拉伸速率对氨纶力学性能的影响研究

氨纶被外力拉伸时,拉伸速率与其内部链段运动速率的差值对氨纶的力学性能影响很大。为研究不同拉伸速率对氨纶力学性能的影响,选用晓星公司生产的线密度为622.2 dtex的氨纶,在仅改变拉伸速率的情况下对氨纶进行一系列的弹性试验,测试其应力松弛率、弹性回复率、塑性变形率等力学性能。结果表明:当定伸长率为300%,拉伸速率分别为500、400、300、200和100 mm/min时,随着拉伸速率的增加,氨纶的应力松弛率不断增加,弹性回复率不断减小,塑性变形率不断增大。     

三种弹性纤维的力学性能及应用特点

实测了氨纶、聚烯烃XLAR、PET/PTT双组分复合纤维以及氨纶毛包芯纱、PET/PTT棉包芯纱的比应力-应变曲线、0.25N负荷下的伸长率、定伸长回复率和定伸长50%三次循环的应力松弛率。通过比较分析得到,氨纶和聚烯烃XLAR具有断裂伸长率大、定负荷伸长率大的特点,必须包芯使用,且反复拉伸易松弛。PET/PTT双组分复合纤维断裂强度较大,定负荷伸长率足够,不需要包覆可直接使用,且定伸长回复率较大,不易松弛,适用于低弹和中弹织物。     

竹笋壳纤维的力学性能

为了了解竹笋壳纤维的基本力学性能,为竹笋壳纤维的进一步开发利用提供理论依据,研究了竹笋壳纤维拉伸断裂性能、松弛性能和定伸长弹性性能等力学指标。结果表明,竹笋壳纤维断裂性能指标拉伸断裂强度、初始模量和断裂伸长率分别为3．21cN／dtex、214．32cN／dtex和2．01％,在湿态下竹笋壳纤维的拉伸断裂强度和初始模量下降较大,分别下降了38．6％和33．1％,断裂伸长率变化不大;比较干、湿态下竹笋壳纤维的抗应力松弛性能和定伸长抗伸回弹性,湿态下的抗应力松弛性能和弹性能力较优。     

长压缩应力-应变平台的聚氨酯弹性体泡沫的研究

制备了具有长压缩应力-应变平台的聚氨酯弹性体泡沫(PUEF),研究了硬段含量、交联度以及密度等对PUEF压缩应力-应变性能、压缩应力松弛及拉伸性能的影响。结果表明,硬段含量及密度对PUEF压缩应力-应变性能的影响较大,而交联度对压缩应力松弛及拉伸性能的影响较大。在相同的原料体系下,随着硬段含量及密度的增加,PUEF的应变降低,应力增大,压缩应力松弛增大,断裂伸长率降低,拉伸强度增大;随着交联点的增加,PUEF的压缩应力松弛降低,断裂伸长率下降,拉伸强度下降。     

TOTM/DOP并用比对CM/EVM共混胶性能的研究

本文主要研究了CM/EVM为70/30配比时,不同TOTM/DOP配比下共混胶料的力学性能、耐热氧老化性能、耐油性能。结果表明:随着TOTM用量的增加,热空气老化前,拉伸强度增大,扯断伸长率基本不变,100%定伸应力呈逐渐上升的趋势;热空气老化后,拉伸强度逐渐下降,扯断伸长率增大,50%定伸应力逐渐减小。在3#标准油中老化后,扯断伸长率呈下降趋势,100%定伸应力增大,拉伸强度增大,体积变化率减小。     

热预处理时间对EPDM/FKM性能影响及共混胶老化过程中交联密度变化的研究

研究了不同热预处理时间对EPDM/FKM性能的影响及共混胶老化过程中性能和交联密度变化。实验证明,随着预处理时间的延长,共混胶拉伸强度先上升后下降,扯断伸长率下降,硬度和100%定伸应力变大。热空气老化中,随着老化时间的延长,当热预处理时间为4 min时,拉伸强度先下降后上升;当预处理时间为6 min时,拉伸强度缓慢上升,扯断伸长率下降,硬度变大,100%定伸应力变大。老化过程中EPDM相交联密度增大,FKM相交联密度缓慢增大,共混胶总交联密度有所增加。综合考虑性能,热预处理时间为6 min较好。     

CM/EVM并用比对共混胶性能的影响

本文研究了CM/EVM不同配比共混胶料的力学性能、耐热氧老化性能和耐油性能,结果表明:随着EVM橡胶用量的增大,热空气老化前,拉伸强度基本不变,拉断伸长率呈现下降趋势,100%定伸应力基本不变;热空气老化后拉伸强度降低,拉断伸长率呈上升趋势;在3#标准油中拉伸强度呈上升趋势,拉断伸长率下降,100%定伸应力增大,△V减小。     

硫化剂TCY用量对CM/ACM共混硫化胶性能影响研究

研究了硫化剂TCY的用量对氯化聚乙烯橡胶(简称CM)/丙烯酸酯橡胶(简称ACM)共混硫化胶性能的影响。结果表明,随硫化剂TCY用量的增加,共混胶焦烧时间基本不变;MH—ML的值逐渐增大,交联程度增加;硫化胶的硬度逐渐增大,撕裂强度先增大后减小;拉伸强度先下降后上升,100%定伸应力增大,扯断伸长率和永久变形增大;老化后拉伸强度先下降后上升,100%定伸应力增大,扯断伸长率和永久变形增大;拉伸强度和100%定伸应力大于老化之前,差值较小。扯断伸长率、永久变形小于老化之前;耐热油老化后拉伸强度、100%定伸应力、扯断伸长率、永久变形的变化趋势与未耐热油老化之前基本相同,且低于耐热油老化之前;体积变化率、质量变化率均下降,耐热油性提高。     

硅橡胶泡沫泡孔结构与压缩应力松弛性能的相关性

中国工程物理研究院的丁国芳等人研究了氧化锌晶须对硅橡胶泡沫泡孔疏密程度及其对材料力学性能的影响,研究了泡沫密度和混合泡孔结构对材料的压缩应力松弛性能的影响。结果表明,随着氧化锌晶须用量的增加,泡孔疏密程度在增大,硅橡胶泡沫材料拉伸强度和扯断伸长率也在变大。硅橡胶泡沫密度在0．56—0．58g／cm^3范围内时压缩应力松弛性能最佳;在相对密度不变的情况,具有混合泡孔结构的硅橡胶泡沫材料的压缩应力松弛性能更好。     

硅橡胶泡沫泡孔结构与压缩应力松弛性能的相关性

中国工程物理研究院的丁国芳等人研究了氧化锌晶须对硅橡胶泡沫泡孔疏密程度及其对材料力学性能的影响,研究了泡沫密度和混合泡孔结构对材料的压缩应力松弛性能的影响。结果表明,随着氧化锌晶须用量的增加,泡孔疏密程度在增大,硅橡胶泡沫材料拉伸强度和扯断伸长率也在变大。硅橡胶泡沫密度在0．56—0．58g／cm^3范围内时压缩应力松弛性能最佳;在相对密度不变的情况,具有混合泡孔结构的硅橡胶泡沫材料的压缩应力松弛性能更好。     

湿热处理对氨纶结构以及性能的影响

使用恒温恒湿老化试验箱,研究了高温高湿环境下氨纶力学性能及微观结构的变化。通过傅里叶变换红外光谱、热力学分析仪、扫描电子显微镜及力学性能分析探究了氨纶湿热老化规律及机制,结果表明:在一定牵伸条件下,随湿度增加,氨纶丝内部氢键化程度变弱且湿度变化对氨纶的影响主要发生在硬链段区域;在高温环境下,氨纶的拉伸强力和拉伸伸长率比在低温环境下的低,而300%拉伸应力比在低温环境下的高,说明氨纶湿热老化本质是湿度对氨纶内部氢键化程度及水解的协同作用,而温度起到加速老化的作用。     

循环拉伸对氨纶结构与性能的影响

为研究多次循环拉伸对氨纶微观结构和尺寸稳定性的影响,选用线密度为622.22 dtex的氨纶丝,分别对其拉伸1 000、4 000、16 000、45 000次。采用傅里叶变换红外光谱、X射线衍射分析、热失重分析、力学性能测试等方法对拉伸后的试验样品进行了表征。结果表明:随着拉伸次数增加,氨纶的氢键化程度一直呈下降趋势;结晶度先上升后下降,拉伸1 000次时得最大值;拉伸次数少于4 000次时,氨纶热稳定性先保持不变,而后下降;力学性能方面,断裂强力与断裂伸长率都是先增后减,拉伸1 000次时都取得最大值;而内耗值一直降低。     

旋转黏度计测定水泥浆体静态屈服应力和黏弹性研究

采用旋转黏度计进行了恒定剪切速率、应力松弛、蠕变试验,研究了水泥浆体的静态屈服应力和黏弹性.结果表明:新拌水泥浆体属于带有屈服应力的黏弹性流体,其静态屈服应力为弹性向黏性转变时所对应的剪切应力;恒定剪切速率、应力松弛、蠕变试验均能测试出水泥浆体屈服应力和黏弹性.在恒定剪切速率试验中,新拌水泥浆体的屈服应力测试依赖于测试时剪切速率的选择,剪切速率过小,水泥浆体不出现静态屈服应力,剪切速率过大,测试的静态屈服应力较小;在应力松弛和蠕变试验中,测试出的水泥浆体静态屈服应力较恒定剪切速率试验的精确.对于一个静态屈服应力未知的新拌水泥浆体,可先采用恒定剪切速率试验粗略选择剪切速率和剪切应力范围,之后采用应力松弛或蠕变试验进行精确测试其静态屈服应力.     

Influence of leather stretching to gain area yield on its stress‐relaxation behavior

Four bovine leathers subjected to five stretching procedures in water at different temperatures between 60 and 80°C and drawing ratios between 1.20 and 1.40 were analyzed to demonstrate the suitability of the generalized Maxwell model to fit the different stress‐relaxation processes related to the structural hierarchy of leather. This consists of a set of three Maxwell units connected in parallel to represent the high‐rate, the medium‐rate, and the low‐rate stress‐relaxation processes, and a Hookean spring in parallel to represent the residual stress at the equilibrium. The high‐rate, the medium‐rate and the low‐rate relaxation times were approximately of 0.6, 10, and 200 s, respectively. Stretching of leather under different conditions to gain area yield in addition to a reduction in thickness also produces a decrease in leather density but not always induce hardening of leather. Leather stretching increases the initial stress when leather is strained 20% for stress‐relaxation tests. The effect of stretching on the residual stress depends on the level of stretching. The hardening effect of stretching measured by relative softness showed a good relationship with the medium‐rate relaxed stress. The harder the stretched leather the higher the decrease in the medium‐rate relaxed stress. Softness also showed a good relationship with the low‐rate relaxation time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 6000–6008, 2006     

Influence of gel and molecular weight on the properties of natural rubber

The influence of gel and molecular weight on the properties of natural rubber has been described. Crystallization, stress relaxation and orientation properties of different samples have been studied. Samples having different gel contents but similar molecular weights and samples having the same gel content with varying molecular weights have been prepared by extracting whole natural rubber. The gel content was varied from 1.5 to 29% and molecular weight from 1 × 10⁵ to 9.9 × 10⁵ g mol⁻¹. The gels reduce the overall crystallinity measured by differential scanning calorimetric experiments after freezing the samples for different time periods at −15°C. They also have a stiffening effect on the tensile relaxation modulus and decrease the rate of relaxation significantly. Birefringence, which measures the orientation and stress-induced crystallization, increases with time at high elongation for samples containing gels. At lower elongation, however, the birefringence of unvulcanized samples decreases with time. This decrease is less for gel samples. Natural rubbers of high molecular weight behave similarly in the above properties when compared to low molecular weight samples. The effect of gel is more predominant than that of molecular weight.     

Relaxation Behavior of Polyethylene Oriented by Various Techniques

Abstract

Stress relaxation of high-density polyethylene extrudates and those crystallized from highly deformed melt (PE-1) have been investigated in a wide range of temperatures (?50 to +120°C) and draw ratios from 5.5 up to 12.2 at the different constant tensile strains from 1 up to 20%. The experimental data obtained have been summarized by the time-temperature superposition principle. Relative intensity of stress relaxation (the stress drop in 10³ s divided by the initial stress) has been observed to increase together with the growth of draw ratio despite the enhancement of the short-term properties. The radiation cross-linking of the PE-1 samples may only decrease the stress relaxation intensity by 30%. The relaxation properties of a number of oriented polyethylene samples produced by various techniques were compared. It has been established that all the investigated materials are characterized by similar values and high relative drops in stress, whereas the short-term properties are essentially different. It points out to the relaxation processes being intensive both in the oriented and unoriented PE.     

Time-dependent birefringence in glassy polymers during stress-relaxation and recovery

The birefringence of polystyrene and polycarbonate have been measured during uniaxial stress relaxation tests. The birefringence of polystyrene shows an instantaneous positive increment on applying a tensile deformation; during stress relaxation the value showed a time-dependent change in the negative direction and the total birefringence eventually became more negative than the value measured prior to the deformation. On unloading, the overall trend was for the birefringence to change towards the level measured in the as-prepared sample prior to deformation but an initial departure in the opposite direction was observed. With polycarbonate, the birefringence changed instaneously on applying the deformation and remained steady during stress relaxation. On unloading the birefringence fell instaneously by an amount less than the increment observed on applying the deformation. Thereafter the birefringence continued to fall, approaching the original value with a time constant similar to that governing stress relaxation. These results are interpreted with reference to the molecular structures, in particular the location of the highly polarizable aromatic rings.