Archive for March, 2010
Rubber Technology and Introduction
The term “rubber” meant the material obtained from the rubber tree Hevea brasiliensis. Today, a distinction is made between crude rubbers and vulcanized rubbers, or elastomers. Elastomer is the preferred term for vulcanized rubber. Other terms that are less frequently used in clued vulcanizate and crosslinked rubber. Elastomers or rubbers are classes of materials like a wood, fibers, metals, glasses, or plastics. The annual consumption of rubber amounts more than 13 million tons. About one third of total global rubber usage is natural rubber that produced in plantations in Thailand, Malaysia, Indonesia as well as in West Africa, and South or Central America. Two thirds of the required rubber is produced synthetically by industrial countries which oil is used to raw material for producing synthetic rubber. More than half of the global production of natural and synthetic rubber is used in tires and great variety of consumer products from motor mounts, fuel hoses over window profiles and heavy conveyor belts to membranes for artificial kidneys. The property of elastomers is the elastic behavior after deformation in compression or tension. It is possible to stretch and elastomer ten times its original length and after removal of the tension it will be return to its original shape and length. Moreover elastomers are characterized by a great toughness under static or dynamic stresses, an abrasion resistance which is higher than steel, by an impermeability to air and water, by a high resistance to swelling solvents and chemicals attack. Rubbers are also capable of adhering to metals and textile fibres. By joining elastomers with metals components which combine the elasticity of elastomers with the rigidity of metals. In combination with fibres such as rayon, polyamide, polyester, glass or steelcord and depenting on the properties of the reinforcing member the tensile strength is increased considerably with and attending reduction in extendibility. This can be of great importance to designers. The properties profile which can be obtained with elastomers depends mainly on the choice of the particular rubber, the compound composition, the production process, and the shape and design of the product. Properties which do justice to elastomers can only by proper compounding with chemicals and other additives of which there are about 20000 different ones, and subsequent vulcanization. Depend on type and chemicals additives and degree of vulcanization given rubber different properties with respect to hardness, elasticity, or strength. And the typical properties of the specific rubber, namely oil, gasoline, aging resistance remain unaltered in the different valcanizates. Read the rest of this entry »
Abbreviations, Chemicals and Trade Names of Rubbers
|BRCRIIR||Butadiene rubbersChloroprene rubbersIsobutene-isoprene rubbers (butyl rubbers)||Buna CB, BudeneNeoprene, BaypreneExxon Butyl|
|NBR||Synthetic Acrylonitrile-butadiene rubbers(nitrile rubbers)||Perbuna, Chemigum|
|HNBR||Hydrogenated nitrile rubbers||Therban, Zetpol|
|NR||Isoprene rubbers, natural (natural rubbers)|
|SBR||Styrene-butadiene rubbers||Buna His, Cariflex S|
|ACM||Polyacrylate rubbers||Cyanacryl, Europrene AR|
|CSM||Cholrosulfonated polyethylene rubbers||Hypalon|
|EPDM||Ethyline-propylene-diene rubbers||Keltan, Nordel|
|EPM||Ethylene-propylene rubbers||Vistalon, Dutral|
|FPM||Fluorocarbon rubbers||Viton, Fluorel|
|VMQ||Vinyl-methyl silicone rubber||Siloprin|
|ECO||Epichlorohydrin rubbers||Hydrin, Epichlomer|
|AU||Polyester urethanes||Urepan, Pellethane|
Natural Rubber (NB)
Many plants produce a milky sap, also referred to latex which is colloidal caoutchoouc dispersion in an aqueous medium. The latex-producing plants of which there are many known belong to different botanical families and they are predominantly found in tropical climates.
Not all caoutchouc-producing plants are harvested for industrial purposes because the yield is either too small the caoutchouc content in the latex too low, or the caoutchouc contains too many resinous impurities.
Early plantation economies used Ficus elastica, de Castilloa, Funtumia and Manihot plants, but they were soon displaced by the Hevea brasiliensis, because the latter gives a much greater yield of a superior caoutchouc.
Modern plantations the Hevea brasiliensis is cultivated. It is a tree up to 20 meters high, with a deep taproot and the first harvests can only be expected from trees which are at least six years old.
In modern plantations the preferred method of propagation is by various vegetative propagation of individual is referred to “clone”. Plants are created by bringing together a wood bud scion and a rootstock to combine in a tree a strong and disease-resistant root system, a stem with a tapresistant bark, and a canopy that is strong and well developed.
Another, albeit limited method of improving yield is the stimulation of latex flow by means of various chemical, such as chloroethyl phosphoric acid or aminotrichloropicolinic acid. After application, these chemical penetrate the bark and produce ethylene within the plant.
The largest producer of NR in Thailand with 1.5 million ha and about 0.6 million tons or 15% production share. In addition, there are Malaysia , Indonesia, India and China, Sri Lanka and numerous other Asiatic, Africal, and Amercan countries located in latitudes of about 100 on either side of the equator.
Tapping of NR latex. In the Hevea, these capillary vessels are longitudinally continuous cells arranged as sheaths concentric with the outer bark. The majority of these vessels is found near the cambium in a 2 to 3 mm thick zone, and the diameter of individual vessels is about 20 to 50 um. If the vessels are severed by a cut in the bark of the tree, latex flows along the cut very slowly and it coagulates after 2 to 5 hours, due to evaporation, thus plugging the vessels to prevent further flow.
The most used system of tapping is the “half spiral” method, which allows tapping every other day. Using gouges and , more recently, specially designed tapping knives, about 0.5 mm to 1-2 mm of cambium is removed. Expert tappers are also capable of making full spiral cuts aroung the complete tree trunk. This very difficult tapping method is called S1/d4 that mean a full spiral cut every fourth day.
Coagulation, Processing of the Coagulate, Sheets and Crepe. Method for recovering the caoutchouc is acid coagulation. The coagulating agent is mostly formic or acetic acid. By collecting the latex in large tanks, agood measure of crossblending of latex from trees of different ages and form different locations is achieved.
Nomally, the latex is diluted with water to a solids content of up to 12 to 18%. The more dilutes the latex, the greater amount of acid is required for coagulation. The iso-electric point is reached at a pH of 5.1 to 4.8 under which conditions the latex coagulates.
The coagulum has to be processed immediately because it changes properties in air under the influence of bacteria.
The coagulation method for the various NR grades differs but little. There are two standard ways of processing the coagulum. First to drying by exposure to hot wood smoke to produce a smoked sheet and air dry to produced pale crepe.
The special grades that have to specific requirements.
Initial Concentration Rubber (ICR) is produced from diluted latex.
Superier Processing Rubber (SP/PA Grades). Containing up to 50% of a crosslinked phase are lebelled SP caoutchouc, e.g. SP20 , SP40 with a 20% and 40% crosslinded blend component.
Oil Extended Natural Rubber (OE-NR).Contains 5 to 40 phr of either, naphthenic or aromatic oil.
Deproteinated Natural Rubber (DP-NR), also referred to as low nitrogen natural rubber(LN-NR)
Heveaplus MG Grades. These are graft copolymer of NR bound with antioxidants polymethylmethacrylate.
Expoxidized Natural rubber (ENR) is a new class of NR and it is available with a degree of epoxidation of 10 to 50% to prevents the ring opening reaction.
Thermoplastic NR is NR and polypropylene which has been crosslinked with peroxide.
Depolymerized NR is a liquid NR of syrup-like consistency. It was specially developed for liquid rubber processing technology.
Powdered or Particulate NR is obtained by grinding NR, but it is of no particular importance.
Peptized NR has proved itselt in the rubber insustry, because it is particularly pure, it can be processed very economically and it gives good vulcanizate properties.
Synthetic Rubber (SR)
For over a century, all rubber goods were manufactured from natural crude rubber which is generated in the rubber tree as a milky liquid known as natural latex. The latter is coagulated and the solid material separated washed and dried to obtain a solid natural crude rubber. After that synthetic crude rubbers were developed and became available in commercial quantities. It’s prepare by reacting certain low molecular weight substances called monomers to form long-chain molecules called polymers that usually obtained and a water emulsion known as synthetic latex, which is similarly coagulated and the solid material separated, washed and dried to obtain solid synthetic crude rubbers.
SR is chemically similar to Natural Rubber (NR) in that they contain olefinic double bonds and could be vulcanized by means of sulphur. These material are obtained through homo or copolymerization of conjugated dienes. In addition mono-olefins and other monomers were increasingly used for the synthesis of saturated polymers which can be crosslinked through reactions other than those involving sulphur. These types of SR can be produced by vinyl polymerization by polycondenzation as well as by polyaddition reactions. Rubbers produced from diene monomers are the most important and widely used ones, whereas saturated rubber constitute as a rule specialty products. By now, the number of SR types offered by the chemical industry to the user has grown very large so that it has become useful to classify the different grades of SR.
Depend on the method used for synthesis one distinguishes chain addition polymerization polycondenzation and polyaddition products.
The polymerization of monomers of the same kind results in homopolymers while that of different monomers produces copolymers. Those copolymers obtained from three different monomers are referred to as terpolymers. The so-called diene rubbers are produced by the homo or copolymerization of conjugated diolefins alone or in combination with olefins so that the polymer chain retains unsaturation which allows vulcanization with sulphur analogous to NR.
By polymerizing simple unsaturated monomers (mono-olefins) and by the polycondensation or polyaddition of saturated components, one obtains fully saturated polymers which cannot be crosslinked with sulphur. By far the greatest number of these polymers are plastics. Howerver, using special methods they can also be loosely crosslinked so that treir physical state can be modified similar to that of diene rubbers through sulphur vulcanization. Thus, these polymers can be used as precursors for producing elastomers.
The different types of rubbers are classified according to ISO R1629 or ASTM-D 1418. Some commercially SR such as butadiene rubber (BR), chloroprene rubber (CR), isobutylene isoprene rubber (IIR), isoprene rubber (IR,synthetic), isoprene rubber (NR, natural rubber), styrene butadiene rubber (SBR).