Chemical structure construction of DOPO-containing compounds for flame retardancy of epoxy resin: A review

Large-scale applications of flame-retardant epoxy resins have caused sustainability concerns on the use of renewable resources and end-of-life wastes. New strategies are required to address chemical recycling of fire-safe epoxy resins. Here, we demonstrated recyclable flame-retardant epoxy resins (FREPs) using itaconic acid-derived hyperbranched epoxy resin (IA-EHBP) and (1,3,5-hexahydro-s-triazine-1,3,5-triyl) benzyl mercaptan (HT-BM). The unique structure enabled a closed-loop chemical recyclable network that can be degraded into monomers with up to 86% yield of recovered monomers. The hyperbranched topological structure of IA-EHBP also contributed to a significant improvement in the strength and toughness of the FREPs. By using positron annihilation lifetime spectroscopy, the effects of free-volume hole size and relative fractional free-volume on the mechanical performance and dynamic mechanical properties of FREPs were studied, and the function relations were therefore established based on the Williams–Landel–Ferry equation. The incorporation of IA-EHBP effectively promoted the formation of char residue and produced phosphorus- and sulfur-containing free radicals to prevent the generation of combustible volatiles, thus enhancing the flame retardancy. The FREPs can be recycled and reused multiple times without loss of performance. The method reported here provides a facile and closed-loop approach for high-efficiency chemical recycling and reuse of end-of-life flame-retardant materials. 

Closed-Loop Recycling of Tough and Flame-Retardant Epoxy Resins

Functionalized halloysite nanotubes endowing epoxy resin with simultaneously enhanced flame retardancy and mechanical properties

Multifunctional epoxy resins with excellent flame retardancy, high strength, high toughness, low dielectric loss and strong fluorescence show great potentials in electrical/electronic applications; however, simultaneously achieving those goals are difficult. To address those challenges, an epoxy resin with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) pendant group and flexible aliphatic spacer (DOPOEP) was developed. V-0 rating in UL-94 test and LOI of 33.1% are achieved at only 0.45 wt% phosphorus loading, benefitting from the flame retardant mechanism in gas and condensed phase. Compared with neat DGEBA, the cured epoxy with 10 wt% DOPOEP shows 13.4% and 37.6% improvements in tensile strength and impact strength, respectively; in addition, the tensile strength of the cured epoxy with 20 wt% DOPOEP is as high as 102.2 MPa, a 35.3% increase, without compromising toughness. The strengthening effects of DOPOEP derive from π-π stacking and hydrogen bonding, and the improved toughness is due to the flexible aliphatic spacer and reduced crosslinking density. Meanwhile, free volume reduction was observed after introducing DOPOEP, which is responsible for the improved modulus. DOPOEP addition also lowers the hydroxyl group concentration, leading to lower dielectric constant and dielectric loss tangent. Moreover, the aggregation-induced emission behavior of DOPOEP was reported, and its application in anti-counterfeiting field was explored. In air, the fluorescence of DOPOEP-containing thermoset steadily diminishes with increasing temperature, however the fluorescence changing trend is complex in vacuum. Besides, the fluorescence of DOPOEP is effective in the visual detection of defects and flaws in cured products. This work provides a new direction for developing high-performance epoxy with multifunction. 

Flame retardant, strong and tough, low dielectric epoxy thermoset enabled by a novel bi-DOPO type epoxy with aggregation-induced emission

A novel aromatic imine-containing DOPO-based reactive flame retardant towards enhanced flame-retardant and mechanical properties of epoxy resin

Efficient flame retardancy, good thermal stability, mechanical enhancement, and transparency of DOPO-conjugated structure compound on epoxy resin

Although a simulator is sometimes useful for training the operation of man-machine system, its application to machine tool operation has been hardly studied so far. In this paper, the authors propose a method for high efficiency skill training of small-diameter deep-hole drilling by using the simulator developed previously. For the training system, a function to display an ideal operation pattern and difference from that of a trainee graphically has been added to the simulator. A new index for evaluating the skill level has been also introduced by using this difference. As the results, it is found that 10 to 15 times of training using the simulator improves trainees' skill. In addition, the training using the simulator which emphasizes sensory display together with the normal simulator is highly effective to overcome saturation of skill level.

High efficiency skill training of small-diameter deep-hole drilling by simulator

Among various manual machine tool operations, it is known that the efficiency and accuracy of small-diameter deep-hole machining with a drilling machine is strongly dependent upon operator's sensitivity and skill learning level. In our previous work, a simulator of the drilling machine operation under no deflection of the drill was developed and its performance as a skill training apparatus was discussed. In this paper, the improvement of the simulator has been investigated. Firstly, the simulation model considering the deflection of the drill has been derived. Secondly, the model parameters have been reasonably identified from the experimental data of actual operation by applying the optimization technique due to genetic algorithm. In addition, the hardware system has been also improved to avoid reduction of realistic feeling which results from complicated calculation of the model.

Drilling Machine Simulator Considering Deflection of Drill

Recently, the role of skilled operators in manufacturing systems is recognized again and the utilization of the mentality and flexibility of human operators to them is focused on with fresh interest. In order to apply the manual operation to a manufacturing system effectively, it is important to investigate the skill learning characteristics of the operators and derive a reasonable skill training method for them. The virtual reality techniques appear to be very useful to carry out this investigation, though the actual manufacturing system is not necessarily suitable for it. In the present paper, the objective manual operation is small-diameter deep-hole machining of an aluminum workpiece with a bench-mounted drilling machine, which is known as a difficult operation requiring operator's sensitivity highly. Firstly, a simple simulator of the drilling machine or a virtual machine has been developed taking into consideration the dynamic characteristics of a real machine. Secondly, its effectiveness to the skill training has been studied through experimental operations by inexperienced testees. A new index for assessing operating performance was also discussed. As the results, it is found that the developed simulator is significantly effective in skill training for the actual drilling machine operation and the operating efficiency continues to improve during early 15 time operations with the simulator.

Skill Training of Manual Machine Operation by Simulator - In the Case of Small-Diameter Deep-Hole Machining with Drilling Machine -

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Papers

Efficient flame retardancy, good thermal stability, mechanical enhancement, and transparency of DOPO-conjugated structure compound on epoxy resin

High efficiency skill training of small-diameter deep-hole drilling by simulator

Drilling Machine Simulator Considering Deflection of Drill

Skill Training of Manual Machine Operation by Simulator - In the Case of Small-Diameter Deep-Hole Machining with Drilling Machine -