Adhesive effect of low-density polyethylene gels on polyethylene moldings

Adhesive effect of low density polyethylene (LDPE) gels in organic solvents such as decalin, tetralin, ando-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, electron microscopy, and DSC measurements. When heated at 110°C for 2 h, all of the gels showed strong adhesive strengths around 30 kg/cm², which is sufficiently strong for practical uses. It has been found that the adhesive strength increases with the heating temperature and that the temperature at which the heated gel begins to exhibit the adhesive effect depends upon solvents and is about 30° lower than that of the HDPE gels.     

Adhesive effect of high density polyethylene gels on polyethylene moldings

Adhesive effect of polyethylene moldings by use of high density polyethylene gels in organic solvents such as decalin, tetralin, ando-dichlorobenzene was investigated by shearing tests, electron microscope, and DSC measurements. All of the gels showed such a strong adhesive strength over 36 kg/cm² that polyethylene plates of 3 mm in thickness gave rise to necking sufficient for practical use, when heated at 120 °C for 2 h. In particular, the gel in tetralin showed a strong adhesive strength when heated at 110 °C. It was found that adhesive strength increases with the heating temperature; the temperatures at which adhesive strength begins to increase differ depending on the type of polyethylene sample and solvent. It is apparent that polyethylene gels exhibit an adhesive effect when they are heated at higher temperatures than the gel melting temperatures, and that the closer the SP values of solvents used for the gelation are to the molded polyethylene, the stronger the adhesion of the polyethylene molding.     

Adhesion of a pair of polyethylene moldings by polyethylene gels

It was found that polyethylene gels in solvents such as benzene, toluene, xylene, decalin, tetralin, tetrachloroethylene, 1,1,2,2-tetrachloroethane, and chlorobenzene are effective for adhesion of a pair of polyethylene plates. In particular, the adhesion strength of polyethylene gels in decalin, tetralin and tetrachloroethylene was strong enough for practical use.Adhesive effect appears due to local dissolution of the surface of polyethylene plate in contact with the gel with increasing temperature, and subsequent recrystallization.     

Dried gels from linear low-density polyethylene: Morphology,thermal behavior,and mechanical properties

Dried gels of a linear low-density polyethylene cast from decalin solutions are investigated with particular attention toward structural, thermal, and mechanical properties. The number-average and weight-average molecular weights are M_n = 32,000 and M_w = 160,000. In the concentration range 1.00–0.20, the swollen gels exhibit nearly isotropic shrinkage upon drying, which is relevant to an ideal crosslinked network behavior. For the concentrations below 0.20, a strong departure from the isotropic shrinkage indicates that the chains begin to disengage from the macromolecular network owing to the dilution effect. The melting behavior of the dried gels shows that crystallization from solution improves the crystal perfection notably as concerns the more defective crystals. The concomitant decrease of the crystal thickness judged from small-angle X-ray scattering is ascribed to a reduction of the surface free energy which is consistent with the build up of regular chain-folded macroconformations. The drawability of the dried gels is considerably improved with increasing dilution as a result of the gradual disentanglement of the coils prior to the crystallization in solution. But beyond concentration 0.20, the drawability drops because of the loss of intermolecular cohesion when the chains begin to disengage from the network. The drastic change of yield behavior between the melt-crystallized and solution-crystallized samples reveals a ductile-to-brittle transition in the mechanism of failure of the crystallites at low strain. This phenomenon is related to the improvement of regular chain-folding.     

Characterizing blends of linear low-density and low-density polyethylene by DSC

Summary A two-step isothermal annealing (TSIA) procedure is described that enables the endothermic peaks of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) and their blends, to be satisfactorily resolved during analysis by differential scanning calorimetry. A modified form of multistep isothermal annealing, the TSIA procedure produces a highly characteristic profile of the blend components by facilitating the segregation of the phases based on branch density. It is proposed that the TSIA procedure may have significant merit in the identification and quantification of the components in an unknown blend as well as increasing the sensitivity in analytical procedures aimed at blend component quantification.     

Adhesion of linear low-density polyethylene and oligomers

A pressure vessel design is put forward taking the form of a two-layer bottle, the inner sealing layer of which is made of linear low density polyethylene by rotational molding and the external power layer of which is made by winding an oligomer impregnated roving or moldable filament on the inner layer. It is shown that, to achieve the highest adhesion strength, it is necessary to preliminarily prepare the surface of the polyethylene sample by obtaining a particular microstructure and to use epoxy or polyester oligomers with a viscosity low enough to provide complete filling of the polyethylene sample surface texture.     

Some simulations on crystallinity in a typical linear low-density polyethylene

Crystallinity in ethylene/1-hexene copolymers, a type of linear low-density polyethylene, was investigated by Monte Carlo simulations. The comonomer distributions generated in the simulated chains and the melting temperatures of real chains were used to estimate the minimum crystallite thickness, which is the critical quantity for simulating crystallization in any type of polymer. Simulated values of this thickness were in good agreement with values calculated from Raman longitudinal acoustic mode (LAM) spectroscopy, except for very low 1-hexene mole fractions, where there were presumably complications from high melt viscosities and chain entanglements. The use of this information in estimating properties of these copolymers is illustrated by some preliminary results on the effects of varying amounts of this comonomer on the sizes and numbers of the polyethylene crystallites.     

Liquid–liquid phase separation in linear low-density polyethylene

The issue of multiple equilibrium phases in compositionally heterogeneous random copolymers is studied with an ethylene-butene copolymer representative of many linear low-density polyethylenes (LLDPE). This material has a dispersed minority phase (volume fraction ?^β ≈ 0.02) of highly branched, amorphous chains. A thermodynamic calculation of the equilibrium liquid state is done using the distribution of chain branching from temperature rising elution fractionation and the Flory-Huggins interaction parameter χ_AB for linear and ethyl branched C₄H₈ repeat units. The calculation indicates that this copolymer is metastable, between the binodal and spinodal at a melt temperature of 150°C. The predicted volume fraction of the second phase, ?^β = 0.016, is in good agreement with experiment. This work is the first to compare directly the observed and calculated two-phase behaviors in a random copolymer. © 1994 John Wiley & Sons, Inc.     

Gamma-oxidation of linear low-density polyethylene: The dose–rate effect of irradiation on chemical and physical modifications

γ-Oxidation of linear low-density polyethylene based on hexene and butene was investigated. Irradiation was performed at different dose rates in air (from 2 rad/s to 100 rad/s). Degraded samples were analyzed with IR combined with NO and SF₄ derivatizations. Our results showed that the lower the dose rate, the higher the degree of oxidation in terms of γ-product formation. Ketone species appeared to be the dominant γ-products. The G values of γ-product formation were very dependent on the dose rate of initiation. Comparison of the G value ratio of different γ-products revealed stoechiometry differences. The complex appearance and disappearance of unsaturations was tentatively explained. The modifications of elongation at break induced by γ-irradiation were monitored by molecular changes in weight. This was not conclusive because changes in elongation at break are inconsistent with changes in M_w/M_n. © 1995 John Wiley & Sons, Inc.     

Thermal, mechanical and electrical properties of copper powder filled low-density and linear low-density polyethylene composites

Low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) with different copper contents were prepared by melt mixing. The copper powder particle distributions were found to be relatively uniform at both low and high copper contents. There was cluster formation of copper particles at higher Cu contents, as well as the formation of percolation paths of copper in the PE matrices. The DSC results show that Cu content has little influence on the melting temperatures of LDPE and LLDPE in these composites. From melting enthalpy results it seems as if copper particles act as nucleating agents, giving rise to increased crystallinities of the polyethylene. The thermal stability of the LDPE filled with Cu powder is better than that for the unfilled polymer. The LLDPE composites show better stability only at lower Cu contents. Generally, the composites show poorer mechanical properties (except Young's modulus) compared to the unfilled polymers. The thermal and electrical conductivities of the composites were higher than that of the pure polyethylene matrix for both the LDPE and LLDPE. From these results the percolation concentration was determined as 18.7 vol.% copper for both polymers.     

Supercritical CO<Subscript>2</Subscript> assisted preparation of open-cell foams of linear low-density polyethylene and linear low-density polyethylene/carbon nanotube composites

The open-cell structure foams of linear low-density polyethylene (LLDPE) and linear low-density polyethylene (LLDPE)/multi-wall carbon nanotubes (MWCNTs) composites are prepared by using supercritical carbon dioxide (sc-CO₂) as a foaming agent. The effects of processing parameters (foaming temperature, saturation pressure, and depressurization rate) and the addition of MWCNTs on the evolution of cell opening are studied systematically. For LLDPE foaming, the foaming temperature and saturation pressure are two key factors for preparing open-cell foams. An increase in temperature and pressure promotes both the cell wall thinning and cell rupture, because a high temperature results in a decrease in the viscosity of the polymer, and a high pressure leads to a larger amount of cell nucleation. Moreover, for the given temperature and pressure, the high pressurization rate results in a high pressure gradient, favoring cell rupture. For LLDPE/MWCNTs foaming, the addition of MWCNTs not only promotes the cell heterogeneous nucleation, but also prevents the cell collapse during cell opening, which is critical to achieve the open-cell structures with small cell size and high cell density.     

The effect of methanethiol on the radiation chemistry of low-density polyethylene

The effect of methanethiol (CH₃SH) on the radiation chemistry of low-density polyethylene was examined by using intrinsic viscosity measurements, low-angle laser light scattering, and infrared spectrophotometry. Radiation-induced intermolecular linking was found to be almost completely inhibited in low-density polyethylene by the presence of less than one weight percent of CH₃SH during irradiation. The net yield of transvinylene was also reduced, probably by reaction of transvinylenes with methylthiyl radicals (CH₃S). The inhibition of radiation-induced intermolecular linking by CH₃SH demonstrates that linking is a relatively slow process involving reactions of alkyl free radicals, and that prompt intermolecular links caused by rapid ionic, ion–molecule, or “hot hydrogen” processes are not significant in the radiation chemistry of low-density polyethylene.     

Solid-state relaxations in linear low-density (1-octene comonomer), low-density,and high-density polyethylene blends

Extensive thermal and relaxational behavior in the blends of linear low-density polyethylene (LLDPE) (1-octene comonomer) with low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have been investigated to elucidate miscibility and molecular relaxations in the crystalline and amorphous phases by using a differential scanning calorimeter (DSC) and a dynamic mechanical thermal analyzer (DMTA). In the LLDPE/LDPE blends, two distinct endotherms during melting and crystallization by DSC were observed supporting the belief that LLDPE and LDPE exclude one another during crystallization. However, the dynamic mechanical β and γ relaxations of the blends indicate that the two constituents are miscible in the amorphous phase, while LLDPE dominates α relaxation. In the LLDPE/HDPE system, there was a single composition-dependent peak during melting and crystallization, and the heat of fusion varied linearly with composition supporting the incorporation of HDPE into the LLDPE crystals. The dynamic mechanical α, β, and γ relaxations of the blends display an intermediate behavior that indicates miscibility in both the crystalline and amorphous phases. In the LDPE/HDPE blend, the melting or crystallization peaks of LDPE were strongly influenced by HDPE. The behavior of the α relaxation was dominated by HDPE, while those of β and γ relaxations were intermediate of the constituents, which were similar to those of the LLDPE/HDPE blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1633–1642, 1997     

Tensile yielding and microstructures of blends of isotactic polypropylene and linear low-density polyethylene

Thin sheets of isotactic polypropylene (iPP) and linear low-density polyethylene (LLDPE) blends were studied by tensile testing, optical microscopy (OM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Eyring's two-process yielding theory was used to analyze the data of yield stress as a function of strain rate and temperature, and satisfactory curve-fitting results were obtained. Furthermore, stress whitening was found to have occurred in the necked regions of tensile specimens under a certain range of yielding conditions. These conditions corresponded to the activation of Process II yielding of Eyring's theory. The whitening was found to be a result of formation of microvoids that initiated at the interface between iPP and LLDPE. © 1995 John Wiley & Sons, Inc.     

Memory effect of low-density polyethylene crystallized in a stepwise manner

Stepwise crystallization and successive melting of Tipolen low density polyethylene was studied by DSC. As a result of the stepwise crystallization of the polyethylene, there were as many peaks on the melting curve as the number of steps used for the crystallization of the sample. Similar results were obtained on Tipolen low-density polyethylene fractionated by gel permeation chromatography, and on Celene low-density polyethylene, too.

---

Zusammenfassung Die stufenweise Kristallisierung und das graduelle Schmelzen des Polyäthylens geringer Dichte Tipolen wurden mittels DSC studiert. Als Ergebnis der stufenweisen Kristallisierung des Polyäthylens befanden sich so viele Peaks in der Schmelzkurve wie die Zahl der zur Kristallisierung der Probe angewandten Stufen. Ähnliche Ergebnisse wurden bei der Fraktionierung des Polyäthylens geringer Dichte Tipolen mittels Gelpermeationschromatographie, sowie im Falle des Polyäthylens geringer Dichte Celene erhalten.

---

Résumé On a étudié, par analyse enthalpique différentielle (DSC) la cristallisation graduelle et la fusion successive du polyéthylène à faible densité Tipolen. La cristallisation graduelle du polyéthylène fournit autant de pics sur la courbe de fusion qu'il n'y en avait d'étapes lors de la cristallisation de l'échantillon. On a obtenu de résultats similaires en effectuant la fractionation, par chromatographie à perméation de gel, du polyéthylène à faible densité Tipolen et aussi en étudiant le polyéthylène Celene, également à faible densité.

---

. , , . - , - , - .

     

Photochemical stabilization of linear low-density polyethylene/clay nanocomposites: Towards durable nanocomposites

This article reports a study of the chemical modifications of LLDPE/nanoblend nanocomposites exposed to UV light in conditions of artificially accelerated ageing and natural weathering. Analysis by infrared spectroscopy of the chemical modifications produced by photoageing shows that the presence of an organo-clay leads to the decrease of the oxidation induction time of the polymer (LLDPE), which results in lower durability of the nanocomposites. Protection against photooxidation was tested with different kinds of UV stabilizers and with a metal deactivator. It is shown that the metal deactivator is very efficient in stabilizing the nanocomposite since it totally cancels the prodegradant effect of the organo-clay. This confirms the role played by iron impurities in natural clays. The use of a metal deactivator offers a new insight into the stabilization strategy for nanocomposites.     

Effect of gamma irradiation on density and thermal expansion changes of uniaxially oriented linear low-density polyethylene

The density and thermal expansion behaviour of linear low density polyethylene, over the temperature range from −95° to +10°C, is measured. The influence of drawing on density and thermal expansion behaviour is related to the chain scission and crosslinking induced by the γ irradiation.     

The influence of morphology on the dielectric and dynamic mechanical behavior of a linear low-density polyethylene

The dielectric and viscoelastic relaxation behavior of linear low-density polyethylene has been investigated. All three usual relaxation regions occur; however, the α process is more dominant than in conventional low-density polyethylene made dielectrically active through the introduction of carbonyl groups. The observed behavior is very sensitive to orientation and morphology. There are significant differences between oriented samples (stretching or extrusion) and a single-crystal texture specimen. Effects are caused by dipolar orientation and dipolar immobilization, the relative importance of each effect varying with specimen preparation. Active dipolar groups are not carbonyls, or any common organic species, but appear to be due to chain ends having residual dipoles caused by the catalyst used in synthesis.     

Radiation chemistry of linear low-density polyethylene. I. Gel formation and unsaturation effects

Quenched and annealed samples of linear low-density polyethylene (LLDPE) were irradiated with ⁶⁰Co γ rays in vacuo at room temperature. The data follow rather accurately Charlesby's equation s = k/r, where s is the soluble fraction, r the dose, and k a constant from which G(X), the G-value for crosslinks, was calculated. Crosslinking in the LLDPE is about twice as extensive at equal doses as in LHDPE. Production of vinylene unsaturation was approximately the same in the two types of polyethylene.