Electrical and rheological properties of carbon black and carbon fiber filled low-density polyethylene/ethylene vinyl acetate composites

2.1 Materials

The matrix polymer used in this study were LDPE and EVA. LDPE (LA-0710, Qatar Petroleum Chemical Co., Ltd, Doha, Qatar) was a commercial grade polymer with a melt flow index of 7 g/10 min and density of 0.918 g/cm³ and Tm of 108°C. EVA (770, E. I. du Pont de Nemours and Company, Wilmington, DE, USA) was also a commercial copolymer with a melt flow index value of 0.8 g/10 min and density of 0.93 g/cm³, and Tm of 96°C and a vinyl acetate content of 9.5%. The CB was acetylene black [average size, 24 nm; di-b-butyl-phthalate value, 260 cm³/100 g; Brunauer-Emmett-Teller value. 63 m²/g, Tianjin LiHuaJin Chemical Industry corporation; Tianjing, China]. The CF was pan-based carbon fiber (average length, 5 mm; average diameter, 7 um; Nanjing WeiDa Composite Material Corporation; Nanjing, China).

2.2 Preparation of conducting composites

All the materials were air-oven-dried before use. Composites were prepared by melt processing in internal mixer (ShangHai KCCK, type XSS-300) at temperature of 180°C and rotation speed of 60 r/min for 30 min. CB and CF contents (in wt%) are listed in Tables 1 and 2.

2.3 Electrical properties

Some 15 mm×15 mm×2 mm shape specimens were compression molded under 10 Mpa force at 180°C for 5 min using platen press and cooled. The resistance was tested with the digital multimeter (VC9808+, Shenzhen Victory Electronic Technology Corporation; Shenzhen, China) when it was lower than 10⁸ and the resistivity was calculated by the following equation:

ρ=R×SL

where R is the resistance of the sample (Ω); S is the cross-sectional area of the sample which is vertical to current direction (cm²); and L is the length of the sample which is parallel to current direction (cm). When the resistance exceeds 10⁸, the high-resistance meter (ZC36, Shanghai Precision Scientific Instrument Corporation; Shanghai, China) was used, and the resistivity was calculated by the following equation:

ρ=R×Sd

where R is the resistance of the sample (Ω); S is the effective area of measuring electrode (19.2 cm²); and d is the thickness of the sample (cm).

2.4 Morphology

The microstructure of composites was analyzed using scanning electron microscope (SEM) (JSM-6510, JEOL). All samples are immersed in liquid nitrogen for 15 min and fractured and then gold-sputtered before observation.

2.5 Thermal properties

Different scanning calorimetry (DSC) measurement was performed by AQ20 (USA TA Instruments). The sample was first heated to 180°C at a rate of 10°C/min and held for 5 min at 180°C for elimination of the thermal history; then cooled down to 40°C at a rate of 10°C/min. Finally it was heated up to 180°C at a rate of 10°C/min for collection of the data.

2.6 Rheological measurements

Dynamic rheological behavior was measured by rotational rheometer (MCR302, Anton Paar GmbH) in oscillatory shear at 1% strain in the parallel plate with 25 mm. The sweep frequency is from 0.1 to 100 rad/s, and the temperature is 180°C.

3.1 Conductivity

Figure 1A gives the dependence of resistivity on CB content for different proportions of LDPE and EVA from 100/0 to 50/50. When the content of CB exceeds 9% the difference of resistivity between composites is subtle. So Figure 2A gives the dependence of resistivity on CB content between 12% and 27% for different proportion of LDPE and EVA from 100/0 to 50/50. It can be seen that with increasing EVA, the resistivity of composites decreased first and reached the lowest point at a proportion of LDPE/EVA being 70/30. Then along with the increase in EVA, the resistivity began to rise.

Figure 1:

The composites resistivity with different EVA contents: (A) Composites resistivity vs. CB content; (B) Composites resistivity vs. CB content from 12% to 27%.

Figure 2:

SEM micrographs of the LDPE/EVA/CB containing 15 wt% CB with different LDPE/EVA mass ratios: (A) LDPE/EVA=100/0; (B) LDPE/EVA=80/20; (C) LDPE/EVA=60/40.

There are three causes attributed to this phenomenon. First the addition of EVA changed the CB particles distribution in matrix, which led to some CB particles transfer from LDPE phase to interface of LDPE phase and EVA phase. Because if the polarity of matrix is not having big difference, CB prefers to locate in the interface or one phase near the interface [17]. It is beneficial to uniformly disperse CB. Figure 2 is the SEM picture of LDPE/EVA/CB composites. Contrast (A) and (B), it could be found that CB particles transferred from LDPE phase to the interface of LDPE phase and EVA phase.

Second the increase in EVA content changes the microstructure of the composites. In Figure 2B, LDPE and EVA form net-like structure when the LDPE:EVA ratio is 70:30. In Figure 2C when the ratio increases to 60:40, LDPE and EVA form co-continuous phase, contrast to net-like structure the co-continuous phase structure is not conductive to CB disperse.

Third some CB particles transfer to the interface, but rest of the CB particles still are present in LDPE amorphous region. From Figure 3, the DSC curves of LDPE/EVA/CB composites with the same CB content but different EVA content, it can be seen that the melting enthalpy of LDPE experienced great decline with the increasing percentage of EVA. So EVA inhibits the crystallization behavior of LDPE, this result is similar to Xu et al.’s [18]; it means the amorphous phase of LDPE is increased, which is equal to dilute CB concentration. The resistivity of composites will increase. The synergistic effect of these three factors causes this phenomenon.

Figure 3:

The DSC curves of LDPE/EVA/CB composites.

Figure 4 shows the dependence of resistivity on CF content for different proportions of LDPE and EVA from 100/0 to 50/50. Along with the rise of EVA content, the resistivity of composites drop monotonously. Before explaining this phenomenon, it is shown in Figure 5 that the SEM pictures of composites, the EVA wrap in the surface of CF improve the compatibility of CF and matrix. So EVA works as a compatilizer, and the composites’ resistivity decreases with an increase in EVA content.

Figure 4:

Resistivity of composites with different ratios of LDPE to EVA vs. CF content.

Figure 5:

SEM micrographs of the LDPE/EVA/CF containing 15 wt% CF with different LDPE/EVA mass ratios: (A) LDPE/EVA=100/0; (B) LDPE/EVA=80/20; (C) LDPE/EVA.

Figure 6 is the contrast of LDPE/EVA/CB and LDPE/EVA/CF composites in electrical properties. When the filler content exceeds the percolation threshold, the resistivity of the samples with CF is much lower than the samples with CB. This is because the chopped CF has large length-diameter ratio, which is beneficial in forming conductive network.

Figure 6:

Contrast of LDPE/EVA/CB and LDPE/EVA/CF composites resistivity with different ratios of LDPE to EVA.

(A) LDPE/EVA 100/0, (B) LDPE/EVA 80/20, (C) LDPE/EVA 60/40, (D)LDPE/EVA 50/50.

3.2 Rheological properties

Figure 7 gives the dynamic rheological curves of LDPE/CB and LDPE/CF composites. In Figure 7A, the curve of complex viscosity η^* vs. sweep frequency ω, all composites’ η^* increased along with the increase in filled concentration. It is because the friction between matrix and filler increased and the chain movement of matrix is restricted. For LDPE/CF composite at low frequency region the complex viscosity was not sensitive to sweep frequency, but along with the increase in sweep frequency the composites exhibited a very strong shear thinning effect, the result is concur with that of Zhang et al.’s [19]. For LDPE/CB composites when the CB reaches 12 wt%, the composites have shear thinning effect in low frequency region; it illustrates dense filler network has formed inside the materials. Figure 6B,C exhibits the function of storage modulus and loss modulus at 180°C. At low frequency the molecular chains of matrices are relaxed and exhibit polymer-like terminal behavior, but at CB loading higher than 12 wt%, the terminal behavior disappears, and the storage modulus and loss modulus of composites are not sensitive with sweep frequency. This phenomenon demonstrates the existence of solid-like structure. But CF filled composites do not have this change, so the CF particle has not effectively restrained the polymer relaxation like CB particle. For LDPE/CF composites, the increase in CF contents improves the complex viscosity of composites because the CF formed net-like structure in matrix. Figure 8 is the residue of composites with different CF contents, all composites experienced 600°C heated for 30 min in muffle furnace. From Figure 8 the increase in CF contents enhanced the density of net-like structure. So the complex viscosity of composites increased with the rise in CF contents. In low frequency region the loss modulus is much higher than storage modulus, the composites exhibit liquid-like property; in high frequency region the value of storage modulus and loss modulus is similar so the composites exhibit semisolid property. In low frequency region when CF contents exceed 6 wt%, the rising rates of complex viscosity, storage modulus, loss modulus are obviously higher, so 6 wt% is also the rheological percolation, and it also shows the change of microstructure for LDPE/CF composites.

Figure 7:

Dynamic rheological curves of LDPE/CB and LDPE/CF composite with different filler content.

(A) Complex viscosity vs. sweep frequency; (B) Storage modulus vs. sweep frequency; (C) Loss modulus vs. sweep frequency.

Figure 8:

Residues of LDPE/EVA composites

(A) LDPE+3% CF; (B) LDPE+6% CF; (C) LDPE+12% CF.

Figure 9A shows the complex viscosity of LDPE/EVA/CB composites increased with the increase of EVA loading. It is caused by the change of microstructure of composites and the change of CB distribution for the added EVA phase. From previous SEM picture (Figure 2), the microstructure of composites transformed from network structure to co-continuous phase structure, and some CB aggregates transferred from interface of LDPE and EVA phase to EVA phase, these changes also caused the complex viscosity of composite to increase. The storage modulus and loss modulus of composites also increased with the increase of EVA concentration. It also proves the previous view that the microstructure of composites changed. The complex viscosity, storage modulus, loss modulus of composites slightly increased in the low frequency region with the increase of EVA phase, but in high frequency region the gap of storage modulus and loss modulus with different EVA concentration composites gradually disappears, so EVA does not affect the CF distribution in matrix. The increase of complex viscosity is because EVA can work as compatilizer, the surface of CF has little oxygenous groups, it has good compatibility with EVA resin which has carboxyl. So the compatibility between CF and matrix improved, the complex viscosity also increased.

Figure 9:

Dynamic rheological curves of LDPE/EVA/CB and LDPE/EVA/CF composites.

(A) Complex viscosity vs. angular frequency; (B) Storage modulus vs. angular frequency; (C) Loss modulus vs. angular frequency.

In order to study the rheological properties of composites more deeply, we chose some samples to take static rheological behavior test and get the curves of apparent viscosity vs. shear rate as shown in Figure 10. Because of the limitations of rotational rheometer, when the shear rate is high the value is not accurate, so we chose the value of log γ range from −1 to 1.5, use origin 8.0 to linear fitting calculate the non-Newtonian index (n) and zero shear viscosity (η₀), the value is listed in Table 3.

Figure 10:

Apparent viscosity as a function of shear rate for composites filled with CB and CF.

From Table 3, it could be found that the n values of both CF filled and CB filled composites are lower than one, exhibit shear thinning and pseudoplastic fluid behavior. The non-Newtonian index of CB filled composites is lower than CF filled composites with the same ingredients; it mean CB filled composites deviated much from Newton fluid. So the CB particles restrain the matrix movement more deeply than CF, and the conductive network of CB filled composites are also large than CF filled composites. This conclusion is correct with the previous one. Besides though contrast the value of zero shear viscosity we can get the similar result that the viscosity of CB filled system is higher than CF filled, and the EVA will increase the viscosity of both types of composites.