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 method for producing a co-extruded polymeric multilayer comprising: i. selecting a first composite layer containing a first polymer, and a second composite layer containing a second polymer, the first composite layer being disposed over the second composite layer, wherein the first composite layer has an extensional viscosity that is increasing at a critical drawdown ratio of the first composite layer, and wherein the second composite layer has an extensional viscosity that is decreasing at a critical drawdown ratio of the second composite layer, wherein the first composite layer extensional viscosity and the second composite layer extensional viscosity is measured by an elongational rheometer RME, ii. co-extruding the first composite layer and the second composite layer at the same time in one common die under temperature, to obtain a co-extruded polymeric multilayer melt, and iii. stretching the co-extruded polymeric multilayer melt to a drawdown ratio of at least 100, wherein the drawdown ratio is the ratio of a cross-sectional area of the co-extruded multilayer melt to a cross-sectional area of the final co-extruded polymeric multilayer, and wherein the final co-extruded polymeric multilayer has a thickness of 1 to 14 ?m, and wherein the co-extruded polymeric multilayer has an improved critical draw down ratio compared to the critical drawdown ratio of the first composite layer and the critical drawdown ratio of the second composite layer.

17. The method of claim 16, wherein the critical draw down ratio of the co-extruded polymeric multilayer is at least 10% greater than of the critical draw down ratio of the first composite layer and the critical drawdown ratio of the second composite layer when extruded separately.

18. The method of claim 16, wherein a third adhesive layer is simultaneously fed between the first and the second composite layers and wherein the co-extruding is carried out by feeding the first composite layer, the second composite layer and the third adhesive layer between the first and the second composite layers at the same time in one common die under temperature, to obtain the co-extruded polymeric multilayer melt, and wherein the third adhesive layer is a tie layer.

19. The method of claim 16, wherein the first composite layer and/or the second composite layer further comprises an adhesive material dispersed in the first polymer and/or in the second polymer, and wherein the adhesive material is dispersed in a concentration ranging from 0.5-60% by weight of the polymer of each composite layer.

20. The method of claim 16, wherein the first polymer and/or the second polymer have a water vapor transmission rate equal to or higher than 1 g mm/m.sup.2 day as measured according to ASTM E96B.

21. The method of claim 16, wherein the co-extruding is carried out by an extrusion technique selected from the group of extrusion-coating, extrusion-lamination or curtain coating extrusion.

22. A co-extruded polymeric multilayer having improved critical drawdown ratio obtained by the method of claim 16.

23. The co-extruded polymeric multilayer of claim 22, wherein the first composite layer and/or the second composite layer further comprises an adhesive material dispersed in the first polymer and/or in the second polymer, and wherein the adhesive material is dispersed in a concentration ranging from 0.5-60% by weight of the polymer of each composite layer.

24. The co-extruded polymeric multilayer of claim 22, wherein a third adhesive layer is simultaneously fed between the first and the second composite layers, wherein the third adhesive layer is a tie layer.

25. The co-extruded polymeric multilayer of claim 22, wherein the first polymer and/or and the second polymer have a water vapor transmission rate equal to or higher than 1 g mm/m.sup.2 day as measured according to ASTM E96B.

26. The co-extruded polymeric multilayer of claim 22, wherein the co-extruded polymeric multilayer has a water vapor transmission rate ranging from 1,000-20,000 g/m.sup.2 day as measured according to ASTM1249.

27. The co-extruded polymeric multilayer of claim 22, wherein the co-extruded polymeric multilayer has a thickness in the range of 1 ?m to 3 ?m.

28. The co-extruded multi-layer structure according to claim 22, wherein the multi-layer structure is obtained by an extrusion technique selected from the group extrusion-coating, extrusion-lamination, or curtain coating extrusion.

29. A device comprising the co-extruded polymeric multilayer obtained by the method of claim 16.

30. The device of claim 29, wherein the device is a diaper back-sheet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0087] 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 ?m and coated on a spun-melt polypropylene non-woven substrate of 15 g/m.sup.2.

[0088] 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 ?m and coated on a spun-melt polypropylene non-woven substrate of 15 g/m.sup.2.

[0089] 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).

[0090] 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). 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

[0091] 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. The interlayer can be obtained by three different ways: [0092] 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. [0093] 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.

[0094] 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.

[0095] Preferable adhesive layer is a tie layer which is made of an adhesive material.

[0096] 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.

[0097] The co-extruded multilayer film comprises two 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.

[0098] 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.

[0099] 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 ?m at equipment take-off speed of more than 500 m/min.

[0100] Coating thickness of coated samples cross-sections can be measured with optical microscope equipped with lens scale.

[0101] 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.

[0102] 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 ?m 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).

[0103] Preferable compositions for preparing the co-extruded multilayer structure can be:

Composition 1)

[0104] Base polymer Polyethylene:

[0105] Extensional thinning layer: linear low density polyethylene MFI preferably from 2 to 30; layer thickness from 0.5 to 6 ?m.

[0106] 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 ?m.

Composition 2)

[0107] Base polymer Polypropylene:

[0108] Extensional thinning layer: PP homo-polymer or co-polymer MFI 2 to 30; layer thickness from 0.5 to 6 ?m.

[0109] 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 ?m.

[0110] 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.

[0111] 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: [0112] 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 ?m. [0113] And, [0114] An extensional thickening layer, optionally containing an adhesive material dispersed therein, selected from the group of: [0115] Low density polyethylene methyl or ethyl acrylate co-polymers or same co-polymers anhydride or acid modified. Layer thickness 0.1 to 2 ?m. [0116] An acrylic or methacrylic acid co-polymer of a Polyolefin. Layer thickness 0.1 to 2 ?m. [0117] An ionomer. Layer thickness 0.1 to 2 ?m. [0118] 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 ?m.

[0119] 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.

[0120] In another embodiment, polyolefin filaments are extruded in a bi-component core-sheath filament extrusion equipment or spun-melt nonwoven equipment with the following compositions:

Composition 3)

[0121] Filament core: 10% to 90% by weight of the total filament section.

[0122] Extensional thickening polyolefin (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. [0123] Filament sheath: 10% to 90% by weight of the total filament section.

[0124] Extensional thinning polyolefin Polypropylene homo-polymer or co-polymer, or linear low density polyethylene MFI from 2 to 30.

[0125] 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.

Composition 4)

[0126] Filament composition:

[0127] Extensional thickening polyolefin, High melt strength polypropylene or Low density polyethylene, MFI from 2 to 10; 10% to 30% of the total weight;

[0128] 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.

[0129] 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.

[0130] In another embodiment, three layers were extruded simultaneously to obtain a blown co-extruded multilayer film with improved bubble stability at high draw-down.

[0131] In this embodiment, the layer composition comprises: [0132] 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 [0133] an outer layer composed of: extensional thinning layer as mentioned above like linear low density polyethylene, polypropylene homo or co-polymers.

EXAMPLES

Example 1

[0134] See cross section in FIG. 1. [0135] Product: Non breathable back-sheet for diapers. [0136] Equipment: Extrusion coating machine 1.5 m width. [0137] Substrate: Polypropylene homo-polymer spun-bond non-woven 15 g/m.sup.2. [0138] layer structure: A-B-substrate [0139] A-layer composition: Extensional thinning polymer linear low density polyethylene Dowlex 2552E MFI 25. [0140] 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. [0141] Process settings at stable running: [0142] Extrusion temperature both layers 220? C. [0143] Die gap (hot) 0.4 mm [0144] Output speed 400 m/min [0145] Air Gap: 345 mm [0146] Total Coating thickness: (FIG. 1) 2.5 ?m [0147] A-Layer thickness: 1.5 ?m. [0148] B-layer thickness: 1 ?m [0149] Draw-Down: 160

Example 2

[0150] 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). [0151] Output speed: 550 m/min [0152] Total Coating thickness (FIG. 2) 1.5 ?m [0153] A-layer thickness: 0.9 ?m [0154] B-layer thickness: 0.6 ?m [0155] Draw down: 266.

Example 3

[0156] Product: Breathable back-sheet for diapers. [0157] Same equipment example 1. [0158] Substrate: Polypropylene homo-polymer spun-bond non-woven 15 g/m.sup.2. [0159] Layer structure: A-B-substrate. [0160] A-layer composition: Extensional thinning polymer, Polyether-Ester block co-polymer Dupont Hytrel DYM350 NC010 MFI 15. [0161] B-layer-Tie layer composition: Extensional thickening polymer Low density Polyethylene-Ethylene acrylate co-polymer resin Dupont Bynel 22E804 [0162] Process settings at stable running. [0163] Extrusion temperature both layers: 270? C. [0164] Die gap (hot) 0.4 mm. [0165] Vacuum box (cooling on cast roll). [0166] Total Coating thickness 3 ?m. [0167] A-layer thickness: 2 ?m [0168] B-layer thickness: 1 ?m [0169] Line speed: 550 m/min. [0170] Draw down: 133. [0171] Breathability (Mocon test ASTM1249): 5,200 g/m.sup.2 day

Example 4

[0172] Product: Breathable back-sheet for diapers. [0173] Same equipment example 1. [0174] Substrate: Polypropylene homo-polymer spun-bond non-woven 15 g/m.sup.2. [0175] Layer structure: A-B-substrate. [0176] A-layer composition: Extensional thinning polymer, Polyamide 6 Zytel ST7301 NC010. [0177] B-layer-Tie layer composition: Extensional thickening polymer Low density Polyethylene-Anhydride modified Ethylene acrylate co-polymer resin Dupont Bynel 21E830. [0178] Process settings at stable running. [0179] Extrusion temperature both layers: 265? C. [0180] Die gap (hot) 0.4 mm. [0181] Vacuum box (cooling on cast roll). [0182] Total Coating thickness 2.5 ?m. [0183] A-layer thickness: 1.5 ?m [0184] B-layer thickness: 1 ?m [0185] Line speed: 550 m/min. [0186] Draw down: 160. [0187] Breathability (Mocon test ASTM1249): 4,200 g/m.sup.2 day

Example 5

[0188] Product: PP Coated raffia for bags. [0189] Same equipment as example 1. [0190] Substrate: woven PP raffia 220 g/m2. [0191] Structure: A-B-Substrate [0192] A-Layer composition: Extensional thinning polymer, polypropylene Homo-polymer Repsol Isplen PP086Y3E MFI 25. [0193] 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. [0194] Process settings at stable running: [0195] Extrusion temperature both layers: 260? C. [0196] Die gap (hot) 0.4 mm. [0197] Air Gap 400 mm. [0198] Total Coating thickness 3 ?m. [0199] A-layer thickness: 2 ?m [0200] B-layer thickness: 1 ?m [0201] Line speed: 550 m/min. [0202] Draw down: 133.

Example 6

[0203] Product: PP spun-bond non-woven [0204] Equipment: Reicofil bi-component spun-bond machine. [0205] Layer structure: core/sheath filament A/B [0206] Core layer A: 90% by weight High Melt Strength Polypropylene Daploy WS420 HMS MFI 22+10% by weight Eastman tackifier Plastolyn R1140. [0207] Sheath layer B: Polypropylene Homo-polymer Repsol Isplen PP086Y3E MFI 25. [0208] Process settings at stable running: [0209] Extrusion temperature: 260? C. [0210] Spinneret capillary diameter sheath: 0.6 mm [0211] Spinneret capillary diameter core: 0.3 mm [0212] Line speed 300 m/min [0213] Resulting filament denier: 0.35 [0214] Draw down ratio: 105