Events
Additives,
Compounding, & Coatings
March 8 - 10, 2010 Atlanta, Georgia
USA
Hilton Airport Hotel |
May 16-20, 2010
Orlando World Center
Marriott Resort &
Convention Center
Orlando, Florida USA |
June 7 - 10, 2010
Fispal Food Service
Expo Center North
São Paulo - Brazil |
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Compounding
The Tek-MixTM process by Stratek is ideal for enhancing plastic compounding, because of its highly effective distributive and dispersive qualities, and because it can operate at lower processing temperatures, lower pressures, and lower shear rates. Consequently, the Tek-MixTM process can be used to:
- mix and blend two or more plastics resins together
- mix a plastic "host" or "matrix" material with sensitive or fragile additives
- mix plastic "host" or "matrix" materials with fillers which tend to aglomerate or which have undesireably large particle sizes
For more information on typical Tek-MixTM process operating temperatures, click here..
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COMPOUNDING: EXAMPLE 1 Creating "Green"
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| Above is the desired color for the mixing experiment covered by this section. While the only successful prior approach to creating this blend involved a 3-step process, Stratek provided this color as a result of a one-step, "one pass", continuous process, using a single screw extruder directly feeding a Stratek Tek-MixTM mixer. |
Problem: Color masterbatches are challenging to compound because of difficulties encountered dispersing the pigments and colorants used. Organic colorants are extremely difficult to disperse in the polymer matrix, and even more challenging when non-polar polymers like Polyethylene and Polypropylene are used.
Additionally, agglomerations can collect in residues in the compounding device, and unless this machine is self-cleaning, it can release small fouling particles repeatedly throughout the run. To counter these problems, often a compounder will utilize wetting agents and waxes to help reduce agglomeration of colorants and to aid in promoting very fine particle distribution.
Within the family of color mixing challenges, the most difficult color application involves dispersing pigments directly into a final product. There is no opportunity to correct an imbalance in a "second pass". This challenge is further complicated when the process requires the use of small, minute quantities of incompatible colorants. Lastly, the challenge becomes even more difficult, when the process forbids the use of any waxes or wetting agents.
In general practice, colorants and pigments in small concentrations, such as under 1% loading, are difficult to directly disperse into a polymer melts to create a uniform, homogeneous color. This is why they are normally processed into masterbatches of 25% to 50% loading then let-down in a second step to create the final product. Additionally, single screw extruders have trouble uniformly processing the powders in which the pigments are provided.
Objective: Evaluate the ability of single screw and twin-screw extruders to create in a single process ("one pass") a challenging color mixture and to compare these results with the same devices after they are equipped with a Stratek Tek-MixTM mixer.
Experimental Design:
The following table shows the four different processing tests conducted. In each case, the materials were processed then pelletized. Randomly selected pellets were then compression molded in a Carver Press to create "press-out" films which are pictured below.
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Test #
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Extruder Used
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Tek-MixTM Device Used
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Compression Molded “Press-out”
Created from Processed Sample Material
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2" Single Screw with Standard Conveying Screw
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None
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2" Single Screw with Standard Conveying Screw and Standard Dynamic Mixer
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None
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30 mm Twin-Screw Extruder
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None
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2” Single Screw Extruder with Standard Conveying Screw Directly Feeding Tek-MixTM Mixer
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50 mm Tek-MixTM
Mixer
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Materials Used to Create the Above Composites
(a) 0.5% Munsell Blue Polyethylene pretumbled as a dry, room temperature blend (organic and inorganic pigments); 50% loaded, MFI = 0.48
(b) 0.5% Yellow Oxide powder
(c) 1.0%Titanium Oxide (TiO2) powder
(d) 98% Petrothene NA 960 LDPE
In aln all the above tests, the materials were "preblended" by tumbling dry, at room temperature, together along with LDPE prior to processing tests, without any surfactants, waxes or any other chemical additive being used.
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♦ Note: Yellow Oxide powder is extremely shear sensitive, furthermore it degrades above 180°C, which is not far below the processing temperature of the Polyethylene and LDPE. When the Yellow Oxide degrades, it produces reddish brown specks, clearly visible in the pellets produced from this material (see "Extruder Only" results, above) This discoloration is unacceptable.
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Test Results: The photos above and below clearly demonstrate that the single screw and the twin-screw extruders were challenged to create homogenous green blends. Furthermore, the reddish-brown specks in some of the samples indicates that the Yellow Oxide had degraded, too. Only the Tek-MixTM configured process yielded acceptable results.
| Below is a photo taken after Test #1 which used a standard 2" single screw extruder equipped with a standard conveying screw. The blue and yellow materials are extremely separated. |
The photo from Test #2 shown below shows the same standard conveying screw after a standard dynamic mixer was attached. Some improvement is shown, but much separation still exists. |
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| The pellets shown at right exhibit the acceptable homogenous green condition (Tex-MixTM samples) and the unacceptable condition (Extruder only) which contains reddish-brown specks of degraded yellow oxide powder. |
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Summary: Only the Tek-MixTM configured equipment provided a satisfactory product in these "single pass", direct compounding tests. The single screw extruder, even when equipped with a state of the art shear mixer, could not achieve a homogenous blend. Even the twin-screw extruder failed the test. Only the Tek-MixTM configured equipment achieved a homogenous blend, as was demonstrated by a perfectly uniform light green color without any streaks, gels, specs or changes in color.
Conclusions: Since using a Tek-MixTM configured single-screw extruder provided a satisfactory product, and since the twin-screw extruder had already surpassed the ability of the single screw extruder, the conclusion is drawn that Tek-MixTM added onto any extruder, including a single screw extruder with a conventional conveying screw only being used, can achieve a perfectly uniform green blend. This avoids entirely having to compound the blue, yellow and white into a masterbatch for further subsequent letdown.
This illustrates the tremendous dispersive power of extensional flow. Alternate high shear techniques would discolor the yellow component, which is shear sensitive, as well as creating streaking requiring multiple pass treatment to achieve uniform dispersion.. Only Tek-Mix TM can directly compound three incompatible additives with a non-receptive LDPE polymer matrix, to produce a uniform compound.
For more information on these tests, or to discuss your own specific requirements, please contact us.
Tek-MixTM - Extensional / Elongational Flow Mixing
The Next Step in Mixing & Blending
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Compounding
