Curing Mechanism Research Articles

The green catalyst: Role of hemp fibers in the curing reaction of bio-based epoxy composites

Understanding the curing behavior of epoxy is a critical aspect in the production of epoxy-based composite and pivotal for process optimization. This study focusses on evaluating the curing kinetic parameters of hemp fiber/bio-epoxy composite at different fiber loading (0–30 wt%). The apparent activation energy was determined using Vyazovkin model. The results revealed an intricate chemistry between hemp fibers and bio-epoxy curing mechanism. The activation energy decreased upon the addition of fibers, indicating the catalytic effect of hemp fibers in the curing kinetics of epoxy resin. Further, at low fiber loading exhibiting augmented curing reaction up to 15 wt% fiber loading, whereas a decreased trend was observed thereafter. As a result, the curing time required to cure the bio epoxy was reduced notably when the fiber loading was less than 15 wt%. Hemp fibers with excellent functional groups such as hydroxyl moieties that actively facilitated the curing reaction, providing insight into their role of augmenting curing efficiency. This research provides crucial insights into tailoring the curing conditions and augment the process control efficiency in the manufacturing of thermoset composites.

Cure behavior and mechanism of cyanate ester with aromatic amines at room temperature

Herein, the catalytic effects of aromatic amines with different structures on the room temperature curing of bisphenol E cyanate (BECy) have been studied using differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy. Our results revealed that the mixed amine comprised of aniline and N,N-dimethyl-p-phenylenediamine (AN/NNDPA) bearing primary amino groups in its structure was a highly effective catalyst for the room temperature curing of cyanate esters. When the cyanate ester and mixed amine were combined at a molar ratio of 100:5 and cured at room temperature for 5 days, the curing degree of BECy resin reached 79.7%. The room temperature curing mechanism of the BECy-AN/NNDPA system was investigated using pyrolysis gas chromatography-mass spectrometry (PY GC-MS). Our results showed that the formation reactions of the intermediate structure and cyclotrimerization reactions occurred during the room-temperature curing process. The activation energies of BECy and different aromatic amine systems at different heating rates were determined using non-isothermal DSC. The results further demonstrated the catalytic effect of AN/NNDPA on the cyanate ester curing reaction. BECy-AN/NNDPA cured at room temperature has suitable heat resistance and a 5% weight loss temperature (Td5) of 361°C.

Mechanical Properties of Fiber-Reinforced Permeable Geopolymer Concrete.

In this paper, permeable geopolymer concrete with high compressive strength and permeability is prepared using alkali-activated metakaolin as a slurry, and its mechanical properties are reinforced by adding steel fibers. The influencing factors of the strength, porosity and permeability coefficient of the fiber-reinforced permeable geopolymer concrete, as well as its microstructure and curing mechanism, are determined by conducting an unconfined compressive strength test, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. The test results show that, under the water permeability required to meet the specification conditions, when the alkali activator modulus is 1.4 and the activation-to-solid ratio is 0.9, the effect of metakaolin activation is the most obvious, and the unconfined compressive strength of the permeable geopolymer concrete is the highest. Moreover, the paste formed via the alkali activation of metakaolin contains a large number of silica-oxygen and aluminum-oxygen bonds with a dense and crack-free structure, which enables the paste to tightly combine with the aggregates; the strength of the permeable geopolymer concrete is early strength, and its strength at a curing age of 3 days is the highest. The strength at a curing age of 3 days can reach 43.62% of the 28-day strength; the admixture of steel fiber can effectively improve the strength of the permeable concrete, and with an increase in the amount of admixture, the strength of the fiber shows a trend of increasing, and then decreasing. Under the conditions of this test, a volume of steel fiber of 0.3% enables the optimum unconfined compressive strength to be reached.

Experimental Study on the Curing Mechanism of Red Mud-Based Stabilized Soil Co-Modified by Nano-SiO2 and Gypsum.

In order to effectively utilize red mud and reduce its occupation of land resources, as well as its impact on the environment, experiments were conducted to develop stabilized soil materials using nano-SiO2 synergistically modified red mud and to investigate the curing mechanism of stabilized soil. The unconfined compressive strength, microscopic morphology, and curing mechanism of the red mud-based stabilized soil materials with different amounts of modified materials were investigated. The test results show that after 7 days of curing, the unconfined compressive strength of red mud-based stabilized soil meets the compressive strength requirement of road base material when nano-SiO2, gypsum, and cement are synergistically modified. In such cases, the soil structure has the lowest fracture rate and the best structural compactness when the amount of nano-SiO2 is 1%. It is found that the needle-like and columnar calcium alumina in the modified red mud-based stabilized soil increases, and the binding energy of hydration product ions in the modified material is improved. The chemical curing mechanism of modified red mud-based stabilized soil includes hydration reaction, pozzolanic reaction, promotion effect of nano-SiO2, and enhancement effect of gypsum. On this base, a model of the early start hydration process of red mud-based stabilized soil promoted by nano-SiO2 is established.

Study of the Self-Polymerization of Epoxy/Phthalonitrile Copolymers and Their High-Performance Fiber-Reinforced Laminates.

Self-polymerization epoxy/phthalonitrile (APPEN) pre-polymers were studied systematically, and then, gelation time and differential scanning calorimetry (DSC) were employed to investigate their curing behaviors. Taking advantage of orthogonal test analysis, the key factors that affected the co-polymerization of APPEN were defined and the appropriate pre-polymerization conditions were analyzed. A possible curing mechanism of APPEN was proposed. Then, the thermomechanical and mechanical properties of glass-fiber-reinforced APPEN laminates (APPEN/GF) prepared at 180 °C were analyzed to understand the cross-linked and aggregation structures. Fracture surface of the composite laminates was also investigated to reveal the copolymerization degree and the interfacial binding. The results indicated that APPEN/GF composites exhibit outstanding mechanical and thermomechanical properties (flexural strength, 712 MPa, flexural modulus, 38 GPa, and Tg > 185 °C). The thermal stability (T5% > 334 °C and IPDT reached 1482 °C) of APPEN/GF composites was also investigated to further reveal the copolymerization between epoxy resin and aminophthalonitrile, which may be beneficial to the application of epoxy-matrix-based composites in the field of high-performance polymer composites.

Low dielectric constant and high thermal stability of liquid crystal epoxy polymers based on functionalized poly(phenylene oxide)

Epoxy resins exhibit poor thermal and dielectric properties because of the disordered entanglement of the chain segments and a large number of hydroxyl groups generated after curing by diamine curing agents. Herein, to prepare high-Tg and low-dielectric epoxy thermosets, two functionalized poly(phenylene oxide) (PPOs) were synthesized by redistribution and esterification reactions and used as curing agents to cure liquid crystal epoxy (t-BPDGE). The modified PPOs (TMPA and TMBPA) contain six active ester functional groups, which not only improved the compatibility and the degree of cross-linking of the PPOs with the epoxy monomers but also avoided the generation of hydroxyl groups after curing. The curing mechanism was investigated using TBB cured glycidyl phenyl ether as a model reaction, and the effects of TMPA and TMBPA on the curing behavior and performances were also studied. Due to the ordered arrangement of the liquid crystal epoxy resins and the presence of PPOs, the overall performances of PPOs-based/t-BPDGE are significantly improved. Specifically, the results showed that TMPA/t-BPDGE possessed a high glass transition temperature (Tg) of 199.47 °C and a char rate of 21.84%, which was 77.09 °C and 3.63 times higher than that of the DDM/t-BPDGE, respectively. Moreover, TMPA/t-BPDGE exhibited a lower moisture absorption (0.23%), dielectric constant and loss (2.94 and 0.00815 at 1 GHz) compared to the DDM/t-BPDGE (0.48%, 3.95 and 0.01898, respectively). The findings provide a new design strategy for preparing high-Tg and low-dielectric epoxy thermosets.

Internal curing mechanism of zeolite pretreated with NaCl under microwave and ultrasonic conditions

Natural zeolite has been used for internal curing to reduce shrinkage of cement-based materials, but the internal curing performance of zeolite needs to be improved. In this study, natural zeolite particles were modified by NaCl solution under conditions of microwave and ultrasonic to improve their internal curing performance. Firstly, the pore structure, chemical bond characteristics, pozzolanic activity, water absorption and water release performance of the modified zeolite were tested. Then, cement paste specimens internally cured with modified zeolite were prepared, the shrinkage, hydration products, and pore structure were tested, and the internal curing mechanism of modified zeolite was discussed. The results show that: (1) Pozzolanic activity of zeolite decreases after NaCl-microwave modification, but increases after NaCl-ultrasonic modification. (2) NaCl modification increases the pore volume and water adsorption of natural zeolite particles. (3) NaCl modified zeolite promotes the formation of ettringite in cement paste, especially for NaCl-microwave modified zeolite. (4) NaCl-microwave modified zeolite has better shrinkage reducing effect and strength improving effect than natural zeolite and NaCl-ultrasonic modified zeolite.

Sustainable biobased composites from norbornylized linseed oil and biomass sorghum fillers

Thermosets and composites sourced from environmentally benign natural sources are in huge demand. In that context, norbornylized linseed oil was used as the key sustainable polymer precursor for thermosets and glass fiber reinforced hybrid biocomposites via three different curing techniques. At first, norbornylized linseed oil was copolymerized by dicyclopentadiene whereas its epoxides were cured via latent cationic initiator, and copolymerization with disulfide containing acid moieties. Further, biomass sorghum fillers were incorporated into the epoxides to fabricate stronger, cheaper, light weight and more sustainable products. The activation energies and crosslink efficiency of the thermosets were calculated first. To evaluate the commercial significance of the products fabricated, synthetic bisphenol-A based epoxides and commercial standards of epoxidized linseed oil-based thermosets and composites were analyzed side by side. Finally, natural flax fiber reinforced composites were also fabricated to make green composites. The thermo-mechanical properties of the composites were investigated using universal testing machine and thermogravimetric analysis. Steric hindrance and ring strain energies seem to have tangible role in the curing process whereas the presence of ring structures increased the crosslink density and mechanical strength. Fine tunability of composite properties by the selection of a specific curing technique was demonstrated by virtue of different curing mechanisms. While outperforming commercial linseed oil epoxides, the norbornylized linseed epoxides-natural fiber composites exhibited comparable performance properties to synthetic bisphenol-A epoxy-glass fiber composites. Therefore, such biocomposites can be viable replacements of harmful bisphenol-A epoxides in the composite industry, especially in the non-structural automotive composite parts.

Evolution and mechanism on shrinkage of high-performance concrete with full aeolian sand cured by superabsorbent polymer

Preparing high-performance concrete with full aeolian sand (FA-HPC) can address the shortage of high-quality aggregates. Its low water-binder ratio (w/b) and high content of cementitious material content endow this concrete with great strength and durability but a high risk of shrinkage cracking. Herein, the effect of superabsorbent polymer (SAP) on the separate shrinkage of FA-HPC, and the mechanism of internal curing (IC) were investigated from various macro–micro perspectives. IC water migration was monitored via nuclear magnetic resonance (1H NMR). The results showed that autogenous shrinkage dominated the total shrinkage of the FA-HPC, accounting for 70%–80%. The SAP with medium particles effectively reduced the total shrinkage in the plastic and hardening stages, with nearly zero autogenous shrinkage at 0.3% content. From the characterization of mechanism, capillary pressure build-up of FA-HPC containing SAP was delayed with significant suppression in internal relative humidity (IRH) drop and moisture loss especially due to self-desiccation. The coupling effect of self-desiccation and evaporation triggered the nonlinear correlation between moisture loss and total shrinkage, whereas SAP amplified the proportion of drying shrinkage. Microscopically, the incorporation of SAP with IC water hindered the early hydration process but resulted in a higher final hydration degree within 72 h bringing a refined pore structure. Moreover, the adaptive release of IC water was used for hydration mainly within 72 h, and it continuously compensated for IRH drop over 28 d based on the supply–demand mechanism in FA-HPC. This study proposes a promising strategy for shrinkage resistance of FA-HPC and promotes the application of aeolian sand-based concrete.

Study on leaching and curing mechanism of heavy metals in magnesium coal based backfill materials

In order to promote the resource utilization of bulk solid waste, it is an effective way to improve the comprehensive utilization level of solid waste and alleviate the environmental pressure caused by storage by using all solid waste as raw material for backfill cementing. In this paper, a magnesium coal based backfill material (MCB) with modified magnesium slag (MMS) and fly ash (FA) as cementing agents, coal gasification coarse slag (CGCS) and coal gangue (CG) as aggregates was proposed. First, the mechanical properties of MCB were studied and then the hydration effect was verified microscopically. Then, the leaching characteristics of raw materials CGCS, CG, and backfill material MCB were characterized. Furthermore, the mechanism of heavy metal leaching and solidifying of MCB was revealed by studying As and Pb. The results show that: (1) The addition of a suitable amount of CGCS with potential pozzolanic activity produces more MCB hydration products such as C-S-H and AFt with a high CG-CGCS ratio. The compressive strength reaches the maximum value of 10.21 MPa when the curing time is 28 days and the CG-CGCS ratio is 3:1. (2) The leaching levels of the target elements (As, Pb, Cr, Mn, Zn, Cu, Cd, Hg, Ag) of CG meet the limits of the class III groundwater quality standard (III-GQSL) in GB/T 14848–2017. The concentration of Pb in CGCS is 12 μg/L, which exceeds the III-GQSL limit of 10 μg/L. The overall leaching risk is CGCS>CG. (3) MCB has a good stabilization/solidification effect on the heavy metal elements Cd, As, and Pb in raw materials. The heavy metal leaching results of MCB with a low CG-CGCS ratio meet the limit of III-GQSL, and the environmental leaching risk is low. (4) In addition to physical encapsulation, the main curing mechanisms of MCB include chemical bonding, alkaline environment, and adsorption. MCB can be used as a potential consolidation agent for the solidification of heavy metal elements. While having good mechanical properties, it can also treat solid waste and effectively stabilize/solidify toxic elements in materials and has the potential for safe application in backfill.

Coordination Compound (2,3,5-Triphenyltetrazolium)2[CuBr4] as Catalyst for the Curing Process of Epoxy Vinyl Ester Binders.

This article presents a study on the synthesis and catalytic properties of copper complex (TPhTz)2[CuBr4] (here TPhTz is 2,3,5-triphenyltetrazolium). The obtained complex was characterized by various spectroscopic methods. The catalytic properties of the complex were evaluated in the curing of an epoxy vinyl ester system and their effectiveness was compared with that of cobalt octoate (its synonyms are known as Co(Oct)2, cobalt(II) 2-ethylhexanoate, cobalt isocaprylate, etc.). The catalyst was added at an amount of 2 w.%. The results showed that a 8 w.% solution of the complex provides catalytic properties with an activation energy of 54.7 kJ/mol, which is 25.2 kJ/mol higher than a standard curing system with Co(Oct)2. Thus, the solution of (TPhTz)2[CuBr4] in THF/DMSO accelerates the initiator decay process at room temperature, but for a longer time. The authors suggest that the curing mechanism may be accelerated by the appearance of (TPhTz)2[CuIBr3] and free bromine in the system. A strength test of fiberglass-reinforced plastic revealed that the addition of this complex did not lead to a decrease in flexural strength and hardness. Thus, use of the complex allowed for the production of polymer composite products using vacuum-assisted resin transfer molding where an extended injection time was needed.

High-performance new photoinitiators for the preparation of functional photocurable polymer composites

Obtaining dental composites with the desirable physical, mechanical and visual properties that polymerise according to an environmentally friendly cationic mechanism is a significant challenge. This is due to the lack of high-performance initiators for this type of material. This is because there are dental composites on the market that have, among other things, toxic monomers such as BisGMA. This paper presents five high-performance, completely new iodonium derivatives of biphenyl-based iodonium salts differentiated by push–pull structure of chromophores determined by positions of methoxy and trisilyl groups in the biphenyl rings. The influence of the methoxy grouping in the biphenyl ring on the push–pull effect was explained, and their spectroscopic and electrochemical properties were determined. For the first time, the possibility of using these compounds as high-performance initiators of cationic and radical polymerization was presented and described. New two-component initiator systems composed of iodonium salts and thiadiazole derivatives for preparing new-generation dental composites have also been proposed. The thiadiazole derivative was an very effective alternative to camphorquinone, commonly used in the dental industry. Completely new organic matrix compositions and a new curing mechanism have also been proposed by introducing monomers that polymerize according to radical and cationic mechanisms. As a result, dental composites with significantly reduced polymerization shrinkage compared to commercially available materials were obtained. Finally, a new photoinitiating system we have developed, makes it possible to obtain new composites that are an exciting alternative to materials currently used in dentistry.

Water activated catechol adhesive allows dip coated antimicrobial coatings

Bioadhesives comprising of catechol crosslinkers have displayed broad utility against both soft and hard substrates. However, catechol's two-part adhesion chemistry requires oxidative chemicals that are detrimental to organic substrates. Herein, a water-activated adhesive with inherent antibacterial properties is prepared by grafting catechol groups onto branched polyethylenimine (PEI-DBA20). The resultant PEI-DBA20 is stable in organic solvents but undergoes curing in the presence of water. The in-built oxidation method relies on the close proximity of catechol/Schiff base functional groups that form tautomers in the presence of aqueous solvents. The curing mechanism is demonstrated by dip coating hydrated substrates, where the grafted dendrimers subsequently crosslink and form thin films. Coated PET films and polyester textiles exhibit an antimicrobial surface with 4–6 log reduction against model Gram-negative bacteria.

In Situ Monitoring of Curing Reaction in Solid Composite Propellant with Fiber-Optic Sensors.

Curing activity in the preparation of solid composite propellants determines the performance of solid rocket motors in operation. Limited by the lack of effective monitoring tools, the complete curing behavior and thermal-induced curing kinetics are rarely disclosed. It is still a challenge to monitor in situ and in real-time the physical and chemical cross-linking reaction during the curing of propellant. Herein, we demonstrate a promising approach based on optical fiber capable of being implanted inside the propellant to monitor the internal stress evolution during the curing process, by taking hydroxyl-terminated polybutadiene propellant as an example. Attributed to the strain and temperature sensitivity of a pair of optical fiber gratings, the thermal-assisted physico-chemical cross-linking states of curing process have been demonstrated in detail. By tracking the stress-induced wavelength shifts of fiber gratings and calculating the curing mechanism function, the complete curing roadmap, including the viscous flow stage, gel stage, hardening stage can be clearly revealed, and the curing completion times are obtained as 154, 81, and 40 h, at the curing temperatures of 60, 70, and 80 °C, respectively. The apparent activation energy of this curing system obtained by calculation is 73.88 kJ/mol. This flexible fiber-based sensor provides an effective tool for unraveling the cure kinetic mechanism, and paves a universal pathway to guide the preparation and applications of versatile composite materials for solid rocket motors.

A highly-crosslinked phthalonitrile modified bismaleimide-triazine resin for PCB substrates: The synergistic effect on curing behavior and properties

In view of the problems of difficult processing and poor toughness, bismaleimide-triazine (BT) resin was modified by the 4-(4-amino-phenoxy)phthalonitrile (4-APN). The effect of 4-APN on the curing behavior of BT resin was investigated, followed by the calculated activation energy. The possible curing mechanism of this ternary system was also proposed, with the further study of the relationship between chemical structures and properties. The results showed that 4-APN can increase the reactivity and decrease the curing temperature of BT resin. Meanwhile, due to the fact that 4-APN exerted a thermal synergistic curing effect on the BT resin, poly(BT/APN)s with a high crosslinking density exhibited the superior overall performance. Concretely, the high heat-resistance of poly(BT/APN)s was displayed, especially the glass transition temperature up to 347 ℃ of poly(BT/30APN). More intriguingly, poly(BT/APN)s with a high breakdown strength possessed better insulation properties, and the dielectric constant and loss reduced to 3 and 0.005 (1 MHz), respectively, while remaining a good stability at high temperature. Additionally, the moisture resistance and toughness of poly(BT/APN)s were also significantly improved. Based on these merits, poly(BT/APN)s can be considered as an ideal material for microelectronics.

Synthesis of diamond nanoparticles/urethane-based bio-adhesive and investigation of its morphology and thermal, physicomechanical, and biological properties

Diamond nanoparticles/urethane-based bio-adhesives (ND-U-BA) were synthesized via in-situ polymerization. Chemical structure and curing kinetics of the samples were studied by Fourier-transform infrared spectroscopy. Results showed NDs controlled the curing profile of samples. Thermal and crystalline properties were studied using differential scanning calorimetry and X-ray diffraction analyses. Results indicated hard segments had higher ordering levels in the presence of NDs which is due to the lower hydrogen bonding among polyurethane chains. Mechanical results showed ND-U-BA had higher adhesion strength due to the formation of chemical crosslinks and a controlled curing mechanism. MTT and biodegradation studies showed ND-U-BA had acceptable cell viability and degradation content. • Urethane and diamond nanoparticles/urethane-based bio-adhesives were synthesized. • Results showed ND nanoparticles had controlling effect on curing kinetic of samples. • Cell viability of bio-adhesives was improved with the ND nanoparticles’ addition. • Incorporation of ND nanoparticles increased the adhesion strength of bio-adhesive.

A Composite Whole-Biomass Tannin–Sucrose–Soy Protein Wood Adhesive with High Performance

Whole-biomass adhesives are the research hotspot of wood adhesives andcan improve the competitiveness of adhesives. The tannin–sucrose adhesive studied by our research group shows good bonding performance, but poor bonding stability induced by low viscosity. In this study, the tannin–sucrose adhesive was modified by isolated soybean protein (SPI), the effect of the SPI substitution ratio for tannin on the properties of the tannin–sucrose–SPI composite adhesive was investigated, and the bonding mechanism was explored using Fourier-transform infrared spectroscopy (FT-IR), thermogravimetry (TG), X-ray diffraction (XRD), and gas chromatography–mass spectroscopy (GC–MS). The results showed that: (1) when the SPI substitution ratio was above 40%, the viscosity of the composite adhesive increased significantly, which effectively avoided adhesive leakage. (2) The tannin–sucrose–SPI composite adhesive displayed high bonding performance and water resistance. (3) The FTIR and GC–MS results revealed that the curing mechanism of the tannin–sucrose–SPI adhesive was very complicated, but it was certain that the conversion of sucrose into furan compounds, especially 5-hydroxymethylfurfural (5-HMF), was the core of the cross-linking reaction of the adhesive when elevating temperature. (4) The macromolecules and high reactivity of SPI compensated for the shortage of high temperature required for the conversion of sucrose into furanic cross-linkers so that the tannin–sucrose–SPI adhesive experienced an efficient curing reaction at a low temperature, and the reaction degree and thermal stability of the curing product increased.

Curing Mechanisms of Polymeric Nano-Copolymer Subgrade.

The mechanical properties of the subgrade have a significant impact on the service life and pavement performance of the superstructure of pavement. By adding admixtures and via other means to strengthen the adhesion between soil particles, the strength and stiffness of the soil can be improved to ensure the long-term stability of pavement structures. In this study, a mixture of polymer particles and nanomaterials was used as a curing agent to examine the curing mechanism and mechanical properties of subgrade soil. Using microscopic experiments, the strengthening mechanism of solidified soil was analyzed with scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier infrared spectroscopy (FTIR), and X-ray diffraction (XDR). The results showed that with the addition of the curing agent, small cementing substances on the surface of soil minerals filled the pores between minerals. At the same time, with an increase in the curing age, the colloidal particles in the soil increased, and some of them formed large aggregate structures that gradually covered the surface of the soil particles and minerals. By enhancing the cohesiveness and integrity between different particles, the overall structure of the soil became denser. Through pH tests, it was found that the age had a certain effect on the pH of solidified soil, but the effect was not obvious. Through the comparative analysis of elements in plain soil and solidified soil, it was found that no new chemical elements were produced in the solidified soil, indicating that the curing agent does not have negative impacts on the environment.

1成分形ウレタン系シーリング材の養生時の温度変化が硬化特性に及ぼす影響

The effect of temperature change during curing of polyurethane sealants on the rate of strength development was investigated. Under a constant temperature environment, the curing rate of the sealant was accelerated at higher temperatures. However, after curing the sealant at a low temperature, the apparent curing rate became lower when the temperature was raised. It was found that at low temperatures, the amount of moisture required for the reaction was insufficient due to the reduced moisture diffusivity. Therefore, it is important to develop materials with a curing mechanism other than reaction with moisture to reduce defects in one-component sealants.

Internal Curing Mechanism of Sepiolite in Cement Paste

Internal Curing Mechanism of Sepiolite in Cement Paste