How is polyethylene created

The average polymer molecule contains carbon atoms, with many short branches. There are about 20 branches per carbon atoms. The relative molecular mass, and the branching, influence the physical properties of LDPE. The branching affects the degree of crystallinity which in turn affects the density of the material. The branches prevent the molecules fitting closely together and so it has low density.

HDPE is produced by three types of process. All operate at relatively low pressures atm in the presence of a Ziegler-Natta or inorganic catalyst. Typical temperatures range between K. In all three processes hydrogen is mixed with the ethene to control the chain length of the polymer. The Ziegler-Natta catalyst, as granules, is mixed with a liquid hydrocarbon for example, 2-methylpropane isobutane or hexane , which simply acts as a diluent.

A mixture of hydrogen and ethene is passed under pressure into the slurry and ethene is polymerized to HDPE. The reaction takes place in a large loop reactor with the mixture constantly stirred Figure 4.

On opening a valve, the product is released and the solvent is evaporated to leave the polymer, still containing the catalyst. Water vapour, on flowing with nitrogen through the polymer, reacts with the catalytic sites, destroying their activity. The residue of the catalyst, titanium IV and aluminium oxides, remains mixed, in minute amounts, in the polymer. Figure 4 The manufacture of poly ethene using the slurry process in a loop reactor.

The second method involves passing ethene and hydrogen under pressure into a solution of the Ziegler-Natta catalyst in a hydrocarbon a C 10 or C 12 alkane. The polymer is obtained in a similar way to the slurry method. Figure 6 Low pressure gas-phase process. A mixture of ethene and hydrogen is passed over a Phillips catalyst in a fixed bed reactor Figure 6. Ethene polymerizes to form grains of HDPE, suspended in the flowing gas, which pass out of the reactor when the valve is released.

Modern plants sometimes use two or more of the individual reactors in series for example two or more slurry reactors or two gas phase reactors each of which are under slightly different conditions, so that the properties of different products from the reactors are present in the resulting polymer mixture, leading to a broad or bimodal molecular mass distribution.

From there, two years later, in , HDPE was produced as pipe. Ziegler was awarded the Nobel Prize for Chemistry. Did you know that polyethylene played a key supporting role during World War II? It was first used as an underwater cable coating and then as a critical insulating material for vital military applications as radar insulation.

This is because it was so light and thin that it made placing radar onto airplanes possible thus vastly reducing the weigh. The substance was a highly guarded secret. After the war, polyethylene became a tremendous hit with consumers It became the first plastic in the United States to sell more than a billion pounds a year. It is currently the largest volume plastic in the world. Different grades of polyethylene and polypropylene offer a wide range of physical properties, such as density, stiffness, flexibility, opacity, melting point, texture, and strength.

Manipulating variables in the reactor, such as monomer, comonomer, catalyst, and cooling media flowrates, can control key quality parameters.

Additives and colorings can modify the appearance of the polymer. Polymer plants are semi-continuous processes. Raw material is fed into a reactor continuously at the front end, while polymer powder and pellets are packaged in batches.

Most sites operate multiple lines with many bins and silos for storage and blending. The plastics are eventually delivered to customers using barges, trucks, or rail cars.

This article describes the different polyolefin production processes, their key operating parameters, and ways to use automation for improved quality control and higher throughput. A polymerization reaction starts with a primary ingredient monomer , such as ethylene or propylene.

Ethylene C 2 H 4 is a stable molecule with two carbon atoms and a double bond. Polyethylene PE is a made by the reaction of multiple ethylene molecules in the presence of catalyst to break the double bond and connect the carbon atoms into a chain Figure 1.

The longer the chain, the higher the molecular weight. Polymers can have molecular weights in the millions. Similarly, polypropylene PP is made by breaking the double bond in a propylene C 3 H 6 molecule, in the presence of a catalyst, to form long chains of three-carbon-atom molecules Figure 2.

The third carbon atom adds a complexity: On which side of the chain will the methyl CH 3 groups fall? These arrangements have different physical properties. Polymerization reactions will also consume hydrogen, which is required to quench the reaction i. Since the concentrations of these components in the reactor affect the probabilities that specific reactions will take place, the composition in the reactor effectively sets the amount of branching and the length of the chain. Ethylene is a stable molecule with two carbon atoms connected by a double bond.

A useful attribute of thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polyethylene liquefy, which allows them to be easily injection molded and then subsequently recycled.

By contrast, thermoset plastics can only be heated once typically during the injection molding process. The first heating causes thermoset materials to set similar to a 2-part epoxy , resulting in a chemical change that cannot be reversed.

If you tried to heat a thermoset plastic to a high temperature a second time, it would burn. This characteristic makes thermoset materials poor candidates for recycling. Different types of polyethylene exhibit wide variability in their crystalline structures.

The less crystalline or amorphous a plastic is, the more it demonstrates a tendency to soften gradually; that is, the plastic will have a wider range between their glass transition temperature and their melting point.

Crystalline plastics, by contrast, exhibit a rather sharp transition from solid to liquid. Polyethylene is an incredibly useful commodity plastic, especially among product design companies. Because of the diversity of PE variants, it is incorporated into a wide range of applications.

For some projects, a part that will eventually be mass-produced in PE can be prototyped with other, more prototype-friendly materials like ABS.