Method and device for detecting the expansion of a char layer of a flame retardant

Abstract

The present application belongs to the field of flame-retardant performance testing, and particularly relates to a method for detecting the expanded carbon layer of a flame-retardant and a testing device. The method for detecting the expanded carbon layer of a flame-retardant comprises: detecting the resistance value of the surface of a sample of a flame-retardant in real time during the combustion of the sample; and determining the formation of the expanded carbon layer of the sample according to the change in the resistance value. Thus, the present application can detect the change in the resistance of the expanded carbon layer in real time during the combustion process, directly reflect the structural change of the carbon layer during the expansion process, and thus realize the real-time detection of the structure of the expanded carbon layer itself, thereby providing reliable data for evaluating the role of the flame retardant in the expanded carbon layer formation process in the synergistic flame-retardant polymer. Furthermore, the sample table for placing the sample of the flame-retardant is a multi-cavity structure, which can avoid the mutual influence between the samples of the flame-retardant when testing multiple samples simultaneously.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant performance testing, specifically relating to a method and testing device for detecting the expanded char layer of flame retardant materials. Background Technology

[0002] With increasing attention to green environmental protection and human health in modern society, halogenated flame retardants, which have high flame retardant efficiency but produce large amounts of smoke and toxic gases during combustion, will gradually be replaced by halogen-free flame retardants. Intumescent flame retardants (IFR), as a type of halogen-free flame retardant, possess environmentally friendly characteristics such as being halogen-free, low-smoke, and low-toxicity, making it one of the effective ways to achieve halogen-free flame retardants. IFR achieves flame retardancy by forming an expanded char layer, which acts as a heat insulation and oxygen barrier. Intumescent flame retardants consist of three parts: an acid source (dehydrating agent), a char source (charring agent), and a gas source (foaming agent). The inorganic acid produced by the thermal decomposition of the acid source catalyzes the dehydration and esterification of the char source, forming a molten layer. The gas source releases a large amount of non-toxic, flame-retardant gas, which dilutes the molten layer and causes it to expand and foam. Subsequently, further temperature increases lead to dehydration, cross-linking, coking, and solidification of the esterified material, ultimately forming the expanded char layer. The formation of a high-quality, closed-cell porous expanded char layer mainly depends on the control of each step in the char layer formation process. The matching between components in an IFR must follow certain rules: for example, the decomposition of the carbon source must be later than the dehydration of the acid source, the matching of melt crosslinking and carbonization rate with the gas source release rate, and the matching of gas source release rate with system viscosity, etc.

[0003] Currently available methods for real-time detection of flame-retardant polymers corresponding to flame retardants include: thermogravimetric-Fourier transform infrared spectroscopy (TG-FTIR) to track the composition of gaseous products during thermogravimetric analysis; pyrolysis-gas chromatography / mass spectrometry (Py-GC / MS) to accurately analyze the composition of gaseous products during the high-temperature pyrolysis of flame-retardant polymers; and cone calorimeters designed based on the oxygen consumption principle to detect information such as combustion intensity and smoke density during the material combustion process in real time.

[0004] The existing real-time testing methods mainly focus on indirect analysis of gas phase components and the surface morphology and composition of the expanded char layer during combustion, rather than direct detection of the structure of the expanded char layer itself. Due to the complexity of the formation process of the expanded char layer and the numerous factors affecting its structure, the accuracy and reliability of the analysis are relatively poor. However, the obscuring effect of the flame and dense smoke generated by the flame-retardant polymer during combustion makes it difficult to accurately detect the structure of the expanded char layer itself in real time using commonly used observation and image acquisition and analysis methods. This makes it difficult to accurately evaluate the role of the flame retardant in the synergistic flame-retardant polymer in the formation process of the expanded char layer. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for detecting the expanded char layer of flame retardants, which solves the problem that existing real-time testing methods for flame retardants are unable to accurately detect the structure of the expanded char layer itself.

[0006] To achieve the above objectives, the present invention provides a method for detecting the expanded char layer of a flame retardant, the method comprising: detecting the resistance value of the sample surface in real time during the combustion of a flame retardant sample; and determining the formation status of the expanded char layer of the sample based on the change in the resistance value.

[0007] Furthermore, the method for determining the formation of the expanded carbon layer in the sample based on the change in resistance value includes:

[0008] When the resistance value of the sample surface is detected to be less than the set resistance threshold, it is determined that a continuous expanded carbon layer has begun to form on the sample surface.

[0009] If the rate of decrease of the resistance value within the set time period is greater than the set rate threshold, it is determined that the density of the expanded carbon layer on the sample surface has increased.

[0010] If the resistance value of the sample surface is detected to be less than the set resistance threshold, and if the resistance value fluctuates repeatedly and the amplitude of the fluctuation is greater than the set change threshold, then it is determined that the expanded carbon layer on the sample surface has cracked.

[0011] Furthermore, the method also includes: simultaneously burning multiple flame retardant samples with the same composition, detecting the resistance values ​​of the surfaces of the multiple flame retardant samples with the same composition in real time, comparing the changes in the resistance values ​​of the surfaces of each flame retardant sample, and determining whether the changes in the detected resistance values ​​are reproducible.

[0012] Furthermore, the method also includes: simultaneously burning flame retardant samples with different compositions, and detecting the surface resistance of the flame retardant samples with different compositions in real time, so as to compare the changes in the surface resistance of the flame retardant samples with different compositions and determine the influence of the composition of the flame retardant on the formation of the expanded char layer.

[0013] This invention provides a novel technical solution for detecting the expanded char layer of flame retardants, as described above. Its beneficial effects include: it can directly reflect the structural changes of the char layer itself during the expansion process by detecting the change in resistance of the expanded char layer in real time during combustion, thereby achieving a relatively accurate real-time detection of the structure of the expanded char layer itself, and providing highly reliable data for evaluating the role of flame retardants in synergistic flame retardant polymers in the formation of the expanded char layer.

[0014] The present invention also provides a testing device for flame retardants, including a sample stage for placing a sample of the flame retardant to be burned; the testing device is used to implement an expanded char layer detection method to test the flame retardant, the method comprising:

[0015] During the combustion of the flame retardant sample, the resistance value of the sample surface is detected in real time; based on the change in the resistance value, the formation of the expanded char layer of the sample is determined.

[0016] Furthermore, the method for determining the formation of the expanded carbon layer in the sample based on the change in resistance value includes:

[0017] When the resistance value of the sample surface is detected to be less than the set resistance threshold, it is determined that a continuous expanded carbon layer has begun to form on the sample surface.

[0018] If the rate of decrease of the resistance value within the set time period is greater than the set rate threshold, it is determined that the density of the expanded carbon layer on the sample surface has increased.

[0019] If the resistance value of the sample surface is detected to be less than the set resistance threshold, and if the resistance value fluctuates repeatedly and the amplitude of the fluctuation is greater than the set change threshold, then it is determined that the expanded carbon layer on the sample surface has cracked.

[0020] Furthermore, the method also includes: simultaneously burning multiple flame retardant samples with the same composition, detecting the resistance values ​​of the surfaces of the multiple flame retardant samples with the same composition in real time, comparing the changes in the resistance values ​​of the surfaces of each flame retardant sample, and determining whether the changes in the detected resistance values ​​are reproducible.

[0021] Furthermore, the method also includes: simultaneously burning flame retardant samples with different compositions, and detecting the surface resistance of the flame retardant samples with different compositions in real time, so as to compare the changes in the surface resistance of the flame retardant samples with different compositions and determine the influence of the composition of the flame retardant on the formation of the expanded char layer.

[0022] Furthermore, the sample stage is a cavity structure, comprising multiple cavities, which are used to place flame-retardant samples; the cavities are provided with openings for the flame-retardant samples placed inside the cavities to come into contact with the flame.

[0023] Furthermore, it also includes a conductive electrode for attaching to the surface of the flame retardant sample to obtain an electrical signal corresponding to the resistance value of the sample surface.

[0024] Furthermore, the conductor electrode is attached to the surface of the flame retardant sample by being embedded in the surface of the flame retardant sample.

[0025] Furthermore, it also includes an adjustment structure for adjusting the distance between the sample stage and the combustion device.

[0026] Furthermore, the surface of the conductor electrode is covered with a substrate with a thickness less than a set thickness threshold, and the material of the substrate is consistent with that of the flame retardant sample.

[0027] This invention provides a novel testing device for flame retardants, as described above. Its advantages include: it can directly reflect the structural changes of the expanded char layer itself during the expansion process by detecting the change in resistance of the expanded char layer in real time during combustion, thereby achieving relatively accurate real-time detection of the structure of the expanded char layer itself, and providing highly reliable data for evaluating the role of flame retardants in synergistic flame retardant polymers in the formation of the expanded char layer. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the method for detecting the expanded char layer of a flame retardant implemented by the flame retardant testing device in an embodiment of the present invention.

[0029] Figure 2 This is a schematic diagram of the structure of the flame retardant testing device in an embodiment of the present invention; in the figure, A is the combustion device; B is the sample stage; C is the conductor electrode attached to the surface of the flame retardant sample; and R is a digital multimeter.

[0030] Figure 3a The resistance value change curve of a flame-retardant SBS sample with an IFR flame retardant content of 40% in the test device embodiment of the present invention during the combustion process.

[0031] Figure 3b The graph shows the change in resistance value of a flame-retardant SBS sample with an IFR flame retardant content of 45% during the combustion process in an embodiment of the flame retardant testing device of the present invention.

[0032] Figure 3c The graph shows the change in resistance value of a flame-retardant SBS sample with an IFR flame retardant content of 50% during the combustion process in an embodiment of the flame retardant testing device of the present invention.

[0033] Figure 3d The graph shows the change in resistance value of a flame-retardant SBS sample with an IFR flame retardant content of 55% during the combustion process in an embodiment of the flame retardant testing device of the present invention.

[0034] Figure 4a The graph shows the resistance change of flame-retardant SBS samples with different contents of APP+PER IFR flame retardant during the combustion process when the gas source consumption is 0 in the test device embodiment of the present invention.

[0035] Figure 4b The graph shows the resistance change of flame-retardant SBS samples with different contents of APP+PER IFR flame retardant during the combustion process when the gas source is MEL and the amount is 1 / 3.

[0036] Figure 4c The graph shows the change in resistance value of flame-retardant SBS samples with different contents of APP+PER IFR flame retardants during the combustion process when the gas source is MEL and the amount is 1 part, in the test device embodiment of the present invention.

[0037] Figure 4d The graph shows the change in resistance value of flame-retardant SBS samples with different contents of APP+PER IFR flame retardant during the combustion process when the gas source is MEL and the amount is 2 parts, in the test device embodiment of the present invention.

[0038] Figure 5a The graph shows the change in resistance value of a flame-retardant FIR / SBS sample with 40% IFR flame retardant content during combustion under different gas source dosages in the test device embodiment of the present invention.

[0039] Figure 5b The graph shows the change in resistance value of a flame-retardant FIR / SBS sample with 45% IFR flame retardant content during combustion under different gas source dosages in the test device embodiment of the present invention.

[0040] Figure 5c The resistance value change curve of a flame-retardant FIR / SBS sample with 50% IFR flame retardant content under different gas source dosages in the test device embodiment of the present invention.

[0041] Figure 5d This is a graph showing the change in resistance value of a flame-retardant FIR / SBS sample with 55% IFR flame retardant content during combustion under different gas source dosages in the test device embodiment of the present invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0043] Example of a testing device for flame retardants

[0044] This embodiment provides a technical solution for a testing device for flame retardants. The testing device includes a sample stage for placing a sample of the flame retardant to be burned. This testing device is used to implement a method for detecting the expanded char layer of flame retardants, thereby testing the flame retardant. (Refer to...) Figure 1The specific method for detecting the expanded carbon layer is as follows:

[0045] During the combustion of flame-retardant samples, the surface resistance of the samples was monitored in real time. The formation of the expanded char layer was determined based on the changes in this resistance. Analysis showed that the surface temperature of flame-retardants exposed to flames is generally 300–1500℃, while incompletely graphitized elemental carbon can form in the samples at 500–700℃. Although the formation of the expanded char layer involves a series of processes including esterification, expansion, cross-linking, and charring, the high flame temperature in a combustion scenario allows for rapid charring of the sample surface, generating elemental carbon. As combustion continues, the amount of elemental carbon in the molten char layer increases. Since elemental carbon is conductive, when the elemental carbon in the char layer forms a continuous conductive network, the originally insulating char layer becomes conductive. Furthermore, the expansion of the char layer during combustion alters the conductive network formed by the elemental carbon, leading to a change in the char layer's resistance. Therefore, this expanded char layer detection method can directly reflect the structural changes of the char layer itself during the expansion process by detecting the change in resistance of the expanded char layer in real time during combustion. This enables a relatively accurate real-time detection of the structure of the expanded char layer itself, providing highly reliable data for evaluating the role of flame retardants in synergistic flame retardant polymers in the formation of the expanded char layer.

[0046] In this embodiment, when the resistance value of the sample surface is detected to be less than a set resistance threshold, it is determined that a continuous expanded carbon layer has begun to form on the sample surface. If the rate of decrease in resistance value within a set time period is greater than a set rate threshold, it is determined that the density of the expanded carbon layer on the sample surface has increased. If, after the resistance value of the sample surface is detected to be less than the set resistance threshold, the resistance value fluctuates repeatedly and the amplitude of the fluctuation is greater than a set change threshold, it is determined that the expanded carbon layer on the sample surface has cracked. Therefore, the formation and development of the expanded carbon layer on the sample surface can be analyzed more accurately and efficiently based on the changes in the resistance value of the sample surface, and it is also convenient to quickly identify important structural changes such as the appearance, optimization (increased density), and failure (cracking) of the expanded carbon layer.

[0047] To avoid abnormalities in the resistance values ​​of flame retardant sample surfaces caused by various influencing factors, which could lead to inaccurate detection results of the expanded char layer, the expanded char layer detection method further includes: simultaneously burning multiple flame retardant samples with the same composition, and detecting the resistance values ​​of the surfaces of the multiple flame retardant samples in real time, so as to compare the changes in the resistance values ​​of each flame retardant sample surface and determine whether the changes in the detected resistance values ​​are reproducible. For example, assuming all flame retardant samples have the same composition, if the resistance values ​​of most flame retardant samples show a consistent trend and the numerical differences are less than the set error threshold, then the resistance value changes of these flame retardant samples are reproducible. This means the resistance value changes can be used as a parameter characterizing the expansion char layer changes of the flame retardant under normal conditions, and thus can be used to determine the formation of the expansion char layer. However, if the resistance value changes of a few individual flame retardant samples differ significantly from those of most samples, then these individual data points are problematic and should be discarded. Alternatively, if only a smaller number of flame retardant samples (less than the set normal value) show small differences in resistance value trends and values, while the resistance values ​​of most other flame retardant samples show large differences, then the resistance value changes of these samples are not reproducible. This may be due to problems with the sample itself, the testing process, the testing environment, or the sample being unsuitable for the testing method. In short, the resistance value changes cannot be used as a parameter characterizing the expansion char layer changes of the flame retardant under normal conditions, and appropriate checks or retesting are required.

[0048] In cases where it is necessary to burn flame retardant samples with different compositions to compare the effects of different compositions on the formation of the expanded char layer, this expanded char layer detection method further includes: simultaneously burning flame retardant samples with different compositions and detecting the surface resistance values ​​of the flame retardant samples with different compositions in real time, so as to compare the changes in the surface resistance values ​​of the flame retardant samples with different compositions and determine the influence of the flame retardant composition on the formation of the expanded char layer. Therefore, testing multiple samples simultaneously ensures that the testing conditions of the multiple samples are basically consistent, thereby eliminating the influence of changes in testing conditions on the electrical signal corresponding to the surface resistance values ​​of the samples and obtaining more reliable resistance value data.

[0049] The structure of the testing device itself is as follows: Figure 2As shown, A is a combustion device for burning flame-retardant samples, and B is a sample stage. This sample stage has a cavity structure, and the cavities are used to place the flame-retardant samples. Specifically, it can include multiple cavities. Each cavity has an opening for the flame-retardant sample placed inside to come into contact with the flame for flame retardant testing. In this embodiment, each cavity is used to place a single flame-retardant sample, thereby avoiding mutual interference between the samples when testing multiple flame-retardant samples simultaneously.

[0050] The testing device also includes conductive electrodes for attaching to the surface of the flame-retardant sample to obtain the electrical signal corresponding to the resistance value of the flame-retardant sample surface, such as... Figure 2 As shown by C in the dashed box (C specifically refers to the rectangular pieces at both ends of the upper surface of the cuboid flame retardant sample; in other embodiments, they can also be attached to other surfaces of the flame retardant sample). To avoid the electrode material affecting the expansion of the char layer and to prevent the electrode from detaching from the char layer during combustion, the conductive electrode is embedded in the surface of the flame retardant sample. In this embodiment, copper sheets or copper mesh are used as electrodes, embedded in the sample surface during the pressing process to ensure firm adhesion. Furthermore, since flames are conductive, to avoid interference with the signal, the electrode surface is covered with a substrate with a thickness less than a set thickness threshold, referred to as a thin substrate, to prevent the flame from conducting through the electrode. To avoid the thin substrate affecting the testing of the flame retardant sample, the material of the thin substrate is the same as that of the flame retardant sample. Specifically, the formulation and material of the thin substrate are exactly the same as those of the flame retardant sample to be tested; that is, the flame retardant sample is pressed into a thin sheet and attached to the electrode surface as a thin substrate. Figure 2 As shown, by connecting the digital multimeter R to the conductor electrode C using wires (i.e., the lines from the two ends of the digital multimeter R to the conductor electrode C), the resistance data corresponding to the sample combustion process can be collected in real time. The data acquisition speed is set to 1 to 50 data points per second.

[0051] Reference Figure 2 The testing device also includes an adjustment structure for adjusting the distance between the sample stage and the combustion device; in this embodiment, the adjustment structure is a height-adjustable fixed frame; when conducting the combustion test of the flame retardant, the sample is burned by a flame generated by the combustion of propane or butane gas. By adjusting the structure, the up and down movement of the sample stage B or the combustion device A can be controlled, changing the distance of the flame applied to the sample, thereby controlling the intensity of the flame applied to the sample.

[0052] Using the testing apparatus described in this embodiment, and following the above-mentioned method for detecting the expanded char layer of flame retardants, the reproducibility of real-time resistance of flame-retardant SBS samples (i.e., flame-retardant samples) with different contents of a certain IFR flame retardant was tested. SBS stands for styrene-based thermoplastic elastomer. The gas source for this IFR flame retardant is MEL, and the dosage is 1 part. This IFR flame retardant uses the composition APP+PER+MEL, where APP is ammonium polyphosphate, PER is pentaerythritol, and MEL is melamine. These three components serve as the acid source, carbon source, and gas source in the IFR composite flame retardant, respectively. Figures 3a-3d The real-time surface resistance changes of flame-retardant SBS samples with different contents of IFR flame retardant during flame application are shown in the figure. The vertical axis represents resistance in MΩ, and the horizontal axis represents combustion time in s. Specifically, under the same test conditions, flame-retardant samples of different contents of IFR flame retardant SBS were tested simultaneously in three chambers. Each group of flame-retardant samples included three flame-retardant SBS samples with the same contents of IFR flame retardant, and the test device tested one group of flame-retardant samples simultaneously. The real-time resistance curves showed consistent trends, indicating that the changes in resistance values ​​of these flame-retardant samples were reproducible. Therefore, the changes in resistance values ​​can be used as a parameter to characterize the changes in the expanded char layer of the flame-retardant under normal conditions, and thus can be used to determine the formation of the expanded char layer of the sample.

[0053] Reference Figures 3a-3d Resistance can only be detected after applying a flame to different samples for a period of time. High flame temperatures rapidly carbonize the flame retardant on the sample surface, generating elemental carbon. Elemental carbon is conductive; when it forms a continuous conductive network in the carbon layer, and the sample surface is covered by a continuous carbon layer, the surface resistance can be detected. Therefore, the phenomenon of detecting surface resistance after applying a flame for a period of time indicates the formation of a continuous, expanded carbon layer on the sample surface. Continued flame application causes the carbon layer resistance to decrease rapidly, and then the rate of decrease slows down. The magnitude of the carbon layer resistance mainly depends on the density of the conductive network formed by the elemental carbon in the carbon layer. An increase in elemental carbon content is beneficial to increasing the carbon layer resistance; however, the expansion of the molten carbon layer caused by gaseous decomposition products leads to tensile deformation of the conductive network, resulting in the destruction of the conductive network and an increase in resistance. Therefore, the rapid decrease in carbon layer resistance indicates the formation of a large amount of elemental carbon in the sample surface carbon layer, increasing the density of the conductive network in the carbon layer. Thus, the rapid decrease in carbon layer resistance indicates an increase in the density of the carbon layer on the sample surface. Since the self-built device tests the surface resistance of the char layer, when the flame retardant on the sample surface is completely decomposed, continuing to extend the flame application time will not cause a large amount of elemental carbon to be generated in the surface char layer, but will only increase the thickness of the char layer, which also causes the rate of decrease in char layer resistance to slow down.

[0054] Figures 4a-4dThe graph shows the real-time surface resistivity changes of flame-retardant FIR / SBS samples with different contents of IFR flame retardant during flame application, with MEL as the gas source. The vertical axis represents resistivity in MΩ, and the horizontal axis represents combustion time in seconds. FIR stands for polyisocyanurate. Figures 4a-4d As shown, when the flame retardant content is low (35%), although the char layer resistance can be detected, it fluctuates drastically with the extension of the applied flame time, even exceeding the instrument's detection range. The detection of char layer resistance indicates the formation of a continuous char layer. The subsequent drastic change in char layer resistance indicates severe deformation of the continuous char layer on the sample surface, or even disruption of its continuity. SEM observation of a porous sample surface further indicates that the char layer ruptured during combustion. This is because low IFR content results in a small char layer with poor insulation, and the large amount of volatiles produced by the decomposition of polymers and flame retardants causes the char layer to expand violently, even rupture, leading to drastic changes in char layer resistance. An unstable, expanded char layer cannot provide effective protection to the matrix, inevitably resulting in poor flame retardant performance. For systems with a higher IFR flame retardant content (≥40%), comparing the char layer resistance curves under different IFR contents reveals that the higher the flame retardant content, the faster the resistance decreases, and the lower the char layer resistance. This indicates that increasing the flame retardant content can improve the density of the char layer. A dense char layer is beneficial to improving the flame retardant properties of the sample, and the fact that the flame retardant properties of the sample increase with the increase of IFR dosage confirms this. Therefore, the faster the surface resistance of the char layer decreases and the lower the resistance is in the initial stage of flame application, the better the density of the formed char layer.

[0055] Figures 5a-5d This chart shows the variation curves of charcoal bed resistance under different MEL (Metal Elastic Gas Source) usage scenarios, where the vertical axis represents resistance in MΩ and the horizontal axis represents combustion time in seconds, assuming the same IFR (Infrared Fume) usage. Figures 5a-5dAs shown, the addition of a gas source increases the surface carbon layer resistance, and the resistance increases with the amount of MEL. Compared with the IFR system without a gas source, the gas source leads to a greater degree of carbon layer expansion, resulting in greater tensile strain in the molten carbon layer, and consequently, an increase in carbon layer resistance. Therefore, the increase in carbon layer resistance caused by MEL is due to the increased degree of carbon layer expansion. Furthermore, the higher the MEL content, the greater the degree of carbon layer expansion, and consequently, the greater the carbon layer resistance. Besides increasing carbon layer resistance, MEL also exacerbates the fluctuations in surface carbon layer resistance, especially at high MEL contents (APP:PER:MEL = 3:1:2), where the fluctuations are more pronounced. These fluctuations in carbon layer resistance indicate changes in the conductive network within the carbon layer, thus suggesting a change in the morphology of the surface carbon layer. Both expansion and rupture of the surface carbon layer cause changes in the conductive network. Therefore, it can be inferred that the fluctuations in surface carbon layer resistance may be caused by the rupture of the expanded carbon layer. Thus, by testing the real-time resistance changes of the expanded carbon layer surface during combustion, the continuity, density, degree of expansion, and even rupture of the carbon layer can be observed.

[0056] Example of a method for detecting the expanded char layer of flame retardants

[0057] This embodiment provides a technical solution for detecting the expanded char layer of flame retardants, as detailed below:

[0058] During the combustion of flame-retardant samples, the surface resistance of the samples was monitored in real time. The formation of the expanded char layer was determined based on the changes in this resistance. Analysis showed that the surface temperature of flame-retardants exposed to flames is generally 300–1500℃, while incompletely graphitized elemental carbon can form in the samples at 500–700℃. Although the formation of the expanded char layer involves a series of processes including esterification, expansion, cross-linking, and charring, the high flame temperature in a combustion scenario allows for rapid charring of the sample surface, generating elemental carbon. As combustion continues, the amount of elemental carbon in the molten char layer increases. Since elemental carbon is conductive, when the elemental carbon in the char layer forms a continuous conductive network, the originally insulating char layer becomes conductive. Furthermore, the expansion of the char layer during combustion alters the conductive network formed by the elemental carbon, leading to a change in the char layer's resistance. Therefore, this expanded char layer detection method can directly reflect the structural changes of the char layer itself during the expansion process by detecting the change in resistance of the expanded char layer in real time during combustion. This enables a relatively accurate real-time detection of the structure of the expanded char layer itself, providing highly reliable data for evaluating the role of flame retardants in synergistic flame retardant polymers in the formation of the expanded char layer.

[0059] In this embodiment, when the resistance value of the sample surface is detected to be less than a set resistance threshold, it is determined that a continuous expanded carbon layer has begun to form on the sample surface. If the rate of decrease in resistance value within a set time period is greater than a set rate threshold, it is determined that the density of the expanded carbon layer on the sample surface has increased. If, after the resistance value of the sample surface is detected to be less than the set resistance threshold, the resistance value fluctuates repeatedly and the amplitude of the fluctuation is greater than a set change threshold, it is determined that the expanded carbon layer on the sample surface has cracked. Therefore, the formation and development of the expanded carbon layer on the sample surface can be analyzed more accurately and efficiently based on the changes in the resistance value of the sample surface, and it is also convenient to quickly identify important structural changes such as the appearance, optimization (increased density), and failure (cracking) of the expanded carbon layer.

[0060] To avoid abnormalities in the resistivity data of the flame retardant sample surface caused by various influencing factors, which would lead to inaccurate detection results of the expanded char layer, the expanded char layer detection method further includes: simultaneously burning multiple flame retardant samples with the same composition, and detecting the resistivity values ​​of the surfaces of the multiple flame retardant samples with the same composition in real time; comparing the changes in the resistivity values ​​of the surfaces of each flame retardant sample, and determining whether the changes in the detected resistivity values ​​are reproducible. For example, assuming all flame retardant samples have the same composition, if the resistance values ​​of most flame retardant samples show a consistent trend and the numerical differences are less than the set error threshold, then the resistance value changes of these flame retardant samples are reproducible. This means the resistance value changes can be used as a parameter characterizing the expansion char layer changes of the flame retardant under normal conditions, and thus can be used to determine the formation of the expansion char layer. However, if the resistance value changes of a few individual flame retardant samples differ significantly from those of most samples, then these individual data points are problematic and should be discarded. Alternatively, if only a smaller number of flame retardant samples (less than the set normal value) show small differences in resistance value trends and values, while the resistance values ​​of most other flame retardant samples show large differences, then the resistance value changes of these samples are not reproducible. This may be due to problems with the sample itself, the testing process, the testing environment, or the sample being unsuitable for the testing method. In short, the resistance value changes cannot be used as a parameter characterizing the expansion char layer changes of the flame retardant under normal conditions, and appropriate checks or retesting are required.

[0061] In cases where it is necessary to burn flame retardant samples with different compositions to compare the effects of different compositions on the formation of the expanded char layer, this expanded char layer detection method further includes: simultaneously burning flame retardant samples with different compositions and detecting the surface resistance values ​​of the flame retardant samples with different compositions in real time; comparing the changes in the surface resistance values ​​of the flame retardant samples with different compositions to determine the influence of the flame retardant composition on the formation of the expanded char layer. Therefore, testing multiple samples simultaneously ensures that the testing conditions of the tested samples are basically consistent, thereby eliminating the influence of changes in testing conditions on the electrical signal corresponding to the surface resistance values ​​of the samples and obtaining more reliable resistance value data.

[0062] This invention has the following characteristics:

[0063] 1) It can directly reflect the structural changes of the expanded char layer itself during the expansion process by detecting the change in resistance of the expanded char layer in real time. This enables more accurate real-time detection of the structure of the expanded char layer itself, providing highly reliable data for evaluating the role of flame retardants in the formation of the expanded char layer in synergistic flame retardant polymers.

[0064] 2) It provides a method for judging the expanded char layer condition corresponding to the change of surface resistance value of flame retardant sample. It can accurately and efficiently analyze the formation and development of the expanded char layer on the sample surface based on the change of surface resistance value. It can also quickly identify important structural changes such as the appearance, optimization (increased density) and failure (cracks) of the expanded char layer.

[0065] 3) Simultaneously burn multiple flame retardant samples with the same composition, compare the changes in the resistance values ​​on the surface of each flame retardant sample, and determine whether the changes in the detected resistance values ​​are reproducible. This can avoid abnormal data on the resistance values ​​of the flame retardant sample surface caused by various influencing factors, which would lead to inaccurate detection results of the expanded carbon layer.

[0066] 4) Simultaneous testing of multiple flame retardant samples for comparison ensures that the testing conditions of the multiple samples are basically consistent, thereby eliminating the influence of changes in testing conditions on the electrical signal corresponding to the surface resistance value of the sample and obtaining more reliable resistance value data.

[0067] 5) The sample stage used to place flame retardant samples has a multi-cavity structure, which can avoid mutual interference between flame retardant samples when testing multiple flame retardant samples at the same time, thus facilitating the simultaneous combustion test of multiple flame retardant samples.

[0068] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or explanatory of the principles of the present invention, and do not constitute a limitation thereof.

Claims

1. A method for detecting the expanded char layer of a flame retardant, characterized in that, The method includes: during the combustion of a flame-retardant sample, real-time detection of the surface resistance of the sample; determining the formation of an expanded char layer on the sample based on changes in the surface resistance; the specific method for determining the formation of the expanded char layer includes: When the resistance value of the sample surface is detected to be less than the set resistance threshold, it is determined that a continuous expanded carbon layer has begun to form on the sample surface. If the rate of decrease of the resistance value within the set time period is greater than the set rate threshold, it is determined that the density of the expanded carbon layer on the sample surface has increased. If the resistance value of the sample surface is detected to be less than the set resistance threshold, and if the resistance value fluctuates repeatedly and the amplitude of the fluctuation is greater than the set change threshold, then it is determined that the expanded carbon layer on the sample surface has cracked.

2. The method for detecting the expanded char layer of flame retardants according to claim 1, characterized in that, The method further includes: simultaneously burning multiple flame retardant samples with the same composition, detecting the resistance values ​​of the surfaces of the multiple flame retardant samples with the same composition in real time, and judging whether the changes in the detected resistance values ​​are reproducible based on the changes in the resistance values ​​of the surfaces of each flame retardant sample.

3. The method for detecting the expanded char layer of flame retardants according to claim 1 or 2, characterized in that, The method further includes: simultaneously burning flame retardant samples with different compositions, and detecting the surface resistance of the flame retardant samples with different compositions in real time, so as to compare the changes in the surface resistance of the flame retardant samples with different compositions and determine the influence of the composition of the flame retardant on the formation of the expanded char layer.

4. A testing device for flame retardants, characterized in that, The testing device is used to implement the method for detecting the expanded char layer of flame retardants as described in any one of claims 1-3, in order to test the flame retardants; it includes a sample stage for placing a flame retardant sample to be burned; and it also includes a conductor electrode for attaching to the surface of the flame retardant sample to obtain an electrical signal corresponding to the resistance value of the sample surface.

5. The testing apparatus for flame retardants according to claim 4, characterized in that, The sample stage is a cavity structure, comprising multiple cavities, which are used to place flame-retardant samples; the cavities are provided with openings for the flame-retardant samples placed inside to come into contact with the flame.

6. The testing apparatus for flame retardants according to claim 4 or 5, characterized in that, The conductor electrode is attached to the surface of the flame retardant sample by being embedded in the surface of the flame retardant sample.

7. The testing apparatus for flame retardants according to claim 4 or 5, characterized in that, It also includes an adjustment structure for adjusting the distance between the sample stage and the combustion device.

8. The testing apparatus for flame retardants according to claim 4 or 5, characterized in that, The surface of the conductor electrode is covered with a substrate with a thickness less than a set thickness threshold, serving as a thin substrate to prevent the flame from conducting through the electrode; the material of this thin substrate is consistent with that of the flame retardant sample.

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