Stycast 4640

Stycast 4640 is a lightweight, temperature-resistant, RTV (Room Temperature Vulcanising) silicone rubber syntactic foam designed for high-performance aerospace and airborne applications. Engineered as a two-component condensation cure system, it yields a flexible, low-density material characterised by a low dielectric constant and dissipation factor. Its primary use cases include potting, sealing, and caulking where weight reduction is critical and superior dielectric properties are required.
The material is typically paired with Catalyst 50 AMB or Catalyst 50-2 AMB. In its mixed, uncured state, it features a viscosity of 45,000 mPa·s and a density of only 0.75 g/cm³, ensuring it remains readily pourable while meeting strict mass constraints. The system offers a 60-minute work life at 25°C, providing ample time for precision application. Because it utilises condensation cure chemistry, it cures upon exposure to moisture and is not subject to the cure inhibition common in other silicone types; however, it is not recommended for use in completely sealed moulds where reaction by-products cannot escape.
The curing profile is flexible, requiring 16 to 24 hours at room temperature or as little as 2 to 4 hours at 65°C. For applications where the operating environment will exceed 125°C, a stepped post-cure schedule is recommended to properly condition the rubber up to its maximum operating temperature of 260°C. Once fully cured, Stycast 4640 maintains a Shore A hardness of 70 and an elongation of 135%, providing the mechanical resilience needed for vibration-prone airborne environments.
Electrically, the cured foam is a superior insulator, boasting a volume resistivity of 1×10¹⁴ ohms-cm and a dielectric strength of 250 volts/mil. Its low dielectric constant of 2.0 and dissipation factor of 0.004 make it ideal for high-frequency electronic protection. While silicone naturally exhibits excellent release properties, durable adhesion to substrates can be achieved by utilising Stycast S 11NC Primer prior to application.