Recent advances in recyclable thermosets and thermoset composites based on covalent adaptable networks

doi:10.1016/j.jmst.2021.03.043

Abstract

Abstract:

Recyclable thermosets and thermoset composites with covalent adaptable networks (CANs, or dynamic covalent networks) have attracted considerable attention in recent years due to the combined merits of excellent mechanical and thermal properties, and chemical stabilities of traditional thermosets and recyclable, remoldable, and reprocessable attributes of thermoplastics. In this paper, we present an overview of the current strategies for synthesizing recyclable thermosets based on CANs, which involve recyclability, reprocessability, and possible rehealability. The recent literature examples are categorized based on the underlying controlled-cleavable linkages such as transesterification, DA/retro-DA chemistry, imine bonds, disulfide metathesis, dynamic B-O bonds, hemiaminals/hexahydrotriazines, and acetal linkages. Various degradation and malleability methods and resulting mechanical properties of the recycled thermosets and thermoset composites are presented. The emerging applications of recyclable thermosets and thermoset composites, with emphasis on their usage in adhesives, biomedical materials, wearable devices, coatings, and 3D printing materials, are also illustrated. Finally, a perspective on the challenges and future perspectives is briefly summarized.

Key words: Covalent adaptable networks (CANs), Recyclable, Crosslink, Degradation, Property

Yihe Zhang, Li Zhang, Guotao Yang, Yalin Yao, Xu Wei, Tianchi Pan, Juntao Wu, Moufeng Tian, Penggang Yin. Recent advances in recyclable thermosets and thermoset composites based on covalent adaptable networks[J]. J. Mater. Sci. Technol., 2021, 92: 75-87.

Figures/Tables 13

Fig. 1. (a) Degradation and repolymerization of the epoxy vitrimer via transesterification. (b) A schematic view of the closed-loop recycling paradigm for epoxy vitrimer CFRP composites [41].

Scheme 1. Scheme view illustrating the EG-assisted dissolution and repolymerization of CANs [49].

Fig. 2. The recycling process of a commercial CFRP propeller to reclaim CF fabric [50].

Fig. 3. Demonstration of thermal recyclability and re-mending ability of DAPUs by hot-compression molding of DAPU100 (two thin disks or granule) on a metal mold at 180 °C for 10 min [21].

Fig. 4. New recycling paradigm of the polyimine-based composites [64].

Fig. 5. (a) Synthetic route of EN-VAN-AP; (b) Dissociative mechanism of imine bond. (c) Tensile properties of wet sample after different periods of heating at 120 °C [65].

Fig. 6. (a) Mechanical recycling of the dynamic epoxy network; (b) Thermoformation of cured composite laminate; (c) Partial dissolution of CFR-epoxy composite in a thiol-containing solution, where the carbon-fiber in contact with the solution was recovered unaltered; (d) Mechanical recycling of dynamic epoxy composites [70].

Fig. 7. The reversibility (a) and solubility (b) of the cured PBNRs. The recycling and regeneration (c) and degradation (d) processes of the GF/PBNR composites with dimensions of 90 mm × 90 mm × 3 mm (L × W × H) in ethanol at room temperature [74].

Fig. 8. (a) Representative digital camera images of the degradable process, immersed in 0.5 mol/L H2SO4 at 90 °C for (i) 0 h, (ⅱ) 0.5 h, (ⅲ) 1 h, and (ⅳ) 2 h [38]; (b) The resin’s first-stage depolymerization time (t1) in different 1 M HCl/THF solutions and the contact angles of their corresponding H2O/THF solutions [75]; (c) The resin’s degradation status in 10 mL 1 M HCl/THF solution at different time [75].

Scheme 2. Schematic pictures of synthesis of acetal-containing epoxy resins, preparation and degradation of acetal-containing CFRPs, and recovery of carbon fibers [39].

Table 1 Summary of mechanical properties, Tg, and reprocessing results of various CANs-based thermosets and thermoset composites.

Degradable bonds or structures	Tensile strength (MPa)	Young'smodulus (MPa)	T_g ( °C)	Recycling (Reprocessing)	Ref.
Transesterification	55	1800	80	240 °C/3 min	[12]
	69.2 ± 2.1	1950±100	187	—	[13]
	6.08-16.62	—	65	10 MPa /160 °C /1 h	[15]
	1.7-2.8	1.8-2.8	12.4-25.0	5 MPa /120 °C /30 min	[16]
	1.05-1.99	2	-21.6 -6.3	15 MPa /200 °C/20 min	[17]
	1.57-52.1	0.46-1970	-30-72	—	[18]
DA/retro-DA chemistry	53	2500	128.5-136	125 °C/20 min	[20]
	11.5-49.8	69.5-229.7	90-148	160-180 °C /5-20 min	[21]
Imine bonds	60.6 ± 1.8	2598± 41	127	0.3 MPa/170 °C/30 min	[23]
	0.63—20.50	—	40.5-75.7	15 Mpa/ 80 °C /5 min	[24]
	11—65	14-1050	47-120	—	[25]
	10-64	0.13-1.0	55-135	45 MPa / 121 °C/1 min	[26]
Disulfide metathesis	1.8-11.4	1.4-106.3	-8.3-10.5	10 MPa/200 °C/10 min	[27]
	13-15	207-1122	43-224	20 MPa /210 °C/2 h	[28]
	0.23	-	about -36	4.5 MPa/25 °C /24 h	[29]
	9.67±0.89	5.04±0.05	29.8	10 MPa/ Sunlight/5 min	[30]
Dynamic B-O bonds	1.75-12.74	2.72-112.45	-55	0.4 kPa/R.T./24 h	[33]
	7-13	42-57	-50-200	10 MPa/200 °C/20 min	[34]
	17.8-32.9	559-768	26-47	hot press /80 °C	[35]
	5.95-31.96	63.9-331.7	6-57	4 MPa/60 °C/15 min	[36]
	28	1.8	98	1.2 MPa/ 180-200 °C /15 s	[73]
Hemiaminals/hexahydrotriazines	80 ± 6	2000± 148	151	—	[38]
	124.7	4800	200.1	about 3 MPa/200 °C /2 h	[75]
Acetal linkages	85 ± 7	3131± 282	169	—	[78]
	71.3 ± 8	3349± 37	184	—	[79]
	27.2-33.3	0.8-1.1	66-71	15 MPa /150 °C/10 min	[80]
	17.3-45.8	1843-2766	103-113	10 MPa/150-180 °C/20-30 min	[81]

Fig. 9. Schematic illustration of rehealability and full recyclability of the e-skin [111].

Fig. 10. An illustration of the crosslinking/dissolution/recrosslinking cycle of recyclable 3D printing of nanoclay-reinforced vitrimer epoxy [51].

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