Manufacturing process of a nanofibrous membrane reinforced composite material and nanofibrous membrane for such a process

Abstract

This invention discloses a composite material and a method for manufacturing such a composite reinforced material. The method includes the following steps: arranging a plurality of layers of reinforcing fibers; impregnating the layers with a resin-based matrix; laminating the reinforcing fiber layers by increasing pressure and / or heat; placing an intermediate layer of polymer nanofibers between the reinforcing fiber layers; wherein, prior to the lamination step, inserting the intermediate layer of polymer nanofibers between the reinforcing fiber layers by placing a polymer nanofiber membrane adhered to a backing substrate; obtaining the polymer nanofiber membrane by direct electrospinning on the backing substrate using a needle-free technique; and wherein, prior to the lamination step, anti-sinking properties are provided to prevent the polymer nanofiber membrane from prematurely sinking into the resin-based matrix.

Description

Method for manufacturing nanofiber membrane reinforced composite materials and nanofiber membranes used in the method This invention relates to a method for manufacturing a composite material having a nanofiber membrane and a nanofiber membrane used in the method. Background: Composite materials are typically materials with reinforcing components, usually composed of high-strength fibers in random or woven form and a matrix component (such as resin) that holds the reinforcing component in place and protects it from external environmental influences. Composite materials can be thermoplastic or thermosetting, depending on the matrix component used. The reinforcing component can take various forms, such as various filament bundles, rovings, tows, chopped fibers, nonwovens, woven fabrics, pads, tapes, microspheres, and nanospheres, with the most commonly used fiber types being carbon / graphite fibers, polyaramid fibers, and glass fibers. For assembling structural components, composite materials are typically laminated, bonding several thin layers (prepreg or resin impregnated during lamination) together and applying pressure and heat to achieve curing or cross-linking. One of the most significant problems arising in composite materials is delamination, the development of fractures propagating between one layer of the laminated composite. Over the years, it has been found that this problem is greatly reduced if a certain amount of microfibers—fibers with a diameter much smaller than that of the main reinforcing fibers (which effectively bind the layers together, acting as fillers and adhesives between the resin matrix and the reinforcing fiber mat)—are arranged between one layer of the laminated composite. The general use of nanofibers obtained by electrospinning from a nozzle has also been proposed. US 6265333 describes, for example, a method for manufacturing prepreg composites, which is expected to include polymeric micro and nanofiber membranes also obtained by electrospinning. Several scientific papers discuss similar techniques in relation to fundamental research. For example, T. Brugo a, R. Palazzetti's "The effect of thickness of Nylon 6,6 nanofibrous mat on Modes I-II fracture mechanics of UD and woven composite laminates," published in Composite Structures 154 (2016), pp. 172-178, describes experimental characteristics of carbon fiber and epoxy resin composites interwoven with nylon 6,6 nanofibers. However, while these prior art documents provide general information on possible techniques for reducing nanofiber membrane peeling, they do not address the practical problems encountered in industrial environments to reduce these theoretical and experimental concepts. In particular, it has been noted that electrospinning technology is almost ideal for obtaining nanofibers that improve the toughness of composite materials, but a satisfactory method for introducing nanofibers into composite materials in a repeatable, rapid manner without harmfully affecting prepreg fabrics (an essential requirement in industrial manufacturing environments) has not yet been found. Further information on the treatment of nanofiber membranes is disclosed in US2011 / 259518.Other examples of nanofiber membranes in the field of composite materials are disclosed in US2016 / 010249, GB2568105, CN112810259 and US2015 / 086743. The objective of this invention is therefore to provide a method for manufacturing composite materials in which polymer nanofibers are introduced simply and inexpensively to improve the toughness of the product with respect to peeling. This objective is achieved using the methods and films disclosed in the basic terminology of the appended claims. Specific and advantageous features are disclosed in the appendices to the claims. Specifically, according to a first embodiment of the present invention, a method for manufacturing a composite reinforcing material is disclosed, comprising the following steps: arranging a plurality of reinforcing fiber layers; impregnating the layers with a resin-based matrix; laminating the reinforcing fiber layers by increasing pressure and / or heat; placing an intermediate layer of polymer nanofibers between the reinforcing fiber layers; wherein, prior to placing a second reinforcing fiber layer and the lamination step, the intermediate layer of the polymer nanofibers is inserted between the reinforcing fiber layers by placing a polymer nanofiber membrane adhered to a backing substrate on the first reinforcing fiber layer; and obtaining the polymer nanofiber membrane by direct electrospinning on the backing substrate using a needle-free technique; and wherein, prior to the lamination step, anti-sinking properties are provided to prevent the polymer nanofiber membrane from prematurely sinking into the resin-based matrix. Preferably, the polymer membrane is obtained from a solution of PA 6 and a solvent containing acetic acid and formic acid. The solution may contain about 12% by weight of PA 6. According to a preferred embodiment, the resin-based matrix is ​​a cross-linked thermosetting resin. In addition, the composite reinforcing material includes a reinforcing fiber layer made of carbon. According to another embodiment, the amount of nanofibers adhered to the backing substrate for forming the polymer film is between 1 and 15 g / m². 2 Preferably, the nanofibers constituting the polymer film have a size of about 100 to 150 nm. According to another preferred embodiment, the backing substrate is an easily peelable backing substrate made of bisiliconized paper web, which is removed before the placement of the second reinforcing fiber layer and the lamination step. Another related embodiment is that the anti-settling property may be achieved by subjecting the polymer nanofiber film to an oil-repellent surface treatment or by treating it with a material having low affinity for the resin-based matrix before the placement step. According to another embodiment of the invention, a polymer film is provided that serves as an intermediate layer between reinforcing fiber layers within a composite resin matrix material, the film comprising electrospun nanofibers spun using a needle-free technique and deposited on a continuous backing substrate having an easily peelable surface. The preferred embodiment of the composite material is prepared in a manner known per se, for example by weaving a fabric layer of reinforcing fibers (such as carbon fibers) and impregnating it with a suitable resin matrix (e.g., a cross-linked thermosetting resin such as epoxy resin). Resin impregnation can be performed during the manufacture of the composite material or at a stage prior to the storage stage, thereby providing a prepreg fabric. The fiber-based component—hereinafter referred to as a pad—can also be constructed in another manner, for example, as a nonwoven fabric of cut fibers. The starting composite material (whether it consists only of reinforcing fibers or is a prepreg) is preferably in the form of a continuous pad roll. In the preparation of the composite product, two or more layers of reinforcing fiber pads are laminated together, inserted or embedded in a resin-based matrix. Before laminating the two layers of composite material, an intermediate component based on nanofibers is inserted between them, preferably by placing the intermediate component on the first layer and then applying the other layers to this first layer. According to the invention, the intermediate component consists of a nanofiber membrane obtained by electrospun polymer (particularly a polymer membrane obtained from continuous electrospinning according to the method described below). In this way, a continuous and uniform film deposited on an easily peelable substrate is obtained, making the industrial incorporation method effective, as will be seen later. To manufacture the nanofiber polymer film, a suitable polymer solution is initially produced. The polymer-solvent pair to be electrolyzed varies depending on the desired polymer: according to a preferred embodiment, a product pair that has proven particularly effective for this application in composites is polyamide 6 (PA6) – for example, Ultramid B24N 03 manufactured by Basf under the trade name – dissolved in a mixture of acetic acid and formic acid – for example, a product sold as a Carlo Erba reagent. An alternative polymer / solvent pair is, for example, polyvinylidene fluoride (PVDF) dissolved in dimethylacetamide or dimethylformamide, but other technical polymers with specific properties can be added, such as, but not limited to, polyimide, polyether ether, polyether ether, and polycaprolactone. A feasible solution method involves preparing the correct amount of the component to be prepared and placing it in a suitable thermo-regulated container, such as a thermoregulated vessel. A precise amount of solvent for preparing the polymer solution is introduced into the container, and a metered amount of polymer in granular or powdered form is added. Preferably, the system can directly recover the used solvent from the raw material tank delivered by the manufacturer and keep them under continuous stirring using an internal stirrer. To promote polymer dissolution in the solvent, the contents are stirred using a mixer, and gentle heating methods can also be applied in the mixer to promote dissolution. In the preferred embodiment disclosed herein, a 12% by weight PA6 solution is suitable.For example, 1000 grams of material uses the following ratio: - 120g PA6 (12 wt%) - 587g glacial acetic acid (58.7 wt%) - 293g formic acid (29.3 wt%) The ratio between acetic acid and formic acid is 2:1 and remains constant even when the general concentration of the solution changes. For example, increasing the total concentration of PA6 provides a proportional reduction in the two acids while maintaining their equal ratio. Once dissolution is complete (clear solution) and the material is kept at room temperature, the solution can be tested before electrospinning. Specifically, it is advantageous to perform three different tests, which are essential for obtaining a stable and repeatable electrospinning method over time using the techniques proposed herein. 1) Assess the solution viscosity (h) using a rotational viscometer. Typically, the viscosity value of polymer solutions is between 50 and 10,000 mPa·s. Preferably, depending on the concentration of the solution, solutions with viscosity values ​​ranging from a minimum of about 120 mPa·s to a maximum of about 900 mPa·s are expected to be used. 2) The percentage concentration of the polymer solution is assessed by using heat equilibrium, which provides direct concentration data, or by oven drying. Typically, for polymer solutions, the concentration ranges from 0.5% to 30% by weight, depending on the type of polymer and the solvent system. Preferably, a solution with a concentration of at least 8% to at most 25% by weight is expected to be used. 3) The conductivity (e) is assessed using a conductivity meter with an immersion probe. The conductivity value must be between 1 and 5000 mS / cm. For industrially optimized manufacturing, it has been found advantageous to use 15 to 30 kg of solution per batch and to employ appropriate methods for solubilizing the polymer in the solvent to avoid aging of the manufactured polymer solution. The resulting polymer solution is used according to the present invention via a continuous electrospinning method utilizing needle-free electrospinning technology to prepare nanofiber membranes on various types of easily peelable substrates. Compared to the needle system mentioned in US 6265333, needle-free electrospinning ensures a high industrial-level manufacturing yield (in m). 2 This technology allows for the production of industrially viable materials. Membranes manufactured using this technique, while exhibiting dimensional uniformity comparable to other microfiber manufacturing techniques (i.e., spunlace and meltblown), also benefit from a larger surface area (due to the nanoscale size of the material) and greater control and handling capabilities in the manufacturing process, even at low weights (maximum 1 g / m³). 2And it has a minimum thickness. These characteristics are crucial so as not to adversely affect the market requirements of composite materials (such as weight and overall thickness). There are four different macroscopic types of suitable backing substrates on which nanofiber layers are fabricated: monofilament fabrics with calibrated mesh, nonwoven fabrics of various types and weights, single / double silicate papers of various weights, and polymer films of various thicknesses and surface finishes (e.g., based on HDPE, LDPE, etc.). For applications in the field of composites, it is particularly important to select the material on which the polymer solution is electrospun. This must only function as a carrier without altering the properties of the product. The best choice for this type of application depends on double silicate paper, in which the release grade on the side in contact with the electrospun material is higher than that on the other side. This process allows for the production of rolls with sufficient internal tension (avoiding unwinding problems). The choice of silicate paper allows for the application of sufficient tensile stress to keep the material completely flat during the coupling phase on the coating line, thereby ensuring perfect adhesion of the nanofiber membrane to the resin used and subsequent removal of the substrate without causing membrane breakage or defects. The peel grade of the substrate must be precisely adjusted to allow for perfect film separation, but also sufficient to ensure that the paper still protects the composite during the subsequent slitting stage, especially in fields such as aerospace where cleanliness is mandatory during this slitting stage. Needle-free electrospinning technology is based on purely physical principles and does not imply any transformation of the material at the level of chemical bonding, whether in solution, in the stage prior to electrospinning, or in subsequent surface treatments (e.g., plasma treatment). As shown in Figure 1, needle-free electrospinning equipment is based on the absence of needles and stationary or rotating metal collectors, but typically includes pairs of steel wires placed at a certain distance from each other, which act as the anode and cathode of the system, respectively. The number of these pairs of wires ranges from a minimum of one pair in pilot plants to a maximum of eight pairs in currently commercially available industrial plants. During the electrospinning process, current flows through the two wires at a voltage difference ranging from a minimum of greater than 0 kV to a maximum of 120 kV. A potential difference is required for the spinning process. In fact, depending on the properties of the polymer solution, the electric field causes the latter to be cold-squeezed from the layer deposited on the cathode wire. The polymer solution moves toward the anode wire that attracts it, becoming thinner and drier during rapid movement due to turbulent motion, until it reaches the point that affects the substrate (which acts as a collector) for each use, and flows over a certain distance. Therefore, between the wire pair acting as the anode and cathode, there exists a virtual plane that cuts off the electric field, along which the substrate operates. The distance between the lower wire (cathode) and the substrate is the working distance of the production line and is always less than the distance between the anode and cathode. To deposit the polymer solution onto the wire acting as the cathode, the system is equipped with a deposition tray. The latter has a through-type housing through which the cathode wire passes.Within each housing, steel assemblies with calibrated feed holes (from 0.5 mm to 0.9 mm) are arranged, within which cathode wires are placed—avoiding contact between various metal components. These feed holes are positioned on a carriage as part of the polymer solution feed piping system. The polymer solution, placed in a special container with a controlled atmosphere (to prevent solvent evaporation), is transferred to the feed holes at a set flow rate using one or more pumps controlled by the production line's control panel. The polymer solution is thus deposited as a thin film onto the cathode wire during the trolley's sliding motion, depending on the opening size of the feed holes, ranging from a minimum of 50 cm to a maximum of 200 cm depending on the production line's extension. The polymer solution is continuously dispensed regardless of the carriage's sliding direction (backward or forward). The substrate has a lateral extension equal to the distance the carriage travels. The substrate moves at a sliding speed perpendicular to the trolley's sliding direction. In the case of a particular substrate, the upper wire acting as the anode can be replaced by a conductive pad, rotating as the substrate moves to ensure a roll-to-roll method. The entire electrospinning zone is located within an area defined as the "electrospinning chamber," which operates under a controlled atmosphere, specifically controlled relative humidity and temperature. The most important parameters that can influence the final properties of the resulting membrane through this electrospinning method are: - Concentration of the polymer solution - Viscosity of the polymer solution - Conductivity of the polymer solution - Distance between electrodes (top and bottom) - Applied electric field - Relative humidity within the electrospinning chamber - Speed ​​of the deposition carriage - Diameter of the steel orifices mounted on the carriage - Diameter of the steel wires on which the polymer solution is deposited - System pump repetition - Substrate displacement velocity - Substrate type - Voltage applied to the substrate - Chamber air recirculation (inlet-outlet flow) During formation, nanofibers move rapidly between conductive wires and collectors and are randomly deposited on the substrate, creating a three-dimensional structure whose weight and thickness depend on the substrate displacement velocity and therefore on the amount of fiber deposited on it. When surface-functionalized for specific applications (e.g., air filtration), such a material with a three-dimensional structure has a weight of approximately 0.1 g / m². 2 When used for materials whose characteristics can be defined as "self-standing" (i.e., can be handled without support), the gradient weight is up to a maximum of 15 g / m³. 2 According to the present invention, for a specific application as a reinforcing interlayer in a composite material, the amount of nanofibers deposited on the substrate is preferably between 3 and 10 g / m², depending on the diameter of the manufactured nanofibers. 2 Between. Experimental tests have shown that approximately 3 g / m in the composite material. 2The amount of nanofibers is sufficient to significantly increase performance by approximately 40% compared to composites without nanofibers. This is achieved when the fiber diameter is approximately 100 to 150 nm, resulting in a g / m³ content of approximately 7 to 8 g. 2 At a weight limit, approximately 94% increase in the fracture resistance of the composite was obtained. It should also be noted that during the composite lamination process, even when the film is at its weight limit (e.g., 8 to 9 g / m³), [the following is unclear and requires further context: "at the weight limit"]. 2The amount of resin used is also completely independent of the weight of the composite resin, as these specific amounts remain below the error threshold typically added to fibrous fabrics. To obtain optimal performance, it is important to define certain membrane and process parameters. In particular, to achieve optimal performance, it is necessary to: - Optimize the amount of resin used in relation to membrane thickness - Optimize the amount of resin based on the diameter of the nanofibers and the porosity of the membrane pores. PA6 nanofibers can have melting temperatures greater than 200°C, for example, 220°C, while polyimide nanofibers can have melting temperatures even greater than 300°C. This makes the resulting membrane suitable for insertion into high-temperature composites used in aerospace, where the high temperatures involved do not allow the use of toughening additives traditionally used in the automotive field. Using the nanofiber membrane obtained according to the present invention, it has been found that the role of the nanofibers occurs primarily at the interface between the resin and the underlying fiber layer (e.g., carbon fiber). The material's very high surface area and inherent porosity (approximately 80% free area, as shown in the SEM images of Figures 2A-2D at different magnifications) and the strong oleophilicity of the nanofiber membrane ensure that, under sufficient pressure during subsequent thermal lamination, it is completely incorporated and fixed within the fluid resin once curing or crosslinking is complete. The benefits of the inserted nanofibers are evident during fracture strength tests conducted at the interfaces between different layers in the subsequent composite (i.e., at the weakest points of the material), where peeling typically occurs between composite layers. The nanofiber-loaded composite, with a matrix at the interfaces between carbon fiber layers containing not only resin but also very long fibers of these nanometer diameters, is uniformly distributed within the resin itself due to the random but consistent and uniform deposition of the nanofibers during electrospinning using needle-free technology. During fracture tests (e.g., via international standard D5528-13), the presence of nanofibers within the resin prevents fracture from propagating uniformly throughout the composite, which would occur in standard composites. This effect is likely due to the fracture line encountering obstacles each time it passes through the layer of nanofibers incorporated into the resin. The better the interconnection between the cured or crosslinked resin and the nanofibers, the more complex the fracture path becomes and the greater the energy consumed during fracture, thus slowing its progress. This non-uniform movement of fracture propagation leads to a significant increase in material strength. The backing substrate or carrier on which the electrospun polymer film is formed and left is easy to roll up and therefore can be stored before use. In particular, once the coupling stage with the pre-impregnated material and subsequent processing (e.g., cutting) are completed, the electrospun film deposited on the silicate paper substrate can be easily peeled off without damaging the film itself and without any issues for the end customer's final handling of the parts. Using double silica paper immediately makes the side on which the film is deposited transparent, so the side used during coupling with the impregnated composite pad (brown polysiloxane paper - white film layer) is immediately visible.During the industrial process of incorporating nanofibers into the composite product, once the reinforcing fiber pad is impregnated with the resin matrix, it can be unfolded and a backing substrate layer (particularly double silica paper) can be placed with the polymer film facing the composite pad. The adhesion of the polymer film to the resin matrix on the composite pad is slightly higher than the separation force between the film and the backing substrate layer: subsequent successive removal of the backing substrate layer leaves the film perfectly placed on the composite, without tearing in the nanostructure of the film or breaking down the reinforcing fibers in the matrix of the composite (which still has low viscosity at this stage). Once the support layer is removed, subsequent layers of the composite can be laminated on top of the first layer equipped with the polymer film, which is also tightly bonded between the two layers by means of lamination pressure and possible lamination heat (which makes the resin more fluid). To optimize the toughening properties of the composite, according to the invention, it is important to ensure that the nanofiber film layer does not sink into the prepreg but remains on the surface of the resin material (the resin film coating the prepreg composite pad), at the interface with which other prepreg layers will be laminated. This potentially dangerous situation should be avoided, as it would reduce the advantages of using the intermediate nanofiber layer. To this end, the manufacturing method of the present invention advantageously provides: arranging anti-settling features that prevent the nanofiber membrane from prematurely settling into the resin matrix. Several different anti-settling measures can be considered for this purpose: ➢ One option may require optimizing the final thickness of the nanofiber layer: a larger thickness will minimize or eliminate settling, thereby minimizing performance loss. ➢ A related embodiment consists of an oil-repellent surface treatment deposited on the nanofiber membrane layer: this solution prevents the aforementioned phenomenon from occurring over time by reducing the resin's ability to wet and bind the nanofibers. ➢ In other embodiments, more oil-repellent high-tech polymers—i.e., those with a surface energy below 30 mN / m—or polymers with a lower inherent affinity for the resin used to manufacture the nanofiber membrane are used, as the chemical properties of these materials prevent the development of this phenomenon. ➢ Finally, another feasible option provides: using an intermediate layer made of a thin, uniform substrate of microfiber material (e.g., a wet-process or melt-blown substrate made of a low surface energy polymer (<30 mN / m)) attached to the nanofiber membrane layer. Thin layers of microfiber material can act as mechanical support for nanofiber membranes, preventing them from sinking into the resin. Furthermore, if appropriately selected, this type of substrate can provide further reinforcement of the final composite and avoid the need for a backing substrate that needs to be removed in subsequent processing steps of the prepreg or composite. To develop new and increasingly industrially effective materials, treatments aimed at modifying the reactivity of the membrane surface are also anticipated. For example, treatments aimed at increasing surface hydrophilicity, or plasma treatments that simultaneously increase surface roughness and generate free radicals or reactive chemicals on the surface, can further improve nanomaterial-resin adhesion, thereby further influencing performance enhancement. It will be understood from the above description that the methods according to the invention can fully achieve the objectives stated in the preamble.Electrospinning and subsequent deposition of polymer nanofiber membranes on easily peelable backing substrates allows for efficient industrial processes. The backing substrate can be peeled off during application, for example when made from release paper, or it can be retained in the composite material, for example when made from microfiber materials. Many adjustable parameters available in the manufacture of polymer nanofiber membranes allow for free adjustment of the membrane's consistency and reactivity based on the properties and particle size distribution of the components present in the composite material. It should be understood that the invention should not be considered limited to the specific embodiments described and illustrated, but rather various variations are possible, all of which are within the skill of a person skilled in the art and do not depart from the scope of protection of the invention itself, which is defined only by the following claims. Other features and advantages of the method and membrane according to the invention will become more apparent from the following detailed description of preferred embodiments, which are provided purely by way of non-limiting example and illustrated in the accompanying drawings, wherein: [Figure 1] is a schematic diagram of an exemplary membrane manufacturing plant according to the invention; and [Figures 2A-2D] are SEM views of the membrane according to the invention at different magnifications.

Claims

1. A method for manufacturing a composite reinforced material, comprising the following steps: arranging a plurality of reinforcing fibers; impregnating the plurality of layers with a resin-based matrix; placing an intermediate layer of polymer nanofibers between the plurality of reinforcing fibers; laminating the plurality of reinforcing fibers by increasing pressure and / or heat; characterized in that, prior to placing a second layer of reinforcing fibers and the lamination step, a polymer nanofiber membrane adhered to a backing substrate is placed on a first layer of reinforcing fibers, and the intermediate layer of polymer nanofibers is inserted between the plurality of reinforcing fibers; after removing the backing substrate, a second layer of reinforcing fibers is laminated onto a first layer equipped with a polymer membrane, and the polymer membrane is tightly bonded between the first and second layers by means of lamination pressure and possible lamination heat. The polymer nanofiber membrane is obtained by direct electrospinning on the backing substrate using a needle-free technique, and is characterized by providing anti-sinking properties before the lamination step to ensure that the layer of the polymer nanofiber membrane does not sink into the impregnated first layer, but remains on the surface of the material of the resin-based matrix at the interface with another impregnated second layer to be laminated therewith.

2. The method of claim 1, wherein the polymer film is obtained from a solution of PA 6 and a solvent containing acetic acid and formic acid.

3. The method of claim 2, wherein the solution contains about 12% by weight of PA 6.

4. The method of claim 1, wherein the resin-based matrix is ​​a cross-linked thermosetting resin.

5. The method of claim 1, wherein the composite reinforcing material comprises a reinforcing fiber layer made of carbon.

6. The method of claim 1, wherein the amount of nanofibers adhered to the backing substrate for forming the polymer film is between 1 and 15 g / m2.

7. The method of claim 1, wherein the nanofibers constituting the polymer film have a fiber diameter of about 100 to 150 nm.

8. The method of claim 1, wherein the backing substrate is an easy-peel backing substrate made of bisiliconized paper web, which is removed before the placement of the second reinforcing fiber layer and the lamination step.

9. The method of claim 1, wherein the anti-settling property comprises: subjecting the polymer nanofiber membrane to an oil-repellent surface treatment or treating it with a material having low affinity for the resin-based matrix prior to the placement step.

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