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A Close-up of Paper Formation
Paper formation in a paper machine, when looked at closely,
is found to be a complex process. When paper is made in a paper
machine, one of the primary events that occur is when the fibers
are made to stick together. This is done on the forming table
in the wire section of the paper machine. Water is gradually
drained away, leaving the fibers behind. While this occurs,
water acts as an intermediate hydrogen bonding agent. When the
water is finally drained away, the fibers become pressed together
to become hydrogen bonded to one another in the press section
of the paper machine.
Below is an illustration showing water(H-O-H) as an intermediary
hydrogen bonding agent(hydrogen bonding is shown with dashed
lines).
Hydrogen bonding is responsible for a multitude of things in
other nature, such as the structure of ice in snowflakes and
protein folding. Hydrogen bonding is partly what give paper
its strength, but sometimes "wet strengtheners" are added as
well. Too little of this bonding, and the paper can fall apart,
too much, and the paper becomes inflexible, so papermakers have
to make sure there is enough bonding to insure integrity, while
simultaneously maintaining flexibility(unless containerboard
is being made).
Below is a micrograph(a microscope photograph) of a Kleenex
tissue paper(colorized for clarity).
At this magnification, one can easily see the pores and the
fibrous and non-uniform
nature of paper. This image is similar to what one would see
if one looked at writing paper, though the fibers would be less
slackened. There are two types of pores in paper:
- Pores between individual fibers, which range from 10-100
µm wide(1 µm is 1/1000th of a meter). Fillers that are added
to paper block some of these pores, so that liquid cannot(or
is less able) to be absorbed by the paper.
- Pores which are inside the fibers themselves, the width
of which range anywhere from 0.001µm to 0.05µm.
The structure of paper is varied depending on the type and it
helps one to understand why different types of paper have such
varied properties, such as tear strength and flexibility.
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