Introduction

Recently, preparation of biocomposites has gained considerable attention to exchange petrochemical materials due to the ecological awareness and the global request for green technology [1, 2]. Natural rubber (NR) is one of the most important renewable polymers according to its desired properties including high mechanical properties, high elasticity, flexibility and resilience. Natural rubber is used in many industrial application including gloves, rubber bands, tires and adhesives [3,4,5].

The natural rubber mechanical properties can be enhanced in different way such as using crosslinkers or addition of reinforcement fillers. Silica [6] and carbon black [7] are the most known reinforcing agents in the rubber manufacturing but the using of these fillers led to high energy consumption and ecological contamination. Consequently, developing eco-friendly fillers is a perfect option to overcome the drawbacks of traditional fillers. Natural fillers extracted from renewable sources can be utilized according to their desired characteristics as low cost, degradability and high mechanical properties [8,9,10,11].

Cellulose nanoparticles are the most common natural polymers and can be obtained from many natural sources including straws, cotton, corncob, peels, bagasse and etc. Many academic researches have verified that nanocellulose can be as reinforcing filler in numerous rubbers, as polyisoprene (natural rubber), nitrile butadiene rubber etc., [4, 12]. Micro and nanocrystalline cellulose are recommended by many researchers to be as green reinforcing fillers according to their remarkable characteristics as renewability, degradability and low cost [13, 14]. Moreover, NCC can be obtained through acid hydrolysis where the amorphous parts of the cellulose such as hemicellulose or lignin are removed [15, 16]. After acid hydrolysis process is completed the resultant NCC has a considerable amount of hydroxyl function groups on its surface which led to more agglomerate and consequently affect on mechanical properties of the composites. Also, several reports introduce methods to improve the dispersion of the nanocellulose in the polymeric matrix including surface modification or chemical grafting of NCC [17,18,19,20,21,22,23,24].

In this manuscript, nanocrystalline cellulose and modified nanocrystalline cellulose were successfully prepared. The obtained NCC and m-NCC were characterized by FTIR spectra and x ray diffraction. The prepared NCC and m-NCC were used as reinforcing fillers in natural rubber biocomposites with different content. The rheometric characteristics, mechanical, thermal stability and electrical properties of prepared biocomposites were also evaluated.

Material and methods

Material

Microcrystalline cellulose was purchased from SDFCL Company. Hydrochloric acid (HCl) 34 wt.% was purchased from El-Nasser pharmaceutical chemical company (Egypt). Hexadecyl-trimethylammonium bromide (CTAB) was provided by Acros organics Company. Smoked sheets natural rubber (NR) provided by Transport and Engineering Company, Egypt with specific gravity 0.913 ± 0.005. All the rubber ingredients zinc oxide, stearic acid, dioctyl phthalate (DOP) and tetramethylthiuram disulfide (TMTD) were of commercial grades. N-cyclohexyl-2-benzothiazole sulfonamide (CBS), 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) and elemental sulfur were gained from Aldrich, Germany.

Preparation of nanocrystalline cellulose

Microcrystalline cellulose was mixed with 5 M of HCl in two neck flask. The mixture was then hydrolyzed at 100 °C for 1 h with continuous stirring. Until time is over, the mixture was placed in ice bath to quench the reaction. Then, washing the resultant mixture with distilled water and centrifuging the mixture for half hour at 3000 rpm. Washing and centrifuging process were repeated for five times. Then the washing process stops when the pH value reaches its neutralization equal 7. Then, prepared NCC was dried and stored at room temperature before analysis and using.

Modification of nanocrystalline cellulose

Fill three-necked flask with equal weight of prepared NCC as well as cationic surfactant (CTAB) at 60 °C for 3 h. After the reaction, the excess CTAB was removed by centrifugation at 4000 rpm/min for 3 min to obtain m-NCC. Modified NCC was dried and before using and analysis.

Preparation of NR/cellulose biocomposites

Nanocrystalline cellulose and modified nanocrystalline cellulose were incorporated into natural rubber as reinforcing filler using a laboratory two-roll mill. Rubber formulations are presented in Table 1. The composites were cured at 140 ± 1 °C using a moving die rheometer (MDR2000).

Table 1 Formulation of natural rubber biocomposites
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Characterizations

Fourier Transforms Infrared (FTIR) spectroscopy

FT-IR spectra of modified and non-modified cellulose were measured at wavenumber range 4000–400 cm−1 with JASCO FTIR-6100E (Japan).

X-ray Diffraction (XRD)

The diffraction angles 2θ for micro, nano, and modified nanocrystalline cellulose were measured using Philip’s X-ray diffractometer PW1390 with Ni-filtered CuKα radiation (wave length of 1.5404 A°) at generator voltage and tube current of 40 KV and 30 A°. Also, the crystallinity was calculated by the following equation:

$${\text{Crystallinity}}\, = \,\frac{{{\text{Area}}\,{\text{of}}\,{\text{crystalline}}\,{\text{peaks}}}}{{{\text{Area}}\,{\text{of}}\,{\text{all}}\,{\text{peaks}}}} \times 100$$
(1)

Scan Electron Microscope (SEM)

Morphology of NR, NR/NCC and NR/m-NCC were studied using Quanta scanning electron microscope (model FEG250, FEI, Hillsboro, Oregon, USA).

Mechanical and thermal oxidative ageing properties

The tensile test of the NR biocomposites was performed using Zwick tensile testing machine (model Z010, Germany), in accordance with ASTM D412. Also, thermal oxidative ageing was performed at 70 ± 1 °C in an oven for 1 week according to ASTM: D 572-04, 2010.

Thermogravimetric Analysis (TGA)

Thermal stability of NR biocomposites was studied using TA Instrument 2910 Series TGA at heating rate 10 °C/minute in nitrogen environment from ambient temperature up to 600 °C.

Dielectric properties of NR biocomposites

LCR meter type AG-411 B (Ando electric Ltd. Japan) was used to evaluate dielectric properties of NR biocomposites at the range of frequency 100 Hz till 100 kHz. The capacitance C, the resistance R and the loss tangent tan d were gained. The rubber composites were molded as discs with 5 cm diameter and 3 mm thickness. Germany ring capacitor (model NFM/5 T Wiss Tech. Werkstatten GMBH,) was service as a measuring cell. Standard materials were used to calibrate the cell, and the experimental errors in permittivity and dielectric loss were establish to be ± 3 and ± 5%, respectively.

Results and discussion

Nanocrystalline and modified nanocrystalline cellulose characterization

FTIR spectra of nanocrystalline obtained by acid hydrolysis and modified nanocrystalline cellulose with CTAB as cationic surfactant are shown in Fig. 1. It can be seen that from this figure, IR absorption bands for NCC appear at 3300, 2912 cm−1 according to the O–H and C–H stretching vibration respectively and absorption peak at 1380 due to C–H and C–O vibrations of polysaccharide [25]. Surface modification of nanocrystalline cellulose was confirmed by two peaks at 2930 and 2860 cm−1 related to CH2 stretches of asymmetrical and symmetrical alkyl chain of CTAB. As well as peak appears at 1450 cm−1 according to methyl group of cationic substituent [26].

Fig. 1
figure 1

FTIR spectra of nanocrystalline and modified nanocrystalline cellulose

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X-ray Diffraction of microcrystalline, nanocrystalline and modified nanocrystalline cellulose are shown in Fig. 2. It is evident from this figure that there are three main peaks at 2θ = 14.75, 16.08 and 22.37 and these peaks are assigning for cellulose. Also, crystallinity was calculated according to Eq. 1 and was found to be 70.39, 78.20 and 68.77 for MCC, NCC and m-NCC respectively. The crystallinity increases from microcrystalline to nanocrystalline cellulose due to acid hydrolysis which can remove amorphous region such as hemicellulose or lignin. Also, modification of NCC with CTAB decrease the crystallinity and this is may be due to increase the amorphous region according to electrostatic interaction between two different charges [27].

Fig. 2
figure 2

X-ray diffraction analysis for micocrystalline, nanocrystalline and modified nanocrystalline cellulose

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NR biocomposites characterization

Rheological characteristic of NR biocomposites

Table 2 clarifies the rheological properties of the NR biocomposites, including maximum and minimum torques (MH and ML, respectively), scorch time (ts2), optimum cure time (Tc90), and cure rate index (CRI). From this table, It is clear that the MH and ML values for NR/mـNCC and NR/NCC composites are higher than those of the blank NR. In addition to, the MH and CRI values increase with increasing cellulosic filler content. Also, the optimum cure time (Tc90) of NR/NCC and NR/mـNCC composites with different content were shown in Fig. 3. It is evident from this figure that (Tc90) decreases with increasing filler content. Moreover, the modification of NCC leads to sharpe decrease in Tc90 more than composites containing NCC [4].

Table 2 Rheometer characteristics for natural rubber biocomposites
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Fig. 3
figure 3

Curing timeTc90 for NR/NCC and NR/m-NC

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Mechanical properties of NR biocomposites

Figure 4a, b revealed the tensile strength and elongation at break of natural rubber composites with different content from NCC and m-NCC respectively. It is clear from this figure that, the mechanical properties of natural rubber were improved gradually with the addition of NCC content up to 5 phr and declined above this ratio. In contrast, the values of elongation at break decreased with increasing NCC content. Also, it can be notice that the addition of m-NCC led to a slight increase in tensile strength when compared to NR and NR/NCC composites, while achieved a noticeable increase in elongation at break which may possibly due to the modification step with CTAB as cationic surfactant. This behavior is parallel to that stated by many researchers [28,29,30,31].

Fig. 4
figure 4

Mechanical properties of NR with different filler content. a Tensile strength, b elongation at break

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Thermal oxidative ageing properties of NR biocomposites

Thermal oxidative ageing is a significant test for polymeric composites which can effect on the composites quality. The effect of adding NCC or mـNCC on thermal ageing resistance was evaluated through the variations in the tensile strength and elongation at break retained values respectively. Figure 5a, b illustrates the retained tensile strength and elongation at break values respectively during ageing periods for NR and NR containing 5phr NCC and mـNCC biocomposites. From this figure, the highest retained value of tensile strength was achieved by NR/5NCC (85.2%) then NR/5 m-NCC (82.5%) and finally NR (79.5%). While the highest retained value of elongation at break was achieved by NR/5 m-NCC (87.9%) then NR/5NCC (85.2%) and finally NR (81.2%). Overall, it can be state that the addition of both NCC and mـNCC to natural rubber enhance the thermal stability of the composites which will be convenient to the life of the end product.

Fig. 5
figure 5

Retained mechanical properties of NR biocomposites after ageing. a Retained tensile strength of NR biocomposites, b retained elongation at break of NR biocomposites

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Thermogravimetic analysis of NR biocomposites

The thermogravimetric curves of the NR, NR/5NCC and NR/5 m-NCC composites are shown in Fig. 6. All composites show same degradation behavior. TGA curve of NR biocomposites clarifies the thermal degradation of the composites in three steps. Firstly, the initial weight loss (up to 200 °C) attributed to removal of moisture and other volatile matter. Secondly, fast decomposition took place when temperature was within the range of 279–371 °C due to cellulose degradation and formation char residual. Finally, last weight loss happened around 371–519 °C and this is may be attributed to degradation natural rubber [32, 33].

Fig. 6
figure 6

TGA curves of NR biocomposites

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Dielectric properties of NR biocomposites

Permittivity ε′ as a function of frequency at room temperature ~25 °C for NR/NCC and NR/m-NCC composites with different filler content is shown in Fig. 7. In both cases it is seen that the permittivity ε′ declines with increasing frequency. It is also seen from this figure that the decline in ε′ is very prominent at low frequencies. In most dielectric materials, the value of ε′ decreases with increasing applied frequency due to the phenomenon of anomalous dispersion.

Fig. 7
figure 7

The permittivity ε′ as a function of frequency at room temperature ~25 °C for NR/NCC and NR/m-NCC

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From point of view, the dielectric relaxation includes the orientation polarization which influenced by molecular arrangement of dielectric material. Consequently, at high frequency, the polar molecules of dielectric move in a rotational motion but are not fast enough to achieve the balance with the field, accordingly dielectric constant appears to decline with increasing frequency because of the existence of polar groups in the natural filler. It was also observed that the increase in the filler content was accompanied by a decrease in the density of the system as well as a delay in the orientation of the dipoles. The modification of NCC with cationic surfactant (CTAB) led to increase in ε′ values of the composite. This increase is due the further addition of polar groups to the network by such modification.

The dielectric loss ε″ variant versus the applied frequency at room temperature ~ 25 °C for NR/NCC and NR/m-NCC composites with different filler content were depicted in Fig. 8. These curves are so complicated reflecting more than one process in addition to the high values of ε″ at low frequency range which may be due to the dc conductivity and the Maxwell Wagner which is due to the difference in permittivities and conductivities of the ingredients of the composites [34, 35]. The values of ε″ are found to be higher for composites containing modified NCC when compared with those for unmodified ones.

Fig. 8
figure 8

The dielectric loss ε″ as a function of frequency at room temperature ~25 °C for NR/NCC and NR/m-NCC

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To understand the behavior of both ε′ and ε″ with NCC and m-NCC content, both values are illustrated graphically versus filler content at fixed frequency and shown in Fig. 9. From this figure, it is seen that both ε′ and ε″ values increase by increasing the filler content in the composite. Also it is seen that both values are pronouncedly high for composites contain m-NCC.

Fig. 9
figure 9

Permittivity ε′ and dielectric loss ε″ versus NR/NCC and NR/m-NCC at frequency = 100 Hz

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The dc electrical conductivity σ were calculated from the measured ac conductivity and the obtained data were illustrated graphically in Fig. 10. The electrical conductivity values were found to be in the order of 10–11 S/cm−1. This finding highly recommend such composites to be used in antistatic applications as the needed range for such application is 10–9–10–14 S/cm−1 [36].

Fig. 10
figure 10

Electrical conductivity σ versus filler content

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Morphology of NR biocomposites

SEM was employed to clarify the surface morphology of NR, NR/5NCC, and NR/5 m-NCC (Fig. 11). Figure 11a shows the uniform and smooth surface of the unfilled NR composite while Fig. 11b, c show the nonuniform dispersion of cellulosic filler in the composites. Also, it is clear that the amorphous structure increase in case of m-NCC than NCC due to penetration of CTAB as cationic surfactant through the successive lamina. This observation is further confirmed by the resultant mechanical properties and XRD.

Fig. 11
figure 11

SEM for NR biocomposites. a Blank NR, b NR/5NCC, c NR/5 m-NCC

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Conclusion

Nanocrystalline and modified nanocrystalline cellulose were successfully prepared and characterized by FT-IR and X-ray diffraction. Also, NCC and m-NCC were incorporated in natural rubber biocomposites with different content. The XRD results illustrated that the crystallinity increased from microcrystalline to nanocrystalline cellulose due to acid hydrolysis which could remove amorphous region such as hemicellulose or lignin. Rheometric properties also idicated that the m-NCC decreased the optimum cure time for the composites more than NCC. In addition to mechanical properties stated that NR/5NCC exhibited the highest tensile strength when compared to composite containing same ratio from m-NCC and NR respectively. Finally mechanical and electrical measurements recommended the composites to be used in antistatic application.