HDPE (High Density Polyethylene) resins are a popular type of plastic suitable for a wide range of applications. This is a general guide to HDPE resins for those with an interest in polymers, for consumers of HDPE resin, or for the prospective purchaser looking to broaden their knowledge. Since their invention in the 1950’s HDPE resins have swept the world, revolutionizing the plastics industry with their strength, durability, chemical resistance, and all round quality. The original discovery of polyethylene was something of an accident in 1898, when a German chemist by the name of Hans von Pechmann, who was heating diazomethane, which produced a white, highly viscous substance. The high density form of polyethylene came later, in the 1950’s where two processes where pioneered by Karl Ziegler, a chemist, and Robert Banks and J. Paul Hogan of Philips Petroleum.
The Making Of HDPE Resin
HDPE resin is produced by a process called polymerization.. Polyethylene is polymerized from ethylene gas, and HDPE is a high density form of this substance, which possesses a very linear chemical structure. The linear structure of this compound makes it not only very dense, but also very strong and fairly chemically resistant. HDPE resin is generally referred to as being inert, which gives it an advantage over many other substances commonly used in various types of commercial applications, which become weakened when exposed to various types of chemicals.
It may surprise you to know that HDPE resin actually begins life as crude oil. From crude oil, a substance called naphtha is extracted by means of distillation. Naphtha is itself a carcinogen, and a very powerful substance used in a wide range of petrochemical applications. To make HDPE resin, naphtha is ‘cracked’, a term which refers to heating it at very high temperatures until it releases ethylene gas.
This ethylene gas is then exposed to a catalyst at high pressure and temperature. The combination of the temperature, pressure, and the catalyst (a substance that helps reactions take place more quickly and easily than they would naturally), forces the ethylene molecules to join together in long chains. The resulting substance is polyethylene.
By changing various elements of the reaction, varying types of polyethylene can be produced. When HDPE resin is first produced, it is a sticky viscous mass with a whitish, translucent, opaque color. Most producers will take this HDPE resin and extrude it through a die, and cut it into small pellets before letting it cool into its solid state.
HDPE resins are also available in reprocessed form. Reprocessed HDPE is made from scrap HDPE, which is gathered from both households and industrial sources. Most HDPE collected from people’s homes is known as post consumer scrap HDPE, whereas HDPE collected from industrial sources is known as post industrial scrap HDPE. One of the extremely useful qualities of HDPE resin is the way that it is able to be easily melted down and formed into new products.
Reprocessed HDPE resins may have small amounts of contamination in them, due to the exposure they have had to contaminants in their prior uses, but every care is taken to try and ensure that reprocessed HDPE resins are as clean as they can possibly be. Some forms of scrap HDPE resin are simply washed and shredded loads of HDPE. These may contain a good deal of contaminants, as not all can be removed just by washing. Some potential contaminants in shredded HDPE resin include labels, other plastics, and sometimes even small pieces of metal. Reprocessed HDPE pellets are much cleaner however, as they are produced from melting down the shredded HDPE and filtering it thoroughly before it is made into pellets.
Properties of HDPE Resins
HDPE resin is a very interesting and unique type of plastic resin. It is regarded as being chemically inert, which means that it makes good containers for many different types of chemicals. It also has a PH resistance from PH 1.5 to PH 14, which makes it suitable for carrying and transporting acids and bases. HDPE resins also demonstrate a fairly wide temperature tolerance, being able to maintain molecular structure in temperatures as low as 148 degrees Fahrenheit, and as high as 248 degrees Fahrenheit. HDPE has an average tensile strength of around 4550 psi, and when under pressure, will bend to accommodate pressure long before it cracks.
HDPE Resin Applications
HDPE resins come in a wide range of grades for various applications. HDPE resins can be formed into bottles, blown into film, made into pipes, or injection molded to make irregularly shaped objects and toys. HDPE resins are found in all areas of daily life, most people now have several products in their homes made of HDPE resins. Shampoo bottles, garden furniture, the piping running under your home, all these things can be made of HDPE resin. HDPE resin is also used in a wide range of commercial applications, from containers, to pipes, to parts for various mechanical processes. With such a wide range of applications, there are also a wide range of types of HDPE available to the manufacturer.. These types of HDPE resins are classified into grades, which are in turn, distinguished by the purpose the grade fits
HDPE Resin Grades
Blow Molding Grade HDPE Resin – Blow molding grade HDPE resin is quite viscous. Blow molding applications require that the HDPE be quite thick and flow slowly, because blow molding involves the HDPE being blown up into the shape of a mold with compressed air. HDPE resin that is too runny would simply gather at the bottom of the mold and produce very badly deformed products.
Injection Molding Grade HDPE Resin – Just the opposite of blow molding grade resin, injection molding grade HDPE resin must be very runny so that it can be quickly injected into a mold, cooled, and then ejected. Injection molding is often used to make items with odd shapes, so an HDPE resin that flows freely into every nook and crevice is essential.
Film Grade HDPE Resin – Film Grade HDPE Resin falls somewhere between injection molding grade HDPE resin and blow molding grade HDPE resin. This is a semi viscous HDPE resin, strong enough to stand being blown into a thin film for carrier bags and other film applications.
Pipe Grade HDPE Resin – Pipe grade HDPE resin is a very strong, dense resin that can stand up to the wear and tear that pipes are exposed to. HDPE pipes are often used both underground and underwater, and sometimes must carry a great deal of weight on top of them. Pipe grade HDPE resin stands up to this task, and produces high quality pipes that stand the test of time.
Off Grade HDPE Resin – Off grade HDPE resin is a type of HDPE resin that does not fall into any particular grade. This resin is typically produced when an HDPE resin production line switches from making one grade of resin to another. The resin produced in the in between stage generally does not have the characteristics of either the resin that was originally produced, or the resin that is being subsequently produced. This resin is not suitable for high intensity applications, but is still useful for applications where high end resin is not necessary. For example, if HDPE pipes were being made to go underneath a road, one would definitely want to be using high quality HDPE pipe grade resin, however, if one is simply making a shampoo bottle inner, it is not necessary to have the highest quality material, as the product will not need to operate under extreme conditions.
For further information on HDPE resins, please refer to the other articles on the site.