Styrene-butadiene-styrene triblock copolymer (SBS) possesses features of both thermoplastics and vulcanized rubber owing to the solubility and thermoplasticity of polystyrene (PS) and the toughness and resilience of polybutadiene (PB). The features result from the incompatibility of thermodynamics between the end of PS molecular chain and the middle of PB molecular chain. The end of PS molecular chain shows glassy state at room temperature, acting as a crosslinking point of rigid network, and displays viscous state at the temperature above viscous flow temperature, enabling plasticity because of the crosslinking destroyed. For this reason, SBS becomes the major material for the production of adhesives.
Solvent-based SBS adhesives have lower viscosity, which enables it to be used for producing adhesive agents with higher solid content and suitable for coating, saving solvent and the energy for solvent volatilization.
Linear SBS is more suitable for being used as the main material of adhesives. The structure of SBS is segmented into linear and radial. Linear SBS possesses low relative molecular weight, good solubility, low viscosity, and lower cohesive strength. Radial SBS has high relative molecular weight, high cohesive strength, high physical crosslinking density, and higher heat resistance and elastic modulus than linear SBS. But if the amount of radial SBS is too large, the shrinkage internal stress of adhesives will increase. The presence of internal stress in adhesive joints will reduce joint strength, resulting in poor toughness and great brittleness of adhesives after curing. In addition, adhesives and adherend molecules need to be made into a solid joint by interdiffusion. Radial SBS are less wettable than linear SBS, and cannot evenly and completely wet bonding materials when bonding.
Toluene, dichloroethane, carbon tetrachloride, vinyl acetate, and cyclohexane are good solvents, while n-hexane, heptane, petroleum ether, solvent gasoline, and acetone are poor solvents for SBS. Single solvent is difficult to meet SBS’s comprehensive requirements. Hence, binary or multi-component mixed solvents made of good solvents and poor solvents are generally used. Poor solvents in the mixed solvents must volatilize at a faster rate than good solvents. Otherwise, the surface of adhesive film will be rough, and its bonding strength will decrease.
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