Bull. Mater. Sci., Vol. 24, No. 5, October 2001, pp. 535–538. © Indian Academy of Sciences.
Interpenetrating polymer networks based on polyol modified castor
oil polyurethane and poly(2-hydroxyethylmethacrylate): Synthesis,
chemical, mechanical and thermal properties
K PRASHANTHA, K VASANTH KUMAR PAI, B S SHERIGARA* and
Department of Industrial Chemistry, Kuvempu University, Jnana Sahyadri 577 451, India
Fosroc Chemicals (India) Limited, Kuluvanahalli, Bangalore 562 111, India
MS received 9 May 2001; revised 20 July 2001
Abstract. Interpenetrating polymer networks (IPNs) of glycerol modified castor oil polyurethane (GC–PU)
and poly[2-hydroxyethylmethacrylate] (PHEMA) were synthesized using benzoyl peroxide as initiator and
N,N-methylene bis acrylamide as crosslinker. GC–PU/PHEMA interpenetrating polymer networks were
obtained by transfer moulding. These were characterized with respect to their resistance to chemical reagents
and mechanical properties such as tensile strength, per cent elongation and shore A hardness. Differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were undertaken for thermal characte-
rization. The changes in NCO/OH ratio and GC–PU/PHEMA composition on the properties of the IPNs were
Keywords. Transesterification; glycerol modified castor oil polyurethane; poly[2-hydroxyethyl methacrylate].
Interpenetrating polymer networks (IPNs) are a new class
of polymer blends in network form in which at least one
component is polymerized and/or crosslinked in the
immediate presence of the other (Sperling 1981). IPNs
possess several interesting characteristics in comparison
to normal polyblends, because the varied synthetic tech-
niques yield IPNs of such diverse properties that their
engineering potential spans a broad gamut of modern
technology (Sperling 1981). IPNs from castor oil based
polyurethane and methacrylate polymers have been
reported by many researchers (Carraher and Sperling
1981; Sperling et al 1981; Sperling and Manson 1983;
Tan and Xie 1984; Sperling 1986, 1993; Patel and Suthar
1987, 1990; Nayak et al 1993; Mallu et al 2000). Though
voluminous literature is available on modified castor oil
polyurethane, the study of IPNs of modified castor oil
polyurethane has not been paid due attention.
As a part of our free radical polymerization studies
(Sherigara et al 1999; Rai et al 2000; Yashoda et al 2000,
2001; Prashantha et al 2001), we present here the synthe-
sis of sequential IPNs from glycerol modified castor oil
polyurethane and poly(2-hydroxyethylmethacrylate). Eva-
luation of their chemical, mechanical and thermal proper-
ties and the effect of NCO/OH ratio of the polyurethane
prepolymer (PPU) on these properties have been
2.1 Characterization methods
2.1a Infrared spectra: Infrared spectra of the synthe-
sized IPNs in KBr pellets were obtained from Shimadzu
FTIR 4200 series spectrophotometer. Whereas in case of
PPU, being a liquid, a thin film was cast over the NaCl
block and its FTIR was recorded.
2.1b Glass transition temperatures: The glass transi-
tion temperatures were determined on a Mettler TA4000
DSC. Temperature and energy calibration were carried
out with indium. Samples weighing between 12 and 15 mg
were used in all cases. The scan rate was 10°C/min in air.
The glass transition temperature (Tg) was calculated as the
inflection point of the jump of heat capacity.
2.1c Mechanical properties: The tensile strength and
elongation at break were measured at room temperature
using Instron Houns-Field universal testing machine
model 4204 as per ASTM D-638 method. Hardness
measurements were made on Shore A hardness test appa-
ratus using ASTM-2240 method.
*Author for correspondence
K Prashantha et al
2.1d Resistance to chemical reagents: Acid, alkali and
solvent resistance were estimated according to ASTM-D-
543-67 method. Samples were hung in the reagent for
seven days and tested for change in weight and for their
2.1e Thermogravimetric analysis: Thermograms were
obtained on Mettler TA 4000 TGA at a heating rate of
10°C/min in air.
British Standard Specification (BSS) grade castor oil
(hydroxyl value 160–162 mg KOH/g) was used without
any purification. Hexamethylene diisocyanate (HMDI)
and 2-hydroxyethylmethacrylate (HEMA) were obtained
from E. Merck (Germany). Triethanolamine and benzoyl
peroxide were obtained from S.D. Fine (India). N,N-
methylene bis acrylamide was procured from Loba Che-
mie (India). Benzoyl peroxide was recrystallized from
chloroform and monomer, HEMA was freed from the
inhibitor before use. All other reagents were of analytical
grade and used without further purification.
2.3a Transesterification castor oil using glycerol (GC–
polyol): A resin kettle equipped with thermometer,
stirrer, nitrogen inlet and reflex condenser were charged
with one equivalent each of castor oil and glycerol along
with catalyst litharge (0⋅05%). Reaction was carried out at
240–250°C for 2 h. Progress of the reaction was moni-
tored by thin layer chromatography (Das and Nirvan
1994) (solvent system: petroleum ether, diethylether and
acetic acid in the ratio of 85 : 15 : 1 by volume, respec-
tively). The resultant polyol was dried at 80°C under
vacuum and had hydroxyl value 290 mg KOH/g, acid
value 2⋅0 mg KOH/g and the viscosity (at 30°C) 483 cps
(Athawale and Kolekar 1998) (scheme 1).
2.3b Polyurethane prepolymer (PPU): A reaction
kettle, under dry nitrogen, was charged with HMDI and
GC–polyol of varying ratio of NCO/OH were added
slowly with stirring. The reaction was carried out at 45°C
for 2 h.
2.3c Sequential interpenetrating polymer network (IPNs):
IPNs were synthesized by charging the isocyanate termi-
nated polyurethane prepolymer in different proportions
and different NCO/OH ratio along with 1% tri-
ethanolamine (for chain extension and curing) to the
mixture of HEMA, 1% N,N-methylene bis acrylamide
crosslinker and benzoyl peroxide (0⋅5% based on HEMA)
in a reaction kettle. The mixture was stirred at room
temperature for 15 min to form a homogeneous solution.
The temperature was increased to 60°C to initiate HEMA
polymerization. After stirring for 1 h, the solution was
poured into a glass mold kept in a preheated oven main-
tained at 60°C. It was kept at this temperature for 24 h
and at 120°C for 4 h to facilitate the complete network
formation. The film thus formed was cooled slowly and
removed from the mold. Nine IPNs were synthesized by
this method (table 1).
3. Results and discussion
3.1 Infrared spectroscopy (IR)
IR spectra of PPU showed characteristic absorption
at 1740 cm–1 and 3410 cm–1 corresponding to urethane
amide (–NH stretching). As the prepolymer is isocyanate
terminate, an intense and sharp band due to NCO is
observed at 2260 cm–1. IR spectra of IPNs showed broad
peak at 3220 cm–1 corresponding to the OH stretching of
the urethane linkage with finite contribution from exten-
sive hydrogen bonding in the system. Further, IPNs does
not show any band at 2260 cm–1 corresponding to NCO
Synthesis, chemical, mechanical and thermal properties of IPNs
because of the reaction of curing agent, 1% triethano-
lamine with terminated NCO. Therefore, there would
be no NCO left for the formation of chemical linkage
between NCO and OH group of PHEMA or HEMA.
3.2 Glass transition temperatures (Tg)
The glass transition temperatures of synthesized IPNs lie
between the Tg of the PPU and PHEMA (table 2). The
appearance of single Tg indicated the formation of an
interpenetrating polymer networks and ruled out the
formation of any phase separation. With increase in
PHEMA concentration in the IPN the Tg of the IPN
shifted towards the Tg of PHEMA.
3.3 The compatibility factor (q)
The compatibility factor value is presented in table 2. It
was calculated from a theoretical equation of DiBeneditto
modified by Xiao et al (1983). A decrease in q implies
greater compatibility caused by increasing NCO/OH ratio.
3.4 Mechanical properties
Mechanical properties such as elongation at break (%),
tensile strength and shore A hardness are furnished in
table 3. The NCO/OH has considerable effect on the
molecular weight of the PPU and the crosslinking density
of the resultant product. It also influences the compati-
bility between polyurethane and PHEMA, hence the pro-
perties of IPNs. High NCO/OH leads to low molecular
weight of PPU (Xiao et al 1983). From table 3 it is
observed that tensile strength and hardness increases
whereas, elongation decreases with increasing NCO/OH
ratio probably due to low molecular weight of PPU at
high NCO/OH ratio. As a result, PU/PHEMA IPNs
exhibit higher tensile strength and hardness. However, an
exactly reverse trend is observed with increasing PU
composition in PU/PHEMA IPNs. It is observed that
glycerol modified castor oil polyurethane and PHEMA
IPNs exhibited better mechanical properties as compared
to unmodified castor oil polyurethane/PHEMA IPNs
(Nayak et al 1997). This may be due to the fact that
glycerol modification of castor oil results in more
crosslinked and stiffer IPNs possessing better mechanical
properties over that of unmodified castor oil polyurethane/
3.5 Chemical resistance
The percentage weight loss of IPNs were determined in
H2SO4, CH3COOH, HCl, HNO3, NaCl and NaOH etc and
the results are furnished in table 4. All the IPNs show
excellent acid and alkali resistance as compared to
unmodified castor oil polyurethane/PHEMA IPNs (Nayak
Table 1. Data on feed composition of individual IPNs.
Table 2. Glass transition temperatures (Tg) of modified castor
oil polyurethane/PHEMA IPN systems.
*Tg = W1Tg1 + W2Tg2, W1 and W2 are the weight fractions of PU
and PHEMA, respectively.
**Compatibility factor (q) was calculated using the equation:
Tg (DSC) – Tg (calcd)/Tg (calcd) = – q/1 + q.
Table 3. Data on mechanical properties.
Tensile strength %
Elongation at break
K Prashantha et al
et al 1997). It is observed that IPNs irrespective of
NCO/OH ratio and PU/PHEMA composition, are stable
in acid and alkali whereas, in methyl ethyl ketone, chloro-
form, carbon tetra chloride and toluene IPNs showed a
varying amount of swelling.
3.6 Thermal analysis
From the thermal analysis it is observed that all the nine
IPNs decompose within 2–4% weight in the temperature
range 0–200°C. About 10% weight loss occurs at 300°C
and about 40% weight loss occurs at 400°C. There is a
rapid weight loss from 40–90% in the temperature range
of 400–500°C. This is due to the decrosslinking of the
two network forms of the IPNs (Sperling 1986). In this
region, the monomer attached to the backbone of the
polyurethane network is most probably detached by a free
radical mechanism from the trunk of the main constituent
polymer. The final weight loss occurs because of the
breakage of the bonds of the homopolymer (PHEMA).
Interpenetrating polymer networks prepared from PHEMA
and the glycerol modified castor oil polyurethane showed
excellent chemical resistance, hardness, elongation, tensile
strength properties and higher compatibility. Hence, it can
be concluded that using the IPN concept, it is possible to
design the most desirable material for a specific end use
One of the authors (KP) thanks the Kuvempu University,
Jnana Sahyadri, for awarding a fellowship.
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Table 4. Chemical resistance test (% weight loss on treatment with different chemical reagents).
reagents IPN-1 IPN-2 IPN-3
IPNs are swelled in solvents like methyl ethyl ketone (MEK), CCl4 and toluene.