Crosslink Density Of Epoxy Resin Research Articles

Hydroxyl-functionalized block co-polyimide enables simultaneously improved toughness and strength of tetrafunctional epoxy resin

Tetrafunctional epoxy resins are widely used as matrix of advanced composite materials due to their excellent high temperature performance. However, the highly crosslinking density of tetrafunctional epoxy resins results in an inherent brittleness, which limits its further application. In this work, hydroxyl-functionalized block co-polyimide (BPADA-ODA/HAB) is first synthesized by the condensation of 3,3′-Dihydroxybenzidine and 4,4′-Diaminodiphenyl ether with 4,4'-(4,4′-isopropylidenediphenoxy) di-phthalic anhydride, and used to toughen tetrafunctional epoxy resins. The co-polyimide molecules provide reactive blocks that participate in the curing reaction of epoxy resin and decrease the crosslinking density of epoxy resin; the co-polyimide can form nano-sized separated phase via self-assembly, which could activate additional energy dissipative mechanisms to enhance the resistance of failure for the epoxy resins. With 5% BPADA-ODA/HAB, the resultant epoxy resins exhibit simultaneous enhancements in impact strength and tensile strength, 184% and 150% higher than those of neat epoxy resin. This study provides an efficient method for preparing tough and strong tetrafunctional epoxy resins, which are expected to be applied more broadly in industries.

A hydrophobic smart coating based on hexagonal boron nitride/metal-organic frameworks for high-performance corrosion protection

In this study, NH2-MIL-125(Ti) was compounded with hexagonal boron nitride (HBN) to prepare nano-composite which had an active and passive corrosion resistance. NH2-MIL-125(Ti) MOFs was synthesized by common methods and it was applied as nanocontainer to encapsulate 2-mercaptobenzothiazole (MBT). Then the composite was modified with 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (PFTS) to make the coating hydrophobic. The UV–vis showed MIL had a good pH responsivity and the loading capacity of MBT reached 17.6 %. The water contact angle of the coating increased from 72.68° to 122.14°. Because of the tetrabutyl titanate in MIL, the crosslinking density of epoxy resin was improved, and the adhesion of coating increased by 81.7 %. In addition, EIS results showed the high corrosion resistance of the composite coating HBN-MIL@MBT/EP in 5 wt% NaCl solution. In conclusion, 0.5 wt% HBN-MIL@MBT can increase the impedance of the coating by 3 orders of magnitude.

Hygrothermal aging behavior and aging mechanism of carbon nanofibers/epoxy composites

In this study, carbon nanofibers (CNFs)/epoxy composites incorporating different contents of CNFs were fabricated and immersed in distilled water for 180 days. Water absorption test, tensile tests, scanning electron microscopy (SEM) observation, dynamic mechanical analysis (DMA), and Fourier transform infrared (FTIR) spectroscopy were performed to evaluate the effect of CNFs content on hygrothermal aging behavior of CNFs/epoxy composites, and to reveal the hygrothermal aging mechanism of such composites. Test results showed that the addition of CNFs into pure epoxy resin could obviously improve its hygrothermal aging behavior. The CNFs/epoxy composites had better resistance to water absorption than pure epoxy because CNFs improved the water barrier properties of the composites. The improvements of glass transition temperature and the storage modulus of the composites before and after aging were attributed to the fact that the addition of CNFs limited the loosening of molecular chains and increased the second cross-link density of epoxy resin. FTIR spectroscopy showed that the number of –OH functional groups of CNFs/epoxy composites was decreased, which revealed that CNFs inhibited the hydrolysis of epoxy resin and water absorption. This study demonstrates that adding appropriate amount of CNFs into epoxy resin has potential to improve the long-term durability of epoxy-based composites in hygrothermal environment.

Study on epoxy resin with high elongation-at-break using polyamide and polyether amine as a two-component curing agent

Abstract This study aimed to improve the flexibility of E-51 epoxy resin by using polyamide/polyether amine as a two-component curing agent. Through solid state nuclear magnetic resonance (SSNMR), it was found that the cross-linking density of epoxy resin could be effectively reduced by adding the polyamide and polyether amine curing agent. The tensile tests showed that the elongation-at-break was remarkably improved. It was found by differential scanning calorimetry (DSC) that the curing behavior of the polyamide epoxy curing system varied with the addition of different polyether amine. Phase contrast microscope showed that phase separation occurred during the reaction of epoxy resin with the polyamide/polyether amine composite curing agent. In this paper, the modified epoxy resin was endowed with high elongation-at-break (>100%) and appropriate tensile strength (10~20 MPa).

Preparation and mechanical properties of thermosetting epoxy foams based on epoxy/ 2-ethyl-4-methylimidazol system with different curing agent contents

Epoxy/2-ethyl-4-methylimidazol system with different curing agent content was completely cured for foaming, and the effect of a systematic variation in 2-ethyl-4-methylimidazol content on the crosslinking density of cured epoxy resins was investigated. It was found that the crosslinking density of completed cured epoxy reduced as the 2-ethyl-4-methylimidazol content increased in certain range of contents (10–50 mol%). Then the precursors were foamed by a batch foaming process with supercritical carbon dioxide. The cellular morphologies of foamed epoxy resins were analyzed by scanning electron microscopy. The results revealed that the reduced crosslinking density would improve the foamability of cured epoxy resin. The microcellular epoxy foams could be obtained by maintaining a moderate crosslinking density, which can be controlled by varying 2-ethyl-4-methylimidazol content. For the completely cured epoxy with different curing agent content, when the crosslinking density of epoxy resin was 232.40 mol m–3 (the 2-ethyl-4-methylimidazol content was 35 mol%) or lower, microcellular structure was obtained by adjusting the foaming conditions. The effects of foaming on the mechanical properties were also discussed. The results indicated that microcellular epoxy foams had higher impact strength but lower tensile strength and tensile modulus, validating that the introduction of microcellular structure in epoxy matrix was conducive to the improvement of the ductility of epoxy foams.

Dynamic mechanical studies on epoxy resins cured by electron beam radiation

Dynamic mechanical analyses on electron beam (EB)-cured epoxy resins were made in the paper. Through the studies on variation rules of gel fraction, tanδ and storage modulus for varied samples, the important effects of EB radiation dosage, initiator dosage, chemical structure, molecular weight and distribution, and heat treatment on curing reaction and properties of epoxy resin systems have been obtained. Under low radiation doses, the gel fraction, glass transition temperature (Tg) and high temperature modulus of cured epoxy resin increase with increasing radiation dose and initiator dosage. The crosslinking density of epoxy resin decreases slightly with increasing molecular weight. When radiation doses increase, the molecular weight has a little influence on the increasing of curing level and an optimal dosage of initiator appears. The experimental results indicate that the radiation reactivity of epoxy resins is directly associated with their chemical structures. Under the same radiation dose, the reaction extent in sample with high polydispersity is higher than that in low polydispersity sample, but the degree of homogeneity in crosslinking structure is lower. When the EB-cured epoxy resin is heated, the crosslinking density is enhanced. If the temperature of heating treatment exceeds the thermal-initiating temperature of initiator, the local thermal-crosslinking network can be formed in resin system.