Acid-induced aggregation and gelation of natural rubber latex particles

Extraction process and characterization of Taraxacum kok-saghyz (TKS) latex

Taraxacum kok-saghyz (TKS) latex is a natural latex produced from its root, and its extraction optimization process is mainly studied in the present paper. The composition of fresh roots of TKS was quantitatively analyzed, and the results showed that the moisture content of the fresh root was approximately 70 %, and the rubber content averaged to 6 % (dry weight ratio). An optimal process route for extracting the TKS latex was finally determined, making the extraction efficiency reach about 80 %, and a new latex extraction process was established and optimized and named "the process of Buffer Extraction TKS Latex (BETL)". Hevea latex, extracted TKS latex and TKS latex collected directly from the broken roots were compared for study. The results showed that, like Hevea latex, the appearance of TKS latex was milky white; and after centrifugation, both showed four layers from top to bottom: rubber particles, Frey-Wyssling particles, C-serum and lutoids. The results of the composition analysis showed that the concentration of TKS latex ranged from 54.54 % to 68.25 %, which is close to that of concentrated Hevea latex; the moisture content of TKS latex was between 31.75 % and 45.46 %. The protein content of TKS latex was 13.51 mg/mL, which was lower than that of Hevea latex at the same rubber hydrocarbon concentration. The molecular structures and properties of Hevea latex, the extracted TKS latex, and the collected TKS latex were characterized by FTIR, 13C NMR, GPC, TG, SEM and LPSA, and the results showed that the main components and structure of the three latexes were similar, which are all cis-1,4-polyisoprene, and include the proteins and lipids. The distributions molecular weights of the three latexes all showed a bimodal distribution, but the molecular weight of the latex collected from TKS was lower, which indicates the larger molecules were difficult to flow outside the root automatically. The Hevea latex and TKS latex rubber particles were both core-shell structure and the size distribution were bimodal, which was consistent with the GPC analysis results.

Chemical Differentiation of Plant Latexes and Their Anti-herbivory Activity against Thrips Frankliniella occidentalis

Despite the extensive studies on latex, some fundamental questions on their chemical specialization and the factors influencing this specialization have yet to be investigated. To address this issue, latexes and their bearing tissues from diverse species were profiled by ¹HNMR and GC-MS. Additionally, the antiherbivory activity of these materials was tested against thrips (Frankliniella occidentalis Pergande, 1895). The multivariate data analysis showed a clear separation between latexes and leaves from the same species. Conversely, the chemical profiles of latexes from different species were highly similar, that is, they displayed much less metabolic species-specificity. These shared chemical profiles of latexes were reflected in their overall higher mortality index (80.4% ± 7.5) against thrips compared with their bearing tissues (55.5% ± 14.9). The metabolites correlated to the antiherbivory activity of latexes were triterpenoids and steroids. However, the activity could not be attributed to any single terpenoid. This discrepancy and the reduction of the latex activity after fractionation suggested a complementary effect of the compounds when in a mixture as represented by the latex. Additionally, aqueous fractions of several latexes were found to possess simple spectra, even with only 1 metabolite. These metabolites were determined to be organic acids that might be involved in the modulation of the rate of latex coagulation, potentially increasing the sealing and trapping effects of the latex.

Facile fabrication of mesostructured natural rubber/silica nanocomposites with enhanced thermal stability and hydrophobicity

Natural rubber (NR)/hexagonal mesoporous silica (HMS) nanocomposites (NRHMS) with enhanced thermal and hydrophobic properties were facilely prepared via in situ sol-gel formation with pH adjustment using a low sulphuric acid (H2SO4) acid concentration. The effect of the amount of 0.5 M H2SO4 (2.5-10 g) added into the pre-synthesis mixture on the physicochemical properties of the obtained NRHMS nanocomposites was investigated. With a small addition of H2SO4 solution, the fabricated NRHMS nanocomposite possessed an improved wormhole-like mesostructure arrangement with a thicker silica wall, which retarded the thermal decomposition of the NR phase, as deduced from the auto-oxidation of NR by thermogravimetric analysis. The H2O adsorption-desorption measurement revealed an increased hydrophobicity of the NRHMS composites, explained by the acid-catalyzed bridging of free silanol groups to siloxane bonds, which was supported by the X-ray photoelectron spectroscopy analysis. Scanning transmission electron microscopy with energy dispersive X-ray spectroscopy elemental mapping revealed a good dispersion of the NR phase within the mesostructured silica. However, a high amount of added H2SO4 solution led to silica-NR phase separation due to the decreased hydrophobic interaction between the silica precursor and rubber chain, as well as an agglomeration of the NR phase itself. The mechanism of NRHMS nanocomposite formation under pH-controlled conditions was proposed to proceed via a cooperative self-assembly route.

BIO-CACO3 FROM RAW EGGSHELL AS ADDITIVE IN NATURAL RUBBER LATEX GLOVE FILMS

Raw hen eggshell powder, a calcium carbonate source, was used as a biofiller in the natural rubber latex compound and latex glove film formation via dipping process. The powder was anticipated to improve the physical (smoothness and thickness of film) and mechanical properties (tensile strength and elongation at break) of latex film and to reduce the extractable protein content on film surface. Eggshell powder ground by a rapid mill was fine particles of approximately 37.48 μm in diameter, suitable for homogeneous and compatible addition into the natural rubber latex compound. Dipping hand mold into the natural rubber latex compound with 50 wt% eggshell added was the best formula to obtain a smooth, clear, thin film surface, with the tensile strength of 23.24 ± 0.745 MPa and the highest elongation at break of 723.99 ± 14.60%, along with a low protein content, a dense film without water leakage, and with a good contact angle. The natural rubber latex glove film possessed good physical-mechanical properties and a low protein content as the results of the raw eggshell powder added as a biofiller.

A Green Approach for Preparing High-Loaded Sepiolite/Polymer Biocomposites

Global industry is showing a great interest in the field of sustainability owing to the increased attention for ecological safety and utilization of renewable materials. For the scientific community, the challenge lies in the identification of greener synthetic approaches for reducing the environmental impact. In this context, we propose the preparation of novel biocomposites consisting of natural rubber latex (NRL) and sepiolite (Sep) fibers through the latex compounding technique (LCT), an ecofriendly approach where the filler is directly mixed with a stable elastomer colloid. This strategy favors a homogeneous dispersion of hydrophilic Sep fibers in the rubber matrix, allowing the production of high-loaded sepiolite/natural rubber (Sep/NR) without the use of surfactants. The main physicochemical parameters which control Sep aggregation processes in the aqueous medium were comprehensively investigated and a flocculation mechanism was proposed. The uniform Sep distribution in the rubber matrix, characteristic of the proposed LCT, and the percolative filler network improved the mechanical performances of Sep/NR biocomposites in comparison to those of analogous materials prepared by conventional melt-mixing. These outcomes indicate the suitability of the adopted sustainable procedure for the production of high-loaded clay⁻rubber nanocomposites with remarkable mechanical features.