Boosting the cross-machine direction shrinkage and extensibility of a fiber product

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention concerns a method for modifying a fiber network of a fiber web in order to increase the drying shrinkage as well as boost especially the cross-machine direction (CD) extensibility of the fiber web product. The invention also concerns the fiber product thus produced.

Description of Related Art

Packaging materials are constantly being developed, and one major aim is to further reduce the amount of plastics used in this packaging. This typically requires an increase in the amount of cellulose-based materials in said packaging.

When producing three-dimensional molded packages from paper products or from other cellulose fiber-based materials, the main bottle neck is the limited extensibility of these materials. Materials with high extensibilities have the highest potential for being used in such packaging products. The problem is most critical in the cross-machine direction (CD) extensibility.

The extensibility issue has been targeted already in the past, but with a focus on the machine direction (MD) extensibility, which can be increased, for example by applying in-plane compaction using technologies like Expanda (see e.g. Ceccato C. et al. 2021) or Clupak (see e.g. U.S. Pat. No. 3,630,837 A). Both these methods involve improving the extensibility of a paper product by mechanically compacting the paper in the drying section of a paper machine, in the nip between two rollers or a roller and a bar where paper experiences an in-plane compaction between a hard surface and an elastomeric surface. The methods thus operate at a high dry matter content, for example around 60-75 w-%.

These methods, however, only provide improvement in the machine direction (MD), and do not cause any significant increase in the cross-machine direction (CD) extensibility.

The CD extensibility has also been mentioned in the past, e.g. in U.S. Pat. No. 3,220,116 A, where an attempt has been made towards improving the CD extensibility via drying the web without restrictive forces after it has been contracted using bowed Mont Hope type of rolls. This is said to cause natural contraction of the web, thus increasing its CD extensibility. However, this contraction takes place through reducing the width of the web, which will change the web dimensions, and will not provide any significant MD extensibility.

Thus, the existing technologies for improving the extensibilities of fibrous webs, such as paper or board products, can still be improved.

SUMMARY OF THE INVENTION

The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

According to a first aspect of the present invention, there is provided a method for modifying the fiber network of a fiber web to increase the extensibility of the web, in either the machine direction (MD) or in the cross-machine direction (CD), or in both directions.

According to a second aspect of the invention, there is provided a method involving processing the web before drying, and then allowing the web to shrink under low restraint during the evaporative drying.

According to a third aspect of the invention, there is provided a method, which involves imposing a three-dimensional pattern in the wet fiber web.

According to a further aspect of the invention, there is provided a fiber product, manufactured according to the method of the invention, having an improved extensibility at least in the CD direction, preferably achieved in part via an increased drying shrinkage.

The present invention is based on imposing a selected pattern, such as a wavy or corrugated pattern, into a wet fibrous web, such as a paper or board web, and allowing the web to shrink under low restraint during evaporative drying. Forming the pattern on a wet web causes local elongations in the web, without affecting the length or width dimensions of the web. The imposed pattern increases the extensibility of the web and also facilitates an advantageous drying shrinkage.

Thus, the imposed pattern together with the achieved increased drying shrinkage cause an improved extensibility of the resulting fiber product.

The invention thus results in an increase of the extensibility in either the MD or CD directions, or both, due to the imposed pattern, combined with the increased drying shrinkage, particularly in the CD direction. An improved extensibility can be achieved via three different routes, i.e. improving the extensibility behaviour of the wet fiber network before processing, imposing a pattern on the web, and allowing drying shrinkage. Since the pattern has been formed in the wet web, it is achieved through the wet elongation of the web instead of a change in the web dimensions, and during the evaporative drying the pattern can be steepened or flattened. In other words, the pattern reduces the resistance against shrinkage.

The improvement in the extensibility of the fiber products thus formed will increase their potential to replace plastics in various applications.

In order to obtain a planar product, the corrugations can be crushed or smoothed out, e.g. by any known procedure that increases the MD extensibility, or by crushing. Flattening the pattern will not remove all the achieved benefits, but since the optional crushing of the corrugations will reduce the drying shrinkage, the aim is typically a compromise between the highly extensible corrugated sheet and the flattened planar sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of the corrugation of a sheet in accordance with at least some embodiments of the present invention, also showing exemplary dimensions of the corrugations.

FIG. 2 is a graph showing the tensile strength of various corrugated paper sheets, when the corrugations have been made and crushed at a selected solids contents.

FIG. 3 is a graph showing the drying shrinkage of various corrugated paper sheets, when the corrugations have been made and crushed at a selected solids contents.

FIG. 4 is a graph showing the strain at break of various corrugated paper sheets, when the corrugations have been made and crushed at a selected solids contents.

FIG. 5 is a graph showing the strain at break of various wet corrugated paper sheets, as a function of the shrinkage. The graph shows the effect of the sole corrugated pattern on strain at break of an unshrunken paper (+15%=ε_restrained) and the additional strain brought about by the shrinkage.

EMBODIMENTS OF THE INVENTION

Definitions

In the present context, the term “extensibility” is used to describe the important characteristic of fibrous products, such as paper or board products, that can be defined as the ability of the fibrous product to increase its linear length upon deformations due to external mechanical forces. The extensibility of the fibrous product is affected, among others, by fiber morphology, interfiber bonding, the structure of the fiber network, drying shrinkage and the straining conditions of the fibrous web.

Particularly, the term “MD extensibility” refers to the machine direction extensibility, while the term “CD extensibility” is used to describe the cross-machine direction extensibility.

The term “drying shrinkage” is intended to define the amount of in-plane shrinkage or contraction that takes place in a sheet or web due to the removal of water from the structure and especially from the swollen wet fibers. It is based on the shrinkage of fibers that takes place almost solely in the transverse direction. The transverse shrinkage of fibers is transmitted to other fibers at the inter-fibre joints. This results also in web shrinkage, but due to the direction, it takes place without affecting the width of the web.

The present invention thus relates to a method of modifying a fiber network of a fiber web, including the steps of imposing a three-dimensional pattern in the wet fiber web, e.g. as shown in FIG. 1, and drying the web while allowing it to shrink without major restraints.

The invention is particularly characterized by imposing the pattern on the web in the forming section or after the forming section before evaporative drying, while known procedures work on webs that have already undergone some drying or in addition to drying are going through a MD compaction process, thus resulting in an improved MD extensibility, but not affecting the CD extensibility to any considerable level. Due to the pattern on the web being formed while the web is still wet, the pattern of the present invention is achieved through the wet elongation of the web, thereby not affecting its dimensions.

In an embodiment, the pattern is imposed using wet press rolls, dandy roll or couch roll. Alternatively, the pattern is imposed in the early areas of the drying section, by pressing the web against a patterned drying roll.

The pulp used in the web is typically mechanical, chemi-mechanical, thermochemimechanical, semi-chemical or chemical pulp, either bleached or unbleached, from softwood, hardwood or non-wood origin, from either virgin or recycled sources, or their mixture, preferably chemical pulp of softwood origin. The pulp is used in forming a web, preferably with a basis weight of >100 g/m², such as 120-250 g/m². An advantageous alternative is to use refined pulp, since unrefined pulp typically results in low strength values and poor elongation at break. Since the patterning requires wet elongation, either low consistency (LC) refined pulp or high consistency (HC) treated pulp can thus be used, or preferably a pulp treated using combination of both methods.

Particularly HC treated pulps are used for their advantageous effect on the wet elongation values. The wet elongation values are typically >10% for these HC and LC treated pulps, while the drainage resistance values remains on low level (SR number below 30, or CSF values above 400 ml). The shrinkage resistance is important for similar reasons as the drying shrinkage. The latter (the drying shrinkage) is resisted by the straightness and stiffness of fibers, which is avoided by HC and LC refining of fibers and wet patterning of the web, i.e. the patterning increases the drying shrinkage by reducing the shrinkage resistance. The shrinkage resistance is additionally reduced by reducing the flatness and buckling resistance of the structure, which is, also, achieved with the help of the patterning.

In an embodiment of the invention the fiber web has thus been formed from a fiber pulp that has been mechanically pre-treated by one or more steps selected from high-consistency refining, kneading or other high consistency treatment, or middle or low-consistency refining, the pre-treatment steps preferably containing at least one high-consistency treatment step that induces fiber deformations, such as curl, kinks, dislocations or microcompressions.

The three-dimensional pattern of the invention is imposed on the web for example by moulding, preferably by pressing the web between two surfaces of which one or both surfaces are shaped, such as forming or wet pressing fabrics, belts or wet pressing rolls on a paper machine, or their combination. The optional smooth surface of the belt or roll may for example be made of an elastic material that flexes with the pattern.

The pattern can be a machine-direction (MD) corrugation of the web, or another similar pattern that provides waves and ridges in the web, particularly in the machine-direction. For example, the pattern can be formed of wave-shaped corrugations or of a folded pattern. Preferably the pattern is formed of wave-shaped smooth corrugations having a flute height of about 0.7 mm and a wave length of 2-4 mm, such as about 2.7 mm.

The dimensions of the pattern are adjusted to be sufficient to cause local buckling of the web or fibers, so that they no longer prevent the drying shrinkage.

In an embodiment of the invention, the dry matter content of the web during the formation of the pattern is 20-60% by weight, or even 10-60% by weight Preferred contents are 35-50% by weight, while even a content as low as 20-40% by weight can be considered advantageous, since it provides the highest wet elongation. The lower end of the range, i.e. 20-30% by weight, is particularly suitable for formation of the pattern on the forming wire.

In addition to the optional mechanical pre-treatment steps described above, one or more chemical pre-treatment steps can be carried out, e.g. by addition of said one or more chemicals to the pulp suspension, or by applying them on or impregnating them in the wet web, or applying them as mixtures of multilayers, or as cellulose solvents, the chemicals being selected from natural or modified polymeric materials with or without cross-linking agents, like micro- or nanofibrillated cellulose material, hemicellulose or cellulose, carboxymethylated cellulose, dialcohol cellulose, starch, xyloglucan, alginate, gelatin, agar, chitosan, guargum, polyamideamine epichlorohydrin (PAE), polyurethane, polylactic acid (PLA), polyvinylacetate (PVAc), polyvinylamine (PVAm), polyethyleneimine (PEI), or polyacrylamide (PAM), preferred chemicals being cellulose microfibrils or nanofibrils (CMF, CNF), or carboxymethylated cellulose (CMC).

The drying step of the method is carried out as an evaporative drying in an unrestrained manner, i.e. while avoiding shrinkage restraints, particularly in the cross machine direction. This will facilitate shrinkage. Shrinkage can, however, also be facilitated further by using chemicals or rollers, or by utilizing stretching in transverse direction. Such a stretching will decrease the MD extensibility, but may increase the CD shrinkage.

Particularly, the evaporative drying is achieved by cylinder drying, impingement drying, or air flotation drying, preferably until a solids content of >80% by weight is achieved, more preferably a solids content of >90%.

In an embodiment of the invention, the drying-induced shrinkage is boosted using contracting rollers.

In another embodiment, the drying shrinkage is boosted using water-soluble or water-swelling chemicals, that have low gel point such as water-soluble or water-swelling natural or modified polymeric materials with or without cross-linking agents, like micro- or nanofibrillated cellulose material, hemicellulose or cellulose, carboxymethylated cellulose, dialcohol cellulose, starch, xyloglucan, alginate, gelatin, agar, chitosan or guargum.

As indicated above, the present invention aims at increasing drying shrinkage. Such drying shrinkage is a type of web shrinkage which, conventionally, is restricted by the axial stiffness of fibers, but this stiffness can, as described above, be reduced by HC and LC refining. Additionally, web shrinkage is, conventionally, restricted by the flatness and bending stiffness of the web. However, as described above, this stiffness is effectively reduced by the pattern imposed on the web.

The dried web can be used as such, or the formed pattern can be smoothed out. The corrugations are typically smoothed out at least partly during the elongation of the web e.g. in 3D moulding. However, smoothening of a non-strained web, to obtain a planar web, can be achieved e.g. by crushing. The crushing can, in turn, take place by using a machine-direction (MD) compacting step, or the crushing step is combined with a machine-direction (MD) compacting step, said MD compacting step preferably being a step, wherein the fiber web is placed in contact with a contracting surface, such as in the Clupak or Expanda process.

The smoothing step can take place before drying, during drying or after drying. However, it is preferred to smooth out the pattern when the web has a dry matter content of 55-95% by weight, more preferably 65-75% by weight.

The smoothing step maintains the advantages achieved by elongation, thus also maintaining the extensibility potential. However, since the smoothing of the pattern reduces the drying shrinkage, a suitable compromise is required between obtaining a smooth product surface and obtaining a particularly advantageous extensibility.

It is also possible to achieve a smoothed out surface of the web by selecting a suitable smooth pattern (e.g. wave-shaped corrugations instead of a folded pattern), or by selecting the dimensions, such as wave lengths, heights or frequencies, to provide a smooth alternative.

The method described above results in a fiber product, having a beneficial MD extensibility, as well as a CD extensibility that is higher than 15%.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In this description, numerous specific details are provided to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details.

While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

The following non-limiting examples are intended merely to illustrate the advantages obtained with the embodiments of the present invention.

EXAMPLES

Example 1—Preparing Corrugated and Optionally Crushed Paper Sheets

Wet paper sheets of 140 g/m² were prepared from pulp refined first at high consistency (HC) and then at low consistency (LC). The sheets were wet pressed to a solids content of about 50%. Some sheets were also moisturized to lower solids contents of 30% and 40%. The sheets had a wet strain at break of 10.4%, while the strain at break of wet sheets made of LC refined pulp is typically 2-4%.

The wet sheets were pressed between two aluminum plates having corrugations with a wave length of 2.7 mm and a flute height of 0.7 mm (as shown in FIG. 1). The over-all dimensions of the sheets did not change during corrugation, but the contour length was increased by approximately 15-16%.

The sheets were allowed to dry and shrink freely, until at selected solids contents the waves were crushed flat. During drying and simultaneous shrinkage, the wave form was maintained. The solids content at crushing was varied from 53 to 87%. The crushing took place at about 23 bar (2.33 MPa) pressure in a hydraulic press.

Two sheets were left without corrugations and one sheet was left corrugated without crushing the corrugations.

Example 2—Properties of the Prepared Paper Sheets

The sheets prepared in Example 1 were tested for their tensile strength, their drying shrinkage and their strain at break.

The applied corrugating and crushing did not essentially weaken the paper sheets, as shown in FIG. 2. The weakest paper was the corrugated, freely dried sheet, which had a tensile index of 41 Nm/g.

The amount of drying shrinkage of the sheets is shown in FIG. 3. The shrinkage varied from 5% to 16% in a freely dried sheet. A reference sheet without corrugations was found to shrink 7.3%. The corrugations thus increased the drying shrinkage considerably. However, as shown by the last reference point of FIG. 3, crushing the corrugations again decreased the drying shrinkage. The results do also indicate that a higher solids content during crushing resulted in a higher drying shrinkage, and that a lower solids content during the corrugation resulted in a higher drying shrinkage. Thus, the optimal alternative is to use a low solids content during the corrugating and a high solids content during the crushing.

The strain at break values are shown in FIG. 4. Generally, these results are explained by the preceding results for the drying shrinkage and the imposed corrugation pattern. Also here the low solids content during corrugation and a high solids content during crushing was found to be beneficial.

For both the drying shrinkage and the strain at break, the highest values were obtained for the corrugated sample that was not crushed at all (see FIG. 4).

As a conclusion, it was shown that the corrugation process increased the contour length of the wet paper by around 15%, yielding a similar increase in the strain at break of unshrunken paper. The corrugations also increased the drying shrinkage by 1-11%-points above the shrinkage of an uncorrugated sheet which was 7.3% (see FIG. 5). The results follow the theoretical dependence:

Strain ⁢ at ⁢ break = Δ ⁢ S + ε Restrained 100 - Δ ⁢ S ,

where ΔS is the percentage shrinkage and ε_Restrained is the strain at break of sample that is not allowed to shrink.

The increased shrinkage can be explained by the reduced resistance of fibers and sheet to the shrinkage forces. In a planar sheet the axially compressed fibers have to buckle under the shrinkage forces before the shrinkage can proceed. Here the imposed corrugations already have caused the fibers to buckle and that way to reduce shrinkage resisting forces.

Since a flat sheet is often desired, a compromise thus needs to be selected between obtaining a flat sheet and obtaining the optimum extensibility. Large part of the corrugations will, however, be stretched out in the converting phase, when the product sheet is moulded into the desired shape.

INDUSTRIAL APPLICABILITY

The present method can be used to prepare cellulose-based fiber products having an increased potential in replacing plastics in packaging products. In particular, the prepared material is useful in increasing the CD extensibility in such cellulose-based materials.

The proposed methods of increasing CD extensibility and exploiting the drying shrinkage can also be utilized in other applications at board machine, where the CD shrinkage today is not essentially restrained.

CITATION LIST

Patent Literature

• U.S. Pat. No. 3,220,116 A
• U.S. Pat. No. 3,630,837 A

Non-Patent Literature

• Ceccato C. et al. 2021