Flocs of fillers combined with cellulose micro and nanofibrils for use in the production of paper products with improved properties

Abstract

The present invention intends to solve the current problem of the need to use synthetic additives, which are costly and environmentally harmful, in the production process of paper products with improved properties, namely the increase of the tensile index (dry and wet-web), with increased filler retention, higher strength and air resistance (suitable for packaging applications), lower roughness, higher opacity and lower capillary water absorption. The invention consists in a product of flocs (1), for application in cellulosic fibres (2), consisting of mineral fillers conjugated with cellulose micro or nanofibrils, and in its use for the production of paper products and on the resultant paper products.

Description

Technical Field

[0001] The present invention relates to the field of the production of paper products, namely on the products constituted by mineral fillers conjugated with cellulose microfibrils, obtained by an enzymatic process, which improve several paper properties, with no use of paper additives usually expensive and harmful for the environment.Background Art

[0002] The competitiveness within the papermaking industry has been exponentially increasing. If the paper properties are to be improved, then underlying processes have to be optimized in such a way that new horizons, as the synthesis of new materials, are in sight. The cellulose nano and microfibrils (CNF and CMF, respectively) are a quite recent material that has proven to significantly improve paper strength. They present unique characteristics, such as reduced size and high specific surface area, high tensile index, crystallinity and transparency, therefore becoming object of great interest, mainly as reinforcing materials in composite structures. Different raw-materials and methodologies can be used to produce these new fibrous structures, usually including mechanical treatments to fibrillate the fibres. To avoid intensive mechanical energy and to overcome some inherent technical difficulties, it is common practice to apply chemical or enzymatic approaches to pre-treat the fibres.

[0003] The use of CNF in papermaking has been reported as they are able to improve strength, filler retention and / or other specific properties such as absorption. In papermaking they are usually combined with additives commonly used in the industry, such as internal strength agents (e.g. cationic starch), internal sizing agents (e.g. alkyl ketene dimer - AKD and alkenyl succinic anhydride - ASA) or retention agents (e.g. cationic polyacrylamides - CPAM). Previous published inventions refer that the mixture of CNF / CMF with the abovementioned additives improves the bonding between the cellulosic fibres and the mineral fillers. In this sense, it is of paramount relevance to thoroughly understand the different and complex interactions and mechanisms between all the paper components.

[0004] The publication WO2018100524A1 is related to an invention where microfibrillated cellulose is mixed with, at least, two retention agents (cationic polymer, such as starch, and micro / nanoparticles, such as silica or bentonite) as a pre-mix before producing paper. The mixing with mineral fillers is not referred to.

[0005] The invention EP2236664A1 refers a mixture of cellulosic fibres with mineral fillers only with the purpose of producing nanofibrillated cellulose, by using a more efficient process.

[0006] The publication WO2016 / 185397A1 refers the production of mineral fillers directly in a microfibrillated cellulose suspension, which can be used to produce paper with improved drainability. No reference can be found in the cited document to the simultaneous improvement of a series of paper properties, such as the improvement of the tensile index, with increased filler retention, higher strength and air resistance, lower roughness, higher opacity and lower capillary water absorption. Additionally, the mineral fillers, and their effect, are associated to the use of a precursor, in this case carbon dioxide with a specific gas bubble size.

[0007] The publication EP 2978894 B1 defines a paper production process consisting in mixing microfibrillated cellulose with a strength agent (e.g. starch) followed by the addition of a microparticle (e.g. silica or bentonite).

[0008] The article He et al. (2017) refers to a composite of CMF, precipitated calcium carbonate (PCC) and cationic starch which originate high dimension flocs allowing an improvement of PCC retention.

[0009] The publication EP 2425 057 B1 discloses a method for preparing an aqueous furnish to be used in paper manufacturing. Said furnish is composed by filler, fibres, nanofibrillated cellulose and a cationic polyelectrolyte (preferentially cationic starch). It is stated that the filler retention and paper strength are improved when the combination of filler, CNF and starch is used.

[0010] The publication WO2015101498A1 discloses a method for providing a pre-treated filler composition containing PCC, cationic polyacrylamide and nanofibrillated cellulose. The aggregates of said pre-treated filler were characterized by Focused Beam Reflectance Measurements and defined as providing the enhancement of paper properties.

[0011] The publication US2017 / 0284030A1 refers to the use of cellulose microfibrils and inorganic particles as a layer at the surface of a paper structure. Although the described surface layer (containing the cellulose microfibrils and the inorganic particles) does not contain additives, the document states that the paper structure must mandatorily contain approximately 2% of additives, such as flocculants, drainage / formation additives, thickeners, starch and retention agents. Additionally, there is no reference to flocs consisting of mineral fillers and cellulosic fibrils with associated specific intrinsic viscosity, reflocculation ability or size. Furthermore, there is no reference to the simultaneous improvement of a series of paper properties, such as the improvement of the tensile index, with increased filler retention, higher strength and air resistance, lower roughness, higher opacity and lower capillary water absorption.

[0012] Considering the published knowledge, there is still the need to globally improve a series of paper properties, by using efficient, economic and environmentally-friendly products and methodologies, being possible to reduce the amount of added additives, or even without the need to use them, which are usually expensive and harmful for the environment.References:

[0013] WO 2018 100524 A1 - Pre-mix useful in the manufacture of a fiber based product EP 2978894 B1 - Process for production of paper or board He et al. (2017), Ming He, Guihua Yang, Byoung-Uk Cho, Yong Kyu Lee, Jong Myoung Won, Effects of addition method and fibrillation degree of cellulose nanofibrils on furnish drainability and paper properties EP 2425057 B1 - Method for producing furnish, furnish and paper WO 2015 101498 A1 - A method for providing a pretreated filler composition and its use in paper and board manufacturing EP 2236664 A1 - Process for the production of nano-fibrillar cellulose suspensions WO 2016 / 185397 A1 - Production of nanosized precipitated calcium carbonate and use in improving dewatering of fiber webs US 2017 / 0284030 A1 - Paper and paperboard products Summary of Invention

[0014] The present invention intends to solve the current problem of the need to use synthetic additives, which are costly and environmentally harmful, in the production process of paper products with improved properties.

[0015] The invention consists in a method for the production of flocs of mineral fillers and cellulose microfibrils comprising the steps of a) disintegrating and refining up to 4000 revolutions in a PFI beater of 0.3 kg of bleached eucalyptus kraft pulp; b) suspending the beaten fibers in water and adjusting the pH to 5 by the addition of sodium citrate buffer; c) heating the suspension to 323.15 K under constant mechanical stirring and adding the enzyme selected from the enzyme "E1" comprising endocellulase, 10 % exocellulase and 5 % hemicellulose and enzyme "E2" comprising endocellulase with 10 hemicellulose; d) stopping the hydrolysis of the cellulose after 7200 s by heating the suspension to 353.15 K for 900 s and cooling the resulting suspension to room temperature; e) washing the enzymatic sample with demineralized water until the conductivity of the filtrate is low; f)mechanically treating the fibres at 1 % consistency, in a high pressure homogenizer, firstly at 5x10 7< Pa and secondly at 10x10 7< Pa, so that the obtained cellulose microfibrils have an intrinsic viscosity from 0.45 to 0.70 m 3< / kg; g) mixing under stirring the resulting aqueous suspension of step f) having a consistency of 0.2 wt% with a 1 wt% aqueous suspension of calcium carbonate until the formation of flocs calcium carbonate and cellulose microfibrils at a 10:1 mass ratio and a total solids concentration of around 0.01 wt%; h) subjecting the flocs produced in step g) to force-breaking by sonication during 900 s, after 1200s of agitation and stirring for further 1800 s after the 900 s sonication step; i) reflocculation to flocs having a median size between 20 and 50 µm. The produced flocs, unexpectedly, lead to a global improvement of several paper properties in the inexistence of any need to add additives usually expensive and harmful for the environment.

[0016] Additionally, the present disclosure relates to paper products produced from the abovementioned flocs of mineral fillers and cellulose microfibrils (obtained by enzymatic hydrolysis). The obtained product presents several globally improved paper properties, such as the increase of the tensile index (dry and wet-web), with increased filler retention, higher strength and air resistance, lower roughness, higher opacity and lower capillary water absorption, eliminating the need of adding additives usually expensive, such as starch, ASA and cationic polyacrylamides.Brief Description of Drawings

[0017] Figure 1. Characterization of the different CNF / CMF produced and of the bleached kraft pulp for comparison. Figure 2. Amount (%) of each component added in each formulation series for the production of bleached eucalyptus pulp handsheets. Figure 3. Evolution of the median of the particle size distribution (d50) of suspensions containing PCC and CNF / CMF. A test with only PCC was conducted for comparison. Figure 4. Evolution of the median of the particle size distribution (d50) of suspensions containing PCC and CMF-E1 produced with increasing number of passes in the HPH. A test with only PCC was conducted for comparison. Figure 5. Filler retention of handsheets produced with the flocs consisting of the different CNF / CMF, and additives. The horizontal line indicates the current reference (handsheets without the flocs and with PCC and all the additives). Figure 6. PCC-tensile factor normalized to the same PCC content in the handsheets produced with flocs consisting of CNF / CMF obtained through different routes and with additives, and comparison with the current reference (without flocs and with PCC and additives). Figure 7. Tensile index of handsheets produced with the flocs of CMF-E1, with and without additives. The extra black column refers to handsheets produced with an extra addition of 5% of PCC. Figure 8. PCC-tear factor normalized to the same PCC content in handsheets produced with flocs consisting of CNF / CMF obtained through different routes and with additives, and comparison with the current reference (without flocs and with PCC and additives). Figure 9. PCC-opacity factor normalized to the same PCC content in handsheets produced with flocs consisting of CNF / CMF obtained through different routes and with additives, and comparison with the current reference (without flocs and with PCC and additives). Figure 10. Wet-web tensile index of handsheets produced without additives and with flocs consisting of CNF-T3 or CMF-E1 and reference handsheets (without flocs and with PCC and additives) . Figure 11. Influence of the addition of flocs consisting of PCC+CMF-E1 on the tensile index of handsheets produced with pulp with different refining degrees, °SR. The PCC-tensile factor was calculated considering the reference of pulp at 29°SR+PCC+additives. Figure 12. Structural properties of handsheets produced with the flocs consisting of the different CNF / CMF and additives. Figure 13. Water retention value (WRV) and Klemm water capillary rise of laboratorial handsheets produced with the flocs consisting of the different CNF / CMF and additives. Figure 14. CNF / CMF production costs and cost of 3% addition to papermaking. *Fibre cost estimated at 700 euro / metric tonne; ** Energy cost assumed to be 0,02 euro / MJ, excluding water consumption cost. Figure 15. Flocs (1) attached to a cellulosic fibre (2), image obtained by field emission scanning electron microscopy of an handsheet produced with cellulosic fibre and flocs. Detailed Description and Description of Preferred Embodiments

[0018] In the present invention enzymatic cellulose microfibrils (CMF) are conjugated with mineral fillers for use in papermaking, which leads to an improvement of the paper properties, being therefore possible to overcome the need to add other paper additives.

[0019] Cellulose microfibrils (CMF) and nanofibrils (CNF) can be synthesized through different routes, including mechanical, enzymatic and / or chemical processes.

[0020] Fibril sizes make it possible to distinguish between nanofibrils and cellulose microfibrils (Kargarzadeh H., Mariano M., Gopakumar D., Ahmad I., Thomas S., Dufresne A., Huang J., Lin N. (2018)) Advances in cellulose nanomaterials. Cellulose 25: 2151-2189). In the TAPPI standard proposal WI 3021 (TAPPI standard proposal WI 3021. Standard terms and their definition for cellulose nanomaterials, draft), cellulose nanofibrils are defined as having a width of 5-30nm and cellulose microfibrils having a width of 10-100nm. Additionally, the nanofibrillation yield was determined, that is, the ratio of nanofibrils in the different samples, Figure 1. For this purpose, centrifugation and gravimetry (as defined in the ISO / CD TS 21346 standard) are used. Samples composed mainly of nanofibrils (that is, with a ratio of nanofibrils - nanofibrillation yield - greater than 50%) and CMF are samples composed mainly of microfibrils (that is, with a nanofibril ratio - nanofibrillation yield - less than 50%).

[0021] In this work were considered, additionally to the cellulose microfibrils (CMF) produced by enzymatic hydrolysis with endoglucanase (according to the methodology reported by Tarrés et al. 2016) and hereby defined as CMF-E1 and CMF-E2, and cellulose nanofibrils (CNF) produced by oxidation mediated by TEMPO, 2,2,6,6-Tetramethylpiperidine 1-oxyl (according to the methodology reported by Saito et al. 2007), commonly designated as CNF-TEMPO, and hereby identified as CNF-T3 and CNF-T9, other CNF / CMF produced through different routes, namely by mechanical refining (hereby identified as CMF-Mec) and by carboxymethylation with distinct monochloroacetic acid amounts, namely 9% and 27% (hereby identified as CNF-C9 and CNF-C27, respectively).

[0022] The enzymatic cellulose microfibrils (CMF) were produced from 0.3 kg of bleached eucalyptus kraft pulp, which was disintegrated and refined up to 4000 revolutions in a PFI beater. The fibres were then subjected to enzymatic hydrolysis by using different commercial enzymes: Enzyme "E1" (endocellulase, 10% exocellulase and 5% hemicellulose) and Enzyme "E2" (endocellulase with 10% hemicellulose). Bovine serum albumin (Sigma-Aldrich, USA) was used to determine their protein concentration, according to the Bradford method (Bradford, 1976) and values of 5.0 and 5.8 kg / m 3< were obtained for enzymes "E1" and "E2", respectively.

[0023] The beaten fibres were suspended in water (3.5% consistency) and the pH was adjusted to 5 by the addition of sodium citrate buffer. The suspension was heated to 323.15 K under constant mechanical stirring and the enzyme was added (3x10 -4< kg per kg of pulp). The cellulose hydrolysis was stopped after 7200 s by heating the suspension to 353.15 K for 900s. The resulting suspension was cooled to room temperature.

[0024] For the CNF produced through oxidation, the beaten fibres were added to an aqueous suspension containing NaBr and TEMPO at room temperature. Afterwards, a sodium hypochlorite (NaClO) solution at a ratio of 3 or 9 mol per kg of fibre was slowly added to the previous mixture, while keeping the pH at 10 with NaOH for 120 s, originating the samples "CNF-T3" and "CNF-T9", respectively.

[0025] Both the enzymatic and the TEMPO samples were thoroughly washed with demineralized water until the conductivity of the filtrate was low. After the enzymatic and oxidative treatments, the fibres were finally mechanically treated, at 1% consistency, in a high-pressure homogenizer ((HPH, GEA Niro Soavi, model Panther NS3006L), firstly at 5x10 7< Pa and secondly at 10x10 7< Pa. This mechanical treatment is intensive, preferentially with two runs (total pressure of 15x10 7< Pa).

[0026] The produced CNF / CMF were characterized by different techniques, namely to determine the nanofibrillation yield, carboxylic groups content, intrinsic viscosity (degree of polymerization) and charge (zeta potential), as shown in Figure 1. A proper characterization of these materials is essential and the parameters to measure may depend on the application intended. Usually the measured parameters include the nanofibrillation yield, the particle size distribution and median particle size, the surface chemistry and the strength properties, but also the specific surface area and suspension rheology accordingly to the end application.

[0027] Intrinsic viscosity measurements were performed for the CNF / CMF suspensions by dissolving them in cupriethylenediamine, according to the ISO standard 5351:2010. The degree of polymerization (DP) was calculated using the Mark-Houwink equation, as described elsewhere (Henriksson et al. 2008).

[0028] The nanofibrils amount depicted in Figure 1 allows distinguishing between the nanofibrillated samples (CNF), which are also the functionalized ones and therefore with high carboxyl groups content and high zeta potential (absolute value), from the microfibrillated samples (CMF).

[0029] In order to properly assess the interactions between all the paper components, laboratorial handsheets were produced with five different series, with distinct amounts of bleached eucalyptus kraft pulp, refined up to 33 °SR (refining degree, Schopper Riegler), CNF / CMF, precipitated calcium carbonate (PCC), cationic starch, alkenyl succinic anhydride (ASA), and / or linear cationic polyacrylamide (CPAM). The former additives are usually added in order to improve the process or paper properties. Figure 2 depicts the amounts used.

[0030] Afterwards, the flocs of PCC combined with the cellulose micro or nanofibrils are produced. Previously, a 1 wt% aqueous suspension of calcium carbonate and a 0.2 wt% aqueous suspension of each of the TEMPO CNF or enzymatic CMF samples were prepared. The calcium carbonate and the CNF or CMF, at a 10:1 mass ratio and a total solids concentration of around 0.01 wt%, are mixed under stirring (2000 rpm). After 1200 s of agitation, sonication is applied during 900 s to break the flocs. After the 900 s the stirring continues for further 1800 s.

[0031] An important factor to consider when improving the paper performance is the evaluation of the interaction of the CNF / CMF with PCC, through flocculation tests. In fact, mineral fillers, such as PCC, are not able to establish strong bonds with cellulosic fibres and therefore during paper formation a great amount of material is lost through the web. In this sense, it becomes essential to efficiently flocculate the filler, in order to promote higher particle sizes in order to avoid losses through the web. Nevertheless, it is also important to control the size in order to not harm the paper formation. As abovementioned, the inorganic fillers are usually flocculated with the aid of synthetic additives, such as cationic polyacrylamides or cationic starch.

[0032] In this sense, the evolution of the flocs size is controlled over time by laser diffraction spectrometry in a Mastersizer 2000 equipment (Malvern Instruments), equipped with the Hydro2000 module, and by applying a PCC refractive index of 1.57 and the Mie theory for the calculations. This procedure was proposed for filler particles (without CNF) by Rasteiro et al. 2008.

[0033] From Figure 3 it is possible to state that the mechanical CMF initially flocculated the PCC particles, but the agitation and sonication applied broke the flocs and therefore values around 8 µm (similar to the normal aggregation of PCC) were obtained after the 5400 s of measurement. On the contrary, the enzymatic CMF led to high PCC flocculation, with floc sizes of ca. 26µm, even after applying the shear forces. In the case of the TEMPO-mediated CNF, floc sizes of around 38µm were obtained if CNF-T3 was used. Finally, for the carboxymethylated CNF, a much stronger flocculation occurred, originating flocs with sizes up to 63 µm. The flocs size is highly dependent on the CNF / CMF properties. For the enzymatic CMF, the intrinsic viscosity must be inferior to 0.75 m 3< / kg, preferably between 0.45-0.70 m 3< / kg, which corresponds to a degree of polymerization between 1000-2000, in order to generate flocs with the above-mentioned sizes. Supplementary studies for this work show that the CNF-TEMPO must have an intrinsic viscosity superior to 0.10 m 3< / kg, preferably between 0.12 - 0.21 m 3< / kg, which corresponds to a degree of polymerization between 300-500, and a carboxyl groups content inferior to 1.4 mol / kg, in order to efficiently flocculate PCC particles, specifically, with floc sizes between 20-50 µm (after flocs breaking and subsequent reflocculation).

[0034] An additional study to assess the influence of the high-pressure homogenizer in the flocculation was conducted with enzymatic CMF-E1 produced with 2, 4 or 6 runs (corresponding to total pressures of 12.5x10 7< , 27.5x10 7< and 42.5x.10 7< Pa, respectively. CMF with decreasing degree of polymerization ((1834, 1747 e 1504, respectively) and decreasing flocculation ability (Figure 4), were obtained.

[0035] The biggest flocs were obtained with CMF-E1 produced with two runs in the HPH (total pressure of 12.5x10 7< Pa). The flocs size presented median values of 28 µm (after breaking the flocs and subsequent reflocculation)

[0036] For the handsheets production, with the formulations and amounts depicted in Figure 1, the produced flocs are added to the eucalyptus bleached beaten fibre, with stirring. After 120 seconds, the remaining paper additives are also added. The cationic starch+ASA mixture is stirred with the abovementioned components for 150 seconds and the cationic polyacrylamide for 5 seconds. The formulations are then poured into the handsheet former, with the following automatic steps of air agitation (5 seconds), decantation (5 seconds) and drainage (time duration dependent on the formulations). The handsheets are pressed, dried and conditioned according to the standard ISO 5269-1. The structural, optical and mechanical properties are measured according to the correspondent standards: basis weight (ISO 536:2011), Gurley air resistance (ISO 5636-5:2003), Bendtsen roughness (ISO 8791-2:2013), tensile index (ISO 1924-2:2008), tear index (ISO 1974:2012), opacity (ISO 2471:2008) and Klemm capillary rise (ISO 8787:1986). The mineral filler retention in the fibrous matrix is evaluated under standard TAPPI T 211 om-93.

[0037] The increase of the amount of mineral fillers used in paper products is usually desirable, not only due to environmental matters since the use of cellulosic fibres can be reduced, but also for economic reasons since fibres are usually much more expensive and papers with higher filler amounts are easier to dry. Besides, their use results in the improvement of several properties, such as opacity, surface smoothness and printability. However, mineral fillers lead to a decrease of the paper strength, since they negatively affect the fibre-fibre bonding. Besides, they are usually lost through the web, as abovementioned, which makes it necessary to add retention agents to the papermaking process. Additional problems related to the paper formation and printability (sheet delamination and / or dusting) are also found. For these reasons, the mineral filler amount is usually limited to values inferior to 30% of the total paper weight.

[0038] In this sense, handsheets were prepared according to the formulations depicted in Figure 2, in a semi-automatic laboratory sheet former (300-1 model, LabTech) using a 120 mesh screen.

[0039] The filler retention was measured as abovementioned, for the 5 different series used for the handsheets production. Figure 5 depicts the PCC retention in the handsheets produced with additives and with the CNF / CMF prepared through the different processes.

[0040] As above explained, with the exception of CNF-T9, the presence of CNF / CMF, and consequent PCC flocculation, leads to a high PCC retention, even in the absence of the paper additives. By comparing the results obtained with the method that recreates the industrial reality of paper production, i.e., using PCC with all the additives, but without CNF / CMF, the results show an increase of the PCC retention with the great advantage of not being necessary to use expensive additives, such as CPAM.

[0041] Paper products should have a high mechanical strength in order to resist all the tensions they are subjected to at the paper machine, as well as at industrial printers. However, there are several factors that negatively influence this parameter, such as the use of recycled fibres and mineral charges, or the need to reduce the paper basis weight (to reduce virgin fibre consumption, for example). The paper strength depends on the strength of the used fibres as well as on the strength of their bonding. Tensile index is the most commonly used property to evaluate paper mechanical strength and is indirectly related to the mineral filler content. In this sense, another way to analyze the obtained results is by using the filler-tensile factor, which consists on a normalization of the tensile index, by considering the effective PCC content on the handsheets: Filler-tensile factor: (tensile index x filler content) with flocs / (tensile index x filler content) with PCC and with additives. If the value obtained is superior to 1, the handsheets have a tensile index superior to that of the reference handsheets, i.e., without flocs and with PCC and with additives.

[0042] The same factor and related considerations were applied for the tear index and opacity.

[0043] The obtained values are depicted in Figure 6, 7 and 8.

[0044] Using the TEMPO nanofibrils with lower charge (CNF-T3) on the flocs production, the tensile index is only slightly increased when comparing to the reference, if no additives are used. In fact, the negative charge of the nanofibrils seems to lead them to preferentially bond with the added cationic additives, therefore the nanofibrils are not available to bond with the fibres, hindering paper strength, as visible at Figure 6 on the series containing additives.

[0045] By using the flocs consisting of enzymatic CMF, the tensile index is always improved when comparing with the reference (factor superior to 1), either in the presence or absence of additives. Furthermore, by removing CPAM, the highest tensile increase is obtained with the enzymatic-CMF flocs, which seems to suggest that the high chain length of these microfibrils is overpassing the effect of CPAM.

[0046] Due to the great improvement of paper properties by using the flocs consisting of the enzymatic CMF "E1", an additional series of handsheets was produced with extra 5% of PCC incorporation, and without additives (Figure 2). The results of the tensile index (Figure 7) reveal that it is possible to produce handsheets with the same filler content than that of the reference (without flocs and with all additives), and with higher tensile index, without the need to add additives.

[0047] The effect of the flocs in the tear index did not follow the same trend (Figure 8). In this case, only the CNF-C27 flocs were able to improve the tear index in the additives absence. However, when using starch+ASA or all additives, the flocs consisting of the non-functionalized CMF (CMF-Mec, CMF-E1 and CMF-E2) lead to a great improvement of this property.

[0048] Additionally, better results were obtained with the flocs consisting of CMF-E1 (either for the tensile or tear indices) than with the flocs of CMF-E2, most probably due to the presence of exocellulase in the composition of enzyme E1.

[0049] The optical properties, evaluated by opacity (Figure 9), were also very much improved, either in the presence or absence of additives.

[0050] The wet-web strength of the handsheets was also significantly improved by using the flocs. Figure 10 depicts the tensile index measured when increasing moisture levels of the handsheets produced without additives and with flocs consisting of CNF-T3, CMF-E1 and CNF-C9. The reference handsheets (without flocs and with PCC and with additives) are also presented for comparison. All of the plotted handsheets have the same mineral filler content (26-28% effective). Furthermore, reference handsheets produced in the same conditions (without additives), but otherwise produced with 25% of softwood fibre, which is usually used to limit web breaks in a paper machine, are also plotted. These results are of extreme importance for the paper machine operation: for moisture levels common for the drying section (7 to 50%), the flocs can contribute to a reduction of web breaks and / or for increased machine speeds, since the wet-web strength is improved. In this sense, Figure 10 also reveals that the flocs can supplant the need for softwood fibre addition, since the wet-web strength is the same than that of the reference handsheets.

[0051] Figure 11 depicts the results obtained for handsheets produced with eucalyptus bleached pulp beaten to different refining degrees (1300, 1800 and 2300 PFI rotations, corresponding to refining degrees of 25, 27 and 29 °SR (Schopper Riegler), respectively). It is concluded that by adding the flocs consisting of PCC and CMF-E1 to laboratorial handsheets without additives, it is possible to reduce more than 4°SR on the refining degree of the pulp, and still maintain the same paper strength. However, the filler retention is slightly affected and, therefore, a filler-tensile factor was computed by considering the reference produced with pulp beaten to 2300 PFI rotations (29°SR) + PCC + additives (SA+P). By analyzing the results it is possible to state that the limit for properties improvement is above 26°SR, meaning that by adding flocs consisting of PCC and CMF-E1 it is possible to produce handsheets with the same filler content and same tensile index than that of the reference handsheets, but by saving energy on pulp beating.

[0052] Figure 12 depicts the handsheets air resistance, measured by the Gurley method, and their Bendtsen roughness, which are relevant structural properties. As well-known, the addition of CNF / CMF to papermaking leads to severe drainability difficulties, since the paper structure becomes more closed. However, when comparing with the performance of flocs consisting of chemical CNF (produced through TEMPO-oxidation or carboxymethylation), the air resistance of the handsheets produced with the enzymatic CMF is not so negatively affected.

[0053] The water retention capacity of the fibrous matrix was evaluated through the water retention value (WRV) and by the Klemm capillary rise (Figure 13). In the additives absence, the handsheets containing the flocs retain the same or less water than the reference but, in the additives presence, the handsheets with flocs retain much more water than the reference, which may be harmful for the drying process at the paper machine. However, by using 5% extra of PCC in the formulation with starch+ASA and with the flocs consisting of CMF-E1, it was possible to obtain the same WRV value than that of the reference handsheets with all additives (WRV=1.22±0.02 kg / kg), besides the aforementioned increased paper strength. On the other side, in the flocs presence, the water absorptiveness capillary rise was always inferior to that of the reference, which means that a lower liquid penetration will occur, e.g., when coating or surface sizing is applied at the paper machine.

[0054] It was also possible to prove that the production of flocs consisting of CMF-E1 and their addition to papermaking is the cheapest among the ones produced, as shown in Figure 13.Examples

[0055] In a preferred embodiment of the invention, the cellulose micro and nano fibrils are obtained through enzymatic hydrolysis, followed by high-pressure homogenization (intensive mechanical treatment with two runs with a total pressure of 15x10 7< Pa). The enzymatic-CMF has an intrinsic viscosity inferior to 0.75 m 3< / kg, preferably between 0.45-0.70 m 3< / kg, which corresponds to degrees of polymerization between 1000 and 2000 The produced flocs are able to reflocculate after forced breaking by using, e.g., sonication, and have a median size (measured by laser diffraction spectrometry) between 20 and 50 µm. By using these flocs, consisting of the microfibrils and nanofibrils abovementioned and mineral fillers, on papermaking there is a global improvement of several paper properties, namely strength increase (increase of 41% and 99% for the dry and wet (50% moisture) tensile indices, respectively, and increase of 20% for the tear index), a slight increase (2.2%) of filler retention, an increase of air resistance, a decrease of surface roughness (65% decrease), an increase of opacity (increase of 0.2%) and a decrease of water absorption (Klemm capillary rise decrease of 24%), when compared to the reference (paper products without flocs and with PCC, starch, ASA and CPAM, commonly used additives which are expensive and potentially environmentally harmful). Additionally, these enhancements are possible even when reducing the amount or not using any other additives.

[0056] The present document thereby discloses the use of the above described flocs (Figure 15) for the production processes of paper products, with a global improvement of several paper properties, being therefore possible to reduce the amount, or without the need to add commonly used paper additives.

[0057] The aforementioned flocs can be used in processes with increased mineral filler retention and with a reduction of the amount of paper additives used, when compared with the paper products produced at the same conditions but without the mentioned flocs.

[0058] The resultant paper products incorporating the described flocs present improved paper properties, namely mechanical, structural and optical properties, with a smaller amount of added paper additives or even without containing any amount of paper additives.

[0059] Therefore, these flocs act as reducers of the amount of internal strength agents, internal sizing agents and retention agents, commonly used in paper products production processes and incorporated in paper products.References:

[0060] Henriksson M., Berglund L. A., Isaksson P., Lindstro T. and Nishino T. (2008) Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 9, 1579-1585. Rasteiro MG, Garcia FAP, Ferreira P, Blanco A, Negro C, Antunes E (2008) Evaluation of flocs resistance and reflocculation capacity using the LDS technique. Powder Technol. 183: 231-238 Saito T, Kimura S, Nishiyama Y, Isogai A (2007) Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules. 8: 2485-2491. Tarrés Q., Saguer E., Pèlach M. A., Alcalà M., Delgado-Aguilar M. and Mutjé P. (2016) The feasibility of incorporating cellulose micro / nanofibers in papermaking processes: the relevance of enzymatic hydrolysis. Cellulose 23, 1433-1445.

Examples

examples

Examples

[0055]In a preferred embodiment of the invention, the cellulose micro and nano fibrils are obtained through enzymatic hydrolysis, followed by high-pressure homogenization (intensive mechanical treatment with two runs with a total pressure of 15x10 7< Pa). The enzymatic-CMF has an intrinsic viscosity inferior to 0.75 m 3< / kg, preferably between 0.45-0.70 m 3< / kg, which corresponds to degrees of polymerization between 1000 and 2000 The produced flocs are able to reflocculate after forced breaking by using, e.g., sonication, and have a median size (measured by laser diffraction spectrometry) between 20 and 50 µm. By using these flocs, consisting of the microfibrils and nanofibrils abovementioned and mineral fillers, on papermaking there is a global improvement of several paper properties, namely strength increase (increase of 41% and 99% for the dry and wet (50% moisture) tensile indices, respectively, and increase of 20% for the tear index), a slight increase (2.2%) of fill...

Technical Field

[0001] The present invention relates to the field of the production of paper products, namely on the products constituted by mineral fillers conjugated with cellulose microfibrils, obtained by an enzymatic process, which improve several paper properties, with no use of paper additives usually expensive and harmful for the environment.Background Art

[0002] The competitiveness within the papermaking industry has been exponentially increasing. If the paper properties are to be improved, then underlying processes have to be optimized in such a way that new horizons, as the synthesis of new materials, are in sight. The cellulose nano and microfibrils (CNF and CMF, respectively) are a quite recent material that has proven to significantly improve paper strength. They present unique characteristics, such as reduced size and high specific surface area, high tensile index, crystallinity and transparency, therefore becoming object of great interest, mainly as reinforcing materials in composite structures. Different raw-materials and methodologies can be used to produce these new fibrous structures, usually including mechanical treatments to fibrillate the fibres. To avoid intensive mechanical energy and to overcome some inherent technical difficulties, it is common practice to apply chemical or enzymatic approaches to pre-treat the fibres.

[0003] The use of CNF in papermaking has been reported as they are able to improve strength, filler retention and / or other specific properties such as absorption. In papermaking they are usually combined with additives commonly used in the industry, such as internal strength agents (e.g. cationic starch), internal sizing agents (e.g. alkyl ketene dimer - AKD and alkenyl succinic anhydride - ASA) or retention agents (e.g. cationic polyacrylamides - CPAM). Previous published inventions refer that the mixture of CNF / CMF with the abovementioned additives improves the bonding between the cellulosic fibres and the mineral fillers. In this sense, it is of paramount relevance to thoroughly understand the different and complex interactions and mechanisms between all the paper components.

[0004] The publication WO2018100524A1 is related to an invention where microfibrillated cellulose is mixed with, at least, two retention agents (cationic polymer, such as starch, and micro / nanoparticles, such as silica or bentonite) as a pre-mix before producing paper. The mixing with mineral fillers is not referred to.

[0005] The invention EP2236664A1 refers a mixture of cellulosic fibres with mineral fillers only with the purpose of producing nanofibrillated cellulose, by using a more efficient process.

[0006] The publication WO2016 / 185397A1 refers the production of mineral fillers directly in a microfibrillated cellulose suspension, which can be used to produce paper with improved drainability. No reference can be found in the cited document to the simultaneous improvement of a series of paper properties, such as the improvement of the tensile index, with increased filler retention, higher strength and air resistance, lower roughness, higher opacity and lower capillary water absorption. Additionally, the mineral fillers, and their effect, are associated to the use of a precursor, in this case carbon dioxide with a specific gas bubble size.

[0007] The publication EP 2978894 B1 defines a paper production process consisting in mixing microfibrillated cellulose with a strength agent (e.g. starch) followed by the addition of a microparticle (e.g. silica or bentonite).

[0008] The article He et al. (2017) refers to a composite of CMF, precipitated calcium carbonate (PCC) and cationic starch which originate high dimension flocs allowing an improvement of PCC retention.

[0009] The publication EP 2425 057 B1 discloses a method for preparing an aqueous furnish to be used in paper manufacturing. Said furnish is composed by filler, fibres, nanofibrillated cellulose and a cationic polyelectrolyte (preferentially cationic starch). It is stated that the filler retention and paper strength are improved when the combination of filler, CNF and starch is used.

[0010] The publication WO2015101498A1 discloses a method for providing a pre-treated filler composition containing PCC, cationic polyacrylamide and nanofibrillated cellulose. The aggregates of said pre-treated filler were characterized by Focused Beam Reflectance Measurements and defined as providing the enhancement of paper properties.

[0011] The publication US2017 / 0284030A1 refers to the use of cellulose microfibrils and inorganic particles as a layer at the surface of a paper structure. Although the described surface layer (containing the cellulose microfibrils and the inorganic particles) does not contain additives, the document states that the paper structure must mandatorily contain approximately 2% of additives, such as flocculants, drainage / formation additives, thickeners, starch and retention agents. Additionally, there is no reference to flocs consisting of mineral fillers and cellulosic fibrils with associated specific intrinsic viscosity, reflocculation ability or size. Furthermore, there is no reference to the simultaneous improvement of a series of paper properties, such as the improvement of the tensile index, with increased filler retention, higher strength and air resistance, lower roughness, higher opacity and lower capillary water absorption.

[0012] Considering the published knowledge, there is still the need to globally improve a series of paper properties, by using efficient, economic and environmentally-friendly products and methodologies, being possible to reduce the amount of added additives, or even without the need to use them, which are usually expensive and harmful for the environment.References:

[0013] WO 2018 100524 A1 - Pre-mix useful in the manufacture of a fiber based product EP 2978894 B1 - Process for production of paper or board He et al. (2017), Ming He, Guihua Yang, Byoung-Uk Cho, Yong Kyu Lee, Jong Myoung Won, Effects of addition method and fibrillation degree of cellulose nanofibrils on furnish drainability and paper properties EP 2425057 B1 - Method for producing furnish, furnish and paper WO 2015 101498 A1 - A method for providing a pretreated filler composition and its use in paper and board manufacturing EP 2236664 A1 - Process for the production of nano-fibrillar cellulose suspensions WO 2016 / 185397 A1 - Production of nanosized precipitated calcium carbonate and use in improving dewatering of fiber webs US 2017 / 0284030 A1 - Paper and paperboard products Summary of Invention

[0014] The present invention intends to solve the current problem of the need to use synthetic additives, which are costly and environmentally harmful, in the production process of paper products with improved properties.

[0015] The invention consists in a method for the production of flocs of mineral fillers and cellulose microfibrils comprising the steps of a) disintegrating and refining up to 4000 revolutions in a PFI beater of 0.3 kg of bleached eucalyptus kraft pulp; b) suspending the beaten fibers in water and adjusting the pH to 5 by the addition of sodium citrate buffer; c) heating the suspension to 323.15 K under constant mechanical stirring and adding the enzyme selected from the enzyme "E1" comprising endocellulase, 10 % exocellulase and 5 % hemicellulose and enzyme "E2" comprising endocellulase with 10 hemicellulose; d) stopping the hydrolysis of the cellulose after 7200 s by heating the suspension to 353.15 K for 900 s and cooling the resulting suspension to room temperature; e) washing the enzymatic sample with demineralized water until the conductivity of the filtrate is low; f)mechanically treating the fibres at 1 % consistency, in a high pressure homogenizer, firstly at 5x10 7< Pa and secondly at 10x10 7< Pa, so that the obtained cellulose microfibrils have an intrinsic viscosity from 0.45 to 0.70 m 3< / kg; g) mixing under stirring the resulting aqueous suspension of step f) having a consistency of 0.2 wt% with a 1 wt% aqueous suspension of calcium carbonate until the formation of flocs calcium carbonate and cellulose microfibrils at a 10:1 mass ratio and a total solids concentration of around 0.01 wt%; h) subjecting the flocs produced in step g) to force-breaking by sonication during 900 s, after 1200s of agitation and stirring for further 1800 s after the 900 s sonication step; i) reflocculation to flocs having a median size between 20 and 50 µm. The produced flocs, unexpectedly, lead to a global improvement of several paper properties in the inexistence of any need to add additives usually expensive and harmful for the environment.

[0016] Additionally, the present disclosure relates to paper products produced from the abovementioned flocs of mineral fillers and cellulose microfibrils (obtained by enzymatic hydrolysis). The obtained product presents several globally improved paper properties, such as the increase of the tensile index (dry and wet-web), with increased filler retention, higher strength and air resistance, lower roughness, higher opacity and lower capillary water absorption, eliminating the need of adding additives usually expensive, such as starch, ASA and cationic polyacrylamides.Brief Description of Drawings

[0017] Figure 1. Characterization of the different CNF / CMF produced and of the bleached kraft pulp for comparison. Figure 2. Amount (%) of each component added in each formulation series for the production of bleached eucalyptus pulp handsheets. Figure 3. Evolution of the median of the particle size distribution (d50) of suspensions containing PCC and CNF / CMF. A test with only PCC was conducted for comparison. Figure 4. Evolution of the median of the particle size distribution (d50) of suspensions containing PCC and CMF-E1 produced with increasing number of passes in the HPH. A test with only PCC was conducted for comparison. Figure 5. Filler retention of handsheets produced with the flocs consisting of the different CNF / CMF, and additives. The horizontal line indicates the current reference (handsheets without the flocs and with PCC and all the additives). Figure 6. PCC-tensile factor normalized to the same PCC content in the handsheets produced with flocs consisting of CNF / CMF obtained through different routes and with additives, and comparison with the current reference (without flocs and with PCC and additives). Figure 7. Tensile index of handsheets produced with the flocs of CMF-E1, with and without additives. The extra black column refers to handsheets produced with an extra addition of 5% of PCC. Figure 8. PCC-tear factor normalized to the same PCC content in handsheets produced with flocs consisting of CNF / CMF obtained through different routes and with additives, and comparison with the current reference (without flocs and with PCC and additives). Figure 9. PCC-opacity factor normalized to the same PCC content in handsheets produced with flocs consisting of CNF / CMF obtained through different routes and with additives, and comparison with the current reference (without flocs and with PCC and additives). Figure 10. Wet-web tensile index of handsheets produced without additives and with flocs consisting of CNF-T3 or CMF-E1 and reference handsheets (without flocs and with PCC and additives) . Figure 11. Influence of the addition of flocs consisting of PCC+CMF-E1 on the tensile index of handsheets produced with pulp with different refining degrees, °SR. The PCC-tensile factor was calculated considering the reference of pulp at 29°SR+PCC+additives. Figure 12. Structural properties of handsheets produced with the flocs consisting of the different CNF / CMF and additives. Figure 13. Water retention value (WRV) and Klemm water capillary rise of laboratorial handsheets produced with the flocs consisting of the different CNF / CMF and additives. Figure 14. CNF / CMF production costs and cost of 3% addition to papermaking. *Fibre cost estimated at 700 euro / metric tonne; ** Energy cost assumed to be 0,02 euro / MJ, excluding water consumption cost. Figure 15. Flocs (1) attached to a cellulosic fibre (2), image obtained by field emission scanning electron microscopy of an handsheet produced with cellulosic fibre and flocs. Detailed Description and Description of Preferred Embodiments

[0018] In the present invention enzymatic cellulose microfibrils (CMF) are conjugated with mineral fillers for use in papermaking, which leads to an improvement of the paper properties, being therefore possible to overcome the need to add other paper additives.

[0019] Cellulose microfibrils (CMF) and nanofibrils (CNF) can be synthesized through different routes, including mechanical, enzymatic and / or chemical processes.

[0020] Fibril sizes make it possible to distinguish between nanofibrils and cellulose microfibrils (Kargarzadeh H., Mariano M., Gopakumar D., Ahmad I., Thomas S., Dufresne A., Huang J., Lin N. (2018)) Advances in cellulose nanomaterials. Cellulose 25: 2151-2189). In the TAPPI standard proposal WI 3021 (TAPPI standard proposal WI 3021. Standard terms and their definition for cellulose nanomaterials, draft), cellulose nanofibrils are defined as having a width of 5-30nm and cellulose microfibrils having a width of 10-100nm. Additionally, the nanofibrillation yield was determined, that is, the ratio of nanofibrils in the different samples, Figure 1. For this purpose, centrifugation and gravimetry (as defined in the ISO / CD TS 21346 standard) are used. Samples composed mainly of nanofibrils (that is, with a ratio of nanofibrils - nanofibrillation yield - greater than 50%) and CMF are samples composed mainly of microfibrils (that is, with a nanofibril ratio - nanofibrillation yield - less than 50%).

[0021] In this work were considered, additionally to the cellulose microfibrils (CMF) produced by enzymatic hydrolysis with endoglucanase (according to the methodology reported by Tarrés et al. 2016) and hereby defined as CMF-E1 and CMF-E2, and cellulose nanofibrils (CNF) produced by oxidation mediated by TEMPO, 2,2,6,6-Tetramethylpiperidine 1-oxyl (according to the methodology reported by Saito et al. 2007), commonly designated as CNF-TEMPO, and hereby identified as CNF-T3 and CNF-T9, other CNF / CMF produced through different routes, namely by mechanical refining (hereby identified as CMF-Mec) and by carboxymethylation with distinct monochloroacetic acid amounts, namely 9% and 27% (hereby identified as CNF-C9 and CNF-C27, respectively).

[0022] The enzymatic cellulose microfibrils (CMF) were produced from 0.3 kg of bleached eucalyptus kraft pulp, which was disintegrated and refined up to 4000 revolutions in a PFI beater. The fibres were then subjected to enzymatic hydrolysis by using different commercial enzymes: Enzyme "E1" (endocellulase, 10% exocellulase and 5% hemicellulose) and Enzyme "E2" (endocellulase with 10% hemicellulose). Bovine serum albumin (Sigma-Aldrich, USA) was used to determine their protein concentration, according to the Bradford method (Bradford, 1976) and values of 5.0 and 5.8 kg / m 3< were obtained for enzymes "E1" and "E2", respectively.

[0023] The beaten fibres were suspended in water (3.5% consistency) and the pH was adjusted to 5 by the addition of sodium citrate buffer. The suspension was heated to 323.15 K under constant mechanical stirring and the enzyme was added (3x10 -4< kg per kg of pulp). The cellulose hydrolysis was stopped after 7200 s by heating the suspension to 353.15 K for 900s. The resulting suspension was cooled to room temperature.

[0024] For the CNF produced through oxidation, the beaten fibres were added to an aqueous suspension containing NaBr and TEMPO at room temperature. Afterwards, a sodium hypochlorite (NaClO) solution at a ratio of 3 or 9 mol per kg of fibre was slowly added to the previous mixture, while keeping the pH at 10 with NaOH for 120 s, originating the samples "CNF-T3" and "CNF-T9", respectively.

[0025] Both the enzymatic and the TEMPO samples were thoroughly washed with demineralized water until the conductivity of the filtrate was low. After the enzymatic and oxidative treatments, the fibres were finally mechanically treated, at 1% consistency, in a high-pressure homogenizer ((HPH, GEA Niro Soavi, model Panther NS3006L), firstly at 5x10 7< Pa and secondly at 10x10 7< Pa. This mechanical treatment is intensive, preferentially with two runs (total pressure of 15x10 7< Pa).

[0026] The produced CNF / CMF were characterized by different techniques, namely to determine the nanofibrillation yield, carboxylic groups content, intrinsic viscosity (degree of polymerization) and charge (zeta potential), as shown in Figure 1. A proper characterization of these materials is essential and the parameters to measure may depend on the application intended. Usually the measured parameters include the nanofibrillation yield, the particle size distribution and median particle size, the surface chemistry and the strength properties, but also the specific surface area and suspension rheology accordingly to the end application.

[0027] Intrinsic viscosity measurements were performed for the CNF / CMF suspensions by dissolving them in cupriethylenediamine, according to the ISO standard 5351:2010. The degree of polymerization (DP) was calculated using the Mark-Houwink equation, as described elsewhere (Henriksson et al. 2008).

[0028] The nanofibrils amount depicted in Figure 1 allows distinguishing between the nanofibrillated samples (CNF), which are also the functionalized ones and therefore with high carboxyl groups content and high zeta potential (absolute value), from the microfibrillated samples (CMF).

[0029] In order to properly assess the interactions between all the paper components, laboratorial handsheets were produced with five different series, with distinct amounts of bleached eucalyptus kraft pulp, refined up to 33 °SR (refining degree, Schopper Riegler), CNF / CMF, precipitated calcium carbonate (PCC), cationic starch, alkenyl succinic anhydride (ASA), and / or linear cationic polyacrylamide (CPAM). The former additives are usually added in order to improve the process or paper properties. Figure 2 depicts the amounts used.

[0030] Afterwards, the flocs of PCC combined with the cellulose micro or nanofibrils are produced. Previously, a 1 wt% aqueous suspension of calcium carbonate and a 0.2 wt% aqueous suspension of each of the TEMPO CNF or enzymatic CMF samples were prepared. The calcium carbonate and the CNF or CMF, at a 10:1 mass ratio and a total solids concentration of around 0.01 wt%, are mixed under stirring (2000 rpm). After 1200 s of agitation, sonication is applied during 900 s to break the flocs. After the 900 s the stirring continues for further 1800 s.

[0031] An important factor to consider when improving the paper performance is the evaluation of the interaction of the CNF / CMF with PCC, through flocculation tests. In fact, mineral fillers, such as PCC, are not able to establish strong bonds with cellulosic fibres and therefore during paper formation a great amount of material is lost through the web. In this sense, it becomes essential to efficiently flocculate the filler, in order to promote higher particle sizes in order to avoid losses through the web. Nevertheless, it is also important to control the size in order to not harm the paper formation. As abovementioned, the inorganic fillers are usually flocculated with the aid of synthetic additives, such as cationic polyacrylamides or cationic starch.

[0032] In this sense, the evolution of the flocs size is controlled over time by laser diffraction spectrometry in a Mastersizer 2000 equipment (Malvern Instruments), equipped with the Hydro2000 module, and by applying a PCC refractive index of 1.57 and the Mie theory for the calculations. This procedure was proposed for filler particles (without CNF) by Rasteiro et al. 2008.

[0033] From Figure 3 it is possible to state that the mechanical CMF initially flocculated the PCC particles, but the agitation and sonication applied broke the flocs and therefore values around 8 µm (similar to the normal aggregation of PCC) were obtained after the 5400 s of measurement. On the contrary, the enzymatic CMF led to high PCC flocculation, with floc sizes of ca. 26µm, even after applying the shear forces. In the case of the TEMPO-mediated CNF, floc sizes of around 38µm were obtained if CNF-T3 was used. Finally, for the carboxymethylated CNF, a much stronger flocculation occurred, originating flocs with sizes up to 63 µm. The flocs size is highly dependent on the CNF / CMF properties. For the enzymatic CMF, the intrinsic viscosity must be inferior to 0.75 m 3< / kg, preferably between 0.45-0.70 m 3< / kg, which corresponds to a degree of polymerization between 1000-2000, in order to generate flocs with the above-mentioned sizes. Supplementary studies for this work show that the CNF-TEMPO must have an intrinsic viscosity superior to 0.10 m 3< / kg, preferably between 0.12 - 0.21 m 3< / kg, which corresponds to a degree of polymerization between 300-500, and a carboxyl groups content inferior to 1.4 mol / kg, in order to efficiently flocculate PCC particles, specifically, with floc sizes between 20-50 µm (after flocs breaking and subsequent reflocculation).

[0034] An additional study to assess the influence of the high-pressure homogenizer in the flocculation was conducted with enzymatic CMF-E1 produced with 2, 4 or 6 runs (corresponding to total pressures of 12.5x10 7< , 27.5x10 7< and 42.5x.10 7< Pa, respectively. CMF with decreasing degree of polymerization ((1834, 1747 e 1504, respectively) and decreasing flocculation ability (Figure 4), were obtained.

[0035] The biggest flocs were obtained with CMF-E1 produced with two runs in the HPH (total pressure of 12.5x10 7< Pa). The flocs size presented median values of 28 µm (after breaking the flocs and subsequent reflocculation)

[0036] For the handsheets production, with the formulations and amounts depicted in Figure 1, the produced flocs are added to the eucalyptus bleached beaten fibre, with stirring. After 120 seconds, the remaining paper additives are also added. The cationic starch+ASA mixture is stirred with the abovementioned components for 150 seconds and the cationic polyacrylamide for 5 seconds. The formulations are then poured into the handsheet former, with the following automatic steps of air agitation (5 seconds), decantation (5 seconds) and drainage (time duration dependent on the formulations). The handsheets are pressed, dried and conditioned according to the standard ISO 5269-1. The structural, optical and mechanical properties are measured according to the correspondent standards: basis weight (ISO 536:2011), Gurley air resistance (ISO 5636-5:2003), Bendtsen roughness (ISO 8791-2:2013), tensile index (ISO 1924-2:2008), tear index (ISO 1974:2012), opacity (ISO 2471:2008) and Klemm capillary rise (ISO 8787:1986). The mineral filler retention in the fibrous matrix is evaluated under standard TAPPI T 211 om-93.

[0037] The increase of the amount of mineral fillers used in paper products is usually desirable, not only due to environmental matters since the use of cellulosic fibres can be reduced, but also for economic reasons since fibres are usually much more expensive and papers with higher filler amounts are easier to dry. Besides, their use results in the improvement of several properties, such as opacity, surface smoothness and printability. However, mineral fillers lead to a decrease of the paper strength, since they negatively affect the fibre-fibre bonding. Besides, they are usually lost through the web, as abovementioned, which makes it necessary to add retention agents to the papermaking process. Additional problems related to the paper formation and printability (sheet delamination and / or dusting) are also found. For these reasons, the mineral filler amount is usually limited to values inferior to 30% of the total paper weight.

[0038] In this sense, handsheets were prepared according to the formulations depicted in Figure 2, in a semi-automatic laboratory sheet former (300-1 model, LabTech) using a 120 mesh screen.

[0039] The filler retention was measured as abovementioned, for the 5 different series used for the handsheets production. Figure 5 depicts the PCC retention in the handsheets produced with additives and with the CNF / CMF prepared through the different processes.

[0040] As above explained, with the exception of CNF-T9, the presence of CNF / CMF, and consequent PCC flocculation, leads to a high PCC retention, even in the absence of the paper additives. By comparing the results obtained with the method that recreates the industrial reality of paper production, i.e., using PCC with all the additives, but without CNF / CMF, the results show an increase of the PCC retention with the great advantage of not being necessary to use expensive additives, such as CPAM.

[0041] Paper products should have a high mechanical strength in order to resist all the tensions they are subjected to at the paper machine, as well as at industrial printers. However, there are several factors that negatively influence this parameter, such as the use of recycled fibres and mineral charges, or the need to reduce the paper basis weight (to reduce virgin fibre consumption, for example). The paper strength depends on the strength of the used fibres as well as on the strength of their bonding. Tensile index is the most commonly used property to evaluate paper mechanical strength and is indirectly related to the mineral filler content. In this sense, another way to analyze the obtained results is by using the filler-tensile factor, which consists on a normalization of the tensile index, by considering the effective PCC content on the handsheets: Filler-tensile factor: (tensile index x filler content) with flocs / (tensile index x filler content) with PCC and with additives. If the value obtained is superior to 1, the handsheets have a tensile index superior to that of the reference handsheets, i.e., without flocs and with PCC and with additives.

[0042] The same factor and related considerations were applied for the tear index and opacity.

[0043] The obtained values are depicted in Figure 6, 7 and 8.

[0044] Using the TEMPO nanofibrils with lower charge (CNF-T3) on the flocs production, the tensile index is only slightly increased when comparing to the reference, if no additives are used. In fact, the negative charge of the nanofibrils seems to lead them to preferentially bond with the added cationic additives, therefore the nanofibrils are not available to bond with the fibres, hindering paper strength, as visible at Figure 6 on the series containing additives.

[0045] By using the flocs consisting of enzymatic CMF, the tensile index is always improved when comparing with the reference (factor superior to 1), either in the presence or absence of additives. Furthermore, by removing CPAM, the highest tensile increase is obtained with the enzymatic-CMF flocs, which seems to suggest that the high chain length of these microfibrils is overpassing the effect of CPAM.

[0046] Due to the great improvement of paper properties by using the flocs consisting of the enzymatic CMF "E1", an additional series of handsheets was produced with extra 5% of PCC incorporation, and without additives (Figure 2). The results of the tensile index (Figure 7) reveal that it is possible to produce handsheets with the same filler content than that of the reference (without flocs and with all additives), and with higher tensile index, without the need to add additives.

[0047] The effect of the flocs in the tear index did not follow the same trend (Figure 8). In this case, only the CNF-C27 flocs were able to improve the tear index in the additives absence. However, when using starch+ASA or all additives, the flocs consisting of the non-functionalized CMF (CMF-Mec, CMF-E1 and CMF-E2) lead to a great improvement of this property.

[0048] Additionally, better results were obtained with the flocs consisting of CMF-E1 (either for the tensile or tear indices) than with the flocs of CMF-E2, most probably due to the presence of exocellulase in the composition of enzyme E1.

[0049] The optical properties, evaluated by opacity (Figure 9), were also very much improved, either in the presence or absence of additives.

[0050] The wet-web strength of the handsheets was also significantly improved by using the flocs. Figure 10 depicts the tensile index measured when increasing moisture levels of the handsheets produced without additives and with flocs consisting of CNF-T3, CMF-E1 and CNF-C9. The reference handsheets (without flocs and with PCC and with additives) are also presented for comparison. All of the plotted handsheets have the same mineral filler content (26-28% effective). Furthermore, reference handsheets produced in the same conditions (without additives), but otherwise produced with 25% of softwood fibre, which is usually used to limit web breaks in a paper machine, are also plotted. These results are of extreme importance for the paper machine operation: for moisture levels common for the drying section (7 to 50%), the flocs can contribute to a reduction of web breaks and / or for increased machine speeds, since the wet-web strength is improved. In this sense, Figure 10 also reveals that the flocs can supplant the need for softwood fibre addition, since the wet-web strength is the same than that of the reference handsheets.

[0051] Figure 11 depicts the results obtained for handsheets produced with eucalyptus bleached pulp beaten to different refining degrees (1300, 1800 and 2300 PFI rotations, corresponding to refining degrees of 25, 27 and 29 °SR (Schopper Riegler), respectively). It is concluded that by adding the flocs consisting of PCC and CMF-E1 to laboratorial handsheets without additives, it is possible to reduce more than 4°SR on the refining degree of the pulp, and still maintain the same paper strength. However, the filler retention is slightly affected and, therefore, a filler-tensile factor was computed by considering the reference produced with pulp beaten to 2300 PFI rotations (29°SR) + PCC + additives (SA+P). By analyzing the results it is possible to state that the limit for properties improvement is above 26°SR, meaning that by adding flocs consisting of PCC and CMF-E1 it is possible to produce handsheets with the same filler content and same tensile index than that of the reference handsheets, but by saving energy on pulp beating.

[0052] Figure 12 depicts the handsheets air resistance, measured by the Gurley method, and their Bendtsen roughness, which are relevant structural properties. As well-known, the addition of CNF / CMF to papermaking leads to severe drainability difficulties, since the paper structure becomes more closed. However, when comparing with the performance of flocs consisting of chemical CNF (produced through TEMPO-oxidation or carboxymethylation), the air resistance of the handsheets produced with the enzymatic CMF is not so negatively affected.

[0053] The water retention capacity of the fibrous matrix was evaluated through the water retention value (WRV) and by the Klemm capillary rise (Figure 13). In the additives absence, the handsheets containing the flocs retain the same or less water than the reference but, in the additives presence, the handsheets with flocs retain much more water than the reference, which may be harmful for the drying process at the paper machine. However, by using 5% extra of PCC in the formulation with starch+ASA and with the flocs consisting of CMF-E1, it was possible to obtain the same WRV value than that of the reference handsheets with all additives (WRV=1.22±0.02 kg / kg), besides the aforementioned increased paper strength. On the other side, in the flocs presence, the water absorptiveness capillary rise was always inferior to that of the reference, which means that a lower liquid penetration will occur, e.g., when coating or surface sizing is applied at the paper machine.

[0054] It was also possible to prove that the production of flocs consisting of CMF-E1 and their addition to papermaking is the cheapest among the ones produced, as shown in Figure 13.Examples

[0055] In a preferred embodiment of the invention, the cellulose micro and nano fibrils are obtained through enzymatic hydrolysis, followed by high-pressure homogenization (intensive mechanical treatment with two runs with a total pressure of 15x10 7< Pa). The enzymatic-CMF has an intrinsic viscosity inferior to 0.75 m 3< / kg, preferably between 0.45-0.70 m 3< / kg, which corresponds to degrees of polymerization between 1000 and 2000 The produced flocs are able to reflocculate after forced breaking by using, e.g., sonication, and have a median size (measured by laser diffraction spectrometry) between 20 and 50 µm. By using these flocs, consisting of the microfibrils and nanofibrils abovementioned and mineral fillers, on papermaking there is a global improvement of several paper properties, namely strength increase (increase of 41% and 99% for the dry and wet (50% moisture) tensile indices, respectively, and increase of 20% for the tear index), a slight increase (2.2%) of filler retention, an increase of air resistance, a decrease of surface roughness (65% decrease), an increase of opacity (increase of 0.2%) and a decrease of water absorption (Klemm capillary rise decrease of 24%), when compared to the reference (paper products without flocs and with PCC, starch, ASA and CPAM, commonly used additives which are expensive and potentially environmentally harmful). Additionally, these enhancements are possible even when reducing the amount or not using any other additives.

[0056] The present document thereby discloses the use of the above described flocs (Figure 15) for the production processes of paper products, with a global improvement of several paper properties, being therefore possible to reduce the amount, or without the need to add commonly used paper additives.

[0057] The aforementioned flocs can be used in processes with increased mineral filler retention and with a reduction of the amount of paper additives used, when compared with the paper products produced at the same conditions but without the mentioned flocs.

[0058] The resultant paper products incorporating the described flocs present improved paper properties, namely mechanical, structural and optical properties, with a smaller amount of added paper additives or even without containing any amount of paper additives.

[0059] Therefore, these flocs act as reducers of the amount of internal strength agents, internal sizing agents and retention agents, commonly used in paper products production processes and incorporated in paper products.References:

[0060] Henriksson M., Berglund L. A., Isaksson P., Lindstro T. and Nishino T. (2008) Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 9, 1579-1585. Rasteiro MG, Garcia FAP, Ferreira P, Blanco A, Negro C, Antunes E (2008) Evaluation of flocs resistance and reflocculation capacity using the LDS technique. Powder Technol. 183: 231-238 Saito T, Kimura S, Nishiyama Y, Isogai A (2007) Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules. 8: 2485-2491. Tarrés Q., Saguer E., Pèlach M. A., Alcalà M., Delgado-Aguilar M. and Mutjé P. (2016) The feasibility of incorporating cellulose micro / nanofibers in papermaking processes: the relevance of enzymatic hydrolysis. Cellulose 23, 1433-1445.

Examples

examples

Examples

[0055]In a preferred embodiment of the invention, the cellulose micro and nano fibrils are obtained through enzymatic hydrolysis, followed by high-pressure homogenization (intensive mechanical treatment with two runs with a total pressure of 15x10 7< Pa). The enzymatic-CMF has an intrinsic viscosity inferior to 0.75 m 3< / kg, preferably between 0.45-0.70 m 3< / kg, which corresponds to degrees of polymerization between 1000 and 2000 The produced flocs are able to reflocculate after forced breaking by using, e.g., sonication, and have a median size (measured by laser diffraction spectrometry) between 20 and 50 µm. By using these flocs, consisting of the microfibrils and nanofibrils abovementioned and mineral fillers, on papermaking there is a global improvement of several paper properties, namely strength increase (increase of 41% and 99% for the dry and wet (50% moisture) tensile indices, respectively, and increase of 20% for the tear index), a slight increase (2.2%) of fill...

Claims

1. Method for the production of flocs of mineral fillers and cellulose microfibrils comprising the following steps: a) disintegrating and refining up to 4000 revolutions in a PFI beater of 0.3 kg of bleached eucalyptus kraft pulp; b) suspending the beaten fibers in water and adjusting the pH to 5 by the addition of sodium citrate buffer; c) heating the suspension to 323.15 K under constant mechanical stirring and adding the enzyme selected from the enzyme "E1" comprising endocellulase, 10 % exocellulase and 5 % hemicellulose and enzyme "E2" comprising endocellulase with 10 % hemicellulose; d) stopping the hydrolysis of the cellulose after 7200 s by heating the suspension to 353.15 K for 900 s and cooling the resulting suspension to room temperature; e) washing the enzymatic sample with demineralized water until the conductivity of the filtrate is low; f) mechanically treating the fibres at 1 % consistency, in a high pressure homogenizer, firstly at 5x107 Pa and secondly at 10x107 Pa, so that the obtained cellulose microfibrils have an intrinsic viscosity from 0.45 to 0.70 m3 / kg; g) mixing under stirring the resulting aqueous suspension of step f) having a consistency of 0.2 wt% with a 1 wt% aqueous suspension of calcium carbonate until the formation of flocs calcium carbonate and cellulose microfibrils at a 10:1 mass ratio and a total solids concentration of around 0.01 wt%; h) subjecting the flocs produced in step g) to force-breaking by sonication during 900 s, after 1200s of agitation and stirring for further 1800 s after the 900 s sonication step; i) reflocculation to flocs having a median size between 20 and 50 µm.

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