Patent Publication Number: US-2022212420-A1

Title: Multi-compound fiber reinforced composites and methods of making the same using frontal polymerization and targeted photosensitizer additives

Description:
INTRODUCTION 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     Light-weight polymeric components, such as reinforced composite materials, have been considered for use as structural and load-carrying components in vehicles. Often such polymeric materials are manufactured by compression molding. However, compression molding, and other similar approaches, to the manufacture of structural composites can be time and energy intensive. Accordingly, it would be desirable to develop methods of preparing reinforced composite materials that are lower in cost and reduce or improve time and energy requirements necessary during the manufacturing process. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     The present disclosure relates to multi-compound fiber reinforced composites and methods of making the same using frontal polymerization and targeted photosensitizer additives. 
     In various aspects, the present disclosure relates to a method for forming a fiber-reinforced composite. The method may include disposing one or more layers in a mold cavity, where each of the one or more layers includes a fiber material and a first compound. The method may further include disposing a second compound in the mold cavity, where the second compound includes a photosensitizer material. Further still, the method may include initiating photopolymerization of the photosensitizer using an ultraviolet light source, removing ultraviolet light source, and completing polymerization of the one or more layers so as to form the fiber-reinforced composite. 
     In one aspect, disposing the one or more layers may include disposing the fiber material in the mold cavity and infusing the fiber material with the first compound. 
     In one aspect, the fiber material may include a first fiber material and a second fiber material, and the first compound may include a first composition and a second composition. 
     In one aspect, disposing the one or more layers may include disposing the first fiber material in the mold cavity, infusing the first fiber material with the first composition, disposing the second fiber material in the mold cavity, and infusing the second fiber material with the second composition. 
     In one aspect, the first and second fiber materials may be the same or different. 
     In one aspect, the first and second compositions may be the same or different. 
     In one aspect, the fiber material may be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers, ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, and combinations thereof. 
     In one aspect, the first compound may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of a thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of a monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of a cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of a diluent. 
     In one aspect, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In one aspect, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     In one aspect, the cationic photoinitiator may be selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     and combinations thereof. 
     In one aspect, the diluent may be selected from the group consisting of: polyfunctional glycidyl ethers, monofunctional aliphatic glycidyl ethers, monofunctional aromatic glycidyl ethers, 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), and combinations thereof. 
     In one aspect, the fiber material may be a first fiber material and the method may further include disposing a second fiber material in the mold cavity on or adjacent to the one or more layers. 
     In one aspect, disposing the second compound may include infusing the second fiber material with the second compound. 
     In one aspect, the second compound may include greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of a photosensitizer material. 
     In one aspect, the photosensitizer material may be selected from the group consisting of: anthracene, perylene, benzophenone, 9,10-diethoxyanthracene, 2,2-dimethoxy-1,2-diphenylethanone, 2-isopropylthioxanthone (ITX), and combinations thereof. 
     In one aspect, the second compound may further include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of a thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of a monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of a cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of an optional diluent. 
     In one aspect, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In one aspect, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     In one aspect, the cationic photoinitiator may be selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     and combinations thereof; and 
     In one aspect, the diluent may be selected from the group consisting of: polyfunctional glycidyl ethers, monofunctional aliphatic glycidyl ethers, monofunctional aromatic glycidyl ethers, 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), and combinations thereof. 
     In one aspect, the method may further include removing the fiber-reinforced composite from the mold cavity. 
     In various aspects, the present disclosure provides a method for forming a fiber-reinforced composite. The method may include disposing a second compound include a photosensitizer material in a mold cavity. The mold cavity may include one or more layers, and each of the one or more layers may include a fiber material and a first compound. The method may further include initiating photopolymerization of the sensitizer using an ultraviolet light source, removing ultraviolet light source, and completing polymerization of the one or more layers so as to form the fiber-reinforced composite. 
     In one aspect, the fiber material may be a first fiber material and the method may further include disposing a second fiber material in the mold cavity on or adjacent to the one or more layers. 
     In one aspect, disposing the second compound may include infusing the second fiber material with the second compound. 
     In one aspect, the method may further include disposing the one or more layers in the mold cavity. 
     In one aspect, disposing the one or more layers may include disposing the fiber material in the mold cavity, and infusing the fiber material with the first compound. 
     In one aspect, the fiber material may include a first fiber material and a second fiber material. The first compound may include a first composition and a second composition. 
     In one aspect, the method further includes disposing the one or more layers in the mold cavity. 
     In one aspect, disposing the one or more layers may include disposing the first fiber material in the mold cavity, infusing the first fiber material with the first composition, disposing the second fiber material in the mold cavity, and infusing the second fiber material with the second composition. 
     In one aspect, the first and second fiber materials may be the same or different, and the first and second compositions may be the same or different. 
     In one aspect, the second compound may include greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of a photosensitizer material; greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of a thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of a monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of a cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of an optional diluent. 
     In one aspect, the first compound may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of a thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of a monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of a cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of an optional diluent. 
     In various aspects, the present disclosure provides a fiber-reinforced composite. The fiber-reinforced composite may include one or more layers, where each of the one or more layers includes a fiber material and a first compound. The fiber-reinforced composite may further include a second compound disposed on or adjacent to the one or more layers. The second compound may include a photosensitizer. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a cross-section illustration of an example fiber-reinforced composite prepared in accordance with various aspects of the current technology; 
         FIGS. 2A-2J  illustrate an example method for forming a fiber-reinforced composite in accordance with various aspects of the current technology; 
         FIGS. 3A-3I  illustrate another example method for forming a fiber-reinforced composite in accordance with various aspects of the current technology; 
         FIGS. 4A-4G  illustrate another example method for forming a fiber-reinforced composite in accordance with various aspects of the current technology; 
         FIGS. 5A-5F  illustrate another example method for forming a fiber-reinforced composite in accordance with various aspects of the current technology; 
         FIG. 6  illustrates an example pressure adding process for use in the formation of a fiber-reinforced composite in accordance with various aspects of the current technology; and 
         FIG. 7  illustrates another example pressure adding process for use in the formation of a fiber-reinforced composite in accordance with various aspects of the current technology. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, elements, compositions, steps, integers, operations, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Although the open-ended term “comprising,” is to be understood as a non-restrictive term used to describe and claim various embodiments set forth herein, in certain aspects, the term may alternatively be understood to instead be a more limiting and restrictive term, such as “consisting of” or “consisting essentially of” Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and/or process steps, the present disclosure also specifically includes embodiments consisting of, or consisting essentially of, such recited compositions, materials, components, elements, features, integers, operations, and/or process steps. In the case of “consisting of,” the alternative embodiment excludes any additional compositions, materials, components, elements, features, integers, operations, and/or process steps, while in the case of “consisting essentially of,” any additional compositions, materials, components, elements, features, integers, operations, and/or process steps that materially affect the basic and novel characteristics are excluded from such an embodiment, but any compositions, materials, components, elements, features, integers, operations, and/or process steps that do not materially affect the basic and novel characteristics can be included in the embodiment. 
     Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated. 
     When a component, element, or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other component, element, or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially or temporally relative terms, such as “before,” “after,” “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures. 
     Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. For example, “about” may comprise a variation of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in certain aspects, optionally less than or equal to 0.1%. 
     In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges. 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
       FIG. 1  is a cross-section illustration of an example fiber-reinforced composite  100 . The fiber-reinforced composite  100  includes a plurality of rows or layers. The plurality of rows or layers includes one or more first layers  140  and at least one second layer  146 . For example, as illustrated, the fiber-reinforced composite  100  may include seven stacked first layers  140  and a second layer  146  disposed on or adjacent to an exposed surface of a first end of the stack of first layers  140 . Each of the first layers  140  includes a first fiber material  120  and a first compound  160 . The second layer  146  includes a second fiber material  126  and a second compound  166 . 
     The first and second fiber materials may be the same or different. In certain variations, the first and second fiber materials  120  may each includes one or more short or continuous fibers. The continuous fibers may be woven (e.g., twill weaved, 5 harness satin, 8 harness satin), non-crimp fabrics, or unidirectional, also including fibers, fiber tows, and fiber tapes. For example, the first fiber material  120  and the second fiber material  126  may each be independently selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     In certain variations, the first compound  160  includes a thermal initiator and a monomer. In other variations, the first compound  160  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the first compound  160  optionally includes a diluent. For example, the first compound  160  includes greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     The second compound  166  includes a photosensitizer. For example, in certain variations, the second compound  166  includes a thermal initiator, a monomer, and a photosensitizer. In other variations, the second compound  166  includes a thermal initiator, a monomer, a cationic photoinitiator, and a photosensitizer. In each instance, the second compound  166  optionally includes a diluent. For example, the second compound  166  includes greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of the photosensitizer; greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     The thermal initiator, monomer, and/or cationic photoinitiator of the second compound  166  may be the same or different from the thermal initiator, monomer, and/or cationic photoinitiator of the first compound  160 . 
     In certain variations, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In certain variations, the monomer includes one or more epoxy thermoset resins, such as diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE′ resin 827, and the like. In other variations, the monomer includes one or more open ring-opening polymerization monomers. In still over variations, the monomer includes non-cyclic monomers, such as vinyl ethers. 
     In each instance, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     The cationic photoinitiator may be suitable for frontal polymerization. For example, in certain variations, the cationic photoinitiator includes one or more photoacid generators (“PAGs”), such as those represented by the following formulas (including the respective trade names and businesses from which such compounds are commercially available, as well as a wavelength of maximum absorbance for UV-Visible Spectroscopy, designated as λ max , where applicable): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     the like, and combinations thereof. 
     In certain variations, the diluent includes polyfunctional glycidyl ethers (such as, HELOXY™ 107, HELOXY™ 48, HELOXY™ 68, and the like), monofunctional aliphatic glycidyl ethers (such as, HELOXY™ 166, HELOXY™ 61, and the like), monofunctional aromatic glycidyl ethers (such as HELOXY™ 62 and the like), 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), the like and combinations thereof. 
     In certain variations, the photosensitizer may be selected from the group consisting of: anthracene, perylene, benzophenone, 9,10-diethoxyanthracene, 2,2-dimethoxy-1,2-diphenylethanone, 2-isopropylthioxanthone (ITX), and combinations thereof. 
     In various aspects, the present disclosure provides methods for forming fiber-reinforced composites (“FRCs”), like fiber-reinforced composite  100 , illustrated in  FIG. 1 . Example methods include disposing one or more fiber materials in a mold cavity and infusing or covering or coating the one or more fiber materials with one or more compounds, where at least one of the one or more compounds includes a photosensitizer. Such methods may further include triggering the photosensitizer so as to induce free radical-induced cationic frontal polymerization (i.e., curing) across the one or more fiber materials. The photosensitizer may be triggered using an ultraviolet light. 
       FIGS. 2A-2J  illustrate an example method for forming a fiber-reinforced composite  290 . The method  200  may be a layer-by-layer process that includes sequentially forming one or more rows or layers  240 ,  242 ,  244 ,  246 , where each layer  240 ,  242 ,  244 ,  246  includes one or more fiber materials  220 ,  222 ,  224 ,  226  and a coating compound  260 ,  262 ,  264 ,  266 . The coating compound  266  of a last layer  246  includes a photosensitizer 
     For example, as illustrated, at  FIG. 2A , the method  200  includes disposing  202  a first fiber material  220  in a mold cavity  232 . In certain instances, the first fiber material  220  may be disposed  202  in the mold cavity  232  using a hand process that is similar to hand layup for pre-impregnated woven materials or dry fabrics (for example, prior to resin infusion) and the like. In other instances, the first fiber material  220  may be disposed  202  in the mold cavity  232  using a robotic process, such as automated tape layup, automated tap placement, automatic fabric layup, automatic fabric placement, and the like. 
     In each instance, the first fiber material  220  defines a first row or layer  240 . Though horizontal rows and layers  240  are illustrated, various other shapes and configurations would be recognized by the skilled artisan, including, for example only, vertical rows. Similarly, the skilled artisan will recognize that that the mold  230  and/or mold cavity  232  may have a variety of other shapes and configurations. In certain variations, the mold  230  may include any material having a low thermal conductivity, including, by way of non-limiting example, steel, aluminum, Invar (FeNi 36 ), austenitic nickel-chromium-based superalloys (such as, INCONEL®), high density tooling foam/board, basic polymers (such as, poly(methyl methacrylate) (PMMA), epoxies, and other thermosets or thermoplastics), glass, and the like. 
     The first fiber material  220  includes one or more short or continuous fibers. The continuous fibers may be woven (e.g., twill weaved, 5 harness satin, 8 harness satin), non-crimp fabrics, or unidirectional, also including fibers, fiber tows, and fiber tapes. For example, the first fiber material  220  may each be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     As illustrated in  FIG. 2B , the method  200  may further include disposing  204  a first compound  260  in the mold cavity  232  so as to infuse or cover or coat the first fiber material  220 . The first compound  260  may be disposed  204  using one or more processes including, by way of non-limiting example, a dropwise addition, a resin infusion process, a rolling process, or the like. Infusion processes may support low volume manufacturing and/or high volume manufacturing using, for example, a high pressure resin transfer molding tool (“HP-RTM”). In certain aspects, a consolidating process can be used that includes pouring the first compound  260  on top of the dry first fiber material  220  and using a roller to spread the first compound  260  across the entire surface of the first fiber material  220 . Such may occur with a thin film placed between the roller and the first compound  260  and the first fiber material  220  so that the roller stays dry of first fiber material  220 . After the rolling is done, the film may be removed and reused for the next layer(s). In other aspects, the first fiber material  220  may be passed through a resin bath. The first fiber material  220  in a resin bath that includes the first compound  260  prior to being disposed in the mold cavity  232 . In each instances, the process may include spreading a first compound  260  on one or more fiber layers (e.g., first fiber material  220 ) and/or placing in a resin bath and disposing in the mold cavity one or more fiber layers (e.g., first fiber material  220 ). 
     In certain variations, the first compound  260  includes a thermal initiator and a monomer. In other variations, the first compound  260  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the first compound  260  optionally includes a diluent. For example, the first compound  260  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     In certain variations, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In certain variations, the monomer includes one or more epoxy thermoset resins, such as diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE′ resin 827, and the like. In other variations, the monomer includes one or more open ring-opening polymerization monomers. In still over variations, the monomer includes non-cyclic monomers, such as vinyl ethers. 
     In each instance, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     The cationic photoinitiator may be suitable for frontal polymerization. For example, in certain variations, the cationic photoinitiator includes one or more photoacid generators (“PAGs”), such as those represented by the following formulas (including the respective trade names and businesses from which such compounds are commercially available, as well as a wavelength of maximum absorbance for UV-Visible Spectroscopy, designated as λ max , where applicable): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     the like and combinations thereof. 
     In certain variations, the diluent includes polyfunctional glycidyl ethers (such as, HELOXY™ 107, HELOXY™ 48, HELOXY™ 68, and the like), monofunctional aliphatic glycidyl ethers (such as, HELOXY™ 166, HELOXY™ 61, and the like), monofunctional aromatic glycidyl ethers (such as HELOXY™ 62 and the like), 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), the like and combinations thereof. 
     As illustrated in  FIG. 2C , the method  200  may further include disposing  206  a second fiber material  222  in the mold cavity  232 . The second fiber material  222  defines a second row or layer  242 . As illustrated, the second layer  242  may be disposed  206  on or adjacent to an exposed surface of the first row  240 . The second fiber material  222  may be the same as or different from the first fiber material  220 . For example, the second fiber material  222  may be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. The second fiber material  222  may be disposed using the same process used to dispose the first fiber material  220 . 
     As illustrated in  FIG. 2D , the method  200  may further include disposing  208  a second compound  262  in the mold cavity  232  so as to infuse or cover or coat the second fiber material  220 . The second compound  262  may be disposed using the same process used to dispose the first compound  260 . 
     The second compound  262  may be the same as or different from the first compound  260 . For example, in certain variations, the second compound  262  includes a thermal initiator and a monomer. In other variations, the second compound  262  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the second compound  262  optionally includes a diluent. Like the first compound  260 , the second compound  262  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     As illustrated in  FIG. 2E , the method  200  may further include disposing  210  one or more other rows or layers  244  in the mold cavity  232 . For example, the method  200  may include subsequently disposing  210  one or more other fiber materials  224  and one or more other compounds  264  in the mold cavity  232 , using method similar to those used to form the first layer  240  and/or the second layer  242 . As illustrated, the method  200  may include disposing  210  five other layers  244  on or adjacent to an exposed surface of the second row  242 . 
     Each of the one or more other fiber materials  224  may be the same or different from the first fiber material  220  and/or the second fiber material  222 . For example, each of the one or more other fiber materials  224  may be independently selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     Similarly, each of the one or more other compounds  264  may be the same or different from the first compound  260  and/or the second compound  262 . For example, each of the one or more other compounds  264  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     As illustrated in  FIG. 2F , the method  200  may further include disposing  212  a final or last fiber material  226  in the mold cavity  232 . As illustrated, the final or last fiber material  226  may be disposed  212  on or adjacent to an exposed surface of the one or more other layers  244  so as to define a last row or layer  246  in the mold cavity  232 . The final fiber material  226  may be the same as or different from the first fiber material  220 , the second fiber material  222 , and/or the one or more other fiber materials  224 . For example, the last fiber material  226  may be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. The final fiber material  226  may be disposed using the same process used to dispose the first fiber material  220 , the second fiber material  222 , and/or the one or more other fiber materials  224 . 
     As illustrated in  FIG. 2G , the method  200  may further include disposing  214  a photosensitizer compound  266  in the mold cavity  232  so as to infuse or cover or coat the final fiber material  226 . The photosensitizer compound  266  may be disposed  214  using the same process used to dispose the first compound  260 , the second compound  262 , and/or the one or more other compounds  264 . 
     In certain variations, the final compound  266  includes a thermal initiator, a monomer, and a photosensitizer. In other variations, the final compound  266  includes a thermal initiator, a monomer, a cationic photoinitiator, and a photosensitizer. In each instance, the final compound  266  optionally includes a diluent. For example, the final compound  266  may include greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of the photosensitizer; greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     The thermal initiator, monomer, and/or cationic photoinitiator of the final compound  226  may be the same or different from the thermal initiator, monomer, and/or cationic photoinitiator of the first compound  260 , as well as the second compound  262  and/or the one or more other compounds  264 . In certain variations, the photosensitizer may be selected from the group consisting of: anthracene, perylene, benzophenone, 9,10-diethoxyanthracene, 2,2-dimethoxy-1,2-diphenylethanone, 2-isopropylthioxanthone (ITX), and combinations thereof. 
     As illustrated in  FIG. 2H , the method  200  may further include using an ultraviolet light source  270  (e.g., UV-LED) so as to initiated photopolymerization  216  of the photosensitizer of the final compound  266 . The ultraviolet light source  270  may have a wavelength around about 365 nm. The ultraviolet light emitted from the ultraviolet light source  270  is unable to penetrate each of the plurality of layers  240 ,  242 ,  244 ,  246 , but is able to initiated the photosensitizer, which may then transfer energy to the cationic photoinitiator, so as to being an exothermic reaction curing of the monomer (e.g., epoxy) so as to form the fiber-reinforced composite  290 . The curing may be aided by the thermal initiator, which helps to keep the reaction progressing through the thickness and length of the plurality of layers  240 ,  242 ,  244 ,  246 . The selection of thermally conducting fibers (e.g., the first fiber material  220 , the second fiber material  222 , the one or more other fiber materials  224 , and/or the last fiber material  226 ), such as carbon fibers, may also help to propagate the thermal front within the plurality of layers  240 ,  242 ,  244 ,  246 . In this manner, a total thickness of the fiber-reinforced composite  290  is not limited by the penetration depth of the ultraviolet light emitted from the ultraviolet light source  270   
     In various aspects, the ultraviolet light source  270  may be positioned at various points relative to the plurality of layers  240 ,  242 ,  244 ,  246 . For example, a single light source  270  placed at the center of a square mold  230  may have a radially expanding cure front, while a single light source  270  placed near the end of the square mold may propagate along the length of the mold  230  as a linear front. In other examples, one or more light sources may be used to accelerate the curing process from different positions. 
     As illustrated in  FIG. 2I , the ultraviolet light source  270  may be removed or turned off  218  after the start of frontal polymerization. In certain aspects, the ultraviolet light source  270  may be moved to another position relative to the mold  230 . Even after the removal of the ultraviolet light source  270 , as illustrated in  FIG. 2J , polymerization continues until polymerization across the plurality of layers  240 ,  242 ,  244 ,  246  is complete and the fiber-reinforced composite  290  is formed. Though not illustrated, in certain variations, the method  200  includes removing the fiber-reinforced composite  290  from the mold  230 . 
       FIG. 3A-3I  illustrate another example method for forming a fiber-reinforced composite  390 . The method  300  may be a layer-by-layer process that includes a fiber free layer. For example, the method  300  may include sequentially forming one or more first rows or layers  340 ,  342 ,  344 , where each of one or more first rows or layers  340 ,  342 ,  344  include a fiber material  320 ,  322 ,  324  and a coating compound  360 ,  362 ,  364 . The method  300  may further include disposing a second row or layer  346  on or adjacent to the one or more first rows or layers  340 ,  342 ,  344 . The second layer  326  includes a photosensitizer. 
     For example, as illustrated, at  FIG. 3A , the method  300  includes disposing  302  a first fiber material  320  in a mold cavity  332 . The first fiber material  320  defines a first row or layer  340 . Though horizontal rows and layers  340  are illustrated, various other shapes and configurations would be recognized by the skilled artisan, including, for example only, vertical rows. Similarly, the skilled artisan will recognize that that the mold  330  and/or mold cavity  332  may have a variety of other shapes and configurations. 
     The first fiber material  320  includes one or more short or continuous fibers. The continuous fibers may be woven (e.g., twill weaved, 5 harness satin, 8 harness satin), non-crimp fabrics, or unidirectional, also including fibers, fiber tows, and fiber tapes. For example, the first fiber material  320  may each be selected from carbon fibers, glass fibers, polyparaphenylene terephthalamide fibers (KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     As illustrated in  FIG. 3B , the method  300  may further include disposing  304  a first compound  360  in the mold cavity  332  so as to infuse or cover or coat the first fiber material  320 . In certain variations, the first compound  360  includes a thermal initiator and a monomer. In other variations, the first compound  360  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the first compound  360  optionally includes a diluent. For example, the first compound  360  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     In certain variations, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In certain variations, the monomer includes one or more epoxy thermoset resins, such as diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE′ resin 827, and the like. In other variations, the monomer includes one or more open ring-opening polymerization monomers. In still over variations, the monomer includes non-cyclic monomers, such as vinyl ethers. 
     In each instance, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     The cationic photoinitiator may be suitable for frontal polymerization. For example, in certain variations, the cationic photoinitiator includes one or more photoacid generators (“PAGs”), such as those represented by the following formulas (including the respective trade names and businesses from which such compounds are commercially available, as well as a wavelength of maximum absorbance for UV-Visible Spectroscopy, designated as λ max , where applicable): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     the like and combinations thereof. 
     In certain variations, the diluent includes polyfunctional glycidyl ethers (such as, HELOXY™ 107, HELOXY™ 48, HELOXY™ 68, and the like), monofunctional aliphatic glycidyl ethers (such as, HELOXY™ 166, HELOXY™ 61, and the like), monofunctional aromatic glycidyl ethers (such as HELOXY™ 62 and the like), 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), the like and combinations thereof. 
     As illustrated in  FIG. 3C , the method  300  may further include disposing  306  a second fiber material  322  in the mold cavity  332 . The second fiber material  322  defines a second row to layer  342 . As illustrated, the second layer  342  may be disposed  306  on or adjacent to an exposed surface of the first row  340 . The second fiber material  322  may be the same as or different from the first fiber material  320 . For example, the second fiber material  322  may be selected from carbon fibers, glass fibers, poly praraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     As illustrated in  FIG. 3D , the method  300  may further include disposing  308  a second compound  362  in the mold cavity  332  so as to infuse or cover or coat the second fiber material  320 . The second compound  362  may be the same as or different from the first compound  360 . For example, in certain variations, the second compound  362  includes a thermal initiator and a monomer. In other variations, the second compound  362  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the second compound  362  optionally includes a diluent. Like the first compound  360 , the second compound  362  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     As illustrated in  FIG. 3E , the method  300  may further include disposing  310  one or more other rows or layers  344  in the mold cavity  332 . For example, the method  300  may include subsequently disposing  310  one or more other fiber materials  324  and one or more other compounds  364  in the mold cavity  332 , using method similar to those used to form the first layer  340  and/or the second layer  342 . As illustrated, the method  300  may include disposing  310  five other layers  344  on or adjacent to an exposed surface of the second row  342 . 
     Each of the one or more other fiber materials  324  may be the same or different from the first fiber material  320  and/or the second fiber material  322 . For example, each of the one or more other fiber materials  324  may be independently selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     Similarly, each of the one or more other compounds  364  may be the same or different from the first compound  360  and/or the second compound  362 . For example, each of the one or more other compounds  364  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     As illustrated in  FIG. 3F , the method  300  may further include disposing  312  a photosensitizer layer  346  in the mold cavity  332 . As illustrated, the photosensitizer layer  346  may be disposed  312  on or adjacent to an exposed surface of the one or more other layers  344  so as to define a last row or layer  346  in the mold cavity  332 . The photosensitizer layer  346  may be disposed  312  using a spray coating process. The photosensitizer layer  346  may have a thickness different from the other layers  340 ,  342 ,  344 . For example, the photosensitizer layer  346  may have a thickness greater than or equal to about 0.01 mm to less than or equal to about 0.5 mm. The first layer  340  and/or the second layer  342  and/or the one or more other rows or layers  344  may each have a cured thickness of greater than or equal to about 30 μm to less than or equal to about 500 μm. 
     In certain variations, the photosensitizer layer  346  includes a thermal initiator, a monomer, and a photosensitizer. In other variations, the photosensitizer layer  346  includes a thermal initiator, a monomer, a cationic photoinitiator, and a photosensitizer. In each instance, the photosensitizer layer  346  optionally includes a diluent. For example, the photosensitizer layer  346  may include greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of the photosensitizer; greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     The thermal initiator, monomer, and/or cationic photoinitiator of the photosensitizer layer  346  may be the same or different from the thermal initiator, monomer, and/or cationic photoinitiator of the first compound  260 , as well as the second compound  262  and/or the one or more other compounds  264 . In certain variations, the photosensitizer may be selected from the group consisting of: anthracene, perylene, benzophenone, 9,10-diethoxyanthracene, 2,2-dimethoxy-1,2-diphenylethanone, 2-isopropylthioxanthone (ITX), and combinations thereof. 
     As illustrated in  FIG. 3G , the method  300  may further include using an ultraviolet light source  370  (e.g., UV-LED) so as to initiated photopolymerization  314  of the photosensitizer of the final layer  346 . As illustrated in  FIG. 3H , the ultraviolet light source  370  may be removed or turned off  316  after the start of frontal polymerization. Even after the removal of the ultraviolet light source  370 , as illustrated in  FIG. 3I , polymerization continues until polymerization across the plurality of layers  340 ,  342 ,  344 ,  346  is complete and the fiber-reinforced composite  390  is formed. Though not illustrated, in certain variations, the method  300  includes removing the fiber-reinforced composite  390  from the mold  330 . 
       FIGS. 4A-4G  illustrate another example method  400  for forming a fiber-reinforced composite  490 . The method  400  may be a two-step infusion process that includes disposing one or more fiber materials  420 ,  426  and one or more coating compounds  460 ,  466 . 
     For example, as illustrated at  FIG. 4A , the method  400  may include disposing  402  a first fiber material  420  in a mold cavity  432 . The first fiber material  420  may be disposed  402  so as to form one or more rows or layers  440 . For example, as illustrated, the first fiber material  420  may be disposed to form seven layers  440 . Though horizontal layers  440  are illustrated here, various other shapes and configurations would be recognized by the skilled artisan, including, for example only, vertical rows. Similarly, the skilled artisan will recognize that the mold  430  and/or mold cavity  432  may have a variety of other shapes and configurations. 
     The first fiber material  420  includes one or more short or continuous fibers. The continuous fibers may be woven (e.g., twill weaved, 5 harness satin, 8 harness satin), non-crimp fabrics, or unidirectional, also including fibers, fiber tows, and fiber tapes. For example, the first fiber material  420  may each be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     As illustrated at  FIG. 4B , the method  400  may further include disposing  404  a first compound  460  in the mold cavity  432  so as to infuse or cover or coat each of the first fiber materials  420  of the one or more layers  440 . 
     In certain variations, the first compound  460  includes a thermal initiator and a monomer. In other variations, the first compound  460  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the first compound  460  optionally includes a diluent. For example, the first compound  460  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     In certain variations, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In certain variations, the monomer includes one or more epoxy thermoset resins, such as diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, and the like. In other variations, the monomer includes one or more open ring-opening polymerization monomers. In still over variations, the monomer includes non-cyclic monomers, such as vinyl ethers. 
     In each instance, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     The cationic photoinitiator may be suitable for frontal polymerization. For example, in certain variations, the cationic photoinitiator includes one or more photoacid generators (“PAGs”), such as those represented by the following formulas (including the respective trade names and businesses from which such compounds are commercially available, as well as a wavelength of maximum absorbance for UV-Visible Spectroscopy, designated as λ max , where applicable): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     the like and combinations thereof. 
     In certain variations, the diluent includes polyfunctional glycidyl ethers (such as, HELOXY™ 107, HELOXY™ 48, HELOXY™ 68, and the like), monofunctional aliphatic glycidyl ethers (such as, HELOXY™ 166, HELOXY™ 61, and the like), monofunctional aromatic glycidyl ethers (such as HELOXY™ 62 and the like), 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), the like and combinations thereof. 
     As illustrated in  FIG. 4C , the method  400  may further include disposing  406  a second fiber material  426  in the mold cavity  432 . As illustrated, the second fiber material  426  may be disposed  412  on or adjacent to an exposed surface of the one or more layers  440  so as to define a last row or layer  446  in the mold cavity  432 . The second fiber material  426  may be the same as or different from the first fiber material  420 . For example, the second fiber material  426  may be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     As illustrated in  FIG. 4D , the method  400  may further include disposing  408  a second compound  466  in the mold cavity  432  so as to infuse or cover or coat the second fiber material  426 . 
     In certain variations, the second compound  466  includes a thermal initiator, a monomer, and a photosensitizer. In other variations, the second compound  466  includes a thermal initiator, a monomer, a cationic photoinitiator, and a photosensitizer. In each instance, the second compound  466  optionally includes a diluent. For example, the second compound  466  may include greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of the photosensitizer; greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     The thermal initiator, monomer, and/or cationic photoinitiator of the second compound  426  may be the same or different from the thermal initiator, monomer, and/or cationic photoinitiator of the first compound  460 . In certain variations, the photosensitizer may be selected from the group consisting of: anthracene, perylene, benzophenone, 9,10-diethoxyanthracene, 2,2-dimethoxy-1,2-diphenylethanone, 2-isopropylthioxanthone (ITX), and combinations thereof. 
     As illustrated in  FIG. 4E , the method  400  may further include using an ultraviolet light source  470  (e.g., UV-LED) so as to initiated photopolymerization  410  of the photosensitizer of the second compound  466 . As illustrated in  FIG. 4F , the ultraviolet light source  470  may be removed or turned off  412  after the start of frontal polymerization. Even after the removal of the ultraviolet light source  470 , as illustrated in  FIG. 4G , polymerization continues until polymerization across the plurality of layers  440 ,  446  is complete and the fiber-reinforced composite  490  is formed. Though not illustrated, in certain variations, the method  400  includes removing the fiber-reinforced composite  490  from the mold  430 . 
       FIGS. 5A-5F  illustrate another example method for forming a fiber-reinforced composite  590 . The method  500  may be a two-step infusion process that includes disposing one or more first layers including one or more fiber materials  520  and one or more coating compounds  560  and a second layer  526  (e.g., fiber free layer) including a second compound  566 . 
     For example, as illustrated at  FIG. 5A , the method  500  may include disposing  502  a first fiber material  520  in a mold cavity  532 . The first fiber material  520  may be disposed  502  so as to form one or more rows or layers  540 . For example, as illustrated, the first fiber material  520  may be disposed to form seven layers  540 . Though horizontal layers  540  are illustrated here, various other shapes and configurations would be recognized by the skilled artisan, including, for example only, vertical rows. Similarly, the skilled artisan will recognize that the mold  530  and/or mold cavity  532  may have a variety of other shapes and configurations. 
     The first fiber material  520  includes one or more short or continuous fibers. The continuous fibers may be woven (e.g., twill weaved, 5 harness satin, 8 harness satin), non-crimp fabrics, or unidirectional, also including fibers, fiber tows, and fiber tapes. For example, the first fiber material  520  may each be selected from carbon fibers, glass fibers, poly paraphenylene terephthalamide fibers (e.g., KEVLAR® fibers), ultra-high molecular weight polyethylene (“UHWMPE”) fibers, basalt fibers, natural fibers, the like and combinations thereof. 
     As illustrated at  FIG. 5B , the method  500  may further include disposing  504  a first compound  560  in the mold cavity  532  so as to infuse or cover or coat each of the first fiber materials  520  of the one or more layers  540 . 
     In certain variations, the first compound  560  includes a thermal initiator and a monomer. In other variations, the first compound  560  includes a thermal initiator, a monomer, and a cationic photoinitiator. In each instance, the first compound  560  optionally includes a diluent. For example, the first compound  560  may include greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     In certain variations, the thermal initiator may be selected from the group consisting of: 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED), benzopinacol bis(trimethylsilyl ether) (TPED-Si), dimethylsulfonylperoxide (DMSP), tert-butylperoxide (TBPO), tert-butylcyclohexylperoxodicarbonate (TBC-PDC), benzoylperoxide (BPO), azo-bis(isobutyronitrile) (AIBN), and combinations thereof. 
     In certain variations, the monomer includes one or more epoxy thermoset resins, such as diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE′ resin 827, and the like. In other variations, the monomer includes one or more open ring-opening polymerization monomers. In still over variations, the monomer includes non-cyclic monomers, such as vinyl ethers. 
     In each instance, the monomer may be selected from the group consisting of: diglycidyl ether bisphenol-A epoxy resin (DGEBA), diglycidyl ether bisphenol-F epoxy resin (DGEBF), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), resorcinyl diglycidyl ether 1,3-bis(2,3-spoxypropoxy)ben), 1,4-butanediol diglycidyl ether, EPIKOTE™ resin 827, vinyl ethers, and combinations thereof. 
     The cationic photoinitiator may be suitable for frontal polymerization. For example, in certain variations, the cationic photoinitiator includes one or more photoacid generators (“PAGs”), s such as those represented by the following formulas (including the respective trade names and businesses from which such compounds are commercially available, as well as a wavelength of maximum absorbance for UV-Visible Spectroscopy, designated as λ max , where applicable): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     the like and combinations thereof. 
     In certain variations, the diluent includes polyfunctional glycidyl ethers (such as, HELOXY™ 107, HELOXY™ 48, HELOXY™ 68, and the like), monofunctional aliphatic glycidyl ethers (such as, HELOXY™ 166, HELOXY™ 61, and the like), monofunctional aromatic glycidyl ethers (such as HELOXY™ 62 and the like), 3-ethyl-3-oxetanemethanol (EOM), 1,4-bis(glycidyloxy)benzene (CHDGE), 1,6-hexanediol diglycidyl ether (HDDGE), neopentyl glycol diglycidyl ether (NPDGE), the like and combinations thereof. 
     As illustrated in  FIG. 5C , the method  500  may further include disposing  506  a photosensitizer layer  546  in the mold cavity  532 . As illustrated, the photosensitizer layer  546  may be disposed  512  on or adjacent to an exposed surface of the one or more other layers  540  so as to define a last row or layer  546  in the mold cavity  532 . The photosensitizer layer  546  may be disposed  506  using a spray coating process. The photosensitizer layer  546  may have a thickness different from the other layers  540 . For example, the photosensitizer layer  546  may have a thickness greater than or equal to about 0.01 mm to less than or equal to about 0.5 mm. 
     In certain variations, the photosensitizer layer  546  includes a thermal initiator, a monomer, and a photosensitizer. In other variations, the photosensitizer layer  546  includes a thermal initiator, a monomer, a cationic photoinitiator, and a photosensitizer. In each instance, the photosensitizer layer  546  optionally includes a diluent. For example, the photosensitizer layer  546  may include greater than or equal to about 0.1 mol % to less than or equal to about 5 mol % of the photosensitizer; greater than or equal to about 0.1 mol % to less than or equal to about 10 mol % of the thermal initiator; greater than or equal to about 20 mol % to less than or equal to about 99 mol % of the monomer; greater than or equal to about 0 mol % to less than or equal to about 10 mol % of the cationic photoinitiator; and greater than or equal to about 0 mol % to less than or equal to about 70 mol % of the optional diluent. 
     The thermal initiator, monomer, and/or cationic photoinitiator of the photosensitizer layer  546  may be the same or different from the thermal initiator, monomer, and/or cationic photoinitiator of the first compound  560 . In certain variations, the photosensitizer may be selected from the group consisting of: anthracene, perylene, benzophenone, 9,10-diethoxyanthracene, 2,2-dimethoxy-1,2-diphenylethanone, 2-isopropylthioxanthone (ITX), and combinations thereof. 
     As illustrated in  FIG. 4D , the method  500  may further include using an ultraviolet light source  570  (e.g., UV-LED) so as to initiated photopolymerization  508  of the photosensitizer of the final layer  546 . As illustrated in  FIG. 4E , the ultraviolet light source  570  may be removed or turned off  510  after the start of frontal polymerization. Even after the removal of the ultraviolet light source  570 , as illustrated in  FIG. 5F , polymerization continues until polymerization across the plurality of layers  540 ,  546  is complete and the fiber-reinforced composite  590  is formed. Though not illustrated, in certain variations, the method  500  includes removing the fiber-reinforced composite  590  from the mold  530 . 
     One or more of the above methods (e.g., method  200 , method  300 , method  400 , method  500 ) may include using one or more other manufacturing processes. For example, as illustrated in  FIG. 6 , each of the methods may further include disposing a low thermal conductivity film  680  across a mold  630  so as to encase the one or more composite layers  640 , which includes at least one layer  646  having a photosensitizer. The low thermal conductivity film  680  may be used to apply vacuum pressure to the one or more composite layers  640 ,  646  in a manner similar to a vacuum bag process and/or autoclave process. In certain instances, adding pressure to the one or more composite layers  640 ,  646  helps to consolidate the final composite (i.e., fiber-reinforced composite). For example, adding pressure may help to reduce or eliminate the porosity of the final composite (i.e., fiber-reinforced composite). 
     Further, in other instances, such as illustrated in  FIG. 7 , each of the methods may further include disposing a mold cover or cap  734  across a mold  730  so as to encase the one or more composite layers  740 , which includes at least one layer  746  having a photosensitizer. The mold cover  734  may be used to apply pressure to the one or more composite layers  740 ,  746  and/or retain heat within the mold  730  such that the polymerization process occurs faster. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.