Walk into any photocopy shop and you will quickly discover that there are literally hundreds of different types of paper. Paper comes in every imaginable color, size, thickness, and quality. How do you know what type of paper to use for what application? What is the difference, for example, between groundwood and woodfree paper, and why does the later cost more when they look identical? The differences lie in how the paper is made. Therefore, in order to understand the differences, you need to understand the basics of the paper making process. This article will attempt to give you a crash course in paper making, explaining the differences in the different types of paper along the way.
Paper is created by pressing together damp fibers, usually cellulose pulp made from plants (such as wood or cotton), and drying them into sheets. To give you a very concise overview, the paper making process consists of six stages: pulping, mixing (optional), pressing, drying, finishing (optional), and cutting.
First, in the pulping stage, organic fibers (most commonly wood) is pulped, or broken down into fibers, and mixed with water. How the wood is pulped determines the paper quality.
In chemical pulping, wood is cooked in a chemical bath that breaks down and separates the lignin (the compound that holds plant cells together) from the cellulose fibers. The lignin is then washed away. The resulting paper is stronger and more durable. However, because over half of the wood (in the form of lignin) is washed away, the yield per paper batch is as low as 40% of the original wood. Consequently, this high quality paper (known, strangely, as “woodfree” paper) is more expensive.
In mechanical pulping, wood chips or logs are fed into rotating stone grinders that crush the wood into fibers. Since the lignin is not removed, the yield per paper batch is up to 95% of the original wood. Consequently, this type of paper (called “groundwood” paper) is cheaper than woodfree paper. The presence of lignin in the paper, unfortunately, makes this type of paper weaker and less durable, causing it to become yellowed and brittle over time.
In the second, optional mixing stage, chemical additives can be added to the fiber pulp. Bleach is commonly added at this point to make the paper white. Fillers, such as chalk or china clay, may be added to make the paper batch better suited for either printing or writing. Finally, sizing chemicals can be added to fine-tune how absorbent the paper is, tailoring it for ink, paint, or other printing mediums.
Third, in the pressing stage, the cellulose fibers are pressed together to form paper. This achieved two things. First, the fibers are pressed into the proper size and thickness for the batch of paper being made. Second, the excess water is squeezed out, leaving behind only the paper fibers.
Fourth, in the drying stage, the paper is fed through a set of ovens until the paper is dried to less than 6% moisture.
Fifth, in the optional finishing stage, the paper may be coated. Coated paper has a thin layer of a material such as china clay applied to one or both sides and then polished. This makes the paper more suitable for high-resolution images. Coated paper may come in matt, semi-matt, or gloss finishes. A glossy finish provides the best optical density for printed images, and thus is commonly used for photographs.
In the final stage, paper is cut into the appropriate lengths. This may be huge rolls for printing presses, or 8½ x 11 inch sheets for photocopiers.
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