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Development of Coffee Biochar Filler for the Production of Electrical Conductive Reinforced Plastic

Development of Coffee Biochar Filler for the Production of Electrical Conductive Reinforced Plastic polymers Article Development of Co ee Biochar Filler for the Production of Electrical Conductive Reinforced Plastic Mauro Giorcelli * and Mattia Bartoli Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-0110-904-327 Received: 15 October 2019; Accepted: 18 November 2019; Published: 21 November 2019 Abstract: In this work we focused our attention on an innovative use of food residual biomasses. In particular, we produced biochar from co ee waste and used it as filler in epoxy resin composites with the aim to increase their electrical properties. Electrical conductivity was studied for the biochar and biochar-based composite in function of pressure applied. The results obtained were compared with carbon black and carbon black composites. We demonstrated that, even if the co ee biochar had less conductivity compared with carbon black in powder form, it created composites with better conductivity in comparison with carbon black composites. In addition, composite mechanical properties were tested and they generally improved with respect to neat epoxy resin. Keywords: electrical properties; mechanical properties; recycling; epoxy resin 1. Introduction Anthropogenic waste stream management is one of the main unresolved problems of industrialized societies [1,2]. In the food waste sector, co ee residuals could be considered not only a waste material but a resource. Recently Christoph Sänger [3] reported that worldwide co ee production was 159.7 million of bags in crop year 2017/18 (about 9.6 MTons), with a mean of 5 kg/capita per year in traditional markets (Germany, Italy, France, USA and Japan) and an increasing consumption in emerging markets (South Korea, Russia, Turkey and China). The co ee waste stream becomes a relevant problem not only after consumption but also during the wet processing of co ee beans when 1 ton of fresh berries results in only about 400 kg of wet waste pulp. Several solutions have been proposed to solve the problem of waste co ee biochar, such as the production of biogas [4] and flavours [5], use as filler in ceramics [6] or as absorbent for the removal of basic dyes from aqueous solutions [7]. Co ee wastes have been also used as feedstock for pyrolytic conversion producing hydrogen-rich gas [8] and fuel-quality biochar [9]. Biochar has been used not only as solid fuel but also as high performance material [10,11], as a flame retardant additive [12,13], for electrochemical [14] and energy storage applications [15] and for production of composites [16–19]. Traditionally, in the realm of carbon fillers in polymer composites, carbon black (CB) plays the main role especially in the automotive field with an estimated consumption of 8.1 MTon/year according to data released by the International CB association [20]. CB has been used for producing conductive composites [21] but, as recently reported by Quosai et al. [22], co ee-based biochar also shows remarkably conductive properties. Furthermore, co ee biochar production has an indisputable advantage if compared with CB. Co ee biochar production uses a food waste stream while oil-based feedstock is required for CB production. This decreases the environmental impact of the production process [23–25]. Among di erent polymers, in this work we focused our attention on epoxy resins doped with these two carbon fillers. As is well known, epoxy resin is a thermoset polymer widely applied in the field of coatings [26], adhesives [27], casting [28], potting [29], composites [30], laminates [31] and encapsulation Polymers 2019, 11, 1916; doi:10.3390/polym11121916 www.mdpi.com/journal/polymers Polymers 2019, 11, 1916 2 of 17 of semiconductor devices [32]. Epoxy resins are used intensively because of their peculiar properties such as high strength, good sti ness, good thermal stability and excellent heat, moisture and chemical resistance [33,34]. Another, unneglectable advantage of epoxy resin is the possibility of being dispersed into the cross-linked polymeric matrix additives, such as micro-encapsulated amines [35,36], that could be realised after material failure promoting the self-healing process of the epoxy composite [37]. In the field of composites materials, production of conductive reinforced plastic materials has attracted an increasing interest in the last few decades [38,39]. Large-scale application fields deserve particular attention. For example conductive epoxy resin has a large-scale application in the field of coatings and adhesives [40]. In these large-scale applications, filler cost is a crucial issue. Epoxy resins have been used as a polymeric host for plenty of carbonaceous materials for the production of conductive reinforced materials [41–44], but the cost of carbon filler has to be take in account. High-cost carbon fillers such as carbon nanotubes and graphene are problematic for large-scale applications. These carbon fillers induce an increment of its electrical and mechanical properties in the host polymer matrix [45–48] but are not a suitable choice for industrial scale production. This is mainly due to the high-cost, up to 300 k$/kg [49], and the problem of low productivity of the plants is well known [50]. Thus, low cost carbon fillers which are not derived from fossil fuels, such as CB, are a topic of relevant interest. In this study, we investigated the use of biochar derived from pyrolytic conversion of the co ee waste stream, such as low cost carbon fillers derived by recycling materials. Results were compared with CB-based composites. Mechanical properties were also investigated for full composite characterization. 2. Materials and Methods 2.1. Carbonaceous Materials Preparation and Characterization Exhausted co ee powder was selected as a real case study. It was collected from Bar Katia (Turin, Italy) supplied by Vergnano (Arabica mixture). Co ee was collected and dried at 105 C for 72 h. Co ee samples (100 g) were pyrolyzed using a vertical furnace and a quartz reactor, heating rate of 15 C/min and kept at the final temperature (400, 600, 800 and 1000 C) for 30 min in an argon atmosphere. Samples were named as C400, C600, C800 and C1000 respectively. Biochar was grinded using a mechanical mixer (Savatec BB90E) for 10 min in order to decrease the particle size. Commercial CB (VULCAN 9 N115) was used to compare with co ee biochar. Ash contents of co ee and carbon-based materials (biochars and CB) were evaluated using a static furnace set at 550 or 800 C respectively for 6 h. All samples were investigated from morphological point of view using a field emission scanning electrical microscopy (FE-SEM, Zeis SupraTM 40, Oberkochen, Germany). The microscope was equipped with an energy dispersive X-ray detector (EDX, Oxford Inca Energy 450, Oberkochen, Germany) that was used to explore the carbon composition of biochars. Particle size distribution of carbon fillers was evaluated using a laser granulometry (Fritsch Analysette 22, Idar-Oberstein, Germany) after a dispersion in ethanol and sonication in an ultrasonic bath for 10 min. Co ee, biochars and CB were analysed through FT-IR (Nicolet 5700, Thermoscientific, Waltham, US) on attenuated total reflectance (ATR) mode (Smartorbit, Thermoscientific) in the range from 500 to 4000 cm . Biochars and CB were analysed through Raman spectroscopy using Renishaw Ramanscope InVia (H43662 model, Gloucestershire, UK). 2.2. Composites Preparation Biochar, derived from coffee, and commercial CB containing epoxy composites were produced using a two component bis-phenol A (BPA) diglycidyl resin (CORES epoxy resin, LPL). Carbonaceous filler (15 wt. %) were dispersed into epoxy monomer using a tip ultrasonicator apparatus (Sonics Vibra-cell) for 15 min. After the addition of the curing agent, the mixture was ultrasonicated for another 2 min Polymers 2019, 11, x FOR PEER REVIEW 3 of 18 Biochar, derived from coffee, and commercial CB containing epoxy composites were produced using a two component bis-phenol A (BPA) diglycidyl resin (CORES epoxy resin, LPL). Carbonaceous filler (15 wt %) were dispersed into epoxy monomer using a tip ultrasonicator Polymers 2019, 11, 1916 3 of 17 apparatus (Sonics Vibra-cell) for 15 min. After the addition of the curing agent, the mixture was ultrasonicated for another 2 min and left into the moulds for 16 h at room temperature. A final thermal curing was performed using a ventilated oven (I.S.C.O. Srl “The scientific manufacturer”) at and left into the moulds for 16 h at room temperature. A final thermal curing was performed using a 70 °C for 6 h. ventilated oven (I.S.C.O. Srl “The scientific manufacturer”) at 70 C for 6 h. 2.3. Electrical Characterization 2.3. Electrical Characterization The measurement set-up was derived from Gabhi et al. [51] and is sketched in Figure 1a for The measurement set-up was derived from Gabhi et al. [51] and is sketched in Figure 1a for fillers and Figure 1b for composites. The instrument was composed of two solid copper cylinders, 30 fillers and Figure 1b for composites. The instrument was composed of two solid copper cylinders, mm in diameter and 5 cm in length, encapsulated in a hollow Plexiglas cylinder with a nominal inner 30 mm in diameter and 5 cm in length, encapsulated in a hollow Plexiglas cylinder with a nominal diameter of 30 mm in the case of filler electrical characterization. In this configuration, the inner inner diameter of 30 mm in the case of filler electrical characterization. In this configuration, the inner diameter was slightly higher so that it was possible to force the copper rods inside the Plexiglas cavity diameter was slightly higher so that it was possible to force the copper rods inside the Plexiglas cavity and the upper rod could slide inside the cylinder during the measurement. This arrangement created and the upper rod could slide inside the cylinder during the measurement. This arrangement created an internal chamber between the two cylinders, where the carbon powder could be inserted. In the an internal chamber between the two cylinders, where the carbon powder could be inserted. In the case of composites, the Plexiglas cylinder was removed and the sample was positioned between the case of composites, the Plexiglas cylinder was removed and the sample was positioned between the aligned copper cylinders. The electrical resistance of the powders or composites was measured at aligned copper cylinders. The electrical resistance of the powders or composites was measured at increasing loads (up to 1500 bar) applied by a hydraulic press (Specac Atlas Manual Hydraulic Press increasing loads (up to 1500 bar) applied by a hydraulic press (Specac Atlas Manual Hydraulic Press 15T). Electrically insulating sheets were placed between the conductive cylinders and the load 15T). Electrically insulating sheets were placed between the conductive cylinders and the load surfaces surfaces in order to ensure that the electrical signal passed through the sample. The resistance of the in order to ensure that the electrical signal passed through the sample. The resistance of the carbon carbon fillers was measured using an Agilent 34401A multimeter. fillers was measured using an Agilent 34401A multimeter. Figure 1. Sketch of measurement set-up for conductivity study of (a) carbon fillers and (b) composite. 2.4. Composites Figure 1. Mechanical Sketch of measurement set-up Characterization for conductivity study of (a) carbon fillers and (b) composite. Carbonaceous materials containing composites were produced as dog-bone shaped according to the ASTM 638 procedure. Samples were tested using a mechanical stress test (MTS) machine 2.4. Composites Mechanical Characterization (MTS Q-test10) in tensile test mode until break point. Data were analysed using a self-developed Carbonaceous materials containing composites were produced as dog-bone shaped according software compiled using Matlab. to the ASTM 638 procedure. Samples were tested using a mechanical stress test (MTS) machine (MTS 2.5. Data Analysis Q-test10) in tensile test mode until break point. Data were analysed using a self-developed software compiled using Matlab. Statistical analysis used were based on t-tests with a significance level of 0.05 (p < 0.05) were carried out using Excel— software (Microsoft Corp.) and the “data analysis” tool. 2.5. Data Analysis 3. Results Statistical analysis used were based on t-tests with a significance level of 0.05 (p < 0.05) were carried out using Excel™ software (Microsoft Corp.) and the “data analysis” tool. 3.1. Carbonaceous Materials Characterization 3. Re Pyr sults olysis of spent grounds co ee proceeded according to the mechanism reported by Setter et al. [52]. The main mechanisms that occurred during the degradative processes were those related with 3.1. Carbonaceous Materials Characterization decomposition of the small lignin fraction [53] and the most abundant polysaccharides (i.e., cellulose and hemicellulose) [54] with the formation of bio-oils rich in anhydrosugars, furans and acetic acid with trace of aromatics [55–57]. Polymers 2019, 11, x FOR PEER REVIEW 4 of 18 Pyrolysis of spent grounds coffee proceeded according to the mechanism reported by Setter et al. [52]. The main mechanisms that occurred during the degradative processes were those related with decomposition of the small lignin fraction [53] and the most abundant polysaccharides (i.e. Polymers 2019, 11, 1916 4 of 17 cellulose and hemicellulose) [54] with the formation of bio-oils rich in anhydrosugars, furans and acetic acid with trace of aromatics [55-57]. Ash content of feedstock and carbonaceous materials were preliminary investigated and Ash content of feedstock and carbonaceous materials were preliminary investigated and summarized in Figure 2. summarized in Figure 2. Figure 2. Ash contents of neat co ee, carbon black (CB) and co ee biochar samples heated at 400, 600, 800 and 1000 C (C400, C600, C800 and C1000 respectively). Columns marked with di erent letters Figure 2. Ash contents of neat coffee, carbon black (CB) and coffee biochar samples heated at 400, 600, were significantly di erent (p < 0.05). 800 and 1000 °C (C400, C600, C800 and C1000 respectively). Columns marked with different letters were significantly different (p < 0.05). The ash content of neat co ee was 1.70 0.14 wt. % and it increased with temperature increments reaching a value around 9 wt. % in the case of C800 (8.92 0.61 wt. %) and C1000 (9.09 0.09 wt. %). The ash content of neat coffee was 1.70 ± 0.14 wt. % and it increased with temperature increments As expected, CB showed a very low ash content (0.07  0.01 wt. %) according to Medalia et al. [58] reaching a value around 9 wt. % in the case of C800 (8.92 ± 0.61 wt. %) and C1000 (9.09 ± 0.09 wt. %). mainly as oxides. Ash content increment at higher temperatures was imputable to advance pyrolytic As expected, CB showed a very low ash content (0.07 ± 0.01 wt. %) according to Medalia et al. [58] degradation of the organic matrix leading to the concentration of inorganic residue [59] that did not mainly as oxides. Ash content increment at higher temperatures was imputable to advance pyrolytic undergo any temperature induced degradation. degradation of the organic matrix leading to the concentration of inorganic residue [59] that did not The e ect of pyrolytic temperature on biochar morphology was studied using FE-SEM as shown undergo any temperature induced degradation. in Figure 3. Neat co ee displayed flaked collapsed structures (Figure 3a, b) that was retained by The effect of pyrolytic temperature on biochar morphology was studied using FE-SEM as shown C400 after pyrolysis at 400 C (Figure 3e,f). With the increase of temperature to 600 C lead to the in Figure 3. Neat coffee displayed flaked collapsed structures (Figure 3a, b) that was retained by C400 formation of porous structures with average diameters close to 30 m separated by carbon lamellae after pyrolysis at 400 °C (Figure 3e, f). With the increase of temperature to 600 °C lead to the formation with a thickness around 1 m (Figure 3g,h). At 800 C, the biochar recovered lost the structure due of porous structures with average diameters close to 30 μm separated by carbon lamellae with a to the massive release of volatile organic matters during the overall pyrolytic process that induced thickness around 1 μm (Figure 3g, h). At 800 °C, the biochar recovered lost the structure due to the the collapse of carbonaceous structures together with an improved grindability [60]. At 1000 C, massive release of volatile organic matters during the overall pyrolytic process that induced the the increased temperature allowed the massive formation of carbon–carbon bonds that promoted the collapse of carbonaceous structures together with an improved grindability [60]. At 1000 °C, the stabilization of the porous architecture with nanoscale lamellae structures. CB showed a typical highly increased temperature allowed the massive formation of carbon–carbon bonds that promoted the aggregate spherule-based shape with average diameter of single particles around 50 nm. stabilization of the porous architecture with nanoscale lamellae structures. CB showed a typical highly aggregate spherule-based shape with average diameter of single particles around 50 nm. Polymers 2019, 11, 1916 5 of 17 Polymers 2019, 11, x FOR PEER REVIEW 5 of 18 Figure 3. FE-SEM captures of (a, b) neat coffee, (c, d) CB, (e, f) C400, (g, h) C600, (i, j) C800 and (k, l) Figure 3. FE-SEM captures of (a,b) neat co ee, (c,d) CB, (e,f) C400, (g,h) C600, (i,j) C800 and (k,l) C1000. C1000. C1000 C800 C600 C400 CB Neat Coffee Polymers 2019, 11, 1916 6 of 17 Polymers 2019, 11, x FOR PEER REVIEW 6 of 18 Organic component of significant carbonaceous materials was also analysed using both FT-IR and Organic component of significant carbonaceous materials was also analysed using both FT-IR Raman spectrometry techniques. Among carbonaceous materials, we reported neat co ee, C1000 and and Raman spectrometry techniques. Among carbonaceous materials, we reported neat coffee, C1000 CB. Results are shown in Figure 4. and CB. Results are shown in Figure 4. Figure 4. FT-IR spectra (ATR mode) of (a) neat co ee, (b) co ee biochar produced at 1000 C and (c) CB in the range of 500–4000 cm . Figure 4. FT-IR spectra (ATR mode) of (a) neat coffee, (b) coffee biochar produced at 1000 °C and (c) −1 CB in the range of 500–4000 cm . The FT-IR spectrum of neat co ee showed the broad band of  (3300–3500 cm ), the bands O–H 1 1 of saturated  (2850–2950 cm ),  (1710–1741 cm ) due to the carboxylic functionalities, C–H C=O –1 The FT-IR spectrum of neat coffee showed the broad band of νO–H (3300–3500 cm ), the bands of (1540–1638 cm ) due to the presence of aromatic structures, saturated and unsaturated C=C C–H –1 –1 saturated νC–H (2850–2950 cm ), νC=O (1710–1741 cm ) due to the carboxylic functionalities, νC=C (1540– 1 1 1 (1370–1440 cm ), saturated  (1243 cm ),  (1030–1148 cm ) and out-of-plane  below C–C C–O O–H –1 – 1638 cm ) due to the presence of aromatic structures, saturated and unsaturated δC–H (1370–1440 cm 700 cm . Those bands clearly identified a lignocellulosic derived matrix with massive presence 1 –1 –1 –1 ), saturated νC–C (1243 cm ), νC–O (1030–1148 cm ) and out-of-plane δO–H below 700 cm . Those bands of polysaccharides and aromatics. C1000 did not show any of the characteristic bands of organic clearly identified a lignocellulosic derived matrix with massive presence of polysaccharides and matrix but show an envelope of bands below 1800 cm due to carbon skeletal movements. Contrary, aromatics. C1000 did not show any of the characteristic bands of organic matrix but show an envelope CB showed low bands intensity below 1000 cm due the lower variety of carbon structure embedded –1 of bands below 1800 cm due to carbon skeletal movements. Contrary, CB showed low bands into particles. –1 intensity below 1000 cm due the lower variety of carbon structure embedded into particles. Raman spectra normalized on G peak are shown in Figure 5. Co ee biochars had the typical Raman spectra normalized on G peak are shown in Figure 5. Coffee biochars had the typical profiles of amorphous materials [61] in contrast to CB which was more graphitic. The graphitic structure profiles of amorphous materials [61] in contrast to CB which was more graphitic. The graphitic for CB could be observed by the deep gorge between D and G peaks and their shaped structure. structure for CB could be observed by the deep gorge between D and G peaks and their shaped An increase of I /I ratio was evident for biochars moving from a pyrolytic temperature of 400 to D G structure. An increase of ID/IG ratio was evident for biochars moving from a pyrolytic temperature of 1000 C. This increase of I /I ratio could be ascribed to the progressively loss of residual functional D G 400 to 1000 °C. This increase of ID/IG ratio could be ascribed to the progressively loss of residual groups with the increase of temperature. This observation was also supported by the decrease of functional groups with the increase of temperature. This observation was also supported by the fluorescence [62]. Due to the loss of less intense parts of these weak interactions, biochar underwent an decrease of fluorescence [62]. Due to the loss of less intense parts of these weak interactions, biochar appreciable disorganization together with aromatic structure formation, in particular up to 600 C, underwent an appreciable disorganization together with aromatic structure formation, in particular without the completion of a proper graphitization process that occurs at higher temperature [63]. up to 600 °C, without the completion of a proper graphitization process that occurs at higher temperature [63]. The evolution of biochar structures due to temperature increment could be monitored though Raman according to Ferrari et al. [61]. Accordingly, the D peaks (Figure 5) showed wave numbers –1 close to 1350 cm that is typical of transition from amorphous carbon to nanocrystalline graphite. At –1 the same time the biochar G peaks showed wavenumbers close to 1580 cm with exception of C1000. Polymers 2019, 11, x FOR PEER REVIEW 7 of 18 –1 Polymers 2019, 11, 1916 7 of 17 This last showed a G peak at 1600 cm due the high amount of nanocrystalline domains not yet rearranged in the ordered structure [64]. Figure 5. Magnification of Raman spectra in the range from 800 to 2000 cm of C400, C600, C800, –1 Figure 5. Magnification of Raman spectra in the range from 800 to 2000 cm of C400, C600, C800, C1000 and CB. C1000 and CB. The evolution of biochar structures due to temperature increment could be monitored though The above mentioned consideration was also supported by EDX analysis that showed the carbon Raman according to Ferrari et al. [61]. Accordingly, the D peaks (Figure 5) showed wave numbers content that significantly increased from C400 to C600–C1000 while oxygen content decreased. close to 1350 Carbon con cm tethat nt of C is 6typical 00–C100of 0 w transition ere not signi fr fic om antamorphous ly different frocarbon m CB even to nanocrystalline if CB showed a mor graphite. e ordered structure. This support the hypothesis that the driving force of the biochar enhanced At the same time the biochar G peaks showed wavenumbers close to 1580 cm with exception of conductivity is the reorganization of nanocrystalline domains and not merely the carbon content, C1000. This last showed a G peak at 1600 cm due the high amount of nanocrystalline domains not shown in Figure 6. Traces of Mg, P, K and Ca were also detected. yet rearranged in the ordered structure [64]. The above mentioned consideration was also supported by EDX analysis that showed the carbon content that significantly increased from C400 to C600–C1000 while oxygen content decreased. Carbon content of C600–C1000 were not significantly di erent from CB even if CB showed a more ordered structure. This support the hypothesis that the driving force of the biochar enhanced Polymers 2019, 11, x FOR PEER REVIEW 8 of 18 conductivity is the reorganization of nanocrystalline domains and not merely the carbon content, shown in Figure 6. Traces of Mg, P, K and Ca were also detected. Figure 6. Carbon content of biochars and CB. Evaluated though energy dispersive X-ray (EDX) analysis. Error values were reported as 5% of the detected values according with Laskin et al. [65]. Columns marked with different letters are significantly different (p < 0.05). Figure 6. Carbon content of biochars and CB. Evaluated though energy dispersive X-ray (EDX) analysis. Error values were reported as 5% of the detected values according with Laskin et al. [65]. Columns marked with different letters are significantly different (p < 0.05). 3.2. Composites Characterization 3.2.1. Electrical Characterization of Carbonaceous Filler and Composites The set-up shown in Figure 1a was used for biochar powders electrical characterization. Around 3 grams of carbonaceous powder, which creates a few millimetres distance between copper cylinders was positioned in the chamber. After the closure of the chamber a pressure was applied with the aim of compacting the powder. The pressure range was from 0 to 1500 bar (step of 250 bar). For each step the stabilized value of resistivity was registered such as the distance between the copper cylinders. The same procedure was repeated for composites of few millimetre thickness. Carbonaceous powders and composites decreased their resistance value during compression until they reached a plateau when high pressure was reached. The decreasing of resistance value could be correlated with the decreasing of space between carbon particles as sketched in Figure 7. In the case of powders, the void among particles collapsed with a production of compact carbon agglomerate as shown in Figure 7a. In the case of composites, Figure 7b shows the mechanism where the polymer chains flow let the carbon particles situate. Polymers 2019, 11, 1916 8 of 17 3.2. Composites Characterization 3.2.1. Electrical Characterization of Carbonaceous Filler and Composites The set-up shown in Figure 1a was used for biochar powders electrical characterization. Around 3 g of carbonaceous powder, which creates a few millimetres distance between copper cylinders was positioned in the chamber. After the closure of the chamber a pressure was applied with the aim of compacting the powder. The pressure range was from 0 to 1500 bar (step of 250 bar). For each step the stabilized value of resistivity was registered such as the distance between the copper cylinders. The same procedure was repeated for composites of few millimetre thickness. Carbonaceous powders and composites decreased their resistance value during compression until they reached a plateau when high pressure was reached. The decreasing of resistance value could be correlated with the decreasing of space between carbon particles as sketched in Figure 7. In the case of powders, the void among particles collapsed with a production of compact carbon agglomerate as shown in Figure 7a. In the case of composites, Figure 7b shows the mechanism where the polymer chains flow let the carbon Polymers 2019, 11, x FOR PEER REVIEW 9 of 18 particles situate. Figure 7. Behaviour of (a) powders and (b) composites during compression. Figure 7. Behaviour of (a) powders and (b) composites during compression. The resistance value, R, with the value of surface S and distance l between copper surfaces were The resistance value, R, with the value of surface S and distance l between copper surfaces were used in Ohm law ( = l/RS) to evaluate the conductivity . The conductivity value of carbon powder used in Ohm law (σ = l/RS) to evaluate the conductivity σ. The conductivity value of carbon powder and composites were evaluated following this procedure: and composites were evaluated following this procedure: 1) A starting value of conductivity was evaluated without any sample in order to measure the (1) A starting value of conductivity was evaluated without any sample in order to measure the value value of resistance of the system. This value was subtracted to the resistance value read with of resistance of the system. This value was subtracted to the resistance value read with samples. samples. (2) The same quantity of carbon powders (CB and biochar) were positioned between copper cylinders 2) The same quantity of carbon powders (CB and biochar) were positioned between copper and kept by the Plexiglas hollow cylinder. The measurement was repeated several times in order cylinders and kept by the Plexiglas hollow cylinder. The measurement was repeated several to have a reliable value. times in order to have a reliable value. (3) Composites were positioned between copper cylinders, in this case the Plexiglas hollow cylinder 3) Composites were positioned between copper cylinders, in this case the Plexiglas hollow was not necessary and the value of conductivity was measured in di erent sample portions. cylinder was not necessary and the value of conductivity was measured in different sample Preliminary results are shown in Figure 8 showing the conductivity of the biochar powders portions. (red line) and percolation curves of related composites. Preliminary results are shown in Figure 8 showing the conductivity of the biochar powders (red line) and percolation curves of related composites. Polymers 2019, 11, 1916 9 of 17 Polymers 2019, 11, x FOR PEER REVIEW 10 of 18 Figure 8. Trends of composites conductivity as function of filler percentage. Red line represents the Figure 8. Trends of composites conductivity as function of filler percentage. Red line represents the biochar conductivity trends while yellow, blue and green show respectively the percolation curves of biochar conductivity trends while yellow, blue and green show respectively the percolation curves of composites containing C400–C600, C800 and C1000. composites containing C400–C600, C800 and C1000. C400 did not show an appreciable conductivity while an increment of pyrolytic temperature up to 600 C400 did C induced not show an appr a conductivity eciab of up le con to d 0.02 uctiv S/im. ty wh Further ile an incremen increments t of of pyro processing lytic tempera temperatur ture up e to 600 °C induced a conductivity of up to 0.02 S/m. Further increments of processing temperature up up to 800 and 1000 C led to a conductivity of up to 0.04 and 35.96 S/m. This remarkable increment of to 8 conductivity 00 and 1000 between °C led to800 a conduc and 1000 tivity of up C was to due 0.04 the and enlar 35.9 gement 6 S/m. This rem of aromatic arkab region le incr formed ement of as conductivity between 800 and 1000 °C was due the enlargement of aromatic region formed as consequence of high temperature carbonization [64]. This deeply a ected the electrical behaviour of consequence of high temperature carbonization [64]. This deeply affected the electrical behaviour of related composites. Consequently, C400 and C600 containing composites were not conductive for related composites. Consequently, C400 and C600 containing composites were not conductive for all all the range of filler percentage investigated. CB composites were not conductive until the filler the range of filler percentage investigated. CB composites were not conductive until the filler concentration of 15 wt. % reaching a conductivity of 5.4 10 S/m with a filler loading of 20 wt. %. –8 concentration of 15 wt. % reaching a conductivity of 5.4 × 10 S/m with a filler loading of 20 wt. %. C1000 composites showed the best performances showing a detectable electrical conductivity with a C1000 composites showed the best performances showing a detectable electrical conductivity with a 15 wt. % of filler and reaching a conductivity of 2.02 S/m with a filler loading of 20 wt. %. 15 wt. % of filler and reaching a conductivity of 2.02 S/m with a filler loading of 20 wt. %. Accordingly, with these data, electrical properties of C1000 and C1000 containing composites Accordingly, with these data, electrical properties of C1000 and C1000 containing composites were studied under a wide range of static pressures comparing with the related CB and CB composites were studied under a wide range of static pressures comparing with the related CB and CB as shown in Figure 9. composites as shown in Figure 9. Polymers 2019, 11, x FOR PEER REVIEW 11 of 18 Polymers 2019, 11, 1916 10 of 17 Figure 9. Trends of CB and C1000 powders and related composites conductivity as a function of pressure applied. Figure 9. Trends of CB and C1000 powders and related composites conductivity as a function of CB powder reached a conductivity around 1700 S/m while in the same conditions C1000 reached a pressure applied. conductivity of 300 S/m. Composites containing of CB and C1000 were conductive but the results showed a different trend compared with the relative powders. CB 15 wt. % reached the value of 4 10 S/m CB powder reached a conductivity around 1700 S/m while in the same conditions C1000 reached and its conductive value showed an influence of applied pressure in the first compression movement. a conductivity of 300 S/m. Composites containing of CB and C1000 were conductive but the results C1000 15 wt. % reached to 10 S/m, with an increment around one order of magnitude if compared showed wita h different tr CB 15 wt. %end c . Thiso dmpared w ifference wi ath the s more re relative levant powders. CB for a filler conce 15 wt. % ntration o reac f 20 hed wt. % the v . In ta hlue of 4 × is case, 5 4 –3 the conductivity of CB-based composites dropped down to 10 –10 S/m in contrast to C1000 which 10 S/m and its conductive value showed an influence of applied pressure in the first compression –2 reached ~10 S/m. The high conductive value for coffee biochar could be due to more uniform filler movement. C1000 15 wt. % reached to 10 S/m, with an increment around one order of magnitude if dispersion inside epoxy resin. Dispersion of the filler inside the epoxy matrix was investigated through compared with CB 15 wt. %. This difference was more relevant for a filler concentration of 20 wt. %. FE-SEM (Figure 10) after samples were cryo-fractured using liquid nitrogen and compared to composites –5 –4 In this case, the conductivity of CB-based composites dropped down to 10 –10 S/m in contrast to with a filler loading of 15 wt. % due to the similarity of conductivity. CB containing composites showed C1000 which reached ~10 S/m. The high conductive value for coffee biochar could be due to more a dark and clear area (Figure 10a) with different compositions. The clear ones were rich in CB aggregates uniform filler dispersion inside epoxy resin. Dispersion of the filler inside the epoxy matrix was (Figure 10b,c) while the darkest were poor (Figure 10d). C1000 containing composites showed smooth investigated through FE-SEM (Figure 10) after samples were cryo-fractured using liquid nitrogen and surfaces with holes (Figure 10e) due the expulsion of embedded C1000 particles during the fracturing compared to composites with a filler loading of 15 wt. % due to the similarity of conductivity. CB (Figure 10e) as clearly shown in Figure 10g. Particles size analysis (Figure 11) showed clearly that C1000 containing composites showed a dark and clear area (Figure 10a) with different compositions. The was composed by two particle populations, one around 100 m and one around 20 m. Considering clear ones were rich in CB aggregates (Figure 10b, c) while the darkest were poor (Figure 10d). C1000 the average size of C1000 particles into the composites was reasonable it was assumed that the bigger ones underwent a disruption during the ultrasonication forming small sized well-dispersed particles. containing composites showed smooth surfaces with holes (Figure 10e) due the expulsion of CB particles size showed also that it would be more appropriate speaking of CB aggregates instead of embedded C1000 particles during the fracturing (Figure 10e) as clearly shown in Figure 10g. Particles single particles [66]. Aggregates could be justified also from Figure 10c where the CB single particles size analysis (Figure 11) showed clearly that C1000 was composed by two particle populations, one were less than 100 nm but they created agglomerates that also the particle size analysis (Figure 11) was around 100 μm and one around 20 μm. Considering the average size of C1000 particles into the not able to detect. composites was reasonable it was assumed that the bigger ones underwent a disruption during the ultrasonication forming small sized well-dispersed particles. CB particles size showed also that it would be more appropriate speaking of CB aggregates instead of single particles [66]. Aggregates could be justified also from Figure 10c where the CB single particles were less than 100 nm but they created agglomerates that also the particle size analysis (Figure 11) was not able to detect. Polymers 2019, 11, 1916 11 of 17 Polymers 2019, 11, x FOR PEER REVIEW 12 of 18 Figure 10. Field emission scanning electrical microscopy (FESEM) of cryo-fractured (a–d) CB and (e–g) Figure 10. Field emission scanning electrical microscopy (FESEM) of cryo-fractured (a–d) CB and (e– C1000 containing composites with a filler loading of 15 wt. %. g) C1000 containing composites with a filler loading of 15wt %. Polymers 2019, 11, x FOR PEER REVIEW 13 of 18 Polymers 2019, 11, x FOR PEER REVIEW 13 of 18 Polymers 2019, 11, 1916 12 of 17 Figure 11. Particle size distribution for CB and C1000. Figure 11. Particle size distribution for CB and C1000. 3.2.2. Composites Mechanical Characterization 3.2.2. Composites Mechanical Characterization Figure 11. Particle size distribution for CB and C1000. With the aim of confirming mechanical consistence of samples, a stress–strain curve was With the aim of confirming mechanical consistence of samples, a stress–strain curve was investigated for composites of 15 wt. % for CB and C1000 compared with neat resin. Mechanical tests 3.2.2. Composites Mechanical Characterization investigated for composites of 15 wt. % for CB and C1000 compared with neat resin. Mechanical tests on dog-bones shaped samples are summarized in the Figure 12. on dog-bones shaped samples are summarized in the Figure 12. With the aim of confirming mechanical consistence of samples, a stress–strain curve was investigated for composites of 15 wt. % for CB and C1000 compared with neat resin. Mechanical tests on dog-bones shaped samples are summarized in the Figure 12. Figure 12. Stress–strain curves of composites containing 15 wt. % of (a) C1000, (b) CB and (c) neat resin. Figure 12. Stress–strain curves of composites containing 15 wt.% of (a) C1000, (b) CB and (c) neat resin. According to data report in Figure 13, maximum elongation of neat resin (3.50%  0.64%) was the highest compared with those of C1000 and CB containing composites (1.16% 0.09% and 1.63% Figure 12. Stress–strain curves of composites containing 15 wt.% of (a) C1000, (b) CB and (c) neat resin. Polymers 2019, 11, x FOR PEER REVIEW 14 of 18 Polymers 2019, 11, 1916 13 of 17 According to data report in Figure 13, maximum elongation of neat resin (3.50% ± 0.64%) was the highest compared with those of C1000 and CB containing composites (1.16% ± 0.09% and 1.63% ± 0.08% respectively). Neat resin showed also a remarkably higher toughness (0.48 ± 0.03 MJ/m ) 0.08% respectively). Neat resin showed also a remarkably higher toughness (0.48  0.03 MJ/m ) compared with composites that showed values not significantly different from each other close to compared with composites that showed values not significantly di erent from each other close to 3 0 0.18 MJ/m . Young′s modulus (YM) showed a significant difference between C1000-based composites 0.18 MJ/m . Young s modulus (YM) showed a significant di erence between C1000-based composites (3258 ± 273 MPa) and CB ones (1940 ± 163 MPa). These last values were quite close to those of neat (3258 273 MPa) and CB ones (1940 163 MPa). These last values were quite close to those of neat resin (1510 ± 160 MPa) and a similar trend was observed with ultimate tensile strength with values resin (1510 160 MPa) and a similar trend was observed with ultimate tensile strength with values of of CB composites not significantly different from those of neat resin (both close to 19 MPa) and higher CB composites not significantly di erent from those of neat resin (both close to 19 MPa) and higher values for biochar-based composites (up to 24.9 ± 1.5 MPa). values for biochar-based composites (up to 24.9 1.5 MPa). Figure 13. Summary of (a) ultimate tensile strength, (b) Young s modulus, (c) toughness and (d) maximum elongation of neat resin, biochar and carbon-based composites. Columns marked with Figure 13. Summary of (a) ultimate tensile strength, (b) Young′s modulus, (c) toughness and (d) same letters were not significantly different (p < 0.05). maximum elongation of neat resin, biochar and carbon-based composites. Columns marked with same letters were not significantly different (p < 0.05). Composites behaviour observed during the mechanical tests enlightened the di erent interaction between di erent carbonaceous filler with epoxy matrix with magnification of filler–resin interaction, Composites behaviour observed during the mechanical tests enlightened the different and in the case of biochar-based composites with an increase brittleness and a reduced elongation. interaction between different carbonaceous filler with epoxy matrix with magnification of filler–resin As reported by Chodak et al. [39] about CB containing poly(propylene) composites, the formation interaction, and in the case of biochar-based composites with an increase brittleness and a reduced of a diffuse particles network is detrimental for the mechanical properties. The same behaviour was elongation. observed in the production of CB-based composites which presented a decrement of Ultimate tensile As reported by Chodak et al. [39] about CB containing poly(propylene) composites, the strength compared with CB1000 ones. CB1000 were very close to the percolation threshold (Figure 7) formation of a diffuse particles network is detrimental for the mechanical properties. The same and this induced a very relevant decrement of maximum elongation. Working below the percolation behaviour was observed in the production of CB-based composites which presented a decrement of threshold allowed the preservation of some of the appealing properties of a brittle resin (i.e., high Young’s Ultimate tensile strength compared with CB1000 ones. CB1000 were very close to the percolation modulus and ultimate tensile strength) together with the magnification of electrical conductivity. threshold (Figure 7) and this induced a very relevant decrement of maximum elongation. Working below the percolation threshold allowed the preservation of some of the appealing properties of a Polymers 2019, 11, 1916 14 of 17 4. Conclusions The co ee waste stream was eciently used as feedstock for pyrolytic conversion at di erent temperatures. The e ect of process temperature on the properties of biochar was investigated and it was observed that further increments of temperature improved the porous stability and conductivity of the material. This phenomenon was probably due to both the formation of new C–C bonds and to the rearrangement of graphitic and quasi-graphitic domains formed during pyrolysis as shown by Raman characterization. The most relevant result of this study was that even if neat biochar produced at 1000 C showed less conductivity with respect to CB when it was dispersed in composite, the electrical properties of a composite containing co ee biochar were some orders of magnitude higher than composites containing CB. In the case of 20 wt. % of C1000, composites showed four orders of magnitudes more that composites containing 20 wt. % of CB. This could be ascribed to the uniform dispersion of co ee biochar, in contrast to CB which creates agglomerations. These agglomerations induced a non-uniform composite structure in the CB containing composites. Mechanical properties of composites with co ee biochar were verified and they were not compromised with respect to composites containing C1000, showing better UTS and YM. Both materials were more brittle than neat resin but C1000 showed some of the properties of high performances resins. Mechanical properties also showed a direct correlation with filler dispersion. Where the filler dispersion was uniform, the mechanical performances were improved. A new era could be at the door for carbon fillers in polymer composites. Considering the sustainability of coffee biochar production, the results reported show how biomass-derived carbon could be a sound replacement for oil-derived carbon fillers such as CB. Author Contributions: Conceptualization, M.G.; methodology M.G., M.B.; formal analysis M.G., M.B.; investigation M.G., M.B.; resources, M.G.; data curation, M.G., M.B.; writing—original draft preparation, M.G., M.B.; writing—review and editing, M.G., M.B.; supervision, M.G. Funding: MODCOMP project (grant 685844, European H2020 program). Acknowledgments: The authors acknowledge Andrea Marchisio for particle size analysis, Salvatore Guastella for FE-SEM and EDX analysis, Massimo Rovere for Raman spectra, Carlo Rosso for mechanical tests and Alberto Tagliaferro for fruitful discussions. Conflicts of Interest: The authors declare no conflict of interest. References 1. Girotto, F.; Alibardi, L.; Cossu, R. Food waste generation and industrial uses: A review. Waste Manag. 2015, 45, 32–41. [CrossRef] 2. Burkhard, R.; Deletic, A.; Craig, A. Techniques for water and wastewater management: A review of techniques and their integration in planning. Urban Water 2000, 2, 197–221. [CrossRef] 3. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Polymers Unpaywall

Development of Coffee Biochar Filler for the Production of Electrical Conductive Reinforced Plastic

Giorcelli, Mauro; Bartoli, Mattia
Polymers – Nov 21, 2019
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polymers Article Development of Co ee Biochar Filler for the Production of Electrical Conductive Reinforced Plastic Mauro Giorcelli * and Mattia Bartoli Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-0110-904-327 Received: 15 October 2019; Accepted: 18 November 2019; Published: 21 November 2019 Abstract: In this work we focused our attention on an innovative use of food residual biomasses. In particular, we produced biochar from co ee waste and used it as filler in epoxy resin composites with the aim to increase their electrical properties. Electrical conductivity was studied for the biochar and biochar-based composite in function of pressure applied. The results obtained were compared with carbon black and carbon black composites. We demonstrated that, even if the co ee biochar had less conductivity compared with carbon black in powder form, it created composites with better conductivity in comparison with carbon black composites. In addition, composite mechanical properties were tested and they generally improved with respect to neat epoxy resin. Keywords: electrical properties; mechanical properties; recycling; epoxy resin 1. Introduction Anthropogenic waste stream management is one of the main unresolved problems of industrialized societies [1,2]. In the food waste sector, co ee residuals could be considered not only a waste material but a resource. Recently Christoph Sänger [3] reported that worldwide co ee production was 159.7 million of bags in crop year 2017/18 (about 9.6 MTons), with a mean of 5 kg/capita per year in traditional markets (Germany, Italy, France, USA and Japan) and an increasing consumption in emerging markets (South Korea, Russia, Turkey and China). The co ee waste stream becomes a relevant problem not only after consumption but also during the wet processing of co ee beans when 1 ton of fresh berries results in only about 400 kg of wet waste pulp. Several solutions have been proposed to solve the problem of waste co ee biochar, such as the production of biogas [4] and flavours [5], use as filler in ceramics [6] or as absorbent for the removal of basic dyes from aqueous solutions [7]. Co ee wastes have been also used as feedstock for pyrolytic conversion producing hydrogen-rich gas [8] and fuel-quality biochar [9]. Biochar has been used not only as solid fuel but also as high performance material [10,11], as a flame retardant additive [12,13], for electrochemical [14] and energy storage applications [15] and for production of composites [16–19]. Traditionally, in the realm of carbon fillers in polymer composites, carbon black (CB) plays the main role especially in the automotive field with an estimated consumption of 8.1 MTon/year according to data released by the International CB association [20]. CB has been used for producing conductive composites [21] but, as recently reported by Quosai et al. [22], co ee-based biochar also shows remarkably conductive properties. Furthermore, co ee biochar production has an indisputable advantage if compared with CB. Co ee biochar production uses a food waste stream while oil-based feedstock is required for CB production. This decreases the environmental impact of the production process [23–25]. Among di erent polymers, in this work we focused our attention on epoxy resins doped with these two carbon fillers. As is well known, epoxy resin is a thermoset polymer widely applied in the field of coatings [26], adhesives [27], casting [28], potting [29], composites [30], laminates [31] and encapsulation Polymers 2019, 11, 1916; doi:10.3390/polym11121916 www.mdpi.com/journal/polymers Polymers 2019, 11, 1916 2 of 17 of semiconductor devices [32]. Epoxy resins are used intensively because of their peculiar properties such as high strength, good sti ness, good thermal stability and excellent heat, moisture and chemical resistance [33,34]. Another, unneglectable advantage of epoxy resin is the possibility of being dispersed into the cross-linked polymeric matrix additives, such as micro-encapsulated amines [35,36], that could be realised after material failure promoting the self-healing process of the epoxy composite [37]. In the field of composites materials, production of conductive reinforced plastic materials has attracted an increasing interest in the last few decades [38,39]. Large-scale application fields deserve particular attention. For example conductive epoxy resin has a large-scale application in the field of coatings and adhesives [40]. In these large-scale applications, filler cost is a crucial issue. Epoxy resins have been used as a polymeric host for plenty of carbonaceous materials for the production of conductive reinforced materials [41–44], but the cost of carbon filler has to be take in account. High-cost carbon fillers such as carbon nanotubes and graphene are problematic for large-scale applications. These carbon fillers induce an increment of its electrical and mechanical properties in the host polymer matrix [45–48] but are not a suitable choice for industrial scale production. This is mainly due to the high-cost, up to 300 k$/kg [49], and the problem of low productivity of the plants is well known [50]. Thus, low cost carbon fillers which are not derived from fossil fuels, such as CB, are a topic of relevant interest. In this study, we investigated the use of biochar derived from pyrolytic conversion of the co ee waste stream, such as low cost carbon fillers derived by recycling materials. Results were compared with CB-based composites. Mechanical properties were also investigated for full composite characterization. 2. Materials and Methods 2.1. Carbonaceous Materials Preparation and Characterization Exhausted co ee powder was selected as a real case study. It was collected from Bar Katia (Turin, Italy) supplied by Vergnano (Arabica mixture). Co ee was collected and dried at 105 C for 72 h. Co ee samples (100 g) were pyrolyzed using a vertical furnace and a quartz reactor, heating rate of 15 C/min and kept at the final temperature (400, 600, 800 and 1000 C) for 30 min in an argon atmosphere. Samples were named as C400, C600, C800 and C1000 respectively. Biochar was grinded using a mechanical mixer (Savatec BB90E) for 10 min in order to decrease the particle size. Commercial CB (VULCAN 9 N115) was used to compare with co ee biochar. Ash contents of co ee and carbon-based materials (biochars and CB) were evaluated using a static furnace set at 550 or 800 C respectively for 6 h. All samples were investigated from morphological point of view using a field emission scanning electrical microscopy (FE-SEM, Zeis SupraTM 40, Oberkochen, Germany). The microscope was equipped with an energy dispersive X-ray detector (EDX, Oxford Inca Energy 450, Oberkochen, Germany) that was used to explore the carbon composition of biochars. Particle size distribution of carbon fillers was evaluated using a laser granulometry (Fritsch Analysette 22, Idar-Oberstein, Germany) after a dispersion in ethanol and sonication in an ultrasonic bath for 10 min. Co ee, biochars and CB were analysed through FT-IR (Nicolet 5700, Thermoscientific, Waltham, US) on attenuated total reflectance (ATR) mode (Smartorbit, Thermoscientific) in the range from 500 to 4000 cm . Biochars and CB were analysed through Raman spectroscopy using Renishaw Ramanscope InVia (H43662 model, Gloucestershire, UK). 2.2. Composites Preparation Biochar, derived from coffee, and commercial CB containing epoxy composites were produced using a two component bis-phenol A (BPA) diglycidyl resin (CORES epoxy resin, LPL). Carbonaceous filler (15 wt. %) were dispersed into epoxy monomer using a tip ultrasonicator apparatus (Sonics Vibra-cell) for 15 min. After the addition of the curing agent, the mixture was ultrasonicated for another 2 min Polymers 2019, 11, x FOR PEER REVIEW 3 of 18 Biochar, derived from coffee, and commercial CB containing epoxy composites were produced using a two component bis-phenol A (BPA) diglycidyl resin (CORES epoxy resin, LPL). Carbonaceous filler (15 wt %) were dispersed into epoxy monomer using a tip ultrasonicator Polymers 2019, 11, 1916 3 of 17 apparatus (Sonics Vibra-cell) for 15 min. After the addition of the curing agent, the mixture was ultrasonicated for another 2 min and left into the moulds for 16 h at room temperature. A final thermal curing was performed using a ventilated oven (I.S.C.O. Srl “The scientific manufacturer”) at and left into the moulds for 16 h at room temperature. A final thermal curing was performed using a 70 °C for 6 h. ventilated oven (I.S.C.O. Srl “The scientific manufacturer”) at 70 C for 6 h. 2.3. Electrical Characterization 2.3. Electrical Characterization The measurement set-up was derived from Gabhi et al. [51] and is sketched in Figure 1a for The measurement set-up was derived from Gabhi et al. [51] and is sketched in Figure 1a for fillers and Figure 1b for composites. The instrument was composed of two solid copper cylinders, 30 fillers and Figure 1b for composites. The instrument was composed of two solid copper cylinders, mm in diameter and 5 cm in length, encapsulated in a hollow Plexiglas cylinder with a nominal inner 30 mm in diameter and 5 cm in length, encapsulated in a hollow Plexiglas cylinder with a nominal diameter of 30 mm in the case of filler electrical characterization. In this configuration, the inner inner diameter of 30 mm in the case of filler electrical characterization. In this configuration, the inner diameter was slightly higher so that it was possible to force the copper rods inside the Plexiglas cavity diameter was slightly higher so that it was possible to force the copper rods inside the Plexiglas cavity and the upper rod could slide inside the cylinder during the measurement. This arrangement created and the upper rod could slide inside the cylinder during the measurement. This arrangement created an internal chamber between the two cylinders, where the carbon powder could be inserted. In the an internal chamber between the two cylinders, where the carbon powder could be inserted. In the case of composites, the Plexiglas cylinder was removed and the sample was positioned between the case of composites, the Plexiglas cylinder was removed and the sample was positioned between the aligned copper cylinders. The electrical resistance of the powders or composites was measured at aligned copper cylinders. The electrical resistance of the powders or composites was measured at increasing loads (up to 1500 bar) applied by a hydraulic press (Specac Atlas Manual Hydraulic Press increasing loads (up to 1500 bar) applied by a hydraulic press (Specac Atlas Manual Hydraulic Press 15T). Electrically insulating sheets were placed between the conductive cylinders and the load 15T). Electrically insulating sheets were placed between the conductive cylinders and the load surfaces surfaces in order to ensure that the electrical signal passed through the sample. The resistance of the in order to ensure that the electrical signal passed through the sample. The resistance of the carbon carbon fillers was measured using an Agilent 34401A multimeter. fillers was measured using an Agilent 34401A multimeter. Figure 1. Sketch of measurement set-up for conductivity study of (a) carbon fillers and (b) composite. 2.4. Composites Figure 1. Mechanical Sketch of measurement set-up Characterization for conductivity study of (a) carbon fillers and (b) composite. Carbonaceous materials containing composites were produced as dog-bone shaped according to the ASTM 638 procedure. Samples were tested using a mechanical stress test (MTS) machine 2.4. Composites Mechanical Characterization (MTS Q-test10) in tensile test mode until break point. Data were analysed using a self-developed Carbonaceous materials containing composites were produced as dog-bone shaped according software compiled using Matlab. to the ASTM 638 procedure. Samples were tested using a mechanical stress test (MTS) machine (MTS 2.5. Data Analysis Q-test10) in tensile test mode until break point. Data were analysed using a self-developed software compiled using Matlab. Statistical analysis used were based on t-tests with a significance level of 0.05 (p < 0.05) were carried out using Excel— software (Microsoft Corp.) and the “data analysis” tool. 2.5. Data Analysis 3. Results Statistical analysis used were based on t-tests with a significance level of 0.05 (p < 0.05) were carried out using Excel™ software (Microsoft Corp.) and the “data analysis” tool. 3.1. Carbonaceous Materials Characterization 3. Re Pyr sults olysis of spent grounds co ee proceeded according to the mechanism reported by Setter et al. [52]. The main mechanisms that occurred during the degradative processes were those related with 3.1. Carbonaceous Materials Characterization decomposition of the small lignin fraction [53] and the most abundant polysaccharides (i.e., cellulose and hemicellulose) [54] with the formation of bio-oils rich in anhydrosugars, furans and acetic acid with trace of aromatics [55–57]. Polymers 2019, 11, x FOR PEER REVIEW 4 of 18 Pyrolysis of spent grounds coffee proceeded according to the mechanism reported by Setter et al. [52]. The main mechanisms that occurred during the degradative processes were those related with decomposition of the small lignin fraction [53] and the most abundant polysaccharides (i.e. Polymers 2019, 11, 1916 4 of 17 cellulose and hemicellulose) [54] with the formation of bio-oils rich in anhydrosugars, furans and acetic acid with trace of aromatics [55-57]. Ash content of feedstock and carbonaceous materials were preliminary investigated and Ash content of feedstock and carbonaceous materials were preliminary investigated and summarized in Figure 2. summarized in Figure 2. Figure 2. Ash contents of neat co ee, carbon black (CB) and co ee biochar samples heated at 400, 600, 800 and 1000 C (C400, C600, C800 and C1000 respectively). Columns marked with di erent letters Figure 2. Ash contents of neat coffee, carbon black (CB) and coffee biochar samples heated at 400, 600, were significantly di erent (p < 0.05). 800 and 1000 °C (C400, C600, C800 and C1000 respectively). Columns marked with different letters were significantly different (p < 0.05). The ash content of neat co ee was 1.70 0.14 wt. % and it increased with temperature increments reaching a value around 9 wt. % in the case of C800 (8.92 0.61 wt. %) and C1000 (9.09 0.09 wt. %). The ash content of neat coffee was 1.70 ± 0.14 wt. % and it increased with temperature increments As expected, CB showed a very low ash content (0.07  0.01 wt. %) according to Medalia et al. [58] reaching a value around 9 wt. % in the case of C800 (8.92 ± 0.61 wt. %) and C1000 (9.09 ± 0.09 wt. %). mainly as oxides. Ash content increment at higher temperatures was imputable to advance pyrolytic As expected, CB showed a very low ash content (0.07 ± 0.01 wt. %) according to Medalia et al. [58] degradation of the organic matrix leading to the concentration of inorganic residue [59] that did not mainly as oxides. Ash content increment at higher temperatures was imputable to advance pyrolytic undergo any temperature induced degradation. degradation of the organic matrix leading to the concentration of inorganic residue [59] that did not The e ect of pyrolytic temperature on biochar morphology was studied using FE-SEM as shown undergo any temperature induced degradation. in Figure 3. Neat co ee displayed flaked collapsed structures (Figure 3a, b) that was retained by The effect of pyrolytic temperature on biochar morphology was studied using FE-SEM as shown C400 after pyrolysis at 400 C (Figure 3e,f). With the increase of temperature to 600 C lead to the in Figure 3. Neat coffee displayed flaked collapsed structures (Figure 3a, b) that was retained by C400 formation of porous structures with average diameters close to 30 m separated by carbon lamellae after pyrolysis at 400 °C (Figure 3e, f). With the increase of temperature to 600 °C lead to the formation with a thickness around 1 m (Figure 3g,h). At 800 C, the biochar recovered lost the structure due of porous structures with average diameters close to 30 μm separated by carbon lamellae with a to the massive release of volatile organic matters during the overall pyrolytic process that induced thickness around 1 μm (Figure 3g, h). At 800 °C, the biochar recovered lost the structure due to the the collapse of carbonaceous structures together with an improved grindability [60]. At 1000 C, massive release of volatile organic matters during the overall pyrolytic process that induced the the increased temperature allowed the massive formation of carbon–carbon bonds that promoted the collapse of carbonaceous structures together with an improved grindability [60]. At 1000 °C, the stabilization of the porous architecture with nanoscale lamellae structures. CB showed a typical highly increased temperature allowed the massive formation of carbon–carbon bonds that promoted the aggregate spherule-based shape with average diameter of single particles around 50 nm. stabilization of the porous architecture with nanoscale lamellae structures. CB showed a typical highly aggregate spherule-based shape with average diameter of single particles around 50 nm. Polymers 2019, 11, 1916 5 of 17 Polymers 2019, 11, x FOR PEER REVIEW 5 of 18 Figure 3. FE-SEM captures of (a, b) neat coffee, (c, d) CB, (e, f) C400, (g, h) C600, (i, j) C800 and (k, l) Figure 3. FE-SEM captures of (a,b) neat co ee, (c,d) CB, (e,f) C400, (g,h) C600, (i,j) C800 and (k,l) C1000. C1000. C1000 C800 C600 C400 CB Neat Coffee Polymers 2019, 11, 1916 6 of 17 Polymers 2019, 11, x FOR PEER REVIEW 6 of 18 Organic component of significant carbonaceous materials was also analysed using both FT-IR and Organic component of significant carbonaceous materials was also analysed using both FT-IR Raman spectrometry techniques. Among carbonaceous materials, we reported neat co ee, C1000 and and Raman spectrometry techniques. Among carbonaceous materials, we reported neat coffee, C1000 CB. Results are shown in Figure 4. and CB. Results are shown in Figure 4. Figure 4. FT-IR spectra (ATR mode) of (a) neat co ee, (b) co ee biochar produced at 1000 C and (c) CB in the range of 500–4000 cm . Figure 4. FT-IR spectra (ATR mode) of (a) neat coffee, (b) coffee biochar produced at 1000 °C and (c) −1 CB in the range of 500–4000 cm . The FT-IR spectrum of neat co ee showed the broad band of  (3300–3500 cm ), the bands O–H 1 1 of saturated  (2850–2950 cm ),  (1710–1741 cm ) due to the carboxylic functionalities, C–H C=O –1 The FT-IR spectrum of neat coffee showed the broad band of νO–H (3300–3500 cm ), the bands of (1540–1638 cm ) due to the presence of aromatic structures, saturated and unsaturated C=C C–H –1 –1 saturated νC–H (2850–2950 cm ), νC=O (1710–1741 cm ) due to the carboxylic functionalities, νC=C (1540– 1 1 1 (1370–1440 cm ), saturated  (1243 cm ),  (1030–1148 cm ) and out-of-plane  below C–C C–O O–H –1 – 1638 cm ) due to the presence of aromatic structures, saturated and unsaturated δC–H (1370–1440 cm 700 cm . Those bands clearly identified a lignocellulosic derived matrix with massive presence 1 –1 –1 –1 ), saturated νC–C (1243 cm ), νC–O (1030–1148 cm ) and out-of-plane δO–H below 700 cm . Those bands of polysaccharides and aromatics. C1000 did not show any of the characteristic bands of organic clearly identified a lignocellulosic derived matrix with massive presence of polysaccharides and matrix but show an envelope of bands below 1800 cm due to carbon skeletal movements. Contrary, aromatics. C1000 did not show any of the characteristic bands of organic matrix but show an envelope CB showed low bands intensity below 1000 cm due the lower variety of carbon structure embedded –1 of bands below 1800 cm due to carbon skeletal movements. Contrary, CB showed low bands into particles. –1 intensity below 1000 cm due the lower variety of carbon structure embedded into particles. Raman spectra normalized on G peak are shown in Figure 5. Co ee biochars had the typical Raman spectra normalized on G peak are shown in Figure 5. Coffee biochars had the typical profiles of amorphous materials [61] in contrast to CB which was more graphitic. The graphitic structure profiles of amorphous materials [61] in contrast to CB which was more graphitic. The graphitic for CB could be observed by the deep gorge between D and G peaks and their shaped structure. structure for CB could be observed by the deep gorge between D and G peaks and their shaped An increase of I /I ratio was evident for biochars moving from a pyrolytic temperature of 400 to D G structure. An increase of ID/IG ratio was evident for biochars moving from a pyrolytic temperature of 1000 C. This increase of I /I ratio could be ascribed to the progressively loss of residual functional D G 400 to 1000 °C. This increase of ID/IG ratio could be ascribed to the progressively loss of residual groups with the increase of temperature. This observation was also supported by the decrease of functional groups with the increase of temperature. This observation was also supported by the fluorescence [62]. Due to the loss of less intense parts of these weak interactions, biochar underwent an decrease of fluorescence [62]. Due to the loss of less intense parts of these weak interactions, biochar appreciable disorganization together with aromatic structure formation, in particular up to 600 C, underwent an appreciable disorganization together with aromatic structure formation, in particular without the completion of a proper graphitization process that occurs at higher temperature [63]. up to 600 °C, without the completion of a proper graphitization process that occurs at higher temperature [63]. The evolution of biochar structures due to temperature increment could be monitored though Raman according to Ferrari et al. [61]. Accordingly, the D peaks (Figure 5) showed wave numbers –1 close to 1350 cm that is typical of transition from amorphous carbon to nanocrystalline graphite. At –1 the same time the biochar G peaks showed wavenumbers close to 1580 cm with exception of C1000. Polymers 2019, 11, x FOR PEER REVIEW 7 of 18 –1 Polymers 2019, 11, 1916 7 of 17 This last showed a G peak at 1600 cm due the high amount of nanocrystalline domains not yet rearranged in the ordered structure [64]. Figure 5. Magnification of Raman spectra in the range from 800 to 2000 cm of C400, C600, C800, –1 Figure 5. Magnification of Raman spectra in the range from 800 to 2000 cm of C400, C600, C800, C1000 and CB. C1000 and CB. The evolution of biochar structures due to temperature increment could be monitored though The above mentioned consideration was also supported by EDX analysis that showed the carbon Raman according to Ferrari et al. [61]. Accordingly, the D peaks (Figure 5) showed wave numbers content that significantly increased from C400 to C600–C1000 while oxygen content decreased. close to 1350 Carbon con cm tethat nt of C is 6typical 00–C100of 0 w transition ere not signi fr fic om antamorphous ly different frocarbon m CB even to nanocrystalline if CB showed a mor graphite. e ordered structure. This support the hypothesis that the driving force of the biochar enhanced At the same time the biochar G peaks showed wavenumbers close to 1580 cm with exception of conductivity is the reorganization of nanocrystalline domains and not merely the carbon content, C1000. This last showed a G peak at 1600 cm due the high amount of nanocrystalline domains not shown in Figure 6. Traces of Mg, P, K and Ca were also detected. yet rearranged in the ordered structure [64]. The above mentioned consideration was also supported by EDX analysis that showed the carbon content that significantly increased from C400 to C600–C1000 while oxygen content decreased. Carbon content of C600–C1000 were not significantly di erent from CB even if CB showed a more ordered structure. This support the hypothesis that the driving force of the biochar enhanced Polymers 2019, 11, x FOR PEER REVIEW 8 of 18 conductivity is the reorganization of nanocrystalline domains and not merely the carbon content, shown in Figure 6. Traces of Mg, P, K and Ca were also detected. Figure 6. Carbon content of biochars and CB. Evaluated though energy dispersive X-ray (EDX) analysis. Error values were reported as 5% of the detected values according with Laskin et al. [65]. Columns marked with different letters are significantly different (p < 0.05). Figure 6. Carbon content of biochars and CB. Evaluated though energy dispersive X-ray (EDX) analysis. Error values were reported as 5% of the detected values according with Laskin et al. [65]. Columns marked with different letters are significantly different (p < 0.05). 3.2. Composites Characterization 3.2.1. Electrical Characterization of Carbonaceous Filler and Composites The set-up shown in Figure 1a was used for biochar powders electrical characterization. Around 3 grams of carbonaceous powder, which creates a few millimetres distance between copper cylinders was positioned in the chamber. After the closure of the chamber a pressure was applied with the aim of compacting the powder. The pressure range was from 0 to 1500 bar (step of 250 bar). For each step the stabilized value of resistivity was registered such as the distance between the copper cylinders. The same procedure was repeated for composites of few millimetre thickness. Carbonaceous powders and composites decreased their resistance value during compression until they reached a plateau when high pressure was reached. The decreasing of resistance value could be correlated with the decreasing of space between carbon particles as sketched in Figure 7. In the case of powders, the void among particles collapsed with a production of compact carbon agglomerate as shown in Figure 7a. In the case of composites, Figure 7b shows the mechanism where the polymer chains flow let the carbon particles situate. Polymers 2019, 11, 1916 8 of 17 3.2. Composites Characterization 3.2.1. Electrical Characterization of Carbonaceous Filler and Composites The set-up shown in Figure 1a was used for biochar powders electrical characterization. Around 3 g of carbonaceous powder, which creates a few millimetres distance between copper cylinders was positioned in the chamber. After the closure of the chamber a pressure was applied with the aim of compacting the powder. The pressure range was from 0 to 1500 bar (step of 250 bar). For each step the stabilized value of resistivity was registered such as the distance between the copper cylinders. The same procedure was repeated for composites of few millimetre thickness. Carbonaceous powders and composites decreased their resistance value during compression until they reached a plateau when high pressure was reached. The decreasing of resistance value could be correlated with the decreasing of space between carbon particles as sketched in Figure 7. In the case of powders, the void among particles collapsed with a production of compact carbon agglomerate as shown in Figure 7a. In the case of composites, Figure 7b shows the mechanism where the polymer chains flow let the carbon Polymers 2019, 11, x FOR PEER REVIEW 9 of 18 particles situate. Figure 7. Behaviour of (a) powders and (b) composites during compression. Figure 7. Behaviour of (a) powders and (b) composites during compression. The resistance value, R, with the value of surface S and distance l between copper surfaces were The resistance value, R, with the value of surface S and distance l between copper surfaces were used in Ohm law ( = l/RS) to evaluate the conductivity . The conductivity value of carbon powder used in Ohm law (σ = l/RS) to evaluate the conductivity σ. The conductivity value of carbon powder and composites were evaluated following this procedure: and composites were evaluated following this procedure: 1) A starting value of conductivity was evaluated without any sample in order to measure the (1) A starting value of conductivity was evaluated without any sample in order to measure the value value of resistance of the system. This value was subtracted to the resistance value read with of resistance of the system. This value was subtracted to the resistance value read with samples. samples. (2) The same quantity of carbon powders (CB and biochar) were positioned between copper cylinders 2) The same quantity of carbon powders (CB and biochar) were positioned between copper and kept by the Plexiglas hollow cylinder. The measurement was repeated several times in order cylinders and kept by the Plexiglas hollow cylinder. The measurement was repeated several to have a reliable value. times in order to have a reliable value. (3) Composites were positioned between copper cylinders, in this case the Plexiglas hollow cylinder 3) Composites were positioned between copper cylinders, in this case the Plexiglas hollow was not necessary and the value of conductivity was measured in di erent sample portions. cylinder was not necessary and the value of conductivity was measured in different sample Preliminary results are shown in Figure 8 showing the conductivity of the biochar powders portions. (red line) and percolation curves of related composites. Preliminary results are shown in Figure 8 showing the conductivity of the biochar powders (red line) and percolation curves of related composites. Polymers 2019, 11, 1916 9 of 17 Polymers 2019, 11, x FOR PEER REVIEW 10 of 18 Figure 8. Trends of composites conductivity as function of filler percentage. Red line represents the Figure 8. Trends of composites conductivity as function of filler percentage. Red line represents the biochar conductivity trends while yellow, blue and green show respectively the percolation curves of biochar conductivity trends while yellow, blue and green show respectively the percolation curves of composites containing C400–C600, C800 and C1000. composites containing C400–C600, C800 and C1000. C400 did not show an appreciable conductivity while an increment of pyrolytic temperature up to 600 C400 did C induced not show an appr a conductivity eciab of up le con to d 0.02 uctiv S/im. ty wh Further ile an incremen increments t of of pyro processing lytic tempera temperatur ture up e to 600 °C induced a conductivity of up to 0.02 S/m. Further increments of processing temperature up up to 800 and 1000 C led to a conductivity of up to 0.04 and 35.96 S/m. This remarkable increment of to 8 conductivity 00 and 1000 between °C led to800 a conduc and 1000 tivity of up C was to due 0.04 the and enlar 35.9 gement 6 S/m. This rem of aromatic arkab region le incr formed ement of as conductivity between 800 and 1000 °C was due the enlargement of aromatic region formed as consequence of high temperature carbonization [64]. This deeply a ected the electrical behaviour of consequence of high temperature carbonization [64]. This deeply affected the electrical behaviour of related composites. Consequently, C400 and C600 containing composites were not conductive for related composites. Consequently, C400 and C600 containing composites were not conductive for all all the range of filler percentage investigated. CB composites were not conductive until the filler the range of filler percentage investigated. CB composites were not conductive until the filler concentration of 15 wt. % reaching a conductivity of 5.4 10 S/m with a filler loading of 20 wt. %. –8 concentration of 15 wt. % reaching a conductivity of 5.4 × 10 S/m with a filler loading of 20 wt. %. C1000 composites showed the best performances showing a detectable electrical conductivity with a C1000 composites showed the best performances showing a detectable electrical conductivity with a 15 wt. % of filler and reaching a conductivity of 2.02 S/m with a filler loading of 20 wt. %. 15 wt. % of filler and reaching a conductivity of 2.02 S/m with a filler loading of 20 wt. %. Accordingly, with these data, electrical properties of C1000 and C1000 containing composites Accordingly, with these data, electrical properties of C1000 and C1000 containing composites were studied under a wide range of static pressures comparing with the related CB and CB composites were studied under a wide range of static pressures comparing with the related CB and CB as shown in Figure 9. composites as shown in Figure 9. Polymers 2019, 11, x FOR PEER REVIEW 11 of 18 Polymers 2019, 11, 1916 10 of 17 Figure 9. Trends of CB and C1000 powders and related composites conductivity as a function of pressure applied. Figure 9. Trends of CB and C1000 powders and related composites conductivity as a function of CB powder reached a conductivity around 1700 S/m while in the same conditions C1000 reached a pressure applied. conductivity of 300 S/m. Composites containing of CB and C1000 were conductive but the results showed a different trend compared with the relative powders. CB 15 wt. % reached the value of 4 10 S/m CB powder reached a conductivity around 1700 S/m while in the same conditions C1000 reached and its conductive value showed an influence of applied pressure in the first compression movement. a conductivity of 300 S/m. Composites containing of CB and C1000 were conductive but the results C1000 15 wt. % reached to 10 S/m, with an increment around one order of magnitude if compared showed wita h different tr CB 15 wt. %end c . Thiso dmpared w ifference wi ath the s more re relative levant powders. CB for a filler conce 15 wt. % ntration o reac f 20 hed wt. % the v . In ta hlue of 4 × is case, 5 4 –3 the conductivity of CB-based composites dropped down to 10 –10 S/m in contrast to C1000 which 10 S/m and its conductive value showed an influence of applied pressure in the first compression –2 reached ~10 S/m. The high conductive value for coffee biochar could be due to more uniform filler movement. C1000 15 wt. % reached to 10 S/m, with an increment around one order of magnitude if dispersion inside epoxy resin. Dispersion of the filler inside the epoxy matrix was investigated through compared with CB 15 wt. %. This difference was more relevant for a filler concentration of 20 wt. %. FE-SEM (Figure 10) after samples were cryo-fractured using liquid nitrogen and compared to composites –5 –4 In this case, the conductivity of CB-based composites dropped down to 10 –10 S/m in contrast to with a filler loading of 15 wt. % due to the similarity of conductivity. CB containing composites showed C1000 which reached ~10 S/m. The high conductive value for coffee biochar could be due to more a dark and clear area (Figure 10a) with different compositions. The clear ones were rich in CB aggregates uniform filler dispersion inside epoxy resin. Dispersion of the filler inside the epoxy matrix was (Figure 10b,c) while the darkest were poor (Figure 10d). C1000 containing composites showed smooth investigated through FE-SEM (Figure 10) after samples were cryo-fractured using liquid nitrogen and surfaces with holes (Figure 10e) due the expulsion of embedded C1000 particles during the fracturing compared to composites with a filler loading of 15 wt. % due to the similarity of conductivity. CB (Figure 10e) as clearly shown in Figure 10g. Particles size analysis (Figure 11) showed clearly that C1000 containing composites showed a dark and clear area (Figure 10a) with different compositions. The was composed by two particle populations, one around 100 m and one around 20 m. Considering clear ones were rich in CB aggregates (Figure 10b, c) while the darkest were poor (Figure 10d). C1000 the average size of C1000 particles into the composites was reasonable it was assumed that the bigger ones underwent a disruption during the ultrasonication forming small sized well-dispersed particles. containing composites showed smooth surfaces with holes (Figure 10e) due the expulsion of CB particles size showed also that it would be more appropriate speaking of CB aggregates instead of embedded C1000 particles during the fracturing (Figure 10e) as clearly shown in Figure 10g. Particles single particles [66]. Aggregates could be justified also from Figure 10c where the CB single particles size analysis (Figure 11) showed clearly that C1000 was composed by two particle populations, one were less than 100 nm but they created agglomerates that also the particle size analysis (Figure 11) was around 100 μm and one around 20 μm. Considering the average size of C1000 particles into the not able to detect. composites was reasonable it was assumed that the bigger ones underwent a disruption during the ultrasonication forming small sized well-dispersed particles. CB particles size showed also that it would be more appropriate speaking of CB aggregates instead of single particles [66]. Aggregates could be justified also from Figure 10c where the CB single particles were less than 100 nm but they created agglomerates that also the particle size analysis (Figure 11) was not able to detect. Polymers 2019, 11, 1916 11 of 17 Polymers 2019, 11, x FOR PEER REVIEW 12 of 18 Figure 10. Field emission scanning electrical microscopy (FESEM) of cryo-fractured (a–d) CB and (e–g) Figure 10. Field emission scanning electrical microscopy (FESEM) of cryo-fractured (a–d) CB and (e– C1000 containing composites with a filler loading of 15 wt. %. g) C1000 containing composites with a filler loading of 15wt %. Polymers 2019, 11, x FOR PEER REVIEW 13 of 18 Polymers 2019, 11, x FOR PEER REVIEW 13 of 18 Polymers 2019, 11, 1916 12 of 17 Figure 11. Particle size distribution for CB and C1000. Figure 11. Particle size distribution for CB and C1000. 3.2.2. Composites Mechanical Characterization 3.2.2. Composites Mechanical Characterization Figure 11. Particle size distribution for CB and C1000. With the aim of confirming mechanical consistence of samples, a stress–strain curve was With the aim of confirming mechanical consistence of samples, a stress–strain curve was investigated for composites of 15 wt. % for CB and C1000 compared with neat resin. Mechanical tests 3.2.2. Composites Mechanical Characterization investigated for composites of 15 wt. % for CB and C1000 compared with neat resin. Mechanical tests on dog-bones shaped samples are summarized in the Figure 12. on dog-bones shaped samples are summarized in the Figure 12. With the aim of confirming mechanical consistence of samples, a stress–strain curve was investigated for composites of 15 wt. % for CB and C1000 compared with neat resin. Mechanical tests on dog-bones shaped samples are summarized in the Figure 12. Figure 12. Stress–strain curves of composites containing 15 wt. % of (a) C1000, (b) CB and (c) neat resin. Figure 12. Stress–strain curves of composites containing 15 wt.% of (a) C1000, (b) CB and (c) neat resin. According to data report in Figure 13, maximum elongation of neat resin (3.50%  0.64%) was the highest compared with those of C1000 and CB containing composites (1.16% 0.09% and 1.63% Figure 12. Stress–strain curves of composites containing 15 wt.% of (a) C1000, (b) CB and (c) neat resin. Polymers 2019, 11, x FOR PEER REVIEW 14 of 18 Polymers 2019, 11, 1916 13 of 17 According to data report in Figure 13, maximum elongation of neat resin (3.50% ± 0.64%) was the highest compared with those of C1000 and CB containing composites (1.16% ± 0.09% and 1.63% ± 0.08% respectively). Neat resin showed also a remarkably higher toughness (0.48 ± 0.03 MJ/m ) 0.08% respectively). Neat resin showed also a remarkably higher toughness (0.48  0.03 MJ/m ) compared with composites that showed values not significantly different from each other close to compared with composites that showed values not significantly di erent from each other close to 3 0 0.18 MJ/m . Young′s modulus (YM) showed a significant difference between C1000-based composites 0.18 MJ/m . Young s modulus (YM) showed a significant di erence between C1000-based composites (3258 ± 273 MPa) and CB ones (1940 ± 163 MPa). These last values were quite close to those of neat (3258 273 MPa) and CB ones (1940 163 MPa). These last values were quite close to those of neat resin (1510 ± 160 MPa) and a similar trend was observed with ultimate tensile strength with values resin (1510 160 MPa) and a similar trend was observed with ultimate tensile strength with values of of CB composites not significantly different from those of neat resin (both close to 19 MPa) and higher CB composites not significantly di erent from those of neat resin (both close to 19 MPa) and higher values for biochar-based composites (up to 24.9 ± 1.5 MPa). values for biochar-based composites (up to 24.9 1.5 MPa). Figure 13. Summary of (a) ultimate tensile strength, (b) Young s modulus, (c) toughness and (d) maximum elongation of neat resin, biochar and carbon-based composites. Columns marked with Figure 13. Summary of (a) ultimate tensile strength, (b) Young′s modulus, (c) toughness and (d) same letters were not significantly different (p < 0.05). maximum elongation of neat resin, biochar and carbon-based composites. Columns marked with same letters were not significantly different (p < 0.05). Composites behaviour observed during the mechanical tests enlightened the di erent interaction between di erent carbonaceous filler with epoxy matrix with magnification of filler–resin interaction, Composites behaviour observed during the mechanical tests enlightened the different and in the case of biochar-based composites with an increase brittleness and a reduced elongation. interaction between different carbonaceous filler with epoxy matrix with magnification of filler–resin As reported by Chodak et al. [39] about CB containing poly(propylene) composites, the formation interaction, and in the case of biochar-based composites with an increase brittleness and a reduced of a diffuse particles network is detrimental for the mechanical properties. The same behaviour was elongation. observed in the production of CB-based composites which presented a decrement of Ultimate tensile As reported by Chodak et al. [39] about CB containing poly(propylene) composites, the strength compared with CB1000 ones. CB1000 were very close to the percolation threshold (Figure 7) formation of a diffuse particles network is detrimental for the mechanical properties. The same and this induced a very relevant decrement of maximum elongation. Working below the percolation behaviour was observed in the production of CB-based composites which presented a decrement of threshold allowed the preservation of some of the appealing properties of a brittle resin (i.e., high Young’s Ultimate tensile strength compared with CB1000 ones. CB1000 were very close to the percolation modulus and ultimate tensile strength) together with the magnification of electrical conductivity. threshold (Figure 7) and this induced a very relevant decrement of maximum elongation. Working below the percolation threshold allowed the preservation of some of the appealing properties of a Polymers 2019, 11, 1916 14 of 17 4. Conclusions The co ee waste stream was eciently used as feedstock for pyrolytic conversion at di erent temperatures. The e ect of process temperature on the properties of biochar was investigated and it was observed that further increments of temperature improved the porous stability and conductivity of the material. This phenomenon was probably due to both the formation of new C–C bonds and to the rearrangement of graphitic and quasi-graphitic domains formed during pyrolysis as shown by Raman characterization. The most relevant result of this study was that even if neat biochar produced at 1000 C showed less conductivity with respect to CB when it was dispersed in composite, the electrical properties of a composite containing co ee biochar were some orders of magnitude higher than composites containing CB. In the case of 20 wt. % of C1000, composites showed four orders of magnitudes more that composites containing 20 wt. % of CB. This could be ascribed to the uniform dispersion of co ee biochar, in contrast to CB which creates agglomerations. These agglomerations induced a non-uniform composite structure in the CB containing composites. Mechanical properties of composites with co ee biochar were verified and they were not compromised with respect to composites containing C1000, showing better UTS and YM. Both materials were more brittle than neat resin but C1000 showed some of the properties of high performances resins. Mechanical properties also showed a direct correlation with filler dispersion. Where the filler dispersion was uniform, the mechanical performances were improved. A new era could be at the door for carbon fillers in polymer composites. Considering the sustainability of coffee biochar production, the results reported show how biomass-derived carbon could be a sound replacement for oil-derived carbon fillers such as CB. Author Contributions: Conceptualization, M.G.; methodology M.G., M.B.; formal analysis M.G., M.B.; investigation M.G., M.B.; resources, M.G.; data curation, M.G., M.B.; writing—original draft preparation, M.G., M.B.; writing—review and editing, M.G., M.B.; supervision, M.G. Funding: MODCOMP project (grant 685844, European H2020 program). Acknowledgments: The authors acknowledge Andrea Marchisio for particle size analysis, Salvatore Guastella for FE-SEM and EDX analysis, Massimo Rovere for Raman spectra, Carlo Rosso for mechanical tests and Alberto Tagliaferro for fruitful discussions. Conflicts of Interest: The authors declare no conflict of interest. References 1. Girotto, F.; Alibardi, L.; Cossu, R. Food waste generation and industrial uses: A review. Waste Manag. 2015, 45, 32–41. [CrossRef] 2. Burkhard, R.; Deletic, A.; Craig, A. Techniques for water and wastewater management: A review of techniques and their integration in planning. Urban Water 2000, 2, 197–221. [CrossRef] 3. 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