Thermoforming Process Effect on Performances of Thermoplastic/Recycled Carbon Fiber Composites

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The topic of composites recycling is very actual nowadays, which makes the paper of potential high interest to the readers. In general, it is well written and scientifically sound, yet there are some issues that should be addressed:

The Abstract should be more compact, eliminating some values and focusing on the fundamental findings and main impact of the research.

Introduction – the sentence in the lines 70-71 makes no sense, maybe a verb is missing, it should be rephrased, anyway. The last two paragraphs would probably fit elsewhere in the paper. The lines 94-103 might fit to the Materials and Methods section, the lines 104-107 to the Results section. I am not convince where to put the lines 108-114, but they do not seem to fit to the introduction.

Other:

- in the line 185, beginning the sentence with “Instead,” is not the best choice (please rewrite, so that the meaning of the paragraph is clear).

- in the line 248 there is a hyperlink error.

- Fig. 9 and 12 – is there any evidence (theoretical formula) that the Elastic modulus vs Porosity dependence should be linear in the whole range? Besides, there is little value of discussing the R square value without performing at least the Pearson coefficient test (especially for as few as five data points).

- Fig. 10 and 11 – there is a scale bar in the BW images, it should be transferred also to the colour ones. The text in the pictures should be bigger (to make it legible).

- the Heading – the section 3. Thermomechanical Characterization… should be probably nested in the section named 3. Results and discussion, or 3 Results (but here I do not strictly insist)

Comments on the Quality of English Language

More or less ok, but having the text reviewed by a native speaker would help to increase the readability, at least in some parts.

Author Response

1. Summary
We appreciate the time and effort you have dedicated to providing your valuable feedback on the manuscript. The grammar have been deeply revised to improve the quality of the paper.
Here is a point-by-point response to the reviewers’ comments and concerns.

2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: The Abstract should be more compact, eliminating some values and focusing on the fundamental findings and main impact of the research.
Response 1: Thank you for pointing this out. The authors agree with this comment, and the abstract has been revised to underline the results achieved in the present work.

Comments 2: Introduction – the sentence in the lines 70-71 makes no sense, maybe a verb is missing, it should be rephrased, anyway. The last two paragraphs would probably fit elsewhere in the paper. The lines 94-103 might fit to the Materials and Methods section, the lines 104-107 to the Results section. I am not convince where to put the lines 108-114, but they do not seem to fit to the introduction.
Response 2: Thank you for pointing-out, we revised the statement (row-70-71) for improve the clarity: ”Carding is a mechanical process in which the discontinuous carbon fibers agglomerates are placed in a series of rotating cylinders covered with wire or pins that work to align and separate the fibers”. 
The discussion at the end of the introduction is focused to present the activities carried out beyond the state of the art. The paragraph has been shortened.  

Comments 3: in the line 185, beginning the sentence with “Instead,” is not the best choice (please rewrite, so that the meaning of the paragraph is clear).
Response 3: Fixed. 

Comments 4: in the line 248 there is a hyperlink error.
Response 4: Fixed. 

Comments 5: Fig. 9 and 12 – is there any evidence (theoretical formula) that the Elastic modulus vs Porosity dependence should be linear in the whole range? Besides, there is little value of discussing the R square value without performing at least the Pearson coefficient test (especially for as few as five data points).
Response 5: Thank you for pointing out, the decrease in mechanical performances due to voids is a relevant topic. We compared data with linear fitting since, as the reviewer noticed, the amount of data is not significant for exploiting high order fitting. By the way the linear dependence, in the author’s opinion, is an appropriate hypothesis since actual defects are voids of air trapped during the processing, therefore the mechanical stiffness loss is related to missing material that would not contribute to sustain loads. Many authors investigated the effect of voids in the mechanical performances of CFRP (i.e. references below) both in tensile and flexural configuration and the linear dependence hypothesis is respected. 

-    Kaibao Wang, Yang Liu, Yao Chen, Hongwei Chen, Huirong Le, Mechanical behaviour of 3D printed carbon fibre reinforced composite metastructure with various filling rates, Composite Structures, Volume 347, 2024, 118421, ISSN 0263-8223, https://doi.org/10.1016/j.compstruct.2024.118421.
-    Yang, P., & El-Hajjar, R. (2012). Porosity Defect Morphology Effects in Carbon Fiber – Epoxy Composites. Polymer-Plastics Technology and Engineering, 51(11), 1141–1148. https://doi.org/10.1080/03602559.2012.689050
-    Stamopoulos A, Tserpes K, Prucha P, Vavrik D. Evaluation of porosity effects on the mechanical properties of carbon fiber-reinforced plastic unidirectional laminates by X-ray computed tomography and mechanical testing. Journal of Composite Materials. 2015;50(15):2087-2098.  doi: 10.1177/0021998315602049.

Comments 6: Fig. 10 and 11 – there is a scale bar in the BW images, it should be transferred also to the colour ones. The text in the pictures should be bigger (to make it legible).
Response 6: Fixed

Comments 7: the Heading – the section 3. Thermomechanical Characterization… should be probably nested in the section named 3. Results and discussion, or 3 Results (but here I do not strictly insist)
Response 7: Fixed. 

4. Response to Comments on the Quality of English Language
Point 1: More or less ok, but having the text reviewed by a native speaker would help to increase the readability, at least in some parts.
Response : The authors have done a deeply English revision to the entire paper, thanks for the suggestion. 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors investigated the properties of two different nonwoven mats (Carbiso TM-PA6/60 and Carbiso TM-MAPP/60) under two processing methods (primary molding and lamination), including porosity, mechanical properties, and matrix-fiber compatibility. It was concluded that the lamination method (Method 2) performed better in two-layer samples, but the difference between the two methods decreased as the number of layers increased. MAPP/60 showed better interfacial bonding due to the presence of maleic anhydride. The paper has some practical significance, but there are still many shortcomings to be improved as a research paper.

1. The paper mainly focuses on the examination of mechanical properties, but little research has been done on morphology, and only very vague morphology pictures are provided. The authors need to add clear SEM images and suggest providing data at several different magnifications so that readers can have a clearer understanding of the material's morphology.
2. the paper lacks the basic performance characterization of the raw materials, which is recommended to be improved.
3. Table 2 shows that the Tg of MAPP/60 is between 45.3 and 53.1°C. Please explain why.
4. In line 211, the author mentions “Additionally, the solid residue at high temperatures (600°C) is around 40% for both materials, a value consistent with the Additionally, the solid residue at high temperatures (600°C) is around 40% for both materials, a value consistent with the reference.”. However, no reference is given to support this. Meanwhile, according to previous reports, the addition of carbon materials or other fillers to the materials will result in the TGA residual carbon content being much higher than the filler content, whereas the authors' results are almost the theoretical value, please explain why. The relevant literature is as follows: ACS Appl. Polym. Mater. 2024, 6, 7794-7800; Polymer Composites. 2024;45:11975-11984.; J Polym Sci. 2024;1 -8.
5. The layout of the images in the paper is too casual, and it is suggested that they be rearranged to make them more aesthetically pleasing.

Author Response

1. Summary        
Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files

2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: The paper mainly focuses on the examination of mechanical properties, but little research has been done on morphology, and only very vague morphology pictures are provided. The authors need to add clear SEM images and suggest providing data at several different magnifications so that readers can have a clearer understanding of the material's morphology.
Response 1: Thank you for comment, the SEM images of the rCF/PA6 and rCF/MAPP were added to figure 7 and figure 8, respectively.

Comments 2: the paper lacks the basic performance characterization of the raw materials, which is recommended to be improved.
Response 2: The basic material considered within the paper are the dry preforms made by rCF/MAPP and rCF/PA6. For both the materials thermal analysis (DSC,TGA,DMA) and morphological analysis have been reported within the paper.

Comments 3: Table 2 shows that the Tg of MAPP/60 is between 45.3 and 53.1°C. Please explain why.
Response 3: 
The slight changes in glass transition probably are related to the process conditions, for both materials we experienced a slight increase of Tg in 2 layer composites, while the 3 layer systems has a Tg similar to the bare material. In the author’s opinion, in the case of 2 layer composite the polymeric phase completely melts and impregnated better the rCF leading to a material with higher temperature stability. 

Comments 4: In line 211, the author mentions “Additionally, the solid residue at high temperatures (600°C) is around 40% for both materials, a value consistent with the Additionally, the solid residue at high temperatures (600°C) is around 40% for both materials, a value consistent with the reference.”. However, no reference is given to support this. Meanwhile, according to previous reports, the addition of carbon materials or other fillers to the materials will result in the TGA residual carbon content being much higher than the filler content, whereas the authors' results are almost the theoretical value, please explain why. The relevant literature is as follows: ACS Appl. Polym. Mater. 2024, 6, 7794-7800; Polymer Composites. 2024;45:11975-11984.; J Polym Sci. 2024;1 -8.
Response 4: Thank you for your comment. The recycled carbon fiber preforms adopted in the present paper are made by the addition of TPC (MAPP/PA6) as filament during the carpet forming. The TGA analyses were carried out to verify the integrity of the material (degradation due to over-heating) and to check if the hot plate compression caused squeeze out of the polymers leading to non-uniform matrix content along the coupons. The reference to the datasheet have been added to the text.

Comments 5: The layout of the images in the paper is too casual, and it is suggested that they be rearranged to make them more aesthetically pleasing.
Response 5: Figures were rearranged to improve the appearance.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Grammar could be improved throughout. This has a negative impact on the clarity of the paper.

Layout / structure of the paper needs to be improved. Sections seem to be mixed and not distinct and becomes hard to follow.

Materials and Method section should focus on what you have done as opposed to the processes from the company. These may fit better in the previous section.

How many layers were the samples?

What thickness were the layers?

What gsm were the layers?  

Are the samples homogeneous? If not, is the small (not given weight) TGA sample representative for the real fibre content? How many times was this repeated?

Paper contains errors: “Error! Reference source not found.):”

Is porosity the only factor that impacts stiffness and viscoelastic behaviour?

How was porosity measured? This is not discussed in the Materials and Method section or the Results sections. Some mention of the Results within the Discussion section but not information on how they were obtained.

Microscopy mentioned in the Discussion section however, this is not discussed in the Materials and Method section or the Results sections. Further information needed.

The paper is primarily written in third person however, on occasion the author switches to first person. Avoid, and keep all in the third person.

Was surface roughness measured? As this is mentioned in discussion.

Were fibre pull out tests undertaken?

Conclusion needs to be more succinct / clear.

Comments on the Quality of English Language

Grammar could be improved throughout. This has a negative impact on the clarity of the paper.

The paper is primarily written in third person however, on occasion the author switches to first person. Avoid, and keep all in the third person.

 

Author Response

1. Summary        
Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files. 

2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: Grammar could be improved throughout. This has a negative impact on the clarity of the paper.
Response 1: Thank you for pointing this out. The authors revised the English language throughout the manuscript to improve its quality.

Comments 2: Layout / structure of the paper needs to be improved. Sections seem to be mixed and not distinct and becomes hard to follow.
Response 2: The paper layout was revised.

Comments 3: Materials and Method section should focus on what you have done as opposed to the processes from the company. These may fit better in the previous section. 
Response 3: The paragraph was modified by moving the literature analysis within the introduction section.
 
Comments 4: How many layers were the samples? What thickness were the layers?
Response 4: Thank you for pointing out this missing information. We have revised the manuscript accordingly to include the number of layers and thickness. The text has been modified as follows:
“In this case, the PA6/60 and MAPP/60 carpets were subjected to two different compression moulding processes to produce multilayer panel: one layer (1L), two layers (2L), and three layers (3L). The carpets were cut to obtain different layers measuring 50 x 50 mm. The thickness of the compression-moulded layer is about 0.5 mm” 

Comments 5: What gsm were the layers?  
Response 5: The nominal surface densities are 500 gsm. The experimental measures reproduces a surface density of 530±10 gsm and 560±30 gsm for rCF/MAPP and rCF/PA6 carpets respectively.

Comments 6: Are the samples homogeneous? If not, is the small (not given weight) TGA sample representative for the real fibre content? How many times was this repeated? 
Response 6: Samples were homogeneous and reproducible, for each test at least three samples were extracted and tested for each coupon.

Comments 7: Paper contains errors: “Error! Reference source not found.):”
Response 7: Fixed 

Comments 8: Is porosity the only factor that impacts stiffness and viscoelastic behaviour?
Response 8: Thank you for your comment, many factor impact on the performance of the material. Basically we found a direct dependence of the mechanical (non-dissipative) stiffness to the porosity, many authors investigated this dependence and achieved similar results (see reference below). The viscoelastic properties are affected by many factors, the defects inside the material, such as voids and the quality of the interface between fibers and matrix.

Comments 9: How was porosity measured? This is not discussed in the Materials and Method section or the Results sections. Some mention of the Results within the Discussion section but not information on how they were obtained.
Response 9: The definition of porosity and the procedure implemented by authors is reported in the results section, paragraph 3.1 (row 244-251)
 
Comments 10: Microscopy mentioned in the Discussion section however, this is not discussed in the Materials and Method section or the Results sections. Further information needed.
Response 10: Materials and Methods section was improved by including the methodology for the OM.

Comments 11: The paper is primarily written in third person however, on occasion the author switches to first person. Avoid, and keep all in the third person.
Response 11: Fixed 
 
Comments 12: Was surface roughness measured? As this is mentioned in discussion.
Response 12: The study of surface roughness of the coupons is out of the scope of the present paper. Actually we noticed by SEM images that the recycled fiber surface has different texture for the rCF/MAPP and rCF/PA6. The hypothesis discussed is that the modification of PP by maleic anhydride improve the interface strength.

Comments 13: Were fibre pull out tests undertaken?
Response 13: Thank you for pointing-out. The interfacial shear strength is a critical feature that can control the load transfer at the interface between fiber and matrix. We did not measured directly the IFSS, but we discussed the quality of the interface by comparing the specific damping capacity (proportional to tanδ) of the materials tested in similar mechanical condition.

Comments 14: Conclusion needs to be more succinct / clear.
Response 14: The conclusion paragraph was revised.

4. Response to Comments on the Quality of English Language
Point 1: Grammar could be improved throughout. This has a negative impact on the clarity of the paper.
The paper is primarily written in third person however, on occasion the author switches to first person. Avoid, and keep all in the third person.
Response 1: Thank you for your valuable feedback. We have carefully revised the manuscript to improve the grammar throughout, enhancing overall clarity. Additionally, we have ensured consistency in the narrative voice by removing all instances of first-person usage and maintaining a third-person perspective throughout the paper, as recommended.   

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The issues mentioned in the previous review round have been addressed, and the paper can be accepted in its present form.

Author Response

We are grateful to the reviewer for constructive comments and suggestions, which have helped us strengthen the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The author answered my concerns.

Author Response

We are grateful to the reviewer for constructive comments and suggestions, which have helped us strengthen the manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

Introduction much improved.

Much improved overall.  

Results and discussion sections still seems to not be distinct. Perhaps consider combining these.

Please include the GSM in the paper.

Please include the weight of the TGA samples used.

Author Response

Introduction much improved.

Much improved overall.  

Thank you for your comments. We are grateful to the reviewers for their constructive comments and suggestions, which have helped us strengthen the manuscript.

Results and discussion sections still seems to not be distinct. Perhaps consider combining these.

The results and discussion have been combined in a unique section.

Please include the GSM in the paper.

Fixed, the materials section was modified by including the nominal GSM, and the actual GSMs were added in the results and discussion section.

Please include the weight of the TGA samples used.

Fixed, the nominal weight of the TGA samples was added to the Experimental Characterization section.