Processing Of Polyolefins Research Articles

Characterization of the abiotic degradation of oxo-biodegradable polypropylene obtained from transition metal-free pro-oxidant

The pro-oxidant additives used in the production of oxo-biodegradable polymers are generally organic salts of transition metals added during the processing of polyolefins. Transition metals accelerate the photooxidation and thermal oxidation of macromolecules, producing oxygenated fragments which are then used as nutrients by microorganisms in the biodegradation process. However, the use of these additives can lead to contamination of the environment by metals, which come free from the reactions. The aim of this study was to obtain an oxo-biodegradable polypropylene by processing flat films with distinct levels of benzoin, an organic substance free of transition metals, which is biodegradable. After extrusion (t = 0 h), samples were submitted to OIT analysis, which showed that benzoin favors the thermal oxidation process. Subsequently, the samples were subjected to accelerated ageing in a UV chamber and were characterized by dilute solution viscometry (DSV), FTIR, TGA, DSC and SEM. Results of the DSV analysis revealed a significant reduction in the viscosimetric average molar mass of the samples containing the additive, which was accompanied by a considerable increase in carbonyl groups, according to the carbonyl indexes calculated from the FTIR spectra. Thermogravimetric analysis of the aged samples containing benzoin indicated a greater loss of thermal stability and the DSC thermograms revealed a considerable reduction in the melting temperature of the oxo-biodegradable PP. The SEM images of the aged samples indicated that the presence of the additive favored the appearance of regions containing flaking and erosion. In general, the results clearly indicated that benzoin accelerated the photooxidation process during ageing by UV irradiation. Unlike organic salts of transition metals, benzoin seems to act in the initiation step of photo- and thermal oxidation, as demonstrated at the end of this study.

Thermal decomposition of polyolefins under different oxygen content. Composition of products and thermal effects

The thermal decomposition of polyethylene and polypropylene was studied under conditions of different oxygen contents in the carrier gas in the temperature range of 200–600 °C. The effect of adding an oxidizing agent to the carrier gas was studied at (O2/N2)mol ratios of 0/100; 1/99; 2/98; 4/96; 6/94; 10/90; 21/79. Combined TG-FTIR and TG-DSC techniques were used in the studies. Based on FTIR spectra, a quantitative and qualitative analysis of the process gaseous products was performed. The study showed that the thermal degradation of polyolefins can lead to mixtures of saturated and unsaturated hydrocarbons, which are an attractive substitute for natural gas for hydrogen production. The molar ratio of saturated/unsaturated hydrocarbons for PE and PP is 3/2. The composition of the gas product mixture as well as the heat balance of the process can be controlled by adjusting the oxygen concentration in the carrier gas. With a content of 3%O2 in the carrier gas for PE and 4%O2 for PP, autothermal conditions of the thermal processing of polyolefins are achieved. The addition of oxygen accelerates the transformation of polyolefins and thus lowers the optimal process temperature. The CxHyO and CO mixture, coexisting with the hydrocarbon mixture in some process variants, can be converted by the water shift reaction into a mixture of H2 and CO2.

Melt memory in random ethylene-1-alkene copolymers.

Chain flexibility or stiffness based polymer conformation plays a crucial role in affecting the dynamics and kinetics of polymers, which is related to the hierarchical architecture of chains. A series of random copolymers of ethylene and 1-alkenes including 1-hexene, 1-octene, and 1-dodecene were synthesized with metallocene catalysts. The crystallization behavior and memory effect in random ethylene-1-alkene copolymers with different side groups were investigated via differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). Rheological tests were performed for understanding their dynamical behavior. The results show that the melting peak and the viscosity decrease but the orthorhombic crystal dimensions increase with co-unit contents increasing in the copolymers. It was found that the scaling relationship between the zero shear viscosity (η0) and molecular weight (Mn) of the copolymers containing ethylene-1-hexene and ethylene-1-octene is 3.6, which is higher than the classical scaling value of 3.4. The memory of crystals in the melt is enhanced with the increase of 1-alkene contents but is independent of the types of 1-alkenes. The enhanced melt memory effect in the copolymers was proposed due to the effect of the 1-alkene based side groups on the dynamics of polymer chains. The present work would be helpful to understand the chain stiffness based polymer dynamics and processing of polyolefins and copolymers prepared with the metallocene catalyst.

Additive Manufacturing of Polyolefins.

Polyolefins are semi-crystalline thermoplastic polymers known for their good mechanical properties, low production cost, and chemical resistance. They are amongst the most commonly used plastics, and many polyolefin grades are regarded as engineering polymers. The two main additive manufacturing techniques that can be used to fabricate 3D-printed parts are fused filament fabrication and selective laser sintering. Polyolefins, like polypropylene and polyethylene, can, in principle, be processed with both these techniques. However, the semi-crystalline nature of polyolefins adds complexity to the use of additive manufacturing methods compared to amorphous polymers. First, the crystallization process results in severe shrinkage upon cooling, while the processing temperature and cooling rate affect the mechanical properties and mesoscopic structure of the fabricated parts. In addition, for ultra-high-molecular weight polyolefins, limited chain diffusion is a major obstacle to achieving proper adhesion between adjunct layers. Finally, polyolefins are typically apolar polymers, which reduces the adhesion of the 3D-printed part to the substrate. Notwithstanding these difficulties, it is clear that the successful processing of polyolefins via additive manufacturing techniques would enable the fabrication of high-end engineering products with enormous design flexibility. In addition, additive manufacturing could be utilized for the increased recycling of plastics. This manuscript reviews the work that has been conducted in developing experimental protocols for the additive manufacturing of polyolefins, presenting a comparison between the different approaches with a focus on the use of polyethylene and polypropylene grades. This review is concluded with an outlook for future research to overcome the current challenges that impede the addition of polyolefins to the standard palette of materials processed through additive manufacturing.

Optimization of Polyethylene Grade Transitions in Fluidized Bed Reactors with Constraints on the Polymer Sticking Temperature

In gas-phase polyolefin processes, it is important to evaluate the melting temperature of particles since exceeding this temperature may cause particle sticking and aggregation. In this work, a mod...

Возможность применения пластометра Брабендера для изучения пероксидной сшивки полиэтилена. Часть 1. Пероксидная сшивка полиэтилена в присутствии триаллилцианурата

In this work, we address the problem of processing of polyolefins in the production of which used cross-linking agents based on peroxides. The peroxide method of production of foamed polymers is one of the most popular in Russia, so the relevance of its improvement is not in doubt. The main problem with working with peroxides is the high content of by-products, which are often toxic and have a negative impact on the staff. In this article, the authors proposed to reduce the concentration of toxic peroxide used by adding a less toxic crosslinking agent – triallylcyanurate to the formulation. The degree of crosslinking of polyethylene is directly proportional to the viscosity of the system, so It was proposed to use the brabender plastometer for studies of the viscosity index of samples. In this work, it was found that the optimal degree of crosslinking has samples with a torque indicator in the range of 1700-1900 g∙m. it was found that adding 2 m.p. to the formulation. triallylcyanurate allows, while maintaining the necessary degree of crosslinking, to reduce the peroxide content by 50%. Lowering the peroxide content directly reduces the amount of volatile acetophenone-the product of thermal decomposition of peroxide, which accumulates in the air of the working area. The dependence of the torque index of samples on the amount of triallylcyanurate and Dicumyl peroxide content was studied. The minimum concentration of Dicumyl peroxide (0.2 m.p.) was determined, which allows to obtain the degree of crosslinking of the required level. It is shown that the replacement of Dicumyl peroxide with the crosslinking agent triallylcyanurate makes it possible to obtain a foam that is not inferior to the original composition in terms of main performance indicators.

The Relevance of Multi‐Injection and Temperature Profiles to Design Multi‐Phase Reactive Processing of Polyolefins

AbstractFor industrial‐scale continuous free radical induced grafting of polyethylene, which involves the contact of a monomer‐ and polymer‐rich phase, it is demonstrated that the functionalization selectivity and grafting density can be significantly improved (e.g., 100%) by considering multiple injection of monomer and/or a temperature profile. With parameters taken from literature, it is highlighted through simulations that step‐wise addition of monomer allows to reduce the homopolymerization rate, and an increased temperature at the start of the reactive processing allows to increase the hydrogen abstraction rate, so that more potential grafting points can be generated. As this increased grafting density goes along with a lower reaction extent, a better process efficiency is obtained albeit at a lowering of the average chain length of the grafts and an increase of the crosslinking density. In any case, a broader span of polymer characteristics (e.g., ranges of grafting/crosslinking densities and functionalization yields) can be achieved by including variations in reactant addition and temperature profiles.

110th Anniversary: An Effective Methodology for Kinetic Parameter Estimation for Modeling Commercial Polyolefin Processes from Plant Data Using Efficient Simulation Software Tools

Polyolefins are one of the most widely used commodity polymers with applications in films, packaging, and the automotive industry. The modeling of polymerization processes producing polyolefins, including high-density polyethylene (HDPE), polypropylene (PP), and linear low-density polyethylene (LLDPE) using Ziegler–Natta catalysts with multiple active sites, is a complex and challenging task. Most of the studies on polyolefin process modeling over the years do not consider all of the commercially important production targets when quantifying the relevant polymerization reaction kinetic parameters based on measurable plant data. Most of the published articles also do not make efficient use of simulation tools, particularly sensitivity analysis, design specifications, and data fit, that are available in commercial modeling software for polymerization processes, such as Aspen Polymers. This paper presents an effective methodology to estimate kinetic parameters that have the most significant impacts on specif...

A two-phase stochastic model to describe mass transport and kinetics during reactive processing of polyolefins

Free radical induced grafting (FRIG) of polyolefins is described at industrially relevant time scales at which phase segregation occurs, leading to the formation of a monomer- and polyolefin-rich phase. The mass transfer of oligomeric species is accounted for using overall average mass transfer coefficients. Model validation is illustrated for FRIG with a non-homopolymerizable monomer based on experimental data on monomer conversion and functionalization degrees. Comparison with an approximate single-phase model highlights that due to phase segregation the reaction rate and the functionalization degree significantly decrease due to a different role of diffusional limitation on termination, whereas the grafting degree and density are unaffected on an overall monomer conversion basis. This mismatch with the single-phase model is enlarged with a homopolymerizable monomer. The polyolefin-rich phase is there characterized by lower monomer concentrations whose grafting kinetics are strongly affected by depropagation, compensating for influences caused by diffusional limitations on termination. For such monomers, the functionalization selectivity can display even a maximum as a function of the overall monomer conversion.

Dynamic simulation of liquid polymerization reactors in Sheripol process for polypropylene

This work executed mathematical modelling and dynamic simulation of a polyolefin process with pre-polymerization and two loop reactors. Transient changes occurring within the reactors were simulated based on a one-dimensional plug flow assumption. Temperature control in the reactors is germane to the realization of the desired products because of the exothermic reactions, and this was accounted for using cooling jackets. The gPROMS package was utilized as a software platform for modelling and simulation.The results identified the physicochemical changes in each reactor. Molecular weight and polydispersity were predicted. Simulation results were validated with commercial data and agree well.

A Novel (SiO2/MgO/MgCl2)·TiClx Ziegler–Natta Catalyst for Ethylene and Ethylene/1‐Hexene Polymerization

A novel (SiO2/MgO/MgCl2)·TiClx Ziegler–Natta catalyst for ethylene and ethylene/1‐hexene polymerization is successfully developed through impregnation of aqueous solution of water‐soluble Mg‐compounds such as magnesium acetate on silica gel, and forming a supported thin layer of magnesium oxide on the surface of SiO2 after high temperature calcination in dry air or oxygen, followed by further reacting with titanium tetrachloride to synthesize the magnesium dichloride carrier in situ and to support the titanium species simultaneously. The method is relatively simple and the resulting catalysts exhibit high activity, good hydrogen response, and copolymerization ability with high 1‐hexene incorporation. Unlike the traditional industrial Ti/Mg Ziegler–Natta catalysts using the relatively expensive, anhydrous and moisture‐sensitive MgCl2, Mg(OEt)2, MgR2 or RMgCl as Mg‐sources, the most unique feature of this novel catalyst is its capability of utilization of any soluble Mg‐compounds and thus shows great potential for application in commercial gas‐phase polyolefin processes. image

INFLUENCE OF NETWORK ON THE INTERFACIAL DIFFUSION OF MULTILAYER POLYETHYLENE BLENDS

Ultrahigh molecular mass polyethylene(UHMWPE) and statistical copolymer of ethylene and 1-hexene(PEH) were used to prepare UHMWPE/PEH blends with relatively low UHMWPE concentrations.UHMWPE forms network structure in the blends with UHMWPE concentrations above a critical value of c=1 wt%.The influence of the formed UHMWPE network on the interfacial diffusion behaviors of multilayer UHMWPE/PEH blends was investigated by applying small oscillatory shear rheology.The interfacial diffusion coefficients of multilayer UHMWPE/PEH blends were obtained from the rheological data.It is found that the diffusion coefficients are time-dependent and two diffusion regions are distinct before the diffusion reaches the plateaus according to the changes of diffusion exponent n in the relation of D-t n,which implies an approximate Fickian diffusion behavior in region Ⅰ(c 1.0 wt%) and an obvious deviation from the Fickian diffusion behavior in region Ⅱ(c1.0 wt%).With the formation of UHMWPE network in the UHMWPE/PEH blends,deviation from the Fickian behavior for diffusion of multilayer UHMWPE/PEH blends becomes significant.The above results help facilitate the industrial processing of multilayer polyolefins,which is related to the interfacial diffusion.

Rheo-chemistry in Reactive Processing of Polyolefin

Abstract A brief review of reactive processing of polyolefins, which are the most widely used polymers today, is given in this paper from the role of flow field in processing and the characterization of the modified chain topologies. The materials are mainly focused on polyethylene (PE), polypropylene (PP) and polyolefin elastomer (POE) here. Aspects of reactions, coupling/crosslinking and scission are discussed both in terms of the mechanism and kinetics under different flow conditions.

Prediction of the melt flow index using partial least squares and support vector regression in high-density polyethylene (HDPE) process

In polyolefin processes the melt flow index (MFI) is the most important control variable indicating product quality. Because of the difficulty in the on-line measurement of MFI, a large number of MFI estimation and correlation methods have been proposed. In this work, mechanical predicting methods such as partial least squares (PLS) method and support vector regression (SVR) method are employed in contrast to conventional dynamic prediction schemes. Results of predictions are compared with other prediction results obtained from various dynamic prediction schemes to evaluate predicting performance. Hourly MFIs are predicted and compared with operation data for the high density polyethylene process involving frequent grade changes. We can see that PLS and SVR exhibit excellent predicting performance even for severe operating situations accompanying frequent grade changes.

Development of polyethylene melt index inferential model

In polyolefin processes the melt index (MI) is the most important control variable indicating product quality. Because of the difficulty in the on-line measurement of MI, a large number of MI estimation and correlation methods have been proposed. In this work, we propose a new MI predicting model in which the reaction heat term in existing prediction models is replaced by temperature. The proposed model is compared with other prediction models as well as operating data to evaluate MI predicting performance. The present model shows least predicting errors compared to existing models.

Prediction and quality control of the melt index during production of high-density polyethylene

In polyolefin processes the melt index (MI) is the most important control variable indicating product quality. Because of the difficulty in the on-line measurement of MI, a lot of MI estimation and correlation methods have been proposed. In this work a new dynamic MI estimation scheme is developed based on system identification techniques. The empirical MI estimation equation proposed in the present study is derived from the 1 st -order dynamic models. Effectiveness of the present estimation scheme was illustrated by numerical simulations based on plant operation data including grade change operations in high density polyethylene (HDPE) processes. From the comparisons with other estimation methods it was found that the proposed estimation scheme showed better performance in MI predictions. The virtual sensor model developed based on the estimation scheme was combined with the virtual on-line analyzer (VOA) to give a quality control system to be implemented in the actual HDPE plant. From the application of the present control system, significant reduction of transition time and the amount of off-spec during grade changes was achieved

Environmental analysis of plastic production processes: Comparing petroleum-based polypropylene and polyethylene with biologically-based poly-β-hydroxybutyric acid using life cycle analysis

Polymers based on olefins have wide commercial applicability. However, they are made from non-renewable resources and are characterised by difficulty in disposal where recycle and re-use is not feasible. Poly-beta-hydroxybutyric acid (PHB) provides one example of a polymer made from renewable resources. Before motivating its widespread use, the advantages of a renewable polymer must be weighed against the environmental aspects of its production. Previous studies relating the environmental impacts of petroleum-based and bio-plastics have centred on the impact categories of global warming and fossil fuel depletion. Cradle-to-grave studies report equivalent or reduced global warming impacts, in comparison to equivalent polyolefin processes. This stems from a perceived CO(2) neutral status of the renewable resource. Indeed, no previous work has reported the results of a life cycle assessment (LCA) giving the environmental impacts in all major categories. This study investigates a cradle-to-gate LCA of PHB production taking into account net CO(2) generation and all major impact categories. It compares the findings with similar studies of polypropylene (PP) and polyethylene (PE). It is found that, in all of the life cycle categories, PHB is superior to PP. Energy requirements are slightly lower than previously observed and significantly lower than those for polyolefin production. PE impacts are lower than PHB values in acidification and eutrophication.

Clustering-Based Hybrid Soft Sensor for an Industrial Polypropylene Process with Grade Changeover Operation

A new methodology is proposed to design a soft sensor for a polypropylene (PP) process with grade changeover operation. In contrast to the general polyolefin process, the PP process usually produces more than 100 different grades of products. Its reaction mechanism, based on seven catalysts, is so complex that neither mechanistic nor empirical models have been successful in describing full-scale industrial applications. The proposed methodology was developed based on the hybrid modeling of novel clustering and black-box and mechanistic models. Clustering based on critical to quality enables the soft sensor to handle the complexity of many different grades. Hybrid modeling offers good predictive power for transient behaviors as well as normal behaviors. The methodology also allows us to reduce the cost of building and updating the model. The developed soft sensor was successfully applied to a real industrial process. The accurate and reliable monitoring of the melt index in the PP process helped to signifi...

A study of grafting reactions during processing of polyolefins

AbstractThe effects of polyolefins composition, that is polyethylene (PE), polypropylene (PP) and a ethylene/propylene copolymer (EPM) of 45/55 monomer ratio, and of operating conditions (screw speed, throughput and screw configuration) on the grafting of maleic anhydride (MA) were investigated. Sampling devices located along the extruder barrel were used to collect samples of modified PE, EPM, and PP that were subsequently characterized in order to monitor the degree of grafting and of crosslinking/degradation. Although a similar trend of MA grafting evolution is observed along the extruder, the final MA graft content depends on the processing parameters, screw configuration, and particularly, throughput have a significant effect on the grafting yield. Taking PE as an example, and by means of a statistical experimental design, an optimal operating window in terms of screw speed, throughput, and geometry was identified, yielding MA graft contents of 2 wt% MA. © 2004 Wiley Periodicals, Inc. Adv Polym Techn 23: 196–210, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20011

EPR and modelling studies of hydrogen-abstraction reactions relevant to polyolefin cross-linking and grafting chemistry.

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233-453 K. in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 x 10(3)-3.7 x 10(5) dm3 mol-1 s-1 per hydrogen) have been determined for C-H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C-H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.