14-10-2016, 04:01 PM
1459076148-PolymersProject.pdf (Size: 265.54 KB / Downloads: 8)
Introduction
Natural polymers (from the Greek poly meaning “many” and meros meaning “parts”) are found in many forms such
as horns of animals, tortoise shell, shellac (from the lac beetle), rosin (from pine trees), asphalt, and tar from
distillation of organic materials. One of the most useful of the natural polymers was rubber, obtained from the sap
of the hevea tree. (Rubber was named by the chemist Joseph Priestley who found that a piece of solidified latex
gum was good for rubbing out pencil marks on paper. In Great Britain, erasers are still called “rubbers”.) Natural
rubber had only limited use as it became brittle in the cold and melted when warmed. In 1839, Charles Goodyear
discovered, through a lucky accident, that by heating the latex with sulfur, the properties were changed making the
rubber more flexible and temperature stable. That process became known as vulcanization.
The first synthetic polymer, a phenol-formaldehyde polymer, was introduced under the name “Bakelite”, by Leo
Baekeland in 1909. Its original use was to make billiard balls. Rayon, the first synthetic fiber was developed as a
replacement for silk in 1911.
Although many polymers were made in the following years, the technology to mass produce them was not
developed until World War II, when there was a need to develop synthetic rubber for tires and other wartime
applications and nylon for parachutes. Since that time, the polymer industry has grown and diversified into one of
the fastest growing industries in the world. Today, polymers are commonly used in thousands of products as
plastics, elastomers, coatings, and adhesives. They make up about 80% of the organic chemical industry with
products produced at approximately 150 kg of polymers per person annually in the United States.
RUBBER, A Natural Polymer
Natural latex is found in the inner bark of many trees, especially those found in Brazil and the Far East. The white
sticky sap of plants such as milkweed and dandelions is also a latex. Latex will turn into a rubbery mass within 12
hours after it is exposed to the air. The latex protects the tree or plant by covering the wound with a rubbery
material like a bandage.
Natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene, see
Figure 1) in the form of folded polymeric chains which are joined in a
network structure and have a high degree of flexibility (See Figure 2).
Upon application of a stress to a rubber material, such as blowing up a
balloon or stretching a rubber band, the polymer chain, which is
randomly oriented, undergoes bond rotations allowing the chain to be
extended or elongated (See Figure 3). The fact that the chains are joined
in a network allows for elastomeric recoverability since the cross-linked chains cannot irreversibly slide over one
another. The changes in arrangement are not constrained by chain rigidity due to crystallization or high viscosity due
to a glassy state.
Since latex will solidify in air, a stabilizer is added to prevent polymerization if the latex is to be stored or shipped in
liquid form. The stabilizer is usually 0.5 to 1% ammonia. When the ammonia is removed by evaporation or by
neutralization, the latex will solidify into rubber.
BAKELITE, The First Synthetic Polymer
Bakelite, a phenol-formaldehyde polymer, was the first completely synthetic plastic, first made by Leo Baekeland in
1907. Baekeland and an assistant started their research in 1904 looking for a synthetic substitute for shellac.
Bakelite was commercially introduced in 1909. Bakelite was first used to make billiard balls, but, later, was used to
make molded insulation, valve parts, knobs, buttons, knife handles, many types of molded plastic containers for
radios and electronic instruments, and more.
Safety Precautions
Wear safety goggles at all times in the laboratory.
Formalin is an irritant to the skin, eyes, and mucous membranes.
Phenol is toxic via skin contact. It is listed as a carcinogen.
Glacial acetic acid is an irritant and can cause burns on contact.
Work under a hood and wear gloves and protective clothing when working with these materials.
Materials needed
Phenol-formaldehyde reaction mixture (freshly prepared solution should be available. The
reaction mixture is made by mixing 25 g 36-40% formaldehyde + 20 g phenol + 55 mL glacial
acetic acid.)
Hydrochloric acid, HCl, concentrated
150-mL beaker
stirring rod
Procedure
Under a hood, measure 25 mL of the phenol-formaldehyde reaction mixture into a 150-mL beaker.
Place the beaker on a white paper towel.
Add 10 mL of concentrated hydrochloric acid, slowly, with stirring.
Add additional hydrochloric acid, dropwise, with stirring. (You will need approximately 2 mL of
HCl.) As the polymerization point is reached, a white precipitate will form and dissolve. At the
point where polymerization begins, the white precipitate will not dissolve.
Continue to stir as the plastic forms and becomes pink in color.
Wash the plastic well before handling.
Describe the properties of the plastic.
RAYON, The First Synthetic Fiber
What may be considered as the first precursor to Rayon, Louis Marie Hilaire Bernigaut, Comte de Chardonnet
worked on a process of producing threads of an ”artifical silk” made from collodion in 1884. Chardonnet’s silk was
finally marketed in 1891, but was short-lived after a young lady’s ball gown when up in a puff of smoke after being
touched by the lighted cigar of her escort. Charles F. Cross and Edward J. Bevan patened their formula for viscose,
a cellulose polymer, in 1892 and were soon manufacturing items of their new Viscoid. The first viscose thread was
made by Charles Topham Jr. and commericially produced in 1899. Rayon, made from regenerated cellulose in
1926, marketed by Du Pont, was finally used as a replacement for silk.