Thermoplastics – A Focus on Polyethylene & Polypropylene

Date Published:   04/15/2020

Thermoplastics are a class of polymers, that with the application of heat, can be softened and melted, and can be processed either in the heat-softened state (e.g. by thermoforming) or in the liquid state (e.g. by extrusion and injection molding). Over 70% of the plastics used in the world are thermoplastics, and the two most commonly used thermoplastics are both olefins, compound made up of hydrogen and carbon that contains one or more pairs of carbon atoms linked by a double bond. These two olefins are polyethylene and polypropylene.

 

Polyethylene

Polyethylene is a tough, light, flexible synthetic resin made by polymerizing ethylene, chiefly used for plastic bags, food containers, and other packaging. It may be of low density or high density depending upon the process used in its manufacturing. It is resistant to moisture and most of the chemicals. It can be heat sealed and is flexible at room temperature (and low temperature), and in additional to its material properties, its low cost contributes to the popularity and high demand of the material.


Major Applications

• Low-density polyethylene (LDPE), recyclable plastic no. 4, is flexible and is used in the production of squeeze bottles, milk jug caps, and retail store bags.
• Medium-density polyethylene (MDPE) is used for sacks, gas pipes, fittings, and packaging film.
• High-density polyethylene (HDPE), recyclable plastic no. 2, is commonly used as milk jugs, liquid laundry detergent bottles, outdoor furniture, margarine tubs, grocery bags, drinking water distribution systems, and water drainage pipes.
• Ultra-high-molecular-weight polyethylene (UHMWPE) is tough and resistant to chemicals. It is used to manufacture moving machine parts, bearings, gears, artificial joints, and bulletproof vests.


Production Process

Polyethylene is made by the polymerization of ethylene, which is the reacting of ethylene molecules together to form a chain of ethylene molecules. The two primary feedstocks for ethylene production are naphtha and natural gas liquids (ethane, propane, butane). Natural gas liquids are used in countries like the United States where natural gas is abundant, and naphtha is used commonly in Asia where producers have access to cheap byproduct naphtha. Ethane is a major bottleneck chemical, so many refineries have furnaces to take the over-abundance of ethane and convert it into the more valuable ethylene.
The first step in the production of ethylene is to take the feedstock and crack it into ethylene in a furnace. This process is called pyrolysis. Also called steam cracking, pyrolysis is the thermal cracking of petroleum hydrocarbons with steam. The production process for the two most common types of polyethylene: low-density polyethylene and high-density polyethylene, will be described below.


Low-density polyethylene (LDPE) is made from ethylene gas under extremely high pressures (up to about 3,500 bar, or 50,000 psi) and high temperatures (up to about 350 °C [660 °F]) in the presence of oxide initiators, which results in a polymer structure with both short and long branches. Because the branches prevent the polyethylene molecules from packing closely together in stiff, hard crystalline formations, LDPE is a very flexible material. Its melting point is approximately 110 °C (230 °F). Principal uses are in packaging film, trash and grocery bags, agricultural mulch, wire and cable insulation, squeeze bottles, toys, and housewares.
High-density polyethylene (HDPE), on the other hand, is manufactured at low temperatures and pressures, using metallocene and Ziegler-Natta catalysts or activated chromium oxide (also known as a Phillips catalyst). The lack of branches in its structure allows the polymer chains to pack tightly, resulting in a highly crystalline, dense material of high strength and moderate stiffness. With a melting point more than 20 °C (36 °F) higher than LDPE, it can withstand repeated exposure to 120 °C (250 °F) for sterilization. Products include blow-molded bottles for milk and household cleaners; blow-extruded grocery bags, construction film, and agricultural mulch; and injection-molded pails, caps, appliance housings, and toys.


Major Technology Providers

Ethylene Plants:

KBR; Technip; Linde; Shaw, Stone & Webster; and Lummus.

Polyethylene Plants:

Sasol; Formosa Plastics; Ineos; and Odebrecht.

 

Polypropylene

Polypropylene, recyclable plastic no. 5, is useful for a plethora of products such as microwave and dishwasher-safe plastic containers, reusable plastic food containers, sanitary pad lining and casing, diaper lining, carpets, ropes, plastic moldings, piping systems, filters for gases and liquids, car batteries, and insulation for electrical cables. Polypropylene sheets are used for stationery folders and packaging and clear storage bins. In medicine, it is used in hernia treatment and to make heat-resistant medical equipment. Although relatively inert, it is vulnerable to ultraviolet radiation and can degrade in direct sunlight. Polypropylene is not as impact-resistant as the polyethylenes (HDPE, LDPE). It is also somewhat permeable to highly volatile gases and liquids.


Production Process

Traditional productions methods are the naphtha steam cracking process and propane dehydrogenation. The naphtha steam cracking process is very similar to the steam cracking process of natural gas liquids; while propane dehydrogenation is a very different process that involves mixing fresh propane feed is with recycled propane and optionally also with recycled hydrogen gas and is fed into a heater to be heated to over 540°C (1000°F) and then enters usually three or four reactors. The propylene-rich reactor effluent is compressed, dried and sent to a cryogenic separator where hydrogen is recovered. The olefin stream then goes to an ethylene column where light-ends are removed prior to the propane-propylene splitter where propylene is recovered. Unconverted feedstock is recycled and combined with the fresh feed.
Propylene could also be generated in FCC (fluid catalytic cracking) units but its yield is relatively low. Another approach is to integrate the propylene production process into refinery or petrochemical plants, and use lower value chemicals made in the process, for the catalytic cracking of mixed C4 olefins (olefins with 4 carbon atoms) and higher carbon olefins.


Major Technology Providers

Propylene Plants:

Lummus Technology; Servomex; Honeywell UOP; and Mitsubishi Chemical.

Polypropylene Plants:

LyondellBasell; Borealis; Chisso; Grace; Mitsui Petrochemical; Krupp Uhde; Linde; Yarsintez/Snamprogetti; Novolen Technologies; Rexene; and Sumitomo Chemical.

 

Conclusion

Demand for thermoplastics exceeds billions of dollars globally, and the two most common originate from hydrocarbons from refineries such as naptha, ethane, propane, and butane. If you already have a refinery, or access to cheap feedstock, or, for example, as a by-product from another chemical plant -- the return on investment of a used thermoplastics plant can be less than 3 years depending on the particulars of your local/global economic positioning and utility costs. Please find a link to our ethylene plants on offer below:

https://www.phxequip.com/plants-subcategory.36.0/ethylene-plants.aspx

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