Stratek Plastic Ltd

Extensional / Elongational Flow Mixing
 

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Home Technology
Technology

The Concept

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Introducing Tek-MixTM

Stratek has developed and patented a technology named Tek-MixTM.   Tek-MixTM is a thermo-mechanically-based process performed by a cavity treatment device  (the Tek-MixTM Processing Machines) which is designed and constructed to take advantage of the natural benefits of Extensional Flow (also known as Elongational Flow) for mixing and blending.    The benefits most commonly realized by customers are greatly improved distribution and dispersion of additives such as anti-oxidants, lubricants, nanoclays, carbon nanotubes, pigments, as well as a marked improvement in the survivability of sensitive additives. Extensional Flow Mixing has been extensively reviewed by Rauwendaal, Manas-Zloczower, Tokihira, Yakemoto, Suzaka, Han, Tadmor, and Utracki, and has shown benefits in reduced energy consumption and reduced shear rates.  Twin-screws also use components of extensional flow.

The Emphasis on Extensional Flow

Shear Force:  Typical shearing methods utilize the difference in velocity between shearing surfaces that create forward and reverse forces to disperse or break-down particles.   Naturally very high shear rates and pressures can be required, leading to highshear heat release, which leads to uncontrollable overheating.  The process of intense shearing and keeping temperature under control requires high energy usage. 
Extensional (Elongational) Flow:   Extensional Flow, resulting from a positive velocity gradient in the flow direction, is at least 400% more efficient than a straight shear field in particle size dispersion due to the application of high stretching forces to the agglomerates, leading to rapid breakup. 
 
It has been demonstrated that the dispersive mixing of two Newtonian liquids is more efficient in extensional flow than in shear flow.    Extensional flow occurs for example when fluid converges from a reservoir into a capillary. In extensional flow, the mixing action only weakly depends on the viscosity ratio, while the effectiveness of shear flow mixers is very dependent on the viscosity ratio.  
Presently there are few deformational systems where the extensional flow field dominates, such as fiber spinning.  It has long been known that extensional flow tends to deform drops into long prolate ellipsoids, which upon cessation of flow disintegrate into a series of micro-drops with diameter about twice as large as the smaller diameter of the prolate ellipsoid. Additionally drops passing through a convergence (capillary flow) are broken into mini-drops with diameter one order of magnitude smaller. For dispersing the minor-phase polymer into a fine droplet system, a series of convergences and divergences with progressively smaller diameters of the restriction could be used.  This is exactly the practice used by Tek-MixTM technology, but in a dynamic high modulation mode.

Primary Benefits of Stratek's Extensional Flow System
  • Low Pressures, High Clearances
  • Four times more energy efficient than high shear systems (avoids strain-hardening)
  • Generates distributive and dispersive mixing
  • Usually Stratek’s system operates at lower temperatures than the premelting extruder
  • No recompaction of particles, which happens often with compressive mixing forces (twin screw)
  • Economically obtained
  • Lower molecular degradation than other methods
     
Tek-MixTM Processing Machines 

Low processing temperatures, coupled with low shear rates which are extensionally applied hundreds of times repetitively to create a dynamic shear field, greatly enhancing dispersive and distributive mixing.   Due to large gaps (over 1 mm) and forward pumping capability, operating pressures are low (less than 20 bar).
 

A unique cavity system in a rotor-stator assembly continuously stretches and relaxes the melt, with hundreds of repetitions, at low shear rates, diverging and converging the melt stream to achieve extensional mixing.


Tek-MixTM - Extensional / Elongational Flow Mixing
The Next Step in Mixing & Blending
 

Extensional Flow - Video

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Below is a Video Window which provides an actual enlarged video depiction of the behavior of a polymer blend during the Extensional Flow process.  This video is provided here courtesy of Compuplast International Inc. 



The video shows the elongation of the droplets of the minor component (black) as they progress into a contraction.  The video also shows that all droplets in the entire passageway are thinned and elongated during the extensional flow convergence in response to the extensional stress created by the increasing velocity.   Such action is repeated in Tek-MixTM hundreds of times as the product passes through the mixer.  This compares  favorably to a standard shear process where the material at the wall is subjected to very high shear stresses that are less effective in dispersing droplets or particles.  The Tek-MixTM process is effective at mixing and blending without requiring high energy (heat, pressure, shear) to accomplish high dispersion and distribution, and all of the matrerial, including midstream, is subjected to enlongational shearing without having the material adjacent to the wall being exposed to high stresses.

 



 
 

Technology: Processing Comparisons

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Mixing is a well-studied art, with requirements being made for both dispersive and distributive mixing. Dispersive mixing requires creating regions of high stress in order to reduce agglomerates and particle size. 
 
Conventionally, the high stresses required for particle breakup have been generated by application of high shear rates to the viscous melt. Often stress is not directly proportional to flow rate, but follow more closely a power-law relationship:

  
 
where τ= shear stress, K = consistency index,  = shear rate, n = power law index, and must be set a fixed value, most conveniently 1 s-1.  The purpose of the latter constand is to simplify the dimensions of K.
 
For pseudoplastic melts--the most common-- n can be as low as 0.1. By solving for the shear rate needed to achieve a critical shear stress,
 
 
We can see that a higher shear rate is needed than with Newtonian fluids (n = 1).  For example, if n = 0.1, the shear rate needed to reach a value of 2K will be 210 = 1024 s-1, whereas if n =1, the shear rate is only 2 s-1.  Where this becomes important is in the shear heating. The rate of energy dissipation in the melt, and thus the heat buildup, depends on the shear stress in the following manner:
 
 W/m3


 
Using the example above, the power density with
Whereas with n = 1, the power density is only
 
Thus, while pseudoplastic behavior can be helpful in some processes such as injection molding, it interferes with the realization of the high stresses needed to disperse agglomerates. If an attempt is made to achieve high stresses by increasing shear rate, the main result is shear heating.  The solution to this dilemma may be the introduction of extensional flow components.

 

Comparisons
Tek-Mix
Alternate Add-on Mixer Technologies
Rotor / Barrel clearance, micrometers
1500
 30 to 300
Typical barrel backpressure generated
20 bar
150 bar
Specific energy consumtion
0.2 HP / kg
0.8 HP / kg
Temperature change in mixing section
reduces more than 20°C, melt is cooled
increases more than 10°C, melt is heated
Dispersive and Distributive mixing
both
rarely both
Dispersion of high filler loadings
readily accomplished
difficult due to low clearances, residues
Typical MFI capability (2.16 kg weight)
0.2 to 40
0.7 to 40
Typical shear rates
200 to 500 s-1
1000 to 5000 s-1
Stress repetitions
Several hundreds
Up to one hundred, often only 5 to 10 events
Typical shear stess required to disperse
30,000 to 60,000 Pascals
100,000 to 300,000 Pascals
Mixing action
extensional and shear flow
usually high shear
Create masterbatches
Yes
No
Direct compound without masterbatches
Yes
No
Mixing performance
independent of feeding extruder
dependent on feeding extruder
Carbon nanotubes
improved percolation
no effect
Nanoclays
improved exfoliation
minor effect on exfoliation
Particle size reduction capability colors
demonstrated maximum particle size 8µm on organic pigment tests
demonstrated maximum particle size 60µm on organic pigment tests
Particle size reduction capability nanoclays (size of random agglomerates before filtering)
demonstrated maximum particle size 20µm on nanoclay tests, without MAH
demonstrated maximum particle size 50- 100µm on nanoclay tests, without MAH
Gel reduction capability
yes, by addressing the Mz and MZ+1 moments of the molecular weight distribution
none
Can run without screenpacks and filters
regularly
never



 


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Newsflash

European Conference Provides Warm Reception
Stratek presented its latest test results  featuring Tek-MixTM technology at a conference in Germany recently.  The presentation covered Stratek’s advances in mixing and blending  additives and fillers including carbon nanotubes (CNT’s), nanoclays, and pigments.   One of the main strengths of the Tek-MixTM technology is its ability to exploit the advantages of Elongational Flow for dispersing and distributing particles without causing excessive degradation to the host resins. For additional information on Stratek’s technical abilities, click here.