Vegetable Oil-Based Hyperbranched Thermosetting Polyurethane/Clay Nanocomposites
© to the authors 2009
Received: 16 February 2009
Accepted: 2 April 2009
Published: 25 April 2009
The highly branched polyurethanes and vegetable oil-based polymer nanocomposites have been showing fruitful advantages across a spectrum of potential field of applications.Mesua ferrea L. seed oil-based hyperbranched polyurethane (HBPU)/clay nanocomposites were prepared at different dose levels by in situ polymerization technique. The performances of epoxy-cured thermosetting nanocomposites are reported for the first time. The partially exfoliated structure of clay layers was confirmed by XRD and TEM. FTIR spectra indicate the presence of H bonding between nanoclay and the polymer matrix. The present investigation outlines the significant improvement of tensile strength, scratch hardness, thermostability, water vapor permeability, and adhesive strength without much influencing impact resistance, bending, and elongation at break of the nanocomposites compared to pristine HBPU thermoset. An increment of two times the tensile strength, 6 °C of melting point, and 111 °C of thermo-stability were achieved by the formation of nanocomposites. An excellent shape recovery of about 96–99% was observed for the nanocomposites. Thus, the formation of partially exfoliated clay/vegetable oil-based hyperbranched polyurethane nanocomposites significantly improved the performance.
Upgradation of the pristine polymers through the formation of nanocomposites is proven to be one of the best ways in recent time [1, 2]. Due to their high aspect ratio, stiffness, and in-plane strength, nanoclays are a choice of major polymer nanocomposite researches , as enhancement of properties like dimension stability, mechanical, thermal, flame retardancy, gas barrier, etc. are significant [4–8].
Again, two-phase morphology of the polyurethane, arising from the non-homogeneity of the hard and soft segments, makes it extraordinarily versatile [9, 10]. A wide range of physical and chemical properties can be tailor made just by judicious variation of composition and structure of three basic building blocks, viz., macroglycol, diisocyanate, and chain extender of polyurethanes or by physical modification like blending or through interpenetrating network formation with other polymers . Thus, polyurethanes are being utilized in a diversified field of applications starting from coating and paint, foam, thermosetting, and thermoplastic elastomer to fiber . However, some advanced applications demand high mechanical strength, chemical resistance, thermostability, low water vapor permeability, etc. Again, as the formation of exfoliated polyurethane/nanocomposites may eliminate the above short comings, in this study attempt has been made to investigate the same.
Again, hyperbranched polymers have been offering their candidature as an advanced polymeric material since the last two decades due to their highly functionalized three-dimensional globular non-entangled inimitable architectural features and unique properties . These macromolecules exhibit many useful properties like improved solubility, low melt and solution viscosity, high reactivity, and so on for their different applications in addition to their single-step preparative techniques [14–17]. Therefore, such novel macromolecules may be a right choice for this research.
Further, the present scenario of environmental issues, problems regarding disposal, reprocess, fast exhaustion of petroleum reserves, etc., have compelled us to use the renewable products including vegetable oils. These vegetable oils have the enormous advantages as they are biodegradable, renewable, sustainable, aptitude to facile modification, non-toxic, and the most importantly environmentally benign . Mesua ferrea plant bears exceptionally high oil-containing (70%) seeds, and the favorable fatty acid composition of the oil enables its use for the preparation of polyurethane [19, 20]. Hence, the investigation of vegetable oil-based highly branched polyurethane nanocomposite is expected to offer high performance materials by coalescing the afore-stated advantages.
Authors herein wish to report the performance characteristics like mechanical, thermal, chemical, adhesive strength, water vapor permeability, etc. properties ofMesua ferrea L. seed oil-based HBPU/nanoclay nanocomposite at different dose levels of nanoclay. The shape memory behavior of the nanocomposites has also been investigated.
Mesua ferrea L. seeds were collected from Darrang, Assam. The seed oil was isolated by solvent-soaking method and purified by the alkali-refining technique. Monoglyceride of the oil was prepared by the standard glycerolysis procedure. Glycerol (Merck, India) and poly(ε-caprolactone) diol (PCL, Solvay Co.,Mn = 3,000 g/mol) were used after drying in an oven prior to use. 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, India) was vacuum distilled and kept in 4A type molecular sieves before use. Octadecylamine-modified montmorillonite clay (MMT) (Sigma Aldrich) of 25–30 wt% was used as nanoclay. Bisphenol-A-based epoxy resin (CY 250) and poly(amido amine) hardener (HY 840) (Ciba Geigy, Mumbai) were used as supplied. Other reagents are of reagent grade and are used without further purification.
Preparation of HBPU and its Modification
The hyperbranched polyurethane was prepared by pre-polymerization technique. At first, the pre-polymer was prepared from 2 mol of poly(ε-caprolactone) diol, 3 mol of monoglyceride ofMesua ferrea L. seed oil, and 7.5 mol of TDI, and the polymer was obtained finally by the reaction of the pre-polymer with 2.5 mol of glycerol in DMF as a solution of 25–30% solid content (w/v). The molecular weight (Mw) of the polymer was 5.3 × 104 g/mol with polydispersity index 2.4, hydroxyl value 37.4 mg KOH/g, and the ratio of hydrodynamic radius with respect to linear analog 0.90.
In order to obtain thermosetting polymer composition, the above HBPU was mixed with 20 wt% (with respect to HBPU) of epoxy resin (100% solid content) and poly(amido amine) hardener (50% by weight with respect to the epoxy resin) at an ambient temperature by mechanical stirring followed by ultrasonication.
Different amounts of the modified dispersed clay (1, 2.5, and 5 wt%) solution was added into the pre-polymer, which was obtained by the same procedure as described above. The nanoclay was dispersed in 1–5 mL of DMF by magnetic stirring followed by ultrasonication before adding into the pre-polymer. The remaining part of the preparation was the same as above. Finally, the nanocomposites films were obtained by solution casting, followed by vacuum degassing and curing at 120 °C for 45 min for further testing and analyses. The cured films were denoted as HBPU1, HBPU2.5, and HBPU5 corresponding to the clay content of 1, 2.5, and 5 wt%, respectively.
Analysis and Characterization Techniques
FTIR spectra of the epoxy-cured hyperbranched polyurethane clay nanocomposites were recorded by a Nicolet (Madison, USA) FTIR Impact 410 spectroscopy using KBr pellets. The thermal analysis was done by a Simadzu, USA thermal analyzer, TG 50, at 10 °C min−1 heating rate under the nitrogen flow rate of 30 mL min−1. The DSC was done by Simadzu, USA, DSC 60, at 3 °C min−1 heating rate under the nitrogen flow rate of 30 mL min−1 from −50 to 80 °C. The measurement of specific gravity, impact resistance, hardness, flexibility (bending), and chemical resistance tests were performed according to the standard methods . Gloss of the films was tested by mini gloss meter, Sheen Instrument Ltd., UK. The mechanical properties such as tensile strength and elongation at break were measured by universal testing machine (UTM) of model Zwick Z010, Germany with a 10-kN load cell and crosshead speed of 50 mm/min. The X-ray diffraction study was carried out at room temperature (ca. 25 °C) on a Rigaku X-ray diffractometer (Miniflex, UK) over the range of 2θ = 1–30° for the above study. Size and distribution of the nanoclay layers were studied by using JEOL, JEMCXII transmission electron microscopy (TEM) at an operating voltage of 80 kV. Ultrasonicator (UP200S, Heishler, Germany) was used at various amplitude and continuous cycle for different time periods. The adhesive strength of the cured thin films was measured by lap shear test as per the standard ASTM D3165-95 procedure by using plywood sheets as the substrate. The water vapor barrier property was measured by taking 10 mL of water in the standard cup of 1 cm2 cut on the lid. The nanocomposite films of 60–70 μm thickness were fixed in the lid, and the systems were kept in the vacuum oven under reduced pressure of 760 mm of Hg and at 25 °C. The initial weight of the cup with water and film is W0. The percent weight loss through the films was calculated by measuring the weight of the cup (W1) at specific intervals during the test using the following equation (W0 − W1)/W0 × 100%.
Sample Preparation for Mechanical Properties
For impact resistance and scratch hardness studies, mild steel strips of 150 × 100 × 1.44 mm3were coated by the polymer nanocomposite solutions. Similarly, tin strips of 150 × 50 × 0.19 mm3were coated for bending and glass strips of 75 × 25 × 1.39 mm3were coated for gloss and chemical resistance test. All the films were found to be in the range of 60–70 μm thickness as measured by a Pentest coating-thickness gauge (Model 1117, Sheen Instrument Ltd., UK). The cured casted nanocomposites samples were cut by the manual sample cutter with dimension as per the ASTM D 412-51T for mechanical testing.
Results and Discussion
Preparation of HBPU and their Nanocomposites
It was observed that with the increase in clay content (1–5 wt%), the degree of phase separation increases (DPS values 47.2, 48.5 and 49.7 for HBPU1, HBPU2.5, and HBPU5, respectively). This may be due to the reaction of the –OH groups of the clay with the –NCO groups of the pre-polymer/TDI. Also, the greater strength of the hydrogen bonding between the –OH of clay and carbonyl group of urethane than intra-hydrogen bonding is attributed to the fact .
Physical and mechanical properties of nanocomposites and pristine polymer
Impact resistance (cm)
Bending (dia. mm)
Scratch hardness (kg)
Tensile strength (MPa)
Elongation at break (%)
Lap-shear adhesion (MPa)
The mechanical properties of the nanocomposites are given in the Table 1. It was observed that the tensile strength of the nanocomposites increases with the increase of the nanoclay content, while the value for elongation at break decreases. This is because of the reinforcement of the matrix by the organo MMT layers which increases the tensile strength . The hydrogen bonding as well as chemical linkages (Scheme 1) between clay layers and HBPU matrix at the interface is responsible for this improvement . In HBPU5, the highest tensile strength of 48 MPa was obtained which may be due to high loading along with exfoliated structure formation as supported by the XRD and TEM results. The formation of exfoliated nanoclay structure enhances the interface interactions through bridge, loop, and tail linkages of the polymer chains with the nanoclay layers via H bonding and polar–polar interactions (Scheme 1). Thus, the optimum strength was obtained for HBPU5. Again, generally, the tensile strength of polymeric composites increases at an expense of its elongation at break value, and it was observed in these nanocomposites also. This decrement is due to the restricted movement of the clay layers in the matrixes by the above interactions. However, the decrement is not very high, as the PCL moiety assists the deformation to a certain extent by its flexible chain .
Another achievement observed in these nanocomposites is the retention of the flexibility (Table 1). The films can be easily bent to 5 mm parallel mandrel without any damage. The long fatty acid chain of theMesua ferrea L. seed oil and PCL moiety are contributed to this flexibility. Mechanical properties like impact and scratch hardness are seen to be increased notably for all the nanocomposites films compared with the HBPU. This may be due to the overall enhancement of the tensile strength of these flexible films.
The adhesive strength of the nanocomposites was investigated on wood substrate (Table 1). An increase in the strength with the loading of the clay was observed. This may be due to the presence of large numbers of end functionality of the hyperbranched polymer along with other polar groups in the systems. The increment of adhesive strength may be due to the strong interactions of polar hydroxyl, epoxy, urethane, ether, and other polar groups of the epoxy-cured HBPU/clay system with the hydroxyl groups of the substrate .
Water Vapor Barrier Property
Thermal properties and shape memory of nanocomposites and pristine polymer
Glass transition temperature (Tg °C)
Melting temperature (Tm °C)
Melting enthalpy (∆Hm J/g)
Shape retention (%)
Shape recovery (%)
Thermal stability data of the nanocomposites and pristine polymer
The nanocomposites with different loading show excellent shape recovery of 96–99%, out of which 90–95% is attained within about 2 min. The values of the shape memory properties of clay nanocomposites were given in Table 2. The increased stored energy due to the incorporation and uniform distribution of clay layers in the HBPU matrix may attribute to this excellent shape memory property . The shape recovery increases with the increase in the clay content which is due to an increase in stored elastic strain energy by clay layers; therefore, while reheating the samples, the film can obtain higher recovery stress due to the release of stored elastic strain .
Chemical Resistance Test
Chemical resistance of the nanocomposites and pristine polymer
The study shows in situ preparation of nanocomposites fromMesua ferrea L. seed oil-based hyperbranched polyurethane and nanoclay offers partially exfoliated structure. The formation of nanocomposites significantly improved the performance characteristics like mechanical properties, thermostability, adhesive strength, shape memory behavior, chemical resistance, etc., without much affecting the impact resistance and ductility. Thus study indicates the potentiality of these materials as advanced materials in various fields.
The authors express their gratitude to the research project assistance given by DST, India through the grant No. SR/S3/ME/13/2005-SERC-Engg, dated 9th April, 2007.
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