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Light Stabilizers for Coatings

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

Introduction

Light and photo-oxidative degradation

2.1 Light

2.1.1 Photo-physical processes

2.2 Photo-chemical degradation processes

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

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

Conclusions

References

Authors

Index

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.

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.