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clay nanocomposites
Progress in Organic Coatings 76 (2013) 1103–1111
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Biodegradation, cytocompatability and performance studies of vegetable oil based hyperbranched polyurethane modified biocompatible sulfonated epoxy resin/clay nanocomposites Gautam Das a , Ranjan Dutta Kalita b , Harekrishna Deka a,1 , Alok K. Buragohain b , Niranjan Karak a,∗ a b
Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Napaam, Assam 784028, India Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Assam 784028, India
a r t i c l e
i n f o
Article history: Received 25 October 2012 Received in revised form 1 February 2013 Accepted 14 March 2013 Available online 8 April 2013 Keywords: Epoxy resin Hyperbranched polyurethane Mesua ferrea L. seed oil Biodegradibility Cytocompatability Renewable resources
a b s t r a c t Lack of degradability and decrease of landfill sites along with growing water and land pollution problems generate a strong concern about the use of synthetic non-biodegradable polymers. Mesua ferrea L. (Ceylon Ironwood) seed oil based diglycidyl sulfone epoxy resin was modified by the same oil based hyperbranched polyurethane. This hyperbranched polyurethane was prepared by using an A2 + B3 approach using monoglyceride of the oil as one of the components, as reported earlier. The epoxy resin was modified by incorporating three different weight percentages of hyperbranched polyurethane viz. 10, 20 and 30 wt%. The hyperbranched polyurethane treated epoxy system was characterized by FTIR, SEM and XRD techniques. The study of performance characteristics reveals that epoxy modified by 30 wt% HBPU is the best composition. Nanocomposites of 30 wt% HBPU based composition were prepared with different dose levels of organo nanoclay (1, 3 and 5 wt%) and were characterized by using wide angle X-ray diffraction (WAXD), SEM, TEM and FTIR techniques. The nanocomposite shows improvement in performance characteristics with the increase of clay content. The nanocomposite with 5 wt% of clay shows an increase of about 230% in tensile strength with respect to the pristine epoxy system. The biodegradability tested on all the samples by Pseudomonas aeruginosa bacterial strain and they exhibited significant degradation after 30 days of inoculation. Thus the resulted nanocomposites have potential to be used as biodegradable an advanced coating materials. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The modification of epoxies to enhance their physical and mechanical properties has been an active area of research for over a few decades. Various modification methods have been use to achieve a tougher and thermally strong epoxy system [1–3]. The physical as well as chemical properties of the modified system, however, largely depend on the degree of compatibility of the used components in the formulation. Among the different types of physical modifications of epoxy resin, blending is one of the most acceptable approaches. The complete miscibility of the components is most desirable because mixing on molecular scale results in superior physical as well as mechanical properties [4]. Epoxy systems are reactive resins that providing a favourable cost-performance
∗ Corresponding author. Tel.: +91 3712 267009; fax: +91 3712 267006. E-mail addresses: [email protected], [email protected] (N. Karak). 1 Present address: Department of Plant Agriculture, University of Guelph, Guelph, Ontorio, Canada N1G2W1. 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.03.007
ratio when compared to other thermosets. These epoxy thermosets have multifaceted applications in surface coating and paints, electronics, structural composites, etc. However, low toughness and brittleness the crosslinked resins with non-biodegradability limits their monolithic applications [5–8]. In recent years, polymers developed from the renewable resources, especially vegetable oils have attracted much attention due to the economical, environmental and societal advantages [9–11]. Vegetable oils have numerous benefits such as biodegradability, renewability, ease of modification, non-toxic, and most importantly environmentally benign [9,10]. Structures of a few fatty acids of vegetable oils are given in Table 1. The large amount of research in the area of synthetic polymers has stimulated new results on nanocomposites based on bio-based polymers (including epoxy thermosets) as matrices with environmental benign reinforcing nanomaterials such as nanoclay. The application of nanotechnology to bio-based polymeric systems may open new possibilities for improving not only the properties but also the cost-price-efficiency at the same time. Owing to the nanometersize particles obtained by dispersion, these nanocomposites can exhibit markedly improved mechanical, thermal, barrier and
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Table 1 Chemical structures of a few fatty acids of vegetable oils. Name of fatty acid
rate of biodegradation and cytocompatability of the nanocomposites were determined in order to examine the applicability of these nanocomposites as advanced coating materials.
Structure
COOH
Behenic
Eicosenoic
2. Experimental
COOH
Capric
6
7
COOH
2.1. Materials
COOH
Gadoleic
COOH
Heptadecanoic
COOH Heptadecenoic
OH
COOH
Isanic
COOH COOH
Linoleic Linolenic
COOH COOH
Myristic Myristoleic
COOH COOH
Oleic Palmitic
COOH
Palmitoleic
COOH Ricinoleic Stearic
OH COOH
physico-chemical properties, when compared with the pristine polymeric systems and conventional composites [12]. Again, there are various approaches to render adequate biodegradability into a polymeric material; however, the one that has gained wide popularity is the use of an already biodegradable component into the non-degradable or less biodegradable polymer [13,14]. Biodegradability of nanocomposites is measured conveniently by broth culture technique where the polymer films are subjected to microbial degradation. P. aeruginosa is well known for its efficiency to degrade polymeric material and hence can be utilized in the present investigation as the microorganism for microbial degradation. Another important facet to examine for biocompatibility of biomaterials is their cytocompatability. Generally, MTT and direct contact tests are used for this purpose [15]. However, non-cytocompatability and shielding efficiency of the polymer to the cells from harmful free radicals can be obtained directly from anti-hemolytic test. Red blood cells (RBC) prone to free radical attack and lose the membrane integrity; consequently, heme protein (haemoglobin) diffusing out of the cell can be estimated by measuring the absorption at wavelength of 415 nm using a spectrophotometer [16]. Although a number of reports on epoxy nanocomposites [17–20] have been published but the work on vegetable oil based epoxy/hyperbranched polyurethane nanocomposites is limited. The lack of sufficient toughness of bio-based epoxy furthers instigate to look for other alternatives ways to improve its performance characteristics. Hyperbranched polyurethane by virtue of its toughness and biocompatibility can be considered as a material of choice for the generation of eco-friendly materials [4]. The present investigation utilized Mesua ferrea L. seed oil (about 70% oil content) based hyperbranched polyurethane modified epoxy of the same vegetable oil as the matrix. In the present communication, the authors wish to report the potentialility of clay nanocomposites of M. ferrea L. seed oil based hyperbranched polyurethane modified sulfone based epoxy of the same oil as advanced coating materials. The performance characteristics like physical, mechanical, thermal properties as well as
M. ferrea L. seeds (Jamugurihat, Assam) were utilized for extraction of the oil. The dried seeds were crushed after dehulling and oil was extracted by solvent soaking method using petroleum ether (60–80 ◦ C). Epichlorohydrin (Merck, India) and 4,4 sulfonlybisphenol or bis(4-hydroxyphenyl) sulfone (bisphenol-S or BPS, Aldrich, Germany) were used without further purification. Poly(amido amine) (HY 840, Ciba Giegy, Mumbai, India) with viscosity of 10,000–25,000 mPas and amine value of 6.6–7.5 equiv/kg was used as received. Bisphenol-A (Burgoyne Burbidges & Co., Mumbai, India) was used after recrystalization from toluene. Organically modified nanoclay, montmorillonite, Nanomer I.30E (octadecylamine modified, Aldrich, Germany) was used as received. Glycerol (Merck, Mumbai) and poly(-caprolactone) diol (PCL, Solvay Co., Korea, Mn = 3000 g mol−1 ) were used after drying at 40 ◦ C under vacuum for 4 h. Lead monoxide (S.D. Fine Chemical Ltd., Mumbai) and 2,4-toluene diisocyanate (TDI, Sigma Aldrich) were used as received. N,N-dimethylformamide (DMF, Merck, Mumbai) was dried over CaO, vacuum distilled, and kept in 4A-type molecular sieves before use. All other reagents used in the present investigation were of reagent grade. 2.2. Methods 2.2.1. Synthesis of diglycidyl ether bisphenol-S epoxy resin (BPSE) The vegetable oil based epoxy resin was prepared by using the similar method as published earlier [21]. Briefly, monoglyceride of the oil (obtained by glycerolysis technique), epichlorohydrine, bisphenol-A (BPA) and bisphenol-S (BPS) were reacted together by maintaining the mole ratio of 1:5:2:1 at (110 ± 5)◦ C for 14 h in slightly alkaline medium. 2.2.2. Synthesis of hyperbranched polyurethane (HBPU) The hyperbranched polyurethane was prepared as reported elsewhere [22]. Briefly, 2.5 mol of poly(-caprolactone)diol, 1.5 mol of monoglyceride of the Nahar oil, and 6.5 mol of TDI were used in the first stage to prepare a prepolymer, and finally, glycerol of 2.5 mol was added to the prepolymer to get the HBPU. The yield of the polymer obtained was 97%. 2.2.3. Modification of epoxy resin by HBPU The composition of the components is given in Table 2. The epoxy resin and the HBPU were mechanically mixed in an oil bath at 80 ◦ C by using a mechanical stirrer. The mixtures were fully stirred for 1 h and then degassed in a vacuum oven to remove any residual gas bubbles or entrapped solvent. Table 2 Compositions of HBPU modified BPSE nanocomposites. Codea
BPSE (g)
HBPU (g)
OMMT (g)
EHBPU10 EHBPU20 EHBPU30 EHPN1 EHPN3 EHPN5
100 100 100 100 100 100
10 20 30 30 30 30
0 0 0 1 3 5
a Digits indicate the hyperbranched polyurethane content in the first three codes and for nanocomposites digits indicate OMMT content.
G. Das et al. / Progress in Organic Coatings 76 (2013) 1103–1111 Table 3 Curing studies of HBPU modified BPSE and its nanocomposites. Sample
Touch free time (min)
Hard dry time (min)
Swelling (%)
Water loss (%)
BPSE EHBPU10 EHBPU20 EHBPU30 EHPN1 EHPN3 EHPN5
45 43 40 35 30 27 17
65 64 63 57 55 47 40
32 30 29 27 25 23 20
1.45 1.12 1.01 0.95 0.85 0.75 0.68
2.2.4. Preparation of nanocomposites The nanocomposites of the 30 wt% hyperbranched polyurethane modified epoxy (EHBPU30) were prepared by incorporating organically modified nanoclay (OMMT) at 1, 3 and 5 wt% (Table 2). The system was heated at 80 ◦ C for 1 h by using a mechanical stirrer. The system was then cooled and sonicated using a single probe sonicator UP200S for 30 min. The temperature of the sample was maintained at (25–30)◦ C by using a water bath. The dispersed nanoclay/resin system was degassed for 30 min under vacuum before further processing. The prepared nanocomposites with 1, 3 and 5 wt% of clay were coded as EHPN1, EHPN3 and EHPN5 respectively (Table 2). 2.2.5. Curing of the hyperbranched polyurethane modified epoxy and nanocomposites By hand stirring for 20 min a homogenous mixture of the epoxy resin, HBPU modified epoxy resin and its nanocomposites with 50 phr (parts per hundred gram with respect to epoxy resin) of poly(amido amine) hardener was prepared, separately, in a glass beaker at room temperature. The degassed mixture was then cast on a glass plate and cured at 100 ◦ C in a muffle furnace to determine both the touch free time (minimum time, when no impression will appear on touching the film) and hard dry time of the resin, modified system and the nanocomposites. The mixtures were also uniformly spread on mild steel plates (150 mm × 50 mm × 1.60 mm), tin plates (150 mm × 50 mm × 0.40 mm) and glass plates (75 mm × 25 mm × 1.75 mm) for impact resistance, gloss and chemical resistance tests respectively. The coated plates were cured at 100 ◦ C for specified period of time as given in Table 3, followed by post curing at 150 ◦ C for 2 h. 2.2.6. Water permeability measurement The water permeability of the nanocomposite films was measured in a desiccating chamber containing CaCl2 as the drying agent under vacuum. Small containers containing weighted amount of distilled water were taken, where middle part of the caps was replaced by experimental nanocomposite film in an airtight manner to avoid any leakage. The containers were then placed in a desiccating chamber. The weight of the container was again taken after a period of 120 h. All the measurements were carried out at (30 ± 1)◦ C and an average of three samples was taken for each measurement. The reduction in water content in the containers was calculated by using the relationship. Percent weight loss of water = [(Wi − Wf )/Wi ] × 100%, where Wi = initial weight and Wf = final weight of the water container. 2.2.7. Biodegradation by broth culture technique A modified broth medium for culture [14] was prepared by dissolving 2.0 g (NH4 )2 SO4 ,·2.0 g Na2 HPO4 , 3.61 g KH2 PO4 , 1.75 g MgSO4 ·7H2 O, 0.2 g CaCl2 ·2H2 O, 50 mg FeSO4 ·7H2 O, 1 mg CuSO4 ·7H2 O, l g MnSO4 ·5H2 O, l g ZnSO4 ·7H2 O, l g H3 BO3 ·5H2 O and l g MoO3 in 1.0 L of demineralized water. 10 mL of this liquid culture media was poured into 100 mL conical flasks and was sterilized.
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The media was then allowed to cool down to room temperature and experimental nanocomposite films were applied to the media under sterile condition. Medium containing no polymer film was also cultured as negative control. 2.2.8. Microbe selection P. aeruginosa strain was selected for the study with strain number MTCC 424. A small inoculums of bacteria containing an approximate 1 × 108 /mL microbes (as calculated from McFarland turbidity method) was inoculated into the conical flask containing 10 mL media for each test. The flasks were then incubated under sterile condition at 37 ◦ C for the degradation study. The samples were collected for spectrophotometric observation at 600 nm against blank culture media on weekly basis under sterile condition. Bacterial growth was calculated from the absorbance data using McFarland turbidity as the standard. 2.2.9. Haemolytic activity assay The haemolytic activity test was done to see if the nanocomposites have any haemolytic activity on the erythrocytes based on the modified protocol as reported by Nair et al. [23]. Briefly, goat blood mixed with the anticoagulant sodium citrate (4%) centrifuged at 2500 rpm for 10 min. The resultant supernatant was discarded and only the erythrocytes were collected. The collected erythrocytes were further washed thrice in PBS (pH 7.4). Now a 10% (v/v) suspension of washed erythrocytes in PBS was prepared in a 50 mL centrifuge tube. 1.9 mL of this erythrocyte solution was taken in a 2 mL centrifuge tube and 100 L of the extract at two different concentrations (1 mg/mL and 250 g/mL) was added into it. The extracts were also prepared in PBS. The tubes were then incubated for 2 h respectively at 37 ◦ C. 2% (v/v) Triton X-100 and PBS were taken as positive and negative control, respectively. After incubation the tubes with 2 mL medium were centrifuged at 2500 rpm for 10 min. 200 L of the supernatant was taken and 2.8 mL of PBS was added to it and then the absorbance was taken at 415 nm in a UV-vis spectrophotometer (Thermo, UK). 2.3. Measurements FTIR spectra of resin, modified system and the nanocomposites were recorded in FTIR spectroscopy (Impact-410, Nicolet, USA) using KBr pellet in the wavelength range of 500–4000 cm−1 at a resolution of 0.01 cm−1 . The surface morphology of the samples was studied by a JEOL scanning electron microscope of model JSM6390LV SEM after platinum coating on the surface. Wide angle X-ray scattering (WAXS) studies were carried out by a powder diffractometer Rigaku X-ray diffractometer (Miniflex, UK) at room temperature (about 25 ◦ C), operated at 30 kV and 15 mA. The scanning rate used was 2.0 min−1 over the range of 2 = 0–30◦ for the above study. The distribution of nanoclay in the polymer matrix was studied by using a JEOL, JSM-1000 CX transmission electron microscope (TEM). A scratch hardness tester (Sheen instrument Ltd, UK) was used to determine the scratch hardness (ASTM D5178/1991) of the cured films. The front impact resistance test was carried out by applying falling ball method with an impact tester (S.C. Dey Co., Kolkata) from a maximum test height of 100 cm. In this test, 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 maximum height up to which the film is not damaged was taken as the impact resistance value. The tensile strength and elongation at break (as per the ASTM D 412-51 T) were measured with the help of a Universal Testing Machine of model Zwick Z010 (Germany) by a 10 kN load cell and at 40 mm/min jaw separation speed. The gloss characteristics of the cured films were found out by a mini glossmeter (Sheen instrument
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Ltd, UK), over the resin and the nanocomposites coated mild steel plates at an angle of incidence of 60◦ . Thermogravimetric (TG) analysis was carried out in Shimazdu TG 50 thermal analyzer at a nitrogen flow rate of 30 mL/min and at the heat rate of 10 ◦ C/min. 3. Result and discussions 3.1. Modification of epoxy The mixing of two polymers to obtain a homogenous system is dependent on many factors. The physical as well as chemical nature of the components plays a decisive role in obtaining a homogeneous phase. Thermodynamics factors such as entropy also must be considered for such system. Mixing of two polymers with low molecular weight can be more conducive for obtaining a uniform system. Further due to the hyperbranched nature of the polyurethane it can provide sufficient functional groups for interaction with epoxy resins. The crosslinking of the system by the hardener may result in a homogenous system with no phase separations [24,25]. M. ferrea L. seed oil modified epoxy resin has glyceride moiety in its structure, this results in the epoxy resin with structural complexity. These long alkyl chains offer epoxy resin with sufficient amount of flexibility, but lacks adequate strength properties. On the other hand, polyurethane of the same oil has structural in-homogeneity with sufficient amount of toughness. As both the polymers have common component of glyceride moiety of the same oil as well as different types of polar groups in the structure, so the resultant product is expected to possess strong interactions and good homogeneity. Further, this may also result biodegradability of the epoxy thermoset. 3.2. Nanocomposites preparation The preparation of suitable nanocomposite of the afore-stated matrix may offer significant improvement of many desirable properties including biodegradation. In this regard, clay plays an effective role in enhancing the rate of biodegradation as it helps in the absorption of the moisture by virtue of its surface hydroxyl groups. This absorbed moisture helps in the hydrolysis of the ester and urethane groups of the matrix and thereby the microbes can easily access the hydrophilic polymer fragments. Thus, clay plays an effective role in enhancing the rate of biodegradation. Thus octadecylamine modified MMT based nanocomposites were prepared in the present investigation. Further, it is expected that hyperbranched polyurethane can help in the exfoliation of the clay layers in the epoxy matrix, because of its unique structural features [14]. Therefore, the modification of epoxy resin by the organically modified clay and hyperbranched polyurethane altogether may result in the enhancement of the performance characteristics including thermo-stability and biodegradability.
Fig. 1. FTIR spectra of (a) BPSE, (b) EHBPU10, (c) EHBPU20 and (d) EHBPU30 before curing.
position shows red shifts of 25 cm−1 for EHBPU10, 10 cm−1 for EHBPU30 and 3 cm−1 for EHBPU20. This shifts may be attributed to the increase in the OH species generated by the ring opening of the oxirane ring. The carbonyl stretching frequency ( C O) broadens and shows shifting to a lower value though the position of the band in all the cases was found to be similar. The broadening was due to the overlap of the NH and C O absorptions band [14]. The shifts however indicates that the C O group of the hyperbranched polyurethane is involved in hydrogen bonding with the OH group and C O group of epoxy resin and vice versa. The epoxide stretching vibrations shows decrease in intensity due to the interaction of the urethane linkage (by NH group) with the oxirane group [26]. In the modified system after curing the absorption bands (in all cases) show significant shift and reduction of the band intensity (Fig. 2). This indicates the formation of crosslinked networks. The crosslinking was further supported by observation of diminishing intensity of band for epoxide stretching at 916 cm−1 in all cases. This suggests extensive amount of interaction present in the system [21]. As observed earlier the OH stretching band for epoxy resin and the NH band of the hyperbranched polyurethane overlaps and appear as a single broad band around 3154–3246 cm−1 .
3.3. FTIR studies of hyperbranched polyurethane modified epoxy and nanocomposites The FTIR spectra of epoxy resin and its modified systems are shown in Fig. 1. The FTIR spectrum of epoxy resin is well documented in our earlier published report [21]. Some of the characteristic bands (cm−1 ) observed are: 3423 ( OH stretching vibrations), 3050 (aromatic C H stretching vibration), 1729 (C O stretching vibration of the triglyceride esters), 1593 (C C stretching vibration), 1300 and 1149 (sulfone stretching vibrations), 1246 and 1106 (C O C stretching vibrations), and 916 and 832 (oxirane ring stretching vibrations). The FTIR spectra of the modified systems show significant shift in the OH stretching vibration, before curing (Fig. 1). The band
Fig. 2. FTIR spectra of (a) EHPN1, (b) EHPN3 and (c) EHPN5 after curing.
G. Das et al. / Progress in Organic Coatings 76 (2013) 1103–1111
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curing there is improper blue shift with simultaneous reduction in the absorption band intensity for C O and red shift of OH absorption band is (Fig. 4) due to significant amount of complex reaction that is occurring during the curing process. Although, the dispersion state of the nanoclay cannot be concluded by the FTIR study. 3.4. XRD analysis
Fig. 3. FTIR spectra of (a) EHBPU10, (b) EHBPU20 and (c) EHBPU30 before curing.
The XRD of the modified system reveals the presence of hyperbranched polyurethane in the modified epoxy resin (Fig. 5i) by the appearance of diffraction peaks at 2 = 21.2◦ (4.19 A◦ ) and 23.4◦ (3.81 A◦ ), which occur due to the crystalline nature of PCL moiety in the structure [29]. The positions of these peaks remain unchanged after modification, though the intensity gradually increases with the increase of HBPU content, which may be due to the increase of the percentage of PCL in the matrix. The state of distribution of clay in the polymer matrix can be obtained from the diffraction pattern in the XRD spectra (Fig. 5ii). In the case of OMMT strong reflection appears at 2 of 4.15◦ resulting from the (0 0 1) crystal surface of the layered silicates. The occurrence of reflection at 20◦ (2) corresponds to (1 1 0) plane. However, in all the nanocomposites, no basal reflection corresponding to (0 0 1) crystal plane was observed. Thus the XRD diffraction peaks for the nanocomposites indicate the possibility of exfoliated nanostructure of clay with different extent of dispersion in the epoxy matrix, though it needs to be confirmed by other tests and analyses like TEM and performance of the thermosets. In case of epoxy resin the effect of clay modifier on the curing has already been reported [21]. The curing rate will be much faster inside the galleries than that outside. As a result of these differences more epoxy chains will be pulled inside the galleries resulting in expanded d-spacing. 3.5. Morphology
Fig. 4. FTIR spectra of (a) EHPN1, (b) EHPN3 and (c) EHPN5 after curing.
This implies that most of the NH groups were hydrogen bonded. The C O absorption intensity decreases in all the cases after curing. This is evident of the extensive amount of interaction between the polar functionalities of the epoxy resin and the hyperbranched polyurethane. The structural changes of the nanocomposites from the pristine polymer can also be analyzed through the FTIR spectra (Figs. 3 and 4) both before and after curing. The bands appearing at about 1034 cm−1 and 550 cm−1 region in the FTIR spectrum (i.e. EHBPU30) is due to Si O and Al O stretching vibrations of nanoclay [27] and the band at about 3446–3627 cm−1 for the OH stretching in Si OH and Al OH moieties located on the surface of the clay (Fig. 3) [27]. The interaction of OH group of clay with the epoxy group and free OH groups of the intercalated polymer chain is indicated by the broadening of OH stretching band in the nanocomposites. The sharp band at about 1620 cm−1 of OMMT corresponding to the absorbed H O H (H2 O) bending vibration (occurring due to hydrophilic nature of the clay) is minimized in the nanocomposites is indicative of the proximity between hydrophobic polymer chains and nanoclay [28]. The C O absorption band prior to curing shows a red shift of about ≈5–10 cm−1 . This red shift indicates lengthening of the C O bond thereby decreasing the absorption frequency by hydrogen bonding. However after
Several factors such as chemical miscibility of the two components, method of mixing, interfacial interaction and crosslinking density, affect the morphology of a two-component system at a given composition [30]. Generally, HBPU shows good compatibility with epoxy resin and among the studied system EHBPU30 showed the best result (Fig. 6). The good homogenization of epoxy resin with hyperbranched polyurethane (Fig. 6, EHBPU30) can be attributed to the polar-polar interaction between the -bonds of aromatic rings in both resins [26]. Enhancement in compatibility is also aided by the network formation through the reaction of amine groups of hardener with ester groups of epoxy resin along with normal crosslinking. Further, the amine hardener may also catalyze reaction of different functional groups of HBPU and epoxy. Thus, the amine hardener also acts as a compatibilizing agent for the system. Similarly, in case of nanocomposites, the addition of nanoclay significantly affects its topological features (Fig. 6b). The SEM image of EHPN3 shows an uneven surface morphology, wherein some protruding white dots or lines were seen on the surface. These changes in surface features can be related to the presence of nanoclay. Observation of protruding dots or lines indicates the adherence of clay layers to the polymeric surface. As observed for the pristine polymeric system the surface is rather smooth. The TEM micrograph of EHPN3 shows well dispersed clay layers in the polymer matrix (Fig. 7), wherein, individual clay layers can be seen. From these observations it can be concluded that XRD and SEM results are complementary by the TEM observation. 3.6. Curing studies The modification of epoxy with hyperbranched polyurethane has profound effect on the curing process of the epoxy resin. The
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Fig. 5. (i) XRD spectra of (a) BPSE, (b) EHBPU10, (c) EHBPU20 and (d) EHBPU30 and (ii) XRD spectra of (a) EHPN1, (b) EHPN3 and (c) EHPN5.
Fig. 6. SEM micrographs for (a) EHBPU30 and (b) EHPN3.
curing time (touch free time as well as drying time) decreases in the modified system with the increase of polyurethane content (Table 3). This may be due to the increase of the possibility of reaction of epoxy/hydroxyl groups of epoxy resin with hydroxyl/urethane groups of polyurethane resins in the presence of amine hardener as the amount of reactants increased [31]. It has been reported [32] that the aromatic moiety of polyurethane resin accelerates the crosslinking reaction of epoxy in an epoxy/amine hardener/urethane reaction system by the formation of an active complex of the hardener with the aromatic moiety of polyurethane with stoichiometric ratio in the curing reaction. Further, the oxirane absorption diminishes in the modified epoxy after curing. The hydroxyl group (of clay) plays a catalytic role in enhancing the cure rate, which can also be justified from the decrease in curing time for the nanocomposites. The cure time decreases with the increases in the clay loading. The alkylammonium ions present in organoclay are also responsible for the catalytic acceleration of the epoxy curing reaction and thus enhancing the cure rate [21].
Fig. 7. TEM micrograph for EHPN3.
3.7. Thermal stability The effect of modification on the thermal stability of the epoxy thermomet was also studied. Fig. 8 shows the thermal profiles of the pristine epoxy and hyperbranched modified epoxy thermosets. However, the initial thermal degradation pattern of the pristine epoxy resin does not show any significant increase after modification. BPSE thermoset exhibit initial degradation temperature of 277 ◦ C, after modification there is a slight increment in the degradation temperature (286 ◦ C). The increase in the thermal stability is due to increase crosslink density thereby bridging the polymer backbone, which results in a tough material [26].
Fig. 8. TGA thermograms for BPSE, EHBPU30 and EHPN5.
G. Das et al. / Progress in Organic Coatings 76 (2013) 1103–1111 Table 4 Performance characteristics of HBPU modified BPSE thermosets.
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Table 5 Mechanical properties of HBPU modified BPSE and its clay nanocomposites.
Sample code
BPSE
EHBPU10
EHBPU20
EHBPU30
Sample code
EHBPU30
EHPN1
EHPN3
EHPN5
Tensile strength (MPa) Elongation at break (%) Impact resistance (cm) Gloss (60◦ ) Scratch hardness (kg)
6 95 100 60 3.4
7.2 97 100 62 4.5
8.2 105 100 62.7 5.1
11.23 110 100 65 5.5
Tensile strength (MPa) Elongation at break (%) Impact resistance (cm) Gloss (60◦ ) Scratch hardness (kg)
11.23 110 100 65 5.5
12.94 67.45 90 70 6
14.98 55.76 87 75 7
19.78 50.14 80 81 10
microphase separation due to more polymer filler interaction [32]. The scratch hardness also augments with the nanoclay loading. The increase in scratch hardness is due to the dissipation of the stress generated at the interface between the moving tip and the film surface throughout the nanocomposite. The stress is than absorbed by the hard clay layers and consequently scratch hardness increase with clay loading in this case.
Consequently, reduction in the segmental mobility may result in increased resistance to thermal decomposition. Higher char content of the HBPU modified BPSE (9.5%) than pristine BPSE (6.5%) also results in shielding of the matrix from thermal decompositions. For HBPU modified BPSE the increase of char residue is attributed to the presence of aromatic both TDI and bisphenol-S and bisphenol-A moieties. Further, it was seen that nanoclay also has a definite role in enhancing the onset degradation temperature (Fig. 8), i.e. up to 309 ◦ C for EHPN5. In addition to restricted segmental motion, clay by virtue of its inorganic nature (SiO2 , Al2 O3 and other metal oxides) also improved the thermal stability of the organic polymer. The well dispersed clay layers thus acts as a barrier preventing volatization of the modified BPSE matrix. The char residue of EHPN5 (10.95%) was also higher than modified as well pristine BPSE at 650 ◦ C. The Hoffman decomposition of the onium modifier of the clay gives protonated montmorillonite that can catalyze the formation of stable carbonaceous residue [21]. However, the onset degradation temperature does not exhibit any significant differences. Although the difference is not too large, there is a slight tendency of increase in the initial degradation temperature with nanoclay content.
The enhancement of barrier properties by the formation of true nanocomposite is a well established phenomenon [34]. The present study on water vapor permeation also shows no exception (Table 3). This can be explained by tortuosity mechanism. The intercalation/exfoliation of clay platelets in the polymer matrix offers a long tortuous path (mean free path) for the guest molecules to pass through and hence the permeation got retarded [35,36]. In the present study, a 2–4-fold reduction in the rate of permeability of water vapor with increase in clay loading from 1 to 5 wt% in the nanocomposites was observed. This is understandable from the increase of tortuosity with the increase of clay loading in the matrix.
3.8. Performance characteristics
3.10. Biodegradation
Vegetable oil based epoxy thermoset lacks desired level of performance. However, the modified BPSE systems were found to exhibit overall good performance characteristics over the pristine system (Table 4). It was also observed that the HBPU content has profound role in optimizing the above performances. The increment of tensile strength was due to increase in the crosslink density; consequently larger stress value is required for their rapture. Thus good compatibility is an important factor for this achievement. The scratch hardness also increases with the increase of the polyurethane content, which is again caused by the increase of crosslinking density, H-bond formation, etc. Although the modified epoxy shows a considerable hardness, still they have enough flexibility as found by the elongation at break values (Table 4). The long fatty acid chains of oil, ester, and ether linkages of PCL render this high flexibility to the films. The impact resistance of all the films shows excellent result as expected from the tensile strength. Nanoclay by virtue of its high aspect ratio and surface area plays a significant role in enhancing the performance characteristics of the HBPU modified BPSE matrix. The tensile strength and scratch hardness of the prepared nanocomposites were found to increase as compared to that of the pristine polymeric system (Table 5). The close proximity of the internally hard clay layers and the soft organic polymer means that the chains were confined by the interface. In addition, as the domain size of the microphase for the system decreases the compatibility between the components increases, this in turn enhances the mechanical properties like tensile strength, modulus, etc. [33]. The mechanical properties gives an idea about the morphological behaviour of the nanocomposites better the mechanical properties, lower is the degree of
One of the most attractive features of polyurethane is its biodegradability, which puts it into the list of potential candidates for many advanced applications [14]. The biodegradability of pristine BPSE, modified epoxy and nanocomposite was tested under control bacterial growth conditions. Pseudomonas sp. being the most prominent organism, as it has high ability to degrade bisphenol-A based epoxy modified vegetable oil based hyperbranched polyurethane [14] so P. aeruginosa (MTCC 424) was taken as the microorganism for the present study. The growth profiling of consortia in the modified broth media lacking dextrose compels the bacteria to use epoxy and modified system as the primary carbon source in each case [37]. The biodegradation of both the pristine BPSE, HBPU modified BPSE and the nanocomposites were quantitatively tested and confirmed by direct exposure to strain of P. aeruginosa bacteria by broth culture technique. After keeping the samples in broth culture media for six weeks, the bacterial OD was determined. The difference in the rate of growth initially for two weeks both for the nanocomposites as well pristine system is not significant as can be seen from the curves after two weeks of bacterial exposure (Fig. 9). However, the bacterial growth rate increases significantly after two weeks, as can be realized from the bacterial count. This observation is indicative of the biodegradation of HBPU modified epoxy and its nanocomposites. After modification of BPSE by HBPU significant enhancement in biodegradation was recorded this accelerated further on addition of nanoclay to the modified epoxy. Again, the rate of biodegradation was found to be higher than the corresponding pristine polymers in both the cases. The growth of P. aeruginosa bacterial strains, modified epoxy system and various nanocomposites films as well as
3.9. Water vapour barrier properties
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Fig. 9. Growth of P. aeruginosa bacterial strain, MTCC 424 on (a) HBPU modified epoxy and (b) nanocomposites.
Fig. 10. SEM micrographs for films after six weeks of bacterial exposure (a) untreated BPSE, (b) inoculated BPSE, (c) EHBPU30 and (d) EHPN5.
on the corresponding pristine polymeric films can be realized from Figs. 9 and 10, respectively. The vegetable oil modified polymeric systems are more prone to bacterial attack [13]. These facts support the above observation
wherein bacterial degradation increases with the hyperbranched polyurethane content. The SEM micrographs (Fig. 10) after six weeks of bacterial exposure exhibit significant topographical changes for polyurethane modified epoxy resin. The bacteria
Fig. 11. RBC protection assay of (a) HBPU modified epoxy and (b) nanocomposites.
G. Das et al. / Progress in Organic Coatings 76 (2013) 1103–1111
are seen to adhere to the film indicating good biocompatibility of the polymeric system. The acceleration of biodegradation is more prominent for the nanocomposites. Clay can cause heterogeneous hydrolysis of the ester groups in presence of microbes by absorbing water. The process has an induction time [34], hence tremendous enhancement of biodegradation was observed only after two weeks of bacterial exposure in this case. The progressive degradation changes the microstructure of the composite film resulting, as can be seen from the SEM images (Fig. 10). 3.11. RBC haemolysis protection assay for cytocompatibility To investigate the effect of nanocomposites on the mammalian blood cells, the haemolysis test was carried out. The nanocomposites exhibit higher inhibition assay as compared to the pristine system (Fig. 11). This observation indicates that the presence of clay has a definite role to play in RBC haemolysis prevention. However, compared to Triton X-100, the nanocomposites did not exhibit any significant haemoglobin release and showed almost similar results as that of the negative control PBS. This indicates that it did not cause any lysis of the erythrocyte membranes. The above observation reveals the non-toxic behaviour of the nanocomposites to the living cells with concomitant prevention of cell damage against any harmful free radicals. 4. Conclusion The study demonstrated significant enhancement in performance characteristics of epoxy resin after its modification by HBPU. Further, the HBPU content in the modified system tuned many properties of the pristine BPSE. The EHBPU30 showed the best performance characteristics among the studied system. Moreover, the addition of nanoclay in the EHBPU30 matrix further enhances the performance characteristics. The SEM and TEM studies reveal a partially delaminated structure of the nanocomposites. All the systems were found to be biodegradable as studied by broth culture technique. P. aeruginosa bacterial strain significantly degrades the system even after 30 days of inoculations. Further, the rate of biodegradation was found to be higher in case of the nanocomposites than the pristine system. The study indicated the potentiality of these materials as advanced biodegradable coating materials. Acknowledgement The authors express their gratitude to the research financial assistance given by DST, India through grant No. SR/S3/ME/0020/2009-SERC dated 9th July 2010. Also the authors express their gratitude to Mr. Joston P Nongkynrih, NEHU, Shillong for helping in TEM analysis. References [1] M.A. Ayman, F.E.K. Ahmed, H.A. Morsy, A. Abdel-Azim, New epoxy resins based on recycled poly(ethylene terephthalate) as organic coatings, Prog. Org. Coat. 58 (2007) 13–22. [2] B. Francisa, P.G. Vanden, F. Posadab, G. Groeninckx, V.R. Lakshmana, R. Ramaswamya, S. Thomas, Cure kinetics and morphology of blends of epoxy resin with poly (ether ether ketone) containing pendant tertiary butyl groups, Polymer 44 (2003) 3687–3699. [3] T. Iijima, M. Tomoi, J. Yamasaki, H. Kakiuchi, Toughnening of epoxy resin by modification with acrylic elastomers containing pendent epoxy groups, Eur. Polym. J. 26 (1990) 145–151. [4] H.L. Frisch, K.C. Frisch, Polyurethane-epoxy interpenetrating polymer networks-barrier and surface properties, Prog. Org. Coat. 7 (1979) 105–111. [5] Y. Ye, H. Chen, J. Wu, L. Ye, High impact strength epoxy nanocomposites with natural nanotubes, Polymer 48 (2007) 6426–6433.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Biodegradation, cytocompatability and performance studies of vegetable oil based hyperbranched polyurethane modified biocompatible sulfonated epoxy resin/clay nanocomposites Gautam Das a , Ranjan Dutta Kalita b , Harekrishna Deka a,1 , Alok K. Buragohain b , Niranjan Karak a,∗ a b
Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Napaam, Assam 784028, India Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Assam 784028, India
a r t i c l e
i n f o
Article history: Received 25 October 2012 Received in revised form 1 February 2013 Accepted 14 March 2013 Available online 8 April 2013 Keywords: Epoxy resin Hyperbranched polyurethane Mesua ferrea L. seed oil Biodegradibility Cytocompatability Renewable resources
a b s t r a c t Lack of degradability and decrease of landfill sites along with growing water and land pollution problems generate a strong concern about the use of synthetic non-biodegradable polymers. Mesua ferrea L. (Ceylon Ironwood) seed oil based diglycidyl sulfone epoxy resin was modified by the same oil based hyperbranched polyurethane. This hyperbranched polyurethane was prepared by using an A2 + B3 approach using monoglyceride of the oil as one of the components, as reported earlier. The epoxy resin was modified by incorporating three different weight percentages of hyperbranched polyurethane viz. 10, 20 and 30 wt%. The hyperbranched polyurethane treated epoxy system was characterized by FTIR, SEM and XRD techniques. The study of performance characteristics reveals that epoxy modified by 30 wt% HBPU is the best composition. Nanocomposites of 30 wt% HBPU based composition were prepared with different dose levels of organo nanoclay (1, 3 and 5 wt%) and were characterized by using wide angle X-ray diffraction (WAXD), SEM, TEM and FTIR techniques. The nanocomposite shows improvement in performance characteristics with the increase of clay content. The nanocomposite with 5 wt% of clay shows an increase of about 230% in tensile strength with respect to the pristine epoxy system. The biodegradability tested on all the samples by Pseudomonas aeruginosa bacterial strain and they exhibited significant degradation after 30 days of inoculation. Thus the resulted nanocomposites have potential to be used as biodegradable an advanced coating materials. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The modification of epoxies to enhance their physical and mechanical properties has been an active area of research for over a few decades. Various modification methods have been use to achieve a tougher and thermally strong epoxy system [1–3]. The physical as well as chemical properties of the modified system, however, largely depend on the degree of compatibility of the used components in the formulation. Among the different types of physical modifications of epoxy resin, blending is one of the most acceptable approaches. The complete miscibility of the components is most desirable because mixing on molecular scale results in superior physical as well as mechanical properties [4]. Epoxy systems are reactive resins that providing a favourable cost-performance
∗ Corresponding author. Tel.: +91 3712 267009; fax: +91 3712 267006. E-mail addresses: [email protected], [email protected] (N. Karak). 1 Present address: Department of Plant Agriculture, University of Guelph, Guelph, Ontorio, Canada N1G2W1. 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.03.007
ratio when compared to other thermosets. These epoxy thermosets have multifaceted applications in surface coating and paints, electronics, structural composites, etc. However, low toughness and brittleness the crosslinked resins with non-biodegradability limits their monolithic applications [5–8]. In recent years, polymers developed from the renewable resources, especially vegetable oils have attracted much attention due to the economical, environmental and societal advantages [9–11]. Vegetable oils have numerous benefits such as biodegradability, renewability, ease of modification, non-toxic, and most importantly environmentally benign [9,10]. Structures of a few fatty acids of vegetable oils are given in Table 1. The large amount of research in the area of synthetic polymers has stimulated new results on nanocomposites based on bio-based polymers (including epoxy thermosets) as matrices with environmental benign reinforcing nanomaterials such as nanoclay. The application of nanotechnology to bio-based polymeric systems may open new possibilities for improving not only the properties but also the cost-price-efficiency at the same time. Owing to the nanometersize particles obtained by dispersion, these nanocomposites can exhibit markedly improved mechanical, thermal, barrier and
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Table 1 Chemical structures of a few fatty acids of vegetable oils. Name of fatty acid
rate of biodegradation and cytocompatability of the nanocomposites were determined in order to examine the applicability of these nanocomposites as advanced coating materials.
Structure
COOH
Behenic
Eicosenoic
2. Experimental
COOH
Capric
6
7
COOH
2.1. Materials
COOH
Gadoleic
COOH
Heptadecanoic
COOH Heptadecenoic
OH
COOH
Isanic
COOH COOH
Linoleic Linolenic
COOH COOH
Myristic Myristoleic
COOH COOH
Oleic Palmitic
COOH
Palmitoleic
COOH Ricinoleic Stearic
OH COOH
physico-chemical properties, when compared with the pristine polymeric systems and conventional composites [12]. Again, there are various approaches to render adequate biodegradability into a polymeric material; however, the one that has gained wide popularity is the use of an already biodegradable component into the non-degradable or less biodegradable polymer [13,14]. Biodegradability of nanocomposites is measured conveniently by broth culture technique where the polymer films are subjected to microbial degradation. P. aeruginosa is well known for its efficiency to degrade polymeric material and hence can be utilized in the present investigation as the microorganism for microbial degradation. Another important facet to examine for biocompatibility of biomaterials is their cytocompatability. Generally, MTT and direct contact tests are used for this purpose [15]. However, non-cytocompatability and shielding efficiency of the polymer to the cells from harmful free radicals can be obtained directly from anti-hemolytic test. Red blood cells (RBC) prone to free radical attack and lose the membrane integrity; consequently, heme protein (haemoglobin) diffusing out of the cell can be estimated by measuring the absorption at wavelength of 415 nm using a spectrophotometer [16]. Although a number of reports on epoxy nanocomposites [17–20] have been published but the work on vegetable oil based epoxy/hyperbranched polyurethane nanocomposites is limited. The lack of sufficient toughness of bio-based epoxy furthers instigate to look for other alternatives ways to improve its performance characteristics. Hyperbranched polyurethane by virtue of its toughness and biocompatibility can be considered as a material of choice for the generation of eco-friendly materials [4]. The present investigation utilized Mesua ferrea L. seed oil (about 70% oil content) based hyperbranched polyurethane modified epoxy of the same vegetable oil as the matrix. In the present communication, the authors wish to report the potentialility of clay nanocomposites of M. ferrea L. seed oil based hyperbranched polyurethane modified sulfone based epoxy of the same oil as advanced coating materials. The performance characteristics like physical, mechanical, thermal properties as well as
M. ferrea L. seeds (Jamugurihat, Assam) were utilized for extraction of the oil. The dried seeds were crushed after dehulling and oil was extracted by solvent soaking method using petroleum ether (60–80 ◦ C). Epichlorohydrin (Merck, India) and 4,4 sulfonlybisphenol or bis(4-hydroxyphenyl) sulfone (bisphenol-S or BPS, Aldrich, Germany) were used without further purification. Poly(amido amine) (HY 840, Ciba Giegy, Mumbai, India) with viscosity of 10,000–25,000 mPas and amine value of 6.6–7.5 equiv/kg was used as received. Bisphenol-A (Burgoyne Burbidges & Co., Mumbai, India) was used after recrystalization from toluene. Organically modified nanoclay, montmorillonite, Nanomer I.30E (octadecylamine modified, Aldrich, Germany) was used as received. Glycerol (Merck, Mumbai) and poly(-caprolactone) diol (PCL, Solvay Co., Korea, Mn = 3000 g mol−1 ) were used after drying at 40 ◦ C under vacuum for 4 h. Lead monoxide (S.D. Fine Chemical Ltd., Mumbai) and 2,4-toluene diisocyanate (TDI, Sigma Aldrich) were used as received. N,N-dimethylformamide (DMF, Merck, Mumbai) was dried over CaO, vacuum distilled, and kept in 4A-type molecular sieves before use. All other reagents used in the present investigation were of reagent grade. 2.2. Methods 2.2.1. Synthesis of diglycidyl ether bisphenol-S epoxy resin (BPSE) The vegetable oil based epoxy resin was prepared by using the similar method as published earlier [21]. Briefly, monoglyceride of the oil (obtained by glycerolysis technique), epichlorohydrine, bisphenol-A (BPA) and bisphenol-S (BPS) were reacted together by maintaining the mole ratio of 1:5:2:1 at (110 ± 5)◦ C for 14 h in slightly alkaline medium. 2.2.2. Synthesis of hyperbranched polyurethane (HBPU) The hyperbranched polyurethane was prepared as reported elsewhere [22]. Briefly, 2.5 mol of poly(-caprolactone)diol, 1.5 mol of monoglyceride of the Nahar oil, and 6.5 mol of TDI were used in the first stage to prepare a prepolymer, and finally, glycerol of 2.5 mol was added to the prepolymer to get the HBPU. The yield of the polymer obtained was 97%. 2.2.3. Modification of epoxy resin by HBPU The composition of the components is given in Table 2. The epoxy resin and the HBPU were mechanically mixed in an oil bath at 80 ◦ C by using a mechanical stirrer. The mixtures were fully stirred for 1 h and then degassed in a vacuum oven to remove any residual gas bubbles or entrapped solvent. Table 2 Compositions of HBPU modified BPSE nanocomposites. Codea
BPSE (g)
HBPU (g)
OMMT (g)
EHBPU10 EHBPU20 EHBPU30 EHPN1 EHPN3 EHPN5
100 100 100 100 100 100
10 20 30 30 30 30
0 0 0 1 3 5
a Digits indicate the hyperbranched polyurethane content in the first three codes and for nanocomposites digits indicate OMMT content.
G. Das et al. / Progress in Organic Coatings 76 (2013) 1103–1111 Table 3 Curing studies of HBPU modified BPSE and its nanocomposites. Sample
Touch free time (min)
Hard dry time (min)
Swelling (%)
Water loss (%)
BPSE EHBPU10 EHBPU20 EHBPU30 EHPN1 EHPN3 EHPN5
45 43 40 35 30 27 17
65 64 63 57 55 47 40
32 30 29 27 25 23 20
1.45 1.12 1.01 0.95 0.85 0.75 0.68
2.2.4. Preparation of nanocomposites The nanocomposites of the 30 wt% hyperbranched polyurethane modified epoxy (EHBPU30) were prepared by incorporating organically modified nanoclay (OMMT) at 1, 3 and 5 wt% (Table 2). The system was heated at 80 ◦ C for 1 h by using a mechanical stirrer. The system was then cooled and sonicated using a single probe sonicator UP200S for 30 min. The temperature of the sample was maintained at (25–30)◦ C by using a water bath. The dispersed nanoclay/resin system was degassed for 30 min under vacuum before further processing. The prepared nanocomposites with 1, 3 and 5 wt% of clay were coded as EHPN1, EHPN3 and EHPN5 respectively (Table 2). 2.2.5. Curing of the hyperbranched polyurethane modified epoxy and nanocomposites By hand stirring for 20 min a homogenous mixture of the epoxy resin, HBPU modified epoxy resin and its nanocomposites with 50 phr (parts per hundred gram with respect to epoxy resin) of poly(amido amine) hardener was prepared, separately, in a glass beaker at room temperature. The degassed mixture was then cast on a glass plate and cured at 100 ◦ C in a muffle furnace to determine both the touch free time (minimum time, when no impression will appear on touching the film) and hard dry time of the resin, modified system and the nanocomposites. The mixtures were also uniformly spread on mild steel plates (150 mm × 50 mm × 1.60 mm), tin plates (150 mm × 50 mm × 0.40 mm) and glass plates (75 mm × 25 mm × 1.75 mm) for impact resistance, gloss and chemical resistance tests respectively. The coated plates were cured at 100 ◦ C for specified period of time as given in Table 3, followed by post curing at 150 ◦ C for 2 h. 2.2.6. Water permeability measurement The water permeability of the nanocomposite films was measured in a desiccating chamber containing CaCl2 as the drying agent under vacuum. Small containers containing weighted amount of distilled water were taken, where middle part of the caps was replaced by experimental nanocomposite film in an airtight manner to avoid any leakage. The containers were then placed in a desiccating chamber. The weight of the container was again taken after a period of 120 h. All the measurements were carried out at (30 ± 1)◦ C and an average of three samples was taken for each measurement. The reduction in water content in the containers was calculated by using the relationship. Percent weight loss of water = [(Wi − Wf )/Wi ] × 100%, where Wi = initial weight and Wf = final weight of the water container. 2.2.7. Biodegradation by broth culture technique A modified broth medium for culture [14] was prepared by dissolving 2.0 g (NH4 )2 SO4 ,·2.0 g Na2 HPO4 , 3.61 g KH2 PO4 , 1.75 g MgSO4 ·7H2 O, 0.2 g CaCl2 ·2H2 O, 50 mg FeSO4 ·7H2 O, 1 mg CuSO4 ·7H2 O, l g MnSO4 ·5H2 O, l g ZnSO4 ·7H2 O, l g H3 BO3 ·5H2 O and l g MoO3 in 1.0 L of demineralized water. 10 mL of this liquid culture media was poured into 100 mL conical flasks and was sterilized.
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The media was then allowed to cool down to room temperature and experimental nanocomposite films were applied to the media under sterile condition. Medium containing no polymer film was also cultured as negative control. 2.2.8. Microbe selection P. aeruginosa strain was selected for the study with strain number MTCC 424. A small inoculums of bacteria containing an approximate 1 × 108 /mL microbes (as calculated from McFarland turbidity method) was inoculated into the conical flask containing 10 mL media for each test. The flasks were then incubated under sterile condition at 37 ◦ C for the degradation study. The samples were collected for spectrophotometric observation at 600 nm against blank culture media on weekly basis under sterile condition. Bacterial growth was calculated from the absorbance data using McFarland turbidity as the standard. 2.2.9. Haemolytic activity assay The haemolytic activity test was done to see if the nanocomposites have any haemolytic activity on the erythrocytes based on the modified protocol as reported by Nair et al. [23]. Briefly, goat blood mixed with the anticoagulant sodium citrate (4%) centrifuged at 2500 rpm for 10 min. The resultant supernatant was discarded and only the erythrocytes were collected. The collected erythrocytes were further washed thrice in PBS (pH 7.4). Now a 10% (v/v) suspension of washed erythrocytes in PBS was prepared in a 50 mL centrifuge tube. 1.9 mL of this erythrocyte solution was taken in a 2 mL centrifuge tube and 100 L of the extract at two different concentrations (1 mg/mL and 250 g/mL) was added into it. The extracts were also prepared in PBS. The tubes were then incubated for 2 h respectively at 37 ◦ C. 2% (v/v) Triton X-100 and PBS were taken as positive and negative control, respectively. After incubation the tubes with 2 mL medium were centrifuged at 2500 rpm for 10 min. 200 L of the supernatant was taken and 2.8 mL of PBS was added to it and then the absorbance was taken at 415 nm in a UV-vis spectrophotometer (Thermo, UK). 2.3. Measurements FTIR spectra of resin, modified system and the nanocomposites were recorded in FTIR spectroscopy (Impact-410, Nicolet, USA) using KBr pellet in the wavelength range of 500–4000 cm−1 at a resolution of 0.01 cm−1 . The surface morphology of the samples was studied by a JEOL scanning electron microscope of model JSM6390LV SEM after platinum coating on the surface. Wide angle X-ray scattering (WAXS) studies were carried out by a powder diffractometer Rigaku X-ray diffractometer (Miniflex, UK) at room temperature (about 25 ◦ C), operated at 30 kV and 15 mA. The scanning rate used was 2.0 min−1 over the range of 2 = 0–30◦ for the above study. The distribution of nanoclay in the polymer matrix was studied by using a JEOL, JSM-1000 CX transmission electron microscope (TEM). A scratch hardness tester (Sheen instrument Ltd, UK) was used to determine the scratch hardness (ASTM D5178/1991) of the cured films. The front impact resistance test was carried out by applying falling ball method with an impact tester (S.C. Dey Co., Kolkata) from a maximum test height of 100 cm. In this test, 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 maximum height up to which the film is not damaged was taken as the impact resistance value. The tensile strength and elongation at break (as per the ASTM D 412-51 T) were measured with the help of a Universal Testing Machine of model Zwick Z010 (Germany) by a 10 kN load cell and at 40 mm/min jaw separation speed. The gloss characteristics of the cured films were found out by a mini glossmeter (Sheen instrument
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Ltd, UK), over the resin and the nanocomposites coated mild steel plates at an angle of incidence of 60◦ . Thermogravimetric (TG) analysis was carried out in Shimazdu TG 50 thermal analyzer at a nitrogen flow rate of 30 mL/min and at the heat rate of 10 ◦ C/min. 3. Result and discussions 3.1. Modification of epoxy The mixing of two polymers to obtain a homogenous system is dependent on many factors. The physical as well as chemical nature of the components plays a decisive role in obtaining a homogeneous phase. Thermodynamics factors such as entropy also must be considered for such system. Mixing of two polymers with low molecular weight can be more conducive for obtaining a uniform system. Further due to the hyperbranched nature of the polyurethane it can provide sufficient functional groups for interaction with epoxy resins. The crosslinking of the system by the hardener may result in a homogenous system with no phase separations [24,25]. M. ferrea L. seed oil modified epoxy resin has glyceride moiety in its structure, this results in the epoxy resin with structural complexity. These long alkyl chains offer epoxy resin with sufficient amount of flexibility, but lacks adequate strength properties. On the other hand, polyurethane of the same oil has structural in-homogeneity with sufficient amount of toughness. As both the polymers have common component of glyceride moiety of the same oil as well as different types of polar groups in the structure, so the resultant product is expected to possess strong interactions and good homogeneity. Further, this may also result biodegradability of the epoxy thermoset. 3.2. Nanocomposites preparation The preparation of suitable nanocomposite of the afore-stated matrix may offer significant improvement of many desirable properties including biodegradation. In this regard, clay plays an effective role in enhancing the rate of biodegradation as it helps in the absorption of the moisture by virtue of its surface hydroxyl groups. This absorbed moisture helps in the hydrolysis of the ester and urethane groups of the matrix and thereby the microbes can easily access the hydrophilic polymer fragments. Thus, clay plays an effective role in enhancing the rate of biodegradation. Thus octadecylamine modified MMT based nanocomposites were prepared in the present investigation. Further, it is expected that hyperbranched polyurethane can help in the exfoliation of the clay layers in the epoxy matrix, because of its unique structural features [14]. Therefore, the modification of epoxy resin by the organically modified clay and hyperbranched polyurethane altogether may result in the enhancement of the performance characteristics including thermo-stability and biodegradability.
Fig. 1. FTIR spectra of (a) BPSE, (b) EHBPU10, (c) EHBPU20 and (d) EHBPU30 before curing.
position shows red shifts of 25 cm−1 for EHBPU10, 10 cm−1 for EHBPU30 and 3 cm−1 for EHBPU20. This shifts may be attributed to the increase in the OH species generated by the ring opening of the oxirane ring. The carbonyl stretching frequency ( C O) broadens and shows shifting to a lower value though the position of the band in all the cases was found to be similar. The broadening was due to the overlap of the NH and C O absorptions band [14]. The shifts however indicates that the C O group of the hyperbranched polyurethane is involved in hydrogen bonding with the OH group and C O group of epoxy resin and vice versa. The epoxide stretching vibrations shows decrease in intensity due to the interaction of the urethane linkage (by NH group) with the oxirane group [26]. In the modified system after curing the absorption bands (in all cases) show significant shift and reduction of the band intensity (Fig. 2). This indicates the formation of crosslinked networks. The crosslinking was further supported by observation of diminishing intensity of band for epoxide stretching at 916 cm−1 in all cases. This suggests extensive amount of interaction present in the system [21]. As observed earlier the OH stretching band for epoxy resin and the NH band of the hyperbranched polyurethane overlaps and appear as a single broad band around 3154–3246 cm−1 .
3.3. FTIR studies of hyperbranched polyurethane modified epoxy and nanocomposites The FTIR spectra of epoxy resin and its modified systems are shown in Fig. 1. The FTIR spectrum of epoxy resin is well documented in our earlier published report [21]. Some of the characteristic bands (cm−1 ) observed are: 3423 ( OH stretching vibrations), 3050 (aromatic C H stretching vibration), 1729 (C O stretching vibration of the triglyceride esters), 1593 (C C stretching vibration), 1300 and 1149 (sulfone stretching vibrations), 1246 and 1106 (C O C stretching vibrations), and 916 and 832 (oxirane ring stretching vibrations). The FTIR spectra of the modified systems show significant shift in the OH stretching vibration, before curing (Fig. 1). The band
Fig. 2. FTIR spectra of (a) EHPN1, (b) EHPN3 and (c) EHPN5 after curing.
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curing there is improper blue shift with simultaneous reduction in the absorption band intensity for C O and red shift of OH absorption band is (Fig. 4) due to significant amount of complex reaction that is occurring during the curing process. Although, the dispersion state of the nanoclay cannot be concluded by the FTIR study. 3.4. XRD analysis
Fig. 3. FTIR spectra of (a) EHBPU10, (b) EHBPU20 and (c) EHBPU30 before curing.
The XRD of the modified system reveals the presence of hyperbranched polyurethane in the modified epoxy resin (Fig. 5i) by the appearance of diffraction peaks at 2 = 21.2◦ (4.19 A◦ ) and 23.4◦ (3.81 A◦ ), which occur due to the crystalline nature of PCL moiety in the structure [29]. The positions of these peaks remain unchanged after modification, though the intensity gradually increases with the increase of HBPU content, which may be due to the increase of the percentage of PCL in the matrix. The state of distribution of clay in the polymer matrix can be obtained from the diffraction pattern in the XRD spectra (Fig. 5ii). In the case of OMMT strong reflection appears at 2 of 4.15◦ resulting from the (0 0 1) crystal surface of the layered silicates. The occurrence of reflection at 20◦ (2) corresponds to (1 1 0) plane. However, in all the nanocomposites, no basal reflection corresponding to (0 0 1) crystal plane was observed. Thus the XRD diffraction peaks for the nanocomposites indicate the possibility of exfoliated nanostructure of clay with different extent of dispersion in the epoxy matrix, though it needs to be confirmed by other tests and analyses like TEM and performance of the thermosets. In case of epoxy resin the effect of clay modifier on the curing has already been reported [21]. The curing rate will be much faster inside the galleries than that outside. As a result of these differences more epoxy chains will be pulled inside the galleries resulting in expanded d-spacing. 3.5. Morphology
Fig. 4. FTIR spectra of (a) EHPN1, (b) EHPN3 and (c) EHPN5 after curing.
This implies that most of the NH groups were hydrogen bonded. The C O absorption intensity decreases in all the cases after curing. This is evident of the extensive amount of interaction between the polar functionalities of the epoxy resin and the hyperbranched polyurethane. The structural changes of the nanocomposites from the pristine polymer can also be analyzed through the FTIR spectra (Figs. 3 and 4) both before and after curing. The bands appearing at about 1034 cm−1 and 550 cm−1 region in the FTIR spectrum (i.e. EHBPU30) is due to Si O and Al O stretching vibrations of nanoclay [27] and the band at about 3446–3627 cm−1 for the OH stretching in Si OH and Al OH moieties located on the surface of the clay (Fig. 3) [27]. The interaction of OH group of clay with the epoxy group and free OH groups of the intercalated polymer chain is indicated by the broadening of OH stretching band in the nanocomposites. The sharp band at about 1620 cm−1 of OMMT corresponding to the absorbed H O H (H2 O) bending vibration (occurring due to hydrophilic nature of the clay) is minimized in the nanocomposites is indicative of the proximity between hydrophobic polymer chains and nanoclay [28]. The C O absorption band prior to curing shows a red shift of about ≈5–10 cm−1 . This red shift indicates lengthening of the C O bond thereby decreasing the absorption frequency by hydrogen bonding. However after
Several factors such as chemical miscibility of the two components, method of mixing, interfacial interaction and crosslinking density, affect the morphology of a two-component system at a given composition [30]. Generally, HBPU shows good compatibility with epoxy resin and among the studied system EHBPU30 showed the best result (Fig. 6). The good homogenization of epoxy resin with hyperbranched polyurethane (Fig. 6, EHBPU30) can be attributed to the polar-polar interaction between the -bonds of aromatic rings in both resins [26]. Enhancement in compatibility is also aided by the network formation through the reaction of amine groups of hardener with ester groups of epoxy resin along with normal crosslinking. Further, the amine hardener may also catalyze reaction of different functional groups of HBPU and epoxy. Thus, the amine hardener also acts as a compatibilizing agent for the system. Similarly, in case of nanocomposites, the addition of nanoclay significantly affects its topological features (Fig. 6b). The SEM image of EHPN3 shows an uneven surface morphology, wherein some protruding white dots or lines were seen on the surface. These changes in surface features can be related to the presence of nanoclay. Observation of protruding dots or lines indicates the adherence of clay layers to the polymeric surface. As observed for the pristine polymeric system the surface is rather smooth. The TEM micrograph of EHPN3 shows well dispersed clay layers in the polymer matrix (Fig. 7), wherein, individual clay layers can be seen. From these observations it can be concluded that XRD and SEM results are complementary by the TEM observation. 3.6. Curing studies The modification of epoxy with hyperbranched polyurethane has profound effect on the curing process of the epoxy resin. The
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Fig. 5. (i) XRD spectra of (a) BPSE, (b) EHBPU10, (c) EHBPU20 and (d) EHBPU30 and (ii) XRD spectra of (a) EHPN1, (b) EHPN3 and (c) EHPN5.
Fig. 6. SEM micrographs for (a) EHBPU30 and (b) EHPN3.
curing time (touch free time as well as drying time) decreases in the modified system with the increase of polyurethane content (Table 3). This may be due to the increase of the possibility of reaction of epoxy/hydroxyl groups of epoxy resin with hydroxyl/urethane groups of polyurethane resins in the presence of amine hardener as the amount of reactants increased [31]. It has been reported [32] that the aromatic moiety of polyurethane resin accelerates the crosslinking reaction of epoxy in an epoxy/amine hardener/urethane reaction system by the formation of an active complex of the hardener with the aromatic moiety of polyurethane with stoichiometric ratio in the curing reaction. Further, the oxirane absorption diminishes in the modified epoxy after curing. The hydroxyl group (of clay) plays a catalytic role in enhancing the cure rate, which can also be justified from the decrease in curing time for the nanocomposites. The cure time decreases with the increases in the clay loading. The alkylammonium ions present in organoclay are also responsible for the catalytic acceleration of the epoxy curing reaction and thus enhancing the cure rate [21].
Fig. 7. TEM micrograph for EHPN3.
3.7. Thermal stability The effect of modification on the thermal stability of the epoxy thermomet was also studied. Fig. 8 shows the thermal profiles of the pristine epoxy and hyperbranched modified epoxy thermosets. However, the initial thermal degradation pattern of the pristine epoxy resin does not show any significant increase after modification. BPSE thermoset exhibit initial degradation temperature of 277 ◦ C, after modification there is a slight increment in the degradation temperature (286 ◦ C). The increase in the thermal stability is due to increase crosslink density thereby bridging the polymer backbone, which results in a tough material [26].
Fig. 8. TGA thermograms for BPSE, EHBPU30 and EHPN5.
G. Das et al. / Progress in Organic Coatings 76 (2013) 1103–1111 Table 4 Performance characteristics of HBPU modified BPSE thermosets.
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Table 5 Mechanical properties of HBPU modified BPSE and its clay nanocomposites.
Sample code
BPSE
EHBPU10
EHBPU20
EHBPU30
Sample code
EHBPU30
EHPN1
EHPN3
EHPN5
Tensile strength (MPa) Elongation at break (%) Impact resistance (cm) Gloss (60◦ ) Scratch hardness (kg)
6 95 100 60 3.4
7.2 97 100 62 4.5
8.2 105 100 62.7 5.1
11.23 110 100 65 5.5
Tensile strength (MPa) Elongation at break (%) Impact resistance (cm) Gloss (60◦ ) Scratch hardness (kg)
11.23 110 100 65 5.5
12.94 67.45 90 70 6
14.98 55.76 87 75 7
19.78 50.14 80 81 10
microphase separation due to more polymer filler interaction [32]. The scratch hardness also augments with the nanoclay loading. The increase in scratch hardness is due to the dissipation of the stress generated at the interface between the moving tip and the film surface throughout the nanocomposite. The stress is than absorbed by the hard clay layers and consequently scratch hardness increase with clay loading in this case.
Consequently, reduction in the segmental mobility may result in increased resistance to thermal decomposition. Higher char content of the HBPU modified BPSE (9.5%) than pristine BPSE (6.5%) also results in shielding of the matrix from thermal decompositions. For HBPU modified BPSE the increase of char residue is attributed to the presence of aromatic both TDI and bisphenol-S and bisphenol-A moieties. Further, it was seen that nanoclay also has a definite role in enhancing the onset degradation temperature (Fig. 8), i.e. up to 309 ◦ C for EHPN5. In addition to restricted segmental motion, clay by virtue of its inorganic nature (SiO2 , Al2 O3 and other metal oxides) also improved the thermal stability of the organic polymer. The well dispersed clay layers thus acts as a barrier preventing volatization of the modified BPSE matrix. The char residue of EHPN5 (10.95%) was also higher than modified as well pristine BPSE at 650 ◦ C. The Hoffman decomposition of the onium modifier of the clay gives protonated montmorillonite that can catalyze the formation of stable carbonaceous residue [21]. However, the onset degradation temperature does not exhibit any significant differences. Although the difference is not too large, there is a slight tendency of increase in the initial degradation temperature with nanoclay content.
The enhancement of barrier properties by the formation of true nanocomposite is a well established phenomenon [34]. The present study on water vapor permeation also shows no exception (Table 3). This can be explained by tortuosity mechanism. The intercalation/exfoliation of clay platelets in the polymer matrix offers a long tortuous path (mean free path) for the guest molecules to pass through and hence the permeation got retarded [35,36]. In the present study, a 2–4-fold reduction in the rate of permeability of water vapor with increase in clay loading from 1 to 5 wt% in the nanocomposites was observed. This is understandable from the increase of tortuosity with the increase of clay loading in the matrix.
3.8. Performance characteristics
3.10. Biodegradation
Vegetable oil based epoxy thermoset lacks desired level of performance. However, the modified BPSE systems were found to exhibit overall good performance characteristics over the pristine system (Table 4). It was also observed that the HBPU content has profound role in optimizing the above performances. The increment of tensile strength was due to increase in the crosslink density; consequently larger stress value is required for their rapture. Thus good compatibility is an important factor for this achievement. The scratch hardness also increases with the increase of the polyurethane content, which is again caused by the increase of crosslinking density, H-bond formation, etc. Although the modified epoxy shows a considerable hardness, still they have enough flexibility as found by the elongation at break values (Table 4). The long fatty acid chains of oil, ester, and ether linkages of PCL render this high flexibility to the films. The impact resistance of all the films shows excellent result as expected from the tensile strength. Nanoclay by virtue of its high aspect ratio and surface area plays a significant role in enhancing the performance characteristics of the HBPU modified BPSE matrix. The tensile strength and scratch hardness of the prepared nanocomposites were found to increase as compared to that of the pristine polymeric system (Table 5). The close proximity of the internally hard clay layers and the soft organic polymer means that the chains were confined by the interface. In addition, as the domain size of the microphase for the system decreases the compatibility between the components increases, this in turn enhances the mechanical properties like tensile strength, modulus, etc. [33]. The mechanical properties gives an idea about the morphological behaviour of the nanocomposites better the mechanical properties, lower is the degree of
One of the most attractive features of polyurethane is its biodegradability, which puts it into the list of potential candidates for many advanced applications [14]. The biodegradability of pristine BPSE, modified epoxy and nanocomposite was tested under control bacterial growth conditions. Pseudomonas sp. being the most prominent organism, as it has high ability to degrade bisphenol-A based epoxy modified vegetable oil based hyperbranched polyurethane [14] so P. aeruginosa (MTCC 424) was taken as the microorganism for the present study. The growth profiling of consortia in the modified broth media lacking dextrose compels the bacteria to use epoxy and modified system as the primary carbon source in each case [37]. The biodegradation of both the pristine BPSE, HBPU modified BPSE and the nanocomposites were quantitatively tested and confirmed by direct exposure to strain of P. aeruginosa bacteria by broth culture technique. After keeping the samples in broth culture media for six weeks, the bacterial OD was determined. The difference in the rate of growth initially for two weeks both for the nanocomposites as well pristine system is not significant as can be seen from the curves after two weeks of bacterial exposure (Fig. 9). However, the bacterial growth rate increases significantly after two weeks, as can be realized from the bacterial count. This observation is indicative of the biodegradation of HBPU modified epoxy and its nanocomposites. After modification of BPSE by HBPU significant enhancement in biodegradation was recorded this accelerated further on addition of nanoclay to the modified epoxy. Again, the rate of biodegradation was found to be higher than the corresponding pristine polymers in both the cases. The growth of P. aeruginosa bacterial strains, modified epoxy system and various nanocomposites films as well as
3.9. Water vapour barrier properties
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Fig. 9. Growth of P. aeruginosa bacterial strain, MTCC 424 on (a) HBPU modified epoxy and (b) nanocomposites.
Fig. 10. SEM micrographs for films after six weeks of bacterial exposure (a) untreated BPSE, (b) inoculated BPSE, (c) EHBPU30 and (d) EHPN5.
on the corresponding pristine polymeric films can be realized from Figs. 9 and 10, respectively. The vegetable oil modified polymeric systems are more prone to bacterial attack [13]. These facts support the above observation
wherein bacterial degradation increases with the hyperbranched polyurethane content. The SEM micrographs (Fig. 10) after six weeks of bacterial exposure exhibit significant topographical changes for polyurethane modified epoxy resin. The bacteria
Fig. 11. RBC protection assay of (a) HBPU modified epoxy and (b) nanocomposites.
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are seen to adhere to the film indicating good biocompatibility of the polymeric system. The acceleration of biodegradation is more prominent for the nanocomposites. Clay can cause heterogeneous hydrolysis of the ester groups in presence of microbes by absorbing water. The process has an induction time [34], hence tremendous enhancement of biodegradation was observed only after two weeks of bacterial exposure in this case. The progressive degradation changes the microstructure of the composite film resulting, as can be seen from the SEM images (Fig. 10). 3.11. RBC haemolysis protection assay for cytocompatibility To investigate the effect of nanocomposites on the mammalian blood cells, the haemolysis test was carried out. The nanocomposites exhibit higher inhibition assay as compared to the pristine system (Fig. 11). This observation indicates that the presence of clay has a definite role to play in RBC haemolysis prevention. However, compared to Triton X-100, the nanocomposites did not exhibit any significant haemoglobin release and showed almost similar results as that of the negative control PBS. This indicates that it did not cause any lysis of the erythrocyte membranes. The above observation reveals the non-toxic behaviour of the nanocomposites to the living cells with concomitant prevention of cell damage against any harmful free radicals. 4. Conclusion The study demonstrated significant enhancement in performance characteristics of epoxy resin after its modification by HBPU. Further, the HBPU content in the modified system tuned many properties of the pristine BPSE. The EHBPU30 showed the best performance characteristics among the studied system. Moreover, the addition of nanoclay in the EHBPU30 matrix further enhances the performance characteristics. The SEM and TEM studies reveal a partially delaminated structure of the nanocomposites. All the systems were found to be biodegradable as studied by broth culture technique. P. aeruginosa bacterial strain significantly degrades the system even after 30 days of inoculations. Further, the rate of biodegradation was found to be higher in case of the nanocomposites than the pristine system. The study indicated the potentiality of these materials as advanced biodegradable coating materials. Acknowledgement The authors express their gratitude to the research financial assistance given by DST, India through grant No. SR/S3/ME/0020/2009-SERC dated 9th July 2010. Also the authors express their gratitude to Mr. Joston P Nongkynrih, NEHU, Shillong for helping in TEM analysis. References [1] M.A. Ayman, F.E.K. Ahmed, H.A. Morsy, A. Abdel-Azim, New epoxy resins based on recycled poly(ethylene terephthalate) as organic coatings, Prog. Org. Coat. 58 (2007) 13–22. [2] B. Francisa, P.G. Vanden, F. Posadab, G. Groeninckx, V.R. Lakshmana, R. Ramaswamya, S. Thomas, Cure kinetics and morphology of blends of epoxy resin with poly (ether ether ketone) containing pendant tertiary butyl groups, Polymer 44 (2003) 3687–3699. [3] T. Iijima, M. Tomoi, J. Yamasaki, H. Kakiuchi, Toughnening of epoxy resin by modification with acrylic elastomers containing pendent epoxy groups, Eur. Polym. J. 26 (1990) 145–151. [4] H.L. Frisch, K.C. Frisch, Polyurethane-epoxy interpenetrating polymer networks-barrier and surface properties, Prog. Org. Coat. 7 (1979) 105–111. [5] Y. Ye, H. Chen, J. Wu, L. Ye, High impact strength epoxy nanocomposites with natural nanotubes, Polymer 48 (2007) 6426–6433.
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