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The effect of process temperatures on joint quality in friction stir butt welding of bimodal high density polyethylene (HDPE) was investigated. Process temperatures were manipulated by varying the rotation speed and welding speed. Further control of the process temperatures was achieved by heating the bottom side of the welded plates using a film heater. Temperatures on the top and bottom surfaces of the welded plates were measured by infrared thermography and thermocouples, respectively. Material flow patterns were also visualized by implementing a novel welding technique where two different colors of HDPE were placed on the advancing and retreating sides, respectively, and the cross section of the welding zone was inspected after welding. It was found that proper welding of this bimodal HDPE takes place when the material is maintained at high enough temperatures in the viscous state throughout its thickness. Under these conditions, proper interphase mixing and fusion of the material takes place. Weld efficiencies in excess of 100% and large elongations in the order of 60% were also obtained at the optimum range of welding temperatures.

In this study we investigate the effect of material temperatures on material flow and weld quality in the friction stir welding of bi-modal high density polyethylene (HDPE). The heat input to the process was controlled by varying the tool rotational speed, welding speed and the material initial temperature. Preheating of the HDPE blanks on the bottom surface of the weld was incorporated in order to increase the material flow in this relatively colder region. Temperatures on the boundary surfaces of the HDPE blanks were measured using an infrared camera and thermocouples. 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 was measured by tensile testing. It was found that material temperatures greatly affect the material flow in the weld zone, which in turn affects the tendency to form defects and the overall joint quality. High joint efficiencies and large elongations in excess of 100% were obtained when the material temperatures across the thickness were in excess of 100 °C.

Abstract We report the electrical, mechanical and electromagnetic interference (EMI) shielding performance of polypropylene random copolymer (PPR)/multi-wall carbon nanotube (MWCNT) nanocomposites enabled via customized fused filament fabrication process. The electro-conductive PPR/MWCNT filament feedstocks were fabricated via shear-induced melt-blending process that allows 3D printing of nanoengineered composites even at higher MWCNT loading (up to 8 wt%). The uniform dispersion of MWCNTs in PPR matrix confirmed via Raman spectroscopy and scanning electron microscopy facilitates better mechanical, electrical and EMI shielding performance. The results furthermore show enhanced shielding properties and higher attenuation for the nanocomposites printed in 90° direction (~ − 37 dB for 8 wt% MWCNT loading). Effective interfacial adhesion between the beads with lesser extent of voids (confirmed via micro-computed tomography) endorsed low transmission loss in nanocomposites printed in 90° direction compared to samples printed in 0° direction. Surface architected structure (frustum shape) reveals higher specific shielding effectiveness (maximum ~ − 40 dBg−1cm3, + 38%) over the plain structure. The realization of excellent shielding effectiveness (~ 99.9% attenuation) of additive manufacturing-enabled PPR/MWCNT nanocomposites demonstrates their potential for lightweight and strong EMI shields. Graphical Abstract

As a continuation of the previous work (Deveci, Antony, & Eryigit, 2018), we investigated the effect of carbon black distribution on the degradation of mechanical properties of butt-fusion joints and overall mechanical integrity of high-density polyethylene pipelines. Polyethylene pipes with similar carbon black concentrations but different distributions were produced with industrial scale compounding and extrusion equipment. Waisted tensile specimens were milled directly from the butt-fused pipe samples and elongated to fracture. Carbon black distributions at the butt fusion interphase, both in axial and radial directions, were investigated using stereo and scanning electron microscopy. A significant decrease in joint integrity, measured as work of fracture, was observed for the welds made with high-density polyethylene pipes with inhomogeneous carbon black distributions. It was found that the width of areas that do not contain carbon black (windows) can significantly enlarge at the butt fusion interphase due to shearing, resulting in brittle failures at the butt fusion interphase with a magnitude related to the level of inhomogeneity.

This investigation elucidates the important role of process temperatures in the friction stir welding of polymers. Measurements of the material temperatures were performed by means of an infrared camera and embedded thermocouples under the weld line. An inverse heat conduction method was also utilized to determine the temperature distribution in the workpiece numerically. The weld quality was determined in terms of the amount of defects present in the stir zone and the tensile strength of the joint. It was found that considerable melting occurred under the rotating shoulder and on the trailing side of the rotating pin. Movement of the molten material by the rotating tool created macro- and micro-voids in the stir zone. Crystallinity and nano-hardness measurements indicated that crystallinity was higher under the tool shoulder due to exposure to high temperatures. Tensile strength of the joint was mainly attributed to fusion welding on the top region and hot forging at the root. Decreasing the welding speed and increasing the penetration depth helped improve the weld quality.

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.

The development of residual stresses in plastic pipes due to post-die cooling rate differences of the polymer melt was investigated using concentric and eccentric circular geometries in the shape of an annular tube. Heat transfer simulations were performed to understand the effect of non-uniform wall thickness on the temperature and residual stress profiles of melt solidified polymers. Residual stress measurements and thermal analysis of the end products and heat transfer simulation of the post die cooling process were evaluated to understand the change in stress state in the polymer products as a function of temperature. This was done through an example of extruded high-density polyethylene pipes using an industrial scale extrusion line. The outcome of this research helps better understanding of the melt solidification process and its effect on application related properties in a way that was never presented before.

In this study, we investigated the effect of carbon black distribution on the degradation of mechanical properties of high-density polyethylene in the form of plastic pipes used in water distribution networks. Polyethylene pipes with similar carbon black concentrations but different carbon black distributions were produced with industrial scale compounding and extrusion equipment. Tensile specimens were directly prepared from extruded pipe samples and elongated to fracture at different strain rates. Carbon black distributions of bulk samples and fracture surfaces were investigated using stereo and scanning electron microscopy (SEM). It was found that the carbon black distributions, fracture surfaces and fracture modes were significantly different in these pipes. Although the yield properties were similar, the post-yield properties of samples were significantly different, dramatically decreasing with the increasing inhomogeneity of carbon black distribution. Pipes with a certain level of heterogeneity in the carbon black distribution showed ductile and brittle fractures in the same fracture plane, whereas homogenous black and natural polyethylene (without carbon black) showed ductile fractures only.

Currently, several testing methods are under development to understand the resistance of polyethylene pipe materials to slow crack growth over comparably short time periods without using aggressive chemicals to accelerate the time to brittle failure. Strain hardening and crack round bar tests have recently been developed and published as ISO testing methods. However, a better understanding of these testing methods is still required with respect to the molecular parameters of the materials. Comparative studies with existing slow crack growth testing methods such as the notched pipe test are of significant interest to the industry. This study discusses correlations of molecular weight, molecular weight distribution, short chain branching and rheological properties of different polyethylene materials with their slow rack growth resistances obtained from the strain hardening and crack round bar tests and their correlations with notched pipe tests.

The effect of carbon black (CB) and microwave-induced plasma graphene (g) on the crystallisation kinetics of the multimodal high-density polyethylene was studied under non-isothermal conditions. The non-isothermal crystallisation behaviour of the multimodal-high-density polyethylene (HDPE), containing up to 5 wt.% graphene, was compared with that of neat multimodal-HDPE and its carbon black based nanocomposites. The results suggested that the non-isothermal crystallisation behaviour of polyethylene (PE)-g nanocomposites relied significantly on both the graphene content and the cooling rate. The addition of graphene caused a change in the mechanism of the nucleation and the crystal growth of the multimodal-HDPE, while carbon black was shown to have little effect. Combined Avrami and Ozawa equations were shown to be effective in describing the non-isothermal crystallisation behaviour of the neat multimodal-HDPE and its nanocomposites. The mean activation energy barrier (ΔE), required for the transportation of the molecular chains from the melt state to the growing crystal surface, gradually diminished as the graphene content increased, which is attributable to the nucleating agent effect of graphene platelets. On the contrary, the synergistic effect resulting from the PE-CB nanocomposite decreased the ΔE of the neat multimodal-HDPE significantly at the lowest carbon black content.

The effect of carbon black (CB) and microwave-induced plasma graphene (g) on the crystallisation kinetics of the multimodal high-density polyethylene was studied under non-isothermal conditions. The non-isothermal crystallisation behaviour of the multimodal-high-density polyethylene (HDPE), containing up to 5 wt.% graphene, was compared with that of neat multimodal-HDPE and its carbon black based nanocomposites. The results suggested that the non-isothermal crystallisation behaviour of polyethylene (PE)-g nanocomposites relied significantly on both the graphene content and the cooling rate. The addition of graphene caused a change in the mechanism of the nucleation and the crystal growth of the multimodal-HDPE, while carbon black was shown to have little effect. Combined Avrami and Ozawa equations were shown to be effective in describing the non-isothermal crystallisation behaviour of the neat multimodal-HDPE and its nanocomposites. The mean activation energy barrier (&Delta, E), required for the transportation of the molecular chains from the melt state to the growing crystal surface, gradually diminished as the graphene content increased, which is attributable to the nucleating agent effect of graphene platelets. On the contrary, the synergistic effect resulting from the PE-CB nanocomposite decreased the &Delta, E of the neat multimodal-HDPE significantly at the lowest carbon black content.

In this study, we have successfully applied constant volume – variable pressure method to measure oxygen gas diffusion coefficient of polypropylene-co-ethylene random copolymer pipes on the bases of Fick's laws of diffusion in cylindrical coordinates. Results showed that oxygen diffusion coefficient of polypropylene-co-ethylene random copolymer in the form of extruded plastic pipe is significantly different from thin films and sheets, mostly due to differences in plastic fabrication process yielding different orientation and crystallinity. Furthermore, long term oxygen diffusion coefficient of polypropylene pipes depends on the type of aging that the polypropylene pipes exposed during its service life. Oxygen transmission rate of single layer polypropylene pipes found higher than permissible value specified in building standards.

Slow crack growth (SCG) is a time-dependent brittle-type failure that polyethylene (PE) pipes suffer from when under low stress levels. This study discusses the relation of morphological, molecular weight and molecular weight distribution, rheological, thermal and tensile properties of different PE materials with SCG behavior obtained from the crack round bar (CRB) test, strain hardening (SH) test, and notched pipe (NPT) test. It was observed that increasing the molecular weight and its distribution, short-chain branches, co-monomer content and length, as well as lateral lamellar area increased the SCG resistance and the subsequent time to failure. This was attributed to the enhanced interlamellar entanglement and tie molecules that resisted deformation for a longer time. In addition, SCG resistance was found to decrease with decreasing lamella thickness and degree of crystallinity (within similar molecular weight range). A decrease in the SCG resistance was observed on increasing the onset degradation temperature. Despite the lack of correlation established from tensile tests in the elastic region, SH and CRB values were found to be directly related. As strain hardening modulus increased, time to failure also increased. Zero-shear viscosities also reflected the SCG resistance properties of the samples, as CRB values increased with increasing zero-shear viscosity.

Friction stir welding of high density polyethylene sheets was performed at different rotational and welding speeds and the material temperature close to the welding zone was monitored by infrared thermography and thermocouple measurements. Welding quality was evaluated by macrostructure analysis and tensile testing. Fracture surfaces of the tensile specimens were also analyzed. It was found that weld quality is highly dependent on the temperature of the material in the welding zone. For some specific welding conditions the welding process was unstable due to elevated temperatures reaching the melting point of HDPE. Instability of the welding processes was characterized by cyclic material temperatures and down forces. Decreasing the welding speed and increasing the rotational speed was found to improve weld quality.

Plastic pipes composed of polypropylene-co-ethylene random copolymers were subjected to accelerated aging by thermo-fatigue loading (i.e., thermal cycling using hot and cold water alternately under pressure), application of hydrostatic pressure at elevated temperatures, and heat aging in an oven. The effects of these accelerated aging techniques on the molecular and crystalline structure of the material were investigated using positron annihilation lifetime spectroscopy, Fourier-transform infrared spectroscopy, and differential scanning calorimetry. It was found that all three types of applied aging techniques decreased the size of the defects in the crystalline–amorphous interphase of the material, increased the density of these defects, and had a negligible impact on the free-volume hole size and intensity of the amorphous phase. Thermo-fatigue loading resulted in decreased lamellae thickness and lamellae thickness distribution; in contrast, hydrostatic pressure loading resulted in increased lamellae thickness and lamellae thickness distribution. The effect of thermo-fatigue loading on the chemical degradation of polypropylene was more pronounced than the effects of hydrostatic pressure and heat aging. POLYM. ENG. SCI., 55:641–650, 2015. © 2014 Society of Plastics Engineers

The production of an innovative, high-performance graphene-based polymer nanocomposite using cost-effective techniques was pursued in this study. Well-dispersed and uniformly distributed graphene platelets within a polymer matrix, with strong interfacial bonding between the platelets and the matrix, provided an optimal nanocomposite system for industrial interest. This study reports on the reinforcement of high molecular weight multimodal-high-density polyethylene reinforced by a microwave-induced plasma graphene, using melt intercalation. The tailored process included designing a suitable screw configuration, paired with coordinating extruder conditions and blending techniques. This enabled the polymer to sufficiently degrade, predominantly through thermomechanical-degradation, as well as thermo-oxidative degradation, which subsequently created a suitable medium for the graphene sheets to disperse readily and distribute evenly within the polymer matrix. Different microscopy techniques were employed to prove the effectiveness. This was then qualitatively assessed by Raman spectroscopy, X-ray diffraction, rheology, mechanical testing, density measurements, thermal expansion, and thermogravimetric analysis, confirming both the originality as well as the effectiveness of the process.