Grades and applications of commodity plastic

Each commodity thermoplastics is available in many grades, either adapted to the requirements of a process, or to the demands of a product type. The major variants will be described and some of the jargon explained. A few of the important applications will be described, while others appear throughout the book. 

The original ICI process for polymerising ethylene, developed in the 1940s, produces low density polyethylene (LDPE) with a density in the range 910-935 kgm. Ethylene, compressed to pressures between 1400 and 2400 bar at a temperature between 200 and 250 °C, is above the critical point where liquids and gases can be distinguished. When it is polymerised using a free 

The mass of crystals in 1 m of polymer is the product of the volume fraction crystallinity, and the crystal density p. The mass of amorphous material is product of the amorphous volume fraction crystallinity (1 — VJ and the amorphous density Pa. Consequently, the polymer density p is linearly related to by 

The melt flow index (MFI) is used as a production control assessment of the average molecular weight. The MFI is an inverse measure of melt viscosity 

The higher melting point of polypropylene (170 °C) compared with HDPE (135 °C) makes it more suitable for fibre applications, and 30% of the production goes into carpets and ropes. About 20% is used in the form of oriented polypropylene film for wrapping potato crisps, cigarettes, etc. where good clarity is required. 

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