水滑石阻燃剂热相变行为及其热力学分析

水滑石(LDH)具有阻燃、消烟、填充和热稳定性等功能，是无机阻燃剂的新品种，以尿素为沉淀剂制备Mg/Al摩尔比为4的镁铝水滑石(MgAl-LDH)阻燃剂。通过X射线衍射光谱(XRD)、傅里叶红外光谱(FT-IR)和热重-差示扫描量热法(TG-DSC)等表征手段，以及结合去卷积及曲线拟合分析，研究了MgAl-LDH阻燃剂的热分解行为，探求其热相变规律，特别是热解相变过程中层板羟基(—OH)的存在形式和层间碳酸根(CO2-₃)的配位变化，并从热力学角度对其热分解过程进行了分析讨论。XRD表征发现，随着温度的升高，LDH阻燃剂主体层板结构发生晶相转变，500 ℃时层间CO2-₃基本脱除生成镁铝双金属混合氧化物(Mg—Al—O)，600 ℃有杂相MgAl₂O₄尖晶石晶相生成。基于FT-IR和TG-DSC表征和曲线拟合分析，发现LDH阻燃剂主体层板—OH存在Al—OH—Al]，Al—OH—Mg]和Mg—OH—Mg]三种形式，各结构单元中的—OH析出难易程度有所不同，Mg—OH—Mg]最难析出，Al—OH—Mg]次之，而Al—OH—Al]较易;层间CO2-₃亦有三种配位方式，即桥联配位、单齿配位和双齿配位。从热力学角度对MgAl-LDH阻燃剂的热力学性质进行了估算，得到了Mg₈Al₂(OH)₂₀CO₃晶体的吉布斯自由能(Δ_rGθ_T)随温度变化的明确表达式。当温度(T)高于228.65 ℃时，吉布斯自由能(Δ_rGθ_T)小于零，MgAl-LDH发生分解，层板中—OH析出过程是自发进行的。该结果与TG-DSC实验结果接近，表明热力学估算所得吉布斯自由能(Δ_rGθ_T)与温度(T)的关系是可靠的。

Phase Transition Behavior and Thermodynamic Analysis of Hydrotalcite Flame-Retardant

The hydrotalcite with the properties of flame-retardant, eliminating smoke, filling and thermostability is a new kind of inorganic flame retardant. In the work, the MgAl hydrotalcite as flame retardant with Mg/Al molar ratio of 4 (MgAl-LDH) was prepared by using urea as the precipitating agent. The thermolysis behavior of the MgAl-LDH flame retardant was investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and thermogravimetry-differential scanning calorimetry (TG-DSC) as well as self deconvolution and curve-fitting analyses. Thermal phase transition of the MgAl-LDH was clarified, especially the characteristics of the hydroxyl groups (—OH) in the brucite-like layers and the changes in coordinate of the carbonate (CO2-₃) from the interlayers. Based on thermodynamic data, thermal decomposition process was discussed. By XRD analysis, it was found that the phase change took place when the decomposition temperature increased. The MgAl-LDH was decarbonated basically to MgAl mixed metal oxides (Mg—Al—O) at 500 ℃, and impurity MgAl₂O₄ phase formed at 600 ℃. According to the analyses of FT-IR, TG-DSC and curve-fitting technique, the hydroxyl groups (—OH) in the brucite-like layers possessed three the ligands such as Al—OH—Al], Al—OH—Mg] and Mg—OH—Mg] modes. Dehydroxylation of the brucite-like layers based on the binding forces, where the Mg—OH—Mg] among the three modes was the most difficult to be removed during the pyrolysis process. In the same way, the CO2-₃ ligands also possessed three modes such as H₂O-bridged CO2-₃, monodentate and bidentate coordination modes. Based on the thermodynamic analysis, the thermodynamic properties of the hydrotalcite as flame retardant were evaluated, and the expressions of the Gibbs free energy (Δ_rGθ_T), as a function of temperature, were derived for the Mg₈Al₂(OH)₂₀CO₃ crystal. Thermodynamic analysis showed that the removal of —OH from the brucite-like layers was spontaneous process, when the Gibbs free energy (Δ_rGθ_T) was under zero at the temperature (T) above 228.65 ℃. The result and datum were close to the experimental result from the TG-DSC analyses, indicating that the relationship between the Gibbs free energy (Δ_rGθ_T) and temperature (T) from thermodynamic analysis was reliable.