How Is Paper Made? Separating Fibres and Letting Them Bond Again

The first problem is lignin, the stiff polymer that binds cellulose fibres in wood and gives a tree its strength. Two approaches exist and they produce very different papers. Mechanical pulping grinds or refines the wood, keeping almost all the material including the lignin, which gives a high yield of around ninety-five percent and a weak, bulky, opaque paper that yellows because lignin darkens on exposure to light and air, which is what newsprint is. Chemical pulping cooks wood chips with chemicals that dissolve the lignin and leave the cellulose, most commonly the kraft process using sodium hydroxide and sodium sulphide, which yields only about half the wood as fibre and produces much stronger, more stable paper, kraft meaning strength in German and Swedish. The kraft process recovers and reuses its chemicals in a closed loop and burns the dissolved lignin for energy, which is why a pulp mill generates most of its own power and also why it smells.

The stock arriving at the paper machine is over ninety-nine percent water, and the machine's whole job is removing it in stages of increasing difficulty:

• •Refining, where fibres are mechanically brushed and fibrillated so they bond better, which trades strength against drainage speed and is the main lever a papermaker has
• •The headbox, which spreads the dilute stock evenly across the full width and controls how the fibres orient
• •The wire, an endless moving mesh where most of the water drains by gravity and suction, forming the wet web; the shaking and speed difference here sets fibre alignment and therefore why paper tears more easily one way than the other
• •The press section, where the web passes through nip rolls that squeeze water out mechanically, which is far cheaper than evaporating it
• •The dryer section, dozens of steam-heated cylinders that evaporate the remaining water and consume most of the machine's energy
• •The calender, where polished rolls press the surface smooth, and the reel, where the finished sheet is wound

Pure fibre alone would make a sheet that is grey, weak when wet and unable to take ink cleanly. Fillers, usually calcium carbonate or clay, occupy up to a third of the sheet, improving brightness, opacity and printability while costing less than fibre. Sizing, historically rosin and now usually synthetic, makes the sheet resist water so that ink sits on the surface instead of feathering into it, which is the difference between writing paper and blotting paper. Retention aids hold fillers and fines in the sheet rather than letting them wash through the wire, optical brightening agents absorb ultraviolet light and re-emit it as blue to make paper look whiter than white, and dyes, wet strength resins and surface coatings of pigment and binder are added for particular grades. The acid rosin sizing used from the nineteenth century until the 1980s left paper mildly acidic, which slowly hydrolyses cellulose, and is the reason books from that period are crumbling while much older rag paper survives intact. Most mills switched to alkaline sizing, and archival paper is now specified as acid free and buffered.

Recovered paper is now a larger raw material than fresh pulp in many countries. The process repulps the waste in water, screens out staples, plastics and grit, and for printing grades removes ink by deinking, in which surfactants detach ink particles and air bubbles float them off in a flotation cell. The limit is physical: every cycle shortens and damages fibres through refining and pressing, reducing their ability to bond, so fibre can typically be recycled something like five to seven times before it is too degraded to form a sheet, and fines are lost at every stage. Recycled systems therefore require a continuous input of fresh fibre, and the two are complements rather than alternatives. Recycling nevertheless saves substantial energy and water relative to making pulp from wood, and diverts a bulky material from landfill where it decomposes anaerobically and releases methane. The material's real advantage is that its raw material regrows, and the argument about paper is properly an argument about how the forests supplying it are managed.

Paper is made by freeing cellulose fibres from the lignin that binds them, either mechanically for a high yield of weak paper that yellows, or chemically for a lower yield of strong stable paper. The fibres are diluted enormously, drained onto a moving mesh, pressed and dried, and they bond to one another without adhesive. Fillers, sizing and brighteners supply opacity, ink resistance and whiteness, and the acidic sizing used until the 1980s is why books of that era are disintegrating. Fibres survive roughly five to seven recycling cycles.