Synthesis of phosphite-type trifunctional cycloaliphatic epoxide and the decrosslinking behavior of its cured network

Polymer 54 (2013) 5182e5187

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Synthesis of phosphite-type trifunctional cycloaliphatic epoxide and the decrosslinking behavior of its cured network Zhuo Chen, Linni Zhao, Zhonggang Wang* State Key Laboratory of Fine Chemicals, Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Zhongshan Road 158, Dalian 116012, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 March 2013 Received in revised form 9 July 2013 Accepted 18 July 2013 Available online 27 July 2013

The motivation of the present work is to design and synthesize reworkable epoxy resin for electronic packaging which are required to be sufficiently stable before 200
C and can rapidly decompose in the temperature range of 200e300
C. For this purpose, a new trifunctional cycloaliphatic epoxide (Epo-A) with a phosphite center linked by three epoxycyclohexyl groups was prepared. The chemical structure was confirmed by FTIR, 1H NMR and 31P NMR spectra. Compared to the phosphate-type analog (Epo-B) and commercial epoxide ERL-4221, Epo-A exhibits the apparently higher curing reactivity. The shearing strength of the cured Epo-A at room temperature is 5.67 MPa, much superior to that of ERL-4221 (3.29 MPa). More importantly, the cured Epo-A can maintain the high shearing strength up to 210
C. Upon further increasing temperature, the network rapidly decomposes, and the strength almost completely losses at around 255
C, just lying in the desirable temperature range for reworking operation. As a result, the dismantlement of the integrated circuit or the replacement of the faulty chip can be realized without damaging the circuit board. In addition, the incorporation of phosphorus in the network results in a significantly increased limiting oxygen index (LOI) from 18.2 (ERL-4221) to 23.2 (Epo-A). The mechanism for the degradation behavior was studied by isothermal and temperature-variable FT-IR spectra in detail. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Cycloaliphatic epoxide Phosphorus Reworkable

1. Introduction Densely crosslinked structure endows the epoxy resins with excellent chemical resistance, high thermal stability, good adhesion strength, and long-term service reliability. On the other hand, however, the three-dimensional network also means that the thermosetting materials are insoluble and infusible. This characteristic makes the dismantlement of a product, such as integrated circuit and optoelectronic device encapsulated with epoxy resin, to be extremely difficult in the event that the repair, replacement and recovery of components are needed. Currently, large amount of electronic wastes are still be simply treated by landfill and incineration, which inevitably bring about serious second pollution to air and ground. Considering the environmental issue, it is strongly desirable to develop new epoxy resins with “reworkable” function, i.e., the cured network can rapidly decrosslink under a controlled

* Corresponding author. E-mail address: [email protected] (Z. Wang). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.07.048

condition so that the products can be conveniently dismantled and recovered [1e6]. From the viewpoint of practical application as electronic packaging materials, this class of epoxy resins should simultaneously meet two primary requirements: (i) the cured products should be thermal stable up to 200
C, and (ii) the epoxy network can rapidly decrosslink in the range of 200e300
C. In fact, the maximum degradation at the temperature of around 250
C is the most desirable. Thus, the reworking operation is energy-saving and can be conducted without damaging the properties of the components and plastic circuit board (PCB) [7e12]. Conventional epoxy resins usually have high thermal stability over 300
C. In the past decade, considerable efforts have been made to reduce the decomposition temperature by incorporating thermally-labile groups into the crosslinked network. Ober synthesized a series of epoxies with primary, secondary, and tertiary ester linkages, and found that the networks containing tertiary esters had the maximum weight-loss temperature at 200e300
C [13,14]. We reported that the introduction of weak tertiary carboneether linkages in the cycloaliphatic diepoxides could significantly decrease degradation temperature [12]. The epoxy resins containing carbamate groups from Wong’s group started to decompose at 220
C

Z. Chen et al. / Polymer 54 (2013) 5182e5187

[15,16]. Musa developed reworkable thermosets materials by utilizing acetal ester groups as thermally degradable linkages [4]. Although the decomposition temperatures of the above network-decrosslinkable epoxy resins fall between 200 and 300
C, their degradation rates are slow and the pyrolysis takes place over a wide temperature range. As a consequence, the disassembly process is time consuming, and the residual fragments adhering on the components and PCB board because of the insufficient decomposition are difficult to be removed. To solve the above problems, recently, cycloaliphatic epoxides with phosphate groups were synthesized in our group [8]. After curing, the product exhibits a rapid degradation, but the temperatures corresponding to the maximum degradation rate are around 280
C, which is still higher than the desired temperature, and needs to be slightly reduced. In addition, the results in our previous study [8] have demonstrated that the CeO bond in the O]PeOeCe is less stable than that in the O]CeOeCe linkage, which suggests that the stability of CeO bond is strongly affected by the electron-withdrawing effect of the neighboring group. Therefore, the replacement of O]PeOeCe with PeOeCe in the triepoxide is expected to further reduce the decomposing temperature of the cured network. As a continuation of the above study, herein, a new trifunctional phosphite-type epoxy resin was designed and synthesized. The comparisons with the phosphate analog (Epo-B) and commercial epoxy resin ERL-4221 on their cure kinetics, thermaldecrosslinking mechanism, flame retardancy and mechanical property of the cured products are conducted. The incorporation of the weaker phosphite linkages in the network will effectively reduce the degradation temperature and accelerate the thermal pyrolysis of the product. Moreover, in the previous papers, the thermal-decrosslinking behaviors are mainly characterized by thermogravimetric analysis method. In this study, the dependency of the adhesion strength of the epoxy resins on temperature was also investigated in detail. These results are especially important for the practical reworking operation.

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2.2. Measurements Fourier-transform infrared spectra (FTIR) were recorded on a Nicolet 5700 spectrometer. Liquid samples were measured by casting film on KBr salt tablets, whereas solid samples were measured using pallets prepared by compressing the dispersed mixture of sample and KBr powder. 1 H NMR and 31P NMR were recoded on an INOVA-400 NMR spectrometer (Varian) using CDCl3 as solvents, tetramethylsilane (TMS) and phosphoric acid as internal standards, respectively. DSC curves were recorded on a NETZSCH DSC204 using N2 as a purge gas (20 ml/min) at a heating rate of 5
C/min, 10
C/min, 15
C/min and 20
C/min respectively. About 5e10 mg cured product was put into the aluminum pan for DSC measurement. TGA measurements were performed on NETZSCH TG209C under N2 atmosphere over a temperature range of 25e800
C with a heating rate of 10
C/min and the gas flow rates of 60 ml/min. Isothermal TGA measurements were performed on NETZSCH TG209C under N2 atmosphere at 240
C. LOI tests were performed according to the standard of ISO45891984 with test specimen bar of 120  6.5  3 mm3. The samples were ignited by a Bunsen burner vertically, and the flame was removed and the timer was started. The concentration of oxygen was raised if the specimen was extinguished before burning 5 cm or 3 min. The oxygen content was adjusted until the limiting concentration was determined. For each sample, ten specimen bars were measured, and the obtained LOI values were averaged. Shearing strength tests were performed on INSTRON-5567A according to the standard of ISO4587-1979. Epoxy resin was cured between the joint of two steel plates with overlapping area (S) of 25 mm  12.5 mm. The sample was carried out with a gradually increased shearing force with a speed of 5 mm/min until the bonded plane was broken. The maximum load (P, N) was recorded, and the shearing strength (r, MPa) was calculated from the equation: r ¼ P=S. For each sample, five specimens were measured, and the obtained values were averaged.

2. Experimental 2.3. Synthesis of tris(cyclohex-3-enylmethyl) phosphite (olefin-phosphite)

2.1. Materials Cyclohex-3-enyl-1-methanol, phosphorus trichloride, and OXONE (a monopersulfate compound: 2KHSO5$KHSO4$K2SO4) were purchased from Aldrich Chemical Company and used without further purification. Hexahydro-4-methylphthalic anhydride (HMPA) and 2-ethyl-4-methy-limidazole (EMI) were used as a curing agent and curing accelerator, respectively. 3,4-Epoxycyclohexylmethyl 30 ,40 -epoxycyclohexane carboxylate (trade name ERL-4221) was purchased from Xinjin chemicals Co., while the phosphate-type trifunctional epoxide Epo-B was synthesized according to the procedure described in the paper [8]. Their chemical structures are shown in Scheme 1.

O O P O

O

O

Cyclohex-3-enyl-1-methanol (56 g, 0.5 mol), triethylamine (70 ml, 0.5 mol), and dichloromethane (150 ml) were added into a 500-ml four-neck round-bottom flask equipped with a mechanical stirrer, a nitrogen inlet, a thermometer and an addition funnel with drying tube. To the above solution, phosphorus trichloride (10 ml, 0.11 mol) dissolved in dichloromethane (20 ml) was added drop wise over a period of 40 min. Then the reaction was allowed to proceed at room temperature for an additional 18 h. The resultant mixture was washed with dilute HCl and Na2CO3 aqueous solutions and finally deionized water to neutrality. The organic phase was dried over MgSO4. After filtration, the filtrate was evaporated on a rotary evaporator. The residue was distilled under reduced pressure to remove the excess cyclohex-3-enyl-1-methanol. The colorless liquid product was obtained with yield of 65%. FTIR (cm1): 3023, 2916, 2839, 1652, 1467, 1263, 1045, 967, 734, 655. 1H NMR (CDCl3/ TMS, ppm): 5.62e5.72 (s, 6H, ]CHe), 3.88e4.02 (m, 6H, eCH2eO), 1.22e2.30 (m, 21H, eCH2e, eCHe).

O

O

2.4. Synthesis of tris(3,4-epoxycyclohexylmethyl) phosphite (Epo-A) O

O

O

Epo-B

ERL-4221

Scheme 1. Chemical structures of Epo-B and ERL-4221.

O

Into a 1000-ml four-necked round-bottom flask quipped with a mechanical stirrer, a pH meter and a dropping funnels were charged olefin-phosphite (20 g, 170 mmol), CH2Cl2 (250 ml), acetone (200 ml), and 18-crown-6 ether (2.5 g). Under the nitrogen atmosphere and vigorously stirring at about 50
C, OXONE (135 g,

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225 mmol) and ethylenediaminetetraacetic acid (0.2 g, 0.6 mmol) in 240 ml of deionized water were added. The reaction was continued for 24 h, and then the organic phase was collected and the aqueous phase was extracted with CH2Cl2. The organic portions were combined, washed with deionized water, and dried over MgSO4. After filtration, the filtrate was evaporated on a rotary evaporator to give 6.4 g liquid with yield of 75%. FTIR (cm1): 3003, 2928, 2835, 1432, 1255, 1039, 975, 825, 785. 1H NMR (CDCl3/TMS, ppm): 3.88e4.02 (m, 6H, eCH2eO), 3.10e3.20 (m, 6H, CH on epoxide ring), 1.01e2.17 (m, 21H, eCH2e, eCHe). 31P NMR (CDCl3/ phosphoric acid, ppm): 9.04 (a single peak).

3025

1652

(b) 1255 2928

825

2988

2835 785

2.5. Curing of epoxides Epo-A and HMPA were mixed with a molar stoichiometric ratio of 1:0.8 at room temperature. Into this mixture 0.5 wt% of EMI was added as a curing accelerator. The mixture was stirred and vacuum deaerated for half an hour, and then it was cured at 140
C for 2 h, 170
C for 4 h, and 190
C for another 2 h. As a comparison, the phosphate-type epoxide Epo-B and commercial epoxide ERL-4221 were also cured under the same condition as above. 3. Results and discussion 3.1. Synthesis and characterization The CleO bonds in phosphorus trichloride are quite active, which could readily react with the hydroxyl groups of cyclohex-3enyl-1-methanol to produce the tricyclohexenyl compound (olefin-phosphite), using triethylamine to adsorb the hydrogen chloride generated in the system. Then, olefin-phosphite was epoxided by inorganic oxidant OXONE to yield the target trifunctional cycloaliphatic epoxy resin Epo-A (Scheme 2). The successful syntheses of Epo-A and its olefin precursor olefin-phosphite were confirmed by FTIR, 1H NMR and 31P NMR spectra. In the FTIR spectra (Fig. 1), for olefin-phosphite, the peak at 1652 cm1 corresponds to the absorption of C]C group. The strong band for PeOeC bond is observed at 967 cm1. After epoxidation reaction, the former C]C absorption completely disappears, while the characteristic bands for cycloaliphatic epoxy groups at 825 cm1 and 785 cm1 appear. In the 1H NMR spectrum

OH

O P O O

P Cl Cl Cl

(a) 4000

3500

3000

2500

1500

1000

-1

Wavenumber (cm ) Fig. 1. FTIR spectra of Epo-A (a) and its olefin precursor (b).

of olefin-phosphite (Fig. 2a), the signals at 5.62e5.72 and 3.88e 4.02 ppm are attributed to the protons on CH]CH and CH2eO, respectively. The other alicyclic protons locate at 1.22e2.30 ppm. The comparison between olefin-phosphite and its epoxided product Epo-A reveals that all the CH]CHs have been converted to epoxy groups (Fig. 2b). Furthermore, in the 31P NMR spectrum of Epo-A (Fig. S1, Supporting Information), only a single peak at 9.04 ppm was observed, indicating that the phosphite center is indeed symmetrically linked with three epoxycyclohexyl groups. 3.2. Curing reaction The thermal curings of Epo-A, Epo-B and ERL-4221 in the presence of curing agent HMPA and curing accelerator EMI were investigated by the Kissinger method [17e20] as below:

i h  d ln b=Tp2 Ea   ¼  R d 1=Tp

(1)

where b is the heating rata, Tp the peak temperature, Ea the activation energy, and R is the gas constant, respectively. All the DSC curves display a single exothermic peak (Fig. S2eS4). The plots of ln ðb=Tp2 Þ vs. reciprocal peak temperature (1/Tp) for the 2-5

6-7

Et3 N/CH2Cl2

2000

5 21

6 7

4

1

O P O

3

O

(Olefin) (b) 6'-7'

O 1' 6' 5' 2'

O P O O

m-CPBA CH2Cl2

O 7' 4'

3'

OP O

O

O

1'

2'-5'

O O

(a) O

7

6

(Epo-A) Scheme 2. Synthetic route to the phosphite-type triepoxide.

5

4

3

2

Chemical Shift(ppm) 1

Fig. 2. H NMR spectra of Epo-A (a) and its olefin precursor (b).

1

Z. Chen et al. / Polymer 54 (2013) 5182e5187

three epoxides are illustrated in Fig. 3, from which the reaction activation energies were calculated. Interestingly, the results show that the Ea value of Epo-A (48.4 kJ/mol) is much lower than those of Epo-B (57.5 kJ/mol) and ERL-4221 (59.5 kJ/mol), indicative of EpoA the apparently higher reaction activity. The faster curing rate of Epo-A is especially favorable for the practical electronic encapsulating process. The catalytic effect of trivalent phosphite could be an important influencing factor for the high reactivity of Epo-A. Another possible reason may be attributed to the difference of electron density surrounding the epoxycyclohexyl groups since the similar phenomena have also been observed in other cycloaliphatic epoxy resins. For example, relative to ether bond, the introduction of ester and imide bonds with strong electron-withdrawing ability results into the epoxides greatly decreased reactivity [21,22]. Compared to ester bond in ERL-4221 and phosphate bond in Epo-B, the lower electron-withdrawing ability of phosphite bond in Epo-A is advantageous for its reactivity with anhydride curing agents. The curing reaction of Epo-A was examined by FTIR spectroscopy. The comparison of the FTIR spectra of HMPA, Epo-A, and the cured Epo-A is presented in Fig. 4. Before curing, the characteristic absorptions at 1863 and 1778 cm1 are attributed to the anhydride group of HMPA, while the peaks at 825 and 785 cm1 are characteristic of the epoxy group of Epo-A (Fig. 4a and b). After curing, all the above bands disappear, and instead, a new strong absorption at 1739 cm1 appears, which is attributed to the ester group formed by the reaction between epoxy and anhydride groups, demonstrating that the curing reaction between HMPA and Epo-A is complete. 3.3. Thermal degradation behavior The comparisons of thermal degradation behaviors between Epo-A, Epo-B and ERL-4221 were studied by TGA and isothermal TGA methods. Fig. 5 and the data in Table S1 (Supporting Information) show that the cured Epo-A, Epo-B and ERL-4221 have the initial decomposition temperatures of 237
C, 257
C and 316
C, and the maximum weight loss occurs at 266
C, 279
C and 368
C, respectively. Relative to ERL-4221, the two phosphoruscontaining epoxy resins exhibit significantly lower degradation temperatures. In addition, compared to other reworkable epoxy resins reported in the literature [12e16], the weak phosphite and phosphate bond are very sensitive to temperature, as reflecting by the very quick weight loss within a very narrow temperature range. Moreover, the initial decomposition temperature of Epo-A is lower

5185

1739

(c) 825 785

(b) 1778

1863

(a)

4000

3500

3000

2500

2000

1500 -1

1000

500

Wavenumber (cm )

Fig. 4. FTIR spectra of HMPA (a), uncured Epo-A (b) and the cured EPo-A (c).

than Epo-B by 20
C, which is advantageous considering the convenient dismantling operation and energy-cost issues. In fact, Epo-A, Epo-B and ERL-4221 were cured with the same curing agent (HMPA) and accelerator (EMI) under the same curing condition. The only difference lies in that the net nodes in Epo-A are phosphite bonds, whereas those in Epo-B and ERL-4221 are phosphates and esters, respectively. The above TGA results indicate that, among the three net nodes, phosphite is the most thermal unstable, leading to that the Epo-A crosslinking network starts to decompose at the lowest temperature. The isothermal TGA tests at 240
C were carried out for the two phosphorus-containing samples to compare their degradation rate and sensitivity to temperature (Fig. 6), and the data are collected in Table S2. As can be seen, the cured Epo-A nearly completely degrades within about 7 min. The time is far shorter than that of EpoB sample (about 40 min), meaning that the disassembly and repair of an electronic product encapsulated with Epo-A are more timeand energy-saving compared to Epo-B and other reworkable epoxy resins. On the other hand, as the measure of the adhesion ability on the substrate, the shearing strengths of the cured Epo-A, Epo-B and ERL-4221 were measured. At 25
C, the values of Epo-A and Epo-B

-9.2

-9.6

ERL-4221

Residual weight (%)

Epo-A Epo-B

2

ln(q/Tp )

100 -9.4

-9.8 -10.0 -10.2

Epo-A Epo-B ERL-4221

80

60

40

-10.4

20 -10.6 -10.8 0.00200

0.00205

0.00210

0.00215

0.00220

0.00225

1/Tp Fig. 3. Kinetics analysis of three epoxides using HMMPA as a curing agent.

0 100

200

300

400

500

600

O

Temperature ( C) Fig. 5. TGA curves the three cured epoxides under air atmosphere.

700

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Z. Chen et al. / Polymer 54 (2013) 5182e5187

3.4. Thermal degradation mechanism

Residual Weight (%)

100

Epo-A Epo-B

90 80 70 60 50 40 30 20 0

5

10

15

20

25

30

35

40

45

50

55

60

Time (min) Fig. 6. Isothermal TGA curves at 240
C air atmosphere for the cured epoxides.

are 5.67 and 5.88 MPa, respectively, much higher than that of commercial ERL-4221 (3.29 MPa). The reason can be attributed to two factors: first, the incorporation of phosphite or phosphate groups results in Epo-A and Epo-B the larger polarity, which is advantageous for the enhancement of interaction force between resin and substrate; second, after curing, the trifunctional Epo-A and Epo-B have the higher crosslinking densities than the difunctional ERL-4221. Furthermore, for the cured Epo-A and Epo-B, the dependency of the shearing strengths on temperature in the range from 25
C to 330
C was examined. As can be seen from Fig. 7, the cured Epo-A can maintain the high shearing strength up to 210
C. However, with the continual growth of temperature, the strengths dramatically decrease from 5.48 MPa (at 210
C) to 0.50 MPa (at 258
C). The results mean that, if this material is applied for electronic packaging, at around 250
C, the faulty chips can be easily disassembled from the circuit board. Epo-B exhibits a similar trend to Epo-A, but the curve apparently shifts toward the higher temperature. For example, the temperature for Epo-B corresponding to the shearing strength of 0.50 MPa has exceeded 320
C, whereas the circuit board cannot resist so high temperature. In this regard, the newly synthesized Epo-A has significant advantage over Epo-B as a reworkable electronic packaging material.

The degraded residues of the cured Epo-A after thermaltreatment at 240
C for different time, and finally at 320
C for 20 min were examined by FTIR spectroscopy. As shown in Fig. 8, before degradation, the sample displays a strong absorption at 1739 cm1 attributed to the ester bond formed by the curing reaction between epoxy group and anhydride curing agent. The bands at 2865 and 2949 cm1 are corresponded to the aliphatic CeH stretching vibration, whereas that at around 1245 cm1 is due to the phosphite linkage. After treatment at 240
C for 5 min, the absorption intensity of phosphite band apparently reduces, and this trend becomes more pronounced with the prolonged degradation time, indicative of the cleavage of OeC bond of phosphite moiety in the network. The generated phosphorous acid is demonstrated by the emerged wide adsorption in the range from 2100 to 3500 cm1 [23,24]. Interestingly, it is found that, after degradation at 240
C for 10 min, the former adsorption at 1739 cm1 almost disappears, which suggests that the strong phosphorous acid generated can catalyze the pyrolysis of the other adjacent ester bonds, and thereby effectively accelerate the collapse of network. The above mechanism can well explain the reason why Epo-A exhibits the significantly faster degradation rate than the other degradable epoxy resins reported previously. Moreover, the comparison between curve c and curve d shows that the samples degraded at 240
C for 10 min and that at 320
C for 20 min have very similar spectra, implying that the network pyrolysis almost complete at 240
C within only 10 min, which is well consistent with the isothermal TGA result. The broadened and enhanced bands in curve d at 1250e1300 cm1 are attributed to the PeOePeO linkage formed by the condensation of phosphorous acid at high temperature. In addition to the residues, the volatilized vapors during isothermal degradation at 240
C for 5 min and 10 min were collected and analyzed, respectively. Their FTIR spectra are presented in Fig. 9. The characteristic absorptions of aliphatic CeH vibration at 2941 and 2858 cm1 mean that the volatilized vapors consist of large amount of cycloaliphatic fragments. The appearance of adsorption at 1704 cm1 due to carboxylic group demonstrates that ester bonds in the network are indeed cleaved by the strong phosphorous acid, supported by the observation in Fig. 7 that the

(d) Epo-A Epo-B

Shear Strength (MPa)

6

(c)

5

4

(b)

3

2

(a)

1

0 30

60

90

120 150 180 210 240 270 300 330 360

Temperature (oC) Fig. 7. Shearing strengths of the cured epoxides after thermal-treatment for 3 min at different temperatures.

4000 3500

3000

2500

2000

1500 -1

1000

500

Wavenumber (cm )

Fig. 8. FTIR spectra of the cured Epo-A after thermal-treatment under air for (a) 0 min/ 240
C, (b) 5 min/240
C, (c) 10 min/240
C, and (d) 20 min/320
C.

Z. Chen et al. / Polymer 54 (2013) 5182e5187

(b)

(a)

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 9. FTIR spectra of the volatilized products of the cured Epo-A after thermaltreatment at 240
C for (a) 5 min, and (b) 10 min.

band of ester bond almost disappears after 10 min of degradation. At high temperature, the carboxylic groups are liable to dehydrate to form anhydride, as indicated by the strong bands at 1788 and 1862 cm1. 3.5. Flame-retardant property The TGA results exhibit that the residual weight of cured Epo-A at 700
C is 10.5 wt%, whereas that of ERL-4221 is only about 0.6 wt% (Table S1). The analysis of FTIR spectra reveals that the solid residue of Epo-A is mainly the condensed product of phosphorous acid fragments cleaved from the network at high temperature, which acts as a protection layer to retard the further combustion of polymers. Therefore, the incorporation of phosphorous element into epoxy networks results into remarkably improved flame-retardant property. For example, the commercial epoxy ERL-4221 has a low LOI value of 18.2, whereas the LOI value of the cured phosphite-type Epo-A is 23.2. In addition, the LOI value of Epo-A is also higher than that of phosphate-type Epo-B (22.7) [8] because there is the slightly higher phosphorus content in Epo-A than Epo-B. 4. Conclusions Novel trifunctional phosphite-type cycloaliphatic epoxide (Epo-A) was designed and synthesized from phosphorus trichloride and cyclohex-3-enyl-1-methanol through the condensation reaction. The FTIR, 1H NMR and 31P NMR spectra confirm the chemical structure. The comparative studies on curing kinetics of the monomers and thermal degradation behavior, mechanical property, flameretardancy of the curing products between Epo-A, phosphate-type

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epoxide Epo-B, and the commercial epoxide ERL-4221 were conducted. The newly synthesized Epo-A had the significantly higher curing activity, as indicated by its much lower activation energy of curing reaction (48.4 kJ/mol) than Epo-B (57.5 kJ/mol) and ERL-4221 (59.5 kJ/mol). The shearing strength of the cured Epo-A was 5.67 MPa, apparently higher than that of ERL-4221 (3.29 MPa). On the other hand, the cured Epo-A could maintain the high shearing strength up to 210
C, but the strengths dramatically decreased to 0.50 MPa (at 258
C), which property is especially desirable for the convenient disassemble or repair of the electronic products. The thermal degradation mechanism of the cured Epo-A was investigated by TGA and FT-IR spectroscopy. The results showed that the network decomposition was initiated by the cleavage of the weak OeC bond of phosphite linkage. The generated strong phosphorous acid could catalyze the pyrolysis of the other adjacent ester bonds, and effectively accelerated the collapse of whole network, leading to the significantly faster degradation rate of the cured Epo-A than the other degradable epoxy resins reported previously. Acknowledgments The support from the Program for New Century Excellent Talents in University of China (Grant No. NCET-06-0280) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2013.07.048. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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