Ayako Torikai

Accelerated photodegradation of poly(vinyl chloride)

This study aimed to find a method for the degradation of waste plastic materials under terrestrial sunlight. Poly(vinyl chloride) (PVC) pre-irradiated with shorter wavelength radiation was irradiated with radiation of longer wavelengths, longer than 290 nm, and the effect of wavelength on the degradation of PVC was investigated by using optical absorption and gel permeation chromatography (GPC). It was found that both the main-chain scission of PVC and degradation product formation were accelerated under the longer wavelength radiation (simulating terrestrial sunlight) by pre-irradiation. These reactions were dependent on the pre-irradiation time. The experimental results show the accelerated photodegradation of PVC under the terrestrial sunlight is possible by pre-irradiation with shorter wavelength radiation.

Spectral sensitivity of chitosan photodegradation

The wavelength sensitivity of photodegradation of solvent-cast chitosan films exposed to monochromatic UV-visible radiation is reported. Measurements were made of changes in absorption spectra, both in the UV-visible region and in the infrared region, as well as changes in dilute solution viscosity of samples, on irradiation at selected wavelengths. Action spectra are reported for these processes. A mechanism of photodegradation based on changes in Fourier transform infrared (FTIR) spectra of irradiated chitosan is presented.

Effects of climate change and UV-B on materials

The outdoor service life of common plastic materials is limited by their susceptibility to solar ultraviolet radiation. Of the solar wavelengths the UV-B component is particularly efficient in bringing about photodamage in synthetic and naturally occurring materials. This is particularly true of plastics, rubber and wood used in the building and agricultural industries. Any depletion in the stratospheric ozone layer and resulting increase in the UV-B component of terrestrial sunlight will therefore tend to decrease the service life of these materials. The extent to which the service life is reduced is, however, difficult to estimate as it depends on several factors. These include the chemical nature of the material, the additives it contains, the type and the amount of light-stabilizers (or protective coatings) used, and the amount of solar exposure it receives. Concomitant climate change is likely to increase the ambient temperature and humidity in some of the same regions likely to receive increased UV-B radiation. These factors, particularly higher temperatures, are also well known to accelerate the rate of photodegradation of materials, and may therefore further limit the service life of materials in these regions. To reliably assess the damage to materials as a consequence of ozone layer depletion, the wavelength sensitivity of the degradation process, dose-response relationships for the material and the effectiveness of available stabilizers need to be quantified. The data needed for the purpose are not readily available at this time for most of the commonly used plastics or wood materials. Wavelength sensitivity of a number of common plastic materials and natural biopolymers are available and generally show the damage (per photon) to decrease exponentially with the wavelength. Despite the relatively higher fraction of UV-A in sunlight, the UV-B content is responsible for a significant part of light-induced damage of materials. The primary approach to mitigation relies on the effectiveness of the existing light stabilizers (such as hindered amine light stabilizers, HALS) used in plastics exposed to harsh solar UV conditions coupled with climate change factors. In developing advanced light-stabilizer technologies, more light-resistant grades of common plastics, or surface protection technologies for wood, the harsh weathering environment created by the simultaneous action of increased UV-B levels due to ozone depletion as well as the relevant climate change factors need to be taken into consideration. Recent literature includes several studies on synergism of HALS-based stabilizers, stabilizer effectiveness in the new m-polyolefins and elucidation of the mechanism of stabilization afforded by titania pigment in vinyl plastics.

Photo-oxidative degradation of polyethylene: Effect of polymer characteristics on chemical changes and mechanical properties. Part 1—Quenched polyethylene

Abstract Photo-oxidative degradation of quenched samples of linear low density (LLD), medium density (MD) and two kinds of high density (HD) polyethylene (PE) films was studied using a medium-pressure mercury lamp. Greater amounts of crosslinking and build up of oxidation products were noticed in LLDPE than the other samples. The primary products of interaction between dienes and oxygen are considered to participate in the initiation of the photo-oxidation reactions. Using the FT-IR difference spectrum technique, the amount of branch concentration in the photo-irradiated PE samples was determined. Oxidation damage at the boundary region between crystalline and amorphous phases is considered to be important in determining the embrittlement time.

Photodegradation of polystyrene and polystyrene containing benzophenone

Abstract The photodegradation of polystyrene has been studied by electron spin resonance, infra-red absorption, viscosity and gel fraction measurements. Irradiations with light from a medium pressure mercury lamp to polystyrene film at 45°C in vacuum, gave rise to the polystyryl radical. When the irradiation was carried out in air under the same conditions as those in vacuum, the peroxy radical was formed. Main-chain scission of polystyrene occurs predominantly in air. Oxygenated products were detected by infra-red spectroscopic measurements and the degradation reaction was found to proceed via peroxide intermediates and result in main-chain scission leading to the formation of carbonyl compounds. When the polystyrene films were irradiated in vacuum, the main reaction was cross-linking through combination of polystyryl radicals. The photodegradation of polystyrene films containing benzophenone has also been investigated. This additive acts as a sensitiser for the photodegradation. A possible mechanism for the radical formation and degradation of polystyrene is proposed.

Photodegradation of polyethylene: Effect of crosslinking on the oxygenated products and mechanical properties

Abstract The photodegradation of three types of polyethylene has been studied by electron spin resonance (ESR), ultraviolet absorption (UV), Fourier transform infrared absorption (FTIR), gel fraction and mechanical properties measurements. Various types of oxygenated products were detected by FTIR spectrometry in photoirradiated polyethylene. The rate of formation of oxygenated products is greater in the case of photoirradiated crosslinked polyethylene. A detailed photodegradation mechanism for polyethylene is proposed on the basis of the experimental results. Tensile tests on photoirradiated polyethylene were also carried out. It was found that crosslinking improved the photostability of polyethylene. The relationship between the amount of oxidation products produced and the decrease of mechanical strength gives a measure of the photostability of polyethylene. We have tried to correlate the carbonyl content with the mechanical properties to predict the photostability of the polymer.

Radiation-induced degradation of polycarbonate: Electron spin resonance and molecular weight measurements

Elementary processes in the γ-irradiated degradation of bisphenol A polycarbonate (PC) have been studied by electron spin resonance (ESR) and molecular weight measurements. γ-Irradiation of PC films at −196°C gave an ESR spectrum attributable to trapped electrons, phenyl and phenoxy type radicals. An ionic species produced in PC was observed by optical absorption measurements. The viscosity average molecular weight, Mv, decreased with increasing irradiation dose both in vacuum and in air. A linear relationship between (1Mn(t) − 1(Mn(0)) and irradiation dose was obtained, where Mn(t) is the number average molecular weight at irradiation time t. Random chain scission processes are suggested for γ-ray induced degradation of PC films. The ratio of the apparent rate constant of chain scission of PC in air to that in vacuum, ks(air)/ks(vac), was estimated to be 2·4. A mechanism for γ-ray induced degradation of PC is proposed on the basis of the experimental results.

Radiation-induced degradation of polyethylene: Role of amorphous region in the formation of oxygenated products and the mechanical properties

Abstract Quenched samples of linear low density, medium density and two kinds of high density polyethylene films were irradiated with γ-rays from a 60 Co source in vacuum and in air at room temperature with irradiation doses ranging from 0 to 100 Mrad. On irradiation in vacuum the extent of crosslinking was about one-and-a-half times greater in the linear low density polyethylene (LLDPE) than in the high density polyethylene (HDPE). On the other hand, irradiation in air produced more crosslinking in high density polyethylene (HDPE). Growth of trans -vinylene unsaturation was found around 10 Mrad in all the samples. Initial increase in elongation and breaking strength (below 5 Mrad) occurred, which was followed by a decrease with increasing dose. LLDPE showed some elongation even at 50 Mrad, while the other samples became brittle and broke at doses far below this value. The mechanism of oxidative degradation is discussed.

Photodegradation of poly(methyl methacrylate) studied by ESR and viscosity measurements

Abstract The photodegradation of poly(methyl methacrylate) has been investigated by electron spin resonance and viscosity measurements. On UV irradiation of poly(methyl methacrylate) film at 40°C in either a vacuum or in air, radicals were produced. Although the observed radical concentration in air is considerably different from that in a vacuum, the number of radicals responsible for the chain scission processes were almost the same in both cases. When the polymer film is irradiated with light of wavelength longer than 290 nm, radical formation is suppressed and the viscosity average molecular weight remains unchanged. The photodegradation has also been examined for poly(methyl methacrylate) films containing benzophenone and 2-hydroxy-4-methoxybenzophenone. Both additives were found to act as protective agents for the polymer. A mechanism of protection by these additives against photodegradation is proposed on the basis of the experimental results.

Elementary processes in the photodegradation of polyvinylchloride

Abstract The elementary processes of photodegradation of polyvinylchloride have been investigated by both electron spin resonance and optical absorption measurements. The final products formed via these elementary reactions have been detected by infra-red absorption measurements. On irradiating polyvinylchloride films at −196°C, alkyl type radicals were produced. This type of radical is shown to be a precursor of the polyenyl type radical which is produced at higher temperatures. The allyl, dienyl and polyenyl type radicals were observed in the course of photodegradation. The peroxy radical was found to be produced by oxidation of the polyenyl type radical. A possible mechanism for the photodegradation of polyvinylchloride films is proposed on the basis of these observations. The mechanism of stabilisation against photodegradation has also been examined for polyvinylchoride films containing 4-methoxy-2-hydroxybenzophenone as an ultra-violet absorber.