WO2003010055A1

WO2003010055A1 - Paper sack

Info

Publication number: WO2003010055A1
Application number: PCT/KR2002/001258
Authority: WO
Inventors: Ku-Hwa Choi; Bong-Jin Bang
Original assignee: Shinduck Ind. Co.,Ltd
Priority date: 2001-07-23
Filing date: 2002-07-03
Publication date: 2003-02-06
Also published as: CN1535232A; CN1258467C

Abstract

A paper sack of novel constitution for packing heavyweight materials is manufactured with the dual medium paper (DMP) or modified medium paper (MMP) manufactured by inserting synthetic fiber and the like between two plies of medium paper in the form of nets, and then pasting them with a water-soluble binder. The sack for packing heavyweight materials is composed of one sheet of kraft paper for the outer ply, one or two plies of paper selected from the group consisting of medium paper, DMP and MMP for the intermediate ply, and one sheet of paper selected from the group consisting of DMP, MMP, BKP and kraft paper for the inner ply.

Description

PAPER SACK

Technical Field

The present invention relates to a paper sack of novel constitution for packing heavyweight material, and more particularly to a paper sack of novel constitution manufactured from medium paper, dual medium paper and modified medium paper for packing cement, sugar, flour, feed, etc.

Background Art

A conventional paper sack for packing heavyweight material such as cement, sugar, flour, feed and the like is manufactured by overlapping multiple sheets of kraft paper, processed from 100% virgin pulp or processed by mixing virgin pulp with 20% or less by weight of waste paper, into three or four-ply. To manufacture a two-ply sack, a special expansible paper such as Clupack (U.S.A., Clupack Inc.), composite paper or the like is used. Although common kraft paper is not as strong as the special paper, it is possible for a kraft paper sack to prevent breakage at the time of loading/unloading or during transportation, moisture permeation by blocking air transmission therethrough during storage, and thus deteriorating its contents therein, by constructing a three to four ply sack.

It is a disadvantage of a common kraft paper sack to have difficulty in matching the air pressure of an air blower and feed rate of the contents in packing. In some cases, air is not discharged smoothly from the sack, thereby generating dust and/or air pockets in the sack, which in turn causes an under-weight pack or sack breakage. In particular, if air remains in the sack, vapor condensation can occur due to a temperature gap between the inside and the outside of the sack since the cement temperature is around 70 ^°C when the cement from a silo is filled into the sack. Such vapor condensation is one of the reasons that cement hardens.

In order to solve such problems, the Korean Utility Model Registration No. 1985-1644 discloses a sack manufactured by interposing a synthetic resin film formed with vents between plies of kraft paper and then heat-sealing them. Such a sack with vents causes deterioration of the contents therein due to moisture permeation if it is preserved for a prolonged time. To improve such a deterioration problem, the Korean Utility Model Registration No. 1996-7485 discloses a sack manufactured with a paper sheet in which a polyethylene film is laminated on kraft paper. In this case, the sack lacks flexibility and thus causes more air pockets therein, and the sack is likely to be broken from impact when dropped during transportation or storage during the winter season. As such, the disclosed sacks are not practically used at present. To make matters worse, in the prior art, it is difficult to separate paper from the films and thus recycling thereof is not achieved, and moreover, environmental pollution is caused since toxic gases, which are very difficult to decompose biologically, are generated from film wastes when disposed.

The Korean Patent Registration No. 1985-457, discloses composite paper for manufacturing sacks by applying a binder of print ink, which allows oxidative polymerization, in the form of a stripe onto the kraft paper surface such as in printing, and then is combined with another kraft paper. The Korean Patent Publication No. 1999-73142 discloses a method for manufacturing a sack by interposing and attaching polyethylene fiber nets between two plies of kraft paper using a water- soluble adhesive to reinforce the sack.

Although these prior arts are said to have improved over the previous prior art, there are still the problems such as the rupture of the sacks due to the low strength of the stitches at the top and bottom of the sacks. In particular, air can not be satisfactorily discharged when cement is packed into the described sacks by air feeding, which in turn causes underweight sacks due to the creation of air pockets inside the sacks. Furthermore, the air remaining in the sack causes the deterioration of its contents (e.g. hardened cement) and breakage of the sacks. Accordingly, these sacks can not be a substitute for the ones manufactured according to the previously disclosed patents due to such disadvantages.

Disclosure of Invention

It is an object of the invention to provide a sack of novel constitution for packing heavyweight material.

It is another object of the invention to provide a sack which makes it possible to expand the recycling limits of a medium paper, by using the medium paper, which has been used only for corrugated cardboard, to manufacture sacks for packing heavyweight material.

It is still another object of the invention to provide a sack for packing heavyweight material that is as good as the Icraft paper sack in the performance and even greater than the kraft paper sack in the stability of loading.

It is yet another object of the invention to provide a sack made of medium paper, that can reduce defective products, such as underweight products, and environmental pollution, such as dust, while packing powder products such as cement.

It is still another object of the invention to provide a sack made of medium paper for packing heavyweight materials, wherein the contents in the sacks are less likely to deteriorate even in prolonged storage due to the low permeability of moisture.

It is still another object of the invention to provide a sack made of medium paper, which has a low risk of breakage or environmental pollution during transportation, unloading and storage.

It is still another object of the invention to provide a sack that prevents it from sliding, characteristic of a sack manufactured with common kraft paper when stacking, by using a novel medium paper that is rigid and stiff in manufacturing the sack.

In order to accomplish the objects of the present invention, the sack in accordance with the invention is manufactured with newly constructed medium paper. Until now, medium paper has been used only for corrugated board.

The present invention is directed towards a sack manufactured with medium paper, dual medium paper (DMP) and modified medium paper (MMP). The sack of the present invention is characterized in that the medium paper, DMP and MMP are used as the outer, intermediate and inner plies of the sack, along with common Icraft paper and B Icraft paper. That is, in a three or four-ply sack composed of outer, intermediate and inner plies, the outer ply consists of common kraft paper or MMP of the invention, the intermediate ply consists of MMP, DMP of the invention and/or common medium paper, and the inner ply is selected from either common kraft paper, B kraft paper, or DMP and MMP. In a two-ply sack composed of only outer and inner plies, the outer ply consists of common kraft paper or MMP, and the inner ply consists of DMP of the invention.

The dual medium paper of the invention is manufactured by overlapping two sheets of medium paper using a water-soluble binder, and the MMP of the invention is manufactured from the mixture of long-fiber pulp whose length-weighted average is about 2.4mm with short-fiber pulp whose length- weighted average is about 1.6mm, refined from waste paper in advance using a fractionator. An effective amount of known paper-strengthening agent may be added to the MMP of the invention.

While the medium paper used for the corrugated cardboard is manufactured with the short- fiber pulp of the repulped paper, the long-fiber pulp drafted from waste paper is used to manufacture newspaper and the like.

In the invention, the raw pulp of medium paper is manufactured by mixing 30wt% or less of long-fiber pulp with the short-fiber pulp. The common kraft paper (KP) is manufactured in accordance with a known manner with 100% virgin pulp or by mixing the virgin pulp with 50wt% or less of the long-fiber pulp. Finally, the B kraft paper (BKP) is of lower quality than the common kraft paper and manufactured by mixing 10~50wt% of virgin pulp with 50~90wt% of long-fiber pulp.

It has been known that, since the medium paper made from the repulped paper is essentially made of short-fiber, the paper can not be used to manufacture a sack for packing heavyweight materials that requires greater strength. There has never been found in any case a sack made of medium paper. However, unlike some concerns that rupture of a sack may occur due to low strength if the medium paper is used instead of kraft paper for the intermediate ply of the sack, rather it is possible to manufacture a sack of better performance than that of the existing kraft paper sack due to the greater thiclcness and stiffness of the medium paper without any problem during the packing of contents, transportation and storage. For example, common kraft paper sacks may often slide or shift to one side when stacked one above another due to the formation of bulge on the central portion of the sack after packing it with heavyweight material such as cement, since the common Icraft paper sack is strong but not very rigid.

However, since the sack in accordance with the present invention is of good rigidity and stiffness, it is possible to fill the sack with cement with the sack remaining in almost exact box form during air-packing, much like a brick after filling. Accordingly, it is possible to prevent sliding and shifting of the sacks to one side and also to maintain a stable state of loading, when packed with heavyweight material such as cement and stored stacked one above another.

The binders used for manufacturing dual medium paper(DMP) in accordance with the invention are water-soluble. The DMP of the invention is for manufacturing a sack use in packing heavyweight materials, and is reinforced by attaching one sheet of medium paper with the binder applied onto another sheet of medium paper, or by inserting and attaching synthetic fiber or the like in the form of a net between the two sheets of the medium paper, so as to improve its strength.

In the present invention, the water-soluble binder is a synthetic resin adhesive composed of ethylenevinylacetate (EVA), polyvinylalcohol (PVA) and/or acryl emulsion. And the softener, polyzol, starch and the like that serves to improve its physical properties will be added as required.

For the reinforcing fiber of the invention, polypropylene (PP) or polyester (PE) fiber is

^•mainly used, but the straw fiber or paper fiber can also be used. The spacing between the weft and warp of the reinforcing fiber is 5mm to 30mm, which can be properly adjusted depending on the desired use of the manufactured products. In the present sack, the reinforcing fiber is in the form of a net attached horizontally and vertically or obliquely.

To manufacture the dual medium paper in accordance with the invention, the composition of a binder, such as the composition of a coating binder with 70wt% of water-soluble acryl emulsion, 29wt% of EVA and lwt% of softener is applied on the upper paper with a paint roller. The fiber net is put and attached on the lower paper moving on a roller, and then the lower paper is laminated with the upper paper while passing through two press rollers. The binder may also be applied on the upper paper only so as to be attached with the lower paper without any fiber net. Subsequently, the attached DMP is dried while passing through four to six heat rollers and finally wound on a winding roller. The water-soluble binder requires a plurality of drying rollers due to low drying rate.

The MMP in accordance with the invention is manufactured from raw pulp of 0 to 70wt%, preferably 40 to 65wt% of short-fiber pulp with 30 to 100wt%, preferably 35 to 60wt% of long-fiber pulp, more preferably 20wt% or less of the virgin pulp instead of the long-fiber pulp.

The medium paper, for example, the one made by adding over 70wt% of short-fiber pulp to the long-fiber pulp, is also used along with the Icraft paper, the DMP or MMP of the invention, and thereby constitutes one component of the sack in accordance with the invention.

However, if the medium paper is entirely used to manufacture the sack, breakage might occur in the sack due to its lower strength. Accordingly, it is necessary to use the kraft paper, and the DMP or MMP of the invention for the outer and inner plies. Although the loss of strength of common medium paper can be complemented if l~2wt% of paper-strengthening agent is added to the raw pulp when making the medium paper, it is not possible to manufacture a sack for packing heavyweight material with the medium paper only in any case.

In general, for the packing paper, the thickness is represented as weight per unit area with respect to raw pulp. For the MMP of the invention, the weight per unit area is about 90~120g/m², the tensile strength (KS M7014) is about 7.1-7.5 kgf/15mm and 3.6-4.0 kgf 15mm in longitudinal and transverse directions, respectively, and disruptive strength (KS M7016) is in the range of 100~120g and 110~124g in longitudinal and transverse directions, respectively, when measured at 20+2 °C in temperature and 65+2% relative humidity.

The paper-strengthening agent generally used in the paper industry can also be used in the present invention, for example, the paper-strengthening agent of epoxy resin {KYMEN 5571 (trademark, Korea Hercules), FINEX-737 (trademark, Taekwang Chemical), TOREZ P-1012 (trademark, Kukdo Chemical), S-6181 (trademark, LyGio Korea), EPOMJNE-WS (trademark, Woojin Industry)}, or the paper-strengthening agent of quaternary ammonium compound {Coltide CQS (trade name, USA CRODA INC.)}, etc.

Although the invention is characterized by a sack manufactured using common medium paper and the modified medium paper, care should be taken when selecting such paper to use as a component of the sack. That is, the common medium paper cannot be used as the outer or the inner ply for the sack. However, it is possible for it to be used as the intermediate ply for the sack only when used along with common kraft paper, B kraft paper, DMP or MMP of outer and inner plies. For example, when the common Icraft paper is used as the outer ply and the MMP is used as the inner ply for the sack, one or two sheets of common medium paper can be used as the intermediate ply for the sack. The DMP of the invention cannot be used as the outer ply for the sack due to its rough surface and two sheets of DMP cannot be used at once due to its thickness. It is possible to use only one sheet of DMP serving both as an intermediate ply and an inner ply due to its rigidity and stiffness for the two-ply sack.

The sack in accordance with the invention can be manufactured in various types, for example, sewing type, adhesive type, pinch bag type, easy-open type, pasted valve bag type and so on, depending on the purpose of use. In manufacturing a sewing type sack for cement, another important factor is a sewing process to seal the top and bottom openings of the sack. According to the invention, the most advantageous results are obtained by the method of sewing to assist the air to be smoothly discharged, in which No. 25 or 26 needles are employed, 37 to 39 stitches are applied and No. 6 to 15 of twisted cotton thread is used.

Generally, cement is filled into a sack together with the air at the pressure of 7kg/cn . At this time, the air is discharged through the stitch holes. If the stitch holes are too smooth without any nap, the cement dust is discharged to the outside together with the air and thus pollutes the sack surface and the air in the working environment.

Since the stitch holes are basically smooth and low in air resistance for the kraft paper, it is not easy to prevent cement dust from being discharged. Therefore it raises dust or creates air pockets during packing, which in turn causes underweight or breakage of the sack or the deterioration of its contents therein. Since the temperature of the inside of the sack is around 70 ^°C when the cement from the silo is added to the sack, some air exists within the sack such as in the form of air pockets and the like, a large temperature gap between the inside and the outside of the sack may cause vapor condensation. It can be one of the reasons that the cement hardens in the sack.

A unique effect of the sack in accordance with the invention is that the sack makes it possible to significantly reduce the amount of raised cement dust compared with the 4-ply kraft paper sack. The present invention can also reduce loss of products by preventing cement hardening during prolonged storage. That is, the medium paper according to the invention is composed of short fiber and has greater thickness, thereby creating a nap around the stitches that serves as a filter, so that the nap allows air to easily pass through but not the cement powder. In addition, since the angled shape of the medium paper sack creates a vortex in the sack when the air is discharged to the outside, which helps the cement powder to die down so as to reduce the amount of raised dust. Moreover, the sliding of the stacked sack can be prevented, and safety can be secured with the reduction of accidents.

Brief Description of the Drawings

These and other features, aspects, and advantages of the present invention will be apparent by the following description, appended claims, and accompanying drawings. In the drawings, cross- sectional views are taken along the transverse line of the central portion of the sack in which reference numerals are designated to each component from the outermost to the innermost:

FIG. 1 is a front-sectional view of a conventional 4-ply kraft paper sack;

FIG. 2 is a cross-sectional view of a laminated structure of the dual medium paper of the example 1 according to the present invention; FIG. 3 is a cross-sectional view of a laminated structure of the dual medium paper of the example 2 according to the present invention;

FIG. 4 is a front-sectional view of the structure of a 2-ply sack of the example 3 according to the present invention;

FIG. 5 is a cross-sectional view of the sack shown in Fig. 4;

FIG. 6 is a front-sectional view of the structure of a 3 -ply sack of the example 4 according to the present invention;

FIG. 7 is a cross-sectional view of the sack shown in Fig. 6;

FIG. 8 is a front-sectional view of the structure of a 4-ply sack of the example 5 according to the present invention;

FIG. 9 is a cross-sectional view of the sack shown in Fig. 8;

FIG. 10 is a front-sectional view of the structure of a 3 -ply sack of the example 6 according to the present invention;

FIG. 11 is a cross-sectional view of the sack shown in Fig. 10;

FIG. 12 is a front-sectional view of the structure of a 3-ply sack of the example 7 according to the present invention;

FIG. 13 is a cross-sectional view of the sack shown in Fig. 12;

FIG. 14 is a front-sectional view of the structure of a 4-ply sack of the example 8 according to the present invention;

FIG. 15 is a cross-sectional view of the sack shown in Fig. 14.

FIG. 16 is a front-sectional view of the structure of a 3-ply sack of the example 9 according to the present invention;

FIG. 17 is a cross-sectional view of the sack shown in Fig. 16;

Fig. 18 is a front-sectional view of the structure of a 4-ply sack of the example 10 according to the present invention; and

Fig. 19 is a cross-sectional view of the sack shown in Fig. 18.

Best Mode for Carrying out the invention

Hereinafter, preferred examples of the present invention will be described in detail with reference to the accompanying drawings. In the present examples, the manufacture of the modified medium paper (MMP) is based on the method of manufacturing conventional common medium paper, however, mixing ratio of the components complies with the present invention.

Comparative Example 1 (Prior Art)

Fig. 1 is a front-sectional view to illustrate the structure of a conventional 4-ply kraft paper sack, in which structure four plies of common kraft paper 3 a are overlapped to construct a sack.

Example 1

Fig. 2 is an embodiment to illustrate a laminated structure of the dual medium paper in accordance with the invention, showing the structure of DMP 20 configured by stacking a medium paper layer 2a, a binder layer 2b, a synthetic resin thread layer 2c, a binder layer 2d and a medium paper layer 2e one after another. The composition of the binder is of 70wt% of water-soluble acryl emulsion, 29wt% of EVA and lwt% of softener.

Example 2

Fig. 3 is an embodiment to illustrate another structure of the dual medium paper in accordance with the invention, showing the structure of the DMP configured by stacking a medium paper layer 2a, a binder layer 2b and a medium paper layer 2e one after another. The binder is of the same composition as that of the example 1.

Example 3

The Figs. 4 and 5 illustrate an embodiment of a two-ply sack in accordance with the invention, which is composed of one sheet of common kraft paper for the outer ply 3 a, one sheet of dual medium paper in accordance with the example 1 for both intermediate and inner plies 20.

Example 4

Figs. 6 and 7 are an embodiment to illustrate a three-ply sack in accordance with the invention, which is composed of one sheet of common kraft paper for the outer ply 3 a, one sheet of dual medium paper in accordance with the example 2 for the intermediate ply 21, and one sheet of the B kraft for the inner ply 3b.

Example 5

Fig. 8 is a front-sectional view to illustrate an embodiment of a four-ply sack in accordance with the invention, and Fig. 9 is a cross-sectional view of the sack shown in Fig. 8, depicting the sack composed of one sheet of common kraft paper for the outer ply 13 a, two sheets of common medium paper for the intermediate ply 12a and one sheet of MMP for the inner ply 12b. The inner ply 12b in the present example is manufactured with the MMP by mixing 60wt% of long-fiber pulp with 40wt% of short-fiber pulp.

Example 6 Fig. 10 is a front-sectional view to illustrate another embodiment of a three-ply sack in accordance with the invention, and Fig. 11 is a cross-sectional view of the sack shown in Fig. 10, depicting the sack composed of one sheet of common Iσaft paper for the outer ply 13 a, one sheet of medium paper containing paper-strengthening agent for the intermediate ply 12c and one sheet of B kraft paper for the inner ply 13b. The intermediate ply 12c in this example is the medium paper containing paper-strengthening agent manufactured by mixing 20wt% of long-fiber pulp with 78wt% of short-fiber pulp and 2wt% of paper-strengthening agent of epoxy resin.

Example 7

Fig. 12 is a front-sectional view to illustrate yet another embodiment of a three-ply sack in accordance with the invention, and Fig.13 is a cross-sectional view of the sack shown in Fig.12, depicting the sack composed of one sheet of common kraft paper for the outer ply 13 a, one sheet of common medium paper for the intermediate ply 12a and one sheet of common kraft paper for the inner ply 13 a.

Example 8

Fig. 14 is a front-sectional view to illustrate another embodiment of a four-ply sack in accordance with the invention, and Fig.15 is a cross-sectional view of the sack shown in Fig. 14, depicting the sack composed of one sheet of MMP for the outer ply 12d, two plies of common medium paper for the intermediate ply 12a and one sheet of MMP for the inner ply 12e. The outer ply 12d in this example is the MMP manufactured by mixing 80wt% of long-fiber pulp with 20wt% of virgin pulp, and the inner ply 12e is the MMP⁺ containing paper-strengthening agent manufactured by mixing 30wt% of long-fiber pulp with 69wt% of short-fiber pulp and lwt% of paper- strengthening agent of epoxy resin.

Example 9

Fig.16 is a front-sectional view to illustrate still another embodiment of a three-ply sack in accordance with the invention, and Fig. 17 is a cross-sectional view of the sack shown in Fig.16, depicting the sack composed entirely of MMP for all of outer, intermediate and inner plies 12f, 12f and 12f. The MMP of the present example is manufactured from 100wt% of long-fiber pulp.

Example 10

Fig. 18 is a front-sectional view to illustrate yet another embodiment of a four-ply sack in accordance with the invention, and Fig.19 is a cross-sectional view of the sack shown in Fig.18, depicting the sack composed of one sheet of common kraft paper for the outer ply 13 a, two sheets of MMP for the intermediate ply 12b and one sheet of common kraft paper for the inner ply

13 a. The intermediate ply 12b in the present example is manufactured with the MMP by mixing

40wt% of long-fiber pulp with 60wt% of short-fiber pulp.

The reference numerals 4 and 14, in the above examples, which have not been yet described, are sewed part of the sack.

The following table 1 shows the percentage of sack breakage that occurred in the conventional four-ply kraft paper sack and the sacks in accordance with the invention, in loading and unloading of respective sacks. The percentage of sack breakage is the rate in which sack breakage occurs from the time of packing of the corresponding material to the time of loading/unloading to/off the freight vehicle. The percentage of sack breakage was measured and averaged for 1,325 sacks that were loaded into one freight car and for 600 sacks that were loaded into one freight truck.

[Table 1]

* KP: common kraft paper

MP: Medium Paper

MP⁺: Medium Paper + Paper-Strengthening Agent

DMP: Dual Medium Paper

MMP: Modified Medium Paper

MMP⁺: Modified Medium Paper + Paper-Strengthening Agent

BKP: B Kraft Paper

As can be seen in table 1, the sack in accordance with the invention improves the percentage of sack breakage, compared with the conventional four-ply kraft paper sack (comparative example 1). Referring to the above results, it is contemplated that though the common medium paper has lower strength than that of the kraft paper, the modified medium paper of greater thickness and high strength in accordance with the invention results in better stiffness and rigidity, which in turn creates a synergistic effect in reducing the percentage of sack breakage when the sack is used for packing heavyweight material.

In addition, the sack in accordance with the invention is better than the conventional kraft paper sacks with respect to physical properties, thereby ensuring easy discharge of air out of the sack and filling of cement into the sack, during the packing of the cement. As a result, dust and air pockets would be significantly reduced, thereby improving the stability of loading and reducing sack breakage.

This can be seen from the fact that the working environment maintains its cleanliness and productivity is improved due to less dust during the packing, since while it takes 2.1 seconds on an average to fill the conventional kraft paper sack with 42kg of cement, it takes only 1.7 seconds on an average to fill the sack of the present invention. Such features of the invention are regarded as the result of formation of the nap around the stitch holes. While the nap, according to the observation with a microscope, normally covers one half of the stitch holes, once the packing operation is initiated, the nap serves as a filter by allowing the air pass through but blocking the cement dust.

As described above, the sack in accordance with the present invention has the effect of improving from working environment, productivity, and recycling of resources to cost saving along with the prevention of the deterioration of the contents therein.

The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred examples of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Claims

1. A three to four-ply sack for packing heavyweight material, characterized in that an outer ply of the sack comprises a kraft paper or modified medium paper (MMP); an intermediate ply of the sack comprises one or two sheets of paper selected from the group consisting of medium paper, dual medium paper (DMP) and MMP; and an inner ply of the sack comprises a sheet of paper selected from the group consisting of DMP, MMP, kraft paper and B kraft paper.

2. The sack as claimed in claim 1, characterized in that said DMP is made by laminating two plies of medium paper with a water-soluble binder composition comprising one or more synthetic resin adhesive selected from the group consisting of ethylenevinylacetate, polyvinylalcohol and acryl emulsion.

3. The sack as claimed in claim 2, characterized in that said DMP is manufactured by inserting a reinforcing fiber between two sheets of medium paper in the form of a net which is selected from the group consisting of synthetic fiber, straw fiber and paper fiber.

4. The sack claimed in claim 2 or 3, characterized in that said water-soluble binder further contains one or more components selected from the group consisting of the softener, polyzol and starch as a physical property improver.

5. The sack as claimed in claim 1, characterized in that said MMP contains 0 to 70wt% of short-fiber pulp and 30 to 100wt% of long-fiber pulp, with respect to the total amount of the raw pulp.

6. The sack as claimed in claim 5, characterized in that said MMP contains 40 to 65wt% of short-fiber pulp and 35 to 60wt% of long-fiber pulp, with respect to the total amount of the raw pulp.

7. The sack as claimed in claim 6, characterized in that said MMP contains 40 to 65wt% of short-fiber pulp, 35 to 40wt% of long-fiber pulp and 0 to 20wt% of virgin pulp, with respect to the total amount of the raw pulp.

8. The sack as claimed in one of the claims 5 to 7, characterized in that said raw pulp for the MMP further contains l~2wt% of a paper-strengthening agent of epoxy resin or quaternary ammonium compound.

9. The sack as claimed in one of the claims 5 to 8, characterized in that said MMP has 90~120g/m² of weight per unit area, and 7.1~7.5kgf/15mm of tensile strength in longitudinal direction and 3.6~4.0kgf/15mm of tensile strength in transverse direction, and 100~120g of disruptive strength in longitudinal direction and 110~124g of disruptive strength in transverse direction when measured at 20+2 °C and 65+2% relative humidity.

10. The sack as claimed in one of the claims 1 to 9, characterized in that said sack is manufactured in accordance with one type selected from the group consisting of sewing type, adhesive type, pinch bag type, easy-open type and pasted valve bag type.

11. A two-ply sack for packing heavyweight material, characterized in that an outer ply of the sack comprises common Icraft paper or modified medium paper (MMP); and an inner ply of the sack comprises dual medium paperfDMP) is made by laminating two plies of medium paper with a water-soluble binder.

12. The sack as claimed in claim 11, characterized in that said water-soluble binder is an adhesive composition selected from the group consisting of ethylenevinylacetate, polyvinylalcohol and acryl emulsion.

13. The sack as claimed in claim 12, characterized in that said water-soluble binder further contains one or more components selected from the group consisting of softener, polyzol and starch as a physical property improver.

14. The sack as claimed in claim 13, characterized in that said water-soluble binder layers further contains one selected from the group consisting of polypropylene fiber, polyester fiber, straw fiber and paper fiber therein, as a reinforcing fiber.

15. The sack as claimed in one of the claims 11 to 14, characterized in that said sack is manufactured in accordance with one type selected from the group consisting of sewing type, adhesive type, pinch bag type, easy-open type, and pasted valve bag type.