聚烯烃老化的时空谱——多因素耦合老化动力学研究

聚烯烃的自然老化是光照、温度、湿度和氧气多种环境因素综合作用的结果,但长期以来,无法通过实验室加速得到模拟综合条件下的老化结果 .本文采用自主设计开发的多因素耦合老化评价系统,可以实现光照、温度、湿度和氧气条件的可控加载,测定高分子材料在不同条件组合下老化时产生的气相降解产物(如CO₂)的生成速率,以此来评价材料的稳定性.以聚丙烯(PP)和聚乙烯(PE)为典型体系,建立了二者关于温度、光照强度、湿度和氧气浓度的单因子老化速率方程,并基于此建立了PP和PE的多因素耦合老化动力学模型.通过测定不同条件下的老化速率,拟合得到了PP和PE的多因素耦合老化动力学方程,并计算得到了PP和PE的全国"老化速率地图"和逐月"老化速率时间谱",对PP和PE的应用提供了重要的指导.进行了PP和PE对不同条件因子的敏感性分析.研究发现,PP对光照强度和湿度的敏感性显著高于PE,对氧气浓度的敏感性略高于PE,而对温度的敏感性要低于PE.     

聚酰亚胺6FDA-mPDA及其非对称中空纤维膜的气体渗透性能

用两步法制备了聚酰亚胺2,2＇-双（3,4-二羧酸苯基）六氟丙烷二酐（6FDA）-1,3-苯二胺（mPDA）．测定了聚合物致密膜的密度、自由体积分率和玻璃化转变温度．制备了不同干纺距离下具有超薄致密皮层的聚酰亚胺中空纤维膜．制备的中空纤维膜在25℃,0．5MPa下,O2的渗透速率为19．10GPU,O2／N2分离系数为5．99,CO2的渗透速率为106．34GPU,CO2／CH4的分离系数为82．00．致密皮层的厚度约为96nm．考察了操作温度对膜性能的影响,结果表明,随着温度的升高,膜的渗透速率增大,分离系数减小．物理老化对膜性能的实验结果表明,随着老化时间的增加,膜的渗透速率减小,分离系数增大．膜的致密层厚度影响膜的老化性能．     

二维相关红外光谱法及标准线性固态模型对天然橡胶热氧老化机理的研究

对天然橡胶复合材料进行了不同时间下的热氧加速老化实验并分析其老化机理.采用红外测试研究天然橡胶在热氧老化过程中可能的基团变化规律,同时对不同老化时间下的红外结果进行二维相关分析,发现老化产物的生成速率依次为酯过氧化物醚;核磁交联密度测试表明总交联密度(XLD)与网链分子量(Mc)及横向弛豫时间(T2)有很好的对应关系,XLD随着老化时间的增加而增加,即天然橡胶热氧老化过程中交联反应占主导地位;示差扫描量热(DSC)测试发现,随着老化时间的增加,玻璃化转变温度(Tg)升高,玻璃化转变区变宽,表明交联反应占主导地位,与核磁交联密度测试结果一致.通过对天然橡胶进行压缩应力松弛实验,采用修正的标准线性固态模型(SLS模型)进行拟合分析,发现在短时热氧老化过程中,老化以氧化和交联反应为主,与核磁交联密度及DSC实验结果一致.     

界面对聚丙烯/碳酸钙复合材料光氧老化的影响

以聚丙烯(PP)/碳酸钙为研究对象,以复合材料界面为着眼点,探讨了不同的填料粒径和表面组成对复合材料光氧老化的影响,并提出了相关机理.首先,文中认为聚合物/无机填料界面面积与界面化学组成是影响老化速率的关键因素.聚合物与碳酸钙的总界面面积的增大会加速PP的光氧老化,不同偶联剂处理会改变碳酸钙的表面化学特性进而对PP老化产生促进或抑制.在此基础上提出了基于界面的老化反应速率概念及相应模型.其次,在老化领域采用了"界面放大"的实验方案,验证了碳酸钙对PP的老化促进很大程度上来源于碳酸钙表面的化学组成.此外,研究发现采用表面处理剂钛酸酯改性的碳酸钙能大大促进PP的老化,且表现为钛酸酯与碳酸钙的协同效果.     

弹道明胶老化初级阶段的模型

用流变震荡模式研究了10 wt%浓度的弹道明胶在冷却和等温下的老化行为.对明胶从sol-gel点冷却到某个目标温度的过程,通过不同冷却速率的实验,建了一个弹性模量与温度的线性关系.不同温度下明胶的等温老化实验表明,弹性模量-时间曲线具有相似的形状.依据Normand提出的二级反应动力学模型,引入一个表征明胶老化行为的速率常数,构建了一个预测弹道明胶在老化初级阶段的弹性模量演化的模型.该模型中模量-温度关系在初始和无穷大时是直线,其他模量-温度线是近似直线,并交于sol-gel点.老化速率常数和过冷度之间符合Flory-Weaver方程.对弹性模量和老化时间进行归一化处理,可将不同温度下的老化曲线叠加成一条主曲线.     

超临界合成Pt修饰的TiO2用于光催化CO2还原制备太阳能燃料

考察了超临界条件下合成TiO2基光催化剂的性质,尤其是在超临界CO2下得到的分散在TiO2上Pt的特性,并与商品化TiO2性能进行了比较.另外,所得催化剂的光催化活性用CO2光还原制太阳能燃料进行了评价.结果表明,该催化剂可得到具有比商用TiO2更好或类似的性能(高比表面积、结晶度、表面羟基浓度,大的孔容、增强的可见光吸收、高的甲烷生成速率)而用于CO2还原制备燃料的反应中.这可归因于该催化剂超临界介质合成过程.     

硬质聚氯乙烯自然老化的变色行为与分子结构变化

以硬质聚氯乙烯(PVC)为研究对象,通过色差实验、FTIR、UV等方法探究了PVC在湿热、辐射低的琼海和暖温、干旱且辐射高的若羌地区自然老化的变色行为与分子结构变化.结果表明,太阳辐射是影响PVC自然老化过程中变色行为和分子结构变化的主要环境因素,PVC在干旱、辐射高的若羌地区比在湿热、辐射低的琼海地区更易变黄和变红,而在两地变暗的程度相近;这是因为随着自然老化时间的延长,PVC逐渐发生脱除氯化氢反应和氧化反应,在干旱且辐射高的若羌,更易发生脱除氯化氢反应,而在湿热且辐射低的琼海,脱除氯化氢反应和氧化反应的速度相近,在若羌老化的PVC样品生成了更多的共轭长度(n)≥8的C=C长共轭序列所致.     

累托石粘土/热塑性聚氨酯弹性体纳米复合材料的热性能研究

以十二烷基季铵盐与累托石 (REC)进行阳离子交换得到有机粘土 (OREC) ,以OREC与热塑性聚氨酯弹性体 (TPUR)采用熔融挤出共混法制备了OREC TPUR纳米复合材料 .用透射电子显微镜 (TEM)表征了复合材料的微相结构 ,测试了复合材料动态热机械性能 (DMA)及热失重 (TG) ,讨论了复合材料的耐热空气老化性能及耐油介质性能等 .结果表明 ,累托石粘土在聚氨酯热塑性弹性体中以纳米尺寸分散 ,纳米复合材料具有较高的动态热机械性能 ,其储能模量最大可提高 7倍多 ,损耗模量最大可提高 4倍多 .复合材料的其他性能均有不同程度的提高 ,特别是OREC添加量为 2 %时 ,复合材料TG、耐油性及耐空气老化性能最高 .其初始分解温度提高 1 5℃ ,在 40 #机油中浸泡 1 68h后拉伸强度保持率达到 86 4% ,1 2 0℃热空气老化箱中老化 72h后拉伸强度保持率达到 87 0 % .     

固定床反应器中添加CO2对费托合成反应的影响

用固定床反应器研究了Fe-Mn催化剂上原料气中添加CO2对费托合成反应的影响.结果表明,在533K与反应总压为1.50MPa时,大量CO2的添加使得生成CO2的选择性迅速降低,烃的生成速率降低,烃的收率也有所降低,产物向轻组分方向偏移,有机含氧化合物的生成速率降低;在593K与合成气(H2 CO)分压为1.50MPa时,随着CO2分压的增加,CO消耗速率变化不大,而烃的生成速率缓慢升高,烃的收率有所升高,CO2的生成速率缓慢降低,H2O的生成速率明显加快.由于CO2的添加,促使水煤气变换反应向逆反应方向进行,降低了催化剂表面氢物种浓度,抑制了加氢反应,低碳烃的烯/烷比有所增大,同时提高了重质烃的选择性,且随着CO2分压的增加,有机含氧化合物的生成速率呈上升的趋势.     

甲醇合成Cu/Zno催化剂前驱体的物相转变

采用共沉淀法制备Cu/ZnO合成甲醇催化剂,重点考察了Cu/Zn母料老化期间pH值的变化情况,并结合X射线衍射(XRD)、红外光谱、微分热重分析、程序升温脱附和扫描电镜等表征于段研究了不同老化时间前驱体物相的变化.结果表明,老化前,前驱体主要以无定形碱式碳酸盐的形式存在;老化过程中,物相由无定形向晶体转变,铜锌开始掺入相应的碱式碳酸盐,此时母液pH值下降,溶液由蓝色变成蓝绿色.随着老化时间的增加,铜锌掺入量增加,晶粒逐渐长大,最终形成大量能生成高活件催化剂的前驱体物相(Cu,Zn)2(CO3)(OH)2和(Cu,Zn)5(CO3)2(OH)6.XRD结果表明,老化时间对催化剂的CuO晶粒大小影响很大.     

Natural aging of shape stabilized phase change materials based on paraffin wax

Natural aging of shape-stabilized phase change materials containing linear low density polyethylene (LLDPE), paraffin wax and expanded graphite (EG) in Qatari climate has been studied. It was found that expanded graphite significantly improved the performance of prepared SSPCMs in multiple ways. Firstly, EG suppressed leakage of paraffin wax from the compact shape of SSPCMs. The addition of 15 wt% of EG to shape stabilized phase change materials (SSPCMs) containing 50 wt% of wax caused a decreasing in the leakage of wax by 50% over 210 days of natural aging.Secondly, ∖expanded graphite enhanced the photochemical stability of the blends; this was confirmed by FTIR analysis, where carbonyl index decreased with EG content.     

Systematic evaluation of textural properties, activation temperature and gas uptake of Cu2(pzdc)2L [L = dipyridyl-based ligands] porous coordination pillared-layer networks

In situ high temperature X-ray diffraction, nitrogen porosimetry and gas adsorption at room temperature were used to elucidate the effect of the degassing or activation temperature on the long-range and micropore textural properties of a series of coordination polymers with pillared-layer structures. Ramp-and-soak thermal gravimetric analysis performed at selected activation temperatures were used to verify the thermal stability of a CPL-n series [Cu(2)(pzdc)(2)L; pzdc = pyrazine-2,3-dicarboxylate; L = 4,4-azopyridine (apy) for CPL-4, 1,2-di-(4-pyridil)-ethylene (bpe) for CPL-5, N-(4-pyridyl)-isonicotinamide (pia) for CPL-6, and 1,2-di-(4-pyridyl)-glycol (dpyg) for CPL-7]. Although the activation temperatures were far below the decomposition point of the complexes, these resulted in significant and unique changes in micropore surface area and volume, even for CPL-4, -5 and -6, which contained pillar ligands with similar dimensions and similar structural long-range order. For the case of CPL-7, however, the framework appeared to be non-porous at any given activation temperature. Pure component equilibrium adsorption data gathered for CO(2), CH(4), and N(2) were used to elucidate the CPL-n materials potential for storage and separations at room temperature. All of the materials exhibited considerable selectivity toward CO(2), particularly at moderate pressures. Meanwhile, CO(2) isosteric heats of adsorption indicated that the pore functionalities arising from the pillar ligands provided similar interactions with the adsorbate in the cases of CPL-4 and -5. For CPL-6, the presence of the carbonyl (C[double bond, length as m-dash]O) group appeared to enhance interactions with CO(2) at low loadings.     

Preferential Carbon Monoxide Oxidation over Copper‐Based Catalysts under In Situ Ball Milling

In situ ball milling of solid catalysts is a promising yet almost unexplored concept for boosting catalytic performance. The continuous preferential oxidation of CO (CO‐PROX) under in situ ball milling of Cu‐based catalysts such as Cu/Cr₂O₃ is presented. At temperatures as low as −40 °C, considerable activity and more than 95 % selectivity were achieved. A negative apparent activation energy was observed, which is attributed to the mechanically induced generation and subsequent thermal healing of short‐lived surface defects. In situ ball milling at sub‐zero temperatures resulted in an increase of the CO oxidation rate by roughly 4 orders of magnitude. This drastic and highly selective enhancement of CO oxidation showcases the potential of in situ ball milling in heterogeneous catalysis.     

Effects of thermal-oxidative aging on the flammability and thermal-oxidative degradation kinetics of tris(tribromophenyl) cyanurate flame retardant PA6/LGF composites

The influence of thermal-oxidative aging on the flame retardancy of the flame retardant long-glass-fiber reinforced polyamide 6 composites (FR/PA6/LGF) with different thermal-oxidative exposure times at 160 °C were studied in this work. The flammability and flame-retardant properties of FR/PA6/LGF were investigated by means of the limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter test (CONE), and scanning electronic microscopy (SEM), before and after thermal-oxidative aging. The thermal-oxidative stability and degradation kinetics of the unaged and aged composites were studied by thermogravimetric analysis (TGA) with the methods of Kissinger and Ozawa in dynamic measurements (10 °C/min–40 °C/min). The results indicated that the flammability properties mirrored the degradation behaviors of these FR/PA6/LGF composites whatever their forms (aged or not). The Ozawa method showed that the causes of the first peak in the heat release rate change by CONE measurement corresponded to the apparent activation energies of the first stage degradation of aged FR/PA6/LGF composites, and the same conclusion with respect to the other heat release rate peak. Moreover, this aging slightly enhanced the solid phase flame-retardant mechanism by a char-promotion function, but had no effect on the gaseous flame-retardant mechanism and the decrease of harmful gas release rates. The existence of a surface migration effect on the flame retardant would endow FR/PA6/LGF composites with better LOI values, a more protective char layer structure, and excellent UL-94 ratings.     

ZnO-SnO2纳米复合氧化物的制备、表征及其气敏性质的研究(英)

本文用可控湿化学共沉淀法研制了ZnO-SnO₂纳米晶体复合气敏材料并考察其对有毒气体CO和NO₂的气敏性质。用TEM、BET和XRD等方法表征了纳米复合物的粒度、形貌、比表面、热稳定性和相稳定性。研究了制备的可控参数，如金属阳离子总浓度、沉淀pH值和老化时间等对复合物气敏性质的影响。研究结果表明，该纳米复合氧化物具有化学均一性，高度热稳定和相稳定性，对CO和NO₂具有高的灵敏度和选择性，其气敏性质依赖于复合物组成、焙烧温度和操作温度。通过2wt％金属Cd的掺杂和10wt% Al2O3氧化物的表面包覆大大提高了气体的灵敏度和选择性。用程序升温吸脱附研究了纳米复合物表面对气体的吸脱附性能，并探讨了气敏机理。     

Low-temperature activation conditions for PAMAM dendrimer templated Pt nanoparticles

Surface immobilized polyamidoamine (PAMAM) dendrimer templated Pt nanoparticles were employed as precursors to heterogeneous catalysts. CO oxidation catalysis and in situ infrared spectroscopy were used to evaluate conditions for dendrimer removal. Infrared spectroscopy showed that PAMAM dendrimer amide bonds begin decomposing at temperatures as low as 75 degrees C. Although the amide stretches are completely removed after 3 h of oxidation at 300 degrees C, 16 h were required to reach maximum catalytic activity. Further treatment under oxidizing or reducing atmospheres did not cause substantial changes in activity. Infrared spectroscopy of the activated materials indicated that organic residues, probably surface carboxylates, are formed during oxidation. These surface species passivate the Pt NPs, and their removal was required to fully activate the catalyst. Substantially less forcing activation conditions were possible by employing a CO/O(2)/He oxidation treatment. At appropriate temperatures, CO acts as a protecting group for the Pt surface, helping to prevent fouling of the nanoparticle by organic residues. CO oxidation catalysis and infrared spectroscopy of adsorbed CO indicated that the low temperature activation treatment yielded supported nanoparticles that were substantially similar to those prepared with more forcing conditions.     

Total luminescence intensity (TLI) offers superior early oxidation detection in unstabilised polyethylene but is no better than FT-IR for stabilised polyolefins

In an earlier study, we have shown that chemiluminescence (CL) and the total luminescence intensity (TLI) method are highly sensitive to oxidation in degradable PE. In this study, stabilised PE and PP were characterised with CL in an inert (TLI) and in an oxygen atmosphere (CL-OIT) and the results were compared to those obtained by the commonly used techniques, FT-IR (carbonyl index (CI)) and thermal analysis (DSC-OIT). PE was aged at a low temperature (80 °C) and PP was aged at temperatures between 60 and 120 °C. Non-Arrhenius behaviour was observed in the oxidation of PP. This showed the importance of aging at a low temperature to obtain realistic results. TLI and CI of stabilised PP and most of the stabilised PE gave comparable results with the same sensitivity for oxidation detection. This was in contrast to our previous results for degradable PE. However, TLI of unstabilised PE showed earlier oxidation detection than CI, which agreed with our earlier results. TLI of PE had a higher sensitivity than CL-OIT, and both TLI and CI of PP were sufficiently sensitive to detect the effect of aging at different temperatures, whereas DSC-OIT was not.     

Effect of molecular weight and temperature on physical aging of thin glassy poly(2,6-dimethyl-1,4-phenylene oxide) films

The effects of polymer molecular weight and temperature on the physical aging of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is examined. Gas permeability and refractive index were monitored during the aging process for PPO film samples at three aging temperatures below the glass transition temperature. Comparisons between the two samples of PPO that differ widely in molecular weight reveal an insignificant difference, which support the notion that above a critical molecular weight range there is little influence on aging rate. Increased temperature, over the limited range of 35–55 °C, results in higher aging rates for films made from both PPO materials. The rate of aging decreases strongly with increasing film thickness over the range examined, ∼0.4–25 μm. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1390–1398, 2007     

Natural rubber composites reinforced with basic magnesium oxysulfate whiskers: Processing and ultraviolet resistance/flame retardant properties

The Magnesium sulfate whiskers (MOSw) were first modified by Stearic acid or Si69, and Natural rubber (NR)/modified-MOSw composites were prepared by blending the modified-MOSw with natural rubber latex. By adding modified-MOS_W into NR, the mechanical properties, the anti-ultraviolet aging property, flammability, and thermal stability of composites were improved obviously. The mechanical properties, crosslink density and thermal stability of composites reach the highest value at 4 wt% Si69-MOS_W. The composite with MOS_W addtion had a higher retention rate after ultraviolet irradiation and the MOS_W could improve the anti-ultraviolet aging property of rubber matrix. The modified MOS_W can effectively improve the oxygen index and the flame retardant grade of rubber composites.     

Combining different analytical approaches to identify odor formation mechanisms in polyethylene and polypropylene

In a previous study, we identified carbonyls as highly odor-active compounds in both unprocessed and processed polypropylene (PP) with higher intensities after processing, indicating a temperature-driven forming mechanism. In the presented work, we studied whether (a) these carbonyls are the major odor drivers to the overall odor of polyolefins, (b) their formation is taking place already at moderate temperatures well below the typical processing temperatures, (c) conventional antioxidants in polyolefins can prevent or reduce their formation, and (d) whether reducing the amount of oxygen present can decrease the overall odor. One polyethylene (PE) and one PP were selected, and both stabilized and unstabilized polymer powder samples were exposed to conditions differing in oxygen concentration and aging time. The changes in the volatile fraction as well as the formation of odor-active compounds were monitored using a multidisciplinary approach by combining analytical methods based on gas chromatography (GC), multivariate data analysis, and sensory methods (GC–olfactometry and a sensory panel). Both investigated materials (PE and PP) showed similar degradation products (aldehydes, ketones, carboxylic acids, alcohols, and lactones) which increased dramatically with increasing aging time and the lack of stabilization. Oxidation products, mainly carbonyl compounds, were responsible for the odor of the investigated materials. The main odor drivers were unsaturated ketones and aldehydes with a chain length between six and nine C-atoms. Interestingly, similar odor patterns were found for both stabilized and unstabilized samples, indicating that similar formation processes take place independent of the stabilization.