Dosing of nanocellulose suspension in gel phase

Abstract

A method of dosing a nanocellulose suspension in gel phase into a second suspension, wherein the method comprises the steps of: providing said nanocellulose suspension in gel phase; providing said second suspension; bringing said nanocellulose suspension in gel phase in contact with said second suspension; wherein the method comprises a step of subjecting said nanocellulose suspension in gel phase to a shear rate of more than 500 l/s, simultaneously with and/or immediately prior to the step of bringing said nanocellulose suspension in gel phase and said second suspension in contact with each other.

Claims

1. A method of dosing a nanocellulose suspension in gel phase into a second suspension, wherein the method comprises the steps of: providing said nanocellulose suspension in gel phase; providing said second suspension wherein said second suspension is selected from a cellulose stock solution, a coating composition and a surface sizing composition; carrying out a high shear rate treatment step comprising subjecting said nanocellulose suspension in gel phase to a shear rate of more than 500 1/s in a high shear mixing device, wherein said nanocellulose suspension in gel phase is brought to a fluidized state through said high shear rate treatment step; and bringing said nanocellulose suspension in gel phase and said second suspension in contact with each other within less than 1 second of said high shear rate treatment step wherein said high shear mixing device is any one of a modified Trump jet apparatus, high pressure liquid injection apparatus, ultrasound apparatus, and a high pressure drop apparatus.

2. The method as claimed in claim 1, wherein the nanocellulose suspension in gel phase has a G′>G″, wherein the G′ is higher than 0.5 Pa when measured at frequency less than 0.1 Hz.

3. The method as claimed in claim 1, wherein the nanocellulose suspension in gel phase has a crowding factor above 60.

4. The method as claimed in claim 1, wherein the nanocellulose suspension in gel phase has a solid content of 1 wt % to 25 wt % based on the total solid content of the suspension when said nanocellulose suspension in gel phase is added to the second suspension.

5. The method as claimed in claim 1, wherein said shear rate is more than 1000 1/s.

6. The method as claimed in claim 1, wherein said nanocellulose suspension in gel phase is subjected to said shear rate treatment step in, or directly in contact with, a dosing zone of a papermaking machine.

7. The method as claimed in claim 1, wherein said second suspension is introduced into said high shear mixing device when said nanocellulose suspension in gel phase is subjected to said shear treatment.

8. The method as claimed in claim 1, wherein the temperature of the nanocellulose suspension in gel phase is at least 25° C.

9. The method as claimed in claim 1, wherein said nanocellulose comprises any one of a microfibrillated cellulose or a nanocrystalline cellulose, or cellulose stock systems.

10. The method as claimed in claim 1, wherein said nanocellulose suspension in gel phase further comprises precursor materials for preventing re-flocculation selected from the group consisting of any one of a debonder, a gas, and nanoparticles.

11. The method as claimed in claim 1, wherein the nanocellulose suspension in gel phase further comprises additives or chemicals.

12. The method as claimed in claim 1, wherein during the step of subjecting said nanocellulose suspension in gel phase to a high shear rate said nanocellulose suspension in gel phase is diluted.

13. The method as claimed in claim 1, wherein a fluidization time of the nanocellulose suspension in gel phase is more than 0.001 seconds.

14. The method as claimed in claim 1 applied in any one of papermaking, paperboard making, including coating, surface sizing and wet end dosing and in manufacture of thin films comprising nanocellulose, or in manufacture of translucent films or substrates/laminates thereof or in tissue manufacturing applications or nonwoven manufacturing applications.

15. The method as claimed in claim 11, wherein the additives or chemicals are selected from the group consisting of any one of a dispersion agent, gelling agent, and a foaming agent.

Description

DESCRIPTION OF EMBODIMENTS

(1) According to one embodiment of the inventive method a nanocellulose suspension in gel phase is dosed into a second suspension wherein said second suspension preferably has a lower solid content than the nanocellulose suspension in gel phase. The nanocellulose suspension in gel phase or highly concentrated suspension comprising fibrous material, also called a “high solid content suspension” of nanocellulose, which hereinafter is called a “gel”, “nanocellulose gel” or “MFC gel”. The second suspension is preferably an aqueous based suspension.

(2) The present invention could be performed in the wet end of a paper making process, for instance into the stock solution. In papermaking, the nanocellulose is usually added in the wet end and short circulation prior to the head box. However, according to alternative embodiments the gel may also be added to long circulation or e.g. during beating of fibers or adding microfibrillated cellulose (MFC) into surface sizes or coating dispersion (after or under preparation of those). The second suspension preferably has a solid content in the range of 0.05 to 75 wt-% based on the total solid content of the suspension. If the second suspension further comprises cellulosic fibers, i.e. the second suspension may be a stock solution, the solid content is preferably between 0.05-10 wt-% based on the total solid content of the suspension. If the second suspension is a surface sizing composition or coating composition the solid content of the second suspension is preferably between 5-75 wt-% based on the total solid content of the suspension.

(3) The nanocellulose suspension in gel phase may be defined by its gel properties or a high crowding factor.

(4) Gels may be defined as a form of matter which is intermediate between solid and liquid and exhibits mechanical rigidity. The shear modulus G, describes the rheological state of the fiber network in the gel or gel phase. The most commonly used definition of gel is a rheological one, obtained from dynamic viscometry. According to this definition, a gel is a viscoelastic system with a ‘storage modulus’ (G′) larger than the ‘loss modulus’ (G″). Typically, the gel phase may be defined using a rheometer and by determining G′ the elastic response and G″ which is the viscous response. The G′ and G″ are determined at a given pH, preferably around 7-8, and given temperature preferable 23° C. and at a controlled ionic strength such as 0.01 NaCl. Pre-shearing of samples and surface roughness and composition of the measuring systems may influence the values. Typically, the rheometer are equipped with cup-cylinder or plate-plate geometries. Further, the gel strength is not linearly dependent on MFC concentration. There are also models on how to estimate critical concentration for fibril entanglement to start.

(5) The gel may be defined by G′>G″ and the loss/phase angle δ. These parameters are very important for the rheological characterization of gels. Essentially, solid characteristics are denoted by G′ while G″ indicates liquid characteristics. For a weak gel, G′>G″, and thus junction zones can be readily destroyed even at very low shear rate and the network structure is destroyed. For strong gel, G′>>G″, and both are independent of frequency; lower tan δ values (<0.1) are observed in this case.

(6) According to one embodiment the G′ for the gel phase is defined or determined to higher than 0.5 Pa, or more preferably higher than 1.0 Pa and most preferably higher than 5.0 Pa when measured at frequency less than 0.1 Hz. The elastic response is now dependent on the concentration of the suspension.

(7) The gel, or hydrogel, is usually formed by weak association between the fibrils and formation of fibril-fibril network (or fiber-fibril) when water is included and “bound to the network”. The exact solid content value for the nanocellulose suspension in gel phase, will be influenced by the above factors affect the gel point and gel behavior.

(8) The nanocellulose suspension in gel phase may also defined by the crowding number or crowding factor (N), is a very useful parameter to indicate the degree of fiber contact in a fiber network. The crowding factor is used by as a parameter to divide fiber suspensions of different degrees of flocculation into different regimes. Each regime covers a range of values for the crowding factor. When N<1, no fiber network can be formed, and all fibers are free to move relative to one another. As all the fibers are free to move both by rotation and translation, they occasionally collide and for a very short moment remain together. With increasing values of N, the fibers have a stronger tendency to collide by translation and, as N becomes larger, collisions also take place as a result of rotational motion. When N=60 the number of contact points per fiber is approximately three, which is enough for a coherent fiber network to be established. The fibers are then no longer free to move relative to one another, either by rotation, or by translation. The fibers are inter-locked in a bent condition, with the frictional forces at the contact points between the fibers giving the network its mechanical strength. When the value of the crowding factor exceeds 60, a fiber network of considerable strength has been established. The reason why N>60 is needed is that, for a fiber to be completely locked into the fiber network, the contact points must be arranged in an alternate manner.

(9) The crowding factor may be expressed as:

(10) $N\approx \frac{5C_m{L}^2}{\omega }$
where C.sub.m is the mass concentration expressed as a percentage, L is the average fiber length in meters, and w is the coarseness (kg/m) (Kerekes and Schell 1992).

(11) If the consistency of the nanocellulose suspension in gel phase, or the nanocellulose gel is 0.5%, and the MFC average coarseness 0.01 mg/m and the MFC average fibril length 0.5 mm the crowing number would be 62.5. See table 1 and 2 below for different crowding factors depending on different characteristics of the nanocellulose or MFC (as disclosed in the tables). The fibril lengths in Table 1 and 2 are estimated based on commercial fiber analyzers, for example Valmet FS5 length weighted average.

(12) TABLE-US-00001 TABLE 1 Crowding factor MFC consistency % 1.0 0.5 0.25 0.1 MFC coarseness mg/m 0.01 0.01 0.01 0.01 Fibril length mm 0.5 0.5 0.5 0.5 Crowding factor 125 62.6 31.5 12.5

(13) TABLE-US-00002 TABLE 2 Crowding factor MFC consistency % 1.0 0.5 0.25 0.1 MFC coarseness mg/m 0.005 0.005 0.005 0.005 Fibril length mm 0.5 0.25 0.35 0.25 Crowding factor 250 62.5 61.25 12.5

(14) According to one embodiment the crowding factor of the gel phase is above 60. The crowding factor may preferably be above 61, or even more preferably above 62. The crowding factor may be in the range of 60 to 15000.

(15) The fibers will not crowd if the fluid viscous forces acting on individual fibers are large, i.e. if the fibers follow the fluid. This phenomenon is governed by the fiber Reynolds Number, Re.sub.F.

(16) ${\mathrm{Re}}_F=\frac{\rho .Math.d.Math.G_e.Math.L}{\mu }$
Where ρ is the fluid density, kg/m.sup.3, G.sub.e is the shear rate, s.sup.−1, L the fiber length and μ the fluid dynamic viscosity, Pa s. The Reyonolds number reflects the ratio of inertial forces to viscous forces acting upon the fiber. When ReF>>1, no flocculation occurs. The rheological properties thus illustrate how the suspension behaves at high shear forces. Rheological properties are shear dependent (dynamic measure), i.e. a given force is needed to break down the structure in order to get the desired effect.

(17) According to one embodiment the nanocellulose suspension in gel phase has a solid content above 1 wt-% based on the total solid content of the nanocellulose suspension when added to the second suspension, preferably a solid content above 3 wt-%, or even more preferably a solid content above 5 wt-%. The solid content of the nanocellulose suspension in gel phase added may preferably be between 3-25 wt-% based on the total solid content of the nanocellulose suspension, even more preferably between 3-10 wt-% based on the total solid content of the suspension.

(18) According to the inventive method the nanocellulose suspension in gel phase is subjected to a high shear rate, or high shear forces just prior to dosing into a second suspension, or flow of the second suspension. The SI unit of measurement for shear rate is s.sup.−1, expressed as reciprocal seconds. The shear rate is defined as time of the scale of the shear forces can be between 1000-10000 1/s.

(19) By high shear rate in this respect is meant a shear rate of at least more than 500/s, or more than 1000 1/s, or more preferred more than 4000 1/s, and most preferred more than 10 000 1/s.

(20) By subjecting the gel to the high shear rate the gel preferably becomes fluidized, or is brought into a fluidized state. This means that the nanocellulose suspension in gel phase is added to the second suspension in a fluidized state.

(21) According to one embodiment the nanocellulose suspension in gel phase is subjected to the high shear rate during a period of time which can be called the fluidization time, for at least 0.001 seconds, preferably more than 0.005 seconds or more preferred at least 0.01 seconds, or most preferred at least 0.05 s.

(22) The high shear rate may be provided by a high shear mixing device.

(23) The high shear mixing device may be any one of a modified Trump jet apparatus, high pressure liquid injection apparatus, ultrasound apparatus, high pressure drop apparatus. By modified Trump jet apparatus is meant a conventional Trump jet which has been modified to provide a high enough shear rate or shear forces. The conventional Trump jet equipment available provides for an effective mixing of streams of material, but is not designed to provide the high shear forces necessary in the present invention in order to fluidize the gel comprising nanocellulose. Ultrasound apparatus may be for instance ultrasonic mixing device. The high pressure injection devices may for instance be narrowed channels or capillaries.

(24) The desired shear rate can also be produced with moving or rotating elements, such as cavitron.

(25) The high shear mixing device may also comprise pipes or tubing having rough walls. In a turbulent flow, the friction, i.e. the roughness of the pipe walls will thus increase the frictional pressure drop. The necessary relative roughness as given in ε/D can be calculated based on the dimensions of the pipe, and the liquid flow.

(26) The high shear rate operation is preferably performed in the proximity of where the nanocellulose suspension in gel phase is to be dosed, a so called dosing zone. Preferably the fluidized gel is brought into contact with the second suspension within less than 1 second, preferably within less than 30 μseconds, preferably less than 15 μseconds, preferably less than 10 μseconds, or even more preferred less than 5 μseconds.

(27) The temperature of the nanocellulose suspension in gel phase may be at least 25° C., or at least 30° C. or at least 35° C.

(28) The nanocellulose suspension in gel phase may also comprise a precursor in the form of debonder, a gas or a nanoparticle. The combination of chemical and mechanical approach enables a higher solid content of the gel. These materials can be any type of polymers or surface active polymer or chemicals that act as debonders or dispersing agents, i.e. prevents re-flocculation of the fibrils. In some cases, it is not possible to add dispersant to MFC and prevent re-flocculation. Strong shearing is needed and then the debonders or dispersants can be added.

(29) The gel may also comprise active or functional additives or chemicals, such as any one of a dispersion agent, gelling agent, and a foaming agent. Other examples of additives that could be co-added with MFC are e.g. dyes, optical brighteners (OBA), hemicellulose e.g. xylan, dispersants such a sodium polyacrylate.

(30) According to one embodiment the functional chemicals may be added into nanocellulose suspension in gel phase at a fluidized state, and this mixture may simultaneously at fluidized state be added to the second suspension.

(31) During the high shear mixing the gel may be diluted, which improves the dispersing effect. By adding water or any other dilution liquid during the high shear mixing of the gel the dispersion of the gel is improved.

(32) The nanocellulose may be microfibrillated cellulose or a nanocrystalline cellulose, or fine materials extracted from the paper machine or stock systems. Such fine materials may for instance be OCC based fines or similar materials. Microfibrillated cellulose (MFC) shall in the context of the patent application mean a nano scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods. The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Cellulose fibres, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 2011, 6:417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril (Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol 53, No. 3), is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).

(33) There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt %) when dispersed in water. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 200 m2/g, or more preferably 50-200 m2/g when determined for a freeze-dried material with the BET method.

(34) Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment step is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CMC), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxydation, for example “TEMPO”), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or nanofibrillar size or NFC.

(35) The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

(36) MFC is produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper. The above described definition of MFC includes, but is not limited to, the new proposed TAPPI standard W13021 on cellulose nanofibril (CMF) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

(37) According to one alternative the MFC is produced and used as a never dried material. This reduces problems with hornification of the cellulose before calendering. The MFC may be produced from never dried pulp, and the MFC is not subsequently dried. Further to this the use of non-hornificated MFC provides for a web or film which is more easily plasticized during calendaring and hence, the desired densification and caliper effect may be achieved.

(38) In the final product, i.e. a film, the formation and evenness of the final product e.g. translucent of the film is clearly improved. Dosing gel-like material without exposing the material to high shear forces will definitely increase risks of MFC-rich and MFC-poor areas in the web.

(39) The flocs of MFC can be identified from the end product which in turns leads to reduced mechanical properties or e.g. reduced optical or barrier properties.

Example

(40) Tests on a pilot paperboard machine were done and two board samples were produced. Both samples comprise MFC and bleached CTMP and they both have a grammage of 150 gsm.

(41) Density was measured in accordance with ISO 534:2005, Scott Bond was measured in accordance with TAPPI UM-403 and z-strength was measured in accordance with SCAN-P 80:98

(42) Board 1:

(43) Microfibrillated cellulose in an amount of 20 kg/t was added at a consistency of 2.3% to a furnish comprising bleached CTMP. The MFC was subjected to a shear force of 5000 1/s in a Trump jet for a period of about 0.1 seconds prior to addition to the furnish. A paperboard ply was thereafter produced from said MFC and furnish mixture. The board produced had a density of 313 kg/m.sup.3 and a Scott Bond of 142 MPa, a z-strength of 225 kPa and the wire retention were 98.8%.

(44) Board 2:

(45) As a comparative sample microfibrillated cellulose in an amount of 20 kg/t was added at a consistency of 2.3% to a furnish comprising bleached CTMP. The MFC were subjected to a MFC at a shear rate below 100 1/s directly prior to addition to the furnish. A paperboard ply was thereafter produced from said MFC and furnish mixture. The board produced had a density of 318 kg/m.sup.3 and a Scott Bond of 106 MPa, a z-strength of 215 kPa and the wire retention were 96.1%.

(46) It is clear from the results from the tests that by subjecting the MFC to high shear forces before addition and mixing with a furnish, results in a board with higher strength. It was found that both the Scott Bond and the z-strength of the board increased. Furthermore, it was also found that the retention on the wire was improved.

(47) In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.