Upcycling Of Epoxy Resins Cured With Amine

Recycling of polymer wastes has been an important subject of environmental protection and resource utilization.According to the quality,performances or value（including economic/environmental value）of the recovered products,the recycling process can be divided into upcycling and downcycling.Owing to the outstanding integrated performances such as high strength and rigidity,high temperature and corrosion resistance,epoxy resin（EP）and its composites have been widely used in various industrial fields including aerospace,new energy,electronic information,insulation,transportation,etc.EP has been occupying about 70%of the whole thermosetting resins.With the increasing demand for EP,its wastes thereby increase.However,EP is infusible,insoluble and hard to be recycled because of its stable three-dimensional network structure.Thus,the research for recycling EP wastes is still in infancy.The existing recycling methods mainly recover EP wastes into materials,oligomers and chemicals with low molecule weight.In view of cost and energy consumption,directly transforming EP wastes into materials is more promising.EP cured with amine is the most widely used among various EP.Based on EP’s structures and performance advantages,as well as the size effect,multiple functional recycled materials from EP wastes were developed,and thus a novel physical upcycling strategy for wastes recycling is established.Herein,we fabricated superhydrophobic surfaces with diverse water adhesion by choosing adhesives with different surface energy for gluing crushed epoxy resin particles（CEPs）,wherein the hydrophobicity and micro-nano structure of CEPs also played important roles.High water adhesion coating with the adhesion of 219μN was synthesized by combining CEPs with EP adhesive,while the adhesive force of coating was 106μN with low surface energy polydimethylsiloxane（PDMS）.The prepared coatings exhibited excellent water droplet storage capacity（50μL）,droplet transportation capacity（170μL）,and fast water collection rate(572.5 m^-2 h^-1),respectively.Besides,we found that amine-cured CEPs showed fast and reversible responsive discoloration to acidic liquids/gases,and successfully applied them in the detection of acids.Taking advantage of the difference in acid response between amine-cured and anhydride-cured CEPs,an acid-induced information storage coating was designed and prepared.In addition,CEPs can also be used as fillers for filter column.Owing to high hydrophobicity and superoleophilicity,CEPs filter column successfully separated the simple oil-water mixtures with high-efficiency.The separation flux is up to 16000 L m^-2 h^-1 and separation efficiency is 97%.Moreover,this new physical upcycling method can also be applied to hydrophobic thermoplastic materials such as PE and PP,with the separation flux of 57325 L m^-2 h^-1and separation efficiency of 99%,showing universal applicability.After acidification,CEPs showed excellent and stable separation performance for various water-in-oil emulsions with micron/nanometers.The separation flux is up to 1000 L m^-2 h^-1 and separation efficiency is 98%.After 10cycling tests,it still maintained effective flux and separation efficiency,indicating excellent reusability.Partial cleavage of chemical bonds in EP can obtain products with high molecular weight.The C-N bonds in EP were selectively broken by HNO₃ digestion under mild conditions,while the benzene rings and other hydrophobic skeletons were retained.So,the linear degraded product（DEP）with high molecular weight（Mw=6764）was obtained.Based on the hydrophobicity and film-forming properties,DEP was coated on melamine foam（MF）by ultrasonic-assisted dip-coating to prepare high-efficiency oil-absorbing material（DEP@MF）.The resulting DEP@MF showed excellent lipophilicity and hydrophobicity（WCA=146.5±1.5^o）,as well as compression cycle stability.Moreover,it also exhibited satisfactory absorption performance for various oils or organic solvents with an ultrafast adsorption rate of 2 s and high oil absorption capacity of 116 g g^-1.By simple mechanical squeezing,the absorbed oils were recovered and DEP@MF can also be recycled.In 10 adsorption-extrusion cycles,the adsorption capacity of the DEP@MF remained stable.Besides,DEP@MF was successfully designed to achieve continuous oil-water separation.By choosing acidic oxidation system,EP was degraded into water-soluble products with smaller molecular weight（Mw=1027）and achieved upcycling via supramolecular interaction among DEP.The relatively eco-friendly and mild recycling method was developed to degrade EP by using H₃PO₄ and H₂O₂.H₃PO₄ reacted with C-N bonds in EP and converted them into ammonium salt,which would reduce the activation energy required for cleaving C-N bonds and facilitate selectivity of chemical bonds fracture.Moreover,H₃PO₄can etch EP to enhance the mass transfer during degradation.The hydroxyl radicals generated from H₂O₂ preferentially attacked C-N bonds,destroying EP’s three-dimensional cross-linking network,but the carbon skeleton structure such as benzene rings and ether bonds remained.Due to the strong oxidizing effect of H₂O₂,the intermediate products were further oxidized into oligomers containing multiple functional groups such as hydroxyl,amino and carboxyl groups.Different from traditional separation and purification that only extracted part of pure products,the mixed degradation products obtained in this work assembled into supramolecular adhesives in the separation process.Due to noncovalent nature and multiple functional groups,DEP is able to respond to temperature,water and p H.With increasing the temperature/water content,the adhesion strength of DEP first increased and then decreased;In alkaline environments,the adhesion strength of DEP decreased more significantly when compared to acidic environments.Multiresponsivity provides more selectivity for controlling adhesive properties of DEP in complex environment.To further simplify the degradation and separation process,EP was degraded into water-insoluble and low molecular weight DEP with H₂O₂ alone.During this one-step oxidative degradation,EP was degraded through the cleavage of C-N cross-linked points and directly converted into high performances adhesive via supramolecular self-assembly.DEP（M_w=593）was a mixture of small molecules with multiple functional groups,mainly including epoxy monomers or dimers/trimers.Supramolecular self-assembly of DEP occurred synchronously with EP’s degradation prevents the intermediate products from being continuously oxidized and degraded into products with lower molecular weight.Because of the excellent hydrophobicity,DEP separated spontaneously from the reaction mixture,avoiding the subsequent complicated separation process.This end-functionalized DEP can be directly used as novel low molecular weight monomers（LMWM）-based supramolecular adhesive without the support of any other components.Strong adhesion performance was realized on various substrates,with a maximum adhesion strength of 4.34 MPa,which is beyond the property space of previously LMWM-based supramolecular adhesives,even higher than previously reported high-molecular-weight molecules（HMWM）based adhesives.Moreover,DEP exhibits excellent and reversible adhesion,good corrosion resistance to water,acid,salt and alkane organic solvents,as well as is easy to be removed from the substrate.In addition,DEP can be used as an underwater adhesive with enviable underwater adhesion strength of 5.88 MPa,which is higher than most of underwater adhesives previously reported.The adhesion strength of DEP does not decay（5.5 MPa）even if placed underwater for 48 h,suggesting a desirable stability of DEP for long-term use.Based on the lipophilicity and hydrophilicity,DEP can also exist spontaneously at air/water or oil/water interface,showing potential application value in the fields of sealing,plugging and oil-water separation.On the basis of above work,we extended the oxidative degradation method to commercially available glass/carbon fiber reinforced epoxy resin composites（GFRP/CFRP）.High degradation efficiency for different amine-cured EP was achieved using H₂O₂under relatively mild condition（90^oC）.The degraded products exhibited excellent thermal stability and showed the potential to form polymer materials through supramolecular interaction.Besides,the microwave-assisted swollen method was used to destroy the dense structure of GFRP/CFRP and thus accelerate their degradation.Long,clean,nondestructive fibers were recovered from the composites,and their tensile strength was comparable to that of virgin fibers.