Fiber is the single largest component in the manufacturing and use of paper and pulp products. With ever increasing demand, the efficient use of wood fiber is a critical issue
What is “fiber yield”?
Fiber yield is the volume of usable fiber derived from either fresh forest fiber or recovered paper. Fiber yield is the proportion of fresh fiber from trees that is converted into pulp for papermaking or it is the proportion of recycled fiber from old newspapers that is converted into new recycled paper. Fiber yield is also an important factor to consider in determining which paper products utilize recycled fiber most efficiently. In addition to environmental considerations, there are also technical considerations—and limitations.
Fiber yield from recovered paper
All fibers get shorter each time they are processed. When fibers become too short or brittle to bond with each another, the physical and performance characteristics of the sheet may be compromised. Paper fibers can be reused anywhere from four to nine times depending on the new paper grade that is being produced. Factors that determine how many times fibers are reused include:
- the ability of the collection system to recover paper
- losses from the deinking process
- the decline in fiber strength with each use
Fiber losses from using recovered paper vary from 10 to 30 percent, depending on the new grade of paper that is being made.
The process of preparing recovered fiber for re-use breaks and stiffens fibers, in some cases resulting in poor product performance. To adjust for better performance, more fiber per sheet may be needed for recycled paper. Using recovered fiber in some grades, like magazine papers for example, may result in lower yields (i.e., a lower efficiency rate) because of the higher chemical applications and processing efforts. In some circumstances, it may not make sense to use higher levels of recovered fiber in those grades that would suffer significant yield losses as a result.
Fiber yield for fresh fiber
Different pulping processes have different fiber yields. Mechanical pulping, which grinds raw wood into pulp, has high fiber yields (above 95%) but requires large amounts of purchased energy. Chemical pulping, which dissovles away the dark compounds in the wood, has lower fiber yields (below 50%) but uses those dissolved compounds to generate most of the energy required for production.
Because not all paper is—or can be—recovered, along with the fact that some fiber is lost in the manufacturing of recycled paper, the input of fresh fiber into the fiber stream will always be necessary. Studies have shown that in North America, without the input of fresh fiber, at current recovery rates, recovered fiber available for paper manufacturing would run out in less than one year and, at maximum recycling rates, in less than 18 months.
The table below illustrates the longevity in months for various paper grades with current and maximum recovery rates. It is clear that some fresh fiber will always necessary in the paper life cycle.
Source: The Fiber Cycle; Metafore, FPAC (2006)1
1. Metafore, Inc. The Fiber Cycle Technical Document. Tech. Metafore, 2006 (http://www.metafore.org/downloads/metafore_reports_fiber_cycle.pdf)