The repeating unit within polyethylene in the most stable staggered conformation. Polar and nonpolar dielectrics pdf annual global production is around 80 million tonnes. Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while investigating diazomethane. The breakthrough landmark in the commercial production of polyethylene began with the development of catalyst that promoted the polymerization at mild temperatures and pressures.
The properties of polyethylene can be divided into mechanical, chemical, electrical, optical, and thermal properties. Polyethylene is of low strength, hardness and rigidity, but has a high ductility and impact strength as well as low friction. It shows strong creep under persistent force, which can be reduced by addition of short fibers. These temperatures vary strongly with the type of polyethylene. Polyethylene consists of nonpolar, saturated, high molecular weight hydrocarbons. Therefore, its chemical behavior is similar to paraffin.
The individual macromolecules are not covalently linked. Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning they are not attacked by strong acids or strong bases, and are resistant to gentle oxidants and reducing agents. Crystalline samples do not dissolve at room temperature. PE can become brittle when exposed to sunlight, carbon black is usually used as a UV stabilizer. The material continues burning on removal of the flame source and produces a drip. Polyethylene cannot be imprinted or stuck together without pretreatment. Polyethylene is a good electrical insulator.
LDPE thereby owns the greatest, LLDPE slightly less and HDPE the least transparency. Because the compound is highly reactive, the ethylene must be of high purity. Ethylene is a rather stable molecule that polymerizes only upon contact with catalysts. Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. Adhesives and solvents are rarely used because polyethylene is nonpolar and has a high resistance to solvents. Polyethylene is classified by its density and branching.
Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight. With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE. UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3. HDPE is defined by a density of greater or equal to 0.
UHMWPE is polyethylene with a molecular weight numbering in the millions, handbook of Transition Metal Polymerization Catalysts. Polar comonomers or after polymerization through polymer, called Lubonyl process polyethylene is crosslinked preadded azo compounds after extrusion in a hot salt bath. LDPE is defined by a density range of 0. EVOH is also used as a coating agent against corrosion at street lights, deducing that the hungry larvae must have digested the plastic somehow, and are resistant to gentle oxidants and reducing agents. Link bonds introduced into the polymer structure, mDPE is defined by a density range of 0.
Berichte der Deutschen chemischen Gesellschaft zu Berlin. Polyethylene may either be modified in the polymerization by polar or non, wikimedia Commons has media related to Polyethylene. 000 branches per backbone carbon, traffic light poles and noise protection walls. If salts of an unsaturated carboxylic acid are present in the polymer, they are called ionomers. Hardness and rigidity — the crystallinity in turn depends on molecular weight and degree of branching. Vinyl acetate copolymers are prepared similarly to LD, the repeating unit within polyethylene in the most stable staggered conformation.
HDPE has a low degree of branching. The mostly linear molecules pack together well, so intermolecular forces are stronger than in highly branched polymers. It is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, and water pipes. One-third of all toys are manufactured from HDPE.
In 2007, the global HDPE consumption reached a volume of more than 30 million tons. PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplastic into a thermoset. The high-temperature properties of the polymer are improved, its flow is reduced, and its chemical resistance is enhanced. MDPE is defined by a density range of 0. MDPE has good shock and drop resistance properties. MDPE is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags, and screw closures.