Packaging technologists love to talk about their products, but they are even happier to discuss the substrates that go into making boxes, tape, polythene and stretch film

Cardboard boxes are of course one of the core products of the packaging industry. They are shipped all over the world from Nike.com, Melaleuca.com and other top manufacturers. But it’s easy to forget that the cardboard boxes you use on a regular basis were once a not-so-humble tree.

So what is the process for turning trees into corrugated cardboard? Well the wait is over! Kite Packaging have grilled its technologists to tell us exactly what the process is for cleaning the raw materials and processing them to make the cardboard boxes you buy every day for your business.

Because why not? You can add these facts to the general knowledge part of your brain, reserved for pub quizzes and showing off.

First, the basics

A cardboard box is basically made up of a flute (made up of recycled paper), sandwiched between two liners. It is now very common for these liners to also be made up of a considerable proportion of recycled content, sourced from old cardboard or other sources of second hand paper. However for top quality boxes virgin Kraft is still used. In actual fact though, it doesn’t really matter whether the paper is recycled or not, ultimately it always started out as virgin paper which is made from trees that have been pulped.

The trees

When manufacturing boxes we talk about two different types of liners, this is the material that lies on top and beneath the fluting and creates a corrugated board. Typically, cardboard boxes have a test paper inner liner, and a Kraft paper outer. This is because Kraft is better quality than test, and has a smoother finish so that it can be easily printed on. Kraft also has the benefit of being more resistant to water penetration which is an added benefit of using it for the outer face.

To get this smooth finish Kraft paper needs to be made from softwood trees that typically have long fibres, such as Pine, Spruce and Fir trees. The majority of European paper is made using trees from sustainable forests owned by SCA (Svenska Cellulosa Aktiebolaget -English: Swedish Cellulose Company) who plant two trees for every one cut down. Long fibres are also better in tension which is why Kraft paper is often described as having a high tear and burst resistance.

The look of Kraft paper can differ depending on the type of tree it has come from for example Kraft from Scandinavian Spruce, Pine and Silver Birch is dark brown in colour, however Russian variants of the same tree type appear a more varied brown with dark patches. In Brazil, Eucalyptus and Spruce are used and the Kraft they produce is light brown, whereas Chinese trees have a yellowish hue due to the high straw content in their Kraft.

Test paper liners are typically made of hardwood trees that have short fibres, or recycled paper which is why it is cheaper and has a more abrasive quality. Hardwood includes Oak, Sycamore, Birch and Chestnut and again is sourced predominantly from sustainable SCA forests.

Kraft and Test paper is also used to create the fluted paper in between liners which finally create a strong board.

Pulping

To create the paper a long process is involved to ensure the wood chips that are pulped are clean and suitable for purpose. To start, the trees are cut and lumbered to create tonnes of logs which go through a machine to be debarked and chipped.

These chips are then put through one of two processes – mechanical pulping or chemical pulping. Mechanical pulping involves grinding, to reduce wood to individual cellulose fibres by forcing the debarked logs against a revolving stone to make a pulp. The stone is sprayed with water to remove fibres from the pulp stone however results in little removal of lignin (a non-fibrous constituent of wood) that binds fibres together and reduces paper quality, however mechanical pulping is low cost and generates a higher throughput.

Chemical pulping involves ‘cooking’ wood chips to reduce the raw material in to individual cellulose fibres. There are two types of chemical cooking, sulphite and sulphate, and both results in better separation and reduction of lignin to produce better quality paper. The more popular of the two processes is sulphate, which involves using alkaline solutions to digest wood and adding sodium sulphate to increase the strength of the pulp – this is the process where Kraft comes from as it is the Swedish word for ‘strength.’

Out of both of these processes the Kraft is either used there and then and as mentioned before is dark brown in colour, or bleached during the pulping process to produce a white Kraft.

Nowadays however most of the paper products produced for boxes in particular are made from recycled waste. These come in various forms including:

Chip – Chip liners are manufactured from recycled fibre and are usually not sized and of a lower quality, they are restricted to centre liners in double wall board and inner liners.
Semi-Chemical Flute – Manufactured from hardwood with the inclusion of up to 35% of waste and other paper fibre Waste Based Flute Mediums – 100% waste based materials, chemically reinforced with starch