Utilization of Torrefied Coffee Grounds as Reinforcing Agent To

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Utilization of torrefied coffee grounds as reinforcing agent to produce highquality biodegradable PBAT composites for food packaging applications Hesham Moustafa, Chamseddine Guizani, Capucine Dupont, Vincent Martin, Mejdi JEGUIRIM, and Alain Dufresne ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02633 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Sustainable Chemistry & Engineering

Utilization of torrefied coffee grounds as reinforcing agent to produce high-quality biodegradable PBAT composites for food packaging applications Hesham Moustafa1, 3, Chamseddine Guizani3, Capucine Dupont4, Vincent Martin5, Mejdi Jeguirim6, Alain Dufresne2, 3* 1

Polymer Metrology & Technology Department, National Institute for Standards (NIS), Tersa Street, El Haram, El-Giza, P.O Box 136, Giza 12211, Egypt. 2 Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France 3 CNRS, LGP2, F-38000, Grenoble, France 4 CEA, Grenoble, France 5 CNRS, LEPMI - UMR 5279, Grenoble, France 6 CNRS, IS2M, Mulhouse, France * Corresponding author: [email protected]; Full mailing address: Pagora-Grenoble INP, CS10065, 461 rue de la Papeterie, 38402 Saint Martin d’Hères cedex, France

ABSTRACT The present study has revealed that torrefied coffee grounds (CG) derived from agriculture commodities can be used as bio-reinforcing agent for biodegradable poly(butylene adipateco-terephthalate) (PBAT) without requiring a compatibilizer. The optimum torrefaction operation was achieved, in order to increase the hydrophobicity of CG. The raw CG was also used as a reference to assess the effect of the torrefaction operation. The structure and morphology of the composites were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The effect of the addition of raw or torrefied CG on the melting temperature and crystallinity of PBAT biocomposites was analyzed by differential scanning calorimetry (DSC). A significant enhancement in the thermomechanical properties for PBAT/torrefied CG composites was observed compared to PBAT/CG composites. Moreover, the hydrophobicity of PBAT composites which was determined by water contact angle was improved when torrefied biomass was added. The thermal stability of the investigated samples was analyzed by thermogravimetric analysis (TGA) and a kinetic model was proposed to describe the thermal degradation of raw CG, torrefied CG, PBAT and their filled composites. The obtained results for these solvent-free

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prepared biocomposites show that they can be potential candidates for food packaging applications. KEYWORDS Torrefaction Coffee grounds; PBAT; Hydrophobicity; Mechanical properties; Thermomechanical properties; TGA INTRODUCTION Coffee derived from roasted coffee beans or grains is one of the largest agriculture crops in the world and the second largest traded commodity after petroleum1, with an annual production of approximately 8 billion kg per year, according to International Coffee Organization (ICO) in 2012. It is the global most favorite beverage after water, with nearly 4 billion coffee cups consumed daily, leaving huge amount of wastes and by-products called coffee grounds (CG). The latter are considered as lignocellulosic materials as they are mainly composed of cellulose, hemicellulose and lignin. They also contain proteins and a non-negligible amount of minerals.2,3 As a consequence, CG have attracted the attention of researchers as “green materials” with many potential uses in a variety of fields such as bioenergy

4,5

, bio-fuels

6-8

or polysaccharides productions.

9,10

In some cases, they can be

used as reinforcing agent in biopolymers to produce sustainable green composites with affordable cost.11,12 However, their hydrophilicity constitutes a real barrier to their incorporation in polymer matrices due to the poor affinity between hydrophobic polymers and hydrophilic CG components, thereby leading to limited usage as bio-reinforcement. Currently, numerous studies have been conducted on CG not only for improving their hydrophobicity for material applications, but also to reduce the risk related to disposal in landfill locations.13 Among these studies, some focused on comptabilizers or coupling agents addition to the polymer matrix,

11,14-17

other on chemical modification of CG by

antimicrobial rosin18, or by torrefaction, which is a mild thermal treatment (200-300°C)

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under nitrogen, with the aim of obtaining a hydrophobic material.19-22 Nowadays, the torrefaction of lignocellulosic biomass played a significant role for improving the biomass properties in terms of higher calorific value, hydrophobicity, grindability, long–term storage and handling.20 The mass loss experienced during the torrefaction step depends on the torrefaction conditions (final temperature, heating rate and holding time). Furthermore, the reduction of O/C and H/C ratios can make torrefied CG a candidate in the production of syngas.23-25 Thus, torrefaction is a promising process to enhance the performance of biomass and wastes for renewable energy applications. The use of CG, which is a widely available, low price, and nontoxic agro-industrial by-product, as possible reinforcing filler or additive can constitute a way for its valorization.26 Besides, the torrefaction treatment can be applied on this waste in order to improve its affinity with the polymeric matrix. Poly(butylene adipate-co-terephthalate) (PBAT) is a synthetic biodegradable thermoplastic biopolyester. It has been currently paid much attention in a variety of disciplines due to not only unique properties when compared to classical petroleum-based polymers, but also its use in extensive applications such as food packaging, biomedical fields, and industrial composting.27,28 However, PBAT has poor mechanical and thermo-mechanical properties, beside its high price; thereby these drawbacks restrict its uses in the industrial sectors. To limit these obstacles, a reinforcing agent shall be added to improve the polymer properties to meet desired applications, and decrease the final price of the product. Nevertheless, the reinforcing effect often depends upon microstructure configuration and interfacial adhesion between the polymer matrix and the filler.29 Berthet et al

30

applied a torrefaction treatment

to wheat straw fibers to increase their hydrophobicity and used them as filler for poly(3hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). They concluded that fiber torrefaction treatment had no significant effect on the mechanical properties of PHBV/wheat straw fiber composites, whereas water vapor permeability was improved by 30% up to 20 wt.% fiber



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content, which was ascribed to the hydrophobic nature of the fibers. On the other hand, torrefied flax shive and sunflower hulls were shown to increase the modulus of polyamide-6, while maintaining the tensile strength similar to that of the neat polymer. Also, the addition of torrefied flax shive imparted decreased moisture absorption for polyamide-6 biocomposites over the neat polymer when soaked for 24 or 72 hours.31 In our previous study

11

, we investigated the possibility of using lignocellulosic CG as a reinforcing agent

for PBAT with the aim to reduce the cost of PBAT based composites and valorize the CG waste. Several application fields can be considered for such composites. Despite promising results, characterization of the composites showed that there would be a need to add a PEG plasticizer to achieve better compatibility and thermo-mechanical properties of the PBAT/CG composites. In the present work, torrefaction pre-treatment of CG is investigated with the aim to enhance the affinity between CG and PBAT by increasing the CG hydrophobicity and hence improve the mechanical and thermo-mechanical properties of the PBAT/CG composites without requiring a compatibilizer with affordable cost to meet eco-friendly purposes. MATERIALS AND METHODS Materials Poly (butylene adipate-co-terphthalate) (PBAT) (PBE 006 resin) was provided from NaturePlast SAS, France. It has a Melt Flow Index (190°C; 2.16 kg) of 4-6 g.10 min-1 with a density of 1.26 g.cm-3. Preparation of coffee ground (CG) Coffee grounds (CG) wastes were collected from local LGP2’s cafeteria at “The International School of Paper, Print Media and Biomaterials” (Grenoble INP-Pagora), Grenoble, France, with an approximate water content of 55% and its preparation was


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described according to our previous work.11 Briefly, the CG wastes were firstly dried in a laboratory oven at 105°C for 48 h to remove water. The dried sample was then sieved to remove any foreign or coarse materials. Afterwards, The TGA analysis under N2 was implemented before the torrefaction procedure to optimize the torrefaction temperatures with sufficient grams of torrefied CG at different temperatures such as 230, 250, 270, 290, and 310°C, as shown in Figure 1(a). CG was heated from room temperature to the torrefaction temperature (230-310°C) using a heating rate of 10°C.min-1, and then the TGA spectrum was recorded after reaching the final temperature for 1 h. FTIR spectra were used to monitor the lignocellulosic CG functional groups for each temperature as shown in Figure 1(b). As displayed in Figure 1, it can be seen that the weight loss of CG powder increased with increasing torrefaction temperature. Meanwhile, FTIR spectra pointed out that the optimum torrefied CG was obtained at 250 °C and 270 °C. Indeed, the intensity of characteristic absorption peaks of –OH and C=O groups at 3345 cm-1 and 1750 cm-1, respectively, for CG powder reduced with increasing the temperature up to 270°C and they disappeared beyond this temperature, evidencing that CG lignocelluloses become hydrophobic materials. For this reason, the CG torrefaction was carried out at 250 °C and 270°C. On the other hand, the two absorption peaks in CG powder at about 2930 and 2850 cm-1 are attributed to the hydrocarbons (C-H asymmetric of CH3 and C-H symmetric of CH2, respectively).32,33 These peaks still appear up to 310 °C because the thermal degradation of lignocellulosic components could be degraded at different temperatures, since lignin is more thermally stable than other components.19



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Figure 1. a) TGA analysis for CG torrefaction performed at different temperatures for 1 h ,and b) FTIR spectra after torrefaction process. Preparation of torrefied CG The CG powder was torrefied at two different temperatures which are 250°C and 270°C Around 30 g of CG sample was placed in a stainless steel home-made reactor with diameter of 7.5 cm and height of 5 cm. This reactor was sealed through a graphite basket and flushed with N2 at a flow rate of 130 L h-1. The reactor was placed in an oven F30400 from Thermolyne with a 33-F30430CM regulator. Five thermocouples enabled to follow the temperature in both gas and bed sample. The sample was firstly dried at 105°C during 1 h, then heated at 5°C.min-1 up to the torrefaction temperature (250°C and 270°C) and maintained at this temperature during 2 h. At the end of the torrefaction process, the torrefied sample was left to cool at ambient temperature, still under nitrogen flow and then collected in clean bottle and stored in a laboratory desiccator for the tests. Particle size analysis The particle size of torrefied CG filler obtained at 250°C and 270°C was thereafter measured and compared with CG powder by using particle analyzer Camsizer XT (Retsch Technology) with X-jet mode, in which the air carriage disperses the CG particles by using 6 Environment ACS Paragon Plus

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air pressure to breakdown the particles agglomerates before enter the measuring area. The particle size analysis of CG and torrefied CG are shown in Figure S1. This figure shows the cumulative particle size distribution as a function of the particle diameter. The results show clearly that the torrefaction treatment causes a decrease of the CG particle size. The particles are likely to shrink upon thermal treatment which leads to the size reduction. For CG, 50% of the particles have size below 0.35 mm, while 90% have size below 0.62 mm. The higher particle size was 1 mm. The average value is estimated at 0.38 mm. Whereas, for the particle size of torrefied CG at 250°C and 270°C, it was found that 50% of the particles have size respectively below 0.29 mm and 0.27 mm, while 90% have size below 0.49 mm and 0.48 mm. The highest particle sizes were respectively 0.8 mm and 0.77 mm, while the average values were estimated at 0.29 mm and 0.27 mm for CG-250°C and CG-270°C, respectively. Altogether, these results show a clear decrease of the CG particle size upon torrefaction treatment. Processing of PBAT/CG biocomposites Prior to the melt compounding, both the PBAT and the CG powder were dried in an oven at 60°C for 12 h. The PBAT and untreated or torrefied CG were mixed using an extruder DSM Xplore Twin-Screw Microcompounder, Netherlands with batch-volume 15 cm3. The extruding temperature profile was set up in six separate heating zones from 160 °C to 165 °C and a screw speed of 100 rpm during the mixing for 5 min. The extruded films (~ thickness from 0.3 to 0.4 mm and width ~ 13 mm) were obtained by a special die which was attached with the mixing chamber. The samples were thereafter cut for characterizations and tests and abbreviated as PBAT/CGX, where X refers to the CG content in the biocomposite either in case of untreated or torrefied CG at 250 and 270°C, as shown in Table 1.



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Table 1. Composition and codification of the prepared formulations for virgin PBAT and its filled composites

Characterization X-ray diffraction (XRD) X-ray diffraction (XRD) measurements for raw CG powders, virgin PBAT and their filled composites with raw and torrefied CG were performed using a PANalytical X-ray diffractometer (X'Pert Pro MPD), Netherlands with CuKα radiation ( 40 kV, 40 mA) by using 2Ɵ range from 10° to 60° at a scan speed rate of 2° min-1, and at a sampling width of 0.02°. Scanning electron microscopy (SEM) The dispersibilty of untreated or torrefied CG particles with different amounts in PBAT matrix was investigated using scanning electron microscopy (SEM, High Resolution Quanta FEI 200, Czech Republic). SEM images were obtained in a high vacuum for both secondary electrons (topography contrast) and backscattered electrons (chemical contrast). The acceleration voltage (5-10 kV) and the electron beam spot size (3-3.5) were carefully chosen in order to optimize the quality of the images. The fractured surfaces were coated with a thin gold layer by using Agar automatic sputter coated prior to the observation.


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Fourier transform infrared spectroscopy (FTIR) The FTIR analysis in transmission mode was used to obtain the optimum conditions for CG torrefaction using a FTIR Perkin-Elmer 1720X spectrometer. The FTIR spectra were recorded in the spectral range between 4000 cm-1 and 500 cm-1 with a resolution of 4 cm-1 and 32 scans. The samples were prepared by pressing 100 mg of dried KBr with 2 mg of sample to obtain a thin disc. The KBr disc was used as a reference. Differential scanning calorimetry (DSC) The thermal analysis of virgin PBAT and its filled composites was conducted using differential scanning calorimetry (DSC Q-100, TA Instruments apparatus), equipped with a liquid nitrogen cooling system (LNCS) unit. Each sample (5-10 mg) was investigated using heating and cooling cycles from -50 °C to 160°C with a heating rate 10 K.min-1 and nitrogen flow rate of 50 mL.min-1. During the heating cycle, the onset melting temperature (Tm_onset) and melting enthalpy (∆Hm, J.g-1) were determined, whereas during the cooling cycle, the onset crystallization temperature (Tc_onset) and crystallization enthalpy (∆Hc, J.g-1) were also used. At least duplicates were recorded for each specimen. Dynamic mechanical analysis (DMA) The DMA analysis for all specimens was conducted using TA Instruments, Model ARESG2 DMA mode from -50°C to 150°C at a heating range of 3 K.min-1, and at a constant frequency of 1 Hz in film tension geometry mode and with a strain of 0.02%. The deformation state of bionanocomposites during DMA test was monitored using high resolution camera which was attached with the equipment and compared with virgin PBAT. Three samples at minimum for each composition were tested. Mechanical properties



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The tensile properties for all investigated samples were conducted using an Instron 5965 Universal Testing Machine, UK with a load cell of 5 kN and a crosshead speed of 50 mm.min-1, in general accordance with ASTM D 638-10. Type 2 dumbbell samples were diecut from the extruded sheets. The standard deviation was calculated from five parallel measurements for each sample. As environmental conditions influence the tensile results, all samples were conditioned in a climate chamber at 23±2°C and 50± 5% relative humidity for at least 20 h prior to the test. Water contact angle (WCA) measurements The WCA for all specimens was performed by depositing different water droplets on the surface of samples by using OCA20 (DataPhysics Instruments GmbH, Germany) that was equipped with Pulnix camera (Dual TAP Accu Pixel). The WCA values were obtained after the deposition during the first few seconds using software module SCA20. All measurements were carried out on dried surface and at 23±2°C and 50±5% humidity. The average value has been taken from at least four trials for each sample. Thermogravimetric analysis (TGA) TGA of the specimens was carried out by using the thermal analyzer Perkin-Elmer TGA-6 equipment, Model STA 6000, Netherlands. The experiments were performed under nitrogen atmosphere with a flow rate of 50 mL.min-1. The samples were heated from 30°C to 700ºC with a heating rate of 10 ºC.min-1 and maintained at this temperature during one hour. The sample mass was chosen carefully between 10 and 15 mg to avoid heat and mass transfer limitations, and therefore, ensuring adequate conditions for the pyrolysis reaction to occur in a chemical regime. RESULTS AND DISCUSSION Morphological analysis for PBAT biocomposites


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X-ray diffraction (XRD) analysis Figure S2 shows the XRD spectra for raw and treated CG powders (panel a), as well as virgin PBAT and its filled composites (panel b). As shown in Figure S2 (a), a broad diffraction peak is observed at 2Ɵ ~ 20.7° for all CG samples, indicating its amorphous state. On the other hand, Figure S2 (b) exhibits the diffraction patterns for virgin PBAT and its filled composites based on untreated and torrefied (at both 250°C and 270°C) CG , with filler contents ranging from 10 to 30 wt.%. From the figure, it is obviously noticed that there is no shift or change in the diffraction peaks between virgin PBAT and its filled composites regardless the CG content. This result is consistent with our previous literature.11 However, the intensity of the diffraction peak at about 2Ө = 22.9° gradually reduced with increasing CG content from 10 to 30 wt. % for all types of fillers when compared to virgin PBAT. This reduction may be attributed to the decrease of PBAT content. SEM analysis SEM can reveal interesting information about the morphology of composites and dispersion quality of the CG filler within the matrix. SEM micrographs representing cut-surfaces for neat PBAT and its CG based composites are shown in Figure 2. The micrograph for neat PBAT shows that it has a smooth and homogeneous surface. With adding 10 wt.% of untreated CG into the matrix, poor dispersion with big voids and empty spaces are observed that are due to the larger particle size of CG particles. For composites based on torrefied CG better filler dispersion in the whole PBAT matrix with different voids, indicating the presence of different particle sizes is observed. This size reduction for torrefied CG could result in improved embedment of the filler within the PBAT matrix strengthened by enhanced filler-matrix interactions. This result is in accordance with the particle size measurements obtained using the CAMSIZER apparatus. However, the values for the size of CG particles seem to quantitatively disagree between particle size measurements (Fig. S1)



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and SEM observations. It is worth noting that, even if a treatment was applied to CG to break the aggregates before particle size analysis, it is likely that extrusion with PBAT is more efficient due to the high shear forces involved. Qualitatively, the number of included CG or torrefied CG particles in the PBAT matrix is observed to increase with CG loading in the SEM micrographs. When increasing the CG content (i.e. 30wt.%), a phase-separation (incompatibility) is observed. This is ascribed to filler aggregation inducing interfacial debonding between CG particles and the polymer matrix, as compared to torrefied samples in which the compatibility is achieved because the torrefied biomass has better grindability, improving compatibility and filler dispersion during the processing, as shown in Figure 2. Interestingly, the good dispersion of the CG based filler was also checked by mapping calcium and potassium using the EDX analysis at the surface of the PBAT composites based on untreated and torrefied CG at 10 wt.% filler content. As these two elements are contained in CG and absent in PBAT, their dispersion is indicative of the dispersion of the filler inside the PBAT matrix. Mapping results showed a quite good and homogeneous dispersion of Ca and K inside the composites, especially in the case of torrefied composites, which again corroborates the good dispersion of the torrefied filler inside the PBAT matrix.


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Figure 2. SEM micrographs of the fractured surfaces for virgin PBAT and its filled composites with various contents of torrefied CG in comparison with untreated CG (at 500x). Mapping images (green color) for PBAT composite based on untreated and torrefied CG at 10 wt.% were taken to show the dispersion quality of CG particles into the matrix 100 µm

(EDX mapping concerns only the calcium element that is only present in CG).

100 µm



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Differential scanning calorimetry (DSC) DSC heating and cooling scans for PBAT biocomposites were applied to explore the effect of untreated and torrefied CG on the melting temperatures and the crystallinity of PBAT. The results are illustrated in Figure 3. The melting and crystallization data for virgin PBAT and its filled composites are summarized in Table 2. From the figure, the heating cycle for virgin PBAT displays two melting endotherms with two associated onset melting temperatures (Tm1_onset and Tm2_onset) centered at 36.5 and 104.1°C, respectively. The corresponding melting enthalpies (∆Hm1 and ∆Hm2) are 3.1 and 6.6 J g-1, respectively. The cooling cycle shows that Tc-onset and ∆Hc are 76.8 °C and 18.0 J g-1, respectively. The global degree of crystallinity of PBAT (corresponding to ∆Hm1 + ∆Hm2) remains roughly unaffected by the addition of CG powder, in the range 9.4-11.0 J.g-1, even if it tends to slightly increase. However, the melting enthalpy associated to the low temperature melting peak (∆Hm1) seems to decrease from 3.1 to 1.6 J.g-1 when adding CG, whereas the melting enthalpy associated to the high temperature melting peak (∆Hm2) displays an increase from 6.6 to 9.4 J.g-1 (see Table 2). This is an indication that the formation of crystals melting at low temperature is hindered by the filler, whereas the formation of crystals melting at higher temperature is promoted. The reason for this observation is unclear but since the two melting peaks probably correspond to different crystalline structures, the crystallization process could be impacted differently by the filler. Moreover, it is observed that Tc-onset shifts from 76.8°C for neat PBAT towards higher temperatures (83-85°C) upon addition of CG. This observation tends to highlight that CG powder facilitates the crystallization of PBAT, probably acting as nucleating agents. When adding torrefied CG to PBAT matrix, Tm1_onset slightly increases, Tm2_onset tends to increase excepted when treatment was performed at 270°C, and Tc-onset increases in particular for CG treated at 270°C, when compared to virgin polymer or untreated CG composites. This complex behavior shows that torrefied CG tends


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to accelerate the crystallization of PBAT, especially when treated at 270°C.34, 33 It can be also observed that the melting enthalpy (∆Hm2) in the composites with CG torrefied at 250°C is notably lower than for the composites with CG torrefied at 270°C. This result is in agreement with that verified elsewhere for PHBV/wheat straw fiber composites.30

Figure 3. DSC curves for PBAT and its filled composites with various untreated and torrefied CG ratios during heating/cooling cycles.



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Table 2. Thermal properties for virgin PBAT and its filled composites obtained from DSC experiments.

Dynamic mechanical analysis (DMA) DMA results pointed out the effect of CG treatment and its content on the dynamic mechanical properties of PBAT and its composites. The evolution of the storage modulus (E’) and loss factor (Tan δ) as a function of temperature is shown in Figure 4 (a-d). A gradual decrease is noticed as expected for E’ for all investigated samples with increasing temperatures. The sharp decrease observed around -30 °C is attributed to the glass transition of PBAT. The corresponding maximum temperature for Tan δ is unaffected upon CG addition and thermal treatment of the filler, except for 30 wt.% CG treated at 270°C for which it significantly shifts towards higher temperatures. This is a strong indication of hindered molecular mobility of amorphous PBAT chains that can result from strong fillermatrix interactions. On the other hand, the E’ values clearly increased with the addition of different CG ratios (for both untreated and torrefied CG) to PBAT matrix. The figure illustrates that there is a significant increase in the E’ values for torrefied PBAT at 30 wt.% filler loading (i.e., PBAT/CG30-250 and PBAT/CG30-270 samples) in comparison


to

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virgin polymer or other composites, particularly in the rubbery zone, as shown in Figure 4 (a & c). However, the filled samples with the same CG filler content, i.e. 30 wt.%, behave differently and the enhancement of E’ is probably associated to not only the reinforcement effect, but also the reduction of particle size in the following order: untreated CG > torrefied CG at 250°C > torrefied CG at 270°C. Similar results have been reported elsewhere.35 Meanwhile, it seems that the introduction of untreated or torrefied CG at 250°C has no significant effect on the peak of Tan δ associated with the glass transition temperature (Tg) of PBAT, but its intensity decreased when compared to neat PBAT as shown in Figure 4 (b). From the figure, a large shift in the Tg for PBAT/CG30-270 around 11°C is observed compared to neat PBAT as already mentioned. The great enhancement in E’ and Tg values might be attributed to the hindered

mobility of polymer chains resulting from strong

interactions between torrefied CG particles (because of higher Ca and K elements in the matrix based on mapping results) and the polymer segments. These findings are in agreement with the results reported elsewhere.36 Moreover, the deformation state of neat PBAT and its reinforced composites with raw CG and torrefied CG was detected during the DMA analysis, as presented in Figure S3. It can be seen that the neat polymer is obviously melted at higher temperatures, whereas no melting or deformation takes place for composites reinforced with 30 wt.% CG for torrefied at 270 °C (PBAT/CG30-270), but only a slight deformation occurred in case of untreated (PBAT/CG30), indicating that the biomass has a significant impact on the thermo-mechanical properties of the material.



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Figure 4. Evolution of the storage modulus and Tan δ as a function of temperature for virgin PBAT and its filled composites with different ratios of untreated CG and torrefied CG at 250°C (a,b) and at 270°C (c,d). Mechanical properties Tensile tests have been performed to investigate the impact of CG addition and torrefaction treatment on the non-linear mechanical behavior at room temperature of PBAT. Typical stress-strain curves for PBAT/CG biocomposites are shown in Figure 5 (a-b) and Table 3. Neat PBAT is a highly elastic polymer with low tensile strength (~ 14.3 MPa) and high strain at break (~ 1545 %). With the introduction of 10 wt.% untreated CG, a significant change for both tensile strength and strain at break is observed and it decreased to ~ 11.12 MPa and 340 %, respectively, as shown in Table 3. When the CG content increased to 30 wt.%, both parameters dramatically reduced to ~ 6.8 MPa and 98%, respectively. The reason


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is most probably due to the poor interfacial adhesion between the lignocellulosic filler and PBAT.

16

Indeed, the effective stress transfer between the matrix and the filler requires an

adequate interfacial bonding. In addition, the poor dispersion of the filler evidenced by SEM observation most probably leads to the formation of weak points in the composites inducing the premature break of the sample. However, when adding 10 wt.% torrefied CG (treated at 250°C, Figure 5 (a), or 270°C, Figure 5 (b)) in PBAT matrix, the tensile strength value distinctly increased to about 17.60 and 18.2 MPa, respectively, compared to neat PBAT. Nevertheless, the strain at break decreased to 1100 %and 1150%, respectively, as shown in Figure 5 (a-b). It was reported that torrefaction treatment had no significant effect on the mechanical properties of PHBV/wheat straw fiber composites, except for 30 wt.% of torrefied fibers.30 It was concluded that improving fiber/matrix adhesion did not lead to a better preservation of mechanical properties, due to the inevitable presence of microscopic defects in the composite materials. When increasing the torrefied CG content to 20 wt.% and 30 wt.%, both the tensile strength and strain at break obviously decreased but the values remain much higher compared to PBAT composites based on untreated CG. In fact the strength value results from a compromise between the stiffness and the ductility of the material. The stiffness should increase when adding a filler but the brittleness induced by the filler can cause premature rupture of the material, therefore reducing the strength. The improvement in tensile properties upon torrefaction may be ascribed to the presence of hydrophobic biomass in the matrix which exhibited a good dispersion and compatibility with the PBAT matrix, as well as their smaller particle size. The latter apparently played a significant role in the enhancement of the tensile strength, as shown in Figure 5(b) for composites obtained with torrefied CG at 270°C. Thus, the tensile properties are consistent with SEM observations.



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Figure 5: Stress-strain curves for virgin PBAT and its filled composites with different ratios of untreated CG and torrefied CG at 250°C (a) and at 270°C (b).

Table 3. Tensile properties of virgin PBAT and its filled biocomposites.

Wettability measurements for biocomposites Contact angle (CA) or wettability measurements were performed to examine the surface properties of torrefied PBAT/CG composites and access the effect of the filler content and its thermal treatment through torrefaction. Figure 6 shows the representative images of CA measurements using distilled water as the probe liquid. From the figure, the CA for neat PBAT is 73° attesting the quite hydrophobic nature of the polymer. It decreases dramatically to 63° and 51° when increasing the untreated CG content to 10 to 30 wt.%, respectively, indicating an increase in the hydrophilic character of the material (due to hydrophilic OH groups borne by the filler). Chen and co-workers 37 have observed similar results and found that the water CA for PBAT nanocomposites decreased with the increase of nanoclay content (hydrophilic filler). In contrast, CA values augmented with the addition of torrefied


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CG, as shown in Figure 6, and this effect is amplified when increasing the treatment temperature. This implies that the hydrophobicity of CG is strongly depended upon the torrefication temperature during the process. In other words, the lignocellulosic components can lose more hydroxyl groups in CG structure when increasing the torrefaction temperature, leading to more hydrophobic composites. With further increase of torrefied CG amount, i.e. at 30 wt.%, the CA values clearly increased to 85° for PBAT/CG30-250, and to 89° for PBAT/CG30-270 compared to PBAT/CG30 (51°) or virgin PBAT (73°).

Figure 6. Water contact angle measurements for virgin PBAT and its filled biocomposites with untreated and torrefied CG.



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Thermogravimetric analysis (TGA) TGA and DTG curves for untreated and torrefied CG, neat PBAT and their filled biocomposites are shown in Figure 7. Focusing first on CG, one can see that the torrefaction step affected the onset temperature of the thermal degradation process, which increased with the severity of torrefaction. This behavior was expected as the less thermally stable compounds were degraded during the torrefaction step. The shape of the DTG curves for CG changed with the torrefaction treatment. The intensity of the first peak decreased with the torrefaction severity due to the degradation of part of the initial CG during the torrefaction step. Also, the char content increased for torrefied CG to 31.7 and 37.5% for torrefaction performed at 250 and 270ºC, respectively, compared to 23.3% for untreated CG. PBAT exhibited the lowest char content. Adding CG or torrefied CG caused the char yield to increase in the different composites. However, the onset degradation temperature shifted to lower values for composite materials filled with CG which are less thermally stable than PBAT. The higher mass loss rate was noticed for PBAT which degrades in a narrow temperature range. Adding CG resulted in a decrease of the maximum loss rate. The temperature at the maximum mass loss rate was almost constant for all composites as the PBAT was the major compound in composites. Small shift of the pyrolysis peak toward lower temperatures can be observed for the highest CG contents. These small shifts are thought to be related to the catalytic effect of potassium contained in CG, which concentration increased with the severity of the torrefaction treatment. The potassium can catalyze the PBAT pyrolysis and lower its degradation peak temperature. The effect can be perceived, as mentioned, for the highest CG content, but remains still small to induce noticeable changes in the thermal degradation behavior for composites. The characteristic features of the thermal decomposition of the raw materials and their biocomposites are summarized in Table 4.


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Figure 7. TGA / DTG curves for CG powders, neat PBAT and its biocomposites filled with untreated and torrefied CG under nitrogen atmosphere. Table 4. Characteristic features of the thermal decomposition of the CG powders, neat PBAT and their filled biocomposites.



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In our previous article, we developed a kinetic model for the simulation of the pyrolysis of CG-PBAT composites, based on the pyrolysis kinetics of PBAT and CG. The same procedure is used in this study and extended to the torrefied CG. The reader can refer to Moustafa et al11 for more details about the pyrolysis kinetic model. An improvement of the optimization procedure was obtained when using both TGA and DTG data to identify the kinetic parameters. The comparison between the experimental results and models for PBAT, CG and torrefied CG are shown in Figure 8. The agreement between the experimental and predicted data attests for the quality of the fit (see Figure S4). The identified kinetic parameters for the pyrolysis of the different raw materials are summarized in Table 5. One can notice an increase of the activation energy for the pyrolysis of Hcell, Cell and Lig with the torrefaction temperature. It may be explained by the fact that more thermally stable fractions of these pseudo-components resides in the torrefied solid after the torrefaction step. The activation energy of Pr decreased with the severity of the torrefaction treatment. The torrefaction step may have affected proteins and made them less thermally stable. It is rather difficult to interpret the identified kinetic parameters evolution after torrefaction as we do deal with a highly complex material. Table 5. Identified kinetic parameters for the pyrolysis of PBAT, CG, CG250 and CG270

The DTG curves for composites are plotted together with the weighted sum of the DTG obtained from the raw materials, as a function of the temperature in Figure 8. One can see


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that the mass loss rate of a composite can be fairly modelled using an additive law where the mass loss rate of the composite is the weighted sum of the mass loss rates of the CG based filler and PBAT. This observation was valid for all composites.

= %CG − filler

+ %PBAT

!"

(1)

The kinetic models developed for the raw materials can be thus directly used to predict the thermal behavior of the composites. These results can also attest for the very good dispersion of the CG-based filler inside the PBAT matrix as a low mass sample used in the TGA analysis (few milligrams) is representative of the global composition of the composites.

Figure 8. Comparison between experimental and predicted DTG curves using an additive law for the different PBAT/CG based composites



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CONCLUSIONS In the present work, torrefied CG was successfully used as hydrophobic reinforcing agent for biodegradable PBAT and showed better affinity with PBAT compared to untreated CG to obtain green composites without requiring a compatibilizer. The CG torrefaction was studied by using TGA and FTIR techniques, which both gave information on the mass loss and the hydrophobicity increase. The PBAT biocomposites based on untreated and torrefied CG filler were prepared by melt compounding. SEM images for PBAT reinforced by torrefied CG exhibited a good filler dispersion and a coalescence between the filler and the polymer matrix when compared to PBAT/untreated CG composites. The crystallinity of PBAT was determined by XRD and DSC and the obtained results showed that it was affected with the addition of untreated or torrefied CG. DMA experiments showed a significant improvement in the storage modulus in the rubbery region, especially with 30 wt.% torrefied CG in the composite. No significant effect of the CG filler on the temperature position of the main relaxation process associated to Tg was reported, except for PBAT/CG30-270 sample for which the Tg was shifted towards higher temperatures by 11°C compared to neat PBAT. Furthermore, the deformation state of the composites was monitored during DMA measurements and it indicated that the torrefied biomass has a significant impact on the thermo-mechanical properties. A large decrease in tensile strength values was observed for PBAT composites when untreated CG was used. In contrast, the addition of 10 wt.% of torrefied CG to PBAT showed better tensile properties compared to neat PBAT, while the strain at break was slightly decreased. Beyond 10 wt.%, the tensile strength and strain at break began to decrease gradually but their values were still better than for untreated CG/PBAT composites. The water CA values were clearly increased when increasing the torrefied CG content in the polymer matrix, resulting in highly hydrophobic biocomposites that could be used for food packaging applications. TGA and DTG analysis


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showed that the thermal stability of the composite samples is nicely described by an additive law involving the thermal degradation of the polymer and the filler. In addition, a kinetic model was suggested to identify the kinetic parameters for the thermal degradation of neat PBAT and its filled composites with untreated and torrefied CG, with an aim to predict the thermal stability of the composites. SUPPORTING INFORMATION Cumulative particle size distribution for untreated CG and torrefied CG; XRD patterns for CG powders and composites; deformation state for PBAT and during DMA test at elevated temperatures; experimental and modeled pyrolysis conversion level as a function of temperature for CG and neat PBAT. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS The authors would like to thank of Higher Ministry of Education & Scientific research, Cairo, Egypt and Laboratoire de Génie des Procédés Papetiers (LGP2), Grenoble, France for the financial support for the postdoctoral fellowship to H. Moustafa. LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir - grant agreement n°ANR-11-LABX-0030) and of the PolyNat Carnot Institut (Investissements d’Avenir - grant agreement n°ANR-11-CARN030-01). REFERENCES (1) Mussatto, S.I.; Machado, E.M.S.; Martins, S.; Teixeira, A. Production, Composition, and Application of Coffee and Its Industrial Residues, Food Bioprocess Technol. 2011,

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For Table of Contents Use Only Utilization of torrefied coffee grounds as reinforcing agent to produce high-quality biodegradable PBAT composites for food packaging applications Hesham Moustafa1, 3, Chamseddine Guizani3, Capucine Dupont4, Vincent Martin5, Mejdi Jeguirim6, Alain Dufresne2, 3 1 Polymer Metrology & Technology Department, National Institute for Standards (NIS), Tersa Street, El Haram, El-Giza, P.O Box 136, Giza 12211, Egypt. 2 Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France 3 CNRS, LGP2, F-38000, Grenoble, France 4 CEA, Grenoble, France 5 CNRS, LEPMI - UMR 5279, Grenoble, France 6 CNRS, IS2M, Mulhouse, France

Graphical abstract:

Synopsis: Torrefied coffee grounds were successfully used as hydrophobic reinforcing agent for biodegradable PBAT to produced sustainable green composites.


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Utilization of Torrefied Coffee Grounds as Reinforcing Agent To

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