Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1998-03-18
2002-09-03
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S304400, C428S308400, C428S315900, C428S318400, C428S319900, C428S500000, C428S515000, C428S516000, C428S523000, C428S910000, C264S138000, C264S152000, C264S153000, C264S157000, C264S288400, C264S290200, C264S297400, C156S244110, C156S244180, C156S250000, C156S285000
Reexamination Certificate
active
06444301
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns labels made from polyolefin films, and a method of making such labels.
2. Description of Related Art
Polyolefin films are increasingly being used to produce labels, both as a substitute for paper labels and to exploit the inherent properties of polyolefin films such as their printability and their ability to be molded into and to adhere to the surfaces of containers as they are being formed. Since polyolefins used to form such films are usually inherently clear and colorless, films formed from them are often rendered opaque and/or colored white, thereby producing a label which is itself opaque.
A variety of techniques have been proposed for making polyolefin films opaque, one being the inclusion of a pigment, for example, titanium dioxide, and another being the formation of microvoids within the films which scatter light and thereby impart opacity to the films. The use of titanium dioxide has the advantage that it generally provides a high degree of opacity but this is accompanied by its additional cost and significant amounts are required in order to obtain the desired opacity. Microvoids have the advantage that they can be formed relatively easily in polypropylene, but the degree of opacity which can be achieved is often insufficient for labels. In addition, the amount of voiding agent which can be added to polypropylene, for example, is limited by the reduction in strength of the resulting voided film. Combinations of a pigment such as titanium dioxide and a voiding agent such as chalk in polypropylene films have therefore been used in an attempt to improve the opacity of labels made from polypropylene.
In certain end uses of labels made from polyolefin films, for example, in in-mold labelling, the film is pre-cut and is converted into individual labels which are then formed into a stack from which individual labels are removed as they are required. However, the problem with removing individual labels from a stack is that they are often difficult to separate from each other as a result of static electricity between adjacent labels, and this often increases as more labels are removed from the stack.
The build up of static charge is also a problem if polyolefin films are cut and fed at high speed from reels of films, for example, in wrap-around or cut-in-place labelling, or pre-formed labels on a release web, for example, in self-adhesive labelling.
The undesirable effects of static charge build up can be reduced, for example, by adding relatively large amounts of antistatic agents to the films, but doing so can also lead to other undesirable physical effects such as reducing print adhesion to the films.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide labels formed from polypropylene polymers that have advantageous properties. It is also an object of the invention to provide methods of making and using such labels.
In accordance with these and other objects, there has been provided a polymeric label formed from a film comprising a layer of propylene polymer resin having microvoids therein, the microvoids having been formed by stretching a web containing the beta-form of polypropylene.
In accordance with the present invention, there is also provided a method of making a label including:
(a) forming a film comprising the beta-form of polypropylene;
(b) stretching the film to form microvoids; and
(c) cutting the film into labels.
In accordance with the present invention, there is also provided a polymeric label formed from a film comprising an opaque layer of propylene polymer that does not include void-initiating particles or opacifying pigments.
Further objects, features, and advantages of the invention will become apparent from the detailed description of preferred embodiments that follows.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
According to the present invention, there is provided a label formed from a film comprising a layer of polypropylene-based resin having microvoids therein, the microvoids having been formed by stretching a web containing the beta-form of polypropylene.
“Polypropylene-based resin” or “polypropylene-based polymer” and “polypropylene” are used synonymously. These terms shall mean polymers containing at least 50% by weight, based on the weight of the polymer, of propylene units.
“Web” shall mean a sheet-like extrudate resulting from extruding the respective polymer melt or melts through a slot die and subsequent cooling of the melt to form the unoriented film.
“Base layer” shall mean either “the layer” in case of a monolayered film or the thickest layer, generally being the innermost, central layer of the multilayer structure.
“Beta form of polypropylene” shall mean that crystalline modification of polypropylene which has a lower melting point and a lower density than the common alpha form of propylene.
“Microvoids” shall mean the hollow vacuoles in the polymer matrix reducing the density of the oriented polypropylene film wherein the reduced density is lower than that of a corresponding film without any voids.
Labels formed from polypropylene films with microvoids therein and produced by stretching a web containing the beta-form of polypropylene have been found to destack readily without the addition of an antistatic agent. In order to reduce static cling still further, an antistatic agent can, if desired, be added to the films to reduce static cling between labels in a stack or static generated on unwinding reels of the films. But in general the amount of antistatic agent can be kept to levels typically used for packaging films. Any desired antistatic agent can optionally be used in the labels.
The beta-form of polypropylene is relatively unstable compared with the corresponding alpha-form under the conditions normally used to produce polypropylene films. Thus, when melts of polypropylene are extruded and then cooled to form a polymeric film, which may then subsequently be stretched, the alpha-form of polypropylene tends to predominate. However, it is known to produce films using polypropylene containing high concentrations of the beta-form of polypropylene by mixing polypropylene containing a high proportion of the alpha-form with a suitable nucleating agent which induces the formation of high concentrations of the beta-form when it is molten and subsequently cooled.
One example of such a process is described in U.S. Pat. No. 4,386,129 which is incorporated herein be reference, in which a variety of so-called beta-nucleators are dispersed in polypropylene following which films are produced therefrom by melting and subsequent cooling, the crystallinity of the resulting cast films being controlled by appropriate adjustment of the cooling conditions. Selective extraction of the beta-form of the polypropylenes from the films leaving a matrix of the alpha-form is then used to impart porosity to the films.
U.S. Pat. No. 5,231,126, which is incorporated herein by reference, describes the use of two component mixtures of beta-nucleating agents to produce microporous films by mono- or biaxial stretching cast polypropylene webs containing a high concentration of the beta-form of polypropylene resulting from the use of the mixture of nucleating agents. It is believed that the porosity results from voids induced by the change of the beta-form into the alpha-form during the stretching process, the alpha-form having a higher density than the beta-form from which it is derived. The development of porosity during the stretching process is accompanied by a significant reduction in apparent film density and the films become opaque with a high degree of whiteness.
More recently it has been proposed in EP 0632095, which is incorporated herein by reference, to use a variety of organic amides as beta-nucleating agents in the formation of mono- and biaxially stretched polypropylene films. A melt of a mixture of polypropylene and the beta-nucleating agent are cast into a film web which is allowed to crystalize at a temperature of 15 to 140° C. to form a soli
Biddiscombe Helen Ann
Davidson Paul Malcolm Mackenzie
Govier Rebecca Karen
Ott Marc Fritz Manfred
Chen Vivian
Hoechst Trespaphan GmbH
Powell Goldstein Frazer & Murphy LLP
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