Hevea rubbers

Hetran teams Hetrazan Heubach method Heulandite Heuristic approach Heuristics Hevea brasiliensis Heveacrumb process Hevea rubber Hexa [100-97-0]... 

Rubber obtained from the shrub, Parthenium argentatum, native to north central Mexico and the adjacent part of Texas. The rubber is not obtained by tapping but by harvesting the shrubs and crushing the woody tissue to separate it from the rubber. Intensive research, started in 1942, has not succeeded in making guayule competitive with Hevea rubber, and since 1962 there has been no commercial production. 

Hevea rubber from uncultivated trees growing in the Amazon Valley of Brazil and exported from Para, a town at the mouth of the river. Para rubber was the best variety of all wild rubber but the advent of plantation rubber steadily reduced its importance until it is now of no significance in world rubber production. 

In the Far East, on the whole, the most troublesome hevea rubber diseases are root decays. It is commonly realized (4, 11, 35, 36, 40, 42, 43, 45) that, left unchecked, rubber root rots would have very soon destroyed commercial production of plantation rubber in the Orient. There is no tree crop that has had its many root rot diseases given such thorough study as has rubber. E. W. Brandes, R. D. Rands. Theodore J. Grant, E. P. Imle, and John B. Carpenter agreed that these troubles have been of practically no concern in tropical America. Langford (19) found little root rot trouble in his considerable observations specifically on rubber in the Western Hemisphere. 

Hevea rubber is undoubtedly one of the unique crops of history and of all agriculture, and one of the most interesting. It is not easy to produce rubber. Research is the tool by which it is possible to grow vast acreages of the tree as a profitable crop. This could never have been done without the past /and present intensive investigations of careful scientists over more than 30 years. Repeated reference in the literature indicates that diseases are the limiting factors in natural rubber production, and that planters owe a debt to disease-control workers. 

The author wishes to make special acknowledgment of the information obtained on rubber disease problems for many years through old contacts while employed by the U. S. Department of Agriculture. The pathological problems of hevea rubber are unique in some respects and have been discussed innumerable times in several different countries with many members of the old Division of Rubber Investigations,... 

Rands, R. D., Hevea Rubber Culture in Latin America, Problems and Pro-... 

Wellman, F. L., Condition of Hevea Rubber Plantations in E1 Recreo, and... 

The chlorination of low molecular weight natural rubber from Guayule (Parthenium Argentatum Grey) has been accomplished. The structure of the chlorinated product is consistent with that of chlorinated Hevea rubber. The use of Azo-bis-isobutyronitrile was as a catalyst resulted in increased chlorine content with a concomitant reduction in molecular weight, thereby allowing the preparation of lower viscosity grades of chlorinated rubber. 

Which of the following are monodisperse polymers with respect to chain length (a) hevea rubber, (b) corn starch, (c) cellulose from cotton, (d) an enzyme, (e) HOPE, (f) PVC, (g) a specific DNA, (h) nylon 6,6, or (i) a specific RNA ... 

Which is a trans isomer (a) gutta percha or (b) hevea rubber ... 

A polymer similar to natural hevea rubber is obtained using the anionic polymerization of isoprene under these conditions. In more polar solvents employing sodium and potassium initiators the amount of cA-1,4 units decreases and trans-, A and trans- i,4 units predominate. 

Block copolymers, with segments or domains of random length, have been produced by the mechanical or ultrasonic degradation of a mixture of two or more polymers, such as hevea rubber and PMMA (Heveaplus). 

FIGURE 9.5 Ball-and-stick models of soft hevea rubber (cw-l,4-polyisoprene) (top) and hard gutta-percha (trfl i-l,4-polyisoprene) (bottom). 

Prior to the discovery of the vulcanization or cross-linking of hevea rubber with sulfur by Goodyear in 1838, Faraday has shown that the empirical formula of this elastomer is CsHg making it a member of the terpene family. The product obtained by pyrolysis of rubber was named isoprene by Williams in 1860 and converted to a solid (polymerized) by Bouchardat in 1879. 

At what temperature would the properties of PS resemble those of hevea rubber at 35 K above its Tg ... 

Naturally occurring antioxidants are present in many plants and trees such as hevea rubber. The first synthetic antioxidants were synthesized independently by Caldwell and by Winkelman and Gray by the condensation of aromatic amines with aliphatic aldehydes. 

Captax (Structure 15.21) is used to the extent of 1% with hevea rubber and accounts for the major part of the over 30,000 t of accelerators used annually in the United States. Other accelerators widely used include 2-mercaptobenzothiazole sulfenamide (Santocure Structure 15.22), used for the vulcanization of SBR dithiocarbamates and thiuram disulfides. Thiuram disulfide (Structure 15.23) is a member of a group called ultra-accelerators, which allow the curing of rubber at moderate temperatures and may be used in the absence of sulfur. 

Before World War II, hevea rubber accounted for over 99% of all elastomers used, but synthetic elastomers account for more than 70% of all rubber used today. NR and many synthetic elastomers are available in latex form. The latex may be used, as such, for adhering carpet fibers or for dipped articles, such as gloves, but most of the latex is coagulated and the dried coagulant used for the production of tires and mechanical goods. 

Till this time, polymer science was largely empirical, instinctive, and intuitive. Several polymers were commercially available prior to World War I celluloid, shellac, Galalith (casein), Bakelite, and cellulose acetate plastics hevea rubber, cotton, wool, and silk rayon fibers Glyptal polyester coatings bitumen or asphalt, and coumarone-indene and petroleum resins. However, as evidenced by the chronological data shown in Table 1.1, there was little... 

This three-component copolymer (terpolymer) (EPDM) contains about 15 double bonds per 1000 carbon atoms in the polymer. This provides enough reactive sites for low-density cross-linking, but unlike Hevea rubber (polyi-soprene), which contains about 200 double bonds per 1000 carbon atoms, EPDM is resistant to ozone. EPDM is used for the sidewalls of tires and for rubber hose but develops too much heat during flexing to be used for heavy-duty tire treads. 

Because of the absence of the methyl pendant group, cto 1,4-polybutadiene is more resistant to abrasion and develops less heat buildup as a result of cyclic flexing than Hevea rubber (polyisoprene NR). Tires made from polybutadiene do not grip the road as well as those made from Hevea elastomers. 

SBR is the most widely used synthetic elastomer. It is an amorphous random copolymer consisting of a mixture of l.2, cis and trans isomers. Cold SBR produced at —20 C consists of 17% 1,2. 6% cis and 77% trans isomers of polybutadiene. This commercial product has a Tt of -60 C, an index of refraction of 1.534S, and a coefficient of linear expansion of 66 X 10 s cm/ cm C. Because of the high percentage of the trans isomer, it is less flexible and has a higher heat buildup, when flexed, than Hevea rubber. Although carbon black-filled or amorphous silica-filled SBR has useful physical and mechanical properties, the SBR gum rubber is inferior to Hevea rubber. 

The chemical and the solvent resistance of SBR are similar to those cited for Hevea rubber. SBR does not crystallize when stretched. However, its abrasion resistance is superior to that of Hem rubber. 

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