Transparent, coated, shrinkable, oriented polypropylene film

Abstract

A transparent, coated, heat-shrinkable film having controlled shrinkability which includes a biaxially oriented substrate having a polypropylene blend core layer and a water based coating. The shrink during the application on packaging machines is achieved through the balanced MD-TD (machine direction and transverse direction) film retraction in and near the sealed areas. The substrate can also include outer layers of a propylene-ethylene-butylene terpolymer disposed on either side of the polypropylene core layer or on either side of additional intermediate layers that may be disposed between the outer layers and the core layer. The invention also includes a process for making this coated heat-shrinkable film. The coated film is formed under controlled temperature and tension conditions to produce a packaging film having shrinkage within a controlled shrinkability range.

Description

BACKGROUND OF INVENTION[0001] The present invention relates to the field of polymer films and, more particularly to a coated heat-shrinkable biaxially oriented polypropylene film.[0002] A shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. The resulting shrinkage of the film results in an aesthetically pleasing, transparent wrapping which conforms to the contour of the product while providing the usual functions required of packaging materials such as protection of the product from loss of components, pilferage, or damage due to handling and shipment. Typical items wrapped in polyolefin shrink films are compact discs, tapes, perfumes, tobacco products, chocolates, toys, games, sporting goods, stationery, greeting cards, household products, office sup...

AI Technical Summary

Benefits of technology

[0013] The substrate is formed as a core layer disposed between two outer layers. The core layer is a blend of polypropylene homopolymer and a copolymer of propylene and ethylene and the outer layers include a propylene-ethylene-butylene terpolymer. The coating is a water-based coating which includes an acrylate, polyvinylidene chloride, PVOH, EVOH, clay filler or combinations thereof. Prior to coating, the substrate may be corona treated and / or a primer may be applied. The drying step may include controlling air flow, temperature profile and tension of the film to simultaneously prevent both shrinkage and wrinkling of the substrate.

[0048] Before applying the coating composition to the appropriate substrate, the surface of the substrate film is treated to ensure that the coating will be strongly adherent to the film thereby eliminating the possibility of the coating peeling or being stripped from the film. This treatment may be accomplished by employing known prior art techniques such as, for example, film chlorination, i.e. exposure of the film to gaseous chlorine, treatment with oxidizing agents such as chromic acid, plasma, hot air or steam treatment, flame treatment and the like. Although any of these techniques may be effectively employed to pretreat the film surface, a particularly desirable method of treatment has been found to be the so-called electronic treatment method which comprises exposing the film surface to a high voltage corona discharge while passing the film between a pair of spaced electrodes. After electronic treatment of the substrate film surface, it may be coated with the coating composition of the present invention which coating will then exhibit a tendency to more strongly adhere to the treated film surface.

Problems solved by technology

Certain applications for heat-shrinkable films, e.g., printed films or packaging of materials such as boxes present unique problems.

When such printed heat-shrinkable films are shrunk, the printing tends to become distorted and unclear.

Therefore, these heat-shrinkable films are unsuitable for applications which require printable films.

Another problem with heat-shrinkable films that are currently being used is that when the films are used on products that have sharp edges, they tend to puncture and when used to cover light weight boxes, they tend to crush the boxes.

Consequently, heat-shrinkable films have not been used for these applications.

However, past attempts to provide a coated film having low shrinkability (e.g., 6% to 12%) have encountered difficulties resulting from the shrinkage of the film during the coating process.

When such a process is used to coat a film having low shrinkability (e.g., 6% to 15%), the drying process shrinks the film to an extent that it is no longer acceptable for use in most packaging applications.

However, E-P copolymer content must be carefully determined inasmuch as the presence of E-P copolymer can cause undesirable film elongation even at lower temperatures, e.g., 60.degree. C.

(140.degree. F.) drying temperatures, which elongation can cause registration problems during converting processes such as printing.