Bimodal High Density Research Articles

Effect of process temperatures on material flow and weld quality in the friction stir welding of high density polyethylene

The material flow in the weld zone of friction stir welded bi-modal high density polyethylene (HDPE) was investigated under different in-process temperatures. The in-process temperatures were controlled by changing the tool rotation speed, welding speed and the material initial temperature. Boundary temperature measurements were recorded on the top and bottom surfaces of the welded plates and the temperature distribution in the workpiece was determined numerically using an iterative inverse heat conduction method. Material flow patterns were observed by welding two different colors of the polymer blanks, white on the advancing side and black on the retreating side. Joint quality was assessed using optical microscopy, and joint strength and elongation were measured by tensile testing. It was found that process temperatures greatly affect the extent of material flow in the weld zone, which in turn affects the tendency to form defects and the overall joint mechanical properties. It was also found that proper fusion across the interfaces occurs when the material in the welding zone was maintained at temperatures in excess of 100 °C across the thickness. The high temperature conditions instigated proper fusion of the material, which in turn resulted in large weld efficiencies and elongations in excess of 100%.

Effect of preheating on joint quality in the friction stir welding of bimodal high density polyethylene

Effect of preheating on joint quality in the friction stir welding of bimodal high density polyethylene

The effect of branched structure on the lamellae evolution under uniaxial extension and SCG resistance of polyolefins

A set of unimodal and bimodal high density polyethylene (HDPE) pipe resins of different grade were characterized to investigate the relation between branched structure and resistance to SCG. The results showed that the SCB of bimodal PE100RC was more likely to incorporate into long chains and the entanglement of tie molecules was higher, thus making thicker lamellae and more complete lamellae network. The strain hardening (SH) modulus proved bimodal PE100RC had better long term performance compare with unimodal and bimodal PE100. Besides, small-angle/wide-angle X-ray scattering (SAXS/WAXS) presented the lamellae evolution of bimodal PE100RC and PE100 under uniaxial extension. In the transition of shish-kebab to fibrillar crystals, the orientation increased with the rising of strain and presented a two-stage process with the turning point at the strain of about 1.0. And the long period curves exhibited a three-stage process which were yeilding, softening and hardening stage. In the yeilding region, the long period of PE100RC grew at a faster rate compared with PE100 as the amorphous region of PE100RC was easier to stretch. At the end of yield region, the lamellae thickness of PE100 was smaller than before, while that of PE100RC became larger. It can be inferred that in PE100, only fragmentation of lamellae can be observed, while in PE100RC, the recrystallization as well as destruction of lamellae occurred.

Molecular Mobility in the Amorphous Phase Determines the Critical Strain of Fibrillation in the Tensile Stretching of Polyethylene

The microstructural development of bimodal high density polyethylene subjected to tensile deformation was investigated as a function of strain after annealing at different temperatures by means of a scanning synchrotron small angle X-ray scattering (SAXS) technique. Two different deformation mechanisms were activated in sequence upon tensile deformation: intralamellar slipping of crystalline blocks dominates the deformation behavior at small deformations whereas a stress-induced crystalline block fragmentation and recrystallization process occurs at a critical strain yielding new crystallites with the molecular chains preferentially oriented along the drawing direction. The critical strain associated with the lamellar-to-fibrillar transition was found to be ca. 0.9 in bimodal sample, which is significantly larger than that observed for unimodal high-density polyethylene (0.4). This observation is primarily due to the fact that the bimodal sample possesses a greater mobility of the amorphous phase and thereby a reduced modulus of the entangled amorphous network. The conclusion of the mobility of the amorphous phase as a determining factor for the critical strain was further proven by the 1H-NMR T2 relaxation time. All these findings contribute to our understanding of the excellent slow crack growth resistance of bimodal polyethylene for pipe application.

Sensitivity of post yield axial deformation properties of high-density ethylene/α-olefin copolymers in relation to molecular structure and slow crack growth resistance

1-butene and 1-hexene based bimodal high density polyethylene samples were produced and post yield axial deformation properties, i.e. natural drawing and strain hardening, were investigated at 20 mm/min test speed and at 80 °C in relation to their molecular structure, such as molecular weight, comonomer type and content, viscosity, lamellae thickness and its distribution. Experimental results were evaluated for understanding the sensitivity of maximum natural drawing ratio and strain hardening modulus to amorphous phase entanglement density and tie molecules in relation to slow crack growth properties of polyethylene. It was found that, natural drawing ratio and strain hardening modulus have strong correlations with molecular parameters. Furthermore, it is demonstrated, for the first time, that natural drawing ratio and strain hardening modulus can be predicted with very high level of confidence by product of weight average molecular weight and comonomer content (mol.%) regardless of comonomer type.

Effect of short chain branches distribution on fracture behavior of polyethylene pipe resins

In this paper, a series of unimodal and bimodal high density polyethylene (HDPE) pipe resins with the similar molecular weight distribution and low short chain branches (SCB) contents were investigated to discover the relationship between short chain branches distribution (SCBD) and fracture behaviors. Three characterization methods, namely temperature rising elution fractionation (TREF), high temperature gel permeation chromatography (HT-GPC) and self-annealing fraction (SSA), have been effectively combined to qualitatively analyze investigate the SCBD of HDPE pipe resins. The results showed the preference of incorporation of SCB into long chains caused the enhancement of yielding stress and strain hardening (SH) modulus, and thus the extension of rupture time. On one hand, this kind of SCB distribution is conductive to induce the nucleation of long chains and form more void volume in the neighborhood of long chains indicating faster migration speed of polymer chains. On the other hand, short chains with less SCBs are easy to rapidly occupy nearby sites on the crystal growth front of long chains to co-crystallize. Whereas, these HDPE resins with the more SCBs incorporated into short chains spent much longer time occupying the growth front, so that parts of these types of short chains form very thinner lamella differentiated from the main crystallizing region. During the initiation and propagation of craze, these parts were firstly broken and their resistance to slow crack growth was based on the incorporating degree of SCBs into short chains. Therefore, besides molecular weight distribution, SCBD is another main cause for long-term properties of HDPE pipe resins.

Flash DSC crystallization study of blown film grade bimodal high density polyethylene (HDPE) resins. Part 2. Non‐isothermal kinetics

ABSTRACTNon‐isothermal ultra‐fast cooling crystallization tests were conducted on three blown film grade bimodal high density polyethylene (HDPE) resins using a fast differential scanning calorimeter, the Flash DSC. Non‐isothermal tests were performed at cooling rates between 50 and 4000°K/s, and the data were analyzed using the modified Avrami model by Jeziorny (Polymer, 1978, 19, 1142). Non‐isothermal data were used to propose a new method named crystallization–time–temperature–superposition, and the two activation energies were obtained for each of the resins. This is very useful for obtaining theoretical crystallization kinetics data at different cooling rates, allowing its use in ultra‐fast cooling polymer processes such as blown film. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1822–1827

Separation of bimodal high density polyethylene using multidimensional high temperature liquid chromatography

High-temperature two-dimensional liquid chromatography (HT 2D-LC) using HT-HPLC as first dimension and HT-SEC as second dimension holds enormous potential to investigate the distribution according to molar mass and chemical composition of bimodal high density polyethylene (BiHDPE), as it avoids drawbacks of crystallization-based techniques. In this study, we have stepwise optimized the chromatographic parameters of 1D, comprising gradient slope and temperature, using model homo- and copolymers of ethylene with the aim to minimize the impact of molar mass on the compositional separation. Then the HT-HPLC was hyphenated to HT-SEC and optimum conditions for the volume of the sample transfer loop were probed with regard to the resolution of BiHDPE into the individual constituents HDPE and LLDPE. A particular important aspect was the use of infrared (IR) detection, and the demands it puts on the chromatographic aspects: We have shown that IR detection can be successfully applied in HT 2D-LC of BiHDPE, which is broadly distributed with regard to short chain branching and molar mass, only when the separation in 2D is optimized with regard to chromatographic resolution. As final result a bimodality is evident in the contour and the 3D surface plots as well as in both HPLC and SEC projections generated from HT 2D-LC.

Effect of Low Molecular Weight Ethylene Homopolymer on Structure and Properties of Bimodal High Density Polyethylene

The effect of low molecular weight (LMW) ethylene homoploymer on the structure and properties of bimodal high density polyethylene (HDPE) was studied by blending two commercial bimodal HDPE resins (tandem reactor) with two types of LWM ethylene homopolymer. The molecule weight and molecule weight distribution, crystsallinity, rheological and mechanical properties of composites were characterized by high temperature gel permeation chromatography (HT-GPC), differential scanning calorimetry (DSC), capillary rheometer respectively. For composites of bimodal HDPE and LMW ethylene homopolymer, the polydispersity index (PDI) and crystallinity can be effectively increased when reduce the molecular weight of first reactor in tandem reactor. The viscosity of composites decreases at high shear rate. Increasing of LMW component has a positive effect on processing properties and tensile strength, while makes composites brittle.

A New Route for Evaluating Short Chain Branching Distribution of High Density Polyethylene by Measuring Crystallizability of Molar Mass Fractions

A bimodal high density polyethylene (HDPE) has been successfully fractionated by analytical size exclusion chromatography into molar mass fractions with M w 's ranging from 3.6 kg/mol to 8 000 kg/mol, and subsequently deposited on germanium disks using the Lab Connections Transform method. After removal of the fractions from the disks, having masses in between 10 µg - 150 µg, differential scanning calorimetry has been successful in measuring the (re)crystallization and melting behavior of these fractions. Comparing the crystallization and melting peak temperatures of the fractions with those of narrow molar mass linear polyethylenes points to the HDPE being linear below and short chain branched above 100 kg/mol respectively. This value coincides roughly with the 'split' between the molar mass distributions resulting from the first and the second polymerization reactor - confirming the addition of 1-butene in the second reactor. http://dx.doi.org/10.5755/j01.ms.17.3.590

Rheological behaviors and molecular weight distribution characteristics of bimodal high‐density polyethylene

AbstractA series of bimodal high density polyethylene (PE) with different molecular weight distributions (MWDs) were prepared by melt blending, and the fitting multipeaks on Gaussian were used to analyze the MWD curves, and the ratio of the areas under unimodal curves was as a tool to characterize the MWD; the phase behaviors and rheological behaviors were studied by dynamic rheological. The results showed that homogeneous bimodal high density PEs could be successfully prepared via melt blending, and the bimodal characteristic could be adjusted as expected. For samples with the MWD peak positions unvaried, the storage modulus, complex viscosity, and zero‐shear viscosity decreased rapidly with the value of AL/U increasing. Especially in the low frequency region, the loss modulus surpassed the storage modulus (G″ > G′) when AL/U > 10.17 and the dynamic cross‐point Gx appeared and increased with increasing AL/U, with an increasing extent much larger than that due to the width of MWD. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Comparison of Fractionation Methods for the Characterisation of Short Chain Branching as a Function of Molecular Weight in Ethylene 1‐Olefin Copolymers

AbstractSummary: Rapid and automated analysis of polyolefins is becoming essential for product development in industry. Quantifying short chain branching in ethylene 1‐olefin copolymers is common practice. Several different methods are available to perform this type of analysis. Preparative fractionation followed by subsequent analysis of the fraction by SEC and NMR, SEC‐FTIR and SEC‐IR were studied towards their applicability in polyolefin research and product development environment. The method of choice is defined by prerequisites such as accuracy, labour and time demands but also in versatility and practicability. The most accurate method is limited in terms of sample throughput and the most practical method is limited towards resolution of very low branching. SEC‐FTIR is capable to measure even heterogeneous low branched samples like bimodal high density polyethylene in rapid and satisfactory matter.

Wybrane wlasciwości folii wytlaczanej z polietylenu bimodalnego duzej gestosci

Wybrane wlasciwości folii wytlaczanej z polietylenu bimodalnego duzej gestosci

Characterization of the Microstructure of Bimodal HDPE Resin

In this work, two industrial bimodal high density polyethylene resins, resin A and resin B, having similar molecular weight (Mw), molecular weight distribution (MWD, and short-chain branching (SCB) content but different mechanical properties, were fractionated through cross-fractionation. The fractions were further characterized by GPC, 13C NMR, DSC and FT-IR techniques. These two resins were firstly fractionated into two fractions, i.e., high-temperature and low-temperature fractions, via preparative solution crystallization fractionation. Resin A with much better mechanical properties contains more high-temperature fractions with longer crystallizable sequences. The SCB content in the low-temperature fraction of resin A is lower than that of resin B. Both low-temperature fractions were then further fractionated using solvent gradient fractionation (SGF). The characterization of SGF fractions indicates that most of the branches fall into the high molecular weight chains in both low-temperature fractions. However, the high molecular weight chains in the low-temperature fraction of resin A contain less SCB than that of resin B.

Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

In order to clarify the effect of high molecular weight component on the crystallization of bimodal high density polyethylene (HDPE), a commercial PE-100 pipe resin was blended with small loading of ultra high molecular weight polyethylene (UHMWPE). The isothermal crystallization kinetics and crystal morphology of HDPE/UHMWPE composites were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The presence of UHMWPE results in elevated initial crystallization temperature of HDPE and an accelerating effect on isothermal crystallization. Analysis of growth rate using Lauritzen–Hoffman model shows that the fold surface free energy (σe) of polymer chains in HDPE/UHMWPE composites was lower than that in neat HDPE. Morphological development during isothermal crystallization shows that UHMWPE can obviously promote the nucleation rate of HDPE. It should be reasonable to conclude that UHMWPE appeared as an effective nucleating agent in HDPE matrix. Rheological measurements were also performed and it is shown that HDPE/UHMWPE composites are easy to process and own higher melt viscosity at low shear rate. Combining with their faster solidification, gravity-induced sag in practical pipe production is expected to be effectively avoided.

Phase Structures of Nascent Polyethylene Powder Studied by Wideline Proton NMR

AbstractThe wideline proton NMR spectra of polyethylene powder samples were analyzed in terms of contributions from three components:(1) a rigid part with immobile chains, (2) a soft region with liquid‐like character which produces a Lorentzian contribution to the spectrum, and (3) an intermediateregion in which the rotation of methylene groups about C–C bonds is partially hindered. The relative mass fractions as well as chain mobilities varied greatly among samples produced by different polymerization techniques. The NMR crystallinity agreed well with that estimated by WAXD and was much higher than DSC crystallinity, indicating an inclusion of the contribution from a crystalline‐amorphous interphase. The crystalline defects in the rigid part could be significantly affected by processing parameters when employing the same type of polymerization technique. The intermediate region in the NMR spectra was analyzed according to the comparison between bimodal high density polyethylene and corresponding linear unimodal one. It was found that the mass fraction of the NMR interphase could be an indication of the percentage of tie molecules between crystalline lamellae and thus may significantly affect the mechanical properties of polymeric material.