Crystallization Of Low Density Polyethylene Research Articles

Morphological and Chemical Analysis of Low-Density Polyethylene Crystallized on Carbon and Clay Nanofillers.

Interest in carbon and clay-based nanofillers has grown in recent years. The crystallization behavior of low-density polyethylene (LDPE) was studied using a variety of notable nanofillers used in engineering applications and prepared using a solution crystallization method. Carbon nanotubes (CNTs), graphene oxide nano-platelets, clay (montmorillonite), and modified clay (surface-modified with trimethyl stearyl ammonium) were used to induce heterogeneous crystallization of LDPE. The crystallized LDPE samples, imaged using scanning and transmission electron microscopy, revealed different microstructures for each nanohybrid system, indicating these various nanofillers induce LDPE lamellae ordering. The underlying interactions between polymer and nanofiller were investigated using FTIR spectroscopy. X-ray diffraction (XRD) was used to determine crystallinity. This work examines how the differences in morphology and chemical structure of the nanofillers induce changes in the nucleation and growth of polymer crystals. These results will provide guidance on functional design of nano-devices with controlled properties.

A facile evaluation on melt crystallization kinetics and thermal properties of low-density polyethylene (LDPE)/Recycled polyethylene terephthalate (RPET) blends

Abstract In this study, we prepare series of low-density polyethylene (LDPE)/recycled polyethylene terephthalate (RPET) blends using melt extrusion. The effects of RPET content on the crystallization behavior and thermal conductive properties of the resultant blends were investigated using DSC, XRD, DMTA, TGA, etc. RPET was found to exert nucleating effect on the melt crystallization of LDPE without changing LDPE's crystal form. A four-parameter model (FPM) was adopted coupled with an in-situ temperature measurement to further disclose the solidification kinetics of LDPE in the presence of RPET. Among three thermal conduction models, the Agari model presented a reasonable prediction of thermal conductivity as a function of LDPE loading. A coefficient of cooling rate (CCR) is proposed and can be applicable as an indicator for thermal conductivity (λ) of the polymer blends.

Influence of Extensional Stress Overshoot on Crystallization of LDPE

Low-density polyethylene (LDPE) shows a stress overshoot in start-up of strong uniaxial extensional flows of constant rate. It is believed that the overshoot is caused by a contraction of the polymer backbone due to alignment of the long chain branches—the consequence being that the molecular strain of the backbone does not increase monotonically with the global strain of the melt. In this study we investigate the semicrystalline morphology of LDPE quenched before, after, and at the overshoot. We do this by combining filament stretching rheometry with ex-situ X-ray scattering. It is found that the overshoot indeed is reflected in the orientation of the crystalline domains of the quenched filaments. In a broader perspective, we show that the final crystalline morphology is determined by the stress at quench—not the strain at quench. With these findings we confirm that the much debated overshoot has a physical origin. More importantly, we conclude that even for complex architectures like branched systems, t...

A processing method with high efficiency for low density polyethylene nanofibers reinforced by aligned carbon nanotubes via nanolayer coextrusion

A method for large-scale processing of low density polyethylene (LDPE) nanofibers reinforced by aligned multi-walled carbon nanotubes (MWCNTs) is achieved via nanolayer coextrusion. The morphology and nano-sized diameter of the nanofibers are observed on scanning electron microscope. The prioritized alignment and dispersion of MWCNTs along the fiber axis are determined by transmission electron microscopy, polarized Raman spectra, and high resolution optical microscopy. The dispersion of MWCNTs in nanofibers is better than that in homologous micron-fibers. In addition, MWCNTs can promote the crystallization of LDPE in varying degrees which depend on the fiber diameter and MWCNTs content. As a result of the alignment of MWCNTs, the nanofibers exhibit enhanced mechanical properties. The tensile strength of LDPE(1 wt% MWCNTs) nanofibers is more than twice of pure nanofibers (without MWCNTs). The volume resistivity of the LDPE(MWCNTs) nanofiber mats containing 9 wt% MWCNTs can be reduced to 100 Ω cm.

Effects of surface morphology on space charge formation in low density polyethylene

The effects of a modified surface morphology on space charge formation in low-density polyethylene (LDPE) were investigated with pulsed electro-acoustics. The change in morphology resulting from LDPE crystallization on atomically flat substrates was imaged by atomic force microscopy. An extensive lamellar morphology was observed, which contrasts with the crystal structure formed between PET films. The regular arrangement was confirmed with wide-angle X-ray diffraction. Space charge accumulation was characterized in samples with different surface morphologies. Charge injection occurs in both films, but is much larger in the LDPE-mica films than in the LDPE-PET films, and the difference increases over time. A suppression of current density in the LDPE-mica films was coincident with space charge formation.

Pressure‐induced crystallization of low density polyethylene on carbon nanotubes and carbon nanofibers

In this study, low‐density polyethylene (LDPE) crystalline structures produced via a solution‐crystallization method on carbon nano‐fillers were characterized. The nano‐fillers used in this study are carbon nanotubes (CNTs) and carbon nanofibers (CNFs). CNTs and CNFs were used to act as a nucleating surface for polymer crystallization. In the case of CNTs, we obtained nanohybrid shish‐kebab architecture by epitaxial crystallization. This is a unique way to bond polymer with nanofillers, in which the CNTs act as shish, while polymer act as kebabs. A pressure‐induced crystallization approach was employed to investigate the effects of crystallization time and pressure on the CNFs/LDPE nanohybrid structure and percent crystallinity. The morphology of the nanohybrids was characterized by scanning electron microscopy (SEM), while differential scanning calorimetry (DSC) was used to quantify the amount of crystalline polymer. The LDPE crystals grown on CNTs had significant morphological differences from those grown on CNFs. The polymer crystals were found to periodically decorate the CNTs with a nanohybrid shish‐kebab (NHSK) architecture, in which CNTs acts as a shish, while LDPE crystals were arranged as circular disks on the CNTs. POLYM. COMPOS., 39:192–200, 2018. © 2016 Society of Plastics Engineers

Graphene Modified Lipophilically by Stearic Acid and its Composite With Low Density Polyethylene

Graphene, prepared by the thermal reduction of graphite oxide (GO), was modified with stearic acid to enhance its lipophilicity. A novel method, using the intrinsic epoxy groups on the graphene, was utilized for reaction with stearic acid to minimize the negative impact of the normal functionalization method on the π-electronic system of graphene. Gravimetric analysis, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) showed that the stearic acid was effectively attached to the graphene. In addition, Raman spectroscopy and electric conductivity of the graphene showed that this novel modification method, utilizing intrinsic defects, did not damage the π-electronic system of the sp2 bonded carbons. The dispersion of graphene in a low density polyethylene (LDPE) matrix was enhanced; consequently, the reinforcing effect in tensile testing was improved by the lipophilic modification. The crystallization behavior observed by differential scanning calorimetry (DSC) showed that the crystallization of LDPE was hindered by dispersed graphene, more evidently when dispersed uniformly.

Gamma irradiated LDPE in presence of oxygen. Part I. Non-isothermal crystallization

Non isothermal crystallization of low density polyethylene (LDPE) samples irradiated under three different atmospheres was analyzed. The used doses were between 33 and 222kGy and the atmospheres content 0, 21 and 100% of oxygen. At a given radiation dose, the amount of produced gel is higher when the oxygen content present in the radiation process is lower.The temperature and the enthalpy of crystallization of the modified polymer decreased almost linearly with the radiation dose. Different classic models were used to predict the development of the relative degree of crystallinity (in the range 0.01–0.3 crystallinity fraction) as a function of temperature and several parameters were analyzed. The values of half-time of crystallization (t1/2) and the parameter Zc, from Avrami's method which characterize the kinetics of non-isothermal crystallization, showed that the crystallization rate, at the same cooling rate, decreased with increasing radiation dose and, at the same dose, with the decrease in the oxygen content. The activation energy for crystallization increased with the radiation dose and with the reduction of the oxygen content, indicating that the increment in molecular crosslinking restricts the crystallization process.

Toward forced assembly of in situ low-density polyethylene composites reinforced with low-Tg phosphate glass fibers: Effects of matrix crystallization and shear deformation

This study is aimed at investigating the feasibility of using facile forced assembly methods (temperature and shear strain-induced orientation of the dispersed phase) to create novel “in situ” low-density polyethylene (LDPE) composites containing fibrillar inorganic phosphate glass (P-glass) reinforcing phase during the composite fabrication. Clearly, the experimental results show that unique thermo-rheological conditions exist under which the “in situ” LDPE composites containing fibrillar P-glass with potential enhanced benefits can be prepared. DSC results showed that the P-glass has a moderate nucleating effect on the LDPE crystallization that restricts in situ deformation of the P-glass during the composite fabrication. Rheo-optical data showed that a 5% P-glass/95% LDPE hybrid composition, subjected to a shear rate of 20 s−1 in the parallel plate configuration and 130°C gave “in situ” LDPE composite samples with the largest amount of P-glass fibers in the limited range of experimental conditions used. This study may spur interests in a better understanding of the potential for the “in situ” reinforcement of engineering plastics with inorganic P-glasses, at the molecular level, to produce novel “in situ” polymer composites with very high aspect ratios of the reinforcing inorganic phase. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Mechanical and thermal properties of thermoplastic elastomer based on low density polyethylene and ultra-fine fully-vulcanized acrylonitrile butadiene rubber powder (UFNBRP)

Ultra-fine fully vulcanized acrylonitrile butadiene rubber powder (UFNBRP) and low density polyethylene (LDPE) were used for preparing thermoplastic vulcanizates (TPVs). A simple blending technique was utilized as an alternative to a complex dynamic vulcanization process typically used for the preparation of TPVs. Blend composition ratio of UFNBRP/LDPE was varied, and the properties of the TPVs were investigated. Morphological result reveals that all blends exhibit a two-phase morphology in which the UFNBRP particles form the dispersed phase in the LDPE matrix. This implies successful TPV preparation. Viscoelastic behavior of the TPVs was found to be governed by UFNBRP loading. As expected, with increasing UFNBRP loading, the elastic contribution increases as evidenced by the decreases in sensitivity of storage modulus to test frequency and in creep compliance. The rheological results are in line with the improvements in rebound resilience and compression set. The presence of chemical crosslinks in UFNBRP in conjunction with the formation of a transient network of rubber particles is thought to be responsible for such increase in elastic response. Additionally, degree of crystallinity and oil resistance of the TPVs were found to decrease with an increase in UFNBRP loading. The suppression of LDPE crystallization is probably caused by the UFNBRP particles locating in the interlamellar spaces of the LDPE matrix.

폐폴리에틸렌 필름의 재활용에 관한 연구

폐폴리에틸렌 필름을 재활용하기 위하여 폐폴리에틸렌과 비산재를 사용하여 콤파운드를 제조하였다. 실험에 사용한 고분자는 생활쓰레기에서 분리수거한 폴리에틸렌(PE)과 포장용 필름을 생산하는 공장에서 발생하는 스크랩에서 얻어지는 저밀도폴리에틸렌(LDPE)과 선형저밀도폴리에틸렌(LLDPE)이다. 폐폴리에틸렌의 물리적 성질을 개량시키기 위하여 화력발전소 굴뚝에서 포집한 석탄재와 생활쓰레기 소각로굴뚝에서 얻어지는 비산재를 사용하였다. 발전소 석탄재를 투입하면 재활용 LDPE와 LLDPE 콤파운드의 인장강도는 감소하고 굴곡모듈러스는 증가하였다. 역청탄재를 혼합한 경우에 비하여 무연탄재를 투입한 LLDPE 콤파운드의 인장강도와 굴곡강도가 약간 높았으며, 이는 무연탄재가 다소 많은 량의 미연소 탄소를 가지기 때문으로 사료된다. 생활쓰레기에서 얻어진 PE 단독에 소각로 비산재를 투입하면 인장강도와 굴곡강도가 모두 증가하였다. 이와 같은 소각로 비산재에 의한 상승효과는 생활쓰레기 PE에 포장용 필름 스크랩의 LDPE를 섞게 되면 Filler 입자가 LDPE의 결정화를 방해하기 때문에 상쇄되는 현상이 관찰되었다. 소각로 비산재를 혼합하여 생활쓰레기 PE를 재활용한 콤파운드는 하수관 등을 제조하는 원료로 사용이 가능하다. 본 연구결과를 폐플라스틱의 재활용에 적용시키면 폐기물처리 비용을 절감하고 환경보존과 자원절약에 기여할 수 있다. The compounds of recycled polyethylene(PE) and fly-ashes were prepared. Polymers used were sorted PE from mixed plastics of household waste and Low Density Polyethylene(LDPE) and Linear Low Density Polyethylene(LLDPE) recycled from the scrap of packaging film plants. Fly-ashes were from the power plant and from the household waste incinerator. The tensile strength of recycled LDPE and LLDPE compounds decreased and the flexural modulus increased with greater amount of the power plant fly-ash. Anthracite fly-ash gave rise to slightly higher tensile and flexural strength of the LLDPE mixtures than bituminous coal fly-ash presumably due to higher content of unburned carbon. The incinerator fly-ash introduced to household waste PE enhanced both tensile strength and flexural modulus of the compounds. When LDPE and household waste PE were used together, the synergistic effect of incinerator fly-ash to household waste PE was offset by reduced crystallization of LDPE due to the filler particle. The compounds of household waste PE and incinerator fly-ash might be applied to structural materials for such as sewage pipe, which reduces the waste treatment cost and conserve the environment and resources.

Shear-induced nonisothermal crystallization of low-density polyethylene

The effect of melt memory on the shear-induced nonisothermal crystallization of a low-density polyethylene melt is analyzed with a shear DTA instrument. The melt state responsible for the saturation of shear-induced isothermal crystallization was identified previously as the steady state in steady shear flows and the strain applied to the melt was identified as the controlling factor for that saturation. Here it is shown that the same strain that saturates the isothermal crystallization also saturates the shear-induced nonisothermal crystallization. Regardless of the cooling rate, the nonisothermal crystallization, starting from the same sheared melt temperature, saturates at the same strain. The contribution of the melt memory effect to the overall nucleation density is estimated, and it is concluded that the majority of nuclei results from the thermal-induced crystallization.

Space charge distribution and crystalline structure in polyethylene blended with EVOH

The space charge distribution in polyethylene samples under direct current (DC) electrical field was measured by pulsed electro-acoustic (PEA) method. It was found that by blending with 5 wt.% of poly(ethylene-co-vinyl alcohol) (EVOH) in low-density polyethylene (LDPE) the amount of accumulated space charges decreased and the field distribution of space charge improved. The differential scanning calorimetry (DSC) study showed that crystallization of LDPE/EVOH started at a higher temperature than LDPE. The results of wide-angle X-ray diffraction (WAXD) and small-angle light scattering (SALS) for LDPE/EVOH indicated that the crystal forms did not change, whereas the spherulites became smaller and imperfective. It can be seen from the results that EVOH played a role of nucleation during the crystallization of LDPE in the blend. The observation of scanning electron microscope (SEM) showed that the domains of EVOH were dispersed in LDPE as particles in diameter of 1 μm. The reduction of space charges in the blend sample can be explained as the results of the trapping of homo-charges at the interface and the dissipation of charges through LDPE matrix consisting of smaller spherulites.

Evaluation of the sample temperature increase during the quiescent and shear-induced isothermal crystallization of polyethylene

The isothermal crystallization of low-density polyethylene is analysed by differential scanning calorimetry and parallel plate rheometry. The temperature increase during the isothermal crystallization, resulting from the polymer sample's thermal resistance and the release of the heat of crystallization, is evaluated for the results obtained with both devices. The implication of this evaluation on the comparative overall crystallization kinetics is also analysed. Two different procedures for evaluating the degree of conversion to the solid phase from rheometry data are discussed. The sample temperature during isothermal DSC and rheometer experiments, these last performed over samples with different thicknesses, is evaluated and the relative crystallization kinetics compared by analysing the dependence of the reciprocal of the half of crystallization time on the estimated sample temperature at this instant. The results obtained indicate that, after performing the temperature corrections, and from an overall kinetics point of view, both procedures yield similar results.

The crystallization of low-density polyethylene: a molecular dynamics simulation

Three models (star-shaped, H-shaped, and comb-shaped polyethylenes) are used to study the crystallization behavior of low-density polyethylene at the molecular level by means of molecular dynamics simulation. It is shown that, for the three types of polyethylene corresponding to the models, the neighboring sequences of trans bonds firstly aggregate together to form local ordered domains, and then they coalesce to a lamellar structure. In the process, the branching sites are rejected to the fold surface gradually. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the number of the branch sites and the length of the branch play an important role in determining the formation of the lamellar structure. The longer the length of the branch and the fewer the number of the branch sites, the more perfect lamellar structure can be formed.

Crystallization of low-density polyethylene- and linear low-density polyethylene-rich blends

The crystallization of a series of low-density polyethylene (LDPE)- and linear low-density polyethylene (LLDPE)-rich blends was examined using differential scanning calorimetry (DSC). DSC analysis after continuous slow cooling showed a broadening of the LLDPE melt peak and subsequent increase in the area of a second lower-temperature peak with increasing concentration of LDPE. Melt endotherms following stepwise crystallization (thermal fractionation) detailed the effect of the addition of LDPE to LLDPE, showing a nonlinear broadening in the melting distribution of lamellae, across the temperature range 80–140°C, with increasing concentration of LDPE. An increase in the population of crystallites melting in the region between 110 and 120°C, a region where as a pure component LDPE does not melt, was observed. A decrease in the crystallite population over the temperature range where LDPE exhibits its primary melting peaks (90–110°C) was noted, indicating that a proportion of the lamellae in this temperature range (attributed to either LDPE or LLDPE) were shifted to a higher melt temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1009–1016, 2000

Differences between crystallization of LDPE and iPP in EPDM matrix

Supermolecular structure of blends made of low density polyethylene (LDPE) or isotactic polypropylene (iPP) and ethylene-propylene- diene rubber (EPDM) was studied with X-ray diffraction (WAXS and SAXS), differential scanning calorimetry (DSC) and positron annihilation spectroscopy. Experimental data reveals different behaviour of the plastomers in the elastomer matrix. The crystalline phase of LDPE is to some extent solvated by the amorphous EPDM, whereas addition of iPP enhances the degree of the blend's crystallinity. This additional crystalline phase is of imperfect nature and probably originates from cocrystallization of propylene monomer units from EPDM onto iPP particles. LDPE recrystallizes in the blends with EPDM at a lower temperature, the higher the plastomer content. Contrary to this, iPP recrystallizes in the rubber matrix at higher temperature. The blend's composition influences the morphology of the crystalline phase. Interlamellar amorphous layer thickness slightly increases with an increase of LDPE content, being simultaneously accompanied by a slight decrease of the crystalline lamella thickness. The long period values remain practically constant. In the case of iPP/EPDM blends, the crystalline lamella thickness increases with an increase of the plastomer content, whereas the interlamellar amorphous layer remains constant, which results in an increase of the long period. The reversed tendency, observed for high-filled samples containing ⩾ 50 phr of iPP, is likely to be associated with too many nucleation centres, finally reducing the spherulite size. Despite annihilation taking place in the amorphous as well as the crystalline phase of polymers, the positron lifetime data stays in good agreement with the postulated supermolecular structure of the blends.

Temperature effect on kinetics of isothermal crystallization of crosslinked filled LDPE—2. Particulate silica with high surface area as a filler

The kinetics of isothermal crystallization of low density polyethylene (LDPE) filled with silica (Ultrasil) and crosslinked by peroxide was investigated by a DSC method. It is suggested that the addition of high-surface-area silica as a filler results in a significant adsorption of LDPE chains on the filler surface, which plays an important role in the kinetics of isothermal crystallization. The isothermal crystallization of filled and crosslinked LDPE is influenced simultaneously by adsorption, nucleation, retardation and formation of pre-ordered structure.

Temperature effect on kinetics of isothermal crystallization of crosslinked filled LDPE—1. Particulate silica with low surface area as a filler

The kinetics of isothermal crystallization of low density polyethylene (LDPE) crosslinked and filled with low surface area silica was investigated by DSC method. The crosslinking initiated by thermal decomposition of peroxide leads to overall retardation of crystallization. The filling of LDPE with low surface area silica leads to the increasing nucleation. The isothermal crystallization of crosslinked and filled LDPE is affected by nucleation, retardation and formation of preordered structure, all effects acting simultaneously making overall dependences sometimes rather complicated.

The influence of crosslinking on isothermal crystallization of LDPE filled with silica

The effect of crosslinking on the isothermal crystallization of low density polyethylene filled with three types of silica was investigated at 375 K. The crosslinking initiated by thermal decomposition of peroxide leads to overall retardation of crystallization. The presence of a filler in the polymer matrix results in the two opposing effects of nucleation and retardation. Crosslinking in the presence of a filler leads to the formation of substantial amounts of an immobile phase resulting in rather unexpected effects, explained by combination of nucleation, retardation and pre-ordering of chains. Comparison of the three types of fillers shows that those with large surface area affect the crystallization to the greatest extent.