Thermoplastic polyurethane/polytetrafluoroethylene composite foams with enhanced mechanical properties and anti-shrinkage capability fabricated with supercritical carbon dioxide
Graphical abstract
Introduction
Thermoplastic polyurethane (TPU) is one of the most widely applied thermoplastic elastomers, which possesses the classical properties of thermoplastic elastomers such as elasticity and low-temperature flexibility. Moreover, it exhibits good mechanical properties such as high elasticity, desirable impact properties at low temperatures, great flexibility, and resistance to aging and abrasion [1,2]. One of the most important commercial TPU products is foam, which has been used extensively in the automotive, packaging, cushioning, oil–water separation, thermal and acoustic isolation fields [[3], [4], [5], [6], [7]]. Chemical foaming is a common foaming method for polymer, including TPU foam. Owing to the pollution caused by the chemical residue, it is more significant to prepare polymer foams with environmentally-friendly foaming agents [8]. Supercritical carbon dioxide (scCO2) was widely used, as a blowing agent, owing to its lower critical pressure and temperature, and non-toxic is the greatest advantage of it [[9], [10], [11], [12], [13], [14]]. Therefore, the scCO2 foaming technology has been widely studied in recent years [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]].
Although TPU foam has already been applied in practice, the problem of the high shrinkage ratio after foaming must be solved, and its mechanical properties should be improved. In the foaming process, the rapid expansion of TPU makes the molecular chains stretched, which leads to internal stress in the TPU foam. Consequently, maintaining the original size of the TPU foam at room temperature is difficult. In addition, a higher concentration of CO2 in TPU foam with respect to that in the atmosphere causes the CO2 inside the TPU foam to diffuse out [28]. Reactive blending is an effective method for decreasing the shrinkage ratio of TPU foam. For example, it was decreased with the addition of acrylonitrile-butadiene-styrene and maleic anhydride to TPU [28]. This result was attributed to the increased melt viscosity and strength of the TPU, which increased the strength of the cell walls and rigidity of the molecular chains in the TPU foam. Some other approaches for improving the melt viscosity and strength have been proposed, such as the addition of carbon nanotubes, graphene, and nano-clay. However, the dispersion of nanofiller is difficult [29]. Polymer blending is considered a cost-effective and practical technique for improving the melt viscosity and adding the desired characteristics to composites. Especially, in-situ fibrillated compounding has attracted increasing attention [[30], [31], [32], [33], [34], [35]]. However, realizing in-situ fibrillated compounding in polymer–polymer composites is difficult owing to the difficulty to deform the unmelted polymer phase during compounding, in addition, a cold drawing process is essential [[33], [34], [35]].
Polytetrafluoroethylene (PTFE) has excellent physical and chemical properties such as good chemical resistance, extremely high temperature stability, and a lower coefficient of friction than other polymers. It was reported that PTFE could deform into a high-aspectratio fibrillar structure under strong shear stress because of the low yield strength of PTFE at elevated temperatures and plastic deformation [[36], [37], [38]]. There were some researches on the effect of in-situ fibrillated PTFE on PPC [39,40], PP [41], and PLA [42], such as the rheological properties and foamability. There was only one report on the effect of fibrillated PTFE on the dynamic mechanical, rheological properties, and foamability of TPU [43]. In-situ fibrillated PTFE can significantly enhance the mechanical, rheological, and foaming properties of TPU [43]. However, it is still unclear whether PTFE or its morphology (fibrillated PTFE) affects the foaming behavior, rheological behavior of TPU, and mechanical properties of TPU foam. Consequently, it is of great significance in enriching the understanding of the effect of the PTFE morphology on TPU by comparing the influence of fibrillated PTFE and PTFE particles on the rheological behavior, foaming properties of TPU, and shrinkage ratio and mechanical properties of TPU foam. In addition, the influence of PTFE particles and fibrillated PTFE on the rheological behavior of TPU in high-pressure CO2, which is significant to control the foaming behavior of TPU, has not been studied up to now.
In this study, a traditional twin-screw extruder was used to prepare TPU/PTFE composites with different morphologies and contents of PTFE. Influence of PTFE on the foaming behavior of TPU, and the shrinkage ratio and mechanical properties of TPU foam was investigated in detail. First, the PTFE morphologies in TPU and cellular structures of TPU/PTFE foams were investigated by scanning electron microscopy (SEM). Subsequently, the effects of different morphologies and contents of PTFE on the rheological behavior of TPU at atmospheric pressure and in high-pressure CO2 and the shrinkage ratio and mechanical performance of TPU foam were investigated. Finally, the mechanisms for the improvement of cell structures and mechanical properties of TPU foam and hydrophobicity of TPU in the presence of PTFE were proposed.
Section snippets
Materials
TPU pellets (Elastollan1185A, BASF Ltd.) used in this experiment was purchased from Suzhou JunXi International Trade Co., Ltd. Commercially available PTFE powders (PTFE 62X) supplied by Dongguan ZhanYang Polymer Material Co., Ltd. and PTFE powders (L170J) supplied by Dongguan Kadar Plastic Material Co., Ltd. were used as additives. In addition, CO2 with a purity of over 99% was used as the blowing agent, which was purchased from Chengdu QiaoYuan Gas Co., Ltd. Two kinds of PTFE powders were used
Morphology of neat TPU and TPU/PTFE composites
To investigate the morphologies of PTFE in the TPU/PTFE composites, the fractured cross-section surfaces of all samples were immersed into DMF to remove the TPU matrix and the SEM images are shown in Fig. 1. Evidently, the surface of the neat TPU was still smooth, and PTFE (L170J) was dispersed as particles in the TPU/PTFE composites. However, PTFE 62X (bigger size than that of PTFE L170J) was fibrillated in the TPU/PTFE composites. PTFE fibrils were formed during melt compounding, which was
Conclusions
In this study, PTFE and TPU were mixed with a twin-screw extruder, and the effect of fibrillar PTFE and PTFE particles on TPU was investigated. It was found that the fibrillated PTFE had a more noticeable augmented effect on TPU. Complex viscosity and storage modulus of TPU-F0.5 were higher than those of TPU-L0.5 and TPU-F0 in high-pressure CO2. As heterogeneous nucleation points, PTFE increased the cell density and decreased the cell size of the TPU foam, which significantly improved the
Declaration of competing interest
None.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (no. 51773138 and 51373103), the State Key Laboratory of Polymer Materials Engineering (sklpme 16-2-03), and the Fundamental Research Funds for the Central Universities of China.
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