Cellulose fiber. The principal raw material for producing paper is cellulose fibers, which are short, threadlike structures. Cellulose fiber is the basic building block of plant matter, and large amounts of it can be extracted from wood. There are four main sources of cellulose fiber used in the manufacture of paper; softwood trees, hardwood trees, recycled fiber, and rag (usually composed of textile cuttings and cotton). As a fifth option, synthetic fibers are sometimes used for specialty papers. Other plants, such as sugar cane or bamboo, are also used as fiber sources.
Softwood and hardwood trees are the most commonly used sources of fiber for sheetfed offset papers. Each source produces fiber with slightly different characteristics. Hardwood trees like poplar, birch, and maples produce shorter fibers, about 1mm in length. Softwood trees like spruce, pine, and fir produce longer fibers, about 3 mm in length. The longer softwood fibers tend to give paper more strength due to better interlocking of the fibers. The shorter hardwood fibers provide paper with bulk and better surface smoothness. See Figure 5-2.
Trees are replenishable, easily harvested, and easily transportable, making them the ideal source of cellulose in papermaking. Cellulose fibers have a very high tensile strength and a great affinity for water, meaning that the fibers can be bonded together strongly in a network to form paper. The size and shape of fibers, which vary with type of tree and even within a given tree, have an important influence on paper properties.
Paper is made up of a variety of ingredients in addition to fiber, including sizing materials, mineral fillers, and coloring matter.
Sizing. Sizing materials include starch and rosin. These ingredients may be added internally, externally, or both. Internal sizing is intended to give the paper water resistance, a key factor in papers used for lithography. Rosin and papermakers' alum are two materials commonly used for internal sizing. Surface sizing controls the absorption of printing ink, allowing for crisper images on the paper surface. Surface sizing also reduces the release of surface fibers onto blankets, a problem called picking. In addition, sizing may also serve as a preliminary treatment for subsequent coating of the paper.
Fillers. Mineral fillers (finely divided, relatively insoluble inorganic materials or minerals) are added to the fiber before the sheet is formed to improve smoothness, opacity, and color. They also reduce strike-through, a condition whereby ink penetrates the paper and shows up on the other side. Fillers also improve the ink receptivity of offset papers. Paper that has been sized but not filled may not accept printing ink quickly enough for good initial setting, especially at high press speeds. Fillers also reduce dot distortion due to its enhanced surface smoothness. In addition, fillers reduce show-through, which occurs when an image printed on one side of the sheet can be seen from the other side due to a lack of opacity. Fillers improve the brightness (whiteness) of paper, which gives printed images more "pop". Clay (from refined natural kaolin clay), titanium dioxide, and calcium carbonate are the most commonly used fillers.
Pigments. Colored paper require the addition of pigments and dyes. Colored papers are fairly common in sheetfed offset lithography. The print designer should understand the adverse effect that colored papers will have on colored inks and images.