Polyurethane/clay nanocomposites with improved helium gas barrier and mechanical properties: Direct versus master-batch melt mixing route Bapan Adak , Mangala Joshi , Bhupendra Singh Butola Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Correspondence to: M. Joshi (E - mail: mangala@textile.iitd.ac.in) ABSTRACT: Thermoplastic polyurethane (TPU)/clay nanocomposite films were produced by incorporation of organo-modified montmo- rillonite clay (Cloisite 30B) in TPU matrix by two different melt-mixing routes (direct and master-batch-based mixing), followed by com- pression molding. In master-batch mixing where the master-batch was prepared by mixing of clay and TPU in a solvent, better dispersion of clay-layers was observed in comparison to the nanocomposites produced by direct mixing. As a consequence, superior mechanical and gas barrier properties were obtained by master-batch mixing route. The master-batch processing resulted in 284 and 236% increase in tearing strength and tearing energy, respectively, with 5 wt % clay-loading. Interestingly, in case of master-batch mixing, the tensile strength, stiffness as well as breaking extension increased simultaneously up to 3 wt % clay-loading. The helium gas permeabil- ity reduced by about 39 and 31% for the TPU/clay nanocomposites produced by mater-batch and direct mixing routes, respectively, at 3 wt % loading of clay. Finally, the gas permeability results have been compared using three different gas permeability models and a good correlation was observed at lower volume fraction of clay. V C 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46422. KEYWORDS: clay; composites; mechanical properties; polyurethane; structure–property relationships Received 16 November 2017; accepted 25 February 2018 DOI: 10.1002/app.46422 INTRODUCTION In recent times, thermoplastic polyurethane (TPU)-based films and coatings are in great demand in various fields due to its versa- tile properties such as excellent flexibility and elastic recovery, excellent shock absorption, good chemical resistance, excellent abrasion resistance, and many more. 1,2 However, the low stiffness and strength of TPU can limit its use in structural applications. In addition to these, high permeability to different gases is another drawback of polyurethane. A good gas barrier property is needed for some special applications such as coated or laminated envelop for inflatables, transparent and flexible packaging, sport balls, bio- mimetic materials, pneumatic tires, corrosion resistant coatings, and many others, where polyurethane-based coatings or films are frequently used. 3–6 Therefore researchers have used many approaches for improving gas barrier property of polyurethane, like modifying structure-morphology 7–12 or incorporating nano- platelets such as nano-clay 11,13–17 or graphene 18,19 in TPU matrix. TPU/clay nanocomposites exhibit enhanced mechanical as well as gas barrier properties in comparison to neat polyurethane. 13–15 TPU is a linear, segmented block copolymer consisting of alter- nate soft and hard segments. The rubbery soft segments are made of linear, long chain diol and the semi-crystalline or glassy hard segments are formed by chemical reaction of alter- nating diisocyanates and chain extenders (short chain diol or diamine). 20 Because of thermodynamic instability among soft and hard domains, phase separation occurs, which is the main reason for higher gas permeability of TPU films. Generally, gas barrier property of TPU improves with increasing hard/soft seg- ment ratio, crystallinity, glass-transition temperature (T g ), ori- entation and packing of polymer chains, and crosslinking density. Many literature 7–11,21 discuss improvement in gas bar- rier property of TPU by changing its structure and morphology via changing the raw materials, their compositions and process conditions. By virtue of inherent impermeability of some layered-structured nanomaterials (nano-clay and graphene, inorganic glasses), they are capable of improving the gas barrier property of polymers sig- nificantly. 22 In polymer/clay nanocomposites, properly dispersed and oriented clay platelets increase the tortuous path length for gas diffusion, which leads to much better gas barrier property compared to pristine polymer and conventional polymeric micro- composites. 23,24 Clays or layered silicates are very commonly used as reinforcing fillers due to its wide availability, low density, eco- nomic competitiveness, high aspect ratio, and high specific sur- face area, which results in remarkable improvement in material V C 2018 Wiley Periodicals, Inc. J. APPL. POLYM. SCI. 2018, DOI: 10.1002/APP.46422 46422 (1 of 12)