A composite with excellent tribological performance derived from oxy-fluorinated UHMWPE particle/polyurethane

Abstract

The thermoplastic polyurethane (TPU)/UHMWPE composite with good compatibility and strong interfacial adhesion is achieved by using direct fluorination surface modification technology. The obtained composite has excellent tribological performance enhancement efficiency with significant decrease in wear volume loss and friction coefficient.

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Introduction

In tribological applications, because of problems of metal corrosion and economical processing of intricate shapes in large quantities for polymer materials, polymer composites are the preferred choice for resistant components. Thermoplastic polyurethanes (TPU) are well known for their good mechanical properties and excellent abrasion resistance, which make them an important class of composite materials for tribological applications such as liners in pipes, tires, bushings and slide coatings.^1,2 However, the available TPU materials so far were not adequate enough to meet the stringent requirement of the advanced composites for tribological application with new technologies and new products rapidly updating.

To further improve the tribological performance of polyurethane-based friction material, one of the most efficient methods is to add various kinds of high wear resistant and self-lubricating fillers into the matrix as reinforcement. Zhaozhu Zhang et al.³ reported that incorporation of the carbon fibers (CF) pre-treated by activation in HNO₃ plus application of toluene-2,4-diisocyanate (TDI) into polyurethane (PU) composite coatings provided much higher wear life and lower friction coefficient in comparison with the unfilled PU coating. It is because those carbon fibers are composed of many laminated graphite planes, which endows them with self-lubricating characteristics. Yuan H. et al.⁴ investigated that the blend of ultra-high-molecular-weight polyethylene (UHMWPE) and TPU using polyethylene-grafted maleic anhydride as a compatibilizer had lower wear rate than the uncompatibilized composite.

UHMWPE demonstrates excellent mechanical properties, especially distinguished wear-resistance and self-lubricating properties, making it an ideal filler candidate to enhance wear resistance of polymer materials.^5,6 However, it is well known that TPU/UHMWPE blends are thermodynamically immiscible and generally have poor ultimate properties due to the nonpolar nature and chemical inertness of UHMWPE.^7,8 Therefore, UHMWPE particles need to be surface-modified to obtain the actual application.

Direct fluorination is a gas-phase chemical reaction of gaseous F₂ and its mixtures with a polymer surface, which is an effective chemical method to modify and control physicochemical surface properties of polymers. Fluorine gas can react with almost all organic compounds in control reactions without further activation by means of catalysts or initiators owing to its high reactivity.^9,10 In applications involving large surface areas such as particles, direct fluorination has significant advantages of improving interfacial adhesion.

In this study, a compatibilized polymer–polymer composite derived from immiscible UHMWPE particle and TPU is obtained by direct fluorination surface modification technology. The compatibility of TPU with UHMWPE particle was investigated through morphological characteristics, X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) measurements. The incorporation of modified UHMWPE particle provides the TPU with excellent tribological performance enhancement with significant decrease in wear volume loss and friction coefficient. Furthermore, the modified composite has higher tear strength than that of virgin TPU, and the mechanical properties of the composite are closed to that of virgin TPU.

Experimental

Direct fluorination of UHMWPE particle (GUR4150-3, American, density: 0.94 g cm⁻³) with diameter ranging from 50–70 μm was carried out in closed stainless steel (SUS316) chamber (10 L) equipped with vacuum line at temperature of 75 °C for 1 h. The gas partial pressures of F₂/N₂ (10 vol% for F₂) mixed gas and O₂ were controlled at 60 kPa and 10 kPa, respectively. The non-fluorinated and oxy-fluorinated samples were denoted as U-UHMWPE and FO-UHMWPE, respectively. The UHMWPE particles were solution blended with a thermoplastic polyurethane (TPU 345X, Bayer, German, Density: 1.21 g cm⁻³) in N,N′-dimethylacetamide (DMAc) solvent to obtain blended composite, denoted as TPU/U-UHMWPE, TPU/FO-UHMWPE.

The chemical composition and structure of oxy-fluorinated UHMWPE particles were characterized by FT-IR and XPS. The abrasion resistance tests of TPU/UHMWPE composites were conducted on a DIN rotating drum abrasion tester. The specimen was tested under constant contact pressure (10 N) and at a constant speed through a defined abrasion distance (40 m) across a specified abrasive cloth attached to the surface of a rotating cylinder. The loss in mass of specimen was converted to volume loss from its calculated density. Net wear volume losses were adopted to evaluate wear-resistance properties.

The static friction coefficient of TPU/UHMWPE composite was obtained through the ratio of the maximum static friction force to the vertical gravity force. The TPU/UHMWPE composite was also investigated through scanning electron microscopy (SEM) and tensile tests, indicating that surface modified UHMWPE particles have good compatibility and strong adhesion with TPU matrix.

Results and discussion

The incorporation of FO-UHMWPE particles in polyurethane gives greatly reduced wear volume loss and static friction coefficient, which is summarized in Fig. 1. Compared with the virgin TPU (100%), the wear volume loss of TPU/FO-UHMWPE composites decrease remarkably, which ranges from 100% to 54.2%, about 45.8% reduction. In contrast, the wear volume loss of TPU/U-UHMWPE composite has a slight decease at 10/100 and then increases to 111.4% at 20/100. The friction coefficient for composite decreases with the increasing content of UHMWPE particles, which is decreased from 0.43 to 0.26 for TPU/U-UHMWPE and 0.19 for TPU/FO-UHMWPE, respectively.

Fig. 1 The wear volume loss and the friction coefficient of TPU, TPU/U-UHMWPE and TPU/FO-UHMWPE composites under different mass ratio.

The above results indicate that the resulted TPU/FO-UHMWPE composite obtains better abrasion resistance properties. It is because that there is only van der Waals force between U-UHMWPE particles and TPU matrix, the particles are easily peeled out from the matrix. There exist two opposite effects: the particles peeled out act as a lubricant that mitigates the surface wear and friction; particles also result in more severe scratched abrasive wear and make wear heavier.¹¹ A large quantity of debris for the non-fluorinated composite was generated during the wear testing. The holes left by the dislodged particles can be seen with SEM shown in Fig. 2e. This indicates that adhesion of the particle to the polyurethane is very poor and contributes little to the abrasion resistance of the composite. Thence, the TPU/U-UHMWPE composite has a slight fluctuation in wear loss with respect to virgin TPU. The behavior of modified particles is different, which is difficult to be separately stripped out from matrix, because it bonds closely with polyurethane matrix. Therefore, during abrasion resistance testing, UHMWPE particles can act as abrasive resistant component material due to their distinguished wear-resistance and self-lubricating properties, when they are located in the interfacial between the specimen and the counterpart cylinder, accordingly reducing the wear loss and friction coefficient shown in Fig. 2f.^12,13

Fig. 2 SEM micrographs of fractured surfaces for TPU/U-UHMWPE composite (a) and TPU/FO-UHMWPE composite (b); particle morphology for UHMWPE particles obtained from TPU/U-UHMWPE composite (c) and TPU/FO-UHMWPE composite (d); typical morphology of wear scar on the composite for TPU/U-UHMWPE (e) and TPU/FO-UHMWPE (f).

Fig. 2 shows the morphology of UHMWPE particles dispersed in the matrix of TPU. In the TPU/U-UHMWPE composite, some particles appear to be easily peeled out from the matrix during the cryo-fracturing, and the two-phase interface is clear (Fig. 2a). Holes left when particles are pulled out, and projecting spherical particles can be seen in cross-section of TPU/U-UHMWPE composites in Fig. 2a, which shows that interfaces in the TPU/U-UHMWPE blends are weak and lack of sufficient adhesion. However, oxy-fluorine-functionalized UHMWPE (FO-UHMWPE) particles yield and break rather than pull out from the matrix (Fig. 2b), which indicates that strong adhesion and good stress transfer occur between two polymer components.

To directly compare the difference of particle morphology in composites, the TPU/UHMWPE specimens stretched in tensile testing were dissolved in DMAc to etch away TPU phase and obtain UHMWPE particles shown in Fig. 2c for TPU/U-UHMWPE and Fig. 2d for TPU/FO-UHMWPE. The particles in TPU/U-UHMWPE composites still maintain the original spherical shape morphology after tensile testing. In contrast, as can be seen from the Fig. 2d, many particles have deformed into elongated spindle shape, which demonstrates strong adhesion occurs between UHMWPE particles and polyurethane matrix. Therefore, the particles deformed with the polyurethane matrix along the stretching direction when the samples were stretched during tensile testing.

ATR-FTIR and XPS characterization were performed to study the surface chemical structure of UHMWPE particles before and after direct fluorination. The activation of fluorine substantially changes the IR spectrum of the virgin UHMWPE particle. The main feature in the IR spectrum of UHMWPE treated with fluorine is a very broad diffuse band over 1000–1300 cm⁻¹ region shown in Fig. 3a, corresponding to –C–F groups. The formation of –C=O bonds is evident from the appearance of the band at near 1736.2 cm⁻¹. The absorption at 1824.4 cm⁻¹ can be attributed to a –C(=O)F groups, which are transformed into –COOH groups by hydrolysis in the presence of atmospheric moisture or water.^14–16

Fig. 3 FTIR spectra (a); surface element content (b), C1s spectra (c) and (d) of virgin and oxy-fluorinated UHMWPE particles.

Fig. 3 also presents the corresponding XPS results. As shown in Fig. 3b, the content of F and O is significantly increased to 23.4% and 22.6%, while the content of C sharply decreases from 96.2% to 54.0% after fluorination, which indicates a large number of fluorine-containing and oxygen-containing polar groups are introduced on the surface of UHMWPE particles by direct fluorination. The polar groups greatly improve the surface energy of UHMWPE, especially for the polar component (about 286.3% improvement) (see Table S1 and Fig. 1S in ESI†).

Fig. 3c and d present C1s spectra of the U-UHMWPE and FO-UHMWPE, respectively. Only strong –C–C– single absorption peak at 284.8 eV in the U-UHMWPE C1s spectrum is observed in Fig. 3c. The FO-UHMWPE C1s spectra are fitted to six peaks corresponding to –C–C– at 284.7 eV, C–C=O/–C–CF_x at 285.9 eV, –C=O at 287.8 eV, and –C–F at 289.4 eV shown in Fig. 3d.^17,18 These chemical groups create an active surface in particles where physical and chemical bonds can form and promote physical and/or chemical interactions between two polymer components.

It is noteworthy that although the surface properties of UHMWPE particles are dramatically changed, the bulk remains unchanged. The direct fluorination results in the formation of a very thin fluorinated surface layer, which is confirmed by the slight decrease in degree of crystallization characterized by XRD after fluorination (see Fig. S2 and Table S2 in the ESI†).

Mechanical properties of TPU/UHMWPE composites are presented in Fig. 4. The tensile strength of TPU/FO-UHMWPE composite is improved from 12.5 MPa to 22.3 MPa, about 78.4% improvement compared with that of TPU/U-UHMWPE composite, and is be close to that of virgin TPU (23.1 MPa). Similar trend can be observed in breaking elongation, which ranges from 523.5% to 892.1%, about 70.4% improvement, approaching to that of virgin TPU (958.9%). It is well known that the filler/matrix adhesion strength plays an important role on the mechanical properties of reinforced polymer composites. The TPU/FO-UHMWPE composite can produce stronger interfaces that are responsible for more efficient stress transfer across interface.

Fig. 4 Mechanical properties and the tear strength of TPU, TPU/U-UHMWPE and TPU/FO-UHMWPE composites.

The incorporation of oxy-fluorine-functionalized UHMWPE particles in polyurethane (PU) increases the tear resistance shown in Fig. 4. Compared with that of TPU/U-UHMWPE composite, the tear strength of TPU/FO-UHMWPE composite is improved from 38.7 kN m⁻¹ to 50.8 kN m⁻¹, about 31.2% improvement, which is higher than that of virgin TPU (48.3 kN m⁻¹). It is hypothesized that the increase in tear resistance is caused by the firmly bonded UHMWPE particles pinning and hindering tearing behavior.¹⁹

Conclusions

The truly compatibilized TPU/UHMWPE composite is achieved by using direct fluorination surface modification technology. Good compatibility and strong adhesion between UHMWPE and TPU are confirmed by morphology characterization and mechanical properties. A great number of polar groups are introduced on the surface of UHMWPE particle and mainly responsible for compatibility. The obtained composite has a higher tribological performance enhancement efficiency with significant decrease in wear loss and friction coefficient. Direct fluorination surface modification technology has been demonstrated as a viable surface treatment method, which produces UHMWPE particles as a wear-resistant additive for composite demonstrated in a polyurethane composite system.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 50973073).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47715k

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