Mechanical properties of polydicyclopentadiene/graphite nanosheet composites prepared by reaction injection moulding

Materials

The powder of expandable graphite with an average diameter of 50 μm was supplied by Qingdao Haida Chemical Industry Co., Ltd. Vinyltrimethoxysilane (DB-171) was bought from Diamond New Material of Chemical Co., Ltd. Diethylaluminium chloride and tungsten hexachloride were obtained from Nanjing Shuanglian Chemical Industry Co., Ltd and Changsha Huajing Powder Material Science and Technology Co., Ltd respectively. 2,6-Ditert-butyl-4-methylphenoxy was supplied by Tianjin Ruijinte Chemical Regent Co., Ltd. Dicyclopentadiene (DCPD) was provided by Zhejiang Hangzhou Yangli Petrochemical Co., Ltd. Liquid paraffin oil was supplied by Yantai Shuangshuang Chemical Co., Ltd.

Preparation of graphite nanosheets

A sample of expandable graphite was held with a crucible in a 900–950°C furnace for 30–60 s to make EG. Then, the sample was cooled to room temperature under protection of nitrogen or argon. Expanded graphite particles were prepared through an ultrahigh speed mixer in alcohol solution, and the nanosheets were dried at 50°C in a vacuum oven for 24 h.

In order to decrease the agglomeration of EG particles and enhance the interfacial compatibility between fillers and matrix, they were treated by coupling agent to get treated EG nanosheets (TEG), and the schematic drawing is shown in Fig. 1. First, silane coupling agent (DB-171) was added into a reactor with anhydrous C₂H₅OH to form 2% alcohol solution. Then, the acidity of the solution was adjusted to pH 3–5 with HCl solution. Expanded graphite particles were put into a round bottom flask with some alcohol in a 70°C water bath, and DB-171 solution prepared previously was added into the reactor with thorough stirring under reflux for 6 h to form a homogeneous solution. Finally, after washing with alcohol several times, the excess alcohol evaporated, and the TEGs were dried at 80°C in a vacuum oven for 24 h.

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Figure 1

Schematic drawing of DB-171 grafted on surface of EG

Preparation of PDCPD/TEG composites

In this study, PDCPD/TEG composites were synthesised by ring opening metathesis polymerisation (Fig. 2) in a simulating RIM system (Fig. 3). First, the TEGs (0, 0·3, 0·5 and 1 wt-% respectively) and DCPD were loaded into a 150 mL conical flask equipped with a magnetic stir bar. Then, a tungsten complex catalyst [WCl₃(2,6-Ditert-butyl-4-methylphenoxy)₃], prepared according to Ref. 14, was added with a syringe from one inlet under nitrogen with strong stirring, while a diethylaluminium chloride activator was injected from the other inlet under nitrogen as well. The final molar ratio of DCPD, catalyst and activator was 1200∶1∶20. At last, the homogeneous solution was drawn into a vacuum mould by a vacuum pump as quickly as possible, and the mould was kept in a 40°C oven for 30 min. The formed blocks were used for testing the mechanical and tribological properties.

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Figure 2

Synthesis of PDCPD/TEG composites by ring opening metathesis polymerisation

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Figure 3

Schematic drawing of RIM system

Mechanical testing

Tensile and bending tests were performed according to Chinese standards GB/T 1040-1992 and GB/T 9341-2000. Notched izod impact tests were performed at room temperature using an izod impact tester (model XJU-22) with an 11 J hammer. Hardness measurements were according to Chinese standard GB 2411-80. All the data reported represented an average of the results on at least five specimens.

Friction and wear testing

Sliding wear tests were carried out on a block on ring friction and wear tester (MM-200). The composite specimens were rotated against a no. 45 steel ring and lasted for 60 min. The applied loads were 100 N for dry sliding and 500 N for liquid paraffin lubricated sliding. The tests were carried out at a linear velocity of 0·42 m s⁻¹ for both conditions. The liquid paraffin lubrication between the sliding surfaces was realised by continuously dropping of liquid paraffin onto the sliding surface at a rate of 10–15 drops/min. The width of the wear tracks was measured with a vernier calliper to an accuracy of 0·02 mm, and the specific wear volumes of the specimens were calculated by the following equation15 (1) In the formula, V is the loss in volume, R is the radius of the carbon steel ring, b is the wear trace width and B is the specimen width. The specific wear rate K₀ (mm³ N⁻¹ m⁻¹) was calculated from the volume loss using the following equation (2) where d is the sliding distance (m), and L is the load (N).

Fourier transform infrared (FTIR) characterisation

Fourier transform infrared spectra in the region of 4000–400 cm⁻¹ were obtained from a Nicolet infrared spectrometer (Nicolet-560). The samples were prepared as pellets using spectroscopic grade KBr.

Microstructure characterisation of wear tracks

Scanning electron microscopy (SEM) (JSM-5610LV) was used to observe the morphology of the worn surfaces of the composites. The voltage was 20 kV, and the surfaces of the samples were coated with a thin layer of gold before observation.

Analysis of dispersion of TEG

It is difficult to get well dispersed suspensions of pristine EG in DCPD monomer because of poor compatibility. As shown in Fig. 4a, when pristine DCPD/EG suspensions were removed from the magnetic stir and sonication bath, the EG remained separated from the DCPD and floated in the surface of the DCPD. To obtain uniform distribution of EG in the monomer, DB-171 was grafted on the surface of EG using the procedures described above (see section on ‘Experimental’), with the expectation of improving the compatibility between the EG and the DCPD monomer. Figure 4b shows the dispersion of EG modified with DB-171 (TEG), and the TEG was dispersed in DCPD easily once magnetic stir and sonication started and the dispersed system remained stable.

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Figure 4

Suspension of EG in DCPD

Analysis of FTIR spectra

Figure 5 shows the FTIR spectra of EG before and after treatment with DB-171. In the spectrum of pristine EG (Fig. 5, A), the broad peak at ∼3440 cm⁻¹ is assigned to the O–H stretches of carboxyl and hydroxyl groups of EG. In comparison with the spectrum of pristine EG, the spectrum of TEG samples (Fig. 5, B) has absorption peaks at 2920 and 2820 cm⁻¹, which is attributed to the resonance of C–H bond, while the peak at 1050 cm⁻¹ corresponds to Si–O–Si stretching vibration, reflecting the success of the surface modification of the original EG.

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Figure 5

Fourier transform infrared spectrum of EG and EG modified with DB-171

Analysis of mechanical properties

Table 1 summarises the variation of mechanical properties of PDCPD/TEG composites. The increase in tensile strength for PDCPD/TEG composites was apparent with the addition of TEG, and tensile strength increased by 22 and 26% for PDCPD/0·3 wt-%TEG and PDCPD/0·5 wt-%TEG composites compared with that of neat PDCPD respectively. Bending strength increased by 28% compared with that of neat PDCPD when the TEG content is 0·5 wt-%. With further addition of TEG, both tensile and bending strengths for PDCPD/1 -wt%TEG slightly decreased. Impact strength reached a maximum when the TEG content was 0·5 wt-%, and 46% improvement was observed. With further addition of TEG, the value of impact strength reduced slightly. Shore hardness of PDCPD/TEG composites decreased monotonically with the increase in TEG loadings in the range tested. According to these results, TEG of low content (<1 wt-%) has an effect on reinforcing the PDCPD matrix.

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Table 1

Mechanical properties of PDCPD/TEG composites

TEG/wt-%	Tensile strength/MPa	Bending strength/MPa	Impact strength/J m−1	Shore hardness
0	27·2±0·5	42·2±2·5	41·8±5·5	60·2±1·5
0·3	33·2±1·2	49·5±3·6	51·1±6·2	65·3±1·2
0·5	34·4±1·5	58·5±2·7	61·2±5·8	72·6±1·0
1	28·9±1·3	56·6±2·5	60·2±4·5	76·5±1·0

Analysis of frictional and wear properties

Variations of friction coefficients and wear rates in dry sliding condition are given in Fig. 6. It is obvious that the unfilled PDCPD had the lowest friction coefficient compared with the filled PDCPD in dry sliding condition. Moreover, the friction coefficients of the composites with different TEG contents were much close to each other within the range of our experiments. The wear rates of the composites were obviously lower than that of the unfilled PDCPD. After inclusion of 0·3 wt-%TEG in the PDCPD composites, the wear rate decreased by 71·2% from 29·8×10^{−5 − −} to 8·59×10⁻⁵ mm³ N⁻¹ m⁻¹. When the TEG content reached 0·5 wt-%, the wear rate slightly increased, and the wear rate for PDCPD/1 wt-%TEG composite almost kept constant. Therefore, TEG played an important role in enhancing the wear resistance of PDCPD in dry sliding, which accorded with the improvement of wear resistance of polytetrafluoroethylene (PTFE)/EG composites.16

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Figure 6

Variations of friction coefficients and wear rates of PDCPD composites as function of TEG loadings under dry sliding condition

Under oil lubricated sliding condition, the entire friction coefficients were lower over an order of magnitude in comparison with those in dry sliding (Figs. 6 and 7), and this is consistent with the results in Ref. 17. The friction coefficient of unfilled PDCPD was only 0·033, and the friction coefficients of all the composites almost keep constant (∼0·04). Overall, liquid paraffin played a significant function of lubrication on the rubbing interface, and the friction coefficients kept a lower level for both unfilled PDCPD and the composites. The wear rates of PDCPD composites in lubricated sliding are listed in Fig. 7 as well. The wear rates of the composites reduced obviously after the inclusion of TEG in the PDCPD composites. The wear rate of PDCPD/0·5 wt-%TEG composite reduced by 75·5% compared with that of unfilled PDCPD, and the wear rate was only 1·15×10⁻⁶ mm³ N⁻¹ m⁻¹. These results indicated the better wear resistance of the composites in the oil lubricated condition.

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Figure 7

Variations of friction coefficients and wear rates of PDCPD composites as function of TEG loadings under lubricated condition of liquid paraffin

In summary, the friction coefficients of unfilled PDCPD were lower than that of PDCPD/TEG composites. The reasons may be as follows. It is commonly believed that frictional heat can exert a dramatic influence on the friction coefficient of neat PDCPD. During continuous friction, the temperature of the interfacial region of two counterparts increased quickly, but the heat was hardly dissipated because of the low thermal conductivity of unfilled PDCPD. The molecular chains of PDCPD became soft and readily contributed to self-lubrication to a degree. However, as for the composites, the TEG filler had the advantages of better thermal conductivity and higher hardness than neat polymer. Kalaitzidou et al.18 have demonstrated the use of exfoliated graphite nanosheets to enhance the thermal and mechanical properties of polymeric resins. Therefore, the thermal conductivities of PDCPD composites containing TEG could increase, and the frictional heat between the interfaces of two counterparts was easily dissipated. Polydicyclopentadiene composites had relatively worse self-lubrication, so the friction coefficients of PDCPD composites increased slightly. As for lubricated sliding of liquid paraffin, the friction coefficient could drop quickly with a layer of oil film adhering to the friction pairs. The main trend of friction coefficients for unfilled PDCPD and PDCPD/TEG composites under paraffin lubricated sliding was almost consistent with the dry sliding condition similarly due to their different effects of frictional heat.

The addition of TEG has enhanced the mechanical properties of PDCPD (according to Table 1) primarily due to the higher stiffness of graphite.19 Therefore, the composites were not easily broken by shear force or had better load carrying capacity, which led to a good antiwear property. The PDCPD composite with 1 wt-%TEG had slightly higher wear rate than the PDCPD composite with 0·5 wt-%TEG. The reason might be due to the higher loadings of TEG causing its aggregates and destroying the uniform structure of the composite.

Analysis of wear mechanisms

Figure 8 shows the SEM images of worn surfaces of the PDCPD composites in dry sliding. The worn surface of the unfilled PDCPD was characterised by severe peeling off and plastic deformation (Fig. 8a and b), and these indicated a severe adhesion wear mechanism. It is speculated that the increased frictional heat deteriorated the mechanical properties of unfilled PDCPD and softened the contact layer, which corresponded to its low wear resistance. For the worn surface of PDCPD composite with 0·3 wt-%TEG (Fig. 8c and d), there were large amounts of rifts on the surface. Graphite has high stiffness and good thermal conductivity,17, 18 which resulted in a stiff surface of PDCPD composite as a sliding counterpart. In addition, the rifts appeared due to the slight surface fatigue caused by continuous sliding.20 The reinforcing effect of TEG on PDCPD contributed to the improvement of its wear resistance, and the morphologies of the worn surface accounted for its wear mechanism.

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Figure 8

Images (SEM) of worn surfaces of PDCPD composites under dry sliding with load of 100 N

When more TEG was loaded, ploughing effect played a dominant role as the TEG content was 0·5 wt-% (Fig. 8e and f). The TEG would be pulled out during sliding, and the quantity of TEG on the sliding surface increased accordingly. Thus, hard abrasive particles readily appeared, which could produce a ploughing effect in the worn surface. The wear resistance of PDCPD composites containing 0·5 and 1 wt-%TEG slightly decreased compared with the composite containing 0·3 wt-%TEG according to Fig. 6. While the TEG content reached 1 wt-%, the peeling off and ploughing became predominant according to the worn surface (Fig. 8g and h). It is conjectured that with large loadings of TEG in the PDCPD matrix, agglomeration of TEG might appear in PDCPD composites. The stress concentration due to the agglomeration of TEG resulted in the brittle surface of the composite. Therefore, the wear mechanism of PDCPD/1 wt-%TEG composite was characterised by fatigue wear.

Figure 9 presents the SEM images of worn surfaces of PDCPD composites in the lubricated condition. It can be seen that the worn surfaces of specimens were smoother than that in the dry sliding condition, and all the composites are subjected to mild damage in contrast with the worn specimens in dry sliding (Fig. 9). According to the SEM images of unfilled PDCPD specimen (Fig. 9a and b), lots of scratches were observed on the surface. The leading wear mechanisms were ploughing effect and slight adhesion wear. However, in dry sliding conditions, the SEM image in Fig. 8b presented serious peeling off, which indicated more abrasion. According to Fig. 9c, it is observed that the ploughing effect dominated for the PDCPD specimen with 0·3 wt-%TEG. However, slight adhesion wear took a certain degree for the appearance with more peeling (Fig. 9d). The SEM images (Fig. 9e and f) of PDCPD/0·5 wt-%TEG composite showed mild crack, less debris and small patches, which indicated the extent of surface damage of the specimens mitigated. Therefore, the reinforcing effect of TEG on the composites worked, and slight fatigue wear played a dominant role in the process of sliding of the PDCPD/0·5 wt-%TEG composite. However, with the increase in TEG content to 1wt-%, the surface morphology of specimens was different from others. It can be seen obviously that some big cracks appear (Fig. 9g and h). It is speculated that high loadings of TEG in the PDCPD matrix agglomerated, and stress concentration appeared. Therefore, the surface morphology might be ascribed to the apparent surface fatigue of the specimen, and the wear form became fatigue wear similar to that in the dry sliding condition.

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Figure 9

Images (SEM) of worn surfaces of PDCPD composites under oil lubricated sliding condition with load of 500 N