Polymers and Graphene-Based Materials as Barrier Coatings: A Quick Review on Their Properties and Synthesis Techniques

Introduction

The impermeable nature of several materials is important to different industrial sectors. If a component in a system is not accurately coated it can result in the catastrophic failure of the entire system. Just the worldwide amount spent on fixing corrosion failure is approximately US$ 2.5 trillion, indicating that effective barriers are vital to the required systems. The most important requirements for materials to be used in barrier applications are good impermeable nature to water vapour and oxygen, high transparency, and good mechanical properties (Sangroniz et al., 2019).

Barrier properties of a material correspond to the specific permeable objects or species such as gases, water vapour, or even light that it is acting as a wall against. Coatings do not only provide barrier properties but can also improve the efficiency of the system, such as the thermal barrier coatings used in turbine engines (Xu et al., 2008). Thermal barrier coatings are a family of coatings specifically used in high temperature applications such as energy generation and storage, and nuclear industries. Aerospace turbine engines use low thermal conducting ceramics or its composites to coat the superalloy, with the first type of coatings used was frit enamels in 1950. This enables a temperature reduction in the surface enabling the engine to operate above the melting temperature of the superalloy, thus improving performance and energy efficiency. Furthermore, coatings are usually multi-layers of different types of physical and chemical properties, with each layer providing its specific function to the system. This includes functions such as corrosion, erosion, or oxidation inhibitors, balancing thermal mismatch between the layers of coatings, or preventing interdiffusion of elements.

Polymers are well known for their barrier properties. Some of the advantageous properties of polymers that make them favorable to be used in multiple industries, which also include, coating for electronics, solar cells, and in the medical industry, are their excellent mechanical properties, and resistance to chemicals and heat. For example, food packaging needs barrier coatings that will prevent the mixing of moisture and oils from the interior and exterior of the packaging as it will compromise the sensory and hygienic integrity of the packed food (Tyagi et al., 2021). The good barrier properties of polymers result from the molecular packing and orientation within the polymer and are influenced by the degree of crystallization of the semicrystalline polymers. The packing and orientation of the molecules determine the fractional free volume (FFV) (Klopffer & Flaconneche, 2001), where a rise in the amorphous density leads to a reduction in the FFV. FFV refers to the volume within the polymer that is available for the penetrant to diffuse into. It does not encompass the volume taken up by the polymer molecules and the volume that is inaccessible by the penetrant (Dhoot et al., 2002). Whereas the presence of crystals within the polymer determines the tortuous pathway that the penetrant must follow, therefore increasing the degree of crystallinity leads to an increase in the barrier properties (Klopffer & Flaconneche, 2001).

Additionally, among the existing methods for corrosion protection, protective coatings are gaining popularity due to their scalable and ease of application. Graphene and its derivatives have caught attention in developing new types of protective coatings due to their thermal stability, good mechanical properties, impermeability to different types of gases, and great chemical resistance (Nine et al., 2015). Graphene provides sufficient protective coatings due to the formation of dense electron clouds that restrict the gaps in the aromatic rings (Sreeprasad & Berry, 2013). Graphene is a nanomaterial, so its thickness is ideal for protecting and maintaining or even enhancing the substrate's properties. To protect against corrosion, the requirement is to either minimize or entirely stop the electrochemical process by blocking access of corrosive ions to the substrate surface (Kumar et al., 2021). Graphene-based materials properties including chemical inertness, small geometric pore, enhanced adsorption capacity, and high surface area, contribute to its capability to be used as effective corrosion inhibitors (Othman et al., 2019a).

This review discusses and introduces current research and challenges found in advanced materials used as barrier and coating applications to accomplish different functionalities such as oil, grease, water, and gas resistance. Detailed advances of polymer and graphene-based materials used for barrier coating applications are concentrated on to give a clearer insight on the developments needed to achieve the highest level of barrier properties for these materials.