The ABC of polyethylene
What is polyethylene?
Polyethylene is the most produced plastic in the world, with which everyone daily comes into contact. From its early days it has been considered a real asset in the world of the materials, although at first its value was only proven as insulation of electrical wiring. At present the power of polyethylene is its discrete reliability, its obvious solidity and its almost unlimited uses. We are so used to this modern material, it has become something common and everyday, and we tend to take it for granted.
Polyethylene can be processed into soft and flexible as well as into tough, hard and strong products. It can be found in articles of all kind of dimensions, from the simplest to the most complex shapes. It is used in everyday appliances, packaging, pipes and toys. Who doesn’t daily use products like cling-foil, squeeze bottle or garbage bag? Without noticing we buy a lot of products in the shop that are packaged in polyethylene. And when we leave the shop, our purchases are put into a carrier bag… from polyethylene. Without realizing it, our existence has become a lot safer as a large part of our pipes; tubes and fuel tanks are made of the solid and reliable polyethylene.
In whatever shape polyethylene is used, there is complete agreement in the excellent characteristics of this material. Polyethylene is a good insulator, it resists caustic materials, it is almost unbreakable and is environment-friendly. Polyethylene is reliable under every circumstance and it can easily deal with tropical temperatures as well as the frosty cold of the polar circle. This tough material is hard wearing. Yet it is remarkably light and it can be processed into all kind of articles without any problem.
The qualities of polyethylene can be summarized into three words: it is strong, it is safe and it is versatile.
The raw material: from naphtha to polyethylene
Naphtha is extracted from crude oil. Naphtha is another word for petroleum. By strongly heating up ("crack”) the naphtha, ethylene is released. In a factory this ethylene is transformed into polyethylene. The word polyethylene means: "a lot of ethylene parts”. These invisible tiny ethylene parts form the building blocks for polyethylene during the production. If we could look into the material during this process, we would see that these building blocks thread together into strings. Once these strings are ready, they look like branches.
The raw material: where do the granules come from?
The ethylene enters the factory as a gas. When the gas is transformed into polyethylene it looks like a warm, fluid pulp. Before it solidifies, the pulp is pushed through a plate with small holes in a constant stream. The solidifying polyethylene strings that come out at the other end are immediately cut into small pieces by a rotating knife. The result is a mass of white, transparent granules that look a lot like coarse hail. These granules will go to companies as raw material, where they are melted and processed into all kinds of products.
Basic features: density and liquidity
During production polyethylene can already be given a certain characteristic. One can choose for a stiffer or for a more elastic type. These features don’t only determine what kind of things can be manufactured from polyethylene, but also how easily it can be done. Whether polyethylene has a stiff or elastic character depends on the "density” of the material and on the "liquidity” in its melted form. The density and liquidity also largely depend on the amount of pressure that is applied during the production of polyethylene. The result of a "low” or on the other hand a "high” pressure is as follows:
- When producing polyethylene at low pressure it gets a high density. The invisible small substance particles form "straight”, robust and tightly packed branches. The result is "dense” polyethylene, with a firm and stiff structure that can be compared with a bundle of straight branches that cannot be pressed further.
Manufacturing polyethylene at high pressure on the other hand leads to a low density. The particles form a crisscross of branches and side branches with, literally, no "line” whatsoever. The weight of this less "dense” polyethylene is lighter. It sticks together more loosely and can be compared to a bundle of sticks of young and elastic wood with a lot of side branches that are also branched off. When you press on such a bundle and let go of it, it bounces back into shape. So elasticity right from the beginning.
- Whether polyethylene has a liquid character or not depends on the so-called "melting index”. This technical word indicates how slowly – or how quickly – the melted mass flows through a gap. It is not surprising that the "dense”, solid polyethylene flows slowly and with difficulty, as it has a stiff and tough character. The "less dense” and looser polyethylene flows much easier. When it is solidified, it feels more flexible and it is more elastic.
Three main types
By making polyethylene more or less "dense” in the factory, there is a suitable type of material available for every application. In practice one of the following types is used in 90% of the applications.
LDPE: "low density” polyethylene
The oldest type. A soft, tough and flexible polyethylene type, used for strong, flexible consumer items, like screw caps and lids. For a long time already, it is also used as insulation material. At present the most popular application is foil, from which carrier bags, packaging material and agricultural plastic are made. During the high water levels in Holland in the last years, the tough strong LDPE foil served as an improvised reinforcement for the dikes.
HDPE: "high density” polyethylene
This is the sturdiest and most inflexible type. Its sturdy and somewhat tough character can be used for a large range of applications. For example the well-known gft-container and a number of everyday domestic products like bottles, clothes pegs and the handle of a washing-up brush. Although HDPE is quite heavy, it can also be used for paper-thin foil that is extremely light and feels crispy. All of us use this type of foil daily; examples are sandwich bags, pedal bin bags or packaging for vegetables, fruit or meats.
LLDPE: a mixture of both previous-mentioned types
With this polyethylene one can go into every direction. It has some features from both of the previous-mentioned types. Both flexible and sturdy products are made from it. LLDPE is generally used in mixtures with one of the previously mentioned materials. Amongst others, even thinner foils can be produced. It is also used for multi-layer packaging. LLDPE is extremely tough and inflexible. These features can be used for the production of larger items, like covers, storage bins and some types of containers.
The development of new types of polyethylene, adapted to modern needs and made possible because of new production techniques, clearly shows that time doesn’t stand still.
- A leading hit amongst the new materials is UHMWPE. This polyethylene is really hardwearing and can resist higher temperatures. This makes it extremely suitable for applications whereby the utmost is required from the material, like gear wheels, gaskets, bearings, filters, cutting boards and hammers.
- The latest is the so-called metallocene (metalloceen) polyethylene. This material has spectacular features because of its very regular pattern of branches and side branches. A special type is the plastomeres (plastomeren). The distinctive feature of this polyethylene is its extremely low density, which makes it very tough. Next to this, it is as clear as glass. In practice, these materials are often used as reinforcement for other plastics.
Processing polyethylene into products – how is it done?
For most of the processing methods the polyethylene granules are put - via a funnel - into a cylinder where they are heated. Inside this cylinder a rotating screw presses the melted mass through an opening at the end, after which the polyethylene can be processed into different products, before it cools off and solidifies. The cylinder with the screw expressing the melted material is called an extruder. The principle of the extruder can be compared to that of a sausage mill. From melted polyethylene objects in many different models can be made, whether they are hollow of massive, large or small. Garden chairs, screw tops, doorknobs or squeeze bottles… the most diverse shapes and dimensions are possible. During the pressing the material can also be flattened into a sheet or stretched into foil.
The making of molded products
- Injection moulds (usually called injection molding). At the end of the extruder a measured quantity of melted polyethylene is pressed into a cooled mould. The content solidifies, the mould opens… and the ready-made product is ejected. This method is suitable for large and small products, like tops, covers, handles, garden furniture, buckets and gft-containers.
- Blow moulds. At the end of the extruder a measured quantity of polyethylene is "put through” and closed off on one side in the shape of a tube. Through the tube opening the polyethylene is blown against the mould side, using compressed air. It immediately solidifies and is than - ready-made – ejected from the mould. This is the designated manner to make bottles.
Rotation moulds. This is a good method for large hollow objects, like containers or toilet booths. The polyethylene is put into a mould as a powder. The mould rotates inside a big hot oven until the powder is melted and an even side is formed. After cooling off the product is finished.
The making of foil
- Blow foil. The melted material is pressed through the opening of the mould using compressed air and rises as a trunk of foil. After cooling off mills flatten the foil to a double layer after which it is rolled up and ready for further processing. This method is very suitable to make foil for bin bags and carrier bags.
- Surface foil. The melted material is pressed through a very narrow opening. This results into one single layer of very thin foil that is rolled up immediately after cooling off. This foil can, in contrast with the blow foil, only be stretched into one direction. Surface foil is often used to stick on layers of other material.
The making of multi-layer foil
Polyethylene foil is very suitable for using in combination with layers of paper, aluminum or other plastics. It is usually used to extra strengthen the food packaging, to be able to print on them and to ensure that the content remains fresher for a longer period. There are three types of multi-layer foil:
- Laminated foil. This is glued on aluminum, paper or other plastic as a layer. An example is the well-known coffee packaging.
Coating. Melted polyethylene is directly pressed onto a layer of aluminum or paper. Such a coated layer is amongst others used for photo paper and packaging for products containing oil or fat.
- Coex foil. Apart from polyethylene this foil can consist of one or more layers of other plastics. In contrast with the laminated foil, all layers of melted material are pressed together, they jointly solidify and become an indestructible foil. An example of this multi-layer foil is cheese packaging.
The making of sheets
This is done in the same manner as with surface foil, only the opening, through which the material is pressed, is wider and in accordance with the desired thickness of the sheet. Polyethylene sheets are often used to make relief shapes. For large, straight pieces, for example side parts, the shapes in the sheet are made with a vacuum machine (vacuumtrekker). For smaller objects, where each groove of the relief has to be visible, a stamp is used.
The making of foam applications for insulation
For a long time already polyethylene is a recognized insulation material, both for electricity and warmth. The foam effect is reached by adding a foam substance or a gas to the melted polyethylene. The material will than get a cell structure that makes it very suitable for warmth insulation. The foamed, warm mass is pressed out off the extruder via a window or tube profile, it cools off immediately and is cut to size.
Polyethylene can sometimes be processed as it is – natural – but most of the time it will need something extra to make it more suitable for certain applications. Substances are added to prevent objects, which are exposed to the open air, from eroding or fading. Sometimes a substance is added that makes the foil even smoother or a substance that prevents the foils from sticking together. Often substances are added that prevents the inflammability. Coloring substances are frequently added. In all cases it are useful and necessary additions.
Polyethylene and the environment
Polyethylene is one of the most environment-friendly materials, since:
- It is an efficient raw material. Per year, not even one percent of the total crude oil and natural gas is used for the global production of polyethylene.
The manufacturing of polyethylene is relatively clean and efficient: the emission of harmful substances is minimal and there is hardly any waste.
- Polyethylene is extremely suitable for recycling. It is a thermoplastic material, which means that it can be melted unlimited times and new products can be made with it. At present, many carrier bags and bin bags are made from recycled polyethylene.
- If polyethylene is collected after use and cannot be processed again, it supplies high-quality fuel for power supply.
Polyethylene: today’s and tomorrow’s material
It is beyond dispute that polyethylene is indispensable in the current world. It obtained a permanent and undisputed position on the materials market. What other material provided so many useful innovations in the field of insulation and packaging?
The question, how to store or transport our valuable food, water and energy without loss or decay – and in the safest possible manner –, is a lot easier to answer since polyethylene is used. It is impossible to imagine life today without polyethylene foils, coatings and cable insulation as well as without the more extensive range of strong, light packaging and domestic applications. Because of the superior features of polyethylene, these products cannot be realized in a better and cheaper manner using other materials.
Tomorrow, undoubtedly, we will demand more stringent requirements from our products than we do today. Polyethylene is the material that can endure the strictest test in the field of durability, safety, hygiene and environmental friendliness. Everything seems to indicate that we can expect a lot more from this valuable material in the future.