Biobased Acrylate Shells for Microcapsules Used in Self-Healing of Cementitious Materials

Round 1

Reviewer 1 Report

Dear Author, 

This paper mainly focused on microcapsules used in self-healing of cementitious materials by biobased acrylate. This will be accepted after the extensive change of that paper. The quality of this paper should be changed a lot. The major review comments and I write in here and yellow mark in the manuscript. 

1. The sentence pattern and grammar should be checked.

2. Abstract should be more precise and informative.

3. Introduction must be rewritten based on the importance of that research.

4. Added something about the self-healing function in the initial of the introduction.

5.  Materials and Methods this part must be subtitles like 2.1 and 2.1.1.

6. Materials and Methods “Very complicated”.

7. All figures should be in the same format.

8. Please describe the results more precisely.  If for all results and discussion.

9. Conclusion should describe more concisely. 

Comments for author File: Comments.pdf

Author Response

Dear Author,

This paper mainly focused on microcapsules used in self-healing of cementitious materials by biobased acrylate. This will be accepted after the extensive change of that paper. The quality of this paper should be changed a lot. The major review comments and I write in here and yellow mark in the manuscript.

We thank Referee 1 for the careful read and positive assessment of the work.

1. The sentence pattern and grammar should be checked.

Response 1: Thanks for your suggestion. The text was carefully revised.

2. Abstract should be more precise and informative.

Response 2: Thanks for your suggestion. The section was rewritten to be more precise and informative.

3. Introduction must be rewritten based on the importance of that research.

Response 3. Thanks for the suggestion. The initial paragraph of the introduction was rewritten.

4. Added something about the self-healing function in the initial of the introduction.

Response 4: Thanks for your suggestion. The section was rewritten to describe the self-healing function.

5. Materials and Methods this part must be subtitles like 2.1 and 2.1.1.

Response 5: Thanks for your suggestion. The section was divided into (L144) “2.1. Selection of biobased acrylates” and (L156) “2.2. Production of microcapsules”.

6. Materials and Methods “Very complicated”.

Response 6: Thanks for your comment. We included a general schematics of experimental plan and procedures (Figure 4 - L220) aiming at simplifying the section.

7. All figures should be in the same format.

Response 7: Thanks for your comment. We modified Figure 5, 6 and 7 to facilitate reading.

8. Please describe the results more precisely. If for all results and discussion.

Response 8. Thanks for the comment. Please accept our apologies for disagreeing with you on this matter.

9. Conclusion should describe more concisely.

Response 9. Thanks for the comment. The conclusion was revised to be more concise (L462): “This study demonstrates for the first time the production of self-healing microcapsules with biobased acrylate shells using microfluidics. A throughout review of the literature was performed, considering a wide range of biobased acrylates according to global production and parameters associated with physical triggering and survival during mixing. According to those parameters, soybean oil was the most suitable contender for the production of acrylated microcapsules. The viscosity is another key parameter governing the applicability of these acrylates for microcapsules production using microfluidics, thus the acrylated epoxidised soybean oil (AESO) viscosity was adjusted to the range of 30-50 mPa.s with 1,6-hexanediol acrylate (HDDA) and biobased isobornyl acrylate (IBOA). However, the use of 75 wt% of the shell as HDDA remains an issue due to the high carbon content derived from fossil fuels. Capsules were successfully produced using IBOA and AESO as shell, with outer diameter of 486 ± 9 μm with calculated shell thickness ranging between 36 and 67 μm. SEM and OM demonstrated a clear core-shell structure and confirmed the calculated shell thickness of the capsules. The retention of mineral oil was confirmed by thermogravimetric analysis of dried capsules and indicated that ~50% of the capsules consisted of the core material. Furthermore, production rates of 2.38g/h were confirmed, resulting in an 92% encapsulation efficiency. The uniaxial tension test of AESO with 75wt% IBOA was conducted, resulting in a Young’s modulus of 1.7±0.4 GPa, ultimate tensile strength of 29.2 ± 7.7 MPa and a strain at failure of 2.1±0.8%. Using DMA, the ductile transition temperature was estimated as 42°C, increasing the likelihood of the capsules surviving mixing. Microcapsules were produced with a size of 481 ± 4 μm and functionalised shells’ surface and embedded in cement paste; upon introducing a crack in the cementitious matrix, several broken capsules were observed, with a measured shell thickness around 6 μm. Furthermore, good interfacial bonds between the capsules and the cement paste were observed, facilitating the rupture, and XCT confirmed a good distribution of the capsules throughout the samples. These results support the extensive application of biobased resins for the creation of novel sustainable microcapsules for self-healing in cementitious materials.”

Author Response File: Author Response.pdf

Reviewer 2 Report

Interesting investigations from authors. 

Manuscript can be accepted.

Author Response

We thank Referee 2 for the positive assessment of the work.

Reviewer 3 Report

Dear Authors,

I have found this manuscript to be very interesting, well-structured and with research ideas worth pursuing. 

However, I do have some observations and recommendations which may be included in the present paper or in future ones (I leave this decision to the Authors):  

- I would have liked to see a more detailed description of the mechanism which causes the microcapsules to fracture and release their active agent. Their basic "working" concept is sound (i.e. a crack in the concrete encounters the microcapsule, leads to its fracture and therefore to the release of the active agent), but some additional details are welcome. For example, is there any way to prove that the capsules are uniformly distributed in a concrete element and do not exhibit any tendency to clump together? Alternatively, is there a high enough possibility that the crack will "encounter" microcapsules and trigger the "healing" process?

- Another suggestion would be to include a (short) explanation of the healing process itself. I believe this can add to the overall quality of the paper.  

- A recommendation would be to investigate if these microcapsules can also be activated by contraction cracks or microcracks. These can emerge without any mechanical work, so it should be investigated if these types of cracks can trigger the "healing" process. It would be ideal for the "healing" process to activate whenever the concrete has a tendency to crack.

- Yet another point of interest would be to test some structural concrete elements in compression, three-point bending etc. These elements should ideally represent elements which tend to crack during the service life of a structure.

I could not identify any typos or other writing mistakes.

Overall, I recommend the publication of this article.

Author Response

We thank Referee 3 for the positive assessment of the work and constructive suggestions.

- I would have liked to see a more detailed description of the mechanism which causes the microcapsules to fracture and release their active agent. Their basic "working" concept is sound (i.e. a crack in the concrete encounters the microcapsule, leads to its fracture and therefore to the release of the active agent), but some additional details are welcome. For example, is there any way to prove that the capsules are uniformly distributed in a concrete element and do not exhibit any tendency to clump together? Alternatively, is there a high enough possibility that the crack will "encounter" microcapsules and trigger the "healing" process?

Response 1: Thanks for your valuable question. Images of the XCT with capsules uniformly distrubuted throughout the matrix were added in Figure 12 together with the following discussion (L432): “A nondestructive XCT scan was performed in order to examine the uniform distribution of microcapsules throughout the cement paste sample. Figure 12(g,h,i) depicts the 2D reconstruction of the samples, where the light grey represents the cement paste and the dark grey circular dots around 500 μm represent the microcapsule distribution. Small dark grey circles ~ 100 μm are also observed and associated with air bubbles [73]. A 3D reconstruction of the sample (Figure 12h) with a green shell surface demonstrates the uniform distribution of microcapsules throughout the sample. According to the literature, capsules can agglomerate within the matrix through two distinct mechanisms: if the capsules cluster during its production, during the photopolymerisation or filtering step, it is possible that they do not separate during mixing and therefore remain in clumps [50]. In this study, however, agglomeration was prevented by collecting the capsules in PVA solution, as shown in Figure 12b, which depicts clearly dispersed capsules. Alternatively, the density differences accentuated by the shaking table may cause the capsules to float and concentrate closer to the cementitious matrix's surface [74]. Adjusting the material's viscosity and/or the capsules' density can reduce this anisotropy. Consequently, a number of authors have reported the uniform distribution of capsules in cementitious matrices for both laboratory tests and in situ applications [49,75,76].” Regarding the “encounter” between the capsules and the crack, the following line was added (L387): “The likelihood of a crack encountering microcapsules and triggering self-healing is largely dependent on three factors: (i) the Young's modulus of the capsules, (ii) the interfacial bonding between the capsule and matrix, and (iii) the tensile strength of the shell [43,70].”

- Another suggestion would be to include a (short) explanation of the healing process itself. I believe this can add to the overall quality of the paper.

Response 2: Thanks for your suggestion. A short explanation of the healing process was added in the first paragraph of the introduction.

- A recommendation would be to investigate if these microcapsules can also be activated by contraction cracks or microcracks. These can emerge without any mechanical work, so it should be investigated if these types of cracks can trigger the "healing" process. It would be ideal for the "healing" process to activate whenever the concrete has a tendency to crack.

Response 3: Thanks for your suggestion. More investigation regarding the crack pattern used to produce the microcapsules will be published soon.

- Yet another point of interest would be to test some structural concrete elements in compression, three-point bending etc. These elements should ideally represent elements which tend to crack during the service life of a structure.

Response 4: Thanks for your suggestion. The mechanical characterisation was out of the scope of current work. 

I could not identify any typos or other writing mistakes.

Overall, I recommend the publication of this article.

Reviewer 4 Report

·        Avoid extra spaces throughout the text!

·        Use correct formatting concerning references in the text!

·        Line 81: cradle-to-gate

·        Line 138: define IBOA and HDDA before using the abbreviation!

Author Response

• Avoid extra spaces throughout the text!

Response 1: Thanks for your suggestion. The text was carefully revised to remove extra spaces.

• Use correct formatting concerning references in the text!

Response 2: Thanks for your suggestion. The references were updated using MDPI reference style.

• Line 81: cradle-to-gate

Response 3: Thanks for your suggestion. The line was altered to “However, a cradle-to-gate life cycle assessment conducted on the production of acrolein from glycerol by Cepsi et al. calculated GWP as 4520 kg CO2/tonne of acrolein [28].”

• Line 138: define IBOA and HDDA before using the abbreviation!

Response 4: Thanks for your suggestion. The line was changed to: “Owing to the high viscosity of the biobased acrylate, 1,6-hexadienol diacrylat (HDDA) and isobornyl acrylate (IBOA) were used as diluting agents to achieve the adequate viscosity for the microfluidic device.”

Reviewer 5 Report

-          It is necessary to cover a paragraph on the philosophy of self-healing of cementitious materials and discus the traditional methods for repairing the cementitious materials other than self-healing method.

-          In page 6 line 194 “Tensile strength”, it is recommended to add the details of the specimen used for the test, shape and dimension, and the name for the standard used.

-          In figure 6 page 8 “Global production of oils”, the bars for Cardanol, Tung, and Grapeseed oils are not clear.

-          In figure 10 page 11 “Mechanical characterisation of AESO:3IBOA. (a) Tensile stress-strain curve and (b)”,   you should complete the caption of (b).        

-          What is the standard specimen for Young’s modulus test?

-           It is recommended to add a methodology section that presents the experimental plan followed in this study. 

Author Response

-          It is necessary to cover a paragraph on the philosophy of self-healing of cementitious materials and discus the traditional methods for repairing the cementitious materials other than self-healing method.

Response 1: Thanks for your suggestion. The following was added to the first line of introduction: “Cementitious materials are prone to fracture, especially when subjected to tensile forces. Once these cracks have formed, they can facilitate the transport of water and other chemicals which can result in corrosion and intensify the cracking process. To minimise crack formation, traditional methods for repair and maintenance are applied, including human intervention to seal cracks. However, these actions are costly; in the United Kingdom alone, approximately 50% of the construction budget is spent on repair and maintenance [1]. An alternative strategy relies on capsule-based self-healing, which aims to distribute small capsules containing a healing agent throughout the matrix. Upon formation of a crack in the matrix, the shell is triggered to release the healing agent and heal the crack without external intervention [2].”

-          In page 6 line 194 “Tensile strength”, it is recommended to add the details of the specimen used for the test, shape and dimension, and the name for the standard used.

Response 2: Thanks for the suggestion. This data was obtained from the literature and the setence was modified for clarity’s sake (L255): “A wide range of biobased acrylates were investigated, and tensile strength data, obtained from previous literature, is summarized in Figure 5, which compares the results to the 10 previously used shell materials and highlighted potential contenders for more sustainable shells.”

-          In figure 6 page 8 “Global production of oils”, the bars for Cardanol, Tung, and Grapeseed oils are not clear.

Response 3: Thanks for your suggestion. The Figure 7 was changed to increase the visibility of Cardanol, tung and grapessed oil production.

-          In figure 10 page 11 “Mechanical characterisation of AESO:3IBOA. (a) Tensile stress-strain curve and (b)”,   you should complete the caption of (b).       

Response 4: Thanks for your suggestion. The line was changed to: “Figure 11. Mechanical characterisation of AESO:3IBOA. (a) tensile stress-strain curve and (b) Modulus–temperature and tangent delta–temperature curves gathered by DMA.

-          What is the standard specimen for Young’s modulus test?

Response 5: Thanks for your suggestion. The following information was added regarding the speciment for the Young’s modulus test (L189): “To quantify the mechanical properties of the polymers, 1.5 mL of acrylate was poured over the negative dog bone shaped mould (70 mm length, 11 mm width, 3 mm height and 6 mm gauge width) made of PDMS.”

-           It is recommended to add a methodology section that presents the experimental plan followed in this study.

Response 6: Thanks for your valuable suggestion. A general schematics of experimental plan was included as Figure 4.

Round 2

Reviewer 1 Report

Can it be a little modified and check the sentence paraphrased?