Synthesis And Properties Study Of Reversible Adhesive Based On Acrylate Modified By Functional Monomers

Facing the problem of difficult cleaning and irreversible use,traditional structural adhesives bring great challenges to the environment and resources and cannot satisfy the transient positioning or allow misplaced positioning during the construction of the adhesive.Therefore,the development of green recyclable adhesives is beneficial to new materials development,environmental protection and resource conservation.At present,reversible adhesives have potential applications in the field of biological organ lap joints(especially teeth or bone connections),electronic component bonding,interior decoration and automotive assembly.In this dissertation,acrylate polymer was selected as a matrix resin,the polyphenol structure,boroxine structure and boronate structure were introduced into the acrylate resin structure to modify the acrylate adhesives via different reaction methods.The resulting adhesives presented thermoplastic reversible adhesion properties based on the self-repair of hydrogen bond and covalent bond.The polyphenol structures were gallic acid(GA)and caffeic acid(CA),and the phenylboronic acid was 4-formylphenylboronic acid(FBA).Reversible acrylate adhesive containing hydrogen bond,which was named as GA-IBH*,was prepared by introducing GA containing polyphenolic structure to the acrylate side chain.The introduction of GA increased the glass transition temperature,thermal failure temperature and cohesive strength of the polymer,but the initial degradation temperature was decreased from 246 °C to 200 °C.In addition,creep resistance and strain recovery rate of GA-IBH* were improved,which was affirmed by the fact that the holding power to stainless steel was increased from 18.0 min to 30.0 min.The introduction of GA improved the adhesion strength of polymer adhesive to carbon steel to 5.2 MPa.The adhesion strength to polypropylene(PP)and polytetrafluoroethylene(PTFE)was also improved.Reversible adhesion strength of GA-IBH* to carbon steel could reach 6.1 MPa.Its reversible adhesion mechanism was attributed to the self-repair of hydrogen bonds based on phenolic hydroxyl.Reversible acrylate adhesive containing covalent self-repair structure,which was named as TFBA-IBH,was synthesized by introducing triformyl boroxine(TFBA)to the acrylate side chain by acetalization reaction.After TFBA being introduced,the number average molecular weight of the polymer showed single molecular branch growth of TFBA,but the weight average molecular weight and the viscosity average molecular weight were increased by orders of magnitude.Compared with IBH,the glass transition temperature of TFBA-IBH had litter change.However,the chemical stability and thermal degradation stability of TFBA-IBH were both increased,and the initial degradation temperature was increased by 49.1 °C.TFBA-IBH had higher cohesive strength and initial tack at room temperature,but its creep resistance was reduced.The visual performance was that the holding power to stainless steel was reduced from 18.9 min to 1.3 min.The adhesion strength of TFBA-IBH to carbon steel(up to 5.86 MPa)was about 4.6 times higher than that of IBH.At the same time,the adhesion strength of TFBA-IBH to PP and PTFE could reach 0.85 MPa and 0.3 MPa,respectively.Moreover,the thermal reversible adhesion strength to carbon steel could reach 9.44 MPa during 10 cycles and its reversible adhesion mechanism was attributed to the self-repair of covalent linkage based on boroxine.Modification of the acrylate adhesive can be achieved in one step by radical copolymerization.However,due to the strong inhibition of phenolic hydroxyl groups,it is difficult to achieve a higher monomer conversion ratio.The protective group of phenylboronic acid was used to prepare the vinyl functional monomer(FBCA).Since the dehydration condensation reaction of phenylboronic acid and catechol was dynamic and reversible,the obtained vinyl functional monomer was the mixture of vinyl boronate(FBCA)and caffeic acid(CA).Both FBCA and CA had been involved in the polymerization,wherein the FBCA content was 0.46 wt%,and the content of CA was 0.11 wt%.Due to the free radical trapping effect of phenolic hydroxyl groups,the number average molecular weight of FBCA-IBH was slightly decreased.However,the weight average molecular weight and viscosity average molecular weight of FBCA-IBH were step-type increased due to the slight oxidative crosslinking of phenolic hydroxyl groups during polymerization.Compared to IBH,FBCA-IBH had higher glass transition temperature,thermal degradation temperature,cohesive strength,strain recovery ratio and holding power.The thermal degradation temperature was raised from 214 °C to 263 °C,and the holding power to stainless steel was increased from 18.9 min to 212.0 min.The adhesion strength of FBCA-IBH to carbon steel could reach 9.37 MPa,which far exceeded that of 502 commercial adhesive.The adhesion strength of FBCA-IBH to PP and PTFE could reach 1.15 MPa and 0.47 MPa,respectively.At the same time,FBCA-IBH showed excellent reversible adhesion performance.The reversible adhesion strength of FBCA-IBH to carbon steel reached 14.36 MPa during 18 cycles.Its reversible adhesion mechanism was ascribed to the self-repair of hydrogen bond and boronate in the presence of Fe3+ and heat.In order to reduce the influence of the phenolic hydroxyl group in the polymerization process and verify the role of phenolic hydroxyl group in the binding process,the adhesive(FBCA-IBA)was prepared by replacing the hydroxypropyl methacrylate(HPMA)in the polymer with the same molar amount of acrylamide(AM).A series of structural characterizations demonstrated that the N-B coordination promoted the formation and stability of boronate so that no free phenolic hydroxyl groups were remained in FBCA-IBA.However,compared with IBA(FBCA-free acrylate-acrylamide copolymer),the molecular weight of FBCA-IBA was reduced,which explained that no cross-linked structure was formed in the polymerization process.In addition,although the glass transition temperature of FBCA-IBA was slightly increased,thermochemical stability and thermal decomposition stability of FBCA-IBA were obviously decreased.Moreover,the cohesive strength,initial tack and creep resistance of FBCA-IBA were also declined.According to the adhesion results,with the introduction of FBCA,adhesion strength of polymer adhesive to 45# steel was decreased from 5.08 MPa to 2.92 MPa.The adhesion strength of FBCAIBA to PP was also reduced,while the adhesion strength to PTFE was slightly increased,because the surface strength was enhanced based on the function of F-B coordination.The adhesion performance of FBCA-IBA only showed the properties of conventional hot melt adhesive and didn't have adhesion enhancement.Comprehensive comparison indicated that FBCA-IBH has the best thermal resistance and adhesion strength to different substrates due to the combination of phenolic hydroxyl,boronate structure and slight crosslink.The reversible adhesion strength value reached 14.36 MPa,which far exceeded the performance of commercial 502 commercial adhesive and even was comparable to the adhesion strength of epoxy resin.