Nanoobjects in wood-protective coatings - Christian Lehringer (EMPA)

Nanoobjects in wood-protective coatings
Christian Lehringer, Klaus Richter
Empa, Swiss Federal Laboratories for Materials
Science and Technology

University College of London, 04.11.2010, Christian Lehringer and Klaus Richter

Topics
 Nano – where size does matter
 Wood – a substrate with special characteristics
 Application of engineered nanoobjects (ENOs) in
wood coatings
 Environmental, health and safety aspects
 Conclusions

Nano – where size does matter
"Nano" = 1 to 100 nm
ISO/TS-27687 (2008)

Nanotechnology
Use of nanomaterials
Anorganic
nanostructures
Organic nanostructures
Nanocomposites
Nanoparticles for drug
delivery
...
Manipulation and
assembly
Nanoparticle
manipulation
Nanostructural
architecture
Nanomanufacturing
Nanoelectronics
...
Disciplines of Nanotechnology

Wood – a substrate with special characteristics
 Bio-Composite-Polymer
 Porous structure
 Heterogenous
 Anisotropic
 Combustible
 Hygroscopic
 Biodegradable
 Sensitive to UV-radiation
 Density depends on species
longitudinal

Key areas of wood surface coating
UV-protection
Hydrophobation
Easy-to-clean Antimicrobial
Hardness
Scratch resistance

Some ENOs currently used for wood
coatings and their application area
*with hydrophobic functionalization
Nanoobject
Aluminum
oxide
(Al2O3)
Iron(III)
oxide
(Fe2O3)
Silver
(Ag)
Titan
dioxide
(TiO2)
Zinc
oxide
(ZnO)
Silizium
dioxide
(SiO2)
Hardness x x
Abrasion resistance x x
Scratch resistance x x
UV-Protection x x x
Antimicrobial x x x
Hydrophobation/
Easy-to-clean
x*

UV-protection
Schilliger 2010

UV-protection
 Nanoscaled pigments of TiO2, Fe2O3, ZnO for
transparent systems
 Absorption and scattering of UV light
 Substitute for organic UV-Absorbers (UVA)
 Combination with lignin stabilizers and hindered
amine light stabilizers (HALS)
 Photocatalytic activity of nanoparticles requires
combination with radical interceptors

UV-protection
www.nanobyk.com

Hydrophobation/ Easy-to-clean

Hydrophobation/ Easy-to-clean
 Sol-gel-technology with organofunctional silanes
or (poly)siloxanes
 Effect: hydrophobic characteristic of substance +
formation of hydrophobic nanostructure
 Liquid water protection feasible, water vapor
protection difficult

Wood-inorganic composites by sol-gel
process
 Two stage process
 Hydrolysis (here Silicic Acid Esters)
 Condensation….
Si
OEt
EtO OEt Si
OEt
HO OH
- 2 EtOH
+ 2 H2O
OE Et O t
Tetraethoxy
silane (TEOS)
OEt
etc.
+ H2O
2 HO ..... (SiO2)nSi
OEt
OEt
HO O OH
OEt
OEt
SiSi
OEt
OH
- H2O
Silicate

3-Isocyanatpropyl
triethoxysilane
β-(3,4 epoxycyclohexyl)
ethyl trimethoxysilane
vinyl trimethoxy
silane
Wood-inorganic composites by sol-gel
process
 … in the cell wall

Antimicrobials
Künniger 2010
Mold Blue stain Algae

Antimicrobials
 Leach-resistance of Ag; TiO2; ZnO for long-term
effect
 Nanoobjects should be released systematically
and preferably continuously in small amounts
from an actively biocidal coating
 Function possibly through ingestion, as contact
poison, by photo-oxidative mechanism or
binding to microbial DNA

Antimicrobials
Hochmannova 2010

Surface hardness/ scratch resistance
 Silicones and nanosized alumina particles (Al2O3;
SiO2) as additive
 Surface modification of nanoparticles by
trialkoxysilanes to improve the dispersibility of in
acrylate media
 In parquet industry, however, until now a broad
application has not been realized

TEM picture of polyacrylate filled with
30 wt.-% nanosized silica.
Bauer 2005

www.nanobyk.com

Challenges and future research
 Homogenous dispersion of nanoobjects in
embedding matrix – avoid agglomeration
 Long-term studies about leaching and
mitigation
 Balance between leach-resistance and
systematic release of nanoobjects
 Polymerization in the wood cell wall and
covalent bonding

Challenges and future research
 Avoid crack formation, yellowing,
depolymerization, improve gloss retention
 Commercial application of sol-gel-
technologies for wood impregnation
 Reactivity of nanoobjects (e.g. Ag  Ag2S)
 Renovation cycles (sanding, coating
removal, new coating)

Design for minimal exposition of engineered
nanoobjects (ENOs)
“Factors of stability”
Stability of ENO integration into coating system
by trend higher by trend lower
Location of ENOs in façade
coating system
in the bottom layers at the surface
Binding type between ENOs
and coating matrix
covalent not covalent
Property of ENOs in organic
coating matrix
not photocatalytic photocatalytic
or mineral coating matrix
wettability high wettability low
Property of façade coating
system
resistance against exterior influ-
ences (e.g. moisture, wind-
abrasion, temperature changes)
low resistance against exte-
rior influences
Som 2010
Stability of ENO integration into coating system
by trend higher by trend lower
in the bottom layers at the surface
covalent not covalent
not photocatalytic photocatalytic
wettability high wettability low
low resistance against exte-
rior influences
“Factors of stability”
Stability of ENO integrat
by trend higher
Location of ENOs in façade
coating system
in the bottom layers
Binding type between ENOs
and coating matrix
covalent
coating matrix
not photocatalytic
or mineral coating matrix
wettability high
Property of façade coating
system

Life cycle of nanocoatings

Estimated influences of ENOs on the environment,
modified after Som et al. (2010)
ENVIRONMENT
Ag
c)
ZnO
c)
TiO2
b)
SiO2
a)
Al2O3
a)
Indication for hazardous effects (with realistic con-
centrations)
+ + + -- --
Solution in water increases the toxic effects (+), re-
duces toxic effects (-)
++ ++ 0 -- ++
Tendency for agglomeration and sedimentation (-) or
no sedimentation (+)
- - -- -/+ --
Waste water facility releases ENO into waters (+),
does not release ENO into waters (-)
- n.i. - - -
Stable during waste incineration (+), burns during
waste incineration (-)
+ + ++ ++ ++
legend: + applies;  weak indices available; - does not apply; n.i, not investigated (high
degree of uncertainty)
The indices represent the overall evaluation of the ENOs: a) rather harmless; b) big uncertainty due to lack of data; c) biological
effect traceable, effect on environment to be expected. These estimations do not represent the effects of nanoobjects that were
generated by unintended actions (e.g. traffic)
*: mostly dependent from contaminants in the samples (transition metals such as Iron, Nickel, Cobalt etc.)
#: Aluminum oxide hydroxide (AlOOH) in the lung was investigated.

Estimated influences of ENOs on the health on basis of
different biological studies, modified after Som et al. (2010)
HEALTH
Ag
a)
ZnO
c)
TiO2
a)
SiO2
a)
amorph
Al2O3
#
b)
Chronic toxicity (long term effects to be expected,
PNEC, PEC), threshold values known
    n.i.
Acute toxicity     
Impairment of DNA     n.i.
Brain damage: damage of the central nervous sys-
tem
n.i. n.i. n.i. n.i. n.i.
Crossing and damaging tissue barriers (e.g. blood-
brain barrier, placenta, lung)
n.i. n.i.   #
Skin     n.i.
Gastrointestinal tract     n.i.
Lung     
legend: + applies;  weak indices available; - does not apply; n.i, not investigated (high
degree of uncertainty)
The indices represent the overall evaluation of the ENOs: a) rather harmless; b) big uncertainty due to lack of data; c) biological
effect traceable, effect on environment to be expected. These estimations do not represent the effects of nanoobjects that were
generated by unintended actions (e.g. traffic)
*: mostly dependent from contaminants in the samples (transition metals such as Iron, Nickel, Cobalt etc.)
#: Aluminum oxide hydroxide (AlOOH) in the lung was investigated.

The "Collingridge dilemma“ (Collingridge 1980)
Decision making in uncertainity
Early phase of development
Influence on
innovations
Technical "log-in effects"
Costs for
corrections
Socio-economic “lock-in” effects
Time and
knowledge
Late phase of developmentEarly phase of development
Influence on
innovations
Costs for
corrections
Time and
knowledge
Late phase of development

Summary
 Wood coating systems with considerable market
relevance
 Thorough risk assessment, life-cycle-analysis,
material characterization, standardized
metrology required
 Public acceptance strongly depends on
transparent security systems, consistent labeling
of nanoproducts and honest communication

Thank you very much for your attention

Nanoobjects in wood-protective coatings - Christian Lehringer (EMPA)

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