Ultrasonic welding of prepreg tape

Abstract

A process for forming a spliced ​​prepreg tape by joining adjacent ends (22, 24) of first and second prepreg tapes (2, 3, 4), each prepreg tape comprising respective first and second prepreg layers (12, 18) comprising a fibrous material and a thermosetting resin, each prepreg tape further comprising respective first and second releasable polymer backing sheets (14), the process comprising the steps of: peeling the first polymer backing sheet from the first prepreg layer to provide a first peeled edge region, and optionally peeling the second polymer backing sheet from the second prepreg layer to provide a first peeled edge region (30) and optionally a second peeled edge region (32), where the prepreg layers are separated from their respective polymer backing sheets; overlapping the first and second prepreg tapes at the first peeled end region and, if present, the second peeled end region to form a pre-spliced ​​joint (34, 38), said pre-spliced ​​joint including first and second adjacent prepreg layers and first and second adjacent polymer backing sheet layers; ultrasonically welding the pre-splice joint (36) to fuse the first and second adjacent prepreg layers together and to fuse the two adjacent polymer backing sheet layers together to form a spliced ​​prepreg tape.

Description

[Technical Field]

[0001] The present invention relates to a process for forming a spliced ​​prepreg tape by joining adjacent ends of first and second prepreg tapes using ultrasonic welding, the spliced ​​tape formed by this process, and a method for laying the spliced ​​prepreg in an automated tape laying process. [Background technology]

[0002] Composite materials have well-documented advantages over traditional structural materials, particularly their ability to provide excellent mechanical properties even at very low material densities. As a result, such materials are widely used, with application areas ranging from industrial and sports and leisure to high-performance aerospace components.

[0003] Prepregs, which comprise an array of fibers or fabrics impregnated with a thermosetting resin such as an epoxy resin, are widely used in the production of such composites. The resin can be combined with the fibers or fabrics in a variety of ways. The resin may be attached to the surface of the fibrous material, but more commonly, the resin partially or completely impregnates the interstices between the fibers. Such prepregs are sometimes referred to as semipregs or towpregs, depending on the degree of resin impregnation, but in the context of this invention, they are considered to fall within the scope of the more general term prepreg.

[0004] Once manufactured, a number of such prepregs are typically "laminated" as desired, and the resulting prepreg stack, i.e., laminate or preform, is cured, typically by exposure to elevated temperatures, to produce a cured composite structure.

[0005] One common method of preparing such laminate structures is to lay down the prepreg as a tape in a so-called automated fiber placement (AFP) process or automated tape laying (ATL) process, which are considered synonymous in the context of this invention and are commonly referred to as "automated tape laying." Generally, these involve automatically laying down prepreg in the form of a relatively narrow tape from a long roll of prepreg. Commonly used widths are 3.2 mm, 6.35 mm, 12.7 mm, and 25.4 mm.

[0006] Such prepreg tapes are typically produced by slitting a wide master, or parent, roll of prepreg. Slitting involves cutting the master roll parallel to the resulting tape length to produce narrower tapes. During or before the slitting process, such parent rolls of prepreg are joined or spliced ​​at their ends to produce longer rolls of elongated prepreg tape. Furthermore, if defects are found in the parent roll during, after, or prior to the slitting process, the defective area is cut out and the remaining prepreg around the defective area is spliced. Traditionally, such defective areas are removed "offline" on a so-called edit table.

[0007] A number of methods of joining or splicing have been proposed, ranging from the simple application of pressure and heat, as disclosed, for example, in EP 3272508, to the use of adhesives, and more recently, ultrasonic welding. Ultrasonic welding has a significant advantage as it has been shown to be capable of producing a spliced ​​joint where the thickness of the joint is essentially the same as the thickness of the individual tapes being joined.

[0008] WO2017 / 220327A1 discloses a process for forming a permanent splice bond between two prepregs by applying ultrasonic welding, without significant change in the below-room-temperature glass transition temperature (Tg) of the fibrous material in the prepregs in the area of ​​the permanent bond.

[0009] However, these prepreg master rolls often have a backing paper, such as silicone-treated paper, to facilitate handling and winding the tape for later transport when slitting and removing defects is required. As is well known, the curable resins in prepregs are generally very sticky, making the backing paper essential for storing the tape in roll form. However, the presence of the backing paper complicates splicing two prepreg tapes, requiring careful manual preparation before welding, which is extremely labor-intensive, especially when done offline on an editing table. Even so, when splicing prepreg layers of such narrow tapes, the cut ends of the backing paper at the splice site are typically simply joined, such as with adhesive tape along the joint, creating a potential failure point during automated tape application.

[0010] Therefore, further improvements in forming splicing tapes suitable for automated tape application are desirable. Summary of the Invention

[0011] In a first aspect, the present invention relates to a process for forming a spliced ​​prepreg tape by joining adjacent ends of first and second prepreg tapes, each prepreg tape comprising respective first and second prepreg layers including a fibrous material and a thermosetting resin, and each prepreg tape further comprising respective first and second releasable polymer backing sheets. This process includes the steps of: peeling a first polymer backing sheet from a first prepreg layer to provide a first peeled edge region, and optionally peeling a second polymer backing sheet from a second prepreg layer to provide a first peeled edge region and optionally a second peeled edge region where the prepreg layers are separated from their respective polymer backing sheets; overlapping the first and second prepreg tapes at the first peeled edge region and, if present, the second peeled edge region to form a pre-splice joint, the pre-splice joint including the first and second adjacent prepreg layers and the first and second adjacent polymer backing sheet layers; and ultrasonically welding the pre-splice joint to fuse the first and second adjacent prepreg layers and fuse the two adjacent polymer backing sheet layers to form a spliced ​​prepreg tape.

[0012] In a second aspect, the present invention relates to a spliced ​​prepreg tape obtainable by the process described herein.

[0013] In a third aspect, the present invention relates to a method of laying a prepreg tape to form a composite material, the prepreg tape comprising a prepreg layer and a polymer backing sheet, the method comprising the steps of removing the polymer backing sheet and laying the prepreg layer by means of an automated tape laying apparatus, the prepreg tape being a spliced ​​prepreg tape according to the second aspect of the invention.

[0014] It has been found that by simultaneously subjecting the polymer backing sheet to an ultrasonic welding process, the two ends of two prepreg tapes can be spliced ​​together in a single step, fusing the respective prepreg layers and the respective polymer backing sheets simultaneously. This is a more efficient procedure than welding the prepreg materials separately and then joining the backing sheets separately. It also produces a backing sheet in the splice area that can withstand higher tensile forces, providing a stronger splice bond and preventing material failure during use, especially during automated tape application. Surprisingly, it has been found that ultrasonic welding can achieve this simultaneous welding without contaminating the polymer backing sheet or silicone release agent with the prepreg layers.

[0015] While a wide variety of polymer backing sheets may be used in the present invention, a backing sheet comprising high-density polyethylene (HDPE) is preferred. This material provides the necessary tensile strength for automated tape application. However, existing ultrasonic welding literature generally indicates that HDPE cannot be effectively ultrasonically welded due to its high degree of crystallinity. The inventors surprisingly discovered that, according to the teachings of the present invention, backing sheets comprising HDPE can be ultrasonically welded. Other polymeric materials, such as MDPE, polypropylene, PTFE, and PLA, may also be used. Blends of these materials and / or layers of combinations of these materials may also be preferred. For example, one preferred backing sheet is constructed with an HDPE core and an MDPE outer layer.

[0016] A spliced ​​prepreg tape will generally be such that, when used in, for example, an automated tape laying process, it can withstand similar tensile forces as the prepreg tapes that are spliced ​​to form the spliced ​​tape. However, such spliced ​​prepreg tapes generally do not inherently perform well when wound into small radius rolls. Because such bending occurs during the automated tape laying process, the spliced ​​prepreg tape must remain intact during such bending deformation.

[0017] Although the polymer backing sheet and the prepreg layers are welded simultaneously in the same single step, it has been found advantageous to select the ultrasonic welding parameters so that the weld has two successive phases.

[0018] In the first phase, some of the energy from the ultrasonic welding is converted into heat within the pre-spliced ​​joint, increasing the temperature and decreasing the viscosity of the resin in the prepreg layers. This is followed by a second phase, in which the temperature of the pre-spliced ​​joint stabilizes and reaches a steady state, during which controlled welding with flow of the thermosetting resin and simultaneous welding of the polymer backing sheet occurs.

[0019] This continuous application of ultrasonic energy, when achieved in a single step, results in a strong fused seam of the prepreg layers as well as a strong weld of the polymer backing sheet, even with HDPE, without burns or holes and without contamination between the polymer backing sheet and the prepreg layers.

[0020] As is known in the art, the sonotrode of an ultrasonic welding machine vibrates perpendicular to the prepreg surface at high frequency, reducing the viscosity of the thermosetting resin and fusing the two layers of prepreg together. At the same time, heat energy is applied to the polymer backing sheet.

[0021] However, care must be taken to ensure that the heat energy supplied to the thermosetting resin is done in a controlled manner to avoid a rapid increase in temperature due to exotherm, which could burn the polymer backing sheet and result in poor results. It has been found that such exotherm can be avoided if the force applied during the welding process is kept below a critical value.

[0022] As is well known to those skilled in the art, there are various parameters that can be varied in an ultrasonic welding process that affect the power applied, such as amplitude, frequency, force, trigger force, etc.

[0023] A suitable frequency range may be 15-70 kHz, preferably 16-35 kHz. A suitable amplitude range may be 3-50 μm, more preferably 15-25 μm, and most preferably 20-25 μm. A suitable applied force value may be 50-2000 N, preferably 200-1000 N, and even more preferably 200-600 N (for a rectangular pre-sewn joint with dimensions of 5.08 cm x 3 cm).

[0024] The maximum power applied has been found to be preferably in the range of 500-900 W (for a rectangular pre-spliced ​​joint measuring 5.08 cm x 3 cm). This provides a good balance between forming a good degree of fusion of adjacent prepreg layers and a good degree of fusion of adjacent polymer layers, and ensuring that the polymer backing sheet is welded over the entire weld area without transferring the polymer material to the prepreg layers. Therefore, a weld time of 1.0-4.0 seconds is appropriate, with 1.5-3.0 seconds being preferred.

[0025] To prevent the pre-splice joint from adhering to any part of the sonotrode, it may be advantageous to place one or two layers of a non-stick material between the pre-splice joint and the sonotrode. Suitable materials include any material that has non-stick properties, but are preferably non-compressible, such as PTFE-coated materials. Examples include Teflon™ (e.g., Tygavac), PTFE-coated glass cloth (e.g., standard grade 7058 Tygadur, available from Taconic Wildcat, UK), and silicone-coated materials such as silicone-coated paper (e.g., 120g GL silicone release coated paper, Mondi, and NSS900 silicone-coated paper, Laufenberg).

[0026] Preferably, the thickness of the splice joint is 50-95%, preferably 70-90%, of the thickness of the pre-splice joint, so that the thickness of the splicing prepreg tape is thinner than the thickness of the two prepreg tapes being spliced ​​together, thus reducing the impact of the splice on the mechanical performance and dimensional tolerances of the cured composite part formed by the automated tape laying equipment.

[0027] The length of the pre-splicing joint has a significant effect on the strength of the spliced ​​joint formed after ultrasonic welding, and therefore the length of the pre-splicing joint is preferably 3 to 200 mm, more preferably 10 to 150 mm, and most preferably 15 to 75 mm.

[0028] Typically, the entire pre-splice joint is ultrasonically welded to more completely weld the adjacent layers of prepreg and polymer backing sheet. However, a cleaner, more effective splice may be achieved if the prepreg tape on both sides of the pre-splice joint is also ultrasonically welded. Therefore, 1 to 50 mm of the prepreg tape on both sides of the pre-splice joint may be ultrasonically welded. This means that the width of the welding tool is wider than the width of the overlapping materials.

[0029] Preferably, the first and second prepreg tapes have the same width, which results in a spliced ​​prepreg tape of constant width and therefore convenient for use with automated tape laying equipment.

[0030] Preferably, the first and second prepreg tapes have the same thickness. This is because the first and second prepreg tapes are usually manufactured in the same process or obtained by cutting to remove defective areas. However, the present invention is equally applicable to joining prepreg tapes of different thicknesses.

[0031] Preferably, the width of the first and second prepreg tapes is less than 30 cm, preferably less than 20 cm, and more preferably less than 10 cm. Such narrow widths allow the resulting spliced ​​prepreg tape to be used in automated tape laying equipment without the need for lengthwise slitting. Furthermore, at these widths, prepreg materials such as resin and fiber material may expand laterally at the pre-spliced ​​joint, resulting in excess width extending beyond the backing sheet. Therefore, the ultrasonic welding step is preferably followed by a step of trimming off the excess width after forming the spliced ​​joint.

[0032] Therefore, there is no need to design the overlapping portion of the prepreg tape to be joined in a complicated manner, and all that is required is to cut it along the width direction. Preferably, the resin or fiber material that flows to the side is removed by cutting after the splice joint is formed.

[0033] As previously explained, the prepreg tape being spliced ​​is either two ends of an existing tape or two newly cut ends of a single tape from which a tape defect has been removed. Therefore, the ability to produce a spliced ​​prepreg tape with simply straight cut ends is particularly advantageous because it greatly simplifies the implementation of automated splicing procedures.

[0034] Therefore, one important advantage of the present invention is that it is suitable for automation because it does not require a separate backing sheet process, and therefore, preferably, the peeling, laminating and ultrasonic welding steps are performed automatically without the need for manual intervention.

[0035] Preferably, the automation includes the use of suction applied to the prepreg tape to cause the peeling and preferably also the overlapping.

[0036] One preferred method of overlapping the first and second prepreg tapes is to peel only the first backing sheet to form a first peeled end region and insert the end of the second prepreg tape into the peeled end region so that the pre-spliced ​​joint includes the first polymer backing sheet, the first polymer backing sheet adjacent to the second polymer backing sheet, the second polymer backing sheet adjacent to the second prepreg layer, and the second prepreg layer adjacent to the first prepreg layer.

[0037] Another preferred method of overlapping the first and second prepreg tapes involves peeling both the first backing sheet and the second backing sheet to form a first peeled end region and a second peeled end region, and inserting the first peeled end region and the second peeled end region into each other so that the pre-spliced ​​joint includes a first polymeric backing sheet, the first polymeric backing sheet adjacent to a second polymeric backing sheet, the second polymeric backing sheet adjacent to the first prepreg layer, and the first prepreg layer adjacent to the second prepreg layer.

[0038] Ultrasonic welding can be performed using any suitable device, but is preferably performed by contacting the pre-spliced ​​joint with a sonotrode while supported on an anvil, which may be in the form of a generally flat plate, a drum, or a roller.

[0039] Commercially available sonotrodes have generally rectangular contact surfaces that should be selected to provide a surface that at least completely covers the area of ​​the pre-spliced ​​joint. The contact surface may be flat or wavy, for example with a dimple pattern, to provide a stronger mechanical lock between the tapes.

[0040] The thermosetting resin in the spliced ​​prepreg tape is essentially chemically unchanged relative to the thermosetting resin in the remainder of the prepreg tape that was not involved in the ultrasonic welding process.

[0041] The thermosetting resin may be selected from those conventionally known in the art, such as phenol formaldehyde resin, urea formaldehyde, 1,3,5-triazine-2,4,6-triamine (melamine), bismaleimide, epoxy resin, vinyl ester resin, benzoxazine resin, polyester, unsaturated polyester, cyanate ester resin, or a mixture thereof. Epoxy resin is particularly preferred. If necessary, a curing agent and, optionally, an accelerator may be included.

[0042] The thermosetting resin is preferably an epoxy resin. The epoxy resin used in preparing the resin composition and / or prepreg of the present invention preferably has an epoxy equivalent weight (EEW) in the range of 10 to 1500, preferably in the range of 50 to 500. Suitable epoxy resins may include mixtures of two or more epoxy resins selected from monofunctional, difunctional, trifunctional and / or tetrafunctional epoxy resins.

[0043] Suitable difunctional epoxy resins include diglycidyl ethers of bisphenol F (bisphenol F epoxy resins), such as Araldite GY281 and GY285 (Huntsman Advanced Materials), diglycidyl ethers of bisphenol A (bisphenol A epoxy resins), such as Epon 825 (DER 332, Dow Chemical Company, Midland, Michigan), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, glycidyl esters, or difunctional epoxy resins based on any combination thereof. Suitable difunctional epoxy resins include GY 281 (also known as LY3581). The difunctional epoxy resins may be used alone or in any suitable combination with other difunctional epoxy resins.

[0044] The difunctional epoxy resin may be selected from diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol A, diglycidyl dihydroxy naphthalene, or any combination thereof.

[0045] Suitable trifunctional epoxy resins include trifunctional epoxy resins based on phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers, aliphatic polyglycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidyl amines, fluorinated epoxy resins, or any combination thereof. Suitable trifunctional epoxy resins are available from Huntsman Advanced Materials (Monthey, Switzerland) under the trade names MY0500 and MY0510 (triglycidyl para-aminophenol) and MY0600 and MY0610 (triglycidyl meta-aminophenol). Triglycidyl meta-aminophenol is available from Sumitomo Chemical Co., Ltd. (Osaka, Japan) under the trade name ELM-120.

[0046] Tetrafunctional epoxy resins are also preferred. The phenyl ring may be further substituted with other suitable non-epoxy substituents. Suitable substituents include, for example, hydrogen, hydroxyl, alkyl, alkenyl, alkynyl, alkoxyl, aryl, aryloxyl, aralkyloxyl, aralkyl, halo, nitro, or cyano radicals. Suitable non-epoxy substituents may be bonded to the para or ortho positions of the phenyl ring, or to meta positions not occupied by epoxy groups.

[0047] Suitable tetrafunctional epoxy resins include N,N,N',N'-tetraglycidyl-m-xylylenediamine (commercially available under the name Tetrad-X from Mitsubishi Gas Chemical Company, Inc., Chiyoda-ku, Tokyo), Erisys GA-240 (manufactured by CVC Chemicals, Moorestown, New Jersey), and N,N,N',N'-tetraglycidylmethylenedianiline (e.g., MY720 and MY0721 from Huntsman Advanced Materials). Other suitable multifunctional epoxy resins include DEN438 (Dow Chemical Company, Midland, Michigan), DEN439 (Dow Chemical Company), Araldite ECN 1273 (Huntsman Advanced Materials), MY722 (Huntsman Advanced Materials), and Araldite ECN 1299 (Huntsman Advanced Materials).

[0048] Preferably, at least one of the multifunctional epoxy resins has at least one meta-substituted phenyl ring in its backbone. Preferred multifunctional epoxy resins are trifunctional or tetrafunctional. Most preferably, the multifunctional epoxy resin is a combination of a trifunctional epoxy resin and a multifunctional epoxy resin. The multifunctional epoxy resin may be saturated, unsaturated, cycloaliphatic, alicyclic, or heterocyclic.

[0049] The resin system may contain a thermoplastic material that is soluble in the epoxy resin, such as polyethersulfone, to improve the toughness of the resin. Examples of thermoplastic toughening agents / particles include the following thermoplastic materials, alone or in combination: polyamide, copolyamide, polyimide, aramid, polyketone, polyetheretherketone, polyester, polyurethane, polysulfone, polyethersulfone, high performance hydrocarbon polymers, liquid crystal polymers, PTFE, elastomers, and segmented elastomers.

[0050] The resin also suitably contains a curing agent and a curing accelerator. The curing accelerator is usually heat-activated and shortens the curing time of the resin. Suitable curing agents include amines including aromatic amines, such as 1,3-diaminobenzene, 1,4-diaminobenzene, and 4,4'-diamino-diphenylmethane, as well as polyaminosulfones such as 4,4'-diaminodiphenylsulfone (4,4'-DDS, Huntsman), 4-aminophenylsulfone, and 3,3'-diaminodiphenylsulfone (3,3'-DDS).

[0051] The present invention is applicable to a wide range of prepreg tapes, and the weight of the structural fiber is 10 to 1200 g / m 2 However, the tape preferably has a basis weight of 100 to 800 g / m 2 , more preferably 200 to 600 g / m 2 is.

[0052] Typically, the fibers in the structural layer generally have a circular or near-circular cross section with a diameter in the range of 3 μm to 20 μm, preferably 5 μm to 12 μm.

[0053] As described below, prepregs are typically manufactured as continuous, woven materials whose length is greater than, and typically much greater than, their width. These prepregs are typically manufactured as prepreg rolls, the length of which is determined by the width of the prepreg. To account for the adhesive properties of prepregs, a polymer backing sheet is provided to allow the prepreg roll to be unrolled during use.

[0054] The fibers may be woven or formed from tows of separate fibers. However, the present invention is more effectively applied when the fibers are individual and not interwoven, and preferably the fibers are unidirectional, i.e., arranged parallel to each other and generally parallel to the longitudinal direction of the prepreg. Unidirectional fibers are more easily spliced. The fibers may be composed of cracked (i.e., stretch-broken), selectively discontinuous, or continuous fibers.

[0055] An example of a unidirectional fiber layer is made from HexTow brand carbon fiber available from Hexcel. HexTow brand carbon fibers suitable for the manufacture of many unidirectional fiber layers include IM5 carbon fiber, available in 6,000, 12,000, and 24,000 filaments; IM7 carbon fiber, available in 6,000 or 12,000 filaments and weighing 0.223 g / m and 0.446 g / m, respectively; IM8-IM10 carbon fiber, available in 12,000 filaments and weighing 0.446 g / m to 0.324 g / m, respectively; and AS7 carbon fiber, available in 12,000 and 24,000 filaments and weighing 0.800 g / m and 1.600 g / m, respectively. The tow, typically 3 mm to 7 mm wide, is fed into an impregnated machine equipped with combs that hold the tow in place and align it in a parallel, unidirectional manner, as described below.

[0056] The fibers may be selected from the list consisting of carbon fibers, glass fibers, graphite fibers, metallized polymers, and mixtures thereof.

[0057] On a weight basis, the prepreg typically contains 15 to 70 wt %, preferably 20 to 65 wt %, more preferably 25 to 50 wt %, and most preferably 25 to 40 wt % of the curable resin. On a volume basis, the prepreg typically contains 15 to 70 vol %, preferably 20 to 60 vol %, and more preferably 30 to 50 vol % of the curable resin.

[0058] On a volume basis, the prepreg typically contains 45 to 75 volume % structural fibers, preferably 55 to 70 volume %.

[0059] The resin and fiber content of uncured prepregs containing unidirectional carbon fibers is determined in accordance with DIN EN 2559 A (Code A). The resin and fiber content of cured composites containing carbon fiber materials is determined in accordance with DIN EN 2564 A.

[0060] Prepreg tapes according to the present invention can be produced by known methods, such as the process described and illustrated in WO 2010 / 150022. Typically, they can be produced in a continuous process involving the movement of thousands of fibers, forming a structural layer of fibers, and passing the structural layer through a series of impregnation stages, typically guided by rollers that act to impregnate the structural layer with resin.

[0061] Before the fibers come into contact with the resin and reach the impregnation zone, they are typically arranged into multi-fiber tows. Each tow contains several thousand filaments, e.g., 12,000. These tows are loaded onto bobbins and first fed through a combing unit to ensure uniform fiber separation. Structural layers are typically formed from multi-fiber tows, which are spread out and meet on a spreader bar before impregnation with the resin.

[0062] To facilitate better resin handling, the resin is usually supported by a backing, such as paper. The resin is usually supplied from a roll and comes into contact with the fibers, with the backing remaining outside the resin-fiber contact area. During the subsequent impregnation process, the backing provides a convenient outer surface for applying pressure, ensuring uniform impregnation of the resin.

[0063] During this impregnation process, the resin passes through the interstices of the fibers. To facilitate resin impregnation into the fibers, impregnation is typically carried out at elevated temperatures, e.g., 60 to 170°C, preferably 100 to 140°C, to reduce the resin viscosity to 0.1 to 100 Pas, preferably 6 to 100 Pas, more preferably 18 to 80 Pas, and even more preferably 20 to 50 Pas. This is most conveniently achieved by heating the resin and fibers to the desired temperature before impregnation, e.g., by passing them through an infrared heater.

[0064] Impregnation is usually followed by a cooling step to reduce the tackiness of the formed prepreg, which may then be followed by further processing steps such as laminating, slitting, separating, etc. Once prepared, the prepreg tape may be wound up for storage over a period of time.

[0065] When a composite material needs to be manufactured, a number of such prepregs are typically stacked together to form a prepreg stack or preform.

[0066] Prepreg tape can be manufactured as rolls of material specially formulated for automated tape laying equipment. Therefore, the prepreg tape with the polymer backing sheet is preferably flexible enough to form rolls less than 20 cm in diameter, preferably less than 10 cm. Known automated tape laying equipment requires rolls to meet specific dimensions. For example, rolls are wound onto cores with an inner diameter of 254 mm or 295 mm, with a tolerance of ±0.5 mm. Rolls can be cut into standard prepreg tape widths, such as 600 mm (24 inches), 300 mm (12 inches), 150 mm (6 inches), 75 mm (3 inches), 50 mm (2 inches), 25 mm (1 inch), 6.34 mm (1 / 4 inch), and 3.18 mm (1 / 8 inch), within a tolerance of ±0.050 mm, and then laminated and cured into multiple layers of tape. Tapes are frequently used in this manner in the manufacture of aircraft parts.

[0067] However, the prepreg tapes according to the present invention are preferably pre-manufactured to the width required for the automated laying equipment, eliminating the need to cut or slit the master prepreg as a separation step, which allows the automation of the splicing procedure according to the present invention to be carried out "on-line", i.e. as part of the prepreg manufacturing process.

[0068] The prepreg tape thus produced is passed to an editing table where defects are cut from the tape by cutting away on both sides along the width of the tape, after which the ends can be spliced ​​together as described above.

[0069] The resulting prepreg tape stack is then typically cured by exposure to elevated temperatures to harden the thermosetting resin and produce a cured composite. The cure cycle used to cure the prepregs and prepreg stacks is a balance of temperature and time, taking into account the reactivity of the resins and the amount of resin and fiber used. This may be carried out under pressure using known methods such as autoclaving. Alternatively, or in addition, curing may be carried out under pressure close to atmospheric pressure using the so-called vacuum bag method.

[0070] As known to those skilled in the art, such curing processes are generally exothermic, and care must be taken to prevent excessive temperatures that could damage the mold or cause decomposition of the resin.

[0071] Typically, the glass transition temperature of the cured resin is 150°C to 200°C, more preferably 160°C to 200°C.

[0072] Once cured, the prepreg or prepreg stack becomes a composite material suitable for structural applications such as aerospace structures. [Brief explanation of the drawings]

[0073] The invention will now be described by way of example and with reference to the following figures: [Figure 1] FIG. 1 is a schematic diagram of a process for forming a splice joint in accordance with the present invention. [Figure 2] FIG. 2 is a schematic diagram of a further process for forming a splice joint in accordance with the present invention. [Figure 3] 3a and 3b are graphs showing the change in compression, applied power and applied force of the pre-spliced ​​joint during ultrasonic welding of Examples 20 and 31, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0074] Referring to the drawings, Figure 1 shows, from top to bottom, various steps in a process for forming a spliced ​​joint according to the present invention. In an initial state, shown in step (a), a side view shows a first prepreg tape 10 comprising a first prepreg layer 12 and a first release polymer backing sheet 14, and a second prepreg tape 16 comprising a second prepreg layer 18 and a second release polymer backing sheet 20. The first prepreg tape 10 has an end 22, and the second prepreg tape 16 has an end 24, the two ends 22, 24 being adjacent to one another and to be spliced ​​together later in the process. The two tapes 10, 16 extend along their lengths outside the view, but are shown truncated for ease of illustration.

[0075] 1, step (b), first polymer backing sheet 14 is peeled from first prepreg layer 12 by peeling edge 22 downward in the direction of the arrow to form first peeled edge region 30. Simultaneously, second polymer backing sheet 20 is peeled from second prepreg layer 18 by peeling edge 24 upward in the direction of the arrow to form second peeled edge region 32. As will be appreciated, such peeling steps can be performed automatically, such as by use of suction.

[0076] 1, the first prepreg tape 10 and the second prepreg tape 16 are overlapped at the first peeled end region 30 and the second peeled end region 32. Specifically, the first peeled end region 30 and the second peeled end region 32 are inserted into each other. As shown in step (d), a pre-spliced ​​joint 34 is formed, which includes the first polymer backing sheet 14, the first polymer backing sheet 14 and the second polymer backing sheet 20 adjacent to each other, the second polymer backing sheet 20 and the first prepreg layer 12 adjacent to each other, and the first prepreg layer 12 and the second prepreg layer 18 adjacent to each other.

[0077] Finally, the pre-seam joint 34 is ultrasonically welded to form the splice joint 36 shown in step (e).

[0078] 2 shows, in a top-to-bottom order, various steps in a further process for forming a spliced ​​joint according to the present invention. In an initial state, shown in step (a), a side view shows a first prepreg tape 10 comprising a first prepreg layer 12 and a first release polymer backing sheet 14, and a second prepreg tape 16 comprising a second prepreg layer 18 and a second release polymer backing sheet 20. The first prepreg tape 10 has an end 22, and the second prepreg tape 16 has an end 24, the two ends 22, 24 being adjacent to one another and to be spliced ​​together later in the process. The two tapes 10, 16 extend along their lengths beyond the view, but are shown truncated for ease of illustration.

[0079] As shown in step (b) of Figure 2, the first polymer backing sheet 14 is peeled downward in the direction of the arrow, and simultaneously the prepreg layer 12 is peeled upward in the direction of the arrow, thereby peeling the first polymer backing sheet 14 from the first prepreg layer 12 to form a first peeled edge region 30.

[0080] 2, the first prepreg tape 10 and the second prepreg tape 16 are overlapped at the first peeled end region 30 and the second peeled end region 32. Specifically, the end 24 of the second prepreg tape 16 is inserted into the first peeled end region 30. As shown in step (d), a pre-spliced ​​joint 38 is formed, which includes the first polymer backing sheet 14, the first polymer backing sheet 14 and the second polymer backing sheet 20 adjacent to each other, the second polymer backing sheet 20 and the second prepreg layer 18 adjacent to each other, and the second prepreg layer 18 and the first prepreg layer 12 adjacent to each other. [Example]

[0081] Two prepreg tapes (individual components available from Hexcel Composites, UK) using 268 gsm, 12k unidirectional carbon fiber (medium modulus, Type A) with 34 wt% M21EV resin system, 0.2 in. (5.08 mm) wide, each with a backing sheet (three layers total, 40 μm thick, containing 60 wt% HDPE sandwiched between two outer layers of 40 wt% MDPE, available from Plasthill, The Netherlands) were combined to form a pre-spliced ​​joint as shown in Figure 2. The tapes were overlapped by 3 cm, and the edges of each tape were flat. Each tape was approximately 270 μm thick, resulting in an initial pre-spliced ​​joint thickness of approximately 540 μm.

[0082] The pre-splice joint was placed on the anvil of a Harman Ultrasonics HiQ Vario benchtop ultrasonic welder with a maximum nominal power of 4800 W. The sonotrode dimensions were 5 cm in the splice length direction and 21.5 cm in the splice width direction, which was larger than the size of the pre-splice joint. The supply voltage was set to 400 V, and the vibration frequency was set to 20,000 Hz. The vibration amplitude (at 60% setting) was set to 14 μm, and the linear vibration velocity was 0.88 m / s.

[0083] The applied force, amplitude, and total energy delivered were systematically varied according to a design of experiments pattern. Ultrasonic welding was then performed to form a seam joint. Ultrasonic welding continued until a predetermined endpoint was reached, e.g., the total energy delivered to the joint, which defined the time it took to form the seam. This was followed by an additional 2.0-second hold period, during which the sonotrode remained in contact with the formed joint after the ultrasonic vibrations had ceased. The force applied during this hold period was the nominal value set at the beginning of the formation period. The thickness of the seam joint was also measured after ultrasonic welding was completed.

[0084] Poly weld strength (PWS) was evaluated and scored as 1 (no strength), 5 (low strength where the poly splice could be separated without stretching the poly), or 10 (high strength where the poly splice could not be separated without stretching and ultimately breaking the splice). Poly weld area (PWA) was assessed by visual inspection to assess the percentage of the area of ​​the splice joint that was welded. Poly transfer (PT) was assessed and scored as 1 (several large charred poly pieces attached to the tape), 2 (several large partially charred poly pieces attached to the tape), 3 (small slightly charred poly pieces attached to the tape), 4 (large transferred pieces corresponding to the poly holes), 5 (partially transferred thin pieces corresponding to the poly holes), 6 (partially transferred thin pieces corresponding to the poly hole), 9 (negligible), and 10 (no visible evidence of poly transfer). The overall poly score was calculated based on the following parameters: (poly weld area x 0.5) + (poly weld strength x 5) + poly transfer x 20. A higher score means a better weld. The results are shown in Table 1 below.

[0085] [Table 1] It can be seen that the results vary widely across parametric space, with no apparent effect of the total energy applied. In practice, only Examples 1 and 2 met the minimum acceptable quality control level, and these were achieved at the maximum amplitude and minimum force settings in this initial set of examples.

[0086] To investigate this further, a second set of experiments was performed using the same prepreg and HDPE backing sheet arrangement, but this time the amplitude was kept at 16.3 μm and the force was gradually reduced from 1300 N to 800 N. The results are shown in Table 2 below.

[0087] [Table 2] A gradual improvement in polyscore with decreasing force was clearly demonstrated. The good results obtained at 800N were reproduced, confirming the consistency of the results. At these values, the consistency is also quite good, although there is room for improvement.

[0088] To investigate this further, a third set of experiments was performed using the same prepreg and HDPE backing sheet arrangement, except this time the amplitude was kept at 17.3 μm and the force was gradually reduced from 1000 N to 600 N. The results are shown in Table 3 below.

[0089] [Table 3] These results further demonstrate that reducing the applied force can result in improved results.

[0090] To investigate this further, a fourth set of experiments was performed using the same prepreg and HDPE backing sheet arrangement, except this time the amplitude was kept at 21.6 μm and the force was gradually reduced from 600 N to 400 N. The results are shown in Table 4 below.

[0091] [Table 4] Since the maximum score for Polyscore is 300, Examples 22 and 23 are essentially the best achievable results.

[0092] The reason Example 20 did not weld successfully was believed to be due to heat generation within the thermoset. Details of this example are shown in Figure 3a. The sudden increase in applied power is clearly visible during the process, which is believed to be the result of a runaway increase in the temperature of the thermoset, resulting in an uncontrolled viscosity drop rather than a smooth, gradual viscosity drop.

[0093] To further investigate the robustness of this combination of amplitude and force, an additional fifth set of experiments was performed using the same prepreg and HDPE backing sheet configuration, except this time the amplitude was kept at 21.6 μm, the force was maintained at 425 N, and the weld time was increased from 1.0 to 3.0 seconds. The results are shown in Table 5 below.

[0094] [Table 5] As can be seen, regardless of the time it took to form the weld, the weld produced was of excellent quality.

[0095] To investigate this further, a sixth set of experiments was performed using the same prepreg and HDPE backing sheet arrangement, except this time the amplitude was kept at 21.6 μm and the force was gradually reduced from 400 N to 325 N. The results are shown in Table 6 below.

[0096] [Table 6] As can be seen, excellent and consistent results were obtained in all examples. Representing the best results obtained, Example 31 is detailed in Figure 3b. It clearly shows a first phase where a change in power occurs as the resin in the prepreg layers begins to warm. This is followed by a distinct second phase where the temperature stabilizes and the thickness of the pre-spliced ​​joint decreases uniformly.

[0097] Mechanical testing A study was conducted to evaluate the mechanical performance of cured laminates of spliced ​​prepreg tapes. Splices were performed using a conventional hot press method (comparative example), ultrasonic welding of the prepreg layers alone (comparative example), and simultaneous welding of the prepreg layers and polyethylene backing sheet (inventive example).

[0098] The resulting excess width was carefully cut off with a knife to ensure that the width of the spliced ​​prepreg tape was uniform.

[0099] The material used for the prepreg was 34% by weight of resin system M21EV containing 0.2 inch (5.08 mm) wide 268 gsm 12k unidirectional carbon fibre (medium modulus type A) (individual components available from Hexcel Composites, UK).

[0100] After splicing, the backing sheet was removed, and a laminate for tensile testing was assembled from four 300 x 300 mm layers of tape. The second layer contained a 30 mm long overlapping splice in the center. All prepreg tapes in each layer were parallel to each other. The first, third, and fourth layers were prepreg tape without a splice. Each layer consisted of six parallel strips of prepreg tape. The laminate was cured in an autoclave at 180°C for 2.5 hours (ramp rate 1°C / min) to form a cured composite laminate.

[0101] Three composite laminates were prepared in this manner: one with a layer of hot-pressed spliced ​​prepreg tape (Series A), one with a layer of ultrasonically welded prepreg tape (without a polymer backing sheet, Series B), and one with a layer of ultrasonically welded prepreg tape (without a polymer backing sheet, Series C). Each composite laminate was then cut into six strips, each consisting of four layers of the original prepreg tape. The tensile strength of each strip was measured according to the EN2561B test method. The results of the tensile strength tests are shown in Table 7 below.

[0102] [Table 7] In tensile tests, the samples containing ultrasonic splices exhibited higher strength: specifically, the sample containing only tape-based ultrasonic splices exhibited a tensile strength of 106% of the reference (100%), while the sample of the present invention exhibited a tensile strength of 111%.

[0103] A second batch of laminates for flexural testing was prepared from eight 200 x 150 mm tape layers. Layer 4 contained a 30 mm long splice in the center. All prepreg tapes in each layer were parallel to each other. Layers 1 through 3 and 5 through 8 were spliceless prepreg tapes. Each layer contained three parallel strips of prepreg tape. The laminates were cured in an autoclave at 180°C (ramp rate 1°C / min) for 2.5 hours to yield cured composite laminates.

[0104] Three composite laminates were prepared in this manner: one with a layer of hot-pressed spliced ​​prepreg tape (Series A), one with a layer of ultrasonically welded prepreg tape (without a polymer backing sheet, Series B), and one with a layer of ultrasonically welded prepreg tape (without a polymer backing sheet, Series C). Each composite laminate was then cut into six strips, each consisting of eight layers of the original prepreg tape. The tensile strength of each strip was measured according to the EN2562 test method. The results of the tensile strength tests are shown in Table 8 below.

[0105] [Table 8] In bending tests, the samples containing ultrasonic seams exhibited lower strength. Lower bending strength is considered advantageous because it indicates less distortion of the laminate. The thickness of the hot-pressed seams was not significant because they were approximately twice as thick as the ultrasonically welded seams and therefore far inferior.

Claims

1. A process for forming a spliced ​​prepreg tape by joining adjacent ends of a first and a second prepreg tape, Each prepreg tape comprises first and second prepreg layers containing fibrous material and thermosetting resin, and each prepreg tape further comprises first and second peelable polymer backing sheets. The process includes the following steps: A process comprising: peeling a first polymer backing sheet from a first prepreg layer to provide a first peeled-edge region; optionally peeling a second polymer backing sheet from a second prepreg layer to provide a first peeled-edge region and optionally a second peeled-edge region, wherein the prepreg layers are separated from their respective polymer backing sheets in the peeled-edge regions; A step of overlapping the first and second prepreg tapes at the first peel-off end region and, if present, the second peel-off end region to form a preliminary joint, wherein the preliminary joint includes the first and second adjacent prepreg layers and the first and second adjacent polymer backing sheet layers; A step of forming a spliced ​​prepreg tape by ultrasonically welding the aforementioned pre-spliced ​​joint portion to fuse the first and second adjacent prepreg layers and to fuse the two adjacent polymer backing sheet layers.

2. The process according to claim 1, wherein the polymer backing sheet is high-density polyethylene.

3. The process according to claim 1, wherein the thickness of the joint portion is 50% to 95%, preferably 70% to 90%, of the thickness of the pre-joint portion.

4. The process according to claim 1, wherein the length of the pre-jointed portion is 3 mm to 200 mm, more preferably 10 mm to 150 mm, and most preferably 15 mm to 75 mm.

5. The process according to claim 1, wherein the first and second prepreg tapes have the same width.

6. The process according to claim 1, wherein the first and second prepreg tapes have the same thickness.

7. The process according to claim 1, wherein the widths of the first and second prepreg tapes are less than 30 cm, preferably less than 20 cm, and more preferably less than 10 cm.

8. The process according to claim 1, wherein ultrasonic welding causes the prepreg layer to spread laterally, resulting in an excess width that extends beyond the backing sheet, and the process is followed by a step of cutting off the excess width after the joint portion has been formed.

9. The process according to claim 1, wherein the end of the prepreg tape is a straight end.

10. The process according to claim 1, wherein the steps of peeling, overlapping, and ultrasonic welding are performed automatically without requiring manual intervention.

11. The process according to claim 10, wherein the automation includes the step of applying suction to the prepreg tape in order to cause the peeling, preferably also to cause the overlapping.

12. The process according to claim 1, wherein only the first backing sheet is peeled off to form a first peeled-off end region, and the end of the second prepreg tape is inserted into the peeled-off end region, so that the preliminary joint portion includes the first polymer backing sheet, the first polymer backing sheet is adjacent to the second polymer backing sheet, the second polymer backing sheet is adjacent to the second prepreg layer, and the second prepreg layer is adjacent to the first prepreg layer.

13. The process according to claim 1, wherein the first backing sheet and the second backing sheet are peeled to form a first peeled end region and a second peeled end region, and the first peeled end region and the second peeled end region are inserted into each other so that the pre-jointed portion includes the first polymer backing sheet, the first polymer backing sheet is adjacent to the second polymer backing sheet, the second polymer backing sheet is adjacent to the first prepreg layer, and the first prepreg layer is adjacent to the second prepreg layer.

14. The process according to claim 1, The prepreg tape manufacturing process is performed first, in which one layer of fiber material and at least one layer of thermosetting resin are brought together, the thermosetting resin is impregnated, the thermosetting resin enters the gaps between the fibers, and first and second prepreg tapes are manufactured without slitting the master prepreg tape along its length. process.

15. The resulting first and / or second prepreg tapes are cut along the width on both sides of the defects in the manufactured tape before the subsequent splicing process. The process according to claim 14.

16. The process according to claim 14, wherein the manufacturing process and the subsequent splicing process are part of a single continuous prepreg tape manufacturing process.

17. A spliced ​​prepreg tape obtained by the process described in any one of claims 1 to 16.

18. A method for forming a composite material by laying prepreg tape, The prepreg tape comprises a prepreg layer and a polymer backing sheet. The method includes the steps of removing the polymer backing sheet and laying the prepreg layer using an automatic tape laying device. The prepreg tape is the spliced ​​prepreg tape according to claim 17. method.

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