SE1900067A1 - Films of microfibrillated cellulose - Google Patents

Abstract

A method is provided for preparing a film of crosslinked microfibrillated cellulose.Phosphorylated microfibrillated cellulose is cast or wet-laid into a film; and then said film is post-treated (e.g. by heat-treatment) to provide crosslinking between the phosphorylated microfibrillated cellulose. Films and hygiene products comprising such films are also described.

Description

FILMS OF MICROFIBRILLATED CELLULOSE A method for providing films of crosslinked microfibrillated cellulose is provided, as Well ascast or wet-laid films of crosslinked phosphowlated microfibrillated cellulose. Productscomprising said films are also described. Such films exhibit desirable properties, e.g. strength (in particular wet-strength), water absorbance and elasticity/flexibility.

BACKGROUND Microfibrillated cellulose (MFC) comprises partly or totally fibrillated cellulose or lignocellulosefibers. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibrildiameter or particle size distribution and/or aspect ratio (length/width) depends on thesource and the manufacturing methods. The smallest fibril is called elementary fibril and hasa diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Nanoscale researchletters 2011, 6:417), while it is common that the aggregated form of the elementary fibrils,also defined as microfibril, is the main product that is obtained when making MFC e.g. byusing an extended refining process or pressure-drop disintegration process (see Fengel, D., ,Tappi J., March 1970, Vol 53, No. 3.). Depending on the source and the manufacturingprocess, the length of the fibrils can vary from around 1 to more than 10 micrometers. Acoarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protrudingfibrils from the tracheid (cellulose fiber), and With a certain amount of fibrils liberated from the tracheid (cellulose fiber).

There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose,nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers,cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibrilaggregrates and cellulose microfibril aggregates. MFC can also be characterized by variousphysical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 Wt%) When dispersed in water.

MFC exhibits useful chemical and mechanical properties. Chemical surface modification ofMFC has the potential to improve the properties of MFC itself, as Well as films wet-laid or cast from MFC, e.g. mechanical strength, Water absorbance and elasticity/flexibility.

An additional problem with chemically modified MFC is that it has increased Water absorptionwhen compared to non-modified MFC, due to its chemical charge, and can start losingintegrity upon contact with water. A balance of mechanical strength and Water absorbance can therefore be difficult to achieve.

Other documents in this technical field include US 4,256,111 and US 6,027,536.

There is therefore a need to improve the properties of films cast from MFC; in particular,(wet) strength, water absorption and elasticity/flexibility properties. Suitably, theimprovement can be achieved in a straightforward manner, Without the use of external modifiers such as crosslinkers.

SUMMARY It has been found by the present inventor(s) that fibrous materials (i.e. films) with desirableelasticity and water absorption can be formed from a cellulose composition comprising phosphorylated microfibrillated cellulose (P-MFC).

A method for preparing a film of crosslinked microfibrillated cellulose is thus provided, saidmethod comprising the steps of: i. casting or wet-laying a cellulose composition comprising phosphorylatedmicrofibrillated cellulose (P-MFC); said cellulose composition additionallycomprising unmodified microfibrillated cellulose and/or chemically-modified microfibrillated cellulose; into a film; ii. heat-treatment of said film to provide crosslinking of the phosphorylated microfibrillated cellulose.A cast film is also provided by the method set out herein. A water-absorbent materialcomprising the film is provided. In a further aspect, a hygiene product comprising the film and/or water-absorbent material is provided.

Further aspects of the invention are provided in the following text and in the dependent claims.

DETAILED DISCLOSURE In a first aspect, the invention provides a method for preparing a film of crosslinkedmicrofibrillated cellulose (MFC).

Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context ofthe patent application mean a nano-scale cellulose particle fiber or fibril with at least onedimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose orlignocellulose fibers. The cellulose fiber is preferably fibrillated to such an extent that the finalspecific surface area of the formed MFC is from about 1 to about 300 mZ/g, such as from 1 to200 mZ/g or more preferably 50-200 mZ/g when determined for a freeze-dried material withthe BET method.

Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysisfollowed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps are usually required in order to make MFC manufacturing both energyefficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose orlignin. The cellulose fibers may be chemically modified before fibrillation, wherein thecellulose molecules contain functional groups other (or more) than found in the originalcellulose. Such groups include, among others, carboxymethyl (CMC), aldehyde and/orcarboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), orquaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or NFC.

The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on theplant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as arefiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, single - ortwin-screw extruder, fluidizer such as microfluidizer, macrofluidizer or fluidizer-typehomogenizer. Depending on the MFC manufacturing method, the product might also containfines, or nanocrystalline cellulose or e.g. other chemicals present in Wood fibers or inpapermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

MFC can be produced from wood cellulose fibers, both from hardwood or softwood fibers. Itcan also be made from microbial sources, agricultural fibers such as wheat straw pulp,bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp includingpulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

The above described definition of MFC includes, but is not limited to, the proposed TAPPI standard W13021 on cellulose nano or microfibril (CMF) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

Phosphorylated microfibrillated cellulose (P-MFC) is typically obtained by reacting cellulosepulp fibers with a phosphorylating agent such as phosphoric acid, and subsequentlyfibrillating the fibers to P-MFC. One particular method involves providing a suspension ofcellulose pulp fibers in Water, and phosphorylating the cellulose pulp fibers in said Watersuspension With a phosphorylating agent, followed by fibrillation with methods common inthe art. Suitable phosphorylating agents include phosphoric acid, phosphorus pentaoxide,phosphorus oxychloride, diammonium hydrogen phosphate and sodium dihydrogen phosphate.

In the reaction to form P-MFC, alcohol functionalities (-OH) in the cellulose are converted tophosphate groups (-OPO32'). In this manner, crosslinkable functional groups (phosphate groups) are introduced to the pulp fibers or microfibrillated cellulose.

In a first general step of the method, cellulose composition comprising phosphorylated microfibrillated cellulose (P-MFC) is formed (e.g. wet-laid or cast) into a film.

In the case that the cellulose composition comprises P-MFC, components other than P-MFCmay be present in the composition. However, the cellulose composition suitably comprisesmore than 25%, preferably more than 50%, such as e.g. more than 75% by weight P-MFC.The cellulose composition comprising P-MFC additionally comprises unmodified (native) MFC.By "unmodified" or "native" MFC is meant microfibrillated cellulose which is the direct resultof fibrillation of native cellulose fibers, i.e. without chemical treatment before or after fibrillation.

Suitably, therefore, the cellulose composition consists of P-MFC and MFC. Alternatively oradditionally, the cellulose composition comprising P-MFC additionally comprises chemically-modified microfibrillated cellulose, such as e.g. dialdehyde-MFC or TEMPO-MFC (i.e. MFCoxidised with 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl). Additional components of thecellulose composition may include natural or synthetic filaments or natural or synthetic staple fibres.

In a second general step of the method, the film from the first step is heat-treated so as to provide crosslinking of the phosphorylated microfibrillated cellulose. Crosslinking suitably takes place without the use of any additional crosslinking agents; i.e. crosslinks are formed directly between the phosphate moieties and other components of the cellulose composition.

Heat treatment in the second general step of the method suitably takes place at atemperature of between 60 and 200 OC, e.g. between 70 and 120 OC. Such temperatures aresufficient to obtain crosslinking, but also limit potential degradation of the MFC. It has beenestablished that heat treatment suitably takes place for a time of between 10 and 180minutes, depending on the temperature used and initial solids content of the material to heattreat. Heat treatment may take place in an oven, but other methods of heat treatment may also be used.

The fibrous material is a film, and the composition is cast or wet-laid using techniquescommon in the art. The present technology provides the possibility of creating P-MFC filmswith water absorption capacity, elasticity and wet-strength. Such films could be used toprepare moulded materials with different shapes upon drying the wetted crosslinked films in different molds.

The general steps of the method (forming, followed by heat-treatment) may be carried outwithout any intervening method steps. Alternatively, one or more intervening method stepsmay be carried out between the forming step and the heat-treatment step. In one particularaspect, the film may be dried before or during the heat-treatment step. Drying can suitablytake place under ambient conditions (e.g. 25 OC). It has been discovered that crosslinkingcan be triggered in film which has been previously dried at ambient conditions, e.g. byputting dried film according to the invention in the oven. This means that one can in principledry the material at ambient conditions (with no crosslinking) and then trigger the crosslinking when desired at a later stage by heat-treatment.

Alternatively, the step of drying the film can take place during the heat-treatment step. Inthis alternative, a dry, crosslinked film is obtained, which can have advantageous water- absorptive and strength properties both in dry and wet conditions.

If hydrated film is required, a further step of hydrating said film with water after the heat- treatment step may be carried out.

It is thought not to be enough to remove the water from the sample at room temperature(i.e. to dry at RT); a heat-treatment is required for the crosslinking. Furthermore, it wasconsidered surprising that some stretchability/elasticity behaviour could be obtained after soaking the heat-treated material in water.

The presence of phosphate crosslinks between MFC fibrils can be ascertained by spectroscopic methods, e.g. 31P NMR.

In a similar manner, a cast film obtained via the method described herein is provided, as well as a cast film of crosslinked phosphorylated microfibrillated cellulose per se.

A hygiene product is provided which comprises the film of the invention and/or a water-absorbent material comprising said film. The hygiene product may be selected from thegroup consisting of a disposable diaper, a sanitary napkin, a wipe, a tampon, an absorbentdressing and a disposable tissue. A method for providing a hygiene product is also provided,said method comprising preparing a film of crosslinked phosphorylated microfibrillatedcellulose according to the invention, and; incorporating said film into a hygiene product. Theskilled person is aware of standard methods for constructing hygiene products, and incorporating fibrous material into such products.

EXAMPLES 1. Preparation of a P-MFC cast film Materials : 1. P-MFC 2: DS=0.86 mmol/g; pH=9.3; ~1.5 wt% A P-MFC film was cast, and subsequently heat-treated in the oven at 105 °C for 20 min. Itpossessed wet-strength, flexibility and even a slight elasticity in wet-state (assessed manually). 2. Swelling of P-MFC films and native MFC films Materials : 1. P-MFC 2: DS=0.86 mmol/g; pH=9.3; ~4.51 wt%2. Native MFC: pH~4.18 wt%3. TEMPO-MFC: DS=1.25 mmol/g; pH=10.5; ~1.94 wt% Two P-MFC films were cast by spreading P-MFC with a spatula on glass petri dishes. One filmwas left at ambient temperature overnight whereas the other one was first dried in the oven for 1 hour at 105 °C before it was left in room temperature overnight. The two films were then soaked in an excess of deionized water for 4.5 hours and their weight was measured again.

The same procedure was performed using native MFC and TEMPO-oxidized MFC (T-MFC).

Drying method Dry weight (g) Weight after Swellingswelling (g) degree (gwater/g) P-MFC 1 hour oven + 0.20 3.68 18.4ambientovernight P-MFC Ambient 0.22 12.23 55.6overnight Native 1 hour oven + 0.21 1.04 5.0ambientovernight Native Ambient 0.21 1.09 5.2overnight T-MFC 1 hour oven + 0.24 0.47 2.0ambientovernight T-MFC Ambient 0.26 0.54 2.1overnight After soaking of the fi|ms in water, the oven-dried P-MFC film showed a good wet strength and some stretchability/elasticity, whereas ambient dried P-MFC and native samples were very fragile and fe|| apart if manually handled. The T-MFC samples had good integrity, both ambient- and oven-dried, but the swelling after drying was very low. The P-MFC had a much higher capacity in terms of water absorbance than the other samples regardless of drying method, although it is apparent that it swells less after oven drying.

Additional conclusions: One P-MFC film was made with the same procedure as above, but taken out of the oven after 30 minutes, before it was completely dry. After drying completely at room temperature, it was soaked in water. The dry/wet strength seemed as poor as for thefilms only dried at room temperature.

One P-MFC film that had been dried in ambient atmosphere Was put in the oven for minutes before it was soaked in water, and exhibited dry/wet strength.

Claims (16)

1. A method for preparing a film of crosslinked microfibrillated cellulose, said method comprising the steps of: i. casting or wet-laying a cellulose composition comprising phosphorylatedmicrofibrillated cellulose (P-MFC), said cellulose composition additionallycomprising unmodified microfibrillated cellulose and/or chemically-modified microfibrillated cellulose; into a film; ii. heat-treatment of said film to provide crosslinking of the phosphorylated microfibrillated cellulose.

2. The method according to claim 1, wherein said cellulose composition additionally comprises unmodified microfibrillated cellulose.

3. The method according to any one of the preceding claims, wherein said cellulose composition additionally comprises chemically-modified microfibrillated cellulose, such as e.g. dialdehyde-MFC Or TEMPO-MFC.

4. The method according to any one of the preceding claims, wherein said cellulosecomposition comprises more than 25%, preferably more than 50%, such as e.g. more than75% by weight P-MFC.

5. The method according to any one of the preceding claims, wherein said heattreatment takes place at a temperature of between 60 and 200 °C, preferably between 70and 120 °C.

6. The method according to any one of the preceding claims, wherein said heat treatment takes place for a time of between 10 and 180 minutes.

7. The method according to any one of the preceding claims, wherein the crosslinking takes place in the absence of any additional crosslinking agents.

8. The method according to any one of the preceding claims, wherein saidphosphorylated microfibrillated cellulose (P-MFC) is obtained by; providing a suspension ofcellulose pulp fibers in water, and phosphorylating the cellulose pulp fibers in said water suspension with a phosphorylating agent, followed by fibrillation.

9. The method according to any one of the preceding claims, further comprising the stepof drying said film, before the heat-treatment step.

10. The method according to any one of the preceding claims, further comprising the step of hydrating said film with water after the heat-treatment step.

11. A cast or wet-laid film obtained via the method of any one of c|aims 1-10.

12. A cast or wet-laid film comprising crosslinked phosphowlated microfibrillated cellulose.

13. A Water-absorbent material comprising the film according to any one of c|aims 11-12.

14. A hygiene product comprising the film according to any one of c|aims 11-12 and/or a water-absorbent material according to claim 13.

15. The hygiene product according to claim 14, selected from the group consisting of adisposable diaper, a sanitaw napkin, a wipe, a tampon, an absorbent dressing and adisposable tissue.

16. A method for providing a hygiene product, said method comprising preparing a film ofcrosslinked microfibrillated cellulose according to any one of c|aims 1-10, and; incorporating said film into a hygiene product.