Asphalt as a Plasticizer for Natural Rubber in Accelerated Production of Rubber-Modified Asphalt

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

See the comments in attachment.

Comments for author File: Comments.pdf

Author Response

Thank you to the reviewers for their critical feedback that helped improve the quality of our manuscript. We have completed the revisions to our manuscript based on your feedback. The following details are included in our response to reviewer file.

Comment : The Introduction is well written and follows a clear scheme. In my humble opinion, however, it is a little bit long. It would be preferable to divide it into two subparagraphs, maybe one related to the Polymer Modified Bitumen, and the following to the natural rubber and related issues (but it is just a personal suggestion to make easier to read and understand).

Response : We greatly appreciate the constructive feedback on the structure and length of the Introduction, and we are pleased that a precise schematic flow can be identified. Regarding the shortening and division suggestions, the current length is essential because the Introduction must comprehensively establish a logical foundation, from the broad context of Polymer-Modified Asphalt (PMA) to a specific focus on Technical Specification Natural Rubber. This depth of content is crucial to justify the major innovation of using 60/70 asphalt as an internal plasticizer by first establishing the fundamental challenges, namely the very long mixing time of TSNR (up to 180 minutes) and the problem of re-agglomeration during storage, as well as to disqualify conventional solutions such as white oil that can be detrimental to the asphalt’s stability and performance. The current structure of the Introduction has been designed to maintain smooth and cohesive narrative transitions, using paragraph breaks to mark sub-themes without breaking the logical flow, an approach we believe is more effective in guiding the reader through the urgency and significance of this research than rigid sub-paragraph divisions.

Comment : I don’t understand the meaning of Specification and Content (numbers) in Table 1. For example:

• Zinc Oxide → Technical specifications with 99% purity means that the Zinc Oxide used shouldhave a purity of 99%? Moreover, does 6% represent the dosage? Is it a percentage by massreferred to? Please clarify the Table since it is not easily understood. Based on the following 2.2section yes, but it should be clarified in the table if not it is useless.

Response : We gratefully acknowledge this valuable feedback and acknowledge that the notation in Table 1 requires explicit clarification for ease of interpretation, which we will revise in the final draft. The “Specification” column refers to the minimum quality or purity standard for the raw material used (e.g., 99% purity for Zinc Oxide). The “Content (%)” column, academically, refers to the mass fraction or relative dosage of that material based on weight of natural rubber (TSNR) per hundred parts of rubber (phr). Therefore, a figure of 6% for Zinc Oxide means that 6 grams of Zinc Oxide are used per 100 grams of TSNR, and 100% TSNR indicates that natural rubber is the basis for the calculation. To eliminate ambiguity, we will revise the column headings to “Content (phr)” or “Content (% weight per TSNR)” and add a footnote explicitly stating that all percentages are calculated based on the weight of TSNR per hundred parts of rubber (phr), so that Table 1 is fully clear and consistent with the methodology details in Section 2.2.

 

Comment : Probably it is my fault, but I can't fully understand the coding used in Table 2. Example:

• CTSNR-430 means that the TNSR used has 4% of natural rubber and 30% of 60/70 pen grade

bitumen? Moreover, the last one should be CTSNR-1050. Finally, I think that numbers are percentage and not grams (as indicated in the table): pay attention that, in some cases, the sum is not 100%.

 

Response : We appreciate your detailed inquiry regarding the sample coding system (CTSNR) in Table 2 and recognize the need for explicit clarification. The coding system (e.g., CTSNR-430) is designed so that the first digit (before the last two digits) represents the percentage of TSNR content to be used in the final asphalt mixture (4%). The last two digits represent the percentage of Asphalt Plasticizer (Asphalt Pen. 60/70) added during compounding (30%). Regarding units, we confirm that the numbers listed in the composition column (e.g., 62.5 for TSNR) do represent mass units in grams (g), which is the actual mass used to compound the compound, not the final percentage of the total mixture, so it is normal for the sum of the masses not to add up to 100%. Finally, you are correct that the last sample code in Table 2 contains a typo, and we will correct it to CTSNR-1050 in accordance with the established formulation pattern. We will add explicit clarification of this coding system and mass units to the description in Section 2.2.1, which precedes Table 2, to improve understanding.

Comment : Penetration and softening point are empirical tools for the characterization of bitumen, providing useful information on its consistency, but rheological properties cannot be defined (for sure not “key rheological properties”). In the DSR test, for a complete analysis the evaluation of G’ and G’’ would be also appropriate. No indication about the replicates tested, it would be interesting to have this information to evaluate data dispersion (which might be a problem with this type of composite material). I believe the section dedicated to rheological characterization with DSR tests is rather limited, or so it appears from what is written. No information is given on the parameters evaluated (other than describing the increase or decrease in the norm of the complex modulus G*), but nothing more. It would be appropriate to adequately expand this section since it represents the core of the paper (given the presence of the term “rheological” in the title). When innovative materials are analyzed, especially those with very complex behavior, empirical tests cannot adequately describe their behavior. A limitation of the study, in my humble opinion, is the absence of low-temperature behaviour (which can be critical for such materials, with improved stiffness and elasticity).

 

Response : We want to thank you for your in-depth review and valuable input regarding rheological characterization. We fully agree that Penetration and Softening Point are empirical properties and not fundamental rheological properties; therefore, we will revise the terms used for these properties to “consistency properties” or “conventional physical properties” in Sections 3.2 and 3.3 to maintain the accuracy of academic terminology. We also acknowledge that the current Dynamic Shear Rheometer (DSR) Section is too brief and focuses only on the shift in the material's viscoelastic behavior, which is the core of rheological behavior. However, our choice of discussion correlates with the limitation of this study regarding the absence of low-temperature testing, this is based on our study focus on the hot climate in Southeast Asia where resistance to rutting (high temperature) is a dominant issue, however, we will accept this limitation as a suggestion that we will consider in further studies in other publications, so that overall, this manuscript will be academically strengthened according to the demands of rheological analysis of composite materials in future publications.

 

Comment : The Authors say: “One of the main obstacles to producing natural rubber-modified asphalt is the difficulty of mixing technical specification natural rubber-based modified asphalt with asphalt, which results in long mixing times and high costs.”. Are they referring to “Technical Specified Rubber”?

Response : We want to thank you for your very timely request for clarification regarding terminology. TSNR stands for Technical Specification Natural Rubber, which is a type of solid natural rubber used in this study as a raw material for asphalt modification. The TSNR used in this study meets standard specifications such as the Indonesian Rubber Standard 20 (SIR 20), with characteristics including 98% dry rubber content, 0.02% impurity content, and a plasticity retention index (PRI) of 50. Although it improves asphalt mechanical properties, such as elasticity and aging resistance, the main challenge in its use is the difficulty and the long time required to mix and distribute it uniformly in hot asphalt.

 

Comment : In my humble opinion, the explanation of mixing procedure for CTSNR is redundant in the Abstract.

Response : We appreciate your feedback. We understand concerns about the length and detail of the Abstract. However, a concise description of the mixing procedure for Compounded Technical Specification Natural Rubber (CTSNR) in the Abstract has been retained for both functional and academic reasons. This research focuses on developing a method to overcome the significant obstacle of long mixing times, and the two-stage procedure (production of CTSNR, followed by production of CTSNRMA) is a key methodological innovation. Mentioning that CTSNR is produced by mastication, and that the addition of an asphalt plasticizer is important because this is the mechanism that allows for the drastic reduction in mixing time (from 180 minutes to 16 minutes). This detail is crucial because the Abstract must clearly link the key experimental variables the addition of asphalt plasticizer at concentrations of 30%, 40%, and 50% with the best results that support claims of process efficiency and improved performance (e.g., CTSNRMA-450 and CTSNRMA-440). Therefore, the explanation of the procedure is not superfluous but rather constitutes essential information that establishes the methodological premises, justifies the main findings, and demonstrates the fundamental differences of this research approach compared to traditional natural rubber asphalt modification methods.

 

Comment : Is it correct “mastication”?

Response : We thank you for your careful attention to terminology. We confirm that the term “mastication” is correct and appropriate engineering terminology in the context of natural rubber and polymer processing. In polymer chemistry and engineering, mastication refers to the intensive mechanical shearing process applied to solid rubber that intentionally shears the polymer molecular chains, thereby reducing molecular weight and, most importantly, the material’s viscosity. This process is a crucial initial step in preparing TSNR compounds, as it helps overcome the main obstacle described in the Introduction: TSNR’s high viscosity, which hinders uniform dispersion in hot asphalt and leads to very long mixing times. Therefore, the use of the term “mastication” accurately describes the viscosity reduction process that precedes the addition of asphalt plasticizers, and its use in the manuscript has been verified as correct and in accordance with scientific principles.

 

Comment : Authors say that the mixing operation is carried out for 1 hour at 150-170°C: isn't there a risk of premature and accelerated aging of the binder which could alter its stiffness and elasticity properties?

Response : We appreciate the careful attention to our operational parameters, particularly regarding the risk of premature asphalt aging during mixing. We confirm that the one-hour mixing operation at a temperature range of 150°C to 170°C was carefully designed to balance the needs for polymer dispersion and mitigate thermal aging. This temperature was specifically chosen to maintain optimal mix viscosity, promote complete dispersion of the natural rubber compound (CTSNR), and reduce the potential for thermal degradation of the TSNR polymer. While we recognize that exposure to heat and time can accelerate aging, the one-hour run at a relatively low temperature range of 150°C to 170°C has proven highly effective for achieving homogenization, thanks to the high efficiency of the asphalt plasticizer. The use of this plasticizer drastically shortens the total time required compared to traditional methods (which can take up to 180 minutes). Short-term aging that occurs during this process is normal in Hot Mix Asphalt (HMA) production and is precisely simulated by RTFOT tests, in which the increase in complex modulus (G*) after RTFOT in some samples (CTSNRMA-440 and CTSNRMA-640) indicates the desired hardening that improves rutting resistance. Therefore, the time and temperature parameters we used are believed to provide an optimal balance between achieving superior homogeneity with an acceptable aging risk within the operational limits of binder production in the field.

 

Comment : Based on my experience, it is a little too simplistic to evaluate the correct dispersion through simple sieving. After the swelling due to maltenes, depolymerization and digestion of rubber particles, the system is represented by a unique phase in which it is not easy to verify the dispersion without specific and appropriate test methods and tools

Response : We appreciate your critical feedback and views regarding the dispersion evaluation method using a simple screening (100-mesh sieve). We fully agree that after the complex process of swelling, depolymerization, and digestion of rubber particles by asphalt maltene, the resulting system (CTSNR Modified Asphalt) is close to a single-phase system, and the screening test may be too simple a method to verify dispersion at the microscale. However, we would like to emphasize that, in the context of this study, the 100-mesh screening method (as per industry standards) is primarily intended to ensure the absence of large rubber agglomerates or incompletely dissolved TSNR particles after the mixing time is stopped, which is a quick indicator of incomplete macro-homogeneity. Meanwhile, the actual dispersion characterization and rheological behavior at a finer scale, including polymer-bitumen interactions that form a rigid network, have been validated and comprehensively analyzed using more sophisticated methods, namely the Dynamic Shear Rheometer (DSR) and Storage Stability testing (which measures the tendency toward phase separation). The results of this test, including the increase in complex modulus (G*) and compliance with storage stability standards, implicitly confirm that the dispersion is sufficient to produce a stable and functional polymer-bitumen matrix, thereby demonstrating that the screening test serves as a rapid operational quality control.

 

Comment : I can't understand the meaning of the histogram in Figure 1. Does it represent the minimum time to achieve homogeneity? If so, it's not at all clear and it would be worth modifying it or replacing it with a table.

Response : We appreciate your feedback regarding the clarity of Figure 1. We confirm that the histogram in Figure 1 accurately represents the Minimum Time Required to Achieve Homogeneity of Compound TSNR (CTSNR) within the asphalt matrix, measured in minutes. We agree that the Y-axis title of the graph (“Mixing Time (minute)”) refers to the duration required for rubber particles to no longer filter through a 100-mesh sieve, indicating the achievement of a uniform macrodispersion. While the data could have been presented in tabular form, this visual representation via a histogram was chosen because it effectively highlights the contrasting comparisons between the various formulations, particularly demonstrating the dramatic decrease in mixing time for samples with higher asphalt plasticizer concentrations (such as CTSNRMA-450 at 16 minutes), which is a central finding of this study. To improve clarity, we will revise the title of Figure 1 and add a caption in Section 3.1 to explicitly define “Mixing Time” as the “Minimum Homogenization Time” required, making it easier for readers to grasp the essence of the findings.

 

Comment : “Flexibility”?

Moreover, Authors say “while reducing its thermal properties”: how can this be deduced from the showed data

Response : We appreciate your careful request for clarification of terminology. Regarding our use of the term “flexibility,” it refers to the increased elasticity and deformability of the modified material, which, in the context of asphalt binders, directly correlates with the increased low-temperature cracking resistance and elasticity observed in DSR tests (although low-temperature data are not presented). The statement “reducing its thermal properties” is imprecise and potentially ambiguous. It should refer to the reduction in viscosity and the practical melting point of TSNR resulting from mastication and the addition of asphalt plasticizers, which collectively facilitate rapid melting and dispersion of TSNR within the asphalt. This reduction (which results in greater melting and dissolution of TSNR) can be indirectly inferred from the presented data: the dramatic reduction in mixing time (from 180 minutes to 16 minutes) and changes in consistency properties (decreased penetration and increased softening point of the modified asphalt) indicate changes in the thermal/rheological characteristics of TSNR that allow it to react more rapidly with the asphalt maltene. We will revise this phrase to “increased dispersibility and decreased viscosity TSNR” to maintain academic accuracy.

 

Comment : It would be appropriate to indicate the measured and reference values for defining the PG value. Which parameters were used to evaluate the results? Moreover, it was stated “Under these conditions, the asphalt must be sufficiently rigid so that traffic loads do not cause it to deform”. It is not just a problem of stiffness, also the elastic properties (so the phase angle) are fundamentals.

Response : We gratefully acknowledge the valuable input on the characterization of Performance Grade (PG) asphalt and the need for clarification of parameters. We fully agree that the determination of high-temperature PG depends not only on stiffness (G*) but also on elastic properties, represented by the phase angle (δ), with the appropriate evaluation parameter being the ratio G*/sin δ. Therefore, we have improved this section. Regarding the stiffness statement, we fully agree that the viscoelastic properties controlled by the phase angle (δ) are fundamental, as the asphalt must exhibit sufficient elastic components for shape recovery, in addition to high stiffness (G*) to resist deformation. This is particularly relevant to the hot climate of Southeast Asia, where extreme pavement temperatures demand binders with high stiffness and a dominant elastic component (reducing the phase angle) to prevent rutting failure under heavy traffic loads.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article by Ibrahim B. et al. evaluates the use of 60/70 penetration-grade asphalt as an internal plasticizer for natural rubber to improve processing efficiency and performance of rubber-modified asphalt. Natural rubbers were prepared with 30%, 40%, and 50% asphalt added during mastication, then blended with hot asphalt (160-170°C) at rubber contents of 4% to 10%. Key findings show that adding 50% asphalt as plasticizer to natural rubber that then added to asphalt in 4% concentration reduced homogenization time from 155 min (no plasticizer) to 16 min. Penetration decreased (from 67.8 to 35 dmm) and softening point increased (from 48 to 58.4 °C) with higher plasticizer content, indicating stiffening. Storage stability improved significantly at 40% plasticizer, while higher rubber contents worsened stability. DSR testing assigned best-obtained products to Performance Grade 76, suitable for hot climates, but some products after aging can become too hard. In general, the study presents valuable experimental data and a promising processing innovation. However, the article needs to be revised before it can be considered for acceptance.

The specific comments are as follows.

Title: “Evaluation of Asphalt as a Plasticizer for Natural Rubber”. The title does not reflect the essence of the article, implying that asphalt was added to natural rubber solely for its plasticization (similar studies do exist). However, the authors also investigate modified asphalt, which is not mentioned in the article's title. Furthermore, the article does not cover mixing dynamics and rheological behavior. The title should be rewritten, for example, “Asphalt as a Plasticizer for Natural Rubber in Accelerated Production of Rubber-Modified Asphalt”.

Line 16: “One of the main obstacles to producing natural rubber-modified asphalt is the difficulty of mixing technical specification natural rubber-based modified asphalt with asphalt.” This phrase makes no sense. There cannot be a difficulty in obtaining something (natural rubber-modified asphalt) when it has already been obtained (natural rubber-modified asphalt).

Line 17: “technical specification natural rubber”. It is unclear what "technical specification" means. What kind of material is "technical specification natural rubber"? A scientific article should operate with terms such as chemical name, molecular weight, crosslink density, chemical name of the curing agent, and so on.

Line 26: “(MBTS), antioxidants (TMQ)”. Abbreviations should be spelled out upon first use. In this case, they can be omitted in the abstract.

Line 30: “CTSNRM-450” - inconsistent naming. Manuscript elsewhere uses CTSNRMA-450. The authors should standardize nomenclature (e.g., CTSNRMA-450 throughout).

Line 31: “In addition, the addition”. This is tautological. The authors should use words with different roots.

Line 35: “CTSNRMA-440… storage stability value of 0.95°C, which is well below the threshold of 2.2°C”. The ASTM D5892 criterion refers to difference in softening point between top and bottom segments after storage—not an absolute value. The wording is ambiguous. The authors should specify “a softening point difference of 0.95°C”.

Line 36: “CTSNRMA-440 sample achieved a Performance Value (PG) of 76”. Performance Value is nonstandard. Correct term is Performance Grade (PG).

Line 39: “compound technical specifications for natural rubber”, “natural rubber modified asphalt”. Keywords should be concise, preferably consisting of no more than two words.

Line 47: “he addition of polymers” - typo (he). The authors should correct to “The addition of polymers”.

Line 54: “…mitigating the environmental impact of carbon emissions[13,14]”. Rubber-modified asphalt does not mitigate carbon emissions per se - it may reduce tire wear or extend pavement life, but net emissions depend on LCA. The authors should rephrase to “potentially contributing to sustainability by valorizing renewable rubber and extending pavement service life”.

Line 59: “…poor compatibility that results in phase separation during storage, reduced rheological properties, and weakened mechanical strength”. This conflates compatibility (thermodynamic) with stability (kinetic). Phase separation reflects instability, not necessarily incompatibility (e.g., kinetically trapped blends may be metastable). The authors should revise for precision, e.g., “thermodynamic incompatibility may lead to macroscopic phase separation during storage…”.

Line 103: “adding a solid natural rubber compound requires approximately 12 hours… NRL requires ~4 hours[28]”. The temperature must be indicated.

Line 110: “can accelerate the depolymerization process”. The alleged depolymerization process is highly questionable for natural rubber under the stated conditions. The authors are likely referring to the depolymerization of vulcanized natural rubber.

Lines 125, 131: “can significantly affect the melting temperature of natural rubber, which accelerates the melting process”. The melting temperature of natural rubber is approximately 30°C. The authors likely mean the decomposition temperature if they are referring to a temperature around 200°C. In that case, using the term "the melting process" is incorrect; it should be "the mixing process."

Line 140: “White oil is a type of plasticizer commonly used… included in the oil-based plasticizer group (paraffinic, naphthenic, and aromatic oils)”. White oil is typically highly refined paraffinic/naphthenic oil - not aromatic. Aromatic oils are discolored and restricted due to PAHs. The authors should clarify: “highly refined paraffinic or naphthenic process oils (often termed ‘white oils’)”.

Line 165–166: “In previous studies, the use of 1–2% asphalt as a plasticizer in natural rubber has been shown to increase mixing time”. Contradicts the paper’s own claim (asphalt reduces mixing time). Likely misphrased - probably decrease mixing time. The authors should correct increase -> decrease, or provide evidence and clarify mechanism.

Line 173: “asphalt is a petroleum-derived material with a high molecular weight, exhibits wax-like characteristics”. Asphalt does not exhibit wax-like characteristics (crystalline state). Furthermore, asphaltenes (as a component of asphalt) have previously been used as plasticizers for synthetic rubbers, e.g., for SIS (see 10.3390/polym14204296), as well as for plasticizing and improving the properties of other polymers, including epoxy resin, polypropylene, polyisobutylene, polystyrene, and so on (see Ignatenko's works). The authors are not the first to use asphalt and its components for polymer plasticization, which should be reflected in the introduction.

Line 192: “TSNR used in this study has specifications”. The average molecular weight of the natural rubber should be provided.

Table 1. “Content (%)”. The total content of all components must equal 100%, not 142%.

Table 2: CTSNR-1040 is listed twice in header, but compositions differ: first has 53.6 g TSNR, second 44.6 g. Presumably a typo—should be CTSNR-1040 and CTSNR-1050. The authors should correct column labels.

Line 228: “mechanically stirred at a constant speed of 500 rpm [5,7,12]”. Mixing speed is process-specific - not a universal standard. Citing literature for this value is inappropriate unless those studies actually used 500 rpm. The authors should delete citation or cite equipment manual/method optimization study. Moreover, the shear rate during mixing (in s⁻¹) should be provided.

Line 231: “to maintain optimal mix viscosity”. This viscosity should be specified.

Line 271: “has been shown to alter the rheology of natural rubber and improve its blending with asphalt, as illustrated in Figure 1”. Figure 1 does not show data for the rheology of natural rubber. The authors should provide data on how the addition of asphalt affects the viscosity of natural rubber.

Figure 1, caption. It is unclear which specific type of CTSNR (e.g., TSNR-430, CTSNR-440, CTSNR-450, etc.) was added to the asphalt. The same applies to Table 3.

Line 281: “CTSNRM-450… 16 minutes… CTSNRMA-400… 155 minutes”. CTSNRMA-400 (0% plasticizer) is not defined in Tables 2–3. No composition provided. The authors should add its formulation to Table 2 or clarify whether this is TSNR directly mixed without compounding.

Line 323: “…swelling process facilitates absorption of… maltene… forming a larger and stronger network”. This overstates—rubber swelling in bitumen forms a gel phase, but does not necessarily create a stronger network unless dissolution with the subsequent crosslinking occurs. The paper shows no evidence of covalent crosslinking. The authors should moderate wording, e.g., “increasing the volume fraction of the rubber-rich phase”.

Line 458: “CTSNRMA-440 and CTSNRMA-640 exhibited… increase in G* by up to twofold… indicating excessive hardening… susceptibility to low-temperature and fatigue cracking”. No low-temperature data (e.g., BBR, m-value) are presented to support low-temperature cracking risk. The authors should either delete the low-temperature crack claim or add supporting data.

Lines 480-524. “Conclusions”. This reads more like a Discussion section rather than Conclusions. The Conclusions section should be concise, highlighting the most important scientific findings of the study, possibly in a list format.

Line 481 and elsewhere: “in Compounded Technical Specification Natural Rubber”. It is unclear why the authors repeatedly use "Technical Specification Natural Rubber" throughout the text. The type of natural rubber only needs to be specified two or three times – in the abstract, introduction, and Materials section. In all other instances, simply "Natural Rubber" is sufficient.

Line 485: “reducing the rubber’s inherently high viscosity”. The authors do not measure viscosity to support such claims. They must either measure the viscosity or justify the assertion of viscosity reduction by citing literature demonstrating that the addition of asphalt or asphaltenes to polymers reduces their viscosity.

Line 490: “CTSNRMA-450… mixing time… 16 minutes… industry standard… 60 to 180 minutes”. “Industry standard” is vague – no source provided. The authors should cite a standard practice document or published benchmark.

References 5, 7, 12, 25: Repeatedly cited for multiple unrelated claims (softening, swelling, degradation, mastication). Over-reliance on a narrow set (mostly same research group). The authors should diversify sources and directly support each mechanistic claim with appropriate literature.

Comments on the Quality of English Language

The English requires editing to remove typos and improve the style of the text.

Author Response

Thank you to the reviewers for their critical feedback that helped improve the quality of our manuscript. We have completed the revisions to our manuscript based on your feedback. The following details are included in our response to reviewer file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Although the discussion regarding rheological properties could be improved, the changes made and the answers given to the questions posed in the reviewing process make the paper ready for publication.

Author Response

Thank you to the reviewers for their critical feedback that helped improve the quality of our manuscript. We have completed the revisions to our manuscript based on your feedback. The following details are included in our response to reviewer file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have implemented minor textual corrections (e.g., expansion of MBTS/TMQ, removal of tautology, nomenclature standardization to CTSNRMA), which is acknowledged. However, the revised manuscript—despite promises made in the cover letter—fails to correct fundamental scientific inaccuracies, omits essential data, ignores prior art, and retains unsubstantiated mechanistic claims. Several responses to reviewer comments read as pro forma acknowledgements without actual implementation. The current version misrepresents polymer physics, overinterprets rheological data, and creates a misleading impression of novelty. Publication would undermine scientific credibility and mislead practitioners.

Below are the outstanding issues. Each is severe, independently sufficient for rejection, and must be fully resolved.

1. Core mechanistic claim—“asphalt reduces rubber viscosity”—remains entirely unsubstantiated, despite explicit promise to provide data or literature support. The article repeatedly assert that asphalt “reduces the rubber’s inherently high viscosity”, “softens TSNR”, “produce a compound with low viscosity”, and “accelerates mixing due to low viscosity”. The cover letter admits this claim lacks direct measurement and pledges to “include relevant viscosity data… or substantiate… with literature citations”. Yet the revised manuscript contains no viscosity data whatsoever for raw TSNR, masticated TSNR, or CTSNR (before mixing with bulk asphalt). No Mw is reported, despite promise. No new citations are added to justify viscosity reduction of polymers by asphaltenes. Furthermore, the key reference of the article (Dong et al., Constr. Build. Mater. 2012, DOI:10.1016/j.conbuildmat.2011.10.021) shows swelling, not plasticization, as the dominant mechanism for rubber and asphalt. The observed reduction in mixing time (155 -> 16 min) is presented as proof of viscosity drop, but time savings may equally stem from improved wettability, reduced agglomeration, or thermal effects—none of which imply true plasticization. This conflation of processing efficiency with rheological change is scientifically indefensible. Either direct rheometry (e.g., Mooney viscosity at 100 °C) must be added, or all claims of “viscosity reduction” must be deleted and replaced with cautious, evidence-based language.
2. Prior art on asphalt/asphaltenes as polymer plasticizers is deliberately omitted, creating false novelty. The reviewer explicitly noted that authors are not the first to use asphaltenes as plasticizers—citing Ignatenko et al. and others authors for plasticization of SIS, PP, epoxy, etc. by asphalt’s components—and demanded this be reflected in the Introduction. The authors promised: “we will revise the Introduction to clarify the status of asphaltene plasticizers in the literature…”. Yet in the revised manuscript, line 202–204 still reads: “asphalt is a high-molecular-weight, petroleum-derived material with good plasticizing ability”—with zero citations. No attempt is made to position this work relative to the established field of asphaltene-polymer interactions. Instead, the text implies originality by omission. This is unacceptable in scholarly work. The Introduction must be rewritten to explicitly acknowledge prior use of asphaltenes for rubber/plastic modification, state that bulk asphalt (not isolated fractions) is used here, and clarify that the novel contribution lies in internal plasticization during TSNR compounding to enable storage and accelerate final blending—not in the plasticizing effect per se.
3. Scientifically erroneous term “melting” at 200 °C is retained despite rejection of correct polymer physics. The cover letter reveals authors refuse correction: “we consider the choice of the words ‘melting temperature’ and ‘melting process’… to be appropriate”. This is categorically false. Natural rubber (amorphous cis-1,4-polyisoprene) has no melting point; its Tg ≈ −70 °C, and at 200 °C it undergoes thermo-oxidative chain scission (e.g., Jiang et al., J. Appl. Polym. Sci. 2024, DOI:10.1002/app.55036), not melting. Yet the article still contain “affect the melting temperature of natural rubber”, “accelerating melting”, and so on. These phrases misrepresent the underlying chemistry and mislead readers unfamiliar with polymer degradation. No journal of materials science can countenance such fundamental error. All instances of melting relative to TSNR above 100 °C must be replaced with thermal softening (≤100 °C, mastication), swelling-induced disintegration, or depolymerization (chain scission) (≥150 °C), with appropriate mechanistic citations.
4. Critical rheological data (low-temperature performance) are absent, yet high-temperature PG assignment implies full grading. The manuscript assigns Performance Grades (PG 76, PG 82) based solely on high-temperature DSR failure temperatures. However, PG classification requires both high- and low-temperature validation (e.g., BBR for m-value and stiffness at −12 °C to −24 °C). Article text (line 520) and its conclusions (lines 571–577) state that excessive post-RTFOT hardening “could potentially compromise long-term reliability” and “increase susceptibility to low-temperature… cracking”—yet no low-temperature data exist. The cover letter admits this and promises to “remove the low-temperature cracking susceptibility claim”, but the revised text retains it verbatim. This overreach misleads engineers who rely on PG for climate-specific binder selection. Either BBR/m-value data must be added, or all reference to PG must be replaced with “high-temperature failure temperature = X °C”, and all low-temperature implications must be deleted with explicit disclaimer: “Low-temperature performance was not evaluated; full PG classification is not supported.”
5. Conclusions section remains a rehashed Discussion, undermining scholarly clarity and concision. The authors rejected the structural critique, claiming their narrative “presents the interconnections among findings”. This is not a justification—it is a failure to follow disciplinary norms. Journals (including Constr. Mater.) require Conclusions to be succinct, forward-looking, and distinct from Results/Discussion. The current 19 lines (539–557) re-describe mixing time, penetration, softening point, and DSR trends—repeating Figure 1–9 content. A proper Conclusions section would occupy ≤6 lines: (1) asphalt plasticizer reduces CTSNR homogenization time to 16 min (91% improvement); (2) CTSNRMA-440 achieves storage stability (ΔSP = 0.95 °C < 2.2 °C) and high-T performance (79 °C failure); (3) high TSNR (>6%) risks phase separation or post-aging softening; (4) mechanism is swelling-driven volume increase, not covalent crosslinking; (5) limitations: no low-T data, no direct rheology of CTSNR; (6) future work: BBR, field trials. The current version wastes reviewer and reader time.
6. Persistent overreliance on narrow self-citation loop weakens scholarly foundation. The cover letter acknowledges “some references… appear frequently” but promises no concrete action beyond vague “ensure diversification”. In the second manuscript version, references [5, 7, 12, 25] (all same group) are cited many times for distinct phenomena: mastication, swelling, degradation, stability. Meanwhile, key independent works (Xia 2021 on degradation kinetics, Guo 2020 on MD compatibility, Fan 2024 on surface-functionalized CRM stability) are absent. This insularity suggests either unawareness of the field or deliberate framing to exaggerate novelty. Citations to [5,7,12,25] must be replaced or supplemented with literature directly supporting the claimed mechanisms, per the reviewer’s specific suggestions.

Thus, this manuscript contains valuable applied data (mixing time reduction, storage stability improvement) but is scientifically immature in its current framing. The authors must rigorously correct the above issues—adding missing data, deleting unsupported claims, citing fairly, and respecting polymer physics.

Comments on the Quality of English Language

The English requires editing to remove typos and improve the style of the text.

Author Response

Thank you to the reviewers for their critical feedback that helped improve the quality of our manuscript. We have completed the revisions to our manuscript based on your feedback. The following details are included in our response to reviewer file.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have improved the article for its publication.