EP0614810A1

EP0614810A1 - Stretch wrap film rolls

Info

Publication number: EP0614810A1
Application number: EP94301428A
Authority: EP
Inventors: James Paul O'sullivan
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1993-02-26
Filing date: 1994-02-28
Publication date: 1994-09-14

Abstract

Stretch wrap film stock which is pre-stretched, prior to winding in roll form, by an amount which leaves at least a functional amount of stretchability remaining in the pre-stretched film stock.

Description

This invention relates to stretch wrap film; that is, film for wrapping loads and which is stretched when wrapped around the load.
Prior to stretch wrap film, loads were bundled by use of a variety of inefficient and/or expensive means, for example, ropes, metal bands and boxes. By using stretch wrap film, pallet loads of goods are effectively bundled so that they can withstand long distance transportation and protect the goods from the elements, while permiting visual identification of the wrapped goods. The merits of transparent stretch wrapping will ensure the continuing use of this type of bundling for many years to come.
In the process of stretch wrap bundling; for example of a pallet load, the end of the film is anchored to the load and either the load is rotated or the roll of film is rotated about the load. During this process a slipping brake action is applied to the roll of film so that the film is applied to the load in a stretched condition. Several overlapping layers of the film are all that are needed to contain the load. Present day stretch wrap film has the ability to cling to itself and other surfaces by virtue of one or both of its surfaces exhibiting tack.
As this bundling technique developed, sophisticated apparatus evolved to apply the stretch wrap film. In one of the systems it was found advantageous to employ a pre-stretch mechanism to the film at a point just before the film was applied to the load. This technique avoided, to a large extent, the use of the load to apply the stretch to the film. This also permitted better control of the longitudinal stretch and provided a means to minimize transverse dimensional decrease or neck-in. The end result included the fact that very little force was needed to wrap a load.
This art is constantly looking for ways to improve the bundling of loads with stretch wrap film and it is an object of the present invention to improve the overall quality of stretch wrap film and the manner of employing such film.
The present invention provides stretch wrap film stock which is pre-stretched, prior to winding in roll form, by an amount which leaves at least a functional amount of stretchability remaining in the pre-stretched film stock.
The present invention also provides a process for the preparation of stretch wrap film, which process comprises :
supplying stretch wrap film stock;
pre-stretching the film stock by an amount which leaves at least a functional amount of stretchability remaining in the pre-stretched film stock; and thereafter winding the film stock in roll form.
The present invention further provides the use of stretch wrap film delivered from a roll of pre-stretched film stock in bundling a load comprising a plurality of members.
The starting material for the present invention is any stretch wrap film in its final commercial form whether already in roll form or during manufacture just prior to winding into roll form. If already in roll form, the film is unwound, pre-stretched to the desired extent and re-wound as a finished product. During manufacture, as indicated, the film is pre-stretched to the desired extent just before wind up.
The stretch wrap film can be a monolayer or multilayer film. It can have cling on one or both sides. One side of the film can be non-cling and also have a slip characteristic in relationship to itself. By this is meant that when two non-cling surfaces are rubbed together there is relative slip between the two. This prevents any destructive drag from occurring between slidingly contacting surfaces.
Examples of polymers which can be employed in film form as stretch wrap film are the polyolefins including all polyethylenes, such as low density polyethylene (LDPE), ethylene copolymerized with another alpha olefin and commonly known as linear low density polyethylene (LLDPE), ethylene vinyl acetate copolymer, ethylene methyl acetate copolymer, polyvinylchloride and polypropylene.
The reasons why pre-stretching of the stretch wrap film has become an option available to pallet load wrappers is that the tension on the wrapping material is not constant. Pallet loads are usually irregular in shape and rotate in an eccentric manner due to inexact centering on a turntable. The film web is wrapped around the load pallet at speeds which vary throughout the wrapping operation. Since the wrapping material is pulled from the feedstock roll by the action of the rotating pallet load, these irregularities cause the tension on the wrapping material to vary. The pallet load, when wrapped in this manner, usually has tight zones and loose zones which may cause the load to be unstable. In addition, the pallet load may consist of relatively light objects which are easily displaced by the uneven pulling forces.
The preparation of the stretch wrap film prior to pre-stretching can be accomplished by any means. Two of the common means are bubble blown film and slot cast film techniques. In both cases the film is extruded and manipulated to a film thickness of from 0.0127 to 0.0381 mm (0.5 to 1.5 mils) or greater. Appropriate additives may be included; for example, cling agents, such as polyisobutylene; indeed, the film stock may be fully formulated prior to the pre-stretching. The films are formed so as have a stretch capability functional for stretch wrapping purposes. Normally this means providing the film with a stretch capability ranging from 50 to 400%. By this is meant that 1 metre of film can be stretched from 1.5 to 5 metres. A commercially available film which can either be pre-stretched during manufacture or unwound after manufacture for pre-stretching and rewinding is Mobilrap XPR a 60 gauge (0.015 mm 0.6 mil) monolayer blend of polyethylenes.
The pre-stretching according to the present invention can be accomplished by any means which, after wind up leaves a functional percentage of stretch remaining in the film. It is desirable that the pre-stretching is effected uniformly throughout the film stock. It is also desirable that the pre-stretching is effected by uniaxial stretching, preferably in the machine direction. In an elementary manner, the film can be pre-stretched by employing a differential speed roller system. With this system two or more transport rollers are used with the downstream roller turning at a higher rate than the upstream roller. This will cause the film to be pre-stretched between the rollers. It is also preferred that means, such as employed in a tenter frame, are utilized to attempt to maintain a predetermined film width and minimize neck-in. By limiting the stretching action to a minimium distance between rollers web neckin will be reduced and the pre-stretched film can be rolled up as a final product. A preferred degree of pre-stretch is to stretch it beyond its yield point and then wind it into rollform.
The phrase "a functional percentage of stretch remaining in the film" means that after the pre-stretched film is wound up, the end user must be able to effect a meaningful degree of stretch in employing commercial equipment in stretch wrapping any objects. This may be at least 20%. The commercial equipment may be hand-held or automatic.
During winding of the pre-stretched film the winding force and/or the degree of stretch must not be so great as to cause blocking of the film which would prevent or hinder unwind during use. These forces also must not cause telescoping (a squeezing out axially of inner layers of film) after winding of the film. Stretch wrap stock according to the invention may suitably be wound onto a core the compressive strength of which is greater than the stress to which it is subjected by the pre-stretching. The ends of the core may be formed as lugs to facilitate manipulation and the lugs may have radial symmetry about the core axis.
By the technique of the present invention pre-stretching can show up any imperfections that may otherwise remain latent. Thus, this technique can give a better quality product to the customer.

Claims

Stretch wrap film stock which is pre-stretched, prior to winding in roll form, by an amount which leaves at least a functional amount of stretchability remaining in the pre-stretched film stock.
Stretch wrap film stock according to claim 1 wherein the pre-stretching is effected uniformly throughout the film stock.
Stretch wrap film stock according to claim 1 or 2 wherein the pre-stretching is effected by uniaxial stretching.
Stretch wrap film stock according to any preceding claim wherein the pre-stretching is effected in the machine direction.
Stretch wrap film stock according to any preceding claim wherein the film stock is fully formulated prior to the pre-stretching.
Stretch wrap film stock according to any preceding claim wherein the roll is wound onto a core the compressive strength of which is greater than the stress to which it is subjected by the pre-stretching.
Stretch wrap film stock according to any preceding claim wherein the ends of the core are formed as lugs to facilitate manipulation.
Stretch wrap film stock according to claim 7 wherein the lugs have a radial symmetry about the core axis.
A stretch wrap film according to any preceding claim wherein the functional amount of stretchability remaining is at least 20%.
A process for the preparation of stretch wrap film, which process comprises :
supplying stretch wrap film stock;
pre-stretching the film stock by an amount which leaves at least a functional amount of stretchability remaining in the pre-stretched film stock; and thereafter winding the film stock in roll form.
Use of stretch wrap film delivered from a roll of pre-stretched film stock in bundling a load comprising a plurality of members.