CA1238752A

CA1238752A - Thermoplastic shipping sack

Info

Publication number: CA1238752A
Application number: CA000478052A
Authority: CA
Inventors: Charles R. Murray
Original assignee: Individual
Current assignee: Smurfit Stone Container Canada Inc
Priority date: 1985-04-01
Filing date: 1985-04-01
Publication date: 1988-07-05

Abstract

Abstract C-I-L 684 "Thermoplastic Shipping Sack"

A single ply tubular shipping sack having walls formed of uni-axially oriented linear low density polyethylene, optionally blended with low density polyethylene, wherein said walls are produced by blowing and cold drawing said polyethylene at a draw ratio to blow ratio of greater than 1:1. The sack has acceptable tear resistance in contrast to the reduced tear resistance expected for uni-axially oriented polyethylene films.

Description

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Thermoplastic Shipping Sack C-I-L 684 This invention relates to tubular thermoplastic shipping sacks formed of uni-axially oriented linear low denslty polyethylene or blends thereof wlth low density polyethylene.
Bulky but lightweight materials such as fiberglass insulation and peat moss are generally shlpped ln compressed form in ~hermoplastic shipping sacks. These sacks are generally known as tubular lnsulation sacks or bags and take the form of an extended envelope or tube sealed at one end prior to its being filled wlth product. For the most part these sacks are produced by the commonly known ln the art "blown film" process, which owes lts popularlty to the fact - that it can be quickly and readily adapted to the productlon of different widths and thicknesses of continuous tubes which can then be easily cut to length and sealed at one end to produce an open top sack.
It will be readily appreclated that the thinner the film thickness (gauge), commensurate with acceptable fllm properties, the less the amount of thermoplastic material required. This downgauging of sack wall thickness ls a most desirable industrial goal. Walls of sac~s produced as tubes by the blown film process, typically, have a fllm thlckness in the range of 3 - 6 mll (75 - 150 X10 4 cm) which ls generally determined by the machine direction (~.D.) tensile strength necessary to handle the package weight, the film stretch resistance required to prevent expansion of the compressed product and the puncture reslstance of the bag for distrlbution handling.
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- 2 - C-X-L 684 The tubes ~rom which these sacks are commonly made are produced with a bubble diameter/die dlameter generally of

3:1 in order to optimize film strength properties.
Although various attempts have been made to use high densi~y polyethylene for the manufacture of downgauged bags because of its high stretch resistance and tensile strength these have largely ~een abandoned because of poor tear resistance and puncture properties. In view of this, lQ polyethylene insulation sacks are most commonly made from resins which have superior tear resistance and puncture properties such as low density or linear low density polyethylene.
It i-~ well known in the art to produce polyethylene films having enhanced puncture, tensile strength and stretch resistance by the process of uni-axially cold drawing the film below its melting point. However, because of the very -~ poor M.D. tear strengths of these orlented films, whlch causes "splittiness", they have been ignored for use ln tubular shipping sacks.
Surprisingly, we have now found that by the correct selection of film blow up ratio (BR) and draw ratio ~DR) of blown and drawn linear low density polyethylene film a tubular uni-axially oriented film having good M.D. tear resistance can be produced suitable for use in a downgauged shipping sack.
Accordingly, the invention provides a tubular shipping sack formed of uni-axially oriented linear low density polyethylene film produced by blowing and cold drawing linear low density polyethylene at a draw ratio to blow ratio (DR/BR) of greater than 1:1.
By the term "Draw Ratio" is meant the ratio of the length of drawn film to the length of undrawn film, and by the term "Blow Ratio~ is meant the ratio of the bubble dlameter to the die diameter. Such terms are well known in the art.

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, ' ~Z3~375~2 ~ e have found that ratios of DR/BR of ~1:1 result in ~ilms having unacceptable M.D. tear properties, whereas values of at least 2:1 are preferred, although ratios >5:1 ; are im~ractica1 to obtain.
Accordingly in a preferred feature, the invention ~rovides a tubular shipping sack formed of uni-axially oriented linear low density polyethylene film produced by ~lowing and cold drawing linear low density polyethylene at a draw ratio to blow ratio of between 2:1 and 5:1.
We have thus found that a shipping sack having improved film stretch resistance and high tensile strength in addition to acceptable tear ~eslstance comparable to that for non-oriented film and ln contrast to the expected usually reduced tear resistance for uni-axially oriented polyethylene film can be manufactured.
We have further found that by blending in a minor amount of high pressure process ~i.e. non-linear) low denslty polyethylene resin with the linear low density polyethylene resin a uni-axially oriented film having further enhanced tear properties can be produced. In addition, we have found that these resin mixtures ~lso provide enhanced bubble stability during the blown film process and facllltate the manufacturing process.
Accordingly, in a preferred feature the invention .urther provides a tubular shipping sack as hereinbefore c`efined wherein said linear low density polyethylene contains a minor amount of low density polyethylene.
The amount of low denslty polyethylene present in the ?olyethylene blend prior to blowing lnto film can be readily determined by the skilled man to be that amount whlch provides acce?table enhanced tear properties. Typically, the blend com?rises 20~ low density polyethylene and offers uni-axially oriented ~ilm ~or use in shipping sacks according to the invention which could be downgauged by 30~.

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4 C-I-L 6~4 The typical blown film process basically comprises the steps of extruding molten thermoplastic material throuyh a circ~lar die, typically of O.OS" diameter gap, to form a tube s which is closed by passing the end through a set of nip/draw ~olls. Air is introduced through the die centre to inflate the tube to the selected width while the speed of the nip rolls is increased to draw the film down to the desired gauge. At this stage the flattened film tube is, optionally, passed to a corona discharge unit to burn the film surface to make it receptive to lnk application when next passed through a flexographlc s~ack press. The tube ls then reinflated by passing it through two sets of nip rolls wlth an air bu~ble trapped between them while the edge of the tube is tucked by forming plates just prior to the second set of nlps in ;order to form any required gusset ln the tube. The tube finally passes to an end seal head where it is heat sealed and guillotined to the required sack length`.
Although it is preferable when extruding to use a -20 maximum die size to obtain highest output rates, low density polyethylene is commonly extruded with much smaller dies to give bLow ratios of between 2 - 3 to provide film impact strength, especially on the edge folds of the flattened tube.
In the blown film process for the manufacture of sacks according to the invention the above general process is modified in several ways. The first change is that the die diameter used is increased by a factor of 2 - 3, depending on the desired bag layflat width, over the die diameter used at present in the industry for the manufacture of thermoplastic tubular sacks. The second change is that a cold drawlng section is introduced between the bubble nip and the corona treatment unit. Such a section may consist of an initial series of heating rolls to raise the film temperature to a point, e.g. ca. 105C, where a minimum of force is required for cold drawing and a series of clo~ely spaced rolls through .. . . -.
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~ 5 - C-I-L 684 which the heated film passes to a set of cooling rolls to bring the film temperature to ambient. The spacing between the closely spaced rolls is, typically, 0.13 mm plus film thickness, The film is then passed to a set of pull nips running at a linear speed, typically, of four times greater than the bubble nip draw rolls.
The blown film thickness is typically four times greater than the required thickness of the finished sack.
Thus, since the desired finished sack film thlckness is typically ~ - 3 mil the initial blown film thlckness`must be 8 - 12 mil. Because it is extremely difficult to control blown film thickness of gauges greater than 12 mil the use of DR/B~ values higher than 5:1 becomes impractical.
15 ~t has thus been found that a suitable open-top tubular polyethylene shipping sack having improved tear resistance can be manufactured using suitably modified conventlonal blown film process apparatus.
In addition, tubular shipping sacks of alternative structure to the simple open-top sack described hereinabove and utilizing the feature o~ the invention to provide the promised advantages may be produced. Such an alternative tub~lar sack is the type known as a "valved bag" shipping sack, which is closed at both ends of the tube and has a self-closing valve structure at an upper side or end.
Such alternative bags may be made by conven~ional processes well-known in the axt suitably modified to provide a sack formed of uni-axially oriented film produced by blowing and cold drawing at the aforesaid draw ratio to blow ratio.
Thus, by the term "tubular shipping sacks" is meant sacks having a resultant shape generally of a tube, optionally provided with gussets, whether made by the speclfic process as hereinbefore described or by alternative processes known in the art which may or may not involve the "back-sealing"
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Tne heat seal produced in the tube, i.e., the two flattened sides (films) o~ the tube, by the end seal head in the process hereinabove described is produced under a combination of pressure and heat, at or above the films' crvstalline melting point, applied to the films in order that they are truly welded at their interfaces such that a clean separation cannot be effected by physical ox chemical means.
It is known that heat build-up during the sealing operation may be sufficient to destroy the orientation of unl-axially oriented films in the vicinity of the heat seal and thus cause serious loss of draw-induced impact strength. We have found that sacks manufactured by the process herPinbefore described have sufficient impact strength suitable for the lntended l; purpose for which the sacks are made. Further, we have found that the most valuable improved tear reslstance properties of the film are not significantly reduced. Thus, an acceptable bridge between the uni-axially oriented film and the body of the seal is formed and permits the manufacture of an improved tear resistant sack having acceptable impact resistance.
Also included within the scope of the invention are those tubular shipping sacks incorporating the feature of the invention wherein the seals or other closures provided in the tubes are formed by adhesive bonding as an alternative to heat sealing. Use of such adhesive bonding provides all the advantages promised hereinabove and also improved impact resistance to the bag. This permits use of such bags for the packaging of bulky and lightweight materials such as, for example, vermiculite insulation material.
Accordingly, the invention provides an open-top tubular shipping sack having a front wall and a back wall, each wall formed of a uni-axially oriented ply of linear low density - , ~ ' .
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The term "linear low density polyethylene" includes linear low density ethylene copolymers with the lower olefins such as, for example, butene, n-hexene, ~-methyl-l-pentene and octene. The term "low density polyethylene"
includes low density ethylene homopolymers and copolymers such as vinyl acetate copolymers, and blends thereof.
While it is generally accepted that all polyethylene film is generally uni-axially oriented to some aegree, the term "uni-axially oriented" when used with reference to linear low density polyethylene in this specification and claims means polyethylene film that has been blown and cold drawn to at least a 2.5-fold extent, preferably to a 4-fold extent, but also up to a 6-fold extent. The orienting of the films may be carried out by the cold drawing of the blown tube as hereinbefore described.
The cold drawn uni-axially oriented film of use in the invention made from linear low density polyethylene resins and low density polyethylene blends thereof can be used in a variety of thicknesses. One particular blend of use in the practice of the invention comprises ]inear low density and low density polyethylenes in the ratic of 4:1.
Also included within the scope of the invention are single ply tubular shipping sacks having walls formed of a co-extruded laminate comprising a layer of uni-axially oriented linear low density polyethylene produced as hereinbefore defined and a layer of a low density ethylene polymer or copolymer compatible with said uni-axially oriented linear low density polyethylene. Examples of such compatible copolymers of use in the invention are e~hylene-vinyl acetate ; 35 copolymers, ethylene-ethyl acrylate copolymers and ethylene-:- -, .. . . .
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methyl metha_rylate copolymers.
It is well-known in the art to co-extrude such a two or more polymer system to form a laminate by means of conventional co-extrusion equipment. However, in the process according to S the invention as is applicable to a laminate the compatible ethylene polymer or copolymer is also subjected to the novel same draw ratio to blow ratio (DR/BR) subsequent to the co-extrusion step as is the linear low density polyethylene.
The compatible ethylene polymer or copolymer layer of the laminate may constitute either the inner surface or the outer surface of the sack to provide additional utility to the sack.
For example, where the compatible polymer or copolymer of the laminate is a soft-flexible copolymer, such as 10~ ethylene-vinyl acetate, providing an external surface of the sac~ it provides superior anti-slip properties. Where a 20% ethylene-methyl acrylate copolymer of the laminate provides the inner layer of the sack, the sack may generally be heat sealed at temperatures as low as 80~C. which reduces the risk and degree of disorientation of the vulnerable oriented layer.
2~ The co-extruded laminate may comprise two or more ~ompatible layers as is deemed appropriate~ Multi-laminated ply may be used wherein one laminate laye~ constitutes a barrier layer to the movement of chemical vapour through the sack walls.
Accordingly, the invention provides an open-top tubular shipping sack as hereinbefore defined wherein said film or ply of uni-axially oriented linear low density polyethylene forms part of a multi-layer laminate with one or more layers of one or more compatible ethylene polymers or copolymers.
An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:
Figure 1 shows a front elevational view, partly cut away, of an open-top sack according to the invention; and Figure 2 is a sectional view along 2 - 2 of Figure 1.

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Figures l and 2 show a generally rectangular single ply tubular sack l having a front wall 2 and a back wall 3 formed of a blown and cold drawn polyethylene blend consisting of linear low density polyethylene (4 parts, density 0.918, melt index 0.5 - ESCORENE 1030* from ESSO CHEMICAL*) and low density polyethylene (l part, density 0.923, melt index 0. 3 - CIL 503*). One end ~ of the tubular sack is heat sealed to form a single poly open-top sack.
The sack is made by extruding the above resin blend and blowing and cold drawing the film on modified conventional equipment as hereinbefore described. The flattened tube (2/" width) is fed to an end seal head where it is heat sealed and guillotined to a bag length of 60". The flattened tube width reduces to a tubular sack width of 16" with the provision of two 5~" gussets. The process is operated with the parameters as given for resin No. 8 in the Table herein-below. The sack has a most valuable tear resistance of ` 240 gm/mil.
A series of films was blown from ESCORENE 1030* (LLDPE) resin and CIL 503* (LDPE) resin at different blow ratios.
These were subsequently cold drawn below their crystalline melting point at different draw ratios and teste~ for M.D.
tear resistance. The process parameters and results are given in the following Table.
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Claims

Claims:

1. A thermoplastic sack comprising film produced by the following process:
extruding a molten linear low density polyethylene through a die having a predetermined diameter to form an extruded tube having a length;
forming an air bubble in said tube, said bubble having a diameter, a ratio of said bubble diameter to said die diameter defining a blow ratio (BR); and cold drawing said tube in a single direction to provide a uni-axially oriented linear low density polyethylene drawn tube with a length, a ratio of said drawn tube length to said tube length defining a draw ratio (DR) such that a DR/BR ratio falls between 2:1 and 5:1.

2. A sack as claimed in Claim 1 wherein said linear low density polyethylene contains a minor amount of low density polyethylene.

3. A sack as claimed in Claim 1 wherein said linear low density polyethylene contains a minor amount of high density polyethylene.

4. A sack as claimed in any one of Claims 1, 2 or 3 wherein said sack comprises a multi-layer laminate with at least one layer of compatible ethylene polymers or copolymers being laminated to said uni-axially oriented linear low density polyethylene tube.

5. A sack as claimed in Claim 4 wherein said compatible ethylene polymer includes a low density polyethylene.

6. A sack as claimed in any one of Claims 1, 2 or 3 wherein said sack comprises a front wall and a back wall, each of said front wall and said back wall comprising a ply of aid uni-axially oriented linear low density polyethylene, and wherein interposed between said front wall and said back wall are two inner plies of low density polyethylene.

7. A sack as claimed in any one of Claims 1, 2 or 3 wherein the sack is a multi-ply sack having a single ply of said uni-axially oriented linear low density polyethylene.

8. A sack as claimed in Claim 7 wherein the different plies of the sack are made of different thermoplastic film.

9. A thermoplastic film produced by the following process:
extruding a molten linear low density polyethylene through a die having a predetermined diameter to form an extruded tubular film having length;
forming an air bubble in said extruded tubular film, said bubble having a diameter, a ration of said bubble diameter to said die diameter defining a blow ratio (BR);
and cold drawing said tubular film in a single direction to provide a uni-axially oriented linear low density polyethylene drawn tubular film with a length, a ratio of said drawn tubular film length to said tubular film length defining a draw ratio (DR) such that a DR/BR ratio falls between 2:1 and 5:1.

10. A thermoplastic film according to Claim 9, wherein said film further includes at least one layer of a compatible ethylene polymer or copolymer.

11. A method of manufacturing a thermoplastic film, comprising the steps of:
extruding a molten linear low density polyethylene through a die having a predetermined diameter to form an extruded tubular film having a length;
forming an air bubble in said extruded tubular film, said bubble having a diameter, a ratio of said bubble diameter of said die diameter defining a blow ratio (BR);

cold drawing said extruded tubular film in a single direction to provide a uni-axially oriented linear low density polyethylene drawn tubular film having a length, a ratio of said drawn tubular film length to said tubular film length defining a draw ratio (DR) such that DR/BR ratio falls between 2:1 and 5:1.

12. A method according to Claim 11 wherein said extruding step includes the step of extruding a molten linear low density polyethylene having a minor amount of low density polyethylene.

13. A method according to Claim 11 wherein said extruding step includes the step of extruding a linear low density polyethylene having a minor amount of high density polyethylene.

14. A thermoplastic sack comprising film formed of uni-axially oriented linear low density polyethylene having a DR/BR ratio of between 2:1 and 5:1, said DR being defined as a ratio of a length of drawn film to a length of undrawn film, said BR being defined as a ratio of a bubble diameter to a die diameter.

15. A sack according to Claim 14 wherein said linear low density polyethylene film contains a minor amount of low density polyethylene.

16. A sack according to Claim 14 wherein said linear low density polyethylene film contains a minor amount of high density polyethylene.