Structure-Processing-Property Relationships of Furanyl Benzoxazine Polymers Derived from Bio-based Feedstocks
Authors:   , Yu Mengwen , Palmese Giuseppe Raffaello 
Published: November 2022

Polybenzoxazine materials are a relatively new class of phenolic polymers with a broad range of interesting properties and advantages, including high char yield and glass transition temperature (Tg), low water absorption, excellent flammability resistance, and near-zero shrinkage upon curing. These versatile traits allow us to tailor the performance properties of benzoxazines for a wide range of applications. Bio-based benzoxazine materials have gained significant attention due to environmental concerns. A variety of natural phenols derived from lignin and furan-based compounds derived from cellulose and hemicellulose can be used to prepare such benzoxazines. In this work, structure-processing-property relationships were developed using bio-derived feedstocks. A series of novel and fully bio-based benzoxazine monomers were synthesized from di-furan diamine (DFDA) and mono-furan amine in combination with several bio-based phenolic compounds. Their processability, curing mechanism, thermal properties and mechanical behavior were investigated using multiple techniques. A structure-processing-property study using DFDA with a series of bio-derived phenolic compounds showed that the phenolic structure affects the curing mechanisms of DFDA-based benzoxazine. Benzoxazines that have available ortho or para positions on the phenolic structures complete curing reactions without mass loss. Phenolic structures with ortho substitution react more slowly than those with para substitution and systems with both ortho and para positions substituted react only after elimination of the para substituent that results in mass loss. Compared with di-benzene analogues based on carcinogenic methylene di-aniline (MDA), incorporation of furan rings into the backbone of the polybenzoxazine network improves processibility, char yield and Tg. Nevertheless, these systems are difficult to process by liquid molding techniques because of their high viscosity and melt temperatures. To improve the processability of current bis-benzoxazine and DFDA-based benzoxazine systems without sacrificing thermal and mechanical performance, two approaches based on monomer structural changes were investigated. First, the hypothesis that asymmetrical bis-benzoxazines should possess lower melting points and lower viscosity was tested by synthesizing asymmetrical DFDA benzoxazine structures using varying ratios of phenol and cardanol. The properties of these materials were compared to equivalent blends of the pure symmetrical benzoxazines to determine that the asymmetrical structures possess significantly lower viscosity and can be liquid at room temperature. An optimum composition of BZ-DFDA-Phenol/Cardanol (80:20) has a viscosity of 9.6 Pa.s at 60 °C and Tg of 237 °C, and it provides useful and potentially commercially viable DFDA-based benzoxazines. Second, mono-furan based benzoxazines based on furfuryl amine and a series of para substituted phenols were evaluated. These monomers were expected to result in benzoxazine systems with a lower viscosity because of the lower molecular weight. A number of monofunctional benzoxazine monomers were found to have low melting points, including BZ-FA-P, BZ-FA-M, and BZ-FA-T-with melting points that ranged from 40 °C to 70 °C-and one of the benzoxazine monomers (BZ-FA-H) is liquid at room temperature with the viscosity of 1035 Pa.s at 20 °C and 1.5 Pa.s at 60 °C. These were not expected to cure effectively on their own because of the functionality; however, it was discovered that the incorporation of a primary aliphatic hydroxy group pendant to the phenolic ring provides a secondary polymerization mechanism that results in cross-linking. In particular, we propose and show that a reaction occurs between the hydroxy group and the 2- or 5- position of furan ring resulting in a cross-linked mono-benzoxazine network. It was found that the extent of this secondary reaction, which influences network structure, is easy to control by selecting post-cure temperature. Thus, thermal and mechanical performance of the cross-linked materials are determined by the post-cure temperature and can be tuned to obtain materials with very high toughness, strength and Tg. For example, BZ-FA-H cured at 220 °C has Tg of 269 °C, modulus of 5 GPa, tensile strength of 96 MPa with 3.7 % elongation at break, as well as K1c of 0.89 MPa*m1/2 and Gic of 141 J/m2. This system can easily be processed by liquid molding and has properties that exceed most commercially formulated systems.