Effect of chain architecture on biaxial orientation and oxygen permeability of polypropylene film

Films of two isotactic propylene homopolymers prepared with different catalysts and a propylene/ethylene copolymer were biaxially oriented under conditions of temperature and strain rate that were similar to those encountered in a commercial film process. The draw temperature was varied in the range between the onset of melting and the peak melting temperature. It was found that the stress response during stretching depended on the residual crystallinity in the same way for all three polymers. Biaxial orientation reduced the oxygen permeability of the oriented films, however, the reduction did not correlate with the amount of orientation as measured by birefringence, with the fraction of amorphous phase as determined by density, or with free volume hole size as determined by PALS. Rather, the decrease in permeability was attributed to reduced mobility of amorphous tie molecules. A single one‐to‐one correlation between the oxygen permeability and the intensity of the dynamic mechanical β‐relaxation was demonstrated for all the polymers used in the study. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of compatibilization on the oxygen‐barrier properties of poly(ethylene terephthalate)/poly(m‐xylylene adipamide) blends

The improvement of the oxygen‐barrier properties of poly(ethylene terephthalate) (PET) via blending with an aromatic polyamide [poly(m‐xylylene adipamide) (MXD6)] was studied. The compatibilization of the blends was attempted through the incorporation of small amounts of sodium 5‐sulfoisophthalate (SIPE) into the PET matrix. The possibility of a transamidation reaction between PET and MXD6 was eliminated by ¹³C‐NMR analysis of melt blends with 20 wt % MXD6. An examination of the blend morphology by atomic force microscopy revealed that SIPE effectively compatibilized the blends by reducing the MXD6 particle size. Thermal analysis showed that MXD6 had a nucleating effect on the crystallization of PET, whereas the crystallization of MXD6 was inhibited, especially in compatibilized blends. Blending 10 wt % MXD6 with PET had only a small effect on the oxygen permeability of the unoriented blend when it was measured at 43% relative humidity, as predicted by the Maxwell model. However, biaxially oriented films with 10 wt % MXD6 had significantly reduced oxygen permeability in comparison with PET. The permeability at 43% relative humidity was reduced by a factor of 3 in compatibilized blends. Biaxial orientation transformed spherical MXD6 domains into platelets oriented in the plane of the film. An enhanced barrier arose from the increased tortuosity of the diffusion pathway due to the high aspect ratio of MXD6 platelets. The aspect ratio was calculated from the macroscopic draw ratio and confirmed by atomic force microscopy. The reduction in permeability was satisfactorily described by the Nielsen model. The decrease in the oxygen permeability of biaxially oriented films was also achieved in bottle walls blown from blends of PET with MXD6. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1361–1370, 2005     

Surface roughness and light transmission of biaxially oriented polypropylene films

The effect of thermal history on the transparency of biaxially-oriented polypropylene (BOPP) films was investigated. Compression molded sheets prepared with different cooling rates were biaxially oriented at several temperatures. Correlations were sought between the light transmission measured at 633 nm and the surface roughness as characterized by atomic force microscopy. It was determined that surface roughness on the 100-μm size scale was responsible for a loss in transparency. Surface roughness on the submicron size scale did not affect the transparency. The clearest films were obtained from compression molded sheets with the most homogeneous texture and by orienting at the lowest temperature. POLYM. ENG. SCI., 47:1658–1665, 2007. © 2007 Society of Plastics Engineers     

Gas transport properties of poly(ethylene‐2,6‐naphtalene dicarboxylate) films: Influence of crystallinity and orientation

H₂ and CO₂ transport properties were investigated on semicrystalline poly(ethylene‐2,6‐dicarboxylate naphtalene) (PEN) films and biaxially stretched films and analyzed in terms of solubility and diffusion. The decrease of the permeability observed on the thermally crystallized samples has been described by Maxwell's law. No change of the sorption properties of the amorphous phase has been noticed as a function of the thermal treatment and the evolution of the diffusion coefficient has been related to a tortuosity effect. The low permeability measured on the biaxially stretched film is related to both a change of the free volume sizes distribution and a tortuosity effect. The interesting barrier properties of the biaxially stretched film are kept even after annealing the film at 250°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1849–1857, 2003     

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.     

Biaxial stretching and structure of various LLDPE resins

The purpose of this study is to investigate the biaxial stretchability, the structure developed, molecular orientation and shrinkage of linear low‐density octene copolymers (LLDPEs) biaxially stretched using a laboratory biaxial stretcher. Seven resins having different molecular characteristics were used in this study. The effect of stretching temperature and rate on stretchability is assessed. Biaxial orientation factors for the crystalline axes as well as that of the amorphous phase were determined using FTIR spectroscopy and shrinkage of oriented films was measured. The results indicate that the high molecular weight tail (Mz) and comonomer content play important roles in orientation of the resins. Higher Mz, MWD and resin content eluting above 90°C (by TREF technique) tends to increase orientation, and finally, some correlation between orientation and Mz, MWD and resin content eluting above 90°C and between shrinkage and amorphous orientation were observed.     

Nanolayer enhancement of biaxially oriented polypropylene film for increased gas barrier

An order of magnitude improvement in the oxygen barrier of biaxially oriented polypropylene (BOPP) films was achieved using a layer-multiplying, forced assembly process. The improvement was achieved without sacrificing clarity and toughness of the films. Sheets with 33 alternating layers of polypropylene (PP) (17 layers) and poly(?-caprolactone) (PCL) (16 layers) were coextruded using layer-multiplication and two thick PP skins were added to the multilayered core as the last step in the continuous coextrusion process. The sheets were subsequently biaxially oriented to draw ratios from 4 × 4 to 6 × 6. Biaxial orientation at elevated temperature reduced the thickness of the melted PCL layers from the microscale to the nanoscale, which created 2-dimensional confinement for subsequent crystallization of the PCL layers. It was anticipated that the PCL layers would recrystallize as highly oriented, in-plane lamellae that would resemble single crystals. However, the PCL lamellae were oriented perpendicular to the film surface, which actually facilitated oxygen permeation through the PCL layers and increased the oxygen permeability of the oriented films. Crystallization as on-edge lamellae was attributed to nucleation by the polypropylene surface. However, the surface nucleation was prevented by inserting buffer polystyrene (PS) layers in between the PCL and PP layers. In this case, the PCL lamellae were oriented in-plane. With the very high aspect ratio lamellar crystals oriented perpendicular to the flux direction, the permeation pathway of oxygen became very tortuous and the oxygen barrier was significantly improved.     

Influence of high power ultrasound on physical–chemical properties of polypropylene films aimed for food packaging: barrier and contact angle features

Investigation was focused on the impact of high power ultrasound (HPUS), also called thermosonication, on the oxygen permeation properties (permeability, solubility and diffusion coefficients) of barrier films aimed for food packaging. For this purpose, biaxially oriented polypropylene (BOPP) coated with acrylic/polyvinylidene chloride (BOPPAcPVDC) and biaxially oriented coextruded polypropylene (BOPPcoex) were used. The physical–chemical profile of the samples was determined using goniometry. There is a significant impact only of extreme HPUS conditions (the longest time and the highest amplitude) on the permeability, solubility and diffusion coefficients of oxygen through the BOPP films. The highest influence on the oxygen permeability in both investigated BOPP samples involved an HPUS with an amplitude of 100% during a 6 min treatment. However, BOPP samples showed different sensitivities at lower HPUS treatments. © 2017 Society of Chemical Industry     

Orientation and property correlations for LLDPE blown films

The orientation and property correlations of biaxially oriented polyethylene (PE) blown films have been studied. A linear low density polyethylene (LLDPE) (DOWLEX ? 2045A) was used to fabricate films at different conditions with blow up ratio, die gap, and frost line height as the variables. The White‐Spruiell orientation factors of crystal unit cells, amorphous chains, and Herman's orientation factors of lamellae were determined from wide‐angle X‐ray diffraction pole figure, birefringence, and small angle X‐ray scattering (SAXS). A general orientation pattern with the crystal unit cell a‐axis preferentially oriented to MD, b‐axis to TD, lamellae stacking along the MD, and amorphous chains preferentially to the MD has been found for all films in this study. A correlation between the orientation of each element of the morphology hierarchy has been revealed. Key mechanical properties including dart impact and Elmendorf tear strength in both MD and TD have been determined. Good correlation has been found among these properties. Most importantly, these properties have excellent correlation to the orientation. These correlations have been linked to underlying morphology and microdeformation mechanisms. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 898–907, 2006     

Thermal properties of biaxially deformed in situ composites

The thermal properties of biaxially blown poly(etherimide) (PEI) films containing a thermotropic liquid crystalline polymer (TLCP) were studied using differential scanning calorimetry, thermogravimetric analysis, wide‐angle X‐ray diffraction and dynamic mechanical thermal analysis. The effects of the compatibilizer and thermal annealing on the thermal properties of biaxially oriented TLCP films were investigated. Though the compatibilizer (poly(ester imide)) improved deformation of the TLCP phase (poly ester amide) and adhesion between the matrix and the TLCP phase, which improved mechanical properties, it did not significantly affect the thermal properties of the in situ composite films. The film degradation behavior corroborated the role of the compatibilizer. Since a relatively small amount of TLCP (10 wt%) was added to the matrix and the matrix PEI was amorphous, the effect of annealing on the TLCP structure was not obvious. By the same token, while the effect of the deformation in the circumferential direction (a change in the blow‐up ratio) was manifest in mechanical property improvements, its effect on the thermal properties was not obvious. All films showed similar thermal expansion behaviors, regardless of the thermal history and of the compatibilizer addition. Thus, there is an optimum amount of the compatibilizer required to obtain optimal mechanical properties for in situ composite films without causing a deterioration of their thermal properties. Polym. Eng. Sci. 44:1419–1428, 2004. © 2004 Society of Plastics Engineers.     

Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003     

Morphology–property relationship in oriented PET films: Microstructural reorganization during heat treatment

Morphology–property relationships for simultaneously biaxially stretched films and heatset with fixed dimensions in the temperature range of 100–240°C have been studied. The observed transition in various properties at 180°C can be explained on the basis of microstructural changes caused by competition among several processes, such as crystallization, solid-state thickening, melting, and molecular relaxation as well as by melting and recrystallization. The resulting structures and, thereby, the properties are different in temperature Regime-II (T_g to T_max) and Regime-III (T_max to T_m). In Regime-II, the high rate of crystallization compared to the rate of molecular relaxation develops a constrained amorphous phase, whereas the predominant melting and recrystallization process in Regime-III generates the relaxed amorphous phase. The structural reorganization during heat treatment is almost the same for uniaxially oriented film, fibers, and biaxially oriented films prepared under similar processing conditions. © 1994 John Wiley & Sons, Inc.     

Morphological, Thermal, Barrier and Mechanical Properties of LDPE/EVOH Blends in Extruded Blown Films

We have investigated the morphological, thermal, barrier, and mechanical properties of low-density polyethylene/ethylene–vinyl alcohol blend (LDPE/EVOH; 85/15 wt%) in highly and biaxially oriented blown films. Maleic anhydride-grafted linear low-density polyethylene (LDPE-g-MAH) in various concentrations (from 0 to 10 phr) was used as the compatibilizer for the immiscible system. Thermal analysis of the blend films shows that their melting temperatures, crystallization temperatures, and heats of fusion stay almost constant upon varying the amount of compatibilizer. The addition of the compatibilizer did not adversely affect the inherent properties of the blends, especially their barrier properties, through constraint effects of the grafted EVOH (EVOH-g-LD). The heat of fusion of EVOH obtained during the first heating is much higher than that of the second as a result of stress-induced crystallization during the blown film process. Oxygen permeation measurements show that the oxygen barrier properties of both highly and biaxially oriented blown films decrease upon increasing the amount of compatibilizer, although morphological analysis showed that the blends exhibit better laminar dispersion of the EVOH phase in the LDPE matrix when LDPE-g-MAH is added. The increase in oxygen permeability results from the presence of microvoids at the interface between the two phases during the process. Mechanical measurements showed that there exists an optimal amount of LDPE-g-MAH for maximizing both the tensile and tear properties in both the machine and transverse directions.     

Thermomechanical properties of a magnetically and mechanically oriented liquid crystalline copolyester,xydar

Elastic moduli and linear coefficients of thermal expansion (CTE) of a random thermotropic liquid crystalline copolyester, oriented in a magnetic field and by mechanical methods were measured in the directions parallel and perpendicular to the orientation direction. The axial elastic modulus of the magnetically oriented film was lower than that of the uniaxially stretched film. The elastic modulus measured in the transverse direction was higher for the magnetically oriented film. In the axial direction, both the mechanically stretched and magnetically oriented films exhibited shrinkage at low temperatures (CTE ≈ -2 · 10^?5 K^?1) and exhibited expansion at elevated temperatures. In the transverse direction, expansion was observed except for the biaxially stretched film at low temperatures. The magnetically oriented film showed the lowest axial CTE at elevated temperatures.     

The barrier properties of polyethylene terephthalate to mixtures of oxygen, carbon dioxide and nitrogen

The diffusion and solubility of oxygen, nitrogen and carbon dioxide have been studied in amorphous and biaxially oriented films of polyethylene terephthalate (PET). To measure the sorption and desorption of each gas simultaneously in cases where mixtures of gases were studied, a mass spectrometer was used as a detector. It was found that the solubility and diffusion of nitrogen in PET were markedly affected by the presence of the other gases, oxygen and carbon dioxide with differences in detail between results for the amorphous and biaxially oriented films. It is of particular interest that the presence of oxygen reduces the solubility and increases the diffusivity of nitrogen.     

Influence of incorporation of ZnO nanoparticles and biaxial orientation on mechanical and oxygen barrier properties of polypropylene films for food packaging applications

In this work, the effect of zinc oxide (ZnO) concentration and shape on processing and properties of new biaxially oriented polypropylene (BOPP)‐ZnO nanocomposites was studied. The use of spherical nanoparticles and nanorods was expected to differently influence the properties of the final material. Films of isotactic polypropylene prepared with different ZnO incorporation were biaxially oriented under conditions of temperature and strain rate that were similar to those encountered in a commercial film process. Scanning electron microscopy analysis was performed to visualize the dispersion degree of the ZnO nanoparticles in the polymer matrix and to observe the surface and the orientation of the elongated nanoparticles. Furthermore, the prepared ZnO‐BOPP nanocomposites were evaluated for both mechanical and oxygen barrier property enhancement. A good combination of mechanical and oxygen barrier properties was obtained for the ZnO‐BOPP films. This result makes the ZnO‐BOPP nanocomposite a proper material for applications such as food packaging. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Characterization of biaxially oriented polypropylene films by atomic force microscopy and microthermal analysis

A new method for evaluating the thermal properties of the films and detecting fabrication failures has been provided. Moreover, this article studies the characterization of biaxially oriented polypropylene films (BOPP) using the Microthermal Analizer (μTA 2990). This instrument combines high‐resolution imaging capabilities of the atomic force microscopy (AFM) with physical characterization by thermal analysis. In the first part of the work, topographic images of the film surfaces were obtained by AFM. They showed that the fabrication process and additives to the films caused differences in the sample topography. In the second part, the thermal conductivity images of multilayer films were obtained by thermal analysis mode. The thickness of each layer was determined for several BOPP films, based on the thermal conductivity signal registered by μTA 2990. Finally, it has been proven that this new technique is valid for detection of thermal transitions in polymer samples. Thus, melting points and glass transitions were measured in the samples with thermal probe. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1553–1561, 2002.     

Preparation of single poly(lactic acid) composites

An approach for making poly(lactic acid) (PLA) single‐polymer composites (SPCs) on the basis of PLA's slowly crystallizing characteristics was investigated. As a slowly crystallizing polymer, PLA can be processed with standard polymer processing techniques into end‐use products with varied crystallinities, from amorphous films to highly crystalline fibers. In this study, amorphous PLA sheets and crystalline PLA fibers/fabrics were laminated and compression‐molded to form an SPC at a processing temperature substantially lower than PLA's melting temperature. The effects of the major process conditions on the performance of the SPC were studied. The processing temperature played a profound role in affecting the fiber–matrix bonding properties. As the processing temperature increased, a drastic improvement in the interfacial bonding occurred at a temperature of around 135°C, which indicated the lower boundary of the process window. The compression‐molded SPC exhibited enhanced mechanical properties; particularly, the tearing strength of the fabric‐reinforced SPC was almost an order higher than that of the nonreinforced PLA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Poly(vinyl alcohol) nanocomposite films: Thermooptical properties,morphology, and gas permeability

Poly(vinyl alcohol) (PVA)/saponite nanocomposites were prepared with various clay concentrations with the solution intercalation method. The intercalations of the polymer chains in the clay were examined with wide‐angle X‐ray diffraction and transmission electron microscopy. The variations of the dispersion, morphology, thermal properties, and gas permeability of the nanocomposites with clay concentrations in the range of 0–10 wt % were examined. Up to a 5 wt % clay loading, the clay particles were highly dispersed in the PVA matrix without any agglomeration of particles. However, some agglomerated structures formed in the polymer matrix above a 7 wt % clay concentration. The thermal stability of the hybrids increased linearly with increases in the clay loading up to 10 wt %. To measure the oxygen permeability and optical properties of the PVA hybrid films, the PVA hybrid solutions were coated onto both biaxially oriented polypropylene and poly(ethylene terephthalate) films, which were used as polymer substrates. The oxygen permeability values monotonically decreased with increases in the clay loading in the range of 0–10 wt %. The optical properties, such as the haze and gloss of the hybrid films when coated onto the matrix films, were nearly constant, that is, independent of the clay loading. These improvements arose because of the largely nanometer‐scale dispersion of the clay layers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 591–596, 2006     

Biaxially oriented poly(propylene-g-maleic anhydride)/phosphate glass composite films for high gas barrier applications

The solid state structure and oxygen transport properties of biaxially oriented poly(propylene-graft-maleic anhydride) (PPgMA) reinforced with a low glass transition temperature (T_g) phosphate glass (Pglass) were investigated. Composites were prepared by melt blending PPgMA with up to 20 volume% Pglass. Melt blended composites were compression molded into monolayer structures and then biaxially stretched at a temperature above the T_g of the Pglass. Scanning electron microscopy confirmed that biaxial stretching transformed the spherical Pglass particles into platelets oriented in the plane of the film. Gas transport measurements revealed a reduction in the oxygen permeability by as much as 2 orders of magnitude compared to the unoriented PPgMA film. The permeability was analyzed according to performance models for dispersions of platelet-like fillers proposed by Cussler and Nielson. Aspect ratios ranging from 15 to 80 were obtained by fitting the experimental data to the models. Mechanical tests revealed that blending with Pglass increased the modulus of the stretched film but reduced the elongation at break only slightly.