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Naturally Occurring Polymers—Plants 285

longer and digesting solutions more concentrated giving a product with less lignin but the overall
yield of cellulose-intense material is lowered by 70%–80%. Further, some degradation of the cel-
lulose occurs.
Most paper is produced by the chemical process where chemicals are employed to solubilize and
remove most of the lignin. While overall yields are lower than the other two main processes, the
product gives good quality writing and printing paper. Three main chemical processes are used.
In the soda process extracting solutions containing about 25% sodium hydroxide and 2.4% sodium
carbonate are used. In the sulfite process the extracting solution contains a mixture of calcium

dihydrogen sulfite and sulfur dioxide. The sulfide process utilizes sodium hydroxide, sodium mono-
sulfide, and sodium carbonate in the extracting solution.

After the chemical treatment, the pulped wood is removed, washed, and screened. Unbleached,
brown-colored paper is made directly for this material. Most whiten or bleached paper is made from
treatment of the pulp with chlorine, chlorine dioxide, hypochlorite, and/or alkaline extraction. In
general, sulfate pulped paper is darker and requires more bleaching and alkaline extraction to give
a “white” pulp.
The sulfi de process, also called the kraft process (the term “kraft” comes from the Swedish word
for strong since stronger paper is produced), is more commonly used. The kraft process is favored
over the sulfite treatment of the paper because of environmental considerations. The sulfi te process

employs more chemicals that must be disposed of-particularly mercaptans, RSHs, which are quite
odorous. Research continues on reclaiming and recycling pulping chemicals.

If pure cellulose was solely used to make paper, the fiber mat would be somewhat water solu-
ble with only particle surface polar groups and internal hydrogen bonding acting to hold the fi bers
together. White pigments such as clay and titanium dioxide are added to help “cement” the fi bers

together and to fill voids producing a firm, white writing surface. This often occurs as part of an
overall coating process.
Most paper is coated to provide added strength and smoothness. The coating is basically an
inexpensive paint that contains a pigment and a small amount of polymeric binder. Unlike most
painted surfaces, most paper products are manufactured with a short lifetime in mind with mod-
erate performance requirements. Typical pigments are inexpensive low-refractive index materials
such as plate-like clay and ground natural calcium carbonate. Titanium dioxide is used only when
high opacity is required. The binder may be a starch or latex or a combination of these. The latexes
are usually copolymers of styrene, butadiene, acrylic, and vinyl acetate. Other additives and color-
ing agents may also be added for special performance papers. Resins in the form of surface coating
agents and other special surface treatments (such as coating with polypropylene and polyethylene)
are used for paper products intended for special uses such as milk cartons, ice cream cartons,
light building materials, and drinking cups. The cellulose supplies the majority of the weight (typi-
cally about 90%) and strength with the special additives and coatings, providing special properties
needed for the intended use.
A better understanding of the nature of paper and films made from synthetic polymers such as

polyethylene can be seen when considering why authorities worry about anthrax escaping from
a paper envelope yet confi ning anthrax in a plastic container with no fear of it escaping. When

you hold good paper up to the light or tear it you will observe the tiny fibers that compose paper.
Even when these fibers appear tiny, they are very large in comparison to individual polymer

chains, be they cellulose or polyethylene. The web of chains for cellulose compose eventually
the cellulose fibers that are put together physically, forming paper with lots of “unoccupied”

spaces between the fibers with the spaces of sufficient size to allow the escape of the anthrax.

By comparison, polyethylene film has little “unoccupied” spaces between the individual chains
with the entire film composed of these individual chains with no large spaces so that the anthrax

cannot escape.
Recycling of paper continues. Today, up to about one half of our paper products are recycled and
this fraction is increasing as we do a better job of collecting and recycling paper products.

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