Evaluation of the Antioxidant Properties of Black Acacia (Acacia mearnsii) Tannin in Vulcanized Natural Rubber Using Different Vulcanization Systems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. In Section 2.4 characterization of natural rubber composites, the used methods are as-known or general information or knowledge. I suggest this part of text should be concise. For example, the corresponding determination methods can be described in a table style. 

2. The typo of some units such as g mol-1 and μg GAE mg-1 should be wrong. Please check all the units. 

3. Figure 2 presents the specific density values of rubber composites in different cross-linking systems. The authors just shown the results, but not discussed the sources of evolution. 

4. The quality of Figure 8 should be improved. And, the standard errors for each data point should be given. I suggest that Fig. 8a and 8b can be merged in one picture. 

5. Elaborate the discussions of the obtained results in the context of Section 3.

6. The conclusions should be concise and redrawn in point-to-point. 

7. The scientific merits and main innovations of this work should be further highlighted in Abstract and Conclusions. 

Comments on the Quality of English Language

Please check or correct all the unit typo. 

Author Response

1. In Section 2.4 characterization of natural rubber composites, the used methods are as known or general information or knowledge. I suggest this part of text should be concise. For example, the corresponding determination methods can be described in a table style.

Dear Reviewer, we sincerely appreciate your suggestion to make Section 2.4 more concise. We understand the importance of brevity in enhancing readability. However, we believe that providing a slightly more detailed description of the methods is crucial to ensuring the reproducibility of our results.

2. The typo of some units such as g mol-1 and μg GAE mg-1 should be wrong. Please check all the units.

Dear Reviewer, we appreciate your comments on the manuscript. As requested, we have carefully reviewed all the units of measurement and corrected any typographical errors.

3. Figure 2 presents the specific density values of rubber composites in different cross-linking systems. The authors just shown the results, but not discussed the sources of evolution.

Dear Reviewer, we appreciate your observation. As suggested, we have expanded the discussion regarding the specific density values presented in Figure 2, exploring the subtle variation in the specific density of the composites that occurs with the addition of tannins. The following is the revised discussion:

“Figure 2 presents the specific density values of rubber composites in different cross-linking systems. It is observed that the tannin content tends to cause a slight increase in specific density. The increase in specific density can be attributed to the higher density of tannin (1.47 g ml^-1) compared to natural rubber (approximately 1.0 g ml^-1). When tannin is incorporated into the same volume of material, it contributes to the overall increase in specific density. This effect, although subtle, reflects the higher concentration of dense molecules within a predominantly less dense elastic matrix.

Additionally, among the different crosslinking systems, the semi-efficient system, fol-lowed by the conventional system, exhibited the highest specific density values, which is related to the small differences in the quantities of reagents added to the composite for-mulation. In the semi-efficient system, the formation of shorter disulfide and monosulfide bonds reduces matrix mobility and limits the permeation of organic solvents, thereby in-creasing the crosslink density. This more compact structure may result in a higher specific density compared to other systems.”

4. The quality of Figure 8 should be improved. And the standard errors for each data point should be given. I suggest that Fig. 8a and 8b can be merged in one picture.

Dear Reviewer, we appreciate your suggestion. Following your recommendation, we have merged Figures 8a and 8b into a single image. Additionally, we have improved the quality of the figure and included error bars for each data point.

5. Elaborate the discussions of the obtained results in the context of Section 3.

Dear Reviewer, we appreciate your suggestion and have included a new section titled "4. Integrated Discussion of Results."

6. The conclusions should be concise and redrawn in point-to-point.

Dear Reviewer, we appreciate your suggestion. However, as we have included a new section (4. Integrated Discussion of Results) that discusses each point in detail, the authors have opted for a more concise conclusion.

7. The scientific merits and main innovations of this work should be further highlighted in Abstract and Conclusions.

Dear Reviewer, we appreciate your observation. Following your recommendation, the abstract and conclusion have been revised.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper has valuable results. But some modifications should be done.

1- What is [Error! Bookmark not defined.] in abstract refer to?

2- Table (3) should be converted to figures. Itis very hard to follow. I can not review this part as it is now.

3- (It is observed that the tannin content tends to cause a slight increase in 346 specific density) ........ why?Discuss.

4- (among the different crosslinking systems, the semi-efficient system exhibited the highest specific density values) ...... Why? Discuss.

5- Figure (7) was only described but not discussed and explained?

6- (Among the vulcanization systems, the efficient system demonstrates the highest abrasion wear.) Why? Discuss.

 

Author Response

1- What is [Error! Bookmark not defined.] in abstract refer to?

Dear Reviewer, we appreciate your observation. We have identified and corrected the undefined marker error in line 91. The line has been updated with the accurate information.

2- Table (3) should be converted to figures. Itis very hard to follow. I cannot review this part as it is now.

Dear Reviewer, we appreciate your feedback and apologize for any difficulty in interpreting the information presented in Table 3. However, the authors have decided to retain the rheometry results in tabular format. We would like to emphasize that rheometry is one of the most relevant tests in the rubber vulcanization process, widely used in both academia and the rubber products industry. Furthermore, the results presented in the form of rheometric curves or tables are widely accepted in the specialized literature.

3- (It is observed that the tannin content tends to cause a slight increase in 346 specific density) ........ why? Discuss.

Dear Reviewer, we appreciate your comment and have included the following discussion:

"The increase in specific density can be attributed to the higher density of tannin (1.47 g ml^-1 – Section 2.1 Materials) compared to natural rubber (approximately 1.0 g ml^-1). When tannin is incorporated into the same volume of material, it contributes to the overall increase in specific density. This effect, although subtle, reflects the higher concentration of dense molecules within a predominantly less dense elastic matrix."

4- (among the different crosslinking systems, the semi-efficient system exhibited the highest specific density values) ...... Why? Discuss.

Dear Reviewer, we appreciate your comment and have included the following discussion:

"In the semi-efficient system, the formation of shorter disulfide and monosulfide bonds reduces matrix mobility and limits the permeation of organic solvents, thereby increasing the crosslink density. This more compact structure may result in a higher specific density compared to other systems."

5- Figure (7) was only described but not discussed and explained?

Dear Reviewer, we appreciate your observation and have replaced the previous description with the following discussion:

“Figure 7 presents the hydroxyl index (HI), a critical measure for evaluating the degree of oxidative degradation in natural rubber composites after exposure to UV-C radiation. This index reflects the formation of hydroxyl groups resulting from the oxidative cleavage of polymer chains. The HI analysis reveals that the antioxidant performance of tannin varies significantly among the vulcanization systems (CV, SEV, and EV) and with the incorporated tannin concentration.

In the conventional system (CV), a substantial increase in HI is observed over the exposure period, suggesting reduced antioxidant protection from tannin in this system. This behavior can be attributed to the predominance of polysulfidic bonds, which are more susceptible to thermal and photo-oxidative degradation. These long bonds fail to stabilize free radicals generated by UV interaction, allowing oxidation to propagate more intensively.

In the semi-efficient system (SEV), the HI shows an initial increase during the first week, followed by stabilization or a slight reduction in the second week, indicating moderate protection from tannin. This response may be linked to the combination of mono- and disulfidic bonds, which confer some chemical stability to the system while still allowing for localized oxidation.

Conversely, in the efficient system (EV), the lowest increase in HI is observed throughout the exposure period, demonstrating the superior antioxidant efficacy of tannin in this system. This outcome is related to the predominance of monosulfidic bonds, which are chemically more stable and less prone to degradation under UV radiation. The interaction between tannin and these shorter bonds creates a more effective antioxidant barrier, limiting free radical formation and propagation.

Furthermore, the efficacy of tannin as an antioxidant is directly related to its concentration. At lower levels (1.0 phr), tannin already exhibits a protective effect, but at higher concentrations (2.0 phr), the hydroxyl index demonstrates greater stability, especially in the EV system. This suggests that tannin effectively neutralizes free radicals and interacts with sulfhydryl groups, delaying oxidative degradation.

These findings underscore the importance of selecting the appropriate vulcanization system to maximize the efficacy of tannin as an antioxidant additive. While the CV system exhibits greater degradation due to less stable bonds, the EV system provides an optimal combination of structural stability and antioxidant protection. This analysis highlights the potential of tannin as a sustainable alternative to prolong the lifespan of rubber composites, particularly in applications exposed to adverse environmental conditions.”

6- (Among the vulcanization systems, the efficient system demonstrates the highest abrasion wear.) Why? Discuss.

Dear Reviewer, we appreciate your comment and have replaced the discussion with a more comprehensive one:

"The efficient system is characterized by the predominance of monosulfidic bonds, which, although more resistant to heat and oxidation, are less flexible and, therefore, more prone to the propagation of microcracks under abrasive stress."

Reviewer 3 Report

Comments and Suggestions for Authors

Manuscript review comments [sustainability-3348884]:

Manuscript Title: Evaluation of the Antioxidant Properties of Black Acacia 2 (Acacia mearnsii) Tannin in Vulcanized Natural Rubber Using 3 Different Vulcanization Systems.

In this manuscript, the authors chose Acacia mearnsii tannin as an antioxidant to present a study of the mechanical, thermal, and structural properties of natural rubber composites Using 3 Different Vulcanization Systems.
tannins are sustainable for the environment because they are natural and biodegradable. Furthermore, earlier research has shown their effectiveness as antioxidants and their capacity to improve the mechanical characteristics and oxidative stability of certain polymeric materials.

I overall recommend publishing this manuscript after addressing the following minor revisions:

Concern #1:   would you clarify with figures the formulation and Preparation of Composites?

Concern#2: Clarify with figures three different vulcanization systems.

Concern#3: Please figure out the result curves of tensile strength tests on NR/Tan composites in their respective curing systems before and after thermal aging.

Author Response

Concern #1:   would you clarify with figures the formulation and Preparation of Composites?

Dear Reviewer, unfortunately, the authors will not be able to provide a drawing, as the formulations of the rubber composites presented in the manuscript are widely known both in the academic field and in the rubber industry. The graphic summary is intended to provide an overview of the process for preparing these composites. Additionally, Table 1 provides a detailed description of the components used and their respective quantities in the manufacturing of the vulcanized composites. However, we have included a brief description for further clarity in Section 2.2, "Formulation and Preparation of Composites":

“The compounds were prepared using an open two-roll mill, following the guidelines of ASTM D3182-21a, with a friction ratio of 1:1.25. Natural rubber, reaction activators (zinc oxide and stearic acid), and the antioxidant tannin were added to the mill in proportions of 0, 1.0, 1.5, and 2.0 phr. After homogenizing the mixture for 20 minutes, it was allowed to rest for 24 hours at an ambient temperature of 23 °C. Subsequently, the mixture was reintroduced into the mill for the addition of the cross-linking agent (sulfur) and vulcanization accelerators (MBTS and TMTD). After homogenizing for 15 minutes, the mixture was left to rest for 2 hours at room temperature. Upon completing these steps, the composite was subjected to rheometric testing and then processed in a hot press to fabricate test specimens.”

Concern#2: Clarify with figures three different vulcanization systems.

Dear Reviewer, unfortunately, the authors will not be able to create the figures, as the conventional, semi-efficient, and efficient vulcanization systems have been standardized by the rubber industry and described in scientific literature over time, based on studies that analyzed the relationships between the chemical components of the formulations and the behavior of vulcanized elastomers.

Conventional system: Involves a high amount of sulfur (2-3 phr) and a low amount of accelerators. It produces networks of polysulfidic bonds, providing good flexibility but lower thermal resistance and aging resistance.

Semi-efficient system: Uses a moderate proportion of sulfur (1-2 phr) and a higher amount of accelerators. It generates a combination of polysulfidic and mono/di-sulfidic bonds, balancing mechanical and thermal properties.

Efficient system: Employs a low amount of sulfur (<1 phr) and a high amount of accelerators. It primarily forms mono and di-sulfidic bonds, resulting in greater thermal stability, aging resistance, and mechanical properties.

Concern#3: Please figure out the result curves of tensile strength tests on NR/Tan composites in their respective curing systems before and after thermal aging.

Dear Reviewer, we appreciate your recommendation. The results related to the tensile strength at break of the NR/Tan composites in their respective curing systems, both before and after accelerated thermal aging, were analyzed. The discussion of the analysis performed has been included in section 3.2.8.

Reviewer 4 Report

Comments and Suggestions for Authors

The article is devoted to an interesting topic of changing the properties of rubber, a comprehensive study of a large list of material properties was conducted. Despite the fact that the authors have done a lot of work, presented in a good article, there are several points that the authors should consider before publishing.
- Line 91. "Error! Bookmark not defined."
- All graphs have a problem with the fonts of the captions, it is necessary to improve the quality of the figures.
- Line 471. The authors indicate that the protective capacity can be increased by adding a larger amount of tannin than was studied in the work. In the text of the manuscript, it is necessary to provide a justification for why the authors took the interval up to 2.0 phr and did not expand it to 5.0 phr.
- Strength properties indicate that SEV allows obtaining samples with the highest hardness, but the lowest tensile strength and plasticity. This result requires a more detailed discussion in the manuscript, since harder and less plastic samples should have higher strength.
- A major drawback of the article is the lack of a comprehensive discussion. To date, the article is a list of various properties. Each of the properties is well researched and described, but no general trends are observed. One example is given in the previous remark - strength and hardness are studied separately, but the phenomenon of simultaneous increase in hardness with decrease in strength is not discussed. The same can be said in general about the integrated approach to the discussion of properties. The authors need to supplement the article with a full-fledged discussion and describe in more detail the theses given in Conclusions.

Author Response

- Line 91. "Error! Bookmark not defined."

Dear Reviewer, we appreciate your observation. We have identified and corrected the undefined marker error in line 91. The line has been updated with the accurate information.

- All graphs have a problem with the fonts of the captions, it is necessary to improve the quality of the figures.

Dear Reviewer, we appreciate your observation. Following your recommendation, the quality of all the figures has been improved.

- Line 471. The authors indicate that the protective capacity can be increased by adding a larger amount of tannin than was studied in the work. In the text of the manuscript, it is necessary to provide a justification for why the authors took the interval up to 2.0 phr and did not expand it to 5.0 phr.

Dear Reviewer, we appreciate your contribution. Following your recommendation, the justification for limiting the tannin content to 2.0 phr has been added to section 3.2.5. In general, the decision to limit the tannin addition to 2.0 phr was made considering experimental feasibility and the scope of the study, which focused on the range presented in the paper. Evaluations of concentrations above 2.0 phr are left as a perspective for future studies, aiming to deepen the understanding and expand the practical applications of the material.

- Strength properties indicate that SEV allows obtaining samples with the highest hardness, but the lowest tensile strength and plasticity. This result requires a more detailed discussion in the manuscript, since harder and less plastic samples should have higher strength.

Dear Reviewer, we appreciate your suggestion. Below is the explanation. The higher hardness in the semi-efficient system (SEV) is associated with the high crosslink density, which reduces the mobility of the polyisoprene chains, resulting in greater rigidity. On the other hand, this structure limits the material's ability to deform elastically under stress, reducing tensile strength and plasticity. It can be inferred that these phenomena are correlated with the type of formation of shorter and more rigid crosslinks, which is characteristic of the semi-efficient system.

- A major drawback of the article is the lack of a comprehensive discussion. To date, the article is a list of various properties. Each of the properties is well researched and described, but no general trends are observed. One example is given in the previous remark - strength and hardness are studied separately, but the phenomenon of simultaneous increase in hardness with decrease in strength is not discussed. The same can be said in general about the integrated approach to the discussion of properties. The authors need to supplement the article with a full-fledged discussion and describe in more detail the theses given in Conclusions.

Dear Reviewer, we appreciate your suggestion and have included a new section: "4. Integrated Discussion of Results."

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript and responses have been well done and can be accepted now. 

Author Response

Dear reviewer,

The authors are grateful for the valuable contributions.

Best regards.

Reviewer 2 Report

Comments and Suggestions for Authors

Table (3) should be converted to figures. It is very hard to follow. I cannot review this part as it is now. I wonder why do not you convert your tables to figures. Please, try to do that. It will be really better for undestanding your results. Moreover, really I cannot review this part as it is now. Thank you.

Author Response

Dear reviewer,

Please find below the revised manuscript with Table 3 converted to a Figure as requested.

 

Best Regards.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

NO COMMENT.