Crepe natural rubber

In appearance, HR resembles natural crepe rubber, since it is an aliphatic, hydrocarbon polymer the density being the minimum (0.91) attainable for elastic materials of this type. In HR, the original unsaturation is very small, and even this low unsaturation is greatly reduced and may even be entirely eliminated during the compounding and curing process. The fact that once vulcanised it is extremely resistant to chemical attack is understandable because it becomes, after vulcanisation, not only a nonthermoplastic strong elastic material, but also essentially a chemically saturated product as well. This means that whilst physically vulcanised HR resembles soft vulcanised natural rubber, chemically it may be considered most similar to ebonite almost devoid of any unsaturation. 

Rubber is a polymer of isoprene (15). Natural rubber is obtained from the bark of the rubber tree as a milky white liquid, which is called latex (Fig. 19.8) and consists of a suspension of rubber particles in water. The rubber itself is a soft white solid that becomes even softer when warm. It is used for pencil erasers and was once used as crepe rubber for the soles of shoes. 

Coagulation of natural rubber latex by stages with the object of removing the yellow colouring matter in the first fraction. The latex in the second fraction produces a white crepe rubber. See Bleaching Agent. 

A modified type of natural mbber which has many processing advantages over normal sheet and crepe rubbers. It shows reduced shrinkage, reduced die swell and much better retention of dimensions especially in complicated extrusions. It is made by adding a quantity of vulcanised latex to normal latex before coagulating. See PA 80. 

Strips of naturally-coagulated rubber removed from the tapping cut, vertical channels and spout prior to making the next tapping. The rubber is washed, dried, creped and sold as blanket crepe. Triallyl Cyanurate... 

An efficient peptising agent for natural rubber it is also used as a bleaching agent in the preparation of pale crepe rubber. 

Watson and coworkers (68,69) studied in details the mechanical reaction of systems of natural rubber plus poly(methyl methacrylate), using a laboratory masticator at 76 rpm, in a nitrogen atmosphere at 15° C. Prior to mastication, the deproteinized crepe rubber was extracted with acetone, imbibed with mdnomer and allowed to homogenize for 16 h, in the absence of light It must be underlined that, with limited mastication and reaction heat dissipation, the actual rubber temperature was much higher the maximum recorded was about 50° C and... 

Vulcanization is the most important natural rubber chemical reaction. Most applications require cross-Unking via vulcanization to increase resiliency and strength. Exceptions are crepe rubber shoe soles and rubber cements. There are a number of methods for sulfur vulcanization, with certain methods producing polysulfide crosslinking and other methods producing the more monosulfidic crosslinks. 

One of the first uses of natural rubber was footwear and much is still used in this product. Probably about 5% of natural rubber production goes into this application. This is especially true in the Far East where rubber and labor costs are cheaper in a labor-intensive competitive industry. Besides its use in the well-known crepe rubber soles, natural rubber is used in sport and protective footwear... 

Fig. 1. Routes by which natural rubber latex is converted into traditional sheet and crepe.

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. 

Oxidation of Natural Rubber (NR) Pale Crepe at 46°C, Accelerated by Ultraviolet (UV) Light... 

A grade of natural rubber inferior to estate brown crepe and formerly known as thick remilled brown crepe cuttings removed in the inspection of smoked sheet rubber are washed, creped and designated smoked blanket crepe . 

Any form of natural rubber which is marketed with the rough surface produced by passing the rubber through a series of mills with grooved rolls travelling at differential speeds. The main types are pale crepe, sole crepe, brown crepe and flat bark crepe. 

The remaining latex is processed into dry rubber as sheets, crepes and bales. There is an International Standard for the Quality and Packing for Natural Rubber grades, the so-called Green Book , published by the Rubber Manufacturers Association. The following grades of NR listed in the Green Book are sold to visual inspection standards only ... 

Raw rubber can be either natural or synthetic. The standard grades of natural rubber are smoked sheets and pale crepe. For high grade products, first grade smoked sheets, namely RMAIX or Pale Crepe, is used. Pale Crepe is practically colourless and is preferred in products where delicate colour, extreme whiteness or transparency... 

Natural Rubbers (NR). The chemical name for NR is polyisoprene, which is a homopolymer of isoprene. It has the cis-1,4 configuration. In addition, the polymer contains small amounts of non-rubber substances, notably fatty acids, proteins, and resinous materials that function as mild accelerators and activators for vulcanization. Raw materials for the production of NR must be derived from trees of the Hevea Brasiliensis species. NR is available in a variety of types and grades, including smoked sheets, air-dried sheets, and pale crepes. 

The elastomer determines most of the physical and chemical characteristics of a rubber compound. Typical elastomers are natural elastomers such as natural rubber (NR), sometimes called crepe, and synthetic elastomers such as butyl (including chlorobutyl and bromobutyl), ethylene propylene diene monomer (EPDM), and styrene butadiene rubber (SBR). A list of commonly used elastomers is shown in Table 2. 

Raw natural rubber shows high green strength by virtue of strain-induced crystallization. Though natural rubber in the raw form is used in a few applications such as adhesives, binders, and sole crepe, it is not truly thermoplastic, primarily due to its high molecular weight [29]. 

Flat bark crepe All types of scrap natural rubber including earth scrap... 

---