Intercrystalline crosslinking of polyethylene

The heat distortion temperature (British Standards Method 102C) and DTA nominal melting point of high density polyethylene have been raised to 140°C by nonrandom crosslinking in which the crosslinks are superimposed on the existing semicrystalline structure. The crosslinking agent was a mixture of dicumyl peroxide and allyl methacrylate, and the crosslinking temperature was 120°C, just below the crystal melting range. In contrast, random crosslinking by the same system above the melting range at 160°C lowered the HDT (heat distortion temperature). Conventional peroxide crosslinking also lowered the HDT. The gel swelling of randomly crosslinked polyethylene was higher than that of nonrandomly crosslinked polyethylene having the same gel content. This dependence of the gel swelling on the crosslink distribution can be correlated with the HDT. The effect of γ-radiation and carbon black loading on the HDT has also been studied.     

Synthesis and application of water‐based urethane acrylate crosslinking agent containing unsaturated group

In this study, we synthesized 1,1,1‐trimethlyolpropane (TMP)‐based diol which has a pendant anionic group and a reactive double bond in the pendant cyclic ring. Then, a water‐based urethane acrylate crosslinking agent (CA) was prepared from isophrone diisocyanate (IPDI), 2‐hydroxyethyl acrylate (2‐HEA), and a low molecular weight diol bearing pendant carboxylic acid. The CA content was varied in order to investigate the influence of the added content on the mechanical and dynamic mechanical properties. The diol used in this study was TMP–cis‐1,2,3,6‐tetrahydrophthalic anhydride (TPA) which was synthesized from the esterification reaction of TMP with TPA. This diol was liquid at room temperature. As the CA increased, the initial modulus and elongation at break decreased. Also, increase of the rubbery plateau modulus was observed by dynamic mechanical analysis (DMA). This result was believed to result from an increase in the crosslinking density in the mixtures. Shift of the T_gh to a higher temperature was also observed by DMA. This is due to the increased hard segments and the phase separation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1216–1223, 2000     

Dielectric relaxation and crosslinking in unsaturated polyester resins

The properties of crosslinked thermosetting resins depend markedly on the completeness of the crosslinking process. Determination of the degree of cure of an unsaturated polyester resin has been studied previously by mechanical, spectroscopic and volume resistivity methods. In this respect the effect of cure time and temperature on the ac dielectric constant and dissipation factor at 1 kc/s and 10 kc/s is considered. The dissipation factor appears to be a most useful parameter for detecting changes in the degree of cure in the later stages of reaction. The electrical properties of the cured resin are discussed, and values for the energy of activation for electrical conduction are compared with literature reports on similar materials.     

Gel formation in polymers undergoing radiation-induced crosslinking and scission

A study was made of the solubility of irradiated polyethylene. The experimental data were treated according to the Saito-Inokuti theory for gel formation in polymers exposed to ionizing radiation. Among other things, this theory is based upon the molecular weight distribution of the unirradiated polymer; in the present work, the actual distributions were determined by high-temperature gel permeation chromatography and corrected for long-chain branching. Under these circumstances, good agreement between theory and experimental data was obtained, which allowed the determination of the radiation yield of main-chain scission and crosslinking.     

Brabender Plasticorder studies of the process of crosslinking polyethylene

The process of crosslinking unfilled thermoplastic polyethylene has been followed with the aid of the Brabender Plasticorder. It is shown that extrusion conditions without any significant scorching can be obtained with this instrument. The maximum torque in the plastograms is discussed and shown not to be a correct criterion of crosslinking efficiency, since its value is influenced by a process of shear grinding of the crosslinked polymer. The crosslinking is further continued after the maximum torque is reached. The dependence of gel content on peroxide concentration is given.     

ESR study of the spin labeled polyethylene crosslinking system

Summary The reactivity ratios for the radical Copolymerization of N-tert-butyl acrylamide with acrylonitrile and with methyl methacrylate were determined at 60°C.     

Modification of polyethylene by unsaturated compounds

Polyethylene was reacted with maleic anhydride and N-phenylmaleimide in solution. The effect of modification on its physico–chemical properties, such as mechanical, oxygen gas transmission, water vapor transmission, and grease resistance properties, was studied.     

Peroxide-initiated crosslinking of polypropylene in the presence of p-benzoquinone

Crosslinking of polypropylene was investigated when initiated by thermal decomposition of organic peroxides. p-Benzoquinone was found to be an effective coagent of crosslinking. The efficiency of benzoquinone is about 5 times higher than that of polyfunctional monomers. Almost complete crosslinking of polypropylene can be reached at proper conditions.     

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.     

Effects of polyethylene molecular structure on peroxide crosslinking of low density polyethylene

The effects of molecular structure on the dicumyl peroxide crosslinking of two low density polyethylenes have been studied. Peroxide efficiency was determined largely by the content of terminal vinyls in the two polymers. Differences in molecular weight distribution and branch content had comparatively little influence on the gel content but hot creep resistance of the vulcanized materials was affected. Scorching was affected, also, by the presence of vinyl unsaturations in the polymer.     

Surface crosslinking of polyethylene and adhesive joint strength

Exposure of polyethylene film to UV radiation at wavelengths of ≤2537 Å is sufficient to induce surface crosslinking and to facilitate the formation of strong adhesive joints to these surfaces with conventional adhesives. Reduction of the vapor pressure in the reaction vessel to about 1 torr apparently maximizes the efficiency of the crosslinking process. Examination of the treated films which have been exposed for times necessary to form strong adhesive joints has revealed an absence of surface oxidation. It appears that crosslinking to improve the mechanical strength of the surface region of the polyethylene is sufficient to allow the formation of strong adhesive joints.     

Modification of unsaturated polyesters using polyethylene glycol

Abstract

The fracture toughness and impact resistance of rigid unsaturated polyesters can be modified by various physical and chemical methods. In this study, flexible unsaturated polyesters were prepared by condensation polymerisation of maleic anhydride, phthalic anhydride and propylene glycol in the presence of polyethylene glycol. By this method, copolymers of unsaturated polyesters with polyethylene glycols of different molecular weights were prepared. A two stage process was used for the synthesis, which resulted in segmental structures containing alternating rigid cross-linked segments and linear soft segments displaced regularly in the polymer chain. Properties such as tensile strength, tensile modulus, elongation at break, toughness, impact strength, surface hardness, abrasion resistance and water absorption were tested after the resin was cured in appropriate moulds and compared with those of the control resin. Polyethylene glycols of lower molecular weight (～ 200) impart flexibility to the polyester chains and increase fracture toughness and impact resistance of rigid unsaturated polyester without seriously affecting other properties. Glass reinforced specimens prepared using the modified resin also show improved toughness and elongation at break.     

High-energy radiation-induced main-chain scission and crosslinking of poly-2.2.2-trichloroethylmethacrylate

Summary Poly-2,2,2-trichloroethylmethacrylate was irradiated with ⁶⁰Co--rays and 16 MeV electrons. The 100 eV yields for main-chain scission and crosslinking were determined as G(S) = 3.7 and G(X) = 0.03, respectively. Neither dose rate (1.6 × 10¹ to 5 × 10¹⁰ rad/s) nor temperature changes (25 to 165 °C) influenced the radiation chemical yields. At 165 °C, thermal crosslinking commenced to compete with radiation-induced main-chain rupture.On sabbatical leave from the Institute of Applied Radiation Chemistry, Technical University of ód, Poland     

Study of crosslinking of unsaturated polyester resins by relaxation methods

Radiation and thermal curing of unsaturated polyester resins with styrene were investigated by combining differential thermal analysis with electric and mechanical relaxation techniques. A microprocessor controlled combined relaxation equipment of special construction was used. By radiation initiated curing the reaction was interrupted at different stages and the products were analyzed. Relaxation and simultaneous differential thermal measurements were also made during the course of gamma radiation and peroxide initiated thermal curing. By this way the shift of the characteristic transitions of the resin as a function of conversion could be studied. Also the change of the phase-structure of the resin caused by the reaction was monitored. By deconvolution of the dielectric spectrum band the physical structure was found to become heterogeneous by crosslinking. Besides the shift of the transition temperatures the oscillator strength of the dielectric transition was found to decrease with conversion. Electrical polarization and depolarization studies were also performed. A special intermittent load thermomechanical technique was used for separating elastic from viscous response of the sample subjected to external mechanical force. The transitions exhibited by the thermomechanical curves were found to shift to higher temperatures by crosslinking and the compliance plateaus decreased.     

Kinetics of crosslinking of linear polyethylene with t-butyl peroxide

The kinetics of the curing of polyethylene with di-t-butyl peroxide have been studied using a Wallace-Shawbury curometer and a Monsanto rheometer. Results obtained for the activation energy of the crosslinking reaction are of comparative value only, because the temperature of the specimen is not known with sufficient accuracy; this applies especially to the rheometer. It was not possible to crosslink polyethylene premixed with carbon black; the addition of carbon black to the mixture of polyethylene and peroxide greatly reduced the degree of crosslinking obtained.     

Synthesis,crosslinking and degradation of some unsaturated polyester resins

Some unsaturated polyester resins were prepared by the ester interchange between p-carbethoxymaleananilic acid and p-carbethoxysuccinanilic acid with saturated and unsaturated diols. Moreover, copolymers were also synthesized by the polyesterification of the aforementioned acids and maleic anhydride with the above-mentioned glycols. The structure of both the unsaturated polyester resins and copolymer resins was established by IR, UV, and NMR spectroscopy. Also, viscosity measurements and molecular weight determinations were further tools for their structural elucidation. All the products were attempted to cure with styrene in the presence of benzoyl peroxide as initiator, and the produced network structures, in the form of films, were tested as surface coating materials for glass and metals. The structure of the cured products was established after their degradation, by physical and spectroscopical means.     

Melting and glass transition of radiation-induced graft polyethylene

Melting and glass transition data are reported employing DSC for styrene-grafted high-density polyethylene obtained by γ radiation. Judging from the data of the melting point and the heat of fusion, the grafted polystyrene had no effect on the polyethylene crystallites, but the half-width of the thermogram was observed to increase slightly, showing an effect on the crystallite size distribution. As no effect was observed on the glass transition temperature by grafting, the amorphous region of the polyethylene apparently was not affected. It is suggested, therefore, that the free volume or segmental mobility will not be decreased by radiation-induced grafting. Very few but long grafted chains had negligible effect on the average polyethylene chain length available for segmental motion, and grafted polystyrene should be expected to differ little from the styrene homopolymer in thermal motions.     

Surface crosslinking of polyethylene using a hydrogen glow discharge

Linear high-density polyethylene is crosslinked by exposing it to a hydrogen glow discharge and alternatively by exposing it to a low-pressure Hg–A ultraviolet lamp in a dry nitrogen atmosphere. For the UV lamp case, the crosslinked gel is measured as a function of the radiation dose. For the plasma case, the crosslinked gel is measured as a function of the exposure time. The two gelation curves are alternatively compared to an exponentially attenuated light theory and a diffusion theory. Excellent agreement exists between the measured gel-versus-dose curve and the theoretical curve based on the light theory for a monochromatic light beam at 1849 Å. The gelation curve for the hydrogen glow discharge case can be explained using the identical theory but applied to a spectrum of light covering the range from 1200 Å to 1900 Å. The different curves can be explained in terms of the different spectra. Energy transfer between the plasma and the polymer due to excited atoms, and/or charged particles, is not required to account for the different gel curves.     

Free volume-based modelling of free radical crosslinking polymerisation of unsaturated polyesters

A new mechanistic kinetic model is presented for the cure behaviour of unsaturated polyester (UP) resins. The model is based on free radical polymerisation mechanism and the free volume concept. The quasi steady-state assumption for the free radical concentration is not used, and the decrease in initiator efficiency with conversion and radical trapping are modelled separately. The glass transition temperature of partially cured samples was measured employing differential scanning calorimetry (DSC) in conjunction with dynamic mechanical analysis (DMA), and the values obtained were incorporated into the model. DSC obtained conversion-time data for a standard commercially available UP resin under isothermal conditions. The kinetic parameters of the model were estimated using parameter optimisation procedures resulting in good agreement between model predictions and experimental data. Modelling in combination with experimental cure data showed that at higher isothermal cure temperatures a greater extent of physical trapping of radicals occurs rendering them inactive.