Melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane and its preparation method

Abstract

The present invention belongs to the field of polymer materials, and relates to a melamine cyanurate (MCA) coated aluminum diethylphosphinate (ADP) intumescent flame retardant thermoplastic polyurethane and a preparation method thereof. The present invention uses a molecular self-assembly technique to prepare the MCA@ADP flame retardant, and uses MCA@ADP as both the acid source and the gas source, and dipentaerythritol as the carbon source to form an intumescent flame retardant system; the melamine cyanurate coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane composite material is prepared by melt blending. Adding MCA@ADP and dipentaerythritol is beneficial to the formation of a dense carbon layer after the combustion of the thermoplastic polyurethane composite material, improves the compatibility between the material and the flame retardant, improves the flame retardant performance of the material, and enhances the fire safety performance of the material.

Description

Technical Field

[0001] The present invention belongs to the field of polymer materials, and particularly relates to melamine cyanurate-coated aluminum diethylphosphinate expandable flame-retardant thermoplastic polyurethane and a preparation method thereof. Background Art

[0002] Thermoplastic polyurethane (TPU) is a multi-functional engineering plastic. As an elastic block copolymer, it is synthesized from a soft segment polyol, a hard segment diisocyanate, and a chain extender. There are many types of polyols in the TPU soft segment, which mainly provide elasticity and toughness for TPU. The TPU hard segment is in a glassy state and mainly provides hardness and modulus for the TPU material. TPU can have different properties by adjusting the ratio between the soft segment and the hard segment, such as wear resistance, tensile properties, tear properties, flexure properties, and low-temperature properties. TPU is being gradually widely used by humans, and it exists in rubber, cable sheaths, tires, and the aviation field. However, the thermal performance of TPU is very poor, and its melting point is very low. The oxygen index (LOI) of unflame-retarded TPU is generally between 16% and 22%. When burning occurs in the presence of an open flame, the internal cross-linked network structure of TPU changes to a linear structure, and the melt viscosity drops to a level that cannot bear the weight of the molten droplets themselves. Therefore, the phenomenon of molten droplet dripping will occur, which is extremely likely to cause secondary fires. Moreover, polyols will decompose into a large amount of toxic smoke at high temperatures, such as carbon monoxide, carbon dioxide, nitrogen oxides, and hydrogen cyanide gas. In summary, it is very important to explore and study the flame-retardant work of thermoplastic polyurethane in the industrial and daily life fields. According to literature reports, the flame retardants studied for TPU flame retardancy mainly include phosphorus-based, intumescent, nanomaterials, nitrogen-based, and silicon-based. Among them, phosphorus-based flame retardants are widely used and have good flame retardant effects. However, when used alone at high temperatures, they will decompose into gases that are extremely harmful to the human body (such as phosphine), which increases the danger of fire escape and rescue work.

[0003] Patent CN113881216A discloses the use of aluminum diethylphosphinate and nano-magnesium hydroxide flame retardants, and patent CN106967230A discloses the use of a melamine cyanurate, aluminum diethylphosphinate, and aluminum hypophosphite expandable flame-retardant system to improve the flame retardant properties of TPU and ethylene-propylene-diene rubber, respectively. However, unmodified aluminum diethylphosphinate is prone to agglomeration, and the composite material containing aluminum diethylphosphinate will release phosphine gas that is toxic to the human body during high-temperature combustion, which will pose a threat to the life safety of escapees. Therefore, improving the compatibility between the flame retardant and the TPU matrix and delaying the release of phosphine gas generated by aluminum diethylphosphinate at high temperatures while improving the flame retardant properties of the TPU composite material in a halogen-free and environmentally friendly expandable flame-retardant system is the technical problem to be solved by the present invention. Summary of the Invention

[0004] Aiming at the problems existing in the background art, one of the purposes of the present invention is to provide a melamine cyanurate-coated aluminum diethylphosphinate intumescent flame-retardant thermoplastic polyurethane, which can not only improve the flame retardancy of thermoplastic polyurethane but also reduce the phosphine gas generated by aluminum diethylphosphinate at high temperatures. At the same time, it can also improve the compatibility between the flame retardant and thermoplastic polyurethane, thereby reducing the deterioration of the mechanical properties of the composite material.

[0005] The melamine cyanurate-coated aluminum diethylphosphinate intumescent flame-retardant thermoplastic polyurethane of the present invention is composed of the following components by mass percentage: 75-80% of thermoplastic polyurethane, 20%-25% of flame retardant, wherein the flame retardant includes 12.5-25% of melamine cyanurate-coated aluminum diethylphosphinate (MCA@ADP), and 0-12.5% of dipentaerythritol (DPER); the coating amount of melamine cyanurate is 10%-20% of the mass of aluminum diethylphosphinate.

[0006] Preferably, the melamine cyanurate-coated aluminum diethylphosphinate intumescent flame-retardant thermoplastic polyurethane is composed of the following components by mass percentage: 75% of thermoplastic polyurethane, 25% of flame retardant, wherein the flame retardant is composed of 12.5-25% of melamine cyanurate-coated aluminum diethylphosphinate (MCA@ADP) and 0-12.5% of dipentaerythritol (DPER); the coating amount of melamine cyanurate is 15% of the mass of aluminum diethylphosphinate.

[0007] More preferably, the melamine cyanurate-coated aluminum diethylphosphinate intumescent flame-retardant thermoplastic polyurethane is composed of the following components by mass percentage: 75% of thermoplastic polyurethane, 12.5% of melamine cyanurate-coated aluminum diethylphosphinate (MCA@ADP), and 12.5% of dipentaerythritol (DPER); the coating amount of melamine cyanurate is 15% of the mass of aluminum diethylphosphinate.

[0008] Another purpose of the present invention is to provide a method for preparing melamine cyanurate-coated aluminum diethylphosphinate (MCA@ADP) by using molecular assembly technology. In a reaction system with ethanol as the solvent, the MCA@ADP flame retardant is synthesized. Cyanuric acid and melamine will form hydrogen bonds in the whole reaction system. Although the two cannot chemically react with ADP, due to intermolecular forces and electrostatic interactions, cyanuric acid will adsorb on the surface of ADP and then react with the melamine added later, thus forming an MCA shell on the surface of ADP. The specific preparation steps are as follows:

[0009] (1) Add anhydrous ethanol to a reactor equipped with a condensing reflux device, heat it in a water bath to 85-90°C, add cyanuric acid (CA), and stir mechanically at 250-300 rpm until CA is completely dissolved;

[0010] (2) Add aluminum diethylphosphinate (ADP) to the solution in step (1), and stir and react at 85 - 90 °C for 10 - 15 min to obtain a suspension;

[0011] (3) Disperse melamine (MEL) in absolute ethanol and ultrasonically disperse it evenly; then slowly drop the melamine - ethanol solution into the suspension in step (2) within half an hour, stir and react to enable the self - assembly of CA and MEL on the surface of ADP into melamine cyanurate (MCA); naturally cool to room temperature, filter, wash, dry, and grind to obtain white powdered melamine cyanurate - coated aluminum diethylphosphinate (MCA@ADP).

[0012] Furthermore, in step (3), the stirring speed of the stirring reaction is 250 - 300 rpm, the reaction temperature is 85 - 90 °C, and the reaction time is 2.5 - 4 h.

[0013] Furthermore, the mass ratio of cyanuric acid to melamine is 1:1; the total mass ratio of cyanuric acid and melamine to the mass of aluminum diethylphosphinate is 2 - 4:20, preferably 3:20.

[0014] The third object of the present invention is to provide a preparation method of melamine cyanurate - coated aluminum diethylphosphinate intumescent flame - retardant thermoplastic polyurethane, and the specific steps are as follows:

[0015] (a) After the mixing temperature of the internal mixer is stable, start feeding materials;

[0016] (b) Pour the dried TPU into the internal mixer from the feeding port, and then add the dried MCA@ADP and DPER. The above - mentioned mixture is fully extruded and sheared under the action of a double - rotor to enable the full mixing of TPU and the MCA@ADP / DPER flame retardant;

[0017] (c) Run for 10 - 15 min, take a sample to obtain a melamine cyanurate - coated aluminum diethylphosphinate intumescent flame - retardant thermoplastic polyurethane composite material.

[0018] Among them, the mixing temperature of the internal mixer is set to 160 - 170 °C.

[0019] The beneficial effects of the present invention are as follows:

[0020] (1) First, through the molecular self - assembly technology, cyanuric acid and melamine synthesize MCA on the surface of ADP through intermolecular forces to obtain MCA@ADP, which is not easy to agglomerate and improves the compatibility with TPU. On the basis of improving its flame - retardant effect, further study the thermal decomposition performance and structural changes of aluminum diethylphosphinate after coating treatment. This preparation method can obtain the required target product through simple chemical reactions, and complex reaction conditions and reaction steps are not required during the experimental process.

[0021] (2) Before the aluminum diethylphosphinate is coated, although it has good flame retardant properties, after combustion tests, it is found that its contribution to the formation of the condensed phase and char residue is very low, and a large amount of phosphine gas will be released under high temperature conditions; after being coated with melamine cyanurate, it not only facilitates the formation of char residue after the composite material burns, but also MCA will inhibit the decomposition of ADP under high temperature conditions, delaying the release of phosphine gas. At the same time, the inert gases (such as nitrogen, ammonia, etc.) generated by MCA as a protective layer at high temperature will dilute the concentration of phosphine and separate the phosphine gas from the air, delaying its reaction with oxygen, thereby reducing the heat release rate and improving the flame retardant properties of the composite material.

[0022] (3) The prepared melamine cyanurate-coated aluminum diethylphosphinate is used as the acid source and gas source of the intumescent flame retardant, and dipentaerythritol (DPER) is used as the carbon source to form a new type of intumescent flame retardant. Through the method of melt blending, it is mixed with TPU through processes such as internal mixing and tablet pressing to prepare the flame retardant TPU composite material, and the entire preparation process is highly efficient. Description of the Drawings

[0023] Figure 1 XPS spectra of ADP and MCA@ADP.

[0024] Figure 2 FT-IR spectra of (a) ADP and (b) MCA@ADP.

[0025] Figure 3 SEM images of (a) ADP and (b) MCA@ADP.

[0026] Figure 4 TG and DTG curves of ADP and MCA@ADP (a) TG and (b) DTG.

[0027] Figure 5 MS spectra of ADP and MCA@ADP.

[0028] Figure 6 TG (a) and DTG (b) curves of pure TPU (TPU-1), 75wt% TPU / 25wt% MCA@ADP / 0wt% DPER mixture (TPU-2) (Example 1), 75wt% TPU / 12.5wt% MCA@ADP / 12.5wt% DPER mixture (TPU-3) (Example 2), 75wt% TPU / 16.67wt% MCA@ADP / 8.33wt% DPER mixture (TPU-4) (Example 3) and 75wt% TPU / 18.75wt% MCA@ADP / 6.25wt% DPER mixture (TPU-5) (Example 4).

[0029] Figure 7 Graphs of heat release rate (a) and total heat release (b) for pure TPU (TPU-1), 75 wt% TPU / 25 wt% MCA@ADP / 0 wt% DPER blend (TPU-2) (Example 1), 75 wt% TPU / 12.5 wt% MCA@ADP / 12.5 wt% DPER blend (TPU-3) (Example 2), 75 wt% TPU / 16.67 wt% MCA@ADP / 8.33 wt% DPER blend (TPU-4) (Example 3), and 75 wt% TPU / 18.75 wt% MCA@ADP / 6.25 wt% DPER blend (TPU-5) (Example 4). Detailed implementation mode

[0030] The present invention will be further described below in conjunction with specific embodiments.

[0031] Example 1

[0032] The melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 75%, MCA@ADP: 25%, DPER: 0%. The preparation process is as follows:

[0033] (1) Add 150 mL of anhydrous ethanol to a 500 mL flask, place it in a water bath at 85 °C, and at the same time provide condensation reflux.

[0034] (2) After the temperature rises to 85 °C, add 1.5 g of cyanuric acid (CA) to the flask in step (1), and then completely dissolve CA under mechanical stirring at 250 rpm; then add 20 g of aluminum diethylphosphinate (ADP) to the mixture and react at 85 °C for 10 min.

[0035] (3) Add 1.5 g of melamine (MEL) and 100 mL of anhydrous ethanol to a beaker, ultrasonicate for 10 min to completely dissolve it, and then slowly drop the MEL-ethanol solution into the flask in step (2) within half an hour, maintaining the original rotation speed and temperature, and react for 3 h. After the entire experimental system cools to room temperature, then carry out filtration, washing, drying, grinding and other operations to obtain white powdery MCA@ADP.

[0036] (4) Place the weighed TPU and MCA@ADP in an oven at 60 °C for 12 h to dry and remove water.

[0037] (5) Set the mixing temperature of the internal mixer to 170 °C, and start feeding after the temperature stabilizes.

[0038] (6) First, slowly pour the dried TPU into the internal mixer from the feeding port, and then slowly add the dried MCA@ADP in sequence. Under the action of the twin rotors, fully extrude and shear it to make the TPU and the flame retardant fully mixed.

[0039] (7) After running for 15 minutes, open the two moving plates of the internal mixer in sequence, take samples, and obtain the TPU / MCA@ADP / DPER (TPU-2) composite material.

[0040] In this embodiment, the TPU matrix material is used as the carbon source, and melamine cyanurate-coated aluminum diethyl phosphinate is used as the acid source and gas source. The properties of the prepared composite material are shown in Table 1.

[0041] Example 2

[0042] The melamine cyanurate-coated aluminum diethyl phosphinate intumescent flame-retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 75%, MCA@ADP: 12.5%, DPER: 12.5%. The preparation process is the same as that of Example 1 to obtain the TPU / MCA@ADP / DPER (TPU-3) material.

[0043] In this embodiment, DPER is used as the carbon source, and melamine cyanurate-coated aluminum diethyl phosphinate is used as the acid source and gas source. The properties of the prepared composite material are shown in Table 1.

[0044] Example 3

[0045] The melamine cyanurate-coated aluminum diethyl phosphinate intumescent flame-retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 75%, MCA@ADP: 16.67%, DPER: 8.33%. The preparation process is the same as that of Example 1 to obtain the TPU / MCA@ADP / DPER (TPU-4) material.

[0046] The properties of the materials in this embodiment are shown in Table 1.

[0047] Example 4:

[0048] The melamine cyanurate-coated aluminum diethyl phosphinate intumescent flame-retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 75%, MCA@ADP: 18.75%, DPER: 6.25%. The preparation process is the same as that of Example 1 to obtain the TPU / MCA@ADP / DPER (TPU-4) material.

[0049] The properties of the materials in this embodiment are shown in Table 1.

[0050] Table 1 Summary of performance characterizations of pure TPU and Examples 1-4

[0051]

[0052] As can be seen from Table 1: When the addition amount of the flame retardant is 25% (see Examples 1-4), the LOI of the novel IFR intumescent flame retardant TPU material composed of the MCA@ADP and DPER compound is higher than that of the TPU without flame retardant modification (22.1%). Among them, the LOI of Example 2 reached 34.5%. After adding the flame retardant, the tensile strength and elongation at break of the composite material both decreased, indicating that adding 25 wt% of the flame retardant will affect the mechanical properties of TPU, but it is still within the acceptable range of the application field.

[0053] Figure 1 are the XPS spectrograms of ADP and MCA@ADP. It can be clearly seen that after coating modification with MCA, the characteristic peak of N element appears at 400 for ADP that originally did not contain N element, and the content increased from 0% to 3.69%, indicating that MCA@ADP contains nitrogen element.

[0054] Figure 2 are the FT-IR spectrograms of ADP and MCA@ADP. According to the spectral line information, the typical absorption peaks of ADP include 2881 and 2998 cm -1 which are the absorption peaks of —CH3, 1153 cm -1 (P=O) and 1078 cm -1 (P-O) stretching vibration absorption peaks. In contrast, the spectrum of MCA@ADP shows some new characteristic absorption peaks of MCA. A strong peak at 3392 cm -1 is due to the —NH2 symmetric stretching vibration of the triazine group. The strong peak near 3234 cm -1 and the medium peak at 2958 cm -1 can be attributed to the formation of hydrogen bonds between —NH2 / NH groups. The stretching vibration of —CO at 1778 cm -1 is due to the cyanate anion. The strong peak at 1740 cm -1 and the weak peak at 1665 cm -1 correspond to the —NH2 shear vibration and —NH2 bending vibration respectively. In addition, the frequencies of the C-N and C=N symmetric stretching bands are 1449 cm -1 and 1538 cm -1 . The above results prove that MCA@ADP has been successfully synthesized in the system.

[0055] Figure 3SEM images of ADP and MCA@ADP. (a) Untreated ADP shows smooth flakes. After coating treatment, multiple ADP flakes are coated by short rod-shaped MCA, as shown in (b). After coating, MCA@ADP has an irregular shape, a rough surface, and is uneven, further indicating the successful preparation of MCA@ADP.

[0056] Figure 4 TG and DTG curves of ADP and MCA@ADP. It can be seen from the TGA and DTG curves that there are obvious differences in the TG curve and DTG curve between MCA@ADP and ADP. From the TG curve, it can be seen that the decomposition temperature of modified ADP is advanced because the outer-coated MCA starts to decompose at about 335 °C. The overlapping of the middle section of the curve is due to the decomposition of ADP. Near 450 °C, ADP gradually stabilizes, while MCA@ADP continues to decompose because in the first stage, MCA decomposes into cyanuric acid (CA), and the triazine compound in it decomposes into N2 and the decomposition of melamine (MEL). The residual mass fractions of ADP before and after modification at 800 °C are 39.25% and 24.71% respectively, which can illustrate that MCA has been successfully coated on the surface of ADP. It can be seen from the DTG curve that MCA@ADP has three obvious thermal weight loss stages: (1) At 356 °C, the melamine cyanurate (MCA) of the outer shell material decomposes into cyanuric acid (CA) and melamine (MEL); (2) At 427 °C, it has the same maximum loss temperature as ADP, indicating that the coated ADP undergoes thermal decomposition (gas phase); (3) At 498 °C, it is because the triazine compound in CA decomposes into N2 and the decomposition of MEL. This process reflects the combined action of the condensed phase and the gas phase.

[0057] Figure 5 Variation curve of the mass spectrometry signal of phosphine (PH3) released by ADP and MCA@ADP with increasing temperature. It can be clearly seen that before 383 °C, the mass spectrometry signal of PH3 released by MCA@ADP is always lower than that of ADP, and the curve has no obvious slope, indicating that the content of PH3 produced by the thermal decomposition of MCA@ADP is significantly less than that of ADP before 383 °C. This is because as the temperature increases, MCA decomposes first, which well protects the internal ADP; then obvious peak fluctuations appear near 400 - 450 °C. Compared with ADP, the signal is delayed, which is because ADP decomposes after MCA decomposes. It can also prove that after coating ADP with MCA, MCA can act as a protective layer to inhibit the decomposition of ADP and delay the generation of PH3.

[0058] Figure 6TG and DTG curves of pure TPU (TPU-1) and MCA@ADP / DPER / TPU composites (Examples 1-4). It can be seen from the TG and DTG curves that there are two thermal decomposition stages of the pure TPU sample: (1) the main chain of TPU breaks; (2) the C-C / C-O bonds on the main chain break. The maximum mass loss temperature of the pure TPU sample is 417.24 °C, and only 3.36% of the mass remains (at 800 °C). For the flame-retardant TPU composites, the maximum mass loss temperature is lower than that of the pure TPU sample; and the residue remaining also increases from 3.36% of the original sample to 6.71%-8.14%. From the TG curve, the composites decompose earlier than TPU, the decomposition rate slows down, and the mass residue increases. It can also be seen from the DTG curve that the composites have an additional decomposition stage around 500 °C compared to the original TPU sample. This is because the triazine compounds in CA formed by the thermal decomposition of MCA in the first decomposition stage and the further decomposition of MEL promote the third decomposition of the composites; the peak value of the pure TPU sample is higher than that of the corresponding flame-retardant sample, indicating that the pure TPU sample has a faster mass loss during thermal decomposition.

[0059] Figure 7 are the heat release rate (HRR) and total heat release (THR) curves of TPU (TPU-1) and MCA@ADP / DPER / TPU composites (Examples 1-4). The thermal combustion performance of polymers is mainly measured by two parameters: HRR and its peak value pHRR. From Figure 7 (a), the pHRR of pure TPU is 1102.46 kW / m 2 , and the pHRR values of composites TPU-3 and TPU-4 are 254.58 kW / m 2 and 241.01 kW / m 2 , respectively, which are decreased by 76.89% and 78.14% compared to pure TPU. Figure 7 (b) shows that the THR of pure TPU is 114.83 MJ / m 2 , and the THR value of composite TPU-3 is 48.01 MJ / m 2 , which is decreased by 58.19% compared to pure TPU. All of the above can prove that after adding the flame retardant, the pHRR and THR of the composites decrease significantly, improving the flame retardant performance of the composites. And when the mass ratio of MCA@ADP to DPER is 1:1, the flame retardant performance is maximally improved compared to the original sample. This is due to the combined action of the inert gas generated by the decomposition of triazine compounds in MCA as the shell and the phosphoric acid free radicals generated by the thermal decomposition of ADP, and the formation of N, P synergistic flame retardancy; after introducing DPER, an intumescent flame retardant system is formed, making the entire flame retardant system play a combined role in the condensed phase and gas phase.

Claims

1. A melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane, characterized in that, The composition by mass percentage is as follows: 75% of thermoplastic polyurethane and 25% of flame retardant; the flame retardant includes 12.5% of melamine cyanurate-coated aluminum diethylphosphinate and 12.5% of dipentaerythritol; the melamine cyanurate-coated aluminum diethylphosphinate is prepared by molecular self-assembly, wherein the coating amount of melamine cyanurate is 15% of the mass of aluminum diethylphosphinate; the preparation method of the melamine cyanurate-coated aluminum diethylphosphinate specifically includes the following steps: (1) Add absolute ethanol to a reactor equipped with a condenser reflux device, heat it in a water bath to 85 - 90 °C, add cyanuric acid, and stir mechanically at 250 - 300 rpm until cyanuric acid is completely dissolved; (2) Add aluminum diethylphosphinate to the solution in step (1), stir and react at 85 - 90 °C for 10 - 15 min to form a suspension; (3) Disperse melamine in absolute ethanol and ultrasonically disperse it evenly; then slowly drop the melamine-ethanol solution into the suspension in step (2) within half an hour, stir and react to enable cyanuric acid and melamine to self-assemble into melamine cyanurate on the surface of aluminum diethylphosphinate; naturally cool to room temperature, filter, wash, dry, and grind to obtain white powdery melamine cyanurate-coated aluminum diethylphosphinate The mass ratio of cyanuric acid to melamine is 1:1; the total mass ratio of cyanuric acid and melamine to the mass of aluminum diethylphosphinate is 3:

20.

2. The melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane according to claim 1, wherein In step (3), the stirring speed of the stirring reaction is 250 - 300 rpm, the reaction temperature is 85 - 90 °C, and the reaction time is 2.5 - 4 h.

3. A preparation method of melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane as described in claim 1, characterized in that, It includes the following steps: (a) Start feeding after the mixing temperature of the internal mixer is stable; (b) Pour the dry thermoplastic polyurethane into the internal mixer from the feeding port, then add the dry melamine cyanurate-coated aluminum diethylphosphinate and dipentaerythritol, and fully extrude, shear, and mix under the action of the twin rotors; (c) Run for 10 - 15 min, take a sample to obtain the melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane composite material.

4. The preparation method of melamine cyanurate-coated aluminum diethylphosphinate intumescent flame retardant thermoplastic polyurethane according to claim 3, characterized in that, The mixing temperature of the internal mixer is set at 160 - 170 °C.

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