Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2000-02-09
2003-03-04
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S319300, C428S515000, C428S516000, C264S045900, C264S173150, C264S173190
Reexamination Certificate
active
06528155
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to opaque polymeric films. More particularly, this invention relates to oriented opaque polymeric films prepared with a cavitating agent comprising a syndiotactic polystyrene polymer.
BACKGROUND OF THE INVENTION
Opaque polymeric films having a polyolefin core layer, e.g., of an oriented isotactic polypropylene (OPP), have been widely used in food and non-food packaging applications, because such films have desirable properties such as resistance to transmission of moisture, air, deleterious flavors, and the like, as well as outstanding mechanical properties.
During the production of these opaque polymeric films, cavitating agents may be used. In cases where polybutylene terephthalate (PBT) is used as the cavitating agent, extrusion plate out can be problem. In particular, PBT may degrade and build up in the film production equipment, forming deposits which further affect the flow patterns of molten polymer in the die. The use of PBT cavitating agents to prepare opaque polymeric films is described in U.S. Pat. No. 4,632,869 to Park et al.
As described in U.S. Pat. Nos. 5,866,246 and 5,861,208, particulate hollow bodies are prepared by dissolving polymer into a solvent and spraying the dissolved polymer into tiny particles. The remainder of the solvent is then removed by drying resulting in hollow particles of polymer. Cavitated film may be prepared by blending these particles with a polymer and extruding the blend, while maintaining the extrusion temperature below the melting or glass transition temperature of the hollow particles in order to retain the hollow shape of the particles. As a practical matter, however, the extrusion operation window is narrow and it is difficult to maintain the hollow particle shape, especially under the extrusion pressure encountered in commercial operation. In particular, uniform opacity is difficult to achieve in the processes described in U.S. Pat. Nos. 5,866,246 and 5,861,208.
SUMMARY OF THE INVENTION
There is provided an opaque polymeric film comprising:
(a) a base layer comprising a polymeric matrix and at least one cavitating agent; and
(b) at least one additional layer;
wherein said cavitating agent comprises solid, non-hollow particles of a syndiotactic polystyrene polymer.
There is also provided a method for producing an opaque polymeric film comprising:
(a) extruding a base layer comprising a polymeric matrix and at least one cavitating agent;
(b) coextruding at least one additional layer on at least one side of the base layer;
(c) cooling the coextruded multi-layer film; and then
(d) orienting the film in at least the machine direction (MD);
wherein said cavitating agent comprises a syndiotactic polystyrene polymer, and wherein said syndiotactic polystyrene polymer is in the form of solid, non-hollow particles during the orienting step (d).
Advantages of the present films include (1) reduced extrusion plate out during manufacture (2) uniform opacity, and (3) resistance to distortion caused by film crease.
DETAILED DESCRIPTION OF THE INVENTION
The base layer of the opaque polymeric film comprises a polymeric matrix, which may be selected from any of the polymers previously used in the art for such purpose. In many cases, such a polymer is a polyolefin having a melting point, for example, of at least about 150° C. and up to, for example, about 167° C. Preferably, the polyolefin of the base layer has a relatively high degree of crystallinity. A particularly desirable polyolefin as the base layer polymer is an isotactic polypropylene homopolymer having a crystallinity of, for example, about 89 to 99% (as measured by
13
C NMR spectroscopy using meso pentads), a melting point of about 155 to about 165° C., and a melt index of about 0.5 to about 15 g/10 minutes (as measured by the standard ASTM D1238 methods).
Other suitable polymeric matrix materials for the base layer include, but are not limited to, syndiotactic polypropylene, ethylene-propylene copolymers, ethylenepropylene-butylene terpolymers, butylene-ethylene copolymers, functionally grafted copolymers, blends of polymers, etc.
At least one cavitating agent in the form of a dispersed phase is provided in the base layer polymeric matrix material before extrusion and orientation of the film. Such dispersed phase comprises particles of a syndiotactic polystyrene polymer. During film orientation, these particles are solid throughout and are not hollow, as distinguished from the hollow particles described in U.S. Pat. Nos. 5,866,246 and 5,861,208. This dispersed phase may also, optionally, comprise at least one additional cavitating agent.
The syndiotactic polystyrene polymer used as a cavitating agent has a high degree of crystallinity. As a result of this high degree of crystallinity, these polystyrene polymers have melting points, as opposed to glass transition temperatures, which are characteristic of amorphous polymers. The melting point of the present syndiotactic polystyrene polymer may be, for example, from about 240° C. to about 280° C.
The degree of syndiotacity may be measured by NMR techniques well known in the art, such as those described in U.S. Pat. No. 5,502,133. The present syndiotactic polystyrene polymers may have, for example, at least 92% racemic pentad (i.e. r-pendat) as measured by NMR spectroscopy.
The degree of syndiotacity can also be measured indirectly as a function of its lack of solubility in various solvents. In particular, amorphous polystryene tends to dissolve in certain solvents, whereas crystalline, syndiotactic polystyrene tends to be insoluble in such solvents. The present syndiotactic polystyrene polymer may be tested by a Soxlet extraction procedure using methylethyl ketone (MEK), as described in U.S. Pat. No. 5,914,375. The MEK-insoluble portion of the syndiotactic polystyrene homopolymer or copolymer may be, for example, greater than 90 wt %.
The present syndiotactic polymer may be a homopolymer or a copolymer of one or more substituted or unsubstituted styrene monomers. These monomers and comonomers and the amount thereof may be selected to result in the formation of a crystalline polymer with a melting point of at least 240° C. Examples of substituted styrenes include para-methylstyrene, meta-methylstyrene, ethylstyrene, butylstyrene, dimethylstyrene, chlorostyrene, bromostyrene, fluorostyrene, methoxystyrene and acetoxy methylstyrene. A preferred substituted styrene is para-methylstyrene.
An example of a particular polystyrene copolymer is a copolymer of para-methylstyrene and unsubstituted styrene. This copolymer may have a para-methylstyrene content of from 1 to 50 wt %, e.g., 1 to 20 wt %, e.g., 1 to 10 wt %, the remainder being unsubstituted styrene.
The optional additional cavitating agent may be a material having a melting point that is higher than the melting point of the polymeric matrix material of the base layer. The optional additional cavitating agent may also be immiscible with polymeric matrix material of the base layer. The optional additional cavitating agent may be any of those described in U.S. Pat. Nos. 4,377,616 and 4,632,869, the entire disclosures of which are incorporated herein by reference. Thus, the optional additional cavitating agent may be selected from a polymer, such as, for example, a polyester (e.g., PBT or polybutylene terephthalate), nylon (e.g., nylon-6), an acrylic resin, or an ethylene norborene copolymer; or an inorganic material, such as, glass, calcium carbonate, metal, or ceramic, or mixtures thereof.
The particle size of cavitating agents in the dispersed phase may be, for example, about 0.1 micron to about 5 microns, more preferably about 0.2 micron to about 2 microns. The dispersed phase may be present in the base layer in an amount of up to about 20 weight percent, for example, from about 5 to about 20 weight percent, based on the entire weight of the base layer.
The cavitating agent may dispersed in the polymeric matrix by blending the cavitating agent and matrix material at a temperature above the melting points of both the matrix material and
Kong Dan-Cheng
Larter John A.
Mount, III Eldridge Milford
Bell Keith A.
ExxonMobil Oil Corporation
James Rick F.
Tarazano D. Lawrence
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