A starch based sustainable tough hyperbranched epoxy thermoset

Article abstract of DOI:10.1039/c5ra09955b

With the growing concern of long term environmental and waste management problems, the recent trend in the polymer industry is aimed to utilize environmentally benign substrates for development of sustainable polymers with the required properties for their potential applications as substitutes for petrochemical derivatives. In this arena, the authors aspired to use starch, a natural renewable polysaccharide obtained from a wide variety of crops, as one of the reactants for a one-pot synthesis of a bio-based sustainable hyperbranched epoxy resin. Nuclear magnetic resonance (¹H NMR and ¹³C NMR), Fourier transformed infrared spectroscopy (FTIR) along with different analytical techniques confirmed the chemical structure of the resin. The poly(amido amine) cured epoxy thermoset exhibited acceptable biodegradation along with desirable properties. It exhibits excellent impact resistance (>100 cm), outstanding scratch hardness (>10 kg), exceptionally high tensile adhesive strength (up to 2906 MPa for aluminum), moderate tensile strength (up to 29 MPa), good elongation at break (up to 38%), high toughness (up to 8.40 MJ m^-3) and very good chemical resistance against a number of chemical environments. Moreover, the thermoset displayed potent biomedical attributes by exhibiting cytocompatibility with erythrocytes as assessed through a hemolytic assay. Thus, the synthesized eco-friendly and sustainable hyperbranched epoxy thermoset with good toughness and exceptional adhesive strength can be a worthy replacement for petroleum-based epoxy thermosets as an advanced engineering material.

Full text of DOI:10.1039/c5ra09955b

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Starch Based Sustainable Tough Hyperbranched Epoxy Thermoset
Rituparna Duarah and Niranjan Karak*
Received 00th January 20xx,
Accepted 00th January 20xx
With the growing concern to long term environmental and waste management problems, the recent trend in polymer
industry is aimed to utilize environmentally benign substrates for development of sustainable polymers with required
properties for their potential applications as substitutes for petrochemical derivatives. In this arena, the authors aspired to
use starch, a natural renewable polysaccharide obtained from a wide variety of crops, as one of the reactants for a one-pot
synthesis of a bio-based sustainable hyperbranched epoxy resin. Nuclear magnetic resonance (1H NMR and 13C NMR),
Fourier transformed Infrared spectroscopy (FTIR) along with different analytical techniques confirmed the chemical
structure of the resin. The poly(amido amine) cured epoxy thermoset exhibited acceptable biodegradation along with
desirable properties. It exhibits excellent impact resistance (>100 cm), outstanding scratch hardness (>10 kg), exceptionally
high tensile adhesive strength ( up to 2906 MPa for aluminum), moderate tensile strength (up to 29 MPa), good elongation
at break ( up to 38 %), high toughness (up to 8.40 MJ m-3) and very good chemical resistance against a number of chemical
environments. Moreover, the thermoset displayed potent biomedical attributes by exhibiting cytocompatibility with the
erythrocytes as obtained through hemolytic assay. Thus, the synthesized eco-friendly and sustainable hyperbranched
epoxy thermoset with good toughness and exceptional adhesive strength can be worthy aspirants for replacing petroleum-
based epoxy thermosets as an advanced engineering material.
DOI: 10.1039/x0xx00000x
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biodegradable starch modified plastics.8-10 Starch based
biodegradable blends of poly(β-hydroxyalkanoates) (PHA),
polylactide (PLA) and poly(ε-caprolactone) (PCL) have been
Introduction
Rapid industrialization through scientific intervention has
revolutionized the concept of sustainable development among
the global communities. Genuine efforts are being made to
generate naturally renewable alternatives with sustainable
attribute with the depletion of the basic raw materials at an
alarming rate, especially petroleum resources,.1-3 In this milieu,
the advancement of bio-based polymers from such renewable
reported to reduce total raw material cost and enhance their
degradability. However, to attain the desired mechanical
properties of such blends some additional agents like glycerol,
sorbitol, urea and formamide have to be incorporated.7 It is
therefore difficult to achieve the expected level of properties
of starch modified polymers to address the demands of many
advanced applications. On the contrary, the use of starch as
one of the reactants to synthesize industrially important
polymer is rarely found in literature, in spite of its many
advantages.
resources contributes
a noteworthy transformation from
conventional petroleum based ones. Further, they minimize
the risk of adverse effect of non-biodegradability of discarded
matter to health and environment, reduce scarcity of raw
material and so on.4-5 Starch, is a versatile and cheap
polysaccharide with reactive primary hydroxyl groups. It is
used as a thickener, water soluble binder, emulsifier, gel
forming agent etc. Further, starch are used in many non-food
items such as paper making, cardboard, textile sizing and
adhesives.6 However, the finished products from native starch
suffers from demerits such as, poor process ability, low
dimensional stability and mechanical properties. Therefore,
various modifications such as blending, derivation and graft
copolymerization have been attempted, to develop starch
based sustainable polymers with enhanced properties.7 One of
such attempts is the development of environmental friendly
Again, amongst different important industrial polymers,
epoxy is a unique class of high-performance thermoset
because of its outstanding attributes like high mechanical
strength, thermostability, chemical resistance, adhesive
strength etc.11 Thus it has garnered widespread attention to
the industrial applications such as coatings, adhesives, binder
in composites and so on. However, the commercial epoxy
thermosets have limited advanced applications, as they
possess disadvantages such as brittleness, low toughness, non-
biodegradability etc.12-14 Hence, in the recent past, a few
attempts have been made to obtain bio-based epoxy with
improved bio-degradability. Efforts have also been made to
develop special architectural hyperbranched epoxy to address
their other shortcomings. Such special structural polymers
possess high reactivity, single step synthetic protocol, low
viscosity and presence of large number of end functionalities
along with internal cavities or free volume.15-17 In almost all
Advanced Polymer and Nanomaterial Laboratory, Center for Polymer Science and
Technology, Department of Chemical Sciences, Tezpur University, Napaam
784028, Assam, India,
E-mail: karakniranjan@gmail.com; Fax: +91-3712-267006; Tel: +91-3712-267009
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such cases, researchers have used vegetable oil-derivatives, Synthesis of hyperbranched epoxy resin
which required additional energy and process step, to achieve
Starch, BPA and epichlorohydrin (1:3 mol ratio with respect to
total hydroxyl groups of starch and BPA) were taken in a two
necked round bottomed flask equipped with a condenser, a
constant pressure dropping funnel, a magnetic stirrer and a
thermometer dipped in an oil bath. The reaction was carried
out at 100 °C for 5 h under constant magnetic stirring. At 60 °C,
5 N aqueous solution of NaOH (equivalent to the hydroxyl
groups) was added drop wise to the reaction mixture through
a dropping funnel, with continuous heating and reached the
desired temperature within 10 min. About 1.5-2 h was
required to complete the addition of NaOH. The reaction was
continued for another 3 h. Then heating was stopped and
allowed to cool down with continuous stirring. The white,
viscous mass was poured into a separating funnel and washed
with brine solution followed by distilled water, three times, to
remove the water soluble impurities and unreacted reagents.
Finally, the resin was dissolved in THF and vacuum dried at 70
°C to remove excess epichlorohydrin and entrapped water.
Using the same technique, three different compositions of
epoxy resins with 5, 10 and 20 wt % of starch were synthesized
and coded as HBSE5, HBSE10, HBSE20, respectively. In addition
to that, DGEBA was also prepared under the same reaction
biodegradability of the resultant thermosets. However,
biodegradability of epoxy thermosets may also be achieved by
the incorporation of hydrolysable bio-based component like
starch, which is biodegradable in
a wide variety of
environments18 to develop epoxy thermosets with sustainable
attribute. Further, starch derivatives like isosorbide, (double
dehydration product of sorbitol), have been reported to
synthesize epoxy resin by etherification with epichlorohydrin
in the presence of aqueous NaOH. Łukaszczyk et al. also
showed the possibility of curing isosorbide based epoxy resins
using typical aliphatic amine based hardeners and acid
anhydrides. However, due to absence of aromatic moiety and
improper crosslinking in the structure, high curing
temperature with longer period of curing time was required.
Also, the properties obtained were found to be inadequate.19
East et al. attempted to cure isosorbide based epoxy resins but
the study for both resins and thermosets were insufficient to
judge the possibility to replace bisphenol A based epoxy
resins.20 Therefore, the design of a hyperbranched epoxy using
starch as one of the reactants is an apt option. Additionally,
such hyperbranched epoxy with combination of aromatic and
aliphatic moieties may lead to excellent performance and
hence may address all the above drawbacks of commercial
epoxy thermosets.21 This type of epoxy may also provide
biocompatibility, which is required for different bio-medical
applications.
conditions for comparison purpose
.
Curing study
For fabrication of HBSE thermosets, proper curing of resin is
Authors, therefore, wish to report here the synthesis, conducted using poly(amido amine), a resinous hardener
characterization and properties evaluation of starch based system. It is a polycondensation product of diethylene triamine
hyperbranched epoxy (HBSE) thermosets to address the with dimmer acid of oleic acid.13 An amount of 50 phr with
shortcomings of commercial epoxy thermosets. The respect to resin of poly(amido amine) hardener was mixed
biodegradability and biocompatibility of the thermosets are with epoxy resins, separately in a glass beaker to obtain a
also delved into. The performance of hyperbranched homogenous mixture of them by hand stirring. A little amount
thermosets is also compared with diglycidyl ether epoxy of of THF was used to facilitate the mixing and stirred vigorously
bisphenol A (DGEBA) thermoset without using starch to prove for about 10 min. The homogenous mixture was then coated
the superiority of the former
.
uniformly on glass plates (75 mm × 25 mm × 1.3 mm) and
commercially available mild steel plates (150 mm × 50 mm
×1.6 mm) for measurement of scratch hardness and impact
strength, respectively. The coated plates were then degassed
in vacuum desiccators to remove entrapped solvent and kept
under room temperature to determine the touch-free time
over a period of 24 h. Further, the films were kept under
vacuum for another 24 h to keep them moisture free. They
were cured at 100 °C followed by post curing at 120 °C for the
required period of time as obtained from their respective
swelling values, in a hot oven.
Experimental
Materials
Soluble starch with molecular weight 342.30 g/mol was
supplied by Sigma Aldrich, Germany and was vacuum dried for
overnight, prior to use. Bisphenol-A (BPA, Sisco Research
Laboratories Pvt. Ltd., India) was used after recrystallization
from toluene. Tetrahydrofuran (THF, SD fine Chem., India) was
used after distillation. Epichlorohydrin, (Sisco Research
Laboratories Pvt. Ltd., India), sodium hydroxide (NaOH,
Rankem, India), sodium sulfate anhydrous, (Merck, Mumbai,
India), hydrochloric acid (Merck, Mumbai, India) and ethanol
(Merck, Germany) were used as received. Poly(amido amine)
hardener (HY 840, Ciba Geigy, Mumbai, India), with an amine
value of 6.6–7.5 equiv/kg, was used as supplied. All other
Characterization
FTIR spectra of the epoxy resins were recorded by a Nicolet
FTIR spectrophotometer (Impact-410, Madison, WI) using KBr
pellet. 1H NMR and 13C NMR spectra of the resins were
recorded by
a 500 MHz AV500 AVANCE-III FT-NMR
reagents used in this investigation were of reagent-grade
.
spectrometer (BROKER, Switzerland), using TMS as the internal
1
standard and d6-DMSO and CDCl3 as the solvents for H NMR
and 13C NMR, respectively. The shear viscosities of epoxy
2 | J. Name., 2012, 00, 1-3
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resins with 10 wt.% of starch at 4 different reaction times electron microscopy (SEM, model JSM-6390LV (JEOL)), after
were determined by using CVO100 Rheometer (Malvern, U.K) platinum coating on the surface.
at room temperature. The physical properties such as epoxy
equivalent, hydroxyl value, solubility, and swelling value of the Biodegradation study
resins were measured by the standard test methods, as
Biodegradation study on the epoxy thermosets were done by
reported in literature.22-23 Scratch hardness test on the cured
McFarland turbidity method using P. aeruginosa and B.
subtilus as the bacterial strains.23-25 A nutrient salt media was
films was carried out by a scratch hardness tester (Sheen
instrument Ltd., U.K.), with stylus accessory and a travel speed
arranged for bacterial growth culture, comprising of 2.0 g
of 30–40 mm/s. Swelling test on the thermosets was done to
(NH4)2SO4, 3.61
CaCl2·2H2O, 2.0
g
g
KH2PO4, 1.75
g
MgSO4·7H2O, 0.2
g
confirm the proper curing of the films, by immersing weighed
amount of the cured films in THF. After 48 h, the final weight
Wf of the swollen film was taken. The swelling value (%) was
determined by the difference in weight between the dried film
and the swollen film, as follows:
Na2HPO4, 50 mg FeSO4·7H2O,
1
mg
CuSO4·7H2O, 50µg MnSO4·5H2O, 70 µg ZnSO4·7H2O, 10 µg
H3BO3·5H2O and 10 µg MoO3 in 1.0 L in distilled water. An
amount of 10 mL of the above prepared liquid culture media
was poured into 100 mL conical flasks and was sterilized using
autoclave machine at 121 °C and 15 lb pressure for 15 min and
allowed to cool down to room temperature. P. aeruginosa and
B. subtilus bacterial strains were cultured in the medium inside
an incubator shaker at 37 °C for 48 h. As calculated by the
McFarland turbidity method, 100 μL (108 microbes/mL) of the
cultured medium was taken in a conical flask containing 10 mL
of the prepared salt medium and was sterilized by exposure to
UV light of wavelength 254 nm. The sterilized films were
incubated inside the medium under sterile condition at 37 °C.
The flask containing mineral salt medium but without any
bacterial strain was used as the control. The extent of
biodegradation was studied from the measurement of weight
loss (%) of the degraded thermoset films, measured after 6
weeks of exposure to the bacterial strains, based on the
following equation.
Swelling % = [(Wf - Wi) / Wi] x 100 -------------------------------- (1)
Where Wi and Wf are the initial and final weights of the
film, respectively. Mechanical properties of the cured films
were measured by a Universal Testing Machine (UTM, Zwick
Z010, Germany) equipped with a 500 N load cell operated at a
crosshead speed of 20 mm/min for tensile strength (ASTM D
638) and elongation at break. The lap shear adhesive strength
(ASTM D 897) of the epoxy resins cured with poly(amido
amine) were taken for the overlapping interfaces of the
adherents (aluminum and plywood) and measured by UTM
equipped with a 10kN load cell at a crosshead speed of 50
mm/min.15 Impact strength of the thermosets was tested by
an impact tester (S. C. Dey Co., Kolkata) as per the standard
falling weight (ball) method (ASTM D 1709) where a weight of
850 g was allowed to fall on the film coated on a mild steel
plate from minimum to maximum falling heights. The impact
resistance is the maximum height up to which the film
remained undamaged. The gloss characteristic of the cured
films was evaluated by using a gloss meter (Minigloss meter,
Sheen, UK), at an angle of incidence, 60°. Thermal properties
of the epoxy thermosets were evaluated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC).
Weight loss, W = (W0 –Wt)/ Wt × 100 ---------------------------- (2)
Where W0 and Wt are the weights of sample before and
after degradation, respectively, at the time of interest, t. The
bacterial growth is indicated by increase in turbidity of the
medium with time. The optical density (OD) of the
microorganism was monitored by measuring the absorbance
of the medium at 600 nm with respect to the control. The
experiment was performed in triplicate. SEM images of the
degraded films were taken to study their surface morphology
after 6 weeks of bacterial degradation.
Thermogravimetric study was carried out by using
a
PerkinElmer 4000 thermal instrument in the temperature
range of 30−700 °C, at a scanning rate of 10 °C/min,
maintaining an inert atmosphere of nitrogen at a gas flow rate
of 30 mL/min. The differential scanning calorimetric study was
done by a PerkinElmer DSC 6000, USA instrument in the
Hemolytic assay
temperature range −30 to +150 °C (starting temperature was 0 To have
°C) following a cycle of heating−cooling−heaꢀng under an cytocompatibility of both HBSE and DGEBA resins and
atmosphere of nitrogen and at a scanning rate of 5 °C/min. thermosets, a hemolytic assay was performed to investigate
a
preliminary understanding about the
The chemical resistance test was determined in different the lysis of red blood cell (RBC) membrane by them. Goat’s
chemical environments, such as aqueous NaOH (5%), aqueous blood was collected from a slaughter house in a heparinized
HCl (10%), aqueous NaCl (15%), aqueous ethanol (20%) and tube containing 4% sodium citrate. It was centrifuged (MPW)
water to study the effect of those chemicals on the at 3000 rpm for 20 min at 4 °C to separate the plasma from
thermosets. The small pieces of cured films were kept in 100 erythrocytes and further washed thrice with phosphate buffer
mL amber glass bottles containing the aforesaid media at 30 saline (PBS). A 5% haematocrit was obtained by re-suspending
°C. The percent of weight loss was measured after 30 days of the packed erythrocytes in PBS (10 mM at pH = 7.4). Different
test. UV-visible spectra were analyzed by a Hitachi (U-2001, concentrations of samples (0.5, 1, 2, 4, and 6 mg mL-1) were
Tokyo, Japan) UV spectrophotometer
.
The surface prepared. Finally, 1900 µL of haematocrit along with 100 µL of
morphologies of the biodegraded polymer films and control the test samples were added in each microfuge tube
films (without bacterial strains) were obtained by scanning (Eppendorf) and incubated at 37 °C for 30 min.
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Fig. 1 1H NMR spectrum of epoxy resin with 10 wt.% just after complete addition of
alkali solution.
The viscosity of the resin increases with the reaction time and
the reactions were carried out for 5 h at 100 °C so that the
hydroxyl groups of starch underwent reaction. This increase in
viscosity was confirmed by determining the variation of shear
viscosity of the epoxy resin with 10 wt.% of starch at 100 °C for
2 h, 3 h, 4 h and 5 h of reaction time. Under a constant shear
stress of 100 Pas, the viscosity increases with reaction time
from 0.8 to 3.2 Pas, with a value of 2.4 Pas at 4 h. The low final
viscosity is due to unique hyperbranched structure with
globular like shape of the epoxy resin.
Scheme 1 Synthesis of HBSE20 resin.
Post-incubation, the cells were kept in an ice bath for 60 s to
halt the incubation of cells, followed by centrifugation for 5
min at 3000 rpm at 4 °C. The haemoglobin concentration was
determined by taking UV-absorbance of the supernatants at
540 nm, as a measure of haemolysis.2, 26 The experiment was
The proposed structure of HBSE20 resin is shown in
Scheme 1. The hydroxyl value, epoxy equivalent and weight
percent of starch are calculated theoretically from the
proposed structure. This structure contains 28 hydroxyl groups
so the calculated hydroxyl value would be 152.55 mg KOH/g
whereas the experimental hydroxyl value is 149.93 mg KOH/g.
This indicates the presence of 26.6 hydroxyl groups which is
close to the theoretical value. Similarly, the weight percent of
starch in the given structure is 22 wt.% with respect to
bisphenol A. This is close to the experimentally used amount of
starch (20 wt.%). Further, the ratio of substituted hydroxyl
group/ unsubstitued hydroxyl group is 0.78. This is again close
performed in triplicate
.
Results and discussion
Synthesis and characterization
HBSE resins were synthesized by a polycondensation reaction
using bisphenol A, epichlorohydrin and starch, keeping the
hydroxyl to epichlorohydrin ratio constant, in presence of
alkali as given in Table 1. At first, DGEBA was obtained as the
reactivity of hydroxyl groups of bisphenol
A towards
epichlorohydrin is greater than that of starch.21 Again, the
reactivity of primary hydroxyl groups of starch is higher than
that of secondary hydroxyl groups. The desired hyperbranched
epoxy resin was then obtained by the reaction between the
DGEBA and starch. The unreacted hydroxyl groups of starch
were reacted with epichlorohydrin, in due course of reaction
time to obtain the final hyperbranched epoxy.
1
to experimental result (0.72, obtained from H NMR spectrum,
as discussed later). However, there are 26 epoxy groups in the
structure which indicates epoxy equivalent of 386.5 g/equiv.
The value deviates from the experimentally observed value of
253.35 g/equiv (39 epoxy groups) which may be due to the
presence of DGEBA moiety with epoxy equivalent of 170
g/equiv (theoretical). Thus, this result indirectly supports the
formation of hyperbranched structure of the starch based
epoxy, though there is certain amount of DGEBA.
Table 1 Composition (mol) of HBSE and DGEBA resins
Further, for better understanding of the reaction
mechanism for the formation of hyperbranched structure, a 1H
NMR spectrum was taken just after complete addition of alkali
solution. This is just to prove the formation of DGEBA at the
initial stage without formation of any starch based epoxy. This
Reactant
Starch
HBSE5
HBSE10
HBSE20
DGEBA
0
0.000730
0.0219
0.00146 0.00292
Bisphenol A
0.0219
0.1679
0.0219
0.2044
0.0219
0.1314
Epichlorohydrin 0.14965
1
1
is indeed found in the H NMR spectrum. The H NMR (Fig. 1)
4 | J. Name., 2012, 00, 1-3
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spectrum shows δH, ppm (500 MHz, d6-DMSO (S), Me4Si): 3.3 The presence of the band observed at 1247 cm−1 is due to the
(1H, CH of oxirane ring), 2.6 and 2.8 (2H, CH2 of oxirane ring), aryl ether linkage in the structure of the resin. The broad and
3.9 (2H, CH2 next to oxirane ring), 6.8 (4H, aromatic protons of strong absorption band in the region of 3252–3547 cm−1 is
bisphenol A), 7.1 (4H, aromatic protons of bisphenol A), 1.6 attributed to the stretching vibration of OH.27 The bands at
(3H, CH3 of bisphenol A), 3.87 (1H, proton of starch attached to 2965-2871 cm−1 and 1161 cm−1 are assigned to the vibrational
C5), 3.83 (1H, proton attached to C6 of starch) 5.01 (1H, proton absorptions of C-H bond and C-O bond of the starch moiety in
attached to C1 of starch), 3.73 (1H, proton attached to C2 of the epoxy structure.27
starch), 3.64 (1H, proton attached to C3 of starch), 3.45 (1H,
H and 13C NMR spectral analysis
proton attached to C4 of starch), 5.48 (1H, OH attached to C4 of
starch), 5.66 (1H, OH attached to C3 of starch), 4.21 (1H, OH
The H NMR spectrum confirmed the simple structure of the
attached to C6 of starch), 4.73 (1H, OH attached to C2 of
1
1
epoxy resin HBSE20. H NMR (Fig. 3a) δH, ppm (500 MHz, d6-
starch). (C1, C2, C3, C4, C5 and C6 of starch are numbered
DMSO (S), Me4Si): 3.3 (1H, CH of oxirane ring), 2.6 and 2.8 (2H,
according to the rules of IUPAC and designated as l, m, n, o, p
and k, respectively in Fig. 1). Thus, the peaks in 1H NMR
CH2 of oxirane ring), 3.9 (2H, CH2 next to oxirane ring), 6.8 (4H,
aromatic protons of bisphenol A), 7.1 (4H, aromatic protons of
spectrum clearly indicate the presence of DGEBA moiety with
bisphenol A), 1.6 (3H, CH3 of bisphenol A), 3.7 (2H, CH2 next to
unreacted starch moiety. No substitution of hydroxyl group of
bisphenol A), 4.1 (1H, CH attached with OH), 2.02 (1H, OH),
starch and opening of epoxide group of DGEBA was observed.
3.64 (1H, proton attached to C5 of starch), 3.74 (1H, proton
Therefore at the initial stage, only DGEBA is formed which is
attached to C6 of starch) 4.95 (1H, proton attached to C1 of
transformed to hyperbranched structure with the reaction of
starch), 3.43 (1H, proton attached to C2 of starch), 3.39 (1H,
hydroxyl groups of starch. However, it is difficult to rule out
proton attached to C3 of starch), 3.35 (1H, proton attached to
the presence of some amount of DGEBA moiety in the
C4 of starch), 5.38 (1H, OH attached to C4 of starch), 5.58 (1H,
hyperbranched resins. Thus, the final structure may be a
OH attached to C3 of starch)28-29,11,21 (C1, C2, C3, C4, C5 and C6 of
mixture of starch based hyperbranched epoxy with DGEBA.
starch are numbered according to the rules of IUPAC and
designated as l, m, n, o, p and k, respectively in Fig. 3a.) The
FTIR spectral analysis
degree of branching of the hyperbranched starch based epoxy
To confirm the different chemical functionalities present in the
was calculated by determining the intensity of the peaks of the
structure of the epoxy resin, FTIR spectroscopy was used, as
substituted and unsubstituted hydroxyl groups from the 1H
shown in Fig. 2. Compared to the IR spectrum of starch, the IR
NMR spectrum of hyperbranched epoxy (Table 2). The
spectra of DGEBA and HBSE5 showed a sharp band for
asymmetric vibration of oxirane ring at 912-825 cm-1.11,12,13,21
considered unsubstituted hydroxyl groups are generated by
The bands at 1604 cm-1 and 3059 cm-1 are due to aromatic C=C
ring opening reaction of oxirane ring of DGEBA. In the same
manner the carbons present in different chemical
stretching vibrations and aromatic C-H vibrations of bisphenol
environments in the structure of hyperbranched epoxy
A moiety in the hyperbranched epoxy HBSE5. The sharp
HBSE20 were confirmed by 13C NMR spectrum (Fig. 3b), δC,
absorption band at 1036 cm−1 confirmed the presence of alkyl–
ppm (CDCl3, S): 31.04 (CH3, bisphenol A), 42.5 and 50 (oxirane
aryl ether groups in the epoxy resin.
ring carbons), 31.04, 114, 128, 143.5, and 156 (bisphenol A
carbons), 68−70 (carbon aꢁached with −OH and ether
linkages),21 105.5 (C1 of starch), 79.5 (C2 of starch), 72.2 (C3 of
starch), 71.1 (C4 of starch), 85.8 (C5 of starch), 67.4 (C6 of
starch),28 (where the carbon atoms C1, C2, C3, C4, C5 and C6 of
the starch moiety are numbered according to the rules of
IUPAC and are designated as o, p, q, r, n and m, respectively,
in Fig. 3b).
Physical properties of the resins
Different compositions of the synthesized starch based
hyperbranched epoxy resins appeared to be odorless and
colorless viscous mass. The physical properties like hydroxyl
value, epoxy equivalent etc. of the resins were determined as
shown in Table 2. It is observed that all three compositions of
epoxy resins possess low epoxy equivalent, which may be due
to the presence of 3 moles of epichlorohydrin with respect to 1
mole of total hydroxyl groups, present in starch and BPA.
However, the resin HBE10 possesses the lowest epoxy
equivalent, and therefore, the highest number of epoxy groups
present in its structure.
Fig. 2 FT-IR spectra of HBSE5, starch and DGEBA resin
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Fig. 3 (a) 1H NMR spectrum of HBSE20 and (b) 13C NMR spectrum of HBSE20.
This may be due to the favorable composition of reactants at The specific gravity of the resins varies from 1 to 1.3 with the
10 wt% of starch, which helps to form the hyperbranched increase of starch content in the resins which may be due to
structure with the highest degree of branching (Table 2). increase of compactness in the structure, as the number of
Further, from the solubility studies, it has been observed that polar groups increases with the same. However, the values are
the resins were soluble in most of the common organic within the normal range of such resins and thermosets (Table
solvents such as dichloromethane, chloroform, acetone, 2).
ethanol, DMAc, DMF, and DMSO etc., and partially soluble in
methanol, xylene and toluene, whereas insoluble in water. Curing study
This high solubility of the resins is mainly due to the presence
For the evaluation of various performance characteristics of
of large numbers of polar functionalities along with flexible
the synthesized HBSE and DGEBA resins, curing was conducted
ether linkage and globular in structure.13 However, compared
with an appropriate hardener, poly(amido amine). During the
to hyperbranched structure, DGEBA resin exhibited less
curing process, the viscous liquid resins are transformed to
solubility in different solvents. The gloss of the thermosets was
solid form by a chemical crosslinking reaction with the
found to be high owing to its highly smooth surface texture
hardener, where high strain oxirane rings of the epoxy resin
and good dimensional stability.
readily open up in the presence of a labile proton of an amine
of the hardener. This hardener contains labile amino protons
that readily react with epoxy groups at high temperature to
Table 2 Physical properties of HBSE and DGEBA resins and thermosets
form crosslinked three- dimensional network structures, which
was monitored by the decrease in the FTIR bands in the region
914–832 cm−1.30
Physical property
Hydroxyl value
(mg KOH/g)
HBSE5
178.46
HBSE10
125.48
HBSE20 DGEBA
149.93
89.37
Epoxy equivalent
(g/equiv)
281.66
206.4
253.35
190.73
Table 3 Curing parameters for HBSE and DGEBA resins
Yield (%)
96.32
96.6
95.25
74 ± 3
1 ±
97.12
71 ± 3
1.05 ±
0.01
1.03 ±
0.01
-
Curing parameter
HBSE5
4
HBSE10 HBSE20
DGEBA
6
Gloss (600)
104 ± 4
0.9 ± 0.01
101 ± 4
1 ± 0.01
Touch free time (at room
temperature) (h)
3
3.5
Specific gravity of
resins
0.02
1.3 ±
0.01
0.72
Curing time at 100 °C (min)
Post curing time at 120 °C
(min)
85
20
75
10
80
15
85
25
Specific gravity of
thermosets
1.06 ± 0.01 1.1 ± 0.01
0.63 0.80
Degree of branching
Swelling value at 25 °C
28
22
25
27
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The curing time of epoxy resin with the poly(amido amine)
hardener depends on the epoxy equivalent of the resin and
the time decreases with the decrease in epoxy equivalent,
which directly indicates the increase in number of epoxy
groups in the structure of resin.21 Among all three
compositions of epoxy resins, HBSE10 took the lowest time to
cure at the given temperature, owing to its lowest epoxy
equivalent. The state of crosslinking or curing was monitored
by measuring the swelling value (Table 3). The curing time is
the time required to attain the swelling value of the epoxy
thermoset in the range of 20−30% (Table 3). Due to the
presence of sufficient crosslinking and its hyperbranched
structure, HBSE took a lesser curing time than DGEBA (Table
3).
Fig. 4 Toughness of thermosets (obtained by integrating stress-strain curves).
Mechanical properties
The mechanical properties of the three HBSE thermosets and
their linear analog were determined (Table 4). The tensile
strength of the thermoset increases with the increase in
aromatic moiety and number of epoxy groups which provides
optimum level of crosslinking in the structure. HBSE10
thermoset has the highest tensile strength as it has the lowest
epoxy equivalent leading to sufficient crosslinking as evident
from its lowest swelling value (Table 3) and highest degree of
branching (Table 2). Although, DGEBA thermoset exhibited
sufficient tensile strength, but possessed low elongation at
break due to its brittle nature, as it contains only an aromatic
For the same reason, HBSE5 exhibited the lowest elongation
whereas HBSE20, the highest elongation at break. It is
noteworthy to mention that HBSE10 thermoset showed the
highest toughness as measured by integrating the area under
the stress-strain curve of it (Fig. 4 and Table 4). This may be
due to the combined effect of high strength and elongation of
the thermoset. The impact resistance was found to be high in
all the hyperbranched epoxy thermosets (impact resistance
was above 1 m as that is the maximum limit of the
instrument). However, DGEBA thermosets exhibited
comparatively low scratch hardness and impact resistance
because of its brittleness character. The adhesive strength of
the epoxy was evaluated for two different substrates viz. ply-
wood and aluminum sheets. However, in all three cases of
HBSE5, HBSE10 and HBSE20, there was substrate failure of the
cellusoic wood substrates, which indicates that their adhesive
strengths were higher than the tensile strength of the wood.
This is attributed to the strong interactions of the polar
hydroxyl, ether and epoxy groups along with the branched
structure with the terminal epoxide groups and the polar
linkages of the poly(amido amine) hardener, with the cellusoic
wood substrates.22 Further, the resins also showed high
adhesive strength with the metal substrates because of their
hyperbranched structures and low viscosity which facilitate the
physical interlocking with the metal substrates.13 In case of
DGEBA, the adhesive strength for both the substrates was less
than the hyperbranched resins. This is attributed to the lesser
number of polar functionalities like hydroxyl groups and ether
groups in its linear structure.
rigid moiety as
a
major constituent in its structure.21
Elongation at break of the thermosets decreased as the
percentages of aromatic moiety and starch content (which
contains amylose and amylopectin moieties) were increased.
Table 4 Mechanical properties of thermosets
Property
Thermoset
HBSE5
HBSE10
29 ± 0.4
HBSE20
26 ± 0.5
DGEBA
Tensile strength
(MPa)
24 ± 0.5
62 ± 0.4
Elongation at break
(%)
28 ± 1.0
5.60 ± 10
>1
36 ± 2.0
8.40 ± 12
>1
38 ± 1.0
7.5 ± 0.6
2.30 ± 8
6.5 ± 5
7.0 ± 0.4
863 ± 5
598 ± 5
Toughnessa
8.28 ±
15
(MJ m-3)
Impact resistance
(m)b
>1
Scratch resistance
(kg)c
>10
>10
>10
Adhesive strength
(MPa) (ply-wood)d
Adhesive strength
(MPa) (aluminium)
1984 ± 5
1086 ± 6
1837 ± 4
2906 ± 4
1337 ± 5
1337 ± 4
Thermal properties
The thermograms and derivatives of the thermograms of the
hyperbranched and DGEBA thermosets are shown in Fig. 5a
and Fig. 5b. The initial degradation temperature, peak
temperatures for first and second stages of degradation and
char residue at 700 °C are given in Table 5. In all the cases it
was observed that the epoxy thermosets exhibited two step
degradation patterns, which can be attributed to the presence
of both aliphatic and aromatic moieties in the structure of the
thermosets.
aCalculated by integrating the area under stress−strain curves, bLimit of the
instrument for impact strength was 100 cm (highest), cLimit of the instrument for
scratch hardness was 10.0 kg (highest), dIn all the cases the used plywood
substrate failed.
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Fig. 5(a) TGA thermograms of thermosets (b) DTG curves of thermosets (c) Glass transition temperatures of thermosets.
A 3-5 wt% of initial loss is due to the loss of volatiles, including (Fig. 5c). This can be attributed to the nature of hyperbranched
water, present in the thermosets as they contain many polar structure in HBSE thermosets and the brittleness character of
groups. The actual initial degradation temperature varies from DGEBA thermosets.11 Further, the Tg value increases with an
262-275 °C as observed from DTG for the thermosets (Table 5). increase in wt.% of starch in the HBSE thermoset. This is due to
The variations of degradation temperature for the thermosets the fact that starch exhibits hydrophilic properties which in
are according to their epoxy equivalent values (Table 1), which turn lead to a large number of intramolecular interactions and
also indicated the adequate degree of crosslink of the strong intermolecular associations via hydrogen bonding
thermosets. The peak temperature for second step of formed by the hydroxyl groups in the structure.7
degradation ranges from 448 to 454 °C due to the degradation
of the thermally stable aromatic moiety and amide linkages in
the epoxy thermoset.11
Table 6 Change in weight (%) of the thermosets in different chemical media
Chemical
HBSE5
HBSE10
HBSE20
DGEBA
Medium
Table 5 Thermal degradation parameters and glass transition temperatures of
10% aq HCl
5% aq NaOH
15% aq NaCl
20% aq EtOH
Water
0.0015
0.0013
0.0012
0.0031
0.0011
0.0013
0.0011
0.0015
0.0025
0.0013
0.0017
0.0014
0.0011
0.0038
0.0012
0.0025
0.0031
0.0024
0.0041
0.0026
thermosets
Parameter
HBSE5
262
HBSE10 HBSE20 DGEBA
Onset temperature (°C)
Peak temperature for 1st
stage degradation (°C)
Peak temperature for 2nd
stage degradation (°C)
Char residue (%)
275
373
266
367
287
374
363
448
451
454
445
Chemical resistance
5.6
3.59
6.39
3.35
Chemical resistance is an essential attribute of epoxy
thermosets. The starch based hyperbranched epoxy as well as
the DGEBA thermosets exhibited very good chemical
resistance in almost all the chemical environments as given in
Table 6. The presence of aromatic moiety and adequate
crosslinking density favor the exceptional chemical resistance
of the epoxy thermoset films.11 However, relatively better
chemical resistance was observed for the hyperbranched
thermosets compared to their linear analog which may be due
to the presence of strong intra- and inter-molecular secondary
interactions such as polar-polar and H-bonding, and the fact
that the former has better crosslinking and unique structural
architecture than the latter.31
Glass transition
64.54
65.15
66.75
71.69
temperature (°C)
However, an overlap of degradation steps was observed which
may be due to the fact that during the degradation of aliphatic
moieties, especially starch based ones, are transformed to
thermostable aromatic moieties through decarboxylation
followed by aromatization. This is indirectly supported by the
observed higher thermal stability of the thermosets with
higher amount of starch, even though it has lower amount of
aromatic moiety based bisphenol-A. The thermosets also
exhibited char residue of 3.35 - 6.39% due to the formation of
carbonaceous products. The glass transition temperatures (Tg)
of the thermosets were measured with the help of DSC given
in Table 5, which indicate that the HBSE thermosets possess
good dimensional stability. The DSC curves indicate that the Tg
of HBSE thermosets are lower than that of DGEBA thermoset
Hemolytic assay
Some of the major requirements of sustainable material are
protection of human health and prevention of permanent
damage to the environment, during its life cycle.
8 | J. Name., 2012, 00, 1-3
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Fig. 6 (a) Anti- haemolytic activity of HBSE and DGEBA thermosets (b) Anti- haemolytic
activity of HBSE and DGEBA resins.
Thus, red blood cell (RBC) hemolysis protection assay was
performed as a biocompatibility representative tool on the
synthesized HBSE and DGEBA, to explore material safety. The
hemocompatibility of both the resins and thermosets of HBSE
and DGEBA were evaluated by detecting the hemolyzation of
erythrocytes by their direct interaction with the erythrocyte
membrane. The toxicity of HBSE and DGEBA thermosets and
resins was evaluated by extent of disruption of the erythrocyte
membrane.2 The RBC protection assay showed that HBSE5,
Fig. 7 (a) Variation of P. aeruginosa bacterial growth against exposure time for the
thermosets (b) Weight loss of the thermosets after 60 days of exposure to the P.
aeruginosa bacterial strain (c) Variation of B. subtilus bacterial growth against exposure
time for the thermosets (d) Weight loss of the thermosets after 60 days of exposure to
HBSE10 and HBSE20 resins and their thermosets possessed the B. subtilus bacterial strain.
good compatibility with the erythrocytes. However, lysis of the
RBC membrane was observed to be the highest in case of
This is due to dissolving or degradation of starch by
DGEBA, and the lowest in case of HBSE20. Again for both resin
and thermoset of HBSE20, the difference is very minute with
the hematocrit as the percentage of biocompatible
component, starch is high. Tween 20, the negative control
displayed drastic destruction of the RBC membrane against the
positive control haematocrit, as evident from the
comparatively large value of hemoglobin absorbance.
Hematocrit exhibited negligible absorbance value. Thus, in-
vitro hemolytic assay confirmed the cytocompatibility of the
tested epoxy with mammalian RBC (Fig. 6).
microorganism attack.8 However, P. aeruginosa bacterial strain
exhibited comparatively higher degradation rate compared to
the B. subtilus bacterial strain, owing to the fact that the
former possesses higher biosurfactant activity and cell surface
hydrophobicity (CSH) than the latter bacterial strain (Fig. 8).33
Hence, a faster colonization and growth of the former bacterial
strain on the polymeric surface was observed than the latter
bacterial strain. This further contributes in biodegradation of
the starch based hyperbranched epoxy thermosets. Also, as
HBSE20 thermoset consisting of the highest amount of starch
in its structure, the growth of the bacteria was found to be the
highest in it. About 26 wt. % and 23 wt. % of weight losses
were found after 6 weeks of their exposure to the P.
aeruginosa and B. subtilus bacterial strains, respectively (Fig.
7b and Fig. 7d). On the other hand, DGEBA thermoset was
found to be biodegradable to a very small extent, owing to the
presence of the non-biodegradable DGEBA moiety as the
major constituent in its structure. The minor amount of
biodegradation of it is due to presence of bio-based
poly(amido-amine) hardener in the thermoset.21 The SEM
images (Fig. 8a and Fig. 8b) of the biodegraded HBSE20
thermoset films revealed significant surface erosion and
bacterial adherence as compared to the control (Fig. 8e) which
further confirmed the bacterial biodegradation. The surface of
DGEBA thermoset films also showed slight surface erosion (Fig.
8c and Fig. 8d) as compared to the control film of DGEBA (Fig.
8f). Thus from the biodegradation study it could be concluded
that the rate of biodegradation of all three hyperbranched
epoxy thermosets depends upon the starch content.
Biodegradation study
From the biodegradation study of HBSE and DGEBA thermoset
films, it was established that they were gradually degraded on
exposure time by P. aeruginosa and B. subtilus bacterial strains
(Fig. 7). The differences in the cell wall structure of the gram
negative bacteria, P. aeruginosa and gram positive bacteria B.
subtilus act as an active barrier which is supposed to control
the degradation of the polymer substrate.32 It was found that
the epoxy thermosets were degraded to a considerable
amount after 6 weeks of inoculations, by the bacterial strains
(Fig 8). This signifies that the polymer (carbon source) acted as
catabolite to the bacteria. The growth of both the bacterial
strains was found to be the highest for HBSE20, as it contains
the highest amount of starch in its structure, and thus provides
a better susceptible surface for bacterial growth, as evident
from the OD curves (Fig. 7a and Fig. 7c). The weight
percentages of weight loss of the epoxy thermosets were
found after 6 weeks of exposure to the bacterial strains which
showed reduction in weight of the thermosets.
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Fig. 8 SEM images of biodegraded (a) HBSE20 by P. aeruginosa (b) HBSE20 by B. subtilus (c) LE by B. subtilus (d) LE by P. aeruginosa and respective controls of (e) HBSE20, and (f)
DGEBA thermoset.
Conclusion
Notes and references
A
novel bio-based biodegradable epoxy thermoset was
1
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Acknowledgement
The authors express their gratitude to Dr. Shaswat Barua for
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Gogoi for his help in the study of bacterial biodegradation and
their valuable suggestions during the work. Dr. P. J. Saikia,
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