Method of manufacturing of a foam-formed cellulosic fibre material, sheet and laminated packaging material

Abstract

The invention relates to a method of manufacturing a new low density foam-formed cellulose material comprising dialcohol-modified celllulose, and to bulk sheets, layers, laminates or moulded articles comprising such material. Furthermore, the invention relates to a laminated packaging material comprising a layer or sheet comprising the low density cellulose material as well as to packaging containers comprising the laminated packaging material. In particular, the invention relates to packaging containers intended for liquid or semi-liquid food packaging, comprising the laminated packaging material.

Claims

1. Method for manufacturing a foam-formed low-density material of cellulose fibres to be used in packaging material for producing packaging containers suitable for liquid and semi-liquid food products, comprising a. forming an aqueous foam comprising water and a foaming agent, b. adding cellulose fibres to the aqueous foam, thus forming a fibrous foam composition, the cellulose fibres added to the aqueous foam comprising both modified cellulose fibres as a cellulose reinforcement fraction and coarse, unmodified cellulose fibres, the modified cellulose fibres containing dialcohol cellulose, the coarse, unmodified cellulose fibres being selected from the group consisting of mechanical, chemi-mechanical, thermomechanical, chemithermo-mechanical pulp (CTMP) fibres and Neutral Sulfite Semi Chemical (NSSC) pulp fibres; c. distributing the fibrous foam composition onto a substrate or into a mould, d. reducing the amount of water in the distributed fibrous foam composition to obtain the foam-formed low-density material of cellulose fibres in its final shape which is usable in the packaging material for producing the packaging containers suitable for liquid and semi-liquid food products, and e. optionally, drying the foam-formed low density material of cellulose fibres; wherein a final content of the dialcohol cellulose, based on the total number of C2-C3 bonds in a final fibrous foam composition, is 0.5 to 5%.

2. Method as claimed in claim 1, comprising mixing unmodified cellulose with the modified cellulose fibres, the mixing of the unmodified cellulose with the modified cellulose fibres taking place before the adding of the cellulose fibres to the aqueous foam.

3. Method as claimed in claim 1, wherein the fibrous foam composition comprises from 0.1 to 7 wt % of cellulose fibre.

4. Method as claimed in claim 1, wherein a further cellulose reinforcement fraction is added, the reinforcement fraction being a heavily refined fibre component chosen from heavily refined chemical pulp having a SR range higher than SR°80 and heavily refined chemithermomechanical pulp (hrCTMP) having a CSF less than 70 mL.

5. Method as claimed in claim 1, wherein the fibrous foam composition comprises from 0.01 to 1000 ppm of the foaming agent.

6. Method as claimed in claim 1, wherein the fibrous foam composition comprises from 20 to 80 volume-% of air.

7. Method as claimed in claim 1, wherein the foaming agent is selected from the group consisting of anionic surfactants.

8. Method as claimed in claim 1, wherein the foaming agent is selected from the group consisting of sodium lauryl (dodecyl) sulphate (SLS, SDS) and sodium laurylethersulfate (SLES).

9. Method according to claim 1, wherein the density of the fibrous foam composition to be reduced from an amount of water is approximately 600-750 kg/m.sup.3, and wherein the average bubble size is 100 μm or below.

10. A foam-formed low density material of cellulose fibres produced by the method as claimed in claim 1, having a density from 100 to 700 kg/m.sup.3.

11. A foam-formed low density material having a density lower than 450 kg/m.sup.3, of cellulose fibres as claimed in claim 10, having a tensile index (MD) of above 20 Nm/g (kNm/kg).

12. A cellulose bulk sheet for a packaging material, comprising the foam-formed material of cellulose fibres as claimed in claim 10, laminated or arranged in layer contact with a further sheet of a different cellulose material.

13. A cellulose bulk sheet for a packaging material, as claimed in claim 12, wherein the further sheet of a different cellulose material is a paper.

14. Laminated packaging material comprising a cellulose bulk sheet as claimed in claim 10, wherein the bulk sheet is laminated to at least one layer of polymer.

15. Laminated packaging material as claimed in claim 14, further comprising an oxygen barrier.

16. Packaging container for liquid- or semi-liquid food comprising the laminated packaging material as defined in claim 14.

17. Method for manufacturing a foam-formed low-density material of cellulose fibres to be used in packaging material for producing packaging containers suitable for liquid and semi-liquid food products, the method comprising a. forming an aqueous foam comprising water and a foaming agent, b. adding cellulose fibres to the aqueous foam, thus forming a fibrous foam composition, the cellulose fibres added to the aqueous foam comprising both modified cellulose fibres as a cellulose reinforcement fraction and coarse, unmodified cellulose fibres, the modified cellulose fibres comprising bleached, chemical pulp fibres and containing dialcohol cellulose, the coarse, unmodified cellulose fibres being selected from the group consisting of mechanical, chemi-mechanical, thermomechanical, chemithermo-mechanical pulp (CTMP) fibres and Neutral Sulfite Semi Chemical (NSSC) pulp fibres; c. distributing the fibrous foam composition onto a substrate or into a mould, d. reducing the amount of water in the distributed fibrous foam composition to obtain the foam-formed low-density material of cellulose fibres in its final shape which is usable in the packaging material for producing the packaging containers suitable for liquid and semi-liquid food products, and e. optionally, drying the foam-formed low density material of cellulose fibres; wherein a final content of the dialcohol cellulose, based on the total number of C2-C3 bonds in a final fibrous foam composition, is 0.5 to 5%.

18. Method as claimed in claim 1, further comprising preparing the modified cellulose fibres by partly oxidizing unmodified cellulose into dialdehyde cellulose, and subsequently reducing the dialdehyde cellulose into dialcohol cellulose, to a conversion degree of about 20-45%, based on the initial number of oxidizable C2-C3 bonds in the modified cellulose.

19. Method as claimed in claim 1, wherein the final content of the dialcohol cellulose, based on the total number of C2-C3 bonds in the final fibrous foam composition, is 0.99 to 3.9%.

20. Method as claimed in claim 17, wherein the final content of the dialcohol cellulose, based on the total number of C2-C3 bonds in the final fibrous foam corn position, is 0.99 to 3.9%.

Description

DESCRIPTION OF THE DRAWINGS

(1) Further advantages and favorable characterizing features will be apparent from the following detailed description, with reference to the appended figures, in which:

(2) FIGS. 1a and 1b are cross-sectional views of a bulk layer for a laminated packaging material according to aspects described herein.

(3) FIGS. 2a and 2b are cross-sectional views of laminated packaging materials according to aspects described herein.

(4) FIG. 3 is a schematic drawing of an extruder, the extruded film, a paper or packaging material web and the rollers arrange to join the plastic and the bulk layer.

(5) FIG. 4 shows an example of a packaging container produced from the packaging material according to embodiments described herein.

(6) FIG. 5 shows the principle of how such packaging containers are manufactured from the packaging material in a continuous forming, filling and sealing process.

(7) FIG. 6 shows a diagram wherein the Scott Bond delamination strength values of a foam-formed cellulose material of the present invention (Example 1), as well as its densities, are plotted versus the content of dialcohol-modified cellulose, based on the added amount of such modified pulp, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(8) FIG. 7 shows a diagram wherein the values of the tensile index, i.e. the tensile strain normalized by grammage weight (g/m.sup.2), of a foam-formed cellulose material of the present invention (Example 1), are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(9) FIG. 8 shows a diagram wherein the tensile strain values of a foam-formed cellulose material of the present invention (Example 1), are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30 of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(10) FIG. 9 shows a diagram wherein the compression strength values in MD and CD of a foam-formed cellulose material of the present invention (Example 1), are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(11) FIG. 10 shows a diagram wherein the Scott Bond, delamination strength, and density values of a foam-formed cellulose material of the present invention (both Examples 1 and 2), are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(12) FIG. 11 shows a diagram wherein the values of the tensile index, i.e. the tensile strain normalized by grammage weight (g/m.sup.2), of a foam-formed cellulose material of the present invention (Example 2), are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(13) FIG. 12 shows a diagram wherein the values of tensile strain of a foam-formed cellulose material as of the present invention (Example 2) are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(14) FIG. 13 shows a diagram wherein the compression strength values in MD and CD of a foam-formed cellulose material of the present invention (Example 2), are plotted versus different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(15) FIG. 14 shows a diagram of the Scott Bond delamination strength of a foam-formed cellulose material with different amounts and types of additives added, at different added amounts of the dialcohol-modified cellulose which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(16) FIG. 15 shows a diagram of the residual strain after compression in the thickness direction, of a foam-formed cellulose material with different amounts and types of additives added, at different added amounts of the dialcohol-modified cellulose, which has about 30% of the total of the initial oxidizable C2-C3 bonds, oxidized and reduced into dialcohol cellulose.

(17) FIG. 16 shows the turbidity of DDJ filtrates of pulp suspensions wherein different retention systems were used at different concentrations of SDS. Filled diamonds represent pulp suspensions to which no retention system was added; filled squares represent pulp suspensions to which CPAM was added before the microparticles were added; filled triangles represent pulp suspensions to which PEO was added before TA was added; and x denote pulp suspensions to which TA was added before PEO was added. The dose of each retention aid component, i.e. CPAM, microparticles, PEO and TA was 0.3 kg/t of dry pulp.

(18) FIG. 17 shows the turbidity of DDJ filtrates of pulp suspensions with different retention systems at high shear forces.

(19) FIG. 18 shows a flowchart illustrating one embodiment of the method according to the invention.

GENERAL DESCRIPTION OF EMBODIMENTS

(20) FIG. 1a schematically shows a cross-section of an example of a bulk layer consisting of the foam-formed cellulose of the invention.

(21) The foam-formed cellulose comprises 1.5 weight % dialcohol cellulose, based on the total cellulose content, and was made according to the following described Examples. It has a density of 301 kg/m.sup.3 and the thickness of the bulk layer made from the foam-formed cellulose is 286 μm. The Scott Bond value measured on the foam-formed cellulose material was 178 J/m.sup.2.

(22) FIG. 1b schematically shows a cross-section of a different example of a bulk layer. The same foam-formed cellulose material as used in FIG. 1a, was according to this example laminated to a further paper, being a Kraft paper having a surface weight of 70 g/m.sup.2, the two celllulose materials thus together forming a bulk layer.

(23) FIG. 2a schematically shows a cross-section of an example of a laminated packaging material. The outer, décor-covering layer (21) is a polyolefin such as a suitable LDPE or PP. The outer layer may be used to provide cover of a printed pattern, a hole and/or weakening (not shown in the figure) which is provided in the bulk layer (22), which layer is arranged on one side of the outer layer (21). Between the outer layer (21) and the bulk layer (22) an additional layer (27) of paper or cellulose is arranged. The bulk layer (22), on the side opposite the outer layer, has a laminate layer (23) selected from suitable polyolefins such as LDPE or PP. The laminate layer provides adhesion to the oxygen barrier (24), which is arranged on the opposite side of the laminate layer (23). The barrier layer (24) provides the desired barrier such as oxygen, light, water and vapour barrier depending on the specific need determined by the product to be packed. The barrier layer can for example be an aluminium foil or a vapour deposited film, such as a metallized or vapour deposition coated film, such as a PECVD (plasma enhanced chemical vapour deposition) coated film. On the side opposite the laminate layer an adhesive polymer (25) is arranged on the barrier layer. The adhesive (25) may for example be applied by extrusion coating. When the barrier layer is aluminium foil the adhesive could be a suitable ethylene (meth)acrylic acid copolymer (E(M)AA) adhesive marketed under the tradename Primacor® or Nucrel®. On the side opposite the barrier layer, the adhesive is provided with a heat-sealable layer (26) such as a suitable polyolefin such as PE or PP or blends thereof. The heat-sealable layer is the layer facing the product in the finished packed packaging container.

(24) FIG. 2b schematically shows a cross-section of a second example of a laminated packaging material. The outer layer (21) (to be directed towards the outside of a package made from the material) is a polyolefin such as a suitable LDPE or PP. The outer layer may be used to provide cover for a printed pattern, a hole and/or weakening (not shown in the figure) which is provided in one or more of the other layers of the laminate. On one side of and adjacent the outer layer, a thin paper (27) of a surface weight of about 100 g/m.sup.2 or lower is arranged. The thin paper layer (27) is laminated to a bulk layer (22), opposite the outer layer, by an intermediate thermoplastic outer binding layer (28). The binding layer (28) may be selected from suitable polyolefins such as LDPE or PP or blends thereof. The binding layer (28) binds the bulk cellulose layer (22) and the thin paper layer (28) together. The bulk layer (22), is further laminated to a laminate layer (23) of thermoplastic polymer, on the side of the bulk layer opposite the side laminated to the binding layer (28). The laminate layer (23) provides adhesion to an oxygen barrier layer (24), which is arranged on the opposite side of the laminate layer (23). The barrier layer (24) provides the desired barrier such as oxygen, light, water and vapour barrier depending on the specific need determined by the product to be packed. The barrier layer can for example be an aluminium foil or a vapour deposited film, such as a metallized or vapour deposition coated film, such as a PECVD coated film. On the side opposite the laminate layer an adhesive polymer (25) is arranged on the barrier layer. The adhesive (25) may for example be applied by extrusion coating. When the barrier layer is aluminium foil the adhesive could be a suitable ethylene (meth)acrylic acid copolymer (E(M)AA) adhesive marketed under the tradename Primacor® or Nucrel®. On the side opposite the barrier layer, the adhesive is provided with a heat-sealable layer (26) such as a suitable polyolefin such as PE or PP or blends thereof. The heat-sealable layer is the layer facing the product in the finished packed packaging container.

(25) A packaging material according to the present invention may be a laminate packaging material which comprises an outermost thermoplastic, heat sealable décor-covering layer which on one side of the layer has a bulk layer comprising the foam-formed cellulosic fibre-material of the invention, which bulk layer on the side opposite the décor-covering layer has a laminate layer, said laminate layer, on the side opposite the bulk layer has an oxygen barrier, and said oxygen barrier, on the side opposite the laminate layer has a heat-sealable layer.

(26) Furthermore, a laminated packaging material may comprise a bulk layer having a density of less than 700 kg/m.sup.3 and comprising foam-formed cellulosic fibre-material according to the present invention. The packaging material further comprises an additional layer arranged by means of a binding layer, such as for example a thermoplastic polymer binding layer, such as a polyolefin-based polymer or copolymer binding layer, wherein the binding layer is arranged between the bulk layer and the additional layer. The additional layer has a decorative printed pattern arranged on the side opposite the binding layer. The bulk layer is provided with a barrier layer on the side opposite the binding layer. A barrier layer may be provided with a heat-sealable layer on the side opposite from the bulk layer. The outermost layer covering the printed decorative pattern is a polyolefin layer, such as an outermost heat-sealable polyolefin layer to be in contact with the surroundings of a packaging container, for example low density polyethylene (LDPE) or polypropylene. The outermost thermoplastic layer provides additional protection, e.g. moisture resistance and scratch/wear resistance, and stability to the packaging container.

(27) FIG. 3 is a schematic illustration of an extruder (31). An extruder of the schematic illustration is suitable for application of the, outer layer (11), laminate layer (13), adhesive (15) and the heat-sealable layer (16). As an example the laminate layer (13) can be applied on the bulk layer (12), whereby the drawing shows a molten plastic film (32) of the polymer to become the laminate layer (13) being arranged by melt extrusion coating onto a bulk layer web (35). The extruder melts and mixes the polymer(s). In case of the layers being polymer blends, the extruder may also be used to blend the polymers which are for example supplied via separate hoppers for the polymer granules. The molten film (32) and the bulk layer are joined in a lamination nip between rollers (33 and 34) which exert a pressure. One of the rollers can be a chilled roller which reduces the temperature of the polymer when in the nip. Similarly the other polymers of the packaging material may be added to the bulk layer (35). The barrier layer (14) may for example be forwarded from a separate roll and fed through the lamination nip together with the laminate layer (13), or with an adhesive.

(28) FIG. 4 shows an example of a packaging container 50a produced from the packaging material described in FIG. 1 or 2. The packaging container is particularly suitable for liquid or semi-liquid food products such as beverages, sauces, soups or the like. Typically, such a package has a volume of from about 100 to about 2000 ml. It may be of any configuration such as those previously described herein, but is for example brick-shaped, having longitudinal and transversal seals 51a and 52a, respectively, and optionally an opening device 53. In another embodiment, not shown, the packaging container may be shaped as a wedge. In order to obtain such a “wedge-shape”, only the bottom part of the package is fold formed such that the transversal heat-seal of the bottom is hidden under the triangular corner flaps, which are folded and sealed against the bottom of the package. The top section transversal seal is left unfolded. In this way the half-folded packaging container is still easy to handle and dimensionally stable (i.e. substantially maintains form and shape) when put on a shelf in the food store or on a table or the like.

(29) FIG. 5 shows the principle as described in the introduction of the present application, i.e. a web of packaging material is formed into a tube 71 by the longitudinal edges 72, 72′ of the web being united to one another in an overlap heat-sealed joint 73. The tube is filled 74 with the intended liquid food product and is divided into individual packages by repeated transversal seals 75 of the tube at a pre-determined distance from one another below the level of the filled contents in the tube.

(30) The packages 76 are separated by incisions in the transversal seals and are given the desired geometric configuration by fold formation along prepared crease lines in the material.

EXAMPLES

Example 1

(31) Modified cellulose pulp was prepared from unmodified wood cellulose fibre pulp by oxidizing part of the fibres in suspension to dialdehyde cellulose at a degree of conversion of about 30%, based on the initial number of oxidizable C2-C3 bonds, and subsequently reducing the dialdehyde cellulose into dialcohol cellulose. The cellulose fibres have an average diameter of at least 1 μm. The average diameter of the fibres of the present disclosure is normally at least 5 μm, such as at least 8 μm, such as at least 12 μm. The average length of the fibres of the present disclosure is preferably at least 0.3 mm, such as 0.3-4 mm. Any fibre length is however conceivable as long as a foam of the fibres may be created, such as up to 50 mm. The fibres of the present disclosure are preferably of lignocellulosic origin. The degree of conversion of cellulose to dialdehyde cellulose can be determined using the method “carbonyl content determination” described below. The fibre suspension used in the method for preparing the fibres of the material may be a suspension of beaten fibres.

(32) Bleached softwood kraft fibres (K46) were supplied by SCA Forest Products (Östrand pulp mill, Timra, Sweden). The material was beaten in a Voith mill to an energy input of 160 Wh/kg (about 30°SR). The fibers were partly oxidized to dialdehyde cellulose by adding 2.5 kg of sodium periodate per 39 kg pulp (dry content 4.5%) to a 50 litre reactor with a stirrer. To limit formation of radicals and unwanted side reactions, the reaction was performed in the dark. After 2 hours of stirring and oxidation, the reaction was stopped by filtration and washing of the fibres. The fibres were then suspended in water and 7.5 kg of ice to a total weight of 37 kg. The dialdehyde cellulose formed was further reduced to dialcohol cellulose: 350 gram sodium borohydride was first dissolved in 2.5 litres of deionized water and the solution obtained was subsequently added to the dialcohol cellulose during 3 minutes and then stirred for 1 hour. The reduction reaction was followed by filtration and thorough washing, resulting in 0.69 kg modified pulp.

(33) The carbonyl content was determined by a protocol based on Zhao et al. (Determination of degree of substitution of formyl groups in polyaldehyde dextran by the hydroxylamine hydrochloride method. Pharm. Res. 8:400-402 (1991)). (The fibers were suspended in water and adjusted to pH 4, followed by dewatering to a gel-like consistency. Then, approximately 0.25 g (dry basis) of these fibers were stirred with 25 ml of 0.25 M hydroxylamine hydrochloride solution at pH 4 for at least 2 h before the fibers were separated from the solution by filtration using a pre-weighed filter paper. The exact mass of the fibers was then determined by oven-drying of the filter paper and the carbonyl amount was determined by titration back to pH 4 with 0.10 M sodium hydroxide.)

(34) A furnish made of CTMP fibres (600 ml CSF) and some of the above modified cellulose pulp (5% and 10% of the dry weight) were mixed gently for 15 min. Then the mixture was moved to an already made SDS-foam and stirred at 3200 rpm until a foam density of 320-380 kg/m.sup.3 was reached and the foam was stabilised. The foam was subsequently decanted into the foam forming mould and filtrated through a wire using vacuum of −0.3 bar. The obtained sheets were wet-pressed and dried with a Kodak drum dryer.

(35) The delamination strength of the sample was measured, and compared with a reference sample, which had no modified pulp added but which was otherwise identical and manufactured in the same way as above. The delamination strength of the sample was much higher than that of the reference sample, and also higher than that of other samples being similar but with the essential difference that they had cellulose nanofibrils (CNF, CMF) added instead of the modified pulp, at amounts up to 20 wt-% of the cellulose content.

(36) The results are visualised in FIG. 6, showing a diagram where the Scott Bond delamination strength, and the density, of the foam-formed cellulose material is plotted versus the content of modified pulp. The actual degree of conversion into dialcohol cellulose, at the addition of 5 and 10 weight-% of modified pulp, respectively, is not exactly known, since the unmodified and the modified cellulose fibres may have had different densities, but could approximately be estimated to lie somewhere between 1 and 3%, as calculated on the total number of oxidizable C2-C3 bonds in the cellulose molecules. The majority of the cellulose fibre composition is accordingly unmodified cellulose, and only a minor part of all available oxidizable bonds of the molecules therein, were converted into dialcohol cellulose.

(37) In addition, the foam-formed cellulose sheet samples obtained remarkably improved tensile strength properties. This is shown in the diagram of FIG. 7, by plotting the tensile strength normalized by grammage weight (g/m.sup.2), i.e. the tensile index, versus the content of modified cellulose pulp. In the diagram of FIG. 8, the tensile strain plotted versus the modified cellulose content is shown.

(38) The diagram of FIG. 9 further shows the improvement of compression strength properties of the samples having 5 and 10 wt % of added modified cellulose. The compression strength in the cross direction (CD) of a bulk layer is supporting the stacking strength of packages made from a laminated material comprising the bulk layers.

(39) The underlying data to the diagrams of FIGS. 6 to 9 are provided in Table 1.

(40) TABLE-US-00001 TABLE 1 95% CTMP + 90% CTMP + 5% cellulose 10% cellulose having 30% having 30% dialcohol- dialcohol- modified modified 100% C2-C3 C2-C3 Property CTMP bonds bonds Grammage (g/m.sup.2) 131.0 86.3 86.1 Thickness (um) 557.0 286.8 269.3 Density (kg/m.sup.3) 236 301 320 Young's modulus MD (MPa) 1016 1023 1217 Young's modulus CD (MPa) 396 892 1056 Tensile strength MD (kN/m) 3.3 2.4 3.0 Tensile strength CD (kN/m) 1.5 2.1 2.6 Tensile strain MD (%) 1.0 1.7 1.9 Tensile strain CD (%) 1.2 1.6 1.8 Compression strength MD 1.8 1.6 1.8 (kN/m) Compression strength CD 0.7 1.0 1.3 (kN/m) z-strength (kPa) — 266 297 Internal bond strength (J/m.sup.2) 60 178 221

Example 2

(41) Similar blends, as made in Example 1, of chemithermomechanical pulp (CTMP, freeness 600 ml) with the modified pulp, and/or with highly refined hardwood pulp, were mixed and foamed. Retention chemicals such as cationic starch (CS) and retention aid (RA) were added to the foamed composition in the order: CS at 0 s, RA 1 (cationic polyacrylamide 0.4 kg/t) at 5 s, RA 2 (microparticles “Perform® SP7200” from Hercules, 0.4 kg/t) at 10 s and mixing was stopped at 15 s. Finally the pulp foam was moved to the sheet mold and sheets were made.

(42) The results concerning Scott Bond delamination strength and tensile strain showed the same improvement trend as in Example 1. Thus, the effect from adding a modified cellulose containing dialcohol cellulose was still evident, independently of additive amounts of starch and other retention additives.

(43) FIG. 10 shows the delamination strength versus the content of dialcohol modified cellulose, and in comparison with the results from Example 1, the delamination strength of the samples from Example 2 seems to start at lower level, but improves at a similar rate with an increasing amount of added modified pulp.

(44) The improvement of the mechanical properties is not equally and generally evident in the second Example trial, however a clear improvement trend is seen regarding tensile index (FIG. 11), the tensile strain (FIG. 12) and compression (FIG. 13). An improvement is achieved in both MD and CD directions, as shown in the figures.

(45) The diagram in FIG. 14 shows a clear increase in the delamination strength with addition of modified pulp containing dialcohol cellulose. The addition of cationic starch further boosts the delamination strength but also slightly increases the density. Adding starch is more efficient in order to further improve the delamination resistance than adding highly refined hardwood fibres. The modified pulp is more efficient as a strengthening additive than the highly refined hardwood fibres, as roughly shown in that a similar delamination resistance can be reached with half the amount of material, i.e. about 5 wt % modified fibres provide the same positive effect as 10 wt % of highly refined hardwood fibres. It can be concluded that the addition of starch works very well together with the addition of dialcohol cellulose, and even with a slightly synergetic effect.

(46) In previous research, the properties of foam-formed cellulose, in the lamination operation during the conversion of packaging materials into a laminated packaging material, was investigated.

(47) In order to analyze the effect of thermal and mechanical loading on the foam-formed cellulose during lamination, tests were conducted on various low density materials. The materials tested were foam-formed cellulose, and foamed polypropylene. Lamination of packaging material structures was done in a flexible lab laminator with two extrusion coating stations. The laminator settings were about 100 m/min web speed, 250-275 N web tension and the reference nip load was 25 N/mm. In each extrusion coating operation, 15-20 g/m.sup.2 of LDPE was melt extruded onto the layer of foam-formed cellulose on the respective sides, at a melt extrusion temperature of about 320° C. The original thickness before lamination and the thickness after lamination were optically measured by using a microscope Olympus BX51. Sample preparation was done by using a microtome.

(48) Generally, it was seen that the remaining thickness of foam-formed cellulose is substantially higher compared to a corresponding laminate variant with foamed polymer materials.

(49) It was also concluded that lamination by extrusion coating works well with foam-formed cellulose having a density of 200 kg/m.sup.3, such as 300 kg/m.sup.3, or higher. Laminates having lower density layers of foam-formed cellulose are more sensitive to lamination heat and pressure, and show higher reduction of the thickness of the foam-formed cellulose material.

(50) Furthermore, it was seen that the thickness reduction of a polymer foam is permanent, due to melting and re-shaping of the heated polymer foam cells, while there is a spring-back effect in the foam-formed cellulose, such that the thickness reduction during lamination is reversed to a final thickness which is only reduced by about 10-15% in a reference nip at densities around 300-400 kg/m.sup.3. The higher the density of a foam-formed cellulose, the better this spring-back effect, or Z-directional compression strength.

(51) With foam-formed cellulose of the present invention, it was seen that the lamination pressure resistance was relatively higher, i.e. the thickness reduction after lamination was significantly lower in the case of the invention, than in the case of samples having different additives, such as soft wood pulp or nano-/micro-fibrillar cellulose (CNF/CMF), for the purpose of improving the delamination resistance. This is illustrated by the diagram in FIG. 15.

(52) Thus, the residual strain after compression in the thickness direction can give an indication of the thickness reduction which can occur during lamination. The lower the residual strain is the more efficient the spacer or bulk layer is.

(53) The residual strain in z-direction (thickness direction) after a specified load on the sample was determined with Lloyd LR10K loading device. The sample area exposed to loading was 15.2 cm.sup.2, with a circular radius of 22 mm. Samples were cut to the same size as the pressing plates. A sample was placed onto the loading table and a maximum pressure of 2 MPa (force 3041 N) was applied on the sample for a period of 1 s. The relative change was calculated with the initial thickness of the sheet (measured with a separate standard device) and the reading of displacement sensor showing the absolute thickness change in the compression. The residual strain was determined as the point where there was no additional clear drop in the loading force. At least five parallel measurements were carried out in separate points.

(54) The addition of dialcohol-modified cellulose pulp and/or highly refined hardwood kraft fibres only slightly increases the residual strain. However if softwood kraft fibres and in particular softwood fibres combined with microfibrillar cellulose are added, the residual strain increases significantly lowering the efficiency of the foam formed cellulose spacer or bulk layer in consequence.

(55) FIG. 16 shows the effect of the SDS surfactant amount on turbidity (value is relative to the amount of solids that went through the 100 mesh wire) for different retention systems. The pulp contained 80% chemithermomechanical pulp (CTMP) and 20% heavily refined hardwood pulp (hrHW). The turbidity was measured using nephelometer and the result is expressed as nephelometric turbidity units (NTU). High turbidity means low retention. The turbidity was the highest (=lowest retention), when retention aids were not used (filled diamonds). The amount of surfactant (SDS) had no effect on turbidity in these test points. With low SDS concentration, 0.1-0.2 g/l, the CPAM+MP system (filled squares) and the PEO+TA system (filled triangles) gave the same retention. However, when the SDS content was increased, the retention decreased (reflected by increased turbidity) for the CPAM+MP system and improved (reflected by decreased turbidity) for the PEO+TA system. In the case where TA was added before PEO (x), the retention was the best and was improved further with increasing the SDS content. The results showed that the non-cationic TA+PEO system is very efficient in foam forming, especially when the SDS content is high.

(56) As can be seen in FIG. 17, the TA+PEO-system seems to work better than the CPAM+MP system when the suspension is subjected to high shear forces. The experiment was performed similarly to the one of FIG. 2, but the stirrer 1 was set close to the wire 3 and the stirrer 1 was on with 1000 rpm during the filtration of foam.

(57) FIG. 18 shows a flowchart illustrating one embodiment of the method according to the invention. 1: Air 2: Surfactant 3: Cellulose fibre (such as coarse CTMP) 4: Cellulose reinforcement fraction (di-alcohol modified cellulose) 5: Foam generation, In a separate unit (such as a tank) 6: Fibre furnish mixing 7: Foam circulation 8: Headbox feed flow 9: Distribution onto a forming wire (headbox) 10: Forming section 11: Wet pressing 12: Drying 13: Foam formed cellulosic fibre material 14: Cationic starch 15: CPAM 16: Microparticle 17: AKD 18: As an alternative retention system instead of using CPAM and microparticle 18a: TA 18b: PEO

(58) Foam generation: surfactant and gas (air) is mixed with water, the foam density will be dependent on the amount of surfactant and mixing energy. Stock preparation: the fibre components are prepared to required freeness (by refining) and mixed to a stock. Foam and stock mixing: Foam and fibre stock are combined (fibrous foam). Headbox feed flow: Fibrous foam is pumped towards the headbox and the needed chemicals are added into the headbox feed flow. Dewatering including drying: Foam is removed in the forming section by using suction boxes and forming wire (fibres retain on the forming wire while the foam goes through the wire and to foam circulation), by mechanical compression in wet pressing and by heating up the moist board, e.g. with drying cylinders.

(59) Packaging containers of the type Tetra Brik® 250 ml were prepared from a laminated packaging material having a bulk layer from the above foam-formed cellulose compositions. The packaging material comprised 12 g/m.sup.2 of an outermost thermoplastic layer of an LDPE, which is arranged to become the outside of the package. Additional layers starting from the outermost layer were: 70 g/m.sup.2 white paper; 15 g/m.sup.2 LDPE as binding layer; a bulk layer of foam-formed cellulose, 332 kg/m.sup.3, 243 μm; a laminate layer of 20 g/m.sup.2 LDPE and a barrier layer of about 6 μm aluminium foil, 6 g/m.sup.2 adhesive (Primacor™ 3440) and 19 g/m.sup.2 heat-sealing layer of a blend of a LDPE (30 w %) and a metallocene catalyzed linear, low density polyethylene. The packaging material was obtained on a roll which was processed in accordance with the conventional manufacturing process in order to generate a 250 ml Tetra Brik® packaging containers containing orange juice. From this test, it was concluded that the amount of cellulose fibres could be reduced by at least 25%, as compared to a corresponding material having one conventional paperboard layer in a corresponding traditional packaging laminate with 12 g/m.sup.2 LDPE outermost layer, 200 g/m.sup.2 paperboard, 20 g/m.sup.2 LDPE laminate layer, 6 um aluminium foil, 6 g/m.sup.2 adhesive (Primacor™ 3440) and 19 g/m.sup.2 heat sealing layer of a blend of a LDPE (30 w %) and a metallocene catalyzed linear, low density polyethylene.

(60) Thus, in addition to further reducing the amount of material used in the laminated packaging material, the bulk layer of the invention is still entirely based on natural, renewable sources, i.e. cellulose.

(61) In order to determine the density of the bulk layer different procedures may be applicable depending on the layers of the packaging material. The density (kg/m.sup.3) of the bulk layer comprising foam-formed cellulose in a multilayered packaging material can be determined by dividing the grammage (kg/m.sup.2) by the thickness (m). The thickness can be obtained by using a standard microscope. The separate grammage can be obtained by a standardized separation procedure using 1 dm.sup.2 circular discs of packaging material. All measurements are performed in a controlled environment of 23° C. and 50% relative humidity. The total grammage of the packaging material is measured using a balance (0.001 g accuracy). The packaging material is split at the foam-formed cellulose layer to obtain two plies. The two plies are place in a beaker containing copper ethylene diamine solution until all the cellulose fibers are easily removed. Thereafter the remaining grammage is determined and the foam-formed cellulose grammage can be calculated by subtracting the remaining grammage from the total grammage. Whenever at least one of the plies contain an aluminium layer the procedure should be to measure the grammage of each ply and use an acetic acid solution instead of the copper ethylene diamine solution and leave the plies for 3 to 4 hours. The layers of the plies of packaging material are split to individual layer and the corresponding individual layer grammage is determined and subtracted from the total grammage. Whenever an additional layer of paper is present the method above is applied but the paper layer is removed, for example by grinding. The weight of the ground material is determined and appropriately corrected in the density calculation of the bulk layer.

(62) The invention is not limited by the embodiments shown and described above, but may be varied within the scope of the claims. Modifications and alterations, obvious to a person skilled in the art, are possible without departing from the concept as disclosed in the appended claims.