Paper cannot be continuously recycled. At one point in its recycling life cycle, the wood fibers in paper have been reprocessed so much they are too short and weak to bond and make new paper. Paper can only be recycled five to seven times before new wood is needed.
The very beginning of a piece of non-recycled paper lies in newly cut wood. The wood is processed into chips, then made into a watery pulp. The pulping process actually separates the wood chips into individual wood fibers, called cellulose. This can be done chemically, by cooking the wood chips with certain chemicals at high pressure to dissolve the lignin bonds holding the cellulose together, or mechanically, by simply pressing wood chips against a grinder. The remaining pulp is then washed, decontaminated and usually bleached. It is then sprayed onto screens so water can drain off and the cellulose fibers in the wood can stick together and bond into a mat on the screen. This mat is rolled between felt cylinders and other rollers to remove more water and also flatten it to the thinness of a sheet of paper. Fresh, never-before-recycled paper is called virgin fiber paper. After it is recycled, the paper is once again made into a pulp, cleaned, pressed and dried.
Each time paper goes through a cycle as above, its cellulose wood fibers become shorter and shorter. The process of making paper requires these fibers to be long and strong enough to actually bond to each other. According to Tappi, the leading technical association for the worldwide pulp, paper and converting industry, wood fibers can only be recycled five to seven times before they become too weak to be made into paper again. As a result, new wood fiber is needed to replace the unusable fibers, which can be washed out of the pulp during recycling.
Almost half of the paper used in the United States is recycled back into new paper products. The recycling process begins with paper collected at local recycling centers, which then get transported to paper mill warehouses. Various paper grades, from newsprint to corrugated cardboard, are separated to make different types of recycled products. When the mill is ready to use the recycled paper, the paper is moved from storage to the pulping machine. Making paper from recycled material is different from making paper from virgin wood in that the pulp must be more thoroughly cleaned. The recycled pulp is not only separated into individual wood fibers but also goes through a screening process in which pulp is squeezed through various sized holes to remove contaminants like bits of glue or plastic. Pulp is also cleaned by being spun in cylinders in which lighter or heavier contaminants can be separated from the top or bottom. Sometimes recycled paper must be de-inked, either by rinsing or by a process called flotation in which soap-like bubbles stick to the ink molecules in the pulp and float up to the surface, where they are removed.
Not all paper is actually fit for recycling in the first place. Certain standards of recycling processing programs separate out paper products that have been contaminated with food waste, hazardous materials like paint or sticky materials. Even things like plastic linings, as in paper cups, or staples in paper cannot be recycled because paper mills cannot process plastic or metal. In fact, just one contaminated item in a batch can send the whole lot to a landfill.