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Polyethylene (PE)

[–CH2–CH2–]n,

Where: n – degree of polymerization (DoP); it exists in two modifications that differ under structure, and, therefore under their properties. 

Both modifications result from ethylene CH2=CH2. 

In one of the forms, monomers are combined, mainly in linear chains 

‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑CH2‑

with DoP usually of 5000 and more.

In another one, chains of 4–6 carbonic atoms are connected to main chain randomly:

Production of High Density Polyethylene (HDPE) is based on the processes of catalytic polymerization of ethylene at low pressure (up to 20 atm). Linear polyethylenes form areas of crystallinity that strongly impact physical properties of samples. This kind of polyethylene is generally called High Density Polyethylene; it represents very hard, strong and rigid thermoplastic that is widely used for injection molding and blowing molding of vessels applied in household and industry. High Density Polyethylene is stronger than Low Density Polyethylene.

PROPERTIES OF HIGH DENSITY POLYETHYLENE

DoP:1000 tо 50 000

Тпл (melt temperature):129 – 135 °С 

Тст: above – 60 °С 

Density: 0,95 – 0,96 g/cm3 

Crystallinity:High 

Solubility:Soluble in aromatic hydrocarbons only at the temperature over 120 °С

Initially, branched polyethylenes have been recovered by way of heating of ethylene (with traces of oxygen as initiating agent) up to the temperatures in the range of 200° С under very high pressure (over 1500 atm). Branches low ability of polyethylene to crystallize; as a result, this sort of polyethylene has following properties:

PROPERTIES OF LOW DENSITY POLYETHYLENE

DoP:800 up tо 80 000 

Тпл:108 – 115 °С 

Тст:below –60 °С 

Density:0,92 – 0,94 g/cm3 

Crystallinity: low 

Solubility:soluble in aromatic hydrocarbons only at the temperature over 80° С

Low Density Polyethylene (LDPE), as for volume of production and application, holds leading position in all countries over the world. Substantially, features of LDPE are defined by degree of branching that is characterized by number of branches per 100 carbonic atoms. Branching of chain prevents tight packing of LDPE macromolecules, and reduces degree of crystallinity that varies within the range of 55-70 %. Other important value that is exposed to branching of chain is a softening temperature. LDPE softening temperature is much lower than water boiling temperature; therefore, this material can not be used for contact with boiling water or steam when sterilization.

HDPE-based films are more rigid, less waxy by touch; they have big density (0,96 g/cm3) compared with LDPE. Elongation and compression strength is higher than for LDPE, and tear and impact resistance is lower. Thanks to tighter packing of macromolecules, penetrability for gases of HDPE is approximately 5-6 times lower than of LDPE. Per water permeability, HDPE yields only to vinyl chloride- and vinyliden chloride-based films. Per chemical stability, HDPE also exceeds LDPE (especially, per stability against oils and fats).

One of the essential scopes of application of HDPE is manufacture of blown extruded hollow vessels (barrels, canisters, bottles) for carriage and storage of acids and alkalis.

Now, Linear Low Density Polyethylene – LLDPE, recovered by low pressure and medium temperature by co-polymerization of ethylene with other olefinic monomer, for instance, butylene CH2=CH‑CH2–CH3, finds more and more application. Where butylene unit is incorporating, short side branch appears:

In this case, packing of chains can not be so tight, as the case is for “pure” linear polyethylene, thus, its density is lower. Properties and amount of second monomer allow to obtain polyethylene with linear medium density molecule.

PROPERTIES OF LINEAR LOW DENSITY POLYETHYLENE

DoP:1000 tо 50 000 

Тпл:120 – 130 °С 

Тст: above – 60 °С 

Density: 0,89 – 0,93 g/сm3 

Crystannility: medium 

Solubility:soluble in aromatic hydrocarbons only at the temperature over 120 °С

Properties of LLDPE are interim between properties of LDPE and HDPE. However, LLDPE is characterized by more uniform distribution of polymer fraction per molecular mass (polydispersity) compared with LDPE. Main advantages of LLDPE compared with LDPE are: higher chemical stability; higher operational features both at low and high temperatures; higher stability against cracking; increased stability against puncture and tearing.

Linear Low Density Polyethylene is a firm, very flexible and slightly elastic thermoplastic, a little bit softener, easier to form and to press as High Density Polyethylene.

Linear Low Density Polyethylene is widely used in manufacture of coverings, packing materials and items made by injection molding, and it is also applied in manufacture of stretch and setting films with low permeability. Films based on this polymer are stable against formation of cracks, have good strength, transparency and barrier properties against oxygen. Application of LLDPE in frozen product packing is caused by its characteristics at low temperatures. Share of application of this polymer in making of stretch films is rapidly growing.

State-of-the-art technologies of producing of polyethylene of any kind maintain sanitary and hygienic safety of application thereof as foodstuff packing material and vessels for keeping of drinking water.

Any industrial grades of polyethylene are not suitable for long-lasting storage of following media: nitric acid (over 25%), sulfuric acid, chloroazotic acid, sulfuric anhydride, chloroform, methyl chloride, ethyl ether, fluorine, hot organic oils fats, organic solvents, trichloroethylene and some other organic solvents.

Cand.Sc. (Chem), Senior Scientist, ИнФОУ НАНУ A. N. Vdovichenko

(Physics of Organic Hydrocarbons, National Academy of Science of Ukraine)

Dr.Sc. (Chem.), Leading Scientist, ИнФОУ НАНУ S.А.Mitchenko

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