edited by: MICHAELR. SLABAUGH
chem I ~upplement
Weber State College Ogden. Utah 84408
The Chemistry of Coatings J a m e s R. Griffith Naval Research Laboratory, Washington. D.C. 20375 Coatings, Coatings Everywhere Just as Nature appears to abhor a vacuum, it seems to love a coating. Everything is quite literally coated in the broad concept of what a coating is, and surface scientists go to great lengths in their efforts to obtain surfaces that are free from any kind of contaminate. Even water of the highest purity contains a surface layer which is in tension due to unbalanced molecular forces, and, in effect, this outermost layer is a coating even though it is materially the same as the water throughout the hulk. Nature produces some spectacular coatings with regard to molecular composition, function, and properties. Consider, for example, grape peels, watermelon rinds, citrus fruit coatings, walnut shells, leopard fur, redwood hark, and human skin. All of these are marvels of molecular engineering which are complex almost beyond imagination and endless in variety-oat hulls, fish scales, eagle feathers, frog eyelids. Bv comoarison humankind is comoosed of Johnnv-comein the field of coatings. he man-made-compolateiy sitions are simplistic; the functions of the coatings are limited largely to decoration and to modest protection. And yet, the modern indoor environment within the advanced countries of the world is almoat totally coated with man-made materials. The design and production of synthetic coatings is a huge industry, and future research and development opportunities abound. One prominent American company is currently sponsoring an advertisement which suggests that the paint on homes may soon convert solar energy into electricity directly! If accomplished, that would not he had for upstarts such as ourselves. An Ancient Art I t has been argued that the purest form of chemistry is the activity known as "synthesis":~lthough many chemists may not agree with this argument, it satisfies the bias of the present author (who is a svnthetic nolvmer chemist). Svnthesis is the " subjecting of some material to a chemical compositional change such that a different material emerges. Another great area of chemistry is "analysis", which is the taking apart of materials chemicallv in order to determine the comoosition in simpler terms. So. where did the chemistrv of coatines start? If some ancient'cave dweller, who was prone to draw pictures of his wife and kids on the wall, ever noticed that the resins (pine tree exudates) from which he made his finger paints were easier to use if he cooked them a little by the campfire, then he started the chemistrv of coatinns without knowine what or of emGirica1 improvement of coatwhy. In fact, that ines hv trial and error was develoned to a vew.hieh state lone he?ore"chemical synthesis as a scieke was conceived. consider the paintings of Michelangelo, for example. He lived from 1475 to 1564, hut John Dalton, upon the atomic theory of whom all synthesis is based. was not born until 1766. Thus. coating as an art existed long hethre synthesis as n science came on the scene, hut the railid development of synthetic materials had to await the latter event.
King Polymer In order to understand the modem chemistry of coatings, it is essential that the concept of apolymer he firmly grasped. "Poly" means many, and "mer" means unit. Thus, a polymer is a many unit molecule put together like a chain. Not all polymers are man-made, and the wonderful coatings of Nature described above are principally composed of natural polymers. Cellulose?proteins, and natural rubber are prominent examples of naturally polymeric materials. In fact, the chemical alteration of cellulose by treatment with nitric acid to produce cellulose nitrate-a process known as early as the 1840's-may have marked the beginning of the chemistry of coatings. It was certainly a synthetic process, although the basic polymer chain did not result from the action of a chemist:
I
sulfuric acid
Cellulose
Cellulose Nitrate
The important difference between cellulose and the nitrated version is that the latter can he dissolved in convenient solvents from which it will deposit as a film on a surface, whereas, the former is not soluble in suitable media. Bv 1923cellulose nitrate lacquers were used as automobile pain& in the United States.
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956
Journal of Chemical Education
This feature presents relevam applications of chemistry to everyday life. The information presented might be used directly In class, posted an bulletin boards w otherwise used to stimulate studem involvement in activities related to chemistry. Contributions should be sent to the feature editor. Mike Slabaugh received his BS degree in chemistry at Purdue Universily in 1965 and his doctorate in arganlc chemistry at Iowa Stete University in 1970. His interest in biochemistry and natural products research (alkaloids)led to r a year of postdoctoral study in biochemistry at Texas A8M University in 1971. Dr. Slabaugh is now Professor of Chemistry at Weber State Coiiepe, where in 1979 he was recognizedas the " ~ k f e s s a rof ihe Year." His I professional interests and goals are directed' toward chemical education and community involvement in science activities. He has been palticularly active in directing regtonal science fairs.
'
t'li
Synthetic Polymers
The first important synthetic coating materials in which the polymer chain was man-made from simple molecules were the phenolic resins which were announced in 1909. Formaldehyde will react with phenol to form additive products illustrated as follows:
6 kH + Z O
r oH
1
I
1
I
1,
I
CHOH -C& -CH* Although the phenolic resins are still used in large quantities, another early class of totally synthetic resins called "alkvds" have had much ereater imoact unon the coatines area. The name is a somewhat imaginative contraction of "alcohol" and "acid". because the materials are oolvesters . . which result from this reaction:
-
0
+
-CH.QH alcohol
HO-C-
It
OCH ,
OCH,
methyl acrylate
OCH,
polymethyl aerylate
+
CH,OH These products can condense with one another under appropriate conditions to form polymeric chains as illustrated below. This is a simplified picture to show the process which in reality is quite complex: CH.oH
an ester linkage as follows, the product is known as an "acrylic" polymer:
-
-
organic acid
The vinyls and the acrylics are used in large quantities in water-based, or latex, paints which have become verypopular. with home owners for internal and external use on houses because of the convenience of cleanup inherent in such systems. In 1970 sales of acrylic paints amounted to nearly 100 million gallons, and only the alkyds were consumed in greater quantity. Since World War I1 two types of coatings which are quite similar in many. reaards-the e~oxiesand the polvure. . . thanes h a w h w m e increaiingly important, particularly for hea\..v-dutyindustrial applicutions. The best coatings of both types are produced fromtwo components, which are blended by the user shortly before application. The components react with each other extensivelv after the coatine is aoolied to a surface, and in both cases tLe reactions are princiiaily simple molecular couolines . " as illustrated hv the followine" tvnical ". examples:
0
-CH,O-C-
1I
+ H20
ester or ''alkyd'' lbkage
The most important components for the polymerization process have been orthophthalic acid and glycerine:
-N-C=O
+
isocyanate group
HOCH,-
-
alcohol
0
II
-NCOCH.H urethane
-
0
glycerine
orthophthalic acid anhydride
The alkyd resins first appeared as commercial coating materials in the late 1920's, and after extensive chemical modification and improvement, they remain among the most widely used coatings components. Drying oil modifications and silicone alkyds represent only two important developments in the history of this class. Unsaturated polyesters combine the alkyd polymer type with that of the vinyl resins, discussed helow, and these are becoming increasingly important in heavy-duty industrial coatings applications, such as linines for storaee tanks. The Lost s i m p i of all synthetic polymers, and the one consumed in the greatesvquantity, is polyethylene: CHdHz ethylene
-
(-CH-CHI-CH-CH-I ~lyethylene
Polyethylene itself is not easy to use as a coating except with specialized application equipment, hut simple derivatives of this parent polymer produce two classes of coatings materials which are used in large quantities. Thus, vinyl chloride is merely ethylene with one chlorine atom per molecule substituted for hydrogen, and when it is polymerized, the product is polyvinyl chloride which is a primary component of the "vinyl" polymers: CH,=CH (-CH,-CH-C%-CH-1
I
vinyl chloride
-
I
I
polyvinyl chloride
If instead of chlorine, the ethylene molecule is substituted with
The coatines that the nolvrners which " svstems are so desiened " . " form on the surface are extremely large in molecular dimension. so-called "infinite networks". and these huae molecules make the epoxies and polyurethanks exceptionaCy tough and durable. Actually, the epoxies are similar to the phenolics in many respects, and the polyurethanes can be considered as special types of alkyds since the coatings resins are largely derived from polyesters. Special Coatings
The domain of coatings and synthetic polymers is so extensive todav that it is not nossihle to adeouatelv review even the major types in a short article. However, mention should he made of the urea and melamine formaldehyde coatings, which are similar in some respects to the phenolics; of the stvrene. acrvlonitrile and hutadiene materials which are special typesbfvinyls; and of the semi-organic substances such as zinc silicates. There is also the entire field of inorganic coatings, including metallics which are commonly applied by electroplating or vacuum metallizing. Within the principally organic series should he included the fluoropolymers and the silicones, although the latter contain sienificant inoreanic oortions. The most imoortant fluoropolymers are rl~l(rrine-iul)slituted derinlives of polyethylene such as polyvinyl fluoride and polyterratl~loroethylene: I-CHI-CH-CH,-CH-1
I
F
I
F
polyvinyl fluoride
1-CF:-CF-CV,-CF-l polytetrafluotoeth~lene (~eflonm)
The substitution of fluorine (the most electronegative element) for hydrogen of the polyethylene chain imparts some Volume 58 Number 11 November 1981
957
profound property changes, and the fluoropolymers are among the most environmentally stable of known coatings. They have high resistance to the ultraviolet light of the sun, to the damaging effects of water, and to microorganisms. The fully fluorinated ~olvmer . . is thermallv stable at exceptionallv. h i..~ h temperatures and has the unlque property of being an exceotional dry film luhricant nirh an anti-adhesive surface. It is &dely used as a coating on cooking utensils because of these properties. The principal factors that have prevented wider use of the fluoropolymersare that they are relatively expensive and they generally are nut convenient to use as general purnone tontines. The silicones are very interesting materials because the molecular chains of which the ~olvmers are comwsed contain . . alternating silicon-oxygen atoms instead of the mhon-carbon linkages of most polymers: L ~ - - -
~~~~~
-~
CH
CH,
I I 1-Si-0-Si-O-Si-O--) I I CH CHI
CH,
I I CH,
p d y d i r n e t h y l Diloxane
The inorganic nature of this "backbone", which is the same as that of ordinary silica sand, imparts unusual properties to the silicones, including high thermal stability and water repellency. As mentioned earlier, some of the alkyd coatings have been improved substantially by chemical modification to include silicone units into the molecular structures. In recent years another type of semi-organicpolymer called polyphosphazene has received considerable research attention. and efforts are underwav to snpplv .. . commercial marketi. 'I'hese n~ntt~rials have nlr~rnating~)h
