NZ198278A

NZ198278A - Polyolefin sleeve for stretch-film packaging

Info

Publication number: NZ198278A
Application number: NZ198278A
Authority: NZ
Inventors: H F Pillman
Original assignee: Illinois Tool Works
Priority date: 1980-09-15
Filing date: 1981-09-04
Publication date: 1984-12-14
Also published as: GB2084168A; AU7482581A; JPS5780026A; SE8105441L; IT1139165B; IE812129L; IE51629B1; GB2084168B; DE3135230A1; FR2490192A1; IT8123965A0; NL8104263A

Description

New Zealand Paient Spedficaiion for Paient Number 1 98278 1 98 2 7 pj:-!".'..'-- "...

Fi!ed: ' bSlZI A,A ... )M DEC 1994 n jn„ - -> •••-. 1I9&5", |i>'Bll &rj B liajffl w Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "POLYMERIC SLEEVE FOR STRETCH-FILM PACKAGING" "1/WE ILLINOIS TOOL WORKS^ INC. a c o p r or a tien- organized under the laws of the State of Delaware, U.S.A. of 8501 W. Higgins Road, Chicago, Illinois 60631, U.S.A. hereby declare the invention, for which I/we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- Case 359S 00 198278 Field ol the Invention 1 This invention relates generally to the packaging arts and more particularly to clear or translucent plastic films that are useful as packaging media.

Background of the Invention It is well known that various polyolefin mfembranes can be stretched 5 in a near-molten state and then rapidly cooled to lock in stress; and it has been possible by this method heretofore to produce films which can be shrink-fit upon various items. In the packaging operation, mild heat is ordinarily applied to release a portion of the stress stored in the film and this in turn reduces the area of the film, causing the shrinkage, while increasing the 10 thickness or cross-section of the film. Items varying from apples to builders' hardware have been packaged in this manner. However, these shrink films have well-recognized shortcomings. For example, exposure to excessive heat totally relaxes the built-in stress and packaging failures result. More importantly, the amount of contractional force that is available per unit cross-15 sectional area of the shrink film is severely restricted by the melt strength and density of the resins employed.

Brief Description of the Invention aging can be overcome by specially devising a plastic film which is applied to the packageable items in a highly stretched condition. The mechanical 20 stretching force is then removed, allowing the film to restore a position of its prestretched force and form as a result of elastic properties, rather than plastic memory. According to the present invention, a polyolefinic homo-polymer is selected to exhibit a stress relaxation index, measured at 30% strain, of at least 300 lbs. per square inch after 72 hours of strain, a density of from 0.91S to 0.923 and a melt index of from Applicant has found that many of the limitations of shrink-film pack- - 2 '12SEP1984 Case- 359S 00 198278 1 0.2 to 0.75. For purposes of this description, it should be understood that the stress relaxation index identifies the resistance of the material to "creep" or "relax" when stretched or strained a predetermined amount under a predetermined force over a predetermined time. This poly-5 mer is then processed in both the molten and solid states in order to establish a selected degree of physical property orientation within the produced film, as determined respectively by the blow-up ratio and the drawdown ratio of a blown film fabricating operation. A short tube of this film is then stretched in the ambient state to receive two or more articles to be 10 packaged.

Applicant's stretch-film possesses greater contractional force about the packaged articles than shrink film and eliminates the expense of the heat tunnels required in a shrink film packaging plant.

Accordingly, a general object of the present invention is to provide a 15 new and improved polymeric sleeve for use in packaging operations.

Detailed Description of the Invention The polymeric sleeve contemplated by the present invention is particularly useful in unitizing or packaging two or more blow-molded beverage bottles for example; and the unstretched circumference of the tube or sleeve is selected to be substantially less than the effective circumference of the 20 array of bottles that is to be encircled. In addition, the film sleeve of the present invention is applied to the items to be packaged in a highly stretched condition, stretched on the order of from about 2 to about 50% of its relaxed state, but not beyond the elastic limits of the material. Stretch-film packaging as practiced using the sleeve of the invention is, in general, less 25 expensive than a comparable shrink film packaging procedure because cheap mechanical force is used to stretch the sleeve of the invention whereas expensive heat energy is required in a shrink film operation. 19827 Case 3598 00 1 The starting material for fabricating the film sleeve of the present invention is a polyolefinic resin, such as polyethylene or polypropylene; and pure or homopolymer material is required, as distinguished from a blend or copolymeric material, although the inclusion of property enhancing additives 5 is contemplated. Such substances as ethylene vinyl acetate, ethylene methyl-acryiate and ethylene ethylacrylate are commonly blended with polyolefinic resins for improving flexibility and toughness. However, applicant has found that such blends are inutile for such stretch-film packaging because films produced from such blended resins display a low modulus of elasticity and 10 poor resistance to creep or low stress relaxation index. As a result, the grip or contractional force exhibited by films of these polymer blends deteriorates relatively rapidly over time, leading to an unacceptably high rate of package failures. One eminently useful resin for practice of the invention is Bakeiite DFDA-0561 Natural 7 which is widely used in manufacturing heavy 15 duty shipping sacks and which is commercially available from Union Carbide Corporation.

Advantageously, a slip additive is incorporated with the homopolymer resin used in the practice of the present invention in order to facilitate fitting of the stretch film sleeve over linear objects in bouquet packaging. 20 For example, a glyceryl ester of erucic acid, derived from rapeseed oil, or oleamide and erucimide or equivalents, may be added while pelletizing the resin. This material migrates to the surface during processing of the resin into film and there it provides a lubrication effect. Anti-oxidants and antiblocking agents may also be incorporated in the resin to advantage.

In order that the fabricated sleeve may function effectively as a packaging element in a stretch-film packaging operation, the substantially homopolymeric polyolefin resin is selected to have a density of from about 0.918 to about 0.923 with values at the higher end of the range being generally preferred. Moreover, the selected resin takes a melt index of i J 98 2 7 Case 3598 00 from about 0.2 to about 0.75 with values at the lower end of the range being preferred. As will be recognized, the melt index of a resin is a measure of its molecular weight, and the melt index, together with density, provides a measure of the modulus of elasticity and also the resistance of a fabricated film to creep. The selected resins of the invention exhibit a generally high modulus of elasticity and other properties appropriate to a stretch-film packaging element.

The molecular orientation in the plastic film and hence the degree of isotropism is important to the successful use of the present tubular packaging element in a stretch-film packaging operation. One convenient indication of the degree of molecular orientation which is suitable for the practice of the present invention is tensile strength; and the instant film material exhibits a tensile strength measured in the transverse direction that is equal to or greater than the tensile strength measured in the machine direction.

The desired degree of molecular orientation is achieved by processing the selected resin in a blown film operation in which the film material is processed using a blow-up ratio of from about 1.9:1 to about 3:1 and using a draw-down ratio of no greater than about 7, preferably about 5. Proper selection of the blow-up ratio is important because, when the molten plastic resin passes through the extrusion die, the molecular chains are oriented preferentially in the machine direction in which the material is being extruded. Blow-up ratios substantially less than 2:1 do not produce the desired degree of orientation, and blow-up ratios of greater than about 4 produce unacceptably unstable film. The draw-down ratio must be properly related to the blow-up ratio because excessive draw-down tends to restore molecular orientation. The desired degree of relationship between the two factors of blow-up ratio and draw-down ratio is denoted by a film material thickness of from about 1 mil to about 2-1/4 mils. 198 2 Case 3598 00 A final parameter of the utility of film tubes for use as stretch-film packaging elements is the stress relaxation index, and film materials having a stress relaxation index measured at 30% strain, of at least about 300 lbs. per square inch over a period of 72 hours assures the desired periods of package permanency.

The manner in which the present invention may be practiced and the purposes to which it may be put are evident from the foregoing descriptions. 198278

Claims

WHAT WE CLAIM IS:

Claim 1. A packaging element for use in a stretch-film package comprising: a sleeve of substantially homopolymeric polyolefinic film material having a density of from 0.918 to 0.923, a melt index of from 0.2 to 0.75 and a stress relaxation index, measured at 30% strain, of at least 3001bs. per square inch, after a 72 hour period.
Claim 2. A packaging element according to claim 1 wherein said film sleeve has a material thickness of from 1 mil to 2-1/4 mils.
Claim 3. A packaging element according to claim 1 wherein said film material is polyethylene.
Claim 4. A packaging element according to claim 1 wherein said film sleeve has a tensile strength measured in the transverse direction that is equal to or greater than the tensile strength measured in the machine direction.
Claim 5. A packaging element according to claim 1 wherein said sleeve is blown film material.
Claim 6. A packaging element according to claim 5 in which the film material is processed using a blow-up ratio of from 1.9:1 to 3:1.
Claim 7. A packaging element according to claim 6 in which the film material is processed using a draw-down ratio of no greater than 7. 19827S
Claim 8. A packaging element according to claim 1 for use in a stretch-film package substantially as hereinbefore described. ILL^O^S TOOL WORKS INC * 12 SEP 1984 ]