Enhancement of Compatibility between Ultrahigh-Molecular-Weight Polyethylene Particles and Butadiene-Nitrile Rubber Matrix with Nanoscale Ceramic Particles and Characterization of Evolving Layer

Abstract

This article examines the modification of surface properties of ultrahigh-molecular-weight polyethylene (UHMWPE) with nanoscale ceramic particles to fabricate an improved composite with butadiene-nitrile rubber (BNR). Adhesion force data showed that ceramic zeolite particles on the surface of UHMWPE modulated the surface state of the polymer and increased its compatibility with BNR. Atomic force microscopy phase images showed that UHMWPE made up the microphase around the zeolite particles and formed the evolving layer with a complex interface. The complex interface resulted in improvements in the mechanical properties of the composite, especially its low-temperature resistance coefficients, thereby improving its performance in low-temperature applications.

Introduction

Many machines operating under low-temperature conditions such as those in Siberia have specific requirements of material properties regarding their operability and reliability. For example, polymeric materials, which are flexible at room temperature, exhibit cold hardening at low temperatures and become brittle. To improve the mechanical properties of polymeric materials, polymer–polymer composites have been intensively studied.1 Polymer–polymer composites can be formed by any combination of a continuous-phase (matrix) and discontinuous polymer particles.2 The properties of composites could be tailored by adjusting important variables such as the type of continuous-phase matrix, the type of polymer particles, amount of polymer particles added etc.3,4 However, when mixing two different polymers, their compatibility at the interface is crucial to achieving a highperformance composite.5 Surface modification of polymer particles changes their surface properties and enables the strong bonding of polymer systems with which they are normally incompatible. For example, reactive-gas processing creates hydroxyl, carboxylate, and halogen groups on ultrahigh-molecular-weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) with highly polar surfaces.6 These polar surfaces allow polymer particles to be well mixed in polar-matrix polymers such as polyurethane, epoxy, and nitrile rubber.2

Surface properties of polymer particles could be also modulated with the addition of ceramic particles at the interface between two polymers.7 To produce high-performance composites without losing the advantage of two mixed polymers, ceramic particles are chemically bonded to surface of the polymer particles and an evolved transition layer is produced between the two polymers.8,9 This evolved transition layer forms when the surface potentials of the two layers are almost similar.10 However, when fabricating the polymer–polymer composites with the addition of ceramic particles, it is a challenge to distribute the ceramic particles homogeneously at the interface without dispersing them inside the polymer matrix. Furthermore, it is also important to study and develop new techniques to characterize the surface potentials across the interface. Using atomic force microscopy (AFM) for structural studies of polymer-based composites makes it possible to estimate the compatibility of two blended polymers by analyzing their interface both quantitatively and qualitatively.11,12

In the study reported in this article, we used zeolite as a surface modifier for UHMWPE polymer particles and butadiene– nitrile rubber (BNR) polymer matrix composites to improve its mechanical properties at low temperature. The zeolite particles were well dispersed at the interface and the evolving layer between the two polymers was characterized using AFM by measuring the adhesion force.

 

Experimental

For the polymer–polymer composites, UHMWPE polymer particles and the BNR polymer matrix were adopted. UHMWPE powder was supplied by Ltd “Tomskneftekhim” (Tomsk, Russia). The mean particle size was 35 ± 5 μm, the density was 0.936 g/cm3, and the average molecular weight was 3.9 × 106 g/mol. BNR was purchased from JSC “KVART” (Kazan, Russia) and its composition is listed in Table 1. Natural clinoptilolite zeolite (Hongurin’s zeolite from Kempendyaysky field, Yakutia, Russia) was used as the surface modifier of UHMWPE. The particle size of the natural zeolite was in the range of 150-1,100 nm and its specific surface area was 11 × 103 m2/g.13 Prior to mixing with UHMWPE, the zeolite particles were activated for 2 min using a planetary mill (AGO-2, Russia); the particle size of the activated zeolite was in the range of 30-900 nm and the specific surface area was increased to 17 × 103 m2/g.14

Table 1.Composition of BNR polymer matrix

In order to modify the surface properties of UHMWPE or to locate the activated zeolite right at the surface of UHMWPE, activated zeolite particles were first mixed with UHMWPE for 2 min using a mixer reactor (Juchheim, Germany).15 UHMWPE polymer particles with zeolite particles were then blended with BNR using an extruder (Brabender, Germany) at 80 ℃ for 10 min. In this experiment, the amount of UHMWPE was 10 wt % of BNR and the amount of zeolite was 5 wt % of UHMWPE. The BNR–zeolite–UHMWPE composites were cured at 155 ℃ for 20 min in an oven. Composite samples were prepared for several tests. For comparison, pure BNR and BNR–UHMWPE composites without added zeolite were also prepared.

The samples for strength and elongation were made in the form of scapulae following the Russian State standard method. The mechanical properties of the composites − the ultimate strength and relative elongation at rupture of the composite samples − were determined following the Russian State standard method (GOST-270-75) using a mechanical tester (Instron, England) with a cross-head velocity of 500 mm/ min. Wear resistance, the volume change (ΔⅤ) resulting from wear, was determined by Russian State standard method (GOST 23509-79) and calculated with ΔⅤ = (m1−m2)/ρ. Here, m1 and m2 are the mass of a sample before and after test, respectively, and ρ is the density of the composite. The coefficient of frost resistance at −45 ℃ was measured following the Russian standard method (GOST-408-78) and calculated with K = L1/L2, where L1 is the sample lengthening at (23 ± 2)℃ and L2 is the sample lengthening at −45 ℃. Dynamic mechanical properties of polymers were investigated in a wide temperature interval from −100 to +150 ℃ with a frequency of 10 Hz.

The morphology and adhesion force of the composite materials were characterized using a multipurpose, scanningprobe, atomic force microscope (AFM; NT-MDT, Ntegra Prima, Russia).16-20 As the composite samples had flat surfaces, it was difficult to determine the interface from the topographical images, the contact mode, force-modulation mode, and phase-contrast mode were utilized to characterize the composite materials as follows.21 Using the closet optics, the polymer phase of BNR was roughly determined. From that point, the surface topography was characterized in the contact mode. To find the interface clearly, the force modulation mode was adopted. As the force modulation mode yielded a different image depending on the rigidity of polymer, a clear interface image was obtained. The adhesion force between the polymer and AFM tip were monitored at 20 points across the interface.22-24 Here, to ensure that capillarity would not influence the adhesion force, a standard cantilever (CSG 10, NSG 20) was held in distilled water.25 The distribution of zeolite in the evolving layer was imaged using the phasecontrast method.19

 

Results and Discussion

The polymer BNR is known to withstand temperatures ranging from −40 to 108 ℃ and is an ideal material for aeronautical applications.26 However, as the outdoor temperature during the winter in Siberia often drops below −40 ℃, the polymer’s frost resistance properties should be further improved. UHMWPE has been recognized as the most suitable matrix polymer for bearing material because of its excellent wear resistance, mechanical properties, and chemical resistance. The mechanical properties of UHMWPE could be improved by increasing the crystallinity percentage and hence its composite with ceramic powders has been widely studied.27 Our previous work showed that ceramic powders changed the surface charge of a polymer and modified its crystallization behavior.28 When fabricating a composite by mixing two different polymers such as BNR and UHMWPE and aiming to obtain their synergetic effect, two polymers should be bonded strongly, especially at the interface. However, two polymers have different surface states in general and special treatment should be applied to increase their compatibility. In this work, zeolite powder was used to modulate the surface state of a polymer. Here, zeolite should be located at the interface of the two polymers rather than inside each polymer. To attach the zeolite powder on the surface of UHMWPE, activated zeolite powder was first mixed with UHMWPE. The UHMWPE with zeolite was then mixed with the BNR matrix polymer to form the composite. In our previous work, the composite from (UHMWPE + zeolite) + BNR showed that Si element from zeolite was found mostly at the interface between UHMWPE and BNR rather than inside either polymer.29

Table 2 lists the mechanical properties of BNR, the BNR– UHMWPE composite, and the BNR–zeolite–UHMWPE composite. While BNR showed an extension coefficient of 238% and 3.7 MPa stress for 100% extension, the composite of BNR and 10 wt % UHMWPE became stiff and exhibited an extension coefficient of 179% and 7.9 MPa for 100% extension. In addition, the wear and frost coefficients were also improved. However, when zeolite was added to BNR–10 wt % UHMWPE, the composite resisted large deformation with an extension coefficient of 186% and became strong, showing 8.6 MPa stress for 100% extension. The wear resistance was also improved from 0.228 to 0.199 cm3. In addition, zeolite improved the coefficient of frost resistance at −45 ℃ from 0.44 to 0.51. The above data indicated that zeolite affected the bonding properties between UHMWPE and BNR.

Table 2.Mechanical properties of BNR–Zeolite–UHMWPE composites

Figure 1 showed the dynamic mechanical properties of polymers in a wide temperature interval from −100 to +150 ℃. The temperature corresponding to the maximum tanδ is the glass temperature of polymer. As adding the UHMWPE and zeolite into BNR, the glass temperature and tanδ decreased. The lower glass temperature and the larger coefficient of frost resistance in Table 2 indicated that zeolite could increase the compatibility of polymer.

To investigate the effect of zeolite on the bonding properties in more detail, the adhesion force between the polymer surface and the AFM probe tip was measured for the composites, and the results are shown in Figures 2 and 4. Here, the adhesion force could be changed depending on the surface state. As shown Figure 2(a), a clear image of the BNR–UHMWPE composite without zeolite was obtained for each phase. The adhesion force scanned across the interface showed the clean-cut interface between BNR and UHMWPE. The adhesive force on the rubber surface turned out to be much higher than the adhesive force on the surface of UHMWPE. The average measured value of the adhesive force in the elastomer matrix was 2.48 nN, while that on the surface of UHMWPE was 1.29 nN, which means the adhesive force on the elastomer matrix was 2.2 times higher.

Figure 1.Dynamic mechanical properties of polymers from −100 to +150 ℃ measued at a frequency of 10 Hz.

Figure 2.(a) AFM image and (b) adhesion force between the probe tip and the surface of the BNR–UHMWPE composite without zeolite.

An AFM image and the adhesion force of the BNR– UHMWPE composite with added compatibilizer are shown in Figure 3. It was determined that the addition of the molecular sieve, which was basically located at the boundary between the surface of UHMWPE particulates and BNR, led to considerable increase of the adhesion force from the UHMWPE surface. The decrease in the difference between the adhesive forces on the surfaces of UHMWPE and the BNR matrix (Fig 3(b).) caused intensification of the materials’ interaction and formed the bonding boundary layer.

As the contact-mode of the AFM provides information on the surface roughness, phase-contrast images of the composite, as shown in Figure 4, were used to analyze the effect of zeolite at the boundary. While the BNR–UHMWPE composite without zeolite showed a clean interface (Fig 4(a).), the BNR–UHMWPE composite with zeolite revealed a complex interface (Fig 4(b).). Lipatov adjusted the adhesion force proximity by adding ceramic particles into polymer composites and proposed the evolving layer between two polymers (Fig 5.).30 In our experiment, the layer formed had a complex structure and consisted of a microphase of particulates of the UHMWPE crystalline polymer. The complex interface indicated that zeolite modified the surface state of UHMWPE and the compatibility between two polymers was increased. The transition layer had an open structure, which gave it the advanced flexural properties of micromolecules that made the relaxation processes faster. It also led to the strong bonding and low-temperature resistance improvement, shown in Table 2.

Figure 3.(a) AFM image and (b) adhesion force between the probe tip and the surface of the BNR–UHMWPE composite with zeolite.

 

Conclusions

To improve the low-temperature mechanical properties of BNR–UHMWPE composites, the surface state of UHMWPE was modified with nanometer-sized ceramic powder prior to mixing with the BNR matrix. Zeolite powder was success-fully distributed at the boundary between the two polymers rather than inside each polymer and promoted the evolving layer. AFM images revealed that zeolite accelerated the formation of microphases at the evolving layer, resulting in a complex interface, which increased the bonding and improved the mechanical properties. The adhesion force measured with the AFM showed that zeolite lowered the adhesion force of UHMWPE and increased its compatibility with BNR.

Figure 4.Tapping-mode AFM images of BNR–UHMWPE composites: (a) without zeolite; (b) with zeolite.

Figure 5.Schematic of the interface in a polymer–ceramic– polymer composite.10