Acknowledgment
The authors express their thanks to H. G. Schutze for his vigorous support and encouragement, M. M . Nicholson for development of the polarographic analysis of DLTDP, A. T. LVatson for the S , method of rating thermal stability, R. T . Moravek for the intrinsic viscosity molecular weight determinations, and the many others a t Humble Research and Development who aided this investigation. The authors are indebted to the Humble Oil 5: Refining Company for permission to publish these results.
literature Cited
(1) Barnard, D., Bateman, L., Cain, ?vi. E., Colclough, T., Cunneen, J. I., J.Chem. Soc. 1961, 5339.
(2) Barnard, D., Bateman, L., Cole, E. R., Cunneen, J. I., Chem. & Znd. (London) 1958,918. (3) Bateman, L., Hargrave, K., Proc. Roy. Sot. (London) A224, 389, 399 (1954). (4) Campbell, T. I V , , Coppinger, G. M., J . A m . Chem. Sot. 74, 1469 (1952). (5) Colkough, T., Cunneen, J. I., Chem. & Znd. (London) 1960, 626. (6) Hargrave, K., Proc. Roy. Soc. (London) A235, 55 (1956). (7) Hawkins, Tt’. L., Lanza. V. L., Loeffler, B. B., Matreyek, Tj-., T%.inslow,F. M., J . Appl. Polymer Scz. 1, 43 (1959). (8) Ingold, K. U., Chem. Rec. 61, 563 (1961). (9) Kharasch, N. L., Potempa, S. J.. Wehrmeister, H. L., Zbzd., 39,269 (1946). (10) Kingsbury, C. A , , Cram, D. J.. J . A m . Chem. Soc. 82, 1810 (1960). RECEIVED for re\iew October 4, 1962 ACCEPTED October 22, 1962 Symposium on Stabilization of Polcmers, Division of Organic Coatings and Plastics Chemistry, 142nd Meeting, ACS, .4tlantic City, N. J., September, 1962.
R E L A T I O N S H I P BETWEEN ULTRAVIOLET ABSORBER S T R U C T U R A L TYPES A N D PHOTOSTABILIZATION OF PLASTICS A.
F. S T R O B E L AND S. C. C A T l N O
Antara Chemicals Division, General Aniline and Film Carp., iVew York 74,N . I:
Lightfastness of ultraviolet-absorbing compounds i s determined qualitatively b y visibly examining thin plastic films containing ultraviolet absorber over brightener-dyed cloth using Hg blacklight, before and after Fade-Ometer exposure. The fluorescence photometer makes the technique more quantitative. The lightfastness of ultraviolet absorbers depends on compatibility with the substrate, long hydrocarbon substituents giving increased stability in polyolefins. A correlation i s established between yellowing on light exposure of cellulose nitrate films containing a series of absorbers and the reaction product of absorber with nitrous acid. The yellowing of polyester-styrene containing various absorbers i s independent of absorber reactions. Metallic driers used in oleoresinous varnishes form colored complexes with phenolic absorbers, while acrylonitrile absorbers do not react with metallic driers.
HE DEGRADATION of plastic materials by ultraviolet light is Twell established (7-3, 8, 9). Almost equally well known is the fact that commercial ultraviolet light absorbers are effective in many cases in stabilizing polymers against degradation by ultraviolet light, but in other instances absorbers are of little value (7, 72, 75). The absorber selected must have strong absorption in the wavelength region to which the polymer is most sensitive. For terrestrial solar irradiation polyethylene is most sensitive to wavelength of 300 m,u, polypropylene to wavelength of 370 mp. Cellulose nitrate is decomposed primarily by radiation of 310 mp a t sea level, while polyester-styrene is yellowed primarily by radiation of 325 mp (5, 6 ) . The absorber must also remain in the plastic, and preferably it should not undergo reaction with the plastic. Performance in these two respects varies over a wide range, and may account for great differences in the success of ultraviolet absorber applications. Less permanent absorbers will suffice for less stable plastics. Thus polypropylene, cellulose nitrate, poly(viny1 chloride),
and oleoresinous varnishes are very sensitive to ultraviolet light and their permanence is improved by absorbers of only moderate stability. O n the other hand, acrylic polymers, polyethylenes, and polyesters are much less sensitive than the first group of polymers, and absorbers of great durability are required to improve their permanence. Ultraviolet absorbers act by a screening process, absorbing the harmful ultraviolet radiation and dissipating the energy as heat. The protective screening action of a n ultraviolet absorber continues until the absorber loses its effectiveness by any or all of four means: photochemical decomposition, exudation and/or sublimation, chemical reaction with materials in the substrate, or removal of absorber in the cleaning operation. Photochemical Decomposition
Many types of organic compounds absorb ultraviolet light strongly, including stilbenes, phenylbenzotriazoles, benzimidazoles, coumarins, and azo and azoxy compounds. However, nearly all compounds which absorb ultraviolet light strongly VOL. 1
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Incorporation of Ultraviolet Light Absorber in Polyethylene. Fifty grams of DYNH-1 (Bakelite) was allowed to form a band on a two-roller mill (preheated, slow roller a t 180" F., fast roller a t 220' F.). The absorber was sprinkled onto the band and the mixture was milled for 10 minutes. The polyethylene band was removed from the rollers, using a knife, and then added again to form a new band. This process of removal and reformation of the band was carried out 20 times. A portion of the band was pressed out in a Carver press between polished stainless steel plates preheated to 250' F. a t 3000-pound pressure. The plates were removed, cooled in ice water, and taken apart to give a polyethylene film 0.015 inch thick. Exposure tests on strips of polyethylene films containing ultraviolet absorbers were carried out in the Atlas Fadeometer. Application of Ultraviolet Light Absorber a n d Cyanoguanidine-Formaldehyde Polymer on Cotton. The 1% dyeings (based on weight of fabric) of both absorber and polymer were made as follows: In a stainless steel dye can were placed 10 ml. of absorber solution (0.5% in acetone), 10 ml. of cyanoguanidine-formaldehyde polymer solution (0.5% in water) 80 ml. of water, and 5 grams of Indianhead cotton fabric. The dye can was shaken for 30 minutes a t 200 O F. in the Atlas Launderometer. The cotton swatch was rinsed with water and dried. The cotton fabric was stapled to cardboard, partially covered, and then exposed to ultraviolet radiation in the Atlas Fade-Ometer. Ultraviolet light reflectance measurements were made on the fabric before and after exposure in the Atlas Fade-Ometer, using the fluorescence photometer (Figure 1). In this case the lower the reflectance reading, the greater the amount of ultraviolet light absorbed by the cotton fabric dyed with absorber and polymer. Incorporation of Ultraviolet Light Absorber in Cellulose Nitrate. A solution of 0.2 gram of absorber in 25 ml. of resin solvent No. 3 (composed of 40 parts by volume of methanol, 27 of ethanol, 18 of toluene, and 15 of ethyl acetate) was mixed with 40 grams of nitrocellulose lacquer [composed of 67.5 parts by weight of half-second nitrocellulose, 19.0 of Glyptal 2477 solution (General Electric Co.), 7.7 of dibutyl phthalate, and 5.8 of 1-butanol]. A wet film 0.003 inch thick was formed on plate glass, using the Bird film applicator. After drying for a n hour the film (0.0015 inch thick) was carefully peeled off the glass plate and stapled to a sheet of paper. Pieces (2 X 6 inches) of the nitrocellulose films stapled to white paper were fastened uniformly to a circular Masonite disk 18 inches in diameter, and a portion of each film was covered with cardboard. The disk was placed on a 33r.p.m. turntable and the films were exposed to radiation from a 275 WRS sunlamp mounted 17 inches above the disk. Incorporation of Ultraviolet Absorber i n PolyesterStyrene Copolymer. A mixture of 50 grams of Polylite 8000 (Reichhold Chemica!s), 0.25 gram of benzoyl peroxide (Luperco ATC: Lucidol Division, Wallace and Tiernan, Inc.), 2.5 grams of dioctyl phthalate, and 125 mg. (0.25%) of absorber was rolled in a stoppered glass jar until a clear solution was obtained. A portion of the solution was poured into a preheated (250' F.) circular mold precoated with silicone oil, covered with a polished stainless steel plate, and subjected to 5000-p.s.i. pressure in the Carver press. The temperature ivas allowed to drop to 225' F. and then raised to 235' F . At this point the mold and plate were quickly removed from the press and separated to give a disk of polyester containing the absorber. which was 0.025 inch thick and 2 inches in diameter. The disk was mounted on a card, covering half of it: and then exposed in the Atlas Fade-Ometer. Incorporation of Ultraviolet Absorbers in Varnishes. To a mixture of 1 pound of Bakelite RR-9432 (phenol-fcrmaldehyde) 0.25 gallon of tung oil and 0.30 gallon of mineral spirits is added lY0 of total weight of ultraviolet absorber. T o a mixture of 1 pound of Teglac Resin 164 (alkyd), 0.1 gallon of soybean oil, body X? 0.2 gallon of dehydrated castor oil. and 0.4 gallon of mineral spirits is added lyOof total weight of ultraviolet absorber. ~
Figure
1.
Fluorescence photometer
OG. Carrara glass reference plate PG. Antireflection coated borosilicate glass cover plate FI. Plus UV Corning 5 8 4 0 7 - 6 0 filter F2. Plus UV Corning 5 8 6 0 7 - 3 7 filter F3. Minus UV Corning 3 3 8 9 3 - 7 3 filter, 2 used PC. Photocell Fl. GE mercury vapor lamp, 6-watt blacklight
are photochemically decomposed by it in a few hours of exposure in the Atlas Fade-Ometer. There are three classes of light-fast ultraviolet absorbing compounds, all offered commercially :
& 8
2-hydroxylbenzoyl derivatives where R = phenoxy or pheny and the benzene rings may be further substituted ;
HO
2-hydroxyphenylbenzotriazoles, Ivhere R RI CS
=
alkyl ;
and
\ / c=c / \
Rz R3 a nonphenolic class of substituted acr) lonitriles \rhere R,. R:! = alkyl or aryl and R3 = a n electronegative substituent. I n many applications even these three classes arc not permanent to light. Experimental Methods of Specimen Preparation
Incorporation of Ultraviolet Light Absorber in Cellulose Acetate. A solution of 0.188 gram of absorber (5% based on the dry weight of cellulose acetate) in 27 ml. of thinner solution (composed of 140 parts by weight of ethanol, 300 of methyl Cellosolve, 400 of ethyl acetate. and 42 of triphenyl phosphate) was mixed Lvith 25 grams of cellulose acetate dope (15% cellulose acetate in acetone). .4 wet film 0.005 inch thick was cast on a clean dry plate glass with the Bird film applicator. The dry film (0.001 inch thick) was stripped from the p1at.e glass and stapled to a sheet of white paper. A portion of the film was exposed to radiation in the Atlas FadeOmeter. 242
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
Preliminary Screening of Ultraviolet-Absorbing Compounds
A 1-mil cellulose acetate sheet containing 5% of absorber is placed: partially exposed and partially covered, in the FadeOmeter for 20 hours. The exposed area is then compared
with the unexposed area for color development. If color has developed, the compound is rejected as a n ultraviolet absorber. If the exposed area has not developed color, a piece of white cotton cloth dyed a t 0.1% owf (on weight of fabric) with a fluorescent brightening agent-say, Blancophor HS-76-is placed behind the entire acetate film and the film is examined under the “black light” of a H g arc. Most compounds lose their ability to absorb ultraviolet light after 10 Fade-Ometer hours; hence in such cases the acetate film becomes brighter in the area exposed in the Fade-Ometer. The fluorescence of the brightener on the cotton in back is excited by the ultraviolet light, which can now penetrate this film area, and the fluorescent light is transmitted back through the film; making it brighter in this area. If the ultraviolet absorber in the film is fast to light, the film will appear uniformly opaque in the exposed and unexposed areas. T h e exposed and unexposed areas will show no line of break for a good lightfast absorber. This technique for rough screening visually can be made more quantitative by use of the fluorescent photometer illustrated schematically in Figure l . This instrument can be used to measure ultraviolet transmittance through a transparent film, or ultraviolet reflectance from a n opaque material.
F1 is a fixed plus ultraviolet filter-i.e., it allows only ultraviolet light to pass through. F:! and F3 are movable filters, F? being similar to F1, and F3 being a minus ultraviolet is, one which absorbs the ultraviolet light and filter-that transmits visible light. With OG pressing down on PG and using Fz,the milliammeter is set a t 100-i.e., all of the ultraviolet light emitted by the two mercury vapor lamps: FL. is allowed to reach the photocell. Next, using Fa in place of F?, the milliammeter will read zero, since F3 absorbs all of the ultraviolet light from the lamps. T o measure the transmittance of ultraviolet light through a plastic film containing a n ultraviolet light absorber, the film containing absorber is placed on PG. O n top of the film is placed a piece of cotton fabric or paper dyed with a fluorescent brightening agent such as Blancophor HS-76. Then OG is pressed down on both the brightened substrate and the film. The ultraviolet light from sources FL which is not screened out by the ultraviolet absorber in the film reaches the brightened cotton or paper and excites the fluorescent brightening agent. T h e visible light emitted by the brightener passes through Fz and strikes the photocell, producing a current which is read as milliamperes on the meter, designated B in the tables. Ultraviolet reflectance is measured by placing a piece of cotton which has been dyed with a n ultraviolet light absorber between plates OG and PG, and using Ff. The absorption of some of the ultraviolet from sources FL by the ultraviolet absorber on the fabric will cause the milliammeter to read lev than 100. designated R in the tables.
Table 1.
Loss of Absorber on Fade-Ometer Exposure, AB
Polyethylene, 85-Hr. Ex@. (0.0002Mole %)
Absorber T
4 - 1 3 ._
i1I IV
1-5 3-7
Photochemical degradation depends on the substrate as well as on the absorber. Thus in Table I , 4.4’dimethoxy-2,2’dihydroxybenzophenone (I), which did not change on light exposure in cellulose acetate, at 0.057,in polyethylene shows poorer lightfastness than in cellulose acetate. The dilauryl ether homolog of the same basic structure (I\-)
?H 0 H?
changes considerably less in polyethylene after 85 hours’ Fade-Ometer exposure. The long hydrocarbon chains built into the absorber have increased the lightfastness in polyethylene over that of the methyl homolog. The dibutyl derivative (111)
7
-r4Ha HO
XI\'
both strong ultraviolet absorbers (but fluorescent), brought about extremely intense development of brown-yellow color in polyester-styrene. .Also benzo) 1 thiophene (X\?
Ivhich does not absorb ultraviolet from 300 to 400 mp appreciably, resulted in extremely brown-yelloir color development, again being a sensitizer. So far the only method of preventing color development is through screening out ultraviolet light from 300 to 360 mp by a nonsensitizing absorber, the best of \vhich is the 2-hydroxyphenylbenzotriazole. The lead, cobalt, and manganese driers used to accelerate air oxidation of oleoresinous varnishes form complexes with the phenolic-type absorbers, but do not react with the nonphenolic substituted acrylonitrile absorbers. Formation of lead and cobalt metal complexes retards the drying time of these varnishes. Often by increasing the proportion of metal driers empirically one can formulate a satisfactory system Xvith the phenolic absorbers. Figure 10 sho\vs the absorption curve of the cobalt complex of 2-hydroxy-4-methoxybenzophenone compared with that of the free absorber (VI). The VOL. 1
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247
Table VII.
technique of achieving permanence to dry cleaning with absorbers is through completely copolymerizing a reactive absorber with the plastic monomer. Two compounds which have been suggested for this purpose are X V I I and X V I I I
Fastness to Dry Cleaning of
Fabric Nylon 200 Spun nylon No. 758 Dacron 54 Dacron 62 Dacron 64
0.12% owf Change in %
Rej%?ctance
48 + 53% 47 + 53% 46 + 51 7o 50 + 59% 44+ 46y0
Dry Cleaning Fastness of Absorbers on Nylon 66
Table VIII.
Absorber VI (1% owf) XVIII (1.2% owf)
AR
46 + 51 42 + 42
considerable color increase of the metal complex is shown by higher absorption in the 400-nip region. Figure 11 shows in similar fashion the greater color of the cobalt complex of 4,4‘-dimethoxy-2~2’-dihydroxybenzophenone compared with the free absorber (I). Five absorbers (111, V, VI: V I I , and X V I ) were compared in varnishes. Absorber X V I is a substituted acrylonitrile R1 CN with maximum absorption a t 325 mp. The
\ / c-c
/ R2
\
R3
stabilizing power of the metal complex of the absorber may still be very good. Thus, in a tung oil-phenolic varnish, which is amber in color, 4,4‘-dibutoxy-2,2’-dihydroxybenzophenone (111) was found superior to the other absorbers in protecting against bleaching in the light. I n a long-oil soybean-alkyd, which is a n almost colorless varnish, 1% of the substituted acrylonitrile absorber with maximum absorption a t 325 mp (XVI) was the most effective of the five absorbers in retaining the color of the varnish in the presence of light. Precipitation of phenolic absorbers by the metal drier, particularly lead, is often a complicating factor in oleoresinous systems. Removal of Absorbers in Dry Cleaning
Fastness to dry cleaning is important where absorbers are used with polymers in fiber form. The ultraviolet absorbers for plastics which have been discussed are water-insoluble, and essentially none of the absorber is removed from the synthetic fiber in normal washing operations. I n dry cleaning-for instance, with perchloroethylene-the fastness of these absorbers is distinctly less than in washing. In Table VI1 are given the ultraviolet reflectance measurements of a series of fabrics dyed with 4,4’-dirnethoxy-2,2’-dihydroxybenzophenone (I), before and after dry cleaning with perchloroethylene according to the AATCC test. The best performance is on Dacron 64. (On calibration, the Nylon 200 results correspond to a loss of 20% of absorber.) One
248
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
(4, 70). I t is not easy to copolymerize such compounds completely, a great deal of absorber usually remaining unpolymerized. Other complications arise in polymerization, because the absorber affects the polymer chain length. Another technique for improving fastness of absorber to dry cleaning involves applying a n absorber containing free carboxylic groups-e.g., the carboxymethyl ether of 2,4dihydroxybenzophenone (XVII1)-to a fiber, say nylon, from an acidic bath. After application to the fiber, the material is alkalized to convert the absorber to the carboxylate form, COONa. The latter form of the absorber, being much less soluble in dry cleaning solvents, is more resistant to dry cleaning. One difficulty here lies in converting the absorber to the salt form without removing it from the fiber. Table VI11 shows improvement of absorber X V I I I over absorber VI in dry cleaning fastness. Conclusion
An ultraviolet absorber can be expected to give satisfactory protection only if it is very compatible with the substrate, undergoes no chemical reaction with it? and is not removed by photochemical or physical means. Literature Cited
(1) Biggs, B. s., Natl. Bur. Std. ( U . S.) Circ. 525, 137-48 (1953). (2) Biggs, B. S.,Hawkins, W. L., M o d . Plastics 31, No. 1, 121-4 (1953). (3)’ Brandes, J., Gewehr, R. (to Vereinigte Glanzstoff-Fabriken), U. S. Patent 2,985,621 (May 23,1961). (4) Clark, G. A. (to Dow Chemical Go.), [bid., 2,937,157 (May
-.,
17 i-,-” q m
(5) Cofma& V.. deVore. H. B.. Nature 123. 87 (1929). (6) Ellis, C.; Wells, A. A:, “Chemical Actioh of Ultrabiolet Rays,” p. 575,Reinhold, New York, 1941. (7) Graham, P. R., Darby, J. R., Katlafsky, B., J . Chem. Eng. Data 4, 372-8 (1959). (8) . , Hoeschele, G. K. (to E. I. du Pont de Nemours & Co.). U. S. Patent 2.984.645 fMav 16. 1961). (9) Kresser, 0.J., ‘LLPolyp;opylene,”pp. 41-2, Reinhold, New York. 1960. (10) Lappin, G. R. (to Dow Chemical Co.), U. S. Patent 2,980,646 (April 18, 1961). (11) Lawton, T. S., Jr., Nason, H. K., Znd. Eng. Chem. 36, 1128?n (\ -1‘ 9’ ‘ ~1 . ~ ) (12) Newland, G. C., Tamblyn, J. R. (to Eastman Kodak Go.), U. S. Patent 3,003,996 (Oct. 10,1961). (13) Nishizawa, Y., J . Chem. Soc. Japan 50, 258, 327,391 (1929). (14) Sutezo, O., Takei, M., Huzita, N., J . SOL.Chem. Ind. Japan 42, Suppl. binding, 54-5 (1939). (15) Van Allan, J. A., Lappin, G. R., Tamblyn, J. W. (to Eastman Kodak Co.), U. S. Patent 2,987,503 (June 6,1961).
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RECEIVED for review October 1, 1962 ACCEPTED October 10,1962 Symposium on Stabilization of Polymers, Division of Organic Coatings and Plastics Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., September 1962.
