Process for production of film comprising microfibrillated cellulose

Abstract

The present invention relates to a process for manufacturing a film comprising high amounts of microfibrillated cellulose (MFC), having haptic properties. According to the present invention, a wet web comprising MFC is formed, followed by addition of particles having an average diameter of at least 1 μm to the wet web, followed by dewatering and/or drying. The wet web may be formed for example by wet laid or cast forming methods. The particles may be added to the wet web for example by cast coating or spraying.

Claims

1. A process for the production of an intermediate substrate or a film comprising the steps of: a) providing a suspension comprising microfibrillated cellulose, wherein a content of the microfibrillated cellulose of said suspension is at least 50 weight-% based on a dry weight of solids of the suspension; b) using the suspension of step a) to form a wet web; c) adding particles having an average diameter of at least 1 μm to the wet web formed in step b); d) dewatering the web, drying the web, or both dewatering and drying the web to form an intermediate substrate or film.

2. The process according to claim 1, wherein at least 50% by weight of the particles added are organic.

3. The process according to claim 1, wherein an amount of particles added to the wet web is at least 1 kg on dry basis per ton of dry solids of the web formed in step b).

4. The process according to claim 1, wherein the wet web is formed by cast forming.

5. The process according to claim 1, wherein the particles are added by curtain coating, cast forming, or spraying.

6. The process according to claim 1, wherein the particles have an average diameter of at least 10 μm.

7. The process according to claim 1, wherein the content of microfibrillated cellulose of the suspension in step a) is at least 60 weight-% based on the weight of solids of the suspension.

8. A film obtained according to the process of claim 1.

9. The film according to claim 8 having a haptic property.

10. The film according to claim 9 wherein the haptic property is texture, an optical effect, or both.

11. The film according to claim 10, wherein more than one sensory effect is achieved.

12. A product comprising a film according to claim 8.

Description

DETAILED DESCRIPTION

(1) The present invention is directed to the production of an intermediate thin substrate or a film comprising the steps of: a) providing a suspension comprising microfibrillated cellulose, wherein the content of the microfibrillated cellulose of said suspension is at least 50 weight-% based on the dry weight of solids of the suspension; b) using the suspension of step a) to form a wet web; c) adding particles having an average diameter of at least 1 μm to the wet web formed in step b); d) dewatering and/or drying the web to form an intermediate thin substrate or film.

(2) The wet web can be prepared for example by wet laid and cast forming methods. In the wet laid method, the suspension is prepared and provided to a porous wire. The dewatering occurs through the wire fabric and optionally also in a subsequent press section. The final drying is usually done using convection (cylinder, metal belt) or irradiation drying (IR) or hot air. A typical wet laid is for example the fourdrinier former used in papermaking. In the cast forming method the wet web is formed for example on a polymer or metal belt and the subsequent initial dewatering is predominantly carried out in one direction, such as via evaporation using various known techniques.

(3) In both techniques, it might be beneficial to prefer less contact drying in order to avoid destruction of the texture. Hence, the substrate should preferably be dried with non-impact drying methods such as infra-red (IR), ultraviolet (UV), electron beam (EB), hot air, hot steam etc. A soft nip dryer or contact dryers can be used depending on the type of deposited particles and texture formed or if a protective coating is used.

(4) The addition of the particles takes place when the wet web has been formed. Thus, at the time of addition of the particles, the dry content of the web is 1-80% by weight, such as 1-60% by weight, such as 1-40% by weight, such as 3-20% by weight. The particles may be added to the full width of the wet web or to a part thereof. The particles can also be a mixture or added in several layers or in sequential steps.

(5) The particles can be added in a defined pattern or randomly, depending on the desired haptic effect.

(6) The microfibrillated cellulose content of the suspension is in the range of from 50 to 99.9 weight-% based on the weight of solids of the suspension. In one embodiment, the microfibrillated cellulose content of the suspension may be in the range of 70 to 99 weight-%, in the range of 70 to 95 weight-%, or in the range of from 75 to 90 weight-%.

(7) Microfibrillated cellulose (MFC) shall in the context of the patent application mean a nano scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods.

(8) The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Cellulose fibres, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 2011, 6:417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril (Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol 53, No. 3.), is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).

(9) There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt %) when dispersed in water. The cellulose fiber is preferably fibrillated to such an extent that the microfibrillated cellulose has a surface area of at least 30 m.sup.2/g or more preferably more than 60 m.sup.2/g or most pref. >90 m.sup.2/g when determined according to nitrogen adsorption (BET) method for a solvent exchanged and freeze dried sample.

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

(11) The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated. MFC is produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

(12) The above described definition of MFC includes, but is not limited to, the new proposed TAPPI standard W13021 on cellulose nanofibril (CNF) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions.

(13) According to another embodiment, the suspension may comprise a mixture of different types of fibers, such as microfibrillated cellulose, and an amount of other types of fiber, such as kraft fibers, fines, reinforcement fibers, synthetic fibers, dissolving pulp, TMP or CTMP, PGW, etc.

(14) The suspension may also comprise other process or functional additives, such as fillers, pigments, wet strength chemicals, retention chemicals, cross-linkers, softeners or plasticizers, adhesion primers, wetting agents, biocides, optical dyes, fluorescent whitening agents, de-foaming chemicals, hydrophobizing chemicals such as AKD, ASA, waxes, resins etc.

(15) The papermaking machine that may be used in the process according to the present invention may be any conventional type of machine known to the skilled person used for the production of paper, paperboard, tissue or similar products.

(16) The dewatering of the wet web according to the wet web can be carried out using methods known in the art. For example, the wet web may be provided on a wire, and be dewatered to form an intermediate thin substrate or film.

(17) The dewatering on wire may be performed by using known techniques with single wire or twin wire system, frictionless dewatering, membrane-assisted dewatering, infrared dewatering, vacuum- or ultrasound assisted dewatering, etc. After the wire section, the wet web may be further dewatered and dried by mechanical pressing including shoe press, hot air, radiation drying, convection drying, etc.

(18) Optionally, wet pressing and/or contact drying can be used to remove moisture from the wet web.

(19) Depending on the dryness of the wet web at the time of adding the particles and depending on the dewatering, the lateral and vertical distribution and infiltration of the particles within the film can be controlled. If the wet web has a high dry content, i.e. relatively low moisture content at the time of adding the particles and if dewatering is predominantly carried out in one direction, the particles will typically not be evenly distributed in the film. The particles will then mostly be present on the side of the film corresponding to the side of the wet web to which the particles were added in the process according to the present invention. Thus, in a cross section of the film, at least 70% of the particles may be present in one half of the cross section, corresponding to the side of the wet web to which the particles were added, and less than 30% of the particles may be present in the other half of the cross section. The distribution of particles may be evaluated by chemical analysis such as FTIR and/or RAMAN spectroscopy, coupled with elementary analysis and/or cross section imaging.

(20) The film or the laminate may also be applied to other paper products, such as food containers, paper sheets, paper boards or boards or other structures that need to be protected by a barrier film.

(21) The film obtained according to the present invention is typically such that it is possible to print on the film using printing methods known in the art.

(22) Advantageously, the film obtained by the process according to the present invention retains its haptic properties when laminated or otherwise applied on other paper or board structures.

EXAMPLES

(23) Films (30 gsm) prepared from MFC dispersion were prepared by vacuum filtration. Samples (see table 1) were added to the wet (5-6 wt-% dry content) or semi-wet (25-30 wt-% dry content) film in the final stage of the vacuum filtration. The samples were added by manually sprinkling onto the wet or semi-wet film. After sample addition, the wet or semi-wet films were dried in a drum drier at 80° C. for at least 90 minutes.

(24) The resulting films were inspected visually before and after a taping test. The taping test was carried by attaching a tape (Scotch crystal) to the surface and subsequently detaching the tape. The films were characterized using a manual sensory analysis (table 1).

(25) TABLE-US-00001 TABLE 1 Samples, appearance Estimated average Sample particle size Appearance Barrisurf LX (kaolin), <2 μm Smooth, soft wet surface Barrisurf LX (kaolin), <2 μm Smooth, soft semi-wet surface Microtalc, semi-wet About 1-8 μm Smooth, slippery, surface soft, some material detaches Arbocel CW 620, semi- >8 μm (fiber diameter) Powdery, soft, some wet <2000 μm (fiber material detaches length) Actigum, semi-wet >10 μm Rough, granular, rough sand-paper Micro-Technik CMC, 10-40 μm (fiber Fine sand-paper, DS = 0.26, semi-wet diameter) rough, hairy Micro-Technik CMC, 10-40 μm (fiber Rough DS = 0.26, semi-wet, diameter) slower addition Particles from Thickness 10-40 μm, Rough, granular, Hansamix refined pine length 0.1-4 mm some material detaches

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