Development of Carbon Dioxide Assisted Thermal Fusion PMMA Bonding Process

Poly-methyl methacrylate (PMMA), which has adventages of high optical transmittance, low-cost, simple fabrication and excellent biocompatibility, is commonly used in manufacturing microfluidic device. This thesis is devoted to the development of effective low-temperature PMMA bonding technology.
Traditional thermal fusion bonding must heat the PMMA to above the glass transition temperature, which may distort the microstructures and induce residual stress. In addition, the pressure distribution is not uniform when the substrates are pressed by hot platens. On the other hand, organic solvent bonding comprises coating and removing solvent, which may induce impurities and harm the microstructure. This research uses carbon dioxide (CO2) as gas solvent and as pressuring agent as well. The bonding temperature thus is lowered and the pressing pressure becomes uniform.
A bonding area of 34.76 % and bonding strength of 619 kPa have been achieved even the PMMA plates are bonded at a temperature as low as 40 ℃. This bonding process has been successfully applied to the packaging of microchannel device with processing temperature below the glass transition temperature (80 ℃).
Besides, in this study an innovative two-stage CO2-assisted thermal fusion bonding process has been developed which takes the soaking and releasing times of CO2 into account. The experimental results shows that this new process significantly enhances the flatness after bonding process and increases bonding area to 86.87 % and bonding strength to 700 kPa.
Furthermore, by coating a layer of PMMA solution on bonding surface, the diffusion number of chain increases, and thus increases the bonding strength up to 860 kPa with the two-stage CO2-assisted process.