International natural rubber type

TABLE 9.2 International Natural Rubber Type and Grade Specification ... 

There are several systems that define the quality and uniformity of natural mbber. One system of grading natural mbber is based on form and visual observation of color and cleanliness. This is known as the International Natural Rubber Specification. The principal types and grades are as follows. There are five other types of mbber classified by this system and many other grades not Hsted here. 

Eight basic natural rubber types are recognized internationally by appearance and description only. These are ... 

In solid form, the natural rubber is graded according to the content of dirt remaining from the precipitation of latex at the plantation. Eight basic NR types have been traditionally recognized internationally. Only the so-called ribbed smoked sheets and the pale crepes are normally used for adhesives. The predominant grade system, the Standard Malaysian Rubber system, has been used since 1965. 

The pneumatic tire has the geometry of a thin-wallcd toroidal shell. It consists of as many as fifty different materials, including natural rubber and a variety ot synthetic elastomers, plus carbon black of various types, tire cord, bead wire, and many chemical compounding ingredients, such as sulfur and zinc oxide. These constituent materials are combined in different proportions to form the key components of the composite tire structure. The compliant tread of a passenger car tire, for example, provides road grip the sidewall protects the internal cords from curb abrasion in turn, the cords, prestressed by inflation pressure, reinforce the rubber matrix and carry the majority of applied loads finally, the two circumferential bundles of bead wire anchor the pressnrized torus securely to the rim of the wheel. 

Since internal mixers can be opened for material feed and compound removal, these are generally simpler to clean when compared to screw-type mixers. It is a common practice to run natural rubber or special high-tack, cohesive compounds through a mixer as a final stage of cleaning. Further, the first batch of compound is typically sacrificed or saved for experimental work following clean-out. 

Kanerva et al. 1996). Natural rubber latex and laboratory animals are probably the most common causes of contact urticaria in the laboratory environment. Other types of noninfectious biological risk may include exposure to animal and plant proteins, e.g., flour, grains or feed, and decorative plants, and enzymes (Tarvainen et al. 1991). Contact urticaria from laboratory animals is more common in persons carrying out experiments with animals, e.g., laboratory technicians and chemists, than in those tending them. The former tasks involve more direct handling of the animals, their secretions, and their internal organs (Susitaival and Hannuksela 1997). 

One of the important families of blended TPEs is thermoplastic natural rubber (TPNR), based primarily on polyolefin and natural rubber (NR). TPNR blends have attracted much attention due to their ease of processability and a broad spectrum of properties available at a competitive price. The TPNR system consists of multi-phases of hard and soft domains from blending thermoplastics with NR. Generally, TPNR is prepared with various type of polyolefin, such as PP, LDPE, LLDPE and HDPE. TPNR is usually prepared by a melt mixing process using an internal mixer or twin-screw extruder. It presents a wide range of useful properties, excellent processing abilities, and can be produced at moderate cost. 

Liotta (12) constructed his artificial heart from a combination of different materials - Lucite, teflon, polyester urethane, and silk. It is difficult to evaluate the thrombogenic potential of this combination of materials because the longest survival was 13 hours in dogs. The principle cause of death in these animals was low cardiac output secondary to inadequate venous return. The following year in a different series of experiments Liotta (13) tried a different combination of materials. Here, although the experiments were acute in scope, thrombus formation at the blood-plastic interface was a major problem. The left ventricular assist device (LVAD) was a tube-type with the housing and valves constructed of Estane. The internal elastic tube was made of either natural rubber. Silastic, or natural rubber covered externally with Silastic. 

It is necessary for the development of suitable methods of product fabrication to have some types of polyurethane elastomer which can be processed as solids using similar methods to those employed for conventional natural and synthetic rubbers, namely two-roll mill or internal mixing and compounding procedures, calendering, extrusion, moulding, fabrication of complex rubber-textile laminates and the like. 

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