Suppression of skin-core structure in injection-molded polymer parts by in situ incorporation of a microfibrillar network
Suppression of skin-core structure in injection-molded polymer parts by in situ incorporation of a microfibrillar network
复制标题
通过原位掺入微纤维网络抑制注塑聚合物部件中的皮芯结构
DOI：
10.1021/ma0604845
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发表时间：
2006-09-19
期刊：
影响因子：
5.5
通讯作者：
Li, Zhong-Ming
中科院分区：
文献类型：
--
作者：
Zhong, Gan-Ji;Li, Liangbin;Li, Zhong-Ming
聚合物，特别是商品聚合物，如聚乙烯（PE）、聚丙烯（PP）等的高性能的实现，仍然是一个重要的开放式研究课题。1聚合物的机械性能可以通过加工过程中聚合物链的取向来增强，例如纺丝，热拉伸，特别是新开发的注塑成型中的剪切控制取向（SCORIM）。2，3对于半结晶聚合物，在注塑过程中的模具填充阶段的剪切流动可能会诱导高分子取向。在极端情况下，定向晶体可以形成互锁串晶结构，这导致注塑部件流动方向上的机械性能显著增强。然而，这些部件通常呈现出不均匀的结构，即皮芯结构。在注射成型期间，接触冷模具壁的热聚合物熔体经历高应变、高应力和大冷却速率，并且因此在壁附近形成具有高取向的表层。这实际上是普通注塑成型的固有问题，因为这些边界条件产生大的温度梯度、剪切速率和应力场。这种非均匀结构由于在厚度方向上不同程度的晶体取向所产生的残余应力而不利于力学性能的提高。此外，不同水平的晶体取向可能导致宏观性能的劣化。6从实用的角度来看，消除皮芯结构有望改善机械性能。如先前所报道的，处理该问题的一种方法7是使用可以有效抑制晶体取向的成核剂。成核剂的存在可以消除皮芯结构，即使在高剪切施加在聚合物流动。然而，在这种情况下，晶体取向通常较低。7-9这并不总是理想的，因为在注塑部件中一定程度的取向对机械性能的改善非常有益。在等规聚丙烯/聚对苯二甲酸乙二酯（iPP/PET）共混物中，通过原位热拉伸可以制备直径为1-10 μm的PET微纤，在iPP基体中构建微纤网络。预期iPP熔体将在此类共混物的注塑期间流过由网络形成的微通道或孔。除了重新定义流场的效果之外，微纤维网络还促进基质聚合物的成核。在原位微纤化的iPP/PET共混物中，剪切流场中晶体成核的三个来源被确定为：（a）剪切诱导的行形核;（B）经典的原纤形核;（c）原纤辅助排列诱导的形核。因此可以推测，原位微原纤网络可以抑制注塑部件的皮芯结构的形成，因为网络可以：（1）有助于使流体在部件厚度上的流速均匀化，（2）充当有效的成核剂以产生典型的横晶层，其中c轴从流动方向偏转，以及（3）辅助剪切流动以形成核。在本说明中，目的是研究原位微纤维网络对注塑件的晶体取向分布的影响。对注塑件内部不同位置的取向参数的分析表明，微纤维网络的存在有效地抑制了皮芯结构。采用SCORIM和热塑性弹性体复合材料可获得高取向度和均匀取向度的注塑制品。
The achievement of high performance in polymers, especially commodity polymers, such as polyethylene (PE), polypropylene (PP), etc., is still an important open research subject. 1 The mechanical properties of polymers can be enhanced through orientation of polymer chains during processing, such as spinning, hot stretch, and especially newly developed shear controlled orientation in injection molding (SCORIM). 2, 3 For semicrystalline polymers, shear flow at the mold filling stage during injection molding may induce high molecular orientation. In the extreme case, the oriented crystals can form an interlocked shish-kebab structure, which results in dramatic enhancement of mechanical properties in the flow direction of injectionmolded parts. 4, 5 However, the parts usually exhibit an inhomogeneous structure, namely a skin-core structure. During injection molding, the hot polymer melt contacting cold mold walls experiences high strain, high stress and large cooling rate, and thus a skin layer with high orientation is formed near the walls. This indeed is an intrinsic problem of normal injection molding because these boundary conditions create large gradients of temperature, shear rate and stress fields. This heterogeneous structure is not favorable to the improvement of mechanical properties due to the residual stress produced by different levels of crystal orientation in the thickness direction. Moreover, the different levels of crystal orientation may cause a deterioration of macroscopic properties. 6 From a practical point of view, elimination of the skin-core structure is expected to improve mechanical properties. One approach to deal with the problem, as has been previously reported, 7 is the use of a nucleating agent that can effectively suppress the crystal orientation. The presence of nucleating agent can eliminate the skin-core structure even with high shearing imposed on the polymer flow. However, in this case, the crystal orientation is generally low. 7-9 This is not always desirable since some level of orientation in the injection molded parts is of great benefit to the improvement of mechanical properties. In isotactic polypropylene/poly (ethylene terephthalate)(iPP/PET) blends, PET microfibrils with diameter of 1-10 µm can be produced through in situ hot-stretch, which constructs a microfibrillar network in the iPP matrix. 10, 11 It is expected that the iPP melt would flow through the microchannels or pores formed by the network during injection molding of such blends. In addition to the effect of redefining the flow field, microfibril networks also promote the nucleation of matrix polymers. In the in situ microfibrillar blend of iPP/PET, three origins for crystal nucleation in shear flow field were identified:(a) the shear induced row nucleation;(b) classical fibril nuclei;(c) nuclei induced by fibril-assisted alignment. 11 One can therefore speculate that the in situ microfibrillar network can suppress the formation of the skin-core structure of the injection-molded parts since the network can:(1) help to homogenize the flow rate of the fluid across the thickness of the part,(2) act as an efficient nucleating agent to generate a typical transcrystalline layers with the c-axis deflected from the flow direction, and (3) assist shear flow to form nuclei. In this Note, the objective is to study the effect of an in situ microfibrillar network on the crystal orientation distribution of injection molded parts. Analyses of orientational parameters of different positions within the injection molded parts show that the presence of the microfibrillar network suppresses the skincore structure effectively. Injection molded parts with high and homogeneous orientation were obtained with a combination of SCORIM and …