Borealis Chemicals Statoil LDPE
BP Chemicals/Amoco Novex LD
Carmel Olefins Ipithene
Chevron Chevron PE
Dow-Carbide Dowlex, DFDA
DSM Stamylan LD
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4. Some Suppliers 5. Trade Names or Trade Marks
Polyethylene (PE) plastics are plastic materials that are based on polymers made mainly with the olefin commonly called ethylene. This usually means no less than 85% ethylene and no less than 95% total olefins. Although a PE homopolymer may be represented as -(CH2-CH2)n-, the simplicity of the for-mula does not convey the tremendous number of variations (or grades) of PE that are available. Such materials may differ in, for example, molecular weight, molecular weight distribution, short chain branching (SCB), long chain branching (LCB) and the presence of catalyst residues. Polymerization using two or more olefin monomers produces copolymers (olefin copolymers).
Because of developments in catalyst technology, copolymers are undergoing a period of very rapid growth. Many commercial PE materials are copolymers that contain a small amount of an alpha olefin, which permits control over SCB and therefore product density. This is important, as the mechanical prop-erties of PE materials are very dependent upon density.
When ethylene is heated at high pressures (up to 3000 bar/43,500 psi) and at temperatures of 100˚C to 300˚C/212˚F to 572˚F, in the presence of a free radical initiator, the plastic material originally known as polythene or polyethylene (PE) is produced. It was the first PE ever produced, and is now better known as low-density polyethylene (LDPE or PELD). LDPE is a semi-crystalline thermoplastic material whose natural color, in thick sections, is a milky white. It has a soft, wax-like feel. Because chain branching (both long and short chain branching are present) interrupts crystallization of the poly-mer chains, the crystallinity level is low. This means that if the extrudate is kept thin and cooled quickly, then film can appear transparent. The material generally has a relatively low density (typically 0.92 g/cm3), but by varying the polymerization conditions, it is possible to produce commercial materials with densities over the range 0.915 to 0.94 g/cm3.
LDPE is a low-cost material with excellent chemical resistance. It is tough, even at low temperatures, has excellent electrical insulation proper-ties, and is capable of being processed by a wide range of techniques.
However, LDPE only has moderate tensile strength, low stiffness, low maxi-mum use temperature, and suffers from creep. Its resistance to light and fire is poor. LDPE has high water vapor and gas (particularly carbon dioxide) per-meability. Permeability to organic vapors is lowest for alcohols and increases in the order: acids to aldehydes and ketones, esters, ethers, hydrocarbons and halogenated hydrocarbons (permeability decreases with density). LDPE is also susceptible to environmental stress cracking (ESC).
By copolymerization of ethylene with a higher alpha olefin (HAO), it is possible to produce a range of very low-density materials. Very low-den-sity polyethylene (VLDPE or PEVLD) may also be known as ultra low-denlow-den-sity polyethylene (ULDPE or PEULD). Sometimes VLDPE is distinguished from ULDPE on the grounds of density. VLDPE may be considered to be a polyeth-ylene (PE) which has a density of 0.915 to 0.90 g/cm3and ULDPE is a PE 184
which has a density below 0.9 g/cm3(say 0.88 g/cm3). Such very low-density HAO grades, which are hardly crystalline, are rubber-like materials that can be used as an alternative to thermoplastic elastomers and for materials modifica-tion (for example, improving the impact strength of PP).
7. Ease of Flow
Both long chain branching (LCB) and short chain branching (SCB) are present in LDPE and, because LCB produces compact molecules, the material flows relatively easily. The ease of flow is rated by what is known as the melt flow index (MFI) or rate (MFR). Both terms refer to the same test. The lower the number, the stiffer the flow since the molecular weight is, generally, higher. At 200˚C/392˚F a plastic material with an MFR of 20 will have approximately twice the spiral flow length of a plastic with an MFR of 2. Low MFR materials exhibit better environmental stress cracking resistance (ESC), solvent resistance, and higher impact strength. Polymers of different density, but with the same melt flow index, do not necessarily have the same molecular weight.
LDPE is more pseudoplastic (shear thinning) than LLDPE because it has a broader molecular weight distribution. If LDPE of a given melt flow rate (index) is compared with LLDPE material of the same melt flow index, it will be found that the LLDPE material has a higher melt viscosity at processing shear rates.
For blown film production, the MFR ranges from 0.2 to 3.0. The low MFR grades are used for heavy-duty sacks, while the high MFR grades are used for packaging applications where good gloss is required. For extrusion coating less viscous grades, with MFR in the range 4 to 12, are employed. In cable coating, the MFR is generally about 0.2.
The following table shows viscosity values for a MFR 2 film grade resin over a range of shear rates and temperatures.
Shear Rate (s-1) Viscosity (Ns/m2)
LDPE is resistant to most solvents at room temperature; however, aromatic and chlorinated hydrocarbons will cause swelling. It is relatively unaffected by polar solvents, (alcohols, phenols, esters and ketones); vegetable oils, water, alkalis, most concentrated acids (at room temperature), and ozone (in absence of UV). It has very low water absorption, even after long immersion times (<0.2%. after one year at 20˚C/68˚F). The addition of carbon black, used to improve weathering, will increase the water absorption. Absorption of other liquids, such as acetone and benzene, will be greater for LDPE than for HDPE.
The best chemical resistance is found with HDPE and cross-linked PE.
Broadly speaking, PE materials exhibit similar chemical resistance in the long term, although, in the short term, LDPE will be attacked more quickly than the higher density materials. Overall, HDPE or LLDPE has the best chemical resistance. HDPE resists aromatic and chlorinated hydrocarbons more than LDPE. In general, PE materials resist fungal and bacterial attack.
They are resistant to hydrolysis and attack by most common organic solvents at room temperature. They are insoluble in organic solvents at temperatures below 50˚C/122˚F. At higher temperatures, they are soluble in hydrocarbons and halogenated hydrocarbons. They can be dissolved in hot solvents, such as toluene, xylene, amyl acetate, trichloroethylene, petroleum ether, paraffin, tur-pentine and lubricating oils. Although there is no solvent for PE at room
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perature, they swell in chlorinated hydrocarbons, aliphatic hydrocarbons, aro-matic hydrocarbons, some esters and oils.
Polyolefin materials are resistant to acids and to alkalies, but are not to oxidizing acids, such as nitric acid, chlorosulfonic acid, chromic acid, and fuming sulfuric acid. They resist acid, basic and neutral salts. The materi-als have good resistance to gases such as carbon dioxide, carbon monoxide, hydrogen sulphide and sulfur dioxide (wet and dry), but have poor resistance to chlorine (wet and dry). The water vapor permeability of all types of PE is low. PE materials are permeable to gases and vapors, but LLDPE and HDPE are less permeable to gases and vapors than LDPE. Permeability for organic vapors is least for alcohols and then increases from acids to aldehydes and ketones, to esters, ethers, hydrocarbons and halogenated hydrocarbons (per-meability decreases with density). Some high molecular weight grades of HDPE and LLDPE are accepted as being suitable for containers for oil and petroleum based products. They are used for fuel tanks. In some cases the formed containers are chemically modified, by fluorination or sulfonation, which makes them almost impermeable to fuels.