Synthesis And Research On Degradation And Recovery Of Unsaturated Polyesters Containing Tertiary Easter For Composite Materials

Thermosetting resins and their composites are low-cost,lightweight,high-strength,and have a rapidly growing demand in aerospace,transportation,construction,energy,medical,sports and other fields.However,due to the stability of the three-dimensional cross-linking network,it is difficult to degrade and recycle thermosetting materials,which not only causes pressure on the environment but also high-value material waste and thus limits their sustainable development.To solve this problem,developing thermoset degradation technology and new degradable and recyclable thermosetting materials has become a research hotspot.In this paper,from the construction of selective cleavage structures,self-crosslinked thermosetting unsaturated polyesters and their composites combined both intrinsic and degradable properties were designed and prepared,and their degradation properties and mechanisms were studied.It provides a new idea for the aqueous phase degradation of thermosetting resins and the recovery of high-value reinforcement fibers.Firstly,a monomer containingβ-carbonyl-tertiary ester,aromatic nucleus and vinyl-terminals was synthesized and a thermosetting unsaturated polyester was self-crosslinked by monomer initiated by azobisisobutyronitrile.Obtained resin showed high C=C conversion rate,well thermal,mechanical properties and solvent resistance.With the introduction ofβ-carbonyl-tertiary ester,designed resin could be degraded in 20%-80%hydrazine aqueous solution at 30-90°C directly.It took only 3.5 h to finish resin deconstruction at condition 90°C/80%.The degradation process had heterogeneous vs homogeneous mode dominated by chemical reaction rate and physical erosion respectively and it is speculated that the selective cleavage was achieved by carbonyl addition activated tertiary ester elimination.The degradation suspension could be easily separated according to water solubility to obtain water-soluble poly-methacrylic acid(salt)-like product and dimethyl sulfoxide-soluble product containing amino and diphenyl methyl structures.Next,the structure design was optimized and a monomer containing both ester andβ-carbonyl-tertiary carbamate was synthesized.By adding ethyl and hydrogen bonds,the flexural properties of obtained unsaturated polyester were significantly improved.After cleavage structure optimization,the resin degraded most efficiently under hydrazine/Na OH molar ratio of 4/1,total concentration of 0.04 mol/m L and80°C.The degradation time was 4 h.For the resin containing both ester andβ-carbonyl-tertiary carbamate,it is speculated that the degradation reaction follows the process of selective cleavage promoting hydrolysis:OH~-in the solution destroyed hydrogen bonds,promoted the carbonyl addition and activated the selective cleavage.Benzene fragments fell from the network.The looser crosslinking network promoted ester alkaline hydrolysis and the residual elastomer further transformed into water-soluble polymethacrylate and ethanolamine,making the resin completely deconstructed.Obtained suspension could also be easily separated and recovered according to water solubility.Finally,carbon fiber composites with well thermal and mechanical properties were prepared based on designed unsaturated polyesters.Under optimal resin degradation conditions,composite degradation was completed faster with fabric structure remaining good.The effect of cleaning post-treatment(soaking duration,times)on the properties of recycled fibers was investigated.When oligomers were present in residues,the filament tensile strength and surface energy of recycled fibers were controllable enhanced.It was speculated that the residual oligomers were re-adsorbed on the fiber surface to form a uniform sizing layer,filling the original defects on fiber surface while introducing abundant active functional groups.When the residues were mainly small molecules,they can be cleaned directly and the properties of the recycled fibers were basically the same as that of virgin carbon fibers.In addition,the interfacial properties of epoxy resin composites prepared with reinforced recycled fibers were significantly enhanced and composites prepared with clean recycled fibers maintained the properties of virgin materials,which verified the excellent remanufacture ability of recycled carbon fibers.