Study On Blends And Copolyester Of PET/PEN

Poly(ethylene terephthalate)(PET)is the most commonly encountered polyester and is used in many everyday applications,mainly due to its low production cost and good mechanical properties. Poly(ethylene naphthalene-2, 6-dicarboxylate)(PEN) has emerged recently as a thermoplastic polyester with outstanding properties,it is quite useful for many packaging applications,due to good mechanical properties,low permeability of oxygen and carbon dioxide,and good thermal properties,high-melting and high glass transition temperature.However,PEN exhibits a high melt viscosity and is also expensive. One potential approach for combining the attractive economics of PET with the better barrier and thermal properties of PEN is through blends of the polymers. The blend is of both academic and industrial interest. Some basic research has been considered by a number of groups. In this article, we made a systematic study on the transesterification reaction of PET and PEN blend, and it's structure, properties and application of blends or copolyester of PET and PEN with mixer, two-screw extruder, differential scanning calorimetry (DSC) and 1H nuclear magnetic resonce (NMR) etc. The influence of the composition of the blend, blending time and temperature, mixer speed and content of the polyester catalysts on the transesterification reactions between PET and PEN blend was quantitative analysised with ~1H NMR.The transesterification level was expressed with the molar content of NET, the randomness and the number-average sequence lengths. The results showed that the physical blends are immiscible, and the transesterification reactions of PET and PEN blend commonly occur in the molten state, which produces first block copolymers and finally random copolymers. The primary factors controlling the transesterification in PEN and PET blend were the blending time and temperature, while the composition of the blend, the content of polyester catalysts, and mixer speed had little effect on the transesterifivation. The higher the annealing temperature was, the faster the transesterification reaction became; The transesterification was a function of blending time, the number-average sequence lengths decrease with the increase of reaction time, the transesterification level and the randomness increase with the increase of reaction time. The kinetics of the transesterification reaction between PET and PEN were studied with 1H NMR.The second-order reversible reaction was accepted for the PET/PEN reactive blend system.The rate constant K of 0.028min-1(280℃),0.044 min-1 (290℃),0.06 min-1 (300℃),the activation energy of 106.98kJ/mol,and induction period of 1.51min(280℃),1.05min(290℃),0.64min(300℃)of the transesterification reaction for the PET/PEN reactive blend were obtained using a kinetic expression based on a second-order reversible reaction mechanism.In the range of research, the transesterification reaction rate of PET and PEN is independent of blend composition; The higher the temperature was, the raiser the blend miscible, the shorter tthe induction period, and the faster the transesterification was. The relationship between transesterification and miscibility was discussed on the induction period. It was found that miscibility leads or enhance to transesterification, that is miscibility is essential condition for transesterification in the polyester blends; The resultant initial block and eventual random copolymers are expected to exhibit enhanced mutual miscibility over the original unreacted components. Moreover, the transreacted chains probably also facilitate and accelerate the compatibilization and reaction of unreacted chains. Transesterification enhances miscibility. They are interrelates and competed. Blends were obtained by solution precipitation at room temperature to avoid transesterification during blend preparation. The thermal properties, miscibility and phase behavior of the physical blends and transesterification products were analyzedby differential scanning calorimetry. It was found that the physical blends are immiscible, the properties of crystallization and dissolve, glass transition temperature of PET and PEN blend has a great change during the transesterification. The degree of transesterification (or randomness) of a polyester blend affects blend clarity and crystallizaton. The copolyester which can improve the blend miscibility was produced by transesterification. When a sufficient number of transesterification reactions occur, the inherent incompatibility between PEN and PET is reduced, and hazy blends of PEN and PET ultimately become miscible and clear when the transesterification level reaches 10%. The blend samples heat treated for shorter time show two glass transitions, which approach closer with the increase of reaction time. And when the extent of transesterification reaches a certain level, the blends are not crystallizable and exhibit a single glass transition temperature between those of starting polymers independent of blend composition, and the glass transition range becomes narrow with the lapse of reaction time. Moreover, the relationship between Tg and composition is consistent with Fox equation, which means PET and PEN were miscible in amorphous phase, but annealing temperature and time has a little effect on Tg. The blending composition, temperature and time have a dramatic effect on the cold crystallization temperature (Tc) and the meltig temperature(Tm). PET/PEN(50/50) has not cold crystallization peak even if annealing time is short. For other compositions, with increasing the mixing time, the cold crystallization temperature increase and the melting temperature decrease which are due to the number-average sequence length decreasing. When the number-average sequence length is shorter than 7,the cold crystallization peak disappear; the melting peak disappear when the transesterification level is about 20%. Tm decrease with the number-average sequence length decrease when LnPET is shorter than 20; however Tm has little change when LnPET is longer than 20. The melting point Tm firstly decreased and then increased as the content of PEN raised, during the increasing of the extent of PEN, crystallization properties of PET/PEN blends became worse and the heat stabilities enhanced. The blending temperature and time can significant affect the intrinsic viscosity for the PET/PEN blend, with prolonged the blendingtime,the intrinsic viscosity of the blend tends to gradually decrease especially at higher temperature,which is associated with the degradation of polymer chains at higher temperature and the change in the sequence structure. The thermal properties, rheologic and dynamic mechanical properties of PET and PEN blends were studied by means of TG and Rheometer etc. The results showed that heat resistance of blend is more than that of PET, but the content of PEN has a little effect on the heat resistance of blend. The apparent viscosity of PET/PEN blends decreased with the increasing of the shear rate. The blends system is non-Newtonian fluid. It's rheological behavior showed the properties of pseudoplastic fluid. The dynamic mechanical properties of PET/PEN blends were studied as a function of blend composition as well as the blending time. For the PET/PEN blends having fTEN of above 10%, all the blends quenched from molten state exhibited the same dynamic mechanical behavior in the glass transition temperature Tg, Tg of the blends was a linear function of the blend composition and independent of the blending time. The blending time did not significantly affect the dynamic mechanical behavior in glass transition region but dramatically changed the behavior of the blends in the cold-crystallization region as a function of blend composition. It was found that the transesterification was prohibited when the blend was performed at a shorter time and lower temperature, both PET and PEN kept their chain regulation well. The influence of the content of PEN and processing technology on the crystallization, gas barrier and heat resistance properties of product were studied with gradient density column, two-stage injection-streth-blow apparatus and gas permeability measurementer. It was found that molecular orientation, crystallinity and copolymerization can improve the gas barrier properties of PET and PEN blends. Crystallinity plays an important role in barrier properties and chain organization, which can determine the path and extent of diffusion. Crystallinity not only reduces permeability by reducing the free volume of the amorphous phase, but also increases the tortuousity of the diffusion path for the penetrant. The estimated amorphous density is higher in the copolymers than in either homopolymer, suggesting thatcopolymerization lends to better chain packing in the amorphous regions of the polymer matrix. PEN has a higher heat distortion temperature and melt viscosity than PET, when processing polyester blends and copolymers that contain PEN, composition, miscibility, transesterification, and crystallization must be considered. If a copolymer or a polyester blend crystallize too slowly or cannot be crystallized to a sufficient level, processing the material may be difficult. The processing condition including barrel temperature, rotations speed of the screw, orientation temperature, composition and stretch ratio had an effect on the gas barrier and heat resistance of PET and PET/PEN copolyester blend. The barrier of product was improved by processing temperature and stretch ratio increasing. The gas barrier and thermal destructive temperature increase but the thermal shrinkage decrease with the increasing of the content of PEN. Heat-resistant and barrier clear bottles of PET and PET/PEN blend using conventional PET molding tool and two-stage injection-streth-blow process can be produced, but it is required modified screw, processing parameter, parison design and stretch rato in order to produce the optimal bottle performance.