Sustainable Sandwich Composites Manufactured from Recycled Carbon Fibers, Flax Fibers/PP Skins, and Recycled PET Core
, Andrew Mills 1, Krutarth Jani 1, Vasudevan Rajamohan 2, Barathan Venugopal 2 and Sameer Rahatekar 1,*Round 1
Reviewer 1 Report
This is a very nice contribution in the field of sustainability, composites and hybridization. This reviewer's only suggestion is a cosmetic one: the equations appear pixelled. Otherwise the paper can be published.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this work, the authors fabricated sustainable sandwich composites using recycled fibres and recyclable matrix. A fabrication procedure that led to sandwich composites with good flexural properties was shown. This work is very important because it tackles the issue of sustainability in industries requiring high performance materials. However, there are too many grammatical mistakes, typos, incorrect sentences in this paper. It gives the impression that the authors did not read the manuscript with attention before submitting it. Also, while I understand from the last paragraph of introduction that your goal is to ameliorate the reduction of bending and tensile properties observed in previous research it might have been interesting to study other properties such as fatigue performance, energy absorption, … of your sandwich structure and see how it stands compared to previous works. Most importantly it is good practice to validate such experimental work with Finite element analysis to improve the quality of the work.
The following modifications and recommendations will help improve the quality of this work:
The work needs to be checked very carefully and thoroughly for typos.
Line 10: Replace Eu by European union
Line 19 -20: replace “for operating” by “to operate”
Line 36: Recently “,’
Line 41: … scientific literature “on sandwich structures?” relies …
Line 46 – 47: Are you talking about the past studies or your present work? The sentence is not clear
Line 55 – 57: Please reformulate the sentence it might not be clear enough because of its length.
Line 59 – 60: The two sentences do not make sense grammatically
Line 60 – 61: Change the sentence as follow: These cores … thermoplastic; they were also reinforced …
Line 64 – 66 You need references for that sentence
Line 66 – 69 That sentence is grammatically incorrect. It is like the second sentence starts without finishing the first one.
Line 71: Replace which were confirmed by “; requirements which were satisfied“
Line 79: of “their” aerospace and …
Line 95: There is an extra space before “Furthermore”
Line 97: a space is needed between combination and [27]
Line 105: It might be good to give some details about how the carbon fibre mat has been recycled or obtained to have an idea of its quality.
Line 113 – 117: Since you are using (i) and (ii) to explain Figure 1 which also contains (i) and (ii) it gives the impression that you are describing respectively the (i) and (ii) parts of the figure. However this is not the case. You might want to change (i) and (ii) by (1) and (2) in the text
The font size of “ (a), (i), …, (ii) “ is too big in Figure 1.
Line 120: 2.2”.” The dot is missing
It might be better to keep the first two paragraphs of section 2.2 together as one paragraph
Line 124: “to ensure maintain the position” is grammatically incorrect
Line 130 – 132: Including the results of the DSC tests would increase the quality of the present work.
Line 136: The space between 6 and KN is missing. Same thing on lines 142 and 143 for 2.8 mm and 3.4 mm
Line 146: 2.3”.” The dot is missing. In addition, why do we have capital C for characterization? In the title of the others subsections only the first word has a capital letter.
Sections 2.3 and 3.2: While Equations (1) – (3) are ok for preliminary analysis, it would have been much better to use FEA modeling and analysis as a prediction tool and to validate your experimental results
Line 162-163: References are needed for that sentence
Section 3.1: SEM characterization of the interface between carbon fibre and flax would increase the quality of this work
Line 187: remove “of” in sentence “The red line … 2”
Line 193: How or in what sense are the trend similar?
Line 205: Reasons or comments on why we have shear failure instead of debonding?
Line 208: 3.4”.” The dot is missing
Line 215: References are missing in the sentence “Some sandwiches … wood skins.”
Section 3.4: While your case study shows the applicability of the sandwich structure developed in this work, it also seems very simplistic since failure is assumed to come only from a deflection stand point. This is not really the case.
Line 245 – 252: It might be best to put this section in the results and discuss how the issues encountered with flax might be solved.
Line 263 -264: Why is the paragraph ending with 3.1. Subsection.
Line 266: The first sentence does not make any sense. What does “carried out swiftly manufactured” mean?
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper addresses both the design and manufacture of fully recyclable sandwich composites made from recycled carbon fibre-based polypropylene faces and recycled thermoplastic PET foam core. Hybrid faces combining flax fibres and recycled carbon fibres are also investigated. A case study of a truck side panel is analyzed.
While this paper demonstrates the benefit of sandwich design using recycled materials, it suffers for a lack of accuracy.
Minor comments
Line 107 : "unidirectional fibres" : do you mean unidirectional fabric? Flax fibres are actually twisted.
Line 111 : The melting point of the adhesive film must be given here
Lines 115-116 : Can you justify why this specific hybrid stack was chosen. Why not alternating carbon and flax plies?
Line 123 : what's the thickness of the PP sheets
What is the thickness of the foam core?
Section 3.3 :
Is the failure mode of sample 1 similar to the one shown in Figure 5?
Major comments
Section 2.2
Did you control whether the flax fibres were degraded or not by the thermo-mechanical cycle you applied (50 bar, 180°C, 10 mins)?
Line 135 : How does PET foam behave under 4 bar at 160°C? Is there any plastic deformation? Are the mechanical properties of the core affected by this cycle?
Why did you design a sandwich beam with thick faces and a relatively thin core? Why not use a thicker foam core and reduce the thickness of the faces?
Section 2.3
Line 155 : h is not the specimen thickness, it is the distance between center lines of the upper and lower faces
Line 164 : The elastic modulus of flax fibres (70 GPa) is overestimated, it is statistically much lower.
Line 164 : why the models of the recycled carbon fibre is a 1/4th of the properties of virgin carbon fibre? Is it because fibres are discontinuous?
Line 165 : Gives the value of the skin and core moduli, otherwise one cannot interpret values reported in table 1.
Section 3.2
Line 192 : Where is the "elastic region (red)" in Figure 4?
Line 193 : "...shows a similar trend as the experimental results." Looking at Figure 4, Prediction does not match experimental value.
Table 1 : Please comments reported values. Why the bending modulus of sample 2 is twice smaller than the one of sample1?
Section 3.4
Line 247-248 : loss of 30% : is it a measured value? How did you obtain that value?
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
All my comments and suggestions were incorporated. All the typos and grammatical mistakes were fixed. The authors added additional results as requested.
Author Response
We would like to thank the reviewers and the editor for their excellent efforts to review our paper.
2nd-Reviewer 2：
We are glad to know that reviewer 2 thinks our work has incorporated all his comments and suggestions and fixed typos and grammatical mistakes.
To the suggestion ‘English language and style are fine/minor spell check required’, we would like to arrange a language improvement via MDPI before publication after the content modification finished.
Reviewer 3 Report
The authors have made substantial changes to their document to take account of my comments.
My final recommendation is to clearly state in the paper that this sandwich construction is unusual because the design involves thick faces and a relatively thin core.
Author Response
Notice: All the line shown in this document is referring to where located inside the revision document when the revision mode of word (which shows who deleted/modified what content) is off. As those deleted content could make a lot of disrupt on the line numbers.
We would like to thank the reviewers and the editor for their excellent efforts to review our paper.
2nd-Reviewer 3:
We are glad to know that reviewer 2 thinks our work has made substantial changes to take account of his comments.
To the suggestion ‘My final recommendation is to clearly state in the paper that this sandwich construction is unusual because the design involves thick faces and a relatively thin core.’
We have added the sentences ‘The recycled PET foam suppliers provide the foams in relatively thin sections (10mm). The skin thickness used in this work after compression moulding was (2.8mm/3.4mm). This combination resulted in sandwich composites structure with relatively small core to skin thickness ratio. Such construction is not a common for sandwich composite panels. In future we will use thick foams structures from different suppliers to achieve high core to skin thickness ratio.’ on line 297-302.
