Review Reports

Round 1

Reviewer 1 Report

The paper presents a study on the effect of various ionic species on the rheological properties of poly(acrylic acid) (PAA) solutions that are interesting for aircraft de-icing. The study clearly deals with an interesting and relevant topic. However, the discussion is rather sparse in my opinion, given the complexity of the different systems that are studied. A detailed discussion how the individual ion species affect the polymer structure would be helpful. For instance, why exactly does the chain undergo multiple structural transitions in the presence of calcium ions and varying sodium concentration (lines 284-286)? What is the difference between sodium and potassium? For these reasons, the guidelines that are given rather seem to be made on an empirical level, and the paper should be revised to provide a deeper understanding.

Furthermore, the following comments should be addressed:

Section 2.3 could be improved. For instance, it is at first unclear if the 50:50 mono-propylene glycol/water mixture was created by weight, or if molar ratios are specified (although this is specified at a later point in the paper). It might also be helpful to list the concentrations of the polymer and all contained ion species for the individual systems in Table 1. In this way, the reader can directly grasp the various systems that have been investigated.

Line 167: How exactly is the critical point in Figure 2 defined, and are the lines in Figure 2 just interpolation lines, or do they originate from some fitting procedure (that is eventually related to the critical point)?

Lines 295-300: The authors state that base liquid can either be chosen to be ethylene or propylene glycol, depending on the intended operating temperature. However, only propylene glycol solutions are studied in this paper, and it is unclear in how far the results can be transfered to ethylene glycol. This is due to the fact that the solvation of both the ions and PAA might be affected by an exchange of one of the solvents, leading to quanitative or even qualitative differences. The authors should therefore formulate their statement more carefully, or provide additional references that show that solvation effects are less severe in this case.

Author Response

Response to points:

The issue of the possible effects which the sodium/potassium formate on the de-icing formations, is presented in line 316:

i)                    The monovalent cation can bind to the carboxyl groups of the Carbomer but in so do they create a ion pair which will block the possibility of hydrogen bonding etc with the groups on the same molecular or between adjacent polymer molecules. The net effect will be that this will inhibit the formation of a stable gel structure.

ii)                   The presence of the sodium or potassium formate will influence the ionic strength of the ‘solution’ and this usually has the effect of lowering the viscosity and hence inhibiting polymer – polymer interaction.

The following is added to the text at the beginning of the Discussion:

Calcium, potassium and sodium are known to interact with PAA (Carbomer A) polymer in various ways. The electrolyte can have two effects: it can shield the acid groups from interaction. The polymer molecule in the limit of complete shielding would appear like a flexible polymer molecule. This would be the case for an electrolyte which contains monovalent cations. In the case of bivalent cations there is the possibility of the creation of bridges between acid groups in the polymer as well as adjacent polymers enhancing interactions. Since potassium belongs to the same main group as sodium, it would be expected to behave similarly to sodium to shield the carboxyl groups. However, due to its larger ionic radius, the potassium cation has a greater binding power, and potassium formate demonstrates an increased ability to suppress the viscosity compared to sodium chloride.

 

Furthermore, the following comments should be addressed:

 

Section 2.3 could be improved. For instance, it is at first unclear if the 50:50 mono-propylene glycol/water mixture was created by weight, or if molar ratios are specified (although this is specified at a later point in the paper). It might also be helpful to list the concentrations of the polymer and all contained ion species for the individual systems in Table 1. In this way, the reader can directly grasp the various systems that have been investigated.

Changed in line 130 to be explicit (ie wt:wt).

The title of the Table 1a has been changed which is a better way to be explicit about the composition of the fluids.

Line 167: How exactly is the critical point in Figure 2 defined, and are the lines in Figure 2 just interpolation lines, or do they originate from some fitting procedure (that is eventually related to the critical point)?

The intersection is used to illustrate the change in behaviour based on two models to fit data as mentioned in Line 159..  

Lines 295-300: The authors state that base liquid can either be chosen to be ethylene or propylene glycol, depending on the intended operating temperature. However, only propylene glycol solutions are studied in this paper, and it is unclear in how far the results can be transfered to ethylene glycol. This is due to the fact that the solvation of both the ions and PAA might be affected by an exchange of one of the solvents, leading to quanitative or even qualitative differences. The authors should therefore formulate their statement more carefully, or provide additional references that show that solvation effects are less severe in this case.

The wording is changed to be more explicit concerning the study being focused on propylene glycol whilst acknowledging the environmental effects of ethylene glycol.

Line 294:

Propylene glycol is more favourable than ethylene glycol from an environmental point of view but limited in terms of its low temperature performance [13,15]. The viscosity of such mixtures at 0^oC and above is insufficient to maintain a stable ice covering prior to the aircraft taxiing and requires to be thickened to achieve the desired rheological profile. The rheological effects of using ethylene glycol are not considered in this study.

Reviewer 2 Report

Dear Authors,

reviewed article is clearly written and presents new, original results. I have found this article well organized and properly prepared.

The introduction contains the appropriate background of the research and give adequate explanation of the aim of reported study. The Experimental section is detailed enough to provide reasonable insight in to conducted procedures.

The results of the manuscript are interesting and thus it can be considered for publication. I suggest only minor revision:

- page 3, line 107: change “poly (acrylic acid)” into “poly(acrylic acid)”

- page 4, line 144: correct “Table 1a fluids s with…” into “Table 1a fluids with…”

- page 4, line 152: add Mark-Houwink coefficients values

- correct in whole manuscript “Pa.s” into “Pa^.s”

- page 4, line 167: “…1 to 2 Pa. Where the concentration….” Change into “…1 to 2 Pa. When the concentration….”

- figure 2: add legend also in figure (not only in figure caption)

- page 7, lines 225-231: explain why [Ca²⁺] = 50 mg^.L^-1 was chosen for an effect of [Na⁺] testing

- page 7, line 232-233: please provide more info about these interactions

- page 8, line 239: correct “…variation with temperature with temperature for…”

- page 9, line 252: “The data presented in Figure 5…” – Figure 7?

Author Response

Dear Authors,

reviewed article is clearly written and presents new, original results. I have found this article well organized and properly prepared.

The introduction contains the appropriate background of the research and give adequate explanation of the aim of reported study. The Experimental section is detailed enough to provide reasonable insight in to conducted procedures.

The results of the manuscript are interesting and thus it can be considered for publication. I suggest only minor revision:

- page 3, line 107: change “poly (acrylic acid)” into “poly(acrylic acid)”

Done

- page 4, line 144: correct “Table 1a fluids s with…” into “Table 1a fluids with…”

Done

- page 4, line 152: add Mark-Houwink coefficients values

Done

- correct in whole manuscript “Pa.s” into “Pa^.s”

Done

- page 4, line 167: “…1 to 2 Pa. Where the concentration….” Change into “…1 to 2 Pa. When the concentration….”

Done

- figure 2: add legend also in figure (not only in figure caption)

Done

- page 7, lines 225-231: explain why [Ca²⁺] = 50 mg^.L^-1 was chosen for an effect of [Na⁺] testing

Added the words to line 223 “ which falls between typical high and low values found in water of 40-80 mgL^-1…

- page 7, line 232-233: please provide more info about these interactions

Added to Line 230: Potassium formate, used in the de-icing of runways, has a different ability to interact with water and with PAA by influencing the inter-polymer interaction of PAA in aqueous solutions. This influences its effective size and thus the rheological properties.

- page 8, line 239: correct “…variation with temperature with temperature for…”

Corrected

- page 9, line 252: “The data presented in Figure 5…” – Figure 7?

Corrected