HDPE plastics are produced from a component of crude oil, naphtha, which is cracked (heated at very high temperatures) in order to extract the ethylene gas which is released when the chemical structure of naphtha deteriorates. The ethylene gas is then polymerized, a process which involves the free gas particles joining together to produce long chains of ethylene molecules.

There are many different types of polyethylene. HDPE is a particular type of polyethylene with a very dense linear structure which gives it an increased tensile and molecular strength. HDPE is normally defined as being polyethylene with minimum density of 0.941 grams per cubed centimeter. Within the category of HDPE plastics, molecular weights range from 100,000 to 500,000, which is where the great variance in HDPE grades arises. Common grades of HDPE plastics include blow molding grade HDPE plastics, injection molding grade HDPE plastics, film grade HDPE plastics, and pipe grade HDPE plastics. Each of these grades have slightly different physical properties, the most notably important one being MFI, or melt flow index. Melt flow index describes how quickly the HDPE plastic flows when in liquid form, applications such as pipe making and blow molding require a highly viscous slow flowing HDPE plastic, whereas injection molding requires a quickly flowing HDPE resin.

HDPE plastics were first invented in the 1950’s, though it took some time to discover ways to reliably produce large amounts of homogeneous HDPE, as the processes under which HDPE is produced must be tightly controlled in order to produce the different grades of HDPE.

Ethylene naturally polymerizes at high temperatures and pressures, but it is not commercially viable to operate facilities under these conditions. So instead, catalysts are used to lower the temperature at which polymerization will take place, and also direct the type of polymer which results. HDPE’s largely linear structure with little molecular branching is commonly produced by Zeigler-Natta catalysts, or chromium catalysts.

HDPE lends itself well to recycling, and a great deal of reprocessed HDPE, recycled HDPE, and regrind HDPE is available on the market. The bulk of recycled HDPE plastics are post consumer scrap reclaimed from kerbside recycling programs. Millions of tons of HDPE plastic milk bottles and other household vessels are collected and recycled every year. For the most part, recycled HDPE plastics are equally as useful as virgin HDPE plastics, though they may not be used in food contact applications. There is some risk that contaminants found in the scrap HDPE from which recycled HDPE is made may pass through the various cleaning and filtration steps and degrade and contaminate the end product. If one is dealing with a reputable supplier, and the specific application is not overly intensive, recycled HDPE is an economically sound alternative to virgin HDPE plastic.

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High Density Polyethylene (HDPE): a rugged material which is easily moulded, has a high resistance to impact and is not affected by most chemicals. Mainly used in blow and injection moulding it may also be vacuum formed or extruded.

Polyvinyl Chloride (PVC): a very common plastic and one of the earliest types developed. PVC’s often contain chemicals called plasticisers which can leach from the plastic over time. You will find it in roofing gutters, electrical insulation and fittings and waste pipes. For food use a special grade has been developed called uPVC and is “unplasticised”. Be careful in selecting a PVC product for food contact. The qualities of PVC vary and they can be flexible or rigid. It is commonly injection moulded, extruded, blow moulded and vacuum formed. PVC is subject to attack by solvents.

Polypropylene (PP): Although mainly used in injection moulding, it may be blow or rotationally moulded, vacuum formed or extruded. PP is an easy to mould material which has medium impact resistance, high structural strength and is resistant to a wide range of chemicals.

Polyethylene Terephthalate (PET): a form of polyester and its most visible use is in fizzy drink bottles and honey jars. Soft drink bottles can be recycled into clothing fibre. It has very high impact resistance and modest structural strength. It is resistant to many solvents and is particularly suitable for food contact. You can freeze it but not boil or microwave. It is commonly injection and blow moulded.

Polypropylene Co-Polymer (PPCO): Because it is a Polypropylene molecule with about 2% HDPE added it has a mixture of the properties of HDPE and Polypropylene including very high impact resistance and moderate structural strength. Primarily used in injection moulding. It is resistant to a wide range of chemicals.

Low Density Polyethylene (LDPE or LLDPE): It is rotationally, blow and injection moulded though it may be vacuum formed or extruded. LDPE has very high impact resistance, modest structural strength and good resistance to a huge range of chemicals.

Acrylonitrile Butadiene Styrene (ABS): It has a shiny impervious surface, good impact resistance and very good structural strength. It is commonly vacuum formed or extruded.

Styrene Acrylonitrile (SAN): It has medium impact resistance, very good structural strength and has a shiny impervious surface. It is mostly injection moulded however it is not the best at high or low temperatures.

Nylon: It generally has excellent structural and impact strength, great working temperature range and is resistant to petrol, oils and solvents. It comes in many and varied forms. It is commonly injection moulded and extruded. It is subject to attack by acids.

High Impact Polystyrene (HIPS): Medium impact resistance and excellent structural strength. It is commonly injection moulded.
Not very good at high or low temperatures.

Polycarbonate (PC): It has an incredibly high resistance to impact. For this reason it is a major component in bullet-proof “glass” and commonly used for beer jugs and conservatories. Subject to attack from solvents. It is commonly injection moulded, extruded, and blow moulded. It has ability to handle high temperatures for that reason it is used in microwave cookware.

Nitrile Rubber (Nitrile): It is synthetic, as opposed to natural latex. Although it is not considered to be a “plastic” it can be injection moulded. Car tyres are made of Nitrile.

Styrene: a generic name given to a range of polymers. It is an organic compound with the chemical formula C6H5CH=CH2. Excellent stuctural strength and clarity offset poor impact resistance and susceptibility to attack from solvents. It is commonly injection moulded.

Plastics

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