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This invention relates to plastics film and bags and sacks made therefrom.
The production of plastics sacks and bags from film, especially blow
extruded tubular film, is well known and has been widely employed for many
years. Plastics compositions based on low density ethylene polymer have
generally been found to provide the most desirable combination of physical
properties and cost but the faces of the bags and sacks formed from such
compositions are very slippery (i.e. have a high slip coefficient) and
this makes their stacking and transport difficult, especially when full.
The art has been aware of this problem for over a decade and many proposals
have been made for solving it. One approach has been to treat the surface
of the film which is to form the outer surface of the sack in order to
roughen it e.g. as proposed in GB-A-1533027 and GB-A-2043527. However,
this requires an additional step in the formation of the film, and is
therefore economically undesirable. Another approach has been to produce
the film in the form of a laminate and to form the layer which is to
provide the outer surface of the sack from a polymeric material having a
relatively high coefficient of friction, e.g. as in U.S. Pat. No.
3,424,649, which proposes a blend of at least 50% up to about 83% by
weight of a specified styrene-butadiene block copolymer, the balance being
polyethylene. However, this requires the use of a relatively large amount
of a special and therefore relatively expensive polymer. Yet another
approach has been to produce the film in the form of a laminate of
ethylene polymer layers and to include in the layer which is to form the
outer surface of the sack a small amount of a tackifier so as to increase
the coefficient of friction of that surface. Materials that have been
proposed as tackifiers for that purpose include hydrocarbon resins, as in
Japanese patent publication No. 79-95683, and ethylene/vinyl acetate
copolymers, as in French patent specification No. 2031801. So far as we
are aware, however, none of these proposals has achieved more than limited
commercial success.
According to the present invention, the problem has been solved
satisfactorily by providing a thermoplastics sack or bag at least one of
the walls of which has an outer layer and an inner layer, each of which is
formed by melt-extrusion from a polyethylene, and the outer layer contains
an isobutylene polymer in an amount sufficient to increase the coefficient
of friction.
The wall may consist of the two layers or may include one or more further
layers lying between said outer and inner layers.
In general both of the walls of the bag or sack will have said multi-layer
construction but this is not essential because the benefit of the
invention can still be obtained when only one of the walls is so
constructed provided that all the sacks (or bags) in a stack are arranged
with the said wall facing in the same direction; i.e. either with said
wall being the upper wall or the underneath wall in all the sacks in the
stack.
The layers of the wall may be separate or they may be fused together, in
which case the wall may be produced in a single operation e.g. by a
process such as coextrusion.
Thus, the invention also provides a multi-layer film comprising a laminate
wherein the external layers, i.e. the layers providing the two faces of
the film, are each formed by extrusion from a polyethylene and one of said
layers contains an isobutylene polymer in an amount sufficient to increase
the coefficient of friction.
The layers of the film are preferably thermally fused together while at
least one of them is still in the heat-softened state as a result of being
melted for extrusion, and most preferably the film is formed by
coextrusion.
The film may be flat film extruded from a slot die but preferably is
tubular film e.g. produced by the blow extrusion process.
Bags and sacks according to the invention may be made from the film with
the layer containing the isobutylene polymer providing the outer layer of
the wall of the bag or sack. The bags and sacks may be formed from the
film in known manner and may be of simple form or, for example, in the
form of pillow sacks or valve sacks.
It has been found that by using an isobutylene polymer as the additive it
is possible to achieve a higher increase in the coefficient of friction
than has been possible hitherto, e.g. to a level of at least 0.8 as
measured by ASTM D-1894 (static test), without producing a surface which
is sticky to the touch or which becomes sticky to the touch on aging.
Therefore, while the films can be processed on downstream machinery as
employed in the production of bags and sacks, do not attract dust and are
not unpleasant to manhandle, bags and sacks formed from them can be
stacked more readily into stable piles which can be palletised and
transported without difficulty by conventional means such as fork lift
trucks.
It has also been found that the coefficient of friction tends to increase
on aging but without any apparent increase in stickiness. In the case of
at least some of the prior art proposals in which tackifiers are employed
as the additives for increasing the coefficient of friction, on the other
hand, either the coefficient of friction tends to decrease on aging or the
additive tends to migrate to the surface and cause stickiness and severe
difficulties in handling.
Adequate results are obtained with even quite small amounts of the
isobutylene polymer, e.g. less than 10%, preferably less than 8% by weight
and more preferably 1 to 6%, based on the weight of polyethylene in the
layer and another unexpected observation is that particularly good results
are obtained at low molecular weights M.sub.w (polyethylene equivalent),
e.g. 1000-5000 or even less, possibly down to 800 or even 500, when
employing polyisobutylene, despite the marked inherent stickiness and
semi-liquid or syruppy state of such material.
Where the bag or sack is formed from a multi-layer film in accordance with
the invention, the film is preferably formed by coextrusion. Co-extrusion
involves extruding two or more different plastics layers through a common
die to produce a multilayer product in a single step. The plastics
materials forming the layers may be extruded through different orifices in
the same die, whereby in the present invention the two external layers are
extruded as separate entities and fuse together downstream of the die, or
they may be extruded through the same orifice. The film of the present
invention may have two layers, or more than two layers, if desired, thus
offering the opportunity of further varying the physical properties of the
film while providing the desired increase in surface coefficient of
friction in one of the external layers.
In general, where the film has been produced by coextrusion both the
external layers will be continuous. However the layer containing the
isobutylene polymer may be discontinuous, e.g. in the form of one or more
stripes or bands which may extend in the direction of extrusion or
alternatively at an angle thereto in which case they will take on a
helical configuration when tubular film is being produced. Alternatively,
this layer may be continuous and the other external layer of the film may
be discontinuous but this is less satisfactory since it may interfere with
facile filling of the bag.
By arranging for the wall of the sack or bag to have two or more layers it
is possible to arrange that the inner layer, and/or any other layer
between the inner and outer layer where the wall has three or more layers,
largely provides the desired physical properties such as puncture and tear
resistance and burst strength, thereby permitting the outer layer to be
formed with a substantially higher coefficient of friction than would
otherwise be possible. Desirably, the outer layer will have a coefficient
of friction (static), as measured in accordance with ASTM D-1894, of at
least 0.8, preferably at least 0.85, more preferably at least 0.87. The
coefficient of friction of the outer layer ideally should be at least
equal to that of paper of the kind used in the outer layers of heavy duty
paper sacks. The inner layer preferably has a coefficient of friction,
measured by the same test, of less than 0.5, more preferably not greater
than 0.4. It is frequently desirable for it to have a value of 0.3 or
less. Such low coefficient of friction may be achieved, for example, by
use of a slip agent, such as a fatty acid amide, in known manner.
By "polyethylene", as used herein, is meant a homopolymer of polyethylene
or a copolymer thereof with a few percent, e.g. up to 5 mole %, of another
.alpha.-olefin, preferably an .alpha.-olefin having from 4 to 8 carbon
atoms, as in butene-1, hexene-1 and octene-1. It will be understood that
where the multilayer film of the invention is to be produced by
coextrusion, the polymers chosen for each of the layers must have similar
processing characteristics so that both may be extruded through the same
die.
In one embodiment, the layers may each comprise low density polyethylene.
In a preferred embodiment, however, the layer which provides the outer
face and which includes isobutylene polymer comprises linear low density
polyethylene and the layer which provides the inner face comprises a
polyethylene which in terms of melt processing characteristics is
compatible with said linear low density polyethylene, e.g. the same or
different linear low density polyethylene, a polyethylene of higher
density or a mixture comprising linear low density polyethylene and a
polyethylene of higher density. Another preferred embodiment has as one
layer low density polyethylene and as the other type, which is preferably
the layer providing the outer face, linear low density polyethylene.
Linear low density ethylene polymer is ethylene polymer having a density
corresponding to that of conventional low density polyethylene, i.e. up to
about 0.925 g/cm.sup.3 but which has been manufactured by a low pressure
process employing a metalbased catalyst such as a Ziegler, Phillips, Union
Carbide, Dow or Standard catalyst, and has fewer CH.sub.3 - groups per
1000 carbon atoms than conventional low density polyethylene made by the
high pressure process using a peroxy catalyst. In general, linear low
density ethylene polymers contain a small percentage of a second
.alpha.-olefin as comonomer. Said second .alpha.-olefin normally has at
least 4 carbon atoms.
The polymer composition from which each layer is formed may also include a
minor proportion of another polymeric material compatible with the
polyethylene employed, e.g. ethylene copolymers containing more than about
5% .alpha.-olefin comonomer, ethylene copolymers with other monomers
and/or propylene polymers (which term includes copolymers).
While it has found that with appropriate choice of the polymer from which
the layer forming the outer face is formed, the bags or sacks of the
invention may be stitched using essentially the same automatic equipment
as is conventionally employed with paper sacks, it may be found
advantageous to coat the area of that layer which contains the isobutylene
polymer and which is to be contacted by stitching or automatic bag
handling machinery, e.g. the shoe of a sewing machine, with a band of a
material which has a lower coefficient of friction than said layer. The
coating may be applied to the bag or sack or to the appropriate face of
the film from which it is to be formed. However one convenient method of
achieving this is to coat the area with a band of printing ink since this
may be effected simultaneously with the printing of any desired legend on
the film or a bag or sack made therefrom.
In one embodiment of the invention, which is particularly desirable where
the bags or sacks are to hold living horticultural produce, e.g. seeds,
bulbs or corms, at least one of the layers contains an infra-red and/or
ultra-violet absorber, e.g. carbon black, to inhibit premature sprouting.
Preferably, the inner layer (or, where the bag or sack comprises more than
two layers, an inner layer) is thus modified, thereby permitting the outer
layer to be distinctively coloured e.g. for identification purposes or for
appearance.
The bags and sacks or the films employed in their production may be
microperforated in known manner e.g. for applications where otherwise the
materials stored in them would sweat.
Bags and sacks may be made according to the invention which exhibit a
combination of puncture resistance, tear strength, burst strength, tensile
strength and low-slip outer surface that enables them to be employed as
acceptable substitutes for heavy duty paper sacks including multi-ply
sacks and provides them with a stacking capability which can exceed that
of the paper sacks. Particularly good results are obtained when the bags
or sacks are made from the multi-layer film of the invention, especially
when the film has been produced by coextrusion.
The invention is now illustrated by the following Example. A black tubular
film having an overall wall thickness of 150 microns was blow coextruded
from a first composition which contained carbon black and a slip agent and
in which the polymeric component was a blend of 75 parts by weight of a
linear low density ethylene polymer having a density of 0.920 and
containing about 3 mole % octene-1 and marketed as DOWLEX 2045 and 25
parts by weight of an ethylene polymer having a density of 0.938 and
containing about 1.2 mole % hexene-1 and marketed as FINA HR 501, and a
second composition in which the polymeric component was a blend having a
density of about 0.919 and marketed as UNIFOS 8006 and comprising (a) a
linear low density ethylene polymer containing about 3 mole % butene-1 and
(b) about 3.6% by weight of a polyisobutylene having a molecular weight
M.sub.w (polyethylene equivalent) in the range of about 1000 to 2000 and a
broad molecular weight spread of from about 300 to about 3000. The first
composition formed the inner layer of the tube and the second formed the
outer which had a thickness of about 38 microns.
The outer face of the walls of sacks made from the film was found to have a
coefficient of friction of 0.85 to 0.87 but was not sticky to the touch
and was not unpleasant to manhandle. The inner face had a coefficient of
friction below 0.5, thereby facilitating filling the sack.
A 40".times.48" pallet was loaded with 1 tonne of produce packed into 40
sacks stacked in 8 layers of 5 sacks per layer. Using the sacks of the
invention, the stack was still stable after tipping the pallet about one
of its 48" edges until the opposite edge was more than 20" off the ground.
There was no evidence of deterioration of the frictional properties or of
stickiness developing in the surface of the sacks of the invention even
after several months storage. Moreover, the sacks were found to stack
better than paper sacks and have higher resistance to water penetration,
improved printability and better inhibition of the germination of seed
products packed in them. They can be stitched like paper sacks or heat
sealed, and can be stored outside.
A method of forming a valve bag in accordance with the invention is now
described with the aid of the accompanying drawings in which
FIG. 1 is an elevational view of a bag made from a single ply of film,
FIG. 2 is a sectional view along line A--A of FIG. 1,
FIG. 3 shows a modification of the bag of FIG. 2,
FIG. 4 is a view corresponding to that of FIG. 2 but of a different form of
bag, and
FIG. 5 is a view corresponding to that of FIG. 2 of a bag similar in
construction to that of FIGS. 1 and 2 but formed from two separate plies.
Referring to FIGS. 1 and 2, the bag 2 has a two-ply back wall 10 and a
front wall made of two-ply partially overlapping panels 12 and 14 which
are integral with the back wall whereby to from therewith a flattened
two-ply tube which is closed at both ends by transverse seals 16 and 18.
Panel 14 lies beneath panel 12 in the area of overlap and the free edge of
its inner ply 20 projects a short distance beyond the free edge of its
outer ply 22. A seal 24 unites both plies of panel 14 with the two plies
of panel 12 and a seal 26 unites the projecting end of ply 20 wih the two
plies of panel 12. The portion left unsealed in the common area of overlap
at the top end of the bag forms a tubular valve sleeve which is delineated
by the top ends of seals 24 and 26 together with seals 28. The projecting
end portion of inner ply 20 acts as a closing flap when the bag has been
filled.
It will be understood that in an alternative embodiment, outer ply 22 may
project beyond inner ply 20. Furthermore, seal 26 is not essential and
seals 28 may be replaced by a continuous seal. In yet another embodiment,
illustrated in FIG. 4, outer ply 22 can extend over panel 12 instead of
beneath it; in other words the portion of panel 12 which overlaps panel 14
lies between plies 20 and 22.
The bag is formed by folding and sealing a flat film or folded slit tubular
film only part of the width of which is provided, over at least a part of
one surface thereof, with the layer containing the isobutylene polymer,
said part of the width being that which provides the external faces of the
walls of the bag. That part of the width of the film which provides the
inner surfaces of the bag is not provided with this layer. Thus, in the
embodiment illustrated in FIGS. 1 and 2 the bag is formed from a film
which comprises a first layer 30 of polyethylene (as herein defined) and a
second layer 32 comprising polyethylene and also containing isobutylene
polymer in an amount which increases the surface coefficient of friction.
This layer is in the form of a band whose width corresponds with that part
of the film which provides the external faces of walls 10, 12 and 14 of
the bag.
Not all of the width of the film which provides the external faces of the
bag need be provided with the layer 32. For example, as illustrated in
FIG. 3, the layer may be provided as two separate bands 32A and 32B which
provide the external faces of panels 10 and 12, respectively.
In the embodiment illustrated in FIG. 5, the bag of FIGS. 1 and 2 is formed
instead from two plies 40 and 42 at least the outer ply 40 being a
multilayer film of this invention with the outer layer containing
isobutylene polymer. The inner ply 42 may comprise the same multilayer
film, in which case the layer which does not contain isobutylene polymer
preferably faces inwards whereby to form the inner face of the bag, or, as
shown in FIG. 5, it may comprise a different film provided it can be
sealed, e.g. by heat sealing, to the outer ply 40.
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