Rubber Latex Particles Research Articles

Hollow natural rubber latex particles as bio-based alternative white pigment for coating applications

Due to the broad particle size distribution as well as non-spherical shape of natural rubber (NR) latex particles, the non-collapse hollow NR latex particles (HL-NRPs) with well-defined void structure were developed aiming to use as an eco-friendly white pigment/additive for coatings. These hybrid HL-NRPs were prepared via the seeded emulsion polymerization of methyl methacrylate/divinylbenzene/acrylic acid (MMA/DVB/AA) monomers on NR core particles in one-pot. Using cumene hydroperoxide (CHP)/tetraethylene pentamine (TEPA) as a redox initiator, the copolymerization occurred at the particle/water interface. Without seed removal, a single void inside NRP was in situ formed by the phase separation process. Hydrophobic and elastic NR became the inner wall which strengthened the HL particles. Whereas its indigenous stabilizers, i.e., proteins-lipids, allowed the osmosis from the aqueous medium to further expand the void. The proposed mechanism was illustrated using oleic acid (OA) as a model fatty acid for non-rubber substances. Since the multiple light scattering inside the hollow structure could enhance their refractive index, the optical performance of HL-NRPs as a low-density opacifying agent in poly(vinyl alcohol) matrix, was examined using UV-Vis spectroscopy. Results showed that the HL-NRPs-based films exhibited high blocking ability over UV and visible regions. Moreover, the HL-NRPs coated with chitosan (CS) enabled these nanocomposites to adsorb the formaldehyde vapor. For sustainable development, HL-NRPs/CS could potentially be a green alternative white pigment/additive for coatings.

Influence of sizes of rubber particles in latex on mechanical properties of natural rubber filled with carbon black

The structures and properties of natural rubber (NR) obtained from latex with small and large rubber particles (SRP and LRP) are obviously different, while carbon black (CB)/SRP or CB/LRP composites have rarely been reported. In this work, SRP and LRP are separated from natural rubber latex (NRL). The properties of obtained NR/CB composites are systematically investigated as a function of SRP and LRP mass ratios. It shows that the nitrogen content of SRP (1.32%) is much higher than that of LRP (0.10%). Mechanical properties increase with the increase of LRP content because plenty of agglomerates exist in SRP/CB composite whereas LRP/CB composite shows a homogeneous dispersion. Furthermore, a possible model is proposed to illustrate the transformations of agglomerates during stretching and dynamic compressing process. This study provides a new perspective for the design of NR materials through the selection of rubber particles in NRL.

Influence of Clones on Relationship between Natural Rubber and Size of Rubber Particles in Latex.

IAN873, Dongfang93114 and Reyan73397, created through vegetative propagation for their high yield and excellent cold resistance, are major clones planted in China. In this work, latexes with rubber particles of the same size from these clones are separated from fresh natural rubber latex, and corresponding rubber films are prepared from each latex. The structure and components of each film are measured. This indicates that the characteristics of the rubbers obtained from latexes with similar particle sizes show some resembling trends among different clones, while for specific samples, those characteristics vary depending on the clone. The molecular weight is generally highest in IAN873 and lowest in Reyan73397. Rubber chains in small rubber particles are longer, and large rubber particles show a wider molecular weight distribution. The gel content of every sample from Reyan73397 is lower than the other two clones. The nitrogen content increases with the size of rubber particles in all clones. The ester content of small rubber particles in IAN873 and Reyan73397 is almost zero. Large rubber particles have more branching points formed via esters. This study provides a new perspective on the influence of clones on the relationship between characteristics of natural rubber and the size of rubber particles in natural rubber latex.

In-situ observation of spatial organization of natural rubber latex particles and exploring the relationship between particle size and mechanical properties of natural rubber

Natural rubber (NR), an important source of elastomeric material, has superior comprehensive properties compared to synthetic high cis-1, 4-polyisoprene, which are attributed to its peculiar architecture. However, it is a challenge to identify the precise structure of NR, especially the structure of NR particles, as there are no direct characterization methods to perform in-situ investigation of structure of NR particles at an intact state. In this study, we used cryogenic transmission electron microscopy (cryo-TEM) to directly explore the morphology of NR particles existing in the latex without drying or chemical fixation. Then, the spatial organization of proteins and phospholipids on the surface of NR particles was visualized by stochastic optical reconstruction microscopy (STORM). The results show that NR particles are spherical particles with an average particle size of 88 ± 15 nm, and the exterior of NR particles is a mixed monolayer of proteins and phospholipids (ca. 4.6 ± 0.77 nm in thickness). Thus, a more complete model for the Hevea brasiliensis rubber particles was proposed. Moreover, we explicated the relationship between the particle size of NR particles, molecular characteristics, and mechanical properties of NR. The Mnand Mw of small rubber particles (SRPs) with a particle size of 106 nm are (1.56 ± 0.06) × 106 g/mol and (2.21 ± 0.07) × 106 g/mol respectively, which are much larger than those of the large rubber particles (LRPs) with a particle size of 628 nm (Mn: (0.41 ± 0.05) × 10 6 g/mol; Mw: (1.63 ± 0.10) × 106 g/mol). After vulcanization, the SRPs have better mechanical properties. Compared with LRPs, SRPs exhibit a 4.3-fold increase in tensile strength (from 5.7 to 24.6 MPa) and a 3.5-fold increase in toughness (from 12.9 to 44.9 kJ/m2). This work not only provides rational design for the new biomimetic synthetic rubber with unique hierarchical structures and excellent properties, but also offers a new possibility for tuning the mechanical properties of NR by preparation of LRPs/SRPs blend of various ratios.

A concise review on design and control of structured natural rubber latex particles as engineering nanocomposites

In this article, we review the modification of natural rubber (NR) latex into the engineered particles for make use of them in functional nanocomposites. The review focuses on tuning nano-/microstructure and/or composition of NR latex particles by means of surface chemical functionalization, structural modification and incorporation of inorganic nanoparticles (NPs). Due to their polydispersity in size and non-spherical shape with complicated indigenous stabilizers, i.e., proteins and lipids, we first describe the fundamental and common methods for the modification of synthetic polymer latex which become a practical guideline for engineering of NR particles to a variety of designs. Subsequently, the examples of successfully modified NR latex and the involved parameters in the preparation are discussed. The different nature and properties of the modified NR latex compared to the synthetic latex are then depicted. Finally, specific examples of these systems used in a range of applications including nanofillers, controlled releasing materials, textiles, electronics and coatings are demonstrated. The research on modification of NR latex particles would be beneficial to the field of materials science and engineering for the valorization of naturally abundant colloidal polymer to the advanced functional materials in the broader utilization/commercialization in the future.

High sensitivity of multi-sensing materials based on reduced graphene oxide and natural rubber: The synergy between filler segregation and macro-porous morphology

Elastomeric conductive composites (ECCs) based on carbonaceous fillers are very attractive and play a significant role in the field of smart sensors due to their excellent flexibility, high and wide-spectrum sensitivity as well as fast response to external stimuli. In this study, a lightweight and multi-sensing composite based on reduced graphene oxide/natural rubber (rGO/NR), is fabricated by a facile and cost-effective approach that combines the rGO assembling on rubber latex particles by graphene oxide in-situ reduction and mild drying of the resulting hydrogels. The resulting composites exhibit a reliable porous structure whereas the rGO is spatially distributed in a three-dimensional segregated morphology that percolates the samples. The composites are characterized by a low percolation threshold (<0.45 vol%), high sensitivity to compression strain (gauge factor equal to 77.64), organic solvents (i.e. toluene and tetrahydrofuran) and temperature (range of detectability is 35–90 °C). The elastomeric composites are proposed for the realization of innovative multi-purpose, wide-spectrum and high-sensitivity wearable sensors for monitoring the motion and temperature of human body in real time.

High Silica Content Graphene/Natural Rubber Composites Prepared by a Wet Compounding and Latex Mixing Process.

The reduced graphene oxide (rGO) modified natural rubber composite (NR) filled with high contents of silica was prepared by a wet compounding and latex mixing process using a novel interface modifier cystamine dihydrochloride (CDHC) with coagulation ability. CDHC acts as a coagulation agent through electrostatic interaction with rGO, SiO2, and latex rubber particles during the latex-based preparation process, while in the obtained silica/graphene/natural rubber composites, CDHC acts as an interface modifier. Compared with the composites prepared by the conventional mechanical mixing method, the dispersion of both rGO and SiO2 in the composites made by a wet compounding and latex mixing process is improved. As a result, the obtained silica/graphene/natural rubber composite prepared by this new method has good comprehensive properties. A Dynamic Mechanical Test suggests that the tan δ values of the composites at 60 °C decrease, indicating a low rolling resistance with increasing the graphene content at a low strain, but it increases at a higher strain. This unique feature for this material provides an advantage in the rubber tire application.

Electric Heating Behavior of Reduced Oxide Graphene/Carbon Nanotube/Natural Rubber Composites with Macro-Porous Structure and Segregated Filler Network.

Conductive polymer composites with carbonaceous fillers are very attractive and play a significant role in the field of electric heaters owing to their lightweight, corrosion resistance, and easy processing as well as low manufacturing cost. In this study, lightweight reduced oxide graphene/carbon nanotube/natural rubber (rGO/CNT/NR) composites were fabricated by a facile and cost-effective approach, which consists of rGO assembling on rubber latex particles and hydrogels formation due to the interaction network established between carbonaceous fillers and subsequent mild-drying of the resulting hydrogels. Thanks to the amphiphilic nature of GO sheets, which can serve as a surfactant, the hydrophobic CNTs were easily dispersed into water under ultrasound. On the basis of both the high stable rGO and CNTs suspension and the assembling of rGO on rubber latex, a three-dimensional segregated network of CNT and rGO were easily constructed in macro-porous composites. Either the segregated network and macro-porous structure endowed the resulting composites with low density (0.45 g cm−3), high electrical conductivity (0.60 S m−1), and excellent electric heating behavior, when the weight content of rGO and CNTs are 0.5% and 2.5%, respectively. For electric heating behavior, the steady-state temperature of the above composites reaches 69.1 °C at an input voltage of 15 V.

Polymer-inorganic hybrid colloids for ultraviolet-assisted direct ink write of polymer nanocomposites

Inorganic-polymer hybrid colloids present a modular and tunable route to fabricate polymer nanocomposites from low viscosity precursors; however, their use in additive manufacturing remains limited. This manuscript describes photocurable “hybrid colloids” to enable layered fabrication of elastomeric nanocomposites, i.e., combination of continuous-phase photocrosslinking chemistry with hybrid colloids of water-dispersible silica nanoparticles and styrene-butadiene rubber (SBR) latex particles. Varying the relative concentrations of polymeric and inorganic particles afforded precise tuning of filler loading in the final nanocomposite and introduced a bimodal particle size distribution with desirable rheological behavior for extrusion-based additive manufacturing. Specifically, ultraviolet-assisted direct ink write (UV-DIW) processing of the photocurable hybrid colloid pastes generated free-standing green bodies, which contained a combination of SBR and silica nanoparticles. Subsequent drying of the green bodies allowed SBR particle coalescence and penetration through the scaffold and surrounding the silica nanoparticles, which yielded a semi-interpenetrating network (sIPN) nanocomposite. Facile tuning of silica concentrations in the hybrid colloid enabled tuning of both the colloidal ink rheology and mechanical properties of the final sIPN nanocomposites to achieve additive manufacturing of silica-SBR nanocomposites with ultimate tensile strains exceeding 300 % and ultimate tensile strengths above 10 MPa.

Chitin nanocrystals reticulated self‐assembled architecture reinforces deproteinized natural rubber latex film

AbstractNatural rubber (NR) latex was initially deproteinized, then, the interactions between chitin nanocrystals (ChNCs) and deproteinized natural rubber (DNR) latex particles in an aqueous system and the properties of DNR/ChNCs composites were investigated. Composite films were obtained by casting and evaporating off the prevulcanized DNR/ChNCs complexation rather than simply mixing of DNR latex and ChNCs suspension. The results of DLS and TEM confirmed that this kind of complexation was formed between negative charged DNR and positively charged ChNCs by self‐assembly. The tensile strength of composite films containing 4 wt% ChNCs was 25.00 ± 1.42 MPa, which was 3.11 multiples of the pure pre‐vulcanized DNR. DNR/ChNCs composite films could provide excellent mechanical properties and avoid allergies caused by proteins in NR latex.

Film formation process of natural rubber latex particles: roles of the particle size and distribution of non-rubber species on film microstructure

Natural rubber (NR) films from various mixing ratios of large- (LRP) and small rubber particles (SRP) were prepared through latex and solution casting methods. Film-forming behaviours of the resulting films were investigated by monitoring their surface characteristics using atomic force microscopy (AFM). When the SRP portion was higher, the surface of the latex-cast films was found to be smoother as smaller particles protruded out of the surface less than larger ones did. AFM phase micrographs revealed a hexagonal shape of packed rubber particles (RPs) of the LRP film, while the RPs in the other samples were sphere-like. After aging under ambient conditions, the packed RPs were flattened while retaining their RP boundaries, creating a supporting framework within the rubber matrix. On the contrary, when the rubber film was cast from a toluene solution, the characteristic of RP boundaries disappeared and became aggregates of the membrane components on the film surface. The latex-cast films performed much better in mechanical strength than the solution-cast films did due to the destruction of the supporting framework arisen from the non-rubber components at the RP boundaries of the solution-cast film. An inhomogeneous density distribution produced by the non-rubber aggregates in the latex-cast films was observed in small angle X-ray scattering measurements.

An anisotropic layer-by-layer carbon nanotube/boron nitride/rubber composite and its application in electromagnetic shielding.

Multifunctional polymer composites with anisotropic properties are attracting interest as they fulfil the growing demand of multitasking materials. In this work, anisotropic polymer composites have been fabricated by combining the layer-by-layer (LBL) filtration method with the alternative assembling of carbon nanotubes (CNTs) and hexagonal boron nitride flakes (hBN) on natural rubber latex particles (NR). The layered composites exhibit anisotropic thermal and electrical conductivities, which are tailored through the layer formulations. The best composite consists of four layers of NR modified with 8 phr (parts per Hundred Rubber) CNTs (∼7.4 wt%) and four alternate layers with 12 phr hBN (∼10.7 wt%). The composites exhibit an electromagnetic interference (EMI) shielding effectiveness of 22.41 ± 0.14 dB mm-1 at 10.3 GHz and a thermal conductivity equal to 0.25 W m-1 K-1. Furthermore, when the layered composite is used as an electrical thermal heater the surface reaches a stable temperature of ∼103 °C in approx. 2 min, with an input bias of 2.5 V.

Pre‐Vulcanization of Large and Small Natural Rubber Latex Particles: Film‐Forming Behavior and Mechanical Properties

AbstractThe pre‐vulcanized large rubber particle (LRP) and small rubber particle (SRP) latices are independently prepared to investigate their film‐forming process and mechanical properties after being cast into films. The surface morphologies and roughness of both LRP and SRP films are found to be dependent on crosslink densities. The networks inside each rubber particle (RP) restrict particle deformation resulting in residual contour of RP within the film surface. For highly crosslinked RP, the collapse of the top surface of the RPs in the LRP films appears to create many “crater‐like” structures within the film surfaces, while they present only protruding particles within the SRP and blend films. This seems to indicate that LRPs are easier to coalesce and form film than SRPs. Additionally, dynamic and mechanical properties and strain‐induced crystallization (SIC) behaviors of the latex films, are effectively enhanced after pre‐vulcanization. The pre‐vulcanized LRP films perform better tensile properties and SIC than the SRP can.

Enhancing the EMI shielding of natural rubber-based supercritical CO2 foams by exploiting their porous morphology and CNT segregated networks.

Natural rubber/carbon nanotubes composite foams (F-NR/CNTs) with high electrical conductivity and excellent electromagnetic interference (EMI) performance were developed through a multi-step process including: (a) CNTs assembled on natural rubber latex particles, (b) pre-crosslinking of natural rubber, (c) supercritical carbon dioxide foaming of pre-crosslinked composite samples and (d) post-crosslinking of foamed composite samples. A closed-cell porous structure and a segregated CNT network are clearly observed in the resulting foams. Due to this morphology, F-NR/CNTs exhibit low density, good mechanical properties, and high electrical conductivity. Owing to the multiple radiation reflections and scattering between the cell-matrix interfaces, the composite foams presented an excellent specific shielding effectiveness (SSE) of 312.69 dB cm2 g-1 for F-NR/CNTs containing 6.4 wt% of CNTs, which is significantly higher than those already published for rubber composites containing comparable filler content. Furthermore, the analysis of EMI SE highlights that absorption efficiency is more significant than reflection efficiency, implying that most of the incident electromagnetic radiation is dissipated in the form of heat. This work provides the fundamentals for the design of innovative light weight and efficient EMI shielding foams characterized by a three-dimensional segregated CNT network with huge potential for use in the electronics and aerospace industries.

Simultaneous reduction and surface functionalization of graphene oxide and the application for rubber composites

ABSTRACTThe simultaneous reduction and functionalization of graphene oxide (GO) was realized through a chemical grafting reaction with a functionalization agent N,N‐bis(3‐aminopropyl)methylamine (APMEL). The reduced and functionalized reduced GO (rGO‐APMEL) sheets can be well dispersed in water without any added surfactant and the formed stable rGO aqueous dispersion can be kept for a long time, which can be used for the preparation of rubber–graphene (GE) composites by latex mixing. The electrostatic interaction between rGO–APMEL (positively charged) and natural rubber latex particles (negatively charged) leads to the formation of NR/rGO–APMEL composites with strong interaction. Compared with blank NR, the tensile strength and modulus for NR/rGO–APMEL increase with the rGO–APMEL loading. Especially, when the filler content is 5 phr, the tensile strength of NR/rGO–APMEL‐5 increases by 32.7%, as a control the tensile strength of NR/GO‐5 and NR/rGO‐5 decrease by 20.1 and 15.6%, respectively. The entanglement‐bound rubber tube model was used to analyze the reinforcing effect of GE on NR/rGO–APMEL nanocomposites at a molecular level. This study may provide us a novel approach to prepare well dispersed and exfoliated rGO–polymer nanocomposites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47375.

Purification and characterization of small and large rubber particles from Hevea brasiliensis.

Natural rubber (NR) is synthesized by the rubber transferase (RTase) on rubber particles (RPs) in latex. Due to the heterogeneity of the RPs in latex, it is difficult to precisely characterize the RTase activity. In this study, we separated the RPs of Hevea brasiliensis with different particle size distributions, via stepwise centrifugations. Analyses of protein compositions and size distributions of NR in the RPs suggest that RPs in Hevea latex can be categorized into two distinct subclasses, the larger RPs (termed 1kRP, 2kRP, and 8kRP) and the smaller RPs (termed 20kRP and 50kRP). Precise enzymatic assays using the RPs revealed that 50kRP showed the highest RTase activity, whereas the larger RPs, which had been regarded to have quite low activity, also exhibited a comparable activity to the smaller RPs. Immunological detections of cis-prenyltransferases in the RPs showed that the abundance of these enzymes correlates with the extent of RTase activity.

Alternative Methods for Accelerating Mechanical Stability of Concentrated Natural Rubber Latex

The mechanical stability time (MST) of latex is an important parameter indicating the colloidal stability of concentrated natural rubber latex (CNRL). This stability is crucial for the transport and storage of CNRL before the product forming processes. A popular method to increase the MST of CNRL in Thailand is by adding laureth soap or other surfactants. Nevertheless, the laureth soap provides a steady but slow increase in the MST without reaching a stable value after 30 days. At the same time, the phospholipids on the surfaces of natural rubber latex particles will naturally undergo hydrolysis reaction leading to even higher MST. For long storage, too high MST leads to coagulation problem in the product-forming step. Therefore, alternative methods were studied in this research to expedite phospholipid hydrolysis, which results in the increase in MST for CNRL without causing problems from excessive negative ions in the product-forming process. The alternative methods including heating in the presence of metal chips and the use of enzyme lipase were compared to the popular laurate soap addition method. It was found that lipase addition was the most effective method to expedite the hydrolysis of phospholipids to reach the standard MST value within 3 days, and reach a plateau within 10 days.

Elastomer Reinforced with Regenerated Chitin from Alkaline/Urea Aqueous System

Novel hybrid elastomer/regenerated chitin (R-chitin) composites were developed, for the first time, by introducing chitin solution (dissolved in alkaline/urea aqueous solution at low temperature) into rubber latex, and then cocoagulating using ethanol as the cocoagulant. During the rapid coprecipitation process, the chitin solution showed rapid coagulant-induced gelation and a porous chitin phase was generated, and the rubber latex particles were synchronously demulsificated to form the rubbery phase. The two phases interlaced and interpenetrated simultaneously to form an interpenetrating polymer network (IPN) structure, which was evidenced by SEM observation. The ensuing compound was also characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), and swelling experiments. The unique porous structure of R-chitin could result in strong physical entanglements and interlocks between filler and matrix, thus a highly efficient load transfer between the filler and the matrix was achieved. Accordingly, R-chitin endows the elastomer with a remarkable reinforcement. We envisage that this work may contribute new insights on novel design of chitin-based elastomer hybrids with IPN structure.

Cytotoxicity and anticancer activity of natural rubber latex particles for cancer cells

To broaden the knowledge of cytotoxicity of natural rubber latex (NRL) nanoparticles we for the first time examined the latex biocompatibility in vitro against a panel of cancer cells (A549, A2780, and MDA-MB-231). Owing to fractionation of NRL nanoparticles by ultra-centrifuge, the effect of the non-rubber constituents (intermediate of 5.8 wt% and sediment of 0.2 wt%) on the cytotoxicity was clarified. For intermediate constituent, the half maximal inhibitory concentration (IC50) values at 24 h was 1.05 mg/mL for A549 cells, which was one order of magnitude higher in toxicity as compared to that for A2780 (0.24 mg/mL) and MDA-MB-231 (0.36 mg/mL) cells. In addition, profound studies including cell cycle arrest abilities and apoptosis induction profiles against cancer cells were discussed in detail. It was found that the constituents exhibit some significant effect on the cell cycle arrest and trigger apoptosis for A2780 cells. This effective apoptosis induction profiles was more prominent in MDA-MB-231 cells incubated with NRL nanoparticles and sediment loading conditions. The percentage of apoptotic cells was ca. 6–8% of the total cells.

Reinforcement effect of soy protein nanoparticles in amine-modified natural rubber latex

Mechanical properties of natural rubber reinforced with soy protein nanoparticles are useful for various rubber applications. However, the properties is further improved by improving interactions between soy protein and rubber. A novel method is used to modify particle surface of natural rubber latex by grafting with diallylamine. The improved polymer-filler interactions caused tensile stress at 300% elongation to match that of carbon black, which is a breakthrough for organic fillers. The grafting was demonstrated with infrared and particle size measurements. The size of natural rubber latex particles increased 8% after the grafting and the infrared spectra showed an increase of amine groups in coagulated NR latex. The curing rate of modified NR reinforced with soy protein matches that of carbon black composite. With the addition of 10% soy protein nanoparticles, the tensile strength of natural rubber and modified natural rubbers increased from 9 to 15MPa to 19–25MPa. Both dynamic mechanical strain and swelling measurements indicate modified natural rubber composites have a higher degree of polymer-filler interaction. The change of reinforcement factors with filler fraction can be described by modified Mooney model that includes anisotropic reinforcement elements. Tanδ at 60°C indicates the modified composite has a better rolling resistance than carbon black.