Thermoplastic sack

A cold-drawn unbalanced biaxially oriented linear low density polyethylene film having a transverse direction draw ratio selected from greater than 1 to less than 3 and a machine direction draw ratio of less than 6 but greater than the transverse direction draw ratio. The film has enhanced tensile and puncture resistance while maintaining acceptable tear properties and is used for shipping sacks.

This invention relates to unbalanced biaxially oriented linear low density 
polyethylene films and, more particularly, to both tubular and heavy duty 
shipping sacks formed therefrom. 
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 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 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 thinner 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 (75-150.times.10.sup.-4 cm) which is generally determined by the 
machine direction (MD) 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 downgauged bags because of its high stretch 
resistance and tensile strength these have largely been abandoned because 
of poor tear resistance and impact puncture properties. In view of this, 
polyethylene insulation sacks are most commonly made from resins which 
have superior tear resistance and impact 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 
uniaxially cold-drawing the film below its melting point. However, because 
of the unbalanced physical properties e.g. poor MD tear strengths, of 
these oriented films, which causes "splittiness", they have been ignored 
for use in tubular shipping sacks. 
In my copending U.S. application Ser. No. 797,918, filed November 14, 1985, 
I have described an improved shipping sack formed of a uniaxially oriented 
polyethylene film having good machine direction tear resistance. The 
uniaxially oriented film is produced by blowing and cold-drawing the 
polyethylene film at a draw ratio to blow ratio (DR/BR) of greater than 
1:1. 
In the process described in Ser. No. 797,918, the film is blown at a 
temperature greater than the crystalline melting point (Tc) of the 
polyethylene resin prior to the cold-drawing of the resultant film, in the 
machine direction only, at a temperature lower than Tc. The effect of this 
is to produce orientation, in essence, in only one direction, namely the 
machine direction to produce uniaxially oriented film. It is understood in 
the art that film orientation refers to the film drawing which occurs 
below the Tc and not to the normal film drawing which occurs in the blown 
film process above Tc. 
Biaxial orientation of thermoplastic films is a well known technique 
wherein a blown or cast film is uniformly cold-drawn in the machine and 
transverse direction at a temperature greater that the glass transition 
temperature (Tg) but less than Tc. 
Typically, biaxially oriented films are cold-drawn uniformly in both 
directions to produce an increase in surface area of about 40 times the 
undrawn film area, with a consequent reduction in film thickness, i.e. 
from 40 mil to 1 mil. This orientation has very beneficial effects in 
improving the tensile and impact properties of the film, typically, by a 
factor of 5 times that of the undrawn film. However, this improvement in 
tensile and impact properties is gained by a corresponding loss in the 
tear properties of the film which is typically reduced to only 10% of the 
undrawn film in both machine and transverse directions. 
While the improved tensile and impact properties of a typical biaxially 
oriented film would be very valuable in increasing the functional strength 
of a plastic shipping sack the extremely poor tear properties of the film 
make it unacceptable for shipping sack use. Plastics shipping sacks 
generally now have punched holes or perforation to allow for the 
evacuation of air from the bag after filling. These perforations become 
the focus for zipper tears in biaxially oriented film when impacted under 
normal handling conditions used for industrial shipping sacks. 
Surprisingly, I have now found that by producing an unbalanced draw 
biaxially oriented film having specific characteristics a greatly improved 
film for use in shipping sacks can be made. I have found that by 
restricting the degree of transverse direction (TD) draw in the film 
relative to the machine direction (MD) draw the dramatic imbalance of 
tensile and tear properties associated with uniaxially orientation and 
associated tendency for film "splittiness" can be avoided. 
Accordingly, the invention provides a thermoplastic shipping sack having 
walls comprising a cold-drawn unbalanced biaxially oriented linear low 
density polyethylene film having a transverse direction draw ratio 
selected from greater than 1 to less than 3 and a machine direction draw 
ratio of less than 6 but greater than the transverse direction draw ratio. 
Preferably, the unbalanced biaxially oriented film has a cold-drawn 
transverse direction ratio of about 2 and a cold-drawn machine direction 
ratio of about 5. This results in a ratio of machine direction draw ratio 
to transverse direction draw ratio of 2.5. 
By the term "draw ratio" is meant the ratio of the length of drawn film to 
the length of undrawn film. 
It is an essential feature of the invention that the film is drawn to a 
greater degree in the machine direction than the transverse direction. 
I have thus found that a shipping sack having improved film stretch 
resistance and high tensile strength in both MD & TD directions in 
addition to acceptable tear resistance comparable to that for non-oriented 
film and in contrast to the usually reduced TD stretch and tensile 
strength resistance for uniaxially 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. 
I have further found that by blending in a minor amount of high pressure 
process (i.e. non-linear) low density polyethylene resin with the linear 
low density polyethylene resin an unbalanced biaxially oriented film 
having further enhanced tear properties can be produced. 
Accordingly, in a more preferred feature the invention further provides a 
thermoplastic sack as hereinbefore defined wherein said linear low density 
polyethylene contains a minor amount of low density polyethylene. 
The amount of low density, polyethylene present in the polyethylene blend 
prior to drawing into film can be readily determined by the skilled man to 
be that amount which provides acceptable enhanced tear properties. 
Typically, the blends may comprise up to 30% high density or low density 
polyethylene, and preferably comprises 20% low density polyethylene. This 
offers unbalanced biaxially oriented film for use in shipping sacks 
according to the invention which could be downgauged by 30%. 
The cold-drawn film process providing the film according to the invention 
basically comprises the steps of extruding molten thermoplastic resin 
through a circular die and drawing the tubular melt over a chilled mandrel 
and subsequently a tapered mandrel by means of a set of nip/ draw rolls. 
The action of the speed of the nip/draw rolls on the film affects 
orientation in the machine direction while expansion of the film tube 
diameter over the tapered mandrel effects orientation in the transverse 
direction, to produce a biaxially oriented film. Adjustment of the pull of 
the nip/draw rolls alters the degree of machine direction orientation 
relative to the transverse direction orientation. A vacuum brake installed 
between the first and second mandrel is used to separate the high tension 
requirements of the cold drawing process from the low tension 
requiremenets of the hot blown film process. 
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 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 uniaxially 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 light duty purpose for which the 
sacks are made. 
It has thus been found that a suitable open-top tubular polyethylene 
shipping sack having improved puncture resistance and (TD) and (MD) 
tensile strengths, while still retaining acceptable tear and edgefold 
impact strength, can be manufactured using suitably modified conventional 
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 sacks of alternative structure to the simple 
open-top sack 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 unbalanced biaxially 
oriented linear polyethylene film produced by cold-drawing at the 
aforesaid TD and MD draw ratios. 
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 heavy 
materials such as, for example, fertilizers and chemicals. 
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 unbalanced biaxially oriented linear 
polyethylene, said ply being produced by cold-drawing said linear 
polyethylene at a TD and MD draw ratio as hereinbefore defined. 
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 duty thermoplastic 
shipping sack. Such sacks may be used for the transportation, packaging 
and storage of 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 fitted with a valved opening. 
Disclosed in our U.S. Pat. No. 4,576,844 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 
uniaxially oriented linear polyethylene film. 
However, I have now found that a much cheaper thermoplastic shipping bag 
than the aforesaid crosslaminated structured bag can be manufactured 
having both excellent heat sealability and puncture resistant properties. 
I have surprisingly found that two layers of low density polyethylene can 
be welded to each other between and to two unbalanced biaxially oriented 
linear polyethylene films 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 unbalanced biaxially oriented film and the body of the heat seal 
is formed. This is to be contrasted with the fact that although two 
uniaxially oriented films in the absence of interposed low density 
polyethylene film could be melted and fused together to produce welded 
bonds, the uniaxially 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. 
It has thus now been found that a suitable thermoplastic shipping bag 
having improved puncture and snag resistance can now be reliably 
manufactured by heat sealing techniques using suitably modified 
conventional equipment. 
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 unbalanced biaxially oriented linear 
polyethylene produced by cold-drawing said linear polyethylene as 
hereinbefore defined; and wherein interposed between said plies are two 
inner layers of non-cold-drawn low density polyethylene. 
Each of the interposed layers of low density polyethylene may constitute 
simply a sheet of polyethylene laminated to a surface of an unbalanced 
biaxially oriented ply and being of sufficient thickness in the heat seal 
area to effect an acceptable bridge between the two unbalanced biaxially 
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 unbalanced biaxially oriented plies. Thus, each of the 
unbalanced biaxially oriented plies comprising the walls of the shipping 
bag have a layer of low density polyethylene laminated thereto. Such a 
structure, of course, does not detract from the requirement that the 
unbalanced biaxially 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. 
Where the layers of low density polyethylene are represented as laminated 
sheets on the unbalanced biaxially oriented plies, each of the sheets must 
be of sufficient thickness to effect an acceptable bridge between the two 
unbalanced biaxially oriented plies. I have found that a mere coating of 
low density polyethylene on each of the unbalanced biaxially plies is not 
sufficient, and that a minimum thickness of 0.5 mil of low density 
polyethylene is required, preferably &gt;1.5 mil. 
I have also found that both of the unbalanced biaxially oriented plies 
constituting the walls of the sack must have a laminated sheet of low 
density polyethylene to provide an acceptable heat seal for heavy duty bag 
use. A single interposed layer of low density polyethylene, represented 
either as a laminated sheet or as a distinct ply, is not satisfactory. 
Thus, a double layer of polyethylene is required. 
In a much preferred form of a sack according to the invention the 
interposed layers of low density polyethylene represent full and distinct 
plies constituting part of the walls of the sack. 
Accordingly, the invention further provides a sack as hereinbefore 
described wherein each of said layers of low density polyethylene 
constitutes an inner ply of the bag. 
In this preferred form of sack each of the walls comprising an unbalanced 
biaxially 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 
unbalanced biaxially 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 
non-oriented low density polyethylene constituting each of the inner 
surfaces or inner walls of the sack such that an unbalanced biaxially 
oriented ply of linear polyethylene does not contact another unbalanced 
biaxially oriented ply of linear polyethylene at a designated heat seal 
area of an inner surface such as to weaken a heat seal when heat seal 
strength is a desired feature. 
In preferred embodiments of the heavy duty sacks according to the invention 
as hereinbefore and hereinafter defined the interposed layer of low 
density polyethylene represented either as a laminated sheet on the 
unbalanced biaxially 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 two-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 unbalanced biaxially 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 copending U.S. application Ser. No. 632,522, 
filed July 19, 1984. 
In other instances it may be preferred to have additional plies of film 
outermost of the unbalanced biaxially oriented ply. Such an outer ply 
could give the benefit resulting from introducing blown low density 
polyethylene film between the gussetted surfaces of unbalanced biaxially 
oriented plies 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. 
An additional benefit to be gained from such an outer layer is that the 
surface can be suitably roughened by the addition of ultra high molecular 
weight HDPE 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 thus lies in the fact that by the introduction of a double 
layer of a non-cold-drawn low density polyethylene film between the mating 
surfaces of two unbalanced biaxially oriented polyethylene films 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 unbalanced biaxially oriented film in conjunction with an inner web 
of blown low density polyethylene 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 our 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. 
The term "low density polyethylene" includes low density ethylene 
homopolymers and copolymers, such as the linear low density polyethylenes, 
vinyl acetate copolymers, and blends thereof. 
The term "linear low density polyethylene" as used within this 
specification and claims includes linear low density ethylene copolymers 
with the lower olefins such as, for example, butene, n-hexene, 4-methyl 
1-pentene and octene. 
While it is generally accepted that all polyethylene film is generally 
oriented to some degree, the term "unbalanced biaxially oriented" when 
used with reference to linear polyethylene in this specification and 
claims means polyethylene film that has been cold-drawn in the transverse 
direction to at least greater than a 1:1 fold extent, preferably to a 
2-fold extent, but also up to a 3-fold extent; and in the machine 
direction to a greater degree than in the transverse direction to a value 
not greater than 6:1. The orienting of the films may be carried out by the 
cold-drawing of the tube as hereinbefoare described. 
The cold-drawn unbalanced biaxially 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 polyethylenes 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 unbalanced biaxially oriented linear polyethylene produced as 
hereinbefore defined and a layer of a low density ethylene polymer or 
copolymer compatible with said unbalanced biaxially oriented linear 
polyethylene. Examples of such compatible copolymers of use in the 
invention are ethylenevinyl 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 is applicable to a 
laminate the compatible ethylene polymer or copolymer is also subjected to 
the novel same MD/TD draw ratios subsequent to the co-extrusion step as is 
the unbalanced biaxially oriented linear polyethylene. 
The compatible ethylene polymer or copolymer layer of the laminate may 
constitute either the inner surface or the outer surface of the sack to 
provide additional utility to the sack. For example, where the compatible 
polymer or copolymer of the laminate is a soft-flexible copolymer, such as 
10% ethylene-vinyl acetate, providing an external surface of the sack it 
provides superior anti-slip properties. Where a 20% ethylene-methyl 
acrylate copolymer of the laminate provides the inner layer of the sack, 
the sack may generally be heat sealed at temperatures as low as 80 degrees 
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 provides an open-top tubular shipping sack as 
hereinbefore defined wherein said film or ply of unbalanced biaxially 
oriented linear low density polyethylene forms part of a multi-layer 
laminate with one or more layers of one or more compatible ethylene 
polymers or copolymers. 
In a further aspect, the invention provides a thermoplastic film suitable 
for use for a shipping sack, said film formed of unbalanced biaxially 
oriented linear low density polyethylene prepared by the cold-drawing of 
said polyethylene at machine and transverse draw ratios as hereinbefore 
defined. 
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.

With reference to FIG. 1, molten thermoplastic resin is extruded from an 
extruder 10 having an 8" annular die 11 having a 0.05" diameter gap. The 
film thickness as it leaves the die is approximately 0.075" and the 
tubular film melt is at a temperature of 220 degrees C. Adjacent the die 
11 is a uniformly cylindrical lower mandrel 12 over and along the surface 
of which lower mandrel is pulled the tubular film by means of at least one 
pair of nip/draw rolls 13. The lower mandrel is maintained at a 
temperature ca. 85 degrees ca. to effect chilling of the film melt. The 
film in the region immediately prior to lower mandrel 12 is at a 
temperature above its crystalline melting point, i.e. ca. 121 degrees C. 
and, thus, no cold-drawing occurs in this region. Acting on the film in 
this region is a cooling air stream directed from an air ring 14, which 
cools the film to a temperature of between 135 degrees C.-150 degrees C. 
As the film moves from the die lips to the part where it is frozen on the 
lower mandrel 12 it is drawn down to 0.025" by means of nip/draw rolls 13. 
Adjacent the upper end of lower mandrel 12 is a tapered mandrel 15 
separated from lower mandrel 12 by a vacuum slot 16. As the film passes to 
and over tapered mandrel 15 a controlled vacuum is applied to the film 
through vacuum slot 16. This allows the generation of a higher tension 
from nip/draw rolls 13 to draw and thin the film over tapered mandrel 15 
to the required thickness of 2.5 mil. i.e. the film tension on lower 
mandrel 12 is thus substantially lower than that generated on tapered 
mandrel 15. As the 0.025" thick tubular film passes vacuum slot 16 the 
vacuum pressure is adjusted to produce sufficient draw tension on the film 
draw nips 13 to pull the film over tapered mandrel 15 and cold-draw it in 
the transverse direction, and, at the same time, in the machine direction 
to a tubular diameter of 17.4". The film temperature is advisably rapidly 
reduced to ca. 60 degrees C. i.e. below its softening point, prior to 
entry between the nip/draw rolls 13 by means of air ring cooling 17 
adjacent tapered mandrel 15. Adjacent mandrel 15 is a metallic reflective 
shield 18 which minimizes the reflective heat lost from the rapidly 
thinning film on tapered mandrel 15. 
The speed of nip/draw rolls 13 is controlled to effect drawing of the film 
to the desired gauge. After exiting nip/draw rolls 13 the flattened film 
tube of 26" width 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 (not shown) 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 provide a 66" x 16" x 10" insulation 
sack. 
The above film has, thus, been drawn to a transverse direction draw ratio 
of 2.2 and a machine direction draw ratio of 4.5. 
EXAMPLE I 
A series of experiments were carried out to evaluate the effect of uniaxial 
orientation on a linear low density polyethylene film. In these 
experiments a polyethylene blend consisting of linear low density 
polyethylene (4 parts, density 0.918, melt index 0.5-ESCORENE 1030* from 
ESSO CHEMICAL) and low density polyethylene (1 part, density 0.923, melt 
index 0.3-CIL 503 *-1% silica) were blown and cold-drawn on modified 
conventional equipment as described in copending U.S. application Ser. No. 
797,918 filed November 14, 1985. The films were blown from the resin at 
different blow ratios and subsequently cold-drawn below their crystalline 
melting point at different draw ratios and tested for MD and TD tear 
resistance. The process parameters and results are given in Table 1. The 
results show the unfavourable TD tensile strength obtained for these 
films. 
EXAMPLE II 
A series of experiments were carried out to evaluate the effect of balanced 
biaxial orientation on a linear low density film. In these experiments 
blown film samples of a 12 mil film produced at a 1:1 blow ratio from a 
resin formulation of an 80/20 blend of EXXON 1030 /CTL 633 * were 
stretched at a temperature of 105 degrees C. on a T. M. Long Co. film 
stretcher following results. EXXON 1030 is a linear low density 
polyethylene-butene copolymer having a melt index of 0.5 g/10 mins and 
density of 0.922 g/cc. CIL 633 is a 2% vinyl acetate - low density 
polyethylene copolymer having a melt index of 0.3 and density of 0.925 
g/cc. The films were cold-drawn equally in the MD and TD below the 
crystalline melting point to form balanced biaxially oriented films. The 
results are shown in Table 2. 
TABLE 1 
__________________________________________________________________________ 
Blown Drawn M D 
Resin Thickness 
Thickness 
Draw 
Blow Tear Tensile 
Mega Pascals 
No. 
(ESCORENE 1030) 
(micron) 
(micron) 
Ratio 
Ratio 
DR/BR 
(gms/mil) 
M D T D 
__________________________________________________________________________ 
1 225 75 3:1 3:1 1:1 20 85 27 
2 150 25 6:1 3:1 2:1 180 175 27 
3 +20% CIL 503 
150 25 6:1 3:1 2:1 220 150 27 
4 225 75 3:1 2:1 1.5:1 
40 85 24 
5 +20% CIL 503 
225 75 3:1 2:1 1.5:1 
70 75 24 
6 225 75 3:1 1:1 3:1 190 75 22 
7 75 25 3:1 1:1 3:1 170 75 22 
8 +20% CIL 503 
225 75 3:1 1:1 3:1 240 70 22 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Draw Ratio Tear (gms/mil) 
Ultimate Tensile 
MD TD MD TD MD TD 
______________________________________ 
2 2 90 120 66 48 
3 3 50 60 80 75 
4 4 20 20 95 97 
5 5 10&gt; 10&gt; 102 100 
______________________________________ 
These results showed the expected trend of rapid decrease in both MD and TD 
tear properties with corresponding increase in tensile strength. Tear 
properties of films with balanced stretch of 3:1 or greater are generally 
unacceptable for shipping sacks. 
EXAMPLE III 
A series of experiments similar to those described in Example 2 with the 
same resin were carried out but wherein the TD draw ratio was maintained 
constant at 2:1. The results are shown in TABLE 3. 
TABLE 3 
______________________________________ 
Draw Ratio Tear (gms/mil) 
Ultimate Tensile 
MD TD MD TD MD TD 
______________________________________ 
2 2 90 120 66 48 
3 2 150 200 80 50 
4 2 200 240 90 52 
5 2 300 320 92 53 
6 2 430 400 89 54 
______________________________________ 
The results show that both the MD and TD tear improved uniformly with 
increased MD draw ratio, i.e. increases MD orientation. 
I have, thus, found that by restricting the degree of TD draw in 
relationship to the MD draw the dramatic imbalance of tensile and tear 
properties associated with uniaxially orientation and associated tendency 
for film "splittiness" can be avoided. In addition, films drawn in this 
fashion while having MD and TD tensile improvement of 100%, have balanced 
and increasing tear properties with increasing orientation. For practical 
purposes TD draw ratios of 2:1 would be typical. TD draw ratio of greater 
than 3:1 would be impractical since undrawn film thickness would be double 
that required for the 2:1 drawn material making control of the 
cold-drawing operation much more difficult to achieve. 
FIGS. 2 and 3 show a generally rectangular single ply tubular sack 1 having 
a front wall 2 and a back wall 3 formed of a cold-drawn polyethylene film 
made from the blend consisting of EXXON 1030 /CIL 633 as described in 
Example III according to the process of manufacture as hereinbefore 
described. The MD draw ratio of the film is 5 and the TD draw ratio is 2. 
One end 4 of the tubular sack is heat sealed to form a single ply open-top 
sack. 
FIGS. 4 and 5 show a generally rectangular 2-ply pillow-type sack 1 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 unbalanced biaxially 
oriented linear low density polyethylene film blend of EXXON 1030 /CIL 633 
as hereinbefore described. 
The sack 1 has, thus, a 2-ply back wall 4, and a 2-ply front 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 2-ply tube. One end of the tube 8 is heat sealed to form a simple 2-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 2-ply tube 18" wide with a 1" 
overlapping portion. The four plies of the overlapping area are then heat 
sealed longitudinally to consolidate the 2-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. 
The open top of the sack is generally heat sealed after 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 airtightness and 
moisture protection, the perforation holes in the inner and outer plies 
are offset typically by 11/2" 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 be readily appreciated that a 2-ply tube of 1.5 mil could 
also be used instead. Indeed, since tubing may be less expensive to 
manufacture the tube could be a preferred option. 
FIGS. 6 and 7 show a generally rectangular 3-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 unbalanced biaxially oriented linear low 
density polyethylene film as for FIG. 4 (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 3-ply tube open only to form a 
self-closing filling sleeve 18. The tube is heat sealed at both ends 19 to 
form a complete valved bag of the type illustrated in our 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. 
FIG. 8 shows a sheet 110 of unbalanced biaxially oriented linear low 
density polyethylene (as for FIG. 4) 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 
unbalanced biaxially 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 2-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 unbalanced biaxially oriented film. 
I have found that the thickness of the inner layers of low density 
polyethylene required to produce an acceptable heat seal will depend 
greatly on the elasticity of the unbalanced biaxially oriented film to be 
used, i.e., the less elastic the unbalanced biaxially oriented 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.