Synthesis and Physicochemical Characteristics of Chitosan-Based Polyurethane Flexible Foams

Round 1

Reviewer 1 Report

This manuscript reports a chitosan-based polyurethane flexible foams. I think a major revision is required before final acceptance. 1. The introduction is too long. It is better to simplify the words. 2. It is better to presnet the chemical structure of Ch in the article. 3. In figure 1, I can’t see the difference among the spectra. It is better to draw some guided lines and mark the important wavenumbers. 4. According to the R.38, an increase in the amount of Ch filler resulted in an increase in the number of open pores. However, in Table 2, the open cell content of PUR+Ch1 is higer than that of PUR+Ch2. The open cell content of PUR+Ch2 is higer than that of PUR+Ch3. Why the results are on the contrary to the reported article? 5. “The glass transition temperature (Tg) of Ch was 143.6 °C.”(line 260) This means that Ch is in glassy state under room temperature. As is well-known, polyurethane is in rubbery state under room temperature. Theoretically, the hardness of PUR are supposed to increase after introduced with Ch. Why the PUR+Chs are softer than PUR? 6. Figure 4 is not clear. 7. It is reported that chitosans are slight soluble in water. In this article, Ch was physically blended in to polyurethane, not chemically bonded. Will parts of Ch lost during the sorption process?

Author Response

Dear Reviewer of Processes,

We would like to thank you for review of our manuscript entitled “Synthesis and physico-chemical characteristics of chitosan-based polyurethane flexible foams”. Thank you for valuable suggestions/questions.

Below, we have included an answer for your suggestions:

1. The introduction is too long. It is better to simplify the words.

We agree with the suggestion that the words are too complex. We have made some changes to the second version to improve its simplicity. We realize that the introduction is a bit long; however, we believe that the issues raised are important in explaining the need to modify traditional foams. Hence, if the reviewer allows, we would like to leave the introduction in only a slightly simplified form.

2. It is better to present the chemical structure of Ch in the article.

We added the chemical structure of Ch in the section “2. 1 Materials”.

3. In Figure 1, I can’t see the difference among the spectra. It is better to draw some guided lines and mark the important wavenumbers.

We agree with the reviewer that the spectra do not appear to add anything essential to the work. However, our intention was to show that, unfortunately, no chitosan molecules were visible on the tested foam surface. We hope to check whether the chitosan molecules have incorporated into the polyurethane structure using the NMR method. For now, however, this is not possible. From the ATR-FTIR analysis, we only see that there are some interactions. It is also important that the general chemical structure of polyurethane has not been changed, hence our idea to leave Figure 2 as it is in the manuscript.

As indicated by the discrete wavenumber shifts, the mentioned interactions between the polyurethane chains and the chitosan particles, are probably of a hydrogen bond nature. Unfortunately, as we wrote, at this stage of the research it cannot be determined whether a chemical reaction has taken place between the OH groups of the chitosan and the NCO groups of the diisocyanate.

4. According to the R.38, an increase in the amount of Ch filler resulted in an increase in the number of open pores. However, in Table 2, the open cell content of PUR+Ch1 is higher than that of PUR+Ch2. The open cell content of PUR+Ch2 is higher than that of PUR+Ch3. Why the results are on the contrary to the reported article?

We fully agree with your suggestion. We formulated this observation imprecisely.
We have corrected this entry in the manuscript. We checked that the results in Table 2 were correct. In our research, we observed that the addition of more than 2 wt% of Ch causes a general deterioration of the examined properties. It has already been noticed during the synthesis that this amount of filler causes Ch to be less distributed throughout the foam volume. Husainie et all also found only generally that the addition of a polysaccharide (including chitosan) causes an increase in the number of open pores. They also noticed that the addition of larger amounts (up to 5%) of chitosan causes the agglomeration of the filler particles.

5. “The glass transition temperature (Tg) of Ch was 143.6 °C.” (line 260) This means that Ch is in glassy state under room temperature. As is well-known, polyurethane is in rubbery state under room temperature. Theoretically, the hardness of PUR are supposed to increase after introduced with Ch. Why the PUR+Chs are softer than PUR?

We thank the reviewer for this comment. We have also considered this phenomenon and it seems to us that the only sensible explanation is that, with such a small amount of filler, the hardness of the samples was more influenced by the number of open pores than by the physical presence of chitosan particles.

6. Figure 4 is not clear.

We have corrected the quality of Figure 5 (previously Figure 4). In this figure, we wanted to present that the PUR+Ch foams with 1-3 wt% of Ch absorbed much more water than unmodified PUR. It may suggest that the addition of Ch as a filler caused an increase in the hydrophilic nature of the foams. However, although the time of incubation of samples in water was long (up to 14 days, 336 hours), equilibrium sorption was not obtained, what is also shown in Figure 5.

7. It is reported that chitosan’s are slight soluble in water. In this article, Ch was physically blended in to polyurethane, not chemically bonded. Will parts of Ch lost during the sorption process?

To our knowledge, the solubility of chitosan in water is negligible. We were rather concerned that the chitosan particles would be washed out of the polyurethane network. However, no floating filler particles were found in the containers after testing. They are therefore well bound to polyurethane chains, which seems to confirm the formation of hydrogen bonds.

We have included all your comment in the second version of the article with all corrections which are highlighted by “Track Changes” function as requested by the editor.

Please let us know of your decision at your earliest convenience.

Yours faithfully,

Authors

Reviewer 2 Report

The article entitled " Synthesis and physico-chemical characteristics of chitosan-based polyurethane flexible foams" presents interesting work on the current topic in the field of sustainable materials development. Most of the discussion based on the experimental evidence is convincing. Some suggestions/questions are raised as follows for authors to consider in order further strengthen the fundamental aspect of this work.

- The authors should add the structural analysis of ROKOPOL®V700 polyol in the materials section in terms of molecular weight and -OH number.

- FT-IR description in Methods section is missing.

- Figure 2 is not clear. The label “a”, “b” , “c”, and “d” are not highlighted in the figure.

- Please check the legend corresponding to curve PUR-Ch1 in Figure 4. The color is missing.

- Did the authors evaluate the density of these foams? This property can confirm the results about hardness.

-  The addition of chitosan (1 or 2 wt%) into the PUR foams results in a noticeable increase of softness, linked to the open cell content. The foams with lower hardness (6.0 ShoreH0) and higher open cell content (95.8) is PUR+Ch1 and not PUR+Ch2 (line 247). Moreover, the authors compare the results with those from reference 23, where the PUR foam synthesized by the two-step method with the addition of 1.5 wt% of Ch was harder than the reference. It is difficult to compare the effect of addition of a filler in such a low content in a PUR foam with so different starting components. In reference 23, the polyol was a poly(Ɛ-caprolactone)diol and  the isocyanate was H12MDI, the diisocyanate (4,4′-methylene dicyclohexyl diisocyanate). I would like to ask the Authors for a comment.

Author Response

Dear Reviewer of Processes,

We would like to thank you for review of our manuscript entitled “Synthesis and physico-chemical characteristics of chitosan-based polyurethane flexible foams”. Thank you for your valuable suggestions/questions.

Below, we have included an answer for your suggestions:

1. The authors should add the structural analysis of ROKOPOL®V700 polyol in the materials section in terms of molecular weight and -OH number.

Thank you for this suggestion. ROKOPOL®V700 polyol had molecular weight 700 g/mol and hydroxyl number value 225–250 mg KOH/g. We have added it in the Materials section.

2. FT-IR description in Methods section is missing.

We apologize for such a basic mistake. We have completed the description as point “2.2.1. Chemical Structure”.

3. Figure 2 is not clear. The labels “a”, “b”, “c”, and “d” are not highlighted in the figure.

We corrected the figure and now it is marked as Figure 3 in the new version of the manuscript. We hope it is clear now.

4. Please check the legend corresponding to curve PUR-Ch1 in Figure 4. The color is missing.

We totally agree with the suggestions of the Reviewer. The quality of Figure 5 (previously Figure 4) was corrected in the new version of the manuscript.

5. Did the authors evaluate the density of these foams? This property can confirm the results about hardness.

In fact, we tried to determine the density of the tested foams, but because of their too high plasticity when the samples were cut, they deformed. Hence, we obtained too large differences between the values for one type of material, and we believe that these results are uncertain. Due to this observation, we have omitted these results from our manuscript.

It seems to us, however, that the connection of hardness with the cell open content allows that, with such a small amount of filler, the hardness of the samples was more influenced by the number of open cells than the physical presence of chitosan particles. This was a reason for such an observation.

6. The addition of chitosan (1 or 2 wt%) into the PUR foams results in a noticeable increase of softness, linked to the open cell content. The foam with lower hardness (6.0 ShoreH0) and higher open cell content (95.8) is PUR+Ch1 and not PUR+Ch2 (line 247). Moreover, the authors compare the results with those from reference 23, where the PUR foam synthesized
   by the two-step method with the addition of 1.5 wt% of Ch was harder than the reference.
   It is difficult to compare the effect of addition of a filler in such a low content in a PUR foam with so different starting components. In reference 23, the polyol was a poly(Ɛ-caprolactone)diol, and the isocyanate was H₁₂MDI, the diisocyanate (4,4′-methylene dicyclohexyl diisocyanate). I would like to ask the Authors for a comment.

- We totally agree with the Reviewer that the softest foam was PUR+Ch1 which also had higher open cell content. We corrected it in the new version of the manuscript.

- It is true that the polyurethane foams with chitosan described in the reference publication [23] had a completely different composition and were obtained by a different method. However, our intention was to indicate that increasing the amount of chitosan above a certain value causes disturbance and deterioration of the expected properties. Hence the reference to the aforementioned paper.

We have included your comment 1-4 in the new version of the manuscript with all corrections which are highlighted by “Track Changes” function as requested by the editor.

Please let us know of your decision at your earliest convenience.

Yours faithfully,

Authors

Round 2

Reviewer 1 Report

The authors haved revised the article carefully.  I have no objection for the publication of this work at this journal.