Mechanisms of Thermal and Photodegradations of Bisphenol A

5

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch005

Mechanisms of Thermal and Photodegradations of Bisphenol A Polycarbonate Arnold Factor General Electric Research and Development Center, PO Box 8, Schenectady,NY12301

This chapter

presents an up-to-date

thermal and photodegradations as new results from drolysis,

review of the literature

on the

of bisphenol A polycarbonate

as well

my laboratory.

The areas described

thermal and thermal oxidative reactions, and

tion. Special emphasis is given to the mechanisms

include

hy

photodegrada-

of these reactions

and to correlating the past work in these areas with more recent results.

BISPHENOL A POLYCARBONATE (BPA-PC; 1) has the desirable combinatior of toughness, transparency, and high heat-distribution temperature. This com bination makes B P A - P C an ideal material for demanding applications where resistance to hydrolytic, thermal, and photochemical degradation are impor tant. A key part of designing more stable systems that allow the reliable use of B P A - P C at the limits of its stability range has been the application of ί detailed understanding of the hydrolytic, thermal, thermal oxidative, and pho tochemical degradation. This chemistry will be reviewed, and an emphasis wil be placed on correlating the past work in these areas with more recent result! and identifying the likely species responsible for discoloration.

Hydrolysis Perhaps the most important but most easily overlooked aspect of BPA-PC stability is its vulnerability to reaction with water (Scheme I). The reaction i: important both during melt processing and in end-use applications involving 0065-2393/96/0249~0059$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


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POLYMER DURABILITY

Scheme I. Hydrolysis of BPA-PC.

exposure to water at elevated temperatures, such as sterilization by autoelaving. The keys to minimizing this reaction with water are as follows: • • • •

removal of catalytic acidic or basic process residues from the resin dur ing manufacture lowering the water content of the resin to ε I

1 ι ι

I

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

5.

FACTOR

67

Thermal and Photodegradations of BPA-PC

The formation of p-hydroxyacetophenone and p-hydroxybenzoic acid is envisioned as coming about via a rearrangement reaction of an initially formed methyl radical to form the more stable benzylie radical (Scheme VI), followed by reaction with 0 to form both products. Even though p-hydroxybenzoic acid was not actually detected in this study, its absence was attributed to the inability of the H P L C to resolve it. This reaction sequence was first postulated by Lee (21) to explain the catalytic effect of 0 on the thermal decomposition of B P A - P C . This sequence also explains the formation of p-hydroxybenzoic acid and p-hydroxyacetophenone derivatives during the photooxidation of B P A - P C (19). 2


2

€Η ·

CH -Ph~

2

~Ph-C-Ph I

CH

•

-Ph-CCH

2

^

I

3

CH -Ph~

2

-Ph-C-OOH

• products

I

3

CH

3

Scheme VI. Proposed pathway for the oxidation of the 2,2-diphenyl propyl unit.

Photoaging of BPA-PC When unstabilized B P A - P C is exposed to U V light, such as encountered dur ing outdoor exposure, the surface of the resin will become yellow and will often erode. The process is a surface phenomenon and generally extends only —25 μιχι into the exposed surface (19). As indicated in Scheme VII, depending on the specific exposure conditions, the chemistry underlying these changes has been ascribed to three general processes: Fries photorearrangement and fragmentation/coupling reactions (2, 22-25), side-chain oxidation (19, 26-32), and ring oxidation (26). Recent spectral studies by Lemaire and co-workers (27, 28) and Pryde (31) clearly illustrated that Fries photoreactions were fa vored when light with λ < 300 nm was used, whereas photooxidation reactions were increasingly more important as U V light of higher wavelengths was used. Nonetheless, the relative roles of each of these reactions and the chemical nature of the compounds responsible for the observed color formation are not clear. To better answer these questions, we recently undertook a product study of an unstabilized sample of B P A - P C that was exposed outdoors to four years of Florida weathering by reductive cleavage of the photoaged material with lithium aluminum hydride ( L A H ) (and lithium aluminum deuteride) followed by analysis by tandem gas chromatography (GC/GC)-high-resolution M S (30). In this way nearly 40 degradation products were characterized. The most im portant ones are fisted in Chart II and are grouped according to the most


POLYMER DURABILITY


68

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1 ο ο.

g

i

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In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996. IPP Dimer Derived Products

Chart II. Key products identified from the GC-GC/MS analysis of BPA-PC aged 4 years in Florida.

Ring Attack PrQduçtS



70

POLYMER DURABILITY

likely mechanism for their production. To analyze for higher molecular weight products, a direct-probe M S experiment was also carried out and revealed the presence of a number of B P A coupling products (Figure 3). The production of these coupling products can best be rationalized by the occurrence of the Fries photofragmentation/couphng reactions illustrated in Scheme VIII. The results of these experiments indicate that the outdoor weathering of B P A - P C is quite complex and involves the operation of at least four processes: side-chain oxidation, ring oxidation, Fries photorearrangement and fragmen tation/coupling reactions, and ring-attack reactions. Ring-attack reactions were not previously reported and are thought to come about by a free-radical re action of phenolic end groups with methyl radicals (30). Most of the com pounds found are not expected to be deeply colored; however, a few, such as the B P A resorcinol derivative listed in Chart II and the ortho-coupled B P A product in Scheme VIII, probably come from L A H reduction of highly colored o-quinone and o-diphenoquinone structures. Even though the products of photooxidation predominate and the Fries photoproducts constitute only a minor part of the product mixture from outdoor-weathered B P A - P C (19, 3032), the Fries photoprocess likely plays a key role in the autocatalytic photooxidative process. During the initiation of the photodegradation process, the

Scheme VIII. Probable mechanism for the formation of higher molecular weigh photoproducts. (Reproduced with permission from reference 39. Copyright 1989 Technomic.)



Figure 3. Direct-probe MS analysis (300-1000 amu, 17 scans merged) of the silyhted LAH reduction product from BPA-P 4 years in Florida.

108.0 -ι



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POLYMER DURABILITY

Fries photoreaetions are probably a major source of free radicals leading to photolabile oxidation products such as hydroperoxides and aromatic ketones, which in turn account for the final autocatalytic stage of B P A - P C photodegradation. In an earlier study of B P A - P C photochemistry, Webb and Czanderna (33) found Fourier transform IR evidence for the presence of hydrogen bonding between free phenolic hydroxyl end groups and the carbonyl of the carbonate group when the phenolic end group concentration exceeded that of the water in the polymer. They proposed that upon photolysis these hydrogen-bonded moieties underwent a hydrogen atom transfer reaction giving rise to reactive free radicals that led to cross-finking (Scheme IX). However, because com mercial BPA-PCs are greater than 97% capped, this pathway probably does not play an important role in the early stages of B P A - P C photogradation. These reactions could very well be important in the later stages, where, as shown by Moore (34), the concentration of phenolic end groups becomes significant. In recent publications by Hoyle and co-workers (35, 36), fluorescence spectroscopy was used as an extremely sensitive tool for the detection of the primary products of the thermal and photodegradation of B P A - P C . For ex ample, short exposure in air to U V fight having a X of —300 nm produced Fries photoproducts, such as salicylic acid and biphenolic type species. Also, as described earlier (7), thermal treatment of B P A - P C in either air or nitrogen at temperatures as low as 250 °C gave rise to a structured fluorescence emis sion mainly due to the formation of dibenzofuran and phenyl-2-phenoxybenzoate. However, photolysis studies of representative B P A - P C thermal degradation products like dibenzofuran, B P A , and xanthone using broad spec trum U V light with a X of 305 nm showed that the presence of these structures in B P A - P C would not greatly effect the photodegradation of B P A P C other than to form photoproducts that subsequently underwent photobleaching. max

max

Crosslinking

Scheme IX. Proposed photoreaction of hydrogen-bonded carbonate groups.


5.

FACTOR

73

Thermal and Photodegradations of BPA-PC


Finally, further proof of the occurrence of the Fries photoprocess during outdoor exposure was provided by an experiment in which an unstabilized molded sample of B P A - P C was sealed in high vacuum (

Mechanisms of Thermal and Photodegradations of Bisphenol A

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