Types Of Crosslinking Reactions Research Articles

Sustainable production of eco-friendly rubberized asphalt binders through chemically crosslinking with polymer modifier

The incorporation of waste tire rubber into asphalt pavement can facilitate the development of sustainable transportation infrastructure. Recently, terminal blend (TB) rubberized asphalt incorporating polymers has gained popularity due to its improved performance and eco-friendliness. However, the crosslinking mechanism between terminal blend (TB) rubberized asphalt and supplemented polymer modifier is not well understood. The present paper aims to investigate the crosslinking mechanism between degraded rubber, styrene-butadiene-styrene (SBS) polymer and crosslinking agent, and to clarify the effect of crosslinking reaction on the properties and production of eco-friendly rubberized asphalt. In this paper, Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), dynamic shear rheological (DSR), multiple stress creep recovery (MSCR) and bending beam rheometer (BBR) tests were employed to investigate the chemical and physical properties of TB rubberized asphalt after incorporating various contents of SBS and crosslinking agent. FTIR test indicates that the conjugated olefins released from crumb rubber have the possibility to react with sulfur. However, sulfur alone fails to greatly improve the rheological performance of TB rubberized asphalt, for the level of crosslinking reaction between sulfur and TB rubberized asphalt is weak. Therefore, it is necessary to further add SBS polymer to enhance the crosslinking reaction. Further study showed that there are two types of crosslinking reactions when involving conjugated olefins. Type I is the reaction of sulfur with conjugated olefins provided by the degraded rubber, which helps the conjugated olefins to crosslink themselves, resulting in increase of the amount of trans olefins. Type II is that sulfur bridges the SBS polymer and degraded rubber through the chemical link between the opened trans olefins double bonds located in SBS and opened conjugated olefins from degraded rubber, which leads to reduction of the amount of trans olefins. Type II crosslinking reaction can effectively improve the high temperature performance of TB rubberized asphalt. The findings of this study not only provides valuable insights into the interaction mechanism of eco-friendly rubberized asphalt, but also promote the recycling of waste tire rubber.

Tailoring Schiff base cross-linking by cyano group toward excellent flame retardancy, anti-dripping and smoke suppression of PET

To improve the flame retardant efficiency of aromatic Schiff base (BA), and adjust the crosslinking temperature of imine groups, a cyano group, an electron-drawing group has been introduced into the chemical structure of aromatic Schiff base to obtain a third monomer for poly(ethylene terephthalate) (PET), named as dimethyl-5-[(2-cyanobenzylidene)amino]-isophthalate (CBAA). Thermogravimetry-differential scanning calorimetry (TG-DSC) and dynamic rheological results prove that the PET-based copolyester containing CBAA (CBAAnPETs) can cross-link at higher temperature than that containing BA, which will not affect the preparing and processing of PET. DSC and wide angle x-ray diffraction (WAXD) results demonstrate CBAAnPETs remain good crystallization ability. Owing to the two types of cross-linking reactions, i. e. the reactions between Schiff base (CHN-) and cyano group (-C≡N), and among -CHN- units, CBAA plays an important role in solid phase as well as gaseous phase endowing CBAAnPETs with excellent flame retardancy. With a low CBAA content (only 6.7 mol%), the copolyesters self-extinguish in 5 s with no melt drips in UL-94 vertical test, and its LOI value also increases to 31.0%. The peak heat release rate and total smoke release of the copolyesters obtained from cone calorimetry also observably decrease.

CROSS-LINKING REACTIONS OF SILICA-FILLED EPOXIDIZED NATURAL RUBBER WITH FUMARIC ACID

ABSTRACT The dual functionality of epoxide and unsaturated sites in epoxidized natural rubber (ENR) permits different types of cross-linking reactions. Besides the normal vulcanization via double bonds, alternative cross-linking reactions could take place via ring opening of epoxide groups by dicarboxylic acids and diamines. We investigated the cross-linking reactions of silica-filled ENR50 with fumaric acid. The cross-linking reactions at 160 °C were assessed using a Monsanto rheometer. Supporting evidence of the reactions was provided by Fourier transform infrared spectroscopy (FT-IR), solvent swelling, and differential scanning calorimetry (DSC) analysis. FT-IR showed new absorbance peaks of hydroxyl and ester groups in agreement with the structures expected to form during the reactions between ENR50 and fumaric acid. The increase in cross-links led to a lower percentage of solvent swelling, higher glass transition temperature, and higher torque. Incorporation of silica filler has further increased the torque due to the stiffening effect of the filler. The amorphous silica used in the current work was prepared from rice husk. Leaching of the rice husk with dilute sulfuric acid before the combustion at 600 °C for 2 h led to the production of high-purity amorphous silica (>99% SiO2).

Stability evaluation of n-alkyl hyaluronic acid derivates by DSC and TG measurement

The thermal and thermooxidative behavior of sodium salt of hyaluronic acid (HA) and its n-hexyl, n-decyl, n-tetradecyl and n-hexadecyl ether derivatives having an equal degree of substitution have been studied by means of differential scanning calorimetry (DSC) and thermogravimetry (TG). Derivatives were prepared by a substitution of H atom at the OH bound to the sixth C of N-acetyl-D-glucosamin of HA unit by n-hexyl, n-decyl, n-tetradecyl and n-hexadecyl chains. Both thermal and thermooxidative degradation of HA and derivatives resulted in multistep process. The main interest of this work was focused on processes occurring in the course of the first decomposition step. Experimental DSC data showed lower stability of derivatives and, remarkably lower heat evolution in comparison with original HA. On the other hand, TG measurement recorded lower mass loss for derivates which indicated appearance of new types of crosslinking reactions. Oxidative stability was evaluated by means of DSC that provided the induction period and the protection factor determination. Derivates showed remarkably lower stability in comparison with original HA; comparing each other, the highest oxidation stability showed n-decyl and n-tetradecyl derivates.

Attachment of Amine‐ and Maleimide‐Containing Ferrocene Derivatives onto Self‐Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross‐Linking Efficiencies and Surface Coverage of Electroactive Groups

AbstractFerrocene derivatives containing primary amines and maleimide groups were attached covalently onto N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol self‐assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross‐linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol SAM. The covalent attachment of β‐ferrocenylethylamine onto a 11,11′‐dithio‐bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross‐linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael‐type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross‐linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross‐linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.

Synthesis and biological evaluation of a pyoverdin-beta-lactam conjugate: a new type of arginine-specific cross-linking in aqueous solution.

Arginine specific reagents such as phenylglyoxal and other alpha-dioxo compounds react with arginine side chains by forming adducts with a stoichiometry of 2:1 or a mixture of 2:1 and 1:1. These adducts are labile in neutral and slightly alkaline aqueous solution. We developed a new type of cross-linking reaction with aliphatic beta-dioxo compounds. They can be used for the well-defined, irreversible covalent attachment of molecules carrying a primary amino group to arginyl residues of water soluble peptides. The reaction proceeds under mild conditions in aqueous solution, essentially without the formation of side products. A pyoverdin-cephalexin conjugate was synthesized in order to promote its cellular uptake by Pseudomonas aeruginosa. Preliminary biological investigations of the conjugate indicated that it enters the bacterial cell via the pyoverdin-mediated iron uptake pathway.

Diffusion hindrance vs. wood‐induced catalytic activation of MUF adhesive polycondensation

AbstractThe reaction of hardening of melamine–urea–formaldehyde (MUF) adhesive resins in the presence of wood and cellulose was confirmed to have a lower energy of activation than the MUF adhesive alone, both in the presence or absence of ammonium chloride hardener, thus both in mildly acid and mildly alkaline environments. DSC exotherms showed that during hardening of melamine to melamine, melamine to urea, and urea to urea crosslinks through methylene bridges occur. Only the earliest reaction, mainly melamine to melamine crosslinking, presents a decrease in energy of activation which can be assigned to catalytic activation by the cellulosic substrate. The other types of crosslinking reactions (i) appear not to occur due to the more favorable and rapid melamine to melamine reaction which precedes them at lower temperature or (ii) do not present catalytic activation by the substrate but rather hindrance by it or (iii) variation of their energy of activation appears to be due to increased diffusion hindrance by the substrate caused by the increasing molecular weight of the resin while hardening. This because the Kissinger equation plots of the resin alone are in the main linear, for all the exotherms, indicating that in hardening of the resin alone diffusion problems appear to be limited. © 1995 John Wiley & Sons, Inc.

Thermal degradation of vinyl acetate—methacrylic acid copolymer and homopolymers. I. An FTIR spectroscopic investigation of structural changes in the degrading material

The thermal degradation in vacuo, up to 500°C, of a vinyl acetate—methacrylic acid (VA—MAA) copolymer having 65% alternating units, and of the corresponding homopolymers, has been studied by reflection—absorption FTIR spectroscopy. Polyvinyl acetate (PVA) crosslinks form due to double bond formation through deacetylation. Crosslinking is due to some extent to aromatic rings formed by intermolecular Diels—Alder condensation, as shown by solubility tests correlated with FTIR spectra. PMAA generates little unsaturation, but crosslinks by intermolecular anhydride links. It decomposes above 350°C when the anhydride breaks down with much chain scission. In VA—MAA copolymer, both types of crosslinking reactions are disturbed by lactone formation between alternating VA and MAA units. The lactone formation ‘protects’ the VA units from deacetylation and competes strongly with anhydride formation. As the temperature increases, the anhydride groups disappear, first with scission and, towards 400°C, with aromatic ring formation, as in PMAA. Lactone groups are more thermally resistant and can be found in the charred polymer at 440°C next to aromatic rings. At 480°C the lactone decomposes, helping the ‘fusion’ of the aromatic rings in the strongly diminished char. It has been concluded that large-scale lactonisation in the VA—MAA copolymer due to its high content of alternating units diverts the degradation from the pathways encountered in the homopolymers.