WATER-SOLUBLE UNBLEACHED BAG PAPER, AND PAPER BAG

Abstract

In a water-soluble unbleached bag paper comprising lignin-containing cellulose fibers as main component and one or more additive(s), at least one surfactant is contained as additive, and the bag paper has a relative wet strength of less than 6% after a 5-minute wetting time. Furthermore, a paper bag comprised of water-soluble unbleached bag paper is described.

Description

The present invention relates to a water-soluble unbleached bag paper comprising lignin-containing cellulose fibers as main component and one or more additive(s), and a paper bag comprised of water-soluble unbleached bag paper.

Bag paper for the production of paper bags and sacks for various contents usually involves high demands on the mechanical tensile properties of the paper both in the machine direction and transverse thereto in order to prevent inadvertent tearing of the paper, whether during filling, stacking, storing or transport, and in addition to good tensile properties it should, moreover, exhibit high porosity and sufficient extensibility both in the long and transverse directions. Porosity is important to avoid a pressure jam in the interior of a bag, for instance when filling the bag, which usually occurs by rapid introduction using machines, and hence tearing of the bag due to a “pressure wave” and, moreover, to ensure that the dust load caused by the filling of powdery materials like cement or the like will be kept as low as possible.

Such a bag paper has been known for quite some time and produced in different qualities for a multitude of applications. Aside from the above-mentioned properties of the bag paper, bag paper has been available in both bleached and unbleached form, wherein bleached bag paper has a considerably lower lignin content than unbleached bag paper, which makes such bag papers perfectly suitable for recycling purposes and, in particular, for instance dissolving in water or water with additives while applying shearing forces. Unbleached bag papers, which have high contents of lignin, are usually not or hardly water-soluble because lignin is a naturally occurring hydrophobing agent, and therefore constitute a relatively big problem, in particular with respect to their disposal. Disposal or recycling by the dissolution of such papers is presently not possible because of the poor dissolution properties, and the introduction of such bags, for instance, into waste incineration would raise problems insofar as, for instance in the event of cement, sacks, residual amounts of raw material or filler material remaining in the bags would eventually cause a concentration of, or increase in, cement or burnt lime or the like in combustion furnaces, which might, in turn, involve problems of removal and the obstruction of important furnace elements.

Water-soluble papers, in contrast, do not show such problems and thus can be disposed of at once. Water-soluble papers, in particular bag papers, are characterized by the complete dissolubility of the paper after treatment, according to norm ISO 5263 “Pulps—Laboratory Wet Disintegration”, and a water penetration rate into the paper determined by a K-value that must be >5000. The R-value in this context is measured by an Emco penetration tester DPM 27. K-value measurements take place at room temperature from 25° C.±2° C. with pure cold water (drinking water quality) having a temperature between 15° C. and 20° C. and no additives whatsoever.

The present invention aims to provide a water-soluble unbleached bag paper that is completely soluble in water according to ISO 5263 and, moreover, comprises a water penetration rate into the paper comparable to that of bleached bag paper, without the cellulose fibers having to be submitted to a bleaching process prior to their use.

To solve this object, the water-soluble unbleached bag paper according to the invention is essentially characterized in that at least one surfactant is contained as said additive and that the bag paper has a relative wet strength of less than 6% after a 5-minute wetting time. The addition of a surfactant allows for a reduction of the surface tension of the thus produced bag paper to such an extent that the effect of lignin as hydrophobing agent will be compensated for, thus rendering the paper water-soluble to the point that it will be dissolved within a few minutes, preferably about 5 minutes, and nothing but cellulose fibers will be detectable. Moreover, the addition of the surfactant does not affect the overall strength of the paper such that the properties of the bag paper are comparable to those of a non-water-soluble unbleached bag paper without having to perform a complex cellulose fiber regeneration procedure for removing the lignin, i.e. a bleaching process, so as to make available a paper that can, in particular, be economically produced.

In that the bag paper has a relative wet strength of less than 6% after a 5-minute wetting time, it has become possible to dissolve the bag paper nearly completely within a short time so as to enable such a bag paper to be, for instance, directly introduced into a concrete mixer along with the construction materials contained therein, e.g. cement, for instance free from residues, and subjected to a mixing process, thus rendering possible the completely residue-free processing of such a bag paper without the thus produced concrete having, for instance, deteriorated properties.

Just by way of comparison, it should be noted that known papers that are also substantially water-soluble, such as toilet paper, which is made from fresh fibers, have wet strengths of about 8% after a 5-minute wetting time, which implies that the paper according to the invention dissolves particularly rapidly and completely.

According to a further development of the invention, the bag paper is devised such that between 0.1% and 2% surfactant is contained. The amount of surfactant in this respect is, in particular, selected such that, on the one hand, the surface tension will not be reduced too much in order to prevent the unlimited and instantaneous water absorption of the paper. Too rapid and too complete a water absorbency of paper, for instance, involves the drawback of such a bag paper being not printable, which is, in particular, disadvantageous for papers and bags used in industrial applications, since, for instance, in the construction industry the content of a bag is usually directly printed on the same. If the absorption of water is too high, such papers will actually be too hygroscopic for flexoprinting, which is usually employed for printing bag papers, and either the paper will not be printable at all or the print will leak or bleed, which makes such a paper undesirable for industrial use.

According to a further development of the invention, the additive of the bag paper is selected such that the at least one surfactant is selected from the group of non-ionic, anionic, cationic or amphoteric surfactants, fatty alcohols, fatty alcohol polylglycol ethers, fatty alcohol sulfates, fatty alcohol ether sulfates, methyl ether sulfates, fatty alcohol ethoxylates or phenol ethoxylates. What type of surfactant is selected for a specific purpose of use has turned out to be noncritical and is to be done as a function of the lignin content of the employed cellulose fibers, the type of wood from which the cellulose fibers are derived, the surface tension of the bag paper and the like, so that basically any of the group of ionic or non-ionic, anionic, cationic or amphoteric surfactants, yet also fatty alcohols, fatty alcohol polylglycol ethers, fatty alcohol sulfates, fatty alcohol ether sulfates, methyl ether sulfates, fatty alcohol ethoxylates or phenol ethoxylates may be chosen.

The paper according to the invention has an absorbency corresponding to a K-value of more than 5000.

The K-value is a measure for the penetration rate of water into paper, and hence the absorbency of the paper. It is usually measured by ultrasonic transmission. During such measurement, the ultrasonic transmitter and the ultrasonic receiver are arranged within a measuring cell in such a manner that the sound wave will directly pass through the paper sample to be checked. During measuring, an ultrasonic wave with a constant frequency and intensity is generated on the transmitter. The interaction of the paper sample with the surrounding medium, i.e. water, causes the ultrasonic intensity to change on the receiver, and this is plotted in a transmission time diagram. The determined K-value will then indicate the gradient of the curve at the time of the highest penetration rate. The measuring procedure applied here is the measuring procedure according to the Emco penetration tester DPM 27.

In order to increase the strength of the bag paper, it can be proceeded according to the invention such that a dry-strength agent is additionally contained. According to the present invention, starches like cationic potato starch, corn starch, tapioca starch are used as dry-strength agents. The printability of the paper will be enhanced by the addition of dry-strength agents.

According to development of the invention, the cellulose fibers used for an unbleached, water-soluble bag paper according to the invention are unbleached softwood fibers. Furthermore, softwood fibers generally show a higher fiber length in comparison to fibers extracted from hartwood, so that by the fibers interlocking among others, a higher strength of the bag paper can be achieved just by use of special fibers, without the need for adding a dry-strength agent or respectively only a very small amount of dry-strength agent would be needed.

According to a further development of the invention, the cellulose fibers are selected from softwood fibers from conifers, in particular fibers from spruce wood and/or pinewood.

To adjust the desired strength, it is proceeded, as in correspondence with a further development of the invention, such that the softwood fibers are used, as high-consistency-beaten (HC-beaten) and/or low-consistency-beaten (LC-beaten) softwood fibers. Depending on the applied beating procedure, the fiber length of the used cellulose is more or less increased, which will also influence the strength of the thus produced, paper. According to the present invention, either high-consistency-beaten softwood fibers or low-consistency-beaten softwood fibers can be used exclusively or in mixtures thereof.

In order to achieve a stable bag paper that is sufficiently strong, in particular in the dry state, the soaked bag is configured such that the longitudinal/transverse ratio of the tensile strength is between 1 and 1.8, in particular 1.3. Bag papers are produced on so-called Clupak units, which enable the production of papers having nearly identical tensile strength properties both in the longitudinal and in the transverse directions. The more uniform the tensile strength properties in the longitudinal and transverse directions, the more stable and more suitable a thus produced paper will be for practical applications, where up to 50 kg cement or more are, for instance, filled into such a bag, and which bag must by no means tear during or after filling. The water-soluble unbleached bag paper according to the invention enables the achievement of a longitudinal/transverse ratio of between 1 and 1.8, in particular 1.3.

In order to, in particular, achieve a good longitudinal/transverse ratio and produce a bag paper having excellent strength properties at a simultaneous water solubility, it is proceeded according to the present invention such that the cellulose fibers are contained as dried, in particular flake-dried, pulp.

The present invention also relates to a paper bag comprised of the water-soluble unbleached bag paper according to the invention. Such a bag is essentially characterized in that the bag paper is glued with an adhesive, in particular a maltodextrin adhesive. Like the paper itself, maltodextrin adhesives are water-soluble so as to enable a filled bag according to the invention to be, for instance, directly introduced into a concrete mixer along with the cement or mortar contained therein, and hence free from residue or just by leaving statistically finely distributed cellulose fibers in the construction material to be produced.

In order to achieve an even higher strength and, in particular, for bags with heavy contents or those dimensioned to be especially large, the paper bag according to the invention is further developed such that it is comprised of a multi-ply bag paper.

In order to achieve barrier properties, the paper bag can be further developed, to the effect that a layer of water-soluble polymers such as polyvinyl alcohol or polyether is additionally applied on at least one paper ply.

In the following, the invention will be explained in more detail by way of exemplary embodiments.

EXAMPLE 1

Sample bags are produced from a water-soluble unbleached bag paper according to the invention and filled with cement. The bags filled with cement are closed and immediately used in a concrete mixer while being subjected to a conventional cement mixing process for 5 minutes and 10 minutes, respectively, whereupon the results were checked.

Used bag paper: non-bleached water-soluble sack/bag Kraft paper with a basis weight according to ISO 536 of 70 g/m².

The cellulose fibers used in the bag paper comprise long fibers of spruce wood that was subjected to an oxidation treatment. The paper does not contain any sizing agent, yet 0.3 weight-% polyalkyleneglycol ethers (fatty alcohol alkoxylate) with MW 588 g/mol was contained as a non-ionic surfactant. The tensile strength of the paper according to ISO 1924-3 was 5.7 kN/m² in the machine direction and 4.2 kN/m² in the direction transverse to the machine running direction, thus corresponding to a tensile strength index of 1.36. From this base paper, three different cement sacks were produced, wherein gluing of the sacks was performed using a maltodextrin adhesive. The first cement sack was comprised of two plies of the above-defined unbleached Kraft paper according to the invention without intermediate ply, the second cement sack comprised a ply of unbleached Kraft paper according to the invention and a second ply of unbleached Kraft paper having a polyvinyl alcohol coating. The coating had a thickness of 30 μm. The third cement sack produced comprised a first layer of unbleached, unsized, water-soluble Kraft paper according to the invention, a bare PVA layer having a thickness of 38 μm, and a second layer of unsized, unbleached Kraft paper according to the invention. Each of the paper layers in all three cement sacks had a basis weight of 70 g/m².

The sacks were examined with respect to non-dissolved paper pieces in the concrete mixer after mixing times of 5 minutes and 10 minutes, respectively, wherein it was found that, as regards sack 1, only very small paper pieces were visible on a sieve at the end of 5 minutes, which hardly changed even after 10 minutes.

With sack 2, a plurality of small paper pieces can still be observed after 5 minutes, which have been significantly reduced after 10 minutes.

With sack 3, a plurality of small paper pieces are still visible after 5 minutes, wherein many paper pieces are still visible even after 10 minutes.

It is thus to be noted that a paper bag made of unsized, unbleached bag paper according to the invention, which paper bag has no additional coating, is able to dissolve almost completely within 5 minutes. Paper sacks 1 to 3 were subsequently filled with cement and subjected to a plurality of handling operations such as rapid filling, closing, stacking, transport, i.e. vibrations, throwing on the ground and the like, wherein it turned out as a result that paper sack 2 was able to withstand throwing on the ground from a height of 1 meter. Paper sack 1 was stable in all handling activities, yet burst open when thrown down from a height of 1 meter. Paper sack 1 withstood dropping from a height of 0.5 meter. The properties of paper bag 3 were comparable to those of paper bag 1.

Claims

1. A water-soluble unbleached bag paper comprising lignin-containing cellulose fibers as main component and one or more additive(s), characterized in that at least one surfactant is contained as said additive, that the cellulose fibers are used as oxidatively treated unbleached softwood fibers, and that the bag paper has a relative wet strength of less than 6% after a 5-minute wetting time.

2. A bag paper according to claim 1, characterized in that between 0.1% and 2% surfactant is contained.

3. A bag paper according to claim 1 or 2, characterized in that the at least one surfactant is selected from the group of non-ionic, anionic, cationic or amphoteric surfactants, fatty alcohols, fatty alcohol polylglycol ethers, fatty alcohol sulfates, fatty alcohol, ether sulfates, methyl ether sulfates, fatty alcohol ethoxylates or phenol ethoxylates.

4. A bag paper according to claim 1, 2 or 3 characterized in that it shows a absorbency corresponding to a K-value of more than 5000.

5. A bag paper according to any one of claims 1 to 4, characterized in that a dry-strength agent is additionally contained.

6. A bag paper according to any one of claims 1 to 5, characterized in that the dry-strength agent is selected from starches like cationic potato starch, corn starch, tapioca starch.

7. A bag paper according to any one of claims 1 to 6, characterized in that the cellulose fibers are used as oxidatively treated, unbleached softwood fibers.

8. A bag paper according to claim 7, characterized in that the cellulose fibers are selected from softwood fibers from conifers, in particular fibers from spruce wood and/or pinewood.

9. A bag paper according to any one of claims 1 to 8, characterized in that the softwood fibers are used as high-consistency-beaten (HC-beaten) and/or low-consistency-beaten (LC-beaten) softwood fibers.

10. A bag paper according to any one of claims 1 to 9, characterized in that the longitudinal/transverse ratio of the tensile strength is between 1 and 1.8, in particular 1.3.

11. A bag paper according to any one of claims 1 to 10, characterized in that the cellulose fibers are contained as dried, in particular flake-dried, pulp.

12. A paper bag comprised of the water-soluble unbleached bag paper according to any one of claims 1 to 11, characterized in that the bag paper is glued with an adhesive, in particular a maltodextrin adhesive.

13. A paper bag according to claim 12, characterized in that it is comprised of a multi-ply bag paper.

14. A paper bag according to claim 12 or 13, characterized in that a layer of water-soluble polymers such as PVA or PE is applied on at least one paper ply.