Page 640 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Technology 603

instance, while calcium carbonate, with a refractive index of about 1.6, is often used as a pigment
in paints, it has a much lower hiding power than titanium dioxide (rutile titanium (IV) oxide), with
a refractive index of about 2.8.
Scattering efficiency increases as the pigment surface area becomes larger, thus smaller particles aid

in increasing the scattering to a lower limit determined by the wavelength of light, thereafter reduced

size produces a rapid loss in scattering efficiency. For good scattering, and good hiding power, the par-
ticles should be dispersed in a homogeneous manner so that dispersing agents are commonly used.
18.12.1.1 Rainwear
When it rains we grab the umbrella and put on a raincoat. Charles Macintosh is credited with the

discovery of waterproof coats, aka raincoats. The first raincoats predated Macintosh’s concept by
hundreds of years. As early as the thirteenth century, South American Indians coated cloth with
natural rubber latex making waterproof caps and footwear. The latex is natural rubber, a nonpolar
material and as such it repels polar water molecules. As this latex was shipped to Europe bacteria
attacked the natural rubber making it useless. In 1748, Francois Fresneau developed a process of
dissolving natural rubber in nonpolar turpentine forming a rubber solution that was poured onto
fabric rendering it “rain” proof. A contemporary of Macintosh, James Syme found that coal tar
naphtha, again a nonpolar liquid, dissolved rubber and could be used to make waterproof fabric. He
turned to over ventures rather than commercializing his process.
Coal tar naphtha is a waste byproduct from the conversion of coal into gas that was used as the
fuel for streetlamps. Macintosh was under contract of the Glasgow Gas Light Company to work with
the waste byproducts. He was extracting ammonia from the byproducts for use in his father’s dye
business. He was left with unusable coal tar naphtha. So, he looked for uses for this waste solvent.
Shortly, he found that it dissolved rubber, which could be poured onto fabric creating water-proofed
material. The material remained sticky and had an unpleasant odor. He conquered the problem of
tackiness by sandwiching the treated fabric between two other fabric layers. The smell remained.
Macintosh patented the idea in 1823 but his rainproof material was not well received, presumable
because of the unpleasant odor. Fortunately for him, the military purchased waterproof fabric from
him, “bad smell and all.”
Improvements continued making the material lighter, more flexible, and less smelly. Use of cold-

cure vulcanization where the rubber was treated with sulfur compounds eliminated the “sticky”
problem allowing a single layer of impregnated fabric to be adequate.
Today, there exists a variety of polymeric materials that offer rainproof umbrellas and clothing,
including “breathable” fabrics. Gore-Tex derived materials offer a combination of layers, including
a breathable polymer membrane made of microporous polytetrafluoroethylene under a layer of PU.

The polytetrafluoroethylene pores provide a layer of air. The PU has a high diffusivity for water
vapor taking away any water that is on the skin, such as water vapor or sweat. The temperature
inside the clothing is generally greater than the external temperature and acts as a “driving force” to
“push” the water through the polytetrafluoroethylene pores and away from the skin.

18.13 ADHESIVES
In contrast to coatings, which must adhere to one surface only, adhesives are used to join two sur-
faces together. Resinous adhesives were used by the Egyptians at least 6,000 years ago for bonding
ceramic vessels. Other adhesives, such as casein from milk, starch, sugar, and glues from animals
and fish, were first used at least 3,500 years ago.

Adhesion occurs generally through one or more of the following mechanisms. Mechanical adhe-

sion with interlocking occurs when the adhesive mixture flows about and into two rough substrate
faces. This can be likened to a hook and eye, where the stiff plastic hooks get caught in the fuzz-

like maze of more fl exible fibers. Chemical adhesion is the bonding of primary chemical groups.

Specific or secondary adhesion occurs when hydrogen bonding or polar (dipolar) bonding occurs.

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