Chemical Modification of NR

However, NR did not attain any commercial value until nearly a century later. The first important chemical modification of NR was vulcanization in 1839, which is generally credited independently to Charles Goodyear (USA) and Thomas Hancock (England). Crosslinks are introduced into NR chains in the process of vulcanization by heating up the NR with addition of sulfur, accelerator, fillers and antioxidants (see Figure 1.2). Vulcanization turns NR into a hard and non-sticky polymeric material with improved tensile strength... 

Mondragon et al ° reported that unmodified and modified NR latex were used to prepare thermoplastic starch/NR/MMT nanoeomposites by twin-screw extrusion. After drying, the nanoeomposites were injection moulded to produce test specimens. SEM of fractured samples revealed that chemical modification of NR latex enhanced the interfacial adhesion between NR and thermoplastic starch (TPS), and improved their dispersion. X-ray diffraction (XRD) showed that the nanoeomposites exhibited partially intercalated/exfoKated structures. Surprisingly, transmission electron microscopy (TEM) showed that clay nanoparticles were preferentially intercalated into the rubber phase. Elastic modulus and tensile strength of TPS/NR blends were dramatically improved from 1.5 to 43 MPa and from 0.03 to 1.5 MPa, respectively, as a result of rubber modification. 

NR will provide better properties if modified chemically. Chemical modification of NR can be categorized into three main categories ... 

Chlorination is one of the early methods of chemical modification of NR. Chlorinated NR (CNR) shows an improvement over the properties of NR such as chemical resistance, flame resistance and thermal stability. Accordingly, CNR has been used as a raw material for paints, adhesives, inks, coatings, etc. The chemical modification of NR by chlorination can be done in solution or latex form. The chlorination of NR in solution form has conventionally been carried out by dissolving in CCI4. However, this has been prohibited in many countries due to serious problems such as environmental pollution, solvent toxicity and the high equipment investment needed. Preparing CNR in latex is an alternative method. The chlorination of NR can be performed directly in latex form under conditions that stabilize the NR particles with non-ionic surfactant. ° Since the NR molecules exist in the rubber particles, the chlorination process in latex form is more difficult than that in solution form, when the reaction occur directly to the NR chains. 

For the NR based blends, therefore, the rheological behaviour must be considered case by case such as blending with thermoplastics and blending with other rubbers. In the case of the chemically modified NR based-blends, it was not only the type of polymer used to blend with NR, but also the variation of the chemical modification of NR would control the different rheological property of the blends. Special-property NR based blends obtained from the... 

ENR prepared from chemical modification of NR latex has recently been commercialized. ENR was claimed to have oil resistance property, low gas permeability and more anti-oxidizing and damping than NR. Up to now, many research works have studied its applications and their blends with other polymers such as in this case, the blend between ENR and neoprene (CR) of which the effect of blending ratio on processability of ENR/CR blend was studied by Chiu et al Mastication of ENR (ENR-50, Malaysian Rubber Producers Research Association), CR (ES-2-16K, Du Pont), vulcanizing agent, accelerator, and other related ingredients was carried out in a pressurized kneader (SYD-5, Star-King Enterprise Co., Taiwan) for 17 min under a rotation rate of 77 rpm. Each sample s composition was listed in Table 18.7. 

K. Dawes and R.3. Rowley, Chemical modification of NR - a new silane coupling agent. Plastics and Rubber Materials and Applications, 3, 23 (1978). 

It has been recognized for many years that chemical modification of NR can be utilized to produce new materials. Chlorinated, hydrochlorinated and cyclized NR have all been produced commercially and poly(methyl methacrylate) grafted" and depolymerized NR are still available. More recent chemical reactions have been restricted to limited levels of modification to change selected properties while retaining the overall rubbery nature of the polymer. These modifications have also been utilized for antioxidant functions, vulcanization sites and coupling to silica fillers. ... 

The epoxidation of NR and other unsaturated polymers has been reported in the literature. However, there are little data on the properties of these materials and, where they do exist, they tend to be conflicting. The above reactions established criteria for the chemical modification of NR, which led to the development of clean epoxidized natural rubber (ENR). These new polymers have improved oil resistance and decreased gas permeability, whilst retaining many of the properties of NR and also exhibiting some novel features. 

The challenge here is the achievement of good compatibility between both polymers because of their dilferent chemical affinities starch is hydrophilic and NR is hydrophobic. With the aim of improving the compatibility, some examples can be found in literature such as the chemical modification of starch by an esterification process and the generation of a graft copolymer in the starch-NR blends by potassium persulfate. ... 

Effect of fiber treatment Chemical modification of fibers decreased the dielectric constant of OPF-sisal fiber-NR hybrid composites [59]. This was due to the decrease in orientation polarization of the composites upon treatment. Chemical treatment results in reduction of hydrophilicity of the fibers leading to lowering of orientation polarization and subsequently dielectric constant. Alkali treatment yielded higher dielectric constant comparing to silane treatment. However, higher concentration of alkali... 

Such modifications of synthetic rubbers can usually be most easily achieved by the copolymerization of a second or third monomer carrying the appropriate functional group. Although he biochemical polymerization process leading to NR is well understood, it has not yet been found possible to induce the enzymes concerned to incorporate functionalized monomer. Therefore modification of NR has to be achieved by direct chemical reactions on the rubber itself. 

NR is essentially a simple olefin and therefore, in theory, amenable to the very many known chemical reactions of such species. The trialkyl-ethylenic double bond is electron rich via inductive and hyper-conjugative effects although somewhat sterically hindered. The better knoi n chemical modification of diene rubbers, including NR, have been reviewed although the literature contains many other examples of more exotic reactions. Most reactions pertinent to a trialkyl ethylene have been tried on NR and, in general, they work with variable degrees of efficiency. However, usually only solution-phase chemistry has been employed. 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

Based on historical data halide modification (HM) of high-molecular compound was carried out in 1859, natural rubber (NR) was exposed to modification and, in addition to that, NR was dissolved in perchlorometh-ane, through which chlorine gas was run through. Modified NR is powder product with content of fixed chlorine not over 62-68% m., which didn t have properties of elastomer [1, 2]. Halide modification of NR may be referred to one of the first attempt of commitment of new properties to polymer with help of carrying out of chemical modification. 

Natural rubber (NR) is an excellent elastomer with a well-balanced set of properties. For this reason, it is used in a wide range of applications, including very demanding engineering components such as structural bearings. Many applications of rubber, however, require special properties for which NR is deficient. Others require that the rubber be of a different physical form than is normally obtained with conventional NR. Thus, modifications of NR either physically or chemically have been an important subject of research carried out by the Malaysian Rubber Research and Development Board. 

Although an excellent elastomer in its own right, the modification of NR either physically or chemically has enabled it to be used as the base polymer in applications where the unmodified rubber would not be suitable. Of the more recent developments in this area, it is expected that TPNR and ENR will be the most commercially important. 

As the name suggests, epoxidised NR is prepared by chemically introducing epoxide groups randomly onto the NR molecule. This chemical modification leads to increased oil resistance, greater impermeability to gases, but an increase in the glass transition temperature, Tg, and damping characteristics the excellent mechanical properties of NR are retained. 

Gopalan Nair et ehemieally modified the chitin whiskers by using different coupling agents, namely phenyl isocyanate (PI), alkenyl succinic anhydride (ASA) and 3-isopropenyl-R, R -dimethylbenzyl isocyanate (TMI). These various chemieal treatments on chitin whiskers improve the adhesion between the chemically modified ehitin whiskers and the NR matrix. However, the mechanical performances of these NR composites markedly decrease after chemical modification. This is due to the partial or total destruction of the three-dimensional network of ehitin whiskers, which is assumed for the unmodified composites. 

Modification by attachment of new chemical groups for example chlorine, epoxy, etc. Attachment of these groups will result addition or substitution reactions at the olefinic double bonds and format chlorinated NR, hydrochlorinated NR and epoxidized NR (ENR). Chlorination and hydrochlorination of NR can be carried out in dry, solution or latex form. Due to its resistance against alkalis and acids, and its wear... 

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