Self-toughening of epoxy resin through controlling topology of cross-linked networks

Effects of Cross-linker Chemistry on Bioelectrocatalytic Reactions in a Redox Cross-linked Network of Glucose Dehydrogenase and Thionine

Enzyme-mediator bioconjugation is emerging as a building block for designing electrode platforms for the construction of biosensors and biofuel cells. Here, we report a one-pot bioconjugation technique for flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) and thionine (TH) using a series of cross-linkers, including epoxy, N-hydroxysuccinimide (NHS), and aldehydes. In this technique, FAD-GDH and thionine are conjugated through an amine cross-linking reaction to generate a redox network, which has been successfully employed for the oxidation of glucose. The bioconjugation chemistry of cross-linkers with the amino groups on FAD-GDH and thionine plays a vital role in generating distinct network structures. The epoxy-type cross-linker reacts with the primary and secondary amines of thionine at room temperature, thereby producing an FAD-GDH-TH-FAD-GDH hyperbranched bioconjugate network, the aldehyde undergoes a rapid cross-linking reaction to produce a network of FAD-GDH-FAD-GDH, while the NHS-based cross-linker can react with the primary amines of both FAD-GDH and thionine, forming an FAD-GDH-cross-linker-TH polymeric network. This reaction has the potential to enable the conjugation of a redox mediator with a FAD-GDH network, which is particularly essential when designing an enzyme electrode platform. The data demonstrated that the polymeric cross-linked network based on the NHS cross-linker exhibited a considerable increase in electron transport while producing a catalytic current of 830 μA cm-2. The cross-linker spacer arm length also affects the overall electrochemical function of the network and its performance; an adequate spacer length containing a cross-linker is required, resulting in a faster electron transfer. Finally, a leaching test confirmed that the stability of the enzyme electrode was improved when the electrode was tested using the redox probe. This study elucidates the relationship between cross-linking chemistry and redox network structure and enhances the high performance of enzyme electrode platforms for the oxidation of glucose.

Review on Frontal Polymerization Behavior for Thermosetting Resins: Materials, Modeling and Application

The traditional curing methods for thermosetting resins are energy-inefficient and environmentally unfriendly. Frontal polymerization (FP) is a self-sustaining process relying on the exothermic heat of polymerization. During FP, the external energy input (such as UV light input or heating) is only required at the initial stage to trigger a localized reaction front. FP is regarded as the rapid and energy-efficient manufacturing of polymers. The precise control of FP is essential for several manufacturing technologies, such as 3D printing, depending on the materials and the coupling of thermal transfer and polymerization. In this review, recent progress on the materials, modeling, and application of FP for thermosetting resins are presented. First, the effects of resin formulations and mixed fillers on FP behavior are discussed. Then, the basic mathematical model and reaction-thermal transfer model of FP are introduced. After that, recent developments in FP-based manufacturing applications are introduced in detail. Finally, this review outlines a roadmap for future research in this field.

Spatial Distribution of the Network Structure in Epoxy Resin via the MAXS-CT Method

We have succeeded in visualizing the spatial heterogeneity of the reaction ratio in epoxy resins by combining medium-angle X-ray scattering (MAXS) and computed tomography (CT). The reaction ratio is proportional to the degree of cross-linking between epoxy and amine in epoxy resins. The reaction ratio and its spatial inhomogeneity affect the toughness of epoxy resins. However, there has been no non-destructive method to measure the spatial inhomogeneity of the reaction ratio, although we can measure only the spatially averaged reaction ratio by Fourier-transform infrared spectroscopy (FT-IR). We found that the scattering peak reflected the cross-linking structures in the q region of MAXS and that the peak intensity is proportional to the reaction ratio. By reconstructing CT images from this peak intensity, we visualized the spatial heterogeneity of the reaction ratio. The application of this method may not be limited to epoxy resins but may extend to studying the heterogeneity of cross-linked structures in other materials.

A Proposal to Improve the Effectiveness of the Deflection Control Method Provided by Eurocodes for Concrete, Timber, and Composite Slabs

Limited deflection of structural members represents an important requirement to guarantee proper functionality and appearance of building and infrastructures. According to Eurocodes, this requirement is ensured by limiting the maximum deflection of horizontal structural members to a fraction of their span. However, each Eurocode provides different maximum deflection limits, which are independent of the type of superstructures considered. Thus, the respect of these limits may not always guarantee the integrity of certain superstructures. In this paper, the reliability of the Eurocode deflection control methods, in guaranteeing the integrity of the superstructures, is assessed and discussed. First, different types of horizontal member, namely rib and clay (hollow) pot, composite steel–concrete, and timber beam slabs are designed to respect the deflection limit enforced by the Eurocodes. Then, the maximum curvature developed by these members is compared with the ultimate (limit) curvatures of various superstructures (e.g., ceramic and stone tile floorings). The results obtained show that the approach adopted by Eurocode 2 may provide non-conservative results, but also that the rules proposed by Eurocodes 4 and 5, albeit more reliable, do not always guarantee the integrity of the superstructure. Based on these results, an alternative method, based on the curvature control, is proposed and its advantages and limitations critically discussed. This method appears simpler and more reliable than the method currently adopted by the Eurocodes.

Cure kinetics and properties of high-performance epoxy thermosets cured with active ester-terminated poly (aryl ether ketone)

Benzene-1,3,5-triyl tribenzoate (TBB), both 3,5-bis(benzoyloxy)benzoate-terminated poly (aryl ether ketone) oligomers (BPAPK and TMPK), containing active ester (Ph−O−(C=O)− structure), were prepared and served as curing agents for dicyclopentadiene novoalc epoxy (DCPD). The curing kinetics and properties of three epoxy thermosets were systematically investigated. The model reaction of TBB and glycidyl phenyl ether was designed to understand the curing mechanism of oxirane ring with active ester. TMPK/DCPD displays the lowest reaction activation energy, which is the result of the combined influence of free volume and diffusion. In addition, TMPK/DCPD has the highest Tg value (218°C), which enhances 34.6% and 42.5% compared with BPAPK/DCPD and TBB/DCPD, respectively. Meanwhile, TMPK/DCPD also shows superior dielectric and water resistance properties due to no secondary alcohol generated after curing and hydrophobic tetramethyl-substituted biphenyl structure. Herein, TMPK/DCPD as high-performance epoxy thermosets has potential applications in electronic packaging fields.

Toughening of Epoxy Systems with Interpenetrating Polymer Network (IPN): A Review

Epoxy resins are widely used for different commercial applications, particularly in the aerospace industry as matrix carbon fibre reinforced polymers composite. This is due to their excellent properties, i.e., ease of processing, low cost, superior mechanical, thermal and electrical properties. However, a pure epoxy system possesses some inherent shortcomings, such as brittleness and low elongation after cure, limiting performance of the composite. Several approaches to toughen epoxy systems have been explored, of which formation of the interpenetrating polymer network (IPN) has gained increasing attention. This methodology usually results in better mechanical properties (e.g., fracture toughness) of the modified epoxy system. Ideally, IPNs result in a synergistic combination of desirable properties of two different polymers, i.e., improved toughness comes from the toughener while thermosets are responsible for high service temperature. Three main parameters influence the mechanical response of IPN toughened systems: (i) the chemical structure of the constituents, (ii) the toughener content and finally and (iii) the type and scale of the resulting morphology. Various synthesis routes exist for the creation of IPN giving different means of control of the IPN structure and also offering different processing routes for making composites. The aim of this review is to provide an overview of the current state-of-the-art on toughening of epoxy matrix system through formation of IPN structure, either by using thermoplastics or thermosets. Moreover, the potential of IPN based epoxy systems is explored for the formation of composites particularly for aerospace applications.

Renewable Coumarin-Derived Network as a Toughening Structure for Petroleum-Based Epoxy Resins

A double-network strategy to toughen epoxy resin system is presented herein. Dihydrocoumarin (DHC), a hexatomic compound extracted from tonka bean, is used as the building block for the construction of the first network, and the diglycidyl ether of bisphenol A epoxy matrix is used as the second network. The resultant double network demonstrates a single glass transition and good compatibility between these two networks. Owing to the firm interfacial adhesion between networks and the effective stress transfer as well as external energy absorption derived from the DHC-based network, the double-network-based epoxy resin shows a significant toughness improvement without trade-offs in the tensile strength and elongation at break. The finding in this study provides a promising way to overcome the intrinsic brittleness of commercial epoxy resin via the utilization of renewable DHC for the construction of a novel double network.

Enhanced toughness and thermal conductivity for epoxy resin with a core-shell structured polyacrylic modifier and modified boron nitride

A new epoxy-based composite with higher toughness and thermal conductivity was developed. First, a poly(n-butyl acrylate)/poly(methyl methacrylate-co-glycidyl methacrylate) (PBMG) core-shell structured latex was prepared by seeded emulsion polymerization to toughen the epoxy resin (EP). Second, boron nitride particles were modified into nano-scale sheets and added to the epoxy/PBMG blend to improve the thermal conductivity of the resulting composite material. The properties of the constituent materials were determined prior to fabrication and testing of the composite product. The monomer conversion in the emulsion polymerization process of the PBMG was checked by determining the solid particle content. The PBMG particle size was characterized by dynamic laser scattering, and the morphology of the particles was characterized by scanning and transmission electron microscopy. The exfoliation of the modified boron nitride (MBN) flakes was verified by TEM and Raman microscopy. The mechanical properties and the thermal conductivity of the EP/PBMG/MBN composite were determined at various constituent contents. Results showed that the unnotched impact strength of the composite increased by 147%, the flexural strength increased by 49.1%, and the thermal conductivity increased by 98% compared with pristine EP at a PBMG content of 5 wt% and MBN content of 7 wt%. With the enhanced properties and ease of fabrication, the developed composite has good potential for application in high-end industries such as microelectronics packaging.

Quantification of anhydride groups in anhydride-based epoxy hardeners by reaction headspace gas chromatography

We demonstrate a reaction headspace gas chromatographic method for quantifying anhydride groups in anhydride-based epoxy hardeners. In this method, the conversion process of anhydride groups can be realized by two steps. In the first step, anhydride groups in anhydride-based epoxy hardeners completely reacted with water to form carboxyl groups. In the second step, the carboxyl groups reacted with sodium bicarbonate solution in a closed sample vial. After the complete reaction between the carboxyl groups and sodium bicarbonate, the CO₂ formed from this reaction was then measured by headspace gas chromatography. The data showed that the reaction in the closed headspace vial can be completed in 15 min at 55°C, the relative standard deviation of the reaction headspace gas chromatography method in the precision test was less than 3.94%, the relative differences between the new method and a reference method were no more than 9.38%. The present reaction method is automated, efficient and can be a reliable tool for quantifying the anhydride groups in anhydride-based epoxy hardeners and related research.

Synergistic charring effect of triazinetrione-alkyl-phosphinate and phosphaphenanthrene derivatives in epoxy thermosets

The component synergistic charring effect of a TAHP/TAD system caused more balanced flame-retardant actions in the gaseous phase and condensed phase.

The Synergy of Double Cross-linking Agents on the Properties of Styrene Butadiene Rubber Foams

Sulfur (S) cross-linking styrene butadiene rubber (SBR) foams show high shrinkage due to the cure reversion, leading to reduced yield and increased processing cost. In this paper, double cross-linking system by S and dicumyl peroxide (DCP) was used to decrease the shrinkage of SBR foams. Most importantly, the synergy of double cross-linking agents was reported for the first time to our knowledge. The cell size and its distribution of SBR foams were investigated by FESEM images, which show the effect of DCP content on the cell structure of the SBR foams. The relationships between shrinkage and crystalline of SBR foams were analyzed by the synergy of double cross-linking agents, which were demonstrated by FTIR, Raman spectra, XRD, DSC and TGA. When the DCP content was 0.6 phr, the SBR foams exhibit excellent physical and mechanical properties such as low density (0.223 g/cm³), reduced shrinkage (2.25%) and compression set (10.96%), as well as elevated elongation at break (1.78 × 10³%) and tear strength (54.63 N/mm). The results show that these properties are related to the double cross-linking system of SBR foams. Moreover, the double cross-linking SBR foams present high electromagnetic interference (EMI) shielding properties compared with the S cross-linking SBR foams.