Paper is made of cellulose fibers bonded together through hydrogen bonds. The fibers can come from softwood or hardwood, as well as recycled paper and broke. Paper may also have pigments, coatings, dyes, sizing agents, and bonding agents to enhance its properties.
To make paper, there are several steps: preparing the pulp, forming the sheets, pressing them, drying them, and finishing the surface. Paper machines today are huge and precise. Some machines are over nine meters wide and can operate at speeds of up to 1800 meters/minute. They can produce a lot of paper – for example, a machine that is eight meters wide can produce around 864 tons of paper per day.
To make paper, cellulose fibers are mixed with water to form a pulp. This pulp is cleaned, blended, and deaerated before being refined or beaten. Refining involves passing the fibers through narrow gaps between spinning disks, which increases the surface area of the fibers and their ability to bond with each other. This process requires a lot of energy and produces some small fiber particles in the final product.
The papermaking process starts with refined fibers that are mixed with fillers, dyes, sizing agents, and strength additives to create a papermaking mix. To make sure that these additives remain in the mix, flocculants are added at various stages during the process. These flocculants are made of cationic starch, poly(acrylamides), poly(ethylene imine), or various aluminum compounds, and are positively charged. Newer retention systems use amphoteric starch, dual systems of cationic and anionic polymers, or small mineral particles, including nanoparticles. The papermaking mix is then diluted to a concentration of 0.3% to 0.8%, deaerated, and delivered to the paper machine.
The pulp mixture is sent through pipes to the headbox of the paper machine, where it is slowed down and spread evenly across the machine. Special equipment is used to prevent swirling and stagnant areas. The mixture then flows onto a screen where water is removed by vacuum and foils.
The paper sheet has a lot of water in it when it comes off the wire. It is very delicate and needs support from felts or fabrics as it goes through the press section. The press section squeezes out more water, leaving the sheet with about half as much water. The water that’s removed is collected and taken out by vacuum. Next, the sheet goes through steam-heated cylinders to dry it. It’s held in place by fabrics during drying to prevent it from shrinking in the wrong direction. Lastly, the paper is collected onto a reel.
The paper undergoes finishing processes to improve its surface, thickness, and density. Calendering is a process that compresses dried paper between cylinders to make it smoother and thinner. This prepares the paper for coating. Supercalendering is a multiple-step process that uses hard and soft rolls to make uncoated paper better for color printing, and coated paper smoother and glossier. New technology uses plastic-covered rolls and high temperatures for even better results.
In the paper machine, surface sizing is done to increase the paper surface strength, primarily its resistance to picking and linting in offset printing. A mixture of binder, pigments, and sizing agents is applied to the paper while it goes through a pair of rolls called the size press. Starch can also be added to the paper using a water box at a calender or by spraying it onto the wire of the machine in other systems.
High-quality printing paper is made by coating the sheet with pigments, binders, and additives. Starch is commonly used as the coating binder. The coating process can be done using a roll coater, a blade coater, or a fountain. The excess coating is removed using a stiff or bent blade, or an air curtain. Coating the paper generates a smooth surface for printing. Board coating involves using an air knife to remove excess coating, and a free-falling curtain is a new development. Starch is also used in large quantities for subsequent converting processes such as corrugating and laminating.