Stock material or miscellaneous articles – Hollow or container type article – Flexible food casing
Reexamination Certificate
2002-12-12
2004-09-07
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Flexible food casing
C428S105000, C428S112000, C156S163000, C264S173120
Reexamination Certificate
active
06787206
ABSTRACT:
Webformed cross-laminates consisting of two or more mutually bonded films, which are unuaxially oriented or unbalanced biaxially oriented, have been commercially available since 1968, but have always presented problems in heatsealing.
These first cross-laminates have been produced with the compositions, structure and method described in GB-A-0,792,976 of Apr. 9, 1958 and GB-A-0,816,607 of Jul. 15, 1959. A tubular film of polyethylene (normally HDPE) or polypropylene has been strongly uniaxially oriented in its longitudinal direction by stretching at a relatively low temperature, then helically cut to a web with its orientation on bias, and two or more such webs have then been laminated, usually by extrusion lamination, with criss-crossing directions of orientation. When such cross-laminates are heatsealed, e.g. by impulse sealing, to form a “peel-type” heatseal, the impact peel strength of the seal has turned out to be exceptionally low (except if the seal is improved by additional material, e.g. by overtaping).
By “peel-type” heatseal is understood a heatseal adapted to withstand peeling forces concentrated in a lineary very narrow zone, as opposed to an “overlap type” heatseal made to withstand the shearing forces, which are distributed over the entire width of the seal.
Impact- or shock-peel strength is particularly needed in heatsealed bags to withstand the shock action when the bag is dropped on one of its edges perpendicular to the linearly extending heatseal.
The poor peeling strength of the seal is due to the fact that the heatsealing ruins the orientation, not only in the bonded part of the seal—where loss of orientation does not matter—but also in immediately adjacent narrow “lines” of unbonded cross-laminate. Upon shock-peeling these “lines” appear weak and tend to rupture. (At lower velocities of peeling, when the material in these lines has time to elongate and orient, the seals may give a satisfactory peel-strength).
The shock-peel strength can be improved, but not sufficiently, by coextruding thin lower melting surface layers (“heatseal layers”) on the films used for manufacture of the cross-laminate. However, when the heatsealing takes place at a temperature at which the orientation in the main layers is maintained, another phenomenon occurs, namely that the two cross-laminates separate from each other since the peeling force is concentrated in the thin bonding layers. To avoid such separation, the films in and immediately adjacent to the bonded zone must be allowed to shrink and thereby grow in thickness, by which the tensions created by the peeling will be reduced.
As described in GB-A-1,526,722 improvements in shock-peel-strength of heatseals (and certain other strength advantages) has been obtained by a different method of producing cross-laminates. In the usual way of practising this method (which has been commercial since 1978) there is first coextruded a tubular film with a generally uniaxial, longitudinal orientation, and with minor surface layers for facilitating the subsequent lamination process (“lamination layers”), and for improving the surface characteristics of the final cross-laminate, especially its heatsealing properties (“heatseal layers”). This tubular film is then helically cut to form a web with melt orientation on bias, and several such webs are continuously cross-“sandwiched” and further oriented by transverse stretching between grooved rollers and—before or after this operation—subjected to conventional longitudinal stretching between ordinary smooth rollers which rotate at different circumferential velocities. The grooved rollers stretching is usually repeated several times, and the longitudinal stretching may also be repeated. For obtaining maximum energy-absorbing properties, also with a view to the shock-peel strength of the heatseal, the longitudinal and transverse stretching steps after bringing the helically cut films together to a “sandwich” is normally carried out at temperatures very much below the melting ranges of the films, and may even be carried out at normal ambient temperature. U.S. Pat. No. 4,629,525 discloses a subsequent heat-stabilization process. I have also suggested arranging the films of such laminates such that one has a main direction of orientation generally following the longitudinal direction of the cross-laminate and the other has a main direction of orientation substantially perpendicular to the longitudinal direction.
There is normally achieved a significant bonding between the cross-“sandwiched” films, so that they will form a laminate, already by the stretching between grooved rollers, and this bonding is increased by the heat-stabilization process. However, a really strong overall bonding is always avoided since this will ruin the tear-propagation resistance, which is a particularly remarkable feature of these cross-laminates.
Improved evenness of the transverse stretching between grooved rollers, and possibilities for higher throughputs, heavier and stiffer cross-laminates is achieved by a method disclosed in EP-B-0,276,100, in which a set of grooved rollers fit extremely fine to each other and act under a high rollers pressure to perform a combined transverse pulling and transverse squeezing. (The example in this present specification makes use of this). The said method is further developed in EP-A-0,624,126.
Practical ways of carrying out the helical cutting are disclosed in EP-B-0,426,702. This patent also explains how a big angle of melt orientation, even 90° to the longitudinal direction of the film, can be achieved by first extruding a tubular film with a “screwed” melt orientation (relative rotation between the extrusion die and the haul-off means) and in a second process helically cutting this tube in a direction which increases the angle of orientation.
Returning to the problem of shock-peel-strength in a heatseal, cross-laminates made according to the method of the above mentioned GB-A-1,526,722 or the above mentioned improvements on this, have since 1978 been used in the manufacture of open-mouth industrial bags sealed in top and/or bottom by a “peel-type” heatseal, and are still used so on a relatively large scale. This includes the use in “form-fill-and-seal” in which the conversion of film to bag takes place in conjunction with the filling operation. However, with the existing need for downgauging the film material for bag making, further improvements are strongly needed. It is hereby important to achieve the improvement in heatseal properties without reducing other important strength properties, in particular the yield tension. The latter is important in connection with stacking of the industrial bags filled with powdered or granular material, since the lower layers of bags in a stack often come under a very high static load.
According to the invention claimed in EP-B-0,338,747 which is developed with a special view to industrial bags made from such cross-laminates, the heatseal is protected by an adjacent row of embossments, which is adapted to act shock absorbing and/or force controlling.
Furthermore according to the invention described in WO-A-98/23434 there is provided a heatseal method—also with a special view to the cross-laminates—which allows a particular high shrinkage of the polymer film, not only in the bonded zone of the heatseal but also in the immediately adjacent, unbonded zones of the film, at the side of the seal which is or is predetermined to come in contact with the contents of the bag. By this high shrinkage the film grows thick in-the said zones, and this compensates for the loss of orientation in these zones. (As mentioned at the beginning of this description the problems in heatsealing of cross-laminates are caused by the loss of orientation in the unbonded zones adjacent to the bonded zone). The invention involves a pivoting of one of the sealing bars.
However, in spite of the proven positive results of these two inventions, the one to protect the seal by means of embossments and the other concerning the special heatseal method, neither of these has met accept
Daniel William J.
Tarazano D. Lawrence
LandOfFree
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