Influence of Commercial Biochar Fillers on Brittleness/Ductility Of

applied sciences

Article Inﬂuence of Commercial Biochar Fillers on Brittleness/Ductility of Epoxy Resin Composites

Mattia Bartoli 1,* , Mauro Giorcelli 1 , Carlo Rosso 2 , Massimo Rovere 1, Pravin Jagdale 3 and Alberto Tagliaferro 1,4

1 Department of Applied Science and Technology, Polytechnic University of Turin, Corso Duca degli Abruzzi 24, 10129 Turin, Italy 2 Department of Mechanical and Aerospace Engineering, Polytechnic University of Turin, Corso Duca degli Abruzzi 24, 10129 Turin, Italy 3 Center for Sustainable Future, Italian Istitute of Technology, Via Livorno 60, 10144 Turin, Italy 4 Faculty of Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON L1G 0C5, Canada * Correspondence: [email protected]; Tel.: +39-011097348

Received: 3 July 2019; Accepted: 30 July 2019; Published: 1 August 2019

Abstract: Production of versatile composites is a very attractive field. Carbon containing epoxy resins are one of the most relevant reinforced plastics used for a wide number of applications. In this research, we studied the influence of five different commercial biochar samples for the selective enhancement of brittleness and ductility of an epoxy based composite. We proved the relationship between biochar morphology and composites mechanical properties with the aid of FT-IR and FE-SEM analysis. We were able to improve the neat resin mechanical properties by doubling its Young’s modulus and ultimate tensile strength using a wheat straw derived material, and to increase its elongation by 40%, we used a Miscanthus derived biochar.

Keywords: biochar; epoxy resin; mechanical properties; pyrolysis; reinforced polymer

Highlights Ultrasonication based procedure was developed to easily disperse biochar into epoxy matrix; • Using wheat straw derived biochar as a ﬁller, the Young’s modulus doubles that of neat epoxy resin; • Using a Miscanthus derived biochar, an increment of 40% of the maximum elongation value • is obtained; A relationship between biochar and composites brittleness/ductility is found and an interpretative • model was proposed.

1. Introduction Epoxy composites field has been deeply investigated because of its common use in several manufactory processes [1]. They play a main role in both aviation and automotive sectors [2,3]. The combination of epoxy matrix with nanostructured carbonaceous fillers (i.e., CNTs, carbon fibers, graphene) is one of the most investigated approaches used for high performance materials production [4,5]. Despite their astonishing properties, the production of nanostructured carbon composites involves some unavoidable and rather critical issues. The most relevant is merely economic and it is due to the high price of the fillers for CNTs that can be as high as thousands of €/kg. Furthermore, nanostructured carbon is not easily dispersible as reported by many authors neither in epoxy resins [6] nor in thermoplastic matrices [7]. Among carbonaceous fillers, a non-traditional sustainable alternative is represented by the carbon residue of biomasses pyrolysis known as biochar [8].

Appl. Sci. 2019, 9, 3109; doi:10.3390/app9153109 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 3109 2 of 13

As recently reported by Khan et al. [9,10], biochar could be used as replacement for traditional high performance carbon fillers such as CNTs. Furthermore, biochars with different thermal histories lead to different properties of the epoxy composites [11]. Moreover, Das et al. [12] enlighten the relationship between different feedstocks and the mechanical properties of biochar. In a further study, Das et al. [13] tested biochar from different feedstocks in a polypropylene matrix showing an appreciable improvement of tensile and flexural properties. Despite these promising properties, biochar production is generally limited to small scale or not reproducible batch after batch. The poor reproducibility has many causes such as the quality of the feedstock and the process parameters. In this article, we reported a study about the incorporation of commercial standardized biochars derived from various feedstocks into an epoxy matrix. The high reproducibility of biochar batches grants the availability of larger amounts of standard materials. Furthermore, the variety of the feedstock used allows us to select the appropriate material for each task. In this study, we correlate the changes in brittleness and ductility of biochar-based composites with the morphological properties of the carbonaceous filler used. A hypothetical interaction mechanism is also proposed.

2. Materials and Methods

2.1. Materials Commercial standardized biochars were purchased from UK Biochar Research Centre. Chosen samples were produced from ﬁve diﬀerent biomasses feedstock (Miscanthus, wheat straw, oil seed rape, rice husk, and mixed softwoods) using a pilot-scale rotary kiln pyrolysis unit [14] setting the highest treatment temperature (HTT) to 550 ◦C. Pyrolysis parameters and yields are summarized in Table1 according to the producer data (further info on the feedstock can be found following the link in reference [15]).

Table 1. Main pyrolytic process parameters adopted by UK Biochar Research Centre for the production of standardized biochar samples.

Heating Rate Residence Time (min) Biochar Yield Feedstock ID (◦C/min) Kiln Residence Time HTT Residence Time (wt.%) Miscanthus straw MSP 65 12 4 22.8 Wheat straw WSP 80 15 5 24.1 Rice husk RH 98 15 8 37.2 Mixed softwoods SWP 78 12 4 21.8 Oil seed rape OSR 78 12 5 28.9

Two components BFA diglycidyl resin were purchased from CORES (Cores epoxy resin, LPL). The curing agent was a mix of primary amines and DMP-30 promoting a two steps polymerization process (see with Figure1). In the ﬁrst step of polymerization, epoxy groups reacted with amines at room temperature producing a low polymerization degree material with aromatic or aliphatic inter chains junction. Afterwards, a thermal curing was carried out to complete the polymerization. During both steps, DMP-30 acted as catalyst and co-curing agent promoting a more uniform polymerization process. Use of small chains amines (1,3-phenylenedimethanamine and N1,N3-dimethylpropane-1,3-diamine) together with the presence of residual hydroxylic functionalities made this material of interest for plenty of applications, such as casting or lamination [16]. Appl. Sci. 2019, 9, 3109 3 of 13 Materials 2019, 12, x FOR PEER REVIEW 3 of 13

FigureFigure 1. 1. PolymerizationPolymerization process process of of epoxy epoxy resin resin with with secondary secondary amine amine ba basedsed curing curing agent.

2.2. MethodsIn the first step of polymerization, epoxy groups reacted with amines at room temperature producingProximate, a low ultimate,polymerization and surface degree analysis material were with run aromatic by UK or Biochar aliphatic Research inter chains Centre junction. (School Afterwards,of Geosciences, a thermal University curing of Edinburgh,was carried Kingsout to Buildings, complete Edinburgh,the polymerization. EH9 3JN During Scotland, both UK) steps, and DMP-30reported acted in the datasheetsas catalyst ofand the co-curing biochars [ 15agent]. Analysis promoting showed a more the high uniform reproducibility polymerization of the materials.process. Use ofThe small morphology chains amines of biochars (1,3-phenylenedimethanamine was investigated using a Field and Emission N1,N3-dimethylpropane-1,3-diamine) Scanning Electron Microscope together(FE-SEM, with Zeis the SupraTM presence 40). of residual hydroxylic functionalities made this material of interest for 1 1 plentyBiochars of applications, were also such analyzed as casting in the or range lamination from 500 [16]. cm − to 4000 cm− using a Fourier transformed infrared (FT-IR) spectrometer (Nicolet 5700, Thermoscientific, Waltham, MA, US) operated in attenuated 2.2.total Methods reflectance (ATR) equipped with a diamond window (Smartorbit, Thermoscientific, Waltham, MA,Proximate, US)). ultimate, and surface analysis were run by UK Biochar Research Centre (School of ® Geosciences,Raman spectroscopicallyUniversity of Edinburgh, analysis was Kings carried Buildings, out using Edinburgh, Renishaw EH9Ramanscope 3JN Scotland, InVia UK) (H43662 and reportedmodel, Renishaw, in the datasheets Gloucestershire, of the biochars UK). [15]. Analysis showed the high reproducibility of the materials.To prepare composites, biochars were mechanically pulverized and subsequently dispersed into the epoxyThe morphology monomer using of biochars a tip ultrasonicator was investigated apparatus using (Sonics a Field Vibra-cell) Emission for 15 Scanning min. Electron MicroscopeParticle (FE-SEM, size distribution Zeis SupraTM was evaluated40). using a laser granulometry (Fritsch Analysette 22, Idar-Oberstein,Biochars were Germany) also analyzed after dispersion in the range in ethanolfrom 500 and cm sonication−1 to 4000 cm in−1 an using ultrasonic a Fourier bath transformed for 10 min. infraredUltrasonication (FT-IR) spectrometer has been proven (Nicolet as a5700, very eThffectiveermoscientific, method to Waltham, realize well MA, dispersed US) operated suspensions in attenuatedof CNTs in total water reflectance [17] or polymeric (ATR) equipped media with [18]. a Despitediamond the window good performances,(Smartorbit, Thermoscientific, ultrasonication Waltham,could induce MA, disruptive US)). phenomena due to ultracavitation effect with localized hot spot up to a thousandRaman◦C[ spectroscopically19]. In order to avoid analysis the was instantaneous carried out temperature using Renishaw rise, ultrasounds® Ramanscope were InVia pulsed (H43662 with model,cycles ofRenishaw, 20 s alternating Gloucestershire, to pause UK). of 10 s to allow a better heat diffusion. After the addition of the curingTo agent, prepare the composites, mixture was biochars further ultrasonicatedwere mechanically for 2 pulverized min and left and into subsequently the moulds for dispersed 16 h at room into thetemperature. epoxy monomer A final using thermal a tip curing ultrasonicator was performed apparatus using (Sonics a ventilated Vibra-cell) oven (I.S.C.O. for 15 min. Srl “The scientific manufacturer”)Particle size at distribution 70 ◦C for 6 was h. evaluated using a laser granulometry (Fritsch Analysette 22, Idar- Oberstein,Biochar Germany) containing after composites dispersion werein ethanol produced and sonication in dog-bone in an shape ultrasonic according bath withfor 10 ASTM min. 638 procedure.Ultrasonication Samples has were been tested proven using as aa MTSvery effect machineive method (MTS Q-test10). to realizeData well dispersed were analyzed suspensions using a ® ofhomemade CNTs in water software [17] compiled or polymeric using media Matlab [18].. AverageDespite the values good of performances, at least three samples ultrasonication were reported. could induce disruptive phenomena due to ultracavitation effect with localized hot spot up to a thousand 3. Results °C [19]. In order to avoid the instantaneous temperature rise, ultrasounds were pulsed with cycles of 203.1. s alternating Biochars Characterization to pause of 10 s to allow a better heat diffusion. After the addition of the curing agent, the mixture was further ultrasonicated for 2 min and left into the moulds for 16 h at room temperature.A summary A final of the thermal biochars curing physiochemical was performed properties using of interest a ventilated in the present oven research(I.S.C.O. is Srl reported “The scientificin Table2 .manufacturer”) at 70 °C for 6 h. Biochar containing composites were produced in dog-bone shape according with ASTM 638 procedure. Samples were tested using a MTS machine (MTS Q-test10). Data were analyzed using a homemade software compiled using Matlab®. Average values of at least three samples were reported.

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Table 2. Main chemical and physical properties of biochar samples.

Proximate Analysis (wt.%) Ultimate Analysis (%) Specific Surface ID Moisture Ash VOCs Fixed Carbon a C HN O b Area (m2/g) MSP 1.83 12.15 11.62 74.40 75.41 2.42 0.78 9.24 33.6 WSP 1.68 10.55 21.25 66.52 68.26 2.10 1.39 6.92 26.4 RH 1.54 47.93 7.48 43.05 48.49 1.24 1.04 2.47 20.1 SWP 1.52 1.25 14.20 83.03 85.53 2.77 0.10 10.36 26.4 OSR 2.61 19.50 16.38 61.51 68.85 1.82 1.59 8.91 7.3 a Calculated as difference, b calculated as difference considering the ash content.

Using the same HTT and similar residence times, the differences in the properties of biochars are due to the differences of the feedstocks [20] as clearly evidenced by TGA, ultimate and proximate analysis [14]. Rice husk (RH) showed a remarkably high ash content, up to 47.93 wt.%, with the ash mainly composed by silica [21]. Oil seed rape (OSR) ash content was particularly high (19.50 wt.%) according to Orlovious et al. [22], while Miscanthus straw (MSP) and wheat straw (WSP) ash content close to 10 wt.%. Mixed softwood (SWP) ash content was the lowest: 1.25 wt.%. Carbon percentages of WSP and OSR were similar (68.26 wt.% and 68.86 wt.%, respectively), while higher carbon contents were found for MSP and SWP (75.41 wt.% and 85.53 wt.%, respectively). In agreement with its high ash content, RH showed a low carbon content: 48.49 wt.%. Nitrogen content was generally between up to 1 wt.% with the exception of MSP and SWP that showed very low values (0.78 wt.% and 0.10 wt.%, respectively). Specific surface area values of RH, WSP, and RH were all close of up to 20 m2/g while MSP and OSR had higher and lower values, respectively (33.6 and 7.3 m2/g, respectively). The trend of surface area values can be understood looking at the FE-SEM captures shown in Figure2. According to Figure2c,d, OSR showed the presence of pores and quasi-collapsed channels filled with small carbon particles (average size below 1 µm) that sensibly reduced the overall available area. MSP showed a similar behavior but, as clearly reported in Figure2b, the 3 to 4 µm wide channels were generally empty and filled with particles only after damage occurred. Open and diffuse channels were present in WSP together with small round shaped inter-channels pores. Inter-channels pores represented a proper wood structure and they became more relevant after pyrolytic treatments [9,23]. According to its softwood original structure [24], SWP showed a very clean surface with isolated pores having an average diameter of 2 µm (Figure2g). RH showed a very interesting morphology with the presence of an external core covered by spikes (Figure2e) enveloping a honeycomb structure (Figure2f) with an average size around 1 µm and wall thickness around 700 nm. Appl. Sci. 2019, 9, 3109 5 of 13

Figure 2. FE-SEM captures of (a,b) Miscanthus straw (MSP), (c,d) oil seed rape (OSR), (e,f) rice husk (RH), (g,h) mixed softwood (SWP), and (i,j) wheat straw (WSP). Materials 2019, 12, x FOR PEER REVIEW 6 of 13

According to Figure 2c,d, OSR showed the presence of pores and quasi-collapsed channels filled with small carbon particles (average size below 1 μm) that sensibly reduced the overall available area. MSP showed a similar behavior but, as clearly reported in Figure 2b, the 3 to 4 μm wide channels were generally empty and filled with particles only after damage occurred. Open and diffuse channels were present in WSP together with small round shaped inter-channels pores. Inter-channels pores represented a proper wood structure and they became more relevant after pyrolytic treatments [9,23]. According to its softwood original structure [24], SWP showed a very clean surface with isolated pores having an average diameter of 2 μm (Figure 2g). RH showed a very interesting morphology with the presence of an external core covered by spikes (Figure 2e) enveloping a Appl.honeycomb Sci. 2019, 9structure, 3109 (Figure 2f) with an average size around 1 μm and wall thickness around6 of700 13 nm. Biochars were also analyzed by FT-IR in ATR mode and spectra were reported in Figure 3. Biochars were also analyzed by FT-IR in ATR mode and spectra were reported in Figure3.

Figure 3. FT-IR attenuated total reﬂectance (ATR) mode spectra of (a) RH, (b) MSP, (c) SWP, (d) OSR, Figure 3. FT-IR attenuated total reflectance 1(ATR) mode spectra1 of (a) RH, (b) MSP, (c) SWP, (d) OSR, and (e) WSP in the region between 500 cm− to 3500 cm− . and (e) WSP in the region between 500 cm−1 to 3500 cm−1. 1 1 νO–H (3000–3500 cm− ) and νC–H (2850–2900 cm− ) bands were not detected in the samples in ν −1 ν −1 agreementO–H (3000–3500 with the low cm amount) and C–H of hydrogen(2850–2900 and cm oxygen) bands (Table were2). not The detected spectral in region the samples between in 1 1 1000agreement cm− and with 2000 the cm low− providesamount of more hydrogen detailed and information oxygen (Table about the2). residualThe spectral functionalities region between of the −1 2000 -1 1 biochars.1000 cm Alland spectra cm evidencedprovides a broad more weakdetailed band information around 1690 abou cmt− the(ν residualC=O) witnessing functionalities the presence of the −1 (ν 1 ofbiochars. residual All ketonic spectra groups evidenced [25]. Alla broad spectra weak showed band aaround medium 1690 intense cm νCC=O= C) witnessingband around the 1580 presence cm− due to aromatic structures embedded in the carbonaceous matrix. The band around 1370 cm 1 (δ ) − O–H was present in all samples but it was weaker in the RH spectrum (Figure2e). A very intense envelop of 1 1 bands can be observed between 1035 cm− and 1070 cm− due to several vibrational modes such as δC–O, νC–O. This region was particularly intense in the RH spectrum probably due to the massive presence 1 of silica which presents an active IR mode (asymmetrical νSi–O–Si) in the region 1020–1110 cm− [26]. Silica is well-known component of rice husk derived biochar as reported by many authors [27–30]. Furthermore, the great amount of silica could be imputable of low intense of RH δO–H band. This was due to –OH reduced mobility induced by strong interactions with silica groups. 1 1 In Figure4, the Raman spectra of the biochars used in the range between 500 cm − to 3500 cm− are reported. Materials 2019, 12, x FOR PEER REVIEW 7 of 13 of residual ketonic groups [25]. All spectra showed a medium intense νC = C band around 1580 cm−1 due to aromatic structures embedded in the carbonaceous matrix. The band around 1370 cm−1 (δO–H) was present in all samples but it was weaker in the RH spectrum (Figure 2e). A very intense envelop of bands can be observed between 1035 cm−1 and 1070 cm−1 due to several vibrational modes such as δC–O, νC–O. This region was particularly intense in the RH spectrum probably due to the massive presence of silica which presents an active IR mode (asymmetrical νSi–O–Si) in the region 1020–1110 cm−1 [26]. Silica is well-known component of rice husk derived biochar as reported by many authors [27–30]. Furthermore, the great amount of silica could be imputable of low intense of RH δO–H band. This was due to –OH reduced mobility induced by strong interactions with silica groups. In Figure 4, the Raman spectra of the biochars used in the range between 500 cm−1 to 3500 cm−1 Appl.are reported. Sci. 2019, 9, 3109 7 of 13

Figure 4. 1 1 Figure 4.Raman Raman spectra spectra of of biochars biochars in in the the range range between between 500 500 cm cm− −1to to 3500 3500 cm cm−−1. All the spectra showed two main peaks in the range 1000 to 2000 cm 1 currently named as D All the spectra showed two main peaks in the range 1000 to 2000 cm−−1 currently named as D (centered around 1360 cm 1) and G (centered around 1590 cm 1)[31]. Less intense peaks were observed (centered around 1360 −cm−1) and G (centered around 1590− cm−1) [31]. Less intense peaks were in the range between 2400 to 3300 cm 1 due to the overtones of the D and G peaks. According to observed in the range between 2400 to −3300 cm−1 due to the overtones of the D and G peaks. According Ferrari et al. [32], the ratio between the area of ID and IG peak could be used to evaluate the order of to Ferrari et al.[32], the ratio between the area of ID and IG peak could be used to evaluate the order of carbonaceous structures. In all the observed spectra, ID/IG ratios were close to each other, ranging from carbonaceous structures. In all the observed spectra, ID/IG ratios were close to each other, ranging 1.0 (SWP) to 1.1 (RH). This was strong proof that the carbonaceous materials are equally disordered from 1.0 (SWP) to 1.1 (RH). This was strong proof that the carbonaceous materials are equally after pyrolysis. Temperature and heating rate adopted in each case were the optimum to induce the disordered after pyrolysis. Temperature and heating rate adopted in each case were the optimum to same level of molecular order in the biochar produced. induce the same level of molecular order in the biochar produced. Before dispersion in polymeric matrix, biochars were mechanically grinded for 10 min and particle Before dispersion in polymeric matrix, biochars were mechanically grinded for 10 min and size distribution were analyzed. The results are reported in Figure5. particle size distribution were analyzed. The results are reported in Figure 5. From these results, OSR showed a particle size distribution with a large distribution centered at well-around 10 µm. RH, WSP, and MSP showed the same peak at 10 µm together with a peak at a higher dimension ranging from around 50 µm to more than 100 µm. SWP showed great particle size with a tailored peak centered at 100 µm. The diﬀerent particle distributions were a consequence of grindability of the feedstock. Considering the residual group detected in Figure4, such distributions were due to the diﬀerent morphologies formed after pyrolytic treatments [33]. According to these observations, it is possible to assume that the SWP particles were hardly grindable because of their smooth surfaces. As highlighted in Figure2e, f, SWP did not show a channeled and porous surface as all the other materials. Accordingly, it is reasonable to assume that pore and channel walls are more fragile than smooth surfaces and undergo a more relevant disruption during the grinding process. Appl. Sci. 2019, 9, 3109 8 of 13 Materials 2019, 12, x FOR PEER REVIEW 8 of 13

Figure 5. BiocharBiochar particles particles size size distribution distribution of of(a) (RH,a) RH, (b) (MSP,b) MSP, (c) SWP, (c) SWP, (d) OSR, (d) OSR, and and(e) WSP (e) WSPafter aftergrinding. grinding.

3.2. MechanicalFrom these Properties results, OSR of the showed Biochar-Based a particle Composites size distribution with a large distribution centered at well-aroundAddition 10 of μm. nanostructured RH, WSP, and carbon MSP showed to an epoxy the same matrix peak involves at 10 μm some together issues with related a peak to at the a dispersionhigher dimension [34]. As rang an example,ing from around good dispersion 50 μm to ofmore graphene than 100 was μm. possible SWP showed only after great derivatization particle size aswith graphene a tailored oxide peak or centered reducing at its 100 concentration μm. The different [35,36]. particle In this research,distributions composites were a consequence were prepared of dispersinggrindability biochars of the feedstock. into epoxy Considering monomer the using residual ultrasonication group detected up to in 2 Figure wt.%. 4, According such distributions to Khan etwere al. [due10], 2to wt.% the different resulted themorphologies best percentage formed to exploitafter pyrolytic the beneficial treatments properties [33]. ofAccording biochar fillers. to these observations,Dog-bone it samplesis possible were to assume produced that according the SWP particles to the methods were hardly described grindable above because using of a fillertheir concentrationsmooth surfaces. of 2As wt.% highlighted and analyzed in Figure through 2e, f, stress SWP testdid producingnot show a the channeled curves reported and porous in Figure surface6 whileas all the mainother mechanicalmaterials. Accordingly, properties are it summarized is reasonable in to Figure assume7. that pore and channel walls are moreNeat fragile resin than has smooth a Young’s surfaces modulus and (YM)undergo of 1286 a more185 relevant MPa, an disruption ultimate tensile during strength the grinding (UTS) ± ofprocess. 17.0 1.4 MPa, a maximum elongation of 4.08 0.08%, and a toughness of 0.565 0.029 MJ/m3. ± ± ± Composites showed two different behaviors leading either to the simultaneous increment of UTS and YM,3.2. Mechanical or to the enhancement Properties of the of Biochar-Based maximum elongation Composites with lesser impact on YM and UTS. OSR and WSP induced a drastic increase of YM (up to 2731 113 MPa and 3074 137 MPa, respectively) and Addition of nanostructured carbon to an epoxy± matrix involves± some issues related to the UTSdispersion up to around[34]. As 33 an MPa. example, Compared good dispersion with these lastof graphene two samples, was MSPpossible and only SWP after showed derivatization lower YM (1669 89 MPa and 1456 79 MPa, respectively) and UTS did not significantly differ from neat resin. as graphene± oxide or reducing± its concentration [35,36]. In this research, composites were prepared Didispersingfferently, biochars MSP and into SWP epoxy induced monomer a maximum using elongation ultrasonication increment up to of 2 up wt.%. to 1.8% According compared to withKhan the et neat resin, while OSR and WSP decreased it to 2.46 0.08% and 1.90 0.44%, respectively. Compared al. [10], 2 wt.% resulted the best percentage to expl±oit the beneficial ±properties of biochar fillers. to theDog-bone properties samples of OSR orwere WSP produced composites, according RH composite to the showed methods lower described improvement above ofusing YM ofa upfiller to concentration of 2 wt.% and analyzed through stress test producing the curves reported in Figure 6 while the main mechanical properties are summarized in Figure 7.

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Neat resin has a Young’s modulus (YM) of 1286 ± 185 MPa, an ultimate tensile strength (UTS) of 17.0 ± 1.4 MPa, a maximum elongation of 4.08 ± 0.08%, and a toughness of 0.565 ± 0.029 MJ/m3. Composites showed two different behaviors leading either to the simultaneous increment of UTS and YM, or to the enhancement of maximum elongation with lesser impact on YM and UTS. OSR and Appl.WSP Sci. induced2019, 9, 3109a drastic increase of YM (up to 2731 ± 113 MPa and 3074 ± 137 MPa, respectively)9 ofand 13 UTS up to around 33 MPa. Compared with these last two samples, MSP and SWP showed lower YM (1669 ± 89 MPa and 1456 ± 79 MPa, respectively) and UTS did not significantly differ from neat resin. 2166 129 MPa with a comparable maximum elongation of 2.70 0.14%. According with data reported Differently,± MSP and SWP induced a maximum elongation increment± of up to 1.8% compared with inthe Table neat2, resin, it is possible while toOSR regroup and WSP the composites decreased in it brittleto 2.46 (OSR, ± 0.08% WSP, and and 1.90 RH) characterized± 0.44%, respectively. by both highCompared YM and to UTSthe properties and ductile of (MSP OSR andor WSP SWP) comp withosites, improved RH composite elongation. showed These lower different improvement behaviors couldof YM be of ascribed up to 2166 to di ±ff 129erent MPa interactions with a comparable between fillers maximum and epoxy elongation matrix. Accordingof 2.70 ± 0.14%. to FT-IR According spectra (Figurewith data3), allreported biochars in Table showed 2, it similar is possible residual to regroup groups, the but composites looking at in FES-SEM brittle (OSR, images WSP, (Figure and RH)2), wecharacterized found clear by diff botherences high among YM and the variousUTS and fillers. ductile Furthermore, (MSP and theSWP) particle with sizeimproved distributions elongation. were quiteThese close different to each behaviors other, withcould exception be ascribed of to RH, diff toerent induce interactions such diff betweenerent behaviors. fillers and Based epoxy on matrix. such evidences,According weto hypothesizedFT-IR spectra the(Figure following 3), all explanationbiochars showed for the similar impact residual of the fillers groups, on the but mechanical looking at propertiesFES-SEM images of the composites. (Figure 2), Consideringwe found clear SWP differenc and MSP,es among neat or the slightly various channeled fillers. Furthermore, surface particles the (Figureparticle1a,b,g,h) size distributions could move were together quite with close the resinto each under other, longitudinal with exception stress. Thisof RH, could to induce induce plane such rearrangementdifferent behaviors. and a Based quasi-laminar on such flowevidences, with an we increase hypothesized in ductility. the following On the other explanation hand, OSR for and the WSPimpact showed of the thefillers massive on the presencemechanical of properties pores that of can the act composites. as anchoring Considering sites forthe SWP polymer and MSP, matrix neat creatingor slightly high channeled tensioned surface structures. particles As a consequence,(Figure 1a,b,g,h) these could materials move showed together lower with yield the resin points under and plasticlongitudinal deformation stress. This range could prior induce to the plane breakage rearrangement than neat resin. and a Composites quasi-laminar were flow analyzed with anthrough increase FE-SEMin ductility. after On fracture the other as shown hand, inOSR Figure and 8WSP. MSP showed containing the massive composites presence underwent of pores a delaminationthat can act as processanchoring after sites the for break. the polymer In this case, matrix resin creating was detached high tensioned from the structures. channel structures As a consequence, (Figure8a,b these in whitematerials circles). showed OSR lower based yield composites points showedand plastic a di ffdeerentformation behavior. range As prior shown to inthe Figure breakage8c,d, thethan resin neat entirelyresin. Composites filled the OSR were porous analyzed structure through showing FE-SEM a remarkably after fracture continuity as onshown the interphases.in Figure 8. In MSP this case,containing cracks propagatedcomposites themselvesunderwent all a arounddelamination the particles process while after using the MSPbreak. they In movedthis case, preferentially resin was alongdetached the channeledfrom the channel structures. structures (Figure 8a,b in white circles). OSR based composites showed a differentAdditional behavior. considerations As shown werein Figure required 8c,d, in the the caseresin of entirely RH based filled composites the OSR thatporous showed structure both planeshowing surfaces a remarkably and spikes continuity and pores. on This the interphases. combination In was this detrimental case, cracks for propagated the ductility themselves and this fact all suggestsaround the the particles more relevant while roleusing of MSP brittleness they moved inducing preferentially elements on along the ductility the channeled inducing structures. ones.

FigureFigure 6.6.Stress Stress versus versus strain strain curves curves of of biochar-based biochar-based composites composites ﬁlled filled with with 2 wt.% 2 wt.% of (a )of WSP, (a) WSP, (b) OSR, (b) (OSR,c) RH, (c ()d RH,) SWP, (d) ( SWP,e) MSP, (e) and MSP, of (andf) neat of ( resin.f) neat resin.

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Figure 7. (a) Young’s modulus, (b) toughness, (c) ultimate tensile strength, and (d) maximum elongation of biocharFigure 7. containing (a) Young’s composites modulus, and(b) neattoughness, resin. ( Datac) ultimate were reportedtensile strength, as average and values (d) maximum of at least threeelongation runs. of biochar containing composites and neat resin. Data were reported as average values of at least three runs.

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Figure 8. FE-SEM captures of composites containing (a,b) MSP and (c,d) WSP after the fracture. Figure 8. FE-SEM captures of composites containing (a,b) MSP and (c,d) WSP after the fracture. FE- FE-SEM. Interphases between biochar particles and resin are highlighted with white circles. SEM. Interphases between biochar particles and resin are highlighted with white circles. 4. Conclusions Additional considerations were required in the case of RH based composites that showed both In this research, the properties of five different commercial biochars were analyzed and their plane surfaces and spikes and pores. This combination was detrimental for the ductility and this fact main morphological aspects were reported. Biochar were dispersed into an epoxy matrix through suggests the more relevant role of brittleness inducing elements on the ductility inducing ones. an ultrasonication method and mechanical properties were tested. MSP and SWP showed the best 4.elongation Conclusions performances with appreciable improvement of the ductility compared with neat resin. OSR, WSP, and RH induced a drastically decrease of the ductility with a magnification of Young’s modulusIn this and research, ultimate the tensile properties strength. of five Considering different commercial the similar biochars residual groupswere analyzed detected and through their mainFT-IR morphological analysis, mechanical aspects properties were reported. could beBiochar ascribed were to dispersed the different into biochar an epoxy morphologies matrix through and notan ultrasonicationto the particle size method distribution. and mechanical Channeled properti and smoothes were surfaces tested. inducedMSP and improvement SWP showed in ductilitythe best elongationcontrary to performances high porous and with irregular appreciable surfaces improvement that promote of higherthe ductility brittleness. compared with neat resin. OSR, WSP, and RH induced a drastically decrease of the ductility with a magnification of Young’s modulusAuthor Contributions: and ultimateConceptualization: tensile strength. Considering M.B. and A.T.; the methodology: similar residual M.B., groups P.J., and detected C.R.; software: through M.R.; FT- IRformal analysis, analysis: mechanical M.B.; investigation: properties M.B.; could resources: be ascr A.T.;ibed C.R.to the data different curation: biochar M.B. and morphologies M.R.; writing—original and not draft preparation: M.B.; writing—review and editing, M.G. and A.T.; supervision: A.T.; project administration: toA.T.; the funding particle acquisition: size distribution. A.T. Channeled and smooth surfaces induced improvement in ductility contrary to high porous and irregular surfaces that promote higher brittleness. Acknowledgments: Authors acknowledge Andrea Marchisio’s help in the particles distribution analysis, and Salvatore Guastella and Mauro Raimondo’s help in FE-SEM analysis. Author Contributions: Conceptualization: M.B. and A.T.; methodology: M.B., P.J., and C.R.; software: M.R.; Conflicts of Interest: The authors declare no conflict of interest. formal analysis: M.B.; investigation: M.B.; resources: A.T.; C.R. data curation: M.B. and M.R.; writing—original draft preparation: M.B.; writing—review and editing, M.G. and A.T.; supervision: A.T.; project administration: A.T.;References funding acquisition: A.T.

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