GB2084168A - Polymeric sleeves for stretch- film packaging - Google Patents

Abstract

A sleeve element for a stretch- film package comprises 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 300 lbs. Per square inch (2100 kN/m<2>) after 72 hours.

Description

SPECIFICATION Polymeric sleeves for stretch-film packag ing The invention relates generally to the packag ing arts and more particularly to clear or translucent plastics films that are useful as packaging media.

It is well known that various polyolefine membranes can be stretched 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 thickness or cross-section of the film.

Items varying from apples to builders' hard ware have been packaged in this manner.

However, these shrink films have well-recog nised shortcomings. For example, exposure to excessive heat totally relaxes the built-in stress and packaging failures result. More impor tantly, the amount of contractional force that is available per unit cross-sectional area of the shrink film is severely restricted by the melt strength and density of the resins employed.

The inventor has found that may of the limitations of shrink-film packaging can be overcome by specially devising a plastics film which is applied to the packageable items in a highly stretched condition. The mechanical 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 pack aging element for use in a stretch-film pack age comprises: a sleeve of substantially homo polymeric 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 300 Ibs. per square inch (2100 kN/m2), after a 72 hour period.

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 predeter mined force over a predetermined time.

The polymer is processed in both the mol ten and solid states in order to establish a selected degree of physical property orienta tion within the produced film, as determined respectively by the blow-up ratio and the draw-down 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 packaged.

The film in a packaging element embodying the invention 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.

A packaging element embodying the present invention is particularly useful in utitizing or packaging two or more blow-moulded containers, 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 array of bottles that is to be encircled. In addition, the film sleeve is applied to the items to be packaged in a highly stretched condition, stretched on the order of from 2 to 50% of its relaxed state, but not beyond the elastic limits of the material. Stretch-film packaging as practiced using a sleeve embodying the invention is, in general, less expensive than a comparable shrink film packaging procedure because cheap mechanical force is used to stretch the sleeve whereas expensive heat energy is required in a shrink film operation.

The starting material for fabricating a film sleeve embodying 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 inclusing of property-enhancing additives is contemplated. Such substances as ethylene vinyl acetate, ethylene methylacrylate and ethylene ethylacrylate are commonly blended with polyolefinic resins for improving flexibility and toughness. However, the inventor has found that such blends are unutile for such stretch-film packaging because films produced from such blended resins display a low modulus of elasticity and 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 that known as "Bakekite DFDA-0561 Natural 7"' which widely is used in manufacturing heavy 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 stietch film sleeve over linear objects in bouquet packaging. 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. Antioxidants and anti-blocking agents may also be incorporated in the resin to advantage.

In order that the fabricated sleeve may function effectively as a packaging element ina stretch-film packaging operation, the substantially homopolymeric polyolefinie resin is selected to have a density of from 0.918 to 0.923, with values at the higher end of the range being generally preferred. Moreover, the selected resin takes a melt index of from 0.2 to 0.75, with values at the lower end of the range being preferred. As will be recognised, 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 used in the practice 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 plastics film, and hence the degree of isotropism, is important to the successful use of a tubular packaging element embodying the invention in a stretchfilm packaging operation. One convenient indication of the degree of molecular orientation which is suitable for the practice of the present invention is tensile strength. Preferably film material of the sleeve 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 preferably 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 1.9:1 and 3:1 and using a draw-down ratio of no greater than 7, preferably about 5. Proper selection of the blow-up ration is important because, when the molten plastics 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 4:1 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 preferably associated with a film material thickness of from 1 mil to 2.25 mils (0.001 inch to 0.00225 inch; 0.025 mm to 0.057 mm).

A final parameter of the utility of film tubes for use as stretch-film packaging elements embodying the invention is the stress relaxation index, and film materials having a stress relaxation index, measured at 30% strain, of 3-t leas 300 Ibs. per square inch (2100 icN/m2) over a period of 72 hours ensure the desired periods of package permanency.

Claims (7)

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 300 Ibs. per square inch (2100 kN/m2), after a 72 hour period.

2. A packaging element according to claim 1, wherein said film sleeve has a matter rial thickness of from 1 mil to 2.25 mils (0.001 inch to 0.0025 inch; 0.025 mm to 0.057 mm).

3. A packaging element according to claim 1 or claim 2 wherein said film material is polyethylene.

4. A packaging element according to any of claims 1 to 3, wherein said film sleeve has a tensile strength measured in the transverse direction that is equal to or greater than the tinsile strength measured in the machine direaction.

5. A packaging element according to any of claims 1 to 4, wherein said sleeve is blown film material.

6. The subject matter of claim 5, in which the film material is processed using a blow-up ration of from 1.9:1 to 3:1.

7. The subject matter of claim 6, in which the film material is processed using a drawdown ration of no greater than 7.