soda solution, was treated via a jet heater at 1 500C using 5.5 atmosphere steam pressure.

The resulting starch paste, after steam condensation, had a solids content of 11% on a commercial basis.

180 gm of this carrier paste was added to a slurry containing 21 0gum water 100gm maize starch, 2.5 gm sodium hydroxide and 3.0 gm powdered borax, and agitated for 10 minutes.

Claims (13)

Adhesive properties: Stein Ilall Viscosity = 52 secs at 200C Solids Content = 25.3% commer cial basis Ratio Carrier Starch: Carried = 1:5.1 WHAT WE CLAIM IS:

1. A starch-based aqueous adhesive having a solids content from 10% to 40% by weight, a Stein Hall viscosity at 350C of from 20 to 100 seconds, and comprising, in a weight ratio of from 1:2 to 1:8 respectively: a) a carrier portion comprising gelatinized molecularly dispersed starch, and b) a non-carrier portion comprising ungelatinized starch having a gelatinization temperature, in said adhesive of from 550C to 75 C.

2. An adhesive as claimed in claim 1, wherein the solids content and weight ratio of carrier to non-carrier portions fall within an area of a linear graph of solids content against weight ratio of carrier to non-carrier portions bounded by the four points: Solids Content 10 20 35 35 Weight Ratio 1:2 1:2 1:6 1:8

3. An adhesive as claimed in claim 1 or claim 2 having a solids content of from 15% to 25% by weight.

4. An adhesive as claimed in any one of claims 1 to 3, having a weight ratio of carrier to non-carrier portion of from 1:2.5 to 1:4.5.

5. An adhesive as claimed in any one of claims 1 to 4, wherein the carrier portion comprises gelatinized molecularly dispersed unmodified maize or wheat starch.

6. An adhesive as claimed in any one of claims 1 to 5. wherein the molecularly dispersed starch has a viscosity of from 100 to 500 centipoises (measured at a solids content of 10% at a temperature of 50"C).

7. An adhesive as claimed in any one of claims 1 to 6, wherein the non-carrier portion comprises ungelatinized unmodified maize or wheat starch having a gelatinization temperature in said adhesive, of from 630C to 660C.

8. An adhesive as claimed in any one of claims 1 to 7, wherein there is also present from 2% to 3.5% of sodium hydroxide and from 0.75% to 3.25% of borax, both by weight on the weight of the starch.

9. A method for the preparation of a starch-based aqueous adhesive as claimed in any one of claims 1 to 8, which method comprises: a) preparing a carrier portion of gelatinized molecularly dispersed starch at a solids content of from 3% to 40% by weight by cooking a starch slurry using a jet cooker operated at a temperature of from 110"C to 1 700C with at least 1.1 times the amount of steam necessary to heat the slurry to the required temperature. b) mixing the carrier portion with a slurry of ungelatinized starch at a solids content of from 10% to 40% by weight, in relative proportion to provide a total solids content of from 10% to 40% by weight and a weight ratio of gelatinized to ungelatinized starch of from 1:2 to 1:8, c) and providing alkali in the resulting mixture in an amount of up to 5%, by weight on the weight of the starch, such that the gelatinization temperature of the ungelatinized starch is from 550C to 750C.

10. A method as claimed in claim 9, wherein the jet cooker is operated in step a) at a temperature of from 1 300C to 1 550C with from 1.5 to 3 times the amount of steam necessary to heat the slurry to the required temperature.

11. A method as claimed in claim 9 or claim 10, wherein there is provided a single raw starch slurry, possibly containing some or all of any sodium hydroxide and borax that may be desired in the final adhesive, this slurry is split into two streams one of which constitutes the non-carrier portion and the other of which is passed through the jet cooker to form the carrier portion, and the two streams are then re-mixed, adding at this stage any further sodium hydroxide or borax that may be desired.

12. A method as claimed in any one of claims 9 to 11 and substantially as hereinbefore described in any one of the Examples -

13. A starch-based aqueous adhesive made by the method of any one of claims 9 to 12.