Page 321 - Carrahers_Polymer_Chemistry,_Eighth_Edition

284 Carraher’s Polymer Chemistry

composition of enzymes present in the two species—humans with the enzyme capability to lyse
or break α-linkages and for cows and termites and other species that contain symbiotic bacteria
in their digestive systems that furnish the enzymes capable to digest or break the β-glucoside
linkages.
While the stomach acid in humans and most animals can degrade polysaccharides to the energy-
giving monomeric units, this is not effi cient unless there is a specifi c enzyme, normally in the gut,
which allows the ready and rapid degradation of polysaccharide. Since these enzymes are somewhat

specific, their ability to degrade is polysaccharide specifi c.
The various crystalline modifications have different physical properties and chemical reactivi-

ties. These variations are a consequence of the properties varying according to plant source, loca-
tion in the plant, plant age, season, seasonal conditions, treatment, and so on. Thus, in general, bulk
properties of polysaccharides are generally measured with average values and tendencies given.
These variations are not important for most physical applications but possibly are important for spe-

cific biological applications where the polysaccharide is employed as a drug within a drug delivery
system or as a biomaterial within the body.

9.3 PAPER
It is believed that paper was invented by Ts’ai in China around the second century AD. The original

paper was a mixture of bark and hemp. Paper was first produced in the United States by William
Rittenhouse in Germantown, PA, in 1690 and this paper was made from rags. Paper was named
after the papyrus plant, Cyperus papyrus.
Paper comes in many forms with many uses. The book you are reading is made from paper, we
have paper plates, paper napkins, newspapers and magazines, cardboard boxes, in fact the amount
of paper items is probably more than twice, by weight, that of all the synthetic polymers combined.
About 30% paper is writing and printing paper. The rest is mainly used for tissues, toweling, and
packaging. If you rip a piece of ordinary paper, not your book page please, you will see that it

consists of small fibers. Most of these cellulosic fibers are randomly oriented, but a small percent-
age are preferentially oriented in one direction because the paper is made from a cellulose-derived
watery slurry with the water largely removed through use of heated rollers that somewhat orient
the fi bers.
Modern paper is made from wood pulp, largely cellulose, which is obtained by the removal of
lignin from debarked wood chips by use of chemical treatments with sodium hydroxide, sodium
sulfi te, or sodium sulfate. Newsprint and paperboard, which is thicker than paper, often contains a
greater amount of residual lignin.
Wood is almost entirely composed of cellulose and lignin. In the simplest paper making scheme,

the wood is chopped, actually torn, into smaller fibrous particles as it is pressed against a rapidly

moving pulpstone. A stream of water washes the fibers away dissolving much of the water-soluble
lignin. The insoluble cellulosic fibers are concentrated into a paste called pulp. The pulp is layered

into thin sheets and rollers are used to both squeeze out much of the water and to assist in achieving
paper of uniform thickness. This paper is not very white. It is also not very strong. The remaining
lignin is somewhat acidic (lignin contains acidic phenolic groups that hydrolyze to give a weakly
acidic aqueous solution) that causes the hydrolytic breakdown of the cellulose. Most of the news-
print is of this type or it is regenerated, reused paper.
Pulping processes are designed to remove the nonsaccharide lignin portion of wood which consti-
tutes about 25% of the dry weight. The remaining is mostly cellulose with about 25% hemicellulose
(noncellulose cell wall polysaccharides that are easily extracted by dilute aqueous base solutions).

Pulping procedures can be generally classified as semichemical, chemical, and semimechanical. In
semimechanical pulping, the wood is treated with water or sulfate, bisulfite, or bicarbonate solution

that softens the lignin. The pulp is then ground or shredded to remove much of the lignin giving
purified or enriched cellulose content. The semichemical process is similar but digestion times are

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