The effect of peroxide crosslinking on thermal,mechanical, and rheological properties of polycaprolactone/epoxidized natural rubber blends

Polycaprolactone (PCL)/epoxidized natural rubber (ENR) blends (PCL/ENR = 70/30, 50/50 wt/wt) were prepared by a melt mixing in an internal mixer in the presence of a small amount (0.5 and 1 phr) of dicumyl peroxide. The effect of peroxide crosslinking on thermal, mechanical, and rheological properties of the blends was investigated by means of DSC, tensile test, and small amplitude oscillating rheometer, respectively. It was revealed that peroxide crosslinking enhanced degree of crystallinity of PCL phase and its non-isothermal melt crystallization temperature. The crosslinked blends behave like a thermoplastic elastomer exhibiting high elongation-at-break and fairly good elastic recovery as well as melt processibility. From melt rheological analysis, the peroxide crosslinked blends showed more pronounced shear thinning effect and higher elasticity compared to simple blends.     

High temperature melted,radiation cross-linked,vitamin E stabilized oxidation resistant UHMWPE with low wear and high impact strength

In a previous communication we showed improvement in the wear resistance and toughness of cross-linked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants by radiation cross-linking after high temperature melting (HTM). In this study, we hypothesized that introduction of vitamin E into UHMWPE before high temperature melting could improve the oxidative stability of these UHMWPEs with low wear and high toughness. Vitamin E was blended with UHMWPE powder at concentrations of 0.1 and 0.2 wt% and consolidated, followed by melting at 300 and 320 °C for 5 h, and subsequent irradiation with electron beam to 150 kGy. These vitamin E/UHMWPE blends showed improved tensile and impact toughness and good wear resistance in comparison with the radiation cross-linked vitamin E/UHMWPE blends. Aggressive accelerated aging with or without pro-oxidant lipids showed that vitamin E-blended, high temperature melted and subsequently irradiated UHMWPE had good oxidation resistance.     

Preparation and characterization of ultrahigh molecular weight polyethylene and polyisoprene solvent-cast blend films

This study covers the preparation of noncrosslinked and crosslinked solvent-cast blend films of ultrahigh molecular weight polyethylene (UHMWPE) and polyisoprene rubber (PIR) and their mechanical, thermal, IR spectroscopic, and morphological characterizations. Solvent-cast films of polymer blends with 0, 10, 20, 35, 50, and 65% PIR composition were prepared by vigorous stirring from a hot decalin solution. The films were crosslinked chemically by using acetophenone as a crosslinking agent under UV radiation. The mechanical properties, measured as ultimate properties and tensile modulus, were found to decrease with PIR content but crosslinking was found to enhance the ultimate strength and tensile modulus. DSC results revealed that melting point of UHMWPE remains almost constant in blends. However, upon crosslinking, the melting point of UHMWPE is depressed almost 5°C. We observed a similar trend in the enthalpy change of the melting of UHMWPE and the variation of percent crystallinity in UHMWPE. Scanning electron microscopy (SEM) studies on the fractured surfaces of the blends showed that the fibrillar texture is present in both crosslinked and noncrosslinked blends. The crosslinking appeared to be through oxygen linkages, which are preferentially conjugated to double bonds, in addition to the possible carbon–carbon crosslinks. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1619–1630, 1998     

SBS/PE-LD的强化交联与形状记忆效应研究

用苯乙烯丁二烯共聚物（SBS）与低密度聚乙烯（PE-LD）共混,采用过氧化二异丙苯（DCP）作为引发剂,3种不同的多官能团单体作为强化交联剂,使共混物发生强化交联形成网状结构,对多官能团单体的强化交联作用进行了分析,测试了交联共混物的拉伸强度与断裂伸长率,探讨了交联共混物的形状记忆效应。结果表明,多官能团单体的加入能够显著提高SBS/PE-LD交联共混物的交联程度,交联SBS/PE-LD共混物具有优异的形状记忆性能,形状固定率和形状回复率均可达100 %。     

Negative synergism of crosslinking agent and antioxidant in polyethylene

The melt flow index and oxidation induction period of low-density polyethylene crosslinked with dicumyl peroxide (DCP) in the presence of tetrakis (methylene β-3, 5-di-butyl-4-hydroxypheny1)-propionate methane (Antioxidant 1010) were determined. The results suggest that there is a negative synergism between this crosslinking agent and antioxindant. Part of the antioxidant terminated the chain propagation by trapping radicals, and thus reduced the efficiency of crosslinking. The antioxidant efficiency was also reduced owing to meaningless consumption.     

Effect of different types of peroxides on properties of vulcanized EPDM + PP blends

Mechanical and tribological behavior of several dynamic vulcanizate blends of polypropylene (PP) with ethylene‐propylene‐diene rubber (EPDM) was examined and compared with those of uncrosslinked blends. Vulcanization was performed using two types of organic peroxides combined with (meth)acrylate coagent. The effect of different types and concentrations of peroxides as crosslinking agents on the properties of the resulting materials were investigated. Dicumyl peroxide (DCP) provides higher reactivity and exhibits nearly the same crosslinking efficiency for both 60/40 and 50/50 blends; almost fully crosslinked samples are obtained if the compound contains 1.0 or 2.0 wt% DCP. These results correlate to the gel content and mechanical properties of our materials. Variation of PP + elastomer ratio does not have a significant influence on friction. From 60/40 group of composites, lower friction values were obtained for samples cured with 0.5 wt% benzoyl peroxide (BP) and 1.0 wt% DCP. The tribological properties of the samples with higher amount of DCP show rubbery rather than a toughened thermoplastic behavior. Wear of the composites decreases with the increasing concentration of the curing agent. Compared to BP, the samples cured with DCP display lower wear. POLYM. COMPOS., 31:1678–1691, 2010. © 2010 Society of Plastics Engineers.     

Radiation-induced crosslinking of UHMWPE in the presence of co-agents: chemical and mechanical characterisation

Radiation-induced crosslinking has been shown to have a beneficial effect on the wear resistance of ultra high molecular weight polyethylene (UHMWPE). Since we postulate that crosslinking takes place through reactions involving terminal double bonds, unsaturated additives were added to UHMWPE in this study to enhance crosslinking. UHMWPE specimens soaked in 1,7-octadiene, methylacetylene and ethylene, respectively, were irradiated with electron beam to different doses in single or multiple passages. FTIR spectroscopy was used for the chemical characterisation of the crosslinked polymer. Tensile tests were performed with all samples in order to monitor changes in the mechanical properties.Gel fraction measurements proved that crosslinking took place in all the irradiated samples, but 1,7-octadiene turned out to be the most effective additive for the present purpose, exhibiting a good efficiency in enhancing crosslinking. The FTIR analyses showed that consumption of vinyl double bonds is proportional to the irradiation dose, with an efficiency which increases with increasing their initial concentration, confirming the hypothesis of the involvement of these groups in the crosslinking process. In the case of UHMWPE with 1,7-octadiene irradiated to high doses in multiple passages, the results of the tensile tests indicated a significant decrease in both elongation at break (up to 65%) and ultimate stress (up to 25%). A possible explanation of the diminished strength of the crosslinked material has also been proposed.     

Facile preparation of a crosslinked hydrophilic UHMWPE membrane

Ultra high molecular weight polyethylene (UHMWPE) filters are widely used in water treatment. In the present work, a facile crosslinking technique is first applied to a UHMWPE flat membrane as a model to realize long-lasting hydrophilicity and improved water treatment efficiency. In the presence of a crosslinker, N, N′-methylene bisacrylamide (MBA), 2-hydroxyethyl methacrylate (HEMA), a hydrophilic modifier, is soaked into the surface of UHMWPE particles by blending. Then, a crosslinking reaction occurs during the initial sintering stage. Finally, sintering is completed at high temperature and pressure. The FTIR and SEM results show that HEMA is successfully crosslinked on the UHMWPE particle surface. Compared to a pristine membrane, the crosslinked UHMWPE flat membrane presents a lower water contact angle and is more rapidly penetrated by water. As a result, the crosslinked membrane can realize pressureless filtration. Another meaningful result is that the water flux recovery rate (FRR) is extremely high both for BSA and sludge filtration tests. Also a parameter K of 1 was obtained to represent the efficiency of the membrane treatment of water under pressureless conditions. These findings demonstrate that the crosslinking strategy is an effective method for realizing long-lasting hydrophilicity and is very promising for UHMWPE filters and other engineering applications.     

Well-mixed blends of HDPE and ultrahigh molecular weight polyethylene with major improvements in impact strength achieved via solid-state shear pulverization

Compared with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding impact strength and crack resistance that make it desirable for a wide variety of applications. Unfortunately, UHMWPE has an ultrahigh viscosity that renders common, continuous melt-state processes ineffective for making UHMWPE products. Attempts to overcome this problem by blending UHMWPE with lower molecular weight high-density polyethylene (HDPE) by melt processing have typically led to poorly dispersed blends due to the vast viscosity mismatch between blend components. Here, we present solid-state shear pulverization (SSSP) as a mild, continuous, and simple approach for achieving effective and intimate mixing in UHMWPE/HDPE blends. These SSSP blends are easily processed by post-SSSP melt extrusion; for an SSSP blend with 50 wt% UHMWPE, we observe more than a factor of 1000 increase in viscosity at a shear rate of 0.01 s⁻¹ but less than a factor of 5 increase at 100 s⁻¹, the latter being more typical of melt-processing operations. Using extensional rheology, we confirm the strain hardening behavior of SSSP blends. Shear rheology and crystallization data show that the mixing between UHMWPE and HDPE can be improved with subsequent passes of SSSP and single-screw extrusion. Finally, we show that blending via SSSP leads to dramatic improvements in impact strength: as compared to literature results, injection-molded sample bars made from SSSP blends with 30–50 wt% UHMWPE exhibit very high values of notched Izod impact strength, 660–770 J/m (the impact strength of neat HDPE was 170 J/m).     

Exploring interfacial interactions,dielectric, ferroelectric and piezoelectric properties of ultrahigh molecular weight polyethylene/graphene oxide nanocomposites

Graphene oxide (GO) incorporated ultra-high molecular weight polyethylene (UHMWPE) nanocomposites were prepared by encapsulating GO by UHMWPE in an aqueous media via high-shear mixing, which were subsequently dried and compression molded. Morphological characterizations via scanning electron microscopy revealed the intercalation of UHMWPE chains in the graphitic stacks corresponding to GO. Further, dielectric permittivity of UHMWPE/GO nanocomposite of 1 wt% GO showed a drastic increase (~61) as compared to pure UHMWPE (~2) due to an enhanced interfacial polarization. A significantly higher value of remnant polarization (~10 nC/cm²) and coercive field (~3 kV/cm) was observed in UHMWPE/GO nanocomposite of 1 wt% GO, which showed a strong hysteresis loop of polarization versus electric field plot as compared to pure UHMWPE, which displayed a very weak hysteresis loop. The piezoelectric coefficient (d₃₃) of ~9.5 pm/V was estimated in UHMWPE/GO nanocomposite of 1 wt% GO via piezoresponse force microscopy. Nanocomposite sensor devices were also fabricated and piezoelectric output voltage of ~6 V was recorded in UHMWPE/GO nanocomposite of 1 wt% of GO. We report here for the first time the unique ferroelectric and piezoelectric properties displayed by UHMWPE/GO nanocomposites.     

Influence of dynamic crosslinking on the morphology,crystallization, and dynamic mechanical properties of PA6,12/EVA blends

This study investigated the effect of dynamic crosslinking of polyamide 6,12 and random copolymers of ethylene and vinyl acetate blends (PA6,12/EVA) on the morphology, crystallinity, and dynamic mechanical properties. The crosslinking agent was dicumyl peroxide (DCP), and the blends were processed in a torque rheometer. The morphology depended on the DCP content, and all blends exhibited the same crystallinity index. However, with increasing crosslinking degree, the interfacial tackiness (E) values increased from 1.8 to 2.7 nm. The lamellar structures of all blends started forming at approximately 160 °C, close to the temperature of pure polyamide. The crosslinked phase enhanced the pseudo‐elastic behavior of the blends and increased their molecular mobility activation energy. Samples with higher crosslinking degree exhibited smaller permanent deformation (0.01%) than those with low crosslinking. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44206.     

动态交联PP/EVA共混物的制备与性能研究

本文将动态交联技术应用于PP/EVA共混体系中,制得动态交联PP/EVA共混物。采用Hakke转矩流变仪研究了动态交联对PP/EVA共混物扭矩的影响;研究了DCP和EVA含量对共混物力学性能的影响;考察了动态交联共混物的维卡软化点。结果表明：加入DCP后,PP/EVA共混物扭矩先升后降,DCP的添加量为EVA含量的1%为宜。随EVA用量的增加,动态交联EVA/PP共混物的冲击强度大幅提高,但拉伸强度有所降低。少量经动态交联的EVA颗粒可以促进共混物中PP的结晶, 提高共混物的维卡软化点。     

Crosslinking of polypropylene–polyethylene blends by peroxide and the effect of pentaerythritol tetrallyl ether

Crosslinking of polypropylene–polyethylene (PP-PE) blends involving 10, 20, 30, 40, 50, 60, 70, 80, and 90% of PP with dicumylperoxide (DCP) or tert-butyl perbenzoate (TBPB) and in the presence of coagent pentaerythritol tetrallyl ether (PETA) was investigated at 180°C. It was found that at lower concentrations of peroxide alone (e.g., 2.5% of DCP) only PE component is crosslinked in all compositions of PP-PE blends. In the crosslinking of PP-PE 50:50 with 4% of TBPB, insoluble gel was obtained, which contained 13% PP and 87% PE. If 2% PETA was also used, the portion of PP in gel increased to 39%; the total yield of gel in PP-PE blend increased from 50 to 70%. The lower crosslinking efficiency of coagent PETA in the PP-PE blends compared with PP alone is associated with better solubility of the coagent in the PE phase in contrast to the PP phase. The coagent does not particularly raise the crosslinking efficiency of peroxide in PE, but increases it in the PP phase. A remarkable decrease in melting temperature and temperature of crystallization of both polymer components depending on peroxide concentration was found by calorimetric measurements.     

An in-situ X-ray scattering study during uniaxial stretching of ionic liquid/ultra-high molecular weight polyethylene blends

An ionic liquid (IL) 1-docosanyl-3-methylimidazolium bromide was incorporated into ultra-high molecular weight polyethylene (UHMWPE) and formed IL/UHMWPE blends by solution mixing. The structure evolution of these blends during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both IL/UHMWPE blends and neat UHMWPE. The elongation-to-break ratios of IL/UHMWPE blends were found to increase by 2-3 times compared with that of pure UHMWPE, while the tensile strength remained about the same. In contrast, during deformation at high temperature (120 °C), no phase transformation was observed. However, the blend samples showed much better toughness, higher crystal orientation and higher tilting extent of lamellar structure at high strains.     

Evolution of rheological properties and morphology development during crosslinking of polyolefin elastomers and their TPV blends with polypropylene

This work investigates the evolution of the rheological properties of elastomeric dynamically vulcanized ethylene‐α‐olefin copolymers (ECs) and their blends with polypropylene (PP), during peroxide initiated crosslinking. Rheological techniques are used in conjunction with gel content measurements to determine the onset of gelation during static crosslinking. The complex viscosity and moduli follow power‐law dependence with respect to frequency at the gel point. The relaxation exponent and corresponding values of tan δ at the gel point are determined from the complex viscosity versus frequency curves and used as criteria for the determination of the instance of gelation. The evolution of morphology of thermoplastic vulcanizate (TPV) blends consisting of EC and PP during dynamic crosslinking is discussed in the context of the evolving rheological properties of the matrix and the dispersed phase that take place upon peroxide modification. TPVs having the crosslinked EC as the matrix present a very fine morphology, whereas the blends containing crosslinked EC particles, present a coarser morphology. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Antioxidant Activities of Free and Liposome-Encapsulated Green tea extracts on canola oil oxidation stability

In the present study, green tea extract was encapsulated in liposomes based on the Mozafari method (with no organic solvents) and characterized for its physicochemical properties (encapsulation efficiency, particle size, and Z-potential). Encapsulation efficiency, particles size, and Z-potential were determined to be 51.34, 419 nm, and -57 mV, respectively. Total polyphenol content of the green tea by Folin-Ciocalteu's phenol reagent was measured as 164.2 mg gallic acid/g extract. Free radical scavenging activities of free and liposomal extracts were 90.6 and 93.4%, respectively, using the DPPH method. Antioxidant activity of the ethanolic extract of green tea in free and liposomal forms with concentrations of 200, 600, and 1000 mg L⁻¹ were assessed on oxidative stability of the canola oil at 60 °C for 0, 4, 8, 12, 16, 20, 24, 28, and 32 days. Results were compared to results of synthetic antioxidant butylated hydroxytoluene at 200 mg L⁻¹. To assess antioxidant activity on canola oil stability, peroxide, thiobarbitoric acid, and anisidine values were assessed as well as the total oxidation value and rancimat test. Results showed that the liposomal green tea extract was more effective than the free extract. Furthermore, a 600 mg L⁻¹ concentration of the green tea extract showed a significant antioxidant activity, compared to other extract concentrations. Increasing storage time and various concentrations of the ethanolic green tea extracts included significant effects on canola oil stability (P ≤ 0.05). Results demonstrated that the green tea extract could be used as an effective antioxidant. Free and liposomal extract (at 600 mg L⁻¹) resulted in stronger functionality than the synthetic antioxidant butylated hydroxytoluene.     

Phase morphology and melt rheological behavior of uncrosslinked and dynamically crosslinked polyolefin blends: role of macromolecular structure

Thermoplastic vulcanizates (TPVs) based on polypropylene (PP) with ethylene–octene copolymer (EOC) and ethylene propylene diene rubber (EPDM) have been prepared by co-agent-assisted peroxide crosslinking system. The study was pursued to explore the influence of two dissimilar polyolefin polymers having different molecular architecture on the state and mode of dispersion of the blend components and their influence on melt rheological properties. The effects of dynamic crosslinking of the PP/EOC and PP/EPDM have been compared with special reference to the concentration of crosslinking agent and ratio of blend components. Morphological analyses show that, irrespective of blend ratio, dynamic vulcanization exhibits a dispersed phase morphology with crosslinked EOC or EPDM particles in the continuous PP matrix. It was found that viscosity ratio plays a crucial role in determining the state and mode of dispersion of blend components in the uncrosslinked system. The lower viscosity and torque values of uncrosslinked and dynamically crosslinked blends of PP/EOC in the melt state indicates that they exhibit better processing characteristics when compared to corresponding PP/EPDM blends.     

Effect of entangled state of nascent UHMWPE on structural and mechanical properties of HDPE/UHMWPE blends

In this study, a synthesized ultra‐high molecular weight polyethylene (UHMWPE) with a less entangled state and a commercial UHMWPE with a highly entangled state were blended with high‐density polyethylene (HDPE) by melt blending, respectively. Rheology, 2D small‐angle X‐ray scattering (2D‐SAXS), differential scanning calorimetry (DSC), and tensile test were used to study the relationship between the microstructure and the mechanical properties of blends. It was demonstrated that the UHMWPE with the less entangled state was easy to be oriented at a given flow. More mechanical networks were achieved among the HDPE matrix and the UHMWPE chains due to the fewer entanglements of synthesized UHMWPE, improving the melting recovery of blends. Furthermore, notably oriented structures (shish‐kebabs) with increased long‐periods were made in the blends of weakly entangled UHMWPE and HDPE. The tensile strength of this blend was thus enhanced, i.e., the tensile strength raised for neat HDPE from 45.7 to 83.1 MPa for HDPE/UHMWPE blends containing 10 wt % of less entangled UHMWPE. However, the phase separation of blends was characterized when more weakly entangled UHMWPE was incorporated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44728.     

化学交联PE-LD/SBS共混物的形状记忆效应研究

分别用3种不同苯乙烯含量的苯乙烯-丁二烯共聚物(SBS)与低密度聚乙烯(PE-LD)混合制成共混物,采用过氧化苯甲酰(BPO)为引发剂,使共混物发生化学交联形成网状结构,对交联共混物的交联度进行了分析,测试了交联共混物的拉伸强度与断裂伸长率,探讨了交联共混物的形状记忆效应。结果表明,在PE-LD/SBS共混物中,随着BPO含量的增加,共混物的交联度增大;BPO含量固定时,随着SBS中丁二烯含量的增大共混物的交联度增大;随着交联度的增加,共混物的拉伸强度与断裂伸长率均下降;当BPO含量为0.5 %～1.5 %时,交联共混物具有良好的形状记忆效应。     

Preparation of HDPE/UHMWPE/MMWPE blends by two-step processing way and properties of blown films

Summary Blends of high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE) were prepared by two-step processing way. Middle molecular weight polyethylene (MMWPE) as a fluidity modifier and compatilizer was added into UHMWPE in the first step, and then modified UHMWPE and HDPE were blending extruded to prepare the HDPE/UHMWPE/MMWPE blends used for blown films. The mechanical test of the blown films revealed that when the content of MMWPE in modified UHMWPE was 40wt% and the content of UHMWPE in the blends was 20 wt%, the film had the optimal mechanical properties. The tensile strength and tear strength of the film increased by 50% and 21%, respectively, compared with those of pure HDPE film. Rheological curves indicated that the melt torque and the apparent viscosity of the HDPE/UHMWPE/MMWPE blends made by two-step processing way both greatly reduced than other blends. The results from DSC suggested that the blends by two-step processing way may form more stable and perfect co-crystallization. PLM (polarized light microscopy) and SEM micrographs revealed that two-step processing way can improve the surface morphology of the films and make the dispersion of UHMWPE particles in HDPE increase.