Polymerization and Physical Properties of Benzoxazine Based Thermosetting Resin and Thermotropic Liquid Crystalline Polyester

Abstract

Epoxy resin has been known as a good candidate for a printed circuit boards (PCBs) material since its excellent mechanical strength and adhesion property to metal. Polybenzoxazine, a thermosetting polymer and strong candidate as an industrially applicable material, has attracted attentions recent years as a novel electrical insulating material displaying excellent thermal stability. Benzoxazine and epoxy/silica nanoparticles were employed to prepare thin film exhibiting ultra-low dissipation factor. In Chapter 2, two types of thermosetting benzoxazine precursors, monobenzoxazine and linear polybenzoxazine, were synthesized in bulk or solution condition, respectively. For monobenzoxazine/silica hybrid film, the epoxy was additionally used to improve its physical stability. With different wt % composition of monobenzoxazine/epoxy, a higher content of monobenzoxazine showed reduced dissipation factor (~ 0.005 frequency/10 GHz) with lower mechanical stability. On the contrary, linear polybenzoxazine/silica hybrid film demonstrated dramatically enhanced mechanical stability possibly due to its higher molecular weight network structure. However, due to its lack of polar functional group, it showed less adhesion ability to copper layer. To overcome these drawbacks, linear polybenzoxazine and epoxy blend system was prepared to increase adhesion to the copper foil and electrical insulation property (dissipation factor = ~ 0.006). In chapter 3, benzoxazine containing cyano group was synthesized. The cyano group in cyano-benzoxazine forms a triazine ring through the thermosetting process. The triazine ring increases the insulating property by diluting dipole concentration and electronegativity. In this study, silica hybrid films were prepared by the thermosetting process using highly insulating cyano-benzoxazine. Since the film obtained through the thermal curing process of cyano-benzoxazine showed brittle properties, epoxy resin was added to enhance the mechanical properties. As the content of cyano-benzoxazine increased, a lower dissipation factor was observed (dissipation factor = ~ 0.004). The thermotropic liquid crystalline polymers represent an anisotropic molten state above the melting point of the polymer. Thermotropic liquid crystalline have high mechanical strength and excellent resistance against heat and chemical. Due to these characteristics, thermotropic liquid crystals are widely used in various industries where electrical components or high mechanical properties are required. Ekonol, which is most common form of thermotropic liquid crystalline polyester (TLCP), requires high temperature for processing due to its high melting point. In order to decrease the melting point of TLCPs, the introduction of non-linear monomers, aliphatic monomers and lateral substituents have been proposed. In chapter 4, several compositions of p-HBA, 6-Hydroxy-2-naphthoic acid (HNA), resorcinol, 4,4-biphenol (BP) and terephthalic acid (TPA) were copolymerized to control melting point and provide optimized mechanical properties. Higher content of resorcinol and HNA in the polymerization of TLCP yield lower melting point which provide enhanced processibility.