Cleaning and liquid contact with solids – Processes – Hollow work – internal surface treatment
Reexamination Certificate
2001-06-06
2003-03-11
Carrillo, Sharidan (Department: 1746)
Cleaning and liquid contact with solids
Processes
Hollow work, internal surface treatment
C134S006000, C134S007000, C134S008000, C134S019000, C134S022110, C134S022140, C134S022190, C134S034000, C134S035000, C134S036000, C134S042000, C134S11500R, C134S16600C, C134S16600C
Reexamination Certificate
active
06530382
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to cleaning of screw extruders. More particularly, the present invention relates to the cleaning of screw extruders by introducing a cleaning compound within and through the screw extruder barrel to thereby clean the same.
BACKGROUND AND SUMMARY OF THE INVENTION
The use of powder coatings for decorative purposes has grown dramatically primarily due to their environmental advantages over liquid coatings. For example, powder coatings do not contain volatile organic solvents that evaporate during application or curing. Expensive, conditioned air from powder paint booths is recycled rather than exhausted because it does not contain solvent vapor—saving energy. Because it contains no solvent, less air has to be exhausted from powder coating drying ovens, saving additional energy. Finally, powder coating overspray is easily captured and recycled without the use of a water-wash system. As a result, there is no paint sludge from booth wash water to landfill. However, in the manufacturing process, liquid coatings do have one key advantage over powder coatings: if a batch of liquid paint is found to be slightly off-color, its color can be adjusted by mixing in additional pigments until the correct color is obtained. In the case of powder coatings, off-spec batches of coating powder have to be scrapped. Often, the reason that batches of coating powder become discolored, or otherwise contaminated, is poor cleaning of the extrusion equipment used to make them.
Thermosetting coating powders are made by first blending the resin and curing agent with up to six other dry ingredients such as colorants, catalysts, flow control additives, fillers, or UV stabilizers in a batch mixer such as a Henschel mixer. This premix is then melt compounded in a single- or twin-screw extruder of the types made by Werner-Pfleiderer, APV Baker, B&P Process Equipment, or Buss. In the extruder, the resin melts, the ingredients are compacted, and the constituents are completely dispersed in the molten resin. The extruder generates enough heat through mechanical shear that little external heating is required to melt the resin. The melt temperatures for mixtures of thermosetting materials can be as high as 180° C., but they typically operate at temperatures from about 60 to about 140° C., only slightly above the melting temperature of the resin. Residence time in the extruder is also short, normally a minute or less. Due to the low processing temperatures and short residence time, little reaction occurs between the resin and curing agents (normally, less than about 5%). As the melt exits the extruder, it is cooled rapidly on a water-cooled drum and then passed to a water-cooled belt. The cooled compound is broken into granules approximately 0.5 in. by 0.5 in. The friable granules are then ground in a hammer mill to a fine particle size, typically 75 &mgr;m or less. The fine particles are screened in a classifier before final packaging.
Normally, one such manufacturing line will produce multiple types of coating powders based on epoxy, polyester, polyurethane, acrylic, or hybrid resins. A line will also produce multiple colors of the various coating powders. Before a manufacturing line can switch from producing one color or resin to another, the entire system must be cleaned to prevent contamination of subsequent batches. By far, the most difficult equipment to clean, or purge, in the manufacturing system is the extruder.
The extruder consists of a barrel and one or more (typically two) screws containing specially designed flights known as feed, transition, mixing, dispersion, and metering “zones”. The screws convey and thoroughly mix the ingredients of the melt through the externally heated barrel and then meter the melt through a die and onto the cooling drum. As mentioned, the temperatures along the barrel are kept only slightly above the melt temperature of the resin, and in some sections, such as the feed section, there is no external heating. However, invariably, some of the ingredients of the coating powder adhere to the barrel, the die, or a section of the screw, leaving particularly tough, abrasion-resistant residue. This abrasion-resistant residue is particularly prevalent in sections that contain scratches or pits. Such residue can contaminate, and even ruin, later batches with a different color it is not completely purged. If the residue remains in the extruder long enough, it can also cross-link and become harder with each batch or eventually degrade into carbon deposits. Highly cross-linked, hardened material that falls off in later batches can sometimes be screened out in classifying equipment because of its larger particle size. However, the classifying equipment has to be operated very slowly, increasing production time and processing costs.
Currently, a few methods of cleaning powder-coating extruders are employed. One method is to disassemble the extruder and clean the screws by sandblasting them and then burning off the remaining residue with a blowtorch. This process normally lasts a little over an hour, but can take up to 6 hours, to complete. The other major drawback of the process is heating the screws. The blowtorch heats the screws in spots to temperatures greater than about 800° F., causing them to become brittle over time. The screws can eventually bend and break. Residue can also be burned off of the screws in a burn-off oven without sandblasting. The screws have to be heated to temperatures above 600° F. for several hours, again shortening their service life. In addition to the degradation caused by heating, there is also the danger of a screw being dropped or chipped by the one or two people who have to disassemble the extruder to remove them. Since one screw on a 65-mm extruder with a ratio of length to diameter (i.e., L:D ratio) of about 15:1 can be quite costly, disassembling the extruder and heating the screws can be too expensive to be a viable cleaning method.
Another method of cleaning the extruder is to disassemble it and immerse the screws in a bath of hot caustic. With time, the hot caustic chemically degrades and breaks up the residue. This is an undesirable method because of the excessive downtime needed to disassemble the extruder and the safety considerations associated with working with hot caustic.
Several commercial cleaning, or “purge”, compounds are used to clean thermoplastic processing equipment such as extruders and injection molding machines. They have also been tried on powder coating extruders. These purge compounds are typically solid thermoplastic carrier resins in pellet, granule, or powder form that contain surface-active agents, abrasive fillers, occasionally amine compounds such as monoethanolamine, or other cleaning agents. Typical thermoplastic carrier resins include acrylic, polyethylene, polypropylene, polystyrene, and styrenic resins such as acrylonitrile-butadiene-styrene (ABS) and styrene-acrylonitrile (SAN). Abrasive fillers include hard acrylic resins, glass fibers, calcium carbonate, mica, aluminum oxide, kaolin, and ceramic spheres. Surface-active agents and other cleaning agents include polyalkylene oxide-based polyol surfactants, neutral salts of alkylbenzenesulfonic acid, and polyethylene and fatty acid amide waxes. Processing equipment does not have to be shut down and disassembled to be cleaned with these thermoplastic purge compounds. The purge compounds are fed to the thermoplastic extruders and injection molding machines in the same manner as production resins, with two differences: they are often fed at a slower rate to allow them to scrape residue, and at times, the screws are stopped and the purge compounds allowed to soak. They are relatively effective at removing thermoplastic residues.
However, these dry, solid thermoplastic purge compounds are normally not effective for purging thermosetting coating powder residues for a few reasons. First, the thermoplastic purge compounds—particularly those containing acrylic resins—have to be processed at higher temperatures (>200
BASF Corporation
Carrillo Sharidan
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