MICROFIBRILLATED CELLULOSE AS RHEOLOGY MODIFIER IN ADHESIVES

Abstract

The present invention relates to starch-based adhesive compositions comprising microfibrillatedcellulose (MFC). In addition to microfibrillated cellulose, these adhesive compositions comprise at least one starch and/or at least one starch derivative.

Claims

1. Adhesive composition comprising: at least one starch and/or at least one starch derivative, in an amount of 5% w/w to 60% w/w, dry matter, of the overall adhesive composition; at least one solvent, said solvent preferably comprising or consisting of water, in an amount of 30% w/w to 95% w/w of the overall adhesive composition; microfibrillated cellulose in an amount of 0.001% w/w to 10% w/w, dry matter, of the overall adhesive composition, preferably of 0.01% w/w to 10% w/w, dry matter, of the overall adhesive composition.

2. Composition according to claim 1, wherein the amount of microfibrillated cellulose in said composition is from 0.01% dry matter, relative to the overall weight of the composition to 10%, preferably from 0.05% to 5%, further preferably from 0.05% to 2%, most preferred from 0.05% to 0.15%, or wherein the amount of microfibrillated cellulose in said composition is from 0.02% w/w relative to the overall weight of the composition to 8% w/w, further preferably from 0.05% w/w to 0.5% w/w and/or wherein the amount of microfibrillated cellulose is from 0.003% w/w to 22% w/w, preferably from 0.02% w/w to 20% w/w, or from 0.04% w/w to 4% w/w or from 0.1% w/w to 2% w/w, further preferably 0.2% w/w to 1.4% w/w, even further preferably 0.2% w/w to 0.6% w/w, as measured relative to the overall amount of starch in the adhesive composition.

3. Composition according to claim 1 or claim 2, wherein the overall amount of starch in said composition is from 15% w/w to 50% w/w, preferably from 25% w/w to 48% w/w or from 22% w/w to 35% w/w, preferably from 30% w/w to 46% w/w, further preferably from 35% w/w to 45% w/w, of the overall adhesive composition.

4. Composition according to any of the preceding claims, wherein the pH value of the adhesive composition is from 8 to 14, preferably from 10 to 13, further preferably from 11.5 to 12.5.

5. Composition according to any of the preceding claims, wherein the at least one starch is a native starch, or a chemically or a physically modified starch, or a starch derivative, or a mixture thereof.

6. Composition according to any of the preceding claims, wherein the microfibrillated cellulose is characterized in that it results in gel-like dispersion that has a zero shear viscosity, .sub.0, of at least 2000 Pa.Math.s, preferably of at least 3000 Pa.Math.s or 4000 Pa.Math.s, further preferably of at least 5000 Pa.Math.s, further preferably at least 6000 Pa.Math.s, further preferably at least 7000 Pa.Math.s, as measured in polyethylene glycol (PEG) as the solvent, and at a solids content of the MFC of 0.65%, wherein the measurement method is as described in the description.

7. Composition according to any of the preceding claims, wherein the microfibrillated cellulose is characterized by a water holding capacity, also often referred to as water retention capacity, of more than 30, preferably more than 40 or 50, further preferably more than 60 or 70 or 75, further preferably more than 80 or 90, further preferably more than 100, as measured by diluting the MFC samples to a 0.3% solids content in water, and then centrifuging the samples at 1000 G for 15 minutes, after which the clear water phase is separated from the sediment and the sediment is weighed, wherein the water holding capacity is given as (mV/mT).sup.1 wherein mV is the weight of the wet sediment and mT is the weight of dry MFC analyzed, wherein the measurement method is as described in the description.

8. Composition according to any of the preceding claims, wherein the microfibrillated cellulose is a non-modified (native) microfibrillated cellulose, preferably a non-modified microfibrillated cellulose derived from plant material.

9. Process for preparing a starch-based adhesive, or an adhesive based on a starch derivative, which process comprises the steps of: (a) mixing at least one starch and/or at least one starch derivative, or a mixture thereof, with at least one solvent, in particular a solvent comprising or consisting of water; (b) optionally adding one or more additives to the mixture from (a); (c) during or after step (a), or during or after optional step (b): adding microfibrillated cellulose, which is preferably present in a solvent, preferably a solvent comprising or essentially consisting of water; (d) optionally adding alkaline, preferably alkali hydroxide, further preferably NaOH, to the mixture of (a) and/or the mixture of (b) and/or the mixture of (c), in a ratio dry alkaline, preferably alkali hydroxide, further preferably NaOH, preferably in an amount relative to dry starch of from 0.1% w/w to 3% w/w, further preferably 0.5% w/w to 2% w/w; (e) optionally adding a further amount of the at least one solvent, in particular a solvent comprising or consisting of water to the mixture of (b) and/or (c) and/or (d); (f) optionally adding a further amount at least one starch/or at least one starch derivative, or a mixture thereof, and/or a further amount of microfibrillated cellulose to the mixture of (b), (c) and/or (d); (g) dispersing the mixture of (c), (d), (e) or (f) until a homogeneous mixture is obtained.

10. Process according to claim 9, wherein the viscosity of the mixture increases by at least 10%, preferably at least 25%, further preferably by at least 35%, in step (c) and relative to viscosity of the mixture of step (a) or step (b).

11. Process according to claim 9 or claim 10, wherein in step (a), first a primary starch is added to a predetermined amount of solvent until a predetermined viscosity is achieved, then, in or after step (b), a secondary starch i.e. a second amount of the same or of different starch or starch derivative, is added.

12. Use of microfibrillated cellulose as a rheology modifying agent in adhesives comprises starch and/or starch derivatives.

13. Use of the adhesive composition from any of claims 1-8 or of the adhesive composition as obtained according to a process from claims 9-11 for the manufacture of corrugated paperboards/cardboards.

14. Process for making corrugated paperboards or cardboards, said process comprising at least the following steps: providing a starch-based adhesive composition according to any one of claims 1-8, and applying said starch-based adhesives to at least a part of the tips of the flutes of a corrugated piece of paper, on at least one side, preferably on both sides; and in a corrugator, applying at least one liner onto said corrugated piece of paper, preferably applying a further liner on the other side of the corrugated piece of paper, and preparing a single, double, triple or further multiple wall cardboard, preferably in a continuous process.

15. Corrugated paperboards or cardboards having at least one flute and at least one liner comprising the starch-based adhesive composition according to any one of claims 1 to 8,

16. Use of the starch-based adhesive composition of any of claims 1-8 in the manufacture of corrugated paperboards or cardboards.

17. Process or use or corrugated paperboard or cardboard according to any one of claims 9-16, wherein the amount of microfibrillated cellulose in said composition is from 0.001% dry matter, relative to the overall weight of the composition to 10%, preferably from 0.01% dry matter, relative to the overall weight of the composition to 10%, preferably from 0.05% to 5%, further preferably from 0.05% to 2%, most preferred from 0.05% to 0.15%, or wherein the amount of microfibrillated cellulose in said composition is from 0.02% w/w relative to the overall weight of the composition to 8% w/w, further preferably from 0.05% w/w to 0.5% w/w, further preferably from 0.05% w/w to 0.15% w/w and/or wherein the amount of microfibrillated cellulose is from 0.003% w/w to 22% w/w, preferably from 0.02% w/w to 20% w/w, or from 0.04% w/w to 4% w/w or from 0.1% w/w to 2% w/w, further preferably 0.2% w/w to 1.4% w/w, even further preferably 0.2% w/w to 0.6% w/w as measured relative to the overall amount of starch in the adhesive composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0096] In accordance with the present invention and as further specified in STM D 907-82, Standard Definitions of Terms Relating to Adhesives, published in Volume 15.06Adhesives, 1984 Annual Book of ASTM Standards, an adhesive is understood to be a material that is applied to the surfaces of articles to join these surfaces permanently by an adhesive bonding process. An adhesive is a substance capable of forming bonds to each of the two parts when the final object consists of two sections that are bonded together. A particular feature of adhesives is the relatively small quantities that are required compared to the weight of the final objects.

[0097] In accordance with the present invention, a starch (also known as amylum) is a polymeric consisting of a large number of glucose units joined by glycosidic bonds. Starch is found in large amounts in foods such as potatoes, wheat, maize (corn), rice, tapioca and sago, among others. Starch typically comprises two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight.

[0098] While amylopectin can be supplied in cold-water-soluble form, amylose is generally insoluble. Amylose can be dissolved with strong alkali, for example by cooking with formaldehyde or by cooking in water at 150-160 C. under pressure. Upon cooling or neutralization, such amylose dispersions typically form gels at concentrations higher than 2% and will precipitate at concentrations lower than 2%. Amylose fractions are never truly soluble in water and in time will form crystalline aggregates by hydrogen bondinga process known as retrogradation, or setback. Retrogradation is the cause of viscosity instability mentioned above and found to a varying degree in starch-based adhesives. Amylopectin is more soluble and less prone to retrogradation.

[0099] In embodiments of the present invention, the starch preferably is unmodified wheat starch, but can be any of the starches commonly used in the adhesive art, that is, all starches and derivatives, in particular dextrins which contain sufficient available hydroxyl and/or functional groups so that a copolymerization reaction can occur between them and the other two reactants.

[0100] A modified starch is a starch that has been chemically modified, for example by hydrolysis, to allow the starch to function properly under conditions frequently encountered during processing or storage, such as high heat, high shear, high pH, freeze/thaw and cooling. Preferred modified starches in embodiments of the present invention are dextrins.

[0101] Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by -(1.fwdarw.4) or -(1.fwdarw.6) glycosidic bonds. Dextrins can be produced from starch using enzymes like amylases or, for example, by applying dry heat under acidic conditions (pyrolysis). Dextrins produced by heat are also known as pyrodextrins. Dextrins are partially or fully water-soluble and typically yield solutions of low viscosity.

[0102] As outlined above, in a second aspect, the present invention relates to a process for preparing a starch-based adhesive, or an adhesive based on a starch derivative.

[0103] Most starches contain 20-30% by weight of amylose, although certain specialty types can have as little as 0% or as high as 80%. Because of the amylose fraction, starch suspended in cold water is initially unable to act as an adhesive because the starch is so tightly bound in crystalline regions. These granules must be opened through processing to obtain adhesive bonding. Heating in water is the simplest method of breaking up starch granules. On heating in water, starch granules first swell and then burst open with a resulting thickening of the suspension. The temperature at which this thickening of the suspension occurs is called the gelation temperature.

[0104] In embodiments of the present invention, the maximum temperature reached in step (i) is 37 degrees Celsius. In embodiments of the present invention, the maximum temperature reached in step (ii) is 35 degrees Celsius.

[0105] The process may comprise the following. In a first embodiment, salts (preferably the chlorides of metals such as calcium, magnesium and zinc) are added to a suspension of the starch (derivative) in the solvent, and the adhesive is produced by controlling temperature and viscosity by way of controlling the time of stirring.

[0106] In case caustic soda is added to the starch suspension; the product may be neutralized with acid (buffer) later in the process.

[0107] In embodiments of the present invention, (modified) starch-based adhesives are formulated with at least one sodium tetraborate (borax), as added in step (ii) or in step (iii). Borax typically provides good adhesion (tack) and machining properties. Borax is generally added in amounts of up to 10% w/w, based on the weight of the dry starch. Sodium hydroxide is preferably added to convert the borax to the more active sodium metaborate.

[0108] Plasticizers are sometimes used to control brittleness of the adhesive line and to regulate the speed of drying. Common plasticizers include glycerin, glycols, sorbitol, glucose and sugar. These types of plasticizers may act as a hygroscopic agent to decrease the drying rate of the film. Plasticizers based on saps, polyglycols and sulfonated oil derivates lubricate the layers within the dried adhesive and, thus, impart flexibility. Urea, sodium nitrate, salicylic acid and formaldehyde plasticize by forming a solid solution with the dried adhesive. All of these additives, any combination thereof, or only one such additive, may added in step (i) or in step (ii).

[0109] In embodiments of the present invention, further additives may be used, such as calcium chloride, urea, sodium nitrate, thiourea and guanidine salts are used as liquefiers to reduce viscosity. These additives may be added at about 5-20% based on dry starch. Improved cold-water resistance may be achieved by adding polyvinyl alcohol or polyvinyl acetate blends. These adhesives will also dissolve in hot water, which is often a benefit. Optimal moisture resistance may be achieved through the addition of thermosetting resins, such as urea formaldehyde or resorcinol formaldehyde.

[0110] Mineral fillers, such as kaolin clay, calcium carbonate and titanium dioxide, may be added in step (i), step (ii) or after step (iii), to reduce cost and control penetration into porous substrates. These additives may be added at concentrations of 5-50%.

[0111] Other additives that may be added in step (i), step (ii) or after step (iii), include but are not limited to preservatives, bleaches, and defoamers. Preservatives that are preferred to prevent microbial activity include formaldehyde (35% solids) at 0.02-1.0%, copper sulfate at about 0.2%, zinc sulfate, benzoates, fluorides and phenols. Preferred bleaching agents include sodium bisulfite, hydrogen and sodium peroxide, and sodium perborate. Organic solvents may be added to improve adhesion to waxed surfaces.

[0112] As discussed above in the third and fourth aspect, microfibrillated cellulose may be advantageously used to modify the rheology of adhesives, in particular starch based (or starch derivative based) adhesives. This use is particularly advantageous for making corrugated cardboards (boxboards).

[0113] Most corrugated boxboard for making cartons is bonded with starch-based adhesives. A fraction of the starch needed to formulate the adhesive is swelled or gelatinized with aqueous caustic. This is blended with a concentrated suspension of unmodified starch. A typical starch-adhesive formulation also includes borax (to increase wet tack and speed curing), and a small quantity of preservatives (for use as a mold inhibitor). The paste is applied to the corrugated flutes and the liners. Upon subsequent exposure to heat, the starch granules swell and burst, forming a strong bond.

[0114] FIG. 7 schematically depicts a continuous production line for making corrugated cardboard (single facer).

[0115] FIG. 8 schematically depicts a layer of cardboard comprising one layer of corrugated paper having the flutes tips coated with adhesive as well as an upper and a lower liner. A schematic illustration of fluted (corrugated) piece of paper, i.e. a piece of paper that has been brought into contact with heat or steam, or both, on corrugating rolls, in order to have a corrugated (fluted) shape is illustrated, which also shows how to exemplary apply glue to the tips of the flutes. In embodiments of the present invention, the glue may be applied along the entire tip or only along parts thereof. The figure also illustrates an upper and a lower liner as applied onto the upper and lower tips of the fluted paper, called single facer and double backer side of the board, resulting in a single walled cardboard.

[0116] Experiments were run on a corrugator machine from BHS (wet end) and Fosber (dry end) (see Example 3 below) for making corrugated cardboard. These experiments have shown that using a starch based adhesive comprising microfibrillated cellulose (as described below in the Examples Section) leads to the following advantages, among others: [0117] an increase in production speed of up to 40%, while achieving equal or better quality cardboard, thus saving time and facilitating the post process steps due to flatter boards. [0118] an increase in bond strength between the flute and liners of the board.

[0119] This translates to saving time and processing costs [less heat (energy) needed for curing due to less water to evaporate when less adhesive is applied; deduced water impact/defects/warp on the paper during process and post process: achieves flatter cardboards].

[0120] Microfibrillated cellulose (MFC) in accordance with the present invention is to be understood as relating to cellulose fibers that have been subjected to a mechanical treatment resulting in an increase of the specific surface and a reduction of the size of cellulose fibers, in terms of cross-section (diameter) and/or length, wherein said size reduction preferably leads to fibrils having a diameter in the nanometer range and a length in the micrometer range.

[0121] Microfibrillated cellulose (also known as reticulated cellulose or as superfine cellulose, or as cellulose nanofibrils, among others) is a cellulose-based product and is described, for example, in U.S. Pat. Nos. 4,481,077, 4,374,702 and 4,341,807. According to U.S. Pat. No. 4,374,702 (Turbak), microfibrillated cellulose has distinct properties vis--vis cellulose products not subjected to the mechanical treatment disclosed in U.S. Pat. No. 4,374,702. In particular, the microfibrillated cellulose described in these documents has reduced length scales (diameter, fiber length), improved water retention and adjustable viscoelastic properties. MFC with further improved properties and/or properties tailor-made for specific applications is known, among others, from WO 2007/091942 and WO 2015/180844.

[0122] In cellulose, which is the starting product for producing microfibrillated cellulose (typically present as a cellulose pulp), no, or at least not a significant or not even a noticeable portion of individualized and separated cellulose fibrils can be found. The cellulose in wood fibres is an aggregation of fibrils. In cellulose (pulp), elementary fibrils are aggregated into microfibrils which are further aggregated into larger fibril bundles and finally into cellulosic fibres. The diameter of wood based fibres is typically in the range 10-50 m (with the length of these fibres being even greater). When the cellulose fibres are microfibrillated, a heterogeneous mixture of released fibrils with cross-sectional dimensions and lengths from nm to m may result. Fibrils and bundles of fibrils may co-exist in the resulting microfibrillated cellulose.

[0123] In the microfibrillated cellulose (MFC) as described throughout the present disclosure, individual fibrils or fibril bundles can be identified and easily discerned by way of conventional optical microscopy, for example at a magnification of 40, and/or by electron microscopy.

[0124] In principle, any type of microfibrillated cellulose (MFC) can be used in accordance with the present invention, as long as the fiber bundles as present in the original cellulose pulp are sufficiently disintegrated in the process of making MFC so that the average diameter of the resulting fibers/fibrils is in the nanometer-range and therefore more surface of the overall cellulose-based material has been created, vis--vis the surface available in the original cellulose material. MFC may be prepared according to any of the processes described in the art, including the prior art specifically cited in the Background-Section above.

[0125] In accordance with the present invention, there is no specific restriction in regard to the origin of the cellulose, and hence of the microfibrillated cellulose. In principle, the raw material for the cellulose microfibrils may be any cellulosic material, in particular wood, annual plants, cotton, flax, straw, ramie, bagasse (from sugar cane), suitable algae, jute, sugar beet, citrus fruits, waste from the food processing industry or energy crops or cellulose of bacterial origin or from animal origin, e.g. from tunicates.

[0126] In a preferred embodiment, wood-based materials are used as raw materials, either hardwood or softwood or both (in mixtures). Further preferably softwood is used as a raw material, either one kind or mixtures of different soft wood types. Bacterial microfibrillated cellulose is also preferred, due to its comparatively high purity.

[0127] In principle, the microfibrillated cellulose in accordance with the present invention may be unmodified in respect to its functional groups or may be physically modified or chemically modified, or both. In preferred embodiments, the microfibrillated cellulose is non-modified or physically modified, preferably non-modified.

[0128] Chemical modification of the surface of the cellulose microfibrils may be achieved by various possible reactions of the surface functional groups of the cellulose microfibrils and more particularly of the hydroxyl functional groups, preferably by: oxidation, silylation reactions, etherification reactions, condensations with isocyanates, alkoxylation reactions with alkylene oxides, or condensation or substitution reactions with glycidyl derivatives. Chemical modification may take place before or after the defibrillation step.

[0129] The cellulose microfibrils may, in principle, also be modified by a physical route, either by adsorption at the surface, or by spraying, or by coating, or by encapsulation of the microfibril. Preferred modified microfibrils can be obtained by physical adsorption of at least one compound. The MFC may also be modified by association with an amphiphilic compound (surfactant).

[0130] In a preferred embodiment of the present invention, the microfibrillated cellulose as used in step (iii) is prepared by a process, which comprises at least the following steps: [0131] (a) subjecting a cellulose pulp to at least one mechanical pretreatment step; [0132] (b) subjecting the mechanically pretreated cellulose pulp of step (a) to a homogenizing step, which results in fibrils and fibril bundles of reduced length and diameter vis--vis the cellulose fibers present in the mechanically pretreated cellulose pulp of step (a), said step (b) resulting in microfibrillated cellulose; [0133] wherein the homogenizing step (b) involves compressing the cellulose pulp from step (a) and subjecting the cellulose pulp to a pressure drop.

[0134] The mechanical pretreatment step preferably is or comprises a refining step. The purpose of the mechanical pretreatment is to beat the cellulose pulp in order to increase the accessibility of the cell walls, i.e. to increase the surface area.

[0135] A refiner that is preferably used in the mechanical pretreatment step comprises at least one rotating disk. Therein, the cellulose pulp slurry is subjected to shear forces between the at least one rotating disk and at least one stationary disk.

[0136] Prior to the mechanical pretreatment step, or in addition to the mechanical pretreatment step, enzymatic (pre)treatment of the cellulose pulp is an optional additional step that may be preferred for some applications. In regard to enzymatic pretreatment in conjunction with microfibrillating cellulose, the respective content of WO 2007/091942 is incorporated herein by reference. Any other type of pretreatment, including chemical pretreatment is also within the scope of the present invention.

[0137] In the homogenizing step (b), which is to be conducted after the (mechanical) pretreatment step, the cellulose pulp slurry from step (a) is passed through a homogenizer at least once, preferably at least two times, as described, for example, in PCT/EP2015/001103, the respective content of which is hereby incorporated by reference.

EXAMPLES

Example 1

[0138] Preparation of Microfibrillated Cellulose (MFC)

[0139] MFC as used to make the compositions in accordance with the present invention is commercially available and commercialized, for example, by Borregaard as Exilva Microfibrillated cellulose PBX 01-V, based on cellulose pulp from Norwegian spruce (softwood).

[0140] The MFC used in the example was present as a paste, having a solids content of 10%, i.e. the dry matter content of microfibrillated fibers in the MFC paste was 10%, while the remaining 90% were water, which was the sole solvent in this case.

Example 2

[0141] Preparation of a Starch Based Adhesive Comprising Borax (Comparative Example)

[0142] A starch-based adhesive as known from the art was prepared based on the following components and using the following steps: [0143] 750 kg of primary water [0144] 180 kg of primary starch (wheat)

[0145] Stirring for 30 sec, temperature 36.5 C.; add: [0146] 100 kg of water [0147] 16.5 kg Primary caustic soda [0148] 80 kg of water

[0149] Stirring for 30 sec

[0150] Viscosity control 1: 10 sec

[0151] Stirring for 840 sec

[0152] Viscosity control 2: 33.8 sec [0153] 260 kg secondary water [0154] Disinfectant: 0.4 kg [0155] 280 kg secondary starch (wheat)

[0156] Stirring for 30 sec at a temperature of 35 C.

[0157] 2.5 kg of borax

[0158] Stirring for 600 sec

[0159] Viscosity control 3, final: 28 sec

[0160] Borax was added after the addition and mixing of the secondary non-swollen starch. The concentration of borax in the final formulation was 0.15%. The Lory viscosity of this starch-based adhesive according to the art including borax was decreasing readily with mixing time, at high shear.

[0161] Preparation of a Starch Based Adhesive Comprising Microfibrillated Cellulose (in Accordance with the Present Invention)

[0162] A starch-based adhesive in accordance with the present invention was prepared based on the following components and using the following steps: [0163] 750 kg of primary water [0164] 180 kg of primary wheat starch

[0165] Stirring for 30 sec, temperature 36.5 C. [0166] 100 kg of water [0167] 16.5 kg Primary caustic soda [0168] 80 kg of water

[0169] Stirring for 30 sec

[0170] Viscosity control 1: 10 sec

[0171] Stirring for 840 sec

[0172] Viscosity control 2: 33.8 sec [0173] 260 kg secondary water [0174] Disinfectant: 0.4 kg

[0175] Temperature 35 C. [0176] 280 kg secondary wheat starch

[0177] Stirring for 30 sec [0178] 2.5 kg of borax

[0179] Stirring for 60 sec [0180] 20 kg of MFC (Exilva PBX 01-V)

[0181] Stirring for 600 sec [0182] 21 kg of water

[0183] Viscosity control 3, final: 32 sec

[0184] Lory viscosity was 34.

[0185] The adhesive consisted of a primary starch portion in which most of the granules are partially swollen, in which uncooked raw starch was suspended. Microfibrillated cellulose was added under high speed stirring (1500 rpm), after the addition and inmix of the borax. The concentration of MFC in the final formulation was 0.12%.

[0186] Lory viscosity was measured with a Lory viscosity cup (Elcometer 2215/1), which is commonly used in the adhesive, paint and coatings industry and which essentially consists of a conventional cylindrical cup with a needle fixed to the bottom. The cup is first dipped into the adhesive, which then empties through an escape hole. The flow time was measured as soon as the point of the needle was visible.

[0187] Stability Test Over Time

[0188] Both for the reference and the starch-based adhesive with MFC, the Lory viscosity and Brookfield viscosity were measured initially, and over time under laboratory conditions, i.e. at 20 C. and under standard ambient conditions. The samples were left on the bench without stirring. For the reference adhesive, the initial Lory viscosity was 36 seconds. After 1 hour, the viscosity was 137 seconds (critical viscosity), and the reference adhesive could no longer be measured by Lory viscosity without being pre-stirred for 30 seconds by a propeller mixer. After 4 hours, the viscosity of the reference adhesive was too high to be measured by Lory viscosity, even with 30 seconds pre-stirring (see FIG. 1).

[0189] For the starch-based adhesive in accordance with the present invention, i.e. the adhesive with MFC, the initial Lory viscosity was 34 and only increased to 43 seconds 1 and 2 hours after preparation. Moreover, the Lory viscosity was still measurable 22.5 hours after preparation and the critical viscosity limit for measuring Lory viscosity was not reached before 25 hours after preparation. After 25 hours, pre-stirring with propeller mixer for 30 seconds had to be performed before the measurements. The final measurement of Lory viscosity was performed 94 hours after the adhesive was prepared (see FIG. 2).

[0190] Brookfield viscosity measurements for the reference starch-based adhesive and the starch-based adhesive with MFC, likewise show a slower increase in viscosity over time with MFC added to the starch-based adhesive (see FIGS. 1 and 2). Brookfield viscosity was measured with Brookfield ViscometerRVT model, spindle no. 4.

[0191] Overall, the viscosity measurements consistently demonstrate that the starch-based adhesive comprising microfibrillated cellulose is far more stable in regard to viscosity and over time than the reference starch-based adhesive without microfibrillated cellulose.

Example 3

[0192] Testing the Starch Based Adhesive in Accordance with the Present Invention in Corrugated Cardboards

[0193] The Lory viscosity and temperature for the starch-based adhesive with MFC were also measured over time in the storage tank, see FIG. 3. To prevent sedimentation and reduce the viscosity of the starch-based adhesives, the glues are stirred for 5 minutes every hour. For the starch-based adhesive with MFC the sufficient time between the stirring was tested: The first 24 hours of storage, the adhesive was stirred for 5 minutes every hour, after 24-48 hours the stirring was 5 minutes every third hour, and from 48-72 hours the adhesive was stirred for 5 minutes every fourth hour. Compared to the reference starch-based adhesive, the frequency of stirring during storage was significantly reduced for the adhesive with MFC.

[0194] The Lory viscosity of the starch-based adhesive with MFC was measured to be 48 seconds after 72 hours storage in tank and the starch-based adhesive could be used directly without adjustment with water for the production of corrugated boards. The temperature of the starch-based adhesive in the tank was 37 C. (see FIG. 3).

[0195] Both the starch-based adhesive with MFC (72 hours) and the reference starch-based adhesive (fresh) were tested on quality BB25b.

TABLE-US-00001 TABLE 1 Standard tests Adhesion Conditions Grammage strength 23 C. 50 RH % g/m.sup.2 N/m ISO 187 1SO 536 Fefco nr.11

[0196] As for making corrugated cardboards, a corrugator from BHS (wet end) and Fosber (dry end) was used, which is a set of machines designed to bring together several sheets of paper to form single, double or triple wall board in a continuous process. The process starts with a paper sheet conditioned with heat and steam on corrugating rolls in order to be given its fluted shape in the single facer.

[0197] Starch-based adhesive is then applied to the tips of the flutes on one side and the inner liner is glued to the fluting (see FIGS. 7 and 8 for a schematic depiction of such a process). The corrugated fluting medium with one liner attached to it (single facer) is then brought to the double backer where the outer liner is glued to the single facer.

[0198] FIG. 4 shows a comparison of the grammage and adhesion strength of corrugated boards, using the reference starch-based adhesive run at 219 m/min (left column) compared to the starch-based adhesive with MFC run at 300 m/min (right column, respectively).

[0199] It is noteworthy that the reference adhesive tested was a fresh glue made the same day as the corrugated boards production, while the glue with MFC was 72 hours old and was used with no addition of water.

[0200] It can be seen from FIG. 4 that the starch-based adhesive containing MFC provides greater adhesion strength to the corrugated boards (on both sides, inner and outer liner, respectively RV and LV), even when the production is run 37% faster. Since the grammage of the cardboard was similar for both adhesives, the improvement of the adhesion strength can be compared and improvements can be attributed to the better performance of the starch-based adhesive with MFC. It was also observed that the boards produced with the MFC starch-based adhesive were flatter than the boards produced with the reference starch adhesive.

[0201] Overall, the viscosity of the starch-based adhesive with MFC is unexpectedly stable over a long period of time, in particular during storage (at least 72 hours) contrary to a starch-based adhesive without MFC, the viscosity of which increases dramatically already after 1 hour.

[0202] Moreover, the starch-based adhesive with MFC is usable for corrugated board production even after 72 hours storage and performs even better than a fresh made reference at high speed production. Therefore production can be run at faster speeds, while better quality and flatter boards are obtained.

[0203] Finally, as can be seen from FIG. 5 (upper curve: starch based adhesive with borax and microfibrillated cellulose; lower curve: starch-based adhesive with borax but no microfibrillated cellulose) and from FIG. 6 (left column: no microfibrillated cellulose), using microfibrillated cellulose as an additive increases the storage modulus of the adhesive (measured by amplitude sweep at 25 C.).

Example 4

[0204] The Effect of MFC Concentration on the Gelatinization Speed and Storage Modulus of the Cured Adhesive

[0205] FIG. 9 and FIG. 10 show the effect of MFC concentration on the gelatinization speed of the starch adhesive, and storage modulus of the cured adhesive. The solid content and caustic soda concentration are equal for the three glues, which do not comprise borax. The MFC content varies from 0.05 to 0.25 w-% of the overall adhesive composition. The higher the MFC concentration, the higher the storage modulus of the cured adhesive and the stronger the cured adhesive becomes (see FIG. 10), which clearly demonstrates that the microfibrillated cellulose in concentrations up to 0.25% w/w is contributing to an increased bond strength. In addition to that, the higher the concentration of MFC, the slower the gelatinization speed and the longer the open time of the adhesive is (see FIG. 9). The advantage of a long open time in a full scale production is that there is more time to adjust for warps on the boards which results in flatter and more stable boards. Furthermore, the longer the open time, the more time for the secondary starch to fully gelatinize, and for the formation of a strong entangled microfibrillar cellulose-starch gel network. In fact, the MFC concentration can be varied to control the bond strength of the adhesive as well as its open time, allowing for a better control of the warps and an overall better quality of the corrugated boards.

Example 5

[0206] The Rheology and Viscosity Stabilizing Effect of MFC Upon High Shearing Impact

[0207] The microfibrillated cellulose is providing an extremely high shear stable viscosity, here shown for a Stein-Hall starch adhesive comprising 0.1% MFC and no borax (FIG. 11 B). After an instant increase in viscosity upon addition of MFC, the viscosity remains constant under high shear stirring for 15 minutes (FIG. 11 B). In comparison, the viscosity of the reference adhesive comprising 0.3% borax (FIG. 11 A), decreased by 27% after 15 minutes of high shear stirring. When preparing a starch adhesive with MFC, a target viscosity of the adhesive can be predetermined and achieved, regardless of stirring time during manufacturing, before it is used on the corrugator or transferred to the storage tank. Having a viscosity stable starch adhesive, which is provided by MFC, the corrugator can be run with the same settings for the adhesive over time, facilitating a continuous production and high production volumes of corrugated boards. This Example shows that MFC can be used advantageously to replace parts or all of the borax as typically used as an additive in starch based adhesives.