Composite Material Recycling Technology—State-of-the-Art and Sustainable Development for the 2020s

Round 1

Reviewer 1 Report

Dear author,

Compliments for addressing such an important theme with a comprehensive literature review. Your paper certainly has potential to be a guideline for many future works. In order to recommend the acceptance of your article, I advise you to conduct a few adjustments:

1. The abstract must be a formal straight-forward part of the text, basically informing: general context + specific context + problem identification + topics that were reviewed + how these topics were structured; in that order. Please leave it that way. Also, leave no words in bold. Furthermore, instead of raising a question in the end, make the statement more formal just explaining as an affirmation that raw material cost is not always imperative to determine the importance of a certain fiber to be recycled, because its market share in volume must also be taken into account (e.g. glass vs carbon).

2. In Table 3, it seems to me that the post-pyrolysis properties of carbon are way too generous. They usually are smaller than that, and this information solely might mislead the reader. Please, cite at least two or more sources to demonstrate a range for the retained tensile strength instead of only a specific value. The same should be done for other EoL options that have data available in the literature.

3. I believe that, when writing a review paper about composites recycling, it is mandatory to approach the topic of self-reinforced thermoplastic composites, given their ultimate recyclability. Studies with polyethylene deserve special attention, given that this is the general-use polymer with higher production worldwide, and that it can be applied in actual high-performance composites being reinforced by UHMWPE, for instance. Please write about it.

4. Since it is a review paper, it is natural that most of the figures may have to be original from other sources. Please, make sure you have copyright permissions for all of them.

5. Please, use 3-levels subsections (e.g. 2.1.1.). Given the large amount of information, it is a bit confusing to read the text as it is. Special attention to sections 1.2 and 2.1. Section 4 could also be easily divided into 4.1 (mechanical), 4.2 (thermal) and 4.3 (chemical).

Author Response

- Dear author,

Compliments for addressing such an important theme with a comprehensive literature review. Your paper certainly has potential to be a guideline for many future works. In order to recommend the acceptance of your article, I advise you to conduct a few adjustments:

Our reply:

We thank you for taking the time to carefully read our manuscript and for the valuable comments and the critical feedback you have provided, which helped us in improving the revised paper that we are re-submitting for review. Please find below our response to each comment.

The authors have performed corrections throughout the manuscript based on reviewer’s suggestions and comments. Most of comments were addressed in the revised manuscript.

1. The abstract must be a formal straight-forward part of the text, basically informing: general context + specific context + problem identification + topics that were reviewed + how these topics were structured; in that order. Please leave it that way. Also, leave no words in bold. Furthermore, instead of raising a question in the end, make the statement more formal just explaining as an affirmation that raw material cost is not always imperative to determine the importance of a certain fiber to be recycled, because its market share in volume must also be taken into account (e.g. glass vs carbon).

Our reply:

The authors would like to thank the reviewer for a lot of insightful comments. The style of the Abstract was chosen to be as such on purpose. The explanation is the following. The manuscript is aimed at both academia and industry. Furthermore, the authors and interested sides are also involved in both academia and industry. The reason for writing the abstract in the following manner is to interest the reader also from the “differently busy” and less formal industrial world (in terms of the language). This was done to increase the readability of the article in the both academic and industrial circles as a compromise. Furthermore, the authors are proponents of the scientific language to become livelier on purpose, i.e., by a mindful use of larger proportion of active voice versus passive voice. This idea was introduced in a book “How to Write a Lot: A Practical Guide to Productive Academic Writing” by a Prof. Paul J. Silvia. Therefore, the authors would prefer that the style of the Abstract stays as is.

All the text in marked in bold was made regular per request of the reviewer.

2. In Table 3, it seems to me that the post-pyrolysis properties of carbon are way too generous. They usually are smaller than that, and this information solely might mislead the reader. Please, cite at least two or more sources to demonstrate a range for the retained tensile strength instead of only a specific value. The same should be done for other EoL options that have data available in the literature.

Our reply:

Excellent comment, thank you! The authors believe that this could be the reason for an average rating in the category “Is the work scientifically sound and not misleading”. In order to improve it, additional sources were referenced, and the range of properties was expanded where it was possible to represent the data accurately.

3. I believe that, when writing a review paper about composites recycling, it is mandatory to approach the topic of self-reinforced thermoplastic composites, given their ultimate recyclability. Studies with polyethylene deserve special attention, given that this is the general-use polymer with higher production worldwide, and that it can be applied in actual high-performance composites being reinforced by UHMWPE, for instance. Please write about it.

Our reply:

Thank you for your valuable suggestion. Information was added in Chapter 3 in Subchapter 3.1.1 Thermoplastic composites and in the Conclusions. An article on the topic was referenced.

4. Since it is a review paper, it is natural that most of the figures may have to be original from other sources. Please, make sure you have copyright permissions for all of them.

Our reply:

The authors have remade a Figure 2 in higher quality per request of another reviewer. The copyright permissions were doublechecked and the authors confirm that all the material in the paper can be used as is.

5. Please, use 3-levels subsections (e.g. 2.1.1.). Given the large amount of information, it is a bit confusing to read the text as it is. Special attention to sections 1.2 and 2.1. Section 4 could also be easily divided into 4.1 (mechanical), 4.2 (thermal) and 4.3 (chemical).

Our reply:

Thank you for an excellent comment that led to good improvement of the manuscript. The authors have structured the document using the 3-levels subsections and improved its readability. All sections have been improved. The weakest category of the manuscript was “Is the work well organized and comprehensively described”, which has been improved due to the comment of the reviewed. Thank you very much.

Author Response File: Author Response.docx

Reviewer 2 Report

The article entitled: “Composite Material Recycling Technology – State-of-the-Art and Sustainable Development for the 2020s.” is appropriate for Journal of Composites Science. The authors present interesting and up-to-date topic supported by literature review. The organization of the article is appropriate. Overall, the paper is well prepared, but it requires some improvements:

- affiliation – please unify all affiliations data of the units - add address data to (4).

- keywords: please consider redefining some keywords, some of them are too generic for example: Society or Technology;

- Methodology:  lack of information about the used methodology of review, such as used database, criteria of the selection the literature, used key phrases for searching the information etc. Detailed information about applied methodology is required.

- Figure 2. – the better quality picture is required.

- Incineration – it needs some definition or additional explanation what kind of process are described in this point, for example pyrolysis could be treat also as the kind of incineration with energy recovery.

- 2.2. Recycling Industry – High Technology Readiness Levels – please consider including it in chapter 4.

- Recycling of Different Types of Composites – the title of the article suggest all types composite such as metal, ceramics etc. please consider it (change of the title? Or definition the aim in abstract)

- Recycling of Different Types of Composites – what about biodegradable composites?

- Thermoplastic composites – this part should be more informative.

- Reinforcement – it do not suites for the classification described in the sub-chapter. Please order the information.

- Summary & Conclusions – most of the chapter is rewriting the sentences use previously in the article. Please make this chapter shorter and include ONLY the most important points.

Author Response

The article entitled: “Composite Material Recycling Technology – State-of-the-Art and Sustainable Development for the 2020s.” is appropriate for Journal of Composites Science. The authors present interesting and up-to-date topic supported by literature review. The organization of the article is appropriate. Overall, the paper is well prepared, but it requires some improvements:

Our reply:

We would like to thank the reviewer for a thorough analysis and positive yet critical feedback on the original submission. The authors have performed corrections throughout the manuscript based on reviewer’s suggestions and comments. Most of comments were addressed in the revised manuscript. We would like to thank you for taking the time to carefully read our manuscript and for the feedback provided. Please find below our response to each comment.

- affiliation – please unify all affiliations data of the units - add address data to (4).

Our reply:

The authors would like to thank the reviewer for his/her suggestion. The affiliations’ style has been unified adding the address data to (4).

- keywords: please consider redefining some keywords, some of them are too generic for example: Society or Technology;

Our reply:

The authors are grateful for the suggestion. The keywords were selected in a systematic way using a combination of the most relevant as well as most popular keywords as indicated via i.e., Google Trends. Therefore, the authors would prefer that the keywords stay unchanged.

- Methodology:  lack of information about the used methodology of review, such as used database, criteria of the selection the literature, used key phrases for searching the information etc. Detailed information about applied methodology is required.

Our reply:

The authors would like to thank the respected reviewer for an interesting suggestion. While there are review articles that show inherent search methodology, others do not provide such details. In case of MDPI, there is no such requirement – as an example, one of the quite influential review articles can be mentioned: Thomason, J.; Jenkins, P.; Yang, L. Glass Fibre Strength—A Review with Relation to Composite Recycling. Fibers 2016, 4, doi:10.3390/fib4020018. This article was cited in the manuscript.

The authors think that in the current case, adding such chapter could hurt the cohesiveness of the current story portrayed in the manuscript, which could hurt the readability of the article. The authors hope that their reasoning can be understood and considered.

- Figure 2. – the better quality picture is required.

Our reply:

The improved quality Figure 2 was made per request of the respected review and included in the revised manuscript.

- Incineration – it needs some definition or additional explanation what kind of process are described in this point, for example pyrolysis could be treat also as the kind of incineration with energy recovery.

Our reply:

The authors thank the reviewer for noting this detail. The definition of incineration was added in the subchapter 2.1.2. Incineration.

- 2.2. Recycling Industry – High Technology Readiness Levels – please consider including it in chapter 4.

Our reply:

We greatly appreciate the reviewer for the insightful suggestion. The weakest rating of the manuscript was in the category “Is the work well organized and comprehensively described”. The authors believe that this comment could relate to the reason of receiving such low rating in the category. The authors hope that their reasoning will help understand the placement of Subchapter 2.2. in Chapter 2 instead of Chapter 4. The authors aimed to provide the current EoL solutions available industrially in Chapter 2, which included Landfilling, Incineration and Recycling. Chapter 2.2. concentrated on the current industrially available Recycling EoL solutions. Therefore, industrially available (high TRL) recycling options were summarized in Chapter 2. The title of the subchapter 2.2. was rephrased to reflect this reasoning better. The new title “Current Recycling End-of-Life Solutions – Industrial Composite Recycling.”. In addition, the following text was added in the revised manuscript in the subchapter 2.2 to make it clear to the reader: “Current EoL recycling solutions available industrially are only those options that have already reached high TRL, and are summarized in this Chapter 2. The in-detail description of respective composite recycling technologies and their detailed spectrum from low to high TRL, as well as the comparison of the techniques, are discussed in Chapter 4 (Recycling Techniques).”

Additionally,  to improve the structure of the manuscript the authors have structured the document using the 3-levels subsections and improved its readability. All sections have been improved.

- Recycling of Different Types of Composites – the title of the article suggests all types composite such as metal, ceramics etc. please consider it (change of the title? Or definition the aim in abstract)

Our reply:

The authors would like to thank the reviewer for his/her suggestion. The scope of this review is limited to fiber reinforced composites. To clarify this, the term “fiber reinforced composites” was used in the revised abstract.

- Recycling of Different Types of Composites – what about biodegradable composites?

Our reply:

The authors would like to thank the reviewer for his/her suggestion. Biodegradable composites are very interesting as they make composite materials more sustainable. Unfortunately, the most widely used composite materials are not biodegradable and therefore need to be recycled. In this paper we have decided to focus on the recycling topic, but we agree that the category of biodegradable composite materials needs to be mentioned.

At the end of chapter 3, the following sentence was added:

“It is worth mentioning that there is also a category of sustainable composite materials, the biodegradable composites. These materials have a relatively short degradation time after EoL and need therefore less recycling effort, as they can be disposed in an environment where they can degrade quickly. As these materials do not need recycling they are not studied in depth in this paper.”

- Thermoplastic composites – this part should be more informative.

Our reply:

A part on thermoplastic composites was expanded per request of the reviewers. In the revised structure of the manuscript, it is Subchapter 3.1.1. Thermoplastic composites. Information on self-reinforced thermoplastics was added and an additional source referenced.

- Reinforcement – it do not suites for the classification described in the sub-chapter. Please order the information.

Our reply:

The chapter 3 was divided into subchapters using 3-level structure in the revised manuscript to make it more structured and easier to read per suggestions of the reviewers. The subchapter discussing types of reinforcements and fibers is now the subchapter 3.2. FRPs can be classified both by matrix and by reinforcement. The subchapter 3.2 describes different available reinforcements commonly used in the FRPs.

- Summary & Conclusions – most of the chapter is rewriting the sentences use previously in the article. Please make this chapter shorter and include ONLY the most important points.

Our reply:

The authors would like to thank the reviewer for a lot of insightful comments. We have chosen this style of Summary and Conclusions on purpose-The explanation is the following. The manuscript is aimed at both academia and industry, as well as the authors and interested sides are also involved in both academia and industry. The reason for writing both Summary and Conclusions is to provide a condensed information for readers from the industry This was done to increase the readability of the article in both academic and industrial circles as a compromise. Therefore, the authors would prefer that the Summary and Conclusions stay in the current style. Thank you very much.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear author,

Since all my previous suggestions have been properly addressed, I hereby recommend the acceptance of your paper.

Kind regards

Reviewer 2 Report

The authors includes most of required changes, however the strong suggestion is add the methodology of the research for the article.