Handbook of UV Degradation and Stabilization

George Wypych

• Publisher: ChemTec Publishing
• Year: 2020
• eBook ISBN: 9781927885581
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Industrial & Technical Chemistry
• Index: Technology & Engineering

Handbook of UV Degradation and Stabilization, Third Edition, discusses different aspects of UV-related phenomena that occur when polymeric materials are exposed to UV radiation. It reviews existing literature, looking at 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 the protection of natural and synthetic polymeric materials. Other chapters look at more specific aspects of UV degradation and stabilization, such as specific polymers and rubbers, analytical methods used in UV stabilization, health and safety, and more. This book is an excellent companion to the Databook of UV stabilizers which has also been published recently. Both books supplement each other without repeating the same information – one contains data another theory, mechanisms of action, practical effects and implications of application.- Provides a practical reference guide for engineers and scientists who design with plastics and formulate plastic materials- Explains the effect 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 vital tissues and food required for their life. 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 (formation of singlet oxygen and superoxide anion radicals that are the precursors of hydrogen peroxide involved in the generation of hydroxyl radical via Fenton-type reactions).¹ DNA has absorption maxima in the UVC range (<280 nm), traces of which are only available in sunlight.¹ Much weaker absorption exists in UVB and UVA.

Directly absorbed UVB causes direct damage to DNA, evidenced by the 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.² Direct UV exposure causes the formation of pyrimidine dimers which are the primary products of DNA degradation.² UVA does not produce pyrimidine dimers but leads to the formation of hydroxyl radicals which react with DNA, causing its damage. Ribonucleic acid, RNA, is also damaged by similar absorption processes. In addition to acting on DNA and RNA, UV light also affects cell membranes by peroxidation of lipids and proteins.²

The above processes show that plants are not only the consumers of UV energy but even they may become severely injured by excessive exposure. It, therefore, becomes interesting to analyze whether plants have any protective mechanisms to help them in the prevention of damage. It is known² 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).² Flavonoids and hydroxycinnamic acids are also antioxidants, but plants synthesize other free radical scavengers, such as ascorbic acid, glutathione, α-tocopherol, and carotenoids.² In addition, some enzymes, such as peroxidase, catalase, glutathione reductase, and superoxide dismutase participate in prevention of unwelcome oxidative reactions.²

The accumulation of UV-absorbing compounds, such as flavonoids and other phenylpropanoid derivatives causes a resultant decrease in the UV transmittance of the epidermis in leaves.³ 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.³ 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%).³ This helps to regulate the dose of UVB accepted by a plant.³ “Higher plants can rapidly adjust their UV sunscreen protection.”³

Olive, pomegranate, green barley, coffee beans, and many other plants contain polyphenols which are currently used in formulations for protection of human skin.⁴ Two polyphenols derived from pomegranate, namely, punicalagin and urolithin A enhanced the repair of UVB-induced DNA damage.⁴

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 the 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.² 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.² Pyrimidine dimers are tolerated, prior to repair, by translesion synthesis or an avoidance mechanism that circumvent DNA damage during replication.² All these processes are not possible in man-made materials. Usually, man-made materials suffer from an accumulation of degradation products (e.g., carbonyl groups) which increase light absorption and accelerate further degradation just after it was initiated.

The absorption of UV-B by plants occurs through a variety of processes and induces responses specific to that absorption, including induction of photomorphogenic responses (light-mediated development leading to the production of nutrients and growth) through perception mechanisms.⁵ The UV-B perception pathway initiated by UV-B-specific DNA damage can influence photomorphogenic growth in plants.⁵ A germinating seedling has little protection from sunlight exposure, and it is vulnerable to UV-B radiation.⁵ Then, production of flavonoids begins, and they offer protection from UV-B light.⁵ The endonucleases have documented functions in DNA repair processes. It is possible that these enzymes are essential for recognition an...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Chapter 1: INTRODUCTION
• Chapter 2: PHOTOPHYSICS
• Chapter 3: MECHANISMS OF UV STABILIZATION
• Chapter 4: UV STABILIZERS
• Chapter 5: STABILITY OF UV STABILIZERS
• Chapter 6: PRINCIPLES OF STABILIZER SELECTION
• Chapter 7: UV DEGRADATION & STABILIZATION OF POLYMERS & RUBBERS
• Chapter 8: UV DEGRADATION & STABILIZATION OF INDUSTRIAL PRODUCTS
• Chapter 9: FOCUS TECHNOLOGY – SUNSCREENS
• Chapter 10: UV STABILIZERS AND OTHER COMPONENTS OF FORMULATIONS
• Chapter 11: ANALYTICAL METHODS IN UV DEGRADATION AND STABILIZATION STUDIES
• Chapter 12: UV STABILIZERS – HEALTH & SAFETY
• INDEX