PREPARATION, PROPERTIES, AND APPLICATIONS OF BENZOXAZINES

Abstract

The motivation behind the present research work is to develop low temperature curing sustainable benzoxazines which exhibit enormous potential to compete with the existing petro-based advance performance. This can be achieved through two approaches i.e., structural modification and physical blending of curing accelerator in the resin. Cardanol was chosen as renewable alternative of petro-based phenol which acts as a reactive diluent and aids solventless synthesis of benzoxazine resins. A microwave assisted synthetic (MAS) route was explored as a sustainable tool for the preparation of benzoxazine resins. In comparison to the conventional methodology, the reaction completion time could be significantly reduced using MAS technique and the sustainability of the procedure was improved. Microwave active bifunctional amines were prepared by the condensation reaction of p-aminobenzoic acid and poly(ethylene glycol)s of different chain lengths to yield amine terminated poly(ethylene glycol)s (ATPEGs). Cardanol, an agro-waste was chosen as the phenolic source, which was reacted with ATPEG to undergo Mannich like condensation resulting in a reactive thermoplastic of telechelic nature. The structure of the resulting monomer was confirmed through Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) spectroscopy. Benzoxazine moieties present at the terminals undergo thermally accelerated ring opening polymerization to form cross-linked networks which was studied using non isothermal differential scanning calorimetry (DSC). The rheological behavior of the resulting polymer suggests that the viscosity of the benzoxazine-endcapped telechelic poly(ethylene glycol)s is sufficiently low to permit solventless processing which can be credited to the presence of flexible polyether linkages. The adhesive properties of

the cross-linked benzoxazine endcapped telechelic poly(ethylene glycol)s have also been studied. Sustainable bis-benzoxazine resins with amide linkages were synthesised where the effect of inclusion of amide linkages in the monomer is expected to result in a monomer with lower polymerisation temperature and better adhesive properties. Polyethylene terephthalate (PET) was chosen as a sustainable feedstock for the amine fraction used to prepare benzoxazine monomer containing amide linkages. Microwave assisted aminolysis of PET was performed to obtain bis-(amino-ethyl) terephthalamide (BAET) and ,-aminoligo(ethylene terephthalamide) (AOET), which were employed as the difunctional amine for the preparation of bis benzoxazines. In comparison to traditional method, microwave assisted aminolysis of PET was found to be significantly faster and the reaction completion time could be brought down appreciably. Mannich like condensation of cardanol with PET derived terephthalamides and paraformaldehyde led to the formation of bis-benzoxazines with amide linkages, the structure of which was confirmed through FT-IR and 1H-NMR spectroscopy. The curing behavior of bis-benzoxazines was studied using non isothermal differential scanning calorimetry. The presence of amide linkages in addition to the polar group formed during the ring opening of benzoxazines led to the improvement in adhesive strength which was quantified in terms of lap shear strength. In addition, urea linkages were introduced in the polybenzoxazine network through ring opening polymerisation of a benzoxazine monomer containing urea linkages. The amine co-reactant for the synthesis of benzoxazine monomer was derived by the additive rearrangement of 4,4'-methylenebis(phenyl isocyanate) (MDI) with ethylene diamine, which underwent Mannich like condensation with cardanol and

paraformaldehyde to yield bio-based benzoxazine monomer containing urea linkages. The structure of the amine and the benzoxazine monomer was characterized by FT-IR and 1H-NMR. Benzoxazine monomer undergoes thermally accelerated ring opening polymerization to form cross-linked networks, which has been demonstrated using rheometry and non-isothermal differential scanning calorimetry. The presence of alternating urea linkages in the benzoxazine network improves the adhesive properties of the resin, which was quantified in terms of Lap shear strength. Thermal degradation of the cross-linked copolymer has also been studied by thermogravimetric analysis (TGA). Curing accelerators were also explored to lower the polymerisation temperature of the bio-based benzoxazine resins. A representative bio-based benzoxazine resin was synthesized by mannich type condensation of cardanol and aniline with formaldehyde under solventless conditions, the structure of which was confirmed using FTIR and 1H-NMR. The curing behaviour of the synthesized resin has been systematically investigated using non-isothermal differential scanning calorimetry. Metal organic frameworks, in view of their high surface area and catalytic activity, are potential candidates for curing accelerators. MOFs have been solvothermally synthesized and characterized using different techniques including powder X-ray diffraction (PXRD), Scanning Electron Microscopy (SEM), TGA, FT-IR and nitrogen physisorption measurements. Introduction of MOFs led to a shift in the curing profile to lower temperature, the extent of which was proportional to the amount of MOFs in the formulation. The activation energy associated with the curing of the resin, has been calculated using Kissinger Akahira Sunose method, was found to concomitantly decrease from 98 to 58 kJ/mol upon addition of MOF-5 (5% w/w).

Most of the curing accelerators are moisture sensitive, which adversely affect the end performance of the polymer. In this context, the potential of stearates based on transition metals as curing accelerators for the polymerization of cardanol based benzoxazine resin has been demonstrated. Metal stearates were formulated with bio based monomer which leads to significant lowering of the curing profiles, the extent of which was proportional to the amount of accelerator. The hydrophobicity associated with the long alkyl chain in stearate bestows it hydrolytic stability, which allows its use under ambient conditions without special caution. Kinetic parameters associated with polymerization of the resin were established using Kissinger Akahira Sunose method. Cardanol-aniline benzoxazine resin was microencapsulated in different polymeric shell walls for temperature triggered healing applications following two approaches. The very first approach which was adopted is solvent evaporation, a “physical” entrapment of monomer and is relatively less complex methodology which is routinely employed for the encapsulation of drugs for pharmaceutical applications. The monomer was encapsulated in poly(styrene) (PS) shells by solvent evaporation technique to obtain spherical microcapsules. The procedure was optimized by studying the effect of operating parameters, particularly stirring speed and PS concentration on microcapsule dimensions and core content. Spherical microcapsules with a core content of ~39 % were obtained when the reaction was carried out at 500 rpm, while maintaining the reaction medium at 60C with 2 % w/v concentration of PS in feed solution. In addition, the resin is successfully microencapsulated in cross linked epoxy microspheres through interfacial engineering. The encapsulation process relies on the preferential reaction of polydimethylsiloxane immiscible epoxy resin and

amine based hardener to form a crosslinked spherical shell at the interface.The microcapsule dimensions and core content could be tailored by modulating the operating parameters, particularly stirring speed and Bz-C:epoxy ratio. Spherical microcapsules with a core content of ~37% were obtained when the reaction was carried out at 600 rpm, while maintaining the reaction medium at 70C with Bz-C: epoxy ratio of 2.3:1.