Transparent materials are fascinating that have potential to revolutionize a wide range of industries as aerospace, defense, electronics, renewable energy and architecture. The usage of optically transparent materials in different sectors increases each day. Transparent armors, windshields of automobiles and airplanes, or protection shield of photovoltaic solar panels are listed as example for the usage areas of transparent materials. A group among transparent materials called ‘transparent composites’ differs and draws attention with their high specific strength and stiffness, impact resistance and electrical resistivity. In this article, we will delve deeper into the topic of transparent composites, exploring the different types of materials that are used to produce them, the advantages they offer over traditional materials, and the challenges that must be overcome in their production.
Traditional Transparent Materials
One of the most commonly used transparent material is glass. Glass has been used for centuries in various applications due to their high transparency and durability. However, traditional glass has several limitations, including brittleness and weight. Another option as a transparent material is polymers (a.k.a. plastics) which are lightweight, flexible and have excellent transparency, but their mechanical properties and susceptibility to scratches and other types of surface defects are low. To overcome these issues in plastics and glass, a new type of material with its unique properties and benefits, named transparent composites, is discovered.
Transparent composites are generally combination of glass-fiber reinforcements and various types of polymeric matrices. Also in literature, there are several studies which uses different types of plant-based fibers with removal of lignin as reinforcement materials, as well. To manufacture transparent composites, glass (A-, C-, S-, T-, or E-glass) or plant-based (wood fiber) fabric reinforcements are impregnated with various types of polymeric resins (epoxies, polystyrenes, polymethylmethacrylates polydimethylsiloxanes, or UV-curable hybrimers) by
using any type of liquid composite molding (LCM) processes. The most commonly used LCM processes for transparent composites are listed as Resin Transfer Molding (RTM), Light RTM (L-RTM), High Pressure RTM (HP-RTM), Vacuum Infusion (VI), Vacuum Bag Molding (VBM), Pressure Bag Molding (PBM), Compression Molding and Autoclave Processing.
One of the key advantages of transparent composites over traditional materials is their enhanced durability and strength. Due to their composite nature, these materials are able to withstand impacts and other types of mechanical stresses that would typically cause traditional materials as plastics and glasses to fail. For instance, using only two layers of glass fabric with epoxy resin will increase the tensile strength of the part up to 600% compared to the part made of epoxy alone. This makes transparent composites ideal for use in applications where durability is critical. Another advantage is their lightweight nature. Traditional materials such as metals and glasses are often heavy and can add significant weight to the structure. Transparent composites, on the other hand, are often much lighter, making them ideal for use in applications where weight is a crucial factor.
Despite the advantages of transparent composites, there are still challenges to reach high optical transmittance over the visible wavelength range (up to 90% measured in between 400-800 nm), therefore manufacturers have to consider the reasons for material and process related potential defects and find solutions to prevent them. One of the key challenges is the refractive index (RI) mismatch that can occur when different materials are combined to form composite material. This can result in optical distortions and reduced transparency. To overcome this issue, various methods are developed to match the refractive index of different materials, such as by adding nano particles, additives (e.g., lowering RI of epoxy by adding polypropylene carbonate), or coatings inside the polymeric resin.
Another challenge in the production of transparent composites is the presence of bulk and surface defects. These defects can reduce the transparency of the composite and compromise its mechanical properties.
Bulk defects can include inter/intra tow porosities, air bubbles, microcracks or uneven distribution of the composite materials. These types of defects can cause variations in the refractive index, which can result in distortions or even opaque areas in the final product. To avoid these issues, careful attention must be paid to the manufacturing process, including the mixing and application of the composite materials. For instance, degassing operation is needed to remove air bubbles from resin mixture before resin injection inside the mold. Also, optimization of resin injection pressure and average flow velocity is crucial to eliminate inter- and intra-tow porosities. In addition, applying additional post-filling operations will be beneficial to decrease the ratio of porosities and air bubbles, also the part thickness variations. For manufacturers, considering the effect of curing cycle of the thermoset used will be beneficial to prevent microcracks.
Surface defects, on the other hand, can include things like scratches, rough spots and orange-peels on the surface of the material. These types of defects can also cause variations in refractive index, as well as reduce the overall clarity of the material. To avoid surface defects, it is important to carefully handle and finish the material during the manufacturing process. This can include techniques such as sanding and polishing to create a smooth surface.
Despite these challenges, the benefits of transparent composites make them an attractive option for a variety of applications. Their light weight, high strength, low thermal expansion and transparency make them ideal for use in aerospace and automotive industries, as well as in architectural and renewable energy applications. In addition, the ability to tailor the properties of the composite materials to specific applications offers a level of customization not found in traditional materials. Overall, transparent composites offer a unique combination of properties that make them a desirable option for a variety of applications. While there are challenges associated with their production, advances in manufacturing techniques and materials science are continually improving their quality and consistency. As the demand for lightweight, high-performance materials continues to grow, it is likely that transparent composites will play an increasingly important role in shaping the materials of the future.