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Overview of Development of Insulating Materials

Wenye Zhang1 and Jun-Wei Zha1,2

1University of Science and Technology Beijing, School of Chemistry and Biological Engineering, No. 30 Xueyuan Road, Haidian District, Beijing, 100083, P. R. China

2North China Electric Power University, State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, School of Electrical and Electronic Engineering, No. 2 Beinong Road, Huilongguan, Changping District, Beijing, 102206, P. R. China

1.1 Introduction

Electrical insulating materials are the foundational components that enable safe and efficient operation of power and electronic systems. By preventing unwanted current flow, insulators allow voltages to be maintained and electric fields to be controlled within equipment. Over the past two centuries, the field of insulating materials has evolved dramatically, encompassing solids such as ceramics, polymers, resins, and paper, liquids such as mineral oils, and gases such as sulfur hexafluoride (SF6), air [1-6]. Historically, the selection of insulating materials has been a key driver for advancements in electrical technology. In the earliest days of telegraphy and power distribution (mid-19th century), engineers relied on natural materials like cotton, silk, paper, rubber, and oils - materials that were readily available but often exhibited limited durability [6-8]. As electrical systems evolved toward higher voltages and more demanding operating environments, the limitations of these early insulating materials became increasingly apparent: Many materials age, degrade, crack, or absorb moisture over time, thereby weakening their performance [9, 10]. This has driven continuous innovation in insulating materials, closely intertwined with the progress in electrical engineering itself. Each generation of insulating materials has addressed the shortcomings of its predecessors and opened up new possibilities for system design, such as enabling higher voltage transmission or reducing equipment size. Therefore, the development of insulating materials can be viewed as a synergistic evolution alongside electrical infrastructure.

In modern times, polymer materials such as plastics and resins, liquid materials such as mineral oil, synthetic esters, and silicone oil, and gas materials such as SF6 have dominated many electrical devices due to their excellent fundamental properties, including dielectric performance, mechanical properties, chemical stability, and thermal properties [8, 11-15]. However, some new challenges that were not fully considered in earlier times have also emerged: environmental sustainability and lifecycle management of these materials have become key concerns [16-18]. In the context of urgent climate agreements, insulating materials now face new challenges and requirements: they must not only exhibit outstanding electrical performance but also remain low-carbon, eco-friendly, and sustainable throughout their entire lifecycle. Many traditional insulating materials, including cross-linked thermosetting polymers, insulating oils, and certain gas or vacuum-based insulating materials, pose recycling or environmental risks when discarded or leaked [19-21]. Additionally, as reliability requirements increase, materials capable of self-monitoring/warning of health status, self-adapting, or even self-healing damage are gaining favor [22-28]. These considerations are driving a new round of research into next-generation insulating materials, which must not only be excellent in performance but also possess low-carbon, eco-friendly, sustainable, and smart characteristics. This chapter will review this evolutionary journey, from the earliest natural insulating materials to the latest low-carbon, eco-friendly, and smart solutions. We will see how early engineers relied on natural materials such as paper, rubber, glass, and oil, and how the emergence of synthetic polymers, specialty fluids, and smart insulating materials revolutionized electrical insulation technology.

1.2 Historical Evolution of Insulating Materials

1.2.1 Early Insulating Materials in Electrical Engineering

The development of electrical insulating materials is closely related to the development of electrical technology. Early researchers and engineers experimented with any material that could separate and protect conductors. Up to about 1925, naturally occurring products such as bitumen, natural rubber, mica sheets, cotton cloth, silk, paper, wood, and ceramics were the standard insulators, as shown in Table 1.1 [32, 33]. For example, cotton and silk tape were used to wrap early wire conductors. Mica, a mineral that can be split into thin flexible sheets, was prized for its high dielectric strength and heat resistance in applications like commutators and early capacitors. Oil-impregnated paper became a widely used insulating material in insulated cables and transformer windings due to its ability to improve dielectric properties by removing air [8]. Even today, some high-voltage direct current (HVDC) cables still use impregnated paper or paper-polypropylene laminates as insulators, although they are gradually being replaced by polymer materials [34, 35]. These materials are widely used because they are simple to obtain and show high electrical impedance, but their performance under electrical and thermal stress is very limited.

One notable natural insulator was natural rubber, derived from latex, which was one of the first insulating materials used for electrical wires in the early 19th century [32]. Unvulcanized rubber was tried as early as 1810, followed by vulcanized rubber after Charles Goodyear's patent in 1844. Natural rubber softens easily at high temperatures and becomes brittle at low temperatures. The vulcanization process greatly improved the stability of rubber by adding cross-links (sulfur bonds) between polymer chains, marking one of the first instances of chemically modified insulating material to enhance performance [30, 36]. Additionally, another crucial early insulator was gutta-percha, a resinous polymer obtained from the Palaquium gutta tree. In 1843-1847, it was introduced as a wire insulating material, especially for submarine telegraph cables. Gutta-percha is a thermoplastic material that can be heated and coated onto the surface of wires, then hardened to form a tough and waterproof coating. It remained the primary insulating material for submarine cables for decades, including the first transatlantic cable in 1858. However, gutta-percha was expensive, available only from Southeast Asia. And it tended to degrade (becoming brittle) when exposed to oxygen and sunlight over time.

Table 1.1 Early commonly used insulating materials, their properties, and applications.

In overhead power lines and early electrical equipment, ceramics and glass were essential insulating materials [37]. By the 1850s, glass insulators were used on telegraph lines, and porcelain, a type of ceramic made from clay, was later found to offer superior mechanical strength and better resistance to weathering. Porcelain insulators were widely used to support high-voltage lines and busbars due to their excellent dielectric strength and weather resistance [6, 38]. These inorganic insulators are essentially permanent, as they do not undergo significant aging, but they are heavy and prone to brittleness. Thus, although polymers have replaced many insulation tasks, ceramics remain crucial in areas with high requirements for stability in extreme environments, such as substation pillar insulators and bushings.

By 1920, the electrical industry had identified a range of natural insulating materials and combinations such as paper oil systems and rubber fabric composites. However, the pursuit of improved performance including higher breakdown voltage, greater temperature resistance, and reduced aging was constrained by the inherent limitations of these natural materials. This laid the...