Role of Air Bubble Inclusion on Polyurethane Reaction Kinetics

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

Reviewer 1

In this paper, the authors reported the Role of air bubbles inclusion on polyurethane reaction kinetics. In general, the results of optical observation, and the reaction kinetics by in-situ FTIR spectroscopy in the manuscript are well-organized and acceptable for publishing. For improving a manuscript, it is recommended to address the following comments:

1-The abstract should be clearer to include the most important results obtained from this work.

AU: We are grateful to the reviewer for this comment that can help to improve the manuscript. We amended the abstract on lines 14-33 in the new version of the manuscript as follows:

"In the case of slow mixing speeds (50 rpm), no air bubbles were included and the early foaming process was characterized by formation of new bubbles (CO₂ nucleation), provided by the blowing reaction. Later on, it was observed that the coalescence affected the overall foaming process, caused by the gelling reaction that, inhibited by the indigent mixing conditions, could not withstand the bubbles expansion. As a result, a PU foam with a coarse cellular structure and an average bubble size of 173 µm was obtained. In this case, the bubbles degeneration rate, dN/dt, was -3095 bubble.cm^-3.s^-1. On the contrary, at 500 rpm, air bubbles were included into the PU reacting system (aeration) and it was observed no formation of new bubbles during the foaming process. After that, the air bubbles underwent growth caused by diffusion of the CO₂ provided by the blowing reaction. As the gelling reaction was not strongly depleted like in case of 50 rpm, the coalescence less affected the bubble growth (dN/dt=-2654 bubble.cm^-3.s^-1), leading to a PU foam with an average bubble size of 94 µm. In the case of foams obtained at 1000 and 2000 rpm, bubbles degeneration was first affected by coalescence and then by Ostwald Ripening and a finer cellular structure was observed (being the average bubble size 62 µm and 63 µm for 1000 rpm and 2000 rpm respectively). During the first foaming stage, the coalescence was less predominant over the bubbles growth (being dN/dt respectively -1838 bubble.cm^-3.s^-1  and -1601 bubble.cm^-3.s^-1) compared to 50 rpm and 500 rpm. This occurrence was ascribed to a more balanced process between the bubbles expansion and the PU polymerization, caused by the more suitable mixing conditions. During the late foaming stage, the Ostwald Ripening only was responsible for the further bubbles degeneration (being dN/dt respectively -89 bubble.cm^-3.s^-1 and -69 bubble.cm^-3.s^-1).”

2-In the introduction, the authors should focus on the benefits and the reason for choosing organofluorine additives (OFAs) than other additives.

AU: We thank the reviewer for this observation and in order to address his comment we amended the new version of the manuscript respectively on lines 106-113 and 118-120:

"In the recent literature, several additives have been extensively studied and used to reduce the bubbles size distribution in PU foams. For instance, solid-type nucleating agents such as talc or organoclay particles induced a significant reduction of the cell size, but they induced several complications due to precipitation of solid particles during the foaming process and the difficulty in reaching a good dispersion [16]. Liquid-type additives such as tetramethylsilane compounds (TEMs) were also used and PU foams with a more uniform and finer cellular structure were obtained. However, these additives presented several limitations such as the high flammability, the high vapor pressure and a high cost [17]. "

“These results, combined with the relative low cost of OFAs and their non-flammability [17], persuaded us to investigate on the possible mechanisms induced by these compounds during the PU foaming process”

3-In the experimental, the authors developed a method that allowed to evaluate the formation of the urethane and the urea groups by deconvolution of the amide I region in the infrared, this method should be verified by one or more of verification tools.

AU: We are grateful to the reviewer for pointing out this aspect and, as we would prefer to not significantly divert from the scope of the paper, we amended the manuscript on lines 207-209 in order to meet his comment as follows:

“The overlapped absorptions in the amide I region (carbonyl region) were deconvoluted by using Gaussian bands and their relative curve fittings were adjusted using verified methods previously reported in [27,28].”

4-It is better to suggest a proposed mechanism for bubbles nucleation associated with the reaction conditions.

AU: In this work, we hinted at the relationship between the products formation and the bubbles degeneration mechanisms. We already proposed a mechanism suitable to describe the bubbles nucleation that would not occur in case of including air bubbles as the blowing agent molecules would prefer to diffuse towards these pre-existing bubbles due to no energy barrier to overcome, as  already reported on lines 275-288:

“The absence of new bubbles (possible BA nucleation) can be explained if one considers how the different nucleation mechanisms may be affected in presence of pre-existing bubbles [25]. Bubble formation mechanisms such as homogeneous (i) and heterogeneous (ii) nucleation are processes that involve the formation of a new gas phase after overcoming a very high energy barrier (radius of the nuclei greater than the critical radius in order to let the bubble spontaneously grow up) [9]. Conversely, when pre-existing bubbles are contained into the system two additional cases may occur: when the radius of curvature of these pre-existing bubbles is smaller than the critical radius (iii) less nucleation energy barrier is required, while, in case of the radius of curvature greater than the critical radius (iv), the required energy lowers to zero [25]. At mixing speeds of 500, 1000 and 2000 rpm, due to the presence of air bubbles within the reacting mixture since the very beginning (included by mixing stage), we may infer that the BA molecules would rather diffuse toward these pre-existing air bubbles (no energy barrier to overcome, iv) than nucleate in gas bubbles into the PU matrix (i).”

AU: Moreover, we have extended the part related to the correlation between the bubbles degeneration mechanisms and data obtained from the evaluation of the kinetics reaction, on lines 518-527 and 540-573 of the new version of the manuscript. Please see also the answer to reviewer 4.

Reviewer 2 Report

Lines 361-373, it seems that in the first part of the paragraph you're sayint he at there is no dependence on any of the variables and XfNCO on mixing speed then at the end of the paragraph you say that there is a slight dependence on mixing speed, slightly confusing. 

Lines 416-417 and the paragraph are confusing.  You say there is no clear  trend.... but I don't understand to what you are referring to.  Please expand.  

Author Response

Reviewer 2

Lines 361-373, it seems that in the first part of the paragraph you're sayint he at there is no dependence on any of the variables and XfNCO on mixing speed then at the end of the paragraph you say that there is a slight dependence on mixing speed, slightly confusing.

AU: We thank the reviewer for this comment that can help to improve the comprehensibility of this paragraph. In practice, k₁ and k₂ have several values that seem to not exhibit an evincible trend with the mixing conditions. As a result, the ratio k₁/k₂ keeps maintaining the same order of magnitude and is characterized by the same trend. On the other hand, the final conversion  slightly increases with the mixing speed, as more isocyanate is consumed. From the analysis of these values, we concluded that the mixing conditions improved the isocyanate consumption, while the behavior of the ongoing gel point of the PU reacting mixture could not be extrapolated. To address the comment of the reviewer, we amended the paragraph on lines 427-433 in the new version of the manuscript as follows:

“The kinetic constants, k₁ and k₂, have several values that seem to not exhibit an evincible trend with the mixing conditions and, moreover, their ratio k₁/k₂ keeps maintaining the same order of magnitude. On the other hand, the final conversion  slightly increases with the mixing speed, as more isocyanate is consumed. From the analysis of these values, we can conclude that the mixing conditions improved the isocyanate consumption, while they seem to indicate no effects on the ongoing gel point of the PU reacting mixture.”

Lines 416-417 and the paragraph are confusing.  You say there is no clear  trend.... but I don't understand to what you are referring to.  Please expand.

AU: In case of products formation (urethane and urea), it can be observed that the diffusion controlled kinetics seems to exert a major influence on the ongoing gel point with the increasing mixing speed. This behavior can be evinced by the ratio k₁/k₂ that, in case of urea formation, keeps decreasing, while this occurrence may not be extrapolated from data in case of urethane formation. We thank the reviewer for clarifying this point and we amended the manuscript on lines 502-506 as follows:

“In case of products formation, it can be observed that the diffusion controlled kinetics seems to exert a major influence on the ongoing gel point with the increasing mixing speed. This behavior can be evinced by k₁/k₂ values that, in case of urea formation, keep decreasing, while this occurrence may not be extrapolated from data in case of urethane formation.”

Reviewer 3 Report

In this manuscript, the authors explored the influence of stir speed on the formation of PU foams. The experiments are well-designed, and their conclusions can be supported by their results. The authors’ findings are also helpful for both academic and industrial studies. Therefore, I recommend its publication after addressing my following comments:

1. the information of reactants is not clear, the authors need to list reactants’ information as clear as possible for other researchers, when mentioning polyols and isocyanates from Dow, which specifically resin did the authors used? The same issue for Amine ad Silicone.
2. In figure 9, the isocyanate conversion slowly increases after 60 min, while the urethane and urea conversion in Figure 11 show almost no changes. Once the isocyanates are consumed, there will be either urethane or urea bonds form, therefore, the authors are expected to observe slowly increased conversion in Figure 11 as well. Can the authors provide some explanation?

3 While it is nice to observe the correlation between stir speed and bubble size. It might be more important to study the effects of viscosity and shear force in the future study, by trying different resins.

Author Response

Reviewer 3

In this manuscript, the authors explored the influence of stir speed on the formation of PU foams. The experiments are well-designed, and their conclusions can be supported by their results. The authors’ findings are also helpful for both academic and industrial studies. Therefore, I recommend its publication after addressing my following comments:

1-the information of reactants is not clear, the authors need to list reactants’ information as clear as possible for other researchers, when mentioning polyols and isocyanates from Dow, which specifically resin did the authors used? The same issue for Amine ad Silicone.

AU: We added trade names for the polyol (including the OH number as additional data) and isocyanate components. Catalysts and surfactants are already contained in these commercial blends that can be purchased from DOW Europe gmbh (CH), we also amended the manuscript on lines 143-151 as follows:

“A formulated mixture of polyether polyols (VORACOR^TM CW 7028, OH number = 370, mixture density = 1.08 g/cm³, viscosity = 6700 mPa.s), with silicone surfactant and catalysts was utilized with polymeric methylene diphenyl diisocyanate (PMDI) (VO-RACOR^TM CE 142, 31.1% NCO, 1.20 g/cm³, 190 mPaּּ.s) to obtain PU foams. Both reactants were formulated and supplied by Dow Italia s.r.l. (Correggio, RE, Italy) and used “as received”. The formulated polyol and isocyanate components implemented in this study can be purchased from DOW Europe GmbH (CH). Water was always utilized as the CBA and was already contained into the polyol formulation. Composition of the formulated polyol is detailed in Table 1.”

2-In figure 9, the isocyanate conversion slowly increases after 60 min, while the urethane and urea conversion in Figure 11 show almost no changes. Once the isocyanates are consumed, there will be either urethane or urea bonds form, therefore, the authors are expected to observe slowly increased conversion in Figure 11 as well. Can the authors provide some explanation?

AU: The chemistry related to PU synthesis may be complex as the isocyanate group is involved in several simultaneous reactions due to its extreme reactivity, as also reported in the introduction on lines 45-48:

“The chemistry involved in the synthesis of rigid PUs is complex because several simultaneous reactions are involved. The simultaneity of several reactions is due to the extreme reactivity of the isocyanate group (-N=C=O) towards the hydrogen active compounds, such as those containing the -OH and -NH functional groups [6].”

AU: Besides the polymerization and the blowing reactions that are the main processes characterizing the PU foaming process, the isocyanate can react with other compounds to give formation to other products as well. For instance, the isocyanate may react with additional -NH functional groups to give allophanate as well as biuret compounds, and may be further involved in dimerization and cyclotrimerization reactions to give dimers and trimers respectively. For this reason, while urethane and urea formation show almost no changes, isocyanate conversion can exhibit a slow increase due to the aforementioned chemical reactions.

We are grateful to the reviewer for this comment that can help to better elucidate the complex picture of the PU chemistry. To this aim, we amended the manuscript on lines 492-500 as follows:

“It is noteworthy that, besides the polymerization and the blowing reactions (the main processes that characterize the PU foaming process), the isocyanate can react with other compounds to give formation to other products as well. Among these, the isocyanate may react with additional -NH functional groups to give allophanate as well as biuret compounds, and may be further involved in dimerization and cyclotrimerization reactions to give dimers and trimers [6]. As a consequence, while urethane and urea formation show almost no changes during the foaming process (Figure 11a and b), isocyanate conversion exhibit a slow increase due to the aforementioned chemical reactions (Figure 9).”

3-While it is nice to observe the correlation between stir speed and bubble size. It might be more important to study the effects of viscosity and shear force in the future study, by trying different resins.

AU: The reviewer is right, viscosity and shear force of the PU reacting mixture are important elements that enrich the comprehension of the fundamental mechanisms that take place during the PU foaming process. In principle, we designed this study in order to give a first hint to the correlation between the bubble formation/degeneration mechanisms that occur during the foaming process and the related chemistry, at different mixing speeds. The study of the rheological properties during the PU foaming process will represent a further step and, moreover, different thermosetting materials can be adequately studied due to the systematicity of this approach.

Reviewer 4 Report

The authors presented an interesting topic on the effect of mixing speed on the foam morphology of PU. The foam process was captured by a high-speed camera while the chemical reaction was characterized by in-situ FTIR spectroscopy. The authors tried to correlate the observed phenomena to the behind-the-scene mechanism, that is, use the mechanism to explain observation. However, I did not see much correlation between these results, except the theoretical explanation. For instance, in the early foaming stage, bubble density increase or decrease rate (Fig. 5 and Fig. 6) may be governed dominantly by one mechanism. If the correlation exists, there should be transitions which reflect the dominance of the gelling and blowing reactions (maybe Fig. 9 and Fig. 11). And these transitions may well be aligned with the bubble density rate change, e.g., on time scale. Could the authors elaborate on this? From my understanding, this might be the key point to prove in this manuscript.               

Other than that, water, itself, is normally treated as PBA instead of CBA since water does not decompose and generate any CO₂ although it served as one reactant in the reaction.

For the bubble density shown in Fig. 5, we know bubbles expands in 3-D during foaming, it may bring difficulty in bubble counting in the image (2-D) since many layers of bubbles might exist. To have a fair comparison, could the authors elaborate on how to overcome the errors? If handled improperly, the error could be huge to cause the data miscalculated and false explanation.    

For the bubble size evolution in Fig. 6, the bubble selection is critical. For fair comparison, the bubble size should be calculated only when the bubble do not interfere with each other. The bubble size rate change could be significantly affected, especially when comparing the cases at low mixing speed and high mixing speeds. It could lead to wrong explanation. Please comment on this.

Author Response

Reviewer 4

The authors presented an interesting topic on the effect of mixing speed on the foam morphology of PU. The foam process was captured by a high-speed camera while the chemical reaction was characterized by in-situ FTIR spectroscopy. The authors tried to correlate the observed phenomena to the behind-the-scene mechanism, that is, use the mechanism to explain observation. However, I did not see much correlation between these results, except the theoretical explanation. For instance, in the early foaming stage, bubble density increase or decrease rate (Fig. 5 and Fig. 6) may be governed dominantly by one mechanism. If the correlation exists, there should be transitions which reflect the dominance of the gelling and blowing reactions (maybe Fig. 9 and Fig. 11). And these transitions may well be aligned with the bubble density rate change, e.g., on time scale. Could the authors elaborate on this? From my understanding, this might be the key point to prove in this manuscript.

AU: We agree with the reviewer, the correlation between the observed bubble features and the quantified products formation rate is a key point in this work. While we proposed a mechanism that suitable describes the observed CO₂ bubbles nucleation in absence of included air bubbles, the reaction kinetics data can be further explored and correlated to the bubbles degeneration mechanism. We are grateful to the reviewer for pointing out this fundamental aspect of the manuscript and we amended the new version on lines 518-527 and 540-573 as follows:

“Data retrieved from the analysis conducted on the reaction kinetics of PU foaming, at the different mixing speeds, can be used to correlate the observed bubbles degeneration with the urethane and the urea formation which, in turn, provide an indirect measure of respectively the polymerization and the blowing reaction rates [6]. During the early foaming stage (stage I), it was observed that coalescence less affected the bubble collapse with the increasing mixing speed. In fact, dN/dt decreased from -3095 to -1601 bubble.cm^-3.s^-1 when evaluating the bubbles degeneration rate from 50 to 2000 rpm respectively. Here, we can infer that, at low mixing speed, the gelling reaction is inhibited by the indigent mixing conditions and, thus, the polymer cannot withstand the bubbles expansion and their relative coarsening.”

“The opposite trends between the two products formation rates strengthen the observation that the higher the mixing speed, the faster the polymerization rate so that the coalescence is progressively depleted [13-15]. Moreover, the trade-off between the polymerization and the blowing reactions, in favor of the former, influences the attainment of the gel point as well. From results obtained by fitting procedure implemented with Equation 7, we can observe that k₁/k₂ goes from 0.0567 to 0.0326 (from 50 to 2000 rpm respectively) in case of urea formation, indicating a shorter time elapsed for the PU reaction to attain the gel point as it indicates the transition from a chemical controlled reaction to a diffusion controlled one [40]. In addition, the observed trend is also in good agreement with the identified time intervals in which dN/dt and Ɗa have been evaluated. Accordingly, the transition between the stages I and II can be associated to the attainment of the gel point [40]. The increasing viscosity during the PU thermosetting reaction leads to a reduced molecular diffusion of the species contained into the reacting system that terminate after the gel point [42]. The molecular diffusion, associated to inter-molecular movements, can be adequately described by data related to the apparent diffusivity, Ɗa [33,34,40,42]. In fact, for stage I, results obtained from Equation 3 show that Ɗa goes from 2.95·10^-5 to 2.22·10^-6 cm²/s when going from 50 to 2000 rpm respectively. However, the reaction continues even after the gel point through intra-molecular interactions that are associated to local movements of the polymeric branches [42]. In case of stage II, Ɗa goes from 9.92·10^-7 to 1.09·10^-7 cm²/s when going from 50 to 2000 rpm respectively. Indeed, the combined results of the apparent diffusivity as well as the gel point effectively confirm that the molecular diffusion is reduced when higher mixing speeds are adopted, leading to a more contained bubbles growth during the foaming process [33,34]. Moreover, it is also highlighted that the coalescence is reduced when the polymer matrix is characterized by a higher polymerization rate in contrast to a lower blowing reaction rate [13-15], confirmed also by FTIR results. We observe that in case of foams obtained at 1000 and 2000 rpm, the Ostwald Ripening appeared to be the responsible for the further cell coarsening in the late foaming process. The design of experiments devoted to elucidate which are the mechanisms that regulate the occurrence of the Ostwald ripening in the PU foaming will be object of a forthcoming paper.”

Other than that, water, itself, is normally treated as PBA instead of CBA since water does not decompose and generate any CO₂ although it served as one reactant in the reaction.

AU: we thank the reviewer for pointing out this incongruence. Accordingly, we amended the text in the new version of the manuscript on line 54.

For the bubble density shown in Fig. 5, we know bubbles expands in 3-D during foaming, it may bring difficulty in bubble counting in the image (2-D) since many layers of bubbles might exist. To have a fair comparison, could the authors elaborate on how to overcome the errors? If handled improperly, the error could be huge to cause the data miscalculated and false explanation.

AU: To overcome the error related in the calculation of the effective number of bubbles when dealing with the 3-D foaming, we adopted a widely used algorithm in the literature proposed by Canny* and then modified by Deriche**. In detail, we predefined a range of threshold values that allowed to evaluate the suitable bubble candidate profiles. Based on this approach, bubbles with a well-defined outline can be considered inside the field of view of the optical camera and can be accounted for the bubbles density evaluation, while bubbles that did not pass the algorithm were not taken into account in the procedure. Accordingly, we amended the manuscript on lines 173-183 as follows:

“For each foaming experiment, the lens of the optical camera was kept on a fixed field of view. Bubbles with a defined edge that is comprised in a predefined range of low and high threshold values can be taken into account for the bubble density evaluation. Bubbles with edges that were not comprised in the aforementioned range were considered out-of-field and then not included in the parameter n. The edge detection is a widely employed built-in function in many commercial IA softwares, the software utilized for the purpose of this work was ImageJ [22]. In the literature, one of the most applied computational approaches for the edge detection is the algorithm proposed by Canny [23] and then modified by Deriche [24]. Here, the Canny edge detector has been applied in order to detect the bubble candidate profiles.”

*J. F. Canny, A Computational Approach to Edge Detection, IEEE Trans. Pattern Anal. Mach. Intell. 8 (1986) 679-698. https://doi.org/10.1109/TPAMI.1986.4767851.

**R. Deriche, Using Canny's Criteria to Derive a Recursively Implemented Optimal Edge, Int. J. Comput. Vision 1 (1987) 167-187. https://doi.org/10.1007/BF00123164.

For the bubble size evolution in Fig. 6, the bubble selection is critical. For fair comparison, the bubble size should be calculated only when the bubble do not interfere with each other. The bubble size rate change could be significantly affected, especially when comparing the cases at low mixing speed and high mixing speeds. It could lead to wrong explanation. Please comment on this.

AU: In the case of bubble size evolution, we wanted to measure the degree of coalescence that affected the bubbles collapse. To this aim, also bubbles that underwent degeneration were taken into account in the evaluation of the average bubble diameter. Moreover, the other objective was to evaluate the apparent diffusivity of the components contained into the dispersed and the continuous phases. In this context, also bubbles that experienced the dry regime were considered in the evaluation of the apparent diffusivity.