The Nanoformulation Toolkit
Dr. Nigel H. Holmes
Senior Project Chemist
MacDermid Autotype Ltd
Wantage
UK
Introduction
• The number of papers that have been produced over the last
decade on the remarkable properties of nanoparticles is huge.
The number of papers providing practical user information is
rather less.
• 1. Is this because the much of the relevant information is buried
within the R & D departments of the users and jealously
guarded? Very possibly true!
• 2. Is this because it is simply not a problem? In the view of the
speaker, no!
• 3. Is it because nanoparticles have not made the jump from
being materials of scientific interest to materials that are
available for industrial use? Yes & no!
• Points 2 & 3 are closely related.
The very big picture
•
When we talk about the potential industrial use of nanoparticles
we are in fact addressing a huge scientific field. A fraction of these are
shown below.
•
Physical strengthening of structures using carbon nanotubes
•
Improving the abrasion resistance of materials using silicon and
aluminium oxide
•
Improving the gas barrier properties of plastics using nanoclays
•
The production of antimicrobial coatings using nanoparticle silver
•
Imparting UV resistance by the addition of oxides of Cerium, Zinc
&Titanium
•
And many, many more.
What you need to consider
Development
Safety
Production
Supply
Cost
Consistency
Do you really need nano?
• Only choose the nanoparticle route if you
believe that the benefits will be measurable
and cost effective.
• Only choose nano if you can identify reliable
suppliers.
• Do not choose the nano-route out of
technological vanity.
Size matters 1
• The magic of nano is defined by size.
• Volume = 4πr3/3 Surface area = 4πr2
• Surface area/Volume = 3/r
• As the radius of a particle decreases then the
relative surface area increases.
• Halving the radius doubles the surface
area/volume ratio.
Size matters 2
• What is the effect of reducing the particle size?
• The result is greater attractive forces between the
particles. There will be a need to prevent
agglomeration.
• The intensity of light scattering is reduced (Rayleigh
Scattering).
• The physical properties of nanoparticles differ
considerably from that of the bulk material. Gold
m.pt. 1064°C (Bulk): Gold m.pt. 750-800°C (5nm
particle).
Size distribution: a warning.
• The volume and therefore the mass of a
100nm particle is 1000x greater than a 10nm
particle.
• Only 1 of the large particles in a mix of 1000
of the smaller particles will contribute 50% of
the total weight.
• Beware of particle size distribution by
weight.
How are my nanoparticles delivered?
• Powder Form
• Dispersion form
• You choose your formulation
(within reason). Very useful for
powder coatings
• You need to formulate with
respect to the dispersion
medium.
• Greater variety of materials
• Fewer dispersions than
powders.
• You will need to disperse and
stabilise the particles.
• The dispersion is already
stabilised.
• Health & safety issues
concerning handling.
• Generally less issues
concerning health & safety, but
care still required.
What is my particle made of?
• d(total) is the hydrodynamic diameter.
d(total)
Core particle
Stabilising shell
d(particle)
More than just the particle
• Without stabilisation nanoparticles will
rapidly agglomerate – not good.
• The stabilising shell plays a role in
determining the bulk properties of the
product.
• We must always consider the particle and
shell as a whole entity and not just the
particle alone.
Refractive index and stabilising shell 1
• The introduction of nanoparticle dispersions
provides a means of modifying the refractive
index of coatings.
• The refractive index values of Al2O3, ZnO &
CeO2 are significantly greater than those of
most organic materials.
• Altering the refractive index of a coating can
alter its optical properties significantly.
The refractive index of mixtures
• There are several empirical models that
can be used to determine the refractive
index of a mixture. The simple volume
fraction model is shown below.
• n(coating)=na(Vol %) + nb(Vol%) + ……
• Where a,b,c, etc. are the components of
the mix.
Refractive index & stabilising
shell 2
• Note: volume % must be used. Most dispersions
quoted as weight % so beware.
•
A ceria nanoparticle dispersion of ~30% by weight
sounds impressive, but this translates to a volume %
of <5%.
• If you use only the refractive index of cerium oxide
you will get the wrong answer! You need to include
the stabilising shell in your calculation.
Effect on refractive index of the
stabilising shell
Weight of Cerium Volume of
oxide
Cerium oxide
100%
100%
Effective
refractive index
1.91
95%
80%
1.80
90%
65%
1.74
80%
45%
1.65
70%
32%
1.59
60%
24%
1.56
Abrasion resistance
• Silica & Alumina are both significantly harder than organic
polymers, therefore significant additions to a formulation
should improve the abrasion resistance of the coating. This has
been shown to be a fact.
•
• If the particles are small enough the clarity of the coating
should not be compromised.
• For ease of formulation it is often more convenient to add
dispersions of nanoparticles directly to the formulation.
• It is possible to combine silica and alumina nanoparticles in UV
curable resins for maximum abrasion resistance.
Defining the network
• Silica nanoparticle
dispersions are readily
available; Alumina a little
less so.
• In some cases the
stabilising shell contains
acrylate groups which
can cross-link with the
resin.
• This generates a dense
cross-linked network.
• The shell surrounding
silica nanoparticles can
be particularly thin.
•
An area of 100nm2 might
contain between 1 -100
stabilising groups.
• Both the nanoparticle and
the cross-linked acrylate
contribute to the overall
abrasion resistance.
The idealised cross-linked network
– Circles represent nanoparticles, rectangles
acrylate molecules
Acrylate photopolymerisation
• Acrylate curing mechanism
Measuring hardness
• Measurements using
different test methods are
not directly comparable.
• The best scientific results
for hardness
measurement are
obtained from
nanoindentation
• Unfortunately most
commercial customers do
not have access to such
equipment.
• Pencil hardness testing is
greatly favoured within
industry and by
customers.
• It’s quick, it’s cheap, it’s
woefully inaccurate!
• A gauge R &R survey of
>1000 pencil hardness
tests by skilled operatives
revealed that the
reproducibility of the test
is zero.
The variability of 2H pencils
Scratch % from 5 pencils: 12x5 individual tests for each pencil
60
Scratch percentage
50
40
Lead 1
Lead 2
30
Lead 3
Lead 4
Lead 5
20
10
0
Number of scratches (0-5 inclusive)
The effect of substrate
• Pencil hardness testing is not
independent of the substrate.
• For the same coating
formulation.
• Metal >2H
• PET 1H
• PC 1B
• The result on flexible
substrates is dependent
on coating thickness and
other factors.
• Thicker coatings give
better results.
• The substrate “bends”
under the pressure of the
test.
Substrates matter
• The effect of substrate on the hallowed pencil
hardness test is a reminder that the substrate can
play a role in product performance.
• For liquid formulations the efficacy of a coating on
polyester film might not be repeated on
polycarbonate.
• Polycarbonate is more solvent sensitive than
polyester; you might end up severely damaging the
substrate if you choose the wrong solvent or
acrylate dispersion.
Volume matters - a reminder
• Most nanoparticle dispersions are
designated as weight percentages.
• A 50% by weight silica nanoparticle
dispersion sound impressive (it is).
• However it’s only about 22% silica by
volume.
• For denser materials the effect is even
greater.
Manufacturing Stability
• You must consider the whole manufacturing
process, not just the formulation in isolation.
• It is perfectly possible for small levels of impurities
in items such as filters to destabilise a formulation
with a consequent loss of production.
• This has been achieved!!
UV stability
• Nanoparticles offer a means of introducing
none fugitive stabilisers to organic coatings.
• The small particle diameter enables the
coating to remain optically clear.
• Good results have been demonstrated with
nanoparticle Cerium Oxide in wood coatings,
Titanium Dioxide & Zinc Oxide in sunscreens
Be Aware!
• Despite its benefits Cerium Oxide can impart a yellow colour to
clear coatings.
• Titanium Dioxide exists in 2 major crystalline forms: Rutile &
Anatase. Both provide UV protection.
•
• The latter is photoactive, the former is not. If you want a selfcleaning coating choose Anatase, but if you want a stable
coating choose Rutile.
• The above comments are a slight simplification as Anatase can
be stabilised.
Zinc Oxide
• Zinc Oxide is not photo-active and
brings no unwanted colour to coatings.
• It is commercially available as a
dispersion from a number of reputable
suppliers.
• It shows a good degree of UV
absorption.
Zinc Oxide 2
• Remember: the nanoparticle shell must also be
considered.
• Different stabilising materials can show markedly
different behaviour with the other ingredients of the
formulation.
• Certain stabilising agents do not have good
compatibility with acrylate oligomers.
• This can lead to agglomeration and loss of
performance.
Conclusion
• Size matters: property enhancement arises out of the increased
surface area.
• Size matters: stability problems arise because of the increased
area.
• Nanoparticles must be stabilised for commercial use.
• If you don’t consider the stabilising shell then you will suffer
problems.
• Possible substrate effects cannot be ignored.
• Despite all of these hurdles it is possible to formulate with
nanoparticles!
Thanks
• Prof. Steve Abbott for many challenging discussions
• My colleagues at MacDermid; especially Andy
Torrens-Burton, Jason Small, Jeremy Gamble and
Lynn Donoghue for their help and patience in the
search for commercially viable nanoparticle
formulations.