Bridging Circular Design Strategies and Natural Fiber Reinforced Polymer Composites: A Preliminary Conceptual Framework

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The current paper presents a framework for systematically integrating Circular Design strategies into Natural Fiber Reinforced Polymer Composite (NFRPC) product development, addressing their unique material properties, lifecycle behavior, and potential for sustainable manufacturing. However, the content, in its current form, is not sufficient for publication in Sustainability.

The novelty of the work is not clearly demonstrated, and the type of paper—whether it is purely a literature review or otherwise—needs to be clarified. The connection between Figure 2 and Circular Design is unclear, as is the purpose of including the research method; while such a method may be important for a research project, its relevance in this paper should be justified. The term “gaps” should be defined—does it refer to material defects or gaps identified in the literature?

In line 228, the statement “To test the applicability and usability of the adapted tools, preliminary validation was conducted through two industrial design engineering bachelor theses” raises concerns about the significance of such validation to the proposed framework. Using students as part of the framework may suggest that the research is limited to an academic context rather than applicable to the industrial field, and it is unclear how individuals with limited experience can effectively assess the framework.

In addition, figure quality and typography should be checked, as there are noticeable extra spaces between words. The paper lacks application to a real case involving a natural fiber to validate the framework. Furthermore, the “Results and Discussion” section contains no actual results or meaningful discussion. Finally, both the abstract and the title fail to accurately reflect the paper’s content.

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript and for the constructive feedback, which has been extremely valuable in improving the quality, clarity and overall contribution of this work. Below we address each of the points raised:

C1: The novelty of the work is not clearly demonstrated, and the type of paper—whether it is purely a literature review or otherwise—needs to be clarified. The connection between Figure 2 and Circular Design is unclear, as is the purpose of including the research method; while such a method may be important for a research project, its relevance in this paper should be justified. The term “gaps” should be defined—does it refer to material defects or gaps identified in the literature?

R1: Clarification of novelty and paper type: The manuscript has been revised throughout to better articulate its objective, scope and contribution. The introduction now explicitly positions the work as a conceptual framework proposal informed by a critical review of literature and adapted tools. The text highlights how this framework specifically addresses the unique challenges and opportunities of NFRPCs, thus bridging the gap between research on sustainable materials and practical design application.

Definition of “gaps”: The term “gaps” is now explicitly defined in the manuscript as referring to unaddressed challenges and opportunities identified in the literature regarding the integration of circular design strategies with NFRPCs, avoiding any confusion with material defects.

Clarification of section 3 (Method) and Figure 2: The Method section has been expanded to clearly describe the step-by-step approach used to develop the framework. Figure 2 has been retained as a visual aid, with an improved caption and explanation in the text to strengthen its connection to Circular Design and justify its relevance within the paper.

C2: In line 228, the statement “To test the applicability and usability of the adapted tools, preliminary validation was conducted through two industrial design engineering bachelor theses” raises concerns about the significance of such validation to the proposed framework. Using students as part of the framework may suggest that the research is limited to an academic context rather than applicable to the industrial field, and it is unclear how individuals with limited experience can effectively assess the framework.

R2: Use of the term “validation”: We acknowledge the ambiguity that the term “validation” may have introduced. To avoid overstating the scope of the study, this has been replaced with “case studies” to describe the first iteration of the framework conducted in an academic environment. This change emphasizes that these activities were exploratory and formative, intended to gather initial feedback and refine the framework, rather than to provide definitive validation.

C3: In addition, figure quality and typography should be checked, as there are noticeable extra spaces between words. The paper lacks application to a real case involving a natural fiber to validate the framework. Furthermore, the “Results and Discussion” section contains no actual results or meaningful discussion. Finally, both the abstract and the title fail to accurately reflect the paper’s content.

R3: Figure quality and typography: All figures have been revised and improved to ensure visual clarity and consistency. Spacing and typographic issues have been corrected.

Results and Discussion section: This section has been significantly revised to provide a more structured and meaningful discussion of the findings, including the insights derived from the case studies and their implications for the applicability and further development of the framework.

Title and Abstract: Both have been rewritten to more accurately reflect the paper’s content, clearly emphasizing the proposal of a conceptual framework, its methodological approach and its potential contribution to advancing Circular Design in the context of NFRPCs.

References: Additional references have been included to strengthen the theoretical basis of the work and support key claims.

We are grateful for the reviewer’s comments, which have led to substantial improvements and greater clarity in the manuscript. We believe these revisions have strengthened the contribution of the paper and improved its suitability for publication in Sustainability.

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript ID: sustainability-3823245

Title: Bridging Circular Design Strategies and Natural Fiber Reinforced Polymer Composites: A Conceptual Framework

 

The present manuscript seeks to establish a linkage between circular design (CD) strategies and natural fiber-reinforced polymer composites (NFRPCs), a subject that is of considerable pertinence to the field of sustainable materials science.  However, major revisions are required to enhance the framework's practicality and scholarly impact.

1. The proposed CD framework for NFRPCs is primarily derived from a synthesis of literature and preliminary testing, which is insufficient to demonstrate its robustness. The validation process should be expanded through the use of diverse case studies, for example industrial-scale NFRPC products in the automotive or construction sectors, and the outcomes of this process should be quantified (for example improved recyclability rates and lifecycle cost reductions) in order to verify its applicability across sectors.
2. Section 4.1 establishes a correlation between CD principles and NFRPCs; however, it does not provide a detailed analysis of how unique NFRPC properties (e.g., biodegradability, moisture sensitivity, fiber-matrix compatibility) directly influence design decisions. For instance, it is necessary to elucidate the manner in which hydrophilic natural fibers influence the selection of reversible joining techniques or end-of-life degradation pathways. This discussion should be supported by references to material science studies on NFRPC ageing and disassembly.
3. The manuscript states that CD tools from the Ellen MacArthur Foundation were "adapted" for NFRPCs, but provides minimal detail on these adaptations. A comparison of the original tools and the modified tools is required, with the modified tools being exemplified by the 'Material Journey Mapping' adjustments for fiber sourcing variability. The changes made must be justified with technical arguments, for example by explaining why standard recyclability metrics fail for bio-based composites.
4. Empirical evidence from structured user studies (e.g., surveys, focus groups with manufacturers or consumers) should be incorporated to quantify factors like acceptance of NFRPCs' aesthetic variability or willingness to engage in repair/maintenance, as these are critical for the viability of the circular business model.
5. The framework does not provide a detailed explanation of how these criteria are applied in a unique manner to NFRPCs. It is imperative to establish a distinction between metrics for bio-based and synthetic polymer matrices. Furthermore, challenges such as the accounting for fiber, agricultural inputs, and biodegradation end-of-life scenarios must be addressed within Life Cycle Assessment (LCA) methodologies.
6. It is evident that key terms such as 'circularity' and 'bio-based' are employed in a inconsistent manner. Explicit definitions must be provided for domain-specific terms, and consistent usage must be ensured throughout the text, particularly in sections linking material properties to design strategies.
7. It is evident that Figures 3 and 4 lack sufficient detail in the text. It is imperative to ensure that the figures are accompanied by clear labels for each framework phase, tool, and decision point, with explanatory text detailing the interaction between these elements.
8. The reference list is inadequate in terms of its robustness. Furthermore, a significant number of references exhibit inconsistent formatting, including the absence of DOI hyperlinks and variations in author or journal name styling. It is imperative that all citations are aligned with the journal's guidelines, whilst also incorporating seminal works on NFRPC lifecycle analysis and circular composite design.

Author Response

We are very grateful to the reviewer for the thorough and constructive feedback, which has been instrumental in refining and strengthening our manuscript. Below, we address each comment in detail and explain the revisions made:

C1: The proposed CD framework for NFRPCs is primarily derived from a synthesis of literature and preliminary testing, which is insufficient to demonstrate its robustness. The validation process should be expanded through the use of diverse case studies, for example industrial-scale NFRPC products in the automotive or construction sectors, and the outcomes of this process should be quantified (for example improved recyclability rates and lifecycle cost reductions) in order to verify its applicability across sectors.

R1: Framework robustness and validation. We fully acknowledge the reviewer’s concern regarding the robustness of the proposed framework. This paper intentionally represents the first iteration of the framework, focusing on synthesizing insights from the literature and exploring their application in an academic setting. To avoid any ambiguity, the term “validation” has been replaced with “case studies”, clarifying that these were exploratory applications conducted through bachelor’s theses to generate formative feedback rather than definitive verification. The conclusions now explicitly outline future work, including the plan to expand testing to a greater number of participants and to real-world contexts, such as local social economy projects in collaboration with the Cabildo of Gran Canaria.

C2: Section 4.1 establishes a correlation between CD principles and NFRPCs; however, it does not provide a detailed analysis of how unique NFRPC properties (e.g., biodegradability, moisture sensitivity, fiber-matrix compatibility) directly influence design decisions. For instance, it is necessary to elucidate the manner in which hydrophilic natural fibers influence the selection of reversible joining techniques or end-of-life degradation pathways. This discussion should be supported by references to material science studies on NFRPC ageing and disassembly.

R2: Linking NFRPC properties with design decisions. We appreciate this valuable suggestion. Section 4.1 has been revised to better articulate how the unique properties of NFRPCs directly inform design decisions, particularly in areas such as joining techniques, durability considerations and end-of-life strategies. Additional references to material science studies on NFRPC have been included to strengthen this discussion.

C3: The manuscript states that CD tools from the Ellen MacArthur Foundation were "adapted" for NFRPCs, but provides minimal detail on these adaptations. A comparison of the original tools and the modified tools is required, with the modified tools being exemplified by the 'Material Journey Mapping' adjustments for fiber sourcing variability. The changes made must be justified with technical arguments, for example by explaining why standard recyclability metrics fail for bio-based composites.

R3: Adaptation of Circular Design tools. The description of each framework phase has been rewritten to provide a clearer explanation of the tool adaptations and their rationale in the context of NFRPCs. We have also prepared complementary material (included as supplementary files).

C4: Empirical evidence from structured user studies (e.g., surveys, focus groups with manufacturers or consumers) should be incorporated to quantify factors like acceptance of NFRPCs' aesthetic variability or willingness to engage in repair/maintenance, as these are critical for the viability of the circular business model.

R4: Empirical evidence and user studies. While comprehensive empirical validation is outside the scope of this first iteration, we recognize its importance for advancing this research. Relevant references have been added to support the importance of user perception and future work will include structured user studies to quantify acceptance of aesthetic variability, willingness to repair and other behavioral factors that influence circular adoption.

C5: The framework does not provide a detailed explanation of how these criteria are applied in a unique manner to NFRPCs. It is imperative to establish a distinction between metrics for bio-based and synthetic polymer matrices. Furthermore, challenges such as the accounting for fiber, agricultural inputs, and biodegradation end-of-life scenarios must be addressed within Life Cycle Assessment (LCA) methodologies.

R5: Application of criteria to NFRPCs. The revised manuscript clarifies the framework’s intention to guide designers in systematically addressing key NFRPC characteristics during early design stages. While we acknowledge variability across different NFRPC systems, the framework includes an experimentation step that encourages designers to set parameters such as fiber type, matrix selection and end-of-life strategy. This approach provides a structured yet flexible pathway to consider these variables rather than overlook them.

C6: It is evident that key terms such as 'circularity' and 'bio-based' are employed in a inconsistent manner. Explicit definitions must be provided for domain-specific terms, and consistent usage must be ensured throughout the text, particularly in sections linking material properties to design strategies.

R6: Terminology consistency. We agree with the reviewer’s observation and have added explicit definitions for key terms such as circularity and bio-based in the introduction. The manuscript has been carefully reviewed to ensure consistency in their use across all sections, especially where material properties are linked to design strategies.

C7: It is evident that Figures 3 and 4 lack sufficient detail in the text. It is imperative to ensure that the figures are accompanied by clear labels for each framework phase, tool, and decision point, with explanatory text detailing the interaction between these elements.

R7: Figures 3 and 4. Figure 3 has been revised to improve clarity and visual readability. Additionally, the text preceding Figure 4 now contains a more detailed explanation of its contents and instructions for interpretation, ensuring that readers can clearly understand the relationship between framework phases, tools and decision points.

C8: The reference list is inadequate in terms of its robustness. Furthermore, a significant number of references exhibit inconsistent formatting, including the absence of DOI hyperlinks and variations in author or journal name styling. It is imperative that all citations are aligned with the journal's guidelines, whilst also incorporating seminal works on NFRPC lifecycle analysis and circular composite design.

R8: References. The entire reference list has been reviewed and reformatted to meet the journal’s style requirements, including the addition of DOI hyperlinks where available. Additional works on NFRPC lifecycle analysis and circular composite design have been incorporated to strengthen the manuscript’s scholarly grounding.

We sincerely thank the reviewer once again for these insightful recommendations, which have substantially improved the clarity, depth and practical relevance of our work. We believe that the revisions have resulted in a more robust and comprehensive manuscript that makes a stronger contribution to the field.

Reviewer 3 Report

Comments and Suggestions for Authors

The submission investigates the conceptual approach to the recycling of natural fiber reinforced polymer composites. The potential of biodegradability and lower environmental impact for artificially fabricated materials attracts the public attention last decades. This makes the work done by authors of interest for readers of MDPI Sustainability.

 

 My remarks are below:

 

1. Describing the circular design (CD) the reader finds a number ideas that usually have been related to ideas of recycling the materials or devices, etc. I believe that the authors have to describe precisely the difference of approaches, the relations between them. Pls. reveal.
2. 1. The authors, for, sure cannot mention in Fig.1 all natural ingredients or all possible matrix polymers. It seems to me that the figure to be reviewed in a way to be done in more general way, now it looks as unfinished one. Pls. correct.
3. Page 4. Four key points of CD are presented, and further in the Pages 6-7 the reader fins key points of CD for NFRPC, names are in the same manner, but in the submission the authors initially discuss filled polymers but not the devices made of it. In view of this at least Assembly and Disassembly & Repair and Maintenance are out of topic of the submission about CD for NFRPC, i.e. they relate to the units made of NFRPC, but no word is related to the polymers themselves. Pls. discuss this issue (here and probably in the others places of the submission) and strengthen the focus the attention on CD for NFRPC.
4. 4.2. is devoted to proposed CD framework and shown the sequence of steps in CD. It would be a really good idea not only to mention two diplomas (with some kind of unclear to the reader images) completed in the accordance with proposed CD but to place them (or its parts related to CD) in Supplemental material section. It will really help to the readers to go into CD conception. Pls. add information in supplemental section.
5. In the submission is not discussed or even mentioned financial aspect of CD. What is the estimation of the CD for NFRPC in this sence, where and when one can expect additional expenses, or time-consuming processes, how they can be estimated, etc. Pls. add such suggestions.

 

 

Thus, the article to be recommended to publication in MDPI Sustainability after major revision.

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript and the constructive remarks, which have helped us clarify and improve the contribution of this work. Please find below our detailed responses and the corresponding revisions made:

C1: Describing the circular design (CD) the reader finds a number ideas that usually have been related to ideas of recycling the materials or devices, etc. I believe that the authors have to describe precisely the difference of approaches, the relations between them. Pls. reveal.

R1: Clarification of Circular Design concepts and their relation to recycling. We appreciate this observation. To avoid confusion, we have added more explicit definitions in the introduction that differentiate Circular Design from recycling-focused approaches. Additionally, we have clarified the relationship between these concepts, emphasizing that Circular Design goes beyond recycling to address strategies for extending product lifespan, enabling reuse and designing for disassembly from the earliest stages of development.

C2: The authors, for, sure cannot mention in Fig.1 all natural ingredients or all possible matrix polymers. It seems to me that the figure to be reviewed in a way to be done in more general way, now it looks as unfinished one. Pls. correct.

R2: Figure 1 completeness and generalization. We agree with the reviewer that it is not feasible to list all natural fibers and polymer matrices. To improve clarity, Figure 1 has been revised to present the materials as broader categories with examples of the most commonly used types. A note has been added in the figure caption explicitly stating that the elements shown are representative examples rather than an exhaustive list.

C3: Page 4. Four key points of CD are presented, and further in the Pages 6-7 the reader fins key points of CD for NFRPC, names are in the same manner, but in the submission the authors initially discuss filled polymers but not the devices made of it. In view of this at least Assembly and Disassembly & Repair and Maintenance are out of topic of the submission about CD for NFRPC, i.e. they relate to the units made of NFRPC, but no word is related to the polymers themselves. Pls. discuss this issue (here and probably in the others places of the submission) and strengthen the focus the attention on CD for NFRPC.

R3: Focus of CD principles on NFRPCs vs. finished products. This is a very valuable point. The text in the section on Assembly and Disassembly has been revised to clarify the distinction between connections at the product level (e.g. joints between parts) and those at the material level (e.g. fiber–matrix interfaces). Similarly, the section on Repair and Maintenance has been substantially rewritten to ensure its focus remains on product design considerations when using NFRPCs. These changes strengthen the coherence between the principles discussed and the topic of Circular Design for NFRPC-based products.

C4: 4.2. is devoted to proposed CD framework and shown the sequence of steps in CD. It would be a really good idea not only to mention two diplomas (with some kind of unclear to the reader images) completed in the accordance with proposed CD but to place them (or its parts related to CD) in Supplemental material section. It will really help to the readers to go into CD conception. Pls. add information in supplemental section.

R4: Supplementary material with case study details. Following the reviewer’s suggestion, we have included supplementary material containing selected work templates used by the students who applied the framework during their bachelor theses. This addition allows readers to see concrete examples of how the proposed approach was used in practice, thereby making the framework more tangible.

C5: In the submission is not discussed or even mentioned financial aspect of CD. What is the estimation of the CD for NFRPC in this sence, where and when one can expect additional expenses, or time-consuming processes, how they can be estimated, etc. Pls. add such suggestions.

R5: Economic aspects of Circular Design. We appreciate this important suggestion. Although economic assessment is not the main objective of this first iteration, we agree that it is a crucial factor in industrial adoption. A note has been added to the conclusion section acknowledging the relevance of financial aspects and highlighting that these will be addressed in future stages of the research as the framework is further developed and tested in real industrial contexts.

Once again, we are grateful for the reviewer’s detailed and constructive feedback. We believe that the revisions have improved the precision, coherence and applicability of our manuscript, making it more valuable to the readers of Sustainability.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author has very well answered all the questions I raised.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have revised and improved the manuscript, thereby satisfactorily addressing the previously expressed concerns.  It is therefore recommended that the manuscript be received.

Reviewer 3 Report

Comments and Suggestions for Authors

I believe that the submission is greately imroved and can be acceped by the jounal in present form