Regulating microstructures of interpenetrating polyurethane-epoxy networks towards high-performance water-lubricated bearing materials
Graphical abstract
Introduction
Oil has long been regarded as the primary lubrication solution for enormous quantities of mechanical systems. However, oil leakage can have a significant impact or even grave threat to local ecosystems. As environmental regulations become increasingly stringent at different levels of governments, replacement of oil-lubricated bearings with water-lubricated ones is an inexorable trend for ship building and hydropower generation industries [[1], [2], [3], [4], [5]].
When water is utilized as lubricating medium, it shows obvious advantages in regards to environmental friendship and energy conservation (small frictional drag). However, owing to the considerably lower viscosity, the load bearing capacity of the water film is greatly reduced when compared to oil. That is, water lubricated bearings are more likely to operate in mixed or boundary lubrication regimes [6]. This poses serious challenges to anti-friction and durability of the bearing materials.
Since the last decades, tribology of polymeric materials has been attracting extensive interests from both academia and industry [[7], [8], [9], [10], [11]]. Polymeric materials are excellent candidates of tribo-materials for water lubrication conditions owing to their high chemical stabilities, self-lubrication and damping characteristics. Modified rubbers, reinforced phenolic resin, new type nylon, special polyurethane (PU), engineering plastics and so on [8,[12], [13], [14]], have been increasingly used for manufacturing water-lubricated bearings.
PU is a type of block polymers comprising alternatingly hard and soft segments along their backbones [15,16]. The soft segments endow PU a high flexibility, while the hard ones provide it with certain rigidity [17,18]. Owing to such a special molecular architecture, PU exhibits excellent properties, e.g. high abrasion resistance, excellent toughness, vibration attenuation and shock absorption characteristics [19,20]. Thanks to the desirable properties, PU has been utilized as matrix material for developing water-lubricated bearings [[21], [22], [23]]. However, some limitations of PU as water-lubricated bearings have been figured out, e.g. low modulus (low load-bearing ability) and significant spirant at low speeds (associated with stick-slip) [18,21]. In particular, when sliding takes place at a heavy load and a low speed, seizure of motion systems can happen. Epoxy resin (EP) is a widely used stiff thermoset with a high modulus, high strength and excellent chemical stability. However, EP is inherently brittle, showing a low fracture toughness and impact resistance [24,25]. Since the recent decades, numerous efforts were dedicated to formulate EP-based composites for various tribological applications [[26], [27], [28], [29]].
Interpenetrating polymer networks (IPNs) are combinations of two or more network polymers synthesized in juxtaposition. Depending on network architectures, IPNs can exhibit superior mechanical properties to those of the individual networks, i.e. a synergistic effect can be achieved by the compatible components [[30], [31], [32]]. PU/EP IPNs, integrating the advantages of PU and EP, compensate the disadvantages of the two components, i.e. low modulus and brittleness, respectively. Hence, PU/EP IPNs has attracts enormous interests from both academia and industry. Up to now, continuous efforts have been dedicated to development of PU/EP IPN composites. It was demonstrated that addition aramid fibers [33], potassium titanate whiskers [34], graphene oxide [35,36], nanodiamond [37] and various nanoparticles [38,39] improved the mechanical and damping properties of PU/EP IPNs.
It is believed that PU/EP IPNs has a high potential for developing water-lubricated bearing materials. Nevertheless, majority of previous works focus on the roles of various fillers in mechanical and damping properties of IPN composites. Solid understanding on the relationship between network structures and properties of PU/EP IPNs is still lacking. Moreover, only a few papers deal with the friction and wear resistance properties [36,37]. In particular, tribological mechanisms of PU/EP IPNs subjected to water lubrication conditions have not been yet comprehensively understood. An obvious gap between theory and tribological application potential of PU/EP IPNs exists.
In the present work, PU/EP IPNs with various stoichiometric ratios of EP and PU were synthetized. The structure-mechanical property relationship of the IPNs was comprehensively investigated. The dependence of the tribological performance under water lubrication conditions on the microstructures of PU/EP IPNs was explored. It is expected that the present work is of significance for disclosing structure-property relation of PU/EP IPNs and pave a route for developing high-performance tribo-materials subjected to water lubrication conditions.
Section snippets
Materials
Diglycidyl ether of bisphenol A-based (DGEBA) epoxy resin precursor (WSR-618, epoxide equivalent weight: 196 g/mol) was supplied by Nantong Xingchen Synthetic Material Co., Ltd. (China). Isocyanate-terminated PU prepolymer (L200) and curing agent 3, 3′-dichloro-4, 4′-diamino diphenyl methane (MOCA) was purchased from Chemtura Shanghai, Co., Ltd. (China). Epoxy resin and PU prepolymer were dried for 8 h before use under vacuum at 100 °C and 70 °C, respectively.
Preparation of PU/EP IPNs
A certain amount epoxy resin
Structures of IPNs
FTIR spectra of EP before and after curing with MOCA, neat PU and PU/EP IPNs were illustrated in Fig. 2. A broad peak of the hydroxyl group (OH) is noticed from the spectrum of cured EP at about 3500 cm−1 in Fig. 2a, indicating that ring-opening reaction of the EP resin with the amino group of MOCA occurred [42]. After adding PU prepolymer into EP, isocyanate (NCO) groups of PU can react with hydroxyl groups of EP resin. First, before the addition of MOCA hardener, the NCO groups can react with
Conclusions
In the present work, microstructures and mechanical properties of PU/EP IPNs as a function of PU contents were investigated. In particular, tribological behaviors of the IPNs in different lubrication regimes were comprehensively studied in water medium. Following conclusions can be drawn:
- 1.
Microstructures of PU/EP IPNs show close dependence on stoichiometric proportions of the components. Our work confirmed the interpenetration of EP and PU networks. TEM and AFM analyses revealed that PU-rich
Acknowledgements
The authors greatly appreciate the financial supports from the National Natural Science Foundation of China (Grant no. 51875552, 51705505), Chinese “Thousand Youth Talents Plan” program and “Innovation Leading Talents” program of Qingdao.
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