Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2000-01-21
2003-02-18
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S516000, C053S441000, C264S173140, C264S173150, C264S173190
Reexamination Certificate
active
06521338
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to stretch wrap film, and its use in the palletisation of loads. In particular, the invention relates to blown stretch wrap film which generates low noise in use.
BACKGROUND OF THE INVENTION
When using a stretch film to wrap goods, either using a hand applicator or a semi- or totally automated machine, considerable noise may be generated as the stretch film is unwound from its supply roll. In some cases, the noise generated may be in excess of that which can be safely tolerated for long term exposure by the human ear. This is particularly disadvantageous when the film is to be unwound by hand or using a semi-automated machine, because of the presence of personnel close to the supply roll.
There have been various proposals to overcome this problem. One such proposal has been to employ a manufacturing method known as chill roll casting. This method involves extrusion of a polymer melt on to water-cooled rollers, thereby achieving rapid cooling of the resultant film. However, it has not proved possible to adapt this technology to the manufacture of the traditionally tougher, stiffer and more crystalline blown films.
Another proposal to overcome the problem of excess unwind noise is disclosed in the Applicant's co-pending patent application WO-A-9629203, in the form of a one-sided cling film having, on one side of the film, a cling layer comprising ultra low density polyethylene (ULDPE), and, on the other side of the film, a non-cling layer comprising low density polyethylene (LDPE) and a solid particulate antiblock additive. The noise levels reported for films of this type range from 85 dB to 96 dB, and therefore except for those films generating a noise level on unwinding at the lower end of this range, it remains necessary, for hearing safety, to limit the period for which an operator is exposed to unwinding of the film.
Furthermore, a problem associated with the use of antiblock additives is that these may be present at the surface of the film during film manufacture and usage, causing undesirable build-up on equipment, necessitating stoppage for cleaning, and diluting the desired effects of those additives in the final film. Some antiblock additives also weaken the film, leading to premature film fracture, and may also detract from film clarity.
WO-A-9515851 discloses blown and cast one-sided cling stretch wrap films which do not require the use of additives or functional polymers to improve their cling properties. These films comprise a cling layer of an homogeneously-branched ethylene polymer having a density of up to about 0.90 g/cm
3
, and a non-cling polyolefin layer having a density of greater than about 0.90 g/cm
3
. However, because WO-A-951581 is not concerned with the production of blown stretch wrap films which, on unwinding from a supply roll, generate comparatively low noise, it does not disclose or give any indication of the noise level generated on unwinding its films.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a multilayer blown stretch film comprises a first layer having cling properties and comprising a copolymer of ethylene and a C
4-8
&agr;-olefin, the copolymer having a density in the range 0.850 to 0.890 g/cm
3
, and a second layer having substantially no cling properties and comprising at least 70 wt. % of a low density polyethylene having a density in the range 0.915 to 0.940 g/cm
3
.
The noise level generated during unwinding of this film is a maximum of 85 dB, and is typically significantly less than this, for instance around 82 dB or less. Accordingly the period for which an operator may be exposed to unwinding of the film is not limited by hearing safety.
According to a second aspect of the present invention, a method of making a multilayer blown stretch film comprises co-extruding a copolymer of ethylene and a C
4-8
&agr;-olefin, the copolymer having a density in the range 0.850 to 0.890 g/cm
3
, with a polymer composition comprising at least 70 wt. % of a low density polyethylene having a density in the range 0.915 to 0.940 g/cm
3
, to form a tube having the ethylene copolymer in one layer and the polymer composition in another layer, and blowing and then cooling the tube, by blown film co-extrusion techniques known to those skilled in the art.
According to a third aspect of the present invention, a film as described above is used in the palletisation of loads.
DESCRIPTION OF THE INVENTION
The multilayer film of the present invention is a stretch film, by which we mean it may be stretched by more than 100%, and typically by more than 200%, of its original, unstretched, length, without damage thereto or loss of one-sided cling properties.
The film may be an A/B film, in which layer A is the cling layer and layer B the non-cling layer. Preferably, the film may include one or more layers in addition, and positioned between, the cling and non-cling layers, to add bulk or to confer other properties on the film. In this case, the properties of the intermediate, or core, layer or layers are typically selected according to the application for which the film is ultimately intended.
The cling layer of the film comprises a copolymer of ethylene and a C
4-8
&agr;-olefin, and has a density in the range 0.850 to 0.890 g/cm
3
, preferably in the range 0.860 to 0.885 g/cm
3
, and more preferably in the range 0.865 to 0.875 g/cm
3
. Suitable ethylene copolymers are very low density polyethylenes (VLDPE), ultra low density polyethylenes (ULDPE), or linear low density polyethylenes (LLDPE), referred to in the industry as “polyolefin plastomers”, abbreviated to “POPs”. Such materials are amorphous in nature. Commercial examples include TAFMER (Mitsui Chemical), AFFINITY and ENGAGE (Dow Chemical), DEX PLASTOMERS (DSM/Exxon JV) and EXACT (Exxon Chemical). Homogeneously-branched copolymers are preferred, and in particular homogeneously-branched POPs.
The term “homogeneously-branched” in the context of ethylene polymers is well understood in the field, and is defined, for instance, in WO-A-9515851. In summary, it is used to define copolymers of ethylene and an &agr;-olefin, for instance butene, hexene or octene, in which the &agr;-olefin monomer is randomly distributed within a given molecule; substantially all the copolymer molecules have the same ethylene: &agr;-olefin monomer ratio; the copolymer has a narrow short chain (i.e. less than 6 carbon atoms) branching distribution; and the copolymer essentially lacks a measurable high density fraction.
The ethylene copolymer for use in the invention may be manufactured by any of the known processes. For instance, the preferred homogeneously-branched ethylene copolymer may be manufactured as described in WO-A-9515851, or the various patents referred to therein.
Preferably, the ethylene copolymer has a melt index in the range 0.5 to 3.0 g/10 min, preferably 0.5 to 1.5 g/10 min, as measured by ASTM norm D1238 (190° C./2.16 kg), for ease of processing.
The polymer composition for use as the non-cling layer comprises at least 70 wt. %, based on the weight of the non-cling layer, of a conventional long chain (i.e. greater than 6 carbon atoms) branched low density polyethylene (LDPE), including ethylene homopolymers and copolymers. Preferably, the non-cling layer comprises at least 80 wt. % of LDPE, more preferably at least 90 wt. %, e.g. 95 wt. % or more, and most preferably the non-cling layer consists essentially entirely of LDPE. By “consists essentially entirely of” we mean that the non-cling layer may contain materials other than LDPE, for instance additives or other ethylene or &agr;-olefin polymers, but in insignificant amounts so as not to have any significant deleterious effect on the absence of cling from the non-cling side of the film, or the low noise level generated on unwinding of the film. Best results are obtained with a film having a 100 wt. % LDPE non-cling layer.
The density of the LDPE is in the range 0.915 to 0.940 g/cm
3
, and is preferably in the range 0.920 to 0.932 g/cm
3
, and is more preferably in the rang
Lowe Hauptman & Gilman & Berner LLP
Mima Films S.C.A.
Tarazano D. Lawrence
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