1735
V O L U M E 2 5 , N O . 11, N O V E M B E R 1 9 5 3 Table 1.
iutoniatic Titration of Dichromate Standards S Taken N Found Precision, 70 0 09810 0 0500 0 0100 0 00500 0 00100 0 1021
0 0 0 0 0
09818 04997
01005 004943 001004 0 1022
+O
67 0 48 0 76 0 G9 0 74 0 36
mubt be essentially as shown in order to prevent pumping of air fiom the side arm into the vessel in place of the solution. Pumping occurs if a bulb is used in place of the long tube or if capillary tuhing is used t o connect the side arm. The amount of solution a-ithdrann is determined by the position of the barrel of the hypodermic syringe in relation to the plunger assembly. From 10 to 20% of the solution should be withdrawn; the rate of stirring, indicator response, titration current, and amount of sample all affect the amount of “overshoot” that must be overcome. -4 cork or rubber cushion bet-ireen the solenoid plunger and the syringe plunger is required to prevent breakage; a similar cushion at the end of the syringe plunger is recommended. Table I gives the results of a set of titrations of dichromate vtandards using the anticipation qystem. It was not possible toper-
form this titration automatic all^ \+ithout end-point anticipation, as the indicator lag is large. The derivative polarographic indicator system ( 4 )was used. The electrolyte was 4 V sulfuric acid0.6.V ferric ammonium sulfate as prescribed by Cooke (2). In the procedure, an unmeasured sample was added to 5 ml. of the electrolyte and titrated to the end point; this pretreated the electrolyte to remove traces of oxidizing or reducing impurity. d series of two to five measured samples (100 J)was then run in succession with this treated electrolyte. The dichromate standards were made from triply distilled water and the dried salt. The precision is the standard deviation of a single value, as calculated from 10 results. The results show that the anticipation system xorks satisfactorily. The titrator is now heing invr3tigated for use in the titration of other oxidizing agents and reducing agents. LITER4TURE CITED
(1) Carson, W. X . , Jr., ASAI.. CHEM.,25, 226 (1953). (2) Cooke, 1%D., ‘. and Furman, S . H , Ibid., 22, 896 (1950). (3) Muller, R. H., IKD. I;SO. C H E \ f . , r l N 4 L . ED., 18, KO. 5 , 24 (Adv. Sect.) (1946). (4) Reilley, C. N., Cooke, W.D., and Furman, N.H., A V I L .CHEY., 23,1223 (1951). RECEIVED for review June 9, 1953. .4ocepted September 8, 1953.
Determination of Polystyrene in Styrenated Alkyd and Styrenated Epoxy 1IELVIN 11. SWAN3 Paint and Chemical Laboratory, Aberdeen Proning Ground, Md.
A quantitative method for determining polystyrene was needed for quality control and acceptance of quick-drying styrene-modified alkyd resins. The styrenated epoxy resins were investigated because of their related modification and their possible future interest to the Ordnance Department. Two procedures for determining the polystyrene have been developed. One is applicable to either the alkyd or epoxy resins. The other, applicable to alkyds only, allows the subsequent determination of the oil acid content of the alkyd resin. Composition requirements of styrenated alkyds can be established and analytical control exercised. The extent to which monomer styrene is converted to polystyrene in the preparation of styrenated resins and oils can be determined.
0
F T H E synthetic resins, the alkyds probably powess the widest range of applicability, especially for automotive
equipment. They are unusually tough and durable and possess excellent solubility, compatibility, adhesion, and gloss retention. Compared to other synthetic resins they are relatively slow-drying. To incorporate the alkyd resins in a quick-drying enamel for ammunition finishing @), they have been modified with styrene. At the present time no analytical control is exercised over the styrene content of these enamels and no composition requirements have been specified, because of the lack of an analytical procedure for determining the styrene. By applying the method of analysis to ten commercially available styrenated alkyds, i t has been found that the polystyrene content varies from 26 to 44% of the nonvolatile portion of the solutions. The composition of some of these resins is shown in Table I. Polystyrene is very resistant to the action of alkalies in alcoholic and nonaqueous media. Because phthalic anhydride in alkyd resins is separated and determined by saponification, styrene, if present, would be found in the filtrate from the phthalic determination. Upon removal of all solvents, the dried
productq, consisting mostly of excess potassium hydroxide, polystyrene, drying oil soaps, and polyhydric alcohols, can be treated with Si% methanol and the styrene separated, dried, and weighed. The product is insoluble in water but slightly soluble in absolute methanol and in aqueous alkali. However, it separate8 quantitatively in filterable form from a solution of 87% methanol, even in the presence of excess alkali. -4s styrene-modified epoxy resins are of current interest, they were tested by the procedure described. It was found that epoxy resins resist saponification by any medium that can subsequently be evaporated to dryness. By controlled fusion with potassium hydroxide pellets, the epoxy resin could be decomposed and the styrene left unchanged. The same technique can be applied to the styrenated alkyds, but not as conveniently as by the method reported here in detail. Interference of certain unsaponifiable resins can be prevented by using the fusion technique. ANALYTICAL PROCEDURE
Method for Styrenated Alkyd Resins. To determine the polystyrene in styrenated alkyds, the phthalic anhydride should first
1736
ANALYTICAL CHEMISTRY Table I.
Composition of Resins
Nonvolatile,
70
Phthalic Anhydride. %
Styrene,
Commercially Available Styrenated Alkyd Resins 51 .o 62.7 61 . O 61.6 50.7 51.5 51 .o 60.5 50.2 50.9
A B C D E F G H I J
24.0 19.1 24.2 21 . A 21.9 21.9 26.3 21.8 22.4 27.3
34.2 42.6 32.8 35.6 35.8 44.4 35.5 32.1 3R.0 26.0
Styrenated Epoxy Resin (Known Styrene Content 33.3%) A 50.0 33.8 33.5 34.0 33.6
be removed by the follon-ing procedure, which is used to measure the phthalic anhydride gravimetrically. The determination can be conducted rithout removal of the phthalic anhydride, but it is not recommended, as the presence of dipotassium phthalate retards the drying.
A sample of the isolated vehicle, containing from 0.3 to 0.6 gram of nonvolatile material (1 to 2 grams of vehicle, S), is weighed into a 500-ml. Erlenmeyer flask having a 24/40 ground joint; 100 ml. of benzene are added, and the samples swirled or warmed until completely dissolved. Then 50 ml. of 1.OS alcoholic potassium hydroxide (66 grams per liter of absolute ethyl alcohol) are added and the sample is refluxed under an air condenser for 1 hour or longer. The condensers are then replaced with loose-fitting corks and the flasks transferred to a 50" C. oven, \There they are allowed to cool to the oven temperature for convenient handling. The warm solution is filtered through a dried and weighed fritted-glass crucible of coarse porosity, previously prepared with a mat of asbestos. The precipitate is transferred and washed, first with benzene and finally with absolute ethyl alcohol. If, during the benzene washing, the filtering rate decreases sharply, some ethyl alcohol should be mixed with the wash medium. The filtrate is retained in a clean, dry suction flask. The dipotassium phthalate in the filtering crucible is dried a t 100" C. for 1 hour, cooled, and weighed The usual calculation for phthalic anhydride is used:
yo phthalic anhydride
=
weight of dipotassium salts X 61.1 weight of vehicle X vehicle solids
The filtrate from the phthalic determination is transferred to a 400-ml. beaker, being m-ashed with small portions of benzene. The solvents are evaporated under a hood using an 80" C. glycol bath and a current of air. When the solvent volume has been reduced to approximately 5 ml., 5 ml. of absolute methanol are added, the sample is agitated, and the evaporation is continued t o apparent dryness. If the sample contains appreciable quantities of styrene monomer, several repeated evaporations r i t h methanol are necessary. Fifteen milliliters of water are thoroughly mixed with 100 ml. of absolute methanol (or 115 ml. of 87% methanol) and added to the sample. A glass stirring rod is inserted and the beaker is covered and warmed in a bath a t 65 C. The sample is stirred every 10 minutes and all portions of the solid matter are loosened from the sides and bottom of the beaker. All lumps or large particles are broken up with the rod. After 30 minutes, the sample is removed from the bath, cooled to room temperature, then filtered through a dried, weighed fritted-glass crucible of coarse porosity, previously prepared with a mat of asbestos. When a 1 of the methanol solution has drained through the crucible, transfer of the sample is continued with water only. When all of the sample has been transferred to the crucible, washing with water is continued until the u-ashings show no alkalinity when tested with indicator paper. The residue is then dried at 110" C., cooled, and weighed. Calculation is made directly on the nonvolatile vehicle:
% polystyrene
=
weight of residue X 100 weight of vehicle X vehicle solids
Method for Styrenated Epoxy Resins (also suitable for alkyd resins). A sample of the isolated vehicle, containing from 0.3 to 0.6 gram of nonvolatile material, is n-eighed into a nickel crucible of 30-ml. capacity. Six grams of potassium hydroxide pellets
are added and the contents of the crucible ale dried in an oven a t 110' C. for 3 hours. A4nelectric heater having a built-in variable transformer instead of a rheostat, such as Precision Scientific Ful Kontrol heater, is used for the fusion of the sample. The heater is fitted with an upper refractory having a 1.25- or 1.5-inch hole, whichever supports the nickel crucible in such manner that the bottom of the crucible does not come in contact Lvith the heating coils. The correct setting or temperature to use must first be established in the folloning manner A nickel crucible of the same dimensions as that used for the sample is placed in the upper refractory of the heater and 5 ml. of n-butyl phthalate are added. The crurible is provided with a small glass cover and heated a t various dial settings for 30 minutes, until the setting is found a t which the temperature of the n-butyl phthalate is 235 C. when tested with a thermometer. Different heaters require different settings, which must be established in the prescribed manner in order to avoid erroneous results. The crucible containing the dried sample is covered with a small glass and placed in the refractory of the preheated heater. After 15 minutes, the cover is removed and a short nickel stirring rod, which does not protrude above the rim of the crucible, is inserted. The melt is stirred with the stirring rod for a few seconds, being handled n i t h tneezers. The nickel rod is left in the sample, the crucible covered, and the heating continued. The stirring is repeated every 15 minutes After 45 minutes' total fusion time, the crucible and rod are transferred to a 400-ml. beaker, the contents are poured out into the bottom of the beaker, 115 ml of 8 i % methanol (15 ml. of water plus 100 ml of absolute methanol) are added, a glass stirring rod is inserted. and the beaker is covered and warmed a t the approximate boiling point (65" C hath) of the mixture until all portions of the sample are loosened from the crucible and beaker and broken into small particleq The mixture is then cooled to room temperature and filtered through a dry fritted-glass crucible of coarse porosity, previously prepared with a mat of asbestos. When all of the methanol solution has drained through the filtering crucible, the transfer of the sample is continued 1% ith water only The nickel crucible and rod are removed, scrubbed, and washed with water. Washing of the contents of the filter crucible is continued with water until the washings show no alkalinity when tested with indicator paper. The residue is then dried at 110" C., cooled, and neighed. As the fusion in nickel crucibles usually results in some foreign matter remaining in the sample, the polystyrene must be dissolved from the crucible to obtain its correct weight. The crucible is filled about four times with XTarm carbon tetrachloride, n-hich is allowed to drip thiough slonly After thorough washing, the crucible is sucked dry, dried at 110' C., then cooled and reweighed. Loss in m i g h t from washing n-ith carbon tetrachloride is taken as polystyrene. The crucible m-ith residue from the nickel crucible can be cleaned with concentrated sulfuric acid. DISCUSS103
Styrene monomer is not measured by either procedure. I t does not undergo polymerization under the conditions of test. In the determination of phthalic anhydride in styrenated alkyd resins, the saponification must be carried out as described by refluxing the resin CI ith alcoholic potassium hydroxide; the oven method ( 1 ) is unsatisfactary. It nil1 cause high results in the phthalic anal3 sis and loa results in the polystyrene determination. The alcoholic potassium hydroxide used should be freshly made and free of excessive yellow color, such as is obtained by using alcohol that has been evposed to air or contains appreciable aldehydes. The yellow color is absorbed by the polystyrene and i d 1 affect results. Oil acids may be determined on the filtrate from the separation of the polystyrene in both methods. To determine the oil acids, the filtrate is heated in a water bath until the methanol is removed, then acidified, and the oil acids are extracted with ether as in the usual procedure. The method outlined for epoxy resins, in ~5 hich the dried resin reacts with melted potassium hydroxide, is very effective in eliminating interference from most other unsaponifiable resins. By this method, the only resins found to interfere are coumaroneindene and petroleum hydrocarbon resins. Qualitative tests for these two resins are not necessary, as their presence is readily recognizable by the dark brown or black, gummy residue found mixed with the white or near-white particles of separated polp-
V O L U M E 2 5 , N O . 11, N O V E M B E R 1 9 5 3 styrene a t the time of filtration. As their presence affects the neight of the separated polystyrene, no means has been found as yet of preventing their interference in the analysis. Resins likely to be used as modification of the alkyds and found not to interfere in the analysis for polystyrene are rosin and rosin ester, acrylic resins, phenylphenol formaldehyde, and other phenolic resins. Accuracy. I n an effort to check the accuracy of the method, three styrenated oils were prepared with special attention to accuracy of composition. .4 styrenated dehydrated castor oil mas prepared with catalyst and a styrenated blown soybean oil without catalyst. Dehydrated castor oil (6.141 grams), 4.961 grams of styrene monomer, and 0.111 gram of benzoyl peroxide (as catalyst) were weighed together into a 50-ml. round-bottomed flask equipped v,ith side-arm thermometer and reflux condenser by means of ground-glass joints. The temperature of the mixture was gradually increased to 220" C., a t which i t remained constant. Sfter 10 minutes i t was cooled and 9.291 grams of xylene were added. The mi.; was then warmed to effect solution and recooled. The nonvolatile matter x a s determined by drying 1-gram sampleq in an oven a t 110" C. for 3 hours. Considering the volatile to be xvleiie and unreacted styrene, a nonvolatile composition of 50 75yo indicated a polymerized styrene content of 39.9yo Using the method outlined for styrenated alkyds, this sample analyzed 39.7% styrene. The styrenated bloirn soybean oil was prepared in a similar manner, omitting the catalyst. Blown sovbean oil (7.000 grams) and 6.994 grams of styrene monomer were weighed together and the temperature was increased slonly over a 3-hour period to 110' C., then to 185" C. in another 2 hours, where the temperature remained constant. After cooling, 13.581 grams of xylene were added and solution was effected. A nonvolatile content of 49.5% indicated 48.i% polymerized styrene content. This sample analyzed 48.1 yo styrene. In addition, two manufacturers' samples, each stated to contain 39% styrene, analyzed 39.4, 39.0, and 39.1 % in one caw anti 40.0, 40.1, and 39.9 in the other. A styrenated epoxy resin with known styrene content of 33.3% analyzed 33.8, 33.5, 34.0, arid 33.6%. In an effort to determine the effect on the method of the socalled "copolymer" formed by reaction of styrene and oils of high conjugated unsaturatiori content, tung oil and styrene monomer were heated together without solvent or catalyst under conditions believed to favor the formation of copolymer and minimize the formation of neutral polystp-ene; 13.116 grams of tung oil arid 12.719 grams of styrene monomer Tvere mixed in a small flask provided with thermometer and reflux condenser, and the trmperature was slon 1y increased to 170" C. over a period of 6 hourq.
1737 Higher temperatures yield products insoluble in the usual solvents. On cooling it was found that the product was 6 6 . i % nonvolatile, indicating the presence of 15.9% polymerized styrene. This sample analyzes 16.7% by the method outlined. The results shon- greater inaccuracy than the other styrenated oils but represent a product t h a t is not encountered comniercially, owing to its instability. These data do not indicate the extent or nature of copolymer formation, or reveal the effect of saponification on such copoljmers, but are presented to demonstrate the results of applying the procedure to a variety of styrenated products. Analysis by infrared spectroscopy indicates the presence of small quantities of acid carbonyl groupings in the separated polystyrene of all samples tested, including the styrenated alkyds. Occluded oils or their soaps could be as responsible for this phenomenon as copolymer formation. The acid number of the separated polymer from all samples tested varies from 2 to 5 . The agreement betv een styrene content by analysis and the theoretical composition indicates that any error due to styrene-oil copolymers is not greater than zt 1% on a nonvolatile basis. These investigations indicate that very little, if any, styrene-oil copolymers are formed, or their effect on the analysis by the methods outlined is negligible. The oil acids of alkyd resins, after separation of the styrene, possess their usual properties. If any appreciable quantity of copolymer passes into the filtrate from the pol3 styrene determination, i t is not extracted with ether as are the oil acids Copolymer formation does not appear to affect the accuracv of the determination of oil acids, but this could be given further study. The polystgrene isolated from oils and resins, ivhen heated in a borosilicate glass test tube, exhibits the complete distillation, negligible char, and odor characteristic of pure polystyrene. Styrenated alkyd samples analyzed by the saponification and the fusion procedures yield the same percentage of polystyrene. Pure polystyrene plastic, dissolved and put through the procedures, is recovered 100%. ACKNOWLEDGMENT
The author appreciates the advisory assistance of C. F. Pickett, chief, Myer Rosenfeld, and H . L. Ammlung of the Paint and Chemical Laboratory. Thanks are due Martha Adams and George Esposito for their help in the analytical work. LITERATURE CITED
(1) Federal Specification TT-P-l4lb, Method 702.1, Procedure B. 12)
Military Specification MIL-E-10687.
(3) Military Specification MIL-E-10687A RECEIVED for review May 21, 1953.
Accepted September 7, 1953.
