- Email: [email protected]
Powder coating of hot dipped galvanized steel
F O C US groups with reactive moieties such as methacrylates. This creates a structure with functional groups at the periphery of the molecules. Several series of different resins have been produced by varying the size, shape, type of dendritic core and the length of the crystalline graft. Also a series of hyperbranched resins without crystalline segments have been synthesized and studied for comparative reasons, by attaching the acrylate groups directly onto the hyperbranched polymer. One obstacle which may occur during synthesis is the formation of strong hydrogen bonds in the hyperbranched polyester. The numerous hydroxyl end groups in combination with hydrogen bond acceptors such as ester carbonyls produce a network of hydrogen bonds with time that renders the polyester difficult to dissolve in most solvents and makes the synthesis/grafting impossible. This aging process is a thermoreversible process and not due to any chemical reaction. The hydrogen bond network is fully degraded if the polyesters are heated above 130°C. Therefore, if the hyperbranched polymer is going to be used in resin synthesis performed in solution, it is better to melt the polymer before dissolution. Experimental details of materials and equipment including test procedures are included in the article. Semi-crystalline resins were photopolymerized using Irgacure 184 by melting the resins at 70°C and irradiating with UV. The main factor affecting the viscosity level of amorphous hyperbranched polymers has been shown to be the polarity of the end groups and high polarity increases the viscosity. Semicrystalline hyperbranched resins show a different rheological behaviour to the amorphous structures. These resins are readily solidified upon cooling without sintering. Control of the grinding process needs careful AUGUST 2002
O N
POWDER
monitoring to avoid heat build up to the melting point. The behaviour on heating is characterized by a high modulus up to the crystalline melting point where a rapid drop in viscosity is obtained in a narrow temperature range. This behaviour gives good storage stability and a low enough melt viscosity to give film formation and levelling. Viscosity of the molten polymer depends on several factors such as core structure, and size, while the dominant factor is graft length in similar manner to the behaviour of star branched polymers. Comparing the viscosity as a function of molecular weight it is shown that a significantly higher molecular weight can be obtained for the star branched polymers compared to the linear types, while retaining a low viscosity. When the same data is presented as a function of the molecular weight of the arms, the critical parameter is not the total molecular weight but the molecular weight of the graft chain. Increasing the number of arms does not greatly affect the viscosity, though the core structure does have a minor affect upon viscosity. The introduction of crystalline grafts changes the behaviour of the resins to depend more on the graft structure instead of the dendritic core. Crosslinked films of the semi-crystalline resins exhibited little or no residual unsaturation in the cured films. Article by M Johansson, E Malmstrom, A Hult and H Claesson entitled “Hyperbranched Polyesters“ published in the European Coatings Journal, Jul 2002, (7-8) 26,28,31-33
Powder coating of hot dipped galvanized steel Powder coating of hot dipped galvanized steel (hdgs) is becoming a popular corrosion resistant combination for use in an increasing range of exterior applications. Pretreatment and surface preparation are most important particularly when the
C O AT I N G S powder coated object is subjected to extremely corrosive environments, such as direct marine influences. Careful attention to surface preparation is also necessary when a two coat application of powder primer and powder finish is used over hdgs. The use of a duplex system of hdgs and powder coating is the subject of a paper by Dr Meyer of Akzo Nobel GmbH. While hot dipped galvanizing alone gives cathodic protection to steel in most environments there are other factors to explain the increasing use of the duplex system. Colour and appearance of the substrate is important for many end use applications while the need to give a much longer service life, and avoidance of zinc emissions to the environment are also increasingly important factors. There are advantages and disadvantages for the duplex system: advantages include the capability to carry out pretreatment and coating in the factory rather than the building site; it is a quick and reliable process; and gives excellent protection against corrosion. Disadvantages are: the limited size of workpieces; curing in ovens is a necessity; and damage in transit requires repair with liquid coating systems. The measurement of corrosion protection of the duplex system is standardized in EN ISO 12944-1 by the loss of mass after one year of exposure, and the durability is expressed in three ranges: low (25 years); medium (5-15 years) and high (>15 years). In calculating the service time of the duplex system the service time of the zinc layer is usually added. The paper contains two tables showing examples of corrosion resistance in different environments. Low and medium resistance Is usually provided by one layer of powder coating on iron phosphated hdgs. On yellow chromate, and to some extent zinc phosphated and passivated hdgs, one coat of powder will satisfy high resistance 3
F O C U S in industrial areas with moderate pollution levels; while very high industrial and marine resistance requires a two coat powder system on hdgs. It is stressed that the paper does not consider problems related to continuous hdgs, electro galvanizing, spray galvanizing and sherardizing. Components which are galvanized in one piece and then powder coated are those dealt with in this paper. Problems can arise because zinc alloy surfaces are highly reactive and may become oxidized. The target is, therefore, to get good adhesion despite oxidation. These zinc layers may also be porous, especially when they are >100 microns thick, and may outgas during the curing of the powder coating. Galvanizing in accordance with EN ISO1461 involves cleaning the steel substrate with alkali, rinse, hydrochloric acid, rinse, followed by fluxing with zinc ammonium chloride, drying and then dipping into the molten zinc, where it reacts with the zinc to form different layers of Zn-Fe-alloys, according to the conditions and chemistry of the melt, different layers are formed. If galvanizing is carried out at around 550°C, a grey non-porous alloy layer of 50100 microns is obtained with all steel chemistries and, if properly prepared, is an excellent substrate for powder coatings. Avoidance of white rust during transportation and storage is important and the components must be transported and stored in dry conditions, ventilated and protected until pretreatment and coating is carried out as soon as possible. Pretreatment of hdgs before powder coating usually involves removal of surface contaminants and oils/greases by wet cleaning followed by a conversion coating. If surface contamination is not too high then sweep blast cleaning would be adequate. For ideal powder coating of hdgs the zinc alloy should be as thin and non 4
O N
POWDER
porous as possible. The hdgs should be cooled in air rather than water. White rust corrosion should be avoided, and high temperature galvanizing at 550°C followed by galvannealing at 550-650°C is the preferred preparation. Pretreatments involve sweeping, possibly chemical treatment with proper drying, and preheating at or above the curing temperature may damage the conversion layers. Powder coatings are specially formulated products with special additives included by the coater to the powder in the fluidized bed. Generally two layers of powder are applied, a primer and a finishing coat. Film thickness needs to be carefully controlled. The practical results in the field of powder coated chromated hdgs with one or two powder layers at the Dutch sea shore are showing no loss of adhesion or corrosion after up to 17 years of service. Powder coated hdgs are mainly used in the building industry with work pieces such as fencing, railings, but heavy parts weighing more than one tonne have also been coated. The best known example of powder coated hdgs is the powder coating of roof racks at the famous “Stade de France” They are also applied to roof racks, bridge parts, platform roofs, pillars, lamp posts, canopies, sound proof barriers, profiles, gates, railings, fencing, facade components and many other components. Paper entitled “Powder Coating of Hot Dipped Galvanized Steel” by Dr Bernd Meyer, of Akzo Nobel GmbH, presented at PCE 2002 in Nuremberg on 15-17 Jan 2002. Bound copies of the conference papers available from the organizers, Vincentz Verlag, Schiffgraben 43, D 30175 Hannover, Germany
In-mould coatings The painting of sheet moulding compound (SMC) for decorative and aesthetic purposes is usually carried out with liquid coatings. The wet paint is applied first as a primer, often followed by a basecoat and clearcoat. Sanding and filling steps are usually
C O AT I N G S necessary. Current outlets for SMC are the sanitary, domestic appliance, automotive and general industrial markets. The market is still growing at an anticipated rate of 10% annually. The problems encountered in wet paint coating of SMC are due to air pockets trapped inside the component which causes popping during the heat curing cycle and high reject rates. In mould coating using wet paint is problematic due to the presence of solvent during the curing process. After mould coating using wet paint is the normal coating method. The part is first primed and sanded, followed by two to three layers of wet paint. Popping problems may result in repeated filling and sanding up to seven times. Powder coating is seldom used for after mould coating although some projects are under way to develop a correct procedure for after mould powder coating. Impress is the name given by Ferro to its in-mould powder coating process and it has been successfully applied for more than 15 years to items such as kitchen sinks. The powder coating system is applied to the interior of the heated mould, at around 150°C and is pressed together with the SMC. Curing takes about 3 to 5 minutes and a perfect one layer coating is achieved on 95% of production. A popular application in the US is the coating of floor panels using a pressing temperature of 105°C. The major use, however, is as a primer coating, allowing the OEM to apply the body colour. In future, after mould coating will be used more widely in small paint shops but solvent free systems such as waterborne or low temperature curing acrylic powder coatings, will be favoured. Developments in in-mould powder coating will be directed to lower curing temperatures of around 80°C which will make the system suitable for a wider range of applications, and materials other AUGUST 2002
O N
POWDER
monitoring to avoid heat build up to the melting point. The behaviour on heating is characterized by a high modulus up to the crystalline melting point where a rapid drop in viscosity is obtained in a narrow temperature range. This behaviour gives good storage stability and a low enough melt viscosity to give film formation and levelling. Viscosity of the molten polymer depends on several factors such as core structure, and size, while the dominant factor is graft length in similar manner to the behaviour of star branched polymers. Comparing the viscosity as a function of molecular weight it is shown that a significantly higher molecular weight can be obtained for the star branched polymers compared to the linear types, while retaining a low viscosity. When the same data is presented as a function of the molecular weight of the arms, the critical parameter is not the total molecular weight but the molecular weight of the graft chain. Increasing the number of arms does not greatly affect the viscosity, though the core structure does have a minor affect upon viscosity. The introduction of crystalline grafts changes the behaviour of the resins to depend more on the graft structure instead of the dendritic core. Crosslinked films of the semi-crystalline resins exhibited little or no residual unsaturation in the cured films. Article by M Johansson, E Malmstrom, A Hult and H Claesson entitled “Hyperbranched Polyesters“ published in the European Coatings Journal, Jul 2002, (7-8) 26,28,31-33
Powder coating of hot dipped galvanized steel Powder coating of hot dipped galvanized steel (hdgs) is becoming a popular corrosion resistant combination for use in an increasing range of exterior applications. Pretreatment and surface preparation are most important particularly when the
C O AT I N G S powder coated object is subjected to extremely corrosive environments, such as direct marine influences. Careful attention to surface preparation is also necessary when a two coat application of powder primer and powder finish is used over hdgs. The use of a duplex system of hdgs and powder coating is the subject of a paper by Dr Meyer of Akzo Nobel GmbH. While hot dipped galvanizing alone gives cathodic protection to steel in most environments there are other factors to explain the increasing use of the duplex system. Colour and appearance of the substrate is important for many end use applications while the need to give a much longer service life, and avoidance of zinc emissions to the environment are also increasingly important factors. There are advantages and disadvantages for the duplex system: advantages include the capability to carry out pretreatment and coating in the factory rather than the building site; it is a quick and reliable process; and gives excellent protection against corrosion. Disadvantages are: the limited size of workpieces; curing in ovens is a necessity; and damage in transit requires repair with liquid coating systems. The measurement of corrosion protection of the duplex system is standardized in EN ISO 12944-1 by the loss of mass after one year of exposure, and the durability is expressed in three ranges: low (25 years); medium (5-15 years) and high (>15 years). In calculating the service time of the duplex system the service time of the zinc layer is usually added. The paper contains two tables showing examples of corrosion resistance in different environments. Low and medium resistance Is usually provided by one layer of powder coating on iron phosphated hdgs. On yellow chromate, and to some extent zinc phosphated and passivated hdgs, one coat of powder will satisfy high resistance 3
F O C U S in industrial areas with moderate pollution levels; while very high industrial and marine resistance requires a two coat powder system on hdgs. It is stressed that the paper does not consider problems related to continuous hdgs, electro galvanizing, spray galvanizing and sherardizing. Components which are galvanized in one piece and then powder coated are those dealt with in this paper. Problems can arise because zinc alloy surfaces are highly reactive and may become oxidized. The target is, therefore, to get good adhesion despite oxidation. These zinc layers may also be porous, especially when they are >100 microns thick, and may outgas during the curing of the powder coating. Galvanizing in accordance with EN ISO1461 involves cleaning the steel substrate with alkali, rinse, hydrochloric acid, rinse, followed by fluxing with zinc ammonium chloride, drying and then dipping into the molten zinc, where it reacts with the zinc to form different layers of Zn-Fe-alloys, according to the conditions and chemistry of the melt, different layers are formed. If galvanizing is carried out at around 550°C, a grey non-porous alloy layer of 50100 microns is obtained with all steel chemistries and, if properly prepared, is an excellent substrate for powder coatings. Avoidance of white rust during transportation and storage is important and the components must be transported and stored in dry conditions, ventilated and protected until pretreatment and coating is carried out as soon as possible. Pretreatment of hdgs before powder coating usually involves removal of surface contaminants and oils/greases by wet cleaning followed by a conversion coating. If surface contamination is not too high then sweep blast cleaning would be adequate. For ideal powder coating of hdgs the zinc alloy should be as thin and non 4
O N
POWDER
porous as possible. The hdgs should be cooled in air rather than water. White rust corrosion should be avoided, and high temperature galvanizing at 550°C followed by galvannealing at 550-650°C is the preferred preparation. Pretreatments involve sweeping, possibly chemical treatment with proper drying, and preheating at or above the curing temperature may damage the conversion layers. Powder coatings are specially formulated products with special additives included by the coater to the powder in the fluidized bed. Generally two layers of powder are applied, a primer and a finishing coat. Film thickness needs to be carefully controlled. The practical results in the field of powder coated chromated hdgs with one or two powder layers at the Dutch sea shore are showing no loss of adhesion or corrosion after up to 17 years of service. Powder coated hdgs are mainly used in the building industry with work pieces such as fencing, railings, but heavy parts weighing more than one tonne have also been coated. The best known example of powder coated hdgs is the powder coating of roof racks at the famous “Stade de France” They are also applied to roof racks, bridge parts, platform roofs, pillars, lamp posts, canopies, sound proof barriers, profiles, gates, railings, fencing, facade components and many other components. Paper entitled “Powder Coating of Hot Dipped Galvanized Steel” by Dr Bernd Meyer, of Akzo Nobel GmbH, presented at PCE 2002 in Nuremberg on 15-17 Jan 2002. Bound copies of the conference papers available from the organizers, Vincentz Verlag, Schiffgraben 43, D 30175 Hannover, Germany
In-mould coatings The painting of sheet moulding compound (SMC) for decorative and aesthetic purposes is usually carried out with liquid coatings. The wet paint is applied first as a primer, often followed by a basecoat and clearcoat. Sanding and filling steps are usually
C O AT I N G S necessary. Current outlets for SMC are the sanitary, domestic appliance, automotive and general industrial markets. The market is still growing at an anticipated rate of 10% annually. The problems encountered in wet paint coating of SMC are due to air pockets trapped inside the component which causes popping during the heat curing cycle and high reject rates. In mould coating using wet paint is problematic due to the presence of solvent during the curing process. After mould coating using wet paint is the normal coating method. The part is first primed and sanded, followed by two to three layers of wet paint. Popping problems may result in repeated filling and sanding up to seven times. Powder coating is seldom used for after mould coating although some projects are under way to develop a correct procedure for after mould powder coating. Impress is the name given by Ferro to its in-mould powder coating process and it has been successfully applied for more than 15 years to items such as kitchen sinks. The powder coating system is applied to the interior of the heated mould, at around 150°C and is pressed together with the SMC. Curing takes about 3 to 5 minutes and a perfect one layer coating is achieved on 95% of production. A popular application in the US is the coating of floor panels using a pressing temperature of 105°C. The major use, however, is as a primer coating, allowing the OEM to apply the body colour. In future, after mould coating will be used more widely in small paint shops but solvent free systems such as waterborne or low temperature curing acrylic powder coatings, will be favoured. Developments in in-mould powder coating will be directed to lower curing temperatures of around 80°C which will make the system suitable for a wider range of applications, and materials other AUGUST 2002