Norman S. Allen a n d J o h n F. McKellar University of Salford Salford, M5 4WT Lancashire. United Kingdom
Polymer Photooxidation An experiment to demonstrate the effect of additives
T h e following experiment is designed t o demonstrate t o undergraduate students t h a t t h e inclusion of appropriate additives can have very marked effects o n t h e physical properties of a polymer. Indeed, many of the man-made polymers in everyday use such as polyolefins, polyamides, a n d polyesters would n o t have their desirable commercial properties unless they contained these additives. Polypropylene, which can be used either i n fiber, film, or bulk form is a good example. Polypropylene, particularly in fiber form, cannot be used commercially without t h e presence of a light stabilizing additive or mixture of additives (I).A typical additive widely used for this purpose is 2-hydroxy-4-octyloxy-henzophenone
This wmpnund ahsorbs light very stnmgly in the wa\,eleligfh r r g i m d s u n l i g h t most d m x ~ g i n gco the polymer (290 350 nm) vet it remain:. photochemically inactive. T h r damaging ral l by s . the dlation t o the D o l ~ l n ein~thus nhsorl,ed ~ r e f e r e n t i a . additive a n d dissipated harmlessly. In recent years, however, t h e ever-increasing use of "disposable" plastic utensils such as drinking cups, wrapping films, and carrier bags has added to t h e problem of environmental pollution. One way of tackling this problem is t o use polymers t h a t contain additive systems t h a t a r e themselves photochemically active, particularly in t h e ultraviolet region (2). Thus, when discarded out-of-doors, t h e polymer has a huilt-in self destruct . DroDertv . . t h a t in time causes t h e d a s t i c utensil t o disintegrate. There is a very wide range of these additives, but. i n this e x ~ e r i m e n t because , of its structural similarity t o (I), henzophenone has been selected
0-a 0
I1
Unlike its ortho-hydroxy derivative, benzophenone is very photoactive in polypropylene and t h u s enhances the degradative effect of sunlight (3). Benzophenone has been shown by Porter and his coworkers (4-7) t o be one of t h e most photochemically active aromatic carbonyl compounds. This photochemical activity of henzophenone also extends t o polymers such a s polypropylene. On t h e other hand, 2-hydroxyhenzophenone is one of t h e most photochemically inactive compounds and this behavior also extends to polymers ( 1 , 3 ) .Thus, there is a marked difference in photochemical activity with only a relatively minor change in structure. Experimental Procedure Preparation of Materials
For the purpose of this experiment polypropylene in the form of powder or chip containing no commercial additives is preferred and may he ohtained from Avi-Sun Corp.. Pennsylvania; Hercules Corp., Delaware; Eastman Corp., -Tennessee; Phillips Petroleum Co., Oklahoma, and E. I. Dupont De Nemours & Co. Ine., Delaware. The presence of any anti-oxidant will prolong the experiment, and, if
suspected, it should be removed beforehand by Saxhlet extraction with ether or acetone. The additives may be incorporated into the polymer by two methods. The first involves solvent-blending the additives (0.1-0.5% wlw) into the polymer powder using dichloromethane. For example, 0.1 g of additive is dissolved in 50 ml of dichloromethane (cold).The solvent is then blended with 50 e of dwropylene powder in a 250-ml beaker. The solvent is then removed %h&"nde; reduced pressure using a rotary evaporator or the powder is spread onto a large tray and allowed to evaporate overnight. The second method involves processing the additives into the polymer melt a t 190°C for about 5 min using any form of standard polymer processing equipment. For example, the polymer and additive may be melt-processed an an open-mill of two rotating, steam-heated, stainless steel rollers. Alternatively, the polymer may be compounded in a Brahender PlastieordeP (Brahender, OHG., Duesburg, W. Germany) which consists of an oil heated mixing head containing two rotating blades. The molten polymer is then removed from the processing equipment and cut, while still hot, into small pieces using a scissors. The molten polymer can be handled quite easilywith asbesto. gloves. Finally, the polymer containing the additives either in the form of powder or small pieces is then pressed into film (usually 2 mm thickness) and then oressine,. the dates on a 10-em ram between . under 20.000 kg.. oressure . t ~ cswam , .,r rlcrtricnll~healed plates S t r d gnu:?+ are plncrd hpturcn I hr. d a c ul tlw plntrs 10 control the rhi~.lin~\s 01 rhr polymer t i l m A omrr.d pulymer idm w i t h w ~add~tlvrimust hr prrpnrerl similady. Photooxidation of the Polymer
A number of suitable light sources are available, but only thase with a relatively high proportion of their output in the ultraviolet region 29&360 nm is preferred. Various light sources or weatherometers that may he used for this purpose are the Carbon Arc Fadeameter, Atlas Electron Devices Co., Chicago; the Sunlighter 11,The Test Laboratory Apparatus Co. Ltd., Somers, Connecticut; the Xenotest-150, Quartzlampen Gm. h.H., Hanau, W. Germany, and the Microseal Lightfastness Tester (Microscal Ltd., London). For the purposes of this experiment the latter was selected. This relatively inexpensive apparatus contains a 500-W mereuryltungsten lamp as its light source. The rate of photooxidation of the polymer can be measured by monitoring the build-up in the non-volatile carhonylic oxidation products using infrared spectroscopy. For this, a Perkin-Elmer 157G infrared spectmphotometer was used. The earhonyl oxidation products arise from the polymer and their formation is best measured by monitoring the increase in the carbonyl absorption band centered at 1710 cm-'. This widely used technique is often referred to as the carhonyl index method. In this particular experiment there is no interference from the ahsorption by the additives as may he seen from Figure 1. The increase in earbonyl absorption (earhonyl index) is given by = [(log~olo/l~)/d] X 100 where I. = intensity of incident light, It = intensity of transmitted light, and d = film thickness in microns. The carbonyl index method has been found to correlate well with changes in the mechanical properties of the polymer ( 1 , 3 ) . Results The effect of additives I and 11 on the formation of carbonylic groups during irradiation of the polymer is shown in Figure 1.It is seen that after 80 h of irradiation the presence of the henzophenone has accelerated the formation of carbonyl groups whereas the 2-hydmxy-derivative hes completely inhibited their formation. The degree (9photmxidation of the polymer films with and without the additives are more easily compared in Figure 2. It is seen that after 30 h of irVolume 56. Number 4. April 1979 1 273
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i \ Figure 2. Rate of photooxidation of commercial polypropylene film (450 fi thickness) containing 0:no addnives. 0: 0 . 1 % w / w benzophenone and 0: 0.1 % w/w 2-hydroxy-4-octylaxy-benraphenone in a Microscal containing a 500-W mercuryltungsten lamp.
WAYENUMBER
(CM-I)
Figure 1. Infrared absorption spectra of commercial polypropylene film (450 w/w benraphsnoneand(C) 0.1 % w/w 2-hydroxy-4-octylaxy benzophenona,before(-)and after I--) 8 0 hr of irradiation in a Microscal with a SO0 W mercuryltungsten lamp. fi thickness)containing (A) no additive?. ( 8 )0.1 %
radiation in the Micmscal the contrul polymer film exhibits no carhonyl fimnatiun. This is communly referred to as an induction periud. The presence of the bemophenone significantly reduces the induction periud to 10 h whereas the 2-hydruxy-derivative prolongs it. Even after 100 hr of irradiation nocarbonyl formation was observed when the 2-hydrouy derivative was present. Discussion Ikrpite their similarity in structure the results show that the two additives have markedly different effects on the light stability of the polymers. As described above 2-hydroxy-4-oetyluxy-benzophenone is yhotoehemically inactive. It is generally believed that the light energy absorhed by the compound is dissipated harmlessly through a nxchanism involving the formation of a reversible siu-membered hydrugen-bonded ring system (3).
274 1 Journal of ChemicalEducation
Benzophenone, however, is photoactive in the polymer and is believed to initiate the photooxidation by the excited triplet state abstracting a hydrogen atom from the polymer matrix (8) "B P-H B-H + P where P-H is the pulymer. Oxidation of the polymer than can proceed by the attackof oxygen on the macroradical P'in the following steps of the well-known Bolland and Gee mechanism (9).
+
-
P'+02-POp' P02.+P-H-POOH+P' The find earbonyl oxidation products, monitored as described above, are formed frum the decomposition ofthe hydroperoxides.
Literature Cited i l l Rsnhy, &and Hahek..J. P.,"Phh~degradatiii, Phottliiddfimand Photaetabilisatiin ulPdymea,".luhnWiley atld Suns, Inc. New Yurk, 1975. Chap. 10, p. 362. (21 Baum, H..and Deanin,H. U..Pul.~m~Plasl. Technu1 E n # . 2. (11.1. (19731. (3) Allon N. S.and McKeliar..l. F. C h r m Soc Rrur.. 4. SYB 119751. (41 PorLer.F.and Wilkinrun. F..Tmna. Forod S u i . 57.1686,(19611. (5) Beckelt, A.and i'artor, li., Tmnr. F e m d . , S o r , 59, 2051, 119651. (61 Porter. (i.and Suppan. P.. Trans. Farod. Ynr 61. 1661. (1965). 171 Partpr. C.and Suppan. P.. ?io!lr Forod Sue. 6'2.3:375. (19661. (81 Harper. U.J , and McKel!ar,d. F.. J. Appl. Pu1.v. S c i . 17.3503. (19731. 191 t(ul1and.d. L. and Oee.G.. Tranr. Forod. S u r . 42. 496.(19461.
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