Stratek Plastic Ltd

Extensional / Elongational Flow Mixing
 

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1.   
 
What exactly is Extensional – Elongational Mixing?
 
 
This is a very well-known characteristic of viscoelastic materials, encountered whenever a viscoelastic polymeric material is hot melted and converged (velocity acceleration) and then diverged (velocity deceleration), creating spiked elongating forces that can break up particles by creating multiple smaller elongated oblates, which then are further reduced by subsequent convergence. The change in velocity creates localized changes in shear rates and stresses which are more efficient at breaking up agglomerates to increase surface area versus conventional sustained high shear alternatives.
2.  
 
 What are the benefits of extensional flow mixing versus conventional mixing?
 
 
Mixing can be accomplished with reduced shear rates, reduced backpressure, reduced energy consumption, reduced temperatures and reduced stresses, typically all of these experiencing a 25% to 75% reduction. Extensional flow stresses do not reduce apparent viscosity the way shear stresses shear thin polymer during processing. Consequently extensional flow mixing can apply stronger hydrodynamic forces to break up and disperse particles.
3. 
 
 Is the extensional mixing concept used commercially today?
 
 
Yes, every orifice encountered in the melt path, twin-screw kneading blocks, Maddock mixing screws, anywhere where melt velocity accelerates and decelerates uses some component of extensional flow. But it’s not used specifically to mix, although most commercial mixers in use today utilize shear stress to disperse but may also apply some extensional flow stress as an indirect consequence of the polymer accelerating and decelerating as it travels through the mixer.
4.   
 
How is Stratek’s system different?
 
 
Stratek’s technology is called Tek-Mix™.  There are three main differences with other devices:
 
a.
Other methods typically use a higher expansion-contraction ratio (defined rheologically as a convergence ratio) with fewer repetitions.
 
b.
Stratek also includes a shearing action that separates and redistributes melts. The action is more gentle but highly repetitive.
 
c.
Stratek operates with:
 
 
·   high clearances (1000 to 1500 µm, versus competitive systems at 15 to 300 µm)
·   low temperatures
·   low backpressure
·   low energy input
 
 
This also allows high load masterbatches to be processed, such as 50% to 70% clay or pigment loading. The combination of these actions results in extremely uniform mixing, at lower temperatures and pressures than alternative systems. These low temperatures and pressures in turn result in a very low energy draw, and that reduces shear rates and shear stresses in the melt. The result is reduced molecular damage, providing improved products and improved physical properties. Stratek uses cavity mixer technology, which has been computer-simulated to arrive at the final configuration, versus other devices generally that rely on orifices or channels.
5.   
 
Where does extensional mixing perform best?
 
 
Extensional Mixing is ideal for:
 
a.   
Handling particle dispersion and particle size reduction where requirements are to break up agglomerates. It lends itself to single- screw compounding, in many cases allowing direct compounding without the need for masterbatching.
 
b. 
Creating excellent dispersion on nanoclays, with full exfoliation, and allowing improved dispersion of carbon nanotubes, from making masterbatches to direct compounding.
 
c. 
In many cases where color pigments are hard to disperse, needing multiple passes to reduce pigment size, extensional mixing can accomplish that in one pass. Tests have shown that 70 µm random particles exiting a twin-screw extruder can be reduced to  10 µm size after passing through a Tek-Mix™mixer.  
 
d.   
Reduction of gels (ie: scissioning of high molecular weight long-chain particles that plug screens and create stress-multipliers in films and parts), tests have shown up to 75% reduction of long-chained molecules that create gels, without affecting the other chains.
 
e.  
Creation of a low temperature addition-treatment zone for temperature sensitive additives (flame retardants), which also aids significantly in reduction of plasticizers and allows higher loading and more efficient use of regrind, and helps enhance blending immiscible polymers.
   
♦
 Extensional Mixing does not replace twin-screw extruders, kneaders, Banbury Mixers or Farrels.  It compliments them and allows more relaxed settings.  Each has a contribution to make to polymer mixing.
6.   
 
What products are not ideally suited to extensional mixing?
 
 
Extensional mixing by nature may cause some chain scission of ultra long-
chained molecules.  These long-chained molecules may need even more 
gentle mixing (batch) if full preservation is required. However most 
thermoplastics and some reactoplastic materials and composites can be
processed.
7.   
 
What equipment is commercially available?
 
a.
Equipment is available from 10 to 4000 lbs/hr, and can be added to any continuous extrusion device, such as single-screw or twin-screw extruders, acting as a barrel extension unit, allowing simplification of the set-up requirements for the single-screw or twin-screw extruder used.
 
 
b.

Equipment to couple directly to injection molding machines is expected to be commercially available in 2010, to provide direct compounding.

 
c.
Present equipment operates on resins that process at ambient temperature up to 320°C, with both alloy steel and anticorrosive materials of construction available.
 8.  
 
 Who manufactures the extensional flow equipment utilizing Stratek technology?
 
 
 Please contact us for the latest information on machine availability.
 
 
 
 9.
 
How flexible is the equipment?
 
 
Stratek has operated prototype models of the Tek-Mix™mixer  for over 7 years and has made every effort to simplify the designs for operation and change-over.   The result is a extremelu flexibile system which is easy to operate.
  a. The same Tek-Mix™mixer can run most products with only a few minor parts being substituted.
  b. The unit is portable and connects quickly to any extrusion device.
  c. Due to the shallow cavities and streamlined flow design, most changeovers can be done in 30 minutes simply by using a special purging compound.
  d. The unit has excellent turndown features to allow throughput flexibility required for many products to match extrusion throughput.
  e.
The 17-4 stainless steel material option and nitrided surface finishing technology allows use with abrasive materials ike glass fibers and talc, or with corrosive materials like PVC. Wear is minimal due to the low shear rates, high clearances, and low temperatures used.
     
 10.
 
What are the economic benefits of using Tek-Mix™ mixers?
 
 
Tek-Mix™ mixers can be added to any compounding or extrusion equipment. The cost benefits are several to the user:
  a. Improvement in certain targeted physical properties
  b. Increased throughputs
  c. Reduction of multi-pass mixing
  d. Commercialization of new products, formulas and additives, with new materials, ability to increase additive loading at reduced temperatures, creating a finer dispersion
  e. Process simplification
 
f.
Waste reduction
 
g.
Reduced material usage to maintain desired physical properties
   
o   Plasticizers, fillers, base matrix material
 
o   Lighter part weights
 11.
 
What is the equipment cost?
    Please contact us for the latest in pricing and availability.
 12. 
 
Who developed Stratek’s technology
   
Stratek is a privately held company innovating in melt flow technology. Stratek’s scientists developed the Tek-Mix™ technology between late 2006 and 2008, patents are pending. Stratek has also worked with several institutes and universities.
 13.
 
 Where is Stratek located?
   
Wallingford, Connecticut, between Hartford and New Haven.