August 1948
1393
INDUSTRIAL AND ENGINEERING CHEMISTRY OPERATION OF TOWER
ACKNOWLEDGMENT
Tests made employing a sensitized paper detector indicated t h a t the fluorine was almost completely absorbed, and the concentration of fluorine (or hydrogen fluoride) in the gases from the 4-foot diameter tower, at fluorine feed rates u p t o 500 pounds per hour, did not exceed 3 p.p.m,, which is believed t o be nontoxic ( 1 ) . SUBSEQUENT EXPERIENCE
After the run described in this paper, the installation operated smoothly for a n extensive period of time; this supported the observations made during the month of testing.
The assistance of H. A. Rehnberg of the Kellex Corporation, under whose supervision the plant was constructed and operated, and W. C. Moore and E. W. Thomas of Ford, Bacon, and Davis, who conducted the tests, is acknowledged with appreciation. This Paper is based on work done for the Manhattan Project under contract, W-7405-Eng. 23, a t the Kelley Corporation. LITERATURE CITED
(1) Landau, R., and R o s e n , R., IND.ENG.CHEM.., 39,281-6 (1947). (2) Turnbull, S. G., Benning, A . F., Feldmann, G . W., Linch, A . L., McHarness, R. C., and Richards, M. K., Ibid., 39, 286 (1947).. Presented as a p&Tt of the Symposiul,l on RECEIVED November 1, 1947. Fluorine Chemistry, Division of Industrial and Engineering Chemistry. a t the 112th Meeting of the ANERICANCHEarIcAL SOCIETY, New York, N, Y .
LACTIC ACID POLYMERS As Constituents of Synthetic Resins and Coatings PAUL D. WATSON Agricultural Research Administration, U . S . Department of Agriculture, Wushington, D . C.
The
paper describes modified lactic acid condensation polymers dqveloped in the Division of Dairy Research Laboratories which may be of interest to the coatings industry. The most useful of these products appears to be a modified polylactylic acid-fatty oil polymer, from which tough, water-resistant coatings may be formulated. Another class of resins is the metal polylactyl lactates derived almost entirely from lactic acid; these may be used for protective and decorative coatings.
T
HE lactic acid of commerce is produced by fermentation of
the carbohydrates present in corn sugar, molasses, and whey. It has been estimated that the whey produced annually as a by-product in t h e manufacture of cheese and casein contains about 500,000,000 pounds of lactose, which is a potential source of about 400,000,000 pounds of lactic acid. Although approximately 6,000,000 pounds of lactic acid (100% basis) produced from all sources are utilized annually by various industries, the relatively high price of the purified acid has apparently retarded greater use. About 400,000 pounds are used in the plastics industry-chiefly as a catalyst and plasticizer in cast phenolic resins. Adoption of the methyl lactate purification process (10) or of the selective extraction method for the recovery of lactic acid esters (3) eventually may lower prices for the high grade acid. With the above considerations in view, research workers in the Division of Dairy Research Laboratories developed a number of resins which are based largely on lactic acid. The can manufacturing industry became interested in their possibilities as a coating for tin cans and sesearch was continued. Then the critical shortage of tin during the war caused serious concern i n the milk industry, and further work was carried out with the object of obtaining a coating for milk cans which would substitute for tin. Shortages of other basic chemicals used in the plastics industry-such as phthalic anhydride, glycerol, and oils-made further studies of lactic acid based resins appear desirable. More detailed information will be found in the patents of reference. The brief survey of lactic acid resins made by Light (9) shows that considerable interest has been directed t o this field over a period of years. Several patents (7, 8, 1.8) mention lactic acid as a constituent of various plastic materials. Stearn, Makower,
and Groggins (11) have investigated lactic acid as a coniponent of alkyd resins. I n considering the economic phases, they expressed the opinion that i t may become a n important. industrial chemical if t h e cost can be made comparable with other resin ingredients. POLYMERIZATION OF LACTIC ACID
Bezzi ( I ) , Hovey and Hodgins ( 6 ) , and Flory ( 5 ) have discussed the theoretical aspects of t h e linear condensation polyiners formed from lactic acid, and the subject need be outlined oiily. When a solution of lactic acid is dehydrated by heating, interesterification occurs by loss of water between the carboxyl and alpha alcohol groups, and monolactyllactic acid, CH3.CHIOH)COOCH(CH8)COOH, dilactyllactic acid, tri-, tetra-, and finally polylactylic acids consisting of long chain molecules of the folloiving type are formed successively: ’
H- [OCH(CHa)CO]z--OH
The reaction may proceed intramolecularly also wit,h the Eormation of the cyclic-dimeric compound, lactide: CHJ-CH-0-CO
1
1
CO-O-CHCH3 Some of the latter compound is usually formed, dependiiig 011 the reaction conditions, and a portion generally escapes because of its ease of sublimation. The water freed during the interesterification reactions evidently hydrolyzes part of the lactide to lactyllactic acid, which then reacts with other lactide molecules by ester interchange. The functional groups apparently react with each other at random, and the polymerization proceeds by the addition of the monomer or cyclic dimer t o the linear chains. The bifunctional nature of lactic acid, therefore, makes it possible t o form resinous polymers by self-esterification. However, these simple resins have very little utility because they are brittle, have a high acid value, and lack water resistanqe- The writer has found t h a t useful resins may be formed by causing polylactylic (polylactyllactic) acid t o react with other a g w t s containing functional groups-such as alcohols, aldehydes, carbohydrates, fatty oils, and certain metal salts. Apparently the functional groups of these compounds react with the hydroxyl
1394
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
or carboxyl groups of the polylactylic acid ~ i t the h elimination of water, lactide, and volatile low polymeis. Improvement in the properties of the resins can be effected also by copolvmerization and esterification with other resinous compounds. PRODUCTION OF POLYLACTYLIC ACID
Polylactylic acid was used a s t'he starting material for the resins described here with t h e except,ion of some of the metallic polylactyl lact,ate polymers. Earlier work on the production of the partially dehydrated acid by means of distillation with various low and high boiling water-mit,hdraming agent,shas been pieviously described (13, 15) by the writer. High boiling solvents-such as cymene, kerosene, turpentine, xylene, and petroleum distillates boiling between 155 a and 216 C.-were found t o be efficient water-withdrawing agents for the production of polylactylic acid of about 110 t o 125% acidity (calculated as lactic acid). The per cent of dehydr?tion (ratio of water distilled to total of free and combined water) averaged about 95%. It required about 6 hours t o dehydrate 3 kg. of 85% lactic acid with xylene and about one half to two thirds of this time using the higher boiling entraining agents. Toward the end of the distillations some acid was distilled over, at the high t,eniperatures used, when t h e still was heated within the temperature range of about 205 O to 225' C.; this caused some error (less than 5'%) in t'hc calculation of the percentage of dehydration. The passing of a st,ream of dry inert gas or air through the still increased somewhat the rate of water removal. The better grades,of lactic acid, which are free from nitrogenous matter, must, be used t80obtain a pale colored polymer. Lactic acid may be dehydrated also under atmospheric pressure by gradually raising the temperature from 100" t o about 180" C. while distilling off the wat,er, or at. lower t.emperat,ures under reduced pressures. The costs involved in the equipment, fuel requirements, and time consumed would be determining factors in the method selected for industrial use. Filachione and Fisher (4) recently have described batch and continuous methods for dehydrating and polymerizing lact,ic acid. ,
POLYLACTYLlC ACII)-DKYl\G
OIL HESIXS
A resinous product' recently patented ( 1 7 ) is a condensation product of polylactylic acid with fatty drying oils. This resin is obtained by reaction of the acid n-ith vegetable oils-such as castor, dehydrated castor, linseed, soybean, or synthetic drzing oils-by heating to a temperature of approximately 265 to 280" C., preferably in the presence of SL catalyst., until a brown, soft, and elastic resin is formed. The proportion of dehydrated acid to oil may be varied over a considerable range. Various catalysts may be used, such as salts and oxides of such metals as aluminum, cobalt, iron, and vanadium. A small amount of fumaric acid or maleic anhydride may be added. The heating requires several hours, depending on size of reaction vessel, speed of stirring, etc., which affect the rate a t which volatile products are removed during t.he heating process. A very insoluble product is obtained if the heating process is not stopped n-hen the product first begins to resinify, as t,hese polymers are of the thermosetting type. Experiments have indicated that this resin may be of value for indust,rialuses, such as t>heimpregnating, gluing, and molding of various art,irles. A valuable property of this resin is its elasticity; the flexibility of the baked coatings makes them resistant to chipping. The polymers may be dissolved in xyleno, tolucne, ketones, or synthetic petroleum solvents with comparable, solvency charactorist,ics. For use on food containers nontoxlc driers should be selected, based on zinc, cobalt, calcium, iron, or manganese. The lacquer bakes fairly hard in 20 minutes at. about 160' C.; but, to obtain a very resistant coating, about 10 to 15 minutes of baki n g a t a t'emperature of 200' C . are required. The well cured coating is resistant to dilute acids, alcohol, water, and dilut'e alkali (1%sodium hydroxide). The coatings resist the action of steam for 2 to 3 hours and n-ithstand the processing necessary in the canning of evaporated milk; this involves autoclaving for 20 minutes a t 116" C. The lacquer may be applied directly to iron, steel, tin, and aluminum without, a priming coat,, but better
Vol. 40, No. 8
adhcsion t o i k n is obtained if the surface fimt is subjected to the hot-dip phosphate process.
A typical formula for the preparation of this lacquer is: Dehydrated Lactic acid (In@% total acidity) Yegetable oil (castor 30 parts, linseed 80 parts) hIaleio anhydride ( 5 parts) '\ Fiiinaric acid ( 5 parts) Catalyst (activated bauxite\
Parts 130 B0 10 9
The soft and brown elastic rtxin is dissolved in xylene to a solids content of about 20'34 resin. h lacquer which produces tt hard, flexible, resistant coating when baked for 15 minutes a t 200" C. is obt,ained by the addition of driers as follows (the percentage indicates the weight of metal added): 0.075% zinc, 0.015% cobalt, and 0 . 0 1 5 ~ omanganese. For many purposes, the durability and appearance of these coatings are enhanced by the addition of a pigment. Chemically inert, heat-stable pigments are preferred, such as titanium dioxide, cobalt blue, iron oxides, and earth colors. The hardness and resistance of the coatings t o steam were improved, particularly on ceramic surfaces, by the addition of about' 570 of cyclohexyl silicate, calculated by weight on a total solids basis. The addition of ethyl silicate was less effective. Field t,est's,conducted during the war, in which inilk was transported in 10-gallon stmeelmilk cans coat'ed with this resin demonstrat,ed that it compared favorably in durability wit,h scveral other commercial organic coat'ings of the phenolic and vinylite types. The test>sshoxved that the lactic acid-fatty oil coating was less likely t o loosen or chip on the interior than these other resins when the exterior of the can was severely dented. I t s superior flexibility and adhesion were demonstrated also by bending thin sheet' iron, which was protected with t8wocoats of this resin, t,hrough a n arc of 180" twenty t,imes without cracking or impairment of the resin. Experiments xere made in these laboratories in which iron cans coated with this resin were used for canning evaporated milk. The lacquer was used also to seal the side seams in place o f solder. Y o off-flavor developed after 2 months of storage.
il food manufacturer after testing this coating on evaporated milk cans in a comparison with a commercial coat)ing wrote t,hat the cans coated wit,h the lactic acid typc o f resin imparted less off-flavor to the milk than the cans coated with the other resin, and did not show the rusting and pitt'ing Tyhich occurred in the latter. A dairy association also reported favorahlc results. A large dairy company reported that the lact,ic acid-fatty oil coating was superior to till 4-heri used on cottage cheese pails. A dairy equipment manufacturer statcd that the hardness of this coating should be acceptable, that the coating was not affected by alkalies in strengths used in daii,ies, and, t,hat, its flexibility vas superior to that of a phenolic typc coaling. An industrial firm stated that thc luetic acid type ficient, resistancc: t o water and wagents to war interior of food cans n-here its nontoxic and superior elastic propert,ies xould be useful. The same firm st'ated t,hat the resin could be inariufactured in a \vel1 controlled process and it had constructed equipment suitable for the product'ion of several million pounds of polymer per year. However, the shortage of vegetable oils and the placing of lactic acid under allocation .halted further industrial development. It is advisable t,o effect ester interchange by heating the coinponents below 250" C. before increasing the temperature in order t o induce molecular polymerization. Castor oil can react directly with polylact.ylic acid without first forming the free fatty acids or monoglycerides. This,facilitates the subsequent incorporation of nonhydroxylated oils, such as linseed or soybean oils, and thereby shortens the heating time necessary for the preparation of the polymer. Particularly rapid combination of fatty oils with polylactylic acid occurs when either conjugated linseed oil or a mixture of hydroxylated and conjugated linseed oils is used. Determination of the molecular weight of the viscous rubbery polymer (acid numbers about 40 to 50) obtained in the condensation of polylactylic acid with fatty oils made by means of boiling point and melting point mrthods indicates that the average molec-
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
ular weight was about 1200 to 1400. The figures will vary somern hat depending on the procedure used in making the resin. The molecular weight of the final baked coatings, which are insoluble and infusible, may be much higher. The flexibility, hardness, and resistance of these coatings may be varied somewhat by using different proportions and mixtures ot the fatty oils. The proportion of oil in the formulation should he at least 20% of the weight of the ingredients to ensure adequate flexibility in the coatings. The baked coatings may be formulated to resist the action of salt water, gasoline, lubricating oil, benzene, ethylene chloride, and methanol. They are resistant also to dilute acids such as acetic, lactic, hydrochloric, and sulfuric. But because of their esterlike nature, they are susceptible to attack by alkalies. Furthermore, the chemical resistance, hardness, drying, and baking properties of the coatings may be improved by certain modifications in the preparation of the resins. An improvement in the characteristics of the polymers containing about, 35 to 70% of polylactylic acid and about 20 t o 50% of fatty drying oils may be effected by the addition of about 10 to 15y0of one or more of the following polymeric substances: oil-soluble olefinic petroleum hydrocarbons; oil-soluble cycloparaffin coal-tar hydrocarbons; oil-soluble alkyl or aryl substituted phenol-formaldehyde resins.
It might be expected that the olefinic and naphthenic polymers would be incompatible with the polyester type of resin formed from polylactylic acid and fatty oils. However, these hydrocarbon polymers may be incorporated with the polyester resin by adding them in the latter stages of the heating process and then raising the temperature to about 270' to 300' C. The product then is heated until an advanced stage of polymerization, ahort of becoming insoluble and infusible, is reached. It is apparent that modification is effected mainly by conjoint polymerization. However, some degree of copolymerization may occur because of the considerable degree of unsaturation of the olefinic hydrocarbons and because of some decomposition of the naphthenic hydrocarbons a t the high temperatures (above 275" C.) used. The removal of water and other volatile products of the condensation may be facilitated by reduced pressures and by the passage of an inert gas through the reaction vessel. This procedure aids also in the production of a light-colored resin, especially when about 5% of maleic anhydride or of fumaric acid, or a mixture of both, is used in the formulation. These agents probably serve to accelerate the resinification process by the presence of their double bonds. Then, too, by this means about 10% of the dark-colored, olefinic hydrocarbons (petropols) may be incorporated in the resin and the color of the final product will be darkened only moderately. The coating is a golden-yellow color when baked on light surfaces. These petroleum derivative polymers sell for a few cents a pound, and their inclusion in the resin lowers the cost. The yield of resinous products generally ranges between 65 and 85%, depending on the time and temperature required to effect condensation; these vary with different formulations. Modified drying oils-for example, conjugated and hydroxylated linseed oils-tend to increase the yield because of their relatively high degree of reactivity. The addition of a condensation or polymerization catalyst appears to confer certain advantages, such as a more rapid reaction and an increase in the yield, hardness, and insolubility of the product. POLYLACTYL LACTATE RESINS
Kovel resinous products developed recently by the writer are derived almost wholly from lactic acid, which furnishes 94 to over 99% of the constituents. Other practical lactic acid resins made heretofore have contained a considerably less proportion of lactic acid. Excellent, tough, water-resistant polymers suitable for protective and decorative coatings can be made from the polylactyl lactates of certain metals. These polymers are made by reacting polylactylic acid with soluble metal salts-such as carbonates, chlorides, and lactatesor by f%st dissolving the metal in the lactic acid, if somewhat soluble, as in the case of iron and zinc, Useful polymers have
1395
been made from the polylactyl lactates of aluminum, chromium, cobalt, copper, iron, lead, manganese, thorium, tin, titanium, zinc, and combinations of these metals. The number of practical polylactyl lactates is limited by the low solubility or inertness of some metal salts and the high cost of others. Some salts, such as calcium carbonate, react readily with polylactylic acid but form brittle polymers with poor resistance to water unless used in conjunction with other metal salts. The metal content of the polymers usually ranges from fractions of 1 to about 6%. Small amounts of some metals are sufficient t o react with polylactylic acid and impart useful properties to the polymers. Larger amounts of other metals may be incorporated by using their more soluble and reactive chlorides. Excessive amounts of metal in the polymers may cause brittleness and increase the infusibility. Three t o six per cent of such metals as aluminum and tin may be combined in polylactyl lactates. Less than 1% of some metals are present in the final polymers because of their low solubility or limited reactivity. The metal salts are dissolved readily in hot lactic acid of various concentrations up to about 85%. The solution then may be dehydrated by heating further under atmospheric or reduced pressures, or by the use of water-entraining agents. CONSTITUTION OF THE POLYMERS. During the heating process the linear polylactyl lactic chains which are formed by interesterification react with the metal salts t o form polylactyl lactates. A molecule of chromium polylactyl lactate, for example, may be shown as follows: O-[OCCH(CHs)O],-H
I
(3-0I
[OCCH(CHa)O],,-H
O-[OCCH(CHa)O],-H This indicates that the linear chains are joined a t the carboxyl end groups by a chromium atom with a valence of three. This polymer sometimes gels during the heating process and becomes thermosetting. The tendency t o become insoluble and infusible on further heating is typical of the behavior of a three-dimensional polymer. Gelation occurs if the functionality of one of the components of a resin is more than two because the resulting cross-linkages between the linear structures promote rigidity. The development of a three-dimensional network structure may be facilitated by means of steric effects and alsq by residual functionalities which form oxygen linkages between metal atoms and ether linkages between adjacent alcohol end groups. Such a polymer containing 0.88% of chromium was prepared by adding chromic lactate t o lactic acid and polymerizing for 16 hours at a temperature of about 190" C. It was then tough, infusible, water-resistant, and insoluble in the common solvents. The average number of repeating units (n)i; each chain may be computed from the percentage of chromium or other metal present by substituting the proper molecular weights in an equation. For example, the equation for a polymer containing 0.86% of chromium may be written as follows: Cr
CrOa
+ [OCCH(CHJO13n + 3H = 0.0086
On solving the equation, n equals 28 units per chain or 84 units per molecule. The average molecular mTeight, calculated on the basis of 84 repeating units per molecule, is about 6000; this is consistent with the properties .of this polymer. It wm assumed in this computation that the polylactylic acid was fully reacted with the metal; this may not always be complete according to data obtained from fractionization experiments and acid number determinations. Experiments have shown that, generally, thermosetting polymers are produced when polylactyl lactates are made with metals
,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
which have a valence of three or more, such as aluminum, chromium, and manganese. However, the polymers obtained with met,als having a valence no greater than tx-0-such as copper, lead, and zinc-are thermoplastic resins of the linear condensation type with a melting point between 80 O and 100 C. The thermosetting type of coating is usually tougher and more solventresist,ant than the thermoplastic type of coating. The met,al polylactyl lact,ate rrsins GENERAL PREPARATIOX. may be prepared in several ways by polymerizing the components: (A) under reduced pressure a t temperatures between 150" and 180" C. for about 10 hours; (B) after removal of water, in layers a,bout, 0.25 inch thick in shallow vessels a t a temperature of about 185" C. for approxiniately 16 hours; ( C ) in an open vessel with rapid stirring at, a temperature of about 200" C. aft,er removal of water and gradually raising it to about 250" to 260" C. for 3 to 5 hours, preferably with the passage of an inert gas, such as carbon dioxide, through the polymer; (D) by the addition of a reactive anhydrous metal chloride t'o polylactylic acid in the presence of a n organic solvent at room temperature or at the boiling temperature of the solvent (below 150" C.) for 1 t,o 3 hours or until evolution of hydrochloric acid gas has ceased. PREPARATION WITH A N H Y D R O C S hIETAL CHLORIDES. K h e n a .reactive anhydrous metal chloride (aluminum chloride) is used as the reactant, it should be added slowly t o polylactylic acid of about 12Oy0 concentration (calculated as lactic acid) which has been previously dissolved in a chlorinated solvent-such as ethylene dichloride, chloroform, methylene chloride, monochlorobenzene, and t,etrachloroet,hane. Xylene or acetone also may be used. However, experiments have shown that the use of benzonc., toluene, and certain other solvents resulted in the formation of soft, fusible resins. Evidently the type of diluent is a factor in the amount of aluminum which reacts with the polylactylic arid. S n amount of anhydrous aluminum chloride equivalcnt to about 5 t o ZOyo of the weight of the polylactylic acid is required, depending on the properties desired in the product. The reaction can take place without the application of heat, but heating speeds up the process. After 1 to 3 hours, if the react,ion is rapid, thc polymer may become insoluble and separate out as a gel. In the slower type of reaction, the resin may remain in solution; it can be separated from the unreacted metal residue by filtration. This filtrate, after removal of hydrochloric acid gas, can be used directly as a lacquer, but a more infusible and st'able product is obtained by curing the polymer for several hours a t a temperature of about 170" C. The resin t,hen may be obt,ained as a pale infusible solid, which may be readily converted into lumps or a n almost colorless powder. The yield of product based on t,he m-eight of the polylactylic acid reacted is usually over 907,. This aluminum polylactyl lactate makes a clear, pract,ically colorless lacquer that dries rapidly in a few minutes and adheres \vel1 to metal and glass. Experiments indicated that a n improved product was obtained in the presence of about 570 of benzyl chloride, which was added to the polylactylic acid solution before the addition of the aluminum chloride. This process has the advantage of requiring very little heat' and, if chlorinated solvents are used throughout for the reaction me. dium and as the lacquer solvent,, the whole process is without fire hazards. The pale color of the product, makes it of interest as a coat,ing for decorative uses. Tin polylactyl lactate and tit,aniuln polylactyl lactate resins may be prepared by reacting,their anhydrous tetrachlorides with polylactylic acid using chlorinated hydrocarbons as the diluent ; this is similar t o the process described for aluminum polylactyl lactate. I n the case of titanium, the best product was produced when a n excess (about 20Cr,) of tit,ariium tetrachloride was added t o the polylactylic acid. PREPARATION WITH S'ARIOUS XETAL SALTS. The preparafioll of metal polylactyl lactate resins from salts other than the very reactive anhydrous metal chlorides may be carried out by dissolving t,he salts in lact,ic acid of about 50% concentration and
Vol. 40, No. 8
heat,ing at, about' 100" C. while stirring. The water then is removed by raising the temperature s l o ~ d yto about 180" C. The mixture is stirred rapidly and carbon dioxide bubbled t,hrough the polymer while the temperature is raised to about 250" C., where it, is held until a sample of the polymer when cooled hardens with little or no tackiness. Generally the process is complete when the polymer is held for approximately 2 hours at, tjhe higher t,emperature. Viscosity measurements during thc process are of value in controlling the degree of polymerization. The yield of product by this method is about 60%, but the yield ma) be increased materially by recycling the volatile by-products of t,he reaction; these, aside from the water, are chiefly lactic acid, lact,ide, and low polymers of a soft or viscous nature. When acenaphthene, in a n amount equal t o about' 57, of the weight, of t,hepolylact'ylic acid present', was added at the beginning of t,he heat polymerization, after removal of the water, it was found that the yield, hardness, and resistance t o alkali of the metal polylactyl lactate resins were improved. A portion of the acenaphthene was removed by sublimat'ion during the process. Presumably it does not' react, but its presence influences the course of the polymerization. Glass-lined vessels should be used in the process if metallic chlorides are the react,ants, but stainless steel is suitable when less react,ive salts are used. These vessels should be designed to allow the free escape of the volatile react'ion products during the polymerization or curing of t'he polymers. Otherwise, inferior resins will be obt'ained. However, when very- reactive rnetal chlorides are used, a closed vessel equipped with a stirrer and provided with a reflux opening for the escape of hydrochloric acid gas is necessary to prevent loss of t'he solvent.
PRACTJCAL APPLICATIOXH. Coating solutions may be prepared readily by adding the solvent before solidification of the polynicr occurs. The polymers are generally soluble in organic solvents of t,he ketone, benzene, or toluene types and also in most chlorinated hydrocarbon solvents. The coatings usually air-dry rapidly when made with volatile solvents. However, for good resistance to water the coatings should be baked for' 15 minutes or longer a t a temperature of about 185' C. Some coatings formulated with chromium, iron, manganese, and thorium could be heated for hours a t about 190' C. without embrittlement. Others made with a.luminum, chromium, and zinc did not darlten appreciably on long heating. Kumerous combinations of t,hese mixed resins are possible by which t,he properties of t,he coatings, such as drying, hardness, water resistance, and toughness, may be varied. I n this way also coatings may be formu1ate.d that have good heat stability and that do not darken excessively when baked a t high temperatures. Pigmented coatings, prepared by using a blended pigment consisting of cobalt' blue and titanium dixoide, were an attractive light b!pe color, hard, tough, glossy, and water-resistant. When baked on iron articles, such as cigaret lighters and pencils, the coatings wore well while in daily use for a period of 8 months. Steel panels coated with the plain polylactyl lactate resin have withstood immersion in 6-ater for weeks without any blushing or softening. These polymers may be used as ext,enderswith other conipatible resins, such as ethylcellulose, vinyl chloride, substihted phenolformaldehyde resins, and melamine-formaldehyde resins. Ylinor percentages of the above resins generally do not cause precipit.ation when added to the polylactyl lactate polymers. hlelamineformaldehyde resin is especially useful for blending because the addition of about 20% of this resin shortens the baking time to less t,han 5 minutes a t 185O C. and improves the hardness.and thermosetting properties of t,he polylaetyl lactatc coatings. These resinous polymers may be mixed with drying oils to formulate fast air-drying and baking varnishes that are useful in coating wood and metal products. When polymerized to a high degree the resins may be coinminuted t,o give a powder which is easy to ship and also may be useful as a constituent in nioldiny compositions. Alone or modified with other resins, plasticizers, and fillers, they arc applicable in the treatment and sizing of paper and wood products and, as sticking agents in fungicides and insecticides, particularly as emulsions.
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
MISCELLANEOUS POLYLACTY LIC ACID RESlN COMBINATIONS
GLYcEnoL AND ALDEHYDE.Resins were prepared by reacting polylactylic acid with glycerol and furfural at a temperature of 150" to 170" C. The reaction may be accelerated by using temperatures of about 225" C. i n the final stages of polymerization; a greater yield is obtained by using at least 20% of furfural. The substitution of about 15% of crotonaldehyde for the furfural and addition of about 15% of glycerol in this formula, on curing, produced a hard, tough, and water-resistant coating with excellent adhesion. This type of resin may be made somewhat elastic by softening it first with ethyl alcohol.and then adding amyl phthalate. The patent desciibing this process (15) also gives examples of other resins made by the addition of dibutyl phthalate, acetic anhydride, and catalytic agents t o polylactylic acid t o formulate coatings which are resistant to water, alcohol, and weak acids and suitable for baking on metal. DISACCHARIDES. A process for making resins in which a disaccharide, such as lactose, reacts with polylactylic acid at temperatures ranging from 160" t o 170" C. has been described in a patent (14). Resins made by reacting furfural with the above ingredients and the use of various catalytic agents are also mentioned. These resins, 1%-hichare hard, infusible, and waterresistant, were formulated t o make coatings suitable for baking on metal surfaces. The reactive (OH) groups of the carbohydrate molecules facilitate the formation of a rigid, infusible molecular structure. CASTOROIL AND CnoToxALmHYm. A mixture was formed, containing 70% of polylactylic acid, 15% of crotonaldehyde, and 15y0of castor oil by weight, which on heating formed a homogeneous solution. This solution was heated for 3 days a t 140' C., and then for 7 days a t 160" C. in shallow, open vessels. The product was then a medium brown, thermoplastic, hard, tough resin. Several soft rubbery polymers also were made by plasticizing this type of resin with a combination of softening agents, such as amyl phthalat,e and alcohols. FVhen the above experiment was carried out by using 517, of polylactylic acid, 36% of crotonaldehyde, and 13oj, of castor oil, the product was a soft and rubbery polymer. The elastic properties were apparently due to the increased proportion of crotonaldehyde in the resin. CARBONATEESTERS.A process which comprises heating polylactylic acid with monomeric esters of carbonic acid, in amounts over 207& until interesterification is effected, produced thermoplastic, light-colored, water-resistant polymers of varying degrees of hardness, depending on the conditions of the reaction. Some of the products were tough and somewhat resistant to boiling water when tested as baked coatings. Esters, such as diethyl carbonate, ethyl chlorocarbonate, and butyl carbonate were used fn the process together with small amounts of ammonium carbonate or sodium succinate as catalysts. FRACTIONATION OF HIGH POLYMERS
Lactic acid condensations are bifunctional in character and, even when carried out with copolymers of a higher functionality, necessarily yield varying amounts of products of relatively low molecular weight unless polymerized to a completely insoluble and infusible state. Consequently these resins generally comprise highly polymerized molecular complexes associated with lower-molecular-weight polymers of a soft and soluble nature. There always will be present some unreacted functional groups because of the difficulty of forcing the condensation reaction t o completion. Then, too, the three-dimensional polymers, even after gelation occurs, consist of.two phases-the soluble sol and the insoluble gel-which constitute the infinite molecular complexes. Procedures for the fractionation of lactic acid resins have been described in a patent (16). The method consists essentially in
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adding to a sdution of the resin a sJit.ablc amount of prccipitant, sufficient to cause separation into two phases but not enough t o cause complete precipitation -of all the dissolved polymer. The precipitant should be a liquid which has only a limited solubility for the polymer, such as petroleum and terpene hydrocarbons. The soft soluble fraction of the resin may be expedit,iously separat,ed from the hard, more highly polymerized fraction, thereby increasing the hardness and insolubility of t,he latter and shortening the period of curing. The soft, soluble fraction may be utilized in adhesives and tackifiers or may be recycled in the polymerization process. I n the preparation of the polylactyl lactate resins, the polylactylic acid may not react completely with the metal, and a softer fraction, containing only a trace of met'al, may be separated by the fractionation procedure. The more highly polymerized fraction containing most of the metal in combination becomes insoluble on the addition of carbon tetrachloride or a light petroleum distillate and may be separated readily. The softer fraction &Is0 may be extracted from the resin by means of liquids in which only the low polymers-such as carbon tetrachloride and alcohols-are soluble. The highly polymerized fraction, usually comprising about Soy0or more of the resin, contains most of the combined metil and makes coatings that have improved water-resistant and drying properties. . It was found also that if a n excess of xylene was used as the diluent in the preparation of a n aluminum polylactyl lactate resin, a n insoluble fraction separated out during the reaction. This fraction could be separated by filtration from the soft soluble portion, which was low in metal content. Fractionation during the process could be effected also by use of a diluent cont d n i n g about 50% of carbon tetrachloride. The fractionation of high polymers has recently been reviewed by Cragg and Hammerschlag (Z), who state t h a t fract,ional precipitation is the most popular method. The extraction of alkyd resins has been described by Wright and DuPuis ( I @ , who are of the opinion that t'he process would be feasible on an industrial scale. LITERATURE CITED
(1) Beeei, S.,Iticcoboni, L., and Sullani, C., Mem. T e a k accad. Italia, Classe sci. fis., mat. e nat., 8, 127 (1937). (2) Cragg, L. H., and Hammerschlag, H., Chem. Reu., 39, 79 (1946). (3) Dietz, A. A., Degering, E. F., and Schopmeyer, H. H., IND. Exn. CHEM.,39, 82 (1947). (4) Filachione, E.M., and Fisher, C. H., Ibid., 36, 223 (1944). ( 5 ) Flory, J., ,Chem. Rev., 39, 137 (1946). (6) Hovey, A. G., and Hodgins, T . S., Paint, Oil Chem. Rev.,102, No, 2, 9, 37, 42 (1940). (7) Hovey, A . G., and Hodgins, T. S. (to Reichhold Chemicals, Inc.), U. S.Patent 2,197,723 (April 16, 1940). (8) Levine, I. E., and Lee, D. D. (to Standard Oil Co. of Calif.), U. S.Patent 2,207,626 (July 9, 1940). (9) Light, L., Paint Manuf., 10, 135 (1940). (10) Smith, L. T., and Claborn, H. \'., IXD. ENG. CHEM..NEWS ED., 17, 641 (1939). (11) Stearn, J. T., Makower, B., and Groggins, P. H., IND.ENG. CHEM.,32, 1335 (1940). (12) Teeters, W. 0. (to E. I. Du Pont de Nemours & Co.), U. S. Patent 2,362,511 (Nov. 14, 1944). (13) Watson, P. D., IND. ENG.CHEM.,32, 399 (1940). (14) Watson, P. D. (to people of the U. S.),U. S. Patent 2,144,352 (Jan. 17, 1939). (15) Watson, P. D. (to the people of the U. S.),U. S.Patent 2,174,491 (Sept. 26, 1939). (16) Watson, P. D. (to the people of the 6. S.),U. S . Patent 2,189,572 (Feb. 6, 1940). (17) Watson, P. D. (to the Secretary of Agriculture), U. S. Patent 2,363,103 (Nov. 21, 1944); Can. Patent 428,333 (June 26, 1945); (to the Secretary of Agriculture), U. S. Patent 2,433,721 (Deo. 30, 1947). (18) Wright, H. J., and DuPuis, R. N., IND.ENG.CHEM.,36, 1004 (1944). RECEIVED June 28, 1947. Presented before the Division of Paint, Varnish, and Plastics,Chemistry at the 112th Meeting of the AMERICAN CHEMICIL SOCIETY,New York, N. Y.
