Powder coatings are not manufactured like conventional paints at all. The procedures and equipment are similar to those used for plastics compounding. Also in its application powder coatings are unique as no solvents are used whatsoever. Hence, pigments have different requirements than those for conventional paints.
Dispersibility: Ease of dispersion is an important property of pigments for powder coatings. The dispersion techniques and conditions are totally different from those of liquid paints, and so direct comparison cannot be made. Dwell time is short and wetting more difficult, so dispersibility becomes more critical. Pigments that show good dispersibility in plastics like polyolefins are quite often suitable for powder coating regarding dispersion properties.
Heat stability: The pigments must withstand temperatures of at least 210°C, and sometimes more, for a minimum of 12 - 15 minutes. Higher temperatures may be required depending on the end-use (barbecue grills, stoves etc.).
Rheology: The flow of the molten coating should not be impaired by the pigment. The coating need good flow properties at low shear, in order to form a smooth continuous film during the melt/cure cycle after application.
Hiding: Good hiding is necessary for powder coatings. The final film should hide at 40 -; 70 ?m, while maintaining good flow and appearance properties.
Weatherability: For exterior applications, a minimum of 1 year Florida exposure is required, with changes no greater than that of the unpigmented system. This applies to both color and gloss retention.
Resistance properties: Chemical resistance properties depend on the end use, e.g. powder coatings for washing machines require alkaline resistance. The following pigments can be recommended for this application: Mixed metal oxides (complex inorganic color pigments) like CI Pigment Yellow 53, Brown 24, Blue 28, Blue 36 but also CI Pigment Yellow 184 (bismuth vanadate), lead containing pigments like CI Pigment Yellow 34 and Red 104, and organic pigments like CI Pigment Yellow 110, Pigment Red 112, 202, 254, 255 and 264 Pigment Violet 19 and 23, Pigment Blue 60 and 15:1. Pigment preparations with e.g. urea aldehyde as a carrier are also offered and can be an interesting alternative to powder pigments.
Phthalocyanine blue pigments come in three different crystal modification forms: the alpha-form (CI Pigment Blue 15), which is a reddish blue with highest color strength, the thermodynamically most stable beta-form (CI Pigment Blue 15:3), which is greenish in its shade and the most reddish epsilon-form (CI Pigment Blue 15:6).
The alpha-form tends to change into the more stable beta-form in certain solvent systems (e.g. xylene). To avoid such a change the alpha-form is stabilized by partial chlorination. This pigment is called solvent stable alpha blue or CI Pigment Blue15:1.
The surface of phthalocyanine pigments is very nonpolar with the result that in most paint systems only weak interactions take place between this blue pigment and the binder. Therefore, stabilization of the dispersions is very difficult, paint systems containing phthalocyanine pigments tend to flocculate or show separation of the liquid paints during storage.
These disadvantages can largely be overcome by special surface treatments or by chemical modifications of the phthalocyanine molecule. The alpha modification which is stabilized towards flocculation and the phase change are registered as CI Pigment Blue 15:2.
In the coatings industry the reddish blue pigments (alpha-form) are preferred to the greenish types (beta-form) for coloristic reasons but also due to their superior dispersibility and flow.
Since resistance to flocculation is a property of the system in which binder and solvents also play a role, it is impossible to make a general statement about which of the flocculation-resistant pigments will perform best in any particular binder system. This has to be tested by the paint manufacturer in the specific system to be used.
Lead containing pigments have been subjected to increasingly restrictive regulations. Paint formulators have typically replaced such lead containing shades by using organic pigments in combination with titanium dioxide. But in certain cases the use of organic pigments in combination with mixed metal oxides (also known as inorganic complex colored pigments) demonstrates a better value-in-use than with TiO2. Because of the inherent chroma, saturation and opacity, mixed metal oxides typically allow the formulator to reduce the more expensive organic pigment component of the formulation and reduce or eliminate the titanium dioxide component as well.
As organic pigments, products are suitable that show a good hiding power and excellent weathering properties. For red these are e.g. CI Pigment Red 48:4, Red 112, Red 170, Red 254, Red 255, Violet 19 for orange e.g. CI Pigment Orange 36 and Orange 73 and for yellow e.g. CI Pigment Yellow 74, Yellow 109, Yellow 110, Yellow 139, Yellow 151, Yellow 154.
In the yellow area also the use of bismuth vanadate pigments is recommended (CI Pigment Yellow 184). Bismuth vanadate containing pigments exhibit a much greater tinting strength than the mixed metal oxide yellows (NiSbTi-Yellow -> CI Pigment Yellow 53), are bright in color, have a very good hiding power (only limited or no addition of TiO2 necessary) with excellent weather fastness and heat resistance. These pigments are considered as non-toxic if the product is non-dusting (finer fractions of dust of this pigment are harmful if inhaled leading to lung lesions).
Segregation of pigments is a well-known phenomenon in the paint industry. Especially in paints where more than one pigment is incorporated it can happen that the pigments in the dried paint surface are not distributed evenly. If there are local concentration differences on the paint surface we are talking of “Floating” or vertical segregation. The paint surface looks streaky or patchy.
If there are no differences in concentration on the paint surface but differences within the paint film (horizontal pigment concentration differences) we are talking of “Flooding” or horizontal segregation. In this case the paint surface shows an even shade and the difference can only be seen if the paint is applied on a glass plate. Segregation is to a large extent linked to the different mobility of the various pigments in the paint formulation. By using polymeric dispersants the effect of segregation can be reduced.
There are various ways to evaluate the segregation process:
During the dispersion process the aim is to surround the pigment particle with sufficient vehicle or resin, such that the particle-to-particle contact is thereafter avoided. On occasions this dispersion process is reversed and reagglomeration or FLOCCULATION occurs.
Reagglomeration means that pigment particles stick together, forming agglomerates. The contact surfaces between the particles contain no binder material. On the contrary during the flocculation process, the single particle do not lose their surrounding binder material. Therefore, flocculates are weakly linked particles, which may be fairly easily separated by low shear forces.
The flocculation of a pigment in an application medium leads to coloristic changes like loss of color strength, gloss and transparency. The tendency to flocculate is a property of the entire pigmented system, which is based on interaction of the pigment surface and the constituents of the medium. Incorporation of small amounts of suitable additives into the application medium can prevent flocculation.
The following parameters have an influence on flocculation:
Grinding gauge: Used as a quick and convenient technique for liquid systems. The HEGMAN gauge consists of a steel block having 2 shallow grooves, which taper from 100 mm to zero. A small “blob” of the medium under test is placed in the deep end of the groove and drawn down with a scrape blade. Equidistant graduations are marked on the shoulders starting with 0 at the deep end of the groove and finishing with 8 (sometimes 10) at the point where the groove reaches the surface level. The graduation mark at which pigment particles begin to be plainly observable as protruding through the surface of the medium in the groove is taken as the index of the degree of dispersion. Normally dispersion will not be considered acceptable until the Hegman gauge reading of at least 7 is reached.
Optical microscope: The use of an optical microscope is quick and the size range observed correlates well with the achievement of color strength. In addition further information as particle type, size and distribution can be seen as well as evidence of flocculation. A small drop of the dispersion is put on a microscope slide and covered with a small glass slide. It is important that not too much pressure is put on the covering glass plate as this could result in an additional dispersion step. The disadvantage of the optical microscope is that the minimum resolution is about 2 mm.
Electron microscope: The big advantage of this equipment is the very high resolution – the ability to see pigment particles in the size range which influences transparency, flow and color. The disadvantage is the cost of the equipment; it is slow and needs experienced operators to interpret the result. Additionally, only dry samples can be measured.
Dispersion, the broad term for the incorporation of pigments in a medium, consists of four processes:
Wetting: In fact, wetting means two separate actions. Primarily wetting means the spreading of a liquid on the powder pigment and eliminating the pigment air interface. Wetting, however, is also the achievement of the softening of the powder pigment agglomerates by means of a liquid.
Breaking down of agglomerates and distribution: In this phase, the agglomerates and remaining aggregates are broken down into the primary particles and by definition are thus distributed throughout the medium. The breaking of the agglomerates is achieved under shear conditions and by impact/attrition. It follows that to achieve maximum effectiveness at this stage, the shear forces and/or the probability of crushing must be at a maximum. This is obtained by the correct selection of the dispersing equipment, depending on the viscosity and type of medium that is to be pigmented.
Stabilization: Once a pigment has been dispersed into the medium, the pigment should be required to stay as primary particles. In media with relatively low viscosity, the dispersed pigment particles, due to attraction forces between them, show a tendency to contract and come together again. This is known as flocculation or reagglomeration, and to eliminate or certainly reduce this tendency, the pigment particles must be stabilized. This stabilization can be achieved by a steric stabilization where the absorbed layer of medium hinders the close approach of the particles or by charge stabilization.
Any or all of the following causes gloss deficiency of a surface:
Light passing through a transparent medium will either go straight through or be reflected from the substrate below without substantial alteration. Light entering an opaque medium will, however, not penetrate and will either be absorbed or reflected.
When discussing pigments, it is not strictly correct to describe them in terms of transparency or opacity. Rather one should describe a pigment in terms of its ability to impart a degree of HIDING (hiding power) or reduction of transfer of light in a particular medium.
Pigmented media achieve hiding power due to two mechanisms: absorption of light and scattering of light. E.g. black absorbs all light, colors absorb selected wavelengths, and white absorbs no light, but scatters the light very effectively.
The measurement of hiding power potential of a pigment relates to the type of medium into which it is incorporated and to the thickness of that applied medium.
The measurement instrumentally of the actual light radiation through a pigmented medium of a given thickness and concentration in comparison with the original light source.
Many applications of paints colored with pigments (or dyestuffs) require that the original color obtained remains unchanged. We define this as the resistance of the product within a given medium to color change when exposed to daylight. Among the radiation that makes up daylight, the most damaging portion is the ultra violet radiation (UV). When we consider Lightfastness, therefore, we are only referring to the resistance to the light component of exterior exposure. The real measure of the resistance of the colored system to exterior conditions should be considered under the term Weather Fastness. As the definition of what is a representative weathering condition is extremely difficult to determine, the measurement of lightfastness without any of the other features of the external environment helps to give a more reproducible indication of the likely resistance of any pigmented paint system.
Exposure during weathering tests is influenced by a whole range of factors over and above visible light; high-energy UV radiation, heat, moisture and various impurities in the atmosphere.
Weathering can be carried out in outdoors exposure test or using so called artificial weathering equipment that is simulating weathering conditions. Exterior exposure is carried out at specific locations, normally selected for their aggressive conditions (high level of sunlight, industrial atmosphere etc.). One of the well-known testing stations in the world is in Florida, USA. Test panels are exposed for a period of 12 or more months included at an angle of 5° facing south.
Many fluids show deviations from this simple relationship:
Rheology is important at the dispersion stage, on storage and at the application stage.
At the dispersion stage: Good flow at this stage normally relates to the lowest viscosity commensurate with the highest possible pigment loading or pigment-to-binder ratio -; depending on the characteristics of the dispersion equipment. Fine pigment particles cause high viscosity because of the large surface area that needs to be covered by the fluid. Large volume fractions (pigment particles plus the surrounding polymer/additive layer) also cause a high viscosity.
On storage: If pigmented systems have to be stored prior to application, there should be no change in the structure of the system such that the rheological properties are affected at the application stage. At the application stage: The ideal rheological behavior is to have Newtonian flow and a lack of thixotropy. The flow characteristics in terms of viscosity relate to the actual requirements of the application process (spraying, roller coating, brushing etc.).
All these polymers are brittle and have to be plasticized for paint use. Thermoplastic acrylic lacquers are primarily used by the automotive paint.
Chlorinated rubbers are used to protect surfaces in highly corrosive conditions, to resist either immersion under water or highly humid conditions (particularly suitable for marine coatings). The solvent systems employed will vary according to exact requirements, but can be expected to be strong (ketones, esters, aromatics); tolerance to these solvents is an important criteria of pigment selection.
Cellulose systems find their main outlet in automotive repair and wood finishes, but are also used for quick drying lacquers including aerosol packs.
The ingredients found within thermosetting paints undergo a chemical reaction on the surface of the article being coated to produce a cross-linked polymer film. This occurs due to a condensation reaction between methylol (-CH20H) attached directly to a nitrogen atom. For this reason the family of resins involved are described as nitrogen resins, although they are more widely known as amino resins. The principle resins used are urea formaldehyde (U/F), melamine formaldehyde (M/F) and acrylamide.
U/F and M/F Finishes
U/F and M/F resins are brittle and have poor adhesion, therefore, they are plasticized by blending with alkyds, polyester or acrylic. Since the cross-linking condensation reaction of U/F and M/F resins proceeds at temperatures of 90 - 18O°C and, with higher levels of stronger acid at temperatures above 15°C, two types of finish can exist:
Stoving Finishes
- Short or medium oil alkyds based on drying or non-drying oils are used for a wide range of general-purpose finishes. Alkyds are used in conjunction with M/F and U/F resins. Where maximum resistance to natural weathering is required, M/F is used. Ratios of the solid resin components normally used are alkyd : M/F = 4:1 by weight and alkyd : U/F = 1.5:1 by weight. Stoving conditions are normally 120 - 130°C for one hour.
Cold Curing Finishes
Alkyd-U/F resin systems can be catalysed by the addition of small amounts of an acid, e.g. hydrochloric, phosphoric and benzene sulphonic acids. These acid catalysed systems are very often used for wood finishes. The curing takes place at room temperature. Similarly, the stoving temperature and time of acrylic amino systems can be reduced by the addition of an acid catalyst (i.e. from 30 min at 120°C to 20 min at 80°C). These systems are of particular interest for automotive finishes. The type of acid used for curing has to be taken in consideration for the pigment selection.
Acrylamide
Unlike U/F and M/F resins, acrylic nitrogen resins need not be blended with another component to obtain flexibility; this can be built into the acrylic copolymer by appropriate choice of monomer. The main outlet for this family of resins is coil coating, particularly for caravan sidings. However, acrylic nitrogen resins can also be modified with other resin (alkyds, polyesters and epoxies), particularly for the domestic appliance market, giving special resistance to fats, detergents and household chemicals.
Polyurethane systems either cure through the reaction between organic isocyanates and hydroxyl containing compounds R-NCO + HO-R gives R-NH-C-OO-R or contain urethane linkages in the molecular structure and dry by one of the other mentioned mechanisms.
A number of paint systems can be mentioned; split into one pack and two pack.
One Pack
Two Pack (Polyhydroxylic resin/isocyanate finishes)
2-pack systems consist of
Epoxy, polyester, polyether and acrylic polymers containing hydroxyl groups are widely used to give 2-pack systems which can be cured at room temperature by urethane formation. Polyol/isocyanate 2-pack systems have wide applications for furniture, floors and boats, as corrosion resistant metal finishes and for coating plastic, rubber, masonry etc. In recent years, acrylic resins with aliphatic polyisocyanates have been used for automotive refinish coatings. For pigment selection, isocyanate curing becomes an additional criterion for two pack and moisture-cured one pack systems. In addition, solvents used will (with the exception of urethane oil and alkyd finishes) not contain -OH groups, so alcohols/ether alcohols are not suitable, rather aromatics, ketones and esters.
Unsaturated polyester resins are normally dissolved in styrene monomer and are able to give films with high-build since the styrene reacts with the polyester base in the presence of a peroxide and a cobalt initiator. This reaction is due to the formation of free radicals which react on c=c bonds.
The basis of the coatings is a 2-pack system containing the following ingredients:
High-build pigmented finishes based on these resins find use in applications such as finishes for kitchen furniture. Pigment selection must take into account reaction with peroxide.
Additionally polymerisation can also be achieved by radiation (UV or electron beam), decomposing "photo initiators" (as part of the paint formulation) to again form free radicals. The formulation of radiation-cured systems in the paint industry (which also includes acrylic based systems) is complex and consideration of pigment choice can now also include radiation-curing inhibition. Radiation-curing systems are ideally suited to coating flat panels (wood, panels for furniture).
