Fall 1999 Vol. 1, No. 1
Additives are an important ingredient in the formulation of powder coatings. For the most part, additives perform the same functions in powder coatings as in liquid coatings. With the exception of wetting, dispersing and foam-control agents, many of the same additives used in liquid coatings are also used in powders.
The ideal powder coating additive should have the following characteristics:
solid, preferably a fine powder, with a crystalline melting point or Tg > 50ºC
specific in function
chemically nonreactive with the binder resins and curatives
effective at low levels.
In cases where the desired additive is a liquid, it can be introduced into the formula as a master batch, either absorbed on a porous carrier, such as silica, or dissolved/dispersed in a compatible resin. While it is possible to add liquids directly to the powder premix prior to extrusion, it is difficult to obtain good distributive mixing of the liquid ingredient and, in most cases, is better avoided.
A brief discussion of the most widely used generic additives in powder coatings follows.
Flow-control additives might more accurately be referred to as surface-tension modifiers, because this is their primary function. However, the term “flow- control additives” is widely used for these materials, which primarily prevent craters in powder coatings and secondarily reduce orange peel. With the exception of special-effect coatings, such as hammertones and veins, a flow-control additive is present in practically every powder coating formulation.
Flow-control additives function to reduce the surface tension of the powder particles as they melt, flow and coalesce at both the coating/substrate and the coating/air surface (1). These additives function by being only partially compatible with the binder. Part of the molecule is soluble in the resin, and another portion is insoluble and orients itself at the resin/substrate or resin/air interface. This molecular orientation promotes a uniform surface tension across the surface of the molten coating, eliminating regions of low surface tension, which can result in craters or poor substrate wetting (2).
The most widely used class of flow-control additives is polyacrylates. Both homopolymers and copolymers are utilized in powder coatings (3). All are moderate- to high-viscosity liquids, and most are supplied as a powdered master batch dispersed on silica particles, usually at an active level of about 60 to 65%. Depending on the particular system of resin and curative, the use level is in the range of about 0.5 to 1.5% active material based on the binder, with an average in the 0.8 to 1.0% range. Pigmented systems usually require a somewhat higher level than clears.
For improved smoothness-i.e. a lower degree of orange peel-higher levels are used than those necessary to prevent cratering. The effectiveness of a particular acrylate additive depends on the composition and molecular mass, as well as the concentration and degree of compatibility (4). If the level is too high, the surface of the coating can become tacky.
In addition to the acrylate types, silicones are also used. These are more effective in their ability to reduce surface tension, and lower amounts are used than in the case of polyacrylates, typically 0.1 to 0.5% based on binder. The straight poly(dimethyl) siloxanes are almost never used because they are extremely active and can cause defects, such as haze, roughness, pinholes or craters in powders utilizing other flow-control additives. This phenomenon is usually referred to as “compatibility” and is a result of the very low surface tension produced by even small amounts of the dimethyl siloxanes. Polyether and polyester-modified poly- siloxanes are therefore preferred (5,6). They also are used at lower levels than polyacrylates and are sometimes preferred in clear coatings. However, the poly- siloxanes are also liquids and normally supplied as a powdered masterbatch on silica particles. Where the highest level of clarity in a coating is required, such as in automotive powder clearcoats, dispersions of active flow-control additives on silica are avoided because the silica particles impart a haze to the clear coating. In this case, 100%-solids flow-control additives are preferred (7).
Flow-control additives based on fluorocarbons are also effective in powder coatings at even lower levels than silicones (8). They are not widely used for several reasons: They normally contain solvents, are expensive and are not readily available in the form of a powder master batch. However, they have excellent recoatability and sometimes are the only flow-control additive that is effective on difficult substrates such as oily or contaminated metal.
Several higher-molecular-weight thermoplastic polymers are effective in preventing craters in powder coatings. Among these are poly (vinyl butyral), cellulose acetate/butyrate and acrylate copolymers. These are seldom used except in functional coatings because they reduce melt flow and increase the orange peel of the film.
A number of additives are used to improve a coating’s appearance. By far, the most widely used additive for this purpose is benzoin.
Benzoin functions as a bubble release or antipopping agent. It has a low melting point and volatizes to some extent during curing (9). It is believed to act as a plasticizer, promoting flow in the resin system and holding the film open to allow volatiles to escape. Studies have shown the addition of benzoin to epoxy resin results in a reduction of both the melt viscosity and the surface tension. However, when benzoin is used in combination with a typical acrylate flow-control additive (Acronal 4F), no reduction of surface tension occurs, other than that imparted by the flow additive alone (10). Regardless of the mechanism, benzoin improves the appearance of powder coatings based on all types of chemistry. It reduces surface haze resulting from micro-defects and improving the distinctness of image (DOI). Unfortunately, most grades of benzoin contain benzil as an impurity, which leads to yellowing, especially on overbake. A number of additives have been developed that do not have the yellowing tendency of benzoin. Among these are Oxymelt A-4 (Estron Chemical), Benzoflex 352 (Velsicol Chem), Surfonyl P-200 (Air Products), Modarex 29-079B (Synthron) and Powdermate EX 542 (Troy).
In highly pigmented/filled systems, a number of additives, which are generically classified as dispersants, have been found effective in improving the gloss, smoothness and DOI of the coating. It has been shown, for example, that at equal pigment levels, a powder with well-dispersed pigments has a significantly lower melt viscosity than one with poorly dispersed pigments (11). Recent work by Bayer shows the surface treatment on a pigment has a considerable effect on the ease of dispersion (12). Some of the additives effective in this regard are Nuosperse 657 (Crea-nova), EX 542 (Troy), Surfonyl P-200 (Air Products) and Rheocin PC (Sud Chemie).
Additives to improve exterior durability are one of the few classes of materials that function exactly the same in paints as in powder coatings. The major types are UV absorbers and hindered-amine light stabilizers (HALS). The majority of these materials are crystalline solids and are added to the powder premix prior to extrusion. As in paints, UV absorbers are primarily used in clear coatings and HALS in pigmented systems. The mechanism of these additives are well-known and described in detail in recent publications (13). In general, the exterior durability of powder coatings is equal or superior to that of conventional coatings of similar chemistry.
As with liquid coatings, the corrosion resistance of powder coatings is, to a significant extent, a function of the chemical composition, molecular weight, cross-link density and other characteristics of the cured binder film. However, while most conventional coatings are air-dry or ambient-cure, powder coatings must always be baked at an elevated temperature to cause the particles to melt, flow and form a continuous film. During this process, the resinous vehicle reaches a low melt viscosity and thoroughly wets the substrate before being cross-linked to form the final coating. Therefore, coatings based on powders generally have better corrosion resistance than liquid paints, for the same reasons conventional baking coatings are superior to air-dry coatings. In addition, powder coatings are usually thicker, typically 2 to 3 mils vs. 1 to 2 mils for liquid coatings, and have superior coverage over sharp edges and projections, both of which contribute to superior corrosion resistance.
Powder coatings function as barrier coatings so that traditional anticorrosion pigments or additives based on chromates, molybdates or other ionic species are generally ineffective in powder coatings. The cured film lacks the water solubility necessary for these materials to function. However, inorganic fillers having good chemical resistance, improve the general chemical resistance of powder coatings by reducing moisture vapor transmission and ionic mobility.
One functional additive that is effective in both liquid paints and powders is zinc dust, which improves corrosion resistance by acting as a sacrificial anode. Powders containing over 75% zinc have been developed (14) and are commercially available from several suppliers. Zinc-rich powders produce coatings that can withstand greater than 4,000 hours scored salt-spray exposure.
A few other additives are useful in improving the chemical and corrosion resistance of powder coatings. Silicone additives migrate to the surface of the coating and resist wetting by corrosive solutions. Teflon powder acts in a similar fashion as do post-blend additives with hydrophobic surfaces. In the final analysis, however, corrosion resistance is primarily a function of the binder and cross-link density of the film.
Charge-control and antistatic additives are used to decrease the surface resistivity of the powder and the applied powder coating. While the additives themselves and the results produced are similar, their purpose is different. Charge-control agents improve the transfer efficiency and ability of the powder to penetrate Faraday cage areas, whereas antistatic agents improve the ability of the coating to conduct extraneous electrical charges to ground.
In a corona-charging system, powder particles are charged by ionic attachment while passing through the ionized air. Charging efficiency is very poor because only a small fraction (around 0.5%) of the ions produced by the corona contributes to the charge on the powder. Most exist as free ions and travel to the workplace where they accumulate in the deposited powder layer (15). Powder particles charge more efficiently if their resistivity is lower. Addition of an antistatic agent to the powder reduces the powder resistivity, resulting in improved charging efficiency. Spraying time can be reduced or the charging-gun voltage decreased. At the same voltage, the powder layer builds more rapidly; at the lower gun voltage, improved penetration in Faraday cage areas results.
A variety of materials are effective in reducing the resistivity of the powder and the final coating. Many are ionic in nature and may result in a coating with reduced water or corrosion resistance. Many of the antistatic agents are quaternary ammonium salts (cationic) or alkyl sulfonates (anionic) based on fatty-acid derivatives. Many are waxy solids, liquids or aqueous solutions, so they must be thoroughly mixed and dispersed in the binder. For the most part, antistatics are added prior to extrusion, although it is possible to coat them on the surface of powder particles by a post-blending operation. Many antistatic agents are also hygroscopic and function by absorbing a layer of water vapor on the surface. Care should be taken to be sure they are well protected from exposure to moisture during storage. Typical use levels range from about 0.5 to 3%, with 1% being a good starting level. The addition of 1 to 2% of most antistatic agents will lower the resistivity of a powder from a level of 1014 to 1015 ohmmeters to 109 to 1012 ohmmeters measured at 20% relative humidity.
For electrostatic charging in a corona gun, it is not desirable to reduce the resistivity much below about 1012 ohmmeters. If the powder is too conductive, it loses its charge too quickly and will not adhere to the substrate. If a more conductive coating is desired, metallic or other conductive pigments must be added.
Some antistatic agents, especially the cationic types, are catalysts for epoxy- containing powders. Also, some have a tendency to yellow when baked. These, as well as any other coating properties that might be affected by ionic/conductive materials, should be thoroughly evaluated when using antistatic agents.
A number of suppliers including Byk-Chemie (ES-80), PPG (Larostat), Henkel (Texaquart 900) and Synthron (Prote-Stat 10) make products suitable for powder coatings. Barium titanate is also used to promote powder-charging characteristics. It has an extremely high dielectric constant and is permanently polarized in a high-voltage field. Small amounts (around 1% on binder) are effective.
Powders designed for tribo-charging usually require additives to achieve a satisfactory degree of tribochargeability. In a tribocharging apparatus, the charge is developed by contacting or rubbing the powder particles against another material, most often PTFE. The powder particles become charged positively, and air ionization is minimized.
Nitrogen-containing compounds are known to improve this characteristic
(۱۶), which is why some organic pigments are effective in this regard. However, nitrogen-containing compounds accelerate or affect the curing rate of many powders coating systems. To reduce this problem, sterically hindered tertiary amines or aminoalcohols are effective, while still providing a marked improvement in tribochargeability (17).
In powder coatings, as in liquid paints, specular gloss is a function of microscopic and macroscopic surface smoothness (19). Gloss in powder coatings can be controlled by both physical and chemical means. Chemical methods usually entail the formation of molecular domains within the coating. The mechanism usually involves binder ingredients with wide differences in reactivity, such as a mixture of two powders (20), two resins with a single curative (21) or one resin with a dual-functional curative (22). Gloss can also be controlled to a certain extent by the use of large-particle-size inorganic fillers, which protrude through the coating, causing a microscopically rough surface.
Two types of additives are primarily effective in gloss control: incompatible ingredients and nonmelting additives. Polypropylene powder is effective in producing a satin finish in the range of 30 to 70 on a 60-degree glossmeter (23). Various types of waxes are also used, often in combination. Because they are incompatible with the binder, they accumulate at the surface of the coating reducing the gloss. They are seldom used at levels greater than 2 to 3%. Higher levels tend to produce a grayness or haze, especially in darker colors. The surface also has a waxy feel and is prone to finger print-ing. Recoatability can also be a problem. For these reasons, waxes are seldom used alone, but in combination with other ingredients.
The best example of a nonmelting additive is Teflon powder. For gloss control, a fine-particle-size powder is used. Teflon powders are often sold in mixtures with waxes for this purpose.
Texturized coatings are achieved by controlling the melt flow and particle size of the powder, the surface tension of the melted coating and the use of insoluble or incompatible additives. Textured coatings, sometimes referred to as structured coatings, are characterized by having a pleasing surface texture with uniform color and gloss. They are distinct from some of the other special finishes, such as hammer-tones, veins, wrinkles and “river textures.” These latter finishes contain several
components, such as metal flakes or several different powders blended together, whereas textured coatings are uniform in appearance with a rough surface.
In some cases, the texture is obtained by controlling the powder’s surface tension. A widely practiced method for producing a structured coating is to eliminate the flow-control additive normally used and replace it with a small amount (usually 0.05 to 0.5%) of a cellulose acetate butyrate (CAB) resin added prior to extrusion. The CAB resin has some effect in lowering the surface tension of the molten powder, but not to the extent necessary to give a smooth coating. This is a condition of “controlled cratering,” in which the coating does not flow out completely, but craters to the substrate do not form either. If higher levels of CAB are used (> 1%), the texture becomes increasingly less pronounced until eventually the film becomes smooth.
Textured coatings using the concept of controlled cratering can also be prepared where the cratering additive is post-blended into the finished powder. In this case, the base powder usually contains no flow- control additive, or at most, a very low level. The cratering additive can be almost any material that has a strong surface- tension lowering effect, such as silicones, acrylates or other materials normally used as flow-control additives. The texture can be controlled by post-blending the surface-tension-lowering additive as a master batch in a resin or absorbed on an inorganic carrier to the finished powder. The character of the texture is also greatly affected by the flow of the base powder. High-flow powders give a glossy, broad texture, while low-flow powders a finer, tighter texture. The texturizing additive can also be pigmented and ground to various particle sizes to give a wide variety of textures.
Another method to produce textured coatings utilizes powders with controlled flow and particle size. A powder is prepared with a relatively low pill flow in the range of 15 to 20 mm. If it is ground to a relatively fine powder and sieved through a 140-mesh screen (100% < 100 µm), the coating will still be relatively smooth with a fine texture. Adding a fraction of larger particles (by grinding coarser powder, around 200 µm, and screening through a 70-mesh screen), will produce a leather-ette or “pebble” texture. In most cases, the low plate flow is obtained through the use of high oil-absorption fillers such as clays or talcs.
Polymeric additives that produce low flow or are infusible in the binder are also used in the formulation of textured coatings. PTFE powders are widely used in this regard. In addition to being nonfusable, they significantly inhibit the flow of a coating. PTFE additives varying in particle size can produce different degrees of texture. Most are very fine powders (from < 10 to 15 µm) and are effective at levels of only 2 to 3%.
Other polymers that do not melt and flow completely, or are incompatible, in the binder resins during curing are also used for texturizing. These include some grades of polyethylene, polypropylene, elastomers, ionomers, nylons and other high melting engineering plastics and also incompatible thermosetting resins, primarily acrylics and silicones.
Most textured coatings have inherently lower gloss as measured on the 60-degree glossmeter. This is primarily a result of the rough surface that the glossmeter reads as lower gloss. In many cases, however, the gloss does not appear significantly reduced to the eye. Because a substantial variation in film thickness often occurs, care must be taken in formulation to use adequate pigmentation levels to produce the desired degree of hiding. It should also be noted that in order to obtain the desired degree of texture, the film thickness is usually greater than for a smooth coating, typically in the range of 75 to 100 µm (3 to 4 mils)
Most additives in powder coatings are blended with the premix and incorporated in the formulation during extrusion. However, several very fine-particle-size additives, typified by colloidal silica or alumina, are mixed with the powder after it has been extruded. When added at this stage in manufacturing, they are referred to as post-blend additives. These additives are extremely fine in ultimate particle size (around 10 to 40 nanometers) and are prepared by a special process. They are also known as pyrogenic or fumed silica or alumina to distinguish them from the more common products of the same composition. They can be added to the chips prior to grinding, or after the powder is already ground.
A problem with blending the additive with the chips before grinding is that some of it gets carried over to the bag house and the exact level in the powder is not known. However, mixing the additive with the powder after it is ground requires a separate operation, so most manufacturers mix the post-blend additive with the chips. This serves to actively distribute the additive in the powder during the grinding operation. The colloidal particles of silica or alumina form a coating on the powder particles, satisfying surface electrical charges and separating them, causing them to act as micro “ball-bearings.” The result is significant improvement in the powder free-flow and application characteristics. The effectiveness of colloidal silica in improving the fluidizing and free-flow characteristics of powders has been known for many years (24).
The addition of 0.1 to 0.5% of a fumed silica/alumina to a powder coating formula results in:
improved transport through hoses
improved resistance to blocking and impact fusion
improved charging characteristics (corona and tribo)
better spray pattern
lower angle of repose (height of cone)
reduced moisture sensitivity
improved corrosion protection (silane-treated grades).
Colloidal aluminum oxide is an effective post-blend additive to improve the tribo-charging characteristics of most powders (25). High levels of post-blended untreated fumed silica (usually 0.3 to 0.7%) act as a thixotrope in thermosetting resin systems. This results in improved edge coverage, desirable in many electric insulation and functional applications but usually less desirable in decorative applications because of reduced flow.
A number of silica grades are treated with organofunctional silanes to make them more hydrophobic. The hydrophobic grades confer a degree of moisture resistance to the powder, reducing the tendency of the powder to absorb atmospheric moisture on storage. They also improve corrosion resistance by presenting a hydrophobic surface to the environment. The hydrophobic surface also serves to improve solvent resistance and reduce staining
giving an improvement in graffiti resistance. For the same reason, recoatability will be adversely affected. The silane-treated grades do not show the same thixotropic properties as the untreated grades, so larger amounts (from 0.5 to 1.0%) can be added without adversely affecting smoothness and appearance. Several hydrophobic grades specifically developed for powder coatings recently have been introduced. (26)
Mar and Slip Additives
Mar and slip additives are only partially compatible, incompatible or insoluble in the binder system. Often, they migrate or “bloom” to the surface of the coating, forming a continuous or discontinuous layer. The most common types used in powder coatings are waxes, silicones and fluoropolymer powders or combinations. The primary function of mar-resistant additives is to help maintain the original appearance of the coating by reducing physical damage. In practical terms, a scratch is the first visible sign of damage. Scratch resistance is a quality that cannot be defined exactly in the physical sense. It is dependent on wear and tear (abrasion), because scratches are localized, whereas abrasion is a global phenomenon. Likewise, scratch resistance is independent of hardness. The fact that an additive migrating to the coating’s surface can increase the scratch resistance indicates scratch resistance is not a function of hardness.
Mar resistance not only implies scratch resistance but also resistance to contamination such as metal marking. The rubbing of a coating surface with a coin exerts considerable pressure over a small contact area, resulting in abrasion marks. This is known as metal marking. Subjective tests for mar resistance include fingernail rubbing (where differences between nail hardness can be extreme), as well as metal marking. The pencil hardness test is a good mar-resistance test because the amount of graphite deposited on the surface provides an indication of relative resistance to contamination by rubbing, even if the surface is not deformed.
Slip is the relative movement between two bodies in contact. If an object is moved along a surface, a resistance occurs acting in the direction opposite the movement, the frictional force. Apart from the surface hardness of the coating, which is formulation dependent, the coefficient of friction between the coating and the abrasive object is the determining factor in predicting scratch resistance.
The oldest and most widely used mar and slip additives in coatings are poly(dimethylsiloxane) fluids. However, these are incompatible with the binder system and can cause problems with pinholes and cratering if they are not adequately dispersed. Fluoropolymer additives are also used to impart mar and slip resistance. Because of their high molecular weight and nonmelting characteristics, fluoropolymer additives remain as a discreet particle in the film, acting more like an inert filler. In most cases, the preferred fluoropolymer is polytetrafluoroethylene (PTFE). Combinations of polyethylene waxes and PTFE are also frequently used because the combination is often more effective than either one alone. Because mar and slip additives significantly alter the surface characteristics of the coating, the formulator needs to take this into consideration during development of a new powder coating. Adhesion of touch-up paints, decals or recoating with the same powder will be adversely affected in many cases and needs to be evaluated if these operations are likely to occur.
On the other hand, the difficulty of recoatability makes these additives good candidates for antigraffiti coatings when easy removal of a second coating is desired. These additives can also be used to improve the stain resistance and chemical resistance of coatings, because the surface is more difficult to wet.
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