Isophthalic Acid


ortho-phthalic isomer has been produced from o-xylene in large quantities for many years, although the main source is from naphthalene. Terephthalic a...
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Isophthalic Acid APPLICATION IN CONDENSATION POLYMERS F. G. LUM

AND

E. F. CARLSTON

California Research Corp., Richmond, Calif.

T

H E three phthalic acid isomers, ortho, meta, and para, may all be derived from the corresponding xylene isomers. The ortho-phthalic isomer has been produced from o-xylene in large quantities for many years, although the main source is from naphthalene. Terephthalic acid has recently become commercially important as a derivative of p-xylene, because the ethylene glycol polymer of terephthalic acid produces an excellent fiber (9). m-Xylene is the most abundant of the xylene isomers, and the prospective availability of large supplies of isophthalic acid from this raw material suggests the desirability of a study of its characteristics in applications of industrial importance. The use of isophthalic acid in drying oil-modified alkyd resins received the most attention in this investigation, and in this field many comparisons with phthalic anhydride and terephthalic acid were made. The first use of isophthalic and terephthalic acids in oil-modified alkyd resins was disclosed in a patent ( 6 ) , which claimed that alkyd resins made from isophthalic and terephthalic acids had better drying characteristics than the corresponding orthophthalic resins, but which did not give any detailed data on the finished products. Hovey and Hodgins (6)discussed the use of isophthalic and terephthalic acids in drying oil-modified alkyd resins and concluded that the differences in drying time claimed could be due t o the presence of antioxidant impurities in phthalic anhydride made from naphthalene. The present work was undertaken t o determine quantitatively the differences t h a t exist between the three isomers when used in semidrying oil-modified alkyd resins.

kettle was equipped with a thermometer, a stirrer of approximately 400 r.p.m., and an outlet for the collection of volatilized material. Carbon dioxide was bubbled through the reaction mixture a t a rate of approximately one fourth liter per hour per 100 grams of charge; this inert gas rate was found t o be sufficient to remove all of the water of esterification and was used in all experiments except where otherwise stated. Alkali-refined soybean oil, double distilled soybean fatty acids, refined glycerol, and a commercial grade of pentaerythritol known as Pentek were used in this work. Isophthalic and terephthalic acids were prepared and purified in these laboratories, and phihalie anhydride was obtained from the Oronite Chemical Co. All of these materials were of 99+% purity. RESULTS

ESTERIFICATION RATES. The esterification of the three phthalic acid isomers in various oil length alkyd resins was studied under different conditions of temperature and excess polyhydric alcohol. Most of this work was done on a resin containing 67% soybean oil and 33% glyceryl isophthalate.

GAS RATESSHOWN ON CURVES ARE LITERS COz PER HOUR PER

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5 30. 2

EXPERIMENTAL

The oil-modified alkyd resins prepared in this work were formulated as shown in the following example:

120.

% rllkyd resin composition desired Soybean oil Glyceryl isophthalate Calculations Isophthalic acid Glycerol Water of esterification Glyceryl isophthalate 15% excess glyoerol

66.7 33.3

2

3

4

5

0

7

8

9

1011

I 2 I 3 1 4 I S

28.9

Figure 1. Eesterification Rates of Phthalic Acid Isomers in 67q0 Soybean Oil Alkyd Resins at 230" C.

33.a

Excess glycerol, 5 %

6.3 1 .e

66.7 12.3

0.05 28.9

All of the glycerol and the soybean oil was cooked until alcoholysis was completed, after which the isophthalic acid was added. This alkyd is described as a 67% soybean oil resin containing 15% excess glycerol. When soybean fatty acids were used instead of soybean oil, all of the materials were charged t o the kettle at once, without catalyst and without preliminary preparation of monoglyceride. When pentaerythritol was used instead of glycerol, the same methods of formulating and cooking were followed.

Two-part glass reaction kettles, flanged to permit easy cleaning and removal of contents, were com letely immersed in a heated and thermostatically controlled DC Eilicone 550 bath to minimize bumping from the condensation of water of esterification. The

I6

TIME IN HOURS

10.7 39.6

Grams Materials charged to kettle Soybean oil Glycerol Litharge Isophthalic acid

I

In Figure 1 are shown the rates of esterification a t 230" C. when 5% excess glycerol is used. Initially, the order of the rate of esterification of the three acids is: phthalic anhydride > isophthalic acid > terephthalic acid. As heating continues, however, the order changes t o isophthalic acid > phthalic anhydride > terephthalic acid. In the case of phthalic anhydride (PA), the esterification rate appears t o increase considerably with an increase in carbon dioxide rate. However, the amount of sublimed phthalic anhydride collected (Table I) suggests that the high esterification rates may be attributed more to the higher loss of phthalic anhydride than to the faster esterification resulting from faster water removal. The volatility of isophthalic acid (IPA) is so low t h a t increasing the carbon dioxide rate to as much as 3 liters per hour per 100 grams does not cause an appreciable loss of acid. The effect of varying the amount of excess polyhydric alcohol on the esterification rate of isophthalic acid is shown in Figure 2 . 1595

INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY

1596

Increasing the excess glycerol from 5 to loyohas a marked effect on the esterification rate, while further increase produces proportionately less change. By using 10 to 15% excess glycerol, instead of 57,, the processing time required to attain an acid numhcr of 10 or less is reduced considerably.

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TIME IN HOURS Figure 2. Effect of Increased Excess Glycerol on Esterification Rate of Isophthalic Acid in a 67yc Soybean Oil Alkyd Resin at 230" C.

The effect of temperature on esterification rate, Figure 3, shows that increased rates result from increasing the cooking temperature; the effect is particularly striking for terephthalic acid (TPA). Increasing the cooking temperature to 275" C. results in a further increase in the rate of esterification of isophthalic acid and t,erephthalic acid, but the rate levels off a t approximately 280" to 290" C. In preparing the isophthalic acid and terephthalic acid alkyd resins a t high temperatures, the temperature was raised slowly after reaching about 220' C. to avoid loss of glycerol. The dott,ed curve in Figure 3 illust,rates what happens when the temperature of an isophthalic acid alkyd cook ie raised too fast. A time of approximately 1 to 11/1 hours between 220" and 260" C. was found sufficient t o eliminate loss of glycerol. This time can be reduced if an efficient steam-heated condenser is used t o return glycerol to the kettle.

Vol. 44, No. 7

and two temperatures. The data on the phthalic anhydride resins were obtained by an azeotrope solvent method to eliminate loss of acid. The differences illustrate the excellent alkyd resin viscosity characteristics that can be obtained with isophthalic acid and terephthalic acid. The maximum amount of glyceryl isophthalate or terephthalate that can be incorporated into an alkyd resin containing normal amounts of excess glycerol is about 40%. Medium and short oil length isophthalic acid and terephthalic acid alkyd resins can therefore be prepared only by using a large excess of glycerol or by modifying the resin with materials having lower functionality, such as ethylene glycol or benzoic and toluic acids. The effect of using pentaerythritol instead of glycerol in isophthalic acid alkyd resins is shown in Figure 6. High viscosity alkyd resins can be made at a soybean oil content of about 72y0 if pentaerythrityl isophthalate is substituted for glyceryl isophthalate. If all of the glycerol in the alkyd resin is replaced with pentaerythritol, the same viscosity can be obtained a t a soybean pentaerythritol oil content of about 76%. The viscosity data presented are the averages of numbers obtained from more than one run a t each oil length. The viscosity of isophthalic acid alkyd resins is influenced considerably by differences in crystal size of the mid; because of this the data on isophthalic acid resin viscosities in Figures 4 and 5 will be displaced upward or downward depending on the crystal size of the isophthalic acid used. A small crystal size isophthalic acid dissolves rapidly in the reaction mixtures and results in somewhat lower viscosities than shown, while a large crystal size isophthalic acid dissolves more slowly, resulting in slightly higher viscosities than shown.

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INERT GAS KATEO N LOSSES TABLE I. EFFECTOF INCREASING O F PHTHALIC ACIDS FROM SOYBEAK OIL X L H Y D RESIN (67% resin at 230' C.) Loss, % COz Rate, Phthalic Isophthalic L./Hr./100 Gm. Anhydride Acid 0.25 2 0.4 0 5 3 1 6 ... 2 11 ... 3 .. 1

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Terephthalic Acid

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Using refluxing solvent to remove the water formed increases the esterification rate. Data on esterification rates obtained by this method of cooking are not presented, however, because of the fact that satisfactory quantitative experiments depend on controlling the rate of reflux, and it was found that reflux rate is much more difficult to control than inert gas rate. Since the esterification of isophthalic acid and terephthalic acid is at first a heterogeneous reaction, it is probable that the data presented here may be displaced if particle size of the acids or rate of agitation is changed. This displacement will probably be slight for isophthalic acid, but for terephthalic acid the difference could be quite large. This is because the solubility of terephthalic acid in the reaction mixture is low, while the solubility of isophthalic acid is comparatively high. VISCOSITY.Figures 4 and 5 show the viscosity characteristics of the three phthalic acids in alkyd resin cooks at two oil lengths

Figure 3. Effect of Increased Temperature on Esterification Rates of Phthalic Acid Isomers in 67Yo Soybean Oil Alkyd Resins Excess glycerol, 15 %

THERMAL STABILITY.Isophthalic acid and terephthalic arid alkyds show little discoloration or decomposition when heated a t high temperatures, such as during prolonged heating under oil bodying conditions up to 305" C. (Table 11). This excellent thermal etability makes possible the preparation of light-colored, fast-drying, high viscosity alkyd resins of high soybean oil content. The data show that a phthalic anhydride alkyd resin decomposes when prepared under these same conditions, resulting in a darkening of the color and an increase in the drying time. FILM PROPERTIES.Soybean oil isophthalic acid and terephthalic acid alkyd resins dry faster than phthalic anhydride resins (Table 11). Laboratory tests of the dried films show that the water and alkali resistance of isophthalic acid and terephthalic acid alkyd resin films are noticeably better after long immersion when compared t o similar phthalic anhydride resin films.

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

July 1952

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POLYAMIDE RESINS

TABLE11. PROPERTIES OF SOYBEAN OIL ALKYD RESINS PREPARED FROM PHTHALIC ACIDS Oil, % Time, Hr. PA 57b .15 .. PA 60 IPA 60 51/t TPA 60 111/z PA 67 14 IPA 67 8 TPA 67 6 PA 65C ,,, PA 65d I P ~ E75 ioi/% PA SO 5

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Because of the absence of literature references on the polymer of isophthalic acid and hexamethylenediamine, a study of the properties of this resin was considered desirable. A polymer of hexamethylenediamine and terephthalic acid has been reported (4). The melting point, however, is so high that it does not appear to be a practical resin. The reaction product of phthalic anhydride and hexamethylenediamine is limited in usefulness because of low melting point and low molecular weight.

Drying Timea, Hr. Dust free Tackfree 21/r 5 11/* 5'/1 11/4 21/2 11/4 21/a zs/4 9 to 10 111/4 23 a/, 21/1 5 21/2 51/2 11/4 21/2 3a/, 7 to 8 41/4 9 to 10 2 1/4 6 to 7 2a/4 6la/' . 9 to 101 Approx. 24 8 to 891/ 41/' 31/r 7 to

Isophthalic acid and hexamethylenediamine were reacted in equivalent proportions in an atPAL 90 G mosphere of oxygen-free nitrogen and in the pres4-4 Y ence of xylenol as a solvent. The water of reaction IPAO 90 4 J was removed a t the reflux temperature of the +4 2-2 xylenol, and after a high polymer was formed the a Wet film thickness = 0.0015 inch; drier = 0.03% oobalt 0.3% lead a8 naphthenates. xylenol was removed under a vacuum of 0.001 to b Commercial resin. 0.01 mm. pressure and a final kettle temperature of 0 Commercial resin, maleic modified. 250" to 260' C. Other polymers were prepared in d Commercial resin, penta modified. e Pentaerythrit 1 phtha!ate. the same manner, but with various amounts of I Bodied linseel oil drying characteristics. terephthalic acid replacing part of the isophthalic acid. Melting point and heat distortion oints of these polymers are shown in Table 111. Fncluded in Table I11 are data on one polymer in which adipic acid replaces part of the isophthalic acid. Outdoor film exposure tests have not progressed far enough to The melting points were determined by measuring the fusion temperature, in an inert atmosphere of finely divided resin reshow differences between paints made from phthalic anhydride, cipitated from xylenol by the addftion of ethanol. The &a isophthalic acid, and terephthalic acid alkyd resins. Figure 7, show that re lacement of 25% or more of the isophthalic acid however, illustrates the kind of differences that may develop with terephtialic acid increases the melting point of the isoin these outdoor exposure tests. A white enamel was prepared phthalic acid polymer, while re lacement of 50% of the isophthalic acid with adipic acid refuces the melting point. from a 67% soybean oil isophthalic acid alkyd resin and compared with the same enamel prepared from three commercial phthalic anhydride alkyd resins. After 6 months of exposure TABLE 111. MELTING AND HEATDISTORTION POINTS OF HEXAin an Atlas Twin Arc Weather-Ometer the results of film breakMETHYLENEDlAMINE ISOPHTHALIC POLYMERS MODIF~ED WITH down showed an outstanding superiority for the isophthalic acid TEREPHTHALIC AND ADIPICACIDS alkyd resin film. Replacement Heat Distortion Melting ZINC OXIDE REACPIVITY.Limited tests indicate that isoof IPA, % Pt.a, 0 c. Pt., O c. phthalic and terephthalic acid alkyd resins have excellent 0 113 114 40 2 5 7$;; viscosity stability in enamels containing zinc oxide. A compari125 155 50% TP.A son of alkyd resins of about the same acid number shows that 50% Adipic 79 isophthalic and terephthalic acid resins are superior to phthalic a Determined by. ASTM method D 648-45T; maximum fiber stresa 66 anhydride resins in this important characteristic. pounds per square inch. IPAc

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230" C.

Figure 4. Viscosity Characteristics of Soybean Oil Alkyd Resins Prepared from Phthalic Acid Isomers Excess glycerol, 15 9%

5 w 5 5 0.10

2

4

6

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14

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Figure 5. Viscosity Characteristics of Soybean Oil Resins Prepared from Phthalic Acid Isomers Excess glycerol, 15%

20

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 44, No. 7

The differences betn-een the isomers that have been reported may be explained by correlation with their structures and physical properties. The low initial esterification rates of isophthalic and terephthalic acids result from their high melting points and low solubilities in the reaction mixtures. Phthalic anhydride reacts almost instantaneously at elevated temperatui es t o the half ester because it is a low melting anhydride, but its subsequent esterification rate is considerably slower. This is probablj- because of steric hindrance and the following de-esterification reaction: 0 //

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