| Biaxially oriented polyethylene terephthalate film (BOPET), with the outstanding dimensional stability and the excellent physical and chemical properties, has been widely used in the fields of packaging, electrical apparatus shell, presswork,decorating and electric industry. However, the BOPET made of common base pellets has been failed to promise an ideal heat-resistance. Moreover, there often happens the broken phenomenon when making such kind of PET films, resulting in the low rate of finished products and the inferior film properties. Therefore, it is fairly important to pay more attentions on improving heat-resistance and processability of the BOPET.In this paper, for the purpose of modifying PET molecular architecture, the copolyester (PETBB) with a relatively high molecular rigidity and the copolyester(GL-PET) with a weak branching degree were synthesized through copolycondensation by incorporating 4,4'-bibenzoic acid and glycerol respectively.The chemical compositions and the molecular sequence structures of copolyesters were characterized, and the glass transitions of crystalline characteristics of PETBBs were investigated in detail, from which the relationship between sequence length and crystallization or melting of the copolyester was revealed. Furthermore, the thermal decomposition kinetics of PETBBs in nitrogen or air atmosphere were studied, and the heat ageing property, the heat-resistant property and so on were analyzed and evaluated for PETBB films. Whereas for GL-PET, the influences of GL dosage on the crystallization rate and the kinetic crystallization ability were discussed.Specifically, besides p-terephthalic acid (PTA) and ethylene glycol (EG), a certain amount (0-55mol% respect to total binary acid) of bibenzoic acid were applied to prepare a series of copolyesters PETBB containing polyethylene terephthalate (PET)and polyethylene bibenzoate (PEBB) segments by using direct esterification technique.Similarly, PTA, EG and 300-1800ppm of glycerol were used to produced weakly branched copolyester, GL-PET. By means of 1H-NMR, 13C-NMR and FT-IR analyses,the products were proved to have expected structures and the real composition was coincident with the feeding ratio. GPC testing showed that the products possessed relatively high molecular weights, i.e., the number average molecular weight of PETBB ranged from 25000 to 29000 g/mol and the intrinsic viscosity from 0.663-0.679 dL/g, moreover, for GL-PET, the number average molecular weight from 15000 to 17800g/mol and the intrinsic viscosity from 0.580 to 0.604 dL/g.The average sequence lengths were calculated for PETBBs from the results of 13C-NMR. It has been found that the sequence length of PET (LPET) decreased while that of PEBB (LPEBB) slightly increased with the increasing BBA use level. In more specific terms, when the content of BBA was increased from 5 to 55mol%, LPET decreased gradually from 33.9 to 2.9, whereas LPEBB increased from 1.2 to 3.6.It was believed the glass transition, crystallization and melting characteristics of PETBB would all depend on its chemical composition and sequence length, to confirm that DSC, X-ray diffraction and small angle X-ray scattering were employed.It was found that when the content of BBA was less than 25%, PETBB showed a crystallization behavior like PET and when BBA more than 35% like PEBB. While the BBA content ranged between 25 and 35mol%, the crystals of PET and PEBB existed together. If PETBB showed a PET-like crystallization behavior, with the increasing BBA content the crystallization temperature from the melting state shifted to low side, i.e., the crystallization was restricted and the melting point reduced.However, if PETBB showed a PEBB-like crystallization characteristic, PETBB melt with high BBA content gave a lower super-cooling degree and the melting point increased. Furthermore, the PET melting point involved in PETBB would decrease almost linearly with the reciprocal of LPET. The evolution of trans-conformation of methylene in PETBB was tracked by means of FT-IR, and result showed that the trans-cis ratio was relative to the composition of copolyester, especially, PETBB with 25mol% or 35mol% of BBA gave the lowest trans-cis ratio, implying that LPET and LPEBB of those two copolyesters were rather short so the crystallization became difficult and the crystallinity was low. The calculated average size of [100] plane of PET from the XRD curves decreased also with the increasing BBA cotent, and when the BBA content was less than 25mol% the long periods obtained from small angle x-ray scattering was almost correlative linearly to the melting point.The heat-resistances in nitrogen or air atmospheres of PETBB and PET were investigated. It was founded that the heat-resistance of PETBB was higher than that of PET, which was enhanced by the increasing BBA content. In the nitrogen, TGA curves of both PET and PETBB showed just one decomposition spinodal,whereas in the air the decomposition behaved in two stages: the first was similar to that in the nitrogen, the second was believed to be a further oxidation. The thermal decomposition kinetics parameters were calculated through Friedman's or Cheng's scheme, the results showed that the decomposition activation energy and the index n of PETBB were higher than those of PET in both nitrogen and air, implying an improved heat-resistance for its rigid molecular chain structures.It was identified that PETBB melt was a shear-thinning one just like PET,however, because of the incorporation of rigid segments in PETBB, the apparent viscosity increased but the flow activation energy decreased. PETBB5 and PETBB 15 showed good processabilities when being cast into films, and the stress-optic relationships of the as-cast sheets were investigated by using the on-line birefringence measurement: It was found when the PET or PETBB sheet was stretched over Tg the resulted stress-optic curve show two linear regions. In the first region the stress-optic coefficient (the slope) was increased with the increased BBA content, however, the slope of second linear region would reduce and the extent increased with the increasing BBA content. That meant the existence of BBA would promote the orientation of polyester molecular chains, and the orientation induced crystallization would happen so as to stabilize the ordered structures more quickly.The accelerated thermal ageing was carried out for biaxially oriented PETBB films, and the intrinsic viscosity, dielectric constant and breakdown voltage of samples with different ageing times were tested respectively. The results showed that both the drop of intrinsic viscosity and the gain of dielectric constant would become slower when more BBA was used, which indicated the enhancement of film heat-resistance. It was deduced from the thermal life curves that the temperature classification of PETBB5 was near to B rank while that of PETBB15 even exceeded B rank.The DSC analysis for GL-PETs indicated that when cooled from melting state the crystallization temperature of GL-PET was high than that of PET, but which was not correlative to the GL content. Specifically, among the GL content ranged from 300 to 1800ppm, GL-PET with 900ppm of GL showed the smallest super-cooling degree and the narrowest crystallization peak. Moreover, the non-isothermal crystallization processes of GL-PET under different cooling rates were investigated. It was found both the Avrami model and the Avrami-Ozawa model could well described the non-isothermal crystallization kinetics on different aspects, and the calculated crystallization rate constants were coincided with the experimental ones, i.e.,GL900-PET possessed the fastest crystallization rate. The kinetic crystallization abilities and activation energies were also calculated by means of Ziabicki or Kissnger model, the results still proved that GL900-PET showed the strongest kinetic crystallization ability and the lowest crystallization activation energy.It should be noted that the results of this paper would promise a strong technique support and theoretical base for the BOPET processing. |
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