Polymeric films

Details Polymeric films Polymeric films

1998-01-16

2001-10-23

Zirker, Daniel (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C428S349000, C428S315700, C428S315900, C428S317900, C428S516000, C428S517000

Reexamination Certificate

active

06306490

ABSTRACT:

FIELD OF THE INVENTION
This invention concerns polymeric films and more particularly polymeric films having voided core layers.
BACKGROUND OF THE INVENTION
Polymeric films having voided core layers typically consist of a core layer of a voided propylene homopolymer with one or more non-voided layers on each surface of the core layer. Such films have found wide acceptance as packaging materials and as label stock, for example for self-adhesive labels and labels which are to be applied to articles by methods such as in-mold labelling.
In general, voiding of the core layer of such films makes them opaque, which in itself can be a desirable property for many end uses such as labelling, and it increases the yield of film from a given amount of propylene homopolymer since a significant proportion of the core layer consists of vacuoles which are formed during the voiding process. Significant reductions in film density can therefore be achieved compared with that calculated from the materials used to form the films.
As with other polymeric films, end uses for polymeric films having voided core layers often include printing of one of their outer surfaces. However, it has been found that doing so can result in curling of the film. Whilst this may not lead to problems with films which are to be used for packaging by so-called form-fill-sealing, it can lead to serious problems with films which are to be used for labelling. This can be especially so with films which are to be used for in-mold labelling where individual labels are held within a mold and against a surface of the mold whilst an article is formed within the mold and the label is thereby incorporated into the surface of the article. Curling leads to label fold and/or the label failing to be correctly incorporated into the articles, e.g. containers lids and other surfaces.
SUMMARY OF THE INVENTION
According to the present invention there are provided biaxially oriented polymeric films comprising a core layer of a voided propylene homopolymer having a density of not more than 0.70 g/cm
3
, and at least one substantially non-voided layer on each surface of the core layer, the ratio of the combined thicknesses of the non-voided layers on the respective surfaces of the core layer being from 2:1 to 1:1.
DETAILED DESCRIPTION OF THE INVENTION
Curling is a particular problem with polymeric films with voided cores having densities of less than 0.70 g/cm
3
, and especially so with relatively thick films such as are often used for labelling. However, by controlling the relative thicknesses of the non-voided layers on either side of the core layer, curling after printing can be brought within acceptable limits for end uses such as in mold labelling.
In general, as the density of the voided core layer of prior art voided films decreases as a result of increased voiding, the problem with curling tends to increase. However, by controlling the relative thicknesses of the layers on either side of the core layer in accordance with the present invention significant reductions in the tendency to curl can be achieved after the films have been printed. For example, films in accordance with the present invention can have cores with densities of less than 0.70 g/cm
3
, preferably less than 0.60 g/cm
3
, more particularly less than 0.55 g/cm
3
, and especially less than 0.50 g/cm
3
and even lower, for example less than 0.48 g/cm
3
.
Although the ratio of the combined thicknesses of non-voided layers on the respective surfaces of the voided core layer should be in the range of from 2:1 to 1:1, it is preferably from 1.8:1 to 1:1, more preferably from 1.6:1 to 1:1, especially from 1.5:1 to 1:1, and more especially from 1.2:1 to 1:1.
Films in accordance with the present invention can be of a variety of overall thicknesses. However, the benefits of the present invention tend to become particularly beneficial with films which are at least 30 &mgr;m thick, and more particularly with films which are at least 50 &mgr;m thick. Although films with a thickness of 50&mgr;m and more can be used as packaging materials, they are often used as labels, for example as self-adhesive labels when a suitable adhesive has been applied or as in-mold labels.
The thicknesses of the non-voided layers on the core layer can also be varied within wide limits, but it is generally preferred that the total thickness of the non-voided layer or layers on one side of the core layer is at least 3.0 &mgr;m. Thicker layers can be used, but as will be appreciated this generally leads to an increased film density due to the voided core layer representing a relatively smaller proportion of the film thickness. Reductions in core density can be effected to maintain overall film density, but this may not be possible if the voided core layers represents only a small proportion of the overall film thickness.
The voided core layer is formed from a propylene homopolymer, which for the purposes of the present invention include polymers containing small amounts of other alpha-olefins, for example up to 2 wt % of ethylene, which have had voids or vacuoles formed therein. Materials and methods which can be used to form the voided structure of the core layer of films of the present invention are well known in the polyolefin film art, suitable materials for the purpose including organic or inorganic particulates with a mean particle size in the range of from 1.0 to 10 &mgr;m. Particular examples of materials which can be used to effect voiding include polyamides, such as nylons, polyesters, e.g. polyethylene terephthalate and polybutylene terephthalate, and chalk.
The various non-voided layers on the voided core layer can in general be selected as desired for the particular end use of the films. They are preferably formed from polyolefins, including blends of polyolefins, and they can be formed from homopolymers, e.g. polyethylene or polypropylene, or copolymers, including terpolymers, containing units derived from two or more polyolefins. Examples of copolymers which can be used include those derived from a major amount of propylene and relatively minor amounts of ethylene and/or butene-1. If desired, at least one outer layer can be of a polar polymer, for example an ethylene/vinyl acetate copolymer, to improve the printability of the films.
Although a single non-voided layer can be present on each surface of the voided core layer, more than one layer can be used. More particularly, it is particularly preferred to use an intermediate layer of a relatively inexpensive polymer such as a propylene homopolymer beneath an outer layer of a relatively expensive copolymer which is used to impart heat seal properties and/or printability to the films, and this enables the total thickness of the non-voided layers on a particular side of the core layer to be increased to a value which serves to assist in reducing the curl of the films.
Although only the core layer of films of the present invention should be voided, the other layers of films of the present invention can include additives provided they don't lead to substantial voiding. For example, one or more of such layers can include a pigment, e.g. sub-micronic titanium dioxide, and they can, if desired, contain one or more additives proposed hitherto in the polyolefin film art, for example antiblock agents, antistatic agents, and slip agents. Examples of antiblock agents which can be used include silica and calcium carbonate. Slip agents which can be used can be migratory and non-migratory, for example fatty acid amides, e.g. erucamide, particulate polyamides, e.g. nylons, and particulate silicone elastomers. As will also be appreciated, the core layer can also contain pigments and/or other additives.
Films in accordance with the present invention can be produced by known methods. However, it is particularly preferred to coextrude melts of the respective polymers plus additives through a slot die and then after cooling to stretch the resulting polymer web sequentially, first in the direction of extrusion and then in the transverse direction.
Since the presen

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