Synthesis Of Biobased Biphenyl Epoxy Oligomer With Low Viscosity And Study On The Properties Of Its Cured Resin

Epoxy resins are extensively applied in composites,electronic appliances,aerospace,and other fields owing to their low shrinkage,excellent mechanical and electrical performances,and chemical resistance.Among them,epoxy resins derived from bisphenol A（BPA）take up dominance.With the development of global industrialization,petroleum resources are becoming increasingly limited.And the raw material BPA for bisphenol A-based epoxy resin is toxic,restricting its further development and application.Additionally,all the traditional petroleum-based epoxy resins exhibit a drawback of high flammability.Currently,bio-based epoxy resins usually have a dilemma between processability,heat-resistance,and flame retardancy,making it difficult to meet the demands of high-temperature and fire-retardant application.Magnolol,which is extracted from magnolia trunks,is renewable and low toxic.It is widely used in Chinese medicine and make-up.Due to the unique biphenyl structure of magnolol moieties,the materials exhibit excellent heat-resistance,superior thermal stability,and high carbon content.Furthermore,the allyl group promotes molecular chain mobility.Consequently,magnolol combines a rigid biphenyl structure with two flexible aliphatic allyl groups,which is expected to obtain a novel biobased epoxy resin with low viscosity,excellent heat-resistance,and intrinsic flame retardancy.The main contents and results are as follows:（1）The bio-based biphenyl epoxy oligomer P-DBP-EP-1 was synthesized using magnolol and epichlorohydrin under alkaline conditions.To investigate the effect of crosslinking density of the cured network on heat-resistance and flame retardancy,P-DBP-EP-1 was cured with the amine curing agent 4,4’-diaminodiphenyl sulfone（DDM）,followed by a thermal addition polymerization of allyl groups.The results showed that the crosslinking density of the cured network gradually increased as the polymerization degree increased.It improved the glass transition temperature（T_g）of the cured resin from 100°C to 300°C,which was 94.8%higher than E51/DDM.The cured resin’s initial decomposition temperature was as high as 421°C,with a char yield of 35.2%at 700°C and a LOI value of 48.0%,passing the UL94 V-0 rating,indicating that the cured resins had outstanding heat-resistance and flame retardancy.（2）To regulate the properties of cured epoxy resins,a series of oligomers P-DBP-EP-n with different polymerization degrees were synthesized using epoxy oligomer and magnolol with tetrabutylammonium bromide（TBAB）as a catalyst.P-DBP-EP-n/DDM cured resins were obtained through two-step curing.To evaluate the effect of polymerization degree on various properties,we characterized a range of properties of P-DBP-EP-n/DDM,including thermal,mechanical,and flame retardant properties.The results showed that,as the degree of polymerization increased,the product changed from a low-viscosity liquid（n=1,11.7 Pa·s at25°C）to a solid（n=3 and n=5）.Increases in the T_gvalues from 300 to 334°C and>350°C further improved the heat resistance.The initial decomposition temperatures(T_5%)exceeded410°C.The increased content of aliphatic chain segments produced by allyl thermal addition reaction increased the tensile strength of the cured resin by 9.7%and 21.7%,and the notched impact strength by 4.0%and 16.0%,respectively.The samples all passed the UL 94 V-0 rating,with LOI values of 48.0%（P-DBP-EP-1/DDM）,41.0%（P-DBP-EP-3/DDM）,and 39.0%（P-DBP-EP-5/DDM）.The material’s outstanding flame-retardant properties are due to the highly crosslinked curing network with rigid structures.（3）The P-DBP-EP-n/DDM system’s second curing conditions are harsh（temperature>300°C,time>8h）,and the cured resin is highly brittle.To reduce the curing temperature of the P-DBP-EP-n/DDM cured resins and further regulate the resin’s mechanical properties,aliphatic thiol esters were introduced to form a thiol-ene network via a click reaction with allyl,building a dual-cured networks with the epoxy-amine network.The results showed that the thiol compound has high reactivity and could react with allyl under mild conditions,thereby lowering the curing temperature of allyl from>300°C to 80～100°C.Tensile strength of the cured resin increased from 20.7 MPa to 42.7 and 47.3 MPa,with a vast increase of 106.3%and 128.5%,respectively.Although the introduction of aliphatic thiols had a negative impact on the thermal stability,the initial degradation temperatures maintained at 350°C.Additionally,the thiol components weakened the rigidity,leading to a significant decrease in storage modulus and a corresponding decrease in T_g to 58°C and 80°C,respectively.However,compared with biobased thiol-ene systems（-25℃g<70℃）,P-DBP-EP-1/Thiol/DDM possessed superior thermomechanical properties.The T_g of the P-DBP-EP-1/PETMP/DDM system is even 53.8%higher than that of the cured resins with dual thiol-ene and thiol-epoxy networks.