Method for manufacturing a composition comprising microfibrillated cellulose

Abstract

The invention discloses a method to form a composition, which method includes fibrillating fibers to form MFC in the presence of an alkali-metal silicate whereby an MFC and silicate mixture is formed The presence of alkali-metal silicate during fibrillation of fibers to MFC, reduces the viscosity and increases the water release behavior, whereby the fibrillation can be accomplished at higher concentrations and a more uniform mixture of MFC-silicate is accomplished. The composition formed by the method of the invention may e.g. be used in paper or paperboard production, in cement production or as an additive in composites.

Claims

1. A method for manufacturing a composition, the method comprising: fibrillating fibers to form microfibrillated cellulose (MFC), wherein the fibrillating is performed in the presence of an alkali-metal silicate, and wherein an MFC and silicate mixture is formed, wherein an initial pH is in a range of 1-5.

2. The method according to claim 1, wherein the alkali-metal silicate is sodium silicate.

3. The method according to claim 1, wherein the method further comprises: mixing the fibers with the alkali-metal silicate prior to or during fibrillation, and wherein the fibers to be mixed with alkali-metal silicate and fibrillated have an SR value of between 15-80.

4. The method according to claim 1, wherein alkali-metal silicate is present in an amount of 1-99 wt % based on the total solid content of fibers.

5. The method according to claim 1, further comprising: dewatering the MFC and silicate mixture to a solid content of at least 3 wt %.

6. The method according to claim 1, wherein pigments are present during the fibrillating.

7. The method according to claim 1, further comprising: adding an acidic media to the MFC and silicate mixture after fibrillation.

8. The method according to claim 1, wherein the method further comprises: mixing the fibers with the alkali-metal silicate prior to or during fibrillation, and wherein the fibers to be mixed with alkali-metal silicate and fibrillated have an SR value of between 25-70.

9. The method according to claim 1, wherein alkali-metal silicate is present in an amount of 1-90 wt % based on the total solid content of fibers.

10. The method according to claim 1, wherein alkali-metal silicate is present in an amount of 1-80 wt % based on the total solid content of fibers.

11. The method according to claim 1, further comprising: dewatering the MFC and silicate mixture to a solid content of at least 5 wt %.

12. The method according to claim 1, further comprising: dewatering the MFC and silicate mixture to a solid content of at least 10 wt %.

Description

DETAILED DESCRIPTION

(1) In accordance with the present invention cellulose fibers are fibrillated to form microfibrillated cellulose, which fibrillation is performed in the presence of an alkali-metal silicate.

(2) 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.

(3) 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).

(4) 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 300 m2/g, such as from 1 to 200 m2/g or more preferably 50-200 m2/g when determined for a freeze-dried material with the BET method.

(5) 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 (CM), 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 fibrils.

(6) 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.

(7) 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.

(8) 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.

(9) In accordance with one embodiment of the invention, a suspension comprising cellulose fibers and an alkali-metal silicate is fibrillated. The alkali-metal silicate may be added to a suspension comprising cellulose fibers prior to or during fibrillation. The alkali-metal silicate may be added as a solution with a solid content of at least 5 wt % or at least 10 or at least 15 wt % to the fiber suspension. The cellulose fibers may be hardwood and/or softwood fibers. The fibrillation can be accomplished by use of e.g. a homogenizator, preferably at a consistency of 1-4% by weight, in a super refiner, preferably at a consistency on of 1-10% by weight or by mechanical treatment in a compactor, shredder, refiner, defibrator, screw, pulper, pump, or high shear mixing devices, at a preferred consistency of between 5-30% by weight.

(10) In one embodiment, the temperature is raised to above 30° C., or above 50° C. or preferably above 75° C. prior to or during the fibrillation.

(11) Optionally, the method further comprise a pre-treatment step, prior to the fibrillating step, which pre-treatment step may comprise enzymatic or mechanical pre-treatment.

Example 1

(12) A trial series was performed in which the dewatering resistance of MFC containing compositions produced according to the invention (sample 2-6) were compared with reference MFC compositions (sample 1 and sample 7). Reference sample 1 is MFC made from 100 wt % kraf fibers, without the addition of water glass. Reference Sample 7 is a mixture of MFC made from kraft fibers and water glass, wherein the MFC and the water glass have been mixed after refining (post-mixing). Reference samples 2-6 are compositions according to the invention wherein kraft fibers have been fibrillated in the presence of water glass.

(13) The experiments were made using fiber suspensions of kraft fibers (pine) which were pretreated by wet disintegration at 3 wt % consistency at 30000 revs (British standard wet disintegrator).

(14) The pH of the fiber suspensions were adjusted to approximately 9.5 before addition of sodium silicate. Water glass (sodium silicate) was added to the samples 2-6 prior to fibrillation. The water glass added was sodium silicate (Dry cont. 50.17 wt %, Be 48-50, Density 1.40-1.52 kg/dm3, SiO2, Na2O molar ratio 2-2.1). The process conditions prior and after fibrillation are shown in table 1. The amount of water glass added is calculated based on the total amount of fibers in the suspensions before fibrillation (20 means 20 wt % of the dry amount of fibers). The “pH, initial” refers to the pH after the addition of sodium silicate but prior to fibrillation. In Sample 6, the pH was adjusted to 3.9 by the addition of sulphuric acid prior to the fibrillation.

(15) The fiber suspensions were fibrillated by fluidization. Fluidization was made by running the suspension 2 times through 400/200 micron chambers and then one time through a 200/100 micron chambers (Microfluidizer). No adjustment of the temperature was done before or after the trials.

(16) TABLE-US-00001 TABLE 1 Sample 1 (ref) 2 3 4 5 6 7 Kraft Fiber 100 100 100 100 100 100 100 [wt %] Water glass 0 5 15 15 15 15 15 [wt %] Fluidization 3x 3x 3x 3x 3x 3x 3x, post mixing pH, initial 9.5 10.7 11.0 11.2 11.4 3.9 PH, final 7.09 10.64 10.81 11.27 11.39 3.8 11.01 Starting 1.0 1.0 1.0 2.96 4.43 4.86 1.5 consistency [wt %]

(17) To investigate the dewatering resistance, the reference MFC Sample 1, the MFC-water glass samples of the invention (samples 2-6) and the reference Sample 7 (post mixing), respectively were dewatered by a vacuum filtration device equipped with 0.65 μm DVPP filter. Prior to the filtration, the samples were diluted to 0.1 wt % consistency using RO water. The mixing were carried out using a rod mixer (30 sec) followed by magnetic stirring for at least 2 minutes. The diluted suspension was poured into the vacuum filtration funnel. The time recorder was started at the same time while initiating the vacuum suction. The time required to the visible water layer to disappear from the top of the fibril pad (film) was monitored (=dewatering time). The results for the references and the samples of the invention are shown in table 2.

(18) TABLE-US-00002 TABLE 2 1 (ref) 2 3 4 5 6 7 Filtration time, 843 373 209 131 130 95 169 [Sec] (30 gsm) Dry content of 25.90 26.23 24.57 27.15 26.78 27.23 26.44 drainage filter cake

(19) The results clearly show that the dewatering resistance is significantly reduced due to the co-fibrillation according to the invention.

Example 2

(20) Another test series was performed to investigate the water retention value (WRV) of the compositions of the invention.

(21) The reference MFC Sample 1, the Sample 4 of the invention and the reference post-mixing Sample 7 (all produced as in example 1 but with a fluidization concentration of 3%) were mixed with bleached never-dried birch kraft pulp (unrefined) in accordance with Table 3, wherein TP 1 refers to a mixture of Sample 1 with kraft pulp, TP 4 refers to a mixture of Sample 4 with kraft pulp and TP 7 refers to a mixture of Sample 7 with kraft pulp.

(22) TABLE-US-00003 TABLE 3 Approximate Test time of cake Point MFC, WRV forming, (TP) [wt %] [%] [min] 1 2.5 188 15 1 5.0 199 27 1 7.5 214 33 1 10.0 222 48 4 2.5 184 6 4 5.0 193 8 4 7.5 195 10 4 10.0 203 10 7 2.5 185 6 7 5.0 197 10 7 7.5 199 12 7 10.0 210 12

(23) Pulp pads for centrifugation were formed using a vacuum filtration device equipped with a 0.65 μm DVPP filter. The cakes with approximately 7-15 wt % dry content were subjected to centrifugation and the water retention value was determined in accordance with SCAN-C 62:00. The results are shown in Table 3.

(24) FIG. 1 shows how the water retention value changes with increased MFC content for the different test points. WRV for the samples comprising 100 wt % MFC were calculated using extrapolation of the trendlines from the chart shown in FIG. 1 (Table 4).

(25) TABLE-US-00004 TABLE 4 TP WRV [%)] 1 646.02 4 409.50 7 492.00

(26) As can be seen in FIG. 1 and in table 4, the water retention value of the compositions made in accordance with the invention (TP 4) are significantly lower than the water retention value of the references (TP4 and TP7), especially at higher MFC contents.