CA1138817A

CA1138817A - Coextruded product and process for the manufacture thereof

Info

Publication number: CA1138817A
Application number: CA000348994A
Authority: CA
Inventors: Warren Baker
Original assignee: Leco Inc
Current assignee: Industries Leco Inc
Priority date: 1980-04-01
Filing date: 1980-04-01
Publication date: 1983-01-04

Abstract

ABSTRACT OF THE DISCLOSURE A process for use in making bags from layered, coextruded film. The film can comprise a layer of HDPE (high density polyethylene) or PP (polypropylene) on a layer of EVA. Normally the HDPE layer cannot be heat sealed to the EVA layer. Both sides of the film are given a fairly high corona treatment. After treatment, a lap joint can be formed with the EVA layer abutting the HDPE layer, and the lap joint can then be heat-sealed together. The invention is also directed to a sleeve or bag made from layered, co-extruded film. One outer layer of the film forming the bag is made from HDPE, the inside layer from one of EVA (ethylene vinyl acetate), EEA, EMA (ethylene maleic anhydride) or EAA. The bag has a heat-sealed lap joint with the inside surface layer overlapping the one outer surface.

Description

This invention is directed toward a process for use in making a bag. The invention is also directed toward a sleeve for use in making a bag, and to the bag itself.
Many bags, particularly those used in the food industry, are now made from layered, coextruded films or sheets to provide certain desirable characteristics. The outer layer of the film normally has good heat resistance for example, while the inner layer of the film has good heat seal characteristics so that the ends of the bag can~be sealed together. Because of these characteristics, however, the longitudinal seam in a bag made from such two-layer co-extruded film must be a fin seal. In a fin seal the inner layer of the film abuts on itself and since the inner layer is made of material which has good heat seal characteristics, the fin seal produced is strong.
It would, however, be preferable to use a lap seal for the longitudinal seam. A lap seal requires 4 to 7% less film material than a in seal thus resulting in a substantial saving. In addition, existing bag filling machines would not have to be changed over from their lap seal configuration, which configuration is most common for bags made from waxed glassine and cellophane film, to a fin seal configuration needed when using layered, coextruded film. A lap seal is not used, however, with layered, coextruded film since the outer layer, which has good heat resistance, normally does not seal to the inner layer at the low heat-sealing tempera-tures used commercially.
It has now been discovered that the layered, coextruded film can be treated in such a manner that the film can produce commercially satisfactory lap seals. More . 1 ~

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particularly, it has been found that if layered, coextruded film is subjected to a fairly high corona treatment on both sides, the heat-resistant outer layer of Eilm will bond to the inner layer of film at relatively low heat-sealing tem-peratures. Thus, lap seals where the inner layer of film abuts the outer layer of film can be successfully used with their attendant advantages instead of fin seals where -the inner layer of film abuts itself.
The invention is particularly directed toward a process for use in making a bag. The process comprises providing a layered, coextruded film with one outer layer of the film made from a first type of thermoplastic material and the other outer layer of the film made from a second type o~thermoplastic material, the layers of the first and second types of thermoplastic material normally not heat-sealable tcgether at low sealing temperatures. A corona treatment is applied to both sides of the film to at least slightly oxidize the outer surfaces of the film. Side edges of the treated film are then overlapped to form a lap joint and the lap joint is heat-sealed together at a low temperature to form a sleeve for use in making a bag.
The invention is also directed toward the sleeve or bag made from layered, coextruded film with a lap joint.
The invention will now be described in detail haviny reference to the accompanying drawings, in which:
FIGURE 1 is a perspective view of a sleeve;
FIGURE 2 is a cross-sectional view of the sleeve taken along line 2-2 of Figure l;
FIGURE 3 is a graph of heat-sealing temperature vs. joint strength for various joined materials;

FIGURE 4 is another graph of hea-t-sealing temperature vs. joint strength for various joined materials; and FIGURE 5 is a further graph of heat~sealing temperature vs. joint strength for polypropylene and EVA.
A sleeve 1 is manufactured in accordance with the present invention, for use in making a bag. A sheet 3 of layered thermoplastic film has its ed~es 5, 7 overlapped to form a lap joint 9. The lap joint 9 is then haat-sealed together as shown at 11. The outer film layer 13 of the sheet is made from one type of thermoplastic material and the inner film layer 15 is made from a second type of t:hermo-plastic material. More particularlyr the outer Eilm layer 13 is made Erom suitable thermoplastic material which is more heat-resistant than the inner film layer 15 while the inner film layer is made from thermoplastic material -which has good heat seal characteristics in comparison to the outer layer. The outer film layer 13 is preferably made from HDPE, although it could also be made from PP r or copolymers of PP in which the propylene is usually present as a major component. The inner film layer 15 is preferably made from EVA although it can also be made from EEA, EMA or ~AA or other similar polymers or copolymers. The two layers are coextruded together to form a one-piece film.
Normally, the HDPE outer layer 13 does not pro-perly heat seal to the EVA inner layer 15 at low temperatures.
In accordance with the present invention the sheet 3 is given a fairly high corona treatment on both sides prior to forming the sleeve. The corona treatment, in the order of 40 dynes/cm.
(which is a measurement of the property of the material), has the effect of oxidizing the surfaces of the layers 13, 15, thus appearing to make them more chemically compatible. The measurements may be ~rom a reading of 36 dynes and up, e.g., up to 55 dynes, but problems may be encountered at higher levels. When the lap joint 9 is heat sealed together, the bond in the joint appears to be more of a chemical type than of a melt fusion type commonly associated with conven-tional heat sealing.
The corona treatment thus permits the overlapped HDPE and EVA surfaces of layers 13, 15 to bond tightly to-gether at low heat-sealing temperatures, whereas without the corona treatment the surfaces would not bond tightly to-gether, particularly at low heat-sealing temperatures.
The improvement in bond strength following corona treatment i9 more clearly shown having reference to Figure 3. Samplas of different ~ilms have been heat-sealed to-gethar in a laboratory at fixed conditions of sealing pressure, sealing bar jaw width, and dwell time. The tem-perature of heat-sealing has, however, been varied to provide the only variable. The seals obtained, at the dif-ferent temperatures, are then pulled apart on a laboratory tensile tester at a fixed separation rate. The maximum force required to pull the seals apart is noted. A plot was then made of force required vs. the heat-sealing temperature for the different seals tested as shown in Figure 3.
The first sample seal tested involved sealing untreated EVA film to EVA film. This sample generally duplicates the condition in actual bag making where normally a two-layered coextruded sheet is used having a HDPE outer ~layer and an EVA inner layer. The two edges of the sheet . ~3~

are brought together with the EVA inner layer abutting to form a fin seal. From Figure 3 it will be seen that the EVA to EVA sample has a great deal of strength at relatively low heat-sealing temperatures.
The second sample seal tested was that of HDPE
film to EVA film. Both films are untreated and the seal generally duplicates the case in bag making where a lap seal is ~ormed from a two-layered coextruded sheet with the inner EVA surface at one edge, lying on top of the o~ter HDPE surface at the other edge. As seen by the plot of "EVA to HDPE (untreated)" in Figure 3, the strength of the seal is very low in comparison to the strength of the "EVA
to EVA (untreated)" seal, particularly in the commercially used sealing temperature range of 200F to 280F.
When the films have been given a fairly high corona treatment, of about 40 dynes/cm., on both sides and then sealed together, the strength of the joint is markedly im-proved. When the treated film is used to duplicate a fin seal, with the EVA surfaces abutting, greater strength is obtained at low temperatures than with untreated ab~tting EVA surfaces, as shown by the plot of "EVA to EVA (treated)"
in Figure 3. A more marked difference, however, is shown by the plot of "EVA to HDPE ~treated)". With this plot, it is seen that adequate seal strength is obtained, even at low sealing temperatures, after the normally incompatible HDPE
and EVA film layers have been corona treated.
The plots in Figure 4 are similar to those in Figure 3, showing the marked improvement in the strength of the seal ai low temperatures for "EVA to EVA" when treated as compared to untrea-ted, and for "EVA to HDPE" when treated.

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The plots of "EVA to HDPE (treated)" in both Figures 3 and 4 show that a lap seal can be satisfactorily used in sleeve or bag manufacture when using co-extruded film consisting of a HDPE layer on an EVA layer provided both sides of the film have been corona treatecl.
With reference to Figure 5, the improved results using polypropylene to treated EVA will be readily evident compared to the results obtained f-or untreated PP
to EVA and thus, very significant increases have been obtained,.as is evident from Figure 5.
While a two-layer film has been the only film discussed, a three-layer film, having HDPE as one outer layer, and one of EVA, or a copolymer thereof, EEA, EM~
or EAA as the other outer layer can also be used.
As used herein, the.preceding abbreviations have their standard meanings in this art, namely, ethylene vinyl acetate, high density polypropylene, ethylene maleic anhydride, etc.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for use in making a bag comprising:
providing a layered, coextruded film, one outer layer of the film made from a first type of thermoplastic material, the other outer layer of the film made from a second type of thermoplastic material, the layers of the first and second types of thermoplastic material normally not being heat-sealable together at low temperatures;
applying a corona treatment to at least one side of the film to at least slightly oxidize the outer surfaces of the film;
overlapping the side edges of the treated film to form a lap joint; and heat-sealing the lap joint together at a low temperature to form a sleeve which can be used to make a container.

2. A process as claimed in Claim 1 wherein the film is two-layered, the outer layer comprising high density polyethylene, the inner layer comprising a member selected from a polymer of ethylene vinyl acetate or a copolymer thereof, a polymer of ethylene ethyl acrylate, a polymer of ethylene maleic anhydride and a polymer of ethylene acrylic acid.

3. A process as claimed in Claim 1 wherein the film is two-layered, the outer layer comprising polypropylene or a copolymer thereof, the inner layer comprising a member selected from a polymer of ethylene vinyl acetate or a copolymer thereof, a polymer of ethylene ethyl acrylate, a polymer of ethylene methylacrylate and a polymer of ethylene acrylic acid.

4. A process as claimed in Claim 1, 2 or 3, wherein the corona is about 40 dynes/cm.

5. A process as claimed in Claim 1, 2 or 3, wherein the lap joint is heat-sealed together at a temperature ranging between 200°F and 280°F.

6. A sleeve for use in making a bag, the sleeve made from a layered, co-extruded film, one outer layer of the film made from high density polyethylene, polypropylene or a copolymer thereof, the inside layer comprising a member selected from a polymer of ethylene vinyl acetate or a copolymer thereof, a polymer of ethylene ethyl acrylate, a polymer of ethylene methylacrylate and a polymer of ethylene acrylic acid; the side edges of the film overlapped to form a lap join-t which is heat-sealed together, the sleeve having the high density polyethylene layer on the outside.

7. A bag made from a layered, co-extruded film the outer layer of the bag made from high density polyethylene, polypropylene or a copolymer thereof, the inner layer made from a member selected from a polymer of ethylene vinyl acetate or a copolymer thereof, a polymer of ethylene ethyl acrylate, a polymer of ethylene methylacrylate and a polymer of ethylene acrylic acid; the side edges of the film overlapped to from a lap joint which is heat sealed together, each end of the bag also heat sealed closed.