Polyolefins

[nanoo16]

Hi,
does anyone have experience in the field of manufactring of polyolefins, or any knowledge about the same.
I am interested in knowing more about manufcturing LDPE / HDPE / polypropylene granules. If you can let me know few articles to read on it, it would be great.

 

[nanoo16]

I am also interested in knowing the scope of reprocessed plastics (LDPE / HDPE / PP granules, scrap plastic flakes etc) industry. I mean if you have any idea about processes which use the above mentioned materials , please let me know.

 

[FarogAlam]

Polyethylene is the most popular of the ethylene petrochemicals, made by the polymerization of ethylene. Polyethylene is the most flexible and widely used plastic in the world. The range of applications is wide: including examples such as packaging, films, bottles, and containers, as well as grocery, trash and shipping bags.
The ethylene polymerization unit contains two reaction systems, each consisting of a fluidized bed reactor, a cycle gas compressor, a cycle gas cooler, and product discharge tanks. Ethylene and other raw materials as well as catalyst are fed continuously to the reactors. Product discharge gases are sent to vent gas recovery, where liquids are recovered and returned to the reactor. Resin is sent to resin degassing and pelletizing.
Polyethylene is classified according to density and the most common products are low density polyethylene (LDPE), high density polyethylene (HDPE), and linear low density polyethylene (LLDPE)


LDPE
Low Density Polyethylene (LDPE) has a specific mass of 0.915 to 0.935. In the past it was the most common polymer but the market is being transferred to LLDPE.
The polymerization method is based on a continuous reaction under high pressure (1200-3000 atm) and temperature (3000?C). The reaction proceeds very rapidly (seconds) and takes place in stirred-vessel autoclaves or tubular reactors. The reaction is followed by polymer/monomer separation and the monomer is recycled. The product is conveyed as a melt into an extruder, emerging as pellets.
LDPE is easily processed by all common fabrication systems for thermoplastics: injection molding, extrusion, blow molding, rotational molding, and thermal forming.

HDPE
High Density Polyethylene (HDPE) has the same chemical structure as LDPE but differs in spatial structure in that it contains very low branching. It has high crystallinity and a specific mass of 0.940 to 0.965. The use of unique catalysts determine structure of HDPE as it is stereo-specific.
The process takes place at low pressures and the main difference between processes lie in choice of catalyst and preparation. Polymerization takes place in solution in a hydrocarbon solvent in a stirred reactor at approximately 100?C and 30 atm. The process is characterized by long contact times on the order of 0.5 to 2 hours. The polymer is precipitated from solvent, separated and dried and can be processed by extrusion, injection, blow molding, rotational molding, or thermal forming.

LLDPE
Linear Low Density Polyethylene (LLDPE) has a specific mass of 0.910 to 0.950 and consists of a controlled structure of short branches. LLDPE is produced by the addition of alpha-olefins - butene, hexene or octene - during the polymerization of ethylene to give a resin with a similar density to LDPE but the linearity of HDPE. LLDPE makes up one third of the total volume consumed of polyethylene.
LLDPE has properties similar to LDPE but at a lower thickness. It has a narrower molecular weight distribution, producing distinct elevation in toughness and tear strength, and improved thermal properties (e. g. melting point). LDPE and LLDPE can be blended to eliminate disadvantages, but then also compromising the properties of the final product.
Some modifications in processing machinery required. Production takes place at relatively low pressures, either in gas-phase or in solution, incorporating energy savings. The reactor operates continuously as a fluidized bed with product obtained as pellets. There is an increase in the associated power requirement for extrusion film blowing. The large majority of LLDPE reactor capacity is gas-phase rather than solution.