Mechanistic proposals for variations in the ...

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Recent Res. Devel. Bioener., 4(2006): ISBN: 81-7895-225-4

Mechanistic proposals for variations in the macrostructure of natural rubber. A review Ehabe Eugene Ejolle1, Nkeng George Elambo2 and Bonfils Frédéric3 1 National Rubber Research Programme, IRAD Ekona Regional Research Centre PMB 25 Buea, Cameroon; 2National Advanced School of Public Works P.O. Box 510 Yaounde, Cameroon; 3IRAD, Département des Cultures Pérennes TA 80/16, 73 rue Jean-François Bréton, 34398 Montpellier, France

Abstract This review covers aspects ranging from the biosynthesis of polyisoprene macromolecules in the natural rubber produced by the Hevea brasiliensis. It analyses the composition of released latex, industrial processing and quality of natural rubber from the Hevea latex, elucidates the separate contributions of the structural components of natural rubber, notably the macrostructure, mesostructure, microstructure and nanostructure, and gives an up-to-date description of mechanisms underlying chain branching and gel for Correspondence/Reprint request: Dr. Ehabe Eugene Ejolle, National Rubber Research Programme, IRAD Ekona Regional Research Centre, PMB 25 Buea, Cameroon. E-mail: [email protected]

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Ehabe Eugene Ejolle et al.

formation in natural rubber. A crucial problem often encounterd during manufacture of natural rubber based goods from the raw product involves the high level of rejects after the mixing process. This problem is equally addressed as structural changes occurring during mastic action of natural rubber are described. Some more or less pertinent criteria used to predict this processability are also presented.

1. Introduction North American pre-Colombian civilisations collected natural rubber by piercing holes through the trunk of certain trees and used this product to manufacture diverse objects [1]. The first scientific description of natural rubber was made by Charles Marie de la Condamine in 1736 while the Hevea “rubber” tree was described for the first time by Fresneau in 1747. Rubber aroused much curiosity and research interest at the time, and such efforts were crowned with important practical discoveries such as the flying balloon by the Montgolfier brothers (1783), the impermeable tissue by Charles Macintosh (1818), mastication by Thomas Hancock (1820), vulcanisation by Charles Goodyear (1839), and the inflatable tyre by Robert Thompson (1845), latter rendered more marketable by John Boyd Dunlop in 1888. The invention of pneumatic tyres in 1850 rendered the global production of natural rubber (1450 tons), harvested exclusively from the Amazonian forests, largely insufficient. As part of the efforts to produce Hevea rubber in S.E. Asia, the English Henry Wickham transported 70,000 Hevea grains (in 1876) from the Amazonian forest to the Kew botanical gardens (London). After acclimatization, 2800 plants were transported to the Perideniya gardens (Sri Lanka), from there to Malaysia and then to almost the whole of S.E. Asia. With genetic improvements, the Hevea plantation took a considerable boom. Today, the Hevea tree has lost its wild character and production from the Amazonia represents only 1% of world production [2]. The total surface area consecrated to Hevea production in the year 2000 stood at about 8.6 million hectares, producing about 7.26 M tons of natural rubber. Today, about three quarters of the world production comes from S.E. Asia, the major producers being Thailand (32%), Indonesia (26%), India (10%) and Malaysia (8.5%). Genetically, Hevea planting material has been much improved with better rubber productivity and resistance to diseases and adverse weather. Another landmark development was the introduction, in the 1960s, of technically specified rubber, as replacement for the traditional crêpe and sheet rubber [3]. About three-quarters of the world’s natural rubber production is consumed in the tyre manufacturing industry [2], characterized by a much automated system and rigorous quality demands. However, natural rubber is a product of biological origin with a particularly complex structure that varies with agronomic parameters associated with the cultivation of the Hevea. Therefore,

Mechanisms for variability of natural rubber

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as compared to its synthetic homologues, the properties of raw natural rubber display an important variability, especially with respect to their flow behaviour during mixing. This paper reviews aspects ranging from the in vivo biosynthesis of the polyisoprene macromolecule (natural rubber) by the Hevea brasiliensis, the composition, molecular structure and quality of the raw rubber, and their eventual implication in the flow behaviour and processability of the rubber, especially during mastication operations.

2. Biosynthesis of rubber by the Hevea brasiliensis Several plant species produce natural rubber, and the latter could be distinguished by the sites of their biosynthetic systems, latex composition, as well as the nature, size and structure of rubber particles they produce [4,5]. The Hevea brasiliensis remains the most suitable plant for industrial exploitation due to its higher dry yields and ease of processing of its rubber [1].

2.1. Laticiferous system and latex flow Plants of the Hevea genus have, in all their parts (roots, crown and trunk cortex), some specialised latex-containing cells called laticifers [4]. For practical reasons, only the tree’s trunk, containing essentially non-differentiated tissue in the cambium, is exploited to obtain latex [6]. At incision of the cortex, in a process termed “tapping”, the intralaticiferous turgo pressure expulses its latex content [6]. Coagulation of rubber particles in the latex progressively obstructs the incision [7], slows down flow, which eventually ceases after a couple of hours. The plugging of the incision is initiated by rupture of lutoids that liberate their contents in the cytosol [8]. Hydrogen ions (H+), cations [7,9] and some intra-lutoidal cationic proteins (some chitinases and glucanases) neutralise the electro-negative charges of rubber particle membranes [10,11]. This destabilises the latex in situ and favours coagulation. Another coagulation mechanism implicates an intralutoidal agglutinine, hevein, which attaches itself unto the rubber particles and links them to each other to form a tri-dimensional network [12].

2.2. In planta synthesis of polyisoprene Biosynthesis of polyisoprene in the Hevea brasiliensis is obvious between successive tapping operations during which latex is regenerated. The rubber particle is composed almost exclusively (> 99%) of poly(cis–1,4-isoprene) macromolecules and as such will be simply referred to as polyisoprene in this review. An early scheme proposed by Lynen [13] for the biosynthesis of polyisoprene, and which remains the reference to this day, distinctly dissociates the biosynthesis of isopentenyl pyrophosphate (IPP) monomer unit from the polymerisation proper [14]:

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• Phase I involves glucose catabolism (glycolysis) during which photosynthesized sucrose from the foliar crown is used to produce acetate molecules (polymer precursor), generate energy (as ATP) and the reduction potential, following reduction of nicotinamide adenine dinucleotide phosphate, NAD(P)H from oxidised NAD(P); • Phase II involves polyisoprene biosynthesis in which energy (ATP) and NADPH are used to elaborate the initial long chain precursor, isopententyl pyrophosphate (IPP), from acetyl coenzyme A. Construction of the long polyisoprene chain is catalysed by two enzymes on the phospholipoproteic membrane of the rubber particles: the “Rubber Elongation Factor” [8,15] and the cis-Prenyl Transferase [5]. The former catalyses elaboration of polyisoprene by successive addition of IPP molecules while the latter catalyses polymerisation of IPP in the cis configuration. Some NMR studies have shown that polyisoprene chains in Hevea rubber are, in theory, composed of three parts as shown in Figure 1 [16]: a terminal group (ω) of two trans groups, a long isoprene chain of cis configuration, and another terminal group (α). H3C

H C C

H H3C CH3 CCH CH 2 CH2 CH2 CH3 C CH CH3 CH2

H

CH3

C CH CH2 CH2

m 2

n

ω trans

cis

CH3 CH2 C CH CH2 OR

α

Figure 1. Theoretical chemical structure of poly(cis-1,4-isoprene) in Hevea brasiliensis rubber.

2.3. Composition of Hevea brasiliensis latex 2.3.1. Main fractions of Hevea latex Hevea latex and natural rubber are distinct products, the former is a stable colloidal dispersion of rubber particles in an aqueous medium while the latter is a solid product obtained following coagulation of the latex, processing and drying of the coagulum. The latex, cytoplasmic content of laticiferous cells, contains 25 to 50% dry matter, about 95% of which is poly(cis-1,4-isoprene) of very high molar mass (5.103 to 107 g/mol). This latex further contains lutoids (10 to 20%) and FreyWyssling particles (≈ 5%) equally dispersed in an aqueous phase or serum of 55 to 65% [17]. Rubber particles of 5 nm to 6 µm, are the most abundant organelles in Hevea latex [18].


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Other biochemical constituents of the latex (lipids, proteins, sugars, etc.) contribute to specific properties of the rubber. Proteins, found mostly in the cytosol and some adsorbed on the rubber, represent about 1% of the mass of latex [19]. Amino acids, also mainly found in the cytosol, constitute ~ 0.15% of the mass of latex [20-22]. The sugars (sucrose, glucose, galactose, fructose and 2-pentoses) and the cyclitols (inositols) represent ~ 2% of the mass of latex, the major compound being the methyl-inositol or quebrachitol [23,24]. The inorganic ions, representing ~ 0.5% of the mass of fresh latex, are mostly phosphates of potassium, chelate-forming cations of heavy metals, and trace amounts of Mg2+, Na+, etc. 2.3.2. Lipids in Hevea latex Much research has been conducted on lipids in Hevea latex and rubber [22,25,26,27,28]. Synthesizing these results is rather difficult as some studies were based on lipids in the latex of different clones (Hevea lineage propagated by budding from mother plants of same genotype) or unidentified clones, others on lipids in differently-processed dry rubber, and even on lipids extracted using different procedures (solvents, extraction techniques and temperatures). This results in considerable variation in results on lipid contents of H. brasiliensis latex (~ 1% w/w) and dry rubber (2.5 - 4.5% w/w). Indeed, the lipid composition of Hevea latex varies with the latex’s clonal origin and fraction: rubber particle, serum or bottom fraction [29]. The rubber particles have some polar lipids (0.89 - 0.95% w/w) and variable quantities of neutral lipids (0.91 - 2.55% w/w). The bottom fraction, obtained after centrifugation of the latex, has a high concentration of both polar lipids (2.5 - 3.7% w/w) and neutral lipids (5.1 - 7% w/w). Therefore, of the lipids in dry natural rubber, a rather little quantity comes from the rubber particles ( ~ 33% w/w latex) and the majority, from lutoids that constitute the main component of the bottom fraction (~ 15 % w/w latex). Lipids represent little (1.65% w/w) of Hevea latex [26]. Of these, about half are neutral lipids (0.88% w/w), one-third are glycolipids (0.54% w/w) and one-seventh are phospholipids (0.23% w/w). Neutral lipids are essentially triacyl glycerols (> 60% w/w), glycolipids are digalactosyl diacyl glycerols (> 60% w/w), and phospholipids are phosphatidyl choline (~60% w/w), phosphatidyl ethanolamine (~21% w/w) and phosphatidyl inositol (~21% w/w). Phospholipids, and essentially phosphatidic acid (> 82% w/w phospholipids), constitute the main lipids associated with lutoids in Hevea latex, representing ~7.4 % of the lutoids’ mass [25]. The influence of clonal origin on total lipid content (1.33 - 3.36% w/w) and composition of different lipid classes has been demonstrated [22,28]. The clones studied could be grouped into three classes based on the neutral and polar lipid contents of their rubber particles:

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• • •

Mainly neutral lipids: clones RRIM 701, RRIM 501 and PB 28/59; Almost equal quantities of polar and neutral lipids: clones RRIM 804 and PR 255; Polar lipids content higher than neutral lipids content: clones RRIM 730, RRIM 703, RRIM 605, RRIM 600, PB 5/63 and Tjir 1.

Triacyl glycerols (TAG) represent ~ 44% of neutral lipids associated with rubber particles [22], the others being carotenoids, fatty acids, sterols, diacyl glycerols, monoacyl glycerols and alcohols, as well as tocotrienols and tocopherols. Analysis of fresh Hevea latex lipid extracts revealed plenty of phosphatidic acid, a bit of phosphatidyl inositol, and trace quantities of phosphatidyl ethanolamine (PE), diphosphatidyl glycerol (DPG) and phosphatidyl choline (PC) [30]. Glycolipids on rubber particles are essentially digalactosyl diacyl glycerols (DGDG, 62.8%), free and acylated glucosterols (21%), as well as monogalactosyl diacyl glycerol (MGDG) and some phytosterols (campesterol, β-sitosterol, stigmasterol and 5-avenasterol, etc.) in trace quantities [26,31]. It is obvious from the literature that biochemical studies on rubber particle bound lipids are quite complex due to the solubility of polyisoprene in most extracting solvents and the rather delicate nature of polar lipids that are often denatured when traditional extraction techniques are employed. An enhanced solvent extraction method is proposed for polar lipids associated with rubber particles from the Hevea brasiliensis appears more suitable for extracting total lipids with optimal glycolipid and phospholipid contents [32].

3. Industrial production and properties of rubber 3.1. Processing of natural rubber Rubber particles in Hevea latex are mostly obtained, after tapping, in liquid form or as coagula. The large processing factories supply the rubber in solid form or as concentrated liquid of ~ 60% dry rubber content [2]. The solid rubber is available, after drying, as either compact granules (55% world production), ribbed-smoked and air-dried sheets (~25% world production), and crepes. Several techniques are available for processing the raw product into increasingly cleaner, drier, and more homogeneous dry rubber. Within each group, there are several “grades” based on the product’s quality [33]. For the concentrate latex, the main process is concentration by centrifugation [34]. For dry natural rubber, the main processing steps are: -

Coagulation, either natural (or spontaneous) or controlled by acidification with formic or acetic acid; Lamination or creping, depending on the type of products, Granulation for the technically-specified rubbers (TSR), Drying, either by smoking or using hot air.


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3.2. Specific properties of Hevea brasiliensis rubber Natural rubber possesses properties that remain to date, rather unequalled by its synthetic homologues. Some of these properties are low heat build-up (essential for heavy duty tyres like those of airplanes), good rigidity in nonvulcanised mixes, excellent adhesive properties, as is necessary during the fabrication of tyres, excellent dynamic properties that render it indispensable for elastic bridges and shock-absorbing systems, good resistance to tear and propagation of fractures, as well as good tensile properties of its pure gum stock (without fillers), necessary for dipped goods (gloves, preservatives, etc.). However, natural rubber is a product of biological origin, an aspect that confers on it a greater variability (as compared to its synthetic homologues) of its properties and its behaviour during processing and moulding of finished articles.

3.3. Quality criteria of technically specified grades The ISO 2000 norm indicates limits of specification criteria (Table 1), defining for each rubber grade (TSR 5, TSR 10, etc.) thresholds and determination procedures. For a rubber lot to belong to a particular class, its dirt, nitrogen, ash and volatile matter contents should be less than the threshold values while the plasticity, PRI and Mooney viscosity should be higher. These quality criteria could be physical (insoluble dirt particles > 45 µm and colour ranging from pale yellow to dark brown), chemical (volatile matter, nitrogen and ash contents) or rheological (initial plasticity - P0, plasticity retention index-PRI Table 1. ISO 2000 specifications of technically-specified rubber (TSR). Limits for the different rubber grades CV

L

5

10

20

50

Normative references

0.05

0.05

0.05

0.10

0.20

0.50

ISO 249

-

30

30

30

30

30

ISO 2007

Plasticity retention index, min.

50

40

30

ISO 2930

Ash, % w/w, max.

0.75

1.0

1.5

ISO 247

Nitrogen, % w/w, max.

< -------------------- 0.60 -------------------->

ISO 1656

Volatile matter, % w/w, max.

< -------------------- 0.80 -------------------->

ISO 248

Characteristics Dirt, % w/w, max. Initial plasticity, min.

Lovibond colour index Mooney viscosity ML(1+4)100

60 ± 5

6

-

-

-

-

ISO 4660 ISO 289

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and the Mooney viscosity - VR). The P0 characterises the aptitude of hot rubber to flow, hence in relation with the amount of energy consumed during high temperature mastication, a process that ruptures the macromolecular chains, thereby reducing its viscosity and facilitates incorporation of other ingredients. The P0 is correlated to the average molar mass of rubber [35]. The PRI indicates the sensitivity of rubber to thermo-oxidation by simulating its degradation under controlled conditions. The PRI is a relative measure, dependent on the P0, but independent of oxygen diffusion [36]. The VR defines the importance of an opposite reaction by the elastomer to a continuous shearing effort generated by a rotor. To a certain extent, a high Mooney viscosity is translated by more energy to be spent during mastication. The VR is equally correlated to the polymer’s average molar mass [37].

4. Structural components of Hevea rubber Natural rubber is an “agro-material” whose quality depends strongly on agronomic factors like the clone [38] and soil/climatic factors like season [39], and this stands as a major inconvenience from the manufacturers’ viewpoint: the variability of its properties, and hence its mixing behaviour which remains difficult to predict, in certain cases, with the available criteria. As earlier indicated, the TSR natural rubber is characterised based on the ISO 2000 norm. Criteria supplied to manufacturers to predict mixing behaviour are the Wallace plasticity (P0), at times the Mooney viscosity (VR) and the Plasticity Retention Index (PRI). The P0 and the VR, measures of the viscoelasticity of the product, would depend on the structure at different scales of the raw material. The PRI gives account of the sensitivity of the product to thermo-oxidation. Knowledge of the chemical and biochemical mechanisms implicated in the variability, is primordial before any efficient means could be instituted to control the variability. This understanding goes, obligatorily, through a methodology that relies on a structure-property approach. Such a measure should be effected upstream (evolution of the product during its “life”, at the exit from the tree and its arrival in the manufacturer’s mixer), and downstream (behaviour and evolution of the product during the various processing phases). In this context, four structural domains are defined – the macrostructure (unmodified raw material, mesostructure (gels and other interactions between polyisoprene chains and non-isoprene compounds), micro- or molecular structure, and the nanostructure or chemical structure.

4.1. Macrostructure The macrostructure involves the global organisation of the material. The criteria that enable scientists/technologists to predict the behaviour of natural rubber during processing shall most certainly be the characteristics measured on the solid material. From the nature of the product and the processing


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techniques used, the main indicators studied are rheological measurements. In this domain, emphasis shall be laid essentially on the Mooney viscometer, an apparatus found in the test laboratories of the majority of TSR rubber producers and which make room for variations of both the shear time and the temperature of the measure.

4.2. Mesostructure and microstructure Natural rubber could be distinguished from its synthetic homologues by a more complex associative structure. This structure is partially destroyed during dissolution in a classical solvent for polyisoprene. In several cases, a portion of rubber termed “gel phase” or “macrogel” remains insoluble in these solvents. The soluble phase contains polyisoprene macromolecules and a variable quantity of rubber micro-aggregates, constituting the microgel. The mesostructure of natural rubber could thus be segmented into two components; microgel and macrogel. A review by Fuller [40] on several studies conducted on synthetic polyisoprenes illustrates the essential role of the polymer’s macromolecular structure on its viscoelasticity. Although this should be the same for other polymers, the problem is more complex for natural rubber due to the presence of macrogel and microgel which seem to disturb the relations between rheological parameters and macromolecular structure [35,41,42]. Early studies on the macromolecular structure of natural rubber consisted of its fractionation by precipitation and determining the average molar masses - number-average ( M n ) by osmometry and viscosity-average ( M n ) of each fraction [43]. On using this procedure, Bristow and Westall [44] suggested a bimodal distribution for crepe natural rubber, prepared from RRIM 501 latex. Westall [45] later showed that the distribution of the inherent molar mass (MMD0; the MMD of natural rubber as would be found at the exit of the tree, not having been subjected to any creping, drying, or other treatment) of natural rubber of this same clone was equally generally bimodal. Subramaniam [46,47] used steric exclusion chromatography to put to light inherent bimodal distributions (Type 1 and Type 2, Figure 2) for most of the clones studied; and the distributions show a shoulder in the low molar masses (Type 3) for some clones. It is likely that, depending on the clone, a large variety of distributions between the two extremes (Type 1 and 3) could be found. These MMDs show two populations of variable magnitudes: one of long chains (350 - 10 000 kg/mol, polystyrene equivalent) and the other of short chains (10 – 350 kg/mol). Following prolonged latex storage, a bimodal distribution could virtually change to a unimodal distribution with shoulder [47]. The low molar mass fraction should be constituted essentially of linear macromolecules and the high molar masses of branched/ramified macromolecules [48]. However, this is not the case for all natural rubber grades as the bimodality seems to persist

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Ehabe Eugene Ejolle et al. PB 330

(Type 3)

PB 310 RRIC 121

(Type 2)

GT 1 PB 311

104

,E+04

(Type 1)

105

1,E+05

106

1,E+06

107

1,E+

Masses molaires (g/mol) Molar masses (g/mol)

Figure 2. Different types of inherent (native) molar mass distributions obtained for natural rubber from different H. brasiliensis clones [51].

even after transesterification reactions which in principle eliminate interactions responsible for the macrogel [49]. Since molar mass measurements are conducted on soluble fractions, the extent of branching of polyisoprene chains would significantly influence natural rubber’s MMD [50]. The inherent molar mass distribution (MMD0) is an important criterion to predict a number of properties of natural rubber obtained after factory processing. The essential feature of an inherent “bimodal” distribution (MMD0) or unimodal distribution with shoulder, will condition the properties of the factory-processed rubber [51]. Natural coagulation of latex and eventual coagula storage could influence the molecular structure and rheological behaviour of processed natural rubber. Depending on the level of metabolism of the Hevea, natural rubber could be more or less predisposed to degradation following maturation and coagula storage [52]. The less metabolically-active the Hevea clone (such as PB 217), the more stable is the molecular structure of its rubber and vice versa.

4.3. Nanostructure or chemical structure All the chemical and physical properties of a polymer depend on its chemical structure, even though there could be a less significant contribution of the material’s macromolecular structure [53]. The density, glass transition temperature, chemical reactivity, solubility, etc., are properties that depend essentially on the chemical structure (molecular scale). It is the smallest scale since it concerns, in general, the monomer unit. In the case of synthetic polymers, problems associated with analysis tend to be focussed on two determinations: identification of constitutive groups and the spatial arrangement of these groups on the macromolecular chain. For natural rubber, these determinations have been the subject of several research operations.


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Other 13C NMR studies on polyisoprene from varied latex-containing plant sources (sunflower, lactarium, Ficus elastica, etc.), as enumerated in the review by Eng and Tanaka [54], have led to a larger extent, to confirm the theoretical structure in Figure 1. The α group is terminated by a hydroxyl (OH) group, or a fatty acid; the ω terminal group, is constituted of a dimethyl allyl group linked to two (or three) trans-isoprene units. For the polyisoprene from the Hevea, the chemical structure of the terminal groups has not been completely identified. A protein could be covalently linked to the dimethyl allyl group (α-group) and a fatty acid, or a phospholipid, at the other extremity (ω) (Figure 3). Irrespective of the plant involved, the pyrophosphate group has not been identified at the chain end [54]. The last option worth exploiting remains identification of the chain’s terminal groups associated with the short chains, and which seem to play an important role in the formation of macrogel and microgel. For natural rubber, the following should also be considered: • •

The main non-isoprene components (lipids, proteins, sugars), and their roles from the biochemical and chemical points of view; The structural irregularities (abnormal groups) on the long chains, which should contribute significantly towards storage hardening and macrogel formation.

Of the non-isoprene constituents, lipids contribute to several phenomena: plasticization, thermo-oxidation, vulcanisation, green strength and macrogel formation. O

-

O

PO O-CH OCH2 2 O O n CHO C (CH2) n CCH3C H H CH2O C (CH2) m CH3

2

HN Protein Protéine téi

di éth l ll t

t

dii

è

h

O h tid l

i 14

l i

Figure 3. Structure of poly(cis-1,4-isoprene) in Hevea brasiliensis rubber [16].

4.4. Gel phase or mesostructure The term “gel” is often used in an ambiguous manner due to the diversity of the structure of materials analysed and the diverse disciplinary backgrounds of researchers who study them: biologists, chemists, physicians. [55] In general, gel formation is a phenomenon easier to recognise than define since it is translated by a change of physical state. The most adapted definition to natural rubber is that of Tanaka [56] who describes it as “a network of branched polymer swollen in a liquid milieu; the properties of which will depend strongly on the interactions between the two constituents.”

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In the framework of the approach adopted in this paper that relies essentially on the interactions at the origin of gel in natural rubber, the definitions of Durand [57] are equally retained concerning the covalent gels, the physical gels, and the temporary gels. A covalent gel leads to the formation covalent inter-molecular bonds. Such a network, that remains insoluble irrespective of the solvent used, is a reversible gel. The formation of this type of network is often due to chemical side reactions, but at times could be induced to improve or modify, the properties of the material (vulcanisation, for instance). Gel formation could be attributed to non-covalent interactions: ionic or polar (hydrogen bonds, Van der Walls bonds, dipole/dipole interactions, etc.). This case involves “physical” gels (or thermo-reversible). Physical gel could be differentiated from covalent gel by the reversibility of the formation process, owing to the presence of specific non-covalent bonds. The bonds are generally not ordered but are constituted of junction zones of a more or less orderly structure. Physical gel could be dissolved, more or less rapidly, in a given solvent and under given conditions. The reversibility of this gel could be expressed during heating cycles (destruction of physical junctions), hence the reason why such systems are usually termed thermo-reversible gels. It has been thought for long that polymers of biological origin (agarose, for instance) were susceptible (capable) of forming this type of network, in aqueous solution, through hydrogen bonds. Furthermore, the same phenomenon has been demonstrated with some synthetic polymers (PVC, isotactic polystyrene, elastomers) in solution in some organic solvents [58,59]. A third type of gel worthy of mention is the “temporary” gel, whose reversible formation, is linked to entanglements, nodes, macromolecular chains. In general, this gel is typical to very long macromolecular chain polymers in the molten state whose molar mass (M) is far greater than the critical entanglement molar mass (Me).

4.5. Mechanisms of gel formation and branching Gel formation in natural rubber could be attributed to several interactions between “abnormal” groups on the polyisoprene molecules and non-isoprene compounds in the latex. At this level, an elucidation of the problem is particularly delicate since several of the mechanisms proposed were not presented in the framework of specific studies on gel (the macrogel content was hardly measured) but during studies to identify reactions responsible for the “hardening during storage” of natural rubber [60]. The storage hardening is a phenomenon, among others, that leads to the formation of macrogel in natural rubber. As concerns studies specific to gels, the works of the team headed by Tanaka [16] were only concerned with mechanisms responsible for macrogel


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formation whereas Voznyakovskii et al. [61] studied only the microgel. On considering interactions responsible for gel formation, the majority of authors think the bonds are essentially covalent, meanwhile others go for non-covalent interactions. It is likely that the both types are implicated. 4.5.1. Storage hardening Storage hardening is a phenomenon typical to natural rubber. It provokes an increase in the viscosity of rubber with a concomitant increase in the microgel content [62,63]. This phenomenon, strongly accelerated at low humidity levels, does not occur in tropical countries due to their high ambient humidity. The main consequence of storage hardening is an increase in macrogel. The treatment of rubber with compounds containing the –NH2 group, such as hydroxylamine and semicarbazide, inhibits storage hardening whereas rubber treated with hydrazine, a bi-functional amine still hardens to an appreciable extent [64 ,65 ,66]. Fresh latex contains metallic ions, some of which could be trapped in the rubber during coagulation [67]. The elimination of these ions by dialysis of latex has been found to reduce the aptitude to hardening of the rubber obtained [68]. Several “abnormal” groups like aldehydes [69,70], epoxides [71,72], lactones [73] and esters [74,75] have been identified on polyisoprene chains, and incriminated for the storage hardening. Other non-isoprene compounds like amino acids [70], proteins [74,76], lipids and fatty acids [49,75] and more recently phospholipids [77], have also been incriminated. 4.5.2. Gel formation by covalent crosslinks Several types of reactions are proposed for the formation of covalent branches, notably: • • •

• •

Radical attacks during oxidation; reactions between aldehydes present on the polyisoprene chain and the diamines of the latex (Figure 4) [65], or/and the amino acids (Figure 5) [70]; reactions between epoxide groups on polyisoprene chain and amino acids or proteins [68], mechanisms criticised by Subramaniam and Wong [78]. More so, a 13C NMR study showed that fresh latex contains no epoxide groups [79], and that these groups should originate from the oxidation of polyisoprene. bi-molecular aldolic condensations between aldehydes (Figure 6) [66]; Reactions between epoxide groups on polyisoprene chains and amino acids or proteins [68]. Recent 13C NMR studies, however indicate that epoxide groups are absent in fresh latex, and only originate from polyisoprene oxidation [79].

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+ H2O

CH2CH N NH2

CH2CHO + NH2 NH2

+

CH2CHO

CH2CH N N

CHCH2

Figure 4. Mechanism of the formation of gels by covalent crosslinks between aldehydes on the polyisoprene and di-amines of the latex [65]. H

H C H2

C O

+

R'

C H2

NH2

+

C NR'

H 2O H

+

C H2

C O

H C H2

C

NHR' CH C

H

O

Figure 5. Mechanism of formation of gels by covalent crosslinks between aldehydes present on the polyisoprene chains and amino acids [70].

O 2

CH2

C H

OH C CH2C

C

H

H

O

O H

H

H C H

C

C C H

H

+

H2O

Figure 6. Mechanism of gel formation by aldolic condensation in a basic milieu.

4.5.3. Gel formation by non-covalent crosslinks Amongst mechanisms implicating non-covalent bonding are proteinpolyisoprene associations through hydrogen bonds between ester groups on polyisoprene molecules and proteins [58 ,74], as well as ionic bonds between metallic cations and amino acids, or carboxylic acids on polyisoprene molecules [68,80]. The enzymatic hydrolysis of proteins in latex leads to a reduction of the macrogel content whereas transesterification of natural rubber eliminates the macrogel entirely [16]. Through transesterification, the macrogel is dissolved and liberates many short polyisoprene chains which should probably have a prime role in the formation of the macrogel itself.


15

These studies led to a proposal of a mechanism (Figure 3) for gel formation involving a covalent bond between a fatty acid or phospholipid and the α-terminal group of the polyisoprene molecule [16]. The other end of the chain (ω-) should be covalently linked to a protein, hence account for the nondetection by NMR of the dimethyl allyl group at this chain’s extremity.

5. Mastication, structure and processability of natural rubber 5.1. Mastication of raw and compounded natural rubber To obtain an article of vulcanised rubber, several processing operations are involved, the mastery and regularity of each of which influences the properties of the final product. The operations range from mastication of raw rubber (or reduction of its viscosity), mixing or incorporation of other formulation ingredients, processing into shape or moulding of mix, to vulcanisation. Much progress has been realised already on vulcanisation (measurement and control of its characteristics) such that a correct choice of accelerators, activators, pre-vulcanisation inhibitors, etc. permits, in principle, a precise control of the process. The situation is however different for the previous steps (mastication to moulding), where the behaviour during mixing, rendered more complex by the interaction with the other ingredients, had not been fully mastered. Indeed, the homogeneity of the rubber and its flow after mastication condition the efficiency of incorporation and dispersion of other ingredients as well as the consistency of the final compound [81]. The need for a tool to characterise the natural rubber’s processability during mastication operations still remains a necessity. Amongst tolls available today (DMTA, RPA 2000, etc.), the choice of the Mooney viscometer is justified based on economic considerations (purchase price and maintenance costs), operational considerations (ease and rapidity of operation, robustness, on-site calibration) and its availability in most rubber production factories. 5.1.1. Mastication on two-roll mills Many researchers have studied the behaviour of natural rubber during mastication on roll mills [82-84]. However, only Shiibashi et al. [74] were interested in the evolution of the structure during this treatment These workers masticated grade RSS 1 natural rubber on a two-roll mill at 50°C and observed that increasing the number of mastication cycles induces an increase in the quantity of soluble fraction (sol) accompanied by a simultaneously significant decrease in their microgel content (from 49 % to 10 %, after 15 passes) and no substantial changes even after 15 passes. They equally observed a decrease in the average molar masses and the extent of branching of the sol phase. The mastication of natural rubber provokes scission of branch points constituting the branched gel structure. As such, the gels are transformed into

16


linear or less branched macromolecules. It is likely that ramification sites in the gel of natural rubber are made up of chains that are relatively weaker than covalent bonds. The branches could be constituted of hydrogen bonds between ester radicals and proteins on the polyisoprene chains.

Efficiency of mastication

5.1.2. Mastication in internal mixers Like in all mastication processes, long chains in natural rubber are equally subjected to some degradation, translated by a modification of their molar masses and the molar mass distribution. This modification occurs in a preferential manner for higher molar mass species. The principal factors that influence the efficiency of mastication are the macromolecular structure [85,86], mastication temperature [87], presence of radical acceptors or oxygen [88], and relative proportion of short to long chains or molar mass distribution [89]. The mobility of macromolecules in natural rubber is rather slow and their disentanglement rather difficult during low temperature mastication (< 100°C) [87]. Degradation of polyisoprene chains is thus solely by mechanical shearing (Figure 7) as new radicals cannot recombine following rapid react with oxygen [88,90]. Cold mastication of natural rubber influences mostly the degree of branching, as measured by increases in Huggins k' constant [86]. This assertion reposes on the law linking the specific viscosity (ηsp), intrinsic viscosity ([η]) and concentration (c): ηsp/c = [η] + k' [η]². The higher the value of k', the higher is the number of branch points in the polymer sample.

Mastication by combined mechanical and oxidative effects

Mechanical degradation

Thermo-oxidative degradation

100°C – 130°C Temperature (°C)

Figure 7. Effect of temperature on the efficiency with which natural rubber is masticated.


17

Between 100 and 130°C, the effect of mastication is minimal, as mechanical degradation and degradation linked with thermo-oxidation are slight. Above 130°C, degradation is essentially due to thermal oxidation. Degradation at this stage could be intensified by the presence of hydro-peroxide radicals generated by mechanical shearing [88,91]. At sufficient partial oxygen pressure, these radicals accelerate chain scission whereas when oxygen is deficient, some radicals form branch points or associate amongst themselves. The effect on the mesostructure of rubber is obvious – reduction in the length of the chains and macrogel content [92]. This phenomenon seems to be different for mastication of raw natural rubber during high-temperature mastication. Due to viscous dissipation, gum temperatures could increase considerably with mastication and remain well above 115°C (Figure 8). At such high temperatures, mastication-induced degradation of NR is caused by thermal oxidation and very little by mechanical scission. Furthermore, it has been shown using rubber samples of different Mooney viscosity values and inherent molar mass distributions, that high-temperature mastication of natural rubber leads to rapid degradation of macrogel into microgel, while the latter could be continuously degraded, and at times completely [89]. Mastication provokes decreases in the average molar mass of polyisoprene chains and changes in their molar mass distributions (MMD). The MMDs evolve differently as mastication progresses, differences that could be linked to the inherent MMD of each rubber (Figure 9). Whereas non-viscosity stabilized rubber grades such as the TSR 10 of monomodal MMD will be progressively degraded by mastication, rubber of the same grade but with a bimodal MMD will degrade much slowly and even stagnate on prolonged mastication. This stagnation is characterized by an offset of the balance between long and short polyisoprene chains due to increases in the quantity of short chains which facilitate the sliding of longer ones. 180 Gum temperature (°C)

TSR 3CV

160 TSR 10

140 Thermo-oxidative degradation

120

Mechanical degradation

100 0

4

8

12

16

20

Mastication time (min)

Figure 8. Effect of internal mixer mastication (initial T = 120°C, N = 100 rpm) on the temperature profiles of grades TSR 10 and TSR 3CV natural rubber [89].

18

Ehabe Eugene Ejolle et al. i). Clone PR 107

ii). Clone PB 217

TSR 10

TSR 10

TSR 3CV

TSR 3CV

3

4

5

7 3

6

4

Log (molar mass, g/mol) Control

1 min

5

6

7

Log (molar mass, g/mol) 20 min

Control

1 min

20 min

Figure 9. Effect of mastication on the molar mass distribution of TSR 10 and TSR 3CV grades of natural rubber from (i) - clone PR 107 and (ii) - clone PB 217.

On using the “Brabender plasticorder” model of an internal mixer, Chandra et al. [93] observed that the degradation of a blend of natural and synthetic rubber (NR/BR and NR/SBR) progresses to a certain critical average molar mass. This phenomenon should be similar to that described above whereby the short chains, when in sufficient quantity, act as internal plasticizers that facilitate the gliding of long rubber chains. On using the “Brabender plasticorder” model of an internal mixer, Chandra et al. [93] observed that the degradation of a blend of natural and synthetic rubber (NR/BR and NR/SBR) progresses to a certain critical average molar mass. This phenomenon should be similar to that described above whereby the short chains, when in sufficient quantity, act as internal plasticizers that facilitate the gliding of long rubber chains.

5.2. Processability prediction using the mooney viscometer 5.2.1. The standard mooney viscosity, ML(1+4)100 Measurement of the Mooney viscosity of natural rubber is governed by the ISO 289 norm which gives prescriptions on the test temperature (100°C), duration of sample pre-heat before shearing starts (1 min) and duration of shearing (4 min). It is denoted conventionally as “ML(1+4)100”, where “L” indicates the “large” rotor. The Mooney viscosity is measured in arbitrary Mooney units (1 Mooney unit = 0.831 Nm). The Mooney viscosity is equally


19

Mooney Viscosity

much used in the synthetic elastomer industry [ML(1+4)125 for the EPDM] and for filled formulations based on natural rubber or other synthetic elastomers. In the case of natural rubber, this measure is mandatory only for the viscositystabilised grades. Generally, on shearing a polyisoprene sample in a Mooney viscometer, the torque rapidly increases (usually a few seconds) to a maximum value (Vmax), decreases very rapidly later to attain a minimal value (Vmin) in several cases, and finally increase gradually again to a plateau [VR or ML(1+4)100] after 2 to 4 min (Figure 10) depending on the sample. It is this final value (VR) that is supplied as a specification criterion. Ong and Subramaniam [94] observed that Vmax could be related to the quantity of energy initially necessary for disentangling polymer chains whereas VR could represent the energy necessary to align macromolecules in the shear direction. As such, macromolecules of a masticated sample (of lower molar mass) would present a lower degree of entanglement and induce thus the lower Vmax and VR values. On the whole, the evolution of the Mooney viscosity during test could be described in three stages: the first which involves an initial increase in torque values to a maximum value (Vmax). The torque then decreases to a minimum value (Vmin). This phenomenon correspond to stress offshoot when shearing starts [95,96], and a third in which the torque relaxes. Once Vmin is reached, the Mooney torque evolves following three trends characteristic of the sample used, as shown in Figure 11 [97].

Vmax

VR

Vmin

Shearing

Relaxation Duration of test

Figure 10. Evolution of the Mooney torque during shearing and during relaxation.

20


VV ML(1+4) ML(1+4) R Rorou 100100

Viscosité Mooney Mooney viscosity

Vmax

Vm in

Type 1

Type 2 Type 3

0

1

2

3

4

Time (min) Temps (min)

Figure 11. Evolution of the Mooney torque ML(1+4) at 100°C of non homogenised rubber samples during shearing in a Mooney viscometer.

• •

•

Type 1: For grade TSR 10 natural rubber (irrespective of the clone), the Mooney torque increases gradually after Vmin to attain a plateau as shearing progresses; Type 2: from Vmin, the Mooney torque increases rapidly to attain a plateau after just about 1 min of shearing. Grade TSR 3CV rubber from clones GT 1 and PB 217 as well as some synthetic polyisoprene like Natsyn 2200 seem to show this trend; Type 3: After Vmax, the Mooney torque stabilises very fast without passing through a minimum. This has been found with grade TSR 3CV rubber from clone PR 107.

5.2.2. Different Mooney criteria used in the rubber industry Several non-conventional criteria related to the Mooney viscosity [ML(5+1), ML(5+60), ML(0+5), ML(0+10), ML(0+60), Vmax/ML(5+4), etc.] have been proposed to characterise the rheology of natural rubber using the Mooney viscometer [94,98,99]. Ong and Subramaniam [94] have obtained, for a pre-heat time of 10 min, an excellent linear relation between VR and Vmax. Storage hardening, mastication (on a roll mill) or the method of coagulation of the rubber had no influence on this correlation. Extending the pre-heat time also bears no influence on VR but provokes a major drop in Vmax, the value of which remains constant after pre-heating for 3 to 5 min depending on the rubber grade tested [98]. This phenomenon has equally been observed elsewhere on natural rubber [94] and on EPDM [100]. Bristow and Sears [98]


21

noted that pre-heating for 5 min leads to a clear improvement of the repeatability of VR and ensures a better discrimination between technically-specified rubber grades. The ML(5+4) decreases with an increase in test temperature from 70 90°C [98]. Above 90°C, equilibrium seems to be attained, giving a rather constant VR. The discrimination between the samples studied was not improved at test temperatures above 100°C on using the ML(5+4), or the Vmax/ML(5+4) ratio. Bristow [99] monitored Mooney torque values as a function of shear time and test temperature, and noted that torque values reduce with an increase in temperature, to give very different torque-time curves. Despite the popularity of the Mooney viscometer, some users have criticised this apparatus for its low shear rate (≈ 1.6 s-1) as compared to those obtained during the transformation phase. As an indication, shear rates of 1 10²/s could be obtained during compression moulding, from 10 - 10²/s during milling on two-roll mills, from 10² - 103/s during extrusion and of 103 - 104/s during injection moulding [101]. Some users believe nonetheless in the applicability of the Mooney measurement as the Mooney viscosity measure (ML1+4)100 has been demonstrated to compare favourably with some highshear rate tests like the dynamic viscosity (η, in Pa.s) obtained with a capillary rheometer operating at 106 s-1. 5.2.3. Relaxation of the Mooney viscosity The Mooney viscosity is a complex measure of a polymer’s viscoelasticity since it does not distinguish between its elastic and viscous components. It is however frequent to find two polymers with the same complex viscosity but completely different elastic and viscous portions. A better discrimination in this case will necessitate a separation of the complex response into its constitutive elastic and viscous components. The decline in viscosity after shearing ceases characterises the relaxation of rubber, and could enable a proper evaluation of the elastic and viscous components of a material. The Mooney relaxation could be an indication of a polymer’s elastic behaviour; a rubber of higher elasticity having a slow relaxation and vice-versa [99,102]. Several descriptors, obtained from measurements of the Mooney viscosity in relaxation, are proposed in the literature to characterise elastomers. Blow and Schofield [102] proposed a parameter “R” characterising the elasticity of rubber at 1 min of relaxation. They then demonstrated relations between the values of “R” and the contraction of rubber after transformation (mastication on roll mills, calendaring or extrusion). Burhin et al. [103] used the “power law” (Mt = M1 t- α) to model Mooney relaxation. M(t) is the torque value (in Mooney units) at time “t”, M1 is the torque value (in Mooney units) at first second of relaxation while α measures the relaxation speed. The parameters “M1” and “α” enable discrimination of natural rubber samples of same Mooney viscosity [ML(1+4)100] and those

22


having different degrees of branching and gel contents [103]. Steeman and Palmen [104] and Struik [105] preferred to characterise relaxation of EPDM in terms of the time required for the VR to reduce by 50 % (t50) or by 80% (t80). It becomes obvious from these studies is that a better assessment of natural rubber quality and processability, would necessitate identification of new attributes. In this wise, Bonfils et al. [106] characterized Mooney torque relaxation, and sought to identify a model that could give a better fit than the traditional power law model (Y = at-b) or the stretched exponential for the experimental decay of Mooney torque over time. In effect, a generalized Maxwell model (Figure 12), and to a slightly lesser extent, an empirical model {Y = 1 - a*ln(t) - b*t/(c+t)], gave the most suitable description of the Mooney relaxation over the traditional models. Moreover, the total relaxation time (τm) given by the generalized Maxwell model showed good correlation with total gel content of samples studied. Good correlations were observed on SBR between ML(0+10) and numberaverage molar masses, M n , or weight-average molar mass, M W [37]: Log ML(0+10) = Log ( M n x M W )1/2. Angier et al. [86] and latter Subramaniam [85] showed that this equation could only be applicable to natural rubber and that the correlation between ML(0+10) and ( M n x M W )1/2 was improved for masticated rubber. This improvement could be attributed to a better homogeneity of samples and a reduction of the molar mass distributions. 0.09

Experimental data Maxwell model

Normalized VR

0.8

Residues

0.06

1.0

0.03 0.00 -0.03 -0.06

0.6

-0.09

time (sec)

0.4 0.2 0.0 0

100

200

300

400

500

time (sec)

Figure 12. Qualitative analysis of fit and plot of residuals (Figure insert) for the generalized Maxwell model.


23

Nair [107] attempted to establish the relations between Mooney viscosity and macromolecular structure of natural rubber (film from fresh latex, or viscosity-stabilised crêpes, from 12 different clones) and obtained, for the films, a linear relationship between VR and intrinsic viscosity, or M n (osmometry) or M W (light diffraction). These relations were not significant for viscosity-stabilised crepes. Bhowmick et al. [108] showed that the rate of relaxation (measured in tension) would increase with macrogel content or M W , the effect of macrogel being more predominant. Afanas'ev and Kozlov [109] studied the relations between relaxation time (τ50) on a Mooney viscometer and intrinsic viscosity [η]and obtained a non-linear relation between the two for some high-viscosity synthetic polymers.

6. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Compagnon, P., 1986, Le caoutchouc naturel, G.P. Maisonneuve et Larose, Paris, France, 595. IRSG, 2001, Rubber Economics Yearbook 2000, International Rubber Study Group, Wembley, UK, 117. Nair, S., Kwong, L.A., and Mun, L.K., 1972, Standard Malaysian Rubber (SMR) and its control, Rubber Research Institute of Malaysia, Kuala Lumpur (Malaysia), 45. de Faÿ, E. and Jacob, J.-L., 1989, Anatomical organisation of laticiferous system in bark, d'Auzac, J., Jacob, J.-L. and Chrestin, H. (Eds), CRC Press Inc., Boca Raton, Florida, 3 Cornish, K., Siler, D.J., Grosjean, O.-K., and Goodman, N., 1993, J. nat. Rubb. Res., 8, 275. Jacob, J.-L., d'Auzac, J., and Prévôt, J.-C., 1993, Clin. Rev. Allergy, 11, 325. Southorn, W.A., 1969, J. Rubb. Res. Inst. Malaya, 21, 494. Southorn, W.A., 1961, Microscopy of Hevea latex, Natural Rubber Research Conference (Eds.), Kuala Lumpur, Rubber Research Institute of Malaysia, p. 766. Ribailler, D., 1972, Doctorat d'Etat en Science Naturelle, Université d'Abidjan, Abidjan, Côte d'Ivoire. Breton, F., Coupé, M., Sanier, C., and D'Auzac, J., 1995, J. nat. Rubb. Res., 10, 37. Subroto, T., van Koningweld, A., Schreuder, A., Soedjanaatmadia, U.S.M., and Benteima, J.J., 1996, Phytochemistry, 43, 29. Gidrol, X., Chrestin, H., Tan, H.-L., and Kush, A., 1994, J. Biol. Chem., 269, 9278. Lynen, F., 1963, Revue Générale du Caoutchouc, 40, 83. Jacob, J.-L., d'Auzac, J., Prévôt, J.-C., and Sérier, J.-B., 1995, La Recherche, 276, 538. Hamzah, S. and Gomez, J.B., 1982, J. Rubb. Res. Inst. Malaysia, 30, 161. Tanaka, Y., 2001, Rubb. Chem. Technol., 74, 355. Moir, G.F.J., 1959, Nature (London), 184, 1626. Gomez, J.B., 1990, J. nat. Rubb. Res., 5, 231. Archer, B.L., 1960, Biochem. J., 75, 236. Altman, R.F.A., 1941, Rubb. Chem. Technol., 14, 659. Ng, T.S., 1960, Isolation and identification of free amino acids in fresh unammoniated Hevea latex, Natural Rubber Research Conference (Eds.), Kuala Lumpur, Rubber Research Institute of Malaysia, p. 809.

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22. Ho, C.C., Subramaniam, A., and Yong, W.M., 1976, Lipids associated with the particles in Hevea latex, International Rubber Conference 1975 (Eds.), Kuala Lumpur, Rubber Research Institute of Malaysia, p. 441. 23. Smith, R.F., 1954, Biochem. J., 57, 140. 24. Bealing, F.J., 1969, J. Rubb. Res. Inst. Malaya, 21, 445. 25. Dupont, J., Moreau, F., Lance, C., and Jacob, J.-L., 1976, Phytochemistry, 15, 1215. 26. Hasma, C.H. and Subramaniam, A., 1986, J. nat. Rubb. Res., 1, 30 27. Hasma, C.H., 1987, J. nat. Rubb. Res., 2, 129. 28. Hasma, C.H., 1991, J. nat. Rubb. Res., 6, 105. 29. Nair, N.U., Thomas, M., Sreelatha, S., Simon, S.P., Vijayakumar, K.R., and George, P.J., 1993, Indian J. Nat. Rubb. Res., 6, 143. 30. Hasma, C.H., 1984, Ph.D. Thesis, University of Ghent, Belgium, Ghent. 31. Nishimura, H., Philp, R.P., and Calvin, M., 1977, Phytochemistry, 16, 1048 32. Bonfils, F., Ehabe, E.E., Aymard, C., Vaysse, L., and Sainte-Beuve, J., 2006, Phytochemical Analysis, In Press. 33. Kuriakose, B. and George, M.K., 2000, Crepe rubbers, George, P.J. and Jacob, K.C. (Eds), Rubber Research Institute of India, Keala, India, 399 34. Blackley, D.C., 1997, Polymer latices science and technology. 2 - Types of latices, Chapman & Hall, London, 560. 35. Bonfils, F., Flori, A., and Sainte-Beuve, J., 1999, J. Appl. Polym. Sci., 74, 3078. 36. Bonfils, F., Laigneau, J.-C., Livonnière, H.d., and Sainte-Beuve, J., 1999, Kaut. Gummi. Kunstst., 52, 32. 37. Kramer, O. and Good, W.R., 1972, J. Appl. Polym. Sci., 16, 2677. 38. Yip, E., 1990, J. nat. Rubb. Res., 5, 52. 39. Le Roux, Y., Ehabe, E.E., Sainte-Beuve, J., Nkengafac, J., Nkeng, G.E., Ngolemasango, F.E., and Gobina, S.M., 2000, J. Rubb. Res., 3, 142. 40. Barnard, D. and Lewis, P.M., 1988, Oxidative ageing, Roberts, A.D. (Eds), Oxford University Press, Oxford, UK, 621. 41. Sambhi, M.S., 1988, J. nat. Rubb. Res., 3, 107. 42. Ngolemasango, F.E., Ehabe, E.E., Aymard, C., Sainte-Beuve, J., Nkouonkam, B., and Bonfils, F., 2003, Polym. Intern., 52, 1365. 43. Carter, W.C., Scott, R.L., and Magat, M., 1946, Journal of the American Chemical Society, 68, 1480. 44. Bristow, G.M. and Westall, B., 1967, Polymer (London), 8, 609. 45. Westall, B., 1968, Polymer, 9, 243. 46. Subramaniam, A., 1972, Rubb. Chem. Technol., 45, 346 47. Subramaniam, A., 1976, Molecular weight and other properties of natural rubber: A study of clonal variations, International Rubber Conference, 1975 (Eds.), Kuala Lumpur, Rubber Research Institute of Malaysia, p. 4. 48. Angulo-Sanchez, J.L. and Caballero-Mata, P., 1981, Rubb. Chem. Technol., 54, 34. 49. Tanaka, Y., Kawahara, S., and Tangpakdee, J.S., 1997, Kaut. Gummi. Kunstst., 50, 6 50. Parth, M., Nicolai, A., and Lederer, K., 2002, Macrom. Symp., 181, 456. 51. Bonfils, F., Char, C., Garnier, Y., Sanogo, A., and Sainte-Beuve, J., 2000, J. Rubb. Res., 3, 164. 52. Ehabe, E.E., Le Roux, Y., Ngolemasango, F.E., Bonfils, F., Nkeng, G.E., Nkouonkam, B., Sainte-Beuve, J., and Gobina, S.M., 2002, J. Appl. Polym. Sci., 86, 703.


25

53. Verdu, J., 1999, Techniques de l'ingénieur: Traité Analyse Chimique et Caracterisation, 3, 1. 54. Eng, A.-H. and Tanaka, Y., 1993, Trends Polym. Sci., 3, 493. 55. Almdal, K., Dyre, J., Hvidt, S., and Kramer, O., 1993, Polym. Gels Netw., 1, 5. 56. Tanaka, T., 1987, Gels, Mark, H.F., Bikales, N.M., Overberger, C.G. and Menges, G. (Eds), John Wiley & Sons Inc., New York, 514. 57. Durand, D., 1991, Les réseaux macromoléculaires et les gels, Polymères, G.F.d. (Eds), Groupe Français d'Etudes et d'Applications des Polymères, Strasbourg, 121 58. Grechanovskii, V.A., 1975, Internat. Polym. Sci. Technol., 2, T/112. 59. Downey, J.S., McIsaac, G., Frank, R.S., and Stöver, H.D.H., 2001, Macromolecules, 34, 4534. 60. Gan, S.-N., 1996, J. Macrom. Sci. - Pure Appl. Chem., A33, 1939. 61. Voznyakovskii, A.P., Dmitrieva, I.P., Klyubin, V.V., and Tumanova, S.A., 1996, Polym. Sci. Series A, 38, 1153. 62. Li, S.-D., Yu, H.-P., Peng, Z., and Li, P.-S., 1998, J. Appl. Polym. Sci., 70, 1779. 63. Li, S.-D., Yu, H.-P., Peng, Z., Zhu, C.-S., and Li, P.-S., 2000, J. Appl. Polym. Sci., 75, 1339. 64. Sekhar, B.C., 1960, J. Polym. Sci., 48, 133. 65. Sekhar, B.C., 1962, Rubb. Chem. Technol., 35, 889. 66. Gorton, A.D.T., 1974, Rubber Industry, 8, 142. 67. Archer, B.L., Barnard, D., Cockbain, E.G., Dickenson, P.B., and McMullen, A.I., 1963, Structure, composition and biochemistry of Hevea latex, Bateman, L. (Eds), Maclaren Press, London, 41. 68. Burfield, D.R., 1986, J. nat. Rubb. Res., 1, 202. 69. Sekhar, B.C., 1962, Abnormal groups in rubber and microgel, 4th Rubber Technology Conference (Eds.), London, p. 460. 70. Gregory, M.J. and Tan, A.S., 1975, Some observations on storage hardening of natural rubber, International Rubber Conference (Eds.), Kuala Lumpur, Rubber Research Institute of Malaysia, p. 28. 71. Burfield, D.R., 1974, Nature (London), 249, 29. 72. Burfield, D.R. and Gan, S.N., 1975, Journal of Polymer Science, Polymer Chemistry Edition, 13, 2725. 73. Gregg, E.C. and Macey, J.H., 1973, Rubb. Chem. Technol., 46, 47. 74. Shiibashi, T., Hirose, K., and Tagata, N., 1989, Kobunshi Robunshu, 46, 465. 75. Eng, A.-H., Ejiri, S., Kawahara, S., and Tanaka, Y., 1994, J. Appl. Polym. Sci.: Polym. Symp., 53, 5. 76. Eng, A.-H., Tanaka, Y., and Gan, S.-N., 1992, J. nat. Rubb. Res., 7, 152. 77. Tangpakdee, J.S. and Tanaka, Y., 1997, J. nat. Rubb. Res., 12, 112. 78. Subramaniam, A. and Wong, S.W., 1986, J. nat. Rubb. Res., 1, 58. 79. Eng, A.-H., Kodama, S., Tangpakdee, J.S., Kawahara, S., and Tanaka, Y., 1998, J. Rubb. Res., 1, 199. 80. Matsuda, H. and Minoura, Y., 1979, J. Appl. Polym. Sci., 24, 811. 81. Freakley, P.K., 2001, Rubber mixing, De, S.K. and White, J.R. (Eds), RAPRA Technology Ltd., Shawbury, 209. 82. Harmon, D.J. and Jacobs, H.L., 1966, J. Appl. Polym. Sci., 10, 253. 83. Kumar, N.R., Roy, S., Gupta, B.R., and Bhowmick, A.K., 1992, J. Appl. Polym. Sci., 45, 937.

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84. 85. 86. 87. 88. 89.


Tokita, N. and White, J.L., 1966, J. Appl. Polym. Sci., 10, 1011. Subramaniam, A., 1980, Rubb. Res. Inst. Malaysia Technol. Bull., 4, 1. Angier, D.J., Chambers, W.T., and Watson, W.F., 1957, J. Polym. Sci., 25, 129. Pike, M. and Watson, W.F., 1952, J. Polym. Sci., 9, 229. Kadivec, I., Trcek, N., and Susteric, Z., 1994, Kaut. Gummi. Kunstst., 47, 178. Ehabe, E.E., Bonfils, F., Sainte-Beuve, J., Collet, A., and Schué, F., 2006, Polym. Eng. Sci., 46, 222. 90. Dunn, J.R., 1977, Aging and degradation, Saltman, W.M. (Eds), John Wiley & Sons Inc., New York, 511. 91. Bartels, H., Hallensleben, M.L., Pampus, G., and Scholz, G., 1990, Angew. Makrom. Chem., 180, 73. 92. Allen, P.W. and Bristow, G.M., 1963, J. Appl. Polym. Sci., 7, 603. 93. Chandra, A.K., Chattaraj, P.P., and Mukhopadhyay, R., 1992, Kaut. Gummi. Kunstst., 45, 390. 94. Ong, E.L. and Subramaniam, A., 1975, Rheological studies of raw elastomers with the Mooney viscometer, International Rubber Conference, 1975 (Eds.), Kuala Lumpur, Malaysia, Rubber Research Institute of Malaysia, p. 39. 95. Grossiord, J.-L. and Quemada, D., 2001, Des concepts aux outils, Coussot, P. and Grossiord, J.-L. (Eds), EDP Sciences & Groupe Français de Rhéologie, Les Ulis, 9. 96. Leblanc, J.-L., 1996, Rhéologie des élastomères et mises en œuvre des polymères, Artel, Paris, France, 376. 97. Ehabe, E.E., 2004, Ph.D. Thesis, Université Montpellier II, Montpellier (France). 98. Bristow, G.M. and Sears, A.G., 1987, J. nat. Rubb. Res., 2, 15. 99. Bristow, G.M., 1990, J. nat. Rubb. Res., 5, 182. 100. Kralj, A., Prodan, T., and Emri, I., 2001, J. Rheol., 45, 929. 101. Popovic, R.S., Plavsic, M., Popovic, R.G., and Ilic, N., 1991, Kaut. Gummi. Kunstst., 44, 702. 102. Blow, C.M. and Schofield, J.R., 1950, Rubb. Chem. Technol., 23, 601. 103. Burhin, H.G., Spreutels, W., and Sezna, J., 1990, Kaut. Gummi. Kunstst., 43, 431. 104. Steeman, P.A.M. and Palmen, J.H.M., 1999, J. Appl. Polym. Sci., 74, 1220. 105. Struik, L.C.E., 1999, J. Appl. Polym. Sci., 74, 1207. 106. Ehabe, E.E., Bonfils, F., Aymard, C., Akinlabi, A.K., and Sainte-Beuve, J., 2005, Polym. Test., 24, 620. 107. Nair, S., 1970, J. Rubb. Res. Inst. Malaysia, 23, 76. 108. Bhowmick, A.K., Cho, J., McArthur, A., and McIntyre, D., 1986, Polymer, 27, 1889. 109. Afanas'ev, S.V. and Kozlov, V.G., 1994, Internat. Polym. Sci. Technol., 21, T/14.