Synthesis, Polymerization, And Performance Of Fluorene Based Benzoxazines

Polybenzoxazines, as a class of thermosetting phenolic resins formed by the thermal ring-opening of the corresponding benzoxazines monomers without any catalyst, have demonstrated various attractive properties such as high thermal stability, high char yields, high glass transition temperature (Tg), near-zero volumetric change upon curing, good mechanical and dielectric properties, low water absorption, and low flammability. These unique characteristics make the polybenzoxazines a better candidate over epoxies and traditional phenolic resins in the electronics, aerospace, and other industries.9,9-bis(4-hydroxyphenyl)-fluorene (BHPF) was synthesized from phenol and fluorenone as raw materials in the presence of liquid acids and solid acids as catalyst by microwave-assisted synthesis technology. A series of difunctional fluorene-based benzoxazine monomers were synthesized from the reaction of BHPF with formaldehyde and primary amines. Their chemical structures were confirmed by FTIR, 1H and 13C NMR analyses. The polymerization behaviors, curing dynamics, and network structure of the precursors and polymers were monitored by differential scanning calorimetry (DSC) and FT-IR. The mechanical performance and thermal properties of neat polybenzoxazines and blending resins were evaluated with DSC, Dynamic Mechanical Analyzer (DMA) and Thermogravimetric Analyzer (TGA).The experimental results show that the yields of BHPF was more than 83% under the optimal process conditions that the molar ratio of phenol to fluorenone was 8 to 1, the mass percentage of catalysts including sulfuric acid and methylsulphonic acid was 2-5% of the total reactants, reaction temperature was 45-55℃and reaction time was 30 min. When solid acids such as phosphotungstic acid and strongly acid cation exchange resin were used as catalyst, the yields of BHPF was more than 70% under the optimal process conditions that the mass percentage of catalysts was 6-9%, reaction temperature was 130℃and reaction time was 90 min. The melting point of BHPF was 223-224℃. The synthetic method using strongly acid cation exchange resin as catalyst will be the direction of development in the futrue.New fluorene-based bezoxazine monomers was synthesized under the optimal process conditions that the molar ratio of BHPF, formaldehyde, and primary amine including aniline, o-toluidine, m-toluidine,p-toluidine,3,5-di-methyl-toluidine, n-butylamine, n-octylamine, cyclohexylamine, allylamine, and propargyl amine was 1:4:2, the mixtures of dioxane and anhydrous ethanol, which the volume ratio is 1:1, were used as solvent, the addition reaction temperature between formaldehyde and primary amine was less than 5℃, the addition reaction time was 2 h, the ring-closing reaction temperature between addition product and bisphenol fluorene was 90-100℃, the ring-closing reaction time was 5-7 h. The yields of fluorene-based bezoxazine monomers were 50-63%.The fluorene based polybenzoxazines show the typical curing characteristic of oxazine ring-opening for difunctional benzoxazines centred at 231-250℃. The chain segments of polymers consist of phenolic Mannich bridge network, arylamine Mannich bridge, arylamine methylene bridge, intramolecular hydrogen bonding, and intermolecular hydrogen bonding. The pendant arylamine rings activated in the para position can undergo an aminomethylation reaction during the polymerization. The curing kinetics and curing craft of fluorene-based bezoxazines was investigated using non-isothermal DSC. and determined by Kissinger, Ozawa and Crane methods. The activation energy of fluorene-based bezoxazines calculated by Kissinger and Ozawa method is 122-174 kJ/mol. The reaction order is approximate 1. The step curing profil of fluorene-based bezoxazines is as follows:170-180℃/2 h,200-220℃/3 h,240-250℃/2 h.The introduction of bulky fluorene moieties into the benzoxazine chains must arise from the higher rigidity of fluorene skeleton in the chain backbone, which restrains the internal rotations and thermal motion of polymer segments. As a result, the Tg values of fluorene-based polybenzoxazines are higher than those of the relative bisphenol A-based polybenzoxazines. In addition, the tighter packing of the polymer chains due to the strong intermolecular and intramolecular hydrogen bonding has significant influence on the Tg of the fluorene containing polybenzoxazines, which confines segmental mobility and contributes to a rigidity of the polymer chains. The aromatic amine-fluorene-based polybenzoxazines are more chemically cross-linked and have tighter packing and higher crosslinking density than the linear aliphatic aminebased polybenzoxazines. The polybenzoxazines containing polymerized groups possess higher crosslinking density. Therefore, the Tg values of polymers increase dramatically. The Tg values of aliphatic amine-fluorene-based polybenzoxazines containing flexible pendant groups significantly decrease as the increases of the length of aliphatic groups.The thermal stability of fluorene-based polybenzoxazines is connected with the fluorenyl structure, substituent structure of amines, and network structure of polymers. The introduction of rigid fluorene skeleton with bulky pendent Cardo moieties into benzoxazine monomers can improve the inherent thermal stability of the thermosets dramatically. Furthermore, the intramolecular hydrogen bonding of the polymer chains, while intermolecular hydrogen bonds typically weaken significantly above Tg, has a tendency to stabilize the molecule. These may contribute to the improvement of thermal stability of the fluorene based polybenzoxazines. The introduction of polymerized groups such as allyl and propargyl into benzoxazine chains can improve the crosslinking density of polymers, and be conductive to the improvement of initial thermo-decomposing temperature and char yield. The thermal stability of propargyl amine-fluorene-based polybenzoxazine is the highest in all fluorene-based polybenzoxazines. The polybenzoxazines derived from aromatic amines attributed to the high aromatic content have the better thermal stability than those derived from aliphatic amines.Epoxy resins can catalyze the ring-opening of benzoxazines to reduce polymerizing temperature, and involve in the crosslinking reaction. The results lead to enhancement of the internal plasticization of polymer molecular, decrease of the rigidity and packing of the polymer chains, drop of the Tg values, and improvement of thermal stability. The decrease of char yields is relative to the dosages of benzoxazines. There is a single decomposition mechanism for the blending resins of benzoxazines/epoxy resins. The introduction of epoxy resins into polybenzoxazine system can improve the thermal stability and processability of polymers at a certain extent.2-ethyl-4-methyl-imidazole (EMI) can catalyze the ring-opening of benzoxazines. The thermal stability and char yields of copolymers consisting of benzoxazines and EMI are higher than those of the neat polybenzoxazines and blending resins of fluorene-based benzoxazines and E-51 epoxy resins. The polymerization resction for ternary blending resins, containing fluorene-based benzoxazines, E-51 epoxy resins, and EMI, undergos three stages. EMI has profit to the formation of homogeneous copolymers and the improvement of crosslinking density and compatibility of blending resins in the ternary system. There is no phenomena of phase separation. The comprehensive performances of the ternary blending resins are better than those of neat corresponding benzoxazines and blending resins of fluorene-based benzoxazines and E-51 epoxy resins without EMI, which may be a good candidate for high performance composite matrices, electronic encapsulation materials, laminate materials, insulation materials and fire resistant materials, etc.