How is Paper Made?
If you really want to know about paper and papermaking, you’ve got to get to know trees.
Trees are all around you. But have you ever really thought about how they’re put together? They’re pretty cool.
Just look at a tree trunk. The bark protects the inner wood from weather, insects and other dangers. Just inside the bark is a thin layer called the cambium, whose cells become both bark and inner wood. Next is sapwood, which carries nourishing sap throughout the tree the same way our blood flows through our bodies to nourish us. Heartwood is the innermost part of the trunk, and even though it isn’t alive, it provides the tree with strength and structure.
All that wood material is formed of fibers, tiny cellulose strands stuck together with a natural adhesive material called lignin. It’s by separating and reorganizing those fibers that we make paper.
Consider the source
Some paper is made brand-new from trees – either small trees harvested just for that purpose, or from sawmill scraps left over when larger trees are made into lumber. A second source of papermaking material is recycled fiber. Each year, more and more paper is recycled – its fibers used a second, third or fourth time. Every year, about 50% of the paper Americans use is recovered for recycling and other uses.
Almost all of the paper you use today is made of wood fibers. Some specialty papers, like stationery and money, are made from linen, cotton, or other plants. Other papers contain a combination of cellulose fibers and synthetics such as latex. Still others are made completely from synthetic materials such as polyolefine. You might find latex in a waterproof mariner’s chart, or polyolefine in a rugged courier envelope. But you’ll find natural fiber paper almost everywhere!
Wood is wood?
Yes, but it’s not that simple. Each tree species grows a certain way, and that affects the way its wood looks and performs. Foresters divide trees into two categories: hardwood and softwood species.
Hardwood trees such as oaks and maples have wood with very short fibers. Paper made from these species is weaker than that made from softwoods, but its surface is smoother, and therefore better to write and print on.
Softwood trees such as pine and spruce have wood with long fibers, and paper made from this type of wood is much stronger. This paper is ideal for making products like shipping containers that require superior strength. But the finish is rougher, and that’s not as good for writing, printing and many other uses.
Happily, we can blend fiber from hardwoods and softwoods into a single paper, getting just the combination of strength, whiteness, writing surface and other characteristics that we want.
And that’s just what we do! Most of the paper you see today is made from both hardwoods and softwoods, a special blend for each purpose.
We make newsprint to be opaque (that means difficult to see through) – so you only see the newspaper’s comics, not the stock market report on the other side!
We make grocery bag paper strong, tissue soft, fine writing paper smooth. Even within the same category, there’s quite a range. Among printing papers, for example, compare the thin sheets of a Bible to the thick, tough pages of a kid’s picture book.
The basic recipe – wood, water and energy – is adjusted to make just the paper that’s needed.
First, workers harvest trees, mostly from special tree-growing areas called tree farms. After the trees are removed, more trees are planted in their place. While they are growing, the young trees produce lots of oxygen, and provide great habitat for deer, quail, turkeys and other wildlife.
The logs are transported to the paper company where they get a bath to rinse away dirt and other impurities before being turned into small chips of wood. The chips are then sorted according to size, and moved to the pulping operation, where they will be turned into pulp for making paper.
In the pulping stage, the individual wood fibers in the chips must be separated from one another. This can be accomplished using one or more pulping techniques. The type of paper that’s being made determines the pulping process that is used. The finished pulp looks like a mushy, watery solution. But if you look at it under a microscope, you will see that the individual wood fibers have all been separated.
Now it’s time to make paper out of our pulp. That mainly means getting the water out of the wood-fiber soup, since this papermaking stock is about 99% water. The first area in which this takes place is called the wet end of the papermaking machine.
First, papermakers spray the stock onto a long, wide screen, called a wire. Immediately, water begins to drain out the bottom of the wire. This water is collected so that it can be reused over and over again. Meanwhile, the pulp fibers are caught on the top side of the wire, and begin to bond together in a very thin mat. The fiber mat remaining on the wire is then squeezed between felt-covered press rollers to absorb more of the water.
Even when this wet end work is over, the pulpy stuff on the wire is still about 60% water. But now it’s time for the dry end.
In the dry end, huge metal cylinders are heated by filling them with steam. The wet paper, which can be up to 30 feet wide, passes through these hot rollers – sometimes dozens of them, and often in three to five groups. Heating and drying the wet sheet seals the fibers closer and closer together, turning them gradually from pulp into paper.
When you look at a piece of paper, can you find any difference in thickness in that single sheet? Probably not, thanks to a part of the paper machine called the calender – big, heavy cast iron rollers that press the drying paper smooth and uniform in thickness.
Sometimes the paper is coated, often with fine clay, to make it glossier and easier to print on.
A bit more drying, then rolled onto a big spool or reel, the pulp – a miraculous mat of fibers from trees – has become paper, ready for a thousand uses.