Oriented structure and anisotropy properties of polymer blown films: HDPE, LLDPE and LDPE

Different types of polyethylene blown films (HDPE, LDPE, LLDPE) differ significantly in the ratio between machine and transverse direction tear resistance. In this paper, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) blown films at different draw-down ratios are studied, and the relation between crystalline structure and anisotropy of blown film properties is investigated. The crystalline morphology and orientation of HDPE, LDPE, LLDPE blown films were probed using microscopy and infrared trichroism. Significant differences in crystalline morphology were found: at medium DDR HDPE developed a row-nucleated type morphology without lamellar twisting, LDPE showed rod-like crystalline morphology and turned out to the row-nucleated structure with twisted lamellae at high draw-down ratio (DDR), while a spherulite-like superstructure was observed for LLDPEs at all processing conditions. They also showed quite different orientation characteristics corresponding to different morphologies. The morphologies and orientation structure for LDPE, LLDPE and HDPE are related to the stress applied (DDR) and their relaxations in the flow-induced crystallization process, which determine the amount of fibrillar nuclei available at the time of crystallization and therefore, the final crystalline morphology. These structure differences are shown to translate into different ratios of machine and transverse direction tear and tensile strengths.     

Characterisation of injected EPBC plaques using the essential work of fracture (EWF) method

The influence of processing induced morphology, thickness and ethylene content (EC) of different ethylene-propylene block copolymers on fracture properties has been studied using the essential work of fracture (EWF) method. To analyse the influence of EC, four different materials were chosen with 0, 5.5, 7.8, and 8.4% in weight EC. Each material was injected in three different thicknesses (1, 2 and 3 mm). The resulting plaques were tested using the EWF method in both main orientations; the melt flow direction and transverse to melt flow direction. Different fracture behaviours have been observed, some of them preventing the applicability of the EWF method. Polarised light microscopy observations have revealed the existence of a skin/core structure, which is reduced with an increase in thickness and EC.     

An essential work of fracture study of the toughness of thermoset polyester coatings

The fracture toughness of a range of thermoset polyester paints with different cross-link densities has been studied, using the essential work of fracture (EWF) method. The glass transition temperature, T_g, of each of the materials was measured using differential scanning calorimetry, and found to lie between 8 and 46 °C. EWF tests were performed on the paint films at a range of temperatures around the measured glass transition temperature of each material. The essential work of fracture, w_e, at T_g was found to decrease with increasing cross-link density from around 20 kJ/m² at a cross-link density of 0.4 × 10⁻³ mol/cm³ to around 5 kJ/m² for cross-link densities of approximately 1 × 10⁻³ mol/cm³ or higher. A maximum in the essential work of fracture was observed at around T_g when w_e was plotted versus temperature, which could be attributed to the effect of an α-relaxation at a molecular level. The polyesters were found to be visco-elastic, and the applicability of the EWF test to the study of these visco-elastic thermoset materials is discussed.     

Orientation characteristics of LLDPE blown films and their implications on Elmendorf tear performance

The orientation features of several linear low-density polyethylene (LLDPE) blown films were characterized and significant insights into the morphological origin of Elmendorf tear resistance were developed. The orientation features of all the LLDPE blown films investigated were described in terms of the Keller–Machin “row” structure. The machine direction (MD) tear resistance was observed to be higher when the non-crystalline chains were closer to equi-biaxial in the plane of the film. Further, the transverse direction (TD) tear resistance was observed to be high when the crystalline lamellae were minimally curved and oriented closer to the film TD. These results indicated that deformations in the interlamellar region and the stresses borne along the lamellar long axes play important roles in distinguishing the MD and TD tear resistances, respectively, of LLDPE blown films.     

Effects of molecular structure on the essential work of fracture of amorphous copolyesters at various deformation rates

The fracture behavior of amorphous copolyesters with different molecular structure was studied with double edge notched tensile loaded specimens (DENT) using the essential work of fracture (EWF) approach. Various deformation rates ranging from 1 to 1000 mm/min were employed. Amorphous poly(ethylene terephthalate) (aPET) exhibited considerably higher specific essential and non-essential work of fracture than the copolyesters containing either cyclohexylenedimethylene (aPET-C) or neopentyl glycols (aPET-N). At high deformation rates, ductile/brittle fracture transition was observed with aPET-C and aPET-N, while aPET always fractured in ductile mode within the entire deformation rate range. These phenomena were ascribed to the different molecular flexibility and entanglement density of the copolyesters. The specific EWF of the aPET as a function of deformation rate went through a minimum. The initial decrease in toughness was caused by the hampered segmental mobility due to the increased deformation rate. The subsequent increase in toughness was attributed to the adiabatic heating induced temperature rise in the process and plastic zones. Strain-induced crystallization of the aPET was observed at ν=500 and 1000 mm/min, which may also contribute to the increase of the specific EWF.     

The application of the essential work of fracture methodology to the plane strain fracture of ABS 3-point bend specimens

The applicability of the EWF methodology to 3-point bend (SEB) specimens under conditions other than plane stress has been assessed experimentally. Different fracture conditions, pure plane strain and plane strain/plane stress transition, were obtained by varying the specimen thickness and testing temperature (20 and 80 °C). Post-mortem fracture surfaces appeared always completely stress-whitened, indicating ductile fracture. The load-line displacement plots are similar over a well-defined range of ligament lengths for which the application of the EWF methodology was in principle possible. Nevertheless, in experiments conducted at room temperature, crack growth was observed to initiate before maximum load and complete ligament yielding. This behaviour was confirmed through plastic collapse analyses. A critical ligament length was found, over which the total specific work of fracture was dominated by edge effects. Below this critical ligament length, EWF methodology was still applicable and it was possible to extrapolate reliable w_Ie values.     

Plane stress fracture toughness of physically aged plasticized PETG as assessed by the essential work of fracture (EWF) method

The plane stress fracture toughness of amorphous copolyester (PETG) sheets plasticized by various amount of neopentylglycol dibenzoate (NPGDB in 0, 5, 10 and 20 wt%) was studied in as-received (AR) and rejuvenated (RJ) states by adopting the essential work of fracture (EWF) method. EWF tests were performed on deeply double-edge notched tensile loaded (DDEN-T) specimens at various deformation rates (2,10 and 100 mm/min) at room temperature. It was established that physical aging strongly affected the EWF terms. The specific yielding-related EWF increased with increasing deformation rate and decreased with increasing plasticizer content. The specific non-essential work and its necking-related constituent, which changed parallel to each other, remained constant up to 10 wt% NPGDB content and decreased afterwards. The plastic zone in the DDEN-T specimens was formed by cold drawing which is governed by the entanglement structure. This was demonstrated by the shape recovery of the plastic zone in the broken DDEN-T specimens after heating them above the T_g of the related PETG compound.     

Effect of the specimen dimensions and the test speed on the fracture toughness of iPP by the essential work of fracture (EWF) method

The fracture parameters of an isotactic polypropylene are studied by the essential work of fracture method. The influence of the specimen height, width and thickness and the effect of the test speed are investigated. Results show that this method is very useful for studying the plane‐stress fracture of this kind of materials in form of films and sheets. Varying the width (30 to 60 mm) and the test speed (2 to 100 mm/min) has no relevant influence, whereas the results are only length independent in a range from 40 to 100 mm. The influence of the thickness is very high, obtaining an important decrease of the specific essential work as the thickness is increased in a range from 38 to 2500 μm. This result is justified with the fracture surfaces obtained, observed by SEM, in which an evolution of the fracture behavior is seen as a function of thickness (38, 100, 500, 1000, 2500 μm). © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 177–187, 1999     

Preparation of HDPE/UHMWPE/MMWPE blends by two-step processing way and properties of blown films

Summary Blends of high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE) were prepared by two-step processing way. Middle molecular weight polyethylene (MMWPE) as a fluidity modifier and compatilizer was added into UHMWPE in the first step, and then modified UHMWPE and HDPE were blending extruded to prepare the HDPE/UHMWPE/MMWPE blends used for blown films. The mechanical test of the blown films revealed that when the content of MMWPE in modified UHMWPE was 40wt% and the content of UHMWPE in the blends was 20 wt%, the film had the optimal mechanical properties. The tensile strength and tear strength of the film increased by 50% and 21%, respectively, compared with those of pure HDPE film. Rheological curves indicated that the melt torque and the apparent viscosity of the HDPE/UHMWPE/MMWPE blends made by two-step processing way both greatly reduced than other blends. The results from DSC suggested that the blends by two-step processing way may form more stable and perfect co-crystallization. PLM (polarized light microscopy) and SEM micrographs revealed that two-step processing way can improve the surface morphology of the films and make the dispersion of UHMWPE particles in HDPE increase.     

Effect of processing conditions on the orientation and properties of polyethylene blown film

The amorphous and crystallite orientation in a number of films produced from characterised grades of high density polyethylene has been examined and some tensile properties measured. The films were produced under commercial production conditions with various blow ratios, freeze line heights and thicknesses. Orientation was assessed using optical and X-ray methods. The relative contributions of the crystallite and amorphous components were calculated for a limited number of the films produced. Amongst the properties measured the ultimate tensile strength and elongation at break appeared to be the most variable. Birefringence was more affected by variation in blow ratio and film thickness than by freeze height changes. Attempts have been made to correlate these variations with crystallite and amorphous orientation as well as the individual behaviour of the polyethylene grades. In carrying out this correlation it became clear that the films could not generally be described by established low or high stress models of crystallite orientation. For these films produced on full-scale commercial equipment it appears that transcrystallisation behaviour is frequently observed, with a tendency towards the low stress model.     

Morphological considerations on the mechanical properties of blown high-density polyethylene films

Blown films having a broad range of morphologies were prepared from high-density polyethylenes (HDPE) with unimodal and bimodal molecular weight distribution under several processing conditions, and the effect of their morphological features on the dart drop impact resistance, Elmendorf tear strength, and tensile properties of the films has been studied. The organization of lamellar stacks seems to play a critical role on the mechanical properties of the blown HDPE films. The dart drop impact resistance of the blown HDPE films is highly dependent on the presence of the network structure of lamellar stacks and the level of the intraconnectivity and interconnectivity of lamellar stacks. The coherent orientation of lamellar stacks leads to significant anisotropy of tear and tensile properties. © 1997 John Wiley & Sons, Inc.     

Structure and properties of starch/poly(ethylene-co-vinyl alcohol) blown films

Blends of native corn starch and poly(ethylene-co-vinyl alcohol) (EVOH), with starch: EVOH ratios of 1 : 1 (SE-50) and 2 : 1 (SE-67A, SE-67B), were processed into blown films. SE-67A had a higher glycerol and water content and was processed at 5°C higher than was SE-67B. The films were conditioned to various moisture contents by equilibrating at a constant relative humidity and by oven drying at 41°C. Equilibrium moisture content, which ranged from 2 to 11%, increased with increasing starch content at a given relative humidity. Mechanical properties depended strongly on starch and moisture content as well as on processing history. The extension to break of SE-50 was only about one-third that of EVOH, while that of the 2 : 1 blends was even lower. SE-67A exhibited a higher extension to break, lower tensile strength and modulus, and greater moisture sensitivity than those of SE-67B. Differential scanning calorimetry and dynamic mechanical analysis revealed evidence of interactions between starch and EVOH, probably indicative of extensive intermixing but not necessarily miscibility. Scanning electron micrographs of fracture surfaces revealed extensive differences in texture with microcracking in SE-50 and SE-67A. The combination of the analytical results provide a basis for explaining many aspects of the mechanical behavior including the marked difference in properties between SE-67A and SE-67B. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2613–2622, 1997     

Determination of plane‐strain fracture toughness of polyethylene copolymer based on the concept of essential work of fracture

Toughness and deformation capacity of six polyethylene copolymers in plane‐strain fracture were characterized using the concept of essential work of fracture (EWF). Two types of regression analysis were considered: one based on the traditional, total work of fracture, while the other on the energy partition to extract the portion that is relevant to the plane‐strain fracture. In particular, the latter analysis excludes energy that is for the final stretch of the surface flanks and produces toughness values that are smaller than those determined based on the total work of fracture. The study found that two types of analysis rank the copolymers in different orders. Based on the results from the energy‐partition approach, the article discusses the influence of material characteristics (molecular weight, branch concentration, density, etc.) on the plane‐strain fracture toughness of the polyethylene copolymers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Influence of annealing treatments on the essential work of fracture of biaxially drawn poly(ethylene terephthalate)

The biaxial sequential stretching process of poly(ethylene terephthalate) produces films with a fibrillar microstructure in which fibrils are parallel to the transverse extrusion direction. The mechanical properties of such films are strongly anisotropic due to both the orientation of crystallites and the properties of the intrafibrillar and interfibrillar amorphous phases. The idea is to modulate the properties of the amorphous phase without altering the fibrillar structure by annealing treatments. The morphology (crystallinity and orientation of the crystalline phase) of the annealed films is characterized and their mechanical properties (tensile tests and essential work of fracture) are tested in the longitudinal direction (parallel to the micro fibrils) and in the transverse direction (perpendicular to the micro fibrils). The crystalline phase orientation is the key parameter governing modulus anisotropy. Concerning crack propagation, annealing treatments lead to opposite evolution of the specific essential work of fracture parameter (w_e) in the longitudinal and transverse directions. Thus, it is possible to erase fracture propagation anisotropy through an adequate annealing treatment. Copyright © 2012 Society of Chemical Industry     

Blown films of nanocomposites prepared from low density polyethylene and a sodium ionomer of poly(ethylene-co-methacrylic acid)

A detailed study of the performance of blown films prepared from nanocomposites based on LDPE and a sodium ionomer of poly(ethylene-co-methacrylic acid) is reported. The organoclay content and film blowing conditions were varied to determine the effect of platelet concentration, exfoliation and orientation on film properties. Mechanical properties including stiffness, puncture resistance, and resistance to tear propagation were evaluated and compared to corresponding properties of unfilled polymer films. Permeability of the films to moisture and common atmospheric gases like oxygen, nitrogen, and carbon dioxide was also measured using standard testing methods.In general, films prepared from nanocomposites based on the ionomer exhibited greater improvements in mechanical and barrier properties over unfilled polymer compared to similar films prepared from nanocomposites based on LDPE. This is due to the greater degree of organoclay exfoliation achieved in the ionomer compared to LDPE. The addition of 3 wt% MMT to the ionomer increased the tensile modulus of blown films by an average of 50% without sacrificing much tear strength, puncture resistance or film extensibility. Gas permeability in these films was lowered by 40% and moisture transmission rate was reduced by 60%.     

Fracture characterization of low‐density polyethylenes by the essential work of fracture: Changes induced by thermal treatments and testing temperature

The tensile deformation and fracture behavior of commercially available low‐density polyethylene (LDPE) films, having different molecular characteristics, was studied. Submitting samples to specific thermal histories controlled the morphological structure of these semicrystalline polymers. Phase‐structure analysis of the resulting materials was performed by DMA and DSC analyses. The plane‐stress essential work of fracture methodology was chosen because the materials used had failed after complete necking of the remaining ligament. Significant differences in behavior, induced by thermal treatments, were found for the tensile yield stress and the specific nonessential work of fracture, but not in the specific essential work of fracture. The results show that the mechanical properties and fracture behavior depend not only on the crystallinity levels and molecular weight characteristics of the samples, but also upon the degree of structural continuity. The β‐relaxation process, associated with the crystal‐amorphous interphase, strongly influences the fracture behavior at testing temperatures chosen below the β‐relaxation temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 781–796, 1999     

Toughness assessment of elastomeric polypropylene (ELPP) by the essential work of the fracture method

The essential work of the fracture (EWF) method was employed to determine the fracture performance of thermoplastic polypropylene (PP) elastomers. Three types of elastomeric polypropylene (ELPP) of homo- and copolymer nature based on different catalysts were involved in this study. Tests were carried out in both I and III fracture modes to check the applicability of the EWF approach for such elastomers. It was found that the trouser tearing test (mode III) overestimates both the specific essential (w_e) and plastic work (w_p) terms when the tearing resistance of the ELPP is higher than the resistance to tensile loading. With decreasing crystallinity (i.e., by decreasing length of the stereoregular chain segments of the block copolymers or increased content of comonomer) w_e increased, whereas in respect to w_p,, an opposite tendency was found. This was interpreted by possible changes in the thermoreversible network structure of the ELPP in which crystalline domains act as network knot points in the amorphous PP matrix. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 873–881, 1998     

Electrical conductivity and self-temperature-control heating properties of carbon nanotubes filled polyethylene films

Electrical properties of polyethylene and carbon nanotube composite films were investigated, when the composite films were set in heating box or under electric field at constant voltage. The composite films were prepared by gelation/crystallization from dilute solution. The mixture of ultra-high molecular weight polyethylene (UHMWPE) and branched low molecular weight polyethylene (LMWPE) was used as matrix, and multi-walled carbon nanotubes (MWNTs) were used as fillers. The filler content was chosen to be 10 wt% (ca. 5.25 vol%) which is a relatively higher loading than the percolation threshold to ensure to act as heating element in plane heater of composite film. The focus was concentrated on the temperature dependences of electric conductivity by external heating and by exothermic effect concerning self-temperature-control heating properties which were measured for the three kinds of UHMWPE-LMWPE composites with the same content of MWNTs in the composites. When a certain voltage was applied to the composite, the surface temperature of film reaches the equilibrium value within less than 100 s. The maximum surface temperature as the equilibrium state of the resultant composite film can be easily controlled by adjusting the composite ratio represented as UHMWPE/LMWPE. The high efficiency of heating and wide adjustability of stable temperature suggested its good application in high efficient plane heater.     

Comparison of fracture behavior of nylon 6 versus an amorphous polyamide toughened with maleated poly(ethylene-1-octene) elastomers

The fracture behavior of an amorphous polyamide (Zytel 330 from DuPont), a-PA, and nylon 6 toughened by maleated poly(ethylene-1-octene) elastomers are reported. The deformation mechanisms during fracture were verified by examining an arrested crack tip and the surrounding regions using transmission electron microscopy analysis. a-PA blends show higher levels of impact strength and lower ductile-brittle transition temperatures than nylon 6 blends. Fracture toughness, characterized by both linear elastic fracture mechanics techniques in terms of the critical strain energy release rate, G_IC, and the essential work of fracture methodology, i.e. the limiting specific fracture energy, u_o, and the dissipative energy density, u_d, using thick (6.35 mm) samples with sharp notches, depends on ligament length, rubber content, rubber particle size and test temperature. In general, a-PA blends show larger values of u_d than do nylon 6 blends while the opposite is seen for u_o. The amorphous polyamide shows a similar critical upper limit on rubber particle size, or interparticle distance, for toughening as the semi-crystalline nylon 6; thus, it is clear that the crystal morphology around the rubber particles must not be the dominant cause of this critical size scale. The deformation mechanisms involved include cavitation of rubber particles followed by some crazing and then massive shear yielding of the matrix.     

Surface properties of polyethylene grafted with triallyl cyanurate in the presence of an electron beam

The surface energy and thermodynamic work of adhesion of polyethylene grafted with triallyl cyanurate in the presence of an electron beam have been determined. The surface energy increased with the grafting level and with the irradiation dose up to 10 Mrad. A similar trend was observed with the work of adhesion. The changes in these surface properties were correlated with the concentration of the polar groups as measured by IR (infrared) spectroscopy and ESCA (electron spectroscopy for chemical analysis) studies.