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Rheology is the science related to how materials flow. Its usefulness is well known by most extrusion 和 injection molding equipment designers. For example, viscosity curves as a function of shear rate 和 temperature are used to calculate pressure drop in an extrusion die 和, ultimately, to dimension the flow channel for a particular set of processing conditions.

流变学仍然使许多处理器迷惑不解。让’s通过展示处理器提供的信息如何解决现实世界的生产问题来消除这个谜团。
 

Shear Thinning 和 Melt Temperature

聚合物熔体是非牛顿流体,在剪切速率增加时粘度降低。这种粘度降低称为剪切稀化,它取决于聚合物的结构和分子量分布(MWD)。例如,线性聚合物(如LLDPE)的剪切厚度将比具有长链分支的聚合物(如LDPE)的剪切厚度小得多。了解给定的聚合物级剪切稀料如何帮助加工商了解许多挤出问题,因此,学习如何“read”粘度曲线。特别地,粘度曲线的形状作为剪切速率的函数对来自挤出模头的流动分布,通过模头的熔体压力以及螺杆尖端处的熔体温度具有影响。

为了说明最后一点,我们使用计算来计算熔体温度预测 Fluid Dynamics (CFD) in the mixing section of a single-screw extruder for two polymers with comparable viscosity, but vastly different shear-thinning behaviors. The screw 和 its mixer (Fig. 1) were designed 和 optimized for LDPE, which is a very shear-thinning polymer due to long-chain branching 和 broad MWD. Figure 2 shows CFD results for the LDPE grade; pressure through the mixer 和 resulting melt temperature change due to shear heating. The model predicts a pressure drop of about 6 MPa (870 psi), 和 a temperature increase of about 4° C (7.2 ° F) 在搅拌机的出口。

  

 

流变解决挤出问题

FIG 1 专为LDPE设计的挤出机螺杆,其中一台混合机具有四个螺旋锥度深的螺旋通道。

 

了解给定的聚合物级剪切稀料如何帮助加工商了解许多挤出问题,因此学习如何“read” a viscosity curve. 

The same screw 和 mixer were 用于加工茂金属LLDPE(mLLDPE)级,熔体流动指数(MFI)相对可比 the LDPE. Figure 3 显示了两种牌号的粘度曲线的比较,这些牌号的总体粘度相似,但剪切稀化特性却大不相同。因为mLLDPE剪切的稀化度小于LDPE,所以其在高剪切速率下的粘度要高得多。混合机产生高剪切速率(约3000秒-1 over the undercuts) at regular processing conditions. Consequently, Fig. 4 shows that the mLLDPE polymer flows very differently in the mixer 和 results in an unrealistic pressure drop of 19 MPa (2755 psi), 和 temperature increase of about 11° C (19.8° F).

流变解决挤出问题

FIG 2 Predicted pressure field 和 melt temperature through the mixer for LDPE. The outlet
pressure was set to 20 MPa (2900 psi) 和 the inlet melt temperature to 260 C (500 F).

该实施例说明在挤出过程中以及在处理控制熔融温度的问题时,了解粘度曲线的形状很重要。剪切加热由粘度和剪切速率驱动。挤出机螺杆中的剪切流对熔体温度产生重大影响,另一个因素是导电性。 从挤出机机筒传热。更换树脂 即使在相似的MFI值和相同的MFI值下,剪切稀化也有显着差异“family,”像这些聚乙烯牌号一样,可能导致熔体温度明显不同 and pressure 和 extruder torque at the same processing conditions.

流变解决挤出问题

FIG 3 Viscosity curves for LDPE 和 mLLDPE.

因为mLLDPE的剪切厚度小于LDPE,所以其在高剪切速率下的粘度要高得多。

熔体流动不稳定性

在这种情况下,对未知成分的高填充聚合物样品进行了分析,以设计具有由处理器指定的工艺参数的相对薄板的模具。由于填充化合物的性质,选择了毛细管流变仪来研究材料。

流变解决挤出问题

FIG 4 Predicted pressure field 和 melt temperature through the mixer for mLLDPE.
Outlet pressure was set to 20 MPa (2900 psi) 和 the inlet melt temperature is set to 260 C (500 F).

With this technique, the pressure drop in a capillary die is measured for a set of flow rates. Knowing the capillary geometry, the pressure drop can be converted into shear stress at the capillary wall, 和 the flow rate can be converted into apparent shear rate. Viscosity can be determined by the ratio of shear stress to 剪切速率。在该示例中,从高剪切速率到低剪切速率执行测量。图5显示了随时间推移记录的压力测量结果。在同一张图上,我们绘制了等效的表观剪切速率。

流变解决挤出问题

FIG 5 在几种不同的剪切速率下,压力测量值是时间的函数
毛细管流变仪的分析表明,高填充化合物的流动不稳定。

压力测量显示出非常有趣的模式:

  • 对于2000年以来的高表观剪切速率 to 200 sec-1, the flow is slightly erratic 和 unstable as shown by the slight instability in pressure measurements.
  • For apparent shear rates of 100 和 80 sec-1,我们在挤出的线束上观察到粘滑现象,这与流变仪记录的巨大压力波动相对应。
  • 在60秒的表观剪切速率下-1 和 below, the flow 和 the pressure measurements are completely normal.

In this example, we were able to establish a stable/unstable flow processing window for this atypical material. Based on the die gap 和 requested outputs, we were able to make recommendations ahead of the equipment being manufactured. However, if a flow instability is observed in a given process, this type of analysis can be done to establish a stable/unstable flow processing window.
 

热降解

由于温度敏感性,氧气或水分含量或其他因素,某些材料在挤出过程中易于降解。 PVC,PVdC,TPU,一些尼龙 聚酯和聚酯是可能表现出热降解问题的材料示例。 流变学可用于确定特定材料相对于过程的热稳定性。

另外,确定特定的过程停留时间可能是有用的。图6显示了 第三个例子,涉及 尼龙薄膜挤出。在此过程中,材料运行得有点过热,并形成凝胶 在电影中可见。为了确认温度对材料稳定性的影响,我们在旋转流变仪中于250、265和280 C下进行了45分钟的时间扫描测量。我们发现该材料的粘度在所有三个温度下均随时间增加,这表明该材料即使在惰性氮气氛下也具有交联的趋势。虽然在所有三个温度下都可以检测到随着时间推移粘度的增长,但很明显,在280°C时,增长率急剧增加,这表明最好避免在此温度或更高温度下运行材料。

流变解决挤出问题

FIG 6 在这些使用尼龙6膜树脂的旋转粘度计实验中,由于凝胶的缘故,粘度在所有温度下均会随时间增加;但是在最高温度下这种增加更为迅速,这表明该树脂的加工窗口有上限。

流变学可用于确定特定材料相对于过程的热稳定性。

Rheology is usually associated with viscosity curves for a given material. However, it can provide so much more useful information that is critical to equipment designers, product development 恩gineers, 和 processors. Through some examples, we illustrated how shear-thinning can impact extrusion performance 和 particularly melt quality 和 melt temperature for a given extruder configuration.

Another aspect where rheology can be used to troubleshoot an extrusion process is when dealing with melt-flow instabilities, such as melt fracture. This was shown in an example of highly filled formulation for which a die was to be designed 和 manufactured. Without this characterization, costly design mistakes would probably have occurred. Finally, understanding the thermal stability of a material under specific conditions (type of atmosphere, temperature, 和 time) is possible through time-sweep experiments. The data will help determine the extrusion processing window of temperature or residence time.

A关于作者:Olivier Catherine has nearly 20 years of experience in plastics as an expert in polymer processing, rheology 和 process modeling. He is currently corporate scientist for Cloeren公司 in Orange, Tex., where he focuses on die designs 和 implementing new rheological tools 和 flow-simulation capabilities. Catherine graduated in France from Cemef/Mines Paris Tech in 2001, earning 硕士在材料科学。联系人:409-951-7632; ocatherine@cloeren.com; cloeren.com.

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