Comparative Investigation of Nano-Sized Silica and Micrometer-Sized Calcium Carbonate on Structure and Properties of Natural Rubber Composites.

Fillers have been widely used in natural rubber (NR) products. They are introduced to serve as a strategy for modifying the final properties of NR vulcanizates. Silica and calcium carbonate (CaCO3) are among the fillers of choice when the color of the products is concerned. In this case, a special focus was to compare the vulcanizing efficiency of NR filled with two different filler types, namely nano-sized silica and micrometer-sized CaCO3. This study focused on the effects of the loading level (10-50 parts per hundred parts of rubber, phr) on the final properties and structural changes of NR composites. The results indicated that increased filler loading led to higher curing torques and stiffness of the rubber composites irrespective of the type of filler used. The better filler dispersion was achieved in composites filled with CaCO3 which is responsible for less polarity of CaCO3 compared to silica. Good filler distribution enhanced filler-matrix interactions, improving swelling resistance and total crosslink density, and delaying stress relaxation. The modulus and tensile strength of both composites also improved over the content of fillers. The CaCO3-filled composites reached their maximum tensile strength at 40 phr, exceeding, by roughly 88%, the strength of an unfilled sample. Conversely, the maximum tensile strength of silica-filled NR was at 20 phr and was only slightly higher than that of its unfilled counterpart. This discrepancy was ascribed to the stronger rubber-filler interactions in cases with CaCO3 filler. Effective rubber-filler interactions improved strain-induced crystallization, increasing crystallinity during stretching and reducing the strain at which crystallization begins. In contrast, large silica aggregates with poor dispersion reduced the overall crosslink density, and degraded the thermomechanical properties, tensile properties, and strain-induced crystallization ability of the NR. The results clearly indicate that CaCO3 should be favored over silica as a filler in the production of some rubber products where high performance was not the main characteristic.

The Use of Kenaf Fibre as a Natural Anti-Degradant in Recycled High-Density Polyethylene and Natural Rubber-Based Thermoplastic Elastomers.

As most plastic materials disintegrate without being properly reused after they are discarded, this present study developed a novel thermoplastic elastomer (TPE) using recycled high-density polyethylene (rHDPE) and natural rubber (NR) with kenaf fibre as a sustainable filler. Apart from being used as filler, this present study aimed to examine the use of kenaf fibre as a natural anti-degradant as well. The results indicated that the tensile strength of the samples was found to have significantly decreased after 6 months of natural weathering and had decreased by a further 30% after 12 months due to the chain scission of the polymeric backbones and the degradation of the kenaf fibre. However, the composites that contained kenaf fibre significantly retained their properties post-natural weathering. In terms of tensile strength and elongation at the break, the addition of only 10 phr of kenaf increased the retention properties by 25% and 5%, respectively. This is noteworthy as kenaf fibre also contains a certain amount of natural anti-degradants. Therefore, as the kenaf fibre improves the weather resistance of composites, plastic manufacturers could use it as either a filler or a natural anti-degradant.

Boosting the Antibacterial Performance of Natural Rubber Latex Foam by Introducing Silver-Doped Zinc Oxide.

Natural rubber (NR) latex foam is one of the rubber products that are increasingly in demand in the market. This is simply because of its lightweight, good thermal insulation, and resilience. The applications of NR latex foam are mostly for pillows and mattresses. This has resulted in these products requiring antibacterial performance which is very important for the safety of the end-users. In this study, the antibacterial NR latex foam was prepared by incorporating the silver-doped zinc oxide (Ag-doped ZnO) into the NR latex foam. Ag-doped ZnO was prepared by microwave-assisted method and then characterized through morphological characteristics and X-ray diffraction (XRD). The content of Ag doped onto ZnO was designed by varying the AgNO3 content at 15 wt%, 50 wt%, and 100 wt% of ZnO. The results confirmed that the Ag was successfully doped onto ZnO. The silver particles were found to be in the 40-50 nm range, where the size of ZnO ranges between 300 and 400 nm, and the Ag attached to the ZnO particles. The XRD patterns of Ag-doped ZnO correspond to planes of hexagonal wurtzite ZnO structure and cubic metallic Ag. This Ag-doped ZnO was further added to NR latex foam. It was observed that Ag-doped ZnO did not affect the physical properties of the NR latex foam. However, it is clear that both the inhibition zone and percent reduction of bacteria (e.g., E. coli and S. aureus) were enhanced by the addition of Ag-doped ZnO. It showed a decrease in the amount of cell growth over contact time. The content of 100 wt% AgNO3 could reduce E. coli and S. aureus up to 64.72% and 58.90%, respectively, when samples were maintained for 24 h. This study provides a scientific understanding of how Ag-doped ZnO could facilitate the development of eventual rubber foam products based on the respective results.

Potent Application of Scrap from the Modified Natural Rubber Production as Oil Absorbent.

The production of raw natural rubber always ends up with leftover latex. This latex is later collected to produce low grades of rubber. The collection of this latex also depends on the latex's quality. However, reproducing the latex may not be applicable if the latex contains many specks of dirt which will eventually be discarded. In this work, an alternative solution was to utilize such rubber in a processable form. This scrap rubber (SR) from the production of natural rubber grafted with polymethyl methacrylate (NR-g-PMMA) production was recovered to prepare an oil-swellable rubber. The rubber blends were turned into cellular structures to increase the oil swellability. To find the suitable formulation and cellular structure of the foam, the foams were prepared by blending SR with virgin natural rubber (NR) at various ratios, namely 0/100, 20/80, 30/70, 50/50, 70/30, 80/20, and 100/0 (phr/phr). The foam formation strongly depended on the SR, as it prevented gas penetration throughout the matrix. Consequently, small cells and thick cell walls were observed. This structure reduced the oil swellability from 7.09 g/g to 5.02 g/g. However, it is interesting to highlight that the thermal stability of the foam increased over the addition of SR, which is likely due to the higher thermal stability of the NR-g-PMMA waste or SR. In summary, the blending NR with 30 phr of SR provided good oil swellability, processability, and morphology, which benefit oil recovery application. The results obtained from this study will be used for further experiments on the enhancement of oil absorbency by applying other key factors. This work is considered a good initiative for preparing the oil-absorbent material based on scrap from modified natural rubber production.

Sustainable Recycling of Waste from Nitrile Gloves: Prolonging the Life Span by Designing Proper Curing Systems.

A massive demand for rubber-based goods, particularly gloves, was sparked by the emergence of the COVID-19 epidemic worldwide. This resulted in thousands of tons of gloves being scrapped due to the constant demand for the items, endangering our environment in a grave way. In this work, we aimed to focus on the utilization of waste nitrile gloves (r-NBR) as a component blended with natural rubber (NR). The life span and other related properties of the blend can be improved by proper control of the chemical recipe. This study assessed three types of crosslinking systems, namely sulfur (S), peroxide (DCP), and mixed sulfur/peroxide (S/DCP) systems. The results indicate that choosing S/DCP strongly affected the tensile strength of the blend, especially at relatively high contents of r-NBR, improving the strength by 40-60% for cases with 25-35 phr of r-NBR. The improvement depended on the crosslink types induced in the blends. It is interesting to highlight that the thermal resistance of the blends was significantly improved by using the S/DCP system. This indicates that the life span of this blend can be prolonged by using a proper curing system. Overall, the S/DCP showed the best results, superior to those with S and DCP crosslinking systems.

Fibre-Reinforced Polymer Composites: Mechanical Properties and Applications.

"Fibre-Reinforced Polymer Composites: Mechanical Properties and Applications" is a newly open Special Issue of Polymers, which aims to publish original and review papers on new scientific and applied research and make boundless contributions to the finding and understanding of the reinforcing effects of various synthetic and natural fibres on the performance of biopolymer composites [...].

Influence of Centrifugation Cycles of Natural Rubber Latex on Final Properties of Uncrosslinked Deproteinized Natural Rubber.

Natural rubber latex (NRL) is a polymer (blend) extracted from the milky sap of para rubber trees. Due to being a natural biopolymer, NRL contains various proteins that may be allergenic to humans when in skin contact. Attempts have been made to use deproteinized natural rubber (DPNR) instead of impure NRL, and the final properties of these two types of rubber tend to differ. Thus, the correlations between their chemistry and properties are of focal interest in this work. DPNR was prepared by incubating NRL with urea, followed by aqueous washing/centrifugation. The physical, mechanical, and dynamic properties of incubated NRL before and after washing/centrifugation were examined to distinguish its influences from those of incubation with urea. According to the findings, the proteins, phospholipids, and chain entanglements were responsible for natural polymer networks formed in the NR. Although the proteins were largely removed from the latex by incubation, the properties of high ammonia natural rubber (HANR) were still maintained in its DPNR form, showing that other network linkages dominated over those contributed by the proteins. In the incubated latex, the naturally occurring linkages were consistently reduced with the number of wash cycles.

Viable Properties of Natural Rubber/Halloysite Nanotubes Composites Affected by Various Silanes.

Natural rubber (NR) is incompatible with hydrophilic additives like halloysite nanotubes (HNT) due to their different polarity. The silane coupling agent is the ideal component to include in such a compound to solve this problem. Many types of silane are available for polymer composites depending on their functionalities. This work aimed to tune it to the composite based on NR and HNT. Four different silanes, namely Bis[3- (Triethoxysilyl)Propyl]Tetrasulfide (TESPT), 3-Aminopropyl triethoxysilane (APTES), N-[3-(Trimethoxysilyl)Propyl] Ethylenediamine (AEAPTMS), and Vinyltrimethoxysilane (VTMS) were used. Here, the mechanical properties were used to assess the properties, paying close attention to how their reinforcement influenced their crystallization behavior after stretching. It was revealed that adding silane coupling agents greatly improved the composites' modulus, tensile strength, and tear strength. From the overall findings, AEAPTMS was viable for NR/HNT composites. This was in direct agreement with the interactions between NR and HNT that silanes had encouraged. The findings from stress-strain curves describing the crystallization of the composites are in good agreement with the findings from synchrotron wide-angle X-ray scattering (WAXS). The corresponding silanes have substantially aided the strain-induced crystallization (SIC) of composites.

Selectively Etched Halloysite Nanotubes as Performance Booster of Epoxidized Natural Rubber Composites.

Halloysite Nanotubes (HNT) are chemically similar to clay, which makes them incompatible with non-polar rubbers such as natural rubber (NR). Modification of NR into a polar rubber is of interest. In this work, Epoxidized Natural Rubber (ENR) was prepared in order to obtain a composite that could assure filler-matrix compatibility. However, the performance of this composite was still not satisfactory, so an alternative to the basic HNT filler was pursued. The surface area of HNT was further increased by etching with acid; the specific surface increased with treatment time. The FTIR spectra confirmed selective etching on the Al-OH surface of HNT with reduction in peak intensity in the regions 3750-3600 cm-1 and 825-725 cm-1, indicating decrease in Al-OH structures. The use of acid-treated HNT improved modulus, tensile strength, and tear strength of the filled composites. This was attributed to the filler-matrix interactions of acid-treated HNT with ENR. Further evidence was found from the Payne effect being reduced to 44.2% through acid treatment of the filler. As for the strain-induced crystallization (SIC) in the composites, the stress-strain curves correlated well with the degree of crystallinity observed from synchrotron wide-angle X-ray scattering.

Potency of Urea-Treated Halloysite Nanotubes for the Simultaneous Boosting of Mechanical Properties and Crystallization of Epoxidized Natural Rubber Composites.

Halloysite nanotubes (HNTs) are naturally occurring tubular clay made of aluminosilicate sheets rolled several times. HNT has been used to reinforce many rubbers. However, the narrow diameter of this configuration causes HNT to have poor interfacial contact with the rubber matrix. Therefore, increasing the distance between layers could improve interfacial contact with the matrix. In this work, Epoxidized Natural Rubber (ENR)/HNT was the focus. The HNT layer distance was successfully increased by a urea-mechanochemical process. Attachment of urea onto HNT was verified by FTIR, where new peaks appeared around 3505 cm-1 and 3396 cm-1, corresponding to urea's functionalities. The intercalation of urea to the distance gallery of HNT was revealed by XRD. It was also found that the use of urea-treated HNT improved the modulus, tensile strength, and tear strength of the composites. This was clearly responsible for interactions between ENR and urea-treated HNT. It was further verified by observing the Payne effect. The value of the Payne effect was found to be reduced at 62.38% after using urea for treatment. As for the strain-induced crystallization (SIC) of the composites, the stress-strain curves correlated well with the results from synchrotron wide-angle X-ray scattering.

Synergistic Effect of Maleated Natural Rubber and Modified Palm Stearin as Dual Compatibilizers in Composites based on Natural Rubber and Halloysite Nanotubes.

The performance of rubber composite relies on the compatibility between rubber and filler. This is specifically of concern when preparing composites with very different polarities of the rubber matrix and the filler. However, a suitable compatibilizer can mediate the interactions. In this study, composites of natural rubber (NR) with halloysite nanotubes (HNT) were prepared with maleated natural rubber (MNR) and modified palm stearin (MPS) as dual compatibilizers. The MPS dose ranged within 0.5-1.5 phr, while the MNR dose was fixed at 10 phr in all formulations. It was found that the mixed MNR/MPS significantly enhanced modulus, tensile strength, and tear strength of the composites. The improvements were mainly due to improved rubber-HNT interactions arising from hydrogen bonds formed in the presence of these two compatibilizers. This was clearly verified by observing the Payne effect. Apart from that, the MPS also acted as a plasticizer to provide improved dispersion of HNT. It was clearly demonstrated that MNR and MPS as dual compatibilizers improved rubber-HNT interactions and reduced filler-filler interactions, which then improved tensile and tear strengths, as well as dynamical properties. Therefore, the mix of MNR and MPS had a great potential to compatibilize non-polar rubber with HNT filler.

Effect of extraction methods on the molecular structure and thermal stability of kenaf (Hibiscus cannabinus core) biomass as an alternative bio-filler for rubber composites.

Lignin from kenaf (Hibiscus cannabinus) core was investigated as an alternative filler for rubber. Three types of extraction methods were used to isolate lignin from kenaf, namely kraft, soda and organosolv process. The particle size, surface area, functionalities changes, molecular weight and thermal properties of the lignin were characterized. The results showed that Kraft lignin (KL) has the smallest particle size (40.41 µm) compared to soda lignin (SL) (63.85 µm) and organosolv lignin (OL) (66.85 µm). This is in good agreement with the BET surface area of 9.52 m2/g, 1.25 m2/g and 2.40 m2/g respectively. However, the smaller surface area of SL compared to OL is due to the smaller pore size and pore volume of SL. KL also showed high hydroxyl content with corresponding high thermal stability as confirmed by NMR and TGA. The thermal stability of the lignin correlates well with the molecular weight (MW). From the overall characteristics, it can be concluded that KL, SL and OL can be used as an alternative filler in rubber compounds to substitute common fillers like silica and carbon.