Diffusional characteristics of coextruded linear low-density polyethylenes prepared from different conditions of processing

The permeability and diffusivity of oxygen, carbon dioxide, nitrogen, and helium have been obtained for a range of linear low density polyethylene (LLDPE) films prepared from the same raw materials but with different processing conditions. The measurements were carried out by means of a permeation technique over the temperature interval where the α-relaxation processes were observed in earlier studies. The temperature dependence of the permeability and diffusion coefficients of gases shows 2 well-differentiated regions in all films. The break temperature of these regions is approximately located at the same temperature as the α-relaxation takes place. Both the permeability and their temperature dependence do not show a noticeable influence on the processing conditions. The effect of processing conditions on the diffusivity seems to be more complex. Differences are observed for different films in the diffusion coefficients, in the case of oxygen, and in their change with the temperature, which is particularly marked in the case of carbon dioxide. Fujita's free volume model has been applied to diffusivity data in order to study the influence of films microstructure in gas permeation properties through them. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 23–37, 1998     

Blends of linear and branched polyethylenes

We employ rheological measurements of polyethylene blends in the melt and solid state, together with thermal analysis, to infer phase behavior. Partial‐miscibility in the melt is characterized by use of the double‐reptation model to define the complex modulus of the continuous phase for input into the emulsion model for the blend; this approach introduces a new fitting parameter, the fraction of the minor component contained in the continuous phase. The results on binary systems suggest the use of HDPE as a compatibilizer for LLDPE/LDPE blends, apparently creating a fully miscible ternary system.     

Effects of polymer structure on the onset of processing defects in linear low density polyethylenes

Linear low density polyethylenes are manufactured by copolymerizing ethylene with 1-alkenes, yielding a linear polyethylene backbone with short side chains. Due to the nature of the catalyst used in the polymerizaton, multimodal branching distributions are typically obtained. In this report, we have investigated the processability of four 1-octene linear low density polyethylenes as a function of the short chain branching distribution. Analytical techniques such as ¹³C nuclear resonance spectroscopy, size exclusion chromatography, differential scanning calorimetry, and temperature rising elution fractionation, in particular were used to elucidate the molecular structure. Processability measurements were made using various extrusion techniques and dynamic mechanical analyses.It was determined that in the absence of any variations in molecular weight, the polymers with the higher proportions of linear polyethylene showed inferior processability In terms of onset of surface imperfections at lower extrusion rates. Polymers with worse processability characteristics also exhibited higher zero shear viscosities.     

Oligoethylenes in high pressure polyethylenes

Summary Elucidation of the mechanism for the production of a variety of oligoethylenes produced in high pressure ethylene polymerization revealed important features of actually occuring radical reactions. Several definite types of radical cyclization reactions are found to be very common in high pressure ethylene polymerization.     

Nonisothermal crystallization kinetics of linear metallocene polyethylenes

The effect of weight‐average molecular weight (M_w) on the nonisothermal crystallization kinetics of linear metallocene polyethylene (m‐PE) was studied with modulated differential scanning calorimetry. Six linear m‐PEs of molecular weights in the range 122–934 kg/mol were prepared by gas‐phase polymerization. The cooling rate (R) was varied in the range 2–20°C/min, and it significantly affected the crystallization behavior. M_w had a weak influence on both the peak crystallization temperature and the crystallization onset temperature. All m‐PEs showed primary and secondary crystallizations. At both low and high R's, the crystallinity showed a significant drop (～ 30%) when M_w was increased from 122 to 934 kg/mol. At low R's (< 10°C/min), the rate parameters in the modified Avrami method [primary rate constant (k_R)] and Mo method [F(T)] of analyses agreed in suggesting that an increased M_w slowed the rate of crystallization. The M_w dependency of k_R followed the Arrhenius type (k_R = k_Roe²⁸¹/M_w, where k_Ro is a rate‐dependent constant). However, at higher R's, k_R approached a constant value. The order parameters obtained by the different methods of analysis (n and α) were independent of M_w, which suggests that the crystal type remained the same. Hoffman–Lauritzen theory was used for data analysis, and activation energy per segment showed a significant decrease, from 225.0 to 11.8 kJ/mol, when M_w was increased from 152 to 934 kg/mol. Finally, all methods of analysis suggested a significant effect of M_w on slowing the overall crystallization process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Morphological parameters and tensile properties of linear polyethylenes

Linear polyethylenes of various molecular weights have been moulded into sheet form and cooled under conditions to produce samples differing in lamella fold length and degree of crystallinity. Their load–extension characteristics have been determined and interpreted in terms of a simple model based on a set of crystalline blocks joined by tie bars which, under uniaxial deformation, rupture and transform to a new microfibrillar structure. An explanation of the tensile properties is obtained in terms of the parameters of chain and fold length, degree of crystallinity, number of crystalline units traversed by one molecule, and number of folds per lamella per molecule.     

性能优异的辐照聚乙烯

数十年来,从事加工聚乙烯电线电缆、收缩管材、冷热水管、片材、泡沫等制品的生产厂家一直采用辐射交联的方法,以提高所生产制品的热性能及机械性能。有部分树脂制造厂商及少数大型塑料制品加工厂也认为,在挤出和注塑加工制品之前,辐照聚乙烯树脂颗粒是一种有效途径,有助于定制     

Crosslinking reactions during processing of silane modified polyethylenes

A study of the crosslinking reactions in silane modified polyethylenes has been completed. Crosslinking of ethylene-vinyl trimethoxy silane copolymers during high temperature melt processing has been investigated using melt rheological and infrared spectroscopic techniques. Two environmental factors are shown to influence crosslinking formation, namely oxidation of the polymer and the presence of di-n-butyl-tin dilaurate, a condensation catalyst. These are believed to initiate high temperature crosslink formation via different non-interacting mechanisms. It is deduced that silane crosslink formation as a result of oxidation is due to intramolecular interaction of oxidised polyethylene and pendant methoxy silane branches. Observation of a hydrogen bonded silanol intermediate species is consistent with this hypothesis.     

Free radical grafting of diethylmaleate on linear low-density polyethylenes

Grafting of preformed polymers is an important method for preparing polymers with functional groups. Lately, the use of extruders as polymerization reactors has increased considerably in industry. However, the knowledge of the total reaction process is still limited. Grafting of diethylmaleate on linear low-density polyethylenes (LLDPEs) was carried out in solution and in a corotating twin screw extruder. The effects of initiator and diethylmaleate concentrations, temperature, and reaction time on graft content and on crosslinking were investigated. The functionalization reaction was also conducted in the presence of an electron donor material to minimize the amount of crosslinked products in the extruder. The grafted products were characterized by means of FTIR and the thermal behavior of LLDPEs and that of its grafted products was determined.     

Thermal,mechanical, and rheological behavior of blends of ultrahigh and normal-molecular-weight linear polyethylenes

Physical Blends of ultrahigh-molecular-weight linear polyethylene (UHMW LPE) and normal-molecular-weight linear polyethylene (NMW LPE) have been evaluated in terms of melt flow rate, tensile stress-strain behavior, heat of fusion, melting temperature, and crystallinity. The behavior of the blends is intermediate between that of the parent polymers; no synergistic effects are observed. The addition of small quantities of NMW LPE does not improve the flow behavior of UHMW LPE sufficiently to render it amenable to conventional melt processing.     

Relationships between processing conditions and rheological behavior of polyethylenes

The rheological properties of high‐density polyethylene melts were found to change drastically after treatment with oxygen or peroxide. Unusual features of the treated melts in shear flow (190°C) included (a) increase in length of time to reach steady state values of shear stress in start‐up experiments; (b) a non‐reproducibility of the low‐shear rate sections of the flow curves measured at increasing and decreasing shear rate; (c) an increase of viscosity at low shear rates compared to the neat sample. Under non‐stationary extensional flow (a regime of constant force) the treatment leads to a change in shape of the strain development with time, an increase of the apparent elongational viscosity, and an increase in time to break. At 150–170°C, the rheological behavior of the treated polyethylenes is completely identical to the corresponding behavior of the untreated. These results, together with data from IR‐spectroscopy and GPC suggest the following mechanism: The oxidation or peroxidation leads to reactive sites in the polymer chain that incorporate a few long branches during the initial contact with oxygen or peroxide. These reactive sites remain in the polymer after cooling/solidification and can become activated again upon heating to 190°C causing additional changes in molecular structure. Formation of the long‐chain branches results in an increased resistance of the melt to extensional deformation, and an improvement in processing behavior, as well as the quality of bottles produced by the blow‐molding process. Polym. Eng. Sci. 44:615–624, 2004. © 2004 Society of Plastics Engineers.     

Reactive processing of polyethylenes on a single screw extruder

The modification of low density polyethylene, linear low density polyethylene, and their blend by dicumyl peroxide at the time of the extrusion on a single screw extruder is reported. The study shows that the optimum conditions of modification can be determined on a torque rheometer and these can then be applied for actual extrusion. A low level of crosslinking can be introduced by reactive extrusion for improving the heat stability without adversely affecting the processing behavior. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2545–2549, 2001     

Tensile creep and recovery in ultra-high modulus linear polyethylenes

The tensile creep and recovery of oriented linear polyethylene (LPE) monofilaments have been studied for a range of samples of different structure. Starting with a comparison of samples of different draw ratio prepared from one grade of polymer, the measurements were extended to examine the effects of molecular weight. Although the viscoelastic behaviour is markedly non-linear it was found valuable to model the creep and recovery at each level of stress by a simple linear solid representation. This representation enabled a clear distinction to be made between recoverable and irrecoverable creep, both of which are affected by draw ratio and molecular weight. When the irrecoverable creep was examined further in terms of an activated Eyring process, a clear distinction between the two molecular weight grades could be made. In particular, the high molecular weight grade displayed a critical stress below which irrecoverable creep fell to a negligible level. This finding could be of considerable importance with regard to the application of ultra-high modulus LPE fibres in reinforcement.     

Elongational behavior of low density/linear low density polyethylenes

Rheological data have been collected in isothermal elongational flow for three different types of blends, made from one low density polyethylene and three linear low density samples. In addition to the transient curves, elongation at break data are also reported. The influence of the composition and of the molecular weight of the linear low density polyethylene is discussed.     

Biaxial orientation of linear polyethylenes using the compressive deformation process

The tensile properties of three grades of linear polyethylene were enhanced by a factor of as much as 15 using a melt/solid phase compressive deformation process that produced equi-biaxial planar orientation in the sheet. Ultra high molecular weight polyethylene with planar isotropy and an in-plane modulus of 10 GPa and a tensile strength of 330 MPa was produced using this method. It was found that the molecular weight had a significant influence on the optimum processing temperature, the ultimate biaxial deformation ratio and hence the ultimate tensile properties. High density polyethylene processed under ideal conditions had a tensile modulus of 2.3 GPa and a tensile strength of 250 MPa. The tensile strength increased linearly with increasing biaxial deformation ratio and the tensile modulus increased non-linearly with increasing biaxial deformation ratio. The deformation rate and the dwell time did not have a significant effect on the tensile properties. Shrinkage tests showed that biaxial deformation was less effective than uniaxial deformation in inducing orientation of the polymer chains, however differential scanning calorimetry results were consistent with the presence of extended chain crystals in very highly oriented ultra high molecular weight polyethylene sheets.     

Flow properties of low density/linear low density polyethylenes

Rheological data have been collected both in shear and non-isothermal elongational flow on three different types of blends, made from one low density polyethylene sample and three linear low density polyethylene samples. In addition to the flow curves, data are presented on the extrudate-swell phenomenon, on the instability arising in capillary flow and on the tensile behavior in the molten state.     

Oligoethylenes in high pressure polyethylenes I identification of homologues

Summary Low molecular weight components which were present in commercial polyethylene resins in minor amounts were analyzed in detail by gas chromatography (GC) and mass spectroscopy (MS). More than forty oligoethylenic homologues were identified, which were consisted of a variety of alkanes, alkenes, cycloalkanes, aromatics, alkanones, alkanol and esters.     

Nitrogen pressure effects on non-isothermal alumina sintering

The densification of a fine grained pure alumina powder was studied during gas pressure sintering. Different nitrogen pressures were applied during non-isothermal sintering runs up to final temperatures between 1150 °C and 1650 °C. Densification, porosity and microstructures have been investigated. A fine alumina powder presents a densification delay during nitrogen pressure sintering mainly due to the gas pressure effect at the beginning of the sintering. The main results of this work concern the influence of nitrogen pressure on non-densifying mechanisms and microstructural evolution, which only depends on densification rate.     

The essential work of plane stress ductile fracture of linear polyethylenes

The specific essential works of plane stress ductile tearing of several high- and ultrahigh-molecular-weight polyethylenes were obtained from deeply edge-notched tension specimens, with either single or double notches, by extrapolating the straight line relationship between the total specific fracture work and ligament length to zero ligament. Provided the fracture morphologies of the torn ligament are not widely different, the specific essential work (w_e) is a material property dependent on thickness but independent of specimen geometry. The specific essential fracture work also can be identified with J_c the critical value of the J-integral along a contour immediately bordering the fracture process zone at the crack tip. There is good agreement between the experimental we values and theoretical J_c estimates for these polyethylene materials.     

Rheological study of linear high density polyethylenes modified with organic peroxide

Four high density polyethylenes were modified using different concentrations of an organic peroxide in order to change their molecular structure. The effects of the presence of vinyl groups in the original polymer molecules and of the peroxide concentration used in the modification process were analyzed. All the concentrations of peroxide used in this study were below the critical concentration that produces a macroscopic molecular network. The weight-average molecular weight of all the polyethylenes augments and the molecular weight distribution gets wider as the concentration of peroxide increases. These results support the general belief that the chain-linking reactions dominate the modification process. Evidence of the important role played by the vinyl groups is found not only in the change of the width of the chromatograms but also in the position of their maximums. The vinyl-containing polymers display the largest molecular changes for a given peroxide content. The magnitude of the viscous and elastic moduli of the polyethylenes goes up as the concentration of peroxide used increases showing the effect of the generated large molecules. The linear viscoelastic response of the modified polymers is thermo-rheologically complex. This complexity can be associated with the generation of branched molecules. For similar molecular weights and peroxide concentration, the flow activation energy displayed by the polyethylenes with larger concentration of vinyl groups is larger. This result suggest that a much more complex molecular structure is formed in the presence of vinyl groups. The dynamic moduli of the polymers were analyzed using the generalized viscoelastic model. The spectrum of relaxation times was determined for each polymer and analyzed as a function of the peroxide concentration.