What is the difference between Polypropylene and Polyethylene?
Some interesting facts about polyethylene and polypropylenePolyethylene and polypropylene belong to the class of thermoplastic polymers (which soften and stretch when heated and harden when cooled, so that it can be moulded anew again and again).
Polyethylene
A brief account of the chemistry:
The ethylene molecule, whose chemical formula is C2H4, is described as H2C=CH2 and drawn as:
two carbons that are joined through a double bond, where each carbon is tied to two hydrogen atoms. Ethylene is a colourless gas with a mild sweet odour. When combining the exact combination of pressure, temperature and suitable catalyst, the double bond of the ethylene molecule will split to form a multi-atom molecule, which is a polyethylene molecule in the following form of:
In its short form it appears as:
It is customary to add an “n” value on the right side of the basic unit (monomer), suggesting a large number of units connected to one another.
By controlling the reactionary conditions, it is possible to control the amount of branching, polymer density, and mechanical properties such as hardness, melting point, impact resistance, and stretching and bending composition.
History:
Polyethylene was first synthesized in a laboratory in 1898 by accident, during the investigation of a different material substance. it was termed then polyethylene. The first industrially polyethylene synthesis was discovered in 1933, but only in 1944, during World War II, that polyethylene was considered a useful material; it was used for coating the cable wires of military radio systems. This is what led to its industrial and commercial production. A significant breakthrough in the industrial production of polyethylene was discovered in 1951 by the American scientists Robert Banks and J. Paul Hogan from the American Phillips Petroleum Company; in 1953, chemists Karl Ziegler from Germany and Giulio Natta from Italy (who later went on to win the Nobel Prize for their work) developed a catalytic system that allowed to produce polyethylene at milder temperatures and pressures.
Polyethylene is known in a number of key forms:
Low-density polyethylene LDPE: (a density range of 0.91-0.94 g/cm3 )
This is a relatively soft material, used to manufacture soft boxes and containers, plastic bags and wrapping sheets.
Linear low-density polyethylene LLDPE: (a density range of 0.915 to 0.925 g/cm3)
This material is stronger than LDPE, more resistant to impact, and can be produced in thinner sheets. Although it is more resistant to environmental conditions, it is harder to process. LLDPE is used to produce thin layers of sheets and bags, toys, lids, buckets, and coatings for cables, wires and pipes.
High-density polyethylene HDPE: (a density range of 0.941 to 0.965 g/cm3)
This material is more rigid than its predecessors, and is highly resistant to environmental conditions. HDPE is used in the production of milk containers, detergent containers, garbage containers, water pipes, pallets and crates. The fact that this material retains its mechanical properties even at low temperatures (below zero) makes it ideal for being used to produce storage products that need to fit into freezers for long periods of time and for multiple use.
Medium-density polyethylene MDPE, Cross-linked polyethylene (PEX), and Ultra-high-molecular-weight polyethylene (UHMWPE), are more types of polyethylene materials used in pallet products.
Polypropylene
A brief account of the chemistry:
The propylene molecule, whose chemical formula is C3H6, is drawn as:
, can be described as follows: Two carbons that are joined through a double bond, one carbon tied to two hydrogen atoms, and the another carbon tied to a hydrogen and to a Methyl. Propylene gas is both colourless and odourless. When combining the exact combination of pressure, temperature and suitable catalyst, the double bond of the propylene molecule will split, to form a multi-atom molecule.
It is customary to add an “n” value on the right side of the basic unit (monomer), suggesting a large number of units connected to one another.
By controlling the reactionary manufacturing conditions (temperature, pressure, and catalyst type), it is possible to cause the ingot molecules to be cast from the same direction all the time, or alternating – once in one direction and once in the opposite direction periodically, or randomly in different directions. The polypropylene produced using each of these three options differ in type (consisting of other properties) and are consequently suitable for different kinds of products.
History:
Polypropylene was first synthesized in 1951 by American scientists Robert Banks and J. Paul Hogan; in 1954, chemists Karl Ziegler and Giulio Natta (who later went on to win the Nobel Prize for their work) developed a catalytic system that allowed to produce polypropylene at milder temperatures and pressures.
Uses:
Polypropylene is used in the making of a variety of plastic items including: toys, household appliances, food packaging, boxes, containers, auto parts, irrigation equipment and parts, transparent sheets, raffia, fibers and carpets, and much more. Polypropylene is found in nearly everything around us. It is relatively inexpensive to produce and is light in weight.
Polypropylene can be both rigid and durable when bent, softer and more impact resistant, transparent or opaque. It is usually limited for use in temperatures above 0ºC, and this is its main shortcoming in comparison with polyethylene.
Polyethylene was first produced 20 years prior to the production of polypropylene, yet while development of polyethylene was slow and nearly at a standstill, polypropylene benefited from much great advancement and attention. Scientists are continually finding new properties contained within this matter, unlocking new and interesting manufacturing possibilities, both in the processing and fabrication of new elements for products made of polypropylene; these include transparency, mechanical resistance, stiffness, shortened cycled times, thin-walled products and more.
Most companies primarily uses high-density polyethylene (HDPE), and all types of polypropylene (PP) in its products.
Polyethylene vs. Polypropylene:
| HDPE | PP | |
| Density range (g/cm3) | 0.941-0.965 | 0.9-0.92 |
| Melting point[ºC] | 132 | 160 |
| Service temperature[ºC] | -70÷ 65 | 0 ÷ 88 |
| Resistance to viruses | Less good | Better |
| Less good | Better |