Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Layered – stratified traversely of length – or multiphase...
Reexamination Certificate
1996-11-27
2001-02-13
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Layered, stratified traversely of length, or multiphase...
C264S173140, C264S173190, C264S177190
Reexamination Certificate
active
06187236
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multilayer film particularly adapted for the packaging of palletized units and to the method of preparation thereof.
In a prior Italian patent application, multilayer films have been disclosed, having high tenacity and good stretchability, obtained from linear low density polyethylene (LLDPE) blended with low density polyethylene (LDPE).
The preparation method disclosed in the prior application comprises the following steps:
extrusion, from a battery of extruders, of LLDPE polyethylene blended with LDPE and at least a polymeric material having adhesion properties;
stratification of the material at the exit of the extruders in function of the extruder it comes from;
exit in atmosphere of the stratified material through the lip of a spreading head and deposition of it on a first cooling roller with high speed of rotation, cooled with water maintained between 10° and 30° C.;
feeding the sheet exiting from the first roller to a second roller cooled with water at a temperature lower than that of the first roller and maintained between 100 and 20° C., the peripheral velocity of the second roller being 10-40% higher than the velocity of the first roller.
The slit of the spreading head, the distance between the lip of the head and the first roller and the lip and the point of contact on the first roller are comprised within well defined ranges.
The film thus obtained presents values of the yield point between 15 and 35 N/mm
2
and ultimate elongation between 200 and 500%.
SUMMARY OF THE INVENTION
It has now been unexpectedly found that it is possible to obtain multilayer films having characteristics of rigidity and stretchability comprised within those of the films disclosed in the prior Italian application and films also having new characteristics with respect to those of the film of the prior Italian application (more rigid but still stretchable) starting from LLDPE polyethylene without addition of LDPE polyethylene.
It has been furthermore found that by suitably regulating the cooling and stretching conditions of the film it is possible to obtain highly rigid but still stretchable films also starting from LLDPE mixed with a minor proportion of LDPE (in quantity from 8 to 40% by weight).
The process of preparation of the film is similar to that disclosed in the prior Italian application.
Some differences, however, are introduced for the preparation of very rigid films.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preparation of the films is carried out by the well known cast-extrusion technique. This technique uses a battery of extruders (two or more), generally monoscrew extruders with screw of variable span to obtain good material homogenization, which extruders feed an apparatus (feed block) that, by means of a mechanical action, stratifies the material coming from the extruders in function of the extruder it comes from.
The sheet coming from the feed block is formed of several layers of which the central layer comes from the extruder of greater diameter and the side layers come from the other extruders.
The bar is then fed to a spreading head, wherefrom, by effect of a mechanical action, it is compelled to exit in the atmosphere through a slit of small opening thus forming a plastic film wherein the breadth prevails on the thickness.
The plastic film is deposited on a first cooled roller, rotating at high velocity, wherein the material is oriented. The film coming from this first roller feeds a second roller also cooled with water.
For the same throughput of the extruder and other conditions, the thickness of the film depends on the peripheral velocity of the first roller: actually, higher the velocity of this roller, lower the thickness of the film.
The film coming from the group of the cooling rollers is then collected on winding reels of adapted breadth and length.
The movement of the film on the winding reel is rendered uniform by constant tensioning of the film realized by a series of motorized rollers each of which presents positive relative rotation.
According to the process of the invention, LLDPE polyethylene and at least a polymeric material compatible with LLDPE and giving the film adhesion properties and, optionally other materials compatible with LLDPE, are extruded from a battery of extruders. The material coming out from the extruders is stratified in function of the extruder it comes from and the stratified sheet is forced to exit through the lip of a spreading head to form a plastic film which is deposited on a first roller rotating at high peripheral velocity (from 300 to 450 m/mn), and cooled with water maintained at a temperature from 10° to 25° C., and then on a second roller, cooled with water maintained at a temperature lower than that of the first roller and comprised between 10° and 20° C.
The second roller has a peripheral velocity 10-40% greater than the velocity of the first roller.
The temperature of the material coming from the spreading head is generally between 230° and 260° C.
The slit ranges between 0.3 and 0.5 mm; the distance between the lip of the spreading head and the first roller is between 2 and 5 mm, preferably 3-5 mm, and the distance between the lip and the contact point on the first roller is between 10 and 15 cm.
The slit is regulated in function of the final thickness of the film and the stretch to which the same may be subjected.
The distance between the lip of the spreading head and the point of contact of the material on the first roller is regulated both by approaching (distancing) the roller to the lip at the exit of the spreading head and by a pressure drop realized with an air aspiration device operating nearby the point of contact of the film on the cooling roller. The obtained pressure drop allows a good adherence of the film on the roller.
By changing the aspiration, the profile with which the material runs the distance from the lip of the spreading head to the cooling roller is changed, and simultaneously, the point of contact.
Greater the distance between the lip and the point of contact, higher the elastic performance of the material.
The operation is such as to avoid sliding of the film between the first and the second rollers.
This result is obtained for example, by subjecting the film to a counterpressure.
For the preparation of the film having a high yield point (between 40 and 80 N/mm
2
) and a still high ultimate elongation (between 50 and 100%) one operates at a temperature of the cooling water of the first roller maintained in the range between 15° and 18° C., while the temperature of the cooling water of the second roller is maintained between 12° and 15° C. The peripheral velocity of the second roller is 20-30% higher than that of the first rollers.
The above-mentioned conditions are applied also to the preparation of the multilayer film having a high yield point (between 35 and 70 N/mm
2
) and ultimate elongation between 60 and 200%, obtained from LLDPE polyethylene mixed with LDPE in quantities between 8 and 40% by weight.
With respect to the film obtained from LLDPE alone, the one obtained from LLDPE mixed with LDPE requires, the other conditions and yield point being equal, a greater difference between the velocities of the cooling rollers.
The thickness of the multilayer film according to the invention is between 2 and 100 micrometers; the preferred thickness ranges between 8 and 20 micrometers.
The LLDPE polyethylene used for the preparation of the film is a well known polymer. It is preparable by using either Ziegler-Natta catalysts or metallocene catalysts.
The LLDPE polyethylene can also be mixed with other polymeric materials in order to improve its processability. Said materials can comprise, for example, partially crystalline copolymers of propylene with minor proportions of butene and/or ethylene.
The LLDPE density is preferably comprised between 0.915 and 0.935 g/cm
3
(ASTM D 1505).
The melt index is preferably comprised between 2 and 3 g/10 (ASTM D 238-65T, E condition).
The LDPE is also a well known polymer. In order to confer the
Eashoo Mark
Josif Albert
Modiano Guido
O'Byrne Daniel
Pentacast S.r.l.
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