CA1080925A - Helical rib reinforced laminate - Google Patents

Abstract

ABSTRACT

A laminated thermoplastic packaging film which is integrally reinforced by means of integral ribs which form a part of the film structure. The ribs on each side are parallel to each other and are oblique to the longitudinal and traverse axes of the film. The ribs on opposite sides of the laminate run in intersecting directions to form a network-like configuration. The ribs may be formed with a tubular extrusion die which has been provided with small notches with rounded corners around the periphery of the die orifice. As the film is being extruded, the notched portion of the die is rotated so that the rib reinforced extruded product is characterized by having ribs, tracking in a helical path, around the extruded film tube. The extruded thermoplastic tubing with the helical ribs may then be flattened while still in a heat-softened condition, by a pair of rollers or other means, to form a two-layer. The resultant laminate is characterized by having a diamond-like network of intersecting reinforcing ribs and may be used in the fabrication of articles such as bags, packaging material and the like.

Description

This invention relates to packaging films.
Plastic bags, particularly disposable plastic bags such as liners Eor containers, trash cans, leaf bags, sand-wich bags, and the like capable of holding moderate loads of the order of 5 to 20 kg. and made of thin polyethylene film, are widely used and have become increasingly popular. Such bags have the advantage that they are highly resistant to j vermin, can be left exposed to the weather when filled, and provide a convenient and inexpensive way of disposing of unwanted trash, or of temporarily storing bulky material, such as leaves or grass clippings.
Disposable plastic bags, particularly when made of thin polyethylene film having a wall thickness of the order of 0.4 to 2 mils (10 to 50 microns) when overloaded, have a tendency to burst; if punctured, they also have a tendency to rip, the rip extending randomly and uncontrollably throughout the film of the bag. Puncture may occur, for example! by sharp objects placed in the bag, cuttings of tin cans, nails, -glass splinters or the like and even a comparatively small 0 .
puncture which is small enough to prevent escape of the con-tents of the bags themselves may, when only slightly stressed ~
(for example upon picking up of the bag) result in a rip, ;~ - -resulting in spillage of the contents. Doubling the bag is no solution, and increasing the wall thickness of the bag rapidly increases the shipping weight of the material, and the costs. The costs for such bags should be kept as low as possible, and the amount of material to be used should be a minimum, consistent with the expected usage, in view of the fact that they are considered to be disposable and may be incinerated.

We have now found a way of improving bags of this type by provlding a reinforcement in the bag structure without, ;-, ~ ,

- 2 -: .
:

however, utilizing additional rnaterials, or markedly increas-ing the cost of manufacture. This is achieved by providing ribs on the film.
It is known that the strength of materials can be increased by forming ribs thereon; ribbed plastic material, as such, has been proposed previously and U.S. Patent 2,750,631 describes a method of manufacturing ribbed sheet plastic. This patent is related, essentially, to an extrusion apparatus which is modified to provide sheet plastic material which has ribs thereon, essentially of square configuration in cross-section.
U.S. Patent 3,193,604 describes an extrusion die for forming ribbed sheeting. The die has recesses to form -~
strands or ribs on one or both sides of the extruded film. If `
ribs are desired on both sides of the film, a dual rotating die arrangement has to be used, with an internally rotating ;
die member inside an externally rotating die member. This arrangement is complicated and it would be desirable to be able to form a film with a pattern of intersecting ribs on the two faces without using such a complicated arrangement.- ;
German Offenlegungsschrift 2406821 of Giuseppe C.
Scarpa, published November ~1, 1974, discloses a similar process for making tubular films with helical reinforcing ` ribs on one side. The disadvantage of having parallel ; helical ribs on only one side of the film is, of course, that ` a rip which starts between the ribs can spread rapidly along the film, running in a helical path between the parallel ribs. It would clearly be desirable to prevent this by provid- -~
ing intersecting ribs which would limit the extent to which a rip could run. The present invention provides a film with such a pattern of ribs and,moreover, enables the ribs to be formed by simple modifications to existing equipment. ;
.. . . .
: - 3 -,; ... : , . . :

The present invention resides in a method of forming a thermoplastic film laminate, which comprises extruding a molten thermoplastic polymer through the annular orifice of a tubular extrusion die the outer member of which die is provided with notches, while continuously rotating said outer member of the extrusion die, whereby a tubular film is disposed having ribs in a helical pattern with respect to the longitudinal axis of the tubular film, flattening the tubular film, and heat laminating together the plies of the flattened film to form a two-layer thermally-bonded laminate in which the ribs on opposite surfaces of the laminate extend in directions which cross each other so that a network of - ~
crossing ribs is obtained. ~ -In a more particular aspect, this invention resides ; in a method as described in the immediately preceding para-graph in which the ribs formed have a thickness ttransverse to the thickness of the film) of about 3.5 mils, are peaked and have on each side thereof a parallel zone of material thicker than the normal wall thickness of the film, the over-all width of the thicker material being approximately 25 mils, the ribs sloping smoothly from their peaks through the `
thicker material to an area of normal wall thickness, and ; the distance between ribs being from 0.1 to 0.5 inch.
The method of the present invention provides a laminate thermoplastic film which comprises at least two layers of : thermoplastic film bonded together. One exterior sur~ace of the laminate has substantially parallel, spaced~apart, peaked ribs which are integral with the laminate surface and formed in the surface and which extend diagonally across the surface. The opposite exterior surface of the laminate also has substantially parallel, spaced-apart, peaked ribs integral with the opposite surface and formed in the opposite surface.

~ 4 ~ `
'' :.' , These ribs extending diagonally across the surface in a direc-tion diagonally opposed to the ribs on the first surface, resulting in the ribs on both surfaces of the laminate inter-secting at a plurality of points producing a network of inter-secting ribs on the laminar film.
The laminate is formed by extruding a molten thermo-plastic resin through the annular orifice of a tubular extru-sion die which is provided with notches either on the interior or exterior member of the extrusion die. The extrusion die is rotated continuously during the course of the extrusion opera-tion. The rotation imparts a twist to the film tube which : . .
results in the ribs formed by the notches advancing in a helical direction around the longitudinal axis of the extruded tube. The tube bearing the helically extending rib elements ; can be cooled and subsequently flattened utilizing conventional collapsing or nip-roller apparatus but to form the laminate the extruded tube bearing the helically extending ribs is flattened while it is still in a heat-softened condition from the heat of extrusion to form the two-layer thermally-bonded ;
laminate. The helically-extending ribs on opposite surfaces of the laminate extend in directions which intersect with each other so that a network of intersecting ribs is formed with ~ rhomboidal areas between the intersecting ribs. Instead of - collapsing the tube while it is in a heat-softened condition from the heat of extrusion to form the laminate, the film tube may be allowed to cool to a non-plastic state and then may be subsequently reheated utilizing conventional means such as infra-red heaters and/or heated collapsing rollers and the like to form the laminate.
By virtue of the helical configuration of the reinforc-ing rib structures along the film, linear propogation of a tear which may have been initiated in the film is stopped or _ 5 _ .. : . . . , : , . . . .
.

blocked by the ribs. For e~ample, if the essentially paral- - :
lel ribs ran in a direction paralleling either the length or the width of the film, linear propogation of an initiated -tear immediate such ribs would not be stopped by such ribs, the tear direction merely paralleling the rib direction.
However, the helical pattern of intersecting ribs will stop propagation of a tear running either along the length or the width of the film since, under normal conditions, the tear will be interrupted by a rib and halted.
Further features and advantages of the inven-tion ~
will become apparent from the following description of pre- ` -ferred embodiments, given by way of example, with reference to the accompanying drawings in which:
Fig. 1 is a schematic side elevation of an extruder and die apparatus which may be employed to produce the reinforced thermoplastic film, -Fig. 2 is an end-elevation of Fig. 1, Fig. 3 is an overhead schematic view of one form of extrusion die which may be employed, ; 20 Fig. 4 is an overhead schematic view of another form of extrusion die which may be employed~
Fig. 5 is a cross-sectional view of the laminar film structure shown in Fig. 1 taken on line 5-5 of Fig. 2 and - Fig. 6 illustrates a bag construction which may be fabricated from the rib-reinforced film.
As shown in Fig. 1, a standard rotating screw type thermoplastic extruder generally designated as 12 is supplied with thermoplastic resin pellets through hopper 11. As the -rotating screw, powered by drive member 10, rotates within the confines of the extruder barrel 12 it causes advancement of the thermoplastic resin particles to adapter 14. The resin particles are heated to a molten condition and thoroughly , .

mixed during advancement through barrel 12. The resultant molten extrudate is expressed through extruder 12 and into adapter 14 and from there into and through annular die member 15. Annular die member 15 has a circular outlet orifice 18 whereby the molten extrudate is expressed from die 15 in the form of a hollow tube 20. Tube 20 is inflated with air sup-plied through the central portion of die 15 via air inlet conduit 13, as conventionally practiced in tubular thermo-plastic extrusion operations. As shown in Fig. 1 annular die 15 is provided with a circular gear wheel 16 which is posi-tioned about the exterior circumference of die 15. A sealed rotary joint 17 is positioned intermediate adaptor 14 and die -15. Rotary joint 17 is chain driven by a drive wheel -~
sprocket attached to a motor 17' in a conventional manner. ~ ~-When sprocket 16 rotates it causes rotation of rotary joint 17, whereby rotary motion is imparted to die 15.
As shown in Figs. 3 and 4, spaced-apart notches 19 are formed in the annular die 15 which circumscribe the cir-cumference of die orifice 18. These spaced-apart notches 19 may be formed either in the external annular lip 25 of die orifice 18 as shown in Fig. 3 or conversely notches 19 may be formed in the internal mandrel portion 24 of die 15 as shown in Fig. 4. In the embodiment shown in Fig. 3, reinforcing ribs 19 will be formed on the exterior surface of tube 20 whereas, utilizing the annular orifice configuration of Fig. 4, rein-forcing ribs 19 will be formed on the interior surface of tube 20. The rotating motion of the extruded film imparted by the rotary die will ensure the production of helical ribs when ~ ;
the mandrel 24 is notched, as shown in Fig. 4.
As shown in Figs. 1 and 2, as the molten thermoplastic material is extruded in the form of an inflated tube 20, ribs 21 are formed in the tube by means of the profiled configuration - 7 ~

.. , . : .

: . :' , - - . - ' , , ,., ., ., ' ' .~ ~' ,:, . ~: ,.: . ::::: ' of extrusion orifice 18. Air ring 9 is positioned adjacent die 15 to surround and cool the extruded tube 20, a conventional cooling technique employed in tubular extrusion of thermoplas- -~
ti~s.
The rib-reinforced material, after it has cooled and solidified, may be collapsed by a se-t of nip rollers 22 to form a flattened tube 23 of rib-reinforced material. Conversely tube 20 may be collapsed while it is in a heat-softened state -so that upon collapsing, the interior surfaces thereof will ; 10 heat-weld together thereby forming a two-layer laminate struc-ture. It will be noted that in the event it is desired to completely solidify and set the tubing after it has been ex-truded, the tubing may be reheated prior to collapsing it so that lamination will occur at the interface of the collapsed tube. This may be accomplished utilizing external heaters such as infra-red heaters (not shown) and/or utilizing inter-nal heating means for rollers 22.
As shown in Figs. 1 and 5, the heat-welded laminar material is characterized by having a series of spaced-apart ` 20 parallel ribs 19 on opposite surfaces of the laminate. The ; ribs on both surfaces extend in a direction which is oblique to the longitudinal axis of the tube from which the laminate was formed. The direction in which the parallel ribs extend on one surface of the laminate is opposed to, or at an angle ~;
with, the direction in which the reinforcing ribs extend on the opposite surface of the laminate. sy virtue of such opposing rib directions on opposite surfaces of the laminate, :
the final product is characterised by having a network of intersecting rib elements with rhomboidal areas between the intersecting ribs. The shape of the rhombus is a function of the speed of the film draw-off rate and the speed of die rota-tion.
':

` . ' :

- . . . . .
.
.

Either the die or the mandrel may be formed with the notches. These are preferably radially-extending, peaked notches which have smoothly rounded corners at their inter-section with the annular extrusion surface of the die or ; mandrel. The peaking of the notches provides the peaked ribs, and the smoothly rounded contour of the notches, as they merge into the die surface provides ribs with smoothly rounded contours extending towards the sheet material. The gap be-tween the mandrel and the extrusion die is substantially wider than the thickness of the wall of the film but the ratio of the depths of the notches with respect to the thickness of ribs is not the same as the ratio of gap width to wall thick-ness; rather, the depths of the notches is substantially less.
The reduction of thickness, during blowing, of the walls from the gap widths to wall widths is greater than the reduction of thickness, upon blowing, of the ridges from notch depths to the overall ridge dimension.
If the quantity of resin material extruded, for ex-ample, for each linear meter of material, is the same for rib-bed as for non-ribbed webs, it is evident that the thickness of ~he material between ribs will be less than that in non-ribbed material, in order to permit the accumulation of material in the ribs. Although the web material, between the ribs, will be somewhat thinner, the overall strength of the bag as measured by tear resistance is enhanced. The conditions which are im-portant to be matched to each other, so that the ribbed struc-ture can be made with its improved tear resistance, without ; additional use of material, are: -(a) Thickness of the ribst that is, overall extent of the ribs, transverse of the thickness of the sheet; this should be about 3.5 mils (about 90 '';';'' ' '', ', ... . . . ..
: ' . . ' .. ' . . ':

.

microns), or slightly less. If substantially ~ ~
less, for example 2 mils (50 microns) then the ~;
ribs will not be thick enough; if thicker, that is, for example 6 mils ~150 microns), additional material will be necessary which increases the total amount of material used without, however, ; substantially enhancing the utility or strength of the bag.
(b) Dlstance between ribs: This should be about 0.1 to 0.5 inches (2.5 to 12.5 mm.). It has been found from actual experience that about 0.25 inches (6 mm.) is the best distance between ribs, although there is very little change in ~ -;
characteristics of the bag within the range of spacing given above.
(c) Slope of the ridges: The slope of the ridge from the peak of a rib to the normal wall thick-ness has an effect on the overall strength. If the slope is too steep, or if the ridge is not , clearly peaked but almost square on top, or ~- trapeze-shaped, a weak point results at the ~-junction between the rib and the web thickness.
This can be compared to the well known notch effect, which should be avoided, since weakness is introduced at the junction between the rib thickness and the rest of the web material, un-, less there is a smooth transition~ This smooth i transition could be compared to the fillet in a ;
:` welded structure. In order to provide smooth ; 30 transition, the die through which the material is extruded, at the side where the ridges are to be formed, should have smoothly rounded edges .:
,; ~

-; :

. ~ . . . .: , , ., ... . ~ , . . , :

which smoothly merge into the circumference of the remainder of the die surface. ~ -(d) Low point between ridges: The average thickness of the web should be as uniform as possible and an undercut adjacent the formation of the rib, with respect to the remainder of the web or film thickness should be avoided. The film thickness should be maintained as uniform as possible. Low points, or undercuts may result if the slope of the peaks (above condition) is -selected to be too steep. For ridges of an`
. , .
overall thickness of about 3.5 mils (about 90 microns) a width of thicker material (measured .. ..
between the regions where the normal- wall thick-ness obtains) of about 25 mils (625 microns) is ~` suitable.
. ~ .
The material itself is not critical, and since its eventual destination is to be discarded, it will normally be used~only for short periods of time. The quality of the mate-;~20~ rial~can range within the wide lim1ts and~the actual composi-tion, and characteristics need not be critically controlled.
A~suitable mater~ial is conventlonal 1iner grade low density polyethylene, that is, polyethylene having a relative density from 0.9 to 0.925. The melt index of the material is likewise ;;
variable within a wide range, and may suitably be in the range of from 0.2 to 6, although a melt index at the lower end of the range is preferred, that is, preferably less than 3, for example 0.3 to 2. ~ ;
The material of which the bags are made is generally referred to in ASTM Standard B 1248-68, "Standard specification . ::`~` for polyethylene plastics molding and extrusion material." -.: . l : ~
: .
. ~ "
-- 1 1 -- . .

~ ' ' ' ; " ' : ' . ' ' ' , . . ' . . ~ ' , , , !, . . . ~,,, . ' .
,. ,, . :' ' " ' , ' ' . ' "'. . ' '' " :: '' ' ' ' ';: ~ . , ' . ' , ' ' ' ' ': ' ' '' ' , ' $
.
With respect to density, the type 1 material is suitable with respect to melt index, material of categories 3, 4 and 5 are -suitable.
The following Example illustrates a specific embod-iment of a method of manufacture of the helically rib-rein-forced film laminar structure.

Low density polyethylene resin having a relative density of 0.919 and a melt index of 1.7 was extruded utilizing a standard 3.5 inch (89 mm.) thermoplastic extruder employing a screw with a 24:1 length to diameter ratio. Melt temperature of the molten polyethylene within the confines of the extruder was about 400F (200C) and the internal pressure within the extruder was about 4550 lbs. per sq. in. (320 kg./cm2). The extrusion die orifice was 12 inches (30 cm) in diameter and had a 0.030 inch (0.762 mm) orifice gap. The die outer lip ; was notched to give 3 to 4 ribs per in. (1 rib per 6.3 to 8.5 ; mm.) in the extruded film product. The die was rotated utiliz-~ ing the apparatus shown in Fig. 1 at a speed of approximately ~ :
- 20 8 revolutions per min. The tube collapsing nip rollers were positioned 14 feet (4.26m.) from the die orifice, a distance -which is lower than normal so that the film tube would be collapsing while the film is at a relatively higher temperature than that of normal tube collapsing operations. Additionally, heat was supplied to the collapsing rollers by infra-red heat lamps which were positioned adjacent to the rolls to attain a tube temperature during the collapsing operation of about 160F.
(70C.). At this temperature, the internal surfaces of the tube ;
were in a heat-softened condition so that ~hen the tube was collapsed, the internal contacting surfaces of the tube were ;~
heat-welded together to form a two-layer laminar structure. ~;~

:

.

The flattened laminar structure was approximately 37 inches (94 cm) in width. The laminate was characterized by having a surface configuration of diagonally intersecting ribs to form a diamond-like pattern as shown in Fig. 2. This laminated sheet was then wound on to hulk rolls ready for product fabrication into articles such as bags or overwrap packaging film material.
Fig. 6 shows a bag fabricated from the laminar film embodiment of the present invention and having the character-istic diamond-like pattern of intersecting ribs.

,, .
',,''.''.

~::

;
.' . :
. ' ', ' .
: -.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of forming a thermoplastic film laminate, which comprises extruding a molten thermoplastic polymer through the annular orifice of a tubular extrusion die the outer member of which die is provided with notches, while continuously rotating said outer member of the extrusion die, whereby a tubular film is disposed having ribs in a helical pattern with respect to the longitudinal axis of the tubular film, flattening the tubular film, and heat-laminating together the plies of the flattened film to form a two-layer thermally-bonded laminate in which the ribs on the opposite surfaces of the laminate extend in directions which cross each other so that a network of crossing ribs is obtained.

2. A method according to claim 1, in which flattening and lamination of the tubular film is effected while the film is still in a heat-softened condition from the heat of extrusion.

3. A method according to claim 1 or 2, in which the ribs formed have a thickness (transverse to the thickness of the film) of about 3.5 mils, are peaked and have on each side thereof a parallel zone of material thicker than the normal wall thickness of the film, the overall width of the thicker material being approximately 25 mils, the ribs sloping smoothly from their peaks through the thicker material to an area of normal wall thickness, and the distance between ribs being from 0.1 to 0.5 inch.