A liquid phosphorus-containing imidazole derivative as flame-retardant curing agent for epoxy resin with enhanced thermal latency, mechanical, and flame-retardant performances

Preparation and Properties of Epoxy Adhesives with Fast Curing at Room Temperature and Low-Temperature Resistance

Developing a highly efficient multifunctional epoxy adhesive is still an enormous challenge, which can rapidly cure at room temperature and has excellent low-temperature resistance performance and is crucial for the epoxy adhesive and electrical sealing fields during severe cold seasons. Herein, diglycidyl phthalate (DP) was synthesized with phthalic anhydride (PA) and epichlorohydrin (ECH) to enhance the curing rate and low-temperature resistance of bisphenol A diglycidyl ether (DGEBA) adhesive. The modified DP/DGEBA adhesives were systematically analyzed by gel time, mechanical properties, and aging resistance (time, temperature, and dry/wet treatment). The results showed that DP with highly active ester groups significantly accelerates the curing speed of DP/DGEBA. DP's rigid aromatic ring-benzene ring and flexible group-ester group gave the adhesive better low-temperature resistance. When the addition of DP was 10 wt % (based on the mass of DGEBA), the gel time of DP/DGEBA epoxy adhesives was reduced by 58 min compared to unmodified DGEBA epoxy adhesive, and after aging at low temperature (-20 °C) for 7 days, the tensile shear strengths of polyvinyl chloride (PVC) and aluminum plate increased by 76.2 and 80.6%, respectively. The results of non-isothermal curing kinetics and dynamic mechanical analysis suggested that when the amount of DP was 10 wt %, the reaction activation energy of DP/DGEBA epoxy adhesive decreased by 4.0%, and the cross-linking density increased by 8.9%. Moreover, the toughness of the modified adhesive was also improved. This study opens up a feasible way for the development of a low temperature-resistant epoxy adhesive cured rapidly at room temperature in practical application.

Synthesis of renewable furan-based phosphate and the superior flame retardancy in biodegradable polylactide

Currently, it has long been considered a challenge to provide sustainable additives for polylactide (PLA) in green way to endow it excellent comprehensive properties. Given the flammability and unsatisfactory crystallization performance of PLA, a furan-based phosphate furfurylamine trimethylphosphate (FATMP) was synthesized from 2-furfurylamine and amino trimethylphosphonic acid by a simple hydration reaction, and the PLA/FATMP composites were prepared by melting blending process. The tensile performance, crystallization behaviors, flame retardancy, and flame-retardant mechanism received special attention. Results showed that the incorporation of only 3 wt% FATMP could indeed increase the LOI value of PLA from 19.8 to 27.3 %, and simultaneously acquired V-0 rating in the vertical burning test owing to the favorable synergistic effect between the vapor phase and the condensed phase. Additionally, the half-crystallization time of PLA was decreased from 12.4 to 5.1 mins with the addition of FATMP, which acted as a nucleating agent. More appealingly, the tensile performance of PLA/FATMP composites was also well maintained. In general, the PLA/FATMP composites we proposed could be promising candidates in application fields where favorable flame retardancy and crystallization ability are required.

Rapid Impregnating Resins for Fiber-Reinforced Composites Used in the Automobile Industry

As environmental regulations become stricter, weight- and cost-effective fiber-reinforced polymer composites are being considered as alternative materials in the automobile industry. Rapidly impregnating resin into the reinforcing fibers is critical during liquid composite molding, and the optimization of resin impregnation is related to the cycle time and quality of the products. In this review, various resins capable of rapid impregnation, including thermoset and thermoplastic resins, are discussed for manufacturing fiber-reinforced composites used in the automobile industry, along with their advantages and disadvantages. Finally, vital factors and perspectives for developing rapidly impregnated resin-based fiber-reinforced composites for automobile applications are discussed.

Neighboring Molecular Engineering in Diels-Alder Chemistry Enabling Easily Recyclable Carbon Fiber Reinforced Composites

Although a variety of dynamic covalent bonds have been successfully used in the development of diverse sustainable thermosetting polymers and their composites, solving the trade-off between recovery efficiency and comprehensive properties is still a major challenge. Herein, a "one-stone-two-birds" strategy of lower rotational energy barrier (Er ) phosphate-derived Diels-Alder (DA) cycloadditions was proposed for easily recyclable carbon fiber (CF)-reinforced epoxy resins (EPs) composites. In such a strategy, the phosphate spacer with lower Er accelerated the segmental mobility and dynamic DA exchange reaction for network rearrangement to achieve high-efficiency repairing, reprocessing of the EPs matrix and its composites and rapid nondestructive recycling of CF; meanwhile, incorporating phosphorus-based units especially reduced their fire hazards. The resulting materials simultaneously showed excellent thermal/mechanical properties, superb fire safety and facile recyclability, realizing the concept of recycling for high-performance thermosetting polymers and composites. This strategy is of great significance for understanding and enriching the molecular connotation of DA chemistry, making it potentially applicable to the design and development of a wide range of dynamic covalent adaptable materials toward practical cutting-edge-tech applications.

Effects of graphene, alumina, and their hybrid on dynamic mechanical behavior of epoxy-based nanocomposites

Graphene has excellent mechanical and thermal properties, while alumina has inexpensive and strong thermal stability, making it ideal for electronic packaging applications. The current work examines the effects of modest loadings of reduced graphene oxide (rGO), alumina (Ala), and their hybrids on the dynamic mechanical characteristics and thermal transitions of epoxy-based nanocomposites. The rGO was made using a modified Hummers’ method. Alumina nanoparticles were used as received. Various epoxy nanocomposite samples with 1 wt% of rGO, 1 wt% of Ala, and 0.5 wt% each of rGO and Ala were created. Then, Raman spectroscopy, XRD, FTIR, XPS, TEM, AFM, and SEM were used to analyze the fillers and their hybrid and epoxy-based nanocomposites. Dynamic mechanical analysis was used to examine the hybrid epoxy-based nanocomposites’ dynamic mechanical behavior. It has been demonstrated that different epoxy nanocomposites with 1 wt% Ala and 1 wt% rGO have higher elastic moduli than those with 0.5 wt% rGO and 0.5 wt% Ala. The rGO-Ala hybrid nanocomposites, on the other hand, exhibit moderate damping and glass transition temperatures. The reason for that may be because Ala filler increases the contact area between the rGO sheets and serves as a bridge between the two fillers, decreasing the resistance to deformation. This may enhance the possibility that these stress concentrations will weaken the composites.

Co-curing preparation of flame retardant and smoke-suppressive epoxy resin with a novel phosphorus-containing ionic liquid

Phosphorus-containing ionic liquid derivatives have been proven to be effective flame retardants for epoxy resin (EP). Flame retardants can accelerate the curing process and improve flame retardancy and smoke suppression of EP composites, which is challenging. In this paper, a novel phosphorus-containing ionic liquid (TPP-PF₆) was synthesized and used both as a co-curing agent with 4,4'-diaminodiphenylmethane (DDM) and as a highly effective flame retardant for EP. It has been found that TPP-PF₆ was conducive to improve the char formation of EP to inhibit the smoke release at high temperatures. For EP/TPP-PF₆ composites, the flame-retardant performance was enhanced rapidly with the increase of TPP-PF₆. With only 2 wt% of TPP-PF₆, EP/2.0TPP-PF₆ reached a UL-94 V-0 rating and a limiting oxygen index of 30.3%. The peak heat release rate, total heat release, and total smoke production values of EP/2.0TPP-PF₆ were reduced by 36.32%, 45.81%, and 15.1% compared with those of pure EP, respectively. The thermal degradation products and flame retardant mechanism in gas and condensed phases were studied. It was found that TPP-PF₆ had flame retardant effect in the barrier effect of the condensed phase and the quenching effect of the gas phase. This work explores the high-efficiency flame retardant and smoke-suppressive structures with co-curing properties for EP, thus promoting the wide application of EP materials.

Growth of copper organophosphate nanosheets on graphene oxide to improve fire safety and mechanical strength of epoxy resins

With the high integration of electronic products in our daily life, high-performance epoxy resins (EP) with excellent flame retardancy, smoke suppression, and mechanical strength are highly desired for applications. In this study, copper organophosphate nanosheets were evenly grown on the surface of graphene oxide (GO) via a self-assembly process based on coordination bonding and electrostatic interactions. The resultant nanohybrid endowed EP with satisfactory flame retardant effect and improved mechanical properties. Incorporating functionalized nanosheets of merely 1 wt% loading, the impact strength of the EP nanocomposites improved by 147% when compared to 1% EP-GO. Additionally, the nanosheets inhibited the smoke and heat release of EP, and the limiting oxygen value of EP-EGOPC reached ∼29%. The mechanism analysis verified that the existence of organophosphate and copper-containing components associated with the physical barrier of GO promoted the hybrid aromatization of the char layer, thereby improving the fire safety of epoxy matrix. This research offers a new interfacial method for designing functional nanosheets with good interface compatibility and high flame-retardant efficiency in polymers.

Advances on Thermally Conductive Epoxy-Based Composites as Electronic Packaging Underfill Materials-A Review

The integrated circuits industry has been continuously producing microelectronic components with ever higher integration level, packaging density, and power density, which demand more stringent requirements for heat dissipation. Electronic packaging materials are used to pack these microelectronic components together, help to dissipate heat, redistribute stresses, and protect the whole system from the environment. They serve an important role in ensuring the performance and reliability of the electronic devices. Among various packaging materials, epoxy-based underfills are often employed in flip-chip packaging. However, widely used capillary underfill materials suffer from their low thermal conductivity, unable to meet the growing heat dissipation required of next-generation IC chips with much higher power density. Many strategies have been proposed to improve the thermal conductivity of epoxy, but its application as underfill materials with complex performance requirements is still difficult. In fact, optimizing the combined thermal-electrical-mechanical-processing properties of underfill materials for flip-chip packaging remains a great challenge. Herein, state-of-the-art advances that have been made to satisfy the key requirements of capillary underfill materials are reviewed. Based on these studies, the perspectives for designing high-performance underfill materials with novel microstructures in electronic packaging for high-power density electronic devices are provided.

An Efficient Cross-Linked Phosphorus-Free Flame Retardant for Epoxy Resins

Epoxy resins (EPs) have been widely used due to their great physical and chemical properties, but their poor flame retardancy limits their further application. In this work, we synthesized a flame retardant containing nitrile groups and a double bond to improve the flame retardancy of EPs. In this way, multiple cross-linking reactions can occur in the EPs to confer better flame retardancy by a simple heat treatment. The UL-94 vertical combustion test, CCT, and limiting oxygen index (LOI) test were used to characterize the flame retardant properties of the cross-linked flame retardant; the results show that with the 10 wt % addition of cross-linked flame retardant, the thermosets can pass the UL-94 V-0 rating. Meanwhile, the contents reached 20 wt %, and the peak heat release rate decreased 40% compared with neat EP.

Epoxy Compositions with Reduced Flammability Based on DER-354 Resin and a Curing Agent Containing Aminophosphazenes Synthesized in Bulk Isophoronediamine

A method for the synthesis of an amine-containing epoxy resin curing agent by dissolving hexakis-[(4-formyl)phenoxy]cyclotriphosphazene in an excess of isophoronediamine was developed. The curing agent was characterized via NMR and IR spectroscopy and MALDI-TOF mass spectrometry, and its rheological characteristics were studied. Compositions based on DER-354 epoxy resin and the synthesized curing agent with different amounts of phosphazene content were obtained. The rheological characteristics of these compositions were studied, followed by their curing. An improvement in several thermal (DSC), mechanical (compression, tension, and adhesion), and physicochemical (water absorption and water solubility) characteristics, as well as the fire resistance of the obtained materials modified with phosphazene, was observed, compared with unmodified samples. In particular, there was an improvement in adhesive characteristics and fire resistance. Thus, compositions based on a curing agent containing a 30% modifier were shown to fulfill the V-1 fire resistance category. The developed compositions can be processed by contact molding, winding, and resin transfer molding (RTM), and the resulting material is suitable for use in aircraft, automotive products, design applications, and home repairs.

Synthesis, Crystal Structures, and Density Functional Theory Studies of Two Salt Cocrystals Containing Meldrum's Acid Group

Two salt cocrystals, C₃₁H₃₄N₄O₈ (DDD) and C₁₇H₂₀N₂O₈ (MDD), were synthesized and their structures were determined by single-crystal X-ray diffraction. DDD is made up of one (C₁₃H₁₃O₈)^- anion, one (C₉H₁₁N₂)⁺ cation, and one 5,6-dimethyl-1H-benzo[d]imidazole molecule. MDD consists of one (C₄H₇N₂)⁺ cation and one (C₁₃H₁₃O₈)^- anion. DDD and MDD belong to the monoclinic, P21/c space group and triclinic, P-1 space group, respectively. A 1D-chained structure of DDD was constituted by N-H···N and N-H···O hydrogen bonds. However, a 1D-chained structure of MDD was bridged by N-H···O hydrogen bonds. Their density functional theory-optimized geometric structures with a B3LYP/6-311G(d,p) basis set fit well with those of crystallographic studies. By calculating their thermodynamic properties, the correlation equations of C ⁰ _p,m , S ⁰ _m , H ⁰ _m , and temperature T were obtained. By comparing the experimental electronic spectra with the calculated electronic spectra, it is found that the PBEPBE/6-311G(d,p) method can simulate the UV-Vis spectra of DDD and MDD. In addition, the fluorescence spectra in the EtOH solution analysis show that the yellowish-green emission occurs at 570 nm (λ_ex = 310 nm) for DDD and the purplish-blue emission occurs at 454 nm (λ_ex = 316 nm) for MDD.

Unsaturated polyester resin modified with a novel reactive flame retardant: effects on thermal stability and flammability

A new reactive flame retardant (DTA), containing phosphaphenanthrene and triazine-trione groups was synthesized and applied to improve the flame retardancy of unsaturated polyester resin. The thermal stability, flame retardancy and combustion behaviors of UP/DTA thermosets were detected by thermogravimeric analysis (TG), limited oxygen index (LOI), vertical burning (UL94) test and cone calorimeter test. According to the research results, the addition of DTA contributed to improving the flame retardancy of UP. After adding 20 wt% DTA, the LOI of UP composite increased from 19.0% of the neat UP to 26.6%, and UL94 rating reached V-0. In addition, compared with pure UP, the peak heat release rate (pk-HRR), average heat release rate (av-HRR) and total heat release rate (THR) of UP/DTA-20 thermosetting material decreased by 44.0, 26.2 and 29.5%, respectively. In the gaseous phase, DTA decomposed to generate nitrogen-containing fragments with diluting effect and phosphorus-containing free radicals with quenching effect to inhibit the combustion. In the condensed phase, phosphaphenanthrene group of DTA decomposed to generate phosphorus-based compounds, which promoted the carbonization of the UP matrix and cooperated with triazine-trione group to increase the char yield. Therefore, DTA plays an important role in flame retardancy in the gas and condensed phases.

Facile Construction of Polypyrrole Microencapsulated Melamine-Coated Ammonium Polyphosphate to Simultaneously Reduce Flammability and Smoke Release of Epoxy Resin

A unique mono-component intumescent flame retardant, named PPy-MAPP, of which melamine-coated ammonium polyphosphate (MAPP) was microencapsulated by polypyrrole (PPy), was synthesized and carefully characterized. The obtained PPy-MAPP was applied to epoxy resin (EP) for obtaining flame-retarded EP composites. The results show that PPy-MAPP imparts better flame retardancy and smoke suppression properties to EP compared to the same addition of MAPP. The EP composite with 15 wt% PPy-MAPP easily passes the UL94 V-0 rating and achieves an LOI value of 42.4%, accompanied by a 61.9% reduction in total heat release (THR) and a 73.9% reduction in total smoke production (TSP) when compared with pure EP. The char residue analysis shows that PPy-MAPP can promote a generation of more phosphorus-rich structures in the condensed phase that improve the integrity and intumescence of char against fire. The mechanical test indicates that PPy-MAPP has a less negative effect on the tensile strength and elastic modulus of epoxy resin due to the good compatibility between PPy-MAPP and the EP matrix, as supported by differential scanning calorimetry (DSC) analyses. In this paper, these attractive features of PPy-MAPP provide a new strategy to prepare satisfactory flame retardant and super flame retarding EP composites.

From herbicide to flame retardant: The lamellar-like phosphorus-bridged amitrole toward high fire safety epoxy resin with light smoke and low toxicity

In an attempt to alleviate the harmful impact of the flammability of epoxy resin on the environment, amitrole, a herbicide, has been converted to a novel flame retardant (PBA) with lamellar morphology through organophosphorus modification. This material has been utilized to fabricate fire safe epoxy thermosets (EP). EP containing 7.5 wt% PBA undergoes quick self-extinguishment upon ignition. This blend displays a high limiting oxygen index (LOI) value of 34%. More importantly, hazardous products (heat, smoke, toxic gases including CO/CO₂) released during combustion of EP, are strongly suppressed in the presence of PBA. The mechanical properties of EP-PBA blends are comparable to those of virgin EP. The tensile strength of EP containing PBA is 90% of that of unmodified EP. The flexural strength of PBA blends is somewhat greater than that for EP containing no additive. A tactful strategy for the transformation of amitrole, a potential environmental contaminant to a benign flame retardant for polymers has been developed.

Construction of bismaleimide resin with enhanced flame retardancy and mechanical properties based on a novel DOPO-derived bismaleimide monomer

It's known that the application of bismaleimide resins (BMI) is limited due to its brittleness and poor flame retardancy. A novel type of BMI monomer (MADQ) based on the typical phosphorus series flame retardant DOPO is designed to improve the fire safety of BMI. Besides, aliphatic long chain structure is introduced in MADQ, which is supposed to be conducive to reducing the rigidity of the BMI cross-linked network and thus to improve the toughness of BMI. It's seen that with the incorporation of 5.24 wt% MADQ, the peak of heat release rate (PHRR) and total heat release (THR) of resultant BMI/MADQ-5 is reduced by 37.7% and 33.9%, respectively. Meanwhile, with modification of 1.07 wt% MADQ, BMI/MADQ-1 possesses UL-94V-0 rating. The relevant mechanism analysis reveals that the phosphaphenanthrene group in MADQ can exert flame retardancy effect both in condensed and gas phase. Besides, the impact strength of the BMI/MADQ is maximally increased by nearly 90.1%. Furthermore, the BMI/MADQ still maintains high tensile strength and thermal stability, which indicates the modification of MADQ did not deteriorate other properties of BMI. An innovative research idea and research basis for the preparation of intrinsic flame-retardant and toughened BMI is provided in this work.