Lecture No (2) Introduction of Rubber

Transcription

1 Lecture No (2) Introduction of Rubber

2 Introduction Polymeric materials could be classified according to behavior with rising temperature (thermosets, thermoplastics and elastomers), the attention has been paid to elastomer (rubber) in this study. The tensile stress-strain diagram for the three types of polymer as shown in (Figure 1). (Figure 1): Tensile Stress-Strain Diagram for three Types of Polymer: Glassy (Thermosets); Crystalline (Thermoplastics); and Rubbery. The rubbers (elastomeric) materials can be classified into two major types according to common use are natural rubber and synthetic rubber. Natural rubber (NR) is exhibit many outstanding properties, such as good oil resistance, low gas permeability, improved rolling resistance, high strength, resilience, elasticity, resistance to abrasion, impact resistance, low cost, and efficient heat dispersion makes it ideal for use in medical and other application. The natural rubber coming from latex that bleeds from the wounding in plants. The drop in natural rubber product can be attributed to the discovery of crude oil, uneconomic size of farmer s holdings, old age of plantations, little agricultural inputs like fertilizer, not found availability for credit facilities, and pesticides, also shortage and high cost of labor in many rubber producing areas. Synthetic rubber is any type of artificially made by man from petrochemical feedstock or polymer material is normally obtained by emulsion addition polymerization and condensation polymerization, which acts as an elastomer. 1

3 Several types of synthetic rubber were made, for some purposes they are better than natural rubber, but, for others, not as good, such as styrene butadiene rubber SBR, nitrile butadiene rubber NBR, butyl rubber IIR, butadiene rubber BR, isoprene rubber IR, neoprene rubber CR etc. The SBR and BR varieties are the most widely consumed type of synthetic rubber. A wide variety of particulate fillers such as carbon black, silica, alumina and clay, which are used in the rubber industry for various purposes, which the most important purposes are reinforcement, to reduction in material costs, improvements in processing, and to improve the mechanical, electrical and fracture properties of rubbers, for examples increase the strength of rubber, increase the hardness, stiffness, tensile modulus, abrasion resistance, tear resistance and increase frictional coefficient, reduced the rebound or resilience. Other fillers, plasticizer are necessarily added to compounding rubber. Due to the rubber is important in the socioeconomic life, many countries developing such as India, Nigeria and Brazil. And great industrial development in transportation field, that lead to many researches that have been made to improve the rubber product that used for examples for the car tires and sport utilities. During the years 1914 to 1940 natural rubber demand had risen from 0.12 tonnes to 1.13 million tonnes for car tire industry, while the forecasted production of natural rubber (12.5 million tons), and the forecasted consumption (13.6 million tons), from all world countries for other uses. While expected a production capacity of rubber in 2020 about 680,000 tons. In 1980, Asia supplies of natural rubber with over 90% and represents the world's largest natural rubber producer, which is based on advantages in terms of available land and labor. History of Rubber Production The history of rubber dates back to the 1525 in the peoples America, and the first known elastomer was natural rubber (NR), were used as rubber balls and use of rubber as mortar. In 1615, the first practical use of rubber in the waterproofing footwear, when peoples of South America extracted this substance from trees 2

4 often called "Hevea Brasiliensis" and "Para Rubber" trees, meaning "weeping wood", these trees represents sources of natural rubber, that begin grew in the Amazon forests in At that time the natural rubber became first demand of a product in world, this is lead to Amazonas becomes the economic heart of Brazil. During the middle of the 18th century to about the end of 20th century, the rubber industry experienced those important steps of development. At that time most rubber tree plantation grew in British and British Colonies in South and Southeast Asia particularly in Thailand, Malaysia, India, China and Indonesia, today rubber tree are grew in Africa such as Nigeria, which became important producers of natural rubber. The difference between latex production techniques in Brazil and Asia, lead to the development of business and increase in productivity. The history of natural rubber in Europeans began from the second half of the 19 th century, when increasing demand of the natural rubber. The rubber studied reaching to the conclusion that this was nothing more than a "type of condensed resinous oil", and suggested that rubber could be used to produce flexible tubes. In 1820, British industrialist produced rubber and attempted to use them in clothing. The first bicycle tire product in 1830, while in 1832, the first factory was set up rubber product. At some time the America was used rubber in all sectors such as particularly automobiles, electric light and waterproof footwear and snow-boots. Charles Goodyear (in England) and Hancock (in America) simultaneously discovered the vulcanization procedure about 1839 by accident, which was the revolution for rubber industry; the vulcanization changed the nature of rubber, make it non-sticky and much more usable. In this process, because of added sulfur, the rubber becomes cross linked and also has better elasticity. In 1845, R.W. Thomson invented the pneumatic tire and the inner tube, in 1869 made the solid rubber balls and hollow rubber balls for the golf and tennis, and in 1888, first use of rubber to form of rain jackets. The Russians and the Germans have work efforts to synthesize rubber, which compete with natural rubber. Because the importance of the rubber industry 3

5 since it first appeared, the rubber is an important raw material that plays in the development of modern civilization, this is prompted much interest from chemists and engineers during Second World War, to discovering its chemical composition of rubber in order to industrialized of synthetic rubber, and discovering that rubber is an isoprene polymer and know how polymerization. This was the springboard for the development of the synthetic rubber industry, and producing elastomers. The major problem of rubber in warm weather rubber becomes sticky and in cold weather it became brittle. While, when heat treating rubber with sulphur during the vulcanization process, rubber was not affected by temperature. Natural Rubber (NR) Natural rubber is a solid product obtained through coagulating the latex (raw material) is usually produced from the rubber tree. Natural rubber is thermoplastic, with a glass transition temperature of 70 C. Nowadays, most natural rubber latex is derived usually from the species (Hevea Brasiliensis) or (Para Rubber) trees which are a commercial tree and economically grown in plantations of Amazon forests or grown in areas with similar conditions to the Amazon. Which, is a quick-growing tree that exceeds 25 m in height, branchless for 10 m or more, and least 50 cm in diameter with smooth bark surface. When a tree matures at the age of 5 to 8 years, the latex is collected during rubber tapping process. Definition of Latex The word latex is commonly used to denote a stable colloidal dispersion of a polymer in an aqueous medium. It has sometimes been extended to include stable colloidal dispersions of polymers in non-aqueous medium in which the polymer is insoluble in it. That is a white, milky fluid that is held in cells that found in the inner layers of the trees. Extraction of Latex Although there a large number of trees that exude secretions similar to latex when the bark is cut, but only a few produce sufficient quantities of a quality rubber. The natural rubber is extracted in the form of latex. 4

6 The natural rubber latex is collected from a crack in the process is known as rubber tapping process, which involves peeling a thin tree layer from part of plants, such as bark, roots, leaves, stems, and fruits, by using a special knife or harvesters to make cracks across the latex vessel, with deep enough without damaging the growing layer, or harming the growth or health of the tree and the tree wound later heals itself. The latex drop from the cut and flows into a collected in small containers for a several hours until it begins to coagulate and the flow stop as shown in (Figure 2). The rubber is product when the coagulated of latex, the latex coagulates occur with the aid of dilute acetic acid, formic acid, and alum. (Figure 2): Typical Rubber Plantation. Rubber Latex Processing Methods After collection the latex, about 70% water, must be taken to a processing plant, where it may be sieved to remove extraneous matter and resinous material, blended (mixed with water), coagulated, rolled into sheets, and then dried in smokehouses. Alternatively, after coagulation, it may be washed, shredded, and granulated under controlled conditions before being dried in bed driers to form a block rubber. Or finally the latex is not utilized in its original due to its high water content and susceptibility to bacterial attack. 5

7 Therefore it is necessary preserve and concentration the latex, at result that the natural rubber is sold as latex concentrate, when water is removed by centrifuging or evaporation process to give stable latex concentrate and contains 60% or more of dry rubber. This required added ammonia with 0.7% percentage is applied to preserve latex coagulation from the bacterial attack, this type of latex uses in many applications as foam, carpet packing, thread and adhesives. Description of Natural Rubber Latex Structure The tapped Hevea latex contains not only rubber particles but also nonrubber particles (such as protein) dispersed in an aqueous medium. The ratio of rubber to non-rubber components varies from source to source. The natural rubber structure is consists of linked chain of isoprene units (C 5 H 8 )n as shown in (Figure 3). And mostly polymerization of the isoprene monomer with a small percentage of impurities to formation repeating alkene double bond in the rubber chain that separated when the latex coagulation. The rubber component from Hevea rubber latex consist of more than 98% of cis 1, 4 polyisoprene and less than 2 % of trans 1, 4 polyisoprene, that represents only natural product in the rubber family, non-crystalline and high elasticity. This structure contains more than 99% of double bonds. (Figure 3): Cis 1, 4 Polyisoprene Units (C 5 H 8 )n in Natural Rubber. History of Synthetic Rubber The first synthetic rubber had been known since 1875, it's expensive, although the structure of synthetic rubber is difference from its natural rubber. The drop in natural rubber production in Brazil coincided with First World War about ( ), lead to need for lower cost with stability products for manufacture 6

8 tires, this was show SBR rubber, which is represented the massive development of the synthetic rubber industry, this product could be vulcanized easily, good costs, good properties and became the important of rubber industry. During Second World War, in 1941, the USA required high natural rubber production, for coat of wire that used in every factory, home, office and military facilities such as warships and tanks. There was no synthetic alternative this is pushed chemists and engineers to develop a synthetic rubber industry. In 1941, the total output of synthetic rubber over 800,000 tons of rubber products. At same time was not any way of producing enough raw materials to produce synthetic rubber, therefore must be obtained in many different ways. The synthetic rubbers were developed since this product was first discovered. Also many researches made to develop the rubber performance in engineering application by enhancing the physical and mechanical properties. Synthetic rubber useful for different application such as car tires, footwear, civil, and medical materials, due to the synthetic rubbers have various properties. 7