Introduction
Composite material is defined as a combination of two or more materials or components that results in overall improved or enhanced properties compared to the individual components used separately. Polymer composite material is an assembly of two or more constituents such as a compatible matrix and a reinforcing component or filler, which yields enhanced characteristics and properties compared to the assembled individual constituents. The compatible matrix components are generally of three types: metal matrices which have high ductility, intermediate stiffness, and intermediate strength; ceramic matrices which have brittleness, but higher strength and stiffness; or polymeric matrices which have lower stiffness and strength than metal and ceramic matrices. The compatible matrix, which is a continuous phase in composite materials, performs several important functions such as holding fibers in the proper direction or orientation, providing protection to the fibers from the environment, and transmitting load at the interface to the fibers by shear loading. Composite materials offer properties that individual constituents do not possess. Constituents of composite materials do not completely merge into each other; however, they are generally divided by boundaries and retain separate mechanical, chemical, and physical properties. The interface between constituents is physically identified, and the properties of the interface typically control the properties of the composite material. The characteristics and properties of composite material cannot be attained by any of the constituents performing alone. There are numerous literatures available providing examples of polymer composites at different scales, ranging from the micro to nano scale, such as mineral filler added ones, fiber-reinforced ones, and nanocomposites and their properties.
Polymer nanocomposite is defined as a material consisting of a nano-crystalline structure containing small nano-crystals, which are incorporated within the nano-scale resin matrix or matrix grains. During the last few years, polymer nanocomposites have been extensively explored, and there has been a lot of interest developed in the field of nanocomposite materials due to their outstanding and unique properties, including improved optical properties, excellent barrier resistance, elastic stiffness, wear resistance, scratch resistance, electrical properties, and flame retardancy. There are many advantages of nanocomposite materials such as high stiffness and strength combined with low density compared to micro-scale or bulk composite materials. Nano-scale composites may offer higher strength than any other micro-scale or bulk materials, and they are considered remarkably better than other composite materials. In the past few years, the synthesis of advanced materials and the processing of smart devices at the nano-scale have enormously increased, and nano-composite materials have contributed tremendously to nano-scale engineering. The intimate presence of nanoparticles, which have a high aspect ratio in the polymer matrices, leads to a complete change in the properties and mechanical behavior of the nanocomposite materials. Typically, polymer nanocomposite materials could be prepared using various types of nano-reinforcing components or fillers because of their small particle size and better intercalation properties.
The reinforcing components or fillers are commonly particulates or fibers, which strengthen the composite material and also provide stiffness. Particulate reinforced composite materials have dimensions approximately equal in all directions. They may be of irregular geometry, regular geometry, or platelets. Particulate composite materials are less stiff and much weaker but lower in cost than fiber-reinforced composite materials. Due to the brittleness and some processing difficulties, particulate composite materials typically contain a low amount of reinforcing component, about 40 – 50 volume %. Fiber-reinforced composite materials are commonly of two types: continuous fiber composite materials or discontinuous fiber composite materials. Continuous fiber composite materials are much stronger than discontinuous or particulate composite materials due to the higher aspect ratio, whereas discontinuous fibers have an aspect ratio shorter than continuous fibers. Continuous fiber composite materials usually have orientation in arranged manners, while discontinuous fiber composite materials have random orientation. Examples of continuous fibers include helical winding, unidirectional, and woven cloth, and examples of discontinuous fibers include random mat and chopped fibers.
The fiber composite material yields high strength due to smaller diameter; the smaller the diameter, the higher the fiber strength, and they also contain only a few surface defects. They also show higher flexibility and ease of processing and fabrication. Common fibers include carbon fiber, aramid fiber, and glass fiber; they may be continuous fiber or discontinuous fiber. The quantity and the type of the reinforcing component or filler determine the overall properties of the polymer nanocomposite materials. In recent years, innovative technologies have been developed to improve the overall properties of nanocomposite materials by using inorganic nanoparticles as a reinforcing component or filler incorporated into the polymeric matrices. Both micro-scale and nano-scale fillers are used for the fabrication of polymer nanocomposite materials, but micro-scale fillers are not completely suitable for polymer nanocomposite materials because they lead to spin-line failure effortlessly due to their size. However, as the size of particles decreases, effectiveness increases due to their higher per unit volume surface area. As the particle size is reduced, the contact area increases, and the interface between compatible matrix and the reinforcing component or filler is maximized. Consequently, loading of nano-filler into the polymeric matrices reduces loadings compared to bulk or conventional composite materials which contain higher loadings of reinforcing component or filler. Inorganic reinforcing components or fillers such as metals or their oxides have been considered more attractive over the past few decades because of their better ability to resist harsh processing conditions. Among the group of inorganic metals or their oxides i.e. TiO2, ZnO, SiO2, MgO, CaO, Titanium dioxide (TiO2) has been broadly studied as a reinforcing component or filler in polymer nanocomposite materials due to its exciting properties. Many literatures reported that TiO2 embedded polymer nanocomposite materials have good mechanical, photocatalytic, dispersion, thermal, optical properties, photoelectric, high dielectric constant, treatment of polluted water and air, oxygen sensitivities, facilitate important environmentally beneficial reactions such as water splitting to generate hydrogen, and viscoelastic properties. Titanium dioxide (TiO2) or titania, mainly exists in three natural morphological crystalline phases: anatase, brookite, and rutile phases. These three morphological crystalline phases of titania (TiO2) have different morphological crystalline structures, such as brookite (orthorhombic), anatase (tetragonal), and rutile (tetragonal). Among these three morphological crystalline phases, rutile is the most stable crystalline phase, whereas, anatase and brookite are metastable crystalline phases, and they can transform into rutile crystalline phase when heated at high temperatures. The rutile crystalline phase has the highest refractive index and ultraviolet absorptivity compared to other two crystalline phases and is employed in pigments, paints, and ultraviolet (UV) absorbents. The crystalline phase, composition, and the surface morphology of titania (TiO2) strongly affect the electronic structure and charge properties. The photocatalytic action of titania (TiO2) strongly depends on the present morphological state and crystalline phase. In polymers photo-degradation, anatase crystalline phase of titania (TiO2) has higher photoactive properties than the other titania (TiO2) phases. There are numerous reported literatures available exposing titania (TiO2) nanoparticles embedded into thermoset or thermoplastic polymers to fabricate polymer nanocomposite materials. Among these, TiO2/epoxy composites have been broadly investigated to improve the performance of epoxy polymer composites such as thermal, mechanical, high resistance to degradation because polymer matrix alone cannot cover all the demanded properties, whereas, some additional reinforcing components or fillers are needed to cover all the desired properties. This review will present a comprehensive literature review of TiO2/epoxy nanocomposite materials. It will begin with the fabrication and properties of nano-scale titania (TiO2), then a detailed literature review of TiO2/epoxy nanocomposites will be presented with processing techniques, characterizations, and applications of TiO2/epoxy nanocomposite materials.
TiO2/Epoxy Nanocomposites
Titanium dioxide (TiO2) nanoparticles are embedded into different types of epoxies to enhance their characteristics and properties, using various types of hardeners. Epoxy resins are thermoset resins typically synthesized by reacting two components: an epoxy resin and a curing agent or hardener. Epoxies are multifunctional resins containing one or several epoxide groups, also known as glycidyl groups or an oxirane ring.
Epoxy resins are mainly divided into two categories: non-glycidyl and glycidyl epoxy resins. Non-glycidyl epoxies are further divided into two sub-categories: cycloaliphatic or aliphatic epoxy resins, which are synthesized by olefinic double bond peroxidation. Glycidyl epoxy resins are further divided into three sub-categories: glycidyl ester, glycidylamine, and glycidyl ether epoxy resins. These glycidyl epoxy resins can be synthesized by the condensation reaction of epichlorohydrin with dibasic acids, dihydroxy compounds, or diamines. The curing reactions involve hydroxyl groups and oxirane groups with the hardeners or catalytic reactions involving cationic polymerization of oxiranes or oxirane groups.
There are various types of epoxy resins available, with Diglycidyl ether of bisphenol-A (DGEBA) being the first commercial epoxy resin and still widely used for many applications. DGEBA epoxy resin is synthesized from excess epichlorohydrin and bisphenol A. Other various types of epoxy materials are summarized in Table 1. The basic difference among various grades of epoxy resin is in their viscosities, which range from 5 – 14 Pa.s at 25°C. Epoxy resins could be prepared commercially using different molar ratios of bisphenol A to epichlorohydrin, resulting in higher or lower molecular weight epoxies.
Epoxy thermoset resins are extensively investigated and used as matrix components in composites for various applications, including adhesives, high-performance coatings, tribological properties such as calender roller covers and slide bearings, and as casting resins. The mechanical properties of epoxy matrices can be influenced by surface modification, such as molecular architecture modification by increasing crosslinking to produce high strength and stiffness. However, highly cross-linked epoxy matrices may exhibit brittleness due to constrained plastic deformation, leading to crack initiation and impulsive failure. Therefore, the focus of many material scientists is to provide high toughness to epoxies without affecting other desired properties such as elastic modulus and mechanical characteristics, which are commonly desired properties for many applications. Traditional tougheners, such as hyper-branched polymers, micro-voids, glass beads, core-shell particles, and particles of spherical rubbers or liquid rubbers, may reduce the desired properties of epoxies such as strength, glass transition temperature (Tg), elastic modulus, mechanical, and thermal properties. Modifiers and tougheners with less rigidity compared to polymer matrices serve an excellent behavior in polymer matrices. For the development of high-performance nanocomposite materials, nanoparticles of inorganic compounds have been applied by researchers to reinforce polymer epoxies. Extensive research has been conducted to embed nano-fillers with high aspect ratios as well as low aspect ratios to enhance the important characteristics and properties of polymer nanocomposite materials such as electrical resistivity, wear resistance, and toughness.
Fillers are typically used to enhance or improve the properties of epoxy composite materials, such as reducing cost, shrinkage reduction on curing, moisture absorption reduction, modulus enhancement, compressive strength enhancement, heat resistance enhancement, thermal enhancement, and electrical conductivity enhancement. However, there are some negatives associated with fillers, such as loss of transparency, reduction in tensile and impact strength, and weight enhancement. Various fillers reported in the literature that could be used with epoxy resins include vermiculite, phenolic microspheres, mica, marble flour, slate flour, calcium carbonate, glass microspheres, alumina, titania, silica, aluminum, and precious metals.
Fabrication of TiO2/Epoxy Composites
Nanoparticles provide a large surface area for interaction between polymer matrices and particles, which significantly controls the properties of polymer composite materials. However, nanoparticles tend to form agglomerates due to electrostatic or Van der Waals forces, resulting in poor dispersion. To harness the beneficial properties and enhance the effectiveness of substantial surface interaction between polymeric matrices and fillers, homogeneous dispersion of nanoparticles is required. Various techniques have been employed to disperse nanoparticles into polymeric matrices, including direct incorporation, surface chemical treatment, ultrasonic pulsed vibrations, and mechanical stirring with high shear forces.
Several techniques have been used by material scientists and researchers for fabricating TiO2/epoxy nanocomposites, including ‘in-situ’ and ‘ex-situ’ techniques. In the ‘ex-situ’ technique, the mixing of inorganic nanoparticles with polymeric matrices is performed either by mechanical mixing or solution mixing, while in the ‘in-situ’ technique, nanoparticles are synthesized within the polymeric matrices. Some of the best synthesis methods are discussed below:
The Torus-mill Dispersion Technique (TML-mill): Nano-powder of titanium dioxide (TiO2) nanoparticles is added directly into the epoxy resin liquid at ambient temperature. The resulting solution, containing a few volume % of TiO2, is stirred mechanically under vacuum to remove bubbles and form a suspension. This leads to the formation of a master batch of TiO2/epoxy nanocomposite material. The master batch is then subjected to further mechanical stirring under vacuum using a torus mill mixing device to achieve good particle dispersion and de-aggregation. The mixture is cast into pre-heated aluminum molds and cured at high temperature.
The ‘Ex-situ’ Technique: In this technique, metal oxide nanoparticles such as TiO2 are prepared by template synthesis in an aqueous solution containing a titanium precursor and a template. The nanoparticles are then dispersed into a polymeric matrix with a planetary mixer under vacuum. The mixture is cast into molds and cured at high temperature.
Properties of TiO2/Epoxy Nanocomposite Materials
Nanocomposite Microstructure
Transmission electron microscopy (TEM) is used to analyze the dispersion state of nanoparticles in epoxy matrices. TEM results of TiO2 nanocomposites show homogeneously distributed nanoparticles with some agglomerates present. The size of these agglomerates ranges between 100 nm and 250 nm.
Mechanical Properties
The tensile behavior of TiO2/epoxy nanocomposite materials is influenced by the rigid reinforcing components of fillers. The Young’s modulus, tensile strength, and tensile strain of the nanocomposite materials at different volume % concentrations of TiO2 with epoxy resin show improvements with the addition of TiO2 nanoparticles. However, excessive addition of TiO2 fillers may slightly reduce the strength of the composite materials. The mechanical behavior and electrical properties of TiO2/epoxy nanocomposite materials have been investigated by various researchers, demonstrating improvements in mechanical moduli, electrical resistivity, and dielectric strength properties.
In summary, TiO2/epoxy nanocomposites offer enhanced mechanical and electrical properties, making them promising materials for various applications in industries such as automotive, aerospace, electronics, and construction.
Crystal Structure of Titanium Dioxide
Titanium dioxide (TiO2) has been extensively studied due to its remarkable properties, including photoelectricity, high dielectric constant, catalytic conversion, humidity treatment of polluted air and water, oxygen sensitivities, and its ability to act as a photocatalyst, facilitating environmentally beneficial reactions such as water splitting to generate hydrogen.
Titanium dioxide (TiO2), or Titania, primarily exists in three natural phases: anatase, brookite, and rutile. These phases have different crystalline structures; brookite is orthorhombic, anatase is tetragonal, and rutile is also tetragonal. Anatase and brookite are metastable phases, while rutile is the most stable and can transform from other phases when heated to higher temperatures. Rutile, with its higher refractive index and ultraviolet absorptivity, is commonly used in pigments, paints, and ultraviolet absorbents. The phase, composition, and surface states of titanium dioxide strongly influence its charge properties and electronic structure, affecting its photocatalytic activity.
Titanium dioxide also exhibits excellent chemical stability and enhances the brightness of colored pigments. Titanium white, known for its high refractive index and optical properties of UV light absorption and scattering, is widely used in sunscreen applications. Proper surface treatment of titanium dioxide particles is crucial to prevent the degradation of organic substrates by the filler particles. Silica coating on titanium dioxide has been extensively studied for its ability to enhance powder durability, although it may lead to opacity loss due to particle agglomeration during liquid-phase manufacturing. Titanium dioxide (TiO2) is a wide bandgap semiconductor, absorbing only a small fraction of solar light. Efforts have been made to extend titanium dioxide’s absorption to the visible region through doping with transition metals, nonmetal atoms, and organic materials. Controlling the morphology and structure of nanoscale titanium dioxide is essential for its various applications. Anatase-phase titanium dioxide nanocrystals, known for their high photocatalytic activity, have garnered attention due to their distinct properties compared to bulk materials. The annual global production of titanium dioxide exceeds 4 million tons, with significant usage in paints, plastics, paper, and various other applications.
Crystal Structure of Titanium Dioxide (TiO2)
Titanium dioxide (TiO2) exists in three natural phases: brookite, anatase, and rutile. These phases have different crystalline structures; brookite is orthorhombic, while anatase and rutile are tetragonal. Both crystals have each Ti atom surrounded by six adjacent O atoms in approximately regular but slightly different octahedral arrangements. Rutile has a more compact structure than anatase, accounting for its higher refractive index. Rutile’s crystal structure consists of corner-shared TiO6 octahedra in a tetragonal cell, while anatase’s structure consists of edge-shared TiO6 octahedra. Rutile is more stable than anatase and is the most abundant form of titanium dioxide.
Properties of Titanium Dioxide (TiO2)
Chemical Properties
Pure titanium dioxide is a colorless, crystalline solid with high stability, non-volatility, and insolubility. It is amphoteric, with more acidic than basic characteristics. Titanium dioxide exists in three fundamental crystal forms: anatase, rutile, and brookite, with rutile being the most stable. Rutile exhibits a higher refractive index, specific gravity, and chemical stability compared to anatase. The high refractive index of titanium dioxide contributes to its hiding power in paints and pigments.
Optical Properties
Titanium dioxide exhibits brightness and whiteness, with reflectance properties similar to a perfect reflecting diffuser. Controlling color during manufacturing is crucial for achieving the desired brightness and whiteness. Flocculation, the formation of loose clumps of titanium dioxide particles in a fluid system, can affect opacity and tinting strength. Titanium dioxide pigments are highly effective in imparting whiteness and brightness to various materials, including paints, plastics, papers, fibers, and enamels. Surface treatments are used to optimize dispersion in different applications.
Applications of Titanium Dioxide
Titanium dioxide finds applications in dye-sensitized solar cells (DSSCs), organic solar cells, photocatalysis, chemical sensors, microelectronics, and electrochemistry. Its ability to scatter visible light and absorb UV light energy enhances the weatherability and durability of polymer products. Titanium dioxide is widely used in paints, plastics, paper, fibers, and enamels due to its high refractive index, brightness, and opacity.