From tree to toilet: engineering loo roll

Engineers get up to many things. You’ll sometimes find them stroking a concrete wall or leaning precariously off the side of a bridge. You might catch one smiling at an aeroplane the way others would at passing dogs or children. Nothing escapes an engineer’s attention. Then there are engineers who, even as you read this, are sitting on the toilet, trapped there by a humble toilet roll. From terrifying machinery to subtle chemistry, engineering touches toilet paper at every stage of its life, from tree to toilet.

Toilet paper is almost entirely cellulose – the main substance that makes up the cell walls of plants. This means that toilet paper decomposes quickly in sewage systems but is not too fragile so as to lose structural integrity at the most crucial moment of use.

How wood is processed into paper

Toilet paper’s raw materials are fresh or recycled wood, or recycled paper products. In the former case, the wood usually comes from pine trees – a large tree can produce up to 1,500 toilet rolls. The process for felling the trees varies by region, from hand-wielded chainsaws to highly technical automated machinery, but the steps are the same: the trees are felled, branches removed, and logs cut to standard lengths ready to be turned into wood chips.

Perhaps the most impressive methods of collecting wood from trees are in Sweden and Finland, where lumberjacks use machines called tree harvesters. The machine’s head grips the base of a tree and a chainsaw cuts across the trunk. The tree harvester’s chainsaws are hydraulically powered, making them much more powerful than handheld devices.

Still gripping the base of the trunk, the head rotates as the tree falls to the ground. Once horizontal, two feed rollers grip the trunk and pull it through the head at great speed. It passes through two or more ‘delimbing knives’, which strip all the branches off the trunk in seconds. Mechanical sensors measure the trunk’s diameter and adjust the delimbing knives accordingly. A wheel measures the length of trunk, and the chainsaw cuts the stripped trunk into logs with predetermined length, ready to go to the woodchipper. Computer algorithms tweak the whole process and adjust the different parameters to maximise how many branch-free logs each trunk produces.

Next, a debarking machine removes the bark. A log is mounted horizontally on a series of rough wheels that grip and slowly spin it under a moving cutting head. The head optically scans the shape of the log and controls the blades to shave off just the right depth of bark to conserve as much of the precious inner wood as possible.

The bare logs are now ready for a chipper to transform them into workable wood chips. These machines are similarly large-scale, often as large as a tractor, and as intimidating as the harvester and debarker. A hopper guides logs into the jaws of the chipper, which can be either a rotating disc with blades on – a bit like a rotating cheesegrater – or a rotating drum with blades on its outer surface. Either way, the chipper ejects the shredded log chips from a chute for storage. Grooved rollers grip and push the logs through the chipper, keeping workers at a safe distance. These powerful chippers, typically hundreds of kilowatts, are usually powered by an internal combustion engine.

Wood is a composite material made from fibrous cellulose (a linear polymer) and hemicellulose (a cross-linked polymer) held together in a matrix of glue-like lignin, a bit like a natural carbon-fibre reinforced plastic composite. Once chipped, the wood is ready to give up its precious cellulose in the pulping process, also known as the kraft process (‘kraft’ meaning ‘strength’ in German). Here, wood chips are heated to about 170°C in digesters under pressure with water, sodium hydroxide, and sodium sulfide. This breaks the chemical bonds between the cellulose, hemicellulose, and lignin components, so the cellulose can then be recovered.

The cellulose fibres are usually 1 to 2 millimetres long, and about 30 micrometres wide – the width of a fine human hair. The straighter and longer the fibres, the stronger the resulting paper. Mechanical action at the pulping stage can shorten the fibres, and introduce kinks and curls, all of which results in weaker paper. However, tissue paper is designed to be easily ripped so damaged fibres aren’t necessarily too much of a problem.

The whole process is made as efficient as possible; the slurry of cellulose fibre pulp that comes out of the kraft process is heated to separate the pulp from the pulping chemicals, which are recovered for reuse. High pressure steam generated by the recovery boiler can feed turbogenerators to provide electricity for the mill or nearby industries, or feed onto the grid for local communities. Once the cellulose is recovered, it is diluted with water to create a suspension of 0.5% fibre. This is then screened, washed in stages and bleached to make it white.

What about recycled toilet paper?

Some toilet paper is made from recycled paper. The recycling process produces damaged fibres, but this is usually an acceptable feedstock for toilet tissue. Recycled paper is often mixed with new pulp to maintain enough mechanical integrity in the end product.

The recycling process mixes wastepaper with water and agitates it in a pulper, a machine not dissimilar to a washing machine. This allows metal or plastic contaminants to sink to the bottom and be removed. That is not the end of the cleanup process for recycled paper. Some inks contain toxic heavy metals such as lead, cadmium, and hexavalent chromium, which must be removed and disposed of safely. (To ensure the safety of recycling processes, these inks are being phased out of the production of recyclable materials.) In the next step, air bubbles are blown through the gloopy pulp. Ink particles attach to air bubbles and form an inky foam that is skimmed off, leaving the rest of the pulp ink-free. Next, the pulp is squeezed through rollers to remove some water and passed through a contraption of revolving knives to break it up.

After this point, the process is the same, regardless of raw material, new or recycled pulp. The watery pulp suspension is sprayed onto a continuously moving screen of a very fine stainless-steel mesh. The process forms a wet sheet of matted fibre over 1.5 metres wide. The fibre sheet runs through hot rollers at over 150°C, which dries it in less than a second. This produces a continuous roll of delicate paper that is just 0.1 millimetre thick. The result is a spool of paper around 75 kilometres long.

A creping process follows to create the required texture and structure for tissue paper. Creping involves a sharp blade scraping the paper off the dryer to create a slightly crinkled sheet. This helps to improve the paper’s flexibility and absorbency. Creping also weakens the paper so that it will disintegrate when wet. Next time you’re in the loo, notice the subtle stretch in your toilet paper – that’s thanks to the creping process. Next, perforating machines stamp rows of holes to perforate the paper into rippable sections. At this stage patterned rollers can also emboss a pattern onto the paper, which also helps with absorption.

On a roll: the finishing touches

Elsewhere in the factory, recycled cardboard inners are made by winding two strips of thin recycled cardboard around a core to make a tube, which the new paper is then wound onto. Two sheets are simultaneously fed onto the tube, one from above and one from below. The sheets come together as they are wrapped onto the cardboard inner to form two-ply toilet paper.

Once the roll has been wrapped to the right size, a long blade cuts the paper and a machine automatically seals the end with glue to stop it unravelling. At this stage, the toilet paper is still on an extremely wide roll. The final stage is for a circular saw to cut the wide roll into standard-width rolls that are then automatically packaged in paper or plastic. In this way, toilet-paper factories can produce millions of rolls every single day.

Like many engineering topics, the production of toilet rolls still has its challenges. For example, materials engineers are studying toilet paper in finer and finer detail to better understand its structure–property relationships, hoping to close the loop further on recycled paper products. Manufacturing engineers seek to make marginal gains in the manufacturing process to make it more resource and energy efficient.

One organisation making a change is Australian startup company Who Gives a Crap. Its plastic-free products are made entirely from recycled feedstocks and the founders donate 50% of the enterprise’s profits to partners including WaterAid, Splash, and Sanergy, who build toilets and improve sanitation in lower-income countries. Their donations have amounted to almost 11 million Australian dollars at the time of writing. About two billion people in the world don’t have access to a toilet and contaminated water contributes to over a million deaths a year, when human waste ends up in local waterways – by being sold in this way, the humble toilet roll is indirectly helping to improve sanitation and save lives globally.

And seismic shifts in consumer priorities, from increased costs of living to higher demands for sustainability, may further shake up the industry as a whole; will there be new products designed to do away with toilet paper all together? These are the thoughts swirling round the heads of the engineers stuck on the loo as you read this, from tree to toilet, and beyond.

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This article has been adapted from "From tree to toilet: engineering loo roll", which appeared in Ingenia 92 (September 2022).