Synthesis and Reprocessability & Flame-retardancy of Benzoxazine Resins Containing Silicon,Boron Elements

Author:Gao Sheng

Supervisor:lu zai jun


Degree Year:2019





The available reprocessable thermosetting resins(RTRs)have relatively poor heat resistance due to that the reversible covalent bonds incorporated into such thermosetting resins undergo reversible reaction at relatively low temperature(less than 200℃),which severely limits their application in the field of high temperature resistance.Therefore,it is urgent to develop high temperature resistant RTRs.The polybenzoxazines(PBZs)possess phenolic hydroxyl groups and Mannich base bridge structures,which endow PBZs with certain heat resistance and flame retardancy.However,this does not meet the requirements of high temperature resistance and green flame retardancy of flame retardant materials in the related fields.In this dissertation,we are committed to develop high temperature resistant reprocessable PBZs and PBZs with high temperature resistance and green flame retardancy.Dynamic aromatic silyl ether(Si-O-Ph)bonds cross-linked reprocessable PBZs and borosiloxane-containing PBZs with high temperature resistance and green flame retardancy were synthesized.The synthesis as well as the rerpocessability,flame retardancy of the two kinds of materials were investigated in detail.The main contents of this dissertation are presented below:Part Ⅰ Synthesis and properties of monofunctional benzoxazine resin crosslinkedvia Si-O-Ph bondsTo overcome the problem that the glass transition temperature(Tg)of RTRs is generally lower than 200℃,we propose a novel way to crosslink benzoxazine resin[poly(P-mdes)]via high temperature resistant dynamic Si-O-Ph bonds.The results show that the dynamic Si-O-Ph bonds are reversible only at high temperature,which enables poly(P-mdes)to be a high temperature resistant thermoset with its Tg(tan θ)reaching 301℃.This Tg is much higher than those of all available RTRs so far.Besides,poly(P-mdes)can be repeatedly reprocessed under hot pressing condition,and still retains 74.0%of the original tensile strength,even after the forth reprocessing procedure.Under acidic conditions(pH=1),poly(P-mdes)is also degraded into oligomers with low molecular weights.Part Ⅱ Synthesis and properties of difunctional benzoxazine resin further crosslinked via Si-O-Ph bondsTo pursuit higher heat resistant property is an eternal motif for investigating thermosetting resins.Based on the work of Part Ⅰ,we have prepared a reprocessable benzoxazine resin with much higher Tg.Siloxane-containing bifunctional benzoxazine(BA-mdes)monomers were synthesized via mannich reaction of bisphenol A,paraformaldehyde and y-aminopropylmethyldiethoxysilane.The BA-mdes monomers were thermally ring-opening polymerized to form PBZ network containing phenolic hydroxyl groups and siloxane side groups,and then the phenolic hydroxyl groups and the siloxane side groups further reacted to form a dynamic Si-O-Ph bonds further crosslinked poly(BA-mdes).The results show that the incorporation of dynamic Si-O-Ph bonds endows poly(BA-mdes)with reprocessability and is quietly effective in improving the thermal properties,maintaining the hydrophobic property and improving the dielectric property.The Tg of PBZ,5%weight loss temperature(Td5)and 10%weight loss temperature(Td10)increase from pristine 170 0C,302 0C and 327℃ to≈400℃,425℃ and 476℃,respectively.The water contact angle of poly(BA-mdes)is 103.5°,and the dielectric constant decreases from 4.94 to 3.10.This work provides a simple and low-cost method for preparing novel PBZ,the poly(BA-mdes)prepared by this method can be applied as reprocessable,high temperature resistant,hydrophobic and low dielectric materials.Part Ⅲ Synthesis and properties of borosiloxane structure-containing benzoxazine resinsThe borosiloxane structure-containing benzoxazine macromonomers(BZ-BSiO-X)were firstly synthesized via condensation reaction between different feed ratios of P-mdes,boric acid(BA),and diphenyldimethoxysilane(DPDMS).Then,poly(BZ-BSiO-X)s were prepared by ring-opening polymerization of the benzoxazine groups in BZ-BSiO-X at elevated temperature.The properties of poly(BZ-BSiO-X)s were investigated by Dynamic mechanical analysis(DMA),Thermogravimetric analysis(TGA),and Oxygen index instrument.The results show that P-mdes enhances the heat resistance of poly(BZ-BSiO-X)s,and enables their Tgs(tan θ)to be higher than 300℃.The borosiloxane part formed by DPDMS and BA is beneficial to improve the thermal stability and flame retardancy of poly(BZ-BSiO-X)s,so that their Td5s are over 400℃,and their limited oxygen index(LOI)values are above 30.This kind of thermosetting resins containing two green flame retardant elements[boron(B)and silicon(Si)]can replace halogen or phosphorus-containing flame retardant materials,and have the potential to be applied as green flame retardant materials in related fields.Part Ⅳ Synthesis and properties of borosiloxane-benzoxazine copolymersAccording to the results from Part Ⅲ,the LOI values were not significantly improved even though the green flame retardancy and high temperature resistance of benzoxazine resins were achieved.Here we propose a way to increase the LOI value by copolymerizing borosiloxane with normal benzoxazine monomers.A series of novel borosiloxane-benzoxazine copolymers were synthesized through the copolymerization of 4,4’-phenylmethanebis(3,4-dihydro-2H-1,3-benzoxazine)(P-ddm)monomers and phenolic hydroxyl groups functionalized borosiloxane(BSi-OH)oligomer.The structures were characterized using Nuclear magnetic resonance(NMR)and Fourier transform infrared(FTIR).The thermal and flame retardant properties of the copolymers were investigated by DMA,TGA,and Oxygen index instrument.The results showed that the addition of BSi-OH oligomer is not only highly efficient in improving green flame retardancy of PBZ,but also enhances their thermal properties Only 25 wt%content of BSi-OH oligomer was able to increase Tg,Td5,Td10,and LOI values from original 211℃,374 0C,395 ℃,and 29.5 to 244℃,408℃,448℃,and 40.1,respectively.This work provides a facile and useful method to prepare new PBZs possessing highly efficient and environmentally-friendly flame retardancy as well as good heat resistance.