Effect of Processing Parameters on Essential Work of Fracture Toughness of LLDPE Blown Films Chin-Fu Lee, 1 Hung-Jue Sue, 1 David M. Fiscus 2 1 Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843 2 ExxonMobil Chemical Company, 5200 Bayway Drive, Baytown, Texas 77522 Blown film process is one of the most attractive proc- esses for producing low-cost, high-performance polyole- fin films. In this study, mode-I essential work of fracture (EWF) analysis and Elmendorf tear test were performed on m-LLDPE films to investigate how different processing conditions influence blown film EWF and tear properties. After the EWF test, the film was carefully characterized, especially within the necked zone. Effects of frost-line height, draw down ratio, blow-up ratio, and haze-zone region on the EWF parameters of the films were deter- mined. Correlation between the EWF parameters and the films’ Elmendorf tear properties was also made. The use- fulness of the EWF test for investigating ductile polymeric blown films fracture performance is discussed. POLYM. ENG. SCI., 55:2403–2413, 2015. V C 2015 Society of Plastics Engineers INTRODUCTION Linear low-density polyethylene (LLDPE) is the most widely used material for film due to its diverse applications such as packaging for snack foods, produce, medical and pharmaceutical products, and stretch and shrink-wraps. The high ductility, strength, durability, and relatively low cost make it attractive for various applications, with 70% of the commercial LLDPE films being produced through the blown film extrusion process [1–3]. Although there has been wide usage of the blown films, few investigations focused on the films’ fracture mechanics and processing–structure–property relationships to-date [4–7]. In the blown film process, molten polymer is extruded through an annular die. Air is fed through an inner tube at the center of the die causing the extruded melt to inflate into a film bubble with diameters several times the die’s diameter, accom- panied by a decrease in film thickness. A concentric air ring cools the film bubble just above the die. The temperature of the melt decreases with increasing distance from the die. The tem- perature reduction increases the viscosity of the melt, induces crystallization, and leads to film solidification [8]. The distance from the die face to where solidification takes place is called the frost-line-height (FLH). At the frost line, the bubble is at its maximum diameter and there is effectively no further stretching. Nip rolls located above the FLH apply tension to the film, pull it away from the die in the machine direction (MD), flatten and collapse the film into a double-layer flat film stock. A single die can make films with many different thicknesses and sizes. The expansion ratio between the die diameter and that of the final film bubble is defined as the blow-up-ratio (BUR). The draw- down-ratio (DDR) is an indicator of the elongation that occurs in the MD, and it is defined as the ratio of die gap to the film’s thickness. The blown film process offers a high level of flexibility for producing a wide variety of high performance films for demand- ing applications with processing variables significantly influenc- ing the film’s morphology and performance. In addition to the resin’s molecular structure, many processing factors, including the melt temperature, speed of cooling, drawing speed, DDR, and BUR, greatly influence the film’s morphology and proper- ties. Blown films generally have a good balance of mechanical properties. Various aspects of the blown film extrusion process have been studied from both modeling and experimental per- spectives [9–12]. Many studies have also attempted to correlate the mechanical properties and microstructures of the polymer film as a function of processing parameters [13, 14]. In packaging application, the plastic film generally experien- ces mode-I and/or mode-III fracture upon loading, where mode- I fracture is usually a weaker mode of fracture. Therefore, it is critical to characterize and improve the film resistance to mode- I fracture. The essential work of fracture (EWF) analysis is a simple, straightforward technique that has been widely utilized for determining the toughness of various ductile polymeric materials, especially for samples in film or sheet forms [15–17]. The EWF method was originally suggested by Broberg [18] and then developed by Cotterell and coworkers [19–21] to character- ize plane-stress fracture toughness of ductile materials. The fun- damental concept of the EWF method is based on the partition of energy, which separates the total fracture energy (W f ) into two components: the EWF (W e ) and the non-EWF (W p ): W f 5W e 1W p 5w e tL1bw p tL 2 (1) w f 5 W f  Lt 5w e 1bw p L (2) where W e represents the energy dissipated in the inner fracture process zone (IFPZ), which is responsible for creation of the fracture surface; W p is the energy dissipated in the outer plastic deformation zone (OPDZ); b is a shape factor associated with the volume of the plastic deformation zone; L is the ligament length; t is the thickness of the specimen (Fig. 1). The specific total work of fracture (w f ) can be obtained by normalizing W f with the cross-sectional area of the ligament where w e is the specific EWF and w p is the specific non-EWF. There is a positive linear dependence between w f and ligament length. The positive intercept (w e ) indicates the resistance to crack propagation, and the slope (i.e., bw p ) indicates the capa- bility of the material to dissipate energy plastically. The Correspondence to: H.-J. Sue; e-mail: hjsue@tamu.edu Contract grant sponsor: ExxonMobil Chemical Company. DOI 10.1002/pen.24129 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2015