Slip agents and polypropylene films prepared therefrom

Details Slip agents and polypropylene films prepared therefrom Slip agents and polypropylene films prepared therefrom

1997-10-09

2002-12-24

Mulcahy, Peter D. (Department: 1713)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C428S516000, C524S229000

Reexamination Certificate

active

06497965

ABSTRACT:

This invention relates to a novel polypropylene composition having specific utility in the manufacture of oriented polypropylene films and to polypropylene films prepared therewith. More specifically, it relates to polypropylene compositions containing certain fatty acid amides not heretofore known to be useful as components in polypropylene compositions.
BACKGROUND INFORMATION
In the manufacture of polypropylene films, it is common practice to include, in the polymer formulation, additives referred to as slip agents. These additives migrate to the surface of the film and decrease the coefficient of friction between the film and the metal rollers over which it is passed during processing, thus facilitating the processing of the film. They also decrease the coefficient of friction between layers of the film when it is wound into rolls thereby facilitating unwinding of the rolls for further processing.
The slip additives employed in most commercial polypropylene films are relatively high molecular weight fatty acid amides. The most widely used fatty acid amides are erucamide, an unsaturated 22 carbon amide (13-docosenamide) and behenamide (docosanamide), the saturated analogue of erucamide. Both of these compounds are readily available, naturally occurring materials. They are normally provided as mixtures containing a small amount of other amides containing about 18 to 20 carbons.
Erucamide works quite well as a slip agent, but it is not favored by some producers and converters as it is relatively volatile and extra care is required during film manufacture and conversion to avoid having quantities of the additive escape from the film and plate out on processing equipment, thereby causing a clean-up problem. For this reason, some producers prefer to use behenamide as the slip agent. As slip agents, erucamide and behenamide appear to perform substantially equally well but behenamide is less volatile and therefore films containing behenamide are easier to handle during processing since the behenamide does not escape and plate out on the processing equipment.
Despite the inconveniences encountered with erucamide-containing film, erucamide is the fatty acid amide slip agent of choice for those who wish to print the film using the well known rotogravure printing technique. In the rotogravure process, ink is applied to the print surface from a gravure cylinder containing the desired image as depressions on its surface. Ink is applied to the cylinder and the surface of the cylinder is wiped by a doctor blade to remove excess ink. The film to be printed is then contacted by the gravure cylinder and the image is thus transferred from the cylinder to the film. While the film is in contact with the cylinder, a small amount of erucamide is deposited on the raised portion of the cylinder. Since erucamide is soluble in the alcohol or ketone ink solvent, it is dissolved by the solvent in the next application of ink and is removed by the next swipe of the doctor blade. Thus, no build-up of erucamide occurs on the doctor blade or on the gravure cylinder.
Behenamide, on the other hand, lacking the carbon-carbon unsaturation found in erucamide, is substantially less soluble in the alcohols and ketones found in printing inks. Thus, when, in the course of rotogravure printing, some of the behenamide is deposited on the printing cylinder, it is not dissolved in the printing ink and wiped off when the next application of ink and wiping by the doctor blade takes place. As a result, the behenamide is merely wiped off by the next swipe of the doctor blade and substantially all of it remains on the blade. After multiple swipes of the blade, a behenamide build-up forms on the doctor blade and on the printing cylinder, eventually reaching a point at which the blade is prevented from wiping the surface of the printing cylinder clean. This build-up causes streaks to form on the printed film, requiring an interruption of the job in order to clean the doctor blade.
It will be immediately apparent that a slip additive that does not cause the film processing problems associated with erucamide but which allows the film to be rotogravure printed without causing the problems associated with behenamide would be a welcome contribution to the polypropylene film art. In accordance with this invention, there has been discovered a slip additive that accomplishes this objective.
It is an objective of this invention to provide oriented polypropylene films that exhibit improved properties in rotogravure printing as compared to films presently known to the art. It is likewise an objective of this invention to provide film-forming compositions for preparing such films.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention, it has been found that certain bis-fatty acid amides exhibit a combination of properties that makes them highly satisfactory as slip agents in oriented polypropylene films. In particular, this combination of properties make them highly satisfactory as slip agents in films that are to be printed in rotogravure printing operations.
Briefly stated, the invention is a composition comprised of a polymer or copolymer of propylene and an N,N′-bis-alkylene fatty acid amide having the general structural formula R—CO—NH—(CH
2
)
n
—NH—CO—R′ wherein R and R′ are the same or different alkyl or alkenyl groups having about 15 to 21 carbon atoms —CO— is a carbonyl group and n is an integer from 2 to 4 said N,N′-bis alkylene fatty acid amide being present in an amount from about 0.05 to about 0.5% by weight based on the combined weight of said amide and the polypropylene.
In another aspect, the invention contemplates an oriented film comprised of a polymer or copolymer of propylene and an N,N′-bis-alkylene fatty amide having the general structural formula R—CO—NH—(CH
2
)
n
—NH—CO—R′ wherein R and R′ are the same or different alkyl or alkenyl groups having about 15 to 21 carbon atoms, n is an integer from 2 to 4 and —CO— is a carbonyl group said N,N′-bis alkylene fatty acid amide being present in an amount from about 0.05 to about 0.5% by weight based on the combined weight of said amide and the polypropylene.
A preferred embodiment of the invention is a composite film comprised of a polypropylene core having, on at least one of its surfaces, a film layer comprised of a composition comprising polypropylene and up to about 0.5% of an N,N′-bis-alkylene fatty amide having the general structural formula R—CO—NH—(CH
2
)
n
—NH—CO—R′ wherein R and R′ are the same or different alkyl or alkenyl groups having about 15 to 21 carbon atoms, n is an integer from 2 to 4 and —CO— is a carbonyl group, said N,N′-bis alkylene fatty acid amide being present in an amount from about 0.05 to about 0.5% by weight based on the combined weight of said amide and the polypropylene.
DETAILED DESCRIPTION OF THE INVENTION
As used in the description of this invention and in the attached claims, the term “film” refers to a stand-alone film, i.e. a film of a sufficient thickness to have the strength and other characteristics required to have utility in packaging and other applications without having to be united with another film to support it. Stand-alone monolayer films are seldom seen in commerce. However, the compositions of the invention are suitable for use in such films.
Oriented polypropylene films of commerce are usually composite, i.e. multilayer, structures in which a core layer of a thickness sufficient to impart stand-alone properties to the overall structure carries one or more thin functional layers on its surface(s). The term “film” will also be used to refer to these thin functional layer films although they do not have sufficient thickness to stand alone.
Composite polypropylene films are typically comprised of a homopolypropylene core having one or more functional layers on its surfaces. Functional layers can act, e.g., as heat or cold seal layers, as receiving layers for printing or other decorative material or as barrier coating receiving layers. In the

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