L17. Paper
So far we've discussed wood mostly in terms of its use as a building material. But wood is also used extensively in making paper. In this lesson, we are going to cover some of the basics of paper, including how its made, some things to think about concerning whether its sustainable, and if recycling paper is worth it.
Where Does Paper Come From?
From our previous lectures we learned that woody plant material is made up of three compounds that make up the plant cell walls: cellulose, hemicellulose, and lignin. Paper is made by separating the cellulose from the lignin and hemicellulose, and by pressing together wet fibers of cellulose pulp. The cellulose used for papermaking can come from just about any fibrous plant material, such as wood, cotton fabrics, fiber crops (including straw or the plant material of food crops like wheat, rice, etc), or waste paper (recycled paper). Up until the mid 1800’s, most paper was made from cotton fabric scraps left over from cloth mills. Since new technologies were introduced in the 1840’s, wood pulp has become the primary substrate for papermaking. Today, paper production in the United States comes is equally split between wood scraps from sawmills, trees cut down specifically for making paper, and from recycled paper (US EPA, 2012).
Different tree types result in different paper qualities. Softwoods (like pines, firs, and spruces) yield long and strong fibers that impart strength to paper and are used for boxes and packaging. Hardwoods, on the other hand, generally have shorter fibers and therefore produce a weaker paper, but one that is smoother, more opaque, and better suited for printing.
How Much Paper Do We Use?
Global paper and paperboard (i.e. cardboard) production was estimated at 410 million tons in 2017 (FAO 2018). As we can see in the graph below, while the global rate of paper production has slowed in the last 10 years, most years show an increase in global paper production.
The US uses around 69 million tons of paper and paperboard per year (US EPA 2015). That is about 430 lbs of paper per person per year or more than a pound per person per day! For reference, a ream of paper weighs 5 lbs. Most paper is used for printing and writing, or wrapping and packaging purposes.
How is Paper Made?
Since wood is the most common material used in papermaking, we are going to focus on it here. Paper making has three main steps, beginning with preparation of the raw material. In this step, wood is chopped up into small pieces and de-barked. Bark contains relatively few useful fibers, and is removed. However, both the bark and any sawdust creating during chipping can be used as fuel to help power the facility.
Once the wood has been prepped, it is turned into wood pulp. In this process, the wood is separated into individual cellulose fibers by removing the lignin. Pulping can be done mechanically or chemically. Mechanical pulping is used to produce paper that is less strong, such as newspaper. This is because mechanical methods physically tear the cellulose fibers from one another. During this processes, the fibers may be cut, resulting in a less strong material for paper making. Mechanical methods make use of the whole wood material. In other words, all of the wood that enters the pulping process can be used; all of the cellulose is kept for making the paper and the separated lignin can be used as fuel.
More commonly, cellulose is separated from lignin using chemical methods. This method, known as chemical pulping, is the most common way of producing wood pulp for paper, with about 75% of wood pulp for paper utilizing chemical methods. Chemical pulping involves using chemical solutions and high temperatures to “un-glue” the fibers by removing the lignin. These chemicals degrade the lignin and hemicellulose into small, water-soluble molecules which can be washed away from the cellulose fibers. By far the most common chemical pulping method is the kraft process which uses sodium hydroxide and sodium sulfide to digest (or break down) the wood. In the kraft processes, the chemicals and water used in digesting the wood chips can be recycled in the process, as discussed in the following video. (And yes, there is a bit of marketing in both of the following videos. Watching videos at faster speeds is always OK.)
The finished wood pulp is often bleached to remove residual lignin. This whitens the paper and helps it resist aging. After bleaching, the final stage of paper making is consolidating the cellulose or wood pulp on a paper machine. This next video has some topics repeated from the first video but has more of a focus on what happens with the pulp.
Is Paper Sustainable?
Asking a sustainability question on a product is hard if there is nothing to compare it to. An LCA of wood for building is only useful in reference to the use for a similar structure of concrete or steel. With paper there are no good alternatives. Digital media might be a good substitute (e.g. is it more sustainable to read a paper book or a book on your laptop?) In general, the two are not equivalent. What we can do is think about the environmental impacts of paper and efforts to reduce that impact.
There are a number of environmental concerns associated with paper occurring at different stages of its life-cycle, including: deforestation, water and air pollution, greenhouse gas emissions, and solid waste pollution.
Deforestation and Land Use: Since most of our paper is made using wood, a major concern associated with paper production is deforestation. Though globally most deforestation is driven by agriculture, logging operations for lumber and paper also contribute. Clear-cutting, which removes all of the trees in a given area, causes a loss of habitat and threatens species survival. Trees are also important in the cycling of water from the soil to the atmosphere, in climate regulation, and as a sink for greenhouse gasses such as carbon dioxide. However, there are ways of managing forests that minimize these risks, such as practices involved in Forest Certification, as discussed in lesson 15.
Pollution from papermaking: Many of the environmental concerns associated with paper result from the pulp and papermaking processes. Even though many pulp mills reuse water during the kraft process (as seen in the video above), papermaking is a water intensive industry, with mills requiring millions of gallons of water a day. Some of this is recycled within the mill (as we saw in the video), but mills still require fresh water inputs.
The pulp and bleaching processes also are responsible for emissions of air pollutants. Burning of the wood and chemical wastes (particularly sulfur and chlorinated compounds) during these processes releases hazardous pollutants into the air, including carbon dioxide (CO2), sulfur dioxide (SO2), nitrous oxides, volatile organic compounds (VOC's), and sulfur containing compounds (such as hydrogen sulfide, dimethyl sulfide, and dimethyl disulfide). Nitrous oxides and sulfur dioxide both contribute to acid rain, though many mills in the US and Europe employ technologies that limit their emissions. VOC's and the sulfur compounds are responsible for strong, bad odors at pulp mills, and represent a potential health risk to people with direct exposure.
Water pollution from the waste materials from pulp and bleaching facilities is also a major concern. Of particular concern are dioxins, a byproduct of the bleaching process which enter water bodies via the wastewater from pulp mills. Dioxins are extremely toxic, being known to cause reproductive and developmental problems, damage the immune system, interfere with hormones, and cause cancer. Though they can be produced by natural processes such as forest fires and volcanic eruptions, they are also produced as a byproduct of pulp making when chlorine is used as a bleaching agent.
Due to the toxic nature of dioxins, elemental chlorine was largely phased out of the bleaching process during the 1990’s, and has been replaced by chlorine dioxide (“elemental chlorine free” bleaching process) or other chemicals (“totally chlorine free” bleaching process). Elemental chlorine free is the most common means of bleaching wood pulp today. The switch from using chlorine to using chlorine dioxide has resulted in a significant reduction in the amount of dioxins entering waters during the bleaching process. However, use of chlorine dioxide still produces some amount of dioxins. Due to its toxicity, there are pushes towards using unbleached paper in more products where paper brightness is not key to the product’s functionality (such as the paper towels pictured here).
End of Life: Though paper products are largely recyclable, they are also a major source of waste in landfills. Paper is biodegradable, however, in landfills its decomposition can produce methane, a potent GHG. Paper and paperboard products represent the largest portion of our municipal solid waste stream (i.e. trash). The total amount of paper waste generated has been declining since 2000 (Figure 1).
"Paper and paperboard products made up the largest percentage of all the materials in MSW, at 25.9 percent of total generation. Generation of paper and paperboard products declined from 84.8 million tons in 2005 to 68.1 million tons in 2015. Generation of newspapers has been declining since 2000, and this trend is expected to continue, partly due to decreased page size, but mainly due to the increased digitization of news. The generation of office-type (high grade) papers also has been in decline, due at least partially to the increased use of the electronic transmission of reports, etc. Paper and paperboard products have ranged between 33 and 27 percent of generation since 2005." (Source USEPA Links to an external site.)
Figure 1. Municipal Solid Waste Generation (Source USEPA Links to an external site.)
In that same year, we recycled about 45 million tons of paper that Americans used (about 67% of total) (EPA Source 2 Links to an external site.). Not only does recycling reduce the amount of solid waste, it can also help reduce some of the environmental impacts associated with paper.
Recycling Paper
To "save the planet" we often think of recycling. As we just learned, we recycle about 45 million tons of paper per year in the US. How does this work and where does this recycled paper go? Perhaps you have seen the terms "pre-consumer" and "post-consumer" on paper products. Pre-consumer recycled paper is made from the scraps of the paper making process or the waste material when they are making the final products. In the videos did you see them trimming the roles of paper? That paper is considered pre-consumer and would would be returned to pulping process. Post-consumer is the more familiar recycling - newspaper, office paper or cardboard that has already been used by the consumer and is then sent to recycling.
In the recycling process, paper or paperboard is re-wetted and reduced to pulp by mechanical means. Inks, adhesives, other contaminants are removed by chemical de-inking and mechanical separation. Here is a great video on the many steps to the process.
Recycled paper generally has different physical properties than that produced directly from wood. Additionally, paper can only be recycled 5-7 times, after which the fibers become too short to bond into new paper. Cardboard egg cartons are at the very end of the line, and are made from the shortest and weakest fibers. At this point, the fibers are just to short to use and go on to another product.
Paper recycling can save water and energy because the starting substrate, paper, requires less preparation than wood. In 2012, the US EPA estimated that recycling 1 ton of paper can:
- Save enough energy to power the average American home for six months
- Save 7,000 gallons of water
- Save 3.3 cubic yards of landfill space
- Reduce greenhouse gas emissions by one metric ton of carbon equivalent (MTCE)
Cellulose Insulation
Not all paper that gets recycled ends up as paper. One of the other big uses is for cellulose insulation. Yes - those old newspapers can be turned into a product that insulates our homes or other buildings. This video is just for fun - no quiz questions. It is however, highly recommended!
Conclusions
This ends our unit on wood. Think about what you know now that you did not know just a few weeks ago. Anything useful? Anything interesting? We hope that you now feel confident enough in your knowledge base to have conversations with family and friends about the pros and cons of wood as a resource.
Our next unit focuses on non-woody biomass, and dives deeper into the processes we use to make bioproducts. If you've ever wondered how ethanol fuel or bioplastic is made - this next unit is for you!
Sources and Links:
FAO 2018. http://www.fao.org/faostat/en/#data/FO/visualize Links to an external site.
US EPA. 2015. Advancing sustainable materials management: 2015 fact sheet. https://www.epa.gov/sites/production/files/2018-07/documents/2015_smm_msw_factsheet_07242018_fnl_508_002.pdf Links to an external site.
US EPA. 2012. https://archive.epa.gov/wastes/conserve/materials/paper/web/html/faqs.html