It’s the cellulose in particular that is responsible for the properties of paper that we value. Cellulose, the most abundant natural polymer, also gives distant cousins of paper—woven cotton textiles such as T-shirts and blue jeans—the durability, comfort, and water absorbency that we look for in clothing.
Strong, but not too strong
To understand how these natural fibers are used in paper, we need to take a look at their structure. Wood fibers generally come in two lengths. Longer fibers from softwoods, which come from evergreens, are 3 to 5 millimeters (mm) long. They are used to reinforce the paper, so it can withstand being pulled apart or torn. Hardwood fibers—which come from deciduous trees, the type that lose their leaves every year—are typically much shorter at about 1.2 mm in average length. They are used to fill in gaps and make paper smooth. Both hardwood and softwood fibers usually have widths less than that of a human hair, which measures about 50 micrometers across.
An even closer look shows us that the chemistry and microstructure of wood fibers are important to paper’s strength and flexibility. First, the long polymeric chains of cellulose, which have about 10,000 units of glucose (C6H12O6), hydrogen bond to each other and form strong, rigid crystalline regions. These structured components are connected to unstructured, flexible regions.
The long, skinny strands of connected cellulose form what are called microfibrils. Within trees and other plants, the microfibrils are found in the plants’ cell walls. The cell walls are composed of many layers of these microfibrils that are wrapped around an open center called the lumen (Fig. 1).