Note: Descriptions are shown in the official language in which they were submitted.
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STIFF WOVEN POLYETHYLENE FABRIC
The present invention relates to a fabric,
made from woven polyethylene tapes, which is useful for
making industrial sacks.
It is known to make industrial sacks from
fabrics, made from woven polyethylene tapes, which have
been extrusion coated with a layer of polyethylene.
When used in some automatic sack making and filling
methods many such commercial fabrics suffer from the
disadvantage of being relatively limp. For example in
one step in an automatic sack filling process a filled
open-mouthed sack is advanced, open mouth upwards,
towards a sewing station where the mouth of the sack is
sewn closed. In such a sewing step it is desirable that
the open mouth of the sack be stiff enough to prevent
sag~ing of the walls of the mouth, which leads to mal-
formation of the sewn mouth or to an incompletely sewn
mouth. In the extreme the problem of an insufficiently
stiff sack mouth may lead to jamming of the apparatus
used to prepare the mouth for sewing ancl/or the sewing
apparatus
Known methods of stiffening the fabric include
using tapes of a higher denier, e.g. using thicker tapes,
tightening the weave of the tapes, or applying a thicker
coating to the woven tape structure. These methods tend
to make the fabric more expensive however.
Another fabric made from polypropylene tapes,
coated with polypropylene possesses sufficient stiff-
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ness to be used in some automatic sack filling methods.
A disadvantage of the polypropylene fabric however is
that seams in the fabric must be joined either by sew-
ing, or by a hotmelt seaming method, which tends to
be costly.
A polyethylene fabric which tends to be sub-
stantially stiffer than fabrics made from woven poly-
ethylene tapes of similar weight per area has now been
found. An advantage of the fabric is that seams may be
made by heat sealing methods such as for example that
disclosed by R.N. ~oole in U.S. patent 3 951 050, which
issued 1976 April 20.
The present invention provides a fabric com-
prising a scrim of woven oriented polyethylene tapes,
said scrim being coated on one side with polyethylene,
and an oriented polyethylene film laminated to the
coating of the scrim.
In a preferred embodiment the polyethylene
film is a monoaxially oriented polyethylene film made
from polyethylene having a density between 0.940 and
0.970 g/cm3 and the direction of orientation of the
polyethylene film follows the direction of either the
warp or weft tapes, preferably the warp tapes of the
woven scrim.
In another embodiment the polyethylene film
is a biaxially oriented polyethylene film made from
polyethylene having a density between 0.940 and
0.970 g/cm3.
In yet another embodiment the scrim is made
from monoaxially oriented tapes of polyethylene having
a density between 0.940 and 0.970 g/cm3 and the coating
is made from polyethylene having a density between
0.910 and 0.955 g/cm3.
In a further embodiment the polyethylene film
is blown film made from polyethylene having a density
between 0.940 and 0.970 g/cm3.
The present invention also provides a sack
made from the fabric of the present invention.
The fabric of the present invention may be
made by a) weaving a scrim from warp and weft tapes
of oriented polyethylene, b) extrusion coatin~ the
scrim with a thin layer of polyethylene and c) laminat-
ing the coated scrim to an oriented polyethylene film.
Each of the three steps may be carried out
using known methods. For example the oriented poly-
ethylene film and the polyethylene coating may be
applied simultaneously to the woven scrim so that the
polyethylene coating acts as a bonding agent to both
the scrim and the film.
The method of simultaneously extrusion coat-
ing the layer of polyethylene and laminating the film
may be illustrated by Figure 1, which shows the method
diagramatically.
A web of scrim 10, made from woven polyethy-
lene tapes, is transported from supply roll 11 to take-
up roll 12 passing over idler rollers 13, 15 and 16
and chill roll 14. A thin layer of polyethylene 17 is
extruded onto scrim 10 from a slot die :L8 of extruder
19, at a position close to or at chill roll 14. Thin
layer 17 is extruded onto the side of scrim 10 which is
furthest away from chill roll 14. Simultaneously, ori-
ented polyethylene film 20 is fed from supply roll 21
and around nip roll 23 onto thin layer 17. Nip roll 22
presses against chill roll 14, thus nipping the three-
layer fabric therebetween. The chill roll causes sol-
idification of this layer 17 and hence bonding of thin
layer 17 to both scrim 10 and oriented film 20. The
thus-laminated fabric 23 is wound up on take-up roll 12.
It is to be understood that the monoaxially
oriented polyethylene ilm may be co-oriented laminated
film. Co-orientation lamination refers to a process
involving essentially simultaneous orientation and
lamination, and may be accomplished by bringing two
polyethylene films into face-to-face contact and orient-
ing the two contacting films longitudinally at a temp-
erature below the melting point of the polymer of the
film having the higher melting point. The polyethylene
films may be the same, or different, and may be homo-
polymers or copolymers of ethylene, provided that the
homopolymers or copolymers have a density between 0.940
and 0.970 g/cm3.
One method for carrying out the co-orienta-
tion lamination involves passing the two contacting
films over a first heated roll and subse~uently over
a second heated roll. The temperature of the first
roll is kept higher than that of the second roll, and
the surface speed of the second roll is at least 1.5
times the surface speed of the first roll, e.g. between
3.0 and 8.0 times the surface speed of the first roll.
It is usual ~or the gap between the first and second
rolls to be in the range of 1.2 to 5.0 mm, measured
between their surfaces in a line ~oining the rotational
centres o~ the rolls.
A preferred method for co-orientation lamin-
ation in which the two polyethylene films are the sameinvolves taking tubular polyethylene film made by the
blown film process, collapsing the film and subsequently
co-orientation laminating as hereinbefore described.
The oriented film may also be blown film made
from polyethylene having a density in the range of
0.g40 to 0.970 g/cm3. Such film may be made by a pro-
cess such as, for example, that disclosed in Canadian
patent 460 963 which issued 1949 November 8 to E.D.
Fuller. The blown film so formed is flattened and slit
at the edges, thus forming two sheets, one of which may
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then be used as the oriented ilm of the present fabric.
Such blown film process is operated preferably at a
blow-up ratio of between about 1.1 to 1 and 4.0 to 1.
The term "blow-up ratio" as used herein means
the ratio of the diameter of the expanded film tubing
to the diameter of the circular die through which the
tubing is extruded.
Biaxially oriented film, useful as the orient-
ed film used in the present invention may be made by
techni~ues known in the art.
It is preferred that the oriented film has a
thickness in the range of 15-50 ~m, most preferably in
the range of 20-30 ~m.
It is to be further understood that the mono-
axially oriented polyethylene tapes used to make the
scrim portion of the present fabric may be made by
first extruding polyethylene film e.g. by the blown
film process or the cast flat film process, slitting
the film longitudinally to form a plurality of tapes
and subsequently orienting the tapes individually in
the machine i.e. longitudinal direction. Alternatively
the monoaxially oriented tapes may be made by forming
monoaxially oriented film e.g. by co-orientation lami-
nation, and thereafter slitting said film into tapes,
as disclosed for example in Canadian patent 1 041 005
to M.J. Wolstencroft, which issued 1978 October 10. The
tapes are then woven into scrim in a known manner.
It is preferred that the polyethylene tapes
used in the scrim have a tape width in the range o 1.2-
6 mm, most preferably in the range of 1.5-3.5 mm. It
is also preferred that the tapes have a linear density in
the range of 550-2200 dtex, most preferably in the
range of 650-1450 dtex.
The selection of the polyethylene used for
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the extrusion coating step is not critical, although
as is known to those skIlled in the art certain poly-
ethylenes may be easier to extrude than others.
It is usual that the weavinq step is done
separately from the extrusion coating and lamlnating
~teps, primarily because the rate of weaving is much
slower than the rate of extrusion coating and laminat-
ing. The last two steps i.e. extrusion coating and
laminating are most conveniently done simultaneously
as hereinbefore described in relation to the descrip-
tion of the drawing. Lamination may however be carried
out separately, using known techniques.
The fabric of the present invention is use-
ful ~or making sacks, particularly those intended to
be filled using automatic sack filling apparatus.
Sacks may be made by folding the fabric longitudinally,
bringing the edges together in abutting relationship
and applying a joining strip over the abutt;ng edges.
A method and apparatus for accomplishing this is dis-
closed by R.N. Poole in U.S. patent 3 951 050. The
tube thus formed is cut into lengths appropriate for
the sacks to be made therefrom. The tube is then flat-
tened and one end of the flattened tube is sealed using
a so-called "tape-over" seal. Essentially, this type
of seal is formed by applying a wide polyethylene
strip or strip of polyethylene-coated tape scrim over
the end of the tube, using a hot melt adhesive.
The sack, when filled may be sealed by sew-
ing, or by the use of an adhesive or hot air sealing.
Sewing tends to make the seal weaker than the other
seals. A tape-over seal is preferred.
The improved stiffness of the present fabric
gives a decided advantage in automatic sack filling
operations. The fabric may however be easier to split
under high stress and sacks made therefrom may be
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unsuitable for some applications where the filled sack
is subjected to extreme stress, e.g. when a filled
45 kg sack is dropped from a height of about 5 m. It
is suitable however for many other applications.
The following examples serve to illustrate
the present invention:
In Example 1 which illustrates the prior art,
a scrim was woven from 2.54 mm wide and 1000 dtex warp
tapes and 3.43 mm wide and 1083 dtex weft tapes. Each
of the tapes was made from oriented polyethylene having
a density of 0.960 g/cm3. The scrim structure had 3.55
ends/cm and 2.75 picksJcm. The scrim was extrusion
coated with a 25 ~m thick layer of polyethylene having
a density of 0.923 g/cm3.
In Example 2, which also illustrates the prior
art, a scrim having 3.55 ends/cm and 3.55 picks/cm was
woven from a 2.54 mm wide and 1089 dtex warp tapes and
3.43 mm wide and 1211 dtex weft tapes. The tapes were
made from the same polyethylene as in Example 1. The
scrim was extrusion coated with a 35 ~m thick layer of
polyethylene having a density of 0.923 g/cm3.
In Example 3, which illustrates one embodi-
ment of the present invention, a scrim havinq 3.55 ends/
cm and 2.28 picks/cm, was woven from 2.54 mm wide and
667 dtex warp tapes and 4.45 mm wide and 1167 dtex
weft tapes. The tapes were made from the same polyethy-
lene as in Example 1. The scrim was extrusion coated
with a 25 ~m layer of polyethylene having a density of
0.923 g/cm3 and a 25 ~m monoaxially co-oriented lami-
nated film of polyethylene having a density of 0.960
g/cm3 was laminated to the coating of the coated scrim.
The direction of orientation of the film was parallel
to the warp tapes.
In Example 4 a coated scrim similar to that
of Example 1, except that the number of weft tapes was
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reduced to 1.58 picks/cm and the coated scrim was lami-
nated as in Example 3, to a 20 ~m layer of blown poly-
ethylene film having a density of about 0.955 g/cm3.
Samples from Examples 1 to 4 were analyzed
for Modified Clark Stiffness and the results are shown
in the table below:
Fabric Weight Modified Clark Stiffness Values
Example(g/m ) Warp Weft
1 91.5 65 50
2 115.3 83 96
3 94.9 144 188
4 98.3 136 75
Modified Clark Stiffness was measured using
the Technical Association of Pulp and Paper Industries
Standard T-451-m45, with the exception that two pieces
of the sample were glued together in face-to-face re-
lationship to minimize the effect of curl of the sample.
A comparison of Examples 1 and 2 show that
the combination of increasing the weave density, the
thickness of the tapes and of the coating gives only
marginal improvement. A comparison of Examples 1 and
3, however, shows that a fabric of the present inven-
tion having about the same fabric weight per square
metre as one of the prior art has a greatly increased
level of Clark Stiffness.
Indeed, the fabric of Example 3 is superior
in respect of Clark Stiffness to a fabric of poly-
propylene comprising a scrim having 4.73 ends/cm and
2.76 picks/cm woven from 2.54 mm and 1111 dtex warp
and weft tapes, coated with a 36 ~m layer of poly-
propylene copolymer.
