SACK

Abstract: In the application, a multilayer paper sack is presented, which is intended to be closed with a transversal seam (1) especially by sewing. At the area of the seam, at least two of the layers of the sack sheath are adhered together with a transverse line of adhesive (2). The line of adhesive reinforces and stiffens the sack.

Description: SACK Field of the technology This invention relates to multi layer paper sacks and particularly to the reinforcing or stiffening of the paper sack mouths. Background to the technology Under stress, sacks closed by sewing tear most easily by the perforated sewn seam. To prevent tearing, these sacks are usually made using stronger paper or a greater number of paper layers than would otherwise be necessary in concern of the other parts of the sack. From patent publication FI-75318, one method of reinforcing the sewn seam of a paper sack closed by sewing is known. According to this method, a separate reinforcing thread of paper is adhered at the area of the seam. Description of the invention A paper sack according to claim 1 has now been invented. Some preferred embodiments of the invention are presented in the other claims. According to the invention at least two sack layers are adhered together at the area of the closing seam. Thus an essentially stronger seam area is provided without adding paper. Furthermore, a nicer result is achieved since no extra paper thread on top of the sack is needed. Adhesion according to the invention also stiffens the sack which is advantageous especially when handling open-mouth sacks in automatic equipment. Preferably the line of adhesive reaches around the entire sack sheath. Adequate strength can nevertheless also be achieved by making two lines at the middle of the sheath side. One application of the invention is an open-mouth sack closed by sewing. Another application is a sleeve valve bag both mouths of which are closed by sewing. Sacks according to the invention can be manufactured on conventional sack production lines which have the generally suitable equipment for forming the lines of adhesive according to the invention. The adhesion can be performed for example in a line printing unit or in a transversal adhesion unit. Adhesion is preferably carried out so that the adhesive does not essentially penetrate the paper. Some applications of the invention are described in more detail in the following. In the drawings of the description - figures la - 1 c represent a two-layer sack and - figure 2 shows how the line of adhesive of a three-layer sack is applied on a line printing machine. Figure la shows an empty and folded two-layer open-mouth sack with a plane bottom closed by adhesion. The mouth is intended to be closed after filling by sewing. The location of the seam is marked with a dotted line 1. At the area of the seam, the paper layers are adhered together over the entire measure around the sack with a Une of adhesive 2. A suitable width for the line of adhesive is approx. 2 - 6 cm. A suitable amount of adhesive is, for example, approx. 1 - 4 g/m 2 calculated as dry matter. Figure 1 b shows a sack opened to be filled and figure 1 c shows a filled sack closed by sewing. Modern sack papers exhibit increased strength properties which has enabled the use of thinner paper and fewer layers than before. However, as a result of this development, the stiffness of the sacks has also decreased which has caused problems with filling machines. For instance, in a conventional closing line of open-mouth sacks, the compressed mouth of a filled sack should remain in an upright position while the sack is transferred to a seaming machine. The line of adhesive 2 acts also as a mouth stiffener. For the purposes of stiffening, the line of adhesive can be made wider than would be necessary for the reinforcement of the seam area For a sack of 50 - 60 cm wide with a height of 80 - 100 cm when filled, a suitable width of the stiffening line is, for example, 15 - 20 cm directed to the head of the compressed area. A stiffened and compressed mouth does not flop down but remains in an upright position. Instead of a line reaching around the entire sack sheath, lines of adhesive made to the middle of the side can be used, the length of the lines being, for example, approx. 25 - 75 % of the side width. Figure 2 shows the manufacture of a three-layer sack by using a conventional line printing machine of a production line. The inner layer is formed from web 3, the middle layer from web 4 and the outer layer from web 5. The webs are run on top of each other and then lead to a sack machine where a tube is formed by adhering 4 the edges together in longitudinal direction, then the tube is cut into pieces and one end of each piece is closed as a bottom. The inner layer web is run through a brush roll 6, an upper creasing roll 7 and a control roll 8. On the surface of the brush roll there is a plate 9 which is used for applying a transverse line of adhesive on the outer surface of the web at intervals equal to the cutting. To the brush roll the adhesive is brought from basin 10 with the help of lift roll 11 and wipe roll 12. Instead of using a rotating roll, it is naturally possible to use a rotating segment of a roll. The adhesive is transferred on the surface of web 3 without pressing essentially so that it does not penetrate the paper. The creasing roll 7 can be moved and thus adjust the web tension and its angle against the brush roll 6. This way the adhesive can always be applied optimally. An applying unit of adhesive as described can easily be created from a so called line printing unit conventionally used in sack lines. When the printing unit is used conventionally for printing, roll 8 is pressed against roll 6. Such a printing unit 13 is also included in the equipment according to figure 2. It is used for printing a desired pattern of one colour on the outer layer web 5. The equipment also comprises a third printing unit 14 which can be used for printing another colour. If desired, this unit can also be transformed into an adhesive applying unit, after which the outer layer and the middle layer can also be adhered together. Naturally the adhesion units are synchronized with the sack machine so that the line of adhesive comes always synchronized to the point of seaming and cutting. Adhesion according to the invention can be also be carried out in a transversal adhesion group of a tube extrusion machine. In this case, there should always be available transversal adhesive groups in use which can be applied to the adhesion according to the same principles as with the line printing equipment described above. Adhesion can be carried out with a suitable starch adhesive which is economical and conventionally used for sack adhesion. Other type of adhesive can nevertheless also be used. With, for example, synthetic dispersion adhesives (such as polyvinyl acetate, PVAC), an even better reinforcing effect is achieved. Mixtures of adhesives can also be used. In the following table 1 , the tear resistance (mN) of the sewn seams of different sacks are compared as an example, both in the machine direction (MD) and in the cross direction (CD) when the seam was unadhered or adhered according to the invention with a starch adhesive or with PVAC. The paper was sack paper, basis weights 80, 95 and 100 g m 2 . All the layers were adhered together. The width of the line of adhesive was approx. 4 cm and the amount of adhesive as dry matter 2 - 4 g/m 2 . Tear resistance was defined according to the SCAN methods. Table 1 Number and basis weight of paper layers 3 * 100 g / m 2 4 * 80 g / m 2 2 * 95 g / m 2 MD CD MD CD MD CD mN mN mN mN mN mN Unadhered 940 1000 1050 1200 650 780 Starch 1200 1520 2260 2060 945 1015 PVAC 1270 1630 2270 2440 It can be noticed that when the paper layers were adhered together, seam strength was essentially improved. In the following table 2, folding resistance of the mouths of different open-mouth sacks are compared, as an example, in MD direction. All sack layers were adhered together. The width of the line of adhesive was approx. 15 cm and the amount of adhesive as dry matter approx. 1 - 3 g/m 2 . Folding resistance was defined according to the SCAN methods. Table 2 Number and basis weight of paper layers 3 * 80 g / m 2 3 * 70 g / m 2 2 * 80 g / m 2 mN mN mN Unadhered 170 105 120 Starch 1385 1 100 475 PVAC 1325 1035 460 It can be noticed that when the paper layers were adhered together, the mouth stiffness was quite essentially improved. When using adhesive reinforcing, thinner papers or fewer paper layers can be used which naturally reduces the material costs.

Claims: Claims 1. A paper sack which has a tubelike sheath consisting of at least two paper layers, which sheath is intended to be closed with a transversal seam (1) so that in the seam the inner surface of the sheath is pressed against inner surface, characterized in that at the area of the seam at least two of the sheath layers are adhered together with a transverse line of adhesive (2). 2. A sack according to claim 1 which is intended to be closed with a transversal sewn seam (1). 3. An open-mouth sack with a plane bottom according to claim 1 or 2 the mouth of which is meant to be closed with a seam (1). 4. A sack according to one of the claims 1 - 3, characterized in that the line of adhesive (2) reaches around the entire sack sheath. 5. A sack according to one of the claims 1 - 4, characterized in that the width of the line of adhesive (2) is 2 - 6 cm. 6. A sack according to one of the claims 1 - 4, characterized in that the width of the line of adhesive (2) is 15 - 20 cm. 7. A method for making a paper sack, in which method a tubelike sheath consisting of at least two paper layers is formed, and onto the sheath a transversal closing area is formed along which the sack is meant to be closed with a seam so that in the seam, the inner surface of the sheath is placed against inner surface, characterized in that the paper layers are adhered together at the area of the closing area with a transverse line of adhesive. 8. A method according to claim 7 where a tubelike sheath is formed from at least two paper webs which are run on top of each other and arranged and joined together from their edges into a tube, characterized in that to form the line of adhesive in at least one paper web, on that surface of the web which will be placed against another paper web, a transverse line of adhesive is applied on regular intervals so that the adhesive remains on the surface of the web without penetrating the paper. 9. A method according to claim 8, characterized in that the line of adhesive is applied with the help of a rotating roll. 10. A method for making, filling and closing a paper sack in which method a tubelike sheath consisting of at least two paper layers is formed, one of the ends of the sheath is closed to form a bottom leaving an open mouth on the other end, the sack is filled through this mouth and the mouth is closed by seaming so that in the seam the inner surface of the sheath is placed against inner surface, characterized in that in the sheath mouth at least two paper layers are adhered together with a transverse line of adhesive and the mouth is closed by seaming by the side of this line of adhesive.

Inventors: RIKASSAARI PENTTI, SIEVERS VALTER, RIKASSAARI, PENTTI, SIEVERS, VALTER
Applicants: UPM KYMMENE OY, UPM-KYMMENE OY
Application year: 1996
Main classification: B31B39
IPC codes: B31B19, B65D33