Rate Of Cross-linking Reaction Research Articles

Research on the vacuum aging performance of HTPB propellant based on the DSC method

The space environment has extreme conditions such as high vacuum and rapid temperature changes, which will lead to problems such as energy drop, irregular burning rate, and abnormal ignition of solid propellants. To obtain the space aging law of HTPB propellant, the aging performance of HTPB (hydroxyl-terminated polybutadiene) propellant in a vacuum environment was studied and compared with that under the atmosphere. According to the DSC test results of the propellant before and after aging, the behaviors of oxidative crosslinking, chain-scission degradation, and oxidant decomposition during the aging process of solid propellant were investigated. The research showed that the low-temperature endothermic peak and “V-type” high-temperature exothermic peak temperature of HTPB propellant increased after aging, and the high-temperature peak bifurcated into “W-type” double peaks. The right peak caused by the “decarboxylation” reaction was more obvious after atmospheric aging while the left peak caused by AP decomposition was remarkable after vacuum aging. In addition, the oxidative crosslinking reaction occurred after aging, and the activation energy characterized by low-temperature peaks continued to increase. In the later stage of atmospheric aging, chain-scission degradation occurred due to OA moisture absorption, and the activation energy decreased. It firstly increased by 126% within 93 days, and then decreased by 23.4%. No degradation and chain-scission reaction occurred in a vacuum environment, the crosslinking reaction rate was high, and the activation energy increased by 40.4% within 49 days. After the aging of the HTPB propellant, the internal AP decomposed, and the activation energy characterized by the high-temperature exothermic right peak continued to increase. The activation energy increased by 40.5% within 49 days after vacuum aging, and increased by 29.9% within 93 days after atmospheric aging, indicating that the decomposition of AP in the HTPB propellant was more obvious under a vacuum environment.

Preparation and Properties of Autocatalytic Biobased Waterborne Polyol from Rosin Based Epoxy Resin

Due to the complex heterogeneous film forming process of two-component waterborne polyurethane (2K-WPU), the crosslinking reaction rate of 2K-WPU cannot meet the demand of efficient application in coatings. In order to improve the crosslinking reaction rate of 2K-WPU, a waterborne polyol containing tertiary amine groups was synthesized from rosin based epoxy resin and secondary amine compound, and then autocatalytic 2K-WPU was prepared by crosslinking the rosin based waterborne amino polyol with polyisocyanate. The structure of the polyol from rosin based epoxy resin was characterized with Fourier infrared (FT-IR) and nuclear magnetic resonance (NMR). The crosslinking kinetics and the crosslinked product of the rosin based waterborne amino polyol were also compared with a commercial acrylic polyol. It was shown from the results that the crosslinking reaction rate of the rosin based waterborne amino polyol was faster than that of the commercial acrylic polyol, which indicated the tertiary amine groups chemically bonded in the rosin based polyols could autocatalyze the crosslinking reaction of 2K-WPUs with catalysts free. The film of the rosin based waterborne amino polyol had excellent impact strength, adhesion, flexibility, hardness, gloss, fullness and solvent resistance, showing a good application prospect in the field of waterborne coatings.

Macrostructural design of highly porous SiOC ceramic foams by preceramic polymer viscosity tailoring

Highly porous SiOC ceramic foams with gradient or uniform macrostructures were obtained through polymer derived ceramic routes. Precuring of preceramic polymers and introduction of SiO 2 powders were used to tailor precursor viscosity and hence SiOC foam macrostructure. Effects of polymer viscosity on porosity, pore size, pore distribution were investigated by light microscopy and micro-computed tomography techniques. SiOC ceramic foams. Foams from one unmodified precursor, showed pore size gradient with small pores located at bottom and large pores at the top. To address this non-uniformity, the viscosity of the precursor was increased by pre-curing the preceramic polymer, which resulted in decrease of the average pore size and improvement in pore size uniformity. For a different system with a self-foaming preceramic polymer, because of the simultaneous release of foaming gases and rapid increase in viscosity during crosslinking, the foam had non-uniform macrostructure with large pores and thick struts at the bottom. By addition of SiO 2 fillers, the crosslinking reaction rate was reduced leading to homogeneous pore nucleation and uniform small pore size foams.

Curing Efficiency of Novolac-Type Phenol–Formaldehyde Resins from Viscoelastic Properties

The curing performance of a homogeneous set of different grades of novolac-type phenol–formaldehyde (PF) resins having different structures is analyzed. The PF resins have a statistical ortho/para substitution at aromatic rings and contain from 5 to 15 wt % hexamethylenetetramine (HMTA) as a hardener. Moreover, the different analyzed grades are PF resins differently modified by addition of boron/phosphorous compounds, or an acrylic rubber, or silicone and xylene/phenol copolymers, or introduction of aralkyl modifications. The curing behavior is compared with that of an unmodified PF resin in which the cross-linking agent (HMTA) is added into the molten polymer to guarantee a good dispersion according to the hexamine adduct phenolic (HAP) technology. The effect of the different modifications is probed by rheological measurements during curing under isothermal conditions. Under similar curing conditions, the modified resins show different curing performances and lower stiffness with respect to the unmodified Ph resin obtained by HAP technology. A method to quantify the curing performance is introduced and suggested based on two relevant parameters derived from rheological measurements, the “curing ratio” that indicates the efficiency to generate the thermal-induced three-dimensional network and a “maximum curing rate” that is related to the cross-linking reaction rate. The results allow classifying the resins as belonging to different classes, depending on the combination of the curing ratio and curing rate.

Studies of crosslinked biopolymer structure for environmental tools in terms of the rheological measurements: part B

The crosslinked biopolymer structure constructed by blending different biopolymers or adding crosslink agent to biopolymers was studied. The rate of crosslink reaction was estimated with rheological measurements, such as elasticity and viscosity, glass transition temperature, crosslinking density, and gelation. Intermolecular and intramolecular association among polymer chains resulted in the formation of a complex three-dimensional (3-D) network as a single, double, or triple helix. These aggregates are progressively disrupted under an influence of applied shear forces that can be heat or pressure for a long period time. Nonetheless, such crosslinked structure exhibited the unique properties that may be found in a crosslinked structure. Therefore, such 3-D crosslinked structure can make an effective tool to stabilize such hazardous materials as heavy metals, toxic organics, and radioactive waste in a subsurface engineering system. In this study, the rate of crosslinking reaction in a biopolymer structure was fundamentally determined with the measurement of rheological properties.

Novel kinetics model for the crosslinking reaction of 1,2,3,4-butanetetracarboxylic acid with cellulose within cotton fabrics

1,2,3,4-butanetetracarboxylic acid (BTCA) has been explored as formaldehyde-free crosslinking agent for many years. In this study, a novel kinetic model for cellulose reacted by BTCA was proposed and the factors that influenced the model parameters were investigated in detail, which could help us understand crosslinking mechanism clearly. Kinetic model, which is a consecutive reaction model, consisting of grafting (esterifying) reaction rate constant and crosslinking reaction rate constant, was deduced. The rate of the whole reaction depends on the rate of grafting reaction, which is related to the diffusion of free BTCA in pores of cellulose. However, the rate of crosslinking reaction depends on thermal vibration of cellulose chain grafted by BTCA, which is more sensitive to the change of curing temperature, especially when the temperature is higher than 150 °C. The Arrhenius activation energies of grafting and crosslinking reactions are 86.3 kJ/mol and 102.8 kJ/mol, respectively, showing high temperature is more conducive to increase the rate of crosslinking reactions comparing with grafting reaction. Moreover, the whole reaction is also affected by catalyst of SHP, which could react with BTCA anhydride to form BTCA-SHP compounds. Due to lower electron cloud density of carbonyl groups, BTCA-SHP compounds have much higher reactivity compared with BTCA anhydrides, which are beneficial to be attacked by hydroxyl groups in cellulose to form ester bonds, imparting excellent wrinkle-resistance to cotton fabrics.

Photoinduced DNA Interstrand Cross-Linking by 1,1'-Biphenyl Analogues: Substituents and Leaving Groups Combine to Determine the Efficiency of Cross-Linker.

Two series of 1,1'-biphenyl analogues with various leaving groups (L=OAc, OCH3 , OCHCH=CH2 , OCH2 Ph, SPh, SePh, and Ph3 P+ ) were synthesized. Their reactivity towards DNA and the reaction mechanism were investigated by determining DNA interstrand cross-link (ICL) efficiency, radical and carbocation formation, and the cross-linking reaction sites. All compounds induced DNA ICL formation upon 350 nm irradiation via a carbocation that was generated from oxidation of the corresponding free radicals. The ICL efficiency and the reaction rate strongly depended on the combined effect of the leaving group and the substituent. Among all compounds tested, the high ICL efficiency (30-43 %) and fast reaction rate were observed with compounds carrying a nitrophenyl group and acetate (2 a), ether (2 b and 2 c), or triphenylphosphonium salt (2 g) as leaving groups. Most compounds with a 4-methoxybenzene group showed similar DNA ICL efficiency (≈30 %) with a slow DNA cross-linking reaction rate. Both cation trapping and free radical trapping adducts were detected in the photo activation process of these compounds, which provided direct evidence for the proposed mechanism. Heat stability study in combination with sequence study suggested that these photo-generated benzyl cations alkylate DNA at dG, dA, and dC sites.

Photoinduced DNA Interstrand Cross-Linking by Benzene Derivatives: Leaving Groups Determine the Efficiency of the Cross-Linker

We have synthesized and characterized two small libraries of 2-OMe or 2-NO2-benzene analogues 2a-i and 3a-i containing a wide variety of leaving groups. Irradiation of these compounds at 350 nm generated benzyl radicals that were spontaneously oxidized to benzyl cations directly producing DNA interstrand cross-links (ICLs). Compounds with a 2-methoxy substituent showed a faster cross-linking reaction rate and higher ICL efficiency than the corresponding 2-nitro analogues. Apart from the aromatic substituent, the benzylic leaving groups greatly affected DNA cross-linking efficiency. Higher ICL yields were observed for compounds with OCH3 (3b), OCH2Ph (3d), or Ph3P+ (3i) as leaving groups than those containing OAc (3a), NMe2 (3e), morpholine (3f), OCH2CH═CH2 (3c), SPh (3g), or SePh (3h). The heat stability study of the isolated ICL products indicated that dGs were the preferred alkylation sites in DNA for the benzyl cations produced from 2a-i, 3c, and 3e-i while 3a (L = OAc), 3b (L = OMe), and 3d (L = OCH2Ph) showed a similar photoreactivity toward dGs and dAs. Although the photogenerated benzyl cations alkylated dG, dC, and dA, ICL assay with variation of DNA sequences showed that the ICL reaction occurred with opposing dG/dC but not with staggered dA/dA.

Diffusion and cure kinetic of sonicated nanoclay-reinforced unsaturated polyester resin

The influence of applying ultrasonic waves on cure kinetic parameters of unsaturated polyester resin (UP) reinforced with Cloisite 10A nanoparticles (10A) via non-isothermal differential scanning calorimetry has been studied. Advanced integral isoconversional method was used to study the influence of sonication on activation energy, pre-exponential factor and reaction model in terms of conversion degree. Furthermore, the kinetic parameters of Vyazovkin and Sbirrazzuoli diffusion factor (VS diffusion factor) and Kamal equations were obtained. The results proved that by applying ultrasound the dominance of the diffusion phenomenon shifted to lower conversion degrees. XRD and TEM tests were performed on samples containing nanoclay before and after the application of ultrasonic waves. The results showed that the application of ultrasonic waves had no significant effect on the intercalation and exfoliation of nanoclay platelets. Ultrasound breaks down agglomerates and thus more styrene adsorbs by nanoclay platelets. It was observed that in the final stage of the cure reaction, the sonication has a great effect on the activation energy and reduces it dramatically. Finally, the influence of sonication on optimal cure cycle of thin composite parts was investigated. The results showed that the application of ultrasonic waves increased the crosslinking reaction rate of the unsaturated polyester resin and decreased the optimum cure cycle time.

Reaction Rate Governs the Viscoelasticity and Nanostructure of Folded Protein Hydrogels.

Hydrogels constructed from folded protein domains are of increasing interest as resilient and responsive biomaterials, but their optimization for applications requires time-consuming and costly molecular design. Here, we explore a complementary approach to control their properties by examining the influence of crosslinking rate on the structure and viscoelastic response of a model hydrogel constructed from photochemically crosslinked bovine serum albumin (BSA). Gelation is observed to follow a heterogeneous nucleation pathway in which BSA monomers crosslink into compact nuclei that grow into fractal percolated networks. Both the viscoelastic response probed by shear rheology and the nanostructure probed by small-angle X-ray scattering (SAXS) are shown to depend on the photochemical crosslinking reaction rate, with increased reaction rates corresponding to higher viscoelastic moduli, lower fractal dimension, and higher fractal cluster size. Reaction rate-dependent changes are shown to be consistent with a transition between diffusion- and rate-limited assembly, and the corresponding changes to viscoelastic response are proposed to arise from the presence of nonfractal depletion regions, as confirmed by SAXS. This controllable nanostructure and viscoelasticity constitute a potential route for the precise control of hydrogel properties, without the need for molecular modification.

Dynamic cross-linking mechanism of acid gel fracturing fluid

In petroleum exploitation, fracturing is a very important technique for the reservoir with low porosity and permeability. The fracturing fluid is the key to fracturing operation. Due to the limitation of lithology complexity, it is difficult for conventional acid-based fracturing fluids to meet technical requirements during the production of carbonate oil and gas resources. In this work, a novel acid gel fracturing fluid was designed. The system has high viscosity which is convenient to carry proppant. Meanwhile, the fluid has good basic properties to meet on-site operation requirements. Since the properties of fracturing fluid depend not only on the molecular structure of thickener, but also on the crosslinked structure formed by polymer and crosslinker. The dynamic cross-linking mechanism of this acid gel fracturing fluid system was focused on. The differences in fluid properties between polymer based fluid and fracturing fluid were investigated by conducting viscoelastic tests. A rheological dynamic model based on the change of viscosity with reaction time was established. The effect of shear rate on cross-linking viscosity and reaction rate was studied. Moreover, the microstructure changes of the system before and after the gelation reaction were revealed through the scanning electron microscopy (SEM) experiment. Based on these experimental results, the cross-linking mechanism of this novel acid gel fracturing fluid can be concluded, which benefits the application in oilfield.

Preparation of highly porous SiC via ceramic precursor conversion and evaluation of its thermal insulation performance

ABSTRACT Porous SiC ceramics were prepared based on precursor conversion method using liquid hyperbranched polycarbosilane (LHBPCS) as the preceramic polymer and expandable microsphere (EMS) as the pore foaming agent. By optimising the content of free radical initiator, the cross-linking process of LHBPCS is synchronised with the foaming process of EMS. The addition of EMS had no serious effect on the cross-linking reaction mechanism and rate of LHBPCS, and the cross-linking of LHBPCS did not significantly confine the foaming of EMS. After 1000°C pyrolysis, EMS decomposed, but its spherical shape with sizes of 20–30 μm homogeneously embedded in the pyrolysed LHBPCS. As a result, a highly porous SiC foam with density of 0.211 g cm–3 and porosity of 91.2% was obtained and its thermal conductivity at room temperature was as low as 0.05 W (m K)–1. In addition, with increasing test temperature, its thermal conductivity increased slowly, indicating a good thermal insulation property.

Self-Catalytic Two-Component Waterborne Polyurethanes with Amino Polyols from Biomass Based Epoxy Resin

In order to improve the crosslinking reaction rate of two-component waterborne polyurethanes (2K-WPUs) in the absence of ungreen external catalysts, waterborne polyols containing different tertiary amines in molecular structure were synthesized from a turpentine-based epoxy resin (TME) and secondary amines (N-benzylethanolamine and diethanolamine). Then self-catalytic 2K-WPUs were prepared with the waterborne amino polyols and a hydrophilically modified hexamethylene diisocyanate tripolymer. The structure and micromorphology of TME based polyols were identified using nuclear magnetic resonance (1H NMR, 13C NMR), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy techniques. The crosslinking reaction of 2K-WPUs was monitored using differential scanning calorimetry (DSC) and FT-IR methods. Kinetics analysis from DSC showed the activation energy of the crosslinking reaction of 2K-WPUs reduced obviously by using amino polyols, which indicated the tertiary amines chemically bonded in the polyols could self-catalyze the crosslinking reaction of 2K-WPUs without adding small molecular toxic catalysts. Small steric hindrance of the tertiary amines led to high catalytic activity. The properties of the crosslinked products of the waterborne TME based polyols were investigated by DSC, thermogravmetric analysis and dynamic mechanical analysis (DMA). The results showed high hydroxyl value of the polyol and aromatic ring in the polyol structure resulted in high storage modulus (G′) and glass transition temperature (Tg) of the crosslinked products. More C–N bond content of the crosslinked products of the waterborne amino polyols decreased the thermal stability in the first degradation stage of the products.

Synthesis of isocyanate microcapsules as functional crosslinking agent for wood adhesive

ABSTRACT In this study, controlled-release isocyanate microcapsules were synthesized as functional crosslinking to slowdown the rate of cross-linking reactions. The isocyanate microcapsules were prepared by In-situ polymerization with polymethylene polyphenyl polyisocyanates (PAPI) as core and Urea formaldehyde resin as shell in oil-in-water emulsions. The particle size distribution, chemical structure, morphology, activity, and stability of the microcapsules were comprehensively characterized. Finally, the microcapsules were applied in a wood adhesive to prepare water-resistant plywood. The results showed that the size of the microcapsules was around 100 μm, the active content of NCO was about 23.5%, and the core content was approximately 80%. Compared with the stability of the bulk isocyanate, the stability of the isocyanate in microcapsules was significantly improved in the wood adhesive system. Furthermore, the isocyanate microcapsules showed highly efficient in plywood at different time, which indicated that isocyanate microcapsules could be controllable released in plywood applications.

Role of chemical reaction and drag force during drop impact gelation process

An instantaneous chemical reaction and solidification have been experimentally explored during sodium alginate drop impact on calcium chloride liquid pool through a high-speed imaging system. Here, different concentrations of sodium alginate are used to study the effect on gelation process. As soon as the alginate drop touches the calcium chloride pool, a crosslinking polymerization reaction starts under the liquid pool surface. The consequence of crosslinking is rapid formation of calcium alginate gel which denotes instantaneous freezing of liquid drop. The drop impact solidification immediately results in the formation of a strong gel layer in front of it which is having significant role in spherical shape recovery. This layer formation is favoured by increasing sodium alginate concentration in the drop and thus crater growth is hindered also. The initial reaction time which is determined as soon as the drop touches the surface, is found to be dependent on the drop kinetic energy. However, the drop gelation time reduces with drop concentration and it controls the final shape and morphology of gel. Such reduction in gelation time is inferred from the enhanced rate of crosslinking reaction. The motion of liquid drop through liquid pool while interacting with the surroundings influences the drag around the spherical drop. The shape evolution in the solidifying gel is governed by the dynamics between the fluid drag and chemical reaction. The morphology of the gel surface has also been studied and its roughness variation has been measured from fractal box counting method.

On the role of nano-silica in the kinetics of peroxide vulcanization of ethylene propylene diene rubber

This work discusses different roles of nano-silica in the kinetics of peroxide vulcanization of ethylene propylene diene (EPDM) rubber for the first time. Initially, it was shown that silica does not participate in the vulcanization mechanism by chemical bonding to other intermediates. Non-isothermal differential scanning calorimeter revealed that the enthalpy of vulcanization, proportional to the crosslinking reaction, goes through a minimum as silica loading increases. Therefore, silica has two roles affecting kinetics of peroxide vulcanization of EPDM: physical effect on the rate of crosslinking reaction through reduced dynamics of rubber chains and catalytic effect on the decomposition of peroxide. In the low-temperature isothermal vulcanization, the enthalpy of vulcanization decreased monotonically, indicating that the physical effect is dominant. The extent of physical effect was correlated to the immobilized bound rubber resulting from the filler-polymer and filler-filler interactions. However, at high-temperature isothermal or non-isothermal vulcanization, the catalytic effect surmounts the physical one.

Soybean‐Oil‐Based Thermosetting Resins with Methacrylated Vanillyl Alcohol as Bio‐Based, Low‐Viscosity Comonomer

AbstractA novel, bio‐based, aromatic monomer (methacrylated vanillyl alcohol, MVA) is synthesized using vanillyl alcohol and methacrylic anhydride in the absence of solvents. The resulting MVA is used as a sustainable comonomer to replace styrene in a maleinated acrylated epoxidized soybean‐oil (MAESO) resin to produce novel thermosets via free radical polymerization. The influence of MVA loading on the viscosity, gelation time, curing extent, thermomechanical properties, and tensile properties of the MAESO–MVA thermoset is investigated. The synthesized MVA exhibits very low volatility, which is beneficial for the development of construction material with low or zero emission. The viscosity of the MAESO–MVA system can be tailored to meet the commercial requirements. Increasing the MVA content accelerates the crosslinking reaction rate and improves thermal and mechanical properties of the MAESO–MVA system. The glass transition temperature increases with increasing MVA content. Soxhlet extraction experiments show that more than 90% of the components are incorporated into the crosslinking network. Overall, the developed MVA monomer shows promising properties to be used as an effective, green comonomer to replace styrene.

Influence of cross-linking rate on the structure of hypercrosslinked networks: Multiscale computer simulation

A multi-scale molecular dynamics simulation was implied to study the formation of hypercrosslinked polystyrene networks at different rates of cross-linking. The rate of cross-linking reaction was controlled by wide-range variation of the probability of chemical bonding between the particles within the reaction distance on individual step of the simulation. The cross-linking was performed in the framework of the coarse-grained model, with the force fields derived from atomistic simulations. The macroscopic properties of network were studied using the atomistic samples restored from the coarse-grained configurations by a reverse mapping procedure. It was shown that the rate of cross-linking influences the structure of “synthesized” hypercrosslinked networks: Slowly cross-linked polymer networks have smaller total specific surface, lower average density, larger pores than those cross-linked at high rate.

Crosslinking and ozone degradation of thermosetting resins based on maleic anhydride/diene copolymer and polyfunctional alcohols

ABSTRACTCrosslinking and de‐crosslinking reactions of an alternating copolymer of maleic anhydride (MAn) and 2,4‐dimethyl‐1,3‐pentadiene (DMPD) by thermal curing with polyfunctional alcohols as the crosslinkers and subsequent ozone degradation are reported in this article. The ring‐opening reaction of an anhydride group by polyfunctional alcohols produces network polymers with an ester linkage. The rate of crosslinking reaction depends on the curing conditions, i.e. the structure of the used alcohols and the curing temperature and time. The crosslinking density of the alcohol‐cured copolymers is low due to a slow reaction between the anhydride and hydroxy groups, being different from the corresponding epoxy‐cured copolymer with a dense network structure reported in a previous article. The insoluble resins are readily de‐crosslinked and solubilized by ozone degradation. The polymer surface modification by ozone is also investigated. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42763.

Comparison of Isothermal with Nonisothermal Kinetics for Ethylene-Vinyl Acetate Cross-Linking Reaction in the Solid State

Cross-linking reaction kinetics of ethylene-vinyl acetate (EVA) copolymer initiated with dicumyl peroxide is studied by differential scanning calorimetry (DSC) and a cross-linking rheometer. As the degree of cross-linking (α) is less than 0.30, EVA can flow relatively freely because of insufficient cross-linking. As α ranges from 0.31 to 1, there is more of the cross-linking reaction, and the polymer becomes fully cross-linked in the later stage of the reaction so that three-dimensional cross-linked nets have been constructed. New kinetics approaches are proposed on the basis of the cross-linking parameters measured with the cross-linking rheometer incorporated into Hill and Avrami mathematical equations. As the cross-linking rheometer is popular in manufacturing the cross-linked EVA, the approaches are simpler and more convenient than the DSC method for fast studying kinetics of the cross-linking reaction of polymer in solid or melting state. The total cross-linking reaction rate changes with α, and the apparent activation energy is 91.2–175.0 kJ/mol.