Properties of uniaxially stretched polypropylene films

Polypropylene (PP) films have been prepared through two different cast extrusion processes: one using a machine direction orientation (MDO) unit and the other stretching the films at the die under high cooling conditions (lab unit). Films for two PP resins different in molecular structure have been prepared using both processing techniques. The effect of the resin structure and the processing conditions on the film properties has been examined. It was found that the MDO unit generated a highly oriented fibrillar crystalline structure with a distribution of elongated thick fibrils while extrusion under high cooling conditions generated an oriented row nucleated lamellar structure. The films showed distinctive tensile responses in stretching, with a strong solid‐elastic response for the oriented MDO films and a steady strain hardening after yielding for the sample obtained from lab unit cast extrusion. It was found that the strength in the transverse direction (TD) was particularly very low for the oriented MDO films made of the bimodal PP. The oxygen permeability was reduced with increasing draw ratio (DR) for the MDO films. The haze property for the MDO samples reduced to a plateau for DR up to 5 while clarity improved continuously with DR.     

Influence of precursor film microstructure on properties of HDPE microporous membranes prepared by stretching

Oriented HDPE films having a stacked lamellar morphology were prepared to develop microporous membranes through cast film extrusion followed by stretching. Applying higher draw ratios (DR) and annealing the cast films improved crystal alignments remarkably. It was shown the improvement in c‐axis orientation upon annealing was more significant for the films prepared with lower DR. Furthermore, applying Raman spectroscopy, the amorphous mass fraction of the films was obtained. Subsequently, considering a three‐phase structure (i.e., crystal, amorphous and interphase), the interphase content of the films was also determined. A distinct DR dependence of the interphase content was noticed. It was observed that in addition to crystal perfection upon annealing, the amorphous phase fraction was also decreased due to possibly increase in free volume, promoting lamellar separation and cavitation during the subsequent stretching step. SEM images of the membranes surfaces as well as their normalized water vapor transmission rate (WVTR) values indicated a more uniform pore formation and increased permeability with DR. Furthermore, it was found that lower interphase content in the films with higher DR favors lamellar separation. However, permeability in the stretched precursor films with DR higher than a certain level (DR = 93) was not improved significantly. Moreover, crystallinity and crystal orientation of produced membranes were investigated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44725.     

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 MDO stretched polypropylene

Two polypropylene cast films of different crystalline structures (one with coexisting small rows of lamellae and spherulites and the other with only a spherulitic structure) were prepared by extrusion. The produced cast films were uniaxially hot drawn at T = 120 °C using a machine direction orientation (MDO) unit and the changes in structure and morphology were examined and related to barrier as well as tear and puncture properties. Structural changes in terms of the degree of crystallinity and crystal size distribution, orientation of the amorphous and crystalline phases, and the deformation behavior at the crystal lattice and lamellae scales were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), respectively. A significant effect of the original crystal morphology on the alignment of the amorphous and crystalline phases was observed from FTIR and WAXD. The results also revealed that the deformation behavior of the crystal structure was dependent on the draw ratio (DR). Our findings showed that by increasing DR the crystal lamellae first broke up and oriented along the drawing direction and then, at large DR, they were deformed and created a fibrillar structure. Morphological pictograms illustrating the effects of original morphology and draw ratio on the stretched film microstructure are proposed. The tear resistance along the machine direction (MD) decreased significantly with increasing DR whereas the puncture resistance increased drastically. Finally, the oxygen transmission rate (OTR) of the MDO stretched films could be correlated with the orientation parameters as well as the β-relaxation peak magnitude of the amorphous tie chains.     

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.     

Tensile properties of linear low density polyethylene (LLDPE) blown films

Various linear low‐density polyethylene (LLDPE; density ～ (0.920 g/cm³)) resins that encompass those polymerized using Ziegler‐Natta, metallocene, and chromium oxide based catalysts were blown into film at similar process conditions, and the tensile properties of the resulting films were investigated in relation to their orientation characteristics. The tensile properties of the subject blown films were observed to be significantly different from those of isotropic/un‐oriented polyethylene specimens of similar density (crystallinity). Further, the tensile properties were different in the machine and transverse directions. These were explained in terms of the orientation and lamellar organization characteristics of the LLDPE blown films. Investigation of the temperature dependence (between ?50°C and +50°C) of these tensile properties indicated an increase in modulus, yield stress and break stress with decreasing temperature pointing to the possible role played by the decreased mobility of the non‐crystalline phase at lower temperatures. Excellent correlation between the Elmendorf tear properties of the LLDPE blown films and their tensile yield characteristics was observed. This added substantial credibility to previous hypotheses that specimen stretching plays a significant role in Elmendorf tear tests and further supported the previously identified structural features and microstructural deformation mechanisms that are deemed responsible for the discernment of LLDPE blown film tear resistance.     

Effect of processing on the crystalline orientation, morphology, and mechanical properties of polypropylene cast films and microporous membrane formation

Cast films of a high molecular weight linear polypropylene (L-PP) were prepared by extrusion followed by stretching using a chill roll. An air knife was employed to supply air to the film surface right at the exit of the die. The effects of air cooling conditions, chill roll temperature, and draw ratio on the crystalline orientation, morphology, mechanical and tear properties of the PP cast films were investigated. The crystallinity and crystal size distribution of the films were studied using differential scanning calorimetry (DSC). It was found that air blowing on the films contributed significantly to the uniformity of the lamellar structure. The orientation of crystalline and amorphous phases was measured using wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The amount of lamellae formation and long period spacing were obtained via small angle X-ray scattering (SAXS). The results showed that air cooling and the cast roll temperature have a crucial role on the orientation and amount of lamellae formation of the cast films, which was also confirmed from scanning electron microscopy (SEM) images of the films. Tensile properties and tear resistance of the cast films in machine and transverse directions (MD and TD, respectively) were evaluated. Significant increases of the Young modulus, yield stress, tensile strength, and tensile toughness along MD and drastic decreases of elongation at break along TD were observed for films subjected to air blowing. Morphological pictograms are proposed to represent the molecular structure of the films obtained without and upon applying air cooling for different chill roll temperatures. Finally, microporous membranes were prepared from annealed and stretched films to illustrate the effect of the PP cast film microstructure on the morphology and permeability of membranes. The observations of SEM surface images and water vapor transmission rate of the membranes showed higher pore density, uniform pore size, and superior permeability for the ones obtained from the precursor films prepared under controlled air cooling.     

Comparative study on development of structural hierarchy in constrained annealed simultaneous and sequential biaxially stretched polylactic acid films

Structural evolution during constrained annealing of PLA films biaxially stretched in simultaneous and sequential biaxial stretching was compared. Annealing of simultaneous biaxially stretched films yields films with in-plane isotropy with (100) crystallographic planes parallel to the surface. The first stage of sequential stretching where the films are stretched in Uniaxial constrained mode was found to yield films exhibiting transverse isotropy. The transverse stretching of these films lead to formation of a distinct second population of chains primarily oriented in transverse direction, generating bimodal orientation texture. When the extent of stretching in two directions are balanced, constrained annealed samples were found to exhibit uniplanar axial (100)[001] texture with the primary chain axial direction now switched to transverse direction. Two new superstructures, along [110] and [100] respectively were discovered in annealed PLA films.     

Preparation of biaxially oriented polyamide 6/polyketone/graphene oxide films with enhanced barrier and mechanical behaviors

In this study, biaxially oriented polyamide 6/polyketone/graphene oxide (PA6/PK/GO) films were prepared by melt blending then simultaneously biaxially stretched process, with the aim of obtaining high barrier properties films and improvements in their mechanical properties. The oxygen transmission rate of biaxially oriented PA6/PK/GO film significantly decreased with addition of polyketone and GO. It is surprising that the biaxially oriented process can excellently improve the barrier properties of biaxially oriented PA6/PK/GO film. For example, there was 94.7% OTR reduction of the film containing 20 wt% PK and 0.08 wt% GO compared with PA6 film at a stretching ratio of 3.3 × 3.3. It is due to more tortuous permeation path of oxygen molecule owing to molecular orientation during biaxially stretching, and higher relative crystallinity with addition of GO. The tensile strength of film was remarkablly improved by stretching orientation and increase GO concentration. However, the elongation at break of film was considerably reduced by increase of stretching ratio. Although addition of GO may slightly improve the elongation at break of film at low stretching ratio, there was dramatic decline of elongation at break with increasing the content of GO at a stretching ratio of 3.3 × 3.3.     

Plastic deformation and tearing in high density polyethylene blown films

Polymer films produced by tubular film blowing have a unique morphology that results from the large elongational flow in melt draw down and biaxial orientation due to bubble blow-up. Three high density polyethylene (HDPE) blown films were produced under similar processing conditions from resins which varied principally in molecular weight (MW) and molecular weight distribution (MWD). Scanning electron microscopy (SEM) showed that the lower MW and narrower MWD resin produced film which had a uniaxial orientation of stacked lamellar crystals. The higher MW (HMW) and broad MWD resins produced films consisting of a network of nearly orthotropically oriented lamellar stacks. Greater high molecular weight fraction (MW > 10⁶) in the resin resulted in more random orientation. The influence of these different structures on properties was studied by examining the plastic zone formation at crack tips and uniaxial tensile deformation with the SEM and comparing them to the macroscopic stress-strain behavior. A continuous deformation of the network structure was observed in the HMW films. Lamellar deformation occurred primarily in regions of stacks oriented parallel to the tensile axis. Macroscopic yield occurred at 6 to 10 percent strain via a shearing and opening the lamellar crystals. Irreversible deformation occurred from ?50 to 400 percent strain by transformation of the oriented lamellae to microfibrils. Eventually all the lamellar stacks in the network become aligned with the tensile axis. This process was found to improve the tear resistance in the crack propagation experiments. The lamellar stacks in the network orient perpendicular to the crack independent of crack propagation direction, insuring a more uniform transmission of stress and preventing local yielding. The tensile modulus, yield stress, and ultimate strength were highest in the film containing more high molecular weight polymer.     

How the biaxially stretching mode influence dielectric and energy storage properties of polypropylene films

The most important polymer film used in commercial capacitors is biaxially oriented polypropylene (BOPP), which could be produced by sequentially or simultaneously biaxial orientation after the melt-extrusion. In order to disclose the influence of the stretching technique on the properties of films, the BOPP films with varied thickness were fabricated by sequential and simultaneous orientation, respectively. Compared to the sequentially biaxially stretched films, the crystal grains in the simultaneously biaxially stretched films are more isotropically dispersed. As temperature increases, all the BOPP films exhibit similar dielectric constant, and the simultaneous films have much lower dielectric loss thanks to the finer blended crystalline and amorphous phases. When the film thickness is smaller than 5 μm, the breakdown field strength, energy density and discharging time of the simultaneous films can be increased by at least 10% comparing to the sequential ones, which is very important for reducing the volume of the film capacitors. All the results suggest the simultaneously biaxial orientation mode shows significant advantages in producing thin BOPP films with better mechanical and electrical properties.     

Lamellar crystalline structure of hard elastic HDPE films and its influence on microporous membrane formation

The development of hard elastic high-density polyethylene (HDPE) precursor films and its influence on the microporous membrane formation have been investigated. As a first step, the HDPE precursor films with ‘row-nucleated lamellar crystalline’ structure were prepared by applying elongation stress to the HDPE melt during T-die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long-period lamellar spacing, crystallite size, and degree of crystallinity. The processing (melt extension and annealing temperature)-structure (lamellar crystalline structure)-property (hard elasticity) relationship of HDPE precursor films was also investigated. The uniaxial stretching of hard elastic HDPE precursor films induces the bending of crystalline lamellae, which leads to the formation of micropores between them. The observation of morphology and air permeability for the HDPE microporous membranes have revealed that the well-developed porous structures characterized by superior air permeability were established preferably from the precursor films prepared by the high stress levels and the high annealing temperatures. Finally, the relationship between the hard elasticity of HDPE precursor films and the air permeability of corresponding microporous membranes was 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.     

Biaxially stretched in comparison with conventionally blown coextruded composite plastic films

Multilayer composite films consisting of polyolefins, either polypropylene, linear low density or medium density polyethylene, a thin adhesive layer, and a polyamide in various ratios were prepared using two basically different processes. The first, a conventional blown film process in which the extrudate is stretched while in the molten state; and the second, a two-stage process in which the quenched extrudate is stretched at a temperature below the polymers' melting point. The films so prepared, having identical composition and similar extents of stretching, were compared on the basis of their tensile properties, thermoelastic shrinkage, oxygen transmission rate, and thermal behavior. The effects of stretching temperature, rate, and extent on the behavior of the composite films and control single layer films were investigated. The stretching temperature was found to be the dominating single process parameter in determining the films' behavior. The overall performance including tensile properties, barrier, and shrink properties of the “cold” stretched films was found markedly superior to that of the conventionally blown films. The stretched composite films possess physical properties that cannot be attained by the conventional process.     

Mechanical properties,stress‐relaxation,and orientation of double bubble biaxially oriented polyethylene films

Biaxially oriented linear low density polyethylene (LLDPE) films were produced using the double bubble process with different machine direction (MD) orientation levels and the same transverse direction (TD) blow‐up ratio. Their mechanical behavior was characterized in terms of the tensile strength and tear resistance. The viscoelastic behavior of oriented films was studied using dynamic‐mechanical thermal analysis (DMTA). The microstructure and orientation were characterized using microscopy, X‐ray diffraction pole figures, and birefringence. The results indicate that MD ultimate tensile strength increases and the TD one decreases with MD stretching ratio. Tear propagation resistance, in general, remained mainly constant in TD and decreased in MD, as the draw ratio was increased. The morphology analyses exhibit a typical biaxial lamellar structure for all samples with different lamellar dimensions. Orientation of c‐axis in crystalline phase, molecular chain in amorphous phase along MD increased with draw ratio. In most crystals, a‐axis was located in the normal direction (ND) and the b‐axis in the ND–TD plane. A good correlation was observed between c‐axis orientation factor and MD mechanical properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3545–3553, 2006     

Effect of uniaxial stretching on thermal,oxygen barrier,and mechanical properties of polyamide 6 and poly(m‐xylene adipamide) nanocomposite films

In this study, the effect of uniaxial stretching on the thermal, oxygen barrier and mechanical properties of aliphatic polyamide 6 (PA6) and aromatic Poly(m‐xylene adipamide) (MXD6) nylon films as well as their in‐situ polymerized nanocomposites with 4 wt% clay were studied. Cast films were prepared by extrusion process and rapidly cooled using an air knife. The precursor films were uniaxially stretched at 110°C with draw ratios varying from 1.5 to 5. DSC results showed that the cold crystallization temperature (T_cc) of the uniaxially stretched MXD6 and MXD6/clay films drastically shifted to the lower temperatures when draw ratio increased. The aromatic nylon films had lower oxygen permeability than those of the aliphatic films, due to more rigidity and chain packing. However, the oxygen permeability of the stretched films increased with draw ratio (DR) up to a critical value for each sample, while further stretching resulted in a reduction in the oxygen permeation. This phenomenon was related to the changes in free volume upon uniaxial stretching. The ability of different geometrical models to describe the experimental relative permeability data was investigated. The Bharadwaj model that took into account clay orientation was the most successful one to predict the oxygen barrier characteristics of the stretched nanocomposites at high draw ratios. The Young's modulus and tensile strength of the aliphatic and aromatic nylons increased with uniaxial deformation, while the flexibility and elongation at break of the former decreased with increasing DR. A larger increase in the Young's modulus of the uniaxially stretched nanocomposite films compared with the neat samples was observed and could be related to the improvement in the clay orientation as well as a better alignment of the crystalline phase due to incorporating the clay platelets in the polymer matrix. In contrast, the flexibility of the stretched MXD6 improved remarkably (ca., 25 times) compared with the precursor film (DR = 1) when the draw ratio increased to 1.5. This could be related to the effect of hot stretching on the enhancement of polymer chains relaxation and mobility at low draw ratios. POLYM. ENG. SCI., 55:1113–1127, 2015. © 2014 Society of Plastics Engineers     

Morphology and texture development of uniaxially stretched poly(ethylene naphthalene‐2,6‐dicarboxylate)

The texture development of PEN films with different semicrystalline morphologies have been studied by X‐ray diffraction. These different structures have been obtained by uniaxially stretching PEN amorphous films at 100 and 160°C (below and above T_g) at different drawing ratios. Samples have also been characterized by DSC to determine the crystallinity ratios, the crystallization, and melting temperatures. To define the orientation of crystallites in the oriented samples, pole figures have been constructed, as a function of temperature and drawing ratio (DR) in the range 1.5–4. In the range from DR = 2 to 4 the orientation is clearly uniplanar‐axial. At T_draw = 100°C the crystallinity shown by DSC analysis is higher than the sample stretched at 160°C. The orientation is also higher when samples are stretched at 100°C. The naphthalene rings mainly stay in the plane of the film with a lower fraction perpendicular to the plane of the film. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 395–401, 2007     

Effects of the biaxial orientation on the mechanical and optical properties and shrinkage of polyamide 6-66–montmorillonite–nanosilica nanocomposite films

Polyamide 6–66 (PA6-66)–montmorillonite (MMT)–nanosilica (NS) nanocomposite films were fabricated through a cast film process and then biaxially stretched on a laboratory stretcher. Uniaxial or biaxial stretching induced the elongated conformation of MMT and NS. The b axis of the α crystals and the amorphous phase were revealed to align along the machine direction (MD) after stretching, with the uniaxial orientation playing a more significant role. Furthermore, the crystallinity of PA6-66 stretching increased with the stretching ratio. Uniaxial stretching gave rise to a significantly enhanced tensile strength along the MD, whereas it slightly decreased the mechanical properties along the transverse direction (TD). In contrast, the films subjected to biaxial stretching exhibited more balanced mechanical properties. Uniaxial and biaxial stretching led to decreased transmittance and increased haze in the PA6-66–MMT–NS films; this could have been due to the elongated nanostructure of the two nanofillers, which inhibited the transmission and facilitated the scattering of visible light. The thermal shrinkage of the films increased with increasing stretching ratio, and the biaxially oriented films presented nearly equal shrinkage in the MD and TD. The addition of nanofillers decreased the shrinkage attributed to the mobility inhibition of the polymer chains during heating. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47504.     

Correlations between processing parameters,morphology, and properties of blown films of linear low‐density polyethylene/low‐density polyethylene blends. I. Crystalline biaxial orientation by IR and mechanical properties

The change of the processing parameters of a blown film operation alters the mechanical and optical properties of the films. This work studied the influence of some of these parameters on the properties of blown films made of blends of linear low‐density polyethylene (LLDPE) and LDPE. Correlations between the crystalline biaxial orientations of these films and the mechanical properties were found. The crystalline biaxial orientation was measured by IR following the Krishnaswamy approach. The a axis of the unit cell was oriented along the machine direction (MD) at all LDPE concentrations, and it was not affected by the blow‐up ratio (BUR). In contrast, the b axis changed its orientation from orthogonal to MD to along the transverse direction (TD), and it was affected by the BUR. Finally, the c axis changed its orientation from equiplanar between the MD and TD to along the thickness of the film, and it was influenced by the BUR. The decrease of the tensile mechanical properties along the MD with the increase in the amount of LDPE in the blends was attributed to the tilting of the c axis toward the film thickness. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3161–3167, 2006     

Microscopic lamellar organization in high-density polyethylene banded spherulites studied by scanning probe microscopy

Surface topography and lamellar aggregation structure of high-density polyethylene (HDPE) banded spherulites were investigated by scanning probe microscopy. HDPE films were prepared by isothermal crystallization at various crystallization temperatures from the melt. Polarizing near-field scanning optical microscopic (NSOM) observations for the HDPE films revealed submicron-scale correlation between surface topography and birefringence of banded spherulites. The height profile of the film surface along the spherulitic radius periodically changed corresponding to the intensity profile of transmitted light along the radius of the extinction ring. This correlation was more clearly observed in the topographic and NSOM images of permanganic etched PE films. Therefore, it was apparently suggested that the crystallographic c-axis of the orthorhombic unit cell was parallel and perpendicular to the film surface at the peak and the valley in the surface corrugation of the banded spherulite, respectively. The band spacing obtained by polarizing NSOM and atomic force microscopy (AFM) was comparable to that determined by polarizing far-field optical microscopic observation under crossed nicols. The band spacing and the peak-to-valley height difference in the corrugation increased with an increase in isothermal crystallization temperature. AFM observations directly indicated local lamellar orientation and stacking manner.