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A criterion for gross melt fracture of polyolefins and its relationship with molecular structure.

机译：聚烯烃总熔体断裂的判据及其与分子结构的关系。

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摘要

In polymer extrusion, a melt instability called gross melt fracture is often observed when the extrusion rate is increased beyond a critical value. Because gross melt fracture limits the production rate, much attention has been devoted to predicting when it will occur. However, there is still no general understanding of the origin of this phenomenon. Several criteria have been proposed for the onset of gross melt fracture (OGMF), but these depend on the procedure used for measuring them and thus are not material properties. The use of a critical extensional stress (CES) is proposed here as a criterion for OGMF. This stress is estimated using the entrance pressure drop analyses proposed by Cogswell and by Binding.;Orifice dies were used to eliminate the complication arising from the presence of a capillary. The entrance angle and contraction ratio were varied to study the effect of die geometry. Various materials including conventional linear low and high density polyethylene and new polyethylenes produced by metallocene and constrained geometry catalysts were used in a study of the effect of molecular structure on OGMF. The latter materials are of special interest, because they have very well-defined molecular structure in terms of molecular weight (MW), molecular weight distribution (MWD), and long chain branching (LCB). In addition, the effect of particulate additives (boron nitride and carbon black) was investigated.;It was found that CES is independent of the die, contraction ratio, and entrance angle, as long as the angle is equal to or greater than 90 degrees. This result suggests that CES is, indeed, a material property. The CES was found to be independent of additive content up to a level of 0.5 wt%. Further, it was found that while CES is independent of MW, increasing levels of LCB and broadening MWD increase CES. Finally, brittle fracture at the entrance of the die is proposed as the cause of gross melt fracture.
机译：在聚合物挤出中，当挤出速率增加到超过临界值时，通常会观察到称为总熔体破裂的熔体不稳定性。由于总的熔体破裂限制了生产率，因此已经将大量注意力投入到预测何时发生。但是，对于这种现象的起源仍然没有普遍的了解。对于总熔体破裂（OGMF）的发生，已经提出了一些标准，但是这些标准取决于用于测量它们的程序，因此不是材料性能。在这里提出使用临界拉伸应力（CES）作为OGMF的标准。使用Cogswell和Binding提出的入口压降分析来估算该应力。使用了分离模具以消除由于毛细管的存在而引起的并发症。改变入射角和收缩率以研究模具几何形状的影响。在研究分子结构对OGMF的影响时，使用了各种材料，包括常规的线性低密度和高密度聚乙烯以及由茂金属和受限几何催化剂生产的新型聚乙烯。后一种材料特别受关注，因为它们在分子量（MW），分子量分布（MWD）和长链支化（LCB）方面具有非常明确的分子结构。此外，还研究了颗粒状添加剂（氮化硼和炭黑）的作用。；发现只要角度等于或大于90度，CES就与模具，收缩率和入射角无关。 。该结果表明，CES实际上是一种物质属性。发现CES与添加剂含量无关，直至0.5wt％的水平。此外，发现虽然CES不依赖于MW，但LCB含量的增加和MWD范围的扩大也增加了CES。最后，提出了在模具入口处的脆性断裂是造成总熔体断裂的原因。

著录项

  • 作者

    Kim, Seungoh.;

  • 作者单位

    McGill University (Canada).;

  • 授予单位 McGill University (Canada).;
  • 学科 Engineering Chemical.
  • 学位 Ph.D.
  • 年度 2000
  • 页码 249 p.
  • 总页数 249
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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