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Natural Rubber (NR):
Natural rubber is the prototype of all elastomers. It is extracted in the form of latex from the bark of the Hevea tree. The rubber is collected from the latex in a series of steps involving preservation, concentration, coagulation, dewatering, drying, cleaning, and blending. Because of its natural derivation, it is sold in a variety of grades based on purity (color and presence of extraneous matter), viscosity, viscosity stability, oxidation resistance, and rate of cure. Modified natural rubbers are also available, with treatment usually performed at the latex stage. These include:
1. epoxidized natural rubber (ENR).
2. deproteinized natural rubber (DNR).
3.
4. process oils have been incorporated.
5. Heveaplus MG rubber ? natural rubber with grafted poly(methyl methacrylate) side chains.
6. thermoplastic natural rubber (TNR) – blends of natural rubber and
polypropylene.
The natural rubber polymer is nearly 100% cis-1,4 polyisoprene with MRwR ranging from 1 to 2.5 x 10P6P. Due to its high structural regularity, natural rubber tends to crystallize spontaneously at low temperatures or when it is stretched. Low temperature crystallization causes stiffening, but is easily reversed by warming. Crystallization gives natural rubber high tensile strength and resistance to cutting, tearing, and abrasion.
Like other high polymers, natural rubber can be pictured as a tangle of randomly oriented sinuous polymer chains. The “length” of these chains is a function of their thermodynamic behavior and is determined as the statistically most probable distance between each end. This chain length
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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