Synthesis of High-Barrier, Amorphous, Heat-Resistant Copolyesters from Naphthalene Dicarboxylic Acid and Neopentyl Glycol

Polyester (PET) has become the focus of research in the shrink film industry due to its good heat resistance, high transparency, and excellent mechanical properties. In order to improve the shrinkage rate of the film, amorphous PET (PETG) is usually obtained by copolymerization modification using some flexible diols and so on. However, introducing flexible structures reduces the glass transition temperature (T_g) and mechanical strength, and importantly, the gas barrier properties of most PETGs are weak. Here, a series of poly(terephthalic acid-naphthalene dicarboxylic acid-ethylene glycol-neopentyl glycol) (PENTN) were prepared by melt polymerization using different contents of 1,4-naphthalene dicarboxylic acid (1,4-NDA) or 2,6-naphthalene dicarboxylic acid (2,6-NDA). The results show that the introduction of 1,4-NDA does not enhance the T_g of the copolyesters significantly, whereas the T_g of the copolyesters increases linearly with the increase in the content of 2,6-NDA. The T_g of 2,6-PENTNs ranges from 82.2 to 90.5 °C. The crystallization is inhibited effectively due to the neopentyl glycol and exhibits good transparency (>85%) and low haze value (<4%), which results in the preparation of copolyesters that are all amorphous polymers. The differences in the effects of 2,6-NDA and 1,4-NDA on the thermal properties of the copolyesters were analyzed. Meanwhile, the copolyesters were prepared into films, and the gas barrier properties were analyzed in detail. The rheological analysis reveals the free volume of the copolyester is rapidly decreasing with the introduction of the naphthalene group structure. The gas barrier property of the copolyesters can be improved to a high level. This work provides an idea for the functionalized modification of packaging materials.