Rheology of LLDPE, LDPE and LLDPE/LDPE blends and its relevance to the film blowing process Predrag Micic 1,2 * and Sati N Bhattacharya 1 1 Rheology and Materials Processing Centre, Royal Melbourne Institute of Technology (RMIT University) Melbourne, Australia 2 Qenos Australia, 611–629 Kororit Creek Road, Altona, Vic 3018, Australia Abstract: The relevance of polymer melt rheology in ®lm blowing process for linear low-density polyethylene (LLDPE) and its blends with three different low-density polyethylenes (LDPEs) has been discussed. The effect of different LDPE components as well as their concentration on shear and elongational viscosity has been investigated. A good correlation has been observed between the extensional rheological parameters of LDPEs measured by different experimental techniques. The molecular structure of parent polymers as well as blend composition play an important role in the rheology of these blends and consequently their performance in the ®lm blowing process. # 2000 Society of Chemical Industry Keywords: polyethylene blends; linear low-density polyethylene; elongational viscosity; shear viscosity; ®lm blowing INTRODUCTION Since its emergence, linear low-density polyethylene (LLDPE) has been challenging low-density polyethy- lene LDPE for market share due to the economic and technological advantages of the manufacturing process as well as to the more superior end-use performance compared with LDPE. However, the supremacy of LLDPE over LDPE, particularly in ®lm applications, has been diminished to some extent by dif®culties during the processing of LLDPE. Higher torque and extrusion pressures are commonly encountered while running LLDPE compared with LDPE intended for the same use. In addition, the much lower melt tension of LLDPE presents a signi®cant problem during the shaping phase of the end product. 1 Higher extrusion torque and pressures while run- ning LLDPE are believed to originate from the higher shear viscosity compared with LDPE. 2 The poor performance of LLDPE in the blowing phase of ®lm making is thought to be related to the low elongational viscosity of LLDPE mostly due to the linear structure of its macromolecules. In contrast, LDPE performs well in such processes due to its high elongational viscosity related to the presence of long chain branched macromolecules in its structure. 3±6 In the past signi®cant effort has been directed towards understanding the effect of the polymer macromolecule structure on both shear and elonga- tional viscosity 7±15 of polyethylenes. A correlation between shear and elongational viscosity and proces- sing behaviour of polymers in the ®lm blowing process has been established. 16±21 It is now recognized that the extruder power consumption and extrusion pressures are the re¯ection of the shear viscosity of polymers. However, the bubble instability problems in ®lm blowing are dictated by the elongational rheology behaviour of the polymer melt, as measured by its elongational viscosity. A qualitative relationship be- tween the elongational viscosity (ie measured in a uniaxial stretching apparatus) and the bubble stability and ®lm thickness uniformity has now been achieved. 22,23 In an attempt to achieve the optimal balance between the processability and end-product proper- ties, the majority of LLDPE used in the ®lm blowing process is blended with a small percentage of LDPE (10±30%). This work investigates how different LDPEs and their blends with LLDPE affect rheology and subse- quently in¯uence the ®lm blowing process. EXPERIMENTAL Materials The LLDPE and LDPE samples used in this work were the commercial grade polymers produced by Qenos, Australia. The gel permeation chromatogra- phy (GPC) traces and calculated values for molecular parameters are given in Fig 1 and Table 1, respec- tively. Melt ¯ow index (MFI) values are also included in the table. The LDPEs were made by autoclave reactor (Received 1 September 1999; revised version received 4 May 2000; accepted 31 May 2000) * Correspondence to: Predrag Micic, Qenos Australia, 611–629 Kororoit Creek Road, Altona, Vic 3018, Australia # 2000 Society of Chemical Industry. Polym Int 0959±8103/2000/$30.00 1580 Polymer International Polym Int 49:1580±1589 (2000)