Understanding Aerospace Chemical: Principles and Applications of Aeronautical and Space Propulsion

The field of aerospace materials can be said to have begun with the production and sale of the Wright Flyer to the U.S. Army in 1909. At that time, the principal consideration for selecting materials was essentially maximum strength with minimum weight.

Understanding Aerospace Chemical

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120 years ago aircraft materials were largely fabric, wood, wire and fasteners. Today, aerospace materials are mostly metal alloys, although they can include polymer-based materials specifically developed for aerospace purposes.

The engineering and manufacture of aerospace materials is defined by the international standards bodies who maintain standards for the materials and processes involved. One of the primary organizations for this is ASTM International, formerly known as American Society for Testing and Materials.

AMS stands for Aerospace Material Specifications that are established by the Society of Automotive Engineers or SAE. AMS specifications are compiled in a comprehensive database of individual directives that standardize procedures, equipment and processes related to aerospace material processing. These are continuously updated and revised to stay up-to-date with advances in material science and processes technologies.

AND10133 is an aluminum extrusion available in many different alloys and finishes such as 7075, 2024 and 6061 in both tempered condition and 0 condition. AND10133 extrusion is used in maintenance applications and new build applications throughout the aviation and aerospace industry.

Three-dimensional printing, or additive manufacturing (AM), is a broad term for a wide range of fabrication methods utilizing materials such as small-molecule, polymer, and metal feedstocks. Each method requires different chemical, physical, and engineering needs to be successful. This article will discuss some of the considerations for polymer-based AM methods. Ultimately, we focus on the chemistries of vat photopolymerization, in which light is used to cure a resin from liquid to solid, to provide an example of how chemical advancements have led to increased speed, resolution, and multimaterial printing capabilities not previously possible.

From personalized medicine to aerospace engineering, the design and fabrication freedoms of three-dimensional (3D) printing have led many to call it an industrial revolution.1 Three-dimensional printing, also termed additive manufacturing (AM), builds objects by adding material generally in a layer-by-layer fashion, until the final structures are achieved. AM enables users to go directly from a digital model to a 3D object without the need for molds or machining. In addition to geometric freedom, AM approaches can also incorporate multiple materials within a single object and allow users to change materials mid-print to produce more complex blends, with tailored mechanical and design properties, compared with other manufacturing approaches. As there are many different types of AM methods, printable materials, including polymers (or plastics), metals, ceramics, glasses, wood, food, and even living cells are possible.2

Polymeric materials are comprised of repeat units, or monomers, covalently bound together to form chains and/or networks. Differences in the properties of these various materials stem from changes in the chemical structure, size distribution, and relative physical orientation of the monomers in the polymer backbone. Polymers provide an essential foundation for modern life and range from naturally derived (i.e., cellulose, proteins, and DNA) to synthesized (i.e., nylon, polystyrene, and polyethylene).

Polymers are generally characterized by two key factors: their response to heat (thermoplastic or thermoset), and response to mechanical deformation (elastic or plastic).21 When heated beyond their glass-transition temperature, Tg, polymeric materials go through reduction in stiffness. As temperature increases, the chains exhibit an increased freedom in mobility, allowing them to disentangle from each other, decreasing viscosity and stiffness. If the polymer chains are linear and not connected to each through chemical linkages, or cross-links, then heating may ultimately lead to melting (Tm, melt transition temperature), which makes the polymer a thermoplastic material. Melting of thermoplastics can enable manufacturing processes such as molding or extrusion. In contrast, thermosets become irreversibly solid or rigid after curing via formation of chemical cross-links. An example of curing is the transition of resins, such as epoxies or dental adhesives, from liquid to solid as polymers within the material cross-link together. The more polymers that are knotted together, the higher the cross-linking density resulting in a network of tightly bound polymeric material. The cross-links in thermoset materials make them more thermally and solvent resistant, and generally thermosets are hard and brittle materials.

At a critical monomer conversion percentage, the polymer will transition from a liquid to a solid, curing the material. This transition is known as gelation, and it is the main chemical parameter for control of object resolution. Chain-growth polymerizations achieve gelation at a relatively low conversion (Figure 5), making it easy to form the object with relatively low curing. After printing, the structure can be cleaned and post-cured for increased conversion and strengthening of end-use object. The tradeoff for early gelation is that the polymer network formed in cross-linkable chain-growth polymerizations is often nonuniform, experiencing regions of high and low-cross-linking that impact the final mechanical properties of the printed part. Network nonuniformity often makes chain-growth acrylate materials brittle.

Chain-growth polymerization mechanisms that do not utilize free radicals, such as cationic polymerizations, provide alternative material options.41 Cationic polymerizations utilize the generation of acid (protons (H+) to facilitate polymerization instead of free radicals (Scheme 3). Cationic polymerizations, in particular cationic ring opening polymerizations of monomers such as epoxides, are already used in VP, and are desired for their higher strength and toughnesses compared to acrylate-based materials.30,42 Epoxides also exhibit good layer adhesion and less shrinkage during polymerization than acrylate materials, which is beneficial for print fidelity. Epoxide polymerizations are generally slower than acrylate polymerizations, as propagation of the heavier acid or proton molecule within the resin is slower than a small, unstable radical.43 Cationic polymerizations are also generally living in character. This means the reaction would continue without termination until all monomer is consumed or is diffusion-limited (the cured object becomes solid, and chains are no longer mobile). In practice this is more complex, as the epoxide heterocycles are susceptible to nucleophilic attack by other electron-rich species. With photoresins that have mixtures of many different chemical components, nucleophilic attack may be possible, but the slow conversion of cationic polymerizations is still its largest limitation preventing more widespread use in VP processes.

Step-growth polymerizations have a very different conversion profile than chain-growth. Polymer is slowly built up from the reaction of multifunctional monomers bit-by-bit, and only at high conversions is gelation achieved (Scheme 4). In photoinitiated chain-growth polymerizations, the number of activated photoinitiators dictates the number of growing chains, and by extension the overall polymer network structure during curing. The growing chains after initiated, exist throughout the propagation steps until terminated. In photo-mediated step-growth polymerizations, photo-mediators or photocatalysts activate a single condensation or addition of two reactive groups. The radical does not propagate along to grow a chain; instead the radical can undergo chain transfer to induce another addition or condensation with other reactive groups within the resin. During VP, radicals are produced where light is present. Gelation and solidification are utilized to limit diffusion and maintain print resolution, similar to cationic polymerizations.44 As gelation occurs later, the network formed from step-growth polymerizations is more uniform.45 Objects formed through step-growth polymerizations thereby have improved mechanical properties compared to similar compositions achieved through chain-growth polymerizations.46 For example, thiol-ene step-growth polymerizations have been investigated within the VP field, and have shown tunable mechanical properties and improved toughness relative to similar acrylate-based resin systems.46 Expanding on this initial work, Schwartz and co-workers found the thiol-ene photoresins produce printed structures with stimuli-responsive shape-memory behavior.47 Ultimately, coupling the wider range of chemical synthetic freedom, network uniformity, oxygen insensitivity, and reduced shrinkage of step-growth polymerizations, researchers have begun to target step-growth polymerization systems for expansion of VP chemistries.

One way that researchers have sought to overcome limitations of different resin chemistries for VP is to utilize mixed resin systems that incorporate multiple polymerization chemistries. Dual-cure networks are an example of this, in which two orthogonal polymerizations occur within the same structure. This creates an interpenetrating network of two different polymers, often having improved mechanical properties compared to their homopolymerized counterparts.55 In VP, dual-cure networks often use mixed photoresins containing acrylates and epoxides to circumvent the slow polymerization of epoxides. The acrylate resin component can be photopolymerized quickly to increase print speed and achieve shape fixity, and the epoxide resin component can be further photo- or thermally polymerized to improve material properties and chemical resistance.56 This has also been shown to be beneficial in mitigating the issues of both radical and cationic polymerizations.43,57 Incorporation of epoxide monomers in acrylate resins can decrease the oxygen inhibition of the free-radical polymerization.57 Similarly, acrylate monomers can decrease the moisture sensitivity of cationic polymerizations.57 These more complex hybrid formulations come with other issues, such as phase separation and shrinkage differences, but they also provide an example of how knowledge of chemical formulations and polymerization mechanisms can help provide a means to move beyond the brittle materials commercially available. For VP processes to progress toward engineering and functional applications, materials with improved properties, and systems that mitigate the chemistry limitations of the polymerizations, are necessary. 2ff7e9595c