Dependence of the foaming window of poly(ethylene terephthalate-co-ethylene 2,5-furandicarboxylate) copolyesters on FDCA content
Graphical abstract
Introduction
Poly(ethylene terephthalate) (PET) is a fossil-based polyester synthesized with chemicals of ethylene glycol (EG) and terephthalic acid (PTA). PET is widely applied in clothes, packaging, and electronic appliances owing to its good chemical resistance, dimensional stability, gas barrier property and electrical insulation, etc [1,2]. Foaming offers a practical means to reduce weight, save energy as well as to bring functions. Owing to the inherent physical properties of PET as well as the functions endowed by the cell structure, PET foams have been widely used in wind energy, construction, transportation and optical display, etc [3,4].
The foaming of PET is much more difficult than that of amorphous polymers, and is severely affected by its low melt strength accompanied by its low crystallization rate [4,5]. It is understandable that the low crystallization rate results in the slow solidification of the cell structure, thus leading to severe coalescence and collapse. Hence, enhancing the melt strength is of great importance to improve the foamability of PET and also to optimize its cell structure. The melt strength of PET is mainly improved via chain extension [[6], [7], [8]], solid-state polycondensation [9], blending [10], incorporating nanoparticles [5] and controlling crystallization [5,10,11], etc.
Crystallization plays a significant role in PET foaming, and prominently affects the cell structure and the physical properties of PET foams. Baldwin et al. [12,13] found that semicrystalline PET foams have much larger cell density and smaller cell size compared to amorphous PET foams. They also found that the cell size of the semicrystalline PET is very sensitive to the foaming temperature, which is attributed to the viscoelastic behavior of semicrystalline polymers. Li et al. [14] reported preparation of sandwich-structure in PET foams via coupling CO2 diffusion and CO2-induced crystallization. Their study showed that with the increase of the saturation time, the cell size decreases, the cell density increases, and the expansion ratio first increases and then decreases. In a following study also from Zhao's group by Xia et al. [11], they used a periodical CO2-renewing process to inhibit the CO2-induced crystallization, and hence obtained PET foams of larger expansion ratio. Jiang et al. [10] reported that the in-situ fibrillation of poly(tetrafluoroethylene) (PTFE) has dual effects on foaming of PET. On one hand, the PEFT fibril networks significantly enhance the melt strength of PET, and on the other, the PTFE fibrils promote the crystallization of PET. Hence, PET foams were formed with small cell size and large cell density. Yao et al. [15] found that the increase in crystallinity of PET foams via post isothermal treatment is favorable for enhancing the tensile properties.
In the past decades, exploration and utilization of biomass-based resources has aroused worldwide attention due to the gradual depletion of fossil-based energy. The biomass derived polyesters such as poly(lactic acid) (PLA) and poly(butylene succinate) (PBS) are the examples of high-value utilization of biomass-based resources [16,17]. 2,5-furandicarboxylic acid (FDCA) is an important bio-based chemical obtained via oxidation of hydroxymethylfurfural (HMF), which can be synthesized from polysaccharides and sugars [18,19]. As the bio-based FDCA is similar in chemical structure with the fossil-based PTA, poly(ethylene 2,5-furandicarboxylate) (PEF) is considered potentially the bio-based substitute for the fossil-based analogue PET [[20], [21], [22], [23]]. Compared to PET, PEF is more rigid in macromolecular mobility due to asymmetric structure and permanent dipole, and has a higher glass transition temperature while a lower melting temperature [24,25]. More importantly, PEF has larger mechanical strength and much better gas barrier properties, which makes it a promising substitute for PET [[25], [26], [27], [28]]. Besides PEF, other furan derived polyesters and the relevant copolyesters have also been synthesized in recent years and studied in many aspects [[29], [30], [31], [32], [33]]. However, the price of PEF is still not affordable nowadays, and hence it is considered that utilization of poly(ethylene terephthalate-co-ethylene 2,5-furandicarboxylate) (PEFT) copolyester is a feasible route for partially realizing bio-based replacement of PET. To date, most of the studies [26,34,35] about PEFT copolyester have focused on its synthesis and crystallization. These studies showed that the crystallization of PEFT is closely related with the comonomer content of FDCA. Hence, it is deduced that the physical properties of PEFT copolyesters would depend closely on the FDCA content as well. PEFT copolyesters can potentially have similar applications as PET does, such as fibers, films and foams, and the FDCA content would play a key role in determining the applications of PEFT. Thereby, it is currently an important task to explore, testify and demonstrate the feasible applications for PEFT copolyesters to make the new materials realize their value in industrial production and end uses. For example, the melt-spinning of PEFT has been studied for preparation of fibers [36]. However, foaming of PEFT, i.e., one potential important application has not been studied so far, and how the copolymerization of FDCA units affects the foaming behavior of PEFT copolyesters still remains a significant but unknown issue.
The objective of the present work was to study the dependence of the foaming window of the PEFT copolyesters on the content of FDCA units. First, the chemical structure of PEFT copolyesters was characterized, and the effect of the content of FDCA units on the sorption of carbon dioxide (CO2) and the crystallization and the melting behavior of PEFT copolyesters were studied. Second, the PEFT copolyesters were foamed via a batch foaming process with high-pressure CO2 as the foaming agent, and the cell structure and the expansion ratio of the PEFT copolyester foams were studied. Third, the dependence of the foaming window of PEFT copolyesters on FDCA content was obtained for different saturation pressures.
Section snippets
Materials
PET, PEF and PEFT copolyesters used in this study were provided by Bio-Based Polymer Materials Team from Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences. The PET, PEF and PEFT copolyesters were synthesized via two-step melt polycondensation, including transesterification and polycondensation. The process of the synthesis was described in detail in the published study [37]. The polyester and the copolyester used in this study have an intrinsic viscosity of
Characterization of FDCA content in PEFT
The chemical structures and 1H NMR spectra of PET, PEF and PEFT copolyesters are shown in Fig. 1a and b. The protons of the benzene ring of PET were located at about 8.2 ppm (a protons), while those of the furan ring of PEF at about 7.5 ppm (b protons) [[35], [36], [37]]. The protons of ethylene diol part in PET and PEF appeared at the chemical shift of around 4.9 ppm (c and d protons), attributed to the similar structure [35]. It can be seen that PEFT copolyesters displayed all the peaks
Conclusions
In the present work, the dependence of the foaming temperature window of PEFT copolyesters as a function of the copolymerized FDCA content was comprehensively studied after different saturation pressures. First, the FDCA content in PEFT copolyesters was characterized, and the effect of FDCA content on the sorption of CO2 and the crystallization behavior of PEFT copolyesters was studied. It was found that with the increase of the FDCA content, the sorption of CO2 in PEFT copolyesters gradually
CRediT authorship contribution statement
Zhijun Wang: Investigation, Formal analysis, Validation, Visualization, Writing – original draft. Jinggang Wang: Resources. Yongyan Pang: Conceptualization, Methodology, Writing – review & editing, Supervision, Project administration, Funding acquisition. Jin Zhu: Resources. Wenge Zheng: Writing – review & editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Financial supports from Ningbo Science and Technology Innovation 2025 Major Special Project (2018B10013), Provincial Key Research and Development Program of Zhejiang (2021C01005) and National Key R&D Program of China (2021YFB3700300) are gratefully acknowledged.
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