TECHNICAL FIELD

The present generally concerns a method for isolating nanofibrils from phosphorylated lignocellulosic fibers.

BACKGROUND

Nanocelluloses are materials with exceptional properties owing to their nanoscopic dimensions and which, as a result, generate a large specific surface area and reaction interface. They can be integrated into a broad spectrum of applications: from packaging to medicine and aerospace industry. However, the major problem that slows down the widespread use of this material at industrial scale is the high cost of production. Nowadays, nanocelluloses are isolated from wood or lignocellulosic biomass by chemical processes, which can cause environmental pollution, and/or by energy-intensive mechanical action on the fiber cell wall.

It is known that phosphorylated lignocellulose includes phosphate substituents that are covalently bonded to the lignocellulose surface increases the accessibility of water to fiber superstructure. The water can go as far as breaking the dense bundle of hydrogen bonds if the phosphorylated fibers are subjected simultaneously to a high- shear mechanical action. As a result, cellulose nanofibrils with an average diameter of 3-5 nanometers can be released. The inventor is aware of several research groups that provided evidence of this process, namely Ghanadpour, M., Carosio, F., Larsson, P.T., Wagberg, L. Phosphorylated cellulose nanofibrils: A renewable nanomaterial for the preparation of intrinsically flame-retardant materials, Biomacromolecules, 16 (2015) 3399-3410 DOI: io.io2i/acs.biomac.5boni7; Ghanadpour, M., Wicklein, B., Carosio, F., Wagberg, L. All-natural and highly flame-resistant freeze-cast foams based on phosphorylated cellulose nanofibrils. Nanoscale, 2018, 10, 4085-4095. DOI: io.iO39/c7nro9243a; Naderi, A., Lindstrom, T., Weise, C.F., Flodberg, G., Sundstrom, J., Junel, K., Erlandsson, J., Runebjork, A.M., Phosphorylated nanofibrillated cellulose: production and properties, Nordic Pulp & Paper Research Journal, 31 (2016) 20-29; Noguchi, Y., Homma, L, Matsubara, Y. Complete nanofibrillation of cellulose prepared by phosphorylation, Cellulose 24 (2017) 1295-1305. DOI 10.1007/810570-017-1191-3; and Noguchi, Y., Homma, I., Matsubara, Y., Fushimi H., Banzashi, G., Preparation and utilization of highly transparent and viscous dispersion of phosphorylated cellulose nanofibers, Advances in Pulp and Paper Research, Oxford. DOI: 10.15376/6-0.2017.2.813. (2017).

The nano-fibrillation process starting from phosphorylated cellulose fibers shows a number of net advantages compared to other known pathways. These include:

1. an esterification reaction, wherein the phosphorylation is simpler and less aggressive toward the lignocellulose fibers than (2,2,6,6-Tetramethylpiperidin-i- yl)oxyl (TEMPO) oxidation and acid hydrolysis chemistries; and

2. a production yield close to 100%, the dissolved fraction is minor and therefore nanofibrils preserve both crystalline and amorphous regions.

However, the nano-fibrillation method proposed by these groups have a number of significant drawbacks. The energy consumption remains quite important despite the fact it is lower compared with other methods. The nano-fibrillation is realized with expensive equipment such as high-pressure homogenizers and microfluidizers that operate at very high pressure. Also, the efficiency of the process drops significantly for phosphorylated lignocellulose fibers with medium and high content of covalently bonded phosphate.

These studies revealed that the overall efficiency of nano-fibrillation significantly decreases if the phosphate content exceeds a specific threshold equivalent to 2500 mmoles/kg of charge. This means that energy consumption increases while the yield, i.e., the fraction of nanofibrils, decreases.

It should be noted that theoretically, each covalently bonded phosphate substituent brings to fiber two equivalent charges that are associated with the two protons, as shown in Figure 1. The total charge is expressed as mmoles/kg and is measured quantitatively by conductometric or potentiometric titration.

One research group concluded that beyond the charge threshold of 2500 mmoles/kg, the fiber crosslinking becomes important, and impairs the nano-fibrillation process. Thus, there is a need for an improved process for producing lignocellulose nanofibrils from phosphorylated fibers.

BRIEF SUMMARY

I have significantly reduced, or essentially eliminated, the problems associated with the above processes by designing a simple yet effective method to produce nanocellulose starting from phosphorylated lignocellulose fibers. The product, known as phosphorylated cellulose nanofibrils, is a type of functionalized nanocellulose which can find various applications in agriculture, forestry, and energy storage. Advantageously, the method used to produce nanocellulose is simple, does not involve chemicals, and has a low energy consumption input.

Accordingly, in one embodiment there is provided a method for producing lignocellulose nanofibrils from phosphorylated fibers, the method comprising: hydrolyzing an amount of phosphorylated fibers in which the phosphorylated fibers include_a phosphorylated lignocellulose of Formula I:

I in which i) lignocellulose is selected from the group consisting of: lignin; hemicelluloses; and cellulose; and ii) n is greater than o but less than 8000 mmoles/kg, the phosphorylated fibers being dispersed in an aqueous medium at a first temperature to produce an aqueous dispersion having a consistency, the phosphorylated fibers being dispersed for a predetermined time so as to enhance /increase water accessibility to the fiber wall, thereby causing fiber swelling.

In one example, the predetermined time is from under 10 minutes up to about 120 minutes. In one example, the consistency is from less than 1% to 10% or more.

In one example, the method further includes: applying a mechanical shear force to the previously treated fibers, in a second aqueous suspension having a pH of between about pH 3.0 to about pH 10.0, thereby isolating the lignocellulose nanofibrils as hydrogel.

In one example, the first temperature is from about 50 degrees Celsius to about 150 degrees Celsius.

In one example, the hydrolyzing is a self-hydrolysis conducted in an open vessel at atmospheric pressure or in a pressure vessel either at atmospheric pressure or above atmospheric pressure.

In one example, the self-hydrolysis can be performed at a consistency (grams of dry phosphorylated fibers/100 grams mixture of fibers and water) % range from less than 1% to 10% or more.

In one example, the phosphorylated lignocellulose fibers have an equivalent charge density from less than 500 mmoles/kg up to about 8000 mmoles/kg.

In another example, the self-hydrolysis is performed for a period from under 10 min to about 120 min.

In another example, the phosphorylated fibers are selected from a group consisting of: sulfite, soda or Kraft pulps, regardless the fiber source, bleachability or refining status; thermomechanical pulp (TMP), old corrugated cardboard (OCC), and dissolving pulp (alpha cellulose).

In one example, the lignocellulose is present in one or more of lignin, hemicelluloses and cellulose, and any combination thereof. The lignocellulose includes lignin present at from o to 30%, m/m. The lignocellulose includes hemicelluloses present at from o to 35%, m/m. The lignocellulose includes cellulose present at from 35 to 95%, m/m.

In still another example, n is an integer from o up to 8000 mmoles/kg wherein the concentration is calculated based on mmoles of equivalent phosphate charges contained in 1 kg of phosphorylated lignocellulose fibers.

In yet another example, the nanofibrils are isolated in water as hydrogel through mechanical action with a device capable to generate a high shear stress. The device includes mixers, homogenizers, microfluidizers or a combination thereof, the device operating at an independent operating pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of that described herein will become more apparent from the following description in which reference is made to the appended drawings wherein:

Figure 1 is a schematic representation of chemical structure of phosphorylated lignocellulose fiber of Formula 1;

Figure 2 is a graphical representation showing the evolution of fiber width with the degree of phosphorylation;

Figure 3 shows images with phosphorylated Kraft fibers (top images) and their isolated cellulose nanofibrils (bottom images) produced using the method described herein; and

Figure 4 shows images with hydrogel of nanocellulose fibrils obtained from phosphorylated fibers following the self-hydrolysis thermal treatment at atmospheric pressure (left image) and above atmospheric pressure (right image).

DETAILED DESCRIPTION

Definitions

Unless otherwise specified, the following definitions apply: The singular forms “a”, “an” and “the” include corresponding plural references unless the context clearly dictates otherwise.

As used herein, the term “comprising” is intended to mean that the list of elements following the word “comprising” are required or mandatory but that other elements are optional and may or may not be present.

As used herein, the term “consisting of’ is intended to mean including and limited to whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory and that no other elements may be present.

As used herein, the term “lignocellulose” is intended to mean a mixture of lignin, hemicelluloses and cellulose main polymers. Examples of the mixtures include, but are not limited to, one or more of lignin (from o to 30% m/m); hemicelluloses (from o to 35%, m/m); and cellulose (from 35 to 95%, m/m). The content of each one of lignin, hemicellulose and cellulose can vary in content %, m/m.

As used herein, the term “aqueous media” is intended to mean water, which is the dispersion medium for lignocellulose fibers. The term dispersion medium means that the lignocellulose fibers do not dissolve in the water, but rather they are dispersed therein.

As used herein, the term “consistency” is intended to mean dispersion of the fibers, described herein, in water. As is known to those skilled in the art of papermaking, the fibers can disperse in water and the consistency, %, is expressed as quantity of dry fibers (g) in 100 g mixture of fibers and water. In the examples described herein, the consistency ranges from less than 1% to 10% or more.

As used herein, “m/m” is intended to mean a mass percentage. For example, cellulose (from 35 to 95%, m/m) means the material contains from 35 to 95 grams of cellulose in 100 grams of lignocellulose fibers. Also, X%, m/m can be written also as X%, w/w, where “w” stands for “weight”. I. Formula:

A phosphorylated lignocellulose of Formula I is illustrated below: in which i) lignocellulose is selected from the group consisting of lignin (from o to 30%, m/m); hemicelluloses (from o to 35%, m/m); and cellulose (from 35 to 95%, m/m); and ii) n is greater than o but less than 8000 mmoles/kg (mmoles of equivalent phosphate charges contained in 1 kg of phosphorylated lignocellulose fibers).

It should be noted that the compound of Formula 1 is acid such that at least one of the two protons can dissociate in water. Advantageously, the protonated form of Formula 1 is capable of self-hydrolysis. Indeed, as I discovered, this is the form, which is capable of self-hydrolysis, i.e., it is the protonated phosphate that is capable of selfhydrolysis and thus there is no requirement to use acid in my proved process.

The lignocellulose is present in one or more of lignin, hemicelluloses and cellulose, and any combination thereof.

In one example, the lignocellulose includes lignin present at from o to 30% m/m.

In one example, the lignocellulose incudes hemicelluloses present at from o to 35% m/m.

In one example, the lignocellulose includes cellulose present at from 35 to 95%, m/m.

In one example, n is an integer from o up to 8000 mmoles/kg wherein the concentration is calculated based on mmoles of equivalent phosphate charges contained in 1 kg of phosphorylated lignocellulose fibers. II. Methodology:

My novel and unobvious method produces lignocellulose nanofibrils from phosphorylated fibers. The method includes hydrolyzing an amount of phosphorylated fibers in which the phosphorylated fibers include_a phosphorylated lignocellulose of Formula I:

I

In Formula 1, the lignocellulose is selected from the group consisting of: lignin; hemicelluloses; and cellulose and n is an integer greater than o but less than 8000 mmoles/kg, The phosphorylated fibers are dispersed in an aqueous_medium_at a first temperature to produce an aqueous dispersion with a consistency. The phosphorylated fibers are dispersed for a predetermined time so as to enhance/increase water accessibility to the fiber wall. This causes the fiber to swell, which in turn enhances water accessibility to the fiber wall. The predetermined time is from under 10 minutes up to about 120 minutes. The consistency is from less than 1% to 10% or more.

A mechanical shear force is applied to the previously treated fibers, in a second aqueous suspension, which has a pH of between about pH 3.0 to about pH 10.0. This allows the lignocellulose nanofibrils to be isolated as a hydrogel.

The first temperature is from about 50 degrees centigrade to about 150 degrees centigrade.

The hydrolyzing is a self-hydrolysis process that is conducted in an open vessel at atmospheric pressure or in a pressure vessel either at atmospheric pressure or above atmospheric pressure. The self-hydrolysis can be performed over a wide consistency range. The consistency (grams of dry phosphorylated fibers/ioo grams mixture of fibers and water) % ranges from under 1% to above 10%.

The phosphorylated lignocellulose fibers have an equivalent charge density from less than 500 mmoles/kg up to about 8ooo mmoles/kg.

The self-hydrolysis is performed for a period from under io min to about 120 min.

The phosphorylated fiber starting material are selected from a group but not limited to sulfite, soda or Kraft pulps, regardless the fiber source, bleachability or refining status; thermomechanical pulp (TMP), old corrugated cardboard (OCC), and dissolving pulp (alpha cellulose).

Alternatively the nanofibrils can be isolated in water as hydrogel through mechanical action with a device that is capable generating a high shear stress. The device includes, for example, mixers, homogenizers, microfluidizers or any combination of those, regardless the operating pressure or other characteristics and manufacturing features.

Examples

As part of my development of the novel and unobvious method, I initially isolated nanofibrils from phosphorylated lignocellulose fibers having a chemical structure of Formula 1, to produce a hydrogel, initially using only kitchen utensils. On the first attempt, I boiled in a regular open pot for approximately 60 minutes a small amount of those fibers in water. I then filtered the fibers to thicken the pulp and added tap water to reach a final consistency between 2-5%. In the example given, a pulp consistency is expressed as % and is intended to mean how many grams of dry fibers are contained in 100 grams of suspension/dispersion/mixture of fibers and water. E.g., 2-5 % means there are between 2 to 5 grams of dry fibers in 100 grams of suspension/dispersion/mixture. The difference to 100 grams, i.e., 98-95 grams, is represented by water. Then, I stirred the pulp for approximately 30 minutes using a home mixer. Although the final product had a hydrogel appearance, I found the process time-consuming and too energy intensive. To improve the nano-fibrillation process, I decided to conduct the thermal treatment in a commercial pressure vessel as a second attempt.

Therefore, the same quantity of fibers and water was submitted for only 15 minutes to this process using the pressure vessel. This observation was wholly unexpected to see that the pulp already developed the characteristics of a hydrogel after such a short thermal treatment under pressure. Moreover, the product completely transformed into a very viscous and homogenous hydrogel after only 10 minutes of stirring. The thermal treatment of fibers in water and under pressure allowed me to efficiently isolate the cellulose nanofibrils, i.e., with a high yield and low energy consumption.

I decided to then carry out a thorough investigation about the interaction of phosphorylated lignocellulose fibers with water at different temperatures. For the beginning, I evaluated the impact of phosphate charge density on fiber swelling in water at room temperature. Thus, I designed a series of phosphorylation reactions on cellulosic Kraft fibers that allowed me to gradually increase the content of phosphate substituents and consequently, the total charge. The method used to phosphorylate the fibers was previously described in Belosinschi, D., Brouillette, F., Shi, Y., Paradis, J., Doucet, J., Phosphorylated lignocellulosic fibers, uses and processes of preparation thereof. International Patent Application WO 2017/214719 Al (2017).

As best illustrated in Figure 2, the degree of swelling is proportional with the fiber width which was measured with an optical device.

As noted, the phosphorylated fibers swell more in water than pristine cellulosic fibers, regardless the quantity of covalently bonded phosphate substituents. This is explained by the fact that phosphate is a substituent with a higher polarity compared to the alcoholic hydroxyl (-OH) groups of cellulose. The width increases the most since this is the only dimension capable of expansion in a lignocellulosic fiber. The width increase is very fast up to around 4000 mmoles/kg of charge after which it slightly decreases.

The phosphorylation reaction is a heterogenous reaction that takes place on the fiber surface. The covalently bonded phosphate starts to build a phosphate layer on the fiber surface with the corresponding hydration cage in a regular aqueous phase. Without wishing to be bound by theory, I understand that above 4000 mmoles/kg of charge, the phosphate density is so high that the water molecules adsorbed on the surface form a compact layer at fiber/water interface. This layer behaves like a physical barrier, preventing the phosphorylated fibers from further water adsorption from the aqueous phase. The layer composed by hydrated phosphate on surface of fibers acts as a lubricant and generates a slippage plane that considerably reduces the shear stress at the fiber interface. As a result, the phosphorylated fibers with a charge density greater than 4000 mmoles/kg develop anti-wear property in water. They are very hard to beat in a wet state and it is difficult to isolate nanofibrils from their structure with a reasonable specific energy consumption.

This analysis demonstrated that the phosphate layer must be cracked to increase the water accessibility to the fiber wall and therefore to allow a facile isolation of nanofibrils. I discovered that the easiest way to realize this is to use a hydrolysis reaction.

As best seen in Figure 1, and Formula 1, the covalently bonded phosphate substituent on cellulose has two protons, one of which can dissociate when in water. The first pKa of covalently bonded phosphate is close to the corresponding pKai of phosphoric acid and has a value of around 3. This value is not low enough to induce a hydrolysis at room temperatures, which is typically about 20 degrees centigrade. However, it proves to be very effective if the activation energy of the hydrolysis reaction is considerably reduced by increasing the temperature. As a result, the phosphorylated fibers are capable of efficient self-hydrolysis once a temperature threshold is exceeded.

My novel and unexpected discovery is based on a simple, efficient, and scalable method to produce hydrogels composed of lignocellulose nanofibrils starting from phosphorylated fibers. Advantageously, the energy expended in the manufacturing process is low and the method is not limited to a low threshold of phosphate charges. Moreover, my discovery now unexpectedly opens the way to produce nanocelluloses with a high phosphate content, which enhance properties such as fire-retardancy, ion exchange, water absorption and the like. The higher the concentration of covalently bonded phosphate substituents, the higher the charge and therefore the nanocellulose is more efficient for the applications described herein. Broadly speaking, the herein invention proposes a self-hydrolysis method to isolate nanofibrils from the phosphorylated lignocellulose fibers:

Self-hydrolysis.

The phosphorylated fibers were submitted to a hydrolysis in hot water. The hydrolysis can be performed at a temperature varying from about 50 degrees Celsius to about 150 degrees Celsius. The rate of hydrolysis increases with the temperature, so the time of hydrolysis decreases accordingly. Hydrolysis can also be carried out at a temperature higher than 100 degrees Celsius. Indeed, I demonstrated this in two examples from Table 2: “Thermal treatment at atmospheric pressure” was realized at 100 C by boiling the fibers in water at atmospheric pressure - 1 atm, and “Thermal treatment above atmospheric pressure” was realized at 110-120 C by boiling the fibers in water above the atmospheric pressure - 2-3 atm in a pressurized cooker vessel. Both processes induce microcracks in the phosphate layer and which later, propagate deeper into the fiber wall. The accessibility of water to the fiber, and implicitly the fiber swelling, increases considerably after the self-hydrolysis.

As best seen in Figure 1 and Formula 1 above, the two protons from the covalently bonded phosphate substituent behave as auto-catalysts for this acid hydrolysis reaction. The self-hydrolysis is crucial because it influences the overall result, from product quality to process efficiency, i.e., the yield and the specific energy consumption of the nanocellulose production.

The isolation of nanofibrils from phosphorylated lignocellulose fibers, also known as “nano-fibrillation”is performed by mechanical action. Any equipment capable of generating a high-shear stress can be used at this stage, regardless the operating pressure and temperature.

Testing results:

Two series of trials were conducted starting from a phosphorylated softwood- bleached-unbeaten Kraft pulp with the chemical structure shown in Figure 1. In the first case, the self-hydrolysis was carried out by boiling the pulp in water under atmospheric conditions (1 atmosphere (atm)), in the second one by thermal treatment of the pulp in water above the atmospheric pressure in a commercial pressure vessel. In the example, the commercial pressure vessel is selected from, for example but not limited to, Ninja Foodi Pressure cooker (Model: OP3O1C); a pressurized still or a pressurized vessel such as a pressure cooker, operating pressure: 2-3 atm. Then, the pulp was thickened by filtration.! on a Buchner funnel. Finally, the pulp was redispersed in tap water and submitted to mechanical action in a high-shear laboratory mixer, for example, a high shear mixer emulsifier, Brand: MXBAOHENG, Model: LYEsooW-Tf oG)). A third sample, composed from the same quantity of phosphorylated fibers and water suspension was submitted directly to the mechanical action. The self-hydrolysis was skipped in this case and the sample was used as blank run for comparison. The conditions and the results of this study are shown in Tables 1 and 2 below:

Table 1

Table 1 shows a comparison of dimensional characteristics of phosphorylated fibers and their isolated cellulose nanofibrils using the method described herein.

Table 2

Table 2 shows the impact of self-hydrolysis on the production efficiency and quality of cellulose nanofibrils hydrogel.

To reiterate, referring to Figure 2: The fiber width increases after phosphorylation because the phosphate is a very hydrophilic moiety and is grafted on the surface;

The increase is sharp until 4000 mmoles/kg of equivalent charge of phosphate, after which it slightly declines;

The phosphate moieties have enough space on the fiber surface, and they adsorb water freely up to 4000 mmoles/kg of charge;

After 4000 mmoles/kg, the density of charge on the fiber surface is quite large and the phosphate moieties start to compete for water;

A compact layer of hydrated phosphate is generated on the surface of fibers beyond this charge threshold, preventing the additional swelling of the fiber; and The phosphate layer must be cracked to further increase the water accessibility to the fiber wall and therefore to allow facile isolation of nanofibrils.

Turning now to Figure 3, phosphorylated Kraft fibers are large filiform elements with sizes (as can be seen in Table 1) of the order of mm by length and pm by width. Their morphology is close to that of pristine Kraft fibers. Nanofibrils isolated from phosphorylated Kraft fibers are filiform elements as well but with sizes (as seen in Table 1) about 1000 times smaller of the order of pm by length and nm by width. They are invisible for the naked eye and form a viscous hydrogel in water.

Turning now to Figure 4, the self-hydrolysis of phosphorylated Kraft fibers by boiling the aqueous pulp suspension at atmospheric pressure leads to a hazier hydrogel (see the left-hand image). This is due to the presence of some fiber fragments along with nanofibrils, suggesting that the self-hydrolysis is less efficient in this case. The selfhydrolysis of phosphorylated Kraft fibers by boiling the aqueous pulp suspension above atmospheric pressure leads to translucent hydrogel (see the right-hand image). This means that the hydrogel is mainly composed of nanofibrils, suggesting that selfhydrolysis is more efficient in this case.

Conclusions

Based on the results provided above, I confirmed the following:

Isolating cellulose nanofibrils from phosphorylated fibers using only a mechanical action is extremely difficult without the use of specialized equipment.

Isolating cellulose nanofibrils from phosphorylated fibers is easily carried out when a self-hydrolysis thermal treatment is performed prior to the mechanical action.

The self-hydrolysis in a pressurized system: a) reduces the duration of the hydrolysis; b) reduces the charge loss; c) reduces the weight loss; d) increases the quantity processed; and e) reduces the duration of mechanical action.

All these aspects translate into an increased productivity and a clear reduction of the overall energy consumption in the preparation of cellulose nanofibrils from phosphorylated fibers when the production process follows the path described in the right-hand column of Table 2 shown above.

Utilities

The product is a hydrogel composition, known as phosphorylated lignocellulose nanofibrils, which is a type of partial-functionalized nanocellulose with various applications in agriculture, forestry, energy storage, healthcare, and environmental remediation.

For example, in agriculture, the composition can be used as a wetting agent for hydrophobic substrates; moisture retention in soil for drought mitigation; organic phosphate-based fertilizer; and encapsulation additive for controlled release of commodity fertilizers.

For example, in forestry, the composition can be used as a fire-retardant agent to prevent and control forest fires; coating additive for lumber and wood-based products, scaffold and antimicrobial agent for control of phytopathogens and as organic phosphate fertilizer for trees.

For energy storage, the composition can be used as a binder for electrode materials; battery additive to buffer the acidity, water, and carbon dioxide traces and to control the SEI (solid electrolyte interphase); gel-electrolyte for aqueous-based batteries; and proton exchange membrane for fuel cell.

For healthcare and environmental remediation, the composition can be used as drug carrier and controlled release by encapsulation, adsorption of heavy metals, dyes, and other drug contaminants from wastewaters.

Other Embodiments

From the foregoing description, it will be apparent to one of ordinary skill in the art that variations and modifications may be made to the embodiments described herein to adapt it to various usages and conditions.