SURFACE ENERGIES, LEWIS ACID-BASE AND LIFSHITZ-VAN DER WAALS FORCES 

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An introduction to powder coatings

Surfaces with powder coatings can be found on a large range of different mainly metal objects in our everyday life, such as furniture or refrigerators. The powder are milled small grains of a formulated polymer material, ready to be used. Binder, pigment, film flow agents and all other additives in the powder form together a coating on the substrate. No solvents are needed, not even water, why powder coatings are more environmental safe than many other paint systems. For a fundamental understanding of powder coating I recommend Powder Coatings Chemistry and Technology, T. A. Misev [1] or a more updated version with the same title but a new author, Pieter Gillis de Lange [2]

How powder coatings works

In short, painting with powders can be described as follows figure 1a-c. The powder is suspended in air. An electrostatic attractive force sticks the dry charged powder particles to the grounded substrate. The powder is heated and melts reaching a low viscosity, spreads and form a film. The film polymerises, cross-links, to a relatively hard thin surface layer. By the choice of polymer additives and formulation the properties of the final coating can be influenced and adjusted to fit a variety of applications.
Figure 1a Milled polymer powder [3].
Figure 1b Powder spread over the surface [4].
Figure 1c Powder heated, Figure 1d The final in this case by IR-light [5]. coating [6].
Painting with powder coatings is a relatively simple and over all a fast process why it is convenient for industrial usage. To be cost effective the series of components should be fairly large since the painting equipment is expensive. An even powder layer is applied to the substrate. This can be done in a variety of ways for example by using a fluidized bed, electrostatic spraying and plasma spraying. For example in a fluidized bed the particles are charged by ionized air, the charged particles repel each other and form a “bed” of powder. A grounded substrate is placed in the “bed” and the particles are drawn to the substrate to form an even layer. When the powder layer is evenly distributed heat is applied to the object for the sintering and coalescence process. The grains melt to form an even and smooth film layer (coating) and curing of the binder can be initialised. The polymerization leads to a cross linkage of the molecules, or in other words more commonly known “the paint is drying”. How the paint hardens or cure depends on the kind of polymer and additives used. A UV-cured polymer takes only seconds to cure while other polymers as thermosetting polymers could crave for higher curing temperatures in an oven, which in that case demands longer processing time and crave substrates that can withstand that high temperatures. Drawbacks with all powder coatings systems are the expensive equipment and the high operating temperatures to melt the powder into a film.
A problem with thermosetting powder coatings, the most commonly used powder today, is the curing temperature of 140-200C. A high curing temperature is necessary to prevent polymerisation and sintering of the particles to take place while storage. Storage stability is related to the reactivity of the components, if too reactive the powder will polymerise during storage and if too little reactive the powder will require a higher temperature when stoving to polymerise. Hence the reactivity must be tuned and that can be understood from the Arrenius equation. The Arrenius equation 1, displays that the higher activation energy (reactivity) demands an increasing temperature for the rate coefficient (of polymerisation) to increase. The different parts of the equation are; rate coefficient K, the constant A, the activation energy Ea, gas constant R and temperature T. ln K is plotted against 1/T and has a straight line with an intercept ln A and the slope –Ea/R [2, 7].
Equation 1 A logarithmic form of the Arrenius eq. that describes the reaction kinetics.
One way of influencing the curing reaction of a thermoplastic powder is the use of catalysts as illustrated in figure 2. Line (1 and 2) that represents different concentrations of the same catalyst gives that a higher concentration brings an earlier curing of the polymer. Line 3 demonstrates a steeper slope, a higher activation energy and pre-exponential parameter. Finally line 4 shows a latent catalyst and the delay of the reaction forms a piecewise linear increasing function. That is for line 4 a low reactivity in the Tstorage temperature range and a higher reactivity of the powder when heated to Tstoving.
Figure 2 Arrenius plot of curing reactions catalyzed with different concentrations of the same catalyst (1 and 2), a catalyst with higher activation energy and pre-exponential parameter (3) and a latent catalyst (4) (arbitrary units) [2].
Heat sensitive objects are very difficult to paint with powder coatings. Wood or plastics that cannot withstand high temperatures are therefore quite difficult to cover with “thermosetting” powder coatings. Heat sensitive substrates such as wood, MDF, plastics and paper needs lower temperatures. A low curing temperature gives problems to achieve good storage stability and/or good film forming properties for thermosetting powders. Therefore a system with a latent catalysis that can be activated after film formation like in a UV-curing system could overcome the problem described above. A good combination of storage stability and high curing speed can be achieved [2].

UV-Curing

There is a variety of polymer material used for powder coatings. Powder formulations can be tailored to fit a certain application, depending on what substrate is to be used or what demands is set on the final coating regarding finish, toughness, scratch resistant, weatherability etc. Over the years development of powder coating has given us a variety of polymer formulations to choose from. One relatively recent development is the introduction of UV-curing techniques for powder coatings.

UV- curing in general

UV-curing of coatings is a process where the polymerization (drying) is initiated by irradiation with UV-light. The UV-light is absorbed by photo-initiators, which are activated upon irradiation to form initiating species that starts the polymerisation. One main advantage with this process is that the coating is stable on storage as long as it is protected from UV-light. The wet-film formation can thus be decoupled from the curing. More details on UV-curing can be found in various textbooks such as Exploring the Science, Technology and Applications of UV and EB Curing, [8].
UV curing commences either by a free radical or a cationic polymerisation that is initiated by a photo initiator.
With radical polymerization photo-initiators forms, during UV radiation, free radicals, which initiate the polymerization of the unsaturated binder system.
With cationic polymerization (only used for epoxy resins) the photo-initiators are onium salts producing cat ions of strong Lewis or Bronstedt acids upon irradiation [2].
Radical Polymerisation Cationic polymerisation
o Shrinking
o No inhibition by water o Inhibition by oxygen
o Very fast curing (in seconds)
Comparing radical and cationic polymerisation
o Very little shrinking o Inhibition by water
o No inhibition by oxygen
o Somewhat longer curing (but also sec.)

UV-curable Powder Coatings

With the help of photo-initiators and UV irradiation to initiate curing of the polymer powder there is an opening for lowering processing temperatures since the polymer blend is stable until exposed to UV-light. By combining UV irradiation with infra-red (IR) heating instead of a conventional oven, the film formation process can be done without affecting heat sensitive substrates. In other words reduced heat impact on the substrate.
In a UV-curing process the first step is to let the powder melt, sinter and the resulting film spread to a smooth homogeneous surface and then the polymer is cured by UV irradiation. In this way the film can level out before curing. Thermally curing systems level and cure partly simultaneously leading to an increased viscosity and more surface defects. Another feature is energy savings when using decreased temperature and a swift processing.

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Film formation

Film formation takes places by heating the powder particles. The individual particles melts and simultaneously spread on the surface and sinter together. Thereafter the melt flows out and level, to a smooth surface. Preferably the cross linking occur as a final step after levelling, to reach a high molecular weight film. This can for example be obtained by UV-curing techniques.
Figure 3 Levelling, spreading of powder coating

Wetting/Spreading of Powder Particles On a Substrate

Spreading of a melted powder particle over a substrate can only occur when the melt has a surface tension that is lower than the surface energy of the substrate. That is when the spreading coefficient (S) is positive in Young’s equation 2. S depends of the following interfacial tensions; the substrate/vapour SV,, the melt (liquid)/vapour LV and the substrate/melt SL., see Figure 3 [9].
How well a liquid wet a substrate surface and spread is distinguished by measuring the contact angle that a liquid droplet in equilibrium forms together with the surface. When the liquid covers the whole surface the contact angle is 0°. If the contact angle is higher than 90° the liquid wetting will be hindered and spreading will not occur [10].
The property that describes the restrictions of the flow is the viscosity that is a higher viscosity means higher restriction for flow.
To obtain as good film formation as possible it is necessary to control how the viscosity and the surface tension of the melt changes with temperature and polymerisation. It is simple to study the viscosity but considerably more difficult to study the surface tension, since the melt has a high temperature [9].

Coalescence

Coalescence is when powder particles sinter together, that is grows or melts together into one solid unit. Several different models exist that describes this process.
Important factors that governs coalescence at a certain temperature are the melting point of the polymer, the viscosity of the melted powder particles and the size of the particles [11]. The Nix and Dodges, equation 3, describes the time needed for separate particles to coalesce. The time t needed for coalescence, at a constant temperature is proportional to the viscosity , the mean radius of curvature Rc and inversely proportional to the surface tension . Rc can to begin with be approximated as the particle diameter [9].
Both these equations however assume that the viscosity and surface tension are constant. The coalescence of powder coating particles however normally coalesce during a temperature increase. This makes a complete description much more complex to describe. One way to improve the understanding is thus to reveal how the surface tension and viscosity vary with temperature.

Film flow and levelling of coatings

After coalescence of the particles, the powder coating forms a continues film layer by spreading over the substrate into an even layer, which is described by Rhodes and Orchards equation 5. The equation describes the process very well [1, 9].
If surface tension and viscosity is assumed constant, at a given temperature, then the velocity of levelling can be described in equation 5 [9]. The constant k = (16 4)/3) [1].
A good flow demands a thick film layer h, high surface tension (driving force) for the melt , low melt viscosity (resisting force) and a small amplitude ai. The amplitude depends on the original size of the particles that is small particles gives a small amplitude. All symbols of the Rhodes and Orchards equation is illustrated in figure 5 [1, 9]. The viscosity depends on the polarity, molecular weight and stiffness of the resin polymer. A high melt viscosity results in a higher resistance of spreading for the polymer melt.

Optimising film formation

Industrial usage of paint systems necessitates swift processing and a reduced time and number of steps in production is favoured. A most important issue is the quality of the end result and an increase of the final surface quality of the coating is favoured.
Different polymer or additive combinations can be used to change film formation behaviour and curing time for powder systems to suite different substrates. Care must be taken in consideration when optimising different production steps such as in the film formation process. If the surface tension or melt viscosity is not in an acceptable range various surface defects might appear. A small change of surface properties could cause a low wettability or coalescence could, as an example, result in a lowering of the gloss of a coated surface.
The many components in a formulated powder coating result in a complex system and there are different parameters that have to be dealt with and dividing them up into smaller parts gives a better understanding of the process. For the ability to observe and govern the different factors effecting the process, as described for the film formation in section 3.3, a look at the reactivity for the system, compatibility between and concentration of the included components could be studied.
It is clear from the equations above regarding coalescence between powder particles and wetting of the powder to the substrate, that the demand for surface tension differs for the two phenomena. A maximum flow demands maximum surface tension, equation 5 and maximum wetting demands minimum surface tension, see equation 2 [9].

Table of contents :

1 INTRODUCTION 
2 AIM OF THIS WORK 
3 THEORY
3.1 AN INTRODUCTION TO POWDER COATINGS
3.2 HOW POWDER COATINGS WORKS
3.2.1 UV-CURING
3.2.1.1 UV- curing in general
3.2.1.2 UV-curable Powder Coatings
3.3 FILM FORMATION
3.3.1 WETTING/SPREADING OF POWDER PARTICLES ON A SUBSTRATE
3.3.2 COALESCENCE
3.3.3 FILM FLOW AND LEVELLING OF COATINGS
3.4 OPTIMISING FILM FORMATION
3.4.1 PARAMETERS INFLUENCING POWDER COATING PROPERTIES
3.5 SUBSTRATE SURFACE
3.6 CALCULATIONS OF POWDER PARTICLE SPREADING
3.7 SURFACE ENERGIES, LEWIS ACID-BASE AND LIFSHITZ-VAN DER WAALS FORCES
3.7.1 THE SUBSTRATE SURFACE
3.7.2 SURFACE TENSIONS OF A POWDER COATING MELT
4 EXPERIMENTAL 
4.1 MATERIALS
4.2 METHODS
4.2.1 MOLECULAR WEIGHT AND DISTRIBUTION
4.2.2 MELTING POINT
4.2.3 PREPARATION OF SUBSTRATE SURFACES
4.2.4 CONTACT ANGLE MEASUREMENTS
4.2.5 POWDER PARTICLE SPREADING
4.2.5.1 Microscope Set-up
4.2.5.2 Heating of the Powder on the Substrates
4.2.6 EVALUATION OF SURFACE ENERGY AND FORCE CHARACTERISTICS
5 RESULTS AND DISCUSSION 
5.1 MOLECULAR WEIGHT AND MOLECULAR WEIGHT DISTRIBUTION
5.2 MELTING POINT
5.3 PREPARATION OF SUBSTRATE SURFACES
5.4 CONTACT ANGLE MEASUREMENTS
5.5 POWDER PARTICLE SPREADING
5.6 EVALUATION OF SURFACE ENERGIES, LEWIS ACID-BASE AND LIFSHITZ-VAN DER WAALS FORCES
5.6.1 SUBSTRATES
5.6.2 POWDER MELT
6 CONCLUSIONS 
7 FURTHER WORK/PROPOSAL FOR FURTHER WORK 
8 ACKNOWLEDGEMENTS 
9 REFERENCES 
APPENDIX I
APPENDIX II

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