Phosphorus-containing liquid cycloaliphatic epoxy resins for reworkable environment-friendly electronic packaging materials

Article abstract of DOI:10.1016/j.polymer.2010.08.039

Novel thermally reworkable, phosphorus-containing, di- and tri-functional liquid cycloaliphatic epoxy resins were designed and synthesized. Their chemical structures were characterized by means of MS, FTIR, ¹H NMR and ³¹P NMR methods

Full text of DOI:10.1016/j.polymer.2010.08.039

Polymer 51 (2010) 4776e4783
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Phosphorus-containing liquid cycloaliphatic epoxy resins for reworkable
environment-friendly electronic packaging materials
*
Wanshuang Liu, Zhonggang Wang , Li Xiong, Linni Zhao
Department of Polymer Science and Engineering, Key Laboratory of Polymer Science and Engineering of Liaoning Province, State Key Laboratory of Fine Chemicals,
School of Chemical Engineering, Dalian University of Technology, Zhongshan Road 158, Dalian, 116012, P. R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel thermally reworkable, phosphorus-containing, di- and tri-functional liquid cycloaliphatic epoxy
resins were designed and synthesized. Their chemical structures were characterized by means of MS,
FTIR, 1H NMR and 31P NMR methods. After curing, the products were transparent and stable up to 220 ꢀC,
while exhibited quick thermal decomposition at the temperature range of 255e280 ꢀC. The removal test
showed that, after heat-treatment at 260 ꢀC in air atmosphere for only 4 min, the residual char on the
glass substrate could be easily wiped off. This unique degradation behavior was attributed to the
synergistic effect of two factors: the thermally-labile phosphate groups evenly distributed within the
three-dimensional network and the in-situ catalyzing of phosphoric acid generated from the cleavage of
phosphate bond on the pyrolysis of adjacent other phosphate and ester groups, as evidenced by the
results of molecular modeling, isothermal TGA and FTIR spectra. In addition, compared to the commercial
cycloaliphatic epoxy resin ERL-4221, the newly synthesized epoxy resins had increased limiting oxygen
index (LOI) by 31%. The combination of excellent reworkability, non-halogen flame retardancy, high glass
transition temperature of 227 ꢀC and high mechanical modulus endows them the potential for envi-
ronment-friendly microelectronic and optoelectronic packaging applications.
Received 6 May 2010
Received in revised form
12 August 2010
Accepted 17 August 2010
Available online 24 August 2010
Keywords:
Epoxy resin
Synthesis
Electronic packaging
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
devices are mounted onto one high-density board. If one chip is
damaged, the whole expensive integrated module has to be dis-
Cycloaliphatic epoxy resins are thermosetting materials widely
employed as encapsulants in microelectronic industry, e.g. direct
chip attach (DCA), chip scale package (CSP) and chip on board for
multichip modules (MCM), or as underfill adhesive in flip chip
technology to help evenly distribute stress caused by thermal
cycling, improve heat shock properties, and enhance the reliability
of the solder interconnects [1e7]. However, after curing, epoxy
resins polymerize to form crosslinking network structure. The
intractability makes them extremely difficult for the disposal,
recycling and reuse of scrapped electronic products. Moreover,
even large amount of electronic wastes have been generated during
the production process since, when the components are encapsu-
lated using conventional epoxy resins, the high decomposition
temperature of over 300 ꢀC and insoluble and infusible nature
make the disassembly and repair almost impossible. This problem
is especially severe for the MCM and integrated optoelectronic
module packagings, in which multiple chips or optoelectronic
carded. It has been reported that the manufacturing loss due to
defective electronic components could be over 30% for MCM
packaging [8]. In this regard, electronic packaging materials, which
can offer reworkability and enable the convenient removal and
replacement of faulty chip without damaging the other compo-
nents and circuit board, are strongly desired to meet environmental
protection and cost-saving requirements [9e13].
To make an epoxy resin reworkable, the method can be adopted
to introduce either chemical or thermally-labile group into the
crosslinking structure. Under controlled condition, the weak bonds
decompose to make network collapse and the decreased physical
properties such as shearing strength and modulus can permit
localized removal of a defective chip. Buchwalter et al reported
epoxy resins containing acetal and ketal linkages, which could be
degraded by applying acid-containing solvent [14], but the
aggressive acid is not convenient for the localized operation of
integrated circuit. Thereafter, Ober [15,16], Shirai [17], and Wong
[18,19] et al developed thermally reworkable epoxies containing
secondary/tertiary ester or carbamate groups, which could
decompose in the favorable temperature range from 200 to 300 ꢀC.
In our previous report, the cured cycloaliphatic diepoxides
* Corresponding author.
E-mail address: zgwang@dlut.edu.cn (Z. Wang).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.08.039

W. Liu et al. / Polymer 51 (2010) 4776e4783
4777
Table 1
containing secondary and tertiary carboneether linkages started to
Chemical structures of ERL-4221, HMPA and EMI used in this study.
decompose at 220 and 239 ꢀC, respectively [20].
Besides the desired temperature, another key target for
reworkable epoxy is the decomposition rate and temperature
sensitivity in order to suit the quick, clean and localized rework
process. Nevertheless, although current reworkable epoxies have
the decomposition temperatures within 200e300 ꢀC, from the
viewpoint of practical application, their degradation rates are still
slow and the pyrolysis temperature ranges are relatively wide.
In addition, as electronic encapsulating materials, a great
concern about cycloaliphatic epoxies is their flammability. Typical
cycloaliphatic epoxy like ERL-4221 only has the LOI value of about
18. Bromine-containing epoxies have satisfied flame retardant
property, but the highly toxic hydrogen bromide, dibenzo-p-dioxin
and dibenzo-furan released during combustion have restricted
their applications due to the environmental reason [21,22]. Alter-
natively, organophosphorus epoxy resins are receiving much
attention because of their good flame retardant ability and the fire-
extinguishing mechanism which the incombustible phosphorous
char serving as an outer flame protection barrier, and therefore
release much less toxic gas and smoke into the atmosphere relative
to the brominated analogues [23e26].
Name
Structure
ERL-4221
HMPA
EMI
On the other hand, it is noteworthy that polymers containing
phosphate bonds usually possess relatively low thermal degrada-
tion temperature [27e29]. Keeping the above suggestions in mind,
the present work was undertaken to design and synthesize new
phosphorus-containing cycloaliphatic di- and tri-functional epoxy
resins with a combination of several favorable characteristics in one
epoxy molecule. After curing, the weak phosphate bond evenly
distributed within the crosslinking network can act as sensitive
temperature trigger to initiate thermal decomposition in the event
of necessity; the introduction of phosphorus element may result in
apparent enhancement in flame retardant property; the liquid state
at room temperature prior to curing and high glass transition
temperature after curing can be expected owing to their cycloali-
phatic structures. Their synthesis and characterizations, thermally
reworkability, flame retardancy and mechanical properties as well
as decomposition mechanism were investigated and discussed in
detail.
solution, phenylphosphoryl dichloride (39.0 g, 0.186 mol) was
added. Then the reaction was allowed to proceed at room
temperature for 36 h. The resultant mixture was washed succes-
sively with dilute HCl and Na2CO3 aqueous solution 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 (at 60e62 ꢀC,
6 Torr) and there was obtained 60.6 g liquid product. Yield: 91%.
FTIR (cmꢁ1): 3026, 2927, 2839, 1651, 1595, 1490, 1435, 1289, 1211,
1046, 1023, 943, 766. 1H NMR (CDCl3/TMS, ppm): 7.19e7.39 (m, 5H,
Ar-H), 5.70 (m, 4H,]CHe), 4.07 (m, 4H, eCH2eO), 1.33e2.18 (m,
14H, eCH2e, eCHe).
2.2.2. Synthesis of bis(3,4-epoxycyclohexylmethyl)phenyl
phosphate (Epoxide I)
Into a 1000 ml four-necked round-bottom flask quipped with
a mechanical stirrer, a pH meter and a dropping funnels were
charged Olefin I (7.6 g, 20 mmol), CH2Cl2 (90 ml), acetone (90 ml),
and 18-crown-6 ether (0.9 g). Under the nitrogen atmosphere and
vigorously stirring at about 50 ꢀC, OXONE (45.0 g, 75 mmol) and
ethylenediaminetetraacetic acid (0.06 g, 0.2 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 filtra-
tion, the filtrate was evaporated on a rotary evaporator to give 6.4 g
liquid. Yield: 74%. HRMS (m/z): calcd for C20H27O6P 394.3975;
found, 394.1586 [M]þ. FTIR (cmꢁ1): 2925, 1595, 1490, 1439, 1279,
1213, 1046, 1025, 943, 904, 808, 788, 769. 1H NMR (CDCl3/TMS,
ppm): 7.16e7.36 (m, 5H, Ar-H), 3.91 (m, 4H, eCH2eO), 3.15 (m, 4H,
CeH on epoxide ring), 1.05e2.13 (m, 14H, eCH2e, eCHe). 31P NMR
(CDCl3/phosphoric acid, ppm): ꢁ6.03 (a single peak).
2. Experimental
2.1. Materials
Phenylphosphoryl dichloride, phosphoryl trichloride, Cyclohex-
3-enyl-1-methanol 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-methyl-
imidazole (EMI) were used as curing agent and curing accelerator,
respectively. Commercial epoxy resin ERL-4221 was employed here
for the purpose of comparison with the newly synthesized two
epoxy resins. Their chemical structures are given in Table 1. Other
chemical reagents were of analytical reagent grade and used as
received.
2.2. Synthesis
2.2.3. Synthesis of tri(cyclohex-3-enylmethyl) phosphate (Olefin II)
The similar epoxidation procedure to that for Olefin I was
carried out except that the phosphoryl chloride compound used
was phosphoryl trichloride instead of phenylphosphoryl dichloride.
A liquid product Olefin II was obtained. Yield: 89%. FTIR (cmꢁ1):
3022, 2914, 2838, 1650, 1436, 1281, 1016, 876, 654. 1H NMR (CDCl3/
TMS, ppm): 5.66 (s, 6H, ]CHe), 3.93 (m, 6H, eCH2eO), 1.30e2.15
(m, 21H, eCH2e, eCHe).
2.2.1. Synthesis of bis(cyclohex-3-enylmethyl)phenyl
phosphate (Olefin I)
Into a 500-ml four-neck round-bottom flask equipped with
a mechanical stirrer, a thermometer and an addition funnel with
drying tube, cyclohex-3-enyl-1-methanol (62.5 g, 0.558 mol),
dichloromethane (50 ml) and triethylamine (38.2 ml, 0.258 mol)
were charged. Under the nitrogen atmosphere, to the above

4778
W. Liu et al. / Polymer 51 (2010) 4776e4783
O
2.2.4. Synthesis of tri(3,4-epoxycyclohexylmethyl) phosphate
(Epoxide II)
Cl
P
Cl
O
O
O
O
O
P
O
O
P
O
The preparation procedure of Epoxide II was similar to that of
Epoxide I except that the olefin compound used was tri(cyclohex-3-
enylmethyl) phosphate instead of bis(cyclohex-3-enylmethyl)
phenyl phosphate. There was obtained 6.8 g liquid. Yield: 78%.
HRMS (m/z): calcd for C21H33O7P 428.4550; found, 428.1859 [M]þ.
FTIR (cmꢁ1): 2928, 1438, 1263, 1019, 960, 979, 810, 786, 745. 1H
NMR (CDCl3/TMS, ppm): 3.82 (m, 6H, eCH2eO), 3.17 (m, 6H, CH on
epoxide ring), 1.03e2.19 (m, 21H, eCH2e, eCHe). 31P NMR (CDCl3/
phosphoric acid, ppm): ꢁ3.54 (a single peak).
O
O
OXONE
O
CH
Cl
/Ace
tone
2
2
Olefin
Epoxide
OH
Cl
C
H
2 Cl
O
/
2
E
t
O
O
O
P
O
3 N
O
P
O
O
O
O
OXONE
CH
P
O
Cl
Cl
C
l /Ac
eton
e
2
2
O
2.3. Curing of epoxides
Olefin
Scheme 1. Synthetic routes of two phosphorous-containing epoxides.
Epoxide
Epoxide and HMPA were homogeneously mixed with a molar
stoichiometric ratio of 1:0.8 at room temperature. Into this mixture
0.5 wt% of 2-ethyl-4-methylimidazole was added as curing accel-
erator. Sample bars for dynamic mechanical analysis (DMA),
differential scanning calorimetry (DSC), thermal gravimetry anal-
ysis (TGA) and LOI tests were obtained by curing in a mold with
specific dimension and cured at 140 ꢀC for 2 h, at 170 ꢀC for 4 h, and
post cured at 190 ꢀC for another 2 h. As a comparison, the
commercial diepoxide ERL-4221 was also mixed with HMPA and 2-
ethyl-4-methylimidazole with the same ratio and cured under the
same conditions.
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. The
standard deviations were found to be within ꢃ0.2.
Viscosities of epoxides were measured using a rotary viscometer
(NDJ-1) over the temperature range of 25e85 ꢀC.
3. Results and discussion
3.1. Synthesis and characterization
2.4. Measurements
The synthesis of two cycloaliphatic epoxides containing phos-
phate ester linkages is presented in Scheme 1, which mainly
consists of two steps: synthesis of olefins and then epoxidations of
olefins to give the epoxides.
Mass spectra were obtained on a Bruker Microtof-Q spectrom-
eter for electrospray ionization in positive and negative modes. The
mass-to-charge (m/z) ratios of the ions were determined with
a quadrupole mass spectrometer, which was scanned from 65 to
2000 amu. The polarity of the ions detected was rapidly switched
between þ and ꢁ, and the data were recorded.
The synthesis of olefin was the typical nucleophilic substitution
reaction of phosphoryl chloride with cyclohex-3-enyl-1-methanol.
Because phosphoryl chlorides were sensitive to water, all the
regents should be purified carefully according to the standard
methods prior to use. Epoxides were prepared according to the
Prileshajev epoxidation reaction of corresponding olefins [30]. The
organic peracids are commonly used oxidation regents, but usually
they are required to be excess to obtain a complete conversion of
C]C double bond into epoxide group, which results in large
amounts of acid waste [31]. Alternatively, in this study, inorganic
oxidant OXONE was adopted to prepare the epoxides. After reaction
was finished, it was very convenient to remove Oxone residues by
simply washing with deionized water.
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.
1H 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 NETZCH DSC 204 thermal
analyzer using N2 as a purge gas (20 ml/min) at a heating rate of
10 ꢀC/min. About 5e10 mg cured product was put into the
aluminum pan for DSC measurement.
DMA measurements were conducted on an apparatus (TA
DMA Q800) at a heating rate of 2 ꢀC/min and at a frequency of
1 Hz under a nitrogen atmosphere. The tests were carried out in
the single cantilever mode using the specimens with dimension
of 2 ꢂ 6 ꢂ 40 mm3.
8-12
O
O P O
O
5
4
1
6-7
2
3
6
7
O
O
1
2-5
8
12
TGA measurements were performed on a NETZSCH TG 209
thermal analyzer under both nitrogen and air 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.
9
11
10
O
6'-7'
8'-12'
2'-5'
5'
1'
2'
3'
6'
7'
1'
O P O
O
Computational calculations were performed using Gaussian
03W program package. The optimized geometries and energies
were calculated with the 6-31G (d)-level calculations based on the
restricted Hartree-Fock calculation method.
LOI tests were performed according to the standard of
ISO4589-1984 with test specimen bar of 12 cm in length, 6.5 mm
in width and 3.0 mm in thickness. 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.
4'
8'
12'
9'
11'
10'
13 12 11 10
9
8
7
6
5
4
3
2
1
Chemical Shift (ppm)
Fig. 1. 1H NMR spectra of Olefin I and Epoxide I.

W. Liu et al. / Polymer 51 (2010) 4776e4783
4779
10
6-7
10
3.0
2.5
2.0
1.5
1.0
0.5
0.0
O
P
5
4
1
2
3
6
7
1
O
O
2-5
O
O
O
8
10
Epoxide
ERL-4221
Epoxide
O
6
10
1'
O
5'
4'
1'
2'
3'
6'
7'
4
O P
O
O
6'-7'
10
2'-5'
2
10
50
100
150
200
250
o
Temperature ( C)
9
8
7
6
5
4
3
2
1
Fig. 4. DMA spectra of cured epoxides.
Chemcial Shift (ppm)
CH]CH disappeared completely, whereas the new signals at 3.17
and 3.15 ppm corresponding to the protons of epoxy rings
appeared, demonstrating that the epoxidation reactions were
complete. Similar signals were assigned in the 1H NMR spectra of
Olefin II and Epoxide II (Fig. 2). Moreover, in the 31P NMR spectra of
Epoxide I and II (Figs. 5S and 6S, Supporting Information), only one
single peak at ꢁ6.03 ppm and ꢁ3.54 ppm was observed, respec-
tively, further confirming the designed structure as depicted in
Scheme 1.
Fig. 2. 1H NMR spectra of Olefin II and Epoxide II.
In the FTIR spectrum of Olefin I, the peak at 1651 cmꢁ1 was
corresponded to the C]C group in the cyclohexenyl ring, the peaks
at 1211 and 1289 cmꢁ1 were due to the absorptions of P]O bond
in eCH2eOeP]O and eAreOeP]O moieties, respectively,
whereas the peaks at 1023 and 919 cmꢁ1 were attributed to the
stretching vibration of eCeOeP in eCH2eOeP and eAreOeP
moieties, respectively, and the absorption of benzene ring was
found at 1589 and 1490 cmꢁ1. Similar characteristic peaks could be
observed in the FTIR spectrum of Olefin II except for the absence
of absorptions of benzene ring, P]O in eAreOeP]O and eCeOeP
in eAreOeP. After epoxidation reactions of Olefin I and II, the
previous C]C absorptions disappeared completely, and new
characteristic peak of epoxy ring at about 825 and 785 cmꢁ1
appeared, indicating that the C]C bonds in the cyclohexenyl rings
were converted into epoxy groups (Figs. 1Se4S, Supporting
information).
3.2. General physical properties
The synthesized Epoxide I and II are liquid at room temperature
and have viscosities at 25 ꢀC of 990 and 2210 mPa s, respectively,
which are higher than that of ERL-4221 due to their multi-
functionality, bulky molecular structure or the presence of aromatic
benzene ring. However, it was found that their viscosity values
rapidly decreased with temperature (Fig. 3). For Epoxide II, upon
raising temperature to 35 ꢀC, its viscosity rapidly decreased to
625 mPa s. Moreover, at 85 ꢀC, Epoxide II had even the similar
viscosity value to ERL-4221. This excellent temperature thixotropic
property is especially favorable for encapsulating of chip on board
and optoelectronic device. In this case, the encapsulants are
required to have low viscosity at increased temperature to enable
convenient operation, and meanwhile, when the temperature is
decreased to room temperature, the high viscosity makes that the
desirable packaging shapes can be maintained. In addition, for flip
chip packaging process, the operation is usually conducted in the
temperature range of 80e90 ꢀC to shorten the underfilling time. At
this temperature, Epoxide I and II have the comparable low
viscosities and flow abilities to the commercially used ERL-4221.
Fig. 4 illustrates the spectra of dynamic mechanical analysis
(DMA) of cured Epoxide I, II and ERL-4221 as a function of
temperature at a heating rate of 2 ꢀC/min and a frequency of 1 Hz.
Fig. 1 shows the 1H NMR spectra of Epoxide I and its precursor
Olefin I. For Olefin I, the signals at 7.19e7.39 ppm could be assigned
to the protons on benzene ring, the peaks at 5.66 and 3.93 ppm
were due to the protons on CH]CH of the cyclohexenyl rings and
CH2eO, respectively, and signals at 1.33e2.18 ppm were corre-
sponded to the protons of other cycloaliphatic CH2 groups. After
epoxidation, the signals at 5.66 and 5.70 ppm due to the protons of
2100
EpoxideII
EpoxideI
ERL-4221
1800
1500
1200
900
600
300
0
Table 2
DMA and DSC results of cured epoxides.
Storage Modulus
Sample
Glassy
Rubber
r
(10ꢁ3
Tg-DSC
(ꢀC)
Tg-DMA
(ꢀC)
regiona (GPa)
regionb (MPa)
mol/cm3)
Epoxide I
Epoxide II
ERL-4221
2.5
2.6
2.3
7.1
28
17
0.66
1.72
0.94
113
199
175
130
227
191
20
30
40
50
60
70
80
90
o
Temperature ( C)
a
Storage modulus at 30 ꢀC.
Storage modulus at Tg-DMA þ 30 ꢀC.
b
Fig. 3. Variation of viscosity as a function of temperature of epoxides.

4780
W. Liu et al. / Polymer 51 (2010) 4776e4783
Air
A
Nitrogen
100
80
60
40
20
0
100
80
60
40
20
0
B
ERL-4221
Epoxide
Epoxide
ERL-4221
Epoxide
Epoxide
100 200 300 400 500 600 700 800 900
Nitrogen
100 200 300 400 500 600 700 800 900
0
-1
-2
-3
-4
-5
0
-1
-2
-3
-4
-5
Air
ERL-4221
Epoxide
Epoxide
ERL-4221
Epoxide
Epoxide
100 200 300 400 500 600 700 800 900
o
Temperature ( C)
100 200 300 400 500 600 700 800 900
o
Temperature ( C)
Fig. 5. TGA and DTG plots the cured epoxides: (A) in Nitrogen and (B) in Air.
The data of Tg values obtained from tan
at both glassy region and rubbery region are summarized in Table 2.
In principle, according to the theory of rubber elasticity, the
d
and the storage modulus
in network. The half-peak width in Fig. 4 showed that, relative to
Epoxide I and ERL-4221, the cured product based on Epoxide II had
an obviously narrow tan
d peak, indicating a dense and uniform
crosslinking density (
its storage modulus in the rubbery region as shown in the equation
r
) of a cured epoxy network is proportional to
network. The reason could be attributed to its highly symmetric
trifunctional structure.
r
¼ E=3RT, where E the storage modulus at Tg þ 30 ꢀC, R the gas
In addition, the cured Epoxide I and II had much higher storage
modulus (about 2.5 GPa) in glassy regions than ERL-4221 (2.3 GPa)
because of the respective contribution of rigid benzene ring in
Epoxide I and trifunctional structure of Epoxide II. The enhanced
modulus values are advantageous for them in electronic packaging
applications.
The glass transition temperatures for the three cured epoxies
were obtained from DMA and DSC methods. Similar to other
polymer systems [6,20,33], in this study, the glass transition
temperatures from DMA (Tg-DMA) are higher than the corre-
sponding values from DSC curves (Tg-DSC), and the Tg values of
constant and T the absolute temperature at Tg þ 30 ꢀC [32,33]. As
can be seen from Table 2, the crosslinking density of cured tri-
functional Epoxide II was higher by 294% and 64.7% than that of
Epoxide I and ERL-4221, respectively.
The analysis of the height and width of the tan
provide information about the crosslinking density and network
homogeneity of cured epoxides. The tan peak is the maximum
ratio of viscous component to elastic component. Since the increase
of segmental mobility in cured epoxy is favorable for viscous
component, it can be assumed that the larger height value of tan
d peak could also
d
d
peak implies a looser network. As illustrated in Fig. 4, the variation
of height of tan peak for the three cured samples exhibited the
d
same trend as that of E value, which further supported the fact that
the cured Epoxide II had the denser crosslinking network, whereas
the network of Epoxide I is looser than the others. On the other
100
Epoxide
Epoxide
hand, considering that tan
glass transition and at this temperature the polymer segments
starts to gain mobility, the width of tan peak can reflect the
d peak is indicative of the occurrence of
80
d
distribution of segmental length between two crosslinking points
60
40
20
0
Table 3
TGA data of cured epoxides.
Sample
Phosphorus
TGA (N2 Atmosphere)
TGA (Air Atmosphere)
content (wt%)
IDTa Tmax
(ꢀC) (ꢀC)
Yield
IDTa Tmax
(ꢀC) (ꢀC)
Yieldc
b
c
b
(%)
(%)
Epoxide I 4.6
Epoxide II 3.7
222 259
257 279
316 368
10.9
10.0
1.3
224 265
260 274
320 363
11.9
8.6
0
ERL-4221
0
0
2
4
6
8
10
12
14
a
IDT is the initial decomposition temperature of cured epoxide.
Temperature of the maximum weight loss rate.
Char yield at 800 ꢀC.
Time (min)
b
c
Fig. 6. Isothermal TGA plots at 260 ꢀC under air atmosphere for the cured epoxides.

W. Liu et al. / Polymer 51 (2010) 4776e4783
4781
Table 4
Isothermal TGA data (260 ꢀC) of cured epoxides.
2364
1648
Sample
Weight loss (%)
(e)
1 min
3 min
5 min
8 min
12 min
Epoxide I
Epoxide II
3.2
2.2
67.3
14.9
84.3
57.7
87.2
66.4
88.8
68.1
(d)
(c)
the three cured samples followed the order: Epoxide
II > ERL-4221 > Epoxide I, which trend was consistent with that
of crosslinking density. The rigid cycloaliphatic structure and
the dense crosslinking network resulted into the cured tri-
functional epoxide II significantly high Tg (199 ꢀC for Tg-DSC and
227 ꢀC for Tg-DMA), which, to our knowledge, is among the
highest glass transition temperature for liquid aliphatic epoxies
(cured with HMPA) reported previously in the literature.
1157
1255
(b)
(a)
1734
2933
2856
1458
3500
3000
2500
2000
1500
1000
500
Wave numbers (cm-1)
3.3. Thermal degradation property and mechanism
Fig. 8. FTIR spectra of pyrolysis products of cured Epoxide II under air atmosphere: (a)
0 min/260 ꢀC, (b) 2 min/260 ꢀC, (c) 10 min/260 ꢀC, (d) 20 min/320 ꢀC, (e) 20 min/
360 ꢀC.
To investigate the thermal decomposition behaviors of cured
epoxides, the TGA and derivative (DTG) curves as a function of
temperature were recorded and presented in Fig. 5, and the ther-
mogravimetric data are summarized in Table 3. As expected, the
phosphate-containing epoxides degraded at the lower temperature
and exhibited higher degradation rate than ERL-4221. In nitrogen
atmosphere, the cured Epoxide I, Epoxide II and ERL-4221 started to
degrade at about 231 ꢀC, 260 ꢀC, and 320 ꢀC, and the temperatures
of the maximum weight loss were 259 ꢀC, 279 ꢀC and 368 ꢀC,
respectively. The decomposition behaviors in air atmosphere are
similar except for a slight change in char yield, meaning that the
oxidative atmosphere had no significant effects on the degradation
of cured epoxides. Moreover, the residual chars of cured Epoxide I
and II at 800 ꢀC (8e12 wt%) were much higher than that of
ERL-4221 (about 0 wt%), which were similar to the previous reports
that the phosphorus-containing polymers usually had the higher
char yield [28].
temperature and, given the same time, the sample based on
Epoxide I had the more weight loss. For example, after degradation
at 260 ꢀC for 3 min, the cured Epoxide I lost weight of 67.3%,
whereas it was only 14.9% for Epoxide II. The reason could be
attributed to two factors. On the one hand, the phosphorus content
of cured Epoxide I (4.6 wt%) was higher than that in Epoxide II
(3.7 wt%), indicative of a higher density of weak phosphate bond in
Epoxide I network. On the other hand, the looser crosslinking
network in cured Epoxide I was advantageous for the diffusion of
phosphorus acid generated in network to catalyze the pyrolysis of
other phosphate bonds. These two factors could account for
Epoxide I the lower degradation temperature and higher degra-
dation rate than Epoxide II.
As a comparison, ERL-4221 had the decomposition temperature
of the maximum weight loss rate of 368 ꢀC, which was far higher
than Epoxide I and II, implying that the primary carboneester bond
was more thermally stable than the phosphate bond. The evidence
from the Hartree-Fock computational calculation using Gaussian
03W program package supported the above experimental results.
The cured Epoxide I and II were thermally stable up to 230 ꢀC,
offering sufficient reliability and resistance to heat shock generated
during the working of integrated circuit or optoelectronic device.
However, once the decomposition temperature was reached, the
degradation process was very quick. The steep and sharp TGA
curves indicated that the thermal decomposition of Epoxide I and II
networks occurred in a narrow temperature range, i.e., their
decomposition processes were very sensitive to temperature. To
further investigate the degradation rate of cured phosphate-con-
taining epoxides, isothermal TGA curves were measured at 260 ꢀC
under air atmosphere (Fig. 6), and the results are summarized in
Table 4. It was seen that the samples based on Epoxide I and II lost
nearly 70% weights within only 3 min and 8 min, respectively,
which phenomena were rarely observed for the previous thermally
reworkable epoxy resins [15e20].
1701
2917
2848
1773
1857
2662
(b)
(a)
The data in Tables 3 and 4 showed that, relative to Epoxide II, the
cured Epoxide I displayed an obviously lower initial decomposition
2929
2857
1704
1732
2665
3500
3000
2500
2000
1500
-1
1000
Wavenumber (cm )
Fig. 7. Optimized geometries and bond lengths of CeO bond by computational
modeling for (A) phosphate group and (B) primary carboneester group.
Fig. 9. FTIR spectra of the condensate of volatilized pyrolysis products at 260 ꢀC with
increasing degradation time: (a) collected at 2 min; (b) collected at 10 min.

4782
W. Liu et al. / Polymer 51 (2010) 4776e4783
Fig. 10. Removal test of cured Epoxide II on a glass slide: (A) transparent cured Epoxide II, (B) heat-treated under air atmosphere at 260 ꢀC for 4 min, (C) wiping with acetone;
(D) a clean slide after wiping.
The optimized geometries and bond lengths of CeO bond in
phosphate and primary carboneester are illustrated in Fig. 7. The
phosphorous groups such as P]O, PeOePeO bonds remained
similar except that the intensity of aliphatic CeH (2938 and
2854 cmꢁ1) and ester group (1734 cmꢁ1) further decreased, indi-
cating that the thermal pyrolysis process was almost complete after
10 min under air atmosphere at 260 ꢀC.
ꢀ
bond length of OeC in phosphate was 1.420 A while that in primary
ꢀ
carboneester bond was 1.415 A, demonstrating that the bond of
OeC in phosphate is weaker than that in primary carboneester.
For the cured Epoxide II, its isothermally degraded process
under air atmosphere with different time at 260 ꢀC, 320 ꢀC and
360 ꢀC, respectively, was followed using FTIR method, and the
obtained spectra are shown in Fig. 8. The comparison between
spectra a, b and c revealed that, after pyrolysis of 2 min at 260 ꢀC,
a wide absorption in the range from 2100 to 3200 cmꢁ1 appeared,
and its intensity became more pronounced after the time was
prolonged to 10 min. A similar evolution trend was also displayed
for the absorption at about 1648 cmꢁ1. These peaks were attributed
to the characteristic absorption of the stretching vibration of OH
and O]P groups in organic phosphorous acid, produced from the
cleavage of OeC bond of phosphate ester moiety in the network.
Moreover, at high temperature, the organic phosphorous acid was
liable to react to form pyrophosphates [34], as confirmed by the
increased intensity of PeOePeO absorption bands lying at
In order to further elucidate the degradation mechanism, for the
cured Epoxide II, the volatilized components with different time of
thermal decomposition at 260 ꢀC were collected and analyzed by
FTIR spectra. As shown in Fig. 9, in the initial 2 min, the FTIR spectra
of the condensate displayed strong characteristic absorptions of
cycloaliphatic moieties at 2929 and 2857 cmꢁ1, carboxylic group at
1701 and 2665 cmꢁ1 and the ester group at 1732 cmꢁ1. As the
degradation time was increased to 10 min, the shoulder peak of
ester bond at 1732 cmꢁ1 disappeared completely, and the new
absorptions at 1857 and 1773 cmꢁ1 belonging to acid anhydride
group appeared. In this study, since hexahydro-4-methylphthalic
anhydride (HMPA) was used as curing agent to react with epoxides
to form ester bond, and its content in cured network was more than
48 wt%, the results in the above FTIR spectra revealed that, in the
initial thermal pyrolysis stage, the break of phosphate bonds and
small amount of ester bonds made the volatilized components
mainly consist of large molecular fragments of cycloaliphatic acids
or cycloaliphatic esters with HMPA skeleton. With the increase of
pyrolysis time, the nearly complete break of ester bonds produced
hexahydro-4-methylphthalic acid, and some of them dehydrated to
yield HMPA in the end.
900e1100 cmꢁ1 and 1300e1250 cmꢁ1
. As revealed by the
isothermal TGA results above, after 10 min of degradation at 260 ꢀC,
more than 60% weight had lost, we believed that, with the collapse
of network, large amount of cycloaliphatic fragments from HMPA
and Epoxide II moieties had volatilized, reflecting in the greatly
decreased peak intensities of absorptions at 2933, 2871 and
1458 cmꢁ1 (CH2), and 1734 cmꢁ1 (COO). After the temperature was
further increased to 320 ꢀC and 360 ꢀC, the characteristic peaks of
Based on the FTIR spectra and TGA analyses as well as the
molecular modeling results, the mechanism of thermal

W. Liu et al. / Polymer 51 (2010) 4776e4783
4783
degradation of this series of phosphor-containing epoxides could
be proposed: at the decomposition temperature, the thermally-
labile phosphate groups firstly pyrolyze to yield strong organic
phosphoric acid, and which then catalyze the cleavage of other
adjacent phosphate or ester bonds, leading to the collapse of
network. The synergistic influence of two factors, i.e., the pres-
ence of weak phosphate bond evenly distributed within the
three-dimensional crosslinking network on the molecular level
and the in-situ catalyzing of organic phosphoric acid on the
decomposition of phosphate or carboneester groups, was
responsible for the suitably low cleavage temperature and rapid
degradation rate.
desirable rework temperature range of 200e300 ꢀC. The low
degradation temperature and rapid degradation rate are attributed
to that the weak phosphate bonds evenly distributed in the
network and the auto-accelerated effect on the cleavages of adja-
cent phosphate and ester bonds by the organic phosphoric acids
generated during thermal decomposition, as demonstrated by the
results of computational calculation and the isothermal FTIR
spectra of solid pyrolysis products and volatized components.
The reworkable test showed that, after heat-treatment for only
4 min at 260 ꢀC, the residue of cured trifunctional epoxide on a glass
slide could be easily wiped off using acetone to obtain a very clean
substrate so that additional grinding and polishing processes are
not necessary. Moreover, the introduction of phosphate group into
the epoxy network led to the cured epoxides apparently enhanced
flame retardant property. Therefore, the combination of several
favorable characteristics such as reworkability, halogen-free flame
retardancy, high glass transition temperature and mechanical
modulus endow the epoxides synthesized promising application
potential as environment-friendly electronic packaging materials.
3.4. Removal test
For the practical application of a reworkable epoxy resin, it is
desired that its cured product can provide enough adhesion
strength on the substrate and sufficient reliability up to 220 ꢀC
equivalent to conventional epoxy packaging material, and mean-
while, in the event of necessity, the epoxy crosslinking network can
collapse rapidly within several minutes in the temperature range of
220e280 ꢀC to allow the faulty chip to be removed. Moreover, the
residue attached on the substrate should be cleaned easily with
some common noncorrosive solvents such as acetone, chloroform
and dichloromethane so that the subsequent surface grinding and
polishing processes are unnecessary.
Acknowledgments
The support from the Program for New Century Excellent Talents in
University of China (Grant No. NCET-06-0280) and the Scientific
Research Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry (Grant No. 2005-546), is gratefully acknowledged.
To evaluate the reworkable property of the epoxies synthesized, in
this study, about 1 g of mixture of trifunctional Epoxide II and HMPA
with the molar ratio of 1:0.8 was cured on a glass slide using 2-ethyl-4-
methylimidazole as catalyst under the same curing condition as the
sample for TGA and DMA measurements. The cured film sample was
transparent with area of about 50 ꢂ 50 mm2 (Fig. 10A). The tempera-
ture of a muffle furnace was pre-determined at 260 ꢀC, in which the
cured samplewas heat-treated for 4 min. The black residue on the glass
slide could be wiped off easily with acetone and a very clean glass slide
was obtained (Fig.10BeD). The test above demonstrated that the quick
degradation at the desired temperature and easy cleaning process of
substrate could be realized using the present epoxides.
Appendix. Supporting information
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2010.08.039.
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