Preparation and Characterization of Eco-Friendly Spent Coffee/ENR50 Biocomposite in Comparison to Carbon Black

polymers

Article Preparation and Characterization of Eco-Friendly Spent Coffee/ENR50 Biocomposite in Comparison to Carbon Black

Gunasunderi Raju 1,2,*, Mohammad Khalid 2,3 , Mahmoud M. Shaban 4 and Baharin Azahari 5

1 School of Distance Education, Universiti Sains Malaysia, 11800 Penang, Malaysia 2 Graphene & Advanced 2D Materials Research Group (GAMRG), School of Engineering and Technology, Sunway University, 47500 Petaling Jaya, Selangor, Malaysia; [email protected] 3 Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia 4 Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt; [email protected] 5 School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia; [email protected] * Correspondence: [email protected]

Abstract: This study investigates the impact of spent coffee biochar (Biochar) compared to carbon black (CB) as a partial replacement for carbon black in epoxidized natural rubber (ENR). Particle size and elemental analysis were used to characterize the biochar and CB. Cure characteristics, tensile, thermal, and morphological properties on the effect of biochar and CB as filler were studied. It was found that incorporating 10 phr of spent coffee biochar could improve the composites’ tensile properties and thermal performance compared to carbon black. However, the addition of biochar significantly affects the maximum torque compared to CB and delays the vulcanization time. SEM study shows that biochar has a strong effect on the morphology of composite films. The FTIR graph reveals no substantial difference between compounds with biochar and CB. According to the thermal calorimetric study, the thermal stability of ENR-Biochar is higher than that of ENR-CB. Additionally, Citation: Raju, G.; Khalid, M.; these findings suggest that the utilization of spent coffee as a sustainable biochar could be further Shaban, M.M.; Azahari, B. Preparation and Characterization of explored, but little has been done in epoxidized natural rubber (ENR). Eco-Friendly Spent Coffee/ENR50 Biocomposite in Comparison to Keywords: biochar; spent coffee; carbon black; epoxidized natural rubber Carbon Black. Polymers 2021, 13, 2796. https://doi.org/10.3390/ polym13162796 1. Introduction Academic Editor: Yung-Sheng Yen Rubber composites have been investigated using renewable, nonpetroleum-based filler sources such as kenaf [1], chitosan [2], starch [3–5], soy proteins [6], and other fibre [7–11]. Received: 30 July 2021 Even though some of these materials have shown potential reinforcing capabilities, the Accepted: 17 August 2021 industry appears hesitant to experiment with filler materials that are fundamentally dif- Published: 20 August 2021 ferent from carbon black (CB) in terms of appearance and source. Global demand for petroleum products is increasing, making it vital to investigate renewable, alternative fuels Publisher’s Note: MDPI stays neutral and other materials to replace them if global petroleum resources run out. Carbon black with regard to jurisdictional claims in is a petroleum product that dominates the rubber composite filler market due to its low published maps and institutional affil- cost [12]. Because of its outstanding reinforcing characteristics, low cost, and ability to be iations. supplied in a highly pure and wide range of agglomerate sizes, carbon black, a petroleum product, has long been the standard filler for rubber composites (with the main market being the tyre industry) [13]. Biochar has recently been considered a possible replacement for carbon black additives in polymeric composites [14–16]. The coffee industry globally Copyright: © 2021 by the authors. produced enormous coffee residues from manufacturing coffee beverages and instant Licensee MDPI, Basel, Switzerland. coffee preparation, with global output estimated at 168.87 million bags (1 bag = 60 kg) This article is an open access article in 2018–2019 [17]. Instead of being thrown away, the coffee residues could be utilized distributed under the terms and for a variety of purposes. New ideas on exploiting this residue and other renewable conditions of the Creative Commons resources should be welcomed as a step toward a more sustainable future. Biochar is a Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ renewable, non-petroleum-based filler substance that resembles CB in appearance. Biochar 4.0/). is carbon-containing charcoal generated by pyrolysis (thermal treatment in an oxygen-free

Polymers 2021, 13, 2796. https://doi.org/10.3390/polym13162796 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 12

sources should be welcomed as a step toward a more sustainable future. Biochar is a re‐ Polymers 2021, 13, 2796 2 of 11 newable, non‐petroleum‐based filler substance that resembles CB in appearance. Biochar is carbon‐containing charcoal generated by pyrolysis (thermal treatment in an oxygen‐ free atmosphere) from various carbon‐containing biomass feedstocks, including hard‐ woods,atmosphere) crop residues, from various and poultry carbon-containing litter [18]. We biomass have recently feedstocks, looked including at using hardwoods, biochar as acrop filler residues, in rubber and composites poultry litter[19–22]. [18 ].ENR We is have a derivative recently lookedof natural at usingrubber biochar which ashas a two filler re in‐ activerubber sites composites that are unsaturated [19–22]. ENR isoprene is a derivative and epoxidized of natural isoprene rubber units. which Biocomposites has two reactive with goodsites biodegradable that are unsaturated properties isoprene can be andderived epoxidized from further isoprene modification units. Biocomposites of ENR [23–26]. with goodThis biodegradable study aims propertiesto see if pyrolysis can be derived can be used from to further convert modification spent coffee of into ENR low [23‐–cost26]. activatedThis carbon. study aims The tofeasibility see if pyrolysis of preparing can be spent used tocoffee convert activated spent coffeecarbon into by physical low-cost activationactivated carbon.was examined The feasibility in this study. of preparing The viability spent of coffee natural activated rubber carbon composites by physical made fromactivation fully renewable was examined fillers in such this as study. biochar The of viability spent coffee of natural (Biochar) rubber and composites epoxidized made nat‐ uralfrom rubber fully renewable (ENR) was fillers investigated such as biochar with the of spent intention coffee to (Biochar) evaluate and its potential epoxidized as natural a sus‐ tainablerubber (ENR) and renewable was investigated substitute with filler the for intention carbon toblack evaluate in rubber. its potential as a sustainable and renewable substitute filler for carbon black in rubber. 2. Materials and Method 2. Materials and Method Epoxidized natural rubber with about 50% epoxy content (designated as ENR50) and Epoxidized natural rubber with about 50% epoxy content (designated as ENR50) and with relative Mw of 3.8 × 105 g/mol was supplied by the Malaysian Rubber Board, Kuala with relative Mw of 3.8 × 105 g/mol was supplied by the Malaysian Rubber Board, Kuala Lumpur, Malaysia. The actual epoxy content was determined using Bruker Avance‐400 Lumpur, Malaysia. The actual epoxy content was determined using Bruker Avance-400 NMR spectrometer and was found to be 51.05%. The spent coffee ground was obtained NMR spectrometer and was found to be 51.05%. The spent coffee ground was obtained from Starbucks, Gelugor Penang. The other compounding ingredients, such as sulphur, from Starbucks, Gelugor Penang. The other compounding ingredients, such as sulphur, zinc oxide, Stearic acid, sulfenamide (CBS), and carbon black N330, were purchased from zinc oxide, Stearic acid, sulfenamide (CBS), and carbon black N330, were purchased from Bayer Ltd. (Penang, Malaysia) and used as received. Bayer Ltd. (Penang, Malaysia) and used as received.

2.1.2.1. Preparation Preparation of of Spent Spent Coffee Coffee Ground Ground and and its Its Derivatives Derivatives SpentSpent coffee coffee grounds grounds obtained obtained from from the the local local coffee coffee shops shops were were washed washed thoroughly thoroughly withwith water water to to remove remove the the impurities, impurities, dried dried in in sunlight sunlight for for two two days, days, oven oven-dried‐dried at at 80 80 °C◦C forfor 24 24 h h and and labelled labelled as as SC SC (Figure (Figure 11a)a) beforebefore beingbeing storedstored inin sealedsealed bags.bags. The drieddried SC waswas carbonized carbonized at at 500 500 °C◦C for for 30 30 min min in in a a furnace furnace and and cooled cooled to to room room temperature temperature once once activationactivation was was complete. complete. The The samples samples were were labelled labelled as as Biochar Biochar Figure Figure 1b1b and kept in aa desiccatordesiccator until until further further use. use.

FigureFigure 1. 1. ((aa)) spent spent coffee; coffee; ( (bb)) biochar. biochar.

2.2. Preparation of Spent Coffee Ground and its Derivatives—ENR Biocomposites 2.2. Preparation of Spent Coffee Ground and Its Derivatives—ENR Biocomposites The preparation of ENR composite was carried out on a laboratory size 12 inches × 6 The preparation of ENR composite was carried out on a laboratory size 12 inches inches× 6 inches two‐roll two-roll mill. Mixing mill. Mixing was done was in done accordance in accordance with ASTM with D3184 ASTM‐80. D3184-80. ENR was ENRfirst masticatedwas ﬁrst masticated for 1 min, forand 1 filler min, was and then ﬁller incorporated was then incorporated into the mixture into the with mixture continued with mixing.continued The mixing. curing agents The curing were then agents sequentially were then added sequentially and mixed. added The and compounding mixed. The compounding process was kept constant for approximately 30 min for all formulations using the formulation given in Table1. The biocomposites obtained are designated herein as ENR-Biochar. Carbon black sample was prepared similarly, as mentioned above and designated herein as ENR-CB. Polymers 2021, 13, 2796 3 of 11

Table 1. Formulation of epoxidized natural rubber biocomposites.

Materials Parts per Hundred of Rubber (phr) ENR50 100 100 Zinc Oxide 3 3 Stearic Acid 2 2 Biochar 0, 2.5, 5, 10, 15, 20 - Carbon Black (N330) - 0, 2.5, 5, 10, 15, 20 CBS 1 1 Sulfur 1.5 1.5

2.3. Chemical and Physical Material Properties Cure characteristics of the mixes at 150 ◦C were determined using a Monsanto Rheome- ter model MDR 2000 according to ASTM D5289-95. Then, the compounds were vulcanized with a heated compression moulding machine at 150 ◦C at their respective cure times. Particle size analysis was performed on a MALVERN Zetasizer Ver. 7.11 (MAL 1029406, Germany) using light scattering on aqueous sample suspensions with a particle size detection limit of 0.6 nm–6000 nm. The measurements were performed in triplicate. Elemental analysis of carbon (C) and hydrogen (H) was carried out using a Perkin Elmer 2400 CHNS/O series II analyzer using acetanilide as standard. Approximately 2 mg of samples were used for each measurement and was done in triplicate. Perkin Elmer System 2000 was used to capture Fourier transform infrared spectra (FT-IR spectra with 16 scans at 4 cm−1 resolutions. The Attenuated Total Reflection (ATR) method was used to perform FT-IR analysis on ENR and ENR biocomposites in the wavenumber range of 4000–650 cm−1. X-ray diffraction (XRD) spectra were recorded using the Siemens D5000 diffractometer in step scan mode using Ni-filtered Cu Kα radiation (0.1542 nm wavelength). The moulded composites were scanned in transmission mode in the angle interval of 2θ = 1–12◦. The Perkin-Elmer TGA-7 thermogravimetric analyzer was used to assess thermal weight loss. The tests were conducted using a stream of dry nitrogen gas (flow rate = 30 mL/min) at temperatures ranging from 40 to 600 ◦C and a heating rate of 10 ◦C/min. The weight of the samples utilized was between 4 and 10 mg. The FEI Quanta 650 FEG Scanning Electron Microscopy was used to examine the tensile-fractured surfaces of biocomposites. All the samples were gold coated to avoid electrostatic charging during the examination.

2.4. Tensile Testing Tensile characteristics were measured using dumbbell-shaped specimens at ambient temperature using an Instron 3366 computerized tensile tester with a 10 kN load cell, according to ASTM D412-93. The Instron machine’s crosshead speed was set at 500 mm/min.

3. Results and Discussion 3.1. Particle Size Analysis Table2 shows the particle size information for various fillers made from spent coffee, biochar, and carbon black, as well as representative SEM images utilized in the analysis. The particle size of biochar after carbonization is considerably smaller than that of spent coffee agglomerates. The biochar particles have a granular, irregular—smooth, and angulated form to them. One of the most significant features in predicting the function of a particular filler in transmitting rubber characteristics is its average particle size. The higher the degree of reinforcement provided to the vulcanizate, the smaller the average particle size of a filler. Filler reinforcing effects on rubber are known to be influenced by three factors: particle size, surface activity, and particle structure [19]. Smaller particles have a greater capacity to absorb rubber molecules, increasing the filler-rubber interfaces and the performance of rubber compounds, which is consistent with our mechanical characteristics that will be discussed. The results of the elemental compositions of the CB and biochar Polymers 2021, 13, 2796 4 of 11

used are presented in Table2. Biochar contained 59.56% carbon and 6.61% hydrogen, with a 30% mass loss, whereas CB had 62.25% carbon and 0.5% hydrogen, along with other components, and a 37.25% mass loss. Pyrolysis resulted in a similar conversion of spent coffee into biochar with a carbon percentage identical to the CB. This is because the lignin content has a value on the order of spent coffee > biochar. Consequently, the study indicated that the higher the lignin level, the greater the carbon content [27]. Because of the high carbon and low ash content, the spent coffee was found to be appropriate for the preparation of carbon black.

Table 2. Particle size analysis and elemental analysis results of the ﬁllers.

Standard % Sample Particle Size (µm) Deviation (µm) CHN SC 0.211 0.171 39.06 1.63 3.21 BioChar 0.081 0.034 59.56 6.61 2.13 CB 0.036 0.012 62.25 0.5 0.45

3.2. Cure Characteristics and Tensile Properties The effect of CB and biochar in the cure characteristics of the ENR matrix is shown in Figures2 and3, respectively. The maximum torque (MH) of the ENR filled rubber composites has steadily increased as the biochar and carbon black loading has increased. The higher MH indicates that the overall network structure, including filler networks, filler-rubber networks, and rubber networks, has increased. This is owing to the greater torque provided by the crosslink density with ENR. The tiny particle size and large specific surface area of carbon black in rubber filled with CB resulted in improved interaction between the filler and matrix, limiting the mobility of rubber molecules and resulting in higher maximum torque than activated carbon-filled rubber. As the crosslinking reaction progressed, the stiffness of rubber mixtures filled with biochar increased rapidly. The highest increase in torque gain during vulcanization was found for composites filled with biochar, which could be associated with an increased tendency for interactions [19]. The cure time (t90) of biochar and CB-filled epoxidized natural rubber compounds decreased with the increasing filler amount. The short cure time of ENR-Biochar and ENR-CB compounds could be due to the extended time on the rolling mill, as more time is needed to homogenize the compound. ENR-CB compounds demonstrated a shorter cure time than Polymers 2021, 13, x FOR PEER REVIEWENR- Biochar, which could be due to the smaller particle size of CB that shortens5 of 12 the cure

time of rubber compounds [28].

8 ENR-Biochar 7.5

7 ENR-CB 6.5 6 5.5

Cure Time/min Cure 5 4.5 4 0 5 10 15 20 Loading /phr

FigureFigure 2. CureCure time time (tc90) (tc90) of ofthe the ENR, ENR, ENR ENR-Biochar‐Biochar and ENR and‐ ENR-CB.CB.

6.5

5.5

5 ENR-Biochar

4.5

Maximum Maximum Torque(MH)/dNm ENR CB 4 0 5 10 15 20 Loading/phr

Figure 3. Maximum torque of the ENR, ENR‐Biochar and ENR‐CB.

Figures 4 and 5 show the tensile strength curves and elongation at the break of EBR‐ Biochar and ENR‐CB under various loading conditions. The tensile strength properties of the biochar‐filled ENR, as shown in Figure 4, increased initially and reached an optimal value when the content was at 10 phr before decreasing as the biochar loading increased. Figure 4 shows that the composites can match the tensile strength of carbon black when loaded with 10 phr of biochar. Even at 15 phr loading, the differential between biochar and CB is minimal at around 3%. This behavior could possibly be due to the chemical interactions and hydrogen bonding between the hydroxyl and epoxide groups in ENR molecules. This property creates the potential to be commercially viable for applications that need a rubbery substance. At that concentration, composites manufactured with the SC biochar fillers would be stronger and more flexible than those made with carbon black. The reinforcing impact of the fine biochar and an increase in the crosslinking density of the films might explain the first rise in tensile strength with increased biochar loading. The aggregation of biochar, which generally includes internal voids capable of absorbing

Polymers 2021, 13, x FOR PEER REVIEW 5 of 12

8 ENR-Biochar 7.5

7 ENR-CB 6.5 6 5.5

Cure Time/min Cure 5 4.5 4 0 5 10 15 20 Loading /phr Polymers 2021, 13, 2796 5 of 11

Figure 2. Cure time (tc90) of the ENR, ENR‐Biochar and ENR‐CB.

6.5

5.5

5 ENR-Biochar

4.5

Maximum Maximum Torque(MH)/dNm ENR CB 4 0 5 10 15 20 Loading/phr

FigureFigure 3. MaximumMaximum torque torque of ofthe the ENR, ENR, ENR ENR-Biochar‐Biochar and ENR and‐CB. ENR-CB.

Figures 4 andand 55 showshow the the tensile tensile strength strength curves curves and andelongation elongation at the atbreak the of break EBR‐ of EBR- BiocharBiochar and and ENR ENR-CB‐CB under under various various loading loading conditions. conditions. The tensile The tensile strength strength properties properties of of the biochar‐filled ENR, as shown in Figure 4, increased initially and reached an optimal the biochar-filled ENR, as shown in Figure4, increased initially and reached an optimal value when the content was at 10 phr before decreasing as the biochar loading increased. value when the content was at 10 phr before decreasing as the biochar loading increased. Figure 4 shows that the composites can match the tensile strength of carbon black when Figureloaded4 with shows 10 phr that of the biochar. composites Even at can 15 phr match loading, the tensile the differential strength between of carbon biochar black when loadedand CB withis minimal 10 phr at of around biochar. 3%. EvenThis behavior at 15 phr could loading, possibly the be differential due to the betweenchemical biochar andinteractions CB is minimal and hydrogen at around bonding 3%. between This behavior the hydroxyl could and possibly epoxide be groups due toin theENR chemical interactionsmolecules. This and property hydrogen creates bonding the potential between to be the commercially hydroxyl andviable epoxide for applications groups in ENR molecules.that need a rubbery This property substance. creates At that the concentration, potential to composites be commercially manufactured viable forwith applications the thatSC biochar need a fillers rubbery would substance. be stronger At and that more concentration, flexible than those composites made with manufactured carbon black. with the SCThe biochar reinforcing fillers impact would of the be strongerfine biochar and and more an increase flexible thanin the those crosslinking made withdensity carbon of black. Thethe films reinforcing might explain impact the of first the finerise in biochar tensile andstrength an increase with increased in the crosslinkingbiochar loading. density of theThe films aggregation might of explain biochar, the which first generally rise in tensileincludes strength internal voids with capable increased of absorbing biochar loading. The aggregation of biochar, which generally includes internal voids capable of absorbing polymer and partially sheltering it from stress when the rubber matrix is deformed, might explain the decrease in tensile strength seen with large biochar loadings [29]. The ENR-CB films’ tensile strength revealed that CB’s reinforcing impact was superior to the other fillers. Due to the high activity of CB and its oxidized surface, which increased with increasing degree of crosslinking of the fillers–elastomers, the improvement in tensile strength after the addition of CB lasted across the entire range studied [30]. They hypothesized that chemical and physical linkages (weak hydrogen bonds and Van der Waals forces) formed between the rubber’s carboxylic and reactive groups on the filler surfaces. However, in the epoxidized natural rubber matrix, the combination of biochar and CB decreased the elongation at break, as seen in Figure5. The reduction is due to a significant interaction between fillers and rubber molecules, which reduced rubber vulcanizate mobility even further. Beyond 10phr loading, however, the elongation of biochar is similar to that of carbon black. Polymers 2021, 13, x FOR PEER REVIEW 6 of 12

Polymers 2021, 13, x FOR PEER REVIEW 6 of 12

polymer and partially sheltering it from stress when the rubber matrix is deformed, might explainpolymer the and decrease partially in sheltering tensile strength it from seenstress with when large the rubberbiochar matrix loadings is deformed, [29]. The mightENR‐ CBexplain films’ the tensile decrease strength in tensile revealed strength that CB’s seen reinforcing with large impactbiochar was loadings superior [29]. to The the ENRother‐ fillers.CB films’ Due tensile to the strength high activity revealed of CB that and CB’s its reinforcing oxidized surface, impact whichwas superior increased to thewith other in‐ creasingfillers. Due degree to the of high crosslinking activity of of CB the and fillers–elastomers, its oxidized surface, the whichimprovement increased in with tensile in‐ strengthcreasing afterdegree the ofaddition crosslinking of CB lasted of the across fillers–elastomers, the entire range the studied improvement [30]. They in hypoth tensile‐ esizedstrength that after chemical the addition and physical of CB lasted linkages across (weak the entire hydrogen range bondsstudied and [30]. Van They der hypoth Waals‐ forces)esized formedthat chemical between and the physical rubberʹ slinkages carboxylic (weak and hydrogenreactive groups bonds on and the Van filler der surfaces. Waals However,forces) formed in the between epoxidized the rubber naturalʹs carboxylicrubber matrix, and reactivethe combination groups on of the biochar filler surfaces. and CB decreasedHowever, thein theelongation epoxidized at break, natural as seen rubber in Figure matrix, 5. theThe combination reduction is dueof biochar to a significant and CB Polymers 2021, 13, 2796 interactiondecreased the between elongation fillers at and break, rubber as seen molecules, in Figure which 5. The reduced reduction rubber is due vulcanizate to a significant mo‐ 6 of 11 bilityinteraction even further. between Beyond fillers and10phr rubber loading, molecules, however, which the elongation reduced rubber of biochar vulcanizate is similar mo to‐ thatbility of even carbon further. black. Beyond 10phr loading, however, the elongation of biochar is similar to that of carbon black.

FigureFigure 4. TensileTensile Properties Properties of ofBiocomposites. Biocomposites. Figure 4. Tensile Properties of Biocomposites.

Figure 5. Elongation at Break of Biocomposites. FigureFigure 5. ElongationElongation at at Break Break of ofBiocomposites. Biocomposites.

3.3. Fourier Transmitted Infrared (FT-IR) Analysis of ENR Biocomposites The FT-IR spectra of the ENR-50, ENR-Biochar and ENR-CB at 10 phr loading are shown in Figure6. The characteristic vibrational absorption bands of ENR: 2961, 2923 and −1 –1 2856 cm are due to C–H stretchings of the –CH3– and –CH2– groups, 1660 cm is due –1 to the C=C stretching, 1448 and 1376 cm are due to C–H bendings of the –CH2– and –1 –CH3– groups, 1259 and 1018 cm are due to C–O–C stretching and bending of the epoxide ring, 873 cm−1 is due to C–H bending attached to the epoxy group [31]. The locations of each frequency of the biocomposites are virtually identical to those of the ENR, as shown in the FT-IR spectra. The absorption peaks of the composite in the wavenumber range 837–1180 cm−1 were similarly larger than the peaks of the ENR vulcanizate spectra. This might result from intermolecular interactions between the polar groups on the biochar and the oxirane groups, or their opened rings products in ENR where there is the possibility of forming a hydrogen bond [32]. Aside from that, because biochars and CB are virtually pure carbon materials, the FTIR spectra do not have any signiﬁcant functional peaks and are extremely similar. Polymers 2021, 13, x FOR PEER REVIEW 7 of 12

3.3. Fourier Transmitted Infrared (FT‐IR) Analysis of ENR Biocomposites The FT‐IR spectra of the ENR‐50, ENR‐Biochar and ENR‐CB at 10 phr loading are shown in Figure 6. The characteristic vibrational absorption bands of ENR: 2961, 2923 and 2856 cm−1 are due to C–H stretchings of the –CH3– and –CH2– groups, 1660 cm–1 is due to the C=C stretching, 1448 and 1376 cm–1 are due to C–H bendings of the –CH2– and –CH3– groups, 1259 and 1018 cm–1 are due to C–O–C stretching and bending of the epoxide ring, 873 cm−1 is due to C–H bending attached to the epoxy group [31]. The locations of each frequency of the biocomposites are virtually identical to those of the ENR, as shown in the FT‐IR spectra. The absorption peaks of the composite in the wavenumber range 837–1180 cm−1 were similarly larger than the peaks of the ENR vulcanizate spectra. This might result from intermolecular interactions between the polar groups on the biochar and the oxirane groups, or their opened rings products in ENR where there is the possibility of forming a Polymers 2021, 13, 2796 hydrogen bond [32]. Aside from that, because biochars and CB are virtually7 pure of 11 carbon materials, the FTIR spectra do not have any significant functional peaks and are extremely similar.

Figure 6. FTIR spectra of ENR 50, ENR-Biochar, ENR-CB at 10phr loading.

3.4. X-ray DiffractionFigure 6. FTIR Analyses spectra of of ENR ENR Biocomposites50, ENR‐Biochar, ENR‐CB at 10phr loading. The XRD patterns of the ENR-Biochar, and ENR-CB at 10 phr loading are given in Figure7. Due to3.4. it having X‐Ray Diffraction the highest Analyses improvement of ENR Biocomposites in tensile, 10 phr loading was chosen as a test specimen forThe morphologyXRD patterns of study. the ENR Therefore,‐Biochar, and it is ENR crucial‐CB at to 10 observe phr loading the filler are given in dispersion in theseFigure samples 7. Due to to understand it having the the highest tremendous improvement improvement in tensile, in 10 their phr loading properties. was chosen The epoxidizedas natural a test specimen rubber filmfor morphology exhibits typical study. amorphousTherefore, it is polymer crucial to behaviour. observe the Afiller dis‐ persion in these samples to understand the tremendous improvement in their properties. broad hump distinguishes it at an angle of 2θ ≈ 18◦. When compared to the ENR-50, the The epoxidized natural rubber film exhibits typical amorphous polymer behaviour. A diffraction peaksbroad for all hump composites distinguishes were almostit at an identical.angle of 2θ With ≈ 18°. increasing When compared filler loading, to the ENR the ‐50, the peaks related todiffraction this form ofpeaks composite for all composites become stronger. were almost This identical. demonstrates With increasing that increasing filler loading, the filler causesthe the peaks composite related material’sto this form global of composite crystallinity become to stronger. rise. With This an demonstrates increase in that in‐ the filler loading,creasing the molecular the filler causes chains the of composite ENR will material’s tweak the global accumulated crystallinity filler to rise. into With a an in‐ closely packed form.crease Diffractionin the filler loading, patterns the are molecular created chains by the of filler ENR being will tweak tightly the packedaccumulated in filler the cavities of the ENR. In Figure7, the traces for spent coffee biochar and carbon black are nearly indistinguishable, as carbon black have slightly greater intensity from 15–30◦ θ 2 . Strong and sharp peaks confirm that their significant crystalline impurities in both biochar and carbon black. It implies that following carbonization, lignin is transformed into biochar with graphitic domains [19]. This will help strengthen its hydrophobicity and affinity for rubber and maintain the increased tensile strength. Polymers 2021, 13, x FOR PEER REVIEW 8 of 12

into a closely packed form. Diffraction patterns are created by the filler being tightly packed in the cavities of the ENR. In Figure 7, the traces for spent coffee biochar and car‐ bon black are nearly indistinguishable, as carbon black have slightly greater intensity from 15–30° 2θ. Strong and sharp peaks confirm that their significant crystalline impurities in Polymers 2021, 13, 2796 both biochar and carbon black. It implies that following carbonization, lignin is trans‐ 8 of 11 formed into biochar with graphitic domains [19]. This will help strengthen its hydropho‐ bicity and affinity for rubber and maintain the increased tensile strength.

500 ENR-Biochar 450 ENR-CB 400 350 300 250 200

Intensity (a.u) 150 100 50 0 0 10 20 30 40 2 Thetha (°)

FigureFigure 7. XRDXRD patterns patterns of of ENR ENR-Biochar‐Biochar and and ENR ENR-CB‐CB at 10 atphr 10 loading. phr loading.

3.5.3.5. Thermal Analysis Analysis of ofENR ENR Biocomposites Biocomposites In Figure 8,8, the the weight weight loss loss versus versus temperature temperature graph graph for ENR for and ENR ENR and filled ENR vul filled‐ vul- canizatescanizates are are presented presented and and the the thermal thermal decomposition decomposition of ENR of composites. ENR composites. At 600 °C, At it 600 ◦C, it cancan be observed that that the the gum gum ENR ENR-50‐50 had had the lowest the lowest final finalweight. weight. All of the All composites of the composites were thermally degraded in a single step. The temperature at which biochar begins to were thermally degraded in a single step. The temperature at which biochar begins to decompose is 340 °C, while the temperature at which the greatest weight loss rate occurs decompose is 340 ◦C, while the temperature at which the greatest weight loss rate occurs is 390 °C.◦ From the data obtained, ENR‐Biochar possessed high thermal stability as it did isnot 390 fullyC. decompose From the upon data heating obtained, up from ENR-Biochar 40–600 °C, resulting possessed in a high higher thermal residue, stability pro‐ as it ◦ didportional not fully to the decompose percentage of upon biocarbon. heating The up remaining from 40–600 solid residueC, resulting of biochar in after a higher heat‐ residue, proportionaling to 600 °C was to the 9.8, percentage whereas carbon of biocarbon. black was 7.7%. The remaining This confirms solid that, residue in terms of of biochar the after heatingthermal todegradation 600 ◦C was mechanism, 9.8, whereas carbon carbon black black and biochar was 7.7%. have This a similar confirms action that, on the in terms of Polymers 2021, 13, x FOR PEER REVIEWtheENR thermal rubber, degradation and biochar composites mechanism, exhibited carbon better black thermal and biochar stability. have a similar9 actionof 12 on the

ENR rubber, and biochar composites exhibited better thermal stability.

120

100

Weight /% Weight 40

0 40 140 240 340 440 540 Temperature/°C

ENR-50 ENR-Biochar ENR-CB

FigureFigure 8. TGATGA of of ENR ENR vulcanizates. vulcanizates.

3.6. Morphological Analysis of ENR Biocomposites As illustrated in Figure 9, scanning electron microscopy was used to compare the morphology of tensile‐fractured surfaces of the composites biochar and CB. For the com‐ posites illustrated in Figure 9c, biochar particles are randomly oriented in planes parallel to the surface and are well dispersed across the whole length of the matrix. The matrix morphology in composites containing biochar shows no evidence of a phase‐separated blend. The fillers are equally adhered to the surface of the rubber, as seen in the images, indicating that the process of interaction between char filler and rubber is comparable to that of conventional carbon black. The primary variations between other fillers and com‐ mercial carbon black are particle shapes, sizes, and distribution. Smaller aggregate size and more homogeneous size distribution of carbon black spheroid particles may benefit during the vulcanization process, resulting in excellent reinforcing. The carbon black seems to disperse well in the ENR due to the higher surface energy of particles of carbon black and resulted in better distribution of fillers than the biochar.

Polymers 2021, 13, 2796 9 of 11

3.6. Morphological Analysis of ENR Biocomposites As illustrated in Figure9, scanning electron microscopy was used to compare the morphology of tensile-fractured surfaces of the composites biochar and CB. For the composites illustrated in Figure9c, biochar particles are randomly oriented in planes parallel to the surface and are well dispersed across the whole length of the matrix. The matrix morphology in composites containing biochar shows no evidence of a phase-separated blend. The fillers are equally adhered to the surface of the rubber, as seen in the images, indicating that the process of interaction between char filler and rubber is comparable to that of conventional carbon black. The primary variations between other fillers and commercial carbon black are particle shapes, sizes, and distribution. Smaller aggregate size and more homogeneous size distribution of carbon black spheroid particles may benefit during the vulcanization process, resulting in excellent reinforcing. The carbon black seems Polymers 2021, 13, x FOR PEER REVIEW 10 of 12 to disperse well in the ENR due to the higher surface energy of particles of carbon black and resulted in better distribution of fillers than the biochar.

Figure 9. ((aa)) SC SC ( (bb)) Biochar Biochar ( (cc)) ENR ENR-BIOCHAR-10‐BIOCHAR‐10 phr ( d) ENR ENR-CB-10‐CB‐10 phr.

4. Conclusions Conclusions The mechanical and physiochemical properties properties of spent coffee biochar and carbon blackblack-based‐based epoxidizedepoxidized natural natural rubber rubber composites composites were were investigated investigated in this in this study. study. Biochars Bio‐ charsmade made from spentfrom spent coffee coffee were ablewere to able replace to replace 10–15% 10–15 of the % of carbon the carbon black asblack it could as it could reach reachalmost almost 99% of99% the of carbon the carbon black black tensile tensile strength strength with with improved improved elongation elongation properties. proper‐ ties.Based Based on the on the mechanical mechanical properties, properties, spent spent coffee coffee can can be be a a partial partial substitute substitute forfor carbon black in rubber composites, which can be a measure to reduce black carbon usage in the future.future. Furthermore, Furthermore, the the thermogravimetric thermogravimetric analysis also confirmed conﬁrmed that the introduction of biochar to the ENR improves the thermal stability of the composites. Scanning electron microscopy further suggestssuggests that that the the homogeneity homogeneity is is greatly greatly enhanced enhanced with with the the introduction introduc‐ tion of spent coffee biochar. This eventually becomes an eye‐opener for the utilization of biomass as sustainable filler in the rubber industry.

Author Contributions: Conceptualization, G.R.; Writing—original draft, G.R.; Writing—review & editing, M.K., M.M.S. and, B.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Universiti Sains Malaysia under Research University Incentive (RUI) [Grant number 1001/PJJAUH/ 8011055]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflicts of interest. The authors declare that they have no known competing financial interests or personal relation‐ships that could have appeared to in‐ fluence the work reported in this paper.

Polymers 2021, 13, 2796 10 of 11

of spent coffee biochar. This eventually becomes an eye-opener for the utilization of biomass as sustainable ﬁller in the rubber industry.

Author Contributions: Conceptualization, G.R.; Writing—original draft, G.R.; Writing—review & editing, M.K., M.M.S. and, B.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Universiti Sains Malaysia under Research University Incentive (RUI) [Grant number 1001/PJJAUH/8011055]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. The authors declare that they have no known competing financial interests or personal relation-ships that could have appeared to influence the work reported in this paper.

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