WO2008100720A1

WO2008100720A1 - Extrusion coated polyolefin based compositions for heat sealable coatings

Info

Publication number: WO2008100720A1
Application number: PCT/US2008/052752
Authority: WO
Inventors: Eva-Maria Kupsch
Original assignee: Dow Global Technologies Inc.
Priority date: 2007-02-12
Filing date: 2008-02-01
Publication date: 2008-08-21
Also published as: AR065321A1; TW200848457A

Abstract

The invention relates to a composition suitable for use in extrusion coating processes to form a heat sealable film. The composition comprises from 50 to 92 percent by weight of a propylene based plastomer or elastomer and from 8 to 50 percent by weight of low density polyethylene. The invention also relates to heat sealable seals, and particularly peelable seals made from the compositions. The compositions of the present invention allow desired physical characteristics in combination with improved processability (low neck-in) as compared to existing homogeneous ethylene copolymer solutions.

Description

EXTRUSION COATED POLYOLEFIN BASED COMPOSITIONS FOR HEAT

SEALABLE COATINGS Field of Invention

The invention relates to polyolefin-based extrusion coating formulations particularly suited for heat sealable films. The invention also relates to polyolefin-based extrusion coating formulations for use in peelable seals, particularly when used with polypropylene based substrates. The invention also relates to methods of making and using the heat sealable films and peelable seals. The invention also relates to formulations that enable a more efficient use in extrusion coating processes. Background and Summary of Invention

It is often desirable to coat an article with another material in order to modify the properties of the article. A particularly desirable coating is that of a heat-sealable film; that is, a film which is capable of being bonded to itself, another film or another substrate with the application of heat and/or pressure. In this manner, the article can be sealed to form structures such as bags or other packaging materials. The resulting seal strength of such bonds determines whether such materials are a peelable seal. Heat sealable and peelable seals (also referred to herein as "peelable films") are employed on a large scale for temporarily closing containers that include, for example, food products or medical devices. During use, a consumer opens the package by separating the heat sealed layers of the peelable film. To gain consumer acceptance, a number of characteristics associated with a heat sealable and peelable film are desired. For example, the film should provide a leak- proof closure of the container or bag. To seal a bag, heat sealing is commonly used. Various apparatus have been constructed for the purpose of forming bags while simultaneously filling the bags with the desired contents. These apparatus are commonly known as vertical form- fill-and-seal and horizontal form-fill-and-seal machines. Other types of forming machines may also be used, as may pre-made bags.

These machines typically have forming collars or bars that shape a flat piece of film into a tubular shape of a bag. Hot metal sealing jaws are moved from an open position to a closed position, contacting the film in order to seal it into a bag shape. During the sealing process, the outer layer of the film comes into direct contact with the hot metal surface of the sealing jaws. Heat is thus transferred through the outer layer of the film to melt and fuse the inner sealant layer to form a seal. Generally, the outer layer has a higher melting temperature than the inner sealant layer. As such, while the inner sealant layer is melted to

- i - form a seal, the outer layer of the film does not melt and is not stuck to the sealing jaws. After the sealing jaws reopen, the film is cooled to room temperature.

Before the inner sealant layer is cooled to room temperature, it should be able to maintain its seal integrity. The ability of an adhesive or sealant layer to resist creep of the seal while it is still in a warm or molten state is generally referred to as "hot tack." To form a good seal, the hot tack of the sealable and peelable film should be adequate and the hot tack initiation temperature should be low enough. The hot tack initiation temperatures is the lowest temperature, at which the strength of the seal reaches 1N/I5mm where the seal is formed with seal bars of 5 mm width at a pressure of 0.5 N/mm² with a dwell time of 0.5 sec and a cooling time of 0.2 sec, and the seal strength is measured with a hot tack tester at a pulling speed of 200 mm/sec. The hot tack temperature window is the temperature range over which the hot tack gives values of at least 1N/I5mm.

Besides adequate hot tack, it is also desirable to have a low heat seal initiation temperature which helps to ensure fast packaging line speeds and a broad sealing window which could accommodate variability in process conditions, such as pressure and temperature. The heat seal initiation temperatures is the lowest temperature, at which the strength of the seal reaches 2 N/15mm measured at least 24 hours after the seal formation where the seal is formed with seal bars of 5 mm width at a pressure of 0.5 N/mm² with a dwell time of 0.5 sec. and the strength of the seal is measured with a tensile tester separating the seal at 100 mm/min in machine direction. The seal temperature window is the temperature range over which the seal strength gives values of at least 2 N/15mm.

A broad sealing window also enables high speed packaging of heat sensitive products, as well as, provides a degree of forgiveness for changes in packaging or filling speeds. For peelable seals, in addition to the "sealable" characteristics described above, the formulation should also have a desired "peelable" characteristic needed to provide an easily openable seal on a package or bag. Peelability generally refers to the ability to separate two materials or substrates in the course of opening a package without compromising the integrity of either of the two. The force required to pull a seal apart is called "seal strength" or "heat seal strength" which can be measured in accordance with ASTM F88-94. The desired seal strength varies according to specific end user applications. For flexible packaging applications, such as cereal liners, snack food packages, cracker tubes and cake mix liners, the seal strength desired is generally in the range of 2 - 25 N/15mm. For example, for easy-open cereal box liners, a seal strength in the range of 4-8 N/15mm is commonly specified, although specific targets vary according to individual manufactures requirements. In addition to flexible packaging application, a sealable and peelable film can also be used in rigid package applications, such as lids for convenience items (for example, snack food such as puddings) and medical devices. Typical rigid packages have a seal strength of 2-13 N/15mm. The seal layer can be on the lid or on the container or both.

Additional desired characteristics for a heat sealable film (including peelable as well as non-peelable seals) include a low coefficient of friction and good abuse resistance. A low coefficient of friction ensures that the sealant layer can be processed smoothly and efficiently on fabrication and packaging equipment and is particularly important for vertical form-fill-and-seal packaging. Good abuse resistance and toughness is desired, for example, in cereal box liners to withstand tears and punctures from irregularly-shaped, rigid cereals. Additional characteristics include taste and odor performance and barrier or transmission properties. Additionally, if the heat seal film is to be applied to the substrate by extrusion coating methods, the processability of the resin in such an environment is also important. "Processability" includes parameters such as improved neck-in, reduced draw instability and power consumption.

Heat sealable and peelable films are generally made from one or more polymeric resins. The resulting characteristics of a heat sealable and peelable film depend largely upon the type of the resins used to form the film. For example, ethylene vinyl acetate (EVA) and ethylene methyl acrylate (EMA) copolymers provide excellent heat sealing properties. However, the seals produced with these copolymers are such that separation usually cannot be achieved without damage to the film, making them unsuitable for peelable seal applications. To alleviate this problem, polybutylene can be mixed with an EVA polymer to produce a heat sealable and peelable film. Although the peelability of the film is improved, the heat sealable and peelable film has some unpleasant odor due to the presence of EVA. In addition to using polybutylene, some ionomers, such as SURL YN®, can be mixed with EVA to produce a heat sealable and peelable film. While the film is peelable, it causes stringiness or "angel hair" upon separation of the film. Moreover, ionomers are generally expensive and may have some odor issues as well.

US Pat No. 6,590,034 describes peelable seals made from a mixture of two immiscible polymers which form a continuous phase and a discontinuous phase wherein the absolute value of the shear viscosity differential of the two polymers is less than 100 percent. Although many potential materials are covered, this reference focuses on the use of homopolymer polypropylene as the discontinuous phase.

Although a number of resins systems have been employed to make a heat sealable film, there continues to exist a need for an improved cost-effective heat sealable film with desired seal strength during processing and transportation as well as during package opening by the end consumer, particularly in the case of peelable seals. It is desirable that the resin system used to produce the heat sealable and peelable film has a relatively lower seal initiation temperature and hot tack initiation temperature and a relatively broad heat sealing window. It is also desirable that the heat sealable and peelable film is relatively age- resistant and has a relatively lower coefficient of friction and good abuse resistance and toughness. It is also desirable that the resin system be suited for extrusion coating the seal onto a substrate, as characterized by relatively low neck-in and relatively high draw down values. It has been discovered that blends and compounds of 50 to 92 percent by weight of the total composition propylene based elastomers or plastomers with from 8 to 50 percent by weight of a particular low density polyethylene allow many of these goals to be met. The particular LDPEs for use in the present invention have a melt index (I₂) (ASTM D1238, condition 190°C/2.16kg) of from 0.2 to 15 dg/min^" and a density (ASTM D792) of from 0.915 to 0.930 g/cm³ and a broad molecular weight distribution such as Mw/Mn of 5.0-13.

The resin compositions of the present invention have been demonstrated to have seal strength in the range that would make them particularly well suited for use as a peelable seal, particularly on polypropylene and polyethylene based substrates.

Brief Description of the Drawings

Fig. 1 shows the comparison of total neck-in data from Example 1 at two different line speeds of an extrusion coating line.

Fig. 2 depicts the heat seal strength measured after 24 hours for the seals of Example 2 obtained at different seal bar temperatures. Fig 3 depicts the heat seal strength measured immediately after sealing for the seals of Example 3 obtained at different seal bar temperatures.

Fig 4 depicts the heat seal strength obtained at different seal bar temperatures measured after 24 hours for the seals of Example 4 to a 50 micron polypropylene film. Detailed Description of the Invention

"Haul-Off" is defined herein to mean the speed at which the substrate is moving, thus stretching or elongating a molten polymer extrudate. ""Neck- in" is the difference between the hot melt width at the die face and the extrudate width on the substrate, measured in millimeters, unless otherwise indicated. The neck- in values reported herein are determined at a haul off rate of 100 meters per minute and 300 meters per min with a coating weight of 25g/m² at an extrusion rate of approximately 113kg/h, at an extruder set temperature profile of about 290 ⁰C (except where otherwise noted) using a 3.5 -inch extruder with a 30:1 L/D screw and an extrusion coating line equipped with a 800 mm wide flat die and having a 0.6 mm die gap.

The term "polymer", as used herein, refers to a compound prepared by polymerizing monomers, whether of the same or a different type. The generic term polymer thus embraces the term "homopolymer", usually employed to refer to polymers prepared from only one type of monomer as well as "copolymer" which refers to polymers prepared from two or more different monomers.

The term "low density polyethylene" may also be referred to as "LDPE", "high pressure ethylene polymer" or "highly branched polyethylene" and is defined to mean that the polymer is partly or entirely homopolymerized or copolymerized in autoclave or tubular reactors at pressures above 14,500 psi (100 MPa) with the use of free-radical initiators, such as peroxides (see for example US 4,599,392, herein incorporated by reference).

The term molecular weight distribution or "MWD" is defined as the ratio of weight average molecular weight to number average molecular weight (M_w/M_n). M_w and M_n are determined according to methods known in the art using conventional GPC. The ratio Mw(absolute)/Mw(GPC) is defined wherein Mw(absolute) is the weight average molecular weight derived from the light scattering area at low angle (such as 15 degrees) and injected mass of polymer and the Mw(GPC) is the weight average molecular weight obtained from GPC calibration. The light scatter detector is calibrated to yield the equivalent weight average molecular weight as the GPC instrument for a linear polyethylene homopolymer standard such as NBS 1475.

The present invention relates to a blend of at least two components, which blends are particularly well suited for use as a heat sealable film. The first component in the blends of the present invention is a propylene-based plastomer or elastomer or "PBPE". These materials comprise at least one copolymer with at least about 50 weight percent of units derived from propylene and at least about 3, or even 5 weight percent of units derived from a comonomer other than propylene. Suitable propylene based elastomers and/or plastomers are taught in WO03/040442, and WO 07/024447, each of which is hereby incorporated by reference in its entirety.

Of particular interest for use in the present invention are reactor grade PBPEs having MWD less than 3.5. It is intended that the term "reactor grade" is as defined in US Patent 6,010,588 and in general refers to a polyolefin resin whose molecular weight distribution (MWD) or polydispersity has not been substantially altered after polymerization. The preferred PBPE will have a heat of fusion (as determined using the DSC method described in US application 60/709668) less than about 90 Joules/gm, preferably less than about 70 Joules/gm, more preferably less than about 50 Joules/gm. When ethylene is used as a comonomer, the PBPE has from 3 to 12 percent of ethylene, or from 4 to 9 percent of ethylene, by weight of the propylene based elastomer or plastomer.

Although the remaining units of the propylene copolymer are derived from at least one comonomer such as ethylene, a C₄_₂o α-olefin, a C₄_₂o diene, a styrenic compound and the like, preferably the comonomer is at least one of ethylene and a C_4-I2 α-olefin such as 1- hexene or 1-octene. More preferably, the remaining units of the copolymer are derived only from ethylene.

The amount of comonomer other than ethylene in the propylene based elastomer or plastomer is a function of, at least in part, the comonomer and the desired heat of fusion of the copolymer. If the comonomer is ethylene, then typically the comonomer-derived units comprise not in excess of about 15 wt percent of the copolymer. The minimum amount of ethylene-derived units is typically at least about 3, preferably at least about 5 and more preferably at least about 9, wt percent based upon the weight of the copolymer. If the polymer comprises at least one other comonomer other than ethylene, then the preferred composition would have a heat of fusion approximately in the range of a propylene-ethylene copolymer with 3 to 20 wt. percent ethylene. Though not intending to be bound by theory, it is thought that attaining approximately similar crystallinity and crystal morphology is beneficial to achieving similar functionality as a peelable seal.

The propylene based plastomer or elastomer of this invention can be made by any process, and include random, block and graft copolymers although preferably the copolymers are of a random configuration. These include copolymers made by Ziegler- Natta, CGC (Constrained Geometry Catalyst), metallocene, and non-metallocene, metal- centered, heteroaryl ligand catalysis. Additional suitable metal complexes include compounds corresponding to the formula:

, where:

R²⁰ is an aromatic or inertly substituted aromatic group containing from 5 to 20 atoms not counting hydrogen, or a polyvalent derivative thereof;

T³ is a hydrocarbylene or silane group having from 1 to 20 atoms not counting hydrogen, or an inertly substituted derivative thereof; M³ is a Group 4 metal, preferably zirconium or hafnium;

G is an anionic, neutral or dianionic ligand group; preferably a halide, hydrocarbyl or dihydrocarbylamide group having up to 20 atoms not counting hydrogen; g is a number from 1 to 5 indicating the number of such G groups; and covalent bonds and electron donative interactions are represented by lines and arrows respectively.

Preferably, such complexes correspond to the formula:

, wherein:

T³ is a divalent bridging group of from 2 to 20 atoms not counting hydrogen, preferably a substituted or unsubstituted, C₃.₆ alkylene group; and Ar² independently each occurrence is an arylene or an alkyl- or aryl-substituted arylene group of from 6 to 20 atoms not counting hydrogen;

M³ is a Group 4 metal, preferably hafnium or zirconium;

G independently each occurrence is an anionic, neutral or dianionic ligand group; g is a number from 1 to 5 indicating the number of such X groups; and electron donative interactions are represented by arrows. Examples of metal complexes of foregoing formula include the following compounds:

where M³ is Hf or Zr;

Ar⁴ is Cβ-₂₀ aryl or inertly substituted derivatives thereof, especially 3,5-di(isopropyl)phenyl, carbazole, 3,5-di(isobutyl)phenyl, dibenzo-lH-pyrrole-1-yl, or anthracen-5-yl, and

T⁴ independently each occurrence comprises a C₃_₆ alkylene group, a C₃_₆ cycloalkylene group, or an inertly substituted derivative thereof;

R²¹ independently each occurrence is hydrogen, halo, hydrocarbyl, trihydrocarbylsilyl, or trihydrocarbylsilylhydrocarbyl of up to 50 atoms not counting hydrogen; and G, independently each occurrence is halo or a hydrocarbyl or trihydrocarbylsilyl group of up to 20 atoms not counting hydrogen, or 2 G groups together are a divalent derivative of the foregoing hydrocarbyl or trihydrocarbylsilyl groups.

Exemplary propylene copolymers include Exxon-Mobil VISTAMAXX™ polymer, and VERSIFY™ propylene/ethylene elastomers and plastomers by The Dow Chemical Company.

The density of the propylene based elastomers or plastomers of this invention is typically at least about 0.850, can be at least about 0.860 and can also be at least about 0.865 grams per cubic centimeter (g/cm³) as measured by ASTM D-792. Preferably the density is less than about 0.89 g/cc. The weight average molecular weight (Mw) of the propylene based elastomers or plastomers of this invention can vary widely, but typically it is between 10,000 and 1,000,000 (with the understanding that the only limit on the minimum or the maximum M_w is that set by practical considerations). For homopolymers and copolymers used in the manufacture of peelable seals, preferably the minimum Mw is about 20,000, more preferably about 25,000. The polydispersity of the propylene based elastomers or plastomers of this invention is typically between 2 and 5. "Narrow polydispersity", "narrow molecular weight distribution", "narrow MWD" and similar terms mean a ratio (M_w/M_n) of weight average molecular weight (M_w) to number average molecular weight (M_n) of less than about 3.5, can be less than about 3.0, can also be less than about 2.8, can also be less than about 2.5.

The PBPEs for use in the present invention ideally have an MFR of from 0.5 to 2000 g/10min, preferably from 1 to 1000, more preferably from 2 to 500, still more preferably from 2 to 40. The particular MFR selected will depend in part on the intended fabrication methods such as blown film, extrusion coating, sheet extrusion, injection molding or cast film processes. MFR for copolymers of propylene and ethylene and/or one or more C₄-C₂₀ α-olefins is measured according to ASTM D-1238, condition L (2.16 kg, 230 degrees C). MFRs greater than about 250 were estimated according to the following correlation:

MFR = 9 XlO¹⁸ Mw^"33584

Mw (grams per mole) was measured using gel permeation chromatography. The overall blends for use in the present invention will also comprise low density polyethylene (LDPE),. The preferred LDPE for use in the present invention has a Melt Index (I₂) (determined by ASTM D1238, condition 190°C/2.16kg), of from 0.2 to 15g/10 min. More preferably the melt index is greater than about 0.5 g/10min. The melt index is preferably less than about lOg/10 min., and can optimally for some applications be between 0.4 and 7.5 10g/min. The preferred LDPE will also have a density (as determined in accordance with ASTM D792) in the range of 0.915 to 0.930 g/cc, preferably 0.915 to 0.925 g/cc. The preferred LDPE will also have a Mw/Mn value as determined with gel permeation chromatography of from 5 to 13.

Such preferred LDPE can be made in an autoclave or tubular reactor as is generally known in the art.

The second component of the present invention may also include LDPE/LDPE blends, for example, blends in which one of the LDPE resins has a relatively higher melt index and the other has a lower melt index and is more highly branched. The component with the higher melt index can be obtained from a tubular reactor, and a lower MI, higher branched, component of the blend may be added in a separate extrusion step or using a parallel tubular/autoclave reactor in combination with special methods to control the melt index of each reactor, such as recovery of telomer in the recycle stream or adding fresh ethylene to the autoclave (AC) reactor, or any other methods known in the art.

Suitable high pressure ethylene polymer compositions for use in preparing the inventive extrusion composition also include low density polyethylene (homopolymer), ethylene copolymerized with at least one α-olefin for example butene, and ethylene copolymerized with at least one α,β-ethylenically unsaturated comonomers, for example, acrylic acid, methacrylic acid, methyl acrylate and vinyl acetate. A suitable technique for preparing useful high pressure ethylene copolymer compositions is described by McKinney et al. in US Patent 4,599,392, the disclosure of which is incorporated herein by reference. These materials may optimally be blended with other LDPE materials, and generally provide seal strengths at the upper end of the desired range, tending to make them less desired for use in peelable seals.

While both high pressure ethylene homopolymers and copolymers are believed to be useful in the invention, homopolymer polyethylene is generally preferred. The compositions of the present invention will comprise at least a propylene based elastomer or plastomer component and a low density polyethylene polymer. The second polymer material will comprise from eight to 50 percent by weight of the overall material. If low heat seal initiation temperature, and/or high hot tack strength is desired, it may be preferred to have the polyethylene comprise less than about 30 percent of the overall composition.

The compositions of the present invention are particularly suited for producing sealing layers by extrusion coating processes. The peelable seal layer can be made in any desired thickness, for example from 1 micron to 3 mm. The sealant layer can be used as a monolayer, but more typically will be one layer of a multilayer structure, for example a 10 micron sealant layer with a 30 micron supporting layer.

When the sealant layer (particularly a sealant layer comprising a majority of PBPE) is coextruded on a substrate which is PP based then the whole structure will be recyclable. Peelable seals made from the blends of the present invention will have an aged seal strength of 2 to 10 N/15mm, preferably 2 to 8 N/15mm as determined by making heat seals at 0.5 sec sealing time and 0.5 N/mm² seal bar pressure on the Kopp heat sealer. Seal strength is measured after at least 24 hour ageing on 15 mm wide samples separated at 100 mm/min in machine direction pulled by a Lloyds tensile tester. It should be understood by

- io - one of ordinary skill in the art that the seal strength may typically be somewhat less for flexible packaging and somewhat higher for rigid packaging.

The peelable seals of the present invention will have a heat seal initiation temperature of less than 120 ⁰C, preferably less than 110 ⁰C, more preferably less than 100 ⁰C, more preferably less than 90 ⁰C. The heat seal initiation temperature is defined as the minimum temperature at which the seal strength of 2N/15 mm is obtained using the Kopp heat sealer making heat seals at 0.5sec sealing time and 0.5 N/mm² seal bar pressure pulled on Lloyds tensile tester at 100 mm/min after 24 hours of welding seal.

It should also be understood that the composition of the present invention may also contain various additives as is generally known in the art. Examples of such additives include antioxidants, ultraviolet light stabilizers, thermal stabilizers, slip agents, antiblock, pigments or colorants, processing aids (such as fluoropolymers), crosslinking catalysts, flame retardants, fillers, foaming agents, etc.

The following Examples further illustrate the present invention. EXAMPLES:

A description of all of the resins used in the Examples is presented in Table 1.

- i i - Table 1

* determined using ASTM D-1238 (2.16kg, 19O⁰C) ** determined using ASTM D-1238 (2.16 kg, 23O⁰C)

Example 1

A series of extrusion coating trials is done and the neck-in measured . The extrusion coating line used has a flat die with a width of 800 mm. The die gap is 0.6 mm. The total neck-in is calculated by taking the difference between the hot melt width at the die face and the coating width on the substrate. The coating weight is 25g/m² and a standard temperature profile of 29O⁰C on the 3.5 inch extruder is used. The neck-in is measured at a line speed of 100 m/min and 300 m/min at a temperature of 290 ⁰C.

Fig 1 shows that the total neck-in of the blend/compound of 70 percent Resin C with 30 percent resin A is significantly less than the pure Resin D at both line speeds, therefore demonstrating improved processability in the extrusion coating process.

Example 2: A series of coating structures is prepared onto oriented polypropylene (OPP) of a thickness of 15 micron with a coating density of 25g/m² at an extruder set temperature profile of 290 ⁰C and a line speed of 100 m/min. The seal strength curves are measured sealing the sealant layer each time to itself using 0.5 N/mm pressure and a dwell time 0.5 seconds at different temperatures as indicated on Figure 2. The seal strength is measured 24 hours after forming the seal. The seal strength is measured using Lloyds Tensile tester pulled at 90° angle from the seal at a crosshead speed of 100 mm/min. The results are depicted in Figure 2. The seal initiation temperature at which the desired 2 N/15mm seal strength are reached at a seal bar temperature of 78 ⁰C for Resin D versus 58°C for 70 percent Resin C/30 percent resin A, demonstrating the lower seal initiation temperatures for the compositions of the present invention. Example 3

A series of coating structures is prepared onto paper (kraft paper 60g/m²) with a coating weight of 25g/ m , and a line speed of 100 m/min. The sealant layer is sealed to itself using 0. 5 N/mm² pressure and a dwell time of 0.5 seconds at different temperatures as indicated on Figure 3. The hot tack strength curves are measured instantaneously (0.2 seconds) after the seal is formed using a J&B Hot Tack Tester 3000 pulled at a 90° angle fro the seal at a crosshead speed of 200 mm per minute.

Figure 3 shows that the hot tack initiation temperature at which the hot tack strength reaches 1 N/15mm is 86 ⁰C for resin D versus 63°C for the 70 percent resin C / 30 percent resin A blend. The hot tack strength of resin D comes down below 1 N/ 15 mm at 160 ⁰C, while the hot tack curve of the resin C/resin A blend remains well above 1 N/ 15 mm until at least 150 ⁰C. Data for the Resin C/resin A blend at 160 ⁰C and higher were not measured but it is safe to assume that the hot tack values at 160 ⁰C and higher would be greater than 1 N/15mm . At any rate it is clear that the formulations of the present invention give a wider hot tack processing temperature window than the comparative material.

Example 4

A series of coating structures is prepared onto bioriented polypropylene (BOPP thickness of 14 micron) with a coating weight of 15g/m², an extruder temperature profile of 320 ⁰C and a line speed of 100 m/min. The seal strength curves are measured sealing the sealant layer each time to a 50 micron film of 100 percent resin E using 0. 5 N/mm pressure and a dwell time of 0.5 seconds at different temperatures as indicated on Figure 4. The seal strength is measured at a minimum of 24 hours after welding the seal. The seal strength is measured using a Lloyds Tensile tester pulled at 90° angle from the seal at a crosshead speed of 100 mm/min. The results are depicted in Figure 4

Seal strength values obtained are in the range of 2 to 6 N/15mm over the temperature range of 85⁰C to 160 ⁰C for the blend of 70 percent resin C /30 percent resin A. Seal strength values obtained are in the range of 2 to 10 N/15mm over the temperature range of 95⁰C to 140 ⁰C for the blend of 70 percent resin B /30 percent resin A. These compare favorably to the seal made with 100 percent of the homogeneously branched LLDPE component, Resin D, which exhibits seal strength values in the range of from 2 to 10 N/15/mm only within the temperature range of from 125⁰C to 16O⁰C. Thus the inventive materials are well-suited for use in peelable seal applications.

Claims

WHAT IS CLAIMED IS:

1. A composition suitable for use in extrusion coating processes to form a heat sealable film, the composition comprising:

a. from 50 to 92 percent by weight of a propylene based plastomer or elastomer

b. from 8 to 50 percent by weight of low density polyethylene.

2. The composition of Claim 1 wherein the propylene based plastomer or elastomer comprises at least about 60 percent by weight of the composition and the low density polyethylene comprises no more than about 40 percent by weight of the composition.

3. The composition of Claim 1 wherein the low density polyethylene has a Melt Index (I₂) of from 0.2 to 15g/10 min.

4. The composition of Claim 1 wherein the low density polyethylene has a Melt Index (I₂) of from 0.3 to lOg/10 min.

5. The composition of Claim 1 wherein the low density polyethylene has a Melt Index (I₂) of from 0.4 to 9 g/10 min.

6. The composition of Claim 1 wherein the low density polyethylene has a Melt Index (I₂) of from 0.4 to 8 g/10 min

7. The composition of Claim 1 wherein the low density polyethylene has a density in the range of 0.915 to 0.930 g/cc, preferably 0.915 to 0.925 g/cc.

8. The composition of Claim 1 wherein the low density polyethylene has an Mw/Mn value in the range of from 5 to 13.

9. The composition of Claim 1 wherein the low density polyethylene has an Mw/Mn value in the range of from 5.5 to 12.5.

10. The composition of Claim 1 wherein the low density polyethylene has an Mw/Mn value in the range of from 6 to 12.

11. The composition of Claim 1 wherein the propylene based elastomer or plastomer contains from 3 percent to 12 percent by weight of the propylene based elastomer or plastomer of units derived from ethylene.

12. The composition of Claim 1 wherein the propylene based elastomer or plastomer has a heat of fusion less than 90 J/gm.

13. The composition of Claim 1 wherein the propylene based elastomer or plastomer has a heat of fusion less than 70 J/gm.

14. The composition of Claim 1 further comprising one or more additives from a group comprising antioxidants, ultraviolet light stabilizers, thermal stabilizers, slip agents, antiblock, pigments or colorants, processing aids (such as fluoropolymers), crosslinking catalysts, flame retardants, fillers and foaming agents.

15. The composition of Claim 1 wherein the low density polyethylene component comprises a blend of two or more different low density polyethylene materials.

16. The composition of Claim 15 wherein a first LDPE resin has a higher melt index than a second LDPE resin and the second LDPE resin is more highly branched than the first LDPE resin.

17. The composition of Claim 15 wherein one of the LDPE resins comprises ethylene copolymerized with at least one α,β-ethylenically unsaturated comonomer.

18. A heat sealable seal made from the composition of Claim 1.

19. The seal of claim 18, wherein the seal is made using an extrusion coating process.

20. The seal of claim 18 wherein the seal has a seal strength in the range of 2 to 12 N/15mm

21. The seal of claim 18 wherein the seal has a seal strength in the range of 1.5 to 10 N/15mm.