Ultra Low Density Polyethylene Research Articles

Synthesis of Ultralow-Density Polyethylene Elastomers Using Triarylnaphthyl Iminopyridyl Ni(II) Catalysts.

Recently, the chain-walking ethylene polymerization strategy has garnered widespread attention as an efficient and straightforward method for preparing polyolefin elastomers. In this study, a series of 2,4,8-triarylnaphthyl iminopyridyl nickel catalysts were synthesized and used in ethylene polymerization. These catalysts demonstrated moderate catalytic activity (105 g mol-1 h-1), producing high-molecular-weight (up to 145.5 kg/mol) polyethylene materials with high branching degrees (75-95/1000C) and correspondingly low melting points. Detailed analysis using 13C NMR spectroscopy revealed that the polyethylenes primarily featured methyl and long-chain branches. Mechanical testing of the polyethylene samples obtained from catalysts Ni1-Ni3 exhibited moderate stress at break (4.64-6.97 MPa) coupled with a very high strain at break (1650-3752%), indicating their very good ductility. Furthermore, these polyethylenes showcased great elastic recovery abilities, with strain recovery values ranging from 72% to 85%. In contrast, the polyethylene produced by Ni4 displayed notably inferior tensile strength (0.16 MPa) and tensile recovery (43%). To the best of our knowledge, this study represents the inaugural utilization of a nickel iminopyridyl catalyst in the preparation of a polyethylene thermoplastic elastomer.

Thermally-triggered multi-shape-memory behavior of binary blends of cross-linked EPDM with various thermoplastic polyethylenes and their potential applications as temperature indicators

Shape-memory polymers (SMPs) are smart materials that can alter their configuration in response to external stimuli. They have shown promise in a number of application areas, including soft robotics or biomedical devices. Frequently, however, the materials needed are expensive, or labor-intensive synthetic processes are involved. In this contribution, we report a versatile and cost-effective manufacturing method for SMPs based on binary elastomer-thermoplastic-blends. These were produced from ethylene-propylene-diene monomer rubber (EPDM) combined with ultra-low-density polyethylene (ULDPE), propylene-ethylene copolymer (PP-c-PE), or high-density polyethylene (HDPE) as thermoplastic components. Atomic force microscopy revealed an immiscible two-phase morphology. Results of dynamic-mechanical thermal analysis showed that all polymer blends with a high thermoplastic load had efficient thermo-responsive dual-shape-memory, also demonstrated on macroscopic specimens. Furthermore, multi-shape-memory of elastomer/thermoplastic (40/60)-blends was investigated. Especially ULDPE-containing blends exhibited particularly promising multi-shape-memory features and stepless, controllable temperature response. Mechanistically, this is based upon the synergistic interaction of the cross-linked elastomer and the thermoplastic switching phase, consisting of different crystalline segments melting over a wide range from 60 to 125 °C. The continuous shape recovery over a broad temperature range could be used to create reusable test strips, e.g., for indicating exposure temperature in transportation chains or overheating protection.

NMR‐Based Cross‐Link Densities in EPDM and EPDM/ULDPE Blend Materials and Correlation with Mechanical Properties

AbstractThe role of cross‐linking in dictating the microstructural and mechanical properties in ethylene‐propylene‐diene‐monomer rubber (EPDM) and EPDM/ULDPE blends cross‐linked by different sulfur amounts is investigated by solid‐state1H time‐domain NMR spectroscopy and tensile‐tests. Analyses of spin‐spin relaxation time (T2), by combining free‐induction decay (FID), magic‐sandwich echo‐FID, and Hahn‐echo experiments demonstrate a reduction in crystal‐amorphous interface regions of pure ultralow‐density polyethylene (ULDPE) upon curative addition. The blends demonstrate a complete loss of these fractions due to curative‐induced plasticization and solvation by polyethylene segments of EPDM. Cross‐link densities, quantified by the magnitude of residual dipolar coupling constant (Dres), arising from topological restrictions to segmental motions, are measured by multiple‐quantum experiments. The entanglement‐dominated EPDMs demonstrate a significant reduction in ultimate tensile properties with increasingDres. The analogous blends yield similarDresvalues up to 0.36 phr of free sulfur. Thereafter, a deviation from the cross‐linking trend of the EPDMs is observed with the blends approaching a cross‐linking limit, thus emphasizing the migration of additives to the amorphous phase of the ULDPE. From the additional contributions of solvation and complex entanglement scenarios in the blends, restoration and even significant enhancement in ultimate tensile strength are achieved. Limitations in applying the popular Mooney–Rivlin analysis are also briefly discussed.

Effects of radical scavenger on space charge accumulation of PP/ULDPE composites for HVDC cable insulation

In this work, the space charge behavior and breakdown strength of polypropylene (PP)/ultralow density polyethylene (ULDPE) blends and three different radical scavenger modified composites are performed at 30, 60 and 90 °C. The trap distribution in the radical scavenger and the composites are investigated based on quantum chemistry calculation and isothermal discharge current test. The results show that, 0.3 wt% addition of radical scavenger has little effect on the thermal and mechanical property of the composites. The radical scavenger can introduce traps with different energy levels, which has greatly inhibited the space charge accumulation aggravated by the increasing temperature in the PP/ULDPE (PU) blends. Although the overall breakdown strength of the samples decreases with the rise of temperature, the performance of radical scavenger modified composites show the best, which suggests a great correlation with the traps they introduce, the molecular structure and chemical reaction of the additives themselves. The PP/ULDPE blends with 0.3 wt% C944 exhibit great potential for HVDC cable insulation.

Effect of graphene nanoplatelets on space charge and breakdown strength of PP/ULDPE blends for HVDC cable insulation

In this paper, polypropylene (PP) is blended with ultra low density polyethylene (ULDPE) content to 5, 15, 25 wt% by melt blending to modify its stiffness and brittleness. The mechanical tests show that both the flexibility and tensile strength of PP are enhanced by the addition of ULDPE with a content of 15 wt%, but the other contents show a weaken elongation at break and tensile strength. Graphene nanoplatelets of 0, 0.005, 0.01 and 0.05 wt% are introduced into the PP/ULDPE (15 wt%) blend to improve its dielectric properties. The DC conductivity, space charge behavior, distribution of trap levels, and breakdown strength of PP/ULDPE/graphene nanocomposites were investigated. The results show that PP/ULDPE/graphene nanocomposites with the nanofiller content of 0.01 wt%, have lower DC conductivity, reduced space charge, and higher breakdown strength. The improved dielectric properties are largely attributable to the deep traps in the interfacial zones between the graphene nanoplatelets and PP/ULDPE blends which restricts the carrier injection and transportation in polymer. It is shown that the PP/ULDPE/graphene nanocomposites with 15 wt% ULDPE and 0.01 wt% graphene nanoplatelets show good dielectric properties for application in HVDC cable insulation.

Rheo‐Raman spectroscopic study of microscopic deformation behavior for ultra‐low‐density polyethylene

AbstractIn situ Raman spectroscopy was applied to ultra‐low‐density polyethylene (ULDPE) to investigate the microscopic deformation behavior under uniaxial stretching. It was found that the crystalline chains of ULDPE show a bimodal molecular orientation parallel and perpendicular to the stretching direction beyond the elastic region. The peak shifts of C − C stretching modes of the crystalline chains obviously depend on the polarization direction, suggesting that the microscopic load sharing is anisotropic. Whereas the stretching stress is applied on the crystalline chains oriented in the stretching direction, the compression stress is applied on those oriented perpendicular to the stretching direction. This anisotropic load sharing is caused by the Poisson shrinkage of the specimen owing to the rubber‐like properties of ULDPE. © 2018 Society of Chemical Industry

Polyethylene green composites modified with post agricultural waste filler: thermo-mechanical and damping properties

The aim of the study was to determine thermo-mechanical properties and applicability of sunflower husk waste as a filler for ultra low density polyethylene composites. The post agricultural waste filler was milled and chemically treated with (3-aminopropyl)triethoxysilane (3-APS). The amount of filler used was 5, 10 and 20 wt%. The mechanical and thermal properties of the composites containing unmodified and modified natural fillers were determined in the course of static tensile test, rebound resilience by Schob method, and dynamic mechanic thermal analysis. The influence of filler loading and chemical modification of the filler on the morphology of natural composites was evaluated by SEM analysis.

Mechanical and thermal properties of impact modified PBT blends and impact modified PBT nanocomposites

Abstract Elastomer toughened poly(butylene terephthalate) (PBT)/organoclay [Cloisite 30B, organo-montmorillonite (OMMT)] nanocomposites were prepared via melt blending using a micro-compounder. In this work, two types of impact modifiers, ultra low density polyethylene grafted glycidyl methacrylate (ULDPE-g-GMA, IM1) and ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA, IM2) were used, and a detailed comparison of the effect of both was made. With respect to the impact strength, 2 wt% of ULDPE-g-GMA produced a better result as compared to 2 wt% E-MA-GMA. Therefore, 2 wt% of ULDPE-g-GMA is considered as the optimized percentage for the preparation of nanocomposites. Being an impact modifier, ULDPE-g-GMA decreases the yield stress, tensile modulus and breaking strength of pure PBT. This issue was addressed in this paper by using organoclay, which may improve the tensile properties of PBT materials. The content of ULDPE-g-GMA was kept constant, whereas organoclay (OMMT) content was varied from 2 to 5 wt% in nanocomposites. The melting and crystallization behavior of pure PBT, impact modified PBT and its nanocomposites were studied by differential scanning calorimetry (DSC). Crystalline morphology was investigated using polarizing optical microscopy (POM) at 185°C, 195°C, and 205°C crystallization temperatures. The optimum increase in tensile modulus of the elastomer toughened PBT nanocomposites was seen with a 3 wt% addition of organoclay.

Elasticity-controlled molecular dynamics of 9,9′-bifluorenyldene as a function of temperature and force

Medium elasticity is an important coupling parameter that influences the molecular motions that occur in solids. In this work, the light-induced twisting motion of 9,9′-bifluorenyldene (BF) was investigated in an elastic polymer, ultra low density polyethylene (ULDPE), as a function of temperature and applied force. Analysis of the BF fluorescence lifetime in accordance with the modulus change of the ULDPE showed that temperature and force significantly affected the excited-state molecular dynamics in the elasticity-controlled regime.

Rheological, physical, and thermal characterization of single‐site catalyst based polyethylenes for seal layer applications

AbstractFour single‐site metallocene catalyst based polyethylenes (mPEs), one ultra low density polyethylene, one conventional linear low density polyethylene (LLDPE), and one low density polyethylene (LDPE) were selected to characterize the effect of side chain branches on physical and mechanical properties. Rheological experiments were carried out to extract complex viscosity and elasticity as a function of frequency. Elongational viscosity tests were also performed to assess long chain branching. For some mPEs, sparse long chain branching improved shear thinning and elasticity of the chains in melt state. During elongation, mPEs with a narrow linear chain distribution showed initially greater melt strength whereas for longer elongation, the mPEs with long chain branching lead over in strength. Cast films were produced from the mPEs and their physical (such as crystallinity, crystal size) and mechanical properties were tested. A double melting peak was observed in the differential scanning calorimetry thermograms of the mPE films. A relatively sharp strain hardening behavior in tensile tests was observed for the mPEs films when compared to LLDPE. Fourier transform infrared was used as an effective and fast method to investigate side chain length. It was found that the positioning of side chain, co‐monomer length, and content influence the melting behavior of mPE films. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers

Laser‐Induced Crosslinking of Ultra‐Low‐ and High‐Density Polyethylene

AbstractThe use of laser radiation to initiate the crosslinking process in ultra low‐density polyethylene (ULDPE) and high‐density polyethylene (HDPE) was evaluated. The process was found to be most effective for pulsed laser irradiation when the polymer was traced with a photoinitiator: 4‐chlorobenzophenone (CBP). The gel content measurements proved that crosslinking took place in all the irradiated samples. The degree of crosslinking was measured for different values of irradiation energy, temperature, photoinitiator concentration and the nature and type of crosslinking agents. The effects of all these parameters on the degree of crosslinking and the consequent effects on mechanical properties of the polymers were analyzed. Also found in the present study is the fact that a better efficiency of crosslinking was achieved at longer laser irradiation wavelength. The ultimate tensile stress and elongation at fracture were measured for all cross‐linked samples and compared with those of the controlled ones.magnified image

Growing Cholesterol-Dependent NS0 Myeloma Cell Line in the Wave Bioreactor System: Overcoming Cholesterol-Polymer Interaction by Using Pretreated Polymer or Inert Fluorinated Ethylene Propylene

Difficulty in growing cholesterol-dependent NS0 cells in the Wave bioreactor using the original low-density polypropylene (LDPE) bags has been encountered. It has been shown that in these bags chemically defined cholesterol is depleted from solution and therefore unavailable for the cells. Our data suggest that the cause of the depletion is not chemical but is due to the physical structure of the polymer. It is proposed that polymer structures with inkbottle pores retain cholesterol, whereas structures with V-shaped pores adsorb cholesterol reversibly. Ultra-low-density polyethylene (ULDPE) bags can support cell growth but need to be pretreated with excess cholesterol. Another material, fluorinated ethylene propylene (FEP) does not need to be pretreated and is found to be superior (negligible cholesterol adsorption) as a result of its inert characteristics.

The Characteristics of Polyethylene Film for Stretch and Cling Film Applications

AbstractPart I. A range of polyethylene films were prepared from metallocene linear low density polyethylene (m‐LLDPE), linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) resins, containing 0 and 8% polyisobutylene (PIB). FTIR, DSC and mechanical analysis techniques were used to investigate the effect of co‐monomer type, density and melt flow index (MFI) on the mechanical performance, orientation and crystallinity of these films. The study established that co‐monomer type and MFI were the greatest factors influencing mechanical performance and crystallinity. Crystallinity was found to be the most influential factor governing PIB migration in these films and this in turn was related to polymer type, density and MFI, High MFI, octene co‐monomer films exhibited the highest orientation, tear resistance and tack strength and would therefore be suitable for stretch film applications. Ultra low‐density polymers gave relatively low tack strength and poor overall mechanical performance.Part II. A range of ethyl vinyl acetate (EVA)/m‐LLDPE/EVA co‐extruded films was manufactured, with vinyl acetate (VA) co‐monomer content of 6, 12 and 18% and PIB content from 0–20%. The films were aged at 45d̀C for up to 28 days, to enable tack (cling) development. The results show that film tack strength improved significantly with ageing. Increased VA concentration in the surface layer also showed significant improvement in film tack strength. The film tensile strength, elongation and tear properties in both machine direction (MD) and transverse direction (TD) were not significantly affected by increase in PIB concentration. However, increased VA content showed slight improvement in MD mechanical performance of the films, TD properties were relatively unaffected. Films with 12 to 18% VA in the surface layers produced high surface tack film and the mechanical performance of these films were comparable to mono‐layer polyethylenes. These films are suitable for stretch wrap applications and have reduced the overall concentrations of tack additives, though high VA films were more difficult to process.

Quantitative determination of short‐chain branching content and distribution in commercial polyethylenes by thermally fractionated differential scanning calorimetry

AbstractA method for rapid quantitative analysis of the content and distribution of short chain branching (SCB) for α‐olefin/ethylene copolymers based on thermally fractionated DSC is presented. Eight commercial polyethylenes, four made with conventional Ziegler‐Natta catalysts and four made with metallocene catalysts, were analyzed by differential scanning calorimetry (DSC), after having been thermally segregated by successive nucleation annealing (SNA). The polyethylenes were also analyzed by temperature rising elution fractionation (TREF) and carbon‐13 nuclear magnetic resonance (13C‐NMR). The SNA‐DSC procedure segregates polyethylenes according to methylene sequence lengths (MSL). The relationship between DSC melting temperature and SCB content was obtained by calibration with linear hydrocarbons; TREF results were not used in the SNA‐DSC calibration. Deconvolution of the SNA‐DSC endotherms yielded estimates of the average SCB contents and SCB distributions. The SCB contents obtained from the SNA‐DSC for linear low density polyethylenes agreed very well with the SCB contents obtained by 13C‐NMR and TREF, and the SCB distributions measured by SNA‐DSC were very similar to those obtained by TREF. The SCB contents obtained by SNA‐DSC for ultra‐low density polyethylenes, made with metallocene catalysts, were about 20% lower than the values obtained by 13C‐NMR; the values obtained by TREF were even lower.

Miscibility and crystallization of metallocene polyethylene blends with polypropylene

AbstractThe crystallization and morphology of very‐low‐density polyethylene (VLDPE) and ultra‐low‐density polyethylene (ULDPE) blends with isotactic polypropylene (PP) were studied by differential scanning calorimetry (DSC) and hot‐stage optical microscopy (HSOM) with polarized light. In particular, the isothermal crystallization of PP in molten PE was investigated. A polypropylene homopolymer was melt‐blended with six types of VLDPEs and ULDPEs, with variations in branch content and length and in molecular weight. All the blends contained 20% PP by mass. It was found that the crystallization temperatures of PP and PE changed in the blends, and the crystallization of PP was affected by branch length and content and by the molecular weight of the PE, indicating a certain degree of miscibility between PP and PE. The isothermal crystallization rate of PP decreased in the blends; in particular, the crystallization rate of PP was slower in the ULDPE with lower MFI, suggesting that crystallization of PP was hindered by PE and that its rate was regulated by the viscosity of ULDPE. HSOM images showed that a portion of the PP crystallized from molten PE, although phase separation was obvious, providing additional information on the miscible behavior between PP and VLDPEs (or ULDPEs). Furthermore, the miscible level between the PP and the ULDPEs was higher than that between the PP and the VLDPEs because the degree of change in the crystallization behavior of the PP and PE was greater in the PP–ULDPE blends. This was possibly a result of the higher branch content in the ULDPE. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1179–1189, 2003

Polymerization Kinetics and Modeling of Solution PE Process with Metallocene Catalysts

Polymerization methods of ethylene include the slurry, solution, and gas phase processes. The solution process for PE not only allows the operation temperature to reach as high as 300°C, but also produces special grade polymers particularly for ultra-low density polyethylene (PE). This study investigates polymerization conditions and kinetics. The commercial constrained geometry catalyst was used for ethylene polymerization. The kinetic studies were conducted for metallocene type polymerization, including catalyst activation, initiation, chain propagation, chain transfer, and termination steps. In addition, kinetic constants were calculated for the polymerization model. Calculation results of catalyst activity and molecular weight were compared with experimental results, indicating their good correlation. Moreover, the conventional polymerization was modified to accurately predict the molecular weight behaviors under various reaction conditions. Exactly how solvent type, reaction temperature, pressure, catalyst concentration, and cocatalyst ratio affect catalyst activity and molecular weight of polymer was also discussed.

Development of novel elastomeric blends containing natural rubber and ultra‐low‐density polyethylene

AbstractThis study sought to develop novel elastomeric compounds using natural rubber (NR) and ultra‐low‐density polyethylene (ULDPE). Blends were prepared by means of a two‐roll mill for three ratios (70/30, 60/40, and 50/50 NR/ULDPE). Conventional vulcanization was performed in a compression mold. The physical and mechanical properties of the blend were determined according to ASTM standards. The results were compared with those obtained from NR blended with styrene‐butadiene rubber (SBR). The morphological examinations with scanning electron microscopy indicated that ULDPE was compatible with NR; thus, the addition of a compatibilizer was not necessary. The cocontinuous phase was dominant in the NR/ULDPE blend containing 50 and 60 wt % NR. The tensile properties, tear resistance, and aging resistance of the NR/ULDPE blends were found to be superior to those of NR/SBR blends. On the other hand, the abrasion and flex cracking resistances of the NR/ULDPE blend were inferior to those exhibited by SBR blends but the Mooney viscosity and resilience of both blends fell in the same range. However, the addition of dicumyl peroxide appeared to have caused crosslinking of the ULDPE phase in the blend, which in turn increased the tensile properties and abrasion and aging resistance. The properties of the tertiary NR/SBR/ULDPE blend were investigated as well. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 650–660, 2001

Compatibility Studies in Binary Blends of PA6 and ULDPE-graft-DEM

In order to study the compatibility promoted in polyamide 6 (PA6) and ultra low-density polyethylene (ULDPE) blends by grafting polar groups into the ULDPE, several blend compositions were prepared in a twin screw extruder. The grafting agent was diethylmaleate (DEM), and the blend compositions prepared were 0, 20, 50, 80 and 100 wt.-% of PA6. The compatibility was evaluated by studying the rheological, thermal, morphological, and spectroscopic (infrared and dielectric) properties of the blends. The formation of a copolymer was observed by infrared spectroscopy after selective extraction of the components, presumably by the interaction of terminal NH 2 groups of PA6 and carbonyl groups of ULDPE(graft-DEM. Thermal properties showed changes due to compatibilization. For instance, fractionated crystallization of the PA6 component was observed when it formed the dispersed phase in reactive blends in view of the enhanced dispersion. Nucleation of the ULDPE component by the PA6 component was observed for reactive and non-reactive blends. The DSC melting results showed the presence of two crystalline forms of the PA6 in the blends. These were the less stable γ-form, predominant over the more stable α-form, in reactive blends, especially for the 20/80 and 50/50 wt.-% blend compositions. Dynamic rheological experiments provided data for fitting the Carreau viscosity model; the results revealed that longer characteristic times are obtained for compatibilized system. This was reinforced by the more elastic behavior that such systems presented in G'-G plots, as compared to the non-reactive ones. Dielectric spectroscopy revealed a noticeable shifting of the α-mode of the PA6 to lower temperatures for the 50/50-g, together with an enhancement of the β over the γ-mode which indicates the presence of tightly bound water. The T g depression could be due to the plasticization effect resulting from the substitution of intramolecular PA6 H-bonds by either water molecules or physical interactions across the interphases.

Food aroma partition between packaging materials and fatty food simulants

By means of thermal desorption experiments, the partition equilibrium (partition coefficient, K) was analysed for six food aroma components (d-limonene, n-decane, ethyl caproate, phenylethanol, n-hexanol and hexanal) between three sealable polymer films suitable for direct food contact (ultra-low density polyethylene, ULDPE; ionomer, ION; and polyester, PET) and four fatty food simulants (ethanol 95%, EtOH; sunflower oil, Oil; n-heptane, HEP; and iso-octane, OCT). The results showed that aroma scalping is highly dependent on the fatty food simulant utilized. Polar aroma components were more sorbed into polymers in the presence of a non-polar fatty food simulant, and vice versa. K values in the presence of Oil were always between those in EtOH and in HEP or OCT. In general, PET was the packaging film which showed the lowest partition coefficient for non-polar components while ULDPE showed the lowest partition for polar aromas. The partition equilibrium of mixed d-limonene, ethyl caproate, and n-hexanol was also determined. The differences in K values between isolated aromas and mixed aromas were small. In general, the most sorbed aroma showed increased partition by mixture while the partition of the least sorbed was reduced.

Food aroma partition between packaging materials and fatty food simulants

By means of thermal desorption experiments, the partition equilibrium (partition coefficient, K) was analysed for six food aroma components (d-limonene, n-decane, ethyl caproate, phenylethanol, n-hexanol and hexanal) between three sealable polymer films suitable for direct food contact (ultra-low density polyethylene, ULDPE; ionomer, ION; and polyester, PET) and four fatty food simulants (ethanol 95%, EtOH; sunflower oil, Oil; n-heptane, HEP; and iso-octane, OCT). The results showed that aroma scalping is highly dependent on the fatty food simulant utilized. Polar aroma components were more sorbed into polymers in the presence of a non-polar fatty food simulant, and vice versa. K values in the presence of Oil were always between those in EtOH and in HEP or OCT. In general, PET was the packaging film which showed the lowest partition coefficient for non-polar components while ULDPE showed the lowest partition for polar aromas. The partition equilibrium of mixed d-limonene, ethyl caproate, and n-hexanol was also determined. The differences in K values between isolated aromas and mixed aromas were small. In general, the most sorbed aroma showed increased partition by mixture while the partition of the least sorbed was reduced.