chem I fupplement
MICHAEL
edited by: R. SLABAUGH
HELENJ. JAMES Weber State College Ogden, Vtah 84408
Polymers and Barnacles James R. Griffith Naval Research Laboratory, Washington, DC 20375 The Crux ot the Problem In a previous publication in THIS JOURNAL (I),I explained that a "poly-mer" is a many-unit molecule most frequently visualized as a chain of atoms. "Mer" comes from the Greek word rneros which means unit or part. I t was also explained that polymer8 may he natural or synthetic and that the understanding required to produce synthetic polymers is principally of the twentieth century. That paper is a recommended preclude to this one. Now we need to determine just how sharp we really are v i s - h i s Nature. Can we devise synthetic polymers good enough to defeat the purpose of a natural polymer-barnacle cement? Lest one think this a trivial task it should be remembered that Nature has had untold millions of years to perfect the barnacle, and i t is about as skillfully designed t o endure as any creature on earth. Why Try?
The ability of ships to move through water with a minimum ex~enditureof enerw is a concern of meat economic importance. The ocean is biimming with creatures, plants andanimals, which become attached to surfaces early in the life wcle and grow to maturity at one spot. Such "fouling organisms." as they are known when nttached ta the hull of a ship, prevent the vessel from sliding through water in streamlined fashion, and propulsive energy is wasted by ineffective movement of water induced by the fouling organisms. Thus, it is necessary to keep hulls clean by some means. For many years "antifouling paints" have been used on the hulls of ships. These paints contain a leachable component which is toxic t o the fouling organisms, and this component is usually copper or tin compounds. Many volumes have been written and extensive research investigations have been performed regarding these important paints, hut recently anew factor has arisen as a consequence of their use. In certain areas of the world where many large ships congregate continuously the concentration of copper in particular is becoming unacceptably high. For example, alarm has been expressed by Egyptian researchers concerning the concentration of copper found in food fishes in the Suez Canal region (2). Possible alternatives to antifouling paints are "fouling release coatings." as we have chosen to-chi them (3). heroncept is similar to that of the antistick frying pan which has a coating sufficiently nonadhesive that b o d is easily removed therefrom. The most dit'ficulty of the myriad fouling orgnn-
isms to deal with in this way is the barnacle because of the tenacity of the bond that its natural cement develops. Figure 1shows a typical collection of volcano-shaped barnacles on the under surface of a vessel coated with ineffective antifouling paint. I b e removal of these creatures entails substantial force because of thestrength ofthe bond holding them to the paint surface. A good fouling release coating would not necesiarily prevent such a barnacle colony, but the individual barnacle could be removed easily such that cleaning would become a practical procedure. The Natural Champlon
I t is not mv oresent ouroose to describe the nhvsiolow of the barnac1e;atask be&-left to those more &owledge-ible in this area. but ratherto ooint out that bv some means Nature allows the barnacle t o a shell which has a base plate superbly bonded to the attachment surface through a proteinaceous cement (4). From the viewpoint of a synthetic polymer chemist, a cement based upon protein normally would not be regarded as a likely "world-beater," however, the infinite variability that is ~ossiblewith rotei in structure bas amarently allowed ~ a t k toexperiment e until a near-perfeci &e has evolved. It is not uncommon for a force of 6 kiloarams per square centimeter to be required for lifting a barnacle veitically ( 5 ) . Many factors determine the soundness of anadhesive bond
This featwe presents relevant applications of chernislw to everyday life. The lnfwmation presented might be used directly in class, posted on bulletin boards or otherwise used to stimulate student invoivement In activities related lo chemistry. Contributions should be sent to the feature editws. Figure 1. A collection at barnacles on a ship hull
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Journal of Chemical Education
Figure 2. Removal of fouling organisms by hosing a release coating.
whether natural or man-made. These include, in addition to the intrinsic quality of the adhesive: the roughness of the surface, the amount and type of contaminant on the surface, the chemical nature of thesurface, and its physical nature; The perfection of the bonding process with regard to voids or cracks in the adhesive can also cause a large effect. The Synthetic Challengers Certain synthetic polymers are known to he difficult to adhere to, and the molecular reasons for this are reasonably well understood. Most polymers are composed of carhon, hydrogen, oxygen, and nitrogen, with those composed of carbon and hydrogen only, such as polypropylene or polyethvlene. . . beine difficult to bond with an adhesive. Those which contain oxygen and/or nitrogen in addition to carhon and hydrogen have what are called "polar" regions whirh can associate with similar regions in an adhesive to produce strong attractions which are basicallv elertrical. Usuallv eood adhesives will bond to these matkrials relatively str&&. In order to obtain the very best in non-adhesive ~ r o ~ e r t i e s i t is necessary to resort to specialty polymers which h e superior to the hydrocarbons. These are the fluoropolymers and the silicones (1).Both of these types are difficult to wet with liquid adhesives, and the bonds which form to the surfaces are usually very weak. The fluoropolymers have been widely used
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as anti-stick coatings on rooking utensils, hut they have been restricted in the breadth of such applications hy the necessity for a high temperature fusion step in processing which is not practical in many instances. In general, the silicones are not very strong compared with other materials of similar properties, and this lack of strength has heen a detriment. However, if one is to defeat so wily a creature as the barnacle at the glue game, means must he found touse the best anti-adhesive surfaces nossihle. In recent years chemical synthesis has produced a series of ~olvmerswhich combine the convenient use nronerties of the &xy resms and the antiadhevive surfaceshf [he fluoropo~ has been demonstrated wlrh these lvmers and sillrones ( f i It new materials that co&ings can be put on the underwater surfaces of ship hulls and that marine foulina- oreanisms have difficulty honding tightly to these fouling release coatings. Figure 2 illustrates the ease with which such a hull can he clraned after it has hecomec~~vcrrd with barnacles and other organisms. Unlike an antifouling coating, this paint does not poison the organisms, and it dnes not rrwlt in pollution of the lord environment as a result of leaching. It ran also he made very durahle with several times the life expectancy of a conventional antifouling p a w . Future Developments Certain experimental fouling release coatings have been tested which seem to be so non-bonding that barnacles fall off spontaneously before they can grow to maturity. One is tempted to say, "Ah, ha! Nature is defeated!" However, those same compositions were apparently quite tasty to a t least one type of fish which persisted in eating the stuff! I t appears to he only a matter of time hefore the ideal fouling release coating can he synthesized, and the broad outline of such a material can now be envisioned hy the synthetic polymer chemist. It-will probably be a fluorine-containing silicone with practical use properties, good coatings characteristics, a physical property mix which favorably augments the non-bonding function, and an acceptahle price. The barnacle has a few million years head start, hut we chemical types are gaining fast. Literature Cited (1) Griffith, J.R., J.CWBM.EDUC.,S~,NO. 11,956 (1981).
(2) Ghanrm, N.A.. El-Awady, N.I., El-Hamouly. W. S.,and El-Amdy, M. M.. J. Caafings Tech.,54, No 684.84 (1982). (3) Gliffith, J.R.,end Bultmsn, J. D.,Nou. En& J, 2,129 (1980). (4) Walker. G., MorimBio., 9.205 (1971). (5) Griffith, J. R, and Buitman, J. D., I d Ens Chem. Pmd. Reg, Den, 17, No,1, S
(1978).
(6) Griffrth. J. R.. Chemfech,290 (1982).
Volume 61 Number 12 December 1984
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