Key Components
Its core components are polyisocyanates and polyols. Polyisocyanates (typically containing two or more highly reactive -NCO functional groups) are the main source of the "rigid segments" in adhesives, providing strength, hardness and heat resistance. Polyols (such as polyether polyols or polyester polyols, which contain multiple -OH groups) form "flexible segments", endowing adhesives with flexibility, elasticity and impact resistance. These two are the basis for synthesizing polyurethane prepolymers or the final adhesive layer.
In addition, the formula often contains chain extenders (low-molecular-weight diols/ amines to enhance the chain structure), catalysts (to accelerate the reaction between -NCO and -OH), solvents (to adjust viscosity), as well as fillers, coupling agents and other additives to optimize workability, adhesion and durability.
Technical Principle
The curing of polyurethane adhesives is mainly based on the high reactivity of isocyanate groups (-NCO), achieving bonding through two main mechanisms:
1. Physical curing (wet curing) : The end of the one-component wet-curing adhesive contains -NCO groups. When exposed to air, -NCO reacts with moisture in the environment or on the substrate surface to form unstable carbamic acid, which then decomposes to release carbon dioxide and form amines. The amines then rapidly react with the remaining -NCO to form a polyurea structure and cross-link and solidify. This process does not require additional heating and is suitable for on-site construction.
2. Chemical curing: The two-component adhesive stores the prepolymer containing -NCO (Component A) separately from the curing agent containing -OH and/or -NH₂ (Component B). When in use, when mixed, -NCO and -OH undergo a direct addition polymerization reaction to form a strong carbamate bond (-NH-COO-), constructing a linear or cross-linked three-dimensional network structure and achieving rapid and high-intensity curing.
The core advantage lies in the designability of its molecular structure. By adjusting the proportion and type of "rigid segments" and "flexible segments", various materials ranging from soft elastomers to rigid plastics can be precisely produced, thereby achieving excellent adhesion to different substrates (such as metals, plastics, wood, fabrics, and rubber), and also featuring good wear resistance, oil resistance, and low-temperature resistance. Its adhesive force not only comes from chemical bonds, but also from its good wettability on the substrate surface and the hydrogen bond effect formed.
