High-pressure Low-density Polyethylene Research Articles

110th Anniversary: Nonideal Mixing Phenomena in High-Pressure Low-Density Polyethylene Autoclaves: Prediction of Variable Initiator Efficiency and Ethylene Decomposition

A segregation–backmixing model is developed to simulate the dynamic operation of multifeed high-pressure low-density polyethylene (LDPE) autoclaves, calculate the specific initiator consumption (gr...

Thermorheology and Crystallization Behaviors of Polyethylenes: Effect of Molecular Attributes

Correlations between polyethylenes of different compositions and branching architectures and the temperature dependence of their viscoelastic behavior as well as the dependence of the nonisothermal crystallization behaviors on the cooling rate were described. To analyze the thermorheological behavior of the various classical polyethylenes, a method proposed by van Gurp and Palmen was utilized and the classical high-pressure low-density polyethylene (LDPE) was found to be thermorheologically complex, while for high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), thermorheological simplicity was observed. The Avrami and Mo methods were applied to describe the nonisothermal crystallization kinetics of the different PEs for various cooling rate. The values of the kinetic parameter F(T), kinetic crystallization rate constant (Zc), and half-time of crystallization (t1/2) indicated that long-chain branching (LCB) had the role of being a heterogeneous nucleating agent and accelerated the ...

Experimental study of the temperature dependence of the permittivity of nanocomposite materials based on a low-density polyethylene matrix with iron nanoparticles

The temperature dependence of the dielectric permittivity of nanocomposites based on a high-pressure (low-density) polyethylene matrix with various concentrations of dispersed iron nanoparticles (2–22 wt %) has been studied. As the temperature increases from 0 to 40°C, the permittivity decreases for all concentrations of nanoparticles in the range studied. The results can be useful for developing thermostable acoustic waveguides.

Investigation of the optical characteristics of composite materials based on cadmium sulfide nanoparticles stabilized in a high-pressure polyethylene matrix

Composite materials based on cadmium sulfide nanoparticles in an inert dielectric matrix of high-pressure (low-density) polyethylene are experimentally studied. It is shown how the size and weight concentration of nanoparticles in the bulk of the stabilizing matrix affect the nanocomposite absorption. The experimental results are interpreted based on the two-flux Kubelka-Munk model.

Prediction of the Bivariate Molecular Weight‐Long Chain Branching Distribution in High‐Pressure Low‐Density Polyethylene Autoclaves

AbstractIn the present study a population balance approach is described to follow the time evolution of bivariate molecular weight‐long chain branching (MW‐LCB) distributions in high pressure low density polyethylene autoclaves. The model formulation is based on a sectional grid method, the so‐called fixed pivot technique (FPT). According to this method, the ‘live’ and ‘dead’ polymer chain populations are assigned to a selected number of discrete points. Then, the resulting dynamic discrete‐continuous molar species equations for ‘live’ and ‘dead’ polymer chains are solved at the specified grid points. It is shown that a very good agreement exists between theoretical results and experimental data which proves the capability of the FPT method in calculating the joint MW‐LCB distribution for branched polymers.magnified image

Effect of blending high‐pressure low‐density polyethylene on the crystallization kinetics of linear low‐density polyethylene

AbstractIt is well known that the addition of a small amount of high‐pressure low‐density polyethylene (HP‐LDPE) to linear low‐density polyethylene (LLDPE) can improve the optical properties of LLDPE, and LLDPE/HP‐LDPE blend is widely applied to various uses in the field of film. The optical haziness of polyethylene blown films, as a result of surface irregularities, is thought to be as a consequence of the different crystallization mechanisms. However, not much effort has been directed toward understanding the effect of HP‐LDPE blending on the overall crystallization kinetics (k) of LLDPE including nucleation rate (n) and crystal lateral growth rate (v). In this study, we investigated the effect of blending 20% HP‐LDPE on the crystallization kinetics of LLDPE polymerized by Ziegler‐Natta catalyst with comonomer of 1‐butene. Furthermore, by combining depolarized light intensity measurement (DLIM) and small‐angle laser light scattering (SALLS), we have established a methodology to estimate the lateral growth rate at lower crystallization temperatures, in which direct measurement of lateral growth by polarized optical microscopy (POM) is impossible due to the formation of extremely small spherulites. This investigation revealed that HP‐LDPE blending leads to enhanced nucleation rate, reduced crystal lateral growth rate, and a slight increase in the overall crystallization kinetics of pure LLDPE. From the estimated crystal lateral growth rate, it was found that the suppression in v from HP‐LDPE blending is larger at lower temperatures than at higher temperatures. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Electrical properties and morphology of polyethylene produced with a novel catalyst

In this paper electrical properties of a novel linear low-density polyethylene synthesized with a catalyst, developed by our unique technique, were measured. It was found that this polyethylene had a higher volume resistivity, a higher breakdown strength and less spacer charge accumulation than conventional high-pressure low-density polyethylene and linear low-density polyethylene. We also measured electrical properties of blends of the new polyethylene and the conventional low-density polyethylene and found that the blends which contained up to 90 wt% of the low-density polyethylene had nearly the same value as the new polyethylene. The morphologies of the polyethylene were observed by using transmission electron microscope (TEM) and found that the blends kept the characteristics shown in the new polyethylene up to 90 wt% of the low-density polyethylene content. The relationship between morphology and electrical properties is discussed and comparison is made between the new polyethylene with those of the conventional low-density polyethylenes.

Multivariate Statistical Monitoring of a High‐Pressure Polymerization Process

The high pressure LDPE (low density polyethylene) industrial process operates under supercritical conditions, and so it is necessary to monitor its performance to prevent abnormal situations. Extre...

HIGH CLARITY LINEAR LOW DENSITY POLYETHYLENES FOR FOOD PACKAGING APPLICATIONS

The food packaging market includes many rapidly developing, increasingly demanding film applications. Important characteristics for food packaging are heat seal and hot tack properties, puncture resistance, and low temperature properties. Each type of polyethylene has unique strengths and weaknesses that contribute to the overall property balance and define specific markets in which the polyethylene can effectively compete. In the past two decades, many new polyethylenes have been introduced into packaging markets. These new polyethylenes have provided increasing toughness, allowing film manufacturers to produce films at reduced gauges, while maintaining physical properties. High pressure low density polyethylene (LDPE) continues to be a key player in many food packaging markets, especially those in which high clarity is required. A major disadvantage of LDPE is poor impact and tear strength. For increased impact and tear strength, film extruders have traditionally turned to linear low density polyethylenes (LLDPE) and have been forced to sacrifice clarity. These new high clarity LLDPE's allow film extruders to economically produce products with both toughness and clarity using existing equipment.

EFFECT OF METALLOCENE POLYETHYLENE ON HEAT SEALING PROPERTIES OF LOW DENSITY POLYETHYLENE BLENDS

Laminations made with blends of metallocene polyethylene (m-PE) and high-pressure low density polyethylene (LDPE) were prepared to investigate the effect of the former resin on heat sealing performance. According to various experimental studies of seal strength, hot tack and differential scanning calorimeter (DSC), it could be concluded that percentages around 15% of m-PE allow the obtainment of the optimal balance for heat sealing properties and cost. Using the interdiffusion theory of heat sealing, a thorough discussion of the results was done.

Dynamic modeling of multizone, multifeed high-pressure LDPE autoclaves

A comprehensive mathematical model is developed to simulate the dy- namic behavior of multizone, multifeed high-pressure ethylene polymerization auto- claves. To describe the complex flow patterns occurring in low-density polyethylene (LDPE) autoclaves, a user-specified multisegment, multirecycle model representation of the actual multizone reactor is established. A general reaction mechanism is em- ployed to represent the kinetics of ethylene polymerization. Dynamic mass, molar species, and energy balances are derived to predict the polymerization rate, monomer conversion, molecular weight developments (e.g., Mn, Mw, long- and short-chain branching), and temperature profile with respect to time and spatial position in the reactor. Detailed results on the start-up and grade transition of a four-zone autoclave reactor are presented and the effects of the macromixing parameters (e.g., number of segments per reaction zone and the total and side external recycle ratios) on the dynamic behavior of the reactor are investigated. It is shown that the model macro- mixing parameters can significantly affect the initiator consumption rate in a reaction zone. The present model is capable of predicting accurately the dynamic behavior of LDPE autoclaves and, thus, can be employed in the design, optimization, and control of these reactors. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2327-2348, 1999

Easy Processing Metallocene Polyethylene

A new family of metallocene polyethylene (mPE) has an outstanding combination of processability, toughness and stiffness. It provides an opportunity to downgauge and improve extrusion output in applications where blends of conventional Ziegler-Natta catalyzed linear low density polyethylene (ZN-LLDPE) and high pressure low density polyethylene (HP-LDPE) are currently used. The processability of this new product was found to be significantly better than that of ZN-LLDPE and comparable to that of ZN-LLDPE/HP-LDPE blends. Its film properties are substantially better than blends of ZN-LLDPE and HP-LDPE. Its dart impact strength is 60% better than that of a comparable ZN-LLDPE product.

Effect of molecular structure in branched polyethylene on adhesion properties with polypropylene

Adhesion properties between branched polyethylene (PE) and isotactic polypropylene (PP) were studied by a peel test and scanning electron microscopy. In this study, two types of branched PEs were used; one is a linear low density polyethylene (LLDPE) and the other is a high pressure low density polyethylene (LDPE). The adhesive strength of the LLDPE/PP is much higher than that of LDPE/PP. Furthermore, the formation of PE influxes between PP spherulites has a small effect on the adhesion. The dynamic viscoelastic measurements for the binary blends composed of branched PE and PP were also carried out to estimate the interfacial tension by using a rheological emulsion model proposed by Palierne. The interfacial tension is 1.0 mN for LLDPE/PP and 2.1 mN for LDPE/PP, suggesting that the interfacial thickness of LLDPE/PP is about twice that of LDPE/PP. The adhesive strength between branched PE and PP will be determined by the interfacial thickness, which represents the entanglements between two polymers. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 457–463, 1998

Supervisory Control of High Pressure LDPE Reactors

A comprehensive mathematical model was employed to simulate the dynamic behaviour of ethylene polymerization in high-pressure autoclave reactors. The model was capable of describing the complex mixing patterns occurring in multizone, multifeed low density polyethylene (LDPE) autoclaves. To represent the kinetics of ethylene polymerization a general reaction mechanism was considered and the method of moments was used to calculate the molecular properties. A two-zone autoclave reactor model was considered for our control studies. The reactor opiated at an unstable steady-state, thus, the temperature in each zone was controlled by manipulating the corresponding initiator flow rate. A Quadratic Dynamic Matrix Controller (QDMC) was designed for controlling the polymerization temperature in a twozone autoclave and its performance was compared to that of two SISO PI controllers, usually employed to control the polymerization temperature. Finally, a supervisory QDMC controller was implemented to a simulated reactor model for the optimal control of polymer quality (e.g., melt index) during reactor start-up and grade transitions.

Oxidation behavior and stabilization of metallocene and other polyolefins

AbstractEXXPOL® metallocene catalyst technology featuring very high‐productivity catalyst systems is expected to provide ethylene polymer compositions that show excellent oxidative stability based on their minimal unsaturation content and low innocuous metal residue of a non‐acidic nature. Metallocene ethylene polymers such as mLLDPE should be comparable to the most stable, commercial polyethylenes such as linear low‐density polyethylenes (LLDPE) from the UNIPOL® gas phase PE process.As background, the oxidation behavior and stabilization case of “total residue” polyolefins based on Ziegler‐Natta or Cr catalysis systems are reviewed. The interacting effects of polymer structure (i.e., tertiary hydrogen and unsaturation) and catalyst residue are discussed in terms of competing degradation reactions ‐ e.g., chain breakage, chain enlargement (or cross‐linking) and chain branching. The metal (Ti, V and Al) and chlorine residues from Ziegler‐Natta catalyst systems and the Cr residue for Cr catalyst systems can play a role in adversely affecting polymer oxidative stability. The residue levels and their nature govern whether there is an appreciable effect on oxidative stability, and therefore the stabilization requirements.Discussed are thermo‐oxidative studies under melt processing conditions which compare selected EXXPOL®/UNIPOL® mLLDPEs with several state‐of‐the‐art Ti/Mg and Cr catalyzed LLDPE from the UNIPOL® gas phase PE process and a high‐pressure, low‐density polyethylene (HP‐LDPE). Standard melt flow and discoloration tests are combined with specialized experiments using dynamic oscillatory shear rheometry and size exclusion chromatographyviscometry to illustrate performance shifts due to polymer composition differences under melt processing conditions. The advantages of dynamic melt rheometry are illustrated in terms of sensitivity to subtle degradation changes in molecular structure. EXXPOL®/UNIPOL® mLLDPE having low unsaturation and metal residue which are non‐catalytic to auto‐oxidation show excellent intrinsic oxidative stability. The discoloration resistance of EXXPOL®/UNIPOL® mLLDPE is high, comparable with HP‐LDPE and the high‐activity Cr catalyzed LLDPE. Stabilization ease for EXXPOL®/UNIPOL® mLLDPE is equivalent to product counterparts (similar MI and density) made using high‐activity, state‐of‐the‐art Cr or Ti/Mg catalyst systems designed for use in the UNIPOL® gas phase PE process.

Non-parametric confidence bounds for process performance monitoring charts

Statistical Process Control (SPC) provides a tool for achieving and maintaining product quality. In today's climate of major data monitoring campaigns there has been an increase in interest in the multivariate statistical projection techniques of principal components analysis and projection to latent structures for process performance monitoring. Within univariate SPC, techniques for identifying when a process is moving out of control are well established. Similar guidelines are required for multivariate statistical process control (MSPC). Two approaches will be discussed - Hotelling's T2 statistic and a new approach, the M2 statistic. Both approaches will be illustrated by application to a high pressure low density polyethylene tubular reactor and to a batch methyl methacrylate polymerisation reactor.

Confidence Bounds for Multivariate Process Performance Monitoring Charts

Statistical Process Control (SPC) provides a tool for achieving and maintaining product quality. Traditional methods have focused on the monitoring of one quality variable at a time. However, in todays climate of major data monitoring exercises this approach is unrealistic and as a result there has been an increase in interest in the multivariate statistical projection techniques of principal components analysis and projection to latent structures for process performance monitoring. There still remain a number of questions which require to be addressed before these new approaches to monitoring can be widely adopted by industry. Within univariate SPC, techniques for identifying when the process is moving out of control are well established and are based both on sound statistical practice and engineering knowledge built up over a number of years. Similar underpinning guidelines require to be established for multivariate statistical process control (MSPC). Two approaches to this problem of defining control limits will be discussed - those based upon Hotelling's T2 statistic and a new approach based upon kernel density estimation and the standard bootstrap, the M2 statistic. Both approaches will be illustrated by application to a high pressure low density polyethylene tubular reactor and to a batch methyl methacrylate polymerisation reactor.

On-line optimization of a high-pressure low-density polyethylene tubular reactor

Low-density polyethylene (LDPE) is a high-volume commodity polymer that is produced in high-capacity tubular and autoclave reactors. The extreme operating conditions employed in the high-pressure free-radical polymerization of ethylene, and the tight profit margins of the LDPE market provide a strong economic incentive to seek the optimal operation of a high-pressure LDPE reactor. In the present work, an on-line multi-level optimization procedure is developed for a high-pressure tubular LDPE reactor based on a comprehensive steady-state mathematical model of the process. Following the well-established separation hypothesis, the problem is hierarchically decomposed in two phases: (i) the parameter estimation phase where certain key model parameters are adjusted, based on selected process measurements, in order to eliminate any process/model mismatch; (ii) the optimization phase where the adapted model is optimized by minimizing a specified objective function defined in terms of the ethylene conversion and the “quality” of the final polymer product. The proposed optimization algorithm is successfully tested against the operation of a large-scale industrial reactor. The most prominent features of the procedure are highlighted. These include the development of a comprehensive mathematical model of the process, the disturbance oriented selection of the adjustable model parameters, the appropriate spatial decomposition of the parameter estimation problem into a number of subproblems and the conditional structuring of the optimization problem.

Sizes of long‐chain branches in a high‐pressure low‐density polyethylene as a function of molecular weight

AbstractThe mean length and the indices of long‐chain branches (LCBs) of a high‐pressure lowdensity polyethylene (HPLDPE) as a function of molecular weight have been determined for its molecular‐weight‐tractionated parts by the 13C‐NMR analysis and the viscosity measurements. The mean LCB length of the fractions was of the order of 200–300 carbons in length. The size of LCBs increases with increasing molecular weight, but the size of LCBs relative to the overall macromolecular size decreases with increasing molecular weight. The LCB sizes as a function of molecular weight determined for the fractions of one parent HPLDPE are in good agreement with those previously reported for HPLDPE whole polymers.

Film Properties and Applications for Lldpe and High Pressure Ldpe Blends

Many papers have been written presenting properties expected from different blends of linear low density polyethylene (LLDPE) and high pressure low density polyethylene (LDPE) [1-3]. This paper attempts to corre late the properties obtained from blends with specific end-use markets (i.e., gar ment bags, shrink wrap, ice bags, and general purpose bags). The effect on ex trusion rate, bubble stability and draw down are discussed for blends of butene and hexene LLDPE with fractional melt index (MI), 2 MI, and 6 MI LDPE homopolymers as well as 3% vinyl acetate (VA)-LDPE copolymers.