Application Example: Feeding & Conveying in Polyolefin Production
Polyolefins are produced by the polymerization of olefins or alkenes (molecules with the general formula CnH2n) such as ethylene, propylene, butene, isoprene, pentene, etc. The name “olefin” means “oil-like” and refers to the oily character of the materials.
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The most important commercial polyolefins are polyethylene (PE), polypropylene (PP) and ethyl vinyl acetate (EVA). Polyethylene is classified according to its density as Very Low Density Polyethylene (VLDPE), Low Density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), Medium Density Polyethylene (MDPE), and High Density Polyethylene (HDPE).
Polyethylene and polypropylene are two of the top polymer resins used in the world today. These resins can be tailored to achieve wide range of mechanical and chemical properties, making it possible to use them in a myriad of applications such as agricultural films, garments, tapes, stretch films, retail bags, bottles, containers, pipes, etc.
The selection of a polyolefin for a particular application depends on the resin’s type and grade, which are determined by the manufacturing technology or process, the catalyst, and the raw feedstock used in production. In turn, the polyolefin grade is determined by key properties such as molecular weight, molecular weight distribution (MWD), crystallinity, branching and density that affect how and where each resin is used.
Two distinctive process sections can be identified in polyolefin production, a Wet-end and a dryend process.
The wet-end process for the manufacturing of polyolefins can be high pressure (autoclave and tubular) or low pressure (slurry, solution or gas phase). It includes:
- Feed preparation
- Catalyst handling
- Polymerization reaction
- Diluent recovery
- Polymer separation, where the polymer is cleaned and dried.
High Pressure Technology
Slurry Phase Process
Gas Phase Process
Wet End Product
Regardless of the process used, the polymerization reaction produces a powder (sometimes called fluff or flake) that is transferred to a bin that serves as a buffer between the Wet End and Dry End processes.
Density and Applications of Polyolefin Products
|Name||Abbr.||SPI Resin ID Code||Density g/cm3||End Use Examples|
|High Density Polyethylene||HDPE||>0.941||Plastic lumber, fuel tanks, furniture, storage sheds, chemical & heat resistant piping & containers|
|Medium Density Polyethylene||MDPE||n/a||0.926 to 0.940||Containers with good shock and drop resistance, gas pipes, shrink film, packaging films|
|Linear Low Density Polyethylene||LLDPE||n/a||0.915 to 0.925||Industrial containers, trash cans, automotive parts, packaging materials under FDA regulations|
|Low Density Polyethylene||LDPE||0.910 to 0.940||Trays & general purpose containers, weldable & machinable parts, computer parts, plastic bags, playground equipment|
|Very Low Density polyethylene||VLDPE||n/a||0.880 to 0.915||Blown films, molded parts, industrial & general rubber, stretch wrap|
|Polypropylene||PP||0.855 to 0.946||Packaging, textile fibers, carpets, ropes, thermal clothing, automotive components, reusable containers|
|Ethyl Vinyl Acetate||EVA||n/a||0.93||Biomedical applications for time release medications, foam padding for sport equipment, flotation devices, sandals, hot melt adhesives|
The polymerized powder (also called “fluff” or “flake”) needs to be cleansed of residual monomers, catalysts and solvents. This can be done using a decanter centrifuge, purge bin, degassing vessel, low-pressure and highpressure separators, or deodorizing silos, depending on the individual process.
The dry section of a polyolefin manufacturing process begins once the clean, dry powder has been stripped of these impurities and is pneumatically transferred with nitrogen (to prevent oxidation) to an intermediate storage or powder surge hopper situated several levels above a melt extruder.
In some technologies, the polymer is transferred in a molten state directly from the reaction area into the melt extruder.