Surface Modification of Nano-Al₂O₃ with Silane Coupling Agent and Its Effect on the Compressive Strength of PI/Al₂O₃ Composites

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors' work is devoted to an interesting and important direction in the production of polymer-based nanocomposites. The work was done at a good level, but for its publication, it is necessary to answer a number of questions.

What is the role of preliminary hydrolysis and condensation 3-aminopropylalkoxysilane in acetic acid? This leads to the preliminary production of oligomers and modification of the surface with them, which again affects the agglomeration and stabilization of the particles obtained later. It is known that amines in silane themselves catalyze heterofunctional condensation, so the question arises:

Why do the authors modify the surface not with a molecule of 3-aminopropylalkoxysilane, but with oligomers? In addition, the authors do not determine the degree of silane grafting in any way, at least by weight gain. It would also be possible to determine what remains in the modifying solution after filtration.

Question about the need for Al2O3 – based on SEM 138 image of Al2O3 powder, The powder is not nano-sized, however, picture c in Figure 4 is not discussed, and conclusions about the particle size are drawn from TEM - Figure 4(d). So what is the actual particle size?

The authors talk at one point about crosslinking occurring at 230 degrees. What is the mechanism of this stitching? Is it possible to draw an approximate stitching mechanism?

Author Response

Dear Reviewer,

Thank you very much for your important comments. We have revised the manuscript according to your suggestions. The responses to your comments are in the word file.

With our best wishes,

J. Cao et al.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article by a team of Chinese authors is devoted to the production of surface modified aluminum oxide nanoparticles with deposited polyimide. Polyimides have excellent thermal stability, which, together with the biocompatibility and chemical stability of these structures, opens up opportunities for widespread research of these structures in order to use them in practice. Nevertheless, the authors note that one of the disadvantages of these structures is a weak mechanical design and low-temperature stability, which greatly limits the possibilities of using compounds. This study aims to solve this problem by fixing the polyimide to an appropriate aluminum oxide matrix. The choice of the carrier was determined by both economic feasibility and suitable parameters such as high thermal and mechanical stability, wear resistance, good conductivity, etc.

In the work, a series of hybrid materials of nano-sized aluminum oxide with applied polyimide was obtained. The application was carried out by the method of hot compression. The cross-link connection of nanoscale aluminum oxide to a polyimide matrix made it possible to reduce the mobility of polyimide chains, thereby increasing the elasticity of the composite. The structural difference in the content of various amounts of aluminum oxide in the composite is also shown and its optimal content is determined. IR spectroscopy and electron scanning and transmission microscopy approaches were used to confirm the modification. Taken together, the approaches allow us to speak with confidence about the successful implementation of the modification. In addition, the paper defines the characteristics of the morphology of the surfaces of the obtained hybrids with different percentages of aluminum oxide, as well as their compressive strength.

In general, the work was done at a good level and there are no critical comments to the authors. Separately, I would like to note that the authors compared the obtained hybrids with both pure polyimide and similar analogues with, for example, silicon oxide, etc. This is very valuable because it gives the reader an understanding of the relevance of the research. Thus, the work corresponds to the subject of the Journal and meets its quality and can be published in its current form.

Author Response

Dear Reviewer,

Thank you very much for your important comments. 

With our best wishes,

J. Cao et al.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Prof. , Editor of Coatings,

Thank you very much for invite me to review the manuscript under the title of “Surface modification of nano-Al₂O₃ by silane coupling agent and its effect on the compressive strength of PI/Al₂O₃ composites”

 I studied this manuscript very well. The manuscript cannot accepted in the present form, it should be revised and improve based on the following comments and suggestions:

1. The English language through all the manuscript should be improved.
2. In the introduction section, the statement in line 27 is in conflict with that in lines 30-31.
3. In page 4, lines 115-116, the authors stated “The appearance of peak at 230 °C indicated the occurrence of cross-linking reactions.” Please check this statement or support it by a suitable reference.
4. In page 4, line 129 “X-ray diffraction spectra” should be “x-ray diffraction patterns”.
5. The authors should add the XRD patterns and the FTIR spectra of PI/Al₂O₃ with and without the coupling agent.
6. The authors should add the JCPDS card number that match with the XRD pattern of pure α-Al₂O_3.
7. In page 5, lines 143-144, and Fig. 5, the authors stated “The increased porosity had a close relationship with the agglomeration of nano-Al₂O₃.” How the authors measured the porosity of the samples?.
8. Why the porosity increased in the nanocomposite with Al₂O₃ content equal to 50 wt.%.?
9. In the caption of Fig. 7, the authors should remove “FTIR spectra and XPS of nano-Al₂O₃”.
10. In page 8, lines 207-208, the authors stated that “and the roughness of the fracture surface increased with the increase of Al₂O₃ content” the authors should add new table contain the roughness values of the PI/Al₂O₃ nanocomposites.

   

   

 

Comments on the Quality of English Language

1. The English language through all the manuscript should be improved.

Author Response

Dear Reviewer,

Thank you very much for your important comments. We have revised the manuscript according to your suggestions. The responses to your comments are in the word file.

With our best wishes,

J. Cao et al.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

I have the following comments regarding this article:

-          Please explain the abbreviations/commercial names in the Introductions sections: APS – line 48; KH550, KH570, NDZ401 – line 52. Please avoid using abbreviations/commercial names without an explanation because it may result in an inability to reproduce your research in the future.

-          Please characterize better the PI and Al2O3 used in this study. For example, please provide the molecular weight or viscosity of PI and the specific surface area or particle size distribution of Al2O3.

-          Judging by the chemical structure presented in Figure 2 KH550 is aminopropyltriethoxysilane – please use the full name before you use the commercial name. The chemical structure presented in Figure 2 is very small and might not be visible to all the readers.

-          How the authors can be sure that acetic acid didn’t react with KH550? Was this checked?

-          The Tg of the PI proposed by the authors based on the DSC results seems to be too high (466 °C). In my opinion, it is more likely that the slow increase of the red curve around 350°C indicates the Tg of the PI. This would be in line with the literature sources, for example:
https://www.degruyter.com/document/doi/10.1515/polyeng-2013-0255/html

https://www.nature.com/articles/pj201637
https://pubs.rsc.org/en/content/articlehtml/2017/ra/c7ra07142f
Especially, if the decomposition temperature T5% of the composites are lower than the Tg! It is just impossible.

-          The Y axis scale in Figure 5 should start form “0” not below “0” otherwise it gives an odd impression of a negative porosity.

-          Please explain the abbreviation RGO – line 254

 

-          The authors state that the reinforcement in the mechanical properties of the composites filled with the surface modified alumina was due to formation of cross-links (Abstract) or three-dimensional network (4. Discussion and 5. Conclusions). The both terms describe formation of chemical links between the modified alumina surface and PI macromolecules. However, the authors didn’t explain what is the nature of these chemical links. The alumina surface is modified with amino-groups, which can interact with the “N” and “O” atoms in the polymer structure via hydrogen bonds between partially positively charged protons and partially negatively charged free electron pairs. However, these are not strong chemical bonds that may be defined as cross-links. In my opinion, formation of the hydrogen bonds is the reinforcing factor here. If the authors have an alternative explanation based on formation of chemical bonds, please elaborate and explain better the mechanism of reinforcement.

Author Response

Dear Reviewer,

Thank you very much for your important comments. We have revised the manuscript according to your suggestions. The responses to your comments are in the word file.

With our best wishes,

J. Cao et al.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have largely made all the corrections and significantly improved the publication. I just still don’t quite understand how the authors determined the degree of silane grafting to the surface of particles using the DSC method?

Author Response

Many thanks for your valuable question. The degree of silane grafting to the surface of particles using the DSC method are revised and added in the manuscript, as follows:

“The DSC method easily reflected the weight loss, especially the loss of organic polymer at high temperatures. In this paper, the silane coupling agent belongs to the organic polymer. The surface grafting modified Al₂O₃ by silane coupling agent will decompose and weight loss after high-temperature treatment. The loss value meant the decrease of silane grafting. Therefore, the difference between the weight loss between the modified Al₂O₃ and the unmodified Al₂O₃ was the grafting specific gravity of the coupling agent. Figure 3 shows the TGA results of original and modified Al₂O₃. The weight loss of nanoparticles <100°C was mainly due to the water desorption. The weight loss between 100 and 800°C was mainly caused by the loss of organic groups grafted the nanoparticles surface [23]. For original Al₂O₃ powder, the total loss was 4.1% and this was due to the loss of absorbed moisture. And the weight loss was up to 8.3% for modified Al₂O₃ powder. Thus the grafting rate of alumina after modification was 4.2 wt.%.”

(Page 2, Section 2.1, Row 86-95, Page 3, Section 2.1, Row 96-98)

Figure 3. TGA curves of original and modified Al₂O₃.

 

Besides, the research further proved that the weight loss is caused by the loss of organic groups grafted the nanoparticles surface between 150 and 600°C. The related explanation and reference is as follows:

“[23] Duan X, Siew WH, Given M, et al. Effect of different surface treatment agents on the physical chemistry and electrical properties of polyethylene nano–alumina nanocomposites. High Volt. 2020;5(4):397-402. doi: 10.1049/hve.2020. 0081.”

(Page 13, Section Reference, Row 392-393)

 

With our best wishes,

1. Cao et al.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors answered all the questions and the manuscript is now suitable for publication.  

Author Response

 Many thanks for your important comments.

 

With our best wishes,

J. Cao et al.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The authors improved the paper and addressed my comments and suggestions. However, there is still one point, with which I can't entirely agree. It is the Response 9. I agree with the explanation regarding covalent bonding between Al2O3 and aminopropylalkoxysilane after its hydrolysis. However, I cannot see a potential covalent bond between the aminopropyl groups and PI. To my knowledge, hydrogen bonds can be formed between them - which can be responsible for the reinforcement of the composites - but not covalent bonds.

Author Response

Many thanks for the important suggestion. The chemical bond between the silane coupling agent and PI is a hydrogen bond rather than a covalent bond. We did not have a thorough understanding of the reinforcement mechanism, and we have made changes in the revised manuscript.

“There are three possible reinforcement mechanisms for the PI/Al₂O₃ composite. In one of the mechanisms, the reinforcement was due to the hydrolysis of silane coupling agents, which produces silicon-oxygen bonds and hydroxide ions. Silicon-oxygen bonds are very stable bonds in silane coupling agents, which can strongly adhere to the surface of inorganic materials. Aminopropyl groups at the other end can form hydrogen bonds with polymers when cured [22,33,34]. Besides, the second reason is attributed to the transfer of shear stresses in the PI/Al₂O₃ interface, and the nanoparticle-matrix interaction is the major factor for its occurrence [35].” (Page 10, Section 4, Row 280-287)

The bonding mechanism is illustrated in the following two references:

“[33] Xianghui Oua, Xuemin Lua, Shuangshuang Chen, et al. Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion. Eur Polym J. 2019: 109368. doi: 10.1016/j.eurpolymj.2019.109368.

https://doi.org/10.1016/j.eurpolymj.2019.109368

[34]Shadpour Mallakpour, Mohammad Dinari. Investigating the nanostructure and thermal properties of chiralpoly (amide-imide)/Al₂O₃ compatibilized with 3-aminopropyltriethoxysilane. Mater Res Bull. 2013;48:3865–3872. doi: 10.1016/j.materresbull.2013.05.095.” http://dx.doi.org/10.1016/j.materresbull.2013.05.095

(Page 13, Section References, Row 413-418)

 

In the Reference [33], Fig. 3(a) shows schematic diagrams of the interfacial interactions between nanosheet and PI matrix and the interphase in the two series of hybrids for different interfacial interactions. The chemical bond between the silane coupling agent and PI is a hydrogen bond.

In the Reference [34], Scheme 1 further demonstrates the reaction mechanism. The chemical bond between the silane coupling agent and PI is a hydrogen bond.

With our best wishes,

J. Cao et al.

 

Author Response File: Author Response.pdf