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19 Photochemical Degradation and Biological Defacement of Polymers P. D. GABRIELE, J. R. GEIB, R. M. IANNUCCI, and W. J. REID Ciba-Geigy Corporation, Additives Department, Ardsley, NY 10502
Engineering plastics and organic coatings are being exposed to increasingly more hostile environments as they replace and protect more wood and metal products, especially in exterior applications. When exposed outdoors these plastics must withstand the concerted effects of moisture, oxygen, heat, ultraviolet light, and micro-organisms in order to perform their designed function. The combined effects of the first four factors, i.e., moisture, oxygen, heat and UV light, can result in photooxidation in the polymer surface. Eventually, erosion and microfracture of the polymer surface ensues, thus creating a microenvironment which is conducive to moisture and "dirt" accumulation. Also, the photooxidation leads to high concentrations of a variety of carbonyl groups which cause the surface to be hydrophilic. This hydrophilicity allows the polymer surface to absorb moisture causing it to swell and result in additional stress cracking. The surface microenvironment provides an ideal atmosphere for mildew growth, i.e., free moisture, carbon and nutrient sources which accumulate during the weathering process. The polymers and coatings which we have examined have had to undergo significant surface deterioration prior to the time that mildew could undergo active growth on the surface. By proper stabilization of the polymers and coatings with benzotriazole UV absorbers, hindered amine light stabilizers (HALS), or combination of the two stabilizers, the surfaces of the polymers and coatings could be maintained and thus active mildew growth prevented. 0097-6156/ 83/0229-0317S06.00/0 © 1983 American Chemical Society
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Engineering p l a s t i c s and organic coatings are being exposed to i n c r e a s i n g l y more h o s t i l e environments as they r e p l a c e and protect more wood and metal products, e s p e c i a l l y i n e x t e r i o r app l i c a t i o n s . When exposed outdoors these m a t e r i a l s must withstand the concerted e f f e c t s of moisture, oxygen, heat, u l t r a v i o l e t l i g h t , and micro-organisms i n order to perform t h e i r designed f u n c t i o n . Photodegradation of p l a s t i c m a t e r i a l s that have been exposed to outdoor c o n d i t i o n s has been studied e x t e n s i v e l y (I). B i o l o g i c a l degradation of p l a s t i c m a t e r i a l s has a l s o been widely studied 02,3^4). Recently, we reported the r e s u l t s o f numerous s t u d i e s o f d i f f e r e n t p l a s t i c m a t e r i a l s that had been exposed outdoors i n F l o r i d a ( 5 ) . The purpose of these s t u d i e s had been to show the e f f e c t i v e n e s s o f hindered amine l i g h t s t a b i l i z e r s (HALS), t y p i c a l l y b i s [ 2 , 2 , 6 , 6 - t e t r a m e t h y l - 4 - p i p e r i d i n y l ] s e b a c a t e (LS I ) , and b e n z o t r i a z o l e UV absorbers, t y p i c a l l y 2(2'-hydroxy5'-methyl-phenyDbenzotriazole (LS I I ) , used alone or i n combin a t i o n with each other i n preventing the photodegradation of p l a s t i c m a t e r i a l s . We had shown that the o p t i m a l l y s t a b i l i z e d samples of a "weatherable" s t y r e n i c terpolymer (an ABS-type p l a s t i c that contained a saturated rubber i n place of the butadiene) s t a b i l i z e d with a combination of 0.5% LS I and 0.5% LS II had been able to maintain i t s c o l o r , appearance, s u r f a c e i n t e g r i t y , and p h y s i c a l p r o p e r t i e s through four years o f outdoor weathering i n F l o r i d a . The c o n t r o l with no l i g h t s t a b i l i z e r s was p i t t e d and h e a v i l y encrusted with mildew. Other samples i n t h i s study which contained only 0.5% HALS or only 0.5% b e n z o t r i a z o l e , were a l s o h e a v i l y encrusted with mildew at t h i s same time p e r i o d , 4 years, thus i n d i c a t i n g that n e i t h e r component of the blend was f u n c t i o n i n g as a f u n g i c i d e . A l s o , mildew was a c t i v e l y growing on the rough cut sides of the o p t i m a l l y l i g h t s t a b i l i z e d samples, again i n d i c a t i n g that n e i t h e r of the l i g h t s t a b i l i z e r s alone or i n combination with each other was a c t i n g as a f u n g i cide. (Figure 1). Samples o f polypropylene s t a b i l i z e d with j u s t LS I had a l s o been able to maintain t h e i r c o l o r , appearance, surface i n t e g r i t y , and p h y s i c a l p r o p e r t i e s through 2.5 years of outdoor weathering i n F l o r i d a . Moreover, t h e r m o p l a s t i c p o l y urethane m a t e r i a l s s t a b i l i z e d with j u s t LS I a l s o had e x c e l l e n t performance through s i x months outdoor weathering i n F l o r i d a . A l l of the above p l a s t i c m a t e r i a l s that contained no l i g h t s t a b i l i z e r s , low amounts of the p r e f e r r e d l i g h t s t a b i l i z e r s , or other types o f l i g h t s t a b i l i z e r s had extensive s u r f a c e e r o s i o n and heavy i n f e s t a t i o n s o f micro-organisms, most notably mildew (Aureobasidium p u l l u l a n s ) , a f t e r the same periods of outdoor exposure. Mildew defacement of organic coatings and p l a s t i c s has long been a major problem of the organic coatings and p l a s t i c s indust r i e s . Aureobasidium p u l l u l a n s i s the major c a u s i t i v e organism
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Figure 1. illustration of the non-biocidal activity of the hindered amine light stabilizer LSI and the benzotriazole UV absorber LS Hon a modified styrenie terpolymer. Key: left, unaged: and right, Florida exposure for 500,000 langleys (approx. 4 years).
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r e s p o n s i b l e f o r t h i s defacement (5-8 ). It was the primary organism i d e n t i f i e d as being r e s p o n s i b l e f o r the b i o l o g i c a l defacement i n the three polymer systems without l i g h t s t a b i l i z e r s that we had p r e v i o u s l y examined. U n t i l now, the approach to solve t h i s problem has been to incorporate a f u n g i c i d e i n t o a c o a t i n g formulation or compound a f u n g i c i d e i n t o a p l a s t i c . Although t h i s technique had success f o r short s e r v i c e time p e r i o d s , the f u n g i c i d e may be vaporized or leached from the coatings and p l a s t i c s over long time p e r i o d s . In a d d i t i o n , the b i o l o g i c a l a c t i v i t y of most non-mercurial organic and organomet a l l i c f u n g i c i d e s are s i g n i f i c a n t l y destroyed by photo-oxidation from the u l t r a v i o l e t l i g h t present i n t e r r e s t r i a l s o l a r r a d i a tion. Once the c o n c e n t r a t i o n of the f u n g i c i d e drops below c r i t i c a l l e v e l s , mildew can attack the coating or p l a s t i c s u r face ( 6 ) . The combined e f f e c t s of moisture, oxygen, heat, and UV l i g h t can r e s u l t i n photooxidation of the polymer s u r f a c e . E v e n t u a l l y , e r o s i o n and m i c r o f r a c t u r e of the polymer ensues, thus c r e a t i n g a microenvironment which i s conducive to moisture and " d i r t " accumulation. A l s o , the photooxidation leads to high concentrations of a v a r i e t y of earbonyl groups which cause the surface to be very h y d r o p h i l i c . This h y d r o p h i l i c i t y allows the polymer surface to absorb moisture causing i t to s w e l l and r e s u l t i n a d d i t i o n a l s t r e s s c r a c k i n g . The surface microenvironment provides an i d e a l atmosphere f o r mildew growth, i . e . f r e e moisture, carbon, and n u t r i e n t sources which accumulate during the weathering process. The polymers and coatings which we have examined have undergone s i g n i f i c a n t surface d e t e r i o r a t i o n p r i o r to the time of mildew appearance. By proper s t a b i l i z a t i o n of the polymers and coatings with b e n z o t r i a z o l e UV absorbers, hindered amine l i g h t s t a b i l i z e r s (HALS), or combination of the two s t a b i l i z e r s , the surfaces of the polymers and coatings could be maintained and thus a c t i v e mildew growth prevented. In a d d i t i o n , by preventing the photooxidation of the surface of the p l a s t i c by the a d d i t i o n of the above l i g h t s t a b i l i z e r s , both the appearance and the p h y s i c a l p r o p e r t i e s of the polymers are preserved f o r s i g n i f i c a n t l y longer periods of time than i f they were u n s t a b i l i z e d or s t a b i l i z e d with other types of s t a b i l i z e r s . In the current study, samples of impact polystyrene that contained a combination of l i g h t s t a b i l i z e r s LS I and LS II and samples that contained no l i g h t s t a b i l i z e r s were weathered outdoors i n F l o r i d a and were monitored f o r changes i n p h y s i c a l appearance and f o r changes i n the chemical s t r u c t u r e of the s u r face by use of a m u l t i p l e i n t e r n a l r e f l e c t a n c e IR s p e c t r o photometer. A l s o , samples of a thermoplastic polyurethane that was s t a b i l i z e d with j u s t LS I and samples that contained no l i g h t s t a b i l i z e r s were weathered and monitored i n the same way as the impact p o l y s t y r e n e . We hope to e s t a b l i s h at what point
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i n the photodegradation of a polymer i t becomes s u s c e p t i b l e to a t t a c k by microorganisms and e s t a b l i s h what e f f e c t the l i g h t s t a b i l i z e r s have i n preventing or d e l a y i n g the photodegradation and subsequent a t t a c k by the microorganisms.
EXPERIMENTAL 1. P r e p a r a t i o n and L i g h t Exposure o f Polymer Samples. (a) Impact P o l y s t y r e n e : L i g h t s t a b i l i z e r s were compounded into commercial impact p o l y s t y r e n e (IPS) by a commerc i a l processor. The samples were i n j e c t i o n molded i n to 1/8 inch t h i c k d i s c s and were subsequently weathered at 45° f a c i n g south i n F l o r i d a . Sample 1 was impact polystyrene which contained no l i g h t s t a b i l i z e r s and Sample 2 was impact p o l y s t y r e n e which contained 0.5% LS I plus 0.5% LS I I . (b) Thermoplastic Urethane F i l m s : Commercial thermoplast i c polyurethane (Estane 5707 from B . F. Goodrich) was d i s s o l v e d i n a 1:1 (w/w) mixture o f DMF/toluene to form a 20% s o l u t i o n . L i g h t s t a b i l i z e r s were separatel y d i s s o l v e d i n a minimum o f the 1:1 solvent mixture and then were added t o the r e s i n s o l u t i o n . Drawdowns of the r e s i n s o l u t i o n s were prepared to o b t a i n a r e s u l t a n t 1.5 m i l t h i c k f i l m a f t e r d r y i n g . The drawdowns were f l a s h e d f o r f i v e minutes at room temperature and then were f o r c e d - a i r d r i e d f o r 15 minutes at 75°C. Subsequently, the f i l m s were exposed at 45° under glass f a c i n g south i n F l o r i d a . Sample 3 was a thermop l a s t i c polyurethane which contained no l i g h t s t a b i l i z e r s and Sample 4 was a thermoplastic polyurethane which contained 0.5% LS I . 2.
Scanning E l e c t r o n Microscope
(SEM).
Each sample was cemented t o a specimen holder and then was coated with approximately 200 8 o f aluminum i n an EFFA Rotary Vacuum Evaporator. The photomicrographs were obt a i n e d from a Cambridge Model S180 Scanning E l e c t r o n Microscope. 3.
M u l t i p l e I n t e r n a l R e f l e c t a n c e (MIR) I n f r a r e d Spectroscopy. A l l samples were run on a P e r k i n Elmer 281 with data s t a t i o n using the m u l t i p l e i n t e r n a l r e f l e c t a n c e attachment (part #186-0382). A 45° r e f l e c t i o n angle was used f o r a l l samples.
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RESULTS The samples of impact polystyrene and a t h e r m o p l a s t i c polyurethane were weathered i n F l o r i d a , and then examined with a scanning e l e c t r o n microscope and with a m u l t i p l e i n t e r n a l r e f l e c t a n c e (MIR) spectrophotometer with the f o l l o w i n g results. W i t h i n s i x months of outdoor exposure i n F l o r i d a , the samples of both IPS and polyurethane that contained no l i g h t s t a b i l i z e r s had a c t i v e growths of mildew p r e s e n t . No mildew was evident on the s t a b i l i z e d samples a f t e r the same period of exposure. ( F i g u r e s 2, 3 , 4) Photos l a and 2a are the photomicrographs of the unexposed impact polystyrene and i l l u s t r a t e the i n i t i a l i n t e g r i t y of the polymer s u r f a c e . Spectra l a i s a MIR s p e c t r a of unexposed neat IPS. Spectra 2a i s a MIR s p e c t r a of the unexposed IPS that c o n t a i n e d l i g h t s t a b i l i z e r s . Note the intense aromatic r i n g b r e a t h i n g modes between approximately 1600 cm and 1400 cm ( 9 ) . Photos lb and 2b are photomicrographs of IPS samples that had been weathered outdoors i n F l o r i d a f o r three months. Spectra 1^ i l l u s t r a t e s the b u i l d u p of earbonyl groups at about 1700 cm a f t e r three months outdoor exposure of t i p IPS. Note the degree of band broadening i n the 1600 cm r e g i o n which i s probably due to a r y l earbonyl f o r m a t i o n . Photo 1c i s a photomicrograph of IPS that contained no l i g h t s t a b i l i z e r s which was exposed outdoors i n F l o r i d a f o r s i x months. Note the t o t a l l o s s of surface i n t e g r i t y i n c l u d ing deep surface c r a z e s . This photo a l s o shows the mildew that was v i s u a l l y present along with t h e i r t y p i c a l hyphal structures. Photo 2c i s a photomicrograph of IPS that contained the combination of l i g h t s t a b i l i z e r s (LS I and LS I I ) which was exposed outdoors i n F l o r i d a f o r s i x months. Note the minimal amount of surface d e t e r i o r a t i o n . Some surface d i r t i s present i n the photo. Spectra 1c corresponds to Photo l c . No|e the intense d e velopment of a earbonyl band around 1620 cm . A f t e r s i x months outdoor weathering the peaks r e l a t e d to the aromatic s k e l e t a l r i n g b r e a t h i n g mode have been masked. Spectra 2c corresponds to Photo 2 c . A s i g n i f i c a n t e a r bonyl peak has formed, but the aromatic s k e l e t a l r i n g b r e a t h ing mode peaks are s t i l l q u i t e apparent. Photos 3a and 4a are the photomicrographs of the unexposed t h e r m o p l a s t i c polyurethanes and i l l u s t r a t e the i n i t i a l i n t e g r i t y of the polymer s u r f a c e . Spectra 3A (neat, no l i g h t s t a b i l i z e r ) and 4a (0.5% LS I) represent the MIR s p e c t r a of the unexposed p o l y e s t e r urethane.
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Figure 2. SEM photomicrographs at 600x (left) and IR spectra (right) of impact polystyrene with no light stabilizers. The samples were exposed in Florida at a 45° angle facing south. SEM shows the presence of mildew.
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