Process for production of film comprising microfibrillated cellulose

Abstract

The present invention relates to a new process for improving dewatering when manufacturing a film comprising high amounts of microfibrillated cellulose (MFC) without negatively impacting the film properties. According to the present invention a high amount of nanoparticles is used as an additive, optionally together with a polymer.

Claims

1. A process for the production of 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 60 weight-% based on the weight of solids of the suspension; b. adding cationic polymer to said suspension, wherein said cationic polymer is selected from a group consisting of: cationic starch, polyaminoamide-epichlorohydrin (PAE), and copolymers thereof; c. adding nanoparticles to said suspension to provide a mixture of said microfibrillated cellulose, cationic polymer, and said nanoparticles and said cationic polymer, wherein said nanoparticles are anionic at neutral or alkaline pH, and wherein an amount of nanoparticles added is between 1.0 to 30 kg on dry basis per ton of dry solids of the suspension, and wherein the weight ratio of cationic polymer to anionic nanoparticles is in the range of from 1:3 to 1:20, wherein said nanoparticles are nanosilicate or nanobentonite particles; d. providing said mixture to a porous wire to form a web; and e. dewatering said web to form an intermediate thin substrate or film having an OTR value, at 50% RH and 23 ° C., between 200 and 7.

2. The process according to claim 1, wherein the weight ratio of cationic polymer to anionic nanoparticles is in the range 1:5 to 1:12.

3. A process for the production of a film comprising the steps of: a. providing a first mixture comprising pulp and nanoparticles; b. fibrillating said first mixture to form a second mixture comprising microfibrillated cellulose, nanoparticles, and a cationic polymer, wherein the content of the microfibrillated cellulose of said second mixture is at least 60 weight-% based on the weight of solids of the second mixture, wherein an amount of nanoparticles added is between 1.0 to 30 kg on dry basis per ton of dry solids of the second mixture, wherein said nanoparticles are nanosilicate or nanobentonite particles, wherein a weight ratio of cationic polymer to nanoparticles is in a range between 1:3 to 1:20, and wherein said cationic polymer is selected from a group consisting of: cationic starch, polyaminoamide-epichlorohydrin (PAE) and copolymers thereof; c. providing said second mixture to a porous wire to form a web; and d. dewatering said web to form an intermediate thin substrate or film having an OTR value, at 50% RH and 23° C., between 200 and 7.

4. The process of claim 3, wherein said nanoparticles are anionic nanoparticles.

5. The process of claim 1, wherein the OTR value, at 50% RH and 23° C. is between 30 and about 7.

6. The process of claim 1, wherein the OTR value, at 50% RH and 23° C. is between 15 and about 7.

7. The process of claim 1, wherein the OTR value, at 50% RH and 23° C. is between 10 and about 7.

8. The process of claim 3, wherein the OTR value, at 50% RH and 23° C. is between 30 and about 7.

9. The process of claim 3, wherein the OTR value, at 50% RH and 23° C. is between 15 and about 7.

10. The process of claim 3, wherein the OTR value, at 50% RH and 23° C. is between 10 and about 7.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: The effect of silica on dewatering rates of 30 GSM films prepared of MFC using a vacuum sheet mold. All test points with silica also include 5 kg/t PAE and 1 kg/t cationic polysaccharide.

DETAILED DESCRIPTION

(2) In one embodiment of the present invention, a film is formed in a paper making machine or according to a wet laid production method, by providing a suspension onto a wire and dewatering the web to form an intermediate thin substrate or said film. According to one embodiment, a suspension comprising microfibrillated cellulose is provided to form said film.

(3) The microfibrillated cellulose content of the suspension may, according to one embodiment be in the range of from 60 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-%.

(4) In one embodiment of the present invention, enhanced dewatering effect of MFC suspension in wet laid production method is achieved by dosing anionic nanoparticles in an early stage of the manufacturing process, not as part of the short circulation retention system in the machine used.

(5) 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.

(6) 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).

(7) There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt %) when dispersed in water. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 300 m.sup.2/g, such as from 1 to 200 m.sup.2/g or more preferably 50-200 m.sup.2/g when determined for a freeze-dried material with the BET method.

(8) 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.

(9) 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.

(10) 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.

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

(12) 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.

(13) 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.

(14) 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.

(15) Subsequent to the wet web being placed onto the wire, it is dewatered to form an intermediate thin substrate or film.

(16) The dewatering on wire may, according to one embodiment be performed by using known techniques with single wire or twin wire system, frictionless dewatering, membrane-assisted dewatering, vacuum- or ultrasound assisted dewatering, etc. After the wire section, the wet web is further dewatered and dried by mechanical pressing including shoe press, hot air, radiation drying, convection drying, etc. The film might also be dried or smoothened by soft or hard nip (or various combinations) calenders etc.

(17) According to one embodiment the wet web is dewatered by vacuum, i.e. water, and other liquids, is sucked from the furnish when it is placed on the wire.

(18) According to one embodiment, the film comprising the microfibrillated cellulose and nanoparticles may be laminated to or with a thermoplastic polymer. The thermoplastic polymer may be any one of a polyethylene (PE), a polyethylene terephthalate (PET) and a polylactic acid (PLA). The polyethylene may be any one of a high density polyethylene (HDPE) and a low density polyethylene (LDPE), or various combinations thereof. By using for instance PLA as the thermoplastic polymer the product may be formed completely from biodegradable materials.

(19) 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.

EXAMPLES

Example 1

(20) A sheet mold equipped with a vacuum pump and commercial SSB wire was used to screen some additives that could be used to enhance the dewatering of MFC suspensions.

(21) Colloidal silica (EKA NP 200) was tested as dewatering agent. In the system containing 5 kg/t PAE (Kymene X-Cel 25) and 1 kg/t cationic polysaccharide with microfibrillated cellulose, silica was found to be very efficient in improving the dewatering of MFC suspension, see FIG. 1. However, unusually high doses (>1.5 kg/t) of silica were required to achieve improvement in dewatering. Dewatering time of 30 gsm MFC film was reduced from 38 seconds (MFC suspension without any chemicals) to 25 seconds with addition of 5 kg/t PAE (Kymene X-Cel 25) and 1 kg/t cationic polysaccharide, and further decreased to 20, 19, or 15 seconds with 1.5, 5.0, or 10.0 kg/t silica (EKA NP 200) addition.

Example 2

(22) Bentonite (Hydrocol SH), silica (EKA NP) and nanobentonite (RXW) were used as fluidization (Microfluidizer MH-110 device) additives with dosage levels of 1 wt-% and 5 wt-%. The pulp and the chemicals where placed into the laboratory wet-disintegrator which was operated for 10000 revs. After disintegration, the pulp/chemical mixture was processed to MFC with Microfluidizer device.

(23) Films were prepared of the fibrillated products in presence of 5 kg/t PAE (Kymene X-Cel 25) and 1 kg/t cationic polysaccharide and it was found that when the suspensions contained 1% (i.e. 10 kg/t) bentonite or silica the dewatering time for 30 gsm MFC films was reduced from approximately 150 seconds to approximately 100 seconds. With the suspension containing 5% (i.e. 50 kg/t) bentonite the dewatering rate was hindered. Nanobentonite provided 10-15% decrease in dewatering time with 1% and 5% dose (i.e. 10 kg/t and 50 kg/t). The dewatering times and film properties are given in Table 1. Surprisingly, in spite of the relatively large content of micro- or nanoparticles (bentonite, silica or nanobentonite) in the films (10 or 50 kg/t) the oxygen barrier properties were remained at an initial level (value 7.9 cc/m.sup.2/day for Ref.) except for the sample containing 5% nanobentonite showing an increased OTR.

(24) Thus, it was found that when introducing micro- or nanoparticles such as silica, bentonite and nanobentonite into the system prior to fibrillation of the pulp to MFC, the dewatering properties of MFC suspensions can be enhanced. Surprisingly high dosages, even up to 50 kg/t, can be used without negative effect on the oxygen barrier properties of the MFC film.

(25) TABLE-US-00001 TABLE 1 the effect of nano- and microparticles on dewatering and properties of 30 gsm films prepared in presence of 5 kg/t PAE and 1 kg/t cationic polysaccharide. Fibrillation of Imatra pretreated pulp was done in presence ot particles. OTR (*) was determined for 20 GSM films at 50% RH and 23° C. temperature. Basis Tensile Dewatering Density, weight, index, time, sec kg/m.sup.3 g/m.sup.2 Nm/g Ref. 152 940 29.9 103 Bentonite, 1% 104 931 30.4 97 Bentonite, 5% 146 915 30.2 91 Silica, 1% 96 936 30.9 98 Silica, 5% 110 907 30.8 88 Nanobentonite, 1% 133 931 30.5 99 Nanobentonite, 5% 129 910 31.5 90 Tensile Elastic stiffness TEA OTR, Stretch, modulus, index, index, cc/(m.sup.2- % Mpa MNm/kg J/kg day)* Ref. 3.8 8486 2.7 2854 7.9 Bentonite, 1% 3.4 8417 2.9 2356 8.7 Bentonite, 5% 2.9 8798 3.1 1946 8.3 Silica, 1% 3.4 8828 2.9 2425 7.6 Silica, 5% 3.1 7730 2.8 2079 8.6 Nanobentonite, 1% 3.5 8626 2.9 2475 8.4 Nanobentonite, 5% 3.5 7802 2.6 2261 40.4 19.5

Example 3

(26) The aim of this trial was to clarify the effect of silica dosage on MFC web dewatering and runnability as well as on resulting product properties, especially barrier properties. The retention system comprised of 4 kg/t wet end starch (Raisamyl 50021), 1.5 kg/t AKD (Aquapel A221), 1 kg/t cationic polysaccharide, 5 kg/t PAE (Kymene X-Cel 25), The tested silica dosages were 0.9, 2.0, 5.0 and 10 kg/t.

(27) The increased silica dosages were found to be beneficial for the web dewatering as expected (see Table 2). With 0.9 kg/t silica dose the water line was 2/2 which was then quite linearly changed closer to the headbox (i.e. improved dewatering) down to 1/3 with the highest silica addition (10 kg/t). With respect to retention, a clear maximum of 98.8% was observed with 2 kg/t silica dose while with all other silica levels the retention was 91.4% or less. Therefore, silica with relatively high dosages can be used to enhance the dewatering of MFC webs in continuous production. Furthermore, the oxygen barrier properties can be preserved in spite of the surprisingly high dosage of silica especially.

(28) The oxygen transmission rates (OTR) showed a clear minimum with the silica dosage of 5 kg/t where OTR was around 10 cc/(m.sup.2-day).

(29) TABLE-US-00002 TABLE 2 wet end conditions and properties of resulting film products in pilot paper machine trials. Water line: the first number is the vacuum box number, the second number is where the water line is in the vacuum box (for example 1/5 means that the water line is in the first vacuum box, in the last part of the vacuum box) P3/1 P3/2 P3/3 P3/4 Fiber source MFC MFC MFC MFC Wet end starch, kg/t 4 4 4 4 Silica, kg/t (dosed 0.9 2 5 10 amount) PAE, kg/t 5 5 5 5 Other additives, kg/t AKD 1.5 AKD 1.5 AKD 1.5 AKD 1.5 cationic cationic cationic cationic polysac- polysac- polysac- polysac- charide charide charide charide O-water temp., ° C. 70 70 70 70 Machine speed, m/min 15 15 15 15 Water line (suction 2/2 2/1 1/5 1/3 boxes) Wire retention, % 83.11 98.82 91.38 90.42 Headbox consistency, % 0.267 0.254 0.290 0.261 pH 8.2 8.1 8.0 7.9 OTR, cc/(m.sup.2-day) 44.9 41.3 9.1 176 50% RH and 23° C. Silica, kg/t 1.1 2.9 4.9 (measured amount)

(30) 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.