Handbook of UV Degradation and Stabilization

George Wypych

• Publisher: ChemTec Publishing
• Year: 2015
• ISBN:
  9781927885017
• Edition: 2
• Topic: Tecnologia e ingegneria
• Subtopic: Scienza dei materiali

This book, the second edition of the first monograph fully devoted to UV degradation and stabilization ever published in English, has 12 chapters discussing different aspects of UV related phenomena occurring when polymeric materials are exposed to UV radiation.

In the introduction the existing literature has been reviewed to find out how plants, animals and humans protect themselves against UV radiation. This review permits evaluation of mechanisms of protection against UV used by living things and potential application of these mechanisms in protection of natural and synthetic polymeric materials. This is followed by chapters with a more detailed look at more specific aspects of UV degradation and stabilization.

• A practical and up-to-date reference guide for engineers and scientists designing with plastics, and formulating plastics materials
• Explains the effects of UV light on plastics, and how to mitigate its effects through the use of UV stabilizers
• Surveys the range of UV stabilizers on the market, and provides advice on their selection and use

1

INTRODUCTION

It will be customary to begin with the historical background of the first studies on UV degradation and stabilization, but chronologically it is more important to review the effect of ultraviolet light on living organisms such as plants, animals and humans, and discuss their methods of prevention of degradation on UV exposure.

Plants survived various levels of UV radiation from the beginning of their existence; moreover, they adapted to use it to harvest energy for biological transformations leading to the production of necessary tissues and food required for their existence. This suggests that plants must have developed various mechanisms of resistance against the destructive energy of UV radiation since they are composed of organic matter, which must be vulnerable to the high energy of UV radiation.

It is known that UVB (280-320) can cause direct and indirect damage of deoxyribonucleic acid, DNA, but UVA (320-400 nm) can only cause indirect damage of DNA. DNA has absorption maxima in the UVC range (<280 nm), traces of which are only available in sunlight. Much weaker absorption exists in UVB and UVA. Directly absorbed UVB causes direct damage to DNA, evidenced by production of DNA lesions which correlate with the amount of UVB energy supplied to the plant. Indirect damage is caused by oxidative processes undergoing in the presence of UVA.¹ Direct UV exposure causes formation of pyrimidine dimers which are the major products of DNA degradation. UVA does not produce pyrimidine dimers but leads to formation of hydroxyl radicals which react with DNA, causing its damage. Ribonucleic acid, RNA, is also damaged by similar absorption processes. In addition to DNA and RNA, UV light also affects cell membranes by peroxidation of lipids and proteins.¹

The above processes show that plants are not only consumers of UV energy but also may become severely injured by excessive exposure. It therefore becomes interesting whether plants have any protective mechanisms to help them in prevention of damage. It is known¹ that plants, similar to man-made materials, apply UV shielding and free radical scavenging.

On exposure to UV radiation, plants accumulate UV-absorbing species, such as flavonoids, hydroxycinnamic acids, and sinapate esters in epidermal cells (external cells of skin).¹ Flavonoids and hydroxycinnamic acids are also antioxidants, but plants synthesize other free radical scavengers, such as ascorbic acid, glutathione, α-tocopherol, and carotenoids.¹ In addition, some enzymes, such as peroxidase, catalase, glutathione reductase, and superoxide dismutase participate in prevention of unwelcome oxidative reactions.¹

The accumulation of UV-absorbing compounds, such as flavonoids and other phenylpropanoid derivatives causes resultant decrease in the UV transmittance of the epidermis in leaves.² This is a primary protective mechanism against the potentially deleterious effects of UV radiation and it is a critical component of the overall acclimation response of plants to changing UV environments.² Some species, such as, for example, okra (Albelmoschus esculentus) exhibit substantial diurnal changes in epidermal transmittance of UVB throughout the day (e.g., predawn – 25%, midday 10%).² This helps to regulate dose of UVB accepted by a plant.² “Higher plants can rapidly adjust their UV sunscreen protection.”²

The above systems bear a close resemblance to man-made protection systems (for details, see Chapter 3), the major difference is in replenishment and delivery. In the case of plants additional quantities can be produced as required, whereas in man-made systems it is not possible. Delivery systems also differ. In the case of man-made materials, the material’s structure is “uniform” and UV stabilizer is delivered to the surface by diffusion and migration. A gradient of concentration must therefore be maintained throughout the cross-section of the product. Plants are built of many different tissues and cells are surrounded by membranes which may participate in protection of intrinsic UV absorbers against their loss to the surroundings.

Plants have other available mechanisms, which so far were not designed by systems developed by people for this purpose. These are reversal, removal, and tolerance.¹ Reversal stands for photoreactions, which are capable of reversing pyrimidine dimers to repair helix-distorting lesions in DNA. It should be taken into consideration that accumulation of pyrimidine dimers may block DNA replication and transcription.¹ Pyrimidine dimers are tolerated, prior to repair, by translesion synthesis or an avoidance mechanism that circumvent DNA damage during replication.¹ All these processes are not possible in man-made materials. Usually, man-made materials suffer from accumulation of degradation products (e.g., carbonyl groups) which increase light absorption and accelerate further degradation just after it was initiated.

There are many confirmed mechanisms of repair which were found to operate in plants. The above scenarios and capabilities are not based on speculations but are the results of a large number of studies. For example, Elsevier’s Science Direct helps to find the results of over 3,500 studies, which were conducted on various plants to determine their resistance to UV damage and mechanisms of their protection.

Climate change affects UV irradiation of plants in both ways. Increased cloud cover causes decrease in radiation doses obtained by plants, whereas ozone holes and other damages to ozone cover drastically increase radiation in some areas of the world.³ This affects plant growth and/or their health. Plants have additional mechanism of mitigation of radiation level not available to the man-made products. They can adjust azimuth and inclination of leaves towards the sun, which drastically changes UVB exposure (even up to 3-4 times).³

If the above mechanisms fail to operate, fungal, bacteria, and viral invasions follow and they compromise nutrient acquisition by the plant, and subsequently the plant is damaged and dies.

There are also differences between species involved in terms of tolerance to increased levels of UV radiation. Considering potential increase of UV radiation in the Arctic, two cyanobacterial mats (Leptolyngbya sp. and Phormidium sp.) were compared in this respect.⁴ The Phormidium mat contained over 25 times the absolute concentration of UV-protecting mycosporine-like amino acid, MAA, and double the concentration of carotenoids compared to the Leptolyngbya mat, but the latter contained a higher ratio of carotenoids+MAAs to chlorophyll.⁴ The Leptolyngbya mat showed significantly lower chlorophyll concentrations under UV enhancement, which could account for the lower photochemical yield in this sample.⁴ Both cyanobacterial species have differing photochemical sensitivity to UVB radiation, which may confer a subtle advantage to the UVB tolerant species over the less tolerant type during a period of high UVB irradiance.⁴

There are also species which are inherently resistant to increased levels of radiation. In one study, the aquatic liverwort Jungermannia exsertifolia was studied under increased levels of radiation in laboratory.⁵ This species lives in mountain streams, where it is exposed to low temperatu...