Study On Degradation Mechanism Of Peek During Melting And The Improvement Of Its Melting Stability

The widespread use of synthetic polymers has revolutionized the manufacturingindustry. Poly (ether ether ketone), a type of thermoplastic resin, is widely usedespecially in aerospace, automotive and health care applications due to the excellentchemical resistance, superior mechanical properties, tribological properties and highservice temperature. However, like other commercial plastics, PEEK will experiencesome types of degradation or cross-linking in the process of extrusion and injectionmolding due to the higher processing temperature which is almost paralleled with its'melting point (around340oC). The degradation will lead to a decline in PEEKproperties, such as embrittlement and difficulty in molding.Little work has been carried out on elucidating the reactions taking place in themelt of PEEK because the determination methods of the degradation are limited. Themethod that tracks the small organic molecules in the gas upon decomposition todeduce the degradation mechanisms is not suitable for the melting PEEK becausethere are nearly no organic molecules released into the gas phase when PEEK ismelted at the temperature lower than450oC. Some methods, like FTIR and13C NMRmeasurements, are not efficiently to investigate the degradation mechanisms preciselysince the chemical changes in the melting PEEK are not as visible as the changeshappened at the higher temperature (over450oC). The excellent solvent resistance ofPEEK is another important factor which brings embarrassment to the analysis ofPEEK because the difficulty to find a fine solvent to dissolve the polymer well thatmany measurements which are normally considered to be proven technique to studythe chemical structure are limited. In this work, the stability of melting PEEK was evaluated by rheological testand thermogravimentric analysis. Dimeric model compound of PEEK have beensynthesized successfully and tested in the conditions paralleled with PEEK to studythe reactions which occurred in the melt. Mass spectrum (MS), FTIR and1H NMRmeasurements were used to observe the changes of chemical structures of thecompounds. By the analysis on the structure changes of model compound, a possiblemechanism of degradation and crosslink of melting PEEK was proposed.The Rheological study of PEEK indicated the melting viscosity of PEEK wouldincrease with the prolongation of the melting time, and the reason was normallyconsidered that some types of crosslink occurred between polymer chains. Tospeculate the reactions which occurred in the melting PEEK accurately, a novelmethod and design of experiment was introduced. The model compound, dimericPEEK, was synthesized and treated at the similar experiment condition paralleled withpolymer PEEK. The model compounds before and after degradation were analyzed bymass spectrum,1H NMR and FTIR to reveal the changes of molecular structurecaused by the heating treatment. The results showed that not only crosslink but alsografting reaction happened during the degradation. When the molecule was heatedabove the melt point, the ether bonds and the end carbonyl groups were more unstablethan the other chemical bonds and easily generate radicals which then grafted to theadjacent molecular on the ortho position of carbonyl or combined with anotherradicals. Meanwhile, a little of crosslink reaction occurred between the molecularchains, causing the increase of viscosity and molecular weight. Finally, a possiblegrafting and crosslink mechanisms of polymer PEEK was proposed.Three types of antioxidants were used to intend to improve the stability of poly(ether ether ketone)(PEEK). To evaluate the effect of the antioxidants on theproperties of PEEK and the stabilization mechanism, some characterization methodswere carried out, such as rheometer, TGA, universal tester and electron spinresonance (ESR). The results indicated that the efficiency of the phosphorousantioxidant (DS9228) in improving the melting stability of PEEK was better than thatof the phenolic antioxidant (3114) and HP136, and the melting stability of PEEK sample containing0.07wt%DS9228was the best among all samples Additionally, noobvious changes could be observed in mechanical properties of PEEK containingantioxidants compared to virgin PEEK.In this dissertation, TG and differential thermogravimetry (DTG) measurementsof Poly (aryl ether ketone) containing naphthalene moieties were reported. Thethermal degradation temperature and the kinetics of polymer were studied throughnonisothermal in Air conditionsElectron spin resonance (ESR) is an effective measurement to observe thestructure and the number of free radicals, which provide a route to study the effect ofthe free radicals on polymer processed at high temperature. the efficiency ofantioxidants for PEEK was evaluated by ESR and the stabilization mechanism ofantioxidants on PEEK resin was discussed.