Co-extruded multi-layer structure, method for obtaining the same, use thereof, and diaper backsheet

Abstract

The present invention relates to a novel coextruded multilayer structure having a critical stretch ratio that is superior to the critical stretch ratio of each of the polymer layers extruded individually. The present invention also relates to a method for obtaining the coextruded multilayer structure. The coextruded multilayer structure obtainable by the method described herein allows the production of low-weight films, filaments, or spunmelt nonwovens at high speeds using conventional extrusion equipment. The coextruded multilayer structure is particularly suitable as a diaper backsheet or flexible packaging coating.

Description

[0001] This application is a divisional application of the invention patent application with the application date of November 5, 2019, application number 2019800727197 (PCT / EP2019 / 080242), and invention name “Co-extruded multilayer structure and method for obtaining the same”. Technical Field

[0002] The present invention relates to the field of extrusion, in particular to the extrusion of polymer melts.

[0003] The present invention relates to novel coextruded multilayer structures and a method for obtaining coextruded multilayer structures.

[0004] The present invention also relates to a coextruded multilayer structure useful as a coating. The coating may be a diaper backsheet or a flexible packaging coating. Background Art

[0005] Polymer extrusion processes like film casting, extrusion coating, blown film, profile extrusion or filament spinning are severely limited in terms of speed, film thickness reduction or filament diameter due to process instabilities related to extrusion speed and draw-down.

[0006] Extrusion speed limitations are related to shear flow instabilities. Critical shear stress leads to the well-known defects of melt sharkskin or sticking lips, and ultimately melt fracture. Under shear flow conditions, extrusion speed limitations are caused by shear stress at the die gap; in addition to polymer rheology, the presence of lubricants, and process conditions, shear stress is highly dependent on the cross-sectional area of the die gap and the melt velocity at the die exit or spinneret capillary.

[0007] Extensional limitations are associated with extensional flow instabilities. These instabilities are known as melt extensional resonance and total melt fracture. The extensional flow behavior of a polymer melt depends on its extensional viscosity.

[0008] We can define the draw ratio as the dimensionless ratio of the extruder die gap cross-sectional area to the final film cross-sectional area, divided by the blow-up rate in the case of blown film, or the ratio of the spinneret capillary cross-sectional area to the final filament cross-sectional area in the case of filament spinning. The draw ratio can also be defined as the ratio of the final output position velocity to the die exit velocity.

[0009] Under extensional flow, the melt behavior of a polymer differs significantly from that under shear flow due to the nature of the stresses associated with molecular chain interactions under deformation.

[0010] The extrusion of polymer melts is mainly dominated by their extensional flow properties rather than their shear flow properties. Therefore, melt instability in extrusion is mainly related to the extensional properties of the melt.

[0011] The extensional flow behavior of a polymer melt depends on its extensional viscosity, which is defined by the ratio of the extensional stress to the elongation: λ = σ / (dε / dt), where σ is the extensional stress and ε is the extensional strain or stretch ratio, which is defined as the ratio between the increase in length and the initial length: Δε = Δl / l0; and for the instantaneous strain: dε = dl / l

[0012] Under steady-state flow conditions, ε is constant.

[0013] Due to experimental and physical limitations, steady-state conditions are difficult to achieve in extensional viscosity measurements; instead, we usually have plots of transient data showing the evolution of the extensional viscosity with time for different strain rates.

[0014] Commercial extensional rheometers based on constant or variable speed of molten polymers are available in the art. Fixtures for extension measurements using shear rheometers are also common. See, for example, "Polymer Melt Rheology and the Rheotens Test", von Anka Bernnat, Institut f"ur Kunststofftechnologie, Universit" at Stuttgart, 2001. The Rheotens experiment is a quasi-isothermal fiber spinning experiment. A polymer melt that has been pre-sheared in a capillary die is stretched under the action of a constant tensile force until the filaments break. The extension diagram obtained from this experiment describes the elongational behavior of the polymer melt and is therefore relevant to many polymer processes such as blow molding, film blowing, and fiber spinning. In addition, the breaking stress of the polymer melt can be calculated, which is important for these industrial applications. In general, melt strength and drawability depend on the material properties of the melt and the processing conditions of the experiment. The existence of the universal Rheotens curve allows the polymer melt properties to be separated from the processing conditions, thereby simplifying the description of the elongational behavior under constant force deformation. The general Rheotens curve reflects the structural differences of polymer melts.

[0015] For an example of determining the extensional viscosity of a polymer, see "Determination of elongational viscosity of polymer melts by RME and Rheotens experiments" by Manfred H. Wagner, Rheol Acta (2002) 41:316-325, DOI 10.1007 / s00397-002-0228-0, which is incorporated herein.

[0016] Three types of purely elongational flow behavior can be observed:

[0017] 1. Extensional viscosity is independent of elongational tensile stress; these materials are called Troutonian materials, and the viscosity is often referred to as Troutonian viscosity.

[0018] 2. Extensional viscosity permanently increases with tensile stress. This behavior is called tension hardening, strain thickening, extensional thickening, or elongational thickening. It is similar to dilatancy in shear flow.

[0019] 3. Extensional viscosity reaches a maximum and then decreases along with the tensile stress. This behavior is called tension thinning, strain thinning, extension thinning, or elongation thinning. It is similar to pseudoplasticity or shear thinning.

[0020] Troutonian materials, i.e., materials with extensional viscosity that is more or less independent of flow rate, include polymethyl methacrylate (PMMA), polystyrene (PS), polyethylene terephthalate (PETE), polycarbonate (PC), and polysulfone (PES), as well as all Newtonian fluids.

[0021] Extensional thickening has been observed for essentially long-branched polymer structures like low-density polyethylene (LDPE), high-melt-strength polypropylene (HMSPP), ethylene vinyl acetate (EVA).

[0022] Linear or substantially short-branched polymers such as high-density polyethylene (HDPE) and polypropylene (PP) exhibit extensional thinning when exposed to extensional flow.

[0023] Thus, depending on the extensional viscosity behavior of the polymer melt, the above-mentioned instability defect under extensional flow is:

[0024] 1. Melt stretch resonance

[0025] 2. Total melt fracture

[0026] Melt stretch resonance

[0027] Christensen (1962) first described this phenomenon in his discussion of extrusion coating of polypropylene as surging or stretching resonance, and subsequently in numerous other publications, including US Pat. No. 4,339,507 by Kurt et al. and US Pat. No. 4,486,377 by Luchessi et al., which describe stretching resonance as a continuous random or periodic oscillation, variation, or pulsation of the polymer melt with respect to velocity and cross-sectional area during melt stretching between the die and the delivery point when the boundary conditions are a fixed velocity at the die exit and a fixed velocity at the delivery point. Stretching resonance occurs in stretch-thinning polymer melts when the stretch ratio exceeds a polymer-specific critical delivery velocity. Stretching resonance is a melt flow instability under extensional flow conditions that manifests as irregularities in the final film or fiber dimensions. Further stretching above the critical value results in melt fracture.

[0028] Stretch-thinning polymers exhibit stretch resonance at their critical stretch ratio.

[0029] Substantially linear or short-chain branched polymers exhibit extension-thinning behavior. Some common examples are linear low-density polyethylene, high-density polyethylene, polypropylene homopolymers and copolymers, thermoplastic polyurethanes, polyether-ester copolymers, linear polyamides, polyamide-ether copolymers, or polylactic acid polymers.

[0030] Common commercial stretch-thinning polymers have a critical draw ratio between 20 and 40.

[0031] Total melt fracture

[0032] Under extensional flow conditions, extension-thickening and Troutonian polymer melts exhibit abrupt total melt fracture at their critical stretch ratio. For each polymer, total melt fracture occurs at a specific critical stretch ratio and depends largely on local stresses, crack distribution, crack size, and crack propagation. In molten, highly entangled polymers, the forces of attraction of individual molecules toward their neighbors are very large because of the large lengths of the molecules. In addition, upon deformation, polymer molecules are oriented and often stretched according to the deformation field. The induced anisotropy can produce large differences in normal stresses, and the inhomogeneities of the material will then depend on this stress difference.

[0033] Polymers with substantially long branched chain structures tend to exhibit extended thickening behavior. Examples include: low-density polyethylene and its copolymers, such as ethylene vinyl acetate or acrylates or polystyrene; polyolefin elastomers and plastomers; high melt strength polypropylene; linear low-density polyethylene partially crosslinked with peroxides; ionomers; acrylic or methacrylic acid copolymers; or chain-extended branched polyesters, polylactic acid polymers, and polyamides.

[0034] Polymers that exhibit extensional thickening behavior under extensional flow conditions typically have a critical draw ratio between 30 and 80.

[0035] The role of cracks or material inhomogeneities as agglomeration sites for cavities and cracks is crucial to fracture mechanics. Chen et al., "Sher fracture of polystyrene melts and solutions - Rheol. Acta 1994," investigated the possible role of agglomeration sites in melt fracture and supported the hypothesis that cavity expansion is crucial for fracture. They also observed that the addition of solvents lowered the critical stress for fracture. They concluded that crack size is an intrinsic property of polymer melts or solutions at a given temperature. Kinlock and Young, "Fracture behavior of polymers - New York Elsevier 1983," also suggested intrinsic crack size to explain the failure of rubbery and glassy polymers.

[0036] US 5688457 explains the effect of boron nitride in the extrusion of fluoropolymers and polyolefins. Boron nitride particles provide many locations for many very small cracks to gather, thereby relieving the stress that causes large-scale cracking before reaching a critical value.

[0037] A general conclusion is that for each polymer, total melt fracture occurs at a specific critical draw ratio and depends strongly on local stress, crack distribution, crack size, and crack propagation.

[0038] The most accurate development for predicting polymer melt extensional properties has been made by The Dow Chemical Company, through the DRI (Dow Rheological Index), published in the Proceedings of Antec 1993. The DRI is independent of shear viscosity and must be calculated for each specific polymer by nonlinear regression of experimental data using the following general formula:

[0039] in:

[0040] τ0 = characteristic relaxation time.

[0041] η0=zero shear viscosity

[0042] The index ranges from 0 (polymer with no long chain branching) to 30.

[0043] Despite all these developments, the extensional behavior of a polymer melt for a specific polymer cannot be predicted and still has to be measured using elongational rheometers (RMErheometrics) or by means of Gottfert Rheotens, which are common equipment for those skilled in the art in the field of extrusion.

[0044] Drawing instabilities also affect the productivity of extrusion coating processes, where the substrate is coated with the desired mechanical properties of the product, in which case high speeds and low coating weights are required. The extremely narrow die gaps required to achieve the targeted low coating weights significantly increase polymer shear stress, forcing the extrusion line to run at low speeds unless it can operate at higher draw ratios.

[0045] Important developments in processes, equipment or melt composition have been disclosed with the aim of increasing the resonant critical stretch ratio of linear polymers by mechanical means at the extrusion equipment, see for example US 4668463, US 4608221 or US 4626574; superheating or rapid melt cooling in US 4859379 and US 4486377; edge encapsulation of linear low density polyethylene by low density polyethylene in US 4348346, or full width encapsulation in "Tension in multilayer film casting of polymer melts" Baigui B. 1998; polymer degradation in US 3247290 and US 4378451; melt mixtures of linear low density polyethylene and low density polyethylene, see for example US 4339507, US 5582923, US 4780264, US5773155, US5674342, US4339507, US5395471, US7846551US33836694A and US 5863665.

[0046] JZ Liang in "Melt elongation strength and drawability of LDPE / LLDPE blends", Beijing University of Chemical Technology, School of Plastic Machinery and Plastic Engineering, teaches the improvement of melt drawability of a melt blend of LLDPE and LDPE with 60% LDPE content.

[0047] Joo Sung Lee, "Stabilization of film casting by an encapsulation method," Korea University, Center for Applied Rheology, Department of Chemical and Bioengineering, February 2003, teaches some stretching and neck-in improvements by encapsulating the edges of a HDPE melt by LDPE in a film extrusion coater.

[0048] Although the prior art teaches improvements in critical stretch ratio compared to the critical stretch ratios of the individual properties of the components, there remains a need to provide a practical method that can substantially improve the critical stretch ratio in a multilayer structure compared to the critical stretch ratios of the individual properties of the polymers forming the layers of the extruded multilayer, which method is suitable for many polymers in an easy, safe and reproducible manner. Summary of the Invention

[0049] The present invention has been made in view of the above-mentioned prior art, and an object of the present invention is to provide a coextruded multilayer structure having an improved critical stretch ratio compared to the critical stretch ratio of each of the polymers of the layers present in the multilayer structure when extruded separately. Thus, the present invention provides a coextruded multilayer structure capable of high elongation or extension under extensional flow conditions without experiencing extensional flow instabilities.

[0050] In order to solve the problem raised by the present invention, in a first aspect, the present invention provides a coextruded multilayer structure, which includes a first composite layer disposed on a second composite layer, and further includes an intermediate layer disposed between the first composite layer and the second composite layer.

[0051] The first composite layer includes a first polymer, the second composite layer includes a second polymer, and the intermediate layer chemically or physically interacts with the first polymer and the second polymer to bond the two layers when either one is in the melt after the polymer solidifies.

[0052] wherein the first polymer and the second polymer differ from each other at least in their extensional viscosity, one polymer maintains or increases its extensional viscosity at its critical stretch ratio under tensile stress, and the other polymer decreases its extensional viscosity at its critical stretch ratio under tensile stress, and wherein the intermediate layer is obtainable by a coextrusion process using a common die, the method comprising the steps of:

[0053] feeding the first and second composite layers to a common die at a temperature so as to simultaneously co-extrude the molten layers, the intermediate layer being formed by chemical interaction between the first polymer and the second polymer, or alternatively, if the third adhesive layer is simultaneously fed to the common die in such a manner as to be interposed between the first and second composite layers, the intermediate layer being formed by physical interaction between the first polymer and the second polymer,

[0054] - stretching the molten coextruded layer under tensile stress after the coextruded layer exits the die, whereby the resulting coextruded multilayer structure has a draw ratio that is higher than the critical draw ratio of each of the first polymer and the second polymer when extruded individually, thereby reducing the cross-sectional area of the coextruded multilayer structure to a lower value,

[0055] - cooling the coextruded multilayer structure to room temperature, and

[0056] - Optionally, a stretching and cooling step is performed to isolate the coextruded multilayer structure from the ambient air.

[0057] According to the invention, the coextruded multilayer structure can have a flat, tubular or profile shape, depending on whether a flat die, a tubular die or a selected profile die is used as a common die in conventional extrusion technology.

[0058] That is, the inventors have discovered that the selection of a first polymer and a second polymer having different extensional viscosity profiles, subject to the proviso that at their individual critical draw ratios, one polymer reduces its extensional viscosity while the other polymer increases its extensional viscosity, results in a melt coextruded multilayer structure under tensile stress having a higher extension / elongation value.

[0059] In an embodiment, the first and / or second polymer may independently be a mixture of polymers.

[0060] According to the invention, the first and / or second polymer mixture has the same properties as the first and / or second polymer, respectively.

[0061] Thus, it is within the scope of the present invention to replace the first polymer with a first polymer mixture and / or the second polymer with a second polymer mixture, with the proviso that the polymer mixture has the properties of the corresponding replaced first and / or second polymer.

[0062] In preferred embodiments, the resulting coextruded multilayer structure has a critical stretch ratio that is 10%, 20%, 50%, 90%, 100%, 150%, 200%, 250%, 275% or 300% better than the critical stretch ratio of each of the first polymer and the second polymer extruded individually.

[0063] The fact that the resulting coextruded multilayer structure has an improved critical stretch ratio is based on the different properties of the selected layers present in the thus formed coextruded multilayer structure. In this property, the stretch-thinning layer is supported by the stretch-densifying layer through its melt strength at its resonant critical stretch ratio, and the stretch-densifying layer is supported by the stretch-thinning layer at its critical stretch ratio. This is due to the fact that small cracks formed at the onset of melt fracture do not propagate because the support of the stretch-thinning layer allows the stretch-densifying layer tension to relax, rather than causing crack propagation throughout the layer and complete rupture of the melt. Therefore, the presence of the stretch-thinning layer in a set of selected layer properties significantly improves the critical stretch for melt fracture.

[0064] Advantageously, the first aspect of the present invention provides a coextruded multilayer structure having a lower thickness or smaller diameter and a lower weight, which makes the coextruded multilayer structure suitable as a thinner coating and therefore has a lower weight.

[0065] Polymers that reduce their extensional viscosity at their critical stretch ratio under tensile stress are polymers whose structure is essentially linear or includes short chain branches, and when the polymers are subjected to tensile stress, their molecular chains become more or less oriented in the shear direction, after which the molecular chains disentangle to a certain extent, which reduces the flow resistance of the polymer.

[0066] The extension-thinning polymer can be selected from the following list, but is not limited to the following list: linear low density polyethylene; polyethylene and polybutylene terephthalate; polyether ester block copolymers; polyether-amide block copolymers; thermoplastic polyurethane; polypropylene homopolymers and copolymers; high density polyethylene; medium density polyethylene linear polyamide; polylactic acid polymers, and their copolymers or mixtures and their mixtures with mineral fillers.

[0067] A polymer that increases its extensional viscosity at its critical stretch ratio under tensile stress is a polymer whose structure is essentially a long-chain branched polymer. When the polymer is subjected to tensile stress, its molecular chains become entangled, thereby preventing relative movement between the molecular chains, which increases the flow resistance of the polymer.

[0068] Troutonian polymer melts exhibit abrupt gross melt fracture under tensile stress at their critical draw ratio, and thus Troutonian polymers can be used as extension-thickening polymers in the present invention.

[0069] The extensional thickening polymer or Troutonian polymer can be selected from the following list, but is not limited to the following list: low density polyethylene; ethylene vinyl acetate and acrylate copolymers; polystyrene; polyolefin plastomers and elastomers; high melt strength polypropylene; cross-linked polypropylene with peroxide or zinc catalyst (such as dymalink 9200 from Cray Valley); partially cross-linked linear low density polyethylene and its copolymers; polyolefin-acid copolymers; ionomers; branched polyamides, polylactic acid polymers and polyesters with chain extenders; and their copolymers or mixtures and their mixtures with mineral fillers.

[0070] Preferably the first polymer and / or the second polymer are selected to have a strength equal to or higher than 1 gmm / m 2 water vapor transmission rate.

[0071] In one embodiment, the coextruded multilayer structure is a film having a thickness as low as 1 μm.

[0072] In various embodiments, the coextruded multilayer structure is a filament having a diameter as low as 1 μm.

[0073] In an embodiment, the water vapor transmission rate of the coextruded multilayer structure ranges from 1,000 to 20,000 g / m2 per day. 2 .

[0074] The coextruded multilayer structures obtainable by the process described herein allow for the production of low weight films, filaments or spunmelt nonwovens at high speeds using conventional extrusion equipment.

[0075] The coextrusion process described herein may be selected from any of the following: cast extrusion, blown film extrusion, extrusion coating, extrusion lamination, curtain coating extrusion, profile extrusion, filament spinning, or spunmelt nonwoven extrusion.

[0076] In a second aspect, the present invention provides a method enabling the coextruded multilayer structure of the first aspect of the invention to be obtained.

[0077] Thus, in a second aspect, the present invention provides a method for obtaining a coextruded multilayer structure comprising a first composite layer arranged above a second composite layer and further comprising an intermediate layer arranged between the first composite layer and the second composite layer by using conventional coextrusion techniques with a common die, the method comprising the steps of:

[0078] - selecting a first polymer and a second polymer, with the proviso that the first polymer and the second polymer differ from each other at least in their extensional viscosity, one polymer increasing its extensional viscosity at its critical stretch ratio under tensile stress and the other polymer decreasing its extensional viscosity at its critical stretch ratio under tensile stress, the extensional viscosity being defined as the ratio between extensional stress and elongation in conventional extrusion techniques,

[0079] feeding the first composite layer and the second composite layer to a common die at a temperature so as to simultaneously co-extrude the molten layers, the intermediate layer being formed by chemical interaction between the first polymer and the second polymer, or alternatively, if a third adhesive layer is simultaneously fed between the first composite layer and the second composite layer to a common die, the intermediate layer being formed by physical interaction between the first polymer and the second polymer,

[0080] - stretching the molten coextruded layer under tensile stress after the coextruded layer exits the die, whereby the resulting coextruded multilayer structure has a draw ratio that is higher than the critical draw ratio of each of the first polymer and the second polymer when extruded individually, thereby reducing the cross-sectional area of the coextruded multilayer structure to a lower value,

[0081] - cooling the coextruded multilayer structure to room temperature, and

[0082] - Optionally, a stretching and cooling step is performed to isolate the coextruded multilayer structure from the ambient air.

[0083] Advantageously, the method defined in the second aspect of the invention saves on the cost of raw material, both in terms of the weight of the processed raw material which may be of smaller size and in terms of the weight of the finished material which is typically rolled, and furthermore produces less waste due to the fact that extensional flow instabilities are more or less prevented, thereby generating smaller amounts of substandard extruded polymer.

[0084] The critical draw ratio of a melt-coextruded layer can be defined as the maximum extension / elongation value under tensile stress without the occurrence of elongational flow instabilities.

[0085] In a third aspect, the present invention provides a coextruded multilayer structure as defined in the first and / or second aspect of the invention for use as a coating on a substrate, wherein the substrate may be selected from the group consisting of a polymer, paper, a textile material, a nonwoven material or a metal film, and wherein the substrate is coated with the coextruded multilayer structure by using a conventional extrusion process selected from extrusion coating, curtain coating extrusion, extrusion lamination, cast extrusion, blown film extrusion, profile extrusion, filament spinning and spunmelt nonwoven extrusion.

[0086] The present invention also relates to a diaper backsheet comprising a coextruded multilayer structure.

[0087] Also contemplated herein are packaging coatings such as nonwoven bags or carton tiles comprising the coextruded multilayer structure defined in the present invention.

[0088] The method for obtaining a coextruded multilayer structure as described herein is particularly suitable for producing low-weight films, filaments and spunmelt nonwovens having a thickness of 1 to 14 microns. Thus, the polymer coatings obtained having a thickness or diameter of 1 to 14 microns are suitable for use as coatings on substrates selected from cellulose, polymer films, nonwovens, metals or textiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 The cross section of the coextruded multilayer structure obtained according to Example 1 of the present invention is shown. The coextruded multilayer structure has a total thickness of 2.5 μm and is coated with 15 g / m 2 The spun melt polypropylene nonwoven substrate is formed.

[0090] Figure 2 The cross section of the coextruded multilayer structure obtained according to Example 2 of the present invention is shown. The coextruded multilayer structure has a total thickness of 1.5 μm and is coated with 15 g / m 2 The spun melt polypropylene nonwoven substrate is formed.

[0091] Figure 3A schematic diagram of a conventional extrusion coating apparatus is depicted. In this embodiment, the conventional extrusion coating apparatus is provided with a common die (D) from which the coextruded multilayer structure (1) exits and is then stretched and cooled in an air gap (A) before being applied as a coating for a substrate (S), which is then rolled in a cooling roller (CR).

[0092] Figure 4 Depicts Figure 3 Schematic diagram of a conventional extrusion coating setup also provided with a vacuum box (V) is shown. For stability purposes, the vacuum box (V) isolates the molten coextruded multilayer structure (1) from the ambient air. In this embodiment, the molten coextruded multilayer structure (1) is cooled by contact with a chill roll (CR). Thereafter, the coextruded multilayer structure (1) is used as a coating for a substrate (S), and the coated substrate is then rolled in the chill roll (CR). DETAILED DESCRIPTION

[0093] Good adhesion of the intermediate layer to the first and second composite layers occurs when the intermediate layer is bonded to the first and second composite layers, both in a molten state, and thereafter when the multilayer structure has solidified. The adhesive strength of the intermediate layer should be at least at the level of the melt strength of the strongest layer; insufficient adhesion levels may only modestly improve stretching and rapidly create interfacial instability through partial or complete delamination of the layers due to significant differences in rheological properties between the layers.

[0094] The middle layer can be obtained in three different ways:

[0095] The intermediate layer can be formed by chemical interaction between the first polymer and the second polymer. In this embodiment, the first polymer and the second polymer are selected to be compatible polymers, i.e., polymers that together form a continuous phase in the molten state and thereafter when the multilayer structure has solidified. At the extrusion temperature, the first polymer and the second polymer each form a mixture matrix, so that the intermediate layer is formed during the extrusion process, in particular when they are in contact at a common die at the temperature.

[0096] If the first composite layer and / or the second composite layer also include a dispersed binder material, an intermediate layer can be formed through chemical interaction between the first polymer and the second polymer. The binder material mixes with the first polymer and / or the second polymer at the extrusion temperature. The binder material dispersed in the first composite layer and / or the second composite layer improves the compatibility between the first polymer and the second polymer. Depending on the degree of compatibility between the first polymer and the second polymer, the binder material can be added in an amount ranging from 0.5 to 10% by weight of the total weight of the polymers when the first polymer and the second polymer tend to mix uniformly when contacted in a molten state, or in an amount ranging from 10 to 60% by weight of the total weight of the polymers when the first polymer and the second polymer tend to mix unevenly when contacted in a molten state.

[0097] The intermediate layer can be formed by physical interaction between the first polymer and the second polymer. In this embodiment, another layer, namely an adhesive layer, is simultaneously applied to a common mold between the first composite layer and the second composite layer. The adhesive layer can be a bonding layer made of an adhesive material. For dissimilar, incompatible polymers, it is preferred to use this additional bonding layer. The bonding layer should be selected to have strong bonding properties for both composite layers.

[0098] A preferred adhesive layer is a tie layer made of an adhesive material.

[0099] In one embodiment, the coextruded multilayer film is extruded onto a substrate using extrusion coating, curtain coating or extrusion lamination equipment. To avoid excessive consumption of polymer while meeting substrate roughness, the coextruded multilayer film is preferably cooled to room temperature before contacting the substrate.

[0100] The coextruded multilayer film comprises three layers, one of which is an elongated thinning polymer, optionally including a binder material dispersed therein, and another of which is an elongated thickening polymer, optionally including a binder material dispersed therein. Preferably, the coextruded multilayer film can also be bonded to a substrate using the same binder material, thereby avoiding the need for a second binder material to bond the coating to the substrate.

[0101] In various embodiments, the coextruded multilayer film comprises three layers, one of which is an extension-thinning polymer, another of which is an extension-thickening polymer, and a third layer of an adhesive material. Preferably, the same adhesive material can also be used to bond the coextruded multilayer film to a substrate, thereby avoiding the need for a second adhesive material as a layer for bonding the coating to the substrate.

[0102] In embodiments, the combination of polymers and layers can result in a critical draw ratio of 260 during extrusion, allowing for total film thicknesses below 3 μm at equipment output speeds greater than 500 m / min.

[0103] The coating thickness of a coated sample cross section can be measured using an optical microscope equipped with a lens scale.

[0104] Preferably, the extended densified layer is not located between the adhesive layer and the equipment roller to prevent the adhesive layer from coming into direct contact with the roller, which could cause it to wrap around and the equipment to stop.

[0105] In an embodiment, as the coextruded multilayer structure (1) is formed or while the coextruded multilayer structure is formed ( Figure 3 ), a curtain coating method is used to coat the substrate (S). When the melt curtain thickness is less than 2 μm, it is disturbed by the air flow generated around the cooling roller (CR). In order to improve the stability of the melt curtain, a vacuum box (V) can be used to support the melt curtain on the surface of the cooling roller (CR) ( Figure 4 ).

[0106] Preferred ingredients for preparing the coextruded multilayer structure may be:

[0107] Ingredient 1)

[0108] -Base polymer polyethylene:

[0109] Extended thinning layer: linear low density polyethylene MFI, preferably from 2 to 30; layer thickness from 0.5 to 6 μm.

[0110] Extension thickening layer: Low density polyethylene MFI 2 to 30 mixed with a hydrocarbon tackifier fully compatible with the mixed polymer. The hydrocarbon tackifier is preferably selected from the following group: average molecular weight from 600 to 3,000, amount from 15% to 60% by weight, preferably from 20 to 40% by weight of the total weight of the extension thickening layer; layer thickness from 0.1 to 6 μm.

[0111] Ingredient 2)

[0112] Base polymer polypropylene:

[0113] Extended thinning layer: PP homopolymer or copolymer MFI 2 to 30; layer thickness from 0.5 to 6 μm.

[0114] Extension thickening layer: long-branched high melt strength polypropylene or in-situ long-branched polypropylene crosslinked, MFI 2 to 30, mixed with a hydrocarbon tackifier that is fully compatible with the base polymer. The hydrocarbon tackifier is preferably selected from the following group: average molecular weight 1,000 to 3,000, proportion 15% to 60% by weight, preferably 20% to 40% by weight of the total weight of the extension thickening layer; thickness from 0.1 to 6 μm.

[0115] When the substrate is a nonwoven or complex packaging coating, the second component is particularly suitable for coated substrates for producing non-breathable diaper backsheets. When the substrate is a textile and especially woven raffia, the substrate can be cellulose paper, a metal layer, especially aluminum, or a polymer layer, and packaging material for bags.

[0116] Preferred ingredients for preparing a water vapor breathable coated polypropylene nonwoven by extruding a coextruded multilayer film onto a polypropylene nonwoven substrate in extrusion coating, curtain coating or extrusion lamination equipment can be:

[0117] - an extended thinning layer, optionally comprising dispersed therein a binder material selected from the group having a porosity greater than 1 g mm / m2 per day as measured by the ASTM E96B method and based on an absorption-desorption (non-porosity) mechanism. 2 The water vapor permeability value of the film is selected from the group consisting of, but not limited to, polyetherester block copolymer elastomers (commercially known as Hytrel from Dupont or Arnitel from DSM); styrene block copolymers; polyamide 6 or 6.6; polyethylene and polybutylene terephthalate; polyethylene oxide block copolymers; ABS; thermoplastic polyurethane; polyether block amides (such as Pebax from Arkema); biopolymers such as PLA; acrylic copolymers; or mixtures or copolymers thereof. The layer thickness is 0.5 to 6 μm.

[0118] as well as,

[0119] - an extended densified layer, optionally comprising dispersed therein a binder material selected from the group consisting of:

[0120] Low density polyethylene methyl or ethyl acrylate copolymers or modified copolymer anhydrides or acids of the same. Layer thickness 0.1 to 2 μm.

[0121] Acrylic or methacrylic acid copolymers of polyolefins. Layer thickness 0.1 to 2 μm.

[0122] Ionomer. Layer thickness 0.1 to 2 μm.

[0123] Ethylene vinyl acetate copolymers having a vinyl acetate content of more than 18%, mixed with a compatible tackifier in a mass proportion of 20 to 60% by weight of the tackifier. Layer thickness 0.1 to 2 μm.

[0124] This combination of polymers and layers can achieve a critical stretch ratio greater than 150 and a Mocon ASTM 1249 water vapor permeability of 3.000 to 20.000 g / m2 per day. 2 between.

[0125] In another embodiment, the polyolefin filaments are extruded in a bicomponent core-sheath filament extrusion apparatus or a spunmelt nonwoven apparatus and have the following composition:

[0126] Ingredient 3)

[0127] - Filament core: 10 to 90% by weight of the total filament segment.

[0128] Extension thickening polyolefin (high melt strength polypropylene or low density polyethylene), 40% to 90% by weight; MFI from 2 to 30; hydrocarbon compatible tackifier, preferably having an average molecular weight between 1,000 and 3,000, 10% to 60% by weight.

[0129] - Filament sheath: 10 to 90% by weight of the total filament segment.

[0130] Extended thinned polyolefin polypropylene homopolymer or copolymer, or linear low density polyethylene, MFI from 2 to 30.

[0131] Similar embodiments can be performed using simple filament extrusion (instead of bicomponent extrusion) and a lower volume percentage of higher viscosity polymer in the core compared to the polymer viscosity and volume percentage in the sheath.

[0132] Ingredient 4)

[0133] -Filament composition:

[0134] Extension thickening polyolefin, high melt strength polypropylene or low density polyethylene, MFI from 2 to 10; 10% to 30% by weight of the total;

[0135] Extended thinning polyolefin, polypropylene homopolymer or copolymer or linear low density polyethylene, MFI 10 to 30; 60% to 85% by weight; compatible tackifier, 5% to 10% by weight.

[0136] With this composition, in a single component extrusion setup, the smaller volume percentage and higher viscosity component goes to the core and the larger volume percentage and lower viscosity component goes to the sheath which allows the core-sheath filament to self-structure.

[0137] In another embodiment, three layers are extruded simultaneously to obtain a blown coextruded multilayer film with improved bubble stability at high stretches.

[0138] In this embodiment, the layer composition includes:

[0139] - an inner layer consisting of an extended thickening layer as described above, like low density polyethylene or high melt strength polypropylene or an in situ branched linear polymer crosslinked 90% to 50% and mixed with a hydrocarbon tackifier 10% to 50%; and

[0140] - an outer layer consisting of an extensible thinning layer as described above, like linear low density polyethylene, polypropylene homopolymer or copolymer.

[0141] Example:

[0142] Example 1: See Figure 1 The cross section in .

[0143] - Product: Non-breathable back sheet for diapers.

[0144] - Equipment: Extrusion coating machine, 1.5m wide.

[0145] -Base sheet: Polypropylene homopolymer spunbond nonwoven, 15g / m 2 .

[0146] -Layer structure: AB-substrate

[0147] - A-layer composition: extended thinning polymer linear low density polyethylene Dowlex 2552E MFI 25.

[0148] - B-layer (tie layer) composition: 75% by weight of extended thickening polymer low density polyethylene Dow LDPEPG7008 MFI 7,7; hydrocarbon tackifier, molecular weight 1200 Eastman Regalite R1125, 25% by weight.

[0149] -Process settings for stable runtime:

[0150] - Extrusion temperature, 220°C for both layers.

[0151] -Mold gap (hot) 0.4mm

[0152] -Output speed 400m / min

[0153] -Air gap: 345mm

[0154] -Total coating thickness: ( Figure 1 )2.5μm

[0155] - A-layer thickness: 1.5 μm.

[0156] - B-layer thickness: 1 μm.

[0157] - Stretch: 160

[0158] Example 2: See Figure 2 Same product and composition as in Example 1, but using a vacuum box and cooling on the casting roll surface ( Figure 4 ).

[0159] -Output speed: 550m / min

[0160] -Total coating thickness ( Figure 2 )1.5μm

[0161] - A-layer thickness: 0.9 μm.

[0162] -B-layer thickness: 0.6μm

[0163] -Stretch:266.

[0164] Example 3:

[0165] -Product: Breathable back sheet for diapers.

[0166] -Same equipment as Example 1.

[0167] -Base sheet: Polypropylene homopolymer spunbond nonwoven 15g / m 2 .

[0168] -Layer structure: AB-substrate.

[0169] - A-layer ingredients: extended thinning polymer, polyether-ester block copolymer Dupont Hytrel DYM350 NC010MFI 15.

[0170] -B layer - adhesive layer composition: extended thickening polymer low density polyethylene - ethylene acrylate copolymer resin Dupont Bynel 22E804

[0171] -Process settings for stable runtime.

[0172] - Extrusion temperature, 270°C for both layers.

[0173] -Mold gap (hot) 0.4mm.

[0174] - Vacuum box (cooling on the casting rolls).

[0175] - Total coating thickness 3 μm.

[0176] -A-layer thickness: 2μm

[0177] -B-layer thickness: 1μm

[0178] -Line speed: 550m / min.

[0179] -Stretch:133.

[0180] -Breathability (Mocon test ASTM1249): 5,200 g / m 2

[0181] Example 4:

[0182] -Product: Breathable back sheet for diapers.

[0183] -Same equipment as Example 1.

[0184] -Base sheet: Polypropylene homopolymer spunbond nonwoven 15g / m 2 .

[0185] -Layer structure: AB-substrate.

[0186] - A-layer composition: extended thinning polymer, polyamide 6 Zytel ST7301 NC010.

[0187] - Layer B - Tie layer composition: Extended thickening polymer low density polyethylene - anhydride modified ethylene acrylate copolymer resin Dupont Bynel 21E830.

[0188] -Process settings for stable runtime.

[0189] - Extrusion temperature, 265°C for both layers.

[0190] -Mold gap (hot) 0.4mm.

[0191] - Vacuum box (cooling on the casting rolls).

[0192] -Total coating thickness 2.5 μm.

[0193] -A-layer thickness: 1.5μm

[0194] -B-layer thickness: 1μm

[0195] -Line speed: 550m / min.

[0196] -Stretch:160.

[0197] -Breathability (Mocon test ASTM1249): 4,200 g / m 2

[0198] Example 5:

[0199] -Product: PP coated raffia for bags.

[0200] -Same equipment as Example 1.

[0201] -Base sheet: Non-woven PP raffia fiber 220g / m 2 .

[0202] -Structure: AB-substrate

[0203] - A-layer composition: extension thinning polymer, polypropylene homopolymer Repsol Isplen PP086Y3EMFI 25.

[0204] - B-layer composition: 89.5% by weight of an extension thickening polymer Repsol Isplen PP086Y3EMFI 25; crosslinked with 0.5% by weight of Cray Valley Dymalink 9200 and 10% by weight of Eastman tackifier Regalite R1 125.

[0205] -Process settings for stable runtime:

[0206] - Extrusion temperature, 260°C for both layers.

[0207] -Mold gap (hot) 0.4mm.

[0208] -Air gap 400mm

[0209] - Total coating thickness 3 μm.

[0210] -A-layer thickness: 2μm

[0211] -B-layer thickness: 1μm

[0212] -Line speed: 550m / min.

[0213] -Stretch:133.

[0214] Example 6:

[0215] -Product: PP spunbond nonwovens

[0216] -Equipment: Reicofil bicomponent spin-bonding machine.

[0217] -Layer structure: core / sheath filament A / B

[0218] - Core layer A: 90% by weight of high melt strength polypropylene Daploy WS420 HMS MFI22 + 10% by weight of Eastman tackifier Plastolyn R1140.

[0219] - Sheath B: Polypropylene homopolymer Repsol Isplen PP086Y3E MFI 25.

[0220] -Process settings for stable runtime:

[0221] -Extrusion temperature: 260℃.

[0222] -Spinneret capillary sheath diameter: 0.6mm

[0223] -Spinneret capillary core diameter: 0.3mm

[0224] -Output speed 300m / min

[0225] - Produced filament denier: 0.35

[0226] -Stretch ratio: 105.

Claims

1. A coextruded multilayer structure having an improved extrusion critical stretch ratio compared to the critical stretch ratio of each of the polymers of the layers present in the multilayer structure when extruded separately, the coextruded multilayer structure comprising a first composite layer disposed over a second composite layer and further comprising an intermediate layer disposed between the first composite layer and the second composite layer, in, The first composite layer includes a first polymer, the second composite layer includes a second polymer, and the intermediate 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 at least in their extensional viscosity, one polymer increasing its extensional viscosity at its own critical stretch ratio under tensile stress, and the other polymer decreasing its extensional viscosity at its own critical stretch ratio under tensile stress, wherein the extensional viscosity is defined as the ratio between extensional stress and elongation in conventional extrusion technology, Among them, the polymer whose extensional viscosity is reduced at its own critical stretching ratio under tensile stress is a polymer whose structure is linear or includes short branched chains, and when the polymer is subjected to tensile stress, the molecular chains of the polymer become oriented in the shear direction, after which the molecular chains of the polymer are disentangled to a certain extent, which reduces the flow resistance of the polymer, and Among them, the polymer that increases its extensional viscosity at its own critical stretch ratio under tensile stress is a polymer having a long branched chain structure. When the polymer is subjected to tensile stress, the molecular chains of the polymer will entangle, preventing the relative movement between the molecular chains of the polymer, which increases the flow resistance of the polymer. And wherein the intermediate layer is obtainable by an extrusion method using a common die, the method comprising the steps of: - feeding the first composite layer and the second composite layer to the one common die at a temperature so as to simultaneously coextrude molten layers, the intermediate layer being formed by chemical interaction between the first polymer and the second polymer, or alternatively, the intermediate layer being formed by physical interaction between the first polymer and the second polymer, with a third adhesive layer being simultaneously fed to the one common die in such a manner as to be positioned between the first composite layer and the second composite layer, wherein the coextruded multilayer structure comprises: a two-layer structure, one layer of the two-layer structure having a polymer that reduces extensional viscosity and the other layer of the two-layer structure having a polymer that increases extensional viscosity; or a three-layer structure, one layer of the three-layer structure having a polymer that reduces extensional viscosity, the second layer of the three-layer structure having a polymer that increases extensional viscosity, and the third layer of the three-layer structure having an adhesive material between the first composite layer and the second composite layer, - stretching the molten coextruded layers under tensile stress after the coextruded layers exit the one common die, whereby the resulting coextruded multilayer structure has a draw ratio that is higher than the critical draw ratio of each of the first polymer and the second polymer when extruded separately, thereby reducing the cross-sectional area of the coextruded multilayer structure to a lower value, the lower value being dependent on the draw ratio of the resulting coextruded multilayer structure, - cooling the coextruded multilayer structure to room temperature, and - Optionally, stretching and cooling steps are performed to isolate the coextruded multilayer structure from the ambient air.

2. The coextruded multilayer structure according to claim 1, wherein The polymer that reduces its extensional viscosity is selected from: linear low density polyethylene; polyethylene and polybutylene terephthalate; polyether ester block copolymers; polyether-amide block copolymers; thermoplastic polyurethane; polypropylene homopolymers and copolymers; high density polyethylene; medium density polyethylene linear polyamide; polylactic acid polymers, and their copolymers or mixtures and their mixtures with mineral fillers.

3. The coextruded multilayer structure according to claim 1 or 2, wherein The other polymer that increases its extensional viscosity is selected from: low density polyethylene; ethylene vinyl acetate and acrylate copolymers; polystyrene; polyolefin plastomers and elastomers; high melt strength polypropylene; cross-linked polypropylene with peroxide or zinc catalyst; Partially cross-linked linear low-density polyethylene and its copolymers; polyolefin-acid copolymers; ionomers; branched polyamides, polylactic acid polymers and polyesters with chain extenders; and their copolymers or mixtures and their mixtures with mineral fillers.

4. The coextruded multilayer structure according to claim 1, wherein The adhesive layer is a bonding layer made of an adhesive material.

5. The coextruded multilayer structure according to claim 1, wherein At least one of the first polymer and the second polymer has a relative humidity equal to or greater than 1 g mm / m2 in days, measured according to ASTM E96B. 2 water vapor transmission rate.

6. The coextruded multilayer structure of claim 1 having a strength in the range of from 1,000 g / m2 in days as measured according to ASTM 1249. 2 Up to 20,000g / m 2 water vapor transmission rate.

7. The coextruded multilayer structure according to claim 1, wherein The coextruded multilayer structure is a film with a thickness down to 1 μm.

8. The coextruded multilayer structure according to claim 1, wherein The coextruded multilayer structures are filaments with diameters down to 1 μm.

9. The coextruded multilayer structure according to claim 1, wherein Such extrusion techniques include cast extrusion, blown film extrusion, extrusion coating, extrusion lamination, curtain coating extrusion, profile extrusion, filament spinning, and spunmelt nonwoven extrusion.

10. A method for obtaining a coextruded multilayer structure according to any one of claims 1 to 9 by using conventional extrusion techniques with a common die, the coextruded multilayer structure comprising a first composite layer disposed on a second composite layer and further comprising an intermediate layer disposed between the first composite layer and the second composite layer, the method comprising the steps of: - selecting a first polymer and a second polymer, with the proviso that the first polymer and the second polymer differ from each other at least in their extensional viscosity, one polymer increasing its extensional viscosity at its own critical stretch ratio under tensile stress and the other polymer decreasing its extensional viscosity at its own critical stretch ratio under tensile stress, said extensional viscosity being defined as the ratio between extensional stress and elongation in conventional extrusion techniques, - wherein the polymer whose extensional viscosity is reduced at its own critical stretching ratio under tensile stress is a polymer whose structure is linear or includes short chain branches, and when the polymer is subjected to tensile stress, the molecular chains of the polymer become oriented in the shear direction, after which the molecular chains of the polymer are disentangled to a certain extent, which reduces the flow resistance of the polymer, and - wherein the polymer whose extensional viscosity increases at its own critical stretching ratio under tensile stress is a polymer having a long-chain branched structure, and when the polymer is subjected to tensile stress, the molecular chains of the polymer become entangled, thereby preventing relative movement between the molecular chains of the polymer, thereby increasing the flow resistance of the polymer, - feeding the first composite layer and the second composite layer to the one common die at a temperature so as to simultaneously coextrude molten layers, the intermediate layer being formed by chemical interaction between the first polymer and the second polymer, or alternatively, the intermediate layer being formed by physical interaction between the first polymer and the second polymer when a third adhesive layer is simultaneously fed to the one common die in such a manner as to be positioned between the first composite layer and the second composite layer, wherein the coextruded multilayer structure comprises: a two-layer structure, one layer of the two-layer structure having a polymer that reduces extensional viscosity and the other layer of the two-layer structure having a polymer that increases extensional viscosity; or a three-layer structure, one layer of the three-layer structure having a polymer that reduces extensional viscosity, the second layer of the three-layer structure having a polymer that increases extensional viscosity, and the third layer of the three-layer structure having an adhesive material between the first composite layer and the second composite layer, - stretching the molten coextruded layers under tensile stress after the coextruded layers exit the one common die, whereby the resulting coextruded multilayer structure has a draw ratio that is higher than the critical draw ratio of each of the first polymer and the second polymer when extruded separately, thereby reducing the cross-sectional area of the coextruded multilayer structure to a lower value, the lower value being dependent on the draw ratio of the resulting coextruded multilayer structure, - cooling the coextruded multilayer structure to room temperature, and - Optionally, the stretching and cooling steps are performed to isolate the coextruded multilayer structure from the ambient air using a vacuum box.

11. The method according to claim 10, wherein: The resulting draw ratio of the coextruded multilayer structure is 10% better than the critical draw ratio of each of the first polymer and the second polymer extruded individually.

12. Use of a coextruded multilayer structure as defined in any one of claims 1 to 9 as a coating on a substrate, wherein The substrate is selected from the group consisting of polymers, paper, textile materials, nonwoven materials or metal films, and wherein the substrate is coated with the coextruded multilayer structure by using a conventional extrusion process selected from the group consisting of extrusion coating, curtain coating extrusion, extrusion lamination, cast extrusion, blown film extrusion, profile extrusion, filament spinning and spunmelt nonwoven extrusion.

13. A diaper backsheet comprising a coextruded multilayer structure as defined in any one of claims 1 to 9.

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