Film and Blends of Polyetheramide Block Copolymer and Ethylene Vinyl Alcohol Copolymer

Ethylene vinyl alcohol copolymer (EVOH) is known to provide an excellent oxygen barrier property when used in making packaging films. And while films containing EVOH have been developed and are being used, the physical properties inherent to EVOH lend certain difficulties to the fabrication and use of EVOH-containing films. EVOH is stiff and punctures easily. EVOH is not readily thermoformed. Orientation of EVOH is accomplished only with difficulty, and within a narrow range of process operating parameters.

These problems are recognized and addressed in U.S. patent application Ser. No. 249,974 Pat. No. 4,347,332 entitled NYLON/EVOH BLENDS, filed on Apr. 1, 1981, in the names of Thomas W. Odorzynski and Jack E. Knott, II; and application Ser. No. 290,172 entitled POLYMERIC BLENDS OF EVOH COPOLYMER AND FILMS MADE FROM THOSE BLENDS, filed on Sept. 20, 1982, in the name of David L. Newsome. Another related application is Ser. No. 290,171, abandoned, entitled ORIENTED EVOH/NYLON BLEND FILM, filed Aug. 5, 1981. The above applications teach improvements in technology relating to EVOH-containing films. And while the improvements do represent advances in technical knowledge, there remain certain problems associated with the use of EVOH polymers.

It is also known that EVOH is susceptible to degradation of its oxygen barrier property in the presence of high humidity, or moisture vapor content, at the EVOH layer. In order to ensure that the EVOH has a low humidity environment within which to operate, the EVOH is usually used in combination with other polymeric layers which are positioned so as to provide a moisture barrier between the EVOH layer and an environment potentially having an unacceptably high moisture content. Thus it is common to find EVOH used in multiple layer structures which include moisture barrier materials such as polyethylenes.

It is further known that EVOH loses some of its sensitivity to moisture, and thus has improved functionality as a barrier component of film, when it is molecularly oriented or otherwise formed without remelting after the making of a film containing EVOH. The process of orienting the EVOH, however, has proved to be difficult because of the physical properties of EVOH. In general, the EVOH, either alone, or in a multiple layer film, has a tendency to split or tear during the orientation process. Although the mechanism is not known for sure, this tendency might be explained in terms of the high degree of crystallinity of EVOH films. Whatever the mechanism, processing of EVOH subsequent to forming a film has proved to be extremely difficult. This difficulty has retarded large scale use of EVOH in the oriented form. While some oriented films may be made which are functionally acceptable, many of those films exhibit a high haze level, which is commercially unacceptable.

One method of improving the processability of EVOH film is taught by copending application Ser. No. 249,975 filed Apr. 1, 1981. In that application, it is taught that certain plasticizers may be blended into the EVOH to improve its performance in the orientation process. While these plasticizers do fulfill the plasticizing role, in that the plasticized EVOH film can be subsequently oriented with ease and the expected reduced moisture sensitivity can be achieved, the plasticizers migrate toward the surface of the film and change the film surface properties over a period of time. While the surface properties may not be critical for some uses, in many cases the changes in surface properties caused by the migration are not acceptable, and another means of modifying the properties of the EVOH layer is desired.

Another method of improving properties of an EVOH film is set forth in U.S. Pat. No. 4,347,332. In that patent, it is proposed to blend nylon, along with a nylon plasticizer, with the EVOH in order to make a clear and desirable film. While a clear and desirable film may be made by this process, it has been found that, after a relatively short processing time in the extrusion of some of the polymer blends to form film, gels begin to appear in the film and get progressively worse until shut-down of the extrusion process is required.

The critical issue addressed by the inventor, then, is that of modifying the EVOH to avoid its undesirable physical characteristics; so that the EVOH layer is susceptible to commercial processes of extrusion and orientation, to produce a commercially acceptable clear barrier film. The perplexing problem is that of discovering a compound or family of compounds which can be blended with EVOH to form a compatible blend and which will also modify the undesirable properties; all this without suffering the problems of migration and surface property changes commonly associated with plasticizers.

It is an object of this invention to provide a novel composition of modified EVOH which can be extruded continuously for long periods of time to produce novel films tougher than similar films having unmodified EVOH.

It is another object of the invention that the novel films be easily susceptible to orientation in the machine direction--uniaxially--and in both the machine direction and cross-machine direction--biaxially.

It is a further object that the resulting oriented film be clear, that is, exhibit a low level of haze. The low haze in the oriented form, then, confirms the functional compatibility of the materials, and, in turn, good layer integrity, while satisfying the appearance requirements essential to its commercial acceptance. The films should exhibit good oxygen barrier properties as exemplary of EVOH, and improved oxygen barrier properties at high humidity, as compared to those in the unoriented form, as exemplary of oriented EVOH.

It is yet another object that the physical properties of the film surface, and particularly surface friction, be more or less constant with time.

In its most general form, the invention is embodied in a composition of matter which is a compatible blend of 5 percent to about 40 percent by weight of a polyetheramide block copolymer (PEBA), and about 60 percent to 95 percent by weight ethylene vinyl alcohol copolymer.

In another form, the invention is a polymeric film which has been made from the compatible blend of the polyetheramide block copolymer and the ethylene vinyl alcohol copolymer. In some preferred embodiments of the film, the film includes, on each surface thereof, a layer of nylon polymer, the result being a film having at least three layers with two layers of nylon and a layer of the blend material therebetween. There may be additional layers juxtaposed on either or both of the nylon layers, as discussed hereinafter. The multiple layer embodiments may provide ease of processing, economy, and abuse resistance.

In the most preferred form of the article of the invention, the film is molecularly oriented in a multiple layer structure. Orientation ratios are 3/1 to 4/1 for uniaxial orientation and 2.times.2 to 4.times.4 for biaxial orientation. In such a preferred article, the overall film thickness is about 1.0 mil, more or less, and the thickness of the blend layer of ethylene vinyl alcohol and polyetheramide block copolymer is about 20% to 50% of the overall thickness of the film.

The invention is further exemplified in a method of making a film. The method comprises forming a mixture of 5 weight percent to about 40 weight percent of a polyetheramide block copolymer and about 60 percent to 95 percent ethylene vinyl alcohol copolymer, extruding the mixture through an extruder and die apparatus to form a film, in which the mixture is present as a layer of blended materials of the mixture, and cooling the film. To make the most preferred form of the article of the invention, the method comprises coextruding the film with a layer of nylon on each surface of the blend to form a three layer film comprising nylon on each surface thereof and a core layer of the blend. The film is finally completed in its most preferred form by reheating it and molecularly orienting it.

The thus produced film may be used directly, or may be combined with other layers in forming a more complex film. It is contemplated that the above-described blend layer may be incorporated into any one member of a large family of films. These films may be constructed by sequential forming and laminating of the various layers. They may preferrably be made by coextrusion of all the polymer layers together, optionally followed by simultaneous orientation of all the layers. Variations on these and other means of forming the films will be obvious to those skilled in the art.

FIG. 1 is a chart representation showing the Shore Hardness of PEBA polymers in relation to other polymers and rubbers.

EVOH copolymers useful in this invention generally contain at least about 55, up to about 80, weight percent vinyl alcohol. Preferred EVOH copolymers generally contain about 60 to 75 weight percent of the vinyl alcohol moiety. The remainder of the molecule consists primarily of ethylene. There is usually present in the EVOH molecule a residual amount of vinyl acetate, since the EVOH is made by an hydrolysis reaction of the ethylene vinyl acetate molecule. Commercially available EVOH is highly hydrolyzed, usually to greater than 99 percent by weight. Especially desirable EVOH copolymers are Soarnol ET and Soarnol D sold by Nippon Gohsei, Japan, and EVAL EP-E and EP-F, sold by Kuraray, Japan.

The PEBA copolymers useful in this invention contain blocks of polyamide and polyether moieties in the polymer chain. A general formula for these polymers is: ##STR1##

The composition ratio PE/PA in the copolymer may vary from 20/80 to 80/20. The polyamide component may be any of the conventional polyamides, such as nylon 6, nylon 66, nylon 6/66, nylon 11 and nylon 12. The polyether component is usually, but not necessarily, selected from polyoxyethylene, polyoxypropylene and polyoxytetramethylene. The melting point of known PEBA's is about 120.degree. to 195.degree. C., and the Shore Hardness is about 65D to 60A as shown on the chart of FIG. 1. Preferred PEBA's are sold by Ato Chemie under the tradename PEBAX. Typical of their polymer resins is PEBAX 5512, which has a melting point of 190.degree. C. and Shore Hardness of 55D. This PEBA is generally desirable because its generally soft "hardness" is conducive to a softening modification of the hard and brittle EVOH when blended with the EVOH. Ohter successfully tested PEBA's are PEBAX 3533 and PEBAX 4033, also sold by Ato Chemie. PEBAX 3533 has a melting point of 162.degree. C. and Shore Hardness of 80A. PEBAX 4033 has a melting point of 168.degree. C. and a Shore Hardness of 40D.

The invention in its broadest sense is represented by a blend composition of EVOH and PEBA. The blend contains, by weight, about 60 percent to 95 percent EVOH and conversely 5 percent to about 40 percent PEBA, wherein the PEBA functions in the role of a modifier for the EVOH-PEBA blend. The preferred range for the common applications is 10 to 20 percent PEBA and 80 to 90 percent EVOH. That preference does vary depending on, for example, the specific EVOH and PEBA polymers used, the fabrication processes, the overall final structure, and the contemplated environments encountered by the finished package. The lower limit of the range, of 5% PEBA, represents a point at which the most desirable PEBA's lose substantial efficiency in the desired affect of the plasticizing function. Depending on the specific PEBA, and the specific EVOH, being used, compositions of up to 40 percent PEBA are contemplated. At the higher concentrations of PEBA, the oxygen barrier properties normally attributed to EVOH may be partially compromised, which is probably attributable to the dilution of the EVOH concentration. Above the upper limit of the range, of 40% PEBA, the beneficial plasticizing effect is being traded for an unacceptable amount of reduction in the oxygen barrier.

Another aspect of the invention is represented by films made from the EVOH-PEBA blends. While single layer unsupported film may be made by conventional extrusion techniques, the film is preferrably formed as part of a coextrusion. A typical coextrusion is /nylon/blend/, 0.4 to 2.0 mils thick, with the blend layer being about 20% to 50% of the thickness of the two-layer composite. Another typical coextrusion is /nylon/blend/nylon/, 0.4-2.0 mils thick, with the blend layer being about 20% to 50% of the thickness of the three layer composite. Constructive to the illustration of the above two and three layer coextrusions, the leading and trailing slashes represent contemplated additional layers which may be positioned on either side, or on both sides, of the coextrusion to provide additional desired functional properties. Structures having exemplary ones of such additional layers are shown hereinafter.

A particularly advantageous property of the EVOH-PEBA blends of this invention, as compared to the previously mentioned EVOH-nylon blends, is that they may be processed continuously through extruder-die apparatus for extended periods of time without gel formation or other polymer degradation. As a secondary testing technique, to compare processability of EVOH-PEBA blends against conventional EVOH-nylon blends, two blends were processed separately in a Brabender mixer. Each combination of blend materials was heated to melting and was mixed continuously for one hour while the torque required for mixing was continuously recorded. In both cases, the mixing torque was initially less than 1 ft. lb. The torque increased steadily with the EVOH-nylon blend; while the torque of the EVOH-PEBA blend remained relatively constant. The blend compositions and the final torques after the one hour of mixing were:

The above test data confirm that material in an extruder and die apparatus processing a conventional EVOH-nylon blend may have process-time limitations; but make no such suggestion for processing blends of EVOH-PEBA. As a primary testing technique, the inventor has subjected the EVOH-PEBA blend to extended conventional extrusion processing and has found no time limitations on extrusion processing of the blend compositions of the invention. Because the PEBA contains a significant proportion of polyamide building blocks, as do all nylons, it would be expected that PEBA's would have extrusive properties similar to those of nylon. As seen in the above test, the PEBA in the blend provides a critical functional capability not necessarily available when nylon alone is blended with EVOH.

As stated earlier, the most efficient use of EVOH as an oxygen barrier, and particularly as regards use in a moist environment, is in the oriented form, either uniaxially oriented or biaxially oriented. The invention is best represented by this most efficient form of the use of EVOH. Recalling first that the normal orientation problems with EVOH are related to its stiffness and rigidity, and that these are commonly attributed to its crystallinity, in those cases where orientation is contemplated, the processing operations should provide minimal opportunity for crystal formation in the extrusion process. Thus, when the film is to be oriented, the film should be cooled quickly upon exiting the extruder die. Exemplary of acceptable processes to accomplish this are those where the film is cast against a chill roll as in cast extrusion or where the film is passed through a quenching water medium as in tubular water quenched extrusion.

Once the film is formed, it may then be reheated and oriented either uniaxially or biaxially. This can be done in an in-line operation or an out-of-line operation. In the orientation process, it appears that the PEBA functions as a plasticizing agent for the EVOH. It apparently accomplishes this, however, without the attendant migration of the PEBA to the surface of the layer, as is common with conventional plasticizers; as no change in surface friction, with time, has been detected.

Acceptable uniaxial orientation ratios are 3/1 to 4/1. For biaxial orientation, acceptable ratios are 2.times.2 to 4.times.4. The higher the orientation ratio, either uniaxial or biaxial, the better are the properties achieved. The preferred ratio, then, is that highest ratio which can be achieved efficiently on available hardware; and that ratio will vary according to the hardware being used. While biaxial orientation has been represented as being the same amount, or uniform, in both axes of orientation, there may be substantial deviation from that sameness, with the only requirement being that the amount of stretch on each axis is 2 to 4 times the original unstretched dimension. Thus 2.times.4 orientation is acceptable, for example, as is a 4.times.2 or a 4.times.3 or a 3.times.4. Thus any recitation of a range of uniform biaxial stretching ratios is meant to include all non-uniform ratios possible within the range. The resulting film in any of these cases is uniform and clear.

In retrospect, it is seen, that the results obtained from making a film from a blend of EVOH and PEBA yield a basic understanding of the operation of the blend material as a polymer. The basic concept derived is that the PEBA functions to lend its softness, flexibility, and toughness to the otherwise relatively brittle and fragile EVOH, thereby modifying its properties to yield a film having improved extensibility and toughness. This concept holds true for all PEBA polymers. Some films which cannot otherwise be successfully oriented become orientable with the inclusion of PEBA. Also, some films which can be oriented without the inclusion of PEBA are easier to orient, and are tougher films, with the inclusion of PEBA. Thus, in some cases the inclusion of the PEBA makes orientation possible, and in other cases it makes orientation easier.

Regarding the selection of the the specific PEBA to be used, the softer PEBA's can provide the necessary extensibility for moderate orientation ratios with the minimum use of PEBA. For the greater toughness required of films which are subjected to higher levels of orientation, and primarily as regards biaxial orientation, the harder PEBA's lend substantially more toughness than the softer PEBA's and are generally preferred. Where toughness is a significant requirement as far as the package protecting the product, the primary purpose of the PEBA may be to impart toughness, and a PEBA having more toughness, as represented by a harder Shore Hardness, will be selected. Thus the selection of the specific PEBA to be used is more or less governed by the amount of orientation and the process and use environments contemplated; although it is seen following, that the selection of the specific PEBA may also be interdependent on other factors.

The primary factors governing the EVOH/PEBA ratio in the blend have to do with the specific EVOH selected and the contemplated end use. Since the functionality of the EVOH as an oxygen barrier is usually the primary purpose of the layer, the selection of the specific EVOH polymer is usually the first step in designing the EVOH-PEBA blends. Given the selection of the specific EVOH, then for any PEBA selected, there is a range of properties available within the range of EVOH/PEBA ratios defined as acceptable. Should that range of properties not satisfy the required parameters, then the selection of at least one of the specific polymers is changed. Since the EVOH is the primary operative polymer; and since the PEBA is primarily used for the purpose of modifying the EVOH to facilitate its processing; then, in most cases, where a specific polymer combination does not provide the required parameters within the range of those available, then the preferred polymer for change, substitution, or modification, is the PEBA. However, if selecting an alternate PEBA, or modifying the PEBA, cannot be made to yield the required parameters, then an alternate EVOH is selected. For example, for use in moist environments, such as for packaging meat or cheese, preferred EVOH has a relatively high ethylene content, such as 40% to 45%. Within the context of all EVOH polymers, those polymers having the higher ethylene content tend to be relatively softer and more extensible; and so typically they will have lower requirements so far as the amount of improvement in softness and extensibility is concerned. The blends designed for these uses generally will fall toward the higher end of the EVOH/PEBA ratios given, as the required amount of modification, to the EVOH, is somewhat less. The specific PEBA selected will, of course, have an affect, too.

For use in dry environments, such as packaging dry snacks like potato chips, the preferred EVOH has a relatively low ethylene content, such as 29% to 35%. Within the context of all EVOH polymers, the polymers having the lower ethylene content tend to be relatively harder and more brittle, and so, typically, will have higher requirements as far as the amount of improvement required in softness and extensibility. The blends designed for these uses generally will fall toward the lower end of the EVOH/PEBA ratios given, as the required amount of modification, to the EVOH, is somewhat more. A larger EVOH/PEBA ratio may, of course, be used where a softer PEBA may be selected while still providing both the required softness and the required toughness.

Regarding specific multiple layer film structures, the EVOH/PEBA mixture is most conveniently fabricated into a film containing the blend by means of a coextrusion process. In most cases, the blend layer is an interior layer in the structure, as compared to the exterior layers, when coextruded. A typical multiple layer structure includes nylon/blend/nylon, which may be a subassembly or a component of the completed film structure. As typical of completed film assemblies, there may be mentioned:

Typical materials used in the tie layers of the foregoing structures are the carboxy modified polymers sold by Chemplex as Plexar 1 and DuPont as CXA 3101. Typical materials used in the sealant layer are ionomer, EVA, and low density polyethylene (LDPE); and indeed blends of some of these materials are typically used. There are only two functional requirements of the sealant layer. First, it must have acceptable adhesion to the adjacent layer. Second, it must have heat sealing properties compatible with heat sealing, preferrably by driving heat from a heat seal bar through all the layers between the sealant layer and the opposite surface of the film.

It is, of course, possible to make certain conventional modifications to the films, or to the processes of making them, without affecting the capabilities of the films to perform their intended functions.

The exemplary film assemblies as shown above may be desirably uniaxially oriented, such as by stretch orienting in the machine direction to make oriented films which are highly desirable for use in packaging dry snack foods. Similarly they may be biaxially oriented for other packaging uses. It is, of course, highly conventional to add layers to the film after the orientation operation. These additional layers, such as OPP, provide additional functional properties.

In the design of some films, it will be found desirable to improve the interlayer adhesion at certain of the layer interfaces. Where the adhesion is needed within a coextruded film, a tie, or adhesive, polymer may be used as a thin layer for enhancing adhesion between the two layers it contacts, as shown in the above listing of typical structures of complete films. As taught in U.S. Pat. Nos. 4,233,367 to Ticknor and Rein, 4,239,826 to Knott and Wang, and 4,254,169 to Schroeder, a number of extrudable adhesives suitable for use are available. Typical adhesives for use in films of this invention are the products sold by Chemplex Company under the PLEXAR designation, the resins sold by Mitsubishi Chemical under the AP designation, and the resins sold by DuPont Company under the CXA designation.

These adhesives are usually olefinic polymers or copolymers which employ carboxy modifications of the general nature ##STR2## which provide the desired bonding capabilities. These adhesive resins are commercially available, and their uses will be obvious to those skilled in the art.

Pellets of Soarnol ET ethylene vinyl alcohol copolymer were mixed with pellets of PEBAX 5512 copolymer in a weight ratio of 80% EVOH to 20% PEBA. The solid pellets were dry blended at room temperature to achieve a uniform mixture. The thus mixed pellets were extruded in a cast coextrusion process with nylon 6 on each side of the blend layer to form a three layer film of nylon/blend/nylon. The thus produced film was 2.85 mils thick. The film was uniaxially oriented at a stretch ratio of 3.5 to 1. The resulting stretched film was about 1 mil thick overall. The blend layer was about 0.4 mil thick. The oriented film was clear, indicating good compatibility. Haze was 1%, according to ASTM D1003. Samples of the oriented film were tested for oxygen permeability and haze. The reported oxygen permeability was 49 cc/m.sup.2 /24 hr. day at 73.degree. F., 100% R.H. Haze was 1.0%. The oxygen permeability of the corresponding unoriented film of the same thickness and thickness ratios was greater than 200 cc/m.sup.2 /day under the same test conditions. At 0% R.H. and 73.degree. F., the oxygen permeability of the test film was only 2.5 cc/m.sup.2 /day.

A film was coextruded and oriented as in EXAMPLE 1 except that the blend ratio was 90% by weight EVOH and 10% PEBA. The oxygen permeability at 73.degree. F., 100% R.H. was 32 cc/m.sup.2 /24 hrs. day, and the haze was 1.0%. The oxygen permeability was less than 1 cc/m.sup.2 /day at 0% R.H. and 73.degree. F.

A control film was made and oriented as in EXAMPLES 1 and 2 except that no PEBA was used. The core layer of the 3-layer film was 100% EVOH. The oxygen permeability was 24 cc/m.sup.2 /day at 73.degree. F., 100% R.H., but the haze measurement was 3.9%. The film was a milky white on the roll. While the film may be functional as a barrier film, its appearance as characterized by the higher haze makes it unacceptable for commercial application, and thus unsaleable.

Pellets of Soarnol ET were dry blended with pellets of PEBAX 5512 in a weight ratio of 80% Soarnol ET to 20% PEBAX, as in EXAMPLE 1. A 4.25 mil film was coextruded. The resulting film was biaxially oriented at a 2.5.times.2.5 stretch ratio. The resulting film had good clarity and tested oxygen permeability of 30 cc/m.sup.2 /24 hr. day at 73.degree. F., 100% R.H.

A 4.25 mil film was made as in EXAMPLE 3 except that the blend ratio was 90% Soarnol ET and 10% PEBAX 5512. The film was biaxially oriented at a 2.5.times.2.5 stretch ratio. The resulting film had good clarity and tested oxygen permeability of 27 cc/m.sup.2 /24 hr. day at 73.degree. F., 100% R.H.

Those skilled in the art will now recognize that minor amounts of nylon may be blended into the EVOH/PEBA blend, and same is contemplated wherever the blend is recited herein.

In testing the films of the invention, including single layer films, no evidence was found of any film surface properties changing with time. Thus it is concluded that the PEBA does not lend variable surface properties to the EVOH-PEBA blend layer; and that the PEBA does not interfere with the functionality of the EVOH.