Unsaturated Polyester Molecular Design For Both Diluent And Prepolymer With Property Study On Its Composites

Unsaturated polyester resins composed of two main igridents:unsaturated polyester prepolymer and active diluent.Styrene was widely used as an active diluent in commercially available unsaturated polyester resins,but suffered problems like high volatility,terrible toxicity,and low sustainablity.Therefore,development of bio-based diluents with low volatility is an important researching direction for unsaturated polyester resin.Ferulic acid can be obtained from lignin,which is widely available.Our previous works showed that the synthesis of bio-based active diluent,4-acetoxy-3-methoxystyrene（AC4VG）from ferulic acid reached a limited conversion rate,and AC4VG failed to result in an unsaturated polyester resin with satisfactory thermal and mechanical properties.In the work,a bio-based active diluent,4-vinyl guaiacol glycidyl ether（DE4VG）was synthesized from ferulic acid via a simple process with two steps:decarboxylation and epoxidation.For mixing with a general unsaturated polyester prepolymer,PMSD,which was synthesized from maleic anhydride,succinic acid,1,3-propylene glycol,DE4VG offered promising merits in terms of viscosity and volatility.The PMSD/DE4VG（3:2）mixture showed a lowest viscosity of 0.85 Pa·s at room temperature,and reduced weight only 2.3%at 40℃for 2 h.DE4VG provides vinyl and epoxy groups,therefore,with the assistance ofinitiator of tert-butyl peroxybenzoate（TBPB）and an epoxy curing agent of 4,4-diaminodiphenylmethane（DDM）,the cured resin resulted in higher crosslinking density（2.59 mol/cm³）,T_g and T_d5%temperatures（195°C and 316°C）,than that of general unsaturated polyester resins.These results demonstrate that DE4VG could effectively improve the thermo-mechanical properties of the unsaturated polyester resin,and therefore has a greater application potential.Generally,unsaturated polyester resins are difficult to be degraded once highly cross-linked,therefore remained a great pressure on the environment.Based on the aforementioned research,we further plan to solve the undegradable problem of unsaturated resin.Thus,2-mercaptoacetic acid was oxidized into a dicarboxylic acid containing disulfide bond,and then used as a monomer for unsaturated polyester prepolymers.A number of cured resins were prepared using styrene as the active diluent.As a result,the presence of a large number of disulfide bonds in the cured resin leads to a completely degradable resin in the 2-mercaptoethanol/N,N-dimethylformamide（β-ME/DMF）solvent mixture.Moreover,when the UPRs were used for preparing composite reinforced by cotton fibers,excellent mechanical properties were obtained.The tensile strength reached to 86.76 MPa,and the cotton fabric could be recycled in the mixed solventβ-ME/DMF（2:8,V/V）at 70°C by the optimized degradation conditions with insignificant changes in terms of mechanical strength.These results suggest that introduction of disulfide bonds into unsaturated polyesters can impart sufficient degradation ability to the cured unsaturated polyester resins,providing a new way for clean treatment of the composite wastes,and for the recycling of the reinforcing fiber materials.