19
Photostability of UV Screeners
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch019
in Polymers and Coatings
James 1
General
E.
1
Pickett
and
James
E.
2
Moore
E l e c t r i c C o m p a n y , C o r p o r a t e R e s e a r c h and D e v e l o p m e n t ,
S c h e n e c t a d y , NY 12301 2
GE
Structured Products,
Mt.
Vernon,
IN
47620
All major classes of commercially available UV screeners undergo photodegradation matrices.
-6
at quantum yields on the order of 1 X
The rates obey zero-order
range and are highly dependent rates of photodegradation
kinetics in the
10
in polar
high-absorption
on the nature of the matrix.
for screeners in cast polymethyl
Typical
methacrylate
films are on the order of 0.3 to 0.7 absorption units per year of outdoor exposure and 0.2 to 0.5 absorption
units per 1000 h of xenon-arc or
QUV exposure. Concentration of the screener appears to play little role. The presence of hindered amine light stabilizers and the nature of the light source might affect the rate of degradation of the matrix. degrading
IS^LOST
Rapidly
matrices seem to cause rapid destruction of the screeners.
PLASTICS
D E G R A D E
W H E N
EXPOSED
OUTDOORS,
and
U V
screeners
often are a d d e d either to the b u l k p o l y m e r o r i n coatings to protect the plas tics. T h e screeners w o r k b y a b s o r b i n g U V light, dissipating t h e e n e r g y h a r m l e s s l y as h e a t ,
a n d thereby
reducing the amount
o f U V light that c a n b e
absorbed b y the polymer. H o w e v e r , to the extent that the q u a n t u m yield o f this p r o c e s s is less t h a n e x a c d y o n e , t h e screeners
are also subject to p h o t o -
d e g r a d a t i o n , a n d t h e i r effectiveness as stabilizers c a n b e lost. M u c h
attention
has b e e n p a i d t o t h e loss o f additives b y extraction o r b l o o m i n g , b u t relatively little w o r k has b e e n d i r e c t e d at t h e i n h e r e n t photostability o f U V screeners a n d t h e factors that affect this photostability. T h e degradation o f both benzophenone
a n d benzotriazole U V screeners
was observed i n polypropylene b y several groups a n d was attributed to r a d i c a l a t t a c k a t d i e c r i t i c a l p h e n o l i c h y d r o x y l g r o u p (1-6)
or through
transfer from excited c h r o m o p h o r e s (7). T h e U V screener was depleted
0065-2393/96/0249-0287$12.00/0 © 1996A m e r i c a n C h e m i c a l Society
In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
free
energy from
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch019
288
POLYMER DURABILITY
the surface of weathered polycarbonate samples, but weathered polycarbonate is known to erode and the screener could have been physically lost (8). The photostability of benzophenone and benzotriazole screeners was studied by flash photolysis and exposure to mercury arcs (9). Recendy, the photodegra dation of benzotriazole screeners was reported in polymethyl methacrylate ( P M M A ) under conditions where leaching could be ruled out (10). Because the findings were fragmentary and the kinetics of the process were ill-defined, we undertook a systematic study of U V screener stability in several matrices with emphasis on the consequences for coatings. Some of this work was re ported previously (II, 12).
Experimental Details All additives were commercial samples obtained from the suppliers. Spectra were taken on a Shimadzu UV-240 spectrophotometer. Rates of degradation were de termined by plotting the absorbance at a maximum vs. time of exposure, drawing a straight line through a portion of the curve, and dividing the slope by the cor rection factor as described subsequendy to arrive at a zero-order rate. The rates are expressed as loss of absorption units per 1000 h of exposure (A/1000 h) unless otherwise specified. Cast Films. P M M A (DuPont Elvacite 2041) or crystal polystyrene were cast from chloroform solution as described previously (II). UV-Cured Acrylics. Resins consisting of hexanedioldiacrylate and a silylated colloidal silica were prepared according to the process described by Lewis and Katsamberis (13). U V screeners were added at 6-7% loadings, and the resins were applied as ca. 5-μπι coatings onto unstabilized 15-mil (368-μιτι) polycarbon ate films and cured under anaerobic conditions using commercially available photoinitiators and Ashdee mercury lamps. Silicone Coatings. Silylated UV-screener derivatives were prepared by hydrosilylating the corresponding allyl derivatives with triethoxysilane and a Pt cat alyst or by treatment of aliphatic alcohol derivatives with 3-trimethoxysilylpropylisocyanate. Coating solutions were prepared by hydrolyzing methyltrimethoxysuane and the silylated U V screener in the presence of aqueous colloidal silica and diluting with a mixture of 2-propanol and 1-butanol. The coating solutions were flow-coated onto glass microscope slides and baked at 130 °C for 1 h to give 4 5-μΐϊΐ coatings. Xenon-Arc Conditions. An Atlas Ci35a weatherometer was run at 0.77 W/m irradiance measured at 340 nm with Type S borosilieate inner and outer filters. The lamp operated in a cycle of 160 min fight at 45 °C dry-bulb temper ature and 50% relative humidity (ca. 65 °C black panel temperature) and 20 min dark. The final 15 min of the dark cycle was with a front and back side water spray. Under these conditions, the samples accumulated 2700 kj/m at 340 nm in 1100 h of exposure. This value is approximately equivalent to one year of Florida 2
2
In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
19.
PICKETT & MOORE Photostability
289
of UV Screeners
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch019
QUV Conditions. Q-Panel Q U V instruments were equipped with FS-4U fluorescent sunlamps. The cycle was 8 h light at 70 °C black panel followed by 4 h of dark condensation at 50 °C. The lamps were rotated or changed at 400-450 h of service according to manufacturer's specifications. The samples were mounted on the back side of a 4 X 4 X 1/16 in. quartz plate by using spring clips so that the samples did not experience the condensation cycle. Mercury Lamp Exposure. The outer glass envelope of a commercial G E HA100 high-pressure mercury street lamp was removed, and the lamp was sus pended in a Pyrex photolysis immersion well. The samples were cast onto quartz plates and hung on a merry-go-round to ensure uniform exposure.
Results and Discussion U V Screener Structure and Function. Even though many doz ens of U V screeners are commercially available, they are all based on just a few chromophores. Representative structures and U V spectra are shown in Chart I and Figures 1 and 2, respectively. If these compounds are to be effective as U V stabilizers they must have high absorption at the wavelengths that cause degradation of the polymer or coating, and they must harmlessly dissipate the energy that they have absorbed. The benzophenone, benzotria zole, and oxanilide classes of screeners are thought to be photostable because their excited states can undergo a rapid internal hydrogen transfer to make higher energy ground-state species as shown in Scheme I (14). The reverse reaction is exothermic, and the heat is dissipated through the matrix. The triazines presumably operate through a similar mechanism. The internal hydrogen bond is key to this process. In a polar environment, some of the hydrogen bonding may be intermolecular with the matrix, and this arrangement would interfere with the mechanism for internal hydrogen transfer. Other reactions leading to destruction of the chromophore could result (25). A second pathway for destruction results if free radicals due to photooxidation of the matrix abstract the phenolic hydroxyl hydrogen of the U V screener leading to oxidation of the chromophore (2, 3). Oxanilides are reported to be unreactive toward free radicals (16). To our knowledge, the photochemistry of cyanoacrylates has not been investigated. One would expect a charge-separated species to be involved in the process at some point (Scheme II). This species, if present, would be subject to attack by nucleophiles such as water. The ground-state species may also be subject to addition of free radicals across the double bond. Either process would result in loss of the chromophore. We subjected Cyasorb 531 (Cyasorb is a trademark of Cytec) to photolysis in air-saturated ethyl acetate solution by using a Pyrex-filtered high-pressure mercury lamp. Whereas many products were observed by gas chromatography, most in very small amounts, benzoic acid was identified as the major product and accounted for about half of the degraded mass. This result is consistent
In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch019
290
POLYMER DURABILITY
Chart I. Structures and classes of the UV screeners. Cyasorb is a trademark of Cytec, Uvinul is a trademark of BASF, and Sanduvor is a trademark of Sandoz. with homolysis of the aryl-carbonyl bond as a major pathway for the degradation of the screener. Photolysis of a benzotriazole screener under similar conditions gave a myriad of products in tiny quantities that we have not yet identified. Kinetics of U V Screener Photodegradation. We described (II) a computer model for the degradation kinetics. In doing the modeling one is faced with the choice of a simple model using a single wavelength of light in a highly absorbing region or a more complicated model including the longer wavelengths of the lesser absorbing "tail". The simple monochromatic model adequately fits the experimental results, at least for the benzophenone and benzotriazole screeners, probably because the longer wavelength fight in the tail has insufficient energy (
