Rapid Removal of Sizing Agent from Carbon Fiber Surface by Liquid-Phase Plasma Electrolysis
and Weiwei Chen 1,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsAbstract
1) P1 L14-15
“TGA further demonstrated that samples treated at 10 mm spacing exhibited negligible weight loss, indicating complete sizing removal.”
- the statement is not logically substantiated.
1. Introduction
2) P1 L40-41
“This method not only has a low efficiency in removing sizing agents…”
- the statement is not substantiated. It is not clear by what criterion the efficiency of sizing removal was assessed?
3) P2 L48-49
“The existence form and quantity of the sizing agent on the surface of carbon fibers were observed by scanning electron microscopy (SEM).”
- My experience shows that SEM does not allow for a reliable determination of the amount of sizing.
​​2. Experimental Section
4) P3 L78
“During LPE treatment, a sodium metasilicate solution… ”
- it is necessary to explain why this particular electrolyte was chosen.
5) It is necessary to explain the process of plasma formation.
6) P4 L111
“It was observed that in the 500x photos of the bare sample, there were irregularly shaped sizing agent layers marked with arrows(Figure 4).”
- there is no evidence that the observed irregularities on the fiber surface are a sizing.
6.1) P4 L90-94
“Thermogravimetric analysis (TGA) was performed with a Thermal Analysis System TGA/DSC 3 (Air 20ml/min,Heating rate 10 K/min from 25 °C to 400 °C,METTLER TOLEDO) under air atmosphere to evaluate the percentage weight loss between sizing-removed and unsized bare samples.”
- The TG analysis shall be carried out in an inert atmosphere, in accordance with ISO 10548:2002 (Metod C). The heating temperature shall not be lower than 800 C for the epoxy sizing.
7) P5 L112-114
“By comparing the 10,000x photos of bare and a-i, it can be found that the monofilament surface of the bare sample is relatively smooth, and its diameter changes along the length direction.”
- The change in the shape of the fiber surface and its diameter in different states is not obvious. It seems that the authors are passing off wishful thinking as reality.
8) P5 L123-125
“The FE-SEM micrographs of 2000x bare clearly demonstrate the macroscopical presence of sizing agent which is marked with arrows between individual fibers.”
- There is no evidence that the observed irregularities on the fiber surface are a sizing.
9) P5 L123-125
“By observing samples a-c, it can be seen that from sample a to sample c, the visible sizing agent on the surface of the carbon fiber becomes less and less…”
- There is no evidence that the observed irregularities on the fiber surface are a sizing.
9) P5 L131-132
“… and the grooves on the surface of the carbon fiber monofilaments become more and more obvious at a ratio of 10,000 times.”
- It's not obvious. It seems that the authors are passing off wishful thinking as reality.
10) P6 L134-138
“At this time, the main influences on the fiber are the temperature field of the electrode and the mechanical vibration during bubble generation. Therefore, when the voltage rises, the temperature of the electrode increases accordingly and the rate of bubble generation accelerates, which speeds up the melting and decomposition of the sizing agent.”
- A detailed description of these processes of action on the fiber should be given in the “Methods” section.
11) P6 L140-146
It can be seen from photos d1-f1 that the surface of the carbon fiber is smooth and the morphology is uniform. However, from the photos d2-f2, point-like sizing agents with relatively high contrast on the surface of the fiber monofilaments can still be observed. When observing the fiber monofilaments at a magnification of 10,000 times, it was found that the axial grooves on the fiber surface were relatively obvious, but there was still a small amount of sizing agent remaining in the form of thin layers or spots.
- All the same comments. The change in the grooves on the fiber surface is not obvious, there is no evidence that the observed irregularities on the fiber surface are a sizing. It seems that the authors are passing off wishful thinking as reality.
12) P6 L149-150
“…the fiber is not only affected by the temperature field of the electrode and the mechanical vibration generated by the bubble, but also directly bombarded by the plasma arc.”
- A detailed description of these processes of action on the fiber should be given in the “Methods” section.
13) P6 L161-164
“From the photos of sample g-i,... “
- the same comments as in (11).
14) P6 L168
“... it can be considered that 10mm is the optimal distance between the carbon fiber and the electrode.”
- the conclusion is not supported by experimental data.
15) P7 L178-181
“This proves that when the spacing is 20 mm, the carbon fibers in LPE are mainly affected by the temperature field of the electrode and the mechanical vibration caused by bubble generation, while the influence of plasma bombardment is not significant. ”
- this fact is not a strict proof of the mentioned phenomena.
16) P7 L185-186
“… samples e and f which is very straight between 25 and 300 °C, indicates that the sizing agent of sample g-i has been removed completely.”
- linearity in the range of 25-300 does not prove complete removal of the sizing. Moreover, the decrease in mass at a temperature above 300 indicates the opposite.
17) P7 L186
“...completely., and… ”
- There is a typo in the text.
18) P7 L188-189
“When the voltage increases to 200 V and 215 V, plasma bombardment begins to become the main factor affecting the removal efficiency of the sizing agent.”
- the statement is not supported by experimental data.
19) P7 L192-194
“It can be seen from the weight loss curve of sample g-i that the curve of sample g-i is very straight between 25 and 300 °C, which indicates that the sizing agent of sample g-i has been removed completely.”
- this is not true, see remark (16).
20) P8 L196-197
“At this time, plasma bombardment is the main factor affecting the removal efficiency of the sizing agent.”
- the statement is not supported by experimental data.
21) P8 L197-199
“By comparing the weight loss curves of samples g-i and e-f, it can be seen that their weight loss curves have the same shape. It can be considered that sample e-i is the weight loss curve of pure carbon fiber without sizing agent.”
- This is not a basis for the statement “sample e-i is the weight loss curve of pure carbon fiber without sizing agent”. The authors need to provide a TG curve for the fiber after annealing above 800 C in an inert atmosphere.
22) P8 L208-210
“Since the weight loss curves of sample e-i are very similar, in order to obtain the optimal parameters for removing the sizing agent, it is necessary to compare the remaining weights of each sample at 300 °C(Figure 8).”
- Why is the diagram given for 300 C and not for 400 C?
23) The authors do not explain the drop in mass at temperatures above 300. This is necessary.
24) P9 L225-226
“It can be seen that the content of C-O bonds on the surface of carbon fibers has 225 decreased after LPE treatment.”
- According to the information in the Introduction section, it appears that LPE treatment does not provide any benefit.
25) P10 L236-238
“The main components of the sizing agent on the surface of carbon fibers include water-soluble and non-water-soluble epoxy resins, dimethyl resin, polyimide resin, polyvinyl alcohol resin, vinyl acetate resin, acrylic resin, etc.”
- The Materials section should contain the exact composition of the sizing.
26) P10 L239-241
“... the non-water-soluble sizing agent has a melting point between 100 °C and 180°C and a flash point between 225 °C and 235 °C”
- This is not true. Complete decomposition of epoxy resin occurs at a temperature of 800 C.
27) P10 L243-244
“When the carbon fiber passes beneath the cathode, the resin on it will first melt under the influence of the high-temperature heat-affected zone of the electrode itself and the plasma.”
- The statement is not supported by experimental data. The temperature under the cathode is unknown.
28)
“Moreover, since it is not soluble with the solvent water and its density is lower than that 245 of water, the molten sizing agent will rise to the plasma arc zone and be ignited and va- 246 porized.”
- The statement is not supported by experimental data. It is not clear how the Archimedes force overcomes the adhesive force between the fiber and the sizing (even if it has melted), as well as the porous medium that the fiber thread is.
29) P10 L250
“high-voltage bubbles”
- the term is not clear, it is necessary to explain in detail the process of formation of these bubbles and their possible influence on carbon fiber.
4. Conclusion
30) P11 L266-270
“In this study, liquid-phase plasma electrolysis (LPE) was successfully employed to remove the sizing agent from T800 carbon fiber surfaces. Through systematic investigation of varying arcing voltages (185–215 V) and electrode spacings (10–20 mm), we determined that an optimal combination of 200 V and 10 mm spacing achieved near-complete sizing removal, as confirmed by SEM, TGA, and XPS analyses.”
- the conclusion is not supported by experimental data.
31) P11 L270-272
At this condition, plasma bombardment dominated the removal mechanism, eliminating sizing residues while exposing the underlying fiber grooves.
- the conclusion is not supported by experimental data.
32) P11 L272-273
TGA further demonstrated that samples treated at 10 mm spacing exhibited negligible weight loss, indicating complete sizing removal.
- the conclusion is not supported by experimental data.
33) P11 L273-277
The results of XPS further confirmed the high efficiency of LPE in the removal of sizing agents(C-O bond content from 41.6% to 26.9%), and the retention of C-O also proved that LPE could maintain the surface activity of carbon fibers, confirming the effectiveness of LPE in decomposing the sizing agent.
- according to the information in the “Introduction” section, LPE treatment does not bring any benefit, only harm in the form of a reduction in C-O bonds.
34) In order to talk about the prospects of this method, in addition to eliminating all the above comments, it is necessary to provide data from mechanical tests of the fiber before and after LPE treatment.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors report liquid plasma electrolysis treatment of commercial carbon fibres studying the changes in the morphology and chemistry of the CF surface with respect of the treatment conditions including voltage and electrode spacing. The idea of using LPE for de-sizing CFs is new, but improvements are needed in every area before the publication can be reconsidered, taking into account the following comments:
- The authors compiled a list of the shortcomings of “conventional” de-sizing processes, including the low efficiency, destruction of active groups and weakened interfacial adhesion force. However, they do not address these issues in relation to their own method. A comparison between the methods should be conducted e.g. by using the heat-treatment method as a benchmark.
- The type of the sizing agent applied to the CF should be specified, as thus is a crucial part of the study. It would also help to understand the results in context.
- The SEM images presented are not suitable for analysing de-sizing efficiency. It seems that the authors wrote about what they wanted to see even though if it was barely visible in the images. For example, the grooves cannot be clearly revealed on any fibres because the images are not sharp enough. The white spots (sizing) are arbitrarily indicated by arrows. For example, I can see several spots on Fig.6. i3, but they are not pointed, probably as they do not in line with the hypothesis.
- The image in Fig.6 is incorrectly referenced in lines 160-161. The figure captions for Figs. 4-6 must include the test parameters and magnifications so that they are sufficiently informative on their own.
- TG curves, especially that of “bare” CF seem to be non-typical due to their linear characteristics instead of showing step-like weight loss. This feature requires some explanation. The correct labelling of the samples in the sentence from lines 183 to 187 must be checked. According to Fig.8, Test “h” was claimed to have the highest sizing removal efficiency. However, it is difficult to reach this conclusion without showing the standard deviations given the many similar values of the other tests.
- XPS analysis would be the most appropriate method for comparing the efficiency of the different tests. Unfortunately, this analysis was only performed on two samples. More problematic is the inaccurate illustration and evaluation of the spectra. Firstly, the X axis settings should be the same on both spectra for better comparison. The deconvoluted graphs must be clearly displayed, with no faded lines. The rather dissimilar FWHM values for sample “h” suggest incorrect deconvolution of the C1s spectrum. In addition, the correct O/C ratio calculation ratio should be confirmed by providing the O1s spectrum and the elemental composition based on the O1s and C1s spectra.
- The description of the hypothesized mechanism of de-sizing process should be clearer. Does the H2 play any role in the process? What do the authors mean by “plasma bombardment”, given that plasma itself is a mixture of various species. Assuming a very low average free path length for the species in LPE, how can bombardment occur? The authors should be more specific about which components of the sizing agent are water-soluble as this does not seem evident given the polymer nature of the sizing agents.
- Finally, in its current form, the discussion of the obtained results and their interpretation does not seem to unambiguously support the conclusions.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe manuscript, in its current form, requires major revision before it can be considered for publication:
1. How does this study improve upon existing plasma or thermal methods for sizing removal beyond offering a "faster" or "more complete" result? Please explicitly position this technique against dry plasma and oxidative desizing methods in the literature.
2. The authors are recommended to expand the literature review, particularly regarding the broader applications of carbon fibers in the first paragraph of the introduction. The current introduction briefly mentions aerospace uses but overlooks important emerging applications such as energy systems and multifunctional surfaces: https://doi.org/10.1016/j.surfin.2016.09.004, https://doi.org/10.1016/j.ijhydene.2019.08.184.
3. What are the advantages of LPE over atmospheric pressure plasma jet (APPJ) or low-pressure RF plasma in terms of process scalability, fiber damage, and fiber–matrix adhesion?
4. The mechanism proposed in Figure 10 lacks experimental verification.
5. Are there quantifiable energy inputs (e.g., plasma density, discharge power) that correlate with removal efficiency? Relying solely on SEM and TG data limits mechanistic insight.
6. Were mechanical properties (e.g., tensile strength, modulus) of carbon fibers tested post-treatment? Plasma treatments are known to damage fiber surfaces under certain conditions.
7. Can the authors confirm that the improved desizing did not degrade fiber integrity?
8. One of the core goals of sizing is to promote interfacial bonding. After removal, how does LPE-treated carbon fiber interact with thermoset or thermoplastic matrices?
9. Why is sample 'a' showing higher weight loss than bare fiber?
Minor suggestions:
1. Ensure that figure captions describe what is shown, not just what was done.
2. Temperature should be expressed consistently (e.g., °C, not C).
3. In Table 2, ensure that all columns have units, and explain what FWHM stands for (full width at half maximum).
4. The conclusions repeat abstract content and do not discuss broader implications. Please include a discussion of potential applications and limitations.
The manuscript has frequent grammatical issues and awkward sentence constructions that impair readability. A thorough language edit is recommended.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for Authors1) P1 L13-15 “TGA further demonstrated that samples treated at 10 mm spacing exhibited negligible weight loss, indicating complete sizing removal.”
- the statement is not logically substantiated. The statement needs to be reformulated.
2) ok
3) ok
4) P3 L93 “During LPE treatment, a sodium metasilicate solution… ”
- it is necessary to explain in the text (!) why this particular electrolyte was chosen
5) It is necessary to explain the process of plasma formation in Experimental Section.
6) ok
6.1) “So, based on this, we supplemented the experiment of heating to 450℃ under nitrogen and maintaining it for 20 minutes. See P14 L246-261 for details.”
- The appropriate changes must be made to the text in Experimental Section.
7) P5 L127-131. « These patterns might be sizing agent. By comparing the 10,000 × photos of the exposed sample and A-I, it can be found that the monofilament surface of the exposed sample has no pattern along the length direction, and the edge of the image is not a straight line either, indicating that there is something wrapped around the cylindrical surface. We believe that this is the sizing agent.»
- The change in the shape of the fiber surface and its diameter in different states is not obvious as before. It seems that the authors are passing off wishful thinking as reality.
8) ok
9) ok
9.1) ok
10) “At this time, the main influences on the fiber are the temperature field of the electrode and the mechanical vibration during bubble generation. Therefore, when the voltage rises, the temperature of the electrode increases accordingly and the rate of bubble generation accelerates, which speeds up the melting and decomposition of the sizing agent.”
- A detailed description of these processes of action on the fiber should be given in the “Experimental Section”.
11) P6 L157-161 «From the photos d2- f2, point-like sizing agents with relatively high contrast on the surface of the fiber mono- filaments can still be observed. When observing the fiber monofilaments at a magnification of 10,000 × it was found that the axial grooves on the fiber surface were relatively obvious, but there was still a small amount of sizing agent remaining in the form of thin layers or spots»
- The change in the grooves on the fiber surface is not obvious, there is no evidence that the observed irregularities on the fiber surface are a sizing. It seems that the authors are passing off wishful thinking as reality.
12) A detailed description of these processes of action on the fiber should be given in the “Experimental Section”.
13) P7 L 181-183«Although there are still some residues of sizing agent in samples g and i, they are much less than those in d-f. This indicates that reducing the distance from 15mm to 10mm is effective for the removal of sizing agent.»
- The authors continue to use the term "sizing" without any evidence. In addition to the decorative words "possibly", "maybe", "probably", the text needs to be completely rewritten.
14) P7 L187-188 «By comparing the samples a-i, when removing the sizing agent, it can be considered that 10mm or 15mm will be the optimal distance between the carbon fiber and the electrode.»
- the conclusion is not supported by experimental data.
15) ok
16) Because the curves in Figures 7 and 8 were obtained in an air atmosphere, Figures 7 and 8 do not provide any useful information and are redundant. Figure 9 should be plotted in the Weight/Temperature axes.
17) ok
18) ok
19) ok
20) ok
21) ok
22) «According to the instructions provided by the manufacturer, the sizing agent for this grade of carbon fiber is sufficient to decompose at 300℃.»
- In the Experiment section, it is necessary to provide a detailed composition of the sizing, according to the instructions provided by the manufacturer. Otherwise this work has no value.
23) ok
24) It still remains unclear what the content of C-O bonds should be?
25) «The composition and content of sizing agents are commercial secrets of each manufacturer. It is impossible for us to find the composition and proportion of specific grades of carbon fibers.»
- In this case, it is necessary to add a section to the work in which the exact composition of the sizing will be determined. Otherwise, the work has no value.
26) My experience shows that absolute decomposition of epoxy resin occurs at 800*C. If the authors have other information, it should be given in the text.
27) ok
28) P12 L311-313 «Moreover, since it is not soluble with the solvent water and its density is lower than that of water, the molten sizing agent will rise to the plasma arc zone and be ignited and vaporized.”
- The statement is not supported by experimental data. As before It is not clear how the Archimedes force overcomes the adhesive force between the fiber and the sizing (even if it has melted), as well as the porous medium that the fiber thread is.
29) ok
30) P11 L266-270 “In this study, liquid-phase plasma electrolysis (LPE) was successfully employed to remove the sizing agent from T800 carbon fiber surfaces. Through systematic investigation of varying arcing voltages (185–215 V) and electrode spacings (10–20 mm), we determined that an optimal combination of 200 V and 10 mm spacing achieved near-complete sizing removal, as confirmed by SEM, TGA, and XPS analyses.”
- the conclusion is not supported by experimental data. SEM data are not representative. Representative DTA data for all modes are not provided.
31) «At this condition, plasma bombardment dominated the removal mechanism, eliminating sizing residues while exposing the underlying fiber grooves»
- the conclusion is not supported by EXPERIMENTAL DATA.
32) «TGA further demonstrated that samples treated at 10 mm spacing exhibited negligible weight loss, indicating complete sizing removal.»
- the conclusion is not supported by experimental data. Seу Comment 30.
33) «The results of XPS further confirmed the high efficiency of LPE in the removal of sizing agents (C-O bond content from 41.6% to 26.9%), and the retention of C-O also proved that LPE could maintain the surface activity of carbon fibers, confirming the effectiveness of LPE in decomposing the sizing agent.»
- It still remains unclear what the content of C-O bonds should be?
34) A detailed description of the method “The tensile strength of fibers” should be given in the “Experimental Section”. In addition, it is necessary to compare the residual strength of the fiber with other methods.
Comments on the Quality of English Language-
Author Response
Please refer to the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe revised version of the manuscript has been improved but there is still a lot that needs further correction and improvement. Although the article contains a lot of interesting information that made it worth publishing, it does not meet the standards of accuracy expected of scientific publications in its current form in several aspects. These are as follows:
Providing a short overview of alternative techniques in the introduction is a good idea but the authors should remain objective. There are several papers available reporting atmospheric plasma treatment of CFs that does not have high requirements for the airtightness.
Unfortunately, the SEM images are of poor quality and not sharp enough to clearly illustrate what the authors explain. I have inserted below a SEM image to show what a focused carbon fibre should look like. I do not think it is possible to draw any conclusions based on these images.
The section on TG measurement has also been improved, but the standard deviations in Fig.8 are still missing. Without these, it is impossible to determine whether there are significant differences between the various samples. If there were no parallel tests performed that the conclusion the authors drawn for the most ideal parameters must be changed to be more general. The results suggest that the performance of the treatment for samples e-i was at the same level. The results could be visualized more clearly in Fig.7 by showing the ordinate values from 90 to 100 only.
Discussion of XPS results has been improved but the revised version also contains inappropriate statements and needs more clarification. Considering that XPS analysis is the most reliable method supporting the conclusions, this section should be unquestionable. Asymmetrical tails are typically caused by the free electrons in the conductive band, typically at metals, graphite or graphene. The p-p* excitation creates a shake-up satellite peak at approximately 6 eV from the main C1s peak. For this reason, in Fig.10.c the decomposition should be made so the C-O (non-conductive bonds) peak is more symmetrical, while the C-C asymmetrical. Some quantitative judgment of O/C ratio can be obtained indeed based on the decomposed C1s spectrum but the correctness of such calculation can be confirmed by also calculating the O/C ratio using both the O1s and the C1s spectrum. In case of different results, the decomposition must be incorrect. The tail of the asymmetric C-C peak and the C-O peak overlap thus consistency between the quantity of the C-O and O species also verifies the proper selection of shape of asymmetric peak. The single type of C-O bonds found on the surface raises suspicions of inadequate resolution, too. Another unusual feature is that there are no sp3 bonds, only sp2, which are more typical of carbon nanotubes. Minor things that the name of the technique must be written with capitals (XPS) everywhere. In line 278, the C-O “keys” have no sense.
It was a good idea to include the measurement of tensile strength. However, the number of parallel measurements must also be given, along with the standard deviation of the results obtained.
Chinese characters appeared several times throughout the paper in reference to figures and tables.
Comments for author File: Comments.pdf
Author Response
plz refer to the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsAccepted in the present form.
Author Response
Thank you for your kindness.
