Analysis of unstable flow behavior in PE⁃UHMW/PE⁃HD blends during capillary extrusion

doi:10.19491/j.issn.1001-9278.2025.11.003

Abstract

Abstract:

This study investigated the unstable flow behavior of ultra⁃high molecular weight polyethylene （PE⁃UHMW）/high⁃density polyethylene （PE⁃HD） blends during capillary extrusion. The surface morphology of extrudates， including pure PE⁃UHMW， pure PE⁃HD， and their blends， was analyzed to characterize flow instabilities at various extrusion temperatures. Key phenomena examined include the critical shear rate for the onset of oscillatory distortion and melt fracture， as well as pressure oscillations and wall slip beyond this critical rate. The results indicated that the extrusion morphology evolved through a sequence of shark skin， oscillatory distortion， and global melt fracture. At an extrusion temperature of 260 ℃， severe pressure oscillations occurred at a shear rate of 116 s⁻¹， accompanied by a dynamic stick–slip transition at the melt–wall interface that induces oscillatory distortion and wall slip. Furthermore， increasing the extrusion temperature delayed the onset of unstable flow， expanding the shear rate range for oscillatory distortion from 116~522 s⁻¹ to 152~686 s⁻¹.

Key words: ultra?high molecular weight polyethylene, capillary extrusion, rheological property, unstable flow

CLC Number:

TQ325.1⁺2

XU Chenglong, WANG Yu, LI Guo, XIE Linsheng, MA Yulu. Analysis of unstable flow behavior in PE⁃UHMW/PE⁃HD blends during capillary extrusion[J]. China Plastics, 2025, 39(11): 13-19.

Figures/Tables 8

Fig.1. Viscosity vs shear rate curves for PE⁃HD， PE⁃UHMW/PE⁃HD blends， and PE⁃UHMW

试样

零剪切黏度η₀/

Pa·s

松弛时间

λ/s

非牛顿指数

Tab.1. Rheological characteristic parameters of PE⁃HD， PE⁃UHMW/PE⁃HD blends and PE⁃UHMW

Fig.2. Material flow curves and evolution micrographs of extrusion morphology

试样	振荡畸变		螺旋熔体破裂	总熔体破裂
试样	临界剪切速率/s^-1	临界剪切应力/kPa	临界剪切速率/s^-1	临界剪切速率/s^-1
PE⁃HD	428	272	774	⁃
PE⁃UHMW/PE⁃HD	116	342	⁃	522
PE⁃UHMW	27	395	⁃	450

Tab.2. Distribution of critical shear rate and critical shear stress for unstable flow

Fig.3. Distribution of critical shear rate for unstable flow at different temperatures

Fig.4. Flow curves of PE⁃UHMW/PE⁃HD blends

Fig.5. Pressure vs time curves during capillary extrusion and rate distribution of viscoelastic⁃to⁃slip transition of melt at pipe wal

Fig.6. Wall slip characteristics parameters critical wall slip velocityvscand critical slip extrapolation length bc vs extrusion temperature curve

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