SYNERGISM BETWEEN PHENOLS AND SULFIDES IN THE STABILIZATION OF POLYOLEFINS TO OXIDATION N O R M A N P. N E U R E I T E R AND D. E. BOWN Humble Oil @ Rejning Co. Research and Development, Baytown, Tex.
The remarkable antioxidant synergism in polypropylene beiween certain hindered phenols and sulfides has been studied with respect to the function of the sulfide. The evidence i s consistent with the proposal that the phenol acts as a peroxy radical scavenger and the sulfide as a decomposer of peroxides. Model reaciions indicate both that the phenol may be continuously regenerated by the sulfide, and that certain sulfoxide structures can decompose with formation of new sulfur-containing materials which are themselves effective as costabilizers. The extent to which these reactions may apply to oxidizing polymer systems i s discussed.
is inhibited separately either with a hindered phenol (0.1%) or with a sulfide such as dilauryl thiodipropionate (DLTDP) (0.37'), the polymer completely degrades within a few hours on exposure to air at 150' C.
contamination of good samples by volatile fragments from samples in the process of degrading. Furthermore, air velocity and temperature generally vary throughout the oven. Degradation of a small plaque often proceeds gradually over a 1- to 2-week period, beginning at one edge, with slow extension throughout the sample. For initial laboratory screenC~~H~~OC-CH~CH~-S-CH~CH~-COCIZHZ~ ing of new inhibitors. the following procedure was developed.
HEN ISOTACTIC POLYPROPYLESE
I1
I1
0
0
Dilauryl Thiodipropionate (DLTDP) OH RI I Rz
'(y i
R3
Hindered Phenol Use of a mixture of the same two inhibitors in the same concentrations, however, successfully inhibits polymer degradation for 70 days. It is not a new concept that a mixture of different inhibitors is more effective than would be predicted by summation of the results for the individual compounds (8). A number of plausible suggestions have been made to explain synergism between sulfur compounds and phenols (8), but the mechanistic facts of the matter remain poorly understood. This paper discusses the authors' conclusions about the nature of the chemical reactions that produce this synergism in polypropylene. Experimental Inhibitor Evaluation
Oxidative Stability. New methods were devised for incorporating inhibitors in polypropylene and for testing their stabilizing efficiency in order to offset disadvantages inherent in the presently accepted techniques. Milling of the polymer in air to incorporate the inhibitors seemed undesirable in that both polymer and inhibitor degradation can occur during the process. The oven-aging technique suffers from the danger of 236
l&EC
PRODUCT RESEARCH AND DEVELOPMENT
One large batch of unstabilized polypropylene was used for all correlated screening tests, since inhibitor response varied markedly with different base resins. The resin had concentrations of 32 p.p.m. titanium, 25 p.p.m. aluminum, 3 p.p.m, iron, 1 p.p.m. nickel, 0 p.p.m. copper, 13 p.p.m. silicon, and 1 p.p.m. sodium (167 p.p.m. total ash as oxides). The molecular weight was 31 6,000. The concentration of metal residues was high enough to give some discoloration with phenolic inhibitors, but both the thermal (oide infru) and oxidative stabilities of the polymer were satisfactory. The inhibited samples Lvere prepared as follows. A 20-gram sample of the dry polymer powder was weighed into a 500-ml. round-bottomed flask, and covered with 150 to 200 ml. of methanol (Baker reagent grade). To this was added the appropriate amount of inhibitor dissolved in methanol (stock solutions of 0.5 Xvt. % were used), and the methanol was removed on a rotating Rinco evaporator a t reduced pressure. The flask was warmed to 60' to 65' C. with a heat lamp for final removal of methanol. By this method, a high degree of dispersion of the inhibitor was achieved. A few inhibitors, insoluble in methanol, were dissolved in ether, and one sample was dissolved in chloroform. While generally quite reproducible results were obtained by the above procedure, occasionally some inhibitor was lost through volatility or poorly dispersed in the polymer as a result of low solubility in cold methanol. LVith a few inhibitors the solution blending gave fe\ver days of oxidative stability than the dry-blending method described below. The inhibited polymer powder was compression molded into a pad of 75-mil thickness (4 X 4 inches) using two pieces of aluminum foil to protect the surface of the pad. Molding was done a t 400' F. for 11/2 minutes a t a constant pressure of 1 to 2 tons; the pad was then transferred to a cold press at room temperature and held for 2l/2 minutes a t 30 tons pressure. Molecular weight breakdown was minimal during the padding operation, except in the cases of thermally unstable inhibitors. This molded pad was cut up into small pieces
about l/le to ‘/s inch square, and these pellets were used in the oxidation test. A method for preparing S-pound inhibited samples, especially for accurate concentration studies, involved a dry blending technique. A few hundred grams of the polymer with enough of the inhibitor (either solid or liquid) to make 5 pounds of the polymer were blended for about 1 minute in a Waring Blendor. Care was taken not to let the powder get too warm during blending. This dry “masterbatch” powder was then blended on a mechanical tumbler with the remainder of a 5-pound sample of the polymer. The final blend was extruded through a 1-inch extruder a t 400’ to 425’ F. The pellets from the extruder were used directly in the oxidation test. I n the test developed for oxidative stability, pellets (3 to 4 grams) were placed in a borosilicate glass U-tube (O.D. 9 mm., height 8 inches), and the tube was suspended in an oil bath maintained a t 150” C., such that the pellets were a t least ‘/2 inch below the surface of the oil. A stream of 10 ml. per minute of air was passed through the tube. The change in molecular weight of samples periodically removed from the test was very gradual until the end of the induction period, after which the polymer rapidly degraded to a dark yellow or brown mixture of liquid and cracked, degraded pellets. The first appearance of cracked pellets was considered the end of the induction period. The oxidation could be accelerated by passage of pure oxygen rather than air through the tube. Occasionally this was done to expedite data collection. Howevex, it was not possible to rely on a simple factor to relate induction periods in air and in oxygen to one another. Certain inhibitor systems, especially those containing high concentrations of phenolic materials, gave rise to bad discoloration very early in the oxidation, without signs of serious polymer degradation Hence, formation of a dark brown color was not a satisfactory end point for the induction period. T h e r m a l or Processing Stability. In order to circumvent the lack of controls in the conventional melt index determination for processability. a thermal stability test was devised which distinguished conveniently between “good” and “bad” resins (unstabilized) and, more importantly, was very useful in evaluating the deleterious effect on the polymer of experimental stabilizers, which otherwise would have been apparent only during subsequent high temperature processing. A glass bulb about 1 inch in diameter on the end of a 5-inch borosilicate glass tube (O.D. 10 mm.) equipped with a groundglass joint for attachment to a high vacuum system, was charged with about 1 gram of the blended polymer-inhibitor powder. The bulb was evacuated until the residual pressure reached a t least 10-6 torr. I t was then sealed off and immersed in a lead bath at 550’ F. (280.8’ C.) for 30 minutes. After cooling, the molecular weight of the polymer was determined and compared to the original molecular weight This molecular weight change was most conveniently expressed as
where Sw is a measure of the number of C-C bonds broken in the polymer chain, assuming random cleavage and no cross linking as is probably the case with polypropylene. This expression has been very useful in these studies of thermal or processing stability, particularly in that it allows one to compare the thermal stability of polymer samples which have different initial molecular weights. Whenever an experimental inhibitor system gave an Sw value greater than 1.0. it was considered unsatisfactory for utilization in polymer. The Sm number of the resin used in the inhibitor screening tests (uninhibited) was 0. Other base resins have varied from 0 to 3.30.
showed widelv different responses to the same inhibitor systems. I n the stabilization of a commercially prepared polymer. one has to contend with more than just the pure hydrocarbon. Secondly, even with the cleanest polypropylene, one can only measure the over-all inhibitor effectiveness. Lvhich is the combination of a basic chemical effect plus several different physical effects-such as solubility or compatibility, volatility, and migratory aptitude. I t is impossible to separate these effects from one another when evaluating a new system by a simple aging test. The physical state of the polymer is no less involved in the inhibition process. A plot of the logarithm of the induction period for oxidation of a sample of stabilized polymer against the reciprocal of the absolute temperature (Figure 1) showed a sharp break as the temperature passed through the region of the polymer melting point. Inhibition was much more efficient below the melting point and much less efficient above.
4.11
I
I
i
2.10
2.20
2.30
2.40
I
2.50
io3 Figure 1. Oxidation of polypropylene containing 0.03% HOR-57: 0.2% DLTDP in 0 2 at 150” to 190’ C. I/T x
f = time to observe polymer decomposition
Extrapolation of the solid polymer line and the liquid polymer line to the melting point (168’ C.) showed clearly the roughly fivefold difference in ease of stabilization of the solid polymer over the liquid a t this temperature. This observation accounts a t least in a qualitative \vay for the difficulties encountered in extending the excellent synergistic results in solid polypropylene to the stabilization of liquid hydrocarbons of similar structure such as 2,6,10,14-tetramethyIpentadecane. A study of the variation in inhibitor effectiveness with phenolic structure is beyond the scope of this report, although there are prodigious differences in the performances of different hindered phenols. Initially, 4,4’-thiobis(Z-tert-butyl-jmethylphenol) (I) was selected as an active phenol, but both the thermal instability of I and the mild synergism contributed by the sulfur atom already present in the molecule made it undesirable.
Scope
There are two very serious complications in drawing conclusions from a study of this type. Variations in the innate stability of different uninhibited polymer samples were often larger than differences among inhibitor systems. Sometimes these differences were relatable to metallic ash residues; more often they were not. Polymers from different producers
1
I
HO
VOL 1
t-Bu
t-Bu
I
I
NO. 4
OH
DECEMBER 1962
237
Finally, HOR-57 was chosen as one particularly effective material. This experimental stabilizer developed a t Humble is a highly alkylated phenol with a molecular weight similar to that of DLTDP. Results
All attempts to identify inhibitor oxidation products formed during the inhibition process were unsuccessful. This obvious approach to the inhibition mechanism was fruitless. Figure 2 shows the effect on polymer stability of variations in the relative concentrations of the two inhibitors. The phenolic component had a greater effect at lower concentrations than did the DLTDP. The degree of protection afforded by a given amount of phenolic steadily increased (almost linearly) with increasing DLTDP concentration up to Z1/2 to 3 times the amount of HOR-57. Above this level, an increase in DLTDP appeared to have little effect. An excess of the phenolic above the DLTDP was of no value. At concentrations above O.jyO, it began to act more as a promoter than a retarder. Even at phenolic concentrations of 0.2%, the polymer became first yellow and then brown as aging proceeded. The lifetime of a given amount of the phenolic appeared to be determined by the amount of DLTDP present. Yet, without any phenol, the DLTDP gave very little stabilization. I n terms of the accepted mechanism of oxidation of hydrocarbons ( 8 ) , these results are consistent with the proposal that the phenol acts as a radical scavenger to interrupt the oxidation chain, and that the sulfide subsequently participates in a different reaction which aids the stabilization. Attempts were made to follow the rate of disappearance of each of the inhibitor components from the oxidizing polymer. DLTDP \vas analyzed by the carbonyl absorption in the infrared; or the partially oxidized polymer pellets were ground to a po\vder, the powder was extracted with benzene, and
the resulting solution analyzed for DLTDP and I by a high speed polarographic technique. The HOR-57 content of samples was determined from the ultraviolet absorption of the phenol nucleus. A series of experiments indicated that the concentration of the phenolic decreased fairly rapidly, while DLTDP disappearance was slower. Although infrared showed little change in the DLTDP carbonyl with time, the polarographic technique which measured the amount of free sulfide showed a slow but steady decrease in the DLTDP concentration. It appeared that DLTDP was acting in a way which conserved the ester carbonyl groups but destroyed the free sulfide. More information was gained by using labeled inhibitors. An oxidizing polymer inhibited with inactive phenolic and DLTDP containing S-35 was assayed periodically. There was a slow but steady loss of activity from the poly-mer, and a steady increase in the amount of activity Tvhich could not be extracted by benzene from ground polymer as osidation proceeded. These results suggested that some C-S bond cleavage was occurring during the inhibition and that some of the sulfur-containing structures liere becoming attached to the polymer chain. A polymer containing I labeled with S-35, along with inactive DLTDP, showed a comparatively rapid loss of activity during oxidation, and also a steady increase in the amount of activity not extractable with benzene. These results indicate some bisphenol decomposition during oxidation and partial attachment of the phenolic residues to the polymer chain. Variation of the chemical structure of the sulfide synergist has been very informative. Table I shows the efficiencies for a number of sulfide structures \then used with either I or HOR-57. Since the oxygen and nitrogen analogs of DLTDP (compounds 4 and 5, Table I) show no synergism, the presence of the sulfur atom is a chemical necessity. Both the oxygen and nitrogen structures should have chelating propercies similar to the sulfide; hence, the validity of the suggestion that metal chelation is the basis of DLTDP activity is seriously jeopardized. The tridecyl ester of thiodipropionic acid (compound 2) showed activity equivalent to DLTDP itself This is corroboration of the fact that activity resides within the thiodipropionate group and is chemically independent of the esterifying alcohol. groups was However, when the direction of the -C-0-
9007 I1 I1
0 just reversed from DLTDP as in compound -. most of the activity was lost. Similarly, with the related thiodiacetate structure (compound 6), all of the synergistic activity with HOR-57 was lost, while some still remained with I . Equally surprising was the difference in activity between dicet) 1 sulfide (compound 3) and DLTDP since the reactivity of the the sulfur atom would be expected to be very similar in the two compounds. While Hawkins had observed synergism between carbon black and 2-thionaphthol (compound 8) in polyethylene stabilization (7), this material was ineffective in the present system. From these experiments, it may be concluded that the sulfur atom is the seat of reactivity, that the ester groups in the DLTDP have significance, and that the position of the sulfur with respect to the ester groups is of importance in determining the relative effectiveness. I1
0.
0.0
I
0.2
0.4
0.6
0.8
1.0
1.2
1.4
% OF INHIBITOR
Figure 2. Polypropylene stability with variations in DLTDP and HOR-57 concentrations
02 was used instead of air 238
to decrease the induction period
l & E C P R O D U C T RESEARCH A N D DEVELOPMENT
Mechanism
The chain propagating species in autoxidations are peroxy radicals and an effective inhibitor must interrupt these chains. This is assumed to be the principal function of the phenolic.
Table 1.
Days of Oxidative Stability in Air at 150'
I Coinhibitor (0.370)
(0.7%)a
C.
HO R57 (0.7%)b
1.
43
70
3 -.
51
78
3.
19
20
4.
5
1
5.
4
2
6.
15
1
7.
10
1
8.
4
3
There are abundant examples of peroxy-radical scavenging by phenols, though the mechanism of the reaction itself is still under discussion (8). \Vhile it may involve formation of the free phenoxy radical, prior complexation of the phenol \\ith a peroxy radical followed by reaction with a second peroxy radical, or initial electron transfer from the phenol to the peroxy radical. the molecular consequences of the reaction are identical in each case: OH 0 Ri 1 Rz Ri 11 R P
\A/ I
R3
R3 OOR I1
0 9.
7
5
I11
The molecular reaction represented by Equation 1 has never been verified at test temperatures of 150' C. for any phenol. However, a t lower temperatures products Lvith structure I1 have been readily isolated (8). The hydroperoxide (111) at 150' C. would be expected to decompose into new initiating radicals. ROOH
A B
'\(A)/
150°
RO
. fHO .
(2)
The fate of I1 structures at 150' C. was not cerrain. .4 sample of IIa was prepared ( 4 ) and dispersed in poly-propylene powder in O.lYc concentration, and the powder was 0
t-Bu
10. Rc-S-Rc 0
9
13
...
1
R-S-S-R
20
23
13. R--8-S-R 0
12
18
1.1. R-S-S-R
7
2
11. R-S-R
0
4
15,
ZI-S-S-S-R
21
35
16.
R-S-SS-S-R
14
38
23
26
1 T. Residue from DLTDP
decompn.
Table II.
Degradation of Polypropylene Containing Initiators
292.000 319 , 0 0 0 292.000
221 ,000
300,000
120,000
221 ,000 70 000 ~
mechanically pressed into a "pill" without heat. Similar pills were prepared containing 0.17, of the well-known free radical initiators, azobisisobutyronitrile (AIBN) and benzo)-l peroxide. These pills were placed in separate tubes, the tubes were evacuated to pressures of less than 1 micron, sealed, and then exposed to a temperature of 150' C. for 2 hours. The negligible efyect on the molecular weight of the polymer is shown in Table 11. However, \vhen these same pills Icere exposed to air at 150' C. for a short time, rapid oxidation took place. Table I1 shoivs conclusively that the peroxydienone structure IIa is an effective free radical initiator for the oxidation of polypropylene. These simple experiments suggest that the function of the sulfur compound is to destroy both the hydroperoxide (111) and the peroxide (11) before they can initiate new oxidative chains. Since the peroxide IIa is thermally unstable at 150' C., any analogous structures from HOR-57 \vould also be unstable under the oxidizing conditions where their formation has been postulated. This difficulty can be resolved by either assuming short, finite lifetimes for these peroxide intermediaws and rapid reaction with the sulfide, or an intercession of the sulfide, or ii sulfide-derived species (zide infra) at an earlier: yet undetermined point in the inhibition process. The reduction of hydroperoxides by sulfides (Equation 3) is known to occur (8),although the detailed mechanism may be complicated (3,6). ROOH
Initial molecular weight of flolymer by intrinsic L'iicosity. method
316.000. .\.feusure?nents are reliable within i 70,000. Average values of 2 runs.
t-Bu
CH3 0 0 - t - B u IIU
0 2
12.
I1
+ R-S-R
+
ROH
- R-S-R
I 0
(3)
Although peroxydienone structures (11) have not been successfull>- prepared from HOR-57, the reaction of the model VOL.
1
NO. 4
DECEMBER 1 9 6 2
239
compound IIa with DLTDP was studied in some detail under a variety of conditions. When IIa was added to a n excess of pure DLTDP under nitrogen at 150’ C., a 41% yield of the parent phenol (IV) was obtained as well as a high yield of is0 butylene. 0 “ 1 f I ,
-+t-Bu,o/t-Bu
OH
+
ROOCCHzCHz-S-CH2CH2COOR 0 V ROOCCHzCH2-S-OH IX 2 IX
X
+
-+
+ CH2=CHCOOR
ROOCCHzCHn-SS-CH2CHzCOOR 0
+ Hz0
(5) (6)
+ ROOCCH2CHs-SS-OH XI
(7)
ROOCCH2CH2-SSSS-CHzCHsCOOR
(8)
CHs=CHCOOR
DLTDP
?
2 XI
I
/\
CH, 0 0 - t - B u IIa
I
H,C-C=CH2
+
0
CH3 IV
XI I ?
XI1 + ROOCCH?CHz-SSS-CHzCH&OOR VI
CHB
+ (DLTDP-Sulfoxide)
(4)
V The DLTDP-sulfoxide (V) was not isolated as such, but was presumed to be an intermediate in the formation of the recovered lauryl acrylate (uide infra). A similar reduction of IIa to I V in 40% yield occurred in refluxing heptane solution. Simple thermal decomposition of IIa gave at best a few per cent of IV. Dicetyl sulfide with IIa gave 11 to 2lY0 yields of the phenol IV. I t was concluded that the sulfide partially regenerates the phenolic stabilizer in addition to destroying deleterious hydroperoxides. Table I shows that DLTDP is unusually good as a synergist. Although there appears to be a difference between DLTDP and dicetyl sulfide in the extent of phenol regeneration in Reaction 4, no chemical justification is apparent for the fact that one sulfide structure should be significantly different from a sterically similar one in the postulated Reactions 3 and 4. Since the sulfoxide was presumably the initial product of “coinhibition,” the reactivity of DLTDP-sulfoxide (V) was examined. While dicetyl sulfoxide remained largely unchanged a t 150” C., V was profoundly unstable. Gas evolution occurred and on cooling the mixture, a white solid and a light yellow liquid were obtained. The gas consisted of SOZ, a little HzS, and HzO. The liquid was lauryl acrylate, and the yellow color seemed due to small amounts of elemental sulfur. The solid residue was a complex mixture which was only partially resolved. Its principal constituent had the surprising trisulfide structure R-SSS-R (VI); and there were smaller amounts of the thiosulfonate RSOzSR (VII), and the disulfide R-SS-R (VIII) (in all of which R=ClzH2500CCHzCHz-). From a decomposition of pure V a t 150’ C. a very small amount of (CnH25OOCCH=CH)2S was recovered; and in a run in dilute benzene (80’ C , ) , the corresponding monolefinic structure was found. Almost coincident with these observations, Kingsbury and Cram (70) showed that sulfoxide pyrolysis analogous to IVoxide decomposition readily occurs when a hydrogen on the beta carbon is activated. Subsequently, Colclough and Cunneen (5), as part of the British Rubber Producers’ outstandingly thorough work on the inhibition of olefin oxidation by sulfides, reported that tert-butyl sulfoxide thermally decomposes to give tert-butyl thiosulfinate and water. I t was found in the present work that the thiosulfinate corresponding to DLTDP (X) is just as unstable as the sulfoxide, giving lauryl acrylate plus essentially the same solid product mixture that was obtained from the sulfoxide. The scheme represented by Equations 5 to 9 is a reasonable, though unproved, path for the decomposition. 240
+
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
+ [SO]
(9)
Most interesting is that the solid decomposition product obtained from the sulfoxide (or thiosulfinate) showed considerable synergistic activity xvhen used as an inhibitor in polypropylene with HOR-57 (compound 17, Table I). Each of the possible individual products was synthesized and screened for activity (Table I, compounds 10 to 16). It is revealing that the principal established component of the solid mixture (the trisulfide, compound 15) was one of the most active members of the series. Only the thiosulfonate (compound 14) and the sulfone (compound 11) showed little or no effect. Conclusions
Since the sulfoxide can decompose with formation of effective synergists, one can say that there is in the DLTDP system the chemical potentiality for continual regcneration not only of a portion of the original phenol, but also of the synergist. I t is tempting to attribute the high activity of DLTDP to this doubly regenerative process; however, the true sequence is likely more sophisticated. From Equations 5 to 9, it can be seen that trisulfide formation during polymer oxidation would involve the sulfoxide concentration to the fourth power, in two bimolecular reactions of two unstable molecules. That this sequence should be possible in solid polymer in the presence of oxygen at the low initial sulfide concentration employed is extremely tenuous. I t is certain, however, that any sulfoxide formed in the polymer rapidly decomposes to lauryl acrylate and the sulfenic acid (IX). The fate of the highly reactive sulfenic acid (9) is uncertain. It may itself be the steady source of the reducing environment that produces the synergism. In the absence of specific evidence, it is pointless to theorize about the function of the sulfenic acid. There are a number of ways in which its participation a t any stage in the inhibition process between the peroxy radical and the peroxydienone could be visualized. The fascinating conclusions recently reported by the British workers ( 7 , 2) may have application to the present systems In the oxidation of squalene a t 70’ C., sulfides and disulfides give rise to sulfoxides and thiosulfinates which are the active oxidation inhibitors. There appears to be a correlation between the inhibiting activity and the thermal instability of the sulfoxides, although the mechanism of action has not been established. So far the authors have assumed participation only of free radical reactions of the phenolic, and ionic or molecular reactions of the sulfides. I t should not be construed from this paper that free radical reactions of sulfides may automatically be ignored. An evaluation of the importance of free radical reactions of sulfur compounds in synergistic inhibition is being undertaken.
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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DECEMBER 1 9 6 2
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