Synthesis Of Pyrogallol-based Benzoxazines And The Study Of Their Ring-opening Polymerization And Flame Retardancy

Polybenzoxazine,a kind of novel thermosetting resin,has gained high attention in academic in recent year due to excellent mechanical and thermal properties,remarkable chemical resistance,minimal moisture absorption,low surface energy and dielectric constant,higher char yield and outstanding fire resistance.In general,benzoxazine can be obtained by Mannich condensation of phenols,primary amines,and formaldehyde or paraformaldehyde,and allows flexible molecular design because of the variety of phenols and primary amines.Polybenzoxazine can be obtained by the heat treatment?upon 200 ??,which restricts the processing of polybenzoxazine.Therefore,the reduction of curing temperature for benzoxazines is still an important subject.Previous studies have shown that the incorporation of phenolic hydroxyl or amide,etc.into benzoxazine is one of the ways to decrease the ring-opening polymerization?ROP?temperature.Benzoxazine with latent catalytic characteristics is due to the formation of intramolecular hydrogen bonding at ambient temperatures,which is beneficial to enhance the shelf life of benzoxazine.With the reducing of petroleum resources and the improvement of environmental awareness,bio-based platform chemical have gained enormous interest in academic and industrial community.Pyrogallol as a candidate from biomaterials can be extracted from tannin or lignin.Thereby,the benzoxazines based on pyrogallol were synthesized and their ROP behaviors were studied,the main contents are as follows:Firstly,pyrogallol,paraformaldehyde,furfurylamine or aniline were used to synthesize twokindsofdifunctionalbenzoxazines,3,7-bis?furan-2-ylmethyl?-3,4,7,8-tetrahydro-2H,6H-benzo[1,2-e:5,4-e?]bis?[1,3]oxazine?-10-ol?PG-FA?and3,7-diphenyl-3,4,7,8-tetrahydro-2H,6-Hbenzo[1,2-e:5,4-e?]bis?[1,3]oxazine?-10-ol?PG-A?.The chemical structures of PG-FA and PG-A were identified by ¹H NMR,¹³C NMR and FTIR tests.Secondly,the ROP behavior of PG-FA and PG-A was monitored by DSC,TGA,FTIR and in situ FTIR measurements.The activation energy of PG-FA and PG-A was calculated via Kissinger and Ozawa methods.The activation energy of PG-FA was 94?Kissinger?and 96.9kJ/mol?Ozawa?.As to PG-A,it was 92?Kissinger?and 94.7 kJ/mol?Ozawa?.With the temperature increasing,the inter-and intramolecular hydrogen bonds between phenolic hydroxyl and oxygen or nitrogen atoms,forming at ambient temperature,were broken and converted into weak-OH??intramolecular hydrogen bonding,which easily released the free phenolic hydroxyl.It accelerated the opening of oxazine ring in PG-FA and PG-A.The carbenium ions could preferentially interact with the activated carbon atom in pyrogallol moiety after the breakage of oxazine ring.The free phenolic hydroxyl in PG-FA and PG-A exhibited the catalytic characteristics as they reached melting condition.Therefore,PG-FA and PG-A monomers display latent active catalytic characteristics.Finaly,four kinds of benzoxazine mixtures were prepared based on PG-FA or PG-A with bisphenol A-aniline-based benzoxazine?Ba?or phenol-4,4-diaminodiphenyl methane-based benzoxazine?PMDA?.The ROP behavior of benzoxazine mixtures has been investigated by DSC and FTIR tests.The ROP temperature of Ba and PMDA gradually decreased with the increase of PG-FA or PG-A contents.In other words,the phenolic hydroxyl in PG-FA or PG-A was beneficial to the ROP of Ba and PMDA.The effects of hydrogen bonds on storage modulus and glass transition temperature were investigated by FTIR and dynamic mechanical analysis?DMA?.The-OH?O?inter?and-OH?O?intra?hydrogen bonds in PG-FA or PG-A made their storage modulus at room temperature increase and also leaded to their glass transition temperature reduce because-OH?O?inter?and-OH?O?intra?hydrogen bonds dissociate at high temperatures.The thermal stabilities and flame retardant properties of polybenzoxazines were studied by TGA,TG-MS,LOI,and vertical flame tests.TGA curves showed that char yield of P?PG-FA/PMDA?and P?PG-A/PMDA?copolymers at 810 ? exceeded 50%under nitrogen,which was higher than that of PPMDA?42%?,excepting for P?PG-FA/PMDA?0.05:1and P?PG-A/PMDA?0.05:1.That is to say,the thermal property of PPMDA was improved by the introduction of PG-FA or PG-A.In the air atmosphere,the TGA test results showed that the oxidative degradation behavior of copolymers was consistent with PPMDA.Vertical burning and limit oxygen index?LOI?test results showed that the flame retardancy of PPMDA was significantly improved by the addition of PG-A and PG-FA.When the ratio of PG-A or PG-FA and PMDA was 0.1:1?mol:mol?,the LOI value of PPMDA increased from 26.6 to 29.6 and 30,respectively.