Molecular Weight Natural Rubber Research Articles

Structure – property relationships of different natural rubber grades

The effects of various natural rubber (NR) grades, namely RSS 3, STR 5L, STR 10, STR 20 and STR 20CV, towards their final properties were investigated. The molecular characteristics of these NR grades were assessed by gel permeation chromatography and by determining protein contents. The highest molecular weight was found for RSS 3, while STR 20CV had the lowest molecular weight. No relationship between molecular weight and protein content was seen in this study. A higher molecular weight rubber provided greater maximum torque and torque difference, and a better resistance to degradation during processing. However, no significant difference was found among molecular weights higher than 1.25 × 106 g/mol. The final torque during processing and the Mooney viscosity of rubber increased with molecular weight due to tie chain entanglements, as later confirmed by a dynamic mechanical analyzer. Thermo-stability and stress relaxation of the various NR grades were also largely governed by their molecular weights. Stability of rubber at an elevated temperature improved with molecular weight. Also, the tensile properties and the strain-induced crystallization varied with the molecular weight. The tensile strength of various NR grades varied from 15 to 18 MPa while the elongation at break was 600%–800%. The molecular weight of NR was found to have a great effect on the processing, thermomechanical, dynamic, and mechanical properties of the vulcanizates. In addition, a narrow molecular weight distribution also affected the mechanical properties. The crystallinities of the NRs during stretching were closely related to their tensile behavior, and the alignment of crystal structures during stretching was almost parallel to the stretching direction in all NR samples. The molecular weight of NR appears to be a key factor associated with the mechanical properties and reinforcement behavior of vulcanizates.

Characterization of natural rubber concerning its components and molecular weight in Taraxacum kok-saghyz Rodin using pyrolysis gas chromatography-mass spectrometry.

Taraxacum kok-saghyz Rodin (TKS) has abundant natural rubber in its root and the molecular weight of its natural rubber is higher than that in Hevea brasiliensis. Thus, TKS is an excellent alternative for the commercial production of natural rubber. The content and molecular weight of natural rubber are two qualitative indicators. Efficient determination for both indicators is still a challenge. In this study, we developed a method to simultaneously determine the content and molecular weight of natural rubber in TKS with pyrolysis－gas chromatography-mass spectrometry. The content of natural rubber was quantified by internal standard method. We optimized the pyrolysis temperature and chromatographic method during content determination. The limits of detection and quantification were 0.47 and 1.56μg, respectively. In addition, the arachidonic acid methyl ester, an unsaturated fatty acid proposed from the α-end group of natural rubber, was quantified to obtain the number of natural rubber polymers. Based on the content and the polymer number, we also quantified the molecular weight of natural rubber. Thus, the content and molecular weight of natural rubber were simultaneously determined in TKS. Our study provides a new perspective for the high throughput analysis of natural rubber.

NOVEL METHOD OF BREAKING DOWN RUBBER MOLECULES USING PEPTIZER AND THE PROPERTIES OF RESULTANT NATURAL RUBBER SOLVENT-BASED ADHESIVE

The huge cost of energy associated with the breaking down of high molecular weight rubber during adhesive production has placed a demand for alternative processing technology. In this study, the use of benzoyl peroxide as a peptizer to lower the molecular weight of natural rubber has been reported. The properties of natural rubber-based solvent adhesive produced by mechanical mastication and peptizer were also compared. Natural rubber (NR) was masticated in a two-roll mill at 80oC for 30 mins and was processed into adhesive using xylene as solvent. Similarly, adhesives were formulated by chemical break down of the heavy natural rubber molecules using benzoyl peroxide as peptizer. The peptizer loading was varied from 0.2 – 1.0 parts per hundred rubber (phr) of rubber at 100oC in a water bath while other compounding additives were kept constants for all samples under study. The adhesives produced were subjected to the following test: viscosity, lap shear test, T-peel test and drying time test. Other properties studied are weathering resistance, heat resistance and effect of moisture on the adhesives bond on different substrates. The result shows that the viscosity of the adhesives increases with increasing peptizer loading and the masticated sample has a viscosity slightly lower than that of 0.8phr and 1.0phr peptizer loading. Furthermore, T-peel test shows that 0.4 phr peptizer loading has a better average T-peel strength for the bonds while the masticated sample had the least average T-peel strength when compared with all samples. The sample with 1.0phr peptizer loading had the least decrease in shear strength while the 0.2 peptizer loading had the maximum decrease in shear strength. Finally, the FT-IR spectra shows similarities in spectra but with difference in the intensity of the peaks, indicating the ability of the benzoyl peroxide to effectively oxidize the natural rubber as compared with the mechanical masticated sample. The peak of 1080 cm-1 in the peptized sample indicates a major difference with the spectra of mechanical masticated sample. Generally, the benzoyl peroxide effectively lowered the molecular weight of the natural rubber which is evident in the viscosity, reduced process time and ease of the adhesive formulation.

Improved adhesion properties of natural rubber-based pressure-sensitive adhesives by incorporating particulate fillers

A new concept of producing natural rubber (NR) based pressure-sensitive adhesive (PSA) using solid particles relatively at a low-cost as tack enhancer has been proposed. The effect of tack enhancer (i.e., xyloglucan (XG), rice flour (RF), calcium carbonate (CaCO3), and silica (SiO2)) on the adhesion properties of the PSA was investigated. Moreover, stress relaxation, tan δ, initial plasticity (Po), and molecular weight of NR which relates to the adhesion properties were also determined. The study found that XG, RF, and SiO2 can be used as tack enhancers for NR-based PSA. The obtained PSA from those tack enhancers can adhere and peel off on a polypropylene (PP) substrate without any adhesive residue. An increase in the adhesion properties of NR-based PSA can be explained by the effect of fillers on the viscoelastic properties and molecular weight of the PSA. Stress relaxation and molecular weight of the NR-based PSA were significantly found to be decreased, while the tan δ was increased by the addition of particulate tack enhancers compared to the masticated neat NR. NR-based PSA with the highest peel strength (2.11 × 102 N/m) was successfully developed by using XG (40 phr). The NR-based PSA with XG was also showed the ability to resist the unfavorable aged hardening of the NR-based PSA in 180 days.

Tensile and structural properties of natural rubber vulcanizates with different mastication times

Mastication reduced the molecular weight of natural rubber (NR). This would affect the tensile properties and strain-induced crystallization of the rubber vulcanizates due to the structural changes of the rubber molecules. In this study, influences of mastication time on tensile response, deformation-induced crystallization, and structural effects of crosslinked NR were investigated. The crystallization behavior and structural changes during stretching were studied by means of wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS). Increased mastication time significantly affected modulus at specified strain and upturn point of strain-induced crystallization of the crosslinked samples while the tensile strength was influenced slightly by mastication. During stretching, degree of crystallinity at given strain was found to decrease with increasing mastication time, while the crystallite size was reduced. Moreover, the size of crosslinked network structures induced by crosslinking also decreased slightly with increasing mastication time, as suggested by SAXS measurement.

Molecular Weight of Liquid Natural Rubber (LNR) Product from the Chemical Depolymerization Process of High Molecular Weight Narutal Rubber Latex

The molecular weight of natural rubber varies greatly, from only a few hundred thousand to more than one million. Liquid natural rubber (LNR) products generally use natural rubber in the form of latex with a relatively low molecular weight, which ranges from a molecular weight of 600,000 g / mol. This study aims to determine the molecular weight of LNR products made from the chemical depolymerization process of natural rubber latex with a molecular weight of more than one million. The natural rubber used latex with 20% Dry Rubber Content (KKK). The natural rubber depolymerization process used NaNO2 and H2O2 degrading agents with a CoCl2 catalyst at 80 °C. H2O2 levels were kept constant at 1 parts per hundred rubber (phr), and NaNO2 levels vary at 0; 2; 3; and 4 phr. After the reactor temperature reaches 80°C, the CoCl2 catalyst was added, which level varied at 0; 1; 3; and 5 phr. The reaction time was varied, which 7, 8, and 9 hours. The results of LNR product was purified to form a semi-gel, and molecular weight was analyzed using gel permeable chromatography (GPC). The results showed that the lowest molecular weight of the resulting LNR product was 108,335 g / mol from the initial molecular weight of 1,384,638 g / mol.

Ultrasonic Extrusion of NR Gum and Its Effect on the Structure and Properties of Unfilled and Silica‐Filled NR

Natural rubber (NR) has been extruded without and with ultrasonic treatment at various ultrasonic amplitudes. The effect of ultrasonic treatment on structure of NR, its compounds with silica and silica/silane and their vulcanizates have been studied. Processing‐structure‐properties relationship of untreated and treated materials has been established. With the increase of ultrasonic amplitude power consumption is increased and die pressure is reduced. Size exclusion chromatography measurements show that the NR molecular weight is reduced with the ultrasonic treatment. The storage modulus and loss modulus, complex viscosity and loss tangent results also confirmed the degradation of NR chains by imposition of ultrasound, especially after treatment at high amplitude. After mixing with silica and silica/silane, it was found that degradation of NR caused a reduction in the rubber–filler interaction, an increase in the filler–filler interaction and flocculation upon heating. Physical properties of vulcanizates including M100, ultimate elongation, and stress at break are also deteriorated. POLYM. ENG. SCI., 60:925–934, 2020. © 2020 Society of Plastics Engineers

Quantitation of isoprenoids for natural rubber biosynthesis in natural rubber latex by liquid chromatography with tandem mass spectrometry

The natural rubber molecule is one of the end products of isoprenoids metabolism in the plant. Dimethylallyl diphosphate (DMAPP) and farnesyl pyrophosphate (FPP) are two typical isoprenoids which control the rate of biosynthesis and the molecular weight of natural rubber. A rapid, nonradioactive method for quantitation of DMAPP and FPP in natural rubber latex by liquid chromatography tandem with mass spectrometry (LC–MS/MS) was reported. DMAPP and FPP were determined in the multiple reaction monitoring mode(MRM)followed by separation with a silica-based C18 column. The external standard quantitative method was established, and the results showed limits of quantitation (LOQs) were 28 ng/ml and 33 ng/ml for DMAPP and FPP, respectively. The concentrations were detected 70–96 ng/ml and 242–375 ng/ml for these two isoprenoids in natural rubber latex. Recoveries of the method were in the range 81–93%. Daytime comparison experiments found that FPP had better stability than DMAPP.

Biodegradation of unvulcanized natural rubber by microorganisms isolated from soil and rubber surface: A preliminary study

ABSTRACTThe Natural rubber (NR) biodegradation by three microorganisms has been evaluated: a yeast (Rhodotorula mucilaginosa) and a bacterium (Pseudomonas sp.), isolated in a liquid culture from soil, and a filamentous fungus (Alternaria alternata), isolated on a solid culture from an NR surface, were tested. The biodegradation was conducted for four months in liquid culture, at 30°C, in agitated and stationary conditions, using a Mineral Salt Medium with NR as the only carbon source. The growth behaviour of the yeast and the bacterium was evaluated by means of optical density measurements (OD650). At the end of the incubation, the dry weight biomass of the microorganisms was measured. R. mucilaginosa showed a higher biomass production in the agitated culture, while a more efficient production was observed in static conditions for the Pseudomonas and A. alternata strains. The highest enzymatic activity of Lignin peroxidase (LiP) and Manganese-dependent peroxidase (MnP) was obtained in static conditions for A. alternata. The laccase production was probed by guaiacol oxidative polymerization on agar plates. The microorganism biodegradation capability was assessed through a combination of SEM analysis, FTIR-ATR spectroscopy, and Size Exclusion Chromatography techniques. An extended mycelium-substrate interphase and a decrease in the NR molecular weight were observed.

The rheological properties of masticated natural rubbers and their linear viscoelastic predictions

Premastication of natural rubber (NR) is a common process to ease the incorporation of fillers and chemicals. This research studied the changes in molecular structure, molecular weight (MW), and linear viscoelastic (LVE) properties of the standard Thai NR block grade STR5L, which is the low contamination grade, after mastication for 20 min at 40, 70, and 100 rpm, respectively. The MWs of unmasticated and masticated NRs were quantified by size exclusion chromatography. The “dual constraint” model, which is known to provide good predictions of the LVE properties from the MW and molecular weight distribution (MWD) of linear polymers, was used to correlate the MWs of NRs to their LVE properties. It was found that the dual constraint model could predict the dynamic moduli of NRs masticated at high speeds (70 and 100 rpm), indicating that the rheological behavior of the masticated NRs at 70 and 100 rpm conformed to linear polymers. On the other hand, the predictions for the unmasticated NR and the NR masticated at a lower speed of 40 rpm (NR40 rpm) deviated from the experimental values, especially in the terminal zone. This deviation might be due to the presence of physical gel and microgel in NR, which might still exist at mastication temperatures below 100 °C.

Chemical Depolymerisation of Natural Rubber in Biphasic Medium

The molecular weight of natural rubber (NR) can be reduced via depolymerization reaction to produce liquid natural rubber (LNR) with a molecular weight less than 50 000 g/mol. In the reaction, hydrogen peroxide and sodium nitrite were added to natural rubber latex to initiate a redox type reaction which then breaks the NR chain. Low permeation of reagents into latex particles allows the degradation to occur greater at the latex particle surface relative to the inner core contributes to high molecular weight distribution (MWD) or polydispersity of the LNR obtained. In this recent works, the reaction was carried out in a biphasic medium consisting of water and toluene phases. Toluene swells latex particles as indicated by the SEM micrographs showing changes in the size of latex particles. This occurrence is suggested to increase the influx of reagents into the latex particles. Consequently, with higher permeation of reagents into the latex particles resulted in the decrease of molecular weight and lower polydispersity of the LNR obtained. Chemical structure analysize showed that the LNRs obtained were attached with hydroxyl and carbonyl groups.

Study on Molecular Weight of Natural Rubber with Different Initial Moisture Contents Dried by Microwave

In this article, the molecular weight of wet Natural Rubber (NR) with different initial moisture contents, 3.80%, 10.05%, 19.76%, respectively, dried by microwave at frequency of 2450MHz was investigated using GPC. It can be seen from the results that an obvious trend of Mnand Mw changes was presented. With the increment of initial moisture contents, Mnand Mw decreased gradually. The molecular weight distribution (d) increased with the initial moisture contents increment, but quite closer to each other.

Natural Rubber-Toughened Polystyrene: Effects of Mixing Procedure and Maleic Anhydride Content on Impact Property and Phase Morphology

The toughening of polystyrene (PS) by melt blending with natural rubber (NR) was prepared in a brabender plasticorder. Two kinds of processing had been used: (a) one-step mixing in which the three components of PS (60 wt%), NR (40 wt%) and maleic anhydride (MA) were simultaneously introduced in the mixer; (b) two-step mixing in which NR and MA were separately premixed before the final mixing with PS. The effect of MA loading (3 and 5 phr) on gel content, impact energy and morphology of PS-NR-MA blend were further investigated. The results showed that the incorporation of NR into PS increased the impact energy of the blend. The results also showed a significant effect of the different mixing procedures on the properties of PS-NR-MA blend. High contents of unreacted MA and gel were found in the blend obtained by the one-step mixing, whereas the blend prepared by the two-step mixing showed lower values in both unreacted MA and gel content. On the other hand, the impact energy of the blend prepared by one-step mixing was slightly higher than that of the blend prepared by two-step mixing. This is due to the decreased molecular weight of NR in the formerly prepared MNR of the two-step mixing. For the effect of MA loading, the decrease of unreacted MA content, gel content and rubber particle size were found at low concentration of MA.

Effect of Microwave Drying on Structure and Properties of Natural Rubber

The morphological structure and properties of wet natural rubber (NR) granulates dried by microwave were studied, which compared with wet NR granulates dried by hot-air. The results show that the molecular weight of NR dried by microwave is higher than that of NR dried by hot-air, and the molecular weight distribution of NR dried by microwave is narrower than that of NR dried by hot-air. The wet NR granules dried by microwave are rough on surfaces and the cutting traces produced during the processing remained on surfaces; whereas the wet NR granules dried by hot-air are smooth on surfaces resulted from melt adhered, and the cutting traces disappeared; the thermal-oxidative resistance of NR vulcanizates dried by microwave improved significantly.

Effect of Molecular Weight of Natural Rubber on the Compatibility and Crystallization Behavior of LLDPE/NR Blends

The compatibility and crystallization behavior of LLDPE blended with the three different molecular weights of NR were studied. The obtained single derivative decomposition peak from TGA measurements for LLDPE blended with the lowest molecular-weight natural rubber clarified the improved compatibility between LLDPE and NR. The reduction of the crystallinity and the change of α-relaxation temperature of LLDPE in the blend were significant. It is interesting that the mechanical properties including the elongation at break and the impact strength of LLDPE were found to be enhanced by adding this low molecular-weight NR without using a compatibilizer.

Natural rubber–isotactic polypropylene thermoplastic blends

AbstractThermoplastic elastomers, prepared by melt blending of natural rubber (NR) and isotactic polypropylene (PP) through a dynamic vulcanization technique, were developed during the later 1970s. However, they have certain drawbacks due to thermal degradation and higher molecular weight of NR. In the study reported here, NR was masticated to different levels prior its addition to isotactic polypropylene to improve the flow properties and to reduce the incompatibility resulting from molecular weight mismatch of NR/PP thermoplastic blends. Mixing energy curves of uncrosslinked blends and those of dynamically vulcanized blends crosslinked using different cure systems were compared. The mixing energy curves of blends containing NR of different molecular weight (Mn) and two grades of PP (injection and film grades) were also compared. Technological and processing properties of the dynamically vulcanized (sulphur and peroxide cure systems) and unvulcanized blends were compared with those of the samples containing unmasticated NR. The results indicated that a number average molecular weight in the range 4 × 105 for NR increased the procoessability without significantly affecting the technological properties of NR/PP thermoplastic blends. Among the three cure systems studied Luperox 101 and dicumyl peroxide gave better technological properties than the sulphur‐cured samples. Two antioxidants, viz. quinoline (TDQ) and imidazole (MBI) type, were tried in NR/PP blends. It was found that TDQ imparts better aging resistance compared to MBI. The improvement in processability due to the reduction in molecular weight of natural rubber by mastication is more noticeable in the case of peroxide vulcanized blends compared to sulphur vulcanized samples. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2063–2068, 2004

Structure and Biosynthesis Mechanism of Rubber from Fungi

Abstract The structure of both terminal units and alignment of the isoprene units give conclusive evidence for the initiation and termination mechanisms of rubber formation in fungi. As shown in Figure 4, the polymerization is estimated to start from trans, trans-farnesyl pyrophosphate and proceeds by successive condensation of IPP to form isoprene units in the cis configuration. The termination is presumed to occur by direct esterification of polyisoprenyl pyrophosphate (PIP-PP) or dephosphorylation of PIP-PP followed by esterification with fatty acids. It is remarkable that the hydroxyl terminal group corresponding to the saponification product from the esterified terminal group was observed in the rubber samples by extraction from sporophores of L. volemus after storage for 2–3 days in a refrigerator without the addition of salt. In these samples, unidentified 13C-NMR signals were also detected. These findings suggest that the direct esterification of PIP-PP is more likely as a termination reaction. It also suggests the occurrence of a chemical or biochemical modification of both terminal groups and the main chain during storage. This is the first information on the termination mechanism of controlling the molecular weight of natural rubber.

The influence of the relative molecular weight of natural rubber on its thermo‐oxidative stability

AbstractThe influence of relative molecular weight on rubber thermo‐oxidation in air at 130 ± 1°C has been studied by means of infrared spectroscopy. The carbonyl group formation in single fractions of natural rubber has been measured. It was found that the development of kinetic curves ΔACO = f(t) corresponds for both fractionated and unfractionated extracted samples. Kinetic curves of natural rubber formation are characterized by an induction period τ. Its magnitude is a function of molecular weight up to the value [η] = 0.2 m3/kg, and then stays invariant to changes in [η]. In the range of low values of the internal viscosity up to 0.2 m3/kg, the dependence τCO = f[η] has an exponential character as a consequence of the statistical character of chains breaking down during the degradation. It has been proved that the reciprocal value dependence of the mean viscosity molecular weight of unfractionated acetone‐extracted natural rubber on the length of induction period is linear. The change in double‐bond amount during the oxidation is not reflected in the induction period determined by infrared spectroscopy.

The Comparative Determination of the Molecular Weight of Rubber by the Methods of Light-Scattering and Osmometry

Abstract 1. An account is given of experiments to determine the molecular weight of natural rubber and other high molecular-weight compounds by light-scattering and osmometry. 2. A new design of osmometer for determining molecular weight is described. 3. It is shown that the light-scattering method gives molecular weight values which agree satisfactorily with those given by the osmotic method.

Some Osmotic Measurements of the Molecular Weight of a High Molecular-Weight Fraction of Rubber Hydrocarbon

Abstract Fractionation of the rubber hydrocarbon in temperate climates has usually resulted in high molecular-weight fractions, with a molecular weight of the order of one million. Bloomfield has shown that fresh latex contains a considerable proportion of hydrocarbon having an intrinsic viscosity (η) of 10 or over and, therefore, a molecular weight of well over 106. The fractionation technique used by Bloomfield in Malaya has now been applied by the writer to smoked sheet and to F rubber, working in the United Kingdom. No very high molecular-weight fractions were found in the smoked sheet, but the F rubber yielded a fraction of (η)=7.3 and a number average molecular weight 6×106, determined osmometrically. The average molecular weight of natural rubber when freshly prepared is probably well over a million, and includes a substantial portion having a molecular weight of several millions. By the time smoked sheet has reached temperate climates, the high molecular-weight portion has probably been converted to gel. F rubber, presumably because of its different method of preparation, retains the major part of its high molecular-weight material during prolonged storage.