A Study on Tribological Properties of Polypropylene Nanocomposites Reinforced with Pretreated HNTs

Liu Zan; Li Yanxiang; Ma Lu; Qin Dunzhong; Cheng Zhilin,

(1. School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002;2. Jiangsu Sinvochem Company, Yangzhou 225002;3. Zhejiang High Technology Research Institute, Yangzhou 225002)

A Study on Tribological Properties of Polypropylene Nanocomposites Reinforced with Pretreated HNTs

Liu Zan1; Li Yanxiang1; Ma Lu1; Qin Dunzhong2; Cheng Zhilin1,3

(1. School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002;2. Jiangsu Sinvochem Company, Yangzhou 225002;3. Zhejiang High Technology Research Institute, Yangzhou 225002)

The polypropylene (PP) nanocomposites filled with pretreated halloysite nanotubes (HNTs) were prepared by the melt-blending method. Before filling, the as-

HNTs powder was at first purified and then modified. The characterization tests showed that the purified HNTs had less impurity and more uniform pore size distribution and the surface hydrophobicity of the modified HNTs was obviously improved. The mechanical and tribological properties of the PP/HNTs nanocomposites were extensively investigated. The results showed that the tensile, bending and notched impact strength of the PP/HNTs nanocomposites was somewhat improved, but the wear resistance of the PP/HNTs nanocomposites was obviously enhanced.

PP; HNTs; nanocomposites; mechanical properties; tribological properties

1 Introduction

Polymer nanocomposites have attracted much attention due to their unique properties in scientific research and industrial applications. Inorganic nano- filler is characteristic of large specific surface area, good surface activity and low cost. The addition of these nano-materials can improve the mechanical strength and thermal resistance of polymer nanocomposites. Carbon black, graphite, silica and silicates are traditional nanofillers[1]. In recent years, researchers have shown interest in nano-materials with large aspect ratio and high strength[2]. Carbon nanotubes (CNTs)have excellent performance such as high aspect ratio,high strength, good corrosion resistance, good thermal conductivity and high electrical conductivity[3], so that the polymer/CNTs nanocomposites have more excellent physical and chemical properties than the conventional polymer-based nanocomposites. However, CNTs are expensive, thus limiting its practical application[4].

Halloysite nanotubes (HNTs) are a type of naturally occurring aluminosilicate (Al2(OH)4Si2O5· nH2O) with nanotubular structures. HNTs are widely deposited in soil in wet tropical and subtropical regions, weathered rocks,and soil generated from volcanic ashes, that are available in abundance in the nature. Due to the nanotubular shape of HNTs, they possess highly meso/macroscopic pore structure and large specific surface area. The length of HNTs ranges from 0.2 μm to 2 μm. The inner diameter and the outer diameter of the tubes range from 10 nm to 40 nm, and from 40 nm to 70 nm, respectively. In contrast to most clay, most of the aluminol (Al-OH)are located in the interior of the HNTs, while the outer portions of the HNTs are primary siloxanes and few silanols/aluminols that are exposed in the edges of the sheets. As an economically available raw material, HNTs as fillers have exhibited many promising applications in polymer due to their inherent hollow nanotube structure and different outside and inside chemistry[5-7]. Deng, et al.[8]prepared the polypropylene-based high halloysite nanotubes containing (33 phr) nanocomposites by using the twin-screw extruder. Due to the size effect, the surface hydrogen bonding effect and the surface electron effect of the HNTs, the phenomenon of agglomeration couldreadily occur, and the modification could be usually carried out before using. HNTs modification was usually achieved by hydrogen bonding and electron transfer mechanism[9].The mechanical properties of the modified HNTs filled into PP have also been reported[10-11]. In the work of Gandhi, et al.[12], the effect of carbon nanotubes (CNTs) on improving wear properties of polypropylene (PP) in dry sliding condition was investigated on a pin-on-disk wear tester.The results indicated that the filling of carbon nanotube(CNTs) could improve the wear properties of PP.

However, the study on the tribological properties of the fiber reinforced polypropylene (PP) composites was rarely involved in the past. In the work of Hufenbach, et al.[13], the experiments showed that the tribo-mechanical properties of PP could be significantly influenced by glass fiber with a suitable reinforcement. Kanny, et al.[14]studied the wear rates and quasi-static mechanical properties of polypropylene (PP) infused with layered organo-modified montmorillonite nanoclays. The testing results showed that PP infused with 2% of organo-modified montmorillonite presented an improved mechanical strength, a higher fracture toughness, and a lower wear rates.

Herein, the PP nanocomposites filled with HNTs were prepared by the melt blending method. Before using,the purification and modification of HNTs powders were adopted. A series of characterizations were used to determine the structure and properties of materials.Finally, their mechanical and tribological properties were measured and discussed.

2 Experimental

2.1 purification of HNTs

Initially, HNTs powder (purchased from the Yangzhou Xigema New Material Co., Ltd., China) was suspended in deionized water under vigorous stirring to obtain a 10%aqueous suspension. Then, this suspension was heated to 60oC for 6 h under stirring, followed by successive washing and centrifugation with deionized water at least three times. Finally, the purified product was dried in the oven at 60oC for 12 h, which was denoted as HNTs-p.

2.2 modification of HNTs

The 95% ethanol aqueous solution with a pH value of 5 was adjusted by acetic acid. 3% of silane coupling(KH550) or 2,2-(1,2-ethene diyldi-4,1-phenylene)bisbenzoxazole (EPB) (purchased from the Sinopharm Chemical Reagent Co., Ltd., China) relative to HNTs-p mass was dropwise added under vigorous stirring. Next, a certain amount of HNTs-p was put into and the resulting suspension was stirred for 10—20 min. Finally, most of the solvent in suspension was evaporated in a common oven at 70oC for 3 h and then was transferred into a vacuum oven operating at the same temperature to be subjected to evaporation until desiccation, and was denoted as HNTs-p/KH550, and HNTs-p/EPB, respectively.

2.3 Preparation of PP/HNTs nanocomposites

Firstly, PP (purchased from the Yan’an Re finery, China)and the above obtained HNTs (5 phr) were violently mixed by a high-speed blender. Then, they were blended in a twin-screw extruder at a melting temperature of 200oC(host speed： 25 Hz, and feeding speed： 5 Hz), and were denoted as PP/HNTs-p, PP/HNTs-p/KH550, and PP/HNTs-p/EPB, respectively.

2.4 Mechanical properties testing

The tensile and flexural properties of the spline were measured on a computer controlled electronic universal testing machine (type WDW-5, made by the Shanghai Hualong Test Instrument Factory). The tensile spline was 33 mm × 4.2 mm × 2 mm and the tensile rate was 10 mm/min. The bending spline had dimensions covering 58 mm in length, 10.4 mm in width and 1.16 mm in thickness made by injection molding. The span length was set to 50 mm and the bending rate was 2 mm/min.

Each specimen was tested for five times to acquire the mean value. The notched impact strength test was performed on a MZ-2056 Izod Impact Tester (made by the Jiangsu Pearl Testing Machinery Co., Ltd.) with an impact energy of 2.75 J and an impact speed of 3.5 m/s.A one-way analysis of variance (ANOVA) was performed to compare the mean values among different groups.Statistical significance was tested at p ＜ 0.05.

2.5 Tribological properties testing

The wear and friction testing for evaluation of PP/HNTs composites was carried out using an MMW-1 wear and friction tester (MMW-1, made by the Jinan Chenda Co.,Ltd., China) under dry condition. The test parameters and environmental conditions covered： a rate of 200 r/min,an applied load of 200 N, a test duration of 60 min, a temperature of 25±2oC, and a relative humidity of 50±10%.The friction coefficient was obtained by the computer automatically. The wear rate K (cm3/h) was calculated according to the following equations：

in which dV and dt are the volume loss and the sliding time, respectively; Δm is the mass loss in mg and ρ is the density of PP composites in g/cm3. The contact schematic diagram of wear tester is shown in Figure 1.

Figure 1 The contact schematic diagram of wear tester

2.6 Characterizations

The pore size distribution was measured by the Sorptomatic 1990 multi-purpose adsorption apparatus(Thermo Scientific, USA), and the pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH)method. The X-ray diffraction (XRD) employed the D8 Advance diffractometer (Cu target; background noise： ＜0.4 CPS; voltage： 40 kV; current： 40 mA;scanning speed： 0.1; scanning range： 5°—70°, Bruker-AXS, Germany,). The weight loss was analyzed by a PyrisTM1 TGA thermogravimetric analyzer (the nitrogen atmosphere was selected and the calcination range was in the range of 30—800oC with a temperature increase rate of 20oC/min, PerkinElmer Company, America).The contact angle was determined with a video optical contact angle meter (type OCA 40, Dataphysics Instruments GmbH, Germany). The SEM images were recorded by a S-4800 scanning electron microscope(Hitachi, Japan).

3 Results and Discussion

3.1 Pretreatment of HNTs

Figure 2 shows the XRD patterns of the as-received HNTs and HNTs-p samples. As for the HNTs-p, the typical characteristic peaks of HNTs at 2θ = 9° and 20° become more distinct and other peaks still remain after purification. This suggests that the purity of HNTs is improved after purification. More interestingly, the intensity of the peak at 2θ = 12° for HNTs-p increases and the peak at 2θ = 9° for the as-received HNTs powder disappears. This should be attributed to the loss of water molecules in the layered structure of HNTs during drying,leading to the change of the layer spacing from 1 nm to 0.7 nm[5].

Figure 2 XRD patterns of HNTs and purified HNTs

Figure 3 represents the BJH pore size distribution of HNTs and HNTs-p. It can be seen that the pore diameter of the as-received HNTs mainly centers at about 12 nm, while that of the purified HNTs-p is about 15 nm. It indicates that the uniformity of HNTs after purification has improved.

Figure 3 The pore size distribution of HNTs and HNTs-p

As shown in Figure 4-a, the mass loss of the KH550-modified HNTs is greater than that of HNTs-p after temperature ending, by more than about 1.87%. It may be ascribed to the amount of KH-550 modifier grafted into HNTs. Similarly, the mass loss of the EPB-modified HNTs has decreased by about 2.48% as compared to the HNTs-p (Figure 4-b), which is ascribed to the amount of EPB grafted into HNTs.

Figure 4 TG curves of samples: a-- HNTs-p/ HNTs-p-KH550, b-- HNTs-p/ HNTs-p-EPB

In contrast to other nanoclays, the multi-layer, tubular nanostructure of HNTs is connected with their relatively weak hydrophilic character. As shown in Figure 5, the contact angle of the unmodified HNTs is about 15°. After modification, that of the KH-550 and EPB modified HNTs increases to 30° and 25°, respectively. It indicates that the hydrophobicity of the modified HNTs is obviously improved.

Figure 6 shows the XRD patterns of PP, HNTs and PP nanocomposites. For neat PP, the lattice planes (110),(040) and (130) are assigned to α-phase. After filling HNTs, the diffraction peaks of the PP nanocomposites are consistent with those of PP and the feature peaks of HNTs do not appear, suggesting that HNTs can form a good dispersion in PP matrix[15-16].

Figure 7 displays the SEM images of the fractured section of PP and PP nanocomposites. Most hydroxyl groups (Al-OH) are located inside the tubes of HNTs,whereas only a few hydroxyl groups (Al-OH and Si-OH) were found on the edges of external surfaces of the tubes[5-7]. Therefore, HNTs should be readily dispersed into the non-polar polyolefin matrices.However, sometimes the weak hydrophilic character of HNTs is insufficient for proper interactions with strong and non-polar polymer chains. In that case, the functionalization processes of HNTs with different organic and inorganic compounds would come into play. As shown in Figure 7, it can be distinctly observed that the fractured sections of the HNTs-filled PP nanocomposites display the morphological features of HNTs, and furthermore the dispersion of the modified HNTs in PP polymer matrix shows more uniform and better orientation than those of the unmodified HNTs. In fact, as for the PP/HNTs-p/ KH550 and the PP/HNTs-p/EPB nanocomposites, the former has a fewer HNTs aggregates in the nanocomposites than those of the latter. Due to the electron transfer interactions between HNTs and EPB, EPB could be adsorbed onto the surface of HNTs and stuck with HNTs together[17]. The size of the EPB-bonded HNTs aggregates is significantly larger than that of the HNTs aggregates formed by the surface hydroxyl groups[12].

Figure 5 Contact angle of: a:HNTs-p, b:HNTs-p/KH550, and c:HNTs-p/EPB

HNTs have a limited possibility of creating strong tube-tube interactions and large area contacts between themselves. This result is originated from geometrical and chemical aspects, such as tubular morphologies featuring a relatively high aspect ratio and few hydroxyl groups located on external surfaces of HNTs. Therefore, the interactions caused by secondary bonds like H-bonding or the van der Waals force between the tubes are relatively weak.Consequently, HNTs can be uniformly dispersed into polymer matrix, which is the crucial factor in obtaining nanocomposites with better properties as compared to unfilled polymers[1].

Figure 6 XRD patterns of PP, HNTs, PP/HNTs, PP/HNTs-p,PP/HNTs-p/ KH550 and PP/HNTs-p/EPB nanocomposites

Figure 7 SEM photos of fractured section of PP and PP nanocomposites

3.2 Mechanical and frictional properties of the HNTs- filled PP nanocomposites

The mechanical properties of the PP/HNTs nanocomposites are shown in Figure 8. Compared with the PP/HNTs nanocomposite, the tensile, flexural and notched impact strength of the PP/HNTs-p nanocomposites has been slightly augmented. More importantly, the tensile, flexural and notched impact strength values of the modified HNTsfilled PP nanocomposites are higher than those of pure PP and the unmodified HNTs- filled PP nanocomposites, with these values increasing about 9.32%, 1.09% and 35.64%for the PP/HNTs-p/KH550 and about 13.37%, 7.15%and 22.55% for the PP/HNTs-p/EPB nanocomposites,respectively. This result is attributed to the fact that the EPB-bonded HNTs aggregates cause more PP molecular chains entangled in the HNTs, thereby increasing the interfacial bonding of PP and HNTs. The result embodies the improvement of the mechanical properties of polymer[5].The mechanical reinforced performance of the fillers in the composites depends on the effective load transfer from the matrix to the fillers, which can be achieved when there are strong interactions at the nanofiller-matrix interface and the nano fillers are dispersed uniformly in the matrix[18-19].There exist three main mechanisms for interactions between matrix and fillers, including the micromechanical interlocking, the chemical bonding, and the van der Waals force[20]. There are effective interactions between HNT walls and PP chains as a result of the existence of hydrogen bonding. The HNTs are held together in bundles by the van der Waals force. Thus, it is crucial to disperse nanotubes well in polymer matrix to acquire satisfactory mechanical performance of the composites.Figure 9 shows the friction properties of PP and the PP nanocomposites. More interestingly, all nanocomposites show high friction coefficient at the initial stage and afterwards drop to a relatively stable value in the later stage. The formation of the transfer film on the steel disc provides a soft polymeric material to replace the surface of the hard metallic material. The contact between the polymer surface and the transfer film further reduces the wear and friction coefficient in contrast with the metal to polymer contact[12]. As shown in Figure 9-a, the friction coefficients of the PP/HNTs and PP/HNTs-p nanocomposites are greater than that of PP in the whole sliding time. After modification, the PP/HNTs-p/EPB and PP/HNTs-p/KH550 nanocomposites exhibit a closer friction coefficient as compared to pure PP with an increasing sliding time. The wear rate of the nanocomposites is shown in Figure 9-b. Obviously,after filling HNTs, the antiwear performance of the nanocomposites is significantly improved. The wear rates of the PP/HNTs, PP/HNTs-p, PP/HNTs-p/KH550, PP/HNTs-p/EPB nanocomposites decrease by 19.4%, 23.6%,34.7%, and 27.8%, respectively.In order to understand the effect of HNTs on the friction and wear properties, Figure 10 displays the SEM photos of the worn surface of PP and the PP/HNTs nanocomposites. It can be seen that the high peaks and deep valleys appear on the worn surface of pure PP during sliding and the severe wear loss has occurred (Figure 10-a). The failure mode of pure PP is mainly adhesion wear, because there are obvious trails of large-scale materials being peeled off from the surface. This is why the un filled PP exhibited very poor wear resistance.The dominant wear mechanism associated with the un filled PP seemed to be dealumination. The observation is in agreement with some earlier researchers[1,5,10].

Figure 8 The mechanical properties of: a- PP, b- PP/HNTs, c-PP/HNTs-p, d- PP/HNTs-p/KH550, and e- PP/HNTs-p/EPB

Figure 9 Friction properties of: a-PP, b-PP/HNTs, c-PP/HNTs-p, d-PP/HNTs-p/ KH550, and e-PP/HNTs-p/EPB nanocomposites

On the other hand, the PP/HNTs nanocomposites are dominated by abrasive wear and adhesion wear in the dry sliding against the #45 carbon steel. The worn surfaces of the nanocomposites are mainly covered by a film containing HNTs aggregates, especially for the PP/HNTs-p, PP/HNTs-p/KH550 and PP/HNTs-p/EPB nanocomposites. In the case of the HNTs-reinforced polymer nanocomposites,nanotubes are held by the PP resin. Upon sliding against the steel counterpart, the load can be transferred to the fibers, the wear resistance of which is much higher. Thus,the wear mechanism of these kinds of composites is mainly manifested in two aspects： the fiber damage and the deterioration offiber-resin bonding[21-22].

Figure 10 SEM photos of worn surface of PP and PP nanocomposites

4 Conclusions

The pretreatment of HNTs was studied before using. A series of the PP/HNTs nanocomposites were prepared by the melt-blending method. The characterizations verified that the purity of HNTs could be improved after purification and the hydrophobicity of HNTs was obviously improved after modification, and the modified HNTs in PP polymer matrix showed more uniform and better orientation. The tensile, flexural and notched impact strength of the modified HNTs- filled PP nanocomposites was higher than those of pure PP and the unmodified HNTs- filled PP nanocomposites, with the strength increasing by about 9.32%, 1.09% and 35.64%for the PP/HNTs-p/KH550, and by about 13.37%, 7.15%and 22.55% for the PP/HNTs-p/EPB nanocomposite,respectively. Furthermore, the PP/HNTs-p/EPB and PP/HNTs-p/KH550 nanocomposites exhibited a lower friction coefficient as compared to pure PP with an increasing sliding time. The wear rates of the PP/HNTs,PP/HNTs-p, PP/HNTs-p/KH550, PP/HNTs-p/EPB nanocomposites decreased by 19.4%, 23.6%, 34.7% and 27.8%, respectively.

Acknowledgment: This work was supported by the Talent Introduction Fund of the Yangzhou University (2012), the Zhejiang High Technology Research Institute of Yangzhou University (2017), the Key Research Project-Industry Foresight and General Key Technology of Yangzhou (YZ2015020), the Innovative Talent Program of Green Yang Golden Phoenix(yzlyjfjh2015CX073), the Yangzhou Social Development Project (YZ2016072), the Jiangsu Province Six Talent Peaks Project (2014-XCL-013), the Jiangsu Province Science and Technology Support Project (BE2014613) and the Jiangsu Industrial-Academic-Research Prospective Joint Project (BY2016069-02). The authors also acknowledge the Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The data of this paper originated from the Test Center of Yangzhou University.

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date: 2017-02-27; Accepted date: 2017-03-16.

Prof. Cheng Zhilin, Telephone： +86-514-87975590-7202, E-mail： zlcheng224@126.com.