Void-containing polyester shrink film

Abstract

Disclosed are polyester shrink films comprising a voiding agent dispersed within a continuous polyester phase. The voiding agent comprises at least one first polymer and at least one second polymer, in which the polymer components have selected physical properties such as glass transition temperature, melting point, tensile modulus, surface tension, and melt viscosity. The resulting shrink films have high opacity, a low coefficient of friction, lower density, low shrink force, and good printability. The films are useful for sleeve label and other shrink film applications, and their lower density allows them to be readily separated from soft drink bottles, food containers and the like during recycling operations. Also disclosed is a process for separating a void-containing polyester from a mixture of polymers.

Description

CROSS REFERENCES TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application Ser. No. 60 / 526,305, filed Dec. 2, 2003.FIELD OF THE INVENTION [0002] This invention pertains to void-containing, polyester shrink film. More specifically, this invention pertains to void-containing shrink films comprising an oriented, continuous polyester phase having dispersed therein a voiding agent comprising at least one first polymer and at least one second polymer, in which the polymer components have selected physical properties such as glass transition temperature, tensile modulus, melting point, surface tension, and melt viscosity. The invention further pertains to a process for a void-containing shrink film and to a process for separating a void-containing polyester from a mixture of different polymers. BACKGROUND OF THE INVENTION [0003] Shaped articles such as, for example, sheet, film, tubes, bottles, sleeves, and labels, are commonly used in various packa...

AI Technical Summary

Benefits of technology

[0012] The film of our invention includes a voiding agent dispersed therein that comprises at least one first polymer and at least one second polymer that are selected on the basis of certain physical properties and the relationship of some of these properties to corresponding properties of the polymer matrix. These parameters include the glass transition temperature (abbreviated herein as “Tg”) or their melting point temperature (abbreviated herein as “Tm”), surface tension, tensile modulus, and melt viscosity. This combination of a first and second polymer provides a voiding agent with superior performance over either polymer component alone and, in particular, gives a shrink film with a higher opacity, reduces cost, improves processability, and reduces shrink forces. Typical polymers that may be used as the first polymer include, but are not limited to, cellulosic polymers, starch, esterified starch, polyketones, polyester, polyamides, polysulfones, polyimides, polycarbonates, olefinic polymers, and copolymers thereof. Similarly, second polymer may comprise one or more polymers selected from polyamides, polyketones, polysulfones, polyesters, polycarbonates, olefinic polymers, and copolymers thereof. In another aspect of the invention, the first polymer comprises one or more of cellulose acetate or cellulose acetate propionate and the second polymer comprises one or more of: polystyrene, polypropylene, or ethylene methyl methacrylate copolymer.

[0013] Our invention also includes a process for a void-containing shrink film, comprising: (i) mixing a polyester and a voiding agent at a temperature at or above the Tg of the polyester to form a uniform dispersion of the voiding agent within the polyester, wherein the voiding agent comprises at least one first polymer and at least one second polymer, wherein the first polymer has a Tg or a Tm greater than the Tg of the polyester, a tensile modulus of at least 1 GPa, and a surface tension that differs from the surface tension of the polymer matrix by an absolute value of 5 dynes / cm or less; and the second polymer has a surface tension that differs from the surface tension of the polyester by an absolute value of at least 5 dynes / cm and a melt viscosity wherein the ratio of melt viscosity of the second polymer to the melt viscosity of the polyester is about 0.1 to about 3.5; (ii) forming a sheet or film; and (iii) orienting the sheet or film of step (ii) in one or more directions. The voided shrink films of our invention may be stretched in one or more directions and may comprise a one or more layers. The films have excellent opacity, good film stiffness, a low shrink force, improved seamability, high surface tension (for printing), and high shrinkage.

[0014] Our void-containing, polyester shrink film may be readily separated from mixtures of polymers and, thus, may be easily recovered and recycled from commercial waste. Thus, another aspect of the instant invention is process for separating a void-containing polyester from a mixture of different polymers, comprising:

Problems solved by technology

For example, during recycling of the plastic material, labels are often separated from the rest of the container because of the presence of inks, glues, and other substances which can contaminant and discolor the recycled polymer.

Unfortunately, the label and container materials used in packaging often have similar densities that prevents the use of such flotation processes.

Although voided films are known, they frequently suffer from a number of shortcomings and often show inferior properties to the corresponding non-voided counterparts such as, for example, poor stiffness, insufficient opacity, high shrink force, and high surface roughness which make them less desirable for many packaging applications.

Increasing the number of voids, however, can increase the surface roughness of the film to the point that the printing quality, texture and feel, and seamability of the label are reduced.

While this approach solves many of the above problems, production of such multilayer films is expensive and requires additional coextrusion or lamination equipment.

Multilayered films also typically have a higher overall film density because of the lack of or decreased voiding on the surface and are not as desirable as monolayer films.

Inorganic agents like calcium carbonate, talc, silica, and the like may be used as voiding agents but, because inorganic substances are typically dense materials, the final density of the shaped article is often too high.

Polyolefins, however, often do not disperse well and may require a compatibilizer such as, for example, a carboxylated polyethylene to obtain a uniform distribution of voids.

When used with polyester polymers to produce voided films, polyolefins also tend to lower the polyester film surface tension and thereby reduce the printability of the film.

Polyolefins are softer than the polyester at room temperature which sometimes lowers the overall film modulus to unacceptable levels.

Finally, polyolefins are relatively inefficient voiding agents and large amounts are required to achieve the necessary density reduction.

As discussed earlier, this leads to poor surface roughness and printing problems, thus making it difficult to use in single layer films.

Other polymeric voiding agents such as, for example, styrenics, polymethyl-pentene, polycarbonate, nylons, cellulosics, and the like, suffer from some of the same voiding efficiency problems as polyolefins.

High modulus styrenics, like atactic polystyrene, are efficient voiding agents, but suffer from outgassing problems when mixed and processed at higher temperatures and, therefore, are useful only at low levels.

Styrenics also tend to embrittle the film.

Crosslinked styrene beads may be used to circumvent this problem, although these beads tend to be expensive.

Cellulosics tend to be hygroscopic and require a separate drying and moisture removal step before incorporation into the polymer matrix.

For voided shrink films, cellulosics also tend to produce undesirably high shrink forces.