new high-strength composite made of "nanoclay" and PVA
Kragen Javier Sitaker
kragen at pobox.com
Fri Oct 5 03:15:17 EDT 2007
I spent three hours reading about this when I should have been either
sleeping in preparation for tomorrow's CaFeConf or reading Craig
Chambers's dissertation. So I thought I'd send out my notes in case
someone else finds them handy.
The Cool New Material
---------------------
Charles Griffiths told me about this October 4 article from Physorg,
"New plastic is strong as steel, transparent":
> http://www.physorg.com/news110727530.html
Apparently, by alternating layers of polyvinyl alcohol and "clay
nanosheets", Nicholas Kotov and a bunch of other people at UMich (many
from his own lab), plus some folks at Northwestern (in some earlier
research; see below) have fabricated an extremely high-strength
composite. It gets its strength from parallel layers of clay
nanosheets glued together with thin layers (monolayers?) of PVA. The
article says:
The [robot] arm dipped the glass [substrate] into the
glue-like polymer solution and then into a liquid that was a
dispersion of clay nanosheets. After those layers dried, the
process repeated. It took 300 layers of each the glue-like
polymer and the clay nanosheets to create a piece of this
material as thick as a piece of plastic wrap.
...
"When you tried to build something you can hold in your arms,
scientists had difficulties transferring the strength of
individual nanosheets or nanotubes to the entire material,"
Kotov said. "We've demonstrated that one can achieve almost
ideal transfer of stress between nanosheets and a polymer
matrix."
The paper is "Ultrastrong and Stiff Layered Polymer Nanocomposites",
in the Oct. 5 issue of Science.
> http://www.sciencemag.org/cgi/content/abstract/318/5847/80
doi:10.1126/science.1143176
Science 5 October 2007: Vol. 318. no. 5847, pp. 80-83.
The authors are Paul Podsiadlo, Amit K. Kaushik, Ellen M. Arruda,
Anthony M. Waas, Bong Sup Shim, Jiadi Xu, Himabindu Nandivada,
Benjamin G. Pumplin, Joerg Lahann, Ayyalusamy Ramamoorthy, and
Nicholas A. Kotov, all of whom are from UMich and five of whom are
from Kotov's lab. The abstract reads:
Nanoscale building blocks are individually exceptionally
strong because they are close to ideal, defect-free
materials. It is, however, difficult to retain the ideal
properties in macroscale composites. Bottom-up assembly of a
clay/polymer nanocomposite allowed for the preparation of a
homogeneous, optically transparent material with planar
orientation of the alumosilicate nanosheets. The stiffness
and tensile strength of these multilayer composites are one
order of magnitude greater than those of analogous
nanocomposites at a processing temperature that is much lower
than those of ceramic or polymer materials with similar
characteristics. A high level of ordering of the nanoscale
building blocks, combined with dense covalent and hydrogen
bonding and stiffening of the polymer chains, leads to highly
effective load transfer between nanosheets and the polymer.
This is interesting because the materials involved are dirt cheap
(literally, in one case) and the process is simple, if slow and
probably expensive.
His lab consists of:
> http://www.engin.umich.edu/dept/che/research/kotov/people.index.htm
Name Email @umich.edu
Ahmet Emrehan Emre emrehan
Angela Antosiewicz amantos
Ashish Agarwal agashish
Bongsup Shim bshim
Edward Jan edjan
Emine Sumeyra Turali esumeyra
G. Daniel Lilly gdlilly
George Carter gmcater
Joong Bahng jbahng
Jungwoo Lee jungwoo
Kevin Critchley kevcritc
Meghan Cuddihy mcuddihy
Nicholas Kotov kotov
Paul Podsiadlo apodsiad
Sudhanshu Srivastava ssrivass
Szushen (Peter) Ho peterho
Thomas Welch tjwii
Vincent Ball vball
Earlier Work I Could Actually Find
----------------------------------
Podsiadlo and Kotov, together with David Paterson of UMich and
Zhongqiang Liu and Phillip B. Messersmith from Northwestern,
published a paper in Advanced Materials earlier this year, "Fusion of
Seashell Nacre and Marine Bioadhesive Analogs: High-Strength
Nanocomposite by Layer-by-Layer Assembly of Clay and
L-3,4-Dihydroxyphenylalanine Polymer":
> http://biomaterials.bme.northwestern.edu/Papers/Clay_Paper_Final%5B1%5D.pdf
doi:10.1002/adma.200602706
Advanced Materials 2007, 19, 949-955
This seems to describe the same technique. Apparently
L-3,4-dihydroxyphenylalanine is more commonly known as DOPA and is an
amino acid found in mussel glue. In this case they used
Na+-montmorillonite for the clay (montmorillonite is a clay that
consists of nanometer-sized sheets).
In this earlier work, they got 100 megapascal ultimate strength and 11
gigapascal Young's modulus. (Higher Young's modulus makes a material
stiffer and consequently more brittle.) For comparison, according to
Wikipedia's "Tensile strength" article, steel typically has yield
stresses in the 200-700 megapascal range and ultimate strengths in the
400-2000 megapascal range; common plastics (HDPE, PP) have yield
stresses in the 10-40 megapascal range and ultimate strengths in the
40-80 megapascal range; carbon fiber's strength is 5650 megapascals,
and Kevlar's yield stress is 3620 megapascals. According to the
"Young's modulus" article, common plastics (LDPE, PP, PET, PS, nylon)
have moduli in the 0.2-7 GPa range, oak's modulus is about 11 GPa,
glass's 72, steel's around 200, and diamond's above 1000.
So the material they ended up with in that experiment was 2-10 times
stronger and 2-10 times stiffer than common plastics, and "comparable
to natural nacre and lamellar bones".
This contrasts with the properties of the nanosheets themselves, with
a Young's modulus, according to the paper, of "250-260 GPa, which is
two orders of magnitude greater than the mechanical properties of most
clay nanocomposites achieved thus far." So in this March paper, they
had gotten to only 1.3 orders of magnitude away, and presumably in the
newer Science paper, they got to within a factor of two or so (0.3
orders of magnitude).
The samples they were testing with in that case were 200 or 300
"bilayers", each bilayer consisting of a layer of clay platelets and a
layer of binder; the 300-bilayer sheets appear to be around 1.2
microns in thickness.
They bought their "Cloisite" brand clay from Southern Clay Products,
then stirred 5 g of clay into a liter of deionized water for a week,
followed by letting it sediment for a day.
Each of the bilayer depositions seems to have taken 16 minutes, partly
due to long periods of washing and drying after each layer.
This work was reviewed in a "Nanowerk" article on 3 April,
"Nanotechnology inspired by mussels and seashells":
> http://www.nanowerk.com/spotlight/spotid=1723.php
I think the "nanotechnology" moniker is fairly misleading.
Raw Materials Pricing
---------------------
You can buy montmorillonite in small quantities in bulk for fairly low
prices, like US$4 per pound.
> http://www.sorbentsystems.com/bulksorbents.html
It sounds like they're specifically using "Cloisite", at least in the
earlier work, which sounds like it might be somewhat expensive because
it seems to be targeted at composite products like these rather than,
say, desiccant:
> http://www.nanoclay.com/benefits.asp
Benefits from Cloisite® technology result in part from the
very high surface area of montmorillonite clay - which is in
excess of 750 m2/gram - and high aspect ratio (about 70 to
150). In dry form, Cloisite® exists in clusters or aggregates
of montmorillonite platelets and very little surface area of
the montmorillonite is exposed, causing very low aspect
ratios. The challenge is to create conditions favorable for
the exposure of all this potential surface area to the
polymer.
And from their "Benefits" page:
> http://www.nanoclay.com/benefits4.asp
Today Southern Clay Products is a leader in supplying
ultra-pure natural montmorillonite and organically modified
montmorillonite to a wide variety of industries.
There was a 2004 article, "Chasing Nanocomposites", in Plastics
Technology:
> http://www.ptonline.com/articles/200411fa2.html
It said about montmorillonite and other "nanoclays":
Polymers reinforced with as little as 2% to 5% of these
particles via melt compounding or in-situ polymerization
exhibit dramatic improvements in thermo-mechanical properties,
barrier properties, and flame retardancy.
...
So far, nanoclays have shown the broadest commercial viability
due to their lower cost --- $2.25 to $3.25/lb --- and their
utility in common thermoplastics like PP, TPO, PET, PE, PS,
and nylon.
The leading nanoclay is montmorillonite, a layered
alumino-silicate whose individual platelets measure around 1
micron diam., giving them an aspect ratio of 1000:1. The two
major domestic producers are Nanocor with its Nanomer line and
Southern Clay Products with its Cloisite line.
The article cites uses such as decreasing gas permeability, increasing
tensile strength, and improving fire retardancy.
Another 2004 article, "Firms lay financial foundation resting on
nanoclay composites", in Small Times, explains a bit more about the
cost structure:
> http://www.smalltimes.com/articles/article_display.cfm?Section=ARCHI&C=Consu&ARTICLE_ID=267740&p=109
The raw nanoclay is cheap -- 40 to 60 cents a pound. But it
needs to be custom-treated to bond with each given type of
polymer, and then dispersed into the resin, two expensive
processes that boost the cost to $3 or $4 a pound.
Polyvinyl alcohol is a major industrial chemical; it supposedly costs
US$1.40 a pound as of 2001:
> http://www.the-innovation-group.com/ChemProfiles/Polyvinyl%20Alcohol.htm
Or you can get it at US$10-US$12 per pound on the web in small
quantities:
> http://www.chemistrystore.com/Polyvinyl_alcohol.htm
Vaguely Related Clay Nanosheet Composites Work
----------------------------------------------
In searching for "clay nanosheets", I found a pending patent
application that sounded similar. I was wondering whether this
pending patent application (priority date 2001, from Chyi-Shan Wang,
Max D. Alexander, Jr., and Chenggang Chen; the authors seem to be
working on high-strength composites at the University of Dayton
Research Institute, and two of them work for the Wright-Patterson Air
Force Base) was likely to restrict the usage of this invention:
> http://www.freepatentsonline.com/20050272847.html
But I think the answer is "no", because at no point during the
layer-by-layer method do the nanosheets and the polymer form a
"substantially homogeneous mixture", which is part of both their
claims 1 and 12.
Another difference is that the 2001-2005 patent application seems to
contemplate low-clay-content composites, with amounts ranging from 1%
to 15%. By contrast, Kotov's layer-by-layer work seems to contemplate
composites similar to nacre, with 95% of the bulk being provided by
the brittle clay, and the other 5% being provided by the binder.
It does seem that this patent, if issued, would cover many of the
existing uses of "nanoclays" profiled in the Plastics Technology
article mentioned above.
Speculation About The New Material
----------------------------------
"Strong as steel" would mean they've managed to increase the ultimate
strength from 100 MPa to 400 MPa or more, probably with a
corresponding increase in stiffness.
To become practical for bulk uses, it needs a more efficient
manufacturing process. The process described in the earlier paper
deposits a 4-nanometer-thick bilayer every 16 minutes, and uses
substantial amounts of deionized water, which must then be repurified.
A hypothetical square-meter layer-by-layer depositing machine would
produce about 15 x 10^-9 cubic meters of material, or 0.015 cubic
centimeters, per hour. It probably would not need a cleanroom
environment, since dust particles would tend to be washed away.
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