Ovarl Nitidandhaprabhas Bank-note Printing Works. Bank of Thailand, and Department of Materials Science Chulalongkorn University Bangkok, Thailand
Chemical Modification of Tung Oil A Diels-Alder experiment for undergraduates
T h e application of t h e Diels-Alder reaction to various syntheseshas heen widely descrihed, a fact which accounts for the merit of the award of the Nobel prize to the discoverers. However, one aspect of the reaction in t h e science of surface coatings, i.e., the enhancement of the alkali receptivity of tung oil by t h e introduction of carhoxyl groups into the chain backbone of the drying oil via t h e Diels-Alder reaction with maleic or fumaric acid. remains little known amone chemists. In view of the simplicity and ease of reaction, it is worthwhile a s a n undereraduate exoeriment desiened t o illustrate both t h e ~ i e l s - ~ i d syntheiis er and the chemical modification of drvine oils. k n i m p o r t a n t drying oil used in surface coatings is tung oil or China wood oil, which can he extracted from the fruits of two species of the family Aleurites, Aleurites Fordii Hemmley and Aleurites montana Wilson. T h e tung tree is a native of t h e Far East, where it has been cultivated for centuries, particularlv alona the hanks of the Yandzeriver in China. As the oil originallycame exclusively from China, i t was known a s China wood oil.' T h e oil is a trial~cerideconsisting mostly of eleosteuric acid (approx. SO%J,.~"18-rarbon aridwith three conjugated douhle bonds. Eleostearic acid is known to exist in two geumetrir forms, alpha and heta. Alpha elemtearic arid is characterized hy the cis, trans, trans geometrical arrangement while beta eleostearic acid posse&es the trans, trans, trans c o n f i g ~ r a t i o n . ~ The presence of three conjugated double bonds in the eleostearic acid moiety of the triglyceride molecules is characteristic of tung oil and accounts for many unique properties of the oil, for example its sensitivity to ultra-violet light, polymerizing to a hard film upon eapasure to uv light, and its ability to dry rapidly--so rapidly that the film may hemme wrinkled. However, an added advantage of the three conjugated double bonds in the tung oil backbone is that the molecule can act as a dime for the Diels-Alder reaction with appropriate dienoohiles.. in oartieular fumaricor maleicacid.. to form an adduct. It . has h e n uhoum that the rearrlon IS I A-ndd~rionin which dirnophile adds t u a trans-trans pmr uf ethylemr bonds and proreeds t h n q h a cis addition. These derivatives contain a cyclohexene nucleus and an exoeyclic double bond with functional carboxyl g r o u ~ sOnce .~ acidic polar groups have been introdueed via the above-mentioned reaction, the oil will manifest a dramatic change in its solubility, becoming water receptive and dissolving appreciably in aqueous alkali solution, a fact whieh can he substantiated by experiments.
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Experimental In a 500-ml three-necked round-bottomed flask fitted with a mechanical stirrer, a thermometer, and a nitrogen inlet are placed 100 g of tung ail and 20 g of fumaric acid. The flask is gradually heated under the atmosphere of nitrogen to 200DCin one hour after whieh the temoerature is raised to 210PCand the mixture maintained at this trrnperature for :30rnmutes. Thwentter the remperafure i reduced to lRO°C in 30 mmutes. T ~ heating P is stopp~cland the mmurc allowed to cool to rwm temperature when a viicuus oil resulLs. It has a visroritv 01235.4 p&es at W C . In omtrait u i t h the original unmadrfied tung oil uhieh only forms rrnulviun when shnkm with aqueous sodium hydroxide, the resulting varnishdissolves appreciably (hut not completely) in the latter without formation of emulsion, showing the presence of earboxyl groups. A thin layer of the varnish will eventually dry toa hard film but it lacks the uv sensitivity characteristicof tungoil. However, if the above procedure is repeated using 10 g instead of 20 g of fumaric acid, a less viscous varnish is obtained whieh becomes wrinkled when exposed to a Hanovia uv lamp, but its alkali solubility is logically inferior to the first one inview of the fact that fewer carhoxyl groups are introduced into the chain haekbone. The above exoeriment has been sueeessfullv oerformed hv a class
action. But the experiment can he performed equally well in any organic laboratory course. The prepared varnish (I) can he mixed with calcium carbonate, titanium dioxide, and requisite pigments and solvents in a ball mill to make a paint whieh when wet can he washed off with an aqueous solution containing 1% each of caustic soda and sulfonated castor oil, the latter acting as a dispersing agent. Following is an example of simple white paint formulation: varnish (I),300 g; titanium dioxide, 200 g; calcium carbonate, 200 g; magnesium silicate, 100 g; white spirit, 400 g; ball-milling for 24 hours. The paint dries slowly. A paint drier (catalyst) such as cobalt naphthenate, if available, will speed up the drying.
H'c=5c=c
