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Development of textured powder coatings
FOCUS
TECHNICAL Development of textured powder coatings There have been many options for formulators of textured or structured powder coatings and each formulator has their own special preferences. In the early days of powder formulation a textured powder was a useful product for working off extrudate residues and batches that had been rejected for bittiness and contamination. This practice still persists at some small costconscious powder producers but the present requirements for consistent reproducibility of quality and appearance allow little opportunity for the modern day powder formulator. In fact, batchto-batch reproducibility is often a problem in the production of textured powder finishes. One method that is still widely used is the incorporation of polymeric materials that are insoluble in the primary resin. These materials have molten viscosities higher than the viscosity of the coating system. When dispersed in the coating, they interfere with the levelling and produce the uneven surface that characterizes the textured finish. The main difficulty with these materials is the lack of uniform dispersion throughout the coating. Another method uses surfactant materials with a powder coating texture base in a post-blending operation and this method is claimed to give a uniform texture across the face of the substrate. Post-blending allows for wider formulating latitude and can save the formulator much time in matching a customer texture pattern. Surfactant texturing does not contaminate the whole process and is more effective than extruded blends of insoluble materials because it uses the difference between the molten surface tension of the additive and that of the powder coating texture base. During the baking 2
ON
POWDER
cycle, surfactant particles distributed throughout the curing film, melt and create areas of low surface tension. It was originally thought that molten coating material flowed away from these areas and accumulated in the space between them. The surface of the film then began to form the irregular bumps typical of a textured finish, and the significant difference in surface tension between the coating and the additive caused an unstable energy condition to exist at the points where they came into contact. It was then concluded that intermixing of the additive and base at this interface initiated surface tension equalization. Recent microscopic examination now indicates that the texture pattern begins to develop as soon as the particles melt and smooth flow begins. The smooth flow continues until the surface tension and molten viscosity of the base coating restrict further levelling. The coating surface moves closer to smoothness but never achieves complete levelling. When smooth flow ceases, texture development begins and the texture density pattern is revealed as the profile grows in vertical amplitude. As the profile becomes more clearly defined it gradually slows in growth due to a balance in interfacial tension being established between the molten surfactant and the partially cured coating film. Gelation overcomes the balanced surface tensions and the texture pattern is established permanently. It is believed that coatings with low molten viscosity or thixotropy may form craters around melted particles of surfactant on the surface of the substrate during the period of smooth flow. This is the time when the difference between the surface tension of the surfactant droplets and the coating would exert the greatest influence on each other. The viscosity is at its lowest and the effect of a low surface tension surfactant would prevent the movement of high surface tension
C OAT I N G S coating material towards it. The best way to formulate texture finishes is described in an article by Larry Waelde of Troy Chemical Corporation. In formulating a texture base the primary ingredients are chosen to match the customers performance or price requirements. The resin system is chosen for its physical and chemical properties, pigments for their aesthetic qualities, and additives to fulfil their particular function. The main difference between a smooth finish and a texture base is the absence of flow additives, since these are unnecessary as they conflict with the function of a surfactant agent. Flow agents will endeavour to create a smooth finish, while the texturing agent attempts to perform the opposite function. Most textured finishes are formulated to match the pattern of a customer standard and the conventional method of producing textured finishes can be a painstaking process for every change in the formulation must be extruded as a separate coating. Post-blending surfactant texturing allows variations in additive concentration to be examined without the need for repetitive extrusions. Additionally, certain thixotropes can be used in a similar manner to modify the flow characteristics of a texture base. This allows rapid testing of various combinations of thixotropes with different levels of surfactant texturing additive. The small particle size of most thixotropes allows them to be dry blended with a texture base without causing a gritty surface appearance. Although these are normally added in the extrusion stage, they can be assessed quickly by this method without the need to produce a new sample coating. The screen test is divided into two main sections: surfactants texture additive ladder; and a thixotrope and additive matrix. The reason for performing two separate evaluations is that some NOVEMBER 2006
FOCUS texture bases do not require a modification of flow characteristics to meet the customer requirement. If the requirements have been satisfied in the first section, the secondary evaluation becomes unnecessary and minimizes the time taken to formulate. A formulation is given in the article for a black texture base. In the first section a portion of the texture base is divided into three equal amounts. Each portion is then blended with increasing amounts of surfactant, eg: 0.5, 1.0 and 2.0% and then blended under low shear conditions using a tumble blender or a roller mixer. Spray and bake each sample on the chosen substrate maintaining identical application conditions. These include: film thickness; gun settings; application distance; and bake schedule. If none of the chosen additive levels gives the desired effect then choose the closest match and repeat using smaller increments of surfactant. The texture pattern also depends on the flow characteristics of the coating, and in the second section post addition of a thixotrope provides a quick method for testing different combinations of flow characteristics with different levels of surfactant. Divide a sample of texture base into two equal portions. Dry blend each portion with the selected level of thixotrope, eg: 3% and 5%. Divide each portion into three equal amounts. Add surfactant at the three levels chosen in the first section, eg: 0.5, 1.0 and 2.0%. Dry blend each mixture under low shear, spray and bake under comparable conditions. If a close match has not been achieved, choose the closest match and repeat the procedure using smaller increments of surfactant. When a close match is achieved, remake the modified texture base including the selected level of thixotrope by blending components, extruding, grinding and sieving. The dry blend in the selected level of surfactant and adjust if necessary. NOVEMBER 2006
ON
POWDER
This procedure can greatly ease the task of the formulator of textured powder coatings. However, if the customer texture standard cannot be matched during the evaluation of section one, then flow modification is necessary. Attapulgite, colloidal or other absorbent clays are good choices for flow modification and a useful table containing nine different thixotropes is included in the article together with pill flow measurements of each product when incorporated at 5% level of addition. Another example of a gold and black veined powder coating is given using a base texture coating containing a 50:50 blend of polyester and epoxy resin with 1% of carbon black. The pill flow of the base was 110 mm. A blend of this base coat with gold pigment and surfactant texturing agent in the ratio 95.5:4.0:0.5 had pill flow of 88 mm. The addition of 5% of thixotrope to this blend had a pill flow of 55 mm. The pill flow test is described in ASTM D 3451 and pill flows between 25 and 65 mm generally produce the most popular texture patterns. A range of products based on the methods given for the second section produced a number of useful patterns although some of the tests did not produce practical texture patterns. The additive and screen test allow a greater flexibility in developing texture finishes for the decorative market. The process of texture matching has become faster and simpler so that textured product lines can be made available more easily than ever before. Article entitled “Development of Textured Powder Coatings” by Lawrence R Waelde of Troy Chemical Corp, published in Paint and Coatings Industry, Oct 2006, 22 (10), 144146,148-153
Powder coating: state of the US industry An editorial review in the Sep 2006 issue of Metal Finishing
C OAT I N G S presents the views and comment of finishers and suppliers engaged in the powder coatings and the general consensus reveals that many companies are seeking to turn challenges into opportunities. The President of a small plating shop in West Springfield expressed the concerns of many small companies. He is facing the loss of more than 70% of his sales to overseas outfits in the last six years, but he is not caving in to the threat of overseas competition. In fact, he has diversified and turned to other finishing services, including powder coating and different plating processes. The company is now concentrating on small to medium lot sizes now that the bigger jobs are going overseas. Powder coating is the fastest growing segment of his business rising from nothing to 38% of total sales in the past three years. CRHP are another surface finisher located in Virginia. Their main challenge is the rising increase in the cost of raw materials and energy associated with the powder coating process. In order to mitigate these costs they are continually searching for comparative quotes and more efficient ways of handling processes to save on materials, time and labour, while still maintaining high product and service levels. At CRHP, whose operation entails 127,000 sq ft and 86 people on full-time payroll, additional funds have been allocated towards the installation of a heat recovery system on its cure/dry-off ovens. It is expected to see a five-year or less, return on investment. Their end-use clients cover a wide range of industries, and the goal is to spread the risks so that when one segment of the market is down the others can help the income to remain steady. They have also ensured that all product services such as pretreatment are carried out on their premises and to become more competitive they are now using chemistry that 3
TECHNICAL Development of textured powder coatings There have been many options for formulators of textured or structured powder coatings and each formulator has their own special preferences. In the early days of powder formulation a textured powder was a useful product for working off extrudate residues and batches that had been rejected for bittiness and contamination. This practice still persists at some small costconscious powder producers but the present requirements for consistent reproducibility of quality and appearance allow little opportunity for the modern day powder formulator. In fact, batchto-batch reproducibility is often a problem in the production of textured powder finishes. One method that is still widely used is the incorporation of polymeric materials that are insoluble in the primary resin. These materials have molten viscosities higher than the viscosity of the coating system. When dispersed in the coating, they interfere with the levelling and produce the uneven surface that characterizes the textured finish. The main difficulty with these materials is the lack of uniform dispersion throughout the coating. Another method uses surfactant materials with a powder coating texture base in a post-blending operation and this method is claimed to give a uniform texture across the face of the substrate. Post-blending allows for wider formulating latitude and can save the formulator much time in matching a customer texture pattern. Surfactant texturing does not contaminate the whole process and is more effective than extruded blends of insoluble materials because it uses the difference between the molten surface tension of the additive and that of the powder coating texture base. During the baking 2
ON
POWDER
cycle, surfactant particles distributed throughout the curing film, melt and create areas of low surface tension. It was originally thought that molten coating material flowed away from these areas and accumulated in the space between them. The surface of the film then began to form the irregular bumps typical of a textured finish, and the significant difference in surface tension between the coating and the additive caused an unstable energy condition to exist at the points where they came into contact. It was then concluded that intermixing of the additive and base at this interface initiated surface tension equalization. Recent microscopic examination now indicates that the texture pattern begins to develop as soon as the particles melt and smooth flow begins. The smooth flow continues until the surface tension and molten viscosity of the base coating restrict further levelling. The coating surface moves closer to smoothness but never achieves complete levelling. When smooth flow ceases, texture development begins and the texture density pattern is revealed as the profile grows in vertical amplitude. As the profile becomes more clearly defined it gradually slows in growth due to a balance in interfacial tension being established between the molten surfactant and the partially cured coating film. Gelation overcomes the balanced surface tensions and the texture pattern is established permanently. It is believed that coatings with low molten viscosity or thixotropy may form craters around melted particles of surfactant on the surface of the substrate during the period of smooth flow. This is the time when the difference between the surface tension of the surfactant droplets and the coating would exert the greatest influence on each other. The viscosity is at its lowest and the effect of a low surface tension surfactant would prevent the movement of high surface tension
C OAT I N G S coating material towards it. The best way to formulate texture finishes is described in an article by Larry Waelde of Troy Chemical Corporation. In formulating a texture base the primary ingredients are chosen to match the customers performance or price requirements. The resin system is chosen for its physical and chemical properties, pigments for their aesthetic qualities, and additives to fulfil their particular function. The main difference between a smooth finish and a texture base is the absence of flow additives, since these are unnecessary as they conflict with the function of a surfactant agent. Flow agents will endeavour to create a smooth finish, while the texturing agent attempts to perform the opposite function. Most textured finishes are formulated to match the pattern of a customer standard and the conventional method of producing textured finishes can be a painstaking process for every change in the formulation must be extruded as a separate coating. Post-blending surfactant texturing allows variations in additive concentration to be examined without the need for repetitive extrusions. Additionally, certain thixotropes can be used in a similar manner to modify the flow characteristics of a texture base. This allows rapid testing of various combinations of thixotropes with different levels of surfactant texturing additive. The small particle size of most thixotropes allows them to be dry blended with a texture base without causing a gritty surface appearance. Although these are normally added in the extrusion stage, they can be assessed quickly by this method without the need to produce a new sample coating. The screen test is divided into two main sections: surfactants texture additive ladder; and a thixotrope and additive matrix. The reason for performing two separate evaluations is that some NOVEMBER 2006
FOCUS texture bases do not require a modification of flow characteristics to meet the customer requirement. If the requirements have been satisfied in the first section, the secondary evaluation becomes unnecessary and minimizes the time taken to formulate. A formulation is given in the article for a black texture base. In the first section a portion of the texture base is divided into three equal amounts. Each portion is then blended with increasing amounts of surfactant, eg: 0.5, 1.0 and 2.0% and then blended under low shear conditions using a tumble blender or a roller mixer. Spray and bake each sample on the chosen substrate maintaining identical application conditions. These include: film thickness; gun settings; application distance; and bake schedule. If none of the chosen additive levels gives the desired effect then choose the closest match and repeat using smaller increments of surfactant. The texture pattern also depends on the flow characteristics of the coating, and in the second section post addition of a thixotrope provides a quick method for testing different combinations of flow characteristics with different levels of surfactant. Divide a sample of texture base into two equal portions. Dry blend each portion with the selected level of thixotrope, eg: 3% and 5%. Divide each portion into three equal amounts. Add surfactant at the three levels chosen in the first section, eg: 0.5, 1.0 and 2.0%. Dry blend each mixture under low shear, spray and bake under comparable conditions. If a close match has not been achieved, choose the closest match and repeat the procedure using smaller increments of surfactant. When a close match is achieved, remake the modified texture base including the selected level of thixotrope by blending components, extruding, grinding and sieving. The dry blend in the selected level of surfactant and adjust if necessary. NOVEMBER 2006
ON
POWDER
This procedure can greatly ease the task of the formulator of textured powder coatings. However, if the customer texture standard cannot be matched during the evaluation of section one, then flow modification is necessary. Attapulgite, colloidal or other absorbent clays are good choices for flow modification and a useful table containing nine different thixotropes is included in the article together with pill flow measurements of each product when incorporated at 5% level of addition. Another example of a gold and black veined powder coating is given using a base texture coating containing a 50:50 blend of polyester and epoxy resin with 1% of carbon black. The pill flow of the base was 110 mm. A blend of this base coat with gold pigment and surfactant texturing agent in the ratio 95.5:4.0:0.5 had pill flow of 88 mm. The addition of 5% of thixotrope to this blend had a pill flow of 55 mm. The pill flow test is described in ASTM D 3451 and pill flows between 25 and 65 mm generally produce the most popular texture patterns. A range of products based on the methods given for the second section produced a number of useful patterns although some of the tests did not produce practical texture patterns. The additive and screen test allow a greater flexibility in developing texture finishes for the decorative market. The process of texture matching has become faster and simpler so that textured product lines can be made available more easily than ever before. Article entitled “Development of Textured Powder Coatings” by Lawrence R Waelde of Troy Chemical Corp, published in Paint and Coatings Industry, Oct 2006, 22 (10), 144146,148-153
Powder coating: state of the US industry An editorial review in the Sep 2006 issue of Metal Finishing
C OAT I N G S presents the views and comment of finishers and suppliers engaged in the powder coatings and the general consensus reveals that many companies are seeking to turn challenges into opportunities. The President of a small plating shop in West Springfield expressed the concerns of many small companies. He is facing the loss of more than 70% of his sales to overseas outfits in the last six years, but he is not caving in to the threat of overseas competition. In fact, he has diversified and turned to other finishing services, including powder coating and different plating processes. The company is now concentrating on small to medium lot sizes now that the bigger jobs are going overseas. Powder coating is the fastest growing segment of his business rising from nothing to 38% of total sales in the past three years. CRHP are another surface finisher located in Virginia. Their main challenge is the rising increase in the cost of raw materials and energy associated with the powder coating process. In order to mitigate these costs they are continually searching for comparative quotes and more efficient ways of handling processes to save on materials, time and labour, while still maintaining high product and service levels. At CRHP, whose operation entails 127,000 sq ft and 86 people on full-time payroll, additional funds have been allocated towards the installation of a heat recovery system on its cure/dry-off ovens. It is expected to see a five-year or less, return on investment. Their end-use clients cover a wide range of industries, and the goal is to spread the risks so that when one segment of the market is down the others can help the income to remain steady. They have also ensured that all product services such as pretreatment are carried out on their premises and to become more competitive they are now using chemistry that 3