Hydrogen bonds are weaker than covalent or ionic bonds but stronger than van der Waals forces, with a typical range of about 4 to 167 kilojoules per mole. They are responsible for the high boiling point of water, the shapes of proteins and DNA, and key properties of materials like paper. In paper, the bonds between cellulose in neighboring fibers hold the sheet together. Cellulose chains are straight and stiff, unlike starch. Cellulose is made of glucose units linked by beta-1,4 bonds, while starch has alpha-1,4 bonds. Cellulose does not coil or branch, so the molecule adopts an extended, rod-like shape, and chains pack side by side. The French chemist Anselme Payen discovered cellulose in 1838. Lignin was named by the Swiss botanist A. P. de Candolle in 1813, from the Latin word lignum, meaning wood. He described it as a fibrous, tasteless material that does not dissolve in water but does dissolve in weak alkaline solutions, which is exactly why alkaline pulping can remove it from wood.

Recycling paper breaks the bonds that hold it together. Mixing paper with water and applying mechanical action breaks the hydrogen bonds and separates the fibers again. Most recycled paper still has some virgin fiber for quality, and de-inked pulp is generally of the same quality as the collected paper or lower. In the kraft cooking liquor, the main reaction that breaks up lignin is the splitting of ether bonds by sulfide or bisulfide ions. At 170 to 176 degrees Celsius, lignin and hemicellulose degrade into fragments that dissolve in the strongly alkaline liquid. Carbon from the black liquor then reduces sodium sulfate back to sodium sulfide in the recovery boiler. The kraft chemical cycle is closed for sodium, sulfur and calcium. Sodium carbonate in green liquor reacts with calcium hydroxide to give sodium hydroxide and calcium carbonate; the calcium carbonate is heated in a lime kiln to make lime, and the lime is slaked with water to remake calcium hydroxide, so each batch of chemicals is used again.

Black liquor starts as only about 15% solids after washing. It is concentrated in evaporators to 20 to 30%, where rosin soap rises and is skimmed off to be made into tall oil, then to 65 or even 80% solids before burning. Higher solids give better energy and chemical efficiency but also thicker liquor that can plug and foul equipment. A modern kraft pulp mill produces more electricity than it uses. Its recovery boiler makes high-pressure steam that feeds turbogenerators, and bark and wood residues are often burned in a separate power boiler; the extra power can go to a neighboring paper mill or onto the local electric grid. Kraft pulp washing is done in several stages. A mill normally has three to five washing stages in a row, and pulp washers use countercurrent flow, so the pulp moves in the opposite direction to the washing water. Thorough washing of the pulp reduces the chemical oxygen demand of the wastewater.

The American Forest & Paper Association, an industry group, says its members' emissions are falling. It says that between 2005 and 2022, combined Scope 1 and 2 emissions of its member U.S. pulp and paper mills fell by 36%, and that 63% of energy used at member facilities in 2022 came from carbon-neutral biomass, mostly the leftovers from making paper. Water is essential to pulp and papermaking, and the industry says it is working to use less. The association says its members aim to reuse water and return clean water to the environment, and reports that since 2005 its members have reduced water use by more than 8% (as of November 2026); these are the industry group's own figures. Worldwide, paper and paperboard production has shifted. FAO figures for 2021 show graphic papers, meaning printing and writing paper, have declined from a mid-2000s peak to below 100 million tonnes a year, while other papers and paperboard, including cardboard and sanitary products, have exceeded 320 million tonnes. E-commerce is one reason cardboard has grown.

Pulp and printing together produced about 1% of world greenhouse gas emissions, according to a source for 2010 (0.9% in 2012). Studies estimate that recycling paper and cardboard emits 0.2 to 1.5 kilograms of CO2-equivalent per kilogram, about 70% of the emissions from making virgin material. In 2019, about 110 million metric tons of paper products were thrown away in the United States. Of that, 56% was landfilled and 38% recycled, and the rest was burned. Paper in landfills breaks down and makes methane, and landfilling paper products has been estimated to lose about $4 billion in value. Plantation forests grow far faster than natural ones. Forests managed for wood commonly yield 1 to 3 cubic meters per hectare per year, while plantations of fast-growing species commonly yield 20 to 30 cubic meters or more. Pine, spruce and eucalyptus are widely planted for industrial use because of fast growth.

Washi is tougher than Western paper made from wood pulp. It is used to repair historic paintings, books and cultural properties at museums and libraries around the world, such as the Louvre and the Vatican Museums, because it is thin and pliable and lasts over 1,000 years thanks to its low impurities. As a Japanese craft, washi is registered as UNESCO intangible cultural heritage. A continuous machine changed more than paper. Charles Kinsey of Paterson, New Jersey, had patented a continuous papermaking machine in 1807 and was making paper with it by 1809, but he switched the mill to cotton and his work was forgotten; the method of continuous production also influenced the continuous rolling of iron and later steel. The main source on Cai Lun is only about 300 characters in volume 78 of the Hou Hanshu, a history written nearly 300 years after his death. It is problematic because of that gap, which is why historians treat the date 105 CE and the story of one inventor with care, and why archaeological finds of older paper matter.

One historian sums up Cai Lun's contribution as improving the skill of papermaking systematically and scientifically and fixing a recipe for it. The recipe, a paper of mulberry and other bast fibers with fishing nets, old rags and hemp waste, reduced the cost of paper, which in the West depended solely on rags. Wasps and hornets were making paper long before people. Social wasps build their nests from plant fiber, mostly wood pulp, and hornets build theirs by chewing wood into a papery pulp; one history says Cai Lun's papermaking method, using rags and other plant fibers in 105 CE, was inspired by wasps and bees.

Words to know

• kraft: strength
• lignin: natural wood glue
• washi: Japanese paper
• recovery: getting back