Synthetic plastics are inexpensive, lightweight and durable materials, easily processed into a variety of products that find use in a wide range of applications. Consequently, the production of plastics has increased markedly over the last 60 years [1
]. Today, plastics are almost completely derived from petrochemicals, produced from fossil oil and gas. Around 4% of world oil and gas production, non-renewable resources, are used as feedstock for plastics and a further 3–4% is expended to provide energy for their manufacture. Approximately 50% of plastics produced each year are used to make disposable items, such as packaging or other short-lived products that are discarded within a year of manufacture. These two observations alone indicate that current use of plastics is not sustainable. In addition, because of the durability of the polymers involved, substantial quantities of discarded end-of-life plastics are accumulating as debris in landfills and in natural habitats worldwide, generating huge terrestrial as well as marine environmental problems. Recycling is clearly a valid waste-management strategy, reducing environmental impact and resource depletion. However, in case of food contact packaging, it is not very desirable and economically advantageous.
On this ground, bioplastics, i.e., plastics obtained from renewable resources and/or biodegradable, may represent a solution to these urgent needs. The use of bioplastics reduces the dependence on fossil resources, reduces greenhouse gas (GHG) emissions, creates renewable energy and increases the resource efficiency [2
]. Part of the volumes of bioplastics produced nowadays is moreover recycled alongside their conventional counterparts (e.g., bio-based PE in the PE-stream or bio-based PET in the PET stream), contributing themselves to a more efficient waste management.
Among the different renewable starting materials that have been used for the preparation of bioplastics, furan-based monomers have attracted considerable attention, the most important example being represented by 2,5-furandicarboxylic acid (2,5-FDCA). Its success is mostly due to its use for the synthesis of poly(ethylene 2,5-furanoate) (PEF), currently considered the most credible bio-based alternative to poly(ethylene terephthalate) (PET), thanks to its very interesting physic/mechanical and barrier properties. In fact, PEF displays improved barrier performances and more attractive thermal and mechanical properties than PET. In particular, it is characterized by a higher Tg
(85 °C vs. 76 °C), a lower Tm
(211 °C vs. 247 °C) [4
], a 1.6 times higher Young’s modulus [4
], 11 times lower oxygen permeability [5
], 19 times lower carbon dioxide permeability [6
] and a 5 times lower water diffusion coefficient [7
]. Lastly, the production of PEF would decrease the non-renewable energy use of about 40–50% and the greenhouse gas emissions of 45–55% ca. with respect to PET [8
Currently, the academic research interest have been also extended to other 2,5-furan dicarboxylate-based polymers, which have been obtained by using aliphatic diols with different length, sugar diols like isosorbide, benzylic structures like 1,4-bishydroxymethyl benzene, and bisphenols like hydroquinone, etc. [9
]. Soccio et al., Vannini et al., and Guidotti et al., investigated the barrier properties of these new 2,5-FDCA-based polyesters, of course comparing them with those of PEF [10
Recently, Tsanaktsis et al. [13
] successfully synthesized poly(neopentyl glycol 2,5-furanoate) (PNF) by melt polycondensation, starting from dimethyl 2,5-furanoate. The new bio-based polyester was mainly subjected to a deep thermal characterization, including melt isothermal crystallization studies.
In the present work, we synthesize PNF starting directly from 2,5-furandicarboxylic acid, and besides the basic physical chemical characterization, mechanical as well as barrier properties of PNF compression molded films were investigated at different temperatures and relative humidity, and correlated to the chemical structure. The functional properties have been then compared to those of both poly(propylene 2,5-furanoate) (PPF), previously prepared in our laboratories, and poly(ethylene 2,5-furanoate) (PEF). Lastly, the permeability behavior after contact with food has been investigated, too.
3. Materials and Methods
2,5-furandicarboxylic acid (2,5-FDCA) 98% was purchased from CHEMOS GmbH & Co. K (Regenstauf, Germany), whereas, neopentyl glycol (NPG), titanium tetrabutoxide (Ti(OBu)4) and titanium tetraisopropoxide (Ti(O-i-Pr)4) from Aldrich (Milan, Italy). FDA, NPG and Ti(O-i-Pr)4 were used as supplied, whereas Ti(OBu)4 was distilled before use.
3.2. Polymer Synthesis
Poly(neopentyl 2,5-furanoate) (PNF) was synthesized in bulk too, starting from 2,5-furan dicarboxylic acid (FDCA) and neopentyl glycol (NPG), using a large excess of NPG (500%) with respect to the acid molar content. Antimony trioxide (Sb2O5) was employed as catalyst (about 7.5 × 10−4 mol). The synthesis was carried out in a 250 mL stirred glass reactor, with a thermostatted silicon oil bath; temperature and torque were continuously recorded during the polymerization. After 30 min, the mixture became transparent, indicating the solubilization of the acid in the glycol. The first stage was conducted at 180 °C under controlled nitrogen flow. In this step, the direct esterification with elimination of water molecules took place (the first phase totally lasted about 4 h). In the second stage, the pressure was gradually reduced to about 0.1 mbar to facilitate the removal of the glycol in excess, and the temperature was risen to 220 °C; the polymerization was carried out until a constant torque value was measured (the second phase totally lasted about 3 h).
The as-synthesized polymer was purified through dissolution in a mixture hexafluoro-2-propanol/chloroform and precipitation in methanol. The purified polymer, in the form of white floccules, was dried at 30 °C under vacuum to constant weight. Thin films of about 150 µm thickness were obtained by compression molding using a Carver press. Purified polymer was melted at 180 °C and kept for 2 min at a pressure of 5 tons/m2. Lastly, the film was cooled to RT in press by tap water.
Film thickness was determined by Sample Thickness Tester DM-G (Brugger Feinmechanik GmbH, Munich, Germany). Reported value represents the mean thickness of three experimental tests, each run on 10 different points on the polymer film surface at RT.
3.3. Molecular, and Thermal Characterization
Polymer structure was checked by 1H-NMR spectroscopy at RT. A Varian Inova 400-MHz (Palo Alto, CA, USA) was used for the measurements.
Molecular weights were determined by gel-permeation chromatography (GPC) at 30 °C with a 1100 HPLC system (Hewlett Packard, Palo Alto, CA, USA) equipped with PLgel 5-μm MiniMIX-C column (Agilent, Milan, Italy). A UV-detector (Hewlett Packard, Palo Alto, CA, USA) was employed as detector. A Hexafluoro-2-propanol/chloroform mixture (5%:95% v/v) was used as eluent with a 0.3 mL/min flow. A molecular weight calibration curve was obtained with polystyrene standards in the range of molecular weight 800–100,000 g/mol.
TGA was carried out under nitrogen atmosphere by means of a Perkin Elmer TGA7 apparatus (Perkin Elmer, Milan, Italy). Gas flow of 30 mL/min and heating scan of 10 °C/min were used for the analysis.
A Perkin Elmer DSC6 (Perkin Elmer, Milan, Italy) was used for the calorimetric measurements. Weighed samples were encapsulated in aluminum pans and heated to about 40 °C above fusion temperature at a rate of 20 °C/min (first scan), held there for 3 min, and then quenched to −40 °C. Finally, they were reheated from −10 °C to a temperature well above the melting at a heating rate of 20 °C/min (second scan).
To evaluate the presence of a rigid-amorphous phase in PNF and PPF, solvent-treated powders characterized by different crystal/amorphous ratio were prepared by partial melting in DSC to various temperatures in the melting range, quickly cooling inside the instrument below the glass transition temperature and reheating at 20 °C/min.
3.4. Water Contact Angle Measurements
Static contact angle measurements were performed on polymer films by using a KSV CAM101 instrument (KSV Instruments, Helsinki, Finland) by recording the side profiles of deionized water drops for image analysis, according to the procedure described by Drelich [33
]. Eight drops were observed on different areas for each film, and water contact angles (WCA) were reported as the average value ± standard deviation.
3.5. Tensile Tests
The tensile measurements were carried out on rectangular films (5 mm wide and 0.2 mm thick) with a crosshead speed of 10 mm/min by using an Instron 4465 tensile testing machine (Darmstadt, Germany), equipped with a rubber grip and a 100 N load cell. A preload of 1 MPa was applied to each specimen prior to testing. At least five replicates were run and the results are provided as the average ± standard deviation.
3.6. Gas Transport Measurements
The determination of the gas barrier behavior was performed by a manometric method, using a Permeance Testing Device, type GDP-C (Brugger Feinmechanik, GmbH, München, Germany), according to ASTM 1434-82 (Standard test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting), DIN 53 536 in compliance with ISO/DIS 15 105-1 and according to Gas Permeability Testing Manual (Registergericht München HRB 77020, Brugger Feinmechanik GmbH).
After a preliminary high vacuum desorption of the up and lower analysis chambers, the upper chamber was filled with the gas test, at ambient pressure. A pressure transducer, set in the lower chamber, records continuously the increasing of gas pressure as a function of the time. The gas transmission rate (GTR) was determined considering the increase in pressure in relation to the time and the volume of the device. All the measurements have been carried out at room temperature (23 °C). The operative conditions were: gas stream of 100 cm3
; 0% RH of gas test, food grade; sample area of 78.5 cm2
(standard measurement area). Films were also analyzed at 5°C, 15°C, and 38 °C. Gas transmission measurements were performed at least in triplicate and the mean value is presented. Method A was used for the analysis, as just reported in the literature [34
] with evacuation of up/lower chambers. Sample temperature was sets by an external thermostat HAAKE-Circulator DC10-K15 type (Thermoscientific, Selangor, Malaysia).
The transport phenomena background followed in the experiment is well described in literature, with a full description of the mathematical equation and interpretation [6
3.7. Simulant Liquids
The food contact simulation was performed in accordance with EU Regulations No. 10/2011 on plastic materials and articles intended to come into contact with food [30
Four solutions were used as food simulants:
Simulant A, Ethanol 10% (v/v), 10 days, 40 °C
Simulant B, Acetic acid 3% (v/v), 10 days, 40 °C
Simulant C, Ethanol 20% (v/v), 10 days, 40 °C
Simulant D1, Ethanol 50% (v/v), 10 days, 40 °C
The measurement was made on a totally immersed 12 cm × 12 cm film specimen. 200 mL of simulant was placed into glass flasks (of 400 mL of volume) containing the film sample and the flasks were then covered with caps. Samples were placed in a stove (Universalschrank UF110, Memmert GmbH + Co. KG, Schwabach, Germany). After the assay time was elapsed, the specimens were removed from the flasks, washed with distilled water two times and dried with blotting paper. Before analysis, the films were kept at room temperature, in dry ambient for at least two weeks. The samples were tested in triplicate.
3.8. Relative Humidity Solution
According to the procedure reported on the Gas Permeability Testing Manual (Registergericht München HRB 77020, Brugger Feinmechanik GmbH)
, the analyses were performed at different relative humidity (RH) obtained with several saturated saline solutions. In particular:
Standard Environment, 23 °C, 85% of RH, with saturated KCl solution;
Tropical Climate, 38 °C, 90% RH, with saturated KNO3 solution;
33% RH, 23 °C, with saturated MgCl2 solution;
57% RH, 23 °C, with saturated NaBr solution;
75% RH, 23 °C, with saturated NaCl solution;
The values for the relative humidity for the saline solutions are taken from DIN 53 122 part 2. In the humid part of the top permeation cell was insert a glass-fiber round filter humidified with the desired saturated saline solution. Method C was used, with gas flow blocked onto the test specimen during evacuation. In this manner, the test gas is humidified inside the permeation cell. This method evacuates only the area of the bottom part of the sample. On the top part of the test specimen, with the humidified gas, the normal ambient pressure is applied.
Poly(neopentyl glycol furanoate), an aromatic polyester derived from renewable resources, has been successful synthesized through melt polycondensation, starting directly from the corresponding dicarboxylic acid. PNF showed superior thermal stability and similar mechanical properties with respect PPF, previously synthesized by us [12
More interestingly, compared to PEF, PNF displays a reduction of permeability to O2
of 11× and 61×, respectively [5
] and revealed to be even better than PPF.
In the case of nitrogen gas and especially in the case of carbon dioxide gas, temperature did not show a significant impact on the gas transmission process, and this is an advantage for storage packaging use. On the contrary, oxygen gas was found to be more sensitive to temperature.
Permeability data after contact with food simulants indicate that no significant change occur in the chemical-physical characteristics of the polymer sample, with a very low effect on the permeability behavior.
The permeability increased with increasing RH, indicating a strong interaction with moisture, as presumable, taking into account the polar nature of the furan ring.
The calculated ratios of permeability were different from the ratios of standard polymers reported in literature [22
]. This result is important for the selection of the correct headspace gas composition for modified atmosphere packaging, in order to avoid collapsing of the package or in order to choice the correct storage atmosphere condition.
In conclusion, PNF can be considered a very important member of the bio-based polyester family, opening up new possibilities in the sustainable packaging.