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GMA acrylics in powder coatings
F O C US were assessed by scanning electron microscopy. Large differences were found in the overall distribution of the pigments on examination at low magnifications that revealed pigment rich and pigment poor areas in the films based on nonpulverized resins. These rich and poor areas were practically absent when the pulverized resin was used although the boundaries between the individual powder particles were still visible in the coating surfaces. Flocculated pigment particles were observed in the films stoved at 200 and 240°C which were more pronounced at 240°C and were absent in films stoved at 150°C. The viscosity of a filled polymer is known to be increased by pigment flocculation as the result of a pigment network structure and the occlusion of polymer material within the pigment flocculates. This network formation can be monitored by measuring the evolution of the elastic modulus G’. It is also noticed that the gloss levels of the two systems seem to be independent of the stoving temperature for both methods while the haze is increased with increasing temperature, agreeing with the observed flocculation at higher temperatures. Coatings prepared from the non-pulverized resin were lower in gloss than those of the coatings formed from pulverized resins. The effect of pigment dispersion on the leveling of the powder coating was studied by evaluating the waviness of the coatings. The results showed that the waviness was not significantly influenced by the heating temperature, and the unexpected high values of waviness at 200 and 240°C indicate that the leveling of the coatings was affected by the observed pigment flocculation at these temperatures. The above tests showed that TiO2 dispersions are not colloidally stable at 200 and JANUARY 2003
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240°C in the absence of dispersants. Further tests were, therefore, carried out to examine the influence of P2VP-b-PCL copolymer dispersants on the pigments’ degree of dispersion. Four different dispersants were evaluated. These differed in the molecular weights of the anchor and buoy blocks and the asymmetry of the block copolymers, and the maximum adsorbed amount of dispersant onto the TiO2 was calculated by the method described in the article. The first trials were based on pigments that had been contacted with dispersants in solution and dried prior to mixing and extrusion. Dispersants 1, 2, and 3 were used at three different concentrations, 0.5 wt%, 1.5 wt% and 2.5 wt% on pigment. Dispersant 4 was used at 1.5 wt% only. Similar results to the preblending experiments showed that the overall distribution of the pigments at the coating surface was inhomogeneous as the consequence of poor mixing due to the large resin flakes. This confirms the earlier findings that the block copolymer dispersants do not influence the dispersive and distributive mixing of the pigments during extrusion under these experimental conditions. The dispersions of the pigments pretreated with 0.5 wt% of block copolymer appeared to be slightly flocculated because the surface is not fully covered with dispersant at this concentration. At the higher levels the pigments did not flocculate at all. Addition of the dispersants significantly improved the gloss and reduced the haze of the coatings, and stable figures for elastic modulus indicated that formation of a pigment network is prevented, and lower levels of waviness were recorded. Additional experiments were carried out in which the blocked copolymer dispersants were first blended with the resin by extrusion, cooled, ground, and then re-extruded together with the
C O AT I N G S other components. Dispersants 1, 3 and 4 were tested at 1.5 wt% and 2.5 wt%. In these tests the dispersants have to diffuse to the pigments during extrusion through the viscous resin melt in order to adsorb onto the pigment surface. No pigment flocculation occurred in the coatings at either dispersant concentration, consequently the elastic modulus and viscosity of the dispersions remained constant in time while the leveling of the coated surfaces remained undisturbed. The tabulated results showed that the coatings had excellent gloss and extremely low haze levels at both 200 and 240°C. Absence of flocculation indicates that the block copolymers had moved to the pigments during extrusion, by diffusion and convection, and had completely covered the pigment surface. The article concludes with a brief discussion on the influence of block copolymer composition on steric stabilization of the dispersions and assesses a steric layer thickness of 2.4-5.0 nm as the minimum required to impart steric stability. Article entitled “Use of Poly(2-vinylpyridine)b-poly(ε-caprolactone) Copolymers as Pigment Stabilizers in Powder Coatings” by F L Duivenvoorde, K Jansen, J Laven, and R van der Linde of Eindhoven University of Technology, Journal of Coatings Technology, Aug 2002, 74(931), 49-57
GMA acrylics in powder coatings The global acrylic powder coating market is estimated at 10,000 tonnes, with 67% usage in North America. In Europe the estimated total is 1,200 tonnes, representing no more than 0.3% of the overall regional market for thermosetting powder coatings. In the Far East the current usage is 2,000 tonnes, mainly in Japan. The annual growth rate predicted for Europe is greater than 25% from 2002-2007, driven by automotive tier 1 topcoat. Current end use applications 3
F O C U S for acrylic powder coatings are: automotive OEM clearcoats and wheels, brass hardware, fixtures, appliances, architectural metals and heat sensitive substrates such as FRP and MDF. The advantages of acrylics, compared to polyesters, are: better reactivity giving lower temperature cure; better weathering; are easier to functionalize and control functionality; have excellent flow, clarity, colour and appearance. A paper by Eric Dumain of Reichhold Inc, presented in slide format, goes on to describe the various types of acrylic resins used in powder coatings. Powder grade acrylic resins can be cured with the same curing agents as their polyester analogues. Hydroxyl functional acrylics cure with blocked isocyanates, while carboxyl functional acrylics are cured with epoxies. GMA acrylics are epoxy functional resins that are cured with diacids or anhydrides. Hydroxylated acrylics are formulated to give acrylic urethane powder coatings with hard, but somewhat brittle films, possessing excellent UV resistance. Combining ratios with blocked isocyanates generally range from 85/15 to 80/20. End uses are general metal applications. Carboxylated acrylics are cured with bis-A epoxies to give excellent detergent and mar resistance and better UV durability than polyester hybrids. Combining ratios of 60/40 are most common. Metal office furniture and appliances are the main end uses. GMA acrylic powders are cured with diacids and polyanhydrides in combining ratios from 75/25 to 88/12. These products are characterized by excellent weathering properties and excellent flow and appearance to give Class A surfaces. They are incompatible with other powder chemistries when the acrylic resin is used as the primary film forming resin, and dedicated manufacturing is necessary. 4
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The paper then describes the uses and potential outlets for GMA acrylic powder coatings. GMA acrylics are being considered for automotive base coats but the earlier types were difficult to disperse due to the lack of residual carboxyl groups to assist pigment wetting, and new resins have been designed by Reichhold for pigmented applications. The paper provides recommended formulations for white automotive basecoats based on two different GMA type acrylics, one cured with 12% polyanhydride and the other 16% dodecanedioic acid, and each containing 25% TiO2. The first formulation gives a slightly higher gloss level and smoothness rating. Cured at 150°C for 20 minutes the films have 2H hardness, good impact, mar, and solvent resistance. Gloss retention on Florida exposure is good and the resistance is enhanced by the addition of a blend of UV absorber and HALS. One important outlet for GMA type acrylics is a clear coat for aluminium wheels where they show superior filiform corrosion resistance than polyesters. It is also observed that increasing the GMA content tends to decrease the filiform corrosion resistance although benefits include improved mar and scratch resistance, better chemical resistance and improved low bake response, especially with the addition of a tin catalyst. A range of GMA acrylic resins were evaluated for physical characteristics and the best compromise of properties was shown by a GMA acrylic having an EEW of 335. Test procedures for assessing filiform corrosion resistance are given in the paper together with the techniques for experimental design. Details are also given of the application of GMA acrylics to heat sensitive substrates such as fibreglass reinforced plastic (FRP), materials which are used
C O AT I N G S in automotive, lawn and garden applications. The problem with powder coating of FRP is the release of air from the substrate and the fact that it is a poor conductor. Adhesion and melt flow properties can also be problematic. Techniques for coating FRP include pre-heating to remove entrapped air, insertion of metal mould or flat shield to promote conductivity, and application of the acrylic powder at low voltages. A baking temperature of 120-150°C is typical. Slide presentation entitled “GMA Acrylic Resins in Powder Coatings” by Eric Dumain of Reichhold Inc, presented at Powder Coating 2002 in Indianapolis on 23-24 Sep 2002. Bound copies of the conference papers available from the organizers, the Powder Coating Institute, Alexandria, VA, USA
Electro-magnetic brush technology update The concept of the electromagnetic brush as an alternative device for applying powder coatings to flat substrates was introduced at the first Powder Coatings Europe conference in 1998. Since that time, DSM Resins has developed this process from the laboratory to the pilot plant stage and a wide range of trials have been carried out with this prototype EMB machine. The EMB process is described briefly for the benefit of readers who are not familiar with this important new development and its potential impact on new market outlets. EMB technology is an alternative powder application process that enables powder coatings to be applied at relatively high speed and in relatively thin coating layers on flat substrates. It can be compared to roller coating application applied to powder coatings rather than liquid paints. The technology is derived from electro-photographic copy and laser printing processes whereby JANUARY 2003
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240°C in the absence of dispersants. Further tests were, therefore, carried out to examine the influence of P2VP-b-PCL copolymer dispersants on the pigments’ degree of dispersion. Four different dispersants were evaluated. These differed in the molecular weights of the anchor and buoy blocks and the asymmetry of the block copolymers, and the maximum adsorbed amount of dispersant onto the TiO2 was calculated by the method described in the article. The first trials were based on pigments that had been contacted with dispersants in solution and dried prior to mixing and extrusion. Dispersants 1, 2, and 3 were used at three different concentrations, 0.5 wt%, 1.5 wt% and 2.5 wt% on pigment. Dispersant 4 was used at 1.5 wt% only. Similar results to the preblending experiments showed that the overall distribution of the pigments at the coating surface was inhomogeneous as the consequence of poor mixing due to the large resin flakes. This confirms the earlier findings that the block copolymer dispersants do not influence the dispersive and distributive mixing of the pigments during extrusion under these experimental conditions. The dispersions of the pigments pretreated with 0.5 wt% of block copolymer appeared to be slightly flocculated because the surface is not fully covered with dispersant at this concentration. At the higher levels the pigments did not flocculate at all. Addition of the dispersants significantly improved the gloss and reduced the haze of the coatings, and stable figures for elastic modulus indicated that formation of a pigment network is prevented, and lower levels of waviness were recorded. Additional experiments were carried out in which the blocked copolymer dispersants were first blended with the resin by extrusion, cooled, ground, and then re-extruded together with the
C O AT I N G S other components. Dispersants 1, 3 and 4 were tested at 1.5 wt% and 2.5 wt%. In these tests the dispersants have to diffuse to the pigments during extrusion through the viscous resin melt in order to adsorb onto the pigment surface. No pigment flocculation occurred in the coatings at either dispersant concentration, consequently the elastic modulus and viscosity of the dispersions remained constant in time while the leveling of the coated surfaces remained undisturbed. The tabulated results showed that the coatings had excellent gloss and extremely low haze levels at both 200 and 240°C. Absence of flocculation indicates that the block copolymers had moved to the pigments during extrusion, by diffusion and convection, and had completely covered the pigment surface. The article concludes with a brief discussion on the influence of block copolymer composition on steric stabilization of the dispersions and assesses a steric layer thickness of 2.4-5.0 nm as the minimum required to impart steric stability. Article entitled “Use of Poly(2-vinylpyridine)b-poly(ε-caprolactone) Copolymers as Pigment Stabilizers in Powder Coatings” by F L Duivenvoorde, K Jansen, J Laven, and R van der Linde of Eindhoven University of Technology, Journal of Coatings Technology, Aug 2002, 74(931), 49-57
GMA acrylics in powder coatings The global acrylic powder coating market is estimated at 10,000 tonnes, with 67% usage in North America. In Europe the estimated total is 1,200 tonnes, representing no more than 0.3% of the overall regional market for thermosetting powder coatings. In the Far East the current usage is 2,000 tonnes, mainly in Japan. The annual growth rate predicted for Europe is greater than 25% from 2002-2007, driven by automotive tier 1 topcoat. Current end use applications 3
F O C U S for acrylic powder coatings are: automotive OEM clearcoats and wheels, brass hardware, fixtures, appliances, architectural metals and heat sensitive substrates such as FRP and MDF. The advantages of acrylics, compared to polyesters, are: better reactivity giving lower temperature cure; better weathering; are easier to functionalize and control functionality; have excellent flow, clarity, colour and appearance. A paper by Eric Dumain of Reichhold Inc, presented in slide format, goes on to describe the various types of acrylic resins used in powder coatings. Powder grade acrylic resins can be cured with the same curing agents as their polyester analogues. Hydroxyl functional acrylics cure with blocked isocyanates, while carboxyl functional acrylics are cured with epoxies. GMA acrylics are epoxy functional resins that are cured with diacids or anhydrides. Hydroxylated acrylics are formulated to give acrylic urethane powder coatings with hard, but somewhat brittle films, possessing excellent UV resistance. Combining ratios with blocked isocyanates generally range from 85/15 to 80/20. End uses are general metal applications. Carboxylated acrylics are cured with bis-A epoxies to give excellent detergent and mar resistance and better UV durability than polyester hybrids. Combining ratios of 60/40 are most common. Metal office furniture and appliances are the main end uses. GMA acrylic powders are cured with diacids and polyanhydrides in combining ratios from 75/25 to 88/12. These products are characterized by excellent weathering properties and excellent flow and appearance to give Class A surfaces. They are incompatible with other powder chemistries when the acrylic resin is used as the primary film forming resin, and dedicated manufacturing is necessary. 4
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The paper then describes the uses and potential outlets for GMA acrylic powder coatings. GMA acrylics are being considered for automotive base coats but the earlier types were difficult to disperse due to the lack of residual carboxyl groups to assist pigment wetting, and new resins have been designed by Reichhold for pigmented applications. The paper provides recommended formulations for white automotive basecoats based on two different GMA type acrylics, one cured with 12% polyanhydride and the other 16% dodecanedioic acid, and each containing 25% TiO2. The first formulation gives a slightly higher gloss level and smoothness rating. Cured at 150°C for 20 minutes the films have 2H hardness, good impact, mar, and solvent resistance. Gloss retention on Florida exposure is good and the resistance is enhanced by the addition of a blend of UV absorber and HALS. One important outlet for GMA type acrylics is a clear coat for aluminium wheels where they show superior filiform corrosion resistance than polyesters. It is also observed that increasing the GMA content tends to decrease the filiform corrosion resistance although benefits include improved mar and scratch resistance, better chemical resistance and improved low bake response, especially with the addition of a tin catalyst. A range of GMA acrylic resins were evaluated for physical characteristics and the best compromise of properties was shown by a GMA acrylic having an EEW of 335. Test procedures for assessing filiform corrosion resistance are given in the paper together with the techniques for experimental design. Details are also given of the application of GMA acrylics to heat sensitive substrates such as fibreglass reinforced plastic (FRP), materials which are used
C O AT I N G S in automotive, lawn and garden applications. The problem with powder coating of FRP is the release of air from the substrate and the fact that it is a poor conductor. Adhesion and melt flow properties can also be problematic. Techniques for coating FRP include pre-heating to remove entrapped air, insertion of metal mould or flat shield to promote conductivity, and application of the acrylic powder at low voltages. A baking temperature of 120-150°C is typical. Slide presentation entitled “GMA Acrylic Resins in Powder Coatings” by Eric Dumain of Reichhold Inc, presented at Powder Coating 2002 in Indianapolis on 23-24 Sep 2002. Bound copies of the conference papers available from the organizers, the Powder Coating Institute, Alexandria, VA, USA
Electro-magnetic brush technology update The concept of the electromagnetic brush as an alternative device for applying powder coatings to flat substrates was introduced at the first Powder Coatings Europe conference in 1998. Since that time, DSM Resins has developed this process from the laboratory to the pilot plant stage and a wide range of trials have been carried out with this prototype EMB machine. The EMB process is described briefly for the benefit of readers who are not familiar with this important new development and its potential impact on new market outlets. EMB technology is an alternative powder application process that enables powder coatings to be applied at relatively high speed and in relatively thin coating layers on flat substrates. It can be compared to roller coating application applied to powder coatings rather than liquid paints. The technology is derived from electro-photographic copy and laser printing processes whereby JANUARY 2003