LIU Yu-yan，LI Li，2，WU Song-quan，TANG Yu-pan

刘宇艳， 李 犁， 吴松全， 唐宇攀

(1.School of Chemical Engineering and Technology，Harbin Institute of Technology，Harbin 150001，China;2.Dept.of Materials Science and Engineering，Carnegie Mellon University，USA)

Epoxy resin and its composites are widely used in various commercial areas，such as automotive industry，aerospace，crude oil industry and so forth［1］.However，due to their crosslink structure，epoxy resins neither melt or dissolve in common solvents，nor degrade by microorganism.Therefore，the recycling of epoxy materials poses a major challenge on the industry.It is considered that the negative impacts on the environment and the low ratio of utilization，pyrolysis［2-4］and other similar approaches have not been regarded as efficient decomposition methods.

One of the present methods used to decompose epoxy resin is chemical degradation［5-8］.Braun succeeded to use of tetrahydronaphthalene as main solvent to decompose the thermosetting epoxy resin［9］.Simultaneously，Yoshiki conducted liquid-phase cracking of epoxy resin at 440℃in the presence of tetralin as well as decalin and explained hydrogenative mechanism clearly［10］.The hardened networks of epoxide-amine from diglycidylether of bisphenol-A(DGEBA)cured with polyamine can be decomposed by glycols in relatively mild conditions［11］.Dang used nitric acid to decompose the epoxy resin cured with amine and recycle the glass fibers successfully［12-13］.In their research，only one hardened networks of epoxy resin was used.

The creativity of this paper lies in using nitric acid to decompose various thermosetting epoxy resins which are cured with different curing agents and different resins respectively.The effects of cross-linked networks on the decomposition properties were investigated.And the mechanism of decomposition was also envisaged.

1 Experiments

1.1 Preparation of Epoxy Resin Samples

Four kinds of epoxy resins were used in the experiments，namely bisphenol-A epoxy resin E-44(Epoxide value=0.41-0.47 eq/100 g)，bisphenol-A epoxy resin E-51(Epoxide value=0.48-0.54 eq/100 g)，N，N，N＇，N＇teraglycidy 4，4＇diaminodiphenyl methane(AG-80)and diglycidyl-4，5-epoxycyclohexane-1，2-dicarboxylate(TDE-85)，and four kinds of curing agents，namely polyamide(PA651)，isophorone diamine(IPDA)，4，4 ＇-diaminodiphenylmethane(DDM)and 2-ethyl-4-methylimidazole(EMI-2，4).The samples to be prepared were divided into two groups:in one group，E-44 was adopted as the matrix resin with a varied series of curing agents，such as PA651，IPDA，DDM，EMI-2，4.Additionally，in the other group，the fixed curing agent IPDA was used with different epoxy resin，such as E-44，E-51，AG-80，TDE-85.Different epoxy resins and curing agents were poured int o a pre-pared 30 cm×30 cm square aluminum mould with fixed molar stoichiometric ratio and then cured.Soxhlet apparatus was employed to distill the samples with acetone at the rate of 0.5 mL/min at 70℃for 24 h.According to the result of distillation，these samples were cured completely.

1.2 Decomposition Experiments of Epoxy Resin

The samples were cut into 30 mm×30 mm×2 mm specimens and put into 500 mL beakers，and then immersed in 100 mL 8 mol/L nitric acid with a 1 g/50 mL(epoxy resin:acid)molar stoichiometric ratio.Then the beaker neck was sealed with PE membrane and kept in a 90℃water bath.After reacting for a while，the specimens got smaller and smaller.The reaction efficiency varied with different time.The residue products， involving solid， liquid and gas phase，were collected for analysis.The solid products were cleaned by 50 mL acetone and 50 mL NaOH，respectively.After drying for 12 h at 120℃the solid products were weighted to calculate the decomposition rate β using the following formulate:

where W0is the weight of sample;W1is the weight of solid residue.

1.3 Analytical Method

FT-IR spectra of the virgin epoxy sample and residue solid products were obtained with a Thermo Nicolet Nexus 670 spectrometer.The sample was ground into powders and mixed with KBr powders，and the mixture was pressed into a disc，which was then placed in test chamber.

The soluble decomposition products of epoxy resins was dried at 35℃for 24 h and then dissolved in acetone.To investigate the structure changes of soluble decomposition products during the reaction，gas chromatography-mass spectrometry(GC-MS) was employed on the HP-6890GC(AGILENT，USA)using N2at the speed of 1.0 mL/min while the heating process from 20℃to 280℃was controlled at the rate of 20℃.All decomposition products were identified using the software of the mass spectrometer.

2 Results and Discussion

2.1 Influence of Different Curing Agents on Decomposition Rate

The time dependence of decomposition rate for specimens(E-44 as the matrix resin)with different curing agents was compared in Fig.1.The results showed that the order of decomposition rate at the same time was as follows:E-44/PA651＞E-44/IPDA＞E-44/DDM ＞ E-44/EMI-2，4.The epoxy resin cured with PA651 could be degraded completely for 2 hours，while the decomposition rate of others were 57.0%(E-44/IPDA)，43.33%(E-44/DDM)and 0.46%(E-44/EMI-2，4).The maximum decomposition rates in this experiment were 100% for 2 hours(E-44/PA651)，92.72%for 4 hours(E-44/IPDA)，100%for 9 hours(E-44/DDM)，and 18.54%for 5 hours(E-44/EMI-2，4).

Fig.1 Decomposition rates of E-44 cured with different curing agents

These results could be explained from different molecular structures of curing agents which led to cross-linked networks of epoxy resins.Comparing these curing agents，it was found that PA651 molecule had a great amount of-CH2-groups，which made the whole molecule chain revolve easily.As is known，the curing agents played a role as the connection points in the whole networks.And the molecule structures of the connection points of E-44/PA651 were easily destroyed，resulting in the collapse of polymeric network easily.This was the reason why the decomposition rate of E-44/PA651 was the highest under the same condition.

Referred to IPDA，the C-N bond and naphthene structure allowed for the higher stability of polymeric network.This was due to that the presence of naphthene structure retards the main chain to revolve.Besides，naphthene structure was also much harder for decomposition.The same reason could be applied to DDM.Benzene ring in the structure of DDM，the rigid and thermal resistant group，restrained revolving of the main chain so that the network was relatively stable.The formation of the conjugate π bonds involves isolated pair electrons at N atoms，which was shared by two benzene rings.Because H in the nitric acid had electroaffinity with the isolated pair electron in the N atom and thus with the increase of strength of electrophilic activities，the isolated pair electrons were constrained more by conjugate function and the polymeric network was stabilized by these conjugate π bonds.Therefore，the decomposition process was performed more difficult.This was the reason why E-44/DDM system hasrelatively lower decomposition rate.

Referring to EMI-2，4，the unsaturated and benzazole structure was the direct reason resulting in that EMI-2，4 as curing agent was harder to be dissolved than other ones.Two N atoms in imidazole were in the conjugate five-ring structure and formed the π - π conjugate effect，ensuring the stability of polymeric network.

2.1.1 FT-IR analysis

Fig.2 shows the IR spectra of residue solid products for four different curing systems.According to these IR spectra，the position of absorbance peak did not change greatly from each other，and it was proved that the solid products with different curing agents had similar chemical structure.The differences of peak strength were due to different kinds of curing agents.A wide and flat peak observed at 3400 cm-1could be recognized as the vibrations of-OH group，which was a symbol of hydrogen bond.A small peak in 2976 cm-1was related to—CH3while the absorption peak at 1700 cm-1in IPDA and EMI-2，4 showed the existence of carbonyl.The characteristic band of benzene rings vibration was observed at about 1660 cm-1in IPDA and DDM.The absorption peak at 822 cm-1，which belonged to the benzene rings，was a symbol peak of epoxy resin.

Fig.2 FT-IR spectra of residue solid products of E-44 cured with four different curing agents

The IR spectra of virgin epoxy sample cured with IPDA is shown in Fig.3.Compared with the results in this figure，the strength of peak at 2976 cm-1in Fig.2，which was the symbol of methyl of cured epoxy-resin chain，had been greatly weakened.This result proved that the polymeric network had been destroyed successfully.The absorption peaks of C-N bonds in Fig.3 at 1031 cm-1and 1257 cm-1indicated that the network of epoxy resin cured by IPDA was formed completely.Additionally，in Fig.2 the peaks in 1031 cm-1and 1257 cm-1were weakened or even disappeared.All of these results demonstrated that the epoxy resin structure had been dissolved.

Fig.3 FT-IR spectrum of virgin epoxy resin

2.1.2 GC-MS analysis

GC-MS analysis was performed to assure the specific reaction products clearly and specific amount of substances was listed in Tab.1 in the residue of E-44/IPDA after reacting for 4 h.

Tab.1 Percentage of dissociation products amount of E-44/IPDA decomposed for 4 h from GC-MS analysis

The presence and percentages of Bisphenol A demonstrated that epoxy resin polymeric network was destroyed completely.Bisphenol A was further degraded into phenol and its homolog.Due to the addition of nitric acid，the majority of phenol was converted into nitrophenol.

Tab.2 lists the results of different amount of substances in the residue of E-44/DDM after decomposition for 10 hours.This analysis showed that the main substances in the reaction production were nitrophenol and 4-hydroxy-3-nitrobenzoic acid.Considering the results of E-44/IPDA，it could be inferred that the bonds of C-O，C-N and the chains between benzene rings in Bisphenol A were cracked.

Besides，the gases produced in the reaction process of E-44/DDM were also characterized by GCMS，and the result is shown in Fig.4.In this spectrum，the main peaks were the ones at 2.87 min，4.39 min and 6.81min，which were analyzed as toluene，butyrolactone and benzoic acid，respectively.According to the aforementioned results，it could be inferred that the bond of C-O and the chains between benzene rings in Bisphenol A were probably destroyed.

Tab.2 Percentage of dissociation products amount of E-44/DDM decomposed for 10 h from GC-MS analysis

Fig.4 GC curve of gas products of E-44/DDM after reacting for 10 h

2.2 Influence of Different Epoxy Resins on Decomposition Rate

In the experiments，four types of epoxy resins，namely E-44，E-51，AG-80 and TDE-85 were chosen to investigate the effect of resin type.IPDA was used as the curing agent.The samples were prepared with a fixed molar stoichiometric ratio of resins and curing agents.Fig.5 shows the decomposition rate of four kinds of samples in nitric acid.It was easily observed that the decomposition rates for 1h were 38.16%(E-44/IPDA)，50.55%(E-51/IPDA)，100%(AG-80/IPDA)and 100%(TDE-85/IPDA)，respectively.The decomposition of these samples decreased as the epoxy value increased progressively from E-44，E-51，AG-80 to TDE-85.

Based on the experiment results，C-N cracking could be considered as the primary reason leading to the different decomposition rate of different cross-linked networks.In the discussion before，the epoxy value of E-51 was higher than E-44 and thus more epoxy group exists for the equal weighting epoxy resin.And the density of C-N bond also rose accordingly with the increase of epoxy value.The functional degree of AG-80 and TDE-85 are four and three，respectively.Therefore，their decomposition rate was greatly higher thanthat of E-44/IPDA and E-51/IPDA.According to the experimental results，it could be concluded that C-N bond cracking was one of the most possible ways for the collapse of the polymeric network.

Fig.5 Decomposition rates of four different kinds of epoxy resins cured with IPDA

2.3 Mechanism Analysis

Considering the experimental result discussed above，the mechanism of decomposing epoxy-resins with nitric acid is envisaged as follows.

2.3.1 C-N bonds cracking

The curing agents used in the experiment are all amine curing agents so that N atoms are brought into the epoxy resin system.N atom has one isolated pair electron，which is inclined to be attacked by H+in the nitric acid，and then C-N bonds crack under this circumstance，the molecular chains of epoxy resin are destroyed in the reaction.

H+is quite active under high temperature and certain acid concentration.It attacks N firstly and because N is too unsteady to exist，the positive charge is transferred quickly to areas with abundant electrons.And this transfer can be performed in two ways.One way is that N+is attached to the alicyclic group to produce the C+ion and then the ring loses one H+ion to form the double bond.The other way is as follows:the N+transfer，to the main chain，will cause it to crack and then CH2+ion is produced.Because CH2+is not stable and easy to lose one H+ion，and thus the double bonds in the network are formed.Besides，the hydroxyl groups in the main chain are apt to be isomerizated to the carbonyl groups.In our opinion，this way of bond cracking is the most possible method of the nitric acid decomposition mechanism(as shown in Fig.6).

Fig.6 Possible degradation mechanism of epoxy resin decomposed by nitric acid mainly in the form of C-N bonds cracking

2.3.2 Ether bonds cracking

Another possible way of the network decomposition is ether bonds cracking(as shown in Fig.7).In the ether bonds，oxygen atom has two isolated pair electrons to be attacked by H+.However，because this atom is affected by conjugation of benzene ring，it is hard to crack this ether bond.But assuming that high temperature and concentration of NO3-ions are provided，the oxygen atom can be polarized and produce the instant dipole moments so that the activation energy of this reaction is decreased.Then the H+ion has enough energy to attack the oxygen atom.After the oxygen atom is attacked，there are two ways for the transfer of positive charges.One way is to transfer to the main chain and then form the carbonyl group，whereas the other one is to transfer to the benzene ring.The conjugation of π bond can disperse positive charge easily.Besides，since benzyl propyl carbocation is more stable，positive charges are apt to transfer to this structure，of which the total energy is lower.Then this positive ion is cracked and propylene phenol is produced.The strong oxidation by acid leads to fragmentation of propenyland produce of4-hydroxy-3-nitrobenzoic acid.After the ether bonds cracking，phenol is formed.Phenol is nitrified in different extent in the thermal and thick decomposition liquor，and produce the nitrophenol，dinitrophenol，trinitrophenol，and so on.

Fig.7 Possible degradation mechanism of epoxy resin decomposed by nitric acid mainly in the form of ether bonds cracking

3 Conclusions

1)On the decomposition condition of 90℃，8 mol/L nitric acid concentration and 1 g:50 mL(resin:resin)，epoxy resins cured with different agents could be decomposed well.Decomposition rate varied greatly with different curing agents and the order was as follows:E-44/PA651＞E-44/IPDA＞E-44/DDM＞E-44/EMI-2，4.

2)With IPDA as the curing agent，four kinds of epoxy resins were chosen to be dissolved in nitric acid.After reacting for 1 hour，AG-80/IPDA and TDE-85/IPDA were decomposed completely while the decomposition rates for E-44/IPDA and E-51/IPDA were38.16%and 50.55%respectively.

3)Based on the decomposition curves，GC-MS analysis and IR spectra，the decomposition mechanism of epoxy resin could be inferred，which was considered including two possible ways:C-N bonds cracking and ether bonds cracking.

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