November 1, 2013! ISSUE 06 N e w s l e t t e r Distributive Mixing Page 2 Why we need improved Distributive Mixing Page 3 How to get there Page 4 and 5 Helpful Hints Page 6 This the sixth issue of of an on-going attempt to provide interesting news to the Powder Industry. It is our belief that we still have a tremendous opportunity for powder coating growth in North America. For example, optimizing extrusion processing conditions can result in opening markets to powder coatings. DS&S Inc. has been active in evaluating improved distributive mixing to help increase appearance and performance properties of powder coatings. We are committed to look beyond supplying catalysts,additives, lab equipment and consulting services to help accelerate the continued growth of the powder coatings market. In this issue, I will reveal the various techniques we investigated that improve the appearance and performance of powder coatings. Each month, or so, we will feature at least one of our products that help grow our industry. STAY TUNED D S & S I n c. N e w s L e t t e r 1 3 9 0 W i n d w a r d L a n e N i c e v i l l e F L 3 2 5 7 8 850-897-3152 www.danickspecialties.com
Distributive Mixing: What is Distributive Mixing and why do we need it when making powder coatings? Distributive mixing is the process of spreading the minor component (catalyst, curative etc.) throughout the matrix in order to obtain good spatial distribution. Reference: Analysis of Mixing in Polymer Processing Equipment Ica Manas-Zloczower Department of Macromolecular Science Case Western Reserve University Cleveland ON 44106
Why we need Improved Distributive Mixing Background: Typically, when melt-mixing, extruding powder coatings, especially in twin-screw extruders, there is a tendency of low visocity components to migrate to the perimeter of the extruder barrel and act as a lubricant rather than getting mixed into the extrudate. Example of powder formulation components that have exhibited this tendency: Blocked isocyanate cross-linkers Uretdione cross-linkers Epoxy component of Hybrid formulations Catalysts such as Dibutyl Tin Dilaurate, Dimethyl Stearyl Amine Dimethyl Lauryl Amine This phenomenon will increase extruder through-put but the resulting extruded product will not be homogeneous and will not have the low viscosity component distributed through-out the extrudate as desired. Results of poor distributive mixing can include: Reduced appearance properties exemplified by: Micro pinholes Micro texture Lower gloss Reduced flow. Reduced performance properties exemplified by: Reduced impact resistance Reduced flexibility Reduced solvent resistance Improved Distributive Mixing can minimize and or eliminate the above problems.
Techniques for Improving Distributive Mixing How to get there: When a powder coating formulation contains components of vastly different viscosities, mixing them to achieve a homogenous product is always a challenge. For example, a typical Hybrid powder formulation contains: Polyester resin (melt viscosity at 200C typically 25-55 poise) Epoxy resin (melt viscosity at 200C typically 2.5-5.5 poise at 200C) Getting these two polymers to be melt-mixed well is more difficult than it seems. At normal extrusion temperatures of 100-125C, the apparent differences in melt viscosities of the polyester and the epoxy are amplfied. If distibutive mixing is not improved, some Hybrid powder formualtions exhibit reduced flow, micro pinholing, etc. One way to help minimize the differences in melt viscosities is to increase extrusion processing temperatures in the second or third zone of the extruder barrel. As the extrudate temperature increases, both components (Polyester and Epoxy) become more liquid-like and can be mixed together easier. This strategy helps minimize the effects of the ten-fold difference in melt-viscosity between the Polyester and Epoxy, since both components become more liquid-like at higher extrusion temperatures. For most powder coating formulations, increasing the second or third extruder barrel zone temperature to 150C will dramatically increase the ability to mix resin components with different melt viscosities. This strategy is not popular since it requires replacing the typical Hot water Heater used for heating the second Barrel Zone with a Hot-oil Heater. Obtaining a good homogeneous mix is even more difficult when liquid catalysts such as Dimethyl Stearyl Amine, Curaid DMS-P (supplied as a dry powder dispersed on a silica carrier) is added to the formulation. This is where improving distributive mixing is really important. Increased turbulence in the extruder and inducing some back-flow are essential to improving distributive mixing in powder coating formulations that have components that are essentially liquid or have very low melt temperatures. To achieve the above improvements that will increase turbulence and create some back-flow and improve distributive mixing, the introduction of some left-handed, reverse pitch, extruder screw elements are necessary. These elements are usually added at a position about 2/3 the way down the exruder screw. They can be chopper elements which are just narrow Kneeding Elements with 45 degree reverse pitch or some 45 degree reverse pitch Kneading Elements. The idea is to minimize loss of through-put while achieving enough back-flow to maximize distributive mixing. Some trial and error evaluation is usually required when selecting which Kneading Elements are best suited.
Techniques for Improving Distributive Mixing How to get there: For example, a typical Polyester/Urethane powder formulation contains: Polyester resin (melt viscosity at 200C typically 30-75 poise) Polymeric blocked isocyante curing agent (melt viscosity at 200C typically 2.5-9.5 poise at 200C) Getting these two polymers to be melt-mixed adequately is more difficult than it seems. At normal extrusion temperatures of 100-125C, the apparent differences in melt viscosities of the polyester and the polymeric blocked isocyanate curative are amplfied. If distributive mixing is not improved, some Polyester/Urethane powder formualtions exhibit reduced flow, micro pinholing, poor impact resistance, etc. One way to help minimize the differences in melt viscosities is to increase extrusion processing temperatures in the second or third zone of the extruder barrel. As the extrudate temperature increases, both components (Polyester and the polymeric blocked isocyanate curing agent) become more liquid-like and can be mixed together easier. This strategy helps minimize the ten-fold difference in melt-viscosity between the Polyester and the Curing agent, since both components become more liquid-like at higher extrusion temperatures. For most powder coating formulations, increasing the second or third extruder barrel zone temperature to 150C will dramatically increase the ability to mix resin components with different melt viscosities. This strategy is not popular since it requires replacing the typical Hot water Heater used for heating the second Barrel Zone with a Hot-oil Heater. Obtaining a good homogeneous mix is even more difficult when liquid catalysts such as Dibutyl Tin Dilaurate, Curaid Tin-D-P (supplied as a dry powder dispersed on a silica carrier) is added to the formulation. This is where improving distributive mixing is really important. Increased turbulence in the extruder and inducing some back-flow are essential to improving distributive mixing in powder coating formulations that have components that are essentially liquid or have very low melt temperatures. To achieve the above improvements that will increase turbulence and create some back-flow and improve distributive mixing, the introduction of some left-handed, reverse pitch, extruder screw elements are necessary. These elements are usually added at a position about 2/3 the way down the exruder screw. They can be chopper elements which are just narrow Kneeding Elements with 45 degree reverse pitch or some 45 degree reverse pitch Kneading Elements. The idea is to minimize loss of through-put while achieving enough back-flow to maximize distributive mixing. Some trial and error evaluation is usually required when selecting which Kneading Elements are best suited.
Helpful Hints Typical Co-rotating Twin Screw Extruder Premix Add some reverse pitch Kneading elements here Techniques for optimizing Distributive Mixing: Increased extruder second zone barrel temperature to 150 C, 302 F to reduce the difference in the viscosities of the various powder formulation components to facilitate better mixing of high and low viscosity components. Incorporate some reverse pitch (left handed) chopper elements about 2/3 down the extruder. barrel to increase turbulence and improve mixing of lower viscosity components with higher viscosity components. Increased extruder RPM to amplify turbulence during extrusion. Remember, slightly reduced extruder through-put is not a weakness when vastly improved product quality is achieved. Note: Good distributive mixing enhances performance of polyester/urethanes formulations to the point that curing agent levels can be reduced to 80% of the stoichiometric amount and still achieve full impact resistance. With Low hydroxyl, 25 OH value, polyesters, good performance properties can be achieved at 93/7 and 92/8 ratios of polyester to curing agent. Another strategy to help improve mixing during the extrusion process is to utilize High Intensity Premixing Conditions rendering all of the powder coating formulation components a fine sugar-like consistency prior to the extrusion process. Utilizing High Intensity Premix makes the extrusion process less like a miracle that has to happen.