*3.3. Tensile Property Results*

*3.3. Tensile Property Results* Tensile results for both uncoated and 10% LA-coated biochar samples are shown in Table 4. The effect of gas treatments to the biochar with air and CO<sup>2</sup> can be seen in the first Tensile results for both uncoated and 10% LA-coated biochar samples are shown in Table 4. The effect of gas treatments to the biochar with air and CO<sup>2</sup> can be seen in the first three rows of Table 4. Tensile strength remains similar with all three samples, however both gas treatments reduce elongation relative to the control.

three rows of Table 4. Tensile strength remains similar with all three samples, however

stiffness of the gas-treated samples is evident in the steeper slopes compared to that of the control. It should be noted that in all stress–strain plots presented in this work, the representative curve that was closest to the average tensile strength and elongation for



BC CO<sup>2</sup> 4 18.6 ± 1.1 447 ± 20 34.9 ± 3.9 *n* = number of replicate runs.

*n* = number of replicate runs.

BC control LA 5 20.7 ± 0.9 532 ± 18 47.9 ± 3.7 BC air LA 4 20.8 ± 1.3 504 ± 25 45.4 ± 5.7 BC CO<sup>2</sup> LA 5 22.1 ± 0.6 536 ± 12 51.6 ± 2.7 By examining the stress–strain curves for these three samples (Figure 3), the higher stiffness of the gas-treated samples is evident in the steeper slopes compared to that of the control. It should be noted that in all stress–strain plots presented in this work, the

each sample was selected and shown for clarity.

representative curve that was closest to the average tensile strength and elongation for each sample was selected and shown for clarity. *C* **2022**, *8*, x FOR PEER REVIEW 6 of 9

**Figure 3.** Stress vs. strain curve for the three uncoated biochar samples, illustrating the effect of air and CO<sup>2</sup> treatment during biochar processing. **Figure 3.** Stress vs. strain curve for the three uncoated biochar samples, illustrating the effect of air and CO<sup>2</sup> treatment during biochar processing.

To show the effect of LA coating, Figure 4 shows each of the stress–strain curves for the BC control, BC air, and BC CO<sup>2</sup> samples with and without LA coating. For the control biochar, LA coating showed no appreciable change in the tensile strength or elongation as the stress–strain curves were nearly identical. For the air-treated biochar sample, tensile strength remained unchanged and there was a slight increase in stiffness relative to the uncoated sample. The effect of LA coating on the CO<sup>2</sup> treated biochar sample showed that stiffness was essentially unchanged, but the tensile strength increased nearly 20%. To show the effect of LA coating, Figure 4 shows each of the stress–strain curves for the BC control, BC air, and BC CO<sup>2</sup> samples with and without LA coating. For the control biochar, LA coating showed no appreciable change in the tensile strength or elongation as the stress–strain curves were nearly identical. For the air-treated biochar sample, tensile strength remained unchanged and there was a slight increase in stiffness relative to the uncoated sample. The effect of LA coating on the CO<sup>2</sup> treated biochar sample showed that stiffness was essentially unchanged, but the tensile strength increased nearly 20%.

**Figure 4.** Stress–strain curves of LA-coated vs. uncoated rubber composite samples for the control (**top**), air-treated (**middle**), and CO2-treated (**bottom**) biochar samples. For each plot the single representative curve closest to the average tensile strength and elongation was chosen for clarity. **Figure 4.** Stress–strain curves of LA-coated vs. uncoated rubber composite samples for the control (**top**), air-treated (**middle**), and CO<sup>2</sup> -treated (**bottom**) biochar samples. For each plot the single representative curve closest to the average tensile strength and elongation was chosen for clarity.

### **4. Conclusions 4. Conclusions**

For this work we wanted to observe two separate surface chemistry modifications to biochar; a gas treatment step followed by the addition of LA. Both treatments are amenable to the rubber processing industry as they only involve gas treatment to the biochar at controlled temperature and dry-milling, two processes that are both easily scalable. Gas treatment of the biochars with air and CO<sup>2</sup> at 300 °C did not appreciably affect the tensile strength of the resulting filled rubber composites, but did increase their stiffness relative to the untreated biochar control. For this work we wanted to observe two separate surface chemistry modifications to biochar; a gas treatment step followed by the addition of LA. Both treatments are amenable to the rubber processing industry as they only involve gas treatment to the biochar at controlled temperature and dry-milling, two processes that are both easily scalable. Gas treatment of the biochars with air and CO<sup>2</sup> at 300 ◦C did not appreciably affect the tensile strength of the resulting filled rubber composites, but did increase their stiffness relative to the untreated biochar control.

Biochar control coated with LA appeared to have no effect on the tensile properties of the resulting filled rubber composite. For the air-treated biochar sample, coating with LA did not appreciably change the tensile strength, but did slightly increase the elongation of the composite, making it slightly softer. The most notable difference in tensile properties occurred when the CO2-treated biochar was then LA-coated; composites made from this biochar sample increased their tensile strength by 19%, and their toughness by 48%. This suggests that the CO<sup>2</sup> gas treatment helps condition the biochar so that LA coating is more effective. CO<sup>2</sup> treatment of biochar increased its carbon content and reduced its oxygen content, reducing the number of charged, oxygen-containing functional groups on the biochar and making it more hydrophobic. Further investigation Biochar control coated with LA appeared to have no effect on the tensile properties of the resulting filled rubber composite. For the air-treated biochar sample, coating with LA did not appreciably change the tensile strength, but did slightly increase the elongation of the composite, making it slightly softer. The most notable difference in tensile properties occurred when the CO2-treated biochar was then LA-coated; composites made from this biochar sample increased their tensile strength by 19%, and their toughness by 48%. This suggests that the CO<sup>2</sup> gas treatment helps condition the biochar so that LA coating is more effective. CO<sup>2</sup> treatment of biochar increased its carbon content and reduced its oxygen content, reducing the number of charged, oxygen-containing functional groups on the biochar and making it more hydrophobic. Further investigation to understand the relative

roles of CO<sup>2</sup> treatment in conjunction with LA coating is needed to help optimize and improve biochar reinforcement of rubber composites.

**Author Contributions:** Conceptualization, S.C.P.; methodology, S.C.P.; investigation, S.C.P. and A.J.T.; resources, A.J.T.; writing—original draft preparation, S.C.P.; writing—review and editing, S.C.P. and A.J.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the U.S. Department of Agriculture, Agricultural Research Service. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

**Acknowledgments:** The authors would like to thank Jason Adkins for collecting ash content data and Kelly Utt for performing FTIR experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
