Hybrid Polyethylene Composites with Recycled Carbon Fibres and Hemp Fibres Produced by Rotational Moulding

Round 1

Reviewer 1 Report

The study assessed polyethene composites reinforced by recycled carbon fibre (RCF) and hemp fibre (HF). The authors did plenty of testing job in porosity, tensile and thermal properties of the composites. However, the presentation must be improved.

1. The novelty must be clearly stated; otherwise, this is a testing report, not a research article;

2. The mechanism, reasoning for the testing results must be given;

3. Add some theory or numerical support to explain the testing results;

4. In Tensile testing, results presentation is poor, e.g. testing apparatus, stress- strain curves, fracture photo etc should be given;

5. Likewise, more processing results should be given for porosity and thermal properties;

6. Unit should be unified: tensile strength in MPa and Young’s modulus in GPa;

7. Figure number has mistake;

8. Figure 7 Scale bar = 50 um? Different from the label in photo. Title of Figure 7 is wrong;

9. Figure 9 is difficult to read, many curves with on legend;

10. Title of Figure 9 is wrong;

All of these details show the authors not taking paper preparation seriously, lead readers unpleasant to read the paper. It is must to rewrite and resubmit.

Author Response

Dear reviewer,

Thank you. We appreciate the time and effort that you have dedicated to providing your valuable feedback on our manuscript. Please find below the response to each of those.

”The study assessed polyethene composites reinforced by recycled carbon fibre (RCF) and hemp fibre (HF). The authors did plenty of testing job in porosity, tensile and thermal properties of the composites. However, the presentation must be improved.”

1. The novelty must be clearly stated; otherwise, this is a testing report, not a research article;

Thanks for the observation. The novelty was stated in the last paragraph of the introduction ”Recycled carbon fibre has been the topic of much research to create applications for recycled materials with high mechanical performance. However, the hybridization of recycled carbon fibre with hemp or any other natural fibre is a topic not yet explored, especially in rotational moulding. Accordingly, the combination of these two fibres in rotationally moulded composites is an innovative approach to produce more sustainable products with good tensile properties at a low cost”.

2. The mechanism, and reasoning for the testing results must be given;
3. Add some theory or numerical support to explain the testing results;

Thank you for your suggestion. We agree that in scientific papers, a clear rationale and theoretical background is necessary. In most cases, we have provided a detailed explanation of the results, including the mechanisms and theory involved.

4. In Tensile testing, results presentation is poor, e.g. testing apparatus, stress-strain curves, fracture photo etc should be given;

In order to improve the discussion of our results, we have added one more picture and a better explanation of the stress/strain behaviour of the rotationally moulded composites.

5. Likewise, more processing results should be given for porosity and thermal properties;

We appreciate your recommendations on items 4 and 5, however, this research has been completed and no more experiments can be performed. We have tried to summarise the main findings of our experiments.

6. The unit should be unified: tensile strength in MPa and Young’s modulus in GPa;

Thanks for the observation, units were updated to GPa.

7. Figure number has a mistake;

Thanks for the observation, It was fixed accordingly.

8. Figure 7 Scale bar = 50 um? Different from the label in the photo. The title of Figure 7 is wrong;

Thanks for the observation, It was fixed accordingly.

9. Figure 9 is difficult to read, many curves with legend;

We agree with you and the recycled carbon fibre and hemp fibre curves were removed as the purpose was to show the thermal behaviour of hybrid compared to hemp composite.

10. The title of Figure 9 is wrong;

Thanks for the observation, It was fixed accordingly.

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

We appreciate the time and effort that you have dedicated to providing your valuable feedback on our manuscript.  Please find below the respective answers to them.

1‐ FTIR: I am wondering if the pellets were dried previously the FTIR? (the presence of water in figure 1 might come from the preparation of the composite pellets)

Although the fibres and KBr were dried prior to the pellet preparation, the band 3384 cm^-1 can be attributed to residual water in the sample. However, all the samples were prepared in the same way and the effect of treatment is evident in the change of peak intensity of band 1052 cm^{- 1}, which corresponds to O-H stretching vibration.

2‐ What did you mean by real density? (If measured, how did you measure this density?)

The true or real density is defined as the ratio of the mass to the volume occupied by that mass (without porosity). The real density of hemp fibre was obtained from the literature and the recycled carbon fibre was obtained from the supplier datasheet.

3‐ Acronyms and multiplicity of acronyms let the paper difficult to read. For instance, does Uuntreated = HF‐10 = URCF?

Thanks for the observation, It was fixed accordingly. H: hemp followed by 10 (fibre content wt.%)

4‐ Table 2. Porosity 6.8 and 7.7 . I would expect a 20‐30% of uncertainty in the measurements. I, therefore, believe that 6.8 = 7.7! This suggests to me that there is no improvement in the pore density!

We agree with you and the following was added “This reduction in void’s size has not reduced the porosity of the final composites (Table 2).” (Line 206)

5‐ Hybridization of hemp fibre with recycled carbon fibre resulted in composites with superior

tensile strength. Perhaps, adding the % of improvement and emphasising the fact that this is a weak improvement if we consider the investment to process the fibres.

Thanks for the observation, it was stated in item 3.3 paragraph “In general, hybridizing hemp with recycled carbon fibre resulted in a stiffer and stronger composite (about 30%) than their natural fibre counterparts, as shown in Figure 8. The hybrid composites showed a higher Young's modulus (about 20%) than pure polyethylene, but no difference in tensile strength”.

6‐ As the second part of the conclusion, I would suggest urging caution in any improvement concerning grafting or surface treatment of the fibre surfaces since improvements are weak (if any). Perhaps a conclusion might be that surface treatments (grafting included) are definitively not appropriate or that the interest of such chemical treatment is lost during the introduction in the matrix or/and too weak to ensure robust cohesion of the fibre with the matrix.

Thanks for your valid comment. It was stated in the paragraph “In conclusion, the treatment of recycled carbon fibre with HNO₃ before rotational moulding is not advantageous considering the small improvement in fibre matrix adhesion, the time consumed, and the hazardous waste generated.” (Line 260)

7‐ *Figure 1: mistake with the number 3 (please: double check)

Thanks for the observation, it was fixed accordingly.

Round 2

Reviewer 1 Report

Compared with the first version, the authors have improved the paper. However, some of the previous comments have not been addressed:

1.  Testing apparatus, stress-strain curves, fracture photo etc, still are not given. The authors explain "this research has been completed and no more experiments can be performed", but these photos/curves are nothing to do with the more experiemnt performed or not. The experiemnt has been completed, these photos/curves should be obtained.

2. The unit are still not to be unified.

Author Response

Dear reviewer, 

Thanks for your valid comments. 

Comment 1: Figure 9 was added.

Comment 2: Units were unified to Young's modulus (GPa) and Tensile Strength (MPa)

Kind regards,

Author Response File: Author Response.docx

Reviewer 2 Report

I am running out of time and the editor is urging me.

I still have questions about the poor (or weak) improvement of grafting fibers. 

Author Response

Dear reviewer 

The poor improvement of fibre-matrix adhesion was attributed to the nature of the rotational moulding process - an absence of external pressure - resulting in high porosity within the final composites.

This information is in paragraph line 191-195.

Thanks for your contribution to this article. 

Best Regards,

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

The authors have improved the paper and it is ready to accept. Before publication, two issues need to be revised:

[1] The added stress-strain cureves show a strong nonlinear of these composites, so it is better to show what criteria to judge the tensile strength.

[2] Then, Page 12 Line 314, Figure 9. Thermal analysis of hybrid and hemp reinforced rotationally moulded PE composites, there is no Figure.