CA2114658A1 - Thermoplastic film and sacks made therefrom - Google Patents
Thermoplastic film and sacks made therefromInfo
- Publication number
- CA2114658A1 CA2114658A1 CA 2114658 CA2114658A CA2114658A1 CA 2114658 A1 CA2114658 A1 CA 2114658A1 CA 2114658 CA2114658 CA 2114658 CA 2114658 A CA2114658 A CA 2114658A CA 2114658 A1 CA2114658 A1 CA 2114658A1
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- Prior art keywords
- sack
- low density
- density polyethylene
- film
- vinyl acetate
- Prior art date
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- Abandoned
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- Manufacture Of Macromolecular Shaped Articles (AREA)
- Bag Frames (AREA)
- Laminated Bodies (AREA)
- Shaping By String And By Release Of Stress In Plastics And The Like (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A shipping sack having walls formed of a uni-axially oriented polyethylene blend film wherein said film is produced by blowing and cold drawing said polyethylene blend at a draw ratio to blow ratio of less than 2:1, preferably, between 1:1 and 2:1. The sack can be used as a tubular shipping sack for bulky, lightweight material or as a heavy duty sack. The sacks have improved physical properties.
A shipping sack having walls formed of a uni-axially oriented polyethylene blend film wherein said film is produced by blowing and cold drawing said polyethylene blend at a draw ratio to blow ratio of less than 2:1, preferably, between 1:1 and 2:1. The sack can be used as a tubular shipping sack for bulky, lightweight material or as a heavy duty sack. The sacks have improved physical properties.
Description
211~6~8 THBRMOPLA8TIC FIL~N AND 8ACR8 NADI~ THEREFROII
Field Of The Invention This invention relates to uni-axially oriented polyethylene films and to thermoplastic, heavy duty shipping sacks made therefrom.
- Background of the Invention Thermoplastic sacks are used in the packaging, transportation or storage of a great variety of materials ranging from powders and granules, bulky and lightweight materials, and agricultural materials such as hay and silage. The thermoplastic sacks according to this invention have general applicability to such products.
Bulky, but lightweight materials such as fiberglass insulation and peat moss are generally shipped in compressed form in thermoplastic sacks. These sacks are generally known as tubular insulation sacks or bags ,1 25 and take the form of an extended envelope or tube sealed at one end prior to its being filled with product. For the most part these sacks are produced by the commonly known in the art ~blown film" process, which owes its popularity to the fact that it can be quickly and readily adapted to the production of different widths and ~` 21146~8 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 appreciated that the thlnner the film thickness (gauge), commensurate with acceptable film properties, the less the amount of thermoplastic material required. This downgauging of sack wall thickness is a most desirable industrial goal. Walls of sacks produced as tubes by the blown film process, typically, have a film thickness in the range of 3-6 mil (75-150X10~ cm) which is generally determined by the machine direction (M.D.) tensile strength necessary to handle the package weight, the film stretch resistance required to prevent expansion of the compressed product and the puncture resistance of the bag for distribution handling. The tubes from which these sacks are commonly made are produced with a bubble diameter/die diameter generally of 3:1 in order to optimize film strength properties.
Although various attempts have been made to use high density polyethylene for the manufacture of down-gauged bags because of its high stretch resistance and tensile strength these have largely been abandoned because of poor tear resistance and puncture properties.
In view of this, polyethylene inæulation 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 is 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 unbalanced physical properties e.g. poor M.D. tear strengths, of these oriented films, - 21146~8 _ 3 _ SL371 which causes "splittiness", they have been ignored for use in tubular shipping sacks.
Claimed in our United States Patent No. 4677007, issued June 30, 1987, are heavy duty shipping sacks and films formed of cold drawn uni-axially oriented linear low density polyethylene film produced by blowing and cold drawing the polyethylene at a draw ratio to blow ratio of between 2:1 and 5:1. This reference, thus, teaches that when blown film is produced at desirable blow ratios for impact improvement, i.e. 2.5 to 1 or greater, the film must be cold drawn to at least 5 to 1 in order to obtain acceptable M.D. tear properties.
While film produced by the process disclosed in USP
4677007, has good impact properties and three times the puncture resistance of an unoriented film, it requires an inner liner of non-oriented film to make it heat sealable for heavy duty shipping sack applications.
The results of experiments disclosed -hereinafter show that linear low density polyethylene films blown at a blow ratio of 2.5 to 1 and cold drawn to 3 to 1 may be satisfactorily heat sealed without the need of an inner liner of non-oriented film. However, the -M.D. tear strength falls to about 30 GMS/MIL, which is unacceptable for heavy duty shipping sack applications.
USP 4677007 teaches against the desirability of making uni-axially oriented polyethylene film of a relatively high blow ratio unless accompanied by a much hlgher draw ratio. Surprisingly, I have now found that a high blow ratio film having a relatively lower draw ratio thani that claimed in USP 4677007 has desirable physical properties to enable satisfactory heavy duty shipping sacks to be manufactured therefrom. Further, I
have found that the film of the present invention does not need to have a high blow ratio or a high draw ratio to possess valuable strength and heat sealability :, :.'..: ,.
.. .: , .. ~
---` 211~6~8 properties. Provided the physical properties as hereinafter defined with reference to the thermographic characteristics of the film are satisfied, such film has utility in the thermoplastic shipping sack art.
Thus, the present invention satisfies the need for a film of use in a heavy duty shipping sack and having acceptable heat sealability and M.D. tear strength.
Summary of the Invention Surprisingly, I have now found that a uni-axially oriented film having good M.D. tear resistance and heat sealability can be produced suitable for use in the production of heavy duty downgauged shipping sacks not requiring an inner liner of non-oriented film.
Accordingly, in one aspect the invention provides a thermoplastic sack comprising heat sealable uni-axially oriented film formed of a blend of 60-95% w/w linea~ low density polyethylene and 5-40% w/w branched ethylene polymer made by the high pressure process selected from the group consisting of branched polyethylene, branched polyethylene copolymer and mixtures thereof, wherein said branched polymer has a differential scanning calorimetric thermogram melting peak temperature that substantially coincides with the thermogram temperature at which 25-35% of the linear low density polyethylene crystallites melt; and wherein said ' 30 film is produced by the following process:
extruding molten said polyethylene blend 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 21146~8 bubble diameter to said die diameter de~ining a blow ratio (BR); and cold drawing said tube in a single direction to provide a drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ration (DR) such that a DR/BR ratio falls below 2:1.
The term "linear low density polyethylene" as used in this specification and claims includes ultra linear low density polyethylene and linear low density ethylene copolymers with the lower olefins such as, for example, butene, n-hexene, 4-methyl-1-pentene and octene. ~-The term "branched ethylene polymer" includes low density ethylene homopolymers and copolymers, such as, for example, acrylic acid, vinyl acetate, methyl methacrylate copolymers, and blends thereof. ~ -~
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 diameter to the die diameter. Such terms are well known in the art.
Preferably, the draw ratio to blow ratio (DR/BR) ratio falls between 1:1 to 2:1. Preferably, the branched polyethylene copolymer is a copolymer of ethylene and an unsaturated compound selected from vinyl acetate, methyl methacrylate and acrylic acid. More preferably, the copolymer is an ethylene-vinyl acetate copolymer having a vinyl acetate context of 6-15% w/w, ' 30 most preferably 9-12% w/w.
By using the correct blend of linear and -~
branched polyethylene and/or branched polyethylene copolymer resins of defined physical characteristics a heat sealable uni-axially oriented film can be manufactured at the lower draw ratios required for heat \. :: ,~
21146~8 sealability and the high blow ratios required for impact strength and still achieve the higher level~ of M.D. tear strength required for the production of heavy duty shipping sacks. The sacks 60 produced typically have twice the puncture resistance of undrawn films and can be downgauged, typically by 30%, while still retaining good M.D. tear properties, typically, 200 GMS/MIL.
The physical properties required of the branched polymer blended with the linear polymer are, ~ -most surprisingly, quite specific and are based on a match of the thermogram melting peak temperature of the branched polymers with the temperature on the thermograph of the linear polymer at which essentially 30% of the latter's crystallites are melted. I have found that the melting peak temperature should be within, preferably, +5 C of the temperature at which 25-35% of the linear -polyethylene crystallites melt.
More preferably, the thermogram melting peak temperature of the branched resin should be within +3 C
of the temperature at which 25-35%, more preferably, approximately 30% crystallites have melted.
The thermogram melting peak temperature of the branched ethylene polymer is the temperature at which the thermogram peaks, typically, when approximately 90% of ;
the polymer has melted.
The thermogram characteristics of resins of use in the practice of the invention can be readily determined by standard methods well-known in the art.
Such characteristic~ can be determined by using a ' 30 differential scanning calorimeter, which measures or charts the temperature rise between a resin material under test and a comparative standard material. Each material is heated, in separate calorimeter cells, typically until the resin under test has fully melte*, and the heat taken up, per unit of time, by each material : '' .. .~ ..
: ~
: . ~
21146~8 is measured. In an alternative differential scanning technique, measurements of temperature against energy input/time for the two materials being heated are determined and take the form of measuring the additional heat supplied to the test material relative to that provided to the standard material for each material to have the same temperature rise.
I have thus found that a shipping sack comprising film having improved film stretch resistance and high tensile strength in addition to acceptable tear resistance comparable to that for non-oriented film and in contrast to the expected usually reduced tear resistance for uni-axially oriented polyethylene film can be manufactured.
The linear low density polyethylene may, optionally also contain a minor amount of high density polyethylene when extra heat resistance is required of the sack.
Thus, by blending, in the defined amounts, a linear polymer with a branched polymer having the defined characteristic of thermogram melting peak relative to the linear polymer thermogram criteria as defined, a uni-axially oriented film having enhanced tear properties can be produced. In addition, I have found that these resin mixtures provide enhanced bubble stability during the blown film process and facilitate the manufacturing process .
The amount of low density polyethylene as hereinbefore defined present in the polyethylene blend prior to blowing into film can be readily determined by the skilled man to be that amount which provides ~-acceptable enhanced tear properties. Typically, the blend comprises 5-40% w/w preferably, 15-20% w/w low density polyethylene and offers uni-axially oriented film . ~, :':
Field Of The Invention This invention relates to uni-axially oriented polyethylene films and to thermoplastic, heavy duty shipping sacks made therefrom.
- Background of the Invention Thermoplastic sacks are used in the packaging, transportation or storage of a great variety of materials ranging from powders and granules, bulky and lightweight materials, and agricultural materials such as hay and silage. The thermoplastic sacks according to this invention have general applicability to such products.
Bulky, but lightweight materials such as fiberglass insulation and peat moss are generally shipped in compressed form in thermoplastic sacks. These sacks are generally known as tubular insulation sacks or bags ,1 25 and take the form of an extended envelope or tube sealed at one end prior to its being filled with product. For the most part these sacks are produced by the commonly known in the art ~blown film" process, which owes its popularity to the fact that it can be quickly and readily adapted to the production of different widths and ~` 21146~8 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 appreciated that the thlnner the film thickness (gauge), commensurate with acceptable film properties, the less the amount of thermoplastic material required. This downgauging of sack wall thickness is a most desirable industrial goal. Walls of sacks produced as tubes by the blown film process, typically, have a film thickness in the range of 3-6 mil (75-150X10~ cm) which is generally determined by the machine direction (M.D.) tensile strength necessary to handle the package weight, the film stretch resistance required to prevent expansion of the compressed product and the puncture resistance of the bag for distribution handling. The tubes from which these sacks are commonly made are produced with a bubble diameter/die diameter generally of 3:1 in order to optimize film strength properties.
Although various attempts have been made to use high density polyethylene for the manufacture of down-gauged bags because of its high stretch resistance and tensile strength these have largely been abandoned because of poor tear resistance and puncture properties.
In view of this, polyethylene inæulation 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 is 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 unbalanced physical properties e.g. poor M.D. tear strengths, of these oriented films, - 21146~8 _ 3 _ SL371 which causes "splittiness", they have been ignored for use in tubular shipping sacks.
Claimed in our United States Patent No. 4677007, issued June 30, 1987, are heavy duty shipping sacks and films formed of cold drawn uni-axially oriented linear low density polyethylene film produced by blowing and cold drawing the polyethylene at a draw ratio to blow ratio of between 2:1 and 5:1. This reference, thus, teaches that when blown film is produced at desirable blow ratios for impact improvement, i.e. 2.5 to 1 or greater, the film must be cold drawn to at least 5 to 1 in order to obtain acceptable M.D. tear properties.
While film produced by the process disclosed in USP
4677007, has good impact properties and three times the puncture resistance of an unoriented film, it requires an inner liner of non-oriented film to make it heat sealable for heavy duty shipping sack applications.
The results of experiments disclosed -hereinafter show that linear low density polyethylene films blown at a blow ratio of 2.5 to 1 and cold drawn to 3 to 1 may be satisfactorily heat sealed without the need of an inner liner of non-oriented film. However, the -M.D. tear strength falls to about 30 GMS/MIL, which is unacceptable for heavy duty shipping sack applications.
USP 4677007 teaches against the desirability of making uni-axially oriented polyethylene film of a relatively high blow ratio unless accompanied by a much hlgher draw ratio. Surprisingly, I have now found that a high blow ratio film having a relatively lower draw ratio thani that claimed in USP 4677007 has desirable physical properties to enable satisfactory heavy duty shipping sacks to be manufactured therefrom. Further, I
have found that the film of the present invention does not need to have a high blow ratio or a high draw ratio to possess valuable strength and heat sealability :, :.'..: ,.
.. .: , .. ~
---` 211~6~8 properties. Provided the physical properties as hereinafter defined with reference to the thermographic characteristics of the film are satisfied, such film has utility in the thermoplastic shipping sack art.
Thus, the present invention satisfies the need for a film of use in a heavy duty shipping sack and having acceptable heat sealability and M.D. tear strength.
Summary of the Invention Surprisingly, I have now found that a uni-axially oriented film having good M.D. tear resistance and heat sealability can be produced suitable for use in the production of heavy duty downgauged shipping sacks not requiring an inner liner of non-oriented film.
Accordingly, in one aspect the invention provides a thermoplastic sack comprising heat sealable uni-axially oriented film formed of a blend of 60-95% w/w linea~ low density polyethylene and 5-40% w/w branched ethylene polymer made by the high pressure process selected from the group consisting of branched polyethylene, branched polyethylene copolymer and mixtures thereof, wherein said branched polymer has a differential scanning calorimetric thermogram melting peak temperature that substantially coincides with the thermogram temperature at which 25-35% of the linear low density polyethylene crystallites melt; and wherein said ' 30 film is produced by the following process:
extruding molten said polyethylene blend 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 21146~8 bubble diameter to said die diameter de~ining a blow ratio (BR); and cold drawing said tube in a single direction to provide a drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ration (DR) such that a DR/BR ratio falls below 2:1.
The term "linear low density polyethylene" as used in this specification and claims includes ultra linear low density polyethylene and linear low density ethylene copolymers with the lower olefins such as, for example, butene, n-hexene, 4-methyl-1-pentene and octene. ~-The term "branched ethylene polymer" includes low density ethylene homopolymers and copolymers, such as, for example, acrylic acid, vinyl acetate, methyl methacrylate copolymers, and blends thereof. ~ -~
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 diameter to the die diameter. Such terms are well known in the art.
Preferably, the draw ratio to blow ratio (DR/BR) ratio falls between 1:1 to 2:1. Preferably, the branched polyethylene copolymer is a copolymer of ethylene and an unsaturated compound selected from vinyl acetate, methyl methacrylate and acrylic acid. More preferably, the copolymer is an ethylene-vinyl acetate copolymer having a vinyl acetate context of 6-15% w/w, ' 30 most preferably 9-12% w/w.
By using the correct blend of linear and -~
branched polyethylene and/or branched polyethylene copolymer resins of defined physical characteristics a heat sealable uni-axially oriented film can be manufactured at the lower draw ratios required for heat \. :: ,~
21146~8 sealability and the high blow ratios required for impact strength and still achieve the higher level~ of M.D. tear strength required for the production of heavy duty shipping sacks. The sacks 60 produced typically have twice the puncture resistance of undrawn films and can be downgauged, typically by 30%, while still retaining good M.D. tear properties, typically, 200 GMS/MIL.
The physical properties required of the branched polymer blended with the linear polymer are, ~ -most surprisingly, quite specific and are based on a match of the thermogram melting peak temperature of the branched polymers with the temperature on the thermograph of the linear polymer at which essentially 30% of the latter's crystallites are melted. I have found that the melting peak temperature should be within, preferably, +5 C of the temperature at which 25-35% of the linear -polyethylene crystallites melt.
More preferably, the thermogram melting peak temperature of the branched resin should be within +3 C
of the temperature at which 25-35%, more preferably, approximately 30% crystallites have melted.
The thermogram melting peak temperature of the branched ethylene polymer is the temperature at which the thermogram peaks, typically, when approximately 90% of ;
the polymer has melted.
The thermogram characteristics of resins of use in the practice of the invention can be readily determined by standard methods well-known in the art.
Such characteristic~ can be determined by using a ' 30 differential scanning calorimeter, which measures or charts the temperature rise between a resin material under test and a comparative standard material. Each material is heated, in separate calorimeter cells, typically until the resin under test has fully melte*, and the heat taken up, per unit of time, by each material : '' .. .~ ..
: ~
: . ~
21146~8 is measured. In an alternative differential scanning technique, measurements of temperature against energy input/time for the two materials being heated are determined and take the form of measuring the additional heat supplied to the test material relative to that provided to the standard material for each material to have the same temperature rise.
I have thus found that a shipping sack comprising film having improved film stretch resistance and high tensile strength in addition to acceptable tear resistance comparable to that for non-oriented film and in contrast to the expected usually reduced tear resistance for uni-axially oriented polyethylene film can be manufactured.
The linear low density polyethylene may, optionally also contain a minor amount of high density polyethylene when extra heat resistance is required of the sack.
Thus, by blending, in the defined amounts, a linear polymer with a branched polymer having the defined characteristic of thermogram melting peak relative to the linear polymer thermogram criteria as defined, a uni-axially oriented film having enhanced tear properties can be produced. In addition, I have found that these resin mixtures provide enhanced bubble stability during the blown film process and facilitate the manufacturing process .
The amount of low density polyethylene as hereinbefore defined present in the polyethylene blend prior to blowing into film can be readily determined by the skilled man to be that amount which provides ~-acceptable enhanced tear properties. Typically, the blend comprises 5-40% w/w preferably, 15-20% w/w low density polyethylene and offers uni-axially oriented film . ~, :':
for use in shipping sacks, according to the invention, which could be downgauged by 30~.
The typical blown film process basically comprises the steps of extruding molten thermoplastic material through a circular die, typically of 0.05"
diameter gap, to form a tube which is closed by passing the end through a set of nip/draw rolls. 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 ink application when next passed - ~
through a flexographic stack press. The tube is then ~-reinflated by passing it through two sets of nip rolls with an air bubble trapped between them while the edge of the tube is tucked by forming plates just prior to the second set of nips in order to form any required gusset ;
in 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 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 manufaature of sacks according to the invention the above general ~ ;
' 30 process is modified in several ways. The first change is that the die diameter u6ed i6 increa6ed 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 sack6. The second change i6 that a cold drawing section is introduced .
~, ~
. . ~ .:
:~, 21146~8 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. 105 C., where a minimum of force is required for cold drawing and a series of closely spaced rolls through 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 linear speed, typically, of three times greater than the bubble nip draw rolls.
The blown film thickness is typically three times greater than the required thickness of the finished sack. Thus, since the desired finished sack film thickness is typically 2-3 mil the initial blown film thickness must be 6-9 mil.
The above general process has been modified in yet several more ways. It is known in the art that when a tubular film is cold drawn it emerges from the draw nip rolls with exceedingly weak (ca. lO gm/mil) edgefold strength as measured by the standard dart impact test.
Such an edgefold strength does not provide tubular film suitable for shipping sack use. I have heat anneal the edgefolds at a temperature ca. 80 C. in order to increase the fold impact strength up to the order of 30 gm/mil, to provide film suitable for tubular shipping sacks.
Excessive "blocking" or internal heat tacking of the walls of the tubular film during the cold drawing process may be overcome by the addition of an inorganic filler e.g. ~ilica or calcium carbonate powder, at a typical concentration of 1% w/w to the formulation. -The heat seal produced in the tube, i.e., the two flattened sides (films) of the tube, by the end seal head in the process hereinabove described is produced 2~146~8 under a combination of pressure and heat, at or above the films' crystalline 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 or chemical means. It is known that heat build-up during the sealing operation may be sufficient to destroy the orientation of uni-axially oriented films in the vicinity of the heat seal and thus cause serious loss of draw-induced impact strength. I have found that sacks manufactured by the process hereinbefore described have sufficient impact strength suitable for the intended duty purpose for which the sacks are made. Further, I have found that the most valuable tear resistance properties of the film are not significantly reduced in contrast to the expected usually reduced tear resistance for uni-axially oriented polyethylene film. 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 without the need for a non uni-axially oriented material inner layer or film.
It has thus been found that a suitable open-top tubular polyethylene shipping sack having improved puncture resistance and M.D. tensile strength, while still retaining acceptable tear and edgefold impact strength, can be manufactured using suitably modified conventional blown film process apparatus.
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 specific process as hereinbefore described or by alternative processes known in the art which may or may not involve the "back-sealing~ of an oriented film.
In addition, tubular shipping sack~ of alternative structure to the simple open-top sack ... . - - , . .
- . : : . .:: ::~ : :
: . . ~: . ~ : : ' ' ' :
21146~8 described hereinabove and utilizing the feature of the invention to provide the promised advantages may be produced. Such an alternative tubular 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 conventional processes well-known in the art suitably modified to provide a sack formed of uni-axially oriented linear polyethylene film produced by blowing and cold drawing a polymer blend having the physical characteristics as hereinabove defined.
Also included within the scope of the invention are those shipping sacks incorporating the feature of the invention wherein the seals or other closures provided in the sacks are formed by adhesive bonding as an alternative to heat sealing. Use of such adhesive bonding provides the advantages promised hereinabove and also improved impact resistance to the sack. This preferably permits use of such sacks for the packaging of bulky and lightweight materials such as, for example, vermiculite insulation material.
Accordingly, the invention provides a thermoplastic sack having a front wall and a back wall, each of said front wall and said back wall comprising a ply of said uni-axially oriented linear polyethylene and branched polyethylene blend as hereinabove defined, said ply being produced by a blowing and cold drawing said polyethylene blend at a draw ratio to blow ratio of less than 2:1, preferably between 1:1 to 2 While the foregoing disclosure has made particular reference to thermoplastic sacks in the form of tubular sacks suitable for use with lightweight and bulky materials, I have found that the aforesaid sacks can be suitably modified to provide an improved heavy ~.
:
.,~'..',~,:, ':: ~: ~. ': .
,~ . - ,' - 12 - S~371 duty thermoplastic shipping sack. Such sacks may be used for the transportation, packaging and storage o~ a wide variety of products in granular or powder form. These sacks may also be of the open-top type, requiring separate provision for closing, or fitting with a valved opening.
Disclosed in commonly assigned U.S. Patent No. 4576844, issued March 18, 1986, are heavy duty shipping sacks comprised of a double layer of non cold-drawn low density polyethylene interposed between two plies of cross-laminated uni-axially oriented linear polyethylene film. However, I have now found that a much cheaper thermoplastic shipping bag than the aforesaid cross-laminated structured bag can be manufactured having both excellent heat sealability and puncture resistant properties. -I have surprisingly found that two layers of uni-axially oriented polyethylene suitably blended with defined branched polyethylene can be welded directly to each other as films, laminates or plies constituting the walls of a shipping bag without there being sufficient ~;
heat build up to cause serious loss of cold draw-induced film strength. Thus, an acceptable bridge between a high strength uni-axially oriented film and the body of the heat seal is formed. This is to be contrasted with the fact that although two uni-axially oriented films of the prior art in the absence of interposed low density polyethylene film could be melted and fused together to produce welded bonds, the uni-axially oriented film immediately adjacent to the welded mass has its cold draw orientation reduced by the heat from the seal with consequent reduction of film strength in this margin area; whereby the seals so produced are sufficiently weak and brittle in the margin area, so as to render them unacceptable for use in heavy duty shipping bags.
...
211~6~8 It ha6 thus now been found that a suitable thermoplastic shipping bag having improved puncture and snag resistance without the need for interposed low density film can now be reliably manufactured by heat sealing techniques using suitably modified conventional equipment when formed of the polymeric blend according to the present invention.
Thus, in a further aspect the invention provides a thermoplastic shipping bag having a front wall and a back wall, each of said front wall and said back wall comprising a ply of uni-axially oriented linear polyethylene blend produced by blowing and cold drawing said linear and branched polyethylene blend as hereinbefore defined. ;~
It is within the scope of the present invention that there may also be present interposed layers of low density polyethylene. Each of the interposed layers of low density polyethylene may constitute simply a sheet of polyethylene laminated to a surface of the uni-axially oriented ply and being of sufficient thickness in the ~ -heat seal area to effect a possible enhancement of the bridge between the two uni-axially oriented plies in this ~-area to form a seal. However, each of these interposed ~ ~ ^
layers of low density polyethylene may extend beyond the heat seal area to represent a laminated layer on the respective full surface of each of the uni-axially oriented plies. Thus, each of the uni-axially oriented plies comprising the walls of the shipping bag may have a layer of low density polyethylene laminated thereto.
Such a structure, of course, does not detract from the requirement that the uni-axially oriented plies need only be heat sealed at designated heat seal areas. These areas constitute those parts of the bag, generally parts of the periphery, where the front and back walls are joined by heat sealing during manufacture.
211~658 Where the layers of low density polyethylene are represented as laminated sheets on the uni-axially oriented plies, each of the sheets must be of sufficient thickness to enhance the bridge between the two uni-axially oriented plies.
In one form of the invention, the sack comprises interposed layers of low density polyethylene constituting full and distinct plies as walls of the sack.
In this form of sack each of the walls comprising a uni-axially oriented ply has an interposing ply of low density polyethylene associated therewith. In this arrangement, each of the interposing plies may be considered as being an inner wall of the sack while the two uni-axially oriented plies are considered as being the two outer walls.
The terms "inner wall" and "inner ply" are meant not to be restricted solely to the actual or true inner wall or ply of the sack which contacts product when the sack is filled. The terms also include the situation, for example, where one or more plies of non-oriented low density polyethylene constitute plies in a multi-wall sack which plies may or may not be adjacent the true inner wall or ply. Similarly, the terms "outer wall" or "outer ply" are meant not to be restricted solely to the most external wall or ply.
Thus, it should be understood that the principles of the invention are applicable also to the fabrication of sacks having walls individually comprising more than two plies. Thus, the invention embraces sacks having three plies, four plies, etc. The important feature for a heavy duty sack is that there must be either a laminated layer of or at least one ply of uni-axially oriented polyethylene blend as hereinabove defined constituting each of the inner surfaces or inner : .
-` 21146~8 walls of the sack such that said uni-axially oriented ply of polyethylene blend contacts another uni-axially oriented ply of polyethylene blend at a designated heat seal area of an inner surface such as to provide a heat seal when heat seal strength is a desired feature.
In embodiments of the heavy duty sacks according to the invention as hereinbefore and hereinafter defined any interposed layer of low density polyethylene represented either as a laminated sheet on the uni-axially oriented ply or as a distinct inner ply or inner wall, is formed of blown linear low density polyethylene. However, it is readily apparent that cast films are also suitable for this application.
A single ply sack is the simplest embodiment of this heavy duty sack. However, in some instances, it is advantageous to have more than two inner plies of non-oriented film constituting the inner layers of the sack, i.e., between the front and back uni-axially oriented outer sides of the sack. An example of this would be a sack of the simplest embodiment with an additional thin true inner ply of linear low density polyethylene in the form of a fine filter mesh to allow air to be filtered from powdered products, as described in our commonly assigned U.S. Patent No. 4672684, issued June 9, 1987.
In other instances it may be preferred to have additional plies of film outermost of the uni-axially oriented ply. Such an outer ply could give the benefit resulting from introducing blown low density polyethylene film between the gussetted surfaces of uni-axially oriented p;lies to give the same improvements in seal quality as created on the innermost parts of the bag.
The squared-off appearance of the final package resulting from this gussetting improves its performance for palletizing and stacking.
'. ''' ' '~ '''' .
2~146~8 An additional benefit to be gained from such an outer layer is that the surface can be suitably roughened by the addition of high molecular weight granules to the film during film extrusion; thus, imparting additional improved handling properties to the sack. As well, the inner surface of this outer ply can be printed and the resulting message thus locked between plies to escape abrasion and distortion during the handling of filled packages. It can easily be seen that the utility of this outer ply can be expanded by using a laminate or coextrusion film to impart special properties to the bag, i.e., oil barrier or grease resistant layers.
The utility of the present invention thus lies in the fact that although plies additional to the uni-axially oriented film according to the invention may be present in the sacks according to the invention for individual and disparate purposes, such additional plies are not necessary to effect satisfactory heat sealing and puncture resistance properties. Thus, both open-top and valved-top type heavy duty shipping sacks, suitable for the packaging of expensive or hazardous materials, can be reliably manufactured using commonly available heat seal ~ -sack making equipment.
The open-top shipping sack for heavy duty use may be made by feeding a web of the uni-axially oriented blend film through commercial side-weld, heat sealed or back seamed and bottom heat sealed bag making equipment.
one particularly useful type of a thermoplastic shipping sack is that known as a valved bag. One such embodiment is described in U.S. Pat. No. 3,833,166.
These bags possess the important commercial advantage of being easily filled through a valve structure with the self-closing of this valve structure after filling. The heavy duty sacks according to the invention are of particular value in the form of a valved bag. `~ ~-~ ' ' ~ : ' ' ,: ' '.', ' ' - :
211~6~8 While it is generally accepted that all polyethylene film i8 generally uni-axially oriented" to some degree, the term "uni-axially oriented" when used with reference to linear polyethylene in this specification and claims means polyethylene film that has been blown and cold drawn to at least a 2.0-fold extent, preferably to a 3-fold extent, but also up to a 6-fold extent, provided that the draw ratio to blow ratio is less than 2:1. The orienting of the films may be carried out by the cold drawing of the blown tube as hereinbefore described.
Cold drawn uni-axially co-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 linear low density and low density polyethylene in the ratio 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 polyethylene produced as hereinbefore defined and a layer of a low density ethylene polymer or copolymer compatible with said uni-axially oriented linear polyethylene. Examples of such compatible copolymers of use in the invention are ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers and ethylene-methyl methacrylate 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 the invention as i8 applicable to a laminate the compatible ethylene polymer or copolymer is also subjected to the novel same draw ratio to blow ratio 21146~8 subsequent to the co-extrusion step as i8 the uni-axially oriented linear polyethylene blend.
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% ethylenevinyl acetate, providing an external surface of the sack it lo provides superior anti-slip properties. Where a 20~
ethylenemethyl 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. The co-extruded laminate may comprise two or more compatible layers as is deemed appropriate.
Also embraced within the scope of this invention are sacks formed of films comprising a laminate formed by adhesive lamination of suitable films. Multi-laminated plies may be used wherein one laminate layer constitutes a barrier layer to the movement of chemical vapour through the sack walls.
Accordingly, the invention further provides an open-top tubular shipping sack as hereinbefore defined wherein said film or ply of uni-axially oriented polyethylene blend forms part of a multi-layer laminate with one or more layers of one or more compatible ethylene polymers or copolymers.
' 30 In a further aspect, the invention provides a thermoplastic film suitable for use for a shipping sack said film formed of uni-axially oriented polyethylene blend prepared by the blowing and cold drawing of said polyethylene at a draw ratio to blow ratio as hereinbefore defined.
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211~6~
The typical blown film process basically comprises the steps of extruding molten thermoplastic material through a circular die, typically of 0.05"
diameter gap, to form a tube which is closed by passing the end through a set of nip/draw rolls. 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 ink application when next passed - ~
through a flexographic stack press. The tube is then ~-reinflated by passing it through two sets of nip rolls with an air bubble trapped between them while the edge of the tube is tucked by forming plates just prior to the second set of nips in order to form any required gusset ;
in 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 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 manufaature of sacks according to the invention the above general ~ ;
' 30 process is modified in several ways. The first change is that the die diameter u6ed i6 increa6ed 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 sack6. The second change i6 that a cold drawing section is introduced .
~, ~
. . ~ .:
:~, 21146~8 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. 105 C., where a minimum of force is required for cold drawing and a series of closely spaced rolls through 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 linear speed, typically, of three times greater than the bubble nip draw rolls.
The blown film thickness is typically three times greater than the required thickness of the finished sack. Thus, since the desired finished sack film thickness is typically 2-3 mil the initial blown film thickness must be 6-9 mil.
The above general process has been modified in yet several more ways. It is known in the art that when a tubular film is cold drawn it emerges from the draw nip rolls with exceedingly weak (ca. lO gm/mil) edgefold strength as measured by the standard dart impact test.
Such an edgefold strength does not provide tubular film suitable for shipping sack use. I have heat anneal the edgefolds at a temperature ca. 80 C. in order to increase the fold impact strength up to the order of 30 gm/mil, to provide film suitable for tubular shipping sacks.
Excessive "blocking" or internal heat tacking of the walls of the tubular film during the cold drawing process may be overcome by the addition of an inorganic filler e.g. ~ilica or calcium carbonate powder, at a typical concentration of 1% w/w to the formulation. -The heat seal produced in the tube, i.e., the two flattened sides (films) of the tube, by the end seal head in the process hereinabove described is produced 2~146~8 under a combination of pressure and heat, at or above the films' crystalline 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 or chemical means. It is known that heat build-up during the sealing operation may be sufficient to destroy the orientation of uni-axially oriented films in the vicinity of the heat seal and thus cause serious loss of draw-induced impact strength. I have found that sacks manufactured by the process hereinbefore described have sufficient impact strength suitable for the intended duty purpose for which the sacks are made. Further, I have found that the most valuable tear resistance properties of the film are not significantly reduced in contrast to the expected usually reduced tear resistance for uni-axially oriented polyethylene film. 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 without the need for a non uni-axially oriented material inner layer or film.
It has thus been found that a suitable open-top tubular polyethylene shipping sack having improved puncture resistance and M.D. tensile strength, while still retaining acceptable tear and edgefold impact strength, can be manufactured using suitably modified conventional blown film process apparatus.
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 specific process as hereinbefore described or by alternative processes known in the art which may or may not involve the "back-sealing~ of an oriented film.
In addition, tubular shipping sack~ of alternative structure to the simple open-top sack ... . - - , . .
- . : : . .:: ::~ : :
: . . ~: . ~ : : ' ' ' :
21146~8 described hereinabove and utilizing the feature of the invention to provide the promised advantages may be produced. Such an alternative tubular 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 conventional processes well-known in the art suitably modified to provide a sack formed of uni-axially oriented linear polyethylene film produced by blowing and cold drawing a polymer blend having the physical characteristics as hereinabove defined.
Also included within the scope of the invention are those shipping sacks incorporating the feature of the invention wherein the seals or other closures provided in the sacks are formed by adhesive bonding as an alternative to heat sealing. Use of such adhesive bonding provides the advantages promised hereinabove and also improved impact resistance to the sack. This preferably permits use of such sacks for the packaging of bulky and lightweight materials such as, for example, vermiculite insulation material.
Accordingly, the invention provides a thermoplastic sack having a front wall and a back wall, each of said front wall and said back wall comprising a ply of said uni-axially oriented linear polyethylene and branched polyethylene blend as hereinabove defined, said ply being produced by a blowing and cold drawing said polyethylene blend at a draw ratio to blow ratio of less than 2:1, preferably between 1:1 to 2 While the foregoing disclosure has made particular reference to thermoplastic sacks in the form of tubular sacks suitable for use with lightweight and bulky materials, I have found that the aforesaid sacks can be suitably modified to provide an improved heavy ~.
:
.,~'..',~,:, ':: ~: ~. ': .
,~ . - ,' - 12 - S~371 duty thermoplastic shipping sack. Such sacks may be used for the transportation, packaging and storage o~ a wide variety of products in granular or powder form. These sacks may also be of the open-top type, requiring separate provision for closing, or fitting with a valved opening.
Disclosed in commonly assigned U.S. Patent No. 4576844, issued March 18, 1986, are heavy duty shipping sacks comprised of a double layer of non cold-drawn low density polyethylene interposed between two plies of cross-laminated uni-axially oriented linear polyethylene film. However, I have now found that a much cheaper thermoplastic shipping bag than the aforesaid cross-laminated structured bag can be manufactured having both excellent heat sealability and puncture resistant properties. -I have surprisingly found that two layers of uni-axially oriented polyethylene suitably blended with defined branched polyethylene can be welded directly to each other as films, laminates or plies constituting the walls of a shipping bag without there being sufficient ~;
heat build up to cause serious loss of cold draw-induced film strength. Thus, an acceptable bridge between a high strength uni-axially oriented film and the body of the heat seal is formed. This is to be contrasted with the fact that although two uni-axially oriented films of the prior art in the absence of interposed low density polyethylene film could be melted and fused together to produce welded bonds, the uni-axially oriented film immediately adjacent to the welded mass has its cold draw orientation reduced by the heat from the seal with consequent reduction of film strength in this margin area; whereby the seals so produced are sufficiently weak and brittle in the margin area, so as to render them unacceptable for use in heavy duty shipping bags.
...
211~6~8 It ha6 thus now been found that a suitable thermoplastic shipping bag having improved puncture and snag resistance without the need for interposed low density film can now be reliably manufactured by heat sealing techniques using suitably modified conventional equipment when formed of the polymeric blend according to the present invention.
Thus, in a further aspect the invention provides a thermoplastic shipping bag having a front wall and a back wall, each of said front wall and said back wall comprising a ply of uni-axially oriented linear polyethylene blend produced by blowing and cold drawing said linear and branched polyethylene blend as hereinbefore defined. ;~
It is within the scope of the present invention that there may also be present interposed layers of low density polyethylene. Each of the interposed layers of low density polyethylene may constitute simply a sheet of polyethylene laminated to a surface of the uni-axially oriented ply and being of sufficient thickness in the ~ -heat seal area to effect a possible enhancement of the bridge between the two uni-axially oriented plies in this ~-area to form a seal. However, each of these interposed ~ ~ ^
layers of low density polyethylene may extend beyond the heat seal area to represent a laminated layer on the respective full surface of each of the uni-axially oriented plies. Thus, each of the uni-axially oriented plies comprising the walls of the shipping bag may have a layer of low density polyethylene laminated thereto.
Such a structure, of course, does not detract from the requirement that the uni-axially oriented plies need only be heat sealed at designated heat seal areas. These areas constitute those parts of the bag, generally parts of the periphery, where the front and back walls are joined by heat sealing during manufacture.
211~658 Where the layers of low density polyethylene are represented as laminated sheets on the uni-axially oriented plies, each of the sheets must be of sufficient thickness to enhance the bridge between the two uni-axially oriented plies.
In one form of the invention, the sack comprises interposed layers of low density polyethylene constituting full and distinct plies as walls of the sack.
In this form of sack each of the walls comprising a uni-axially oriented ply has an interposing ply of low density polyethylene associated therewith. In this arrangement, each of the interposing plies may be considered as being an inner wall of the sack while the two uni-axially oriented plies are considered as being the two outer walls.
The terms "inner wall" and "inner ply" are meant not to be restricted solely to the actual or true inner wall or ply of the sack which contacts product when the sack is filled. The terms also include the situation, for example, where one or more plies of non-oriented low density polyethylene constitute plies in a multi-wall sack which plies may or may not be adjacent the true inner wall or ply. Similarly, the terms "outer wall" or "outer ply" are meant not to be restricted solely to the most external wall or ply.
Thus, it should be understood that the principles of the invention are applicable also to the fabrication of sacks having walls individually comprising more than two plies. Thus, the invention embraces sacks having three plies, four plies, etc. The important feature for a heavy duty sack is that there must be either a laminated layer of or at least one ply of uni-axially oriented polyethylene blend as hereinabove defined constituting each of the inner surfaces or inner : .
-` 21146~8 walls of the sack such that said uni-axially oriented ply of polyethylene blend contacts another uni-axially oriented ply of polyethylene blend at a designated heat seal area of an inner surface such as to provide a heat seal when heat seal strength is a desired feature.
In embodiments of the heavy duty sacks according to the invention as hereinbefore and hereinafter defined any interposed layer of low density polyethylene represented either as a laminated sheet on the uni-axially oriented ply or as a distinct inner ply or inner wall, is formed of blown linear low density polyethylene. However, it is readily apparent that cast films are also suitable for this application.
A single ply sack is the simplest embodiment of this heavy duty sack. However, in some instances, it is advantageous to have more than two inner plies of non-oriented film constituting the inner layers of the sack, i.e., between the front and back uni-axially oriented outer sides of the sack. An example of this would be a sack of the simplest embodiment with an additional thin true inner ply of linear low density polyethylene in the form of a fine filter mesh to allow air to be filtered from powdered products, as described in our commonly assigned U.S. Patent No. 4672684, issued June 9, 1987.
In other instances it may be preferred to have additional plies of film outermost of the uni-axially oriented ply. Such an outer ply could give the benefit resulting from introducing blown low density polyethylene film between the gussetted surfaces of uni-axially oriented p;lies to give the same improvements in seal quality as created on the innermost parts of the bag.
The squared-off appearance of the final package resulting from this gussetting improves its performance for palletizing and stacking.
'. ''' ' '~ '''' .
2~146~8 An additional benefit to be gained from such an outer layer is that the surface can be suitably roughened by the addition of high molecular weight granules to the film during film extrusion; thus, imparting additional improved handling properties to the sack. As well, the inner surface of this outer ply can be printed and the resulting message thus locked between plies to escape abrasion and distortion during the handling of filled packages. It can easily be seen that the utility of this outer ply can be expanded by using a laminate or coextrusion film to impart special properties to the bag, i.e., oil barrier or grease resistant layers.
The utility of the present invention thus lies in the fact that although plies additional to the uni-axially oriented film according to the invention may be present in the sacks according to the invention for individual and disparate purposes, such additional plies are not necessary to effect satisfactory heat sealing and puncture resistance properties. Thus, both open-top and valved-top type heavy duty shipping sacks, suitable for the packaging of expensive or hazardous materials, can be reliably manufactured using commonly available heat seal ~ -sack making equipment.
The open-top shipping sack for heavy duty use may be made by feeding a web of the uni-axially oriented blend film through commercial side-weld, heat sealed or back seamed and bottom heat sealed bag making equipment.
one particularly useful type of a thermoplastic shipping sack is that known as a valved bag. One such embodiment is described in U.S. Pat. No. 3,833,166.
These bags possess the important commercial advantage of being easily filled through a valve structure with the self-closing of this valve structure after filling. The heavy duty sacks according to the invention are of particular value in the form of a valved bag. `~ ~-~ ' ' ~ : ' ' ,: ' '.', ' ' - :
211~6~8 While it is generally accepted that all polyethylene film i8 generally uni-axially oriented" to some degree, the term "uni-axially oriented" when used with reference to linear polyethylene in this specification and claims means polyethylene film that has been blown and cold drawn to at least a 2.0-fold extent, preferably to a 3-fold extent, but also up to a 6-fold extent, provided that the draw ratio to blow ratio is less than 2:1. The orienting of the films may be carried out by the cold drawing of the blown tube as hereinbefore described.
Cold drawn uni-axially co-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 linear low density and low density polyethylene in the ratio 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 polyethylene produced as hereinbefore defined and a layer of a low density ethylene polymer or copolymer compatible with said uni-axially oriented linear polyethylene. Examples of such compatible copolymers of use in the invention are ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers and ethylene-methyl methacrylate 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 the invention as i8 applicable to a laminate the compatible ethylene polymer or copolymer is also subjected to the novel same draw ratio to blow ratio 21146~8 subsequent to the co-extrusion step as i8 the uni-axially oriented linear polyethylene blend.
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% ethylenevinyl acetate, providing an external surface of the sack it lo provides superior anti-slip properties. Where a 20~
ethylenemethyl 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. The co-extruded laminate may comprise two or more compatible layers as is deemed appropriate.
Also embraced within the scope of this invention are sacks formed of films comprising a laminate formed by adhesive lamination of suitable films. Multi-laminated plies may be used wherein one laminate layer constitutes a barrier layer to the movement of chemical vapour through the sack walls.
Accordingly, the invention further provides an open-top tubular shipping sack as hereinbefore defined wherein said film or ply of uni-axially oriented polyethylene blend forms part of a multi-layer laminate with one or more layers of one or more compatible ethylene polymers or copolymers.
' 30 In a further aspect, the invention provides a thermoplastic film suitable for use for a shipping sack said film formed of uni-axially oriented polyethylene blend prepared by the blowing and cold drawing of said polyethylene at a draw ratio to blow ratio as hereinbefore defined.
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211~6~
In yet a further aspect, the invention provides a thermoplastic film as hereinabove defined and wherein said film forms part of a multi-layer laminate with one or more layers of one or more compatible ethylene polymers or copolymers. The layer of the compatible ethylene polymer is at least 0.5 mil thick and, preferably, at least 1.5 mil thick. Preferably, the compatible ethylene polymer is low density polyethylene.
Brief Descrition of the Drawings Several embodiments of this invention will now be described by way of example only with reference to the accompanying drawings, in which:
Fig.1 shows a front elevational view, partly cut away, of an open-top tubular sack according to the invention;
Fig.2 is a sectional view along 2-2 of Fig.1;
Fig.3 is a front elevational view of an open-top heavy duty sack according to the invention;
Fig.4 is a sectional view along line 2-2 of Fig.3;
Fig.5 i8 a front elevational view of a heavy duty valved bag according to the invention; -Fig.6 is a sectional view along the line 4-4 of Fig.5;
Fig.7 is a diagrammatic view of a section through a heat seal used in the practice of the invention;
Fig.8 is a cross-sectional view of a preferred laminate of a thermoplastic material according to the invention; and Fig. 9 shows differential scanning calorimetric thermograms of several polyethylene resins according to and outside of the invention.
21146~8 Detailed Descri~tion of the Preferred Embodiment Figs.l and 2 show a generally rectangular single ply tubular sack 1 having a front wall 2 and a back wall 3 formed of a co-extruded and cold drawn polyethylene blend consisting of linear low density polyethylene (3 parts by weight, melt index 1.0; m.p.
121 C., - DOWLEX 2045 from Dow Chemical Company) and low density polyethylene (1 part by weight, melt index 0.8, m.p. 100 C. - AT1080 resin, copolymer of ethylene-vinyl acetate (91% w/w - 9% w/w) - AT Plastics Inc., Brampton, Ontario). One end of the tubular sack is heat sealed to form a single ply open-top sack.
The sack is made by extruding the above resin blend and blowing, cold drawing and heat annealing the film on modified conventional equipment. The flattened tube (27" 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 5-1/2~ gussets. The process is operated with the parameters as given for resin No. 7 in the Table hereinbelow. The sack has a most valuable tear resistance of 300 gm/mil and an edgefold impact strength of 30 gms/mil, while having improved puncture resistance and M.D. tensile strength.
A series of resin blends comprising DOWLEX 2045 resin and branched low density polyethylene and polyethylene vinyl acetate copolymers were blown at a blow ratio of 2.5 to 1 to produce a series of 6 MIL
films. The films were cold drawn through a range of draw ratios and measured for Elmendorf tear strengths. The process parameters and results are given in the following Table.
. ,.,.,, ~.. .. .
211~6~8 TAB,L,,E,, SAMPLE RESIN (LLDPE- DRAW DRAW RATIO/ ELMENDORF
NO. DOWLEX 2045) RATIO BLOW RATIO TEAR (GMS/M
1 100% LLDPE 2.0:1 0.8 40 100% ~ 3.0:1 1.2 50 100% n 4.1:1 1.64 50 100% ~ 5.5:1 2.2 80 100% ~ 6.4:1 2.55 190 2 80% LLDPE
20% AT540E 2.0:1 0.8 50 " " 3.0:1 1.2 60 n ~ 3.8:1 1.52 80 5.2:1 2.08 220 " ~'' 6.6:1 2.64 180 3 80% LLDPE
20% AT1520 2.1:1 0.95 60 ~ 3.2:1 1.28 40 n ~ 5.3:1 2.12 30 n ~ 6.4:1 2.55 70 4 80% LLDPE
20% AT1211 2.0:1 0.8 80 ~ 3.1:1 1.24 150 n ~ 4.2:1 1.68 240 ~ 5.5:1 2.2 290 90% LLDPE
10% AT1211- 2.2:1 0.9 60 n ~- 3.0:1 1.2 100 n ~ 4.0:1 1.6 100 6 60% LLDPE
40% AT1211 2.0:1 0.8 80 3.0:1 1.2 150 ~ 4.1:1 1.64 280 7 75% LLDPE
25% AT1080 2.1:1 0.95 80 n ~ 3.0:1 1.2 230 n ~ 3.8:1 1.52 300 8 80% LLDPE
20% AT1060~3.6:1 1.44 80 : -., ~;
9 80% LLDPE
20% ULLLDPE~ 4.0:1 1.6 40 21146~8 80% LLDPE
20% AT1230t~ 3.7:11.48 200 11 80% LLDPE
20% UCC6862- 3.4 1.36 70 AT540E has M.I. 0.8, N.P. 111 C., low density polyethylene - AT Plastics Inc., Brampton, Ontario.
", AT1520 has M.I. 2.0; M.P. 87 C., low density ethylene-vinyl acetate copolymer (15% vinyl acetate) - AT Plastics Inc., Brampton, Ontario.
~--AT1211 has M.I. 0.3; M.P. 95C., low density ethylene-vinyl acetate copolymer (12% vinyl acetate) - AT Plastics Inc., Brampton, ontario AT1080 has M.I. 0.8; M.P. 100C., low density ethylene-vinyl acetate copolymer (9% vinyl acetate) - AT Plastics Inc., Brampton, Ontario #AT1060 has M.I. 1.0; M.P. 105C., low density ethylene-vinyl acetate copolymer (6% vinyl acetate) - AT Plastics Inc., Brampton, ontario ##ULLLDPE is ATTANE 4001 -##tAT1230 has M.I. 3.0; M.P. 95C., low density ethylene-vinyl acetate copolymer (12% vinyl acetate) - AT Plastics Inc., Brampton, Ontario ~UCC6862 has M.I. 0.15; M.P. 109C., low density ethylene (PEROMONT UCC 6862 - Union Carbide -`
Inc. Donbury. CT) For an informative review of differential scanning calorimetry see "Differential Scanning Calorimetry", J.L. McNaughton and C.T. Mortimer, "IRS; ~-Physical Chemistry", Series 2, 1975, Volume 10, Butterworths, London, England.
: - ~
:
21146~8 Thermograms using a Perkin-Elmer DSC-4 differential scanning calorimeter, system 4 thermal analysis processor and AD 2Z autobalance were obtained for the following resins and provided the following data.
Dowlex 2045: ~ -WT: 4.16 mg.
SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 127.17 CAL/GRAM: 27.04 ENTERED AREA: 27.04 TEMP %
50.13 100 91.8 88.2 83.7 77.9 -100 70.4 105 60.6 110 48.3 127.3 0 Thermogram melting peak temperature: 121C
Temperature at which 70% of crystallites melted: 100C.
. ~, - ::
AT540E:
PEAK FROM: 50.13 TO: 115.95 CAL/GRAM: 28.38 ENTERED AREA: 28.38 TEMP
50.13 100 85.7 ; 85 ~80.3 - ~-73.8 65.9 100 56.3 105 44.2 116.08 0 Thermogram melting peak temperature: 111C
, :,': ., -` 21146~8 -ATls20:
WT: 4.21 mg.
SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 112.2 CAL~GRAM: 11.55 ENTERED AREA: 11.84 TEMP %
50.13 100 61.6 47.5 26.3 .5 100 -2.01 105 -.8 110 -. 1 112.2 0 Thermogram melting peak temperature: 87C
, AT1211:
;
WT: 4.15 mg. -SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 111.58 CAL/GRAM: 15.65 ENTERED AREA: 15.73 ;
TEMP %
50.13 99.5 63.6 915 28.5 100 2.2 105 -.4 110 0 :,, 111.71 0 Thermogram melting peak temperature: 95C ~ - ~
. , ~, .
-` 2114658 WT: 4.16 mg.
SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 112.28 CAL/GRAM: 17.92 ENTERED AREA: 18 TEMP %
50.13 99.5 79.8 70.6 58.5 42.7 100 13.3 105 -.4 110 -. 01 ,, 112.2 0 Thermogram melting peak temperature: 95C
.
-AT1060:
WT: 4.20 mg. -SCAN RATE: 10.00 deg/min PEAK FROM: 50.15 -~
TO: 113.75 CAL/GRAM: 20.96 ENTERED AREA: 20.97 TEMP %
50.15 99.
86.2 79.6 go 71.1 60.2 100 46.1 105 20.5 -1'10 113.74 o ~. :
Thermogram melting peak temperature: 105C
~ 21146~8 UCC6862:
WT: 4O18 mg.
SCAN RATE: 10.00 deg/min - PEAK FROM: 50.13 TO: 111.89 CAL/GRAM: 26.52 ENTERED AREA: 26.52 TEMP %
50.13 100 85.2 79.2 71.6 61.9 100 49.4 -105 30.7 110 .2 112.02 0 Thermogram melting peak temperature: 109C
AT1230:
WT: 4.19 mg.
SCAN RATE: 10.00 deg/min - -PEAK FROM: 49.98 TO: 106.23 -CAL/GRAM: 13.81 ENTERED AREA: 13.82 TEMP %
49.98 99.9 86.
79.1 ,~
69.1 `
56.9 40.9 ~-14.7 106.24 0 Thermogram melting peak temperature: 95C
. - , ,,,.:
Reference is now made to the Table and Fig. 9 which, by way of example, represents the differential ~ ;
, . ~, -,:
: , - - ., .-~
i - ~
~.: ' ' :
~-~` 21146~8 scanning calorimetric thermograms of the following resins denoted as follows:
(I) : DOWLEX 2045 (used in Sample 1) (II) : AT 1080 (used in Sample 7) (III) : AT1520 (used in Sample 3) (IV) : AT540E (used in Sample 2) Points IA, IIA, IIIA and IVA on the graphs denotes the respective, thermogram melting peak temperature of each of the resins. Point B denotes the thermogram temperature at which 30% of the linear low density polyethylene (I) crystallites melt, and the range C denotes the range +5OC of point B. In the thermogram shown B is 100C and range C is 95-105C.
Thus, polymer blends of use in the practise of the invention comprising branched polyethylene resin AT1080 having the thermogram characteristics denoted as II; which resin has a melting peak temperature of 100C, which i8 well within the range C of 95-105C required for use with linear polyethylene (I).
In contrast, branched resins AT1520 and AT540E
have melting peak temperatures of 87C and 111C, respectively, and, thus, fall outside the scope of the present invention.
Thus, from the results shown in the TABLE it can readily be seen that the effective onset in improvement of the tear at low draw ratios occurs fairly sharply when the melting peak temperature of the branched blending resin goes from a M.P. of 87C to one of 95C.
Then the effect tapers off sharply when the M.P. goes from 100C to 105C. The effect is seen to be relatively independent of melt index (M.I.) Specifically, samples 4, 6, 7 and 10 at draw ratio: blow ratio values of less than 2 have excellent - 2~ - SL371 tear resistance, whereas Sample 5 has a lower, but valuable tear strength.
FIGS. 3 and 4 show a generally rectangular two-ply pillow-type sack l-having an inner wall 2 formed of blown linear low density polyethylene film (3 mil) manufactured from "2045" linear low density polyethylene resin (Dow Chemical Co.), and an outer ply 3 (3.5 mil) of uni-axially oriented linear low density polyethylene film made from blowing and cold drawing a polyethylene blend having the parameters as given for resin No. 7 in the Table.
The sack 1 has thus a two-ply back wall 4, and a two-ply iron wall 5 made up of first and second partially overlapping panels 6 and 7. The outer ply 3 of back wall 4 is continuous with the outer wall 3 of front wall 5 except where separated and joined together by heat sealing with layer 2 in the overlapping panels 6 and 7. Thus, the walls 4 and 5 are integral and form a two-ply tube. One end of the tube 8 is heat sealed to form a simple two-ply open-top bag.
The sack is made by feeding a web of 37" film 3 into a longitudinal folding frame with a web of film 2 and forming a two-ply tube 18" wide with a 1" overlapping portion. The four plies of the overlapping area are then heat sealed longitudinally to consolidate the two-ply tubing which is then passed to a transverse heat seal unit to make the bottom seal 8. A 26" length of tube with the heat seal present is cut from the web by a guillotine to form the open top bag 1.
' 30 To test the strength of the heat seals, sack 1 was filled with 50 pounds of granular salt, heat sealed at its open end by a "Dough boy" heat sealer, and drop tested on each side, edge and butt from a height of 10 feet. There was no rupture of any film or seal. The open top of the sack is generally heat sealed after S~
21146~8 filling with product to produce an airtight and watertight package. Because it is extremely difficult to exclude all air from the filled package prior to the heat sealing operation, it is preferable to perforate the walls of the bags with pinholes typically 0.025" in diameter to facilitate air release, the number of holes required depending on the amount of air left in the bag and the type of product being packaged. In those cases where it is critical that the package retains its maximum value for air tightness and moisture protection, the perforation holes in the inner and outer plies are offset typically by 1~" to create an indirect path to air-product mixes during the venting period.
Although the inner ply 2 of the sack is described as a single ply of sheeting it can readily appreciated that a two-ply tube of 1.5 mil could also be used instead. Indeed, since tubing may be less expensive to manufacture the tube could be the preferred option.
FIGS. 5 and 6 show a generally rectangular three-ply pillow type bag 10 having a front side 11 and a back side 12 joined together around the entire periphery of the bag. Front side 11 consists of an inner wall 13 and an outer wall 14 formed of blown linear low density polyethylene (4 mil), and a middle wall 15 of the same uni-axially oriented low density polyethylene film as for FIG. 3 (3.5 mil). Back side 12 is of an identical construction.
Front side 11 has partially overlapping panels 16 and 17 heat sealed together longitudinally to form a ' 30 three-ply tube open only to form a self-closing sleeve 18. The tube is heat sealed at both ends 19 to form a complete valved bag of the type illustrated in aforesaid U.S. Pat. No. 3,833,166. In the embodiment shown the bag has its lateral edges 20 tucked in and heat sealed in the longitudinal region 21 through twelve layers of film.
-` 211~658 It will be noted that in this embodiment all the heat seal areas the uni-axially oriented film ie never sealed to itself but always has a double layer of non-oriented fiIm between mating seal surfaces even in the twelve-ply heat seal areas 19. It is, of course, desirable to have this tucked-in multiply seal area to give the filled bag a squared configuration. Again, it will be appreciated that tubing could be substituted for sheeting in layers 13 and 14. -Besides the advantage of allowing the bag to be ~ -gusset sealed, the outer ply can be reverse printed to -lock the print between plies 14 and 15 to protect it from abrasion in transit. Additionally, a small amount (0.5%) of 40 mesh high molecular weight high density polyethylene resin can be incorporated in layer 14 during extrusion to produce a pebbled surface to confer excellent handling properties on the filled bags.
FIG. 7 shows a polyethylene heat seal of a bag -loaded with product wherein the seal is under tension due to the product acting in a manner tending to separate the plies. The figure shows a polyethylene heat seal mass 50 resulting from the fusion of part of the two oriented high density polyethylene films 51 and the two non-oriented linear low density polyethylene films 52.
Integral with heat seal mass 50 at heat seal margin 53 are laminated seals 54, extending along each of the two adjacent plies 51 and 52. There is a relatively minor lamination seal 55 between the two plies 52.
The presence of the laminated seal inteqral with the heat seal can be accomplished by the application -~
of a gradient heat seal bar unit to the films whereby the -~
lamination is effected at the same time as the heat seal.
Alternatively, it can be accomplished in a two-stage operation wherein a laminated seal is first made, typically of a 1~ width, by joining the plies at a ' ''~' ' - 21146~8 temperature lower than the melting pint of the uni-axially oriented polyethylene (to prevent destruction of the orientation), typically 240F. Subsequently, a side weld heat seal is made through the laminated section by the application of temperature and pressure.
FIG. 8 shows a sheet 110 of uni-axially oriented linear density polyethylene (as for FIG. 3) of 1.5 mil thickness and a sheet 111 of low density polyethylene of 0.25 mil thickness laminated thereto.
The laminated sheets may be prepared by extrusion lamination.
It is preferred that the low density polyethylene in contact with the uni-axially oriented ply has as low a melting point as possible and be as fluid as possible when melted. These characteristics are generally achieved using low density polyethylene polymers with relatively low tensile yield strength. It is, therefore, desirable that the inner layer of the two-ply structure be a co-extrusion with only a thin layer, typically 0.25 mil thick, of low melt temperature, high melt index film on the layer in direct contact with the uni-axially oriented film.
I have found that the thicknes6 of the inner layers of low density polyethylene required to produce an acceptable heat seal will depend greatly on the elasticity of the uni-axially oriented film to be used, i.e., the less elastic the uni-axially film the thicker the low density polyethylene film must be. Relative thicknesses of all the polyethylene layers can be readily determined by the skilled man.
Thus it is seen that the present invention readily achieves the ends and advantages mentioned as well as those inherent therein. While certain preferred embodiments of the invention have been illustrated and described for purposes of the present disclosure, ., . .:, ~ .
~ 21146~8 - 32 - 8L3~1 numerous changes in the arrangement and construction o~
elements thereof may be made by those skilled in the art, which changes are encompassed within the scope and 6pirit of the present invention as defined by the appended claims.
','` :'.
. ,,, ..' -' .' ':
Brief Descrition of the Drawings Several embodiments of this invention will now be described by way of example only with reference to the accompanying drawings, in which:
Fig.1 shows a front elevational view, partly cut away, of an open-top tubular sack according to the invention;
Fig.2 is a sectional view along 2-2 of Fig.1;
Fig.3 is a front elevational view of an open-top heavy duty sack according to the invention;
Fig.4 is a sectional view along line 2-2 of Fig.3;
Fig.5 i8 a front elevational view of a heavy duty valved bag according to the invention; -Fig.6 is a sectional view along the line 4-4 of Fig.5;
Fig.7 is a diagrammatic view of a section through a heat seal used in the practice of the invention;
Fig.8 is a cross-sectional view of a preferred laminate of a thermoplastic material according to the invention; and Fig. 9 shows differential scanning calorimetric thermograms of several polyethylene resins according to and outside of the invention.
21146~8 Detailed Descri~tion of the Preferred Embodiment Figs.l and 2 show a generally rectangular single ply tubular sack 1 having a front wall 2 and a back wall 3 formed of a co-extruded and cold drawn polyethylene blend consisting of linear low density polyethylene (3 parts by weight, melt index 1.0; m.p.
121 C., - DOWLEX 2045 from Dow Chemical Company) and low density polyethylene (1 part by weight, melt index 0.8, m.p. 100 C. - AT1080 resin, copolymer of ethylene-vinyl acetate (91% w/w - 9% w/w) - AT Plastics Inc., Brampton, Ontario). One end of the tubular sack is heat sealed to form a single ply open-top sack.
The sack is made by extruding the above resin blend and blowing, cold drawing and heat annealing the film on modified conventional equipment. The flattened tube (27" 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 5-1/2~ gussets. The process is operated with the parameters as given for resin No. 7 in the Table hereinbelow. The sack has a most valuable tear resistance of 300 gm/mil and an edgefold impact strength of 30 gms/mil, while having improved puncture resistance and M.D. tensile strength.
A series of resin blends comprising DOWLEX 2045 resin and branched low density polyethylene and polyethylene vinyl acetate copolymers were blown at a blow ratio of 2.5 to 1 to produce a series of 6 MIL
films. The films were cold drawn through a range of draw ratios and measured for Elmendorf tear strengths. The process parameters and results are given in the following Table.
. ,.,.,, ~.. .. .
211~6~8 TAB,L,,E,, SAMPLE RESIN (LLDPE- DRAW DRAW RATIO/ ELMENDORF
NO. DOWLEX 2045) RATIO BLOW RATIO TEAR (GMS/M
1 100% LLDPE 2.0:1 0.8 40 100% ~ 3.0:1 1.2 50 100% n 4.1:1 1.64 50 100% ~ 5.5:1 2.2 80 100% ~ 6.4:1 2.55 190 2 80% LLDPE
20% AT540E 2.0:1 0.8 50 " " 3.0:1 1.2 60 n ~ 3.8:1 1.52 80 5.2:1 2.08 220 " ~'' 6.6:1 2.64 180 3 80% LLDPE
20% AT1520 2.1:1 0.95 60 ~ 3.2:1 1.28 40 n ~ 5.3:1 2.12 30 n ~ 6.4:1 2.55 70 4 80% LLDPE
20% AT1211 2.0:1 0.8 80 ~ 3.1:1 1.24 150 n ~ 4.2:1 1.68 240 ~ 5.5:1 2.2 290 90% LLDPE
10% AT1211- 2.2:1 0.9 60 n ~- 3.0:1 1.2 100 n ~ 4.0:1 1.6 100 6 60% LLDPE
40% AT1211 2.0:1 0.8 80 3.0:1 1.2 150 ~ 4.1:1 1.64 280 7 75% LLDPE
25% AT1080 2.1:1 0.95 80 n ~ 3.0:1 1.2 230 n ~ 3.8:1 1.52 300 8 80% LLDPE
20% AT1060~3.6:1 1.44 80 : -., ~;
9 80% LLDPE
20% ULLLDPE~ 4.0:1 1.6 40 21146~8 80% LLDPE
20% AT1230t~ 3.7:11.48 200 11 80% LLDPE
20% UCC6862- 3.4 1.36 70 AT540E has M.I. 0.8, N.P. 111 C., low density polyethylene - AT Plastics Inc., Brampton, Ontario.
", AT1520 has M.I. 2.0; M.P. 87 C., low density ethylene-vinyl acetate copolymer (15% vinyl acetate) - AT Plastics Inc., Brampton, Ontario.
~--AT1211 has M.I. 0.3; M.P. 95C., low density ethylene-vinyl acetate copolymer (12% vinyl acetate) - AT Plastics Inc., Brampton, ontario AT1080 has M.I. 0.8; M.P. 100C., low density ethylene-vinyl acetate copolymer (9% vinyl acetate) - AT Plastics Inc., Brampton, Ontario #AT1060 has M.I. 1.0; M.P. 105C., low density ethylene-vinyl acetate copolymer (6% vinyl acetate) - AT Plastics Inc., Brampton, ontario ##ULLLDPE is ATTANE 4001 -##tAT1230 has M.I. 3.0; M.P. 95C., low density ethylene-vinyl acetate copolymer (12% vinyl acetate) - AT Plastics Inc., Brampton, Ontario ~UCC6862 has M.I. 0.15; M.P. 109C., low density ethylene (PEROMONT UCC 6862 - Union Carbide -`
Inc. Donbury. CT) For an informative review of differential scanning calorimetry see "Differential Scanning Calorimetry", J.L. McNaughton and C.T. Mortimer, "IRS; ~-Physical Chemistry", Series 2, 1975, Volume 10, Butterworths, London, England.
: - ~
:
21146~8 Thermograms using a Perkin-Elmer DSC-4 differential scanning calorimeter, system 4 thermal analysis processor and AD 2Z autobalance were obtained for the following resins and provided the following data.
Dowlex 2045: ~ -WT: 4.16 mg.
SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 127.17 CAL/GRAM: 27.04 ENTERED AREA: 27.04 TEMP %
50.13 100 91.8 88.2 83.7 77.9 -100 70.4 105 60.6 110 48.3 127.3 0 Thermogram melting peak temperature: 121C
Temperature at which 70% of crystallites melted: 100C.
. ~, - ::
AT540E:
PEAK FROM: 50.13 TO: 115.95 CAL/GRAM: 28.38 ENTERED AREA: 28.38 TEMP
50.13 100 85.7 ; 85 ~80.3 - ~-73.8 65.9 100 56.3 105 44.2 116.08 0 Thermogram melting peak temperature: 111C
, :,': ., -` 21146~8 -ATls20:
WT: 4.21 mg.
SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 112.2 CAL~GRAM: 11.55 ENTERED AREA: 11.84 TEMP %
50.13 100 61.6 47.5 26.3 .5 100 -2.01 105 -.8 110 -. 1 112.2 0 Thermogram melting peak temperature: 87C
, AT1211:
;
WT: 4.15 mg. -SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 111.58 CAL/GRAM: 15.65 ENTERED AREA: 15.73 ;
TEMP %
50.13 99.5 63.6 915 28.5 100 2.2 105 -.4 110 0 :,, 111.71 0 Thermogram melting peak temperature: 95C ~ - ~
. , ~, .
-` 2114658 WT: 4.16 mg.
SCAN RATE: 10.00 deg/min PEAK FROM: 50.13 TO: 112.28 CAL/GRAM: 17.92 ENTERED AREA: 18 TEMP %
50.13 99.5 79.8 70.6 58.5 42.7 100 13.3 105 -.4 110 -. 01 ,, 112.2 0 Thermogram melting peak temperature: 95C
.
-AT1060:
WT: 4.20 mg. -SCAN RATE: 10.00 deg/min PEAK FROM: 50.15 -~
TO: 113.75 CAL/GRAM: 20.96 ENTERED AREA: 20.97 TEMP %
50.15 99.
86.2 79.6 go 71.1 60.2 100 46.1 105 20.5 -1'10 113.74 o ~. :
Thermogram melting peak temperature: 105C
~ 21146~8 UCC6862:
WT: 4O18 mg.
SCAN RATE: 10.00 deg/min - PEAK FROM: 50.13 TO: 111.89 CAL/GRAM: 26.52 ENTERED AREA: 26.52 TEMP %
50.13 100 85.2 79.2 71.6 61.9 100 49.4 -105 30.7 110 .2 112.02 0 Thermogram melting peak temperature: 109C
AT1230:
WT: 4.19 mg.
SCAN RATE: 10.00 deg/min - -PEAK FROM: 49.98 TO: 106.23 -CAL/GRAM: 13.81 ENTERED AREA: 13.82 TEMP %
49.98 99.9 86.
79.1 ,~
69.1 `
56.9 40.9 ~-14.7 106.24 0 Thermogram melting peak temperature: 95C
. - , ,,,.:
Reference is now made to the Table and Fig. 9 which, by way of example, represents the differential ~ ;
, . ~, -,:
: , - - ., .-~
i - ~
~.: ' ' :
~-~` 21146~8 scanning calorimetric thermograms of the following resins denoted as follows:
(I) : DOWLEX 2045 (used in Sample 1) (II) : AT 1080 (used in Sample 7) (III) : AT1520 (used in Sample 3) (IV) : AT540E (used in Sample 2) Points IA, IIA, IIIA and IVA on the graphs denotes the respective, thermogram melting peak temperature of each of the resins. Point B denotes the thermogram temperature at which 30% of the linear low density polyethylene (I) crystallites melt, and the range C denotes the range +5OC of point B. In the thermogram shown B is 100C and range C is 95-105C.
Thus, polymer blends of use in the practise of the invention comprising branched polyethylene resin AT1080 having the thermogram characteristics denoted as II; which resin has a melting peak temperature of 100C, which i8 well within the range C of 95-105C required for use with linear polyethylene (I).
In contrast, branched resins AT1520 and AT540E
have melting peak temperatures of 87C and 111C, respectively, and, thus, fall outside the scope of the present invention.
Thus, from the results shown in the TABLE it can readily be seen that the effective onset in improvement of the tear at low draw ratios occurs fairly sharply when the melting peak temperature of the branched blending resin goes from a M.P. of 87C to one of 95C.
Then the effect tapers off sharply when the M.P. goes from 100C to 105C. The effect is seen to be relatively independent of melt index (M.I.) Specifically, samples 4, 6, 7 and 10 at draw ratio: blow ratio values of less than 2 have excellent - 2~ - SL371 tear resistance, whereas Sample 5 has a lower, but valuable tear strength.
FIGS. 3 and 4 show a generally rectangular two-ply pillow-type sack l-having an inner wall 2 formed of blown linear low density polyethylene film (3 mil) manufactured from "2045" linear low density polyethylene resin (Dow Chemical Co.), and an outer ply 3 (3.5 mil) of uni-axially oriented linear low density polyethylene film made from blowing and cold drawing a polyethylene blend having the parameters as given for resin No. 7 in the Table.
The sack 1 has thus a two-ply back wall 4, and a two-ply iron wall 5 made up of first and second partially overlapping panels 6 and 7. The outer ply 3 of back wall 4 is continuous with the outer wall 3 of front wall 5 except where separated and joined together by heat sealing with layer 2 in the overlapping panels 6 and 7. Thus, the walls 4 and 5 are integral and form a two-ply tube. One end of the tube 8 is heat sealed to form a simple two-ply open-top bag.
The sack is made by feeding a web of 37" film 3 into a longitudinal folding frame with a web of film 2 and forming a two-ply tube 18" wide with a 1" overlapping portion. The four plies of the overlapping area are then heat sealed longitudinally to consolidate the two-ply tubing which is then passed to a transverse heat seal unit to make the bottom seal 8. A 26" length of tube with the heat seal present is cut from the web by a guillotine to form the open top bag 1.
' 30 To test the strength of the heat seals, sack 1 was filled with 50 pounds of granular salt, heat sealed at its open end by a "Dough boy" heat sealer, and drop tested on each side, edge and butt from a height of 10 feet. There was no rupture of any film or seal. The open top of the sack is generally heat sealed after S~
21146~8 filling with product to produce an airtight and watertight package. Because it is extremely difficult to exclude all air from the filled package prior to the heat sealing operation, it is preferable to perforate the walls of the bags with pinholes typically 0.025" in diameter to facilitate air release, the number of holes required depending on the amount of air left in the bag and the type of product being packaged. In those cases where it is critical that the package retains its maximum value for air tightness and moisture protection, the perforation holes in the inner and outer plies are offset typically by 1~" to create an indirect path to air-product mixes during the venting period.
Although the inner ply 2 of the sack is described as a single ply of sheeting it can readily appreciated that a two-ply tube of 1.5 mil could also be used instead. Indeed, since tubing may be less expensive to manufacture the tube could be the preferred option.
FIGS. 5 and 6 show a generally rectangular three-ply pillow type bag 10 having a front side 11 and a back side 12 joined together around the entire periphery of the bag. Front side 11 consists of an inner wall 13 and an outer wall 14 formed of blown linear low density polyethylene (4 mil), and a middle wall 15 of the same uni-axially oriented low density polyethylene film as for FIG. 3 (3.5 mil). Back side 12 is of an identical construction.
Front side 11 has partially overlapping panels 16 and 17 heat sealed together longitudinally to form a ' 30 three-ply tube open only to form a self-closing sleeve 18. The tube is heat sealed at both ends 19 to form a complete valved bag of the type illustrated in aforesaid U.S. Pat. No. 3,833,166. In the embodiment shown the bag has its lateral edges 20 tucked in and heat sealed in the longitudinal region 21 through twelve layers of film.
-` 211~658 It will be noted that in this embodiment all the heat seal areas the uni-axially oriented film ie never sealed to itself but always has a double layer of non-oriented fiIm between mating seal surfaces even in the twelve-ply heat seal areas 19. It is, of course, desirable to have this tucked-in multiply seal area to give the filled bag a squared configuration. Again, it will be appreciated that tubing could be substituted for sheeting in layers 13 and 14. -Besides the advantage of allowing the bag to be ~ -gusset sealed, the outer ply can be reverse printed to -lock the print between plies 14 and 15 to protect it from abrasion in transit. Additionally, a small amount (0.5%) of 40 mesh high molecular weight high density polyethylene resin can be incorporated in layer 14 during extrusion to produce a pebbled surface to confer excellent handling properties on the filled bags.
FIG. 7 shows a polyethylene heat seal of a bag -loaded with product wherein the seal is under tension due to the product acting in a manner tending to separate the plies. The figure shows a polyethylene heat seal mass 50 resulting from the fusion of part of the two oriented high density polyethylene films 51 and the two non-oriented linear low density polyethylene films 52.
Integral with heat seal mass 50 at heat seal margin 53 are laminated seals 54, extending along each of the two adjacent plies 51 and 52. There is a relatively minor lamination seal 55 between the two plies 52.
The presence of the laminated seal inteqral with the heat seal can be accomplished by the application -~
of a gradient heat seal bar unit to the films whereby the -~
lamination is effected at the same time as the heat seal.
Alternatively, it can be accomplished in a two-stage operation wherein a laminated seal is first made, typically of a 1~ width, by joining the plies at a ' ''~' ' - 21146~8 temperature lower than the melting pint of the uni-axially oriented polyethylene (to prevent destruction of the orientation), typically 240F. Subsequently, a side weld heat seal is made through the laminated section by the application of temperature and pressure.
FIG. 8 shows a sheet 110 of uni-axially oriented linear density polyethylene (as for FIG. 3) of 1.5 mil thickness and a sheet 111 of low density polyethylene of 0.25 mil thickness laminated thereto.
The laminated sheets may be prepared by extrusion lamination.
It is preferred that the low density polyethylene in contact with the uni-axially oriented ply has as low a melting point as possible and be as fluid as possible when melted. These characteristics are generally achieved using low density polyethylene polymers with relatively low tensile yield strength. It is, therefore, desirable that the inner layer of the two-ply structure be a co-extrusion with only a thin layer, typically 0.25 mil thick, of low melt temperature, high melt index film on the layer in direct contact with the uni-axially oriented film.
I have found that the thicknes6 of the inner layers of low density polyethylene required to produce an acceptable heat seal will depend greatly on the elasticity of the uni-axially oriented film to be used, i.e., the less elastic the uni-axially film the thicker the low density polyethylene film must be. Relative thicknesses of all the polyethylene layers can be readily determined by the skilled man.
Thus it is seen that the present invention readily achieves the ends and advantages mentioned as well as those inherent therein. While certain preferred embodiments of the invention have been illustrated and described for purposes of the present disclosure, ., . .:, ~ .
~ 21146~8 - 32 - 8L3~1 numerous changes in the arrangement and construction o~
elements thereof may be made by those skilled in the art, which changes are encompassed within the scope and 6pirit of the present invention as defined by the appended claims.
','` :'.
. ,,, ..' -' .' ':
Claims (30)
1. A thermoplastic sack comprising heat sealable uni-axially oriented film formed of a blend of 60-95% w/w linear low density polyethylene and 5-40% w/w branched ethylene polymer made by the high pressure process selected from the group consisting of branched polyethylene, branched polyethylene copolymer and mixtures thereof, wherein said branched polymer has a differential scanning calorimetric thermogram melting peak temperature that substantially coincides with the thermogram temperature at which 25-35% of the linear low density polyethylene crystallites melt; and wherein said film is produced by the following process:
extruding molten said polyethylene blend 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 drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ration (DR) such that a DR/BR ratio falls below 2:1.
extruding molten said polyethylene blend 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 drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ration (DR) such that a DR/BR ratio falls below 2:1.
2. A sack as claimed in claim 1 wherein said DR/BR ratio falls between 1:1 and 2:1.
3. A sack as claimed in claim 1 or claim 2 wherein said thermogram melting peak temperature coincides with the thermogram temperature at which essentially 30% of the linear low density polyethylene crystallites melt.
4. A sack as claimed in claim 1, 2 or 3 wherein said branched ethylene polymer is an ethylene-vinyl acetate copolymer.
5. A sack as claimed in claim 4 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 6-15% w/w.
6. A sack as claimed in claim 4 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 9-12% w/w.
7. A sack as claimed in claim 1 wherein said linear low density polyethylene contains a minor amount of high density polyethylene.
8. A sack as claimed in any one of claims l to 7 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 blend tube.
9. A sack as claimed in claim 8 wherein said compatible ethylene polymer or copolymer includes a low density polyethylene.
10. A sack as claimed in any one of claims 1 to 9 wherein said sack comprises a front wall and a back wall, each of said front wall and said back wall comprising a ply of said uni-axially oriented linear low density polyethylene blend and wherein interposed between said front wall and said back wall are two inner plies of low density polyethylene.
11. A sack as claimed in any one of claims 1 to 10 wherein the sack is a multi-ply sack having a single ply of said uni-axially oriented linear low density polyethylene blend.
12. A sack as claimed in claim 11 wherein the different plies of the sack are made of different thermoplastic film.
13. A thermoplastic heat sealable uni-axially oriented film formed of a blend of 60-95% w/w linear low density polyethylene and 5-40% w/w branched ethylene polymer made by the high pressure process selected from the group consisting of branched polyethylene, branched polyethylene copolymer and mixtures thereof, wherein said branched polymer has a differential scanning calorimetric thermogram melting peak temperature that substantially coincides with the thermogram temperature at which 25-35% of the linear low density polyethylene crystallites melt; said film produced by the following process:
extruding molten said polyethylene blend 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 drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ratio (DR) such that a DR/BR ration falls below 2:1.
extruding molten said polyethylene blend 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 drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ratio (DR) such that a DR/BR ration falls below 2:1.
14. A film as claimed in claim 11 wherein said DR/BR ratio falls between 1:1 and 2:1
15. A film as claimed 13 or claim 14 wherein said thermogram melting peak temperature coincides with the thermogram temperature at which essentially 30% of the linear low density polyethylene crystallites melt.
16. A film as claimed in any one of claims 13, 14 or 15 wherein said branched ethylene polymer is an ethylene-vinyl acetate copolymer.
17. A film as claimed in claim 16 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 6-15% w/w.
18. A sack as claimed in claim 16 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 9-12% w/w.
19. A method of manufacturing a thermoplastic film formed of a 60-95% w/w linear low density polyethylene and 5-40% w/w branched ethylene polymer made by the high pressure process selected from the group consisting of branched polyethylene, branched polyethylene copolymer and mixtures thereof, wherein said branched polymer has a differential scanning calorimetric thermogram melting peak temperature that substantially coincides with the thermogram temperature at which 25-35% of the linear low density polyethylene crystallites melt; said method comprising the steps of extruding molten said polyethylene blend 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 drawn tube of said uni-axially oriented linear low density polyethylene with a length, a ratio of said drawn tube length to said tube length defining a draw ratio (DR) such that a DR/BR
ration falls below 2:1.
ration falls below 2:1.
20. A method as claimed in claim 19 wherein said DR/BR ratio falls between 1:1 to 2:1.
21. A method as claimed in claim 19 or claim 20 wherein said thermogram melting peak temperature coincides with the thermogram temperature at which essentially 30% of the linear low density polyethylene crystallites melt.
22. A method as claimed in claim 19, 20 or 21 wherein said branched ethylene polymer is an ethylene-vinyl acetate copolymer.
23. A method as claimed in any one of claims 19 to 22 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 6-15%
w/w.
w/w.
24. A method as claimed in any one of claims 19 to 22 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 9-12%
w/w.
w/w.
25. A thermoplastic sack comprising film formed of a uni-axially oriented polyethylene blend of 60-95% w/w linear low density polyethylene and 5-40% w/w branched ethylene polymer made by the high pressure process selected from the group consisting of branched polyethylene, branched polyethylene copolymer and mixtures thereof, wherein said branched polymer has a differential scanning calorimetric thermogram melting peak temperature that substantially coincides with the thermogram temperature at which 25-35% of the linear low density polyethylene crystallites melt; said film having a DR/BR ratio of less than 2:1; said DR
being defined as a ratio of a length of undrawn film, said BR being defined as a ratio of a bubble diameter to a die diameter.
being defined as a ratio of a length of undrawn film, said BR being defined as a ratio of a bubble diameter to a die diameter.
26. A thermoplastic sack as claimed in claim 25 wherein said DR/BR ratio falls between 1:1 to 2:1.
27. A thermoplastic sack as claimed in claim 25 or claim 26 wherein said thermogram melting peak temperature coincides with the thermogram temperature at which essentially 30% of the linear low density polyethylene crystallites melt.
28. A thermoplastic sack as claimed in any one of claims 25 to 27 wherein said branched ethylene polymer is an ethylene-vinyl acetate copolymer.
29. A thermoplastic sack as claimed in claim 28 wherein said ethylene-vinyl acetate copolymer has a vinyl acetate content of 6-15% w/w.
30. A thermoplastic sack as claimed in claim 28 wherein said vinyl acetate content falls between 9-12% w/w.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US1966993A | 1993-02-19 | 1993-02-19 | |
| US08/019,669 | 1993-02-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2114658A1 true CA2114658A1 (en) | 1994-08-20 |
Family
ID=21794428
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2114658 Abandoned CA2114658A1 (en) | 1993-02-19 | 1994-01-31 | Thermoplastic film and sacks made therefrom |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2114658A1 (en) |
| MX (1) | MX9401282A (en) |
-
1994
- 1994-01-31 CA CA 2114658 patent/CA2114658A1/en not_active Abandoned
- 1994-02-18 MX MX9401282A patent/MX9401282A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| MX9401282A (en) | 1994-08-31 |
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| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |