cis-4-cyclohexene-1,2-dicarboxylic anhydride, and the catalytic
hydrogenation of the intermediate. The former reaction requires
strict conditions to prevent the homopolymerization of the
diene and the copolymerization between diene and succinic
anhydride; while the latter reaction requires a highly selective
catalyst to avoid the side reactions on the anhydride groups
such as condensation, hydrogenation and crosslinking. BTCA
is prepared by the liquid-phase air-oxidation of pseudocumene
to form trimellitic acid, which is subsequently dehydrated by
heating crude trimellitic acid with vanadium pentoxide. The
oxidation is conducted in acetic acid with Co(OAc)2 and
Mn(OAc)3 as major catalysts and tetrabromoethane as co-
catalyst. Therecovery of acetic acid is problematicin this process,
and the trace amount of bromine anions in the product lead to
the reduction of the dielectric constant of the cured epoxies. In
view of the unique chemical structures of rosin acids, they can
be converted to the analogues of CHDB and BTCA by simple
reaction processes under mild reaction conditions.
In this study, two rosin derivatives, maleopimaric acid (MPA)
and methyl maleopimarate (MMP) which resemble BTCA
and CHDB in structure and functionality, respectively, were
synthesized (Scheme 1) and studied as curing agents for the
curing of a commercial epoxy resin. In comparison, BTCA and
CHDB were also studied for the curing of the same epoxy.
The major objectives of this study are to identify simple and
effective synthesis methods for MPA and MMP, investigate
the curing characteristics of the novel bio-based curing agents
and determine the positive effects of the bulky hydrogenated
phenanthrene ring structure of rosin acid on the properties of
the cured epoxies. Furthermore, the results from this study could
provide important information for the evaluation of potential
scale-up of the laboratory synthesis in industrial process and for
the feasibility of replacing some existing petrochemical curing
agents with rosin-based compounds.
(98.5%), 1,2-cyclohexanedicarboxylic anhydride (97%, CHDB),
1,2,4-benzenetricarboxylic anhydride (97%, BTCA) and 2-ethyl-
4-methylimidazole (95%), were also obtained from Aldrich.
Potassium carbonate (99%, anhydrous, granular) was obtained
from B. T. Baker. Magnesium sulfate (anhydrous, reagent grade)
was obtained from Fisher. Solvents of reagent grade for synthesis
(acetic acid, dimethylformamide, ethyl ether, chloroform) were
used as received.
Synthesis of curing agents
Synthesis of methyl abietate20. To a 100 mL flask was charged
5.8 g of powdered K2CO3 (42 mmol) and 90 mL of anhydrous
DMF. The mixture was stirred for 30 min at room temperature,
and then 5.0 g of abietic acid (12 mmol) and 5.7 g of iodomethane
(30 mmol) were added. After the reaction was continued for 12 h
at room temperature, another 5.7 g iodomethane (30 mmol) was
added and the reaction was continued for another 12 h at room
temperature. The salt precipitate was removed via filtration, and
the filtrate was diluted with 300 mL of ethyl ether and washed
with water three times. The ethyl ether layer was then dried
with anhydrous MgSO4 and concentrated in vacuum. A white
1
product weighing 3.16 g was obtained (yield 81%). H NMR
(CD3Cl, d ppm) 5.77 (s, 1H), 5.37 (s, 1H), 3.61 (s, 3H), 2.23–
1.56 (m, 11H), 1.25–1.18 (m, 6H), 1.02–1.00 (m, 7H), 0.82 (s,
3H). 13C NMR (CD3Cl, d ppm) 178.81, 145.39, 135.71, 122.62,
120.81, 51.99, 51.17, 46.79, 45.32, 38.56, 37.34, 35.10, 34.75,
27.68, 25.88, 22.69, 21.64, 21.08, 18.36, 17.24, 14.26. FT-IR
(cm-1) 897, 1182, 1230, 1242, 1390, 1452, 1721, 2950. ESI-MS
m/z 317.6, [M + H+].
Synthesis of MMP. In a 100 mL three-necked round flask
equipped with a heating bath, a magnetic stirrer and a reflux
condenser, 15 g of methyl abietate (45 mmol) was slowly heated
to 180 ◦C and then maintained at this temperature for 3 h to
complete the isomerization from the abietic structure to the
pimaric structure under an Ar atmosphere. The reaction was
cooled to 120 ◦C before 50 mL of acetic acid was added. To
this solution were added 4.4 g of maleic anhydride (45 mmol)
and 0.85 g of p-toluene sulfonic acid (PTS) (4.5 mmol). The
reaction mixture was refluxed for 12 h and then cooled to room
temperature to receive the crude methyl maleopimarate. The
crude product was recrystallized from acetic acid twice to obtain
17.0 g white crystals (yield 90%). 1H NMR (CD3Cl, d ppm) 5.52
(s, 1H), 3.66 (s, 3H), 3.11 (m, 2H), 2.70–2.74 (d, 1H), 2.47–2.52
(m, 1H), 2.25 (m, 1H), 1.78–1.24 (m, 13H), 1.15 (s, 3H), 1.00–
0.98 (d, 6H), 0.59 (s, 3H). 13C NMR (CD3Cl, d ppm) 179.29,
173.13, 171.26, 148.22, 125.36, 53.51, 53.47, 52.39, 49.51, 47.21,
45.97, 40.58, 38.21, 37.89, 36.62, 35.58, 35.00, 32.69, 27.09,
21.85, 20.79, 20.17, 17.20, 16.95, 15.76. FT-IR (cm-1) 795, 850,
922, 947, 1000, 1080, 1140, 1246, 1388, 1458, 1718, 1776, 1843,
2860, 2960. Acid value: theoretical: 258 mg KOH g-1; found:
270 mg KOH g-1. ESI-MS m/z 415.4, [M + H+].
Scheme 1 The chemical structures of curing agents used in this study.
Experimental
General
Abietic acid (75% by HPLC) was obtained from Aldrich and
used as received. It was actually a mixture of abietic acid and
other rosin acids with most neutral compounds removed. A
liquid epoxy (2,2-bis[4-(glycidyloxy)phenyl]propane), with an
epoxide equivalent weight of 171–175 g eq-1 (DER 332), was
obtained from Dow Chemical Company. Maleic anhydride
(powder, 95%), iodomethane (99.5%), p-toluene sulfonic acid
Synthesis of MPA23. In a 100 mL three-necked round flask
equipped with a magnetic stirrer and a reflux condenser, 10 g
of abietic acid (75% purity, 24 mmol) was heated to 180 ◦C
and maintained at this temperature for 3 h to complete the
isomerization from the abietic structure to the pimaric structure
under an Ar atmosphere. The reaction was cooled to 120 ◦C,
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1018–1025 | 1019
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