Study on organics-accelerated benzoxazine polymerization and the structure-property relationship

Polybenzoxazine constitutes a new generation of thermosets, which inherits the advantages from phenolic resins while overcomes the drawbacks. The other unique properties, such as low curing shrinkage, low water absorption and good molecular design flexibility, make it a potential candidate in adhesives, microelectronic packaging as well as aerospace engineering. However, conventional benzoxazines are mostly solid under room temperature and require high curing temperature, which complicate their processing and limit their commercialization. Brønsted acids and Lewis acids are efficient accelerators for benzoxazine polymerization. But they are prohibited in electronics, metal adhension and outdoor applications due to the halogen and metal elements, corrosive acidic attributes and moisture sensitivity. Organics like phenol and thiol are effective accelerators without the aforementioned disadvantages. However, when they are used as the accelerators, the properties of the materials are worse than that of their thermally cured counterparts. Amine is a newly discovered accelerator for benzoxazine. By carefully designing the molecular structure of amine, it is possible to accelerate benzoxazine polymerization as well as enhance material properties. Unfortunately, the mechanism of benzoxazine/amine reaction is still not very clear to date. Thus, it is of significant importance to investigate the reaction mechanism and establish the structure-property relationship in order to provide the guideline for accelerator design. Therefore, the aim of the thesis is to 1) develop new benzoxazine and benzoxazine/epoxy system with good processability and material properties; 2) establish the structure-property relationship; 3) investigate the reaction mechanism of benzoxazine and amine 4) introduce a new methodology for developing accelerators to benefit both polymerization and material properties. The first part of the thesis reports the development of a new benzoxazine through convenient preparation method with inexpensive raw materials to overcome the challenges of high liquefying and curing temperature. The product is liquid under room temperature with a low viscosity suitable for resin transfer molding. The incorporated amine accelerates the polymerization at low temperature, which is much faster than that of conventional benzoxazine. By copolymerization with epoxy, the obtained copolymers show better material performances, which include higher glass transition temperature, higher char yield, high flexural modulus and strength. The second part focuses on establishing the structure-property relationship of benzoxazine and amine. Phenylenediamine isomers are selected in this part of study and the directing effect of amino group on aryl ring is highlighted. Other than the conventional amine-attacking path when any kind of amine is used, aryl-attacking path is found only when meta-isomer is applied. The new path siginificantly changes the reactivity of meta-isomer with benzoxazine and dramatically influences the material properties. Due to the aryl-attacking path, the prepared material with meta-isomer shows higher crosslink density, higher glass transition temperature and better thermal stability than the materials prepared with other amines. Besides, when the isomers are applied in the benzoxazine/amine/epoxy ternary system, only the copolymer with meta-isomer shows comparable glass transition temperature to the conventional epoxy, reduction in glass transition temperature is observed when other amines are applied. The third part studies the reaction mechanism of benzoxazine and amine. This part first reports the room temperature reaction between benzoxazine and amine. By analyzing the reaction rate and conversion at equilibrium, it is clear that amine-attacking is a reversible process while aryl-attacking is an irreversible process. The influence of cationic counterpart on the reaction is also confirmed. By further introducing the conceptual density functional theory, the nucleophilicty of the amine is proved to determine the reactivity of benzxoazine and amine. Thus, the reaction mechanism between benzxoazine and amine is clarified. The last part provides a new methodology on developing accelerators for benzoxazines. Unlike tranditional method, of which the accelerators are applied as small molecules, the new method describes the utilization of accelerator oligomers to react with benzoxazine. To investigate the key factor on thermal stability, a chain-type benzoxazine is selected for comparison simultaneously. The results indicate that although both small molecular accelerator and accelerator oligomer can accelerate polymerization, only when accelerator oligomer is used, the thermal properties of the materials (glass transition temperature, thermal stability and char yield) are enhanced. The small molecular accelerator is the dominant factor that induces the reduction in thermal stability of the material. In conclusion, the thesis introduces a new benzoxazine and benzoxazine/epoxy systems with better processability and material properties than conventional benzoxazine. Meanwhile, it also establishes the structure-property relationship of benzoxazine and amine and clarfies the reaction mechanism. Based on this, a new methodology is provided for accelerator design, which is helpful for the development of benzoxazine accelerator in the future.

Topic: Epoxy Resin Curing Processes
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences