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Processing of powder coatings in a reciprocating single screw extruder
Peter Franz is R&D Manager with Buss Compounding Systems AG, Switzerland, the manu-facturer of the reciprocating single screw extruder, and Senior Technology Consultant with Buss America Inc., the American sales and engineering subsidiary. He graduated as a mechanical engineer at the Federal Technical University of Zurich, Switzerland. After graduation he made post-studies in chemical engineering. He is member of the German Society of Engineers (VDI) and was heading the group of Plastic Compounding as chairman for ten years.
Edmund Meier joined Buss America Inc., Bloomingdale, Illinois, in 1982. He is Senior Product Manager for the application of Reciprocating Single Screw Extruders for Powder Coatings, Toners and Food.
This article describes setup and operation of the reciprocating single screw extruder system as well as its impact on modern production technology of powder coatings. Detailed and practical hints for users are given.
1. The development of powder coating technology Since 1951 reciprocating single screw extruders have been used in the paint and lacquer industry for the continuous production of pigments, pigment pastes and lacquers. In the early 60's up-coming safety regulations began to put the industrial use of solvent-based paints under constraint. Water-borne paints were not considered to be a competitive, economical alternative, as water requires a significant amount of energy for evaporation. Thus, the only way out was the development of paints without solvents and water: powder coatings. It was the reciprocating single screw extruder pioneering this development work and contributing to the successful breakthrough of powder coatings. As early as 1971, the continuous production of powder coatings using reciprocating single screw extruders was presented in literature [1].
Figure 1evidences the successful growth of powder coatings: within 20 years the volume increased by a factor of 8.4, that is within periods of 6.5 years in average the volume doubled.
The past three decades of powder coatings are characterized by a tremendous amount of R&D work being carried out, both in chemistry and process technology. Interdependent, the chemistry stimulated the improvement of the process technology and the latter opened up new windows for the chemistry.
At the beginning, epoxy resins were exclusively used as binder. Meanwhile, the chemistry added polymers like acrylics and polyesters. Hybrid systems were developed. Curing temperatures and times were reduced. The film gauges got thinner. The applications spread out over technical and household appliances, the building and the automotive industries; each domain having its own specific requirements and quality standards. The objectives of today's R&D work focus mainly on:
* coil and sheet coating
* thin film applications
* can and container coatings
* automotive primers and top coatings
* coatings with metallic effects and special texture
* high gloss and wet look coatings
* super durable coatings, UV-curable powder coatings
* low temperature curing systems for the coating of plastics, wood and paper.
Weather-resistant powder coatings based on polyester and TGIC (TriGlycidyl IsoCyanurate) have already been under fire for some years because of the possible mutagenicity of TGIC. In 1996 the European authorities imposed a more stringent labeling requirement (skull and crossbones) for TGIC itself and any product containing TGIC. This stipulation is to be implemented in all EU countries by March 1998. The chemical industries developed substitutes resulting in even better characteristics but which need more residence time and energy during processing.
The process technology had and has to keep pace and has to cope with these dynamically changing requirements. In addition to these processing aspects, the R&D work in process technology resulted in an improved economy. Over the last 20 years, the output rates of the reciprocating single screw extruders were increased by a factor of 6 (Figure 2) following almost the growth of the annual usage of powder coatings. In parallel, the screw speed increased by a factor of 4 and the output rate per revolution by a factor of 1.5.
These facts only show the tip of the iceberg seen from the surface. Hidden behind is a tremendous amount of R&D work that no longer could be carried out empirically, but needed the scientific backup. By means of math-modeling, an in-depth understanding of the operating principle of the reciprocating single screw extruders was developed. Computer simulations helped to optimize the energy balance of each individual processing step. This scientific approach in combination with the experimental work led to an overall plant design that meets the current and future requirements of the powder coating market. Some of these results are discussed in this paper.
2.Operating principle of reciprocating single screw extruders The reciprocating single screw extruder was invented by a German engineer, Heinz List in the early 1940's. At that time List had been a member of the engineering group of I.G. Farbenindustrie in Germany. The objective of this group was to develop continuously operating machines for processing high viscosity materials. By the end of World War II List moved to Switzerland to concentrate on his concept of a reciprocating single screw extruder. This concept first was described publicly in List's Swiss Patent filed on August 8, 1945 [3] followed by a U.S. patent in 1946 [4] and a German patent filed in 1949 [5]. List's first paper was published as early as 1950 in "Kunststoffe" [6], a European magazine for the plastic industry. Since then many authors have dealt with the operating principle of the reciprocating single screw extruder [7,8,9,10], to mention only a few of them.
The reciprocating single screw extruder (Figure 3) is a continuously operating single-screw machine, but its unique operating principle and its special screw and barrel design differentiates it from traditional single-screw extruders:
The screw of a common single-screw extruder has a continuous spiral. With the reciprocating single screw extruder this spiral is broken by three gaps per revolution, resulting in the kneading flights.
Three rows of kneading pins or kneading teeth cooperating with the kneading flights are individually inserted in the barrel, at radial intervals of 120°.
The screw of a common single-screw extruder merely rotates. The motion of the screw of the reciprocating single screw extruder is different: an axial stroke or oscillation is superimposed to the rotation. A special gear box generates this characteristic motion. The mechanism ensures that each revolution of the screw is accompanied by one full stroke forward and backwards.
Figure 3 additionally reveals a design feature appreciated by R&D engineers, as well as maintenance people and operators: the vertically split barrel that easily can be opened and closed.
In order to explain the operating principle, a section of the screw is projected into a flat plane (Figure 4).
The kneading flights appear as three rows of stretched rhomboids and the kneading teeth as three rows of diamonds. (When unfolding the screw into the plane one of the rows of kneading teeth to which tooth #3 and #4 belong to is cut in half.)
The rotation of the screw corresponds to a movement from bottom to top. Hence, the stock is conveyed from the right hand side to the left.
The starting position of Figure 4 is chosen at will. To begin with, only one single kneading flight is viewed, e.g. the gray shaded one in the center. The four gray shaded kneading teeth #1, #2, #3 and #4 are cooperating with the gray shaded kneading flight.
The operating principle depends only on the movement of the kneading flights relative to the kneading teeth. Therefore, in order to better visualize the operating principle, in this and the following figures, the screw is supposed to be stationary while the kneading teeth move from top to bottom.
From its starting position, the screw in Figure 5 has completed one quarter of a full revolution, i.e. a rotation of 90°. On its way the kneading tooth #1 wipes past the left-hand long flank of the gray shaded kneading flight, thus generating a shear gap. The relative speed of the kneading flight and the kneading tooth results in a shear gradient. According to the power laws of non-Newtonian viscous materials, this shear gradient develops shear stress in the stock. Hence, dispersive and distributive mixing takes place. The shear stress developed in the stock is in equilibrium with the shear stress on the flank of the kneading flight and the kneading tooth. Consequently, the flanks are cleaned. The kneading teeth #3 and #4 do not yet interact with the shaded kneading flight but with others; an aspect that will be discussed later.
As the screw rotates another 90°, so that it has completed half of a full revolution from its starting position (Figure 6) , the kneading tooth #4 wipes past the right-hand short flank of the shaded kneading flight. Again a shear gap is generated and shear stress developed, resulting in dispersive and distributive mixing. In parallel, the flanks of the shaded kneading flight and the kneading tooth are cleaned respectively.
During this motion the kneading teeth #1, #3 and #4 pass through the corresponding gap between the kneading flights. Thus, the stock is split apart and partially pushed backwards against the main direction of flow from the right to the left. This once was described as the "pilgrim step": Two steps forward and one step backwards. This procedure superimposes a unique axial mixing effect to the radial one; a mixing efficiency normally missing with standard twin-screw and single screw extruders. This is the reason that reciprocating single screw extruders need only a short processing length to perform homogeneous compounding. Field experience shows that the processing length of that reciprocating single screw extruder is only about half or one-third of that of standard twin-screw or single-screw extruders. This is confirmed scientifically when calculating the striation thickness that will be discussed later.
A further rotation by 90°, three-quarter turns is now completed (Figure 7), makes the kneading tooth #3 wiping past the left-hand short flank of the shaded kneading flight. Shear energy is dissipated in the shear gap, dispersive and distributive mixing performed and the corresponding flanks of the kneading flight and the kneading tooth cleaned respectively. Kneading tooth #3 and #4 are plowing through the gaps of the kneading flights, splitting the stock apart for axial mixing.
In the final rotation of 90° (Figure 8) the screw now completes a full revolution of 360° and the kneading tooth #2 wipes past the right-hand long flank of the shaded kneading flight. Again the stock is sheared, dispersive and distributive mixing takes place and the flank of the kneading flight and the kneading tooth is cleaned. As the kneading tooth #2 passes the gap between the kneading flights it splits the stock apart and pushes it partially forward resulting again in axial mixing.
During one full revolution, the four kneading teeth viewed so far cooperate with the four flanks of the shaded kneading flight. That is, four finite shear elements are generated which might be compared with four miniaturized two-roll mills. On two-roll mills stock-blenders are installed in order to perform lateral mixing in addition to the radial one. Analogous to these stock-blenders the kneading teeth plowing through the gaps between the kneading flights superimpose axial mixing to the radial one and re-orientate the stock.
Figure 9 also reveals that what has been discussed so far for the shaded kneading flight also takes place at all other kneading flights.
A larger section (Figure 9) confirms this statement: the orbits or paths traced by the kneading teeth completely cover the entire screw surface. There are no dead areas or volumes left; the reason for the acknowledged performance of the reciprocating single screw extruder with regard to dispersive and distributive mixing as well as its self-cleaning efficiency.
The micro-mixing efficiency of an extruder scientifically can be evaluated by calculating the striation thickness. This is a rather abstract name. But it can be understood as if a sausage of a certain length would be cut in as many slices as the striation thickness stipulates: the greater the number of the striation thickness the thinner the slices, that is the better the micro-mixing efficiency.
Calculations made for the reciprocating single screw extruder anticipating a short processing length of 4 L/D only [11] resulted in a striation thickness of 248 or 2.8 times 1014 or 280,000,000,000,000.
This number scientifically confirms the surprisingly short processing length necessary for homogenization and compounding. It also corresponds to the field experience stated in the comments to Figure 6.
3.Design features of the reciprocating single screw extruder
Figure 3 shows schematically the vertically split barrel and Figure 10 displays the real machine with opened barrel:
For inspection purposes and maintenance, there is no need to pull screws or barrels. The barrel easily opens and closes like opening and closing a door or a window. The barrel and the screw remain fitted and positioned and no cooling circuit has to be disconnected. The barrel is lined and the liners are longitudinally subdivided in segments. In case of wear, only the worn out liner segments are exchanged. Hence, there is no need to move a barrel to a workshop for refurbishing.
The screw is made up of individual elements mounted on a central shaft. They are tightened by a nut at the screw tip. For replacement the barrel is opened up, the screw nut removed and the elements pulled from the central shaft. Again, there is no need to pull the entire screw or barrel section in order to get access to the screw. The screw remains positioned and the temperature control loop connected to it.
Standard kneading teeth or kneading pins individually inserted in the liner segments can be replaced by thermocouple pins (Figure 11).
A hole is drilled into a standard kneading pin down to the tip. Only a thin metal layer is left which protects the thermocouple from wear and damage. A thermocouple is inserted into this bore and firmly pressed to the end of the bore by a spring. As the tip of the pin completely extends into the stock and its thermal capacity is low the thermocouple reads accurately the real stock temperature. This feature significantly adds to the accuracy of the overall process control.
The contour of such a thermocouple pin is identical to that of a standard kneading pin. Consequently, neither the mixing process nor the self-cleaning efficiency is changed.
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