PERMEABLE SACK

Abstract: This application presents a permeable sack which has an air permeable inner layer (1) and an outer layer (2; 2'). On the outer surface of the inner layer (1) there are channels (4) along which air that has flown through the inner layer during the filling of the sack is allowed to escape from the space between the layers.

Description: PERMEABLE SACK Field of the technology The invention relates to multi layer sleeve valve sacks. Particularly the invention concerns the escape of air from the sack during the filling of the sack. Background to the technology During the filling of sacks, air must be allowed to escape from the sack. During the filling of sleeve valve sacks, the valve surrounds the filling pipe in a relatively airtight manner and therefore most of the air must escape through the sack sheath. Finely-divided materials, such as cement, are rationed to the sacks by air-blowing which noticeably increases the amount of air that escapes through the sheath. The escape of air can be encouraged by using especially porous sack paper or by perforating the sack sheath. With sacks of 40 - 50 kg made of conventional papers, this usually does provide satisfactory air permeability. However, if air permeability could be further improved, filling pressure and speed could be further increased. The rate of escape of air through the sack sheath is parallel to the sheath surface area. To reach the same filling speed with smaller sacks of, e.g. 20 - 25 kg, as with bigger sacks, the air permeability of the sheath should be correspondingly improved. When the sack has a moisture barrier, air permeability of the sheath is naturally further reduced from what is was before. To improve air permeability, the moisture barriers of conventional sacks were abundantly perforated which resulted in correspondingly reduced moisture barrier properties. Yet even perforation does not always provide satisfactory air permeability from the point of view of the filling percentage, not even with sacks of 50 kg. Some products, such as cement, are packed in sacks equipped with moisture barriers only if the storage conditions so require. Under the circumstances, it is impossible to utilize the entire capacity of the sack filler, dimensioned for sacks with conventional air permeability. Description of the invention Now a sack according to claim 1 has been invented. Some of the preferred embodiments are presented in the other claims. A sack according to the invention is essentially characterized in that there are channels on the outer surface of the inner layer, along which the air that has moved through the inner layer is allowed to escape from the space between the layers. Thus the air permeability of the sheath is essentially improved without perforation of the sack. When no perforation or at least perforation to an essentially smaller extent is carried out, a more airtight sack is provided which will, for example, reduce linting. Understandably, "air" from the point of view of the invention, can refer to any gas used during the filling stage. Channels between the layers also increase the friction between the layers thus adding to sack strength. The invention is particularly useful when a moisture barrier is used in the sack. It is placed outside the channels. Depending on the circumstances, the moisture barrier can even be lightly perforated to improve air permeability yet maintaining adequate moisture barrier properties. Preferably the inner layer is porous and strong sack paper with good air permeability. Paper which is porous enough needs no perforation which means than no material leakage occurs, not even between the layers. The outer layer may be of any material suitable for the purpose. The channels are preferably made to the paper before the final drying in the process of manufacture, since then the channels do not essentially weaken the paper by breaking the fibres. Some of the embodiments of the invention are described in more detail in the following. In the drawings of the description Figure 1 shows the sheath layers of a two-layer sack according to the invention, Figure 2 shows an alternate embodiment of the sack in Fig. 1 , and Figure 3 shows a bottom of a sack according to the invention. In a sack sheath according to Fig. 1 , there is an inner paper layer 1 , an outer paper layer 2 and, between these layers, a moisture barrier layer 3. On the outer surface of the inner layer 1 there are parallel channels 4. The inner layer 1 has good air permeability. Its air permeability resistance according to the Gurley standard is, e.g., 6 ± 2 s. During the filling of the sack, air moves through the inner layer 1 to the channels 4 and flows along the channels away from the space between the layers. If the basis weight of the inner layer 1 is 90 g m 2 , the thickness of the layer is approx. 70 μm of which the depth of the channels 4 is, e.g., approx. 15 - 20 μm. The width of the channels is, e.g., 2 - 5 mm and the space between them is, e.g., 1 - 2 mm. The moisture barrier 3 is of suitable plastic, such as polyethylene of 10 - 15 μm. It is adhered only from the mouth regions of the sack to the paper layers 1 and 2 by light dot adhesion. The moisture barrier can nevertheless also be a layer laminated on the paper surface. In this case, the moisture barrier is preferably on the inner surface of the outer layer. If desired, the moisture barrier can be lightly perforated to improve air permeability. However, suitable perforation quantity is, for example, only 30 - 40 % of what is conventionally used. The inner layer may also consist of several sublayers. In this case, the channels are preferably placed on the outer surface of the innermost sublayer. The other sublayers may also be channelled. The outer layer is made of paper with strength properties suitable for the purpose. The outer layer may also consist of several sublayers. In the embodiment according to figure 2, the outer layer 2' is made of channelled paper in which the channels 5 are on the inner surface. This way the over- pressure in the channels 4 pushes the moisture barrier film 3 into the channels 5 on the outer layer, and the cross-sectional area of the air flow channel between the film and the inner layer 1 correspondingly grows. The air escapes from the channels 4 at the bottom areas of the sack. Figure 3 shows the bottom of a sack in a filling situation. The front side of the sack is folded to the bottom area to form a topmost overlap flap 6. Air escapes in the longitudinal direction of the sack from the channels 4 at the end 7 of the overlap flap. On top of the bottom, a bottom cover sheet 8 is further attached. The bottom cover sheet 8 is attached with adhesive seams 9, 9.1 and 10 in the longitudinal direction of the bottom, which seams may be either continuous or broken. At the area of the end of the flap 7 of the front side, the bottom cover sheet does not have any adhesive applied to it, as a result of which a channel 1 1 in the longitudinal direction of the bottom is formed at this area, along which channel air is allowed to escape from the bottom ends. To encourage the escape of air, it is possible to make perforations 12 at the area of the channel 11 on the bottom cover sheet 8, or to use a broken line of adhesive in the seam 9' next to the channel. Furthermore, the bottom cover sheet is preferably made a little shorter than the bottom so that during the filling stage the end 13 of the sheet does not close the air channel of the bottom against the end fold 14 of the bottom. On the other end of the sack, the overlap flap of the back side of the sack is folded topmost to the bottom area, which allows the air to escape from both ends of the sack.

Claims: Claims 1. A permeable sack with an air permeable inner layer (1 ) and an outer layer (2; 2'), the mouths of which sack are closed to form bottoms, and in which sack the outer surface of the inner layer (1) has channels (4) along which air that has flown through the inner layer during the filling of the sack is allowed to escape from the space between the layers, characterized in that the air escapes from the channels (4) from the space between the inner layer (1 ) and the outer layer (2; 2') from their edge. 2. A sack according to claim 1, characterized in that the channels (4) are essentially in the longitudinal direction of the sack. 3. A sack according to claim 1 or 2, characterized in that the sack is closed by folding the ends to form a plane bottom, and that on one end of the sack the overlap flap (6) of the front side is folded topmost, and on the other end of the sack the overlap flap of the back side. 4. A sack according to one of the claims 1 - 3, characterized in that a bottom cover sheet (8) is attached on top of the bottom. 5. A sack according to claim 4, characterized in that the bottom cover sheet (8) is shorter than the bottom. 6. A sack according to claim 4 or 5, characterized in that the bottom cover sheet (8) is attached with lines of adhesive in the longitudinal direction of the bottom so that at the area of the end (7) of the flap folded on top there is no adhesive but instead an air outlet channel (11). 7. A sack according to claim 6, characterized in that at the area of the end (7) of the flap there is an air outlet channel (11 ) in the longitudinal direction of the bottom. 8. A sack according to claim 6 or 7, characterized in that in the space between the end (7) of the flap and the edge of the bottom there is a broken line of adhesive 9.1. 9. A sack according to one of the claims 4 - 8, characterized in that there are perforations (12) on the bottom cover sheet. 10. A sack according to claim 6 and 9, characterized in that there are perforations (12) on the bottom cover sheet at the area of the end of the side flap folded on top. 11. A sack according to one of the claims 1 - 10, characterized in that there is a moisture barrier layer (3) between the inner layer and the outer layer. 12. A sack according to claim 11 , characterized in that the moisture barrier layer (3) is an unattached film between the inner layer and the outer layer. 13. A sack according to claim 11 , characterized in that the moisture barrier layer (3) is attached to the inner surface of the outer layer. 14. A sack according to one of the claims 1 - 13, characterized in that there are channels (5) on the inner surface of the outer layer (2').

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