Polypropylene: The Core Dielectric Material for Capacitors and Its Development Path

Polypropylene (PP) has been the core dielectric material for capacitors since the 1980s, replacing capacitor paper due to its high breakdown strength, low dielectric loss, and processing properties. With a density of just 0.89-0.91 g/cm³, it is one of the lightest general-purpose plastics.

The properties of polypropylene are closely related to the stereoconfiguration of its molecular chains. In isotactic polypropylene (iPP), all methyl groups are on the same side of the molecular chain, forming a highly regular helical structure with a crystallinity of 50-70%, resulting in high tensile strength (35-40MPa) and a high melting point (160-170°C). In syndiotactic PP (sPP), the methyl groups alternate sides, granting high transparency and impact resistance. Atactic PP (aPP) has randomly distributed methyl groups, is amorphous, and is often used in adhesives and asphalt modification. The isotacticity of iPP directly determines its crystallinity, which in turn affects its mechanical properties: for every 10% increase in crystallinity, the tensile strength increases by 15-20MPa.

As a dielectric, polypropylene performs exceptionally well: its dielectric constant is stable at 2.2-2.36 (1kHz), its dissipation factor is below 0.0002, its volume resistivity exceeds 10^16 Ω·cm, and it can withstand high fields up to 600V/μm. It offers thermal stability with a broad continuous operating temperature range (-50°C to 120°C). Furthermore, metallized film based on PP possesses self-healing properties; upon breakdown, it evaporates the electrode to restore insulation, withstanding over 100 breakdowns per square meter with less than 0.5% capacitance loss.

To increase the energy storage density of capacitors, current technological pathways focus primarily on material innovation: firstly, optimizing the aggregated structure of pure PP, reducing ash content, and molecular modification; secondly, developing composite PP, such as nanocomposites, chemical grafting, blends, and multilayer structures; and thirdly, exploring entirely new materials with high dielectric constants or high-temperature resistance. Through continuous structural optimization and compositing, polypropylene continues to advance capacitor technology.