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Basics ofCoating Technology

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BasicsofCoating Technology

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2 BASF Handbook on Basics of Coating Technology

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3BASF Handbook on Basics of Coating Technology

BASF HANDBOOK ON

Basicsof CoatingTechnology

Prof. Dr. Artur Goldschmidt/Dr. Hans-Joachim Streitberger

2nd revised edition

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Foreword to the second revised edition

Glasurit’s ”Paints and Coatings” manual has been a standard work in the field of coatingmaterials since it was first published in 1934. Since then a number of editions in Ger-man were published. The latest edition being first time published also in English in2003 was well received on a worldwide basis. This second revised and corrected editionwas sponsored by BASF Coatings. Compared to the first edition based on the Germanedition of 2002 market data were actualized, some passages revised, more Englishreferences introduced and corrections made.The manual starts logically with a brief historical summary of painting, its currenteconomic and technical significance and the social framework underpinning the indus-try. This is followed by a chapter dealing with product composition, raw materials,principles of product formulation and the relevant production processes for these prod-ucts. Descriptions are then given of the properties of the liquid and solid coating mate-rials for the respective application types, after which a chapter follows that deals withtheoretical aspects of coating compositions. Applications are then explained for thereader, taking due account of the current significance of the coating processes. In thischapter the substrate plays a major role because of its impact on the coating process andoutcome. The demands on the environment, health, safety and quality as specified in thecurrent legislation in Europe and Germany are dealt with in the next chapter. A journeyis then undertaken through the various industrial sectors of the paint and coatingsindustry with the aid of a large number of examples and illustrations. This chapter is ofparticular interest for newcomers to the industry and practically-minded readers. Stand-ard tables, general performance indicators, a selection of relevant websites, definitionsof terms and a comprehensive index round off the book.When reading the manual, please bear in mind that abbreviations are defined the firsttime they are used. In addition, you will find a complete list of all abbreviations used inthe book in chap. 9.3. Environmentally compatible coating materials are introduced inchap. 5.8 referring to more detailed technical information on these materials in theindividual chapters. In any case, the comprehensive index will help you get to theinformation you need quickly.Literature references are cited to enable readers to obtain more in-depth information.These are assigned to the individual chapters and may therefore be repeated on occa-sion. Many comments and scenarios are based on experience gained by BASF Coatingsand are therefore not necessarily assigned to specific quotations.Statutory provisions and mostly DIN EN standards have been followed for the termsand units used. Readers will therefore find that many expressions traditionally used inthe industry have not been employed. By using standardized expressions, however, wewish to contribute to a uniform specialist terminology and therefore to the prevention ofambiguity or misunderstandings. An appendix of acronyms is included for reference.Manufacturers, users, the academic community, institutes and authorities have variedexpectations for the manual. The publishers have attempted to address a broad range of

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topics, based on the response to the previous edition and on the current technical andsocial developments, but have consciously avoided the in-depth treatment of specialistaspects to prevent the book’s scope from becoming too unwieldy.The scope of the book nevertheless meets the objective of publishing a work of refer-ence that provides satisfactory answers to paint-related issues for both newcomers andexperts.We wish to express our gratitude to the many employees of BASF Coatings AG whohave contributed valuable suggestions and detailed information. Our special thanks gostill to Klaus-Peter Löbbe and for the second edition to the management team of BASFCoatings and to Raimar Jahn. We also thank Dr. P. Bachhausen and Dr. W. Kreis fortheir assistance as well as the publishers for their patient advices.

Prof. Dr. A. Goldschmidt and Dr. H.-J. StreitbergerWiedenbrück, July 2007

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Content

1 Introduction

2 Coating Material

3 Coatings

4 Coating Technology

5 Safety, Environmental Protection and Health

6 Principles of Quality Management

7 Coating Industries

8 Industrial Standards

9 Appendix

10 Index

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Content

1 Introduction ...................................................................................................................... 151.1 Definition, Tasks and Economic Importance .................................................................... 151.2 Retrospective ...................................................................................................................... 211.3 Bibliography ....................................................................................................................... 25

2 Coating Material .............................................................................................................. 272.1 Raw Materials .................................................................................................................... 272.1.1 Film Forming Agents ......................................................................................................... 302.1.1.1 Natural Materials ................................................................................................................ 332.1.1.2 Synthetic Resins ................................................................................................................. 492.1.2 Solvents .............................................................................................................................. 1252.1.2.1 Theory of Solubility ........................................................................................................... 1272.1.2.2 Physical Properties ............................................................................................................. 1302.1.2.3 Chemical and Physiological Properties ............................................................................. 1332.1.2.4 Important Solvents for Coatings ........................................................................................ 1332.1.3 Pigments and Extenders ..................................................................................................... 1372.1.3.1 Physical Principles for Chroma and Hiding Power ........................................................... 1422.1.3.2 The Most Important Pigments for Coating Materials ....................................................... 1452.1.3.3 Extenders ............................................................................................................................ 1652.1.4 Plasticizers and Additives .................................................................................................. 1682.1.4.1 Plasticizers .......................................................................................................................... 1682.1.4.2 Additives ............................................................................................................................. 1722.1.5 Summary ............................................................................................................................. 1922.2 From Raw Material to Coating Material ........................................................................... 1932.2.1 General Rules on Drawing up Formulations ..................................................................... 1942.2.1.1 Purpose and Quality ........................................................................................................... 1942.2.1.2 Production Resources, Application Systems and Object to be Painted ............................ 1942.2.1.3 Function in Paint System ................................................................................................... 1952.2.1.4 Cost Effectiveness and Availability ................................................................................... 1962.2.1.5 Occupational Health, Safety and Environmental Protection Regulations ........................ 1972.2.2 Material Flow in a Paint Factory ....................................................................................... 1972.2.3 Theory of Dispersion ......................................................................................................... 2002.2.3.1 Pigment Specific Properties for Dispersion Processes ..................................................... 2002.2.3.2 Wetting and Decomposition of Agglomerates ................................................................... 2032.2.3.3 Stabilization of Pigment Dispersions ................................................................................ 2082.2.3.4 Optimum Mill Base Formulation ....................................................................................... 2172.2.4 Production of Coating Materials ........................................................................................ 2202.2.4.1 Processing Sequences in a Paint Plant ............................................................................... 2212.2.4.2 Agitation and Agitators ...................................................................................................... 2222.2.4.3 Dispersersing and Dispersers ............................................................................................. 2282.2.4.4 Separation Processes in the Manufacture of Paint ............................................................ 2462.2.5 Summary ............................................................................................................................. 2532.3 Characterization of Coating Materials ............................................................................... 2542.3.1 Measurement Accuracy ...................................................................................................... 2562.3.2 Testing Raw Materials and Coating Materials .................................................................. 2592.3.2.1 Safety and Environment Related Performance Indicators ............................................... 2592.3.2.2 Chemical Characterization ................................................................................................. 2642.3.2.3 Physical Data ...................................................................................................................... 2742.3.3 Pigment Specific Tests ....................................................................................................... 3002.3.3.1 Indicators of the Pigment as a Raw Material .................................................................... 3012.3.3.2 Tests with the Pigmented Coating Material ....................................................................... 3112.3.4 Processibility of Coating Materials ................................................................................... 3152.3.5 Summary ............................................................................................................................. 3182.4 Literature ............................................................................................................................ 318

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Content

3 Coatings ............................................................................................................................. 3233.1 From Coating Material to Coating : The Film Formation ................................................. 3233.1.1 Wetting and Flow ............................................................................................................... 3233.1.2 Solidification of the Film ................................................................................................... 3253.1.2.1 Physical Drying .................................................................................................................. 3273.1.2.2 Chemical Curing ................................................................................................................ 3313.1.2.3 Flows in the Solidifying Film ............................................................................................ 3323.1.3 Film Shrinkage ................................................................................................................... 3333.1.4 Special Features of High Solid Paints and Waterborne Paints .......................................... 3343.1.5 Monitoring the Film Forming Process with Measuring Instruments ................................ 3363.1.5.1 Flow and Sag ...................................................................................................................... 3373.1.5.2 Film Formation of Air Drying Paints ................................................................................ 3383.1.5.3 Indirect Methods ................................................................................................................ 3383.1.6 Summary ............................................................................................................................. 3403.2 Properties and Testing the Coatings .................................................................................. 3403.2.1 Film Thicknesses ................................................................................................................ 3413.2.1.1 Wet Films ............................................................................................................................ 3423.2.1.2 Dry Films ............................................................................................................................ 3423.2.2 Dry Film Density ................................................................................................................ 3493.2.3 Measurement of Voids in Coating Films ........................................................................... 3503.2.4 Visual Properties ................................................................................................................ 3513.2.4.1 Gloss ................................................................................................................................... 3523.2.4.2 Color and Coloristic ........................................................................................................... 3603.2.5 Mechanical Properties ........................................................................................................ 3723.2.5.1 Adhesion ............................................................................................................................. 3733.2.5.2 Elasticity ............................................................................................................................. 3853.2.5.3 Hardness ............................................................................................................................. 3963.2.5.4 Abrasive Resistance and Mar Resistance .......................................................................... 4033.2.5.5 Other Tests .......................................................................................................................... 4053.2.6 Summary ............................................................................................................................. 4063.3 Durability of Coatings ....................................................................................................... 4063.3.1 Fundamental Features of Ageing ....................................................................................... 4063.3.2 Ageing Tests ....................................................................................................................... 4143.3.2.1 Test Methods for Surface Durability ................................................................................. 4153.3.2.2 Corrosion Protection Tests ................................................................................................. 4243.3.2.3 Chemical Resistance .......................................................................................................... 4313.3.3 Summary ............................................................................................................................. 4333.4 Literature ............................................................................................................................ 433

4 Coating Technology .......................................................................................................... 4374.1 Substrate Impact ................................................................................................................. 4394.1.1 Wood and Wood Materials ................................................................................................. 4404.1.1.1 Wood as a Material ............................................................................................................. 4404.1.1.2 Wood as a Workpiece ......................................................................................................... 4424.1.1.3 Pretreatment of Wood ......................................................................................................... 4444.1.1.4 Surface Refinement of Wood Panels ................................................................................. 4454.1.2 Metallic Materials .............................................................................................................. 4464.1.2.1 Properties of Metals ........................................................................................................... 4464.1.2.2 From the Material to the Workpiece .................................................................................. 4514.1.2.3 Pretreatments ...................................................................................................................... 4524.1.3 Plastics ................................................................................................................................ 4644.1.3.1 Plastic Types and Properties .............................................................................................. 4644.1.3.2 Pretreatment ........................................................................................................................ 4674.1.4 Mineral Substrates .............................................................................................................. 4704.1.4.1 Concrete, Mortar, Plaster ................................................................................................... 4704.1.4.2 Glass ................................................................................................................................... 472

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Content

4.1.5 Other Substrates ................................................................................................................. 4724.1.5.1 Rubber ................................................................................................................................ 4724.1.5.2 Leather ................................................................................................................................ 4734.1.6 Work Piece Design and Coatability ................................................................................... 4734.1.7 Summary ............................................................................................................................. 4754.2 Paint Processing ................................................................................................................. 4764.2.1 Processing of Wet Paints .................................................................................................... 4774.2.1.1 Dip Coating (Object Taken to Paint) ................................................................................. 4784.2.1.2 Brushing, Manual and Mechanical Roller, Flood and Curtain-Coating Methods

(Paint Applied Directly to the Object) ............................................................................... 5004.2.1.3 Spray Processes (Paint Applied Indirectly to the Object) ................................................. 5064.2.1.4 Paint Supply Systems ......................................................................................................... 5424.2.1.5 Booth Conditioning and Overspray Elimination ............................................................... 5594.2.1.6 Automated Systems and Robots for Paint Processing ...................................................... 5654.2.1.7 Conveyors ........................................................................................................................... 5684.2.1.8 Paint Removal .................................................................................................................... 5704.2.2 Processing Powder Coatings .............................................................................................. 5714.2.2.1 Retrospective ...................................................................................................................... 5714.2.2.2 Plant and Equipment Details .............................................................................................. 5734.2.2.3 Powder Coating Materials .................................................................................................. 5834.2.2.4 Powder-Specific Test Methods .......................................................................................... 5844.2.3 Summary ............................................................................................................................. 5864.3 Drying and Curing .............................................................................................................. 5874.3.1 Film Formation by Heat Transfer ...................................................................................... 5874.3.1.1 Theory ................................................................................................................................. 5884.3.1.2 Oven Design ....................................................................................................................... 6004.3.2 Curing by UV and Electron-Beam Radiation .................................................................... 6074.3.3 Summary ............................................................................................................................. 6144.4 Literature ............................................................................................................................ 614

5 Safety, Environmental Protection and Health .............................................................. 6195.1 Legislative Framework ...................................................................................................... 6195.2 Safety .................................................................................................................................. 6245.2.1 Manufacturing .................................................................................................................... 6255.2.2 Storage ................................................................................................................................ 6265.2.3 Transportation .................................................................................................................... 6275.2.4 Application ......................................................................................................................... 6285.3 Environmental Protection .................................................................................................. 6305.3.1 Exhaust Air ......................................................................................................................... 6305.3.2 Wastewater ......................................................................................................................... 6365.3.3 Recycling and Disposal ...................................................................................................... 6385.4 Health ................................................................................................................................. 6405.5 Ecobalances ........................................................................................................................ 6425.6 Environment Compatible Paints and Coatings .................................................................. 6465.6.1 High Solids ......................................................................................................................... 6475.6.2 Waterborne Paints .............................................................................................................. 6475.6.3 Slurry .................................................................................................................................. 6505.6.4 Powder Coatings ................................................................................................................ 6515.6.5 Radiation Curable Paints and Coatings ............................................................................. 6535.6.6 Other Coating Systems ....................................................................................................... 6555.7 Economy of Coating Processes .......................................................................................... 6565.8 Literature ............................................................................................................................ 658

6 Principles of Quality Management ................................................................................. 661

6.1 Evolution of Quality Concepts over Time ......................................................................... 6616.2 Defects in Coating Processes and Applied Coatings ......................................................... 663

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6.2.1 Identifying Defects and their Causes ................................................................................. 6646.2.2 The Most Frequent Causes of Surface Defects ................................................................. 6686.2.3 Summary ............................................................................................................................. 6766.3 Material Control ................................................................................................................. 6766.4 Defect Prevention by Process Control and Control Loops ............................................... 6786.5 Quality Management .......................................................................................................... 6806.6 Literature ........................................................................................................................... 685

7 Coating Industries ............................................................................................................ 6877.1 Automotive OEM Coating ................................................................................................. 6887.1.1 Pretreatment ........................................................................................................................ 6927.1.2 Electrocoating ..................................................................................................................... 6947.1.3 Seam Sealant and Underbody Protection .......................................................................... 6987.1.4 Primer Surfacer .................................................................................................................. 6987.1.5 Topcoat Application ........................................................................................................... 7027.1.6 Repair in Automotive OEM Lines ..................................................................................... 7077.1.7 Outlook ............................................................................................................................... 7087.2 Automotive Refinishing ..................................................................................................... 7107.3 Automotive Supply Industry .............................................................................................. 7177.4 Coil Coating ....................................................................................................................... 7237.5 Commercial Vehicles ......................................................................................................... 7277.6 Mechanical Engineering .................................................................................................... 7307.7 White Goods ....................................................................................................................... 7317.8 Building Supplies ............................................................................................................... 7327.9 Rail Vehicles ....................................................................................................................... 7347.10 Wood Coating ..................................................................................................................... 7357.11 Other Fields of Application ................................................................................................ 7397.11.1 Protection of Structures ...................................................................................................... 7397.11.2 Steel Furniture .................................................................................................................... 7407.11.3 Aviation Industry ................................................................................................................ 7417.11.4 Electrically Insulating Coatings ......................................................................................... 7427.11.5 Communication Equipment ............................................................................................... 7437.11.6 Road Marking Paints .......................................................................................................... 7447.11.7 Shipbuilding ....................................................................................................................... 7457.11.8 Miscellaneous Coating Applications ................................................................................. 7467.12 Literature ............................................................................................................................ 747

8 Industrial Standards ........................................................................................................ 7518.1 General Information on Standardization Work ................................................................. 7518.2 Actual Industrial Standards and Specifications for Coatings ........................................... 7548.3 Literature ............................................................................................................................ 754

9 Appendix ........................................................................................................................... 7559.1 List of Physical Constants .................................................................................................. 7559.1.1 Latin Symbols .................................................................................................................... 7559.1.2 Greek Symbols ................................................................................................................... 7579.2 Web-Addresses of Interest for the World of Coatings ...................................................... 7589.2.1 Organizations ...................................................................................................................... 7589.2.2 Publishing/General Informations ....................................................................................... 7609.2.3 Research Institutes/Universities/Colleges ......................................................................... 7619.2.4 Miscellaneous ..................................................................................................................... 7629.3 Acronyms/Abbreviations ................................................................................................... 7629.4 Picture Sources ................................................................................................................... 768

10 Index .................................................................................................................................. 769

Content

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BASF Handbook on Basics of Coating Technology 15

Fig. 1.1.1Coated objects of daily life

Definition, tasks and economic importance

1 Introduction

1.1 Definition, Tasks and Economic ImportanceThe task of coating technology is to provide surface protection, decorative finishes andnumerous special functions for commodities and merchandise by means of organic coat-ings. Many everyday products are only made usable and thus saleable because of theirsurface treatment. To achieve this, relevant coating formulations, their production plant,the coating material and suitable coating processes for the product must be available.However, the quality to be achieved by means of the coating process is not the onlyfunction of the coating material used. The object to be painted or coated itself with itsspecific material and design and anappropriate application process arefurther variables which play a sig-nificant role. In addressing the on-going tasks of quality optimizationand rationalisation while minimiz-ing the impacts for humans and theenvironment, it is vital that the de-pendencies mentioned above be notonly recognized but also taken intoaccount as the framework definingthe conditions in which work is car-ried out from development to appli-cation. Coating technology, there-fore, is an interdisciplinary subject.Paints and coating materials are notend products, but merely initial orintermediate products which, for theabove mentioned reasons, require a skilled and conscientious user if they are to be con-verted into the actual end product, the coating itself. Only the cured coating, in manycases a system consisting of several individual coats, may meet the wishes invested inand the requirements demanded of the coated products.Two of the most important of the many functions which coatings have to meet are pro-tection and decoration. Other noteworthy features are the informative tasks and theachievement of special physical effects. The conspicuousness of emergency servicevehicles, the camouflaging of military equipment, and road or airport markings are justsome of the informative tasks required of coatings. Color markings enable areas orspaces to be clearly signed or divided. Color coding helps to indicate the contents ofcontainers or the material being conveyed in pipes. Optical effects induced by coloredor metallic pigments lend a coating a particular optical attraction. Deliberately gener-ated surface textures such as scars or wrinkles expand the range of effects which can beachieved. The use of color schemes for rooms and machines based on known physi-

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BASF Handbook on Basics of Coating Technology16

Introduction

ological and psychological effects of colors also contributes in various ways toimproved working conditions and enhanced safety. Functional pigments produce tem-perature dependent colors, for example as a result of their thermochromic properties,and therefore, indirectly permit the temperatures of objects to be measured.The most important task for coatings, in economic terms, is surface protection. Thuscoatings help to retain value and improve the usability properties of almost all productsand are therefore, of huge economic significance. Particular mention should be made ofthe protection of goods made of metals which only gain lasting anticorrosive protectionwhen they are painted.

It is vital in this regard, for ex-ample, in the automotive sec-tor, for the resistance of thecoating system to external,sometimes aggressive naturaland anthropogenic atmosphericinfluences such as tree resins,bird droppings, acids, alkalis,salts and organic solvents, to beguaranteed.The protective function ofpaint on cars must not be im-paired even under extreme me-chanical impacts such as stonechippings thrown up from theroad by traffic or by brush ac-tion in carwashes.Furthermore, coatings must

withstand combined, i.e. physical and chemical, effects to which objects are subjected,for example, in the different weather conditions. The interaction of sunshine, rain, heatand frost combined with emissions from heating systems and internal combustion en-gines, by ozone and saline fog makes great demands on a coating’s resistance and pro-tective properties.However, a surface protection coating can also be applied in order to meet quite differ-ent requirements. Floors and steps can be made nonslip by means of rough or high gripcoatings, thereby increasing their utility value. By contrast, surface friction can be re-duced by use of smooth coatings to produce a high degree of nonadhesiveness. Flam-mable materials can be rendered safe by means of flame retardant coatings. Antibacte-rial coatings help maintain sterile surfaces in production and storage facilities in dairiesand breweries or prevent the growth of barnacles and algae on ships’ hulls. In the elec-trical engineering sector insulating coatings provide effective and lasting insulation forwire, windings and condenser materials. On the other hand, conductive coatings can beused to make insulating substrates electrically conductive or even to print electricalcircuits. Furthermore, organic coatings can help to reduce noise pollution. Acousticalinsulation coatings for machines and underbody protection coatings for passenger carsare examples of this.

Fig. 1.1.2Factors determining the quality of coatings

Process Substrate

Coating Design

EcologicalRequirements

EconomicalRequirements

Final Quality

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This broad spectrum of require-ments explains why no single coat-ing material can satisfy every wishsimultaneously and in the sameway. The goal of providing coat-ing materials for the durable pro-tection, decoration and impro-vement of objects made of wood,metal, plastic or mineral materialsat reasonable prices can only bemet by adopting different formu-lations using a range of materialsand material combinations. Eachof these combinations targets alimited field of substrates, a se-lected application method and aspecific profile of film properties.Coating technology is used in metal processing, in the manufacture of plant and ma-chinery and in the electrical engineering industry. All kinds of road and rail vehicles,ships and aircraft are important objects which require painting or coating. Effectivesurface protection by means of paints and coatings is also indispensable in the civilengineering sector, for steel and concrete structures and in wood processing. Even plas-tics and leather require coating in many cases. Modern paper, plastic or sheet metalpackaging materials are inconceivable without the protection and decoration affordedby coatings.The worldwide paint and coatings market reflects economic developments in the re-gions. It is most highly developed, for example, in the so called triade (North America,Europe, South East Asia). The per capita consumption of paints and coatings in theseregions is approx. 4.5 kg. The growth in coating consumption is determined by theeconomic development in the individual regions or countries [1.3.1].The broad field of applications for coatings and their widespread use are explained bythe high value and great benefits which they offer. The fact that there are few objectswhich do not require coating is an indication of the enormous importance of coatingtechnology. Calculating this importance merely in terms of the quantity of coating ma-terials manufactured annually worldwide of 28.6 million tonnes in 2006 with a value ofsome 80 billion euros (fig. 1.1.3) gives an incomplete picture.Although quoting the quantities of coating materials is not a direct indication of theadded value of industrial commodities, it does permit the area which can be protected ordecorated by means of coating materials to be calculated, taking due account of the filmthickness to be applied. Assuming an annual production quantity of 28.6 million tonnesand a wet film thickness of 100 µm (0.1 mm), a surface area of some 315,000 km2 canbe coated. That represents about 3/4 of the surface area of Germany. A 10 m wide stripwith a film thickness of 100 µm, on the other hand, coated using the same quantity ofpaint would stretch about 100 times from the earth to the moon or go round the world600 times.

Fig. 1.1.3World coatings market of 28.6 mio tons in 2006 by regions

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Introduction

A more meaningful means of evaluation would be to use the value of the effectivelyprotected and improved products. Assuming that an added value of 20% of the pro-duced goods is achieved by painting or coating them in the form of an extended servicelife and increased attractiveness, this means EUR 200 billion for the German market in2000. This represents 50 times the sales value of the paints and coatings.

The division of the mar-ket into branches or seg-ments is not uniformaround the world. Anumber of breakdowns,however, seem agreed onusing certain segments,such as decorative paints,general industrial paints,automotive paints, andprinting inks.Decorative paints repre-sent the largest market for

paints and coatings at 53%. This is followed, at 29%, by the market for the industrialcoating of a huge range of objects, from compact discs via plastic bumpers for cars torail vehicles (see chapter 7). Automotive coating lines and refinishing bodyshops areeach clearly defined segments with a high technological value, though of less signifi-cance in terms of volume sales. Printing inks represent approx. 4% of the worldwidedemand for coating materials and are a separate segment in technological and market-ing terms, though not from the point of view of their composition.The size of the European market was 9.1 million tonnes in 2006. There is a slight shifttowards industrial coatings and printing inks compared with the sectoral division in therest of the world. Germany is the leader with a consumption of approx. 1.6 milliontonnes ahead of Italy, France, the UK and Spain, which are all in the range between 0.7and 0.8 million tonnes. The size of the Northamerican market was 21.2 Bio. US $ in2002 served by about 835 companies [1.3.2]The graphic below gives an overview of the economic development of the paint andcoatings industry in Germany since 2000 which is closely linked to the country’s over-all economic development.One of the characteristic features of coating technology in addition to coating consump-tion is the still high energy consumption for processing coatings which is estimated atapprox. 200 billion kWh annually worldwide. This figure is the equivalent of the energycontent of approx. 30 million tonnes of crude oil. If the raw materials required for paintproduction are also added to this figure in the form of crude oil equivalents, the result isa total crude oil requirement of approx. 100 million tonnes for the worldwide manufac-ture and processing of coating materials, or some 3 to 4% of the annual crude oil extrac-tion worldwide.The legal requirements imposed on environment compatible coating processes haveresulted in greater use of appropriate coating materials in the last 20 years. Theseinclude, in particular, solvent free powder coatings, waterborne paints, in which organic

Fig. 1.1.4Worldwide coatings market in 2006 by segments (source : VdL)

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solvents are replaced inwhole or in part by water,high solids paints and radia-tion curable paints, which areprocessed either in an aque-ous solution or completelywithout conventional sol-vents with the aid of low mo-lecular reactive thinners (seechapter 5). Statistics from theAssociation of the GermanPaint Industry (VdL) showthat these coatings haveachieved the greatest growth,with a 10% annual increasein the last 10 years.Ongoing improvements inour knowledge of the toxicol-ogy of the raw materialsentail a regular replacement of raw materials, which is associated with the correspond-ing development costs, if we wish to maintain the quality standard achieved.As far as energy consumption is concerned, there is still a need to be more economicalin the use of raw materials and energy. A proportion of the material is lost en route to thefinished coating. Spray application, which is specified for many objects to be paintedbecause of its optical attractiveness and range of colours, has a particular role in thisregard. As far as coating wood and plastics is concerned, the more effective electrostaticspray methods have not yet found universal acceptance. In addition, the paintlines losesubstantial quantities of heat energy. In recent years a number of developments haveincreased the efficiency of coating processes to such an extent that growth in the paintand coatings market in the industrially developed countries has been below the growthlevel of the gross domestic product (GDP).As a result of the use of solvents as an application aid for coatings, an additional hydro-carbon emission load of approx. 200,000 tonnes is estimated annually in 2007 for Ger-many alone for the currently standard average organic solvent content of 50% [1.3.3].Whereas the organic emissions from motor vehicles have been successfully reduced toless than 1/3 of their previous level in the last 20 years by the introduction of the cata-lytic converter, the successes in coating technology have been more modest to date bycomparison. Despite the introduction of waterborne paints and powder coatings, ap-proximately half of all the industrially processed coating materials used in Germany arestill in the form of conventional, i.e. solventbased materials.From a European perspective, the solvent processing industries, including the paint andcoatings industry, are now the leading emitters of organic compounds and solvents.European legislators responded to this fact by issuing a 1999 EU directive (see chapter5.3) in an attempt to force even small to medium sized companies to adopt more strin-gent measures to avoid emissions.

Fig. 1.1.5Production of coatings in Germany from 2000 to 2006

In more than 200 manufacturing plants with a total of 20,000people about 2.3 million metric tons of coatings have beenproduced representing a value of 4.8 billion Euros in the year2006.

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Introduction

Significant success has beenachieved by manufacturers andprocessors of paints and coat-ings with regard to occupationalhealth and safety. The chemicalindustry, for example, has ledthe accident statistics in Ger-many for the industry with theleast number of incidents.An analysis of this situation re-veals that manufacturers andconsumers of paints and coat-ings, though occupying differentvalue added stages, are extrem-

ely closely connected. Manufacturers develop and supply coatings materials to the con-sumers who in turn modify the product while processing them in physical, physico-chemical and chemical processes and convert them in this way into a bound, mechani-cally solid and, at the same time, flexible coating.The path of chemistry from the raw material to the finished coating starts at the rawmaterials or paint manufacturer and is then consciously interrupted before it is taken upagain during processing by the paint consumer.Although the performance profile of a coating is initially shaped by the paint and thusby the paint manufacturer, it is the processor who actually generates the finished prop-erties. The industrial scale coating of consumer goods is therefore a joint effort betweenpaint and coatings manufacturers and processors.

Fig. 1.1.6German coatings market in 2006 by product classes(source : VdL)

ProcessSequence

LineSpeed

ApplicationEquipment

ApplicationParameters

Flash OffTime /

Air Conditions

Layer DistributionRatio

Filmbuilt1 or 2 Coat

Appearance

Substrate Flash Off /Curing

PaintMaterial

ReciprocationMovement

VoltageFlow Rates

ObjectTemperature

Distanceto Target

R.P.M.Bell

ClimateConditions

VolumeAtomizing Air

VolumeShaping Air

AirlessAirmix

ESTATribo

PneumaticHVLP

High RotationMicro-/Ecobel

SurfaceRoughness

SurfaceStructure

SurfaceTension

Material

Conductivity

Pretreatment

Time

OvenCurve

Temperature

HeatingDirect / Indirect

Temperature

Rheology

SolidContent

EvaporationKinetics

Viscosity

SurfaceTension

Fig. 1.1.7Various factors determining the final coating result of the spray application process

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Paint manufacturers who really know their job are nowadays responsible not only fordeveloping, manufacturing and selling paint. Their task also includes providing the con-ditions for successful painting by their permanent technical presence and support. Thisrelates primarily to materials and processes, though includes detailed environmentalprotection and occupational safety issues. Paint manufacturers offer a package, as itwere, in which the material is just one component among many (see fig. 1.1.7). Apartfrom the technical tasks of manufacturing and processing coating materials, particularattention has to be paid to quality assurance methods.Quality assurance links production with R&D and sales within a company. Productionmust be capable of reproducibly providing the quality demanded by the customer, whilesales must identify the total costs to achieve the appropriate prices.However, paint manufacturers are faced with specific problems since they are expectedto produce constant material quality and at the same time paints with constantprocessibility. Only this provides the best conditions for achieving a uniform result inthe painted article. This means that production paint and coatings entails more thanmerely manufacturing a product whose composition is identical to a defined standard.Rather, since physical variables can only rarely be applied as criteria for the practicalproperties of coatings, paint testing of necessity includes simulating the applicationmethod used by the processor of these materials. This gives rise to a large number ofdifferent test methods because of the very wide range of specification conditions andthe different requirements on the coating process resulting from them. Standardisingthese tests and reducing their overall number is also a priority task for all concerned.Quality and costs of a coating are defined, as mentioned earlier, not only by the paintmaterial and an application method appropriate to it, but also and significantly by thesubstrate, i.e. by the material and the design of the object to be painted. It is thereforeclear that it is extremely important to address surface treatment, material selection andits design properties during product design and to integrate these features in the overallplanning.Coating technology is therefore a complex marriage of chemical, physical, process-engineering, environmental, toxic and economic variables. This discipline is in constantmove as a result of technical progress and further accelerated by legislative require-ments. Companies are therefore called upon to link well known features with newknowledge. Industrial coating technology can only be fully understood if, in addition todetailed knowledge of paint processing, the properties of the coating material and of theobject to be coated are known as well and also if all the quality shaping variables withinthe range of economic and environmental requirements are addressed.

1.2 RetrospectiveThe aim of the following look back at the materials and painting methods of the past isto highlight coating technology’s entangled paths by illustrating a few key events inorder to make its progress more transparent.The early coating materials were natural resins. Dioscorides and Pliny report, amongother things, on the use of countless exotic natural resins from the time of the Greeksand Romans. Later papers describe the importance of colophony, copals, shellac and

Retrospective

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Introduction

dammar. Later still, in the 12th century, come reports of the combination of hardenednatural resins such as amber with resinifying, i.e. chemically hardening natural oils[1.3.4].The application methods up to that point consisted solely of brushing or wipe tech-niques, without significant emissions, waste water or paint waste.

Fig. 1.2.1First recipe of a coating material by Theophilius

The monk Rodgerus von Helmershausen reports in his book Schedula DiversariumArtium for the first time in detail around the end of the first millennium of the composi-tion and manufacture of the then standard paint [1.3.5]. He is therefore, regarded as theperson who specified the first relevant paint formulation with detailed productioninformation.Linseed oil and amber (then termed ”rubber” in German), are boiled together in a ratioof 2:1, with the hardened resin as a nonvolatile reactive thinner, and the linseed oil as achemically crosslinking component. Solvents to regulate viscosity were not used at thattime because of their insufficient availability.In the 10th century it was the Arabs with the doctor Abu Mansur who taught the Europe-ans the art of distillation [1.3.6]. This art was then used in Europe for, among otherthings, extracting turpentine oil. When the von Eyk brothers extended Rodgerus vonHelmershausen’s formulation, which was the first to be systematically described, byadding turpentine oil in the early 15th century, physical drying and thus emissions hadbeen invented [1.3.7]. This outcome is noteworthy, on the one hand, because the use ofpainting expanded substantially as a result and, on the other hand, because the con-nected environmental problems are still tying up a considerable part of the paint spe-cific research capacity to this day.After the memorable ”invention” of emissions the „invention“ of waste followed verymuch later. By this is meant the introduction of the continuous production line by HenryFord in the early 20th century which represents the start of industrial scale paintingtechnology. The production line has brought the world huge benefits in terms of speedand therefore also economical production systems. However, full use could only bemade of the faster coating processes which this required by employing quick dryingpaints and a new application method. The consequence of this was that the oil paints

Rodgerus von Helmershausen, also called Theophilius, describes the manufacturing ofcoatings and gives detailled recommendations for formulas in chapter 21 of his book“Schedula Diversarium Artium“ around 1000 AC :

• Bring linseed oil into a small pot and add rubber, which is called “Forniß“ …

• In a different step collect 4 stones … over these put an empty pot and fill inthe entire and mentioned rubber-“Forniß“, which is called „glassa“ in latin …

• Then bring carefully fire beneath until the rubber is melted.

• Have a third pot nearby on the coal in which warm linseed oil is filled …

• Pour the warm oil on top and stir with the iron …

• Related to the mass make sure that 2 parts are of oil and one third of rubber …

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Retrospective

which took several weeks to cure were superseded by quick drying cellulose paints withsolvent contents of up to 80%. At the same time the wastefree brush method which hadbeen standard up to that point was replaced by the spray method. However, the intro-duction of the spray gun, which had been invented by de Vilbiss back in the 19th cen-tury, also ushered in a new problem because of its extremely poor material applicationefficiency of less than 50%: the generation of paint waste and, at the same time, a dras-tic increase in organic emissions because of the considerably higher solvent contents ofthe cellulose paints.1920 introduced therefore a completely new situation in coating technology without anappropriate approach for the new problem. This approach had to be developed in thecourse of the following decades. The production line meant technical progress in theform of increased coating quality as a result of improved application and a more eco-nomical production system, though at the expense of the environment in the form ofwaste and emissions.The introduction of the production line therefore shared the problems of emissions andwaste which had been known since the time of the van Eyk brothers and was also thestart of the metamorphosis of a craftsmenship into a segment of industrial productiontechnology.Until people finally becameaware of this situation, re-sulting in appropriate con-certed activity, governmentaction was necessary in theform of environmental leg-islation. The innovationswhich occurred subsequent-ly were no longer the resultsof individual ideas butwere increasingly achievedthanks to cooperationamong all those involved inthe finished coating.The first goal was to ration-alise the processes andoptimise quality on the ba-sis of conventional, i.e.solventborne, coating tech-nology. Later, from the1960s onwards, researchstarted in a new direction from several different locations simultaneously, initially sepa-rated but later with a common goal in view: the development of low solvent and lowwaste painting methods [1.3.7].The results of this work are new waterbased materials, radiation curable systems andwaste free powder coatings in association with new processing methods. Electrocoatpainting, roller and curtain coating e.g. for coil coating and the application of solid

Fig. 1.2.2Historical highlights in the evolution of coating technology

EuropeanLacquer Art

Romans, Greek

EastasianLacquer Art

1100 TheophiliusRosins, oil and first recipe

1400 van Eyk BrothersRosins, oil, additional serpentine oil

1550 Martin BrothersLead soap as siccative

1920 ProductivityHenry Ford: Assembly line

and nitrocellulose paint

1960 EcologyWaterborne paints, powder coating

1990 EcoefficiencyCombining ecology with economy

2. Environmental Impact:Waste

About 1500: Transfer of

Eastasian lacquer know-how

1. Environmental ImpactEmission of solvents

“Eco paradise”(No emission, no waste)

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Introduction

powder coatings are the most noteworthy innovations of the last 50 years. Environmen-tal protection, on the one hand, but occupational safety issues also, on the other hand,were beginning to become much more structured like the National Environmental PolicyAct (NEPA) in USA from 1969. Research into environment compatible paint technolo-gies was expanded to include the aspect of minimizing consumption of resources at thelatest with the passage of Germany’s Federal Environmental Protection Act in March1974 and the oil crises.The various trade associations and technical bodies of the American and European paintand coatings industries were also involved in this. The efforts in this direction werestrengthened by the voluntary commitment (“responsible care”) of the paint and coat-ings industry in almost every region of the world.Successful examples of such a concerted approach to dealing with a problem by anumber of different groups and bodies include the development of the new paint proc-ess for the Mercedes A-Class in which a paint manufacturer, a paint processor and aplant manufacturer cooperated, and also the “Expansion of the application fields forpowder coating technology” research project funded by Germany’s Federal Ministryfor Education and Research (BMBF) and coordinated by the German Research Associa-tion for Surface Treatment (DFO) together with a total of 20 companies from the fieldsof paint production, paint processing, plant engineering, the raw materials industry andcountless research institutes. The industry supported LEPC (Low Emission PaintConsortium) under the umbrella of USCAR (United States Council for AutomotiveResearch) is another example [1.3.8].The globalization of the world economy, particularly manufacturing industry, is cur-rently confronting the paint and coatings industry with new challenges. Since qualityrequirements have been standardized worldwide as a result of consolidation, in the au-tomotive industry for example, the major paint suppliers have recognized this and nowguarantee the same quality standard for coatings everywhere. No end to this develop-ment is presently in sight. In addition, sales channels have been expanded for manypaint suppliers by the introduction of e-commerce, the electronic transaction of pur-chases over the Internet. Of necessity, all aspects are steps in the direction of consolida-tion.The quality requirements, particularly by the automotive industry in the form of ISOstandards, VDA standards and the QS9000 series mean that the entire process from theproduct idea, via development, scale up, production and sale to the final application isfalling more and more wholly within the paint and coatings companies’ sphere of re-sponsibility. It now seems that the entire process chain starting with the production ofthe paint materials to the finished (coated) article will become a key competency of themajor paint companies.It is therefore, guaranteed that the active players in this field will continue to growcloser together. Only in this way will it be possible to develop efficient processes for therespective sectors or applications.The conditions for coating technologies with a zero environmental impact and makingthe most efficient use of resources are largely in place. Future success will depend onwhether holistic concepts can be found. Product life cycle or ecoefficiency audits [1.3.9,1.3.10] with the aim of achieving sustainable development will therefore provide the

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Bibliography

principles underpinning research and development programmes more and more fre-quently [1.3.11].Soundly based knowledge is an essential factor in the successful introduction of envi-ronment compatible coating technologies with the simultaneous implementation of newideas in processing technologies. This includes details on correlations and interdepen-dencies between the individual scientific disciplines and thus of the various sectorsinvolved in the paint process. This book has been written not least for this reason.

1.3 BibliographyReferences

1.3.1 A. Mehta, PPCJ 194 (4481), p20 (2004)1.3.2 P. Phillips, Coatings World 11/2002, p221.3.3 “Lack im Gespräch”, DLI Publication No. 73, VDL, Frankfurt, 20021.3.4 Kittel, Lehrbuch der Lacke und Beschichtungen, Band 1, p17,

Hirzel Verlag, Stuttgart, 19981.3.5 Rodgerus von Helmerhausen, schedula diversarium artium, Hrsg. V. Albert Ilg,

Verlag W. Braumüller, Wien, 18741.3.6 O. Krätz, 7000 Jahre Chemie, p 15, Nikol Verlagsgesellschaft, Hamburg 19901.3.7 Die Lack Story, 100 Jahre Farbigkeit zwischen Schutz, Schönheit und Umwelt,

Verband der Lackindustrie aus Anlass seines 100-jährigen Bestehens, 20001.3.8 T. Meschiewitz, Y. Rahangdale and P. Pearson, Proceedings XVII. Surcar, Cannes, 19951.3.9 S. Papasavva, S. Kia, J. Claya, R. Gunther, JCT 74 (925), p65 (2002)1.3.10 D. Engel, T. May, R. R. Matheson, P. Q. Nguyen ECJ 1-2/2004, p401.3.11 Anonymus, F+L 106(6), p48 (2000)

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Coa

ting

Mat

eria

ls

Raw Materials

2 Coating Material

In accordance with the old DIN 55 945 or with European standard DIN EN 971-1 whichhas been in force since 1996, and in general agreement with the English speaking expertsthe correct designation of materials for organic coatings providing surface protection iscoating materials. Paints, on the other hand, is the standard term for coating materialsfeaturing particular properties. In the English speaking world the term ”coatings” isoften synonymous for paint, the layer on the coated products, as well as the paintingprocess.The standard defines paints as liquid, paste or powder coating materials which enableoptically opaque coatings to be produced with decorative, protective and, if necessary,also specific technical properties. It should be noted that many terms in general use,such as varnish or vehicle, are different from the terms specified in internationalstandards (see chapter 8). Paints are therefore just one part of the large class of coatingmaterials, but nevertheless have to meet a broad spectrum of extremely different require-ments. They can be sub-divided in accordance with various principles: by the functionof the coating (clearcoat, metallic paint, solid paint), the particular layer in the coatingsystem (primer, primer surfacer, topcoat), by the purpose of the paint (car paint,decorative paint, industrial paint), by its degree of environmental compatability (waterbased paint, high solids paint, radiation curable systems, powder coating), by thechemistry of film forming agent (alkyd resin paint, acrylic resin paint, cellulose nitratelacquer) or by the processing conditions (baking enamel, oxidatively curable coatingmaterial), to name the major classification criteria.The selection of the right coating materials for an optimum performance profile thereforedepends equally on the quality requirements, the specified application conditions, thecuring process, the design features and the various materials of the object to be painted.All coating materials, however varied their use may be, must have the same propertiesof wetting substrates, transforming into a closed film, flowing and then solidifying sothat the desired mechanical and chemical protection of the object is achieved.The performance profile of the paints therefore has to be adjusted not only to theapplication but also to the subsequent film formation. A fresh paint finish applied byspray gun must exhibit sufficient flowability so the droplets seal all pores. At the sametime or subsequently it must equalize all peaks and troughs in the film surface caused bythe application. Surface tension and viscosity are the most important paint parametersin this regard. Furthermore, special effects such as a high gloss finish, chroma andoriented reflection (metallic effect) may have to be achieved. Clearcoats for woodenpanelling therefore have a completely different composition compared with those ofanticorrosive paints or coating materials for color designs on leather.

2.1 Raw MaterialsResins as film forming agents, often incorrectly termed binders, additives, solvents,pigments and extenders are the usual ingredients of liquid coating materials. Binders in

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Coating MaterialC

oati

ngM

ater

ials

vehicle

clea

r coat, solventborne or waterborne

powder coatingpigmented paint, solventborne or waterborne

solvents(organic or/and

water)

pigmentsand

extenders

solidaddi-tives

transient

film formingagent

accordance with DIN EN 971-1 are the pigment free and extender free parts of the driedor cured coating. They are, therefore, made up of the film forming agent and thenonvolatile part of the additives (see figure 2.1.1).If there are no pigments and extenders, the material is a clearcoat, and if there are nosolvents when higher molecular film forming agents are used, the coating material canbe ground to produce a powder coating. If the film forming agents are lower molecularliquid products, it is also possible to dispense partially or wholly with the use of organicsolvents. Water soluble resins or dispersions permit water to be substituted for thesolvents.

Irrespective of the application, a film forming agent which solidifies as a result ofphysical or chemical processes is the indispensable component of a coating material.Classic film forming agents are exclusively oligomeric to polymeric organic materialsbecause of the range of requirements relating to adhesion to the substrate, to mechanicalstrength with simultaneous elasticity and resistance to ambient effects. They can bemanufactured from natural substances after chemical modification or by industrialsynthesis.Despite the existence of a broad range of synthetic resins, the natural substances ormodified natural substances have lost little of their importance. Cyclorubber orchlorinated rubber, cellulose esters and alkyd resins based on natural fatty acids are justa few examples of the technically important group of renewable paint raw materials.

Fig. 2.1.1Typical composition of coatings

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Coa

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Mat

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ls

Raw Materials

Film forming agents can be divided into two basic groups, irrespective on theirderivation. The first group is the higher molecular, physically curing film forming agentssuch as cellulose nitrate, cellulose acetobutyrate, thermoplastic acrylates and also PVCcopolymers. These resins set by giving off solvents, without undergoing a chemicalchange during film formation.For high-grade decorative and industrial paints, on the other hand, chemically reactivefilm forming agents or film forming agent combinations are used. The crosslinkingreaction initiated by heat or catalysts after application enables lower molecular andthus high solid base resins to be used. An example of this are baking enamels based onacrylicamino resin, alkyd amino resin or phenolic resin combinations. The oxidativelycuring alkyd resins, incorrectly termed ”air-curing”, start to crosslink at room temper-ature, though much more slowly. Modern polyurethane and epoxy resin paints cure veryquickly, even at room temperature. The latter can generally only be used in the form of2-component materials because of their high chemical reactivity.A coating material formulation usually also contains solvents in addition to a filmforming agent. Only when the correct materials are selected and combined is it possibleto achieve troublefree processing and film formation. The most common organicsolvents are aromatic hydrocarbons such as xylene and solvent naphtha, aliphatichydrocarbons such as mineral spirit or vernice, esters such as ethyl acetate or butylacetate, alcohols such as propanol or butanol, ethers such as butyl glycol, ether esterssuch as ethyl or butyl glycol acetate and various ketones. Suitable combinations of thehuge range of available solvents have to be chosen, depending on the resin type. It isespecially important, with regard to the application process, to choose solvent combina-tions with the correct evaporation properties. Only in this way can a balance beguaranteed between the flowability and stability of paints freshly applied to verticalsurfaces. Water as a solvent for environmental compatible paints plays a different role(see chapter 6.6.2).Diverging from the classic principle of using solvents solely to regulate viscosity, theycan also react chemically with the paint resins during film formation, i.e. have a role inaddition to their function as a thinner. These socalled reactive thinners therefore remainin the solidifying film and thus contribute to a reduction in organic emissions. Paintswhich can be cured by means of UV or electron radiation also deserve to be mentionedin this context, as do unsaturated polyesters cured by organic peroxides.The pigments used in the coating materials give the coating its chroma and hidingpower. Extenders influence application properties, such as sandability and hardness, butalso increase the nonvolatile proportion of the coating material. In many cases pigmentsand extenders prevent premature degradation of the substrate to be protected by paintingand the polymer protective coating itself by the absorption of UV radiation. A furtherimportant task for certain special pigments is to optimize protection against corrosion.The many types of pigment are divided into organic and inorganic absorption pigments,metallic and interference pigments and anticorrosive pigments. The first two groupscited consist of extremely finely dispersed, generally crystalline solids. Titanium andiron oxides and the mixed-phase pigments are the most important among the inorganicpigments. There is a huge variation in the composition of organic pigments. Modern,synthetically manufactured organic pigments also offer excellent fastness to light and

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Coating MaterialC

oati

ngM

ater

ials

longterm outdoor exposure properties. Their particular strength lies in their brillianceand their high tinting strength. At high temperatures, however, with certain exceptions,they are inferior to the inorganic pigments in terms of heat stability.Special optical effects are achieved by means of pearlescent, interference or metallicpigments. Coatings whose color or luminosity vary with the viewing angle are ofparticular interest in the automotive sector because of their attractiveness.The final coating material components to be mentioned are the additives andplasticizers. With the exception of the plasticizers, these substances are added to theresin/solvent or pigment mixture in sometimes minute quantities. Despite this lowconcentration, they have a significant impact on the properties of coating materials andcoatings. Additives can facilitate dispersion of the pigments during production. In theready-to-use paint they suppress the tendency for the pigments to sediment, enhanceflowability when the paint is applied, improve flow during film formation and improvethe applied properties by influencing the surface smoothness or roughness. They preventthe pigments from floating to the surface, accelerate curing, also have an effect on thegloss, increase resistance to harmful UV radiation and reduce premature degradation bybacteria or mildew which attack paint.

2.1.1 Film Forming Agents

Whereas it is possible for a coating material to dispense with the raw materials such assolvents, additives or pigments mentioned in the first section, a film forming agent or afilm forming agent combination is absolutely essential if the specified tasks as a paintare to be met. The film forming agent must meet numerous criteria. These relate essen-tially to adhesion to the substrate and the film’s mechanical strength (cohesion) whileretaining its elasticity at the same time. With such a range of requirements it is necessaryto use macromolecular polymeric materials. Their molecular structure enables them tomeet many requirements by mechanical linking of the molecular chains. It is just asfeasible to achieve these properties with synthetically manufactured oligomeric orpolymeric materials as with appropriate natural polymers, though the latter may requirechemical modification to enable them to be used in practice. However, optimum coatingproperties can only be obtained if the film forming agent can wet the substrate as a resinsolution, as a dispersion or as a molten powder coating and can be transformed into avisually attractive, smooth film by means of subsequent flow.The mechanical strength can therefore be formed by the release of solvents (physicaldrying) or by molecular enlargement with simultaneous crosslinking of the film formingagent molecules (chemical hardening).All film forming agents which solidify by physical drying, i.e. purely by releasingsolvents, therefore remain sensitive to solvents and can be liquefied again by heattreatment. These film forming agents are thermoplastic. Consequently, coatings madeusing them are also thermoplastic.Chemically reactive film forming agents, on the other hand, perform differently. Theseare chemically crosslinked with reactive groups of the resin molecules during filmformation and thereby transformed into an insoluble duromeric film which also cannotbe reliquefied.

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