Epoxy resin bearing diacylhydrazine moiety as a degradable adhesive for traceless oxidative removal

Article abstract of DOI:10.1016/j.polymer.2016.06.066

Bisphenols functionalized with diacylhydrazine moieties and ester groups were prepared from 5-hydroxyisophthalic acid by esterification and partial hydrazination, followed by oxidative coupling of the obtained hydrazide. Ester groups with long alkyl chains or polyether increased the solubility of bisphenols in the epoxy resin. The epoxy resin was cured by heating with bisphenols in the presence of a catalytic amount of imidazole, with more rapid curing observed for more soluble bisphenols. The cured resin, with T_d5?≈?300?°C, decomposed rapidly when exposed to sodium hypochlorite solution. The above resin could be used as a strong and tough adhesive for metal and glass, whereas it can be easily removed by treatment with sodium hypochlorite solution without any trace. The observed dismantling rate positively correlated with bisphenol solubility.

Full text of DOI:10.1016/j.polymer.2016.06.066

Polymer 99 (2016) 83e89
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Epoxy resin bearing diacylhydrazine moiety as a degradable adhesive
for traceless oxidative removal
Takahiro Oguri, Akie Kawahara, Nobuhiro Kihara*
Department of Chemistry, Faculty of Science, Kanagawa University, Tsuchiya, Hiratsuka 259-1293, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Bisphenols functionalized with diacylhydrazine moieties and ester groups were prepared from 5-
hydroxyisophthalic acid by esterification and partial hydrazination, followed by oxidative coupling of
the obtained hydrazide. Ester groups with long alkyl chains or polyether increased the solubility of
bisphenols in the epoxy resin. The epoxy resin was cured by heating with bisphenols in the presence of a
catalytic amount of imidazole, with more rapid curing observed for more soluble bisphenols. The cured
resin, with Td5 z 300 ꢀC, decomposed rapidly when exposed to sodium hypochlorite solution. The above
resin could be used as a strong and tough adhesive for metal and glass, whereas it can be easily removed
by treatment with sodium hypochlorite solution without any trace. The observed dismantling rate
positively correlated with bisphenol solubility.
Received 3 May 2016
Received in revised form
24 June 2016
Accepted 28 June 2016
Available online 2 July 2016
Keywords:
Oxidatively degradable polymer
Epoxy resin
© 2016 Elsevier Ltd. All rights reserved.
Adhesive
1. Introduction
weather resistance.
We have reported poly(diacylhydrazine) as an oxidatively
Adhesives should have two opposite properties: sufficient
bonding strength during use and their amenable removal after use.
Even though bonding strength has been conventionally given more
importance, novel functional adhesives that lose their strength in
response to external stimuli on demand have been extensively
studied. Among such stimuli, photo-irradiation [1e5] and heating
[6e12] have been widely studied, causing decomposition or poly-
merization of certain functional groups and the associated loss of
adhesive strength. Thermal expansion of microcapsules is another
principle for formulating thermally removable adhesives [13,14].
Acid-catalyzed isomerization or decomposition of ester or acetal
groups has also been studied as dismantling stimuli [15e20].
Electrolysis is another dismantling stimulus that can be used
[21,22]. Electrostatic interaction between ionic surfaces can be
attenuated by the treatment with salt or hot water [23,24]. How-
ever, removing an adhesive from a material surface without leaving
any residue is generally difficult when the adhesive exhibits strong
bonding. Furthermore, since the stimuli utilized for dismantling,
e.g., light, heat, acids, and water, are naturally ubiquitous, acci-
dental dismantling may happen. Since sensitive dismantling re-
quires a low threshold, amenable removal is incompatible with
degradable polymer [25e28]. When diacylhydrazine is oxidized by
certain oxidants such as sodium hypochlorite, the corresponding
carboxylic acid and nitrogen gas are formed rapidly and quantita-
tively after hydrolysis. However, diacylhydrazine is inert to oxygen
even at 300 ꢀC. Poly(diacylhydrazine), a class of polyamide, is
thermally and chemically (excluding specific oxidants) as stable as
polyamide (nylon), a typical engineering plastic. Since sodium hy-
pochlorite is an easily available but nonnatural compound, poly(-
diacylhydrazine) exhibits both high weather resistance and high
degradability. These special features of poly(diacylhydrazine)
prompted us to introduce the diacylhydrazine moiety into adhe-
sives (Scheme 1).
Among the various adhesives, we paid especial attention to
epoxy resins, one of the strongest and toughest adhesives [29,30].
Epoxy resins are polyaddition-type polymers, and hence, the
diacylhydrazine moiety can be introduced on the polymer back-
bone. Epoxy resins can be cured by bifunctional nucleophiles such
as bisphenols in the presence of catalysts such as imidazole. Thus,
bisphenols bearing the diacylhydrazine moiety can act as curing
agents for epoxy resins, and the resins cured with such bisphenols
are expected to be oxidatively degradable (Scheme 2).
In this paper, preparation of novel bisphenols with diacylhy-
drazine moiety is described. Curing of epoxy resins and oxidative
degradation of the cured resin were examined. Furthermore, the
use of the cured resin as an adhesive and its traceless oxidative
* Corresponding author.
E-mail address: kihara@kanagawa-u.ac.jp (N. Kihara).
http://dx.doi.org/10.1016/j.polymer.2016.06.066
0032-3861/© 2016 Elsevier Ltd. All rights reserved.

84
T. Oguri et al. / Polymer 99 (2016) 83e89
1-ol (350 mL) was refluxed for 3 days. After the removal of the
solvent in vacuo, the residue was dissolved in ethyl acetate, washed
with sodium bicarbonate solution and brine, dried over magnesium
sulfate, and evaporated to give 10Bu (8.10 g, 100%) as a brown solid.
1H NMR (300 MHz, DMSO-d6):
d
10.30 (s, 1H), 7.94 (t, J ¼ 1.5 Hz,
1H), 7.57 (d, J ¼ 1.5 Hz, 2H), 4.28 (t, J ¼ 6.5 Hz, 4H), 1.75e1.64 (m,
4H), 1.49e1.36 (m 4H), 0.94 (t, J ¼ 7.3 Hz, 6H) ppm.
2.4. 10Oc
Scheme 1.
A solution of 5-hydroxyisophthalic acid (21.81 g, 120 mmol),
octan-1-ol (40 mL, 437 mmol), and p-toluenesulfonic acid mono-
hydrate (3.45 g, 18.1 mmol) in toluene (70 mL) was refluxed with
Dean-Stark apparatus for 3 days. After the removal of the solvent in
vacuo, the residue was dissolved in ethyl acetate, washed with
sodium bicarbonate solution and brine, dried over magnesium
sulfate, and evaporated to give 10Oc (47.57 g, 98%) as an off-white
solid.
removal were studied.
2. Experimental section
2.1. General
1H NMR (300 MHz, DMSO-d6):
d
10.30 (s, 1H), 7.94 (t, J ¼ 1.5 Hz,
NMR spectra were recorded on a JEOL JNM-ECP300, a JNM-
ECA400, a JNM-ECS400, a JNM-ECP500, and a JNM-ECZ600 spec-
trometers using tetramethylsilane as an internal standard. IR
spectra were recorded on a JASCO FT/IR-4100 spectrometer. Ther-
mal gravimetric analysis (TGA) was performed on a Rigaku Thermo
plus TG-8120 instrument using an aluminum pan under air. Dif-
ferential scanning calorimetry (DSC) was performed on a RIGAKU
Thermo plus DSC-8230 using a crimped aluminum pan under air.
Tension was measured using an Instron 5565 Tester. All chemicals
and solvents were reagent grade, and were used without further
purification.
1H), 7.56 (d, J ¼ 1.5 Hz, 2H), 4.27 (t, J ¼ 6.5 Hz, 4H), 1.76e1.65 (m,
4H), 1.46e1.18 (m, 20H), 0.90e0.80 (m, 6H) ppm.
2.5. 10PE
A solution of 5-hydroxyisophthalic acid (40.18 g, 221 mmol), 2-
(2-methoxyethoxy)ethanol (40 mL, 340 mmol), and p-toluene-
sulfonic acid monohydrate (4.14 g, 21.8 mmol) in toluene (50 mL)
was refluxed with Dean-Stark apparatus for 3 days. After the
removal of the solvent in vacuo, the residue was dissolved in ethyl
acetate, washed with sodium bicarbonate solution and brine, dried
over magnesium sulfate, and evaporated to give 10PE (85.24 g,
100%) as a pale yellow oil.
2.2. 10Et
1H NMR (300 MHz, DMSO-d6):
d
10.35 (s, 1H), 7.96 (t, J ¼ 1.6 Hz,
A solution of 5-hydroxyisophthalic acid (6.17 g, 33.8 mmol) and
p-toluenesulfonic acid monohydrate (2.38 g, 14 mmol) in ethanol
(800 mL) was refluxed for 2 days. After the removal of the solvent in
vacuo, the residue was dissolved in ethyl acetate, washed with
sodium bicarbonate solution and brine, dried over magnesium
sulfate, and evaporated to give 10Et (7.92 g, 100%) as a white solid.
1H), 7.59 (d, J ¼ 1.6 Hz, 2H), 4.43e4.37 (m, 4H), 3.78e3.72 (m, 4H),
3.62e3.56 (m, 4H), 3.48e3.42 (m, 4H), 3.24 (s, 6H) ppm.
2.6. 11Et
1H NMR (500 MHz, DMSO-d6):
d
10.29 (s, 1H), 7.95e7.93 (m, 1H),
A solution of 10Et (37.61 g, 161 mmol) and hydrazine mono-
hydrate (7.9 mL, 160 mmol) in ethanol (50 mL) was stirred for 3
days. The reaction mixture was poured into water, and the pre-
cipitate was collected by filtration, washed with water followed by
ethyl acetate, and dried in vacuo to give 11Et (18.07 g, 50%) as a
white solid.
7.57 (d, J ¼ 1.4 Hz, 2H), 4.32 (q, J ¼ 7.1 Hz, 4H), 1.32 (t, J ¼ 7.1 Hz, 6H)
ppm.
2.3. 10Bu
1H NMR (300 MHz, DMSO-d6):
d 10.08 (s, 1H), 9.87 (s, 1H), 7.85
A solution of 5-hydroxyisophthalic acid (5.01 g, 27.5 mmol) and
p-toluenesulfonic acid monohydrate (712 mg, 3.74 mmol) in butan-
(t, J ¼ 1.5 Hz, 1H), 7.48e7.43 (m, 2H), 4.49 (s, 2H), 4.31 (q, J ¼ 7.1 Hz,
2H), 1.32 (t, J ¼ 7.1 Hz, 3H) ppm.
2.7. 11Bu
A solution of 10Bu (1.17 g, 3.98 mmol) and hydrazine mono-
hydrate (0.2 mL, 4.1 mmol) in ethanol (20 mL) was stirred at 40 ꢀC
for 2 days. The reaction mixture was poured into water, and the
precipitate was collected by filtration, washed with water followed
by ethyl acetate. The crude product was dissolved in methanol.
Silicagel was added, and the solvent was evaporated in vacuo to
charge the products on the silicagel. The products were chroma-
tographed (eluent: dichloromethane-methanol, 40/1, v/v) to give
11Bu (386 mg, 38%) as a white solid.
1H NMR (500 MHz, DMSO-d6):
d 10.10 (s, 1H), 9.88 (s, 1H),
7.85e7.84 (m, 1H), 7.48e7.46 (m, 1H), 7.45e7.43 (m, 1H), 4.53 (s,
2H), 4.26 (t, J ¼ 6.4 Hz, 2H), 1.73e1.64 (m, 2H), 1.47e1.37 (m, 2H),
0.93 (t, J ¼ 7.3 Hz, 3H) ppm.
Scheme 2.

T. Oguri et al. / Polymer 99 (2016) 83e89
85
2.8. 11Oc
A solution of 10Oc (19.98 g, 49.1 mmol) and hydrazine mono-
mixture was poured into water. The precipitate was collected by
filtration, washed with water, dried in vacuo to give 12Oc (1.41 g,
93%) as a white solid.
hydrate (2.4 mL, 49.4 mmol) in ethanol (30 mL) was stirred at 40 ꢀC
for 2 days. The reaction mixture was poured into ether, and the
precipitate was collected by filtration. The crude product was dis-
solved in methanol. Silicagel was added, and the solvent was
evaporated in vacuo to charge the products on the silicagel. The
products were chromatographed (eluent: dichloromethane-
methanol, 30/1, v/v) to give 11Oc (7.28 g, 48%) as a white solid.
1H NMR (300 MHz, DMSO-d6):
d
10.64 (s, 2H), 10.21 (s, 2H),
7.97e7.92 (m, 2H), 7.57e7.51 (m, 4H), 4.29 (t, J ¼ 6.3 Hz, 4H),
1.78e1.65 (m, 4H), 1.46e1.18 (m, 20H), 0.92e0.81 (m, 6H) ppm. 13
NMR (150 MHz, DMSO-d6): 166.23, 166.06, 158.75, 135.43, 132.44,
C
d
120.06, 119.84, 65.93, 32.24, 29.67, 29.63, 29.18, 26.47, 23.08,
14.89 ppm. IR (KBr): 3428, 3306, 2955, 2925, 2855,1719,1696,1589,
1559, 1496, 1391, 1332, 1243, 1105, 998, 984, 889, 773, 756, 737, 674,
1H NMR (300 MHz, DMSO-d6):
d
10.10 (s, 1H), 9.88 (s, 1H),
654, 605 cmꢁ1
2.13. 12PE
To a mixture of ethanol (20 mL) and methanol (20 mL) was
.
7.86e7.82 (m,1H), 7.48e7.42 (m, 2H), 4.54 (s, 2H), 4.25 (t, J ¼ 6.5 Hz,
2H), 1.75e1.63 (m, 2H), 1.45e1.15 (m, 10H), 0.88e0.78 (m, 3H) ppm.
2.9. 11PE
dissolved 11PE (4.26 g, 15.1 mmol), and PhI(OAc)2 (4.87 g,
15.1 mmol) was added. After stirring at 50 ꢀC for 2 days, the solvent
was evaporated in vacuo. The residue was dissolved in methanol.
Silicagel was added, and the solvent was evaporated in vacuo to
charge the products on the silicagel. The products were chroma-
tographed (eluent: dichloromethane-methanol, 20/1, v/v) to give
12PE (3.29 g, 77%) as a pale yellow gummy solid.
A mixture of 10PE (37.24 g, 96.4 mmol) and hydrazine mono-
hydrate (4.8 mL, 99 mmol) was stirred for 3 days. The reaction
mixture was dissolved in methanol. Silicagel was added, and the
solvent was evaporated in vacuo to charge the products on the
silicagel. The products were chromatographed (eluent:
dichloromethane-methanol, 50/1, v/v) to give 11PE (13.05 g, 48%)
as a white solid.
1H NMR (300 MHz, DMSO-d6):
d 10.65 (s, 2H), 10.25 (s, 2H), 7.96
1H NMR (300 MHz, DMSO-d6):
d 10.13 (s, 1H), 9.88 (br, 1H), 7.85
(t, J ¼ 1.4 Hz, 2H), 7.55 (d, J ¼ 1.4 Hz, 4H), 4.44e4.37 (m, 4H),
(t, J ¼ 1.4 Hz, 1H), 7.49e7.46 (m, 1H), 7.46e7.43 (m, 1H), 4.50 (d,
J ¼ 3.9 Hz, 2H), 4.41e4.36 (m, 2H), 3.77e3.72 (m, 2H), 3.62e3.56 (m,
2H), 3.48e3.43 (m, 2H), 3.23 (s, 3H) ppm.
3.78e3.72 (m, 4H), 3.62e3.56 (m, 4H), 3.48e3.42 (m, 4H), 3.24 (s,
6H) ppm. 13C NMR (150 MHz, DMSO-d6):
d 166.39, 166.32, 158.92,
135.62, 132.47, 120.32, 120.18, 72.42, 70.85, 69.51, 65.49, 59.18 ppm.
IR (KBr): 3231, 2945, 1718, 1643, 1597, 1527, 1443, 1334, 1308, 1251,
1105, 1036, 1000, 978, 960, 884, 871, 846, 777, 755, 675, 613, 540,
2.10. 12Et
501 cmꢁ1
2.14. Curing
The epoxy resin was heated with an equimolar amount of
.
To a mixture of acetonitrile (3.5 mL), water (3.5 mL), and NMP
(3.5 mL) was dissolved 11Et (1.31 g, 5.83 mmol), and Oxone (3.59 g,
5.84 mmol) was added. After stirring for 3 days, the reaction
mixture was poured into water. The precipitate was collected by
filtration, washed with water, dried in vacuo to give 12Et (1.21 g,
100%) as a white solid.
bisphenol at 70e130 ꢀC to prepare a homogeneous mixture. When
the bisphenol did not completely dissolve in the epoxy resin even at
130 ꢀC, the minimum amount of N-methylpyrrolidinone (NMP) was
added to make the mixture homogeneous. Imidazole (10 mol% with
respect to the epoxy resin) was added to the mixture, which was
allowed to stand at the above temperature for 30 min to obtain the
cured resin.
1H NMR (500 MHz, DMSO-d6):
(s, 2H), 7.54 (s, 4H), 4.34 (q, J ¼ 7.1 Hz, 4H), 1.34 (t, J ¼ 7.1 Hz, 6H)
ppm. 13C NMR (150 MHz, DMSO-d6):
166.16, 166.03, 158.68,
d 10.64 (s, 2H), 10.23 (s, 2H), 7.96
d
135.39, 132.45, 120.00, 119.79, 62.01, 15.12 ppm. IR (KBr): 3560,
3491, 3246, 3007, 1707, 1638, 1593, 1513, 1452, 1394, 1372, 1331,
1306, 1283, 1248, 1219, 1109, 1027, 1002, 980, 883, 796, 774, 721,
506 cmꢁ1
2.11. 12Bu
To a mixture of acetonitrile (1.5 mL), water (1.5 mL), and NMP
.
2.15. Adhesion and oxidative dismantling
A small amount of the cured resin was placed between two flat
specimens of the tested material, which were heated on a heating
plate at 140 ꢀC for 1 h. After cooling to room temperature, the
bonded specimen was immersed in a 5% sodium hypochlorite
solution.
(1.5 mL) was dissolved 11Bu (268 mg, 1.06 mmol), and Oxone
(701 mg, 1.14 mmol) was added. After stirring for 2 days, the re-
action mixture was poured into water. The precipitate was collected
by filtration, washed with water, dried in vacuo to give 12Bu
(244 mg, 98%) as a pale yellow solid.
2.16. Adhesion strength
1H NMR (500 MHz, DMSO-d6):
d 10.66 (s, 2H), 10.27 (s, 2H),
7.96e7.93 (m, 2H), 7.56e7.52 (m, 4H), 4.30 (t, J ¼ 6.5 Hz, 4H),
The epoxy resin was heated with an equimolar amount of
bisphenol at 80e130 ꢀC to prepare a homogeneous mixture. After
the addition of imidazole (10 mol%), the mixture was heated at
140 ꢀC until it became homogenous. The mixture was spread be-
tween two aluminum plates (25 mm wide, 1.0 mm thick), and the
adhesion area was set to 625 mm2 (25 mm length). The plates were
placed on a heating plate, with a weight of 1.2 kg placed on top.
After heating at 140 ꢀC for 3 h, the aluminum plates were cooled to
room temperature to prepare single lap joint. The two plates were
pulled from both sides to apply tensile-shear, and the maximum
load at bond-breaking was measured for the calculation of adhe-
sion strength.
1.74e1.67 (m, 4H), 1.48e1.39 (m, 4H), 0.95 (t, J ¼ 7.4 Hz, 6H) ppm.
13C NMR (150 MHz, DMSO-d6):
d 166.16, 165.99, 158.66, 135.38,
132.39, 119.97, 119.73, 65.62, 31.16, 19.69, 14.56 ppm. IR (KBr): 3304,
2961, 2874, 1721, 1697, 1648, 1592, 1559, 1496, 1445, 1394, 1332,
1251, 1110, 1061, 999, 984, 889, 772, 739, 674, 588, 555 cmꢁ1
.
2.12. 12Oc
To a mixture of acetonitrile (3.5 mL), water (3.5 mL), and NMP
(3.5 mL) was dissolved 11Oc (1.59 g, 5.17 mmol), and Oxone (3.19 g,
5.20 mmol) was added. After stirring for 2 days, the reaction

86
T. Oguri et al. / Polymer 99 (2016) 83e89
Scheme 4.
epoxy resins. To avoid steric hindrance, the positions ortho to the
hydroxy group were left vacant. Furthermore, to avoid the forma-
tion of phthalazine [33], the ester moiety was introduced at the
meta position of the hydrazide group. Thus, bisphenol 12 was
rationally designed and synthesized from the corresponding
diester 10 (Scheme 4). Ethyl (10Et), butyl (10Bu), octyl (10Oc), and
2-(2-methoxyethoxy)ethyl (10PE) esters were used. Hydrazination
of 10 with 1 equiv of hydrazine monohydrate gave a mixture of 10,
11, and the corresponding bishydrazide, which can be removed
from the reaction mixture due to its high solubility in water. Since
the polarity of the hydrazide group is considerably higher than that
of the ester group, 11 could be easily separated from 10. Oxidative
coupling of monohydrazide 11 was effectively carried out with
Oxone® [34], except for 11PE, because 12PE was slightly soluble in
water. For 11PE, PhI(OAc)2 was used as the oxidant, furnishing 12PE
in moderate yield. The structure of 12 was confirmed by a charac-
teristic 1H NMR signal of the phenolic OH group at 10.3 ppm and
that of the diacylhydrazine NH group at 10.7 ppm.
The oxidation of diacylhydrazine 12 was examined. When 12
was treated with 5% sodium hypochlorite solution, rapid decom-
position accompanied by the evolution of nitrogen gas was
observed, resulting in a clear brown solution, which is indicative of
phenol group oxidation.
The solubility of 12 in equimolar amounts of BPAER and BDER
was examined. The ethyl and butyl esters, 12Et and 12Bu, respec-
tively, did not form homogeneous mixtures even at 200 ꢀC,
although homogeneity could be achieved by the addition of a small
amount of NMP. The amount of NMP required for the dissolution of
12Bu was smaller than that for 12Et. The octyl (12Oc) and polyether
esters (12PE) formed homogeneous mixtures when heated. While a
temperature of 120 ꢀC was required for 12Oc, 12PE dissolved
already at 40 ꢀC. Thus, the solubility increased if an ester group with
a long alkyl chain was introduced, and was further enhanced by the
polyether functionality.
Scheme 3.
3. Results and discussion
3.1. Design, synthesis, and solubility of bisphenols bearing
diacylhydrazine moieties
Simple bisphenols bearing the diacylhydrazine moiety were
prepared from phenolic carboxylic acid esters 1, 4, and 7, as shown
in Scheme 3. The above esters readily reacted with hydrazine hy-
drate to afford phenolic hydrazides 2, 5, and 8, respectively.
Oxidative coupling of these hydrazides with bis(acetylox-
y)(phenyl)-l
3-iodane [31] (PhI(OAc)2) gave the corresponding
diacylhydrazines 3, 6, and 9 in good yields.
The solubility of the prepared diacylhydrazines in epoxy resins
was examined. Equimolar amounts of resins based on bisphenol A
(BPAER) and butane-1,4-diol (BDER) were used (Chart 1). Both
simple diacylhydrazines 3a and 6 and methoxy-substituted diac-
ylhydrazine 3b were insoluble in all epoxy resins, even at high
temperatures, which was rationalized by strong hydrogen bonding
between diacylhydrazine moieties [32]. The flexible aliphatic
diacylhydrazine 9 exhibited better solubility, dissolving in BDER
above 80 ꢀC, but still being insoluble in BPAER, even at high
temperatures.
3.2. Curing of epoxy resins and their properties
Ester groups were introduced onto the bisphenol scaffold to
increase the solubility and prepare homogeneous mixtures with
Curing was performed with 10 mol% of the imidazole catalyst
added to the mixture of epoxy resin and bisphenol 12, followed by
heating to 130e140 ꢀC. The curing temperature, the range for the
exothermic reaction, was measured by DSC, with a heating rate of
5 ꢀC/min. A small amount of NMP was added in the case of 12Et and
12Bu to make the mixture homogeneous. A typical DSC profile is
shown in Fig. 1. The results are summarized in Table 1.
The curing temperature decreased slightly when an ester group
with a long alkyl chain was introduced, the drop being even more
pronounced when 12PE was used as the curing agent. It seems that
bisphenol solubility is responsible for these changes, with the
curing inhibited by the aggregation of diacylhydrazine moieties due
to strong hydrogen bonding. The aggregation effect is supposedly
Chart 1.

T. Oguri et al. / Polymer 99 (2016) 83e89
87
Fig. 1. DSC profile for the mixture of 12Et, BPAER, 10 mol% of imidazole, and NMP at the heating rate of 5 ꢀC/min.
less significant for the more soluble diacylhydrazines.
the thermal stability. In general, the epoxy resin cured by 12 shows
comparable thermal stability to that of the resin cured by the
nonfunctionalized bisphenol.
Oxidative degradation of the cured resin was investigated.
When immersed in 5% sodium hypochlorite solution, every resin
gradually decomposed with the evolution of nitrogen gas to give
clear colorless solutions. Since brown color was not observed, the
phenol groups in the curing agent were consumed during the
curing process, and resin decomposition occurred by oxidative
degradation of the diacylhydrazine moiety. The degradation rate
could be roughly estimated from the rate of gas evolution. The
epoxy resin cured by 12PE decomposed most rapidly, and the one
cured by bisphenol with a short alkyl ester moiety decomposed
slower. The order of the decomposition rates was independent of
the hydrophilicity of the curing agent, correlating with its solubility
in the epoxy resin. Therefore, the dispersity of the diacylhydrazine
moiety in the cured resin, rather than the attack of the hypochlorite
3.3. Adhesion and dismantling
Equimolar amounts of the epoxy resin and bisphenol 12 were
mixed with 10 mol% of imidazole. For 12Et and 12Bu, a minimal
amount of NMP was added, and the mixture was heated under
stirring until homogenization. The resulting paste was placed be-
tween two flat materials, and the resin was cured at 140 ꢀC for 1 h
before the investigation of adhesive properties. The results are
summarized in Table 3.
Glass was strongly and toughly bonded by all samples tested.
Metal was also strongly and toughly bonded when BPAER was
used. However, the adhesive became brittle when using BDER, with
strong adhesion possible only when 12Et was used as the curing
agent. Natural rubber plates were not bonded with either epoxy
resin. In the case of wood and paper, the resin infiltrated the ma-
terial and did not work as an adhesive.
Adhesion strength was measured using aluminum plates and
BPAER. To evaluate the intrinsic strength of the epoxy resin, 12Oc
and 12PE were used as curing agents; the NMP component was
unnecessary for these bisphenols. Bisphenol A was used as a
reference curing agent. Two aluminum plates of 1 mm thickness
were bonded with a homogeneous mixture of BPAER, bisphenol,
and imidazole in a molar ratio of 1.0:1.0:0.1 to prepare single lap
joint. A 1.2 kg weight was put on the plates, and the epoxy resin was
cured at 140 ꢀC for 3 h. The curing condition was not optimized. The
plates were pulled from both sides to break the adhesive, with
interfacial failure observed in every case. The tension was moni-
tored by TMA to obtain the maximum load, which was divided by
adhesion area to give adhesion strength [29,30]. The results are
summarized in Table 4. The epoxy resin cured by 12 exhibited high
adhesion strength, although it was reduced to 30e40% when
diacylhydrazine moieties were introduced into bisphenol. The high
crystallinity of the diacylhydrazine moiety makes the epoxy resin
brittle, reducing the adhesive toughness.
anion and water, has
decomposition.
a profound effect on the rate of
Thermal stability of the cured resin was evaluated as the 5%
weight loss temperature Td5 measured by TGA in air, with a heating
rate of 10 ꢀC/min. A typical TGA profile is shown in Fig. 2. The results
are summarized in Table 2; the thermal stability of the epoxy resin
cured by bisphenol A is also listed for comparison.
Since the epoxy resin cured with 12Et exhibited a higher Td5
value than that cured with bisphenol A, the introduction of the
diacylhydrazine moiety increased the thermal stability of the resin,
probably due to its strong hydrogen bonding ability. However, the
ester group with a long alkyl chain or the polyether chain decreased
Table 1
Curing temperature of epoxy resin.a
Curing agent
Curing temperature (ꢀC) for
BPAER
BDER
12Et
12Bu
12Oc
12PE
122e189
129e191
121e190
39e153
123e192
129e195
123e194
41e151
a
BPAER or BDER was mixed with equimolar amount of 12 and 10 mol% of
imidazole. The mixture was heated with monitoring heat flow by DSC (5 ꢀC/min).

88
T. Oguri et al. / Polymer 99 (2016) 83e89
Fig. 2. TGA profile of BPAER cured by 12PE and 10 mol% of imidazole at the heating rate of 10 ꢀC/min in air.
Oxidative dismantling was investigated as follows. Two glass
dismantling are listed in Table 5.
slide plates were bonded and placed in a sodium hypochlorite so-
lution. Gas evolution started immediately, and the plates were
spontaneously separated after a certain period. Even though the
adhesive layer was very thin, the decomposition completed
without any stirring, since the degradation products were small
molecules. The mixing effect of the evolving nitrogen gas might
help the infiltration of sodium hypochlorite solution into the space
between the glass plates. The residual epoxy resin deposit
remaining on glass finally disappeared after further treatment with
sodium hypochlorite solution. The times for spontaneous
Oxidative degradation of BDER was faster than that of BPAER,
probably due to the more hydrophilic nature of BDER. The curing
agent had a large effect on the rate of oxidative degradation:
dismantling occurred faster when a more degradable resin was
used as the adhesive.
4. Conclusion
Novel bisphenols bearing diacylhydrazine moieties were
developed as curing agents for epoxy resins. Functionalizing the
bisphenol with an ester group strongly enhanced its solubility, and
homogeneous solvent-free mixtures were obtained for octyl and
polyether esters. The epoxy resin cured by these bisphenols
showed not only oxidative degradability by sodium hypochlorite
solution, but also strong adhesion to glass and metal. The materials
Table 2
Thermal stability (Td5) of cured epoxy resin.a
Curing agent
Td5 (ꢀC)
BPAER
BDER
Table 4
12Et
12Bu
12Oc
12PE
358
312
296
305
322
337
318
287
300
e
Adhesion strength of cured resin for aluminum.a
Curing agent
Adhesive strength (MPa)
bisphenol A
12Oc
12PE
bisphenol A
1.5
1.1
3.8
a
Measured by TGA (10 ꢀC/min in air).
a
BPAER is used as the epoxy resin. Two aluminum plates were
adhered at 140 ꢀC for 3 h with 1.2 kg of weight. The curing condition
was not optimized.
Table 3
The adhesive property of cured epoxy resin.a
Epoxy resin
Curing agent
Material
Glass
Metalb
Rubber
Wood
Paper
Table 5
The period elapsed until the spontaneous debonding.a
BPAER
BPAER
BPAER
BPAER
BDER
BDER
BDER
BDER
12Et
12Bu
12Oc
12PE
12Et
12Bu
12Oc
12PE
T
T
T
T
T
T
T
T
T
T
T
T
T
B
B
B
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Curing agent
Elapsed time (h)
BPAER
BDER
12Et
12Bu
12Oc
12PE
30
30
18
6
20
20
12
4
a
a
T: tough, B: brittle, N: no adhesion.
Aluminum, copper, and iron.
Two slide glass plates were adhered by cured epoxy resin, and stood in sodium
hypochlorite solution.
b

T. Oguri et al. / Polymer 99 (2016) 83e89
89
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dismantled by treatment with sodium hypochlorite solution. The
adhesion strength and oxidative degradability were dependent on
the miscibility of the curing agent with the epoxy resin: while long
alkyl and polyether chains increased both the curing agent misci-
bility and the degradability of the cured resin, these side chains also
slightly decreased the thermal stability and adhesion strength of
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Supporting information
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Acknowledgements
We thank the financial support of Grant-in-Aid for Scientific
Research from Japan Society for the Promotion of Science
(23550141) and MEXT-Supported Program for the Strategic
Research Foundation at Private Universities, 2013e2017 (Creation
of new fusion materials by integration of highly-ordered nano
inorganic materials and ultra-precisely controlled organic
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