I-VDUSTRIAL AiVD ENGINEERING CHEXISTRY
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darkening. Calcium oxide showed no color and no darkening. Ferrous sulfate formed a yellowish green mixture. Ferrous sulfide is black, but ferric sulfide is yellowish green. Ozone present around the light has probably oxidized the material. Mercuric chloride caused the mixture to turn black after exposure. The mixture was yellow before exposure. From these examples it seems that the sulfide of the metal is formed when possible under the conditions of the experiment. If this sulfide is dark, it increases darkening, if light, it may partly overcome the darkening due to metallic zinc. The color of the mixture depends on two factors, color of sulfide and quantity of metallic zinc present. Conclusions
From the results of these experiments, in which the darkening of lithopone has been driven as near as possible to its
349
ultimate end by means of intense ultra-violet light in the presence of moisture, the following conclusions might be inferred: The darkening of lithopone is due to the reduction to metallic zinc and probably free sulfur. The whitening is due t o oxidation of zinc to zinc oxide. The redarkening may be due in part to the influence of the free sulfur, which causes the zinc oxide to darken, and in part to the decomposition of more zinc sulfide. Metals present tend t o form the sulfide and darken in the case of dark-colored sulfides, but protect in the case of light-colored sulfides. Such darkening does not bleach out as does zinc darkening. Quantities of metal used in driers (0.5 per cent or less) are sufficient to cause sulfide darkening. The darkening with a metal present is probably due to both causes, the formation of metallic zinc and a dark-colored sulfide.
Alkyd Resins as Film-Forming Materials‘ R . H. Kienle and C. S. Ferguson GENERAL ELECTRIC COMPANY, SCHENECTADY, N. Y.
VERY material t h a t In discussing the use of alkyd resins as film-forming of the film-forming possibilimaterials, differentiation is made between three general ties of these resins. They approduces a transparent, homogelleous film types, namely, heat-non-convertible, heat-convertible, p a r e n t l y were content to and oxygen-convertible. The preparation of satisfactory study the reaction as such and upon e\Taporation of the solvents from its solution sooner solutions of the last two types and the properties of the were satisfied when they got or later comes under investiresultant films are described. It is shown that the a resin. oxygen-convertible resins bear serious consideration as D u r i n g 1911-1915 Callagation as to its applicability in the paint, v a r n i s h , a n d film-forming materials as they possess both the quickhan7 a t the Pittsfield works drying characteristics of nitrocellulose lacquers and the l a b o r a t o r y , together with l a c q u e r industries. If the film-building properties of oil-base varnishes. A r s e m , * D a w ~ o n , a~n d material is a resin 01: possesses Howelllo a t the Schenectady resinous characteristics, it is customary to examine it as a film-forming material (1) when research laboratory of the General Electric Company, carried used alone; ( 2 ) in conjunction with other resins or gums; (3) out an extensive investigation into the glycerol-phthalic blended with drying oils; (4)as an ingredient in nitrocellu- anhydride reaction, and as a result new and useful resins were lose lacquers. The alkyd resins are no exception and are made. They became particularly interested in the resins bebeing examined from all these angles. However, a t this time cause of their heat irreversibility. they will be discussed only from the standpoint of their filmCallahan devoted most of his attention to the glycerolforming ability when used by themselves. phthalic anhydride reaction and its possible application. Alkyd resins include all those complexes resulting primarily hrsem and his co-workers studied the alkyd reaction as a from the inter-reaction of a polyhydric alcohol and a poly- whole and the preparation of numerous other resins based basic acid, and the term has therefore a definite scientific on this reaction-i. e., they replaced the phthalic anhydride meaning. in whole or in part with other polybasic acids and in part with some monobasic acids. In addition they studied flexiHistorical bilization. working chieflv with castor oil as the flexibilizine The formation of alkyd resins has long been known. Ber- agent. They ascertained many characteristics of the resins and pointed out the possibility of using them as film-forming zelius reported a resin from tartaric acid and glycerol. Van materials. They found the resins to be extraordinarily good Bemmelnz probably did the first systematic work. He prepared resins from succinic acid, from citric acid, from a mix- stickers and, working with solutions of the resins, they obture of benzoic and succinic acid, by heating with glycerol. tained very adherent, tough, varnish-like films on metals if Other early investigators were D e b ~ sL, o~~ r e n c oand , ~ Funaro these films were properly baked. They only worked with and Danesi.j Smith6 was the first to prepare a resin from a few simple solvents, such as acetone, alcohol-benzene, and glycerol and phthalic anhydride. His product, however, on coal-tar oil-alcohol. With these solvents films of poor bodyheating puffed up into a brittle, vesicular mass, which was ing characteristics and with decided tendencies to pull up, owing to high surface-tension effects, were the best they obuseless from an industrial standpoint. tained. In only one case did they obtain a satisfactory reNone of these investigators seem to have been cognizant sult. Dawson, working with alcohol-benzene solutions of a Presented before the Division of Paint and Varnish Chemistry at glycerol-phthalic anhydride-oleic acid resin, was able to the 76th Meeting of the American Chemical Society, Swampscott, Mass., obtain smooth, tough, adherent films on metals, but the filmSeptember 10 to 14, 1928. building properties of this varnish were poor. Neverthe2 Van Bemmeln, J . prakt. Chem., 69, 84 (1856).
E
Y
1
Mag., 4, 1 6 ; Jahrb. for!. Chem.. 1856, 431. Lourenco, A n n . chim phys., 8, 63, 313 (1883). 5 Funaro and Danesi, Jahrb. for!. Chem., 1880, 799. ‘ S m i t h , J . SOC.Chem. Znd., 20, 1073 (1901). a Debus, P h d .
4
Callahan, U. S. Patent 1,108,329 (1914), et al. SArsem, U. S. Patent (1914). Dawson, U. S. Patent 1,141,944 (1915). lo Howell, U. S. Patent 1,098,728 (1914). 7
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less, it found commercial application as a coating for oxide film lightning-arrester plates, as it gives exactly the necessary adhesiveness, toughness, and electrical properties required in this application, something not obtained with the other varnishes tested. Recent Developments During the war the introduction of the Gibbs processll for the manufacture of phthalic anhydride by catalytic oxidation of the vapors of naphthalene resulted in cheap phthalic I
1
hard, with a tendency to he brittle in their early stages. They generally have a high acid value (85-130) and only moderate degrees of esterification (75-80 per cent). On heat-converting they invariably become very tough, strong, and hard. The polyhydric alcohol used in the heat-convertible resins can be replaced in part by a mono- or di-alcohol, the polybasic acid with a monobasic acid, or both. As long as the replacement is not complete, the resulting resin retains its heat-convertibility. The presence of the lower valent alcohol or acid in the resin functions (1) to retard the time of gelation in preparation; (2) to reduce the effect of temperature on the reaction; (3) to decrease the acid value and to increase the degree of esterification. Data illustrating these points are given in Table I. Table I-Comparison
of Reaction Data of Various Convertible Alkyd Resins
Temperature of preparation Gel time Acid value Esterification
anhydride. Following this, nitrocellulose lacquers were developed, which led to the commercial availability of many types of new solvents. These two developments awakened a new interest in the alkyd resins as film-forming materials. Colloid-chemical studies on the alkyd resins led to certain other possibilities in the alkyd resins. A principle of flexibilization was uncovered and two new general types of alkyd resins were introduced. This work was discussed in some detail by one of the writers a t the conference on synthetic resins a t the A. C. R. Institute of Chemistry at Evanston, Ill., in 1928.
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GLYCERYL WITH 1870 PHTHALATE GLYCOL 243' C. 243' C. 25 min. 30 min. 118 60 77.770 84.3%
WITH30% FATTYACID 243" C. 155 min. 40 86.8%
OXYGEN-COWERTIBLE REsINs-These resins are prepared by replacing part of the polybasic acid with the necessary amount of an oxidizable, unsaturated, fatty acid or acidsfor example, linoleic, linolenic, oleostearic acid, or, what is more economical, with the mixed fatty acids of any one of the drying oils. These alkyd resins are usually pale yellow to brown, and have acid values from 4 to 60, depending upon their composition and method of manufacture, and degrees of esterification ranging from 85 to 99 per cent. They are softer, more rubbery than the heat-convertible resins in the earlier stages, and attain greater toughness and elasticity upon gel conversion. All three types of alkyd resins can be plasticized by addition of inert, low vapor-pressure substances, as is done with nitrocellulose in the nitrocellulose lacquers.
Types of Alkyd Resins The earlier alkyd resins were all of the heat-convertible type; that is, on heating they were converted from a readily fusible condition to a non-fusible condition, with lyophilic gellike properties. It has also been found possible to prepare both heat-nonconvertible and oxygen-convertible alkyd resins. The oxygen-convertible resins are particularly interesting to the paint and varnish chemist as in them is synthesized a base which simulates, not only physically and mechanically, but chemically as well, the natural-resin, drying-oil blends of the varnish kettle. These resins on addition of oxygen are converted into gels analogous to drying oils. Preparation and General Properties HEAT-NON-CONVERTIBLE REsms-These resins are prepared by the inter-reaction of dihydric alcohols with polybasic acids. Examples are the resins prepared from ethylene glycol, diethylene glycol, or propylene glycol and phthalic anhydride. I n general, these resins have low acid values, can be easily carried to a high degree of esterification ( a p proaching 100 per cent), and are obtainable either hard and brittle like ordinary resin or soft and gummy like a balsam. HEAT-CONVERTIBLE RESINS-Resins of this type are prepared essentially by the inter-reaction of the higher polyhydric alcohols (tri or above) with polybasic acids. The best known example is glyceryl phthalate. These resins are quite 1'
Gibbs, J. IND. END. CHEM.,11, 1031 (1919).
As the maximum physical and mechanical properties of film-forming materials seem to develop upon a lyophilic solgel transition, but little work has been done with the heatnon-convertible alkyd resins, although adhesive transparent films can be formed. Types of Resin Solutions Two types of resin solutions can be prepared using the alkyd resins as a base-namely, baking and air-drying. BAKINGSoLuTroNs-The baking type is prepared from the heat-convertible alkyd resins as the base; the air-drying type,
from the oxygen-convertible alkyd resins. Films made from the former definitely require heating to develop their gel structure which gives them their maximum properties. On the other hand, films made from the air-drying type require only exposure to the air; heat simply assists in the gel conversion, its action being the same as in the case of oil base varnishes. Typical baking curves for these two types of resin solutions are given in Figures 1 and 2. The data for both these curves were obtained by determining the time a t several temperatures necessary to develop a tack-free, non-flowing, rubbery condition. T o insure uniformity in the films a standard sheet of aluminum and a dipping machine were used. It is apparent on comparing the two curves how the introduction of oxygen-convertibility in alkyd resins speeds up the gel conversion a t any one temperature, oxygen linkage being superimposed on natural linkage. Solvents and Solutlons. The proper choice of solvents is very important to obtain the best film-building properties; thus, the early work with the baking solutions invariably gave solutions of poor film-building characteristics. It has only been by careful investigation of a large number of solvents that this has been overcome. I n general, the heat-convertible alkyd resins, used in the preparation of this type of solution, are soluble in polar solvents. I n this respect the resins are similar to nitrocellulose. Some of the solvents applicable to the preparation of baking solutions are as follows: Acetone Acetone oil Alcohol-coal-tar naphthas Butyl acetate-butyl alcohol Glycol ethers Monoethyl ethylene glycol Monobutyl ethylene glycol
Carbitol Butyl carbitol Glycol diacetate Benzyl acetate Ethyl acetate Phthalate esters
Table 11-Exposure AGENT Gasoline Turpentine Benzene Kerosene 5'7 NaOH "01 5 $ NaCl Water Steam
59
Tests of Films from Baking Solution and Baked Oil-Base Varnish ALKYDBAKINGSOLUTION OIL-BASEVARNISH Soft Peels Soft Peels Disintegrated Peels 0. K. 0. K. Cracked badly
AIR-DRYING SoLuTroNs-Solvent a d Solutions. The solubility characteristics of the oxygen-convertible alkyd resins used in the preparation of the air-drying resin solutions are essentially the same as those described for the baking solutions, except that considerable dilution with aliphatic solvents-e. g., mineral spirits-can be effected. However, care must be exercised in diluting with such solvents as mineral spirits because they tend t o increase the viscosity. As some of the air-drying properties of these solutions depend upon the solvents, solvent combinations with a suitable rate of volatility must be chosen. An excellent solvent combination is the following: Parts b y =,eight Petroleum naphtha Coal-tar naphtha Butanol
4 4
1
The coal-tar naphtha and butanol constitute the true solvent portion, the petroleum naphtha, the diluent. The presence of an alcohol, such as butanol, is particularly effective in reducing the viscosity of the solution. The film-building properties of the air-drying solutions are particularly good. The following data compare an alloil insulating varnish with an air-drying resin solution a t the same viscosity: VISCOSITY
Hot cutting with these solvents is preferable, although solutions can be made in the cold. Proper choice of the resin is important in order to minimize a tendency in this type of solution to give a high surface tension during the baking process, which sometimes causes the film to pull on itself. I n preparing a suitable baking solution it is advisable to use solvents of graded boiling points, the least volatile solvent preferably being a true solvent. The following is given as representative of :t typical alkyd resin baking solution: Glyceryl oleic phthalate resin Glyceryl phthalate resin Ethyl lactate Butyl acetate
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Parts b-y weight Parts by weight 10 Butyl alcohol 60 100 Alcohol 130 30 Benzene 130
40
Pigmentation. Some discretion must be used in pigmenting these solutions, particularly when basic pigments and carbon blacks are incorporated. Excellent, clear colored films may be prepared by using heat-resisting dyes or by baking a t low temperatures with ordinary dyes. Film Charackristics. The films prepared from baking solutions are invariably very adherent, hard, and tough. P r o p erly baked, a very good gloss results. The films can be made exceedingly flexible for their hardness. Most of the baking solutions change but little in color on baking, hence true colors can be produced and easily duplicated. I n fact, it has been possible to prepare a white which remained white even though the baking temperature was 200" c. The films are resistant to most chemical reagents, being particularly inert to mineral oils. Table I1 summarizes a series of 96-hour exposure tests made to compare the resistance to various reagents of films from a typical baking solution and a baked oil-base varnish.
CP. Standard oil varnish Air-drying resin solution
45 45
TOTALSOLIDS Per cent 18-22
35
Pigmentation. There is considerably less difficulty in the pigmentation of the air-drying solutions than when baking solutions are used. Again light tints and even whites can be made. However, baking cannot be carried to the same extent as with the baking solutions; on the other hand, as the baking curves in Figures 1 and 2 show, this is not necessary. Film Characteristics. Films from air-drying resin solutions are very adhesive and smooth. They generally have a quick initial set-up and become quite hard, more so than oil films but probably not quite so hard as lacquer. They are flexible and usually glossy. They are very resistant to mineral oils. The solutions can be brushed, sprayed, or dipped, smooth films resulting in all cases. They can be applied on cloth, paper, wood, or metal. Table 111-Comparison
PAINT Good film-building
of Paint Nitrocellulose Lacquer, and AirDrying Resin Films LACQUER AIR-DRYINGRESIN Very poor film-building Good film-building
Fair adhesive properties Poor adhesive properties
Outstanding adhesive properties
Surface not very hard
Surface hard: resists abrasion
Harder than paint; not so hard as lacquer
Does not resist oil, solvent, alkali or steam
Resists oil well, but-peels easily from surface, especially with steam
Resists a s well as lacquer; does not peel
Dries slowly
Dries fast
Dries fast
Good whites can be obtained
Excellent whites can be made
As good as paint a t present
For more than a year exposure tests have been in progress upon certain of these air-dried pigmented films, using standard iron test plates, exposed 45 degrees to the south a t
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Schenectady. To date the films are in excellent condition. Further tests are in progress, iticluding a number upon wood.
A comparison of the characteristics of paint, nitrocellrtlose lacquer, and air-drying resin films is giveii in Tahle 111. Conclusion
The film-forming cliaracteristics of tlie alkyd resins are important to tlie paint and varnish chemist. Three types of alkyd resins can be. prepared differing in their conversion to a non-fusible, jelly-like state, namely(1) heat-noli-convertible, (2) heaGeonvertible, and (3) oxygenconvertible. Each produces resin solutiolis that yield films with singular
7-01. 21, No. 4
characterist,ics. In gemral, we can divide these solutions into (1) baking, and (2) air-drying. The former require heat to develop their inaximuni properties, outstanding of which are touglmess, adhesiveness, flexibility, oil resistance. The latter require primarily reaction with oxygen, although lieat can also be used, in which case its function is essentially to speed up tlie oxygeti reaction. These air-drying films possess t.lie same outstanding properties as the films from the baking solutions, t,ogcther with an additional pronounced film-building characteristic. The air-drying solutions can in a sense be considered as truly synthetic varnishes, as they simulate, not only physipally and ineclianieally, but chernically as well, the Mends from the old-time varnisli kettle.
Corrosion-Note on an Apparent Relation of Protective Film to Microstructure'
T HAS been pointed out2 that the concept of protective film format,ionshould play an important part in the d e velopment of corrosion-resistait alloys. This paper Dresents a case illustrative of one possible aspect of such a11 application. The case wax oiie of corrosion in a small gear piiinp used to force a sodium xmtliate solution of cellrrlose ilito the spinnerets of a rayon mill. The gears were of rhrome-manganesc steel with 0.2 per cent carhoii and rotated between plates of cast iron having the following analysis: silicon 2.20, sulfur 0.10, manganese 0.50, phosphorus 0.55,total rarhon 3.50 per cent. With certain import a n t exceptions, all parts of the apparat.iis cirming into cont,art with the solution showed a tliin but dense and a d h e r e n t eonting of ferrous sulfide, which clearly had r o n s t i t u t , e d an efficient protective film a g a i n s t progressive corrosion. The cxceptions were t h o s e portions of t,he cast,iron sideplat,es a d jacent to the periphe r i e s oF tlie g e a r s . These showed strong corrosion, the iron lieing roughened. The site of this action is also the site of the greatest relabive movement b e twe eii these parts, and the
I
I Received December 21. 1928.
whit
m o . . a,
419
a " , Chern.
(iwmi.
Figure I-Segment of Cast-Iron Side Plate Showin8StrongCorraalon Adjacent to Perloher) of Revolving Gear. 3 x
logical conclusion is that the protective sulfide film had liere bcen continoously slicarcd or rubbed off, the rather sharp dividing line between tlie pit,ted circumferential zone and the inner relatively unaffectcd portion indicat,ing the point at which the rate of shear became greater tlian tlie film could resist. (Figure 1) Especially notable is tlie fact tliat the opposing faces of the germ tliemselves showed an effective film and no corrosion. The same was true (if the periphery of the gears and t,he upposing surfaces of the housing, between which the clearance v a s extremely close. Unlike the side plates, however, these parts were made of high-grade steel as indicated aboi-e. The microscope revealed a t once that the roughened or pitted apyearance here was due to attack along the grain boundaries, leaving the grains tliernselves in strong relief. (FiLqre 2) This clearly suggests that the segregation of impurities along grain boundaries characteristic of cast iron was i n some way primarily responsible for the destruction of the protectivc film, prubably hg causing coincident lines of weakness in the film and thus Facilitating rnecliariical (lisruptioii. On this hypot,hesis s e g r e g a t i o n along grain boundaries may play a vital part in the resistance of alloys in general to combined riirrosive a n d mechanical attack, and may aceoitnt fur the effect of r e l a t i v e l y minute percent,agesof c o m p o n e n t s . The e f f e c t m a y be coiis t r i i c t i v e q u i t e :is easily as destructive, FWure 2-Corraded Area in Figure 1 s m ~ ethe segregated Enlarged, Showing Aireck along Grain matter can be thought ~ ~ ~ ~ d ~ r i ~ of as forming a resistant as well as a iion-resirtant deposit against chemical attack.
