NR-based nanocomposites

This is the most widely used naturally occurring rubber. The literature search shows that many research groups have prepared nanocomposites based on this rubber [29-32]. Varghese and Karger-Kocsis have prepared natural rubber (NR)-based nanocomposites by melt-intercalation method, which is very useful for practical application. In their study, they have found increase in stiffness, elongation, mechanical strength, and storage modulus. Various minerals like MMT, bentonite, and hectorite have been used. 

FIGURE 2.7 Effect of various clays on natural rubber (NR)-based nanocomposites (at 4 phr loading). (From Bhattacharya and Bhowmick, Unpublished data.)... 

They have studied the properties of NR-epoxidized natural rubber (ENR) blend nanocomposites also. Vulcanization kinetics of natural mbber-based nanocomposite was also smdied. The effect of different nanoclays on the properties of NR-based nanocomposite was studied. The tensile properties of different nanocomposites are shown in Figure 2.7 [33]. 

Production cost of NR-based nanocomposites has to be optimized to the performance of the NR goods in order to capture market share. NR industries, in general, are comparatively conservative in nature compared to SR industries. Positive effects of nanotechnology on NR goods may become visible to the industries after extended studies. 

A synergistic effect was also found between clay and CB N330 from Cabot, in NR based nanocomposites prepared by emulsion compounding.It was observed that both fillers were dispersed at nanoscale and randomly in the NR matrix the space between the clay layers was filled with CB particles. The mechanical properties of the nanocomposites, such as moduli at 100% and 300% elongation, tensile strength and tear strength were much improved, at the same total filler level, by the hybrid filler system, with respect to composites containing only one filler. 

Some NR-based nanocomposites with bio-based nanoreinforcements like chi-tin whiskers, starch nanocrystals, cellulose whiskers. A lot of work on bionanocomposites of NR has been reported by Alain Dufresne and coworkers.But most of these studies use latex blending technique without vulcanization, for the bionanocomposites preparation. There are less reports available on vulcanized rubber-based bionanocomposites prepared by master batch processing and two-roll mill mixing, which have the potential to be adapted for commercial use. ... 

Table 23.1 Techniques typically used to analyse the multi-scale clay organization in NR-based nanocomposites.

Table 23.2 Microscopic techniques typically used to characterize CNTs in NR-based nanocomposites.

Figure 3. TEM photographs of a non-layered clay (a) and sodium fluorohectorite-filled NR-based nanocomposite (b) produced by latex compounding. Silicate content of 10 phr.

Recently a lot of attention is being given to the field of latex-based nanocomposites. Various organoclays as well as pristine clays have been intercalated in aqueous medium with NR latex, SBR latex, NBR latex, as well as carboxylated nitrile mbber (XNBR) latex [184—187], to achieve a good degree of dispersion. 

In modern vulcanization processes, NR is generally compounded (either under high or low temperature) with 0.5 to 1 wt% of accelerators, different concentrations of CBs (which act as a filler) (up to 45 wt% for tyre manufacturing), low concentrations of aromatic amines and phenols for antioxidation purposes and 5-8 wt% of sulfur. These types of vuleanized NR are commonly known as NR composites (when the filler dimension is on the microscale) or NR nanocomposites (when the filler dimension is on the nanoscale). NR-based composites or nanocomposites will be discussed in more detail in the following sections. 

Carbon black (CB) is indisputably the most widely used reinforcing filler in NR formulations. It improves tensile and tear strengths, modulus and hardness, abrasion and thermo-oxidative resistance, etc. of NR-based materials. CB is manufactured by a variety of processes, including the channel process, to produce furnace black, thermal black, lamp black and acetylene black. NR-based composites and nanocomposites with the addition of CB exhibit the monotonous black colour to the finished goods. 

Sustainability of the materials becomes one of the prime concerns in the research development in industries and academic institutions. The search for nonpetroleum-based fillers has accelerated the research into bio-nanofillers from biomass. In some cases, bio-based nanocomposites show unique advantages over traditional inorganic nanoparticles. Commonly used bio-nanofillers in NR are whiskers of cellulose, chitosan, nanocrystals of starch etc. The studies on these fillers are aimed at competitive production cost and equivalent properties as compared to other petroleum-based fillers. In addition, biocompatibility and biodegradability of the bio-based fillers are hoped to be retained after dispersion in the NR matrix. 

A remarkable non-linear viscoelastic behaviour, known as the Payne effect was observed for NR/CNT nanocomposites. A sample with 10 phr of CNT displayed a great Payne effect. It was reported that, below 3.8 wt% of CNT, the Payne effect was not observable.It was reported as well a reduction of Payne effect for a composite with 9.1 wt% CNT, by increasing the temperature, from 298 K to 353 K. For this composite, the Payne effect was however much higher than for a composite with the same content of CB. It was commented that at this concentration CNT was above the percolation threshold. Remarkable Payne effect was observed also in IR based nanocomposites,from strain sweep tests performed in the shear mode it was noticed also below the percolation threshold of CNT (7.2 phr, see above) and it became much more evident above said threshold. 

Table 7.2 Summary of some silica modified via admicellar polymerization in NR- or polyisoprene-based nanocomposites.

Much effort has gone into preparing rubber-clay nanocomposites based on NR as described in the earlier section. However, the dispersion of such nanoclays in non-polar NR matrices was very poor. In this section we would like to outline the preparation and characterization of NR-clay nanocomposites in the presence of certain eompatibilizers added into the system externally. For instance, Teh et al. prepared NR-OMt nanocomposites by a melt compounding method. ENR 25 and ENR 50 were used as compatibilizers. Pristine MMT was modified with oetadecyltrimethylamine and abbreviated as MMT-ODTMA. The amount of organoclay was only 2 phr while the amount of ENR was varied. NR-MMT-ODTMA showed mostly intercalated structure of the... 

Jamal et al. determined the eomplex dielectric permittivity and magnetic permeability of NR-Ni nanocomposites in the X band of mierowave frequencies from 7 to 12 GHz. The real part of dielectrie permittivity does not change with the frequency but shows a steady increase with the inerease in loading of Ni nanoparticles. The dieleetrie loss shows an inerease with the frequency in general and for NR based eomposites there is a shallow relaxation peak around 8.25 GHz. In both type of eomposites, the dieleetrie loss increases... 

Comparative Study of SBR and NR Based Calcium Carbonate Nanocomposites... 

Natural rubber/cw-1,4-polybutadiene (NR/BR) blends (70/30 mass ratio) have been widely used in the tire industry. Many nanocomposites based on organo-montmorillonite (OMMT)/rubber blends have been investigated. However, relatively little attention had been paid to binary rubber hybrids/ montmorillonite nanocomposites, and according to Zheng Gu et ah, no studies existed dealing with OMMT/NR/BR nanocomposites. So, the authors described the preparation of OMMT/NR/BR nanocomposites by direct mechanical blending and determined the cure characteristics, static mechanical properties, dynamic mechanical properties, and thermal stability of the nanocomposites. OMMT/NR/BR nanocomposites had exactly the same onset decomposition temperature and lower thermal degradation rate as the NR/BR blends. 

Fig. 7 Dynamic mechanical analysis of pure NR and cellulose nanofiber-based nanocomposites...

Very interesting studies of natural rubber reinforcement with ZnO nanoparticles were performed by scientists from India, under the direction of Sabu Thomas [62]. The goal of these studies was to characterize the viscoelastic behavior and reinforcement mechanism of ZnO nanoparticles introduced into the rubber matrix. They have presented a constrained polymer model based on a rubbery region and a ZnO nanoparticle. Very interestingly, the authors presented a core-shell morphology model and constrained polymer model to explain the constrained polymer chains in NR/ZnO nanocomposites [62]. Thanks to this research and the proposed models, it is possible to understand the behavior of nanofillers in the polymer matrix and maybe in the future to develop an ideal nanofiller for use in the rubber matrix. 

Fig. 21 Representative DMA thermograms of NR/XSBR based nanocomposites filled with different CNT loadings (a) tan 5 and (b) E [100]...

Polymeric blends are popularly known to show desired properties based on the components. But their nanocomposites have been prepared to further modify the properties. Various blends like NR-ENR and PP-EPDM [188,189] with nanofillers have also been smdied. 

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