Insights into the Synthesis of Poly(ethylene 2,5-Furandicarboxylate

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Insights into the synthesis of poly(ethylene 2,5-furandicarboxylate) from 2,5-furandicarboxylic acid: steps toward environmental and food safety excellence in packaging applications. Maria Barbara Banella, Jacopo Bonucci, Micaela Vannini, Paola Marchese, Cesare Lorenzetti, and Annamaria Celli Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00661 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Insights into the synthesis of poly(ethylene 2,5-furandicarboxylate) from 2,5-furandicarboxylic acid: steps toward environmental and food safety excellence in packaging applications. Maria Barbara Banella,a Jacopo Bonucci,a Micaela Vannini,a* Paola Marchese,a Cesare Lorenzettib Annamaria Celli,a a Department

of Civil, Chemical, Environmental and Materials Engineering, University of Bologna,

Via Terracini 28, 40131 Bologna, Italy b Tetra

Pak Packaging Solutions AB, Ruben Rausing Gata, SE-221 86 Lund, Sweden

*corresponding

Author (email address: [email protected])

Abstract Poly(ethylene 2,5-furandicarboxylate) (PEF) is considered today as a very promising bio-based polymer for packaging applications. Most often scientific literature describes synthetic procedures based on the transesterification of the dimethylester of the 2,5-furandicarboxylic acid, whereas this paper aims at studying the possibility of a practical and profitable synthetic route for PEF comprising a direct esterification stage of 2,5-furandicarboxylic acid. In that respect two catalysts, Zinc Acetate and Aluminum acetylacetonate, chosen for their compatibility with food contact applications and for featuring a potentially reduced environmental impact, were investigated. The synthesis was performed by using tight reaction conditions: low excess of diol and short reaction time. Interesting results were obtained in terms of final PEF viscosity, colour and di-ethylene glycol content, that may strongly influence thermal and barrier properties. Furthermore, the obtained amorphous polymers are potentially suitable for the manufacturing of various packaging articles such as oriented films and bottles.

Keywords: poly(ethylene 2,5-furandicarboxylate), FDCA, catalyst, food packaging

INTRODUCTION Poly(ethylene 2,5-furandicarboxylate) (PEF) is today considered as a very promising bio-based aromatic polyester, derivable from C6 sugar-based resources: it can find applications in the manufacturing of bottles and a variety of food packaging materials thanks to some interesting characteristics and properties. Indeed, PEF features a very low permeability: in comparison with 1 ACS Paragon Plus Environment

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poly(ethylene terephthalate) (PET). PEF is characterized by permeability,

11-fold reduction in oxygen permeability and

2-fold reduction in water

19-fold reduction in carbon dioxide

permeability.1 Such a behaviour can be ascribed to the higher rigidity of the furan ring with respect to the terephthalate unit2 and therefore to the reduction of the segmental motions of the 2,5furandicarboxylic moieties.1,3 Such characteristics induce a reduced diffusion inside the material amorphous phase.4 This interesting characteristic is supported by the consideration that the replacement of PET and other gas barrier polymers with PEF can lead to significant environmental benefits. Eerhart et al. for instance report that the use of PEF in the bottle market could save about 440–550 PJ of nonrenewable energy (NREU) and 20 -35 Mt of CO2 equivalent in GHG emissions.5 Although PEF has been known for more than 70 years6 and many different synthetic recipes to produce it have been analyzed, in terms of precursors (diacid or diester), catalysts, diacid/diol stoichiometries, temperatures and pressures, some problems remain unsolved and most likely the study of the synthetic process will still require great space. It is known that progresses in the synthetic way to prepare PEF are recent, mainly thanks to improvements in technology to produce high purity 2,5-furandicarboxylic acid (FDCA) on a large scale. Different approaches were used to prepare PEF, beginning with solution or interfacial polymerizations7,8 and then testing even the ring opening polymerizations.9,10 In any case, the literature mainly describes PEF polymers obtained by melt polymerization11-24 and generally starting from the dimethylester of FDCA (DMFD).11-20 The diester is indeed more easily purified than the free acid13,25 and the transesterification of the diesters can be performed under milder conditions than the direct Fischer esterification of the diacid. On the other hand, from an industrial perspective, the use of FDCA as a monomer for the synthesis of PEF seems to be the most advantageous and environmental friendly process: no solvents and no halogenated monomers are used, the additional step of esterification of FDCA to dimethyl FDCA is bypassed, water (not alcohol) is managed as a condensation by-product.26 Furthermore, it is possible to speculate on the possibility of retrofitting the synthesis of PEF from FDCA into existing PET polycondensation plants from terephthalic acid (PTA). However, there are some drawbacks. The synthesis of PEF from FDCA has been generally carried out starting from a large molar ratio of diol vs. diacid: typically the molar ratio EG/FDCA is in the 1.3-3 range, due to the low solubility of FDCA in diols. Recently, Joshi et al. showed that this parameter can be optimized as the solubility of FDCA in EG is enhanced at temperatures ranged from 180 to 190°C.27 A large excess of diol is not economically advantageous in industrial terms as 2 ACS Paragon Plus Environment


it generally requires high chemical volumes and large EG recycling. Furthermore, a high amount of diol usually results in an increased content of di-ethylene glycol (DEG) units along the polymeric chain. The problem of the ether-bridge formation during polymerization, despite being seldom discussed in the literature,16,18,24,27 is nevertheless a key-point. In fact, the formation of DEG affects the final material properties: in the case of PET, for instance, it leads to a decrement of melting temperature, crystallization rate, glass transition temperature, degradation resistance, and barrier properties.28-30 Moreover, the synthetic procedure often undergoes lengthy polymerization times that increase the occurrence of unwanted side reactions, with the formation of yellow, greenish or brown colours of the final material.15,16,31 This process was especially associated to the decarboxylation of FDCA under the reaction conditions.11,32 Furthermore, the final colour can also be due to impurities in the sugars, from which FDCA derives, and eventually in FDCA itself.33 This latter problem is one of the reasons why the use of the dimethyl ester (DMFD) is generally suggested.11,32 For example, absorbance at 400 nm (measured in a 5 mg/ml solution in dichloromethane/hexafluoroisopropanol 8/2) is 0.228 and 0.042 for the polymers obtained from FDCA and DMFD, respectively, where a value below 0.05 is considered acceptable.15,16 Then, the final PEF is often characterized by relatively low molecular weights that determine poor properties. Therefore, a solid state polymerization (SSP) has been considered as a final polymerization step by some authors.11,13,19,20,30,31,34-36 This technique, quite common in the industrial production of high molecular weight PET used in injection blow molded containers, was already exploited with PEF samples. Indeed, after 3 days under vacuum at 180°C, the number average molecular weight of a PEF prepared by Knoop and co-workers starting from DMFD increased by 10 times.13 Finally, bearing in mind the food packaging applications for which the PEF has been developed, even the choice of a suitable catalyst becomes an important point to be considered. Indeed, catalysts based on Titanium, Tin and Antimony and Germanium did prove to be suitable for the synthesis of PEF, but feature significant drawbacks. Titanium based catalysts invariably lead to an unacceptable discoloration, while the use of organo-Tin and Antimony compounds are troublesome with respect to the green environmental profile. The use of these catalysts is also questionable from a food contact safety perspective;37-39 in fact, if on the one side the current national regulations still allow their use with certain limits, on the other, an increasing attention of potential contamination has pushed major brands to issue policies restricting their use in packaging solutions. Last but not least, the use of Germanium based catalysts, once very popular in PET manufacturing, has been

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constantly declining because of its cost, restricting its use to few specialty polyesters, mainly for the Japanese market. On the basis of the above issues, the present paper aims at investigating the synthesis of PEF starting directly from FDCA and focusing on the identification of a catalyst that can achieve many goals: i) food safety and environmental sustainability; ii) decrement of glycol excess and polymerization time, minimizing the presence of ether-bridges (DEG moieties); iii) reduced discoloration. The final purpose is to define a potential industrial way to produce an excellent material for food packaging.

EXPERIMENTAL PART Materials 1,2-Ethylene Glycol, Titanium (IV) butoxide (TBT), Zinc Acetate (ZnAcO) were purchased from Sigma-Aldrich. Aluminum acetylacetonate (Al(acac)3) was supplied from Alfa Aesar. 2,5furandicarboxylic acid (FDCA) was provided by Tetra Pak. All the reagents supplied featured a high purity level and were therefore used without further purifications.

Synthesis in glass reactors and solid state polymerization (SSP) 25.76 g (0.165 mol) of FDCA, 25.57 g (0.412 mol) of EG and a defined amount of catalyst were charged into a glass reactor, equipped with a stirrer, a temperature programmer, a vacuum controller and a torque meter which gives an indication of the viscosity of the reaction melt. The amount of the catalyst was calculated as a ppm ratio between the weight of the metal contained in the catalyst alone and the total theoretical weight of the polymer obtained. The reactor was immersed into a salt bath preheated to 190°C. After 60 min, the temperature was increased up to 200°C and the reactor was maintained at this temperature for 60 min. The first stage was conducted at atmospheric pressure, under nitrogen atmosphere and under stirring (150 rpm) with continuous removal of water. In the second stage the reactor was heated from 200 to 240°C while the pressure was gradually reduced to 0.07 mbar. These conditions were reached within 90 min, using a linear gradient of temperature and pressure, and maintained for 120 min until a maximum torque value was reached. The first stage was extended by one additional hour when the catalyst was added in the second stage. The solid state experiments were carried out in an oven preheated at a temperature of 200°C and connected to a vacuum pump to remove the volatile by-products generated during SSP. The samples were previously powdered and crystallized at T=110°C for 15 hours and then treated at 4 ACS Paragon Plus Environment


T=165°C for 48 hours. These two steps require a time t1=63 hours. Finally, the samples were kept 15 hours at 200°C, leading to an overall process time (t2) equal to 78 hours. Characterization and film preparation The 1H NMR analysis was performed at room temperature on samples dissolved in a mixture of deuterated chloroform/deuterated trifluoroacetic acid 80/20 (V/V). The 1H NMR spectra were recorded at 400 MHz with a Varian Mercury 400 spectrometer. The calorimetric analysis was performed by a Perkin–Elmer DSC-6 calibrated with high purity standards. The samples (about 10 mg) were subjected to the following thermal treatment: 1st scan: from 30°C to 250°C at 20°C min-1, and 1 min of isotherm at 250°C; cooling scan: from 250°C to 20°C at 10°C min-1 and 1 min of isotherm at 20°C; 2nd scan: from 20°C to 250°C at 10°C min-1. During the 2nd scan, the glass transition temperature (Tg) was determined and its error was equal to ±0.2°C. The inherent viscosities were measured at 30°C with an Ubbelohde viscometer using a solution phenol/1,1,2,2-tetrachloroethane (TCE) 60/40 (wt/wt) at a concentration of 0.5 g dL-1. The determined viscosities are affected by an error evaluable as ±0.01 dL/g. The inherent viscosity J of each PEF sample was calculated using the following equation: J = [ ln (t/t0) ] / c where t is the flow time of solution, t0 is the flow of pure solvent, and c is the concentration of the solution, expressed as g dL-1. For each sample, the average value was determined after performing almost five different measurements. The ATR FT-IR spectra (reported in the Supporting Information) were recorded over the wavenumber range 650-4000 cm-1 using a Perkin Elmer Spectrum One FT-IR spectrometer equipped with a Universal ATR sampling accessory. UV-VIS characterization was carried out according to the following procedure: a solution of 5mg/mL was prepared in a mixture of dichloromethane/1,1,1,2,2,2-hexafluoro-2-propanol 80/20 v/v and the absorbance was measured at 400nm with a spectrophotometer UV-VIS. All measurements were repeated at least three times, to determine the related standard deviations. For the film preparation 1.20 g of PEF powder were weighted and scattered on a Teflon foil. The foils were placed between the plates of the Carver press and heated at 240°C, under 5-6 bars over 5 minutes. Then the film was quickly cooled to room temperature, led to room pressure and finally separated by the Teflon foils.


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RESULTS AND DISCUSSION Set up of the synthesis conditions and polymerization tests With the aim of studying the PEF synthesis starting from FDCA, some reaction parameters have been set up, according to literature, and the analytical approach to understand relationships between polymerization conditions and final polymer properties has been defined. In particular, the EG/FDCA ratio has been set to 2.5 taking into account the losses of EG due to evaporation. At lower ratios an overbalancing of the monomer content determines a low polymerization degree, whereas, at higher EG/FDCA ratios an increment in DEG units is highly probable. Indeed, the DEG units are formed as by-products derived from the etherification side reaction of EG or hydroxyethyl ester end-groups,18 mostly catalysed by acidic medium, not avoidable when FDCA is used as starting material. It is necessary to remember that the determination of the DEG amount is significant as it can affect important properties, such as Tg and melting point in PET, where ether units are segregated in the amorphous regions.28 Moreover, ether blocks are characterized by high chain mobility and are then responsible of low barrier performances.29 In this regard the amount of DEG units was calculated by 1H NMR analysis.24,27 Figure 1 shows an example of 1H NMR spectrum of a sample of PEF: the peak attributions of the protons of the repeating unit, of the end-groups and of the DEG units, are reported. In particular, the DEG amount has been calculated as a molar ratio between the normalized peak of protons “d” and the sum of the normalized protons “d”, “a” and “f”. The temperature range used during the first stage (190-200°C) matches the high solubility conditions of FDCA in EG and the low DEG production.27. Moreover, following few preliminary tests, the total polymerization time has been established at 5.5 hours, a very short time with respect to the polymerization conditions described in literature.14,18,20,24 In this case, it is necessary to balance two opposite factors: the need to achieve a high molecular weight, that requires long reaction time, and the need to avoid degradation and yellowing of the material, as well as to limit the DEG formation, that calls for a short reaction time.



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O

O O

b

a O

O

a

a

b b

CHCl3 O O

c

O

O

O

c

O

O

O

d

O

d

O

b b

b b O

O O

e O

O

OH

f

b b c,e f

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

O

d

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0 ppm

Figure 1: 1H NMR spectrum of a sample prepared by using Al(acac)3 catalyst with the assignments of signals due to the DEG and terminal units.

Two specific catalysts have been tested, Aluminum acetylacetonate (Al(acac)3) and Zinc Acetate (ZnAcO): this choice is justified by the fact that they are suitable for food applications. Both of them have been largely exploited in the PET synthesis.40-42 In the synthesis of PEF only ZnAcO has been tested combined with Antimony trioxide;16,43 indeed, ZnAcO is commonly used as a catalyst for the transesterification reaction in the first stage of polyester synthesis from the diester and EG and, therefore, in the second stage the addition of a further catalyst, active in the polycondensation process, is carried out.44 On the contrary, Al(acac)3 has never been exploited for PEF preparation. Gruter et al. tested the Al(acac)3 in the poly(butylene 2,5-furandicaboxylate) preparation starting from the DMFD diester31 and obtained in a moderate yield a polymer characterized by low inherent viscosity and high absorbance.


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Al(acac)3 is a compound that we have assumed does not leave traces in the final PEF since hydrolysis reactions occurring during the PEF synthesis are likely to leave only Aluminum oxide and hydroxide traces, i.e. compounds deemed to have an outstanding food safety profile at the actual concentration. Accordingly, Al(acac)3 has been already used as catalyst for food packaging applications.45 In the same way, the ZnAcO is a compound easily hydrolysable in the aqueous media to Zn oxide and hydroxide and, for this reason, it is similarly usable in the polymerizations intended to develop materials for the food sector. In order to compare the results obtained by using Al(acac)3 and ZnAcO catalysts, polymerizations with Titanium (IV) butoxide (TBT) have also been carried out. TBT is not approved for the food contact, but is the most used in the PEF syntheses: therefore, the results obtained can be useful as a reference for the present studies. In general, the amount of catalyst, used in this study, ranges from 20 to 400 ppm. Then, also the possibility of adding it at the beginning of the first or the second polymerization stage has been considered. Finally, in order to investigate the efficiency of the polymerization process, three independent characteristics of the final materials have been studied: (i) the inherent viscosity, as an indication of polymeric chain length, (ii) the absorbance at 400 nm, that refers to the yellowing degree, as an estimation of by-side reaction extent, (iii) the DEG content, determined by 1H NMR analysis. A basic characterization of all the samples has been carried out by NMR and DSC to confirm the polymeric structure and to study the phase behaviour. As an example, from Figure 1, where a NMR spectrum of a PEF polymer here synthesized is reported, it is possible to observe that the reaction is successful: the PEF structure is confirmed and no peaks evidencing degradation reactions are present. Moreover, the Tg values are reported for all the samples and, even if not commented, they are useful data to verify the relatively high molecular weights and the low DEG content of the samples. Table 1 reports the results of the polymerization tests carried out according to the above fixed conditions: three different catalysts (TBT, ZnAcO, Al(acac)3) have been tested and used at the first stage of the polymerization, i.e. at the beginning of the synthetic process. The final polymers, obtained using 400 and 50 ppm of TBT, are clearly characterized by a positive high value of inherent viscosity: however, the colour tends to black, as confirmed by a very high value of absorbance. Moreover, the DEG content is significantly high, varying from 4.4 to 5.0 mol%. Comparatively, the DEG amount in commercial PET samples usually runs between 2.8 (for PET film grade) and 3.9 mol% (PET bottle grade). Tg values, that depend on both viscosity and DEG amount, are around 82°C, that is a typical value found in literature for PEF.14 Other thermal 8 ACS Paragon Plus Environment


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transitions are not evident in the DSC diagram (entirely reported in the Supporting Information), confirming that PEF is an amorphous polymer.

Table 1: characteristics and results of PEF polymerization experiments where catalyst is added in the first step Sample Code

Catalyst

Amount

5

(ppm)

(dl/g)

A*

DEG

Tg

(mol%)

(°C)

1

TBT

400

0.36

0.53±0.03

4.7

82.1

2

TBT

50

0.32

0.35±0.01

5.0

82.6

3

ZnAcO

400

0.34

0.15±0.00

3.7

83.6

4

ZnAcO

50

0.25

0.12±0.00

2.7

84.2

5

Al(acac)3

400

0.19

0.07±0.01

3.2

80.2

6

Al(acac)3

100

0.23

0.08±0.01

2.6

81.3

7

Al(acac)3

50

0.25

0.07±0.01

3.3

83.3

8

Al(acac)3

21

0.27

0.12±0.01

3.0

82.5

*Absorbance at 400 nm

From Table 1 it is notable that the Zinc-based catalyst leads to a relatively high molecular weight, as already observed by other authors.18,31,46 More specifically, the use of 400 ppm of ZnAcO seems very interesting for the high viscosity value of the final polymer. On the other hand, the use of Al(acac)3 does not allow the polymer to reach viscosities comparable with those obtained with ZnAcO. Moreover, it is noteworthy that inherent viscosities tend to decrease with the increment of the amount of catalyst. To confirm this trend, a high number or syntheses has been performed with Al(acac)3, whose amount varies from 21 to 400 ppm; the results indicate that

varies from 0.27 to 0.19 correspondingly.

In front of these results, it is necessary to underline that the study of the catalysis mechanism in the PEF polymerization is not an object of this work; however, we can speculate that for the ZnAcO the mechanism is analogous to that in the PET synthesis, which is known and already reported. The zinc ion coordinates the carbonylic oxygen and therefore increases the double bond polarization, favouring the nucleophilic attack.47,48 The Al(acac)3 seems to work in the same way, but it is characterized by a lower kinetic constant,49 probably due to the higher steric hindrance of acetylacetonate groups, which could prevent the metal coordination. Moreover, to explain the observed trend between amount of Al(acac)3 and viscosity, it is possible to hypothesize that high 9 ACS Paragon Plus Environment

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amount of catalyst can foster the reverse reaction, leading to opposite results, as observed for other catalytic systems;44,48 in any case, the behaviour must be studied more in depth. Moreover, it is notable that both ZnAcO and Al(acac)3 induce a relatively low DEG amount, significantly lower than those obtained with the use of TBT. Concerning the colour, whereas TBT is responsible for the important PEF yellowing, ZnAcO and Al(acac)3 cause a reduced discoloration, with a very low value of absorbance. From these first results, it is possible to assume that both ZnAcO and Al(acac)3 represent interesting new candidates as catalysts for the PEF preparation starting from FDCA and using a low EG/FDCA ratio (2.5) and a short polymerization time (5.5 hours). Therefore, to confirm this outcome, new polymerization experiments have been carried out, testing the possibility of adding the catalyst only at the second reaction stage. In fact, the first polymerization stage (i.e. the direct esterification of the diacid) is autocatalytic and consequently it is often carried out without catalyst (and preferably under pressure).44,50 Moreover, the catalyst can be sensitive to an aqueous medium, thus producing a hydrolysis reaction and losing its catalytic efficacy. The related data are summarized in Table 2.

Table 2: characteristics and results of PEF polymerizations with ZnAcO and Al(acac)3 as catalyst, tested in different amounts, combination and in different addition time. PEF obtained with TBT is used as a reference material Sample Code Catalyst

Amount

Stage

of (ppm)

catalyst

of 5

A

(dl/g)

DEG

Tg

(mol%) (°C)

introduction 9

TBT

400

2

0.33

0.50±0.02

6.1

79.8

10

ZnAcO

400

2

0.36

0.05±0.00

2.9

83.5

11

ZnAcO

100

2

0.26

0.10±0.01

3.0

80.8

12

ZnAcO

50

2

0.24

0.09±0.01

2.9

80.9

13

Al(acac)3

400

2

0.27

0.16±0.00

4.6

81.8

14

Al(acac)3

50

2

0.26

0.07±0.00

2.7

82.9

15

Al(acac)3

21

2

0.33

0.22±0.01

3.6

83.2

16

ZnAcO/

50/50

1

0.26

0.08±0.01

2.6

80.9

50/50

2

0.28

0.09±0.00

2.9

82.3

400/50

2

0.17

0.10±0.01

3.0

79.2

Al(acac)3 17

ZnAcO/ Al(acac)3

18

ZnAcO/



Al(acac)3

Firstly, the addition of TBT in the second stage has been tested, in order to have the related PEF as reference. It is remarkable that the use of TBT only at the second stage causes a slight decrement of viscosity and an increment of the DEG content with respect to sample 1, therefore it is not so successful for the PEF synthesis. Then, the addition at the second stage of an amount of ZnAcO varying from 400 to 50 ppm has been investigated. The data listed in Table 1 and 2 show that 400 ppm of ZnAcO, allows the polymer to reach the highest values of inherent viscosity regardless of the stage at which the catalyst is added. However, the best performances are obtained in sample 10, where the catalyst is present only in the second stage. In this case, the value of absorbance is very low, while the DEG amount is good (2.9 mol%). This success is due to the fact that ZnAcO is a catalyst for the transesterification reaction (and not for the direct esterification), meaning that its highest catalyst activity is carried out after the first stage, when acidic groups are already consumed.51 Moreover, it is known that the catalyst used for the transesterification could catalyse the polycondensation due to high similarity of the two reactions.52 On the other hand, from Table 2 it is confirmed that in the presence of Al(acac)3 viscosities tend to decrease as the amount of the catalyst is increased, regardless of the stage at which it is added. Furthermore, with 50 ppm of Al(acac)3 added in the first or in the second stage (see 7 and 14 samples), the results appear very interesting: the PEF samples are characterized by a good colour (A= 0.07), low DEG content (equal and lower than 3.3 mol%). The viscosity is relatively low, but a step of solid state polymerization can be applied to try to increase the molecular weight of the sample (see next section). Finally, since the mixing of different metal catalysts is a common practice to improve the performances of the single catalysts,41,42,53 the combination of Al(acac)3 and ZnAcO has also been tested. Table 2 reports the obtained results with a mixture of catalysts (samples 16-18). Sample 16 has been prepared by adding in the first stage the same weighted amount (50 ppm) of metal for both the catalysts. In this case the final polymer is characterized by an intermediate value of viscosity (0.26 dl/g), a low DEG content (2.6 mol%) and good absorbance (0.08). Such a promising result encouraged a more in-depth study for the possible existence of a positive interaction between the aluminum- and zinc-based catalysts. However, by adding the combined catalysts in the second stage, a slight worsening has been recorded in all the analyzed properties. Moreover, the mixing of the amounts of catalyst which produced the best results in the single addition of the two catalysts has been tested: 400 ppm for ZnAcO (see sample 10) and 50 ppm of 11 ACS Paragon Plus Environment

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Al(acac)3 (see sample 14 for Al(acac)3) added in the second stage. Even in this case the combination provided values that were not encouraging: a synergistic effect of the two catalysts was excluded and not further investigated in this work. Figure 2 reports the trend of the absorbance vs. the inherent viscosity for all the samples. It is notable that sample 10, prepared with ZnAcO, presents very interesting characteristics in terms of inherent viscosity (>0.3 dl/g) and colour (A 0.3 dl/g

14

7

10

0 0.15

0.2

0.25

0.3

0.35

0.4

Inherent viscosity (dl/g)

Figure 2: graphical summary of A vs. J for all the prepared PEF samples

Solid State Polymerization (SSP) In order to increase molecular weight and, then, obtain filmable materials, the PEF sample prepared with 50 ppm of Al(acac)3, added in the first stage, (sample 7) has been subjected to a stage of SSP, by following the protocol reported in the experimental part, where treatments at 110, 165, and 200°C are carried out. Two different times have been chosen for the sample analysis: t1 and t2, that correspond to 63 and 78 h of SSP. Sample 7 has been chosen for its good performances in terms of low absorbance (see Figure 2) and also for its low DEG content (Table 1). Furthermore, a comparative SSP experiment has been carried out on PEF prepared with catalyst based on titanium 12 ACS Paragon Plus Environment


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2

TBT

50

0.32

0.35±0.01

5.0

82.6

2-t2

TBT

50

0.30

0.97±0.02

4.5

80.6

It is remarkable that, with respect sample 7, the SSP carried out on PEF prepared with TBT (sample 2) did not produce favourable results: indeed, the viscosity decreased, as well as the Tg, probably due the occurrence of some degradation reactions. This behaviour was confirmed by a noteworthy yellowing of solid stated TBT polymer (sample 2-t2) and it was already reported by other authors.35 In any case, the applied SSP protocol was not optimized for the different catalysts present inside the samples and it simply try to increase the molecular weight of the sample and then prepare PEF films. Other tests on SSP will be performed in order to better understand the behaviour of the samples. Thanks to the achieved viscosity, the sample 7 presents a better appearance. It was submitted to compression moulding and a film, with good optical properties, was obtained, as shown in Figure 4. To better observe the colour difference, even the films deriving from samples 1 (TBT, A: 0.53) and 3 (ZnAcO, A: 0.15) has been reported as a comparison. The good results obtain with sample 7 are notable.

a)

b)

c)

Figure 4: 70 microns films on PEF writing to show the discoloration of a) sample 1 (TBT 400 ppm, A: 0.53; b) sample 3 (ZnAcO 400 ppm, A: 0.15; c) sample 6 (Al(acac)3 50 ppm,, A: 0.07)

CONCLUSIONS The synthesis of PEF, a bio-based material developed for packaging applications, has been investigated under different point of views, using FDCA as reactant monomer and with the aim of solving some drawbacks that characterize the synthetic process. In particular, starting from fixed conditions, related to a low glycol/FDCA ratio and a short polymerization time, other variables were taken into consideration. In particular, the amount of catalyst and its addition time have been 14 ACS Paragon Plus Environment


investigated and optimized with respect to a laboratory procedure including an atmospheric pressure direct esterification stage. It results that ZnAcO and Al(acac)3 are very promising catalysts for the PEF polymerization from FDCA, regardless of the stage at which they are inserted in the polymerization medium: the use of ZnAcO allows to reach relatively high inherent viscosity in a usual polymerization procedure, while the use of Al(acac)3 needs a final step of SSP to reach good viscosity data. The final PEF samples are characterized by very low discoloration and low DEG content. The prepared samples were therefore filmed, showing low yellowing degree and high transparency, confirming the possibility to exploit these materials in the food packaging field, thanks to the catalysts that may be considered as particularly suitable for food contact applications. The authors also believe that the studied catalytic systems may be conveniently transferred to an industrial PEF production. In that respect further reduction of discoloration phenomena and reduced DEG may arise from the adoption of a first esterification stage at high pressure (5 bar) and T>240°C. This approach may indeed increase FDCA solubility in MEG, further decreasing the optimal MEG/FDCA molar ratio, associated to DEG formation.

Supporting Information DSC curves of all the PEF samples (included the samples undergone to SSP), FT-IR spectra of PEF samples prepared with the three catalysts (TBT, ZnAcO and Al(acac)3) in different amounts (400 and 50 ppm).

Acknowledgments Financial support from Tetra Pak (Suisse) SA is gratefully acknowledged.

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