Krisda Suchiva

Hemicellulosic polymer from Vetiver grass and its physicochemical properties

Abstract Chemical composition of two ecotypes of the Vetiver grass ( Vetiveria zizanioides nash ) leaves has been investigated using the gravimetric method. The principal polysaccharides found in samples were hemicelluloses (ca. 38%), followed by cellulose (ca. 27%). Protein content determined by bicinchoninic acid assay using bovine serum albumin (BSA) as a standard was approximately 5%. Lignin content determined by acid chlorite method was approximately 10%. Ash content was 3% consisting of mainly silica (ca. 50%). The obtained hemicelluloses with the molecular weight of 30,000 showed the decomposition temperature at 310 °C. In order to determine an optimum condition for extraction of hemicelluloses, effects of process parameters: alkaline type (NaOH, KOH, Ca(OH) 2 and Ba(OH) 2 ), alkaline concentration (0.025–4 M), extraction time (2–18 h) and temperature (25–60 °C) were studied. It was found that all parameters influenced on the hemicellulose yield and properties in various degrees. Finally, an optimum condition giving a maximum yield of 35% hemicelluloses was recommended as 4 M NaOH at ambient temperature for 8 h.

Structure properties of purified natural rubber

Purifiednatural rubber (PNR) is natural rubber (NR) from which most of the nonrubber constituents are removed by repeated centrifugation. The study of PNR is of interest for two reasons. First, it has been reported that deproteinized natural rubber exhibited improved dynamic mechanical properties including heat buildup and flex-cracking resistance. However, the explanations for the observed improvement have not been given. Second, NR uncontaminated by nonrubber substances (mainly proteins and lipids) might be more suitable for medical applications than normal NR, which contains potential allergy-causing compounds, e.g., proteins. The present work was carried out with dual objectives: to understand the effect of nonrubber constituents on the network structures and properties of NR vulcanizates and to make the first assessment of the mechanical properties of the prepared PNR vulcanizates. The vulcanization system used was N-cyclohexylbenzothaizole-2-sulphenamide (CBS)-accelerated sulfur vulcanization system. Both the efficient vulcanization (EV) and conventional vulcanization (CV) systems were studied. It was found that vulcanization of PNR was strongly inhibited compared with normal NR, indicating significant influences of nonrubber compounds. For unfilled PNR, their tensile and tear properties were generally smaller than those of NR containing nonrubber constituents (WNR). PNR vulcanizates were also softer than WNR vulcanizates. Vulcanized PNR, however, exhibited distinct superiority in flex-cracking resistance than its WNR counterpart. Analysis of the network structures of the vulcanizates studied showed that for the EV system, the type of crosslinks [polysulfidic (Sx), disulfidic (S2), and monosulfidic (S)] in PNR vulcanizates were more evenly distributed than in WNR samples. The % Sx, S2, and S crosslinks were respectively 36.4, 25.0, and 38.7 in PNR samples compared with 6.6, 29.7, and 64.1 in WNR samples. For the CV vulcanization system, the differences in sulfur crosslink type were not as great but the tendency toward the formation of shorter sulfur crosslinks persisted in PNR vulcanizates. The more uniformly distribution of sulfur crosslink type was thought to be responsible for the observed superiority in flex-cracking resistance of PNR vulcanizates. For carbon black-filled PNR vulcanizates, similar trends existed with respect to their properties. Properties of PNR vulcanizates were generally lower than those of WNR vulcanizates, particularly when the CV vulcanization system was employed. The EV vulcanization system gave PNR properties comparable to those of WNR samples except for heat buildup where PNR showed better properties. Flex-cracking resistances of CB-filled PNR vulcanizates, however, still maintained their superiority over those of WNR counterparts of similar crosslink density. It was concluded, therefore, that the improved dynamic properties of filled PNR vulcanizates over those of normal NR are also likely to be due to more balanced formation of sulfidic crosslinks of different lengths, thus better cyclic load-bearing properties.

Influence of nonrubber constituents on tack of natural rubber. I. At very short times of contact (pendulum test)

The effect of nonrubber constituents such as proteins and lipids on tack of natural rubber (NR) was studied. Tack or adhesion at short contact time was determined for contact between crosslinked rubbers and glass plates. Various types of natural rubber, with or without nonrubber constituents, were prepared. Synthetic polyisoprene was used as a reference because of its chemical similarity. In this work, the impact pendulum test was carried out to study very short contact times tc of the order of milliseconds. The results show an increase of tack energy with tc for all the rubbers. This is attributed to an increase in true contact area with time and to a modification of the interfacial energy. Furthermore, the natural rubber without lipids and also polyisoprene exhibited the highest tack values while whole natural rubber remained always the less tacky. These results cannot be explained by differences in either the bulk viscoelastic properties of the crosslinked materials or their overall surface energy. The presence of a thin layer of varying composition at the surface of the rubbers appears to be the essential factor that affects the tack properties of the crosslinked materials.

Study of tack properties of uncrosslinked natural rubber

The purpose of this study was to better understand the effect of non-rubber substances (mainly proteins and lipids) on adhesion (against glasses) and self-adhesion tack properties of uncrosslinked natural rubber (NR). Various types of NR, with or without non-rubber constituents, were prepared. Synthetic polyisoprene rubber was also used as a reference material. All the rubbers were first characterized by many techniques (FT-IR, DSC, GPC, etc.). Two experimental tests were specially utilized to measure the level of adhesion and self-adhesion: (i) at very short contact times (from a few milliseconds up to 0.1 s) the impact of a pendulum and (ii) for longer contact times (from 0.1 s to a few h) the contact of a probe using a tensile testing machine. The tack energy increased with contact time for all the rubbers studied. Natural rubber which did not contain proteins and lipids exhibited the highest adhesion and self-adhesion tack abilities. In contrast, whole natural rubber, containing both proteins and lipids, showed the lowest tack property. In each case, self-adhesion levels are higher than those of adhesion, presumably due to interdiffusion of macromolecular chains or chain segments at the interface.

Long-chain branching and mechanism controlling molecular weight in Hevea rubber

Abstract The linear character of transesterified deproteinized natural rubber (DPNR-TE) was confirmed by the analysis of terminal groups with NMR and viscometric analyses. The branch content of DPNR rubber from fresh latex was found to range from 0.3 to 1.3 and 0.7 to 3.2, based on tri- and tetra-functionalities, respectively. The plot between the number of branch-points and molecular weight (MW) can be divided into three fractions: (A) the rubber fractions in MW ranging from 2.4×105 to 1.9×106; (B) between 1.9×105 and 2.4×105; and (C) those of MW less than 1.9×105. The fraction (A) showed the number of branch-points per a branched molecule (m) higher than that of fractions (B) and (C). This plot is superimposable with the bimodal molecular-weight distribution (MWD) of Hevea rubber, showing a good coinciding of peak-tops at the high and low MW fractions. It seems likely that there is a close relationship between the number of branch-point and bimodal MWD of natural rubber.

Origin of phase shift in atomic force microscopic investigation of the surface morphology of NR/NBR blend film.

Atomic force microscopy (AFM) was used to study the morphology and surface properties of NR/NBR blend. Blends at 1/3, 1/1 and 3/1 weight ratios were prepared in benzene and formed film by casting. AFM phase images of these blends in tapping mode displayed islands in the sea morphology or matrix-dispersed structures. For blend 1/3, NR formed dispersed phase while in blends 1/1 and 3/1 phase inversion was observed. NR showed higher phase shift angle in AFM phase imaging for all blends. This circumstance was governed by adhesion energy hysteresis between the device tip and the rubber surface rather than surface stiffness of the materials, as proved by force distance measurements in the AFM contact mode.

Effects of Blend Ratio and SBR Type on Properties of Carbon Black-Filled and Silica-Filled SBR/BR Tire Tread Compounds

This work aimed at investigating the effects of blend ratio between styrene butadiene rubber (SBR) and butadiene rubber (BR) and SBR type (E-SBR and S-SBR) on properties of SBR/BR tire tread compounds. Influences of these parameters on properties of the tread compounds reinforced by 80 parts per hundred rubber (phr) of carbon black (CB) and silica were also compared. Results reveal that hardness, strengths, and wet grip efficiency were impaired whereas rolling resistance was improved with increasing BR proportion. Surprisingly, the presence of BR imparted poorer abrasion resistance in most systems, except for the CB-filled E-SBR system in which an enhanced abrasion resistance was observed. Obviously, S-SBR gave superior properties (tire performance) compared to E-SBR, particularly obvious in the silica-filled system. Compared with CB, silica gave comparable strengths, better wet grip efficiency, and lower rolling resistance. Carbon black, however, offered greater abrasion resistance than silica.

Study of Rheological Behavior and Extrudate Surface Quality of Rubber Compounds

Protocol of capillary test for a characterization of rheological properties in various rubber compounds was implemented. By the use of capillary die specially designed by Michelin, the surface characteristics of the extrudate of various rubber compounds were investigated. Parameters investigated were: types of rubber matrix (NR, BR and SBR), loadings of carbon black and types of processing aids. The Index of Cohesion (IC), gained from the analysis of the extrudate surface severity, was used for predicting the cohesive force within the rubber molecules. The high IC value implied low magnitude of extrudate surface severity, i.e., high melt strength of the rubber compounds. Also, the investigation on extrudate surface appearance yielded the information on flow regimes, i.e., smooth surface, sharkskin instability and gross melt fracture. Furthermore, the extrudate surface quality was determined by the use of the rubber extruder equipped with the Garvey die as a standard tool for evaluating the extrudability of unvulcanized rubber compounds. The interconnection between IC value and Garvey die extrudability of rubber compounds was established. Result obtained revealed significant difference in extrudate surface characteristics of rubber compounds with different types and loadings of carbon black and processing aids. The SBR extrudates showed smooth extrudate surfaces whereas the BR extrudates exhibited sharkskin instability at any given output rate. Moreover, with increasing the black content, the SBR compounds showed the increment of the IC value meaning the reduction in magnitude of extrudate surface severity.

Preparation and Characterizations of Natural Rubber from Natural Rubber Latex by Using Thermal Drying Method °"√»÷'—'''' πÈâ''È‚¥¬°"√"™ââ

Natural rubber prepared from thermal drying technique was achieved. The range of drying temperature covered from 70 I150 Ilocal-made machine. The cure characteristic at 155 I mooney viscosity, mechanical properties (before and after aging) were investigated as compared to control rubber (derived from acid coagulation) It was found that mooney viscosity of thermal dried natural rubbers (TNR) was higher than that of acid coagulated rubber (ANR). T 2 and T 90 of TNR for all vulcanizing system were shorter than that of ANR. The average mechanical properties of TNR showed slightly lower tensile strength as compared to ANR. The differences in obtained properties should be claimed by the content of non rubber in rubber composition.

Effect of Mixing Conditions on Phase Morphology of NR/EPDM Blends

A number of mixing parameters including mixing temperature, rotor speed, fill factor, mixing time, and loading sequence have strong influences on mixing quality. In this work, an in-house developed co-rotating batch mixer equipped with the MX2 rotors, which providing a combination of shear and extensional flows, was used to prepare NR/EPDM blends under various mixing temperatures, rotor speeds, and mixing times. Phase morphology and magnitude of coefficient of dispersive mixing (CDM) were used as qualitative and quantitative determination of mixing quality, respectively. It was found that the lower the mixing temperature, the greater the mixing quality would be obtained. The optimum rotor speed was observed at 60 rpm which was probably caused by the counter-balancing effect of shear stress and shear heating.