Synthesis and characterization of organically modified clay/castor oil based chain extended polyurethane nanocomposites

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Abstract

A series of 1,4-butane diol chain extended polyurethane nanocomposites based on castor oil, and 4,4′-diphenylmethane diisocyanate (MDI) were synthesized with modified clay (Cloisite 30B) as filler. The synthesis was carried out in bulk and without catalyst via a two-step polymerization. The clay percentage was varied from 0% to 5% by weight of the nanocomposite. The prepared nanocomposites were characterized using TEM, SEM, WAXD, FTIR, Thermogravimetric Analysis (TGA), mechanical properties and Moisture absorption behavior. TEM and XRD results confirmed successful exfoliation of clay in the polyurethane matrix. Thermogravimetric Analysis (TGA) results showed that the thermal stability of nanocomposites improved with increased percentage of clay. Moreover, char percent also increased from 5.6% to 12% as clay percent increased from 0% to 5%. The Young’s modulus improved more than 300% with addition of 5% clay filler. These nanocomposites exhibited lower water absorption and diffusivity values as compared to neat polyurethane.

Introduction

Polyurethanes have been extensively used due to their excellent physical properties (e.g. high tensile strength, abrasion and tear resistance, oil and solvent resistance, low flexibility, etc.) and high versatility in chemical structures. The properties of various types of urethane polymers are dependent upon molecular weight, degree of cross-linking, effective intermolecular forces, stiffness of chain segments and crystallinity. Due to many structural variations that are possible in their formation, urethanes may be considered the most versatile polymers.

Although there are host of raw materials used for making polyurethanes, continuous depletion of natural resources has made it imperative to identify alternate resources that are renewable or have plant origin [1]. Polyurethane based on polyols derived from different vegetable oils, like castor [2], [3], [4], linseed [5], soybean [6], rice bran and coffee meal oil [7] with or without modifications are being used these days due to their excellent properties derived from hydrophobic nature of triglycerides. If the triglyceride structure can be preserved, thermosetting polyurethane networks can be obtained, whose network properties depend upon on the crosslinking density, and position of fatty acid chain. Among all these oils, castor oil is the most popular due to inherent hydroxyl group. Commercially castor oil consists of triglycerides that contain 90% ricinoleic acid residues and 10% non-functional acid residues, so that castor oil has effective hydroxyl functionality of 2.7 [8]. The long pendant chains of fatty acids impart flexibility and hydrolytic resistance to the network, whereas double bond present in castor oil serves as grafting centers.

The disadvantage of castor oil include: low hydroxyl number leading to low modulus material; sluggish rate of curing of secondary hydroxyl groups [2] and structural irregularity due to steric hindrances offered by the long pendant fatty acid chains during polyurethane formation, resulting in low tear strength. These disadvantages can be offset in numerous ways: (a) making polyurethane interpenetrating network with various polymers like polystyrene [9], poly methyl methacrylate [10], polyacrylonitrile and organic silicons [11], (b) castor oil can be transesterified or alcoholyzed with polyhydroxy alcohols, most commonly by glycerol and trimethylol propane [12], (c) adding nano-reinforcement like exfoliated clay, carbon nanotubes, carbon nanofibers, exfoliated graphite (graphene), nanocrystalline metals and a host of additional nanoscale inorganic filler to pristine polymers to prepare nanocomposite that have improved properties.

Among nano-reinforcements, clay has generated much interest due to its high aspect ratio and available surface area which results in enhancement of mechanical properties, thermal stability, flame redundancy, and barrier properties of the matrix polymers [13]. Since natural clay is hydrophilic and lacks affinity with hydrophobic organic polymers, it needs to be modified by introducing organic cationic molecules. Organic modifiers can impart hydrophilicity to clay, thereby improving compatibility of clay with polymers. Since pioneering work in the area of polyurethane/clay nanocomposites by Wang et al. [14] numerous studies have been reported [15], [16], [17], [18], [19], [20]. But none of these studies have focused on castor oil based polyurethanes.

The objective of this work was to synthesize and characterize a series of chain extended polyurethane nanocomposites based on biobased polyol i.e. castor oil and commercially available organically modified clay, Cloisite 30B. The soft segment was castor oil and the hard segment structure was 1,4-butanediol (BD) chain extender. Synthesis was carried out in bulk and without catalyst via a two-step polymerization varying the amount of clay and keeping hard segment ratio constant. Nanocomposites synthesized were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Wide angle X-Ray diffraction (WAXD).

Section snippets

Material used

Castor oil was commercial grade and was purchased from the local market. It was dehydrated at 105 °C in nitrogen atmosphere and characterized for hydroxyl value (148), acid value (2) and moisture content (0.379%). Modified montmorillonite clay Cloisite 30B was procured from Southern Clay Products, USA with methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium (MT2EtOH) as modifier, modifier concentration of 90 meq/100 g clay and moisture content <2%. Chain extender, 1,4-butane diol was procured

Morphology and dispersion of polyurethane nanocomposites

XRD is commonly used for characterization of the structure of nanocomposites. For an intercalated structure, the (0 0 1) characteristic peak tends to shift to lower angle regime due to expansion of the basal spacing [21]. Although the layer spacing increases, there still exists an attractive force between the silicate layers to stack them in an ordered structure. In contrast, no peaks are observed in the XRD pattern of exfoliated polymer nanocomposites due to loss of structural registry of the

Conclusions

Castor oil based chain extended polyurethane nanocomposites were synthesized with organoclay, Cloisite 30B as filler. The synthesized nanocomposites were characterized using TEM, SEM, WAXD, FTIR, TGA, mechanical and barrier properties. XRD and TEM confirmed exfoliation of clay platelets in the polyurethane matrix. The results of TGA and FTIR experiments indicated an interaction between the clay and polymer improving thermal stability. A reduction in water sorption compared to the pure matrix

Acknowledgements

We gratefully acknowledge financial support rendered by All India Council of Technical Education (AICTE), India and University Grants Commission (UGC), India for the research work.

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