Photoinitiated crosslinking of EPDM/inorganic filler blends and characterization of related properties

<正>Photoinitiated crosslinking of ethylene-propylene-diene terpolymer(EPDM) blends filled with calcium carbonate, talc and calcined kaolin(CK) in the presence of benzil dimethyl ketal as photoinitiator and trimethylolpropane triacrylate as crosslinker and their related properties have been studied by different analytical methods.The results from gel content and heat extension determination show that the efficiency of photocrosslinking of EPDM increases with increasing the content of diene and its molecular weight.The EPDM blends with 100 phr different inorganic fillers can be photocrosslinked to gel content of above 60%by 5 s UV-irradiation under optimum conditions.Under the same conditions of irradiation,the orders of photocrosslinking rate and final gel content are EPDM/CaCO_3EPDM/talcEPDM/CK.The data from thermogravimetric analysis,dynamic mechanical thermal analysis,electrical properties,mechanical tests and scanning electron microscopy show that UV irradiation crosslinking apparently enhances the thermal stability,mechanical properties and electrical properties of the photocrosslinked EPDM/inorganic filler samples.Although the attenuated total-reflection FTIR data show that inorganic fillers can promote the surface photo-oxidation of EPDM/inorganic filler samples with increasing the irradiation time,the above related properties of the photocrosslinked EPDM blends irradiated within 5 s are enough to satisfy many applications in the cable industry.     

Chemistry of dicumyl peroxide-induced crosslinking of linear polyethylene

The course of intermolecular reactions induced by peroxide decomposition in linear polyethylene has been determined and compared with similar reaction systems in which the substrate is a lowmolecular-weight alkane. Efficiency of intermolecular coupling is 40% for an alkane and 25–30% for the polymer system. Competing reactions, which reduce coupling efficiency, are mainly disproportionation of the initially formed radicals and to a minor extent further reactions involving the products from the peroxide decomposition. The low efficiencies found are similar to those well studied when initial radicals are produced by radiolysis in polymer and alkane systems.     

The crosslinking of polyethylene irradiated in a multilayer system

Polyethylene films characterized by different thickness and crosslinking efficiencies were irradiated with 1.5 MeV electrons at an average current density of 0.016 mA/cm². The samples were in the form of film stacks with or without distance supports between the polymer layers. Some of the samples were irradiated in an N₂ atmosphere. The absorbed dose in a given polymer layer was estimated from the gel content. From the dose-gel dependence estimated for each polymer layer and the dose-depth dependence based on the literature data, the gel-depth and dose-depth curves were plotted. From the comparison of these curves with experimental data it was concluded that the absorbed dose depends on the polymer film thickness and the presence or absence of the gas phase between polymer layers.     

Influence of crosslinking on physical properties of low density polyethylene

Influence of crosslinking on physical properties of low density polyethylene was studied.The results indicated that,at low degrees of crosslinking,the network hardly affects the crystallinity,elastic modulus(E) and yielding stress while it improves the tensile strength and strain at break simultaneously.Tensile strength reaches a maximum of about 24 MPa at 1.5 phr dicumyl peroxide(DCP) then decreases to a constant value of about 18 MPa due to decrease of crystallinity.E reaches its maximum at 0.5 phr DCP corresponding to gel fraction of about 75%without marked change in crystallinity.The crosslinked polyethylene exhibits two yielding processes,and both yielding stresses approximately linearly depend on crystallinity.     

Effect of solvent on radiation grafting and crosslinking of polyethylene

Crosslinking of polyethylene influences its swelling properties. It could be expected that pre-crosslinking of polyethylene influences the rate and yield of grafting as well. This is demonstrated by pre-crosslinking of polyethylene and by its subsequent grafting with styrene after the trapped radicals had been annealed out.In order to obtain more direct information about the influence of swelling agent on polyethylene crosslinking, the elastic modulus of the crosslinked polyethylene was investigated. Stress–strain curves of polyethylene samples irradiated in different environments were recorded in molten state at 165 °C. The results show that irradiation of swollen polyethylene produces fewer effective crosslinks than does irradiation of dry polymer.     

Photoinitiated grafting polymerization of acrylic acid onto the surface of polyethylene under pulsed laser radiation

Photoinduced grafting polymerization on the surface of PE films induced by nanosecond pulsed laser radiation is studied. The grafting is performed from the liquid phase composed of acrylic acid and a photoinitiator (benzophenone) dissolved in it. Pulsed laser radiation with a wavelength of 355 nm, a pulse duration of 11 ns, and a repetition rate of 10 Hz is used. Formation of the surface-graft polymer is followed by IR-ATR spectroscopy and contact-angle measurements. It is found that the time of laser treatment sufficient for the efficient modification of the PE surface with the grafted poly(acrylic acid) is in the range from 0.5 to 1.0 s at a laser-pulse energy density of 200–500 mJ/cm². At energy densities beyond this range, the efficiency of the reaction decreases rapidly. The results on laser grafting are compared with the results of grafting during UV irradiation with a lamp at a wavelength of 365 nm.     

Role of minority structures and mechanism of peroxide crosslinking of polyethylene

Minority structures are considered to be defect structures that are formed during polyethylene (PE) preparation and during the crosslinking process in PE. The minority structures that play the predominant role in PE crosslinking are vinyl double bonds. Moreover, the decomposition of dicumyl peroxide in PE does not proceed according to first-order kinetics, but induced peroxide decomposition also takes part. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 675–688, 2004     

Effect of hydrophobic oxides on radiation crosslinking of low density polyethylene

Effect of hydrophobic oxide, containing Si–SH=CH₂ groups, on the radiation crosslinking of low density polyethylene (LDPE) has been studied. It was found that mechanical stability of irradiated LDPE containing improved SiO₂ is higher than that of samples containing unimproved SiO₂.     

CPS K 843 Radiation-induced crosslinking: Effect on structure of polyethylene

Branched polyethylene irradiated (0–400 Mrad) with a Co⁶⁰ source at room temperature under vacuum was studied by density, wide- and small-angle X-ray scattering (WAXS and SAXS) measurements. The radiation effects on the structure of bulk, branched polyethylene are quite similar to those observed by others on single crystals or oriented preparations. These effects include changes in bulk density, crystallinity(w _c orv _c) and¯d ₁₀₀ and¯d ₂₀₀ spacings as a function of irradiation. A decrease in crystallinity is seen to begin at radiation dose 100 Mrad whereas lattice expansion indicating onset of an orthorhombic-hexagonal transition can begin as low as 10 Mrads. The decrease in crystallinity can be attributed to additional lattice distortions primarily introduced by the crosslinks occurring at the lateral grain boundaries, while lattice expansion can be associated with the same crosslinking mechanism which begins at the defects both within the crystals as well as those outside the crystals at the lateral grain boundaries. Strong evidence for a primary crosslinking-at-the-defects mechanism has also come from _c and _a data obtained in this study as a function of radiation dose. The same data have also led to an excellent correspondence between the measured density crystallinityv _c and the measured WAXS crystallinityw _c. Without consideration of the effects of crosslinks on _c and _a one would have obtained a divergence of the two crystallinities, especially at radiation doses greater than 100 Mrads.Dedicated Prof. Dr. R. Bonart on the occasion of his 60^th birthday     

Accelerated water tree aging of crosslinked polyethylene with different degrees of crosslinking

The effect of crosslinking degree on accelerated water tree aging in crosslinked polyethylene (XLPE) was investigated. The peroxide-crosslinking process was adopted to make XLPE specimens with different degrees of crosslinking by controlling the doping content of dicumyl peroxide (DCP) in low-density polyethylene (LDPE). The water blade electrode method was applied to accelerate water-tree aging of LDPE and XLPE specimens (hereafter referred to as the specimens), and their morphologies were observed using an optical microscope. The variation of crystalline morphology and anti-cracking performance of the amorphous region in the specimens were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and an electronic universal testing machine. Based on the experimental results, it was found that XLPE has great anti-water-treeing performance compared to LDPE. In addition, the higher the crosslinking degree, the better the anti-water-treeing performance. Although crystal growth is inhibited due to the crosslinking reaction, the density of tie molecular chains greatly increases in the amorphous region and exhibits significantly tighter lamellar stacking, which is the reason that water tree growth is restrained with increasing crosslinking degree.     

The thermomechanical method for evaluation of the crosslinking parameters in irradiated polyethylene

The measurement of penetration deformation above the melting point was used for determination of viscoelastic parameters, compliance and retardation time, in γ-irradiated polyethylene systems. The time-dependent penetration deformation was followed over two creep and recovery cycles. The five-element mechanical model equation was used for calculation of amplitudes, time factors and remained deformations. The modulus of elasticity, derived from the equilibrium compliance was used to obtain M_c, the value of the average molecular weight between crosslinks.     

ESR study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent

The reaction mechanisms of thermal crosslinking of polyethylene with dicumyl peroxide (DCP) at high temperatures were investigated by electron spin resonance (ESR). The crosslinking reactions were induced by the alkyl radicals formed by the thermal decomposition of DCP above 120°C. The kinetics of the free radical reaction were followed during crosslinking reactions at temperatures between 145 and 180°C. © 1997 John Wiley & Sons, Inc.     

Influence of chemical crosslinking on the creep behavior of ultra-high molecular weight polyethylene fibers

In this study, the effect of chemical crosslinking on the creep behavior of high-strength fibers, obtained by gel-spinning and subsequent hot-drawing of ultra-high molecular weight polyethylene (UHMWPE), is examined. In the first part of the paper, the general aspects of the creep behavior of these fibers are discussed. The second part deals with UHMWPE fibers that are crosslinked by means of a) chlorosulfonation and b) dicumyl peroxide treatment followed by UV irradiation. The latter technique leads to an improvement of the creep resistance of the UHMWPE fibers without affecting their high tensile strengths. In spite of the fact that the network formation is fairly high, the creep cannot be completely removed. The results indicate that the creep process in UHMWPE fibers is associated with a deformation mechanism in the crystalline regions of the fiber, which are not affected by chemical crosslinking.     

The effect of post-irradiation annealing on the crosslinking of high-density polyethylene induced by gamma-radiation

Samples of two high-density polyethylenes having different crystallinity levels were gamma irradiated under vacuum at doses ranging from 20 to 300 kGy. Subsequently, the vials containing the irradiated samples were exposed to different post-irradiation treatments. Parts of the specimens were annealed while still under vacuum. The annealing time was 4 h and the annealing temperatures 110 °C or 150 °C. Others were exposed directly to air opening the vials without any thermal treatment. It was verified that in all cases the dosage to produce an incipient gel increases with the crystallinity of the initial sample. The amount of gel produced after exposing specimens of the same polymer to a given dose increases with the annealing temperature. The largest increment in the amount of gel produced at the completion of the post-irradiation treatment was found on the samples with the highest initial crystallinity level.Evidence of oxidation was found in all irradiated samples. The extent of oxidation depends on the initial crystallinity of the sample, the irradiation dose and the type of post-irradiation treatment. The heat of fusion measured in the annealed samples decreases with the gel content while the fusion temperature was slightly affected. Ductile or brittle behaviors were observed after testing specimens under tensile stress. The yield stress increases proportionally to the crystallinity level that, in turn, depends on the total dosage applied to the samples. The extensibility of ductile samples is determined by the amount of gel produced regardless of the degree of initial crystallinity and the type of annealing process applied to each sample.     

The mechanism of action of crosslinking additives in polyethylene vulcanisation: an EPR investigation

The nature and kinetics of the free radicals intermediates in the peroxide induced thermal vulcanisation of low density polyethylene and the mechanism of action of two crosslinking co-agents with the structure of alkenyl aromatic hydrocarbons have been elucidated by EPR spectroscopy. At low peroxide concentration, the crosslinking mechanism in the pure low density polyethylene (LDPE) matrix is based, as expected, on the H abstraction from the chains and the coupling of the resulting macroalkyl radicals. The latter reaction takes place in competition with the disproportionation causing the formation of chain unsaturations that accumulate with the reaction time. At relatively large (>2%) peroxide concentrations, such unsaturations reach high enough levels to scavenge all the initiating and macroalkyl radicals giving macroallyl radicals as the dominant intermediates and determining changes in the structure of the crosslinks during the course of the reaction. At the concentration used, the activities of the co-agents 1,3-diisopropenylbenzene and 2,4-diphenyl-4-methyl-1-pentene are sufficient to scavenge all the initiating and macroalkyl radicals by double bond addition; this mechanism, when involving macroalkyl radicals, leads to the attachment of saturated and unsaturated pendants on the LDPE chains respectively. The unsaturated pendants are thought to be responsible for the crosslinking promotion effect by the 1,3 diisopropenylbenzene coagent. The lack of the second unsaturation causes 2,4-diphenyl-4-methyl-1-pentene to act essentially as a mild antiscorch agent.     

Photoinitiated anti-Hydropentafluorosulfanylation of Terminal Alkynes

A photoinitiated anti-hydropentafluorosulfanylation of terminal alkynes using SF₅Cl and (TMS)₃SiH as the hydrogen atom donor is reported. This transformation generates selectively (Z)-(1-alken-1-yl)pentafluoro-λ⁶-sulfanes (Z:E : >85:15), thus allowing the preparation of this previously unknown geometrical isomer. DFT calculations highlight that the selectivity is due to the intrinsic preference of SF₅-substituted vinylic radicals to adopt a cis geometry, and to increased steric contacts during the transition structures leading to the minor (E)-products.     

Photocrosslinking of polyethylene. I. Photoinitiators,crosslinking agent,and reaction kinetics

High density polyethylene has been photocrosslinked in the melt by using 4-chlorobenzophenone (4-CBP) as photoinitiator and triallylcyanurate (TAC) as crosslinking agent. Various factors affecting the crosslinking process (photoinitiator used and its concentration, irradiation temperature and time, atmosphere, UV light source, light intensity) were examined. By optimizing the irradiation conditions, it was found that samples up to 2 mm thick could readily be crosslinked to high gel content (≈ 90%) with satisfactory homogeneity within very short irradiation times (≈ 15 s). The kinetic analysis of the crosslinking process showed a linear relation between log S and log t (S is the sol fraction t is the irradiation time). An induction period was observed, the length of which decreased with increasing concentration of 4-CBP and increasing light intensity. An upper limit of gel content was also found for long irradiation times at a level depending on the concentration of 4-CBP. The crosslinking rate was proportional to the second order of light intensity at lower intensity and changed to first order at higher light intensity. A mechanism based on the recombination of chain radicals through both allyl and alkyl type radicals of TAC was proposed.     

Radiation-induced crosslinking: II. Effect on the crystalline and amorphous densities of polyethylene

Small-angle x-ray scattering (SAXS) was used to determine the structural changes in polyethylene induced by radiation. The changes in densities of the crystalline and amorphous phases, _c and _a , were calculated after direct determination of the mean square density fluctuation <²>. _a increases with increasing radiation dose for both linear and branched polyethylene. This accounts for the serious discrepancy between crystallinities determined from wide-angle x-ray scattering and density measurements. This study confirms our previous proposal that crosslinks occur primarily in the noncrystalline phase, most likely at the defects in the lateral grain boundary regions.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday.