【作者】 高传慧；

【导师】 李先国；

【作者基本信息】 中国海洋大学 ， 海洋化学工程与技术， 2010， 博士

【摘要】 碱式硫酸镁晶须因其增强、阻燃、抑烟、填充等功能而被广泛应用于塑料等复合材料中,是一类极有发展前景的镁盐升级换代产品。本文以氯化镁或海水制盐副产卤水为原料,加入硫酸镁和氨水,采用无模板水热法一步合成152型碱式硫酸镁(MHSH, MgSO4·5Mg(OH)2·2H2O)晶须。利用X射线衍射(XRD)、差热-热重分析(TG-DTA)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)等分析手段对MHSH晶须合成过程中温度和时间诱导的相变、形态演化和生长机理等进行了较为系统的研究,通过考察改性前后MHSH晶须表面接触角的变化以及晶须对聚丙烯(PP)复合材料力学和阻燃性能的影响,对晶须的表面改性方法进行了评价。研究结果表明,在190℃下,采用一步法,无模板水热反应5h所合成的MHSH晶须呈纤维状,直径为0.5-2μm,长径比为50-200,且一端聚集成扇形。晶须表面光滑且纤细匀整,选区电子衍射(SAED)和能谱(EDS)分析表明扇形晶须的尖端与束状处的结构与组成一致。以氯化镁或海水制盐副产卤水作为原料所合成的晶须外观形貌以及晶须长度和长径比差别不大；一步法与两步法合成的晶须组成一致,形貌差别亦不大；添加乙醇作为晶形控制剂的晶须分散性较好,聚集成扇形的数量明显减少；添加乙酸作为晶形控制剂的晶须扇形聚集更加明显,并且晶须的长度有了明显的增加,长径比增长较大。对不同反应时间所得的产品进行SEM分析发现,晶须尖端的二维台阶是由晶须生长过程中螺型位错形成的,这些台阶可作为晶须晶体的生长点。MHSH晶须的生长过程实质上就是生长基元Mg-O6从周围环境中不断通过界面进入晶格位点的过程,晶体生长是晶体内部缺陷——螺型位错延伸的结果,位错的延伸促使了晶须的生长。从晶体生长热力学角度分析,MHSH晶须的水热合成属于溶解再结晶机制,MgCl2、MgSO4和氨水反应所得的Mg(OH)2在水热体系中随温度的升高首先溶解,然后溶液中的Mg2+、SO42与OH-发生反应生成MHSH晶须。从动力学角度,通过对晶体几种可能的生长机理模型进行分析发现,低温和高温反应的动力学模型不同,反应温度为170℃、180℃、190℃时的结晶机理为多核控制表面生长,结晶速率由表面反应控制；反应温度为200℃和210℃时,结晶机理为单核控制表面生长。利用热重法研究了MHSH晶须在程序升温条件下的热降解失重过程,并用Satava法对其热分解机理进行了分析,发现三步分解反应的反应机理是不同的,其积分函数分别为Avrami-Erofeev方程G(α)=[-ln(1-α)]1/2、Prout-Tomkins方程G(α)=ln[α/(1-α)]和Mampel power方程G(α)=α1/2；分别对应于成核与生长、无规则成核和成核机理。利用Kissinger法对热重数据进行分析处理,获得了热分解反应的动力学参数,结果表明,在MHSH的三步热分解反应中,第一步脱除结晶水的表观活化能Ea最小,为277.6 kJ/mol,第二步分子内水的脱除和第三步脱除S03所需的活化能较大,分别为492.3和797.6 kJ/mol。第一步反应较快,第二步反应较慢,第三步是热分解反应的控制步骤,反应最慢。首次采用甲基丙烯酸甲酯(MMA)乳液原位聚合包覆法改性MHSH晶须,获得了表面包覆聚甲基丙烯酸甲酯(PMMA)的晶须。傅里叶变换红外光谱(FT-IR)、SEM、EDS、TG表征结果表明,原位聚合成功获得了以MHSH晶须为核,PMMA为壳的核壳式结构。与表面活性剂硬脂酸(SA)和硅烷偶联剂KH-570的改性相比,乳液原位聚合包覆法改性效果最好,改性后的晶须与水的表面接触角由12.71°增加为87.32°,增长幅度最大。三种方法改性后的MHSH晶须表面均由亲水性向亲油性转变。通过对MHSH晶须／PP复合材料的力学和阻燃性能的分析,评价比较了三种方法的表面改性效果。添加晶须后PP复合材料的拉伸和弯曲性能均明显提高,改性晶须的增强效果要优于未改性晶须,添加改性晶须的复合材料力学性能增强幅度可达到80%。但由于镁盐晶须是高强度脆性材料,自身伸长率较小,复合材料的断裂伸长率随晶须含量的增加而呈下降趋势。在不影响PP复合材料力学性能的情况下,晶须在复合材料中的含量越高阻燃效果越好；表面改性对晶须阻燃性能影响不大。上述研究结果表明,利用海水制盐副产卤水、氨水和硫酸镁制备的镁盐晶须材料对聚合物基体具有较好的增强阻燃性能和广阔的潜在应用前景。本研究首次以海水制盐副产卤水作为原料,采用一步法制得了MHSH晶须,为多途径开发利用苦卤镁资源和高值化、精细化镁盐产品的生产提供了新思路。更多还原

【Abstract】 Magnesium hydroxide sulfate hydrate (MHSH) whiskers have attracted much attention because of their potential application as resin additives of flame retardant, fillers, or reinforcers. In this thesis, we report preparation of MHSH whiskers using one-step hydrothermal synthesis with magnesium chloride (or bittern), ammonia and magnesium sulfate as raw materials and no additional template. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), thermal analysis (TG-DTA), and scanning electron microscope (SEM) were employed to characterize the phase transition induced by temperature and time, morphology evolution and growth mechanism during the hydrothermal process. The products were modified to enhance the compatibility with polypropylene (PP). The surface modification methods were evaluated by investigating the changes of surface contact angle of whiskers, mechanical and flame retardant properties of the composites.The MHSH whiskers prepared at 190℃for 5 hours exhibited a randomly arranged, fanlike morphology with an average diameter of 0.5-2μm and aspect ratios between 50 and 200. The SAED and EDS analysis showed that the elementary composition at the intersection point was consistent with that in the main body of the fanlike whiskers. The morphology of whiskers prepared using magnesium chloride or bittern as raw materials was very similar without obvious difference. Moreover, no distinct difference was observed for products from one-step and two-step techniques.When using ethanol as crystal control agent, most of the whiskers presented as single fibers, while the whiskers aggregated more severely with increased aspect ratios when adding acetic acid as crystal control agent.In the synthesis process, many two-dimensional steps were observed at tips of the whiskers, as seen from SEM images. It was speculated that these steps were formed during crystal growth as a result of crystal dislocations. The growth of MHSH whiskers was essentially the process of Mg-06 entering the crystal lattice through the interface. It was exactly the extension of dislocations that made the growth of the whiskers.From a thermodynamic view, the formation of MHSH whiskers might follow the dissolution-recrystallization mechanism in which the interaction of Mg2+, SO42- and OH- ions, after the dissolution of the initially formed particulate Mg(OH)2, resulted in the production of the whiskers. From a dynamic point of view, the mechanism for crystal growth was different when the products were synthesized at different temperatures. When the reaction occurred at 170℃,180℃and 190℃, the crystallization was controlled by multi-core surface growth, while it changed to be controlled by single-core surface growth when the reaction temperature was 200℃or 210℃.Thermo-gravimetric analyzer was used to characterize the thermal behavior of the whisker products. It was indicated from Satava method that the first step of the thermal decomposition of the MHSH whiskers was in accord with the equation of Avrami-Erofeev with integral form of G(a)=[-ln(1-a)]1/2. The second step was in accord with the equation of Prout-Tomkins with integral form of G(a)=ln[a/(1-a)], and the final step was Mampel power with integral form of G(a)=a1/2. The corresponding mechanism was nucleus formation and growth, branching nuclei and nucleus formation, respectively. Based on DTA data, the kinetic parameters for each step were calculated using a non-isothermal Kissinger method. The activation energy of the first decomposition step was 277.6 kJ/mol, indicating the crystal water moieties can be easily lost. The activation energies of the second and third decomposition steps were 492.3 and 797.6 kJ/mol.This paper fist reported the method of surface-initiated in-situ polymerization to prepare PMMA modified MHSH whiskers with a well defined core-shell hybrid structure. SEM, EDS, TG and FT-IR were used to characterize the effect of surface modification. Compared with the modified effect of stearic acid (SA) and KH-570, the surface contact angle of the PMMA modified whiskers increased most greatly and the whiskers surface was all changed from hydrophilic to lipophilic after modification. The mechanical and flame retardant properties of the composites were investigated in order to evaluate the three different modification method. It was found that the mechanical properties of PP composites can be improved by 80 percent after adding modified whiskers, and the reinforce effect of modified whiskers was better compare to those unmodified. While the elongation at break of PP composites decreased because of the brittleness of MHSH whiskers. The flame retardant effect was better when the content of whiskers was higher. The main impact of surface modification on the composite materials was improvement of mechanical properties, and the effect on flame retardancy of the whisker was little.The research above showed that MHSH whiskers prepared using bittern, ammonia and magnesium sulfate as starting raw materials had wide potential application prospect due to its good flame retardant effect and reinforce properties for polymer matrix.In this paper, the MHSH whiskers were prepared for the first time using one-step method with bittern as raw material. It also provided a new approach for multi-channel development and utilization of magnesium in the bittern resources and production of magnesium products with high additional value.更多还原