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The role of additives in powder coatings
F O C U S curatives need to split off the blocking agents at elevated temperatures to release the highly reactive isocyanates, whereas the internally blocked (uretdiones) do not release volatile blocking agents. While the use of uretdiones was obviously the favoured option, for many years it was found to be impossible to reduce the curing temperature because no known catalyst was effective at temperatures below 180°C. This was a limiting factor in the wider acceptance of polyurethane powder coatings since the trend with other powder systems was to steadily reduce curing temperatures or stoving times and, additionally, new market outlets for the coating of wooden substrates and aluminium alloys required much lower curing schedules. The breakthrough occurred with the discovery of a new catalyst, which activated the uretdione in a novel curing sequence to make possible curing temperatures as low as 120°C. The catalyst, tetra-alkyl ammonium carboxylate (TAAC) overcomes the problem of kinetic inhibition of the cleavage reaction by catalyzing a reaction exotherm in the range of 140°C, which is only possible when all acidic residues have been removed from the hydroxylated polyester binder. A small addition of an epoxy functional compound serves as a suitable acid scavenger, and this reaction is also catalyzed by TAAC. The curing mechanism differs from the normal uretdione conversion of the hydroxylated polyester due to the partial formation of an allophanate structure, and the amount of allophanate conversion is related to the NCO:OH ratio of the system. The multiple reactions occurring within this powder system require a careful balance of the stoichiometric ratios since they have a profound influence on the final coating performance. While the early work was directed to the primary reaction 2
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involving the uretdione, the importance of the polyester resin should not be neglected. In low temperature curing powder systems, it is often necessary to use a polyester resin with a low glass transition temperature (Tg) that gives a lower melt viscosity to improve flow during the powder melting stage. However, powder particles with a lower Tg have a greater tendency to agglomerate during transport and storage of the powder coating. During this initial investigation it was apparent that certain polyesters negatively affected the reactive storage stability of the system, and this effect was often related to a low acid value that tended to inhibit the reaction and improve long term storage stability. It has also been shown that certain monomer compositions in the polyester resin could positively affect the storage stability of these, low temperature, curing systems. The paper then illustrates the beneficial effects of formulation modifications and typical starting formulations are given, together with details of comprehensive testing schedule applied to the cured films. Paper entitled “Empowering Powder – Polyurethanes for Low Temperature Cure Powder Coatings” by Corey King and associates from the Evonik Degussa, US and German divisions, presented at the recent Coatings 08 Congress held in Charlotte, NC, 2-4 Jun 2008. Copies of the congress papers in CD format are available from the organizers, Vincentz Network. Website: http://www.coatings.de
The role of additives in powder coatings A recent issue of the PCI journal was predominantly devoted to the 7th Annual Additives Guide and it provided a reminder of the importance of additives in optimizing the application and performance qualities of powder coatings. The complete absence of volatile components in powder coatings does reduce their dependence on additives in comparison to the much wider
C OAT I N G S range of additives required in liquid coatings. However, the different physical characteristics of powders provided new challenges to the additives industry, and the success of their efforts has been a positive factor in the rapid growth of the powder coatings market. The latest guide is, therefore, a good opportunity to review the current role of additives in powder coatings. Additives for powder coatings should ideally be: a fine powder with a Tg above 50°C; specific in its function and 100% active; must not combine with binder or curing agent; and be effective at low levels of addition. Unfortunately many useful additives are liquids and are difficult to disperse unless they are added as a masterbatch in the binder resin or absorbed on a solid with a high surface area/mass ratio, ie fumed silica’s. The one additive that finds a place in all powder formulations is the flow control additive. Its function is to prevent cratering in the molten film and improve surface appearance. Film formation and flow of thermosetting powders are controlled by two main parameters: melt viscosity and a high surface tension, which must however be lower than the surface tension of the substrate to facilitate adequate wetting of the substrate. Additionally, the surface tension at the air interface must be homogeneous. Polymeric acrylates have proved to be useful flow additives but their presence in the film does cause a noticeable haze even at the usual dosage of around 1% on total charge. In clear coatings it is advisable to use a modified polysiloxane. In functional powder coatings a polyvinylbutyral resin with relatively high melt viscosity is the preferred flow additive. Another favoured option in conventional powder coatings is cellulose acetobutyrate (CAB). Degassing agents serve as solid solvents to keep the molten SEPTEMBER 2008
F O C U S film open long enough to allow air bubbles to escape from the film before the commencement of curing. Benzoin is the favourite degassing agent although it is costly and prone to yellowing due to the gradual conversion of benzoin to the highly coloured benzil. A number of alternatives are available, but my own personal preference, which is not included in these options, is a low cost ketone resin with melting point in the range 80-100°C. Exterior durable powder coatings often contain UV light absorbers (UVA) for clear powder coatings and sterically hindered amine light stabilizers (HALS) for pigmented systems to protect the film against the destructive influence of direct sunlight. Some formulators use both UVA and HALS as a synergistic combination to give extra protection against loss of gloss, cracking delamination and colour change. Antioxidants are added for protection against yellowing in overbake conditions, and these are usually combinations of a sterically hindered antioxidant and an organophosphite. Resin chemists often include these items during manufacture of the polyester binder. I am aware that many powder coating formulators incorporate pigment dispersing aids into highly pigmented formulations, but I have never found these additions to give appreciable benefits. Modern organic pigments have been engineered by the suppliers to cope with the poor dispersive quality of hot melt blending processes. Antistatic and charge control additives are essential modifiers for the reduction of electrical surface resistivity. Although the results are similar the action of the two types is different. An antistatic agent improves the ability of the applied coating to conduct charge to ground, while the charge control additive improves the ability of the powder to penetrate Faraday cage areas SEPTEMBER 2008
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and, therefore, the transfer efficiency. Antistats are commonly cationic quaternary ammonium salts or anionic alkyl sulfonates. They are usually incorporated prior to extrusion but can be post blended. Tribo charging agents are generally nitrogen containing chemicals and HALS are preferable since they do not accelerate the cure of epoxy hybrid powder coatings. The problem is most prevalent with polyester based powder systems. Anticaking additives such as the fumed silicas, or colloidal alumina may be added to the flaked extrudate before grinding or after the powder has been ground. Addition levels of about 0.1% are found to be effective with some of the new generation of fumed silicas. Mar resistance and slip additives are the one area where significant changes have occurred during the past year. The original purpose of these additives was to improve lubricity (slip), scratch and abrasion resistance, and anti blocking properties, which was achieved by putting lubricants into the film surface so that abrasive forces would simply glide over the surface without causing mechanical damage. Waxes and silicones were widely specified for this purpose. Now the emphasis has changed to the use of additives that actually make the film surface much harder and less prone to damage and penetration by graffiti. The recommended materials are nano-structured inorganics such as silicates and aluminates. The main problem is how to incorporate these ultra fine materials uniformly by powder coating hot melt compounding, which is known to be a notoriously poor method of breaking down agglomerates. Unless they can be added as dispersions during the manufacture of the binder resin it is difficult to see how they can be of benefit to powder coatings formulators. Until this problem is resolved, mar resistance is achieved by the addition of finely
C OAT I N G S divided silica and alumina, although nepheline syenite is a less costly alternative. The one area in which powder additives excel is the formation of textured or structured effect finishes. Powder coatings are the ideal medium for designing uniform textured effects, which give excellent uniformity of finish on both vertical and horizontal surfaces. Uniform hammertone finishes are easily formed with powder compositions containing metallic flake or mica pigments by simply leaving out the flow additive and adding a little CAB to restrict the cratering effect from exposing the metal substrate. Alternatively, the same effect can be produced by adding flow agent absorbed on silica as a post additive without the use of CAB. There are many ways of producing effect finishes in powder coatings and the latest option is the use of liquid crystal flakelets to produce the image of shifting colour patterns. Matting additives are also recommended for powder coatings but the most reproducible matting effects are best achieved by blending incompatible binder resins or combining curing agents with differing reaction rates. Paint & Coatings Industry (PCI), Jun 2008, 24 (7th Annual Additions Guide), 38-123
INDUSTRY NEWS Rohm and Haas to reduce US activities Rohm and Haas Inc is launching a major restructure which will initially involve the loss of 925 jobs (almost 6% of world workforce). Most of the job cuts will be in N America. A pie chart gives Rohm and Haas workforce by geographical area. Rohm and Haas is to cut emulsion production capacity by 30%. It will 3
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involving the uretdione, the importance of the polyester resin should not be neglected. In low temperature curing powder systems, it is often necessary to use a polyester resin with a low glass transition temperature (Tg) that gives a lower melt viscosity to improve flow during the powder melting stage. However, powder particles with a lower Tg have a greater tendency to agglomerate during transport and storage of the powder coating. During this initial investigation it was apparent that certain polyesters negatively affected the reactive storage stability of the system, and this effect was often related to a low acid value that tended to inhibit the reaction and improve long term storage stability. It has also been shown that certain monomer compositions in the polyester resin could positively affect the storage stability of these, low temperature, curing systems. The paper then illustrates the beneficial effects of formulation modifications and typical starting formulations are given, together with details of comprehensive testing schedule applied to the cured films. Paper entitled “Empowering Powder – Polyurethanes for Low Temperature Cure Powder Coatings” by Corey King and associates from the Evonik Degussa, US and German divisions, presented at the recent Coatings 08 Congress held in Charlotte, NC, 2-4 Jun 2008. Copies of the congress papers in CD format are available from the organizers, Vincentz Network. Website: http://www.coatings.de
The role of additives in powder coatings A recent issue of the PCI journal was predominantly devoted to the 7th Annual Additives Guide and it provided a reminder of the importance of additives in optimizing the application and performance qualities of powder coatings. The complete absence of volatile components in powder coatings does reduce their dependence on additives in comparison to the much wider
C OAT I N G S range of additives required in liquid coatings. However, the different physical characteristics of powders provided new challenges to the additives industry, and the success of their efforts has been a positive factor in the rapid growth of the powder coatings market. The latest guide is, therefore, a good opportunity to review the current role of additives in powder coatings. Additives for powder coatings should ideally be: a fine powder with a Tg above 50°C; specific in its function and 100% active; must not combine with binder or curing agent; and be effective at low levels of addition. Unfortunately many useful additives are liquids and are difficult to disperse unless they are added as a masterbatch in the binder resin or absorbed on a solid with a high surface area/mass ratio, ie fumed silica’s. The one additive that finds a place in all powder formulations is the flow control additive. Its function is to prevent cratering in the molten film and improve surface appearance. Film formation and flow of thermosetting powders are controlled by two main parameters: melt viscosity and a high surface tension, which must however be lower than the surface tension of the substrate to facilitate adequate wetting of the substrate. Additionally, the surface tension at the air interface must be homogeneous. Polymeric acrylates have proved to be useful flow additives but their presence in the film does cause a noticeable haze even at the usual dosage of around 1% on total charge. In clear coatings it is advisable to use a modified polysiloxane. In functional powder coatings a polyvinylbutyral resin with relatively high melt viscosity is the preferred flow additive. Another favoured option in conventional powder coatings is cellulose acetobutyrate (CAB). Degassing agents serve as solid solvents to keep the molten SEPTEMBER 2008
F O C U S film open long enough to allow air bubbles to escape from the film before the commencement of curing. Benzoin is the favourite degassing agent although it is costly and prone to yellowing due to the gradual conversion of benzoin to the highly coloured benzil. A number of alternatives are available, but my own personal preference, which is not included in these options, is a low cost ketone resin with melting point in the range 80-100°C. Exterior durable powder coatings often contain UV light absorbers (UVA) for clear powder coatings and sterically hindered amine light stabilizers (HALS) for pigmented systems to protect the film against the destructive influence of direct sunlight. Some formulators use both UVA and HALS as a synergistic combination to give extra protection against loss of gloss, cracking delamination and colour change. Antioxidants are added for protection against yellowing in overbake conditions, and these are usually combinations of a sterically hindered antioxidant and an organophosphite. Resin chemists often include these items during manufacture of the polyester binder. I am aware that many powder coating formulators incorporate pigment dispersing aids into highly pigmented formulations, but I have never found these additions to give appreciable benefits. Modern organic pigments have been engineered by the suppliers to cope with the poor dispersive quality of hot melt blending processes. Antistatic and charge control additives are essential modifiers for the reduction of electrical surface resistivity. Although the results are similar the action of the two types is different. An antistatic agent improves the ability of the applied coating to conduct charge to ground, while the charge control additive improves the ability of the powder to penetrate Faraday cage areas SEPTEMBER 2008
O N
POWDER
and, therefore, the transfer efficiency. Antistats are commonly cationic quaternary ammonium salts or anionic alkyl sulfonates. They are usually incorporated prior to extrusion but can be post blended. Tribo charging agents are generally nitrogen containing chemicals and HALS are preferable since they do not accelerate the cure of epoxy hybrid powder coatings. The problem is most prevalent with polyester based powder systems. Anticaking additives such as the fumed silicas, or colloidal alumina may be added to the flaked extrudate before grinding or after the powder has been ground. Addition levels of about 0.1% are found to be effective with some of the new generation of fumed silicas. Mar resistance and slip additives are the one area where significant changes have occurred during the past year. The original purpose of these additives was to improve lubricity (slip), scratch and abrasion resistance, and anti blocking properties, which was achieved by putting lubricants into the film surface so that abrasive forces would simply glide over the surface without causing mechanical damage. Waxes and silicones were widely specified for this purpose. Now the emphasis has changed to the use of additives that actually make the film surface much harder and less prone to damage and penetration by graffiti. The recommended materials are nano-structured inorganics such as silicates and aluminates. The main problem is how to incorporate these ultra fine materials uniformly by powder coating hot melt compounding, which is known to be a notoriously poor method of breaking down agglomerates. Unless they can be added as dispersions during the manufacture of the binder resin it is difficult to see how they can be of benefit to powder coatings formulators. Until this problem is resolved, mar resistance is achieved by the addition of finely
C OAT I N G S divided silica and alumina, although nepheline syenite is a less costly alternative. The one area in which powder additives excel is the formation of textured or structured effect finishes. Powder coatings are the ideal medium for designing uniform textured effects, which give excellent uniformity of finish on both vertical and horizontal surfaces. Uniform hammertone finishes are easily formed with powder compositions containing metallic flake or mica pigments by simply leaving out the flow additive and adding a little CAB to restrict the cratering effect from exposing the metal substrate. Alternatively, the same effect can be produced by adding flow agent absorbed on silica as a post additive without the use of CAB. There are many ways of producing effect finishes in powder coatings and the latest option is the use of liquid crystal flakelets to produce the image of shifting colour patterns. Matting additives are also recommended for powder coatings but the most reproducible matting effects are best achieved by blending incompatible binder resins or combining curing agents with differing reaction rates. Paint & Coatings Industry (PCI), Jun 2008, 24 (7th Annual Additions Guide), 38-123
INDUSTRY NEWS Rohm and Haas to reduce US activities Rohm and Haas Inc is launching a major restructure which will initially involve the loss of 925 jobs (almost 6% of world workforce). Most of the job cuts will be in N America. A pie chart gives Rohm and Haas workforce by geographical area. Rohm and Haas is to cut emulsion production capacity by 30%. It will 3