Compatibility of HDPE/postconsumer HDPE blends using compatibilizing agents

Mechanical properties, molecular weight, X‐ray diffraction, and differential scanning calorimetry (DSC) characterization of blends of virgin high‐density polyethylene (HDPE) with two types of recycled material were investigated. The recycled came from urban plastic waste; one kind was only washed and grounded and the other was extruded and pelletized to remove most of contaminant particles. Starting with the 30/70 virgin/grounded recycled and 50/50 virgin/pelletized recycled blends the recycled content was increased in both blends and compatibilizing agents were used to increase the blend performance. A mixture of phenolic antioxidants and phosphite costabilizers under the name of Recycloblend?, ethylene vinyl acetate (EVA) copolymer, low‐density polyethylene (LDPE), and linear low density polyethylene (LLDPE) were used as compatibilizers. The effect of these additives and the recycled content on the performance of extrusion blow‐molded bottles was determined. The results suggest that blends of virgin/grounded recycled and virgin/pelletized recycled HDPE, in general, were not significantly different among each other and both had a quite similar behavior than the virgin HDPE when compatibilizing agents were used. The addition of compatibilizing agents yielded a material with properties similar to those for the virgin HDPE, helping to reduce the effect of polymers degradation on the rheological and mechanical behavior, with Recycloblend and LLDPE being the most effective for the blends with grounded recycled material, and LLDPE y EVA, for the blends with pelletized recycled. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3696–3706, 2006     

Degradation of polyethylene during extrusion. II. Degradation of low‐density polyethylene,linear low‐density polyethylene,and high‐density polyethylene in film extrusion

The degradation of different polyethylenes—low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (HDPE)—with and without antioxidants and at different oxygen concentrations in the polymer granulates, have been studied in extrusion coating processing. The degradation was followed by online rheometry, size exclusion chromatography, surface oxidation index measurements, and gas chromatography–mass spectrometry. The degradations start in the extruder where primary radicals are formed, which are subject to the auto‐oxidation when oxygen is present. In the extruder, crosslinking or chain scissions reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is overall dominating for the more linear LLDPE and HDPE resins. Additives such as antioxidants react with primary radicals formed in the melt. Degradation taking place in the film between the die orifice, and the quenching point is mainly related to the exposure time to air oxygen. Melt temperatures above 280°C give a dominating surface oxidation, which increases with the exposure time to air between die orifice and quenching too. A number of degradation products were identified—for example, aldehydes and organic acids—which were present in homologous series. The total amount of aldehydes and acids for each number of chain carbon atoms were appeared in the order of C5>C4>C6>C7?C2 for LDPE, C5>C6>C4>C7?C2 for LLDPE, and C5>C6>C7>C4?C2 for HDPE. The total amounts of oxidized compounds presented in the films were related to the processing conditions. Polymer melts exposed to oxygen at the highest temperatures and longest times showed the presence dialdehydes, in addition to the aldehydes and acids. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1525–1537, 2004     

Some aspects of structural electrophysics of irradiated polyethylenes

In the case of the insulation polymeric materials, such as polyethylenes, it is of essential interest to understand correlations between structural changes and (di)electric properties. The dielectric behavior of different polyethylenes, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE), irradiated to different absorbed doses of gamma radiation, was studied through dielectric loss (tan δ) analysis. Dielectric relaxation behavior is related to the changes in the initial structure of different polyethylenes and to the radiation-induced processes of oxidative degradation and crosslinking. Differential scanning calorimetry (DSC), IC spectroscopy and gel measurements were used to determine the changes in the crystal fraction, oxidative degradation and degree of network formation, respectively.     

Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low‐density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular‐weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short‐chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short‐chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HDPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short‐chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2542–2549, 2004     

Effect of Metallocene-Catalyzed Polyethylene on the Physicomechanical Properties of Blends with High-Density Polyethylene or Low-Density Polyethylene

The physicomechanical properties of polymer blend formulations comprising different grades of metallocene-catalyzed linear low-density polyethylenes (mLLDPEs) with high-density polyethylenes (HDPEs) or a low-density polyethylene (LDPE) were investigated. For blends with HDPE, the addition of mLLDPE improves the Izod impact strength and some tensile properties. For blends with LDPE, adding mLLDPE increases the ductility and the percent elongation at break.     

Photo‐induced scission and crosslinking in LDPE,LLDPE, and HDPE

The molecular degradation characteristics of three different polyethylenes were determined by deriving chain scission and crosslinking concentrations from gel permeation chromatography molecular weight distributions obtained after 3 weeks and 6 weeks laboratory ultraviolet exposure. Injection‐molded bars (3 mm thick) made from a low‐density polyethylene (LDPE), a linear low‐density polyethylene (LLDPE), and a high‐density polyethylene (HDPE) were used and all showed strong depth variations in degradation. Degradation was rapid near the exposed surfaces but very little change occurred in the bar centers, due to oxygen starvation. The most rapid rises in scission and crosslink concentrations were observed with LDPE, for which the concentrations after 6 weeks exposure were approximately double those measured after 3 weeks. With LLDPE and HDPE the scission and crosslink concentrations after 6 weeks exposure were very much greater than twice those after 3 weeks. Scission dominated over crosslinking at all depths and for all materials the scission/crosslink ratio was always ≥3, with a value of ～9 recorded for HDPE near the exposed surface after 6 weeks exposure. POLYM. ENG. SCI., 45:579–587, 2005. © 2005 Society of Plastics Engineers     

Thermal and mechanical properties of silane‐grafted water crosslinked polyethylene

The effects of linear low density polyethylene (LLDPE) grafting with vinyltrimethoxysilane by different types and contents of peroxide were studied. When grafting silane onto LLDPE, with 0.10 phr of Dicumyl peroxide (DCP) or 0.05 phr content of 2,5‐Dimethyl‐2,5‐di (tert‐butyl‐peroxy)‐hexane (DHBP), it was found that the grafting effect was improved; however, as Di(2‐tert‐butylperoxypropyl ‐(2))‐benzene (DIPP) or excess DHBP was used, LLDPE was supposed to cause self‐crosslinking, which reduced the grafting effect of silane and was invalid in the processing of extrusion. In this study, vinyl trimethoxysilane (VTMS) was grafted onto various polyethylenes (HDPE, LLDPE, and LDPE) using DCP as an initiator in a twin screw extruder. The grafted polyethylenes were able to crosslink utilizing water as the crosslinking agent. The effects of varied crosslinking time on the mechanical properties of the crosslinked polyethylenes were studied. It was found that the HDPE and LLDPE were apt to crosslink during the grafting process and thus decreased the grafting ratio. Multiple melting behavior was observed for crosslinked LDPE and LLDPE. Mechanical and thermal properties of the crosslinked PE are much better than that of uncrosslinked PE. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2383–2391, 2005     

EVA对农膜再生料的力学性能及流变性能的影响

用熔融共混法制备了EVA与农膜再生料（RPE）的共混材料,研究了EVA对农膜再生料的改性作用,并与新料低密度聚乙烯（LDPE）进行了对比研究。对再生料、改性材料和新料进行了力学性能分析、旋转流变分析、转矩流变分析和形貌分析。结果表明,EVA可以显著提高农膜再生料的断裂伸长率,对拉伸强度影响不大,当EVA用量为50%时,改性材料的断裂伸长率和拉伸强度都和新料相当。EVA可以提高再生料的相容性,改善其流变性能和加工性能,使再生料的流变行为接近新料,更容易加工。     

Effects of ultrasonic oscillations on processing behavior and mechanical properties of metallocene‐catalyzed linear low‐density polyethylene/low‐density polyethylene blends

The influences of ultrasonic oscillations on rheological behavior and mechanical properties of metallocene‐catalyzed linear low‐density polyethylene (mLLDPE)/low‐density polyethylene (LDPE) blends were investigated. The experimental results showed that the presence of ultrasonic oscillations can increase the extrusion productivity of mLLDPE/LDPE blends and decrease their die pressure and melt viscosity during extrusion. Incorporation of LDPE increases the critical shear rate for sharkskin formation of extrudate, crystallinity, and mechanical properties of mLLDPE. The processing behavior and mechanical properties of mLLDPE/LDPE blends were further improved in the presence of ultrasonic oscillations during extrusion. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2522–2527, 2004     

Phase structural analyses of polyethylene coatings on high‐density papers. III. Determination of the crystallite thicknesses by T1 measurements

Two different extrusion‐coating qualities of polyethylene, namely LDPE and HDPE, were coated on high‐density papers. Differences were observed with respect to their response to storage and low temperature heat treatment. HDPE does not respond to storage at ambient temperature and heat treatment in the same way as LDPE. The LDPE‐coating exhibits an increase in the monoclinic crystalline fraction at the paper surface as a result of heat treatment. The nature of this response appears to be a result of adhesion to a paper surface, the properties of this surface, orientation of polymer chains, and chain mobility differences. The increase of the monoclinic fraction is shown to relate to an increase of the mean crystallite thickness and initiation of new crystallites at the paper surface. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 235–241, 2004     

Investigation of polyethylene/sisal whiskers nanocomposites prepared under different conditions

In this work, sisal nanowhiskers (SNWs) extracted from sisal fibers were used to reinforce high‐density polyethylene (HDPE) and low‐density polyethylene (LDPE). The nanocomposites were prepared by solution casting from toluene and melt mixing, both followed by melt pressing. In the case of melt mixing, the surfaces of the SNW were also chemically modified with 1 phr of vinyl triethoxy silane to improve their dispersibility and compatibility with the matrices. The SNW had an average length of 197 nm and diameter of 12 nm, and a crystallinity index of 89%. Fourier transform infrared confirmed the surface chemical modification of the SNW. The whiskers were fairly well dispersed in the matrices, regardless of the treatment or preparation method. The presence of whiskers, as well as nanocomposite preparation method, had an observable influence on the storage modulus of LDPE, but very little influence on that of HDPE. There was, however, no significant influence on the degradation behavior of both polymers. The crystallization behavior of the polymers was found to strongly depend on their morphologies. The melting and crystallization behavior of the LDPE nanocomposites were almost unchanged, while an increase in crystallinity was observed for all the HDPE nanocomposites. The tensile properties depended on the type of polymer, the treatment, and the preparation method. Generally there was an improvement in tensile modulus, and a decrease in elongation at break, but the stress at break only improved for the HDPE nanocomposites. POLYM. COMPOS., 35:2221–2233, 2014. © 2014 Society of Plastics Engineers     

Cocrystallization and miscibility studies of blends of ultrahigh molecular weight polyethylene with conventional polyethylenes

Ultrahigh molecular weight polyethylene (UHMWPE) was mechanically mixed with conventional polyethylenes (LLDPE, HDPE, and LLDPE) using an internal mixer. Rheological studies of these blends suggest that UHMWPE seems to be miscible with LLDPE, HDPE, and LDPE in the melt state. Yield characteristics are observed in all blend systems, particularly in high UHMWPE blend compositions. Differential scanning calorimetry and small-angle light scattering studies show that cocrystallization takes place in the blends of UHMWPE/LLDPE and UHMWPE/HDPE blends. However, separate crystals are formed in UHMWPE/LDPE. The formation of separate crystals may be attributed to long chain branching of conventional low-density polyethylene. Tensile properties of the former two blends vary almost linearly with blend compositions, while deviations are seen in the latter UHMWPE/LDPE blends.     

Polyethylenes/PET blend compatibilization with maleic anhydride modified polyethylenes obtained by a UV preirradiation process

The compatibilization of HDPE/LDPE/LLDPE/PET blend during reactive extrusion, using compatibilizing agents, such as modified high, low, and lineal low density polyethylenes with maleic anhydride, was carried out. The agents were prepared in our laboratory by using a UV preirradiation process, containing different grafting and crosslinking degrees. The materials were compared with same maleic anhydride modified polyethylenes prepared by the traditional peroxide method in our laboratory and with a commercial maleic anhydride modified lineal low density polyethylene. The mechanical and thermal properties, as well as their morphology, were evaluated in the compatibilized blends and changes in crystallization phases recorded. The elongation at break and impact strengths increased with compatibilization level and morphology was markedly more homogenous. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 560–567, 2007     

Rheological and mechanical properties of composites made from wood flour and recycled LDPE/HDPE blend

The effect of compounding method is studied with respect to the rheological behavior and mechanical properties of composites made of wood flour and a blend of two main components of plastics waste in municipal solid waste, low-density polyethylene (LDPE) and high-density polyethylene (HDPE). The effects of recycling process on the rheological behavior of LDPE and HDPE blends were investigated. Initially, samples of virgin LDPE and HDPE were thermo-mechanically degraded twice under controlled conditions in an extruder. The recycled materials and wood flour were then compounded by two different mixing methods: simultaneous mixing of all components and pre-mixing, including the blending of polymers in molten state, grinding and subsequent compounding with wood flour. The rheological and mechanical properties of the LDPE/HDPE blend and resultant composites were determined. The results showed that recycling increased the complex viscosity of the LDPE/HDPE blend and it exhibited miscible behavior in a molten state. Rheological testing indicated that the complex viscosity and storage modulus of the composites made by pre-mixing method were higher than that made by the simultaneous method. The results also showed that melt pre-mixing of the polymeric matrix (recycled LDPE and HDPE) improved the mechanical properties of the wood–plastic composites.     

Tensile behavior of irradiated recycled polyolefin plastics

Solid wastes represent a potential source for raw materials in the world. In Brazil, municipal solid waste (MSW; 2–%) is expected to grow at a rate per year higher than the worldwide rate (1%). On the other hand, the consumption of polymer blends increases at a rate more than twice that of all plastics. Therefore, the recycling of polymeric blends has gained increasing attention in the world due to economic and environmental considerations. A two‐step process, developed at the Institute (IMA/UFRJ), allows one to recover plastic residues and permits the production of materials with controllable composition and homogeneous characteristics. The compression behavior of polyblends, composed of typical polymers that appear in domestic wastes—low and high polyethylenes—can be improved by gamma irradiation. In the present work, the tensile behavior of recycled 75/25 blends of low‐density polyethylene (LDPE) and high‐density polyethylene (HDPE), after exposure to gamma rays in the air, was investigated. Tensile testing, scanning electron microscopy, infrared and solid‐state ¹³C‐nuclear magnetic resonance spectroscopy, as well as gel content were used to study the effect of gamma irradiation on the polymer blends. The tensile strength was found to increase with radiation dose while the elongation at break decreased. Our experimental results indicate that the gamma irradiation degradation process involves crosslinking at lower doses and chain scission at higher doses. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 899–909, 2000     

Effect of hydroperoxide decomposer and slipping agent on recycling of polypropylene

Polypropylene (PP) recycling has always been challenging because the polymer is highly susceptible to thermooxidative degradation during extrusion. Recycled (degraded) PP is normally blended with virgin PP to achieve reasonable mechanical properties after reprocessing operations. However, impurities present in recycled PP tend to degrade even the virgin PP in this process. In this study, standard recycled PP was produced in a laboratory by repeated extrusion and pelletization operations of virgin PP. This material was blended with virgin PP in a ratio from 3 : 7 to 7 : 3. An attempt was made to stabilize the recycled blend by adding a peroxide decomposer (triphenylphosphite, TPP) and a slipping agent (zinc stearate) in contrast to radical scavengers normally used in reprocessing. It was found that by using 0.3–0.5 wt % of TPP and 2 wt % of zinc stearate, this degradation could be effectively attested. Compared to the tensile strength retention of 68% (based on strength of pure virgin PP) of a 60 : 40 (recycled : virgin) PP blend without any stabilizer, a value of 77% was obtained for the same blend with the above‐mentioned stabilizers. This stabilization effect was attributed to decomposition of unstable hydroperoxides to stable compounds in the recycled materials by TPP, and lower generation of new radicals in the presence of zinc stearate. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3247–3251, 2004     

Degradation of HDPE and LLDPE in closed mixing chamber: a comparison

Summary Degradation of linear high density polyethylene (HDPE) and butyl branched linear low density polyethylene (LLDPE) was studied during moulding in a closed mixing chamber. At the beginning of the process the rate of oxidative degradation was found faster for LLDPE than for HDPE but later this relation reversed. The degradation mechanism was the same for both types of polyethylenes but the rate of elementary steps depended on the chemical structure of the polymer chain. The differences were attributed to the structural differences in the original materials and the products formed during degradation.     

Crystalline-state extrusion of low density polyethylenes

Three low density polyethylenes, one long branched (A) and two linear (B and C), have been solid-state-extruded at several constant temperatures from ambient to 80°C and to draw ratios ? 8. The initial densities and melt indices of A, B, and C are 0.920, 0.920, and 0.935 g/cm³, and 1.9, 0.8, and 1.2, respectively. Melt-crystallized cylindrical billets were extruded through conical dies in an Instron Capillary Rheometer. The linear polymers were found to draw by extrusion more readily than the branched; all three strain-harden. Density, birefringence, tensile, and thermal properties have been evaluated as functions of extrusion temperature and draw ratio. Despite a measured loss via die swell, substantial orientation takes place during solid-state extrusion as evidenced by increases in transparency, birefringence, and tensile modulus (up to 4.5 times that of the original isotropic polymer). Depending on the polymer and the draw temperature, density does go through a minimum or shows a monotonic increase with draw by extrusion. A minimum in modulus is also observed at low draw and at all draw temperatures for all three polymers. The highest tensile moduli achieved are 0.73, 0.46, and 1.5 GPa for A, B, and C, respectively, at their highest draw ratio. The melting point for polymer B decreases with extrusion draw ratio, whereas it remains constant after a small initial drop, for the two others. For all three low density polyethylenes, birefringence increases rapidly with extrusion draw and then levels off at high draw. The birefringence limit is similar for A and B, i.e., 0.046 ± 0.004, but higher for C, i.e., 0.068 ± 0.009. This work extends beyond others in that it studies the effect of short as well as long branches in solid-state extrusion by comparing the linear and long branched LDPE polymers and LDPE with prior evaluations of HDPE.     

Mechanical properties-structure relationships of crosslinked blends based on LDPE/HDPE waste

Low-density polyethylene (LDPE) waste was blended with high-density polyethylene (HDPE) waste of different degrees of degradation. Structural, mechanical and rheological properties of these blends were investigated. It was found that 2 wt.-% of dicumyl peroxide improves simultaneously the tensile strength and elongation at break without serious decrease of the melt elasticity of separate PE wastes and their binary blends in comparison with unmodified PE. It was shown by DSC analyses that modification of the blends leads to better compatibility between LDPE and HDPE.     

Experimental investigation on reprocessing of extruded wood flour/HDPE composites

This article presents an experimental study on the change in the properties of wood–plastic composites (WPCs) when reprocessed. The degree of properties degradation upon reprocessing, for recycling purpose, can be considered as a key factor to choose an alternative against discarding into the environment. A material which retains its properties when recycled, or at least exhibits insignificant reduction in its properties, is favorable in environmental point of view. To investigate the reprocessing effect on the WPC properties, in this study, cylindrical profiles of WPC, with 60 wt% of wood content, were produced using a twin screw extruder, at first stage (virgin WPC). These profiles were then chopped into granules and used in the reproduction of the same shaped product (recycled WPC). For the measurement of mechanical properties, tensile and three‐point bending tests were conducted. Differential scanning calorimetry (DSC) test was performed to compare thermal behavior of the neat HDPE, virgin and recycled composites. Scanning electron microscopy (SEM) images were also produced to observe the adhesion quality of the components and changes in wood particles size. Physical properties such as density and water uptake were also measured. A reduction in strength was observed upon recycling which was accompanied with the decrease in density, while an increase in the flexural modulus was noticed. The results also indicate that the recycled samples exhibit a higher water uptake. Analysis of thermal behavior showed a slight increase in the melting temperature of the reprocessed composite and decrease in the degree of crystallinity especially at the first stage of the HDPE process. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers