A MOULDABLE FIBROUS SHEET AND A PRODUCTION METHOD THEREOF

Abstract

The present invention relates to a web of fibrous cellulosic material derived from wood pulp, said web being suitable for three-dimensional moulding to form a packaging product, wherein the web comprises >40 wt % of soft wood chemical pulp and at least one strength enhancement agent, wherein the web has a grammage less than 400 g/m.sup.2, and wherein the cellulose fibers of said soft wood chemical pulp comprise a fiber curl of >9%.

Claims

1. A web of fibrous cellulosic material derived from wood pulp, said web being suitable for three-dimensional moulding to form a packaging product, wherein the web comprises: >40 wt % of soft wood chemical pulp comprising cellulose fibers, and at least one strength enhancement agent, wherein the web has a grammage less than 400 g/m.sup.2, and wherein the cellulose fibers of said soft wood chemical pulp comprises a fiber curl of >9%.

2. The web according to claim 1, wherein the web comprises a density between 600-875 kg/m.sup.3.

3. The web according to claim 1, further comprising: between 0.3-10% by weight of microfibrillated cellulose, based on a total fiber weight of the web.

4. The web according to claim 1, wherein said strength enhancement agent is-comprises a natural binder agent in the form of a starch, at an amount of 5-75 kg/tn.

5. The web according to claim 4, wherein said starch is selected from a group consisting of: native starch, cooked starch, cationic starch, native chemically modified starch, physically modified polymer grafted starch, enzyme modified starch, anionic starch, amphoteric starch, crosslinked starch, pre-gelled starch,. and swelled starch.

6. The web according to claim 1, wherein said web has been subjected to hydrophobic sizing.

7. The web according to claim 1, wherein the web comprises a geometrical mean tensile strength index of >5 Nm/g.

8. The web according to claim 1, wherein the web comprises a geometrical mean stretch, referring to a percentage elongation of the web before rupture, of >4% as measured by standard tensile test ISO 1924-2 with a span length of 20 mm and a test speed of 2 mm/minute.

9. The web according to claim 1, wherein the web comprises a stretch, referring to a percentage elongation of the web before rupture, in the cross direction (CD), of >6.5% as measured by standard tensile test ISO 1924-2 with a span length of 20 mm and a test speed of 2 mm/minute.

10. A multiply laminate material comprising: at least two layers, wherein one layer of the least two layers is a web according to claim 1.

11. The multiply laminate material according to claim 10, further comprising: at least one barrier layer forming a barrier against liquid, gas, aroma, or grease, oil and/or fat.

12. A method for producing a molded, three-dimensional fiber-based product, comprising at least the steps of: providing a fibrous cellulosic web material according to claim 1; providing a forming tool with at least one three-dimensional mold; and using the forming tool for forming a three-dimensional product out of the fiber-based material.

13. The method according to claim 12, wherein said forming is selected from a group consisting of: vacuum forming, thermoforming, cold forming, pressing, or hydroforming.

14. A three-dimensional, fiber-based product obtained by the method according to claim 12.

15. A three-dimensional, fiber-based product comprising: a laminate with multiple layers, whereof at least one layer of said multiple layers is made of the web of fibrous cellulosic material according to claim 1.

16. The web according to claim 1, wherein the web comprises a three-dimensional packaging product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 shows a micrograph of a web material made from the suspension according to the invention;

[0040] FIG. 2 shows a micrograph of a web material which has been stretched apart;

[0041] FIG. 3 shows a perspective view of the web material in FIG. 2; and

[0042] FIG. 4 illustrates kink and curl of a cellulose fiber.

DESCRIPTION OF EMBODIMENTS

[0043] The present description is directed to a a web of fibrous cellulosic material (i.e. sheet material) derived from wood pulp, said material having properties which makes it suitable for 3D forming fiber-based products, such as trays, food packages and/or containers. The description is also directed to the 3D forming of said sheet material into said fiber-based products.

[0044] The present invention is based on the insight that use of pulp containing curled cellulose fibers, leads to improvements upon three-dimensional forming a sheet material made from said pulp. The curled fibers that are present in pulp gives strong network and flocculation when forming a sheet, leading in its turn to improved moldability of the resulting dried sheet material. According to the invention, the cellulose fibers of the dried sheet material comprises a fiber curl that is higher than 9%. The fiber curl of the dried sheet material is determined by means of re-pulping or disintegrating a portion of the sheet, and thereafter measure the fiber curl by standard methods, such as in a Valmet FS5 image analyzer. Disintegration of the sheet into pulp can be done by standard methods, such as by the procedure specified by ISO 5263 describing laboratory wet disintegration.

[0045] One way of obtaining a pulp suspension with a portion of curled fibers is high consistency refining (HCR). This is a well-known technique for making fibrillated thermomechanical pulp (TMP). “High consistency” refers to a discharge consistency of a refined pulp suspension that is higher than 20 wt %.

[0046] Due to high transfer of stresses between the fibers at HCR, microcompressions are imparted leading to that curled and kinked fibers are created. Curly fibers produce high flocculation, and 3D flocks have relatively high strength and flock stretchability compared to stiff non-curly fibers due to mechanical interlocking.

[0047] FIG. 1 shows a micrograph of a dried fiber-based sheet material according to the invention, having a curl higher than 9%. FIGS. 2 and 3 show an example of the same material as in FIG. 1, which has been subjected to stretch to cause a rupture. It is seen in FIG. 2 that the curled fibers in the web material are oriented such that they overlap the ruptured portion, thus withholding the material and leading to a material with improved tolerance to tensions—i.e. a material with increased stretch.

[0048] The skilled person understands that “curled” is referring to curved cellulose fiber and “kink” is referring to sharp changes in an axial direction of a cellulose fiber. The curl % is measured by means of a fiber image analyzing instrument, such as Valmet FSS, and is determined by measuring individual fiber contours and projected lengths. FIG. 4 shows an illustration of fibre curl and fibre kink. Curl % is based on length-weighed curl of cellulose fibers and is calculated as 100%*(I/L) where I is the fiber contour length and L is the projected end-to-end distance of the fiber, i.e. the distance between the two points on the fibre that are furthest apart.

[0049] According to one embodiment of the invention, microfibrillated cellulose (MFC) is added to the pulp suspension to form a mixture which is then used for producing the web of fibrous cellulosic material in a traditional paper-making process (e.g. wherein pulp is fed to a paper machine where it is formed as a paper web and the water is removed from it by pressing or drying). MFC may for instance be added before, during or after a step of HC refining. The microfibrillated cellulose is preferably microfibrillated cellulose produced from mechanical, thermomechanical or chemical pulp, preferably produced from Kraft pulp. The microfibrillated cellulose preferably has a Schopper Riegler value)(SR° of more than 80. According to another embodiment the MFC may have a Schopper Riegler value)(SR° of more than 93. According to yet another embodiment the MFC may have a Schopper Riegler value)(SR° of more than 85. The Schopper-Riegler value can be obtained through the standard method defined in EN ISO 5267-1. This high SR value is determined for a pulp, with or without additional chemicals, thus the fibers have not consolidated into a film or started e.g. hornification. The SR value specified herein, is to be understood as an indication but not a limitation, to reflect the characteristics of the MFC material itself.

[0050] A natural binder agent is preferably added to the pulp suspension intended to be used for producing the web, said natural binder agent preferably being in the form of native, cooked or swelled cationic starch. Swelling of the starch can be done by cross-linking starch, swelling in a solvent or party cooking.

[0051] The fibrous sheet material according to the invention is preferably used for 3D-forming a product intended for containing foodstuff. Such product may comprise multiply layers in addition to said fibrous sheet material. For example, the product may comprise a barrier layer intended to be in contact with the foodstuff when the product is in use, where such barrier layer may be e.g. a polymer barrier layer. Such polymer layer can be applied as a coating onto the fibrous sheet material, or extruded in an extrusion process or laminated as a film resulting in a multilayer board which then can be subjected to 3D forming into a product. It is also conceivable that the barrier layer is applied onto the product during the 3D-forming step, in which case a polymer film may be put into contact with the product and adhered thereto by means of a combination of vacuum and heat.

EXAMPLE

[0052] A pulp suspension was prepared by means of HC refining cellulose fibers from Scotts pine at a consistency of 20 wt %. A reference sample, corresponding to sample number 1 in Table 1, was prepared by means of LC refining cellulose fibers from Scotts pine at a consistency of 3.5 wt %.

[0053] The HC refined pulp suspension was used to prepare four samples, corresponding to samples number 2-5 in Table 1. Natural binder in the form of swollen starch was added to samples 2, 4 and 5. The starch was prepared by swelling by cooking for 3 min starting from temperature 75° C. (max 85° C.) whereafter it was cooled by dilution with cold water to consistency of 2%.

[0054] Each of the samples 1-5 was used for preparing a dried fibrous sheet material with a grammage of 150 g/m2.

[0055] Each of the sheet materials (samples 1-5) was tested with regards to geometrical tensile index, geometrical stretch, stretch in cross direction (CD) and fiber curl %.

[0056] Each sheet material was also subjected to three-dimensional forming by means of thermoforming with Variovac Primus thermoforming-line using constant pre-heating and heating temperatures, both at 90° C. The heating time and forming time were varied between 0.5 s and 2 s. The forming was done with vacuum assist. The forming pressure was varied between 0.4 and 1 bar. A mould depth of 20 mm with optimised mould geometry was used. Three packages in one forming cycle (3.1 forming) were formed.

[0057] Moisture content of the sheets were between 5.4 and 7.3% (samples were pre-conditioned).

[0058] Even if vacuum can be applied, good results might also be obtained only with temperature and pressure. Forming can further be made with or without corona pretreatment. After molding, the packaging can be filled after which a lid is sealed on the packaging.

[0059] The stretch was measured by means of a standard tensile test (ISO 1924-2 with a span length of 20 mm), wherein the sheet to be tested was stretched with test speed of 2 mm/minute until a point where it ruptured. The stretch then corresponds to the percent elongation when rupturing, i.e. to what extent in % the sheet material deforms without breaking upon being subjected to stretching.

[0060] In Table 1, convertability result “poor” means that the 3D-formed product had visible, open cracks in the material; “moderate” means that the 3D-formed product had small but visible cracks; and “good” means that no cracks were visible in the material of the product.

TABLE-US-00001 TABLE 1 Property Unit 1 2 3 4 5 Refining LC HC HC HC HC Pulp Pine Pine Pine Pine Pine Starch kg/tn 0 15 0 15 30 SR level 30 30 40 40 40 Sheet properties Grammage g/m2 150 150 150 150 150 Geom tens Nm/g 59.1 57.1 45.7 52.7 57.1 index Geom stretch % 3.8 6 5.4 5.7 6.2 Stretch CD % 5.7 9.6 9 9.2 9.9 Fiber curl FS5 % 8.7 12.5 13.3 12.7 12.4 Convertability test Vacuum Poor Moderate Moderate Moderate Good forming

Results

[0061] It can be seen in Table 1 that HC refining provides higher curl% compared to LC refined treatment, and further that samples 2-5 have higher geometrical stretch as well as higher stretch in CD compared to sample 1. In line herewith, the convertability test also showed that the 3D formability of all samples 2-5 are better than for sample 1. Sample 1, prepared with LC refining has good tensile strength, but low fiber curl resulting in low stretch. Sample 3, prepared with HC refining but no starch has good stretch, but poor tensile strength. The samples 2, 4 and 5 prepared in accordance with the invention using a strength enhancement agent in the form of starch exhibit both good tensile strength and but high fiber curl resulting in high stretch.