ANISOTROPIC THIN FOAMED POLYETHYLENE SHEET AND APPLICATIONS THEREOF

Abstract

An anisotropic multilayer film comprising one or more foam layers is described herein.

Claims

1. An anisotropic multilayer foamed film comprising one or more foam layers, wherein at least one layer comprises 10 to 100 percent by weight LLDPE with a melt index of 0.2 to 2 g/10 min, and the resulting film has a surface with an average Sheffield smoothness of less than 100.

2. The film of claim 1 which is produced by the blown film process, cast film process, or any method known in the art.

3. The film in claim 1 which is produced using an extrusion die with a die gap of 0.45 to 1.3 mm.

4. The film in claim 1 wherein the film is produced by the blown film process using an annular extrusion die and a blow-up ratio of 2:1 to 3.5:1.

5. The film of claim 1 wherein a supercritical physical blowing agent is introduced into the molten resin inside the mixing section of the extruder during the process.

6. The film of claim 5 wherein the supercritical blowing agent used is either nitrogen, carbon dioxide or a mixture of nitrogen and carbon dioxide.

7. The film of claim 5 wherein the supercritical blowing agent is introduced inside the mixing section of the extruder at the injection pressure greater than 34 bar, preferably greater than 70 bar, more preferably greater than 240 bar, and most preferably greater than 380 bar.

8. The film of claim 1 wherein a nucleating agent is used to produce a foamed layer with an average cell size of 10 to 1000 m.

9. The film of claim 8 wherein the nucleating agent is 0.05 to 15 percent by weight of an inorganic additive, an organic additive or a mixture of an inorganic and an organic additive.

10. The film of claim 1 wherein the cell density in the foam layer is 10.sup.2 to 10.sup.9 cells/cm.sup.3 and the film density is 0.1 to 0.9 g/cm.sup.3.

11. The film of claim 1 wherein at least one layer contains some apt amounts of other additives to include pigments, antistatic agents, UV stabilizers, and antioxidant.

12. The film of claim 1 wherein at least one layer is a solid layer.

13. The film of claim 1 wherein the film has an average Sheffield smoothness of less than 70, preferably less than 50, more preferably less than 30, and most preferably less than 10.

14. An anisotropic multilayer foamed film comprising one or more foam layers, wherein at least one layer comprises 10 to 100 percent by weight LLDPE with a melt index of 0.2 to 2 g/10 min, and the resulting film has a puncture propagation tear resistance greater than 500 g/mil.

15. The film of claim 14 which is produced by the blown film process, cast film process, or any method known in the art.

16. The film in claim 14 which is produced using an extrusion die with a die gap of 0.45 to 1.3 mm.

17. The film in claim 14 wherein the film is produced by the blown film process using an annular extrusion die and a blow-up ratio of 2:1 to 3.5:1.

18. The film of claim 14 wherein a supercritical physical blowing agent is introduced into the molten resin inside the mixing section of the extruder during the process.

19. The film of claim 18 wherein the supercritical blowing agent used is either nitrogen, carbon dioxide or a mixture of nitrogen and carbon dioxide.

20-39. (canceled)

40. An article prepared using the film of claim 1.

41-42. (canceled)

Description

DETAILED DESCRIPTION

[0017] The singular forms a, an, and the include plural referents unless the context clearly dictates otherwise.

[0018] As used in the specification and in the claims, the term comprising may include the embodiment consisting of and consisting essentially of.

[0019] All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of from 2 grams to 10 grams is inclusive of the end points, 2 grams and 10 grams, and all the intermediate values)

[0020] As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as about and substantially, may not be limited to the precise value specified. The modifier about should also be considered as disclosing the range defined by the absolute value of the two endpoints. For example, the expression from about 2 to about 4 also discloses the range from 2 to 4.

[0021] The present disclosure relates to multilayer or monolayer polyethylene foam film suitable to be used in a wide range of applications such as collation shrink, pouches, lamination, pet food bags, surface protection, agricultural films, geomembrane, packaging, trash bag, grocery bags, food wrap, pallet wrap, shrink film, labels, and pouches for FFS packaging. The fabrication of either a monolayer or a multilayer film is within the scope of this invention and technique and one or more layers of the film may be foamed.

[0022] The term anisotropic refers to the fact that certain properties of the foamed film differs depending on the direction along which the property is measured. For the purpose of this disclosure, properties were measured against two direction (in-flow or machine direction (MD), and cross-flow or transverse direction(TD)) which are perpendicular to each other.

[0023] The term foam in this invention refers to a cellular structure formed when a gas is blown into the molten polymer, and bubbles nucleate when the gas diffuses out of the polymer, right after the application of a thermodynamic instability, as the polymer solidifies.

[0024] Generally, the desired application of a thin film determines the essential physical and mechanical properties of the film which subsequently concludes the best resin or a blend of a few resins and additives for processing. Furthermore, the processing properties is a crucial factor in material selection. More specifically, in the blown film process of this invention where the head pressure is high because of a very narrow gap, the melt fracture should be avoided, and the resin should have a good thermal stability and high enough melt strength. Various thermoplastics can be used in the blown film process of this invention such as polyethylene (PE), polypropylene (PP), polystyrene (PS), ethylene vinyl acetate (EVA), ethylene vinyl alcohol (EVOH), polyvinyl chloride (PVC), Polyvinylidene chloride (PVDC), polyamide (PA), polyurethane (PU), or any of the resins known as TPE family such as, but not limited to, propylene-ethylene copolymer, Thermoplastic Olefin (TPO), Thermoplastic Polyurethane (TPU). The LLDPE copolymer in this invention can include an -olefin co-monomer such as butene, hexane, or octene.

[0025] The choice of material in this invention could be determined in separate ways based on processing requirements, product properties, and material specification. The processing requirement in a blown film process dictates the use of a high melt strength resin to get a stable bubble. Low-density polyethylene (LDPE) is relatively easy to process resin at a lower processing temperature compared to HDPE. Because of the existence of long chain branching, all LDPE grades show a rather high processing melt strength. Therefore, an entirely stable bubble with a relatively low frost line height can be examined with a PE blend the majority of which is LDPE. Moreover, LDPE shows an elongational thickening behavior which can further increase the melt strength and can cause strain hardening. On the other hand, generally LDPE presents a rather low tear strength along the machine direction (MD).

[0026] Typically, film manufacturers capitalize on a blend of LDPE and high-MI LLDPE, while the blend is an immiscible blend in many cases, wherein LDPE improves the processing ability and ductility while the LLDPE enhances the modulus and strength. In a conventional blown film line, the production of the film from LLDPE is mostly avoided because of the extensive amount of difficulties in processing. The most important of which is forming a stable bubble using a high MI LLDPE that exhibits a poor melt strength. In this invention, we are using LLDPE with a very low melt index, or fractional melt index, which can offer an enhanced MD tear strength, improved gas and moisture permeation properties compared to that of LDPE, far better environmental stress cracking resistance, and an enhanced tensile strength at yield.

[0027] In blown film extrusion, typically sharkskin melt fracture occurs when LLDPE is processed through a narrow die gap. Furthermore, LLDPE with low melt index, which offers an improved toughness, shows a higher tendency to melt fracture than a high-MI LLDPE, as do metallocene catalyzed resins (mLLDPE) which have a narrow molecular weight distribution than conventional LLDPEs. In this invention, a very small and precise amount of supercritical gas, as a processing aid and blowing agent, is injected into the molten polymer at a high pressure, for example greater than 34 bar, inside an efficient and effectual mixer, e.g., cavity transfer mixer, as an extension to the extruder's barrel. For example, the supercritical blowing agent used in this invention can be either nitrogen, carbon dioxide or a mixture of nitrogen and carbon dioxide. The temperature of the mixer could be accurately controlled within 1 C. The inclusion of a very small amount of gas could offer a few important advantages in blown film extrusion. First, it could reduce the back pressure which allows processing at a higher throughput and delayed any bubble instability. Therefore, melt fracture could be reduced significantly. Second, the possibility of using a very low melt index LLDPE with narrow molecular weight distribution, to include metallocene -olefin copolymers, became viable for processing to improve the film properties.

[0028] In some embodiments, The LLDPE component of the resin has a melt index in the range 0.1 to 1.2 g/10 min as is determined by ASTM D1238. The LLDPE component of the resin can have a density of 0.915 to 0.935 g/cm.sup.3. The polyethylene resin may be comprised of 70 to 90 percent by weight copolymer and from 30 to 10 percent by weight homopolymer, corresponding to 100 percent by weight. The LLDPE component of the resin may be a copolymer containing one or more of the -olefins; 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. In another case, the polyethylene resin can be a blend of 51 to 99 percent by weight LLDPE with a very low MI in the range of 0.2 to 1 g/10 min, as determined by ASTM D1238, and 1 to 49 percent by weight LDPE. In some cases, the polyethylene blend may contain up to 10 percent by weight HDPE that might improve the miscibility of the blend. Also, for example, the LLDPE component of the resin may be a blend of mLLDPE and LLDPE.

[0029] In some cases, at least one layer of the film has 0.05 to 15 percent by weight of an inorganic additive, an organic additive or a mixture of an inorganic and an organic additive. In some other cases, the polymer composition of each layer may comprise some apt amounts of other additives to include pigments, antistatic agents, UV stabilizers, and antioxidant. In another embodiment, at least one layer includes a clarifying agent at less than 1 percent by weight, preferably less than 0.5 percent by weight, more preferably less than 0.1 percent by weight, and mostly preferably less than 0.05 percent by weight.

[0030] A multilayer film, comprising at least one foam layer, fabricated using this invention in a blown film process has sets of significantly improved physiomechanical properties compared to the film articles of the prior art, to the best of applicant's knowledge, as in particular puncture propagation tear (PPT) resistance, acc. to ASTM D2582, can be greater than 650 g/mil, more specifically can be greater than 750 g/mil, and most specifically can be greater than 900 g/mil. In some cases, the MD tear resistance, acc. to ASTM D9929, can be greater than 100 g.Math.mil, more specifically can be greater than 200 g/mil, and most specifically can be greater than 300 g/mil. In some embodiments, the foam film resulting from the present invention has a far better cellular morphology compared to the existing prior arts, as it can have uniformly distributed cells with an average cell size of 10-100 m, an average cell density of 10.sup.2-10.sup.9 cells/cm.sup.3, and an expansion ratio of the foamed layer from 1 to 9. in one other case, the expansion ratio of any foamed layer is less than 1.1 compared to the non-foamed layer of the same composition.

[0031] All the equipment used in this invention are very well-known to the skilled persons in the art and well labeled and extensively described in the literature. In some cases, the film can be produced by the blown film process using an annular die with a die gap from 0.45 to 1.3 mm and a blow-up ratio ranging from 2:1 to 3.5:1. Higher blow-up ratios can result in a more balanced MD/TD orientation which improved overall film toughness. The die geometry and specification may be manufactured according to the patent with the application number of US 2012/0228793 A1.

[0032] As it was explained earlier, a very small amount of a physical blowing agent in a super critical condition can be injected into the molten resin, at a very precisely controlled rate, inside a mixer with a very effectual distributive and dispersive element before entering the annular die. This unit is controlled as a separate temperature zone with an accuracy of 1 C. and a gas injection pressure variation below 1%. The plasticization effect of the gas results in a viscosity change of the molten resin, specifically the aforementioned low-MI PE resin, which enhances the processibility of the resin at a lower temperature compared to the processing temperature which is used conventionally. This also may benefit and assist ones to manipulate the crystallization kinetics of the resin to improve a few properties.

[0033] In the blown film process of this invention, because of the overall high specific heat capacity of polyethylene, the transverse stretch of the bubble could be delayed until the film became cooler, which enhanced the bubble stability and the frost line height. This caused the bubble expansion in transverse direction to occur at a lower temperature and shorter period of time which is very effective to increase the molecular orientation in the transverse direction, therefore, to improve the MD tear strength.

[0034] In some other embodiments the film in this invention can be produced by the blown film process, cast film process, or any method known in the art.

[0035] The following examples demonstrate the process of the present disclosure. The examples are only demonstrative and are intended to put no limit on the disclosure with regards to the materials, conditions, or the processing parameters set forth herein.

Examples

[0036] Samples of multilayer film (three layers) were produced on a blown film line comprising one 80 mm main extruder and two 70 mm co-extruders. The core extruder was equipped with a supercritical gas injection unit, capable of injecting nitrogen or carbon dioxide, and a 90 mm MuCell Transfer Mixer, both from MuCell Extrusion LLC.

[0037] All samples were produced with a polyethylene blend comprising LLDPE copolymer with an -olefin co-monomer comprises 1-octene with MFI=1 and the density of 0.935. The foamed core layer of all samples contains talc as the cell nucleating agent which was added in the form of a 67% talc filled LDPE masterbatch. The solid layers comprised from 53.5 wt % to 86 wt % the aforementioned LLDPE of the total composition.

[0038] Supercritical nitrogen was used as a physical blowing agent and was injected into the MuCell Transfer Mixer (MTM) at the concentration from 0.0662% to 0.0737%, very accurately, into the molten polymer. To characterize the tear and impact resistance of the foam films a Spencer Impact tester from Thwing-Albert, capable of performing the Elmendorf tear resistance test and Spencer impact test, was used. The smoothness of the products (Sheffield unit) was evaluated using a Gurley 4340 Automatic Densometer & Smoothness Tester.

TABLE-US-00001 Sample Sample E Sample F Sample G Density (gr/cm.sup.3) 0.78 0.76 0.81 Thickness (um) 102 94 92 Throughput (kg/hr) 422.4 421.7 422.5 PBA % 0.0662 0.0737 0.0727 Basic weight (gr/m.sup.2) 79.6 71.4 74.5 Line speed (m/min) 17.8 19 18.2 Die Gap (mm) 0.6 0.6 0.6 Skin Skin Skin Skin Skin Skin Layers A Core B A Core B A Core B LLDPE wt %, MFI 1 76 63.5 55 76 53.5 55 86 53.5 55 67% talc filled LDPE 0 15 0 0 15 0 0 15 0 Layer Thickness (um) 25.5 50.3 26.3 23.2 47.7 23.2 24.1 43.8 24.1 Layer Density (gr/cm.sup.3) 0.97 0.59 0.95 0.97 0.56 0.95 0.97 0.64 0.95 Layer Throughput (kg/hr) 131 158.7 132.7 132.5 158.8 130.4 132.5 159.6 130.4 Layer Volume Ratio (%) 25 49.3 25.7 24.6 50.7 24.7 26.2 47.6 26.2 Tear Strength MD (grf) 1137.1 1055.5 866.1 Tear Strength TD (grf) 1574.4 1715 1518.9 Tensile Str at Break MD (MPa) 19.4 18.2 19.3 Tensile Str at Break TD (MPa) 17.3 17.5 16.8 Yield Stress MD (MPa) 11.1 10.1 10.5 Yield Stress TD (MPa) 10.3 10.25 9.7 Elongation at Break MD (%) 942 892 918 Elongation at Break TD (%) 970 928 928 1% Sec. Modulus MD (MPa) 325 285 294 1% Sec. Modulus TD (MPa) 299 317 287 Toughness MD (in. Ibf/in.sup.3) 15988 14509 15731 Toughness TD (in. Ibf/in.sup.3) 15432 14343 13549 PPT MD (grf) 2604.3 2989.8 2603.3 PPT TD (grf) 5072.4 4390.3 4309.8 PPT MD (gr/mil) 667.1 902.4 783.7 PPT TD (gr/mil) 1299.2 1325.2 1297.4 Tear length in PPT MD (cm) 2.9 3.2 3.4 Tear length in PPT TD (cm) 1.6 1.8 1.4 Smoothness (Sheffield Unit) 9.8 6.9 10.7

[0039] All the samples listed in the table above were made with the same processing condition and the same throughput. Sample F was made based on the sample E but with a lower weight per unit area and the same resin composition by the inclusion of a higher amount of supercritical gas up to 0.0737%. The Sheffield smoothness of the film in sample F improved, e.g. down to 6.9 Sheffield unit, as well as the puncture propagation tear resistance of the film, e.g. up to about 900 gr/mil. Sample G was made based on sample F, but with the inclusion of higher amount of LLDPE in the Skin A which resulted in a little higher PPT. The tensile properties, including the yield strength, as well as the tear strength in the machine direction for all the samples were comparable to, or in some case surpasses, that of the similar foam films or equivalent solid counterparts in existing prior art.