Stock material or miscellaneous articles – Structurally defined web or sheet – Nonplanar uniform thickness material
Reexamination Certificate
1999-05-21
2002-02-05
Garbe, Stephen P. (Department: 3727)
Stock material or miscellaneous articles
Structurally defined web or sheet
Nonplanar uniform thickness material
C156S312000, C156S583400, C206S521000, C383S107000
Reexamination Certificate
active
06344258
ABSTRACT:
The invention concerns methods of supplying a sandwich of polymer films with a linear heat-seal, the apparatus for carrying out and the products resulting from such methods.
A first aspect of the invention concerns the achievement of a high shock-peal-strength, which in particular is needed when the seal is the bottom or top seal of so called heavy duty or industrial bag, which must be constructed to resist the impact of accidental falls which may occur during transport or storage of the filled bag. In industrialised countries it is normal to request that the filled bag must be able to pass drop cycles on each of its faces from at least 2 m height, while in developing countries, where the treatment of the bags often is much rougher, the requirements are often for 4 m drop height.
A distinction is drawn between “flat” drops, i.e. drops on one of the two major faces, “edge drops” i.e. drops on one of the two minor faces perpendicular to top and bottom, “top drops” and “bottom drops”, i.e. drops on to the top and bottom, respectively.
When a filled, heat-sealed bag falls on top or bottom, the shock-action on top or bottom seals is negligible.
“Edge drops” causes a straight peal action on both top and bottom seals.
The panel action by “flat drops” is almost negligible if the bag is a simple “pillow bag” without gussets. However the trend in industrial packaging n bags has for many years been towards use of gusseted bags, now more and more practised by “form-fill-and-seal”, which process starts with a gusseted tube from reel and in one machine line in sequence makes the bottom seal, cuts the tube into lengths to make open-mouth bags, fills the bag and makes the top seal.
When a gusseted, filled, heat-sealed industrial bag falls “flat” and without special precautions being taken by the sealing, there occurs a strong bias type of pealing, which also can be described as tearing, in the four spots where the inner-folds of the gussets interact with the heat-seals. This is because the contents of the bag spread cut horizontally by the impact, thereby tearing in the gussets with forces concentrated around the gussets' inner-folds.
This means that the locations in the top and bottom seals where these seals intersect with the gusset inner folds are subjected to particularly strong shock-peel or shock-tear forces. The situation is aggravated by the fact that in these locations the seals are relatively weak due to the change from “2-ply” to “4-ply” bag material.
The conventional way of counter acting this is by two so called “K-seals” in each corner, seals angled at about 45° to the bottom and top, starting in the mentioned spots of intersection and bonding each outer-ply to the adjacent ply in the gusset but without bonding gusset-ply to gusset-ply.
Reverting to the straight shock-peeling occurring during “edge-drops” this is most critical in the “2-ply” parts of the seal where these border on “3-ply” or “4-ply” parts, i.e. the longitudinal seam (if such a seam is present) and at the gussets.
When bags accidentally fall, the velocity by which pealing (tearing) takes place will often exceed 5 ms
−1
. The standardised laboratory tests for heat-seal-strength are carried out at much lower velocities, and I have found them being without any value for evaluation of the practical performance when a bag falls. For this purpose these tests often give directly misguiding results when different polymer materials or different types of seals are compared. For my comparisons I apply a simplified shock-peel test and a simplified shock-tear test at velocity about 5,5 ms
−1
. This test is further explained in the example herein.
When rationally making the (generally linear) top seals and bottom seals in a bag there is always aimed at improved shock-peel strength by promoting swelling through contraction in its plane of the material in the bonded zone and in the immediate adjacent zones of unbonded film material. It is clear that this is needed only on the side of the seal which is predetermined for high shock-peel-strength, i.e. the side adjacent to the contents of the bag. This is conventionally achieved by tapering the edges of the sealing bands or in a similar way making a smooth change between bonded and unbonded zones of the film-sandwich. More precisely, the positive effect of smoothing is that a boundary zone of the heat-seal, which is not bonded, participates in the swelling through contraction in a perpendicular direction to the linear seal. (In my terminology I consider all which has been molten as “seal”, not limiting this term to the bonded part of the film-sandwich).
However, with the need for downgauging the film materials for bag making, which is a result of ecological and power saving considerations, there has come and will further come a need for much more efficient increase of shock-heat-seal-peel strength. The first industrial realisation of such downgauging has been based on use of stiffer polymer compositions and of higher degrees of malt orientation, in particular highly melt oriented, coextruded films combining HDPE and LLDPE. A later step in such developments, now being introduced by the industry, combines a similar coextrusion technique with cross-lamination (lamination with main directions of orientation criss-crossing) and subsequent biaxial stretching. A survey over inventions involved in this technology is made in the introduction of WO93/14938.
It is clear that the downgauging in itself means reduced shock-peel strength. In addition this strength strongly depends on the stiffness of the material and as mentioned the downgauging requires the use of increased stiffness which reduces the shock-peel strength even more. The higher the stiffness, the lower this strength. A reason for this is that the shock-peel strength depends on the capability in the material to deform elastically and to deform permanently in the “peel line” instead of rupturing, and to undergo such permanent deformation at a sufficient rate. (if the seal falls by shock-peeling, this is normally due to a rupture and not to “delamination”). Furthermore, the stiffer the film material the lower its capability to take up some of the energy of the shock by elastic elongations in the surroundings of the seal.
Further problems are connected with the orientation, which is an important factor in the downgauging.
FIG. 1
illustrates this. The orientation is lost in the seal including its unbonded boundaries. In the bonded part this does not matter, because the thickness has become doubled, but in the unbonded parts the elimination of orientation reduces the shock-strength. (It does not necessarily reduce the strength at lower velocities of peeling, when the material has time to elongate the orient).
A major limiting factor in the downgauging is “flimsiness” of the film which makes bag production or the handling of an unfilled bag difficult. In the above mentioned WO93/1428 I disclose how I strongly improve on this by a special cold stretching method, which produces a waved cross-section with thickened top-portions. In the present set of drawings I show this as a microphoto, FIG.
4
. (It concerns the film material actually used in example 1). From this it is immediately understood that this structure, which is needed for a strong downgauging, due to the thickness variations also necessitates an improvement of the structure of the seal. (The thickness differences have no significant influence on the general strength properties of the film, since the thinner portions are stretched more strongly).
In experiments preceding the present invention I have tried to improve on the shock-seal-peel strength by use of flat sealing surfaces placed under an angle of 5-15 degrees to each other with the angle opening towards the side where the peal strength is wanted so as to promote the swelling in this side. This gave very improved results when sealing a film-sandwich of a relatively even thickness, but was insufficient or directly harming in the case of significant thickness variations, e.g. at the change around the inne
Daniel William J.
Garbe Stephen P.
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