Linear Low Density Polyethylene Resins Research Articles

Single-screw extrusion performance for LLDPE resin as a function of compression ratio

High-quality polyethylene film and sheet must be free of solid polymer fragments and degraded resin. Moreover, the products must be produced at high rates to minimize conversion costs. Thus, the extruder must produce a homogenous extrudate at the proper discharge temperature and pressure. The screw design and process operating parameters are key. A key screw design parameter is the compression ratio. This paper experimentally studies how the compression ratio affects a linear low-density polyethylene (LLDPE) resin extrusion process. A 2.8 compression ratio screw was optimal for LLDPE resin. Significant solid polymer fragments were observed in the extrudate for 2.8 to 3.6 compression ratio screws at 110 rpm and higher. For 2.0 and 2.4 compression ratio screws at less than 50 rpm, the internal pressures in the extruder were relatively low. At screw speeds 50 rpm and higher, the specific rates for all screws were higher than the calculated specific rotational rate, creating negative axial pressure gradients in the metering sections.

Tuning tear and slow crack growth resistance of linear low-density polyethylene resins through partial replacement of 1-butene with 1-hexene

The present study encompasses the utilization of a mixture of 1-butene and 1-hexene comonomers for the large-scale production of a film-grade linear low-density polyethylene (LLDPE) resin. The goal involves assessing the impact stemming from the partial replacement of 1-butene with 1-hexene on the thermal, physical, mechanical and rheological properties of the resultant resin, juxtaposed against an LLDPE counterpart exclusively produced by 1-butene as the comonomer. Although the comprehensive examination of key physical, thermal, rheological and mechanical characteristics indicates nominal influence due to the substitution, a marked escalation materializes in both the tear resistance and slow crack growth (SCG) resilience of the LLDPE resins formulated through the amalgamation of these comonomers. Specifically, the tear and SCG resistance of the proposed resin exhibit an extraordinary surge exceeding 60% and 30%, respectively. This discernible enhancement underscores the viability of the proposed technique as a propitious avenue for augmenting the requisite mechanical properties, obviating the need for an all-encompassing shift from 1-butene to longer-chain α-olefin comonomers.

Composition analysis of post‐consumer recycled blends of linear low‐ and low‐density polyethylene using a solution‐based technique

AbstractBlends of linear low‐density polyethylene (LLDPE) and low‐density polyethylene (LDPE) are commonly used in film applications, including those made with post‐consumer recycled (PCR) film resins. The ratio of LLDPE/LDPE in PCR resins can vary based on the complexity of the waste stream, making it essential to determine their composition before incorporating them into virgin resins through mechanical recycling. In this study, we propose a quantitative analysis using a solution‐based crystallization elution fractionation (CEF) technique to determine the composition of PCR LLDPE/LDPE blends. The results were compared with those obtained by a standard differential scanning calorimetry (DSC) technique. A series of Ziegler–Natta catalyzed LLDPE resins were blended with LDPE to establish the CEF calibration curves. The CEF calibration curves were found to be similar for 1‐butene LLDPE/LDPE and 1‐hexene LLDPE/LDPE blends. Fourier transform infrared spectroscopy and nuclear magnetic resonance were used as needed to determine the type of comonomer in the LLDPE component for the DSC methodology, which varies the calibration curve. We found that the CEF and DSC techniques provide comparable results in terms of LDPE content for neat polymer blends, depending on the density of each component. However, the proposed CEF technique provides more accurate and reliable results than previously published techniques in estimating LDPE content in PCR LLDPE/LDPE blends that may contain inorganic compounds.

Synthesis of silylamine and siloxyamine compounds: A novel approach to flame retardancy of polypropylene and Epoxy resins

A series of innovative flame retardants containing N-Si and N-O-Si bonds have been synthesized. Their chemical structures were verified by nuclear magnetic resonance (NMR) spectroscopy and thermal stabilities determined by thermogravimetric analysis (TGA). The results indicate that their thermal and hydrolytic stabilities could be adjusted by fine tuning their silylamine (N-Si) and siloxyamine (N-O-Si) skeletons. Thus, the substituents bonded to the phthalimides, such as diphenylmethylsilyl, dimethylphenylsilyl, diphenyl‑tert-butylsilyl, triisopropylsilyl or diphenylmethylsiloxy; whichever was attached to the imide played a significant role on its thermal and hydrolytic properties. Upon thermal dissociation, the N-Si and N-O-Si compounds were shown to yield aminyl, silyl and oxygen-centered radicals that provided fire proofing effect in polypropylene (PP) films alone and a synergistic effect in combination with conventional phosphorous flame retardant in PP, linear low-density polyethylene (LLDPE) and epoxy resins (EP). Their efficacy as flame retardants alone in thin PP films were evaluated according to the DIN4102-B2 test. Both N-Si and N-O-Si significantly slowed down the burning of PP films. Moreover, their potential as adjuvants with a conventional phosphorous based flame retardant, i.e., spirocyclic phosphonate (AFLAMMIT® PCO 900) in PP and EP resins was evaluated according to the UL-94 V fire standard. A synergistic effect was observed when the phosphorous flame retardant was combined with either N-Si or N-O-SI containing compounds, since none of the flame retardants by themselves could provide V-0 rating in the UL94-V test in PP, LLDPE or EP resins even at much higher loadings. The thermogravimetric analysis carried out under inert atmosphere revealed enhanced and earlier onset of decomposition and thermal activation of phosphorous based flame retardant in the presence of N-Si or N-O-Si in comparison to spirocyclic phosphonate, N-Si or N-O-Si used alone, respectively. The interaction of the phosphorus flame retardant with N-Si was verified by 31P NMR.

A rheology and processing study on controlling material and process defects in polymer melt extrusion film casting using polymer blends

The primary objective of this research paper is to control the material and process defects in polymer melt extrusion film casting (EFC) process for linear chain architecture polyethylene (PE) resins through polymer blending methodology. Extrusion film casting is a well-known industrially important manufacturing process that is used to manufacture thousands of tons of polymer/plastic films/sheets and coated products. In this research, the necking defect in an EFC process has been studied experimentally for a linear low density polyethylene (LLDPE) resin and attempts have been made to control its necking by blending in a long chain branched (LCB) low density polyethylene (LDPE) resin. The blending methodology is based on the understanding that a LDPE resin displays enhanced resistance to necking as compared to the LLDPE resin. It is found that added LDPE resin enhances necking resistance for the primary LLDPE resin. Further, as the LDPE concentration increases in the blend formulation, the necking is further reduced as compared to pure LLDPE. Analogous to past studies on EFC of linear and long chain branched architecture containing PEs, it is observed that as the LDPE is increased in the blend formulations, the formulations displayed enhanced melt elasticity and extensional strain hardening in rheological studies. It is concluded from this study that polyethylene resins having linear chain architecture can be made amenable to enhanced resistance to necking using appropriate amount of a long chain branched resins. Finally, process defects such as the draw resonance onset could be shifted to higher draw ratios as the LDPE level is increased in the LLDPE-LDPE blend formulation.

Different Dependence of Tear Strength on Film Orientation of LLDPE Made with Different Co-Monomer.

Crystal orientations, tear strength and shrinkage of Linear Low-Density PolyEthylene (LLDPE) films made with different processes (compressed, cast and blown) were investigated. The films were made with three different LLDPE resins, respectively, which have similar density and molecular weight but are made with different comonomers (1-butene, 1-hexene and 1-octene), in order to investigate if tear strength in Machine Direction (MD) of the LLDPE films made with different comonomer has similar dependence on crystal orientation. Our study indicates that the films made of 1-hexene and 1-octene based LLDPE resins have significantly higher intrinsic tear strength and less decrease in MD tear strength for a given film orientation. That is, for a given orientation in MD, the MD tear drops dramatically for films made with butene-based resin but much less decrease for the films made with hexene and octene-based resins. The shrinkage property at high temperature shows a good correlation with crystal orientation and the fraction of the crystals melted at this temperature.

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

Mechanism of particle build-up on gas-solid fluidization column wall due to electrostatic charge generation

Particle build-up on gas-solid fluidization column wall due to electrostatic charging causes significant operational challenges including reactor shutdown in industrial processes such as gas-phase ethylene polymerization to produce polyethylene. It is well acknowledged that in fluidization process electrostatic charges are generated as a result of continuous particle-particle and particle-vessel wall contacts. However, the mechanism of charged particle attraction towards the fluidization column wall and their adhesion has received minimal attention. This work proposes a mechanism for particle build-up on the column wall by experimentally investigating the fouled particles charge distribution using a charged particle separator apparatus. The experiments were carried with two types of linear low-density polyethylene resins in a pressurized pilot-scale gas-solid fluidization system. Experimental results showed that the polyethylene layer built-up on the column wall contained both positively and negatively charged particles. A mechanism was proposed indicating that charged particle migration towards the metallic column wall was due to the image and electrostatic forces. The image forces were attributed to the particle-wall contacts while the electrostatic forces were between the charged particles fouled on the column wall and those oppositely charged within the bulk of the bed. In addition, the net space charge of the bulk particles contributed to the migration of the particles towards the column wall.

Crystallization kinetics modeling of high density and linear low density polyethylene resins

AbstractThis paper describes isothermal and nonisothermal crystallization kinetics of a Ziegler‐Natta catalyzed high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins. Standard techniques such as differential scanning calorimetry (DSC) and light depolarization microscopy (LDM) techniques were used to measure isothermal kinetics at low supercoolings. DSC was also used to measure nonisothermal crystallization kinetics at low cooling rates. Extrapolation of isothermal crystallization half‐times of Z‐N catalyzed LLDPE resin using the isothermal half‐time analysis led to erroneous predictions, possibly due to Z‐N LLDPE consisting of a mixture of molecules having different amounts of short chain branching (comonomer). However, predicted reciprocal half‐times at high supercoolings, using isothermal half‐time analysis and using nonlinear regression of nonisothermal crystallization kinetics measured at low cooling rates using the differential Nakamura model, of the HDPE were similar to measured reciprocal half times at high supercoolings of a similar HDPE by Patki and Phillips. It is also shown that the differential Nakamura model can be effectively used to model nonisothermal crystallization kinetics of HDPE resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Performance artificial turf components — fibrillated tape

Artificial turf surfaces with “soft” yarns are increasingly used because of haptic attractiveness and player friendliness. Linear low density polyethylene (LLDPE) with low density is preferred to achieve the desired softness. The choice of LLDPE material is guided by the performance requirements outlined in the FIFA Quality Concept for football turf . Different LLDPE materials were extruded into fibrillated tape yarns on pilot and production scale machinery. A key mechanical property of fibrillated yarns is the tear resistance in machine direction influencing the fibrillation process and the ultimate yarn splitting and resulting wear in the field. A high tear resistance makes fibrillating the yarn difficult however improves the ultimate split and wear resistance in the field. Unfibrillated oriented yarns of different composition were analyzed for tear and other mechanical properties. The results are related to previous work on durability of certain yarn compositions which demonstrate the benefits of low density LLDPE resins in terms of durability and wear resistance . As proof of concept one metallocene LLDPE resin with one of the highest tear resistance values currently commercially available was extruded into oriented yarn and subsequently fibrillated. Processing conditions and choice of particular LLDPE greatly influence yarn fibrillation and performance. As already demonstrated in monofilament yarns , shrinkage is identified as an important measure to characterize residual stress in low density LLDPE yarns. We discuss strategies to minimize shrinkage for fibrillated tape made with low density LLDPE and refer to its relation to performance in the Lisport test used to determine the durability of an artificial turf yarn.

Polymer processing and characterization of LLDPE films loaded with α‐tocopherol, quercetin, and their cyclodextrin inclusion complexes

AbstractNatural antioxidant additives were compounded into linear low‐density polyethylene (LLDPE) using a twin‐screw counter‐rotating mixer and compression molded into films. Manufactured LLDPE films contained 2715 mg kg−1 α‐tocopherol in its free and β‐cyclodextrin complexed form and 1950 mg kg−1 quercetin in its free and γ‐cyclodextrin complexed form. Both cyclodextrin complexes were loaded into films at 1.5% by weight. These natural antioxidants were incorporated into LLDPE resins with two different catalyst types, Ziegler‐Natta and metallocene. Films were characterized by optical microscopy, oxidation induction time (OIT), oxygen transmission rate, contact angle analysis, and atomic force microscopy (AFM). All antioxidant additives increased the oxidative stability of LLDPE as measured by increased OIT, particularly quercetin. Natural antioxidants and their cyclodextrin inclusion complexes may provide a dual function in packaging to protect the polymer from oxidative degradation during melt processing and to delay the onset of oxidation of the packaged food during storage. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Simultaneous Deconvolution of the Bivariate Distribution of Molecular Weight and Chemical Composition of Polyolefins Made with Ziegler‐Natta Catalysts

Polyolefins made with Ziegler-Natta catalysts have non-uniform distributions of molecular weight (MWD) and chemical composition (CCD). The MWD is usually measured by high-temperature gel permeation chromatography (GPC) and the CCD by either temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF). A mathematical model is needed to quantify the information provided by these analytical techniques and to relate it to the presence of multiple site types on Ziegler-Natta catalysts. We developed a robust computer algorithm to deconvolute the MWD and CCD of polyolefins simultaneously using Flory's most probable distribution and the cumulative CCD component of Stockmayer's distribution, which includes the soluble fraction commonly present in linear low-density polyethylene (LLDPE) resins and have applied this procedure for the first time to several industrial LLDPE resins. The deconvolution results are reproducible and consistent with theoretical expectations.

Dart Impact Testing of Polyethylene Film: Mechanical Interpretation and Model

A physical model of dart impact tests of film manufactured from semi-crystalline polyethylene resins is developed. The models treat the dart impact tests as a special case of the standard high-speed stress/strain measurement performed on a polymer sample with a linearly changing cross section. They describe the dart impact strength as a complex function of the parameters that characterize the stress-strain curve of the resin: stresses and strains at the yield, necking and breaking points. The models correctly predict the range of the dart impact strength (ASTM D1709, ISO 7765) and are suitable for semi-quantitative characterization and ranking of linear low density polyethylene resins for film applications.

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

Crystalline spherulitic growth kinetics during shear for linear low-density polyethylene

The Linkam shearing cell was used in conjunction with a polarized light microscope to study the isothermal crystallization process of linear low-density polyethylene resins under simple shear flow. The growth of spherulitic morphology was observed under slow shear rates (less than 1 s−1). The crystalline spherulitic growth rate increases, as the shear rate increases. This has been attributed to the decrease of the activation diffusion energy. A relationship between the activation diffusion energy and the shear rate is proposed, under the experimental conditions employed. The modified Hoffman–Lauritzen equation successfully describes spherulitic crystallization kinetics under shear conditions, when the appropriate value of activation diffusion energy is employed. POLYM. ENG. SCI. 46:1468–1475, 2006. © 2006 Society of Plastics Engineers

Diffusion of limonene in polyethylene

Diffusion coefficients of limonene in various linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) resins have been determined from sorption data using a thermogravimetric methodology. From these data, one can determine whether polymer synthesis parameters such as the choice of catalytic process or co-monomer result in substantial differences in how much food packaging additives might migrate to food. For example, LLDPE is currently manufactured using either one of two distinct catalytic processes: Ziegler–Natta (ZN) and metallocene, a single-site catalyst. ZN catalysis is a heterogeneous process that has dominated polyolefin synthesis over the last half-century. It involves a transition metal compound containing a metal-carbon bond that can handle repeated insertion of olefin units. In contrast, metallocene catalysis has fewer than 20 years of history, but has generated much interest due to its ability to produce highly stereospecific polymers at a very high yield. In addition to high stereospecificity, metallocene-catalysed polymers are significantly lower in polydispersity than traditional ZN counterparts. Absorption and desorption testing of heat-pressed films made from LLDPE and LDPE resins of varying processing parameters indicates that diffusion coefficients of limonene in these resins do not change substantially.

Natural weathering test for films of various formulations of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE)

Natural (outdoor) weathering test was performed to investigate the UV stability of thin films (0.06 mm) of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). The PE films were prepared from various formulations of LLDPE and LDPE resins. Some of these films contained a single high molecular mass HALS only, along with a primary antioxidant (i.e. Irganox 1010) and a secondary antioxidant (i.e. Irgafos 168 or Alkanox TNPP), while others contained HALS and UVA (i.e. Chimassorb 81 or Tinuvin P or Tinuvin 326) along with these antioxidants. The HALS used was either an oligomeric or a synergistic mixture of a high molecular mass (HMM) hindered amine stabilizer and co-additives. The UV stability was investigated by exposing the prepared films at 45° towards south in the direct sunshine up to 365 days. Fifty percent of tensile strength retention was determined for all these exposed films and it was found that the films containing a single HALS gained improved UV stability by about two to 12 fold over the pure films. On the other hand, films that contained a combination of HALS and UVA obtained further improved UV stability over the films containing a single HALS (both have antioxidants). Films containing a single HALS reached 50% TS retention within 205 days, whereas, films containing a combination of HALS and UVA reached 50% TS retention within 590 days, which is about three times further improvement in UV stability.

Does the length of the short chain branch affect the mechanical properties of linear low density polyethylenes? An investigation based on films of copolymers of ethylene/1-butene, ethylene/1-hexene and ethylene/1-octene synthesized by a single site metallocene catalyst

Three nearly identical linear low density polyethylene resins based on copolymers of ethylene with 1-butene (B), 1-hexene (H) and 1-octene (O) were utilized to investigate the effect of short chain branch length on the mechanical properties of blown and compression molded (quenched and slow cooled) films. The content of short chain comononer in the three copolymers was ca. 2.5–2.9mol% that corresponded to a density of 0.917–0.918g/cm3. Within a given series, the tensile properties of these films do not show any significant difference at slow deformation rates (up to 510mm/min), even though the DSC and TREF profiles of ‘H’ and ‘O’ differed slightly in comparison to ‘B’. However, at higher deformation rates (ca. 1m/s), the breaking strength of these films was found to increase with increasing short chain branch length. In addition, the Spencer impact and Elmendorf tear strength of the blown films were also observed to increase with increasing short chain branch length. Further, dart impact strength and high-speed puncture resistance (5.1m/s) of 1-octene and 1-hexene based samples was also observed to be higher than that based on 1-butene. The blown films displayed low and comparable levels of equivalent in-plane birefringence and crystalline orientation by wide angle X-ray scattering. This confirms that the differences in mechanical properties in the blown film series are not attributable to differences in molecular orientation. The deformation behavior of both the compression molded and blown films were also investigated in a well-defined controlled regime by analyzing their essential work of fracture. It was found that the essential work of fracture of films based on 1-hexene and 1-octene was higher than that of films based on 1-butene. While the origin of these differences in mechanical properties with increasing short chain branch length is not fully understood, the present investigation confirms this effect to be pronounced at high deformation rates for both the blown and compression molded quenched films.

Determination of the Percentage of Homopolymer Component in Ziegler/Natta Catalyst Linear Low-Density Polyethylene Resins Using High-Temperature Cell Fourier Transform Infrared and Partial Least Squares Quantitative Analysis Technique

A new method for the determination of the percentage of homopolymer component, using high-temperature cell Fourier transform infrared (FT-IR) by partial least squares (PLS) quantitative analysis technique, was developed and applied to Ziegler Natta linear low-density polyethylene (LLDPE). The method is based on the IR spectrum changes between the 730 cm(-1) band and 720 cm(-1) band at the temperature of 110 degrees C, which is near the melting point of the polyethylene. The HD % (the percentage of high-density component, i.e., the percentage of homopolymer component) results obtained by CTREF (CRYSTAF in TREF mode) technique are used as the input data together with the respective FT-IR spectra for PLS analyses to establish a calibration curve. The PLS quality is characterized by a correlation coefficient of 0.997 (cross-validation) using four factors and a root mean square error of calibration (RMSEC) of 0.772. The HD% of the unknown can then be predicted by the PLS software from the unknown FT-IR spectrum. A control resin was tested seven times by CTREF and FT-IR. The HD% of the control resin was 28.59+/-0.88% by CTREF and 29.05+/-2.37% by FT-IR. It was found that the method was applicable for the same comonomer type of LLDPE within a melt index range and density.

Metallocene LLDPE Films for Heavy-Duty Sack Applications

This article describes (1) the applications for heavy-duty sacks, (2) the technical requirements for the sacks, and (3) the films and resins used to meet the critical requirements for this industry. It also describes how the technical requirements of the sack are determined. Finally, it shows how gas phase metallocene-catalyzed hexene copolymer linear low density polyethylene resins are used to produce downgauged sacks with tailored properties to meet the needs of this industry.