Light Stabilizers for Coatings E-Book

Andreas Valet

• Herausgeber: Vincentz Network
• Kategorie: Fachliteratur
• Serie: European Coatings LIBRARY
• Sprache: Englisch

Seitenzahl: 186

Veröffentlichungsjahr: 2017

Beschreibung

The durability of coatings is one of the most important quality characteristics and can be achieved by incorporating light stabilizers in the paint. These substances are comprehensively described in this book, which also deals with chemical and applicational aspects.

Leseprobe

Andreas Valet

Adalbert Braig

Light Stabilizers for Coatings

Cover: Coloures-pic/Fotolia

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Andreas Valet, Adalbert Braig

Light Stabilizers for Coatings

Hanover: Vincentz Network, 2017

European Coatings Library

ISBN 978-3-86630-132-0

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European Coatings Library

Andreas Valet

Adalbert Braig

Light Stabilizers for Coatings

Foreword

Ah, happy he who still can hope to rise,

Emerging from this sea of fear and doubt!

What no man knows, alone could make us wise;

And what we know, we well could do without.

Goethe, Faust

Almost twenty years after the first edition of this book, the editor was asked by colleagues in the coating industry whether it would be possible to publish an up-dated second edition. When the editor asked us whether we would be interested in working on such a second edition, we agreed with great pleasure.

Although some time has passed since the first edition, the problems facing the industry are still the same: paint flaking off an object remains a serious concern. The object has lost its protection and is exposed to the elements. It has lost the colour that made it attractive. It has become insignificant and unnoticed.

The purpose of this book is to explain the underlying principles of paint degradation, demonstrate, with the help of numerous examples, how paint films can be protected and serve as a practical guide for formulators when selecting light stabilizers for their paint formulation.

For a more precise analysis of the mechanism of paint degradation and its prevention, the reader is referred to the extensive bibliography at the end of the book, which covers the subject comprehensively.

Hermann Hesse wrote “Blue, yellow, white, red and green – what wonderful colours” [1]. When properly stabilized, colourful finishes ARE wonderful.

We would like to take this opportunity to thank our colleagues from the former Ciba Specialty Inc. and BASF Switzerland AG. Without their collaboration and support it would have never been possible to carry out the research, over a period of more than two decades, on which this book is based. We would also to thank Dr. Godwin Berner and Hans-Jürgen Berger who pioneered, built-up and led this successful business for so many years. Special thanks goes to Allan Cunningham for his great support editing our English text.

Basle, Switzerland June 2016

Andreas Valet and Adalbert Braig

Contents

1 Introduction

2 Light and photo-oxidative degradation

2.1 Light

2.1.1 Photo-physical processes

2.2 Photo-chemical degradation processes

3 Stabilization options

3.1 UV absorbent pigments

3.2 UV absorbers

3.2.1 UV absorber classes

3.2.2 Mode of action of UV absorbers

3.2.2.1 Phenolic UV absorbers

3.2.2.2 Non-phenolic UV absorbers

3.2.3 Examples of UV absorbers

3.3 Free-radical scavengers

3.3.1 Antioxidants

3.3.2 Sterically hindered amines

3.3.2.1 Mode of action of HALS

3.4 Quenchers

3.5 Peroxide decomposing agents

4 Stabilization of coatings

4.1 Automotive coatings

4.2 Light stabilization of automotive coatings

4.2.1 Two-coat systems

4.2.2 Specific requirements of UV absorbers in coatings

4.2.2.1 Solubility and compatibility of UV absorbers

4.2.2.2 Volatility of UV absorbers

4.2.2.3 Reactable UV absorbers

4.2.2.4 Effect of UV absorbers on coating colour

4.2.2.5 Unwelcome side reactions

4.2.2.6 UV absorbers and photoinitiators

4.2.3 Specific requirements on HALS in coatings

4.2.3.1 Solubility and compatibility of HALS

4.2.3.2 Volatility of HALS

4.2.3.3 Reactable HALS

4.2.3.4 Effect of HALS on coating colour

4.2.3.5 Unwelcome side reactions

4.2.4 Weathering results for two-coat systems

4.2.4.1 Weathering tests

4.2.4.2 Results for solvent-borne clear coats

4.2.4.3 Results for water-borne clear coats

4.2.4.4 Results for powder clear coats

4.2.4.5 Results for UV-curable clear coats

4.2.4.6 Coatings on plastic substrates

4.2.4.7 UV protection of epoxy-based fibre reinforced plastics

4.2.4.8 Effect of additional basecoat stabilization

4.2.4.9 Exposure results for one-coat finishes

4.3 Light stabilization of industrial coatings

4.3.1 Stabilization of paints for metal substrates

4.3.2 Stabilization of clear wood coatings

4.4 Stability of light stabilizers

4.4.1 Photo-chemical stability of UV absorbers

4.4.2 Long-term stability of HALS

5 Conclusions

6 References

Authors

Index

1 Introduction

In the dictionary, paints are defined as “liquid or powdered, solid substances which are applied thinly to objects and which dry by chemical reaction and/or physical changes to form a solid film whose function may be decorative and/or protective” [2].

Although applied only thinly, paints alter the appearance and increase the durability of many everyday objects. The efficiency of a paint, i.e. its ability to protect the coated object, is governed by the nature of the binder used in its formulation. Binders are also often referred to as film-forming agents, surface coating resins or synthetic resins. Paints contain organic solvents and/or water, or are completely solvent-free, depending on the kind of binder used. Paints may also contain pigments, fillers and other additives.

Paint films are exposed to all conditions arising in daily life, including mechanical stresses, chemicals and weathering, against which they must protect the coated object.

In 1860, A. Hofmann stated [3] that “the characteristic change which occurs in gutta-percha (rigid natural latex) when it has been in contact with air for some time, is well known. It becomes brittle and irreversibly loses its texture.” Investigations had shown this change to be due to oxidation of the gutta-percha when exposed to air and Hofmann’s statement is probably the first reference in the literature concerning the chemical reactions that alter polymer properties.

As a result, efforts began to protect polymers against these chemical reactions. New polymers required stabilizers to prevent them from deterioration in practical use. This formed the basis for the development of stabilizers for polymers. The term “stabilizer” refers to any additive that prevents or delays polymer degradation, irrespective of what kind of degradation mechanism is involved. The development of light stabilizers is described in the following publications and patent specifications.

– Remarks on the Change of Gutta Percha under Tropical Influences [3]

– Verfahren, um das Erhärten und Brüchigwerden von Kautschuk, Guttapercha, Balata und ähnlichen Gummiarten zu verhindern (Methods of preventing the hardening and embrittlement of rubber, gutta-percha, balata and similar rubbers) DP 221310; W. Ostwald, 1908)

– The Chain Reaction Theory of Negative Catalysis[4]

– Autoxidation von Kohlenwasserstoffen: Über ein durch Autoxidation erhaltenes Tetrahydronaphthalin-Peroxid (Autoxidation of hydrocarbons: tetrahydronaphthalene peroxide obtained through autoxidation)[5]

– Der Kettenmechanismus bei der Autoxidation von Natriumsulfidlösungen (The chain mechanism during the autoxidation of sodium sulphide solutions) [6]

– Pellicle and the Manufacture thereof (USP 2,129,131; E. Du Pont de Nemours, 1938

– Vinylidene Chloride Composition Stable to Light (USP 2,264,291; The Dow Chemical Company, 1941)

– Lichtschutzmittel und ihre Beurteilung (Light Stabilizers and their Assessment) [7]

– Weather resistance of Cellulose Ester Plastics Compositions [8]

– 4-Benzoylresorcinol as an Ultraviolet Absorbent (USP 2,568,894; General Aniline & Film Corporation, 1951)

– Verwendung von 2-Phenylbenzotriazol-Verbindungen zum Schützen von organischem Material gegen ultraviolette Strahlung (Use of 2-phenylbenzotriazole compounds to protect organic materials from UV radiation) (DE 1185610; Ciba-Geigy AG, 1957)

– Free Radical Reactions involving no Unpaired Electrons [9]

– Stabilization of Synthetic Polymers (USP 3,542,729; Sankyo Ltd., 1970)

The preceding references formed the basis for the development of light stabilizers for coatings.

In this book, in addition to paint, two other terms are widely used in the industry will appear, namely coating and varnish.

2 Light and photo-oxidative degradation

2.1 Light

Light is generally described as radiation visible to the human eye comprising wavelengths between 400 and 750 nm[2]. But “visible” light is only a part of the electromagnetic radiation to which the earth is exposed. Electromagnetic radiation can be divided into different groups as shown in Figure 2.1. Figure 2.2 shows the sub-division of “ultraviolet to infrared”.

Figure 2.1: Classification of electromagnetic radiation with wavelengths λ of 10-15 to 103 m[10]

Figure 2.2: Classification of radiation with wavelength range of 100 to 4000 nm

Table 2.1: Relationship between wavelength λ and dissociation energy of some organic model compounds[11, 12, 13]

λ [nm]

Bond

Type of bond

Dissociation energy [kJ/mol]

UV-B

230

-C-C-

aromatic

520

286

R-O-H

alcohol

420

290

R-CR2-H

prim./sec./tert. H

410/395/385

310

C-O-H

alcohol

385

320

-C-O-

ether

365 to 390

UV-A

340

R-CH2-CH3

aliphatic

335 to 370

350

-CR2-Cl

aliphatic chlorides

330 to 350

360

-CH2-NR2

amine

330

400

-O-O-

peroxide

270

Most of the energy-rich electromagnetic radiation (λ < 290 nm) is absorbed by the earth’s atmosphere, primarily by the ozone layer in the stratosphere. Although only 6 % of the light reaching the earth’s surface is the ultraviolet light (“UV light”) it is responsible for most of polymer degradation due to weathering.

Figure 2.3: Sunburn of the human skin caused by UV radiation (UV-A, UV-B) [14]

Table 2.1 shows the relationship between wavelength and dissociation energy in different molecules [11, 12, 13]. These figures demonstrate that the UV light reaching the earth’s surface is sufficiently rich in energy to break covalent bonds present in polymers.

Only light that is absorbed is capable of initiating photo-chemical processes. Pure polymers such as polymethyl methacrylate, polyethylene or aliphatic polyesters are photochemically stable between 300 and 400 nm because they do not absorb light. The emphasis here is on the word “pure”. However, if the polymer contains impurities that absorb light, e.g. catalyst residues and other substances added during production, or oxidation products, it becomes sensitive to UV light. Polymers whose basic structure already contains UV absorbent groups are inherently light sensitive and are likely to undergo photo-chemical degradation even in the absence of such impurities. Typical examples include polystyrene and styrene copolymers, aromatic polyurethanes, polyesters and polyepoxides.

Human beings, too, experience the harmful – and very painful – effects of light. Here, the skin, whose function, after all, is to protect the body, is attacked by UV-B, UV-A, and even visible light [14, 15]. Damage and eventual destruction of the skin can seriously affect the underlying tissue.

Figure 2.3 shows the sunburn of the human skin. Reflected light, water, snow and droplets of perspiration, for example, can further magnify the effect of sunlight.

By analogy, the damage of polymers and paint films by light can also be classified as a kind of “sunburn”, the difference being that polymers, unlike the human skin, are not capable of regeneration.

2.1.1 Photo-physical processes

The first step in a photo-chemical reaction is the absorption of light that promotes the molecule into an energy-rich, excited state. Molecules can exist in two electronic states:

– singlet state S (paired electron spins)

– triplet state T (unpaired electron spins)

According to Hund’s first law, electronic states with greater spin multiplicity are more stable, i.e. the triplet state is generally lower in energy than the corresponding singlet state.

Upon absorption of light, molecules are promoted from their singlet ground state So to a first energy-rich excited state S1 or T1.

The probability of a chemical reaction in an excited state increases with the lifetime of the state. As the lifetime of the excited triplet state T1 is longer than that of the corresponding singlet state S1, most photo-chemical reactions occur in the excited triplet state [16]. Reactions in the shorter-lived singlet state S1 can occur if they are thermodynamically and kinetically favoured. The kinetic factor in particular is dependent upon the substrate. A molecule in the excited singlet state S1 has more possibilities to dissipate energy. These are shown in Figure 2.4 in the so-called Jablonski diagram [17].

Energy release or deactivation is possible with or without radiation. The lowest triplet state T1 is formed by a radiation-less transition from S1 to T1, described as “inter-system crossing”. The transition T1 to T2, T3 etc. is only possible if the molecule absorbs light in the T1 state. Transitions from S1 to T1 or from T1 to S0 break the spin selection rules (change of spin multiplicity) but can occur if sufficient spin-orbit coupling is present [18].

The smaller the energy difference between S1 and T1, the greater the possibility of inter-system crossing.

2.2 Photo-chemical degradation processes

Photo-oxidative degradation processes, in which chain cleavage, chain branching and oxidation reactions occur, can be divided into the stages shown in Figure 2.5 [19]. Chain initiation, equation (a), involves energy transfer from a photo-activated donor D* to an acceptor A present in the ground state. This energy transfer can take place inter-molecularly or intra-molecularly [12].

In intra-molecular energy transfer, a functionality in an excited state (D*) transfers energy to a functionality in the ground state (A) within the same polymer molecule. This process is important in those molecules (e.g. copolymers) in which parts are present which can easily be photo-activated.

In the case of inter-molecular energy transfer, a molecule not belonging to the polymer itself but being in an excited state, transfers its energy to the polymer [19]. In Section 2.1 it was pointed out that pure polymers such as polyethylene and polymethyl methacrylate are photo-chemically stable. However, impurities present in these polymers, e.g. catalyst residues, can initiate photo-chemical degradation.

Chain initiation (a) in Figure 2.5 leads to free radicals on the polymer chain that react with oxygen (photo-oxidation). The speed of this photo-oxidation depends on the type of polymer and coating (preparation, composition, impurities, additives). At the same time, purely thermal processes may also take place (oxidation).

Given the enormous quantities of paints and polymers used throughout the world, stabilization against photo-chemical degradation is crucial from an economic as well as ecological point of view. After all, the destruction of paint films and polymers requires their replacement, especially when the coating serves not only for decorative reasons but to protect the underlying surface, be it metal, wood or plastic. Once the protective coating deteriorates and ultimately breaks down, the revealed surface is exposed to the elements: metal will corrode and wood will be degraded by humidity and UV light.

Figures 2.6 to 2.9 show typical examples of photo-oxidatively damaged polymers and coatings.

Figure 2.4: Jablonski diagram [17], IC: Internal Conversion; ISC: Inter System Crossing

Figure 2.5: Schematic representation of photo-oxidation of polymers (P)

1 may contain catalyst residues, hydro peroxides etc.

Figure 2.6: Embrittlement of polymer material caused the destruction of the greenhouse film

Figure 2.7: Loss of adhesion of a 2-coat automotive finish caused by weathering

Figure 2.8: Micrograph of cracking in an automotive clear coat

Figure 2.9: Outdoor weathering tests with red HD polyethylene. Weathering: Florida, 45° S