CO-EXTRUDED MULTILAYER STRUCTURE AND METHOD FOR OBTAINING THEREOF

Abstract

The present invention is directed to a novel co-extruded multilayer structure that possess a draw down ratio superior than the critical draw-down ratio of each one of the polymeric layers, extruded individually. The present invention is also directed to a method for obtaining the co-extruded multilayer structure.

The co-extruded multilayer structure obtainable by the method described herein allows preparing films, filaments or spun-melt non-wovens of low weight at high speed using conventional extrusion equipments. The co-extruded multilayer structure is especially suitable as diaper back-sheets or flexible packaging coatings.

Claims

1-15. (canceled)

16. A co-extruded multi-layer structure having an improved extrusion critical draw down ratio compared with the critical draw down ratio of each one of the layer's polymers present in the multi-layer structure, the co-extruded multi-layer structure comprising a first composite layer disposed over a second composite layer, and further comprising an inter-layer disposed between the first and the second composite layer, wherein the first composite layer contains a first polymer, the second composite layer contains a second polymer, and the inter-layer chemically or physically interacts with the first polymer and the second polymer, wherein the first polymer and the second polymer differ from each other in at least its extensional viscosity, one polymer increasing its extensional viscosity at itself critical draw down ratio under tensile stress and the other polymer decreasing its extensional viscosity at itself critical draw down ratio under tensile stress, wherein the extensional viscosity is the ratio between extensional stress and extensional rate in a conventional extrusion technique, and wherein the inter-layer is obtainable by an extrusion method comprising using a die, wherein the die is one common die, and the method comprising the steps of: dispersing an adhesive material in the first polymer and/or in the second polymer of the first composite layer and/or the second composite layer, respectively, feeding the first and the second composite layers to the one common die under temperature in order to co-extrude the molten layers simultaneously, whereby the inter-layer is formed by chemical interaction between the first and the second polymers when they contact in molten state, or alternatively, adding a third adhesive layer between the first and the second composite layers, thereby forming a multi-layer, and feeding the multi-layer to the one common die under temperature in order to co-extrude it, whereby the inter-layer is formed by physical interaction of the molten first and second polymers with the third adhesive layer simultaneously fed between the first and the second composite layers to the one common die, the method further comprising:— when the molten co-extruded layers leave the die, stretching the molten co-extruded layers under tensile stress, whereby the resulted co-extruded multi-layer structure has a draw-down ratio which is higher than the critical draw-down ratio of each one of the first and of the second polymers, individually extruded, thereby decreasing the cross-sectional area of the co-extruded multi-layer structure to a lower value, and cooling down the co-extruded multi-layer structure to room temperature.

17. The co-extruded multi-layer structure according to claim 16, wherein the adhesive material is present either in a concentration ranging from 0.5-10% by weight when the first and the second polymers have a tendency to homogeneously mix together when contacted in a molten state, or either in a concentration ranging from 10-60% by weight when the first and the second polymers do not homogeneously mix together when contacted in a molten state.

18. The co-extruded multi-layer structure according to claim 16, wherein the adhesive layer is a tie layer which is made of an adhesive material.

19. The co-extruded multi-layer structure according to claim 16, wherein at least one of the first polymer and the second polymer has a water vapor transmission rate equal to or higher than 1 g mm/m.sup.2 day as measured according to ASTM E96B.

20. The co-extruded multi-layer structure according to claim 16, having a water vapor transmission rate ranging from 1,000-20,000 g/m.sup.2 day as measured according to ASTM1249.

21. The co-extruded multi-layer structure according to claim 16, wherein the co-extruded multi-layer structure is a film with a thickness as low as 1 μm.

22. The co-extruded multi-layer structure according to claim 16, wherein the co-extruded multi-layer structure is a filament with a diameter as low as 1 μm.

23. The co-extruded multi-layer structure according to claim 16, wherein the extrusion technique is selected from the group of cast extrusion, blown film extrusion, extrusion-coating, extrusion-lamination, curtain coating extrusion, profile extrusion, filament spinning and spun-melt non-woven extrusion.

24. The co-extruded multi-layer structure according to claim 16, wherein polymers decreasing their extensional viscosity at their own critical draw down ratio under tensile stress are polymers whose structures are substantially linear or contains short branched chains that when they are subjected to tensile stress their molecule chains became more or less oriented in the shear direction, thereafter the molecule chains disentangle to a certain extent, which lowers their flow resistance.

25. The co-extruded multi-layer structure according to claim 16, wherein polymers increasing their extensional viscosity at their own critical draw down ratio under tensile stress are polymers those structure is substantially long branched chain that when they are subjected to tensile stress their molecule chains tangle and prevent relative motion between the molecule chains, which increases their flow resistance.

26. The co-extruded multi-layer structure of claim 16, further comprising isolating the steps of stretching and cooling of the molten co-extruded multiplayer structure from ambient air by using a vacuum box.

27. A method for obtaining the co-extruded multi-layer structure of claim 16, comprising using a die, wherein the die is one common die, and the method comprises the steps of: dispersing an adhesive material in the first polymer and/or in the second polymer of the first composite layer and/or the second composite layer, respectively, feeding the first and the second composite layers to the one common die under temperature in order to co-extrude the molten layers simultaneously, whereby the inter-layer is formed by chemical interaction between the first and the second polymers when they contact in molten state, or alternatively, adding a third adhesive layer between the first and the second composite layers, thereby forming a multi-layer, and feeding the multi-layer to the one common die under temperature in order to co-extrude it, whereby the inter-layer is formed by physical interaction of the molten first and second polymers with the third adhesive layer simultaneously fed between the first and the second composite layers to the one common die, the method further comprising:— when the molten co-extruded layers leave the die, stretching the molten co-extruded layers under tensile stress, whereby the resulted co-extruded multi-layer structure has a draw-down ratio which is higher than the critical draw-down ratio of each one of the first and of the second polymers, individually extruded, thereby decreasing the cross-sectional area of the co-extruded multi-layer structure to a lower value, and cooling down the co-extruded multi-layer structure to room temperature.

28. The method according to claim 27, wherein the draw-down ratio is 10% superior to the critical draw-down ratio of each one of the first polymers and of the second polymer, extruded individually.

29. The method according to claim 27, further comprising isolating the steps of stretching and cooling of the molten co-extruded multiplayer structure from ambient air by using a vacuum box.

30. A coated substrate comprising the co-extruded multi-layer structure of claim 16.

31. The coated substrate of claim 30, wherein the substrate of the coated substrate is selected from the group consisting of a polymer, paper, textile material, non-woven material or metal film, and wherein the coated substrate is obtained by using a conventional extrusion method selected from the group of extrusion-coating, curtain-coating extrusion, extrusion-lamination, cast extrusion, blown film extrusion, profile extrusion, filament spinning and spun-melt non-woven extrusion.

32. The coated substrate of claim 30, wherein the coated substrate is a diaper back-sheet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] FIG. 1 shows a cross section of the co-extruded multilayer structure obtained according to example 1 of the invention having a total thickness of 2.5 μm and coated on a spun-melt polypropylene non-woven substrate of 15 g/m.sup.2.

[0090] FIG. 2 shows a cross section of the co-extruded multilayer structure obtained according to example 2 of the invention having a total thickness of 1.5 μm and coated on a spun-melt polypropylene non-woven substrate of 15 g/m.sup.2.

[0091] FIG. 3 depicts a schematic view of a conventional extrusion-coating equipment. In this embodiment, a conventional extrusion-coating equipment is provided with a common die (D) from which leaves the co-extruded multilayer structure (1), then stretched and cooled within the air gap (A), and thereafter used as a coating of a substrate (S), then the coated substrate is rolled in a cooling roller (C-R).

[0092] FIG. 4 depicts a schematic view of the conventional extrusion-coating equipment shown in FIG. 3, further provided with a vacuum box (V). The vacuum box (V) isolates the molten co-extruded multilayer structure (1) from the ambient air for stability purposes. In this embodiment, the molten co-extruded multilayer structure (1) is cooled in contact with a cooling roller (C-R).

[0093] Thereafter, the co-extruded multilayer structure (1) is used as a coating of a substrate (S), then the coated substrate is rolled in a cooling roller (C-R).

DETAILED DESCRIPTION OF THE INVENTION

[0094] A good adhesion of the interlayer with the first and second composite layers results when the interlayer is bonded to the first and the second composite layers both in molten state and thereafter when the multilayer structure has been solidified. Interlayer adhesion strength should be, at least, at the level of melt strength of the strongest layer; insufficient adhesion level could only give moderate improvements in draw-down and quickly produces interfacial instabilities by partial or total layer delamination caused by a significant difference in rheological behaviour between layers.

[0095] The interlayer can be obtained by three different ways: [0096] The interlayer can be formed by chemically interaction between the first and the second polymers. In this embodiment, first and second polymers are selected to be compatible polymers between them, that is, polymers that in molten state and thereafter when the multilayer structure has been solidified form together a continuous phase. Under extrusion temperatures, both the first and the second polymers form a blend matrix, thereby the interlayer being formed during the extrusion method, particularly when they contact under temperature at the one common die. [0097] The interlayer can be formed by chemically interaction between the first and the second polymers, provided that the first composite layer and/or the second composite layer further contain a dispersed adhesive material therein. Under extrusion temperatures, the adhesive material blends with the first polymer and/or with the second polymer. The adhesive material dispersed in the first composite layer and/or in the second composite layer improves the compatibility between the first and the second polymers. Depending on the compatibility degree between the first and the second polymers, the adhesive material can be added in an amount ranging from 0.5-10% by weight of the total weigh of the polymer when the first and the second polymers have a tendency to homogeneously blend together when contact in a molten state, or either in an amount ranging from 10-60% by weight of the total weigh of the polymer when the first and the second polymers do not homogeneously blend together when contact in a molten state. [0098] The interlayer can be formed by physically interaction between the first and the second polymer. In this embodiment, a further layer, which is an adhesive layer, is simultaneously fed between the first and the second composite layers to the one common die. The adhesive layer can be a tie layer which is made of an adhesive material. With dissimilar non-compatible polymers, it is preferable to use this additional tie layer. The tie layer should be selected to have strong adhesion properties to both composite layers.

[0099] Preferable adhesive layer is a tie layer which is made of an adhesive material.

[0100] In one embodiment, a co-extruded multilayer film is extruded using an extrusion-coating, curtain coating or extrusion laminating equipment onto a substrate. It is preferable cool down the co-extruded multilayer film to room temperature before contacting the substrate in order to avoid excessive consumption of polymer in fulfilling the substrate roughness.

[0101] The co-extruded multilayer film comprises three layers, one of an extensional thinning behaviour polymer, optionally containing an adhesive material dispersed therein, and another of an extensional thickening behaviour polymer, optionally containing an adhesive material dispersed therein. Preferably, the same adhesive material can be also used for bonding the co-extruded multilayer film onto the substrate, thereby avoiding the usage of a second adhesive material for bonding the coating with the substrate.

[0102] In a different embodiment, the co-extruded multilayer film comprises three layers, one of an extensional thinning behaviour polymer, another of an extensional thickening behaviour polymer, and the third layer of an adhesive material. Preferably, the same adhesive material can be also used for bonding the co-extruded multilayer film onto the substrate, thereby avoiding the usage of a second adhesive material as a layer for bonding the coating with the substrate.

[0103] In an embodiment, the combination of polymers and layers can results in a critical draw down ratio of 260 in the extrusion process, thereby allowing a film total thickness below 3 μm at equipment take-off speed of more than 500 m/min.

[0104] Coating thickness of coated samples cross-sections can be measured with optical microscope equipped with lens scale.

[0105] It is preferable that the extensional thickening layer do not be situated in a position between the adhesive layer and the equipment rolls to prevent direct contact of the adhesive layer with this roll that may cause wrapping around it and equipment stops.

[0106] In an embodiment, curtain coating method is employed for coating a substrate (S) with and simultaneously as the co-extruded multilayer structure (1) is formed (FIG. 3). When the melt curtain thickness is lower than 2 μm it becomes to be disturbed by the air flows generated around the cooling roll (C-R). In order to improve the melt curtain stability, a vacuum box (V) can be used to support the melt curtain onto the cooling roll (C-R) surface (FIG. 4).

[0107] Preferable compositions for preparing the co-extruded multilayer structure can be:

[0108] Composition 1)

[0109] Base Polymer Polyethylene:

[0110] Extensional thinning layer: linear low density polyethylene MFI preferably from 2 to 30; layer thickness from 0.5 to 6 μm.

[0111] Extensional thickening layer: low density polyethylene MFI 2 to 30 blended with a hydrocarbon Tackifier, which is fully compatible with the blended polymer. The hydrocarbon tackifier is preferably selected from the group of an average molecular weight from 600 to 3,000 in an amount of 15% to 60% by weight, preferably from 20 to 40% by weight of the total weight of the extensional thickening layer; layer thickness from 0.1 to 6 μm.

[0112] Composition 2)

[0113] Base Polymer Polypropylene:

[0114] Extensional thinning layer: PP homo-polymer or co-polymer MFI 2 to 30; layer thickness from 0.5 to 6 μm.

[0115] Extensional thickening layer: long branched High melt strength polypropylene or in situ long branched polypropylene via crosslinking, MFI 2 to 30; blended with a hydrocarbon tackifier, which is fully compatible with the base polymer. The hydrocarbon tackifier is preferably selected from the group of an average molecular weight from 1,000 to 3,000 in a proportion of 15% to 60% by weight, preferably from 20% to 40% by weight of the total weight of the extensional thickening layer; thickness from 0.1 to 6 μm.

[0116] This second composition is especially suitable for coated substrates for the production of non-breathable diaper back-sheets when the substrate is a non-woven or complex packaging coatings. Substrate can be a cellulose paper, a metal layer, especially aluminium, or a polymeric layer, and packaging material for bags when the substrate is a textile and especially woven raffia.

[0117] Preferable compositions for preparing a water vapour breathable coated polypropylene non-woven extruding a co-extruded multilayer film in extrusion-coating, curtain coating or extrusion-laminating equipment onto a polypropylene non-woven substrate can be: [0118] an extensional thinning layer, optionally containing an adhesive material dispersed therein, selected from a group having values of water vapour permeability measured with ASTM E96B method higher than 1 g mm/m.sup.2 day and based on mechanisms of absorption-desorption (no porosity), for example, but not exclusively: Polyether-ester block co-polymer elastomer (commercially known as Hytrel from Dupont or Arnitel from DSM); styrene block co-polymers; polyamides 6 or 6.6; Polyethylene and Polybutylene terephthalate; Polyethylene oxide block-copolymers; ABS; thermoplastic polyurethanes; polyether block amides (like Pebax from Arkema); bio-polymers like PLA; acrylic-co-polymers; or its blends or co-polymers. Layer thickness 0.5 to 6 μm.

[0119] And, [0120] An extensional thickening layer, optionally containing an adhesive material dispersed therein, selected from the group of: [0121] Low density polyethylene methyl or ethyl acrylate co-polymers or same co-polymers anhydride or acid modified. Layer thickness 0.1 to 2 μm. [0122] An acrylic or methacrylic acid co-polymer of a Polyolefin. Layer thickness 0.1 to 2 μm. [0123] An ionomer. Layer thickness 0.1 to 2 μm. [0124] Ethylene vinyl-acetate co-polymer with more than 18% vinyl-acetate content blended with a compatible tackifier in a mass proportion of 20-60% by weight of tackifier. Layer thickness 0.1 to 2 μm.

[0125] This combination of polymers and layers can reach critical drawdown ratios higher than 150 and Mocon ASTM 1249 water vapour breathability between 3.000 and 20.000 g/m.sup.2 day.

[0126] In another embodiment, polyolef in filaments are extruded in a bi-component core-sheath filament extrusion equipment or spun-melt nonwoven equipment with the following compositions:

[0127] Composition 3) [0128] Filament core: 10% to 90% by weight of the total filament section.

[0129] Extensional thickening polyolef in (High Melt Strength Polypropylene or Low density Polyethylene) 40% to 90% by weight; MFI from 2 to 30; hydrocarbon compatible tackifier preferably with an average molecular weight between 1,000 and 3,000, 10% to 60% by weight. [0130] Filament sheath: 10% to 90% by weight of the total filament section.

[0131] Extensional thinning polyolefin Polypropylene homo-polymer or co-polymer, or linear low density polyethylene MFI from 2 to 30.

[0132] A similar embodiment is performed by using a simple filament extrusion (instead the bi-component one) and a higher viscosity polymer at lower volume percent in the core than the polymer viscosity and volume percent in the sheath.

[0133] Composition 4)

[0134] Filament Composition:

[0135] Extensional thickening polyolefin, High melt strength polypropylene or Low density polyethylene, MFI from 2 to 10; 10% to 30% of the total weight;

[0136] Extensional thinning polyolefin, Polypropylene homo or co-polymer or Linear low density polyethylene, MFI 10 to 30; 60% to 85% of the total weight; compatible Tackifier from 5% to 10% by weight.

[0137] With this composition, in single component extrusion equipment, the minor volume percent and higher viscosity component goes to the core and the major volume percent and lower viscosity component goes to the sheath self-structuring the core-sheath filament.

[0138] In another embodiment, three layers were extruded simultaneously to obtain a blown co-extruded multilayer film with improved bubble stability at high draw-down.

[0139] In this embodiment, the layer composition comprises: [0140] an inner layer composed of: extensional thickening layer as mentioned above like low density polyethylene or high melt strength polypropylene or in-situ branched linear polymers via cross-linking 90% to 50% blended with a hydrocarbon tackifier 10% to 50%; and [0141] an outer layer composed of: extensional thinning layer as mentioned above like linear low density polyethylene, polypropylene homo or co-polymers.

EXAMPLES

Example 1: See Cross Section in FIG. 1

[0142] Product: Non breathable back-sheet for diapers. [0143] Equipment: Extrusion coating machine 1.5 m width. [0144] Substrate: Polypropylene homo-polymer spun-bond non-woven 15 g/m.sup.2. [0145] layer structure: A-B-substrate [0146] A-layer composition: Extensional thinning polymer linear low density polyethylene Dowlex 2552E MFI 25. [0147] B-layer (tie layer) composition: 75% by weight Extensional thickening polymer Low density polyethylene Dow LDPE PG7008 MFI 7, 7; Hydrocarbon tackifier molecular weight 1200 Eastman Regalite R1125, 25% by weight. [0148] Process settings at stable running: [0149] Extrusion temperature both layers 220° C. [0150] Die gap (hot) 0.4 mm [0151] Output speed 400 m/min [0152] Air Gap: 345 mm [0153] Total Coating thickness: (FIG. 1) 2.5 μm [0154] A-Layer thickness: 1.5 μm. [0155] B-layer thickness: 1 μm [0156] Draw-Down: 160

Example 2: See Cross Section in FIG. 2. Same Product and Composition than Example 1 but Using a Vacuum Box and Cooling on Cast Roll Surface (FIG. 4)

[0157] Output speed: 550 m/min [0158] Total Coating thickness (FIG. 2) 1.5 μm [0159] A-layer thickness: 0.9 μm [0160] B-layer thickness: 0.6 μm [0161] Draw down: 266.

Example 3

[0162] Product: Breathable back-sheet for diapers. [0163] Same equipment example 1. [0164] Substrate: Polypropylene homo-polymer spun-bond non-woven 15 g/m.sup.2. [0165] Layer structure: A-B-substrate. [0166] A-layer composition: Extensional thinning polymer, Polyether-Ester block co-polymer Dupont Hytrel DYM350 NC010 MFI 15. [0167] B-layer-Tie layer composition: Extensional thickening polymer Low density Polyethylene-Ethylene acrylate co-polymer resin Dupont Bynel 22E804 [0168] Process settings at stable running. [0169] Extrusion temperature both layers: 270° C. [0170] Die gap (hot) 0.4 mm. [0171] Vacuum box (cooling on cast roll). [0172] Total Coating thickness 3 μm. [0173] A-layer thickness: 2 μm [0174] B-layer thickness: 1 μm [0175] Line speed: 550 m/min. [0176] Draw down: 133. [0177] Breathability (Mocon test ASTM1249): 5,200 g/m.sup.2 day

Example 4

[0178] Product: Breathable back-sheet for diapers. [0179] Same equipment example 1. [0180] Substrate: Polypropylene homo-polymer spun-bond non-woven 15 g/m2. [0181] Layer structure: A-B-substrate. [0182] A-layer composition: Extensional thinning polymer, Polyamide 6 Zytel ST7301 NC010. [0183] B-layer-Tie layer composition: Extensional thickening polymer Low density Polyethylene-Anhydride modified Ethylene acrylate co-polymer resin Dupont Bynel 21 E830. [0184] Process settings at stable running. [0185] Extrusion temperature both layers: 265° C. [0186] Die gap (hot) 0.4 mm. [0187] Vacuum box (cooling on cast roll). [0188] Total Coating thickness 2.5 μm. [0189] A-layer thickness: 1.5 μm [0190] B-layer thickness: 1 μm [0191] Line speed: 550 m/min. [0192] Draw down: 160. [0193] Breathability (Mocon test ASTM1249): 4,200 g/m.sup.2 day

Example 5

[0194] Product: PP Coated raffia for bags. [0195] Same equipment as example 1. [0196] Substrate: woven PP raffia 220 g/m2. [0197] Structure: A-B-Substrate [0198] A-Layer composition: Extensional thinning polymer, polypropylene Homo-polymer Repsol Isplen PP086Y3E MFI 25. [0199] B-Layer composition: 89.5% by weight Extensional thickening polymer Repsol Isplen PP086Y3E MFI 25; crosslinked with 0.5% by weight of Cray valley Dymalink 9200 and 10% by weight Eastman tackifier Regalite R1125. [0200] Process settings at stable running: [0201] Extrusion temperature both layers: 260° C. [0202] Die gap (hot) 0.4 mm. [0203] Air Gap 400 mm. [0204] Total Coating thickness 3 μm. [0205] A-layer thickness: 2 μm [0206] B-layer thickness: 1 μm [0207] Line speed: 550 m/min. [0208] Draw down: 133.

Example 6

[0209] Product: PP spun-bond non-woven [0210] Equipment: Reicofil bi-component spun-bond machine. [0211] Layer structure: core/sheath filament A/B [0212] Core layer A: 90% by weight High Melt Strength Polypropylene Daploy WS420 HMS MFI 22+ 10% by weight Eastman tackifier Plastolyn R1140. [0213] Sheath layer B: Polypropylene Homo-polymer Repsol Isplen PP086Y3E MFI 25. [0214] Process settings at stable running: [0215] Extrusion temperature: 260° C. [0216] Spinneret capillary diameter sheath: 0.6 mm [0217] Spinneret capillary diameter core: 0.3 mm [0218] Line speed 300 m/min [0219] Resulting filament denier: 0.35 [0220] Draw down ratio: 105