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Brainstorm invites questions
etec@extrudertimes.com
Readers and practitioners may send their
questions to ET. The questions must state
clearly the background of the problem /
issue. Selected questions with answers will
appear in subsequent issues of ET.
P l e a s e s e n d y o u r q u e s t i o n s t o
Q1. Apart from creating new PET bottles &
containers, what are the best three
'high
value end uses' of recycle PET plastics ?
Q2. What would be the ideal 'residence time’
in a Co-rotating Twin-Screw Extruder ? How
does this actually influence the outcome of
the final product?
The best answer will be published. Readers
are
to send-in their answers with
appropriate drawings / pictures / illustrations
to etec@extrudertimes.com
The ET expert panel, would SELECT the most
appropriate answer. ET's decision is final.
The participant to send full name and address
particulars along with their answers.
LAST DATE to receive the entry is
March 15th, 2008.
INVITED
Published by
www.extrudertimes.com
www.steer.in
SCS
STEER CONSULTANCY SERVICES
How to improve the dispersion in a 40% loading of carbon black of grades N220 / N330
with LDPE or LLDPE ?
Mr. Sharad Rathi -
Hyderabad.
Dispersion - the termmeans “breaking up" - as in "disperse" a crowd.
In order to improve dispersion, as in the case of agglomerated organic pigment particles,
the best strategy is to 1) Increase RPM without increasing feed rate 2) Reduce
temperature in all zones after polymer has melted, except in metering zone going down as
low as possible and 3) Increase kneading blocks in screw design.
We always thought that with Carbon black, the problem is not of dispersion but that of
agglomeration due to compression at the time of melting. Kneading is required to wet
every particle of a filler with the polymer. This is very true with CaCO3 and TiO2. The
question is 'Is this correct with respect to Carbon black?" since to avoid dusting, Carbon
Black manufacturers use a sugar solution to pelletise into fixed size particles. However, the
bond strength of these agglomerates are low and the individual grains can be easily
separated. Before kneading or wetting is accomplished, the unwet carbon black
agglomerates bind themselves strongly. This makes the task of mixing 40% Carbon black
into a polymer into a very interesting problem. The primary task is only of wetting or
kneading. Therefore, completely different kneading and melting strategy is called for -
lowering of viscosity, avoiding pressure build-up, enough folding action or back mixing
with high clearances and elongational flows with fractional elements. [PS : At SCS this
subject is under closer scrutiny. In the course of a few weeks, a more conclusive view
would emerge.]
Q: What is the difference between shear, shear rate, and shear heating?
a. SHEAR : is a force that causes two parts of a body to slide past each other in a
parallel direction.
b. SHEAR
RATE : The overall velocity over the cross section of a channel with which
molten polymer layers are gliding along each other or along the wall in
laminar flow.
velocity cm/sec.
Shear Rate =
=
= sec. -1
clearance cm
c. SHEAR
HEATING : is due to friction caused by flow of melt through narrow passages in the
die plate or die hole during filling phase
Response received from : Mr. Prakash Hadimani, Bangalore
Reader’s Queries on Extruders, Elements,
Polymer-Compounding etc are answered
by ET’s Export Group
Answer for ET 4
QUESTION YOU
You can accomplish anything in life, provided that
you do not mind who gets the credit.
1
- Harry S. Truman