Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Patent
1997-10-24
1999-10-26
Zirker, Daniel
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
4283159, 4283173, 4283177, 4283179, 428349, 428515, 428517, B32B 2700, B32B 712
Patent
active
059724900
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns polymeric films and more particularly polypropylene films having good gloss.
2. Description of Related Art
It has been proposed hitherto in EP0004633-A to produce opaque polypropylene films, the opacity resulting from microvoids within a core layer of a three layer film structure. The outer heat sealable layers of these films impart a degree of gloss to the films. However, higher degrees of opacity combined with high gloss values have been sought in the art.
One proposal for increasing the opacity of polypropylene films has been to use titanium dioxide as a filler. GB2195947-A, for example, describes biaxially oriented polypropylene films having a base layer of a voided propylene homopolymer with an outer heat sealable polymeric layer containing titanium dioxide thereon. The titanium dioxide in the outer layer imparts improved opacity to the films compared with analogous voided films such as are described in EP 0004633-A, but the surface gloss of the films is reduced.
It has been proposed in EP 0517109-A to use a layer of a propylene homopolymer containing finely divided titanium dioxide between a propylene homopolymer core layer and a polymeric outer layer containing no filler, and such films are said to have high gloss.
SUMMARY OF THE INVENTION
According to the present invention there are provided biaxially oriented polyolefin films comprising a core layer of a propylene polymer, an intermediate layer of a non-voided, substantially non-pigmented propylene polymer on the core layer, and an outer skin layer of a polyolefin including titanium dioxide as a pigment.
It has surprisingly been found that by including titanium dioxide in the outer skin layer, films with particularly high gloss can be achieved compared, for example, with films having titanium dioxide in the intermediate layer and no titanium dioxide in the outer skin layer. This is also particularly surprising since films analogous to films of the present invention but without the intermediate layer between the core layer and a titanium dioxide containing skin layer show low gloss. The low gloss of these latter films is not surprising since the use of titanium dioxide in the outer surface of polyolefin films has been proposed hitherto for making synthetic papers with writable surfaces, for example J49012815 and J61228053.
The core layer of films of the present invention is of a propylene polymer, and it is preferably of a propylene homopolymer or a copolymer of propylene with low amounts, e.g. up to 2 wt %, of ethylene. The core layer can be voided or non-voided, and known organic or inorganic voiding agents can be used to effect voiding using known methods, for example using polymeric particles e.g. polyesters such as polyethylene terephthalate or polybutylene terephthalate, or polyamides such as nylons, or inorganic particulates such as chalk. The voiding agent should be of a size such that it can initiate voids when the films of the present invention are biaxially oriented.
In addition to or alternatively to a voiding agent, the core layers of films of the present invention can include one or more pigments, a particularly preferred pigment being titanium dioxide.
The core layer will usually include one or more additives which impart antistatic and slip properties to the films, examples of materials for imparting such properties being known in the art.
The intermediate layer is of a non-voided, substantially non-pigmented propylene polymer. The preferred propylene polymers are propylene homopolymers or copolymers of propylene with low amounts, e.g. up to 2 wt %, of ethylene. It is also preferred that the propylene polymers have an isotactic content of at least 90%.
The outer skin layer is of a polyolefin, and preferably a propylene polymer. However, it is generally preferred that the outer skin layer should be heat sealable, for example to itself, and it is therefore preferred to use copolymers derived from two or more a-olefins including ethylene. For example, the
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Biddiscombe Helen A
Crighton Allan J
Hoechst Trespaphan GmbH
Zirker Daniel
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