M. P. Stevens Robert College Istanbul, Turkey
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Preparation and Crosslinking of an Unsaturated ~ o l ~ e s t e r
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A n organic chemistry experiment
Unsaturated polyesters are ideally suited for introducing students to polymer chemist,ry in the laboratory because they are easy to prepare, they utilize both condensation and addition polymerization, and they serve to demonstrate the techniques involved in preparing the most widely used of the polymers or plastics of the glass fiber-reinforced type. The following experiment requires a three-necked flask, stirrer, and heating mantle, in addition to the usual laboratory equipment. Conceivably, the experiment could be done without ground glass equipment and with a Bunsen burner in place of the heating mantle, but the three-necked flask and stirrer are essential. Ingredients used in the condensation step are phthalic anhydride, maleic anhydride (Eastman White Label), and 1,6pentanediol (practical grade). The anhydrides are the two most commonly used in commercial preparations. iflaleic anhydride is usually sold in the form of briquets. It i s a very toxic ch~micaland students should be careful to avoid coming in contact with it. Briquets should be handled with tongs. They can be added to the flask in crushed or molten form. If molten form is used, the briquet should be melted in a beaker covered with a watchglass in the hood, and care should be taken to avoid inhaling fumes. This procedure results in some loss of maleic anhydride due to hold-up in the beaker and evaporation while pouring into the reaction flask; however, exact amounts of ingredients are not critical for the success of this experiment. Whichever method is used, it might be pointed out that polyester manufacturers almost without exception handle maleic anhydride in molten form. The glycol, 1,5-pentanediol, was chosen for several reasons. Its two primary alcohol groups esterify rapidly, so the experiment can he completed easily in two three-hour laboratory periods. Also, the final product is of low enough viscosity that it can be readily poured, while hot, from the reaction vessel, and the vessel can be quickly cleaned of residual polyester with acetone. Finally, the crosslinked product is firm enough to have the look and feel of a hard plastic. A small excess (5 mole percent) of glycol is used to increase esterification rate and to compensate for glycol loss due to distillation. Usually, polyesterifications are run in an inert atmosphere to minimize the formation of colored impurities caused by the high reaction temperature and to preclude premature crosslinking. With the short reaction time involved here, however, we have no problems of gelation, and the final product is clear and only a pale yellow in color. 160
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Journal o f Chemical Education
The nnmber of residual carboxyl groups (acid number1) can be determined by titrating with a previously standardized base solution. This property, toget,her with viscosity, is normally used in process control of polyesters. Hydroxyl numberZmay also be determined, but the procedure is more complicated. Molecular weight of the coridensat~ionproduct can be calculated if both values are known. Styrene, the most common commercial monomer for unsat,urat,edpolyesters, is used for the crosslinking step. We use phenyl disulfide to photochemically initiate crosslinking because of difficulties in obtaining peroxides in Turkey. Another procedure is to dissolve ly0 benzoyl peroxide3by weight based on polyester-styrene solution and to heat it in an oven a t 100DCfor about an hour, or to add 1% henzoyl peroxide and 0.5% N,Ndimet,hylaniline as promoter for room temperature curing. To save glassware and to facilitate cleaning, it is recommended that crosslinking experiments be performed in paper cups. If styrene solutions are to be stored for long periods, they should be stabilized with about 200 parts per million of a suitable inhibitor such as hydroquinone. Books which deal in detail with processing and fabrication of unsaturated polyesters are available.' The Experiment
Condensation. A 300-ml three-necked flask is fitted with a stirrer, heating mantle, thermometer, and distillation assembly consisting of a distilling head with thermomet,er, condenser, and receiver. Ground joints and stirrer braring are lubricated with glycol. (Ground joints have less tendency to freeze when lubricated with glycol than with stopcock grease.) The stirrer is started and 87.4 g (0.84 mole) of 1,s-pentanediol, 39.2 g (0.40 mole) of maleic anhydride, and 59.2 g (0.40 mole) of pht,halic anhydride are added to the flask. An exoAcid number is defined as the number of milligrams of base required to neutralize 1 g of polymer. It is cslculaled as follows: Acid number = ml of base X normality X equivalent weight of base weight of sample A value of 40-50 should be obtained in this experiment. W. K., et al., J . Appl. P o l p m Sei. 9, 1681 See: SEIFERT, (1965). ' Aeonvenient, safe form of benzoyl peroxide that disperses readily in styrene-polyester solution is Luperco ATC, a 50% mixture of peroxide in a plasticiser, available from Wallace and Tieman, Inc., Lueidol Division, Buffalo, New York. 'See for example: BOENIG,H. V., "Unsaturated Polyesters," Elsevier Publishing Co., New York, 1964; LAWRENCE, J. R., "Polyester Resins," Reinhold Publishing Corp., New York, 1960.
thermic reaction causes the temperature to rise to around 100°C. (For larger scale preparations, the anhydrides should be added slowly enough that the rate of t,he exothermic reaction does not become excessive.) The reaction mixture is heated as rapidly as possible to 200°C. It becomes homogenous at about 140°C, and the first water of esterification distils over at about lSO°C. The overhead temperature initially rises to about 120°C hut falls rapidly as more glycol is consumed in thc reaction. A small amount of glycol is lost here. Reaction temperature is maintained until the t,heoretical volume of water has been collected (1.5-2 hrs). The product is cooled to 150°C and poured into a 250 ml beaker or, if possible, a paper cup. A small (1-2 g) sample of the product is weighed into a beaker using a previously weighed spatula. The sample is dissolved in acetone and titrated to a phenolphthalein end point with 0.1 M sodium or potassium hydroxide to determine acid number. Crosslinking. A sample of polyester is poured into a
beaker or paper cup, and sufficient styrene is added to make a 70:30 by weight po1yester:styrene solution. The mixture is stirred, with warming if necessary, until solution is complete. Phenyl disulfide, 1% by weight, is dissolved in the polyester solution, and the sample is placed in sunlight or under an ultraviolet lamp until it cures to a hard product (1-2 hours). The surface will remain tacky due to inhibition of the crosslinking reaction by atmospheric oxygen. Different amounts of phenyl disulfide may be used to show the effect of concentration on gelation time. (See preceding discussion for comments on crosslinking initiated with benzoyl peroxide.) Alternate procedure. The mole ratio of maleic to phthalic anhydride can be varied to demonstrate the effect of increasing the crosslinking density on the properties of the final product. By using 91.7 g (0.88 mole) of 1,5-pentanediol, 55.9 g (0.56 mole) of maleic anhydride, and 41.4 g (0.28 mole) of phthalic anhydride and following the procedure outlined above, a stiffer plastic is obtained.
Volume 44, Number 3, March 1967
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