Polythene film – how is it made?

Polythene is one of the world’s most ubiquitous plastics but like most everyday products little thought is given to how it is made, and how it is turned into polythene film.

Raw materials

Ethylene gas is the primary ingredient for polyethylene, but this is not the start of the process. It typically starts with petroleum, commonly referred to as crude oil.

Crude oil is of little use in its natural state and requires refining into its various components. Having been transported from its extraction point to the refinery it undergoes a distillation process. This process causes it to separate into various usable products. These include liquid petroleum gas (LPG), gasoline, naphtha, kerosene, diesel fuels, fuel oil, lubricating oils, paraffin wax, bitumen and petroleum coke.

The product we need is the Naphtha fraction. This is passed through a hydrotreater which uses hydrogen to remove the sulphur. The desulphurized Naphtha fraction then undergoes a further refining process known as cracking. Ethylene gas being one of the outputs of this process.

Green or bio-ethylene

Whilst the conventional route to ethylene gas has always been from crude oil, an alternative source is ethanol (ethyl alcohol). Ethanol is produced by fermenting sugar or sugar rich organic material. Once purified the ethanol undergoes dehydration using a catalyst to produce ethylene gas.

Not only is this process sustainable, but it has the potential of producing a polymer that has a negative carbon footprint.

From gas to solid

At the polythene manufacturing plant the ethylene gas is passed into a reactor vessel at high pressure. An initiator, such as oxygen or organic peroxide is added to start the polymerisation process. The result of this process is polyethylene which at this point looks like a warm, pulpy fluid.  Before it cools down and solidifies, it is forced through a plate with small holes. This turns it into long thin strands which are then cut into pellets or granules which resemble hailstones or grains of rice. The various grades of polyethylene (i.e. LDPE, MDPE, HDPE) result from adjustments to the polymerisation process conditions.

Typically packed and palletised in 25kg sacks, the finished granules are ready for shipment around the world.

Polyethylene granules are turned into products in a number of other ways including moulding and casting, but at Hanmere we manufacture blown polythene film.

Granules to polythene film

Pallets of granules are booked in on arrival at Hanmere. The delivery and batch codes being recorded to enable traceability. Bagged material is used rather than silos as it gives us greater flexibility at the next stage of the process; blending.

As a customer order nears completion our operators start blending a batch of polymer for the next. Each blend is a mix of different polythene grades and is designed to give the performance characteristics needed for a given application. With the blending done the granules are conveyed to the extrusion line feed hopper by a vacuum feed system.

The extruder feed-screw draws in the granules from the feed hopper. A second smaller feed injects any custom colours that are required for the order. As the feed-screw drives the granules through the extruder the high temperatures and pressures return the polyethylene to its molten state.

Fully liquid and under pressure the polymer exits vertically upward through a circular slot in the extrusion die. As it exits the die the tube is inflated like a giant bubble. The tube is inflated for two reasons. Firstly, it reduces the thickness from several millimetres to a few tens of microns. Secondly, it increases the diameter to the size required for the finished product. Once inflated the continuous tube of polythene film passes up the centre of the extrusion tower, cooling and solidifying as it goes. At the top of the tower the inflated tube is collapsed between rollers into continuous lay-flat tube (LFT).

Film to product

The flattened tube is guided down to the conversion equipment where it is turned into the finished product. Conversion can be a simple as reeling the LFT on to large rolls but we have the facility to finish the final product in range of forms to suit most applications.

Once it has been converted, the film is packed and palletised ready for shipment.

We may have boiled this all down a bit to simplify the explanation.  However, in reality we create many different products, ranging from paper-like to gossamer thin.  We manufacture and print one of the widest ranges of polythene film in the UK, and are at the forefront of the industry.  Glimpse some of our processes in action in our promotional video.

So, next time you are handling some polythene, we hope you will enjoy having this insight into the many processes it had to undergo to become your bespoke product.

Sources:

http://www.plasticseurope.org/information-centre/education-portal/resources-room/abc-of-plastics/the-abc-of-polyethylene.aspx

/* */