A Staff Report from the Straight Dope Science Advisory Board

How is paper made?

November 22, 2005

Dear Straight Dope:

My question is about something that has plagued me since I was a kid. How is paper made? I know it's made from trees but what is the actual process? I've also wondered how many sheets of normal-sized paper come from one tree.

Pulp and papermaking is an industry that impacts pretty much everyone on the planet, employing about 1.15 million workers in the U.S. alone, yet few know how the process works, other than having a vague sense that the sequence of events is:

  1. Trees
  2. ???
  3. Paper.

Papermaking typically does begin with trees as the raw material, although many non-woody plants can be used. Plants used for papermaking include cotton, wheat straw, sugar cane waste, flax, bamboo, linen rags, and hemp (no, really, dude! You can also make clothes out of it, and hammocks, and, um, flags and stuff). Cotton is often used in high-quality papers, such as for professional printing and resume applications; in addition it can be blended with wood to make mid-range paper products. Cotton is used to make United States currency, which is 75% cotton and 25% linen, according to the Treasury Department.

However suitable these other sources may be, nearly 95% of the raw material for papermaking comes from trees. Softwood trees most used for papermaking include spruce, pine, and fir; common hardwood trees used include oak, maple, and birch. What makes a tree or plant suitable for paper is cellulose fiber. Trees are generally a composite of cellulose fibers bonded together with lignin, plus sugars and other organic compounds. Depending on species, about 40-50% of the tree consists of cellulose suitable for papermaking (efforts are underway to create transgenic trees with as much as 55-60% cellulose, but I digress).

Since only the cellulose is needed, the first step in papermaking is to separate the lignin and other materials from the cellulose, a process called pulping, which produces, as one might surmise, a substance called wood pulp. There are two primary methods of making this pulp, each with advantages and disadvantages--mechanical and chemical. There are other pulping processes, such as semi-mechanical and so forth, but we'll focus on the primary two.

Mechanical pulping can be done in several ways, but all are based on the same principle--finely grinding or chopping the wood to separate the cellulose fibers from everything else. Sometimes this is done by grinding alone; other times steam and chemicals aid in the process. Mechanical pulping is very efficient, and can convert 90% or more of the wood into pulp. However, the resulting pulp contains most of the lignin it started with, causing the resultant paper to turn yellow or brown when exposed to the sun. The process also tends to produce fibers that are short and stiff, and which yield paper that isn't very strong. For that reason, mechanical pulps are mainly used for packaging, newsprint, and other low-strength applications. Mechanical pulp is sometimes blended with chemical pulp to produce a middle-level product with low cost and reasonable strength and color properties.

Chemical pulping uses chemicals, heat, and pressure to dissolve the lignin in the wood, freeing the cellulose fibers. In the "kraft" process, the wood and chemicals are cooked in a digester to remove the sugars, about 90-95% of the lignin, and anything else you don't want in the final product. The waste from the digester is known as "black liquor," and it's often burned at the paper mill as an energy source. Kraft mills account for about 75-80% of all pulp production in the U.S. An older sulfite process is used in a handful of mills (fewer than 2%), where acid is used to modify the chemical structure of the lignin, which is then washed out of the cellulose.

A note about recycled paper processing: Since paper contains the same fibers as the original plants, these fibers can be re-used to make new paper. As in the case of virgin material, the process starts with pulping. You typically start by putting the recycled paper into a large vessel filled with water, then chopping it up to separate the fibers and wash out the inks and other contaminants. This chopped paper slurry then goes through several other mechanical and chemical treatments to recover as much fiber as possible and continue removing ink, colors, and coatings. As you might expect, the extra mechanical and chemical stress damages and shortens the fibers, limiting the number of times that paper can be recycled. A common rule of thumb is that paper can be recycled about three times, but that isn't really correct, as even under ideal circumstances paper fibers are lost at each step of the process, and some types of paper recycle better than others. It's safer to say, depending on the type of paper, that fibers (as opposed to paper) can be recycled 3-6 times on average.

For paper with a high level of whiteness, the pulp must be bleached. Due to its lower lignin content, chemically-produced pulp is generally easier to bleach. Bleaching typically uses some combination of chlorine, sodium hydroxide, and hydrogen peroxide as whitening agents or lignin removers. Pulp and paper mills used to be a significant source of chlorine, dioxin, and other hazardous emissions, so many have switched from pure chlorine to chlorine dioxide, and some have eliminated chlorine entirely. The products of sulfite mills are typically light-colored to start with, so they sometimes can be bleached without using chlorine at all. Recycled paper commonly requires more bleaching because it's made from a mix of low- and high-lignin papers containing inks, dyes, and so on. As a result, 100%-recycled paper often has an off-white color, and some may be light yellow or gray.

Once the pulp is ready, it's time to make the paper. The basic process dates back 1,900 years according to Kew Botanical Gardens in the UK:

In AD 105 a Chinese court official, Ts'ai Lun, produced a paper web from a slurry of paper mulberry (Broussonetia papyrifera) fibres in water. A small amount of the slurry was lifted up in a rectangular sieve consisting of a sheet of silk surrounded by a frame. The sieve was shaken gently to spread the fibres evenly and, as the water drained off, they settled to form a sheet which was then dried in the sun. This process produced a long-lasting, high quality paper, as can be seen from the samples preserved in the British Museum.

That's essentially what's done today. The pulp is highly diluted with water (sometimes 99%-plus), and the mixture is sprayed onto a moving mesh screen in layers to make a mat. It's like spraying multiple layers of paint to produce a thicker coat. The mat then goes through several mechanical and vacuum processes to dewater, compact, and dry it, and then is sent through heated rollers to squeeze out any remaining moisture and compress the mat into paper. Infrared dryers may be used to speed up the process or achieve a more consistent product. The mat can be quite large, as wide as 10 yards, and is generally laid out as a long, continuous roll--according to the German Pulp and Paper Association, the sheets can end up in a continuous roll 60 kilometers in length!

After the paper has the proper thickness and moisture content (but typically before it's rolled), it may be coated with synthetic binders to increase its strength and water resistance. It may also be colored at this point or given a light plastic coating to give it a glossy texture and remove any paper odor. The paper rolls are then cut to size and packaged for shipping to another facility for secondary processing.

Papermaking is a high-speed process--the paper typically moves at more than 45 miles per hour. Having seen it in person, I'm amazed that it works so well without tearing, and that the consistency of the paper is so uniform. I've worked for several pulp and paper mills, and touring the papermaking portion of the mill is always the most interesting thing I do while I'm there.

Pulp and paper mills use a lot of resources--plant and process efficiencies vary greatly, but in my experience it takes roughly 1-1.5 pounds of coal to make 1 pound of finished paper from trees delivered to the mill. Pulp and paper mills require a lot of electricity, water, natural gas, and related chemicals to make paper--some mills I've been to use as much electricity as nearly 100,000 average American homes. Many mills generate a large part of their electricity from burning waste--wood scrap, leaves and needles, bark, waste pulp, and black liquor. By any measure the pulp and paper mill industry is a major "green power" producer, which is good considering that in 1998 the total energy consumption of U.S. pulp and paper mills was estimated at an incredible 2.75 quadrillion Btu (the equivalent of about 470 million barrels of petroleum). Between 46-55% of that is self-generated using biomass and biomass waste, and the rest either purchased from the local utility or generated on-site from fossil fuels.

How many sheets of paper come from a single tree? I tried doing the calculations myself and ended up with far too many variables and assumptions to yield a confident answer, so I turned to the web. The best site I found,  run by the Conservatree, has a statistics page that tells us:

  • 1 ton of uncoated virgin (non-recycled) printing and office paper uses 24 trees
  • 1 ton of 100% virgin (non-recycled) newsprint uses 12 trees
  • A pallet of copier paper (20-lb. sheet weight, or 20#) contains 40 cartons and weighs 1 ton. Therefore:
  • 1 carton (10 reams) of 100% virgin copier paper uses 0.6 trees
  • 1 tree makes 16.67 reams of copy paper or 8,333.3 sheets
  • 1 ream (500 sheets) uses 6% of a tree (and those add up quickly!)
  • 1 ton of coated, higher-end virgin magazine paper (used for magazines like National Geographic and many others) uses a little more than 15 trees (15.36)
  • 1 ton of coated, lower-end virgin magazine paper (used for newsmagazines and most catalogs) uses nearly 8 trees (7.68)

On submitting this paper to my faithful peer-reviewers (what, you think I just dash these things off?), I was asked a couple of related questions and pointed to an interesting resource:

Why does a paper mill smell so bad? My Dad engineered for a couple, and all I can remember is that horrible, nausea producing stench.

Having worked in paper mills, I can attest that the smell can be daunting. The smell is only partially a result of the sulfur compounds used in the kraft process; mostly, it's a result of the cooking out of the lignins and sugars in the wood. Remember too that trees contain some natural sulfur compounds, which are liberated during the pulping process. The smell is sickly sweet, reminiscent of what I encounter at large grain processors. The good thing is that, though annoying, it's not very hazardous. After a week at a plant you stop noticing it partially because your hair, clothes, and hotel room take on the same smell.

My own paper questions are about the "weight" of paper (60 pound, 20 pound, etc.) and cotton content. More of both is supposed to be better, I think. But when does the cotton get in the paper?

The cotton pulp is blended in with the wood pulp before it is applied to the moving mesh screen. It could be applied in separate layers, but that could lead to segregation of the fibers and the paper peeling or developing flaws. Using more cotton yields a higher-quality, more expensive paper. In the U.S. a paper's weight is expressed in pounds per ream (500 sheets) of uncut paper, with greater weight generally meaning better paper more suitable for fine printing. Since bond paper has a basic sheet size of 17x22 inches, the weight of a ream of 8.5x11 paper would be 1/4 the uncut weight. Thus 60-pound "letter" sized paper would weigh 15 pounds per ream, and 20-pound paper would weigh 5 pounds per ream.

A note on glossy paper: It's a little different from normal paper as it contains a clay compound called kaolin. Since kaolin contains slightly elevated levels of thorium and uranium, someone has naturally done some science on the half life of Playboy magazine.

www.orau.org/ptp/collection/consumer%20products/magazines.htm

I don't think I can add much to that.

References

Much of my research consisted of on-site interviews of pulp and paper mill personnel I work for, but here are some selected references I used for parts of this report:

American Forest and Paper Association, www.afandpa.org

Conservatree, www.conservatree.com/learn/EnviroIssues/TreeStats.shtml

Enzyme Processes for Pulp and Paper: A Review of Recent Developments. William R. Kenealy and Thomas W. Jeffries, Institute for Microbial and Biochemical Technology, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory

International Paper Ream Weight Calculator, www.internationalpaper.com

Paper Task Force Recommendations for Purchasing and Using Environmentally Preferable Paper. The Paper Task Force (Duke University, Environmental Defense Fund, Johnson & Johnson, McDonald's, The Prudential Insurance Company of America, Time Inc.), final report, 1995.

"Paper-Making: Information on Raw Materials and Paper-Making," German Pulp and Paper Association.

Smook, Gary, Handbook for Pulp & Paper Technologists, 2nd ed., 1992.

U.S. Department of Energy, "Forest Products Industry Analysis Brief, 1998."

U.S. Department of Energy, "Manufacturing Energy Consumption Survey, 1998."

U.S. Department of Energy, "Monthly Energy Review, September 2005: Thermal Conversion Factors."

U.S. Environmental Protection Agency, Development Document for Proposed Effluent Limitations--Guidelines and Standards for the Pulp, Paper and Paperboard Point Source Category, report number EPA-821-R-93-019, 1993.

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