Insights into high molecular weight poly(ethylene 2,5-furandicarboxylate) with satisfactory appearance: Roles of in-situ catalysis of metal zinc
Graphical abstract
High molecular weight biobased polyester PEF with satisfactory appearance and faster crystallization rate was obtained by metal zinc powder as an efficiently in-situ catalyst.
Introduction
Driven by the increasing concern for environmental issues and rapid crude oil consumption, the biobased poly(ethylene 2,5-furandicarboxylate) (PEF), synthesized from 2,5-furandicarboxylic acid (FDCA) and EG, has attracted more attention due to its excellent properties and great potential to be the substitute of the petroleum-based polyethylene terephthalate (PET) [1], [2]. Especially, PEF possesses extraordinary barrier properties, that is 13–19 and 6–11 times less permeable to CO2 and O2 than PET [3], [4], which endows PEF to be suitable for the packaging of beverage bottles. By contrast with PET, PEF has lower melting temperature of 210–215 °C [5], [6], [7], that reduces the processing temperature with no thermal decomposition until Td ∼ 389.3 °C [8]. Furthermore, PEF exhibits a higher Young’s modulus than PET (ca. 2.0 and 1.3 GPa, respectively) [9], [10], which brings about a mechanically more resilient material for final applications. Importantly, production of PEF as a candidate to PET could reduce nonrenewable energy use and greenhouse gas emissions by 40%–50% and 45%–55%, respectively [11]. It is therefore PEF is an ideal alternative to PET [12], [13], and is potentially suitable for extensive fields of applications such as oriented food and beverage packaging, clothing, and in the car industry [14], [15].
Despite its great potential, the development and practical application of PEF still face some problems, such as its yellowish-brown coloration [12], [16] and slow crystallization [17], [18], [19], [20]. It is well known that the yellowish-brown color of PEF is mainly from three aspects: (a) sugar-based impurities in the FDCA monomer [21], (b) side reactions and especially the decarboxylation of FDCA monomer taking place during the polymer synthesis, (c) the presence of various additives, such as manganese, cobalt, germanium, titanium [12], [16] and organic catalysts [6], [7] and so on. To mitigate the color of PEF, ring-opening polymerization and acyl chloride solution method [18], [22] can be used to prepare white PEF with Mn (20.8 and >30 kg mol−1) under benign conditions. However, the complicated technique and a large number of organic solvents that could cause environmental hazards [22], [23], are not applicable for industrial production. Many researchers [24], [25], [26], [27] convert FDCA into DMFD to synthesize PEF via transesterification method with EG, it is found that the influence of FDCA monomer on the color of PEF can be avoided effectively, yet, PEF can be easily contaminated by colored catalyst or the unstable ligands [28]. Moreover, melt polycondensation process is time and temperature dependent. To pursuit the high molecular weight for the desired material properties, the long exposure to the high temperatures (around 230−250 °C) not only increases the production costs, but even more importantly leads to discolouration of PEF, making it unsuitable for the market [26], [27], [28]. It is therefore full of great significant to synthesize high molecular weight PEF with good appearance via transesterification method from DMFD and EG under an efficient, colorless and stable catalyst system.
Additonally, PEF as a semicrystalline polymer, its very slow crystallization adversely affects its spinning performance, compared with PET [19], [29]. To tailor the properties, numerous pathways have been considered to improve the crystallization rate, such as, copolymerization [30], [31], [32], blending [33] and nanocomposite compounding of PEF [34], [35], [36], [37], [38]. Among them, nanocomposite technology has proved to be an effective way to improve the crystallization of polymers. Martino et al.38] prepared nanocomposite materials composed of PEF and organo-modified montmorillonite (OMMT) clays by solvent casting method. Although the presence of clays induced a slight nucleating effect on the PEF crystallization, this method used a large amount of hexafluoroisopropanol which was highly toxic and corrosive, will seriously endanger human health and ecological environment. Xie et al. [39] reported that PEF nanocomposites containing 2.5 wt% montmorillonite (MMT) showed significantly improved melt crystallization. Nevertheless, the in-situ synthesis of PEF/MMT nanocomposites was carried out in a three-step process and it lasted as long as 18 h. Codou et al. [40] proposed that crystallization of PEF composited with cellulose by twin screw extrusion was enhanced and the nucleating effect increased with the cellulose concentration. Although the above methods provided guidance for improving the crystallinity of PEF, it might be complex, time-consuming and environmental polluting. It is reported that, some metal salts, such as zinc phenylphosphonate [41], zinc citrate [42], and zinc phenylmalonate [43], are found to be the most effective NAs for polymers. Recently, Liang et al. [44] proposed a strategy of in situ catalysis and nucleation of zinc salts of amino acids towards poly(l-lactide) (PLA). The result revealed that the in situ zinc salts of amino acids not only acted as a catalyst, but also served as an NA enhancing the crystallization rate of PLA greatly. To our best knowledge, the crystallization rate of PEF strongly depended on catalysts, molecular weight and the content of diethylene glycol furandicarboxylate unit (DEGF) in chain segment [6], [19]. It will be interesting and challenging if a simple and cost-effective strategy could be developed, which can simultaneously improve the molecular weight, color and crystallization rate of PEF.
Calcium (Ca), magnesium (Mg) and zinc (Zn) as common and inexpensive elements are biocompatible and essential elements in human body. These metals have been widely used as catalysts for polyester synthesis in the form of metal salts, metal oxides or metal alkoxides [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]. Among them, Zn-based catalysts are reported to be effective catalysts for bulk polymerization of PEF and polylactide (PLA) with good appearance [55], [56]. However, there are few reports on the preparation of polyesters with metal powder as catalysts. Schwach et al. [45] prepared PLA with metal zinc powder as catalyst for the first time. It was found that zinc lactate, which was from the reaction of zinc metal and lactic acid (the hydrolysate of lactide), was the actual catalytic active specie for the chain growth of lactide via coordination insertion mechanism. Huang et al. [53] successfully synthesized block copolymers of ε-caprolactone and d,l-lactide in the presence of EG or PEG, using metal zinc powder as catalyst in stead of stannous octoate or other catalysts due to its biocompatibility and low cost of metal zinc. The above of works imply that metal powder can be directly used as an in-situ catalyst, avoiding the tedious catalyst synthesis process and the problem of thermochromism of unstable ligands. Therefore, it is worth exploring the preparation of PEF by using metal powder as in-situ catalyst and investigating the molecular weight, color and properties of PEF induced by this in-situ catalyst.
In present work, we focused on the preparation of a high molecular weight PEF material with good appearance and faster crystallization rate in trace presence of metal zinc powder, to maximize the potential of PEF in the feild of fiber materials. That is, series of experiments on PEFs polymerized from DMFD and EG via transesterification method with metal zinc powder as in-situ catalyst were successfully carried out. Compared with other active metals (Ca, Mg, Fe), metal zinc showed an excellent catalytic activity and it was suitable for bulk polymerization of a series of high molecular weight furan based polyesters with good appearance. On the basis of experimental results and data of DSC, PEF catalyzed by metal zinc powder had faster crystallization rate than that of PEF catalyzed by stannous oxalate. It was verified that metal zinc was versatile, which can not only act as an in-situ catalyst for the polymerization of PEF, but also as an NA for the products of PEF. The efficiency and universality of zinc powder as in-situ catalyst was proved in the preparations of other furan-based polyesters from DMFD and diols with different methylene groups (3, 5, 6 and 8). This study develops a simple and cost-effective approach for the production of PEF with good appearance and faster crystallization rate.
Section snippets
Chemicals
Dimethyl furan-2,5-dicarboxylate (DMFD, 99.0%, Inn Chem), terephthalate (TPA, 99.0% Inn Chem) ethylene glycol (EG, 99.0%, China National Medicines), 1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, 1-8-octanediol (98%, Aladdin) Calcium, magnesium, iron and zinc (powder, 100-200 mesh, 99.9%, Inn Chem), Methanol (99.5%, Inn Chem) Hexafluoroisopropanol (HFIP, 99% J&K Scientific Ltd.), dimethyl sulfoxide (DMSO, >95%Tian in Fuyu Fine Chemical Co., Ltdj). Tetrabutyl titanate (TBT, 99%, Tianjin No1
Results and discussion
Preparation of PEF with different metal powder as catalysts
Metal zinc powder was used as catalyst to polymerize DMFD with EG via transesterification method for the first time. It is satisfactory that a high number-average molecular weight (4.80 × 104g mol−1) PEF (PEF-Zn) with light color was obtained as listed in Table 1 and Fig. 1. From the 1H NMR of PEF (Fig. S1), the signal peaks of DEGF in PEF were observed at 4.52 and 4.00 ppm, and the molar fractions of DEGF was calculated to be 2.91% by
Conclusions
Bio-based polyesters PEF, with high molecular weight (Mn, 5.40 × 104 g mol−1), good appearance and faster crystallization rate, was obtained with zinc powder as an in-situ catalyst via transesterification method for the first time. The catalytic active species Zn (II), formed from by-product FDCA with metal Zn in the polymerization system, can not only exhibit excellent catalytic activity, but efficaciously inhibit the influence of by-product FDCA on color of PEF. The crystallization rate of PEF
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Science and Technology Development Project of Jilin Province (China, No. 20180201069SF) and the National Science Foundation of China (No. 51703220)
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