Polyolefins have traditionally been polymerized via the heterogeneous Ziegler Natta (Z-N) catalysts since their discovery in the 1950s, but the later developments of the homogeneous single site metallocene/methylaluminoxane (MAO) catalysts have opened the possibility for the synthesis of polyolefin polymers and copolymers (commercialized in the 1990s) with highly defined microstructure and great degree of control of their molecular architecture resulting in some unique beneficial properties compared to their Z-N counterparts. The main aim of the work reported here was to examine critically the effects of the processing conditions during multipass extrusions on the molecular and rheological characteristics of metallocene-catalyzed linear low density polyethylene and a Ziegler-Natta-based counterpart polymer and on the thermoxidative (in the melt) stability of their melts and their stabilization. Both polymers examined here were based on ethylene-1-hexene copolymers and produced by the same manufacturer with similar density and melt index (MI) values. The melt stability and rheological properties of the polymers were determined from melt flow and capillary rheometric measurements. The molecular characteristics were investigated using GPC for molecular weight determination, ¹³C-NMR for short chain branching, ¹H-NMR and FTIR for the type and concentration of unsaturation groups, FTIR for carbonyl-containing compounds and chemical analysis for hydroperoxide determination. The processibility of the two polymers was compared through examination of different processing conditions (die temperatures 210–285°C, screw speeds 50–200 rpm) used for a multipass extrusion process. Further, the effects of the processing conditions on the melt thermoxidation of the differently catalyzed LLDPE polymers were investigated. The results demonstrated that both the catalyst type and the processing conditions have a great effect on both the rheological and the themoxidative behavior of the polymers as well as on the balance between the competing oxidative reactions taking place in the polymer melt, for example, chain scission and crosslinking reactions. The results obtained for the different multipass extrusion conditions of the two polymers have illustrated clearly that the degradation mechanisms during their melt processing are substantially different. The melt degradation route of the Ziegler-catalyzed polymer was shown to be dominated by chain scission reactions under all the processing conditions examined, and more so at the higher shear rates and temperatures. By contrast, the evidence clearly showed that the metallocene-catalyzed polymer gave rise to crosslinking reactions that predominated even under the more severe processing conditions, that is, higher temperatures and shear rates.

中文翻译：

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加工条件和催化剂类型对茂金属和齐格勒催化乙烯-1-己烯共聚物热氧化降解机理和熔体稳定性的影响

本文报道的工作的主要目的是严格检查多道次挤压过程中的加工条件对茂金属催化的线性低密度聚乙烯和齐格勒-纳塔基对应聚合物的分子和流变特性以及热氧化（在熔体）其熔体的稳定性及其稳定性。此处检查的两种聚合物均基于乙烯-1-己烯共聚物，由同一制造商生产，具有相似的密度和熔体指数 (MI) 值。聚合物的熔体稳定性和流变性能由熔体流动和毛细管流变测量确定。使用 GPC 进行分子量测定研究了分子特征，此处检查的两种聚合物均基于乙烯-1-己烯共聚物，由同一制造商生产，具有相似的密度和熔体指数 (MI) 值。聚合物的熔体稳定性和流变性能由熔体流动和毛细管流变测量确定。使用 GPC 进行分子量测定研究了分子特征，此处检查的两种聚合物均基于乙烯-1-己烯共聚物，由同一制造商生产，具有相似的密度和熔体指数 (MI) 值。聚合物的熔体稳定性和流变性能由熔体流动和毛细管流变测量确定。使用 GPC 进行分子量测定研究了分子特征，¹³ C-NMR 用于短链支化，¹H-NMR 和 FTIR 用于不饱和基团的类型和浓度，FTIR 用于含羰基化合物，化学分析用于氢过氧化物测定。通过检查用于多道次挤出工艺的不同加工条件（模具温度 210–285°C，螺杆速度 50–200 rpm）来比较两种聚合物的加工性能。此外，研究了加工条件对不同催化 LLDPE 聚合物熔体热氧化的影响。结果表明，催化剂类型和加工条件对聚合物的流变和热氧化行为以及聚合物熔体中发生的竞争性氧化反应（例如断链）之间的平衡都有很大影响和交联反应。两种聚合物在不同多道次挤出条件下获得的结果清楚地表明，它们在熔融加工过程中的降解机制有很大不同。Ziegler 催化的聚合物的熔体降解路线在所有检查的加工条件下都以断链反应为主，在更高的剪切速率和温度下更是如此。相比之下，证据清楚地表明，即使在更苛刻的加工条件，即更高的温度和剪切速率下，茂金属催化的聚合物也会产生占主导地位的交联反应。Ziegler 催化的聚合物的熔体降解路线在所有检查的加工条件下都以断链反应为主，在更高的剪切速率和温度下更是如此。相比之下，证据清楚地表明，即使在更苛刻的加工条件，即更高的温度和剪切速率下，茂金属催化的聚合物也会产生占主导地位的交联反应。Ziegler 催化的聚合物的熔体降解路线在所有检查的加工条件下都以断链反应为主，在更高的剪切速率和温度下更是如此。相比之下，证据清楚地表明，即使在更苛刻的加工条件，即更高的温度和剪切速率下，茂金属催化的聚合物也会产生占主导地位的交联反应。