Assembly Considerations
| Solvent Bonds |
Solvent bonding is most commonly used with amorphous thermoplastic components. It is a fast, simple, and relatively inexpensive method of joining. Properly bonded joints, made from components with bonding surfaces held to tight tolerances, may approach the strength of base resins. Fluid-tight seals can be produced with warp-free surfaces. Solvent bonding can produce cleaner and more aesthetically acceptable joints. Semi-crystalline thermoplastics are usually not good candidates for this technique as they have good solvent resistance at room temperature.
Solvents separate the molecular chains of the thermoplastics to be bonded on contact and cause softening of the surfaces being bonded. For bonding, the softened surfaces are pressed against each other to cause the molecular chains to intermingle. Clamp time ranges from a few seconds to minutes, depending on the solvent selection. The bond gains strength as the solvent evaporates. Heat can be used to accelerate the evaporation of the solvent. Full curing may take a few days depending on the type of solvent.
Selection of solvents is based on the solvent properties and the resins to be bonded. Using methylene chloride to bond Makrolon polycarbonate helps prevent solvent-vapor entrapment due to its low boiling point and faster evaporation rates. For complex assemblies, ethylene dichloride is used to allow more time required for fixturing etc. A mixture of the two solvents is commonly used to achieve assembly times in the middle range (see figure)
Because of low viscosities of solvents, the bonding surfaces have to be warp-free for perfect contact. For gap filling quality, the solvents are sometimes doped with parent resins. However, this could cause longer evaporation times and the possibility of solvent-vapor entrapment in the joint.
The joint should be free of solvent after bonding. Trapped solvent vapors in the joint area, or elsewhere in the assembly, can cause crazing and cracking at the joint, or at other parts of the assembly in contact with the solvent vapors. For this reason, avoid deep tongue and groove joints in solvent bonding. Assemblies should not be packaged for storage or shipping until the solvent has completely evaporated. Trapped solvent vapors inside the package may still attack the assembly.
Solvents pose health and environmental risks and should be handled appropriately. For this reason, solvent bonding is only used when other methods are not suitable.
