Understanding the diverse types of adhesives is crucial for effectively joining composite materials in various industries. This introduction provides insight into four primary adhesive categories: epoxy, polyimide, bismaleimide, and silicone. Each type offers unique properties and applications, making them integral components in composite material assembly.
Epoxy Adhesives
Originating in the 1950s, epoxy adhesives have witnessed steady growth in acceptance and usage since their inception. Widely employed in bonding both metal and composite materials, epoxy-based adhesives offer a diverse range of mechanical properties depending on formulation and curing. They exhibit minimal by-product evolution during cure, except for phenolic-modified types, and feature low shrinkage along with excellent adhesion to various substrates.
Polyimide-Based Adhesives
Noted for their high-temperature capability, reaching up to 300°C, polyimide adhesives find extensive use in applications such as radomes. However, their curing process involves the evolution of volatiles, necessitating extraction or high processing pressures.
Bismaleimide-Based Adhesives
Evolved from BMI resins for fiber-reinforced composites, bismaleimide adhesives offer high-temperature capabilities (200°C to 230°C) and excellent electrical properties, making them ideal for high-energy radome applications. With no volatiles evolved during cure, they simplify processing and reduce bondline porosity, although they may exhibit poor peel resistance due to stiffness. Modified BMI adhesives can offer improved peel resistance.
Silicone-Based Adhesives
Derived from synthetic polymeric materials with a basic polysiloxane structure, silicone adhesives offer a wide range of properties. They boast excellent temperature resistance (-120°C to +300°C), along with resistance to UV, IR radiation, and oxidation. Silicone adhesives find extensive use in “soft-structural” applications requiring adhesive strength to maintain component integrity, flexibility to accommodate strains or vibrations, and additional properties like thermal or electrical conductivity. An example application is the assembly of solar cells. The different chemistries in commercial silicone products can affect processing conditions and, in some cases, result in the release of chemical by-products.
In this table it is shown different adhesive types, its shear strength and peel strength:
