An Attempt to Find a Suitable Biomass for Biochar-Based Polypropylene Biocomposites

Figure 1 indicates the TGA curves of all the four biomasses compared with biochar and neat PP. Figure 1 also presents the TGA curves of the composites made from the biomasses and biochar. Table 2 also illustrates the TGA results for all the samples, such as the onset of decomposition temperature (T_onset) and the temperatures at the maximum mass loss rates (T_max1 and T_max2 obtained from the derivatives of the mass loss curves) wherever applicable. From Fig. 1a, it can be clearly observed that biochar was the most thermally stable having negligible decomposition events and retaining the highest amount of carbonaceous residues (Das et al. 2017). Neat PP, on the other hand, was the least thermally stable as it rapidly decomposed leaving almost no residue. Amongst the four biomasses, wool was found to have the highest amount of residue which was due its charring ability (Kim et al. 2015). Rice and coffee husk retained somewhat similar amounts of residues, which was slightly higher than the residues produced by the wood. The onset of thermal decomposition of all the biomass appeared earlier compared to those of the biochar and neat PP (Table 2). The extent of decomposition was the lowest in biochar due to its thermal stability. The stable aromatic nature of carbon molecules and C-C covalent bonds amongst the molecules most probably imparted a resistance towards thermal degradation. The neat PP was seen to have a high magnitude of decomposition with a single decomposition maximum at ~389 °C. Rice husk, coffee husk, and wood all had dual decomposition maxima, one at ~300–330 °C and another one at ~440–478 °C. The first decomposition was represented by the degradation of cellulose in these biomasses while the second decomposition was marked by the degradation of lignin (Das et al. 2015a). The wool, lacking the aforementioned biopolymers, had a single decomposition maximum centered around 338 °C. The thermal decomposition in wool was dominated by the disruption of the microfibril-matrix structure along with the disulfide bonds and the peptides in the wool’s protein (Kim et al. 2017). The coffee husk had the highest T_max2 (478 °C) amongst all the four biomasses. Relatively high amount of nitrogen released from the caffeine in coffee husk (Saenger et al. 2001) can suppress the volatile gases from the biomass and slow down the reaction of material, so decomposition can be delayed.

From Fig. 1b it can be observed that all the composites had onset temperatures which were higher than the individual onset temperatures of the biomasses. This is because the thermally stable biochar in the composite delayed the start of degradation. The biomasses, when added with biochar in PP composite produced similar amounts (~27 wt%) of residues which were higher than the individual residues produced by the biomasses (although the neat wool produced a similar residue of ~26 wt%) and the neat PP (~0 wt%). This can also be attributed to the very high amount (~78 wt%) of residues left by the thermally stable biochar (Fig. 1a). All the composites had two events of decomposition maximum, one at ~330–370 °C and the other at ~412–490 °C. The first decomposition maximum was due to the degradation of cellulose in the biomasses (and breakage of microfibril-matrix structure in the wool). The decomposition maximum of neat PP has been shifted from ~389 °C to ~412–490 °C. This behavior is attributed to the inclusion of thermally stable biochar in the composite. Hence, it demonstrates that the application of biochar can induce beneficial thermal stability in composites.

Figure 2 shows a comparison of the reaction-to-fire properties of individual biomasses and biochar as obtained from the cone calorimeter. Figures 2a, 2b, 2c and 2d represent HRR, CO₂ production, CO production, and mass loss rate, respectively. Table 3 summarizes the other important fire properties of biomasses and biochar. The peak heat release rate (PHRR) is one of the most important parameters to judge the fire characteristics of a material whereas the HRR curve describes the overall combustion behavior of the material. The time to ignition (TTI) and time to PHRR (TPHRR) provide the time a material takes to catch fire and produce the maximum heat, respectively under radiative heat. Therefore, material ignition can be analyzed by TTI. Both the HRR and TTI depend on the exterior heat flux, degree of ventilation, and the extent of destruction of the tested material whereas the total heat release (THR) represents the internal energy of the material irrespective of the environmental factors (Schartel and Hull 2007). All the biomasses ignited rapidly (~3–8s) and also had flameout within 100 s. The wool had the highest peak heat release rate (PHRR) of 430 kW/m² whereas the coffee husk had the lowest (188 kW/m²). The lowest T_max2 of wool and the highest T_max2 of coffee husk (amongst the biomasses) might have contributed to their respective PHRRs in cone calorimeter (Table 2). The rice husk exhibited a PHRR value of 200 kW/m² whereas the wood had a PHRR of 250 kW/m². The wood had the highest TTI and TPHRR amongst all the other biomasses which are positive attributes. Due to rapid combustion and early flameout, the THR of all the biomasses were fairly low (below 20 MJ/m²). From the HRR curves, the initial rapid combustion of wool can be clearly seen as a sharp peak at about 25 s. The disassociation of the microfibril-matrix structure and disruption of disulfide bonds triggered the char formation and once the char was built-up, the HRR went down. Under the continuous heat radiation, the char started breaking its structure and underlying wool continued decomposition and combustion, consequently resulting in another subdued heat release peak at ~45 s. However, the HRR reduced after that due to the formation of char and the extinction of fuel and continued a receding pattern until flameout at ~160 s. The rice husk also exhibited two subdued peaks and probably the second peak was a result of underlying un-burnt virgin biomass being available to the incoming O₂ resulting in an increase in HRR. Both coffee husk and wood have a single peak implying a uniform combustion of the tested biomasses under radiative heat. The CO₂ production of wool was the highest followed by wood, rice husk, and coffee husk (Fig. 2b). On the contrary, wool and wood had lower CO production compared to those of coffee and rice husks (Fig. 2c). From the mass loss curves (Fig. 2d), although wool was seen to leave carbonaceous residues in TGA (Fig. 1a), the continuous radiative heat caused oxidation of the chars formed, leading to complete loss of mass. The biochar left the highest amount of residues and the residues remaining in rice and coffee husk were slightly more than wood, which corroborates the TGA mass loss results observed in Fig. 1a.

It was observed that the biochar did not undertake ignition under the radiative heat whereas all the biomasses ignited rapidly (Das et al. 2017). Moreover, the PHRR values of the biomasses were quite high (~188–430 kW/m²) compared to that of the biochar (~33 kW/m²) and the time it took (25–57 s) to reach PHRR was also much faster than that of the biochar (~305 s). Biochar also produced the lowest amount of CO₂ amongst all other biomasses. The somewhat inverse trend of CO₂ and CO production can be observed in Fig. 2c. This is attributed to the greater extent of combustion (hence oxidation) of samples producing more CO₂, and consequently, less CO. The TPHRR of biochar was difficult to measure owing to the steady state progression of its combustion cycle. The reaction-to-fire properties of the biochar are described in detail elsewhere (Das et al. 2017). In brief, owing to the high temperature of biochar production, all the flammable volatiles escaped. In addition, the high production temperature caused gradual condensation of carbon into aromatic clusters thereby making the C-C covalent bonds strong and stable. As a consequence, the tested biochar was inert under radiative heat and did not ignite.

Table 4 presents the cone calorimeter parameters for biomass added biochar composites whereas Fig. 3 shows the reaction-to-fire properties of the same composites. The cone calorimeter results of the composites are compared with that of the neat PP. From Table 4 and Fig. 3a, it can be observed that neat PP burned rapidly with a sharp HRR peak due to the fast degradation of its hydrocarbon backbone. The PHRR of the wool based composite was the highest (596 kW/m²), and the PHRR of coffee husk added composite was the lowest (394 kW/m²), amongst all the biomasses. The high PHRR of neat wool and low PHRR of neat coffee husk (Table 3) might have contributed towards the respective PHRRs in the composites. On the other hand, the neat rice husk, although, having lower PHRR than neat wood, its composite exhibited higher PHRR (501 kW/m²) than the wood-based composite (449 kW/m²). The possible explanation for this behavior is explained as follows. The biochar was made from the same wood, which is used as one of the four biomasses. Production of biochar retains the structure of the parent feedstock (in this case, wood) (Sun et al. 2012). Therefore, it is possible that the biochar and wood were physically in close contact with each other in the PP matrix (like pieces in a puzzle) (Fig. 4). The rice husk with a different morphology with the biochar in the PP matrix, combusted with a greater severity (compared to wood-based composite) without the shielding effect of biochar and consequently increased the overall PHRR of the composite. The prolonged combustion of wool (due to char formation) resulted in a THR similar to that of the neat PP (97 MJ/m²) whereas wood, rice, and coffee husk had similar THR of ~90 MJ/m² (Table 4). All the biomass-based composites experienced shorter TPHRR and TTI than those of the neat PP, most probably due to an early onset of thermal decomposition and rapid ignition of the biomasses (compared to neat PP) as observed in Tables 2 and 3. The CO₂ production curves (Fig. 3b) of the biomass-based biochar composites mirrored the HRR curves because CO₂ was produced as long as the composites were burning. The peak rate of CO₂ production was similar to the values of PHRR corresponding to each of the biomass in the composites. Hence, wool based composite had the highest rate of CO₂ production whereas coffee husk had the lowest. Wood and rice husk based composite had more or less similar rate of CO₂ production. The CO production increased beyond the discontinuance of CO₂ production in all the biomass-based composites. This can be attributed to the biochar in the composite providing an insulating carbonaceous layer which facilitated an incomplete combustion of polymer to produce less CO₂ and more CO. It is interesting to note that the HRR and gas production of the biomasses and biochar-based composites were significantly lower than those of the neat PP. Moreover, the severity of mass loss of the biomass/biochar composites is much less than that of the neat PP. Therefore, the addition of thermally stable biochar in biomass added composites is beneficial when reaction-to-fire properties are concerned. This is evident as wood added composite (without biochar) was previously reported to have a PHRR of 568 kW/m² (Zhang et al. 2012) and the addition of biochar reduced the PHRR to ~21 % in the present study.

The tensile (tensile strengths and moduli) and flexural properties (flexural strengths and moduli) of all the composites and neat PP are summarized in Table 5. Among the different biomass added composites, the wood-based sample had the highest tensile/flexural strength values (36 MPa/67 MPa) and flexural modulus (4.4 GPa). Its tensile modulus (4.73 GPa) was close second to that of the rice husk based composite (4.87 GPa). The tensile strength of rice husk composite was similar to that of the neat PP (~32 MPa) but its flexural strength was higher (62.6 MPa). In fact, the flexural strength and tensile/flexural moduli of all the biomass-based composites were higher than that of the neat PP. The good dispersion of the biochar in the PP matrix and the biochar pore infiltration by PP enhanced the stress transfer efficiency between the matrix and the constituents. In addition, the inclusion of biochar with the biomasses reduced the deformability of the PP which consequently enhanced the flexural strengths and tensile/flexural moduli. The tensile properties of the composites can depend on the individual chemical composition of the biomasses (Fidelis et al. 2013). The cellulose is the main component responsible for the resistance of the biomass towards stress. This strong and stiff constituent of biomass (i.e. cellulose) remains aligned with longitudinal axis of the fibers which contribute to the fiber’s mechanical properties. The average amount of cellulose in the lignocellulosic biomasses is as follows: pine wood 40–45% (Fengel and Grosser 1975); rice husk 25–35 % (Luduena et al. 2011); coffee husk 19–26 % (Bekalo and Reinhardt 2010). Therefore, the wood-based composite had the highest resistance (consequently highest tensile strength) due to the highest amount of cellulose in it. Similarly, the higher amount of cellulose in rice husk compared to coffee husk resulted in better mechanical properties. In this research, short wool fibers (average length: 2.8 mm) were used to manufacture the composites. This length was lower than the critical length of the wool (3.6 mm), which is essential for maximizing the strength properties of the resulting composites (Kim et al. 2014).

The addition of coffee husk and wool did not enhance the tensile and flexural strengths of the composite. However, it improved the modulus (both tensile and flexural). This was due to the reduction of intra-particle distances as a consequence of the inclusion of the higher amount of reinforcing constituents (Ho et al. 2015). Application of rice husk to biochar/PP composite had a positive correlation with flexural strength and tensile/flexural moduli. Out of all the biomasses, wood seemed to have an overall favorable effect on all the tested mechanical properties of biochar/PP composites. In past investigations where only the biomass was used (loading amount of 30 wt%- similar to current study) along with PP in the composites generally obtained inferior mechanical properties compared to what is observed in the present study. Stark and Rowlands (2003) prepared wood/PP composites and they reported the tensile/flexural strengths and tensile/flexural moduli to be 24.3 MPa/41.4 MPa and 3.46 GPa/2.89 GPa, respectively. In another study by Rodríguez et al. (2003), coffee husk/PP composites were reported to have a very low tensile strength and modulus of 12.4 MPa and 1.94 GPa, respectively. Rice husk/PP composites exhibited a tensile strength of ~ = <25 MPa (Premalal et al. 2002; Yang et al. 2004) whereas wool/PP composites had tensile strength and modulus of 34 MPa and 2.4 GPa (Kim and Bhattacharyya 2016), respectively. In the current investigation, the average tensile/flexural strengths and tensile/flexural moduli for all the biomass and biochar-based composites (combined) were 31.9 MPa/61.7 MPa and 4.3 GPa/4.05 GPa, respectively. Therefore, it can be observed that except for wool, all other biomasses, when used to make PP composites without biochar possess inferior mechanical properties. These observations point out the fact that biochar is indeed beneficial for enhancing the mechanical properties of a biomass-based composite. In addition, biochar plays an additional role in resisting flame propagation upon composite combustion.

Scanning electron microscopy (SEM) was performed to gain an insight into the state of manifestation of the biomasses in the PP matrix. From Fig. 5, it can be observed that the rice husk had reasonable interfacial bonding with the PP aided by 4 wt% MAPP, whereas the bonding of wood and PP was good (the wood+ BC image). The bonding between wood and PP is also demonstrated in Figure S1 in supplementary information. The SEM image (Figure S1) is from a different sample set that contained ammonium polyphosphate, nevertheless, the image supports the argument of bonding between wood and PP. It can be observed from Figure S1, that part of the wood has been fractured off under tensile stress whereas the remaining wood particle is still adhering to the PP matrix. On the other hand, a clear de-bonding from the PP matrix was seen with coffee husk at several places in the sample (an example is in the coffee husk+ BC image). In the wool added composite, the biochar particles were seen to be embedded within the wool fibers potentially hindering the interaction of wool and PP. Hence, the tensile strength of the wood composite was the highest followed by rice husk and then coffee husk and wool (Table 5). The mechanical interlocking of biochar with PP occurred in all the biomass added composites, exemplified here with the rice husk added sample (Fig. 6). This interlocking is the reason for the enhanced tensile/flexural moduli of the composites compared to the neat PP (Table 5).

The structural integrities of the chars obtained after combustion in the cone calorimeter were also analyzed through SEM. The SEM images of combustion chars of the biomass added composites are illustrated in Fig. 7. The chars of coffee husk and wood added with biochar seemed to be more compact and dense whereas the chars of wool and rice husk were observed to have more holes and fissures. These holes allowed the transport of O₂ between the ambient environment and the underlying materials. This might have been the reason for coffee husk and wood-based composites to have better reaction-to-fire properties (especially PHRR) than wool and rice husk based composites (Table 4).

The use of wood fibres, as the adequate biomass for biochar composites performance, also leads to improved environmental sustainability. It is known that wood-based thermoplastic polymer composites can be recycled at the end of their lives. Bourmaud and Baley (2007) have studied the effect of recycling on mechanical properties of composites based on cellulose fibers and PP, and identified that tensile modulus of the composite is preserved after reprocessing cycles. Moreover, biochar is also known as a sustainable material, thus the hybrid of wood and biochar in the composites leads towards sustainability and environmental innocuousness (Das et al. 2017). Furthermore, the natural fibers emit much less toxic gases, such as CO and NO_x, consume less energy and have more disposal options compared to those of synthetic materials, namely glass and carbon fibers, during their manufacturing and incineration. Therefore, the bio-based composites have shown better performance in life cycle assessment than synthetic fiber based composites (Joshi et al. 2004).

As mentioned in Introduction section, the biomasses used in the current study are organic wastes. Employment of the wastes and biochar in composites manufacturing can significantly reduce total production cost, as the price of biochar (NZD 0.22/kg) is much less than natural and synthetic fibers. The economic advantages of the materials can also enhance potential of the hybrid of biomass and biochar for mechanically sound and fire resistant composites.