Commercial Linear Low Density Polyethylene Research Articles

Memory of crystallization in the melt of commercial linear low density polyethylenes processed in an open twin-screw extruder

Compared with a constant recrystallization rate of commercial linear low density polyethylenes (LLDPE) as a function of melt temperature, copolymers reprocessed in an open twin-screw extruder, display the expected strong melt-memory features upon melting followed by recrystallization. Precipitation from dilute solution had the same effect on melting-recrystallization of commercial copolymers. In the twin-screw melt re-processing, a small content of long-chain branches and crosslinks are incorporated to the copolymer's structure, but especially a further consumption of additives uncovers self-nucleation features that speed up recrystallization and are related to the major copolymer microstructure. The observed difference in re-crystallization behavior between metallocene type and Ziegler-Natta (ZN) type copolymers, both with equivalent average comonomer content and molecular mass, is due to the inter-chain comonomer distribution. While for metallocene-made and low density polyethylenes, below a critical melt temperature the recrystallization rate increases continuously with decreasing melt temperature, in broadly distributed ZN copolymers there is an inversion of the temperature gradient of the rate at the onset of liquid-liquid phase separation (LLPS). The mass fraction of highly branched molecules required to observe the onset of LLPS via melting followed by recrystallization was evaluated from the TREF profiles, and the effect of self-nucleated melts at different levels on the overall spherulitic morphology was observed via polarized optical microscopy.

Strategy for reducing molecular ensemble size for efficient rheological modeling of commercial polymers

We develop a method for efficient prediction of linear and nonlinear rheology of polydisperse polymers by judicious selection of a small number of representative polymer molecules from the large ensemble of chains comprising the molecular weight and branching distribution. Specifically, we use a numerical inversion of the double reptation model to select five or six representative molecular species to optimally fit the linear rheology of commercial polymers and then use the regressed parameters to compute the nonlinear rheology via the Rolie-double-poly (RDP) model. The method is tested for several model systems, namely, two polydisperse linear polystyrene (PS) samples described in Shivokhin et al. [Polym. Eng. Sci. 56, 1012–1020 (2016)] and Munstedt [J. Rheol. 24, 847–867 (1980)], a commercial linear low density polyethylene (LLDPE), and a low density polyethylene (LDPE) whose rheological properties are measured in the current study. For the linear polymers including the PS samples and the LLDPE, the predictions of the “representative” RDP model of the start-up extensional rheology are comparable to those of the “full” RDP model based on all the species drawn from the gas permeation chromatography characterization. The method then successfully predicts the linear rheology of blends of LLDPE and LDPE using the same representative molecules found by fitting each of the pure polymers. Further fitting the extensional rheology of the LDPE requires using the “priorities” q i and stretch relaxation times τ s , i of the representative molecules as adjustable parameters, whose values are then held fixed when predicting the extensional rheology of blends of the LDPE with the LLDPE roughly as successfully as does branch-on-branch model. The reduction in the number of representative polymer species offers new opportunities for faster simulations of flowing polymers, as well as for the prediction of segmental orientation to be used in the modeling of flow-induced crystallization.

The in-line detection method of sharkskin melt flow instability during polyethylene extrusion based on pressure analysis

Abstract This paper is concerned with the issues of occurrence and detection of melt flow instabilities forming during polyethylene extrusion. A novel method of in-line detection and evaluation of sharkskin melt flow instability occurring during the extrusion of linear low density polyethylene (LLDPE) was developed. The application of the dynamic pressure analysis procedure with three different signal processing methods allows to obtain both quantitative and qualitative information about the phenomena which caused the distortion of the surface of the extrudate during the technological process. Investigations were carried out with two commercial LLDPE grades with different molecular weight distributions. In-line evaluation of the efficiency of the novel measuring system was preceded by off-line rheological experiments which allowed to obtain broad information about the influence of the macromolecular structure of selected polyethylenes on their properties in the molten state and their potential ability to create sharkskin instability. The preliminary multi-criteria assessment of the structure-rheological properties relationship allowed the verification of the sensitivity of the novel measuring system and the definition of proposals for the selection of appropriate signal processing methods for the different types of melt fracture instabilities formed on the extrudate surface.

Thermo-Mechanical Behavior and Hydrolytic Degradation of Linear Low Density Polyethylene/Poly(3-hydroxybutyrate) Blends

In this work, a commercial linear low density polyethylene (LLDPE) utilized for packaging applications was melt compounded with different amounts (from 10 wt% up to 50 wt%) of poly(3-hydroxybutyrate) (P(3HB)), with the aim to evaluate the possibility to partially replace LLDPE with a biodegradable matrix obtained from renewable resources. The processability, microstructural and thermo-mechanical behaviour of the resulting blends was investigated. Melt flow index (MFI) values of the LLDPE matrix were not much affected until a P(3HB) content of 20 wt%, while for higher P(3HB) concentrations an evident decrease of the viscosity was detected. Scanning electron microscope (SEM) observations on the blends highlighted that at limited P(3HB) concentrations the secondary phase was homogeneously dispersed in form of isolated domains, while at a P(3HB) content of 50 wt% a continuous layered morphology could be detected. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) did not evidence any chemical or physical interaction between the two polymer phases. Quasi-static tensile tests and dynamical mechanical analysis showed that the introduction of P(3HB) led to a pronounced stiffening effect, while the progressive drop of the yield and ultimate mechanical properties could be attributed to the weak interfacial adhesion and poor compatibility between the two matrices. The resistance to hydrolytic degradation of the LLDPE/P(3HB) blends was evaluated over a period of 100 days of immersion in water at 50 °C. It was observed that the weight variation and the decrease of the tensile properties due to the hydrolytic process on the biodegradable phase were evident only for a P3HB content of 50 wt%. In conclusion, this work showed that the partial replacement of LLDPE with a biobased P(3HB) could lead to the development of an innovative blend with good processability and mechanical properties, until a P(3HB) amount of 20 wt%.

Smart Packaging in the Sustainability Challenge: Eumelanin as a UV-Absorption Enhancer of Polymers

The black-brown biopigment eumelanin screens the harmful ultraviolet (UV) radiation in the human skin. In industry, the absorption of plastics in the UV range is improved by the inclusion of additives. Our work reports an evaluation of the biopigment eumelanin as a nature-inspired additive for polymers to improve their UV-absorption. Eumelanin, being non-toxic and produced by natural sources, represents an interesting candidate for sustainable plastic additives. We fabricated eumelanin-including films of commercial linear low-density polyethylene (LLDPE), a plastic ubiquitous in packaging applications. The films were obtained by melt-compounding and compression molding. The melanin free acid treatment (MFA) was used to enhance the biopigment dispersion in the films. We observed that amounts of eumelanin as low as 0.8% wt. bring about an increase of the UV absorption up to one order of magnitude in the UVA range. We also assessed the effect of eumelanin on the film stability under prolonged UV irradiation: the biopigment proved to work both as UV-absorption enhancer but also as a photo-prooxidant, as inferred by thermogravimetric analysis (TGA) and infrared (IR) spectroscopy.

Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes.

Group 10 metal catalysts have shown much promise for the copolymerization of nonpolar with polar alkenes to directly generate functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)-P(III) chelating ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization catalysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP) Pd catalysts are competent to generate linear, functional polyethylenes with Mw up to ca. 2 × 105 g mol-1, significantly higher than prototypical catalysts in this field, and with polar content up to ca. 9 mol %. Functional groups are positioned by these catalysts almost exclusively along the main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectroscopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species, which may correlate to their complementary catalytic properties versus privileged catalysts such as electrophilic α-diimine (Brookhart-type) or neutral phosphine-sulfonato (Drent-type) complexes. Though steric blocking within the catalyst coordination sphere has long been a reliable strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complementary concept less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.

Charge generation and saturation on polymer particles due to single and repeated particle-metal contacts

The development of an electrostatic model for particle-wall contact charging requires a good understanding of the charge generation mechanism for insulative particles. Single and repeated-contact experiments were performed with various particles, including commercial linear low-density polyethylene, to determine their rates of charging as well as their charge saturation limits. Plotting the charge saturation value of the particles against their respective surface areas revealed a linear trend which could be used to calculate the charge saturation of the particle for a given particle size. Additional studies include the effect of initial charge, collision frequency, particle type and impurities on particle charging.

Clarification of linear low‐density polyethylene using Bis‐oxalamide compounds

The clarifying effect of using bis‐oxalamide compounds on linear low density polyethylene (LLDPE) has been investigated for the first time. It was demonstrated that the haze of a commercial LLDPE resin, DOWLEX™ 2045G, can be reduced up to 50% by adding 0.2 wt % of a bis‐oxalamide clarifying agent. The investigations on the crystalline morphology revealed that the density of spherulite cores increased with increasing concentration of bis‐oxalamide N5. Through an investigation of structure‐property relationships based on the tunable bis‐oxalamide structure, it was demonstrated that the combination of linear alkyl core and cyclohexyl pendant groups showed the best clarification. The interactions of clarifying molecules were simulated using dynamic molecular simulation. The simulation suggested that the rigidity of the core functionality played a role on the hydrogen bonding in the intermolecular network, which may contribute to the macroscopic clarification performance. POLYM. ENG. SCI., 58:142–149, 2018. © 2017 Society of Plastics Engineers

Improvement of Water Vapor Permeability of LLDPE/EVA Film with Zeolite A as Filler

In this work, water vapor permeability of linear low density polyethylene (LLDPE)/ethylene vinyl acetate copolymer (EVA) film is improved by incorporation of zeolite A as filler (0-25%wt) for prolonging shelf-life of fresh produces. All films were characterized by SEM, DSC, tensile testing, contact angle measurement and tested for water vapor permeability (WVP). The shelf-life of Bird’s eye chili in the film samples was also tested at 10°C for 21 days. It was found that zeolite A particles were virtually dispersed in EVA phase. Accordingly, crystallinity and tensile properties of LLDPE/EVA/Zeolite A films is independent to zeolite loading. Despite, when zeolite loading was increased, the dispersion became low. The film’s wetting behavior was enhanced by increasing zeolite content in the LLDPE/EVA/Zeolite A films. The observed increase in the film’s polarity significantly enhances the WVP. Therefore, less water condensed can be found inside the package made with LL80E20Ze25 film, as compared to LLDPE or LLDPE/EVA film. The LL80E20Ze25 also possesses comparable tensile properties to the commercial LLDPE film and hence can used as packaging for extending the shelf-life of fresh produces.

Linear low density polyethylene: Microstructure and sealing properties correlation

The microstructures of four commercial linear low density polyethylenes (LLDPE) were evaluated and correlated with their sealing properties. Atomic Force Microscopy (AFM), Temperature Rising Elution Fractionation (TREF), Differential Scanning Calorimetry (DSC) and Crystallization Analysis Fractionation (CRYSTAF) experiments revealed that the comonomer distribution is one of the main factors that influence the sealing properties. The superior sealing performance showed by metallocene LLDPE samples in comparison to Ziegler-Natta LLDPE samples can be attributed to their well balanced chemical composition distribution.

Branch length distribution in TREF fractionated polyethylene

Commercial polyethylene is typically heterogeneous in molecular weight as well as in molecular topology due to variability in catalyst systems and catalyst activity. Further, processing of polyethylene after polymerization may also result in changes to the structure. While quantification of molecular weight is routine using gel permeation chromatography (GPC); quantification of the heterogeneity in molecular topology and microstructure is more difficult. In this paper, a novel method is used to examine the structure and branch content of a linear low-density polyethylene (LLDPE). The method uses a scaling model to analyze small-angle neutron scattering (SANS) data from dilute solutions of a series of LLDPE fractions. The scaling approach quantifies short-chain and long-chain branch content in polymers concurrently, thereby illustrating the distribution of these branches in the polyethylene fractions. Additionally, new quantities such as the average long-chain branch length and hyperbranch content are measured to provide further insight into the structure of these polymers. LLDPE used in this study is fractionated using temperature rising elution fractionation (TREF). Results from the analysis of these fractions show evidence of long-chain branching in commercial LLDPE which could be partly attributed to post-synthesis processing conditions.

Synthesis of new 1‐decene‐based LLDPE resins and comparison with the corresponding 1‐octene‐ and 1‐hexene‐based LLDPE resins

AbstractThis article extends the composition of linear low‐density polyethylene (LLDPE) resins to that containing 1‐decene comonomer units, and examines the effects of comonomer (type and concentration) to copolymerization and physical properties of LLDPE resins. CGC metallocene technology, under high temperature and high pressure (industrial reaction condition), was used to prepare three types of well‐defined LLDPE copolymers containing 1‐hexene, 1‐octene, and 1‐decene units. They show high molecular weight with narrow molecular weight and composition distributions, comparative catalyst activities, and similar comonomer effects. However, 1‐decene seems to exhibit significantly higher comonomer incorporation than 1‐hexene and 1‐octene, which may be associated with its high boiling point, maintaining liquid phase during the polymerization. The resulting LLDPE copolymers show a clear structure–property relationship. Melting temperature and crystallinity of the copolymer are governed by mole % of comonomer. The increase of branch density linearly decreases the LLDPE melting point and exponential reduction of its crystallinity. On the other hand, the density of the copolymer decreases with the increase of comonomer weight %, which shows a sharp linear relationship in the low comonomer content. The tensile properties of 1‐decene‐based LLDPE are very comparative with those of the commercial LLDPE resins with similar compositions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 639–649, 2007

Thermo-mechanical properties of LLDPE/SiO 2 nanocomposites

Two series of linear low density polyethylene (LLDPE)/SiO 2 nanocomposites were prepared. They were based on two types of commercial LLDPE, one prepared by metallocene (mLLDPE) and the other by traditional Ziegler–Natta (zLLDPE) catalysts, and silica nanoparticles surface treated with dimethyldichlorosilane. The silica nanonparticles used have an average diameter of 16 nm, and their weight fraction varied from 2 up to 10%. The structure and thermal-mechanical features of the nanocomposites were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMA) as well as tensile tests. The effect of nanoparticles on crystallinity, and hence to the morphology of the materials was studied. The secondary transitions were also affected by the filler presence, while the tensile properties were reinforced with varying the nanoparticle weight fraction. The addition of the nanofillers brought up an increase in the elastic modulus and the tensile strength of mLLDPE accompanied by an unusual dramatic increase in the elongation at break. The same trend, although to a lesser extent, was observed for the zLLDPE/SiO 2 composites. The increment of the elastic modulus of the composites with increasing filler content was simulated with three micromechanical models developed in previous works. The model which assumes an effective interface between the matrix and the nanoparticles provided the best fitting with the experimental data of mLLDPE/SiO 2.

Grafting of a LLDPE using gamma irradiation

In this investigation, the grafting of a commercial linear low-density polyethylene (LLDPE) with different concentrations of diethyl maleate (DEM, 5 and 15wt.%) was carried out at different absorbed doses from a cobalt-60 source of gamma rays (0, 15, 30, 50, 100, 200kGy). This process was performed in a decalin solution at 10% w/v to obtain a homogeneous dispersion of the monomer into the polyethylene matrix. The grafting degree was estimated by means of FTIR using a calibration curve reported in literature. Thermal properties of the functional polymers were studied by thermogravimetric analysis (TGA). Melt flow index (MFI) values were also taken.The results found indicate that the grafting degree increases as the concentration of DEM in the reaction mixture and the absorbed doses are increased upto 100kGy, as expected. However, the behavior at higher doses is attributed to secondary reactions such as long-chain branching and/or crosslinking, which are faster than radical reactions responsible for the grafting of the DEM onto the polymeric chain. This fact was ascertained by the decrease of the MFI values as the applied irradiation was increased, irrespective of the quantity of DEM used in the grafting reaction. Therefore, in order to obtain a high grafting degree, the absorbed dose should be estimated carefully. Initial degradation temperatures of the grafted PEs decreased when the gamma irradiation dose was higher than 100kGy. This indicates that the thermal stability decreases as higher doses are applied to the material, which is associated to branching and crosslinking. The grafting degree never exceeded 0.3mol%, which demonstrates the low efficiency of the functionalization procedure here presented.

Novel characterization of the crystalline segment distribution and its effect on the crystallization of branched polyethylene by differential scanning calorimetry

Based on a thermal segregation treatment, a novel semiquantitative method for the characterization of the crystalline segment distribution in branched polyethylene copolymers was established by the results of differential scanning calorimetry being treated with the Gibbs–Thomson equation. The method was used to describe the segment distribution of Ziegler–Natta-catalyzed linear low-density polyethylene (Z–N LLDPE), metallocene-catalyzed linear low-density polyethylene (m-LLDPE), and a commercial linear low-density polyethylene with a wide molecular weight distribution. The isothermal crystallization kinetics of Z–N LLDPE and m-LLDPE were studied to assess the effect of different segment distributions. According to their molecular characteristics, the crystallization behaviors were analyzed. They indicated that the different segment distributions of the two polymers resulted in different crystallization processes, including the nucleation and growth of crystals under various crystallization conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2107–2118, 2002

Effect of boron nitride on melt fracture phenomena and processability of lldpe during extrusion

AbstractMelt fractures related to processing instabilities limit processing rates in many commercially important polymer processing operations, such as fiber spinning, film blowing, extrusion, and various coating flows. Therefore, melt fracture is responsible for deteriorating the quality and the mechanical properties of final products for rates greater than a critical processing one at which melt fracture occurs. In this study, a commercial linear low‐density polyethylene (LLDPE) was modified by adding a small amount of boron nitride (BN) during extrusion in order to improve processability. Capillary rheometry was used to assess processability at various temperatures, levels of applied shear rate, and the length‐to‐diameter (L/D) ratio for both the pure resin and resins containing boron nitride. Also, parallel‐plate rheometry was used to evaluate the dynamic rheological properties of these resins. The relationship between the characteristic relaxation time and the critical shear rate for the onset of melt fracture and slip is discussed.

Blown Film Characterization of Binary Blends of Metallocene-Catalyzed LLDPES

We have explored the effects of blending metallocene-catalyzed linear low-density polyethylenes (LLDPE) of reasonably differing molecular weights on the performance and orientation characteristics of blown films. The presence of relatively longer molecules in a metallocene-catalyzed LLDPE resulted in blown films with relatively lower machine direction (MD) tear resistance, lower impact strength and higher transverse direction (TD) tear resistance. While all of the blown films investigated displayed the Keller-Machin “row” structure, the degree of orientation was observed to increase with increasing concentration of long molecules (or high molecular weight blend component). The observed influence of orientation in the crystalline and non-crystalline phases on the Elmendorf tear resistance of the blown films was observed to be consistent with previous investigations on a wide spectrum of commercial LLDPE blown films [15]. Morphological investigations of the blend films indicated the potential significance of microstructural anisotropy on blown film impact performance. This calls into question the wide-spread notion that co-monomer distribution profile is largely responsible for distinguishing the impact performance of various classes of LLDPE blown films.

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

A new technique for characterization of linear low-density polyethylene (LLDPE) is presented in this report. The molecular structure of two commercial LLDPEs, produced by copolymerization of ethylene with 1-butene over a Ziegler-Natta and a metallocene catalyst, was investigated. The LLDPE resins were fractionated by temperature rising elution fractionation (TREF), and the TREF fractions were further analyzed by size exclusion chromatography and differential scanning calorimetry (DSC) coupled with successive nucleation/annealing (SNA). The cross-fractionation techniques provided detailed information about the molecular structure of different types of LLDPEs; of particular interest is the TREF-SNA-DSC cross-fractionation which allowed a direct observation of methylene sequence distribution and thus short chain branch (SCB) distribution. TREF-size exclusion chromatography cross-fractionation showed that the molar mass of the Ziegler-Natta LLDPE increased monotonically with decreasing SCB, whereas the plot of Mw vs SCB for the metallocene LLDPE showed a maximum. TREF-SNA-DSC cross-fractionation clearly showed that the metallocene LLDPE only had intramolecular heterogeneity in SCB distribution, whereas the Ziegler-Natta LLDPEs exhibited both intermolecular and intramolecular heterogeneity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 960–967, 2000

Effect of Peroxide Modification on Melt Fracture of Linear Low Density Polyethylene during Extrusion

A commercial linear low density polyethylene (LLDPE) was modified by adding a small amount dicumyl peroxide (DCP) during extrusion at moderate processing temperature (200°C). The effects of peroxide modification were to change the molecular weight and rheological properties, as evident by increase in high molecular portion, shear sensitivity and melt viscosity at low frequency region. These effects were confirmed by gel permeation chromatography (GPC) and Rheometric dynamic spectrometer (RDS). One outstanding feature of peroxide modification was the remarkable delay in the onset of melt fracture upon extrusion. The dilution of DCP in polypropylene (PP) powder was more effective for improvement of these properties than by direct loading of DCP. The observed remarkable delay in melt fracture is believed to be due to increase in high molecular portion by producing long chain branch and crosslinking. That is, DCP modification results in delay in the onset of melt fracture through increase in high molecular portion which can resist high elongational stress when leaving the die.