Molecular structure and storage hardening of natural rubber: Insight into the reactions between hydroxylamine and phospholipids linked to natural rubber molecule

The effect of hydroxylamine on the molecular structure and storage hardening of natural rubber (NR) was investigated by the treatment of deproteinized NR (DPNR) latex with hydroxylamine. The hydroxylamine treatment decreased the content of long‐chain fatty acid ester groups in DPNR from about 2–0.7 mol per rubber molecule. The molecular weight and molecular weight distribution changed apparently after treatment with hydroxylamine. The relative intensity of the ¹H NMR signals corresponding to phospholipids at the α‐terminal group decreased after the hydroxylamine treatment. The Huggins ‘k’ constant of hydroxylamine‐treated DPNR showed the liberation of linear rubber molecules caused by decomposition of branch points derived from phospholipids. The absence of storage hardening in hydroxylamine‐treated DPNR was observed to be caused by not only the reaction of hydroxylamine and aldehyde groups but also the removal of phospholipids as well as the breakdown of phospholipid aggregations as a result of hydroxylamime, contributing to the establishment of a newly proposed mechanism of hydroxylamine on the inhibition of storage hardening in NR. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43753.     

Reducing and stabilizing the viscosity of natural rubber by using sugars: Interference of the Maillard reaction between proteins and sugars

This work is an extension of previous work elucidating the reduction and stabilization of solid natural rubber (NR) viscosity by using sugars. Various amounts of glucose, fructose, sucrose, and maltose were incorporated into fresh NR (FNR), deproteinized NR (DPNR), and synthetic polyisoprene (IR) latexes. The results revealed that all sugars cannot decrease the Mooney viscosity of FNR, while only monosaccharides, that is, glucose and fructose, can significantly decrease the Mooney viscosity of both DPNR and IR by way of a lubrication mechanism. The proteins in FNR can diminish the capability of glucose and fructose to decrease the Mooney viscosity. Furthermore, glucose was found to reduce the occurrence of storage hardening in DPNR by interacting with polar groups of phospholipids at the rubber chain ends. Measurements of browning intensity as well as analysis of Maillard reaction products together with the NR protein–glucose model compound were utilized to confirm that the reduction and stabilization of the viscosity of NR using monosaccharides were interfered by the Maillard reaction between the proteins in NR and the monosaccharides.     

Controlled degradation of cured natural rubber by encapsulated benzophenone as a photosensitizer

The photodegradation of raw natural rubber and natural rubber compound film were studied using an artificial solar energy simulator. The properties of degraded rubber sheets containing benzophenone (BP) were determined by solution viscosity, ¹H‐NMR, and FTIR analyses. In the case of rubber compounds containing BP, the changes of tensile strength and crosslinking density were determined. It was found that BP could amply accelerate the photodegradation of rubber. To control the release rate of BP, it was necessary to encapsulate BP with urea–formaldehyde as a matrix. The encapsulated BP or capsule was formed by an interfacial polycondensation reaction between formaldehyde and urea. The kinetic of release rate of BP from urea–formaldehyde capsule was markedly observed within 15 days of release time; after that the rate of BP released from urea–formaldehyde microcapsule was very slow. At the same concentration of BP, the degradation rate of rubber compound by adding BP directly was faster than that of the rubber containing encapsulated BP. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 297–305, 2003     

Compatibility enhancement of silica and natural rubber compound using UVA-induced silane-grafted saponified skim natural rubber

Silane coupling agents are potential reagents widely used to improve the compatibility between silica and less polar rubber, especially natural rubber (NR). Nevertheless, high temperature is generally required to generate the interaction between the components during the mixing process. Accordingly, an alternative method by grafting the silane coupling agent onto the rubber molecules would be a desirable approach to develop a compatibilizer for the silica-filled NR compound. In this work, skim NR was used as a starting material due to its linear structure. The optimal conditions of the grafting reaction were found to be 1 phr of an alkoxy silane and 5 phr of benzoyl peroxide under 8 min of UVA irradiation time. These conditions were applied for producing the rubber material used in the mixing process of STR 5L and silica. The cure characteristics, silica dispersion and mechanical properties of the rubber compounds were improved, suggesting that the modified rubber was an efficient material for increasing the compatibility between silica and NR.     

Hyperdeproteinized natural rubber prepared with urea

Preparation of hyperdeproteinized natural rubber was made from fresh latex and preserved high‐ammonia latex by treatment with urea in the presence of sodium dodecyl sulfate. Concentration of urea, temperature, and time for the incubation were investigated to remove the proteins effectively. Under the best conditions, the total nitrogen content and amount of allergenic proteins for the deproteinized rubbers were 0.005 wt % and 1.0 μg/ml, respectively, which were less than those of natural rubber deproteinized with proteolytic enzyme. The hyper‐deproteinized natural rubber was proved through FT IR spectroscopy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 555–559, 2004     

Characterization of biodegradable semi‐interpenetrating polymer based on poly(vinyl alcohol) and sodium alginate containing natural neem (Azadirachta indica) for controlled release application

A biodegradable novel semi‐interpenetrating polymer network based on poly(vinyl alcohol) (PVA) and sodium alginate containing neem (Azadirachta indica) in the presence of azadirachtin‐A (neem Aza‐A) as well as glutaraldehyde as a crosslinking agent was prepared for use in the controlled released of neem Aza‐A. This is necessary because neem Aza‐A is not stable in the environment. The neem Aza‐A‐containing beads were prepared using various experimental parameters, such as the extent of crosslinking and the amount of loading, in order to optimize the process variables. The chemical structure of the capsule wall was evaluated through X‐ray diffraction. In addition, the swelling behaviour of the capsules and their thermal stability were investigated. The strength of the capsule wall depended on the PVA in the matrix and the crosslinking density. Scanning electron microscopy, electron probe microanalysis and atomic force microscopy data indicated that the structure of the bead walls is rough and nonporous. Swelling results indicated that swelling of the polymeric beads decreases with increasing exposure time to the crosslinking agent. At particular intervals, the remaining concentration of neem Aza‐A was analysed using high‐performance liquid chromatography. The release data were fitted to an empirical equation to estimate the kinetic parameters. The degree of release of neem Aza‐A was controlled by the parameter conditions. Copyright © 2010 Society of Chemical Industry     

Cationic cyclization of purified natural rubber in latex form with a trimethylsilyl triflate as a novel catalyst

Cyclization of deproteinized natural rubber (DPNR) or purified natural rubber latex was effectively performed in latex phase by using trimethylsilyl‐trifluoromethane sulfonate or trimethylsilyl triflate (TMSOTF) as a novel catalyst, which is still not reported in the case of natural rubber latex. Various cyclization conditions affecting the degree of cyclization were studied, such as dry rubber contents, temperature, TMSOTF concentrations, and time. The cyclized products were characterized by FTIR, Raman, ¹H‐, and ¹³C‐NMR spectroscopies, as well as DSC and TGA. The degree of cyclization was estimated by ¹H‐NMR spectrum. It was found that the degree of cyclization in NR was a function of cyclization conditions. The thermal stability of cyclized DPNR increased with the degree of cyclization. Solubility of the obtained rubber was good in chloroform, toluene, cyclohexanone, and cyclohexane, and bad in tetrahydrofuran. The average number molecular weight of cyclized DPNR with 76% degree of cyclization was about 4.2 × 10⁴ g/mol. On the basis of FTIR, Raman, ¹H‐, and ¹³C‐NMR, the C?C of cyclized DPNR dramatically decreased after prolonged reaction time. In addition, the topology of cyclization DPNR particles was rough on its rubber particle as analyzed by TEM. The mechanism for this reaction will also be discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007.     

Development of a controlled release neem capsule with a sodium alginate matrix,crosslinked by glutaraldehyde and coated with natural rubber

At present, the use of huge quantities of synthetic pesticides in conventional agriculture has lead to some major environmental problems. Natural pesticide are now being developed to avoid such problems. Neem (Azadirachtin A) seed oil hereafter designated as neem Aza-A, is one such natural pesticide known to be a powerful insect antifeedant and growth-regulating substance yet limited in application because of its rapid degradation in the environment. Therefore, encapsulation of neem Aza-A within membranes to control its release and improve its stability in the environment may improve its effectiveness. Controlling the release of the pesticide was achieved by utilization of glutaraldehyde–alginate gel capsules modified by coating with a natural rubber (NR) layer. The optimization of the properties of the neem Aza-A containing beads was achieved by changing variables such as the extent of crosslinking, the amount of loading and NR layer. The SEM data indicated that the walls of the beads are smooth and nonporous. The swelling results indicated that swelling of the polymeric beads decreased with increasing exposure time to glutaraldehyde and reduced the rate of release of the pesticide. The data on the rate of release of neem Aza-A from the differently prepared capsules into an aqueous environment was analyzed by HPLC and fitted into an empirical equation to estimate the kinetic parameter. The degree of release of neem Aza-A from capsules was controlled by their condition of formation.     

In vitro synthesis of high molecular weight rubber by Hevea small rubber particles

Hevea brasiliensis is one of few higher plants producing the commercial natural rubber used in many significant applications. The biosynthesis of high molecular weight rubber molecules by the higher plants has not been clarified yet. Here, the in vitro rubber biosynthesis was performed by using enzymatically active small rubber particles (SRP) from Hevea. The mechanism of the in vitro rubber synthesis was investigated by the molecular weight distribution (MWD). The highly purified SRP prepared by gel filtration and centrifugation in the presence of Triton^® X-100 showed the low isopentenyl diphosphate (IPP) incorporation for the chain extension mechanism of pre-existing rubber. The MWD of in vitro rubber elongated from the pre-existing rubber chains in SRP was analyzed for the first time in the case of H. brasiliensis by incubating without the addition of any initiator. The rubber transferase activity of 70% incorporation of the added IPP (w/w) was obtained when farnesyl diphosphate was present as the allylic diphosphate initiator. The in vitro synthesized rubber showed a typical bimodal MWD of high and low molecular weight fractions in GPC analysis, which was similar to that of the in vivo rubber with peaks at around 10⁶ and 10⁵ Da or lower. The reaction time independence and dependence of molecular weight of high and low molecular weight fractions, respectively, indicated that the high molecular weight rubber was synthesized from the chain extension of pre-existing rubber molecules whereas the lower one was from the chain elongation of rubber molecules newly synthesized from the added allylic substrates.