WO2019148084A1 - Tackifier for resin infusion family processes - Google Patents

Abstract

A process of forming a composite structure by providing a plurality of layers having one or more of a fiber or a fabric; applying a tackifier to each of the layers, the tackifiers comprising sPEKK in amount selected to on provide tackification for the lay-up process and to serve as a release agent; assembling a preform with the plurality of layers; infusing the preform with resin.

Description

UNITED STATES PATENT APPLICATION

Of

Scott DeFelice

60 Cleveland Street

Holyoke, MA 01040

and

Anthony DeCarmine

681 Beaumont Highway

Lebanon, CT 06249 for

TACKIFIER FOR RESIN INFUSION FAMILY PROCESSES

Attorney for Applicants Walter B. Welsh, Registration No. 64,424

WHITMYER IP GROUP LLC

600 Summer Street Stamford, CT 06901 203 703-0800 TACKIFIER FOR RESIN INFUSION FAMILY PROCESSES

TECHNICAL FIELD

[0001] The present invention generally relates to the field of resin transfer molding (“RTM”) processes and vacuum assisted resin transfer molding (“VARTM”) processes used for molding composite articles, and more particularly relates to advanced vacuum assisted resin transfer molding (“A- VARTM”) processes. The invention also relates to carbon fiber reinforced plastics prepared using prepreg / autoclave processes.

BACKGROUND

[0002] There are many industries producing fiber-reinforced resin composite parts. For instance, composite parts are commonly used in the automotive, marine, industrial, and aerospace industries.

[0003] Carbon fiber reinforced plastics (CFRP) comprising carbon fibers (CF) and plastics such as an epoxy resin are lightweight and excellent in mechanical properties. Therefore, they have been used in wide applications including aircraft parts, automotive parts, building structures and sporting goods.

[0004] For fabricating CFRP, various processes can be selected.

Among them, prepreg / autoclave process shown in Figure 5A have been a major process to fabricate high performance CFRP adequate for aircraft structures.

[0005] Another commonly known method is the resin transfer molding (“RTM”) process in which reinforcing materials (e.g. glass fibers, carbon fibers, etc.) are placed into a closed mold and then impregnated at high pressure (e.g. 400 psi and higher) with a liquid matrix (e.g. a polymer resin). 2

[0006] In a variant of the RTM process, the closed mold is put under vacuum prior to the injection, at atmospheric pressure, of the matrix to impregnate the reinforcing materials. Such a process is generally known as a vacuum assisted resin transfer molding (“LIGHT RTM”) or (“VARTM”) process. The VaRTM process is illustrated in Figure 5B. It has the advantage to fabricate CFRP of large scale and complex shape at lower cost.

[0007] In line with the VARTM process is the advanced VARTM (“A- VARTM”) process. In A-VARTM process, the mold is usually open and light weight compared to other RTM or VARTM processes. To compress the layers of reinforcement materials on a complex mold shape, a flexible vacuum bag is used. When the bag is put under vacuum, the atmospheric pressure insures the proper compaction of the reinforcing materials and removes air in the bag. After impregnation of the reinforcing fibers with the matrix, the pressure on the bag becomes neutral.

[0008] Often used in this process are varieties of tackifiers, often thermoplastics, that are used to assist densification of the dry fiber preform by tacking fibers and fiber layers into place prior to infiltration of curing resins.

The use of tackifiers enhances the density of the fiber preform by keeping otherwise loose fibers in close proximity, thereby leading to a higher performing composite structure. This disclosure generically uses the term enabler to refer to an additive used in the resin transfer process.

[0009] In addition to tackifiers, other additives such as processing and handling aids are applied to the dry fiber preforms for a variety of different purposes. Although additives are generally used in relatively low quantity by weight compared to resins, reinforcements and fillers, they perform critical functions. While additives and modifiers often increase the cost of the basic material system, these materials always improve cost/performance.

[0010] There are a number of additives that are used to modify and enhance resin properties that become a part of the polymer matrix. These - 3 - additives include: thixotropes, pigments & colorants, fire

retardants, suppressants, uv inhibitors & stabilizers, conductive additives, and release agents.

[0011] Release agents facilitate removal of parts from molds. These products can be added to the resin, applied to molds, or both. Zinc stearate is a popular mold release agent that is mixed into resin for compression molding. Waxes, silicones and other release agents may be applied directly to the surface of molds. The release agents are applied to inhibit the composite from adhering to the mold after the infusion process. To facilitate releasing the finished component after curing, the mold or outside of the composite is usually coated with a release agent such as a suitable wax so that the matrix material does not bond with the mold, which would make it effectively impossible to remove the component from the mold without damaging either one. The release agent is applied to the mold and or composite during the laying up process. Known release agents are polyvinyl alcohol, silicone wax, slip wax, etc. The release agent must be applied in a uniform thickness in order to ensure a smooth outer surface of the hardened component.

However, it is not easy to apply the release agent so that these requirements are met, and, if improperly applied, an uneven release agent layer can result in an uneven or dimpled surface. Furthermore, the types of release agent generally used contain volatile solvents, which pose a health risk to anyone exposed to them. Another main disadvantage of having to use such a release agent is that, after curing, hardened remnants of the release agent can adhere composite.

[0012] A disadvantage of known systems and methods is that the cost is increased by requirement for multiple additive systems, such as a tackifiers, stabilizers, and additives.

[0013] Another disadvantage of known systems and methods is that multiple additive systems in the resin infusion process create complexity and - 4 - the opportunity for failure from unforeseen interactions between the various additive systems, which are generally serving conflicting purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a schematic representation of a dry preform;

[0015] FIG. 2 is a schematic representation of the preform of FIG. 1 with interlaced toughener/tackifier between layers of the preform;

[0016] FIG. 3 is a schematic representation of a composite having the preform with the interlaced toughener/tackifiers between the layers of the preform and infused with resin.

[0017] FIG. 4 is a schematic representation of the process used to form the composite of FIG. 3.

[0018] FIG. 5A is a schematic of a prepreg / autoclave process.

[0019] FIG. 5B is a schematic of a VARTM process.

[0020] FIG. 6 is a schematic of a portion of the VARTM process shown in FIG. 5B

DETAILED DESCRIPTION

[0021] Referring now to FIG. 1 , the process described herein makes use of dry reinforcements/preforms 10. The preforms 10 may have two outer layers 14 and one or more intermediate layers 14. The dry

reinforcement/preforms 10 include, but are not limited to, layers 14 formed from one or more of tow, woven fabric, unidirectional fabric, braid, fiber, or 3D woven preforms. For example, the layers 14 of the preform 10 could be formed from carbon fibers, Fiberglass® E-or S glass fibers, Quartz, or Silicon Carbide (inorganic material in a fibrous condition or in the form of a loose mass of filaments or fibers), and/or Kevlar® fibers (man-made fibers). - 5 -

[0022] In accordance with the process described herein, the fiber or fabric preform is tackified or coated with sPEKK. To obtain sPEKK, a PEKK polymer is functionalized. In functionalizing the polymer, certain chemical groups are added to the basic structure of the molecule. The functionalization step alters the wetting and solubility behaviors of the polymer. For example, by functionalizing a PEKK family material, it is possible to make the material water soluble. The functionalized PEKK polymer can be blended with water to form a sizing composition.

[0023] An example of a functionalization for use with the present invention includes, but is not limited to sulfonation. Sulfonation of PEKK material results in a PEKK material that is soluble in water. It is possible to vary functionalization of the PEKK material so as to vary the solubility. In some embodiments, the functionalized PEKK material is soluble to the point of deliquescence. The functionalization step can be performed on the PEKK material after polymerization or in the synthetic path. After the PEKK material is functionalized, it is blended with water to form the sizing composition. It is possible to vary the viscosity of the sizing composition by adjusting the ratio of functionalized PEKK material to water. PEKK is functionalized to form sPEKK. Suitable polyetherketoneketones are available from commercial sources, such as, for example, certain of the polyetherketoneketones sold under the brand name OXPEKK by Oxford Performance Materials, South Windsor,

Connecticut, including OXPEKK-SP polyetherketoneketone.

[0024] U.S. Publication No. US20040131910 (the‘910 application) discloses a method of producing sPEKK. The disclosure of the‘910 application is incorporated herein in its entirety. US Publication No.

US201 10294943 discloses of method of sizing with PEKK the disclosure of which is incorporated by referenced. US Publication No. US20150274588 discloses a method of sizing with sPEKK, the disclosure of which is incorporated by reference. 6

[0025] The tackifier 12 can be dispersed on the layers 14 of the reinforcement/preform 10 as a powder, in a film form coated on the layers 14 of the reinforcement, or as a veil laminated to the surfaces 40, 42, 44 and 46 of the layers 14 forming the preform 10. The percentage of the tackifier 12 used may depend on the fabric and/or fiber used for the layers 14 of the preform 10. Typically, the tackifier 12 would be present in an amount in the 5 to 10 wt % range based on the areal weight of the preform. The percentage of tackifier can also be changed to put more where it is needed to increase the toughness of the composite and less in areas where toughness is not a requirement. Furthermore, the tackifier 12 may be applied so as to cover all of the length of the surfaces 40, 42, 44, and 46 or so as to only cover selected regions of a respective surface 40, 42, 44, and/or 46, less than the entire length of the respective surface. In some embodiments, the sPEKK is blended with water to form a sizing composition.

[0026] In some embodiments, the tackifiers 12 is dispersed on the layers as a sizing agent.

[0027] Sufficient heat is applied so that the sizing composition dries and creates a solid layer on at least a portion of a surface of the fiber. At this point, the applied sizing composition is still water soluble as before the application because the applied PEKK is functionalized. The applied PEKK preferably forms a relatively thin coating on the layer. For example, the coating may be from about 1 to about 50 microns thick. In one embodiment of the invention, the surfaces are completely covered by the coating, although in other embodiments certain portions of the fiber surfaces remain uncoated. Typically, the coating may comprise from about 0.01 to about 10% by weight of the sized fibers.

[0028] The presence of tackifier 12 helps in laying up a part that has contour. The tackifier 12 can be locally heated during layup to facilitate in - 7 - forming desirable shapes and contours. The tackifier 12 can be locally subject to pressure to facilitate in forming desirable shapes and contours

[0029] Referring now to FIG. 4, once the composite or preform layup is complete and the tackifier 12 has been added, as shown in FIG. 2, the composite with the preform 10 and the tackifier 12 is sealed in a vacuum bag. Flow distribution media may be provided on top and bottom surfaces 18 and 20 of the composite to help improve the permeability in the through thickness direction. The flow distribution media may be any open weave material which will survive the final cure temperature of the resin system, and has a much greater permeability than the preform being infused.

[0030] The vacuum bag can be attached to a heat source such as an integrally heated tool or can be placed into an autoclave or similar vessel.

Main resin 22 may be infused into the preform 10 by drawing a vacuum on the fiber and/or fabric layup. Once the preform 10 fills, the main resin 22 appears in the outlet line, which is then closed along with the main resin inlet. The composite is heated to a temperature that increases the main resin viscosity to a level that is higher than typical for infusion but is still low enough to allow the plies/layers 14 of the composite to be consolidated under pressure.

External pressure may be applied to the composite, and the main resin outlet from the bag is opened. The opening of the bag allows residual main resin or entrapped gas to escape the composite. The external pressure may be in the range of low pressures from 50 psi to 100 psi to high pressures in the range of thousands of psi. The external pressure causes compaction of the composite to the target fiber volume. The composite is then heated up to the cure temperature.

[0031] The composite, as shown in FIG. 3, may comprise layers 14 of fibers and/or fabric, the tackifier 12 joined to the layers 14, and layers 22 of main resin intermediate the layers 14 with the tackifier 12. 8

[0032] Fiber volumes in excess of 60% and void volumes less than 2% may be achieved using the process described herein. This makes the process acceptable for structural composite components that require impact resistance and damage tolerance. For example, the process described herein could be used to form lighter weight fan containment cases.

[0033] As can be seen from the foregoing discussion, by selecting an appropriate level of the tackifier, the properties of the overall composite performance can be tailored. Segments of the part can be locally toughened to increase damage tolerance, while other areas that do not need this requirement can be unmodified. This can be done by providing the tackifier 12 in selected portions of the preform 10 where toughening or strengthening is required. By having a functionally graded preform, a designer can achieve a mechanical performance while optimizing the structure for weight.

[0034] The main resin 22 may comprise any suitable thermoset resin matrix type known in the art including, but not limited to, an epoxy resin, a bismaleimide resin, a polyimide resin, and mixtures thereof.

[0035] Some embodiments of the present invention include the step of heating the applied sizing composition to a temperature at or above the defunctionalization temperature of the PAEK polymer, for example in reference to sPEKK to between 300°C - 400°C. This heating causes the functional groups to detach from the PAEK polymer, resulting in an applied sizing composition that is less water soluble. In this way, it is possible to recover the desirable solvent resistant behaviors in the original polymer after it has been applied to the fiber.

[0036] In some embodiments, the applied sizing composition is heated to a temperature at or above the fusion point of the PAEK polymer, thereby enhancing the bonds of certain systems. 9

[0037] In yet other embodiments of the present invention, the applied tackifiers is subjected to heat and pressure prior to assembly of the preform. The inventors have discovered that an sPEKK-based tackifier enables the formation of near net thickness and net shape rigid preforms by the

simultaneous application of heat and pressure (either mechanically or by vacuum) that can then be assembled into complex components prior to resin infusion by either vacuum or closed mold high pressure resin transfer allowing for the fabrication of highly complex structural composite products.

[0038] In yet other embodiments of the present invention a non-woven veil is place between layers of the preform. The non-woven veil may serve as a tackifier. The non-woven veil is fabricated using sPEKK as a feedstock. It is known that very high temperature thermoplastics are difficult to form into non- wovens, especially thin non-wovens, due to oxidation at high temperatures or viscosity issues typical to such resins. The inventors have discovered that the use of sPEKK as a feedstock to make very fine non-woven veil provides a workaround for these issues.

[0039] As sPEKK is soluble in water and other innocuous solvents, it can be prepared to various solution viscosity options and processed variously.

[0040] In one embodiment of the invention a fiberizing method is used prepare the sPEKK veil. This approach uses a pneumatic fiberizing system, in the manner of systems supplied by Nordson - typically for hot glue

application, to deposit a web either directly onto a fiber preform or onto a transfer carrier sheet or film. Such systems are capable to generate highly regular patterns, highly random patterns or something in-between. Generally, a very thin layer of evenly distributed liquid is the goal. During the process, the sPEKK must be dried and possibly defunctionalized / fused. Use of warm/hot dry air in the fiberizer will drive the fibers to a more dry state (preferably not entirely dry as at last some tack may be desired to form the non-woven), but a heat set will be required. Said heat may be applied by any typical means. 10

[0041] In one embodiment of the present invention an alternate manner of preventing the soluble sPEKK to be re-dissolved requires the use of a saturated solution carrying an available metal ion (group I or II metals have been used but this is no limitation) to form the acid salt from the acid form of sPEKK. This has the effect of reducing or eliminating solubility without thermal process or defunctionalizing the polymer. Swelling may occur during the process and is needed to carry the metal to the acid locations, but this can be subsequently dried post process.

[0042] In one embodiment of the present invention electrospinning is used to form the sPEKK veil. Electrospinning is the formation of nano-scale fiber bodies (as opposed to the formation of micron scale fiber bodies with a fiberizer). This known process uses an intense electrical field to assist the formation of ultra-fine fibers. A solution is ejected from a small opening as a liquid jet - this jet is engaged by the intense electrical field in the spinning machine and pulled faster than the ejection velocity, resulting in a pulling effect, drawing the fiber smaller than a physical orifice could be made.

Submicron fibers are readily made this way. The impact location for the drawn fiber is usually one of the electrodes creating the field - which may be stationary, rotating or traversing such that the deposit is an evenly distributed nano-structured fiber veil. This process is aided by the use of volatile solvents (which may include water, if the temperature and pressure are appropriate) that allow the fiber to deposit dry or nearly so, as with the prior method. Post- processes are similar.

[0043] Veil made by these routes are useful for other purposes.

Surgical mesh is one such alternate use, as is filtration media and structures for spacecraft - the latter possibly benefitting from an applied metal coating to protect the veil from ionizing radiation, heat, atomic oxygen and so forth.

Enhanced reflection would make such a space veil more effective for solar sails or laser propelled vehicles. - 1 1

[0044] A blend of sPEKK and pinning aids (like CNT or Halloysite) can also be applied to create a composite PEKK/CNT interface layer with a thickness measured in microns and rich in pinning aids to further the usefulness of the system by enhancing in-service performance. The application may be a contiguous layer or a sprayed pattern to prevent occlusion of the prepreg (if that is desired).

[0045] In yet other embodiments, the present invention is applicable in the prepreg /autoclave process illustrated in FIG. 5A. Laminates structures are rate limited, in general, by their inter-laminar strength. Various approaches have been attempted to improve this inter-laminar behavior. Sometimes, tiny pinning inclusions like CNT are included into the matrix resin to provide added engagement at the interface. This mode of addition cannot provide the expensive additives to the desired location - selectively introducing to the surface of the prepreg is preferable.

[0046] Applying such pinning additives to the intended interface surfaces can be difficult. Materials like CNT cannot be safely handled as raw, dry product, suggesting the use of a carrier material. This carrier is desirably very thin, at most a few percent of the thickness of the prepreg. In addition, any such carrier must not interfere with the adhesion of layers - preferably the carrier must be an adhesive of at least the holding power of the laminate or must obscure only a very small fraction of the interlaminar area.

[0047] A carrier material that suits is sPEKK. sPEKK is water soluble derivative of the high performance polymer PEKK. Pinning additives like CNT or Halloysite are readily blended into such aqueous solutions. sPEKK can be converted to PEKK thermally. PEKK is a known strongly adhesive material used in laminate systems and is affine for various high temperature thermoplastics.

[0048] An sPEKK-based tackifier enables the formation of near net thickness and net shape rigid performs by the simultaneous application of 12 heat and pressure (either mechanically or by vacuum) that can then be assembled into complex components prior to resin infusion by either vacuum or closed mold high pressure resin transfer allowing for the fabrication of highly complex structural composite products.

[0049] A blend of sPEKK and pinning aids can be applied to a prepreg tape, the tape can be heated (locally or bulk) thru the conversion temperature to create a composite PEKK/CNT interface layer with a thickness measured in microns and rich in CNT. The application may be a contiguous layer or a sprayed pattern to prevent occlusion of the prepreg (if that is desired).

[0050] A similar process may be affected against an appropriate carrier substrate and transferred to a prepreg.

[0051] It should be apparent from the foregoing detailed description that the objects set forth hereinabove have been successfully achieved.

Moreover, while there is shown and described present preferred embodiments of the invention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

- 13 - What is claimed is,

1. A preform assembly for use in VARTM or A-VARTM, comprising, a plurality of layers formed from at least one of a fiber and a fabric, the plurality of layers defining an outer surface of the preform and further defining a plurality of opposing surfaces between each of the plurality of layers;

a tackifier applied to the preform assembly on at least of portion of the outer surface of the preform assembly and on at least a portion of the opposing surfaces, the tackifier applied in an amount sufficient to maintain the layers of the preform together during the remainder of molding process and in amount sufficient to serve as a release agent for the preform,

wherein the tackifier comprises sulfonated polyetherketoneketone (sPEKK).

2. The preform assembly of claim 1 , the sPEKK is selected so that is desulfonated during a heat treatment of the preform.

3. The preform assembly of claim 1 , wherein the tackifier consists essentially of sPEKK at all time priors to a heat treatment of the preform that is sufficient for desulfonation of the sPEKK.

4. The perform assembly of claim 3, wherein the tackifier ties opposing layers of the preform after the resin transfer process.

5. A process of forming a composite structure comprising the steps of providing a plurality of layers comprising one or more of a fiber or a fabric;

applying a tackifier to each of the layers, the tackifiers comprising sPEKK in amount selected to on provide tackification for the lay-up process and to serve as a release agent;

assembling a preform with the plurality of layers; - 14 - infusing the preform with resin.

6. The process of claim 5, wherein said tackifier applying step comprises applying said tackifier to selected locations on the layers, which selected locations are each shorter than a length of each layer.

7. The process of claim 5, wherein said plurality of layers includes two outer layers and at least one intermediate layer and said tackifier applying step comprises applying said tackifier to an outer surface of the two outer layers, an inner surface of each of said outer layers, and to two opposed surfaces of said at least one intermediate layer.

8. The process of claim 5, wherein said infusing step comprises infusing said preform with a thermoset resin matrix.

9. The process of claim 1 , wherein said infusing step comprises infusing said preform with a resin selected from the group consisting of an epoxy resin, a bismaleimide resin, a polyimide resin, and mixtures thereof.

10. The process of claim 8, further comprising the step of heating the preform to a temperature at or above the defunctionalization temperature of the applied sPEKK.

1 1. The process of claim 10, wherein the functional groups detach from the sPEKK in the preform.

12. The process of claim 1 1 , wherein said infusing step comprises,

sealing said preform with said applied tackifier in a vacuum bag;

placing the vacuum bag in contact with a heat source;

infusing the main resin into the preform with the applied tackifier by drawing a vacuum on the preform with the applied tackifier; - 15 - closing the flow of the main resin when said preform is filled;

heating the preform with the applied tackifier and the infused main resin to a temperature that increases resin viscosity and allows the layers of the preform to be consolidated under pressure;

applying external pressure to the preform; and

opening the vacuum bag to allow residual main resin or entrapped gas to escape.

13. A process of forming a composite structure comprising the steps of providing plurality of layers comprising one or more of a fiber or a fabric;

applying a sizing composition to the plurality of layers, the sizing composition comprising sPEKK and water;

assembling a preform with the plurality of layers.

14. The process of claim 13, wherein the sizing composition further comprises a pinning aid.

15. The process of claim 14, wherein the pinning aid ties adjacent layers in the preform.

16. The process of claim 15, wherein the pinning aid comprises one or more of carbon nanotubes or Halloysite nanotubes.

17. The process of claim 16, wherein the plurality of layers comprise a prepreg material.

18. The process of claim 14, wherein said applying step comprises applying said sizing composition to selected locations on the layers, which selected locations are each shorter than a length of each layer. - 16 -

19. The process of claim 18, wherein said plurality of layers includes two outer layers and at least one intermediate layer and said sizing composition applying step comprises applying said sizing agent to an outer surface of the two outer layers, an inner surface of each of said outer layers, and to two opposed surfaces of said at least one intermediate layer.

20. The process of claim 13, wherein said infusing step comprises infusing said preform with a thermoset resin matrix.