MICROFIBRILLATED CELLULOSE FOR CONTROLLING VISCOSITY AND GEL TEMPERATURE IN STARCH-BASED ADHESIVES

Abstract

The present invention relates to starch-based adhesive compositions comprising microfibrillated cellulose (“MFC”). In addition to microfibrillated cellulose, these adhesive compositions comprise at least one starch and/or at least one starch derivative. The adhesive compositions have a reduced amount of caustic soda vis-a-vis adhesive compositions that do not comprise MFC as an additive. Borax or boric acid (or any derivative thereof) may be partially or fully replaced with microfibrillated cellulose, wherein MFC (contrary to borax) surprisingly does not significantly affect the gel temperature of the overall adhesive.

Claims

1. An 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 of a total weight of the adhesive composition; at least one solvent in an amount of 30% w/w to 95% w/w, of the total weight; microfibrillated cellulose in an amount of 0.001% w/w to 10% w/w of the total weight; alkaline in a total amount that is from 0.05% w/w to 0.8% w/w of the adhesive composition and/or in a total amount of 0.1% w/w to 2.7% w/w as measured relative to an overall amount of starch in the adhesive composition.

2. The composition according to claim 1, wherein the amount of microfibrillated cellulose in said composition is from 0.02% w/w relative to the total weight to 8% w/w and/or wherein the amount of microfibrillated cellulose is from 0.02% w/w to 20% w/w as measured relative to the overall amount of starch in the adhesive composition.

3. The composition according to claim 1, wherein the overall amount of starch in said composition is from 15% w/w to 50% w/w of the total weight of the adhesive composition.

4. The composition according to claim 1, wherein the at least one starch and/or at least one starch derivative is a native starch, or a chemically or a physically modified starch, or a mixture thereof.

5. The composition according to claim 1, wherein the pH value of the adhesive composition is from 8 to 14.

6. The composition according to claim 1, wherein said composition comprises no or only trace amounts of boric acid, or derivatives thereof.

7. The composition according to claim 1, wherein the microfibrillated cellulose (MFC) is characterized in that it results in gel-like dispersion that has a zero shear viscosity, η.sub.0, of at least 2000 Pa*s as measured in a solvent of polyethylene glycol (PEG), and at a solids content of the MFC of 0.65%.

8. The composition according to claim 1, wherein the microfibrillated cellulose (MFC) is characterized by a water holding capacity of more than 30 as measured by diluting MFC samples to a 0.3% solids content in water, and then centrifuging the samples at 1000 G for 15 minutes, after which a resulting clear water phase is separated from a sediment and the sediment is weighed, wherein the water holding capacity is given as (mV/mT).sup.−1 wherein mV is a weight of wet sediment and mT is a weight of dry MFC analyzed.

9. A process for preparing a starch-based adhesive, or an adhesive based on a starch derivative, the process comprising 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 comprising water; (b) during or after step (a): adding microfibrillated cellulose; (c) adding alkaline to the mixture of (a) and/or the mixture of (b); (d) dispersing the mixture of (c) until a homogeneous mixture is obtained.

10. The process according to claim 9, wherein a viscosity of the mixture increases by at least 10% in step (b) and relative to viscosity of the mixture of step (a).

11. The process according to claim 9, wherein no or only trace amounts of boric acid, or derivatives thereof, are added in any of steps (a) through (d).

12. The process according to claim 9, 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 (a), a secondary starch is added, wherein the secondary starch is the same or a different starch or starch derivative as the primary starch.

13. A method comprising using microfibrillated cellulose as a gel temperature stabilizing agent and/or as a viscosity stabilizing agent in an adhesive comprising a starch and/or starch derivative.

14. (canceled)

15. An starch-based adhesive composition comprising microfibrillated cellulose as a partial or complete replacement for boric acid or any derivative thereof the composition has an increased viscosity vis-a-vis an otherwise same composition that comprises boric acid or a derivative thereof in the same amount as the microfibrillated cellulose.

16. The adhesive composition according to claim 15, wherein (i) the composition has a gel temperature that does not differ from a gel temperature of an otherwise same adhesive composition without either boric acid, or a derivative thereof, nor MFC, by more than 2 K or (ii) the adhesive composition has a gel temperature lowered by at least 1 K relative to an otherwise same composition that comprises boric acid or a derivative thereof in the same amount as the microfibrillated cellulose.

17-18. (canceled)

19. A corrugated paperboard or cardboard having at least one flute and at least one liner and comprising an adhesive composition according to claim 1.

20. (canceled)

21. The process according to claim 9, wherein the amount of microfibrillated cellulose in said composition is from 0.001%% w/w, relative to the total weight of the composition to 10% w/w and/or the amount of microfibrillated cellulose is from 0.02% w/w to 20% w/w as measured relative to the overall amount of starch in the adhesive composition.

22. The adhesive composition of claim 1, wherein the alkaline is alkali hydroxide and/or NaOH.

23. The adhesive composition of claim 1, wherein the alkaline is in a total amount that is from 0.1% w/w to 0.5% of the adhesive composition and/or in a total amount of 0.3% w/w to 1.5% as measured relative to the overall amount of starch in the adhesive composition.

24. The adhesive composition of claim 2, wherein the amount of microfibrillated cellulose in said composition is from 0.05% w/w relative to the total weight to 2% w/w and/or wherein the amount of microfibrillated cellulose is from 0.1% w/w to 2% w/w as measured relative to the overall amount of starch in the adhesive composition.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0124] In the following, the present invention, and aspects thereof, is/are illustrated by way of Figures, which are not meant to limit the invention but only to illustrate the same.

[0125] FIG. 1 shows the bond strength (according to the pin adhesive test, PAT) for the same board quality processed with, on the one hand, the MFC starch adhesive in accordance with the invention and, on the other hand, the reference (only) borax adhesive, not in accordance with the invention, i.e. not comprising any MFC;

[0126] FIG. 2 shows the edge crush test (ECT) and the torsional strength (stiffness) for the same board quality processed with, on the one hand, the MFC starch adhesive in accordance with the invention and, on the other hand the reference borax adhesive (no MFC) not in accordance with the invention;

[0127] FIG. 3 shows the viscosity behavior (lack of stability) of an adhesive not in accordance with the invention, i.e. a Minocar native wheat starch adhesive as known from the art, which comprises borax as additive, but not MFC;

[0128] FIG. 4 shows the viscosity behavior of an adhesive in accordance with the invention, i.e. a Minocar native wheat starch adhesive, which comprises MFC as an additive;

[0129] FIG. 5 shows the Lory viscosity and the temperature of a starch based adhesive in accordance with the present invention (i.e. comprising MFC) over time during storage (in a storage tank);

[0130] FIG. 6 shows a comparison of the grammage and adhesion strength of an adhesive in accordance with the present invention, as used in corrugated cardboards (right column, respectively) compared to an adhesive known from the art (left column, respectively);

[0131] FIG. 7 shows how using MFC as an additive in starch based adhesives increases the storage modulus of the uncured adhesive;

[0132] FIG. 8 shows that MFC is a more efficient thickener for a starch adhesive than borax;

[0133] FIG. 9 shows how adding MFC to the starch adhesive, and removing borax altogether, stabilizes the gel temperature between 56° C. and 58° C., even when varying the MFC content from 0.1% dry matter to 0.5% dry matter, at a similar NaOH content, respectively;

[0134] FIG. 10 shows the effect of NaOH on the gel temperature of a starch adhesive with MFC and the difference in gel temperature between a MFC starch adhesive vs a borax starch adhesive prepared according to the Stein-Hall process with native corn starch;

[0135] FIG. 11 shows the dependence of the gel temperature on the NaOH content for different types of corn and wheat starches comprising MFC.

[0136] FIG. 12 shows the effect of MFC on the gel temperature and the storage modulus of the starch adhesive prepared with modified wheat starch and according to Corrtech process;

[0137] FIG. 13 shows the effect of MFC and borax on the storage modulus and curing profile of the starch adhesives in dependence of the temperature;

[0138] FIG. 14 shows the effect of the MFC concentration on the gelatinization speed of the starch adhesive (Stein-Hall, native wheat);

[0139] FIG. 15 shows the effect of the MFC concentration on the storage modulus of the cured starch adhesive (Stein-Hall, native wheat);

[0140] FIG. 16 schematically depicts a continuous production line for making corrugated cardboard (single facer);

[0141] FIG. 17 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;

[0142] FIG. 18 shows the effect of MFC on the bond strength of the double backer side of BB24b quality corrugated boards compared to the bond strength of the borax reference boards; the boards with the MFC starch (Stein-Hall) adhesive are produced at higher production speed than the reference boards with borax starch (Stein-Hall) adhesive;

[0143] FIG. 19 shows the effect of MFC on glue consumption for production of BB24b quality corrugated boards compared to the consumption of the borax reference adhesive run at lower production speed;

[0144] FIG. 20 shows the effect of MFC on the thickness, edgewise crush resistance and torsion strength (stiffness) of the corrugated (BB24b) boards, compared to the reference adhesive comprising borax run at lower production speed:

DETAILED DESCRIPTION OF THE INVENTION

[0145] 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 15.06—Adhesives, 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.

[0146] In accordance with the present invention, a starch (also known as “amylum”) is a polymer 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, pea, 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.

[0147] 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 bonding—a 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.

[0148] In embodiments of the present invention, the starch preferably is unmodified wheat starch or corn starch, but may be any of the starches commonly used in the adhesive art, that is, all starches and derivatives which contain sufficient available hydroxyl and/or functional groups so that a copolymerization reaction can occur between them and the other two reactants.

[0149] 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.

[0150] 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 α-(1.fwdarw.4) or α-(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.

[0151] 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.

[0152] 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.

[0153] In embodiments of the present invention, the maximum temperature reached in step (a) is 42 degrees Celsius. In embodiments of the present invention, the maximum temperature reached in step (e) is 32 degrees Celsius.

[0154] 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.

[0155] Since alkaline, in particular caustic soda is added to the starch suspension; the suspension may be neutralized with acid (buffer) later in the process.

[0156] 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 be added in step (a) or in step (b), or in step (c), or in step (d), or after any of these steps.

[0157] 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.

[0158] Optimal moisture resistance may be achieved through the addition of thermosetting resins, such as urea formaldehyde or resorcinol formaldehyde.

[0159] Mineral fillers, such as kaolin clay, calcium carbonate and titanium dioxide, may be added in step (a), step (b), step (c), step (d), step (e), step (f) or in step (g), or after any of these steps, or in any combination or in all these steps, in order to reduce cost and control penetration into porous substrates. These additives may be added at concentrations of 5-50%.

[0160] Other additives that may be added in step (a), step (b), step (c), step (d), step (e), step (f) or in step (g), or after any of these steps, or in any combination or in all these steps, 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.

[0161] As discussed above, microfibrillated cellulose may be advantageously used to modify the rheology of adhesives, in particular starch based (or starch derivative based) adhesives, and in particular to control the gel temperature. The possibility to control the gel temperature is of particular importance for making corrugated cardboards (boxboards), as the adhesive needs to be applied to various layers, in particular on to the flutes peaks of the corrugated paper layer, in a controlled manner, followed by a gelatinization reaction to form a strong bond between the papers. If the gelatinization happens to slow or to rapid, the bonding and quality of the boards will be poor, due to insufficient (too low gel temperature) or excessive (too high gel temperature) penetration of the glue into the papers. Furthermore, with microfibrillated cellulose, the gel temperature is stable with storage time, which is of high importance for the production of corrugated boards, whereas it increases the production quality and capacity, when the same glue can be run on Monday, as on Friday, with the same corrugator settings.

[0162] Most corrugated cardboard (boxboard) for making cartons is bonded with starch-based adhesives. A fraction of the starch needed to formulate the adhesive (called the carrier) is swelled or gelatinized with aqueous caustic. This mixture is blended with a concentrated suspension of raw starch. The paste is applied onto the corrugated flutes and the liners are attached. Upon subsequent exposure to heat, the starch granules swell and burst, forming a strong bond.

[0163] Experiments on a line for making corrugated cardboard (see below) 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: [0164] an increase in production speed of up to 37%, while achieving equal or better quality cardboard, thus saving time and facilitating the post process steps due to flatter boards. [0165] an increase in bond strength between the flute and liners of the board. [0166] a reduction in the amount of adhesive required to arrive at cardboards that have the same strength and performance as cardboards using conventional adhesives (that comprise borax)

[0167] 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].

[0168] “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.

[0169] 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-à-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.

[0170] 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 μm (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 μm may result. Fibrils and bundles of fibrils may co-exist in the resulting microfibrillated cellulose.

[0171] 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×, or by electron microscopy.

[0172] 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-à-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.

[0173] 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.

[0174] 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.

[0175] In principle, the microfibrillated cellulose in accordance with the present invention may be unmodified (non-modified) 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.

[0176] 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.

[0177] 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).

[0178] However, in preferred embodiments, the microfibrillated cellulose is not physically modified.

[0179] In a preferred embodiment of the present invention, the microfibrillated cellulose of any embodiment as disclosed above is prepared by a process, which comprises at least the following steps: [0180] (1) subjecting a cellulose pulp to at least one mechanical pretreatment step; [0181] (2) subjecting the mechanically pretreated cellulose pulp of step (1) to a homogenizing step, which results in fibrils and fibril bundles of reduced length and diameter vis-à-vis the cellulose fibers present in the mechanically pretreated cellulose pulp of step (1), said step (2) resulting in microfibrillated cellulose; [0182] wherein the homogenizing step (2) involves compressing the cellulose pulp from step (1) and subjecting the cellulose pulp to a pressure drop.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] In the homogenizing step (2), 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

[0187] Preparation of Microfibrillated Cellulose (MFC)

[0188] 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).

[0189] The MFC used in the example was present as a paste, having a solids content of 9-11%, i.e. the dry matter content of microfibrillated fibers in the MFC paste was 9-11%, while the remaining ca. 90% were water, which was the sole solvent in this case.

Example 2

[0190] Preparation of a Stein-Hall Starch Adhesive Comprising Borax (Comparative Example)

[0191] A starch-based adhesive as known from the art was prepared based on the following components and using the following steps: [0192] 400 kg of primary water [0193] 42 kg of primary starch (native wheat)

[0194] Stirring for 15 sec, temperature 42° C.; add: [0195] 70 kg of water [0196] 19 kg Primary caustic soda (31%)

[0197] Stirring for 1200 sec [0198] 650 kg secondary water [0199] Disinfectant: 2 kg [0200] Temperature of 32° C. [0201] 400 kg secondary starch (native wheat)

[0202] Add 4.3 kg of borax

[0203] Stirring for 1100 sec

[0204] Viscosity control final: 40 sec.

[0205] The ratio NaOH/starch was 1.3% w/w.

[0206] The Lory viscosity was measured with a Lory viscosity cup (here: Elcometer 2215), which comprises 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.

[0207] Borax was added after the addition and mixing of the secondary non-swollen starch. The concentration of borax in the final formulation was 0.27%. The Lory viscosity of this starch-based adhesive according to the art including borax was decreasing readily with mixing time, at high shear.

[0208] Preparation of a Stein-Hall Starch Adhesive Comprising Microfibrillated Cellulose (in Accordance with the Present Invention)

[0209] The process for preparing a starch-based corrugated paperboard adhesive, comprising MFC, is given below. The viscosities at different process steps were measured online by a viscometer, and controlled manually by measuring the Lory viscosity.

[0210] An adhesive in accordance with the present invention was prepared based on the following ingredients and manufactured according to the following steps: [0211] 400 kg of primary water [0212] 55 kg of primary starch (native wheat)

[0213] Stirring for 15 sec, temperature 42° C.; add: [0214] 70 kg of water [0215] 12 kg Primary caustic soda (31%)

[0216] Stirring for 1200 sec [0217] 630 kg secondary water [0218] Disinfectant: 2 kg

[0219] Temperature of 32° C. [0220] 400 kg secondary starch (native wheat)

[0221] 20 kg of microfibrillated cellulose (Exilva PXB 01-V)—9% dry content

[0222] Stirring for 1100 sec

[0223] Viscosity control final: 40 sec

[0224] The pH of the final adhesive was 11.9. The caustic soda concentration required was reduced by 37% compared to the reference borax adhesive. The ratio NaOH/starch was 0.8% w/w.

[0225] Unless explicitly stated otherwise, all measurements as described herein were taken at standard laboratory conditions, i.e. a temperature of 25° C., an ambient pressure of standard pressure and at an ambient humidity of 50%.

[0226] The adhesive consisted of a primary starch portion, the carrier, in which most of the granules were fully swollen, in which uncooked raw starch was suspended.

[0227] Microfibrillated cellulose was added under high speed stirring (1500 rpm), after the addition and in mix of the secondary portion of unswollen starch. Microfibrillated cellulose was easily dispersed in the mixture. The concentration of MFC (dry) in the final formulation was 0.12%. The dry mass fraction of the MFC crosslinker was 0.4% (the ratio polymer to MFC as crosslinker was 228 to 1).

[0228] Based on the presence of Microfibrillated Cellulose, the Lory viscosity of the adhesive did remain stable, and did not drop with increasing mixing time at high shear. Under alkaline conditions as present, the MFC was cross-linking the starch polysaccharide by hydrogen bonds and additionally stabilizes the mixture by forming a physical network composed of entangled fibrils, thus protecting the starch from high shear degradation, and also from further reaction by caustic soda.

[0229] The Stein-Hall starch adhesive in accordance with the present invention was tested in the production of corrugated boards HB26c (c-flute) quality, on both the single facer (SF) and the double backer (DB) sides of the boards, and the properties of the boards were compared with the borax reference board properties which were produced the same day. The corrugator settings were equal in both trials. The production speed and the gap were 205 m/min and 0.13 mm respectively, for the processes with both starch adhesives. The gel temperature of the MFC adhesive in accordance with the present invention with reduced caustic soda concentration was 56° C., whereas the gel temperature for the borax reference was 54° C. The solid contents were 26.3 and 24%, respectively.

[0230] The bond strength measured with the Pin Adhesion Test (PAT) was considerably higher for the boards with the MFC starch adhesive compared to the borax reference boards, see FIG. 1. In fact, with MFC an increase in bond strength for the double backer of 22% was achieved, whereas an increase of 5% was measured for the single facer side of the boards (FIG. 1). Moreover, the edgewise crush resistance (ECT) and torsional strength were somewhat higher for the boards with the MFC starch adhesive, compared to the borax reference adhesive, see FIG. 2. PAT and ECT were measured according to the standard references given in Table 4. The torsional strength/stiffness (TSmd, bpi) was measured according to GTm34024.

Example 3

[0231] Adhesive Stability Over Time: Laboratory Test

[0232] Both for a Minocar (native wheat) starch adhesive comprising 0.15% borax (reference adhesive) and the same adhesive with the addition of 0.12% MFC (adhesive in accordance with the invention), 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. 3).

[0233] 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 measureable 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. 4).

[0234] 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. 3 and 4). Brookfield viscosity was measured with Brookfield Viscometer—RVT model, spindle no. 4.

[0235] 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.

[0236] Adhesive Stability Over Time: Testing the Starch Based Adhesive in Accordance with the Present Invention in Corrugated Cardboards

[0237] The Lory viscosity and temperature for the starch-based adhesive with MFC were also measured over time in the storage tank, see FIG. 5. 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.

[0238] 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. 5).

[0239] Both the starch-based adhesive with MFC (72 hours) and the reference starch-based adhesive (fresh) were tested on quality BB25b (180 g/m.sup.2 EK liner/110 g/m.sup.2 SC fluting/180 g/m.sup.2 EK liner).

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

[0240] As for making corrugated cardboards, a corrugator 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.

[0241] 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. 16 and 17 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.

[0242] FIG. 6 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).

[0243] 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.

[0244] It can be seen from FIG. 6 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.

[0245] 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.

[0246] 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.

[0247] The stable viscosity of the adhesive comprising MFC also means that there is no need for additional additions of water over time, since the adhesive quality remains intact. This also means that the solid content remains unchanged and the adhesive can be formulated and kept over a long period of days, for example for over several days or weeks. This opens up the possibility of a continuous adhesive production line.

[0248] Finally, as can be seen from FIG. 7 (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

[0249] Effect of MFC on the Lory Viscosity of the Starch Adhesive Prepared According to Stein-Hall Process with Corn Starch

[0250] FIG. 8 shows the effect of MFC on the Lory viscosity of the starch adhesive, and how MFC is a more efficient thickener than borax. The starch adhesives were prepared according to Stein Hall process with native corn starch with equal amount of caustic soda or NaOH (0.5 w-% of the overall adhesive composition) and equal solid content (25 w-% of the overall adhesive composition) for all adhesives. In order to achieve the same viscosity as the borax starch adhesive, 72% less (dry) MFC is needed.

[0251] Effect of MFC on the Gel Temperature for Stein-Hall Corn Adhesives

[0252] As can be seen from FIG. 9, adding MFC as an additive (and removing borax) stabilizes the gel temperature of the adhesive composition between 56° C. and 58° C., even when varying the MFC content from 0.1% dry matter to 0.5% dry matter. The caustic soda concentration was equal for all of the adhesive compositions. The reference starch adhesive (open triangle in the Figure), with the same amount of caustic soda and 0.5% borax (but no MFC) had a gel temperature of 61° C.

[0253] As can be seen from FIG. 9, varying the MFC content in this concentration range does not significantly affect the gel temperature of the starch adhesive. This is important, since in adhesive processing, the gel temperature is a key process parameter that needs to be controlled and that should not vary to any significant extent.

[0254] As can be seen from FIG. 10, for a starch adhesive comprising 0.1% MFC (and no borax), the gel temperature of the adhesive is essentially solely a function of the caustic soda (NaOH) concentration in the formulation. By using linear regression, the concentration of caustic soda providing the desired gel temperature of the adhesive can be easily calculated. The MFC in the formulation will furthermore stabilize the gel temperature of the adhesive under process and during storage.

[0255] The Dependence of Gel Temperature on NaOH Content for Different Types of Corn and Wheat Starches, Native and Modified, Comprising MFC.

[0256] FIG. 11 shows the gel temperature of starch adhesives prepared with different kinds of starches, different solid contents and all of them comprising MFC. FIG. 11 shows that no matter the process type, the recipe parameters or the nature of starch, there is always a linear relationship between the gel temperature and the caustic soda content. As a result, it is always possible to calculate the amount of caustic soda needed for the desired gel temperature, based on linear regression, when using MFC (and replacing at least some, preferably most borax) in the starch adhesive.

Example 5

[0257] The determination of the curing temperature of the starch adhesive with borax and the starch adhesive with MFC was performed on a rheometer (Anton Paar Physica MCR 102). A concentric cylinder geometry was used. To determine the curing temperature, a temperature sweep from 25° C. to 70° C. was performed in the linear viscoelastic region i.e. at a deformation of 0.1% and frequency of 1 Hz. The storage modulus was measured as a function of temperature. The gel temperature was determined as the temperature of the onset of the steep increase in the storage modulus.

[0258] FIG. 12 shows that the adhesive containing MFC as a crosslinker has a lower curing temperature than the adhesive containing borax as a crosslinker. The caustic soda concentration as well as the solid content are equal in both glues. With MFC replacing borax, the caustic soda amount is reacting with the starch units, whereas MFC is stabilizing the starch both physically and chemically, thus stabilizing the viscosity and gel temperature of the adhesive.

[0259] It is believed that the formation of the MFC-starch gel-network is the result of strong intermolecular hydrogen bonds as well as physical entrapment of the starch molecules by entangled fibrils network.

[0260] After curing of the starch adhesives, this new gel-structure introduced by MFC was evident, when it was found that the gel is more malleable and a softer textured gel compared to the borax crosslinked starch that is a self-standing and brittle viscoelastic hydrogel once it is cooled down. A prolonged open time for adjustments of warps and water defects during the corrugating production process provided by MFC is beneficial, enhancing the quality of the corrugated boards.

[0261] FIG. 12 furthermore shows the effect of MFC on the gel temperature and the storage modulus of the starch adhesive prepared with modified wheat starch and according to Corrtech process. The gel temperature of the starch adhesive comprising 0.21% MFC of the overall adhesive composition is lower than that of the starch adhesive comprising 0.28% borax, at similar caustic soda (0.36% caustic soda of the overall adhesive composition) and starch content (28% of the overall adhesive composition). The speed of gelatinization that is illustrated by the slope of the curve is more gentle for the starch adhesive comprising MFC meaning that the latter will take slightly more time to cure compared to the starch adhesive comprising borax. The advantage is that the release of water will be slower and thus the open time longer resulting in a more controlled drying time and more stable boards.

[0262] In addition to that, the storage modulus of the cured starch adhesive comprising MFC is higher than that of the starch adhesive comprising borax, indicating that the slower release of water for the MFC-starch adhesive upon heat, may improve the gelatinization of the secondary starch, which together with the microfibrillated cellulose are providing a stronger adhesion.

[0263] The effect of MFC was further investigated by comparing the starch adhesive comprising neither borax nor MFC with the starch adhesive comprising MFC and the starch adhesive comprising borax (FIG. 13). The caustic soda content is the same in the starch adhesive comprising MFC and the starch adhesive comprising borax. However the caustic soda content is lower than the one used in the preparation of the adhesives presented in FIG. 12 (0.20% against 0.36% of the overall adhesive composition). This study confirms that upon application of heat, MFC somewhat prolonges the time for the gelatinization reaction of the starch adhesive, whereas borax shortens it, even at this lower caustic soda content. In addition to that, the storage modulus of the cured starch adhesive comprising MFC has increased by 27% at this lower caustic soda content. In fact FIG. 13 clearly demonstrates that MFC, unlike borax, also increases the storage modulus of the adhesive in the liquid state (before curing) and hence is building a stronger network structure compared to the borax reference adhesive.

Example 6

[0264] The Effect of MFC Concentration on the Gelatinization Speed and Storage Modulus of the Cured Adhesive

[0265] FIG. 14 and FIG. 15 show the effect of MFC concentration on the gelatinization speed of the starch adhesive, and storage modulus of the cured adhesive. Caustic soda concentration and solid content are equal for the three glues. The MFC content is varying from 0.05 to 0.25 w-% of the overall adhesive composition.

[0266] The higher the MFC concentration, the higher the storage modulus of the cured adhesive and the stronger the cured adhesive becomes (see FIG. 15), 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. 14). 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 7

[0267] Comparison of a Minocar Native Wheat Adhesive with MFC to the Borax Reference

[0268] After 24 hours of storage, the viscosity and solid content of the MFC (0.12% w/w of total formulation) starch adhesive and the borax (0.15% w/w of total formulation) reference adhesive were measured, and the values are given in Table 2. The caustic soda concentration was the same for both adhesives, 0.3% w/w on total formulation.

TABLE-US-00002 TABLE 2 Properties of starch adhesives crosslinked by either borax (reference adhesive) or MFC, after 24 hours of storage. Initial viscosities were 28 and 29 sec., respectively. Solid Temp. Visc. Brookfield Txt. content Sample (° C.) (sec.) (mPa .Math. s) (B/v) (%) Ref. adhesive 32 34 1740 51 25.5 with borax Adhesive with 33 29 1160 40 24.6 MFC

[0269] The starch adhesive with MFC had a stable viscosity after 24 hours storage at 37° C., and could be used as it was for cardboard production with no extra addition of water. During storage, the adhesive with MFC was stirred for 5 minutes every 4.sup.th hr. In contrast, the starch adhesive with borax was less stable, and showed an increase in viscosity from 28 seconds to 34 seconds after 24 hours storage and had to be stirred for 5 minutes every hour to reduce the viscosity of the adhesive and prevent sedimentation.

[0270] Both adhesives, the starch adhesive crosslinked by MFC and the starch adhesive crosslinked by borax, were used on corrugated boards BB25c quality. Both adhesives were run with the same process parameters. For the starch-MFC adhesive the production was run at normal speed as well as high speed (see Table 3).

TABLE-US-00003 TABLE 3 Overview of process parameters for running corrugated board BB25c quality. Sample Layer Speed m/min Ref. adhesive with borax Single Facer 207 (Inner Liner) Adhesive with MFC Single Facer 207 (Inner Liner) Adhesive with MFC Single Facer 250 (Inner Liner)

[0271] Samples were analyzed in the laboratory according to the standard test methods given in Table 4.

TABLE-US-00004 TABLE 4 Standard references. Water Conditions Grammage Thickness absorption Humidity 23° C. - g/m.sup.2 m.m. Cobb.sub.60 - g/m.sup.2 % 50RH % ISO 187 ISO 536 ISO 3034 ISO 535 ISO 287 Bursting Edge wise PAT Bending Box strength crush resistance compression resistance kPa kN/m N/m Md/cd - Nm BCT - N ISO 2759 ISO 3037 Fefco nr.11 ISO 2493 ISO 12048

[0272] The adhesive with MFC crosslinked starch gave flatter corrugated boards. The pin adhesion method (PAT) was used to measure the adhesion strength between the flutes and liners of corrugated board. In particular, the starch-MFC adhesive gave better bonding strength of the boards, both at similar and higher production speeds. (Adhesion strength of the MFC adhesive compared to the reference borax adhesive measured by pin adhesion test (PAT) on BB25c boards run at 207 and 250 m/min.).

Example 8

[0273] Testing the Adhesives Prepared According to a Stein-Hall Process on Native Wheat on Corrugated Boards

[0274] The starch adhesive crosslinked by MFC (0.13% w/w on total formulation) and the starch adhesive crosslinked by borax (reference, 0.27% w/w on total formulation), both prepared according to the Stein Hall process, were used on corrugated boards BB24b quality. The concentration of caustic soda was the same for both adhesives, 0.4% w/w on total formulation.

[0275] The adhesives were applied on the double backer (DB) side (outside of the box). The production of the corrugated boards with the reference adhesive was run at 232 m/min, whereas the production of the boards with the adhesive crosslinked by MFC was run at 250 m/min (see Table 5). The glue gap was set to 0.08 mm for the both adhesives.

TABLE-US-00005 TABLE 5 Overview of process parameters for running corrugated boards BB24b quality Sample Layer Speed m/min Ref. adhesive with borax DB (Outer Liner) 232 Adhesive with MFC DB (Outer Liner) 250

[0276] Samples were analyzed in the laboratory according to the standard test methods given in Table 6. The resulting values for both the reference adhesive with borax and the adhesive with MFC are shown in FIGS. 18, 19 and 20.

TABLE-US-00006 TABLE 6 Standard test references Conditions Grammage Thickness PAT 23° C. - g/m.sup.2 m.m. N/m 50RH % ISO 187 ISO 536 ISO 3034 Fefco nr.11 % kN/m BCT - N Md/cd - Nm ISO 287 ISO 3037 ISO 12048 ISO 2493

[0277] The adhesive with MFC crosslinked starch gave remarkably flatter corrugated boards. The pin adhesion method (PAT) was used to measure the adhesion strength between the flutes and liners of corrugated boards. As can be seen in FIG. 18, the starch-MFC adhesive gave a better bonding strength of the boards compared to the reference adhesive. In fact, at 8% higher production speed, the bond strength of the boards with the Stein-Hall adhesive with MFC is 27% greater than the bond strength of the boards with the reference adhesive. Moreover, with MFC as a crosslinking agent in the starch adhesive, the adhesive consumption was reduced with 33% (FIG. 19), compared to the reference adhesive crosslinked with borax. The glue consumption in g/m.sup.2 was calculated for 6 parallels of each sample according to the following method and equation: Weight of airdry (constant temperature and humidity conditions) corrugated board—ideal weight of the paper=difference=glue consumption. From FIG. 20, it can be seen that the thickness, edgewise crush resistance (ECT) and torsional strength (stiffness) were comparable for both adhesives, even if the boards with the MFC adhesive were produced at 8% higher production speed.

[0278] From this test it can be concluded that by using microfibrillated cellulose as a crosslinker in starch adhesives, less adhesive can be applied and stronger bonds are formed, as well as improved production speeds and flatter boards compared to the reference adhesive with borax.

[0279] Overall, it can be concluded that by using MFC as a cross linker in a starch-based adhesive, completely or partly replacing borax, the following advantages may be observed, either all of these advantages, or at least a sub-set thereof: [0280] efficient and instant thickening of the starch adhesive [0281] stable adhesive viscosity and gel temperature during process and storage [0282] allows for running corrugating process at lower temperatures [0283] stable quality of the adhesive during storage (no or less sedimentation) [0284] improved texture and a shear thinning rheological behavior which are improving the application properties of the adhesive [0285] higher storage modulus of the uncured (liquid) and cured (solid) starch adhesive [0286] increased open time of the adhesive [0287] improved bond strength of the corrugated boards [0288] higher production speed [0289] flatter and more stable boards from corrugator and post-process [0290] reduced glue consumption [0291] reduced water defects [0292] equal or better ECT and torsional strength values within specifications [0293] increased production capacity and/or reduced waste