Note: Descriptions are shown in the official language in which they were submitted.
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Thermoplastic Shipping Sack C-I-L 684
This invention relates to tubular thermoplastic shipping
sacks formed of uni-axially oriented linear low denslty
polyethylene or blends thereof wlth low density polyethylene.
Bulky but lightweight materials such as fiberglass
insulation and peat moss are generally shlpped ln compressed
form in ~hermoplastic shipping sacks. These sacks are
generally known as tubular lnsulation sacks or bags and take
the form of an extended envelope or tube sealed at one end
prior to its being filled wlth product. For the most part
these sacks are produced by the commonly known ln the art
"blown film" process, which owes lts popularlty to the fact
- that it can be quickly and readily adapted to the productlon
of different widths and thicknesses of continuous tubes which
can then be easily cut to length and sealed at one end to
produce an open top sack.
It will be readily appreclated that the thinner the film
thickness (gauge), commensurate with acceptable fllm
properties, the less the amount of thermoplastic material
required. This downgauging of sack wall thickness ls a most
desirable industrial goal. Walls of sac~s produced as tubes
by the blown film process, typically, have a fllm thlckness
in the range of 3 - 6 mll (75 - 150 X10 4 cm) which ls
generally determined by the machine direction (~.D.)
tensile strength necessary to handle the package weight,
the film stretch resistance required to prevent expansion
of the compressed product and the puncture reslstance of
the bag for distrlbution handling.
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The tubes ~rom which these sacks are commonly made are
produced with a bubble diameter/die dlameter generally of
3:1 in order to optimize film strength properties.
Although various attempts have been made to use high
densi~y polyethylene for the manufacture of downgauged bags
because of its high stretch resistance and tensile strength
these have largely ~een abandoned because of poor tear
resistance and puncture properties. In view of this,
lQ polyethylene insulation sacks are most commonly made from
resins which have superior tear resistance and puncture
properties such as low density or linear low density
polyethylene.
It i-~ well known in the art to produce polyethylene films
having enhanced puncture, tensile strength and stretch
resistance by the process of uni-axially cold drawing the
film below its melting point. However, because of the very
-~ poor M.D. tear strengths of these orlented films, whlch
causes "splittiness", they have been ignored for use ln
tubular shipping sacks.
Surprisingly, we have now found that by the correct
selection of film blow up ratio (BR) and draw ratio ~DR)
of blown and drawn linear low density polyethylene film a
tubular uni-axially oriented film having good M.D. tear
resistance can be produced suitable for use in a downgauged
shipping sack.
Accordingly, the invention provides a tubular shipping
sack formed of uni-axially oriented linear low density
polyethylene film produced by blowing and cold drawing
linear low density polyethylene at a draw ratio to blow
ratio (DR/BR) of greater than 1:1.
By the term "Draw Ratio" is meant the ratio of the length
of drawn film to the length of undrawn film, and by the term
"Blow Ratio~ is meant the ratio of the bubble dlameter to the
die diameter. Such terms are well known in the art.
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~ e have found that ratios of DR/BR of ~1:1 result in
~ilms having unacceptable M.D. tear properties, whereas
values of at least 2:1 are preferred, although ratios >5:1
; are im~ractica1 to obtain.
Accordingly in a preferred feature, the invention
~rovides a tubular shipping sack formed of uni-axially
oriented linear low density polyethylene film produced by
~lowing and cold drawing linear low density polyethylene
at a draw ratio to blow ratio of between 2:1 and 5:1.
We have thus found that a shipping sack having
improved film stretch resistance and high tensile strength
in addition to acceptable tear ~eslstance comparable to
that for non-oriented film and ln contrast to the expected
usually reduced tear resistance for uni-axially oriented
polyethylene film can be manufactured.
We have further found that by blending in a minor amount
of high pressure process ~i.e. non-linear) low denslty
polyethylene resin with the linear low density polyethylene
resin a uni-axially oriented film having further enhanced
tear properties can be produced. In addition, we have found
that these resin mixtures ~lso provide enhanced bubble
stability during the blown film process and facllltate the
manufacturing process.
Accordingly, in a preferred feature the invention
.urther provides a tubular shipping sack as hereinbefore
c`efined wherein said linear low density polyethylene contains
a minor amount of low density polyethylene.
The amount of low denslty polyethylene present in the
?olyethylene blend prior to blowing lnto film can be readily
determined by the skilled man to be that amount whlch provides
acce?table enhanced tear properties. Typically, the blend
com?rises 20~ low density polyethylene and offers uni-axially
oriented ~ilm ~or use in shipping sacks according to the
invention which could be downgauged by 30~.
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The typical blown film process basically comprises the
steps of extruding molten thermoplastic material throuyh a
circ~lar die, typically of O.OS" diameter gap, to form a tube
s which is closed by passing the end through a set of nip/draw
~olls. Air is introduced through the die centre to inflate
the tube to the selected width while the speed of the nip
rolls is increased to draw the film down to the desired
gauge. At this stage the flattened film tube is, optionally,
passed to a corona discharge unit to burn the film surface to
make it receptive to lnk application when next passed through
a flexographlc s~ack press. The tube ls then reinflated by
passing it through two sets of nip rolls wlth an air bu~ble
trapped between them while the edge of the tube is tucked
by forming plates just prior to the second set of nlps in
;order to form any required gusset ln the tube. The tube
finally passes to an end seal head where it is heat sealed
and guillotined to the required sack length`.
Although it is preferable when extruding to use a
-20 maximum die size to obtain highest output rates, low density
polyethylene is commonly extruded with much smaller dies to
give bLow ratios of between 2 - 3 to provide film impact
strength, especially on the edge folds of the flattened tube.
In the blown film process for the manufacture of sacks
according to the invention the above general process is
modified in several ways. The first change is that the die
diameter used is increased by a factor of 2 - 3, depending
on the desired bag layflat width, over the die diameter used
at present in the industry for the manufacture of thermoplastic
tubular sacks. The second change is that a cold drawlng
section is introduced between the bubble nip and the corona
treatment unit. Such a section may consist of an initial
series of heating rolls to raise the film temperature to a
point, e.g. ca. 105C, where a minimum of force is required
for cold drawing and a series of clo~ely spaced rolls through
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which the heated film passes to a set of cooling rolls to
bring the film temperature to ambient. The spacing between
the closely spaced rolls is, typically, 0.13 mm plus film
thickness, The film is then passed to a set of pull nips
running at a linear speed, typically, of four times greater
than the bubble nip draw rolls.
The blown film thickness is typically four times
greater than the required thickness of the finished sack.
Thus, since the desired finished sack film thlckness is
typically ~ - 3 mil the initial blown film thlckness`must be
8 - 12 mil. Because it is extremely difficult to control
blown film thickness of gauges greater than 12 mil the use of
DR/B~ values higher than 5:1 becomes impractical.
15 ~t has thus been found that a suitable open-top tubular
polyethylene shipping sack having improved tear resistance
can be manufactured using suitably modified conventlonal
blown film process apparatus.
In addition, tubular shipping sacks of alternative
structure to the simple open-top sack described hereinabove
and utilizing the feature o~ the invention to provide the
promised advantages may be produced. Such an alternative
tub~lar sack is the type known as a "valved bag" shipping
sack, which is closed at both ends of the tube and has a
self-closing valve structure at an upper side or end.
Such alternative bags may be made by conven~ional
processes well-known in the axt suitably modified to
provide a sack formed of uni-axially oriented film produced
by blowing and cold drawing at the aforesaid draw ratio to
blow ratio.
Thus, by the term "tubular shipping sacks" is meant sacks
having a resultant shape generally of a tube, optionally
provided with gussets, whether made by the speclfic process
as hereinbefore described or by alternative processes known
in the art which may or may not involve the "back-sealing"
of an oriented film.
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Tne heat seal produced in the tube, i.e., the two
flattened sides (films) o~ the tube, by the end seal head in
the process hereinabove described is produced under a
combination of pressure and heat, at or above the films'
crvstalline melting point, applied to the films in order that
they are truly welded at their interfaces such that a clean
separation cannot be effected by physical ox chemical means.
It is known that heat build-up during the sealing operation
may be sufficient to destroy the orientation of unl-axially
oriented films in the vicinity of the heat seal and thus cause
serious loss of draw-induced impact strength. We have found
that sacks manufactured by the process herPinbefore described
have sufficient impact strength suitable for the lntended
l; purpose for which the sacks are made. Further, we have
found that the most valuable improved tear reslstance
properties of the film are not significantly reduced. Thus,
an acceptable bridge between the uni-axially oriented film
and the body of the seal is formed and permits the manufacture
of an improved tear resistant sack having acceptable impact
resistance.
Also included within the scope of the invention are
those tubular shipping sacks incorporating the feature
of the invention wherein the seals or other closures
provided in the tubes are formed by adhesive bonding as
an alternative to heat sealing. Use of such adhesive
bonding provides all the advantages promised hereinabove
and also improved impact resistance to the bag. This
permits use of such bags for the packaging of bulky and
lightweight materials such as, for example, vermiculite
insulation material.
Accordingly, the invention provides an open-top tubular
shipping sack having a front wall and a back wall, each wall
formed of a uni-axially oriented ply of linear low density
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polyethylene optionally blended with a minor amount of low
density polyethylene, said ply being produced by blowing and
cold drawing said linear low density polyethylene, optionally
blended with said low density polyethylene, at a draw ratio
to blow ratio of greater than 1:1.
The term "linear low density polyethylene" includes
linear low density ethylene copolymers with the lower
olefins such as, for example, butene, n-hexene, ~-methyl-
l-pentene and octene. The term "low density polyethylene"
includes low density ethylene homopolymers and copolymers
such as vinyl acetate copolymers, and blends thereof.
While it is generally accepted that all polyethylene
film is generally uni-axially oriented to some aegree, the
term "uni-axially oriented" when used with reference to
linear low density polyethylene in this specification and
claims means polyethylene film that has been blown and cold
drawn to at least a 2.5-fold extent, preferably to a 4-fold
extent, but also up to a 6-fold extent. The orienting of the
films may be carried out by the cold drawing of the blown
tube as hereinbefore described.
The cold drawn uni-axially oriented film of use in the
invention made from linear low density polyethylene resins
and low density polyethylene blends thereof can be used in a
variety of thicknesses. One particular blend of use in the
practice of the invention comprises ]inear low density and
low density polyethylenes in the ratic of 4:1.
Also included within the scope of the invention are
single ply tubular shipping sacks having walls formed of a
co-extruded laminate comprising a layer of uni-axially
oriented linear low density polyethylene produced as
hereinbefore defined and a layer of a low density ethylene
polymer or copolymer compatible with said uni-axially oriented
linear low density polyethylene. Examples of such compatible
copolymers of use in the invention are e~hylene-vinyl acetate
; 35 copolymers, ethylene-ethyl acrylate copolymers and ethylene-
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methyl metha_rylate copolymers.
It is well-known in the art to co-extrude such a two or
more polymer system to form a laminate by means of conventional
co-extrusion equipment. However, in the process according to
S the invention as is applicable to a laminate the compatible
ethylene polymer or copolymer is also subjected to the novel
same draw ratio to blow ratio (DR/BR) subsequent to the
co-extrusion step as is the linear low density polyethylene.
The compatible ethylene polymer or copolymer layer of the
laminate may constitute either the inner surface or the outer
surface of the sack to provide additional utility to the sack.
For example, where the compatible polymer or copolymer of the
laminate is a soft-flexible copolymer, such as 10~ ethylene-
vinyl acetate, providing an external surface of the sac~ it provides
superior anti-slip properties. Where a 20% ethylene-methyl
acrylate copolymer of the laminate provides the inner layer
of the sack, the sack may generally be heat sealed at
temperatures as low as 80~C. which reduces the risk and
degree of disorientation of the vulnerable oriented layer.
2~ The co-extruded laminate may comprise two or more
~ompatible layers as is deemed appropriate~ Multi-laminated
ply may be used wherein one laminate laye~ constitutes a
barrier layer to the movement of chemical vapour through
the sack walls.
Accordingly, the invention provides an open-top
tubular shipping sack as hereinbefore defined wherein said
film or ply of uni-axially oriented linear low density
polyethylene forms part of a multi-layer laminate with
one or more layers of one or more compatible ethylene polymers
or copolymers.
An embodiment of the invention will now be described by
way of example only with reference to the accompanying
drawings in which:
Figure 1 shows a front elevational view, partly cut
away, of an open-top sack according to the invention; and
Figure 2 is a sectional view along 2 - 2 of Figure 1.
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Figures l and 2 show a generally rectangular single
ply tubular sack l having a front wall 2 and a back wall 3
formed of a blown and cold drawn polyethylene blend consisting
of linear low density polyethylene (4 parts, density 0.918,
melt index 0.5 - ESCORENE 1030* from ESSO CHEMICAL*) and
low density polyethylene (l part, density 0.923, melt
index 0. 3 - CIL 503*). One end ~ of the tubular sack is
heat sealed to form a single poly open-top sack.
The sack is made by extruding the above resin blend and
blowing and cold drawing the film on modified conventional
equipment as hereinbefore described. The flattened tube
(2/" width) is fed to an end seal head where it is heat
sealed and guillotined to a bag length of 60". The flattened
tube width reduces to a tubular sack width of 16" with the
provision of two 5~" gussets. The process is operated with
the parameters as given for resin No. 8 in the Table herein-
below. The sack has a most valuable tear resistance of
` 240 gm/mil.
A series of films was blown from ESCORENE 1030* (LLDPE)
resin and CIL 503* (LDPE) resin at different blow ratios.
These were subsequently cold drawn below their crystalline
melting point at different draw ratios and teste~ for M.D.
tear resistance. The process parameters and results are
given in the following Table.
* Trademark
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