Buckwheat Hulls/Perlite as an Environmentally Friendly Flame-Retardant System for Rigid Polyurethane Foams

Round 1

Reviewer 1 Report

The manuscript by A. Strakowska et al. describes on "Buckwheat Hulls/Perlite as an environmentally friendly flame retardant system for Rigid Polyurethane Foams". Overall, it is well-written manuscript. In short, rigid polyurethane (PU) foams were modified with a flame retardant system consisting of a component derived from waste biomass - buckwheat hulls and a volcanic mineral - perlite powder in a total amount of 15 parts by weight per hundred parts of polyol. The impact of flame retardants on selected properties of PU composites including structure, physical and mechanical properties (density, compressive strength, flexural strength, hardness), insulating properties (thermal conductivity), thermal properties (temperature of thermal decomposition stages) and flame reducing properties flammability (ignition time, total heat release, total smoke release, average CO and CO₂ yield, and maximum average heat release rate). These results suggested that the introduction of selected fillers into the rigid PU foam significantly affected the properties of the composites prepared. These additives caused a change in the morphology of the foams - the structure became more heterogeneous with a greater number of open pores, which resulted in a change in density and hardness, and a slight decrease in the compression or flexural strength. The resulting foams with the use of flame retardants were also characterized by reduced hydrophobicity, in particular with a higher mass of buckwheat hulls. These studies also demonstrated that the change in structure caused by the addition of an additional component increased thermal conductivity. On the other hand, buckwheat hulls and perlite worked very well as flame retardants, because the addition of buckwheat/perlite modifiers improved both the thermal stability and the burning behavior of composite foams. Therefore, I find the contents of this manuscript suitable for publication in Polymers as its present form.

 

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,
thank you very much for such a favorable and informative review.

Reviewer 2 Report

1 Please give detail stress-strain curves and explain how to obtain the toughness of those foams.

2 Toughness is highly depedent on the temperature. Please give more information on toughness variations at high temperature.

3 Please provide more information on all the tests given in the manuscript so that the readers are convinent to understand your tests and make your solutions repeatability.

 

Author Response

Dear Reviewer,

Thank you for your valuable comments in the review. I hope our answers will be sufficient.

 

1 Please give detail stress-strain curves and explain how to obtain the toughness of those foams.

Unfortunately, currently in the testing machine, we do not have the ability to export data to Excel and we cannot prepare stress - strain diagrams. The generated report is in the form of a pdf file with a table containing the necessary data. The course of the stress-strain relationship that we observe during the measurement is typical for foams and individual samples are characterized by similar values, which we average to the value presented in the manuscript. As for the hardness of the obtained foams, it is typical for this type of system. Standard foam without additives, made of the components described in the materials section, according to the manufacturer's recommendations, usually reaches a hardness of about 70 Shore degrees. In turn, each additive interferes with the resulting structure, causing greater heterogeneity, greater or lesser porosity, different pore sizes or the thickness of the ribs between the pores, so that changes in foam hardness after modification are always measurable and visible.

 

2 Toughness is highly depedent on the temperature. Please give more information on toughness variations at high temperature.

Unfortunately, we always measure at room temperature and we have no information on how the hardness of the material will change with increasing temperature. The devices for measuring the hardness of polymeric materials of different hardness, which we have at our disposal, are adapted to measure hardness at ambient temperature.

 

3 Please provide more information on all the tests given in the manuscript so that the readers are convinent to understand your tests and make your solutions repeatability.

We have analyzed the entire part describing the research methodology, we have made corrections and we hope that in this form all information will be legible and available to the recipient.

Author Response File: Author Response.docx

Reviewer 3 Report

In this work, developing flame retardant polyurethane foams using natural waste in the form of buckwheat hulls in combination with perlite. The current manuscript needs minor revision before acceptance and some recommendations are listed.

1. Could you provide a scheme for the preparation of PU foam?

2. From Table 5, the PU-12.5B-2.5P shows the lowest THR, TSR, COY, and CO2Y values. But from the TGA data, this sample does have the highest char residue values. Why it showed good flame-retardant performance?

Author Response

Dear Reviewer,

Thank you for your valuable comments in the review. I hope our answers will be sufficient.

 

In this work, developing flame retardant polyurethane foams using natural waste in the form of buckwheat hulls in combination with perlite. The current manuscript needs minor revision before acceptance and some recommendations are listed.

 

1. Could you provide a scheme for the preparation of PU foam?

We have prepared a diagram of the preparation of foams and included it in the manuscript. We hope it is readable enough. Thank you for this suggestion, in fact such a scheme always better illustrates the technology of preparing foams.

 

2. From Table 5, the PU-12.5B-2.5P shows the lowest THR, TSR, COY, and CO2Y values. But from the TGA data, this sample does have the highest char residue values. Why it showed good flame-retardant performance?

It seems to us that this sample, due to the high content of buckwheat hulls rich in phosphorus compounds, passed the flammability tests well. An additional small amount of mineral additive in the form of pearlite enhanced this effect giving a positive feedback effect. This is confirmed by the TGA results, in which the sample was characterized by higher temperatures at specific mass losses. On the other hand, in the char residue content, a certain tendency to increase the content as a function of the mineral filler content can be noticed, which is obviously caused by the very nature of the additives.

Reviewer 4 Report

   The article presents an innovative approach to the flame retardancy of rigid polyurethane foams using natural waste in the form of buckwheat hulls in combination with a inorganic additive - perlite.

1. In the introduction, the author should analyze the reasons for the flammability of polyurethane and explain the importance of flame retardant modification.

2. Synthesis of phosphorus containing polyols for intrinsic flame retardancy of polyurethane is an important flame retardant modification method, and its advantages and disadvantages should be introduced. At the same time, the author should compare the modification method of this article with the existing research to reflect the progressiveness and innovative nature of this research.

3. The author should specifically analyze the content of different elements such as phosphorus, nitrogen, and magnesium in buckwheat husks.

4. The author should first use test methods such as vertical combustion and oxygen index to more intuitively characterize the flame retardancy of polyurethane.

5. From the heat release and smoke release data, it can be seen that PU-12.5B-2.5P is the best flame retardant sample. Have the authors attempted to further increase the content of flame retardants in combination with the oxygen index value of this sample to further enhance its flame retardancy.

6. The author has not conducted a detailed study of the flame retardant mechanism, but should further analyze the composition and structure of the gaseous phase products of the material, as well as the condensed phase products, to analyze the flame retardant mechanism of buckwheat hulls and perlite.

Author Response

Dear Reviewer,

Thank you for your valuable comments in the review. I hope our answers will be sufficient.

The article presents an innovative approach to the flame retardancy of rigid polyurethane foams using natural waste in the form of buckwheat hulls in combination with a inorganic additive - perlite.

1. In the introduction, the author should analyze the reasons for the flammability of polyurethane and explain the importance of flame retardant modification.

The sentence in the introduction has been added

The weak point of polyurethane foams from the usability point of view is their flammability. Not only do they ignite from a small fire source, they burn at a high speed. During their combustion, a lot of heat is released, smoke and toxic gases are produced. Thermal decomposition of the polyurethane composite occurs at temperatures above 200°C. The decomposition products are primarily hydrogen cyanide and carbon monoxide, but also nitrogen oxides, nitriles, hydrogen chloride and carbon dioxide are present. The simplest compounds are formed at temperatures exceeding 800°C as a result of defragmentation of previous decomposition products [5-8]. For this reason, much attention has been paid to reducing the flammability of PU foams. Currently, flame retardant properties can be improved by adding flame retardants. The selection of an effective flame retardant substance for polymer composites requires the fulfillment of appropriate requirements. The desired properties effectively prevent fires, and thus also do not cause the emission of toxic substances during combustion and smoke [5]. Until recently, the use of halogen flame retardants was popular, but due to the release of large amounts of smoke and toxic gases into the atmosphere during their combustion, these additives were abandoned…..

2. Synthesis of phosphorus containing polyols for intrinsic flame retardancy of polyurethane is an important flame retardant modification method, and its advantages and disadvantages should be introduced. At the same time, the author should compare the modification method of this article with the existing research to reflect the progressiveness and innovative nature of this research.

To our knowledge, there have been no previous studies on the flame retardant properties of such a synergistic system. There is very little information on the flame retardant properties of perlite, while the literature data on buckwheat hulls mainly concerns their anti-aging properties. In addition, you can find many articles on attempts to reduce the flammability of polyurethane foams with various additives, but the majority of these are reports on synthetic or mineral additives, and additives of natural origin / waste biomass are in much smaller amounts. Therefore, it seems to us that our approach is quite effective and innovative.

3. The author should specifically analyze the content of different elements such as phosphorus, nitrogen, and magnesium in buckwheat husks.

Thank you for your valuable attention. In fact, such an analysis would enable a more thorough determination of the flame-retardant effect of the foams by the addition of buckwheat hulls. Unfortunately, at present we do not have the possibility to directly perform elemental analysis of elements, EDS or ICP in buckwheat hulls, but in the future we will certainly try to obtain the results of such tests

4. The author should first use test methods such as vertical combustion and oxygen index to more intuitively characterize the flame retardancy of polyurethane.

We tried to determine the OIT using DSC, but unfortunately polyurethane foams are quite fickle material for testing, they expand in the crucible, making it difficult to determine the oxygen index using this method. On the other hand, during the vertical combustion of the beam, we obtained a very large dispersion of results. Therefore, the most reliable result for us is to carry out combustion tests in a cone calorimeter, the more so that we already have a lot of knowledge in this type of materials and we can more or less determine how the combustion process will be for a reference sample.

5. From the heat release and smoke release data, it can be seen that PU-12.5B-2.5P is the best flame retardant sample. Have the authors attempted to further increase the content of flame retardants in combination with the oxygen index value of this sample to further enhance its flame retardancy.

So far, we have not attempted to increase the amount of flame retardants due to concerns about the structure of the rigid foam. From our experience, we know that too much additives significantly affect the structure, causing greater heterogeneity and the formation of more open cells, which translates into later properties, including deterioration of insulation. But of course, this is a valuable remark and nothing stands in the way of experimentally checking the effect of a higher content of flame retardant additives.

6. The author has not conducted a detailed study of the flame retardant mechanism, but should further analyze the composition and structure of the gaseous phase products of the material, as well as the condensed phase products, to analyze the flame retardant mechanism of buckwheat hulls and perlite.

Unfortunately, as in the case of the elemental analysis of buckwheat hulls, and in the case of the analysis of the structure and composition of the gas phase products of the material and condensed phase products, we do not have access to appropriate instrumental methods to conduct a detailed analysis. Of course, this would be valuable information that would enable us to study the mechanism of combustion in depth. For now, we rely on literature data containing information on the ingredients contained in the additives that can effectively reduce flammability in the tested composites.

Round 2

Reviewer 4 Report

Accept in present form