Light Stabilization of Bisphenol A Polycarbonate - Advances in

20

Light Stabilization of Bisphenol A

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.ch020

Polycarbonate

T. Thompson and P.P. Klemchuk Additives Division, Ciba-Geigy Corporation, Ardsley, NY 10502

Bisphenol

A polycarbonate

and phosgene with primarily

(BPA-PC)

nols are added to the polymerization are incorporated

as end groups.

mer with unique properties

is a copolymer

is a major commercial

such as high impact

tributes these excellent properties ronments.

It absorbs terrestrial

strength,

H

E

poly-

glass-like

Its aromatic content con-

and makes it resistant to most envisunlight sufficiently

photooxidation,

to undergo

and chain scission.

tion that accompanies exposure to sunlight detracts from the

T

A

phe-

for molecular weight control and

BPA-PC

clarity, and high glass transition temperature.

phototransformations,

of bisphenol

carbonate linkages. Monofunctional

appearance.

Ultraviolet-light-absorbing

the polymer

to reduce the rate of

Fries

Discolorapolymer's

additives are usually added to

discoloration.

PRIMARY P H O T O T R A N S F O R M A T I O N S of bisphenol A polycarbonate ( B P A -

P C ) reported i n the literature are Fries phototransformations, and photooxidation (Schemes

I—III). T h e s e r e a c t i o n s a r e

chain scission,

wavelength-depend

ent, a n d unfortunately m a n y studies r e p o r t e d i n t h e literature w e r e at w a v e l e n g t h s b e l o w t h o s e o f t e r r e s t r i a l s u n l i g h t . T h e r e f o r e , t h e

conducted findings

are

not necessarily relevant to what m a y occur d u r i n g natural weathering.

Review

of

Literature on

Photoprocesses.

BPA-PC

Photodegradation

I n o n e o f the earliest investigations o f the

photo

d e g r a d a t i o n o f B P A - P C , B e l l u s a n d c o - w o r k e r s (I) e x p o s e d c h l o r o f o r m s o l u tions o f p o l y m e r (no characterization information was provided) to unfiltered light from a 1 0 0 - W medium-pressure

m e r c u r y arc. O n t h e basis o f U V a n d

I R spectra they postulated the formation o f p o l y m e r i c phenylesters o f salicylic

0065-2393/96/0249-0303$12.00/0 © 1996 A 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.

304

POLYMER DURABILITY


hv

I

OH

Scheme I. Fries photorearrangement. acid (absorbance at 315 nm) and polymeric dihydroxybenzophenones (absorbances at 355 nm and 1630 c m , U V and IR, respectively). By using alkaline hydrolysis of the photolyzed polymer, they isolated a bright yellow substance with 7% yield after 120-h exposure. This material was thought to be a sub stituted dihydroxybenzophenone on the basis of U V and IR spectra peaks at 360-362 nm and 1635 c m , respectively, and on the basis of the U V and IR spectra of low molecular weight 2,2'-dihydroxybenzophenones at 335-360 nm and 1630 c m . These results constitute one of the first findings of Fries rearrangements in the photolysis of B P A - P C . The relevance of these results to the natural weathering of B P A - P C is questionable in view of the light source used. However, it is a beginning to understanding polycarbonate photochem istry. In a subsequent study, Mullen and Searle (2) investigated the wavelength sensitivity of 0.1-mil solution-cast and 10-mil extruded P C films by using spec trally dispersed xenon light from 230 to 630 nm. The films were scanned with a U V spectrophotometer at predetermined wavelengths of 320, 360, and 400 nm, which are the absorbance wavelengths of polymeric phenyl salicylates, dihydroxybenzophenones, and yellow products, respectively. Activation spectra - 1

- 1

- 1


20.

THOMPSON & K L E M C H U K

Light Stabilization of BPA-PC

305


h v j

1 CO + ·

Scheme II. Chain scission.

1 Scheme III. Initiation of photooxidation. derived from the data showed the solution-east film was sensitive to wave lengths from 230 to 320 nm, and especially to the region 280-290 nm. The range of wavelengths causing photodegradation of the 10-mil ex truded film was 230-430 nm, which was considerably greater than for the thinner film and was attributed to the greater thickness. The increased film thickness caused a red shift of the activation peak by about 5 nm and increased



306

POLYMER DURABILITY

the photodegradative effect of long-wavelength U V radiation. The results with both films indicated sensitivity to U V radiation in the vicinity of 280-295 nm and supported a two-stage Fries photorearrangement: first to polymeric phenyl salicylates and subsequently to dihydroxybenzophenones. The results provided evidence that other products were also formed. Although direct correlation between the formation of dihydroxybenzo phenones and yellowing was not found, the activation spectra of the solutioncast films for absorbance at 360 and 400 nm were very similar. That similarity and the postulation of Bellus and co-workers regarding the identity of the alkaline hydrolysis product from photorearrangement (substituted dihydroxy benzophenones) suggest the dihydroxybenzophenone functionality may play a role in the yellowing of B P A - P C on exposure to U V light. In addition to those already mentioned, many papers (3-11) were pub lished in which evidence was presented for chain scission and Fries photorearrangements occurring simultaneously in B P A - P C undergoing exposure to U V radiation. Nearly all reports indicated the Fries photorearrangements were favored at lower wavelengths (e.g. 254-290 nm). Both processes appear to arise from C - O bond scission in the carbonate groups on absorption of fight. Chain scission appears to take place more frequently toward the ends of poly mer chains and suggests that when a terminal carbonate group absorbs a photon and cleaves, the fragments can move apart because they are not as restricted as when they are within the polymer chain. Scissions that occur within the polymer chain in glassy regions have a greater chance to recombine. Many studies dealing with the photooxidation of P C were published. In early stages of photooxidation the geminal dimethyl groups were believed to be involved; in later stages ring oxidation was found to take place. Papers by Clark and Munro (12, 13), Factor and co-workers (14, 15), and Rivaton and co-workers (7) are among the most informative and provide a body of infor mation that is essential for understanding the photooxidation of B P A - P C .

I n f l u e n c e o f E n d G r o u p s . Chain terminators such as phenol and f-butylphenol are used to control the molecular weight of B P A - P C and serve to cap the polymer chains. Different manufacturers most likely have their own proprietary practices; therefore, commercial polymers will vary in the type and degree of capping. The degree of capping of the end groups is of signif icance to the polymer because during the high temperature processing that is mandatory with P C , terminal phenolics react with carbonate linkages and cause polymer transformations. The photostability of B P A - P C is also depend ent on the degree of capping of the end groups (the more capping, the more stable the polymer), because free phenolic groups absorb U V radiation at about 290 nm, which is known to cause degradation of the polymer. Polymer terminal groups are measured by IR spectroscopy: terminal phenolic groups absorb at 3595 c m and terminal phenyl groups absorb at 1383 c m . - 1


- 1

20.



307

Webb and Czanderaa (16-18) are virtually the only investigators who have looked at the influence of end groups on the photostability of PCs. They worked mainly with three polymers differing in capping and defined as follows: I, uncapped polymer of low molecular weight ( M , 2500) with 100% free phenolic groups; IV, a commercial polymer with 12% free phenolic groups and capped with 88% phenyl end groups ( M , 18,360); and V , acetylated I with only 1% free phenolic groups ( M , 3120). The wavelength dependence of the photodegradation of thin films was investigated with monochromatic laser U V radiation at 265, 272, 285, 287, and 308 nm. Changes in the vibrational spectra of the capped and uncapped films were measured quantitatively by in situ Fourier transform IR and reflection-ab sorption spectroscopy. The spectra showed that phenolic end groups in the uncapped polymer, if present in concentrations exceeding the water content of the polymer, were hydrogen-bonded to the backbone carbonyl groups. The correspondence of changes in molecular weight to changes in the vibrational spectra of the exposed films was investigated by size exclusion chromatogra phy. The results indicated that free phenolic end groups sensitized P C to some photodegradation reactions (such as cross-linking) at 287 and 265 nm while inhibiting Fries photorearrangements. High concentrations of terminal phe nolic groups in I induced a cross-linking reaction that predominated at 287 nm and competed with chain scission at 308 nm. The quantum yield for chain scission in I was lower than for IV and V, evidently because of the competing cross-linking reactions of terminal phenolic groups. n

n


n

Surface Photodegradation. B P A - P C strongly absorbs U V light be low 290 nm i n 3-mil films and 125-mil thick plaques (Figure 1). The absorb ance tails into the near-UV toward the visible region. Because 290 nm is at the tail end of terrestrial sunlight, the polymer absorbance in that region means U V light-induced reactions will occur i n the polymer during natural weathering. The intensity of the polymers absorbance establishes competition for photons with UVAs. The UVAs can mitigate the harmful effects of U V light on B P A - P C only in proportion to the fraction of U V light of relevant wavelengths that they absorb. Even though the additive's extinction coeffi cients are likely to be much greater than the polymer's in most of the U V region, the concentration differentials on surfaces and in thin sections are likely to favor photochemistry of the polymer on exposure to sunlight. In addition to phototransformations, Webb and Czanderna (16-18) also made observations regarding the photooxidation of B P A - P C , its impact on properties, and photostabilization. They expressed the view that many prop erties such as transparency, tensile strength, impact resistance, and rigidity are adversely influenced by reactions at the surface of the solid polymer, where solar U V absorption and uptake of oxygen and water are highest. In their opinion, the additions of U V stabilizers and antioxidants to the bulk polymer



308

POLYMER DURABILITY

Wavelength (nm)

Figure 1. UV absorbance of BPA-PC: 500 mg/mL chloroform solution, 3-milfilm, and 125-mil plaque. have been generally unsuccessful in extending outdoor service life beyond three years. H y d r o l y s i s . Because B P A - P C is a carbonate ester, it is sensitive to degradation by hydrolysis, mainly at elevated temperatures. The polymer is resistant to hydrolysis by water at ambient temperatures. The solubility of water in the polymer is very low. However, enough water dissolves in B P A P C when immersed in boiling water so that, when cooled to ambient tem perature, the polymer appears hazy due to small droplets of water that are released. Ram et al. (19) examined B P A - P C for hydrolytic stability. They found immersion in water at room temperature for one year had no effect on the mechanical properties of Lexan PC143, a UV-stabilized grade polymer with M of 27,300 and dispersity of 1.63. However, after 30 days in water at 40 °C this polymer lost 16% of initial elongation and after the same period at 60 °C it lost 55% of initial elongation. Testing at higher temperatures revealed total tensile breakdown and severe impact loss after immersion for 2 weeks in water at 80 °C. Boiling water had a catastrophic effect on chain length and mechanical properties. Although not believed to be an important mechanism in normal circumstances, hydrolysis plays a significant role if moisture is not excluded during processing and if high temperatures are a feature of end use. w


20.



309


Evaluation of Light Stabilizers Little information has been published about the effectiveness of additive classes other than UVAs in protecting the polymer against photodegradation. This study was undertaken to investigate the photostabilization of B P A - P C by the main classes of stabilizing additives. Several chemical classes of UVAs, hindered amine light stabilizers (HALS), a hindered phenolic benzoate, and a nickel-containing stabilizer were evaluated. The major objective of this investigation was to determine the influence of stabilizing additives on several polymer properties during exposure of 1-mil thick solution-cast films to artificial light sources or sunlight. The properties of interest were color; tensile strength and elongation; U V , IR, and N M R spectra; molecular weight; and rate of oxygen consumption. Evaluating stabilizers in solution-cast films offers two distinct advantages: First, thermal degradation of the additive or the polymer will not lead to erroneous light-stabilizer activity. Second, a thin film mimics what would be expected to happen at the surface of the polymer, which is the most difficult area to stabilize and arguably the most important.

Experimental Materials. The additives used were of commercial quality and were used without additional treatment. The polymer used for most of the work was Lexan 141-111, which was used without additional treatment. Stabilizers. The stabilizers used were as follows: UVA-1, 2-(2'-hydroxy s', 5'-di(dimethylbenzyl)benzotriazole; UVA-2, 4-octadecyl-4'-ethoxyoxanilide; UVA-3, 2-ethylhexyl-2 -cyano-3 ,3 -diphenylacrylate; HALS-1, bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate; HALS-2, bis(l,l-octyloxy-2,2,6,6-tetramethyl-4piperidinyl)sebacate; NiStab, nickel bis[emyl-(3,5-di-teft-butyl-4-hydroxyberizyl) pnosphonate];and benzoate, hexadecyl-3,5-di-ferf-butyl-4-hydroxybenzoate. ,

,

,

Preparation of Specimens for Exposure. BPA-PC solutions were pre pared containing 20% BPA-PC in spectrograde methylene chloride. The appro priate amounts of additive were added to the polymer solutions. Films were cast with an 8-path wet-film applicator (Gardco, 2-in. width, 8-mil wet thickness) on 8 X 10 X 1/4 in. glass plates to provide 10 X 2 X 0.001 in. specimens. The films were mounted in infrared cards that were modified with 1.5 X 1.5 in. openings. Property Measurements. Yellowness index (ASTM1925) measurements were made on the films with the Applied Color Systems, Inc. model CS-5 spec trophotometer using the 2° observer and the large area opening with specular reflector included. The U V measurements were made with a Gilford RESPONSE UV-vis spectrophotometer; IR spectra were run with the Perkin Elmer model No. 683 IR spectrophotometer; NMR spectra were run with the Varian Unity 500 N M R system. Molecular weight analysis was carried out by gel permeation chro-


310

POLYMER DURABILITY


matography using P C standards (Aldrich). In all cases duplicate determinations were made. Tensile testing was carried out with an Instron model No. 1123 on 6 X 0.25 X 0.001-in. specimens (4-6 pulls were averaged). The following conditions were used: gauge length, 50.8 mm (2 in.); crosshead speed, 10 mm/min; and full-scale load, 5.0 kg. Exposures. Most of this work was carried out in a custom-built exposure device (referred to as 340 F L ) in which forty 48-in. UV-A fluorescent lamps (340 nm) were mounted vertically in the form or a cylinder with a diameter of 34 in. Samples were exposed vertically 2 in. away from and parallel to the bulbs on aluminum shelves that were rotated at 1.25 rpm. A limited number of samples were exposed in an Adas Xenon Arc Weather-ometer (XAW), model Ci-65, 0.35 W/m irradiance, Corad filters, and no spray cycle. Films supported on glass plates were exposed in Florida facing south at a 45° angle for one year. Irradiance measurements were made with an International Light IL1700 ra diometer for the two accelerated exposure devices at two relevant wavelengths of 290 and 365 nm. The X A W had a greater proportion of far-UV light than r i d the 340 F L exposure. The ratio of UV-B to UV-A for the X A W was consistent with published data (Table I). 2

Oxygen Uptake Procedure. BPA-PC solutions were prepared with spectrograde methylene chloride in Pyrex test tubes, 8 X 3/4 in. o.d. Each tube was rolled horizontally for 2 h to form a uniform film of BPA-PC on the inner wall of the tube. The tubes were filled with pure oxygen after a minimum of six complete evacuations. An initial pressure of 500-600 mm H g was established to bring at tention to any leaks in the closed system. The tubes were exposed in an Applied Photophysics Multilamp photoreactor equipped with fluorescent lamps centered at 320 nm and modified for concurrent exposure of six samples. Oxygen uptake was monitored with pressure transducers interfaced with an IBM-XT personal computer. Samples were run in triplicate for each formulation. The oxygen versus nitrogen exposure was carried out in a Rayonet photoreactor equipped with 310nm fluorescent lamps. Table I. Irradiance of Light Source Irradiance Measured* 290 nm 365 nm Ratio, 290/365 Published^ 250-320 nm (UV-B) 320-400 nm (UV-A) Ratio, UV-B/UV-A

Sunlight

340 FL

XAW

ND ND ND

0.173 13.14 1:76

2.41 38 1:16

1.4 25.3 1:18

ND ND ND

3.3 54.2 1:16

NOTE: Values are in W/m . "XAW was controlled at 0.35 W/m . Data were published by Atlas Electric Devices Co. XAW was controlled at 0.55 W/m . 2

2

fo

2


20.



311


Results and Discussion Yellowness Index and Tensile Property Changes, 340 F L Exposure. Formulations containing UVAs significantly reduced yellowing of the 1-mil B P A - P C films on exposure for 1223 h in the 340 F L exposure device. Non-UV-absorbing light stabilizers, alone or in combination with U V A - 1 , of fered little or no improvement in yellowness index (YI). The breaking strength of formulations containing UVAs changed little after the final interval of this test of 1223 h exposure in the 340 F L exposure device. Formulations con taining only non-UV-absorbing stabilizers underwent a significant loss i n breaking strength. Table II summarizes the results. YI and Tensile Property Changes, Comparison of Light Exposures. The YI measurements (7-8 for all) of B P A - P C films for a blank and 1 or 2% U V A - 1 showed little differentiation after 1 year of exposure in Florida. This result was not the case for the exposures in the 340 F L and X A W ; lower rates of color development resulted in the films with UVA-1 than in the blank films. The 340 F L and X A W developed about the same degree of yellowing i n the samples. The YI proved to be a very useful method for following degradation on photolysis of B P A - P C . The blank and stabilized films underwent considerable loss of elongation after 6 months of exposure i n Florida. The stabilized formulations retained elongation only slightly better than unstabilized ones. These films, when com pared with films of similar YIs after exposure in the 340 F L or X A W , showed much greater loss in elongation than the films exposed in the indoor devices. The greater loss of elongation during exposure in Florida may have been due Table II. Comparison of Light Stabilizer Classes

Stabilizer

Yellowness Breaking Index (1925) Strength (kg) 1223 h Oh 1223 h Oh

None UVA-1 UVA-2 UVA-3 HALS-1 HALS-2 NiStab Benzoate UVA01/HALS-1 UVA-l/HALS-2 UVA-l/NiStab UVA-l/Benzoate

3.1 3.2 3.1 3.1 3.2 3.1 4.7 3.1 3.3 3.2 3.6 3.2

18.1 5.7 6.8 6.5 13.1 11.7 21.8 15.4 5.6 4.8 5.5 4.6

7.4 7.8 7.2 8.0 6.7 7.3 7.1 7.2 6.3 7.6 6.6 8.1

0.4 6.2 6.5 6.2 0.5 0.7 0.3 0.8 7.3 6.0 6.2 6.0

NOTE: Samples were 1-mil BPA-PCfilmsexposed to 340 F L . All UVAs were 2 wt% and non-UVAs were 1 wt%.


POLYMER DURABILITY

312


to hydrolysis effects combined with photolysis effects. Elongation losses were greater in the X A W exposure than in the 340 F L exposure. Even though the X A W was controlled at a constant relative humidity of 30%, it probably had more moisture present than the 340 F L . This difference again implicates hy drolysis as a possible contributor to loss of tensile properties, although the greater portion of far-UV light in the X A W may also have contributed. Tables III-V summarize YI data, and Tables VI-VIII summarize percent elongation data for the accelerated and outdoor exposures. U V and IR Spectral Changes, 340 F L Exposure. O n exposure to U V light, unstabilized B P A - P C underwent a broad increase i n absorption Table III. Yellowness Index of 1-mil BPA-PC Film on Exposure in 340 FL Formulation Oh 285 h 535 h 797 h 973 h 1254 h 1473 h Blank 1% UVA-1 2% UVA-1

3.1 3.2 3.2

4.5 3.9 3.7

8.1 4.9 4.6

6.5 4.5 4.0

10.7 5.4 4.9

NA NA 6.5

15.4 6.6 NA

NOTE: N A is not available.

Table IV. Yellowness Index of 1-mil BPA-PC Film on Exposure in XAW 1277 h Formulation 538 h 780 h 1019 h Oh 308 h Blank 1% UVA-1 2% UVA-1

3.1 3.1 3.1

6.0 4.6 3.9

4.9 3.8 3.6

7.9 NA NA

NA 9.1 6.5

11.0 6.2 4.9

NOTE: N A is not available.

Table V. Yellowness Index of 1-mil BPA-PC Film on 45 °South Exposure in Florida Formulation 0 mo 1 mo 2 mo 3 mo 4 mo 5 mo 6 mo 9 mo 12 mo Blank 1% UVA-1 2% UVA-1

3.1 3.1 3.1

3.3 3.3 3.3

3.8 3.5 3.4

4.2 3.9 3.9

4.6 4.3 4.1

6.1 5.4 5.2

5.7 5.5 5.2

8.2 7.4 7.2

Fail. 10.4 9.5

Table VI. Percent Elongation of 1-mil BPA-PC Film on Exposure in 340 F L Formulation Oh 285 h 535 h 797 h 973 h 1254 h 1473 h Blank 1% UVA-1 2% UVA-1

90 96 95

36 97 118

18 64 70

6 NA NA

1 16 63

NA 6 NA

NOTE: Five replicates pulled for each formulation. N A is not available.


NA NA 11

20.



313

Table VII. Percent Elongation of 1-mil BPA-PC Film on Exposure in XAW Formulation

Oh

Blank

308 h

538 h

780 h

1019 h

1277 h

116

12

15

4

1

ΝΑ

1% U V A - 1

99

77

37

ΝΑ

3

3

2%

95

49

56

ΝΑ

5

3

UVA-1


NOTE: Four replicates pulled from each formulation. N A is not available.

Table VIII. Percent Elongation of 1-mil BPA-PC Film on 45 °South Exposure in Florida Formulation 0 mo I mo 2 mo 3 mo 4 mo 5 mo 6 mo 9 mo 12 mo 116

67

29

11

5

3

2

Fail.

Fail.

1%

UCA-1

99

50

45

27

23

12

6

3

2

2%

UVA-1

95

52

47

35

14

6

9

4

3

Blank

NOTE: Six replicates pulled for each formulation.

in the near-UV region and tapered off into the visible region. This increase is in direct relation to increases in YI, which is a valuable technique for following photolysis of P C films that do not contain UVAs. The unstabilized films also underwent a significant broad increase in absorbance in the 3500 c m region, presumably due to the formation of phenolic species. The IR spectra changed little for the films stabilized with UVA-1. IR analysis is a good qualitative technique for following photolysis of B P A - P C ; however, quantification of a broad IR peak resulting from several species is not a simple task. - 1

N M R Spectral Changes, 340 F L Exposure. Analysis by 500M H z N M R spectroscopy was used to follow the photolysis of unstabilized P C films. Two types of protons were identified. Protons at 6.73 and 6.80 ppm in the ortho position relative to hydroxyl were a measure of the formation of phenolic species and accounted for about 2% of the aromatic protons after 1200 h exposure to 340 F L . Additionally, protons observed at 8.05 and 8.15 ppm that were ortho to a carbonyl were a measure of the amount of Fries photo-products formed. These protons accounted for about 0.5% of the aro matic protons after 1200 h exposure to 340 F L . This technique was not able to differentiate the performances of UVAs during short exposure periods: the N M R spectra of the films containing UVAs changed little during a typical accelerated exposure study. Longer exposures are needed to differentiate be tween UVAs by this technique. Molecular Weight Changes, 340 F L Exposure. Films stabi lized with UVA-1 maintained their molecular weights, whereas those of un stabilized B P A - P C films were reduced considerably during 1200 h exposure. (Table IX, 340 F L exposure). Difficulties in reproducibility were encountered


314

POLYMER DURABILITY

Table IX. Molecular Weight and Chain Scission Analysis of 1-mil Films on Exposure to 340 FL Formulation

Oh

285 h

Blank 1% UVA-1 2% UVA-2

1.60 1.60 1.60

1.34 1.44 1.58

Blank 1% UVA-1 2% UVA-2

0 0 0

0.19 0.11 0.01

M X I0" 1.04 1.17 1.31 1.50 1.49 1.52 1.55 1.54 1.61 Chain Scissions 0.54 0.37 0.22 0.07 0.05 0.07 0.04 0.03 -0.01 n


973 h

797 h

535 h

1254 h

1473 i

0.74 1.32 ΝΑ

ΝΑ ΝΑ 1.35

1.16 0.21 ΝΑ

ΝΑ ΝΑ 0.19

4

NOTE: Values reported are averages of duplicate G P C determinations. The precision is ± 10% relative to polycarbonate standards. Values for M were calculated based on P C standards. n

with the gel permeation chromatography method that was used for molecular weights. Changes in molecular weight can provide valuable insights into the photodegradation of B P A - P C .

Oxygen Uptake Results. UVA-1 was evaluated to determine its ef fect on the photooxidation of B P A - P C . F i l m samples coated on the interior of Pyrex tubes were exposed under an oxygen atmosphere for approximately 600 h in an Applied Photophysics photoreactor equipped with 320-nm fluo rescent lamps. Oxygen uptake was monitored continuously. U V A - 1 was very effective at retarding photooxidation of P C : Oxygen uptake values for a blank and B P A - P C sample treated with U V A - 1 (1 wt%) were 52,000 and 23,000 mmol/h, respectively.

Investigations with Poly(methyl methacrylate) Filters. B P A P C producers have responded to the difficulty in stabilizing the bulk polymer by focusing on stabilizing the surface of the polymer. In a limited investigation, a 10-mil poly(methyl methacrylate) filter containing 2% UVA-1 effectively sta bilized the surface of B P A - P C . During 1600 h exposure in the 340 F L , results with the films behind filters without U V A - 1 were reminiscent of results with blank, unfiltered, unstabilized films; they increased in YI from 3.5 at the start to 19.5. O n the other hand, the films behind filters with 2% U V A - 1 did not show any significant increase in YI. The same was true of changes in U V absorption spectra: The U V absorbance of the blank-filtered samples increased significantly, whereas those exposed to UV-filtered light had not changed at all in U V absorption characteristics. These results demonstrate the importance of stabilizing the surface of the polymer and the feasibility of a filtering ap proach to the light stabilization of B P A - P C .



20.



315

Comparison of Results with Oxygen and Nitrogen Atmospheres. Unstabilized 1-mil B P A - P C films were exposed for 500 h in a Rayonet photoreactor equipped with fluorescent lamps centered at 310 nm, under both nitrogen and oxygen atmospheres, to compare the color development attributed to photooxidation with color development from Fries photorearrangements. O n the basis of changes in UV-vis absorbance spectra, the color formation under nitrogen was slightly greater than under oxygen; therefore, photooxidation does not play a major role in the yellowing of B P A - P C (Figure 2).

Conclusions • UVAs were found to be the most effective stabilizers against the photo degradation of B P A - P C . Other classes of stabilizing additives were gen erally ineffective when used alone and they contributed only marginally to the stabilization provided by a U V A . • YI measurements of 1-mil B P A - P C films proved to be a reliable and reproducible measure of photodegradation of the polymer; difficulties with reproducibility were encountered with tensile testing of the 1-mil films.

250

300

350

400

450

Wavelength (nm) Figure 2. UV absorbance of BPA-PC for oxygen vs. nitrogen atmosphere. UNEX is unexposed polymer.



316

POLYMER DURABILITY

• The 340 F L exposure device was suitable for evaluating light stabilizers in B P A - P C . • Monitoring changes i n U V absorption spectra of B P A - P C on light ex posure was useful for evaluating the photodegradation of films not containing UVAs. • Natural weathering in Florida proved to be more severe than the indoor exposure devices. Percent elongation dropped faster, relative to YI, for the films exposed to natural weathering than for the films exposed in the 340 F L and the Xenon Arc weatherometer. • N M R analysis at 500 M H z was useful for following photodegradation of unstabilized films. This technique was not useful for differentiating be tween UVAs during our typical exposure periods. • IR spectroscopy was useful for qualitative monitoring of hydroxyl for mation in B P A - P C during fight exposure. • Similar typical increases of U V absorption in spectra of B P A - P C were found with unstabilized films exposed to fight in both oxygen and nitro gen. However, the exposure of the films in a nitrogen atmosphere caused slightly more color development than exposure in oxygen. • Limited oxygen uptake results indicated UVAs significantly reduced the rate of photooxidation of B P A - P C . • The absorbance of B P A - P C in the far-UV region of terrestrial sunlight made its photostabilization challenging because the polymer competed with UVAs for U V photons. • The most effective stabilization of B P A - P C was obtained by exposing specimens behind P M M A filters containing a U V A .

References 1. 2. 3. 4.

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28, 1994.

for review December 6, 1993.

ACCEPTED

revised manuscript November


Light Stabilization of Bisphenol A Polycarbonate - Advances in

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