关键词：导电复合材料;老化;应力松弛;SEM/EDS;XRD;结晶度 Abstract Conductive polymer composites, with good flexibility, adjustable conductivity, easy forming and low production cost, can be used as functional material in many fields for its antistatic properties, electromagnetic shielding/microwave absorbing properties, and pressure/temperature sensitivity. However, in the process of processing, storage and use, due to comprehensive influence of many factors, aging will inevitably occur which will lead to deterioration of the properties. In this paper, PP/SSFs (stainless steel fibers) conductive composites were prepared by melt-blending and injection molding. The specimens were subjected to accelerated hygrothermal aging and UV aging. Stress relaxation curves, resistivity and crystallinity were experimentally measured. Micromorphology and elemental distribution of specimens before and after aging have been observed and dectected by scanning electron microscope (SEM) and energy spectrometer (EDS). The results show that the stress relaxation curves display three-stages in characteristics. And the stress reduces after hygrothermal aging due to the breaking and cross-linking of molecular chain caused by aging. The initial resistivity of PP/SSFs composites decreases with the increase of filler content, while it will increased with aging time. Due to the piezoresistive effect of the conductive polymer, the resistivity of the specimens decreases significantly with the increase of initial load, and then it tends to a stable value and fluctuation in a smaller range. The results of SEM/EDS analysis show that with the increase of aging time, the oxygen content on the specimens surface increases, and it will decreases with distance (depth) to the surface of specimen. XRD results show that the crystallinity of composites decreased with the increase of SSFs content and aging time. The present research will provide an experimental basis for the study of aging properties of conductive polymer composites.
Keywords：conductive composites;aging;stress relaxation;SEM/EDS;XRD;crystallinity -->0 PDF (590KB)元数据多维度评价相关文章收藏文章 本文引用格式导出EndNoteRisBibtex收藏本文--> 朱振华, 邵柏军, 王俊, 邵宇, 陈建康, 张明华. 老化对PP/SSFs导电复合材料结构及 应力松弛性能的影响[J]. 力学学报, 2018, 50(3): 517-526 https://doi.org/10.6052/0459-1879-18-080 ZhuZhenhua, ShaoBaijun, WangJun, ShaoYu, ChenJiankang, ZhangMinghua. EFFECT OF AGING ON STRUCTURE AND STRESS RELAXATION OF PP/SSFs COMPOSITES[J]. Chinese Journal of Theoretical and Applied Mechanics, 2018, 50(3): 517-526 https://doi.org/10.6052/0459-1879-18-080
图3是PP/SSFs复合材料老化前后应力松弛曲线,从图中可以看出应力松弛有明显的三阶段特征,松弛初期应力随时间迅速下 降(快速松弛),第二阶段应力下降速度减缓(减速松弛),最后阶段应力趋于平稳(稳定松弛). 在刚发生应变时的很短 时间内,分子没有足够的时间进行大范围的调整,只能是分子间距离的改变,这一改变需要相当高的能量,所需要的应力最大, 此时处于玻璃态;随着时间延长,链段可以重新进行调整,使局部 应力得以松弛,从而模量下降几个数量级,材料趋于变软,此时是黏弹态. 应力随时间的延长而降低,在足够长的时间内,应力会衰减到一个有限值,所需要的时间即为松弛时间. 在松弛过后,链受到拉伸,缠结链的平均末端距增大,阻止远程运动的远程相互作用使其保持着一伸长,此时模量又进入一个平台, 这是橡胶态[9,11-12]. 显示原图|下载原图ZIP|生成PPT 图3老化前后应力松弛结果对比 -->Fig. 3Comparison of stress relaxation results before and after aging -->
湿热老化与未老化的材料相比,松弛后的应力水平呈现先降低后增加的趋势. 相同SSFs含量的复合材料老化20 d时应力水平最低,这是由于材料中的分子链断裂所致,而老化40 d后的应力水平较20 d有所升高甚至 高于未老化试样,分析认为这是由于材料分子链发生了交联使其应力水平反而有所回升. 图4是应力松弛过程中复合材料电阻率随时间的变化,从图中可以看出在加载至初始应力的过程中材料的电阻率迅速降低,随后在松弛 阶段下降速度趋于平稳并在很小的范围内波动. 并且从图中可以看出,该体系材料的初始电阻率随着湿热老化时间的延长而增大,在预加载阶段SSFs含量为6%和7%的复合材料的 电阻率出现了较为明显的波动,这是由于材料导电填料含量相对较低,并且SSFs含量为6%的复合材料电阻率波动比7%含量更加明显,另外湿热老化对材料松弛阶段电阻率影响不大. 显示原图|下载原图ZIP|生成PPT 图4应力松弛过程中材料电阻率变化 -->Fig. 4Comparison of resistivity during stress relaxation for specimens before and after aging -->
2.2 老化前后试样的微观形貌分析
未老化 试样的微观形貌如图5所示. 从图5中可以看出 SSFs 在 PP 基体中分布的比较均匀,随着导电填料含量增加单位面积内出 现 SSFs 的数量也随之增多,形成导电通路的几率随之增大. 显示原图|下载原图ZIP|生成PPT 图5不同SSFs含量的PP/SSFs复合材料的微观形貌图 -->Fig. 5Microstructure of PP/SSFs composites with different SSFs content -->
经能谱分析仪测得湿热老化和紫外老化300 h, 600 h, 900 h的试样表面的氧元素含量(质量百分比),如图6所示. 从图中可以看出 试样表面氧元素含量经湿热和紫外老化后均明显增加,且在相同老化时间下紫外老化的试样比湿热老化试样的氧元素含量增加更多. 聚丙烯在紫外光照射和湿热条件下容易发生氧化反应,生成含氧的羰基和羟基等[33],且羰基和羟基随老化时间的延长而增长. 分析认为,试样中氧元素含量的增加主要是因为试样中羰基、羟基等含氧官能团的增加. 显示原图|下载原图ZIP|生成PPT 图6试样表面氧元素含量变化 -->Fig. 6Changes of oxygen content on the surface of the specimens -->
图7是老化试样中氧元素含量随试样深度的变化规律,图7(b)和图7(e)中的(1) (4) 分别是图9(a) 和图9 (d)脆断面SEM图中按箭头方 向(从试样表层指向试样心部)的4个点的能谱,从图中可以看出,氧元素含量随老化层深度的增加而明显减小直至减为零,图7(c)和图7 (f)即为试样中氧元素含量随距试样表层距离的变化规律,从图中可以看出,氧元素含量随距表面距离增加而单调降低,即老化程度减弱. 图中还可以看出湿热老化900 h后的深度约为150 m,紫外老化900 h的深度约为500 m. 分析认为,聚丙烯试样在强紫外光长时间照射下,试样表层分子链率先发生断裂,然后发生降解最终导致粉化,试样表层粉化后进一 步促进了试样深层的氧化作用,而湿热老化900 h后试样表面仍未出现粉化现象. 显示原图|下载原图ZIP|生成PPT 图7氧元素含量随老化深度的变化 -->Fig. 7Changes of oxygen content with aging depth -->
显示原图|下载原图ZIP|生成PPT 图7氧元素含量随老化深度的变化(续) -->Fig. 7Changes of oxygen content with aging depth(continued) -->
2.3 老化前后试样的XRD分析
未老化试样的XRD图谱如图8(a)所示,经MDI Jade 6分析软件计算出试样的结晶度,其结晶度随SSFs含量的变化趋势如图8(b)所示. 从图8(a)中可以看出,随着SSFs的加入,第2个衍射峰强度明显减弱,其他衍射峰强度也有所减小. 从图8(b)中可以看出,结晶度随SSFs含量的增加而减小,分析认为填料的加入对PP基体的结晶过程有阻碍作用,填料含量越多复合材 料的结晶度越小. 显示原图|下载原图ZIP|生成PPT 图8未老化试样的XRD图谱及其结晶度变化 -->Fig. 8XRD patterns and crystallinity changes of unaged samples -->
同理可测出导电填料含量为0%, 4%, 6%, 8%的试样的结晶度随老化时间的变化如图9所示. 从图中可以看出,随填料含量增加, 复合材料结晶度降低,且随老化时间延长结晶度呈减小趋势. 外界环境的温度、湿度、辐照强度以及老化时间等均会导致聚丙烯试样的分子结构发生支化、断链、交联及氧化等反应[34]. 老化过 程中的部分非晶链段在高温下会重新排列成有序的晶区结构[35],这会导致结晶度的增加;但是随着老化时间的延长,材料中分子链 薄弱处易发生断裂,形成自由基发生耦合作用,导致交联反应的发生,这会限制聚丙烯分子链的运动,抑制结晶能力,从而使复合材料结晶 度下降. 所以在老化过程中重结晶反应和交联反应两个过程同时存在,湿热老化初期重结晶反应占主导,材料的结晶度会有所增加,当老化时间延 长交联反应占主导时,材料的结晶度会呈下降趋势. 显示原图|下载原图ZIP|生成PPT 图9试样的结晶度随老化时间的变化 -->Fig. 9Changes of the crystallinity of the specimen with the aging time -->
3 结 论
(1)松弛实验中,达到初始应力前的恒工程应变率加载过程中,应力迅速增加,材料的电阻率迅速降低. 松弛阶段应力有明显的三阶段特征, 电阻率在松弛初期稍有下降,在松弛后续阶段,电阻率趋于恒定,只在很小范围内波动. (2)湿热老化后与未老化的试样相比,松弛后的应力水平呈现先降低后增加的趋势. 对相同导电填料含量的试样,老化20 d后材料分子 链发生断裂使应力衰减较明显,而老化40 d的材料由于分子链的交联现象使其应力水平反而有所回升. 另外湿热老化使材料初始电阻率增加,并且老化时间越长,材料的初始电阻率越高,但对松弛阶段的电阻率变化规律影响不大. (3)随着湿热老化和紫外老化时间的延长,复合材料会发生氧化产生羰基和羟基等含氧官能团,导致复合材料外表面的氧元素含量增加. 从试样脆断面的能谱分析得出,氧元素含量随距试样表面距离的增加而减小,在本实验条件下老化时间相同时,紫外老化深度比湿热老化深度大得多. (4) PP/SSFs复合材料的结晶度随填料含量的增加而减小,且在老化过程中复合材料的重结晶过程和交联过程同时存在. 湿热老化初期重结晶反应占主导,材料的结晶度有所增加,随着湿热老化和紫外老化时间的延长交联反应占主导复合材料的结晶度呈减小趋势. The authors have declared that no competing interests exist.
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