Effect Of Carbon Black Loading On Linear Low-Density Polyethylene Properties

The mechanical, thermal, electrical, reheology, and morphological properties of composites made from linear low-density polyethylene and carbon black (CB) content of 5%, 10%, and 20% by weight were investigated. The optimum mechanical and electrical properties performance was achieved with the adding of 5% carbon black. The drop in properties after adding more CB is due to agglomeration and poor dispersion of carbon particles in the polymer matrix. CB resulted in higher dynamic viscosity and storage modulus at low frequencies, although this phenomenon was reversed at high frequencies. CB resulted in more shear thinning of LLDPE at high shear rates. irradiation important the properties at 250 kGy. This paper discuss the impact of CB on the mechanical, electrical, thermal stability and rheology of LLDPE. The relation between the microstructure and dispersion of CB is explained.


INTRODUCTION
Linear low-density polyethylene is commonly utilized polymer in the industry because of its unique structure and excellent overall performance. It is produced by polymerizing ethylene with a slight amount of an α-olefin. LLDPE, however, is not photodegradation resistant and its relatively low mechanical properties and thermal stability can sometimes limit its application in industry. The development of particulate reinforced polymer composites is currently viewed as one of the highly promising methodologies in the area of next generation engineering products [1][2][3].
The addition of CB to LLDPE has been investigated by many authors due to its low cost and conductive properties [18][19][20] [18][19][20][21]. Pin Zhouet et al. [18] experiment the electrical properties of the CB filled LLDPE/EMA composites and higher percolation threshold due to the good interaction between EMA and CB. The stability of CB filled LLPE is important and studded by V.M Goldberg et al. [18] for thermo oxidative degree and artificial weathering by M.liu et al. [19] both showed that CB has a positive impact on the stability. Mechanical properties of CB filled LLDPE was studied by A.Ahmed et al. [21] and showed the electron irradiation important the properties at 250 kGy. This paper discuss the impact of CB on the mechanical, electrical, thermal stability and rheology of LLDPE. The relation between the microstructure and dispersion of CB is explained.

Materials
The materials utilized in this research were Linear Low-Density Polyethylene (LLDPE) provided as pellets by Qatar Petrochemical Company (QAPCO) with the following properties, a Melt Flow Index (MFI) of 1.0 g/10 min (190 ºC, 2.16 kg) and a density of 0.918 g/cm 3 . The average particle size of CB was about 30 nm and a density of 2.5 g/cm 3 . The Materials were completely dried before the combination that reduce the impact of moisture [22].

Composite sample preparation
Composite samples were arranged in a two-step process that included compounding and molding. First LLDPE and CB powders were tumbled mixed in the dry state in the right proportions before being compounded in a Laboratory size FARREL 2 Roll Mill. The mill temperature was set at 160 o C and samples were fed twice into the Mill for a total compounding time of about 12 min. The second step consisted of molding 2 mm thick composite sheets in a Compression Molding Press Machine. The molding cycle consisted of the following: 5 min at a temperature of 180 o C and zero pressure, followed by 5 min at T = 180 o C and P = 1400 psi, and finally about 5 min cooling at a rate of about 15 o C/min. Table 1 indicates the defined symbol for each method and the subsequent compositional ratio of each component.

Differential Scanning Calorimeter (DSC) Measurements
Differential scanning calorimeter assessment was carried out applying a Perkin Elmer Instrument Pyris 6 (DSC) with a sample weight of 5 to 8 mg. All samples were heated at a rate of 10 ºC/min to 170 ºC, next held for 5 min at 170 ºC to remove the earlier thermal history, and then cooled at a rate of 10 ºC/min to 30 ºC, then held for 5 min at 30 ºC and heated again at a rate of 10 ºC/min to 170 ºC under nitrogen atmosphere (20 ml/min) [22]. The cold crystallization temperature (Tc ºC), melting temperature (Tm ºC), and heat of fusion were revealed from the second heat scan. The crystallinity of samples (Xc % ) was determined employing the subsequent expression [23]: The crystallinity was determined from the ratio among the melting enthalpy (ΔH) amount from the second scan of the DSC assessment and the melting enthalpy of the 100% crystalline phase (ΔHf) from the literature [24]. For polyethylene a ΔHf value of 289 J/g was utilized [24], and W is the weight fraction of LLDPE in the composites. Half crystallization time (t1/2) is the time necessary to 50% of crystallization to be done, which represent the speed of crystallization rate [25], t1/2 can be obtained by the following equation [26]: Where, (Tc.onst) is the oncet temperature of crystallization process, (Tc) is the crystallization temperature, and (r) is the cooling rate (r).

Thermogravimetric Analysis (TGA) Measurements
Thermogravimetric (TG) and Differential Thermogravimetric (DTG) measurements were performed with Perkin Elmer Pyris Analyzer 6 [22]. The specimens of LLDPE and composites with weight of about 10 mg were heated from 30 ºC to 550 ºC at a heating rate of 10 º C/min [27].
Samples were arranged in a ceramic pan and the tests were done in a nitrogen atmosphere with N2 being provided at a flow rate of 20 ml/min. In this work, comprehensive and precise aspects identifying the thermal stability based on the temperature at which sample losses 50% of its weight (T50%) and the temperature of maximum rate of weight loss (Tpeak) (from DTG curves) are carried as a measure of thermal stability of the composites [28].

Tensile test Measurements
The tensile properties of the composites were calculated as per the standard test method ASTM D 638-02 [29]. The tensile strength of the composite samples were tested by Lloyd instrument's material analysis machine related to a remote microcomputer for data acquisition and evaluation. The load was assessed by a load cell of 50 kN capacity, although the displacement was evaluated applying an internal extensometer. The speed of examination was 100 mm/min [30]. Samples were examined under the similar requirement for each category of samples. The values described were the average values of three individual amounts [31].

Hardness Test Measurements
A micro -hardness analysis was organized with all samples by HRF -Rockwell Hardness F Scale by application of load 60 kg and the provided measurement values were of indentation hardness of a total of 10 indentation points on the sample surface [32]. The slandered was utilized to calculate the hardness of the compounds corresponding to ASTM D785-08.

Morphological Observations
The microstructures of composite samples were examined applying a scanning electron microscope (Nova Nano SEM 450) [33] . These samples were fractured in liquid nitrogen into pieces with a surface area of 2mm, each surface of which was coated with a thin layer of gold and then positioned into the SEM. The microstructure of the samples was examined at an accelerating voltage of 3kV. Through the microstructural examination, the interactions among CB and virgin polymer were found [34].

Rheology Characterization
The viscoelastic behavior of the neat polymer and CB reinforced samples was analyzed by melt rheology in an AERS Rheometer. Samples were first molded into 2 mm thick disks and then rheology measurements were presented to define the storage modulus (G'), loss modulus (G'') and dynamic viscosity (η*). Experiments were finalized over a frequency sweep range among 0.01 and 100 rad/s at a temperature of 210 o C [35] .

Electrical Conductivity Measurement
The electrical conductivity was assessed by Kithely 2400 source meter using the two-probe method. The electrical measurement was carried out at room temperature 25 o C. The samples were formulated as disks of 2 mm thickness and covered with silver sticker on both sides of the disc to ensure good contact of the sample surface with the two aluminum electrodes (1 mm diameter) which fixed on the opposite sides of the sample [36]. Figure 1 illustrates the measurement scheme. The specific conductivity σ was measured by means of a device measuring very high resistance. The procedure was set different values of current for measuring the voltage and calculating the resistance (R) [10]. Equation (3) is used to calculate the conductivity where A is the area of discs samples and L is length of disk. Figure 1: Illustration of the measurement for the electrical conductivity.

Differential Scanning Calorimeter (DSC) Measurements
The heating and cooling thermograms of the LLDPE/CB composites are shown in Figure 2. The DSC findings in terms of melting point (Tm, ºC), crystallization temperature (Tc, ºC) and the percentage crystallinity (Xc) are summarized in Table 2. The DSC measurements indicate that the melting point of composites is not considerably influenced by variations in the CB substance.
Furthermore, the Xc of composites is not affected by a CB addition of 5 wt% but then decreased with 10 and 20% of CB addition. A low shoulder might be observed in the low point of temperature side of the melting bend of the LLDPE indicating paracrystalline structure. A virgin LLDPE has a relatively wide molecular weight distribution and small series branching arrangement. The branches are rather found in the lower molecular weight chains; hence, LLDPE performs as is a combination of higher molecular weight linear and lower molecular weight branched particles [37]. This heterogeneous distribution of polymer chains is considered as a wide melting zone. The CB fragments are recognized to nucleate the polymer crystallization, growing its crystallinity and affecting the nature of the lamella in the crystallite [38]. As can be seen, from Figure 2 and Table 2 this nucleating function of the carbon particles in the crystallization of LLDPE is small. At a CB particle of extra than 5 wt% the reduction to crystallinity level can be justified by the existence of an extreme amount of CB contents that can obstruct the movement of the polymer chain parts and therefore, prevent crystal growing [28,20]. This can be explained by the fact that crystallization is composed of both nucleation and growth.
Even though nucleation is enhanced by the CB particles, growth is significantly reduced due to restrictions on mobility of the polymer chains in the occurrence of a high concentration of CB particles. The half crystallization time t1/2 was enhanced with the addendum of CB content and is generally in agreement with the crystallinity results discussed earlier. It can be concluded that the nucleation started and the crystallization procedure progressed in a relatively short period for the neat polymer [25]. The smaller time of t1/2 represent the faster crystallization rate [39] The slow crystallization rate of CB composites can be illustrated by the week CB-polymer interactions [40] and slow crystal growth in the presence of CB particles.

Thermogravimetric Analysis (TGA) Measurements
DTGA curves of pure LLDPE and LLDPE composites with various CB loadings are exhibited in Figure 3. It is viewed that the thermal decomposition of all mixtures happens below 500 ºC. The temperature at which a sample drops 50% of its weight (T50%), these were taken from TG curves (not shown), and temperature of max rate of weight loss (Tend) are carried as an amount of thermal stability of the composites and represented in Table 3. It is apparent from Table 3 that, adding CB improves the thermal stability of the composites. The noted growth in thermal stability is believed to be attributable to the limitation of mobility of segmental progress of LLDPE, because of the improved interface among the CB and the polymer matrix [40][41][42][43].   Figure 4 indicates the difference in the tensile strength with the addition of CB. The tensile strength increases significantly by adding CB at 5 wt% and then it decreases rapidly when CB content is more than 5 wt%. This indicates that the optimum carbon black loading in LLDPE is about 5 wt%. The degree of reinforcement of LLDPE composites increases with filler loading and the extent of polymer-filler interaction. The reduction of tensile strength beyond 5 wt% loading can be explained in terms of particle agglomeration at higher content of CB [44,45]. These agglomerated, hard particles, create concentrated stress points and therefore reduce the stress transfer from polymer chains to carbon particles [20]. So, at higher content, CB particles will exhibit a poor dispersion and it is evidently difficult to shape strong adhesion, weak surface energy, with polymer matrix. This will be shown later in the morphological study.

Tensile test Measurements
The influence of CB loading on the samples modulus is shown in Figure 5. The neat polymer, LLDPE, is of low tensile modulus 141 MPa, after adding CB the tensile modulus increases with increasing CB contents, which reveals enhanced stiffness caused by increasing filler substance. A change of distortion performance from ductile to more brittle occurred at higher fillings of CB.
As the amount of CB particles increases in the LLDPE matrix, some portions of the polymer are trapped inside the filler network, which increases the effective volume ratio of the solid particles in the composite. On the other hand, polymer-filler interface might cause in the absorption of the polymer chains on the filler surface and reduces its progress. Consequently, the polymer viscosity and modulus will be increased [46,47].
From the point of view of packaging technology, agricultural film applications and extension at break is a critical factor explaining the ductility of materials below tension. Figure

Hardness Test Measurements
Surface hardness is usually examined as one of the extremely vital elements associated to the wear resistance of materials. Figure 6 shows the hardness as a function of CB content in LLDPE.
It shows that the hardness of the composite is almost linearly enhanced with rising CB content.
This indicates that addition of CB particles is beneficial for improving surface hardness of LLDPE composites. CB particles acted as stress concentration points. It is expected; therefore, beyond 5 wt% of CB content, polymers chains have penetrated into the CB particles agglomerates [49].

Rheology Characterization
The rheology characterization of the various composite samples gives a very good indication of their behavior in an extrusion or a molding process. The rheology also provides further understanding of the effect of CB on the LLDPE composites. As we can see in Figure 8, the flow behavior of LLDPE is significantly affected by the addition of CB. This is true both at low and high frequencies. The storage modulus of the LLDPE melts (G') increases by about 50 times just by the addition of 5% CB, in the case of 20% CB it jumps by 130 times. This elastic behavior of the melt is reversed at high frequencies and the 20% CB composite shows the lowest G' at the highest frequency. This again supports the earlier observation regarding the agglomeration of the CB aggregates at the high loading of 20%. The 5% composite is the closest in behavior to the virgin polymer in terms of its melt processing due to the good dispersion of the CB particles in the composite matrix. Figure 9 shows the changes in dynamic viscosity as a function of increasing frequency for the different composites. Even though the viscosity of LLDPE is increased by the addition of CB at low shear, this is not the case at high shear rates where the neat polymer maintains a higher viscosity than the composites. CB actually increases the shear thinning of LLDPE as the frequency goes up as it is seen on the right-hand side of Figure 9. Shear thinning is a known phenomenon in polyethylene [51]. The 5% addition shows the best uniformity and distribution morphology when it comes to the melt processing of LLDPE composites, and the higher loadings of CB significantly change the processability of the polymer.

Electrical Conductivity Measurement
To investigate the effects of the filler and LLDPE content on the conductivity. Figure 10 indicates the reliance of electrical resistivity at room temperature, as a role of the comparative carbon black-amount. As observed, a greater composition of the filled CB particles findings in a lower electrical conductivity. The level of diffusion of CB in the 5CB matrix is higher than in the 10CB and 20CB. Here particular, the variation in crystallinity of the polymers performs a main part in clarifying the resistivity of the CB/thermoplastic polymer compounds explored [52]. The 20 CB have greater resistivity because of its smaller crystallinity. The rising in the filler loading more typically has slightly influence on the combined electrical resistivity. The conductivity of the mixture falls significantly with rising quantity of conducting filler in the polymer format, and the combination felt an insulator-to-conductor transition at a specific vital filler substance. This quick break down, indicating the accumulation of conducting atoms to shape systems, is welldefined as the percolation transition [13]. In an exciting research by Du and coworkers [53], they discovered that the level of alignment necessary to achieve highest electrical conductivity in the path of nanofiller alignment declines with rise in nanofiller amounts. Figure 10: Influence of CB content on conductivity.

CONCLUSION
In this research, composites produced by mixing varying contents of CB nanoparticles and LLDPE were investigated. The results show that: a-These findings can be very useful in the design and operation of LLDPE industrial compounding technology.
b-Thermal analysis of the composites indicated a decrease in the quantity of crystalline phase upon addition of CB. The decrease is significant for PE10CB and PE20CB samples, indicative of a lower quantity of the ordered phase with increasing filler content due to agglomeration and limited mobility in the melt dusring crystallization. The thermal stability is slightly improved for CB filled LLDPE composites.
c-5 CB is considered good conductor, increasing the fillers do not increase the conductivity.
d-The addition of CB to thermoplastics causes a large increase in viscosity, especially at low shear rate, where at sufficiently high additives loadings, the viscosity may become unbounded due to strong interaction between the CB particles. It was demonstrated that CB increases the shear thinning behavior of LLDPE.
e-The overall analysis of thermal, mechanical, and rheology results, leads us to recommend that the optimal loading of CB in the LLDPE composite should be around 5%.

ACKNOWLEDGMENTS
The authors would like to thank the support from the Center for Advanced Materials (CAM) at Qatar University and the kind support from Qatar Petrochemical Company (QAPCO). Also want to thank Dr. Noorulnissa Khanim for fruitful discussion.