Heat shrinkable films containing single site catalyzed copolymers having long chain branching.

Field of application of the invention

The present invention relates generally to heat-shrinkable films which comprise single-site catalyzed copolymers having at least one long-chain branch.

Background of the invention

Shrinkable thermoplastic films are useful in many applications for packaging meat, cheese, poultry and many other food and non-food products. There is a constant search for an improvement in these films to give them better impact resistance, better optical properties and better shrinkability. For example, in US Pat. 4 640 856 (Ferguson et al.) Describes a thermoplastic heat-shrinkable multilayer film having improved shrink, toughness and barrier properties. The film contains at least one layer of very low density polyethylene and a gas barrier layer of a vinylidene chloride copolymer or ethylene vinyl alcohol. It has been found that the film is particularly suitable for making pouches for packaging large pieces of fresh red meat.

U.S. Patent Nos. 5,059,481, 4,976,898 and 4,863,769 (all Lustig et al.) Disclose a heat-shrinkable film suitable for packaging food products, such as frozen poultry, main cuts of meat and processed meat products wherein the film is a biaxially stretched monolayer film of a very low density polyethylene copolymer or a multilayer film containing very low density polyethylene.

U.S. Patent No. 4,457,960 (Newsome) describes the use of linear low density polyethylene in molecular oriented multilayer films.

In each of the above patents, the incorporation of conventional ethylene / alpha-olefins made using Ziegler-Natta catalyst systems into heat-shrinkable films is described. Catalytic Ziegler-Natta processes are widely used in the polymer industry and have a long history dating back to the year of about 1957.

These systems are often referred to as heterogeneous because they consist of many types of catalytic species, each in different metal oxidation states and in different coordination environments with ligands. Examples of heterogeneous Ziegler-Natta systems include metal halides activated by an organometallic co-catalyst, for example titanium or magnesium chlorides complexed to trialkylaluminum, and those disclosed in patents such as U.S. Patents 4,302,565 and 4,302 566 can be found. Since these systems contain more than one catalytic species, they have polymerization centers or sites with different activities and varying abilities to incorporate a comonomer into a polymer chain.

The result of this "multi site" chemistry is a product with poor control of polymer chain building both within the sequence of a single chain and compared to an adjacent chain. In addition, differences in catalyst efficiency at some sites (centers) give a high molecular weight polymer and elsewhere a low molecular weight polymer. Thus, copolymers made using these systems result in polymer products that are chain mixtures, some of which have a high comonomer content and other nearly no comonomer content. For example, conventional Ziegler-Natta "multi-site" catalysts may yield a linear ethylene / α-olefin copolymer (HDPE, LLDPE, VLDPE, ULDPE) having an average comonomer percentage of 10, but in which the comonomer content in the individual chains is in the range of 0 to 40%. This, coupled with the diversity of chain lengths, results in a truly heterogeneous mixture that also has a broad molecular weight distribution (MWD).

Linear low density polyethylene (LLDPE) has proven to be very successful as a feedstock of choice for packaging films. The term LLDPE is generally understood to describe copolymers of ethylene and one or more other alpha-olefin monomers that have been polymerized at low pressure using a Ziegler-Natta catalyst to achieve a density in the range of about 0.915 to about 0.940. Although no clear standard exists, LLDPE polymers are often marketed with density subgroups, for example, as linear medium density polyethylene (LMDPE), linear low density polyethylene (LLDPE), very low density linear polyethylene (VLDPE), or ultra-low density linear polyethylene (ULDPE). These classifications are used for marketing and vary from supplier to supplier.

These materials are very different from high density low density polyethylene (LDPE), which is generally understood to be a highly branched, single low melting point homopolymer. For example, an LDPE having a density of 0.92 typically has a melting point of about 112 ° C, while an LLDPE having a corresponding density has melting points of 107 ° C, 120 ° C, and 125 ° C. With LLDPE, several melting points are usually found and are a consequence of the above-mentioned heterogeneous incorporation of a comonomer.

Recently, a new type of ethylene copolymer has been introduced which is the result of a new catalyst technology. Examples of introductory journal articles include "Exxon Cites" Breakthrough "in Olefins Polymerization" in "Modern Plastics", July 1991, p. 61; "Polyolefins Gain Higher Performance from New Catalyst Technologies" in "Modern Plastics", Oct. 1991, p. 46; "PW Technology Watch" in "Plastics World", Nov. 1991, p. 29; and an article in "Plastics Technology", Nov. 1991, p. 15th

These new resins are made using metallocene catalyst systems whose novelty is based on the steric and electronic equivalence of each catalyst position. Metallocene catalysts are characterized as containing a single stable chemical type rather than a volatile mixture of states as discussed for conventional Ziegler-Natta catalysts. This results in a system consisting of catalyst positions having singular activity and selectivity. For this reason, metallocene catalyst systems are often referred to as "single site" because of their homogeneous nature, and with their help, polymers and copolymers are produced which are often referred to by their suppliers as "single site resins".

Generally, metallocene catalysts are organometallic compounds containing one or more cyclopentadienyl ligands bonded to metals such as hafnium, titanium, vanadium or zirconium. To improve the catalytic activity is often a cocatalyst such. B., without the invention being limited thereto, oligomeric methylalumoxane used. By varying the metal component and the cyclopentadienyl ligand, a variety of polymer products can be made to have molecular weights in the range of about 200 to more than 1,000,000 and molecular weight distributions of from about 1.5 to about 15. The choice of co-catalyst affects the effectiveness and thus the production rate, yield and cost. Examples of metallocene catalysts are disclosed in US Pat. 4 701 432, 4 306 041, 5 088 228, 4 935 397, 5 084 534, 3 161 629, 5 055 438, 5 057 475 and JP 63/175004 and JP 1 101 315.

As a consequence of the single site system provided by metallocenes, ethylene / alpha-olefin copolymer resins can be prepared in which each polymer chain actually has the same structure. Therefore, the copolymer chains made from single site systems are consistent not only in terms of chain length but also in terms of average comonomer content and even in terms of regularity of comonomer spacing or incorporation along the chain ,

In contrast to the above-mentioned Ziegler-Natta polymers, these single-site metallocene polymers are characterized by having a narrow molecular weight distribution (MWD) and a narrow composition distribution (CD). While conventional polymers have MWD's of about 3.5 to 8.0, for metallocenes MWD is in the range of about 1.5 to about 2.5, and most preferably about 2.0. The MWD refers to the width of the molecular weight distribution of the polymer chains, and is a value obtained by dividing the number average molecular weight by the weight average molecular weight. Due to the low CD or regularity of the side branch chains along a single chain and their equality in the distribution and length of all other chains, the low molecular weight and high molecular weight "ends" are greatly reduced. These features reduce the extractable materials resulting from poor LMW control and improve the optical properties by removing the linear, ethylene-rich moieties contained in conventional heterogeneous resins.

Conventional Ziegler-Natta systems thus provide heterogeneous resins that represent the differential nature of their multiple site catalysis, while metallocene systems yield homogeneous resins that in turn reflect the nature of their single site catalysis.

Another distinguishing feature of single site catalyzed ethylene copolymers is their melting point range. The narrow CD of metallocenes gives a narrow melting point range as well as a lower differential scanning calorimeter (DSC) peak melting point peak. Unlike conventional resins, which retain a high melting point over a wide range of densities, the metallocene resin has a direct relationship with the melting point. For example, an ethylene / butene copolymer having a density of 0.905 g / cc prepared using a metallocene catalyst has a peak melting point of about 100 ° C, while a copolymer of ethylene having a somewhat lower density and butene, prepared using a conventional Ziegler catalyst, reveals its heterogeneous nature at a melting point of about 120 ° C. The DSC shows that the Ziegler resin has a much broader melting point range and actually melts higher despite its lower density.

It should be noted that at least some of the heretofore available linear ethylene-based polymers are closer to the physical and compositional properties achieved with the metallocene-catalyzed polyolefins of this invention. For example, in "Sequence and Branching Distribution of Ethylene / 1-Butene Copolymers Prepared with a Soluble Vanadium Based Ziegler-Natta Catalyst" in "Macromolecules", 1992, 25, 2820-2827, it is confirmed that a soluble catalytic Ziegler-Natta catalyst Vanadium-based system VOCl 3 / Al 2 (C 2 H 5) 3 Cl 3. acts essentially as a single site catalyst, although VOCl & sub3; is not a metallocene. Homogeneous copolymers prepared using such a catalyst system have been commercially available for several years. An example of this is the resins sold under the trade name Tafmer (TM) by Mitsui.

US Patent No. 4,501,634 (Yoshimura et al.) Describes an oriented multilayer film containing a tafmer as a blend component in at least one layer. Japanese Kokoku 37907183 (Gunze Limited) relates to a heat-sealable biaxially oriented composite film in which the heat-sealable layer contains Tafmer in a mixture.

The above patents disclose homogeneous ethylene-α-olefins having densities below 0.90 g / cc. Heretofore, such resins having densities at and above 0.90 g / cc have generally not been available because of the limitations of the VOCl 3 process. Nevertheless, U.S. Patent 1,209,825 (DuPont) describes homogeneous copolymers having densities of up to about 0.920 and prepared with single site Ziegler catalysts. When extruded into a film, the resins have improved physical and optical properties compared to films of heterogeneous copolymers. However, these homogeneous copolymers with densities above 0.90 g / cm³ were previously not commercially available. It is believed that process limitations have precluded the production of these resins in other than laboratory quantities. However, metallocene catalysts can give these homogeneous copolymers with a wide range of densities in commercial quantities. The advantages of homogeneity can now be imparted to copolymers having densities analogous to those of the conventional VLDPEs and LLDPEs.

Although improved physical properties such as optical properties, low extractables content, and improved impact resistance can be achieved, the narrow composition distribution of some typical metallocene catalyzed resins can lead to some processing difficulties. It has been found that these processing problems are avoided if some limited long-chain branching is introduced. That is, a typical metallocene-catalyzed ethylene-α-olefin may be considered to be a collection of linear chains, each having a substantially identical length, each having about the same number of short chain (comonomer) branches that occur at regular intervals Distances are arranged along this chain. Splicing of an abbreviated linear chain having the same regular comonomer distribution on each of the linear chains or on at least some of the chains in the aggregate results in an ethylene-α-olefin having substantially all the physical properties of the original copolymer, but with improved "Body" or melt strength for improved processability including improved extrudability; Orientation speed and sensitivity for irradiation has.

One way of quantifying the improved processability of the new homogeneous branched ethylene-α-olefin copolymers is by the melt flow ratio I 10 / I 2 described in ASTM D-1238. Such copolymers are described in WO 93/08221 (Dow), and it is noted that the I 10 / I 2 ratio for them is an indication of the degree of long chain branching. For linear polyolefins, the I 10 / I 2 ratio increases with increasing molecular weight distribution. The molecular weight distribution can be defined as the ratio of weight average molecular weight (Mw) and number average molecular weight (Mn) or Mw / Mn. Conventional heterogeneous linear ethylene-α-olefin copolymers generally have a high I 10 / I 2 ratio and a correspondingly high molecular weight distribution. Linear homogeneous ethylene-α-olefin copolymers have a low molecular weight distribution and a corresponding low I10 / I2 ratio. Homogeneous ethylene-α-olefin copolymers having a long chain branching, e.g. Such as those supplied by Dow under the trade name Insite have low molecular weight distributions but high I 10 / I 2 ratios. In part, Dow defines those copolymers as having an I.sub.10 / I.sub.2 ratio of 5.63 and a Mw / Mn ratio of .ltoreq.1 (I.sub.10 / I.sub.2) -4.63. This high I 10 / I 2 ratio is an indication of the ease of processability discussed above, while the homogeneity for which the Mw / Mn ratio is a measure gives improved physical properties as also discussed above. A more detailed description of the chemistry and in particular of the rheology of these branched homogeneous copolymers can be found in WO 93/08221, which is incorporated herein by reference.

EP 0 416 815 (Dow) describes the preparation of ethylene / α-olefin copolymers using monocyclopentadienylsilane complexed to a transition metal. The homogeneous ethylene copolymers which can be prepared using the catalyst have better optical properties than typical ethylene polymers as indicated therein and are well suited for films or injection molding.

In WO-A-92114784, EXXON describes metallocene-catalyzed ethylene / α-olefin copolymers which are stated to be suitable for producing films having various desirable properties, but which do not disclose any heat-shrinkable films (films) and no homogeneous ethylene / α-olefin copolymers with a long-chain branching described.

The subject of EP-A-0 495 099 (Mitsui Petrochemical) is an olefin copolymer prepared with a bis-cyclopentadienyl metallocene catalyst and for molding shaped articles, such as films, and in particular for the production of inflation foils can be used.

EP-A-0 416 815 (Dow) discloses a homogeneous ethylene / α-olefin copolymer prepared using a monodentate metallocene catalyst, the copolymer being said to be suitable for the production of films (Films) by blow molding process, without reference to the production of heat-shrinkable films.

EP-A-0 452 920 (Mitsui Petrochemical) describes a process for producing an ethylene copolymer and an olefin polymer using a multidentate metallocene catalyst, and no reference is made to a homogeneous ethylene / α-olefin copolymer with a long-chain branching and not on heat-shrinkable films.

The subject of GB-A-2 206 890 (Grace) is a heat shrinkable film made from a heterogeneous ethylene / α-olefin copolymer.

US-A-5,132,074 (Isozaki) describes a process for producing a heat-shrinkable film using ethylene / α-olefin copolymers. However, it does not mention homogeneous resins having a long chain branching therein.

As indicated below, it has been found that resins made by the Dow process have improved physical properties characteristic of single site catalyzed resins, but also have processability similar to that of conventional Ziegler-Natta Copolymers is. It is believed that the dow metallocene resins have the limited long chain branching discussed above.

The aim of the present invention is therefore to provide a film structure with improved physical properties, for example improved optical properties and improved impact strength and low levels of extractable materials, and in particular a heat-shrinkable impact-resistant multi-layer film, which is suitable for packaging ,

Another object of the present invention is to provide a film which is easily extrudable and workable.

Summary of the invention

These and other objects are achieved with a heat-shrinkable, multi-layer, impact-resistant film suitable for packaging which comprises a homogeneous single-site-catalyzed copolymer of ethylene and an alpha-olefin having from 3 to 10 carbon atoms, said single-site catalyzed copolymer comprising one long chain Branching and having a density of from about 0.86 to about 0.95 g / cc.

Detailed Description of the Preferred Embodiments

The present invention relates to a heat-shrinkable, multi-layer impact film which is suitable for packaging purposes and comprises a homogeneous single-site catalyzed copolymer of ethylene and an α-olefin having 3 to 10 carbon atoms, said single-site catalyzed copolymer has a long chain branching and a density of about 0.86 to about 0.95 g / cc.

According to a preferred embodiment, the present invention relates to a heat-shrinkable, impact-resistant multilayer film having at least one inner core layer comprising a homogeneous single-site-catalyzed copolymer of ethylene and an α-olefin having 4 to 10 carbon atoms and having a long chain branching, said copolymer having a density of about 0.89 to about 0.91 g / cm³.

A further embodiment of the invention relates to a heat-shrinkable, impact-resistant multilayer film which has at least two core layers, each of said core layers comprising a homogeneous single-site-catalyzed copolymer of ethylene and an α-olefin having 4 to 10 carbon atoms and having a long-chain branching said copolymer has a density of from about 0.89 to about 0.91 g / cc.

The term "copolymer" as used herein refers to polymers of two or more comonomers. Therefore, although generally only ethylene / alpha-olefin copolymers are discussed in the present specification, this term also encompasses copolymers of ethylene with one or more alpha-olefins or ethylene with an alpha-olefin and another comonomer.

As used herein, the term "oriented" is interchangeable with the term "heat shrinkable", which terms refers to a material that has been stretched and fixed by cooling, substantially maintaining its stretched dimensions. An oriented (ie, heat-shrinkable) material tends to return to its original unstretched (unstretched) dimensions when heated to an appropriate elevated temperature.

The film according to the invention is preferably oriented and is produced by extrusion processes, in particular generally known coextrusion processes. Initially, it is cooled to a solid state, for example by cascade water or cooled by quenching with air, after which it is reheated to a value within its orientation temperature range and oriented by stretching. The orientation orientation can be achieved in a variety of ways, for example, by using "inflated bubble" or "frame-on". These methods are well known to those skilled in the art and relate to orientation methods in which the material is heated to its softening temperature and then drawn in the transverse or transverse (TD) and / or longitudinal or machine direction (MD) directions , After heating and stretching, the film is quenched while substantially maintaining its stretched dimensions to rapidly cool the film and fix or block it in the oriented molecule configuration.

The film layers may be made by coextrusion, with further layers subsequently being extrusion coated thereon to form multilayer films. Multilayer tubing may also be made, with one of the tubing subsequently being applied by extrusion coating or lamination on top of the other. The extrusion coating process in film making is preferred over the coextrusion of the entire film when it is desired to subject one or more layers of the film to a treatment which may be detrimental to one or more of the other layers. This may be the case when it is desired to irradiate one or more layers of high energy electron foil when the foil is a barrier layer of one or more copolymers of vinylidene chloride (eg, Saran (TM)) such as vinylidene chloride and vinyl chloride or vinylidene chloride and methyl acrylate, and vinylidene chloride with ethyl acrylate or acrylonitrile.

Films of this type include, for example, those in which the barrier layer is a saran (TM) layer, in addition to or instead of an EVOH layer. It is well known to those skilled in the art that high energy electron irradiation is generally detrimental to these Saran (TM) barrier layer compositions, as the radiation can degrade and discolor Saran (TM), turning brown. Now, when preparing a film having a barrier layer having a Saran (TM) layer by full coextrusion and then irradiating the multilayer structure with high energy electrons, the irradiation should be performed with low values ​​and with care. However, this can be avoided by extruding one or more first layers, subjecting the layer or layers to high energy electron beam exposure, and then sequentially apply the Saran (TM) barrier layer and optionally other layers (which may or may not have been irradiated) on the outer surface of the extruded, previously irradiated hose applies. This sequence allows irradiation of the first layer or layers with high energy electrons without exposing the Saran (TM) barrier layer to deleterious discoloration.

As used herein, the term "extrusion" or "extrusion" thus includes coextrusion, extrusion coating or combinations thereof.

As previously indicated, the heat shrinkable film of the present invention may optionally be subjected to high energy radiation treatment, including, but not limited to, corona discharge, plasma, flame, ultraviolet and high energy electron treatment. The irradiation is most preferably performed prior to orientation and in the case of a saran-containing barrier structure prior to extrusion coating with the barrier layer component. The radiation doses are here expressed by the irradiation unit "RAD", where one million RAD or 1 megarad is referred to as "MR". A suitable radiation dose of high energy electrons is in the range of up to about 12 MR, more preferably from about 2 to about 9 MR. However, irradiation after orientation, regardless of structure, and irradiation performed at lower dose levels is also within the scope of the present invention.

In the heat-shrinkable multilayer film of the present invention, each layer generally performs a function or gives the overall structure a characteristic property. The sealant layer composition is selected in consideration of the ease of heat sealing and depending on the intended end use, and other factors, e.g. As the resistance to grease, may be of importance. The outer layer composition may be selected in consideration of abuse resistance or, if a given end use requires a fold-over overlap seal, taking into consideration the sealability with the sealant layer. If a barrier is required, it will be chosen depending on the degree of gas or moisture impermeability required for the final product to be packaged therewith. Other inner layers can serve to impart bulk to the film, improve shrinkability, improve interlayer adhesion, or give any combination of these properties.

It has been found that, for the purposes of the present invention, the use of homogeneous ethylene-α-olefin copolymers having a long chain branching in at least one core layer of a heat shrinkable multilayer film provides improved impact properties and excellent shrinkage properties. In particular, for a gas barrier layer material containing a core layer of a polymeric material having gas barrier properties, for example, a vinylidene chloride copolymer or an ethylene-vinyl alcohol copolymer, further inner layers of these homogeneous branched ethylene-α-olefin copolymers have been found sandwiching the barrier layer, provide a processable barrier film having improved impact resistance and improved free shrinkage. That is, a heat-shrinkable impact-resistant multi-layer film having the general structure given below is within the scope of the present invention.

Sealing / core / barrier / core / abuse (wear)

wherein each of the core layers comprises the same homogeneous long chain branched single site catalyzed copolymer of ethylene and an alpha-olefin having from 4 to 10 carbon atoms, said copolymer having a density of from about 0.89 to about 0.91 g / cc.

A preferred embodiment of this heat-shrinkable impact-resistant multilayer film comprises:

a) a sealing layer;

b) a first core layer comprising a homogeneous ethylene-octene copolymer having an I10 / I2 ratio of 5.63 and a Mw / Mn ratio of ≤ (I10 / I2) -4, 63 and has a density of from about 0.89 to about 0.91 g / cc;

c) a barrier layer;

d) a second core layer comprising a homogeneous ethylene-octene copolymer having an I10 / I2 ratio of 5.63 and a Mw / Mn ratio of ≤ (I10 / I2) -4, 63 and has a density of from about 0.89 to about 0.91 g / cc; and

e) a layer of abuse or wear.

The respective compositions of the seal-barrier and abuse layers may be selected from those polymeric resins or resin blends which provide the required functional properties. Examples of materials suitable for use in the sealant layer include EVA, LLDPE, VLDPE, EAA, EMAA, ionomers, homogeneous linear ethylene-α-olefin copolymers, and homogeneous branched ethylene-α-olefin copolymers. Examples of materials suitable for use in the abuse layer include EVA, LLDPE, VLDPE, homogeneous linear ethylene-α-olefin copolymers, homogeneous branched ethylene-α-olefin copolymers, polypropylene, nylon and High density polyethylene. Examples of materials suitable for use in the barrier layer include vinylidene chloride vinyl chloride copolymers, vinylidene chloride-methyl acrylate copolymers, ethylene vinyl alcohol, acrylonitrile, and nylon. But also other materials which have the required properties in terms of sealability and sealing integrity, abuse or Wear resistance and low gas permeability, are also suitable for use in the sealing layer, in the abuse or wear layer and in the barrier layer of a film according to the invention. Each of the two core layers contains a homogeneous ethylene-α-olefin copolymer, either alone or in admixture with another (further) polymeric material.

A preferred embodiment of such a heat-shrinkable multilayer film having the above-mentioned general construction is a heat-shrinkable impact-resistant multilayer film comprising:

a heat-shrinkable, impact-resistant multilayer film comprising:

a) a sealing layer;

b) a first core layer comprising a homogeneous long chain branched ethylene-α-olefin copolymer having a density of from about 0.89 to about 0.91 g / cc;

c) a barrier layer;

d) a second core layer comprising a homogeneous long chain branched ethylene-α-olefin copolymer having a density of from about 0.89 to about 0.91 g / cc; and

e) a layer of abuse or wear;

wherein the homogeneous branched ethylene-α-olefin copolymer of the first core layer is different from that of the second core layer.

A preferred embodiment of this type of heat-shrinkable impact-resistant multilayer film has the general structure:

Sealing / core / barrier / core / abuse (wear)

each of the two core layers comprising the same homogeneous copolymer of ethylene and octene having an I10 / I2 ratio of 5.63 and a Mw / Mn ratio of ≤ (I10 / I2) - 4.63 wherein said copolymer has a density of from about 0.89 to about 0.91 g / cc.

For those applications where the homogeneous branched ethylene-α-olefin copolymer gives adequate sealing and abuse properties, there is a film comprising:

a) a sealant layer comprising a homogeneous single-site catalyzed copolymer of ethylene and an alpha-olefin having from 4 to 10 carbon atoms, said copolymer having a long-chain branching and a density of from about 0.89 to about 0.91 g / cc ;

b) a barrier layer; and

c) a wear layer comprising a homogeneous single-site catalyzed copolymer of ethylene and an α-olefin having 4 to 10 carbon atoms, said copolymer having a long-chain branching and a density of about 0.89 to about 0.91 g / cm³;

also within the scope of the present invention, wherein the sealant and abuse layer each contain said homogeneous branched ethylene-α-olefin copolymer either alone or in admixture with another (further) polymeric material.

For those applications where a barrier layer is not required, there is a film with the general structure:

Seal / core / abuse

wherein the core layer comprises a homogeneous long chain branched single site catalyzed copolymer of ethylene and an α-olefin having from 4 to 10 carbon atoms, said copolymer having a density of from about 0.89 to about 0.91 g / cc;

also within the scope of the present invention, wherein the core contains said homogeneous branched ethylene-α-olefin copolymer either alone or in admixture. Other core layers of branched homogeneous ethylene-α-olefin copolymers or other polymeric materials may also be included therein.

For each of the above general structures, it should be noted that additional inner layers may be provided to enhance inner layer adhesion or to impart bulk, if required.

Examples of such embodiments of the present invention are multilayer films as defined above, said film comprising:

(A) a first layer comprising an ethylene-vinyl acetate copolymer;

(B) a second layer comprising a vinylidene chloride-methyl acrylate copolymer;

(C) a third layer comprising a blend of a homogeneous ethylene-octene copolymer and an ethylene-butyl acrylate copolymer; and

(D) a fourth layer comprising a blend of an ethylene-vinyl acetate copolymer and a linear low density polyethylene; or

(A) a first layer comprising an ethylene-vinyl acetate copolymer;

(B) a second layer comprising a vinylidene chloride-methyl acrylate copolymer;

(C) a third layer comprising a blend of a homogeneous ethylene-octene copolymer and an ethylene-methacrylic acid copolymer; and

(D) a fourth layer comprising a blend of an ethylene-vinyl acetate copolymer and a linear low density polyethylene; or

(A) a first layer comprising an ethylene-vinyl acetate copolymer;

(B) a second layer comprising a vinylidene chloride-methyl acrylate copolymer;

(C) a third layer comprising a blend of a homogeneous ethylene-octene copolymer and an anhydride grafted ethylene-vinyl acetate copolymer; and

(D) a fourth layer comprising a blend of an ethylene-vinyl acetate copolymer and linear low density polyethylene; or

(A) a first layer comprising an ethylene-vinyl acetate copolymer;

(B) a second layer comprising a vinylidene chloride-methyl acrylate copolymer;

(C) a third layer comprising a blend of a homogeneous ethylene-octene copolymer and an ethylene-vinyl acetate copolymer; and

(D) a fourth layer comprising a blend of an ethylene-vinyl acetate copolymer and a linear low density polyethylene; or

(A) a first layer comprising an ethylene-vinyl acetate copolymer;

(B) a second layer comprising a vinylidene chloride-methyl acrylate copolymer;

(C) a third layer comprising an ethylene-vinyl acetate copolymer;

(D) a fourth layer comprising a homogeneous ethylene-octene copolymer; and

(E) a fifth layer comprising an ethylene-vinyl acetate copolymer.

It has been found that a preferred method for producing a film according to the invention is an extrusion coating process as described above. In this process, a part of the film structure is extruded, cooled and subjected to irradiation with the other layers before extrusion coating and then oriented. Preferably, the sealant layer and a first core layer are extruded and irradiated prior to extrusion coating with the barrier layer, the second core layer and the wear layer and the subsequent orientation. Most preferably, the sealant layer, the first core layer, and a third layer selected to promote adhesion to the barrier layer are extruded and irradiated, followed by extrusion coating with the barrier layer, a subbing adhesion-enhancing layer, the second core layer and the wear layer and then performed an orientation. As a preferred process of this invention, it has been found that the use of homogeneous branched ethylene-α-olefin copolymers in the core layers reduces extrusion head pressure and allows for increased orientation speeds.

The following examples are representative of the preferred embodiments of the inventive films containing homogeneous branched ethylene-α-olefin copolymers. The following tests were used to evaluate these slides:

Tensile strength: A measure of the force required at constant elongation until breakage (rupture) of a sample of the film; determined according to ASTM D 882.

Elongation: A measure of the percentage of elongation (extension) required to break (tear) a sample of the film; determined according to ASTM D882.

Modulus: The ratio between the change in force and the change in stretch in the straight section of an Instron Tensile Test curve; determined according to ASTM D 882 method A.

Crack Extension: The force required to prolong a crack resulting from a minute cut made with a sharp blade in a sample of the foil; determined according to ASTM D 1938.

Free Shrinkage: percentage of dimensional change for a 10 cm x 10 cm sample of film when subjected to selected heating; determined according to ASTM D 2732.

Bursting a Ball: The energy required to burst and penetrate a clamped film sample; determined according to ASTM D 3420.

Instrument Impact Value: The energy required to pierce a clamped sample of the film, similar to the ball site test defined above. However, the instrumental impact test device has the ability to measure the tensile / strain curve until fracture. The "gradient" is the ratio between the change in force and the change in stretch in the rectilinear portion of the curve. The "peak" is a measure of the maximum force exerted on the sample to rupture it. The "impact energy" is a measure of the energy that is absorbed by the sample before it is ruptured. The instrumental impact value is determined according to ASTM D 3763.

Haze: percentage of transmitted light that is scattered forward as it passes through a sample; determined according to ASTM D 1003 method A.

Clarity: A measure of the distortion of an image viewed through a sample; determined according to ASTM D 1746.

Gloss: The surface reflection or surface gloss of a sample; determined according to ASTM D 2457.

Parallel Plate: A bag is placed between two plates with a specific spacing in between and inflated until its seal fails. The pressure inside the bag at the time of failure is a measure of the seal quality. The results are given in inches (cm) water pressure (IOWP).

LRHB (linearly rising hot water bursting pressure): A clean, sealed bag is inflated to a specific static pressure and the sealing surface is immersed in 182 ° F (83 ° C) hot water. After 5 seconds, the pressure inside the bag increases at a rate of 2 inches (5.1 cm) of water. The time to pressure increase to failure and burst is a measure of seal quality. The test results are given in s and inch (cm) water pressure (IOWP).

LRHB-G (linearly increasing hot water burst pressure - grease): This procedure is the same as in the LRHB test described above, except that peanut oil is first applied to the sealing surface.

VPHB (hot water burst pressure variable): as in the LRHB test described above, a clean, sealed bag is inflated to a specific static pressure and the sealing surface is immersed in 182 ° F (83 ° C) hot water. After 5 seconds, the pressure inside the bag increases at a specific speed in the range of 1 to 7 inches (2.54 to 18 cm) of water. Again, the test results are given in seconds and inch (cm) water pressure (IOWP).

VPHB-G (Variable Hot Water Burst Pressure - Grease): the procedure is the same as in the VPHB test described above, except that peanut oil is first applied to the sealing surface.

Gel: A measure of the relative amount of ultrahigh molecular weight polymer contained in a sample. Gel measurement can be an indication of the degree of crosslinking present in a sample as the amount of polymer collected as a gel increases with crosslinking. The gel is determined by solvent extraction with boiling toluene. In this method, a sample weighing 0.4 g is extracted in a cellulose ring for 21 hours, taken out, dried and weighed again. The percentage of gel is calculated by dividing the remaining polymer (toluene insoluble fraction) by the original weight. However, a 0% gel value can not reliably indicate that no meshing occurred. Instead, the degree of cross-linking may not be great enough to give a measurable gel.

DSC: The Differential Scanning Calorimeter (DSC) is an instrument that measures the heat flow to a polymer sample during its programmed heating at 10 ° C per minute. The curve obtained with the DSC can be used to characterize the beginning and end of the melting point of a sample and its (its) peak melting point (melting points.

Polydispersity (Mw / Mn): A measure of the uniformity of chain length within a polymer sample. It defines the width of the molecular weight distribution. It is obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn).

The Mw and Mn are determined by gel permeation liquid chromatography.

example 1

An extruded two-layer precursor film having an inner layer / outer layer structure was produced. The inner layer was a blend of 90 wt% NA 295-000, a 6.7% vinyl acetate EVA supplied by Quantum, and 10 wt% Dowlex 2045. The outermost layer was a blend of 85% by weight of XUR-1567-48562-B9, a homogeneous ethylene-octene copolymer having a density of 0.904 g / cm³ and a MI of 1.0 with a limited long chain branching distributed on one Development base of DOW, and 15% by weight of EA 719-009, an ethylene-butyl acrylate copolymer with 18.5% by weight of butyl acrylate, supplied by Quantum. After exposure, the precursor film was coated with a vinylidene chloride-methyl acrylate copolymer barrier from Dow and an outer abuse layer from a mixture of 92.5 wt% LD 318.92 ex Exonon, a 9th % vinyl acetate EVA, and 7.5% by weight Dowlex 2045 coated by extrusion.

The resulting four-layer film was then oriented from hot water using an inclusion bubble method.

Example 2

The procedure set forth in Example 1 was repeated, except that the outermost layer of the precursor film was 85% by weight from XUR-1567-48562-B9 from Dow and 15% by weight from Nucrel 1202HC Ethylene-methacrylic acid copolymer supplied by DuPont.

Example 3

The procedure given in Example 1 was repeated, except that the outermost layer of the precursor film was 85% by weight of XUR-1567-48562-B9 from Dow and 15% by weight from Bynel CXA 3101, an anhydride grafted EVA with 18.4% vinyl acetate supplied by DuPont.

Example 4

The procedure set forth in Example 1 was repeated, except that the outermost layer of the precursor film was 85% by weight from XUR-1567-48562-B9 from Dow and 15% by weight from LD 318.92 from U.S. Pat Exxon, an EVA with 9% vinyl acetate, existed.

Example 5

For purposes of comparison, the procedure set forth in Example 1 was repeated, except that the outermost layer of the precursor film was 85% by weight Dowlex 2045 and 15% by weight EA 719-009, an EBA of 18.5 % Butyl acrylate.

Examples 6-10

The oriented multilayer films of Examples 1-5 were tested for instrumental impact, free shrinkage and "peel", a measure of the bond strength between the outermost layer of the precursor film and the barrier layer that had been extrusion coated thereon. The results are shown in Table I below. The instrumental impact value and the T-peel numbers for the films of Examples 1 to 4 are favorable compared to those of Comparative Example 5. The free shrinkage of the films according to the invention are better than those of the structure according to the prior art. It should be noted that the numerical values ​​given for Comparative Example 5 are representative of a series of production runs. Table I

Example 11-19

The films of Examples 11-19 were each formed from a precursor film having an inner layer of 90% NA 295-000, a 6.7% EVA supplied by Quantum, and 10% Dowlex 2045, a core layer, and an outermost adhesive layer EP 4062-3, a 15% vinyl acetate EVA supplied by DuPont. After irradiation of the precursor film at a selected dose, it was coated with a Dow vinylidene chloride-methyl acrylate copolymer barrier layer and an outermost abuse layer of 92.5% LD 318.92 and 7.5% Dowlex 2045.

Thereafter, the whole structure was oriented in hot water using a sealed bubble process under constant orientation preheat and hot water bath temperatures of 195 ° F (91 ° C).

The core layer composition and the irradiation dose for each structure are shown in Table II below. Also indicated are the orientation speeds. It should be noted that for the films of Examples 11-16, the orientation rates increase with increasing exposure dose, while this relationship is not apparent for Comparative Examples 17-19.

The resins used in the core layer are XUR-1567-48562-B9, a homogeneous ethylene-octene copolymer having a density of 0.904 g / cc and a MI of 1.0 with a limited long chain branching supplied by Dow, represented below by B9, XUR-1567-48562-B4, a homogeneous ethylene-octene copolymer having a density of 0.909 g / cc and a MI of 1.0 with a limited long-chain branching supplied by Dow, shown below through B4, and Attane 4203, a heterogeneous ethylene-octene copolymer having a density of 0.905 9 / cc supplied by Dow and represented by 4203.

The film build and percentage target thickness per layer for each film were as follows:

Inner / core / adhesive // ​​barrier / abuse

11.9 49.6 6.0 8.7 23.8 Table II

Examples 20 to 28

The oriented multilayer films of Examples 11-19 were tested for instrumental impact and free shrinkage. The results are shown in Table III below. The films of Examples 11 to 16 are advantageous compared to all the values ​​of those of Comparative Examples 17 to 19. Table III

Example 29

A co-extruded three-layer precursor film having the structure: inner layer / core layer / outermost layer was prepared. The inner layer was a blend of 90 wt% NA 295-000, a 6.7% vinyl acetate EVA supplied by Quantum, and 10 wt% Dowlex 2045, a heterogeneous linear ethylene-octene copolymer having a density of 0.920 Dow. The outermost layer was from EP 4062-2, a 15% vinyl acetate EVA supplied by DuPont. The core layer was from XU59220.01, Lot 427833, a homogeneous ethylene-octene copolymer having a density of 0.9016 g / cc and a Ml of 0.9 with a long chain branching sold on a development basis from Dow. The resin had an I10 / I2 ratio of 10.9 and a Mw / Mn ratio of 2.03.

After irradiation, the precursor film was coated with a Dow vinylidene chloride-methyl acrylate copolymer barrier layer blended with epoxidized soybean oil and an ethylene methyl acrylate, an Elvax 3175GC adhesive layer, a DuPont 28% vinyl acetate EVA, a core layer of XU 59220.01 , Lot 427833, Dow Company, and an outer wear layer of a blend of 92.5 wt% LD 318.92 from Exxon, a 9% vinyl acetate EVA. and 7.5% by weight of Dowlex 2045 by extrusion.

The resulting seven-layer film was then oriented in hot water using an inclusion-bubble method. The preheat temperature was 192 ° F (89 ° C) and the hot water bath was at 197 ° F (92 ° C). The oriented final structure had a thickness of 1.8 mils (0.046 mm) with the individual layers having the following nominal thicknesses:

Seal / core / outer // barrier / adhesive / core / abuse

0,28 0,75 0,13 0,18 0,13 0,18 0,15

Example 30

The procedure given in Example 29 was repeated with a final oriented structure having a thickness of 2.2 mils (0.056 mm). The individual layers had the following thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,33 1,0 0,14 0,18 0,14 0,24 0,17

Example 31

The procedure given in Example 29 was repeated with a final oriented structure having a thickness of 2.7 mils (0.069 mm). The individual layers had the following nominal thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,36 1,39 0,15 0,18 0,15 0,30 0,17

Example 32

The procedure given in Example 29 was repeated with the following exceptions: the preheat temperature was 191 ° F (88 ° C) and the temperature of the hot water bath was 184 ° F (84 ° C). The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following nominal thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,36 1,59 0,15 0,18 0,15 0,39 0,17

Example 33

The procedure of Example 32 was repeated with the sole exception that other orientation conditions were used. The preheat temperature was 199 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 34

The procedure set forth in Example 29 was repeated with the core layers distributed by XU59243.00, a homogeneous 0.904 g / cm3 ethylene-octene copolymer and a 0.8 long-chain MI, distributed on a development basis of the company Dow, were replaced. The resin had an I10 / I2 ratio of 9.2 and a Mw / Mn ratio of 2.17. For orientation, the preheat temperature was 194 ° F (90 ° C) and the temperature of the hot water bath was 190 ° F (88 ° C). The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following nominal thicknesses:

Seal / core / outer // barrier / adhesion / core / abuse

0.36 1.59, 0.1 & sup5; 0.1 8 0.1 5 0.39 & liter; & sub7;

Example 35

The procedure of Example 34 was repeated, with the sole exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 36

The procedure set forth in Example 29 was repeated with the core layers coated by XU 59220.00, a homogeneous 0.940 g / cm3 ethylene / octene copolymer, and a 0.9 long MI branched on a development basis of U.S. Pat Company Dow, have been replaced. The resin had an I 10 / I 2 ratio of 11.3 and a Mw / Mn ratio of 2.4. The preheat temperature was 194 ° F (90 ° C) and the temperature of the hot water bath was 190 ° F (88 ° C). The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had nominal thicknesses, as indicated in Example 32 above.

Example 37

The procedure of Example 36 was repeated, with the only exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 38

The procedure set forth in Example 29 was repeated with the core layers distributed by XU 59220.01, Lot 421733, a homogeneous ethylene / octene copolymer having a density of 0.9028 and an M1 of 0.9 with a long chain branching, on a development basis from the company Dow, were replaced. The resin had an I.sub.10 / I.sub.2 ratio of 10.5 and a Mw / Mn ratio of 2.4. The preheat temperature was 194 ° F (90 ° C) and the temperature of the hot water bath was 190 ° F (88 ° C). The finish-oriented structure had a thickness of 3.0 mils (0.076 mm) and the individual layers given in Example 32 above.

Example 39

The procedure of Example 38 was repeated, with the sole exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 40

The procedure set forth in Example 29 was repeated with the core layers coated by XU 59220.02, a homogeneous ethylene / octene copolymer having a density of 0.906 and a Ml of 0.8 with a long chain branching, marketed by Dow, have been replaced.

The resin had an I10 / I2 ratio of 11.8 and a Mw / Mn ratio of 2.2. The preheat temperature was 197 ° F (92 ° C) and the temperature of the hot water bath was 194 ° F (90 ° C). The final oriented structure with the individual layers had a thickness of 3.0 mils (0.076 mm) and the individual layers were as indicated in Example 32 above.

Example 41

The procedure of Example 40 was repeated, with the sole exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 42

The procedure set forth in Example 29 was repeated with the core layers coated by XU 59220.03, a homogeneous 0.988 ethylene-octene copolymer and a 0.9 MI with a long chain branching, marketed by Dow, have been replaced. The resin had an (I 10 / I 2 ratio of 10.5 and a Mw / Mn ratio of 2.0. The preheat temperature was 187 ° F (86 ° C) and the temperature of the hot water bath was 181 ° F (83 ° C). The final structure and the individual layers had thicknesses as indicated in Example 32 above.

Example 43

The procedure of Example 42 was repeated, with the sole exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 44

The procedure given in Example 29 was repeated replacing the core layers with XU 59220.04, a homogeneous ethylene octene copolymer having a density of 0.896 and a Ml of 0.9 with long chain branching, marketed on a Dow development basis were. The resin had an I.sub.10 / I.sub.2 ratio of 10.5 and a Mw / Mn ratio of 2.17. The preheat temperature was 188 ° F (87 ° C) and the temperature of the hot water bath was 182 ° F (83 ° C). The final oriented structure and the individual layers had thicknesses as indicated in Example 32 above.

Example 45

The procedure of Example 44 was repeated, with the sole exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Example 46

The procedure set forth in Example 29 was repeated replacing the core layers with Insite C24, a homogeneous branched ethylene-octene copolymer having a density of 0.904 g / cc and a Ml of 0.9 distributed on a Dow development basis were. The resin had an I10 / I2 ratio of 9.2 and a Mw / Mn ratio of 2.0. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C). The final structure was 3.0 mils (0.076 mm) thick and the individual layers had nominal thicknesses as reported in Example 32 above.

Example 47

For purposes of comparison, the procedure set forth in Example 29 was repeated replacing the core layers with Attane 4203, a Dow heterogeneous octene copolymer having a density of 0.905 g / cc and a MI of 0.8. The resin had an I 10 / I 2 ratio of 8.2 and a Mw / Mn ratio of 3.8. The preheat temperature was 194 ° F (90 ° C) and the temperature of the hot water bath was 190 ° F (91 ° C). The final oriented structure had a thickness of 3.0 mils (0.076 mm) and the individual layers had target thicknesses as indicated in Example 32.

Example 48

For purposes of comparison, the procedure of Example 47 was repeated with the sole exception that other orientation conditions were used. The preheat temperature was 200 ° F (93 ° C) and the temperature of the hot water bath was 195 ° F (91 ° C).

Examples 49 to 68

To evaluate their physical properties, the oriented multilayer films of Examples 29-48 were tested for instrumental impact and free shrinkage. Examples 34 and 35 are based on core layers of XU 59243.00, a homogeneous branched ethylene-octene copolymer from Dow having a density of 0.904 g / cc and a Ml of 0.8, having a density and a melt index substantially the same as those of Attane 4203, a heterogeneous ethylene-octene linear copolymer used in Comparative Examples 47 and 48. However, the film structures of Examples 34 and 35 show greatly improved peak stress and energy to break compared to the films of Examples 47 and 48, respectively. The results are shown in Table IV below. Table IV

Examples 69 to 88

To evaluate the processability of the homogeneous branched ethylene-octene copolymers found in each of the above-mentioned film structures, the orientation rate for each structure was recorded at each orientation temperature. Surprisingly, the film structures containing Dow's homogeneous branched ethylene-octene copolymers and having improved toughness in Examples 49-66 above were oriented faster than the films of Comparative Examples 47 and 48. The results are shown in Table V below specified. Table V

Examples 89 to 90

In order to evaluate the processability of the homogeneous long chain branched ethylene-octene copolymers incorporated in the film structures of the present invention, the head pressures were recorded during the extrusion of both the substrate and the films of Examples 34 and Comparative Example 47. Examples 29 to 33 and 36 to 45 are not included in Table VI below because valid comparisons can only be made between resins having the same melt index. Example 46 is not included because it was not made under comparable extrusion conditions. It should be noted that although the films of Examples 35 and 48 contain the same polymeric materials as those of Examples 34 and 47, which are evaluated below, they are not included in the table because they result from the same extrusion trials , That is, Example 35 has the orientation of the unoriented stripe structure as described in Example 34 under orientation conditions other than those indicated in Example 34. The same applies to Example 48 with respect to Example 47. The homogeneous branched ethylene-octene copolymer used in Example 34 gave lower extrusion head pressures than the heterogeneous linear ethylene-octene copolymer of Example 47. Table VI

Example 91

The procedure described in Example 29 was repeated with the two core layers constituted by a mixture of 80% by weight XU59243.00, a homogeneous ethylene / octene copolymer having a density of 0.904 g / cm3 and a MI of 0.8 with long chains Branch, distributed on a development basis by Dow, and replaced by 20% by weight of LD-318.92, a 9% VA ethylene-vinyl acetate copolymer sold by Exxon. The XU59243.00 resin had an I10 / I2 ratio of 9.2 and a Mw / Mn ratio of 2.17. The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following nominal thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,36 1,59 0,15 0,18 0,15 0,39 0,17

Example 92

The procedure described in Example 29 was repeated replacing the two core layers with a mixture of 90% by weight of XU 59243.00 and 10% by weight of LD-318.92 from Exxon.

The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following nominal thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,36 1,59 0,15 0,18 0,15 0,39 0,17

Example 93

The procedure set forth in Example 29 was repeated replacing the two core layers with a blend of 80% by weight of XU59243.00 and 20% by weight of Elvax 3175GC, a 28% VA ethylene-vinyl acetate copolymer from DuPont ,

The final oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following nominal thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,36 1,59 0,15 0,18 0,15 0,39 0,17

Example 94

The procedure given in Example 29 was repeated replacing the two core layers with a mixture of 90% by weight of XU59243.00 and 10% by weight of Elvax 3175GC from DuPont.

The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following nominal thickness:

Seal / core / outer // barrier / adhesion / core / abuse

0,36 1,59 0,15 0,18 0,15 0,39 0,17

Example 95

The procedure set forth in Example 29 was repeated with the two core layers constituted by a blend of 85% by weight of XU59243.00 and 15% by weight Escorene LD-720.92, a 19% VA ethylene-vinyl acetate copolymer ex Exon , have been replaced.

The finished oriented structure had a thickness of 3.0 mils (0.076 mm). The individual layers had the following specified thickness:

Inside / core / outer // barrier / adhesion / core / abuse

0,36 1,59 0,15 0,18 0,15 0,39 0,17

Examples 96 to 100

The oriented multilayer films of Examples 91-95 were tested for instrumental impact and free shrinkage. The results are given in Table VII below. Table VII

The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description of the invention. It is not exhaustive and the invention is not limited to the particular form disclosed and modifications and changes are possible in light of the above teachings or may be apparent from the practice of the invention. The embodiments have been chosen and described in order to explain the principles of the invention and its practical applications, in order to enable those skilled in the art to embody the invention in various embodiments and with various modifications suitable for the particular use contemplated. exploit. The scope of the invention is defined by the following claims and their equivalents.